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Introduction

What is an engineering thermoplastic? Such a definition is difficult to arrive at andvery subjective, but, for the purposes of this Profile, any thermoplastic that can beformed into a load-bearing shape that might otherwise be formed from, for example,steel or wood will be classed as an engineering thermoplastic. This review isrestricted to unreinforced and short fibre reinforced thermoplastics where thereinforcing fibre (usually glass or carbon) is typically less than 2-3 mm in length.Long fibre reinforced thermoplastics are dealt with in the Profile ThermoplasticComposites .

Thermoplastics offer many advantages over traditional materials, including: lowdensity, low energy for manufacture, low processing costs and the ability to makecomplex shapes relatively easily.

Thermoplastic Characteristics

Thermoplastic materials generally fall within two classes of molecular arrangement,amorphous and semi-crystalline (see Fig.1). Table 1 lists a selection of amorphousand semi-crystalline polymers.

Fig.1: Molecular arrangement of polymer chains

CORROSION

Justin Furness, Materials Information Services

Table 1: Amorphous and semi-crystalline polymers

Generally, fully amorphous polymers are stiff, brittle and clear in the virgin state. Thetemperature and stress state have a profound effect on the molecular arrangementand hence the properties of a polymer. Under the action of sufficient stress, thepolymer chains can uncoil and align over a period of time. At elevated temperatures,polymer chains have enough energy to rotate and coil up further.

Fig.2 shows schematically the effect of temperature on the elastic modulus of anamorphous and semi-cyrstalline polymer. Below a temperature known as the glasstransition temperature, Tg, the structure of amorphous polymers is termed glassy ,with a random arrangement of the polymer chains, similar to the random moleculararrangement found in glass. As the temperature increases to Tg, the polymer chainshave sufficient thermal energy to rotate, resulting in a drop in modulus. Onedefinition of Tg is the temperature at which molecular rotation about single bandsbecomes favourable as the temperature increases. At temperatures above Tg butbelow the melting temperature, Tm, there is a rubbery region , where the materialcan exhibit large elongations under relatively low load.

Amorphous thermoplastics are generally used at temperatures below their Tg, wherethey can be brittle, just like glass. There are, however, certain exceptions.Polycarbonate is amorphous yet it is considered tough at temperatures well below itsTg. When it does fracture below its Tg, it does so in a brittle manner, but thisrequires a large amount of energy and so PC is considered tough, finding use inapplications requiring impact resistance, e.g. safety helmets and bullet proof glazing.This behaviour is due to the chemical bonds in polycarbonate rather than thearrangements of the polymer chains.

Semi-cyrstalline materials such as polyamides do not exhibit a clear Tg or rubberyregion, although one is often quoted as the amorphous parts of the structure willundergo some transition. For these polymers the main transition occurs at Tm whenthe crystalline regions break down (see Fig.2). Some chain rotation in theamorphous regions will occur below Tm, giving some impact resistance at thesetemperatures. Values of Tg and Tm for a number of polymers are given in Table 2.

Fig.2: The effect of temperature on the elastic modulus (assuming bothtypes of polymer have the same Tg and Tm).

Table 2: Values of Tg and Tm for selected polymers (data for guidance only).

Chemical Resistance

The resistance to chemicals of polymers is usually determined by immersingspecimens in the media in question at various temperatures. After a given time haselapsed, the specimens are removed and changes in their mass, dimensions andmechanical properties are measured. However, this is only of limited practical use indetermining the suitability of a polymer for a given application, because the internaland external stresses experienced in-service can have a dramatic effect on thenature of the chemical attack.

When embrittlement of polymers occurs under these conditions it is termedenvironmental stress cracking (ESC) and the environment is frequently a liquid orvapour that would not attack the polymer if the stress were absent. ESC, a time-dependent phenomenon, often first makes itself apparent on the polymer surface inthe form of fine hairline cracks that reflect light. These find cracks are in fact bridgedby extended polymer chains which are still able to transfer some load. However, ifsubjected to impact loads, they will act as stress-concentrators, resulting in failure.Over a long period of time they can develop into cracks in their own right.

