Modern Plastics

Handbook

Modern Plasticsand

Charles A. Harper Editor in ChiefTechnology Seminars, Inc.

Lutherville, Maryland

McGraw-HillNew York San Francisco Washington, D.C. Auckland Bogotá

Caracas Lisbon London Madrid Mexico City MilanMontreal New Delhi San Juan Singapore

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0267146_FM_Harper_Plastics_MHT 2/24/00 4:39 PM Page iii

Library of Congress Cataloging-in-Publication Data

Modern plastics handbook / Modern Plastics, Charles A. Harper (editor in chief).p. cm.

ISBN 0-07-026714-61. Plastics. I. Modern Plastics. II. Harper, Charles A.

TA455.P5 M62 1999668.4—dc21 99-056522

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Copyright © 2000 by The McGraw-Hill Companies, Inc. Printed in theUnited States of America. Except as permitted under the United StatesCopyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base orretrieval system, without the prior written permission of the publisher.

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Information contained in this work has been obtained by The McGraw-Hill Companies, Inc. (“McGraw-Hill) from sources believed to be reliable.However, neither McGraw-Hill nor its authors guarantee the accuracy orcompleteness of any information published herein and neither McGraw-Hill nor its authors shall be responsible for any errors, omissions, ordamages arising out of use of this information. This work is publishedwith the understanding that McGraw-Hill and its authors are supplyinginformation but are not attempting to render engineering or other pro-fessional services. If such services are required, the assistance of anappropriate professional should be sought.

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Contributors

Anne-Marie Baker University of Massachusetts, Lowell, Mass. (CHAP. 1)

Carol M. F. Barry University of Massachusetts, Lowell, Mass. (CHAP. 5)

Allison A. Cacciatore TownsendTarnell, Inc., Mt. Olive, N.J. (CHAP. 4)

Fred Gastrock TownsendTarnell, Inc., Mt. Olive, N.J. (CHAP. 4)

John L. Hull Hall/Finmac, Inc., Warminster, Pa. (CHAP. 6)

Carl P. Izzo Consultant, Murrysville, Pa. (CHAP. 10)

Louis N. Kattas TownsendTarnell, Inc., Mt. Olive, N.J. (CHAP. 4)

Peter Kennedy Moldflow Corporation, Lexington, Mass. (CHAP. 7, SEC. 3)

Inessa R. Levin TownsendTarnell, Inc., Mt. Olive, N.J. (CHAP. 4)

William R. Lukaszyk Universal Dynamics, Inc., North Plainfield, N.J.(CHAP. 7, SEC. 1)

Joey Meade University of Massachusetts, Lowell, Mass. (CHAP. 1)

James Margolis Montreal, Quebec, Canada (CHAP. 3)

Stephen A. Orroth University of Massachusetts, Lowell, Mass. (CHAP. 5)

Edward M. Petrie ABB Transmission Technology Institute, Raleigh, N.C.(CHAP. 9)

Jordon I. Rotheiser Rotheiser Design, Inc., Highland Park, Ill. (CHAP. 8)

Susan E. Selke Michigan State University, School of Packaging, EastLansing, Mich. (CHAP. 12)

Ranganath Shastri Dow Chemical Company, Midland, Mich. (CHAP. 11)

Peter Stoughton Conair, Pittsburgh, Pa. (CHAP. 7, SEC. 2)

Ralph E. Wright Consultant, Yarmouth, Maine (CHAP. 2)

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Preface

The Modern Plastics Handbook has been prepared as a third memberof the well-known and highly respected team of publications whichincludes Modern Plastics magazine and Modern Plastics WorldEncyclopedia. The Modern Plastics Handbook offers a thorough andcomprehensive technical coverage of all aspects of plastics materialsand processes, in all of their forms, along with coverage of additives,auxiliary equipment, plastic product design, testing, specifications andstandards, and the increasingly critical subject of plastics recyclingand biodegradability. Thus, this Handbook will serve a wide range ofinterests. Likewise, with presentations ranging from terms and defin-itions and fundamentals, to clearly explained technical discussions, toextensive data and guideline information, this Handbook will be use-ful for all levels of interest and backgrounds. These broad objectivescould only have been achieved by an outstanding and uniquely diversegroup of authors with a combination of academic, professional, andbusiness backgrounds. It has been my good fortune to have obtainedsuch an elite group of authors, and it has been a distinct pleasure tohave worked with this group in the creation of this Handbook. I wouldlike to pay my highest respects and offer my deep appreciation to all of them.

The Handbook has been organized and is presented as a thoroughsourcebook of technical explanations, data, information, and guide-lines for all ranges of interests. It offers an extensive array of propertyand performance data as a function of the most important product andprocess variables. The chapter organization and coverage is well suitedto reader convenience for the wide range of product and equipmentcategories. The first three chapters cover the important groups of plas-tic materials, namely, thermoplastics, thermosets, and elastomers.Then comes a chapter on the all important and broad based group of additives, which are so critical for tailoring plastic properties.Following this are three chapters covering processing technologies and

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equipment for all types of plastics, and the all important subject ofauxiliary equipment and components for optimized plastics process-ing. Next is a most thorough and comprehensive chapter on design ofplastic products, rarely treated in such a practical manner. After this,two chapters are devoted to the highly important plastic materials andprocess topics of coatings and adhesives, including surface finishingand fabricating of plastic parts. Finally, one chapter is devoted to thefundamentally important areas of testing and standards, and onechapter to the increasingly critical area of plastic recycling andbiodegradability.

Needless to say, a book of this caliber could not have been achievedwithout the guidance and support of many people. While it is not pos-sible to name all of the advisors and constant supporters, I feel that Imust highlight a few. First, I would like to thank the Modern Plasticsteam, namely, Robert D. Leaversuch, Executive Editor of ModernPlastics magazine, Stephanie Finn, Modern Plastics Events Manager,Steven J. Schultz, Managing Director, Modern Plastics WorldEncyclopedia, and William A. Kaplan, Managing Editor of ModernPlastics World Encyclopedia. Their advice and help was constant.Next, I would like to express my very great appreciation to the teamfrom Society of Plastics Engineers, who both helped me get off theground and supported me readily all through this project. They areMichael R. Cappelletti, Executive Director, David R. Harper, PastPresident, John L. Hull, Honored Service Member, and Glenn L. Beall,Distinguished Member. In addition, I would like to acknowledge, withdeep appreciation, the advice and assistance of Dr. Robert Nunn andDr. Robert Malloy of University of Massachusetts, Lowell for theirguidance and support, especially in selection of chapter authors. Last,but not least, I am indebted to Robert Esposito, Executive Editor of the McGraw-Hill Professional Book Group, for both his support andpatience in my editorial responsibilities for this Modern PlasticsHandbook.

It is my hope, and expectation, that this book will serve its readerwell. Any comments or suggestions will be welcomed.

Charles A. Harper

xii Preface

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Contents

Contributors ixPreface xi

Chapter 1. Thermoplastics 1.1

1.1 Introduction 1.11.2 Polymer Categories 1.41.3 Comparative Properties of Thermoplastics 1.791.4 Additives 1.791.5 Fillers 1.821.6 Polymer Blends 1.83References 1.85

Chapter 2. Thermosets, Reinforced Plastics, and Composites 2.1

2.1 Resins 2.12.2 Thermosetting Resin Family 2.22.3 Resin Characteristics 2.102.4 Resin Forms 2.102.5 Liquid Resin Processes 2.132.6 Laminates 2.292.7 Molding Compounds 2.38References 2.88

Chapter 3. Elastomeric Materials and Processes 3.1

3.1 Introduction 3.13.2 Thermoplastic Elastomers (TPEs) 3.13.3 Melt Processing Rubbers (MPRs) 3.243.4 Thermoplastic Vulcanizates (TPVs) 3.263.5 Synthetic Rubbers 3.333.6 Natural Rubber 3.483.7 Conclusion 3.49References 3.50

v

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Chapter 4. Plastic Additives 4.1

4.1 Introduction 4.14.2 Scope 4.24.3 Antiblock and Slip Agents 4.24.4 Antioxidants 4.64.5 Antistatic Agents 4.124.6 Biocides 4.164.7 Chemical Blowing Agents 4.194.8 Coupling Agents 4.234.9 Flame Retardants 4.264.10 Heat Stabilizers 4.364.11 Impact Modifiers 4.414.12 Light Stabilizers 4.464.13 Lubricants and Mold Release Agents 4.494.14 Nucleating Agents 4.544.15 Organic Peroxides 4.584.16 Plasticizers 4.624.17 Polyurethane Catalysts 4.66

Chapter 5. Processing of Thermoplastics 5.1

5.1 Material Concepts 5.25.2 Extrusion 5.185.3 Estrusion Processes 5.555.4 Injection Molding 5.84References 5.121

Chapter 6. Processing of Thermosets 6.1

6.1 Introduction 6.16.2 Molding Processes 6.26.3 Techniques for Machining and Secondary Operations 6.246.4 Postmolding Operations 6.286.5 Process-Related Design Considerations 6.296.6 Mold Construction and Fabrication 6.346.7 Summary 6.37

Chapter 7. Auxiliary Equipment

7.2 Raw Material Delivery 7.57.3 Bulk Storage of Resin 7.87.4 Bulk Resin Conveying Systems 7.227.5 Bulk Delivery Systems 7.287.6 Blending Systems 7.357.7 Regrind Systems 7.387.8 Material Drying 7.437.9 Loading Systems 7.507.10 System Integration 7.57

vi Contents

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7.2

Section 1. Material Handling 7.3

7.1 Introduction 7.3

Section 2. Drying and Dryers 7.63

7.11 Why Do We Dry Plastic Materials? 7.637.12 Hygroscopic and Nonhygroscopic Polymers 7.657.13 Drying Hygroscopic Polymers 7.667.14 How Physical Characteristics of Plastics Affect Drying 7.717.15 How Dryers Work 7.727.16 Critical Dryer Components 7.777.17 Monitoring Drying Conditions 7.857.18 Drying System Configurations 7.897.19 Gas or Electric? 7.937.20 Handling Dried Material 7.94

