Specifications

resistance, and mechanical spacing of conductors in low voltage applications. Often the properties

of such fabrics are upgraded by impregnation with a varnish after application. Better results are

generally obtained if the fabric is impregnated prior to application.

1.8 PLASTICS : A plastic in a broadest sense is defined as any non-metallic material that can

be moulded to shape. The most common definition for plastics is that they are natural or synthetic

resins, or their compounds which can be moulded, extruded, cast or used as films or coatings.

Most of the plastics are of organic nature composed of hydrogen, oxygen, carbon and nitrogen.

The synthetic plastic development dates from 1909 when Dr Bakeland announced the production

of phenol-formaldehyde. Since then several new plastics have been developed.

The plastics possess infinite variety of properties. Properties common to most of the plastics

are given below :

1. Light weight. 2. Low thermal conductivity 3. A wide range of colours

4. Resistance to deterioration by moisture 5. Low electrical conductivity

Plastics, most commonly, are classified as(1).Thermoplastic and (2) Thermosetting.

Thermoplastic materials are those which soften on the application of heat, with or without

pressure but they require cooling to set them to shape.

Thermosetting materials are those plastics which require heat and pressure to mould

them into shape.

Thermoplastic Materials:

Polyethylene or Polythene : Polythenes are obtainable as viscous liquids, gums and tough

flexible solids suitable for moulding. They have waxlike in appearance, semi-transparent, odourless

and one of the lightest plastics. Flexible over a wide temperature range. High resistivity and

dielectric strength. Chemically resistant. Do not absorb moisture. Dielectric losses and dielectric

constant are low. They are relatively cheaper in cost.

Polyvinyl chloride (PVC) : The vinyl chlorides are formed from hydrochloric acid, limestone,

and natural gas or coal. The forms of vinyl chloride are almost unlimited. PVC is used in electric

and electronic equipment such as circuit boards, cables, electrical boxes, computer housing,

insulation and adhesive tapes.

The flexible types are strong, tear resistant and have good ageing properties. The rigid

types have good dimensional stability and are water resistant. They are resistant to acids and

alkalies. It becomes soft beyond 80

C. It is self extinguishing when ignited and the source of

flame is removed. It offers more resistance to oxygen, ozone and sunlight.

Softening temperature ..……. 120

Insulation resistance… ….….. 10

-10

Dielectric strength [kV/mm] ……. 30

For example PVC is difficult to ignite and in the absence of a powerful external flame will not