Plastics Technology Handbook Fifth Edition

Manas Chanda

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2018 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed on acid-free paper International Standard Book Number-13: 978-1-4987-8621-8 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged, please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright .com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Names: Chanda, Manas, 1940- author. Title: Plastics technology handbook / Manas Chanda. Description: Fifth edition. | Boca Raton, FL : CRC Press, Taylor & Francis, 2018. | Series: Plastics engineering series | Includes bibliographical references. Identifiers: LCCN 2017035500| ISBN 9781498786218 (hardback : acid-free paper) | ISBN 9781315155876 (ebook) Subjects: LCSH: Plastics. Classification: LCC TA455.P5 C46 2018 | DDC 668.4–dc23 LC record available at https://lccn.loc.gov/2017035500 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Dedicated to the hallowed memory of my father and mentor Narayan Chanda (1911–2005), a renowned author and a decorated teacher, who showed by example the value of dedication and perseverance in academic pursuits.

http://taylorandfrancis.com

Contents Preface ............................................................................................................................................. xxvii Author .............................................................................................................................................. xxxi

Characteristics of Polymers and Polymerization Processes 1.1 1.2

1.3

1.4

1.5 1.6 1.7 1.8 1.9

What Is a Polymer?.................................................................................................................................... 1 Molecular Weight of Polymers................................................................................................................ 3 n ) .............................................................................. 3 1.2.1 Number-Average Molecular Weight (M 1.2.1.1 End-Group Analysis .....................................................................................................3 1.2.1.2 Ebulliometry (Boiling-Point Elevation)....................................................................4 1.2.1.3 Cryoscopy (Freezing-Point Depression) ..................................................................4 1.2.1.4 Membrane Osmometry ................................................................................................4 1.2.1.5 Vapor-Phase Osmometry ............................................................................................5 w ) ............................................................................... 7 1.2.2 Weight-Average Molecular Weight (M 1.2.2.1 Light-Scattering Method ..............................................................................................7 1.2.2.2 Low-Angle Laser Light Scattering (LALLS) ............................................................9 v )............................................................................. 9 1.2.3 Viscosity-Average Molecular Weight (M 1.2.3.1 Dilute Solution Viscometry...................................................................................... 10 1.2.4 Polydispersity Index ...................................................................................................................12 1.2.4.1 Gel Permeation Chromatography........................................................................... 14 Polymerization Reactions........................................................................................................................ 20 1.3.1 Addition or Chain Polymerization .........................................................................................20 1.3.2 Coordination Addition Polymerization .................................................................................24 1.3.3 Step Polymerization....................................................................................................................28 1.3.4 Supramolecular Polymerization ...............................................................................................34 1.3.5 Copolymerization ........................................................................................................................35 Polymerization Processes ........................................................................................................................ 37 1.4.1 Process Characteristics...............................................................................................................37 1.4.1.1 Bulk, Solution, and Suspension Polymerization.................................................. 38 1.4.1.2 Emulsion Polymerization.......................................................................................... 39 1.4.1.3 Microemulsion Polymerization ............................................................................... 43 1.4.2 Industrial Polymerization..........................................................................................................49 1.4.2.1 Heat Removal .............................................................................................................. 49 1.4.2.2 Reactor Agitation........................................................................................................ 50 1.4.2.3 Residence Time ........................................................................................................... 50 1.4.2.4 Industrial Reactors ..................................................................................................... 51 Conﬁgurations of Polymer Molecules.................................................................................................. 58 Conformations of a Polymer Molecule................................................................................................ 58 Polymer Crystallinity ............................................................................................................................... 60 1.7.1 Determinants of Polymer Crystallinity ..................................................................................60 The Amorphous State.............................................................................................................................. 62 Structural Shape of Polymer Molecules............................................................................................... 63

Contents

1.10 Thermal Transitions in Polymers ......................................................................................................... 65 1.10.1 Tg and Tm ....................................................................................................................................65 1.10.2 Regions of Viscoelastic Behavior ...........................................................................................68 1.10.3 Factors Affecting Tg ..................................................................................................................70 1.10.3.1 Chain Flexibility ...................................................................................................... 70 1.10.3.2 Steric Effects.............................................................................................................. 71 1.10.3.3 Conﬁgurational Effects ........................................................................................... 71 1.10.3.4 Effect of Cross-Linking .......................................................................................... 71 1.10.4 Factors Affecting Tm .................................................................................................................71 1.10.4.1 Symmetry .................................................................................................................. 72 1.10.4.2 Intermolecular Bonding......................................................................................... 72 1.10.4.3 Tacticity ..................................................................................................................... 73 1.10.4.4 Branching, Chain Flexibility, and Molecular Weight ..................................... 74 1.10.5 Relation between Tm and Tg ...................................................................................................74 1.11 Designing a Polymer Structure for Improved Properties................................................................ 74 1.12 Cross-Linking of Polymer Chains......................................................................................................... 76 1.12.1 Reactions of Functional Groups.............................................................................................76 1.12.2 Vulcanization..............................................................................................................................78 1.12.3 Radiation Cross-Linking ..........................................................................................................83 1.12.4 Photochemical Cross-Linking.................................................................................................84 1.12.5 Ionic Cross-Linking ..................................................................................................................85 1.13 Solubility Behavior of Polymers ............................................................................................................ 86 1.13.1 Solubility Parameter..................................................................................................................86 1.14 Effects of Corrosives on Polymers ........................................................................................................ 94 1.15 Thermal Stability and Flame Retardation ........................................................................................... 94 1.15.1 Thermal Degradation ...............................................................................................................98 1.15.2 Ablation .....................................................................................................................................102 1.15.3 Flame Retardation ...................................................................................................................103 1.16 Deterioration of Polymers .................................................................................................................... 103 1.16.1 Chemical Deterioration..........................................................................................................104 1.16.2 Degradation by Radiation......................................................................................................106 1.16.3 Microbiological Deterioration...............................................................................................106 1.17 Stabilization of Polymers ...................................................................................................................... 107 1.17.1 Antioxidants and Related Compounds ..............................................................................109 1.17.2 Chemical Structures of Antioxidants ..................................................................................110 1.17.3 Stabilization of Selected Polymers .......................................................................................110 1.17.3.1 Polypropylene.........................................................................................................112 1.17.3.2 Polyethylene............................................................................................................114 1.17.3.3 Polystyrene..............................................................................................................115 1.17.3.4 Acrylonitrile–Butadiene–Styrene Copolymers................................................115 1.17.3.5 Polycarbonate .........................................................................................................115 1.17.3.6 Nylons......................................................................................................................116 1.17.3.7 Thermoplastic Elastomers ...................................................................................116 1.17.3.8 Polyacetal.................................................................................................................117 1.17.3.9 Poly(Vinyl Chloride) ............................................................................................117 1.17.3.10 Rubber......................................................................................................................118 1.18 Metal Deactivators.................................................................................................................................. 119

Contents

1.19 Light Stabilizers ....................................................................................................................................... 120 1.19.1 Light Stabilizer Classes ...........................................................................................................121 1.19.1.1 UV Absorbers.........................................................................................................122 1.19.1.2 Quenchers ...............................................................................................................123 1.19.1.3 Hydroperoxide Decomposers .............................................................................124 1.19.1.4 Free-Radical Scavengers.......................................................................................124 1.20 Light Stabilizers for Selected Plastics ................................................................................................. 128 1.20.1 Polypropylene...........................................................................................................................128 1.20.2 Polyethylene..............................................................................................................................128 1.20.3 Styrenic Polymers ....................................................................................................................129 1.20.4 Poly(Vinyl Chloride) ..............................................................................................................130 1.20.5 Polycarbonate ...........................................................................................................................130 1.20.6 Polyacrylates .............................................................................................................................131 1.20.7 Polyacetal...................................................................................................................................131 1.20.8 Polyurethanes ...........................................................................................................................131 1.20.9 Polyamides ................................................................................................................................131 1.21 Diffusion and Permeability................................................................................................................... 131 1.21.1 Diffusion ....................................................................................................................................132 1.21.2 Permeability ..............................................................................................................................132 1.22 Polymer Compounding ......................................................................................................................... 133 1.22.1 Fillers ..........................................................................................................................................135 1.23 Plasticizers ................................................................................................................................................ 137 1.23.1 Phthalic Acid Esters................................................................................................................139 1.23.2 Phosphoric Acid Esters ..........................................................................................................140 1.23.3 Fatty Acid Esters......................................................................................................................140 1.23.4 Polymeric Plasticizers .............................................................................................................141 1.23.5 Miscellaneous Plasticizers......................................................................................................142 1.24 Antistatic Agents..................................................................................................................................... 142 1.24.1 External Antistatic Agents.....................................................................................................143 1.24.2 Internal Antistatic Agents .....................................................................................................144 1.24.3 Chemical Composition of Antistatic Agents.....................................................................144 1.24.3.1 Antistatic Agents Containing Nitrogen............................................................144 1.24.3.2 Antistatic Agents Containing Phosphorus ......................................................146 1.24.3.3 Antistatic Agents Containing Sulfur.................................................................146 1.24.3.4 Betaine-Type Antistatic Agents..........................................................................147 1.24.3.5 Nonionic Antistatic Agents.................................................................................147 1.25 Flame Retardants .................................................................................................................................... 147 1.25.1 Halogen Compounds..............................................................................................................148 1.25.2 Phosphorus Compounds .......................................................................................................149 1.25.3 Halogen–Antimony Synergetic Mixtures...........................................................................149 1.25.4 Intumescent Flame Retardants.............................................................................................150 1.26 Smoke Suppressants ............................................................................................................................... 151 1.27 Colorants .................................................................................................................................................. 151 1.28 Antimicrobials ......................................................................................................................................... 152 1.29 Toxicity of Plastics.................................................................................................................................. 152 1.29.1 Plastic Devices in Pharmacy and Medicine.......................................................................153 1.29.1.1 Packing ....................................................................................................................153 1.29.1.2 Tubings and Blood Bag Assemblies..................................................................153 1.29.1.3 Implants...................................................................................................................154 1.29.1.4 Adhesives.................................................................................................................154

xii

Contents

1.29.1.5 Dental Materials ....................................................................................................154 1.29.1.6 Nanomedicines and Drug Delivery...................................................................155 1.29.2 Biodegradable Plastics and Bioplastics ...............................................................................155 1.29.3 Oxo-Biodegradable Plastics ...................................................................................................156 1.29.4 Toxicity of Plastic Combustion Products ..........................................................................157 1.29.5 Toxicity Testing .......................................................................................................................157 References........................................................................................................................................................... 157

Fabrication Processes 2.1 2.2

2.3

2.4

2.5

2.6 2.7

Types of Processes ..................................................................................................................................161 Tooling for Plastics Processing............................................................................................................161 2.2.1 Types of Molds.........................................................................................................................162 2.2.2 Types of Dies ............................................................................................................................162 2.2.3 Tool Design ...............................................................................................................................163 Compression Molding ..........................................................................................................................164 2.3.1 Open Flash ...............................................................................................................................165 2.3.2 Fully Positive ............................................................................................................................165 2.3.3 Semipositive .............................................................................................................................165 2.3.4 Process Applicability ...............................................................................................................165 Transfer Molding ...................................................................................................................................166 2.4.1 Ejection of Molding.................................................................................................................168 2.4.2 Heating System.........................................................................................................................168 2.4.3 Types of Presses .......................................................................................................................169 2.4.4 Preheating ..................................................................................................................................169 2.4.5 Preforming.................................................................................................................................170 2.4.6 Flash Removal...........................................................................................................................170 Injection Molding of Thermoplastics.................................................................................................170 2.5.1 Types of Injection Units.........................................................................................................171 2.5.2 Clamping Units ........................................................................................................................172 2.5.3 Molds ..........................................................................................................................................173 2.5.3.1 Mold Designs............................................................................................................173 2.5.3.2 Number of Mold Cavities......................................................................................175 2.5.3.3 Runners......................................................................................................................175 2.5.3.4 Gating.........................................................................................................................176 2.5.3.5 Valve Gates ...............................................................................................................178 2.5.3.6 Venting ......................................................................................................................179 2.5.3.7 Parting Line ..............................................................................................................179 2.5.3.8 Cooling.......................................................................................................................179 2.5.3.9 Ejection ......................................................................................................................180 2.5.3.10 Standard Mold Bases ..............................................................................................180 2.5.4 Structural Foam Injection Molding .....................................................................................180 2.5.5 Co-Injection (Sandwich) Molding .......................................................................................180 2.5.6 Gas-Assisted Injection Molding ...........................................................................................181 Injection Molding of Thermosetting Resins.....................................................................................182 2.6.1 Screw-Injection Molding of Thermosetting Resins..........................................................182 Extrusion...................................................................................................................................................185 2.7.1 Extruder Capacity ....................................................................................................................186 2.7.2 Extruder Design and Operation ...........................................................................................186 2.7.2.1 Typical Screw Construction ..................................................................................186 2.7.2.2 Screw Zones ..............................................................................................................187

Contents

2.8

2.9 2.10

2.11 2.12

2.13

2.14

xiii

2.7.2.3 Motor Drive............................................................................................................ 187 2.7.2.4 Heating..................................................................................................................... 187 2.7.2.5 Screw Design .......................................................................................................... 188 2.7.3 Multiple-Screw Extruders..................................................................................................... 189 2.7.4 Blown-Film Extrusion........................................................................................................... 190 2.7.5 Flat Film or Sheet Extrusion ............................................................................................... 192 2.7.6 Pipe or Tube Extrusion ........................................................................................................ 194 2.7.7 Wire and Cable Coverings................................................................................................... 195 2.7.8 Extrusion Coating .................................................................................................................. 195 2.7.9 Proﬁle Extrusion..................................................................................................................... 196 Blow Molding .........................................................................................................................................196 2.8.1 Extrusion Blow Molding ...................................................................................................... 197 2.8.2 Injection Blow Molding........................................................................................................ 198 2.8.3 Blow Molds.............................................................................................................................. 198 Calendering .............................................................................................................................................199 Spinning of Fibers .................................................................................................................................200 2.10.1 Melt Spinning.........................................................................................................................202 2.10.2 Dry Spinning ..........................................................................................................................202 2.10.3 Wet Spinning..........................................................................................................................202 2.10.4 Cold Drawing of Fibers........................................................................................................204 Electrospinning of Polymer Nanoﬁbers...........................................................................................204 Thermoforming .....................................................................................................................................208 2.12.1 Vacuum Forming ..................................................................................................................208 2.12.2 Pressure Forming...................................................................................................................209 2.12.3 Mechanical Forming .............................................................................................................210 Casting Processes ..................................................................................................................................210 2.13.1 Simple Casting........................................................................................................................210 2.13.2 Plastisol Casting.....................................................................................................................210 2.13.2.1 Dip Casting.......................................................................................................... 212 2.13.2.2 Slush Casting ....................................................................................................... 212 2.13.2.3 Rotational Casting.............................................................................................. 213 Reinforcing Processes...........................................................................................................................213 2.14.1 Molding Methods .................................................................................................................. 214 2.14.1.1 Hand Lay-Up or Contact Molding ................................................................ 214 2.14.1.2 Spray-Up .............................................................................................................. 215 2.14.1.3 Matched Metal Molding ................................................................................... 215 2.14.1.4 Vacuum-Bag Molding ....................................................................................... 216 2.14.1.5 Pressure-Bag Molding ....................................................................................... 216 2.14.1.6 Filament Winding .............................................................................................. 217 2.14.1.7 Pultrusion............................................................................................................. 218 2.14.1.8 Prepreg Molding................................................................................................. 220 2.14.2 Fibrous Reinforcements .......................................................................................................223 2.14.2.1 Glass Fibers.......................................................................................................... 223 2.14.2.2 Graphite/Carbon Fibers, the Beginning ........................................................ 224 2.14.2.3 Manufacture of Graphite (Carbon) Fibers ................................................... 225 2.14.2.4 Graphite/Carbon Fibers and Fabrics ............................................................. 227 2.14.2.5 Graphite/Carbon Fiber-Reinforced Plastics ................................................. 229 2.14.2.6 Manufacture of CFRP Parts............................................................................. 230 2.14.2.7 Applications of CFRP Products ...................................................................... 231 2.14.2.8 Aramid Fibers ..................................................................................................... 234

xiv

2.15

2.16 2.17 2.18

2.19 2.20

2.21

Contents

2.14.2.9 Applications.........................................................................................................235 2.14.2.10 Extended-Chain Polyethylene Fibers .............................................................235 Reaction Injection Molding.................................................................................................................. 237 2.15.1 Machinery ...............................................................................................................................238 2.15.2 Polyurethanes.........................................................................................................................239 2.15.3 Nylons......................................................................................................................................239 Structural Reaction Injection Molding .............................................................................................. 240 2.16.1 Applications............................................................................................................................241 Resin Transfer Molding ........................................................................................................................ 241 Foaming Processes.................................................................................................................................. 242 2.18.1 Rigid Foam Blowing Agents ...............................................................................................244 2.18.2 Polystyrene Foams ................................................................................................................244 2.18.2.1 Extruded Polystyrene Foam.............................................................................244 2.18.2.2 Expandable Polystyrene ....................................................................................245 2.18.2.3 Structural Foams.................................................................................................246 2.18.3 Polyoleﬁn Foams ...................................................................................................................246 2.18.4 Polyurethane Foams .............................................................................................................251 2.18.4.1 Flexible Polyurethane Foams...........................................................................251 2.18.4.2 Rigid and Semirigid Foams..............................................................................254 2.18.5 Foamed Rubber .....................................................................................................................255 2.18.6 Epoxy Resins ..........................................................................................................................255 2.18.7 Urea-Formaldehyde Foams.................................................................................................256 2.18.8 Silicone Foams .......................................................................................................................257 2.18.9 Phenolic Foams .....................................................................................................................258 2.18.10 Poly(Vinyl Chloride) Foams ..............................................................................................258 2.18.11 Special Foams ........................................................................................................................261 Rapid Prototyping/3D Printing ........................................................................................................... 263 Rubber Compounding and Processing Technology ....................................................................... 265 2.20.1 Compounding Ingredients ..................................................................................................265 2.20.1.1 Processing Aids.................................................................................................. 268 2.20.1.2 Fillers.................................................................................................................... 269 2.20.2 Mastication and Mixing ......................................................................................................270 2.20.2.1 Open Mill............................................................................................................ 270 2.20.2.2 Internal Batch Mixers....................................................................................... 271 2.20.3 Reclaimed Rubber .................................................................................................................272 2.20.4 Some Major Rubber Products............................................................................................274 2.20.4.1 Tires.......................................................................................................................274 2.20.4.2 Belting and Hoses...............................................................................................276 2.20.4.3 Cellular Rubber Products .................................................................................278 Miscellaneous Processing Techniques................................................................................................ 278 2.21.1 Coating Processes..................................................................................................................278 2.21.1.1 Fluidized Bed Coating .......................................................................................279 2.21.1.2 Spray Coating......................................................................................................280 2.21.1.3 Electrostatic Spraying ........................................................................................280 2.21.1.4 Smart Coatings....................................................................................................281 2.21.1.5 Electrografted Coatings.....................................................................................285 2.21.2 Powder Molding of Thermoplastics .................................................................................287 2.21.2.1 Static (Sinter) Molding......................................................................................287 2.21.2.2 Rotational Molding ............................................................................................287 2.21.2.3 Centrifugal Casting ............................................................................................288

Contents

2.21.3

Adhesive Bonding of Plastics............................................................................................288 2.21.3.1 Solvent Cementing ............................................................................................ 289 2.21.3.2 Adhesive Bonding ............................................................................................. 289 2.21.3.3 Joining of Speciﬁc Plastics............................................................................... 292 2.21.4 Welding..................................................................................................................................294 2.21.4.1 Hot-Gas Welding .............................................................................................. 295 2.21.4.2 Fusion Welding.................................................................................................. 295 2.21.4.3 Friction Welding................................................................................................ 295 2.21.4.4 High-Frequency Welding ................................................................................ 295 2.21.4.5 Ultrasonic Welding........................................................................................... 296 2.21.5 Joining Polymer–Metal Hybrids ......................................................................................296 2.22 Decoration of Plastics.........................................................................................................................299 2.22.1 Painting..................................................................................................................................299 2.22.2 Printing ..................................................................................................................................300 2.22.2.1 Gravure Printing................................................................................................ 300 2.22.2.2 Flexography ........................................................................................................ 300 2.22.2.3 Screen Process Printing.................................................................................... 300 2.22.2.4 Pad Printing........................................................................................................ 301 2.22.2.5 Flex Printing ....................................................................................................... 301 2.22.3 Hot Stamping .......................................................................................................................301 2.22.4 In-Mold Decorating ............................................................................................................302 2.22.5 Embossing .............................................................................................................................302 2.22.6 Electroplating........................................................................................................................303 2.22.7 Vacuum Metallizing............................................................................................................303 References........................................................................................................................................................... 304

Plastics Properties and Testing 3.1 3.2

Introduction.............................................................................................................................................307 Mechanical Properties ..........................................................................................................................307 3.2.1 Stress and Strain .......................................................................................................................308 3.2.2 Stress–Strain Behavior.............................................................................................................310 3.2.3 Viscoelastic Behavior of Plastics ...........................................................................................313 3.2.3.1 Modulus and Compliance .....................................................................................313 3.2.4 Stress–Strain–Time Behavior.................................................................................................314 3.2.4.1 The WLF Equations................................................................................................316 3.2.5 Creep Behavior .........................................................................................................................317 3.2.6 Maxwell Model .........................................................................................................................318 3.2.6.1 Stress–Strain Relation .............................................................................................318 3.2.6.2 Generalized Maxwell Model .................................................................................320 3.2.7 Kelvin or Voigt Model ............................................................................................................322 3.2.7.1 Stress–Strain Relation .............................................................................................322 3.2.8 Four-Element Model ...............................................................................................................324 3.2.9 Zener Model ..............................................................................................................................325 3.2.10 Superposition Principle ...........................................................................................................326 3.2.11 Isometric and Isochronous Curves.......................................................................................327 3.2.12 Pseudoelastic Design Method................................................................................................328 3.2.13 Effect of Temperature..............................................................................................................330 3.2.14 Time–Temperature Superposition........................................................................................331 3.2.15 Dynamic Mechanical Properties ...........................................................................................333

xvi

Contents

3.2.16

3.3

3.4

Rheological Behavior ..............................................................................................................334 3.2.16.1 Classiﬁcation of Fluid Behavior .........................................................................335 3.2.16.2 Effect of Shear Rate on Viscosity.......................................................................338 3.2.16.3 Effect of Molecular Weight on Viscosity .........................................................338 3.2.16.4 Effect of Temperature on Polymer Viscosity..................................................339 3.2.16.5 Effect of Pressure on Viscosity ...........................................................................340 3.2.16.6 Weissenberg Effects...............................................................................................340 3.2.16.7 Irregular Flow or Melt Fracture.........................................................................340 3.2.17 Measurement of Viscosity .....................................................................................................341 3.2.17.1 Rotational Viscometers ........................................................................................342 3.2.17.2 Capillary Rheometers ...........................................................................................343 3.2.18 Plastics Fractures .....................................................................................................................344 3.2.19 Impact Behavior of Plastics...................................................................................................345 3.2.20 Fatigue of Plastics ....................................................................................................................348 3.2.21 Hardness ....................................................................................................................................351 3.2.22 Indentation Hardness .............................................................................................................351 3.2.22.1 Brinell Hardness Number ...................................................................................351 3.2.22.2 Vickers Hardness Number..................................................................................351 3.2.22.3 Knoop Hardness Number ...................................................................................352 3.2.22.4 Rockwell Hardness Number ...............................................................................352 3.2.22.5 Barcol Hardness.....................................................................................................352 3.2.22.6 Durometer Hardness ............................................................................................353 3.2.23 Rebound Hardness ..................................................................................................................354 3.2.24 Scratch Hardness .....................................................................................................................355 3.2.25 Stress Corrosion Cracking of Polymers .............................................................................356 Reinforced Plastics..................................................................................................................................356 3.3.1 Types of Reinforcement..........................................................................................................356 3.3.2 Types of Matrix ........................................................................................................................357 3.3.3 Analysis of Reinforced Plastics .............................................................................................357 3.3.3.1 Continuous Fibers ...................................................................................................357 3.3.3.2 Discontinuous Fibers ..............................................................................................360 3.3.3.3 Fiber Length Less than lc .......................................................................................362 3.3.3.4 Fiber Length Equal to lc ........................................................................................363 3.3.3.5 Fiber Length Greater than lc ................................................................................363 3.3.4 Deformation Behavior of Fiber-Reinforced Plastic ..........................................................364 3.3.5 Fracture of Fiber-Reinforced Plastics ..................................................................................365 3.3.5.1 Tension ......................................................................................................................365 3.3.5.2 Compression.............................................................................................................365 3.3.5.3 Flexure or Shear.......................................................................................................366 3.3.6 Fatigue Behavior of Reinforced Plastics..............................................................................366 3.3.7 Impact Behavior of Reinforced Plastics ..............................................................................366 Electrical Properties................................................................................................................................366 3.4.1 Dielectric Strength ...................................................................................................................367 3.4.2 Insulation Resistance...............................................................................................................368 3.4.3 Arc Resistance...........................................................................................................................369 3.4.4 Dielectric Constant ..................................................................................................................370 3.4.4.1 Polarization and Dipole Moment ........................................................................372 3.4.4.2 Dielectric Constant versus Frequency ................................................................374 3.4.4.3 Dielectric Constant versus Temperature............................................................374

xvii

Contents

3.4.4.4 Dielectric Losses.......................................................................................................375 3.4.4.5 Dielectric Losses of Polar Polymers ....................................................................376 3.5 Optical Properties ...................................................................................................................................377 3.5.1 Optical Clarity ..........................................................................................................................377 3.5.2 Index of Refraction ..................................................................................................................378 3.5.3 Piped Lighting Effect ...............................................................................................................380 3.5.4 Stress-Optical Characteristics ................................................................................................380 3.6 Thermal Properties ................................................................................................................................381 3.6.1 Specific Heat..............................................................................................................................381 3.6.2 Thermal Expansion .................................................................................................................382 3.6.3 Thermal Conductivity.............................................................................................................382 3.6.4 Transition Temperatures and Temperature Limitations ................................................383 3.6.5 Standard Properties of Plastics .............................................................................................384 3.7 Identification of Common Plastics .....................................................................................................384 3.7.1 Behaviors on Heating and Ignition......................................................................................394 3.7.2 Tests for Characteristic Elements.........................................................................................395 3.7.3 Specific Tests .............................................................................................................................397 3.8 Plastics Analysis by Instrumental Methods ......................................................................................402 3.8.1 IR Spectroscopy ........................................................................................................................402 3.8.1.1 Methods of Measurement......................................................................................403 3.8.1.2 Instruments ...............................................................................................................404 3.8.1.3 Sample Preparation .................................................................................................404 3.8.1.4 Fourier Transform IR Spectroscopy....................................................................406 3.8.1.5 Qualitative Analysis ................................................................................................407 3.8.1.6 Quantitative Analysis..............................................................................................415 3.8.2 NMR Spectroscopy ..................................................................................................................417 3.8.2.1 General Principles ...................................................................................................418 3.8.2.2 Chemical Shift ..........................................................................................................421 3.8.2.3 Shielding Mechanisms............................................................................................423 3.8.2.4 Spin–Spin Coupling ................................................................................................426 3.8.2.5 Applications in Polymer Analysis .......................................................................426 References........................................................................................................................................................... 430

Industrial Polymers 4.1 4.2

xviii

Contents

4.2.2

4.3

Oleﬁn Copolymers...................................................................................................................464 4.2.2.1 Styrene–Butadiene Rubber ....................................................................................464 4.2.2.2 Nitrile Rubber...........................................................................................................465 4.2.2.3 Ethylene–Propylene Elastomer.............................................................................465 4.2.2.4 Butyl Rubber.............................................................................................................467 4.2.2.5 Thermoplastic Elastomers .....................................................................................467 4.2.2.6 Fluoroelastomers......................................................................................................470 4.2.2.7 Styrene–Acrylonitrile Copolymer ........................................................................472 4.2.2.8 Acrylonitrile–Butadiene–Styrene Terpolymer...................................................472 4.2.2.9 Ethylene–Methacrylic Acid Copolymers (Ionomers)......................................474 4.2.3 Acrylics .......................................................................................................................................475 4.2.3.1 Polyacrylonitrile .......................................................................................................475 4.2.3.2 Polyacrylates .............................................................................................................476 4.2.3.3 Polymethacrylates....................................................................................................477 4.2.3.4 Polyacrylamide .........................................................................................................479 4.2.3.5 Poly(Acrylic Acid) and Poly(Methacrylic Acid) ..............................................480 4.2.3.6 Acrylic Adhesives ....................................................................................................481 4.2.4 Vinyl Polymers .........................................................................................................................482 4.2.4.1 Poly(Vinyl Acetate).................................................................................................482 4.2.4.2 Poly(Vinyl Alcohol) ................................................................................................483 4.2.4.3 Poly(Vinyl Acetals) .................................................................................................484 4.2.4.4 Poly(Vinyl Cinnamate) ..........................................................................................485 4.2.4.5 Poly(Vinyl Ethers)...................................................................................................485 4.2.4.6 Poly(Vinyl Pyrrolidone).........................................................................................486 4.2.4.7 Poly(Vinyl Carbazole) ............................................................................................486 Part II: Condensation Polymers ..........................................................................................................486 4.3.1 Polyesters ...................................................................................................................................487 4.3.1.1 Poly(Ethylene Terephthalate) ...............................................................................487 4.3.1.2 Poly(Butylene Terephthalate) ...............................................................................488 4.3.1.3 Poly(Dihydroxymethylcyclohexyl Terephthalate)............................................490 4.3.1.4 Unsaturated Polyesters...........................................................................................491 4.3.1.5 Aromatic Polyesters ................................................................................................497 4.3.1.6 Wholly Aromatic Copolyester..............................................................................499 4.3.1.7 Polycarbonates .........................................................................................................500 4.3.2 Polyamides.................................................................................................................................503 4.3.2.1 Aliphatic Polyamides ..............................................................................................503 4.3.2.2 Aromatic Polyamides..............................................................................................511 4.3.2.3 Polyimides .................................................................................................................513 4.3.3 Formaldehyde Resins ..............................................................................................................519 4.3.3.1 Phenol–Formaldehyde Resins...............................................................................519 4.3.3.2 Urea–Formaldehyde Resins...................................................................................524 4.3.3.3 Melamine–Formaldehyde Resins .........................................................................527 4.3.4 Polyurethanes............................................................................................................................529 4.3.4.1 Polyurethane Rubbers and Spandex Fibers .......................................................529 4.3.4.2 Flexible Polyurethane Foam..................................................................................534 4.3.4.3 Rigid and Semirigid Polyurethane Foams .........................................................535 4.3.4.4 Polyurethane Coatings ...........................................................................................536 4.3.5 Ether Polymers .........................................................................................................................537 4.3.5.1 Polyacetal...................................................................................................................538 4.3.5.2 Poly(Ethylene Oxide)..............................................................................................540

xix

Contents

4.3.5.3 Applications ..............................................................................................................541 4.3.5.4 Poly(Propylene Oxide) ...........................................................................................544 4.3.5.5 Epoxy Resins.............................................................................................................545 4.3.5.6 Poly(Phenylene Oxide)...........................................................................................557 4.3.6 Cellulosic Polymers .................................................................................................................559 4.3.6.1 Regenerated Cellulose.............................................................................................560 4.3.6.2 Cellulose Nitrate ......................................................................................................560 4.3.6.3 Cellulose Acetate......................................................................................................561 4.3.6.4 Other Cellulose Esters ............................................................................................562 4.3.6.5 Cellulose Ethers........................................................................................................562 4.3.7 Sulfide Polymers.......................................................................................................................563 4.3.7.1 Polysulfides ...............................................................................................................563 4.3.7.2 Poly(Phenylene Sulfide) .........................................................................................564 4.3.8 Polysulfones...............................................................................................................................565 4.3.8.1 Properties...................................................................................................................566 4.3.9 Polyether Ketones ....................................................................................................................567 4.3.10 Polybenzimidazole ...................................................................................................................569 4.3.11 Silicones and Other Inorganic Polymers ...........................................................................570 4.3.11.1 Silicones ...................................................................................................................570 4.3.11.2 Polyphosphazenes .................................................................................................576 4.3.11.3 Polythiazyl...............................................................................................................577 4.3.12 Polyblends..................................................................................................................................577 4.3.12.1 Prediction of Polyblend Properties ...................................................................581 4.3.12.2 Selection of Blend Components.........................................................................582 4.3.12.3 Compatibilization of Polymers...........................................................................584 4.3.12.4 Industrial Polyblends............................................................................................586 4.3.12.5 Nanoblends .............................................................................................................586 4.3.13 Interpenetrating Polymer Networks ....................................................................................587 4.3.13.1 Industrial IPNs.......................................................................................................589 References........................................................................................................................................................... 590

Polymers in Special Uses 5.1 5.2

5.3

5.4

Contents

5.4.2

5.5

5.6 5.7 5.8 5.9 5.10 5.11

5.12

5.13 5.14

5.15

Inherently Conductive Polymers..........................................................................................618 5.4.2.1 Doping .......................................................................................................................620 5.4.2.2 Conducting Mechanisms .......................................................................................625 5.4.2.3 Applications ..............................................................................................................627 5.4.3 Photoconductive Polymers ....................................................................................................635 Electroactive Polymers...........................................................................................................................636 5.5.1 Ionic EAPs .................................................................................................................................636 5.5.1.1 Polymer–Metal Composites ..................................................................................636 5.5.1.2 Ionic Polymer Gels..................................................................................................637 5.5.1.3 Carbon Nanotubes ..................................................................................................637 5.5.1.4 Conductive Polymers..............................................................................................639 5.5.2 Electronic EAPs........................................................................................................................640 5.5.2.1 Ferroelectric Polymers............................................................................................640 5.5.2.2 Polymer Electrets.....................................................................................................640 5.5.2.3 Electrostrictive Polymers........................................................................................641 5.5.2.4 Dielectric Elastomers ..............................................................................................642 Polymers in Fiber Optics ......................................................................................................................644 Polymers in Nonlinear Optics .............................................................................................................648 Langmuir–Blodgett Films......................................................................................................................648 Piezo- and Pyroelectric Polymers .......................................................................................................650 5.9.1 Applications...............................................................................................................................651 Polymeric Electrolytes............................................................................................................................653 Polymers in Photoresist Applications ................................................................................................656 5.11.1 Negative Photoresists..............................................................................................................659 5.11.2 Positive Photoresists ...............................................................................................................661 5.11.2.1 Near-UV Application .......................................................................................... 661 5.11.2.2 Mid- and Deep-UV Photoresists ...................................................................... 663 5.11.3 Electron Beam Resists.............................................................................................................667 5.11.4 Plasma-Developable Photoresists.........................................................................................667 Photoresist Applications for Printing.................................................................................................669 5.12.1 Printing Plates ..........................................................................................................................669 5.12.1.1 Relief or Raised-Image Plates ............................................................................ 669 5.12.1.2 Photolithography/Planographic Plates ............................................................ 670 5.12.1.3 Photogravure ......................................................................................................... 670 5.12.2 Photoengraving ........................................................................................................................671 5.12.3 Printed Circuits........................................................................................................................671 5.12.4 Collotype and Prooﬁng Systems ..........................................................................................671 Optical Information Storage.................................................................................................................672 Polymers in Adhesives...........................................................................................................................673 5.14.1 Solvent-Based Adhesives........................................................................................................674 5.14.2 Water-Based Adhesives..........................................................................................................675 5.14.3 Hot Melt Adhesives ................................................................................................................676 5.14.4 Radiation-Curable Adhesives................................................................................................677 Degradable Polymers .............................................................................................................................678 5.15.1 Packaging Applications ..........................................................................................................679 5.15.2 Medical and Related Applications .......................................................................................680 5.15.2.1 Synthetic Polymers............................................................................................... 680 5.15.2.2 Controlled Release Agents.................................................................................. 682 5.15.2.3 Tissue Engineering ............................................................................................... 683

Contents

xxi

5.16 Ionic Polymers......................................................................................................................................... 685 5.16.1 Physical Properties and Applications..................................................................................686 5.16.1.1 Ionic Cross-Linking ..............................................................................................686 5.16.1.2 Ion-Exchange..........................................................................................................687 5.16.1.3 Hydrophilicity ........................................................................................................689 5.16.2 Ionomers ....................................................................................................................................691 5.16.2.1 Polyethylene Ionomers.........................................................................................691 5.16.2.2 Elastomeric Ionomers...........................................................................................694 5.16.2.3 Ionomers Based on Polytetraﬂuoroethylene ...................................................695 5.16.2.4 Ionomers Based on Polysulfones .......................................................................696 5.16.3 Polyelectrolytes .........................................................................................................................697 5.16.3.1 Ion-Exchangers ......................................................................................................697 5.16.3.2 Polycarboxylates ....................................................................................................705 5.16.3.3 Integral Polyelectrolytes.......................................................................................706 5.17 Synthetic Polymer Membranes............................................................................................................ 707 5.17.1 Membrane Preparation...........................................................................................................707 5.17.1.1 Wet-Extrusion Process.........................................................................................708 5.17.1.2 Hollow Fiber Membranes....................................................................................709 5.17.2 Membrane Modules ................................................................................................................712 5.17.3 Applications...............................................................................................................................713 5.18 Hydrogels and Smart Polymers........................................................................................................... 714 5.18.1 Smart Polymers........................................................................................................................716 5.19 Dendritic Polymers................................................................................................................................. 722 5.19.1 Applications ..............................................................................................................................724 5.20 Shape Memory Polymers ...................................................................................................................... 725 5.21 Microencapsulation ................................................................................................................................ 727 5.21.1 Processes for Microencapsulation ........................................................................................728 5.21.1.1 Complex Coacervation.........................................................................................728 5.21.1.2 Polymer–Polymer Incompatibility ....................................................................729 5.21.1.3 Interfacial and In Situ Polymerization .............................................................729 5.21.1.4 Spray Drying ..........................................................................................................733 5.21.1.5 Fluidized-Bed Coating..........................................................................................733 5.21.1.6 Co-Extrusion Capsule Formation......................................................................734 5.21.1.7 Other Processes......................................................................................................734 5.21.2 Applications...............................................................................................................................735 5.22 Nanosize Polymers ................................................................................................................................. 737 5.22.1 Polymer Nanoparticles ...........................................................................................................737 5.22.2 Polymer Nanospheres.............................................................................................................738 5.22.3 Polymer Nanoﬁbers ................................................................................................................739 5.22.4 Polymer Nanowires, Nanotubes, and Nanorods..............................................................740 5.23 Polymer Nanocomposites ..................................................................................................................... 742 5.24 Polymer–Clay Nanocomposites........................................................................................................... 743 5.25 Polymer–Carbon Nanocomposites ..................................................................................................... 746 5.25.1 Graphite-Based PNCs .............................................................................................................747 5.25.2 CNT-Based PNCs ....................................................................................................................747 5.25.2.1 Nanotube Functionalization ...............................................................................747 5.25.2.2 Nanocomposite Fabrication Methods...............................................................748 5.25.2.3 Nanotube Alignment in Composites ................................................................750 5.25.2.4 Properties of Nanocomposites ...........................................................................751

xxii

Contents

5.25.3 Graphene-Based PNCs ........................................................................................................753 5.25.3.1 Technical Production of Graphene ............................................................... 754 5.25.3.2 Preparation of Nanocomposites..................................................................... 760 5.25.3.3 Properties of Graphene–PNCs ....................................................................... 762 5.26 Microfibrillar/Nanofibrillar Polymer Composites.........................................................................764 5.26.1 Microfibrillar/Nanofibrillar Polymer–Polymer Composites .......................................765 5.26.1.1 Manufacturing Steps......................................................................................... 766 5.26.1.2 Properties and Applications............................................................................ 768 5.26.2 Microfibrillar/Nanofibrillar SPCs......................................................................................770 5.26.2.1 Manufacturing Steps......................................................................................... 770 5.26.2.2 Properties and Applications............................................................................ 771 5.27 Wood–Polymer Composites..............................................................................................................774 5.27.1 WPC Feedstocks....................................................................................................................775 5.27.1.1 Wood.................................................................................................................... 775 5.27.1.2 Plastics ................................................................................................................. 775 5.27.1.3 Compounded Pellets......................................................................................... 776 5.27.1.4 Additives.............................................................................................................. 776 5.27.2 Manufacture of WPC Products .........................................................................................776 5.27.2.1 Compounding .................................................................................................... 777 5.27.2.2 Extrusion ............................................................................................................. 777 5.27.3 Properties of WPC Products..............................................................................................779 5.27.4 Applications of WPC Products .........................................................................................780 References........................................................................................................................................................... 781

Recycling and Disposal of Waste Plastics 6.1 6.2 6.3

6.4

6.5

Introduction.............................................................................................................................................795 Outline of Recycling Methods.............................................................................................................799 Recycling of Poly (Ethylene Terephthalate) .....................................................................................804 6.3.1 Direct Reuse ..............................................................................................................................804 6.3.2 Reuse after Modiﬁcation.........................................................................................................806 6.3.2.1 Glycolysis...................................................................................................................806 6.3.2.2 Methanolysis.............................................................................................................808 6.3.2.3 Ammonolysis............................................................................................................808 6.3.2.4 Hydrolysis .................................................................................................................809 6.3.2.5 Depolymerization in Supercritical Fluids...........................................................809 6.3.2.6 Enzymatic Depolymerization................................................................................810 6.3.3 Incineration ...............................................................................................................................810 Recycling of Polyurethanes ..................................................................................................................811 6.4.1 Thermopressing Process .........................................................................................................811 6.4.2 Kneader Process .......................................................................................................................812 6.4.3 Hydrolysis ..................................................................................................................................812 6.4.3.1 Glycolysis...................................................................................................................813 6.4.3.2 Ammonolysis............................................................................................................813 Recycling of Poly (Vinyl Chloride) ....................................................................................................815 6.5.1 Characterization of Used PVC..............................................................................................816 6.5.2 In-Line PVC Scrap...................................................................................................................816 6.5.3 PVC Floor Coverings ..............................................................................................................818 6.5.4 PVC Rooﬁng Sheets.................................................................................................................818 6.5.5 Post-Consumer PVC ...............................................................................................................819 6.5.6 Vinyloop and Texyloop Processes........................................................................................819

xxiii

Contents

6.6 6.7

Recycling of Cured Epoxies................................................................................................................820 Recycling of Mixed Plastics Waste ...................................................................................................821 6.7.1 Direct Reuse ............................................................................................................................822 6.7.2 Homogeneous Fractions.......................................................................................................824 6.7.3 Liquefaction of Mixed Plastics............................................................................................825 6.8 Post-Consumer Polyethylene Films..................................................................................................825 6.9 Recycling of Ground Rubber Tires ...................................................................................................826 6.10 Recycling of Car Batteries ...................................................................................................................828 6.11 Plastic Recycling Equipment and Machinery .................................................................................828 6.11.1 Plastocompactor.....................................................................................................................829 6.11.2 Debaling and Initial Size Reduction..................................................................................829 6.11.2.1 Shredder ...............................................................................................................830 6.11.2.2 Cutter or Guillotine ...........................................................................................830 6.11.2.3 Screw Shredder ...................................................................................................830 6.11.2.4 Granulators ..........................................................................................................830 6.11.2.5 Fine Grinding ......................................................................................................831 6.11.3 Cleaning and Selection .........................................................................................................831 6.11.3.1 Dry Separation ....................................................................................................832 6.11.3.2 Wet Separation ...................................................................................................834 6.11.3.3 Other Methods....................................................................................................834 6.11.4 Resin Detectors: Type and Conﬁguration........................................................................834 6.11.5 Automatic Sortation..............................................................................................................836 6.11.5.1 PVC/PET and Commingled Plastics Sortation ...........................................836 6.12 Upcycling of Waste Plastics ...............................................................................................................838 6.13 Plastics Waste Disposal in Landﬁlls .................................................................................................839 6.14 Energy Recovery from Waste Plastics .............................................................................................841 6.15 Disposal of E-Waste Plastics ..............................................................................................................842 References........................................................................................................................................................... 843

Trends in Polymer Applications 7.1 7.2

7.3

Introduction.............................................................................................................................................847 Polymers in Packaging ..........................................................................................................................848 7.2.1 Retorting.....................................................................................................................................850 7.2.2 Asceptic Packaging ..................................................................................................................850 7.2.3 Hot-Filling .................................................................................................................................851 7.2.4 Controlled-Atmosphere Packaging ......................................................................................851 7.2.5 High-Barrier Films...................................................................................................................851 7.2.6 Oxygen Scavenger-Based Packaging ....................................................................................852 7.2.7 Plastic Bottles ............................................................................................................................852 7.2.8 Chemical Containers ...............................................................................................................853 7.2.9 Dual Ovenables.........................................................................................................................853 7.2.10 Closures ......................................................................................................................................854 7.2.11 Biodegradable Packaging ........................................................................................................854 7.2.12 Pharmaceutics Packaging and Nanomedicines .................................................................855 7.2.13 Wood–Plastic Composites in Packaging.............................................................................856 Polymers in Building and Construction...........................................................................................856 7.3.1 Rooﬁng .......................................................................................................................................857 7.3.2 Flooring ......................................................................................................................................857 7.3.3 Windows ....................................................................................................................................858 7.3.4 Pipes ............................................................................................................................................859

xxiv

Contents

7.3.5 7.3.6

7.4

7.5

7.6

7.7

7.8

7.9

7.10

Insulation ...................................................................................................................................859 Polymer–Concrete Composites.............................................................................................860 7.3.6.1 Fiber-Reinforced Polymer in Concrete...............................................................862 7.3.7 Wood–Plastic Composites .....................................................................................................863 7.3.8 Smart Healable Polymer Composites..................................................................................863 7.3.9 Biodegradable Composites.....................................................................................................864 Polymers in Corrosion Prevention and Control .............................................................................864 7.4.1 Flue Gas Desulfurization ........................................................................................................865 7.4.2 Chemical Resistant Masonry.................................................................................................865 7.4.3 Piping Systems..........................................................................................................................866 7.4.4 Boiler and Cooling Water Treatment .................................................................................866 7.4.5 Biodegradable Scale Inhibitor ...............................................................................................866 7.4.6 Reinforcing Steel in Concrete................................................................................................867 Plastics in Automotive Applications ..................................................................................................868 7.5.1 Exterior Body Parts .................................................................................................................868 7.5.2 Interior Components...............................................................................................................869 7.5.3 Load-Bearing Parts ..................................................................................................................870 7.5.4 Under-the-Bonnet (Hood).....................................................................................................870 7.5.5 Future Trends ...........................................................................................................................871 7.5.6 Polymer Nanocomposites ......................................................................................................871 7.5.7 “Green” Composites ................................................................................................................872 Polymers in Aerospace Applications..................................................................................................873 7.6.1 Carbon Fibers ...........................................................................................................................873 7.6.2 Resins ..........................................................................................................................................874 Polymers in Electrical and Electronic Applications........................................................................875 7.7.1 Wire and Cable Insulation.....................................................................................................876 7.7.2 Polymer Insulators...................................................................................................................877 7.7.3 Printed Circuit Boards ............................................................................................................877 7.7.4 Connectors.................................................................................................................................877 7.7.5 Enclosures ..................................................................................................................................878 7.7.6 Optical Fibers............................................................................................................................878 7.7.7 Information Storage Discs .....................................................................................................879 7.7.8 Polymeric FET and LED ........................................................................................................880 7.7.9 Polymer-Based Solar Cells .....................................................................................................881 Polymers in Agriculture and Horticulture........................................................................................883 7.8.1 Plastic Film ................................................................................................................................884 7.8.2 Plastic Crates.............................................................................................................................885 7.8.3 Building ......................................................................................................................................885 7.8.4 Pipes and Hoses .......................................................................................................................885 7.8.5 Greenhouses ..............................................................................................................................885 Polymers in Domestic Appliances and Business Machines ..........................................................886 7.9.1 Large Appliances ......................................................................................................................886 7.9.2 Small Appliances ......................................................................................................................888 7.9.3 Business Equipment ................................................................................................................888 7.9.4 Air Filters ...................................................................................................................................889 7.9.5 Solar Systems ............................................................................................................................889 Polymers in Medical and Biomedical Applications ........................................................................890 7.10.1 Medical Packaging ...................................................................................................................890 7.10.2 Nontoxic Sterilizable Items....................................................................................................890 7.10.3 Biodegradable Polymers .........................................................................................................892

Contents

xxv

7.10.4 Conducting Polymer Nanotubes.......................................................................................... 892 7.10.5 Biomimetic Actuators.............................................................................................................893 7.10.6 Dental Resin Composites.......................................................................................................894 7.10.7 Appliances .................................................................................................................................894 7.10.8 Disposables................................................................................................................................894 7.11 Polymers in Marine and Offshore Applications.............................................................................. 895 7.11.1 Cables .........................................................................................................................................895 7.11.2 Coatings .....................................................................................................................................895 7.11.3 Other Applications..................................................................................................................896 7.12 Polymers in Sport ................................................................................................................................... 896 7.12.1 Synthetic Surfaces....................................................................................................................896 7.12.2 Footwear ....................................................................................................................................897 7.12.3 Equipment.................................................................................................................................897 7.13 Renewable Synthetic Polymers ............................................................................................................ 898 References ............................................................................................................................................................ 900

Appendices

A1 Trade Names for Some Industrial Polymers ................................................................ 903 A2 Commonly Used Abbreviations for Industrial Polymers .......................................... 915 A3 Formulations of Flame-Retarded Selected Thermoplastics ....................................... 919 A4 Formulations of Selected Rubber Compounds............................................................. 923 A5 Formulations of Selected PVC Compounds ................................................................. 927 A6 Formulations of Polyurethane Foams ............................................................................ 931 A7 Conversion of Units ........................................................................................................... 935 A8 Typical Properties of Polymers Used for Molding and Extrusion .......................... 937 A9 Typical Properties of Cross-Linked Rubber Compounds.......................................... 943 A10 Typical Properties of Representative Textile Fibers .................................................... 947 Index.................................................................................................................................................. 949

http://taylorandfrancis.com

Preface In answer to a question about what he thinks to be chemistry’s biggest contribution to science and to society, Lord Todd, president of the Royal Society of London, said (as quoted in Chem. Eng. News 50, 40, 1940): I am inclined to think that the development of polymerization is, perhaps, the biggest thing chemistry has done, where it has had the biggest effect on everyday life. The world would be a totally different place without artificial fibers, plastics, elastomers etc. Even in electronics what would you do without insulation? And you come back to polymers again. Polymers are made of macromolecules, which are large molecules made by joining together many smaller ones while processable compositions based on polymers are, by common usage, called plastics. The term plastics thus includes all of the many thousands of grades of commercial materials, ranging in application from squeeze bottles, bread wrappers, fabrics, paints, adhesives, rubbers, wire insulators, foams, greases, oils, and films to automobiles and aircraft components, and missile and spacecraft bodies. As plastics thus touch everybody’s life in many different ways, in many different forms, and for many different needs, under diverse conditions of use and applications, a comprehensive source of information about different aspects of plastics, such as synthesis and manufacture, processing and fabrication, properties and testing, stability and degradation, or recycling and disposal is needed. Plastics Technology Handbook was thus conceived and published, in its first edition, way back in 1987, in recognition of this need. As the plastics world has progressed, expanded, and changed rapidly with time, this handbook also has been constantly revised, expanded, and updated, in tune with this change, to reach the present stage of fifth edition, in which, however, the textbook-style approach and objective presentation of the earlier editions have been retained, while enlarging, upgrading, and updating several areas of greater interest and adding many new topics of current focus and potential importance. Starting with basic concepts, definitions, and terminologies of polymer science, the opening chapter of the book describes different types of average molecular weights of polymers, theories behind them and experimental methods of determination, different types of polymerization processes (addition, condensation, coordination, and supramolecular), reaction mechanisms and kinetics, industrial methods of polymerization (bulk, solution, suspension, emulsion, and microemulsion), and polymerization reactors (batch-type, plug flow, cascading, etc). Molecular characteristics and structural shapes of polymer molecules are analyzed, focusing on polymer configurations and conformations with effects on polymer crystallinity and solid state morphology that greatly influence physical and chemical properties of polymer materials. Thermal transitions (glass transition and melting) of polymers and their relation to molecular structure and morphology are rationalized. Viscoelastic behavior, cross-linking effects of polymer chains, polymer solubility and its predictability, as well as effects of corrosives are discussed. Other facets of polymer characteristics including thermal stability, flame retardation and ablation, and thermal, chemical, and radiation degradation are explored. Chemical methods of stabilization of polymers are explained, focusing on all common polymers, individually. Since polymers are not useful without the addition of additives (i.e., polymer compounding), various additives, namely, fillers, plasticizers, antistatic agents, flame retardants, smoke suppressants, colorants, and antimicrobials, are highlighted. Toxicity of plastics is also discussed with focus on plastic devices used in pharmacy, medicines, nanomedicines, drug delivery, and toxicity testing. Beginning with a general discussion of tooling for plastics processing—especially molds, dies, and tool design—the second chapter of the book presents, in a simple yet elaborate manner, essential features of the most common methods of processing thermosetting plastics (namely, compression and transfer molding) and thermoplastics (namely, extrusion, injection, blowing, and calendering), besides other processes, such as thermoforming, slush molding, and spinning with persistent focus on functioning and methodology of the processes. The significance of new developments in mold design in respect to runners and gates is discussed. All important fabrication processes are fully illustrated and lucidly explained. These include plunger xxvii

xxviii

Preface

injection molding, screw injection molding, foam injection molding, blown film extrusion, flat film extrusion, pipe extrusion, extrusion blow molding, injection blow molding, calendering, conventional fiber spinning, electrospinning (of nanofibers), thermoforming, and casting processes. All aspects of reinforced plastics—both glass-reinforced and graphite/carbon-reinforced plastics—and their molding techniques are described and illustrated with diagrams. The processes of pultrusion, prepreg molding, reaction injection molding, resin transfer molding, conventional foaming, and syntactic foaming are presented. Rubber compounding and processing technology, including rubber reclamation and fabrication of rubber-based products, are discussed, followed by a host of other processing techniques, which include spray coating, fluidized bed coating, electrostatic spraying, stimuli-responsive coating, powder molding, centrifugal casting, adhesion bonding, high-frequency welding, ultrasonic welding, and plastics decorative processes (e.g., gravure printing, flexography, screen printing, in-mold decorating, embossing, electroplating, and vacuum metallizing). More recent developments in polymer fabrication and processing, such as joining polymer– metal hybrids, smart coatings, rapid prototyping/3D printing, and flex printing, are also presented. Besides the remarkable ease and scope of plastics fabrication, it is the wide range of properties inherent in plastics or imparted by various means that gives plastics the dominant place among all materials. Chapter 3 discusses plastics properties under four main headings—mechanical, electrical, optical, and thermal—giving theoretical derivations where necessary and rationalizing them on molecular and structural basis. Mechanical properties considered include elastic stress–strain, viscoelastic stress–straintime, creep (with mathematical modeling), pseudoelastic design, time–temperature superposition principle, dynamic mechanical properties, hardness, impact strength, and stress–corrosion cracking behavior. The mechanical behavior of fiber-reinforced plastics is separately discussed and mathematically modeled. The electrical behavior of plastics is evaluated in terms of the specific properties, such as dielectric strength, dielectric constant, insulation resistance or resistivity, and arc resistance. Characteristic aspects of light transmitting ability and optical clarity, refractive index, and light piping, as well as stress-optical behavior of specific plastics, are discussed. Thermal properties are highlighted by comparing different plastics in respect of specific heat, thermal expansion, thermal conductivity, and thermal transitions with specific focus on application aspects. An important facet of materials development and proper materials selection is testing and standardization for various properties. The latter part of Chapter 3 is devoted to this aspect. A number of standard test methods for plastics are presented schematically (in simplified form), highlighting the principles of the tests and methods of measurements. The last part of the chapter is devoted to chemical identification and analysis of common plastics. A layman yet systematic procedure, involving heating and ignition, detection of hetero-elements (N, S, halogens), solubility observation, and other tests, is presented followed by exact identification via specific chemical tests. Instrumental methods of plastics analysis, both qualitative and quantitative, are also presented, an elaborate and illustrative coverage being given to the most commonly used methods, namely, IR and NMR spectroscopy. Industrial polymers, that is, those that are produced on very large, large, and relatively large scales— including the so-called engineering polymers that possess superior mechanical properties for engineering applications—are discussed in Chapter 4. These polymers have been classified into three broad categories— addition polymers (i.e., chains consisting entirely of C–C bonds), condensation polymers (i.e., chains with hetero-atoms, e.g., O, N, S, Si, in the backbone chain), and special polymers (i.e., products with special properties, such as temperature and fire resistance, photosensitivity, electrical conductivity, and piezoelectric properties). Further classification of addition, condensation, and special polymers has been made on the basis of monomer composition, and all important polymers in each class have been discussed in detail with focus on production methods, characteristic properties, and principal applications. The main classes thus considered are polyolefins, acrylics, vinyl polymers, polyesters, polyamides, formaldehyde resins, polyurethanes, ether polymers, cellulosic polymers, and sulfide polymers. Polysulfones, polyether ketones, polybenzimidazole, silicones, and other inorganic polymers are included under the special polymers category. Blend polymers, including industrial polyblends, nanoblends, and interpenetrating network polymers, are also given due coverage.

Preface

xxix

Besides a handful of high-volume polymers like PE, PP, PVC, PS, PC, nylon, polyesters, and so on, which are very visible in everyday life, there are hundreds of other polymers, polymer derivatives, and polymeric combinations that play special and often critical roles in diverse fields of human activity. Chapter 5 is devoted to such polymers. For a systematic discussion, these polymers have been placed in different groups according to their properties and/or areas of uses, namely, high-temperature and fireresistant polymers, liquid crystal polymers, electroactive polymers, photoresist polymers, shape-memory polymers, conducting polymers, photoconductive polymers, polymers in fiber optics, optical information storage, and nonlinear optics, piezo- and pyro-electric polymers, ionic and ion-exchange polymers, packaging polymers, adhesive polymers, and biodegradable polymers. A wide range of novel applications of polymers are highlighted. These include electrostatic discharge (ESD) protection, rechargeable batteries, polymer solar cells, actuation and robotics, sensors, “smart” coatings, fuel cells, micro- and nanolithography, micro- and nano-electronics, controlled release and drug delivery, tissue engineering, reverse osmosis, solar desalination (by the “Sirotherm” process), oil–surfactant–water separation, hemodialysis, microencapsulation, plasmapheresis, oxygenator, carbonless copying, and light-sensitive color imaging. Nanotechnology being one of the most promising technologies of the present century, an elaborate coverage is given to polymer nanomaterials (nanoparticles, nanospheres, nanofibers, nanowires, nanotubes, and nanorods), polymer–clay nanocomposites, polymer–carbon nanocomposites and the exciting new domain of graphene-based polymer nanocomposites. Microfibrillar/nanofibrillar polymer– polymer and single-polymer composites—a fascinating development of recent years based on the novel concept of “converting instead of adding”—are also highlighted in Chapter 5, at the end of which the socalled green composites or wood–polymer composites are discussed with focus on recent developments. While plastics recycling has been practiced for many years with focus on homogeneous industrial scraps and homogeneous post-consumer plastics, the industry also accepted the challenge of recycling heterogeneous plastics waste based on new technologies of separation and processing. Chapter 6 deals with both these aspects of recycling and discusses various methods of waste plastics disposal, used in practice, with their merits and demerits. Different recycling methods currently available for predominant polyethylene terephthalate (PET) wastes are discussed, as well as those of polyurethanes, poly(vinyl chloride), and cured epoxies. For mixed wastes of plastics, methods considered are various direct use technologies, separation into single materials for reuse, and liquefaction for converting into oil. Considering its magnitude, importance, and several unique problems that it presents, rubber tire recycling is treated in detail. Recycling methods of car batteries are reviewed. Equipment and machinery commonly used for various steps in waste plastics recycling operation, including automatic sortation, are described. The new concept of upcycling (as opposed to “downcycling” or recycling) of waste plastics is reviewed. Focus is also given on issues associated with plastics waste disposal, advantages and disadvantages of landfill, gas recovery prospects, impact of biodegradable plastics, composting, and energy generation by incineration. While there are continuous improvements in the established uses of polymers, new applications are being developed in diverse areas of human activity. Chapter 7, which is the last chapter of the book, presents a comprehensive overview of new developments in polymer applications in recent years. The areas considered are packaging, building and construction, corrosion prevention and control, automotive, aerospace applications, electrical and electronic applications, agriculture and horticulture, domestic appliances and business machines, medical and biomedical applications, marine and offshore applications, and sports. Furthermore, a state-of-the-art account of emerging new areas such as single-polymer composites, polymer solar cells, and renewable synthetic polymers, which are believed to hold much promise for the future, is given. In writing a book of this kind, I accumulated indebtedness to a wide range of people, not the least to the authors of earlier publications in the field, which include such classic reference books as Modern Plastics Handbook (Charles A. Harper, ed., McGraw-Hill, New York, 2000) and Plastics Materials (J. A. Brydson, Butterworth-Heinemann, Oxford, UK, 1999). My faculty colleagues in the Department of Chemical Engineering at the Indian Institute of Science (IISc), Bangalore, and innumerable students and academic

xxx

Preface

associates in other universities and colleges have provided much welcome stimulation and direct help. I have also received much support and encouragement from Professor Ganapathy Ayappa, the departmental chairman. I am greatly indebted to all of them. I also acknowledge with gratitude the painstaking work of Ms. B. G. Girija in preparing computer graphics of a large body of figures, diagrams, and illustrations that adorn the pages of the book. My thanks are due to Mr. Ravichandra, Manager of the Campus Xeroxing and Services Center at IISc, for providing very valuable and crucial help without which preparation of this new edition would not have been possible. While it is not possible to mention the names of all students who have been part of my academic career and a source of strength, special mention must be made of two among them—Dr. Amitava Sarkar and Dr. Ajay Karmarkar—who have provided me invaluable help and assistance in many ways in my long journey of authorship. Among my academic friends abroad, I would especially mention Prof. G. L. Rempel (University of Waterloo, Waterloo, Canada), Prof. Stoyko Fakirov (Auckland University, Auckland, New Zealand), and Prof. Kenneth J. Wynne (Virginia Commonwealth University, Richmond, Virginia) for the valuable help and support that I received from them. At the end, I would record with gratitude the significant contribution of Dr. Salil K. Roy, my erstwhile college-mate and formerly a professor in the postgraduate program in civil engineering of the Petra Christian University, Surabaya, Indonesia, who motivated me to write this book on plastics and provided me with a good amount of valuable literature in the initial phase of the project. I would like to especially thank Mrs. Allison Shatkin, Materials Science and Chemical Engineering Editor at CRC Press (Taylor & Francis), for her deep sensitivity, understanding, and stimulating encouragement in my work. The excellent cooperation that I have received from Ms. Florence Kizza, Engineering and Environmental Sciences Editor, and Ms. Teresita Munoz, the Editorial Assistant, is highly appreciated. Finally, I should thank Ms. Adel Rosario of Manila Typesetting Company (Philippines) and her highly efficient team for contributing so well to the shaping of the book. I am grateful to my wife Mridula, daughter Amrita, and granddaughter Mallika for their extraordinary tolerance and ungrudging support without which this voluminous manuscript could not have been prepared. Manas Chanda

Author Manas Chanda was formerly a professor in the Department of Chemical Engineering, Indian Institute of Science, Bangalore, India. He also worked as a summer-term visiting professor at the University of Waterloo, Ontario, Canada, with regular summer visits to the university from 1978 to 2000. He guided the PhD research of 15 students and about 20 Masters dissertations. A five-time recipient of the International Scientific Exchange Award from the Natural Sciences and Engineering Council, Canada, Dr. Chanda is the author and coauthor of more than 100 scientific papers, articles, and books. The books he authored include Engineering Materials Science (3 volumes), 1979, published by Macmillan (New Delhi, India); Introduction to Polymer Science and Chemistry: A Problem Solving Approach, 2nd Edition, 2013; and Plastics Technology Handbook, 5th Edition, 2018, the last two published by CRC Press (Boca Raton, Florida). His biographical sketch is listed in Marquis’ Who’s Who in the World Millennium Edition (2000) by the American Biographical Society. A Fellow of the Indian National Academy of Engineers (New Delhi) and a member of the Indian Institute of Chemical Engineers (Calcutta) and Indian Plastics Institute (Mumbai), he received his BS (1959) and M. Tech (1962) degrees from Calcutta University and his PhD degree (1966) from the Indian Institute of Science, Bangalore, India.

xxxi

http://taylorandfrancis.com

1 Characteristics of Polymers and Polymerization Processes 1.1 What Is a Polymer? A molecule has a group of atoms which have strong bonds among themselves but relatively weak bonds to adjacent molecules. Examples of small molecules are water (H2O), methanol (CH3OH), carbon dioxide, and so on. Polymers contain thousands to millions of atoms in a molecule which is large; they are also called macromolecules. Polymers are prepared by joining a large number of small molecules called monomers. Polymers can be thought of as big buildings, and monomers as the bricks that go into them. Monomers are generally simple organic molecules containing a double bond or a minimum of two active functional groups. The presence of the double bond or active functional groups acts as the driving force to add one monomer molecule upon the other repeatedly to make a polymer molecule. This process of transformation of monomer molecules to a polymer molecule is known as polymerization. For example, ethylene, the prototype monomer molecule, is very reactive because it has a double bond. Under the influence of heat, light, or chemical agents this bond becomes so activated that a chain reaction of selfaddition of ethylene molecules is generated, resulting in the production of a high-molecular-weight material, almost identical in chemical composition to ethylene, known as polyethylene, the polymer of ethylene (Figure 1.1). The difference in behavior between ordinary organic compounds and polymeric materials is due mainly to the large size and shape of polymer molecules. Common organic materials such as alcohol, ether, chloroform, sugar, and so on, consist of small molecules having molecular weights usually less than 1,000. The molecular weights of polymers, on the other hand, vary from 20,000 to hundreds of thousands. The name polymer is derived from the Greek poly for many and meros for parts. A polymer molecule consists of a repetition of the unit called a mer. Mers are derived from monomers, which, as we have seen for ethylene, can link up or polymerize under certain conditions to form the polymer molecule. The number of mers, or more precisely the number of repetitions of the mer, in a polymer chain is called the degree of polymerization (DP). Since the minimum length or size of the molecule is not specified, a relatively small molecule composed of only, say, 3 mers might also be called a polymer. However, the term polymer is generally accepted to imply a molecule of large size (macromolecule). Accordingly, the lowermolecular-weight products with low DP should preferably be called oligomers (oligo = few) to distinguish them from polymers. Often the term high polymer is also used to emphasize that the polymer under consideration is of very high molecular weight. Because of their large molecular size, polymers possess unique chemical and physical properties. These properties begin to appear when the polymer chain is of sufficient length—i.e., when the molecular weight exceeds a threshold value—and becomes more prominent as the size of the molecule increases. The 1

Plastics Technology Handbook

H Heat/light Catalyst

C — — C H

H *

C — — C H

Activated ethylene H

H H

+ C

H *

C — — C H H

C* + n C

n+2

Polyethylene

Intermediate steps during formation of polyethylene.

FIGURE 1.1

dependence of the softening temperature of polyethylene on the degree of polymerization is shown in Figure 1.2a. The dimer of ethylene is a gas, but oligomers with a DP of 3 or more (that is, C6 − or higher paraffins) are liquids, with the liquid viscosity increasing with the chain length. Polyethylenes with DPs of about 30 are greaselike, and those with DPs around 50 are waxes. As the DP value exceeds 400 or the molecular weight exceeds about 10,000, polyethylenes become hard resins with softening points about 100°C. The increase in softening point with chain length in the higher-molecular-weight range is small. The relationship of such polymer properties as tensile strength, impact strength, and melt viscosity with molecular weight is indicated in Figure 1.2b. Note that the strength properties increase rapidly first as the chain length increases and then level off, but the melt viscosity continues to increase rapidly. Polymers with very high molecular weights have superior mechanical properties but are difficult to process and fabricate due to their high melt viscosities. The range of molecular weights chosen for commercial polymers represents a compromise between maximum properties and processability.

Tensile strength, melt viscosity

Softening temperature (°C)

100

Tensile strength

Melt viscosity Commercial range

0 500 (a)

1000

1500

Degree of polymerization (DP)

Molecular weight (b)

FIGURE 1.2 Polymer properties versus polymer size. (a) Softening temperature of polyethylene. (b) Tensile strength, and melt viscosity. (Adapted from Seymour, R. B. and Carraher, C. E. Jr., 1992. Polymer Chemistry. An Introduction. Marcel Dekker, New York.)

Characteristics of Polymers and Polymerization Processes

1.2 Molecular Weight of Polymers In ordinary chemical compounds such as sucrose, all molecules are of the same size and therefore have identical molecular weights (M). Such compounds are said to be monodisperse. In contrast, most polymers are polydisperse. Thus a polymer does not contain molecules of the same size and, therefore, does not have a single molecular weight. In fact, a polymer contains a large number of molecules—some big, some small. Thus there exists a variation in molecular size and weight, known as molecular-weight distribution (MWD), in every polymeric system, and this MWD determines to a certain extent the general behavior of polymers. Since a polymer consists of molecules of different sizes and weights, it is necessary or an average degree of polymerization (DP). to calculate an average molecular weight (M) The molecular weights commonly used in the characterization of a polydisperse polymer are the number average, weight average, and viscosity average molecular weights. Consider a sample of a polydisperse polymer of total weight W in which N = total number of moles; Ni = number of moles of species i (comprising molecules of the same size); ni = mole fraction of species i; Wi = weight of species i; wi = weight fraction of species i; Mi = molecular weight of species i; xi = degree of polymerzation of species i.

n) 1.2.1 Number-Average Molecular Weight (M From the deﬁnition of molecular weight as the weight of sample per mole, we obtain X n = W = M N X =X

Ni Mi N

Wi =Mi

wi =Mi

ni Mi

1 wi =Mi

(1.1)

(1.2)

n by the mer weight M0, we obtain a number-average degree of polymerization, DPn , where Dividing M X n Ni x i M DPn = = X M0 Ni

(1.3)

n is obtained by end-group analysis or by measuring a colligative property such as The quantity M elevation of boiling point, depression of freezing point, or osmotic pressure [1,2]. 1.2.1.1 End-Group Analysis End-group analysis can be used to determine Mn of polymer samples if the substance contains detectable end groups, and the number of such end groups per molecule is known beforehand. End-group analysis has been applied mainly to condensation polymers, since these polymers, by their very nature, have reactive functional end groups. The end groups are often acidic or basic in nature, as exempliﬁed by the carboxylic groups of polyesters or the amine groups of polyamides; such groups are conveniently esti n is derived according to mated by titration. From the experimental data, M n = f w e M a

(1.4)

where f is the functionality or number of reactive groups per molecule in the polymer sample, w is the weight of the polymer, a is the amount of reagent used in the titration, and e is the equivalent weight of the reagent.

Plastics Technology Handbook

1.2.1.2 Ebulliometry (Boiling-Point Elevation) In applying this method, the boiling point of a solution of known concentration is compared to that of the solvent at the same pressure. For ideally dilute solutions, the elevation of the boiling point, T – Tb, is related to the normal boiling temperature of the solvent Tb, its molar latent heat of evaporation Le, and molecular weight M1, as well as to the molecular weight of the solute M2, and relative weights of solvent and solute W1 and W2, respectively, by DTb = T – Tb =

RT 2b W2 M1 Le W1 M2

(1.5)

For convenience, Equation 1.5 is rewritten as DTb =

RT b2 M1 1000W2 = k e m2 1000Le W1 M2

(1.6)

where ke is the molal boiling-point elevation constant of the solvent given by ke = (RT 2b M1)/(1000Le) and m2 is the solute molality (in units of moles per kilogram), given by m2 = ð1000W2 Þ=ðW1 M2 Þ To determine a molecular weight, one measures DTb for a dilute solution of solute in solvent and calculates m2 from Equation 1.6. The molecular weight M2 of the solute is then equal to the value of (1000W2)/(W1m2). Ebulliometry, like end-group analysis, is limited to low-molecular-weight polymers. 1.2.1.3 Cryoscopy (Freezing-Point Depression) Calculation of the freezing-point depression of the solvent and hence the molecular weight of the solute by this method proceeds exactly the same way as for the boiling-point elevation. For cryoscopy of ideal solutions, equations corresponding to those for DTb and ke are DTf = −kfm2 and kf = (RT 2f M1)/(1000Lf), where DTf ≡ T − Tf is the freezing-point depression, Tf is the freezing point of pure solvent, and Lf is the molar latent heat of fusion. The solvent’s molal freezing-point depression constant kf is calculated in the same way as ke is calculated in ebulliometry. Some kf values so obtained are as follows: water, 1.8; acetic acid, 3.8; benzene, 5.1; succinonitrile, 20.3; camphor, 40. The large kf of camphor makes it especially useful in molecular weight determinations. Like ebulliometry, the cryoscopic method is also limited to relatively n up to 50,000. low-molecular-weight polymers with M 1.2.1.4 Membrane Osmometry Osmotic pressure is the most important among all colligative properties for the determination of molecular weights of synthetic polymers. To explain osmotic pressure, let us imagine a box (Figure 1.3) that is divided into two chambers, one containing a polymer solution and the other containing pure solvent, separated by a semipermeable membrane (typically cellophane) that allows solvent to pass through but not polymer, because the diffusion rate of the much larger polymer molecules through the pores of the membrane is negligibly small. As the solvent enters the solution side to establish equilibrium, the pressure on the solution side must be greater. Either by waiting till equilibrium is reached or by measuring and compensating for pressures automatically, osmotic pressure p can be measured at several concentrations of the polymer solution. According to thermodynamic theory, p c

c!0

RT M

(1.7)

where p = osmotic pressure (g/cm2) = hr; h = difference in liquid levels at equilibrium (cm); r = density of solvent (g/cm3); c = concentration (g/cm3); T = absolute temperature (K); M = molecular weight (g/mole); and R = gas constant, 8.48 × 104 (g cm/mole K).

Characteristics of Polymers and Polymerization Processes

h3 h2

Osmotic head (h)

h1 h2 h3

Solution of B in A

Pure A

Semipermeable membrane

FIGURE 1.3 Schematic diagram showing the development of osmotic head as a function of time, where h1 represents the initial liquid levels, h2 denotes the levels after some time, and h3 represents the levels when equilibrium is attained.

Equation 1.7 holds only at inﬁnite dilution, where the osmotic pressure is best represented by an attenuated power series: p M = 1 + A2 Mc + A3 M 2 c c RT

(1.8)

where A2 and A3 are the second and third virial coefﬁcients, respectively. Often, A3 can be taken as equal to (A2/2)2, so that Equation 1.8 can be rewritten as p 1=2 c

RT M

1=2 A Mc 1+ 2 2

(1.9)

Plots of (p/c)1/2 versus concentration are usually linear and can be extrapolated to infinite dilution (c = 0). The second virial coefficient is obtained from such plots by dividing the slope by the intercept and by M/2. In static osmometers, the heights of liquid in capillary tubes attached to the solvent and solution compartments (Figure 1.3) are measured. At equilibrium, the hydrostatic pressure corresponding to the difference in liquid heights is the osmotic pressure. The main disadvantage of this static procedure is the considerable length of time required for attainment of equilibrium. However, this can be overcome by using commercially available automatic osmometers (Figure 1.4) that operate on the null-point principle (i.e., solvent pressure is adjusted by a servo mechanism until a sensor can detect no tendency for solvent to flow through the membrane in either direction) and equilibrium can be reached in 5 to 10 min. With care, molecular weight of 10,000 to 500,000 can be measured with about 1% accuracy. 1.2.1.5 Vapor-Phase Osmometry Vapor-phase osmometry is based on vapor pressure lowering, which is a colligative property. The method n . There is no membrane in a vapor-pressure osmometer. Instead, there are two matched therefore gives M thermistors in a thermostated chamber that is saturated with solvent vapor (Figure 1.5). With a hypodermic syringe, a drop of solution is placed on one thermistor and similarly a drop of solvent of equal size on the other thermistor. The solution has a lower vapor pressure than the solvent at the same temperature,

Plastics Technology Handbook

Servo motor

Ampliﬁer

Solution Membrane

h Elevator

Solvent Solvent reservoir Optical detector

Light source Bubble

FIGURE 1.4 Schematic diagram of essential components of a high-speed membrane osmometer (Hewlett-Packard Corp., Avondale, Pennsylvania).

Wheatstone bridge circuit Galvanometer

Syringe (solution)

Syringe (solvent)

Thermistor beads

Chamber

Solvent cup Thermal block

FIGURE 1.5

Schematic diagram of a vapor-phase osmometer.

and so the solvent vapor condenses on the solution droplet. The solution droplet, therefore, starts getting diluted as well as heated up by the latent heat of condensation of the solvent condensing on it. In a steady state, the total rise in temperature DT can be related by an equation, DT 1 = ks + Bc + Cc 2 + … c Mn

(1.10)

where c is the solute concentration and ks is an instrument constant, which is normally determined for a given solvent, temperature, and thermistor pair, by using solutes of known molecular weight.

Characteristics of Polymers and Polymerization Processes

The temperature difference between the two thermistors can be measured very accurately as a function of the bridge imbalance output voltage, DV. The operating equation is DV K = + KBc c M

(1.11)

where K is the calibration constant. A plot of DV/c versus c (where c is the solution concentration) is made and extrapolated to zero concentration to obtain the ordinate intercept (DV/c)c!0. The calibration constant K can be computed using the equation K = M ðDV=cÞc!0

(1.12)

where M is the molecular weight of the known standard sample. To determine the molecular weight of an unknown sample, solutions of the sample are made in different concentrations in the same solvent used for the standard sample and the whole procedure is repeated to obtain the ordinate intercept. The molecular weight of the unknown sample is then given by n = K=(DV=cÞc!0 M

(1.13)

The upper limit of molecular weights for vapor-phase osmometry is considered to be 20,000. Development of more sensitive machines has extended this limit to 50,000 and higher.

w) 1.2.2 Weight-Average Molecular Weight (M n , the molecular weight of each species is weighted by Equation 1.1 indicates that in the computation of M the mole fraction of that species. Similarly, in the computation of weight-average molecular weight the molecular weight of each species is weighted by the weight fraction of that species: X X Wi Mi w = M wi Mi = X (1.14) Wi X = X

Ni Mi2 Ni M i

(1.15)

w , by the mer weight: The weight-average degree of polymerization, DPw , is obtained by dividing M X w Wi x i M DPw = = X (1.16) M0 Wi w can be determined by measuring light scattering of dilute polymer solution [3,4]. M w is always M n . Thus for a polymer sample containing 50 mol% of a species of molecular weight 10,000 higher than M n = and 50 mol% of species of molecular weight 20,000, Equation 1.1 and Equation 1.15 give M w = ½(10,000)2 + (20,000)2 =½10,000 + 20,000 = 17,000. 0:5(10,000 + 20,000) = 15,000 and M 1.2.2.1 Light-Scattering Method The measurement of light scattering by polymer solutions is an important technique for the determination of weight-average molecular weight, Mw. It is an absolute method of molecular weight measurement. It also can furnish information about the size and shape of polymer molecules in solution and about parameters that characterize the interaction between solvent and polymer molecules. The experimental

Plastics Technology Handbook

technique is, however, exacting, mainly because of the large difference in intensity of the incident beam and light scattered by the polymer solution. If a polymer is dissolved in a solvent, the light scattered by the polymer molecules far exceeds that by the solvent, and this provides an absolute measure of molecular weight (Figure 1.6). The Debye relationship, which provides the basis of determining polymer molecular weight from solution scattering is [5] Hc 1 = + 2A2 c k MP(q)

(1.17)

where H is a lumped constant including the refractive index of the solvent and the change in refractive index of the solution with polymer concentration (determined separately using a differential refractometer). The intensity of light is measured at angle q and concentration c. The second virial coefﬁcient A2 is determined from the data, while P(q), called the particle scattering factor (a complex function of molecular shape), is simply the ratio of the scattering intensity to the intensity in the absence of interference, measured at the same angle q. The light intensity factor k is derived from the “raw” galvanometer readings Ig and Igs when the sample cell contains the solution and the solvent, respectively, with the photocell of Figure 1.6 positioned at an angle q in both cases. The equation used is [5] k=

(Ig − Igs ) sin q : 1 + cos2 q

(1.18)

Since it is known that P(q) = 1 at q = 0, it is customary to extrapolate both q and c to 0 so that one can then obtain the molecular weight M from Equation 1.17. This can be done by plotting Hc/k versus concentration at constant values of q and then plotting the intercept 1/MP(q) versus sin2(q/2) to give the intercept 1/M. However, this double extrapolation to zero q and zero c can be effectively done on the same plot by the Zimm method. Zimm plots (Figure 1.7) consist of graphs in which Hc/k is plotted against [sin2 (q/2) + bc], where b is a constant arbitrarily chosen to give an open display of the experimental data and to enable the two extrapolations to c = 0 and q = 0 to be carried out with comparable accuracy. (It is often convenient to take b = 100.) In practice, intensities of scattered light are measured at a series of concentrations and at several angles for each concentration. The Hc/k values are plotted, as shown in Figure 1.7. The extrapolated points on the q = 0 line, for example, are the intersections of the lines through the Hc/k values for a ﬁxed c and various q values with the ordinates at the corresponding bc values. Similarly, the c = 0 line is drawn through the intersections of the lines through the Hc/k values, for a ﬁxed q and various c values, with the corresponding sin2 (q/2) ordinates. The q = 0 and c = 0 lines intersect on the ordinate and the intercept is equal to 1/M, that is, the reciprocal of the molecular weight. It can be shown Focused monochromatic light beam (λ)

Sample cell Transmitted beam

θ = 0°

θ r θ = 135°

θ = 45° Movable photo cell

θ = 90° iθ To galvanometer

FIGURE 1.6 Arrangement of the apparatus required to measure light scattered from a solution at different angles with respect to the incident beam.

Characteristics of Polymers and Polymerization Processes

c = 0.002 c = 0.002 θ = 135° c = 0.002 θ = 120° c = 0.002 θ = 105° c = 0.002 c = 0.002 θ = 90° θ = 150° c = 0.002 θ = 75° c = 0.002 θ = 60° c = 0.0015 c = 0.002 c = 0.002 θ = 30° θ = 45° θ = 150° θ = 0° c = 0.0010 θ = 150°

Hc 6 –1 κ × 10 (mol g )

4.0

3.0

θ=0 line

2.0

c = 0 line Constant c line

c=0 θ = 0°

Constant θ line

c = 0.00075 θ = 150° c = 0.0005 θ = 150° c=0 θ = 150°

c in g/cm3

1.0 0

0.2

0.4

0.6

0.8

1.0

1.2

sin2 (θ/2) + 100c

FIGURE 1.7 A typical Zimm plot. The concentration units employed are g/cm3. The symbols o represent extrapolated points. (From Chanda, M. Introduction to Polymer Science and Chemistry. A Problem Solving Approach 2013. CRC Press, Boca Raton, FL.)

that this molecular weight obtained by the light scattering method is the weight average molecular weight w ), as deﬁned by Equation 1.14. of the polymer (M 1.2.2.2 Low-Angle Laser Light Scattering (LALLS) In some commercial light-scattering instruments, conventional light sources have been replaced by helium-neon (He-Ne) lasers (l = 6328 Å). Because of the high intensity of these light sources, it is possible to make scattering measurements at much smaller angles (2°–10°) than with conventional light sources and also for smaller samples at lower concentrations. Since, at low angles, the particle scattering factor P (q) approaches unity, Equation 1.17 effectively reduces to Hc 1 = + 2A2 c k M

(1.19)

Therefore, the intercept of a linear plot of Hc/k versus c at a single (small) angle gives 1/M (and hence w ), while one-half of the slope gives the second virial coefﬁcient A2. The method is thus much simpler M and it avoids the laborious double extrapolation of the aforesaid Zimm plot of the conventional light scattering method. One disadvantage of the LALLS method, however, is that chain dimensions cannot be obtained since scattering is measured only at a single angle (q).

v) 1.2.3 Viscosity-Average Molecular Weight (M The viscosity-average molecular weight is deﬁned by the equation v = M

wi Mia

i1=a

.X i1=a Ni Mi1+a Ni Mi

(1.20)

v = M w and for a = −1, M v = M n . Thus, M v falls between M w and M n , and for many polymers For a = 1, M it is 10%–20% below Mw . Mv is calculated from the intrinsic viscosity [h] by the empirical relation va ½h = K M

(1.21)

Plastics Technology Handbook

where K and a are constants. [h] is derived from viscosity measurements by extrapolation to “zero” concentration [5,6]. In correlating polymer properties (such as reactivity) which depend more on the number of molecules n is a more useful parameter than M w or M v . Conin the sample than on the sizes of the molecules, M versely, for correlating polymer properties (such as viscosity) which are more sensitive to the size of the w or M v is more useful. polymer molecules, M Because it is easy to determine, the melt index often is used instead of molecular weight in routine characterization of polymers. It is defined as the mass rate of polymer flow through a specified capillary under controlled conditions of temperature and pressure. The index can often be related empirically to some average molecular weight, depending on the specific polymer. A lower melt index indicates a higher molecular weight, and vice versa. 1.2.3.1 Dilute Solution Viscometry V is known as the Mark–Houwink–Sakurada (MHS) equation. Equation 1.20 used for the calculation of M The classical method for determining K and a values of this equation involves fractionation of a whole polymer into subspecies, or fractions, with narrow molecular weight distributions. An average molecular n ) or light scattering (M w ) and, if the weight can be determined on each such fraction, by osmometry (M V of monodisperse polymer. fractions are narrow enough, the measured average can be approximated to M V The intrinsic viscosities measured at a constant temperature for a number of such fractions of known M are fitted to the equation [5] V) ln½h = ln K + a ln (M

(1.22)

to obtain the MHS constants K and a for the particular polymer/solvent system at the temperature w V is closer to M of viscosity measurement. (Since actual fractions are not really monodisperse and M n , it is a better practice to determine the molecular weight by light scattering than by than to M osmometry.) The two constants K and a are derived from the intercept and slope of a linear least-squares fit to [h] − M data for a series of fractionated polymers. The method assumes that K and a are fixed for a given polymer type and solvent and do not vary with polymer molecular weight. (This is not strictly true, however, and the MHS constants determined for higher-molecular-weight species may depend on the molecular weight range. Tabulations of such constants therefore usually list the molecular weights of fractions for which the particular K and a values were determined.) The determination of intrinsic viscosity is performed very easily with simple glass viscometers. Since the viscosity of a liquid depends markedly on temperature, viscosity measurements must be made at a carefully controlled temperature (within ±1°C). Before a measurement, the viscometer is therefore equilibrated in a carefully controlled thermostatic bath at the required temperature. Two popular viscometers are of the Ostwald and Ubbelohde types, shown in Figure 1.8. To operate the Ostwald viscometer, a given volume of liquid is introduced into bulb B through stem A and is drawn up by suction till it fills the bulb C and moves beyond the fiducial mark a. The suction is then released and the time taken by the liquid meniscus to pass between the fiducial marks a and b across the bulb C is measured. The liquid obviously flows under a varying driving force proportional to the changing difference (h) in the levels of the liquids in the two tubes. To ensure that this driving force is the same in all cases, the same amount of liquid must always be taken in bulb B. The above condition of always using the same volume of liquid does not apply, however, in the case of the Ubbelohde suspended-level viscometer, shown in Figure 1.8b. A modified design of the Ubbelohde viscometer is shown in Figure 1.8c. For measurement with the Ubbelohde viscometer, a measured volume of polymer solution with a known concentration is pipetted into bulb B through stem A. This solution is transferred into bulb C by applying a pressure on A with compressed air while column D is kept closed. When the pressure is released, the solution in bulb E and column D drains back into bulb B and the end of the capillary remains free of

Characteristics of Polymers and Polymerization Processes

C C

Capillary

Capillary Capillary

E B

(a)

(b)

(c)

FIGURE 1.8 Common types of glass viscometers. (a) Ostwald viscometer. (b) Ubbelohde suspended-level viscometer. (c) A modiﬁed Ubbelohde suspended-level viscometer (see text for description).

liquid. The solution flows from bulb C through the capillary and around the sides of the bulb E into bulb B. The volume of liquid in B has no effect on the rate of flow through the capillary because there is no back pressure on the liquid emerging from the capillary as the bulb E is open to atmosphere. The flow time t for the solution meniscus to pass between the fiducial marks a and b on bulb C above the capillary is noted. Since the volume of solution in B has no effect on the flow time t, the solution in B can be diluted in situ by adding a measured amount of solvent through A. The diluted solution, whose concentration is easily calculated from the solvent added, is then raised up into C, as before, and the new flow time is measured. In this way, the concentration of the solution in B can be changed by successive dilution with measured volumes of solvent and the corresponding flow times can be determined. Denoting the terms related to solvent with subscript zero, a ratio of viscosities of solution (h) and solvent (ho) in terms of respective flow times (t and to) for the same volume and through the same capillary is given by h r t = ho ro to

(1.23)

Equation 1.23 can be derived [5] from the Hagen–Poiseuille equation relating liquid viscosity to volumetric flow rate for flow through a capillary tube of given length and radius and under a given pressure difference between the ends of the tube. The ratio h/ho is known as the relative viscosity (hr). Other terminologies commonly used for solution viscosity are as follows: specific viscosity (hsp) = hr − 1; reduced viscosity (hsp/c) = (hr − 1)/c; inherent 1 h −1 . viscosity (hinh) = (ln hr)/c; intrinsic viscosity (½h) = limc!0 c ho For dilute solutions, r is very close to ro and Equation. 1.23 simplifies to h t = ho to

(1.24)

Thus, the ratio of viscosities needed for the determination of intrinsic viscosity [h], defined as above, can be obtained simply from flow times without measuring absolute viscosities. Since, however, the h/ho ratios are obtained from Equation 1.24 with measurements made at finite concentrations of the solution, it becomes necessary to extrapolate the data to zero concentration in order to satisfy the definition of [h].

Plastics Technology Handbook

ηinh × 10–2 (cm3 g–1)

(ηsp/c) × 10–2 (cm3 g–1)

1.6

1.4

1.2

1.0

0.4 0.1 0.2 0.3 Concentration, c × 102 (g cm–3)

FIGURE 1.9 A typical plot of hsp/c and hinh against c. (From Chanda, M. 2013. Introduction to Polymer Science and Chemistry. A Problem Solving Approach, CRC Press, Boca Raton, FL.)

There are a variety of ways to carry out this extrapolation. The variation in solution viscosity (h) with increasing concentration (c) can be expressed as a power series in c. The equations usually used are the Huggins equation [5] hsp 1 h = − 1 = ½h + kH ½h2 c + k0H ½h3 c2 + … c c ho

(1.25)

and the Kraemer equation [5] hinh =

ln (h=ho ) = ½h – k1 ½h2 c – k1′ ½h3 c – … c

(1.26)

It is easy to show that both equations should extrapolate to a common intercept equal to [h]. The usual calculation procedure thus involves a double extrapolation of Equations 1.25 and 1.26 on the same plot V from the MHS equation. (see Figure 1.9) to determine [h] and hence M

1.2.4 Polydispersity Index The ratio of weight-average molecular weight to number-average molecular weight is called the dispersion or polydispersity index (I). It is a measure of the width of the molecular-weight distribution curve (Figure 1.10) and is used as such for characterization purposes. Normally I is between 1.5 and 2.5, but it may range to 15 or greater. The higher the value of I is, the greater is the spread of the molecular-weight distribution of the polymer. For a monodisperse system (e.g., pure chemicals), I = 1. There is usually a molecular size for which a given polymer property will be optimum for a particular application. So a polymer sample containing the greatest number of molecules of that size will have the optimum property. Since samples with the same average molecular weight may possess different molecular-weight distributions, information regarding molecular-weight distribution is necessary for a proper choice of polymer for optimum performance. A variety of fractionation techniques, such as fractional precipitation, precipitation chromatography, and gel permeation chromatography (GPC), based on properties such as solubility and permeability, which vary with molecular weight, may be used for separating polymers of narrow size ranges.

Characteristics of Polymers and Polymerization Processes

0.5

Weight fraction

0.4 0.3 0.2 0.1 0

FIGURE 1.10

12 16 20 24 28 32 Mean molecular weight

40 × 103

Molecular-weight distribution of a polymer.

Example 1: A sample of poly(vinyl chloride) is composed according to the following fractional distribution (Figure 1.10). Wt fraction

0.04

0.23

0.31

0.25

0.13

0.04

Mean mol. wt × 10−3

n, M w , DPn , and DPw . (a) Compute M (b) How many molecules per gram are there in the polymer? Answer: (a) Mean mol. wt (Mi)

Wt fraction (wi)

wi × Mi

wi/Mi

0.04

7,000

280

0.57 × 10−5

0.23

11,000

2,530

2.09 × 10−5

0.31 0.25

16,000 23,000

4,960 5,750

1.94 × 10−5 1.90 × 10−5

0.13

31,000

4,030

0.42 × 10−5

0.04 S

39,000

1,560 19,110

0.10 × 10−5 6.21 × 10−5

From Equation 1.2 n = M

1 = 16,100 g=mole 6:21 10−5

From Equation 1.14 w = 19,110 g=mole M 1 mer weight of vinyl chloride (C2H3Cl) = (2)(12)+(3)(1)+35.5 = 62.5 g/mer DPn =

16,000 g=mole = 258 mers=mole 62:5 g=mer

DPw =

19,110 g=mole = 306 mers=mole 62:5 g=mer

Plastics Technology Handbook

(b) Number of molecules per gram =

X wi (Avogadro number) Mi

= (6:21 10−5 )(6:02 1023 ) = 3:74 1019 molecules=g 1.2.4.1 Gel Permeation Chromatography Gel permeation chromatography (GPC) is an extremely powerful method for determining the complete molecular weight distribution (MWD) and average molecular weights [5]. It is essentially a process for the separation of polymer molecules according to their size. The separation occurs as a dilute polymer solution is injected into a solvent stream, which then passes through a column packed with porous gel particles, with the porosity being typically in the range 50–106 Å. GPC is also known as gel ﬁltration, gel exclusion chromatography, size-exclusion chromatography, and molecular sieve chromatography. The principle of GPC is simple. It is shown schematically in Figure 1.11. A schematic layout of a typical GPC system is shown in Figure 1.12. Consider a stationary column packed with ﬁnely divided solid particles, all having the same pore size. The smaller polymer molecules that are able to enter the pores (tunnels) of the gel particles will have longer effective paths than larger molecules and will hence be “delayed” in their passage (elution) through the column. On the other hand, larger polymer molecules with a coil size greater than the pore diameter will be unable to enter the pores and will thus be swept along with the solvent front to appear in the exit from the GPC column (Figure 1.11) and reach the detector (Figure 1.12) ahead of the smaller molecules. The volume of solvent thus required to elute a particular polymer species from the point of injection to the detector is known as its elution volume. Molecular weight can be calculated from the GPC data only after calibration of the GPC system in terms of elution volume or retention time with polymer standards of known molecular weights (see below). The most common type of column packing used for analysis of synthetic polymers consists of polystyrene gels,

Substrate particle with pores (tunnels)

Out

FIGURE 1.11 A schematic of the principle of separation by gel permeation chromatography. Black circles represent molecules of coil sizes ≤ pore diameter, while crosses represent molecules of coil sizes > pore diameter. If a sample with molecular size distribution enters the column at the same time, the molecules will emerge from the column sequentially, separated according to molecular size, from larger to smaller.

Characteristics of Polymers and Polymerization Processes

Pump with pressure gauge

Eﬄuent Detector

Recorder Collector Solvent reservoir

Filter

Columns Sample injection

FIGURE 1.12 Schematic layout of a typical gel permeation chromatography apparatus. It is, however, normal practice to use a set of several columns each packed with porous gel particles having a different porosity, depending on the range of molecular sizes to be analyzed. (Adapted from Allcock, H. R. and Lampe, F. W., Contemporary Polymer Chemistry, 2nd ed., 1990. Prentice Hall, Englewood Cliffs, NJ.)

called styragel particles (hence the term gel permeation). These are highly porous polystyrene beads and are highly cross-linked. For GPC in aqueous systems, the common packings are cross-linked dextran (Sephadex) and polyacrylamide (Biogel). Porous glass beads can be used with both aqueous and organic solvents. After passage through the column(s), the solvent stream (eluant) carrying the size-separated polymer molecules passes through a detector, which responds to the weight concentration of polymer in the eluant. The most commonly used detector is a differential refractometer. It measures the difference in refractive index between the eluted solution and the pure solvent. This difference is proportional to the amount of polymer in solution. Spectrophotometers are also used as alternative or auxiliary detectors. The elution volume (also called the retention volume) is the volume of solvent that has passed through the GPC column from the time of injection of the sample. It is conveniently monitored by means of a small siphon, which actuates a marker every time it ﬁlls with eluant and dumps its contents. The raw GPC data are thus available as a trace of detector response proportional to the amount of polymer in solution and the corresponding elution volumes. A typical GPC record (gel permeation chromatogram) is shown in Figure 1.13. It can yield a plot of the MWD, since the ordinate corresponding to the detector response can be transformed into a weight fraction of total polymer while the elution volume axis can be transformed into a logarithmic molecular weight scale by suitable calibration (explained below). Using a baseline drawn through the recorder trace, the chromatogram heights are measured for equal small increments of elution volume. The weight fraction corresponding to a particular elution volume is taken as the height of the ordinate divided by the sum of the heights of all the ordinates under the trace. This process normalizes the chromatogram (see Example 3). Let us now consider how the elution volume axis of a chromatogram, such as shown in Figure 1.13, can be translated into a molecular weight scale. This necessitates calibration of the particular GPC column using monodisperse polymer samples. The main problem encountered in this task is that monodisperse or very narrow distribution samples of most polymers are not generally available. However, such samples are available for a few speciﬁc polymers. For polystyrene, for example, anionically polymerized samples of narrow MWDs with polydispersity index less than 1.15 are commercially available in a wide range of molecular weights (103 to 106). Using such narrow MWD samples, a polystyrene calibration of molecular

Plastics Technology Handbook

Elution volume

FIGURE 1.13 A typical gel permeation chromatogram. The lower trace with short vertical lines is the differential refractive index while the upper curve is an absorption plot at a ﬁxed ultraviolet frequency. The short vertical lines are syphon dumps counted from the time of injection of the sample. The units of the ordinate depend on the detector, while those of the abscissa can be in terms of syphon volumes (counts) or volume of solvent.

weight versus elution volume can be easily obtained for the given GPC column (or columns) and the given GPC solvent. A problem that would then remain is to establish a relationship for the particular GPC column between the elution volume and molecular weight of some other chemically different polymer. An approach to solving this problem is described below. A series of narrow MWD polystyrene samples used with the particular GPC column and the GPC solvent yield a set of GPC chromatograms, as shown in Figure 1.14. The peak elution volumes and the corresponding molecular weights thus provide a polystyrene calibration curve (Figure 1.15) for the particular GPC column and solvent used. In the next step, known as universal calibration, the polystyrene calibration curve is translated to one that will be effective for another given polymer in the same apparatus and solvent. To extend the calibration to other polymers, a calibration parameter that is independent of the chemical nature of the polymer, that is, a universal calibration parameter, is required. Experimentally, it has been found that such a parameter could be the product of the intrinsic viscosity and molecular weight (i.e., [h]M). Thus, as shown in Figure 1.16, the logarithm of the product [h]M plotted against elution volume, with tetrahydrofuran used as the solvent, provides a single curve for a wide variety of polymers, which thus suggests that a universal calibration procedure may be possible. (Such a single curve for different polymers is not obtained, however, by simply plotting logM against elution volume.) A theoretical validity of the aforesaid experimental observation can also be obtained from a consideration of the hydrodynamic volume of the polymer, as shown below.