Poor chemical resistance is often considered the Achilles heel of amorphousthermoplastics. Crystalline resins tend to have much better chemical resistance, andin particular solvent resistance, because of their insoluble nature. However,crystalline polymers are particularly susceptible to ESC.

Mechanical Properties

It must be made clear that polymers respond to applied loads very differently tometals, making direct comparisons of strength and stiffness of limited use. Forexample, a large number of design formulae have been derived for beams, platesand columns dependent on the elastic modulus as a fundamental measure of theresponse of the material to stress, the implicit assumption being that the materialbehaves in a linear, elastic manner until yielding. For most metals, this is true; forpolymers, the stress-strain curve is rarely linear, there is no true proportional limitand the behaviour is greatly affected by strain rate and temperature. Because apolymer behaves in a manner that seems to combine elastic solid and highly viscousbehaviour, they are termed viscoelastic .

The deformation of polymer components subjected to external stress is composed ofthree elements (see Fig.3):

• instantaneous (reversible) elastic deformation;

• time-dependent viscoelastic deformation;

• time-dependent irreversible plastic deformation.

This time-dependent behaviour means that test methods must be carefully definedand data sheet values must be used with care. (ISO 10350 and ISO 11403 arecurrent international standards for single and multi-point data respectively).

Fig.3: Breakdown of the strain response of a typical polymeras a function of stress and time

Viscoelastic materials creep under steady stress and relax under steady strain. Inorder to design a component that will not creep more than say, 1% over its projectedlifetime, it will be necessary to examine creep curves from the material supplier toascertain the stress level that will restrict creep to this limit.

Tensile test results may be useful in the preliminary selection of materials, but areonly really of any further use to the designer if the strain rate experienced by thecomponent in service is similar to that used in the test. Typical stress-strain curvesfor a number of polymers are shown in Fig.4.

Fig.4: Typical stress-strain curves for various types of polymer

The designer of a polymer component always needs to be aware of the anisotropicnature of these materials, particularly those reinforced with fibres. As aconsequence of the manufacturing process, alignment of polymer chains and anyfillers or fibres tend to occur in the direction of flow. While mechanical properties inthat direction may be very good, the properties at right angles to the flow directionmay be very poor and will almost certainly be different. Of course, this can beviewed as an advantage such that by appropriate mould design, the alignmentoccurs in regions of high stress where it is needed.

The Izod Charpy impact values provide a useful indication of the material stoughness. Comparative figures for notched and unnotched specimens also enablenotched impact resistance to be assessed. Actual component behaviour cannot bedetermined from such data because of variations in loading pattern and partgeometry. Also, materials may withstand a blow of impact energy below thatdeemed necessary to result in fracture, but a second blow at that energy level maybe enough to cause fracture. Amorphous polymers such as PC are susceptible tothis behaviour, whereas semi-crystalline materials such as Pas and POMs usuallyperform well under repeated impact. The impact strength of polymers is highlytemperature dependent.

Thermal Properties

The service temperature is the range between the maximum and minimumtemperatures that the polymer is subjected to in-service. The maximum temperatureis limited by softening and burning: the minimum temperature by embrittlement.Insight into the ability of a polymer to withstand short-term exposure to elevatedtemperatures under load can be gained by referring to the heat deflectiontemperature (HDT). The HDT does not give an absolute measure of the thermalstability of a material.

One of the most common measures of long-term temperature resistance is theUnderwriters Laboratory (UL) thermal index. This index is derived by exposing testspecimens to different temperatures and testing them after various time intervals.When property levels drop to 50% of their initial values, this is deemed a failure.Oxidation of the polymer is the most common cause of degradation at hightemperatures.

The molecular weight of the polymer decreases as the long-chain molecules un-zip ,and the physical strength and impact strength drop. As degradation continues, soother properties are affected, including electrical properties. Hence, long-termtemperature resistance is often rated according to the application, e.g. mechanicalwith impact, mechanical without impact and electrical. An indication of HDT and ULindex values are given in Table 3.

