Modern Plastics Handbook 2011 Part 1 pptx

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

Modern Plastics Handbook

Modern Plastics

and Charles A Harper Editor in Chief

Technology Seminars, Inc Lutherville, Maryland

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

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pro-fessional services If such services are required, the assistance of an

appropriate professional should be sought.

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, East Lansing, 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)

The Modern Plastics Handbook has been prepared as a third member

of the well-known and highly respected team of publications which

includes Modern Plastics magazine and Modern Plastics World

Encyclopedia The Modern Plastics Handbook offers a thorough and

comprehensive 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 suited

to 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

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 Plastics

team, namely, Robert D Leaversuch, Executive Editor of Modern

Plastics magazine, Stephanie Finn, Modern Plastics Events Manager,

Steven J Schultz, Managing Director, Modern Plastics World

Encyclopedia, and William A Kaplan, Managing Editor of Modern Plastics 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 and

Dr 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 and

patience in my editorial responsibilities for this Modern Plastics

Handbook.

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

Charles A Harper

Chapter 4 Plastic Additives 4.1

6.5 Process-Related Design Considerations 6.29

Chapter 7 Auxiliary Equipment

7.3 Bulk Storage of Resin 7.8

Section 2 Drying and Dryers 7.63

7.12 Hygroscopic and Nonhygroscopic Polymers 7.65

7.14 How Physical Characteristics of Plastics Affect Drying 7.71

7.22 Simulation and Polymer Processing 7.101

7.24 History of Injection-Molding Simulation 7.106 7.25 Current Technology for Injection-Molding Simulation 7.109

Chapter 8 Design of Plastic Products 8.1

9.3 Assembly of Plastics Parts—General Considerations 9.16

9.7 Recommended Assembly Processes

11.4 Test Methods for Acquisition and Reporting of Property Data 11.13

11.6 Misunderstood and Misused Properties 11.70

Appendix 11.1 Selected ISO/IEC Standards/Documents 11.77

Appendix 11.4 Some Unit Conversion Factors 11.92

Appendix B Some Common Abbreviations Used

Appendix C Important Properties of Plastics

Appendix D Sources of Specifications and

Index follows Appendix E

A.-M M Baker

Joey Mead

Plastics Engineering Department

University 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 applications

is 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 well

as 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

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 unaware

of the true nature of the materials they had prepared For many years

scientists believed they were colloids—aggregates of molecules with a

particle size of 10- to 1000-nm diameter It was not until the 1920sthat Herman Staudinger showed that polymers were giant molecules

or 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 development

of polypropylene, high-density linear polyethylene, and other ospecific polymers

stere-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 to

the location of its glass transition temperature (T g ) A plastic has a T g

above room temperature, while a rubber will have a T g below room

temperature T gis most clearly defined by evaluating the classic tionship of elastic modulus to temperature for polymers as presented

rela-in Fig 1.1 At low temperatures, the material can best be described as

a glassy solid It has a high modulus and behavior in this state is acterized ideally as a purely elastic solid In this temperature regime,materials most closely obey Hooke’s law:

char-  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 orders

of magnitude for amorphous polymers The temperature at which thepolymer behavior changes from glassy to leathery is known as the

glass transition temperature, T g The rubbery plateau has a relatively

stable modulus until as the temperature is further increased, a bery flow begins Motion at this point does not involve entire mole-cules, but in this region deformations begin to become nonrecoverable

rub-as permanent set takes place As temperature is further increrub-ased,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

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 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-

molecu-merized, the resulting polymer is termed a copolymer such as ethylene

vinylacetate

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 ing on the structure of the polymer chain as well as the processing con-ditions The polymer may exist in a random unordered structure

depend-termed an amorphous polymer An example of an amorphous polymer

Thermoplastics 1.3

Figure 1.1 Relationship between elastic modulus and temperature.

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.2

The polymer formed is insoluble and is removed as the reaction ceeds Thermal degradation of the acetal resin occurs by unzippingwith the release of formaldhyde The thermal stability of the polymer

pro-is increased by esterification of the hydroxyl ends with acetic dride An alternative method to improve the thermal stability is copoly-

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

merization with a second monomer such as ethylene oxide The mer is prepared by cationic methods.3This 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

copoly-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.

