PLASTICS
1. A
plastic in a broadest sense is defined as any non-metallic material that can be
moulded to shape.
2. 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.
3. Most
of the plastics are of organic nature composed of hydrogen, oxygen, carbon and
nitrogen.
4. The
synthetic plastic development dates from 1909 when Dr Bakeland announced the
production of phenol-formaldehyde.
5. Since
then several new plastics have been developed. The plastics possess infinite
variety of properties.
6. 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
7. Low
electrical conductivity Plastics, most commonly, are classified as
(1)
Thermoplastic and (2) Thermosetting.
8. 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.
9. Thermoplastic Materials:
a.
Polyethylene
or Polythene: Polythenes are obtainable as viscous
liquids, gums and tough flexible solids suitable for moulding. They have wax like
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.
b.
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 800 C. It is self extinguishing when ignited and the source of flame is
removed. It offers more resistance to oxygen, ozone and sunlight.
1.
Softening temperature..……. 1200C
2.
Insulation resistance…….….. 1012-1013
3.
Dielectric strength [kV/mm] ……. 30
c.
For example PVC is difficult to ignite
and in the absence of a powerful external flame will not continue to burn. This
is due to its chlorine compound. This makes it an ideal construction and cable
material. The incineration (burning) of PVC causes the release of toxic
chemicals like dioxins and other chemicals that are harmful to humans.
10. Thermosetting Plastics Aminos
a.
Urea
formaldehyde resins: They are derived from the reaction of
urea with formaldehyde or its polymers. These resins cannot offer high
resistance to heat.
b.
Melamines:
When the resin is used with asbestos or glass fibre as filler material, its
heat resistance is in the range of 2000 C. It is highly resistant to chemicals.
Possess outstanding electric arc resistance. Excellent resistance to water.
Available in full range of transluscent or opaque colours. Boards are made from
these material are used as distributor heads, casings for electric devices,
terminal boards.
c.
Phenolics
[Phenol formaldehyde resin]: They are made by a
reaction between phenol and formaldehyde. They are probably the most widely
used and cheapest of thermosetting plastics. Strong, rigid and dimensionally
stable. Heat and solvent resistant. Non-conductors of electricity. Used as
Electrical appliance handles, TV and Radio cabinets
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