There are thousands of insulating materials available in the market. Insulations technology is one of the few branches where the number of materials is very large. Any special requirement can be served by some special material. The requirement of good insulating materials can be classified as electrical, mechanical and chemical. Electrically the insulating material has high resistivity to reduce the leakage current to reduce the high dielectric strength to enable it to withstand higher voltage without punctured or broken down. Since the insulators are use on the basis of volume and not weight, A low density is preferred similarly a uniform viscosity for liquid insulates insures uniform thermal electrical properties. Chemically the insulators should be resisting of oils, liquid gas, fumes, acids, and alkalis. It should not deteriorate by the action of chemical in the acids or by contact with other metals. Insulating materials must have some mechanical properties on the basis of the use to which they are put. Thus when used for electric machine insulation the insulator should have sufficient mechanically strength to vibration. Hence an ideal insulating material should possess:
(1)Dielectric strength as good as that of mica,
(2)Volume and surface resistivity equal to that of sulphur,
(3)Mechanical strength as good as that of steel,
(4)Crushing resistance as that as good as granite,
(5)Ease of machining as that as good as that of wood,
(6)Fire proofing as good AS that of silica,
(7)Chemical inertness is equal to that of platinum,
(8)Surface finish like that of ebonite,
(9) Water proofing quality as good as that of paraffin wax.