Section 3. CAD, CAM, CAE 7.99

7.21 Introduction 7.997.22 Simulation and Polymer Processing 7.1017.23 The Injection-Molding Process 7.1047.24 History of Injection-Molding Simulation 7.1067.25 Current Technology for Injection-Molding Simulation 7.1097.26 The Changing Face of CAE 7.1237.27 Machine Control 7.1267.28 Future Trends 7.129References on CAD, CAM, CAE 7.131

Chapter 8. Design of Plastic Products 8.1

8.1 Fundamentals 8.18.2 Design Fundamentals for Plastic Parts 8.498.3 Design Details Specific to Major Processes 8.79References 8.116

Chapter 9. Finishing, Assembly, and Decorating 9.1

9.1 Introduction 9.19.2 Machining and Finishing 9.29.3 Assembly of Plastics Parts—General Considerations 9.169.4 Methods of Mechanical Joining 9.179.5 Adhesive Bonding 9.359.6 Welding 9.709.7 Recommended Assembly Processes for Common Plastics 9.809.8 Decorating Plastics 9.91References 9.105

Chapter 10. Coatings and Finishes 10.1

10.1 Introduction 10.110.2 Environment and Safety 10.510.3 Surface Preparation 10.610.4 Coating Selection 10.1110.5 Coating Materials 10.1810.6 Application Methods 10.42

Contents vii

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10.7 Curing 10.5510.8 Summary 10.58References 10.59

Chapter 11. Plastics Testing 11.1

11.1 Introduction 11.111.2 The Need for Testing Plastics 11.111.3 Diverse Types of Testing 11.211.4 Test Methods for Acquisition and Reporting of Property Data 11.1311.5 Uniform Reporting Format 11.6611.6 Misunderstood and Misused Properties 11.7011.7 Costs of Data Generation 11.73Appendix 11.1 Selected ISO/IEC Standards/Documents 11.77Appendix 11.2 Selected ASTM Standards 11.84Appendix 11.3 List of Resources 11.87Appendix 11.4 Some Unit Conversion Factors 11.92References 11.92Suggested Reading 11.94

Chapter 12. Plastics Recycling and Biodegradable Plastics 12.1

12.1 Introduction 12.112.2 Overview of Recycling 12.1412.3 Design for Recycling 12.2212.4 Recycling of Major Polymers 12.2412.5 Overview of Plastics Degradation 12.7112.6 Natural Biodegradable Polymers 12.8012.7 Synthetic Biodegradable Polymers 12.9212.8 Water-Soluble Polymers 12.9712.9 Summary 12.99References 12.100

Appendix A. Glossary of Terms and Definitions A.1

Appendix B. Some Common Abbreviations Used in the Plastics Industry B.1

Appendix C. Important Properties of Plastics and Listing of Plastic Suppliers C.1

Appendix D. Sources of Specifications and Standards for Plastics and Composites D.1

Appendix E. Plastics Associations E.1

Index follows Appendix E

viii Contents

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1.1

Thermoplastics

A.-M. M. BakerJoey MeadPlastics Engineering DepartmentUniversity of Massachusetts, Lowell

1.1 Introduction

Plastics are an important part of everyday life; products made fromplastics range from sophisticated products, such as prosthetic hip andknee joints, to disposable food utensils. One of the reasons for thegreat popularity of plastics in a wide variety of industrial applicationsis due to the tremendous range of properties exhibited by plastics andtheir ease of processing. Plastic properties can be tailored to meet spe-cific needs by varying the atomic makeup of the repeat structure; byvarying molecular weight and molecular weight distribution; by vary-ing flexibility as governed by presence of side chain branching, as wellas the lengths and polarities of the side chains; and by tailoring thedegree of crystallinity, the amount of orientation imparted to the plas-tic during processing and through copolymerization, blending withother plastics, and through modification with an enormous range ofadditives (fillers, fibers, plasticizers, stabilizers). Given all of theavenues available to pursue tailoring any given polymer, it is not sur-prising that such a variety of choices available to us today exist.

Polymeric materials have been used since early times, even thoughtheir exact nature was unknown. In the 1400s Christopher Columbusfound natives of Haiti playing with balls made from material obtainedfrom a tree. This was natural rubber, which became an important

Chapter

1

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product after Charles Goodyear discovered that the addition of sulfurdramatically improved the properties. However, the use of polymericmaterials was still limited to natural-based materials. The first truesynthetic polymers were prepared in the early 1900s using phenoland formaldehyde to form resins—Baekeland’s Bakelite. Even withthe development of synthetic polymers, scientists were still unawareof the true nature of the materials they had prepared. For many yearsscientists believed they were colloids—aggregates of molecules with aparticle size of 10- to 1000-nm diameter. It was not until the 1920sthat Herman Staudinger showed that polymers were giant moleculesor macromolecules. In 1928 Carothers developed linear polyestersand then polyamides, now known as nylon. In the 1950s Ziegler andNatta’s work on anionic coordination catalysts led to the developmentof polypropylene, high-density linear polyethylene, and other stere-ospecific polymers.

Polymers come in many forms including plastics, rubber, and fibers.Plastics are stiffer than rubber, yet have reduced low-temperatureproperties. Generally, a plastic differs from a rubbery material due tothe location of its glass transition temperature (Tg). A plastic has a Tgabove room temperature, while a rubber will have a Tg below roomtemperature. Tg is most clearly defined by evaluating the classic rela-tionship of elastic modulus to temperature for polymers as presentedin Fig. 1.1. At low temperatures, the material can best be described asa glassy solid. It has a high modulus and behavior in this state is char-acterized ideally as a purely elastic solid. In this temperature regime,materials most closely obey Hooke’s law:

� � Eε

where � is the stress being applied and ε is the strain. Young’s modu-lus, E, is the proportionality constant relating stress and strain.

In the leathery region, the modulus is reduced by up to three ordersof magnitude for amorphous polymers. The temperature at which thepolymer behavior changes from glassy to leathery is known as theglass transition temperature, Tg. The rubbery plateau has a relativelystable modulus until as the temperature is further increased, a rub-bery flow begins. Motion at this point does not involve entire mole-cules, but in this region deformations begin to become nonrecoverableas permanent set takes place. As temperature is further increased,eventually the onset of liquid flow takes place. There is little elasticrecovery in this region, and the flow involves entire molecules slippingpast each other. Ideally, this region is modeled as representing viscousmaterials which obey Newton’s law :

� � � �ε

1.2 Chapter One

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Plastics can also be separated into thermoplastics and thermosets.A thermoplastic material is a high molecular weight polymer that isnot cross-linked. A thermoplastic material can exist in a linear orbranched structure. Upon heating a thermoplastic, a highly viscousliquid is formed that can be shaped using plastics processing equip-ment. A thermoset has all of the chains tied together with covalentbonds in a network (cross-linked). A thermoset cannot be reprocessedonce cross-linked, but a thermoplastic material can be reprocessed byheating to the appropriate temperature. The different types of struc-tures are shown in Fig. 1.2.

A polymer is prepared by stringing together a series of low molecu-lar weight species (such as ethylene) into an extremely long chain(polyethylene) much as one would string together a series of beads tomake a necklace. The chemical characteristics of the starting low molecular weight species will determine the properties of the finalpolymer. When two different low molecular weight species are poly-merized, the resulting polymer is termed a copolymer such as ethylenevinylacetate.

The properties of different polymers can vary widely, for example,the modulus can vary from 1 MN/m2 to 50 GN/m2. Properties can bevaried for each individual plastic material as well, simply by varyingthe microstructure of the material.

In its solid form a polymer can take up different structures depend-ing on the structure of the polymer chain as well as the processing con-ditions. The polymer may exist in a random unordered structuretermed an amorphous polymer. An example of an amorphous polymer

Thermoplastics 1.3

Figure 1.1 Relationship between elastic modulus and temperature.

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is polystyrene. If the structure of the polymer backbone is a regular,ordered structure, then the polymer can tightly pack into an orderedcrystalline structure, although the material will generally be onlysemicrystalline. Examples are polyethylene and polypropylene. Theexact makeup and details of the polymer backbone will determinewhether or not the polymer is capable of crystallizing. This microstruc-ture can be controlled by different synthetic methods. As mentionedpreviously, the Ziegler-Natta catalysts are capable of controlling themicrostructure to produce stereospecific polymers. The types ofmicrostructure that can be obtained for a vinyl polymer are shown inFig. 1.3. The isotactic and syndiotactic structures are capable of crys-tallizing because of their highly regular backbone. The atactic formwould produce an amorphous material.

1.2 Polymer Categories

1.2.1 Acetal (POM)

Acetal polymers are formed from the polymerization of formaldehyde.They are also known by the name polyoxymethylenes (POM). Polymersprepared from formaldehyde were studied by Staudinger in the 1920s,but thermally stable materials were not introduced until the 1950swhen DuPont developed Delrin.1 Homopolymers are prepared fromvery pure formaldehyde by anionic polymerization, as shown in Fig.1.4. Amines and the soluble salts of alkali metals catalyze the reaction.2The polymer formed is insoluble and is removed as the reaction pro-ceeds. Thermal degradation of the acetal resin occurs by unzippingwith the release of formaldhyde. The thermal stability of the polymeris increased by esterification of the hydroxyl ends with acetic anhy-dride. An alternative method to improve the thermal stability is copoly-

1.4 Chapter One

Figure 1.2 Linear, branched, cross-linked polymer structures.