Amount of polymer in eluant

Increasing molecular weight

Elution volume

FIGURE 1.14 Gel permeation chromatography elution curves for polymer standards having very narrow molecular weight distribution.

Characteristics of Polymers and Polymerization Processes

100

Molecular weight, Mx × 10–4

10 4 1.0 0.4 0.1 100

120

140

160

180

Peak elution volume, Ve (cm3)

FIGURE 1.15

A typical polystyrene standard calibration curve for GPC.

109

(η) M

108

107

106

105

Polystyrene (“linear”) Polystyrene (“comb”) Polystyrene (“star”) Poly(methyl methacrylate) (linear) Poly(vinyl chloride) Poly(butadiene) Poly(phenyl siloxane) Poly(styrene-co-methyl methacrylate) Poly(styrene-g-methyl methacrylate) Poly(styrene-g-methyl methacrylate) (comb)

18 20 22 24 26 28 Elution volume (5-mL counts, THF solvent)

FIGURE 1.16 A universal calibration curve for several polymers in tetrahydrofuran. (Drawn with data of Grubisic, Z., Rempp, P., and Benoit, H. 1967. Polymer Lett. 5, 753.)

Plastics Technology Handbook

As long ago as 1906, it was shown by Einstein that the viscosity of a dilute suspension of spherical particles relative to that of the suspending medium is given by the expression [5] h=ho − 1 = 2:5j

(1.27)

where f is the volume fraction of the suspended material. If all polymer molecules exist in solution as discrete entities, without overlap and each solvated molecule of a monodisperse polymer has an equivalent volume (or hydrodynamic volume) V and molecular weight M, then the volume fraction f of solvent-swollen polymer coils at a concentration c (mass/volume) is j = c V NAV =M

(1.28)

where NAV is Avogadro’s number. Combination of Equations 1.27 and 1.28 yields 1 h − ho 2:5VNAV = ho M c

(1.29)

Using this equation and applying the deﬁnition of intrinsic viscosity [h] given earlier, 1 ½h = limc!0 c

h − ho ho

2:5NAV limc!0 V M

(1.30)

Multiplying both sides by M and taking logarithm gives logð½hM Þ = log (2:5NAV ) + logðlimc!0 V Þ

(1.31)

The product [h]M is thus seen to be a direct function of the hydrodynamic volume of the solute at inﬁnite dilution. Since studies of GPC separations have shown that polymers appear in the eluate in inverse order of their hydrodynamic volumes in the particular solvent, it may thus be stated that two different polymers that appear at the same elution volume in a given solvent and particular GPC column at a given temperature have the same hydrodynamic volumes and hence the same [h]M characteristics [5]; that is, logð½hx Mx Þ = logð½hs Ms Þ

(1.32)

where the subscripts x and s indicate the unknown polymer X (i.e., polymer with unknown molecular weight) and the standard polymer S, respectively. If each intrinsic viscosity term in Equation 1.27 is replaced by its MHS expression (Equation 1.21), one obtains for the two polymers at equal elution volumes log Kx Mx ax+1 = log Ks Ms as+1

(1.33)

Solving for log Mx gives log Mx =

1 1 + ax

log

Ks 1 + as + log Ms Kx 1 + ax

(1.34)

The elution volume (Ve) that corresponds to a GPC peak in the unknown polymer is used to obtain a value of log Ms from the polystyrene standard curve (Figure 1.15) that has been obtained in the same column and solvent, and Mx is then calculated from Equation 1.34. Alternatively, a number of values of Ve can be chosen and a new calibration curve for the polymer X can be constructed using a standard curve

Characteristics of Polymers and Polymerization Processes

such as Figure 1.15 and Equation 1.34. The procedure is applicable only if the MHS constants, Ks, as, Kx, and ax, are known. Values of Ks and as are available in the literature for the standard polymer in various solvents, and in many cases, values of Kx and ax may also be available. However, if the desired MHS constants are not available for the polymer under study in the GPC solvent or for the standard in the same solvent, they can be determined by measuring the intrinsic viscosities, as described earlier. Example 2: A series of narrow distribution polystyrene standards dissolved in chloroform were injected into a GPC column at 35°C yielding a set of chromatograms. The following data of peak elution volumes and corresponding sample molecular weights were reported (Dawkins, J. V. and Hemming, M. 1975. Makromol. Chem. 176, 1777): s 10−3 (g/mol) M Ve (cm3)

867

670

411

160

98.2

19.8

10.3

3.7

122.7

126.0

129.0

136.5

141.0

147.0

156.5

162.5

170.0

Using the above data for polystyrene standards, construct a calibration curve for the molecular weight elution volume of polymer X in chloroform at 35°C. The MHS constants in chloroform at 35°C may be taken as K = 4.9 × 10−3 cm3/g, a = 0.79 for polystyrene and K = 5.4 × 10−3 cm3/g, a = 0.77 for polymer X. s versus Ve gives the polystyrene calibration curve (cf. Answer: A semilogarithmic plot of M Figure 1.15) for the given GPC column, solvent, and temperature. Substitution of the MHS constants in Equation 1.33 gives the expression log Mx = −0.0238 + 1.0113 log Ms. To construct an elution calibration curve (Mx vs. Ve) for polymer X, various values of Ve are assumed and corresponding to each of them the Ms value is ﬁrst obtained from the polystyrene calibration curve (Figure 1.15) and then Mx from the above expression. A semilog plot of Mx versus Ve gives the required calibration curve (Figure 1.17).

Standard molecular weight, Ms × 10–4

Example 3: The GPC column of Example 2 was used for the determination of molecular weight of a sample of the same polymer X. After injecting a chloroform solution of the polymer into the GPC

100 40 10 4

1 0.4

0.1 100

140

180

Peak elution volume, Ve (cm3)

FIGURE 1.17 Elution calibration curve (Mx vs. Ve) for polymer X (Example 2) derived from polystyrene calibration curve and MHS constants.

Plastics Technology Handbook

column, the refractive index difference (Dň) between the eluted solution and pure solvent was measured as a function of elution volume (Ve), which yielded the following data [5]: Dň × 105 Ve (4-mL count)

0.6 40

3.4 39

12.4 38

15.0 37

9.9 36

3.0 35

0.4 34

n and M w and the polydispersity Using the calibration curve obtained in Example 2, calculate M index of the sample. Answer: The molecular weight corresponding to each elution volume is determined from the elution calibration curve for this polymer in Figure 1.17. The corresponding weight fraction wi is computed from the refractive index difference by the following relation based on the assumption that Dň is proportional to concentration and the proportionality factor is independent of molecular weight: wi = Dň/SDň. The results are tabulated below.

wi = Dňi/SDň

Mi × 10−3 (from Figure 1.17)

0.009

182

1620

0.049

35 36

0.067 0.221

117 76

7851 16,834

0.573 2.914

0.336

16,780

6.712

38 39

0.277 0.076

33 21

9154 1598

8.406 3.624

0.013

187

0.957

S = 54 × 103

S = 23.236

Elution volume (4-mL count)

wiMi

(wi/Mi) × 106

n = 1=S(wi =Mi ) = 43 103 g mol−1; polydispersity w = Swi Mi = 54 103 g mol−1M Therefore, M index = Mw =Mn = 1:25.

1.3 Polymerization Reactions There are two fundamental polymerization reactions. Classically, they have been differentiated as addition polymerization and condensation polymerization. In the addition process, no by-product is evolved, as in the polymerization of vinyl chloride (see below); whereas in the condensation process, just as in various condensation reactions (e.g., esteriﬁcation, etheriﬁcation, amidation, etc.) of organic chemistry, a lowmolecular-weight by-product (e.g., H2O, HCl, etc.) is evolved. Polymers formed by addition polymerization do so by the successive addition of unsaturated monomer units in a chain reaction promoted by the active center. Therefore, addition polymerization is called chain polymerization. Similarly, condensation polymerization is referred to as step polymerization since the polymers in this case are formed by stepwise, intermolecular condensation of reactive groups. Another polymerization process that has now appeared as a new research area of considerable interest is supramolecular polymerization (see later).

1.3.1 Addition or Chain Polymerization In chain polymerization, a simple, low-molecular-weight molecule possessing a double bond, referred to in this context as a monomer, is treated so that the double bond opens up and the resulting free valences

Characteristics of Polymers and Polymerization Processes

join with those of other molecules to form a polymer chain. For example, vinyl chloride polymerizes to poly(vinyl chloride): H2C

Polymerization

CH2

(1.35)

Cl Poly(vinyl chloride)

Vinyl chloride

It is evident that no side products are formed; consequently the composition of the mer or repeating unit of the polymer (–CH2–CHCl–) is identical to that of the monomer (CH2═CHCl). The identical composition of the repeating unit of a polymer and its monomer(s) is, in most cases, an indication that the polymer is an addition polymer formed by chain polymerization process. The common addition polymers and the monomers from which they are produced are shown in Table 1.1. Chain polymerization involves three processes: chain initiation, chain propagation, and chain termination. (A fourth process, chain transfer, may also be involved, but it may be regarded as a combination of chain termination and chain initiation.) Chain initiation occurs by an attack on the monomer molecule by a free radical, a cation, or an anion; accordingly, the chain polymerization processes are called free-radical polymerization, cationic polymerization, or anionic polymerization. (In coordination addition or chain polymerization, described below separately, the chain initiation step is, however, assumed to be the insertion of the ﬁrst monomer molecule into a transition metal–carbon bond.) A free radical is a reactive substance having an unpaired electron and is usually formed by the decomposition of a relatively unstable material called an initiator. Benzoyl peroxide is a common free-radical initiator and can produce free radicals by thermal decomposition as O

R C O O C R

O + R + CO2

(1.36)

(R = Phenyl group for benzoyl peroxide initiator)

Free radicals are, in general, very active because of the presence of unpaired electrons (denoted by dot). A free-radical species can thus react to open the double bond of a vinyl monomer and add to one side of the broken bond, with the reactive center (unpaired electron) being transferred to the other side of the broken bond: O

H + H2C

R C O

R C O CH2 C

(1.37)

(X = CH3, C6H5, Cl, etc.)

The new species, which is also a free radical, is able to attack a second monomer molecule in a similar way, transferring its reactive center to the attacked molecule. The process is repeated, and the chain continues to grow as a large number of monomer molecules are successively added to propagate the reactive center: H

H R

O CH2 C

Successive addition of monomer

R C O

O ( CH2 C )m

(1.38)

This process of propagation continues until another process intervenes and destroys the reactive center, resulting in the termination of the polymer growth. There may be several termination reactions depending on the type of the reactive center and the reaction conditions. For example, two growing

Vinylacetate

CH2

CH3

CH2

Acrylonitrile H2C

Styrene

Propylene

Ethylene CH2

Monomer

O C

CH3 Poly(vinyl acetate)

Polyacrylonitrile

Polystyrene (PS)

Polypropylene (PP)

Polyethylene (PE)

TABLE 1.1 Typical Addition Polymers (Homopolymers)

CH2

O CH3

CH3

CH2

Polymer

(Continued)

Emulsion paints, adhesives, sizing, chewing gum, e.g., Flovic, Mowilith, Mowicoll.

Widely used as ﬁbers; best alternative to wool for sweaters, e.g., Orlon, Acrilan.

Transparent and brittle; used for cheap molded objects, e.g., Styron, Carinex, Hostyren, Lustrex. Modiﬁed with rubber to improve toughness, e.g., High impact Polystyrene (HIPS) and acrylonitrile–butadiene– styrene copolymer (ABS). Expanded by volatilization of a blended blowing agent (e.g., pentane) to make polystyrene foam, e.g., Styrocell, Styrofoam.

High density polyethylene (HDPE) and low density polyethylene (LDPE); molded objects, tubing, ﬁlm, electrical insulation, used for household products, insulators, pipes, toys, bottles, e.g., Alkathene, Lupolan, Hostalen, Marlex. Lower density, stiffer, and higher temperature resistance than PE; used for water pipes, integral hinges, sterilizable hospital equipment, e.g., Propathene, Novolen, Moplen, Hostalen, Marlex.

Comments

22 Plastics Technology Handbook

11.

10.

CH2

Butadiene CH2

Isoprene

Isobutylene CH2

CH3

CH2

CH3

Methyl methacrylate

Tetraﬂuroethylene CF2

CH2

OCH3

CH3

CF2

CH Cl

Vinyl chloride CH2

CF2

cis-1,4-Polybutadiene

cis-1,4-Polyisoprene

Polyisobutylene (PIB)

CH2

CH CH2

CH3

CF2

CH3

CH2

Poly(methyl methacrylate) (PMMA)

Polytetraﬂuroethylene

CH2

Polymer Poly(vinyl chloride) (PVC)

Typical Addition Polymers (Homopolymers)

Monomer

TABLE 1.1 (CONTINUED)

OCH3

CH3 n

Tires and tire products, e.g., Cis-4, Ameripol-CB, Diene.

Tires, mechanical goods, footwear, sealants, caulking compounds, e.g., Coral, Natsyn, Clariﬂex I.

Lubricating oils, sealants, copolymerized with 0.5– 2.5 mol% isoprene to produce Butyl rubber for tire inner tubes and inner liners of tubeless tires.

Transparent sheets and moldings; more expensive than PS; known as organic glass, used for aeroplane windows; e.g., Perspex, Plexiglass, Lucite, Diakon, Vedril.

High temperature resistance, chemically inert, excellent electrical insulator, very low coefficient of friction, expensive; moldings, films, coatings, used for non-stick surfaces, insulation, gaskets; e.g. Teflon, Fluon.

Water pipes, bottles, gramophone records, plasticized to make PVC film, leather cloth, raincoats, flexible pipe, tube, hose, toys, electrical cable sheathing, e.g., Benvic, Darvic, Geon, Hostalit, Solvic, Vinoflex, Welvic.

Comments

Characteristics of Polymers and Polymerization Processes 23

Plastics Technology Handbook

radicals may combine to annihilate each other’s growth activity and form an inactive polymer molecule; this is called termination by combination or coupling: H

H R

C O

C .m + . C

CH2

C O

CH2 n O C

CH2

(1.39)

C m

CH2 n O C

A second termination mechanism is disproportionation, shown by the following equation: H R

O R

C O

CH2

O C n O

CH2 C m

CH2 C m–1 CH2 C H + C X X X

(1.40)

H CH

C X

CH2

n–1

O C R O

In chain polymerization initiated by free radicals, as in the previous example, the reactive center, located at the growing end of the molecule, is a free radical. Similarly, in chain polymerizations initiated by ionic systems, the reactive center is ionic, i.e., a carbonium ion (in cationic initiation) or a carbanion (in anionic initiation). Regardless of the chain initiation mechanism—free radical, cationic, or anionic—once a reactive center is produced it adds many more molecules in a chain reaction and grows quite large extremely rapidly, usually within a few seconds or less. (However, the relative slowness of the initiation stage causes the overall rate of reaction to be slow and the conversion of all monomers to polymers in most polymerizations requires at least 30 min, sometimes hours.) Evidently, at any time during a chain polymerization process the reaction mixture will consist only of unreacted monomers, high polymers and unreacted initiator species, but no intermediate sized molecules. The chain polymerization will thus show the presence of high-molecular-weight polymer molecules at all extents of conversion (see Figure 1.18). In certain ionic chain polymerizations, which feature a fast initiation process coupled with the absence of reactions that terminate the propagating reactive centers, molecular weight increases linearly with conversion. This is known as “living” ionic chain polymerization.

1.3.2 Coordination Addition Polymerization Many polymers are now manufactured on a commercial scale using Ziegler–Natta catalysts, an outstanding example being polypropylene of high molecular weight which cannot be made by commercial processes of free-radical or ionic chain polymerization. Perhaps the best known Ziegler–Natta systems are those derived from TiCl4 or TiCl3 and an aluminum alkyl. The catalyst systems appear to function by formation of a coordination complex between the catalyst, growing chain, and incoming monomer. Hence the process is referred to as coordination addition polymerization and the catalysts as coordination catalysts. Polymers with stereoregular structures (see later) can be produced with these catalysts. The efﬁciency or activity of the early Ziegler–Natta catalyst systems was low. The term activity usually refers to the rate of polymerization, expressed in terms of kilograms of polymer formed per gram of catalyst. Thus a low activity meant that large amounts of catalyst were needed to obtain reasonably high yields of polymer, and the spent catalyst had then to be removed from the product to avoid contamination. This problem effectively disappeared with the advent of subsequent generations of catalysts leading to large increases in activity without loss of stereospeciﬁcity. This was achieved by increasing the

Characteristics of Polymers and Polymerization Processes

Molecular weight

(b)

(c)

(a) 0

100

% Conversion

FIGURE 1.18 Variation of molecular weight with conversion in (a) step polymerization, (b) free-radical polymerization, and (c) ionic chain polymerization.

effective surface area of the active component by more than two orders of magnitude through impregnation of the catalyst on a solid support such as MgCl2 or MgO. For example, in contrast to a typical TiCl3–AlR3 catalyst which yields about 50–200 g of polyethylene per gram of catalyst per hour per atmosphere of ethylene, as much as 200,000 g of polyethylene and over 40,000 g of polypropylene per gram titanium per hour may be produced using a MgCl2-supported catalyst, thus obviating the need to remove the spent catalyst (a costly step) from the product. Such catalyst systems are often referred to as high-mileage catalysts. Stereospeciﬁcity of the catalyst is kept high (>90%–98% isotactic dyads) by adding electron-donor additives such as ethyl benzoate. The catalyst complex of the TiCl3/AlR3 system essentially acts as a template for the successive orientation and isotactic placement of the incoming monomer units. Though a number of structures have been proposed for the active species, they fall into either of two general categories: monometallic and bimetallic, depending on the number of metal centers. The two types can be illustrated by the structures (I) and (II) for the active species from titanium chloride (TiCl4 or TiCl3) and alkylaluminum (AlR3 or AlR2Cl). R

(R)CI TI (R)CI

AI CI (I)

(R)CI

CI CI

(R)CI

TI CI (II)

Structure (I), representing a bimetallic species, is the coordination complex that arises from the interaction of the original catalyst components (titanium and aluminum compounds) with exchange of R and Cl groups. The placing of R and Cl groups in parentheses signiﬁes that the exact speciﬁcation of the ligands on Ti and Al cannot be made. Structure (II), representing a typical monometallic species, constitutes an active titanium site at the surface of a TiCl3 crystal. Besides the four chloride ligands that the central Ti atom shares with its neighboring Ti atoms, it has an alkyl ligand (received through exchange reactions with alkyl aluminum) and a vacant orbital (□).

Plastics Technology Handbook

The truly active bimetallic catalysts are complexes that have an electron-deﬁcient bond, e.g., Ti⋯C⋯Al in (I). Chain propagation by the bimetallic mechanism [7] occurs at two metal centers of the bridge complex as shown in Figure 1.19. With the chain growth taking place always from the metal end, the incoming monomer is oriented, for steric reasons, with the ═CH2 group pointing into the lattice and the CH3 group to one side, with the result that the process leads to the formation of an isotactic polymer. While a limited amount of experimental evidence does lend support to the bimetallic concept, majority opinion, however, favors the second and simpler alternative, the monometallic mechanism (described next). It is generally accepted that the d-orbital in the transition element is the main source of catalytic activity and that it is the Ti-alkyl bond that acts as the polymerization center where chain growth occurs. For aTiCl3 catalyst the active center [8] is formed by the interaction of aluminum alkyl with an octahedral vacancy around Ti, as shown in Figure 1.20. To elaborate, the ﬁve-coordinated Ti3+ on the surface has a vacant d-orbital, represented by -□, which facilitates chemisorption of the aluminum alkyl and this is followed by alkylation of the Ti3+ ion by an exchange mechanism to form the active center TiRCl4-□. The vacant site at the active center can accommodate the incoming monomer unit, which forms a p-complex with the titanium at the vacant d-orbital and is then inserted into the Ti-alkyl bond. The sequence of steps is shown in Figure 1.21 using propylene as the monomer.

CH CH (a)

CH3

Incoming monomer

CH3

CH2 Al

Ti Cl

CH2

Partial delocalization of alkyl CH 3 bridge

CH2

CH CH2 Monomer insertion

CH2 (d)

CH2 Al

Cl CH3

Another sequence of monomer addition

CH3

CH2 CH

Six-membered ring transition state

Ti Sequences

CH3

CH2

Repeated Al

(c)

Polymer chain attached to metal end

CH3

CH2 (e)

CH3

CH2 CH

(b)

Bridge complex

CH3

(f)

FIGURE 1.19 Bimetallic mechanism for stereospeciﬁc polymerization. (Adapted from Patat, F. and Sinn, H. 1958. Angew. Chem., 70, 496.)

Characteristics of Polymers and Polymerization Processes

Chemisorbed C H C H 2 5 2 5 Al-alkyl

Vacant d-orbital Cl

Al Cl

Al(C2H5)3

Alkylation of Ti3 ion

C2H5

TiCl3 C2H5 Cl ClAl(C2H5)2

Cl Cl

Vacant site

FIGURE 1.20 Interaction of aluminum alkyl with an octahedral vacancy around Ti in the ﬁrst stage of monometallic mechanism. (After Cossee, P. 1967. The Stereochemistry of Macromolecules, A. D. Ketley, ed., Vol. 6. Marcel Dekker, New York.)

C2H5 CI

C2H5

CI CH

CH3

Vacant site

Monomer

CH CI

C2H5 CI

CH3 CI Migration of alkyl chain

π-complex of monomer

Active center at Ti

CH2

CH3

New active center

CI C

H CH3

CH2

CH CI CH 3

CH2

CH CH3

C2H5

CH3

C2H5

Momomer insertion between Ti-alkyl bond

Another sequence of steps CH

C2H5

C CI

Transition state before insertion of monomer

Growing polymer chain

Ti Vacant site at original position

FIGURE 1.21 Monometallic mechanism for stereospeciﬁc polymerization. (After Cossee, P. 1967. The Stereochemistry of Macromolecules, A. D. Ketley, ed., Vol. 6. Marcel Dekker, New York.)

After the monomer is inserted into the Ti-alkyl bond, the polymer chain migrates back to its initial position, while the vacant site migrates to its original position to accept another monomer molecule. This migration is necessary, as otherwise an alternating position would be offered to the monomer leading to the formation of a syndiotactic polymer instead of an isotactic polymer. The termination of a polymer chain growing at an active center may occur by various reactions, as shown below with propylene as the example.

Plastics Technology Handbook

1. Chain Transfer to Monomer Ti CH2

CH(CH3)

+ CH3 CH

CH2

ktr,M

Ti CH2CH2CH3

(1.41)

+ CH2 C(CH3)

Ti CH2

CH(CH3)

+ CH3 CH

CH2

ktr,M

Ti CH + CH3

CH CH3

(1.42)

CH(CH3)

where •–Ti represents the transition metal active center on the catalyst site at which chain propagation takes place. Note that it is the methylene carbon atom from the monomer that is bonded to the transition metal atom (cf. Figure 1.49). 2. Chain transfer to the Group I–III metal alkyl: Ti CH2

CH(CH3)

+ Al(C2H5)3

Ti CH2CH3

+ (C2H5)2Al – CH2

(1.43)

CH(CH3)

3. Spontaneous intramolecular b-hydride transfer: Ti CH2

CH(CH3)

Ti H + CH2 C(CH3)

(1.44)

4. Chain transfer to an active hydrogen compound such as molecular hydrogen (external agent): Ti CH2

CH(CH3)

+ H2

ktr,H2

Ti H + CH3

CH(CH3)

(1.45)

The above reactions terminating the growth of polymer chains are indeed chain transfer reactions since in each case a new propagating chain is initiated. The relative extents of these reactions depend on various factors such as the monomer, the initiator components, temperature, concentrations, and other reaction conditions. Under normal conditions of polymerization, intramolecular hydride transfer is negligible and termination of propagating chains occurs mostly by chain transfer processes. Being a highly effective chain transfer agent, molecular hydrogen is often used for polymer molecular weight control.

1.3.3 Step Polymerization Step polymerization occurs by stepwise reaction between functional groups of reactants. The reaction leads successively from monomer to dimer, trimer, tetramer, pentamer, and so on, until ﬁnally a polymer molecule with large DP is formed. Note, however, that reactions occur at random between the intermediates (e.g., dimers, trimers, etc.) and the monomer as well as among the intermediates themselves. In other words, reactions of both types, namely, n mer + monomer ! (n + 1) mer and n mer + m mer ! (n + m) mer occur equally. Thus, at any stage the product consists of molecules of varying sizes, giving a range of molecular weights. The average molecular weight builds up slowly in the step polymerization process, and a high-molecular-weight product is formed only after a sufﬁciently long reaction time when the conversion is more than 98% (see Figure 1.18a). Since most (though not all) of the step polymerization processes involve poly-condensation (repeated condensation) reactions, the terms step polymerization and condensation polymerization are often used synonymously. Consider, for example, the synthesis of a polyamide, i.e., a polymer with amide

Characteristics of Polymers and Polymerization Processes

(–CONH–) as the characteristic linkage. If we start with, say, hexamethylenediamine and adipic acid as reactants, the ﬁrst step in the formation of the polymer (nylon) is the following reaction producing a monoamide: =

H2N – (CH2)6 – NH2 + HO – C – (CH2)4 – C – OH O

(1.46) =

H2N – (CH2)6 – NH – C – (CH2)4 – C – OH + H2O O

The reaction continues step-by-step to give the polyamide nylon-6,6. The overall reaction may thus be represented as n H2N

CH2

NH2 + n HO C

CH2

NH CH2

NH C

CH2

C OH O

O 4

(1.47)

+ n OH H2O

O O Poly(hexamethylene adipamide)

We see that the composition of the repeating unit (enclosed in square brackets) equals that of two monomer molecules minus two molecules of water. Thus a condensation polymer may be defined as one whose synthesis involves elimination of small molecules or whose repeating unit lacks certain atoms present in the monomer(s). With the development of polymer science and the synthesis of new polymers, the previous definition of condensation polymer is inadequate. For example, in polyurethanes (Table 1.2), which are classified as condensation polymers, the repeating unit has the same net composition as the two monomers (i.e., a diol and a diisocyanate), which react without eliminating any small molecule. To overcome such problems, chemists have introduced a definition which describes condensation polymers as consisting of structural units joined by internal functional groups such as

–

CO– ester –C–O– , amide – C –NH – , imide – N , urethane – O –C – NH – , O O CO– O O sulfide –S– , ether –O– , carbonate – O – C – O – , and sulfone – S – linkages. O

A polymer satisfying either or both of the above definitions is classified as a condensation polymer. Phenol–formaldehyde, for example, satisfies the first definition but not the second. Some condensation polymers along with their repeating units and condensation reactions by which they can be synthesized are shown in Table 1.2. Some high-performance polymers prepared by polycondensation are listed in Table 1.3. The ring-opening polymerizations of cyclic monomers, such as propylene oxide, H3C

O CH CH2

CH2 CH O n CH3

(1.48)

or –caprolactam O C CH2

NH CH2

(1.49) 5

Polyurethane (PU)

Polyester

Polyamide (PA)

n HO

n H2N

n HO

n H2N

COH

OH + n OC N

OH + n R"O C

+ n Cl C

n HOC

OH + n HO C

COH

NH2

NH2 +

TABLE 1.2 Typical Condensation Polymers

NCO

C OR"

NH R

COH

C OH

(n–1) H2O

NHC

(n–1) H2O

R OC NH

Polymerization Reactiona

+ (2n–1) HCl

OH + (2n–1) H2O

OC NH R'

+ (2n–1) R"OH

+ (2n–1) H2O

O (n–1)

NCO

(Continued)

Rubbers, foams, coatings; e.g., Vulkollan, Adiprene C, Chemigum SL, Desmophen A, Moltopren.

Textile ﬁbers, ﬁlm, bottles; poly(ethylene terephthalate) (PET) e.g., Terylene, Dacron, Melinex, Mylar.

Moldings, ﬁbers, tirecord; poly(hexamethylene adipamide) (Nylon 6,6) e.g., Ultramid A; polycaprolactam (nylon-6), e.g., Ultramid B, Akulon, Perlenka, poly (hexamethylene sebacamide) (Nylon-6,10), e.g., Ultramid S, Zytel.

Comments

30 Plastics Technology Handbook

H2N n

n H2N

+ O

n CH2

NH2 + n CH2 O

C NH2

OH CH2

Cl + n Na2 Sx

+ n CH2

n HO

n Cl

R O

Typical Condensation Polymers

Sx n

NH C

C N

C NH2

NH C

(n–1)

NHCH2 n

NH CH2

+ (n–1) H2O

+ n H2O

+ 2n NaCl

H + (n–1) H2O

CH2

Polymerization Reactiona

R, R′, R″ represent aliphatic or aromatic ring. The repeating unit of the polymer chain is enclosed in parentheses.

Melamine– formaldehyde (MF)

Urea–formaldehyde (UF)

Phenol–formaldehyde (PF)

Polysiloxane

Polysulphide

TABLE 1.2 (CONTINUED)

Particle-board binder resin, paper and textile treatment, molding compounds, coatings, e.g., Beetle, Resolite, Cibanoid. Dinnerware, table tops, coatings, e.g., Formica, Melalam, Cymel.

Plywood adhesives, glass– ﬁber insulation, molding compound, e.g., Hitanol, Sirfen, Trolitan.

Elastomers, sealants, ﬂuids, e.g., Silastic, Silastomer, Silopren.

Adhesives, sealants, binders, hose, e.g., Thiokol.

Comments

Characteristics of Polymers and Polymerization Processes 31

Polyetheretherketone (PEEK)

Polyethersulfone (PES)

Polycarbonate (PC)

n KO

n HO

OK + n F

CH 3

Polymer Type and Polycondensation Reaction

OH + n Cl C

TABLE 1.3 Some High-Performance Condensation Polymers

(n–1) KCl n +

CH 3

O C

(Continued)

Moldings and sheets; transparent, tough and physiologically inert: HCl + (2n–1) used for safety glasses, lenses, n screens and glazings, electrical and electronics, appliances, compact discs, e.g. Merlon, Baylon, Jupilon. Moldings, coatings, membranes; rigid, transparent, self-extinguishing, resistant to heat deformation: used for electrical components, molded circuit boards, appliances operating at high temperatures, e.g., Victrex PES. Moldings, composites, bearings, coatings; very high continuous use + (2n–1) KF temperature (260°C): used in n coatings and insulation for high performance wiring, composite prepregs with carbon ﬁbers, e.g., Victrex PEEK.

Comments

32 Plastics Technology Handbook

Polyimide

Poly(p-phenylene terephthalamide)

Poly(phenylene sulphide) (PPS)

n O

n Cl

O + n H2N NH2

N O

O NH2

C Cl + n H2N

+ (2n–1) NaCl

Cl + n Na2S S

Some High-Performance Condensation Polymers

Polymer Type and Polycondensation Reaction

TABLE 1.3 (CONTINUED)

C N

O + (2n–1) NaCl

Films, coatings, adhesives, laminates; outstanding in heat resistance, ﬂame resistance, abrasion (2n–1) H O 2 + resistance, electrical insulation resistance, resistance to oxidative n degradation, high energy radiation and most chemicals (except strong bases): used in specialist applications, e.g., Kapton, Vespel.

Moldings, composites, coatings; outstanding in heat resistance, flame resistance, chemical resistance and electrical insulation resistance: used for electrical components, mechanical parts, e.g., Ryton, Tedur, Fortron. High modulus fibers; as strong as steel but have one-fifth of weight, ideally suited as tire cord materials and for ballistic vests, e.g., Kevlar, Twaron.

Comments

Characteristics of Polymers and Polymerization Processes 33

Plastics Technology Handbook

proceed either by chain or step mechanisms, depending on the particular monomer, reaction conditions, and initiator employed. However, the polymers produced in Equation 1.48 and Equation 1.49 will be structurally classified as condensation polymers, since they contain functional groups (e.g., ether, amide) in the polymer chain. Such polymerizations thus point out very clearly that one must distinguish between the classification based on polymerization mechanism and that based on polymer structure. The two classifications cannot always be used interchangeably. Both structure and mechanism are usually needed in order to clearly classify a polymer.

1.3.4 Supramolecular Polymerization Supramolecular polymers are a relatively new class of polymers in which monomeric repeating units are held together with directional and reversible (noncovalent) secondary interactions, unlike conventional macromolecular species in which repetition of monomeric units is mainly governed by covalent bonding. A schematic comparison of a covalent polymer and a supramolecular polymer is shown in Figure 1.22. The directionality and strength of the supramolecular bonding, such as hydrogen bonding, metal coordination, and p–p interactions, are important features resulting in polymer properties in dilute and concentrated solutions, as well as in the bulk. It should be noted that supramolecular interactions are not new to polymer science, where hydrogen bonding and other weak reversible interactions are important in determining polymer properties and architectures. However, for linear supramolecular polymers to form, it is a prerequisite to have strong and highly directional interactions as a reversible alternative for the covalent bond. Hydrogen bonds between neutral organic molecules, though they hold a prominent place in supramolecular chemistry because of their directionality and versatility, are not among the strongest noncovalent interactions. Hence, either multiple hydrogen bonds with cooperativity must be used or hydrogen bonds should be supported by additional forces like excluded volume interactions [9]. Though the concept has been known for years, it was not known how to incorporate such sufﬁciently strong but still reversible interactions. However, in the past two decades following the development of strong hydrogen-bonding dimers, several research groups have applied these dimers for the formation of hydrogen-bonded supramolecular polymers. Thus the ﬁnding by Sijbesma et al. [10] that derivatives of 2-ureido-4[1H]-pyrimidinone (UPy, 1 in Figure 1.23) are easy to synthesize and they dimerize strongly (dimerization constant>106 M−1 in CHCl3) by self-complementary quadrupole (array of four) hydrogen bonding (2 in Figure 1.23) prompted them to use this functionality as the associating end group in reversible self-assembling polymer systems. A difunctional UPy compound, 4 in Figure 1.24, possessing two UPy units can be easily made in a one step procedure, from commercially available compounds, methylisocytosine (R═CH3) and hexyldiisocyanate (R═C6H12). The compound forms very stable and long polymer chains (5 in Figure 1.24) in solution as well as in the bulk [9,11]. Dissolving a small amount of the compound in

(a)

(b)

FIGURE 1.22 Schematic representation of (a) a covalent polymer and (b) a supramolecular polymer. (After Brunsveld, L., Folmer, B. J. B., Meijer, E. W., and Sijbesma, R. P. 2001. Chem. Rev., 101, 4071. With permission.)

Characteristics of Polymers and Polymerization Processes

NH2

O = C = N - R' N

N H

O H

Solvent

1 UPy

R H N

N H R

FIGURE 1.23 Synthesis of a monofunctional 2-ureido-4[1H]–pyrimidinone (UPy) (1) and dimerization of 1 in solution forming a quadrupole hydrogen-bonded unit. (After Sijbesma, R. P., Beijer, F. H., Brunsveld, L., Folmer, B. J. B., Hirschberg, J. H. K., Lange, R. F. M., Lowe, J. K. L., and Meijer, E. W. 1997. Science, 278, 1601. With permission.)

chloroform gives solutions with high viscosities, while calculations show that polymers with molecular weights of the order of 106 can be formed. Deliberate addition of small amounts of monofunctional compounds (1 in Figure 1.23) results in a sharp drop in viscosity, proving that linkages between the building blocks are reversible and unidirectional and that the monofunctional compounds act as chain stoppers. For the same reason, the supramolecular polymers show polymer-like viscoelastic behavior in bulk and solution, whereas at elevated temperatures they exhibit liquid-like properties [9]. The quadrupole hydrogen-bonded unit can be employed in the chain extension of telechelic oligomers such as polysiloxanes, polyethers, polyesters, and polycarbonates [11]. Thus the electrophilic isocyanate group (–NCO) of “synthon” (3 in Figure 1.24) can be reacted with common nucleophilic end groups (–OH or –NH2) of telechelic oligomers, resulting in supramolecular polymers by chain extension (Figure 1.25). Thus the material properties of telechelic polymers have been shown to improve dramatically upon functionalization with synthon, and materials have been obtained that combine many of the mechanical properties of conventional macromolecules with the low melt viscosity of oligomers [9]. In contrast to conventional high-molecular-weight polymers, supramolecular (reversible) polymers with a high “virtual” molecular weight show excellent processability due to the strong temperature dependency of the melt viscosity [11]. Moreover, hybrids between blocks of covalent macromolecules and supramolecular polymers can be easily made.

1.3.5 Copolymerization All the addition polymers we have considered so far (Table 1.1) contain only one type of repeating unit or mer in the chain. Polymers can also be synthesized by the aforesaid processes with more than one type of

Plastics Technology Handbook

NH2

R"–NCO

+ OCN – R"–NCO

O H

(a) UPy

NCO

N H

R 4 UPy

(b)

FIGURE 1.24 Preparation of (a) UPy possessing an isocyanate functional group (3) and (b) a difunctional UPy compound (4) which forms a supramolecular polymer (5) by hydrogen bonding (cf. Figure 1.23). (After Brunsveld, L., Folmer, B. J. B., Meijer, E. W., and Sijbesma, R. P. 2001. Chem. Rev., 101, 4071 and Folmer, B. J. B., Sijbesma, R. P., Versteegen, R. M., van der Rijt, J. A. J., and Meijer, E. W. 2000. Adv. Mater., 12, 12, 874. With permission.)

(UPy -NCO)

FIGURE 1.25 Schematic representation of the formation of supramolecular polymer (7) by chain extension of reactive telechelic oligomer with UPy. (After Folmer, B. J. B., Sijbesma, R. P., Versteegen, R. M., van der Rijt, J. A. J., and Meijer, E. W. 2000. Adv. Mater., 12, 12, 874. With permission.)

mer in the chain. Such polymers are called copolymers. They are produced by polymerizing a mixture of monomers (copolymerization) [12] or by special methods. Copolymers can be of different types, depending on the monomers used and the speciﬁc method of synthesis. The copolymer with a relatively random distribution of the different mers in its structure is referred to as a random copolymer. Representing, say, two different mers by A and B, a random copolymer can be depicted as ABBABBBAABBAABAAABBA

Characteristics of Polymers and Polymerization Processes

(a)

(b)

(c)

(d)

FIGURE 1.26 Copolymer arrangements. (a) Two different types of mers (denoted by open and ﬁlled circles) are randomly placed. (b) The mers are alternately arranged. (c) A block copolymer. (d) A graft copolymer.

Other copolymer structures [13] are known: alternating, block, and graft copolymer structures (Figure 1.26). In the alternating copolymer the two mers alternate in a regular fashion along the polymer chain: ABABABABABABABABABAB A block copolymer is a linear copolymer with one or more long uninterrupted sequences of each mer in the chain: AAAAAAAAAABBBBBBBBBB A graft copolymer, on the other hand, is a branched copolymer with a backbone of one type of mer to which are attached one or more side chains of another mer. AAAAAAAAAAAAAAAA B B B B B Copolymerization, which may be compared to alloying in metallurgy, is very useful for synthesizing polymers with the required combination of properties. For example, polystyrene is brittle and polybutadiene is flexible; therefore copolymers of styrene and butadiene should be more flexible than polystyrene but tougher than polybutadiene. The general-purpose rubber GRS (or SBR), the first practical synthetic rubber, is a copolymer of styrene and butadiene.

1.4 Polymerization Processes 1.4.1 Process Characteristics Free-radical chain or addition polymerizations are commonly carried out by four different processes: (a) bulk or mass polymerization, (b) solution polymerization, (c) suspension polymerization, and (d) emulsion polymerization. Processes (c) and (d) are essentially of the heterogeneous type containing a large proportion of nonsolvent (usually water) acting as a dispersion medium for the immiscible liquid monomer. Bulk and solution polymerizations are homogeneous processes, but some of these homogeneous systems may become heterogeneous with progress of polymerization owing to the polymer formed being insoluble

Plastics Technology Handbook

in its monomer (for bulk polymerization) or in the solvent used to dilute the monomer (for solution polymerization). Step polymerizations such as polyesterification and polyamidation are carried out in bulk or solution phase with provision to remove the by-products of condensation reactions. 1.4.1.1 Bulk, Solution, and Suspension Polymerization The kinetic schemes for free-radical polymerizations in bulk monomer, solution, or suspension (but not in emulsion) are the same, the rate of polymerization (Rp) being given by [5] − d ½M=dt = Rp = kp =kt 1=2 ð fkd ½IÞ1=2 ½M (1.50) where [M] and [I] are the concentrations of monomer and initiator, respectively; kd, kp, and kt are the rate constants for initiator decomposition, chain propagation, and chain termination, respectively; f is the initiator efficiency or efficiency of initiation. The general ranges of values of these quantities are [14] as follows: [M]: 10 to 10−1 mol L−1, [I]: 10−2 to 10−4 mol L−1, Rp: 10−4 to 10−6 mol L−1 s−1, kp: 104 to 102 L mol−1 s−1, kd: 10−4 to 10−6 s−1, kt: 108 to 106 L mol−1 s−1, and f: 0.3 to 0.8. The above equation shows that the rate of polymerization depends directly on the monomer concentration and on the square root of the rate of initiation. Thus, doubling the monomer concentration doubles the polymerization rate, while doubling pffiffiffi the rate of initiation or initiator concentration increases the polymerization rate only by the factor 2. The overall extent of polymerization or (fractional) monomer conversion (p) over a period of time (t) is obtained by integration of Equation 1.50. This gives [5] − lnð 1 − pÞ = 2 kp =kt 1=2 ð f ½Io =kd Þ1=2 1 − e−kd t=2

(1.51)

where [I]o is the initial concentration of the initiator. This equation can be used to calculate the amount of polymer produced (i.e., the moles of monomer converted to polymer) in time t at a given temperature or for determining the time needed to reach different extents of conversion for actual polymerization systems where both [M] and [I] decrease with time. Polymerization in bulk, that is, of undiluted monomer, minimizes any contamination of the product. But bulk polymerization is difficult to control because of the high exothermicity and high activation energies of free-radical polymerization and the tendency toward the gel effect (in some cases). By carrying out the polymerization of a monomer in a solvent (solution polymerization), these disadvantages of the bulk process can be avoided. The solvent acting as a diluent reduces the viscosity gain with conversion and allows more efficient agitation or stirring of the medium, thereby enabling better transfer and dissipation of heat. Solution polymerization is, however, advantageous only if the polymer formed is to be applied in solution (avoiding the need for solvent removal at the end), such as for making coating (lacquer) grade poly(methyl methacrylate) resins from methyl methacrylate and related monomers. Suspension polymerization combines the advantages of both the bulk and solution polymerization techniques. It is used extensively in the mass production of vinyl and related polymers. In suspension polymerization (also referred to as bead or pearl polymerization), the monomer is suspended as droplets by efficient agitation in a large volume (continuous phase) of nonsolvent, commonly referred to as the dispersion or suspension medium. Water is used as the suspension medium for water insoluble monomers because of its obvious advantages. Styrene, methyl methacrylate, vinyl chloride, and vinyl acetate are polymerized by this suspension process. The size of the monomer droplets in suspension polymerization usually ranges between 0.1 and 5 mm in diameter. Suspension is maintained by mechanical agitation and addition of stabilizers. Low concentrations of suitable water-soluble polymers, such as carboxymethyl cellulose or methyl cellulose, poly (vinyl alcohol), gelatin, and so on, are used as suspension stabilizers. They raise the medium viscosity and effect stabilization by forming a thin layer on the monomer/polymer droplets. Water-insoluble inorganic compounds such as bentonite, kaolin, magnesium silicate, and aluminum hydroxide, in finely divided

Characteristics of Polymers and Polymerization Processes

state, are sometimes used to prevent agglomeration of the monomer droplets. Initiators soluble in monomer, such as organic peroxides, hydroperoxides, or azocompounds—often referred to as oil-soluble initiators—are used. Each monomer droplet in a suspension polymerization thus behaves as a miniature bulk polymerization system and the kinetics of polymerization within each droplet are the same as those for the corresponding bulk polymerization (Equations 1.50 and 1.51). At the end of the polymerization process, the monomer droplets appear in the form of tiny polymer beads or pearls (hence the term bead or pearl polymerization). As mentioned previously, the two main termination processes for propagating chain radicals in all freeradical polymerizations are combination or coupling (Equation 1.39) and disproportionation (Equation 1.40). The degree of polymerization (DP) of polymer chain, and hence polymer molecular weight, depends on which termination process is prevalent. It can be shown [5] that the number average degree of polymerization (DPn ) is related by DPn =

kp 2 ½M2 1 (2 − etc ) kt Rp

(1.52)

where etc is the fraction of propagating chains terminating by coupling, while kp, kt, Rp, and [M] are as in Equation 1.50. It is evident from Equation 1.52 that the degree of polymerization, and hence polymer molecular weight, decreases with the increase in the polymerization rate in free-radical chain polymerization (not when the reaction is carried out in emulsion [see below]). 1.4.1.2 Emulsion Polymerization In emulsion polymerization, monomers are polymerized in the form of emulsions and the polymerization in most cases involves free-radical reactions. Like suspension polymerization, the emulsion process uses water as the medium. Polymerization is much easier to control in both these processes than in bulk systems because stirring of the reactor charge is easier owing to lower viscosity, and removal of the exothermic heat of polymerization is greatly facilitated with water acting as the heat sink. Emulsion polymerization, however, differs from suspension polymerization in the nature and size of particles in which polymerization occurs, in the type of substances used as initiators, and also in mechanism and reaction characteristics. Emulsion polymerization normally produces polymer particles with diameters of 0.1–3 mm. On the other hand, polymer nanoparticles of sizes 20–30 nm are produced by microemulsion polymerization (see later). The original theory of emulsion polymerization is based on the qualitative picture of Harkins [15] and the quantitative treatment of Smith and Ewart [16]. The essential ingredients in an emulsion polymerization system are water, a monomer (not miscible with water), an emulsifier, and an initiator that produces free radicals in the aqueous phase. Monomers for emulsion polymerization should be nearly insoluble in the dispersing medium but not completely insoluble. A slight solubility is necessary as this will allow the transport of monomer from the emulsified monomer reservoirs to the reaction loci (explained later). Emulsifiers are soaps or detergents, and they play an important role in the emulsion polymerization process. A detergent molecule is typically composed of an ionic hydrophilic end and a long hydrophobic chain. Some examples are as follows: Anionic detergent: Sodium laurate: CH3(CH2)10COO− Na+ Sodium alkyl aryl sulfonate: CnH2n+1C6H4SO3− Na+ Cationic detergent: Cetyl trimethyl ammonium chloride: C16H33N+(CH3)3 Cl− Anionic and cationic detergent molecules may thus be represented by —•− and —•+, respectively, indicating hydrocarbon (hydrophobic) chains with ionic (polar) end groups. Let us now consider the locations of the various components in an emulsion polymerization system. A micelle of an anionic detergent can be depicted as a cluster of detergent molecules (—•−) with the

Plastics Technology Handbook

hydrocarbon chains directed toward the interior and their polar heads in water (see Figure 1.27). In the same way, detergent molecules get adsorbed on the surface of an oil droplet suspended in water. Such materials are therefore said to be surface active and are also called surfactants. When a relatively water-insoluble vinyl monomer, such as styrene, is emulsiﬁed in water with the aid of an anionic surfactant and adequate agitation, three phases result (see Figure 1.27): (1) an aqueous phase in which small amounts of both monomer and surfactant are dissolved (i.e., they exist in molecular dispersed state); (2) emulsiﬁed monomer droplets that are supercolloidal in size (>10,000 Å), stability being imparted by the reduction of surface tension and the presence of repulsive forces between the droplets since a negative charge overcoats each monomer droplet; and (3) submicroscopic (colloidal) micelles that are saturated with monomer. This three-phase emulsion represents the initial state for emulsion polymerization (Figure 1.27). Stage I (see Figure 1.27) begins when a free radical–producing water-soluble initiator is added to the three-phase emulsion described above. The commonly used initiator is potassium persulfate, which decomposes thermally to form water-soluble sulfate radical ions: 50−60° C

S2 O8 2− ! 2SO4 − •

(1.53)

The rate of radical generation by an initiator is greatly accelerated in the presence of a reducing agent. Thus, an equimolar mixture of FeSO4 and K2S2O8 at 10°C produces radicals by the reaction S2 O8 2− + Fe2+ ! Fe3+ + SO4 2 – + SO4 − •

(1.54)

about 100 times as fast as an equal concentration of the persulfate alone at 50°C. (Redox systems generally ﬁnd use for polymerizations only at lower temperatures.)

Initial stage

Stage I

M I

Monomer accumulation inside micelle

M MM M MM

A Water-soluble initiator I

In M

M A

I A

added

Out

2 A

M MM

Soap molecule out

M MM M MM

Out

Micelle converts into monomer-polymer particle

Out A

In Monomer droplet (emulsiﬁed)

A Soap-like free radical in

M M : Dissolved monomer molecule;

Dissolved soap molecule;

Monomer droplet (emulsiﬁed)

Soap-like free radical

FIGURE 1.27 Schematic of emulsion polymerization showing three phases present. (After Williams, D. J. 1971. Polymer Science and Engineering, Prentice Hall, Englewood Cliffs, NJ.)

Characteristics of Polymers and Polymerization Processes

The sulfate radical ions generated from persulfate react with the dissolved monomer molecules in the aqueous phase to form ionic free radicals (–•): SO4 − • + ðn + 1ÞM!− SO4 ðMÞn M•

1.55

These ionic free radicals can be viewed as soap-like anionic free radicals as they are essentially made up of a long hydrocarbon chain carrying an ionic charge at one end and a free-radical center at the other (represented as −A—· in Figure 1.27). They thus behave like emulsifier molecules and because of the existence of a dynamic equilibrium between micellar emulsifier and dissolved emulsifier, they also can at some stage be implanted in some of the micelles. Once implanted this way in a micelle, a soap-like anionic free radical initiates polymerization of the solubilized monomer in the micelle. The micelle, thus “stung,” grows in size by reacting with the solubilized monomer, and to replenish it, more monomer enters the micelle from monomer droplets via the aqueous dispersion phase. The “stung” micelle is in this way transformed into a monomer–polymer (M/P) particle (see Figure 1.27). Thus, in Stage I, the system will consist of an aqueous phase containing dissolved monomer, dissolved soap-like free radicals, micelles, “stung” micelles, M/P particles, and monomer droplets. The rate of overall polymerization increases continuously since nucleation of new particles (i.e., conversion of micelles into M/P particles) and particle growth occur simultaneously. For the same reason, a particle size distribution occurs during Stage I. However, at 13%–20% monomer conversion, nearly all the emulsifier will be adsorbed on the M/P particles and the micelles will disappear. Since new particles mostly originate in micelles, with the disappearance of micelles, the nucleation of new M/P particles essentially ceases. This marks the end of Stage I. In Stage II (Figure 1.28) that follows, there occurs a continued growth of the existing M/P particles in the absence of any new particle nucleation. Free radicals enter only the M/P particles where polymerization takes place as the particles are supplied with monomer from the emulsified monomer droplets via the aqueous phase. Stage II thus features a constant overall rate of polymerization (Figure 1.28). It is followed by Stage III, which begins when the overall rate of polymerization begins to deviate from linearity and nonlinear growth rate is observed. The nonlinearity may appear (a) in the form of a decrease in rate as a result of dwindling monomer concentration inside M/P particles, or (b) in the form of an increase, if the gel effect (generally attributed to a greater decrease of the termination rate constant kt, as

Conversion (%)

Trommsdorﬀ eﬀect

Stage III: Nonlinear growth rate (diﬀusioncontrolled regime) 60% Stage II: • No new particle nucleation • Constant rate of growth

}25% to 30% Emulsiﬁed monomer droplets disappear ~15% Stage I: Simultaneous particle nucleation and growth Time

FIGURE 1.28 Schematic conversion–time curve for a typical emulsion polymerization showing three main stages of the polymerization process.

Plastics Technology Handbook

compared to the propagation rate constant kp, owing to increased viscosity at higher conversions) becomes important with the monomer reservoirs having already disappeared in Stage II. A simple analysis of emulsion polymerization kinetics is based on the idealized emulsion system as depicted in Figure 1.27. However, the treatment centers only around Stage I and Stage II, as no general theory for Stage III is available. From theoretical considerations, it can be shown [5,16] that the number (Np) of M/P particles at the end of Stage I is Np = 0:53ðas ws Þ0:6 ðRr =uÞ0:4

(1.56)

where as is the area occupied by the unit weight of surfactant, ws is the weight concentration of surfactant, Rr is the rate of radical generation, and u is the rate of volume growth of a M/P particle. Equation 1.56 indicates that the particle number depends on the 0.6 power of the surfactant concentration and on the 0.4 power of the initiator concentration, since the rate of radical generation, Rr, by thermal dissociation of initiator is given by Rr = 2NAv kd ½I

(1.57)

where NAv is Avogadro’s number (6.023 × 1023 molecules mol−1), [I] is the initiator concentration (mol/L) in aqueous phase, and kd is the thermal dissociation rate constant (s−1) of the initiator, with two radicals being produced from each initiator molecule. (Typical values [17] of the quantities in Equation 1.56 are as follows: Np ~ 1015–1016 per cm3 of aqueous phase, Rr ∼ 1012–1014 radicals (number) cm−3 s−1, asws ∼ 105 cm2/cm3 aqueous phase, u ~ 10−20 cm3 s−1.) For estimating the rate of polymerization in Stage II, one may generally assume that free radicals enter the particles singly, that is, one at a time. For example, considering typical values pertinent to emulsion polymerization, if the rate of generation of free radicals (Rr) in the aqueous phase is 1014 per second per milliliter and the value of the number of M/P particles is 1015 per milliliter, then assuming that all the radicals generated eventually enter M/P particles (since there are no micelles), the rate of radical entry into a particle will average out to about 0.1 radical per second, that is, one radical every 10 s. When a (soap-like) free radical enters a M/P particle, it initiates the polymerization of monomer in the particle, but the polymerization is terminated when another free radical enters the same particle (since, as a simple calculation using known kt values shows, two radicals cannot coexist in the same particle and they would terminate mutually within a few thousandths of a second). The particle thereafter remains inactive till another free radical enters and initiates the polymerization afresh. Thus, if a radical enters a M/P particle every 10 s as calculated above, the particle will experience alternate periods of activity (growth) and inactivity (no growth), each of 10 s duration. In other words, each M/P particle will remain active for half of the total time and inactive for the other half. This situation will remain unchanged even if there is a change in the rate of radical entry into the particle. The average number of active radical per particle can thus be considered to be ½ and independent of the rate at which radicals enter the particle. (These concepts are known collectively as the Smith–Ewart theory). The rate of polymerization in a M/P particle, Rpp (mol s−1), is thus given by [5] Rpp =

kp ½M 2NAv

(1.58)

where kp is the propagation rate constant (L mol−1 s−1), [M] denotes the monomer concentration (mol/L) in a M/P particle, and NAv is Avogadro’s number. If Rpp is constant and the number of particles per unit volume, Np (L−1), is constant, then the overall rate of emulsion polymerization per unit volume, Rp (mol L−1 s−1), is simply given by −

Np kp ½M d½M = Rp = dt 2NAv

(1.59)

Characteristics of Polymers and Polymerization Processes

Substituting for Np from Equation 1.56 gives Rp = 0:53ðas ws Þ0:6

0:4 kp ½M Rr u 2NAv

(1.60)

According to the simple Smith–Ewart model described above, the rate of polymerization in Stage II will thus depend on the 0.6 power of the surfactant concentration (ws) and the 0.4 power of the rate of radical generation in aqueous phase, Rr, which, in turn, is related to the aqueous phase (Stage I) initiator concentration, [I], through Equation 1.57. Since chain termination takes place as soon as a radical enters an active M/P particle, the rate of chain termination can be equated to the rate of radical entry into the particles (Rr/Np) and the degree of polymerization, which is given by the ratio of the rate of chain propagation to the rate of chain termination, can be expressed as [5] DPn =

kp ½M N k ½M = p p Rr Rr =Np

(1.61)

Equation 1.61 shows that the degree of polymerization DPn , like the rate of polymerization Rp (Equation 1.56), is directly dependent on the number of particles. Thus, unlike polymerization by the bulk, solution, and suspension techniques, polymerization by the emulsion technique permits simultaneous increase in rate and degree of polymerization by increasing the number of polymer particles (Np), that is, by increasing the surfactant concentration, at a fixed rate of initiation. This possibility of combining high molecular weight with high polymerization rate is one reason for the popularity of the emulsion technique. The Smith–Ewart kinetic scheme, described above, is highly idealized, though valuable for its simplicity. It explains adequately only a small part of the vast literature on emulsion polymerization. Thus, it works well for monomers such as styrene, butadiene, and isoprene, which have very low water solubility (0.1–0.15 mm in diameter), which can accommodate more than one growing chain simultaneously. 1.4.1.3 Microemulsion Polymerization According to IUPAC definition, microemulsions are dispersions (made of water, oil, and surfactants) that are optically isotropic and thermodynamically stable and have dispersed droplet diameters varying approximately from 1 to 100 nm (usually 10 to 50 nm, i.e., 100 to 500 Å). The small particle size leads to a translucent or even to a transparent system if the particle size is a few hundred angstroms. In comparison, the average diameter of droplets in an ordinary emulsion (macroemulsion), discussed above, is in the micron range. Microemulsions, like micellar dispersions, are liquid dispersions containing surfactant aggregates. However, whereas in micellar dispersions the aggregates are made of surfactant only and are usually dispersed in water, in microemulsions, the aggregates are much larger and have large liquid cores (oil or water) surrounded by a surfactant monolayer that stabilizes the dispersion. In many cases, the micellar aggregates are spherical, but they can also be tubular and, in a few cases, they can also grow very long and entangle like polymers [18]. While the aqueous component of microemulsions may contain salt(s) and/or other ingredients, the “oil” component may actually be a mixture of different hydrocarbons and olefins. Microemulsions form upon simple mixing of the components and do not require the high-shear conditions generally used to make ordinary (macro) emulsions. Besides the optical clarity (or translucency) of a microemulsion and the small (10–50 nm) droplet size of the dispersed phase, an additional feature that distinguishes it from ordinary emulsions is that the average drop size does not grow with time owing to thermodynamic stability.

Plastics Technology Handbook

There are three basic types of microemulsions, namely, direct (oil dispersed in water, denoted by “o/w”), reversed or inverse (water dispersed in oil, denoted by “w/o”), and bicontinuous (i.e., regions of water and oil). The domains of the dispersed phase are either globular or interconnected (giving a bicontinuous microemulsion), as shown schematically in Figure 1.29. These are stabilized by an interfacial film of surfactant (usually in combination with a cosurfactant), its molecules being oriented at the interface such that the hydrophilic ends are in the aqueous phase and the hydrophobic ends are in the oil phase. To prepare a microemulsion, in a simple procedure, milky emulsions can be first prepared using water, oil, and surfactant and then lower alkanols (butanol, pentanol, and hexanol) can be added in controlled amounts so as to obtain transparent or translucent solutions comprising dispersions of either water-in-oil (w/o) or oil-in-water (o/w) in nanometer or colloidal dispersions. The lower alcohols added are called cosurfactants. They lower the interfacial tension between oil and water sufficiently for almost spontaneous formation of the aforesaid microheterogeneous systems. Various surfactant-to-cosurfactant ratios can be used in the preparation. The miscibility of oil, water, and amphiphile (i.e., surfactant plus cosurfactant) depends on the overall composition, which, in turn, depends on the system. Ternary (water/surfactant/oil), pseudo-ternary (water/amphiphile/oil), or explicitly quaternary (water/surfactant/cosurfactant/oil) phase diagrams are usually employed to describe the phase manifestation that is essential in the study of microemulsions. These phase diagrams help define the microemulsion areas. Samples from the best combinations, that is, those that produce the largest area of microemulsion, can be subjected to further studies. The knowledge of phase manifestations of the pseudo-ternary (water/amphiphile/oil) or explicitly quaternary (water/surfactant/cosurfactant/oil) mixtures has been systematized. According to Winsor [19], four types of microemulsion phases exist in equilibrium. These phases are commonly referred to as Winsor phases; they are Winsor I: with two phases, the lower o/w microemulsion phase in equilibrium with the upper excess oil; Winsor II: with two phases, the upper w/o microemulsion phase in equilibrium with excess water; Winsor III: with three phases, middle microemulsion phase (o/w plus w/o, called bicontinuous) in equilibrium with upper excess oil and lower excess water; Winsor IV: in single phase, with oil, water, and surfactant homogeneously mixed. Interconversion among these phases can be achieved by varying the proportions of the components. Figure 1.30 gives a composite representation of the aforesaid features of microemulsion forming systems. Being thermodynamically stable, “nano-dispersions” of water-in-oil or oil-in-water, microemulsions can be considered as microreactors to carry out chemical reactions and, in particular, to synthesize nanomaterials. Microemulsions thus have received much recent attention as media for synthesis of nanoparticles like Pt, Pd, Rh, and Ir (by reducing the corresponding salts in the water micropools of w/o microemulsions with hydrazine or hydrogen gas) and as media for polymerization to produce thermodynamically stable latexes in the nanosize range (106) can be produced from monomers at fast reaction rates in both emulsion and microemulsion processes, it is only in the latter that stable latexes with polymer particles smaller than 50 nm can be easily obtained, since in this case polymerization occurs in the monomer reservoir encapsulated in a nanosize space [20]. While microemulsions exhibit a wide variety of microstructures, it is the spherical oil-in-water (o/w) or water-in-oil (w/o) microstructures that have provoked the greatest interest to carry out polymerization in practice. The phase diagram concept provides a reliable basis for the use of microemulsion in polymerization systems. The phase diagram maps the thermodynamically stable regions of o/w and w/o microemulsions. Experimental determination of single-phase (microemulsion) regions thus precedes microemulsion polymerization. In fact, a thorough study involving composition and characterization of phase diagrams of various systems, including those that contain monomer, must be done before performing any microemulsion polymerization. To give an example, for polymerization of methyl acrylate in microemulsions using sodium dodecyl sulfate (SDS) (C12H25SO4Na) as surfactant and pentanol (C5H11OH) as cosurfactant, Stoffer and Bone [21], in their pioneering work on microemulsion polymerization, determined boundaries of single-phase region by titrating with water various surfactant/cosurfactant/monomer mixtures, first to the point of dissolution of surfactant (which marked one limit of the solubility region) and then to the appearance of turbidity (which defined another limit). Complementary information was obtained by titration of water/surfactant mixtures with a cosurfactant/monomer solution. Figure 1.31 thus shows a microemulsion region that contains 25% methyl acrylate. While Stoffer and Bone used w/o microemulsions stabilized by SDS and pentanol to carry out polymerization of methyl acrylate and methyl methacrylate at 50°C using oil-soluble initiator benzoyl peroxide or AIBN and hence observed kinetic similarity to solution polymerization, Atik and Thomas [22] carried out polymerization of styrene in o/w microemulsion and provided the first account of a microemulsion polymerization that produces nanosize spherical latex particles (Figure 1.32). They, however, found that the stability of the microemulsion was limited by the solubility of the polymer formed.

Plastics Technology Handbook

D (H2O) 10

50 30

90 B 10 (Surfactant)

C 90 (Cosurfactant)

FIGURE 1.31 A microemulsion region (schematic) containing water, surfactant (sodium dodecyl sulfate), cosurfactant (pentanol), and 25% monomer (methyl methacrylate). (After Stoffer, J. O. and Bone T. 1980. J. Polym. Sci. Polym. Chem. Ed., 18, 2641.)

200 nm

FIGURE 1.32 Electron micrograph of radiation-polymerized styrene microemulsion stabilized by surfactant cetyl trimethyl ammonium bromide and cosurfactant hexanol. (Reprinted with permission from Atik, S. S. and Thomas, K. J. 1981. J. Am. Chem. Soc., 103, 4279. Copyright 1981 American Chemical Society.)

Guo et al. [23] also carried out microemulsion polymerization of styrene in an o/w system with SDS surfactant and 1-pentanol cosurfactant using a water-soluble K2S2O8 or an oil-soluble 2,2′-azobis-(2methylbutyronitrile) (AMBN) initiator at 70°C. The reaction yielded stable latex of small size (20–30 nm) and high molecular weight (1.2 × 105), which implied that each latex particle consisted of two or three polystyrene molecules. The maximum polymerization rate and number of particles varied with the 0.47 and 0.40 powers of K2S2O8 concentration and 0.39 and 0.38 powers of AMBN, respectively, in agreement with the 0.4 power predicted by Smith–Ewart theory, Case II (see above). This consistency was attributed to the comparable size of microemulsion droplets and micelles. Since the work of Atik and Thomas in 1981, most work was done in four- or ﬁve-component (including cosurfactant) microemulsion. However, the presence of a fourth component, such as alcohol cosurfactant, substantially limits the utility of microemulsion, mainly for two reasons. First, the cosurfactant complicates the phase behavior of the microemulsion system, whereas the ternary microemulsion formulations

Characteristics of Polymers and Polymerization Processes

are considerably easier to deal with. Second, alcohol present as a cosurfactant can act as a chain transfer agent, interfering with the desired polymerization and reducing the polymer molecular weight. In this context, the first report on polymerization in ternary microemulsions (without cosurfactant alcohol) by P’erez-Luna et al. [24] using cationic surfactant dodecyl trimethylammonium bromide (DTAB) and styrene monomer assumes significance. The one-phase microemulsion region was determined visually by styrene titration of aqueous micellar solution of DTAB at a fixed temperature to obtain phase boundaries that were also checked by preparing samples with compositions below and above the titration determined phase boundaries. In the one-phase microemulsion region of styrene/water/DTAB mixtures thus determined at 60°C (using a few parts per million of hydroquinone to inhibit thermal polymerization), styrene polymerization was carried out with K2S2O8 (1 wt% with respect to the monomer) as initiator. In unpolymerized microemulsions, two apparent particle sizes were always observed— a small size of ca. 0.6–0.8 nm in hydrodynamic radius and a larger composition-dependent size (6–15 nm), believed to be DTAB micelles and styrene-swollen droplets, respectively. The microlatexes produced by polymerization remained stable with respect to coagulation for months and the particles were spherical with radii in the range 20–30 nm and apparently monodisperse when observed with TEM (see Figure 1.33). The anionic surfactant sodium bis(2-ethylhexyl sulfosuccinate) (AOT, shown below) produces microemulsion without the addition of any cosurfactant. CH3

CH2

CH3

(CH2)3

CH2

CH3

(CH2)3

CH2

CH3

CH2

SO3Na

CH2

(AOT)

In the ﬁrst account of inverse microemulsion polymerization reported in 1982, Leong and Candau [25] used inverse (w/o) microemulsion consisting of water–acrylamide mixture dispersed in toluene and stabilized by AOT surfactant without requiring the presence of any cosurfactant. (The microemulsion, however, needed the presence of a large amount of the surfactant.) Photopolymerization of the inverse

100 nm

FIGURE 1.33 Transmission electron micrograph of a microemulsion containing 13.8 wt% DTAB, 8 wt% styrene, and 78.2 wt% water after polymerization at 60°C with K2S2O8 initiator (1 wt% with respect to monomer). (Reprinted with permission from P’erez-Luna, V. H., Puig, J. E., Castano, V. M., Rodriguez, B. E., Murthy, A. K., and Kaler, E. W. 1990. Langmuir, 6, 1040. Copyright 1990 American Chemical Society.)

Plastics Technology Handbook

microemulsion was rapid with total conversion to polymer in less than 30 min, producing lowpolydispersity polyacrylamide of high molecular weight (∼3 × 106) conﬁned in small (FAA. On the other hand, if FAA or FBB>FAB, the system will be incompatible and the molecules will separate, forming two phases. In the absence of any speciﬁc interaction (e.g., hydrogen bonding) between solvent and solute, we can reasonably assume the intermolecular attraction forces between the dissimilar molecules to be approximately given by the geometric mean of the attraction forces of the corresponding pairs of similar molecules; that is, FAB = (FAAFBB)1/2. Consequently, if FAA and equal, FAB will also be similar and the materials should be soluble.

1.13.1 Solubility Parameter A measure of the intermolecular attraction forces in a material is provided by the cohesive energy. Approximately, this equals the heat of vaporization (for liquids) or sublimation (for solids) per mol. The cohesive energy density in the liquid state is thus (DEv/V, in which DEv is the molar energy of vaporization and V is the molar volume of the liquid. The square root of this cohesive energy density is known as the solubility parameter (d), that is, d = (DEv =V)1=2

(1.84)

If the vapor behaves approximately like an ideal gas, Equation 1.84 can be written as d = ½(DHv − RT)=V1=2 = ½(DHv − RT)r=M1=2

(1.85)

where DHv is the molar enthalpy of vaporization and r is the density of liquid with molecular weight M. For a volatile liquid cohesive energy density and, hence d, can be determined experimentally by measuring DHv and r.

Characteristics of Polymers and Polymerization Processes

Example 7: Calculate an estimate of the solubility parameter for water at 25°C from its heat of vaporization at the same temperature, given by H2 O(l) = H2 O(g),

DH25°C = 10:514 kcal

Answer: From Equation 1.85, d 2 = ½(10, 514 cal mol−1 ) − (1:987 cal mol−1 K−1 )(298 K)(1 g cm−3 )=(18 g mol−1 ) = 551:2 cal cm−3 = 2:3 109 J m−3 d = 23:5 (cal cm−3 )1=2 = 48:0 103 (J m−3 )1=2 = 48 MPa1=2 [Conversion factors: 1 cal cm−3 = 4.184 × 106 m−3 = 4.184 × 106 Pa = 4.184 MPa. Hence, 1 (cal cm−3)1/2 = 2.045 MPa1/2] Hildebrand [37] ﬁrst used the solubility parameter approach for calculating estimates of the enthalpy of mixing, DHmix, for mixtures of liquids. The equation employed is DHmix = Vmix f1 f2 (d 1 − d 2 )2

(1.86)

where Vmix is the molar volume of the mixture, and d1 and d2 are the solubility parameters of components 1 and 2, respectively. A necessary requirement for solution and blending compatibility is a negative or zero Gibbs free energy change (DGmix) when the solution or blend components are mixed, that is, DGmix = DHmix − TDSmix ≤ 0

(1.87)

Since the ideal entropy of mixing (DSmix) is always positive, the components of a mixture can be assumed to be miscible only if DHmix≤TDSmix. Solubility therefore depends on the existence of a zero or small value of DHmix, only positive (endothermic) heats of mixing being allowed, as in Equation 1.86. Miscibility or solubility will then be predicted if the absolute value of the ((d1−d2) difference is zero or small [38]. Speciﬁc effects such as hydrogen bonding and charge transfer interactions can lead to negative DHmix but these are not taken into account by Equation 1.86, and separate considerations must be applied in order to predict their effect on miscibility and solubility [39]. Solubility parameters of solvents can be correlated with the density, molecular weight, and structure of the solvent molecule. According to the additive method of Small [40], the solubility parameter is calculated from a set of additive constants, F, called molar attraction constants, by the relationship d=

r X F M

(1.88)

X where F is the molar attraction constants summed over the groups present in the compound; r and M are the density and the molar mass of the compound. The same procedure is applied to polymers and EquationX 1.88 is used, wherein r is now the density of the amorphous polymer at the solution temperature, Fi is the sum of all the molar attraction constants for the repeat unit, and M is the molar mass of the repeat unit. Values of molar attraction constants for the most common groups in organic molecules were estimated by Small [40] from the vapor pressure and heat of vaporization data for a number of simple molecules. A modiﬁed version of a compilation of molar attraction constants [41,42] is reproduced in Table 1.7. An example of the use of the tabulated molar attraction constants is given in the problem worked out below.

Plastics Technology Handbook TABLE 1.7 Group Molar Attraction Constants Group

Molar Attraction, F (cal cm3)1/2 mol−1

–CH3

147.3

–CH2–

131.5

>CH– >C
C═(oleﬁn)

121.53 84.51

–CH═(aromatic)

117.12

–C═(aromatic) –O–(ether, acetal)

98.12 114.98

–O–(epoxide)

176.20

–COO– >C═O

326.58 262.96

–CHO

292.64

(CO)2O –OH–

567.29 225.84

–OH aromatic

170.99

–NHs –NH–

226.56 180.03

–N– –C≡N –N═C═O

61.08 354.56 358.66

–S–

209.42

Cl2 –Cl (primary)

342.67 205.06

–Cl (secondary)

208.27

–Cl (aromatic) –Br

161.0 257.88

–Br (aromatic)

205.60

–F

41.33

Structure Feature Conjugation Cis

23.26 −7.13

Trans

−13.50

5-membered ring 6-membered ring

20.99 −23.44

Ortho substitution

9.69

Meta substitution Para substitution

6.6 40.33

Source: Hoy, K. L. 1970. J. Paint Technol., 42, 76 and Brandrup J. and Immergut E. eds., 1975. Polymer Handbook, 2nd Ed. Wiley Interscience, New York.

Characteristics of Polymers and Polymerization Processes

Example 8: Calculate an estimate of the solubility parameter for the epoxy resin DGEBA (diglycidyl ether of bisphenol A) having the repeat unit structure as shown below and density 1.15 g/cm3. CH3 O

CH2

CH3

CH2 n

Answer: M (for repeating unit) = 284 g/mol. F (cal cm3)1/2/mol

Groups –CH3

147.3

294.60

–CH2

131.5

263.00

1 1

85.99 32.03

>CH– >C
> ;

nonradical products

(1.125)

In the light of the above photooxidation scheme there are four possibilities of protection against UV light. These are based on (1) prevention of UV light absorption or reduction of the amount of light absorbed by the chromophores; (2) reduction of the initiation rate through deactivation of the excited states of the chromophoric groups: (3) intervention in the photooxidative degradation process by transformation of hydroperoxides into more stable compounds, without generation of free radicals, before hydroperoxides undergo photolytic cleavage; and (4) scavenging of the free radicals as soon as possible after their formation, either as alkyl radicals or as peroxy radicals. According to the four possibilities of UV protection mechanism described above, the light stabilizer classes can be designated as (1) UV absorbers, (2) quenchers of excited states, (3) hydroperoxide decomposers, and (4) free radical scavengers. It must be mentioned, however, that this classiﬁcation is a simpliﬁcation and that some compounds may be active in more than one way and often do so. 1.19.1.1 UV Absorbers The protection mechanism of UV absorbers is based essentially on absorption of harmful UV radiation and its dissipation in a manner that does not lead to photosensitization, i.e., conversion to energy corresponding to high wavelengths or dissipation as heat. Besides having a very high absorption themselves, these compounds must be stable to light, i.e., capable of absorbing radiative energy without undergoing decomposition. Hydroxybenzophenones and hydroxyphenyl benzotriazoles are the most extensively studied UV absorbers. Though the main absorptions of 2-hydroxybenzophenone are situated in the uninteresting wavelength domain around 260 nm, substituents such as hydroxy and alkoxy groups push this absorption towards longer wavelengths, between 300 and 400 nm, and at the same time total absorption in the UV absorbers (XII) are essentially derived from 2,4-dihyrdoxybenzophenone (X, R═H). Through choice of adequate alkyl group R in the alkoxy groups it is possible to optimize the protective power and the compatibility with the plastics to be stabilized. O

C R = H, CH3 to C12H25 OR (XII)

Derivatives of 2-hydroxybenzophenone have highly conjugated structures and a capacity to form intramolecular hydrogen bonds that exert a decisive inﬂuence on the spectroscopic and photochemical properties of these compounds. It has been shown with 2-hydroxybenzophenone (XIII) that on exposure to light (XIII) is transformed into enol (XIV), which turns back into its initial form (XIII) on losing thermal energy to the medium (Reaction 1.126): O

OH hν

(1.126)

(–heat) (XIII)

(XIV )

123

Characteristics of Polymers and Polymerization Processes

The light energy consumed by the UV absorber corresponds to the quantity of energy needed to break the hydrogen bond. This explanation is supported by the fact that compounds that cannot lead to the formation of intramolecular hydrogen bonds (benzophenone or 2-methoxybenzophenone) do not absorb in the UV wavelength range. Hydroxyphenyl benzotriazoles have the structure (XV) where X is H or Cl (chlorine shifts the absorption to longer wavelengths), R1 is H or branched alkyl, and R2 is CH3 to C8H17 linear and branched alkyl (R1 and R2 increase the afﬁnity to polymers). Some technically important materials in this class are 2-(2′-hydroxy-5′-methyl-phenyl)-benzotriazole, 2-(2′-hydroxy-3′-5′-di-tert-butyl-phenyl)-benzotriazole, and 2-(2′-hydroxy-3′,5′-di-tert-butyl-phenyl)-5-chlorobenzotriazole. In comparison with 2-hydroxyben zophenones, the 2-(2′-hydroxyphenyl) benzotriazoles have higher molar extinction coefﬁcients and steeper absorption edges towards 400 nm. HO

N N N

(XV)

The exact mechanism of light absorption by hydrobenzotriazoles is not known. However, the formation of intramolecular hydrogen bond, as in (XVI), and of zwitter ions having a quinoid structure, as in (XVII), may be responsible for the transformation of light radiation energy into chemical modiﬁcations: H

N N X

+ N

(1.127)

NR2

R2 (XVII)

(XVI)

It may be noted that the tendency to form chelated rings by the creation of hydrogen bonds between hydroxide and carbonyl groups [as in (XIII)] or groups containing nitrogen [as in (XVI)] is a characteristic property of all UV absorbers. A fundamental disadvantage of UV absorbers is the fact that they need a certain absorption depth (sample thickness) for good protection of a plastic. Therefore, the protection of thin section articles, such as films and fibers, with UV absorbers alone is only moderate. 1.19.1.2 Quenchers Quenchers (Q) are light stabilizers that are able to take over energy absorbed from light radiation by the chromophores (K) present in a plastic material and thus prevent polymer degradation. The energy absorbed by quenchers can be dissipated either as heat (Reaction 1.130) or as fluorescent or phosphorescent radiation (Reaction 1.131): K + hn ! K*

(1.128)

K + Q ! K + Q*

(1.129)

Q* ! Q + heat

(1.130)

Q* ! Q + hn0

(1.131)

124

Plastics Technology Handbook

For energy transfer to occur from the excited chromophore K* (donor) to the quencher Q (acceptor), the latter must have lower energy states than the donor. The transfer can take place by two processes: (1) longrange energy transfer or Föster mechanism and (2) contact, or collisional, or exchange energy transfer. The Förster mechanism is based on a dipole-dipole interaction and is usually observed in the quenching of excited states. It has been considered as a possible stabilization mechanism for typical UV absorbers with extinction coefficients greater than 10,000. The distance between chromophore and quencher in this process may be as large as 5 or even 10 nm, provided there is a strong overlap between the emission spectrum of the chromophore and the absorption spectrum of the quencher. However, for an efficient transfer to take place in the contact or exchange energy transfer process, the distance between quencher and chromophore must not exceed 1.5 nm. From calculations based on the assumption of random distribution of both stabilizer and sensitizer in the polymer, it is thus concluded that exchange energy transfer cannot contribute significantly to stabilization. This would not apply, however, if some kind of association between sensitizer and stabilizer takes place (for example, through hydrogen bonding). Considering the dominant role of hydroperoxides in polyolefin photooxidation [cf. Equation 1.121 and Equation 1.122], quenching of excited –OOH groups would contribute significantly to stabilization. However, since the –OOH excited state is dissociative, i.e., its lifetime is limited to one vibration of the O–O bond, contact energy transfer during this very short time (about 10–3 s) appears highly unlikely if the –OOH group is not already associated with the quencher. The quenching action being thus independent of the thickness of the samples, quenchers are specifically useful for the stabilization of thin section articles such as films and fibers. Metallic complexes that act as excited state quenchers are used to stabilize polymers, mainly polyolefins. They are nickel and cobalt compounds corresponding to the following structure: O H C

N Ni O

H O (XVIII)

Metallic complexes based on Ni, Co, and substituted phenols, thiophenols, dithiocarbamates, or phosphates are used. Typical representatives are nickel-di-butyldithiocarbamate, n-butylamin-nickel2,2′-thio-bis-(4-tert-octyl-phenolate), nickel-bis-[2,2′-thio-bis-(4-tert-octyl-phenolate)] and nickel-(Oethyl-3,5-di-tert-butyl-4-hydroxy-benzyl)-phosphonate. But their use is not as widespread as for other UV absorbers because they tend to be green. 1.19.1.3 Hydroperoxide Decomposers Since hydroperoxides play a determining role in the photooxidative degradation of polymers, decomposition of hydroperoxides into more stable compounds, before the hydroperoxides undergo photolytic cleavage, would be expected to provide an effective means of UV protection. Metal complexes of sulfurcontaining compounds such as dialkyldithiocarbamates (XIX), dialkyldithiophosphates (XX) and thiobisphenolates (XXI) are very efficient hydroperoxide decomposers even if used in almost catalytic quantities. Besides reducing the hydroperoxide content of pre-oxidized polymer films, they also can act as very efficient UV stabilizers. This explains the fact that an improvement in UV stability is often observed on combining UV absorbers with phosphite or nickel compounds. 1.19.1.4 Free-Radical Scavengers Besides the absorption of harmful radiation by UV absorbers, the deactivation of excited states by quenchers, and the decomposition of hydroperoxides by some phosphorus and/or sulfur containing

125

Characteristics of Polymers and Polymerization Processes

compounds, the scavenging of free-radical intermediates is another possibility of photostabilization, analogous to that used for stability against thermooxidative degradation. It has been shown that compounds (XXI), (XXII), (XXIII), and (XXIV) are effective radical scavengers. The radicals generated by hemolytic cleavage of hydroperoxide (Equation 1.132):

M 2+

(C 4H 9) 2 NC

CN(C 4H 9 ) 2 M = Zn, Ni

S (XIX)

(i–C 3H 7) 2 N

S Ni

P (i–C 3H 7) 2 N

N(i–C 3 H 7) 2

P S

N(i–C 3 H 7) 2

(XX) NH 2C 4H 9

Ni O

O S R = tert. octyl

R (XXI)

PPOOH ! PPO• +• OH

(1.132)

HO• + InH ! ½InH…• OH

(1.133)

PPO• + InH ! PPOH + In•

(1.134)

may be removed by reactions, such as

with a radical scavenger InH. The latest development in the ﬁeld of light stabilizers for plastics is represented by sterically hindered amine-type light stabilizers (HALS). A typical such compound is bis-(2,2,6,6-tetramethyl-4-piperidyl)sebacate (XXV). Since it does not absorb any light above 250 nm, it cannot be considered a UV absorber or a quencher of excited states. This has been conﬁrmed in polypropylene through luminescence measurements.

126

Plastics Technology Handbook

C4H9O

CH2 P

OC4H9 P CH2

Ni O

(

= tert butyl) (XXII) O

OC12H25

HO (XXIII)

(XXIV)

O O C (CH2)8

(XXV)

A low-molecular weight HALS such as (XXV), denoted henceforth as HALS-I, has the disadvantage of relative volatility and limited migration and extraction resistance, which are undesirable in special plastics applications (for example, in ﬁne ﬁbers and tapes). For such applications, it is advantageous to use polymeric sterically hindered amines such as poly-(N-b-hydroxyethyl-2,2,6,6-tetramethyl-4hydroxypiperidyl succinate) represented by (XXVI) and a more complex polymeric hindered amine represented by (XXVII). In later discussions, they will be designated as HALS-II and HALS-III, respectively. Though they do not reach completely the performance of the low-molecular weight HALS-I, they are nevertheless superior to the other common light stabilizers used at several-fold higher concentrations.

CH 2 CH 2 OCCH 2 CH 2 C O

(XXVI)

( CH2 )6

NH tert octyl

(XXVII)

The protection mechanisms of HALS, known so far mostly from studies with model systems, can be summarized as follows: From ESR measurements it is concluded that, under photooxidative conditions,

127

Characteristics of Polymers and Polymerization Processes

HALS are converted, at least in part, to the corresponding nitroxyl radicals (XXVIII). The latter, through Reaction 1.135, are thought to be the true radical trapping species. R

O (XXVIII)

OR (XXIX)

(1.135)

Another explanation of the UV protection mechanism of HALS involves the hydroxylamine ethers (XXIX) formed in Reaction 1.135. There is indirect evidence that (XXIX) can react very quickly with peroxy radicals, thereby regenerating nitroxyl radicals (Reaction 1.136). Reaction 1.135 and Reaction 1.136, which constitute the “Denisov cycle,” result in an overall slowdown of the usual chain oxidation Reaction 1.119 and Reaction 1.120. R

+ RO 2R

RO 2

OR (XXIX)

O (XXVIII)

(1.136)

Nitroxyl radicals may also react with polymer radical to form hydroxylamine (Reaction 1.137), and the latter can react with peroxy radicals and hydroperoxides according to Reaction 1.138 and Reaction 1.139:

NO +R

NOH + RO 2

NOH +ROOH

NOH + C = C

(1.137)

NO + RO 2H

(1.138)

NO + H 2O + RO

(1.139)

The formation of associations between HALS and hydroperoxides followed by reaction of these associations with peroxy radicals (Reaction 1.140 and Reaction 1.141) represents another possibility of retarding photooxidation. NH + HOOR

[ NH... HOOR]+ RO 2

[ NH... HOOR]

ROOR+ H 2O + NO

(1.140)

(1.141)

128

Plastics Technology Handbook

The hydroperoxides associated with HALS may undergo photolysis producing hydroxy and alkoxy radicals in close proximity to the amines. The radicals may then abstract a hydrogen atom from the amine and form hydroxylamine and hydroxylamine ethers (Reaction 1.142):

OH NH ... HOOR

NOH + ROH

(1.142)

N H OR

NOR + H 2O

Still other mechanisms have been postulated, e.g., the interaction of HALS with a,b,-unsaturated carbonyl compounds and the formation of charge-transfer complex between HALS and peroxy radicals. However, despite extensive publications in the ﬁeld, a complete knowledge of the process occurring in polymer photooxidation in the presence of HALS is still not available.

1.20 Light Stabilizers for Selected Plastics In choosing a light stabilizer for a given plastic, several factors, in addition to its protective power, play an important role. In this respect, one may cite physical form (liquid/solid, melting point), thermal stability, possible interaction with other additives and ﬁllers that may eventually lead to discoloration of the substrate, volatility, toxicity (food packaging), and above all, compatibility with the plastics material considered. An additive can be considered as compatible if, during a long period of time, no blooming or turbidity is observed at room temperature and at the elevated temperatures that may be encountered during projected use of the plastic material. Among the light stabilizer classes available commercially, only a few may be used in a broad range of plastics. Thus, nickel compounds are used almost exclusively in polyoleﬁns, whereas poly(vinyl chloride) is stabilized with UV absorbers only.

1.20.1 Polypropylene For the light stabilization of polypropylene, representatives of the following stabilizer classes are mainly used: 2-(2′-hydroxyphenyl)-benzotriazoles, 2-hydroxy-4-alkoxybenzophenones, nickel-containing light stabilizers, 3,5-di-tert-butyl-4-hydroxybenzoates, as well as sterically hindered amines (HALS). Nickelcontaining light stabilizers are used exclusively in thin sections, such as ﬁlms and tapes, whereas all other classes may be used in thin as well as thick sections, though UV absorbers have only limited effectiveness in thin section. Nickel-containing additives are also used as “dyesites” because they allow the dying and printing of polypropylene ﬁbers with dyestuffs susceptible to complexation with metals.

1.20.2 Polyethylene As all polyoleﬁns, polyethylene is sensitive to UV radiation, although less than polypropylene. For outdoor use polyethylene needs special stabilization against UV light. The light stabilizers for polyethylene are in principle the same as for polypropylene. On accelerated weathering, HALS show much better performance in HDPE tapes than UV absorbers, despite the latter being used in much higher concentrations. The comparison between HALS is, however, in favor of the polymeric HALS-III, which has the same performance when added at a concentration of 0.05% as HALS-I and HALS-II at 0.1%. Among the numerous commercial light stabilizers only a few are suitable for low density polyethylene (LDPE). This is mainly due to the fact that most light stabilizers are not sufﬁciently compatible at levels of

Characteristics of Polymers and Polymerization Processes

129

concentration necessary for the required protection and so they bloom more or less rapidly. Initially, UV absorbers of the benzophenone and benzotriazole types were used to protect LDPE materials. With the development of nickel-quenchers, significant improvement in the light stability of LDPE films has been achieved. For reasons of economy, however, combinations of nickel-quenchers with UV absorbers are mainly used. The service life of LDPE films may be increased by raising the concentrations of light stabilizers. Additive contents close to 2% may be found in greenhouse films thought to last up to 3 years outdoors. A further improvement in UV stability of LDPE was expected with the development of HALS. However, LDPE compatibility of HALS available in the early years was insufficient, resulting in relatively poor performance on outdoor weathering. It was only with the development of polymeric HALS that these difficulties were overcome. Tests have shown that HALS-II is significantly superior to UV absorbers and Ni-stabilizers so that the same performance can be achieved with much smaller concentrations. However, use of combinations of the polymeric HALS-II with a UV absorber leads to a significant improvement of the efficiency in comparison with the HALS used alone at the same concentration as the combination. A further boost of the performance can be achieved through use of the polymeric HALS-III. For example, the performance of HALS-II can be reached by using HALS-III at about half the concentration. The superiority of HALSIII becomes even more pronounced in films of thickness below 200 mm. In linear low density polyethylene (LLDPE) also, the polymeric HALS-II and HALS-III show much better performance than other commercial light stabilizers. Blooming is observed with low molecular weight HALS-I, similar to that found with LDPE. In more polar substances such as ethylene-vinyl acetate copolymers (EVA), the low molecular weight HALS-I can be used. However, in this substrate too, the polymeric HALS-II and HALS-III are significantly superior to the low molecular weight HALS.

1.20.3 Styrenic Polymers Double bonds are not regarded as the chromophores responsible for initiation of photooxidation in polystyrenes because they absorb below 300 nm. However, peroxide groups in the polymer chain, resulting from co-polymerization of oxygen with styrene, are deﬁnitely photolabile. Moreover, oxidation products such as aromatic ketones of acetophenone type, which have been detected by emission spectroscopy, are formed during processing of styrene polymers at high temperatures. Aromatic ketones (AK) in the triplet state are able to abstract hydrogen from polystyrene (PSH) (Reaction 1.143): AK ! AK* AK* + PSH ! AKH• + PS•

(1.143)

This reaction is considered the most important initiation mechanism for styrene photooxidation in the presence of aromatic ketones. Styrenic plastics such as acrylonitrile/butadiene/styrene graft copolymers (ABS) and impact-resistant polystyrenes are very sensitive towards oxidation, mainly because of their butadiene content. Degradation on weathering starts at the surface and results in rapid loss of mechanical properties such as impact strength. Because of the lack of efﬁcient light stabilizers, ABS has not been used outdoors on a large scale. However, by combining two light stabilizers with different protection mechanisms, e.g., a UV absorber of the benzotriazole class and the sterically hindered amine HALS-I, it is possible to achieve good stabilization even in ABS. This is a case of synergism in which the UV absorber protects the deeper layers, while HALS-I assures surface protection. At the same time discoloration of the ABS polymer is also reduced signiﬁcantly. The same holds for polystyrene and styrene–acrylonitrile copolymers (SAN), and the best

130

Plastics Technology Handbook

protection is obtained with HALS/UV absorber combinations. Light stabilization is necessary for articles of these polymers for which UV exposure can be expected (e.g., covers for ﬂuorescent lights).

1.20.4 Poly(Vinyl Chloride) “Pure” poly(vinyl chloride) (PVC) does not absorb any light above 220 nm. Different functional groups and structural irregularities that may arise during polymerization and processing have thus been considered as possible initiating chromophores. They include irregularities in the polymer chain as well as hydroperoxides, carbonyl groups, and double bonds. Thermal stabilizers used in PVC also confer some degree of light stability. Ba/Cd salts and organic tin carboxylates, for example, confer already some UV stability to PVC on outdoor exposure. However, for transparent and translucent PVC articles requiring high UV stability, the light stability conferred by thermal stabilizers is not sufﬁcient. The addition of light stabilizers in such cases is therefore mandatory. So far, UV absorbers yield the best results in practical use. HALS-I has almost no effect.

1.20.5 Polycarbonate Bisphenol-A polycarbonate (PC) absorbs UV light below 360 nm but its absorption is intense only below 300 nm. Insufficient light stability of PC on outdoor use is manifested by yellowing, which increases rapidly. Studies indicate that on absorption of UV light, PC undergoes photo-Fries rearrangement, which gives first a phenyl salicylate, and after absorption of a second photon and subsequent rearrangement, it gives 2,2′-dihydroxybenzophenone groups (Figure 1.76). The absorption of these groups reaches into the visible region, and PC yellowing has been essentially attributed to them. In addition to the Fries reaction, the formation of O2-charge transfer complexes in PC, similar to those found in polyolefins and leading to the formation of hydroperoxides, is considered to contribute significantly to photooxidation in the early stages. Among the stabilizer classes, only UV absorbers are in use for stabilization of PC. In choosing a UV absorber, intrinsic performance, volatility, adequate thermal stability at the elevated processing temperatures (about 320°C), and effect on initial color of the PC articles should be considered. Benzotriazole, oxanilide, and cinnamate-type UV absorbers are effective photostabilizers for PC with benzotriazoles giving the best performance among the three types.

OH hν

C O

FIGURE 1.76

Photo-Fries rearrangement on polycarbonate.

Characteristics of Polymers and Polymerization Processes

131

1.20.6 Polyacrylates Poly(methyl methacrylate) (PMMA) is highly transparent in the UV region, and thus much more light stable than other thermoplastics. UV absorbers may therefore be used to confer a UV ﬁlter effect to PMMA articles. PMMA window panes for solar protection containing 0.05%–0.2% 2-(2′-hydroxy-5′methylphenyl)-benzotriazole are well-known examples of this application. The rear lights of motor cars, electric signs, and covers for ﬂuorescent lights are some applications for which PMMA is UV-stabilized. The excellent light-stabilizing performance of HALS is also found with PMMA.

1.20.7 Polyacetal Polyacetal is markedly unstable towards lights because even UV radiation of wavelengths as high as 365 nm may initiate its degradation. Polyacetal cannot therefore be used outdoors if it does not contain any light stabilizers. Even after a short weathering, surface crazes and pronounced chalking are observed. Carbon black (0.5%–3%) is a good stabilizer for polyacetal when sample color is not important. Other possibilities for stabilization are the use of 2-hydroxybenzophenone and, especially, hydroxyphenylbenzotriazole-type UV absorbers. Stabilization with HALS/UV absorber is superior to that with UV absorber alone.

1.20.8 Polyurethanes Lights stability of polyurethanes depends to a large extent on their chemical structure, and both components (i.e., isocyanate and polyol) have an inﬂuence. Polyurethanes based on aliphatic isocyanates and polyester diols show the best light stability if yellowing is considered, whereas polyurethanes based on aromatic isocyanates and polyether diols are worst in this respect. Light stabilizers are used mainly in the coatings industry (textile coatings, synthetic leather). In addition to some UV absorbers of the 2-(2′-hydroxyphenyl)-benzotriazole type, HALS used alone or in combination with benzotriazoles are especially effective stabilizers.

1.20.9 Polyamides The absorption of aliphatic polyamides in the short wavelength region of sunlight is attributed largely to the presence of impurities. Direct chain scission at wavelengths below 300 nm and photosensitized oxidation above 300 nm have been considered a long ago as responsible for photooxidation. Antioxidants used in polyamides often confer good light stability as well. However, enhanced performance is obtained with the combination of an antioxidant with a light stabilizer. The sterically hindered amines are signiﬁcantly superior. For example, molded polyamide samples stabilized with HALS (0.5%) are found to exhibit approximately twice the light stability of the samples stabilized with a phenolic antioxidant (0.5%) or combination of the latter with a UV absorber (0.5%).

1.21 Diffusion and Permeability There are many instances where diffusion and permeation of a gas, vapor, or liquid through a plastics material is of considerable importance in the processing and usage of the material. For example, dissolution of a polymer in a solvent occurs through diffusion of the solvent molecules into the polymer, which thus swells and eventually disintegrates. In a plastisol process the gelation of PVC paste, which is a suspension of PVC particles in a plasticizer such as tritolyl phosphate, involves diffusion of the plasticizer into the polymer mass, resulting in a rise of the paste viscosity. Diffusion processes are involved in the production of cellulose acetate ﬁlm by casting from solution, as casting requires removal of the solvent. Diffusion also plays a part in plastic molding. For

132

Plastics Technology Handbook

example, lubricants in plastics compositions are required to diffuse out of the compound during processing to provide lubrication at the interface of the compound and the mold. Incompatibility with the base polymer is therefore an important criterion in the choice of lubricants in such cases. Permeability of gases and vapors through a ﬁlm is an important consideration in many applications of polymers. A high permeability is sometimes desirable. For example, in fruit-packaging applications of plastics ﬁlm it is desirable to have high permeability of carbon dioxide. On the other hand, for making inner tube and tubeless tires, or in a child’s balloon, the polymer used must have low air permeability.

1.21.1 Diffusion Diffusion occurs as a result of natural processes that tend to equalize the concentration gradient of a given species in a given environment. A quantitative relation between the concentration gradient and the amount of one material transported through another is given by Fick’s ﬁrst low: dm = −D

dc A dt dx

(1.144)

where dm is the number of grams of the diffusing material crossing area A (cm2)of the other material in time dt (s). D is the diffusion coefﬁcient (cm2/s) whose value depends on the diffusing species and the material in which diffusion occurs, and dc/dx is the concentration gradient, where the units of x of and c are centimeters and grams per cubic centimeter. Diffusion in polymers occurs by the molecules of the diffusing species passing through voids and other gaps between the polymer molecules. The diffusion rate will therefore heavily depends on the molecular size of the diffusing species and on the size of the gaps. Thus, if two solvents have similar solubility parameters, the one with smaller molecules with diffuse faster in a polymer. On the other hand, the size of the gaps in the polymer depend to a large extent on the physical state of the polymer—that is, whether it is crystalline, rubbery, or glassy. Crystalline structures have an ordered arrangement of molecules and a high degree of molecular packing. The crystalline regions in a polymer can thus be considered as almost impermeable, and diffusion can occur only in amorphous regions or through region of imperfection: hence, the more crystalline the polymer, the greater will be its tendency to resist diffusion. Amorphous polymers, as noted earlier, exist in the rubbery state above the glass transition temperature and in the glassy state below this temperature. In the rubbery state there is an appreciable “free volume” in the polymer mass, and molecular segments also have considerable mobility, which makes it highly probable that a molecular segment will at some stage move out of the way of the diffusing molecule, the contributing to a faster diffusion rate. In the glassy state, however, the molecular segments cease to have mobility, and there is also a reduction in free volume or voids in the polymer mass, both of which lower the rate of diffusion. Thus, diffusion rates will be highest in rubbery polmers and lowest in crystalline polymers.

