are materials which are softened by heat and set into lasting form when
shaped in a mould. Some are natural; some are semi-synthetic, the result
of chemical action on a natural substance; some are synthetic, built up
from the constituents of oil or coal. All are based on the chemistry of
Carbon, with its capacity for forming chains. The molecules that compose
them (monomers) link together in the setting or curing process to form chains
(polymers), which give plastics their flexible strength. Some plastics retain
their ability to be softened and reshaped: like wax, they are thermoplastic.
Others set permanently in the shapes they are given by heat and pressure:
like eggs, they are thermosetting.
first fully synthetic plastics did not appear until the 1900's, natural
plastics which can be softened by heat and pressed into shape have existed
for centuries. These include horn, bone and tortoiseshell and have a similar
molecular structure to the later synthetic plastics.
In 1862 the
first semi-synthetic or manufactured plastic, initially called Parkesine
and later Celluloid, was formed from natural materials developed from
cellulose. Again many of the products created from this material were
decorative fashion accessories but also more mundane items such as shoehorns
and door handles.
in the development of plastics took place between the two world wars and
it is in this period that many of the plastics which are widespread today
(polystyrene, acrylic, PVC, nylon) and the techniques used to shape them
(injection moulding patented in 1926) evolved.
Thermoplastics can be shaped at high temperatures, cooled, reheated and reshaped many times without permanently changing their structure and properties. They consist of chain molecules (polymers) with weak bonding between adjacent chains. It is these bonds between the molecule chains which weaken to allow the plastic to deform when heated. They also give thermoplastics a certain flexibility at room temperature.
Plastics or thermosets have strong cross-linking bonds between polymer
chains. This cross-linking can be the result of both a chemical reaction
and heat, and the net result is a plastic which actually hardens as it
is heated and therefore offers good resistance to heat. For this reason
it is often used in applications such as pan handles and electrical fittings.
The following handout lists a number of the most common Thermosetting Plastics and Thermoplastics, their forms, properties and common uses. It is important that you are able to discuss, with confidence, a number of these plastics as they are extremely common-place in the area of Household Appliances which will be a topic for the terminal exam.
For Exam questions on this subject, click here