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merization with a second monomer such as ethylene oxide. The copoly-mer is prepared by cationic methods.3 This was developed by Celaneseand marketed under the tradename Celcon. Hostaform is anothercopolymer marketed by Hoescht. The presence of the second monomerreduces the tendency for the polymer to degrade by unzipping.4

There are four processes for the thermal degradation of acetalresins. The first is thermal or base-catalyzed depolymerization fromthe chain, resulting in the release of formaldehyde. End capping thepolymer chain will reduce this tendency. The second is oxidativeattack at random positions, again leading to depolymerization. Theuse of antioxidants will reduce this degradation mechanism.Copolymerization is also helpful. The third mechanism is cleavage ofthe acetal linkage by acids. It is, therefore, important not to processacetals in equipment used for polyvinyl chloride (PVC), unless it hasbeen cleaned, due to the possible presence of traces of HCl. The fourthdegradation mechanism is thermal depolymerization at temperatures

Thermoplastics 1.5

Figure 1.3 Isotactic, syndiotactic, and atactic polymer chains.

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above 270°C. It is important that processing temperatures remainbelow this temperature to avoid degradation of the polymer.5

Acetals are highly crystalline, typically 75% crystalline, with a melt-ing point of 180°C.6 Compared to polyethylene (PE), the chains packcloser together because of the shorter C�O bond. As a result, the poly-mer has a higher melting point. It is also harder than PE. The highdegree of crystallinity imparts good solvent resistance to acetal poly-mers. The polymer is essentially linear with molecular weights (Mn) inthe range of 20,000 to 110,000.7

Acetal resins are strong and stiff thermoplastics with good fatigueproperties and dimensional stability. They also have a low coefficientof friction and good heat resistance.8 Acetal resins are considered sim-ilar to nylons, but are better in fatigue, creep, stiffness, and waterresistance.9 Acetal resins do not, however, have the creep resistance ofpolycarbonate. As mentioned previously, acetal resins have excellentsolvent resistance with no organic solvents found below 70°C, howev-er, swelling may occur in some solvents. Acetal resins are susceptibleto strong acids and alkalis, as well as oxidizing agents. Although theC�O bond is polar, it is balanced and much less polar than the car-bonyl group present in nylon. As a result, acetal resins have relativelylow water absorption. The small amount of moisture absorbed maycause swelling and dimensional changes, but will not degrade the poly-mer by hydrolysis.10 The effects of moisture are considerably less dra-matic than for nylon polymers. Ultraviolet light may causedegradation, which can be reduced by the addition of carbon black. Thecopolymers generally have similar properties, but the homopolymermay have slightly better mechanical properties, and higher meltingpoint, but poorer thermal stability and poorer alkali resistance.11Along with both homopolymers and copolymers, there are also filledmaterials (glass, fluoropolymer, aramid fiber, and other fillers), tough-ened grades, and ultraviolet (UV) stabilized grades.12 Blends of acetalwith polyurethane elastomers show improved toughness and are avail-able commercially.

Acetal resins are available for injection molding, blow molding, andextrusion. During processing it is important to avoid overheating or theproduction of formaldehyde may cause serious pressure buildup. Thepolymer should be purged from the machine before shutdown to avoidexcessive heating during startup.13 Acetal resins should be stored in a

1.6 Chapter One

CH2H2C OOnn

Figure 1.4 Polymerization of formaldehyde to polyoxymethylene.

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dry place. The apparent viscosity of acetal resins is less dependent onshear stress and temperature than polyolefins, but the melt has lowelasticity and melt strength. The low melt strength is a problem forblow molding applications. For blow molding applications, copolymerswith branched structures are available. Crystallization occurs rapidlywith postmold shrinkage complete within 48 h of molding. Because ofthe rapid crystallization it is difficult to obtain clear films.14

The market demand for acetal resins in the United States andCanada was 368 million pounds in 1997.15 Applications for acetalresins include gears, rollers, plumbing components, pump parts, fanblades, blow-molded aerosol containers, and molded sprockets andchains. They are often used as direct replacements for metal. Most ofthe acetal resins are processed by injection molding, with the remain-der used in extruded sheet and rod. Their low coefficient of frictionmake acetal resins good for bearings.16

1.2.2 Biodegradable polymers

Disposal of solid waste is a challenging problem. The United Statesconsumes over 53 billion pounds of polymers a year for a variety ofapplications.17 When the life cycle of these polymeric parts is complet-ed they may end up in a landfill. Plastics are often selected for appli-cations based on their stability to degradation, however, this meansdegradation will be very slow, adding to the solid waste problem.Methods to reduce the amount of solid waste include either recyclingor biodegradation.18 Considerable work has been done to recycle plas-tics, both in the manufacturing and consumer area. Biodegradablematerials offer another way to reduce the solid waste problem. Mostwaste is disposed of by burial in a landfill. Under these conditions oxy-gen is depleted and biodegradation must proceed without the presenceof oxygen.19 An alternative is aerobic composting. In selecting a poly-mer that will undergo biodegradation it is important to ascertain themethod of disposal. Will the polymer be degraded in the presence ofoxygen and water, and what will be the pH level? Biodegradation canbe separated into two types—chemical and microbial degradation.Chemical degradation includes degradation by oxidation, photodegra-dation, thermal degradation, and hydrolysis. Microbial degradationcan include both fungi and bacteria. The susceptibility of a polymer tobiodegradation depends on the structure of the backbone.20 For exam-ple, polymers with hydrolyzable backbones can be attacked by acids orbases, breaking down the molecular weight. They are, therefore, morelikely to be degraded. Polymers that fit into this category include mostnatural-based polymers, such as polysaccharides, and synthetic mate-rials, such as polyurethanes, polyamides, polyesters, and polyethers.

Thermoplastics 1.7

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Polymers that contain only carbon groups in the backbone are moreresistant to biodegradation.

Photodegradation can be accomplished by using polymers that areunstable to light sources or by the use of additives that undergo photo-degradation. Copolymers of divinyl ketone with styrene, ethylene, orpolypropylene (Eco Atlantic) are examples of materials that are sus-ceptible to photodegradation.21 The addition of a UV-absorbing mate-rial will also act to enhance photodegradation of a polymer. Anexample is the addition of iron dithiocarbamate.22 The degradationmust be controlled to ensure that the polymer does not degrade pre-maturely.

Many polymers described elsewhere in this book can be consideredfor biodegradable applications. Polyvinyl alcohol has been consideredin applications requiring biodegradation because of its water solubil-ity. However, the actual degradation of the polymer chain may beslow.23 Polyvinyl alcohol is a semicrystalline polymer synthesizedfrom polyvinyl acetate. The properties are governed by the molecularweight and by the amount of hydrolysis. Water soluble polyvinyl alco-hol has a degree of hydrolysis 87 to 89%. Water insoluble polymersare formed if the degree of hydrolysis is greater than 89%.24

Cellulose-based polymers are some of the more widely available, nat-urally based polymers. They can, therefore, be used in applicationsrequiring biodegradation. For example, regenerated cellulose is used inpackaging applications.25 A biodegradable grade of cellulose acetate isavailable from Rhone-Poulenc (Bioceta and Biocellat), where an addi-tive acts to enhance the biodegradation.26 This material finds applica-tion in blister packaging, transparent window envelopes, and otherpackaging applications.

Starch-based products are also available for applications requiringbiodegradability. The starch is often blended with polymers for betterproperties. For example, polyethylene films containing between 5 to10% cornstarch have been used in biodegradable applications. Blendsof starch with vinyl alcohol are produced by Fertec (Italy) and used inboth film and solid product applications.27 The content of starch inthese blends can range up to 50% by weight and the materials can beprocessed on conventional processing equipment. A product developedby Warner-Lambert, called Novon, is also a blend of polymer andstarch, but the starch contents in Novon are higher than in the mate-rial by Fertec. In some cases the content can be over 80% starch.28

Polylactides (PLA) and copolymers are also of interest in biodegrad-able applications. This material is a thermoplastic polyester synthe-sized from the ring opening of lactides. Lactides are cyclic diesters oflactic acid.29 A similar material to polylactide is polyglycolide (PGA).

1.8 Chapter One

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PGA is also a thermoplastic polyester, but one that is formed from gly-colic acids. Both PLA and PGA are highly crystalline materials. Thesematerials find application in surgical sutures, resorbable plates andscrews for fractures, and new applications in food packaging are alsobeing investigated.