1.21.2 Permeability Permeation of gas, vapor, or liquid through a polymer film consists of three steps: (1) a solution of permeating molecules in the polymer, (2) diffusion through the polymer due to concentration gradient, and (3) emergence of permeating molecules at the outer surface. Permeability is therefore the product of solubility and diffusion; so where the solubility obeys Henry’s law one may write P = DS where P is the permeability, D is the diffusion coefficient, and S is the solubility coefficient [64].

(1.145)

Characteristics of Polymers and Polymerization Processes

133

Hence, factors which contribute to greater solubility and higher diffusivity will also contribute to greater permeability. Thus, a hydrocarbon polymer like polyethylene should be more permeable to hydrocarbons than to liquids of similar molecular size but of different solubility parameter, and smaller hydrocarbons should have higher permeability. The permeabilities of a number of polymers to the common atmospheric gases [65,66], including water vapor, are given in Table 1.17. It appears that regardless of the film material involved, oxygen permeates about four times a fast as nitrogen, and carbon dioxide about 25 times as fast. The fact that the ratios of the permeabilities for all gases, apart from water vapor, are remarkably constant, provided there is no interaction between the film material and the diffusing gas, leads one to express the permeability as the product of three factors [65]: one determined by the nature of the polymer film, one determined by the nature of the gas, and one accounting for the interaction between the gas and the film; i.e., Pi,k = Fi Gk Hi,k

(1.146)

where Pi,k is the permeability for the system polymer i and gas k; Fi, Gk and Hi,k are the factors associated with the ﬁlm, gas, and interaction respectively. When Hi,k≈1, there is little or no interaction, and as the degree of interaction increases, Hi,k becomes larger. With little or no interaction, Equation 1.146 becomes Pi,k = Fi Gk

(1.147)

It then appears that the ratio of the permeability of two gases (k,l) in the same polymer (i) will be the same as the ratio between the two G factors Pi,k Gk = Pi,l Gl

(1.148)

If the G value of one of the gases, usually nitrogen, is taken as unity, G values of other gases and F values of different polymers can be calculated from Equation 1.148 and Equation 1.147. These values are reliable for gases but not for water vapor. Some F and G values for polymers are given in Table 1.18. Evidently, the F values correspond to the ﬁrst column of Table 1.17, since G for N2 is 1. The G values for O2 and CO2 represent the averages of PO2/PN2 and PCO2/PN2 in columns 5 and 6, respectively.

1.22 Polymer Compounding In many commercial plastics, the polymer is only one of several constituents, and it is not necessarily the most important one. Such systems are made by polymer compounding—a term used for the mixing of polymer with other ingredients. These other ingredients or supplementary agents are collectively referred to as additives. They may include chemicals to act as plasticizing agents, various types of filling agents, stabilizing agents, antistatic agents, colorants, flame retardants, and other ingredients added to impart certain specific properties to the final product [61,67,68]. The properties of compounded plastics may often be vastly different from those of the base polymers used in them. A typical example is SBR, which is the largest-volume synthetic rubber today. The styrenebutadiene copolymer is a material that does not extrude smoothly, degrades rapidly on exposure to warm air, and has a tensile strength of only about 500 psi (3.4 × 105 N/m2). However, proper compounding changes this polymer to a smooth-processing, heat-stable rubber with a tensile strength of over 3,000 psi 20 × 10 N/m2). Since all properties cannot be optimized at once, compounders have developed thousands of specialized recipes to optimize one or more of the desirable properties for particular applications (tires, fan belts, girdles, tarpaulins, electrical insulation, etc.). In SBR the compounding ingredients can be (1) reinforcing fillers, such as carbon black and silica, which improve tensile strength or tear strength; (2) inert fillers and pigments, such as clay, talc, and

11 23 13.0

2.9

– 3.12

64.5

80.8

Polystyrene

Polypropylene (d = 0.910) Butyl rubber

Polybutadiene

Natural rubber

7.8

1,310

1,380

92 51.8

35 352

1.6 10

1.7

0.72 1.53

0.29

CO2 (30°C)

– 4.1 3.0 2.9

– –

3.8

3.9 2.9

2.8

3.8 3.0

3.8

3.3 4.4

5.6

PO2 =PN2

680 –

12,000

130 800

75,000

7,000 1,560

240

2.9 1,300

H2O (25°C, 90% RH)

Source: From Stannett, V. T. and Szwarc, M. 1955. J. Polym. Sci., 16, 89 and Paine, F. A. 1962. J. Roy. Inst. Chem., 86, 263.

233

191

10.6 55

2.8

2.7 19

Polyethylene (d = 0.954, 0.960) Polyethylene (d = 0.922)

0.38 1.20

0.30

0.10 0.22

0.053

O2 (30°C)

Cellulose acetate

0.08

0.10 0.40

Polyamide (Nylon 6) Poly(vinyl chloride) (unplasticized)

0.03 0.05

Rubber hydrochloride (Plioﬁlm ND)

0.0094

Polychlorotriﬂuoro-ethylene Poly(ethylene terephthalate) (Mylar A)

N2 (30°C)

Poly(vinylidene chloride) (Saran)

Polymer

Permeability (P × 1010 cm3/cm2/mm/s/cm Hg)

TABLE 1.17 Permeability Data for Various Polymers

16.2

21.4

– 16.2

13 19

16 25

24 31

PCO2 =PN2

–

– –

4,100

48 42

2,680

70,000 3,900

3,000

97 26,000

1,400

PH2 O =PN2

Rubbery

Crystalline Rubbery

Glassy

Crystalline Semicrystalline

Glassy

Crystalline Semicrystalline

Crystalline

Crystalline Crystalline

Crystalline

Nature of Polymer

Ratios (to N2 Permeability as 1.0)

134 Plastics Technology Handbook

135

Characteristics of Polymers and Polymerization Processes TABLE 1.18 F and G Constants for Polymers and Gases Polymer

Poly(vinylidene chloride) (Saran)

0.0094

Poly(chlorotriﬂuoroethylene)

0.03

Poly(ethylene terephthalate) Rubber hydrochloride (Plioﬁlm)

Gas

1.0

0.05 0.08

O2 H2S

3.8 21.9

Nylon 6

0.1

CO2

24.2

Cellulose acetate (+15% plasticizer) Polyethylene (d = 0.922)

5 19

Ethyl cellulose (plasticized)

Natural rubber Butyl rubber

80.8 3.12

Nitrile rubber

2.35

Polychloroprene Polybutadiene

11.8 64.5

Source: From Stannett, V. T. and Szwarc, M. 1955. J. Polym. Sci., 16, 89.

calcium carbonate, which make the polymer easier to mold or extrude and also lower the cost; (3) plasticizers and extenders, such as mineral oils, fatty acids, and esters; (4) antioxidants, basically amines or phenols, which stop the chain propagation in oxidation; and (5) curatives, such as sulfur for unsaturated polymers and peroxides for saturated polymers, which are essential to form the network of cross-links that ensure elasticity rather than flow. Polymer applications which generally involve extensive compounding are rubbers, thermosets, adhesives, and coatings. Fibers and thermoplastic polymers (with the exception of PVC) are generally not compounded to any significant extend. Fibers, however, involve complex after-treatment processes leading to the final product. PVC, which by itself is a rigid solid, owes much of its versatility in applications to compounding with plasticizers. The plasticize content varies widely with the end use of the product but is typically about 30% by weight. Of the compounding ingredients, fillers and plasticizers are more important in terms of quantities used. Other additives used in smaller quantities are antioxidants, stabilizers, colorants, flame retardants, etc. The ingredients used as antioxidants and light stabilizers, and their effect have been discussed previously. Fillers, plasticizers and flame retardants are described next.

1.22.1 Fillers Fillers play a crucial role in the manufacture of plastics. Alone many plastics are virtually useless, but they are converted into highly useful products by combining them with fillers. For example, phenolic and amine resins are almost always used in combination with substances like wood flour, pure cellulose, powdered mica, and asbestos. Glass fiber is used as a filler for fiber-reinforced composites with epoxy or polyester resins. Another extremely important example is the use of carbon filler for rubber. Rubber would be of little value in modern industry were it not for the fact that the filler carbon greatly enhances its mechanical properties lie tensile strength, stiffness, tear resistance, and abrasion resistance. Enhancement of these properties is called reinforcement, and the fillers which produce the strengthening effect are known as reinforcing fillers. Other fillers may not appreciably increase strength, but they may improve other properties of the polymer, thus, making it easier to mold, which reduces cost. Fillers used in plastics can be divided into two types: particulate and fibrous. Typical fillers in these two categories and the improvements they bring about are summarized in Table 1.19. In some instances they are added to perform one or more prime functions or to provide special properties. Asbestos, for example,

136

Plastics Technology Handbook

Calcium carbonate

Calcium silicate Carbon black

+ +

S S/P +

+ + +

Graphite

Jute Kaolin

+ +

+ + +

S S/P

Carbon ﬁber Cellulose Cotton (macerated/ chopped ﬁbers) Fibrous glass

Recommend forUseina

Processability

Moisture Resistance

Thermal Conductivity

Aluminum powder Asbestos

Electrical Conductivity

Electrical Insulation

Tensile Strength

Hardness

Dimensional Stability

Aipha cellulose

Impact Strength

Heat Resistance

Alumina

Fillers

Stiffness

Chemical Resistance

Properties Improved

Lubricity

TABLE 1.19 Some Fillers and Their Effects on Plastics

+ +

S/P

S S/P S S/P S

+ +

S/P

+ +

S S/P

Kaolin (calcined)

S/P

Mica Molybdenum disulfide Nylon (macerated/ chopped fibers) Acrylic fiber (Orion)

+ +

S/P P

S/P

S/P S/P

S/P

Rayon Silica, amorphous TFE-ﬂuorocarbon Talc Wood ﬂour

Note: P = in thermoplastics only; S = in thermosets only; S/P = in both thermoplastics and thermosets.

provides high-temperature resistance and improves dimensional stability. Mica improves the electrical and thermal properties of all compounds. Glass fibers produce high strength. Carbon black is the only important reinforcing filler for most elastomers. It also imparts UV resistance and coloring. Beryllium oxide-filled resins gain high conductivity without loss of electrical properties. Metal particles have been used as fillers for plastics to improve or impart certain properties. Thus, aluminum has been used for applications ranging from making a decorative finish to improving thermal conductivity. Copper particles are used in plastics to provide electrical conductivity. Lead is used because it dampens vibrations, acts as barrier to gamma-radiation, and has high density. Of the new space-age products used as reinforcing fillers, carbon fibers and boron fibers have the greatest potential for use in high-strength advanced composites. Carbon fibers made by pyrolizing organic fibers such as rayon and polyacrylonitrile have tensile strengths approaching 3.3 × 105 psi (2.3 (109 N/m2). Boron fibers made by depositing boron from a BCl3–H2 mixture onto tungsten wire have tensile strengths approaching 5 × 105 psi (3.5 × 109 N/m2). The specific strengths and specific moduli of polymer composites made with these fibers are far above those attainable in monolithic structural materials such s highstrength aluminum, steel, and titanium. These composites can thus lead to significant weight savings in

Characteristics of Polymers and Polymerization Processes

137

actual applications. A relatively recent addition to the high-performance fiber field is the organic polymeric fiber Kevlar-49, developed by DuPont. It has a higher specific strength than glass, boron, or carbon. Furthermore, Kevlar, a polyamide, is cheap, having one-sixth the price of acrylic-based graphite fibers. The extensive range of fillers and reinforcing agents used nowadays indicates the importance that these materials have attained. The main difference between inert and reinforcing fillers lies in the fact that modulus of elasticity and stiffness are increased to a greater or les extent by all fillers, including the spherical types such as chalk or glass spheres, whereas tensile strength can be appreciably improved only by a fiber reinforcement. The heat deflection temperature, i.e. stiffness at elevated temperatures, cannot be increased by spherical additives to the same extent as by fiber reinforcement. On the other hand, fillers in flake form, such as talc or mica, likewise produce a marked improvement in the heat deflection temperature but lead to a decrease in tensile strength and to elongation at break. The influences of common fillers on the properties of polyolefins are compared in Table 1.20.

1.23 Plasticizers Plasticizers are organic substances of low volatility that are added to plastics compounds to improve their flexibility, extensibility, and processability. They increase flow and thermoplasticity of plastic materials by decreasing the viscosity of polymer melts, the glass transition temperature (Tg) the melting temperature (Tm), and the elasticity modulus of finished products [69]. Plasticizers are particularly used for polymers that are in a glassy state at room temperature. These rigid polymers become flexible by strong interactions between plasticizer molecules and chain units, which lower their brittle-tough transition or brittleness temperature (Tb) (the temperature at which a sample breaks when struck) and their Tg value, and extend the temperature range for their rubbery or viscoelastic state behavior (see Figure 1.52). Mutual miscibility between plasticizers and polymers is an important criterion from a practical point of view. If a polymer is soluble in a plasticizer at a high concentration of the polymer, the plasticizer is said to be a primary plasticizer. Primary plasticizers should gel the polymer rapidly in the normal processing temperature range and should not exude from the plasticized material. Secondary plasticizers, on the other hand, have lower gelation capacity and limited compatibility with the polymer. In this case, two phases are present after plasticization process—one phase where the polymer is only slightly plasticized, and one phase where it is completely plasticized. Polymers plasticized with secondary plasticizers do not, therefore, deform homogeneously when stressed as compared to primary plasticizers. The deformation appears only in the plasticizer-rich phase and the mechanical properties of the system are poor. Unlike primary plasticizers, secondary plasticizers cannot be used alone and are usually employed in combination with a primary plasticizer. Plasticizer properties are determined by their chemical structure because they are affected by the polarity and flexibility of molecules. The polarity and flexibility of plasticizer molecules determine their interaction with polymer segments. Plasticizers used in practice contain polar and nonpolar groups, and their ratio determines the miscibility of a plasticizer with a given polymer. Plasticizers for PVC can be divided into two main groups [67] according to their nonpolar part. The first group consists of plasticizers having polar groups attached to aromatic rings and is termed the polar aromatic group. Plasticizers such as phthalic acid esters and tricresyl phosphate belong to this group. An important characteristic of these substances is the presence of the polarizable aromatic ring. It has been suggested that they behave like dipolar molecules and form a link between chlorine atoms belonging to two polymer chains or to two segments of the same chain, as shown in Figure 1.77a. Plasticizers belonging to this group are introduced easily into the polymer matrix. They are characterized by ability to produce gelation rapidly and have a temperature of polymerplasticizer miscibility that is low enough for practical use. These plasticizers are therefore called solvent-type plasticizers, and their kerosene extraction (bleeding) index is very low. They are, however, not recommended for cold-resistant materials.

Talc Mica

Glass spheres (550 2

40 46

220 65

500

Elongation (%)

– 30

0.95 1.16

1.14

1.11

1.14 1.16

1.13

1.26 1.11

0.92

Density (g/cm3)

Polyethylene Glass ﬁber

High density

30 30

Asbestos

–

40 30

Chalk Glass ﬁber

Filler Content (% by wt)

Polyethylene

Low density

Material

TABLE 1.20 Inﬂuence of Fillers on the Properties of Polyoleﬁns

138 Plastics Technology Handbook

139

Characteristics of Polymers and Polymerization Processes

PVC chains O

O C

PVC chains

Screened Cl atom

O (a)

FIGURE 1.77

(b)

Action of (a) a polar aromatic plasticizer and (b) a polar aliphatic plasticizer on PVC chains.

The second group consists of plasticizers having polar groups attached to aliphatic chains and is called the polar aliphatic group. Examples are aliphatic alcohols and acid or alkyl esters of phosphoric acid (such as trioctyl phosphate). Their polar groups interact with polar sites on polymer molecules, but since their aliphatic part is rather bulky and flexible other polar sites on the polymer chain may be screened by plasticizer molecules. This reduces the extent of intermolecular interactions between neighboring polymer chains, as shown in Figure 1.77b. Polar aliphatic plasticizers mix less well with polymers than do polar aromatics and, consequently, may exude (bloom) from the plasticized polymer more easily. Their polymer miscibility temperature is higher than that for the first group. These plasticizers are called oil-type plasticizers, and their kerosene extraction index is high. Their plasticization action is, however, more pronounced than that of polar aromatic plasticizers at the same molar concentration. Moreover, since the aliphatic portions of the molecules retain their flexibility over a large temperature range, these plasticizers give a better elasticity to finished products at low temperature, as compared to polar aromatic plasticizers, and allow the production of better cold-resistant materials. In PVC they also cause less coloration under heat exposure. In practice, plasticizers usually belong to an intermediate group. Mixtures of solvents belonging to the two groups discussed above are used as plasticizers to meet the requirements for applications of the plasticized material. Plasticizers can also be divided into groups according to their chemical structure to highlight their special characteristics. Several important plasticizers in each group (with their standard abbreviations) are cited below.

1.23.1 Phthalic Acid Esters Di(2-ethyl hexyl) phthalate (DOP) and diisooctyl phthalate (DIOP) are largely used for PVC and copolymers of vinyl chloride and vinyl acetate as they have an affinity to these polymers, produce good solvation, and maintain good flexibility of finished products at low temperature. The use of n-octyl-ndecyl phthalate in the production of plastics materials also allows good flexibility and ductility at low temperature. Diisodecyl phthalate (DDP), octyl decyl phthalate (ODP), and dicapryl phthalate (DCP) have a lower solvency and are therefore used in stable PVC pastes. Butyl octyl phthalate (BOP), butyl decyl phthalate (BDP, and butyl benzyl phthalate (BBP) have a good solvency and are used to adjust melt

140

Plastics Technology Handbook

viscosity and fusion time in the production of high-quality foams. They are highly valued for use in expandable plasticized PVC. Dibutyl phthalate (DBP) is not convenient for PVC plasticization because of its relatively high volatility. It is a good gelling agent for PVC and vinyl chloride-vinyl acetate copolymer (PVCA) and so is sometimes used as a secondary plasticizer in plasticizer mixers to improve solvation. DBP is mainly used for cellulose-based varnishes and for adhesives. It has a high dissolving capacity for cellulose nitrate (CN). Dimethyl phthalate (DMP) also has high dissolving capacity for CN. It has good compatibility with cellulose esters and are used in celluloid made from CN and plastic compounds or ﬁlms made from other cellulosic polymers, cellulose acetate (CA), cellulose acetate-butyrate (CAB), cellulose acetate-propionate (CAP), and cellulose propionate (CP). It is light stable but highly volatile. Diethyl phthalate (DEP) possesses properties similar to DMP and is slightly less volatile.

1.23.2 Phosphoric Acid Esters Tricresyl phosphate (TCP), trioctyl phosphate, diphenyl 2-ethylhexyl phosphate, and tri(2-ethylhexyl) phosphate (TOP) are used as plasticizers as well as ﬂame retardants. They have a low volatility, resist oil extraction well, and are usually combined with other plasticizers. TCP is a good ﬂame retardant plasticizer for technical PVC, PVAC, NC, and CAB. It is used for PVC articles especially in electrical insulation, but it is not recommended for elastic materials to be used at low temperature. Trioctyl phosphate is a better choice in low temperature applications but it offers a lower resistance to kerosene and oil extraction. Diphenyl 2-ethylhexyl phosphate is a good gelling agent for PVC. It also can be used for materials designed for low temperature application. TOP gels NC, PVCA, and PVC. It has markedly higher volatility than DOP, and it gives plastisols of low viscosity.

1.23.3 Fatty Acid Esters The esters of aliphatic dicarboxylic acids, mainly adipic, azelaic, and sebacic acid, are used as plasticizers for PVC and PVCA. Di-2-ethylhexyl adipate (DOA), benzyl butyl adipate, di-2-ethylhexyl azelate (DOZ), and di-2-ethylhexyl sebacate (DOS) are good examples. They give the polymer outstanding lowtemperature resistance and are distinguished by their high light stability. Another characteristic is their low viscosity, which is valuable in the manufacture of PVC plastisols. The solvating action of these esters on PVC at room temperature is weak. This also has a favorable effect on the initial viscosity and storage of plastisols, which is observed even when, for example, plasticizer mixtures of DOP and an aliphatic dicarboxylic ester with DOP contents of up to 80% are used. Such combinations of plasticizers help to improve gelation. DOS exceeds the low-temperature resistance of all other products in the group. It has the least sensitivity to water and has a relatively low volatility. DOS is used most often because of these properties and because of its high plasticization efﬁciency. Monoisopropyl citrate, stearyl citrate, triethyl citrate (TEC), butyl and octyl maleates, and fumarates are other important plasticizers for the preparation of stable PC pastes and of low-temperature-resistant PVC products. Triethyl citrate and triethyl acetylcitrate are among the few plasticizers to have good solvency for cellulose acetate. In comparison with diethyl phthalate, with which they are in competition in this application, there are slight but not serious differences in volatility and water sensitivity. The interest in citrate esters is due to a favorable assessment of their physiological properties. They are intended for plastic components used for packaging of food products. Butyl and octyl maleates, being unsaturated, are used for copolymerization with vinyl chloride, vinyl acetae, and styrene to provide internal plasticization. In the production of poly(vinyl butyral), which is used as an adhesive interlayer ﬁlm between glass plates for safety glass, triethyleneglycol di(2-ethyl) butyrate is an extremely valuable plasticizer that has a

141

Characteristics of Polymers and Polymerization Processes

proven record of success over many years. It has the required light stability and a plasticizing effect tailored to the requirements of poly(vinyl butyral) that gives the ﬁlms suitable adhesion to glass ensuring good splinter adhesion over the temperature range from −40 to +70°C. The ethylhexanoic esters of triethylene glycol and polyethylene glycol are good plasticizers for cellulose acetate butyrate (CAB). They, however, have only limited compatibility with poly(vinyl butyral). Through the action of peracids on esters of unsaturated fatty acids, oxygen adds to the double bound forming an epoxide group. For example, epoxidized oleates have the following structure: H3C

(CH2)7

Since the epoxide group reacts with acids, epoxidized fatty acid esters have become popular as plasticizer for PVC with a good stabilizing effect. Such compounds are not only able to bind hydrogen chloride according to the reaction

+ HCI

But they are also able to substitute labile chlorine atoms in PVC under the catalytic inﬂuence of metal;

PVC

Zn2+ or Cd2+

PVC

O Cl

O R

R′

1.23.4 Polymeric Plasticizers Polymeric plasticizers can be divided into two main types. Oligomers or polymers of molecular weight ranging from 600 to 8,000 belong to the ﬁrst group. They include poly(ethylene glycol) and polyesters. High-molecular plasticizers comprise the second group. Poly(ethylene glycol) is used to plasticize proteins, casein, gelatin, and poly(vinyl alcohol). Polyester plasticizers are condensation products of dicarboxylic acid with simple alcohols corresponding to the following two general formulas: Ac – G – AcD – G – Ac Al – AcD – G – AcD – Al where Ac, monocarboxylic acid; G, glycol; AcD, dicarboxylic acid; Al, monofunctional alcohol In practice, esters from adipic, sebacic, and phthalic acid are frequently used as polyester plasticizers. The value of n may vary from 3 to 40 for adipates and from 3 to 35 for sebacates. Polyester plasticizers are seldom used alone. They are used in combination with monomeric plasticizers to reduce the volatility of the mixed solvents. They offer a higher resistance to plasticizer migration and to extract by kerosene, oils, water, and surfactants. Polyester plasticizers are used specially in PVC-based blends and in nitrocellulose varnishes.

142

Plastics Technology Handbook

Ethylene-vinyl acetate (EVA) polymers (containing 65%–70% by weight of vinyl acetate) are of industrial interest as high-molecular weight plasticizers for PVC, mainly because of their low cost. A polymeric plasticizer PB-3041 available from DuPont allows the preparation of a highly permanent plasticized PVC formulation. It is believed to be a terpolymer of ethylene, vinyl, acetate, and carbon monoxide. Also, butylene terephthalate-tetrahydrofuran block copolymers, with the trade name of Hytrel (DuPont), are used as excellent permanent plasticizers of PVC.

1.23.5 Miscellaneous Plasticizers Hydrocarbons and chlorinated hydrocarbons (chloroparaffins) belong to the secondary plasticizer type. Aromatic and aliphatic hydrocarbons are used as extenders, particularly in the manufacture of PVC plastisols that must maintain as stable a viscosity as possible for relatively long periods of time (dip and rotational molding). The petroleum industry offers imprecisely defined products with an aliphaticaromatic structure for use as extenders. They are added to plastisols in small amounts as viscosity regulators. Dibenzyl toluene also serves the same purpose. A disadvantage of this type of extenders is the risk of exudation if excessive quantities are used. The normal liquid chlorinated paraffins used as plasticizers for PVC have viscosities ranging from 100 to 40,000 MPa.s at 20°C. Products with chlorine contents ranging from 30 to 70% are on the market. Compatibility with PVC increases with increasing chlorine content but the plasticizing effect is reduced. The low viscosity products (chlorine content 30%–40%) are used as secondary plasticizers for PVC. They have a stabilizing effect on viscosity in plastisols. Chlorinated paraffins can be used up to a maximum of 25% of the total plasticizer content of the PVC plastisol without the risk of exudation. As chlorinecontaining substances, these plasticizers also have a flame-retarding effect. At a chlorine content of 50%, chlorinated paraffins are sufficiently compatible with PVC to be used alone, i.e., as primary plasticizers, in semirigid compounds under certain circumstances. Products with extremely good compatibility, however, have low plasticizing action, requiring larger amounts to be used. Among other plasticizers, o- and p-toluene-sulfonamides are used to improve the processibility of urea and melamine resins, and cellulose-based adhesives. For the same reason, N-ethyl-o-toluenesulfonamide and N-ethyl-p-toluenesulfonamide are used for polyamides, casein, and zein. Diglycol benzoates are liquid plasticizers rather like benzyl butyl phthalate (BBP) in their plasticizing action in PVC. In the manufacture of PVC floor coverings, the benzoates offer the advantage of a highly soil resistant surface. In this respect, they are even superior to BBP, which itself performs well here. Pentaerythritol esters can be used instead of phthalic acid esters when low volatility is one of the major factors in the choice of plasticizer for PVC. Pentaerythriol triacrylate can be used to plasticize PVC and to produce-cross-links under UV radiation. Such plasticized cross-linked PVC has some application in the cable industry. Commonly used plasticizers for some polymers beside PVC are listed below: Cellulosics: Dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), dimethylcyclohexyl phthalate, dimethyl glycol phthalate, trichloroethyl phosphate, cresyldiphenyl phosphate, some glycolates, some sulfonamides. Poly(vinyl butyrals): Triethyleneglycol di(2-ethyl)butyrate, triethyleneglycol propionate, some adipates, some phosphates. Polyurethanes: Dibutyl phthalate (DBP), diethyleneglycol dibenzoate (DEGB), and most of the plasticizers used for PVC are preferred for their good plasticization action.

1.24 Antistatic Agents By virtue of their chemical constitution, most plastics are powerful insulators, a property that makes them useful for many electrical applications. A disadvantage of the insulation property, however, is the accumulation of static electricity, which is not discharged fast enough due to the low surface conductivity of

Characteristics of Polymers and Polymerization Processes

143

most plastics—a difference between plastics and metals. On plastics and other nonmetallic materials, frictional contact is necessary to generate static charges. On immobile objects, for example, static electricity may build up simply by friction with the ambient air. Electrostatic charges can also be produced on the surfaces of polymeric materials in the course of processing operations such as extrusion, calendaring, and rolling up of plastic sheets or films. The superficial electrical potential generated by friction may reach values up to a few tens of kilovolts, and this presents serious difficulties for practical applications and to users. Spark formation through heavy charging with subsequent ignition of dust or solvent/air mixtures have been a cause of many destructive explosions. In the service life of plastics products, static charging can give rise to many other troublesome effects, as for example, interference with the sound reproduction of records by dust particles picked up by electrostatic charges, production delays due to clinging of adjacent films or sheets, lump formation during pneumatic transportation, and static build-up on people passing over synthetic fiber carpeting or plastic floor-coverings, with a subsequent “shock” as the charge flows off, usually when the person touches a door handle. There are many ways to eliminate surface electrostatic, for example, by increasing the humidity or the conductivity of the surrounding atmosphere, or by increasing the electric conductance of materials with the use of electroconducting carbon blacks, powdered metals, or antistatic agents. Electroconducting carbon blacks are largely utilized to increase the electric conductivity of organic polymers. The electric conductivity of carbon blacks depends, inter alia, on the capacity to form branched structures in the polymer matrix, and on the size and size distribution of carbon black particles. The branched and tentacular structures of carbon in the polymer matrix are responsible for the electric conductivity, as is the case for lamp, acetylene, and furnace carbon blacks. The specific resistance of the carbon particles decreases with their size and then increases with further diminution of the size. A wide size distribution is believed to favor the formation of branched structures contributing to greater conductivity. In spite of the effectiveness of some carbon blacks in reducing surface charges on plastics materials, the use of antistatic agents have increased steadily. The simplest antistatic agent is water. It is adsorbed on the surface of objects exposed to a humid atmosphere, and it forms a thin electroconducting layer with impurities adsorbed from the air. Such a layer is even formed on the surface of hydrophobic plastics, probably because of the existence of a thin layer of dirt. Antistatic agents commonly used are substances that are added to plastics molding formulations or to the surface of molded articles in order to minimize the accumulation of static electricity. In general terms, antistatic agents can be divided according to the method of application into external and internal agents.

1.24.1 External Antistatic Agents External antistatic agents are applied to the surface of the molded article from aqueous or alcoholic solution by wetting, spraying, or soaking the plastic object in solution, followed by drying at room temperature or under hot blown air. Their concentration varies between 0.1 and 2% by weight. Almost all surface active compounds are effective, as well as numerous hygroscopic substances such as glycerin, polyols, and polyglycols, which lack the surface activity feature. The most important external antistatic agents from the practical point of view are quaternary ammonium salts and phosphoric acid derivatives. The advantage of external agents lies in the fact that, in their performance, the properties of compatibility with the polymer and controlled migration in the polymer, which play an important part in the performance of internal antistatic agents described later, are not of any consideration. It is generally assumed that surface-active molecules accumulate on the surface and are oriented such that the hydrophobic part containing the hydrocarbon chain extends into the plastic, and the hydrophilic part points outwards there it is able to absorb water on the surface. The phase boundary angle between water and plastic is reduced by the surface active antistatic agents thus allowing the absorbed water to be uniformly distributed on the plastic surface. A water ﬁlm forms on the surface thus increasing the

144

Plastics Technology Handbook

conductivity by means of an ion conduction process. This also explains why surface conductivity, and hence the antistatic action, are found to increase with increasing atmospheric humidity. For the ion conduction in the surface ﬁlm, a conductivity mechanism that is similar to protonic conductivity in water has been suggested:

H H+

H H

H+

This mechanism is based on a comparison of the conductivities of substances of different chemical structure. For instance, primary amines are efﬁcient as antistatic agents but secondary amines are not. The conductivity of tertiary amines, on the other hand, depends on the nature of N-hydroxyalkyl substituents. Among amides, only N,N-disubstituted derivatives derivatives and mainly those having two hydroxyalkyl substituents are effective. The presence of many OH groups in the molecule makes the efﬁciency of the antistatic agent less dependent on the humidity. Antistatic agents bearing OH or NH2 groups are able to associate in chain form via hydrogen bonding and display antistatic activity even at low atmospheric humidity, unlike compounds that are able to form only intramolecular hydrogen bonds. Many antistatic agents also show hygroscopic properties, thereby intensifying the attraction of water to the surface. At constant atmospheric humidity, a hygroscopic compound attracts more water to the surface and so increases antistatic effectiveness.

1.24.2 Internal Antistatic Agents Internal antistatic agents are incorporated in the polymer mass as additives, either before or during the molding process. However, to be functional, they must be only partially miscible with the polymer so that they migrate slowly to the surface of the plastics material. Their action therefore appears after a few hours and even a few days after compounding, depending on the mutual miscibility of the agent and the polymer. The concentration of internal antistatic agents in plastics varies from 0.1 to 10% by weight. Interfacial antistatic agents are all of interfacially active character, the molecule being composed of a hydrophilic and hydrophobic component. The hydrophobic part confers a certain compatibility with the particular polymer and is responsible for anchoring the molecule on the surface, while the hydrophilic part takes care of the binding and exchange of water on the surface.

1.24.3 Chemical Composition of Antistatic Agents The selection of an antistatic agent for a given polymer is based on its chemical composition. Accordingly, antistatic agents can be divided in many groups, as presented below. 1.24.3.1 Antistatic Agents Containing Nitrogen Antistatic agents containing nitrogen are mainly amines, amides, and their derivatives, such as amine salts, and addition compounds between oxiranes and aminoalcohols. Pyrrolidone, triazol, and polyamine derivatives also belong to this group. A few representative examples belonging to this group are (CH2CH2O)x H Ethoxylated amines of the type R

N (CH2CH2O)x H

145

Characteristics of Polymers and Polymerization Processes

R1 Amine oxides of the type R2

O Fatty acid polyglycolamides of the type R

NH(CH2CH2O)3H

The following two commercial products are typical examples: (1) Alacstat C-2 (Alcolac Chemical Corp., U.S.A.). This is an N,N-bis(2-hydroxyethyl)-alkylamine used for polyoleﬁns at 0.1% by weight. (2) Catanac 477 (American Cyanamid Co., U.S.A.). It is N-(3-dodecyloxy-2-hydroxypropyl)ethanolamine (C12H25OCH2CHOHCH2NHCH2CH2OH) used for linear polyethylene (0.15% by weight), for polystyrene (1.5% by weight), and for polypropylene (1% by weight). These compounds are not recommended for PVC. Other compounds containing nitrogen and used as antistatic agents are quaternary ammonium salts, quaternized amines, quaternized heterocycles obtained from imidazoline and pyridine, and amides of quaternized fatty acids. Some of these compounds can be utilized for PVC, for example, +

R2 Ammonium salts

X–

N R4

R2 Quaternized ethoxylated amine

(CH2CH2O)xH

X–

O Amide of quaternized fatty acid

Some typical examples of commercial products are Catanac SN, Catanac 609, Catanac LS, and Catanac SP, all of American Cyanamid Co. Catanac SN is stearamidopropyl-dimethyl-b-hydroxyethylammonium nitrate, while Catanac 609 is N,N-bis(2-hydroxyethyl)-N-(3′-dodecyloxy-2′-hydroxypropyl) methylammonium methylsulfate: CH2CH2OH [C12H25OCH2CHOHCH2

CH3]+ CH3SO4–

CH2CH2OH

It is supplied mainly as a 50% by weight solution in a water-propanol mixture. It is applied as an external antistatic agent at 2% by weight concentration (pH 4–6) and is recommended for phonograph records and other products made of PVC and its copolymers.

146

Plastics Technology Handbook

Catanac LS is 3-lauramidopropyl-trimethylammonium methylsulfate: CH3 CH3]+ CH3SO4–

[C11H23CONHCH2CH2CH2

CH3

It exists in a crystalline form with a melting point of 99°C–103°C. Catanac SP is stearamidopropyl-dimethyl-b-hydroxyethyl-ammonium dihydrogenphosphate: CH3 [C17H35CONHCH2CH2CH2

CH2CH2OH]+ H2PO4–

N CH3

supplied as a 35% by weight solution in a water-isopropanol mixture (pH 6–8). It beings to compose slightly at 200°C, and at 250°C its decomposition is rapid. It is soluble in water, acetone and alcohols, and is not corrosive to metals even during prolonged contact. Catanac SP can be used as an internal and external antistatic agent at 1%–3% by weight. 1.24.3.2 Antistatic Agents Containing Phosphorus Antistatic agents that contain phosphorus can be used for all polymers, although they are recommended mainly for PVC with which they also function as plasticizers. They are phosphoric acid derivatives, phosphine oxide, triphosphoric acid derivatives, and substituted phosphoric amides. Typical examples are the following: Phosphoric acid esters O = P(OR)3 Ethoxylated alcohols and phosphoric acid esters O = P½O(CH2 CH2 O)x − R3 +

– Ammonium salts of phosphoric acid esters

O O O

R1 R2

H N H

1.24.3.3 Antistatic Agents Containing Sulfur Antistatic agents that contain sulfur include compounds such as sulfates, sulfonates, derivatives of aminosulfonic acids, condensation products of ethyleneoxide and sulfonamides, and sulfonates of alkylbenzimidazoles, dithiocarbamides, and sulﬁdes. Sulfur-containing antistatic agents are recommended mainly for PVC and PS because they do not interfere with heat stabilizers. They are not suitable for PMMA, polyoleﬁns, and polyamides. As examples, there are alkylpolyglycolether sulfates, RO(CH2 CH2 O)x SO−3 Me+ , which are marketed under the trade name Statexan HA (Bayer).

Characteristics of Polymers and Polymerization Processes

147

1.24.3.4 Betaine-Type Antistatic Agents Betaines are trialkyl derivatives of glycine, which exist as dipolar ions of the formula R+3 NCH2 CO−2 . Typical examples of betaine-type antistatic agents are stearylbetaine and dodecyldimethyl–ethanesulfobetaine. They are used mainly for polyolefins. 1.24.3.5 Nonionic Antistatic Agents Nonionic antistatic agents are nonionizable, interfacially active compounds of low polarity. The hydrophilic portion of the molecule is usually represented by hydroxyl groups and the hydrophobic portion by organic groups. This group of antistatic agents includes the following: polyhydroxy derivatives of glycine, sugars, and fatty acids, sometimes modified by addition of oxiranes, most of the products being nontoxic and hygienically acceptable but having low antistatic effects; heavy alcohol derivatives such as alkylpolyglycol ethers, which are recommended for polyolefins and PVC; alkylphenolpolyglycol ethers having good heat stability, which are used mainly for polyolefins; polyglycol ethers ether obtained from the reaction of glycols with oxiranes, which are used for polyolefins; and fatty acid polyglycol esters RCO2 (CH2CH2O)xH, which are used for many type of plastics. Nonionic antistatic agents are supplied for the most parting liquid form or as waxes with a low softening region. The low polarity of this class makes its members ideal internal antistatic agents for polyethylene and polypropylene. A number of substances that cannot be included in any of the groups above are also good antistatic agents. They are silicone copolymers, organotin derivatives, oxazoline derivatives, organoboron derivatives, and perfluorated surfactants. Some of them show excellent heat stability.

1.25 Flame Retardants Efforts to develop flame-retarding plastics materials have been focusing on the increasing use of thermoplastics. As a result, flame-retarding formulations are available today for all thermoplastics, which strongly reduce the probability of their burning in the initiating phase of a fire. The possibility of making plastics flame-retardant increases their scope and range of application. (It must be noted, however, that in the case of fire, the effectiveness of flame retardants depends on the period of time and intensity of the fire. Even a product containing the most effective flame-retardants cannot resist a strong and long-lasting fire) [70]. Flame retardants are defined as chemical compounds that modify pyrolysis reactions of polymers or oxidation reactions implied in combustion by slowing them down or by inhibiting them. The combustion of thermo-plastics (see “Fire Resistant Polymers” in Chapter 5) is initiated by heating the material up to its decomposition point through heat supplied by radiation, flame, or convection. During combustion, free radials are formed by pyrolysis and, besides noncombustible gases such as carbon dioxide and water, combustible gases are formed–mainly hydrocarbons, hydrogen, and carbon monoxide. The radicals combine with oxygen and combustible gases in a radical chain reaction, thereby causing heat release and further decomposition of the plastics material. For continued combustion it is necessary to have sufficient oxygen as well as combustible gaseous compounds. The combustion will be slowed down or inhibited if free radicals, which are evolved by pyrolysis, are blocked. For example, it is presumed that the following reactions take place when thermoplastics containing organobromine compounds as flame retardants are used: Exothermic propagation Chain branching Chain transfer

HO• + CO = CO2 + H•

(1.149)

H• + O2 = HO• + O•

(1.150)

O• + HBr = HO• + Br•

(1.151)

148

Plastics Technology Handbook

Inhibition

HO• + HBr = H2 O + Br•

(1.152)

RH + Br• = R• + HBr

(1.153)

Regeneration

The radical chain reaction is interrupted when the highly reactive HO• radical, which plays a key role in the combustion process, is replaced by the less reactive Br• radical (Reaction 1.152). While Equation 1.151 and Equation 1.152 show reactions with HBr (which is released during combustion), experiments have shown that halogen-containing organic compounds may also react as undecomposed molecules. Flame retardants, in view of the above discussion, can be defined as chemical compounds that modify pyrolysis reactions of polymers or oxidation reactions implied in the combustion by slowing these reactions down or by inhibiting them. In practice, mixtures of flame retardants are often used to combine different types of retardancy effects. In spite of a few thousand references on flame retardants, only a small number of compounds are produced commercially. They are mainly chlorine, bromine, phosphorus, antimony, aluminum, and boron-containing compounds. Most flame retarding agents may be divided into three groups: (1) halogen compounds, (2) phosphorous compounds, and (3) halogen–antimony synergetic mixtures.

1.25.1 Halogen Compounds As flame retardants, bromine compounds show superiority over chlorine compounds. For example, in case of combustion of n-heptane/air mixture, the minimum concentrations (in vol. %) found necessary for the extinction of flame are 5.2% for dibromomethane and 17.5% for trichloromethane. The efficiency of halogen-containing agents depends on the strength of the carbon-halogen bond. Thus the relative efficiency of aliphatic halides decreases in the following order: Aliphatic bromides > aliphatic chlorides = aromatic bromides > aromatic chlorides It is also known that aromatic bromides have a higher decomposition temperature (250°C–300°C) than aliphatic bromides (200°C–250°C). Bromine-containing compounds on a weights basis are at least twice as effective as chlorine-containing ones. Because of the smaller quantities of bromine compounds needed, their used hardly influences the mechanical properties of the base resins and reduces markedly the hydrogen halide content of the combustion gases. Aliphatic, cycloaliphtic, as well as aromatic and aromatic-aliphatic bromine compounds are used as flame retardants. Bromine compounds with exclusively aromatic-bound bromine are produced in large quantities. Occupying the first place is tetrabromobios-phenol-A, which is employed as a reactive flame retardant in polycarbonates and epoxy resins. Similarly, tetrabromophthalic anhydride has been used in the production of flame-retarded unsaturated polyester resins. Brominated diphenyls and brominated diphenylethers—for instance, octabromodiphenyl ether (melting point 200°C–290°C) and decabromodiphenyl ether (melting point 290°C–310°C)-have gained great significance. Possessing excellent heat stability, they are excellent flame retardants for those thermoplastics that have to be processed at high temperatures, such as linear polyesters and ABS. Aromatic-aliphatic bromine compounds, like the bis(dibromopropyl)-ether of tetrabromobisphenol A, or bromoethylated and aromatic ring-brominated compounds, such as 1,4-bis-(bromoethyl)tetrabromobenzene, combine the high heat stability of aromatic-bound bromine with the outstanding flame retardancy of aliphatic-bound bromine. They are used mainly as flame retardants for polyolefins, including polyolefin fibers. Polymeric aromatic bromine compounds, such as poly(tribromostyrene), poly(pentabromobenzyl acrylate) or poly(dibromophenylene oxide) are suitable for application in polyamides, ABS, and polyesters.

Characteristics of Polymers and Polymerization Processes

149

Oligomeric bromine-containing ethers on the basis of tetrabromoxylene dihalides and tetrabromobisphenol-A (molecular weight between 3,000 and 7,000) are useful flame retardants for polypropylene, polystyrene, and ABS. The following cycloaliphatic compounds are suitable as flame retardants in polyolefins: hexabromocyclododecane and bromine-containing Diels-Alder reaction products based on hexachloro cyclopentadiene.

1.25.2 Phosphorus Compounds Whereas halogen acids, which are released during the combustion of thermoplastics containing organic halogen compounds, interrupt the chain reaction (e.g., Reaction 1.133), the flame-retarding action of phosphorus compounds is not yet fully understood. It is supposed, however, that in case of fire, phosphorus compounds facilitate polymer decomposition, whereby the very stable poly(meta-phosphoric acid) formed creates an insulating and protecting surface layer between the polymer and the flame. The phosphoric acid also reacts with the polymer and produces a char layer protecting the surface. Phosphorus-containing flame retardants may be divided according to the oxidation state of the element into phosphates [(RO)3PO], phsophites [(RO)3P], phosphonites [(RO)2PR′], phosphinates [(RO) R2′PO], phosphines [R3P], phosphine oxides [R3PO], and phosphonium salts [R4PX]. The halogenated phosphorus-containing compounds represent a very important group of flame retardants with high efficiency A few examples are tris-(tribromoneopentyl)-phosphite, tris-(dibrompropyl)-phosphate, tris(di-bromophenyl)-phosphate, and tris-(tribromophenyl)-phosphate. A similar flame retardancy may also be obtained by the combinations of phosphorus-containing compounds with various halogen derivatives. These combinations are relatively universal in their action. Phosphorus-containing flame retardants are suitable for polar polymers such as PVC, but for polyolefins their action is not sufficient. In this case they are used together with Sb2O3 and with halogenated compounds. Alkyl-substituted aryl phosphates are incorporated into plasticized PVC and modified PPO (Noryl) to a great extent; for other plastics they are less important. Quaternary phosphonium compounds are recommended as flame retardants for ABS and polyolefins.

1.25.3 Halogen–Antimony Synergetic Mixtures Antimony oxide is an important component of flame retarded thermoplastics containing halogen compounds. Used alone, the flame-retardant effect of antimony oxide is not sufficient, but the effect increases significantly as halogen is added to the system. The basis of this antimony-halogen synergism is probably the formation of volatile flame-retardant SbX3 (see below), where X is Cl or Br. Antimony–halogen combinations have be widely used in polymer for flame retardation, and the interaction of antimony (mostly as antimony oxide) with halogenated polymers or polymers containing halogenated additives represents the classic case of flame-retardant synergism. Experimental results indicate that the best Sb:X ratio is 1 to 3. More effective are ternary synergetic mixtures such as Sb2O3− pentabromotoluenechloroparaffin. The antimony oxide-alkyl (aryl) halide system functions on heating according to the following mechanism (shown for chloride): RCl ! HCl + R0 CH = CH2

(1.154)

Sb2 O3 + 6HCl ! 2SbCl3 + 3H2 O

(1.155)

Sb2 O3 + 2HCl ! 2SbOCl + H2 O

(1.156)

150

Plastics Technology Handbook

Volatile SbCl3 reduces the formation of radicals in the ﬂame and also affects the oxidation process strongly, thus terminating the combustion. As with phosphorus compounds, it promotes carbonization of the polymer. It is believed that SbOCl and SbCl3 already function as dehydrogenation agents in the solid phase of the polymer in ﬂame. At 240°C–280°C, SbOCl is transformed to higher oxychlorides and SbCl3, and at temperatures around 500°C the higher antimony oxychlorides are converted back into antimony oxide by releasing again SbCl3: 240°C–280°C

⟶

5SbOCl

4Sb4 O5 Cl2

Sb4 O5 Cl2 + SbCl3

410°C–475°C

3Sb3 O4 Cl

⟶

5Sb3 O4 Cl + SbCl3

475°C–565°C

⟶

4Sb2 O3 + SbCl3

(1.157)

(1.158)

(1.159)

The flame retardation is also attributed to the consumption of thermal energy by these endothermic reactions, to the inhibition reactions in the flame, and to the release of hydrochloric acid directly in the flame. The pigmentation effect of mixtures containing Sb2O3 is a disadvantage that limits their use for light-colored and transparent products. The demands on a flame retardant and thermoplastics formulated with such agents are manifold. The flame retardant should provide a durable flame-retarding effect by the addition of only small quantities of the additive; it should be as cheap as possible and the manner of incorporation should be easy; it should be nontoxic and should not produce fire effluents with increased toxicity; it should not decompose at the processing temperatures; it should not volatilize and smell; and the mechanical, optical, and physical properties of the thermoplastics should be affected as little as possible. It is understandable that these farranging demands cannot be satisfied by only one flame retardant and for all thermoplastics with their manifold applications. One is therefore forced to seek the optimum flame retarding formulation for each thermoplastic and the specific application. Examples of typical formulations of several flame-retarded thermoplastics are given in Appendix A3.

1.25.4 Intumescent Flame Retardants While halogenated compounds are good fire-retardant additives for polyolefins, especially in synergistic combination with antimony trioxide, there are serious disadvantages, such as evolution of toxic gases and corrosive smoke. In the search for halogen-free flame retardants, intumescent flame-retardants (IFRs) have received considerable attention recently, and they have shown particularly good efficiency in the flame retardation of polypropylene (PP) [71]. IFRs that contain ammonium polyphosphate (APP) and pentaerythritol (petol) work mainly through a condensed phase mechanism [72]. APP is most often used as the acid source and petol as a carbon source. These APP-based products also have a low smoke density and do not emit corrosive gases during combustion. Recently, the synergistic activity of IFRs with heavy metal ions has received wide attention as this can render an IFR more efficient at lower concentrations [73]. It has been found that divalent metal compounds and their derivatives have an excellent synergistic effect when used together with the polyphosphate and petol. Thus, using nickel formate, Ni(HCOO)2 the limiting oxygen index (LOI) of PP/ APP/petol (90: 6.75: 3.25 by wt.) has been found [74] to have the highest value of 30% at 2–3 wt.% Ni (HCOO)2. It is proposed that during burning, the petol is first phosphorylated by APP, with the release of water and ammonia, while the phosphate ester is pyrolyzed with the formation of double bonds that subsequently cross-link, resulting in a three-dimensional structure.

151

Characteristics of Polymers and Polymerization Processes

1.26 Smoke Suppressants In recent years, the hazards of smoke and toxic gas production by burning have received much greater recognition. Since burning of a large amount of polymer represents a fuel rich combustion, it produces large amounts of toxic carbon monoxide and carbon-rich black smoke, causing death and inhibiting escape from the burning environment. A comparison of smoke production from burning different polymers is given in Table 1.21. The problem of smoke suppression has not yet been solved satisfactorily, because the demands on flame retardancy and reduced smoke development are principally of antagonistic nature. Most of the developmental work on smoke suppressants has been carried out in the field of PVC. For this polymer, highly dispersed aluminum trihydrate is widely used as a flame retardant and as a smoke suppressant. Antimony trioxide-metal borates (barium borate, calcium borate, and zinc borate) are good smoke reducers. Zinc borate of the formula 2ZnO:3B2O3:3.5H2O has shown the best effect with regard to reduced flammability and reduced smoke development of PVC. It is possible to reduce smoke development of rigid PVC significantly by adding rather small quantities of molybdic oxide. For example, 2% of molybdic oxide reduces the smoke density to about 35%, but does not decrease the limiting oxygen index value of rigid PVC. Several methods have been proposed for the determination of the smoke density. Reproducible values are obtained with the laboratory method worked out by the National Bureau of Standards, whereby a specimen of 7.6 × 7.6 cm is heated by radiation. The smoke density is then measured optically.

1.27 Colorants A wide variety of inorganic acid organic materials are added to polymers to impart color. For transparent colored plastics materials, oil-soluble dyes or organic pigments (such as phthalocyanines) having small particle size and refractive index near that of the plastic are used. Others, including inorganic pigments, impart opaque color to the plastic. Some of the common colorants for plastics, among many others, are barium sulfate and titanium dioxide (white), ultramarine blues, phthalocyanine blues and greens, chrome green, molybdate organs, chrome yellow, cadmium reds and yellows, quinacridone reds and magentas, and carbon black. Flake aluminum is added for a silver metallic appearance, and lead carbonate or mica for pearlescence. The principal requirements of a colorant are that it have a high covering power-cost ratio and that it withstand processing and service conditions of the plastic. The colorant must not catalyze oxidation of the polymer nor adversely affect its desirable properties. Colorants are normally added to the powered plastic and mixed by tumbling and compounding on a hot roll or in an extruder. TABLE 1.21 Smoke Emission on Burning (NBS Smoke Chamber, Flaming Condition) Polymer ABS Poly(vinyl chloride)

Maximum Smoke Density (Dmax) 800 520

Polystyrene

475

Polycarbonate Polyamide-imide

215 169

Polyarylate

109

Polytetraﬂuoroethylene Polyethersulfone

95 50

Polyether ether ketone

152

Plastics Technology Handbook

1.28 Antimicrobials Antimicrobials impart protection against mold, mildew, fungi, and bacterial growth to materials. Without antimicrobials, polymers can experience surface growths, producing allergic reactions, unpleasant odor, staining, embrittlement, and even permanent product failure. The effectiveness of an antimicrobial depends on its ability to migrate to the surface of the product where microbial attack first occurs. Antimicrobials are thus usually carried in plasticizers which are mobile. High mobility, however, can result in leaching of the additives due to weather exposure. The durability of protection is, therefore, determined by a proper balance between migration and leaching. Of the hundreds of chemicals that are effective as antimicrobials, only a few are used in plastics applications. The most common group of active ingredients consists of 10,16-oxybisphenoxy arsine (OBPA, tradenames: Intercide, Akzo Chemical; Vinyzene, Morton International), 2-n-octyl-4isothiazolin-3-one (tradenames: Micro-Check 11, Ferro Corporation; Vinyzene IT, Morton International), N-trichloro-methylthio-4-cyclohexene-1,2-dicarboximide (Captan, tradenames: Vancide 89, R.T. Vanderbilt; Fungitrol C, Hüls America), and N-(trichloromethylthio) phthalimide (Flopet, trade name; Fungitrol 11, Huls America). Other antimicrobial active ingredients include amine-neutralized phosphate and zinc 2-pyridinethiol-1-oxide. OBPA is the most toxic and requires approximately 0.04% active ingredient in the final product. Less toxic active ingredients such as Flopet require a loading of 1.0% to achieve a similar level of protection. Antimicrobials are generally formulated with a carrier into concentrations of 2–5% active ingredient and are available to plastic processors in powder, liquid, or solid-pellet form. The carrier is usually a plasticizer, such as epoxidized soybean oil or diisodecyl phthalate. Among the antimicrobials, OBPA has broad-spectrum effectiveness against gram-positive and gramnegative bacteria and fungi. Another additive, 2-n-octyl-4-isothiazolin-3-one, is a fungicide only and is commonly used to prevent molds in paints. Captan is an agricultural fungicide that has important plastics applications. The majority of all antimicrobial additives are used in flexible PVC. The remaining portion is used in polyurethane foams and other resins. PVC applications using antimicrobials include flooring, garden hose, pool liners, and wall coverings.

1.29 Toxicity of Plastics With the expanding use of plastics in all walks of life, including clothing, food packaging, and medical and paramedical applications, attention has been focused on the potential toxic liability of these man-made materials. Toxic chemicals can enter the body in various ways, particularly by skin absorption, inhalation, and swallowing. Although some chemicals may have an almost universal effect on humans, others may attack few persons. The monomers used in the synthesis of many of the polymers are unsaturated compounds with reactive groups such as vinyl, styrene, acrylic, epoxy, and ethylene imine groups. Such compounds can irritate the mucous membranes of the eyes and respiratory tract and sensitize the skin. They are suspected of including chemical lesions and carcinogenic and mutagenic effects. It was long ago that carcinogenic properties of the monomers vinylchloride and chloroprene were reported, and there may be more trouble ahead in this respect. However, the monomers used in the production of polyester or polyamide resins are usually less reactive and may be expected to be less harmful. Although many monomers are harmful chemicals, the polymers synthesized from them are usually harmless macromolecules. But then one has to take into account that the polymers may still contain small amounts of residual monomer and catalyst used in the polymerization process. Moreover, polymers are rarely used as such but are compounded, as we saw earlier, with various additives such as plasticizers, stabilizers, curing agents, etc., for processing into plastic good. Being relatively smaller in molecular size,

Characteristics of Polymers and Polymerization Processes

153

the residual monomer, catalyst residues, and the additives can migrate from the plastic body into the environment. They may eventually migrate into food products packed in plastic containers, or they may interact with the biological substrate when plastics materials are used as parts of tissue and organs implanted into humans. The toxic potential of thermodegradation and combustion products of plastics when these materials are burned, either deliberately or by accident, is also an important consideration in view of the widening use of plastics as structural and decorative lining material in buildings, vehicles, and aircraft.

1.29.1 Plastic Devices in Pharmacy and Medicine Within the past two decades there has been an increase in the use of a variety of plastic materials in the pharmaceutical, medical, dental, and biochemical engineering fields. Such plastic devices can be classified according to use into six basic groups: (1) collection and administrative devices—e.g., catheters, blood transfusion sets, dialyzing units, injection devices; (2) storage devices—e.g., bags for blood and blood products, containers for drug products, nutritional products, diagnostic agents; (3) protective and supportive devices—e.g., protective clothes, braces, films; (4) implants having contact with mucosal tissue— e.g., dentures, contact lenses, intrauterine devices; (5) permanent implants—e.g., orthopedic implants, heart valves, vascular grafts, artificial organs, and (6) nanomedicines and drug delivery. The toxic potential of plastic devices becomes more relevant in those applications which involve long periods of contact with the substrate. For short periods of contact, however, a great number of plastic materials manufactured today as medical and paramedical devices will produce little or no irritant response. With polyethylene, polypropylen, Teflon, Dacron, polycarbonate, and certain types of silicone rubbers, the migration of additives from the material is so small that a biological response cannot be detected. Besides, many of these materials contain additives only in extremely low concentrations, some perhaps having only a stabilizing agent in concentrations of less than 0.1 wt.%. Incidence of tissue response increases when the plastic materials require greater concentrations of various types of additives. It should be stressed, however, that the toxic effects of a material will depend on the intrinsic toxicity of the additives and their ability to migrate from the material to the tissue in contact with it. It is important to know precisely the toxic potential of each of the ingredients in the final polymerized and formulated material to be used in an implantable or storage device. 1.29.1.1 Packing Many pharmaceutical manufacturers have now taken to packaging their drug product in plastic containers. The plastics used most in these applications are those manufactured from polyethylene and polypropylene, though other materials have also been used. Since both polyethylene and polypropylene contain extremely small amounts of additives (mostly as antioxidants and antistatic agents), the possibility of their release in sufficient concentrations to endanger to patient is negligible. 1.29.1.2 Tubings and Blood Bag Assemblies Plastic tubings are used for many medical applications, such as catheters, parts of dialysis and administration devices, and other items requiring clear, flexible tubings. The most successful tubing is PVC, which has been plasticized to give it the desired flexibility. Plasticized PVC is also the chief material used in America for making bags for blood and products. In Europe polyethylene and polypropylene have been mainly used. The plasticizers used most for flexible PVC are the esters of phthalic acid; one that is most employed from this group is di-2-ethylhexyl phthalate. Long-term feeding studies have demonstrated the nontoxic nature of the plasticizer under the experimental conditions used. Organotin compounds, which are one of the best groups of stabilizers for vinyl polymers, have been used to stabilize PVC, but, in general, their toxicity has decreased their use in medical applications.

154

Plastics Technology Handbook

1.29.1.3 Implants Man-made materials have been used for making implants to save human life. It is possible that these materials, depending on their specific nature and the site of implant, will degrade with long periods of contact and release polymer fragments in the body. These in turn may elicit one or more biological responses, including carcinogenesis. However, no well-substantiated evidence has been reported showing that man-made materials have caused cancer in humans, although the real answer will be available only when these materials have been implanted for periods of time exceeding 20 to 30 years. Perhaps the most commonly used implantable material is silicone rubber. This material, if properly prepared by the manufacturer, does not cause local toxic response. Various types of epoxy polymers and polyurethane materials also have found one or more medical applications. 1.29.1.4 Adhesives In most medical and paramedical applications of plastics, the materials used are those already produced by the manufacturers in a polymerized or formulated form. Certain surgical and dental applications, however, require that the material be polymerized or formulated just prior to use. Surgical cements and adhesives, a host of dental filling materials, materials for dentures, cavity liners, and protective coatings for tooth surfaces are in this category. Cyanoacrylates have become very useful as tissue adhesives in surgical applications, because they polymerize rapidly in contact with moisture and create an extremely tenacious film. Methyl cyanoacrylate, used initially, has now fallen out of favor because of its toxic properties. The butyl and heptyl analogs are, however, quite satisfactory and do not produce objectionable tissue response. They also degrade at a much slower rate than the methyl compound in a biological environment. 1.29.1.5 Dental Materials Sometimes known as white filling or synthetic porcelain, polymeric dental composites are commonly used as a tooth-colored restorative material, for example, in the fabrication of fillings and veneers, and the cementation of crowns. Dental composites are complex mixtures generally containing an organic resin matrix, reinforcing inorganic filler (such as Sr-glass, 0.7 mm), a silane coupling agent, which connects the filler and the resin matrix, and stabilizers to maximize storage [75]. Composites without the filler and coupling agent are commonly used as sealants, which effectively isolate pits and fissures to help prevent caries. The dental resins are commonly based on the highly viscous bisphenol A glycidyl methacrylate (abbreviated as bis-GMA, also known as Bowen’s monomer after the inventor) have been used for over four decades. Because of its high viscosity (1,369 Pa.s), bis-GMA is blended with diluent, lower molecular weight monomers, to provide a workable matrix resin for composites, for example, bisphenolA dimethacrylate (bis-DMA), ethylene glycol dimethacrylate (EGDMA), and triethylene glycol dimethacrylate (TEGDMA). Camphorquinone (CQ) is traditionally used as photosensitizer for dental composites. It undergoes a redox reaction with a tertiary amine to produce radicals for free-radical polymerization of the acrylate resin. In a typical procedure, a mixture of bis-GMA and TEGDMA with 1 wt% of CQ and tertiary amine is photopolymerized using visible light device with an intensity of 900 mW/cm2. The photopolymerization of the dental resin is fast with most of the reaction taking place within 40 s, causing gelation and vitrification accompanied by shrinkage of about 8%. Shrinkage continues, as also stiffening, during post-polymerization at a much slower rate (see Table 1.22). Based on data reported in several studies involving application of sealant to teeth, it appears that low levels of bisphenol A (BPA) may be released from certain sealants, although only during a short time period immediately after application of the sealant. Further, no detectable levels of BPA have been found in blood after application of a sealant that releases low levels of BPA into saliva and the sealant possesses no risk to human health [76].

155

Characteristics of Polymers and Polymerization Processes TABLE 1.22 Young’s Modulus, Density, and Shrinkage of Dental Resin Matrix after Photopolymerization Young Modulus (GPa)

Density (g/cm3)

Shrinkage (%)

2.22

1.203

8.02

3.27

1.220

9.44

3.31

1.222

9.67

Time after Photopolymerization (s)

Source: From Trufﬁer-Boutry, D., Demonstier-Champagne, S., Devaux, J., Brebuyck, J.-J., Larbanois, P., Mestdagh, M., and Leloup, G. 2005. Eur. Cell Mater., 9, Suppl. 1, 60.

1.29.1.6 Nanomedicines and Drug Delivery The term “nano” refers to all molecules and devices/technologies in the size range 1–1,000 nm (a nanometer is 10−9 m). Nanomedicine has been defined as “the science and technology of diagnosing, treating, and preventing disease, relieving pain, and preserving and improving human health by using molecular tools and molecular knowledge of the human body.” New drugs and new delivery systems both thus come under the umbrella of nanomedicines. Innovative devices and drug delivery systems are needed to exploit many of the drugs developed from advances in molecular biology, to guide the therapeutic to its correct location of action and ensure that pharmacological activity is maintained for an adequate duration once it is there. In many cases, water soluble polymers are used to carry the active drug into the body without affecting the sites other than the target sites, where the local environment breaks down the polymer releasing the active drug. “Nanoparticles” are solid colloidal polymeric carriers (less than 1 mm in size) that have received much attention over the recent years due to their ability to control drug release and distribution and due to their biodegradability [77]. Furthermore, these systems have proven their potential to administer peptides or other drugs either by intravenous or oral routes, increasing their bioavailability and protection of the drug against degradation, and reducing the associated adverse effects [78,79]. Most commonly used methods for preparing polymer-based nanoparticles include emulsion evaporation [80], in situ monomer polymerization [81], a method based on the salting out effect [82] and nanoprecipitation [83]. The last named method represents an easy and reproducible technique, which has been widely explored for producing both vesicle and matrix type nanoparticles, also termed nanocapsule and nanosphere, respectively [84]. For the last decade, surfactant-free nanoparticles formation has been investigated by many researchers. Fessi et al. [83] developed surfactant-free nanocapsules of poly(D,Llactide) (PLA) by the nanoprecipitation technique, using a novel and simple procedure which involved interfacial deposition of a preformed, well-defined and biodegradable polymer following displacement of a semi-polar solvent miscible with water from a lipophilic solution. Nanocapsules of PLA containing indomethacin as a drug model were thus prepared [83]. More recently, poly(lactide-co-glycolide) nanoparticles were prepared by using poly(ethylene glycol)-based block copolymers as substitutes for conventional surfactants [85]. Progress in the development of nano-sized hybrid therapeutics and nano-sized drug delivery systems over the last decade has been remarkable. A growing number of products have already secured regulatory authority approval and, in turn, are supported by a healthy clinical development pipeline. They include products used to treat cancer, hepatitis, muscular sclerosis, and growth hormone deficiency. (See “Therapeutics Packaging and Nanomedicines” in Chapter 7.)

1.29.2 Biodegradable Plastics and Bioplastics The ability to undergo biodegradation producing nontoxic by-products is a useful property for some medical applications. Biodegradable polymers [86] have been formulated for uses such as sutures, vascular grafts, drug delivery devices, and scaffolds for tissue regeneration, artiﬁcial skin, orthopedic implants, and others. The polymers commonly known in the medical ﬁeld for such applications include poly

156

Plastics Technology Handbook

(a-hydroxy esters), poly(ϵ-caprolactone), poly(ortho esters), polyanhydrides, poly(3-hydroxybutyrate), polyphosphazenes, polydioxanones, and polyoxalates (see Chapter 5). The homopolymers poly(L-lactic acid) (PLLA) and poly(glycolic acid) (PGA) as well as poly (DLlactic-co-glycolic acid) (PLGA) copolymers are poly(a-hydroxy esters). They are biocompatible, biodegradable and are among the few biodegradable polymers with Food and Drug Administration (FDA) approval for human clinical use. PLLA, PLGA copolymers, and PGA are produced by a catalyzed ring— opening polymerization of lactide and/or glycolide. Their degradation in vivo is brought about by simple, non-enzymatic hydrolysis of the ester bond leading, eventually, to lactic acid and/or glycolic acid. These acids are processed through normal metabolic pathways and eliminated from the body ultimately through the respiratory system as carbon dioxide. PGA is a highly crystalline, hydrophilic poly(a-hydroxy ester) and was one of the first synthetic degradable polymers to find application as a biomaterial. One of the first applications of PGA was as a biodegradable suture material. PLLA is one of the strongest known biodegradable polymers and has therefore found many applications in areas, such as orthopedics. Although organ transplantation has saved many lives, the harsh reality remains that the need for donor organs far outweighs the supply. It is recognized that tissue engineering may provide an alternative to organ transplantation. Tissue engineering involves the creation of natural tissue with the ability to restore missing organ function. This may be achieved either by transplanting cells seeded into a porous material or, in some cases, by relying on ingrowth of tissue and cells into such a material. PGA, PLLA and PLGA copolymers have been used as an artificial scaffold [87] in cell transplantation and organ regeneration (see Chapter 5). Two different groups of products fall under the term “bioplastics,” namely, biobased plastics and compostable plastics. Biobased materials or plastics are partly or entirely made of renewable raw materials, a few examples being polylactic acid (PLA), polyhydroxyalkanoate, starch, cellulose, chitin, and gelatin. Biobased plastics can be biodegradable, but they are not so always, examples of the latter being composites of natural fiber, wood, and plastic. Compostable plastics, on the other hand, can be completely biodegraded by microorganisms. Special bacteria release enzymes capable of breaking down the polymer chains of compostable plastics into small parts, which are then digested by the bacteria together with other organic matter present in the waste, leaving behind water, CO2, and biomass. The biodegradability of a compostable polymer depends entirely on its chemical structure and the polymer can be produced from renewable raw materials or from fossil sources. A commercial example of compostable bioplastics is Ecovio marketed by BASF. It consists of the biodegradable BASF polymer Ecoflex and PLA, derived from corn, and also consists partially of renewable raw materials. Compared to simple starch-based bioplastics, Ecovio is more resistant to mechanical stress and moisture.

1.29.3 Oxo-Biodegradable Plastics Discarded conventional plastics remain in the environment for decades. They block sewers and drains, disﬁgure the streets, beaches and countryside, and kill wildlife on land, in rivers and oceans. To overcomethese problems increasing attention has been paid to the development of degradable plastics: (1) starch-based, biodegradable, (2) aliphatic polyesters, biodegradable, (3) photodegradable, and (4) oxobiodegradable. The starch-based plastics do not degrade totally, since only the starch constituent is consumed by microbial activity, and the plastic residues can be harmful to the soil and to birds and insects. Aliphatic polyesters, described above, are relatively expensive. In the same way as starch, they rely on microbial activity in compost or soil to degrade. Both these products degrade by a process of hydrodegradation. Photodegradable plastics degrade after prolonged exposure to sunlight, so will not degrade if buried in a landﬁll, a compost heap, or other dark environment, or if heavily overprinted. Oxo-biodegradable

Characteristics of Polymers and Polymerization Processes

157

plastics (Symphony Plastic Technologies, Borehamwood, Herts, U.K.) are a new development. The plastics degrade by a process of oxo-degradation. The technology is based on a small amount (typically 3%) of prodegradent additive (proprietary) being introduced into the conventional manufacturing process, thereby making the plastic degradable. The degradation, which does not rely on microbes, starts immediately after manufacture and accelerate when exposed to heat, light or stress. The process is irreversible and continues in air until the whole material reduces to CO2 and water, leaving no fragments in the soil. The oxobiodegradable plastics (OBPs) are also consumed by bacteria and fungi after the additive has broken down the molecular structure sufficiently, allowing access to living micro-organisms. So organic wastes in homes, restaurants, hospitals etc. can be put into oxo-biodegradable plastic sacks for disposing straight into the composting plant without emptying the sacks. Even if OBPs were eaten by cow, deer, turtle or other animal whilst still intact, they would degrade even faster due to the temperature and bacteria present in the gut without causing blockage, unlike conventional plastic bags which could kill the animal. OBPs have an advantage over plastics produced from starch or other agricultural products in that they biodegrade and can be composed but do not need to be buried in a compost heap or landfill in order to degrade. The fact that they can degrade in a normal environment is a significant factor in relation to litter, because a large amount of plastic waste on land and at sea cannot be collected and buried. OBPs can be made transparent and can be used for direct food contact. The length of time OBP takes to degrade totally can be “programmed” at the time of manufacture by varying the amount of additive and can be as little as a few months or as much as a few years.

1.29.4 Toxicity of Plastic Combustion Products Fires involving plastics produce not only smoke but also other pyrolysis and combustion products. Since most of the polymeric materials contain carbon, carbon monoxide is one of the products generated from the heating and burning of these materials. Depending on the material, the temperature, and the presence or absence of oxygen, other harmful gases may also evolve. These include HCl, HCN, NO2, SO2, and fluorinated gases. Presently, various flame-retarding agents are added to plastics to reduce their combustion properties. When heated or subjected to flame, these agents can change the composition of the degradation products and may produce toxic responses not originally anticipated [73].

1.29.5 Toxicity Testing Ideally, materials for medical and paramedical applications should be tested or evaluated at three level: (1) on the ingredients used to make the basic resin, (2) on the ﬁnal plastic or elastomeric material, and (3) on the ﬁnal device. Organizations such as the Association for the Advancement of Medical Instrumentation, the U.S.A. Standard Institute, and the American Society for Testing and Materials (F4 Committee) have developed toxicity testing programs for materials used in medical applications. The American Dental Association has recommended standard procedures for biological evaluation of dental materials [89].

References 1. Seymour, R. B. and Carraher, C. E. 1992. Polymer Chemistry, An Introduction. Marcel Dekker, New York. 2. Billmeyer, F. W. 1969. Appl. Polymer Symposia, 10, 1. 3. Debye, P. 1947. J. Phys. Coll. Chem., 51, 18. 4. Price, B. A., Halwer, M., and Speiser, R. 1950. J. Opt. Soc. Am., 40, 768. 5. Chanda, M. 2013. Introduction to Polymer Science and Chemistry, A Problem Solving Approach. CRC Press, Boca Raton, FL. 6. Onyon, P. F. 1951. Viscometry, Chap. 6. In Techniques of Polymer Characterization, P. W. Allen, ed. Butterworth, London, UK.

158

Plastics Technology Handbook

7. Patat, F. and Sinn, H. 1958. Angew. Chem., 70, 496. 8. Cossee, P. 1967. The mechanism of Ziegler-Natta polymerization II. Quantum-chemical and crystalchemical aspects, Chap. 3. In The Stereochemistry of Macromolecules, Vol. 6, A. D. Ketley, ed., Marcel Dekker, New York. 9. Brunsveld, L., Folmer, B. J. B., Meijer, E. W., and Sijbesma, R. P. 2001. Chem. Rev., 101, 4071. 10. Sijbesma, R. P., Beijer, F. H., Brunsveld, L., Folmer, B. J. B., Hirschberg, J. H. K., Lange, R. F. M., Lowe, J. K. L., and Meijer, E. W. 1997. Science, 278, 1601. 11. Folmer, B. J. B., Sijbesma, R. P., Versteegen, R. M., van der Rijt, J. A. J., and Meijer, E.W. 2000. Adv. Mater., 12, 12, 874. 12. Lenz, R. W. 1970. Ind. Eng. Chem., 62, 2, 54. 13. Wall, F. T. 1944. J. Am. Chem. Soc., 66, 2050 14. Odian, G. 2004. Principles of Polymerization, Wiley, New York. 15. Harkins, W. D. 1947. J. Am. Chem. Soc., 69, 1428. 16. Smith, W. V. and Ewart, R. W. 1948. J. Chem. Phys., 16, 592. 17. Rudin, A. 1982. The Elements of Polymer Science and Engineering, Academic Press, Orlando, FL. 18. Langevin, D. 1992. Annu. Rev. Phys. Chem., 43, 341. 19. Winsor, P. A. 1954. Solvent Properties of Amphiphilic Compounds, Butterworth, London, UK. 20. Chanda, M. 2016. Direct synthesis of nano-size polymers by microemulsion polymerization, Chap. 3. In Nano-Size Polymers, Preparation, Properties, Applications, S. Fakirov, ed. Springer Nature, New York. 21. Stoffer, J. O. and Bone, T. 1980. J. Polym. Sci. Polym. Chem. Ed., 18, 2641. 22. Atik, S. S. and Thomas, K. 1981. J. Am. Chem. Soc., 103, 4279. 23. Guo, G. S., El-Asser, M. S., and Vanderhoff, J. W. 1989. ACS Symp. Ser., 384, 86. 24. P’erez-Luna, V. H., Puig, J. E., Castano, V. M., Rodriguez, B. E., Murthy, A. K., and Kaler, E. W. 1990. Langmuir, 6, 1040. 25. Leong, Y. S. and Candau, F. 1982. J. Phys. Chem., 86, 2269. 26. Candau, F., Leong, Y. S., and Fitch, R. M. 1985. J. Polym. Sci. Polym. Chem. Ed., 23, 193. 27. Gan, L. M., Chew, C. H., Lim, J. H., Lee, K. S., and Gan, L. H. 1994. Colloid. Polym. Sci., 272, 1082. 28. Pinto Reis, C., Neufeld, R. J., Rebeiro, A. J., and Veiga, F. 2006. Nanomedicine, 2, 1, 6. 29. Levenspiel, O. 1979. The Chemical Reactor Omnibus, OSU Book Stores, Corvalis, OR. 30. Flory, P. J. 1944. Chem. Rev., 35, 51. 31. Nielsen, L. C. 1969. J. Macromol. Sci. Rev. Macromol. Chem. C369. 32. Boyer, R. F. 1963. Rubber Chem. Tech., 36, 1303. 33. Boyer, R. F. 1954. J. Appl. Phys., 25, 825. 34. Beamen, R. G. 1952. J. Polym. Sci., 9, 470. 35. Mark, H. F. 1967. Sci. Am., 217(3), Sept. 19, 148, see also p. 156. 36. Overberger, C. G. and Moore, J. A. 1970. Fortschr. Hochpolym. Forsch., 7, 113. 37. Hildebrand, J. H., Prausnitz, J. M., and Scott, R. L. 1970. Regular and Related Solutions, Van Nostrand Reinhold, New York. 38. Burrell, H. 1955. Ofﬁcial Dig., 27, 726. 39. Nelson, R. C., Hemwall, R. W., and Edwards, G. D. 1970. J. Paint Technol., 42, 636. 40. Small, P. A. 1953. J. Appl. Chem., 3, 71. 41. Hoy, K. L. 1970. J. Paint Technol., 42, 76. 42. Brandrup, J. and Immergut, E., eds. 1975. Polymer Handbook, p. 251. Wiley Interscience, New York. 43. Burrell, H. 1975. Solubility parameter values. In Polymer Handbook, 2nd Ed., J. Brandrup and E. Immergut, eds., p. 251. Wiley Interscience, New York, pp. IV-337–359. 44. Krause, S. 1972. J. Macromol. Sci. Macromol. Rev., C7, 251. 45. Hansen, C. M. 1967. J. Paint Technol., 39, 104, see also p. 511. 46. Barton, A. F. M. 1983. Handbook of Solubility Parameters and Other Cohesion Parameters. CRC Press, Boca Raton, FL.

Characteristics of Polymers and Polymerization Processes

159

47. Hansen, C. M. and Beerbower, A. 1971. Kirk Othmer Encycl. Chem. Technol., 2nd Ed., A. Standen, ed., Suppl. 889, 910, Wiley Interscience, New York. 48. Teas, J. P. 1968. J. Paint Technol., 40, 519. 49. Bagley, E. B., Nielsen, T. P., and Scigliano, J. M. 1971. J. Paint Technol., 43, 35. 50. Anon. 1968. Plastics World, 26, 3, 30. 51. Lee, H., Stoffey, D., and Neville, K. 1967. New Linear Polymers, McGraw-Hill, New York. 52. Pielichowski, K. and Njuguna, J. 2005. Thermal Degradation of Polymeric Materials. Rapra Technology, Shrewsbury, Shropshire, UK. 53. Bhuiyan, A. L. 1982. Adv. Polym. Sci., 47, 43. 54. Schmidt, D. L. Mod. Plastics, 37, 131 (Nov.1960), 147 (Dec. 1960). 55. Kuryla, W. C. and Papa, A. J., eds. 1973. Flame Retardancy of Polymeric Materials, 3, p. 129. Marcel Dekker, New York. 56. Halim Hamid, S., Amin, M. B., and Maadhah, A. G. eds. 1992. Handbook of Polymer Degradation, p. 129. Marcel Dekker, New York. 57. Rabek, J. F. 1996. Photodegradation of Polymers, Springer, Berlin-Heidelberg, New York. 58. Rivaton, A., Gardette, J.-L., Mailhot, B., and Morlat-Therlas, S. 2005. Macromol. Symp., 225, 129. 59. Grassie, N. and Scott, G. 1985. Polymer Degradation and Stabilization. Cambridge Univ. Press, Cambridge. 60. Hawkins, W. L. 1972. Polymer Stabilization. Wiley, New York. 61. Gächter, R. and Müller, H. eds. 1987. Plastics Additives Handbook, p. 89. Hanser Publishers, Munich/ New York. 62. Henman, T. J. 1982. World Index of Polyoleﬁn Stabilizers, Kogan Page Ltd., London, UK. 63. Ranby, B. and Rabek, J. F. 1975. Photodegradation, Photooxidation, and Photostabilization of Polymers, Interscience, New York. 64. Massey, L. 2002. Permeability Properties of Plastics and Elastomers, 2nd Ed. ChemTec Publishing, Toronto-Scarborough. 65. Stannett, V. T. and Szwarc, M. 1955. J. Polym. Sci., 16, 89. 66. Paine, F. A. 1962. J. Roy. Inst. Chem., 86, 263. 67. Seymour, R. B. 1978. Additives for Plastics., Vol. 2., Academic Press, New York. 68. Stepak, J. and Daoust, H. 1982. Additives for Plastics, Springer, New York. 69. Wypych, G. 2004. Handbook of Plasticizers, ChemTec Publishing, Toronto-Scarborough. 70. Dufton, P. W. 2003. Flame Retardants for Plastics, ChemTec Publishing, Toronto-Scarborough. 71. Camino, G. and Delobel, R. 2000. Fire Retardancy of Polymeric Materials, A. F. Grand and C. A. Wilke, eds., p. 217. Marcel Dekker, New York. 72. Baljinder, K. K. and Horrocks, A. R. 1996. Polym. Degrad. Stab., 54, 289. 73. Lewin, M. and Endo, M. 2003. Polym. Adv. Technol., 14, 3. 74. Chen, X., Ding, Y., and Tang, T. 2005. Polym. Int., 54, 904. 75. Soderholm, K. and Mariotti, A. 1999. J. Am. Dent. Assoc., 130, 201. 76. Fung, E. Y. K., Ewoldsen, N. O., St. Germain, H. S., Marx, D. B., Miaw, C., Siew, C., Chou, H., Grunniger, S. E., and Meyer, D. M. 2000. J. Am. Dent. Assoc., 131, 51. 77. Kreuter, J. 1991. J. Controlled Release, 16, 169. 78. Maincent, P., Le Verge, R., Sado, P., Couvreur, P., and Devissaguet, J. P. 1986. J. Pharma. Sci., 75, 955. 79. Couvreur, P., Grislain, L., Lenaerts, V., Brasseur, P., Guiot, P., and Biernacki, A. 1986. Polymeric Nanoparticles and Microspheres, P. Guiot and P. Couvreir, eds., pp. 27–93. CRC Press Inc., Boca Raton, FL. 80. Zambaux, M. F., Bonneaux, F., Gref, R., Maincent, P., Dellacherie, E., Alonso, M. J., Labrude, P., and Vigneron, C. 1998. J. Controlled Release, 50, 31. 81. Hármia, H., Speiser, P., and Kreuter, J. 1986. J. Microencapsul., 3, 3. 82. Alle´man, E. Gumy, E., and Doelkar, E. 1992. Int. J. Pharm., 87, 247. 83. Fessi, H., Puisieux, F., Devissaguet, J.-P., Ammoury, N., and Benita, S. 1989. Int. J. Pharm., 55, R1.

160

Plastics Technology Handbook

84. Chacon, M., Berges, L., Molpaceres, J., Aberturass, M. R., and Guzman, M. 1996. Int. J. Pharm., 141, 81. 85. Chacon, M., Berges, L., Molpaceres, J., Aberturass, M. R., and Guzman, M. 1996. Int. J. Pharm., 141, 81. 86. Siggs, L. J. and Mikos, A. G. 1996. Synthetic biodegradable polymers for medical applications. In Physical Properties of Polymers Handbook, J. E. Mark, ed., Chap. 44, Am. Inst. of Physics, Woodbury, New York. 87. Thomson, R. C., Wake, M. C., Yaszemski, M. J., and Mikos, A. G. 1995. Adv. Polym. Sci., 122, 245. 88. Autian, J. 1970. J. Fire Flammability, 1, 239. 89. American Dental Association. 1972. J. Am. Dent. Assoc., 84, 382.

2 Fabrication Processes 2.1 Types of Processes As indicated in Chapter 1, the family of polymers is extraordinarily large and varied. There are, however, some fairly broad and basic approaches that can be followed when designing or fabricating a product out of polymers or, more commonly, polymers compounded with other ingredients. The type of fabrication process to be adopted depends on the properties and characteristics of the polymer and on the shape and form of the final product. In the broad classification of plastics there are two generally accepted categories: thermoplastic resins and thermosetting resins. Thermoplastic resins consist of long polymer molecules, each of which may or may not have side chains or groups. The side chains or groups, if present, are not linked to other polymer molecules (i.e., are not cross-linked). Thermoplastic resins, usually obtained as a granular polymer, can therefore be repeatedly melted or solidified by heating or cooling. Heat softens or melts the material so that it can be formed; subsequent cooling then hardens or solidifies the material in the given shape. No chemical change usually takes place during this shaping process. In thermosetting resins the reactive groups of the molecules from cross-links between the molecules during the fabrication process. The cross-linked or “cured” material cannot be softened by heating. Thermoset materials are usually supplied as a partially polymerized molding compound or as a liquid monomer–polymer mixture. In this uncured condition they can be shaped with or without pressure and polymerized to the cured state with chemicals or heat. With the progress of technology the demarcation between thermoplastic and thermoset processing has become less distinct. For thermosets processes have been developed which make use of the economic processing characteristics or thermoplastics. For example, cross-linked polyethylene wire coating is made by extruding the thermoplastic polyethylene, which is then cross-linked (either chemically or by irradiation) to form what is actually a thermoset material that cannot be melted again by heating. More recently, modified machinery and molding compositions have become available to provide the economics of thermoplastic processing to thermosetting materials. Injection molding of phenolics and other thermosetting materials are such examples. Nevertheless, it is still a widespread practice in industry to distinguish between thermoplastic and thermosetting resins. Compression and transfer molding are the most common methods of processing thermosetting plastics. For thermoplastics, the more important processing techniques are extrusion, injection, blow molding, and calendaring; other processes are thermoforming, slush molding, and spinning.

2.2 Tooling for Plastics Processing Tooling for plastics processing deﬁnes the shape of the part. It falls into two major categories, molds and dies. A mold is used to form a complete three-dimensional plastic part. The plastics processes that use molds are compression molding, injection molding, blow molding, thermoforming, and reaction injection 161

162

Plastics Technology Handbook

molding (RIM). A die, on the other hand, is used to form two of the three dimensions of a plastic part. The third dimension, usually thickness or length, is controlled by other process variables. The plastics processes that use dies are extrusion, pultrusion and thermoforming. Many plastics processes do not differentiate between the terms mold and die. Molds, however, are the most predominant form of plastics tooling.

2.2.1 Types of Molds The basic types of molds, regardless of whether they are compression, injection, transfer, or even blow molds, are usually classified by the type and number of cavities they have. For example, Figure 2.1 illustrates three mold types: (a) single-cavity, (b) dedicated multiple-cavity, and (c) family multiple-cavity. Single-cavity mold (Figure 2.1a) represents one of the simplest mold concepts. This design lends itself to low-volume production and to large plastic part designs. The multiple-cavity molds may be of two types. A dedicated multiple-cavity mold (Figure 2.1b) has cavities that produce the same part. This type of mold is very popular because it is easy to balance the plastic flow and establish a controlled process. In a family multiple-cavity mold (Figure 2.1c), each cavity may produce a different part. Historically, family mold designs were avoided because of difficulty in filling uniformly; however, recent advances in mold making and gating technology make family molds appealing. This is the case especially when a processor has a multiple-part assembly and would like to keep inventories balanced.

2.2.2 Types of Dies Within the plastics industry, the term die is most often applied to the processes of extrusion (see “Extrusion”). Extrusion dies may be categorized by the type of product being produced (e.g., film, sheet, profile, or coextrusion), but they all have some common features as described below. 1. Steel. The extrusion process being continuous, both erosion and corrosion are significant factors. Hence the dies must be made of a high-quality tool steel, hardened so that the areas that contact the

(a)

(b)

(c)

FIGURE 2.1

Three basic types of molds. (a) single-cavity; (b) dedicated multiple-cavity; (c) family multiple-cavity.

Fabrication Processes

3. 4.

163

plastic material do not erode. Additionally, many dies have a dense, hard chrome plating in the area where plastic melt contacts the die. Heaters. Extrusion dies are to be heated in order to maintain a melt flow condition for the plastic material. Most of the heaters are cartridge-type elements that slip fit into the die at particular locations. In addition to the heaters, the dies have to accommodate temperature sensors, such as thermocouples. Melt Pressure. Many sophisticated dies are equipped with sensors that monitor melt pressure. This allows the processor to better monitor ad control the process. Parting Line. Large extrusion dies must be able to separate at the melt flow line for easier fabrication and maintenance. Smaller extrusion dies may not have a parting area, because they can be constructed in one piece. Die Swell Compensation. The polymer melt swells when it exits the die, as explained previously. This die swell is a function of the type of plastic material, the melt temperature, the melt pressure, and the die configuration. The die must be compensated for die swell so that the extruded part has the corrected shape and dimensions. Molds and dies for different fabrication processes will be described later in more detail when the processes are discussed.

2.2.3 Tool Design The design of the tooling to produce a specific plastics part must be considered during the design of the part itself. The tool designer must consider several factors that may affect the fabricated part, such as the plastics material, shrinkage, and process equipment. Additionally, competitive pressures within the plastics industry require the tool designer to consider how to facilitate tool changeovers, optimize tool maintenance, and simplify (or eliminate) secondary operations. Historically, plastics molds and dies were built by toolmakers who spent their lives learning and perfecting their craft. Today the void created by the waning numbers of these classically trained toolmakers is being filled by the development of numerically controlled (NC) machinery centers, computer-based numerically controlled (CNC) machinery centers, and computer-aided design (CAD) systems. Molds and dies can now be machined on computer-controlled mills, lathes, and electric discharge machines that require understanding of computers and design, rather than years of experience and machining skills. The quality of tool components is now more a function of the equipment than of the toolmaker’s skill. The high costs of molds and the fact that many production molds are built under extreme time constraints leave no room for trial and error. Though prototyping has been widely used to evaluate smaller part designs when circumstances and time allow, prototyping is not always feasible for larger part designs. There are, however, several alternatives to prototyping, e.g., CAD, finite-element analysis (FEA), and rapid prototyping. While CAD allows a tool designer to work with a three-dimensional computer model of the tool being designed and to analyze the design, FEA allows the tool to be evaluated (on a computer) for production worthiness. The mold is then fabricated from the computer model, a process called computeraided manufacturing (CAM). Rapid prototyping is a relatively new method of producing a plastics part by using a three-dimensional computer drawing. A sophisticated prototyping apparatus interprets the drawing and guides an articulating laser beam across a specific medium such as a photopolymer plastic or laminated paper, the result being a physical representation of the computer-based drawing. Prototyped parts can be produced in less than 24 h, and part designs can be scaled to fit the size of the prototyping equipment. Another trend is the introduction of molds that accept interchangeable modules. Modules take less time to manufacturing, and in turn, cut down on the delivery time and costs. In addition, it usually takes less time to change the module than the entire mold frame.

164

Plastics Technology Handbook

2.3 Compression Molding Compression molding is the most common method by which thermosetting plastics are molded [1–3]. In this method the plastic, in the form of powder, pellet, or disc, is dried by heating and then further heated to near the curing temperature; this heated charge is loaded directly into the mold cavity. The temperature of the mold cavity is held at 150°C–200°C, depending on the material. The mold is then partially closed, and the plastic, which is liquefied by the heat and the exerted pressure, flows into the recess of the mold. At this stage the mold is fully closed, and the flow and cure of the plastic are complete. Finally, the mold is opened, and the completely cured molded part is ejected. Compression-molding equipment consists of a matched mold, a means of heating the plastic and the mold, and some method of exerting force on the mold halves. For severe molding conditions molds are usually made of various grades of tool steel. Most are polished to improve material flow and overall part quality. Brass, mild steel, or plastics are used as mold materials for less severe molding conditions or shortrun products. In compression molding a pressure of 2,250 psi (158 kg/cm2)–3,000 psi (211 kg/cm2) is suitable for phenolic materials. The lower pressure is adequate only for an easy-flow materials and a simple uncomplicated shallow molded shape. For a medium-flow material and where there are average-sized recesses, cores, shapes, and pins in the molding cavity, a pressure of 3,000 psi (211 kg/cm2) or above is required. For molding urea and melamine materials, pressures of approximately one and one-half times that needed for phenolic material are necessary. The time required to harden thermosetting materials is commonly referred to as the cure time. Depending on the type of molding material, preheating temperature, and the thickness of the molded article, the cure time may range from seconds to several minutes. In compression molding of thermosets the mold remains hot throughout the entire cycle; as soon as a molded part is ejected, a new charge of molding powder can be introduced. On the other hand, unlike thermosets, thermoplastics must be cooled to harden. So before a molded part is ejected, the entire mold must be cooled, and as a result, the process of compression molding is quite slow with thermoplastics. Compression molding is thus commonly used for thermosetting plastics such as phenolics, urea, melamine, an alkyds; it is not ordinarily used for thermoplastics. However, in special cases, such as when extreme accuracy is needed, thermoplastics are also compression molded. One example is the phonograph records of vinyl and styrene thermoplastics; extreme accuracy is needed for proper sound reproduction. Compression molding is ideal for such products as electrical switch gear and other electrical parts, plastic dinnerware, radio and television cabinets, furniture drawers, buttons, knobs, handles, etc. Like the molding process itself, compression molding machinery is relatively simple. Most compression presses consist of two platens that close together, applying heat and pressure to the material inside a mold. The majority of the presses are hydraulically operated with plateau ranging in size from 6 in. square to 8 ft square or more. The platens exert pressures ranging from 6 up to 10,000 tons. Virtually all compression molding presses are of vertical design. Most presses having tonnages under 1000 are upward-acting, while most over 1,000 tons act downward. Some presses are built with a shuttle-clamp arrangement that moves the mold out of the clamp section to facilitate setup and part removal. Compression molds can be divided into hand molds, semiautomatic molds, and automatic molds. The design of any of these molds must allow venting to provide for escape of steam, gas, or air produced during the operation. After the initial application of pressure the usual practice is to open the mold slightly to release the gases. This procedure is known as breathing. Hand molds are used primarily for experimental runs, for small production items, or for molding articles which, because of complexity of shape, require dismantling of mold sections to release them. Semiautomatic molds consist of units mounted firmly on the top and bottom platens of the press. The operation of the press closes and opens the mold and actuates the ejector system for removal of the molded article. However, an operator must load the molding material, actuate press controls for the molding sequence, and remove the ejected piece from the mold. This method is widely used.

Fabrication Processes

165

Fully automatic molds are specially designed for adaptation to a completely automatic press. The entire operation cycle, including loading and unloading of the mold, is performed automatically, and all molding operations are accurately controlled. Thermosetting polymers can be molded at rates up to 450 cycles/h. Tooling must be of the highest standard to meet the exacting demands of high-speed production. Automatic molds offer the most economical method for long production runs because labor costs are kept to a minimum. The three common types of mold designs are open ﬂash, fully positive, and semipositive.

2.3.1 Open Flash In an open flash mold a slight excess of molding powder is loaded into the mold cavity (Figure 2.1a) [4]. On closing the top and bottom platens, the excess material is forced out and flash is formed. The flash blocks the plastic remaining in the cavity and causes the mold plunger to exert pressure on it. Gas or air can be trapped by closing the mold too quickly, and finely powdered material can be splashed out of the mold. However, if closing is done carefully, the open flash mold is a simple one, giving very good results. Since the only pressure on the material remaining in the flash mold when it is closed results from the high viscosity of the melt which did not allow it to escape, only resins having high melt viscosities can be molded by this process. Since most rubbers have high melt viscosities, the flash mold is widely used for producing gaskets and grommets, tub and flash stoppers, shoe heels, door mats, and many other items. Because of lower pressure exerted on the plastic in the flash molds, the molded products are usually less dense than when made using other molds. Moreover, because of the excess material loading needed, the process is somewhat wasteful as far as raw materials are concerned. However, the process has the advantage that the molds are cheap, and very slight labor costs are necessary in weighing out the powder.

2.3.2 Fully Positive In the fully positive molds (Figure 2.2b) no allowance is made for placing excess powder in the cavity [4]. If excess powder is loaded, the mold will not close; an insufficient charge will result in reduced thickness of the molded article. A correctly measured charge must therefore be used with this mold—it is a disadvantage of the positive mold. Another disadvantage is that the gases liberated during the chemical curing reaction are trapped inside and may show as blisters on the molded surface. Excessive wear on the sliding fit surface on the top and bottom forces and the difficulty of ejecting the molding are other reasons for discarding this type of mold. The mold is used on a small scale for molding thermosets, laminated plastics, and certain rubber components.

2.3.3 Semipositive The semipositive mold (Figure 2.2c and d) combines certain features of the open flash and fully positive molds and makes allowance for excess powder and flash [4]. It is also possible to get both horizontal and vertical flash. Semipositive molds are more expensive to manufacture and maintain than the other types, but they are much better from an applications point of view. Satisfactory operation of semipositive molds is obtained by having clearance (0.025/25 mm of diameter) between the plunger (top force) and the cavity. Moreover, the mold is given a 2–3° taper on each side. This allows the flash to flow on and the entrapped gases to escape along with it, thereby producing a clean, blemish-free mold component.

2.3.4 Process Applicability Compression molding is most cost-effective when used for short-run parts requiring close tolerances, high-impact strength, and low mold shrinkage. Old as the process may be, new applications continue to

166

Plastics Technology Handbook

Plunger

Land Parting line

Cavity depth

Molded piece

Clearance Loading space Depth of cavity

Cavity Knockout pins (b)

(a)

Knockout pins

Cavity

Plunger

a b

Land a

Knockout pin (c)

Positive portion

Cavity well

Cavity (d)

FIGURE 2.2 Compression molds. (a) A simple ﬂash mold. (b) A positive mold. Knockout pins could extend through plunger instead of through cavity. (c) Semi-positive mold as it appears in partly closed position before it becomes positive. Material trapped in area b escapes upward. (d) Semipositive mold in closed position.

evolve compression molding. For example, in the dental and medical fields, orthodontic retainers, and pacemaker casings are now mostly compression molded because of low tool costs. Injection molding tools to produce the same part would cost as much as eight times more. Manufacturers of gaskets and seals who started out with injection-molded products to take advantage of the faster cycle times, are now switching back to compression molding to maintain quality level required for these parts. The use of compression molding has expanded significantly in recent years due to the development of new materials, reinforced materials in particular. Molding reinforced plastics (RPs) requires two matched dies usually made of inexpensive aluminum, plastics, or steel and used on short runs. Adding vacuum chambers to compression molding equipment in recent years has reduced the number of defects caused by trapped air or water in the molding compound, resulting in higher-quality finished parts. Another relatively new improvement has been the addition of various forms of automation to the process. For example, robots are used both to install inserts and remove finished parts.

2.4 Transfer Molding In transfer molding, the thermosetting molding powder is placed in a chamber or pot outside the molding cavity and subjected to heat and pressure to liquefy it [1–6]. When liquid enough to start ﬂowing, the material is forced by the pressure into the molding cavity, either by a direct sprue or though a system of runners and gates. The material sets hard to the cavity shape after a certain time (cure time) has elapsed. When the mold is disassembled, the molded part is pushed out of the mold by ejector pins, which operate automatically. Figure 2.3 shows the molding cycle of pot-type transfer molding, and Figure 2.4 shows plunger-type transfer molding (sometime called auxiliary raw transfer molding). The taper of the sprue is pot-type transfer is such that, when the mold is opened, the sprue remains attached to the disc of material left in the pot, known as cull, and is thus pulled away from the molded part, whereas the latter is lifted out of the cavity by the ejector pins (Figure 2.3c). In plunger-type transfer molding, on the other hand, the cull and the sprue remains with the molded piece when the mold is opened (Figure 2.4c).

