Page 18 - Plastics Engineering

CHAPTER 1 - General Properties of Plastics

1.1 Introduction

It would be difficult to imagine our modem world without plastics. Today
they are an integral part of everyone’s lifestyle with applications varying from
commonplace domestic articles to sophisticated scientific and medical instru-
ments. Nowadays designers and engineers readily hun to plastics because they
offer combinations of properties not available in any other materials. Plastics
offer advantages such as lightness, resilience, resistance to corrosion, colour
fastness, transparency, ease of processing, etc., and although they have their
limitations, their exploitation is limited only by the ingenuity of the designer.
The term plastic refers to a family of materials which includes nylon,
polyethylene and PTFE just as zinc, aluminium and steel fall within the family
of metals. This is an important point because just as it is accepted that zinc
has quite different properties from steel, similarly nylon has quite different
properties from ITFE. Few designers would simply specify metal as the mate-
rial for a particular component so it would be equally unsatisfactory just to
recommend plastic. This analogy can be taken still further because in the same
way that there are different grades of steel there are also different grades of,
say, polypropylene. In both cases the good designer will recognise this and
select the most appropriate material and grade on the basis of processability,
toughness, chemical resistance, etc.
It is usual to think that plastics are a relatively recent development but in
fact, as part of the larger family called polymers, they are a basic ingredient of
animal and plant life. Polymers are different from metals in the sense that their
structure consists of very long chain-like molecules. Namal materials such as
silk, shellac, bitumen, rubber and cellulose have this type of structure. However,
it was not until the 19th century that attempts were made to develop a synthetic
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