188 HANDBOOK OF ELECTRICAL ENGINEERING
9.2.1.3 Polymer
A polymeric compound contains several different molecules e.g., carbon, hydrogen, oxygen, silicon,
chlorine, sulphur. These molecules combine in small groups usually with a carbon molecule in the
middle. The group is repeated linearly many times in the form of a chain.
Polymers can be natural or synthetic materials, which include but are not limited to rubbers,
elastomers and plastics.
When two polymers are combined the resulting compound is called a copolymer.9.2.1.4 Plastic
This is a very widely used term to describe typical household, automobile and industrial components
that are moulded from man-made chemical compounds. In the electrical engineering field the term
has a more specific definition, especially where insulation materials are being described.
Plastic materials are those that are formed from synthetic compounds e.g., polymers or natural
compounds that have previously been modified, for example hydrocarbons refined from crude oil,
natural gas or derivatives of ethane, methane and naphtha. By definition the manufacturing of a
plastic component should include a viscous flowing process that usually requires heat and pressure,
for example extrusion or injection moulding.
Two important groups of insulating and sheathing materials that also come within the definition
of plastic are the thermoplastic and thermoset polymeric compounds.
9.2.1.5 Resin
The preferred term for synthetic polymers is resin polymers or simply resins. Hence the two main
groups are thermoplastic resins and thermoset resins.
9.2.1.6 Thermoplastic resins
Thermoplastic resins (elastomers and polymers) are plastics that retain their flexibility and chemical
composition when heat is applied and removed. The applied heat is only sufficient to steadily melt
the resin.
The most widely used thermoplastic resins used in cables are polyvinyl chloride (in various
forms), polyethylene (in several forms), polypropylene and polytetrafluoroethylene.
In general these resins in their basic forms do not have sufficiently good properties to make
them attractive as materials for cables. The necessary properties for insulation are usually different
from those required for sheathing and bedding, even though the same basic resin may be used for these
purposes. Other substances are polymerised or mixed with the basic resin during its manufacture.
These substances are used to improve or provide:-
- High dielectric strength.
- Low dielectric loss angle.
- High insulation resistivity.