Table 3: Thermal properties of unfilled polymers(data for guidance only — absolute values are a function of grade)

Polymers tend to become brittle at low temperatures, and the variation of impactstrength or notched impact strength with temperature can be used to determineminimum service temperature.

Additives

A large number of additives are available for the modification of polymer properties,with plasticisers the most dominant additives in terms of tonnage and value.Plasticisers are typically added to PVC compounds for increased elastomericproperties. A typical formulation for a colourless, flame-retardant PVC sheet is:53.5% PVC suspension, 32.0% plasticisers, 8.5% extenders, 5.0% flame retardants,0.5% lubricants and 0.5% stabilisers.

The fastest growing additives are the flame retardants, largely a result of theincreasingly stringent legislation on the flame and smoke toxicity of materials used inpublic places. Of the halogen-free flame retardants, aluminium trihydrate (ATH) isprobably the most common.

While certain monomer units that go to make up polymers do not absorb uv light(e.g. the monomer units of PVC, PMMA and PS), these polymers almost invariablycontain traces of uv absorbing impurities. Hence, for prolonged outdoor use, mostpolymers contain light stabilisers; hindered amine light stabilisers (HALS) andcarbon black are frequently used. Long-term exposure to light causes ageing of thepolymer due to the breaking of polymer bonds, cross linking or oxidation (see Table

4). The consequences are: embrittlement, cracking and changes in colour ortransparency. Coatings are also frequently applied to prevent attack from sunlight aswell as other environmental agents. The design of coatings for polymers could formthe basis of a Profile in itself.

Antioxidants, typically hindered phenolic compounds, are used to retard thermal andoxidative degradation of polyolefins during polymerisation, processing and end-use.Some suppliers offer combined uv stabiliser and antioxidant packages.

Other additives include: impact modifiers, antistatic agents, blowing agents,pigments and dyes.

Table 4: Resistance to weathering (data for guidance only)

Fillers

While fillers were originally added to polymers primarily to reduce the cost of thefinished product, they are now also used to enhance mechanical properties, andthere is often little distinction between fillers and reinforcement. Mineral fillers suchas calcium carbonate, mica and silica have been used for many years as low costfillers that improve mechanical properties. Other mineral fillers include talc, clay andwollastonite.

• Stabilised with carbon black# Weathering restricted to surface layer

Taken from: Important Physical Properties and Aspects of Plastics Technology,Willi Bartholomeyzik, BASF, 1988

Reinforcements

Reinforcements are typically high strength, high modulus, inert fibrous materials thatform a good bond with the polymer, either neat or with the aid of wetting agents.Glass fibres are the most common reinforcing material. For more demandingautomotive and aerospace applications, carbon fibre reinforcement may be moreappropriate. They are used primarily to improve strength stiffness and dimensionaland thermal stability.

Blends and Alloys

Considering that ABS, polyamides and polycarbonates account for about 75% of theapplications of engineering thermoplastics, in some respects there is little scope fornew polymers. Polysulphone, polyethersulphone, polyetherimide, polyamideimideand polyetheretherketone only account for a few percent of the total market.

One way of developing new materials without all the risks of formulating an entirelynew polymer is to blend two or more polymers together. Hopefully, the blend willdisplay the best qualities of each polymer and none of their worst. In practice,blends do not often work very well as polymers are not generally miscible. However,a number of miscible polymer blends are available commercially, including PPO/PS.PVC/PA66 and PS/PC.

In the case of PPO/PS blends, the PPO imparts high-temperature resistance andsome toughness; PS contributes to a lower cost. This blend is used in theautomotive industry and extensively for business machine housings. Reinforcedgrades have replaced metals in many areas because of low cost and ease ofprocessibility.

ABS is only partially miscible in PC, yet ABS/PC blends have found commercialsuccess as there is good interfacial adhesion between the different phases. ABS isin itself a two-phase mixture, basically consisting of rubber modified SAN (seeGlossary). ABS/PC blends are also used for car trim.