167

Fabrication Processes

Cull Sprue Molding compound

Sprue bush

Molded part

(a)

(b)

(c)

FIGURE 2.3 Molding cycle of a pot-type transfer mold. (a) Molding compound is placed in the transfer pot and then (b) forced under pressure when hot through an oriﬁce and into a closed mold. (c) When the mold opens, the sprue remains with the cull in the pot, and the molded part is lifted out of the cavity by ejector pins. (After Frados, J. ed. 1976. Plastics Engineering Handbook, 4th Ed., Van Nostrand Reinhold, New York.)

Another variation of transfer molding in screw transfer molding (Figure 2.5). In this process the molding material is preheated and plasticized in a screw chamber and dropped into the pot of an inverted plunger mold. The preheated molding material is then transferred into the mold cavity by the same method as shown in Figure 2.4. The screw-transfer-molding technique is well suited to fully automatic operation. The optimum temperature of a phenolic mold charge is 240°F ± 20°F (155°C ± 11°C), the same as that for pot-transfer and plunger molding techniques. For transfer molding, generally pressures of three times the magnitude of those required for compression molding are required. For example, usually a pressure of 9,000 psi (632 kg/cm2) and upward is required for phenolic molding material (the pressure referred to here is that applied to the powder material in the transfer chamber). The principle of transferring the liqueﬁed thermosetting material from the transfer chamber into the molding cavity is similar to that of the injection molding of thermoplastics (described later). Therefore the same principle must be employed for working out the maximum area which can be molded—that is, the projected area of the molding multiplied by the pressure generated by the material inside the cavity must be less than the force holding the two halves together. Otherwise, the molding cavity plates will open as the closing force is overcome. Transfer molding has an advantage over compression molding in that the molding powder is ﬂuid when it enters the mold cavity. The process therefore enables production of intricate parts and molding around thin pins and metal inserts (such as an electrical lug). Thus, by transfer molding, metal inserts can be molded into the component in predetermined positions held by thin pins, which would, however, bend or break under compression-molding conditions. Typical articles made by the transfer molding process are terminal-bloc insulators with many metal inserts and intricate shapes, such as cups and caps for cosmetic bottles.

168

Plastics Technology Handbook

Transfer ram

Molding compound

Molded part

Cull

(a)

(b)

(c)

FIGURE 2.4 Molding cycle of a plunger-type transfer mold. (a) An auxiliary ram exerts pressure on the heatsoftened material in the pot and (b) forces it into the mold. (c) When the mold is opened, the cull and sprue remain with the molded piece. (After Frados, J. ed. 1976. Plastics Engineering Handbook, 4th Ed., Van Nostrand Reinhold, New York.)

2.4.1 Ejection of Molding Ejection of a molded plastic article from a mold can be achieved by using ejector pins, sleeves, or stripper plates. Ejector pins are the most commonly used method because they can be easily fitted and replaced. The ejector pins must be located in position where they will eject the article efficiently without causing distortion of the part. They are worked by a common ejector plate or a bar located under the mold, and operated by a central hydraulic ejector ram. The ejector pins are fitted either to the bottom force or to the top force depending on whether it is necessary for the molding to remain in the bottom half of the female part or on the top half of the male part of the tool. The pins are usually constructed of a hardened steel to avoid wear.

2.4.2 Heating System Heating is extremely important in plastics molding operations because the tool and auxiliary parts must be heated to the required temperature, depending on the powder being molded, and the temperature must be maintained throughout the molding cycle. The molds are heated by steam, hot waters, and induction heaters. Steam heating is preferred for compression and transfer molding, although electricity is also used because it is cleaner and has low installation costs. The main disadvantage of the latter method is that the heating is not fully even, and there is tendency to form hot spots.

169

Fabrication Processes

Clamping ram

Molded piece Hopper

Mold halves

Drive motor

Plasticizer screw

Heater Transfer ram

FIGURE 2.5 Drawing of a screw-transfer molding machine. (After Frados, J. ed. 1976. Plastics Engineering Handbook, 4th Ed., Van Nostrand Reinhold, New York.)

2.4.3 Types of Presses Presses used for compression and transfer molding of thermosets can be of many shapes and designs, but they can be broadly classified as hand, mechanical, or hydraulic types. Hand presses have relatively lower capacity, ranging from 10 to 100 tons, whereas hydraulic presses have considerably higher capacity (500 tons). Hydraulic presses may be of the upstroke or downstroke varieties. In the simple upstroke press, pressure can be applied fairly quickly, but the return is slow. In the downstroke press fitted with a prefilling tank, this disadvantage of the upstroke press is removed, and a higher pressure is maintained by prefilling with liquid from a tank. The basic principles of hydraulics are used in the presses. Water or oil is used as the main fluid. Water is cheap but rusts moving parts. Oil is more expensive but it does not corrode and it does lubricate moving parts. The main disadvantage of oil is that it tends to form sludge due to oxidation with air. The drive for the presses is provided by single pumps or by central pumping stations, and accumulators are used for storing energy to meet instantaneous pressure demand in excess of the pump delivery. The usual accumulator consists of a single-acting plunger working in a cylinder. The two main types of accumulators used are the weight-loaded type and the air-loaded type. The weight-loaded type is heavy and therefore not very portable. There is also an initial pressure surge on opening the valve. The pressuresurge problem is overcome in the air- or gas (nitrogen)-loaded accumulator. This type is more portable but suffers a small pressure loss during the molding cycle.

2.4.4 Preheating To cut down cycle times and to improve the finished product of compression molding and transfer molding, the processes of preheating and performing are commonly used. With preheating, relatively thick sections can be molded without porosity. Other advantages of the technique include improved flow of resin, lower molding pressures, reduced mold shrinkage, and reduced flash.

170

Plastics Technology Handbook

Preheating methods are convection, infrared, radio frequency, and steam. Thermostatically controlled gas or electrically heated ovens are inexpensive methods of heating. The quickest, and possibly the most efﬁcient, method is radio-frequency heating, but it is also the most expensive. Preheaters are located adjacent to the molding press and are manually operated for each cycle.

2.4.5 Preforming Preforming refers to the process of compressing the molding powder into the shape of the mold before placing it in the mold or to pelleting, which consists of compacting the molding powder into pellets of uniform size and approximately known weight. Preforming has many advantages, which include avoiding waste, reduction in bulk factor, rapid loading of charge, and less pressure than uncompacted material. Preformers are basically compacting presses. These presses may be mechanical, hydraulic, pneumatic, or rotary cam machines.

2.4.6 Flash Removal Although mold design takes into consideration the fact that flash must be reduced to a minimum, it still occurs to some extent on the molded parts. It is thus necessary to remove the flash subsequent to molding. This removal is most often accomplished with tumbling machines. These machines tumble molded parts against each other to break off the flash. The simplest tumbling machines are merely wire baskets driven by an electric motor with a pulley belt. In more elaborate machines blasting of molded parts is also performed during the tumbling operation.

2.5 Injection Molding of Thermoplastics Injection molding is the most important molding method for thermoplastics [7–9]. It is based on the ability of thermoplastic materials to be softened by heat and to harden when cooled. The process thus consists essentially of softening the material in a heated cylinder and injecting it under pressure into the mold cavity, where it hardens by cooling. Each step is carried out in a separate zone of the same apparatus in the cyclic operation. A diagram of a typical injection-molding machine is shown in Figure 2.6. Granular material (the plastic resin) falls from the hopper into the barrel when the plunger is withdrawn. The plunger then pushes the material into the heating zone, where it is heated and softened (plasticized or plasticated). Rapid heating Plunger

Back pressure plate Clamp pressure

Hopper

Gating Torpedo Barrel

Clamp cylinder Mold

Nozzle Runner Mold cavity

Ram pressure

Resin

Heater Injection chamber Cooling zone Hydraulic cylinder

FIGURE 2.6 Cross section of a typical plunger injection-molding machine. (After Lukov, L. J. 1963. SPE J., 13(10), 1057.)

171

Fabrication Processes

takes place due to spreading of the polymer into a thin film around a torpedo. The already molten polymer displaced by this new material is pushed forward through the nozzle, which is in intimate contact with the mold. The molten polymer flows through the sprue opening in the die, down the runner, past the gate, and into the mold cavity. The mold is held tightly closed by the clamping action of the press platen. The molten polymer is thus forced into all parts of the mold cavities, giving a perfect reproduction of the mold. The material in the mold must be cooled under pressure below Tm or Tg before the mold is opened and the molded part is ejected. The plunger is then withdrawn, a fresh charge of material drops down, the mold is closed under a locking force, and the entire cycle is repeated. Mold pressures of 8,000–40,000 psi (562–2,812 kg/cm2) and cycle times as low as 15 sec are achieved on some machines. Note that the feed mechanism of the injection molding machine is activated by the plunger stroke. The function of the torpedo in the heating zone is to spread the polymer melt into thin film in close contact with the heated cylinder walls. The fins, which keep the torpedo centered, also conduct heat from the cylinder walls to the torpedo, although in some machines the torpedo is heated separately. Injection-molding machines are rated by their capacity to mold polystyrene in a single shot. Thus a 2-oz machine can melt and push 2 oz of general-purpose polystyrene into a mold in one shot. This capacity is determined by a number of factors such as plunger diameter, plunger travel, and heating capacity. The main component of an injection-molding machine are (1) the injection unit which melts the molding material and forces it into the mold; (2) the clamping unit which opens the mold and closes it under pressure; (3) the mold used; and (4) the machine controls.

2.5.1 Types of Injection Units Injection-molding machines are known by the type of injection unit used in them. The oldest type is the single-stage plunger unit (Figure 2.6) described above. As the plastic industry developed, another type of plunger machine appeared, known as a two-stage plunger (Figure 2.7a). It has two plunger units set one on top of the other. The upper one, also known as a preplasticizer, plasticizes the molding material and feeds it to the cylinder containing the second plunger, which operates mainly as a shooting plunger, and pushes the plasticized material through the nozzle into the mold. Later, another variation of the two-stage plunger unit appeared, in which the ﬁrst plunger stage was replaced by a rotation screw in a cylinder (Figure 2.7b). The screw increases the heat transfer at the walls Preplasticizing cylinder Torpedo Heaters Valve Screw Preplasticizing plunger

Heaters

Shooting plunger 3-Way (a) valve

Injection chamber

(b)

Injection chamber

Shooting plunger

FIGURE 2.7 Schematic drawings of (a) a plunger-type preplasticizer and (b) a screw-type preplasticizer atop a plunger-type injection molding machine. (After Lukov, L. J. 1963. SPE J., 13(10), 1057.)

172

Plastics Technology Handbook

Hopper Injection chamber

Heating (a) Sprue

Cylinder Heater

(b)

FIGURE 2.8 Cross section of a typical screw-injection molding machine, showing the screw (a) in the retracted position and (b) in the forward position. (After Lukov, L. J. 1963. SPE J., 13(10), 1057.)

and also does considerable heating by converting mechanical energy into heat. Another advantage of the screw is its mixing and homogenizing action. The screw feeds the melt into the second plunger unit, where the injection ram pushes it forward into the mold. Although the single-stage plunger units (Figure 2.6) are inherently simple the limited heating rate has caused a decline in popularity: they have been mostly supplanted by the reciprocating screw-type machines [10]. In these machines (Figure 2.8) the plunger and torpedo (or spreader) that are the key components of plunger-type machines are replaced by a rotating screw that moves back and forth like a plunger within the heating cylinder. As the screw rotates, the ﬂights pick up the feed of granular material dropping from the hopper and force it along the heated wall of the barrel, thereby increasing the rate of heat transfer and also generating considerable heat by its mechanical work. The screw, moreover, promotes mixing and homogenization of the plastic material. As the molten plastic comes off the end of the screw, the screw moves back to permit the melt to accumulate. At the proper time the screw is pushed forward without rotation, acting just like a plunger and forcing the melt through the nozzle into the mold. The size of the charge per shot is regulated by the back travel of the screw. The heating and homogenization of the plastics material are controlled by the screw rotation speed and wall temperatures.

2.5.2 Clamping Units The clamping unit keeps the mold closed while plasticized material is injected into it and opens the mold when the molded article is ejected. The pressure produced by the injection plunger in the cylinder is transmitted through the column of plasticized material and then through the nozzle into the mold. The

Fabrication Processes

173

unlocking force, that is, the force which tends to open the mold, is given by the product of the injection pressure and the projected area of the molding. Obviously, the clamping force must be greater than the unlocking force to hold the mold halves closed during injection. Several techniques can be used for the clamping unit: (1) hydraulic clamps, in which the hydraulic cylinder operates on the movable parts of the mold to open and close it; (2) toggle or mechanical clamps, in which the hydraulic cylinder operates through a toggle linkage to open and close the mold; and (3) various types of hydraulic mechanical clamps that combine features of (1) and (2). Clamps are usually built as horizontal units, with injection taking place through the center of the stationary platen, although vertical clamp presses are also available for special jobs.

2.5.3 Molds The mold is probably the most important element of a molding machine. Although the primary purpose of the mold is to determine the shape of the molded part, it performs several other jobs. It conducts the hot plasticized material from the heating cylinder to the cavity, vents the entrapped air or gas, cools the part until it is rigid, and ejects the part without leaving marks or causing damage. The mold design, construction, the craftsmanship largely determine the quality of the part and its manufacturing cost. The injection mold is normally described by a variety of criteria, including (1) number of cavities in the mold; (2) material of construction, e.g., steel, stainless steel, hardened steel, beryllium copper, chromeplated aluminum, and epoxy steel; (3) parting line, e.g., regular, irregular, two-plate mold, and three-plate mold; (4) method of manufacture, e.g., machining, hobbing, casting, pressure casting, electroplating, and spark erosion; (5) runner system, e.g., hot runner and insulated runner; (6) gating type, e.g., edge, restricted (pinpoint), submarine, sprue, ring, diaphragm, tab, flash, fan, and multiple; and (7) method of ejection, e.g., knockout (KO) pins, stripper ring, stripper plate, unscrewing cam, removable insert, hydraulic core pull, and pneumatic core pull. 2.5.3.1 Mold Designs Molds used for injection molding of thermoplastic resins are usually flash molds, because in injection molding, as in transfer molding, no extra loading space is needed. However, there are many variations of this basic type of mold design. The design most commonly used for all types of materials is the two plate design (Figure 2.9). The cavities are set in one plate, the plungers in the second plate. The sprue bushing is incorporated in that plate mounted to the stationary half of the mold. With this arrangement it is possible to use a direct center gate that leads either into a single-cavity mold or into a runner system for a multi-cavity mold. The plungers are ejector assembly and, in most cases, the runner system belongs to the moving half of the mold. This is the basic design of an injection mold, though many variations have been developed to meet specific requirements. A three-plate mold design (Figure 2.10) features a third, movable, plate which contains the cavities, thereby permitting center or offset gating into each cavity for multicavity operation. When the mold is opened, it provides two openings, one for ejection of the molded part and the other for removal of the runner and sprue. Moldings with inserts or threads or coring that cannot be formed by the normal functioning of the press require installation of separate or loose details or cores in the mold. These loose members are ejected with the molding. They must be separated from the molding and reinstalled in the mold after every cycle. Duplicate sets are therefore used for efficient operation. Hydraulic or pneumatic cylinders may be mounted on the mold to actuate horizontal coring members. It is possible to mold angular coring, without the need for costly loose details, by adding angular core pins engaged in sliding mold members. Several methods may be used for unscrewing internal or external threads on molded parts: For high production rates automatic unscrewing may be done at relatively low cost by the use of rack-and-gear mechanism actuated by a double-acting hydraulic long-stroke cylinder.

174

Plastics Technology Handbook

1 3

4 6

8 10

9 19

16 13

FIGURE 2.9 A two-plate injection-mold design: (1) locating ring; (2) clamping plate; (3) water channels; (4) cavity; (5) sprue bushing; (6) cavity retainer; (7) gate; (8) full round runner; (9) sprue puller pin; (10) plunger; (11) parting line; (12) ejector pin; (13) stop pin; (14) ejector housing; (15) press ejector clearance; (16) pin plate; (17) ejector bar; (18) support plate; (19) plunger retainer.

Runner and sprue Cavity plate

Molded part Force

FIGURE 2.10

A diagram of a three-plate injection mold.

Fabrication Processes

175

Other methods of unscrewing involve the use of an electric gear-motor drive or friction-mold wipers actuated by double-acting cylinders. Parts with interior undercuts can be made in a mold which has provision for angular movement of the core, the movement being actuated by the ejector bar that frees the metal core from the molding. 2.5.3.2 Number of Mold Cavities Use of multiple mold cavities permits greater increase in output speeds. However, the greater complexity of the mold also increases significantly the manufacturing cost. Note that in a single-cavity mold the limiting factor is the cooling time of the molding, but with more cavities in the mold the plasticizing capacity of the machine tends to be the limiting factor. Cycle times therefore do not increase prorate with the number of cavities. There can be no clear-cut answer to the question of optimum number of mold cavities, since it depends on factors such as the complexity of the molding, the size and type of the machine, cycle time, and the number of moldings required. If a fairly accurate estimate can be made of the costs and cycle time for molds with each possible number of cavities and a cost of running the machine (with material) is assumed, a break-even quantity of the number of moldings per hour can be calculated and compared with the total production required. 2.5.3.3 Runners A thermoplastic resin is melted in the barrel of the injection molding machine and is injected into the mold. The channels, or the pathways, through which the melted resin enters the gate areas of the mold cavities are called runners. There are two general types of plastic injection molds, defined by the types of runners used, namely, hot or cold. In a hot runner mold, the runner is internal in the mold and, during operation, is kept constantly at a temperature above the melting point of the plastic so that the resin always remains in a liquid state within the tool except when it passes through the gate into the mold cavity. This means that the material in the channel, also referred to as runner, is not ejected with the finished part. Instead, it stays in the mold ready to fill the cavity to make the next part. Hot runner molds thus eliminate runners entirely and the mold runs automatically, eliminating variations caused by operators. In a long-running job, hot runner molding (also known as “runnerless” molding) is the most economical way of molding as there is no regrinding (of runners) with its attendant cost of handling and loss of material. Hot runner molds consist of two plates and include a heated manifold and a number of heated nozzles. The manifold distributes the melted plastic to the various nozzles, which then meter it precisely to the injection points of the cavities. There are several types of hot runner systems. In general, they fall into two main categories, namely, externally heated and internally heated. The externally heated systems are well suited to polymers that are sensitive to thermal variations. The internally heated systems, however, allow better flow control. The advantages of hot runner systems are as follows: (1) they eliminate runners and avoid potential waste; (2) they allow potentially shorter cycle time (i.e., time required to mold a part); (3) they are better for high-volume production; and (4) they can accommodate larger parts. The disadvantages are as follows: (1) they have more expensive molds; (2) they have higher maintenance costs; (3) the material and color cannot be changed easily; and (4) they may not be suited to certain types of engineering plastic resins and thermally sensitive thermoplastic resins. Cold runner molds usually consist of either two or three plates that are held within the mold base. The plastic is injected into the mold via the sprue and runners that lead to the parts in the cavity. In two-plate molds, the material in the runner system and the parts remain attached, and an ejection system is used to separate the pair from the mold. Two-plate molds have one parting line along which the mold is split into two halves. In three-plate molds, in contrast, the runner is contained on a separate plate, leaving the parts to be ejected alone. These molds have two parting lines. When a molded part is ejected, the mold splits into three

176

Plastics Technology Handbook

sections. It is important that the runner dimension is thicker than the component because this ensures that the molten plastic can be packed into the component as it cools without any restriction. While in both two-plate and three-plate systems the runner is reground and recycled to reduce plastic waste, a three-plate cold runner system offers greater design flexibility and allows gates to be installed according to application requirements. Tunnel gates (submarine gates, see below) are the most frequently used in combination with cold runner systems because trimming of the gate from the part takes place automatically. Typically, the action of either the mold opening or ejecting the part also removes the gate from the part. The main advantages of cold runner systems are as follows: (1) they are comparatively cheaper to produce and maintain; (2) they can accommodate a wide variety of polymers; (3) material and color changes can be made easily; and (4) short cycle time and fast production rate can be achieved by using robotic assist in removing runners. The disadvantages are as follows: (1) cycle times are longer than hot runner systems; and (2) plastic waste is generated from runners (if they cannot be reground and recycled). 2.5.3.4 Gating The gate provides the connection between the runner and the mold cavity. It must permit enough material to flow into the mold to fill out the cavity. The type of the gate and its size and location in the mold strongly affect the molding process and the quality of the molded part. There are two types of gates: large and restricted. A restricted (pin-pointed) thermal gate is a very small orifice between runner and cavity. It is usually circular in cross section and, for most thermoplastics, does not exceed 0.060 in. in diameter. The apparent viscosity of a thermoplastic is a function of shear rate—the viscosity decreases as the shear rate and, hence, the velocity increases. The use of the restricted gate is therefore advantageous, because the velocity of the plastic melt increases as it is forced through the small opening; in addition, some of the kinetic energy is transformed into heat, raising the local temperature of the plastic and thus further reducing its viscosity. The passage through a restricted area also results in higher mixing. Hot-tip gates are identified as restricted thermal gates, which are typically located at the tip of the part rather than on the parting line and are ideal for round or conical shapes where uniform flow is necessary. Hot tip gates are only used with hot runner molding systems. The gate leaves a small raised nub on the surface of the part. The most common type of gate in injection molding is the edge gate (Figure 2.11a), where the part is gated either as a restricted or larger gate at some point on the edge of the part. The edge gate is easy to construct and is best suited for flat parts. Edge gates are ideal for medium and thick sections and can be used on multicavity two-plate molds. This gate will leave a scar on the parting line. The edge gate can be fanned out for large parts or when there is a special reason. Then, it is called a fan gate (Figure 2.11b). A fan gate reduces stress concentration in gate area by spreading the opening over a wider area. With this type of gate, less warping of parts can usually be expected. When it is required to orient the flow pattern in one direction, a flash gate (also known as film gate) may be used (Figure 2.11c). It is very thin, involves extending the fan gate over the full length of the part, and has parallel runners before the gate. It is generally used in thin and flat requirement, like flat mobile phone cap, ipod cap, and others. The most common gate for single-cavity molds is the direct gate or sprue gate (Figure 2.11d). It is located in the center of the plastic part, has a circular cross section, is slightly tapered, and merges with its largest cross section into the part. It is suitable for a big part with a deep cavity. No runner is required. As the gate feeds directly from the nozzle of the machine into the molded part, the pressure loss is minimum. But the sprue gate has the following disadvantages: a bigger cross-sectional gate area and a high stress concentration around the area, the need for gate removal, lack of a cold slug, more difficult degating, and degating relics affecting product appearance since the gate mark is visible in molded components, for example, bucket molding (backside cylindrical gate mark visible). [It may be mentioned that pin gate, especially used for three-part molds, is a reverse taper sprue gate. The runner channel is

177

Fabrication Processes

Top view

Side view (a)

(b)

(d)

(c)

(e)

(f ) Runner Plastic

Tab Gate (g)

Runner (h)

Knockout pins

FIGURE 2.11 Gating design: (a) edge; (b) fan; (c) ﬂash; (d) sprue; (e) diaphragm; (f) ring; (g) tab; (h) submarine.

located in a separate runner plate. The melt flow is divided into several directions and led into the cavity by separate gate locations. The gate point is designed to be very small. It is trimmed off by the action of injection mold opening (see Figure 2.10).] A diaphragm gate (Figure 2.11e) has, in addition to the sprue, a circular area leading from the sprue to the piece. The molten material flows from the center toward the outer circumference. A diaphragm gate is thus used in symmetrical cavity filling to reduce weld line formations and improve filling rate. This type of gate is suitable for gating concentric molded products, such as annular and tubular articles, hollow tubes, and so on. The diaphragm eliminates stress concentration around the gate because the whole area is removed, but the cleaning of this gate is more difficult than a sprue gate. Ring gates (Figure 2.11f) are annular gates, which accomplish the same purpose as gating internally in a hollow tube, but from the outside. The gate is between the runner and the cavity. It encircles the core to permit the melt to first move around the core before filling the cavity. Its easy filling and venting characteristics could avoid weld lines and reduce stress. Ring gates are thus most suitable for cylindrical components to eliminate weld line defect. When the gate leads directly into the part, there may be surface imperfection attributed to jetting. This may be overcome by extending a tab from the part into which the gate is cut. This procedure is called tab gating (Figure 2.11g). The tab has to be removed as a secondary operation. A submarine gate (Figure 2.11h) is one that goes through the steel of the cavity. It is very often used in automatic molds as a type of gate having a structure that automatically cuts the molded item and gate at the time of opening and closing the parting surface. The submarine gate allows one to gate away from the parting line, allowing more flexibility to place the gate at an optimum location on the part. The gate leaves a pin-sized scar on the part.

178

Plastics Technology Handbook

The tunnel gate is a variation of the submarine gate. Classifying tunnel gates broadly into those provided on the fixed half and those on the moving half of the parting surface, there are four patterns of gate and runner combinations. When a tunnel gate is provided on the fixed half, the molded product is cut off from the gate at the time of opening the parting surface. On the other hand, when a tunnel gate is provided on the moving half, the molded product is cut off from the gate at the time that the runner is ejected by the runner ejector pin. On the basis of trimming modes, the various gates fall into two main types, namely, manually trimmed gates and automatically trimmed gates. Manually trimmed gates are those that require an operator to separate parts from runners. These gates are used if the required gate is too large to be sheared from the part, if the part is too thin, or if the material is shear sensitive such that the high shear rate of automatic trimming would damage the part. Gate types trimmed from the part manually include sprue gate, diaphragm gate, flash or film gate, edge gate, fan gate, ring gate, and tab gate. Automatically trimmed gates incorporate features in the tool to break or shear the gate as the mold is opened to eject the part. This type of gate is used to avoid the need for a press operator, decrease cycle time, maintain consistent cycle times for all shots, and minimize gate scrap. Automatically trimmed gates include pin gates, submarine (tunnel) gates, sub gates, hot runner gates, and valve gates (see below). 2.5.3.5 Valve Gates In injection molding with thermal gating or hot tip gating, the melt in the gate area (after cavity fill) cools and solidifies, forming a small slug inside the gate, which remains in the gate during the phases of mold open, part ejection, and mold close motion. During the next injection, the pressurized melt flushes the cold slug into the melt stream that fills the empty cavity, while the slug liquefies from shear heating and mixes with the melt. A part molded with thermal gating retains a standing vestige at the gate interface. Thermal gate vestige is highly dependent on the gate diameter, a larger gate producing a larger vestige. Gate cooling optimization is critical in thermal gate design since mold open cannot occur until the gate is sealed enough to break clearly from the part as well as “hold back” the melt in the hot runner. Mold open before complete gate solidification results in drool (extrudation or leakage of molten resin) or stringing (insufficient cooling of melt in area between finished part and sprue), whereas excessive gate cooling can produce a frozen gate to prevent or delay gate opening and result in short shots or unfilled cavities. All molding parameters (pressure, temperature, and time) that determine the quality of a molded part are also responsible for the formation of the thermal gate at every cycle to avoid drooling and stringing of the gate. If gate quality, as discussed above, is a critical factor and gate vestiges are unacceptable, valve-gated hot runner systems are generally recommended, as they offer more processing control. In valve-gated hot runner systems, the flow of plastic into the mold cavity is controlled with a valve stem or mechanical shutoff pins. Through mechanical action, the valve stem moves forward and seals the gate orifice. The valve stem remains in the closed position during mold open and part ejection, preventing drool and stringing. A typical example is polyamide (PA). It may drool when processed in a thermally gated hot runner, but any possibility of drooling is eliminated if it is processed in a valve-gated hot runner. Another drawback of thermal gated systems is that melt decompression, which can lead to splay (“splash-like” appearance) and other imperfections on the surface of the molded part, is often required on these systems to relieve pressure in the manifold. With valve-gated nozzles, however, melt decompression is not needed since the seal is robust even if the hot runner manifold remains pressurized. The molded part separates from the valve gate without breaking or shearing plastic. Any discoloration or deformation as a result of gate break is thus unlikely. However, valve gate nozzles leave a small witness mark (of the same size as the gate diameter) on the part. Nowadays, the commonly used valve gates are pneumatic valve gates, hydraulic valve gates, and electrical valve gates (eGate). Pneumatic valve gates are the most widely used and are common for small parts in packaging, electronic,

Fabrication Processes

179

and medical applications. Though the pneumatic valve gates are proven and are cleaner than the hydraulic ones, they have certain limitations like slow response (owing to the high compressibility of air), lower closing force, and proneness to inconsistent opening/closing times, no monitoring capability, single speed, and single stroke. Hydraulic valve gates are the second most common actuation tool often used in large part molding in automotive applications. Their main advantages are that they are quicker and more powerful compared to pneumatic valve gates and have a smaller piston size, while their limitations are that they are service intensive with need for oil changes, bleeding air, and replacement of seals, besides being single speed and single stroke, and lacking the ability to monitor pin position and repeatability. They are also energy intensive, consuming about nine times more energy compared to electric valve gates (eGate), which are a new alternative, having features that were available never before with conventional valve gates. Featuring adjustable pin position, speed, acceleration, stroke, immediate response (100 rpm). A few typical mixing section designs are shown in Figure 2.20. The fluted-mixing-section-barrier-type design (Figure 2.20a) has proved to be especially applicable for extrusion of polyolefins. For some mixing problems, such as pigment mixing during extrusion, it is convenient to use rings (Figure 2.20b) or mixing pins (Figure 2.20c) and sometimes parallel interrupted mixing flights having wide pitch angles (Figure 2.20d). A later development in extruder design has been the use of venting or degassing zones to remove any volatile constituents from the melt before it is extruded through the die. This can be achieved by placing an obstruction in the barrel (the reverse flights in Figure 2.21) and by using a valved bypass section to step down the pressure developed in the first stage to atmospheric pressure for venting. In effect, two screws are used in series and separated by the degassing or venting zone. Degassing may also be achieved by having a deeper thread in the screw in the degassing section than in the final section of the first screw, so the polymer melt suddenly finds itself in an increased volume and hence is at a lower pressure. The volatile vapors released from the melt are vented through a hold in the top of the extruder barrel or through a hollow core of the screw by way of a hole drilled in the trailing edge of one of the flights in the degassing zone. A vacuum is sometimes applied to assist in the extraction of the vapor. Design and operation must be suitably controlled to minimize plugging of the vent (which, as noted above, is basically an open area) or the possibility of the melt escaping from this area. Many variations are possible in screw design to accommodate a wide range of polymers and applications. So many parameters are involved, including such variables as screw geometry, materials characteristics, operating conditions, etc., that the industry now uses computerized screw design, which permits analysis of the variables by using mathematical models to derive optimum design of a screw for a given application. Various screw designs have been recommended by the industry for extrusion of different plastics. For polyethylene, for example, the screw should be long with an L/D of at least 16:1 or 30:1 to provide a large area for heat transfer and plastication. A constant-pitch, decreasing-channel-depth, metering-type polyethylene screw or constant-pitch, constant-channel-depth, metering-type nylon screw with a compression

Feed section

Screw hub

Screw shank

189

Fabrication Processes

Out

In (a)

(b)

(c)

(d)

FIGURE 2.20 Mixing section designs: (a) ﬂuted-mixing-section-barrier type; (b) ring-barrier type; (c) mixing pins; (d) parallel interrupted mixing ﬂights. (After Frados, J. ed. 1976. Plastics Engineering Handbook, 4th Ed., Van Nostrand Reinhold, New York.)

Reverse ﬂights Valve

Vent Stage 2

Stage 2

Bypass Pressure gauge

Valve

FIGURE 2.21 A two-stage vented extruder with a valved bypass. (After Fisher, E. G. 1971. Blow Molding of Plastics. Iliffe, London.)

ratio between 3–1 and 4–1 (Figure 2.22) is recommended for polyethylene extrusion, the former being preferable for ﬁlm extension and extrusion coating. Nylon-6, 6 melts at approximately 260°C (500°F). Therefore, an extruder with an L/D of at least 16:1 is necessary. A screw with a compression ratio of 4:1 is recommended.

2.7.3 Multiple-Screw Extruders Multiple-screw extruders (that is, extruders with more than a single screw) were developed largely as a compounding device for uniformly blending plasticizers, fillers, pigments, stabilizers, etc., into the polymer. Subsequently, the multiple-screw extruders also found use in the processing of plastics. Multiple-screw extruders differ significantly from single-screw extruders in mode of operation. In a single-screw machine, friction between the resin and the rotating screw, makes the resin rotate with the screw, and the friction between the rotating resin and the barrel pushes the material forward, and this also generates heat. Increasing the screw speed and/or screw diameter to achieve a higher output rate in a single-screw extruder will therefore result in a higher buildup of frictional heat and higher temperatures. In contrast, in twin-screw extruders with intermeshing screws the relative motion of the flight of one screw inside the channel of the other pushes the material forward almost as if the machine were a positivedisplacement gear pump which conveys the material with very low friction.

190

Plastics Technology Handbook

Metering section

Compression section

Feed section

Screw diameter Land width of the ﬂights (a)

Lead Channel or ﬂight depth

Compression section Metering section

Feed section

(b)

FIGURE 2.22 (a) Constant pitch, decreasing channel depth, metering-type polyethylene screw. (b) Constant pitch, constant-channel-depth, metering-type nylon screw (not to scale). (After Frados, J. ed. 1976. Plastics Engineering Handbook, 4th Ed., Van Nostrand Reinhold, New York.)

In two-screw extruders, heat is therefore controlled independently from an outside source and is not inﬂuenced by screw speed. This fact become especially important when processing a heat-sensitive plastic like poly(vinyl chloride) (PVC). Multiple-screw extruders are therefore gaining wide acceptance for processing vinyls, although they are more expensive than single-screw machines. For the same reason, multiple-screw extruders have found a major use in the production of high-quality rigid PVC pipe of large diameter. Several types of multiple-screw machines are available, including intermeshing corotating screws (in which the screws rotate in the same direction, and the ﬂight of one screw moves inside the channel of the other), intermeshing counterrotating screws (in which the screws rotate in opposite directions), and nonintermeshing counterrotating screws. Multiple-screw extruders can involve either two screws (twin-screw design) or four screws. A typical four-screw extruder is a two-stage machine, in which a twin-screw plasticating section feeds into a twinscrew discharge section located directly below it. The multiple screws are generally sized on output rates (lb/h) rather than on L/D ratios or barrel diameters.

2.7.4 Blown-Film Extrusion The blown-film technique is widely used in the manufacture of polyethylene and other plastic films [14,15]. A typical setup is shown in Figure 2.23. In this case the molten polymer from the extruder head enters the die, where it flows round a mandrel and emerges through a ring-shaped opening in the form of a tube. The tube is expanded into a bubble of the required diameter by the pressure of internal air admitted through the center of the mandrel. The air contained in the bubble cannot escape because it is sealed by the die at one end and by the nip (or pinch) rolls at the other, so it acts like a permanent shaping mandrel once it has been injected. An even pressure of air is maintained to ensure uniform thickness of the film bubble. The film bubble is cooled below the softening point of the polymer by blowing air on it from a cooling ring placed round the die. When the polymer, such as polyethylene, cools below the softening point, the

191

Fabrication Processes

Pinch rolls

Collapsing plate Wind up Gusset bars Blown tube

Guide rollers

Mandrel Frost line Cooling ring Air inlet Extruder

Adjustable section of die Die Valve Air supply

FIGURE 2.23

Typical blow-ﬁlm extrusion setup.

crystalline material is cloudy compared with the clear amorphous melt. The transition line which coincides with this transformation is therefore called the frost line. The ratio of bubble diameter to die diameter is called the blowup ratio. It may range as high as 4 or 5, but 2.5 is a more typical figure. Molecular orientation occurs in the film in the hoop direction during blowup, and orientation in the machine direction, that is, in the direction of the extrudate flow from the die, can be induced by tension from the pinch rolls. The film bubble after solidification (at frost line) moves upward through guiding devices into a set of pinch rolls which flatten it. It can then be slit, gusseted, and surface-treated in line. (Vertical extrusion, shown in Figure 2.23, is most common, although horizontal techniques have been successfully used.) Blown-film extrusion is an extremely complex subject, and a number of problems are associated with the production of good-quality film. Among the likely defects are variation in film thickness, surface imperfections (such as “fish eyes,” “orange peel,” haze), wrinkling, and low tensile strength. The factors affecting them are also numerous. “Fish eyes” occur due to imperfect mixing in the extruder or due to contamination of the molten polymer. Both factors are controlled by the screen pack. The blown-film technique has several advantages: the relative ease of changing film width and caliber by controlling the volume of air in the bubble and the speed of the screw; the elimination of the end effects (e.g., edge bead trim and nonuniform temperature that result from flat film extrusion); and the capability of biaxial orientation (i.e., orientation both in the hoop direction and in the machine direction), which results in nearly equal physical properties in both directions, thereby giving a film of maximum toughness. After extrusion, blown-film is often slit and wound up as flat film, which is often much wider than anything produced by slot-die extrusion. Thus, blown-films of diameters 7 ft. or more have been produced, giving flat film of widths up to 24 ft. One example is reported [16] of a 10-in. extruder with 5-ft diameter and a blowup ratio of 2.5, producing 1,100 lb/h of polyethylene film, which when collapsed and slit in 40 ft wide. Films in thicknesses of 0.004–0.008 in. are readily produced by the blown-film process. Polyethylene films of such large widths and small thicknesses find extensive uses in agriculture, horticulture, and building.

192

Plastics Technology Handbook

2.7.5 Flat Film or Sheet Extrusion In the flat-film process the polymer melt is extruded through a slot die (T-shaped or “coat hanger” die), which may be as wide as 10 ft. The die has relatively thick wall sections on the final lands (as compared to the extrusion coating die) to minimize deflection of the lips from internal melt pressure. The die opening (for polyethylene) may be 0.015–0.030 in.—even for films that are less than 0.003 in. thick. The reason is that the speed of various driven rolls used for taking up the film is high enough to draw down the film with a concurrent thinning. (By definition, the term film is used for material less than 0.010 in. thick, and sheet for that which is thicker.) Figure 2.24 illustrates the basic components of a sheet extrusion die. The die is built in two halves the part for easier construction and maintenance. The use of a jack bolt facilitates separation of the die halves when the die is full of plastic. The die lip can be adjusted (across the entire lip length) to enable the processor to keep the thickness of the extruded sheet within specification. Figure 2.25 illustrates a T-type die and a coat-hanger-type die, which are used for both film and sheet extrusion. The die must produce a smooth and uniform laminar flow of the plastic melt which has already been mixed thoroughly in the extruder. The internal shape of the die and the smoothness of the die surface are critical to this flow transition. The deckle rods illustrated in Figure 2.25 are used by the processor to adjust the width of the extruded sheet or film. Body bolt

Jack bolt

Adjusting bolt Die lip

Heater holes

Adapter face Plastic melt

FIGURE 2.24

Sheet extrusion die.

V-shaped cross section Adjustable jaw (a)

(b)

FIGURE 2.25

Adjustable jaw

Deckle channel Die land

Die land

Internal deckle

External deckle

Schematic cross sections (a) T-type and (b) coat-hanger-type extrusion dies.

193

Fabrication Processes

Following extrusion, the film may be chilled below Tm or Tg by passing in through a water bath or over two or more chrome-plated chill rollers which have been cored for water cooling. A schematic drawing of a chill-roll (also called cast-film) operation is shown in Figure 2.26. The polymer melt extruded as a web from the die is made dimensionally stable by contacting several chill rolls before being pulled by the powered carrier rolls and wound up. The chrome-plated surface of the first roll is highly polished so that the product obtained is of extremely high gloss and clarity. In flat-film extrusion (particularly at high takeoff rates), there is a relatively high orientation of the film in the machine direction (i.e., the direction of the extrudate flow) and a very low one in the traverse direction. Biaxially oriented film can be produced by a flat-film extrusion by using a tenter (Figure 2.27). Polystyrene, for example, is first extruded through a slit die at about 190°C and cooled to about 120°C by Nip roll (rubber) Extruder Nip roll (stainless steel)

Treater bar Slitter

Die

Powered carrier Idler rolls rolls Nip roll (rubber)

Polished chill rolls (water-cooled)

Nip roll (stainless steel) Wind up

FIGURE 2.26

Sketch of chill-roll ﬁlm extrusion. (After Lukov, L. J. 1963. SPE J., 13, 10, 1057.) Across stretch

Extruder

Extruded ﬁlm

Along stretch

Heater

Stretched ﬁlm cooling

To winder

Heater

Slit die Water cooled rolls

Edge grips mounted on endless chain belt

Rollers

FIGURE 2.27 Plax process for manufacture of biaxially stretched polystyrene ﬁlm. (After Brydson, J. A. 1982. Plastics Materials, Butterworth Scientiﬁc, London, UK.)

194

Plastics Technology Handbook

passing between rolls. Inside a temperature-controlled box and moving sheet, rewarmed to 130°C, is grasped on either side by tenterhooks which exert a drawing tension (longitudinal stretching) as well as widening tension (lateral stretching). Stretch ratios of 3:1–4:1 in both directions are commonly employed for biaxially oriented polystyrene film. Biaxial stretching leads to polymers of improved tensile strength. Commercially available oriented polystyrene film has a tensile strength of 10,000–12,000 psi (703–843 kg/cm2), compared to 6,000–8,000 psi (422–562 kg/cm2) for unstretched material. Biaxial orientation effects are important in the manufacture of films and sheet. Biaxially stretched polypropylene, poly(ethyleneterephthalate) (e.g., Melinex) and poly(vinylidene chloride) (Saran) produced by flat-film extrusion and tentering are strong films of high clarity. In biaxial orientation, molecules are randomly oriented in two dimensions just as fibers would be in a random mat; the orientationinduced crystallization produces structures which do not interfere with the light waves. With polyethylene, biaxial orientation often can be achieved in blown-film extrusion.

2.7.6 Pipe or Tube Extrusion The die used for the extrusion of pipe or tubing consists of a die body with a tapered mandrel and an outer die ring which control the dimensions of the inner and outer diameters, respectively. Since this process involves thicker walls than are involved in blown-film extrusion, it is advantageous to cool the extrudate by circulating water through the mandrel (Figure 2.28) as well as by running the extrudate through a water bath. The extrusion of rubber tubing, however, differs from thermoplastic tubing. For thermoplastic tubing, dimensional stability results from cooling below Tg or Tm, but rubber tubing gains dimensional stability due to a cross-linking reaction at a temperature above that in the extruder. The high melt viscosity of the rubber being extruded ensures a constant shape during the cross-linking. A complication encountered in the extrusion of continuous shapes in die swell. Die swell is the swelling of the polymer when the elastic energy stored in capillary flow is relaxed on leading the die. The extrusion of flat sheet or pipe is not sensitive to die swell, since the shape remains symmetrical even through the dimensions of the extrudate differ from those of the die. Unsymmetrical cross sections may, however, be distorted. Adaptor for connecting to extruder

Insulator Water in

Water out

Spiral baﬄes

Tapered mandrel (cooling and sizing)

FIGURE 2.28 An extrusion die ﬁtted with a tapered cooling and sizing mandrel for use in producing either pipe or tubing. (After Fisher, E. G. 1971. Blow Molding of Plastics, Iliffe, London.)

195

Fabrication Processes

2.7.7 Wire and Cable Coverings The covering or coating of wire and cable in continuous lengths with insulating plastics is an important application of extrusion, and large quantities of resin are used annually for this purpose. This application represented one of the ﬁrst uses of extruders for rubber about 100 years ago. The wire and cable coating process resembles the process used for pipe extrusion (Figure 2.28) with the difference that the conductor (which may be a single metal strand, a multiple strand, or even a bundle of previously individually insulated wires) to be covered is drawn through the mandrel on a continuous basis (Figure 2.29). For thermoplastics such as polyethylene, nylon, and plasticized PVC, the coating is hardened by cooling below Tm or Tg by passing through a water trough. Rubber coatings, on the other hand, are to be cross-linked by heating subsequent to extrusion.

2.7.8 Extrusion Coating Many substrates, including paper, paperboard, cellulose film, fireboard, metal foils, and transparent films, are coated with resins by direct extrusion. The resins most commonly used are the polyolefins, such as polyethylene, polypropylene, ionomer, and ethylene–vinyl acetate copolymers. Nylon, PVC, and polyester are used for a lesser extent. Often combinations of these resins and substrates are used to provide a multiplayer structure. [A related technique, called extrusion laminating, involves two or more substrates, such as paper and aluminum foil, combined by using a plastic film, (e.g., polyethylene) as the adhesive and as a moisture barrier.] Coatings are applied in thicknesses of about 0.2–15 mils, the common average being 0.5–2 mils, and the substrates range in thickness from 0.5 to more than 24 mils. The equipment used for extrusion coating is similar to that used for the extrusion of flat film. Figure 2.30 shows a typical extrusion coating setup. The thin molten film from the extruder is pulled down into the nip between a chill roll and a pressure roll situated directly below the die. The pressure between these two rolls forces the film on to the substrate while the substrate, moving at a speed faster than the extruded film, draws the film to the required thickness. The molten film is cooled by the watercooled, chromium-plated chill roll. The pressure roll is also metallic but is covered with a rubber sleeve, usually neoprene or silicone rubber. After trimming, the coated material is wound up on conventional windup equipment. Polymer melt

Bare wire

Coated wire

Die

Crosshead

FIGURE 2.29

Crosshead used for wire coating.

196

Plastics Technology Handbook

Extruder Water-cooled chill roller (driven)

Wind-up

Slitter (driven) Reel of uncoated paper

Die

Coated paper

Pressure roll (idler)

FIGURE 2.30

Sketch of paper coating for extrusion process.

2.7.9 Profile Extrusion Profile extrusion is similar to pipe extrusion (Figure 2.28) except that the sizing mandrel is obviously not necessary. A die plate, in which an orifice of appropriate geometry has been cut, is placed on the face of the normal die assembly. The molten polymer is subjected to surface drag as it passes through the die, resulting in reduced flow through the thinner sections of the orifice. This effect is countered by altering the shape of the orifice, but often this results in a wide difference in the orifice shape from the desired extrusion profile. Some examples are shown in Figure 2.31.

2.8 Blow Molding Basically, blow molding in intended for use in manufacturing hollow plastic products, such as bottles and other containers [17]. However, the process is also used for the production of toys, automobile parts, accessories, and many engineering components. The principles used in blow molding are essentially similar to those used in the production of glass bottles. Although there are considerable differences in the process available for blow molding, the basic steps are the same: (1) melt the plastic; (2) form the molten plastic into a parison (a tubelike shape of molten plastic); (3) seal the ends of the parison except for one area through which the blowing air can enter; (4) inflate the parison to assume the shape of the mold in which it is placed; (5) cool the blowmolded part; (6) eject the blow-molded part; Extruded Orifice (7) trim flash if necessary. section Two basic processes of blow molding are extrusion blow molding and injection blow molding. These processes differ in the way in which the parison is made. The extrusion process utilizes an unsupported parison, whereas the injection process utilizes a parison supported on a metal core. Orifice Extruded section The extrusion blow-molding process by far accounts for the largest percentage of blowmolded objects produced today. The injecFIGURE 2.31 Relationships between extruder die orifice and tion process is, however, gaining acceptance. extruded section.

197

Fabrication Processes

Although any thermoplastic can be blow-molded, polyethylene products made by this technique are predominant. Polyethylene squeeze bottles form a large percentage of all blow-molded products.

2.8.1 Extrusion Blow Molding Extrusion blow molding consists basically of the extrusion of a predetermined length of parison (hollow tube of molten plastic) into a split die, which is then closed, sealing both ends of the parison. Compressed air is introduced (through a blowing tube) into the parison, which blows up to ﬁt the internal contours of the mold. As the polymer surface meets the cold metal wall of the mold, it is cooled rapidly below Tg or Tm. When the product is dimensionally stable, the mold is opened, the product is ejected, a new parison is introduced, and the cycle is repeated. The process affords high production rates. In continuous extrusion blow molding, a molten parison is produced continuously from a screw extruder. The molds are mounted and moved. In one instance the mold sets are carried on a rotating horizontal table (Figure 2.32a), in another on the periphery of a rotating vertical wheel (Figure 2.32b). Such rotary machines are best suited for long runs and large-volume applications. In the ram extrusion method the parison is formed in a cyclic manner by forcing a charge out from an accumulated molten mass, as in the preplasticizer injection-molding machine. The transport arm cuts and holds the parison and lowers it into the waiting mold, where shaping under air pressure takes place (Figure 2.33). A variation of the blow-extrusion process which is particularly suitable for heat-sensitive resins such as PVC is the cold perform molding. The parison is produced by normal extrusion and cooled and stored until needed. The required length of tubing is then reheated and blown to shape in a cold mold, as in conventional blow molding. Since, unlike in the conventional process, the extruder is not coupled directly to the blow-molding machine, there is less chance of a stoppage occurring, with consequent risk of holdup and degradation of the resin remaining in the extruder barrel. There is also less chance of the occurrence of “dead” pockets and consequent degradation of resin in the straight-through die used in this process than in the usual crosshead used with a conventional machine.

Extruder Knife Die head Parison

Eject Extruder Cooling

Closed

Knife

Die head Eject Parison

Blowing (a)

Rotation

Blowing

Cooling

(b)

FIGURE 2.32 Continuous extrusion blow molding. (a) Rotating horizontal table carrying mold sets. (b) Continuous vertical rotation of a wheel carrying mold sets on the periphery. (After Morgan, B. T., Peters, D. L., and Wilson, N. R. 1967. Mod. Plastics, 45, 1A, Encyl. Issue, 797.)

198

Plastics Technology Handbook

Die head Parison transport Parison of semimolten plastic

Air pressure Mold closed around parison

Bottle

Mold

FIGURE 2.33 Continuous extrusion blow molding with parison transfer. The transport arm cuts the extruded parison from the die head and lowers it into the waiting mold. (After Morgan, B. T., Peters, D. L., and Wilson, N. R. 1967. Mod. Plastics, 45, 1A, Encyl. Issue, 797.)

2.8.2 Injection Blow Molding In this process the parison is injection molded rather than extruded. In one system, for example, the parison is formed as a thick-walled tube around a blowing stick in a conventional injection-molding machine. The parison is then transferred to a second, or blowing, mold in which the parison is inﬂated to the shape of the mold by passing compressed air down the blowing stick. The sequence is shown in Figure 2.34. Injection blow molding is relatively slow and is more restricted in choice of molding materials as compared to extrusion blow molding. The injection process, however, affords good control of neck and wall thicknesses of the molded object. With this process it is also easier to produce unsymmetrical molding.

2.8.3 Blow Molds Generally, the blow mold is a cavity representing the outside of a blow-molded part. The basic structure of a blow mold consists of a cast or machined block with a cavity, cooling system, venting system, pinchoffs, ﬂash pockets, and mounting plate. The selection of material for the construction of a blow mold is based on the consideration of such factors as thermal conductivity, durability, cost of the material, the resin being processed, and the desired quality of the ﬁnished parts. Commonly used mold materials are beryllium, copper, aluminum, ampcoloy, A-2 steel, and 17-4 and 420 stainless steels. Beryllium–copper (BeCu) alloys 165 and 25 are normally used for blow molds. These materials display medium to good thermal conductivity with good durability. Stainless steels such as 17-4 and 420 are also frequently employed in blow molds where durability and resistance to hydrochloric acid are required. Heat-treated A-2 steel is often used as an insert in pinchoffs where thermal conductivity is not a concern and high quality parts are required. For blow molding HDPE parts, aluminum is commonly employed for the base material, with BeCu or stainless steel inserts in the pinchoff areas. For PVC parts BeCu, ampcoloy, or 17-4 stainless steel are used as the base material, with A-2 or stainless inserts in the pinchoff area. For PET parts, the base mold is typically made of aluminum or BeCu, with A-2 or stainless steel pinchoffs. The production speed of blow-molded parts is generally limited by one of two factors: extruder capacity or cooling time in the mold. Cooling of mold is accomplished by a water circuit into the mold. Flood cooling and cast-in tubes are most common in cast molds; drilled holes and milled slots are the norm in machined blow molds.

199

Fabrication Processes

(a)

(b)

(c)

Injection cycle

(d)

FIGURE 2.34

(e)

(f )

Sketch of injection-blow-molding process.

In multiple-cavity molds, series and parallel cooling circuits are used. Series cooling enters and cools one cavity, then moves to the next until all the cavities are cooled. The temperature of the water increases as it moves through the mold, and this results in non-uniform cooling. Parallel cooling, on the other hand, enters and exists all cavities simultaneously, thereby cooling all cavities at a uniform rate. Parallel cooling is thus the preferred method but it is not always possible due to limitations.

2.9 Calendering Calendering is the leading method for producing vinyl film, sheets, and coatings [18]. In this process continuous sheet is made by passing a heat-softened material between two or more rolls. Calendering was originally developed for processing rubber, but is now widely used for producing thermoplastic films, sheets, and coatings. A major portion of thermoplastics calendered is accounted for by flexible (plasticized) PVC. Most plasticized PVC film and sheet, ranging from the 3-mil film for baby pants to the 0.10 in “vinyl” tile for floor coverings, is calendered. The calendaring process consists of feeding a softened mass into the nip between two rolls where it is squeezed into a sheet, which then passes round the remaining rolls. The processed material thus emerges as a continuous sheet, the thickness of which is governed by the gap between the last pair of rolls. The surface quality of the sheet develops on the last roll and may be glossy, matt, or embossed. After leaving the calender, the sheet is passed over a number of cooling rolls and then through a beta-ray thickness gage before being wound up. The plastics mass fed to the calender may be simply a heat-softened material, as in the case of, say, polyethylene, or a rough sheet, as in the case of PVC. The polymer PVC is blended with stabilizers,

200

Plastics Technology Handbook

Feed Hot roll Fixed roll

Hot roll

Feed Cold roll (a)

(b) Engraved (replaceable)

Uneven speed

Even or uneven speed

Even speed

Uneven speed

Bank

Calendered sheet

Bank

Fabric

Feed

Conveyor

Friction (c)

(d)

FIGURE 2.35 Typical arrangements of calender rolls: (a) single-ply sheeting; (b) double-ply sheeting; (c) applying rubber to fabrics; (d) proﬁling with four-roll engraving cylinder.

plasticizers, etc., in ribbon blenders, gelated at 120°C–160°C for about 5–10 min in a Banbury mixer, and the gelated lumps are made into a rough sheet on a two-roll mill before being fed to the calender. Calenders may consist of two, three, four, or five hollow rolls arranged for steam heating or water cooling and are characterized by the number of rolls and their arrangement. Some arrangements are shown in Figure 2.35. Thick sections of rubber can be made by applying one layer of polymer upon a previous layer (double plying) (Figure 2.35b). Calenders can be used for applying rubber or plastics to fabrics (Figure 2.35c). Fabric or paper is fed through the last two rolls of the calender so that the resin film is pressed into the surface of the web. For profiling, the plastic material is fed to the nip of the calender, where the material assumes the form of a sheet, which is then progressively pulled through two subsequent banks to resurface each of the two sides (Figure 2.35d). For thermoplastics the cooling of the sheet can be accomplished on the rolls with good control over dimensions. For rubber, cross-linking can be carried out with good control over dimensions, with the support of the rolls. Despite the simple appearance of the calender compared to the extruder, the close tolerances involved and other mechanical problems make for the high cost of a calendaring unit.

2.10 Spinning of Fibers The term spinning, as used with natural fibers, refers to the twisting of short fibers into continuous lengths [19–21]. In the modern synthetic fiber industry, however, the term is used for any process of producing continuous lengths by any means. (A few other terms used in the fiber industry should also be defined. A fiber may be defined as a unit of matter having a length at least 100 times its width or diameter. An individual strand of continuous length is called a filament. Twisting together filaments into a strand gives continuous filament yarn. If the filaments are assembled in a loose bundle, we have tow or roving. These can be chopped into small lengths (an inch to several inches long), referred to as staple. Spun yarn is made

201

Fabrication Processes

by twisting lengths of staple into a single continuous strand, and cord is formed by twisting together two or more yarns.) The dimensions of a filament or yarn are expressed in terms of a unit called the “tex” which is a measure of the fineness or linear density. One tex is 1 gram per 1,000 meters or 10−6 kg/m. The tex has replaced “denier” as a measure of the density of the fiber. One denier is 1 gram per 9,000 meters, so 1 denier = 0.1111 tex. The primary fabrication process in the production of synthetic fibers is the spinning—i.e., the formation—of filaments. In every case the polymer is either melted or dissolved in a solvent and is put in filament form by forcing through a die, called spinneret, having a multiplicity of holes. Spinnerets for rayon spinning, for example, have as many as 10,000 holes in a 15-cm-diameter platinum disc, and those for textile yarns may have 10–120 holes; industrial yarns such as tire core might be spun from spinnerets with up to 720 holes. Three major categories of spinning processes are melt, dry, and wet spinning [19]. The features of the three processes are shown in Figure 2.36, and the typical cross sections of the fibers produced by them are shown in Figure 2.37. The products of all the above processes are micro-size fibers. In contrast, ultrafine fibers or nanofibers are produced by electrospinning, which is described in Section 2.11.

Polymer solution

Polymer chips Heating grid

Pump

Polymer melt Pump

Filter and spinnerette

Fiber cooling and solidiﬁcation

Air diﬀuser

Hot chamber

Steam chamber Roll and guide

Yarn drawing

(a)

Bobbin

Polymer solution

(b)

Packaging

Pump Fiber solidiﬁcation by precipitation

Filter and spinnerette

Coagulation bath

(c)

FIGURE 2.36 Schematic of the three principal types of ﬁber spinning: (a) melt spinning; (b) dry spinning; (c) wet spinning. (After Carraher, C. E., Jr. 2002. Polymer News, 27, 3, 91.)

202

Plastics Technology Handbook

2.10.1 Melt Spinning In melt spinning, which is the same as melt extrusion, the polymer is heated and the viscous melt is pumped through a spinneret. An inert atmosphere is provided in the melting chamber before the pump. Special pumps are used to operate in the temperature range necessary to (a) (b) (c) produce a manageable melt (230–315°C). For nylon, for example, a gear pump is used to feed FIGURE 2.37 Typical cross section of fibers produced by the melt to the spinneret (Figure 2.36a). For a different spinning processes: (a) melt-spun nylon from polymer with high melt viscosity such as polyvarious shaped orifices; (b) dry-spun cellulose acetate from propylene, a screw extruder is used to feed a round orifice; (c) wet-spun viscose rayon from round heated spinneret. Dimensional stability of the orifice. fiber is obtained by cooling under tension. Typical melt spinning temperatures are given in Table 2.1.

2.10.2 Dry Spinning In dry spinning, a polymer is dissolved in a solvent and the polymer solution (concentration on the order of 20–40%) is filtered and then forced through a spinneret into a chamber through which heated air is passed to achieve dimensional stability of the fiber by evaporation of the solvent (Figure 2.36b). For economical reasons, the gas is usually air, but inert gases such as nitrogen and superheated steam are sometimes used. The skin which forms first on the fiber by evaporation from the surface gradually collapses and wrinkles as more solvent diffuses out and the diameter decreases. The cross section of a dryspun fiber thus has an irregularly lobed appearance (Figure 2.37). Recovery of the solvent used for dissolving the polymer is important to the economics of the process. Cellulose acetate dissolved in acetone and polyacrylonitrile (PAN) dissolved in dimethylformamide are two typical examples. The hot solution (dope) of PAN in DMF is extruded directly into a hot stream of nitrogen at 300°C. The residual DMF is recovered in subsequent water washing steps. Dry spun fibers have lower void concentrations than wet spun fibers. This is reflected in greater densities and lower dyeability for the dry spun fibers.

2.10.3 Wet Spinning Wet spinning also involves pumping a solution of the polymer to the spinneret. However, unlike dry spinning, dimensional stability is achieved by precipitating the polymer in a nonsolvent (Figure 2.36c). For example, PAN in dimethylformamide can be precipitated by passing a jet of the solution through a bath of water, which is miscible with the solvent but coagulates the polymer. For wet-spinning cellulose TABLE 2.1 Typical Spinning Temperatures for Selected Polymers Polymer

Melting Point (°C)

Typical Spinning Temperature (°C)

Nylon-6

220

280

Nylon-6,6 Poly(ethylene terephthalate)

260 260

290 290

∼130

220–230

170 120–140

250–300 180

Polyethylene Polypropylene Poly(vinylidene chloride) copolymers

Source: Carraher, C. E. Jr. 2002. Polymer News, 27(3), 91.

203

Fabrication Processes

triacetate a mixture methylene chloride and alcohol can be used to dissolve the polymer, and a toluene bath can be used for precipitation of the polymer. In some cases the precipitation can also involve a chemical reaction. An important example is viscose rayon, which, is made by regenerating cellulose from a solution of cellulose xanthate in dilute alkali. S R

CS2, NaOH

R O C S– Na+ + H2O Cellulose xanthate H2O

H2SO4 NaHSO4

R OH + CS2 Cellulose

+ Na+ (HSO4–)

If a slot die rather than a spinneret is used, the foregoing process would yield cellulose film (cellophane) instead of fiber. Wet spinning is the most complex of the three spinning processes, typically including washing, stretching, drying, crimping, finish application, and controlled relaxation to form tow material [22]. A simplified sketch of the Asahi wet spinning process for making PAN (acrylic) fiber tows is shown in Figure 2.38. The polymer solution (dope) is made in concentrated HNO3 (67%) at low temperatures using pulverizer and mixer, filtered, and deaerated. The dope, containing 14–15% polymer and maintained at −7°C, is extruded at a pumping pressure of 10–15 atm through the spinnerets immersed in the coagulation bath. Each spinneret has about 46,000–73,000 holes (different sizes for different grades). The acid concentration in the bath is 37% and the temperature is −5°C. The filaments from 5 spinnerets are collected into a tow. The spinning speed in this step is about 7 meters per minute. The dilute nitric acid from the bath goes to the concentration section where 67% HNO3 is obtained for re-use in dope preparation. In the next pre-finishing step, the fibers (tows) are repeatedly washed with water by spraying and immersion to remove the acid. The fiber is stretched in three stages, a 1:10 stretch being obtained in the last hot water bath at 100°C. In the finishing section, the tows pass through a water bath containing 1% oil to impart antistaticity and reduce friction. The tows are then dried in a hot air (135°C) dryer over rollers. Subsequent treatments include a second finishing oil spray, crimping, second hot air drying, plaiting, thermosetting, and cutting into staple fibers. Wash and stretching

Polymer Acid

Finishing Dope preparation

Water + oil

3–4 stages

Slurry tank Acid out

Out

In N2

Heat treatment

FIGURE 2.38

Crimping

Drying

A simpliﬁed sketch of the Asahi wet spinning process for polyacrylonitrile ﬁber.

204

Plastics Technology Handbook

Fibers made from wet spinning generally have high void contents in comparison to all of the other processes giving them increased dyeability and the surface is rougher with longitudinal serrations. Hollow ﬁbers for gas and liquid separation are prepared by passing air through the material just prior to entrance into the non-solvent bath.

2.10.4 Cold Drawing of Fibers Almost all synthetic fibers are subjected to a drawing (stretching) operation to orient the crystalline structure in the direction of the fiber axis. Drawing orients crystallites in the direction of the stretch so that the modulus in that direction is increased and elongation at break is decreased. Usually the drawing is carried out at a temperature between Tg and Tm of the fiber. Thus, polyethylene (Tg = −115°C) can be drawn at room temperature, whereas nylon−6,6 (Tg = 53°C) should be heated or humidified to be drawn. Tg is depressed by the presence of moisture, which acts as a plasticizer. The drawing is accomplished by winding the yarn around a wheel or drum driven at a faster surface velocity than a preceding one.

2.11 Electrospinning of Polymer Nanofibers Nanofibers are defined as fibers with diameters of less than 100 nm. In the textile industry, however, this definition is often extended to include fibers with diameters as large as 1000 nm. Polymer nanofibers are generally produced by electrospinning, which is a simple yet versatile method for producing ultrathin fibers from a variety of materials and is currently the most important technique for converting polymers into nanosized fiber materials. When the dimensions of polymer fiber materials are reduced from micrometers (e.g., 10–100 µm) to submicrons or nanometers (e.g., 10 × 10−3 to 100 × 10−3 µm), there appear several outstanding properties, such as very large surface area-to-volume ratio (for a nanofiber, this can be as large as 103 times that of a microfiber) and superior mechanical properties (e.g., stiffness and tensile strength) compared with any other form of the material. These features make polymer nanofibers ideally suited for many important applications. In recent years, a number of processing techniques have been developed to produce polymer nanofibers such as drawing [23], templating [24], self-assembly [25], and electrospinning [26]. Though the drawing process can produce one-by-one very long single nanofibers, only a viscoelastic material that can undergo strong deformations during pulling can be made into nanofibers through drawing. The template method, besides depending on the use of a nanoporous membrane as template to make nanofibers, cannot produce one-by-one continuous nanofibers. The self-assembly, on the other hand, is time-consuming in processing continuous polymer nanofibers. It thus appears that the electrospinning process is the only method that can be developed for mass production of one-by-one continuous nanofibers from solutions or melts of various polymers. Although the term “electrospinning” (derived from “electrostatic spinning”) was used relatively recently (around 1994), its fundamental idea, namely, production of polymer filaments using an electrostatic force, dates back more than 70 years earlier. This is evident from a series of patents [27] that appeared from 1934 to 1944 describing an experimental setup for the production of polymer filaments (from polymer solution) between two electrodes bearing electrical charges of opposite polarity. One of the electrodes was placed into the solution and the other was placed onto a collector. On being ejected through a small hole of a metal spinneret, the charged solution jets evaporated to become solid fibers and deposited on the collector. The potential difference to be applied between the electrodes depended on the properties of the spinning solution, such as polymer molecular weight and solution viscosity. Subsequently, over the years, numerous variations of electrospinning setup were devised and used, and nearly 100 different polymers, mostly dissolved in solvent (and some heated to melts), were spun into ultrafine fibers using this technique. A schematic diagram of the basic setup for electrospinning of polymer nanofibers is shown in Figure 2.39. It has three basic components: a high-voltage (usually direct current, DC) power supply, a

205

Fabrication Processes

Syringe Polymer solution Needle

Taylor cone

Liquid jet

High voltage power supply

Smaller diameter ﬁber Fiber jet

Nonwoven nanoﬁber mat

Pore

Larger diameter ﬁber

Collector

FIGURE 2.39 Schematic diagram of the basic setup for electrospinning. The inset shows a diagram of the electriﬁed Taylor cone and SEM image (schematic) of the randomly oriented nonwoven mat of polymer nanoﬁbers deposited on the collector.

spinneret (a metallic needle), and a metal collector (grounded conductor). The spinneret is connected to a syringe containing the polymer solution (or melt), which is fed through the spinneret at a constant and controllable rate with the use of a syringe pump. When a high voltage (usually in the range of 1 to 30 kV) is applied, the pendant drop of liquid that is held by surface tension at the tip of the spinneret becomes highly electrified with the induced charges evenly distributed over the surface. The drop then experiences two major types of electrostatic forces, namely, repulsion between the surface charges and the coulombic force exerted by the external field. Under the influence of these two forces, the hemispherical surface of the drop elongates to form a conical shape, known as the Taylor cone. Further increasing the electric field, a critical value is attained at which the repulsive electrostatic force can overcome the surface tension of the polymer solution (or melt) and force the ejection of a charged jet of the fluid from the tip of the Taylor cone. The electrified jet then undergoes a stretching and whipping (rapid bending) process leading to the formation of a very long and thin thread. During this process, the solvent (of polymer solution) also evaporates, leaving behind a charged polymer fiber, while its diameter can be greatly reduced from hundreds of micrometers to as small as tens of nanometers. This charged fiber is often deposited as a randomly oriented, nonwoven mat (see Figure 2.39) on the grounded metal collector placed under the spinneret. This relatively simple and straightforward technique has been used to process more then 50 different types of organic polymers. Polymers melted at a high temperature can also be processed into nanofibers by electrospinning. Instead of a solution, the polymer melt is introduced into the capillary tube and the electrospinning process is performed in a vacuum condition, which means that the whole assembly of capillary tube, the charged jet ejection, and the collector must be fully encapsulated within a vacuum. A number of variations [28] of conventional electrospinning of polymer solution or melt have been developed and used for different objectives, a few of which are briefly highlighted below. Electroblown spinning (EBS), or electroblowing, is an electrospinning process in the presence of a controlled airflow. In this process, two forces are simultaneously applied for producing nanofibers,

206

Plastics Technology Handbook

namely, electrical force and air-blowing shear force. The method is especially useful for highly viscous solutions in which applying high voltage alone is not sufficient to overcome surface tension. Centrifugal electrospinning (CFS) makes combined use of an electrical field and a centrifugal field. Compared with pure centrifugal spinning that is required to rotate at thousands of revolutions per minute (rpm), the rotational speed in CFS can decrease to 300–600 rpm with more aligned fibers. Combining centrifugal and electrical forces into the spinning process, CFS leads to further orientation of polymer chains in nanofibers and a higher production rate can be achieved at a lower working voltage or lower rpm. Near-field electrospinning (NFES) is a potential approach for easier and more predictable location control for the deposition of nanofibers, which is impossible to be achieved by conventional electrospinning owing to the whipping action of fibers. By reducing the distance between the nozzle and the substrate to less than a few millimeters, NFES allows the fibers to land on the substrate before the onset of whipping so that nonwoven nanofibers can be deposited precisely along a predetermined pattern. Coaxial electrospinning (CES) is an innovatively extended form of electrospinning that uses two concentrically aligned capillaries to enforce fiber formation with a core–shell structure. CES is of particular interest for those core materials that cannot form fibers via electrospinning by themselves, such as conductive polymers, metals, or some natural polymers. While the process is conceptually similar to that of conventional electrospinning, two dissimilar materials can be delivered independently through the coaxial capillary and drawn to generate nanofibers in a core–shell configuration. The shell and the core may not be miscible owing to the short process duration at which the jet becomes solidified into fibers. Emulsion electrospinning (EES) is similar to normal solution electrospinning, except that the solution is replaced by an emulsion (water-in-oil or oil-in-water type). Using water-in-oil type of emulsions comprising a water phase that is a drug/protein dissolved in water, and an oil phase, which is a polymer dissolved in an organic solvent, the process allows for the encapsulation of a wide range of bioactive molecules (with different solubilities) into polymeric nanofibers. For a polymer that can be electrospun into nanofibers, the ideal targets of manufacturing would be the following: (1) consistent and controllable fiber diameters, (2) defect-free or defect-controllable fiber surface, and (3) continuous single nanofibers. However, researches so far have shown that these three targets are not easily achievable. This is because many parameters can influence the transformation of a polymer solution into nanofibers by electrospinning. These include (a) solution properties such as viscosity, elasticity, surface tension, and electrical conductivity; (b) operating variables such as hydrostatic pressure in the spinneret, electric potential at the tip of the spinneret, and the gap between the tip and the collector; and (c) ambient parameters such as solution temperature, humidity, and air velocity in the electrospinning chamber. Since nanofibers result from evaporation or solidification of polymer fluid jets, the fiber diameters should depend primarily on the jet sizes and the polymer contents in the jets. Since the polymer solution viscosity is proportional to the polymer concentration, one of the most significant parameters influencing the fiber diameter is the solution viscosity. A higher viscosity results in larger fiber diameter. The parameters other than the solution viscosity that affect the fiber diameter are strength of applied potential, solution conductivity, polymer feeding rate, capillary size, and the distance between the capillary and the collector. Polymer solutions with high conductivity have high surface charge density that leads to finer fibers. A challenge with electrospinning lies in the fact that the fiber diameters obtained are seldom uniform. Another problem is that defects such as beads and pores (voids) may occur in polymer nanofibers. Bead formation can be reduced by using higher polymer concentrations for electrospinning [29]. Fibers without beads may also result from reduction of surface tension [30] that is more likely to be a function of solvent compositions and not polymer concentration [31]. Adding some filler material into the polymer solution for electrospinning can also result in fibers free of voids [32]. This apart, the idea of incorporating fillers was used to prepare composite nanofibers by dispersing carbon SWNTs (single-wall nanotubes) in polyacrylonitrile solution that was electrospun into ultrafine fibers.

Fabrication Processes

207

The basic setup for electrospinning being very simple, it has found widespread use in many laboratories for making ultrathin fibers. Because electrospinning is a continuous process, the fibers could be as long as several kilometers and comparable to fibers manufactured by conventional drawing or spinning techniques. However, because of the bending instability associated with a spinning jet, electrospun fibers are mostly deposited on the surface of a collector in randomly oriented, nonwoven form that can be useful, however, for a relatively small number of applications, such as filtration [33], tissue scaffolds [34], implant coating film [35], and wound dressing [36]. Since applications can be expanded only when continuous single nanofibers or uniaxial fiber bundles are available, considerable research has been focused on devising possible means to align electrospun nanofibers. Over the last decade, a number of approaches have been demonstrated to directly produce electrospun nanofibers as uniaxially aligned arrays. Two of the earliest such approaches involved the use of a rotating drum (or frame) or a pair of split electrodes as the collector. Thus, aligned poly(glycolic acid) nanofibers [37] and collagen nanofibers [38] were obtained by using a rotating cylindrical collector at 1000 and 4500 rpm, respectively. It was demonstrated that by using a collector consisting of two conductive strips separated by a void gap of variable widths (up to several centimeters), electrospun fibers could be axially aligned over long length scales during the spinning process [39]. Bundles of uniaxially aligned PAN nanofibers were prepared by this method using a solution of 15 wt% PAN/DMF for electrospinning. [Note: Electrospun PAN nanofibers can be used to make carbon nanofibers by a series of heat treatments. See Section 2.14.2.3.] The lengths of the bundles were 3 cm, which was ∼85% of the width of the gap between the collectors [40]. This method allows the aligned fibers to be transferred onto other solid substrates for further processing steps and applications. Single fibers can also be collected across the gap and transferred onto a substrate for the fabrication of single-fiber-based devices [39]. Though electrospinning is a well-established process capable of producing nonwoven webs as well as single or well-aligned arrays of continuous nanofibers with controlled morphology and size, the major challenge associated with the process is its production rate, compared with that of conventional fiber spinning. Of the two main parameters related to production efficiency, namely, flow rate and fiber diameter, the flow rate in electrospinning is largely determined by the strength of the electric field, which, in turn, is limited by the electric breakdown strength of the spinning atmosphere (usually air) [41], while fiber diameters are two to three orders of magnitude smaller than conventional polymeric fibers. Thus, the throughput of a single electrospinning needle is typically 0.1–1.0 g h−1 by fiber weight or 1.0–5.0 mL h−1 by flow rate, depending on the polymer solution, while, in comparison, millions of tons of fiber are produced per annum by conventional spinning methods. Since the low production rate of conventional needle electrospinning setup, as mentioned above, hinders commercialization and limits the application scope of electrospun nanofibers, productivity enhancement on a comparable industrial scale to that of conventional polymeric fibers has been under active investigation over the last 15 years. The emphasis in this drive has been on multi-jet electrospinning, which is the straightforward way to increase the throughput (as the fiber productivity can be simply increased by increasing the jet number). In order to obtain multi-jets, an array of multiple needles can be used as the spinneret [39]. However, in such multi-jet electrospinning, a strong repulsion occurs among the jets, and this may lead to reduced fiber production as the jets have to be set at an appropriate distance from one another to reduce the jet repulsion, thus requiring a large space to accommodate the needles for the mass nanofiber production. Kim et al. [42] have shown that by using an extra-cylindrical electrode as an auxiliary electrode to cover the multi-jet spinneret, the fiber deposition area can be dramatically reduced, thus improving the fiber production rate. It, however, leads to the formation of coarser fibers. Needle configuration, needle number, and needle spacing are three key parameters for designing the needle array in a multi-needle electrospinning setup, while the needle configuration is of two types, namely, linear arrays and twodimensional (i.e., square, circular and elliptic, hexagonal and triangular) arrays with significant effects on flow rate and electric field distribution [41]. The adoption of an array of needles as the spinneret for

208

Plastics Technology Handbook

electrospinning can also facilitate the production of multicomponent blend nanofibrous mats from different polymer solutions. Besides using multi-needle spinneret, many other devices have been used as electrospinning spinneret. Methods have also been explored to fabricate arrayed capillaries, such as porous membranes with patterned arrays of micrometer-sized channels, or an assembly of tens to hundreds of polyimide-coated glass capillaries of the type used in electrophoresis. Many research efforts have also been directed at improving the electrospinning productivity and/or quality of the electrospun fiber by replacing the needle spinneret with other spinnerets, such as conical wire coil, plate, splashing spinneret, rotary cone, and bowl edge. More recently, a number of methods have been developed to enhance the electrospinning throughput that can be roughly classified as needleless electrospinning methods. Among these, the most successful design for practical applications is the upward needleless electrospinning, which has been shown to have the ability to mass produce nanofibers. In a typical upward needleless electrospinning setup, a two-layerfluid system is used [43], in which the lower fluid layer is a ferromagnetic suspension and the upper layer is a polymer solution to be spun. During electrospinning, when a normal magnetic field is applied to the system from a permanent magnet or coil, steady vertical spikes are formed perturbing the interlayer interface as well as the free surface of the uppermost polymer layer. Then, as a result of applying high voltage to the fluid at the same time, the perturbations of the free surface become sites of jetting directed upward. As thousands of jetting eject upward, they undergo strong stretching (by the electric field) and bending instability, solvent evaporates, and solidified nanofibers deposit on the upper counterelectrode. A complicated setup is, however, required for the process and the nanofibers formed have a large diameter and a wide diameter distribution. Though nanofibers prepared by electrospinning usually exhibit a solid interior and smooth surface, nanofibers with some specific secondary (e.g., core-sheath, hollow, and porous) structures can also be prepared if appropriate processing parameters or new designs of spinnerets are employed. For example, CES (described earlier) has been used to produce nanofibers with core-sheath structures [44].

2.12 Thermoforming When heated, thermoplastic sheet becomes as soft as a sheet of rubber, and it can then be stretched to any given shape [45]. This principle is utilized in thermoforming processes which may be divided into three main types: (a) vacuum forming, (2) pressure forming (blow forming), and (3) mechanical forming (e.g., matched metal forming), depending on the means used to stretch the heat softened sheet. Since fully cured thermoset sheets cannot be resoftened, forming is not applicable to them. Common materials subjected to thermoforming are thermoplastics such as polystyrene, cellulose acetate, cellulose acetate butyrate, PVC, ABS, poly(methyl methacrylate), low- and high-density polyethylene, and polypropylene. The bulk of the forming is done with extruded sheets, although cast, calendered, or laminated sheets can also be formed. In general, thermoforming techniques are best suited for producing moldings of large area and very thin-walled moldings, or where only short runs are required. Thermoformed articles include refrigerator and freezer door liners complete with formed-in compartments for eggs, butter, and bottles of various types, television masks, dishwasher housings, washing machine covers, various automobile parts (instrument panels, arm rests, ceilings, and door panels), large patterned diffusers in the lighting industry, displays in advertising, various parts in aircraft industry (windshields, interior panels, arm rests, serving trays, etc.), various housing (typewriters, Dictaphones, and duplicating machines), toys, transparent packages, and much more.

2.12.1 Vacuum Forming In vacuum forming, the thermoplastic sheet can be clamped or simply held against the RIM of a mold and then heated until it becomes soft. The soft sheet is then sealed at the RIM, and the air from the mold cavity

209

Fabrication Processes

(a) Heaters active, stock heating

(b)

Heaters active, stock heating

FIGURE 2.40

Stock on mold, healer’s idle

Stock on mold, heaters idle

Vacuum applied, stock cooling

Plug assist loweredvacuum applied

(a) Vacuum forming. (b) Plug-assist forming using vacuum.

is removed by a suction pump so that the sheet is forced to take the contours of the mold by the atmospheric pressure above the sheet (Figure 2.40a). The vacuum in the mold cavity is maintained until the part cools and becomes rigid. Straight cavity forming is not well adapted to forming a cup or box shape because as the sheet, drawn by vacuum, continues to ﬁll out the mold and solidify, most of the stock is used up before it reaches the periphery of the base, with the result that this part becomes relatively thin and weak. This difﬁculty is alleviated and uniformity of distribution in such shapes is promoted if the plug assist is used (Figure 2.40b). The plug assist is any type of mechanical helper which carries extra stock toward an area where the part would otherwise be too thin. Plug-assist techniques are adaptable both to vacuum-forming and pressure forming techniques. The system shown in Figure 2.40b is thus known as plug assist vacuum forming.

2.12.2 Pressure Forming Pressure forming is the reverse of vacuum forming. The plastic sheet is clamped, heated until it becomes soft, and sealed between a pressure head and the RIM of a mold. By applying air pressure (Figure 2.41), one forces the sheet to take the contours of the mold. Exhaust holes in the mold allow the trapped air to escape. After the part cools and becomes rigid, the pressure is released and the part is removed. As compared to vacuum forming, pressure forming affords a faster production cycle, greater part deﬁnition, and greater dimensional control. A variation of vacuum forming or pressure forming, called free forming or free blowing, is used with acrylic sheeting to produce parts that require superior optical quality (e.g., aircraft canopies). In this process the periphery is deﬁned mechanically by clamping, but no bolt is used, and the depth of draw or height is governed only by the vacuum or compressed air applied.

210

Plastics Technology Handbook

Compressed air open

Clamps

(a)

(b)

Vent holes

FIGURE 2.41 Pressure forming: (a) heated sheet is clamped over mold cavity; (b) compressed air pressure forces the sheet into the mold.

2.12.3 Mechanical Forming Various mechanical techniques have been developed for thermoforming that use neither air pressure nor vacuum. Typical of these is matched mold forming (Figure 2.41). A male mold is mounted on the top or bottom platen, and a matched female mold is mounted on the other. The plastic sheet, held by a clamping frame, is heated to the proper forming temperature, and the mold is then closed, forcing the plastic to the contours of both the male and the female molds. The molds are held in place until the plastic cools and attains dimensional stability, the latter facilitated by internal cooling of the mold. The matched mold technique affords excellent reproduction of mold detail and dimensional accuracy.

2.13 Casting Processes There are two basic types of casting used in plastics industry: simple casting and plastisol casting.

2.13.1 Simple Casting In simple casting, the liquid is simply poured into the mold without applying any force and allowed to solidify. Catalysts that cause the liquid to set are often added. The resin can be a natural liquid or a granular solid liquefied by heat. After the liquid resin is poured into the closed mold, the air bubbles are removed and the resin is allowed to cure either at room temperature or in an oven at low heat. When completely cured, the mold is split apart and the finished casting is removed. In the production of simple shapes such as rods, tubes, etc., usually two-piece metal mold with an entry hole for pouring in the liquid resin is used. For making flat-cast acrylic plexiglass or lucite sheets, two pieces of polished plate glass separated by a gasket with the edge sealed and one corner open are usually used as a mold. Both thermosets and thermoplastics may be cast. Acrylics, polystyrene, polyesters, phenolics, and epoxies are commonly used for casting.

2.13.2 Plastisol Casting Plastisol casting, commonly used to manufacture hollow articles, is based on the fact that plastisol in fluid form is solidified as it comes in contact with a heated surface [46]. A plastisol is a suspension of PVC in a liquid plasticizer to produce a fluid mixture that may range in viscosity from a pourable liquid to a heavy paste. This fluid may be sprayed onto a surface, poured into a mold, spread onto a substrate, etc. The plastisol is converted to a homogeneous solid (“vinyl”) product through exposure to heat [e.g., 350°F (176°C)], depending on the resin type and plasticizer type and level. The heat causes the suspended

211

Fabrication Processes

resin to undergo fusion—that is, dissolution in the plasticizer (Figure 2.42)—so that on cooling, a flexible vinyl product is formed with little or no shrinkage. The product possesses all the excellent qualities of vinyl plastics. Dispersion-grade PVC resins are used in plastisols. These resins are of fine particle size (0.1–2 mm in diameter), as compared to suspension type resins (commonly 75–200 mm in diameter) used in calender and extrusion processing. A plastisol is formed by simply mixing the dispersion-grade resin into the plasticizer with sufficient shearing action to ensure a reasonable dispersion of the resin particles. (PVC plasticizers are usually monomeric phthalate esters, the most important of them being the octyl esters based on 2-ethylhexyl alcohol and isooctyl alcohol, namely dioctyl phthalate and diisooctyl phthalate, respectively.) The ease with which virtually all plastisol resins mix with plasticizer to form a smooth stable dispersion/paste is due to the fine particle size and the emulsifier coating on the resin particles. (The emulsifier coating aids the wetting of each particle by the plasticizer phase.) The liquid nature of the plastisol system is the key to its ready application. The plastisol may be spread onto a cloth, paper, or metal substrate, or otherwise cast or slushed into a mold. After coating or molding, heat is applied, which causes the PVC resin particles to dissolve in the plasticizer and form a cohesive mass, which is, in effect, a solid solution of polymer in plasticizer. The various changes a plastisol system goes through in the transformation from a liquid dispersion to a homogeneous solid are schematically shown in Figure 2.43. At 280°F (138°C) the molecules of plasticizer begin to enter between the polymer units, and fusion beings. If the plastisol were cooled after being brought to this temperature, it would give a cohesive mass with a minimum of physical strength. Full fusion occurs and full strength is accomplished when the plastisol is brought to approximately 325°F (163°C) before cooling. The optimum fusion temperature, however, depends on resin type and plasticizer type. For coating applications it is common practice to add solvent (diluent) to a plastisol to bring down viscosity. This mixture is referred to as organosol. It may be applied by various coating methods to form a film on a substrate and then is heated to bring about fusion, as in the case of plastisol. Female die

Press

Heater

Plastic sheet Sheet clamp (a)

Male die Press Forming plastic sheet Sheet clamp

Female die

(b)

FIGURE 2.42

Male die

Matched mold forming: (a) heating; (b) forming.

212

Plastics Technology Handbook

27°C

54°C

80°C

Resin particles

Gelation: Swollen particles touch; all plasticizer taken by resin

Plasticizer Pre-gelation: Resin particle swollen with absorbed plasticizer

Liquid plastisol

160°C Fusion: Plasticizer uniformly distributed along polymer chain

FIGURE 2.43 solid.

140°C Partial fusion: Plasticizer begins to dissolve polymer

Various changes in a plastisol system in the transformation from a liquid dispersion to a homogenous

Unlike coating applications, there are some applications where it is desirable to have an infinite viscosity at low shear stress. For such applications, a plastisol can be gelled by adding a metallic soap (such as aluminum stearate) or finely divided filler as a gelling agent to produce a plastigel. A plastigel can be cold molded, placed on a pan, and heated to fusion without flow. The whole operation is like baking cookies. A rigidsol is a plastisol of such formulation that it becomes a rigid, rather than a flexible, solid when fused. A very rigid product can be obtained when the plasticizer is polymerized during or right after fusion. For example, a rigidsol can be made from 100 parts of PVC resin, 100 parts of triethylene glycol dimethacrylate (network forming plasticizer) and 1 part of di-tert-butyl peroxide (initiator). This mixture has a viscosity of only 3 poises compared with 25 poises for phthalate-based plastisol. However, after being heated for 10 min at 350°F (176°C), the resin solvates and the plasticizer polymerizes to a network structure, forming a hard, rigid glassy solid with a flexural modulus of over 2.5 × 105 psi (1.76 × 104 kg/cm2) at room temperature. Three important variations of the plastisol casting, are dip casting, slush casting, and rotational casting. 2.13.2.1 Dip Casting A heated mold is dipped into liquid plastisol (Figure 2.44a) and then drawn at a given rate. The solidified plastisol (with mold) is then cured in an oven at 350°F–400°F (176°C–204°C). After a cools, the plastic is stripped from the mold. Items with intricate shapes such as transparent ladies’ overshoes, flexible gloves, etc., can be made by this process. The dipping process is also used for coating metal objects with vinyl plastic. For example, wire dish drainers, coat hangers, and other industrial and household metal items can be coated with a thick layer of flexible vinyl plastic by simply dipping in plastisol and applying fusion. 2.13.2.2 Slush Casting Slush casting is similar to slip casting (drain) of ceramics. The liquid is poured into a preheated hollow metal mold, which has the shape of the outside of the object to be made (Figure 2.44b). The plastisol in immediate contact with the walls of the hot mold solidifies. The thickness of the cast is governed by the time of stay in the mold. After the desired time of casting is finished, the excess liquid is poured out and the solidified plastisol with the mold is kept in an oven at 350°F–400°F (176°C–204°C). The mold is then opened to remove the plastic part, which now bears on its outer side the pattern of the inner side of the

213

Fabrication Processes

Plastisol (solidiﬁed)

Heated mold

Liquid plastisol

Liquid plastisol Split mold

(a) (b)

FIGURE 2.44

Split mold (c)

Plastisol casting processes: (a) dip casting; (b) slush casting; (c) rotational casting.

metal mold. Slush molding is used for hollow, open articles. Squeezable dolls or parts of dolls and boot socks are molded this way. 2.13.2.3 Rotational Casting In rotational casting a predetermined amount of liquid plastisol is placed in a heated, closed, twopiece mold. The liquid is uniformly distributed against the walls of the mold in a thin uniform layer (Figure 2.43c) by rotating the mold in two planes. The solidiﬁed plastisol in the mold is cured in an oven; the mold is then opened, and the part is removed. The method is used to make completely enclosed hollow objects. Doll parts, plastic fruits, squeeze bulbs, toilet ﬂoats, etc. can be made by rotational casting of plastisols.

2.14 Reinforcing Processes An RP consists of a polymeric resin strengthened by the properties of a reinforcing material [47]. Reinforced plastics occupy a special place in the industry. They are at one and the same time both unique materials into themselves and part and parcel of virtually every other segment of the plastics industry. Reinforced plastics are composites in which a resin is combined with a reinforcing agent to improve one or more properties of the resin matrix. The resin may be either thermosetting or thermoplastic. Typical thermosetting resins used in RPs include unsaturated polyester, epoxy, phenolic, melamine, silicone, alkyd, and diallyl phthalate. In the field of reinforced thermoplastics (RTPs), virtually every type of thermoplastic material can be, and has been, reinforced and commercially molded. The more popular grades include nylon, polystyrene, polycarbonate, polyporpylene, polyethylene, acetal, PVC, ABS, styrene-acrylonitrile, polysulfone, polyphenylene sulfide, and thermoplastic polyesters. The reinforcement used in RP is a strong inert material bound into the plastic to improve its strength, stiffness, or impact resistance. The reinforcing agent can be fibrous, powdered, spherical, crystalline, or whisker, and made of organic, metallic, or ceramic material. Fibrous reinforcements are usually glass, although asbestos, sisal, cotton and high-performance fibers (discussed later) are also used. To be structurally effective, there must be a strong adhesive bond between the resin and the reinforcement. Most reinforcements are thus treated with sizes or finishes to provide maximum adhesion by the resins. Although by definition, all RPs are composites (i.e., combinations of two materials—resin and reinforcement—that act synergistically to form a new third material RP with different properties than the original components) the term advanced composites, or high-strength composites, has taken on a special meaning. The term is applied to stiffer, higher modulus combinations involving exotic reinforcements such as graphite, boron, or other high-modulus fibers like aromatic polyamide fibers (Nomex and Kevlar) and extended-chain polyethylene fibers (Spectra ECPE). And resins like epoxy or some of the newer high

214

Plastics Technology Handbook

heat-resistant plastics—polyamides, polyamideimide, polyquinoxalines, and polyphenylquinoxalines. Prime outlets for these materials are in the aerospace and aircraft industries.

2.14.1 Molding Methods As mentioned above, either thermosetting and thermoplastic resin can be used as the matrix component in RPs. Today, the RTP have become an accepted part of the RPs business, although smaller than the reinforced thermosets. RTP are generally made available to the processor in the form of injection molding pellets (into which glass has been compounded) or concentrates (also a pellet but containing a much higher percentage of reinforcement, and designed to be mixed with nonreinforced pellets). It is also possible for the processor to do his own compounding of chopped glass and thermoplastic powder in injection molding. In rotational molding and casting techniques, the processor usually adds his own reinforcement. For thermoforming, glass-RTP laminates are sold commercially. Structural foam molding of glass-RTPs and reinforcing molded urethane foams appeared as later developments in the field of RTPs. Among thermosetting resins, unsaturated polyesters are by far the most widely used in RPs, largely because of their generally good properties, relatively easy handling, and relatively low cost. For special uses, however, other types are significant: epoxies for higher strength, phenolics for greater heat resistance, and silicones for their electrical properties and heat resistance. All these resins must be used in conjunction with a system of catalysts or curing agents in molding thermoset composites. The type and amount strongly affect the properties, working life, and molding characteristics of the resin. The polyester and epoxies are most often mixed with a catalyst just prior to molding. The most widely used catalyst for polyesters is benzoyl peroxide. Where heat is not available for curing, special catalyst-promoter systems can be used. With epoxies, an amine curing agent that reacts with the resin is most often used. However, there are many other types to choose from (see Chapter 4). The polyester, epoxy, and thermosetting acrylic resins are usually thick liquids that become hard when cured. For this reason, they are most often combined with the reinforcement, by the molder, by dipping or pouring. There are available, however, preimpregnated reinforcements (prepregs) for the molder who wants to keep the operations as simple as possible. Several methods are employed to make RPs. Although each method has the characteristics of either molding or casting, the process may be described as (1) hand lay-up. (2) spray-up, (3) matched molding, (4) vacuum-bag molding, (5) pressure-bag molding, (6) continuous pultrusions, (7) filament winding, and (8) prepreg molding. 2.14.1.1 Hand Lay-Up or Contact Molding A mold is first treated with a release agent (such as wax or silicone-mold release), and a coating of the liquid resin (usually polyester or epoxy) is brushed, rolled, or sprayed on the surface of the mold. Fiberglass cloth or mat is impregnated with resin and placed over the mold. Air bubbles are removed, and the mat or cloth is worked into intimate contact with the mold surface by squeegees, by rollers, or by hand (Figure 2.45a). Additional layers of glass cloth or mat are added, if necessary, to build up the desired thickness. The resin hardens due to curing, as a result of the catalyst or hardener that was added to the resin just prior to its use. Curing occurs at room temperature, though it may be speeded up by heat. Ideally, any trimming should be carried out before the curing is complete, because the material will still be sufficiently soft for knives or shears. After curing, special cutting wheels may be needed for trimming. Lowest-cost molds such as simple plaster, concrete, or wood are used in this process, since pressures are low and little strength is required. However, dimensional accuracy of the molded part is relatively low and, moreover, maximum strength is not developed in the process because the ratio of resin to filler is relatively high.

215

Fabrication Processes

Resin brushed on

Chopped ﬁber Spray gun

Roller

Molding cures in air Mold

(a)

FIGURE 2.45

Layers of resin and ﬁbers

(b)

Molding

(a) Basic hand lay-up method. (b) Spray-up technique.

The hand lay-up process can be used for fabricating boat hulls, automobile bodies, swimming pools, chemical tanks, ducts, tubes, sheets, and housings, and for building, machinery, and autobody repairs. 2.14.1.2 Spray-Up A release agent is first applied on the mold surface, and measured amounts of resin, catalyst, promoter, and reinforcing material are sprayed with a multiheaded spray gun (Figure 2.44b). The spray guns used for this work are different from those used for spraying glazes, enamels, or paints. They usually consist of two or three nozzles, and each nozzle is used to spray a different material. One type, for example, sprays resin and promoter from one nozzle, resin and catalyst from another, and chopped glass fibers from a third. The spray is directed on the mold to build up a uniform layer of desired thickness on the mold surface. The resin sets rapidly only when both catalyst and promoter are present. This method is particularly suitable for large bodies, tank linings, pools, roofs, etc. 2.14.1.3 Matched Metal Molding Matched metal molding is used when the manufacture of articles of close tolerances and a high rate of production are required. Possible methods are perform molding, sheet molding, and dough molding. In preform molding the reinforcing material in mat or fiber form is preformed to the approximate shape and placed on one-half of the mold, which was coated previously with a release agent. The resin is then added to the perform, the second half of the mold (also coated previously with a release agent) is placed on the first half, and the two halves of the mold are then pressed together and heated (Figure 2.46). The resin flows, impregnates the perform, and becomes hard. The cured part is removed by opening the mold. Because pressures of up to 200 psi (14 kg/cm2) can be exerted upon the material to be molded, a higher ratio of glass to resin may be used, resulting in a stronger product. The cure time in the mold depends on the temperature, varying typically from 10 min at 175°F (80°C) to only 1 min at 300°F (150°C). The cure cycle can thus be very short, and a high production rate is possible. The molding of sheet-molding compounds (SMC) and dough-molding compounds (DMC) is done “dry”—i.e., it is not necessary to pour on resins. SMC, also called prepreg, is basically a polyester resin mixture (containing catalyst and pigment) reinforced with chopped strand mat or chopped roving and formed into a pliable sheet that can be handled easily, cut to shape, and placed between the halves of the heated mold. The application of pressure then forces the sheet to take up the contours of the mold. DMC is a doughlike mixture of chopped strands with resin, catalyst, and pigment. The charge of dough, also called premix, may be placed in the lower half of the heated mold, although it is generally wise to perform it to the approximate shape of the cavity. When the mold is closed and pressure is applied, DMC flows readily to all sections of the cavity. Curing generally takes a couple of minutes for mold temperatures

216

Plastics Technology Handbook

Press ram Press platen Steam cores Female mold Molding Resin Preform Guide pins Stops Male mold Press platen

(a)

FIGURE 2.46

(b)

Matched metal molding: (a) before closing of die; (b) after closing of die.

from 250 to 320°F (120°C–160°C). This method is used for the production of switch gear, trays, housings, and structural and functional components. 2.14.1.4 Vacuum-Bag Molding In vacuum-bag molding the reinforcement and the resin mixed with catalyst are placed in a mold, as in the hand layup method, and an airtight flexible bag (frequently rubber) is place over it. As air is exhausted from the bag, atmospheric air forces the bag against the mold (Figure 2.47). The resin and reinforcement mix now takes the contours of the mold. If the bag is placed in an autoclave or pressure chamber, higher pressure can be obtained on the surface. After the resin hardens, the vacuum is destroyed, the bag opened and removed, and the molded part obtained. The technique has been used to make automobile body, aircraft component, and prototype molds. 2.14.1.5 Pressure-Bag Molding In pressure-bag molding the reinforcement and the resin mixed with catalyst are placed in a mold, and a flexible bag is placed over the wet lay-up after a separating sheet (such as cellophane) is laid down. The bag is then inflated with an air pressure of 20–50 psi (1.4–3.5 kg/cm2). The resin and reinforcement follow the contours of the mold (Figure 2.48). After the part is hardened, the bag is deflated and the part is removed. The technique has been used to make radomes, small cases, and helmets.

Clamp

To vacuum

Gasket

Molded part

Glass resin lay-up Flexible bag

Flexible bag

Mold

(a)

FIGURE 2.47

(b)

Vacuum-bag molding: (a) before vacuum applied; (b) after vacuum applied.

217

Fabrication Processes

Rubber bag (not inﬂated)

Rubber bag (inﬂated)

Air pressure line Pressure back-up plate

Clamps

Glass resin lay-up

Air

Molded part

Cellophane Mold (a)

FIGURE 2.48

(b)

Pressure-bag molding: (a) during lay-up; (b) during curing.

