What is
a plastic?
When we talk of a substance
being “plastic” (the adjective) most
people would agree on its meaning. It can flow or
be moulded, it is ductile or it can be shaped,
but when we turn to defining “plastics”
(the noun) we have problems. Almost any simple
definition will exclude materials which everyone
would agree should be included so we have to turn
to scientific terms and start with the comment
that plastics are all polymers (poly = many).
Natural polymers
have been with us since time began but synthetic
polymers are much more recent, their origins
being traced to Alexander Parkes and his exhibits
of Parkesine at the International Exhibition of
London in1862. Natural polymers include shellac,
tortoiseshell and horn, as well as many resinous
tree saps. All these have been processed with
heat and pressure into articles such as hair
combs and items of jewellery for many centuries.
A polymer
is simply a very large molecule made up of many
smaller units joined together, generally end to
end, to create a long chain. The smallest “building
block” of a polymer is called a monomer
(mono = one) and if all the monomers are
chemically the same, then that polymer is called
a homopolymer.
Monomers generally
contain carbon and hydrogen with, sometimes,
other elements such as oxygen, nitrogen, chlorine
or fluorine. Perhaps the most common example of a
synthetic homopolymer today is polyethylene or
“polythene” whilst other common
materials are polypropylene, polystyrene and poly(vinyl
chloride), more commonly know as “PVC”.
Sometimes two monomers are used, monomer “A”
and monomer “B” which join together
chemically in an alternating sequence: -A-B-A-B-A-B-
etc. to give a copolymer. Examples of this type
are the nylon family, the most common member
being called nylon 66 for the simple reason that
“A” and “B” both contain 6
carbon atoms in their respective monomers. From
here on there are a number of possibilities.
“A” and “B” could couple
randomly, short chains of “A” could
couple to short chains of “B” and there
is even the possibility of the inclusion of a
third monomer, monomer “C”. These can
be very simply visualised by thinking of
Christmas “paper chains” with each link
being one monomer and different colours used to
illustrate each type. Another example would be
the classic plastic necklace made of “poppets”.
Polymers are divided into
two distinct groups: thermoplastics and
thermosets. The thermoplastics
are those which, once formed, can be heated and
reformed over and over again. This property
allows for easy processing and facilitates
recycling.
Thermosets
cannot be reformed or remoulded. Once these
polymers are formed in a particular shape, that
is it! The earliest of all synthetic plastics,
Parkesine, was of the former type whilst the
first of all truly synthetic plastics, Bakelite,
belongs to the latter category. Thermosets differ
from thermoplastics chemically in that heating
the former introduces a three-dimensional network
to the long chains so that they are no longer
able to flow freely past one another like they
can in the case of the thermoplastics.
Whilst all plastics are
polymers, not all polymers are plastics. Rubbers,
more properly today called “elastomers”
are also polymers and everything written above
about the structure of plastics applies to
elastomers. Whilst everyone would claim to be
able to distinguish between a rubber and a
plastic, there are many plastics on the market
which have some elastic character and the
distinction becomes blurred at the interface.
Plasticized PVC is elastic enough to be used in
applications where rubbers would be an equally
valid choice whilst the Kratons
are polymers of the (“A-A-A-A-A-A-”
coupled to “B-B-B-B-B-B-B-”) type where
the “A”’s are plastic segments and
the “B”’s rubbery ones. These
behave as thermoplastic materials on heating but
like vulcanized rubbers at ambient temperature.
Raw elastomers are of course thermoplastic but
become thermosets after vulcanization.
|