Page 19 - Plastics Engineering

2 General Properties of Plastics
polymeric material and the first success was based on cellulose. This was a
material called Purkesine, after its inventor Alexander Parkes, and although
it was not a commercial success it was a start and it led to the development
of Celluloid. This material was an important breakthrough because it became
established as a good replacement for natural materials which were in short
supply - for example, ivory for billiard balls.
During the early 20th century there was considerable interest in these new
synthetic materials. Phenol-formaldehyde (Bakelite) was introduced in 1909,
and at about the time of the Second World War materials such as nylon,
polyethylene and acrylic (Perspex) appeared on the scene. Unfortunately many
of the early applications for plastics earned them a reputation as being cheap
substitutes. It has taken them a long time to overcome this image but nowadays
the special properties of plastics are being appreciated, which is establishing
them as important materials in their own right. The ever increasing use of
plastics in all kinds of applications means that it is essential for designers and
engineers to become familiar with the range of plastics available and the types
of performance characteristics to be expected so that these can be used to the
best advantage.
This chapter is written as a general introduction to design with plastics. It
outlines the range of plastics available, describes the type of behaviour which
they exhibit and illustrates the design process involved in selecting the best
plastic for a particular application.

1.2 Polymeric Materials
Synthetic large molecules are made by joining together thousands of small
molecular units known as monomers. The process of joining the molecules
is called polymerisation and the number of these units in the long molecule
is known as the degree of polymerisation. The names of many polymers
consist of the name of the monomer with the suffix poly-. For example, the
polymers polypropylene and polystryene are produced from propylene and
styrene respectively. Names, and symbols for common polymers are given
in Appendix F.
It is an unfortunate fact that many students and indeed design engineers
are reluctant to get involved with plastics because they have an image of
complicated materials with structures described by complex chemical formulae.
In fact it is not necessary to have a detailed knowledge of the structure of
plastics in order to make good use of them. Perfectly acceptable designs are
achieved provided one is familiar with their performance characteristics in
relation to the proposed service conditions. An awareness of the structure of
plastics can assist in understanding why they exhibit a time-dependent response
to an applied force, why acrylic is transparent and stiff whereas polyethylene
is opaque and flexible, etc., but it is not necessary for one to be an expert