2.14.1.6 Filament Winding In the filament-winding method, continuous strands of glass fiber are used in such a way as to achieve maximum utilization of the fiber strength. In a typical process, rovings or single strands are fed from a reel through a bath of resin and wound on a suitably designed rotating mandrel. Arranging for the resin impregnated fibers to tranverse the mandrel at a controlled and predetermined (programmed) manner (Figure 2.49) makes it possible to lay down the fibers in any desired fashion to give maximum strengths in the direction required. When the right number of layers have been applied, curing is done at room temperature or in an oven. For open-ended structures, such as cylinders or conical shapes, mandrel design is comparatively simple, either cored or solid steel or aluminum being ordinarily used for the purpose. For structures with integrally wound end closures, such as pressure vessels, careful consideration must be given to mandrel design and selection of mandrel material. A sand-poly(vinyl alcohol) combination, which disintegrates readily in hot water, is an excellent choice for diameters up to 5 ft (1.5 m). Thus, a mandrel made of sand with water-soluble poly(vinyl alcohol) as a binder can be decomposed with water to recover the filament-wound part. Other mandrel materials include low-melting alloys, eutectic salts, soluble plasters, frangible or breakout plasters, and inflatables. Because of high glass content, filament-wound parts have the highest strength-to-weight ratio of any reinforced thermoset. The process is thus highly suited to pressure vessels where reinforcement in the Rotating mandrel

Traversing resin bath

Supply of roving

FIGURE 2.49

Sketch of ﬁlament winding.

218

Plastics Technology Handbook

highly stressed hoop direction is important. Pipe installation, storage tanks, large rocket motor cases, interstage shrouds, high-pressure gas bottles, etc., are some of the products made of ﬁlament winding. The main limitation on the process is that it can only be used for fabricating objects which have some degree of symmetry about a central axis. 2.14.1.7 Pultrusion

FIGURE 2.50

Pultruded FRP

Pull mechanism

Die and heat source

Resin soaked ﬁber

Resin impregnator

Tension roller

Reinforcement material (ﬁbers, or woven or braided strands)

Pultrusion is the process of “pulling” raw composites (i.e., resin-impregnated fiber or cloth) through a heated die, creating a continuous composite profile of high fiber content. The term pultrusion combines the words pull and extrusion to signify that material is forced through the die by pulling, instead of pushing. In the process (Figure 2.50), continuous reinforcement materials like fibers or oven mat or braided strands are impregnated with resin and then pulled through a long, heated stationary die, where the resin undergoes polymerization, while the die controls the resin content. The impregnation is done either by pulling the reinforcement through a resin bath or by injecting the resin into an injection chamber that is connected to the die. Resin can also be injected directly into the die in some pultrusion systems. Most often, the reinforcement is glass fiber, but it can also be carbon, aramid, or a mixture. The impregnating resin is almost always a thermosetting resin with unsaturated polyesters accounting for nearly 90% and epoxies for the balance. Other thermosetting resin types, such as polyurethane and vinyl esters, are also used. Though economic and environmental factors favor the use of a thermoplastic resin as matrix, the high viscosity of thermoplastic melts makes high productivity and high degree of resin impregnation of the fiber difficult to achieve. In the standard pultrusion process, the raw fiber is pulled off the racks/rolls and guided through a resin bath or resin impregnation system whereby the fiber reinforcement becomes fully impregnated (wetted out) with the resin such that all the fiber filaments are thoroughly saturated with the resin mixture. The uncured composite material is then guided through a series of tooling (known as “pre-former”) that helps arrange and organize the fiber into the correct shape, while excess resin is squeezed out (known as “debulking”). The raw composite then passes through a precisely machined steel die, which is heated to a constant temperature, and may have several zones of temperature throughout the length to cure the thermosetting resin. The profile that exits from the die is a cured, pultruded FRP. The process yields straight constant cross-section parts of virtually any shippable length with high unidirectional strength (e.g., I-beams, rods, and shafts).

Schematic of the pultrusion process. (After https://en.wikipedia.org/wiki/Pultrusion.)

219

Fabrication Processes

The pultrusion technology is not limited to thermosetting polymers. It has been used successfully with thermoplastic polymers, such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), by impregnating the glass fiber with resin powder or surrounding it with sheet material of the thermoplastic resin, followed by heating to fuse the resin [48]. Further, in a novel adaptation of the pultrusion process for thermoplastic polymers, known as reactive thermoplastic pultrusion (RTP), very low viscosity reactive monomers of thermoplastic polymers have been used to enable impregnation of a great quantity (up to 85%) of fibers in the composite. One of the first companies to pioneer the RTP process is the French company “CQFD Composites,” which commercially developed this technology to produce linear or curved structural profiles. The process (Figure 2.51) has two steps. In Step 1, a proprietary formulation of low-viscosity monomer (caprolactam), containing catalyst, activator, additives, and suitable fibers, is introduced under pressure into a pultrusion die. In Step 2, a thermoplastic polymer (nylon-6) is synthesized (see Section 4.3.2.1) in situ among the fibers in the pultrusion die, while shaping of the profile takes place under heat and pressure. (The polymerization reaction being sensitive to humidity and oxygen, it requires a well-controlled process to proceed efficiently.) Structural profiles of composites can thus be produced with extremely high content (up to 85 wt%) of reinforcing fibers, which can generate strategic advantages in construction, transport, or electrical applications. With modulus ranging from 50 to 60 GPa, such RTP profiles, unlike conventional thermoset profiles, are also heat-deformable and hence can be post-shaped under heat. Structural RTP profiles have been used as insert to be the structural “backbone” of injection-molded parts, especially in the automobile industry. Since the standard pultrusion process involves pulling the materials through a stationary die, it is suited to manufacture only straight profiles (see Figure 2.52b). In a later modification of the process, developed by Thomas GmbH + Co.Technik + Innovation KG, the die is not stationary but moves back and forth along the profile to be manufactured. The modified process, known as Radius-Pultrusion, enables production of two- and three-dimensional curved profiles in endless circles and arches of any radius (see Figure 2.52c). The profiles can be reinforced with endless fibers (glass, carbon, or natural) in a unidirectional or with the help of netting or webbing [strong fabric commonly made of synthetic fibers, such as nylon, polypropylene, or polyester, and woven as a flat strip (Figure 2.52a) or tube of varying width, often used in place of rope] in a bidirectional way. The Radius-Pultrusion technology allows nearly unlimited application of fiber-reinforced materials for engineers and architects in various fields, for example, automotive and transportation industry (e.g., fixtures for bumpers; dashboards; structural profiles; springs; frame and body for lorries, buses, and trains; container walls; aircraft bodies; and naval architecture), building industry/architecture (e.g., window profiles, arched profiles, bridges, scaffolding

Fibers

Catalyst + activator

Monomer

Additives

Composite

Pultrusion die (heated)

FIGURE 2.51

Schematic of the reactive thermoplastic pultrusion process of CQFD composites.

220

Plastics Technology Handbook

(a)

(c)

(b)

FIGURE 2.52 (a) Webbing, (b) pultruded straight proﬁles from the standard pultrusion process, (c) pultruded curved proﬁles from the Radius-Pultrusion process.

and ladders, stairs and rails, and grids), and sports (e.g., tennis rackets, squash courts, golf clubs, battens for surfing and sailing, surfboards, and fishing rods). 2.14.1.8 Prepreg Molding Optimal strength and stiffness of continuous fiber–reinforced polymeric composites is obtained through controlled orientation of the continuous fibers. One means to achieve this is by prepreg molding [49]. In this process, unidirectionally oriented layers of fibers are pre-impregnated with the matrix resin and cured to an intermediate stage of polymerization (B-stage). When desired, this pre-impregnated composite precursor, called a prepreg, can be laid up in the required directions in a mold for quick conversion into end components through the use of hot curing techniques. Prepregs can thus be described as pre-engineered laminating materials for the manufacture of fiber–reinforced composites with controlled orientation of fibers. For the designer, a precisely controlled ply of prepreg represents a building block, with well-defined mechanical properties from which a structure can be developed with confidence. For the fabricator, on the other hand, prepregs provide a single, easy-to-handle component that can be applied immediately to the lay-up of the part to be manufactured, be it aircraft wing skin or fishing rod tube. The prepreg has the desired handleability already built in to suit the lay-up and curing process being utilized, thus improving efficiency and consistency. Prepregs have been used since the late 1940s, but they have only achieved wide prominence and recognition since the development of the higher performance reinforcing fibers, carbon, and kevlar. The quantum leap in properties provided by these new fibers generated a strong development effort by prepreg manufacturers and there is no doubt that new developments made in this area have been as significant, if not as evident, as that of the introduction of the new fibers. The use of prepreg in the manufacture of a composite components offers several advantages over the conventional wet lay-up formulations: 1. Being a readily formulated material, prepreg minimizes the material’s knowledge required by a component manufacturer. The cumbersome process of stocking various resins, hardeners, and reinforcements is avoided.

Fabrication Processes

221

2. With prepreg a good degree of alignment in the required directions with the correct amount of resin is easily achieved. 3. Prepreg offers a greater design freedom due to simplicity of cutting irregular shapes. 4. Material wastage is virtually eliminated as offcuts of prepregs can be used as random molding compounds. 5. Automated mass-production techniques can be used for prepreg molding, and the quality of molded product is reproducible. 6. Toxic chemical effects on personnel using prepregs are minimized or eliminated. A flowchart showing the key stages in the fabrication of composite structures from raw materials with an intermediate prepregging step is given in Figure 2.53. It can be seen that there are two basic constituents to prepreg—the reinforcing fiber and the resin system. All of the advanced reinforcing fibers are available in continuous form, generally with a fixed filament diameter. The number of filaments that the supplier arranges into a “bundle” (or yarn) varies widely, and is an important determinant of the ability to weave fabric and make prepregs to a given thickness. In the majority of cases, the yarn is treated with a size to protect it from abrasion during the weaving or prepregging process. Often, the size is chosen such that it is compatible with the intended resin system. Resin systems have developed into extremely complex multi-ingredient formulations in an effort to ensure the maximum property benefit from the fiber. Normally there are four methods of impregnation: (1) solution dip; (2) solution spray; (3) direct hot-melt coat; and (4) film calendaring. The solution dip and solution spray impregnation techniques work with a matrix resin dissolved in a volatile carrier. The low viscosity of the resin solution allows good penetration of the reinforcing fiber bundles with resin. In solution dipping, the fiber, in yarn or fabric form, is passed through the resin solution and picks up an amount of solids dependent upon the speed of through-put and the solids level. With solution spraying, on the other hand, the required amount of resin formulation is metered directly onto the fiber. In both cases, the impregnated fiber is then put through a heat cycle to remove the solvent and “advance” the chemical reaction in the resin to give the correct degree of tack. Direct hot melt can be performed in a variety of ways. In one method, the reinforcing fiber web is dipped into a melt resin bath. A doctor blade, scraper bar, or metering roller controls the resin content. Alternately, the melt resin is first applied to a release paper, the thickness of the resin being determined by a doctor blade. The melt resin on the release paper is then brought into contact with a collimated fiber bundle and pressed into it in a heated impregnation zone. In film calendaring, which is a variation of the above method, the resin formulation is cast into a film from either hot-melt or solution and then stored. Thereafter in a separate process, the reinforcing fiber is sandwiched between two films and calendered so that the film is worked into the fiber. The decision of which method to use is dependent upon several factors. Holt melt film processes are faster and cheaper, but certain resin formulations cannot be handled in this way, and hence solution methods have to be used. The solution dip method is often preferred for fabrics as the need to squeeze hot-melt and film into the interstices of the fabric can cause distortion of the weave pattern. In a process based on a biconstituent two impregnation concept [49], the polymeric matrix is introduced in fibrous form and a comingled two of polymer and reinforcing fibers are fed into a heated impregnation zone (Figure 2.54). In this method, it may be possible to effect better wetout of the reinforcing fibers with the matrix polymer, especially with high viscosity thermoplastic matrix polymers, through intimate comingling. The machines necessary to accomplish the above prepregging procedures are many and varied. There are three distinct aspect to quality control: raw material screening, on-line control, and batch testing. All three are obviously important, but the first two are more critical. Fibers and base resins are supplied against certificates of conformance and often property test certificates. On-line control during the manufacture of prepreg revolves around the correct ratio of fiber to resin. This is done by a traversing Beta-gauge, which scans the dry fiber (either unidirectional of

Elastomers

Epoxy Phenolic Polyester Polyimide Solvents Hardeners Thermoplastic Tape Thickness 50–300 μm Width 25–1200 mm Single yam Crossplied packs Resin contents 34% – 45%

Woven Thickness 100–500 μm Width 500–1500 mm Resin contents 30% – 60%

STD weaves Bias weaves Triaxial Hybrids

Prepregs

Woven fabrics

Intermediate

Tape winding

Automatic cutting Hand lay-up Tape laying

Lay-up

Oven/bag

Autoclave

Press

Curing

Fastening

Bonding

Drilling

Trimming

Finishing

Structure

FIGURE 2.53 Flow chart showing key stages in the fabrication of composite structures from raw materials by prepreg molding. (After Lee, W. J., Seferis, J. C., and Bonner, D. C. 1986. SAMPE Q., 17, 2, 58.)

Resin systems

Fibers

Glass Kevlar Carbon Boron Others

Raw material

222 Plastics Technology Handbook

223

Fabrication Processes

Heat

Polymer ﬁber

Reinforcing ﬁber

Polymer matrix

Reinforcing ﬁber

FIGURE 2.54 Biconstituent tow impregnation using a comingled tow of polymer and reinforcing ﬁbers. (After Lee, W. J., Seferis, J. C., and Bonner, D. C. 1986. SAMPE Q., 17(2), 58.)

fabric) and then the impregnated fiber and provides a continuous real-time plot of the ratio across the width of the material. This can be linked back to the resin system application point for continuous adjustment. Batch testing is carried out to verify prepreg properties, such as resin content, volatile level, and flow. The resin “advancement” (chemical reaction) is monitored via a Differential Scanning Calorimeter (DSC) and the formulation consistency by testing the Tg via DSC or Dynamic Mechanical Analyzer (DMA). The laminate properties are also determined. All are documented and quoted on a Release Certificate. Commercial prepregs are available with different trade names. Fibredux 914 of Ciba–Geigy is a modified epoxy resin preimpregnated into unidirectional fibers of carbon (HM or HT), glass (E type and R type), or aramid (Kevlar 49) producing prepregs that, when cured to form fiber reinforced composite components, exhibit very high strength retention between −60°C and +180°C operational temperatures. “Scotchply” brand RPs of Industrial Specialties Division of 3M Company are structural-grade thermosetting molding materials, consisting of unidirectional nonwoven glass fibers embedded in an epoxy resin matrix. The product is available both in prepreg form in widths up to 48 in. (1.22 m) and in flat sheet stock in sizes up to 48 in. (1.22 m) × 72 in. (1.83 m). The cured product is claimed to possess extraordinary fatigue life, no notch sensitivity, high ultimate strength, and superior corrosion resistance. The range of application of the RP includes vibratory springs, sonar housings, landing gear, picker blades, snowmobile track reinforcement, helicopter blades, seaplane pontoons, missile casing, and archery bow laminate.

2.14.2 Fibrous Reinforcements Although many types of reinforcements are used with plastics, glass fibers predominate. Fibrous glass reinforcements are available in many forms (described below). Asbestos is used in the form of loose fiber, paper, yarn, felt, and cloth. The two largest uses of asbestos in plastics are with PVC in vinyl asbestos tile and with polyesters and polypropylene. Most natural and synthetic fibers do not have the strength required for a RPs part. However, when intermediate strengths are satisfactory, they can be used. In this category are nylon, rayon, cotton fabrics, and paper. Sisal fibers have also found use as a low cost reinforcing material in premix molding compounds. High-modulus graphite and carbon fibers, aramid fibers and ECPE fibers are playing a more and more important role in RPs. Boron filaments, with outstanding tensile strengths, are usually used in the form of prepreg tapes and have been primarily evaluated for the aerospace and aircraft industry. 2.14.2.1 Glass Fibers A high-alkali “A-glass” and a low-alkali “E-glass” are used as reinforcements for polymer composites, the latter being used most often. Since the modulus of E-glass is 10.5 × 106 psi and the tensile strength

224

Plastics Technology Handbook

upwards of 250,000 psi, it is not surprising that the stiffness and strength of most plastics can be increased by compounding with glass. A more chemical resistant glass, sodium borosilicate (C-glass), and a highertensile-strength glass, S-glass, are also available. E-glass is a calcium alumino-silicate, and S-glass is a magnesium aluminosilicate. Fiberglass is available as a collection of parallel filaments (roving), chopped strands, mat, and woven fabric. Glass filaments are produced by melting a mixture of silica, limestone, and other reactants, depending on the type of glass and forcing the molten product through small holes (bushings). The hot filaments are gathered together and cooled by a water spray. These multiple glass filaments are gathered together into a bundle, called a strand, which is wound up on a coil. Short fibers (staple) are produced by passing a stream of air across the filaments as they emerge from the bushings. Rovings are rope-like bundles of continuous untwisted strands for use in such processes as perform press molding, filament winding, spray-up, pultrusion, and centrifugal casting. They can also be converted into chopped strand mats or cut into short fibers for molding compounds. Chopped strands of glass 1/32–1/2 in. in length can be incorporated in thermoset or thermoplastic materials about as easily as the particulate fillers. Each strand may be made up of 204 individual filament whose diameter is 2–7.5 × 10−4 in. Chopped strands several inches long can be loosely bound as a mat that is porous and in which the strands are randomly oriented in two dimensions. This form is suitable for impregnation by a liquid polymer. After polymerization or cross-linking (curing) under pressure, the composite will comprise a polymer-network matrix in which the individual strands are embedded. Chopped strand mats provide nondirectional reinforcement (i.e., strengths in many directions, as contrasted to unidirectional forms which are continuous fibers, like roving, that provide strength in one direction). These mats are available in a variety of thicknesses, usually expressed in weight per square foot. In order to hold the fibers together, a resin binder is generally used, the type depending on the resin and molding process. In some cases, the mats are stitched or needled, instead of using the resin binder. A woven glass fabric (cloth) might be used in place of the mat or in combination with it. In this case there will be a variation in strength with the angle between the axis of the fibers and the direction of stress. Twisted yarns are generally woven into fabrics of varying thicknesses and with tight or loose weaves, depending upon the application. Most are balanced weaves (i.e., equal amounts of yarn in each direction), although some are unidirectional (more fibers running in one direction). Although costlier, they offer a high degree of strength. Rovings can also be woven into a fabric that is less costly than the woven yearn fabrics, coarser, heavier, and easier to drape. For optimum adhesion at the interface between the fiber surface (stationary phase) and the resin matrix (continuous phase), the glass fibers must be treated with coupling agents to improve the interfacial adhesion. The pioneer coupling agent (linking agent) was methacrylatochromic chloride (Volan). This has been supplanted by organosilanes, organotitanates, and organozirconates. These coupling agents contain functional groups, one of which is attracted to the fiber surface and the other to the resin (Figure 2.55). 2.14.2.2 Graphite/Carbon Fibers, the Beginning Graphite carbon fibers are the predominant high-strength, high-modulus reinforcing agent used in the fabrication of high-performance polymer composites. In general, the term graphite fiber refers to fibers that have been treated above 1,700°C (3,092°F) and have tensile moduli of elasticity of 5 × 105 psi (3,450 MPa) or greater. Carbon fibers are those products that have been processed below 1,700°C (3092°F) and consequently exhibit elastic moduli up to 5 × 105 psi (3,450 MPa) [47]. A further distinction is that the carbon content of carbon fibers is 80%–95%; and that of graphite, above 99%. However, the industry has universally adopted the term “graphite.” It will therefore be used to describe both product forms in this section. Graphite fibers were first utilized by Thomas Edition in 1880 for his incandescent lamps. The filaments were generated by the carbonization of bamboo in the absence of air. When tungsten filaments replaced the graphite in lamps, interest in graphite materials waned until the mid 1950s when rayon-based graphite

225

Fabrication Processes

CH = CH2 Si

+ Cl

CH = CH2

Si Slightly hydrolyzed glass surface

FIGURE 2.55

Si +

Vinyl trichlorosilane

Attachment to glass surface

Mechanism of functioning of a glass surface ﬁnish.

fibers were created. These products exhibited relatively high tensile strengths of about 4 × 105 psi (2,760 MPa) and were designed for rocket/missile ablative component applications. A significant event that led to the development of today’s graphite industry was the utilization of PAN as a graphite precursor material by Tsunoda in 1960 [50]. Subsequent work led to continued improvement of PAN-based graphite fiber properties by numerous researchers. These developments focused on stretching the PAN precursor to obtain a high degree of molecular orientation of the polymer molecules followed by stabilizing it under tensile load, carbonization, and graphitization. PAN-based graphite fibers are now available with tensile moduli of up to 1.2 × 106 psi (8,280 MPa) and tensile strengths above 8 × 105 psi (5,516 MPa). Pitch was first identified as a graphite precursor by Otani in 1965 [51]. These fibers are made by melt spinning a low-cost isotropic molten (petroleum) pitch and then oxidizing the filaments as they are spun. This step is followed by carbonization at 1,000°C (1832°F) in an inert atmosphere. Process modifications to improve the fiber properties evolved through the 1970s until pitch-based (including mesophase liquid crystal pitch) graphite fibers with tensile strengths up to 3.75 × 105 psi (2,590 MPa) and tensile moduli to 1.2 × 106 psi (8,300 MPa) were achievable. 2.14.2.3 Manufacture of Graphite (Carbon) Fibers The pyrolysis of organic fibers used as graphite precursors is a multistage process. The three principal graphite precursors are PAN, pitch, and rayon, with PAN as the predominant product. The commercial production of PAN precursor fiber is based on either dry or wet spinning technology. In both instances, the polymer is dissolved in either an organic or inorganic solvent at a concentration of 5–10% by weight. The fiber is formed by extruding the polymer solution through spinneret holes into hot gas environment (dry spinning) or into a coagulating solvent (wet spinning). The wet spinning is more popular as it produces fibers with round cross section, whereas the dry spinning results in fiber with a dog bone cross section. The wet-spun precursor making process [52] includes three basic steps: polymerization, spinning, and after treatments (Figure 2.56). Acrylonitrile monomer and other comonomers (methyl acrylate or vinyl acetate) are polymerized to form a PAN copolymer. The reactor effluent solution, called “dope,” is purified, the unreacted monomers removed, and the solid contaminants filtered off. The spinning process next extrudes the purified dope through holes in spinnerettes into a coagulating solution. The spun gel fiber then passes through a series of after-treatments such as stretching, oiling, and drying. The product is the PAN precursor. In order to produce high-strength, high-modulus graphite fibers from the PAN precursor, it is essential to produce preferred molecular orientation parallel to the fiber axis and then “stabilize” the fiber against relaxation phenomena and chain scission reactions that may occur in subsequent carbonization steps. A typical step-by-step PAN-based graphite manufacturing process begins with the aforementioned

226

Plastics Technology Handbook

Comonomers + catalysts Solvent

Polymerization Filtration

Precursor ﬁber

FIGURE 2.56

Winding or piddling

Extrusion coagulation

Washing

After treatments (stretching, oiling, drying)

Typical PAN precursor manufacturing steps based on solution polymerization and wet spinning.

precursor stabilization which is followed by carbonization, graphitization, surface treatment, and sizing, as shown schematically in Figure 2.57. The stabilization of the PAN precursor involves “preoxidation” by heating the fiber in an air oven at 200°C–300°C (392°F–572°F) for approximately one hour while controlling the shrinkage/tension of the fiber so that the PAN polymer is converted into a thermally infusible aromatic ladder-like structure. The next step is the process of carbonization, which pyrolyzes the stabilized PAN-based fibers until they are transformed into graphite (carbon) fibers. The carbonization treatment is done in an inert atmosphere (generally nitrogen) at temperatures greater than 1,200°C (2,192°F). This step removes hydrogen, oxygen, and nitrogen atoms from the ladderlike polymers whose aromatic rings then collapse into a graphitelike polycrystalline structure. It is during this stage that high-mechanical-property characteristics of graphite fibers are developed. The development of these properties is directly related to the formation and orientation of graphitelike fibers or ribbons within each individual fiber. Graphitization performed at temperatures above 1,800°C (3,272°F) is an optional treatment. Its purpose is to improve the tensile modulus of elasticity of the fiber by improving the crystalline structure and orientation of graphitelike crystallites within each individual fiber. The higher heat-treatment temperature used in graphitization also results in a higher carbon content of that fiber. The final step in the process of producing graphite or carbon fiber is surface treatment and sizing prior to bobbin winding the continuous filaments. The surface treatment is essentially an oxidation of the fiber surface to promote wettability and adhesion with the matrix resin in the composite. Sizing improves handleability and wettability of the fiber with the matrix resin. Typical sizing agents are poly(vinyl alcohol), epoxy, and polyimide. Pitch-based graphite fibers are produced by two processes. The precursor of one of these processes is a low-softening-point isotropic pitch and the process scheme includes the following steps: (1) melt-spin isotropic pitch; (2) thermoset at relatively low temperatures for long periods of time; (3) carbonize in an inert atmosphere at 1,000°C (1,832°F); (4) stress graphitize at high temperatures 3,000°C (5,432°F). The high-performance fibers produced in this manner are relatively expensive because of the very long thermosetting time required and the need for high-temperature stress graphitization. The commercially more significant process for making pitch-based fibers is the mesophase process, which involves the following steps: (1) heat treat in an inert atmosphere at 400–450°C (752–842°F) for an extended period of time in order to transform pitch into a liquid-crystalline (mesophase) state; (2) spin the mesophase pitch into fibers; (3) thermoset the fibers at 300°C (572°F) for 2½ h; (4) carbonize the fibers at 1,000°C (1,832°F); and (5) graphitize the fibers at 3,000°C (5,432°F). Since long thermosetting times and

227

Fabrication Processes

Pan precursor

Carbonization

Stabilization

Surface treatment

Sizing

Carbon fiber

Graphitization

O Stabilization N

(200°C – 300°C in air)

Pan precursor

H2N

C N

Carbonization 1,000 – 3,000°C in inert gas

Graphite (carbon) fiber

FIGURE 2.57

Schematic of a typical step-by-step PAN-based graphite manufacturing process.

stress graphitization treatment are not required, the high-temperature graphite fibers produced by this process are lower in cost. The process by which rayon precursor is converted to graphite fibers includes four steps: (1) fiber spinning; (2) stabilization at 400°C (752°F) for long periods of time; (3) carbonization at 1,300°C (2,372°F); and (4) stress graphitization at high temperatures 3,000°C (5,432°F). The rayon-based graphite fibers produced in this manner tend to be relatively expensive because of the very long stabilization times required and the need for stress graphitization at high temperatures. 2.14.2.4 Graphite/Carbon Fibers and Fabrics The excellent properties of graphite are directly attributable to the highly anisotropic nature of the graphite crystal. The standard-grade PAN-based graphite fibers, which make up the largest part of both the commercial and aerospace markets, have tensile strengths ranging from 4.5 × 105 to 5.5 × 105 psi (3100– 3800 MPa) and moduli of approximately 340,000 psi (2345 MPa). Further, a family of intermediatemodulus/high-strain fibers with tensile strengths up to 7 × 105 psi (4800 MPa) and modulus above 4 × 105 psi (2760 MPa) have been developed to meet high-performance aerospace requirements. The highmodulus fibers (both PAN- and pitch-based), used in high-stiffness/low-strength applications, such as space hardware, have tension moduli ranging from 5 × 105 to 1.2 × 106 psi (3450–8280 MPa) and strain to failures generally greater than or equal to 1%. Graphite fibers are available to the user in a variety of forms—continuous filament for filament winding, braiding or pultrusion, tow for creation of fiber cloth and fabrics, chopped fiber for injection or compression molding, impregnated woven fabrics for lay-ups, and unidirectional tapes for lamination. [It may be mentioned that man-made fibers are usually extruded into filaments, which are then converted into filament yarn (composed of continuous filaments assembled with or without twist), staple (cut lengths from filaments), or tow (a large strand of continuous fiber filaments collected in loose, rope-like form).]

228

Plastics Technology Handbook

A carbon fiber filament is much thinner than a human hair. The strands of carbon fiber are bundled together to create what is known as tow and the tow is used to create the carbon fiber cloth. Carbon fiber filaments created in different ways by different processes have their own set of structural values and properties like strength and stiffness as noted above. There are several variations of tow sizes, the most common sizes being 3k, 6k, and 12k, where “k” stands for thousands (thus, there are 3000 individual filaments in a 3k tow). While there are several types of carbon fiber, differing in strength and modulus, there are several tow size differences and many different cloth patterns [53], such as twill, satin, plain, unidirectional (Uni), triaxial, and so on to choose from. A common way carbon fiber cloth is listed is similar to the following two examples: (a) T700S-12k, 7 oz plain weave and (b) AS4-6k, 11 oz harnesssatin 4 weave. While the tags “T700S” and “AS4” refer to the brand or type of carbon fiber, the numbers “12k” and “6k” relate to the tow size; the “7 oz” and “11 oz” are usually the weights of the cloth per yard; the “plain weave” and “4 harness-satin weave” indicate how the tow is woven into cloth. The more underlapping and overlapping (or “bumpy”) the fibers within the weave, the weaker it will be because as the cloth begins to come under tension, the straightening tow in it is subjected to a shear force and it snaps. Thus, a plain weave will be more prone to breaking than a 4 harness-satin weave using the same type of tow. A harness-satin 4 weave also has fewer overlapping bumps than a 2 × 2 twill weave, which has less overlapping than a plain weave. The strongest clothes are the nonwoven types like unidirectional cloth. [A unidirectional fabric is one in which the primary fibers (i.e., tow bundles) run in one direction only, usually in 0° direction (i.e., the warp direction), while a small amount of fiber or other material may run in other directions mainly to hold the primary fibers in position.] Among the weaves, the plain weave is the most difficult to drape (the ability of a fabric to conform to a complex surface). Woven fabrics are produced by the interlacing of warp (0°) fibers and weft (90°) fibers in a regular pattern or weave style. Weaves are generally referred and defined by notation such as 1 × 1, 2 × 2, 4 × 4, and 3 × 1. The first number in a set refers to how many strands are crossed “over” before going “under” the perpendicular strands (in a 90° weave). The second number refers to how many strands are crossed “under” before going back “over” the perpendicular strands. A plain weave, which is defined as a 1 × 1 weave, would thus run as over, under, over, under, over, under, and so on. Similarly, a 3 × 1 weave would run as over, over, over, under, over, over, over, under, over, over, over, and so on. A twill weave is defined as a set of identical number (>1) of weave both under and over, as for example, 2 × 2, 4 × 4 (see Figure 2.58). This produces the visual effect of a straight or broken diagonal “rib” to the fabric. A twill weave thus has a 3D “look” to it, which is so often desired. It is also much easier to bend around complex curves than a plain weave because its weave is more loose. Superior wet out and drape is seen in the twill weave over the plain weave with only a small reduction in stability. A satin weave is characterized by three or more weft yarns passing (“floating”) over a warp yarn or vice versa. A satin fabric tends to have a high luster because of the high number of “floats” (i.e., missed interlacings) on the fabric. The “harness” number used in the designation of satin (typically 4, 5, and 8) is the total number of fibers crossed and passed under, before the fiber repeats the pattern. A 3 × 1 harnesssatin (see Figure 2.58) is referred to as a harness-satin 4, H4, or 4HS and a 4 × 1 harness-satin is referred to as a harness-satin 5 (5HS or H5). Satin weaves are very flat and have good wet out and a high degree of drape. A harness-satin bends over complex curves better than either a plain or a twill weave, and it almost always has more weaves per inch than the other two. For applications of carbon fibers, the most important properties to consider are the tensile strength, modulus, and elongation/strain. Tensile strength is how hard it is to break by pulling it apart from end to end. The higher the tensile strength, the harder it is to break. Modulus is a measure of the stiffness. The higher the modulus, the stiffer the carbon fiber is. A high-modulus carbon fiber will be stiffer but also weaker in tensile strength than a low or standard modulus fiber. Thus, a high-modulus carbon fiber will give stiffer but weaker and more brittle parts, whereas a high-strength/low-modulus carbon fiber will give stronger but flexible parts. To consider a typical application such as bike frames, one may choose highmodulus carbon fiber as this will give a stiffer bike frame while using less material (hence making the frame lighter) and because the loss of strength is not large enough to compromise the safety of the frame.

229

Fabrication Processes

(a)

(b)

(c)

FIGURE 2.58 Different types of woven fabrics: (a) a plain (1 × 1) weave, (b) a twill (2 × 2) weave, and (c) a satin (3 × 1) weave.

With low-modulus carbon fiber, on the other hand, to get a frame that is as stiff as a high-modulus frame, one will need more carbon fiber and the frame will be heavier. In general, if there are no complex curves to be covered and aesthetics are not important, a plain weave is the best option. If, however, aesthetics are very important, a twill weave is generally selected, while for a sophisticated look, a harness-satin H7 or H8 is often used, the latter being the best choice for very complex curves. 2.14.2.5 Graphite/Carbon Fiber-Reinforced Plastics Graphite (or carbon) fiber-reinforced plastics (CFRP is preferred to the term GFRP, which also stands for glass fiber-reinforced plastics) having graphite/carbon, or simply carbon, fibers as reinforcement are strong and light, relatively more expensive to produce, but are commonly used wherever high strength-to-weight ratio and rigidity are required, such as aerospace, automotive, sports goods, and many other consumer and technical applications. The matrix or binding polymer is often a thermoset resin, such as epoxy, but other thermoset or thermoplastic polymers, such as polyester, vinyl ester, or nylon are sometimes used. Besides carbon fiber as reinforcement, CFRPs may also contain other fibers such as aramid (e.g., Kevlar, Twaron), aluminum, ultrahigh-molecular-weight polyethylene (UHMWPE), and additives, such as silica, rubber, and carbon nanotubes. Because CFRP consists of two distinct components, namely, carbon fiber reinforcement, which provides the strength, and a polymer resin (matrix), which binds the reinforcements together, the material properties of the composite will depend on the respective properties of these two components. However, unlike isotropic materials like metals and alloys, CFRP will have directional strength properties depending on the layouts of the carbon fiber and the properties of the carbon fibers relative to the polymer matrix. The two different equations governing the net elastic modulus of a composite (using the properties of the fiber reinforcement and the matrix resin) can also be applied to CFRP. Thus, the following equation (see Equation 3.129 for derivation) Ec = Em fm + Ef ff

(2.3)

is valid for CFRP with the fibers oriented in the direction of the applied load; Ec is the modulus of the composite; fm and ff are the volume fractions of the matrix and fiber, respectively, in the composite; and Em and Ef are the elastic moduli of the matrix and fibers, respectively. The other extreme case of the elastic modulus of the composite with the fibers oriented transverse to the applied load can be found from the following equation: Ec =

Em Ef Em ff + Ef fm

(2.4)

Although CFRPs with epoxy have high strength and elastic modulus, they exhibit virtually no plasticity, with less than 0.5% strain to failure. Efforts to toughen CFRPs include replacing epoxy with alternative

230

Plastics Technology Handbook

matrix resins. One such resin with high promise is PEEK, which exhibits an order of magnitude greater toughness with similar elastic modulus and strength. However, PEEK is considerably more difficult to process and is also more expensive. While having the advantage of high initial strength-to-weight ratio, compared to common structural materials, CFRP has a design limitation that arises from the fact that because of its complex failure modes, the fatigue failure properties are difficult to predict and design for. As a consequence, when using CFRP for critical cyclic-loading applications, considerable strength safety margins should be provided in design to ensure adequate component reliability during service. Like other polymer-based composites, CFRPs can also be profoundly affected by humidity and temperature, their combined action leading to degradation of the composites’ mechanical properties, particularly at the matrix–fiber interface, where the diffusing moisture plasticizes the polymer matrix. The carbon fibers, moreover, can cause galvanic corrosion when CFRP parts are attached to aluminum. 2.14.2.6 Manufacture of CFRP Parts The process by which most CFRP products are made varies depending on the pieces being created, the finish (outside gloss) required, and the number of the particular piece to be produced. The molding methods described in Section 2.14.1 for FRPs, in general, can also be used for producing CFRP parts. Thus, using the hand lay-up method, highly corrosion-resistant, stiff, and strong CFRP parts can be made by layering sheets of carbon fiber cloth into a mold (see Figure 2.45) in the shape of the final product, choosing the alignment and weave of the cloth fibers to optimize the strength and stiffness of the product. The carbon cloth or mat is impregnated with the matrix resin and is air- or heat-cured. Parts used in less critical areas are manufactured by draping cloth over a mold, with the resin either pre-impregnated into it (known as prepreg) or sprayed over it. High-performance parts using single molds are often vacuum-bagged (see Figure 2.47) or autoclavecured in order to avoid the presence of small air bubbles in the material as such bubbles reduce the strength. Using vacuum-bagged (atmospheric) oven-cured material systems for secondary aircraft structures (such as flaps, fairings, etc.) is well established. However, to produce composite materials with less than 1% void content and superior mechanical properties required for aerospace primary structures, such as wings, fuselage, and empennage components with integrated stiffeners, autoclave curing is employed. An alternative to the autoclave method is to use internal pressure via inflatable air bladders or expandable polystyrene foam inside the noncured laid-up carbon fabric. Vacuum-bagging is generally used for CFRP molding when simple objects and relatively few pieces (1–2 per day) are to be made. Typically, an aluminum mold is polished and waxed, and a release agent is applied, followed by carbon fabric and resin. Vacuum is then applied and the assembly is set aside to allow the piece to harden (cure). The resin can be applied in three ways to the fabric in a vacuum mold. One is wet lay-up, where the two-part epoxy resin is mixed, poured, and spread on the carbon fabric, and the fabric, then referred to as “wet prepreg cloth,” is laid in the mold and placed in the bag. The other is infusion, where the fabric and the mold are placed inside the bag while the resin is pulled in by vacuum through a tube and then through a device that causes even spreading of the resin throughout the fabric. The third method of applying resin for vacuum-bagging is a dry lay-up, where a carbon fiber prepreg (“dry prepreg cloth”) is laid in the mold in a way similar to adhesive film. The assembly is then placed in a vacuum and allowed to cure (∼120°C). [Note that a carbon fiber prepreg, usually referred to as “dry carbon cloth,” is carbon cloth that has been pre-impregnated with epoxy, which is not wet like a regular lay-up epoxy, but it is usually a bit sticky at room temperature. The dry carbon cloth is usually stored in a freezer.] As there is no excess resin bleed-out and resin waste is minimal in the process, the resulting prepreg parts are generally lighter and have fewer pinholes than parts made by wet lay-up. However, to achieve still better pinhole elimination by purging gases along with minimal resin use, autoclaving is generally required.

Fabrication Processes

231

Compression molding, also known as matched metal molding (Section 2.14.1.3), is a high-volume, high-pressure method that can be used for molding complex, high-strength CFRP parts. The process uses a two-piece (male and female) mold (see Figure 2.46), commonly made of aluminum or steel, that is pressed together with the fabric and resin between the two, while curing takes place under heat and pressure. The cured part is removed by opening the mold. Close tolerance of the molded parts and very high rate of productions can be achieved. However, initial cost may be high since the molds require machining of very high precision, such as computer numerical control (CNC) prototype machining. [Note: CNC is the automation of machine tools that are operated by precisely programmed commands encoded on a storage medium, as opposed to manual controlling. Machine movements that are controlled by cams, gears, levers, or screws in conventional machines are directed by computer and digital circuitry in CNC machines ensuring high precision [54].] For difficult and convoluted shapes, filament winding (Section 2.14.1.6) can be used to create strong and durable CFRP structures. Filament winding (Figure 2.49) is the process of winding resin-impregnated fiber on a mandrel surface in a precise geometric pattern. This is accomplished by rotating the mandrel while a delivery head precisely positions the carbon fibers or filaments on the mandrel surface. Thus, CFRP structures can be made with properties stronger than steel at much lighter weights. Filamentwound CFRP pressure vessel has been one of the most effective solutions for high-pressure storage. From pressure vessels to tubing to intricate components for medical devices to aerospace and military parts, filament winding can be used to meet high-performance demands of many critical applications. The pultrusion process (see Section 2.14.1.7) can be used to produce CFRP profiles, such as rods, angles, tubes, and sheets with maximum rigidity and minimum mass. The profiles are produced by pulling carbon fibers and resins through a heated die in a continuous process (Figure 2.50) that aligns the fibers lengthwise (in the direction of pulling). Carbon being 70% lighter than steel, 40% lighter than aluminum, and having three times the stiffness of either for the same weight, the aligned carbon fibers contribute greatly to the rigidity, while minimizing weight. For example, 0.125-in-diameter pultruded carbon rods with 67% fiber volume and bisphenol epoxy vinyl ester as the matrix resin has the following typical properties: density, 1.5 g/cm3; tensile strength, 2.34 GPa; tensile modulus, 134 GPa; compressive strength, 1.90 GPa; compressive modulus, 131 GPa; ultimate tensile strain, 1.30%; and glass transition temperature, 100°C [55]. 2.14.2.7 Applications of CFRP Products Graphite composites have exceptional mechanical properties that are unmatched by other materials. The principal advantage of graphite composites are high specific stiffness (stiffness divided by density), high specific strength (strength divided by density), and extremely low coefficient of thermal expansion (CTE). Graphite composites are also nonpoisonous, biologically inert, and transparent to x-rays. Historically, graphite composites have been very expensive, which limited its use to only special applications. However, over the past 15 years, the manufacturing processes have improved and the prices of graphite composites have steadily declined. Consequently, graphite composites are now economically viable in many applications, such as sporting goods and high-performance vehicles, boats, and industrial machinery. Table 2.2 gives a comparison of costs and mechanical properties of graphite composites and several other materials. The properties are listed in ranges as there are a wide variety of graphite fibers and resins, thus allowing numerous combinations and variation of properties [56]. For example, PAN-based carbon fiber has higher strength than pitch-based carbon fiber, while the latter has higher stiffness and lower (negative) CTE than the former. Any material that is strong and light is characterized by a high strength-to-weight ratio (also known as specific strength). While strength is resistance to breaking, rigidity or stiffness (measured by Young’s modulus) is resistance to bending or stretching. It is seen from Table 2.2 that both specific strength and specific stiffness of graphite composites are several times higher than those of common structural materials like fiber glass composites, aluminum, and steel. CFRP is thus the material of choice for applications where lightweight structures need to carry extremely high loads, such as components of spacecraft, fighter aircraft, and race cars.

0.050 1 × 106 to 1.8 × 106 160 × 106 to 200 × 106 1 × 10−6 to 2 × 10−6

0.050

1.8 × 106 to 4 × 106 200 × 106 to 1,000 × 106

−1 × 10−6 to 1 × 10−6

Density (lb/in3)

Speciﬁc strength(strength/density) Speciﬁc stiffness (Stiffness/Density)

CTE (in/in-°F)

Source: www.performancecomposites.com/about-composites-technical-info/124-designing-with-carbon-ﬁber.html. Note: Conversion factors: 1000 psi = 6.895 MPa; 1 lb/in3 = 27,675 kg/m3; 1 in/in-°F = 1.8/K.

50,000–90,000 8 × 106 to 10 × 106

90,000–200,000 10 × 106 to 50 × 106

$5–$20

$20–$250+

Strength (psi) Stiffness (psi)

Graphite Composite (Commercial Grade)

Cost $/lb

Graphite Composite (Aerospace Grade)

TABLE 2.2 Comparison of Properties of Graphite Composites with Those of Other Structural Materials

6 × 10−6 to 8 × 10−6

363,640–636,360 18 × 106 to 27 × 106

0.055

20,000–35,000 1 × 106 to 1.5 × 106

$1.50–$3.00

Fiber Glass Composite

13 × 10−6

350,000 100 × 106

0.10

35,000 10 × 106

Aluminum 6061 T-6

7 × 10−6

200,000 100 × 106

0.30

60,000 30 × 106

$0.30

Steel, Mild

232 Plastics Technology Handbook

Fabrication Processes

233

Carbon fiber has been described as a “game-changing material.” This could not be truer as it is in sporting goods. The high-strength and lightweight properties of carbon fiber have taken sporting goods to the next level of performance. Thus, golf shafts, racquets, skis, snowboards, hockey sticks, fishing rods, and bicycles have all been advanced through CFRP. As carbon fiber reduces weight without sacrificing strength, its use in marine applications means that yachts, cruisers, and racing vessels will be lighter and stronger when made with CFRP. Tough and durable CFRP material stands up to the extremes of marine environments and its high specific stiffness lends itself well to use in such applications as masts, hulls, and propellers. CTE is a measure of how much a material expands and contracts when the temperature goes up or down. As the data in Table 2.3 show, carbon fiber can have a broad range of CTEs, −1 to +8. The variation occurs because of the direction of measurement, the fabric weave, and the precursor material (e.g., higher CTE of PAN-based fiber and lower CTE of pitch-based fiber). A negative CTE for graphite fiber means that when the fiber is heated, it will shrink. So when such graphite fiber is put into a resin matrix (positive CTE), the composite can be tailored to have zero or very small CTE (see Table 2.2). Such graphite composites are therefore used for applications where small movements owing to temperature change can be critical, for example, telescope and other optical machinery, high-precision antennas, and scanning and imaging machines. The application of carbon fiber in the wind energy industry has led to the development of a lighter, longer, stiffer, and stronger wind turbine blade, pushing the industry to higher levels of performance, as longer blades mean more energy output per revolution. Being nonpoisonous, biologically inert, and x-ray permeable, carbon fiber is useful in medical applications, for example, prosthesis, implants, tendon repair, x-ray accessories, surgical instruments, and so on. However, the matrix, either epoxy or polyester, can be toxic and proper care needs to be exercised. Carbon fiber is used for 3D printing applications in both milled and chopped grades. In combination with a wide range of resins, parts that have exceptional mechanical properties can be created. While the major markets for advanced graphite fiber composites, as mentioned above, are aerospace, marine, automotive, industrial equipment, and recreation, military aerospace applications dominate the market and military consumptions are slated to increase rapidly as programs, which utilize a very high percentage of composites, move from development to large-scale production. Graphite fiber usage in space applications is in a large measure linked to space station programs and production activities. Examples of non-aerospace military applications include portable, rapid deployment bridges for the army, and propeller shafts for submarines. Fiber usage in the commercial aerospace sector is also growing. Commercial planes such as Boeing 767 and the Airbus A320 utilize two to three times the graphite fiber per plane that is used in older commercial models. The biggest industrial market potential of graphite fibers is in the automotive sector. The graphite composite usage in this area should increase as lower-cost fibers become available. A major and growing use of chopped graphite fibers in the industrial market is as a reinforcement for thermoplastic injection molding compounds. The advantages of such use include greater strength and stiffness, higher creep and fatigue resistance, increased resistance to wear, higher electrical conductivity, and improved thermal stability and conductivity. Continuous tow, chopped fibers, and milled fibers are produced for both general plastics (e.g., nylons, polycarbonates) and high-temperature engineering thermoplastics (such as PEI, PEEK, PPS, etc). Carbon fiber-reinforced carbon (also known as carbon–carbon or C/C) is a composite material consisting of carbon fiber reinforcement and graphite matrix. It was developed for the nose cones of intercontinental ballistic missiles and wing leading edges of the Space Shuttle Orbiter. It has also been used in the brake systems (brake discs and pads) of Formula One racing cars since 1976. The carbon–carbon composite is well suited to structural applications at high temperatures, or where thermal shock resistance and/or a low coefficient of thermal expansion is needed. It, however, lacks impact resistance and this must be taken into consideration in designing so as to avoid the likelihood of violent impacts.

234

Plastics Technology Handbook TABLE 2.3 Comparison of Thermal Expansion Coefﬁcients Material

CTE (in/in-°F)

Steel

Aluminum

Kevlar Carbon ﬁber woven

3 or lower 3 or lower

Carbon ﬁber unidirectional

−1 to +8

Fiber glass Brass

7–8 11

Source: www.christinedemerchant.com/carboncharacteristics.html.

A carbon–carbon composite material is made in three stages [57]. In the first stage, carbon filament or cloth, surrounded by an organic binder such as plastic or pitch, is first laid-up in its intended final shape. In the second stage, the lay-up is heated so that the binder undergoes pyrolysis to relatively pure carbon. This is, however, accompanied by the formation of voids. The void formation is reduced by the addition of coke or some other carbon aggregate in the initial lay-up but is not eliminated. In the third stage, the voids are gradually filled by forcing a carbon-forming gas such as acetylene through the material at a high temperature over several days. This long heat treatment process, which also helps carbon to form into larger graphite crystals, is the main reason for the high cost of carbon–carbon composite. The way the initial carbon fiber scaffold is laid up and the quality of the matrix filler strongly influence the properties of the final material such as hardness and thermal properties like resistance to thermal expansion, temperature gradients, and thermal cycling. 2.14.2.8 Aramid Fibers Aramid fiber is the generic name for aromatic polyamide fibers. As defined by the U.S. Federal Trade Commission, an aramid fiber is a “manufactured fiber in which the fiber forming substance is a long chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings”: O

O H

n (I)

n (II)

Among the commercially available aramid fibers are DuPont’s Nomex (I) and Kelvar (II); in fact these trade names are commonly used in lieu of the generic name. Kelvar 49 is a high-modulus aramid fiber and is the most widely used reinforcing aramid fiber. Kevlar 29 has a lower modulus and Kevlar 149 has a higher modulus than Kevlar 49. Aromatic polyamides are described in greater detail in Chapter 4. Aramid fibers can be used to advantage to obtain composites having lighter weight, greater stiffness, higher tensile strength, higher impact resistance, and lower notch sensitivity than composites incorporating E-glass or S-glass reinforcement. Weight savings over glass result from the lower specific gravity of aramid fibers, 90.4 lb/in.3 (1.45 g/cm3) versus E-glass, 159.0 lb/in.3 (2.55 g/cm3). Higher stiffnesses are reflected in a Young’s modulus up to 19 × 106 psi (1.31 × 105 MPa) for Kevlar 49 and 27 × 106 psi (1.86 × 105 MPa) for Kevlar 149, compared to 107 psi (6.9 × 104 MPa) for E-glass and 12 × 106 psi (8.6 × 104 MPa) for S-glass. Aramid composites are more insulating than their glass counterparts, both electrically and thermally, more damped to mechanical and sonic vibrations, and are transparent to radar and sonar.

Fabrication Processes

235

Despite their outstanding mechanical properties, these high-modulus organic fibers have the processability normally associated with conventional textiles. This leads to wide versatility in the form of the reinforcement, e.g., yarns, rovings, woven and knit goods, felts, and papers. 2.14.2.9 Applications Fabrics woven from Kevlar 49 aramid fiber are often used as composite reinforcement, since fabrics offer biaxial strength and stiffness in a single ply. The mechanical properties of Kevlar 49 aramid are dependent on the fabric construction. The composite properties are functions of the fabric weave and the fiber volume fraction (typically 50%–55% with ply thickness 5–10 mils, depending on fabric construction). In 1987, DuPont introduced high-modulus Kevlar 149. Compared to Kevlar 49 it has higher performance (47% modulus increase) and lower dielectric properties (65% decrease in moisture regain). Over the past three decades, Kevlar has gained wide acceptance as a fiber reinforcement for composites in many end uses, such as tennis rackets, golf clubs, shafts, skis, ship masts, and fishing rods. The boating and aircraft industries make extensive use of advanced composites. The advanced composites have allowed innovative designers to move ahead in designing aircraft with unprecedented performance. The cost of high-modulus aramid fibers is higher than E-glass and equivalent to some grades of S-glass on a unit-weight basis. Price differences versus glass are, however, reduced by about half on a unit volume basis when lower density of the aramids is taken into account. For many applications, fabrics containing more than one fiber type offer significant advantages. Hybrids of carbon and Kevlar 49 aramid yield greater impact resistance over all-carbon construction and higher compressive strength over all-Kevlar construction. Hybrids of Kevlar 49 and glass offer enhanced properties and lower weight than constructions containing glass as the sole reinforcement and are less expensive than constructions using only Kevlar 49 reinforcement. 2.14.2.10 Extended-Chain Polyethylene Fibers Extended-chain polyethylene (ECPE) fibers are relatively recent entrants into the high-performance fibers field. Spectra ECPE, the first commercially available ECPE fiber and the first in family of extended chain polymers manufactured by Allied-Signal, Inc. was introduced in 1985. ECPE fibers are arguably the highest modulus and highest strength fibers made. These are being utilized as a reinforcement in such applications as ballistic armor, impact shields, and radomes to take advantage of the fiber’s unique properties. Polyethylene is a flexible molecule that normally crystallizes by folding back on itself (see Chapter 1). Thus fibers made by conventional melt spinning do not possess outstanding physical properties. ECPE fibers, on the other hand, are made by a process that results in most of the molecules being fully extended and oriented in the fiber direction, producing a dramatic increase in physical properties. Using a simple analogy, the structure of ECPE fibers can be described as that of a bundle of rods, with occasional entangled points that tie the structure together. Conventional PE, by comparison, is comprised of a number of short-length chain folds that do not contribute to material strength (see Figure 2.59). ECPE fibers are, moreover, made from ultrahigh molecular weight polyethylene (UHMWPE) with molecular weight generally 1–5 million that also contributes to superior mechanical properties. Conventional PE fibers, in comparison, have molecular weights in the range 50,000 to several hundred thousand. ECPE fibers exhibit a very high degree of crystalline orientation (95%–99%) and crystalline content (60%–85%). High-modulus PE fibers can be produced by melt extrusion and solution spinning. The melt extrusion process leads to a fiber with high modulus but relatively low strength and high creep whereas solution spinning in which very high-molecular-weight PE is utilized yields a fiber with both high modulus and high strength. The solution spinning process for a generalized ECPE fiber starts with the dissolution of polyethylene of approximately 1–5 million molecular weight in a suitable solvent. This serves to disentangle the polymer chains, a key step in achieving an extended chain polymer structure. The solution must be fairly dilute to facilitate this process, but viscous enough to be spun using conventional melt spinning equipment. The cooling of the extrudate lends to the formation of a fiber that can be continuously dried to

236

Plastics Technology Handbook

Conventional ﬁber

Extended-chain ﬁber

(a)

(b)

FIGURE 2.59 Fiber morphology of polyethylene. (a) Conventional PE ﬁber characterized by relatively low molecular weight, moderate orientation, and crystalline regions chain folded. (b) Extended-chain PE ﬁber characterized by very high molecular weight, very high degree of orientation, and minimum chain folding.

remove solvent or later extracted by an appropriate solvent. The fibers are generally postdrawn prior to final packaging. The solution spinning process is highly flexible and can provide an almost infinite number of process and product variations of ECPE fibers. Fiber strengths of (3.75–5.60) × 105 psi (2,890–3,860 MPa) and tensile moduli of (15−30) × 106 psi [(103−207) × 103 MPa] have been achieved. The properties are similar to other high-performance fibers; however, because the density of PE is approximately two-thirds that of high modulus aramid and half that of high-modulus carbon fiber, ECPE fibers possess extraordinarily high specific strengths and specific moduli. Figure 2.60 compares the specific strength versus specific modulus for currently available fibers. Traditional binders and wetting agents are ineffective in improving resin adhesion to polyethylene. For ECPE fibers, this characteristic is actually advantageous in specific areas. For instance, ballistic performance is inversely related to the degree of adhesion between the fiber and the resin matrix. However, for applications requiring higher levels of adhesion and wetout, it has been shown that by submitting ECPE fiber to specific surface treatments, such as corona discharge or plasma treatments, the adhesion of the fiber to various resins can be dramatically increased. The chief application areas being explored and commercialized for ECPE fibers are divided between traditional fiber applications and high-tech composite applications. The former include sailcloth, marine ropes, cables, sewing thread, nettings, and protective clothing. The latter includes impact shields, ballistics, radomes, medical implants, sports equipment, pressure vessels, and boat hulls. ECPE fibers (such as Spectra 1,000) are well suited for high-performance yachting sails, offering, in addition, resistance to sea water and to typical cleaning solutions used in the sailing industry, such as

Speciﬁc strength (106 in.)

Spectra 1000 Spectra 900

Kevlar 29

‘S’ glass

Kevlar 149

Kevlar 49

‘E’ glass

HT graphite

Boron

HM graphite

Steel

FIGURE 2.60

100

300

500 700 Speciﬁc modulus (106 in.)

Comparative properties of various reinforcing ﬁbers.

900

Fabrication Processes

237

bleach. The major sport equipment applications to date have been canoes, kayaks, and snow and water skis. Numerous other sport applications are under development. The high-strength, lightweight, low-moisture absorption and excellent abrasion resistance of ECPE make it a natural candidate for marine rope. In marine rope applications, load, cycling, and abrasion resistance are critical. Thus a 12-strand ECPE braid, for example, is reported to withstand about eight times the number of cycles that cause failure in 12-strand aramid braid. Specially toughened and dimensionally stabilized ECPE yarn has been used in a revolutionary new line of cut-resistant products. ECPE fibers are being used to produce cut-resistant gloves, arm guards, and chaps in such industries as meat packing, commercial fishing, and poultry processing and in sheet metal work, glass cutting, and power tool use. ECPE’s high strength and modulus and low specific gravity offer higher ballistic protection at lower density per area than is possible with currently used materials. The significant applications include flexible and rigid armor. Flexible armor is manufactured by joining multiple layers of fabric into the desired shape, the ballistic resistance being determined by the style of the fabric and the number of layers. Traditional rigid armor can be made by utilizing woven ECPE fiber in either thermoset or thermoplastic materials. Ballistics are currently the dominant market segment. Products include helmets, helicopter seats, automotive and aircraft armor, armor radomes, and other industrial structures. The radome (radome protective domes) market is also important for ECPE fibers. ECPE composite systems act as a shield that is virtually transparent to microwave signals, even in high-frequency regimes.

2.15 Reaction Injection Molding A new type of injection molding called Reaction Injection Molding (RIM) has become important for fabricating thermosetting polymers [58,59]. RIM differs from the conventional injection molding in that the finished product is made directly from monomers or low-molecular-weight polymeric precursors (liquid reactants), which are rapidly mixed and injected into the mold even as the polymerization reaction is taking place. Thus, synthesis of polymers prior to molding is eliminated, and the energy requirements for handling of monomers are much less than those for viscous polymers. For RIM to be successful, the monomers or liquid reactants must be fast reacting, and the reaction rates must be carefully synchronized with the molding process. Thus the polymers most commonly processed by RIM are polyurethanes and nylons though epoxies, and certain other polymers such as polycyclopentadiene have been processed by RIM. The process uses equipment that meters reactants to an accuracy of 1%, mixes them by high-pressure impingement, and dispenses the mixture into a closed mold. The mold is, in fact, a chemical reactor. The reaction in RIM takes place in a completely filled mold cavity. Reinforcing fillers are sometimes injected into the mold along with the reactants, a process called reinforced reaction injection molding (RRIM). In the mold the functional groups of the liquid reactants react to form chemical linkages, producing solid polymers, which comprise polymeric chains or networks depending on the starting materials. The temperature of the mold plays a vital role in the polymerization of reactants. To produce a molded part by the RIM process requires precise but realistic process control. Figure 2.61 shows a simplified schematic of the RIM process. The important elements of the process are conditioning, metering, mixing, and molding. All the liquid reactants require precise temperature control. The flow property (viscosity) of the liquid reactants usually varies with temperature as does the density. For accurate metering the temperature must be controlled within very narrow limits. This is usually accomplished by recirculating reactants from conditioning or storage tanks designed to maintain raw material temperatures specified by the system supplier. These conditions are normally quite moderate (30°C–38°C) for polyurethanes. For some polymerization systems, such as nylon which is processed at a high temperature, the machines are designed with heated lines and temperature control devices for pumps, mixers, and other components. The molds are also designed to control temperatures as the reaction characteristics of the RIM process are exothermic. A higher temperature increases the reaction rate, which results in an decrease of cycle

238

Plastics Technology Handbook

Liquid component B

Recirculation

Liquid component A

Conditioning

Metering

Mixing Reaction/molding

FIGURE 2.61

Schematic of RIM process.

time. For polyurethane, however, speeding the reaction rate by operating the mold at elevated temperatures is to be avoided, as this changes the types of linkages produced (see “Polyurethanes” in Chapter 4). Polyurethane RIM systems have been commercial in the United States for about 60 years and a bit longer in Europe. It is still a rapidly growing ﬁeld of technology. The automotive industries in the United States account for most of the commercial RIM production. A later development for RIM polyurethane, and to a lesser extent RIM nylons, is the application for housings of various instruments and appliances: computer housings, business machine housings, TV and radio cabinets, instrument cases, and similar electronic product enclosures. While elastomeric RIM is most commonly used in these applications, some housings are also molded from RIM structural foam. Though systems suppliers do not always clearly differentiate between elastomeric and structural RIM, elastomeric RIM is molded in thin cross section (usually 0.125 in.) at high density while structural foam has an interior foam structure, a density about one-third that of elastomeric RIM, and is molded in thicker cross section (usually 0.375 in.).

2.15.1 Machinery Conditioning and temperature control are accomplished by recirculating reactants from storage tanks which are jacketed and/or contain tempering coils to maintain the process temperature required by the chemical system. RIM parts (especially polyurethanes) are usually removed from the mold before the chemical reactions that develop the physical properties are complete. The part is placed on a support jig that holds it in its final shape until it is fully cured. In some cases this is done by simply setting the supported part aside for 12–24 h. More often the supported part is postcured in an oven for several hours at temperatures of about 180°F (82°C). Nylons are completely reacted in the mold and postcuring is not necessary. RIM polyurethanes made with aromatic isocyanates (such as pure or polymeric MDI) have a tendency to darken as a result of the effect of UV light on the chemical ring structure of the MDI component. Soft white limestone or fine carbon black is often used as filler to mask the effect of this color change. Polyurethanes manufactured with aliphatic isocyanates are light stable, and products are molded in a wide range of bright colors. Especially interesting is the development of equipment to add color concentrate, usually dispersed in a polyol, directly into the mixing head attached to a given mold. The basic urethane formula is adjusted to compensate for the additional reactive polyol. Using this technique with a

239

Fabrication Processes

multiclamp RIM line, it is possible to mold different colors using a single RIM machine. In some cases, aromatic RIM systems are molded in color then painted the same color. This technique eliminates the need to touch up every dent or scratch which would otherwise show up tan or white. There are several systems for painting RIM parts. Usually, RIM parts are simply primed and painted. Before painting, however, the part is cleaned to remove mold-release agents. The most common mold releases are metal stearates (or soaps) that can be removed from the part by a water wash and paraffin waxes that are usually removed by solvent vapor degreasing. Silicone mold releases are to be avoided as they are very difficult to remove from the part, and paint will not stick to the silicone surface film.

2.15.2 Polyurethanes The most common chemicals used in the RIM process for poyurethanes are isocyanates containing two or more isocyanate (–N═C═O) groups and polyols, which contain two or more hydroxyl (–OH) groups. These reactive end-groups, so named because they occur at the ends of the chemical structure, react chemically to form a urethane linkage:

(

)

The chemical system, must be adjusted so that the number of isocyanates and hydroxyls balance and that all reactive end-groups are used in the formation of urethane linkages. The number of polymer structures that can be formed using the urethane reaction is quite large. There are ways to produce polyurethanes having different physical properties (see “Polyurethanes” in Chapter 4). If linear polyols are reacted with diisocyanates, a flexible polyurethane will be formed. If a low boiling liquid, such as Refrigerant-11 (R-11), is incorporated into the system, the heat of reaction will produce a cellular structure. The resulting product will be flexible polyurethane foam. The physical properties of these materials can be varied by selecting polyols with shorter or longer polyol chains. The most common polyol or macroglycol chains are polyethers and polyesters (Chapter 4). The composition of these thermoplastic chains also plays a role in the physical properties of the end product. These chain segments in the block copolymer are often referred to as “soft” blocks, or segments, while the polyurethane segments formed by the reaction of diisocyante with glycol are referred to as “hard” blocks or segments. In addition to changing the chain composition and length, the physical properties can be varied by blending up to approximately 10% of a long-chain triol (such as a triol adduct of ethylene oxide and propylene oxide with glycerol) into the basic resin system formulation. This produces branching in the “soft” segment of the block copolymer. Excessive triol modification may, however, diminish physical properties. Use of short-chain triols such as glycerol will produce cross-linking in the “hard” segments of the polymer chain. The formation of hard blocks and cross-linking in the hard block tend produce a stiffer, more rigid product. The hard blocks tend to be crystalline and reinforce the amorphous polymer, improving its strength.

2.15.3 Nylons RIM nylons, like polyurethanes, form polymers very rapidly by the reaction of chemical end-groups. The linkages produced are as follows: Polyesteramide prepolymer + (CH2)5

CONH

Caprolactam

Nylon block copolymer

240

Plastics Technology Handbook

Equipment used to manufacture RIM products must be extensively rebuilt to process RIM nylons. Because the viscosity of nylon RIM systems is low and the ingredients are quite reactive, leakage at the seals and the volumetric efficiency of the metering pumps (that is, the amount of material actually pumped divided by the volume displaced by the metering pumps) may cause problems, which require special attention. Hence, most manufacturers recommend having the machine designed specifically for nylon RIM systems. The first commercial product made from nylon RIM was a front quarter panel (fender) for the Oldsmobile Omega Sport. Because of the excellent impact strength of nylon RIM, it has been used for bumper covers and automobile fascia. It also finds application in housings for business machines and electronics.

2.16 Structural Reaction Injection Molding Structural reaction injection molding (SRIM) may be considered as a natural evolution of RIM. It is a very attractive composite manufacturing process for producing large, complex structural parts economically. The basic concepts of the SRIM process are shown in Figure 2.62. A preformed reinforcement is placed in a closed mold, and a reactive resin mixture that is mixed by impingement under high pressure in a specially designed mix head (like that in RIM) flows at low pressure through a runner system to fill the mold cavity, impregnating the reinforcement material in the process. Once the mold cavity is filled, the resin quickly completes its reaction. A completed component can often be removed from the mold in as little as one minute. SRIM is similar to RIM in its intensive resin mixing procedures and its reliance on fast resin reaction rates. It is also similar to resin transfer molding (RTM) (discussed later in this chapter) in employing performs that are preplaced in the cavity of a compression mold to obtain optimum composite mechanical properties. The term structural is added to the term RIM to indicate the more highly reinforced nature of the composite components manufactured by SRIM. The key to SRIM is the perform. It is a preshaped, three-dimensional precursor of the part to be molded and does not contain the resin matrix. It can consist of fibrous reinforcements, core materials, metallic inserts, or plastic inserts. The reinforcements, cores, or inserts can be anything available that meets the economic, structural, and durability requirements of the parts. This tremendous manufacturing freedom allows a variety of alternative perform constructions. Most commercial SRIM applications have been in general industry or in the automotive industry. The reinforcement material most commonly used has been fiberglass, due to the low cost. Fiberglass has been used in the form of woven cloth, continuous strand mat, or chopped glass. Space-shaping cores can be used in the SRIM process to fabricate thick, three-dimensional parts with low densities. Specific grades of urethane-based foams, having densities of 6–8 lb/ft3 and dimensional stability at SRIM molding temperatures, are commonly used as molded core materials. Fiberglass reinforcements and inserts can be placed around these cores, resulting in SRIM parts, molded in one piece, that are very light-weight and structurally strong and stiff. Metallic inserts can be used in SRIM parts as local stiffeners, stressed attachment points, or weldable studs. The metallic material of choice is usually steel. SRIM is a very labor-intensive process, and the consistency from preform to preform is usually poor. However, for very low manufacturing volumes this process can be cost-effective. Most SRIM resins have several characteristics in common: their liquid reactants have roomtemperature viscosity below 200 cps; their viscosity-cure curves are sigmoidal in shape, the typical moldfill time being 10–90 sec; and their demold time is from 60 to 180 sec, varying with catalyst concentration. The low viscosity of SRIM resins and their relatively long fill times are crucial in allowing them to penetrate and flow through their reinforcing performs. The design of the gating and runner configuration (if any) is usually kept proprietary by the molder. However, it appears that most SRIM parts are center-gated, with vents located along the periphery of the part. This configuration allows the displaced air in the mold cavity to be expelled uniformly.

241

Fabrication Processes

Tank (component A)

Metering cylinder

Recirculation

Air pressure

Tank (component B)

Metering cylinder Static mixer

Mixing head Preform

FIGURE 2.62

Schematic of structural reaction injection molding (SRIM).

2.16.1 Applications The ability of SRIM to fabricate large, lightweight composite parts, consisting of all types of precisely located inserts and judiciously selected reinforcements, is an advantage that other manufacturing processes find difficult to match. Moreover, large SRIM parts can often be molded in 2–3 min, using clamping pressures as low as 100 psi. The capital requirements of SRIM are thus relatively low. The first commercially produced SRIM part was the cover of the sparetire well in several automobiles produced by General Motors. Since then, SRIM automotive structural parts have included foamed door panels, instrument panel inserts, sunshades, and rear window decks. Nonautomotive applications include satellite dishes and seat shells for the furniture market.

2.17 Resin Transfer Molding RTM is similar to SRIM. In its common form, RTM is a closed-mold, low-pressure process in which dry, preshaped reinforcement material is placed in a closed mold and a polymer solution or resin is injected at a low pressure, ﬁlling the mold and thoroughly impregnating the reinforcement to form a composite part. The mold pressure in the RTM process is lower than in both SRIM and RIM/RRIM and the molding cycle time is much longer. The reinforcement and resin may take many forms, and the low pressure combined with the preoriented reinforcement package, affords a large range of component sizes, geometries, and performance options. RTM is an excellent process choice for making prototype components. It allows representative prototypes to be molded at low cost, unlike processes such as compression molding and injection molding, which require tools and equipment approaching actual production level. When prototyping with RTM, less reactive resins are generally used, allowing long ﬁll times and easier control of the vents. Sizes can range from small components to very large, complex, three-dimensional

242

Plastics Technology Handbook

structures. RTM provides two ﬁnished surfaces and controlled thickness, while other processes used for prototyping, such as hand lay-up and wet molding, give only a single ﬁnished surface.

2.18 Foaming Processes Plastics can be foamed in a variety of ways. The foamed plastics, also referred to as cellular or expanded plastics, have several inherent features which combine to make them economically important. Thus, a foamed plastic is a good heat insulator by virtue of the low conductivity of the gas (usually air) contained in the system, has a higher ratio of flexural modulus to density than when unfoamed, has greater loadbearing capacity per unit weight, and has considerably greater energy-storing or energy-dissipating capacity than the unfoamed material. Foamed plastics are therefore used in the making of insulation, as core materials for load-bearing structures, as packaging materials used in product protection during shipping, and as cushioning materials for furniture, bedding, and upholstery. Among those plastics which are commercially produced in cellular form are polyurethane, PVC, polystyrene, polyethylene, polypropylene, epoxy, phenol-formaldehyde, urea-formaldehyde, ABS, cellulose acetate, styrene-acrylonitrile, silicone, and ionomers. However, note that it is possible today to produce virtually every thermoplastic and thermoset material in cellular form. In general, the basic properties of the respective polymers are present in the cellular products except, of course, those changed by conversion to the cellular form. Foamed plastics can be classified according to the nature of cells in them into closed-cell type and opencell type. In a closed-cell foam each individual cell, more or less spherical in shape, is completely closed in by a wall of plastic, whereas in an open-cell foam individual cells are inter-connecting, as in a sponge. Closed-cell foams are usually produced in processes where some pressure is maintained during the cell formation stage. Free expansion during cell formation typically produces open-cell foams. Most foaming processes, however, produce both kinds. A closed-cell foam makes a better buoy or life jacket because the cells do not fill with liquid. In cushioning applications, however, it is desirable to have compression to cause air to flow from cell to cell and thereby dissipate energy, so the open-cell type is more suitable. Foamed plastics can be produced in a wide range of densities—from 0.1 lb/ft.3 (0.0016 g/cm3) to 60 lb/ft.3 (0.96 g/cm3)—and can be made flexible, semirigid, or rigid. A rigid foam is defined as one in which the polymer matrix exists in the crystalline state or, if amorphous, is below its Tg. Following from this, a flexible cellular polymer is a system in which the matrix polymer is above its Tg. According to this classification, most polyolefins, polystyrene, phenolic, polyycarbonate, polyphenylene oxide, and some polyurethane foams are rigid, whereas rubber foams, elastomeric polyurethanes, certain polyolefins, and plasticized PVC are flexible. Intermediate between these two extremes is a class of polymer foams known as semirigid. Their stress–strain behavior is, however, closer to that of flexible systems than to that exhibited by rigid cellular polymers. The group of rigid cellular polymers can be further subdivided according to whether they are used (1) for non-load-bearing applications, such as thermal insulation; or as (2) load-bearing structural materials, which require high stiffness, strength and impact resistance. The description of cellular foams as low, medium or high density is very common in practice. This is, however, not exact as the different density ranges which correspond to each of these items are not strictly defined. The following figures can, however, serve as a rough general guide: lb/ft.3

Kg/m3

Low density

0.1–3

2–50

Medium density

3–21

50–350

High density

21–60

350–960

243

Fabrication Processes

(Note that the density of a polymer foam refers to its bulk density, deﬁned by the ratio of total weight/total volume of the polymer and gaseous component. Obviously the gas phase contributes considerably to the volume of the end product, while the solid component contributes almost to the entire weight.) Obtained forms of foamed plastics are blocks, sheets, slabs, boards, molded products, and extruded shapes. These plastics can also be sprayed onto substrates to form coatings, foamed in place between walls (i.e., poured into the empty space in liquid form and allowed to foam), or used as a core in mechanical structures. It has also become possible to process foamed plastics by conventional processing machines like extruders and injection-molding machines. Polymer foams may be homogeneous with a uniform cellular morphology throughout or they may be structurally anisotropic. They may have an integral solid polymer skin or they may be multicomponent in which the polymer skin is of different composition to the polymeric cellular core. Schematic representations of the different physical forms of cellular polymers are given in Figure 2.63. Some special types of foams, namely, structural foams, reinforced foams, and syntactic foams are represented by Figure 2.63c to Figure 2.63f. These are described in a later section. Foaming of plastics can be done in a variety of ways. Most of them typically involve creating gases to make foam during the foaming cycle. Once the polymer has been expanded or “blown,” the cellular structure must be stabilized rapidly; otherwise it would collapse. Two stabilization methods are used. First, if the polymer is a thermoplastic, expansion is carried out above the softening or melting point, and the form is then immediately cooled to below this temperature. This is called physical stabilization. The second method—chemical stabilization—requires the polymer to be cross-linked immediately following the expansion step. Common foaming processes are the following: 1. Air is whipped into a dispersion or solution of the plastic, which is then hardened by heat or catalytic action or both. 2. A low-boiling liquid is incorporated in the plastic mix and volatilized by heat.

(a)

(b)

(c)

(d)

(e)

(f )

FIGURE 2.63 Schematic representations of section through different types of cellular polymer. (a) Low-density open-cell foam. (b) High-density closed-cell foam. (c) Single-component structural foam with cellular core and integral solid skin. (d) Multicomponent structural foam. (e) Fiber-reinforced closed-cell foam. (f) Syntactic foam.

244

Plastics Technology Handbook

3. Carbon dioxide gas is produced within the plastic mass by chemical reaction. 4. A gas, such as nitrogen, is dissolved in the plastic melt under pressure and allowed to expand by reducing the pressure as the melt is extruded. 5. A gas, such as nitrogen, is generated within the plastic mass by thermal decomposition of a chemical blowing agent. 6. Microscopically small hollow beads of resin or even glass (e.g., microballoons) are embedded in a resin matrix. Foams can be made with both thermoplastic and thermosetting plastics. The well known commercial thermoplastic foams are polystyrene, PVC, polyethylene, polypropylene, ABS copolymer, cellulose acetate. The thermosetting plastics which may be mentioned, among others, are phenol-formaldehyde, urea-formaldehyde, polyurethane, epoxy, and silicone. The methods of manufacture of some of these polymeric foams are given below.

2.18.1 Rigid Foam Blowing Agents There are four types of polymers typically used for rigid foam production, namely, polystyrene, polyurethane, polyolefin, and phenolic. Within the polyolefin segment, rigid foams can be produced using polyethylene or polypropylene. Following the implementation of the Montreal Protocol of 1989, chlorofluorocarbons (CFC-11 and CFC-12) which had been the primary blowing agents for both flexible and rigid foams, were no longer available. Hydrochlorofluorocarbons (HCFCs) were one of the primary blowing agents that were then adopted, specifically HCFC-141b, HCFC-142b, and HCFC-22. Insulating foam products (with some exceptions) generally utilize HCFCs due to the superior insulation properties that they impart. The non-ozone depleting (i.e., not CFCs or HCFCs) blowing agents that are currently in use and will be substituted for ozone depleting HCFCs as the latter being still ozone depletants are phased out are: (i) hydrofluorocarbons (HFCs); (ii) hydrocarbons (e.g., pentanes, butanes); and (iii) carbon dioxide. Non-insulating foam products typically utilize hydrocarbons, such as isobutane, pentane, isopentane, and hexane. The use of CO2 (either water-based or liquid) is a major identified option to reduce the emission of non-HCFC blowing agents from polyurethane foam and extruded polystyrene boardstock applications. However, the thermal insulation properties of CO2-blown foam are significantly compromised when compared to halocarbon-blown foam. Halocarbons (i.e., HCFCs, HFCs) are thus expected to be used in insulation foam manufacture for several years into the future. The primary HCFC replacements in these sectors are expected to be the liquid HFCs, which may see extensive use once HCFCs can no longer be used.

2.18.2 Polystyrene Foams Polystyrene, widely used in injection and extrusion molding, is also extensively used in the manufacture of plastic foams for a variety of applications. Polystyrene produces light, rigid, closed-cell plastic foams having low thermal conductivity and excellent water resistance, meeting the requirements of low-temperature insulation and buoyancy applications. Two types of low-density polystyrene foams are available to the fabricator, molder, or user: (1) extruded polystyrene foam and (2) expandable polystyrene for molded foam. 2.18.2.1 Extruded Polystyrene Foam This material is manufactured as billets and boards by extruding molten polystyrene containing a blowing agent (nitrogen gas or chemical blowing agent) under elevated temperature and pressure into the atmosphere where the mass expands and solidiﬁes into a rigid foam. Many sizes of extruded foam are available, some as large as 10 in. × 24 in. × 9 ft. The billets and boards can be used directly or cut into different forms. One of the largest markets for extruded polystyrene in the form of boards is in low-temperature insulation

245

Fabrication Processes

(e.g., truck bodies, railroad cars, refrigerated pipelines, and low-temperature storage tanks for such things as liquefied natural gas). Another growing market for extruded polystyrene boards is residential insulation. Such boards are also used as the core material for structural sandwich panels, used prominently in the construction of recreational vehicles. 2.18.2.2 Expandable Polystyrene Expandable polystyrene is produced in the form of free-flowing pellets or beads containing a blowing agent. Thus, pellets chopped from an ordinary melt extruder or beads produced by suspension polymerization are impregnated with a hydrocarbon such as pentane. Below 70°F (21°C) the vapor pressure of the pentane dissolved in the polymer is low enough to permit storage of the impregnated material (expandable polystyrene) in a closed container at ordinary temperature and pressure. Even so, manufacturers do not recommend storing for more than a few months. The expandable polystyrene beads may be used in a tabular blow-extrusion process (Figure 2.64) to produce polystyrene foam sheet, which can subsequently be formed into containers, such as egg cartons and cold-drink cups, by thermoforming techniques. Expandable polystyrene beads are often molded in two separate steps: (1) Preexpansion or prefoaming of the expandable beads by heat, and (2) further expansion and fusion of the preexpanded beads by heat in the enclosed space of a shaping mold. Steam heat is used for preexpansion in an agitated drum with a residence time of a few minutes. As the beads expand, a rotating agitator prevents them from fusing together, and the preexpanded beads, being lighter, are forced to the top of the drum and out the discharge chute. They are then collected in storage bins for aging prior to molding. The usual lower limit of bulk density for bead preexpansion is 1.0 lb/ft.3 (0.016 g/cm3), compared to the virgin bead bulk density of about 35 lb/ft.3 (0.56 g/cm3). Molding of preexpanded (prefoamed) beads requires exposing them to heat in a confined space. In a typical operation (Figure 2.65) prefoamed beads are loaded into the mold cavity, the mold is closed, and steam is injected into the mold jacket [60]. The prefoamed beads expand further and fuse together as the temperature exceeds Tg. The mold is cooled by water spray before removing the molded article. Packages shaped to fit their contents (small sailboats, toys, drinking cups, etc.) are made in this way. Special machines have been designed to produce thin-walled polystyrene foam cups. Very small beads at a

Pinch rolls

Low density foam bubble

Tubular die

FIGURE 2.64

Expandable polystyrene beads

Extruder

Tubular blow extrusion for production of low-density polystyrene foam sheet.

246

Plastics Technology Handbook

Filling: mold ﬁll

Fusion: steam on

Steam

Drains open

Cool: water spray on

Steam

Drains closed

Water

Part ejection: mold open

Water

Drains open

FIGURE 2.65 Molding of preexpanded (prefoamed) polystyrene beads. (Adapted from PELASPAN Expandable Polystyrene, Form 171–414, Dow Chemical Co., 1966.)

prefoamed density of approximately 4–5 lb/ft.3 (0.06–0.08 g/cm3) are used, which allow easy flow into the molding cavity and produce a cup having the proper stiffness for handling. 2.18.2.3 Structural Foams Structural foam is the term usually used for foam produced in an injection molding press and made of almost many thermoplastic resin. Structural foam is always produced with a hard integral skin on the outer surfaces and a cellular core in the interior, and is used almost exclusively for production of molded parts. The process is thus ideally suited for fabrication of parts such as business machine housings (commonly for ABS), and similar parts or components in which lightweight and stiffness are required. The structural foam injection molding process (Figure 2.66), by which a product with a cellular core and a solid skin can be molded in a single operation, gets its name from the application of its product rather than the mechanism of the process itself. In a manner directly opposite to the vented extruder (Figure 2.21), a blowing agent, often nitrogen, is injected into the melt in the extruder. The polymer melt, injected with gas, is then forced into the accumulator where it is maintained at a pressure and temperature high enough to prevent foaming (Figure 2.66a). When a sufficient charge has accumulated it is transferred into the mold (Figure 2.66b). The melt foams and fills the mold at a relatively low pressure (1.3–2.6 MPa) compared to the much higher pressure in the accumulator. The lower operating pressures of the molds make the molds less expensive than those used for conventional injection molding. However, the cycle times are longer because the foam being a good insulator, takes longer time to cool. Structural foams can also be made using a chemical blowing agent (discussed later) rather than an inert gas. In that case, a change in pressure or temperature on entering the mold triggers gas formation. Today structural foam injection molding is a very fast-growing polymer processing technique that can be used to modify the properties of thermoplastics to suit specific applications.

2.18.3 Polyolefin Foams Polyolefin foams can be produced with closely controlled density and cell structure. Generally the mechanical properties of polyolefins lies between those of a rigid and a flexible foam. Polyolefin foams have a very good chemical and abrasion resistance as well as good thermal insulation properties. Crosslinking improves foam stability and polymer properties. A variety of foams can be produced from various types of polyethylenes and cross-linked systems having a very wide range of physical properties, and foams can be tailor-made to a specific application.

247

Fabrication Processes

Accumulator Blowing agent

Mold

Valve

Extruder

Hydraulic press (a)

Accumulator Blowing agent Mold

Valve

(b)

Extruder

Hydraulic press

FIGURE 2.66 Structural foam process. (a) Filling the accumulator. The blowing agent (usually nitrogen) is injected into the melt in the extruder before it is passed into the accumulator. (b) Filling the mold. The accumulator ram injects the melt into the mold where the reduced pressure allows the gas to foam the resin.

Polypropylene has a higher thermostability than polyethylene. The production volume of polyolefin foams is not as high as that of polystyrene, polyurethane, or PVC foams. This is due to the higher cost of production and some technical difficulties in the production of polyolefin foams. The structural foam injection molding process, described previously for polystyrene, is also used for polyethylene and polypropylene structural foams (see Figure 2.66). Commercial extrusion processes for polyolefin foam products are derived from the original Dow process which basically involves five steps, namely, extrusion, mixing, cooling, expansion, and aging (see Figure 2.67). These steps of the extrusion process may be performed on equipment of several different configurations such as single-screw extruders, twin-screw extruders, and tandem-extruder lines. Singlescrew extruders must be equipped with a multistage long screw of high length-to-diameter ratio capable of performing all the aforesaid extrusion steps. Twin-screw extruders, on the other hand, have low shear rate and high mixing ability both of which are desirable in foam extrusion. Except where the extrusion rate is low such as for products having a small cross section, most polyolefin foam products are made with tandem-type extruders, as shown in Figure 2.68. The primary extruder consisting of a two-stage screw melts the resin and then mixes the melt with the solid additives and liquid blowing agent, whereas the second extruder, usually larger than the primary one and designed to provide maximum cooling efficiency, cools the molten polymer mixture to the optimum foaming temperature. In some equipment, however, the second extruder is designed to perform both as mixer for the blowing agent and as cooler. An alternative to large tandem extruders is the accumulating extrusion system which provides a high instantaneous extrusion rate. It is commonly employed for producing large plank products. In the simple system, shown in Figure 2.69, the foamable melt is fed into an accumulator by a single screw extruder and

248

Plastics Technology Handbook

Aging

FIGURE 2.67

Expansion

Cooling

Blowing agent

Polymer, solid additives

Mixing

Extrusion

Block diagram of polyoleﬁn foam manufacture by extrusion process. Blowing agent

Primary extruder

Cooling extruder

FIGURE 2.68

Polymer, solid additives

Schematic diagram of tandem extruder.

Primary extruder

Hydraulic cylinder Accumulator

FIGURE 2.69

Sliding gate

Schematic of accumulation extrusion system.

pushed out by a ram through a die orifice. The process is discontinuous, resulting in a loss of yield, but it has the advantage of low capital requirement. The shape of a polyolefin foam product is determined largely by the shape of the die. Thus a circular die is used for a rod, an annular die for a tube or sheet product, and a slit die for a plank product. To make polyolefin foam sheet, the extruded tubular foam, expanding at the annular die, is guided over a sizing mandrel, slit, laid flat, and wound into a roll [61]. Cool air is blown in at the nose of the mandrel to reduce the friction between the hot expanding foam and the mandrel. The extruded polyolefin foam must be dimensionally stabilized by aging, since the foam deforms according to the internal cell pressure, which changes with time as air and gaseous blowing agent diffuse into and out of the foam at different rates. If the rates are equal, the cell pressure, and hence the foam dimensions, will remain constant, as is found for the LDPE/CFC-114 system. Most blowing agents, however, permeate through polyolefins faster than air and, as a result, the foams shrink. The aging time required for the shrunken foam to recover and stabilize depends on the properties of the polymer and the

249

Fabrication Processes

physical attributes of the foam, such as the open-cell content, foam density, and foam thickness. The aging time may range from less than a week for a thin sheet to several weeks for a thick plank. The production line of a typical process to manufacture thin ultra-low-density (ULD) polypropylene foam sheet, consists of tandem extruders, an accumulating vessel, and an annular die. The secondary extruder is designed to mix a large amount of blowing agent into the polymer and then to cool the mixture. The blowing agent consists of a large proportion (90%) of a highly soluble blowing agent which provides the heat sink necessary for foam stabilization and a small proportion (10%) of a low-permeability blowing agent which serves as an inflatant. The accumulating extrusion system allows the high extrusion rate required for the production of ULD foam sheet. There are several processes for the production of moldable polyolefin beads. In the BASF process, LDPE foam strands are extruded out of a multi-hole die and granulated to beads by a die-face cutter. Inexpensive butane is used as the blowing agent and the foam beads are then cross-linked by electronbeam. As the beads have atmospheric cell pressure, a special technique is required to develop the necessary cell pressure for molding [62]. In the Kanegafuchi process, most widely used to manufacture LDPE foam beads, dicumyl peroxide is impregnated into finely pelletized LDPE beads suspended in water in an autoclave with the help of a dispersant such as basic calcium tertiary phosphate and sodium dodecylbenzene sulfonate. The beads are then heated to cross-link. The cross-linked beads are impregnated with a suitable blowing agent (e.g., a non-ozone depleting replacement of CFC-12), cooled, discharged from the autoclave and immediately expanded with steam to make foam beads. For molding, the foam beads are charged into a mold and heated with superheated steam (>140 kPa) to expand and weld. The majority of cross-linked polyolefin foam sheet products are made by one of the four Japanese processes: Sekisui, Toray, Furukawa, and Hitachi, the first two of which use the radiation method and the latter two a chemical method for cross-linking. The flow diagram of the radiation cross-linked polyolefin foam sheet process is shown in Figure 2.70. The key steps of the process include a uniform mixing of polymer and blowing agent (powder), manufacturing void-free sheet of uniform thickness, cross-linking the sheet to the desired degree by irradiation with a high-energy ray, and then softening and expanding the sheet in a foaming chamber (oven) using a suitable support mechanism. The Sekisui process employs a vertical air oven like the one shown in Figure 2.71 for expanding the foamable sheet. The oven consists of a horizontal preheating chamber and a vertical foaming chamber. The rapidly expanding sheet supports itself by gravity in the vertical direction, while a specially designed tentering device keeps the sheet spread out. In the Toray process, the foamable sheet is expanded while afloat on the surface of molten salts. The process is suitable for producing crosslinked PP foam sheet as well as PE foam sheet. The flow diagram of the chemically cross-linked polyolefin foam sheet process is shown in Figure 2.72. Unlike in the radiation cross-linking process, a peroxide cross-linking agent is incorporated in the polymer along with the blowing agent. Therefore, a tighter temperature control must be maintained in

Oven heating

Foaming

Electron beam irradiation

Crosslinking

Polyethylene

Sheet forming

Mixing

Blowing agent

FIGURE 2.70

Flow diagram of radiation cross-linked polyoleﬁn foam sheet forming process.

250

Plastics Technology Handbook

Infrared heaters

Preheating chamber (150°)

Foamable sheet

Infrared heaters Foaming chamber (200°) Hot air

Hot air

Sheet tentering device

FIGURE 2.71

Schematic of Sekisui vertical foaming oven for cross-linked polyoleﬁn foam sheet.

Polyethylene Two-stage oven Foaming

Crosslinking

Sheet forming

Mixing

Cross-linking agent Blowing agent

FIGURE 2.72

Flow diagram of chemically cross-linked polyoleﬁn foam sheet forming process.

the sheet manufacturing steps to prevent premature cross-linking by the peroxide. In the oven, on the other hand, the cross-linking of the polyolefin sheet must be thermally effected without causing the blowing agent to decompose. Consequently, both the oven design and the selection of raw materials are more difficult in the chemical cross-linking process. Both the Furukawa and Hitachi processes employ horizontal air ovens consisting of at least two sections, the preheating section and the foaming/ forming section, and having one or more non-stick conveyors to support the sheet during heating and expansion [61]. Polyolefin foams have many and varied applications due to their unique properties which include buoyancy, resiliency, energy absorption, low thermal conductivity, resistance to chemicals, thermoformability, and ease of fabrication. The major application areas of polyolefin foams are cushion packaging (pads and saddles, encapsulation, case inserts, etc.), construction (expansion joint filler, closure strips, floor underlayment etc.), automotive (headliner, door trim, instrument panel, trunk liner, air conditioner liner,

251

Fabrication Processes

etc.), insulation (insulation of pipe, storage tanks), and sports and leisure (life vests, surfboards, swim aids, ski belts, gym mats, etc.). Thin polyoleﬁn foam sheet products are used primarily as wrapping materials to protect the surfaces of articles from minor dents and abrasion during handling and shipping.

2.18.4 Polyurethane Foams Polyurethane foams, also known as urethane foams or U-foams, are prepared by reacting hydroxylterminated compounds called polyols with an isocyanate (see Figure 1.62). Isocyanates in use today include toluene diisocyanate, known as TDI, crude methylenebis(4-phenyl-isocyanate), known as MDI, and different types of blends, such as TDI/crude MDI. Polyols, the other major ingredient of the urethane foam, are active hydrogen-containing compounds, usually polyester diols and polyether diols. It is possible to prepare many different types of foams by simply changing the molecular weight of the polyol, since it is the molecular backbone formed by the reaction between isocyanate and polyol that supplies the reactive sites for cross-linking (Figure 1.62), which in turn largely determines whether a given foam will be flexible, semirigid, or rigid. In general, high-molecular-weight polyols with low functionality produce a structure with a low amount of cross-linking and, hence, a flexible foam. On the other hand, low-molecular-weight polyols of high functionality produce a structure with a high degree of cross-linking and, consequently, a rigid foam. Of course, the formulation can be varied to produce any degree of flexibility or rigidity within these two extremes. The reactions by which urethane foam are produced can be carried out in a single stage (one-shot process) or in a sequence of several stages (prepolymer process and quasi-prepolymer process.) These variations led to 27 basic types of products or processes, all of which have been used commercially. In the one-shot process, all of the ingredients—isocyanate, polyol, blowing agent, catalyst, additives, etc.—are mixed simultaneously, and the mixture is allowed to foam. In the prepolymer method (Figure 1.62), a portion of the polyol is reacted with an excess of isocyanate to yield a prepolymer having isocyanate end groups. The prepolymer is then mixed with additional polyol, catalyst, and other additives to cause foaming. The quasi-prepolymer process is intermediate between the prepolymer and one-shot processes. 2.18.4.1 Flexible Polyurethane Foams The major interest in flexible polyurethane foams is for cushions and other upholstery materials. Principal market outlets include furniture cushioning, carpet underlay, bedding, automotive seating, crash pads for automobiles, and packaging. The density range of flexible foams is usually 1–6 lb/ft.3 (0.016–0.096 g/cm3). The foam is made in continuous loaves several feet in width and height and then sliced into slabs of desired thickness. 2.18.4.1.1 One-Shot Process The bulk of the flexible polyurethane foam is now being manufactured by the one-shot process using polyether-type polyols because they generally produce foams of better cushioning characteristics. The main components of a one-shot formulation are polyol, isocyanate, catalyst, surfactant, and blowing agent. Today the bulk of the polyether polyols used for flexible foams are propylene oxide polymers. The polymers prepared by polymerizing the oxide in the presence of propylene glycol as an initiator and a caustic catalyst are diols having the general structure

CH3

CH OH

CH2

O ( CH2

CH CH3

O )n CH2

CH OH

CH3

252

Plastics Technology Handbook

The polyethers made by polymerizing propylene oxide using trimethylol propane, 1,2,6-hexanetriol, or glycerol as initiator are polymeric triols. For example, glycerol gives HO ( C3H6O )n CH2

CH(OH)

CH2 ( C3H6O )n OH

The higher hydroxyl content of these polyethers leads to foams of better loadbearing characteristics. Molecular weight in the range 3,000–3,500 is found to give the best combination of properties. The second largest component in the foam formulation is the isocyanate. The most suitable and most commonly used isocyanate is 80:20 TDI—i.e., 80:20 mixture of tolylene-2,4-diisocyanate and tolylene2,6-diisocyanate. One-shot processes require sufficiently powerful catalysts to catalyze both the gas evolution and chain extension reaction (Figure 1.62). Use of varying combinations of an organometallic tin catalyst (such as dibutyltin dilaurate and stannous octoate) with a tertiary amine (such as alkyl morpholines and triethylamine), makes it possible to obtain highly active systems in which foaming and cross-linking reactions could be properly balanced. The surface active agent is an essential ingredient in formulations. It facilitates the dispersion of water (see below) in the hydrophobic resin by decreasing the surface tension of the system. In addition, it also aids nucleation, stabilizes the foam, and regulates the cell size and uniformity. A wide range of surfactants, both ionic and nonionic, have been used at various times. Commonly used among them are the watersoluble polyether siloxanes. Water is an important additive in urethane foam formulation. The water reacts with isocyanate to produce carbon dioxide and urea bridges (Figure 1.62). An additional amount of isocyanate corresponding to the water present must therefore be incorporated in the foaming mix. The more water that is present, the more gas that is evolved and the greater number of active urea points for cross-linking. This results in foams of lower density but higher degree of cross-linking, which reduces flexibility. So when soft foams are required, a volatile liquid such a trichloromonofluoromethane (bp 23.8°C) may be incorporated as a blowing agent. This liquid will volatilize during the exothermic urethane reaction and will increase the total gas in the foaming system, thereby decreasing the density, but it will not increase the degree of crosslinking. However, where it is desired to increase the cross-link density independently of the isocyanatewater reaction, polyvalent alcohols, such as glycerol and pentaerythritol, and various amines may be added as additional cross-linking agents. A typical formulation of one-shot urethane foam system is shown in Table 2.4 [63]. Most foam is produced in block form from Henecke-type machines (Figure 2.73) or some modification of them. In this process [64], several streams of the ingredients are fed to a mixing head which oscillates in a horizontal plane. In a typical process, four streams may be fed to the mixing head: e.g., polyol and fluorocarbon (if any); isocyanate; water, amine, and silicone; and tin catalyst. The reaction is carried out with slightly warmed components. Foaming is generally complete within a minute of the mixture emerging from the mixing head. The emergent reacting mixture runs into a trough, which is moving backward at right angles to the direction of traverse of the reciprocating mixing head. In this way the whole trough is covered with the foaming mass. Other developments of one-shot flexible foam systems include direct molding, where the mixture is fed into a mold cavity (with or without inserts such as springs, frames, etc.) and cured by heat. In a typical application, molds would be filled and closed, then heated rapidly to 300°F–400°F (149°C–204°C) to develop maximum properties. A good deal of flexible urethane foam is now being made by the cold-cure technique. This involves more reactive polyols and isocyanates in special foaming formulations which would cure in a reasonable time to their maximum physical properties without the need for additional heat over and above that supplied by the exothermic reaction of the foaming process.

253

Fabrication Processes TABLE 2.4 Urethane One-Shot Foam Formulation Ingredient

Parts by Weight

Poly(propylene oxide), mol. wt. 2000 and 2OH/molecule

35.5

Poly(propylene oxide) initiated with trifunctional alcohol, mol. wt. 3000 and 3 OH/molecule

35.5

Toluene diisocyanate (80:20 TDI) Dibutyltin dilaurate

26.0 0.3

Triethylamine

0.05

Water Surfactant (silicone)

1.85 0.60

Trichloromonoﬂuoromethane (CCl3F)

12.0

Final density of foam = 1.4 lb/ft.3 or 0.022 g/cm3 (2.0 lb/ft.3 or 0.032 g/cm3 if CCl3F is omitted) Source: One-Step Urethane Foams, 1959. Bull, F40487, Union Carbide Corp.

Motor Mixing head

Reciprocating motion

Deposited foam

Direction of motion

Trough in motion Stationary frame

FIGURE 2.73 shot process.

Schematic of a Hanecke-type machine for production of polyurethane foam in block form by one-

Cold-cure foaming is used in the production of what is known as high-resilient foams having high sag factor (i.e., ratio of the load needed to compress foam by 65% to the load needed to compress foam by 25%), which is most important to cushioning characteristics. True cold-cure foams will produce a sag factor of 3–3.2, compared to 2–2.6 for hot-cured foams. 2.18.4.1.2 Prepolymer Process In the prepolymer process the polyol is reacted with an excess of isocyanate to give an isocyanateterminated prepolymer which is reasonably stable and has less handling hazards than free isocyanate. If water, catalysts, and other ingredients are added to the product, a foam will result. For better load-bearing and cushioning properties, a low-molecular-weight triol, such as glycerol and trimethylolpropane, is added to the polyol before it reacts with the isocyanate. The triol provides a site for chain branching. Although the two-step prepolymer process is less important than the one-shot process, it has the advantage of low exotherms, greater ﬂexibility in design of compounds, and reduced handling hazards.