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 ing point of 180°C.6Compared to polyethylene (PE), the chains packcloser together because of the shorter CO 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-

melt-mers The polymer is essentially linear with molecular weights (M n) 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 coefficient

of friction and good heat resistance.8Acetal resins are considered ilar to nylons, but are better in fatigue, creep, stiffness, and waterresistance.9Acetal 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-

sim-er, swelling may occur in some solvents Acetal resins are susceptible

to strong acids and alkalis, as well as oxidizing agents Although the

CO bond is polar, it is balanced and much less polar than the 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.10The 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.11

car-Along with both homopolymers and copolymers, there are also filledmaterials (glass, fluoropolymer, aramid fiber, and other fillers), tough-ened grades, and ultraviolet (UV) stabilized grades.12Blends 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.13Acetal resins should be stored in a

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.17When the life cycle of these polymeric parts is complet-

ed they may end up in a landfill Plastics are often selected for 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 recycling

appli-or biodegradation.18Considerable work has been done to recycle 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 presence

plas-of oxygen.19An alternative is aerobic composting In selecting a 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 can

poly-be 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.20For 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

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.21The 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 considered

in applications requiring biodegradation because of its water 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

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

nat-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 Blends

of 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 developed

by 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 able applications This material is a thermoplastic polyester synthe-sized from the ring opening of lactides Lactides are cyclic diesters oflactic acid.29A similar material to polylactide is polyglycolide (PGA)

biodegrad-PGA is also a thermoplastic polyester, but one that is formed from 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.

gly-Polycaprolactones are also considered in biodegradable applicationssuch as films and slow-release matrices for pharmaceuticals and fer-tilizers.30Polycaprolactone is produced through ring opening polymer-ization of lactone rings with a typical molecular weight in the range of15,000 to 40,000.31It 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 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-

polyhydroxybutyrate-ria Alcaligenes eutrophus, which is fed a carbohydrate The bactepolyhydroxybutyrate-ria

produce polyesters, which are harvested at the end of the process.33

When 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 a

melting point between 173 and 180°C and a T g near 5°C.35

Copolymers 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 ural polysaccharide produced by FMC, Chitin, another polysaccharidethat is insoluble in water, and Chitosan, which is soluble in water.37

nat-Chitin 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

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.40Plants 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, regardless

of 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

TABLE 1.1 Selected Mechanical Properties of Common Celluloses

Tensile strength, MPa Ultimate elongation, %

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

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 1.2.43Propertiescan 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.44Cellulose 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 CAP

is injection-molded into steering wheels, telephones, appliance ings, flashlight cases, screw and bolt anchors, and is extruded into

hous-TABLE 1.2 Selected Mechanical Properties of Cellulose Esters

Cellulose Cellulose acetate Cellulose acetate

Percent moisture absorption

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 tion-molding applications which require many postfinishing steps.47

injec-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 This

is due to the strength and stability of the carbon-fluorine bond.48Thefirst 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) in

1938.49In addition to their high-temperature properties, 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.50The compressive properties of fluoropolymers are generallysuperior to their tensile properties In addition to their high-

fluoropoly-Thermoplastics 1.13

OCOCH2CO-Na+

O

OCH

methyl-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).

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.55FEP has good chemical resistance,

a low dielectric constant, low friction properties, and low gas 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.57

perme-Extrusion 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

resistance of ECTFE, however, and will decompose and melt whenexposed to a flame.60The polymer has good abrasion resistance for a flu-orine-containing polymer, along with good impact strength The polymer

is used for wire and cable insulation where its high-temperature ties are important ETFE finds application in electrical systems for com-puters, aircraft, and heating systems.61

proper-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 that

it may be processing using most thermoplastic processing techniques.64

Typical processing temperatures are in the range of 230 to 290°C.65

PCTFE 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 ized from tetrafluoroethylene by free radical methods.67The reaction

polymer-is 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 9,000,000.68

PTFE is a linear crystalline polymer with a melting point of 327°C.69

Because of the larger fluorine atoms, PTFE assumes a twisted zigzag

in 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.71The polymer does notabsorb significant quantities of water and has low permeability to gas-

es and moisture vapor.72PTFE is a tough polymer with good insulatingproperties It is also known for its low coefficient of friction, with values

in the range of 0.02 to 0.10.73PTFE, like other fluoropolymers, has 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 process

excel-by either molding or extrusion PFTE is processed in powder form excel-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 result

in cleavage planes After sintering, thick parts should be cooled in anoven at a controlled cooling rate, often under pressure Thin parts may

be 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

F2C CF2n

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 lubricant

is 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.76Dispersions 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 procgas-essed underhigh cleanliness standards because the presence of any organic matterduring the sintering process will result in poor properties as a result

of the thermal decomposition of the organic matter This includes bothpoor visual qualities and poor electrical properties.77The 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 Graphite

or 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 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

vari-Polyvinylindene fluoride. Polyvinylindene fluoride (PVDF) is crystallinewith a melting point near 170°C.80The 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

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

PVDF can be melt processed by most conventional processing niques The polymer has a wide range between the decomposition tem-perature and the melting point Melt temperatures are usually 240 to260°C.83Processing 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

tech-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 1997.88Nylons, alsoknown as polyamides, are synthesized by condensation polymeriza-tion methods, often reacting an aliphatic diamine and a diacid Nylon

is a crystalline polymer with high modulus, strength, impact ties, low coefficient of friction, and resistance to abrasion.89Althoughthe 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

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 ber of carbon atoms There are many different types of nylons thatcan be prepared, depending on the starting monomers used The type

num-of nylon is determined by the number num-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 in

the diamine used (a) and the second number refers to the number of carbon atoms in the diacid monomer (b 2), due to the two carbons

in the carbonyl group.92

The amide groups are polar groups and significantly affect the 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.93This structure also gives polymers that are tough above theirglass transition temperature.94

poly-Thermoplastics 1.19

NH

O C

C

O

C O

+ n

b a

n

n b a

Synthesis of nylon.