Polycaprolactones are also considered in biodegradable applicationssuch as films and slow-release matrices for pharmaceuticals and fer-tilizers.30 Polycaprolactone is produced through ring opening polymer-ization of lactone rings with a typical molecular weight in the range of15,000 to 40,000.31 It is a linear, semicrystalline polymer with a melt-ing point near 62°C and a glass transition temperature about �60°C.32

A more recent biodegradable polymer is polyhydroxybutyrate-valerate copolymer (PHBV). These copolymers differ from many ofthe typical plastic materials in that they are produced through bio-chemical means. It is produced commercially by ICI using the bacte-ria Alcaligenes eutrophus, which is fed a carbohydrate. The bacteriaproduce polyesters, which are harvested at the end of the process.33When the bacteria are fed glucose, the pure polyhydroxybutyratepolymer is formed, while a mixed feed of glucose and propionic acidwill produce the copolymers.34 Different grades are commerciallyavailable that vary in the amount of hydroxyvalerate units and thepresence of plasticizers. The pure hydroxybutyrate polymer has amelting point between 173 and 180°C and a Tg near 5°C.35Copolymers with hydroxyvalerate have reduced melting points,greater flexibility and impact strength, but lower modulus and ten-sile strength. The level of hydroxyvalerate is 5 to 12%. These copoly-mers are fully degradable in many microbial environments.Processing of PHBV copolymers requires careful control of theprocess temperatures. The material will degrade above 195°C, soprocessing temperatures should be kept below 180°C and the pro-cessing time kept to a minimum. It is more difficult to processunplasticized copolymers with lower hydroxyvalerate contentbecause of the higher processing temperatures required. Applicationsfor PHBV copolymers include shampoo bottles, cosmetic packaging,and as a laminating coating for paper products.36

Other biodegradable polymers include Konjac, a water-soluble nat-ural polysaccharide produced by FMC, Chitin, another polysaccharidethat is insoluble in water, and Chitosan, which is soluble in water.37Chitin is found in insect exoskeletons and in shellfish. Chitosan can beformed from chitin and is also found in fungal cell walls.38 Chitin isused in many biomedical applications, including dialysis membranes,bacteriostatic agents, and wound dressings. Other applicationsinclude cosmetics, water treatment, adhesives, and fungicides.39

Thermoplastics 1.9

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1.2.3 Cellulosics

Cellulosic polymers are the most abundant organic polymers in theworld, making up the principal polysaccharide in the walls of almostall of the cells of green plants and many fungi species.40 Plants producecellulose through photosynthesis. Pure cellulose decomposes before itmelts, and must be chemically modified to yield a thermoplastic. Thechemical structure of cellulose is a heterochain linkage of differentanhydroglucose units into high molecular weight polymer, regardlessof plant source. The plant source, however, does affect molecularweight, molecular weight distribution, degrees of orientation, andmorphological structure. Material described commonly as “cellulose”can actually contain hemicelluloses and lignin.41 Wood is the largestsource of cellulose and is processed as fibers to supply the paper indus-try and is widely used in housing and industrial buildings. Cotton-derived cellulose is the largest source of textile and industrial fibers,with the combined result being that cellulose is the primary polymerserving the housing and clothing industries. Crystalline modificationsresult in celluloses of differing mechanical properties, and Table 1.1compares the tensile strengths and ultimate elongations of some com-mon celluloses.42

Cellulose, whose repeat structure features three hydroxyl groups,reacts with organic acids, anhydrides, and acid chlorides to formesters. Plastics from these cellulose esters are extruded into film andsheet, and are injection-molded to form a wide variety of parts.Cellulose esters can also be compression-molded and cast from solu-tion to form a coating. The three most industrially important celluloseester plastics are cellulose acetate (CA), cellulose acetate butyrate(CAB), and cellulose acetate propionate (CAP), with structures asshown below in Fig. 1.5.

These cellulose acetates are noted for their toughness, gloss, andtransparency. CA is well suited for applications requiring hardnessand stiffness, as long as the temperature and humidity conditionsdon’t cause the CA to be too dimensionally unstable. CAB has the bestenvironmental stress cracking resistance, low-temperature impact

1.10 Chapter One

TABLE 1.1 Selected Mechanical Properties of Common Celluloses

Tensile strength, MPa Ultimate elongation, %

Form Dry Wet Dry Wet

Ramie 900 1060 2.3 2.4Cotton 200–800 200–800 12–16 6–13Flax 824 863 1.8 2.2Viscose rayon 200–400 100–200 8–26 13–43Cellulose acetate 150–200 100–120 21–30 29–30

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Figure 1.5 Structures of cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate.

OCOCH3

OCOCH3

OCOC4H9

OCOC4H9H9C4OCO OCOC3H7

OCOC3H7

H7C3OCOH3COC O

CH

CH CH

CHCH CH

CH

CH

CH CH

CH

CH

OCH OCH

O O

OCH

O

1.11

0267146_Ch01_Harper 2

/24/00 5:01 PM P

age 1.11

strength, and dimensional stability. CAP has the highest tensilestrength and hardness. Comparison of typical compositions and prop-erties for a range of formulations are given in Table 1.2.43 Propertiescan be tailored by formulating with different types and loadings ofplasticizers.

Formulation of cellulose esters is required to reduce charring andthermal discoloration, and typically includes the addition of heat sta-bilizers, antioxidants, plasticizers, UV stabilizers, and coloringagents.44 Cellulose molecules are rigid due to the strong intermolecu-lar hydrogen bonding which occurs. Cellulose itself is insoluble andreaches its decomposition temperature prior to melting. The acetyla-tion of the hydroxyl groups reduces intermolecular bonding, andincreases free volume depending upon the level and chemical nature ofthe alkylation.45 Cellulose acetates are thus soluble in specific sol-vents, but still require plasticization for rheological properties appro-priate to molding and extrusion processing conditions. Blends ofethylene vinyl acetate (EVA) copolymers and CAB are available.Cellulose acetates have also been graft-copolymerized with alkylesters of acrylic and methacrylic acid and then blended with EVA toform a clear, readily processable, thermoplastic.

CA is cast into sheet form for blister packaging, window envelopes,and file tab applications. CA is injection-molded into tool handles,tooth brushes, ophthalmic frames, and appliance housings and isextruded into pens, pencils, knobs, packaging films, and industrialpressure-sensitive tapes. CAB is molded into steering wheels, toolhandles, camera parts, safety goggles, and football nose guards. CAPis injection-molded into steering wheels, telephones, appliance hous-ings, flashlight cases, screw and bolt anchors, and is extruded into

1.12 Chapter One

TABLE 1.2 Selected Mechanical Properties of Cellulose Esters

Cellulose Cellulose acetate Cellulose acetateComposition, % acetate butyrate propionate

Acetyl 38–40 13–15 1.5–3.5Butyrl — 36–38 —Propionyl — — 43–47Hydroxyl 3.5–4.5 1–2 2–3

Tensile strength at fracture, 23°C, MPa 13.1–58.6 13.8–51.7 13.8–51.7

Ultimate elongation, % 6–50 38–74 35–60Izod impact strength, J/m

notched, 23°C 6.6–132.7 9.9–149.3 13.3–182.5notched, �40°C 1.9–14.3 6.6–23.8 1.9–19.0

Rockwell hardness, R scale 39–120 29–117 20–120Percent moisture absorption

at 24 h 2.0–6.5 1.0–4.0 1.0–3.0

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pens, pencils, tooth brushes, packaging fim, and pipe.46 Celluloseacetates are well suited for applications which require machining andthen solvent vapor polishing, such as in the case of tool handles, wherethe consumer market values the clarity, toughness, and smooth finish.CA and CAP are likewise suitable for ophthalmic sheeting and injec-tion-molding applications which require many postfinishing steps.47

Cellulose acetates are also commercially important in the coatingsarena. In this synthetic modification, cellulose is reacted with analbrecht halide, primarily methylchloride to yield methylcellulose orsodium chloroacetate to yield sodium cellulose methylcellulose(CMC). The structure of CMC is shown in Fig. 1.6. CMC gums arewater soluble and are used in food contact and packaging applica-tions. Its outstanding film-forming properties are used in paper siz-ings and textiles and its thickening properties are used in starchadhesive formulations, paper coatings, toothpaste, and shampoo.Other cellulose esters, including cellulosehydroxyethyl, hydrox-ypropylcellulose, and ethylcellulose, are used in film and coatingapplications, adhesives, and inks.

1.2.4 Fluoropolymers

Fluoropolymers are noted for their heat-resistance properties. Thisis due to the strength and stability of the carbon-fluorine bond.48 Thefirst patent was awarded in 1934 to IG Farben for a fluorine-con-taining polymer, polychlorotrifluoroethylene (PCTFE). This polymerhad limited application and fluoropolymers did not have wide appli-cation until the discovery of polytetrafluorethylene (PTFE) in1938.49 In addition to their high-temperature properties, fluoropoly-mers are known for their chemical resistance, very low coefficient offriction, and good dielectric properties. Their mechanical propertiesare not high unless reinforcing fillers, such as glass fibers, areadded.50 The compressive properties of fluoropolymers are generallysuperior to their tensile properties. In addition to their high-

Thermoplastics 1.13

OCOCH2CO-Na+

O

OCH

CH

CH CH

HO OH

OCH

Figure 1.6 Sodium cellulose methyl-cellulose structure.

0267146_Ch01_Harper 2/24/00 5:01 PM Page 1.13

temperature resistance, these materials have very good toughnessand flexibility at low temperatures.51 A wide variety of fluoropoly-mers are available, PTFE, PCTFE, fluorinated ethylene propylene(FEP), ethylene chlorotrifluoroethylene (ECTFE), ethylene tetraflu-oroethylene (ETFE), polyvinylindene fluoride (PVDF), and polyvinylfluoride (PVF).

Copolymers. FEP is a copolymer of tetrafluoroethylene and hexa-fluoropropylene. It has properties similar to PTFE, but with a meltviscosity suitable for molding with conventional thermoplastic pro-cessing techniques.52 The improved processability is obtained byreplacing one of the fluorine groups on PTFE with a trifluoromethylgroup as shown in Fig. 1.7.53

FEP polymers were developed by DuPont, but other commercialsources are available, such as Neoflon (Daikin Kogyo) and Teflex(Niitechem, formerly USSR).54 FEP is a crystalline polymer with amelting point of 290°C, which can be used for long periods at 200°Cwith good retention of properties.55 FEP has good chemical resistance,a low dielectric constant, low friction properties, and low gas perme-ability. Its impact strength is better than PTFE, but the other mechan-ical properties are similar to PTFE.56 FEP may be processed byinjection, compression, or blow molding. FEP may be extruded intosheets, films, rods, or other shapes. Typical processing temperaturesfor injection molding and extrusion are in the range of 300 to 380°C.57Extrusion should be done at low shear rates because of the polymer’shigh melt viscosity and melt fracture at low shear rates. Applicationsfor FEP include chemical process pipe linings, wire and cable, andsolar collector glazing.58 A material similar to FEP, Hostaflon TFB(Hoechst), is a terpolymer of tetrafluoroethylene, hexafluoropropene,and vinylidene fluoride.

