Your guide to the right plastic

Polystyrenes are clear, easily coloured and have a good resistance to acids and bases. Although brittle, they have a smooth surface for printing.

HIPS (High Impact Polystyrene) is a blend of polystyrene and rubber which, because of its toughness, is typically used in casings for cameras, TVs, and vacuum cleaners. Disadvantages in manufacture include a tendency to stress crack and a vulnerability to solvent attack. SAN is a tougher co-polymer of polystyrene which offers transparency.

ABS is a rigid thermoplastic with very high impact strength.

PMMA is transparent, strong and UV resistant, but brittle – and more generally known as perspex. It is normally available in sheet form for thermoforming and as a powder for conventional processing. Its use in the manufacture of car light covers exemplifies a successful combination of properties to deliver a product which is transparent, UV resistant, dimensionally stable, impact resistant and chemically resistant to car washes.

Polycarbonate is tough and transparent with low flammability. However, its chemical resistance is not great and its UV resistance normally has to be enhanced with additives. Care must be taken with the design and processing of this polymer to avoid mechanical failure due to large frozen-in stresses.

The simplest polymer, HDPE, is tough and chemical resistant and can be converted by most production processes. Its main uses are in packaging, pipes and automotive components such as fuel tanks.

LDPE and LLDPE are less rigid forms of polyethylene and used in film and domestic items such as buckets and bowls, and small pharmaceutical bottles.

Polypropylene has similar properties to polyethylene but a higher melting point and good fatigue properties. Its excellent fatigue properties permit the use of an integral hinge in mouldings.

Polymers also encompass polyamides of which nylon 66 is the most commercially important. Its filaments are cold drawn to orientate chains to give certain properties such as high strength, good elasticity, abrasion and solvent resistance. Aromatic polyamides include Kevlar which has a high melting point and good thermal stability.

PET is widely used to produce film, fibres and, most commonly, carbonated drinks bottles. Key properties include transparency, low taint and a good dimensional stability.

PBT is a more recent polyester which offers high rigidity and good dimensional stability at high temperatures. Its uses, especially when glass filled, include housings and functional parts for electrical motors, door mirror components, and capacitor housings.

Typical applications for rigid or unplasticised PVC include pipes, rainwater goods, curtain rails and small bottles – it can, however, be modified for tougher applications. With plasticisers added, PVC becomes more flexible with a wide range of properties.

Of the two fluoropolymers, polytetrafluoroethylene (PTFE) has a high melting point, is chemically inert and has a low coefficient of friction. Although difficult to process by normal means, it is used for non stick coatings, bearings and medical applications. The second, polyvinylidene fluoride (PVdF) has many of the same properties as PTFE but with a lower melting point and it can be processed by conventional means. Because of its chemical resistance, a common use is in pipes and pumps.

Finally, polysulphone is a speciality polymer which, because of its ability to retain its properties at temperatures above 200 degrees C, is used for heated water pump impellers, and microwave and other cookware.