Figure 1.11 General structure of nylons.

Nylons are relatively insensitive to nonpolar solvents, however,because of the presence of the polar groups, nylons can be affected bypolar solvents, particularly water.95The presence of moisture must beconsidered in any nylon application Moisture can cause changes inpart dimensions and reduce the properties, particularly at elevatedtemperatures.96As 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.98The degree

of 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 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

mois-Nylon properties are affected by the amount of crystallinity This can

be controlled, to a great extent, in nylon polymers by the processing ditions A slowly cooled part will have significantly greater crystallinity(50 to 60%) than a rapidly cooled, thin part (perhaps as low as 10%).101

con-Not 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 surface

to internal sections.102Nucleating agents can be utilized to create

small-er sphsmall-erulites in some applications This creates matsmall-erials with highsmall-ertensile yield strength and hardness, but lower elongation and impact.103

The 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 ondary importance to the crystalline melting behavior Dried polymers

sec-have T g values near 50°C, while those with absorbed moisture may

have T gs in the range of 0°C.105The glass transition temperature caninfluence the crystallization behavior of nylons; for example, nylon 6/6

may be above its T g at room temperature, causing crystallization atroom temperature to occur slowly leading to postmold shrinkage This

is less significant for nylon 6.106

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 tendency

to drool due to their low melt viscosity Special nozzles have beendesigned for use with nylons to reduce this problem.108Nylons 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 6.110Specialty 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.112Nylon 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

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.116Nylon 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 class

of materials is their tendency to decompose before melting.118However,certain aromatic polyamides have gained commercial importance Thearomatic polyamides can be classified into three groups: amorphous

copolymers with a high T g , crystalline polymers that can be used as a

thermoplastic, and crystalline polymers used as fibers

The copolymers are noncrystalline and clear The rigid aromatic

chain structure gives the materials a high T g One of the oldest types is

poly(trimethylhexamethylene terephthalatamide) (Trogamid T) Thismaterial has an irregular chain structure, restricting the material from

crystallizing, but with a T gnear 150°C.119Other glass-clear polyamides

include Hostamid with a T g also near 150°C, but with better tensilestrength than Trogamid T Grilamid TR55 is a third polyamide copoly-

mer with a T gabout 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 lene adipamide It has a T g near 85 to 100°C and a T m of 235 to240°C.122To obtain high heat deflection temperature the filled gradesare normally sold Applications include gears, electrical plugs, andmowing machine components.123Crystalline 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,

poly-m-xyly-is Nomex, which can be used to give flame retardance to cloth whenused as a coating.124

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 tance to solvents, high rigidity, and low gas permeability.127Althoughthe 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

resis-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.129One copoly-mer currently available is Barex (BP Chemicals) The copolymer hasbetter barrier properties than both polypropylene and polyethyleneterephthalate.130Acrylonitrile is also used with butadiene and styrene

to form ABS polymers Unlike the homopolymer, copolymers of lonitrile can be processed by many methods including extrusion, blowmolding, and injection molding.131

Preparation of polyacrylonitrile.

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.132Acrylic 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 tic that has been available since the 1970s under the trade name ofTorlon.134PAI can be produced from the reaction of trimellitic trichlo-ride with methylenedianiline, as shown in Fig 1.14

thermoplas-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.136Polyamide-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 between

330 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.141

The 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

presence of the aromatic rings gives the polymer a high T g and goodtemperature resistance The temperature resistance of polyarylateslies between polysulfone and polycarbonate The polymer is flameretardant and shows good toughness and UV resistance.145

Polyarylates 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 260

to 382°C with mold temperatures of 65 to 150°C Extrusion and blowmolding grades are also available Polyarylates can react with water

at 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 used

in 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.147The 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

N H

C O

O

C O

C O

C

O

C O

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).149The reactants are heated to form a soluble prepolymer that isconverted to the insoluble polymer by heating at temperatures above300°C.150The 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 Polybenzimidazole

is usually amorphous with a T gnear 430°C.151Under 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.152The 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

H N

N

N

N H

n

General structure of polybenzimidazoles.