Glossary of Materials

This Glossary is designed to give the reader a feel for the properties and applicationsof engineering thermoplastics and does not attempt to be exhaustive.

Acrylonitrile butadiene styrene (ABS)ABS polymers are amorphous polymers readily synthesised by adding styrene andacrylonitrile to a polybutadiene latex. In the resulting polymer, some styrene-acrylonitrile (SAN) is grafted on to the polybutadiene backbone chain while theremainder of the SAN forms a continuous matrix. It is the SAN grafted onto thepolymer chains that makes the two phases compatible. This essentially gives ABSits strength and toughness. A wide range of ABS materials can be formulated fromdifferent combinations of the above three components. It is particularly important to

take into account the tendency ABS polymers have to creep, particularly at elevatedtemperatures. ABS materials also have poor solvent resistance, but they have highimpact strength and lend themselves to processing by injunction moulding.

Applications of ABS include: computer and household appliance housings, crashhelmets and electroplated bathroom furniture.

FluoropolymersThese polymers are essentially those in which some or all of the hydrogen atoms inpolyethylene have been replaced by flourine, with polytetraflouroethylene (PTFE)perhaps the best known. PTFE is a tough, flexible, crystalline polymer that retainsductility down to —1500C. Its solvent and chemical resistance is the best of all thethermoplastics and it has the lowest coefficient of friction of any known solid (0.02).On the downside, it has to be moulded by a powder sintering technique, although itcan be extruded very slowly, and it is very expensive with low strength and stiffness.Applications of PTFE are therefore limited to those that make use of its specialproperties, for example, bearings, chemical vessel linings, gaskets and non-stickcoatings. Other flouropolymers include: polyvinyl flouride (PVF), polyvinylideneflouride (PVDF), perfluoroalkoxy tetraflouroethylene (PFA) and polychlorotriflouroethylene (PCTFE).

These are largely the results of attempts to derive polymers with all the benefits ofPTFE that are melt-processable. These attempts have met with some success,although none of the melt-processable flouropolymers can match the chemicalinertness of PTFE. PVDF has useful piezoelectric properties.

Polyamides (PAs – nylons)Polyamides have good strength and toughness with excellent fatigue resistance.However, they are prone to absorb moisture, ranging from 8 — 10% for PA6 andPA66 to 2 — 3% for PA11 and PA12 at saturation. Mechanical properties areaffected by moisture; toughness is improved by absorbed moisture whereasmodulus is reduced. Polyamides are resistant to hydrocarbons, esters and glycols,but swell and dissolve in alcohols. They are also attached by acids but generallystable to alkalis. PA6 and PA66 are mainly used in textiles, but they also findapplication where toughness is a requirement, for example, zip fastener teeth, gears,wheels and fan blades. PA11 is more flexible than PA66 and is typically used forpetrol and hydraulic hose as well as powder coatings for metals. Strength andrigidity of these materials can be dramatically enhanced by the addition of glass orcarbon fibre reinforcement; the level of saturation water absorption is also reduced.However, the designer needs to be aware of the anisotropic properties that canresult in mouldings due to the flow and alignment of the reinforcing phase that occursduring moulding.

Glass reinforced polyamides are the material of choice for applications such aspower tool housings. Transparent amorphous polyamides are available and findapplication in sterilisable medical components and sight glasses.

Polyarylates (PAryls)Polyarylates are a family of aromatic polyesters that are considered tough, resistantto uv radiation, resistant to heat and, in amorphous form, transparent. The basic

mechanical properties of amorphous Paryls are similar to polycarbonates. Theelastic rebound of Paryls is exceptional which makes it a logical candidate for snap-fit applications. They are susceptible to environmental stress cracking in thepresence of aliphatic or aromatic hydrocarbons and their properties deterioraterapidly in water.

They find application in solar panels, lighting, fire helmets and electrical connectors.

