Page 20 - Plastics Engineering

General Properties of Plastics 3
in polymer chemistry in order to use plastics. Those who wish to to have a
general introduction to the structure of plastics may refer to Appendix A.
The words polymers and plastics are often taken as synonymous but in
fact there is a distinction. The polymer is the pure material which results from
the process of polymerisation and is usually taken as the family name for
materials which have long chain-like molecules (and this includes rubbers).
Pure polymers are seldom used on their own and it is when additives are
present that the term plastic is applied. Polymers contain additives for a number
of reasons. The following list outlines the purpose of the main additives used
in plastics.
Antistatic Agents. Most polymers, because they are poor conductors of
current, build up a charge of static electricity. Antistatic agents attract mois-
ture from the air to the plastic surface, improving its surface conductivity and
reducing the likelihood of a spark or a discharge.
Coupling Agents. Coupling agents are added to improve the bonding of the
plastic to inorganic filler materials, such as glass fibres. A variety of silanes
and titanates are used for this purpose.
Fillers. Some fillers, such as short fibres or flakes of inorganic materials,
improve the mechanical properties of a plastic. Others, called extenders, permit
a large volume of a plastic to be produced with relatively little actual resin.
Calcium carbonate, silica and clay are frequently used extenders.
Flame Retardants. Most polymers, because they are organic materials, are
flammable. Additives that contain chlorine, bromine, phosphorous or metallic
salts reduce the likelihood that combustion will occur or spread.
Lubricants. Lubricants such as wax or calcium stearate reduce the viscosity
of the molten plastic and improve forming characteristics.
Pigments. Pigments are used to produce colours in plastics.
Plasticisers. Plasticisers are low molecular weight materials which alter the
properties and forming characteristics of the plastic. An important example is
the production of flexible grades of polyvinyl chloride by the use of plasticisers.
Reinforcement. The strength and stiffness of polymers are improved by
adding fibres of glass, carbon, etc.
Stabilisers. Stabilisers prevent deterioration of the polymer due to environ-
mental factors. Antioxidants are added to ABS, polyethylene and polystyrene.
Heat stabilisers are required in processing polyvinyl chloride. Stabilisers also
prevent deterioration due to ultra-violet radiation.
There are two important classes of plastics.

(a) Thermoplastic Materials
In a thermoplastic material the very long chain-like molecules are held
together by relatively weak Van der Waals forces. A useful image of the struc-
ture is a mass of randomly distributed long strands of sticky wool. When the
material is heated the intermolecular forces are weakened so that it becomes
soft and flexible and eventually, at high temperatures, it is a viscous melt.