This Glossary is designed to give the reader a feel for the properties and applications of engineering thermoplastics and does not attempt to be exhaustive.
Acrylonitrile Butadiene Styrene (ABS)
ABS polymers are amorphous polymers readily synthesised by adding styrene and acrylonitrile to a polybutadiene latex. In the resulting polymer, some styrene acrylonitrile (SAN) is grafted on to the polybutadiene backbone chain while the remainder of the SAN forms a continuous matrix. It is the SAN grafted onto the polymer chains that makes the two phases compatible. This essentially gives ABS its strength and toughness. A wide range of ABS materials can be formulated from different combinations of the above three components. It is particularly important to take into account the tendency ABS polymers have to creep, particularly at elevated temperatures. ABS materials also have poor solvent resistance, but they have high impact strength and lend themselves to processing by injection moulding.
Applications of ABS include: computer and household appliance housings, crash helmets and electroplated bathroom furniture.
These polymers are essentially those in which some or all of the hydrogen atoms in polyethylene have been replaced by fluorine, with polytetrafluoroethylene (PTFE) perhaps the best known. PTFE is a tough, flexible, crystalline polymer that retains ductility down to -150°C. Its solvent and chemical resistance is the best of all the thermoplastics 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 it can 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 special properties, for example, bearings, chemical vessel linings, gaskets and non-stick coatings. Other fluoropolymers include: polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), perfluoroalkoxy tetrafluoroethylene (PFA) and polychloro trifluoroethylene (PCTFE).
These are largely the results of attempts to derive polymers with all the benefits of PTFE that are melt processable. These attempts have met with some success, although none of the melt processable fluoropolymers can match the chemical inertness 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 and PA66 to 2 - 3% for PA11 and PA12 at saturation. Mechanical properties are affected by moisture, with toughness improving with the absorption of moisture whereas modulus is reduced. Polyamides are resistant to hydrocarbons, esters and glycols, but swell and dissolve in alcohols. They are also attacked by acids but generally stable to alkalis. PA6 and PA66 are mainly used in textiles, but they also find application 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 for petrol and hydraulic hose as well as powder coatings for metals. Strength and rigidity of these materials can be dramatically enhanced by the addition of glass or carbon fibre reinforcement; the level of saturation water absorption is also reduced. However, the designer needs to be aware of the anisotropic properties that can result in mouldings due to the flow and alignment of the reinforcing phase that occurs during moulding.
Glass reinforced polyamides are the material of choice for applications such as power tool housings. Transparent amorphous polyamides are available and find application in sterilisable medical components and sight glasses.
Polyarylates are a family of aromatic polyesters that are considered tough, resistant to UV radiation, resistant to heat and, in amorphous form, transparent. The basic mechanical properties of amorphous PAryls are similar to polycarbonates. The elastic rebound of PAryls is exceptional which makes it a logical candidate for snap fit applications. They are susceptible to environmental stress cracking in the presence of aliphatic or aromatic hydrocarbons and their properties deteriorate rapidly in water.
They find application in solar panels, lighting, fire helmets and electrical connectors.
Polycarbonate is unusually tough, due to the nature of its chemical bonding. It is also transparent and almost self extinguishing, with a relatively high continuous use temperature of around 115°C. Chemical resistance is not outstanding and it needs the addition of light stabilisers for any UV resistance. Glass fibres enhance the stiffness but reduce toughness, as might be anticipated. Polycarbonate is a versatile blending material, with blends of PC/PET and PC/ABS available commercially. Applications of polycarbonate include: glazing panels, light fittings, safety helmets and medical components.
Thermoplastic Polyesters (PET, PBT)
Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are the most common thermoplastic polyesters. They are similar to PA6 and PA66 in many respects but with much lower water absorption. However, they are prone to hydrolysis, and prolonged contact with water at temperatures as low as 50°C has a detrimental effect on properties.
PET is used in the manufacture of biaxially oriented film and bottles, the latter suitable for carbonated drinks. The purpose of the orientation is to enhance rigidity, strength and toughness and also to improve barrier properties, which allows thinner bottles to be made.
PBT displays a good combination of stiffness and toughness and can withstand continuous service at 120°C. The most important grades are those reinforced with glass. Applications for PBT include electrical connectors, pump components, and gears, 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 their mechanical properties to 250°C. They exhibit low flammability and smoke emission characteristics and offer the lowest minimum service temperature of thermoplastics. They are relatively expensive and can be difficult to process. Thermoplastic polyimide requires high temperatures and pressures and is usually processed by autoclave or compression moulding. They are susceptible to attack by halogenated solvents.
Polyamideimide (PAI) was initially developed as a high temperature enamel but was later modified for processing by injection and compression techniques. No other commercially available unreinforced thermoplastic is as strong as PAI over its operating range. Applications include valves, bearings, gears, electrical connectors and jet engine parts. If wear resistance is important, grades filled with graphite and PTFE are available.
