Important thermoplastic materials are described as follows: 1. Polyethylene or Polythene 2. Polyvinylchloride 3. Polypropylene 4. Teflon or Polytetrafluoroethylene 5. Polystyrene 6. Acrylics 7. ABS (Acrylonitric Butadiene Styrene) 8. Silicones 9. Polyvinylidene Chloride 10. Polyamides 11. Bitumen.
1. Polyethylene or Polythene (C2H4)n:
This material originated in England was produced commercially in the United States in 1943.
Polyethylenes are obtainable as various liquids, gums and tough flexible solids suitable for moulding.
Properties:
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(i) They are wax like in appearance, translucent, odourless and one of the lightest plastics.
(ii) Flexible over a wide temperature range.
(iii) High resistivity and dielectric strength.
(iv) Chemically resistant.
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(v) Do not absorb moisture.
(vi) Dielectric losses and dielectric constant are low.
(vii) They are relatively low in cost.
Uses:
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(i) High voltage (upto 30 kV) applications.
(ii) Coaxial cables.
(iii) Packaging.
(iv) Moisture proofing.
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(v) Coating ice-cube trays.
(vi) As insulation in submarine cables and radar lines.
(vii) Lining for lagoons to avoid seepage of polluted water into the underground.
(viii) Fan and blower casing (gas filled polythene).
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(ix) Pipes and tanks for water storage.
Two types of polyethylene are manufactured depending upon the condition of polymerization:
(a) High density polyethylene (HDPE).
(b) Low density polyethylene (LDPE).
(a) High Density Polyethylene (HDPE):
a. This is obtained by low pressure polymerisation process.
b. It has specific gravity 0.96 and softening temperature 120-130°C.
Typical properties of HDPE are given in Table 9.1.
(b) Low Density Polyethylene (LDPE):
This is obtained by high pressure process.
It has specific gravity 0.90 and softening temperature 86°C.
Polythene is prone to degradation and embrittlement by sunlight due to cross linking but performance can be improved by addition of black pigments such as carbon black, which absorbs rays and prevents damage.
Underground polythene is not affected by salts in the soil and has excellent resistance to salts and bacteria. However, it is attacked by strong acids. It is a good electrical insulator.
Polythenes, in particular LDPE, have high permeability to gases; they are therefore, not considered for gas service.
2. Polyvinylchloride (PVC):
This is the cheapest and most widely used plastic.
The vinylchlorides are formed from hydrochloric acid, limestone and natural gas or coal. The forms of vinylchloride are almost unlimited.
It is manufactured by addition polymerisation of vinylchloride. Polymerisation is carried out in the presence of catalyst.
It is different from polyethylene. In PVC one in four hydrogen atoms is replaced by a chlorine atom, hence it has greater rigidity.
The reaction is indicated below:
PVC can be manufactured in expanded or cellular form. It is available in two forms namely flexible and rigid. It can be easily extruded and moulded into the desired form. The joints are obtained by solvent welding.
Properties:
(i) The flexible types are strong, tear resistant, and have good ageing properties.
(ii) The rigid types have good dimensional stability and are water resistant. They are resistant to acids and alkalies.
(iii) It is attacked by aromatics and soluble in ketones and esters.
(iv) Its resistance to impact invariably deteriorates with time.
(v) It becomes soft beyond 80°C.
(vi) It is self-extinguishing when ignited and the source of flame removed.
(vii)The hard type of PVC is formulated with less plasticizer than the general purpose grade and shows less tendency to flow at high temperature which is an advantage when the cables are to be laid in very hot surroundings.
(viii) Although its electrical properties are not so good as those of rubber, it offers more resistance to oxygen, ozone and sunlight.
(ix) PVC has a tendency to decompose when it is heated or exposed to sunlight with the liberation of hydrochloric acid gas. A small amount of lead salt, lead silicate or lead stearite is blended into the polymer at the compounding stage to prevent the same.
The properties of PVC (flexible) are given in Table 9.1.
Uses:
(i) Cable jackets.