ECTFE is an alternating copolymer of chlorotrifluoroethylene andethylene. It has better wear properties than PTFE along with goodflame resistance. Applications include wire and cable jackets, tank lin-ings, chemical process valve and pump components, and corrosion-resistant coatings.59

ETFE is a copolymer of ethylene and tetrafluoroethylene similar toECTFE, but with a higher use temperature. It does not have the flame

1.14 Chapter One

C C

F F

F F n

C C

F CF3

F FFigure 1.7 Structure of FEP.

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resistance of ECTFE, however, and will decompose and melt whenexposed to a flame.60 The polymer has good abrasion resistance for a flu-orine-containing polymer, along with good impact strength. The polymeris used for wire and cable insulation where its high-temperature proper-ties are important. ETFE finds application in electrical systems for com-puters, aircraft, and heating systems.61

Polychlorotrifluoroethylene. Polychlorotrifluoroethylene (PCTFE) ismade by the polymerization of chlorotrifluoroethylene, which is pre-pared by the dechlorination of trichlorotrifluoroethane. The polymer-ization is initiated with redox initiators.62 The replacement of onefluorine atom with a chlorine atom, as shown in Fig. 1.8, breaks up thesymmetry of the PTFE molecule, resulting in a lower melting point andallowing PCTFE to be processed more easily than PTFE. The crys-talline melting point of PCTFE at 218°C is lower than PTFE. Clearsheets of PCTFE with no crystallinity may also be prepared.

PCTFE is resistant to temperatures up to 200°C and has excellentsolvent resistance with the exception of halogenated solvents or oxygencontaining materials, which may swell the polymer.63 The electricalproperties of PCTFE are inferior to PTFE, but PCTFE is harder and hashigher tensile strength. The melt viscosity of PCTFE is low enough thatit may be processing using most thermoplastic processing techniques.64Typical processing temperatures are in the range of 230 to 290°C.65PCTFE is higher in cost than PTFE, somewhat limiting its use.Applications include gaskets, tubing, and wire and cable insulation.Very low vapor transmission films and sheets may also be prepared.66

Polytetrafluoroethylene. Polytetrafluoroethylene (PTFE) is polymer-ized from tetrafluoroethylene by free radical methods.67 The reactionis shown in Fig. 1.9. Commercially, there are two major processes forthe polymerization of PTFE, one yielding a finer particle size disper-sion polymer with lower molecular weight than the second method,which yields a “granular” polymer. The weight average molecularweights of commercial materials range from 400,000 to 9,000,000.68PTFE is a linear crystalline polymer with a melting point of 327°C.69

Thermoplastics 1.15

n

C C

F F

F ClFigure 1.8 Structure of PCTFE.

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Because of the larger fluorine atoms, PTFE assumes a twisted zigzagin the crystalline state, while polyethylene assumes the planar zigzagform.70 There are several crystal forms for PTFE, and some of thetransitions from one crystal form to another occur near room temper-ature. As a result of these transitions, volumetric changes of about1.3% may occur.

PTFE has excellent chemical resistance, but may go into solution nearits crystalline melting point. PTFE is resistant to most chemicals. Onlyalkali metals (molten) may attack the polymer.71 The polymer does notabsorb significant quantities of water and has low permeability to gas-es and moisture vapor.72 PTFE is a tough polymer with good insulatingproperties. It is also known for its low coefficient of friction, with valuesin the range of 0.02 to 0.10.73 PTFE, like other fluoropolymers, has excel-lent heat resistance and can withstand temperatures up to 260°C.Because of the high thermal stability, the mechanical and electricalproperties of PTFE remain stable for long times at temperatures up to250°C. However, PTFE can be degraded by high energy radiation.

One disadvantage of PTFE is that it is extremely difficult to processby either molding or extrusion. PFTE is processed in powder form byeither sintering or compression molding. It is also available as a dis-persion for coating or impregnating porous materials.74 PTFE has avery high viscosity, prohibiting the use of many conventional process-ing techniques. For this reason techniques developed for the process-ing of ceramics are often used. These techniques involve preformingthe powder, followed by sintering above the melting point of the poly-mer. For granular polymers, the preforming is carried out with thepowder compressed into a mold. Pressures should be controlled as toolow a pressure may cause voids, while too high a pressure may resultin cleavage planes. After sintering, thick parts should be cooled in anoven at a controlled cooling rate, often under pressure. Thin parts maybe cooled at room temperature. Simple shapes may be made by thistechnique, but more detailed parts should be machined.75

Extrusion methods may be used on the granular polymer at very lowrates. In this case the polymer is fed into a sintering die that is heat-ed. A typical sintering die has a length about 90 times the internal

1.16 Chapter One

F2C CF2n

n

C C

F F

F FFigure 1.9 Preparation of PTFE.

0267146_Ch01_Harper 2/24/00 5:01 PM Page 1.16

diameter. Dispersion polymers are more difficult to process by thetechniques previously mentioned. The addition of a lubricant (15 to25%) allows the manufacture of preforms by extrusion. The lubricantis then removed and the part sintered. Thick parts are not made bythis process because the lubricant must be removed. PTFE tapes aremade by this process, however, the polymer is not sintered and a non-volatile oil is used.76 Dispersions of PTFE are used to impregnate glassfabrics and to coat metal surfaces. Laminates of the impregnated glasscloth may be prepared by stacking the layers of fabric, followed bypressing at high temperatures.

Processing of PTFE requires adequate ventilation for the toxic gas-es that may be produced. In addition, PTFE should be processed underhigh cleanliness standards because the presence of any organic matterduring the sintering process will result in poor properties as a resultof the thermal decomposition of the organic matter. This includes bothpoor visual qualities and poor electrical properties.77 The final proper-ties of PTFE are dependent on the processing methods and the type ofpolymer. Both particle size and molecular weight should be considered.The particle size will affect the amount of voids and the processingease, while crystallinity will be influenced by the molecular weight.

Additives for PTFE must be able to undergo the high processingtemperatures required, which limits the range of additives available.Glass fiber is added to improve some mechanical properties. Graphiteor molybdenum disulfide may be added to retain the low coefficient offriction while improving the dimensional stability. Only a few pig-ments are available that can withstand the processing conditions.These are mainly inorganic pigments such as iron oxides and cadmi-um compounds.78

Because of the excellent electrical properties, PTFE is used in a vari-ety of electrical applications such as wire and cable insulation andinsulation for motors, capacitors, coils, and transformers. PTFE is alsoused for chemical equipment, such as valve parts and gaskets. The lowfriction characteristics make PTFE suitable for use in bearings, moldrelease devices, and antistick cookware. Low molecular weight poly-mers may be used in aerosols for dry lubrication.79

Polyvinylindene fluoride. Polyvinylindene fluoride (PVDF) is crystallinewith a melting point near 170°C.80 The structure of PVDF is shown inFig. 1.10. PVDF has good chemical and weather resistance, along withgood resistance to distortion and creep at low and high temperatures.Although the chemical resistance is good, the polymer can be affected byvery polar solvents, primary amines, and concentrated acids. PVDF haslimited use as an insulator because the dielectric properties are fre-quency dependent. The polymer is important because of its relatively

Thermoplastics 1.17

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low cost compared to other fluorinated polymers.81 PVDF is unique inthat the material has piezoelectric properties, meaning that it will gen-erate electric current when compressed.82 This unique feature has beenutilized for the generation of ultrasonic waves.

PVDF can be melt processed by most conventional processing tech-niques. The polymer has a wide range between the decomposition tem-perature and the melting point. Melt temperatures are usually 240 to260°C.83 Processing equipment should be extremely clean as any cont-aminants may affect the thermal stability. As with other fluorinatedpolymers, the generation of HF is a concern. PVDF is used for appli-cations in gaskets, coatings, wire and cable jackets, and chemicalprocess piping and seals.84

Polyvinyl fluoride. Polyvinyl fluoride (PVF) is a crystalline polymeravailable in film form and used as a lamination on plywood and otherpanels.85 The film is impermeable to many gases. PVF is structurallysimilar to polyvinyl chloride (PVC) except for the replacement of achlorine atom with a fluorine atom. PVF exhibits low moisture absorp-tion, good weatherability, and good thermal stability. Similar to PVC,PVF may give off hydrogen halides in the form of HF at elevated tem-peratures. However, PVF has a greater tendency to crystallize and bet-ter heat resistance than PVC.86

1.2.5 Nylons

Nylons were one of the early polymers developed by Carothers.87

Today, nylons are an important thermoplastic with consumption inthe United States of about 1.2 billion pounds in 1997.88 Nylons, alsoknown as polyamides, are synthesized by condensation polymeriza-tion methods, often reacting an aliphatic diamine and a diacid. Nylonis a crystalline polymer with high modulus, strength, impact proper-ties, low coefficient of friction, and resistance to abrasion.89 Althoughthe materials possess a wide range of properties, they all contain theamide (�CONH�) linkage in their backbone. Their general struc-ture is shown in Fig. 1.11.

1.18 Chapter One

n

C C

H F

H FFigure 1.10 Structure of PVDF.

