The main requirements of the insulating materials used for power apparatus are:
1. High insulation resistance
2. High dielectric strength
3. Good mechanical properties i.e tenacity and elasticity
4. It should not be affected by chemicals around it
5. It should be non-hygroscopic because the dielectric strength of any material goes very much down with moisture content
VULCANIZED RUBBER: Rubber in its natural form is highly insulating but it absorbs moisture readily and gets oxidized into a resinous material; thereby it loses insulating properties. When it is mixed with sulphur along with other carefully chosen ingredients and is subjected to a particular temperature it changes into vulcanized rubber which does not absorb moisture and has better insulating properties than even the pure rubber. It is elastic and resilient.
The electrical properties expected of rubber insulation are high breakdown strength and high insulation resistance. In fact the insulation strength of the vulcanized rubber is so good that for lower voltages the radial thickness is limited due to mechanical consideration.
The physical properties expected of rubber insulation are that the cable should withstand normal hazards of installation and it should give trouble-free service.
Vulcanized rubber insulated cables are used for wiring of houses, buildings and factories for low-power work.
There are two main groups of synthetic rubber material.
1) General purpose synthetics which have rubber-like properties and
2) Special purpose synthetics which have better properties than the rubber e.g., fire resisting and oil resisting properties.
The four main types are: (i) butyl rubber, (ii) silicon rubber, (iii) neoprene, and (iv) styrene rubber.
BUTYL RUBBER: The processing of butyl rubber is similar to that of natural rubber but it is more difficult and its properties are comparable to those of natural rubber. The continuous temperature to which butyl rubber can be subjected is 85°C whereas for natural rubber it is 60°C. The current rating of butyl insulated cables is approximately same as those of paper or PVC insulated cables. Butyl rubber compound can be so manufactured that it has low water absorption and offers interesting possibilities for a non-metallic sheathed cable suitable for direct burial in the ground.
SILICONE RUBBER: It is a mechanically weak material and needs external protection but it has high heat resistant properties. It can be operated at temperatures of the order of 150°C. The raw materials used for the silicon rubber are sand, marsh gas, salt, coke and magnesium.
NEOPRENE: Neoprene is a polymerized chloro-butadiene. Chloro-butadiene is a color less liquid which is polymerized into a solid varying from a pale yellow to a darkish brown color. Neoprene does not have good insulating properties and is used up to 660 V AC but it has very good fire resisting properties and therefore it is more useful as a sheathing material.
STYRENE RUBBER: Styrene is used both for insulating and sheathing of cables. It has properties almost equal to the natural rubber.