(ii) Lead-wire insulation.
(iii) Rubber substitute.
(iv) Fabric coating.
(v) Rainwater goods.
(vi) Corrugated roofing.
(vii) Flooring and ceiling panels.
(viii) Gramophone records.
(ix) Cladding.
(x) Pipes and fittings for water service (unsuitable for hot water service because of low softening point).
3. Polypropylene (PP):
It is manufactured by addition polymerization of propylene.
It is stronger and more rigid material than polyethylene owing to the presence of CH3 group attached to the linear molecular chains.
Properties:
(i) Its resistance to chemicals is better than that of polyethylene.
(ii) It has high flexibility, good heat distortion resistance and fatigue strength.
(iii) It is attacked by strong acids.
(iv) They are brittle at low temperature, translucent and flammable.
(v) They have poor resistance to ultra violet rays and weathering.
Other properties are given in Table 9.1.
Uses:
(i) Vacuum flasks
(ii) Hair dryers
(iii) Filaments and fibers
(iv) Refrigerator parts
(v) Electrical insulators
(vi) Films and sheets for packing
(vii) Flash light casings
(viii) Automotive parts
(ix) Vacuum cleaner bodies
(x) Washing machine parts
(xi) Pipes, tanks, spray nozzles
(xii) Radio and television cabinets.
4. Teflon or Polytetrafluoroethylene (PTFE):
The mer of PTFE is represented as CF2 = CF2.
PTFE is manufactured by addition polymerisation technique. Polymerisation is carried out under organic peroxide or oxygen as catalyst.
It can be fabricated into desired shapes by hot processing and sintering the polymer powder.
Properties:
(i) PTFE is completely inert, chemically (owing to the presence of strong carbon fluorine covalent bonds of extreme stability).
(ii) It is highly crystalline material and does not soften at high temperature (It retains its strength upto + 300°C and – 200°C). It loses its crystallinity on heating above 325°C.
(iii) It is non-flammable.
(iv) It is translucent/opaque in nature.
(v) It is a good electrical insulator.
(vi) It has very low coefficient of friction and it does not stick to other materials.
(vii) It has excellent resistance to most chemicals and solvents.
(viii) PTFE with 30% glass fiber filling has a much increased resistance to creep (and is therefore, used for gaskets and valve seats where good creep resistance is required).
Other properties are given in Table 9.1.
Uses:
(i) Non-stick coatings
(ii) Chemical pipes
(iii) Bearing bushes
(iv) Piston rings
(v) Anti-corrosive seals
(vi) High temperature electronic parts
(vii) Mouldings in aircraft
(viii) Seals
(ix) Pipes
(x) Gaskets
(xi) Laboratory equipments, and
(xii) Lining of chutes.
5. Polystyrene:
A high strength plastic is obtained by addition polymerisation in presence of SnCl4 catalyst.
It can be toughened by mixing another polymeric material acrylonitrile butadiene to give ABS plastics.
Properties:
(i) It has good electrical properties and negligible water absorption.
(ii) Hard and brittle.
(iii) Low dielectric constant.
(iv) High resistivity.
(v) It is transparent in nature.
(vi) Freedom from taste and colour and physiologically harmless nature makes polystyrene suitable for use in contact with foodstuffs.
(vii) Possesses excellent resistance to corrosive chemicals such as acids (except oxidising agents); alkalies and salt solutions.
Uses:
(i) Lenses
(ii) Refrigerator-cabinet compounds
(iii) Radar components
(iv) Electrical insulation
(v) Instrument panels
(vi) Food containers
(vii) Light fittings
(viii) Automobile dash blanks
(ix) Office items like desk calendars, pen stands, trays etc.
(x) Housewares like water jugs, mugs, tumblers, plates, trays etc.
(xi) Sheets and films for packaging.
6. Acrylics (Polymethyl Methacrylate PMMA):
They are the polymer of methyl methacrylate having trade name Perspex.
PMMA is manufactured by addition polymerisation of methyl methacrylate in presence of a peroxide catalyst.
