Many thermoplastics are now accepted as engineering materials and some are distinguished by the loose description engineering plastics. The term probably originated as a classification distinguishing those that could be substituted satisfactorily for metals such as aluminium in small devices and structures from those with inadequate mechanical properties. This demarcation is clearly artificial because the properties on which it is based are very sensitive to the ambient temperature, so that a thermoplastic might be a satisfactory substitute for a metal at a particular temperature and an unsatisfactory substitute at a different one.
A useful definition of an engineering material is that it is able to support loads more or less indefinitely. By such a criterion thermoplastics are at a disadvantage compared with metals because they have low time-dependent
moduli and inferior strengths except in rather special circumstances. However, these rather important disadvantages are off-set by advantages such as low density, resistance to many of the liquids that corrode metals and above all, easy processability . Thus, where plastics compete successfully with other materials
in engineering applications it is usually because of a favourable balance of properties rather than because of an outstanding superiority in some particular respect, although the relative ease with which they can be formed into complex shapes tends to be a particularly dominant factor. In addition to conferring the
possibility of low production costs, this ease of processing permits imaginative designs that often enable plastics to be used as a superior alternative to metals rather than merely as a tolerated substitute.
Currently the materials generally regarded as making up the engineering plastics group are Nylon, acetal, polycarbonate, modified polyphenylene oxide (PPO), thermoplastic polyesters, polysulphone and polyphenylene sulphide. The newer grades of polypropylene also possess good basic engineering performance and this would add a further 0.5 m tonnes. And then there is unplasticised polyvinyl chloride (uPVC) which is widely used in industrial pipework and even polyethylene, when used as an artificial hip joint for example, can come into the reckoning. Hence it is probably unwise to exclude any plastic from consideration as an engineering material even though there is a sub-group specifically entitled for this area of application.
In recent years a whole new generation of high performance engineering plastics have become commercially available. These offer properties far superior to anything available so far, particularly in regard to high temperature performance, and they open the door to completely new types of application for plastics.
The main classes of these new materials are
(i) Polyarylethers and Polyarylthioethers
polyarylethersulphones (PES)
polyphenylene sulphide (PPS)
polyethernitrile (PEN)
polyetherketones (PEK and PEEK)
(ii) Polyimides and Polybenzimidazole
polyetherimide (PEI)
thermoplastic polyimide (PI)
polyamideimide (PAI)
(iii) Fluompolymers
fluorinated ethylene propylene (FEiP)
perfluoroalkoxy (PFA)
A number of these materials offer service temperatures in excess of 200°C and
fibre-filled grades can be used above 300°C.
PLASTICS
ENGINEERING
Third Edition
R.J. Crawford, BSc, PhD, DSc, FEng, FIMechE, FIM
Department of Mechanical, Aeronautical
and Manufacturing Engineering
The Queen’s University of Belfast
l E I N E M A N N
OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS
SAN DlEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO
Butterworth-Heinemann
An imprint of Elsevier Science
Linacre House. Jordan Hill, Oxford OX2 8DP
225 Wildwood Avenue, Woburn, MA 01801-2041
First published 1981
Second edition 1987
Reprinted with corrections 1990. 1992
Third edition 1998
Reprinted 1999.2001, 2002
Copyright 0 1987, 1998 R.J. Crawford. All rights reserved
The right of R.J. Crawford to be identified as
the author of this work has been asserted in
accordance with the Copyright. Designs and
Patents Act 1988