254

Plastics Technology Handbook

2.18.4.1.3 Quasi-Prepolymer Process In the quasi-prepolymer process a prepolymer of low molecular weight and hence low viscosity is formed by reacting a polyol with a large excess of isocyanate. This prepolymer, which has a large number of free isocyanate groups, is then reacted at the time of foaming with additional hydroxy compound, water, and catalyst to produce the foam. The additional hydroxy compound, which may be a polyol or a simple molecule such as glycerol or ethylene glycol, also functions as a viscosity depressant. The system thus has the advantage of having low-viscosity components, compared to the prepolymer process, but there are problems with high exotherms and a high free-isocyanate content. Quasi-prepolymer systems based on polyester polyols and polyether polyols are becoming important in shoe soling, the former being most wear resistant and the latter the easiest to process. 2.18.4.2 Rigid and Semirigid Foams The flexible foams discussed in previous sections have polymer structures with low degrees of crosslinking. Semirigid and rigid forms of urethane are products having higher degree of cross-linking. Thus, if polyols of higher functionality—i.e., more hydroxyl groups per molecule—are used, less flexible products may be obtained, and in the case of polyol with a sufficiently high functionality, rigid foams will result. The normal density range for rigid and semirigid foams is about 1–3 lb/ft.3 (0.016–0.048 g/cm3). Some packaging applications, however, use densities down to 0.5 lb/ft.3 (0.008 g/cm3); for furniture applications densities can go as high as 20–60 lb/ft.3 (0.32–0.96 g/cm3), thus approaching solids. At densities of from 2 lb/ft.3 (0.032 g/cm3) to 12 lb/ft.3 (0.19 g/cm3) or more, these foams combine the best of structural and insulating properties. Semirigid (or semiflexible) foams are characterized by low resilience and high energy-absorbing characteristics. They have thus found prime outlet in the automotive industry for applications like safety padding, arm rests, horn buttons, etc. These foams are cold cured and involve special polymeric isocyanates. They are usually applied behind vinyl or ABS skins. In cold curing, the liquid ingredients are simply poured into a mold in which vinyl or ABS skins and metal inserts for attachments have been laid. The liquid foams and fills the cavity, bonding to the skin and inserts. Formulations and processing techniques are also available to produce self-skinning semirigid foam in which the foam comes out of the mold with a continuous skin of the same material. Rigid urethane foams have outstanding thermal insulation properties and have proved to be far superior to any other polymeric foam in this respect. Besides, these rigid foams have excellent compressive strength, outstanding buoyancy (flotation) characteristics, good dimensional stability, low water absorption, and the ability to accept nails or screws like wood. Because of these characteristics, rigid foams have found ready acceptance for such applications as insulation, refrigeration, packaging, construction, marine uses, and transportation. For such diverse applications several processes are now available to produce rigid urethane foam. These include foam-in-place (or pour-in-place), spraying, molding, slab, and laminates (i.e., foam cores with integral skins produced as a single unit). One-shot techniques can be used without difficulty, although in most systems the reaction is slower than with the flexible foam, and conditions of manufacture are less critical. Prepolymer and quasi-prepolymer systems were also developed in the United States for rigid and semirigid foams, largely to reduce the hazards involved in handling TDI where there are severe ventilation problems. In the foam-in-place process a liquid urethane chemical mixture containing a fluorocarbon blowing agent is simply poured into a cavity or metered in by machine. The liquid flows to the bottom of the cavity and foams up, filling all cracks and corners and forming a strong seamless core with good adhesion to the inside of the walls that form the cavity. The cavity, of course, can be any space, from the space between two walls of a refrigerator to that between the top and bottom hull of a boat. However, if the cavity is the interior of a closed mold, the process is known as molding.

255

Fabrication Processes

Rigid urethane foam can be applied by spraying with a two-component spray gun and a urethane system in which all reactants are incorporated either in the polyol or in the isocyanate. The spraying process can be used for applying rigid foam to the inside of building panels, for insulating cold-storage rooms, for insulating railroad cars, etc. Rigid urethane foam is made in the form of slab stock by the one-shot technique. As in the Henecke process (Figure 2.73), the reactants are metered separately into a mixing head where they are mixed and deposited onto a conveyor. The mixing head oscillates in a horizontal plane to insure an even deposition. Since the foaming urethane can structurally bond itself to most substrates, it is possible (by metering the liquid urethane mixture directly onto the surface skin) to produce board stock with integral skins already attached to the surface of the foam. Sandwich-construction building panels are made by this technique.

2.18.5 Foamed Rubber Although foamed rubber and foamed urethanes have many similar properties, the processes by which they are made differ radically. In a simple process a solution of soap is added to natural rubber (NR) latex so that a froth will result on beating. Antioxidants, cross-linking agents, and a foam stabilizer are added as aqueous dispersions. Foaming is done by combined agitation and aeration with automatic mixing and foaming machines. The stabilizer is usually sodium silicoﬂuoride (Na2SiF6). The salt hydrolyzes, yielding a silica gel which increases the viscosity of the aqueous phase and prevents the foam from collapsing. A typical cross-linking agent is a combination of sulfur and the zinc salt of mercaptobenzothiazole (accelerator). Cross-linking (curing or vulcanization) with this agent takes place in 30 min at 100°C. When making a large article such as a mattress, a metal mold may be ﬁlled with the foamed latex and heated by steam at atmospheric pressure. After removing the foamed rubber article from the mold, it may be dewatered by compressing it between rolls or by centrifuging and by drying with hot air in a tunnel dryer. In foamed rubber formulation a part of the NR latex can be replaced by a synthetic rubber latex. One such combination is shown in Table 2.5.

2.18.6 Epoxy Resins Any epoxy resin can be made foamable by adding to the formulation some agent that is capable of generating a gas at the curing temperature prior to gelation. Such foaming agents may be low-boiling liquids which vaporize on heating (e.g., CFCs such as Freons) or blowing agents that liberate a gas when TABLE 2.5 Foamed Rubber Formulation Ingredient

Parts by Weight

Styrene-butadiene latex (65% solids)

123

Natural rubber latex (60% solids) Potassium oleate

33 0.75

Sulfur

2.25

Accelerators Zinc diethyldithiocarbamate

0.75

Zinc salt of mercaptobenzothiazole

1.0

Trimene base (reaction product of ethyl chloride, formaldehyde, and ammonia) Antioxidant (phenolic)

0.8 0.75

Zinc oxide

3.0

Na2SiF6

2.5

Source: Stern, H. J. 1967. Rubber: Natural and Synthetic, Palmerton, New York.

256

Plastics Technology Handbook

heated above 70°C, such as 2,2′-azobis(isobutyronitrile) or sulfonylhydrazide, which decompose evolving nitrogen. A foaming gas can also be generated in situ by adding a blowing agent that reacts with amine (curing agent) to form the gas. A typical such system consists of an epoxy resin, a primary amine (hardener), and a hydrogen-active siloxane (blowing agent). The siloxane reacts with the amine, evolving hydrogen as a foaming gas. Instead of using amines it is also possible to use other hydrogen-active hardeners such as phenols and carboxylic acids. The reaction of gas evolution occurs immediately after mixing resin and hardeners and before the mixture begins to cure in the mold. This is essential for the formation of closed-cell foam structure during the curing, which takes place under a definite expansion pressure against the mold wall, leading to formation of dense casting. In the production of expanded laminates of sandwich configuration, this yields very tough and impact-resistant structures. Expanded materials with excellent high-temperature properties are obtained when cresol novolacs are used as hardeners. A typical formulation is based on mixtures of bisphenol A resins and epoxy novolac resins which are cured by cresol novolacs and accelerated by suitable nitrogen-containing agents. Applications of epoxy foams can be categorized in three areas, namely, (1) unreinforced materials, (2) glass-fiber-reinforced materials, and (3) sandwich constructions [61]. Because of their light weight and absence of shrinkage, foamed epoxies are used in the production of large-scale patterns. Having excellent dielectric properties, epoxy foams find applications in electronics such as for casting and sealing electronic components like small transformers and capacitors, and in insulating cables. The light weight properties of foamed epoxies are utilized in fiber-reinforced materials for which glass fiber mats and unidirectional rovings are most suitable, with the majority of applications involving sandwich constructions. A few practical examples are foamed epoxy windsurfing board, epoxy rotor blades for wind energy generators, and automotive spoilers.

2.18.7 Urea-Formaldehyde Foams Urea-formaldehyde (UF) foams are basically two-component systems as the production of the foams requires mainly a UF resin and a foam stabilizing agent. The UF resin (see Chapter 4) is produced by the condensation of urea and formaldehyde in the range of mole ratios of 2: 3–1: 2 in the presence of alkaline catalysts (pH∼8), which yield only short-chain oligomers, and under weakly acidic conditions (pH 4–6), which result in a higher molecular weight mixture of oligomers (solubilized by attached methylol groups). Though numerous substances have been proposed as foam-stabilizing agents for the commercial foam system, aqueous solutions of the sodium salts of dodecylbenzene sulfonic acid and dibutylnaphthalene sulfonic acid have proven to be of value. Aqueous solutions of strongly dissociating organic and inorganic acids with a pH range of 1–1.5 are used as hardening agents. However, phosphoric acid is preferred because of its negligible corrosive action. The hardening agent is preferentially added to the foam stabilizing agent and the concentration of the hardener is chosen so that the final foam gels within 30–90 sec at 20–25°C after mixing the components. In a stationary process for making UF foams, a mixture of water and foam-stabilizing agent is introduced into a vessel equipped with tubular vanes. Foaming takes place upon feeding air and a ureaformaldehyde resin solution (see “Urea-Formaldehyde Resin” in Chapter 5). The generated foam is guided by the action of the tubular vanes to the outlet channel of the vessel from where the foam exits as a rectangular slab and is transported on a conveyor belt until the foam structure hardens sufficiently. Blocks are cut from the foam slab, dried at about 40°C for about 2 h, and then pressed mechanically into sheets of the required dimensions [61]. For on-site production of foam, the raw materials are transported by pumps into a foaming machine. A dispersion of foam stabilizing agent is formed in water in the machine and the resin is introduced into the mixing chamber through jets. The finished foam emerges from the plastic pipe and can be used immediately at the site. One thousand liters (1 m3) of foam can be produced on-site from 20 liters of resin and 18 liters of foam solution.

257

Fabrication Processes

The UF foam plastics are open-cell cellular materials with the capability of absorbing oils and solvents. UF foam is non-toxic, nonflammable, and stable with respect to almost all organic solvents, light and heavy mineral oils, but is decomposed by dilute and concentrated acids and alkalis. It exhibits extraordinary aging stability, has good sound-absorbing properties, and has the lowest thermal conductivity, despite the open cells. These properties coupled with its light weight (bulk density 11 kg/m3) and low manufacturing cost, make UF foam suitable for a wide spectrum of applications, including industrial filling materials, insulating in enclosed cavities, plant substrates (soil-free cultivation), and medical applications. UF foam is used on-site to fill cavities of all shapes and sizes, whether of natural origin or resulting from construction wall or other applications. It has been successfully employed in mining for over 50 years. The complete filling of cavities eliminates the hazard of methane accumulation and reduces the danger of fire and explosion. UF foam filling is an inexpensive, rapid, and excellent heat-insulated lightweight construction method. When insulation is retrofitted, the foam is introduced into the existing cavities through sealable small holes. When UF foam is formed, formaldehyde is released. It is important to make sure that the proper ratio of components is employed and suitable construction measures are taken, as otherwise the problems of formaldehyde release from foam over short term or long term may be encountered. With present day technologies, it is possible to satisfy strict conditions that a formaldehyde level of 0.1 ppm should not be exceeded in the air of a room used continuously for dwelling purposes. Geo- and hydroponics, as well as plastoponics are terms referring to the cultivation and breeding of plants with foam as flakes or in solid form. Beans, potatoes, carrots, tomatoes and ornamental plants have been grown extensively in foam. Its suitability for land recovery in desert and semi-desert regions has been established through extensive testing.

2.18.8 Silicone Foams Silicone foams result from the condensation reaction between ≡SiH and ≡SiOH shown below: ≡ SiH + ≡ SiOH + Catalyst ! ≡ Si – O – Si ≡ +H2

(2.5)

When these three components (that is, ≡SiH-containing cross-linker, ≡SiOH-containing polymer, and catalyst) are mixed together, both blowing (generation of hydrogen gas) and curing or cross-linking, that is, formation of siloxane linkage (≡Si–O–Si≡) occur. It should be noted, however, that a cross-linked product forms if the functionality of the ≡SiH-containing component is 3 or greater and the ≡SiOH-containing compound has a functionality of at least 2. These reactions are heat accelerated, but they occur readily at room temperature in the presence of catalyst. These RTV (room temperature vulcanizing) foams are thus two-pack systems. Generally, the ≡SiH (such as methylhydrogen siloxane) and catalyst make up the second component. A variety of catalysts can be used to promote the reaction. Chloroplatinic acid or other soluble platinum compound is most commonly used because it imparts ﬂame retardancy to the formulation. If a vinyl endblocked polymethylsiloxane is used in place of polydimethylsiloxane in the above formulation, then another competing reaction can also occur as shown below: ≡ SiH+ ≡ SiCH = CH2 + Catalyst ! ≡ Si – CH2 – CH2 – Si ≡

(2.6)

The reaction is also catalyzed by platinum. The addition of some vinyl-containing polysiloxane can thus improve properties such as density, tensile strength, cure rate, and so forth. Although hydrogen generation [Equation 2.5] is the most prevalent method of blowing silicone foam, there are other approaches. Adding a gas at a high enough pressure (so its volume is low before it expands) is an easy way to make foam. Gases commonly used are N2, CO2, while previously air-pressurized liqueﬁed gases such as CFCs were used. One may also use chemical blowing agents (see later), which decompose generating gas when heat is applied or pH is changed. Nitrogen-liberating organic blowing agents are used extensively for foaming silicone gum.

258

Plastics Technology Handbook

Silicones in general are inert to most environmental agents and have many unique properties (see “Silicones” in Chapter 4). When siloxane polymers are processed into foams they carry with them most of their durability characteristics and characteristic properties. Silicone foams are thus used in a wide range of applications. Flexible foam sheet is used in airplanes as the material for ﬁre blocking, insulation of air ducts, gasketing in engine housing compartments, and shock absorbers. Silicone foam is a popular choice in the construction industry, because of its weatherability, thermal insulation, and sealing capability.

2.18.9 Phenolic Foams Phenolic foam is a light weight foam created from phenolic resins. It is used in a large range of applications, such as flower foam blocks, building thermal insulation, fire protection, damping, and civil engineering in a wide variety of shapes: blocks, sheets, and sprayed foams. Phenolic foams are generally made using a resol-type phenolic resin (or a resin blend typically containing 60% of a resol-type resin and 40% of a resorcinol-modified novolac resin), surfactants, blowing agents, catalysts, and additives. Surfactants and additives are mixed into the resin and the blend is then mixed with the liquid blowing agent(s) and finally with an acid catalyst, e.g., H3PO4. Surfactants are used to control cell size and structure. The most common surfactants are siloxaneoxyalkylene copolymers, polyoxyethylene sorbitan fatty acid esters, and the condensation products of ethylene oxide with castor oil and alkyl phenols. A commonly added additive is urea which is used as a formaldehyde scavenger. Very fine particle size inorganic fillers can be added to act as nucleating sites and to promote finer, more uniform cell structure, as well as increased compressive strength, but at a cost of higher density. The most common blowing agents used for making phenolic foams are organic liquids that have boiling points approximately in the range 20°C–90°C. Suitable blowing agents include HFCs, HCFCs and others, and hydrocarbons having from about 3–10 carbon atoms such as pentane, hexane and petroleum ether. Hydrocarbons such as isopentane, isobutane and hexane are the preferred blowing agents for flower blocks. In insulating foam sector, however, the non-hydrocarbon blowing agent alternatives are not as viable and Foranil fluorochemical blowing agents have been mainly used for their insulating properties and non-flammability. Phenolic foams can also be prepared without the use of CFC or hydrocarbon blowing agents. In a typical preparation [65], resol 200, ethoxylated castor oil 8, boric anhydride accelerator 36, and SnCl2.2H2O 36 parts are mixed and heated for about 4 minute at 120°C to obtain a foam having density 0.003 g/cm3 and 29% closed cells. In another method [66], 1 mol phenol, 2.6 mol formaldehyde, and 5% dimethylaminoethanol are heated at 70°C–100°C for 4 h to obtain a liquid (70%–80% solids content) which is mixed with 2%–3% silicone foam regulator and 5 parts NaHCO3. Toluenesulfonic acid (20 parts, 80% aq.) is then added to obtain a stable rigid foam.

2.18.10 Poly(Vinyl Chloride) Foams A number of methods have been devised for producing cellular products from PVC, either by a mechanical blowing process or by one of several chemical blowing techniques. PVC foams are produced in rigid or flexible forms. The greatest interest in rigid PVC foam is in applications where low-flammability requirements prevail. It has an almost completely closed cell structure and therefore low water absorption. The rigid PVC foam is used as the cellular layer of some sandwich and multi‐layer panels. Plastisols are the most widely used route to flexible expanded PVC products. Dolls, gaskets, and resilient covers for tool handles, for example, are produced from expandable plastisol compounds by molding, while varied types of upholstery, garment fabrics, and foam layer in coated-fabric flooring are made from coatings with such compounds. Figure 2.74 shows a schematic representation of the German Trovipor process for producing flexible, mainly open cell, and low to medium density (60–270 kg/m3, 3.75–16.87 lb/ft.3) PVC foam. It is normally produced in the form of continuous sheet (Figure 2.74).

259

Fabrication Processes

PVC

Plasticizer Additives

(a) (b) Mixer Autoclave Paste feed container

Paste Gas stream Pump Cooler Perforated plate

Insert gas Spray tower (c)

Paste feed pump

To cutter Heating(hf )

After heating

Cooling tunnel

Conveyor belt

FIGURE 2.74 Schematic representation of the Trovipor process. (a) PVC paste preparation. (b) Gasiﬁcation of PVC paste. (c) Spraying and fusion.

Many chemical blowing (foaming) agents have been developed for cellular elastomers and plastics, which, generally speaking, are organic nitrogen compounds that are stable at normal storage and mixing temperatures but undergo decomposition with gas evolution at reasonably well-defined temperatures. Three important characteristics of a chemical blowing agent are the decomposition temperature, the volume of gas generated by unit weight (“gas number,” defined as the volume of gas, in cm3, liberated by the transformation of 1 g of the blowing agent per minute), and the nature of the decomposition products. Since nitrogen is an inert, odorless, nontoxic gas, nitrogen-producing organic substances are preferred as blowing agents. Several examples of blowing agents [67] especially recommended for vinyl plastisols are shown in Table 2.6; in each case the gas generated in nitrogen. To produce uniform cells, the blowing agent must be uniformly dispersed or dissolved in the plastisol and uniformly nucleated. It should decompose rapidly and smoothly over a narrow temperature range corresponding to the attainment of a high viscosity or gelation of the plastisol system. The gelation involves solvation of the resin in plasticizer at 300°F–400°F (149°C–204°C), the temperature depending on the ingredients employed in the plastisol. The foam quality is largely determined by the matching of the decomposition of the blowing agent to the gelation of the polymer system. If gelation occurs before gas evolution, large holes or fissures may form. On the other hand, if gas evolution occurs too soon before gelation, the cells may collapse, giving a coarse, weak, and spongy product. Among the blowing agents listed in Table 2.6, azobisformamide (ABFA) is the most widely used for vinyls because it fulfills the requirements efficiently. ABFA decomposition can also be adjusted through proper choice of metal organic activators so that the gas evolution occurs over a narrow range within the wide range given in Table 2.6. Though the gas number of ABFA is normally 220–260 cm3/g, it can go up to 420 cm3/g in the presence of catalysts. Azodicarbonamide is recommended for foaming of PVC, polyolefins, polyamides, polysiloxanes, epoxides, polymers and compolymers of acrylonitrile and acrylates, and rubbers. Diazoaminobenzene (DAB) is one of the first organic blowing agents to find industrial application. Its decomposition point (95°C–150°C) and gas number (115 cm3/g) depend on the pH of the medium; in

260

Plastics Technology Handbook

TABLE 2.6 Commercial Blowing (Foaming) Agents Chemical Type Azo compounds Azobisformamide (Azodicarbonamide) (ABFA) Azobisisobutyronitrile (AIBN)

Decomposition Temperature in Air (°C)

Decomposition Position Range in Plastics (°C)

Gas Yield (mL/g)

195–200

160–200

220

115

90–115

130

103

95–100

115

105

90–105

126

195

130–190

265

Benzenesulfonylhydrazide (BSH) Toluene-(4)-sulfonyl hydrazide (TSH)

>95 103

95–100 100–106

130 120

Benzene-1, 3-disulfonyl hydrazide (BDH)

146

115–130

4, 4′-oxybis (benzenesulfonylhydrazide) (OBSH)

150

120–140

125

Diazoaminobenzene (DAB) N-Nitroso compounds N, N′-Dimethyl-N, N′ -dinitrosoterephthalimide (DMTA) N, N′ -Dinitrosopentamethylenetetramine (DNPA) Sulfonyl hydrazides

Source: Lasman, H. R. 1967. Mod Plastics, 45, 1A, Encycl. Issue, 368.

acidic media it decomposes at lower temperature and more completely. DAB is used in foaming phenolic and epoxy resins, PVC, rubber and other high poymers. N,N′-Dinitrosopentamethylenetetramine (DNPA) is the cheapest (except for urea oxalate) and most widely used organic blowing agent accounting for 50% of all blowing agents used. It however disperses poorly in mixtures and is sensitive to shock and friction (explosive). Because of the relatively low temperature of decomposition, DTA can be used to make foams with a uniform cellular structure without deterioration of the polymer. The disadvantages of DTA are, however, poor dispersive ability in mixtures and sensitivity to moisture. Nevertheless, DTA is used in foaming PVC (especially for thin walled articles), polyurethane, polystyrene, polyamides, and siloxane rubbers. BSH is used for foaming rubbers, polystyrene, epoxy resins, polyamides, PVC, polyesters, phenolformaldehyde resins, and polyolefins. However, the thermal decomposition of BSH yields not only nitrogen but also a nontoxic residue (disulfide and thiosulfone) which may degrade to give thiophenol and thus an unpleasant odor to the foams. OBSH is one of the best blowing agents of the sulfonylhydrazide class. Its gas liberation characteristics (no stepwise change up to 140°C) makes it possible to obtain foams with small, uniform cells. It is nontoxic and does not impart color and smell to articles. OBSH is used for foaming PVC, polyolefins, polysulfides, microporous rubber, or foamed materials based on mixtures of polymers with rubbers. In the last case, OBSH acts additionally as a cross-linking agent. Closed-cell foams result when the decomposition and gelation are carried out in a closed mold almost filled with plastisol. After the heating cycle, the material is cooled in the mold under pressure until it is dimensionally stable. The mold is then opened, and the free article is again subjected to heat (below the previous molding temperature) for final expansion. Protective padding, life jackets, buoys, and floats are some items made by this process. The blowing agents given in Table 2.6 can be used to make foamed rubber. A stable network in this product results from the cross-linking reaction (vulcanization), which thus corresponds to the step offusion in the case of plastisols. Some thermoplastics also can be foamed by thermal decomposition of blowing agents even though they do not undergo an increase in dimensional stability at an elevated temperature. In

Fabrication Processes

261

this case the viscosity of the melt is high enough to slow down the collapse of gas bubbles so that when the polymer is cooled below its Tm a reasonably uniform cell structure can be built in. Cellular polyethylene is made in this way.

2.18.11 Special Foams Some special types of foams are: (1) structural foams; (2) syntactic foams and multifoams; and (3) reinforced foams. Structural foams (Figure 2.63c and d), which possess full-density skins and cellular cores, are similar to structural sandwich constructions or to human bones, which have solid surfaces but cellular cores. Structural foams may be manufactured by high pressure processes or by low-pressure processes (Figure 2.66). The first one may provide denser, smoother skins with greater fidelity to fine detail in the mold than may be true of low-pressure processes. Fine wood detail, for example, is used for simulated wood furniture and simulated wood beams. Surfaces made by low-pressure processes may, however, show swirl or other textures, not necessarily detracting from their usefulness. Almost any thermoplastic or thermosetting polymer can be formulated into a structural foam. In the case of syntactic foams (or spheroplastics), instead of employing a blowing agent to form bubbles in the polymer mass, hollow spherical particles, called microspheres, microcapsules, or microballoons, are embedded in a matrix of unblown polymer. (In multifoams, microspheres are combined with a foamed polymer to provide both kinds of cells.) Since the polymer matrix is not foamed, but is filled mechanically with the hollow spheres, syntactic materials may also be thought of as reinforced or filled plastics, with the gas-containing particles being the reinforcing component. Synthetic wood, for instance, is provided by a mixture of polyester and small hollow glass spheres (microspheres). The cellular structure of the syntactic foam depends on the size, quantity, and distributive uniformity of the microspheres. Since the microspheres have continuous shells, the final material will, as a rule, have completely enclosed cells, and thus can be called absolute foamed plastics or “absolute” closed-cell foams. This, together with the absence of microstructural anisotropy (because the microspheres have practically all the same size and are uniformly distributed in the matrix), gives a syntactic material its valuable properties. They have better strength-to-weight ratios than conventional foamed plastics, absorb less water, and can withstand considerable hydrostatic pressures. Using hollow sphere means that the final material is lighter than one containing a compact filler, such as glass powder, talc, kaolin, quartz meal, or asbestos. Figure 2.75a and b are graphical presentations of syntactic foam structure in which the two components, microspheres and resin fill completely the whole volume (no dispersed voids) and the density of the product is thus calculated from the relative proportion of the two. Measured density values often differ from the calculated ones due to the existence of some isolated or interconnected irregularly shaped voids, as shown in Figure 2.75c. The voids are usually an incidental part of the composite, as it is not easy to avoid their formation. Nevertheless, voids are often introduced intentionally to reduce the density below the minimum possible in a close-packed two-phase structure. Syntactic foams exhibit their best mechanical behavior in the compressive mode. The spheres themselves are an extremely strong structure and hence can withstand such stresses very well. Syntactic materials consisting of hollow glass microspheres in epoxy resin are used for sandwich structures and as potting compounds for high-density electronic modules and other units likely to encounter hydrostatic pressures. Hollow glass microspheres and powdered aluminum in resin are used as core materials for sandwich construction and radomes. Hollow glass microspheres in aluminum matrix are used for aerospace and extreme hydrostatic pressure (oceanographic) applications in view of low weight and high compressive strength. Polymer foams may be reinforced, usually with short glass fibers, and also other fibers such as asbestos or metal, and other reinforcements such as carbon black. The reinforcing agent is generally introduced into the basic components and is blown along with them, to form part of and to reinforce the walls of the cells (Figure 2.63e). When this is done, it is not unusual to obtain increases in mechanical properties of 400–500% with fiberglass content up to 50% by weight, especially in thermosettings. The principal

262

Plastics Technology Handbook

(a)

(b)

(c)

FIGURE 2.75 Graphical representations of syntactic foam structures. (a) Two-phase composite with random dispersion of spheres. (b) Two-phase composite with hexagonal close-packed structure of uniform sized spheres (74% by vol.). (c) Three-phase composite containing packed microspheres, dispersed voids and binding resin.

advantages of reinforcement, in addition to increased strength and stiffness, are improved dimensional stability, resistance to extremes of temperature and resistance to creep. Two processes for the manufacture of glass-reinforced foam laminates used as building materials, namely, free-rise process and restrained rise process are presented in Figure 2.76 and Figure 2.77. The glass fiber reinforcement is a thin (0.25–1.25 mm) mat supplied in roll form. It consists of layers of relatively long (1.5–4 m) glass fibers, the fibers in one layer being at an acute angle to the fibers in each next adjacent layer. A small amount of silane-modified polyester, or other binder is present, at a level of 2%–10% by weight. This type of glass mat is relatively porous to the passage of liquids and is also capable of expanding within a mixture of rising foam chemicals to provide a uniform, three-dimensional reinforcing network within the final foamed laminate. The glass fiber reinforcement is functionally effective when used at levels of 4–24 g per board foot of the laminate [68]. Various types of facing sheets may be

Isocyanate, surfactant, blowing agent

Top facing sheet

Polyol

Catalyst

Glass ﬁber mat Guillotine knife

Mixing head 65 –120°C

Matering rolls

Oven

Bottom facing sheet

FIGURE 2.76

Schematic of a free-rise process for manufacture of glass-reinforced foam laminates.

263

Fabrication Processes

Isocyanate, surfactant, blowing agent

Top facing sheet

Polyol

Catalyst

Glass ﬁber mat Guillotine knife

Mixing head

Matering rolls

Oven

Bottom facing sheet

FIGURE 2.77

Schematic of a restrained rise process for manufacture of glass-reinforced foam laminates.

used, such as aluminum foil for building insulation products, asphalt-saturated felts for roof insulation or any other material, e.g., paper and plastic ﬁlms.

2.19 Rapid Prototyping/3D Printing Rapid prototyping [69,70] refers to the process of fabricating a physical model from 3D digital data by using additive manufacturing (also known as 3D printing) technology. A 3D printer, which is a type of industrial robot, lays successive layers of materials onto a build tray to create a 3D object that can be of almost any shape and geometry. It enables designers to cost-effectively translate their ideas into 3D models for concept evaluation and improvement. Using a multi-material 3D printer, functional parts of machines, vehicles, or equipment can be rapidly prototyped and tested in real working conditions, thereby enabling form, fit, and functional testing to improve design and product quality. Additive manufacturing (AM) is the name to describe technologies like 3D printing that build 3D objects by adding layer upon layer of material, whether the material is plastic, metal, or concrete. The terms 3D printing and additive manufacturing are synonymous umbrella terms for all AM technologies [71]. Common to AM technologies is the use of a computer, 3D modeling software (computer-aided design or CAD), machine equipment, and layering material. In a typical AM process, once a CAD sketch (model) is produced, it is processed by a software called “slicer,” which converts the model into a series of thin layers and produces a G-code file containing instructions tailored to a specific type of 3D printer. The AM equipment (3D printer) reads in data from the CAD file or G-code file and lays down or adds successive layers of liquid, powder, sheet material, or other, in a layer-upon-layer fashion to fabricate a 3D object [71]. In a 3D printing process involving the use of thermoplastic polymer, the latter is injected through indexing nozzles onto a platform. The nozzles trace the cross-section pattern for each particular layer with the molten polymer hardening before the application of the next layer. The process is repeated until the build or model is completed. The printer resolution describes layer thickness and X–Y resolution in dots per inch (DPI) or micrometers (µm). Typical layer thickness is around 100 µm (250 DPI), although some

264

Plastics Technology Handbook

machines can print layers as thin as 16 µm (1,600 DPI). The particles (3D dots) are around 50 to 100 µm (500 to 250 DPI) in diameter [72]. [It may be mentioned that while the term 3D printing originally referred to a process employing standard and custom inkjet print heads, the technology used by most 3D printers to date is fused deposition modeling, a special application of plastic extrusion (see below).] A large number of additive processes are now available. The main differences between processes are in the way layers are deposited to create parts and in the materials that are used. In some methods, the material is softened or melted to produce the layers, for example, selective laser melting (SLM), direct metal laser sintering (DMLS), selective laser sintering (SLS), fused deposition modeling (FDM), or fused filament fabrication (FFF), while others cure liquid materials into solid using different technologies, such as stereolithography based on photopolymerization [72]. In FDM, the model or part is produced by extruding small beads of material that harden immediately to form layers. A thermoplastic filament or metal wire is unreeled from a coil to supply material to an extrusion nozzle head (3D printer extruder). The nozzle head heats the material and turns the flow on or off, while stepper or servo motors are used to move the extrusion head and adjust the flow. The printer usually has three axes of motion. Polyjet 3D printing is similar to inkjet printing, but instead of jetting drops of ink onto paper, Polyjet 3D printers jet tiny droplets of liquid photopolymer onto a build tray. The tiny droplets are instantly UV-cured, forming fine layers that accumulate on the build tray to create a precise 3D model or part. Where overhangs or complex shapes require support, the 3D printer jets a removable gel-like support material that can be easily removed by hand or with water. Models, prototypes, parts, or tooling thus produced are ready to use and do not need post-curing. With 16-micron layer resolution and accuracy as high as 0.1 mm, a Polyjet 3D printer can produce thin walls and complex geometries using a wide range of materials. High-resolution plastic prototypes with fine details and moving parts can be produced via 3D printer and many different polymers can be used, including acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), polylactic acid (PLA), high-density polyethylene (HDPE), PC/ABS, polyphenylenesulfone (PPSU), and high-impact polystyrene (HIPS). In general, the polymer is in the form of a filament fabricated from virgin resins [72]. The prototypes made by AM or 3D printing can be subjected to rigorous product testing to gain confidence in the design before investigating in an injection mold tool. Plastic prototypes can accurately reflect what is achievable with injection molding. The use benefits lie in the design freedoms that the additive approach engenders. Its immense design flexibility gives the potential of multifunctional components and structures. Typical industrial applications are in automotive, aerospace, and prosthetic medical fields. Developing products frequently require multiple materials that can make the development process and rapid prototyping, in particular, complicated. The Objet process of Rutland Plastics (www.sys-uk.com) that uses multiple materials simultaneously overcomes this problem. The Objet Connex printer offers the unique ability to print parts and assemblies made of multiple materials, with different mechanical properties, all in a single build. With this particular 3D printing and rapid prototyping process it is also possible to undertake low-volume production. Traditional techniques like injection molding are usually less expensive than 3D printing for manufacturing polymer products in high quantities, but AM can be faster and less expensive, in addition to being more flexible, when producing relatively small quantities of parts [72]. Three-dimensional printers, moreover, give designers and concept development teams the ability to produce parts and concept models using a desktop-size printer. The first 3D printer designed to operate in zero gravity, Zero-G Printer, was built in 2014 under a joint partnership between NASA and the Marshall Space Flight Center (MSFC). Its applications for space offer the ability to print parts or tools on site in space instead of using rockets to bring pre-manufactured items to the site of use. Three-dimensional printing of nanosized objects can also be performed. It uses microelectronic device fabrication methods. Such printed objects are typically grown on a solid substrate (e.g., silicon carbide).

Fabrication Processes

265

2.20 Rubber Compounding and Processing Technology 2.20.1 Compounding Ingredients No rubber becomes technically useful if its molecules are not cross-linked, at least partially, by a process known as curing or vulcanization [73–76]. For NR and many synthetic rubbers, particularly the diene rubbers, the curing agent most commonly used is sulfur. But sulfur curing takes place at technically viable rates only at a relatively high temperature (>140°C) and, moreover, if sulfur alone is used, optimum curing requires use of a fairly high dose of sulfur, typically 8–10 parts per hundred parts of rubber (phr), and heating for nearly 8 h at 140°C. Sulfur dose has been substantially lowered, however, with the advent of organic accelerators. Thus, incorporation of only 0.2–2.0 phr of accelerator allows reduction of sulfur dose from 8–10 to 0.5–3 phr and effective curing is achieved in a time scale of a few minutes to nearly an hour depending on temperature (100°C–140°C) and type of the selected accelerator. The low sulfur dose required in the accelerated sulfur vulcanization has not only eliminated bloom (migration of unreacted sulfur to the surface of the vulcanizate), which was a common feature of the earlier nonaccelerated technology, but also has led to the production of vulcanizates of greatly improved physical properties and good resistance to heat and aging. The selection of the accelerator depends largely on the nature of the rubber taken, the design of the product, and the processing conditions. It is important to adopt a vulcanizing system that not only gives a rapid and effective cross-linking at the desired vulcanizing temperatures but also resists premature vulcanization (scorching) at somewhat lower temperatures encountered in such operations as mixing, extrusion, calendaring, and otherwise shaping the rubber before final cross-linking. This may require the use of delayed-action type accelerators as exemplified by sulfenamides. Other principal types of accelerators with different properties are guanidines, thiazoles, dithiocarbamates, thiurams, and xanthates (Table 2.7). Accelerators are more appropriately classified according to the speed of curing induced in their presence in NR systems. In the order of increasing speed of curing, they are classified as slow, medium, semiultra, and ultra accelerators. The problem of scorching or premature vulcanization is very acute with ultra or fast accelerators. Rubber stocks are usually bad conductors of heat and therefore flow of heat to the interior of a vulcanizing stock from outside is very slow. As a result, in thick items the outer layers may reach a state of overcuring before the core or interior layers begin to cure. For such thick items, a slow accelerator (Table 2.7) is most suitable. For butyl and EPDM rubbers, which have very limited unsaturations, slow accelerators are, however, unsuitable and, fast accelerators should be used at high temperatures for good curing at convenient rates. Since butyl rubber is characterized by reversion a phenomenon of decrease of tensile strength and modulus with time of cure after reaching a maximum, duration of heating at curing temperatures must be carefully controlled, and prolonged heating must be avoided. For rubbers with higher degree of unsaturations, an ideal accelerator is one that is stable during mixing, processing, and storage of the mix, but that reacts and decomposes sharply at the high vulcanization temperature to effect fast curring. These requirements or demands are closely fulfilled by the delayedaction accelerators typical examples of which are given in Table 2.7. The spectacular effects of modern organic accelerators in sulfur vulcanization of rubber are observed only in the presence of some other additives known as accelerator activators. They are usually twocomponent systems comprising a metal oxide and a fatty acid. The primary requirement for satisfactory activation of accelerator is good dispersibility or solubility of the activators in rubber. Oxides of bivalent metals such as zinc, calcium, magnesium, lead, and cadmium act as activators in combination with stearic acid. A combination of zinc oxide and stearic acid is almost universally used. (Where a high degree of transparency is required, the activator may be a fatty acid salt such as zinc stearate.) Besides speeding up the rate of curing, activators also bring about improvements in physical properties of vulcanizates. This is highlighted by the data given in Table 2.8.

266

Plastics Technology Handbook

TABLE 2.7 Accelerator for Sulfur Vulcanization of Rubbers Accelerator Type and Formula

Chemical Name

Accelerator Activity

Guanidines Diphenyl guanidine (DPG)

Medium accelerator

Triphenyl guanidine (TPG)

Slow accelerator

Mercaptobenzothiazole (MBT)

Semi-ultra accelerator

Mercaptobenzothiazyl disulﬁde (MBTS)a

Semi-ultra (delayed action) accelerator

N-Cyclohexyl benzothiazyl sulfenamide (CBS)

Semi-ultra (delayed action) accelerator

N-Oxydiethylenebenzothiazyl sulfenamide (NOBS) or 2-Morpholinothiobenzothiazole (MBS)

Semi-ultra (delayed action) accelerator

N-t-Butylbenzothiazyl sulfenamide (TBBS)

Semi-ultra (delayed action) accelerator

Zinc diethyl Dithiocarbamate (ZDC)

Ultra accelerator

NH C

NH Thiazoles

N C

S N

N C S

S Sulfenamides

C S

N C

S NH

S N C

C4H9

S Dithiocarbamates C 2H 5

C 2H 5

Zn2+

S– 2

(Continued)

267

Fabrication Processes TABLE 2.7 (CONTINUED) Accelerator Type and Formula

C 2H 5

Accelerator for Sulfur Vulcanization of Rubbers Chemical Name

Accelerator Activity

Sodium diethyl dithiocarbamate (SDC)

Ultra accelerator, water soluble (used for latex)

Tetramethyl thiuram disulﬁde (TMTD, TMT)a

Ultra accelerator

Tetraethyl thiuram disulﬁde (TETD, TET)a

Ultra accelerator

Tetramethyl thiuram monosulﬁde (TMTM)

Ultra accelerator

Sodium isopropyl xanthate (SIX)

Ultra accelerator, water soluble (suited for latex)

Zinc isopropyl xanthate (ZIX)

Ultra accelerator

S– Na+

C 2H 5 Thiuram sulﬁdes

CH3 N

CH3 N CH3

CH3 C2H5

S S

C2H5 N C2H5

C2H5 CH3 N

CH3

S C

N CH3

CH3 Xanthates

CH3 CH

S– Na+

CH3 CH3 CH

S O

CH3 a

Zn2+

S– 2

Sulfur donors.

A sulfur-curing system thus has basically four components: a sulfur vulcanizing agent, an accelerator (sometimes combinations of accelerators), a metal oxide, and a fatty acid. In addition, in order to improve the resistance to scorching, a prevulcanization inhibitor such as N-cyclohexylthiophthalimide may be incorporated without adverse effects on either the rate of cure or physical properties of the vulcanizate. The level of accelerator used varies from polymer to polymer. Some typical curing systems for the diene rubbers (NR, SBR, and NBR) and for two oleﬁn rubbers (IIR and EPDM—see Appendix A2 for abbreviations) are given in Table 2.9. In addition to the components of the vulcanization system, several other additives are commonly used with diene rubbers. Rubbers in general, and diene rubbers in particular, are blended with many more additives than is common for most thermoplastics with the possible exceptions of PVC. The major additional classes of additives are 1. Antidegradants (antioxidants and antiozonants) 2. Processing aids (peptizers, plasticizers, softeners, and extenders, tackiﬁers, etc.) 3. Fillers

268

Plastics Technology Handbook TABLE 2.8 Effect of Activator on Vulcanization Tensile Strength, Psi (MPa) ZnO (phr) Time of Cure (min)

0.0

5.0

100 (0.7)

2300 (16)

400 (2.8)

2900 (20)

60 90

1050 (7.2) 1300 (9.0)

2900 (20) 2900 (20)

Note: Base compound: NR (pale creep) 100; sulfur 3; mercaptobenzothiazole (MBT) 0.5. Temperature of vulcanization 142°C.

TABLE 2.9 Components of Sulfur Vulcanization Systems Rubber a

Additive (phr)

SBR

NBR

IIR

EPDM

Sulfur

2.5

2.0

1.5

2.0

1.5

Zinc oxide Stearic acid

5.0 2.0

5.0 1.0

3.0 2.0

5.0 1.0

TBBS

0.6

1.0

–

MBTS MBT

– –

1.0 –

0.5 –

– 1.5

TMTD

–

0.1

1.0

0.5

See Table 2.7 for accelerator abbreviations.

4. Pigments 5. Others (retarders, blowing agents) The use of antioxidants and antiozonants has already been described in Chapter 1. 2.20.1.1 Processing Aids Peptizers are added to rubber at the beginning of mastication (see later) and are used to increase the efficiency of mastication. They act chemically and effectively at temperatures greater than 65°C and hasten the rate of breakdown of rubber chains during mastication. Common peptizers are zinc thiobenzoate, zinc-2-benzamidothiophenate, thio-b-naphthol, etc. Processing aids other than the peptizer and compounding ingredients (additives) are added after the rubber attains the desired plasticity on mastication. Common process aids, besides the peptizer, are pine tar, mineral oil, wax, factice, coumarone-indene resins, petroleum resins, rosin derivatives, and polyterpenes. Their main effect is to make rubber soft and tacky to facilitate uniform mixing, particularly when high loading of carbon black or other fillers is to be used. Factice (vulcanized oil) is a soft material made by treating drying or semidrying vegetable oils with sulfur monochloride (cold or white factice) or by heating the oils with sulfur at 140–160°C (hot or brown factice). The use of factice (5–30 phr) allows efficient mixing and dispersion of powdery ingredients and gives a better rubber mix for the purpose of extrusion. Ester plasticizers (phthalates and phosphates) that are used to plasticize PVC (see Chapter 1) are also used as process aids, particularly with NBR and CR. Polymerizable plasticizers such as ethylene glycol dimethacrylate are particularly useful for peroxide curing of rubbers. They act as plasticizers or tackifiers during mixing and undergo polymerization by peroxide initiation during cure.

Fabrication Processes

269

The diene hydrocarbon rubbers are often blended with hydrocarbon oils. The oils decrease polymer viscosity and reduce hardness and low temperatures brittle point of the cured product. They are thus closely analogous to the plasticizers used with thermoplastics but are generally known as softners. Three main types of softners are distinguished: aliphatic, aromatic, and naphthenic. The naphthenics are preferred for general all-round properties. NRs exhibit the phenomenon known as tack. Thus when two clean surfaces of masticated rubber are brought into contact the two surfaces strongly adhere to each other, which is a consequence of interpenetration of molecular ends followed by crystallization. Amorphous rubbers such as SBR do not display such tack and it is necessary to add tackifiers such as rosin derivatives and polyterpenes. 2.20.1.2 Fillers The principles of use of inert fillers, pigments, and blowing agents generally follow those described in Chapter 1. Major fillers used in the rubber industry are classified as (1) nonblack fillers such as china clay, whiting, magnesium carbonate, hydrated alumina, anhydrous, and hydrated silicas and silicates including those in the form of ground mineral such as slate powder, talc, or French chalk, and (2) carbon blacks. Rather peculiar to the rubber industry is the use of the fine particle size reinforcing fillers, particularly carbon black. Fillers may be used from 50 phr to as high as 100–120 phr or even higher proportions. Their use improves such properties as modulus, tear strength, abrasion resistance, and hardness. They are essential with amorphous rubbers such as SBR and polybutadiene that has little strength without them. They are less essential with strain-crystallizing rubbers such as NR for many applications but are important in the manufacture of tires and related products. Carbon blacks are essentially elemental carbon and are produced by thermal decomposition or partial combustion of liquid or gaseous hydrocarbons to carbon and hydrogen. The principal types, according to their method of production, are channel black, furnace black, and thermal black. Thermal black is made from natural gas by the thermatomic process in which methane is cracked over hot bricks at a temperature of 1,600°F (871°C) to form amorphous carbon and hydrogen. Thermal black consists of relatively coarse particles and is used principally as a pigment. A few grades (FT and MT referring to fine thermal and medium thermal) are also used in the rubber industry. Most of the carbon black used in the rubber industry is made by the furnace process (furnace black), that is, by burning natural gas or vaporized aromatic hydrocarbon oil in a closed furnace with about 50% of the air required for complete combustion. Furnace black produced from natural gas has an intermediate particle size, while that produced from oil can be made in wide range of controlled particle sizes and is particularly suitable for reinforcing rubbers. Quite a variety of grades of furnace blacks are available, e.g., fine furnace black (FF), high modulus (HMF), high elongation (HEF), reinforcing (RF), semireinforcing (SRF), high abrasion (HAF), super abrasion (SAF), intermediate super abrasion (ISAF), fast extruding (FEF), general purpose (GPF), easy processing (EPF), conducting (CF), and super conducting furnace black (SCF). Channel black is characterized by lower pH, higher volatile content, and high surface area. It has the smallest particle size of any industrial material. A few grades of channel blacks (HPC, MPC, or EPC corresponding to hard, medium, or easy processing channel) are used in the rubber industry. For carbon-black fillers, structure, particle size, particle porosity, and overall physico-chemical nature of particle surface are important factors in deciding cure rate and degree of reinforcement attainable. The pH of the carbon black has a profound influence. Acidic blacks (channel blacks) tend to retard the curing process while alkaline blacks (furnace blacks) produce a rate-enhancing effect in relation to curing, and may even give rise to scorching. Another important factor is the particle size of the carbon black filler. The smaller the particle size, the higher the reinforcement, but the poorer the processability because of the longer time needed for dispersion and the greater heat produced during mixing. Blacks of the smallest particle size are thus unsuitable for use in rubber compounding.

270

Plastics Technology Handbook

For carbon black fillers the term structure is used to represent the clustering together and entanglement of fine carbon particles into long chains and three-dimensional aggregates. High-structure blacks produce high-modulus vulcanizates as high shear forces applied during mixing break the agglomerates down to many active free radical sites, which bind the rubber molecules, thereby leading to greater reinforcement. In nonstructure blacks the aggregates are almost nonexistent. Most of the nonblack fillers used in rubber compounds are of nonreinforcing types. They are added for various objectives, the most important being cost reduction. Precipitated silica (hydrated), containing about 10–12% water with average particle size ranging 10–40 nm, produce effective reinforcements and are widely used in translucent and colored products. Finely ground magnesium carbonate and aluminum silicate also induce good reinforcing effects. Precipitated calcium carbonate and activated calcium carbonate (obtained by treating calcium carbonate with a stearate) are used as semireinforcing fillers. Short fibers of cotton, rayon, or nylon may be added to rubber to enhance modulus and tear and abrasion resistance of the vulcanizates. Some resins such as “high styrene resins” and novolac-type phenolic resins mixed with hexamethylene tetramine may also be used as reinforcing fillers or additives. Whereas SBR has a styrene content of about 23.5% and is rubbery, styrene-butadiene copolymers containing about 50% styrene are leatherlike whilst with 70% styrene the materials are more rigid thermoplastics but with low softening points. Both of these copolymers are known in the rubber industry as high styrene resins and are usually blended with a hydrocarbon rubber such as NR and SBR. Such blends have found use in shoe soles, car wash brushes and other moldings, but in recent years have suffered increasing competition from conventional thermoplastics and thermoplastic rubbers.

2.20.2 Mastication and Mixing A deficiency of NR, compared with the synthetics, is its very high molecular weight, which makes mixing of compounding ingredients and subsequent processing by extrusion and other shaping operations difficult. For NR it is thus absolutely necessary, while for synthetic rubbers it is helpful, to subject the stock to a process of breakdown of the molecular chains prior to compounding. This is effected by subjecting the rubber to high mechanical work (shearing action), a process commonly known as mastication. Mastication and mixing are conveniently done using two-roll mills and internal mixers. The oxygen in are plays a critical role during mastication. Rubber that has been masticated is more soft and flows more readily than the unmasticated material also allows preparation of solutions of high solids content because of the much lower solution viscosity of the degraded rubber. Rubber is also rendered tacky by mastication, which means that the uncured rubber sticks to itself readily so that articles of suitable thickness can be built up from layers of masticated rubber or rubberized fabric without the use of a solvent. 2.20.2.1 Open Mill The mainstays of the rubber industry for over 80 years has been the two-roll (open) mill and the Banbury (internal) mixer. Roll mills were first used for rubber mixing over 120 years ago. The plastics and adhesives industries later adopted these tools. The two-roll mill (Figure 2.78) consists of two opposite-rotating rollers placed close to one another with the roll axes parallel and horizontal, so that relatively small gap or nip (adjustable) between the cylindrical surface exists. The speeds of the two rolls are usually different, the front roll having a slower speeds. For NR mixing, a friction ratio of 1:1.2 for the front to back roll may be used. For some synthetic rubbers or highly filled NR mixes, friction ratios close to 1.0 produce good results. The nip is adjusted so that when pieces of rubber are placed between the rolls, they are deformed by shearing action and squeezed through the nip to which they are returned by the operator. On repeated passage through the nip, the around the front roll and a moving “bank” above the nip. As the rolls keep on rotating, the operator uses a knife to cut through the band on the front roll, removes the mass in parts from time, and places it is a new position to ensure uniform treatment and mixing through the nip.

271

Fabrication Processes

Feed/bank Back roll

Front roll

Rubber band

Nip (adjustable)

FIGURE 2.78

Section showing the features of a two-roll open mill.

With most rubbers other than NR, addition of different compounding ingredients may be normally started soon after a uniform band is formed on the roll and a bank is obtained. For NR, however, milling is usually continued to masticate the rubber to the desired plasticity, and mixing of compounding ingredients is started only after the adequate mastication. Mixing is effected by adding the different ingredients onto the bank. They are gradually dispersed into the rubber, which is cut at intervals, rolled over, and recycled through the nip of the moving rolls to produce uniform mixing. Since the rate of mastication is a function of temperature, time and temperature of mastication have to be controlled or kept uniform from batch to batch in order to get the desired uniform products from different batches. Roll mills vary greatly in size from very small laboratory machines with rollers of about 1 in. in diameter and driven by fractional horsepower motors to very large mills with rollers of nearly 3 ft. in diameter and 7 or 8 ft. in length and driven by motors over 100 hp. 2.20.2.2 Internal Batch Mixers Internal batch mixers are widely used in the rubber industry. They are also used for processing plastics such as vinyl, polyolefins, ABS, and polystyrene, along with thermosets such melamines and ureas because they can hold materials at a constant temperature. The principle of internal batch mixing was first introduced in 1916 with the development of the Banbury mixer (Figure 2.79a). A Banbury-type internal mixer essentially consists of a cylindrical chamber or shall within which materials to be mixed are deformed by rotating blades or rotors with protrusions. The mixer is provided with a feed door and hopper at the top and a discharge door at the bottom. As the rubber or mix is worked and sheared between the two rotors and between each rotor and the body of the casing, mastication takes place over the wide area, unlike in a open mill where it is restricted only in the area of the nip between the two rolls. The rotor blade of the Banbury mixer is pear shaped, but the projection is spiral along the axis and the two spirals interlock and rotate in opposite directions (Figure 2.79b). The interaction of rotor blades between themselves, in addition to producing shearing action, causes folding or “shuffling” of the mass, which is further accentuated by the helical arrangement of the blade along the axis of the rotor, thereby imparting motion to the mass in the third, or axial, direction. This combination of intensive working produces a highly homogeneous mix. An important and novel feature of the Banbury mixer is a vertical ram to press the mass into contact with the two rotors. Rubbers, fillers, and other ingredients are charged through the feed hopper and then held in the mixing chamber under the pressure of the hydraulic (or manual) ram. As a result, incorporation of solids is more rapid. Both the cored rotors and the walls of the mixing chamber can be cooled or heated by circulating fluid. Because of the large power consumption of such a machine (up to 500 hp) the cylinder walls are usually water-cooled by sprays (Figure 2.79a).

272

Plastics Technology Handbook

Feed hopper door Cored rotors

Follower Cooling sprays

(a)

Sliding discharge door

(b)

FIGURE 2.79

(a) Cross section of a Banbury mixer; (b) Roll mixing blades in Banbury mixer.

Other major machines in use in the rubber industry are the Shaw intermix and the Baker-Perkins shear-mix. Both the Banbury-type and the intermix mixers have passed through modiﬁcations and reﬁnements at different points in time. Mechanical feeding and direct oil injection in measured doses into the mixing chamber through a separate oil injection port are notable features of modern internal mixers. Higher rotor speeds and, in the Banbury type, the higher ram pressures, are used for speedy output. In Banbury-type mixers, the rotors run at different speeds, while in intermix mixers the rotor speeds are equal but the kneading action between the thicker portion of one rotor and the thinner portion of the other produces a frictional effect. The operation of internal mixers is power intensive, and a given job is performed at a much higher speed and in a much shorter time on a two-roll open mill. However, the major vulcanizing agent (such as sulfur) is often added later on a two-roll mill so as to eliminate possibilities of scorching. And even if this is not practiced, the mix, after being discharged from the internal mixer, is usually passed through a two-roll mill in order to convert it from irregular lumps to a sheet form for convenience in subsequent processing.

2.20.3 Reclaimed Rubber The use of reclaimed rubber in a fresh rubber mix not only amounts to waste utilization but offers some processing and economic advantages to make it highly valued in rubber compounding. Though waste

Fabrication Processes

273

vulcanized rubber is normally not processable, application of heat and chemical agents to ground vulcanized waste rubber leads to substantial depolymerization whereby conversion of the rubber to a soft, plastic processable state is effected. Rubber so regenerated for reuse is commonly known as reclaimed rubber or simply as reclaim. Reclaimed rubber can be easily revulcanized. Worn-out tires and scraps and trimmings of other vulcanized products constitute the raw material for reclaimed rubber. Therefore a good reclaiming process must not only turn the rubber soft and plastic but also must remove reinforcing cords and fabrics that may be present. There are a number of commercial processes [75] for rubber regeneration: (1) alkali digestion process, (2) neutral or zinc chloride digestion process, (3) heater or pan process, and (4) reclaimator process. Tires are most commonly reclaimed by digestion processes. For processes (1) and (2), debeaded tires and scraps, cut into pieces, are ground with two-roll mills or other devices developed for the purpose. Two-roll mills generally used for grinding tires turn at a ratio of about 1:3, thus providing the shearing action necessary to rip the tire apart. The rubber chunks are screened, and the larger material is recycled until the desired size is reached. The ground rubber is then mixed with a peptizer, softener, and heavy naphtha, and charged into spherical autoclaves with requisite quantities of water containing caustic soda for process (1) or zinc chloride for process (2). The textile is destroyed and mostly lost in the digestion process. Steam pressure and also the amount of air or oxygen in the autoclave greatly influence the period necessary for rubber reclaiming. On completion of the process, the pressure is released, the contents of the autoclave discharged into water, centrifuged, pressed to squeeze out water, and dried. The material is finally processed through a two-roll mill during which mineral fillers and oils may be added to give a product of required specific gravity and oil extension. Butyl and NR tubes and other fiber-free scrap rubbers are reclaimed by means of the heater or pan process. Brass tube fittings and other metal are removed from the scrap. The scrap is mechanically ground, mixed with reclaiming agents, loaded into pans or devulcanizing boats, and autoclaved at steam pressures of 10–14 atm (1.03–1.40 MPa) for 3–8 h. The reclaim is finally processed much the same way as in the digester process. The reclaimator process is more attractive than the above processes. The reclaimator is essentially a high-pressure extruder that devulcanizes fiber-free rubber continuously. Ground scrap is mechanically treated in hammer mills to remove the textile material, mixed with reclaiming oils and other materials, and then fed into the reclaimator. High pressure and shear between the rubber mixture and the extruder barrel walls effectively reclaim the rubber mixture. Devulcanization occurs at 175–205°C in a few minutes and turns the rubber into reclaim that issues from the machine continuously. The whole regeneration, which is a dry process, may be completed in about 30 min. The reclaiming oils and chemicals are complex wood and petroleum derivatives that swell the rubber and provide access for breaking the rubber bonds with heat, pressure, chemicals, and mechanical shearing. Approximately 2–4 parts of oil are used per 100 parts of scrap rubber. Some examples of reclaiming oils include monocyclic and mixed terpenes, i.e., pine-tar products, saturated polymerized petroleum hydrocarbons, aryl disulfides in petroleum oil, cycloparafinic hydrocarbons, and alkyl aryl polyether alcohols. Reclaimed rubber contains all the fillers present in the original scrap or waste rubber. It shows very good aging characteristics and is characterized by less heat development during mixing and processing as compared to fresh rubber. The use of reclaim in a fresh rubber mix is advantageous not on consideration of physical and mechanical properties but essentially for smooth processing and reduced cost. Radial tires (see later) do not use reclaimed rubber because they require higher abrasion resistance that cannot be attained by mixing reclaimed rubber. Better processes for the production of higher quality reclaimed rubber are needed in order to use it for radial tires. To improve the quality of reclaimed rubber, cross-links in vulcanizates should be severed selectively during a devulcanization process and no lowmolecular-weight compound such as swelling solvent should remain in the reclaimed rubber after the devulcanization process.

274

Plastics Technology Handbook

A devulcanization process that utilizes supercritical CO2 as a devulcanization reaction medium in the presence of diphenyl disulfide as a devulcanizing reagent has been reported [77]. The process devulcanizes unfilled NR vulcanizates effectively. Further, a comparison of measured sol/gel components as well as dynamic mechanical properties of the devulcanized rubber products of filled and unfilled NR vulcanizates has indicated that the presence of carbon black in the vulcamizate does not disturb the devulcanization in supercritical CO2.

2.20.4 Some Major Rubber Products The most important application of rubbers is in the transport sector, with tires and related products consuming nearly 70% of the rubber produced. Next in importance is the application in belting for making flat conveyor and (power) transmission belts and V-belts (for power transmission), and in the hose industry for making different hoses. Rubber also has a large outlet in cellular and microcellular products. Other important and special applications of rubber are in the areas of adhesives, coated fabrics, rainwear, footwear, pipes and tubing, wire insulation, cables and sheaths, tank lining for chemical plants and oil storage, gaskets and diaphragms, rubber mats, rubber rollers, sports goods, toys and balloons, and a wide variety of molded mechanical and miscellaneous products. Formulations of a few selected rubber compounds are given in Appendix A4. 2.20.4.1 Tires Tire technology is a very specialized area, and a tire designer is faced with the difficult task of trying to satisfy all the needs of the vehicle manufacturer, the prime factors of consideration, however, being safety and tread life. Figure 2.80 shows the constructions of a standard bias (diagonal) ply tire and a radial ply tire. The major components of a tire are: bead, carcass, sidewall, and tread. In terms of material composition, a tire on an average contains nearly 50% of its weight in actual rubber; for oil extended rubbers (typically containing 25 parts of aromatic or cycloparaffinic oils to 75 parts of rubber), it is less. The remainder included carbon black, textile cord, and other compounding ingredients plus the beads. The bead is constructed from a number of turns or coils of high tensile steel wire coated with copper and brass to ensure good adhesion of the rubber coating applied on it. The beads function as rigid, practically inextensible units that retain the inflated tire on the RIM. The carcass forms the backbone of the tire. The main part of the carcass is the tire cord. The cords consist of textile threads twisted together. Rayon, nylon, and polyester cords are widely used. Steel cords are also used. While the practice of laying rubber-impregnated cords in position is still followed, the use of woven fabric is more widespread. To promote a good bond with rubber, the fiber or cord is treated with an adhesive composition such as water-soluble resorcinol-formaldehyde resin and aqueous emulsion of a copolymer of butadiene, styrene, and vinyl pyridine (70:15:15). The resincoated cord or fabric is dried and coated with a rubber compound by calendaring, whereby each cord is isolated from its neighbor. For conventional tires, the rubber-coated fabric is then cut to a predetermined width and bias angle. The biascut plies are joined end-to-end into a continuous length and batched into roll form, interleaved with a textile lining to prevent self-adhesion. The sidewall is a layer of extruded rubber compound that protects the carcass framework from weathering and from damage from chafing. Together with the tread and overlapping it in the buttress region, the sidewall forms the outermost layer of the tire. It is the most highly strained tire component and is susceptible to two types of failure—flex cracking and ozone cracking. The tread is also formed by extrusion, but different rubber compounds are used for sidewall and tread (see Appendix A4). When making the side-wall and the tread in two separate extrusion lines, it is useful to take the extruded sidewall to the second extruder, which would deliver the tread to the sidewall. To simplify the tire-building operation, the sidewall and the tread may also be produced as a single unit by simultaneous extrusion of the two compounds in a single band-the tread over the sidewall-with two

275

Fabrication Processes

Casing plies

Filler

Chafer Casing plies

Wall rubber Chafer (a)

Casing plies

Bead wires Bread wrap Tread pattern Thread bracing layers Radial plies

Inner lining

Wall rubber

Chafer Chafer strip

(b)

FIGURE 2.80

Bead ﬁller

Apex strip Beap wrapping Beap coil

Diagram showing. (a) bias (diagonal) ply tire; (b) radial ply tire.

extruders arranged head-to-head with a common double die. The band is rapidly cooled to avoid scorching and then cut to the appropriate length. The tread receives its characteristic pattern from the mold when the subsequently built tire is vulcanized. While building the tire, a layer of specially compounded cushion rubber may be used to keep heat development on flexing to a minimum and achieve better adhesion between the tread and the carcass. One or more layers of fabric, known as breakers or bracing layers, may be placed below the cushion. The bracing layers raise the modulus of the tread zone and level out local blows to the tread as it contacts the road. The tire building is carried out on a flat drum, which is rotated at a controlled speed. The plies of rubber-coated cord fabric are placed in position, one over the other, and rolled down as the drum rotates, the inner lining being placed next to the drum and the ends of the drum being flanged to suit the bead configuration of the tire. The plies of rubber-coated textile are assembled in three basic constructions— bias (diagonal), radial, and bias belted. Bias tires have an even number of plies with cords at an angle of 30–38° from the tread center line. Passenger-car bias tires commonly have two or four plies, with six for heavy duty service. Truck tires are often built with six to twelve plies, although the larger earthmover types may contain thirty or more.

276

Plastics Technology Handbook

In the radial-ply tire, one or two plies are set at an angle of 90° from the center line and a breaker or belt or rubber-coated wire or textile is added under the tread. This construction gives a different tread-road interaction, resulting in a decreased rate of wear. The sidewall is thin and very flexible. The riding and steering qualities are noticeably different from those of a bias-ply tire and require different suspension systems. The bias-belted tire, on the other hand, has much of the tread wear and traction advantage of the radial tire, but the shift from bias to bias-belted tires requires less radical change in vehicle suspension systems and in tire building machines. These features make the bias-belted tire attractive to both automobile and tire manufacturers. When the tire building on the drum is complete, the drum is collapsed and the uncured (“green”) tire is removed. The cylindrical shape of the uncured tire obtained from the building drum is transformed into a toroidal shape in the mold resulting in a circumferential stretch of the order of 60%. For shaping and curing, the uncured tire is pressed against the inner face of the heated mold by an internal bag or bladder made of a pre-cured heat-resistant rubber and inflated by a high-pressure steam or circulating hot water. Different types of molding presses are in use. In the Bag-O-Matic type of press, the uncured tire is placed over a special type of bag or bladder, and the operation of shaping, curing, and ejection of the cured tire are accomplished by an automatic sequence of machine operation. Pneumatic tires require an air container or inner tube of rubber inside the tire. The manufacture of inner tubes is done essentially in three steps: extrusion, cutting into length, and insertion of value and vulcanization. The ends of the cut length of the tube are joined after insertion of the valve prior to vulcanization. Tubeless tires have, instead of an inner tube, an inner liner, which is a layer of rubber cured inside the casing to contain the air, and a chafer around the bead contoured to form an airtight seal with the RIM. 2.20.4.2 Belting and Hoses Uses of flat conveyor and (power) transmission belts and V-belts (for power transmission) are to be found in almost all major industries. V-belts for different types cover applications ranging from fan belts for automobiles, belts for low-power drives for domestic, laboratory, and light industrial applications, to high-power belts for large industrial applications. Textile cords or fabric and even steel cords constitute an important part of all rubber belting and hoses. The various types of cords used in the tire industry are also in use in the belting industry. Essential steps in making conveyor belts are: (1) drying the fabric; (2) frictioning of the hot fabric with a rubber compound and topping to give additional rubber between plies and the outer ply and cover, using a three- or four-roll calender; (3) belt building; and (4) vulcanization. The process of belt building essentially consists or cutting, laying, and folding the frictioned (a) and coated fabric to give the desired number of plies. The construction may be straight-laid or graded-ply types (Figure 2.81) with the joints in neighboring plies being staggered to eliminate weakness and failure. Finally, the cover coat is (b) applied by calendaring. Vulcanization of conveyor belts may be carried out in sections using press cure or continuously by means of a Rotocure equipment. In press cure, vulcanization is done by heating in (c) long presses, the belt being moved between FIGURE 2.81 Construction of conveyor belts: (a) straight successive cures by a length less than the length of the press platens. Since in this process the end laid, (b) folded jacket, and (c) graded ply.

277

Fabrication Processes

or overlap zones receive an additional cure, it is desirable to minimize damage or weakness due to overcure by using flat cure mixes and allowing water cooling at each end section of the platen. In each step of vulcanization, the section of the belt to be vulcanized is gripped and stretched hydraulically to minimize or eliminate elongation during use. The difficulties of press cure may, however, be avoided by adopting continuous vulcanization with a Rotocure equipment in which the actual curing operation is carried out between an internally steam heated cylinder and a heated steel band. Rotocure is also useful for the vulcanization of transmission belts and rubber sheeting. The most common feature of V-belts is their having a cross section of the shape of a regular trapezium with the unparallel sides at an angle of 40° (Figure 2.82). The V-belts usually consist of five sections: (1) the top section known as the tension section, (2) the bottom section, called the compression section, (3) the cord section located at the neutral zone, (4) the cushion section on either side of the cord section, and (5) one or two layers of rubberized fabric, called the jacket section covering the whole assembly. Three different rubber compounds are required for use in the above construction of a V-belt: (1) a base compound, which is the major constituent on a weight basis, (2) a soft and resilient cushion compound required for surrounding and protecting the reinforcing cord assembly, and (3) a friction compound used for rubberizing the fabric casing of the belt (see Appendix A4). Relatively short length V-belts are built layer by layer on rotatable collapsible drum formers. The separate belts are then cut out with knives and transferred to a skiving machine that imparts the desired V-shape. The fabric jacket is then applied, and the belts are vulcanized in open steam using multi-cavity ring molds for smaller belts. Long belts are built similarly on V-groove sheave pulleys using weftless cord fabric in place of individually would cord. They are vulcanized endlessly by molding in a hydraulic press under controlled tension. A rubber hose has three concentric layers along the length. While the innermost part consisting of a rubber lining or tube is required to resist the action of the material that would pass through the top layer is meant to play the role of a protective layer to resist weathering, oils, chemicals, abrasion, etc. Between the inner lining and the outer cover is given a layer of reinforcement of textile yarn or steel wire applied by spiraling, knitting, braiding, or circular loom weaving. A cut woven fabric wrapped straight or on the bias may also be used to reinforce the inner lining or tube. Essentially, the process of hose building consists of extruding the lining or tube, braiding or spiraling the textile around the cooled tube, and applying an outer cover of rubber to the reinforced hose using a cross-head extruder. Several methods are employed for vulcanization. In one process, the built hose is passed through a lead press or lead extruder to give a layer of lead cover to the hose. The hose is then wound on a drum, filled with water or air, and the ends are sealed. The whole assembly is then heated to

Width

Thickness

Tension section Cord section Compression section

Cushion section

Cover or jacket section

40°

FIGURE 2.82

Cross section showing V-belt construction.

278

Plastics Technology Handbook

achieve vulcanization. The water or air inside expands, and the vulcanizing hose is pressed against the lead acting as the mold. On completion of cure, the sealed ends are cut open, the lead cover is removed by slitting lengthwise in a stripping machine, and the cured hose is coiled up. 2.20.4.3 Cellular Rubber Products Cellular rubber may be described as an assembly of a multitude of cells distributed in a rubber matrix more or less uniformly. The cells may be interconnected (open cells) as in a sponge or separate (closed cells). Foam rubber made from a liquid starting material such as latex, described earlier, is of open-cell type. Cellular products made from solid rubber are commonly called sponge (open cell structure) and expanded rubber (closed cell structure). The technology of making cellular products from solid rubber is solely dependent on the incorporation of a blowing agent, usually a gas such as nitrogen or a chemical blowing agent, into the rubber compound. The most widely used chemical blowing agent for this application is dinitroso pentamethylene tetramine (DNPT). The curing is carried out either freely using hot air or steam or in a mold that is only partially filled with the molding compound. Synthetic rubbers, particularly SBR, are preferred as they allow precise control over level of viscosity required for obtaining consistent product quality. The sponge and expanded rubber products include carpet backing, sheets, profiles, and molding. The development of microcellular rubber has brought a revolution in footwear technology. Microcellular rubber with an extremely fine noncommunicating cell structure and very comfortable wearing properties, is the lightest form of soling that can be produced. Density of soling as low as 0.3 g/cm3 may be obtained with a high dose (8–10 phr) of DNPT at a curing temperature of 140–150°C. For common solings, the density normally varies between 0.5 and 0.8 g/cm3. High hardness and improved abrasion resistance along with low density, desired in microcellular soling, can be achieved by using SBR and high styrene resins with NR in right proportions. Higher proportions of high styrene resins give products of higher hardness and abrasion resistance and lower density. Silicious fillers such as precipitated silica and aluminum or calcium silicate also give higher hardness, abrasion resistance, and split tear strength. Microcellular crumbs can be used in considerable quantity along with china clay and whiting to reduce the product cost. Higher proportions of stearic acid (5–10 phr) are normally used in microcellular compounds in order to bring down the decomposition temperature of DNPT type blowing agents (see Appendix A4). Post-cure oven stabilization of the microcellular sheets, typically at 100°C for 4 h, reduces the delayed shrinkage after cure to a minimum.

2.21 Miscellaneous Processing Techniques 2.21.1 Coating Processes A coating is thin layer of material used to protect or decorate a substrate. Most often, a coating is intended to remain bonded to the surface permanently, although there are strippable coatings which are used only to afford temporary protection. An example of the latter type is the strippable hotmelt coating with ethyl cellulose as the binder [78], which is used to protect metal pieces such as drill bits or other tools and gears from corrosion and mechanical abrasion during shipping and handling. Two of the principal methods of coating substrates with a polymer, namely extrusion coating and calendaring have already been dealt with in this chapter. Other methods of coating continuous webs include the use of dip, knife, brush, and spray. Dip coating, as applied to PVC, has already been described in previous section on plastisols. In knife coating the coating is applied either by passing the web over a roll running partly immersed in the coating material or by running the coating material onto the face of the web while the thickness of the coating is controlled by a sharply proﬁled bar (or knife). This technique, also referred to as spreading, is

Fabrication Processes

279

used extensively for coating fabrics with PVC. The PVC is prepared in the form of a paste, and more than one layer is usually applied, each layer being gelated by means of heat before the next layer is added. Lacquers are a class of coatings in which film formation results from mere evaporation of the solvents(s). The term “lacquer” usually connotes a solution of a polymer. Mixtures of solvents and diluents (extenders which may not be good solvents when used alone) are usually needed to achieve a proper balance of volatility and compatibility and a smooth coherent film on drying. Some familiar examples of lacquers are the spray cans of touch-up paint sold to the auto owner. These are mostly pigmented acrylic resins in solvents together with a very volatile solvent [usually dichlorodifluoromethane (CCl2F2)] which acts as a propellant. A typical lacquer formulation for coating steel surfaces contains polymer, pigment, plasticizer (nonvolatile solvent), and volatile solvents. Latex paints or emulsion paints are another class of coatings which form films by loss of a liquid and deposition of a polymer layer. The paints are composed of two dispersions: (1) a resin dispersion, which is either a latex formed by emulsion polymerization or a resin in emulsion form, and (2) a dispersion of colorants, fillers, extenders, etc., obtained by milling the dry ingredients into water. The two dispersions are blended to produce an emulsion paint. Surfactants and protective colloids are added to stabilize the product. Emulsion paints are characterized by the fact that the binder (polymer) is in a water-dispersed form, whereas in a solvent paint it is in solution form. In emulsion systems the external water phase controls the viscosity, and the molecular weight of the polymer in the internal phase does not affect it, so polymers of high molecular weight are readily utilized in these systems. This is an advantage of emulsion paints. The minimum temperature at which the latex particles will coalesce to form a continuous layer depends mainly on the Tg. The Tg of a latex paint polymer is therefore adjusted by copolymerization or plasticization to a suitable range. The three principal polymer latexes used in emulsion paints are styrenebutadiene copolymer, poly(vinyl acetate), and acrylic resin. Although the term “paint” has been used for latex-based systems as well as many others, traditionally it refers to one of the oldest coating systems known—that of a pigment combined with a drying oil, usually a solvent. Drying oils (e.g., linseed, tung), by virtue of their multiple unsaturation, behave like polyfunctional monomers which can polymerize (“dry”) to produce film by a combination of oxidation and free-radical propagation. Oil-soluble metallic soaps are used to catalyze the oxidation process. Combinations of resins with drying oils yield oleoresinous varnishes, whereas addition of a pigment to a varnish yields an enamel. The combination of hard, wear-resistant resin with softer, resilient, drying-oil films can be designed to give products with a wide range of durability, gloss, and hardness. Another route to obtaining a balanced combination of these properties is the alkyd resin, formed from alcohols and acids (and hence the “alkyd”). Alkyds are actually a type of polyester resin and are produced by direct fusion of glycerol, phthalic anhydride, and drying oil at 410°F–450°F (210°C–232°C). The process involves an esterification reaction of the alcohol and the anhydride and transesterification of the drying oil. A common mode of operation today is thus to start with the free fatty acids from the drying oil rather than with the triglycerides. 2.21.1.1 Fluidized Bed Coating Fluidized bed coating is essentially an adaptation of dip coating and designed to be used with plastics in the form of a powder of fine particle size. It is applied for coating metallic objects with plastics. The uniqueness of the process lies in the fact that both thermoplastics and thermosetting resins can be used for the coating. Uniform coating of thicknesses from 0.005 to 0.080 in. (0.13–2.00 mm) can be built on many substrates such as aluminum, carbon steel, brass, and expanded metal. The coating is usually applied for electrical insulation and to enhance the corrosion resistance and chemical resistance of metallic parts. Low-melting polymers are most appropriate for fluidized beds. Highermelting polymers must have a sufficiently low melt viscosity that the particles can flow and fuse together to form a continuous coating. Thermoplastic polymers in common use include nylon, PVC, acrylics, polyethylene, and polypropylene.

280

Plastics Technology Handbook

Other possible materials are thermoplastic urethane, silicones, EVA, polystyrene, or any other lowmelting, low-viscosity polymer. Thermosetting polymers are limited largely to epoxy and epoxy/polyester hybrids since other thermosets, such as phenolic and urea-formaldehyde resins, give off volatile byproducts that can create voids in the coating. The powder resin particles range in size form about 20–200 microns. Particles larger than 200 microns are difficult to suspend. Particles smaller than 20 microns may create excessive dusting and release of particles from the top of the bed. The actual coating process is uncomplicated; however, achievement of a uniform coating requires considerable skill. The metal object to be coated of powdered resin fluidized by the passage of air through a porous plate. The bed is not heated; only the surface of the object to be coated is hot. As the powder contacts the hot substrate, the particles adhere, melt, and flow together to form a continuously conforming coating. The object is removed from the bed when the desired coating thickness is obtained. On cooling, the coated resin resumes its original characteristics. In the case of a thermosetting resin, additional time at elevated temperature may be required to complete the cure. The ability of the fluidized bed to continuously conform to and coat parts having unusual shapes and sizes permits a high degree of flexibility in application. This is invaluable to the processor who wishes to coat one-of-a-kind products. In general, the coatings are smooth and glossy, with excellent adhesion to the substrate, providing the hermetic seal necessary for proper maintenance. Fluidized bed coating is certainly the most efficient method of applying a thick coating in a single step. Thicknesses of 0.1 in. (2.5 mm) or greater are easily attained. Probably the single biggest advantage of powder coating is the nearly 100% utilization of the coating resin, without the hazard or expense of solvents. Almost all the coatings applied by this process have a definite function, chiefly electrical insulation, but it can be used for applications that simply require a thick coating with powder. Examples of electrical applications in include small motor stators and rotors, electronic components (capacitors or resistors), transformer casings, covers, laminations, and busbar. Other items coated using fluidized beds include valve bodies used in chemical industries, refinery equipment, and appliance and pump parts. 2.21.1.2 Spray Coating Spray coating is especially useful for articles that are too large for dip coating or fluidized bed coating. The process consists of blowing out fine polymer powders through a specially designed burner nozzle, which is usually flame heated by means of acetylene or some similar gas, or it can be heated electrically. Compressed air or oxygen is used as the propelling force for blowing the polymer powder. 2.21.1.3 Electrostatic Spraying The electrostatic spraying of polymer powders utilizes the principle that oppositely charged particles attract, a principle that has been used for many years in spraying solvent-based paints. In electrostatic spraying, polymer powder is first fluidized in a bed to separate and suspend the particles. It is then transferred through a hose by air to a specially designed spray gun. As the powder passes through the gun, direct contact with the gun and ionized air applies an electrostatic charge to the particles of powder. The contact area may be a sleeve that extends the length of the gun or merely small pins that extend into the passageway of the powder. For safety the gun is designed for high voltage but low amperage. The part to be coated is electrically grounded, attracting the charged particles. This produces a more even coating and reduces overspray. Parts to be coated may be preheated, thereby forming the coating immediately, or they may be coated cold as the electrostatic charge will hold the particles in place until heat is applied. Once heat is applied, the particles melt and flow together, forming a continuous protective coating. Coatings 50–75 microns thick can be applied electrostatically to cold objects and coatings up to 250 microns thick to hot objects. The polymers used in spraying of powders are the same as those used in fluidized beds. The key characteristic for any polymer, thermoplastic or thermoset, applied as a

Fabrication Processes

281

powder is low melt viscosity, which enables polymer particles to flow together and form a continuous coating. The chief characteristic of electrostatically applied powder polymer is the ability to produce a thin coating. It is the preferred method of producing a coating of 0.001–0.002 in. (0.025–0.050 mm) thickness. It is a continuous process and suited to automated assembly-line production. With a well designed recovery system and fully enclosed spray booth, the process permits full utilization of the powder. The particle size of sprayed powders is smaller than that of powders used in fluidized bed coating. The average particles size is 30–60 microns. Much of what is termed decorative powder coating is applied by electrostatic spray. Appliances, laboratory instruments, transformer housings, engine parts, and chain-link fences are among the many types of products so coated for decorative purposes. Coatings that provide electrical insulation are also applied by electrostatic spray. Objects coated include electrical motor armatures and stators, electrical switchgear boxes, and magnet wire. Corrosion-resistant coatings are designed to prevent the corrosion of the underlying substrates. These include pipe, fencing, concrete reinforcing bars, valves, conduit, and pumps. Polymers used for corrosion protection are usually thermosetting, particularly epoxies because of their superior adhesion characteristics. 2.21.1.4 Smart Coatings Whereas traditional coatings only protect the substrate to which they are applied by providing a barrier between the surface and the environment, smart coatings go much further as they are able to sense a change in conditions in the environment and respond to that change in a predictable and noticeable manner to mend or eliminate the problem. Stimuli for smart coatings can be any of a number of changes in environmental conditions, such as heat, pressure, pH, impact, vibrations, presence of pathogens and other organisms, certain chemicals (such as corrosive materials), humidity, electronic and magnetic fields, sunlight and other radiations, and others. The functional ingredient within the intelligent coating can be the resin itself or a variety of additives including microencapsulated ingredients, pigments, antimicrobial agents, enzymes or other bioactive species, nanomaterials (nanoparticles, nanotubes, nanocapsules, etc.), microelectromechanical devices, and radio-frequency identification devices. The potential applications for these numerous types of smart coatings are broad and varied, including corrosion control, camouflage, bio-weapon detection and destruction, and other safety applications. The need for smart coatings and functional surfaces exists in diverse industries, including aerospace, marine, automotive, construction, communication, textile, biomedical, electronics, energy, environmental protection, personal safety, and many others. Smart polymer coatings can be broadly classified into two categories, depending on the type of sensors used, as those based on color response and those based on noncolor response. For smart sensors based on color response, the response may be visible color change, fluorescence, or phosphorescence as a result of a variety of stimuli that include pH change, redox reactions, presence of heavy metals, sorption of chemicals, radiation, mechanical action, temperature variation, and electrical current [79]. A few selected smart coatings based on color response are listed in Table 2.10, indicating types of stimulus, response, sensor used, sensing mechanism, and application. The smart functionality may be imparted to the coatings by additives or colorants that are added separately into the coating before application on a given substrate. The colorants may be simple dyes or pigments as shown for systems 1–5 in Table 2.10. Smart functionality may also be built into the polymeric structure, as shown for systems 6–8 in Table 2.10. Examples of sensors in system 6 are acrylic polymers with long crystallizable, hydrophobic side chains that can act as temperature-activated crystallinity/permeability switches [79]. In a typical application as smart seed coating, germination is prevented at soil temperatures below about 13°C because the seed coating is then crystalline and provides a barrier to moisture, while at higher temperatures, the coating assumes an amorphous structure that enhances moisture penetration and allows seed germination [80]. Examples of sensors in system 7 are conductive films of polyaniline (Pani), polypyrrole (Ppy), and polythiophene (Pth), which undergo changes in properties, such as color, electrical conductivity,

Color change

pH change

Oxidation

Mechanical force

Light, temperature

Heavy metal, radioactive contamination

Temperature

Oxidation

UV light, g-rays, electrons Color change

Side-chain crystallizable polymer

Colorimetric dye

Polymerization

Source: Feng, W., Patel, S. H., Young, M-Y., Zunino III, J. L., and Xanthos, M., 2007. Adv. Polym. Tech., 26, 1.

Diacetylenes

Moisture barrier (for controlling seed germination)

Surface decontamination with strippable coatings

Battery tester, printing inks, coatings

Radiation detection

Switch between charged and neutral Sensing, actuation, corrosion resistance states

Crystalline to amorphous transformation

Colored metal complex

Photochromic dyes, thermochromic Structural transitions pigments, LCP and dyes

Film or CaCO3 formation (healing); Self-healing; crack detection dye release (crack sensing)

U/F capsules with ﬁlm former, or Ca(OH)2 or marker dye

Corrosion detection

Application

Redox reaction—ﬂuorescent oxidized form

Ionic transition

Sensing Mechanism

Fluorescein, Schiff bases

pH indicator

Sensor

Effect on optical properties Conducting polymer ﬁlms of Pani, Ppy, Pth

Color change

Capsule rupture-healing, color change

Fluorescence

Response

System Stimulus

TABLE 2.10 Polymeric Smart Coatings Containing Functional Colorants

282 Plastics Technology Handbook

Fabrication Processes

283

permeability, density, and charge density, during reversible redox and pH switching reactions. These conducting polymers can be used in smart coatings for corrosion protection on steel, stainless steel, aluminum, and copper. The conductive polymer can respond to oxidants in a corrosive environment and protect the metal anodically by retarding or inhibiting corrosion through the formation of passivating metal oxide films. Examples of radiation detection sensor in system 8 are diacetylenes (R–C≡C–C≡C–R, where R is a substituent group). Diacetylenes are colorless solid monomers. They usually form red- or blue-colored polymers [=(R)C–C≡C–C(R)=]n when irradiated with high-energy radiations, such as x-ray, g-ray, electrons, and neutrons. As exposure to radiation increases, the color of the sensing strip composed of diacetylenes intensifies in proportion to the dose. By using a proper diacetylene and thickness of coating, one can monitor doses even lower than 1 rad (e.g., 0.1 rad or even lower). SIRAD dosimeters, for example, are able to monitor as low as 0.01 rad by using more sensitive diacetylenes, thicker sensor and scanners, and CCD camera-type equipment [81]. [Note: A CCD, or charge coupled device, is an integrated circuit etched onto a silicon surface forming light-sensitive elements called pixels. Photons incident on this surface generate charges that can be read electronically and converted into a digital copy of the light pattern falling on the device.] Some examples of smart polymer coatings based on noncolor response are shown in Table 2.11. These are compared on the basis of stimulus, response, sensor type, sensing mechanism, and application. Examples [79] include modification of absorptive characteristics of coatings on biomaterials (systems 1 and 2), self-healing of cracks in coatings (system 3), temporary protective coatings that can be removed on demand via dissolution in appropriate reagent (system 4), and monitoring durability of coatings through dielectric sensors (system 5). The smart functionality may be built in the polymeric structure of a coating or may be provided through incorporation of suitable additives to the coating before its application on a given substrate. Since poly(N-isopropylacrylamide) (PNIPAM) has a lower critical solution temperature (LCST) of 31°C in an aqueous environment, below 31°C, the polymer becomes hydrated and assumes a random coil configuration, while above 31°C, the polymer chains take on a much more compact configuration by sudden dehydration and increased hydrophobic interaction between the polymer chains. When grafted onto solid surfaces, the polymer therefore provides a smart coating (system 1) with varying properties that can be controlled by applying an external stimulus—temperature. Below the phase transition temperature, the PNIPAM-grafted surfaces are hydrophilic, swollen, and nonprotein adsorptive (nonfouling), while above the transition temperature, the grafted polymer chains collapse and the surface becomes hydrophobic and protein-retentive. The LCST of PNIPAM being close to the body temperature, PNIPAM-grafted surfaces offer possibilities for a number of novel applications. Examples [82] include smart and thermally responsive coatings as cell culture substrates to control the attachment and detachment of cells, the recovery of cultured cells, a biofouling releasing coating, temperature-responsive membranes, controlled release of drugs, and temperature-responsive chromatography. Different methods have been reported for grafting PNIPAM on surfaces. These include activated substrates and functionalized polymers [83], e-beam irradiation [84], photoinitiated grafting of functionalized polymer [85], and plasma-induced grafting [86]. Microcapsules containing a small amount of “healing agent” that will be released by crack propagation have been incorporated into polymer coatings. This process has been used for self-healing in polymer composites through release of a polymerizable healing agent that would bridge cracks after reaction with appropriate catalysts [79], as shown in Figure 2.83. The microcapsules in self-healing polymers not only store the healing agent in normal conditions of use but also provide a mechanical trigger for the self-healing process when damage occurs in the host material and the capsules rupture. For this to happen, the microcapsules must possess long shelf life, excellent bonding to the host material, and sufficient strength to remain intact during processing of the host polymer and yet rupture when the polymer is damaged. In the example cited (system 2), these

Temperature

Crack owing to mechanical action

Neutralizing agents [NH3, (NH4)2CO3]

Aging deterioration

Light

Surface wettability

Azobenzene derivatives

Frequency dependent dielectric measurement

Carboxylated copolymer

Dissolution in water

Changes in molecular mobility of ions and dipoles

U/F microcapsules containing DCPD

Poly(N-isopropyl acrylamide)

Sensor

Capsule rupture and healing

Hydrophilicity, hydrophobicity

Response

Source: Feng, W., Patel, S. H., Young, M-Y., Zunino III, J. L., and Xanthos, M., 2007. Adv. Polym. Tech., 26, 1.

Stimulus

System

TABLE 2.11 Polymeric Smart Coatings Based on Noncolor Response

Azobenzene cis–trans photoisomerization

Dielectric sensing of changes in mobility at molecular level

Coating for photo-control of cell adhesion

Monitor durability of coatings

Temporary protective coating

Self-healing of cracks

Catalyzed polymerization of DCPD Neutralization of carboxylic groups

Coating on biomaterials

Application

Transition from hydrophobic (protein retentive) to hydrophilic (nonprotein adsorptive) below LCST (31°C)

Sensing Mechanism

284 Plastics Technology Handbook

285

Fabrication Processes

Catalyst

Microcapsule

(a) Crack

(b)

(c)

FIGURE 2.83 Autonomic healing of a polymer composite containing an embedded, microencapsulated healing agent and a catalytic chemical trigger (see text for functioning mechanism). (After White, S. R., Sottos, N. R., Geubelle, P. H., Moore, J. S., Kessler, M. R., Sriram, S. R., Brown, E. N., and Viswanathan, S. 2001. Nature, 409, 794.)

combined characteristics are achieved by microencapsulation of dicyclopentadiene (DCPD) healing agent utilizing acid-catalyzed in situ polymerization of urea with formaldehyde in an oil-in-water emulsion to form capsule wall [87,88]. The addition of the resulting free-flowing microcapsules (50–200 mm) to an epoxy matrix also provides significant toughening to the composite system in addition to self-healing property. For a structural polymeric material (such as epoxy) that has the ability to autonomically heal cracks, the material incorporates a microencapsulated healing agent and a catalytic chemical trigger within the polymer matrix (Figure 2.83a). The microcapsule shell provides a protective barrier between the catalyst and DCPD to prevent polymerization during the preparation of the composite. An approaching crack ruptures embedded microcapsules, releasing the healing agent into the crack plane through capillary action (Figure 2.83b). Polymerization of the healing agent is triggered by contact with the embedded catalyst, resulting in bonding of the crack faces (Figure 2.83c). The damage-induced triggering mechanism provides site-specific autonomic control of repair [87]. On being released into the crack plane, DCPD undergoes ring-opening metathesis polymerization (ROMP) in the presence of a transition metal catalyst (Grubbs’ catalyst) at room temperature to yield a tough and highly cross-linked polymer network in several minutes. The use of living (i.e., having unterminated chain ends) polymerization catalysts, as above, also enables multiple healing events. 2.21.1.5 Electrografted Coatings Achieving permanent adhesion between very dissimilar materials such as polymers and metals is a very challenging task. However, synthetic polymers can now be chemisorbed on a variety of conducting surfaces by cathodic electrografting of acrylic monomers [89]. Electrografting is merely implemented in a standard electrochemical assembly, as shown in Figure 2.84. The conducting surface to be coated is cathodically polarized in an oxygen- and water-free solution of the monomer and a conducting salt in an organic solvent under a dry and inert atmosphere. Upon application of the appropriate cathodic

286

Plastics Technology Handbook

ΔE

O CH2 CH

R O Counter electrode (anode) (a)

Substrate (cathode)

Reference

(b)

FIGURE 2.84 (a) Electrochemical setup used for electrografting of acrylates. (b) Electrografted acrylate on the cathodic substrate. (After Gabriel, S., Jérôme, R., and Jérôme, C. 2010. Prog. Polym. Sci., 35, 113.)

potential to the working electrode for a few seconds, a strongly adhering thin polymer film is deposited on the cathode surface, which is accordingly passivated. A proposed mechanism [89] consists of the transfer of one electron from the substrate (cathode) to the monomer, with the bonding of the radical-anion species to the substrate. Chain initiation is thus an electrochemical event, in contrast to the chain propagation that proceeds through the repeated addition of the monomer to the chemisorbed anionic species, for example, S–CH2CH(COOR)− for S/CH2=CH(COOR) system, where S is a conductive substrate. Common metals such as Fe, Ni, and Cu have been successfully coated by cathodic electrografting. These must be pretreated, however, for removing any oxide formed at the surface. Semiconductors, such as n- and p-doped silicon, chemically pretreated with hydrofluoric acid to remove their oxide layer, are well suited to electrografting that results in chemisorption of polymer chains through quite stable Si−C bonds. Electrografting has also been performed on electrically conducting powders and (nano)particles, which are placed in a zinc container immersed in (and filled by) the electrochemical bath and used as a cathode. (Zinc fulfills the critical requirement of the process, i.e., the inability of the container to be electrografted.) While the electrografting process, as initially developed, commonly used readily available monomers, namely, acrylonitrile and (meth)acrylates, new monomers were later synthesized that contained in the ester a large variety of substituents, which do not interfere with electro-polymerization. These substituents may be an initiator of radical or ring-opening polymerization, a monomer that is anodically polymerizable, a preformed polymer, and so on. Since electrooxidation of aromatic monomers, such as pyrrole and thiophene (or their derivatives), which is a classical technique to prepare intrinsically conducting polymers, has the drawback of poor adhesion of the film to the substrate, to tackle this problem, cathodic electrografting has been combined with anodic synthesis of conjugated polymers. In a first strategy, dual monomers, that is, acrylates that contain a pyrrole or thiophene unit in the ester group, for example, N-(2-acryloyloxyethyl)pyrrole (PyA) [formula: CH2=CHCO2CH2CH2N(C4H4)] and 3-(2-acryloyloxyethyl)thiophene (ThiA) [formula: CH2=CHCO2CH2CH2(C4H3S)], have been synthesized. The parent polyacrylates are easily chemisorbed under cathodic polarization, followed by a polarization inversion that results in the polymerization of the pyrrole (Figure 2.85) or thiophene substituents, thus making the electrically conducting film strongly adhering. Pyrrole or thiophene can also be added to the electrochemical bath with the purpose of increasing the thickness of the electrically conducting film.

287

Fabrication Processes

O Ec O

Ea N

H N m

(PyA)

FIGURE 2.85 Electrografting of N-(2-acryloyloxyethyl)pyrrole (PyA) followed by electrooxidation of pyrrole. (After Gabriel, S., Jérôme, R., and Jérôme, C. 2010. Prog. Polym. Sci., 35, 113.)

As an alternative, PyA or ThiA can be polymerized by conventional or controlled radical polymerization and—before electrooxidation of pyrrole or thiophene—the polyacrylates (polyPyA or polyThiA) so obtained can be dissolved in the electrochemical bath or cast onto an electrode that has been previously modified by electrografting of PyA or ThiA. Electroactive films of a controlled thickness (up to several microns) can thus be obtained, depending on the anodic polarization time. This process can be instrumental in improving the performance of various high-tech devices, such as light-emitting diodes, anticorrosion coatings, electrochromic windows, and electrochemical sensors that depend on the stability and adherence of a conjugated polymer coating on a conducting solid surface [89]. A very large variety of organic films have been successfully deposited with strong adhesion onto various substrates by cathodic electrografting with the possibility of having a second layer of chains grown from the first one under controlled/living conditions. In this case, the electrografted film is thin and plays the role of an anchoring “primer” [89], whereas the covalently bonded top layer can be designed to have desired molecular structures, composition, and thickness. The unique flexibility of this strategy of coating and multilayered film deposition has generated high value-added practical applications, such as biocompatibilization of medical implants, bactericidal coatings, surface functionalization of electronic devices, and stimuli-responsive coatings (smart coatings). The evolving shift from micro- to nanoelectronics needs the ability to perform very local surface modification, molecular handling of polymer chains, and sensing at the nanometric scale with respect to spatial detection and spatial deposition. Electrografting can prove to be very instrumental for successfully facing such challenges generated by the emerging nanotechnologies.

2.21.2 Powder Molding of Thermoplastics 2.21.2.1 Static (Sinter) Molding The process is often used with polyethylene and is limited to making open-ended containers. The mold which represents the exterior shape of the product is filled with powder. The filled mold is heated in an oven, causing the powder to melt and thus creating a wall of plastics on the inner surface of the mold. After a specific time to build the required wall thickness, the excess powder is dumped from the mold and the mold is returned to the oven to smooth the inner wall. The mold is then cooled, and the product is removed. The product is strain free, unlike pressure-molded products. In polyethylene this is especially significant if the product is used to contain oxidizing acids. 2.21.2.2 Rotational Molding Rotational molding (popularly known as rotomolding) is best suited for large, hollow products, requiring complicated curves, uniform wall thickness, a good finish, and stress-free strength. It has been used for a variety of products such as car and truck body components (including an entire car body), industrial

288

Plastics Technology Handbook

containers, telephone booths, portable outhouses, garbage cans, ice buckets, appliance housing, light globes, toys, and boat hulls. The process is applicable to most thermoplastics and has also been adapted for possible use with thermosets. Essentially four steps are involved in rotational molding: loading, melting and shaping, cooling, and unloading. In the loading stage a predetermined weight of powdered plastic is charged into a hollow mold. The mold halves are closed, and the loaded mold is moved into a hot oven where it is caused to simultaneously rotate in two perpendicular planes. A 4:1 ratio of rotation speeds on minor and major axes is generally used for symmetrically shaped objects, but wide variability of ratios is necessary for objects having complicated conﬁgurations. In most units the heating is done by air or by a liquid of high speciﬁc heat. The temperature in the oven may range from 500°F–900°F (260°C–482°C), depending on the material and the product. As the molds continue to rotate in the oven, the polymer melts and forms a homogenous layer of molten plastic, distributed evenly on the mold cavity walls through gravitational force (centrifugal force is not a factor). The molds are then moved, while still rotating, from the oven into the cooling chamber, where the molds and contents are cooled by forced cold air, water fog, or water spray. Finally, the molds are opened, and the parts removed. (Rotational molding of plastisol, described earlier, is similar to that described here. However, in this case the plastic is charged in the form of liquid dispersion, which gels in the cavity of the rotating mold during the heating cycle in the oven.) The most common rotational molding machine in use today is the carousel-type machine. There are three-arm machines consisting of three cantilevered arms or mold spindles extending from a rotating hub in the center of the unit. In operation, individual arms are simultaneously involved in different phases (loading, heating, and cooling) in three stations so that no arms are idle at any time (Figure 2.86). Modern rotational-molding machines enable large parts to be molded (e.g., 500-lb car bodies and 500-gal industrial containers). For producing small parts an arm of a carousel-type machine may hold as many as 96 cavities. 2.21.2.3 Centrifugal Casting Centrifugal casting is generally used for making large tube forms. It consists of rotating a heated tube mold which is charged uniformly with powdered thermoplastic along its length. When a tubular molten layer of the desired thickness builds up on the mold surface, the heat source is removed and the mold is cooled. The mold, however, continues to rotate during cooling, thus maintaining uniform wall thickness of the tube. Upon completion of the cooling cycle, the plastic tube, which has shrunk away from the mold surface, is removed, and the process is repeated. Usual tube sizes molded by the process range from 6–30 in. in diameter and up to 96 in. in length.

