Thermoplastic Polymers

Thermoplastic polymers soften and supple on heating and 'solidify' on cooling. The heating and cooling cycle within a certain temperature limit can be applied to these materials several times without affecting their properties. Polymers which generally have a linear structure fall in this category. The synthetic thermoplastic materials. used in electrical engineering for insulation purpose are; polyethylene (PE), polyvinylchloride (PVC), polypropylene (PP) and polyamide (PA). The extent of their application in the industry is in the order of their above mentioned sequences. Such materials have relatively poor thermal resistance and their properties deteriorate rapidly at higher temperature.

Thermoset Polymers

The polymers which soften when heated for the first time resulting into cross-linking reaction (network formation), are known as thermoset polymers. This reaction, leading to the formation of network structure, is also known as curing or setting of the polymer. All monomers having functionality equal to two would give rise to linear polymers and, therefore, thermoplastic materials. However, when they have functionality more than two, the resulting polymer has a network structure. The polymers used for electrical insulation purpose are desired to retain their rubbery (flexible) properties. Hence, these are defined as 'lightly cross-linked polymers'.

The chemical reaction leading to cross-linking is achieved with the help of an additive, known as 'agent'. The term 'cross-linking agent' is very general. It refers to (a) molecules which bridge two polymer molecules, for example, vulcanizing agents; such as sulphur, selenium and sulphur monochloride for rubbers; and polyamines in expoxide resins, (b) molecules which initiate a cross-linking reaction, for example, peroxides which initiate a double bond polymerisation in polyesters and dicumyl peroxide in low density polyethylene (LDPE); (c) those which are purely catalytic in their action such as, acids for phenolic resins, amines in epoxides; and (d) active site generators, for example, peroxides which abstract protons from the polymer chains.

The thermal, mechanical and electrical properties of different cross-linked materials vary considerably from each other. However, in general, after cross­linking a material does not soften on reheating, instead it becomes thermally more stable compared to thermoplastic materials. Cross-linked polymer resins are, for example; polyester-resin, phenolresin, siliconresin and the most widely used in electrical engineering are the expoxyresins. Among rubbers, typical examples used for electrical insulation are; natural rubber also known as 'India rubber', and a large number of synthetic rubbers like silicon rubber (SiR), ethylene-propylene rubber (EPR), etc.

The bulk properties of a polymer can be altered suitably by incorporation of a number of additives. Variations in the choice of additives can produce widely differing products. This is true in particular with PVC. In terms of functions, there are a large number of groups of additives, of which the following most important are commonly used in preparing polymer compounds for electrical insulation purpose:

1. Fillers-usually applied to modify physical properties, mainly mechanical, of a polymer.
2. Plasticisers and Softeners-to lower the melt viscosity and also to change physical properties (softness, flexibility).
3. Colorants-normally soluble colorants(dyestuffs) are used.
4. Anti-ageing additives to prevent structural degradation due to chemical reactions like oxidation, ozone attack, dehydroclorination (especially with PVC) and ultra-violet attack, e.g. sunlight.
5. Flame retarders - to improve the degree of fire resistance of polymers.
6. Cross-linking agents-to achieve intermolecular combination at the chain ends.
   

Some very high quality solid insulating systems have been produced by combining different dielectric materials. Common composites of organic and inorganic materials are; fibre glass reinforced plastics, mica based plastic tapes, quartz, fibre and mica mixed with synthetic resins, such as epoxyresin used in electrical machines, etc. Oil and wax based compound impregnated insulation systems with paper and polymer tapes are commonly applied in power cables, capacitors and transformers. 'Uniaxially oriented polyethylene' (UOPE) tape, developed recently has been found to have very good compatibility with oils besides having good mechanical properties. In such composite insulating systems, the advantages of inherent properties of their constituent  materials are made use of. Thus, it is possible to create insulating systems having better thermal, mechanical and electrical properties.