0267146_Ch01_Harper 2/24/00 5:01 PM Page 1.18

There are five primary methods to polymerize nylon. They arereaction of a diamine with a dicarboxylic acid, condensation of theappropriate amino acid, ring opening of a lactam, reaction of adiamine with a diacid chloride, and reaction of a diisocyanate with adicarboxylic acid.90

The type of nylon (nylon 6, nylon 10, etc.) is indicative of the num-ber of carbon atoms. There are many different types of nylons thatcan be prepared, depending on the starting monomers used. The typeof nylon is determined by the number of carbon atoms in themonomers used in the polymerization. The number of carbon atomsbetween the amide linkages also controls the properties of the poly-mer. When only one monomer is used (lactam or amino acid), thenylon is identified with only one number (nylon 6, nylon 12). Whentwo monomers are used in the preparation, the nylon will be identi-fied using two numbers (nylon 6/6, nylon 6/12).91 This is shown inFig. 1.12. The first number refers to the number of carbon atoms inthe diamine used (a) and the second number refers to the number ofcarbon atoms in the diacid monomer (b � 2), due to the two carbonsin the carbonyl group.92

The amide groups are polar groups and significantly affect the poly-mer properties. The presence of these groups allows for hydrogenbonding between chains, improving the interchain attraction. Thisgives nylon polymers good mechanical properties. The polar nature ofnylons also improves the bondability of the materials, while the flexi-ble aliphatic carbon groups give nylons low melt viscosity for easy pro-cessing.93 This structure also gives polymers that are tough above theirglass transition temperature.94

Thermoplastics 1.19

NH

O

CCH2

n

5

CH2 CH2

CH2 CH2

H2N NH2 HO OH

NH NH

C

O

C

O

C

O

C

O

+nba

n

nba

Figure 1.12 Synthesis of nylon.

Figure 1.11 General structure of nylons.

0267146_Ch01_Harper 2/24/00 5:01 PM Page 1.19

Nylons are relatively insensitive to nonpolar solvents, however,because of the presence of the polar groups, nylons can be affected bypolar solvents, particularly water.95 The presence of moisture must beconsidered in any nylon application. Moisture can cause changes inpart dimensions and reduce the properties, particularly at elevatedtemperatures.96 As a result, the material should be dried before anyprocessing operations. In the absence of moisture nylons are fairlygood insulators, but as the level of moisture or the temperatureincreases, nylons are less insulating.97

The strength and stiffness will be increased as the number of carbonatoms between amide linkages is decreased because there are morepolar groups per unit length along the polymer backbone.98 The degreeof moisture absorption is also strongly influenced by the number ofpolar groups along the backbone of the chain. Nylon grades with few-er carbon atoms between the amide linkages will absorb more mois-ture than grades with more carbon atoms between the amide linkages(nylon 6 will absorb more moisture than nylon 12). Furthermore, nylontypes with an even number of carbon atoms between the amide groupshave higher melting points than those with an odd number of carbonatoms. For example, the melting point of nylon 6/6 is greater thaneither nylon 5/6 or nylon 7/6.99 Ring-opened nylons behave similarly.This is due to the ability of the nylons with the even number of carbonatoms to pack better in the crystalline state.100

Nylon properties are affected by the amount of crystallinity. This canbe controlled, to a great extent, in nylon polymers by the processing con-ditions. A slowly cooled part will have significantly greater crystallinity(50 to 60%) than a rapidly cooled, thin part (perhaps as low as 10%).101Not only can the degree of crystallinity be controlled, but also the size ofthe crystallites. In a slowly cooled material the crystal size will be larg-er than for a rapidly cooled material. In injection-molded parts wherethe surface is rapidly cooled the crystal size may vary from the surfaceto internal sections.102 Nucleating agents can be utilized to create small-er spherulites in some applications. This creates materials with highertensile yield strength and hardness, but lower elongation and impact.103The degree of crystallinity will also affect the moisture absorption, withless crystalline polyamides being more prone to moisture pickup.104

The glass transition temperature of aliphatic polyamides is of sec-ondary importance to the crystalline melting behavior. Dried polymershave Tg values near 50°C, while those with absorbed moisture mayhave Tgs in the range of 0°C.105 The glass transition temperature caninfluence the crystallization behavior of nylons; for example, nylon 6/6may be above its Tg at room temperature, causing crystallization atroom temperature to occur slowly leading to postmold shrinkage. Thisis less significant for nylon 6.106

1.20 Chapter One

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Nylons are processed by extrusion, injection molding, blow molding,and rotational molding among other methods. Nylon has a very sharpmelting point and low melt viscosity, which is advantageous in injec-tion molding, but causes difficulty in extrusion and blow molding. Inextrusion applications a wide molecular weight distribution (MWD) ispreferred, along with a reduced temperature at the exit to increasemelt viscosity.107

When used in injection-molding applications, nylons have a tendencyto drool due to their low melt viscosity. Special nozzles have beendesigned for use with nylons to reduce this problem.108 Nylons show highmold shrinkage as a result of their crystallinity. Average values are about0.018 cm/cm for nylon 6/6. Water absorption should also be considered forparts with tight dimensional tolerances. Water will act to plasticize thenylon, relieving some of the molding stresses and causing dimensionalchanges. In extrusion a screw with a short compression zone is used,with cooling initiated as soon as the extrudate exits the die.109

A variety of commercial nylons are available including nylon 6,nylon 11, nylon 12, nylon 6/6, nylon 6/10, and nylon 6/12. The mostwidely used nylons are nylon 6/6 and nylon 6.110 Specialty grades withimproved impact resistance, improved wear, or other properties arealso available. Polyamides are used most often in the form of fibers,primarily nylon 6,6 and nylon 6, although engineering applications arealso of importance.111

Nylon 6/6 is prepared from the polymerization of adipic acid andhexamethylenediamine. The need to control a 1:1 stoichiometric bal-ance between the two monomers can be improved by the fact thatadipic acid and hexamethylenediamine form a 1:1 salt that can be iso-lated. Nylon 6/6 is known for high strength, toughness, and abrasionresistance. It has a melting point of 265°C and can maintain proper-ties up to 150°C.112 Nylon 6/6 is used extensively in nylon fibers thatare used in carpets, hose and belt reinforcements, and tire cord. Nylon6/6 is used as an engineering resin in a variety of molding applications,such as gears, bearings, rollers, and door latches, because of its goodabrasion resistance and self-lubricating tendencies.113

Nylon 6 is prepared from caprolactam. It has properties similar tothose of nylon 6/6, but with a lower melting point (255°C). One of themajor applications is in tire cord. Nylon 6/10 has a melting point of215°C and lower moisture absorption than nylon 6/6.114 Nylon 11 andnylon 12 have lower moisture absorption and also lower melting pointsthan nylon 6/6. Nylon 11 has found applications in packaging films.Nylon 4/6 is used in a variety of automotive applications due to its abil-ity to withstand high mechanical and thermal stresses. It is used ingears, gearboxes, and clutch areas.115 Other applications for nylonsinclude brush bristles, fishing line, and packaging films.

Thermoplastics 1.21

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Additives, such as glass or carbon fibers, can be incorporated toimprove the strength and stiffness of nylon. Mineral fillers are alsoused. A variety of stabilizers can be added to nylon to improve the heatand hydrolysis resistance. Light stabilizers are often added as well.Some common heat stabilizers include copper salts, phosphoric acidesters, and phenyl-�-naphthylamine. In bearing applications self-lubricating grades are available which may incorporate graphitefillers. Although nylons are generally impact resistant, rubber is some-times incorporated to improve the failure properties.116 Nylon fibers dohave a tendency to pick up a static charge, so antistatic agents areoften added for carpeting and other applications.117

Aromatic polyamides. A related polyamide is prepared when aromaticgroups are present along the backbone. This imparts a great deal ofstiffness to the polymer chain. One difficulty encountered in this classof materials is their tendency to decompose before melting.118 However,certain aromatic polyamides have gained commercial importance. Thearomatic polyamides can be classified into three groups: amorphouscopolymers with a high Tg, crystalline polymers that can be used as athermoplastic, and crystalline polymers used as fibers.

The copolymers are noncrystalline and clear. The rigid aromaticchain structure gives the materials a high Tg. One of the oldest types ispoly(trimethylhexamethylene terephthalatamide) (Trogamid T). Thismaterial has an irregular chain structure, restricting the material fromcrystallizing, but with a Tg near 150°C.119 Other glass-clear polyamidesinclude Hostamid with a Tg also near 150°C, but with better tensilestrength than Trogamid T. Grilamid TR55 is a third polyamide copoly-mer with a Tg about 160°C and the lowest water absorption and densi-ty of the three.120 The aromatic polyamides are tough materials andcompete with polycarbonate, poly(methyl methacrylate), and polysul-fone. These materials are used in applications requiring transparency.They have been used for solvent containers, flowmeter parts, and clearhousings for electrical equipment.121

An example of a crystallizable aromatic polyamide is poly-m-xyly-lene adipamide. It has a Tg near 85 to 100°C and a Tm of 235 to240°C.122 To obtain high heat deflection temperature the filled gradesare normally sold. Applications include gears, electrical plugs, andmowing machine components.123 Crystalline aromatic polyamides arealso used in fiber applications. An example of this type of material isKevlar, a high-strength fiber used in bulletproof vests and in compos-ite structures. A similar material, which can be processed more easily,is Nomex, which can be used to give flame retardance to cloth whenused as a coating.124

1.22 Chapter One

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1.2.6 Polyacrylonitrile

Polyacrylonitrile is prepared by the polymerization of acrylonitrilemonomer using either free-radical or anionic initiators. Bulk, emul-sion, suspension, solution, or slurry methods may be used for the poly-merization. The reaction is shown in Fig. 1.13.

