Page 34 - Plastics Engineering

General Properties of Plastics 17

Table 1.4
Physical characteristics of thermoplastic rubbers
5Pe Olefinic Polyurethane Polyester Styrenic Polyamide

Hardness (Shore A-D) 60A to 60D 60A to 60D 40D to 72D 30A to 45D 40D to 63D
Resilience (%) 30 to 40 40 to 50 43 to62 60 to 70 -
Tensile strength
(MN/m2) 8 to 20 30 to 55 21 to 45 25 to 45
Resistances
Chemicals F PIG E E PIE
Oils F E E F -
Solvents PIF F G P PIE
Weathering E G E PIE E
Specific gravity 0.97- 1.34 1.1 1 - 1.21 1.17-1.25 0.93-1.0 1.0- 1.12
Service temperature ("C) -50-130 -40-130 -65- 130 -30- 120 -65-130

(d) Thermosetting Plastics
Aminos. There are two basic types of amino plastics - urea formaldehyde
and melamine formaldehyde. They are hard, rigid materials with good abra-
sion resistance and their mechanical characteristics are sufficiently good for
continuous use at moderate temperatures (up to 100°C). Urea formaldehyde is
relatively inexpensive but moisture absorption can result in poor dimensional
stability. It is generally used for bottle caps, electrical switches, plugs, utensil
handles and trays. Melamine formaldehyde has lower water absorption and
improved temperature and chemical resistance. It is typically used for table-
ware, laminated worktops and electrical fittings.
Phenolics. Phenol-formaldehyde (Bakelite) is one of the oldest synthetic
materials available. It is a strong, hard, brittle material with good creep resis-
tance and excellent electrical properties. Unfortunately the material is only
available in dark colours and it is susceptible to attack by alkalis and oxidising
agents. Typical applications are domestic electrical fittings, saucepan handles,
fan blades, smoothing iron handles and pump parts.
Polyurethanes. This material is available in three forms - rigid foam, flex-
ible foam and elastomer. They are characterised by high strength and good
chemical and abrasion resistance. The rigid foam is widely used as an insula-
tion material, the flexible foam is an excellent cushion material for furniture
and the elastomeric material is used in solid tyres and shock absorbers.
Polyesters. The main application of this material is as a matrix for glass
fibre reinforcement. This can take many forms and is probably most commonly
known as a DIY type material used for the manufacture of small boats, chemical
containers, tanks and repair kits for cars, etc.
Epoxides. Epoxy resins are more expensive than other equivalent thermosets
(e.g. polyesters) but they an generally out-perform these materials due to better