Polycarbonate (PC)Polycarbonate is unusually tough, for reasons that have already been explained. Itis also transparent and almost self-extinguishing, with a relatively high continuoususe temperature of around 1150C. Chemical resistance is not outstanding and itneeds the addition of light stabilisers for any uv resistance. Glass fibres enhance thestiffness but reduce toughness, as might be anticipated. Polycarbonate is a versatileblending material, with blends of PC/PET and PC/ABS available commercially.Applications of polycarbonate include: glazing panels, light fittings, safety helmetsand medical components.

Theroplastic polyesters (PET, PBT)Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are the mostcommon thermoplastic polyesters. They are similar to PA6 and PA66 in manyrespects but with much lower water absorption. However, they are prone tohydrolysis, and prolonged contact with water at temperatures as low as 500C has adetrimental effect on properties.

PET is used in the manufacture of biaxially oriented film and bottles, the lattersuitable for carbonated drinks. The purpose of the orientation is to enhance rigidity,strength and toughness and also to improve barrier properties, which allows thinnerbottles to be made.

PBT displays a good combination of stiffness and toughness and can withstandcontinuous service at 1200C. The most important grades are those reinforced withglass. Applications for PBT include electrical connectors, pump components, andgears, as well as under-bonnet and exterior parts for cars.

Thermoplastic polyimides (PI, PAI, PEI)Polyimides (PI) are noted for their high temperature performance, retaining theirmechanical properties to 2500C. They exhibit low flammability and smoke emissioncharacteristics and offer the lowest minimum service temperature of thermoplastics.They are relatively expensive and can be difficult to process; thermoplasticpolyimide requires high temperatures and pressures and is usually processed byautoclave or compression moulding. They are susceptible to attack by halogenatedsolvents.

Polyamideimide (PAI) was initially developed as a high temperature enamel but waslater modified for processing by injection and compression techniques. No othercommercial available unreinforced thermoplastics is as strong as PAI over itsoperating range. Applications include valves, bearings, gears, electrical connectorsand jet engine parts. If wear resistance is important, grades filled with graphite andPTFE are available.

Polyetherimides (PEI) are amorphous, high-performance thermoplastics with acontinuous use temperature of around 1700C. PEI resins can also be melt-processed using typical equipment for high-volume production. The strength , creepand impact properties of PEIs make them ideal for under-bonnet components. Theyare also used in high temperature switchgear and electrical connectors. A number ofmedical equipment components are manufactured using PEIs, taking advantage oftheir excellent resistance to repeated sterilisation using steam, autoclave, gammaradiation or ethylene oxide. Microwave cookware is another application.

Polyoxymethylene (POM-acetal)These polymers are high crystalline thermoplastics that are commercially availableas homopolymers or copolymers. This crystallinity is responsible for their excellentsolvent resistance, fatigue resistance, surface finish and predictable mechanicalproperties over a wide temperature range. POMs are superior to Pas in stiffness,creep resistance, fatigue strength and water absorption, but have inferior impact andabrasion resistance. In general, the copolymers are thermally more stable, withsimilar mechanical properties to the homopolymer, albeit with slightly reduced tensileproperties.

The greater stiffness and strength of the homopolymer have promoted its use incams, gears and exterior car door handles. The copolymer is preferred wherechemical resistance and resistance to hydrolysis in particular are important.Examples of the applications of POM copolymer are electric kettles, was basics andshower heads, as well as snap fit components and toys.

Polyphenylene oxide (PPO)PPO is a high strength, tough and heat resistant polymer, but in the unmodified stateit is extremely difficult to process. It is also relatively expensive. Fortunately, it ismiscible with polystyrene, and the resulting amorphous blends are easily processedand cheaper than PPO with little loss in mechanical properties. Stiffness andstrength are approximately 50% higher than high impact ABS, with similar creepbehaviour. Modified PPO grades are also self-extinguishing when ignited.Resistance to solvents is poor, a characteristic of styrene-based polymers. As wellas glass fibre reinforced grades, these materials are available in structure foamgrades.

Modified PPOs are used for electrical fittings, car fascia panels, TV components, andcomputer housings. Foamed modified PPO is particularly suited to the last example.