Polyacrylonitrile will decompose before reaching its melting point,making the materials difficult to form. The decomposition tempera-ture is near 300°C.125 Suitable solvents, such as dimethylformamideand tetramethylenesulphone, have been found for polyacrylonitrile,allowing the polymer to be formed into fibers by dry and wet spinningtechniques.126

Polyacrylonitrile is a polar material, giving the polymer good resis-tance to solvents, high rigidity, and low gas permeability.127 Althoughthe polymer degrades before melting, special techniques allowed amelting point of 317°C to be measured. The pure polymer is difficult todissolve, but the copolymers can be dissolved in solvents such asmethyl ethyl ketone, dioxane, acetone, dimethyl formamide, andtetrahydrofuran. Polyacrylonitrile exhibits exceptional barrier proper-ties to oxygen and carbon dioxide.128

Copolymers of acrylonitrile with other monomers are widely used.Copolymers of vinylidene chloride and acrylonitrile find application inlow gas permeability films. Styrene-acrylonitrile (SAN polymers)copolymers have also been used in packaging applications. Althoughthe gas permeability of the copolymers is higher than for pure poly-acrylonitrile, the acrylonitrile copolymers have lower gas permeabilitythan many other packaging films. A number of acrylonitrile copoly-mers were developed for beverage containers, but the requirement forvery low levels of residual acrylonitrile monomer in this applicationled to many products being removed from the market.129 One copoly-mer currently available is Barex (BP Chemicals). The copolymer hasbetter barrier properties than both polypropylene and polyethyleneterephthalate.130 Acrylonitrile is also used with butadiene and styreneto form ABS polymers. Unlike the homopolymer, copolymers of acry-lonitrile can be processed by many methods including extrusion, blowmolding, and injection molding.131

Thermoplastics 1.23

CH CHH2C CH2

C N C N

nn

Figure 1.13 Preparation of polyacrylonitrile.

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Acrylonitrile is often copolymerized with other monomers to formfibers. Copolymerization with monomers such as vinyl acetate, vinylpyrrolidone, and vinyl esters gives the fibers the ability to be dyedusing normal textile dyes. The copolymer generally contains at least85% acrylonitrile.132 Acrylic fibers have good abrasion resistance, flexlife, toughness, and high strength. They have good resistance tostains and moisture. Modacrylic fibers contain between 35 and 85%acrylonitrile.133

Most of the acrylonitrile consumed goes into the production of fibers.Copolymers also consume large amounts of acrylonitrile. In addition totheir use as fibers, polyacrylonitrile polymers can be used as precur-sors to carbon fibers.

1.2.7 Polyamide-imide

Polyamide-imide (PAI) is a high-temperature amorphous thermoplas-tic that has been available since the 1970s under the trade name ofTorlon.134 PAI can be produced from the reaction of trimellitic trichlo-ride with methylenedianiline, as shown in Fig. 1.14.

Polyamide-imides can be used from cryogenic temperatures to nearly260°C. They have the temperature resistance of the polyimides, butwith better mechanical properties, including good stiffness and creepresistance. PAI polymers are inherently flame retardant with littlesmoke produced when they are burned. The polymer has good chemicalresistance, but at high temperatures it can be affected by strong acids,bases, and steam.135 PAI has a heat-deflection temperature of 280°C,along with good wear and friction properties.136 Polyamide-imides alsohave good radiation resistance and are more stable than standardnylons under different humidity conditions. The polymer has one of thehighest glass transition temperatures, in the range of 270 to 285°C.137

Polyamide-imide can be processed by injection molding, but specialscrews are needed due to the reactivity of the polymer under moldingconditions. Low compression ratio screws are recommended.138 Theparts should be annealed after molding at gradually increased tem-peratures.139 For injection molding the melt temperature should benear 355°C, with mold temperatures of 230°C. PAI can also beprocessed by compression molding or used in solution form. For com-pression molding, preheating at 280°C, followed by molding between330 and 340°C with a pressure of 30 MPa, is generally used.140

Polyamide-imide polymers find application in hydraulic bushingsand seals, mechanical parts for electronics, and engine components.141The polymer in solution has application as a laminating resin forspacecraft, a decorative finish for kitchen equipment, and as wireenamel.142 Low coefficient of friction materials may be prepared byblending PAI with polytetrafluoroethylene and graphite.143

1.24 Chapter One

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1.2.8 Polyarylate

Polyarylates are amorphous, aromatic polyesters. Polyarylates arepolyesters prepared from dicarboxylic acids and bisphenols.144Bisphenol A is commonly used along with aromatic dicarboxylic acids,such as mixtures of isophthalic acid and terephthalic acid. The use oftwo different acids results in an amorphous polymer; however, thepresence of the aromatic rings gives the polymer a high Tg and goodtemperature resistance. The temperature resistance of polyarylateslies between polysulfone and polycarbonate. The polymer is flameretardant and shows good toughness and UV resistance.145Polyarylates are transparent and have good electrical properties. Theabrasion resistance of polyarylates is superior to polycarbonate. Inaddition, the polymers show very high recovery from deformation.

Polarylates are processed by most of the conventional methods.Injection molding should be performed with a melt temperature of 260to 382°C with mold temperatures of 65 to 150°C. Extrusion and blowmolding grades are also available. Polyarylates can react with waterat processing temperatures and they should be dried prior to use.146

Polyarylates are used in automotive applications such as door handles,brackets, and headlamp and mirror housings. Polyarylates are also usedin electrical applications for connectors and fuses. The polymer can beused in circuit board applications because its high-temperature resis-tance allows the part to survive exposure to the temperatures generatedduring soldering.147 The excellent UV resistance of these polymers allowsthem to be used as a coating for other thermoplastics for improved UVresistance of the part. The good heat resistance of polyarylates allowsthem to be used in applications such as fire helmets and shields.148

Thermoplastics 1.25

C

NH

CO

O

C

OC

O

C

N CH2 + HCl

O

C

O

Cl

Cl

ClH2N NH2CH2+

n

Figure 1.14 Preparation of polyamide-imide.

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1.2.9 Polybenzimidazole

Polybenzimidazoles (PBI) are high-temperature resistant polymers.They are prepared from aromatic tetramines (for example, tetraamino-biphenol) and aromatic dicarboxylic acids (diphenylisophtha-late).149 The reactants are heated to form a soluble prepolymer that isconverted to the insoluble polymer by heating at temperatures above300°C.150 The general structure of PBI is shown in Fig. 1.15.

The resulting polymer has high-temperature stability, good chemicalresistance, and nonflammability. The polymer releases very little toxicgas and does not melt when exposed to pyrolysis conditions. The polymercan be formed into fibers by dry-spinning processes. Polybenzimidazoleis usually amorphous with a Tg near 430°C.151 Under certain conditionscrystallinity may be obtained. The lack of many single bonds and thehigh glass transition temperature give this polymer its superior high-temperature resistance. In addition to the high-temperature resistance,the polymer exhibits good low-temperature toughness. PBI polymersshow good wear and frictional properties along with excellent compres-sive strength and high surface hardness.152 The properties of PBI at ele-vated temperatures are among the highest of the thermoplastics. In hot,aqueous solutions the polymer may absorb water with a resulting loss inmechanical properties. Removal of moisture will restore the mechanicalproperties. The heat-deflection temperature of PBI is higher than mostthermoplastics and this is coupled with a low coefficient of thermalexpansion. PBI can withstand temperatures up to 760°C for short dura-tions and exposure to 425°C for longer durations.

The polymer is not available as a resin and is generally notprocessed by conventional thermoplastic processing techniques, butrather by a high-temperature and pressure sintering process.153 Thepolymer is available in fiber form, certain shaped forms, finishedparts, and solutions for composite impregnation.

PBI is often used in fiber form for a variety of applications such asprotective clothing and aircraft furnishings.154 Parts made from PBIare used as thermal insulators, electrical connectors, and seals.155

1.26 Chapter One

HN

N

N

NH

n

Figure 1.15 General structure of polybenzimidazoles.

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1.2.10 Polybutylene (PB)

Polybutylene polymers are prepared by the polymerization of 1-buteneusing Ziegler-Natta catalysts The molecular weights range from770,000 to 3,000,000.156 Copolymers with ethylene are often prepared aswell. The chain structure is mainly isotactic and is shown in Fig. 1.16.157

The glass transition temperature for this polymer ranges from �17to �25°C. Polybutylene resins are linear polymers exhibiting goodresistance to creep at elevated temperatures and good resistance toenvironmental stress cracking.158 They also show high impactstrength, tear resistance, and puncture resistance. As with other poly-olefins, polybutylene shows good resistance to chemicals, good mois-ture barrier properties, and good electrical insulation properties. Pipesprepared from polybutylene can be solvent welded, yet the polymerstill exhibits good environmental stress cracking resistance.159 Thechemical resistance is quite good below 90°C, but at elevated temper-atures the polymer may dissolve in solvents such as toluene, decalin,chloroform, and strong oxidizing acids.160

Polybutylene is a crystalline polymer with three crystalline forms.The first crystalline form is obtained when the polymer is cooled fromthe melt. The first crystalline form is unstable and will change to a sec-ond crystalline form after standing over a period of 3 to 10 days. Thethird crystalline form is obtained when polybutylene is crystallizedfrom solution. The melting point and density of the first crystallineform are 124°C and 0.89 g/cm3, respectively.161 On transformation tothe second crystalline form, the melting point increases to 135°C andthe density is increased to 0.95 g/cm3. The transformation to the sec-ond crystalline form increases the polymer’s hardness, stiffness, andyield strength.

Polybutylene can be processed on equipment similar to that used forlow-density polyethylene. Polybutylene can be extruded and injection-molded. Film samples can be blown or cast. The slow transformationfrom one crystalline form to another allows polybutylene to undergopostforming techniques such as cold forming of molded parts or sheet-ing.162 A range of 160 to 240°C is typically used to process polybutylene.163The die swell and shrinkage are generally greater for polybutylene than

Thermoplastics 1.27

Figure 1.16 General structure for polybutylene.

CH2

CH2

CH3

CHn

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for polyethylene. Because of the crystalline transformation, initiallymolded samples should be handled with care.

