Page 203 - Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 187

9.2 ELECTRICALLY NON-CONDUCTING MATERIALS USED IN THE
CONSTRUCTION OF CABLES

9.2.1 Definition of Basic Terminology

Some terms are used loosely in various engineering disciplines e.g. plastic, rubber. However,
they have particular connotations in electrical engineering, especially in the field of cable
manufacturing.

9.2.1.1 Rubber

Rubber is obtained in two basic forms, natural rubber and synthetic rubber. Natural rubber is the sap
of the particular species of trees called Hevea brasiliensis, see References 3 and 4, which is sticky
when at tropical temperature, reasonably hard at low temperatures and oxidises when exposed to
the atmosphere. Natural rubber is a naturally occurring compound of carbon and hydrogen, and is
of little use as a basic material. It is therefore mixed with other chemical compounds, filler mate-
rials such as carbon black and then vulcanised to produce ‘vulcanised rubber’ or more generally
called simply ‘rubber’. The vulcanising process requires sulphur to be added and the application of
heat and pressure. The molecules of rubber are formed in long chains. Individual chains are not
bonded to adjacent chains; hence the chains can slide alongside each other with little resistance
to movement. This gives processed rubber the ability to recover without permanent deformation.
Natural rubber does not necessarily recover to its original shape, since its stability depends on
its ambient temperature. Vulcanising or ‘curing’ causes the sulphur to cross-bond adjacent chains,
which stiffens the material thereby making it more useful. By increasing the sulphur content or
extending the vulcanising time, or a combination of both functions, the rubber becomes progres-
sively harder with higher tensile strength. Increasing additives such as carbon black can reduce
the dielectric strength, thereby making the rubber a poorer insulator. Carbon by itself is of course
a conductor.
Synthetic rubbers are also composed of carbon and hydrogen molecules, but they are com-
bined by manufacturing processes. A synthetic rubber, which closely resembles natural rubber, is
polyisoprene, which has the same chemical composition.
Reference 5 also describes many types of insulating materials.

9.2.1.2 Elastomer

The term ‘elastomer’ is the most appropriate technical term for rubber, and is generally applied to
synthetic rubbers, e.g. ethylene propylene rubber. It derives its name from the well-known elastic
property of rubber.

However some non-rubber compounds are also called elastomers if they exhibit a non-
deforming elastic property similar to rubber at room temperature, even if the compound is relatively
hard. The two main groups of non-rubber elastomers are thermoplastics, e.g., polyvinyl chloride,
polypropylene and thermosets, e.g., ethylene propylene rubber, cross-linked polyethylene. These two
groups are also covered by the term ‘plastic’.