Page 21 - Plastics Engineering

4 General Properties of Plastics
When the material is allowed to cool it solidifies again. This cycle of softening
by heat and solidifying on cooling can be repeated more or less indefinitely
and is a major advantage in that it is the basis of most processing methods for
these materials. It does have its drawbacks, however, because it means that the
properties of thermoplastics are heat sensitive. A useful analogy which is often
used to describe these materials is that, like candle wax, they can be repeatedly
softened by heat and will solidify when cooled.
Examples of thermoplastics are polyethylene, polyvinyl chloride, polysty-
rene, nylon, cellulose acetate, acetal, polycarbonate, polymethyl methacrylate
and polypropylene.
An important subdivision within the thermoplastic group of materials is
related to whether they have a crystalline (ordered) or an amorphous (random)
structure. In practice, of course, it is not possible for a moulded plastic to
have a completely crystalline structure due to the complex physical nature of
the molecular chains (see Appendix A). Some plastics, such as polyethylene
and nylon, can achieve a high degree of crystallinity but they are probably
more accurately described as partially crystalline or semi-crystalline. Other
plastics such as acrylic and polystyrene are always amorphous. The pres-
ence of crystallinity in those plastics capable of crystallising is very depen-
dent on their thermal history and hence on the processing conditions used
to produce the moulded article. In turn, the mechanical properties of the
moulding are very sensitive to whether or not the plastic possesses crys-
tallinity.
In general, plastics have a higher density when they crystallise due to the
closer packing of the molecules. Qpical characteristics of crystalline and amor-
phous plastics are shown below.

I Amorphous Crystalline

Broad soflening range - thermal Sharp melting point - the
agitation of the molecules breaks regular close-packed structure
down the weak secondary bonds. results in most of the
The rate at which this occurs secondary bonds being broken
throughout the formless structure down at the same time.
varies producing broad a Usually opaque - the
temperature range for softening. difference in refractive indices
Usually transparent - the looser between the two phases
structure transmits light so the (amorphous and crystalline)
material appears transparent. causes interference so the
Low shrinkage - all material appears translucent or
thermoplastics are processed in opaque.
the amorphous state. On High shrinkage - as the
solidification, the random material solidifies from the