Properties:
(i) It is hard, rigid and transparent; it possesses high impact strength.
(ii) It is attacked by acetone, petrol etc., but it is resistant to household chemicals.
(iii) It is highly resistant to weathering. However, it is attacked by concentrated acids, but not by alkalies.
(iv) It has low abrasion resistance and low softening temperature.
(v) It is a good electrical insulator.
Uses:
(i) Sanitary ware
(ii) Sheeting
(iii) Sinks, baths
(iv) Display signs
(v) Lenses
(vi) Hospital equipment
(vii) Roof lights and domes
(viii) Aircraft light fixtures
(ix) Light weight garments, and
(x) Manufacture of fibers.
7. ABS (Acrylonitric Butadiene Styrene):
Properties:
(i) ABS materials are tough, hard and rigid.
(ii) They possess hard surface, dimensional stability, chemical resistance and heat resistance.
(iii) They have low water absorption and resistance to creep or cold flow.
(iv) They are unaffected by water, inorganic salts, alkalies and many acids.
(v) They are soluble in ketones, aldehydes, esters and some chlorinated hydrocarbons.
Uses:
(i) Extruded sheets.
(ii) Extruded pipes.
(iii) Safety helmets.
(iv) Refrigerator lines.
(v) Protective and decorative housings for telephones, radios, business machines, portable appliances.
(vi) Electrical parts.
8. Silicones:
The basic ingredients consist of silica and coke which are mixed and placed in an electric furnace. The silica is reduced to silicon, which is one of the starting materials. The other reactant is methyl chloride, prepared by chlorinating methane gas. The two reactants are mixed in a reaction at a high temperature and moderate pressure, using copper as a catalyst. The desired products are then hydrolysed and condensed to form silicon polymers.
They are a chemical hybrid, a cross between organic and inorganic materials. They have both the stability of inorganic products and the versatility of organic.
As a moulding compound they have good qualities and the ability to withstand high heat (upto 260°C).
They are mechanically weak and have poor chemical resistance.
They are water repellent.
Uses:
They are used as high heat resistant insulations.
9. Polyvinylidene Chloride:
Properties:
(i) Acid resistant.
(ii) Adequately tough.
(iii) Non-flammable.
(iv) Solvent resistant.
Uses:
(i) Seat covers.
(ii) Bells and gaskets.
(iii) Woman fabrics.
(iv) Acid-resistant tubing.
10. Polyamides:
The general name of linear polyamides is nylon. It is produced by series of condensation reaction between an amine and organic acids. Nylon is produced from the condensation polymerisation reaction between hexaethylene diamine and adipic acid. In this process both the reactions, i.e., hexamethylene diamine and adipic acid contain six carbon atoms, thus, the, nylon so produced is called 6.6. For all nylons similar nomenclature is used.
Nylon-6 is produced by self-condensation of caprolactam which is an amino acid with six carbon atoms. The condensation reaction takes place between the acid group at one side and amino group at other side of the same molecule.
Nylon-11 is produced by self-condensation of 11 amino undecanove acid with a linear monomer containing eleven carbon atoms.
Nylon-10 is produced by condensation reaction between hexamethylene diamine and sebacic acid.
Properties of Nylon:
(i) All nylons possess good abrasion resistance.
(ii) They are very tough and strong but flexible too.
(iii) They possess high impact strength.
(iv) They absorb water which causes reduction in strength and impact properties.
(v) They are resistant to most of the solvents and chemicals.
(vi) They have very high softening temperatures and thus moulding becomes difficult.
Uses:
(i) The main use of nylon is in the manufacture of fiber and yarn.
(ii) The nylons are also used for moulding gears, valves, bearings, containers and electrical equipment.
11. Bitumen:
It is the oldest known plastic and was employed about 3000 B.C.
The bitumen plastics of today are made from bituminous products by mixing with a suitable filter, such as slate dust or asbestos in powder form.
They have good electrical insulating properties.
Uses:
They have been used for battery-cell plugs, stoppers, electrical-mouldings of the cheaper class etc.