Page 32 - Plastics Engineering

General Properties of Plastics 15
Polytetrafluoroethylene (PTFE). The major advantages of this material are
its excellent chemical resistance and its extremely low coefficient of friction.
Not surprisingly its major area of application is in bearings particularly if the
environment is aggressive. It is also widely used in areas such as insulating
tapes, gaskets, pumps, diaphragms and of course non-stick coatings on cooking
utensils.
Thermoplastic Polyesters. These linear polyesters are highly crystalline and
exhibit toughness, strength, abrasion resistance, low friction, chemical resis-
tance and low moisture absorption. Polyethylene terephthalate (PET) has been
available for many years but mainly as a fibre (e.g. Terylene). As a moulding
material it was less attractive due to processing difficulties but these were
overcome with the introduction of polybutylene terephthalate (PBT). Applica-
tions include gears, bearings, housings, impellers, pulleys, switch parts, bumper
extensions, etc. and of course PET is now renowned for its success as a replace-
ment for glass in beverage bottles. PBT does not have such a high performance
specification as PET but it is more readily moulded.
Polyetheretherketone. This material, which is more commonly known as
PEEK, is one of the new generation plastics which offer the possibility of high
service temperatures. It is crystalline in nature which accounts in part for its
high resistance to attack from acids, alkalis and organic solvents. It is easily
processed and may be used continuously at 200°C where it offers good abrasion
resistance, low flammability, toughness, strength and good fatigue resistance.
Its density is 1300 kg/m3. Applications include wire coatings, electrical connec-
tions, fans, impellers, fibres, etc.

(b) Amorphous plastics
Polyvinyl Chloride (PVC). This material is the most widely used of the
amorphous plastics. It is available in two forms - plasticised or unplasticised.
Both types are characterised by good weathering resistance, excellent electrical
insulation properties, good surface properties and they are self extinguishing.
Plasticised PVC is flexible and finds applications in wire covering, floor tiles,
toy balls, gloves and rainwear. Unplasticised PVC (uPVC) is hard, tough, strong
material which is widely used in the building industry. For example, pipes,
gutters, window frames and wall claddings are all made in this material. The
familiar credit cards are also made from uPVC.
Polymethyl Methacrylate (PMMA). This material has exceptional optical
clarity and resistance to outdoor exposure. It is resistant to alkalis, detergents,
oils and dilute acids but is attacked by most solvents. Its peculiar property of total
internal reflection is useful in advertising signs and some medical applications.
Qpical uses include illuminated notices, control panels, dome-lights, lighting
diffusers, baths, face guards, nameplates, lenses and display models.
Polystyrene (PS). Polystyrene is available in a range of grades which
generally vary in impact strength from brittle to very tough. The non-pigmented