CABLES, WIRES AND CABLE INSTALLATION PRACTICES 189- High melting temperature.
- High tensile strength and resistance to mechanical impact.
- Good flexibility.
- Good handling properties for installation and termination.
- Good resistance to ultraviolet light.
- Good dimensional stability.
- Long service life.
- Low water absorption.
- Low emission of smoke and acid gases during fire situations.
- Low flammability.
- Low solubility in mineral oils (drilling mud), acids, alkalis, organic compounds and solvents.
- Good extrusion performance.
- High resistance to ozone.
Not all of the above can be optimised for a particular type of cable. Some sacrifices need
to be made when a particular property or overall performance is to be obtained. For example good
performance during a fire inside a building where people are present and smoke and gas must
be minimised.
9.2.1.7 Thermoset resins
Thermoset resins do not melt when heated, and are irreversibly changed after the heat is removed.
They are produced from a two-stage chemical process. The second stage is called ‘cross-linking’, and
is similar to vulcanising. The process requires heat, pressure, catalysts, or irradiation, or a combination
of these additives to produce the desired material.
Thermoplastic resins can be further processed by ‘cross-linking’ techniques to produce ther-
mosets.
The thermoset elastomers most frequently used for cables are ethylene propylene rubber and
cross-linked polyethylene.
Additives are used in a similar manner to produce the same improvements as listed in sub-
section 9.2.1.6.
Thermosets are widely used as sheet type insulating materials, adhesives, jointing compounds
for cables and solid mouldings such as post insulators.
See Table 9.5 for electrical and physical properties of thermoset resins.9.2.1.8 Electrical and physical properties of thermoplastic and thermoset resins
There are many thermoplastic and thermoset resins but only a few developed for use in manufacturing
cables. Table 9.5 summarises the main properties of the more frequently used resins.