Polyaryletherketones (PEEK, PEK)These semi-crystalline polymers have excellent mechanical properties, good thermalstability and good chemical resistance. Despite a Tg of 1450C, the continuousservice rating of PEEK is 2500C. PEEK is inherently fire retardant; it is easier toburn a hole through an aluminium sheet than through one made from PEEK. Thesematerials are, however, very expensive and difficult to process.

They find application in high temperature wire covering and printed circuit boards.Fibre reinforced grades are used in demanding applications that include valves,pumps and missile nose cones.

Polysulphones (PSul, PES)Polysulphone (PSul) is an amorphous, transparent polymer with good heatresistance and stiffness. It can be processed by conventional thermoplastictechniques despite the fact that it has a continuous use temperature of 1500C.Resistance to ionising radiation is good but to uv radiation is poor. PSul issusceptible to stress cracking in certain solvents.Polyethersulphone (PES) has a high continuous use temperature (1800C) and amodified PES has been shown to operate for tens of thousands of hours at 2000Cwithout significant loss in properties. This high temperature stability is not matchedby weathering or uv resistance which is poor. The polysulphones are used inelectrical and electronic applications, medical components requiring repeatedsterilisation and microwave cookware and underbonnet and aerospace components.

Polyphenylene sulphide (PPS)PPS is a crystalline material, usually supplied reinforced with glass fibres or glassfibres and mineral fillers. The chemical and ionising radiation resistance of PPS isexcellent and the maximum recommended service temperature for PPS is about2000C, although it will withstand 3500C for short periods of time. While PPS will burnand char in the presence of a flame, it is self-extinguishing and any smoke that doesform is lower in toxicity compared to that given off by many polymers. There aresome similarities between PPS and polysulphones, with PPS usually the cheaperoption. Uses of PPS include chemically resistant coatings, chemical pumps andelectrical components.

Liquid crystal polymers (LCPs)While the strength and stiffness of polymers can be enhanced by the addition offibres such as glass or carbon, much research has been carried out into a class formaterials whose molecules have a tendency to align themselves and remain in thatalignment. These materials are known as liquid crystal polymers.

They comprise a diverse family although most are based on polyesters andpolyamides. In their molecular structure, LCPs do not fit into the conventionalpolymer categories of amorphous and semi-crystalline, displaying a high degree ofcrystallinity in the melt phase, hence liquid crystal . LCPs are essentially composedof long, rod-like molecules that align themselves in the direction of material flow.This alignment is maintained as solidification takes place, hence they are referred toas self-reinforcing . However, this does lead to anisotropic properties.

Despite offering the best high temperature and fire resistance properties of all thethermoplastics, with certain grades able to operate at temperatures around 3000C,LCPs are relatively easy to process, although the higher the temperature resistancethe more difficult the processing. The crystalline nature imparts excellent resistanceto solvents, industrial chemicals and uv and ionising radiations. They are expensiveand, apart from dual-use (conventional and microwave oven) cookware, productionvolumes are anticipated to be low.

Further uses of LCPs are envisaged in electronic and automotive markets, replacingdie cast and machined metal parts as well as thermosets.

Sources of Further Information

Handbook of Plastic Materials and TechnologyEd. II Rubin, John Wiley and Sons, 1990

The Structure and Properties of Polymeric MaterialsD W Clegg and A A CollyerThe Institute of Materials 1993

Engineered Materials Handbook, Vol 2 – Engineering PlasticsASM International, 1988

Plastics Materials: Properties and ApplicationsA W Birley and M J Scott, Leonard Hill, 1982

Materials Selector, Vol 3Eds. N A Waterman and M F Ashby, Chapman and Hall, 1991

Where to get advice

Engineering thermoplastics is a vast subject and there are many centres offeringexpertise. It is recommended you contact the Materials Information Service in thefirst instance (see cover). If they cannot help directly, then they will be happy todirect you to someone who can. Future Profiles will deal with aspects ofthermoplastic materials not covered here, including selection and design.