2.21.3 Adhesive Bonding of Plastics Adhestives are widely used for joining and assembling of plastics by virtue of low cost and adaptability to high-speed production. They can be subdivided into solvent or dope cements, which are suitable for most thermoplastics (not thermosets), and monomeric or polymerizable cements which can be used for most thermoplastics and thermosets. Solvent cements and dope cements function by attacking the surfaces of the adherends so that they soften and, on evaporation of the solvent, will join together. The dope cements, or bodied cements, differ from the straight solvents in that they contain, in solution, quantity of the same plastic which is being bonded. On drying, these cements leave a ﬁlm of plastic that contributes to the bond between the surfaces to be joined. Monomeric or polymerizable cements consist of a reactive monomer, identical with or compatible with the plastic to be bonded, together with a suitable catalyst system and accelerator. The mixture polymerizes either at room temperature or at a temperature below the softening point of the thermoplastic being joined. In order to hasten the bonding and to reduce shrinkage, some amount of the solid plastic may also

289

Fabrication Processes

Charging area

Powder

Mold halves

Oven Cooling chamber

FIGURE 2.86

Basics of continuous-type three-arm rotational molding machine.

be initially dissolved in the monomer. Adhesives of this type may be of an entirely different chemical type than the plastic being joined. A typical example is the liquid mixture of epoxy resins and hardener which by virtue of the chemical reactivity and hydrogen bonding available from the epoxy adhesive provide excellent adhesion to many materials. In joining with solvents or adhesives, it is very important that the surfaces of the joint be clean and well matched since poor contact of mating surfaces can cause many troubles. The problem of getting proper contact is aggravated by shrinkage, warpage, flash, marks from ejector pins, and nonflat surfaces. 2.21.3.1 Solvent Cementing Solvent cementing or solvent welding basically involves softening the bonding area with a solvent or a solvent containing small quantities of the parent plastic, referred to as dope or bodied cement, generally containing less than 15% resin. The solvent or cement must be of such composition that it will dry completely without blushing. Light pressures must be applied to the cemented joint until it has hardened to the extent that there is no movement when released. Structural bonds of up to 100% of the strength of the parent material are possible with this type of bonding. Table 2.12 gives a list of typical solvents selected as useful for cementing of plastics. A key to selection of solvents in this table is how fast they evaporate: a fast-evaporating solvent may not last long enough for some assemblies, while evaporation that is too slow could hold up production. It may be noted that solvent bonding of dissimilar materials is possible were the materials can be bonded with the same solvents (see Table 2.12). The solvent is usually applied by the soak method in which pieces are immersed in the solvent, softened, removed, quickly brought together, and held under light pressure for some time. Areas adjoining the joint area should be masked to prevent them from being etched. For some applications where the surfaces to be cemented fit very closely, it is possible to introduce the cementing solvent by brush, eyedropper, or hypodermic needle into the edges of the joint. The solvent is allowed to spread to the rest of the joint by capillary action. 2.21.3.2 Adhesive Bonding Adhesive bonding is a process in which the adhesive acts as an agent to hold two substrates together (as opposed to solvent cementing where the parent materials actually become an integral part of the bond) and the adhesion is chemical.

290

Plastics Technology Handbook

TABLE 2.12 Typical Solvents for Solvent Cementing of Plastics Plastics

Solvent

ABS

Methylene chloride, tetrahydrofuran, methyl ethyl ketone, methyl isobutyl ketone

Acetate

Methylene chloride, chloroform, acetone, ethyl acetate, methyl ethyl ketone

Acrylic Cellulosics

Methylene chloride, ethylene dichloride Acetone, methyl ethyl ketone

Nylon

Aqueous phenol, solutions of resorcinol in alcohol, solutions of calcium chloride in alcohol

PPO PVC

Methylene chloride, chloroform, ethylene dichloride, trichloroethylene Cyclohexane, tetrahydrofuran, dichlorobenzene

Polycarbonate

Methylene chloride, ethylene dichloride

Polystyrene Polysulfone

Methylene chloride, ethylene dichloride, trichloroethylene, methyl ethyl ketone, xylene Methylene chloride

Table 2.13 gives a list of various adhesives and typical applications in bonding of plastics [90]. This table is not complete, but it does give a general idea of what types of adhesives are used and where. It should be remembered, however, that thousands and thousands of variations of standard adhesives are available off the shelf. The computer may shape up as an excellent selection aid for adhesives. Selection is made according to the combination of properties desired, tack time, strength, method of application, and economics (performance/cost ratio). The form that the adhesive takes (liquids, mastics, hot melts, etc.) can have a bearing on how and where they are used. The anaerobics, which can give some very high bond strength and are usable with all materials except polyethylene and fluorocarbons, are dispensed by the drop. A thin application of the anaerobics is said to give better bond strength than a thick application. The more viscous, mastic-type cements include some of the epoxies, urethanes and silicones. Epoxies adhere well to both thermosets and thermoplastics. But epoxies are not recommended for most polyolefin bonding. Urethane adhesives have made inroads into flexible packaging, the shoe industry, and vinyl bonding. Polyester-based polyurethanes are often preferred over polyether systems because of their higher cohesive and adhesive properties. Silicones are especially recommended where both bonding and sealing are desired. Hot melts—100% solids adhesives that are heated to produce a workable material—are based on polyethylene, saturated polyester or polyamide in chunk, granule, pellet, rope, or slug form. Saturated polyesters are the primary hot melts for plastics; the polyethylenes are largely used in packaging; polyamides are used most widely in the shoe industry. Application speeds of hot melts are high and it pays to consider hot melts if production requirements are correspondingly high. Pressure sensitives are contact-bond adhesives. Usually rubber based, they provide a low-strength, permanently tacky bond. They have a number of consumer applications (e.g., cellophane tape), but they are also used in industrial applications where a permanent bond is not desirable or where a strong bond may not be necessary. The adhesive itself is applied rapidly by spray. Assembly is merely a matter of pressing the parts together. Film adhesives require an outside means such as heat, water, or solvent to reactivate them to a tacky state. Among the film types are some hot melts, epoxies, phenolics, elastomers, and polyamides. Film adhesives can be die cut into complicated shapes to ensure precision bonding of unusual shapes. Applications for this type of adhesive include bonding plastic bezels onto automobiles, attaching trim to both interiors and exteriors, and attaching nameplates on luggage. While most plastics bond without trouble once the proper adhesive has been selected, a few, notably polyolefins, fluorocarbons and acetals, require special treatment prior to bonding. Untreated polyolefins adhere to very few substrates (a reason that polyethylene is such a popular material for packaging adhesives). Their treating methods include corona discharge, flame treating (especially for large, irregularshaped articles), and surface oxidation by dipping the articles in a solution of potassium dichromate and

2,3

1–4

–

14 –

–

10,14 –

–

4,10

4,8,10 10

– 10 10

– –

–

4,8,10 10

–

– –

10 –

–

– 10

– –

–

2,9,10,13

– 10 –

2,3,7,9

–

– 4,7 10

– –

–

3,4,6,13 3,4,6,13

–

10 10

3,4,6,10

10 10

3,4

3,4,7

– 3,10,13

2,3,7,10,13,14

PVC

–

– –

–

– –

2,3,7,9 –

2,3,7,9

8,10 10

3,10

3 2,3

2,10

10 9,10

3,4,13 –

9,10 10

9,10

1–4

– 3

2,3

Nylon

–

– – –

–

– –

10,12

– –

–

– 10

–

10 –

– 2

2,12

– –

–

– 15

–

– –

–

4,10 –

10,13

4,13

13 10,13

–

– –

–

– 10

–

– –

–

4,12,13 12,13

–

– –

–

4,10 –

–

10,12 – – 4,5,10,12,13

–

– –

–

– –

–

– –

1 1

–

3,8,10,11 –

2,3,10,12,13

– –

–

– –

8,10

– –

–

– –

2,3,14

– 2,3

3,14

–

– –

–

10 –

8,10

– –

1,4

– 2

Polyesters

–

2–4,10,12,14 – – 3,10,12,13

–

– 4,10,14

–

4,7 –

8,10,14

10 –

4,8,10

– 10

2,3

– 2

3,4,10,13

– –

–

– –

–

– –

–

– 10

4,10

– –

4,13

4,13 13

3,4

Source: Adapted from O'Rinda Trauernicht, J. 1970. Plastics Technology, Reinhold Publishing, New York. Note: Elastomeric: 1, Natural rubber. 2, Neoprene. 3, Nitrile. 4, Urethane. 5, Styrene-butadiene. Thermoplastic: 6, Poly(vinyl acetate). 7, Polyamide. Thermosetting: 8, Phenolformaldehyde. 9, Resorcinol, Phenol-resorcinol/formaldehyde. 10, Epoxy. 11, urea-formaldehyde. Resin: 12, Phenolic-poly(vinyl butyral). 13, Polyester. Other: 14, Cyanoacrylate. 15, Solvent.

PF Polyester

–

– –

–

– –

PP PS

3,4,6,10 –

2,3,7,9

– –

10 –

10 10

8,10

– 10

PVC PC

– –

Nylon

Cellulosic Fluorocarbons

2,4,6,10 2,4,6,10

–

Ceramics

Acrylic

Rubber

– 2,3

2,3

10,14 2,4,6,10 2,4,6,10 2,4,6,10 3,10 10

3,10 10

– 10

Acrylic Cellulosics Fluorocarbons

ABS Acetal

3,10

Paper Wood

Acetal

Metals

ABS

TABLE 2.13 Typical Adhesives for Bonding Plastics

Fabrication Processes 291

292

Plastics Technology Handbook

sulfuric acid. Fluorocarbons can be prepared for bonding by cleaning with a solvent such as acetone and then treating with a special etching solution. For acetals, one pretreatment method involves immersing articles in a special solution composed mainly of perchloroethylene, drying at 120°C (250°F), rinsing, and then air drying. 2.21.3.3 Joining of Specific Plastics The same basic handling techniques apply to almost all thermoplastic materials. In the following section, however, a few thermoplastics will be treated separately, with mention of the specific cements most suitable for each. The section on acrylics should be read in connection with the cementing of any other thermoplastics. 2.21.3.3.1 Cast Acrylic Sheeting Articles of considerable size and complexity can be fabricated from methyl methacrylate plastics by joining sections together by solvent welding. The technique described here applies to cast sheeting. With articles made from methyl methacrylate molding powders or extruded rod, tubing or other shapes, joining is generally not as satisfactory as with cast sheeting. With care and practice, the transparency of acrylic resin can be retained in joints with the formation of a complete union of the two surfaces brought into contact. Usually one of the two surfaces to be joined is soaked in the cementing solvent until a soft, swollen layer (cushion) has been formed upon it. This soft surface is then pressed against the surface to be attached and held in contact with it so that the excess solvent contained in the soaked area softens it also. For some purposes, it may be desirable to dissolve clean savings of methyl methacrylate resin in the solvent in order to raise its viscosity so that it can be handled like glue. The most universally applicable type of solvent cement is the polymerizable type, comprising a mixture of solvent and catalyzed monomer. These are mobile liquids, volatile, rapid in action, and capable of yielding strong sound bonds. An example of these is a 40–60 mixture of methyl methacrylate monomer and methylene chloride. Before using this cement. 1.2 grams of benzoyl peroxide per pint of solvent, should be added. Heat treatment or annealing of joints made with solvent cements is highly desirable because it greatly increases the strength of the joint. 2.21.3.3.2 Cellulosics The cements used with cellulosic plastics are of two types: (1) solvent type, consisting only of a solvent or a mixture of solvents; (2) dope type, consisting of a solution of the cellulosic plastic in a solvent or mixture of solvents. Acetone and mixture of acetone and methyl “cellosolve” are commonly used as solvent cements for cellulose acetate. Acetone is a strong solvent for the plastic, but evaporates rapidly. The addition of methyl “cellosolve” retards the evaporation, prevents blushing, and permits more time for handling the parts after application of the cement. A cement of the dope type leaves upon drying a film of plastic that forms the bond between the surfaces to be joined. These cements are generally used when an imperfect of the parts requires filling. A typical composition of the dope-type cement for cellulose acetate is Parts by Weight Cellulose acetate Acetone

130 400

Methyl “cellosolve”

150

Methyl “cellosolve” acetate

Fabrication Processes

293

Other cellulosics, cellulose acetate butyrate and propionate are cemented in accordance with the technique described for cellulose acetate. In the case of dope cements, the plastic to be dissolved in solvents is cellulose propionate. Similarly for ethyl cellulose plastic, the strongest bonds are made by solvents or by solvents bodied with ethyl cellulose plastic. 2.21.3.3.3 Nylon The recommended cements for nylon-to-nylon bonding are generally solvents, such as aqueous phenol, solutions of resorcinol in alcohol, and solutions of calcium chloride in alcohol, sometimes “bodied” by the inclusion of nylon in small percentages. Aqueous phenol containing 10–15% water is the most generally used cement for bonding nylon to itself. The bond achieved by use of this cement is water resistant, flexible, and has strength approaching that of the nylon. Calcium–chloride–ethanol solution bodied with nylon is recommended for nylon-to-nylon joints where there is possibility of contact with foods or where phenol or resorcinol would be otherwise objectionable. For bonding nylon to metal and other materials, various commercial adhesives, especially those based on phenol-formaldehyde and epoxy resins, are sometimes used. Epoxy adhesives (in two-part systems), for example, have been used to produce satisfactory joints between nylon and metal, wood, glass and leather. 2.21.3.3.4 Polycarbonate Solvent cementing of parts of polycarbonate may be effected by the use of a variety of solvents or light solutions of polycarbonate in solvents. Methylene chloride, a 1–5% solution of polycarbonate in methylene chloride, and a mixture of methylene chloride and ethylene dichloride (with a maximum of 40% ethylene dichloride) are commonly recommended. Solvent should be applied to only one of the bonding surfaces while the other half remains dry and ready in the clamping fixture. As soon as the two parts have been put together, pressure should be applied immediately. Pressure between 200 and 600 psi is suggested for best results. Holding time in the pressure fixture is approximately 1–5 min, depending on the size of the bonding area. For bonding molded parts of polycarbonate to other plastics, glass, wood, aluminum, brass, steel, and other materials, a wide variety of adhesives can be used. Generally, the best results are obtained with solventless materials, such as epoxies and urethanes. 2.21.3.3.5 Polyethylene The good solvent resistance of polyethylene and other olefins precludes the use of solvent-type cements. Several commercial rubber-type adhesives produce moderate adhesion with polyethylene that has been surface treated. One technique for surface treatment is to dip polyethylene in a chromic acid bath (made up of concentrated sulfuric acid 150 parts by weight, water 12 parts, and potassium dichromate 7.5 parts) for about 30 sec at 70°C. The parts are rinsed with water after this treatment. Still another effective surface treatment for producing cementable surfaces on polyethylene is electrical discharge. The open oxidizing flame method is also used extensively for this purpose. 2.21.3.3.6 Polystyrene Complex assemblies of polystyrene, usually molded in section, may be joined by means of solvents and adhesives. Polystyrene is soluble in a wide variety of solvents. According to their relative volatilities they may be divided into three groups—fast drying, medium drying, and slow drying. Methylene chloride, ethylene dichloride, and trichloroethylene are some of the fast-drying solvents that produce strong joints. However, they are unsatisfactory for transparent articles of polystyrene because they cause rapid crazing. “Medium-drying” solvents such as toluene, perchloroethylene, and ethyl benzene that have higher boiling temperature are less apt to cause crazing.

294

Plastics Technology Handbook

High-boiling or “slow-drying” solvents such as amylbenzol and 2-ethylnaphthalene often require excessive time for development of sufficient bond strength, but they will not cause crazing to appear so quickly. Up to 65% of a fast- or medium-drying solvent may be added to a slow-drying solvent to speed up the development of initial tack without greatly reducing the time before crazing appears. A bodied, or more viscous, solvent may be required by certain joint designs and for producing airtight or watertight seals. These are made by dissolving usually 5%–15% of polystyrene by weight in a solvent. Solvent-based contact cements provide the strongest bond between polystyrene and wood. These adhesives all have a neoprene (polychloroprene) base and a ketonic-aromatic solvent system. 2.21.3.3.7 Poly(Vinyl Chloride) and Copolymers On account of the relative insolubility of PVC and the markedly increased effect of solvents with the increasing content of vinyl acetate in the copolymer resins, there exists among the vinyl chloride-acetate copolymer system a great diversity of composition and of ability to be cemented by solvents. The copolymer resins are most rapidly dissolved by the ketone solvents, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. Propylene oxide also is a very useful solvent in hastening solution of copolymer resins, especially those of high molecular weight, and of straight PVC. This solvent penetrates the resins very rapidly and, in amounts up to about 20–25%, improves the “bite” into the resin. The chlorinated hydrocarbons also are excellent solvents for the vinyl chloride-vinyl acetate copolymer resins, and are suitable for use in cements. 2.21.3.3.8 Thermosetting Plastics Adhesive bonding is, for various reasons, the logical method of fastening or joining cross-linked and reinforced thermoset plastics of themselves, or to other materials. Since most thermoset plastics are quite resistant to solvents and heat, heat-curing solvent-dispersed adhesives may be used. Such adhesives consist of reactive or thermosetting resins (e.g., phenolics, epoxies, urea-formaldehydes, alkyds, and combinations of these), together with compatible film-formers such as elastomers or vinyl-aldehyde condensation resins. Isocyanates are frequently added as modifiers to improve specific adhesion to surfaces that are difficult to bond. These adhesives may be applied not only in solvent-dispersed form, but also in the form of film, either unsupported, or supported on fabric, glass-mat, and so forth. A great many of outstanding adhesive formulations are based on epoxy resins. A broad spectrum of adhesive formulations with a wide range of available properties have resulted from the use of polymeric hardeners such as polyamides and polyamines, phenolics, isocyanates, alkyds, and combinations of amines with polysulfide elastomers, and the “alloying” of the epoxy with compatible polymeric filmformers, such as poly(vinyl acetate) and certain elastomers. In cemented assemblies of thermoset plastics and metals, where structural strength is generally desired, the adhesive must be more rigid than those used for bonding plastic to plastic, i.e., one with modulus, strength, and coefficient of thermal expansion between those of the plastic and the metal. In many cases, such adhesives are stronger than the plastic itself.

2.21.4 Welding Often it is necessary to join two or more components of plastics to produce a particular setup or to repair a broken part. For some thermoplastics solvent welding is applicable. The process uses solvents which dissolve the plastic to provide molecular interlocking and then evaporate. Normally it requires closeﬁtting joints. The more common method of joining plastics, however, is to use heat, with or without pressure. Various heat welding processes are available. Those processes in common commercial use are described here.

295

Fabrication Processes

2.21.4.1 Hot-Gas Welding Hot-gas welding, which bears a superficial resemblance to welding of metals with an oxyacetylene flame, is particularly useful for joining thermoplastic sheets in the fabrication of chemical plant items, such as tanks and ducting. The sheets to be joined are cleaned, beveled, and placed side by side so that the two beveled edges form a V-shaped channel. The tip of a filler rod (of the same plastic) is placed in the channel, and both it and the adjacent area of the sheets are heated with a hot-gas stream (200–400°C) directed from an electrically heated hot-gas nozzle (Figure 2.87a), which melts the plastics. The plastics then fuse and unite the two sheets. The hot gas may be air in PVC welding, but for polyethylene an inert gas such as nitrogen must be used to prevent oxidation of the plastics during welding. 2.21.4.2 Fusion Welding Fusion or hot-tool welding is accomplished with an electrically heated hot plate or a heated tool (usually of metal), which is used to bring the two plastic surfaces to be joined to the required temperature. The polyfusion process for joining plastic pipes by means of injection-molded couplings is an example of this type of welding. The tool for this process is so shaped that one side of it fits over the pipe while the other side fits into the coupling. The tool is heated and used to soften the outside wall of the pipe and the inside wall of the coupling. The pipe and coupling are firmly pressed together and held until the joint cools to achieve the maximum strength of the weld. The tool is chrome plated to prevent the plastic sticking to its surfaces. 2.21.4.3 Friction Welding In friction or spin-welding of thermoplastics, one of the two pieces to be jointed is fixed in the chuck of a modified lathe and rotated at high speed while the other piece is held against it until frictional heat causes the polymer to flow. The chuck is stopped, and the two pieces are allowed to cool under pressure. The process is limited to objects having a circular configuration. Typical examples are dual-colored knobs, molded hemispheres, and injection-molded bottle halves. 2.21.4.4 High-Frequency Welding Dielectric or high-frequency welding can be used for joining those thermoplastics which have high dielectric-loss characteristics, including cellulose acetate, ABS, and PVC. Obviously, polyethylene, polypropylene, and polystyrene cannot be welded by this method. The device used for high-frequency welding is essentially a radio transmitter operated at frequencies between 27 and 40 MHz. The energy obtained from the transmitter is directed to electrodes of the welding apparatus. The high-frequency field causes the molecules in the plastic to vibrate and rub against each other very fast, which creates frictional heat sufficient to melt the interfaces and produce a weld. Filler rod pressed into weld bed

Welding gun nozzle

Ultrasonic tool Welded joint

(a)

FIGURE 2.87

Shaded surface on rod and material must be molten

Fixed anvil (b)

Welding of plastics: (a) hot-gas welding; (b) ultrasonic contact welding.

296

Plastics Technology Handbook

2.21.4.5 Ultrasonic Welding In ultrasonic welding the molecules of the plastic to be welded are sufficiently disturbed by the application of ultrahigh-frequency mechanical energy to create frictional heat, thereby causing the plastics to melt and join quickly and firmly. The machinery for ultrasonic welding consists of an electronic device which generates electrical energy at 20/50 kHz/sec and a transducer (either magnetostrictive or piezoelectric) to convert the electrical energy to mechanical energy. In the contact-welding method (Figure 2.87b) the ultrasonic force from the transducer is transmitted to the objects (to be welded) through a tool or “horn,” generally made of titanium. The amplitude of the motion of the horn is from 0.0005 to 0.005 in. (0.013–0.13 mm) depending on the design. The method is generally used for welding thin or less rigid thermoplastics, such as films, or sheets of polyethylene, plasticized PVC, and others having low stiffness.

2.21.5 Joining Polymer–Metal Hybrids Polymer–metal hybrid (PMH) structures are increasingly used in industrial components, especially because of associated weight savings. Compared to polymers and polymeric composites, PMH structures are more difﬁcult to join by traditional joining methods, mostly because of the strong dissimilar physical– chemical features of the joining partners. However, constant efforts on developing improved alternative joining techniques for these hybrid structures, such as FricRiveting and injection over molding, have contributed to increasing use of such structures in industrial applications. FricRiveting is a recently patented technique for joining PMH structures. In the simplest process variant, a rotating cylindrical metallic rivet is inserted in a thermoplastic base plate. As the high rotation speed and pressure increase friction, heat is generated and it builds up owing to low polymer thermal conductivity. The local increase in temperature induces the formation of a softened/molten polymeric layer around the tip of the rotating rivet. When the desired penetration depth is achieved, rotation is decelerated and the forging pressure is applied so that the plasticized rivet’s tip is deformed into a paraboloidal shape by the opposite reactive forces of the colder polymer and becomes anchored to the polymeric base after cooling (Figure 2.88). Several other joint conﬁgurations that allow fabrication of hermetic sealed joints by FricRiveting are seen in Figure 2.89. Metal–polymer overlap joints can also be produced by creating friction-welded seams between the metallic part and the rivet (Figures 2.89a and 2.89b), in addition to the mechanical anchoring

Metallic rivet

Plasticized material

Plastic (a)

(b)

FIGURE 2.88 Schematic of friction-riveting technique. (a) Positioning of the components; (b) insertion of the threaded rivet (e.g., aluminum 6060-T6) in the plastic component; (c) forging of the plasticized tip of the rivet, a joint being formed after consolidation of the molten plastic. (After Amancio-Filho, S. T. and dos Santos, J. F. 2009. Polym. Eng. Sci., 49, 1461.)

297

Fabrication Processes

Weld seam

Metal

Plastic Plastic

Metal

(a)

Weld seam

(b)

Plastic

(c)

FIGURE 2.89 Examples of polymer–metal joints made by friction riveting: (a) Lap joint of plastic and metallic (e.g., polyetherimide/aluminum 2024-T351) components; (b) hermetic sealed sandwich joint; and (c) hermetic sealed lap joint of metallic and plastic components. Polyetherimide/aluminum 2024-T351 joints with plain cylindrical rivets have displayed only very little amount of thermally degraded polymer and the mechanical performance of the joint has been found to be as high as 95% of the tensile strength of the metallic partner. (After Amancio-Filho, S. T. and dos Santos, J. F. 2009. Polym. Eng. Sci., 49, 1461.)

effect of the deformed tip of the rivet into the polymeric part. In the process, only a very little amount of thermomechanical degradation of the polymer is observed, although high temperatures within 50%–95% of the metallic rivet melting temperature are observed [91]. The accompanying thermomechanical flaws are small and do not strongly influence the overall mechanical strength of these joints. The main advantages of FricRiveting, as compared to traditional joining techniques, are as follows: (a) little or no surface cleaning or preparation of the joining partners is required; (b) only single side accessibility is required; (c) hermetic sealed joints can be obtained by choosing adequate joint geometry; (d) a wide range of materials can be joined; (e) joints have good mechanical strength and performance; and (f) robotic applications are possible. On the other hand, FricRiveting has a few limitations: (a) allows only spot-like joints; (b) not generally applicable to thermoset polymers; and (c) joints are not reopenable. There are three main PMH technologies currently being employed by the automotive original equipment manufacturers and their tier one and tier two suppliers: (a) injection over-molding technologies, (b) metal over-molding technologies combined with secondary joining operations, and (c) adhesively bonded PMH technologies [91]. These are briefly described below. In injection over-molding technologies, the polymer (typically glass fiber reinforced nylon) is injected around a metal stamping profile that has been placed in an injection mold so that the plastic wraps around the edges of the sheet metal and/or through carefully designed extruded holes or buttons. No secondary operations are required in the process and the drawing oils/greases do not need to be removed from the metal stamping. In metal over-molding technologies, a steel stamping is placed in an injection mold in order to coat its underside typically with a thin layer of reinforced nylon. The polymer-coated surface of the metal insert is then ultrasonically welded, in a secondary operation, to an injection-molded nylon subcomponent. In this process, a closed-section structure with continuous bond lines is produced that affords a high loadbearing capability. The hollow core of the part permits functional integration like cable housings and air or water channels. In adhesively bonded technologies, glass-fiber-reinforced polypropylene is typically joined to a metal stamping using an adhesive that is applied by high-speed robots. Adhesive bonding minimizes stress

298

Plastics Technology Handbook

concentrations and acts as a buffer to absorb contact stresses between the metal and polymer components of the joint. Moreover, the joining process enables creation of closed-section structures that offer high load-bearing capabilities and permit functional integration like direct mounting of air bags in instrumentpanel beams or incorporation of air or water channels inside door modules. The aforesaid PMH technologies, though used widely in the manufacturing of various nonstructural and load-bearing automotive body-in-white (BIW) components, are known to display some signiﬁcant shortcomings. [Note: BIW refers to the stage in automotive manufacturing in which a car body’s sheet metal components have been welded together, but before doors, hoods, motor, trim, etc. have been added and before painting has been done.] For example, to achieve polymer-to-metal interlocking, the injection over-molding technologies rely on the presence of holes and free edges in the metal stamping, but the holes may compromise the structural integrity of the stamping and edge over-molding may often be restricted. The disadvantages of adhesively bonded technologies are the adhesive cost, long curing time, and limited ability of the adhesive to withstand aggressive chemical and thermal environments encountered in BIW pretreatment in paint shops. Moreover, the secondary (joining) operations needed in both metal over-molding and adhesively bonded PMH technologies are associated with additional cost. To overcome the aforesaid shortcomings of the PMH technologies and to help meet the needs of automotive equipment manufacturers for a cost-effective, robust, reliable PMH technology that can be used for the manufacturing of load-bearing BIW components, a new approach, the so-called direct adhesion PMH technology, was proposed [92]. The approach uses direct adhesion and mechanical interlocking, and is named “clinch-lock PMH technology.” No holes and free edges are required in the process and a structural adhesive is not used. In the clinch-lock PMH technology, ﬁrst, shallow millimeter-size impressions/indentations within the metallic stamping are produced using a simple stamping process and the impressions are used to anchor the subsequently injection-molded plastic ribs to the metal stamping [93]. To ensure an effective metal/ polymer interlocking, the indentations should have a “dove tail” shape (Figure 2.90a). The joint provides

(a)

(b)

FIGURE 2.90 Schematic of two types of polymer–metal clinch-locking. (a) Indentations have a “dove tail” shape; (b) plastic is injected around metal-stamping protrusion. (After Grujicic, M., Sellappan, V., Arakere, G., Ochterbeck, J. M., Seyr, N., Obieglo, A., Erdmann, M., and Holzleitner, J. 2010. Multidiscipline Modeling in Materials and Structures, 6(1), 23.)

Fabrication Processes

299

effective metal/polymer connectivity by at least two distinct mechanisms, namely, mechanical interlocking and enhanced adhesion owing to an increased metal/polymer contact surface area. In Figure 2.90b, an alternative method of polymer–metal clinch-locking is shown in which the plastic is injected around the metal-stamping protrusion instead of into the metal-stamping impression. When compared with the existing competing PMH technologies, the new clinch-lock PMH technology offers two main advantages, namely, no holes in the metal stamping or over-molding of stamping ﬂanges are required and no expensive adhesive is needed to obtain the required level of polymer–metal mechanical interconnectivity.

2.22 Decoration of Plastics Commercial techniques for decorating plastics are almost as varied as plastics themselves. Depending on end-use applications or market demands, virtually any desired effect or combination of effects, shading of tone, and degree of brightness can be imparted to flexible or rigid plastics products. The primary decorating technique is raw-materials coloring achieved at the compounding stage. Although most thermoplastics are produced in natural white or colorless transparent form, color is usually added by directly blending colorants into the base resin prior to the processing stage. These colorants (or color concentrates) are available in a wide range of stock shades with precise tinctorial values. Colors can also be matched to exact customer specifications and these specifications kept in computer memory to ensure batch-to-batch or order-to-order consistency. Color blending can also be utilitarian, as in color-coded wire- and cable-sheathing. Besides basic raw-materials coloring, mentioned above, designers have a large palette of decorating media at their disposal. Plastics can be decorated in various ways, which include painting processes, direct printing, transfer decoration, in-mold decoration, embossing, vacuum metallizing, sputtering, and electroplating. Most of these processes require bonding other media, such as inks, enamels, and other materials to the plastics to be decorated. Some plastics, notably polyolefins and acetals, are, however, highly resistant to bonding and need separate treatment to activate the surface. Commonly used treatment processes are flame treatment, electronic treatments such as corona discharge and plasma discharge, and chemical treatment. In flame treatment, plastic objects such as bottles and film are passed through an oxidizing gas flame. Momentary contact with the film causes oxidation of the surface, which makes it receptive to material used in decorating the product. In the corona discharge process the plastic film to be treated is allowed to pass over an insulated metal drum beneath conductors charged with a high voltage. When the electron discharge (“corona”) between the charged conductors and the drum strikes the intervening film surface, oxidation occurs and makes the surface receptive to coatings. Molded products are also treated in a similar manner, often by fully automatic machinery. In the plasma process [94], air at low pressure is passed through an electric discharge, where it is partially dissociated into the plasma state and then expanded into a closed vacuum chamber containing the plastic object to be treated. The plasma reacting with the surfaces of the plastic alters their physicochemical characteristics in a manner that affords excellent adhesion to surface coatings. The process can be used for batch processing of plastics products, including films which may be unreeled in the vacuum chamber for treatment. Acetal resin products are surface treated by a chemical process consisting of subjecting the product to a short acid dip that results in an etched surface receptive to paint.

2.22.1 Painting Virtually all plastics, both thermoplastic and thermosetting, can be pained, with or without priming or other preliminary preparation procedures. The process, however, requires special consideration of the

300

Plastics Technology Handbook

resin-solvent system to achieve adhesion, adequate covering, and chemical resistance. Painting operations have the advantage of being as simple or as sophisticated as the application may dictate. Plastics parts or materials can be coated manually by brushing, dipping, hand-spray painting, flow coating or roller coating; they can be automatically spray-painted with rotating or reciprocating spray guns, and electro-statically painted using a conductive precoating procedure. Painting operations have the advantage of offering almost unlimited color options as well as great variety of surface finishes and final surface properties to meet such needs as gloss, UV resistance, abrasion resistance, and chemical resistance.

2.22.2 Printing The primary printing presses used in plastics are gravure printing, flexography, silk-screen printing, and pad printing. 2.22.2.1 Gravure Printing Gravure printing is a process that requires the use of an engraved metal cylinder or roller. Rotogravure is thus an appropriate title for this printing process. The engraving or etching process on the surfaces of the metal cylinder results in recessed areas that pick up ink or liquid coatings from a reservoir. With proper formulation of printing ink, the gravure process can be applied to a great variety of plastic substrates. Virtually all thermoplastic film or sheet applications are printable by this process. A good example of the capabilities of the gravure process is the printing of woodgrain patterns on carrier foil for use in hot-stamping applications (described later). Woodgrain patterns may require the application of several coatings to achieve the proper effect. Several engraved cylinders can be used in sequence for continuous printing. 2.22.2.2 Flexography Flexography uses a flexible printing plate, typically a metal-silicone rubber-bonded combination with the rubber surface processed to leave the printing surface raised over the back-ground area. The raised and recessed areas on the surface can be fabricated through photographic etching and/or engraving. After transferring the ink from a reservoir through a roller-doctor blade system onto the curved flexible plate, the ink is transferred off the raised portions to the material to be printed. The process is suitable for a variety of applications, ranging from simple label film to decoration on molded parts such as plaques, medallions or wall tile. However, the flexible printing plates used in flexography do not permit the very fine detail that can be achieved on metal surfaces such as used in gravure printing. There are also limitations to the size and shape of articles that can be printed. 2.22.2.3 Screen Process Printing The process derived its name from the use of silk cloth or silk screen in the transfer of printing ink to articles to be printed. Integral to the process is the use of a suitable open-weave cloth or screen (silk is still commonly but not exclusively used) stretched over a framework. Screens made of nylon or other synthetic material are often employed, as also stainless steel or other metallic screens. The stretched screen is selectively coated through the use of a stencil corresponding to an art copy of the image to be printed (see “Silkscreen Printing” in Chapter 5); this coated (closed) area resists the passage of printing ink, which can only penetrate through the uncoated (open) areas of the screen. There are various ways to prepare the screen for printing, other than stenciling. Polyolefins such as polyethylene and polypropylene must be surface treated before being printed. The most effective way is in an integrated machine where surface treatment takes place right before printing. A time lapse will mean that the treatment will lose some of its effect. Three methods are used; flame treatment, corona discharge, and chemical treatment. Flame treatment is considered the most practical and most widely used.

Fabrication Processes

301

2.22.2.4 Pad Printing Pad printing uses printing principles and techniques from letterpress and flexography. The uniqueness of the process has to do with the use of a smooth silicone pad that picks up ink impression from an engraved or etched plate and transfers it to the product to be decorated. The engraved plate, known as a cliche, is produced in a manner similar to that of printing plates for offset or gravure roller printing. The silicone pickup pad can be designed to meet almost any shape and configuration of the product part. This ability has prompted tremendous growth in pad printing. An additional capability of the process is that it can print several colors and impressions within one cycle of operation. Coatings can be layered when wet to accomplish multicolor designs with very accurate registration and impression quality. 2.22.2.5 Flex Printing Flex is a sheet of poly(vinyl chloride) (PVC), or simply “vinyl,” widely used to produce high-quality digital print for outdoor hoardings and banners. The weights of banner substrate may range from as light as 9 oz to as heavy as 22 oz per square yard, and may be double sided or single sided. A vinyl banner can also be reinforced with nylon for added strength and durability. Large banners (which can be so large as to cover the side of a building) are printed on a special mesh material to allow passage of air. Such mesh banners are a great solution for long-term use in windy outdoor locations. Usually, matte vinyl is chosen for indoors and glossy is chosen for outdoors. Hoardings and banners are commonly printed with large format inkjet printers (manufactured by companies such as HP, EFi Vutek, Mimaki, Roland, Mutoh, or one of many Chinese and Korean manufacturers) in CMYK mode. CMYK refers to the four inks typically used in color printing, namely, cyan, magenta, yellow, and black. (The last letter in “black” is chosen because “B” means blue.) The ink is typically applied in the order of the abbreviations. In CMYK printing with color halftoning (also called screening), which allows for less than full saturation of the printing colors, a full continuous range of colors can be produced. However, without halftoning, the three primary colors could be printed only as solid blocks of color, and therefore only seven colors can be produced, namely, the three primaries themselves plus three secondary colors produced by layering two of the primaries (cyan and yellow producing green, cyan and magenta producing blue, and yellow and magenta producing red) plus layering all three of them resulting in black. Very large vinyl banners may be produced using large-format inkjet printers of >2.5 m width, or computer-controlled airbrush devices that print the ink directly onto the banner material. Readily available banner templates may be used or a color graphics file can be created with standard tools for highresolution digital printing. Banners are also produced by applying individual elements cut from selfadhesive vinyl by a computer-driven vinyl cutter. A vinyl cutter (or vinyl plotter) is a computer-controlled plotting device with a blade instead of a pen. When a vector-based design created in a software program is sent to the vinyl cutter, the latter cuts along the vector paths laid out in the design from vinyl sheet or other material. The design may then be removed from the sheet and used for scrapbooking, card making, sign making, sewing, or many other crafts. Thus, computer-designed vector files with patterns and letters can be directly cut on a roll of vinyl that has been mounted and fed into the vinyl cutter. Large-format vinyl cutters are available for those using the machine to make large signs or other large-format designs. These machines tend to cut vinyl only.

2.22.3 Hot Stamping Hot stamping is one of the original methods of decorating plastics materials. Though familiarly known as hot stamping, the terminology “coated foil transferring” might be more appropriate since in this process the printed coating on a carrier ﬁlm is transferred onto a plastic surface. Secure adhesion is accomplished with the use of heat, pressure, and time under controlled conditions.

302

Plastics Technology Handbook

The key to the process is the use of a carrier film (usually a polycarbonate, polyester, or cellophane) upon which various coatings provide the desired decorative effect. The coated foil is placed over the plastic to be decorated, and a heated die forces the foil onto the plastic. The proper control of heat, pressure, and time transfers the coating off the carrier foil onto the plastic. The hot-stamping process is a versatile tool for plastics decoration. A wide variety of coatings can be deposited on the carrier film which allows the process to be used on almost any thermoplastic material and many thermosets. Metallic effects can be imparted by depositing microthin coatings of gold or silver or chrome; multiple coatings can be applied to the carrier film to achieve such effects as woodgraining, marbleizing, or multicolored designs, Three-dimensional decorative effects can also be achieved by embossing the surface of the carrier medium coating.

2.22.4 In-Mold Decorating As the name implies, in-mold decorating is a process in which a predecorated overlay (film), or decal, is placed in the mold, where the decorated element is fused to the molded part during the heating/cooling cycles of the molding operation. Since the decorated coating is bonded between the plastic part and the film (which will be the exterior surface), thus forming an integral part of the product, it produces one of the most durable and permanent decorations. High-quality melamine dinnerware is decorated by this method, and so are a host of other household and hardware plastics goods. In-mold decoration can be done with either injection molding of thermoplastics or compression molding of thermosets. Thermosetting plastics are decorated with a two-stage process. For melamine products, for example, the mold is loaded with the molding powder in the usual manner and closed. It is opened after a partial cure, and the decorative “foil” or overlay is placed in position. The mold is then closed again, and the curing cycle is completed. The overlay consists of a cellulose sheet having printed decoration and covered with a thin layer of partially cured clear melamine resin. During the molding cycle the overlay is fused to the product and becomes a part of the molding. The process is relatively inexpensive, especially when a multicolor decoration is required. For in-mold decoration of thermoplastic products, single-stage process is used. The foil or overlay is thus placed in the mold cavity prior to the injection of the polymer. It is held in place in the mold by its inherent static charge. Shifting is prevented during molding by inducing an additional charge by passing the wand of an electronic static charging unit over the foil after it is properly positioned. The overlay, in all cases, is a printed or decorated film (0.003–0.005 in. thick) of the same polymer. Thus, polystyrene film is used for a polystyrene product, and polypropylene film for a polypropylene product. A similar procedure may also be used for decorating blow-molded products.

2.22.5 Embossing Embossing is used for producing a tactile texture or pattern on plastics sheet or ﬁlm. As the process involves the use of heat and pressure to texture a semiﬁnished substrate, embossing is largely limited to thermoplastic materials. However, it can be adapted to thermoset composites, such as melamineimpregnated sheet stock. Embossing is most commonly done with a two-roller system, in which one roller carries the embossing pattern and the other provides the essential pressure backup and feeding actions. Texture or pattern can be applied to the embossing or surface roller through a variety of processes, including conventional engraving, chemical engraving, etching, and laser cutting. Embossing can also be performed without rollers, e.g., using textured aluminum foil in one-time use, or stainless steel plates with engraved textures can be used in the press cycle time and again, offering multiple impression economies.

Fabrication Processes

303

Embossing is most frequently used as a method of decorating nonslip packaging materials, vinyl wall coverings, furniture laminates, building-panel laminates, textured foil for hot stamping, and other applications where the innate quality of three-dimensional printing is of value.

2.22.6 Electroplating Electroplating is a chemical process for depositing heavy metals on plastics to achieve decorative effects and/or upgraded functionality. Since plastics are nonconductors of electricity, electroplating requires that the surface be properly conditioned and sensitized to receive metallic coatings. The principle of electroplating is to electrically conduct metal atoms such as copper, nickel and chrome off anodes placed within the plating baths through the plating solutions and onto the plastic production part. The target, i.e., the production part, acts as a cathode via connection to conductive plating racks, the part being attached to the plating rack with metal holding devices, spring-loaded contacts or prongs. The point of contact between the plating rack and the plastic part forms the continuity of the current flow from anode through the solution onto the plastic part. The process of electroplating begins with the plastic part attached to the plating rack being subjected to preplate procedure, which is designed to create a surface on the plastic parts that will develop a bond between the plastic and the first nickel or copper deposit. These initial deposits are extremely thin, in the micron (10−6 mm) range. This first deposit is designed to increase conductivity uniformly over the plastic surface. When preplating is completed (and the plastic articles have a conductive coating), it is possible to proceed to the electroplating operation, which is very similar to conventional electroplating on metal. Electroplating of plastic products provides the high-quality appearance and wear resistance of metal combined with the light weight and corrosion resistance of plastics. Plating is done on many plastics, including phenolic, urea, ABS, acetal, and polycarbonate. Many automotive, appliance, and hardware uses of plated plastics include knobs, instrument cluster panels, bezel, speaker grilles, and nameplates. In marine searchlights zinc has been replaced by chrome-plated ABS plastics to gain lighter weight, greater corrosion resistance, and lower cost. An advantage of plastics plating is that, unlike metal die castings, which require buffing in most cases after plating, plastics do not ordinarily require this extra expensive operation. The use of plated plastics also affords the possibility of obtaining attractive texture contrasts.

2.22.7 Vacuum Metallizing Vacuum metallizing is a process whereby a bright thin film of metal is deposited on the surface of a molded product or film under high vacuum. The metal may be gold, solver, or most generally, aluminum. The process produces a somewhat delicate surface compared to electroplating. The metallizing process can be used on virtually all properly (surface) prepared thermoplastic and thermosetting materials. Small clips of the metal to be deposited are attached to a filament. When the filament is heated electrically, the clips melt and, through capillary action, coat the filament. An increased supply of electrical energy then causes vaporization of this metal coating, and plating of the plastic product takes place. To minimize surface defects and enhance the adhesion of the metal coating, manufacturers initially give the plastics parts a lacquer base coat and dry in an oven. The lacquered parts are secured to a rack fitted with filaments, to which are fastened clips of metal to be vaporized. The vaporization and deposition are accomplished at high vacuum (about 1/2 micron). The axles supporting the part holding the fixtures are moved so as to rotate the parts during the plating cycle to promote uniform deposition. The thickness of the coating produced is about 5 × 10−6 in. (127 nm). After the deposition is completed, the parts are removed and dipped or sprayed with a top-coat lacquer to protect the metal from abrasion. Color tones, such as gold, copper, and brass may be added to this coating if desired.

304

Plastics Technology Handbook

Vacuum metallizing of polymer films, such as cellulose acetate, butyrate, and Mylar, is performed in essentially the same way. Film rolls are unreeled and rewound during the deposition process to metallize the desired surface. A protective abrasion-resistant coating is then applied to the metallized surface in an automatic coating machine. Vacuum metallizing is a versatile process used in a great variety of applications. Examples range from highly decorative cosmetic closures to automotive grilles and instrument clusters. Vacuum metallized plastic parts can replace metal parts with large saving in manufacturing costs and weight. The process can also serve functional needs, such as lamp reflectors or diffusion grids for overhead fluorescent lighting. Vacuum metallizing on interior surfaces of computer or communication equipment provides a degree of radio frequency interference shielding.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

Modern Plastics Encyclopedia, Vols. 42–45, McGraw-Hill, New York, 1964–1967. Bikales, N. M., ed. 1971. Molding of Plastics, p. 737, Interscience, New York. Butler, J. A. 1964. Compression Transfer Molding, Plastics Institute Monograph, Iliffe, London. Vaill, E. W. 1962. Modern Plastics, 40, 1A, Encycl. Issue, 767. Maiocco, A. L. 1964. Transfer molding, past, present, and future, SPE Tech. Pap., 10, XIV-4. Frados, J., ed. 1976. Plastics Engineering Handbook, 4th Ed., Van Nostrand Reinhold, New York. Rubin, I. I. 1973. Injection Molding of Plastics, Wiley, New York. Brown, J. 1979. Injection Molding of Plastic Components, McGraw-Hill, New York. Dyn, J. B. 1979. Injection Molds and Molding, Van Nostrand Reinhold, New York. Lukov, L. J. 1963. Injection molding of thermosets, SPE. J., 13, 10, 1057. Morita, Y. 1966. Screw injection molding of thermosets, SPE. Tech. Pap., 12, XIV-5. O’Brien, J. C. 1976. Business injection molding thermosets, Plast. Engl., 32, 2, 23. Schenkel, G. 1966. Plastics Extrusion Technology and Theory, Elsevier, New York. Bikales, N. M. 1971. Extrusion and Other Plastics Operations, Interscience, New York. Fischer, E. G. 1976. Extrusion of Plastics, Newness-Butterworth, London. Van, R. T., De Hoff, G. R., and Bonner, R. M. 1968. Mod. Plastics, 45, 14A, Encycl. Issue, 672. Fisher, E. G. 1971. Blow Molding of Plastics, Iliffe, London. Elden, R. A. and Swan, A. D. 1971. Calendering of Plastics, Plastics Institute Monograph, Iliffe, London. Mark, H. F., Atlas, S. M., and Cernia, E., eds. 1967. Man-Made Fibers: Science and Technology, Vol. 3, p. 91. Interscience, New York. Moncrieff, R. W. 1963. Man-Made Fibers, Wiley, New York. Riley, J. L. 1956. In Polymer Processes, C.E. Schildknecht, ed., p. 91. Interscience, New York, Chap. XVIII. Carraher, C. E. 2002. Polym. News, 27, 3, 91. Ondarcuhu, T and Joachim, C. 1998. Europhys. Lett., 42, 2, 215. Martin, C. P. 1996. Chem. Mater., 8, 1739. Whitesides, G. M. and Grzybowski, B. 2002. Science, 295, 2418. Deitzel, J. M., Kleinmeyer, J., Hirvonen, J. K., and Beek, T. N. C. 2001. Polymer, 42, 8163. Formalas, A. 1934. US Patent 1,975,504; Formalas, A. 1944. US Patent, 2,349,950. Fakirov, S., ed. 2016. Nano-size Polymers: Preparation, Properties, Applications, Springer International Publishing AG Switzerland. Fong, H. and Renekar, D. H. 1999. J. Polym. Sci.: Part B Polym. Phys., 37, 24, 3488. Doshi, J. and Renekar, D. H. 1995. J. Electrostatics, 35, 2–3, 151. Liu, H. Q. and Hsieh, Y. L. 2002. J. Polym. Sci. Part B: Polym. Physics, 40, 2119. Zussman, E., Yarin, A. L., and Weihs, D. 2002. Experiments in Fluids, 33, 315. Gibson, P. W., Schreuder-Gibson, H. L., and Riven, D. 1999. AIChE J., 45, 1, 190.

Fabrication Processes

305

34. Fertala, A., Han, W. B., and Ko, F. K. 2001. J. Biomed. Mater. Res., 57, 48. 35. Buchko, C. J., Chen, L. C., Shen, Y., and Martin, D. C. 1999. Polymer, 40, 7397. 36. Jin, H. J., Fridrikh, S., Rutledge, G. C., and Kaplan, D. 2002. Abstracts of Papers Am. Chem. Soc., 224, 1–2, 408. 37. Boland, E. D., Simpson, D. G., Polanski, K. J., and Bowlin, G. L. 2001. J. Macromol. Sci. Pur. Appl. Chem., A38, 12, 1231. 38. Mathews, J. A., Wnek, G. E., Simpson, D. G., and Bowlin, G. L. 2002. Biomolecules, 3, 2, 232. 39. Li, D. and Xia, Y. 2004. Adv. Mater., 16, 14, 1151. 40. Jalili, R., Morshed, M., Abdol Karim, S., and Ravandi, H. 2006. J. Appl. Polym. Sci., 101, 4350. 41. Zhou, F.-L., Gong, R.-H., and Porat, I. 2009. Polym. Int., 58, 331. 42. Kim, G., Choo, Y. S., and Kim, W. D. 2006. Europ. Polym. J., 42, 9, 2031. 43. Yanin, A. L. and Zussman, E. 2007. Polymer, 45, 9, 2977. 44. Moghe, A. K. and Gupta, B. S. 2008. Polymer Rev., 48, 353. 45. Butzko, R. L. 1958. Plastics Sheet Forming, Van Nostrand Reinhold, New York. 46. Sarvetnick, H. A., ed. 1972. Plastisols and Organosols, p. 58, Van Nostrand Reinhold, New York. 47. Lubin, G., ed. 1982. Handbook of Composites, p. 58, Van Nostrand Reinhold, New York. 48. https://en.wikipedia.org/wiki/Pultrusion. 49. Lee, W. J., Seferis, J. C., and Bonner, D. C. 1986. Prepreg processing science, SAMPE Q., 17, 2, 58. 50. Tsunoda, Y. 1966. U.S. Patent 3,286,969. 51. Otani, S. 1965. Carbon, 3, 213. 52. Daumit, G. P. 1987. Latest in carbon fibers for advanced composites, Performance Plastics ‘87 First International Ryder Conference on Special Performance Plastics and Markets, Feb., pp. 11–13. Atlanta, Georgia. 53. www.sollercomposites.com/fabricchoice.html 54. http://encyclopedia2.thefreedictionary.com/computer+numerical+control 55. http://www.tapplastics.com 56. http://www.performancecomposites.com/about-composites-technical-info/124-designing-with -carbon-fiber.html 57. https://en.wikipedia.org/wiki/Reinforced_carbon-carbon 58. Becker, W. E., ed. 1979. Reaction Injection Molding, p. 368, Van Nostrand Reinhold, New York. 59. Kresta, J. E., ed. 1985. Reaction Injection Molding, ACS Symp. Ser., Vol. 270, p. 368, American Chemical Society, Washington, DC. 60. PELASPAN Expandable Polystyrene, Form 171–414, Dow Chemical Co., 1966. 61. Klempner, D. and Frisch, K. C., eds. 1991. Handbook of Polymeric Foams and Foam Technology, p. 368, Hanser, Munich. 62. Zizlsperger, J., Statny, F., Beck, G., and Tatzel, H. 1970. U.S. Patent 3504 068 (to BASF). 63. One-Step Urethane Foams, Bull. F40487, Union Carbide Corp., 1959. 64. Phillips, L. N. and Parker, D. B. V. 1964. Polyurethanes: Chemistry, Technology, and Properties, Iliffe, London. 65. Harris, T. G. 1981. U.S. Patent 4281069 1981 10728 (to Armstrong World Industries, U.S.A.). 66. Chandra, P. and Kumar, A. 1979. Indian Patent 147107 19791117 (to Bakelite Hylam Ltd., India). 67. Lasman, H. R. 1967. Mod. Plastics, 45, 1A, Encycl. Issue, 368. 68. Gluck, D. G., Hagan, J. R., and Hipchen, D. E. 1980. In Advances in Urethane Science and Technology, Vol. 7, K.C. Frisch and D. Klempner, eds., p. 137, Technomic Publ. Co., Lancaster, Pennsylvania. 69. Noorani, R. I. 2005. Rapid Prototyping: Principles and Applications. Wiley, New York. 70. Chua, C. K., Leong, K. F., and Lim, C. S. 2010. Rapid Prototyping: Principles and Applications, World Scientific. 71. http://www.eos.info/additive_manufacturing/for_technology-interested 72. https://en.wikipedia.org/wiki/3D_printing 73. Schidrowitz, P. and Dawson, T. R., eds. 1952. History of Rubber Industry, p. 137, IRI, London.

306

Plastics Technology Handbook

74. Hoffman, W. 1967. Vulcanization and Vulcanizing Agents, Maclaren, London. 75. Nourry, A., ed. 1962. Reclaimed Rubber, Its Developments, Applications and Future, p. 137, Maclaren, London. 76. Ghosh, P. 1990. Polymer Science and Technology of Plastics and Rubbers, Tata McGraw-Hill, New Delhi. 77. Kojima, M., Tosaka, M., Ikeda, Y., and Kohjiya, S. 2005. J. Appl. Polym. Sci., 95, 137. 78. Parker, D. H. 1965. Principles of Surface Coating Technology, Wiley, New York. 79. Feng W., Patel S. H., Young M.-Y., Jurino III J. L., and Xanthos M. 2007. Advances in Polym. Tech., 26, 1, 1. 80. Intelligent Technology, www.ﬁelderschoicedirect.com 81. Patel, G. N. 2009. U.S. Patent 2009/0224176 A1, Sep 10. 82. Pan, Y. V., Wesley, R. A., Luginbuhl, R., Denton, D. D., and Ratner, B. D. 2001. Biomacromolecules, 2, 32. 83. Ito, Y., Chan, G., Guam, Y. Q., Imanishi, Y. 1997. Langmuir, 13, 2756. 84. Takei, Y. G., Aoki, T., Sanui, K., Ogata, N., Sakurai, Y., and Okano, T. 1994. Macromolecules, 27, 6163. 85. Peng, T. and Chen, Y. L. 1998. J. Appl. Polym. Sci. 70, 2133. 86. Lee, Y. M. and Shim, J. K. 1997. Polymer, 38, 1227. 87. White, S. R., Sottos, N. R., Geubelle, P. H., Moore, J. S., Kessler, M. R., Sriram, S. R., Brown, E. N., and Viswanathan, S. 2001. Nature, 409, 794. 88. Brown, E. N., Kessler, M. R., Sottos, N. R., and White, S. R. 2003. J. Microencapsulation, 20, 6, 719. 89. Gabriel, S., Jérôme, R., and Jérôme, C. 2010. Prog. Polym. Sci., 35, 113. 90. O’Rinda Trauernicht, J. 1970. Bonding and joining plastics, Plastics Technology, Reinhold Publishing, New York. 91. Amancio-Filho, S. T. and dos Santos, J. F. 2009. Polym. Eng. Sci., 49, 1461. 92. Grujicic, M., Sellappan, V., Omar, M. A., Seyr, N., and Erdmann, M. 2008. J. Matls. Processing Tech., 197, 363. 93. Grujicic, M., Sellappan, V., Arakere, G., Ochterbeck, J. M., Seyr, N., Obieglo, A., Erdmann, M., and Holzleitner, J. 2010. Multidiscipline Modeling in Materials and Structures, 6(1), 23. 94. Harris, R. M., ed. 1999. Coloring Technology for Plastics, Chemtec Publishing, Toronto.

3 Plastics Properties and Testing 3.1 Introduction There are two stages in the process of becoming familiar with plastics. The first is rather general and involves an introduction to the unique molecular structures of polymers, their physical states, and transitions which have marked influence on their behavior. These have been dealt with in Chapter 1. The second stage, which will be treated in this chapter, is more specific in that it involves a study of the specific properties of plastics which dictate their applications. Besides the relative ease of molding and fabrication, many plastics offer a range of important advantages in terms of high strength/weight ratio, toughness, corrosion and abrasion resistance, low friction, and excellent electrical resistance. These qualities have made plastics acceptable as materials for a wide variety of engineering applications. It is important therefore that an engineer be aware of the performance characteristics and significant properties of plastics. In this chapter plastics have been generally dealt with in respect to broad categories of properties, namely, mechanical, electrical, thermal, and optical. In this treatment the most characteristic features of plastic materials have been highlighted. An important facet of materials development and proper materials selection is testing and standardization. The latter part of this chapter is therefore devoted to this aspect. It presents schematically (in simplified form) a number of standard test methods for plastics, highlighting the principles of the tests and the properties measured by them.

3.2 Mechanical Properties Several unfamiliar aspects of material behavior of plastic need to be appreciated, the most important probably being that, in contrast to most metals at room temperature, the properties of plastics are time dependent [1–4]. Then superimposed on this aspect are the effects of the level of stress, the temperature of the material, and its structure (such as molecular weight, molecular orientation, and density). For example, with polypropylene an increase in temperature from 20 to 60°C may typically cause a 50% decrease in the allowable design stress. In addition, for each 0.001 g/cm3 change in density of this material there is a corresponding 4% change in design stress. The material, moreover, will have enhanced strength in the direction of molecular alignment (that is, in the direction of ﬂow in the mold) and less in the transverse direction. Because of the inﬂuence of so many additional factors on the behavior of plastics, properties (such as modulus) quoted as a single value will be applicable only for the conditions at which they are measured. Properties measured as single values following standard test procedures are therefore useful only as a means of quality control. They would be useless as far as design in concerned, because to design a plastic component it is necessary to have complete information, at the relevant service temperature, on the 307

308

Plastics Technology Handbook

time-dependent behavior (viscoelastic behavior) of the material over the whole range of stresses to be experienced by the component.

3.2.1 Stress and Strain Any force or load acting on a body results in stress and strain in the body. Stress represents the intensity of the force at any point in the body and is measured as the force acting per unit area of a plane. The deformation or alteration in shape or dimensions of the body resulting from the stress is called strain. Strain is expressed in dimensionless units, such as cm/cm, in./in., or in percentage. Corresponding to the three main types of stress—tensile, compressive, and shear—three types of strain can be distinguished. Thus, tensile strain is expressed as elongation per unit length (Figure 3.1a), e = D‘=‘0 = (‘ − ‘0 )=‘0

(3.1)

and compressive strain as contraction per unit length (Figure 3.1b), e = D‘=‘0 = (‘0 − ‘)=‘0

(3.2)

If the applied force or load, F, is tensile or compressive, the resulting tensile or compressive stress, s, is deﬁned by s = F=A

(3.3)

where A is the cross-sectional area perpendicular to the direction in which the force acts (Figure 3.1a). The shearing stress is deﬁned by a similar equation t = Fs =A

(3.4)

where Fs is the shearing force acting on an area A, which is parallel to the direction of the applied force (Figure 3.1c). Shear strain is measured by the magnitude of the angle representing the displacement of a certain plane relative to the other, due to the application of a pure shear stress, such as a in Figure 3.1c. The corresponding shear strain g may be taken equal to the ratio aa′/ab (=tan a). A shear strain is produced in torsion, when, for example, a circular rod is twisted by tangential forces, as shown in Figure 3.1d. For small deformations the shear strain, g, can be calculated from the triangle ABC g = BC=AB = rq=‘

(3.5)

where r is the radius and q is the angle of twist. An ideal elastic material is one which exhibits no time effects. When a stress is applied the body deforms immediately, and it recovers its original dimensions completely and instantaneously when the F

Fs a a´ d´

d´

(b)

(c)

d´ α

r r

b (a)

A d

(d) Fs

B C

FIGURE 3.1 (a) Tensile or longitudinal strain, e = (‘ − ‘0 )=‘0 . (b) Compressive strain, e = (‘0 − ‘0 )=‘0 . (c) Shear strain, g = aa′/ab. (d) Shear strain in torsion g = rq=‘.

309

Plastics Properties and Testing

stress is removed. When the ideal elastic body is subjected to tensile (or compressive) stress, the proportionality is expressed as s=E·e (3.6) where s is the applied stress (tensile or compressive) in lbf/in.2, kgf/cm2 or other appropriate units of force per unit cross-sectional area (Equation 3.3), e is the axial strain (Equation 3.1 and Equation 3.2), and E is the modulus of elasticity, commonly known as the Young’s modulus. The proportionality law as deﬁned above is known as Hooke’s law. Likewise, if the ideal solid is subjected to a shear stress (t), then the shear strain (g ) developed as a function of stress applied is given by the expression t=G·g

(3.7)

Here, the proportionality constant G is known as the shear modulus, also called the modulus of rigidity. The elastic constants in tensile deformation and shear deformation are summarized and compared below: Tensile (Figure 3.1a)

Shear (Figure 3.1c)

Stress

s = F/A

t = F/A

Strain Modulus

e = (‘ − ‘0 )=‘ E = s/e

g = tan a G = t/g

Compliance

D = e/s

J = g /t

It may be noted that for an ideal elastic body compliance is the inverse of modulus. The modulus of elasticity, E, and the modulus of rigidity, G, as deﬁned above, apply under longitudinal and shear forces, respectively. When a hydrostatic force is applied, a third elastic modulus, the modulus of compressibility or bulk modulus, K, is used. It is the reciprocal of compressibility, b, and is deﬁned as the ratio of the hydrostatic pressure, sh, to the volumetric strain, DV/V0: K=

sh 1 = DV=V0 b

(3.8)

As indicated in Figure 3.1, an elongation (or compression) in one direction, due to an axial force, produces a contraction (or expansion) in the lateral direction, i.e., at right angles to the direction of the force. The ratio of the lateral strain to the longitudinal strain is called Poisson’s ratio v. It is an important elastic constant, For instance, a tensile stress, sx, which produces a tensile strain, ex in the x-direction will also produce a contractive strain, ey, in the y-direction, the two being related by v = −ey =ex

(3.9)

Combining Equation 3.9 with Equation 3.6 and rearranging yields ey = −(v=E)sx

(3.10)

Equation 3.10 thus deﬁnes the contribution (ey) of the stress sx in the x-direction to the total strain in the y-direction. Poisson’s ratio, v, as deﬁned above, is a fourth elastic constant. For small deformations, the bulk modulus and modulus of rigidity can be calculated from the modulus of elasticity and Poisson’s ratio by the following equations: E K= (3.11) 3(1 − 2v) G=

E 2(1 + v)

(3.12)

310

Plastics Technology Handbook

The elastic modulus can also be calculated from the bulk modulus and the modulus of rigidity by the relation 1 1 1 = + E 9K 3G

(3.13)

For soft materials such as gels, pastes, putties and many colloidal systems, which do not compress to the extent to which they are deformed under stress, K is very large compared to G, and therefore from Equation 3.13, E = 3G. For other materials as metals, ﬁbers, and some plastics, however, K must be considered.

3.2.2 Stress–Strain Behavior The stress–strain behavior of plastics measured at a constant rate of loading provides a basis for quality control and comparative evaluation of various plastics. The diagram shown in Figure 3.2a is most typical of that obtained in tension for a constant rate of loading. For compression and shear the behavior is quite similar except that the magnitude and the extent to which the curve is followed are different. In the diagram, load per unit cross section (stress) is plotted against deformation expressed as a fraction of the original dimension (strain). Even for different materials the nature of the curves will be similar, but they will differ in (1) the numerical values obtained and (2) how far the course of the typical curve is followed before failure occurs. Cellulose acetate and many other thermoplastics may follow the typical curve for almost its entire course. Thermosets like phenolics, on the other hand, have cross-linked molecules, and only a limited amount of intermolecular slippage can occur. As a result, they undergo

Elongation at failure E = σ/

2 Ultimate strength

Stress (σ)

(a)

Strain

Tangent modulus Initial tangent modulus

2% secant modulus 0 (b)

FIGURE 3.2

2 3 Strain (%)

(a) Nominal stress–strain diagram. (b) Typical moduli values quoted for plastics.

311

Plastics Properties and Testing

fracture at low strains, and the stress–strain curve is followed no further than to some point below the knee, such as point 1. Ultimate strength, elongation, and elastic modulus (Young’s modulus) can be obtained from the stressstrain study (Figure 3.2a). For determining the Young’s modulus (E) the slope of the initial tangent, i.e., the steepest portion of the curve, is measured. Other moduli values are also used for plastics (see Figure 3.2b). The appearance of a permanent set is said to mark a yield point, which indicates the upper limit of usefulness for any material. Unlike some metals, in particular, the ferrous alloys, the drop-of-beam effect and a sharp knee in the stress–strain diagram are not exhibited by plastics. An arbitrary yield point is usually assigned to them. Typical of these arbitrary values is the 0.2% or the 1% offset yield stress (Figure 3.3a). Alternatively, a yield stress can be defined as that at which the ratio of total stress to total strain is some selected amount, say 50% or 70% of the elastic modulus (Figure 3.3b). In the first case the yield stress is conveniently located graphically by offsetting to the right the stated amount of 0.2% (or 1%) and drawing a line paralleling that drawn for the elastic modulus. The point at which this line intersects the observed stress–strain line defined the yield stress. In the second case also the point of intersection of the line drawn with a slope of 0.7E, for instance, with the observed stress–strain line determines the yield stress. Up to point 1 in Figure 3.2a, the material behaves as an elastic solid, and the deformation is recoverable. This deformation, which is small, is associated with the bending or stretching of the inter-atomic bonds between atoms of the polymer molecules (see Figure 3.4a). This type of deformation is nearly instantaneous and recoverable. There is no permanent displacement of the molecules relative to each other.

Strain

.7E e=0 Slop

Stress Slope =E

=E Slope

Stress

Yield

Strain

Oﬀset (a)

FIGURE 3.3

(b)

Location of a yield value.

(a)

(b)

(c)

FIGURE 3.4 Deformation in plastics. (a) Stretching of polymer molecule. (b) Staightening out of a coiled molecular chain. (c) Intermolecular slippage.

312

Plastics Technology Handbook

0 (a)

Strain (ε)

0 (b)

0 (c)

Stress (σ)

Between points 1 and 2 in Figure 3.2a, deformations have been associated with a straightening out of a kinked or coiled portion of the molecular chains (see Figure 3.4b), if loaded in tension. (For compression the reverse is true.) This can occur without intermolecular slippage. The deformation is recoverable ultimately but not instantaneously and hence is analogous to that of a nonlinear spring. Although the deformation occurs at stresses exceeding the stress at the proportional limit, there is no permanent change in intermolecular arrangement. This kind of deformation, characterized by recoverability and nonlinearity, is very pronounced in the rubber state. The greatest extension that is recoverable marks the elastic limit for the material. Beyond this point extensions occur by displacement of molecules with respect to each other (Figure 3.4c), as in Newtonian flow of a liquid. The displaced molecules have no tendency to slip back to their original positions, therefore these deformations are permanent and not recoverable. Poisson’s ratio is a measure of the reduction in the cross section accompanying stretching and is the ratio of the transverse strain (a contraction for tensile stress) to longitudinal strain (elongation). Poisson’s ratio for many of the more brittle plastics such as polystyrene, the acrylics, and the thermoset materials is about 0.3; for the more flexible plasticized materials, such as cellulose acetate, the value is somewhat higher, about 0.45. Poisson’s ratio for rubber is 0.5 (characteristic of a liquid); it decrease to 0.4 for vulcanized rubber and to about 0.3 for ebonite. Poisson’s ratio varies not only with the nature of the material but also with the magnitude of the strain for a given material. All values cited here are for zero strain. Strain energy per unit volume is represented as the area under the stress–strain curve. It is another property that measures the ability of a material to withstand rough treatment and is related to toughness of the material. The stress–strain diagram thus serves as a basis for classification of plastics. Strong materials have higher ultimate strength than weak materials. Hard or unyielding materials have a higher modulus of elasticity (steeper initial slope) than soft materials. Tough materials have high elongations with large strain energy per unit volume. Stress–strain curves for type cases are shown in Figure 3.5. It must be emphasized that the type behavior shown in Figure 3.5 depends not only on the material but also very definitely on conditions under which the test is made. For example, the bouncing putty silicone is putty-like under slow rates of loading (type curve a) but behaves as an elastic solid under high rates of impact (type curve b or d). Figure 3.6a shows that at high extension rates (>1 mm/sec) unplasticized PVC is almost brittle with a relatively high modulus and strength. At low extension rates ( s1 > s3 and E(t)≠1/D(t). E(t) and G(t) are obtained directly only from stress relaxation measurements, while D(t) and J(t) require creep experiments for their direct observation.

3.2.4 Stress–Strain–Time Behavior When a mass of polymer is stressed the deformation produced may be considered as a sum of the following three deformations (see Figure 3.4): 1. A deformation due to bond bending and stretching which is instantaneous and independent of temperature (ordinary elastic deformation, eoe). 2. A deformation due to chain uncoiling which is not instantaneous and whose rate depends on temperature (high elastic deformation, ehe). 3. A deformation due to slippage of polymer molecules past one another (viscous deformation ev). It is often assumed that the rates of such viscous deformation do not change with time if the applied stress is constant. Figure 3.7 shows schematically the above types of deformational response as a result of a ﬁxed stress imposed on a body showing ordinary elastic deformation only (Figure 3.7b), a second body showing high elastic deformation only (Figure 3.7c), and a third body showing viscous deformation only (Figure 3.7d). In each case, the stress is imposed at time t0 and held at a constant value until time t1, when it is removed. Real polymers exhibit deformation-time curves which are a combination of the three basic responses, and a simple relationship for a combined or total strain e e = eoe + ehe + ev can be used to analyze the deformation under a given stress.

315

Load

Strain

Plastics Properties and Testing

t0 (b)

t0 (c)

Time

Strain

Time

Strain

(a)

t1 Time

(d)

Time

FIGURE 3.7 Types of deformational response as a result of (a) a ﬁxed load being imposed between times t0 and t1: (b) ordinary elastic material; (c) highly elastic material; (d) viscous material.

The combined response, however, differs widely among polymers. Figure 3.8 shows typical deformationtime curves. It will be noted that, given sufﬁcient time, ehe will reach a constant value while ev continues to increase with time. On release of stress, ehe will eventually disappear but ev will remain constant. An important conclusion resulting from this is that since both the high elastic and the viscous components of strain depend on both time and temperature the total strain also will depend on time and temperature. This has been shown to be an important factor affecting many polymer properties. It is therefore proposed to consider the background to this fact in greater detail in the following section.

Recoverable

Total strain (ε) Strain

εhe εoe

Irrecoverable εv t0

t1 Time

(a)

Recoverable

Strain

Total strain (ε) εhe εoe

Irrecoverable t0 (b)

t1 Time

FIGURE 3.8 Strain-time curves: (a) material showing substantial ordinary elastic, high elastic, and viscous components of strain; (b) material in which high elastic deformation predominates.

316

Plastics Technology Handbook

3.2.4.1 The WLF Equations For a polymeric segment to move from its position to its adjacent site there must be some holes in the mass of the material into which the segment can move—and simultaneously leave a vacant space into which another segment may move. The important point is that molecular motion cannot take place without the presence of holes. These holes, collectively, are called free volume. One interpretation of the glass transition temperature (Tg) is that it is a temperature below which the free volume is really too small for much molecular movement. However, at or above Tg the molecules have sufficient energy for movement, jostling occurs and the free volume increases quite sharply with an increase in temperature. It is usual to express the temperature coefficient of the free volume as being the difference between the thermal expansion coefficients above and below Tg. This may be expressed mathematically by the equation f = fg + (aa − ab )(T − Tg ) = fg + Da(T − Tg )

(3.22)

where f is the fractional free volume at temperature T, fg is the fractional free volume at Tg, and aa and ab are the coefﬁcients of thermal expansion above and below the Tg, respectively. The value of Da is simply (aa − ab). Now it has been shown that the viscosity is related to the fractional free volume by an expression of the form 1 = Ke−A=f hT

(3.23)

1 = Ke−A=fg hTg

(3.24)

so that

where K and A are constants. Combining these one may write ! hT 1 1 loge = − hTg f fg Substituting for f from Equation 3.22, this expression yields ! −(T − Tg ) hT log10 = 2:303fg ½(fg =Da) + (T − Tg ) hTg

(3.25)

(3.26)

Experimental data on a large range of polymers have demonstrated the approximate general validity of the equation ! −17:44(T − Tg ) hT log10 (3.27) = 51:6 + (T − Tg ) h Tg known as the Williams, Landel, and Ferry Equation (WLF equation). Solving Equation 3.26 and Equation 3.27 one obtains fg = 0.025 and Da = 4.8 × 10−4 deg−1. Equation 3.27 implies that if we know the viscosity at some temperature T we can estimate the viscosity at Tg, and from this estimate the viscosity at another temperature T1. The WLF equation thus gives the effect of temperature on viscosity. There are also other applications of the WLF equation. In essence, if the value of material property changes with temperature, and if this change arises from changes in the viscosity of the system, then it may well be possible to apply the WLF equation to the property change.

317

Plastics Properties and Testing

One example of this is in relation to stress relaxation. If a polymer is deformed to a fixed strain at constant temperature, the force required to maintain that strain will decay with time due to viscous slippage of molecules. One measure of this rate of decay or stress relaxation is the relaxation time l, which may be defined as the time taken for the stress to decrease to 1/e of its initial value on application of strain (discussed later). In this case, it is found that ! −17:44(T − Tg ) lT log10 (3.28) = 51:6 + (T − Tg ) lTg which is of the same form as Equation 3.27. For experiments performed in shear, there is a rather complicated relation between the time-dependent stress relaxation shear modulus G(t) defined by Equation 3.19 and the time-dependent creep compliance J(t) defined by Equation 3.21. But if the slope of log G(t) versus log t is −m, then, to a good approximation, G(t) · J(t) =

sin mp mp

(3.29)

for m < 0.8. Not only are G(t) and J(t) related, but the former in turn is related to the tensile modulus which itself is related to the stress relaxation time l. It is therefore possible in theory to predict creep-temperature relationships from WLF data, though in practice these are best determined by experiments.

3.2.5 Creep Behavior Except for a few exceptions like lead, metals generally exhibit creep at higher temperatures. Plastics and rubbers, however, possess very temperature-sensitive creep behavior; they exhibit signiﬁcant creep even at room temperature. In creep tests a constant load or stress is applied to the material, and the variation of deformation or strain with time is recorded. A typical creep curve plotted from such a creep test is shown in Figure 3.9a. The ﬁgure shows that there is typically an almost instantaneous elastic strain AB followed by a time-dependent strain, which occurs in three stages: primary or transient creep BC (stage 1), secondary or steady-state creep CD (stage II), and tertiary or accelerated creep DE (stage III). III 2.8

m2 6

2.0

f/c

2.4 Stress = σ1

Strain

gf/ c

35 7k

Strain

(a)

Strain (%)

A Time

Stress = σ1

1.6 1.2

0.8

143

0.4

71.4 k

0 10–1 (b)

Log time

/cm

gf 4k

kgf/

gf/cm

100

101 102 103 Log time (h)

104

(c)

FIGURE 3.9 Typical creep curve (a) with linear time scale and (b) with logarithmic time scale. (c) Family of creep curves for poly(methyl methacrylate) at 20°C (1 kgf/cm2 = 0.098 MPa).

318

Plastics Technology Handbook

The primary creep has a rapidly decreasing strain rate. It is essentially similar in mechanism to retarded elasticity and, as such, is recoverable if the stress is removed. The secondary or steady-state creep is essentially viscous in character and is therefore nonrecoverable. The strain rate during this state is commonly referred to as the creep rate. It determines the useful life of the material. Tertiary creep occurs at an accelerated rate because of an increase in the true stress due to necking of the specimen. Normally a logarithmic time scale is used to plot the creep curve, as shown in Figure 3.9b, so that the time dependence of strain after long periods can be included. If a material is linearly viscoelastic (Equation 3.17), then at any selected time each line in a family of creep curves (with equally spaced stress levels) should be offset along the strain axis by the same amount. Although this type of behavior may be observed for plastics at low strains and short times, in most cases the behavior is nonlinear, as indicated in Figure 3.9c. Plastics generally exhibit high rates of creep under relatively low stresses and temperatures, which limits their use for structural purposes. Creep behavior varies widely from one polymer to another; thermoset polymers, in general, are much more creep resistant than thermoplastic polymers. The steadystate creep in plastics and rubbers (often referred to as cold flow) is due to viscous flow, and increases continuously. Clearly, the material cannot continue to get larger indefinitely, and eventually fracture will occur. This behavior is referred to as creep rupture. The creep strength of these materials is defined as the maximum stress which may be applied for a specified time without causing fracture. The creep strength of plastics is considerably increased by adding fillers and other reinforcing materials, such as glass fibers and glass cloth, since they reduce the rate of flow. The creep and recovery of plastics can be simulated by an appropriate combination of elementary mechanical models for ideal elastic and ideal viscous deformations. Although there are no discrete molecular structures which behave like individual elements of the models, they nevertheless aid in understanding the response of plastic materials.

3.2.6 Maxwell Model The Maxwell model consists of a spring and dashpot connected in series (Figure 3.10a). When a load is applied, the elastic displacement of the spring occurs immediately and is followed by the viscous ﬂow of liquid in the dashpot which requires time. After the load is removed, the elastic displacement is recovered immediately, but the viscous displacement is not recovered. 3.2.6.1 Stress–Strain Relation The spring is the elastic component of the response and obeys the relation s1 = Ee1

(3.30)

s1 and e1 are the stress and strain, respectively, and E is a constant.

Strain ε

σ2, ε2 η

FIGURE 3.10

εo = σo/E Recovery

Relaxation (ε constant) σo

εo = σo/E Time t

Stress σ (a)

ep Cre

Strain σ

Unloaded

σ1, ε1 E

(b)

Time t (c)

(a) The Maxwell model. (b), (c) Responses of the model under time-dependent modes of deformation.

319

Plastics Properties and Testing

The dashpot (consisting of a piston loosely ﬁtting in a cylindrical vessel containing a liquid) accounts for the viscous component of the response. In this case the stress s2, is proportional to the rate of strain e_ 2 ; i.e., s2 = he_ 2

(3.31)

where h is a material constant called the coefﬁcient of viscous traction. The total strain, e, of the model under a given stress, s, is distributed between the spring and the dashpot elements: e = e1 + e2

(3.32)

From Equation 3.32 the rate of total displacement with time is e_ = e_ 1 + e_ 2

(3.33)

and from Equation 3.30 through Equation 3.32, e_ =

1 1 s_ + s E 1 h 2

(3.34)

But both elements are subjected to the entire stress, s, s = s1 = s2

(3.35)

Therefore Equation 3.34 can be written as e_ =

1 1 s_ + s E h

(3.36)

which is the governing equation of the Maxwell model. It is interesting to consider the responses that this model predicts under three common time-dependent modes of deformation. Equation 3.36 is commonly rearranged as follows: s_ = Ee_ −

1 s l

(3.37)

where l = h/E is the ratio of the viscosity h of the dashpot and the tensile modulus E of the spring. Note that l has the units of time and that it characterizes the viscoelastic nature of the element very concisely, as the ratio of the viscous portion of the response to the elastic portion. This naturally occurring parameter is taken to be the response time or the relaxation time of the model. Equation 3.37 is the governing equation of the Maxwell model. It is interesting to consider the responses that this model predicts under three common time-dependent modes of deformation. 1. Creep. If a constant stress s0 is applied, then Equation 3.37 becomes s0 1 e0 = E l l

(3.38)

e = e0 (1 + t=l)

(3.39)

e_ = Integration yields

which indicates a constant rate of increase of strain with time—i.e., steady-state creep (Figure 3.10b). Equation 3.39 describes the creep response of the Maxwell element. 2. Relaxation. If the strain is held constant, then Equation 3.37 becomes s_ + s=l = 0

(3.40)

320

Plastics Technology Handbook

Solving this differential equation with the initial condition s = s0 at t = t0 gives s = s0 expð−t=l)

(3.41)

This indicates an exponential decay of stress with time (Figure 3.10c). The stress will relax and approach zero monotonically. The relaxation time l is thus the time required for the stress to decay to 1/e, or 0.37, of its initial value. Since the strain remains constant in a relaxation experiment, Equation 3.41 can also be written as E(t) = E0 expð−t=l)

(3.42)

where E(t) is the tensile modulus of the Maxwell element at time t, and E0 the modulus at the initial time of deformation. The corresponding equation for the Maxwell element in shear is G(t) = G0 expð−t=l)

(3.43)

where G(t) is the shear modulus of the Maxwell element at time t, and G0 is the modulus at t = 0; the relaxation time l is now the ratio of the viscosity h of the viscous component and the shear modulus G of the elastic component of the model. 3. Recovery. When the initial stress, s0, is removed, there is an instantaneous recovery of the elastic strain, e0, and then, as shown by Equation 3.36, the strain rate is zero, so there is no further recovery (Figure 3.10b). 3.2.6.2 Generalized Maxwell Model The behavior of a polymer system is so complicated that we cannot represent it with the response time of a single Maxwell element. In other words, the simple model described above cannot approach the behavior of a real system. In 1893, Weichert showed that stress–relaxation experiments could be represented as a generalization of Maxwell’s equation. The mechanical model according to Weichert’s formulation is shown in Figure 3.11; it consists of a large number of Maxwell elements coupled in parallel. Since the strain in each element is common, we sum the forces acting on the individual elements to obtain the total stress as a function of time, i.e., X s(t) = si (3.44) For relaxation with constant strain e0 we combine Equation 3.41 and Equation 3.44 to obtain X t s(t) = e0 Ei exp − li

(3.45)

where li = hi/Ei is the relaxation time of the ith element. The overall modulus as a function of time, E(t), is thus E(t) =

s(t) X t = Ei exp − e0 li

(3.46)

The synthesis of E(t) from known values of Ei and li is simpliﬁed by the use of semilog paper. σ (t), εo

E1 η1

FIGURE 3.11

E2 η2

E3 η3

Generalized Maxwell model (Weichert’s formulation).

Ei ηi

λi = ηi/Ei

321

Plastics Properties and Testing

Example 3.1: Derive the overall time-dependent modulus E(t) for 0 < t < 200 sec when i

E0 [(dynes/cm2) × 10−8]

li (sec)

1.000

100

0.667

0.333

Answer: For each element, Ei(t) is given by a straight line on semilog paper (Equation 3.42) with intercept (Ei)0 and a negative slope of li (Figure 3.12). Adding the curves arithmetically gives E(t) directly: t Ei (t) = (Ei )0 exp − li E(t) =

3 X Ei (t) i=1

Note that E(t) is not a straight line in Figure 3.12. As can be seen, the Maxwell–Weichert model possesses many relaxation times. For real materials we postulate the existence of a continuous spectrum of relaxation times (li). A spectrum-skewed toward lower times would be characteristic of a viscoelastic fluid, whereas a spectrum skewed toward longer times would be characteristic of a viscoelastic solid. For a real system containing crosslinks the spectrum would be skewed heavily toward very long or infinite relaxation times. In generalizing, l may thus be allowed to range from zero to infinity. The concept that a continuous distribution of relaxation times should be required to represent the behavior of real systems would seem to follow naturally from the fact that real polymeric systems also exhibit distributions in conformational size, molecular weight, and distance between crosslinks. If the number of units is allowed to become infinite, the summation over the differential units of the model (Equation 3.44) can be explained by an integration over all relaxation times. Thus as i!∞ in Figure 3.11, the range of allowable relaxation times becomes zero to infinity. From the notion that the stresses in the individual elements, si, are functions of time and relaxation times, si = si(t, li), we

ΣEi (t)

109

1.00 exp(– t ) 100 Ei 0.667 exp(– t ) 50

108

0.333 exp(– t ) 25

107

50 Time (sec)

100

FIGURE 3.12 Time-dependent modulus for individual Maxwell elements and for the sum of three elements in parallel ∑Ei(t).

322

Plastics Technology Handbook

deﬁne a continuous function s (t, l) such that the total stress, s (t), is given by the following (compare Equation 3.44): ð∞ s(t) =

s(t, l)dl

(3.47)

and for relaxation with constant strain, ϵ0, it is given by the following (compare Equation 3.45): ð∞ (3.48) s(t) = e0 E(l)e−t=l dl 0

Since E(t) = s(t)/e0, we ﬁnd that we have developed an expression suitable for representing the time dependence of the relaxation modulus, i.e., ð∞ E(t) =

E(l)e−t=l dl

(3.49)

The function E(l) is referred to as the distribution of relaxation times or the relaxation spectrum. In principle, once E(l) is known, the result of any other type of mechanical experiment can be predicted. In practice E(l) is determined from experimental data on E(t). Since the distribution of relaxation times is so broad, it is more convenient to consider ln l. Hence we introduce the function H(ln l), where the parenthesis denotes functional dependence, to replace E(l) as E(l) =

H(ln l) l

(3.50)

Then Equation 3.49 becomes (note the change of limits): +∞ ð

H(ln l)e−t=l d(ln l)

E(t) =

(3.51)

−∞

and all relaxation times are considered as ln l. What we desire now is a means to determine H(ln l) from data obtained as E(t) versus ln t. This is virtually impossible to do directly, and a number of approximate methods have been devised. These approximations are discussed in the advanced reference works of Ferry [5] and Tobolsky [6].

3.2.7 Kelvin or Voigt Model In the Kelvin or Voigt model the spring and dashpot elements are connected in parallel, as shown in Figure 3.13a. This model roughly approximates the behavior of rubber. When the load is applied at zero time, the elastic deformation cannot occur immediately because the rate of ﬂow is limited by the dashpot. Displacements continue until the strain equals the elastic deformation of the spring and it resists further movement. On removal of the load the spring recovers the displacement by reversing the ﬂow through the dashpot, and ultimately there is no strain. The mathematical relations are derived next. 3.2.7.1 Stress–Strain Relation Since the two elements are connected in parallel, the total stress will be distributed between them, but any deformation will take place equally and simultaneously in both of them; that is, s = s1 + s2

(3.52)

e = e1 + e2

(3.53)

323

Plastics Properties and Testing

σ1, ε1

σ2, ε2 η

Stress (σ)

(a) Unloaded

Relaxation

(b)

ee Cr

Recovery

Time

Stress (σ)

Strains (ε)

ε = σ0/ E ε constant σ0

(c)

Time

FIGURE 3.13 (a) The Kelvin or Voigt model. (b), (c) Responses of the model under time-dependent modes of deformation.

From Equation 3.30, Equation 3.31, and Equation 3.52, s = Ee1 + he_ 2 or, using Equation 3.53, s = Ee + he_

(3.54)

which is the governing equation for the Kelvin (or Voigt) model. Its predictions for the common timedependent deformations are derived next. 1. Creep. If a constant stress, s0, is applied, Equation 3.54 becomes s0 = Ee + he_ The differential equation may be solved for the total strain, e, to give s E e = 0 1 − exp − t E h

(3.55)

(3.56)

This equation indicates that the deformation does not appear instantaneously on application of stress, but it increases gradually, attaining asymptotically its maximum value e = s0/E at inﬁnite time (Figure 3.13b). The Voigt model is thus said to exhibit retarded elastic deformation in creep experiments. The quantity h/E = l is called a retardation time. It is the time (t = l) at which the deformation is retarded by 1/e of its maximum value. (The physical meaning of h/E for Maxwell and Voigt models should not be confused.) By comparison with Equation 3.56 the creep equation under a constant shear stress t0 may be written as g=

t0 ½1 − exp( − t=l) G

(3.57)

where g is the time-dependent shear strain of the Voigt element and G is the shear modulus of its elastic component; l is the retardation time (=h/G).

324

Plastics Technology Handbook

Equation 3.57 is conveniently written as J = J0 ½1 − exp( − t=l)

(3.58)

where J is the shear creep compliance (=g /t0) at time t, and J0 is the shear creep compliance at the time of stress application. 2. Relaxation. If the strain is held constant; then Equation 3.54 becomes s = Ee 3. Recovery. If the stress is removed, Equation 3.54 becomes 0 = Ee + he_ This differential equation may be solved with the initial condition e = e0 at the time of stress removal to give E e = e0 exp − t (3.59) h This equation represents an exponential recovery of strain which, as a comparison with Equation 3.56 shows, is a reversal of the predicted creep. The Kelvin (or Voigt) model therefore gives an acceptable ﬁrst approximation to creep and recovery behavior but does not predict relaxation. By comparison, the previous model (Maxwell model) could account for relaxation but was poor in relation to creep and recovery. It is evident therefore that a better simulation of viscoelastic materials may be achieved by combining the two models.

3.2.8 Four-Element Model A combination of the Maxwell and Kelvin models comprising four elements is shown in Figure 3.14a. The total strain is e = e1 + e2 + e_ k

(3.60)

σ1, ε1 E1

σ2, ε2 η1

η2

Stress (σ)

(a)

e ep Cr

ε0 = σ0 /E Recovery

ε0 = σ0 /E

Relaxation (ε constant) Stress (σ)

Strain (ε)

Unloaded

Time (b)

σ0

Time (c)

FIGURE 3.14 (a) Four-element model. (b), (c) Responses of the model under time-dependent modes of deformation.

325

Plastics Properties and Testing

where ek is the strain response of the Kelvin model. From Equation 3.30, Equation 3.31, and Equation 3.56, s st s E e = 0 + 0 + 0 1 − exp − 2 t (3.61) E1 h1 E2 h2 Thus the strain rate is e_ =

s0 s0 E2 t + exp − h1 h2 h2

(3.62)

The response of this model to creep, relaxation, and recovery situations is thus the sum of the effects described previously for the Maxwell and Kelvin models and is illustrated in Figure 3.14b. Though the model is not a true representation of the complex viscoelastic response of polymeric materials, it is nonetheless an acceptable approximation to the actual behavior. The simulation becomes better as more and more elements are added to the model, but the mathematics also becomes more complex.

3.2.9 Zener Model Another model, attributed to Zener, consists of three elements connected in series and parallel, as illustrated in Figure 3.15, and known as the standard linear solid. Following the procedure already given, we derive the governing equation of this model: h3 s_ + E1 s = h3 (E1 + E2 )e_ + E1 E2 e

(3.63)

This equation may be written in the form a1 s_ + a0 s = b1 e_ + b0 e

(3.64)

where a1, a0, b1, and b0 are all material constants. A more general form of Equation 3.64 is an

∂n s ∂n−1 s + ⋯ +a0 s n + an−1 ∂t ∂ t n−1 = bm

σ1, ε1 E1

σ2, ε2 E2

σ3, ε3

∂m e + ⋯ +b0 e ∂ tm

The modern theory of viscoelasticity favors this type of equation. The models described earlier are special cases of this equation. Hookean body. All constants a and b except a0 and b0 are zero. Equation 3.65 becomes a0 s = b 0 e

η3

(3.65)

(3.65a)

Maxwell element. All constants a and b except a0, a1, and b1 are zero. Equation 3.65 becomes FIGURE 3.15

The standard linear solid.

a0 s + a1

∂s ∂e = b1 ∂t ∂t

(3.65b)

326

Plastics Technology Handbook

This is the spring and dashpot in series and applies to stress relaxation at constant strain. Voigt element. All constants a and b except a0, b0, and b1 are zero. Equation 3.65 becomes ∂e (3.65c) ∂t This is the spring and dashpot in parallel and applies to strain retardation at constant stress. a0 s = b0 e + b1

3.2.10 Superposition Principle Each of the creep curves in Figure 3.9c depicts the strain response of a material under a constant stress. However, in service, materials are often subjected to a complex sequence of stresses or stress histories, and obviously it is not practical to obtain experimental creep data for all combinations of loading. In such cases a theoretical model can be very useful for describing the response of a material to a given loading pattern. The most commonly used model is the Boltzmann superposition principle, which proposes that for a linear viscoelastic material the entire loading history contributes to the strain response, and the latter is simply given by the algebraic sum of the strains due to each step in the load. The principle may be expressed as follows. If an equation for the strain is obtained as a function of time under a constant stress, then the modulus as a function of time may be expressed as E(t) =

s e(t)

(3.66)

Thus if the applied stress is s0 at zero time, the creep strain at any time, t, will be given by e(t) =

1 s E(t) 0

(3.67)

On the other hand, if the stress, s0, was applied at zero time and an additional stress, s1, at time u, the Boltzmann superposition principle says that the total strain at time t is the algebraic sum of two independent responses; that is, e(t) =

1 1 s0 + s E(t) E(t − u) 1

(3.68)

For any series of stress increments this equation can be generalized to e(t) =

u=t X

u=−∞

1 E(t − u)

(3.69)

The lower limit of the summation is taken as −∞ since the entire stress history contributes to the response. As an illustration, for a series of step changes in stress as in Figure 3.16a, the strain response predicted by the model is shown schematically in Figure 3.16b. The time-dependent strain response (creep curve) due to the stress s0 applied at zero time is predicted by Equation 3.66 with s = s0. When a second stress, s1, is added to s0, the new curve will be obtained, as illustrated in Figure 3.16b, by adding the creep due to s1 to the anticipated creep due to s0. Removal of all stress at a subsequent time u2 is then equivalent to removing the creep stain due to s0 and s1, independently, as shown in Figure 3.16b. The procedure is repeated in a similar way for other stress changes. To take into account a continuous loading cycle, we can further generalize Equation 3.69 to ðt e(t) = −∞

1 ds(u) du E(t − u) du

(3.70)

327

Stress (σ)

Plastics Properties and Testing

σ1

σ0 σ3 0

Creep due to σ0

C Creep due to σ1

0 (b)

FIGURE 3.16

Time

A+B

Strain (ε)

(a)

u1 t1

u2 t1

Creep due to σ3

Time

(a) Stress history. (b) Predicted strain response using Boltzmann’s superposition principle.

In the same way the stress response to a complex strain history may be derived as ðt de(u) du s(t) = E(t − u) du

(3.71)

−∞

When the stress history has been deﬁned mathematically, substitution in Equation 3.70 and integration within limits gives the strain at the given time. The stress at a given time is similarly obtained from Equation 3.71.

3.2.11 Isometric and Isochronous Curves Isometric curves are obtained by plotting stress vs. time for a constant strain; isochronous curves are obtained by plotting stress vs. strain for a constant time of loading. These curves may be obtained from the creep curves by taking a constant-strain section and a constant-time section, respectively, through the creep curves and replotting the data, as shown in Figure 3.17. An isometric curve provides an indication of the relaxation of stress in the material when the strain is kept constant. Since stress relaxation is a less common experimental procedure than creep testing, an isometric curve, derived like the preceding curves from creep curves, is often used as a good approximation of this property. Isochronous curves, on the other hand, are more advantageously obtained by direct experiments because they are less time consuming and require less specimen preparation than creep testing. The experiments actually involve a series of mini creep and recovery tests on the material. Thus a stress is applied to a specimen of the material, and the strain is recorded after a time t (typically 100 sec). The stress

328

Plastics Technology Handbook

Creep curves Strain

σ5 σ4 σ3

ε' σ2 σ1

Log time

σ4

σ5

Stress

t´

σ4 σ3

σ3 σ2 σ1

Time = t´

Strain = ε (a)

FIGURE 3.17

Log time

(b)

Strain

(a) Isometric and (b) isochronous curves from creep curves.

is then removed and the material is allowed to recover. This procedure is repeated unit there are sufﬁcient points to plot the isochronous curve. Note that the isochronous test method is quite similar to that of a conventional incremental loading tensile test and differs only in that the presence of creep is recognized and the “memory” of the material for its stress history is overcome by the recovery periods. Isochronous data are often presented on log–log scales because this provides a more precise indication of the nonlinearity of the data by yielding a straightline plot of slope less than unity.

3.2.12 Pseudoelastic Design Method Due to the viscoelastic nature of plastics, deformations depend on such factors as the time under load and the temperature. Therefore the classical equations available for the design of structural components, such as springs, beams, plates, and cylinders, and derived under the assumptions that (1) the modulus is constant and (2) the strains are small and independent of loading rate or history and are immediately reversible, cannot be used indiscriminately. For example, classical equations are derived using the relation Stress = constant strain where the constant is the modulus. From the nature of the creep curves shown in Figure 3.17a, it is clear that the modulus of a plastic is not constant. Several approaches have been developed to allow for this fact, and some of them also give very accurate results; but mathematically they are quite complex, and this has limited their use. However, one method that has been widely accepted is the pseudoelastic design method. In this method appropriate values are chosen for the time-dependent properties, such as modulus, and substituted into the classical equations.

329

Plastics Properties and Testing

The method has been found to give sufficiently accurate results, provided that the value of the modulus is chosen judiciously, taking into account the service life of the component and the limiting strain of the plastic. Unfortunately, however, there is no straightforward method for finding the limiting strain of a plastic. The value may differ for various plastics and even for the same plastic in different applications. The value is often arbitrarily chosen, although several methods have been suggested for arriving at an appropriate value. One method is to draw a secant modulus which is 0.85 of the initial tangent modulus and to note the strain at which this intersects the stress–strain curve (see Figure 3.2b). But this method may be too restrictive for many plastics, particularly those which are highly crystalline. In most situations the maximum allowable strain is therefore decided in consultations between designer and product manufacturer. Once an appropriate value for the maximum strain is chosen, design methods based on creep curves and the classical equations are quite straightforward, as shown in the following examples. Example 3.2: A plastic beam, 200 mm long and simply supported at each end, is subjected to a point load of 10 kg at its mid-span. If the width of the beam is 14 mm, calculate a suitable depth so that the central deflection does not exceed 5 mm in a service life of 20,000 h. The creep curves for the material at the service temperature of 20°C are shown in Figure 3.18a. The maximum permissible strain in this material is assumed to be 1%. Answer: The linear elastic equation for the central deflection, d, of the beam is d=

PL3 48EI

where P, load at mid-span; L, length of beam; E, modulus of beam material; I, second moment of area of beam cross section The second moment of area is I=

bd3 14d 3 = mm4 12 12

m2 gf /c

1.5 3 14

200

f kg

Stress (kgf/cm2)

Strain (%)

2.0

gf/

cm 2 cm 2

2.5

f/c

g 7k

1.0

10 m gf/c 71 k

Initial modulus = 9490 kgf/cm2

160 120 80

1% secant modulus = 9285 kgf/cm2

0.5 40 0 10–1 (a)

101 102 103 Log time (h)

104

105

0 (b)

1.0

2.0 Strain (%)

3.0

4.0

FIGURE 3.18 (a) Creep curves for material used in illustrative examples. (b) Isochronus curve at 20,000 h service life (1 kgf/cm2 = 0.098 MPa).

