Basic Technology of Plastics - Introduction to Polymeric Composites with Rice Hulls - page 14

At one time, all organic compounds came from plants or animals. However, today
coal, oil and natural gas are good sources of carbon for the chemicals used in the
manufacture of plastics. Many organic hydrocarbons are obtained directly from
animal or plant sources. Oils such as cottonseed, linseed, soybean oils, and lard yield
hydrocarbons for the production of plastics. Scientists and chemists are constantly
finding new sources that can be used as bases. In the field of hydrocarbons, there are
two major organic groups to consider: (a) aliphatic or straight-chain molecules, and
(b) cyclic or ring-shaped molecules. Aliphatic hydrocarbons may be divided further
into three types of hydrocarbon: (a) alkanes or paraffins; (b) alkenes or olefins; and
(c) alkynes or acetylenes.
2.2.4 Polymers
If a large number of molecules bond together in a regular pattern, a polymer is formed.
A polymer is made up of many units repeated in a pattern. This repeating unit (mer)
should be considered to be bi-functional or di-functional (two reactive bonding sites).
The chemical reaction that joins monomers (mers) together is called polymerisation .
If the polymer consists of similar repeating units, it is known as a homopolymer
(meaning same). If two different monomers are polymerised, the resulting polymer
is called a co-polymer. If three different monomers are polymerised, a terpolymer is
the product. Polymers are the basic building blocks for the manufacture of the vast
range of plastics. Plastics are divided broadly into two main categories: thermoplastic
(soften on heating and can be re-used) and thermosetting (harden on heating and
cannot be re-used).
2.2.5 Polymerisation
Polymerisation processes can be classified into two-step polymerisation and chain
polymerisation . Linear polymerisation and non-linear step-growth polymerisation are
processes in which the polymerisation occurs with more than one molecular species.
Also, chain-growth polymerisation processes occur with monomers with a reactive end
group. Chain-growth polymerisation processes include free-radical polymerisation,
cationic polymerisation, ring-opening polymerisation and metallocene-catalysis
polymerisation.
The atomic composition of polymers consists primarily of non-metallic elements
such as carbon (C), hydrogen (H) and oxygen (O). In addition, recurrent elements
are nitrogen (N), chlorine (C), fluorine (F) and sulfur (S). Polymers may also contain
other non-metallic elements such as silicon (Si) in silicone or beryllium (B).