Polyetherimides (PEI) are amorphous, high performance thermoplastics with a continuous use temperature of around 170° C. PEI resins can also be melt processed using typical equipment for high volume production. The strength, creep and impact properties of PEIs make them ideal for under bonnet components. They are also used in high temperature switchgear and electrical connectors. A number of medical equipment components are manufactured using PEIs, taking advantage of their excellent resistance to repeated sterilisation using steam, autoclave, gamma radiation or ethylene oxide. Microwave cookware is another application.
Polyoxymethylene (POM Acetal)
These polymers are highly crystalline thermoplastics that are commercially available as homopolymers or copolymers. This crystallinity is responsible for their excellent solvent resistance, fatigue resistance, surface finish and predictable mechanical properties over a wide temperature range. POMs are superior to PAs in stiffness, creep resistance, fatigue strength and water absorption, but have inferior impact and abrasion resistance. In general, the copolymers are thermally more stable, with similar mechanical properties to the homopolymer, albeit with slightly reduced tensile properties.
The greater stiffness and strength of the homopolymer have promoted its use in cams, gears and exterior car door handles. The copolymer is preferred where chemical resistance and resistance to hydrolysis in particular are important. Examples of the applications of POM copolymer are electric kettles, wash basins and shower heads, as well as snap fit components and toys.
Polyphenylene Oxide (PP0)
PPO is a high strength, tough and heat resistant polymer, but in the unmodified state it is extremely difficult to process. It is also relatively expensive. Fortunately, it is miscible with polystyrene, and the resulting amorphous blends are easily processed and cheaper than PPO, with little loss in mechanical properties. Stiffness and strength are approximately 50% higher than high impact ABS, with similar creep behaviour. Modified PPO grades are also self extinguishing when ignited. Resistance to solvents is poor, a characteristic of styrene-based polymers. As well as glass fibre reinforced grades, these materials are available in structural foam grades.
Modified PPOs are used for electrical fittings, car fascia panels, TV components, and computer housings. Foamed modified PPO is particularly suited to the last example.
Polyaryletherketones (PEEK, PEK)
These semi-crystalline polymers have excellent mechanical properties, good thermal stability and good chemical resistance. Despite a Tg of 145°C, the continuous service rating of PEEK is 250°C. PEEK is inherently fire retardant. It is easier to burn a hole through an aluminium sheet than through one made from PEEK. These materials 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 heat resistance and stiffness. It can be processed by conventional thermoplastic techniques despite the fact that it has a continuous use temperature of 150°C. Resistance to ionising radiation is good but to UV radiation is poor. PSul is susceptible to stress cracking in certain solvents.
Polyethersulphone (PES) has a higher continuous use temperature (180°C) and a modified PES has been shown to operate for tens of thousands of hours at 200°C without significant loss in properties. This high temperature stability is not matched by weathering or UV resistance which is poor. The polysulphones are used in electrical and electronic applications, medical components requiring repeated sterilisation, microwave cookware and under bonnet and aerospace components.
Polyphenylene sulphide (PPS)
PPS is a crystalline material, usually supplied reinforced with glass fibres or glass fibres and mineral fillers. The chemical and ionising radiation resistance of PPS is excellent and the maximum recommended service temperature for PPS is about 200°C, although it will withstand 350°C for short periods of time. While PPS will burn and char in the presence of a flame, it is self extinguishing and any smoke that does form is lower in toxicity compared to that given off by many polymers. There are some similarities between PPS and polysulphones, with PPS usually the cheaper option. Uses of PPS include chemically resistant coatings, chemical pumps and electrical components
Liquid Crystal Polymers (LCPs)
While the strength and stiffness of polymers can be enhanced by the addition of fibres such as glass or carbon, much research has been carried out into a class of materials whose molecules have a tendency to align themselves and remain in that alignment. These materials are known as liquid crystal polymers.
They comprise a diverse family although most are based on polyesters and polyamides. In their molecular structure, LCPs do not fit into the conventional polymer categories of amorphous and semi crystalline, displaying a high degree of crystallinity in the melt phase, hence ‘liquid crystal’. LCPs are essentially composed of 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 to as ‘self reinforcing’. However, this does lead to anisotropic properties.
Despite offering the best high temperature and fire resistance properties of all the thermoplastics, with certain grades able to operate at temperatures around 300°C, LCPs are relatively easy to process, although the higher the temperature resistance the more difficult the processing. The crystalline nature imparts excellent resistance to solvents, industrial chemicals, and UV and ionising radiations. They are expensive, and, apart from dual use (conventional and microwave oven) cookware, production volumes are anticipated to be low.
Further uses of LCPs are envisaged in electronic and automotive markets, replacing die cast and machined metal parts as well as thermosets.