An important application for polybutylene is plumbing pipe for bothcommercial and residential use. The excellent creep resistance of poly-butylene allows for the manufacture of thinner wall pipes compared topipes made from polyethylene or polypropylene. Polybutylene pipe canalso be used for the transport of abrasive fluids. Other applications forpolybutylene include hot melt adhesives and additives for other plastics.The addition of polybutylene improves the environmental stress crack-ing resistance of polyethylene and the impact and weld line strength ofpolypropylene.164 Polybutylene is also used in packaging applications.165

1.2.11 Polycarbonate

Polycarbonate (PC) is often viewed as the quintessential engineeringthermoplastic, due to its combination of toughness, high strength, highheat-deflection temperatures, and transparency. The worldwidegrowth rate, predicted in 1999 to be between 8 and 10%, is hamperedonly by the resin cost and is paced by applications where PC canreplace ferrous or glass products. Global consumption is anticipated tobe more than 1.4 billion kg (3 billion lb) by the year 2000.166 The poly-mer was discovered in 1898 and by the year 1958 both Bayer inGermany and General Electric in the United States had commencedproduction. Two current synthesis processes are commercialized, withthe economically most successful one said to be the “interface” process,which involves the dissolution of bisphenol A in aqueous caustic sodaand the introduction of phosgene in the presence of an inert solventsuch as pyridine. The bisphenol A monomer is dissolved in the aque-ous caustic soda, then stirred with the solvent for phosgene. The waterand solvent remain in separate phases. Upon phosgene introduction,the reaction occurs at the interface with the ionic ends of the growingmolecule being soluble in the catalytic caustic soda solution and theremainder of the molecule soluble in the organic solvent.167 An alter-native method involves transesterification of bisphenol A withdiphenyl carbonate at elevated temperatures.168 Both reactions areshown in Fig. 1.17. Molecular weights of between 30,000 and 50,000g/mol can be obtained by the second route, while the phosgenationroute results in higher molecular weight product.

The structure of PC with its carbonate and bisphenolic structureshas many characteristics which promote its distinguished properties.The para substitution on the phenyl rings results in a symmetry andlack of stereospecificity. The phenyl and methyl groups on the quarte-nary carbon promote a stiff structure. The ester-ether carbonategroups �OCOO� are polar, but their degree of intermolecular polar

1.28 Chapter One

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HO

HO

OH

OH

OH

O

+

+

+

+

C

CH3

CCl2

CH3

C

CH3

CH3

C 2 HClOCO

O

2OCO

O

O

OCO

CH3

CH3

C

CH3

CH3

(a)

(b)

Figure 1.17 Synthesis routes for PC: (a) interface process and (b) transesterification reaction.

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age 1.29

bond formation is minimized due to the steric hindrance posed by thebenzene rings. The high level of aromaticity on the backbone, and thelarge size of the repeat structure, yield a molecule of very limitedmobility. The ether linkage on the backbone permits some rotation andflexibility, producing high impact strength. Its amorphous nature withlong, entangled chains, contributes to the unusually high toughness.Upon crystallization, however, PC is brittle. PC is so reluctant to crys-tallize that films must be held at 180°C for several days in order toimpart the flexibility and thermal mobility required to conform to astructured three-dimensional crystalline lattice.169 The rigidity of themolecule accounts for strong mechanical properties, elevated heat-deflection temperatures, and high dimensional stability at elevatedtemperatures. The relative high free volume results in a low-densitypolymer, with unfilled PC having a 1.22-g/cm3 density.

A disadvantage includes the need for drying and elevated tempera-ture processing. PC has limited chemical resistance to numerous aro-matic solvents, including benzene, toluene, and xylene and has aweakness to notches. Selected mechanical and thermal properties aregiven in Table 1.3.170

Applications where PC is blended with acrilonitrile butiadienestyrene (ABS) increases the heat distortion temperature of the ABSand improves the low-temperature impact strength of PC. The favor-able ease of processing and improved economics makes PC/ABSblends well suited for thin-walled electronic housing applicationssuch as laptop computers. Blends with polybutylene terephthalate(PBT) are useful for improving the chemical resistance of PC topetroleum products and its low-temperature impact strength. PCalone is widely used as vacuum cleaner housings, household appli-ance housings, and power tools. These are arenas where PC’s highimpact strength, heat resistance, durability, and high-quality finish

1.30 Chapter One

TABLE 1.3 PC Thermal and Mechanical Properties

30% glass-filled Makroblend Xenoy, CL101Polycarbonate polycarbonate PR51 Bayer GE

Heat-deflection temperature, °C, method A 138 280 90 95

Heat-deflection temperature, °C, method B 142 287 105 105

Ultimate tensile strength, N/mm2 65 70 56 100

Ultimate elongation, % 110 3.5 120 100

Tensile modulus, N/mm2 2300 5500 2200 1900

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justify its expense. It is also used in safety helmets, riot shields, air-craft canopies, traffic light lens housings, and automotive batterycases. Design engineers take care not to design with tight radiiwhere PC’s tendency to stress crack could be a hindrance. PC cannotwithstand constant exposure to hot water and can absorb 0.2% of itsweight of water at 33°C and 65% relative humidity. This does notimpair its mechanical properties but at levels greater than 0.01%processing results in streaks and blistering.

1.2.12 Polyester thermoplastic

The broad class of organic chemicals, called polyesters, is character-ized by the fact that they contain an ester linkage,

O�

��C � O��and may have either aliphatic or aromatic hydrocarbon units. As anintroduction, Table 1.4 offers some selected thermal and mechanicalproperties as a means of comparing polybutylene terephthalate (PBT),polycyclohexylenedimethylene terephthalate (PCT), and poly(ethyleneterephthalate) (PET).

Liquid Crystal Polymers (LCP). Liquid crystal polyesters, known as liq-uid crystal polymers, are aromatic copolyesters. The presence ofphenyl rings in the backbone of the polymer gives the chain rigidity,forming a rodlike chain structure. Generally, the phenyl rings arearranged in para linkages to yield rodlike structures.171 This chainstructure orients itself in an ordered fashion both in the melt and inthe solid state, as shown in Figure 1.18. The materials are self-rein-forcing with high mechanical properties, but as a result of the orient-ed liquid crystal behavior, the properties will be anisotropic. Thedesigner must be aware of this in order to properly design the part andgate the molds.172 The phenyl ring also helps increase the heat distor-tion temperature.173

The basic building blocks for liquid crystal polyesters are p-hydroxy-benzoic acid, terephthalic acid, and hydroquinone. Unfortunately, theuse of these monomers alone gives materials that are difficult toprocess with very high melting points. The polymers often degradedbefore melting.174 Various techniques have been developed to give mate-rials with lower melting points and better processing behavior. Somemethods include the incorporation of flexible units in the chain (copoly-merizing with ethylene glycol), the addition of nonlinear rigid struc-tures, and the addition of aromatic groups to the side of the chain.175

Liquid crystal polymers based on these techniques include Victrex(ICI), Vectra (Hoescht Celanese), and Xydar (Amoco). Xydar is based

Thermoplastics 1.31

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TABLE 1.4 Comparison between Thermal and Mechanical Properties of PBT, PCT, PCTA, PET, PETG, and PCTG

30% glass- 30% glass- 30% glass- PET 30% glass- PETG PCTGPBT unfilled filled PBT filled PCT filled PCTA unfilled filled PET unfilled unfilled

Tm, °C 220–267 220–267 — 285 212–265 245–265 — —Tensile modulus, MPa 1,930–3,000 8,960–10,000 — — 2,760–4,140 8,960–9,930 — —Ultimate tensile strength, MPa 56–60 96–134 124–134 97 48–72 138–165 28 52

Ultimate elongation, % 50–300 2–4 1.9–2.3 3.1 30–300 2–7 110 330Specific gravity 1.30–1.38 1.48–1.54 1.45 1.41 1.29–1.40 1.55–1.70 1.27 1.23HDT, °C

264 lb/in2 50–85 196–225 260 221 21–65 210–227 64 6566 lb/in2 115–190 216–260 260 268 75 243–249 70 74

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age 1.32

on terephthalic acid, p-hydroxybenzoic acid, and p,p′-dihydroxy-biphenyl, while Vectra is based on p-hydroxybenzoic acid and hydroxynaphthoic acid.176 These materials are known for their high-temperature resistance, particularly heat-distortion temperature. Theheat-distortion temperature can vary from 170 to 350°C. They alsohave excellent mechanical properties, especially in the flow direction.For example, the tensile strength varies from 165 to 230 MPa, the flex-ural strength varies from 169 to 256 MPa, and the flexural modulusvaries from 9 to 12.5 GPa.177 Filled materials exhibit even higher val-ues. LCPs are also known for good solvent resistance and low waterabsorption compared to other heat-resistant polymers. They have goodelectrical insulation properties, low flammability with a limiting oxy-gen index in the range of 35 to 40, but a high specific gravity (about1.40).178 LCPs show little dimensional change when exposed to hightemperatures and a low coefficient of thermal expansion.179

These materials can be high priced and often exhibit poor abrasionresistance, due to the oriented nature of the polymer chains.180 Surfacefibrillation may occur quite easily.181 The materials are processable ona variety of conventional equipment. Process temperatures are nor-mally below 350°C, although some materials may need to be processedhigher. They generally have low melt viscosity as a result of theirordered melt and should be dried before use to avoid degradation.182LCPs can be injection-molded on conventional equipment and regrindmay be used. Mold release is generally not required.183 Part design forLCPs requires careful consideration of the anisotropic nature of thepolymer. Weld lines can be very weak if the melt meets in a “butt” typeof weld line. Other types of weld lines show better strength.184

Liquid c