330

Plastics Technology Handbook

So from the expression for d, d3 =

PL3 56Ed

The only unknown on the right side is E. For plastic this is time dependent, but a suitable value corresponding to the maximum permissible strain may be obtained by referring to the creep curves in Figure 3.18a. A constant-time section across these curves at 20,000 h gives the isochronous curve shown in Figure 3.18b. Since the maximum strain is recommended as 1%, a secant modulus may be taken at this value. It is 9285 kgf/cm2 (=92.85 kgf/mm2). Using this value in the above equation gives d3 =

10(200)3 56 92:85 5 d = 14:5 mm

Example 3.3: A thin-wall plastic pipe of diameter 150 mm is subjected to an internal pressure of 8 kgf/cm2 at 20°C. It is suggested that the service life of the pipe should be 20,000 h with a maximum strain of 2%. The creep curves for the plastic material are shown in Figure 3.18a. Calculate a suitable wall thickness for the pipe. Answer: The hoop stress, s, in a thin-wall pipe of diameter d and thickness h, subjected to an internal pressure, P, is given by s=

Pd 2h

so h =

Pd 2s

A suitable design stress may be obtained from the creep curves in Figure 3.18a. By referring to the 20,000-h isochronous curve (Figure 3.18b) derived from these curves, the design stress at 2% strain is obtained as 167.7 kg/cm2. (Note that a similar result could have been obtained by plotting a 2% isometric curve from the creep curves and reading the design stress at a service life of 20,000 h.) Substituting the design stress into the equation for h gives h=

8 150 = 3:58 mm 2 167:7

It may be seen from the creep curves (Figure 3.18a) that when the pipe is first pressurized, the strain is less than 1%. Then as the material creeps, the strain increases steadily to reach its limit of 2% at 20,000 h. In both examples it has been assumed that the service temperature is 20°C. If this is not the case, then creep curves at the appropriate temperature should be used. However, if none are available, a linear extrapolation between available temperatures may be sufficient for most purposes. Again, for some materials like nylon the moisture content of the material has a significant effect on its creep behavior. In such a case creep curves are normally available for the material in both wet and dry states, and appropriate data should be used, depending on the service conditions.

3.2.13 Effect of Temperature Many attempts have been made to obtain mathematical expressions which describe the time and temperature dependence of the strength of plastics. Since for many plastics at constant temperature a plot of

331

Plastics Properties and Testing

stress, s, against the logarithm of time to failure (creep rupture), t, is approximately linear, one of the expressions most commonly used is t = Ae−Bs

(3.72)

where A and B are constants. In reality, however, they depend on factors such as material structure and on temperature. The most successful attempts to include the effects of temperature in a relatively simple expression have been made by Zhurkov and Bueche, who used an equation of the form [7] −gs U t = t0 exp 0 RT

(3.73)

where t0 is a constant which has approximately the same value for most plastics, U0 is the activation energy of the fracture process, g is a coefﬁcient which depends on the structure of the material, R is the molar gas constant, and T is the absolute temperature. A series of creep rupture tests on a given material at a ﬁxed temperature would permit the values for U0 and g for the material to be determined from this expression. The times to failure at other stresses and temperatures could then be predicted. The relative effects of temperature rises on different plastic materials depend on the structure of each material and, particularly, whether it is crystalline or amorphous. If a plastic is largely amorphous (e.g., polymethyl methacrylate, polystyrene), then it is the glass transition temperature (Tg) which will determine the maximum service temperature, since above Tg the material passes into the rubbery region (see Figure 1.19). On the other hand, in plastics which have a high degree of crystallinity (e.g., polyethylene, polypropylene), the amorphous regions are small, so Tg is only of secondary importance. For them it is the melting temperature which will limit the maximum service temperature. The lowest service temperatures which can be used are normally limited by the brittleness introduced into the material. The behavior of plastics materials at room temperature is related to their respective Tg values. This aspect has been dealt with in Chapter 1.

3.2.14 Time–Temperature Superposition In engineering practice, it is often necessary to design for the use of a material over a long period of time— many years, for example. A common parameter to use in design work is the elastic modulus. We know, however, that for polymers the modulus decreases with increasing time under load. Accumulation of long-term data for design with plastics can be very inconvenient and expensive. A method is thus needed to extrapolate data from shorter time studies at higher temperature to longer times over several decades of time scale at the desired temperature so that a lower limit of the modulus can be determined for use in design. On the other hand, it is sometimes difﬁcult to obtain data over a very short time scale. One must then extrapolate data obtained under practicable experimental conditions to these short time scales. An empirical method for such extrapolations is available for amorphous polymer systems and, in general, for polymer systems where structure does not change during the period of testing. The aforesaid extrapolations make use of a time-temperature superposition principle which is based on the fact that time and temperature have essentially equivalent effects on the modulus values of amorphous polymers. Figure 3.19 shows modulus data taken at several temperatures for poly(methyl methacrylate) [8]. Because of the equivalent effect of time and temperature, data at different temperatures can be superposed on data taken at a speciﬁed reference temperature merely by shifting individual curves one at a time and consecutively along the log t axis about the reference temperature.

332

Plastics Technology Handbook

40°C

Log E(t ), (dynes/cm2)

80°C 9

100°C 110°C 115°C Stress relaxation of PMMA

8 135°C 7

0.001 0.0001 0.1

1 10 Time (hr)

100

1000

FIGURE 3.19 Logarithm of tensile relaxation modulus versus logarithm of time for unfractionated poly(methyl v = 3:6 106 . (After McLoughlin, J. R. and Tobolsky, A. V. 1952. J. Colloid Sci., 7, 555.) methacrylate) of M

This time–temperature superposition procedure has the effect of producing a single continuous curve of modulus values extending over many decades of log t at the reference temperature. A curve constructed in this way, as shown in Figure 3.20 (with a reference temperature 115°C), is known as the master curve. The time-temperature superposition can be expressed mathematically as E(T1 ,t) = E(T2 ,t=aT )

(3.74)

for a tensile stress relaxation experiment (T2 > T1). The procedure asserts that the effect of changing the test temperature on viscoelastic properties is the same as that of multiplying or dividing the time scale by a constant quantity (aT) at each temperature. The quantity aT is called the shift factor, and it must be obtained directly from the experimental curve by measuring the amount of shift along the log t scale 40°

Log E(t) (dynes/cm2)

10 100° 110°

115° 8 Master curve 135° 7 –10

–5

Log t (h at 115°C)

FIGURE 3.20 Modulus-time master curve based on time–temperature superposition of data in Figure 3.19. Times referred to temperature of 115°C.

333

Plastics Properties and Testing

necessary to match the curve. The parameter aT is chosen as unity at the reference temperature and is a function of the temperature alone, decreasing with increasing temperature. It is common practice now to use the glass transition temperature (Tg) as the reference temperature for master curve construction. For most amorphous polymers, the shift factor at any other temperature T is then given fairly well by log10 aT = log

t(T) −17:44(T − Tg ) = t(Tg ) 51:6 + (T − Tg )

(3.75)

Equation 3.75 is known as the WLF equation (see Equation 3.27) after the initials of the researchers who proposed it [9]. The expression given holds between Tg and Tg + 100°C. However, if a different reference temperature is chosen an equation with the same form as Equation 3.75 can be used, but the constants on the right hand side must be re-evaluated. The significance of the WLF generalization (Equation 3.75) cannot be over-emphasized. Again and again, one finds in the literature methods of superposing time and temperature for mechanical and other properties in amorphous and partially amorphous materials. Whatever modifications are introduced usually reduce the behavior back in the direction of Equation 3.75.

3.2.15 Dynamic Mechanical Properties A complete description of the viscoelastic properties of a material requires information over very long times. To supplement creep and stress relaxation measurements which are limited by experimental limitations, experiments are therefore performed in which an oscillating stress or strain is applied to the specimen. These constitute an important class of experiments for studying the viscoelastic behavior of polymeric solids. In addition to elastic modulus, it is possible to measure by these methods the viscous behavior of the material in terms of characteristic damping parameters. Damping is an engineering material property and the observed response is much more sensitive to the polymer constitution than in step-function experiments. Oscillatory experiments (also referred to as dynamic mechanical experiments) thus offer a powerful technique to study molecular structure and morphology. A significant feature is the breadth of the time-scale spectrum available with these methods, e.g., 10−5–108 cycles/sec. In a dynamic experiment, the stress will be directly proportional to the strain if the magnitude of the strain is small enough. Then, if the stress is applied sinusoidally the resulting strain will also vary sinusoidally. (The same holds true if the strain is the input and the stress the output.) At sufficiently low frequencies, the strain will follow the stress in phase. However, in the general case the strain will be out of phase. In the last instance, the strain can be factored into two components—one of which is in phase with the stress and the other which lags behind the stress by p/2 rad. Alternatively, the stress can be decomposed into a component in phase with the strain and one which leads the strain by p/2 rad. This is accomplished by use of a rotating vector scheme, as shown in Figure 3.21. The magnitude of the stress at any time is represented by the projection OC of the vector OA on the vertical axis. Vector OA rotates with a frequency w equal to that of the sinusoidally varying stress. The length of OA is the stress amplitude (maximum stress) involved in the experiment. The strain is represented by the projection OD of vector OB on the vertical axis. The strain vector OB rotates in the same direction as OA with frequency w but it lags OA by an angle d. The loss tangent (discussed later) is defined as tan d. The strain vector OB can be resolved into vector OE along the direction of OA and OF perpendicular to OA. Then the projection OH of OE on the vertical axis is the magnitude of the strain which is in phase with the stress at any time. Similarly, projection OI of vector OF is the magnitude of the strain which is p/2 rad (one quarter cycle) out of phase with the stress. The stress can be similarly resolved into two components with one along the direction of OB and one leading the strain vector by p/2 rad. The ratio of the in-phase stress to the strain amplitude (maximum

334

Plastics Technology Handbook

C E

D δ ω

FIGURE 3.21

Decomposition of strain vector into two components in a dynamic experiment.

strain) is called the storage modulus. In a shear deformation experiment this quantity is labeled G′(w). The ratio of the out-of-phase stress to the strain is the loss modulus G″(w). If, on the other hand, the strain vector is resolved into its components, the ratio of the in-phase strain to stress amplitude (maximum stress) is the storage compliance J′(w) and the ratio of the out-of-phase strain to the stress amplitude is the loss compliance J″(w). It is evident from the above description that G′(w) and J′(w) are associated with the periodic storage and complete release of energy in the sinusoidal deformation process. The loss parameters G″(w) and J″w), on the other hand, reflect the nonrecoverable use of applied mechanical energy to cause viscous flow in the material. At a specified frequency and temperature, the dynamic response of a polymer in shear deformation can be summarized by any one of the following pairs of parameters: G′(w) and G″(w), J′(w) and J″(w), or absolute modulus |G| and tan d.

3.2.16 Rheological Behavior Rheology is the science of deformation and flow of matter. Essentially, all thermoplastic resins (and many thermosetting resins) are required to undergo flow in the molten state during the course of product manufacture. Important fabrication processes such as injection, extrusion, and calendering all involve the flow of molten polymers. In plastics fabrication, it is important to understand the effect, on melt viscosity, of such factors as temperature, pressure, rate of shear, molecular weight, and structure. It is also equally important to have reliable means of measuring viscous properties of materials. The flow behavior of polymeric melts cannot be considered to be purely viscous in character. The response of such materials is more complex, involving characteristics that are both viscous and elastic. This is only to be expected when one is trying to deform variously entangled long-chain molecules with a distribution of molecular weights. During flow, polymer molecules not only slide past each other, but also tend to uncoil—or at least they are deformed from their equilibrium, random coiled-up configuration. On release of the deforming stresses, these molecules tend to revert to random coiled-up forms. Since molecular entanglements cause the molecules to act in a cooperative manner, some recovery of shape corresponding to the recoiling

335

Plastics Properties and Testing

occurs. In phenomenological terms, we say that the melt shows elasticity in addition to viscous flow. The elastic—that is to say, time-dependent—effects play a most important part in die swell, extrusion defects, and melt fracture, which will be considered later in this section. 3.2.16.1 Classification of Fluid Behavior Although one can measure deformation in a solid, one cannot normally do this in a liquid since it undergoes a continuously increasing amount of deformation when a shear stress is applied. But one can determine the deformation rate (the shear rate) caused by an applied shear stress, or vice versa, and fluid behavior can be classified on this basis. We begin by making a reference to Figure 3.22, which schematically illustrates two parallel plates of very large area A separated by a distance r with the space in between filled with a liquid. The lower plate is fixed and a shear force Fs is applied to the top plate of area A producing a shear stress (t = Fs/A) that causes the plate to move at a uniform velocity v in a direction parallel to the direction of the force. It may be assumed that the liquid wets the plates and that the molecular layer of liquid adjacent to the stationary plate is stationary while the layer adjacent to the top plate moves at the same velocity as the plate. Intermediate layers of liquid move at intermediate velocities, and at steady state in laminar flow a velocity distribution is established as indicated by the arrows in the diagram. The velocity gradient between the two plates is dv/dr. It is defined as the shear rate and is commonly given the symbol g_ i.e., g_ = dv=dr

(3.76)

If the liquid is ideal and it is maintained at a constant temperature, the shear stress is linearly and directly proportional to the shear rate such that one may write t = h(dv=dr) = hg_ or

(3.77)

h = t=g_

(3.78)

where h is the coefficient of viscosity or simply the viscosity or internal friction of the liquid. The linear relationship between t and g_ given by Equation 3.77 or Equation 3.78 is known as Newton’s law and liquids which behave in this manner are called Newtonian fluids or ideal fluids. Other fluids which deviate from Newton’s law are described as non-Newtonian. For such fluids, the viscosity defined by Equation 3.78 is also known as the apparent viscosity. In practice, the Newtonian behavior is confined to low molecular weight liquids. Polymer melts obey Newton’s law only at shear rates close to zero and polymer solutions only at concentrations close to zero. The most general rheological equation is _ h = f (g,T,t,P,c, … …)

(3.79)

where the variables are g_ = shear rate (itself a function of the shear stress), T = temperature, t = time, P = pressure (itself a function of volume), c = conMoving plate of area A centration, and the multiple dots which follow and velocity v include, for example, molecular parameters (such v Shear as molecular weight and molecular weight distridr force (F5) bution), compositional variables (crystallinity and r the presence of additives), and factors that relate to processing history. Such an equation is clearly dv unrealistic, so we shall consider here some of the Stationary plate principal variables, one at a time, assuming that the others remain constant. Several common types of rheological behavior FIGURE 3.22 Velocity distribution of a liquid between are shown in Figure 3.23 based upon t vs. g_ curves. two parallel plates, one stationary and the other moving.

336

Plastics Technology Handbook

Pseudoplastic

Shear stress τ

Newtonian

. Shear rate γ

(a)

(b)

Bingham

Dilatant

. γ

(c)

FIGURE 3.23

. γ

(d)

. γ

_ for different types of ﬂuid material. Flow curves (t versus g)

These flow phenotypes are named Newtonian, pseudoplastic, dilatant, and Bingham. In Newtonian liquids, the viscosity is constant and independent of shear rate. In pseudoplastic and dilatant liquids the viscosity is no longer constant. In the former it decreases and in the latter it increases with increasing shear rate; that is to say, the shear stress increases with increasing shear rate less than proportionately in a pseudoplastic and more than proportionately in a dilatant. Pseudoplastics are thus described as shear-thinning and dilatants as shear-thickening fluid systems. These two flow phenotypes can be described by an Equation the “power law”: t = hN g_ n

n>1

Dilatant Newtonian

Log τ

n=1

nd /4

(b)

Impact

(a) (c)

FIGURE 3.33 Impact test. (a) Schematic diagram of Charpy impact testing machine. (b) Arrangement of Charpy impact specimen. (c) Mounting of Izod impact specimen.

346

Plastics Technology Handbook

20 40 High density polyethylene 15 Impact strength (ft-lbf/in2)

Dry nylon 10

Acetal

Impact strength (kJ/m2)

PVC

ABS 5

n Polystyre

Acr ylic 0

0.5

1.0

1.5

2.0

Notch tip radius (mm)

FIGURE 3.34

Variation of impact strength with notch radius for several thermoplastics.

When the sharp notch (0.25-mm radius) is used, it may be assumed that the energy necessary to initiate the crack is small, and the main contribution to the impact strength is the propagation energy. On this basis Figure 3.34 would suggest that high-density polyethylene and ABS have relatively high crackpropagation energies, whereas materials such as PVC, nylon, polystyrene, and acrylics have low values. The large improvement in impact strength observed for PVC and nylon when a blunt notch is used would imply that their crack-initiation energies are high. On the other hand, the smaller improvement in the impact strength of ABS with a blunt notch would suggest that the crack-initiation energy is low. Thus the benefit derived from using rounded corners would be much less for ABS than for materials such as nylon or PVC. Temperature has a pronounced effect on the impact strength of plastics. In common with metals, many plastic materials exhibit a transition from ductile behavior to brittle as the temperature is reduced. The variation of impact strength with temperature for several common thermoplastics is shown in Figure 3.35. The ranking of the materials with regard to impact strength is seen to be influenced by the test temperature. Thus, at room temperature (approximately 20°C) polypropylene is superior to acetal; at subzero temperatures (e.g., −20°C) polypropylene does not perform as well as acetal. This comparison pertains to impact behavior measured with a sharp (0.25-mm) notch. Note that notch sharpness can influence the impact strength variation with temperature quite significantly. Figure 3.36 shows that when a blunt (2-mm) notch is used, there is indeed very little difference between acetal and polypropylene at 20°C, whereas at −20°C acetal is much superior to polypropylene. It may be seen from Figure 3.35 and Figure 3.36 that some plastics undergo a change from ductile or tough (high impact strength) to brittle (low impact strength) behavior over a relatively narrow temperature change. This allows a temperature for ductile-brittle transition to be cited. In other plastic materials this transition is much more gradual, so it is not possible to cite a single value for transition temperature. It is common to quote in such cases a brittleness temperature, TB(1/4).

347

Plastics Properties and Testing

8 0.25-mm notch

Nylon (wet)

ABS 10 Polypropylene

Impact strength (kJ/m2)

Impact strength (ft-lbf/in2)

Acetal 5 PVC

Acrylic 0 –40

–20

Test temperature (°C)

FIGURE 3.35

Variation of impact strength with temperature for several thermoplastics with sharp notch.

This temperature is defined as the value at which the impact strength of the material with a sharp notch (1/4-mm tip radius) is 10 kJ/m2 (4.7 ft-lbf/in2). When quoted, it provides an indication of the temperature above which there should be no problem of brittle failure. However, it does not mean that a material should never be used below its TB (1/4), because this temperature, by definition, refers only to the impact behavior with a sharp notch. When the material is unnotched or has a blunt notch, it may still have satisfactory impact behavior well below TB (1/4). Other environmental factors besides temperature may also affect impact behavior. For example, if the material is in the vicinity of a fluid which attacks it, then the crack-initiation energies may be reduced, resulting in lower impact strength. Some materials, particularly nylon, are significantly affected by water, as illustrated in Figure 3.37. The absorption of water produces a spectacular improvement in the impact behavior of nylon. Note that the method of making the plastic sample and the test specimen can have significant effect on the measured values of the properties of the material. Test specimens may be molded directly or machined from samples which have been compression molded, injection molded, or extruded. Each processing method involves a range of variables, such as melt temperature, mold or die temperature, and shear rate, which influence the properties of the material. Fabrication defects can affect impact behavior for example, internal voids, inclusion, and additives, such as pigments, which can produce stress concentrations within the material. The surface finish of the specimen may also affect impact behavior. All these account for the large variation usually observed in the results of testing one material processed and/or fabricated in different ways. It also emphasizes the point that if design data are needed for a particular application, then the test specimen must match as closely as possible the component to be designed. In some applications impact properties of plastics may not be critical, and only a general knowledge of their impact behavior is needed. In these circumstances the information provided in Table 3.1 would be

348

Plastics Technology Handbook

10 2-mm notch

6 PVC 10 4

Polypropylene

Impact strength (kJ/m2)

Impact strength (ft-lbf/in2)

eta Ac

5 c Acr yli

0 –40

–20

0 40

Test temperature (°C)

FIGURE 3.36

Variation of impact strength with temperature for several thermoplastics with blunt notch.

adequate. The table lists the impact behavior of a number of commonly used thermoplastics over a range of temperatures in three broad categories [19].

3.2.20 Fatigue of Plastics A material subject to alternating stresses over long periods may fracture at stresses much below its maximum strength under static loading (tensile strength) due to the phenomenon called fatigue. Fatigue has been recognized as one of the major causes of fracture in metals. Although plastics are susceptible to a wider range of failure mechanisms, it is likely that fatigue still plays an important part in plastics failure. For metals the fatigue process is generally well understood and is divided into three stages crack initiation, crack growth, and fracture. Fatigue theory of metals is well developed, but the fatigue theory of polymers is not. The completely different molecular structure of polymers means that three is unlikely to be a similar type of crack initiation process as in metals, though it is possible that once a crack is initiated the subsequent phase of propagation and failure may be similar. Fatigue cracks may develop in plastics in several ways. If the plastic article has been machined, surface ﬂaws capable of propagation may be introduced. However, if the article has been molded, it is more probable that fatigue cracks will develop from within the bulk of the material. In a crystalline polymer the initiation of cracks capable of propagation may occur through slip of molecules. In addition to acting as a path for crack propagation, the boundaries of spherulites (see Chapter 1), being areas of weakness, may thus develop cracks during straining. In amorphous polymers cracks may develop at the voids formed during viscous ﬂow.

349

Plastics Properties and Testing

8 0.25-mm notch

4-week immersion

10 1-week immersion

2-week immersion

Impact strength (kJ/tm2)

Impact strength (ft-tbf/in2)

2 Dry

–40

FIGURE 3.37

–20 0 20 Test temperature (°C)

Effect of water content on impact strength of nylon.

TABLE 3.1 Impact Behavior of Common Thermoplastics over a Range of Temperatures Temperature (°C) −20

−10

Polyethylene (low density)

Polyethylene (high density) Polypropylene

B C

B B

Polystyrene

Poly(methyl methacrylate) ABS

C B

C A

Plastic Material

Acetal

Teﬂon PVC (rigid)

B B

A B

A A

Polycarbonate

Poly(phenylene oxide) Poly(ethylene terephthalate)

B B

A B

Nylon (dry)

Nylon (wet) Glass-ﬁlled nylon (dry)

B C

A C

A B

Polysulfone

Note: A, tough (specimens do not break completely even when sharply notched); B, notch brittle; C, brittle even when unnotched.

350

Plastics Technology Handbook

A number of features are peculiar to plastics, which make their fatigue behavior a complex subject not simply analyzed. Included are viscoelastic behavior, inherent damping, and low thermal conductivity. Consider, for example, a sample of plastic subjected to a cyclic stress of ﬁxed amplitude. Because of the high damping and low thermal conductivity of the material, some of the input energy will be dissipated in each cycle and appear as heat. The temperature of the material will therefore rise, and eventually a stage will be reached when the heat transfer to the surroundings equals the heat generation in the material. The temperature of the material will stabilize at this point until a conventional metal-type fatigue failure occurs. If, in the next test, the stress amplitude is increased to a higher value, the material temperature will rise further and stabilize, followed again by a metal-type fatigue failure. In Figure 3.38, where the stress amplitude has been plotted against the logarithm of the number of cycles to failure, failures of this type have been labeled as fatigue failures. This pattern will be repeated at higher stress amplitudes until a point is reached when the temperature rise no longer stabilizes but continues to rise, resulting in a short-term thermal softening failure in the material. At stress amplitudes above this crossover point there will be thermal failures in an even shorter time. Failures of this type have been labeled as thermal failures in Figure 3.38. The fatigue curves in Figure 3.38 thus have two distinct regimes—one for the long-term conventional fatigue failures, and one for the relatively short-term thermal softening failures. The frequency of the cyclic stress would be expected to have a pronounced effect on the fatigue behavior of plastics, a lower frequency promoting the conventional-type fatigue failure rather than thermal softening failure. Thus it is evident from Figure 3.38 that if the frequency of cycling is reduced, then stress amplitudes which would have produced thermal softening failures at a higher frequency may now result in temperature stabilization and eventually fatigue failure. Therefore, if fatigue failures are required at relatively high stresses, the frequency of cycling must be reduced. Normally, fatigue failures at one frequency on the extrapolated curve fall from the fatigue failures at the previous frequency (Figure 3.38). As the cyclic stress amplitude is further reduced in some plastics, the frequency remaining constant, the fatigue failure curve becomes almost horizontal at large values of the number of stress cycles (N). The stress amplitude at which this leveling off occurs is clearly important for design purposes and is known as the fatigue limit. For plastics in which fatigue failure continues to occur even at relatively low stress amplitudes, it is necessary to deﬁne an endurance limit—that is, the stress amplitude which would not cause fatigue failure up to an acceptably large value of N. 400 35 T F T

300

T 25

T T

F 20

200

F F

Stress amplitude (Mpa)

Stress amplitude (kgf/cm2)

100 103

104

105 106 Log cycles to failure

107

FIGURE 3.38 Typical fatigue behavior of a thermoplastic at several frequencies, F, fatigue failure; T, thermal failure, ○, 5.0 Hz; D, 1.67 Hz; □, 0.5 Hz. (Adapted from Crawford, R. J. 1981. Plastics Engineering, Pergamon, London.)

351

Plastics Properties and Testing

3.2.21 Hardness Hardness of a material may be determined in several ways: (1) resistance to indentation, (2) rebound efficiency, and (3) resistance to scratching. The first method is the most commonly used technique for plastics. Numerous test methods are available for measuring the resistance of a material to indentation, but they differ only in detail. Basically they all use the size of an indent produced by a hardened steel or diamond indentor in the material as an indication of its hardness—the smaller the indent produced, the harder the material, and so the greater the hardness number. The measured hardness is defined as macroor micro-hardness according to the load applied on the indenter, the load being more than 1 kg for macrohardness and 1 kg or less for microhardness tests. Hardness tests are simple, quick, and nondestructive, which account for their wide use for quality control purposes.

3.2.22 Indentation Hardness The test methods used for plastics are similar to those used for metals. The main difference is that because plastics are viscoelastic allowance must be made for the creep and the time-dependent recovery which occurs as a result of the applied indentation load. 3.2.22.1 Brinell Hardness Number A hardened steel ball 10 mm in diameter is pressed into the ﬂat surface of the test specimen under load of 500 kg for 30 sec. The load is then removed, and the diameter of the indent produced is measured (Figure 3.39). The Brinell hardness number (BHN) for macrohardness is deﬁned as BHN =

Load applied to indentor (kgf ) 2P pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi = Contact area of indentation (mm2 ) pD(D − D2 − d 2 )

(3.95)

where D is the diameter of the ball and d is the diameter of the indent. Tables are available to convert the diameter of the indent into BHN. Although the units of Brinell hardness are kgf/mm2, it is quoted only as a number. A disadvantage of the Brinell hardness test when used for plastics is that the edge of the indent is usually not well deﬁned. This problem is overcome in the following test. 3.2.22.2 Vickers Hardness Number The Vickers hardness test differs from the Brinell test in that the indentor is a diamond (square-based) pyramid (Figure 3.39) having an apex angle of 136°. If the average diagonal of the indent is d, the hardness number is calculated from Load applied to indentor (kgf ) P Vickers hardness number = (3.96) = 1:854 2 Contact area of indentation (mm ) d2 Tables are available to convert the average diagonal into Vickers number used for both macro- and microhardnesses. Load P 172°30'

136° D

d (a)

FIGURE 3.39

(b)

(c)

Indentation hardness tests. (a) Brinell test. (b) Vickers test. (c) Knoop test.

130°

352

Plastics Technology Handbook

3.2.22.3 Knoop Hardness Number The indentor used in the Knoop hardness test for microhardness measurement is a diamond pyramid, but the lengths of the two diagonals, as shown in Figure 3.39, are different. If the long diagonal of the indent is measured as D, the hardness number is obtained from P Knoop hardness number = 14:23 2 (3.97) D Time-dependent recovery of the indentation in plastics is a problem common to all three tests. To overcome this problem, allow a fixed time before making measurements on the indent. 3.2.22.4 Rockwell Hardness Number The Rockwell test used to measure macrohardness differs from the other three tests because the depth of the indent rather than its surface area is taken as a measure of hardness. A hardened steel ball is used as the indentor. A major advantage of the Rockwell test is that no visual measurement of the indentation is necessary, and the depth of the indent is read directly as a hardness value on the scale. The test involves three steps, as shown in Figure 3.40. A minor load of 10 kg is applied on the steel ball, and the scale pointer is set to zero within 10 sec of applying the load. In addition to this minor load, a major load is applied for 15 sec. A further 15 sec after removal of the major load (with the minor load still on the ball), the hardness value is read off the scale. Since creep and recovery effects can influence readings, it is essential to follow a defined time cycle for the test. Several Rockwell scales (Table 3.2) are used, depending on the hardness of the material under test (Table 3.3). The scale letter is quoted along with the hardness number e.g., Rockwell R60. Scales R and L are used for low-hardness number, e.g., Rockwell R60. Scales R and L are used for low-hardness materials, and scales M and E when the hardness value is high. When the hardness number exceeds 115 on any scale, the sensitivity is lost, so another scale should be used. 3.2.22.5 Barcol Hardness The Barcol hardness tester is a hand-operated hardness measuring device. Its general construction is shown in Figure 3.41. With the back support leg placed on the surface, the instrument is gripped in the hand and its measuring head is pressed firmly and steadily onto the surface until the instrument rests on the stop ring. The depth of penetration of the spring-loaded indentor is transferred by a lever system to an indicating dial, which is calibrated from 0 to 100 to indicate increasing hardness. To allow for creep, one normally takes readings after 10 sec. The indentor in the Barcol Tester Model No. 934-1 is a truncated steel cone having an included angle of 26° with a flat tip of 0.157 mm (0.0062 in.) in diameter. The values obtained using this instrument are

1 Minor load

2 Minor + major loads

3 Minor load only

Measurement taken as indication of hardness

FIGURE 3.40

Stages in Rockwell hardness test: 1, minor load; 2, minor and major loads; 3, minor load only.

353

Plastics Properties and Testing TABLE 3.2 Rockwell Hardness Scales Scale

Major Load (kg)

Dia. of Indentor (in.)

1/2

1/4

M E

100 100

1/4 1/8

TABLE 3.3 Choice of Hardness Test Methods Based on Modulus Range of Plastics Material Low modulus

" #

Test Method

Rubber

Shore A or BS 903

Plasticized PVC

Shore A or BS 2782

Low-density polyethylene Medium-density polyethylene

Shore D Shore D

High-density polyethylene

Shore D

Polypropylene Toughened polystyrene

Rockwell R Rockwell R

ABS

Rockwell R

Polystyrene Poly(methyl methacrylate)

Rockwell M Rockwell M

High modulus

Indicating dial

Spring

26°

Indentor

Lever

Support leg

0.006 in. dia. (0.156 mm)

Plunger tip

FIGURE 3.41

General construction of Barcol hardness tester.

found to correlate well to Rockwell values on the M scale. This instrument is used for metals and plastics. Two other models, No. 935 and No. 936, are used for plastics and very soft materials, respectively. 3.2.22.6 Durometer Hardness A durometer is an instrument for measuring hardness by pressing a needle-like indentor into the specimen. Operationally, a durometer resembles the Barcol tester in that the instrument is pressed onto the sample surface until it reaches a stop ring. This forces the indentor into the material, and a system of levers transforms the depth of penetration into a pointer movement on an indicating dial, which is calibrated from 0–100.

354

Plastics Technology Handbook

Type A

Type B

822 g

10 lbf 1.27

1.27

3 35°

2.5 0.8

30° Stop ring

FIGURE 3.42

Two types of Shore durometer.

TABLE 3.4 Some Typical Hardness Values for Plastics Material

Brinell

Vickers

4 7

2 6

Polystyrene

M83

Poly(methyl methacrylate) Poly(vinyl chloride)

20 11

5 9

M102 M60

90 80

Poly(vinyl chloride-co-vinyl acetate)

60 80

High-density Polyethylene Polypropylene

Polycarbonate Nylon Cellulose acetate

Knoop

Rockwell

Barcol

R40 R100

M75

7 5

M70 M75

M64

Shore D 70 74

The two most common types of durometers used for plastics are the Shore Type A and Shore Type D. They differ in the spring force and the geometry of the indentor, as shown in Figure 3.42. Due to creep, readings should be taken after a ﬁxed time interval, often chosen as 10 sec. Typical hardness values of some of the common plastics measured by different test methods are shown in Table 3.4. The indentationbased techniques cannot be applied to soft, rubber-like polymers, for which, particularly when dealing with blends, copolymers, etc., one can use the Fakirov equation: H = 1.97Tg – 571 with H (microhardness) in MPa and Tg (glass transition temperature) in °K. Combining the rule of mixtures (H = SHi ji) and this equation, it is possible to calculate the H-value of materials comprising soft component and/or phase [20].

3.2.23 Rebound Hardness The energy absorbed when an object strikes a surface is related to the hardness of the surface: the harder the surface, the less the energy absorbed, and the greater the rebound height of the object after impact. Several methods have been developed to measure hardness in this way. The most common method uses a Shore scleroscope, in which the hardness is determined from the rebound height after the impact of a diamond cone dropped onto the surface of the test piece. Typical values of Scleroscope hardness together with the Rockwell M values (in parentheses) for some common plastics are as follows: PMMA 99 (M 102), LDPE 45 (M 25), polystyrene 70 (M 83), and PVC 75 (M 60).

355

Plastics Properties and Testing

3.2.24 Scratch Hardness Basically, scratch hardness is a measure of the resistance the test sample has to being scratched by other materials. The most common way of qualifying this property is by means of the Mohs scale. On this scale various materials are classiﬁed from 1 to 10. The materials used, as shown in Figure 3.43, range from talc (1) to diamond (10). Each material on the scale can scratch the materials that have a lower Mohs number; however, the Mohs scale is not of much value for classifying plastic materials, because most common plastics fall in the 2–3 Mohs range. However, the basic technique of scratch hardness may be used to establish the relative merits of different plastic materials from their ability to scratch one another.

10,000 Diamond

2000

1000

Corundum or sapphire 9 Nitrided steels Topaz 8

Cutting tools File hard

500 110 100 200 80 100

60 40 20 0 Rockwell B

20 0 Rockwell C

140 120

Easily machined steels

Brinell hardness

Orthoclase Apatite

6 5

Fluorite Calcite

4 3

Gypsum

Talc

Most plastics

120 100

100

60 130

Quartz

Brasses and aluminum alloys

80 20

80 60 40 Rockwell

20 Rockwell M

Mohs hardness

FIGURE 3.43

Comparison of hardness scales (approximate).

356

Plastics Technology Handbook

Scratch hardness is particularly important in plastics used for their optical properties and is usually determined by some of mar-resistance test. In one type of test a specimen is subjected to an abrasive treatment by allowing exposure to a controlled stream of abrasive, and its gloss (specular reﬂection) is measured before and after the treatment. In some tests the light transmission property of the plastic is measured before and after marring.

3.2.25 Stress Corrosion Cracking of Polymers Stress corrosion cracking of polymers occurs in a corrosive environment and also under stress [21,22]. This kind of crack starts at the surface and proceeds at right angles to the direction of stress. The amount of stress necessary to cause stress corrosion cracking is much lower than the normal fracture stress, although there is a minimum stress below which no stress corrosion cracking occurs. The stress corrosion resistance of polymers depends on the magnitude of the stress, the nature of the environment, the temperature, and the molecular weight of the specimen. Ozone cracking is a typical example of stress corrosion cracking of polymers. The critical energy for crack propagation (tc) in ozone cracking varies very little from one polymer to another and is about 100 erg/cm2 (0.1 J/m2). This value is much lower that the tc values for mechanical fracture, which are about 107 erg/cm2 (104 J/m2). In ozone cracking very little energy is dissipated in plastic or viscoelastic deformations at the propagating crack, and that is why tc is about the same as the true surface energy. The only energy supplied to the crack is that necessary to provide for the fresh surfaces due to propagation of the crack, because in ozone cracking chemical bonds at the crack tip are broken by chemical reaction, so no high stress is necessary at the tip. The critical energy tc is about 4,000 erg/cm2 for PMMA in methylated spirits at room temperature, but the value is lower in benzene and higher in petroleum ether. Thus tc in this case is much higher than the true surface energy but still much lower than that for mechanical crack propagation.

3.3 Reinforced Plastics The modulus and strength of plastics can be increased significantly by means of reinforcement [23–25]. A reinforced plastic consists of two main components—a matrix, which may be either a thermoplastic or thermosetting resin, and a reinforcing filler, which is mostly used in the form of fibers (but particles, for example glass spheres, are also used). The greater tensile strength and stiffness of fibers as compared with the polymer matrix is utilized in producing such composites. In general, the fibers are the load-carrying members, and the main role of the matrix is to transmit the load to the fibers, to protect their surface, and to raise the energy for crack propagation, thus preventing a brittle-type fracture. The strength of the fiber-reinforced plastics is determined by the strength of the fiber and by the nature and strength of the bond between the fibers and the matrix.

3.3.1 Types of Reinforcement The reinforcing filler usually takes the form offibers, since it is in this form that the maximum strengthening of the composite is attained. A wide range of amorphous and crystalline materials can be used as reinforcing fibers, including glass, carbon, asbestos, boron, silicon carbide, and more recently, synthetic polymers (e.g., Kevlar fibers from aromatic polyamides). Some typical properties of these reinforcing fibers are given in Table 3.5. Glass is relatively inexpensive, and in fiber form it is the principal form of reinforcement used in plastics. The earliest successful glass reinforcement had a low-alkali calcium–alumina borosilicate composition (E glass) developed specifically for electrical insulation systems. Although glasses of other compositions were developed subsequently for other applications, no commercial glass better than E glass

357

Plastics Properties and Testing TABLE 3.5 Typical Properties of Reinforcing Fibers Tensile Strength Fiber

Density (g/cm )

10 kgf/cm

Tensile Modulus GPa

10 kgf/cm2

GPa

E Glass

2.54–2.56

3.5–3.7

3.4–3.6

7.1–7.7

70–76

Carbon

1.75–2.0

2.1–2.8

24.5–40.8

240–400

Asbestos Boron

2.5–3.3 2.6

2.1–3.6 3.0–3.6

2.1–3.5 3.0–3.5

14.3–19.4 40.8–45.9

140–190 400–450

Silicon carbide

3.2–3.4

3.0–3.7

3.0–3.6

46.9–50.0

460–490

Kevlar-49

1.45

3.0–3.6

13.2

130

Source: Crawford, R. J. 1981. Plastic Engineering, Pergamon, London.

has been found for plastics reinforcement. However, certain special glasses having extra high-strength properties or modulus have been produced in small quantities for specific applications (e.g., aerospace technology). Glass fibers are usually treated with finishes. The function of a finish is to secure good wetting and to provide a bond between the inorganic glass and the organic resin. The most important finishes are based on silane compounds—e.g., vinyltrichlorosilane or vinyltriethoxysilane.

3.3.2 Types of Matrix The matrix in reinforced plastics may be either a thermosetting or thermoplastic resin. The major thermosetting resins used in conjunction with glass-ﬁber reinforcement are unsaturated polyester resins and, to a lesser extent, epoxy resins. These resins have the advantage that they can be cured (cross-linked) at room temperature, and no volatiles are liberated during curing. Among thermoplastic resins used as the matrix in reinforced plastics, the largest tonnage group is the polyoleﬁns, followed by nylon, polystyrene, thermoplastic polyesters, acetal, polycarbonate, and polysulfone. The choice of any thermoplastic is dictated by the type of application, the service environment, and the cost.

3.3.3 Analysis of Reinforced Plastics Fibers exert their effect by restraining the deformation of the matrix while the latter transfers the external loading to the fibers by shear at the interface. The resultant stress distributions in the fiber and the matrix tend to be complex. Theoretical analysis becomes further complicated because fiber length, diameter, and orientation are all factors. A simplified analysis follows for two types of fiber reinforcement commonly used, namely, (1) continuous fibers and (2) discontinuous fibers. 3.3.3.1 Continuous Fibers We will examine what happens when a load is applied to an ideal fiber composite in which the matrix material is reinforced by fibers which are uniform, continuous, and arranged uniaxially, as shown in Figure 3.44a. Let us assume that the fibers are gripped firmly by the matrix so that there is no slippage at the fibermatrix interface and both phases act as a unit. Under these conditions the strains in the matrix and in the fiber under a load are the same (Figure 3.44b), and the total load is shared by the fiber and the matrix: Pc = Pm + Pf

(3.98)

where P is the load and the subscripts c, m, and f refer, respectively, to composite, matrix and ﬁber.

358

Plastics Technology Handbook

Fib e

Load ΔL

Fiber

σf Stress

Matrix

Matr

σm (a)

Strain, (ΔL/L)

(b)

(c)

FIGURE 3.44 (a) Continuous-ﬁber reinforced composite under tensile load. (b) Iso-strain assumption in a composite. (c) Arrangement of ﬁbers in a cross-plied laminate.

Since the load P = sA, Equation 3.98, expressed in terms of stresses (s) and cross-sectional areas (A), becomes sc Ac = sm Am + sf Af Rearranging gives

sc = sm

Am Ac

+ sf

Af Ac

(3.99) (3.100)

Since the ﬁbers run throughout the length of the specimen, the ratio Am/Ac can be replaced by the volume fraction Fm = Vm/Vc, and similarly Af/Ac by Ff. Equation 3.100 thus becomes sc = sm Fm + sf Ff

(3.101)

Equation 3.101 represents the rule of mixture for stresses. It is valid only for the linear elastic region of the stress–strain curve (see Figure 3.2). Since Fm + Ff = 1, we can write sc = sm (1 − Ff ) + sf Ff

(3.102)

Since the strains on the components are equal, ec = em = ef

(3.103)

Equation 3.101 can now be rewritten to give the rule of mixture for moduli Ec ec = Em em Fm + Ef ef Ff i.e., Ec = Em Fm + Ef Ff

(3.104)

Equation 3.103 also affords a comparison of loads carried by the ﬁber and the matrix. Thus for elastic deformation Equation 3.103 can be rewritten as sc sm sf = = Ec Em Ef or Pf E = r Pm Em

Af Am

Ef Em

Ff Fm

(3.105)

Because the modulus of ﬁbers is usually much higher than that of the matrix, the load on a composite will therefore be carried mostly by its ﬁber component (see Example 3.4). However, a critical volume

359

Plastics Properties and Testing

fraction of fibers (Fcrit) is required to realize matrix reinforcement. Thus for Equation 3.102 and Equation 3.105 to be valid, Ff > Fcrit. The efficiency of reinforcement is related to the fiber direction in the composite and to the direction of the applied stress. The maximum strength and modulus are realized in a composite along the direction of the fiber. However, if the load is applied at 90° to the filament direction, tensile failure occurs at very low stresses, and this transverse strength is not much different than the matrix strength. To counteract this situation, one uses cross-plied laminates having alternate layers of unidirectional fibers rotated at 90°, as shown in Figure 3.44c. (A more isotropic composite results if 45° plies are also inserted.) The stress–strain behavior for several types of fiber reinforcement is compared in Figure 3.45. As already noted, if the load is applied perpendicularly to the longitudinal direction of the fibers, the fibers exert a relatively small effect. The strains in the fibers and the matrix are then different, because they act independently, and the total deformation is thus equal to the sum of the deformations of the two phases. Vc ec = Vm em + Vf ef

(3.106)

Dividing Equation 3.106 by Vc and applying Hooke’s law, since the stress is constant, give s sFm sFf = + Em Ef Ec

(3.107)

Dividing by s and rearranging, we get Ec =

Em Ef Em Ff + Ef Fm

(3.108)

The ﬁber composite thus has a lower modulus in transverse loading than in longitudinal loading.

0°

45°

90° Unidirectional

0°

Bidirectional

Random

0° 90°

Stress

0°,45°,90°

45° 45° 45°

FIGURE 3.45

Strain

Stress–strain behavior for several types of ﬁber reinforcement.

360

Plastics Technology Handbook

Example 4: A unidirectional fiber composite is made by using 75% by weight of E glass continuous fibers (sp. gr. 2.4) having a Young’s modulus of 7 × 105 kg/cm2 (68.6 GPa), practical fracture strength of 2 × 104 kg/cm2 (1.9 GPa), and an epoxy resin (sp. gr. 1.2) whose modulus and tensile strength, on curing, are found to be 105 kg/cm2 (9.8 GPa) and 6 × 102 kg/cm2 (58.8 MPa), respectively. Estimate the modulus of the composite, its tensile strength, and the fractional load carried by the fiber under tensile loading. What will be the value of the modulus of the composite under transverse loading? Answer: Volume fraction of glass fibers (Ff) =

0:75=2:4 = 0:60 0:75=2:4 + 0:25=1:2 Fm = 1 − 0:6 = 0:4

From Equation 3.104, Ec = 0:4(105 ) + 0:6(7 105 ) = 4:6 105 kg=cm2 (45 GPa) From Equation 3.101, sc = 0:4(6 102 ) + 0:6(2 104 ) = 1:22 104 kg=cm2 (1:2 GPa) Equation 3.105, on rearranging, gives Pf Et 7 105 = Ff = 0:6 = 0:91 Pc Ec 4:6 105 Thus, nearly 90% of the load is carried by the ﬁber, and the weakness of the plastic matrix is relatively unimportant. For transverse loading, from Equation 3.108, Ec =

105 (7 105 ) = 2 105 kg=cm2 (19:6 GPa) 0:6 105 + 0:4(7 105 )

Equation 3.102 and Equation 3.104 apply to ideal fiber composites having uni-axial arrangement of fibers. In practice, however, not all the fibers are aligned in the direction of the load. This practice reduces the efficiency of the reinforcement, so Equation 3.102 and Equation 3.104 are modified to the forms sc = sm (1 − ff ) + k1 sf ff

(3.109)

Ec = Em (1 − ff ) + k2 Ef ff

(3.110)

If the fibers are bi-directional (see Figure 3.45), then the strength and modulus factors, k1 and k2, are about 0.3 and 0.65, respectively. 3.3.3.2 Discontinuous Fibers If the fibers are discontinuous, the bond between the fiber and the matrix is broken at the fibers ends, which thus carry less stress than the middle part of the fiber. The stress in a discontinuous fiber therefore varies along its length. A useful approximation pictures the stress as being zero at the end of the filler and as reaching the maximum stress in the fiber at a distance from the end (Figure 3.46a).

361

Plastics Properties and Testing

Fiber d l σMax Stress in ﬁber

F3 F1

F2 dx

(a)

lc/2

lc/2 τ

Fiber

Tensile stress σ (b)

σ Matrix

FIGURE 3.46 Composite reinforced with discontinuous fibers. (a) A total length lc, at the two ends of a fiber carries less than the maximum stress. (b) Interfacial strength of the matrix fiber.

The length over which the load is transferred to the fiber is called the transfer length. As the stress on the composite is increased, the maximum fiber stress as well as the transfer length increase, as shown in Figure 3.46a, until a limit is reached, because the transfer regions from the two ends meet at the middle of the fiber (and so no further transfer of stress can take place), or because the fiber fractures. For the latter objective to be reached, so as to attain the maximum strength of the composite, the fiber length must be greater than a minimum value called the critical fiber length, lc. Consider a fiber of length l embedded in a polymer matrix, as shown in Figure 3.46b. One can then write, equating the tensile load on the fiber with the shear load on the interface, spd 2 = tpdl 4

(3.111)

where s is the applied stress, d is the ﬁber diameter, and t is the shear stress at the interface. The critical ﬁber length, lc, can be derived from a similar force balance for an embedded length of lc/2. Thus, lc =

sff d 2ti

and lc sff = d 2ti

(3.112)

where sff is the fiber strength and ti is the shear strength of the interface or the matrix, whichever is smaller. So if the composite is to fail through tensile fracture of the fiber rather than shear failure due to matrix flow at the interface between the fiber and the matrix, the ratio lc/d, known as the critical aspect ratio, must be exceeded, or, in other words, for a given diameter of fiber, d, the critical fiber length, lc, must be exceeded.

362

Plastics Technology Handbook

If the fiber length is less than lc, the matrix will flow around the fiber, and maximum transfer of stress from matrix to fiber will not occur. Using Equation 3.112, we can estimate the value of lc/d from the values of sff and ti, and vice versa. Typical values of lc/d for glass fiber and carbon fiber in an epoxy resin matrix are 30–100 and 70, respectively. If the fibers are discontinuous, then, since the stress is zero at the end of the fiber, the average stress in the fibers will be less than the value sfmax, which it would have achieved if the fibers had been continuous over the whole length of the matrix. The value of the average stress will depend on the stress distribution in the end portions of the fibers and also on their lengths. If the stress distributions are assumed to be as shown in Figure 3.46a, then the average stress in the fibers may be obtained as follows. Considering a differential section of the fiber as shown in Figure 3.46a, we obtain pd 2 F1 = sf 4 dsf pd 2 F2 = sf + dx dx 4 F3 = tpd dx For equilibrium, F1 = F2 + F3 so sf

pd 2 = 4

2 ds pd sf + f dx + tpd dx dx 4 d ds = −t dx 4 f

Integrating gives d 4

s ð1

(3.113)

ðx dsf = −

t dx l=2

sf =

4t(l=2 − x) d

(3.114)

Three cases may now be considered. 3.3.3.3 Fiber Length Less than lc In this case the peak stress occurs at x = 0 (Figure 3.47a). So from Equation 3.114, 2tl d The average fiber stress is obtained by dividing the area of the stress-fiber length diagram by the fiber length; that is, sf =

f = s

(l=2)2tl=d tl = l d

The stress, sc, in the composite is now obtained from Equation 3.109 sc = sm (1 − Ff ) +

tlk1 f d f

(3.115)

363

Plastics Properties and Testing

l < lc

l = lc

Stress

σf max

Stress σf max σf

2τlc d

σf

2τl d

σf

l > lc

lc /2

(a)

FIGURE 3.47

(b)

(c)

Stress variation for short and long ﬁbers.

3.3.3.4 Fiber Length Equal to lc In this case the peak value of stress occurs at x = 0 and is equal to the maximum ﬁber stress (Figure 3.47b). So 2tl sf = sfmax = c (3.116) d f = Average fiber stress = s

1 lc (2tlc =d) 2 lc

i.e., f = s

tlc d

So from Equation 3.109,

sc = sm (1 − ff ) + k1

tlc ff d

(3.117)

3.3.3.5 Fiber Length Greater than lc 1. For l/2 > x > (l − lc)/2 (Figure 3.47c), 4t 1 l−x sf = d 2 2. For (l − lc)/2 > x > 0 (Figure 3.47c), sf = constant = sfmax =

2tlc d

The average ﬁber stress, from the area under the stress-ﬁber length graph is f = s

(lc =2)sfmax + (l − lc )sfmax = l

lc 1− s 2l fmax

(3.118)

364

Plastics Technology Handbook

So from Equation 3.109, l sc = sm (1 − ff ) + k1 ff 1 − c ffmax : 2l

(3.119)

It is evident from Equation 3.118 that to get the average fiber stress as close as possible to the maximum fiber stress, the fibers must be considerably longer than the critical length. At the critical length the average fiber stress is only half of the maximum fiber stress, i.e., the value achieved in continuous fibers (Figure 3.47c). Equations such as Equation 3.119 give satisfactory agreement with the measured values of strength and modulus for polyester composites reinforced with chopped strands of glass fibers. These strength and modulus values are only about 20%–25% of those achieved by reinforcement with continuous fibers. Example 5: Calculate the maximum and average fiber stresses for glass fibers of diameter 15 mm and length 2 mm embedded in a polymer matrix. The matrix exerts a shear stress of 40 kgf/cm2 (3.9 MPa) at the interface, and the critical aspect ratio of the fiber is 50. Answer: lc = 50 15 10−3 = 0:75 mm Since l > lc, then smax =

2tlc = 2 40 50 = 4 103 kgf =cm2 ( = 392 MPa) d

Also, lc 0:75 f = 1 − s (4 103 ) = 3:25 103 kgf =cm2 ( = 318 MPa) s = 1− 22 2l fmax

3.3.4 Deformation Behavior of Fiber-Reinforced Plastic As we have seen, the presence of fibers in the matrix has the effect of stiffening and strengthening it. The tensile deformation behavior of fiber-reinforced composites depends largely on the direction of the applied stress in relation to the orientation of the fibers, as illustrated in Figure 3.45. The maximum strength and modulus are achieved with unidirectional fiber reinforcement when the stress is aligned with the fibers (0°), but there is no enhancement of matrix properties when the stress is applied perpendicular to the fibers. With random orientation of fibers the properties of the composite are approximately the same in all directions, but the strength and modulus are somewhat less than for the continuous-fiber reinforcement. In many applications the stiffness of a material is just as important as its strength. In tension the stiffness per unit length is given the product EA, where E is the modulus and A is the cross-sectional area. When the material is subjected to flexure, the stiffness per unit length is a function of the product EI, where l is the second moment of area of cross section (see Example 3.2). Therefore the stiffness in both tension and flexure increases as the modulus of the material increases, and the advantages of fiber reinforcement thus become immediately apparent, considering the very high modulus values for fibers.

365

Plastics Properties and Testing

3.3.5 Fracture of Fiber-Reinforced Plastics Although the presence of the reinforcing fibers enhances the strength and modulus properties of the base material, they also cause a complex distribution of stress in the materials. For example, even under simple tensile loading, a triaxial stress system is set up since the presence of the fiber restricts the lateral contraction of the matrix. This system increases the possibility of brittle failure in the material. The type of fracture which occurs depends on the loading conditions and fiber matrix bonding. 3.3.5.1 Tension With continuous-fiber reinforcement it is necessary to break the fibers before overall fracture can occur. The two different of fracture which can occur in tension are shown in Figure 3.48. It is interesting to note that when an individual fiber in a continuous-fiber composite breaks, it does not cease to contribute to the strength of the material, because the broken fiber then behaves like a long short fiber and will still be supporting part of the external load at sections remote from the broken end. In short-fiber composites, however, fiber breakage is not an essential prerequisite to complete composite fracture, especially when the interfacial bond is weak, because the fibers may then be simply pulled out of the matrix as the crack propagates through the latter. 3.3.5.2 Compression In compression the strength of glass-fiber reinforced plastics is usually less than in tension. Under compressive loading, shear stresses are set up in the matrix parallel to the fibers. The fiber aligned in the loading direction thus promote shear deformation. Short-fiber reinforcement may therefore have advantages over continuous fibers in compressive loading because in the former not all the fibers can be aligned, so the fibers which are inclined to the loading plane will resist shear deformation. If the matrixfiber bond is weak, debonding may occur, causing longitudinal cracks in the composite and buckling failure of the continuous fibers.

Matrix Fiber

(a)

(b)

FIGURE 3.48 Typical fracture modes in ﬁber-reinforced plastics. (a) Fracture due to strong interfacial bond. (b) Jagged fracture due to weak interfacial bond.

366

Plastics Technology Handbook

3.3.5.3 Flexure or Shear In flexure or shear, as in the previous case of compression, plastics reinforced with short fibers are probably better than those with continuous fibers, because in the former with random orientation of fibers at least some of the fibers will be correctly aligned to resist the shear deformation. However, with continuousfiber reinforcement if the shear stresses are on planes perpendicular to the continuous fibers, then the fibers will offer resistance to shear deformation. Since high volume fraction (ff) can be achieved with continuous fibers, this resistance can be substantial.

3.3.6 Fatigue Behavior of Reinforced Plastics Like unreinforced plastics, reinforced plastics are also susceptible to fatigue. There is, however, no general rule concerning whether glass reinforcement enhances the fatigue endurance of the base material. In some cases the unreinforced plastic exhibits greater fatigue endurance than the reinforced material; in other cases, the converse is true. In short-fiber glass-reinforced plastics, cracks may develop relatively easily at the interface close to the ends of the fibers. The cracks may then propagate through the matrix and destroy its integrity long before fracture of the composite takes place. In many glass-reinforced plastics subjected to cyclic tensile stresses, debonding many occur after a few cycles even at modes stress levels. Debonding may be followed by resin cracks at higher stresses, leading eventually to the separation of the component due to intensive localized damage. In other modes of loading, e.g., flexure or torsion, the fatigue endurance of glass-reinforced plastics is even worse than in tension. In most cases the fatigue endurance is reduced by the presence of moisture. Plastics reinforced with carbon and boron, which have higher tensile moduli than glass, are stiffer than glass-reinforced plastics and are found to be less vulnerable to fatigue.

3.3.7 Impact Behavior of Reinforced Plastics Although it might be expected that a combination of brittle reinforcing fibers and a brittle matrix (e.g., epoxy or polyester resins) would have low impact strength, this is not the case, and the impact strengths of the fibers or the matrix. For example, polyester composites with chopped-strand mat have impact strengths from 45 to 70 ft-lbf/in2 (94–147 kJ/m2), whereas a typical impact strength for polyster resin is only about 1 ft-lbf/in2 (2.1 kJ/m2). The significant improvement in impact strength by reinforcement is explained by the energy required to cause debonding and to overcome friction in pulling the fibers out of the matrix. It follows from this that impact strengths would be higher if the bond between the fiber and the matrix is relatively weak, because if the interfacial bond is very strong, impact failure will occur by propagation of cracks across the matrix and fibers requiring very little energy. It is also found that in short-fiber-reinforced plastics the impact strength is maximum when the fiber length has the critical value. The requirements for maximum impact strength (i.e., short fiber and relatively weak interfacial bond) are thus seen to be contrary to those for maximum tensile strength (long fibers and strong bond). The structure of a reinforced plastic material should therefore be tailored in accordance with the service conditions to be encountered by the material.

3.4 Electrical Properties The usefulness of an insulator or dielectric ultimately depends on its ability to act as a separator for points across which a potential difference exists. This ability depends on the dielectric strength of the material, which is deﬁned as the maximum voltage gradient that the material withstands before failure or loss of the material’s insulating properties occurs.

Plastics Properties and Testing

367

Besides permittivity (dielectric constant), dielectric losses, and dielectric strength, another property used to define the dielectric behavior of a material is the insulation resistance, i.e., the resistance offered by the material to the passage of electric current. This property may be important in almost all applications of insulators. This resistivity (i.e., reciprocal of conductivity) of a plastic material with a perfect structure would tend to be infinite at low electric fields. However, the various types of defects which occur in plastics may acts as sources of electrons or ions which are free to contribute to the conductivity or that can be thermally activated to do so. These defects may be impurities, discontinuities in the structure, and interfaces between crystallites and between crystalline and amorphous phases. Common plastics therefore have finite, though very high, resistivities from 108 to 1020 ohm-cm. These resistivity values qualify them as electrical insulators. Polymeric materials have also been produced which have relatively large conductivities and behave in some cases like semiconductors and even photoconductors [26]. For example, polyphenylacetylene, polyaminoquinones, and polyacenequinone radical polymers have been reported with resistivities from 103 to 108 ohm-cm. It has been suggested that the conductivity in these organic semiconductors is due to the existence of large number of unpaired electrons, which are free within a given molecule and contribute to the conduction current by hopping (tunneling) from one molecule to an adjacent one (see “Electroactive Polymers” in Chapter 5).

3.4.1 Dielectric Strength Dielectric strength is calculated as the maximum voltage gradient that an insulator can withstand before puncture or failure occurs. It is expressed as volts (V) per unit of thickness, usually per mil (1 mil = 1/1,000 in.). Puncture of an insulator under an applied voltage gradient results from small electric leakage currents which pass through the insulator due to the presence of various types of defects in the material. (Note that only a perfect insulator would be completely free from such leakage currents.) The leakage currents warm the material locally, causing the passage of a greater current and greater localized warming of the material, eventually leading to the failure of the material. The failure may be a simple puncture in the area where material has volatilized and escaped, or it may be a conducting carbonized path (tracking) that short circuits the electrodes. It is obvious from the cause of dielectric failure that the measured values of dielectric strength will depend on the magnitude of the applied electric field and on the time of exposure to the field. Since the probability of a flaw and a local leakage current leading ultimately to failure increases with the thickness of the sample, dielectric strength will also be expected to depend on the sample thickness. The measurement of dielectric strength (Figure 3.49a) is usually carried out either by the short-time method or by the step-by-step method. In the former method the voltage is increased continuously at a uniform rate (500 V/sec) until failure occurs. Typically, a 1/8-in. thick specimen requiring a voltage of about 50,000 V for dielectric failure will thus involve a testing period of 100 sec or so. In step-by-step testing, definite voltages are applied to the sample for a definite time (1 min), starting with a value that is half of that obtained by short-time testing, with equal increments of 2,000 V until failure occurs. Since step-by-step testing provides longer exposure to the electric field, dielectric strength values obtained by this method are lower than those obtained by the short-time test. Conditions to stepby-step testing correspond more nearly with those met in service. Even so, service failure almost invariably occurs at voltages below the measure dielectric strength. It is thus necessary to employ a proper safety factor to provide for the discrepancy between test and service conditions. Increase in thickness increases the voltage required to give the same voltage gradient, but the probability of a flaw and a local leakage current leading ultimately to failure also increases. The breakdown voltage increases proportionally less than thickness increases, and as a result the dielectric strength of a material decreases with the thickness of specimen (Figure 3.49b). For this reason, testing of insulation plastic should be done with approximately the thickness in which it is to be used in service.

368

Electrodes Test piece

Breakdown voltage (v)

15,000

FIGURE 3.49

(b)

4000

2000

5000 Dielectric strength

0 (a)

down Break To 450 v per mil at 1"/8 in

10,000

4 6 8 10 Thickness (mils)

0 12

Dielectric strength (volts/mil)

Plastics Technology Handbook

(a) Dielectric strength test. (b) Dependence of dielectric strength on thickness of sample.

It is seen from Figure 3.49b that the dielectric strength increases rapidly with decreasing thickness of the sample. A rule of thumb is that the dielectric strength varies inversely with the 0.4 power of the thickness. For example, if the dielectric strength of poly(vinyl chloride) plastic is 375 V/mil in a thickness of 0.075 in., it would be 375(75/15)0.4 or about 700 V/mil in foils only 15 mils thick. The fact that thin foils may have proportionally higher dielectric strength is utilized in the insulation between layers of transformer turns. The dielectric strength of an insulation material usually decreases with increase in temperature and is approximately inversely proportional to the absolute temperature. But the converse is not observed, and below room temperature dielectric strength is substantially independent of temperature change. Mechanical loading has a pronounced effect on dielectric strength. Since a mechanical stress may introduce internal flaws which serve as leakage paths, mechanically loaded insulators may show substantially reduced values of dielectric strength. Reductions up to 90% have been observed. Dielectric strength of an insulating material is influenced by the fabrication detail. For example, flow lines in a compression molding or weld lines in an injection molding may serve as paths of least resistance for leakage currents, thus reducing the dielectric strength. Even nearly invisible minute flaws in a plastic insulator may reduce the dielectric strength to one-third its normal value.

3.4.2 Insulation Resistance Test piece Guard

Galv.

Electrode

Applied voltage

FIGURE 3.50

Insulation resistance test.

The resistance offered by an insulating material to the electric current is the composite effect of volume and surface resistances, which always act in parallel. Volume resistance is the resistance to leakage of the electric current through the body of the material. It depends largely on the nature of the material. But surface resistance, which is the resistance to leakage along the surface of a material, is largely a function of surface ﬁnish and cleanliness. Surface resistance is reduced by oil or moisture on the surface and by surface roughness. On the other hand, a very smooth or polished surface gives greater surface resistance. A three-electrode system, as shown in Figure 3.50, is used for measurement of insulation resistance. In this way the surface and volume leakage currents are separated. The applied voltage must be well below the dielectric strength of the material. Thus, in practice, a voltage gradient less than 30 V/mil is applied. From the applied voltage and the leakage current, the leakage resistance is computed. Since

369

10–5

FIGURE 3.51

105 1010 Speciﬁc resistance (ohm/cm)

Porcelain Polyethylene Mica Polytetraﬂuoroethylene Amber Sulfur Polystyrene Fused quartz

Glass

Cellulose acetate

Pure water

Rubber

Sea water

Graphite

Gold

Copper Iron Nichrome

Plastics Properties and Testing

1015

The resistivity spectrum.

the measured value depends, among other things, on the time during which the voltage is applied, it is essential to follow a standardized technique, including preconditioning of the specimen to obtain consistent results. The insulation resistance of a dielectric is represented by its volume resistivity and surface resistivity. The volume resistivity (also known as specific volume resistance) is defined as the resistance between two electrodes covering opposite faces of a centimeter cube. The range of volume resistivities of different materials including plastics is shown in Figure 3.51. Values for plastics range from approximately 1010 ohm-cm for a typical cellulose acetate to abut 1019 ohm-cm for a high-performance polystyrene. The surface resistivity (also known as specific surface resistance) is defined as the resistance measured between the opposite edges of the surface of a material having an area of 1 cm2 It ranges from 1010 ohm for cellulose acetate to 1014 ohm for polystyrene. The insulation resistance of most plastic insulating materials is affected by temperature and the relative humidity of the atmosphere. The insulation resistance falls off appreciably with an increase in temperature or humidity. Even polystyrene, which has very high insulation resistance at room temperature, becomes generally unsatisfactory above 80°C (176°F). Under these conditions polymers like polytetrafluoroethylene and polychlorotrifluoroethylene are more suitable. Plastics that have high water resistance are relatively less affected by high humidities.

3.4.3 Arc Resistance The arc resistance of a plastic is its ability to withstand the action of an electric are tending to form a conducting path across the surface. In applications where the material is subject to arcing, such as switches, contact bushes, and circuit breakers, resistance to arc is an important requirement. Arcing tends to produce a conducting carbonized path on the surface. The arc resistance of an insulator may be deﬁned as the time in seconds that an arc may play across the surface without burning a conducting path. A schematic of an arc-resistance test is shown in Figure 3.52. Plastics that carbonize easily (such as phenolics) have relatively poor arc resistance. On the other hand, there are plastics (such as methacrylates) that do not carbonize, although they would decompose and give off combustible gases. There would thus be no failure in the usual sense. Special arc-resistant formulations involving noncarbonizing mineral ﬁllers are useful for certain applications. But when service conditions are severe in this respect, ceramics ought to be used, because they generally have much better arc resistance than organic plastics. Related to arc resistance is ozone resistance. This gas is found in the atmosphere around high-voltage equipment. Ignition cable insulation, for example, should be ozone resistant. Natural rubber is easily

370

Plastics Technology Handbook

Electrodes

Arc between electrodes

Test piece

FIGURE 3.52

Arc-resistance test.

attacked and deteriorated by ozone. Fortunately, most synthetic resins have good ozone resistance and are satisfactory from this point of view.

3.4.4 Dielectric Constant The effect of a dielectric material in increasing the charge storing capacity of a capacitor can be understood by considering the parallel-plate type sketched in Figure 3.53. If a voltage V is applied across two metal plates, each of area A m2, separated by a distance, d m, and held parallel to each other in vacuum, the electric ﬁeld established between the plates (Figure 3.53a) is E=

−V d

(3.120)

The charge density, Q0/A, where Q0 is the total charge produced on the surface area A of each plate, is directly proportional to the electric ﬁeld. Q0 V = −e0 E = e0 A d

(3.121)

Ae0 V = C0 V d

(3.122)

or Q0 =

Bound charge

+ + V

–

Q = Q0 + Q '

Area of plate (A)

Charge (Q0) +

+ + + + + + + +

E = –V/d

d (Vacuum) –

(a)

–

Free charge

–

+ +

Net negative charge, –Q' at surface Region of no net charge

(b)

FIGURE 3.53 Schematic illustration of the effect of dielectric material in increasing the charge storing capacity of a capacitor.

371

Plastics Properties and Testing

The proportionality constant, e0, is called the dielectric constant (or permittivity) of a vacuum. It has units of Q =A coul=m2 coul=V farad e0 = 0 = = = V=m V=d m m and a value of 8.854 × 10−12 farad/m. The quantity C0 in Equation 3.122 is the capacitance of a capacitor (condenser) with a vacuum between its plates. It can be deﬁned as the ratio of the charge on either of the plates to the potential differences between the plates. Now if a sheet of a dielectric material is inserted between the plates of a capacitor (Figure 3.53b), an increased charge appears on the plates for the some voltage, due to polarization of the dielectric. The applied ﬁeld E causes polarization of the entire volume of the dielectric and thus gives rise to induced charges, or bound charges, Q′, at its surface, represented by the ends of the dipole chains. These induced charges may be pictured as neutralizing equal charges of opposite signs on the metal plates. If one assumes, for instance, that the induced charge −Q′ neutralizes an equal positive charge in the upper plate of the capacitor (Figure 3.53b), the total charge stored in the presence of the dielectric is Q = Q0 + Q′. The ratio of the total charge Q to the free charge Q0 (which is not neutralized by polarization) is called the relative dielectric constant or relative permittivity, er, and is characteristic of the dielectric material. er =

Total charge Q = Free charge Q0

(3.123)

Obviously, er is always greater than unity and has no dimensions. For most materials er exceeds 2 (Table 3.6). Dividing both the numerator and denominator of Equation 3.123 by the applied voltage V and applying the deﬁnition of C from Equation 3.122, we obtain er =

e C = e0 C0

(3.124)

The relative dielectric constant or relative permittivity is thus deﬁned as the ratio of the capacitance of a condense with the given material as the dielectric to that of the same condenser without the dielectric.

TABLE 3.6 Dielectric Properties of Electrical Insulators tan d Material

er at 60 Hz

60 Hz

106 Hz

Dielectirc Strengtha (V/mil)

Ceramics Porcelain

0.010

–

Alumina Zircon

9.6 9.2

– 0.035

CF2 stretching vibration. This band is a multiplet and consists of three peaks at 1240, 1215, and 1150 cm−1. Other major bands are located at 641, 554, and 515 cm−1, and these are assigned to the >CF bending modes. In the IR spectrum of poly(vinyl chloride) (PVC), another common polymer with halide groups and having the general formula –[–CH2–CHCl–]–, the peaks found in a range of 2800–3000 cm−1 (Figure 3.93a) correspond to –C–H stretch. The peak at higher wavenumber is assigned to the asymmetric stretching and the lower peak is assigned to the symmetric stretching vibration of –C–H. The peaks around 1400 cm−1 are assigned to –C–H aliphatic bending vibration, while the peak at 1250 cm−1 is attributed to the bending vibration of –C–H near Cl. The –C–C– stretching vibration of the PVC backbone chain occurs in a range of 1000–1100 cm−1. Finally, the peaks in a range of 600–650 cm−1 correspond to the C–Cl bond. Hydroxyl Groups. A careful examination of the hydroxyl stretching region (Table 3.11) can often be valuable in the determination of structure of unknown polymeric compounds. Thus, the presence of a band in the 3700–3150 cm−1 (2.7–3.2 mm) region is a very reliable indication of the presence of hydroxyl groups. [Note, however, the following two exceptions. Since water shows strong absorption in this region, the sample must be fully dry for hydroxyl observation. Halide powders being rarely dry, halide disks for sample preparation are thus best avoided for measurement of hydroxyl groups. Also, N–H groups, if present, can cause interfering absorptions in the hydroxyl region (see Table 3.11).] For illustration, Figure 3.93b shows the IR absorption spectrum of poly(vinyl alcohol) (PVA) with the general formula –[–CH2–CH(OH)–]–. While strong hydroxyl bands of free alcohol, that is, nonbonded –O–H stretching bands, usually occur at 3600–3650 cm−1, PVA exhibits a bonded –O–H stretching band at 3200–3570 cm−1, which would be attributed to the formation of intramolecular and intermolecular hydrogen bonds among PVA chains owing to high hydrophilic forces. The band at 2940 cm−1 is assigned to –C–H stretching, whereas the bands (doublet) at 1330–1440 cm−1 are likely to arise from mixed –C–H and –O–H in-plane bending vibrations [38]. Carbonyl Groups. For compounds containing carbonyl (C=O) groups, the precise range of absorption frequency 1700–1850 cm−1 is often sufﬁcient to determine their presence. [However, the carbonyl group

410

Plastics Technology Handbook

1.0

(a)

Absorbance

0.8 0.6 0.4 0.2 1.0

(b)

Absorbance

0.8 0.6 0.4 0.2 1.0

(c)

Absorbance

0.8 0.6 0.4 0.2 0 4000

3000

2000 Wavenumber (cm–1)

1000

400

FIGURE 3.93 IR absorption spectra of (a) poly(vinyl chloride), (b) poly(vinyl alcohol), and (c) poly(vinyl acetate). (Adapted from NICODOM IR Libraries, http://www.ir.spectra.com.)

may bond so strongly to an existing hydroxyl group in its vicinity that the band of the latter in the usual range 3150–3700 cm−1 could become difﬁcult, or impossible, to observe. For example, in carboxylic acids, it is evident only as a broadening of the C–H band at 2870 cm−1.] For illustration, the IR spectrum of poly (vinyl acetate) (PVAc), with the general formula –[–CH2–CH(OCOCH3)–]–, is shown in Figure 3.93c. The most prominent band in this spectrum is seen at 1736 cm−1, which may be assigned to the carbonyl (–C=O) stretching vibration (see Table 3.11). The adjacent region from 1700 to 500 cm−1 is complex. It is composed of stretching –C–O vibration, rocking, wagging, and twisting vibrations of –CH2 groups, out-of-plane bending vibrations of the –C–H chain, and one or more stretching vibrations of the polymer chain [39]. Nitrogen-Containing Groups. If the elements test on an unknown polymeric compound reveals the presence of nitrogen, it then becomes imperative to conduct IR spectral runs to determine the type of

411

Plastics Properties and Testing

nitrogen-containing group in the polymer. Nitrogen-containing groups commonly found in synthetic polymers are amines, amides, imide, nitrile, and urethane. In IR spectra, primary amines (–NH2) exhibit two N–H peaks, one near 3350 and one near 3180 cm−1, from asymmetric and symmetric stretching vibrations, respectively. Secondary amines (–NHR), however, give rise to one –N–H stretch peak at 3300. All amides produce a very strong –C=O peak at 1680–1630 cm−1 (usually with the frequency lowered due to hydrogen bonding, if present). In addition, primary amides (–CONH2) and secondary amides (–CONH–) exhibit IR absorptions owing to –N–H bending at 1640–1550 cm−1, with primary amides showing two spikes, as in the case of amines, and secondary amides showing only one spike. Figure 3.94a, for example, shows the IR spectrum of nylon-6, which is a secondary polyamide with the general formula –[–(CH2)5–CONH–]–. The spectrum features, as expected, three strong absorption bands at about 1.0

(a)

Absorbance

0.8 0.6 0.4 0.2 1.0

(b)

Absorbance

0.8 0.6 0.4 0.2 1.0

(c)

Absorbance

0.8 0.6 0.4 0.2 0 4000

3000

2000 Wavenumber (cm–1)

1000

FIGURE 3.94 IR absorption spectra of (a) nylon-6, (b) polyurethane, and (c) polydimethylsiloxane. (Adapted from NICODOM IR Libraries, http://www.ir.spectra.com.)

412

Plastics Technology Handbook

3294 cm−1 (hydrogen bonded >N–H stretch), 1645 cm−1 (C=O), and 1545 cm−1 (CONH). The two moderate peaks at 2932 and 2862 cm−1 are attributed to CH2 asymmetric stretching and CH2 symmetric stretching, respectively. In addition, a number of small peaks appear in the 1400–500 cm−1 region. These are attributed to NH, CO, CONH, CN, methylene sequences, and crystalline forms. It may be mentioned that the major bands of nylon IR spectra are characteristic for polyamides and make identification of the generic class a simple task. However, the features distinguishing the subgeneric classes, such as nylon-6 and nylon-6,6, are more subtle. For example, nylon-6 and nylon-6,6 have similar IR spectra and are differentiated by the presence of a weak crystalline band near 935 cm−1 in the nylon-6,6 spectrum. For polyimides, the main bands in the IR spectrum are those attributed to the carbonyl (C=O) groups in the imide [–(–CONRCO–)–] ring, which appear as a doublet at 1780 and 1720 cm−1. The band at 1780 cm−1 is sharp, while that at 1720 cm−1 is broader and stronger. The spectra of polyamide–imides show the characteristic bands of the carbonyl groups of both the amide and imide structure. One can thus see a series of four bands of relatively high intensity in the 1600–1800 cm−1 region, which is a clear indication that the unknown resin sample is a polyamide–imide. The nitrile (–C≡N) group produces an absorption band at 2240–2280 cm−1, a region that is relatively free of other well-known absorption bands and the band can therefore be easily observed. The major characteristic of the IR spectrum of polyacrylonitrile with the general formula –[–CH2–CH(CN)–]– is thus the nitrile band at 2240 cm−1. Polyurethanes containing the urethane linkage [–NH–CO–O–] exhibit the prominent band pair noted earlier for the secondary amide structure, but the pair now occurs at about 1540 and 1700 cm−1. These bands are easily recognized in simple urethanes and in polyether and polyester urethane rubbers. Figure 3.94b, for example, shows a typical polyurethane IR spectrum. The characteristic band at 1730 cm−1 in this spectrum is associated with the C=O group in polyurethane, while other bands are assigned as follows. The absorption band at 3320 cm−1 corresponds to >NH stretching and the sharp peaks at 2860 and 2940 cm−1 are associated with –CH2– stretching, while other modes of –CH2– vibrations give rise to the bands in the region 1500–1300 cm−1. The bands at 1540 cm−1 are attributed to the group of –NH– vibrations. Silicon-Containing Groups. Polymeric organosilicon compounds are commonly referred to as silicones, the most widely used silicone resin being polydimethylsiloxane, with the general formula –[–Si(CH3)2– O–]–. Methyl groups attached to a silicon atom undergo the same C–H stretching and bending vibrations as a CH3 attached to a carbon atom, but the positions of the bands for a Si–CH3 group are different from those for a C–CH3 group, because of electronic effects. The absorption attributed to the umbrella mode (symmetric bend) vibration of the Si–CH3 group produces a very intense band at 1260 ± 5 cm−1, and when a silicon atom has two methyl groups attached to it, denoted as Si(CH3)2, there appears a strong methyl rocking mode band at 800 ± 10 cm−1. The pattern of bands in the spectrum of polydimethylsiloxane is very characteristic—a series of four intense bands between 1200 and 800 cm−1 (see Figure 3.94c). Few other materials give rise to this pattern. The band attributed to the Si–H group occurs at about 2200 cm−1 and is exceedingly prominent. The absorption owing to the Si–O linkage, which forms the backbone of silicone resins, occurs between 1100 and 1000 cm−1, producing a broad, complex, and intense band. Sharp bands arise near 1250 and 1430 cm−1 owing to Si–Me (as noted above) and Si–Phenyl groups, respectively. The group most frequently encountered, however, is the Si–Me group at 1250 cm−1. This band is particularly easy to observe, even in the presence of other materials absorbing in this region of the spectrum. Silicones containing the Si–H group are also easily recognized in mixtures, since few other substances have significant absorption in the region where the strong Si–H band appears (2100– 2220 cm−1). The absorption band attributed to the OH group attached to Si is, however, similar to that of the alcoholic OH group. Applications. An IR spectrum may be looked upon as a “fingerprint” of a sample in the form of absorption peaks that correspond to the frequencies of vibrations of the bonds between the atoms making up the material. Because each different material is a unique combination of atoms, no two compounds

Plastics Properties and Testing

413

produce exactly the same IR spectrum. Therefore, IR spectroscopy can provide a positive identification of every different kind of material, though it can be reasonably expected that unambiguous identification of hydrocarbon polymers by IR spectral comparison alone will be difficult because of the similarity of spectra of various possible isomers. However, polymers containing various groups with special elements such as O, S, N, and so on are analyzed relatively easily because they may be analyzed via functional groups. Since the IR spectrum can be looked upon as the molecular fingerprint of a sample, IR spectral analysis can be used in many cases to identify unknown materials and check the quality or consistency of a sample. Moreover, since the size of the peaks in the spectrum is a direct indication of the amount of material present, IR spectroscopy can also be used for quantitative analysis (explained later). Since many functional groups can be easily detected and quantified by IR spectroscopy, this technique can be conveniently used in many cases to monitor the quality and state of polymeric materials exposed to environmental and other conditions. For example, many polymers easily undergo oxidation, which is indicated by the appearance of an absorption band of the C=O group near 1720 cm−1. Thermal and photo-chemical (UV-induced) oxidations of polyethylene plastics and fabrics may lead to the formation of hydroxyl (of hydroperoxide/alcohol) and carbonyl (of carboxyl and anhydride) groups, which are easily identified and measured by IR spectroscopy. Since the quality and performance of plastic products depend on the quality of polymer components used in their manufacture, proper identification and quality testing are critically important for the plastics industry. IR spectral data can be used for identification of polymer samples, qualitative analysis of polymer starting materials, or analysis of in-process samples and product quality control. Comparison of measured spectral data with spectral reference databases provides a rapid and effective identification tool for all types of polymer materials, and all sizes and forms including pellets, parts, opaque samples, fibers, powders, wire coatings, and liquids. In appropriate cases, chemical reactions, such as analysis of esterification of cellulose by carboxylic acids, can be monitored by IR spectroscopy. The aim of qualitative analysis of polymer mixtures is to determine the presence of individual components and, in the case of copolymers, to determine the presence of individual monomer units. This can be accomplished by considering that the spectrum of a mixture is additively composed of the spectra of the individual components and that all absorption bands of the spectrum should be ascribed to individual components, no band being in surplus and none missing. This is exemplified in Figure 3.95, which presents the IR spectra of butadiene–styrene, butadiene–acrylonitrile, and butadiene–styrene– acrylonitrile copolymers. In the first and the third spectra, the characteristic absorption bands of styrene units can be seen at about 700, 760, and 1500 cm−1, while in the second and third spectra, the absorption band at 2250 cm−1 shows the presence of the acrylonitrile unit. The most reliable evidence for the identity of two compounds can be obtained from differential spectrophotometry. In this method, the compound under investigation is inserted into the path of the sample beam and a known compound into the path of the reference beam. If the two compounds are identical, the spectrum will be free of absorption bands. However, if there is a different content of the same group in the two compounds, the spectrum obtained will contain only the bands of this group. Polymer blends are a mixture of chemically different polymers or copolymers with no covalent bonding between them. The IR spectrum will thus be expected to contain absorption bands of all the individual polymers of the blends, the relative intensities of the peaks being dependent on the relative proportions of the constituent polymers. However, if there is a chemical interaction between the polymers, this leads to a considerable difference (shift in peak position) in the blend spectrum. As an illustration, Figure 3.96 shows the FT-IR spectra of two neat resins, polystyrene (PS) and poly(methyl methacrylate) (PMMA), while the FT-IR spectra of their blends in various proportions are shown in Figure 3.97. There are no shifts of the peaks of any group in the blend spectra, which signifies that there is no chemical interaction between the constituent polymers and they remain as a physical mixture. A careful analysis of the IR spectra of the PS/PMMA blends (Figure 3.97) shows that there is a decrease in the transmittance of carbonyl (C=O) and methoxyl (–OCH3) stretchings (at 1732 and 1149 cm−1, respectively) with an increase of PS content, while

414

Plastics Technology Handbook

5000

2000

Wavenumber (cm–1) 1500 1000

700

Transmission (%)

(a)

0 80

(b)

0 80

20 0

10 Wavelength (μm)

FIGURE 3.95 IR spectra of (a) a butadiene–styrene copolymer; (b) a butadiene–acrylonitrile copolymer, and (c) a butadiene–styrene–acrylonitrile terpolymer. 80 Transmittance (%)

70 60 50 40 30 20 10 0 4500

4000

3500

3000 2500 2000 Wavenumber (cm–1)

1500

1000

500

4000

3500

3000 2500 2000 Wavenumber (cm–1)

1500

1000

500

(a) 80 Transmittance (%)

70 60 50 40 30 20 10 0 4500 (b)

FIGURE 3.96

IR spectra of (a) polystyrene and (b) poly(methyl methacrylate).

415

Plastics Properties and Testing

500

Transmittance (%)

400

300

200

100

20:80

40:60

50:50

60:40

80:20

0 4500

FIGURE 3.97

4000

3500

3000 2500 2000 Wavenumber (cm–1)

1500

1000

500

IR spectra of blends of polystyrene and poly(methyl methacrylate) in different proportions.

there is an increase in transmittance of these peaks with an increase of PMMA content, clearly indicating the formation of polymer blends. 3.8.1.6 Quantitative Analysis IR spectroscopy is widely used for quantitative analysis in the ﬁeld of synthetic resins. The analysis is based on Lambert–Beer’s law. If an absorption band in the IR spectrum arises from a particular component in the sample, the concentration of the component is related to the absorbance (A) or percentage transmission at the band maximum by this law, written as A = log10

100 = kcl Percentage transmission

(3.134)

where c is the concentration of the absorbing component, l is the thickness of the sample, and k is a constant. The sample can be prepared by hot pressing the resin between Teﬂon-coated stainless steel plates into sheets of thickness ranging, typically, from 0.2 to 3 mm or solvent casting ﬁlms (typically 0.03 mm thick). The value of k can be determined by measuring the IR absorbance of samples containing a known concentration of the absorbing component. For the determination of absorbance, a base-line calibration is normally used, the base line being drawn tangentially between the minima occurring at each side of the absorption peak. Considering, for example, the absorption band of the acetate group of PVAc at 2.15 mm, shown in Figure 3.98, the absorbance is measured by drawing the straight line background CD above the band and calculating log10(EG/EF). Plotting this absorbance against the thickness of PVAc calibration sample, the graph obtained should be linear and pass through the zero point. From the slope of the linear plot, the constant k in Equation 3.134 can be evaluated using the measured or calculated value of the acetate content of the resin. Using this value of k and the measured absorbance of the band at 2.15 mm, quantitative determination of vinyl acetate in a copolymer, such as vinyl chloride/vinyl acetate copolymer, can be performed. For copolymers containing up to 20% acetate, a sample of about 10 mm thickness gives a band of suitable intensity for measurement. An alternative (and simpler) method for determining the acetate content in vinyl chloride/vinyl acetate copolymer is based on the measurement of the absorbance ratio at 5.8 mm/7.0 mm (i.e., –C=O to –CH2–

416

Plastics Technology Handbook

Transmission

Wavelength

FIGURE 3.98 Measurement of absorbance of the absorption band at 2.15 mm of the acetate group in poly(vinyl acetate) by the base-line calibration method.

ratio). In this method, the sample thickness need not be measured as the acetate content can be determined simply by referring to a calibration curve of 5.8 mm/7.0 mm absorbance ratio versus acetate content. In many cases, some free vinyl acetate (residual monomer) may be present in vinyl acetate polymers or copolymers. Since the band at 2.15 mm is attributed to the acetate group, both free and combined vinyl acetate will absorb at this wavelength. For free vinyl acetate, however, the spectrum can be measured at 1.63 mm (using a relatively thick specimen), where the band is only attributed to the vinyl double bond (–CH=CH2), and hence the monomer content of the sample can be determined. Copolymers of vinyl chloride with acrylates (most commonly, methyl acrylate and ethyl acrylate) produce an ester carbonyl band at 5.8 mm, but the C–O band near 8.5 mm is different in the two cases. Moreover, ethyl acrylate even at low concentrations shows a band at 9.75 mm, which is not present in the methyl acrylate system. IR spectra of the blends of PVC and poly(methyl methacrylate) are similar to those of vinyl chloride/acrylate copolymers. The presence of aliphatic ether in vinyl chloride polymers becomes evident from the appearance of a relatively strong ether band near 9.0 mm. The spectra of vinyl chloride/vinyl isobutyl ether copolymers (containing more than 10% vinyl isobutyl ether), for example, show a prominent, though rather broad, ether band near 9.0 mm, but also a doublet band owing to –CH(CH3)2 at 7.3 mm. The latter is, however, not a conclusive proof for the isobutyl group. IR spectroscopy may provide a simple means of analyzing the composition of polymer blends in many cases. With well-homogenized and unfilled blends, the IR spectroscopic measurement can be performed directly on the sample. On the other hand, if the blend is inhomogeneous and/or filled, an indirect method may be used in which the polymer blend component is separated from the filler by solvent extraction and IR spectroscopy is then applied to the extracted polymer mixture. Considering, as an example, the analysis of polyethylene–polyisobutene (unfilled) blend composition by IR spectroscopy using the direct method, a series of standard samples are prepared by blending known amounts of polyethylene and polyisobutene by milling and then hot-pressing the blend into films of about

417

Plastics Properties and Testing

X´

Transmission

Y´

B E (2.39 μm) A 10

D 11

12 2.25 Wavelength (μm)

2.50

FIGURE 3.99 Measurement of absorbance of the absorption bands of a polyethylene-polyisobutene blend at 10.53 and 2.4 mm band maxima by the base-line calibration method.

0.3 mm thickness, which is suitable for polyisobutene contents between 5% and 25% (w/w). The IR spectra of all standard and unknown samples are measured over 9.0–12.5 mm and 2.1–2.8 mm wavelength ranges. The absorbances at the band maxima (∼10.5 and ∼2.4 mm) are measured by the base-line method. With straight line backgrounds XX′ and YY′ drawn as shown in Figure 3.99, the absorbances are calculated as log10(AC/AB) and log10(DF/DE). Denoting the latter as A10.5 and A2.4, respectively, the ratio A10.5/A2.4 is plotted against wt% polyisobutene contents of the blend standards to obtain a straight line passing through the origin. This calibration curve can be used to determine the polyisobutene content in an unknown polyethylene–polyisobutene blend from the absorbance ratio A10.5/A2.4 measured with a film of the blend. The above method uses a reference band (2.4 mm) as a substitute for film thickness. The method proves useful when the film thickness is difficult to measure (e.g., for rubbery samples). However, if the samples are sufficiently rigid for thickness measurement (such as with a micrometer gauge), the measurement of the reference band absorbance may be omitted and the A10.5/sample thickness ratio is used in a similar type of procedure. If films of controlled thickness can be prepared by hot-pressing with, say, 0.05 mm feeler gauges as spacers between stainless steel plates (preferably coated with Teflon) and spectra are recorded over the wavelength range 9.5–15 mm, absorbances can then be measured at 10.5 and 13.9 mm band maxima by drawing base lines XX′ and YY′, as shown in Figure 3.100 and applied to Equation 3.134, leading to the following relation: Absorbance at 10:5 mm log10 AC=BC = Absorbance at 13:9 mm log10 DF=EF (3.135) wt% polyisobutene in blend =K wt% polyethylene in blend The value of K can be determined from this relation using the known proportions of polyisobutene and polyethylene of the standard blend. Using the determined value of K and the measured absorbances on the spectrum of an unknown sample, the ratio of the wt% contents of polyisobutene and polyethylene in the blend can be determined.

3.8.2 NMR Spectroscopy NMR spectroscopy [34] is now established as an important technique for characterization and testing of polymers. NMR spectra can be observed from a number of atomic nuclei, but for the organic chemist, the spectra of the 1H nucleus, that is, the 1H NMR spectra or proton resonance spectra, are of the greatest practical importance, the reason being that most polymers are organic compounds containing hydrogen and a great deal can be learned about their structure if the relative positions and chemical environment of the hydrogen atoms in the molecule can be established. This information can be derived from proton

418

Plastics Technology Handbook

Y X

Transmission

X´

13.9 μm

10.5 μm F

C 10

Y´

12 13 Wavelength (μm)

FIGURE 3.100 Measurement of absorbance of the absorption bands of a polyethylene–polyisobutene blend at 10.5 and 13.9 mm by the base-line calibration method.

resonance spectra. Though, in principle, complementary information would be obtained from carbon NMR spectra, for the majority of organic compounds, the naturally abundant isotope 12C is inactive, while the 13C nucleus, which is magnetically active, has low natural abundance and gives only weak resonance signals. The NMR spectroscopy of organic compounds is thus confined mainly to proton resonance spectra. Unless otherwise mentioned, NMR spectroscopy/spectra in this book will always mean proton NMR (or 1H NMR) spectroscopy/spectra. 3.8.2.1 General Principles Certain atomic nuclei like H, D, 13C, F, and so on possess an intrinsic mechanical spin. Since a charged particle spinning about its axis is equivalent to a circular electric current, which, in turn, gives rise to a magnetic field, a spinning nucleus behaves as a tiny bar magnet whose axis is coincident with the axis of the spin, and its potential energy (Un) in a magnetic field of strength H is Un = −Hm cos q

(3.136)

where m is the magnetic moment of the nucleus (i.e., the strength of the nuclear dipole) and q is the angle between the magnetic moment vector and the direction of the magnetic field. According to quantum mechanics, the angle q is not a continuous variable but can have only certain discrete values; in other words, in an applied magnetic field, the spin angular momentum vector for a nucleus cannot point in any arbitrary direction, but can have only a discrete set of orientations. This is the result of a phenomenon, known as space quantization. The angular momentum vector can point only such that its components along the direction of the magnetic field are given by mI(h/2p), where the quantum number mI can have any of the values I, I − 1, …, −(I − 1), −I, with I representing the spin of the nucleus. Thus, for I = 1, the possible values of mI are 1, 0, and −1. and the nucleus can have three spin orientations. For proton, however, I = 1/2 and mI can be only +1/2 and −1/2, and so proton can have only two spin orientations. Each orientation represents a spin state and transition of a nucleus from one spin state to an adjacent one may occur by absorption or emission of an appropriate quantum of energy. From a classical point of view, the behavior of a spinning proton (pictured as a tiny bar magnet rotating about its axis) is analogous to that of a gyroscope spinning in frictionless bearings. It is a known fact that

419

Plastics Properties and Testing

when a force (torque) is applied to a spinning gyroscope, its axis does not tilt but merely precesses about the direction of the force. Similarly, a spinning proton, behaving as a magnetic gyroscope, will precess about the direction of the applied ﬁeld (see Figure 3.101), keeping the angle q constant. The expression for the precessional frequency (w) of such a spinning proton has the same form as the frequency of the precessional motion of an orbiting electron (known as Larmor frequency), namely, w=

Magnetic moment H radian=s Angular moment mH cycles=s = 2ppn

where pn is the spin angular momentum of the nucleus given by h pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pn = I(I + 1) 2p

(3.137)

(3.138)

So long as the angle q between its spinning axis and the field direction is constant, the potential energy of the proton in the field, given by Equation 3.136, will also be constant and no energy will be absorbed from the field. In order to cause a change of the angle q, a second magnetic field is to be applied perpendicular to the main field. The secondary magnetic field must not be a stationary one, however, as otherwise its effect in one half-cycle of the precessional motion will be cancelled by the effect in the other half-cycle. To produce a net effect, the secondary field must also rotate about the direction of the main field with a frequency equal to that of the precessing proton. The secondary magnetic field can then interact with the precessing proton and energy can be exchanged; if the frequencies differ, there will be no interaction. Thus, when the frequency of the rotating secondary field and the frequency of the precessing nucleus are equal, they are said to be in resonance, since in this condition, transition from one nuclear spin state to another can readily occur, giving rise to absorption or emission of energy. A rotating magnetic field can be produced in a simple way by sending the output current of a radiofrequency crystal oscillator through a helical coil (solenoid) of wire. In NMR spectroscopy, the sample under investigation is taken in a small glass tube placed between the pole faces of a dc electromagnet (Figure 3.102). The coil that transmits the radio-frequency field is placed with its axis perpendicular to the direction of the main field produced by the electromagnet. The coil is made in two halves to allow N

H S

FIGURE 3.101 A spinning proton, behaving as a magnetic gyroscope, precesses about the direction of the applied magnetic ﬁeld H, with a constant angle q.

420

Plastics Technology Handbook

Sweep current

Test tube

Poles of 15,000 G magnet

Receiver coil

Radio receiver

Transmitter coil Radio frequency transmitter (typical frequency = 60 Mc/s)

Recorder

FIGURE 3.102

Schematic representation of an NMR spectrometer.

the insertion of the sample holder. The electric current passing through the coil produces in it a magnetic field directed along its axis, and this field reverses its direction with the same frequency as the current. It is a general property of vectors that the resultant of two identical vectors rotating with the same frequency in opposite directions is equivalent to a vector having constant direction and periodic change to magnitude, which, in fact, represents an alternating motion. The alternating magnetic field along the axis of the coil, described above, is thus equivalent to two magnetic fields rotating with the same frequency but in opposite directions. Of these two rotating magnetic fields, the one whose direction of rotation is the same as the direction of the precessional motion of the nucleus will act as the secondary magnetic field. The other field rotating in the opposite direction can be ignored, since its average effect is zero. To obtain a condition of resonance (absorption or emission of energy), the magnetic field at the proton and the frequency of the alternating current supplied to the coil from the oscillator must be such that Equation 3.137 is satisfied. The nuclei can then absorb energy from the secondary field. Coils located within the pole gap or wound about the poles of the magnet allow a sweep to be made through the applied magnetic field to bring about this condition of resonance. When resonance absorption of energy takes place, it can be thought of as producing nuclei in the excited state, which will then tend to return to the lower level in order to approach the Boltzmann distribution ratio. The radiation emitted in this process is picked up by the receiver coil, which is a separate radio-frequency coil consisting of a few turns of wire wound tightly around the sample tube. The receiver coil is perpendicular to both the magnetic field and the radio-frequency transmitter coil in order to minimize pickup from these fields. It is the sample that provides this coupling between the receiver and the transmitter. The signal from the receiver coil can be displayed on an oscilloscope or a recorder chart. Experimentally, the resonance condition may be obtained in two alternative ways. We might vary either the field strength of the electromagnet or the frequency of the oscillator, keeping the other fixed. Suppose we apply a fixed magnetic field and the Larmor frequency produced by it is, say, 60 Mc/s; if the frequency of the oscillator is then varied over a range including 60 Mc/s, resonance absorption will occur exactly at that frequency. On the other hand, we can fix the oscillator frequency at 60 Mc/s and vary the applied field over a range until absorption occurs. The latter arrangement is simple and widely used in practice. Most NMR spectrometers in use today employ a fixed oscillator frequency of either 60 Mc/s, 100 Mc/s or 220 Mc/s.

421

Plastics Properties and Testing

3.8.2.2 Chemical Shift A very important characteristic of the NMR technique is that it can distinguish protons in different molecular environments. If the resonance frequencies of all protons in a molecule were the same, as given by Equation 3.137, then the NMR spectrum would show only one peak for the compound, and as such would be of little use to the organic chemist. However, we must consider the fact that the field strength represented in Equation 3.137 is the field strength experienced by the protons in the sample and is not the same as the strength of the applied magnetic field. Protons whether in hydrogen atoms or molecules are surrounded by an electromagnetic charge cloud having approximately spherical symmetry. A magnetic field induces electronic circulations in the charge cloud in a plane perpendicular to the applied field and in such a direction as to produce a field opposing the applied field, as shown in Figure 3.103. The induced field is directly proportional to the applied field H and so can be represented by sH, where s is a constant. The effective magnetic field experienced by the proton is therefore Heff = ðH − s H Þ = H ð1 − s Þ

(3.139)

We can thus say that proton is shielded from the external ﬁeld by diamagnetic electron circulation and s represents the shielding constant. The extent of shielding of a proton depends on the electron density around it in a molecule. A molecule may con

Plastics Technology Handbook Fifth Edition - PDFCOFFEE.COM (2024)