They are however adversely affected by ultraviolet light and thus have to be

They are however adversely affected by ultraviolet

This preview shows page 64 - 66 out of 73 pages.

They are however adversely affected by ultraviolet light and thus have to be used with a stabiliser. Typical applications are pipe fittings, parts for water pumps and washing machines, car instrument housings, bearings, gears, hinges and window catches, seat belt buckles, etc. THERMOSETTING POLYMERS, THEIR PROPERTIES AND USES 1. POLYESTERS These polymers are produced by the polycondensation of polycarboxylic acids and polyhydric alcohols. They are either produced as thermosets or thermoplastics. The thermoset form is mainly used with glass or other fibres to form hard and strong composites. They have a maximum service temperature of the order of 200 °C. They have excellent dielectric properties and surface hardness. They are widely used in paper, mat, cloth, insulation for wire and cable, etc. Composites are used for boat hulls, architectural panels, car bodies, panels in aircraft, and stackable chairs. 2. PHENOLICS These polymers are produced by the polycondensation of phenol and formaldehyde. They have a highly cross-linked structure.
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~ Page 65 of 73 ~ They have good dielectric properties and surface hardness. Phenol formaldehyde, also known as Bakelite, was the first synthetic polymer. The polymer is opaque and initially light in colour. It does, however, darken with time and so is always mixed with dark pigments to give dark coloured polymer materials. It is supplied in the form of a moulding powder, including the resin, fillers and other additives such as pigments. When this moulding powder is heated in a mould, the cross-linked polymer chain structure is produced. The fillers account for some 50 to 80% of the total weight of the moulding powder. Wood flour, a very fine softwood sawdust, when used as a filler increases the impact strength of the polymer material, whilst asbestos fibres improve the heat properties and mica the electrical resistance. Phenolics are widely used in making electric iron handles, socket boxes, fan-motor housings, switch covers, etc. Phenol formaldehyde mouldings are used for electrical plugs and sockets, switches, door knobs and handles, camera bodies and ash trays. Composite materials involving the phenolic resin being used with paper or an open weave fabric, e.g. a glass fibre fabric, are used for gears, bearings, and electrical insulation parts. 3. UREA FORMALDEHYDE These polymers are produced by the chemical combination of urea and formaldehyde. They are highly cross-linked polymers. Like the phenolics they are used as moulding powders. Cellulose and wood flour are widely used as fillers in these materials. They have good bonding quality, mechanical properties and dielectric properties. Hard, rigid, high-strength materials are produced. They are widely in making instrument dials, electric mixer housings, cosmetic boxes, distributor heads, etc.
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