Page 254 - Mechanical Engineer's Data Handbook

242 MECHANICAL ENGINEER’S DATA HANDBOOK

6. I3 Plastics

The term ‘plastic’ is used for materials based on cooling. The process can be repeated continuously.
polymers to which other materials are added to give Thermosetting polymers (or thermosets) cannot be
the desired properties. ‘Fillers’ increase strength, ‘plas- softened and reshaped by heating. They are plastic at
ticizers’ reduce rigidity, and ‘stabilizers’ protect some stage of processing but finally set and cannot be
against ultraviolet radiation. resoftened. Thermosets are generally stronger and
‘Thermoplastic’ polymers soften when heated and stiffer than thermoplastics.
can be reshaped, the new shape being retained on

6.13. I Thermoplastics Cellulose

Acetal and polyacetal ‘Cellulose nitrate’ is inflammable and has poor per-
formance in heat and sunlight. Its uses are currently
These combine very high strength, good temperature limited. Cellulose acetate has good strength, stiffness
and abrasion resistance, exceptional dimensional sta- and hardness and can be made self-extinguishing.
bility and low coefficient of thermal expansion. They Glass-filled grades are made. Cellulose acetobutyrate
compete with nylon (but with many better properties) (CAB) has superior impact strength, dimensional
and with diecastings (but are lighter). Chemical resis- stability and service temperature range and can be
tance is good except for strong acids. Typical applica- weather stabilized. Cellulose proprionate (CP) is simi-
tions are water-pump parts, pipe fittings, washing lar to CAB, but has better dimensional stability and
machines, car instrument housings, bearings and can have higher strength and stiffness. Ethyl cellulose
gears. has better low-temperature strength and lower density
than the others. Processing of cellulose plastics is by
Acrylics (methylmethacrylate, PMMA) injection moulding and vacuum forming. Applications
include all types of mouldings, electrical insulation,
These are noted for their optical clarity and are and toys.
available as sheet, rod, tubing, etc., as Perspex (UK)
and Plexiglas (USA, Germany, etc.). They are hard Ethylene-vinyl acetate (EVA)
and brittle and resistant to discolouring and weather-
ing. Applications include optical lenses and prisms,
transparent coverings, draughting instruments, reflec- This material gives tough flexible mouldings and
tors, control knobs, baths and washbasins. They are extrusions suitable for a wide temperature range. The
material may be stiffened by the use of fillers and is also
available in a wide range of transparent and opaque
colours. used for adhesives. Applications include all types of
mouldings, disposable liners, shower curtains, gloves,
inflatables, gaskets, and medical tubing. The material
Acrylonitrile-butadiene-styrene (ABS)
is considered competitive with polyvinyl chloride
(PVC), polythene and synthetic rubbers, and is also
This combination of three materials gives a material used for adhesives and wax blends.
which is strong, stiff and abrasion resistant with good
properties, except out of doors, and ease of processing.
The many applications include pipes, refrigerator Fluorocarbons
liners, car-instrument surrounds, radiator grills, tele-
phones, boat shells, and radio and television parts. These have outstanding chemical, thermal and electri-
Available in medium, high and very high impact cal properties. The four main types are described
grades. below.