Q.4 Define rubbers (elastomers). Explain their characteristics. Also explain various types of rubber.
Ans. A chemical polymer which sustains reversible elongation of several hundred percent is known as elastomer (rubber). Elastomer is derived from the words elastic and mer.
Characteristics of Elastomer: -
(a) Material is very soft and its modulus of elasticity is low.
(b) A very high straining is possible.
(c) Stress is reversible.
(d) Elastomer is non-crystalline and is above its glass transition temperature.
Classification of Rubber: -
(i) Natural Rubber: -
It is a hydrocarbon polymer of isoprene units. It is obtained from latex which is a milky emulsion tapped from the bark of trees. Solids in the latex are coagulated by the addition of either acetic acid or formic acid. They are then separated from liquid by squeezing through the rolls. The rubber sheets are then washed out and dried up. Crude rubber is then vulcanized by combining it chemically with certain amount of sulphur at high temperatures. Vulcanization improves the physical properties of rubber to a greater extent, giving it much higher tensile strength, a smaller permanent set, high resistance to solvents and lesser susceptibility to changes in temperatures. Natural rubber has great resistance to abrasion, fatigue and possesses high frictional properties.
Application: -
Natural rubber are used in the manufacturing of tires, seals, couplings, etc.
(ii) Synthetic Rubbers: -
(a) Butadiene-styrene (SBR): -
Rubbers of this kind are co-polymers of butadiene and styrene. This can be vulcanized with sulphur. These rubbers are highly resistant to abrasion and weathering,
Application: -
Car tyres, shoe soles, flooring, electric insulation, etc.
(b) Neoprene: -
They are the polymers of chloroprene. In Neoprene, the basic raw material is butadience. They are vulcanized with metal oxides and cannot be vulcanized to hard rubber. Neoprene have outstanding resistance to ozone, weathering, a variety of chemicals and oils.
Applications: -
They are used in producing oil resistant appliances, hose, low voltage insulation, adhesives, tank linings, etc.
(c) Butyl Rubbers: -
Butyl rubbers consist of isobutalene with small amounts of isoprene or butadiene. These rubbers have great impermeability of gases and have high resistance to aging, great resistance to oxiding agents.
Applications: -
Butyl rubbers are used in producing tyres, inner tubes, adhesives, tank linings, wire and cable insulation, etc.
(d) Nitrile Rubbers: -
These are the co-polymers of cryolonitrile and butadience. These rubbers have a very good resistance to oil, grease and solvent and also to abrasion and aging.
Application: -
Nitrile rubbers are used for tank linings, conveyor belts and hose. Since these are cold resistant, therefore they are selected for use in aircraft design for flying at high altitudes.
(e) Silicon Rubbers: -
These are linear condensation polymer based on a dimethyl silicon polymer. These rubbers have poor physical properties and are difficult to be processed. These rubbers are unaffected by ozone, and are highly resistant to hot oils and have excellent electrical properties.
Applications: -
They find application in wire and cable insulation, coatings, tubing, gaskets, packaging, etc.
(f) Polyurethane Rubbers: -
Polyurethane are the elastic polymers containing urethane linkage. These rubbers have good strength, resilience and offer resistance to abrasion, ultraviolet light, oxygen, ozone, solvents and flex cracking.
Applications: -
They are extensively used in producing truck and passenger tires, soles and heels, solid tires, films, conveyor belts, etc.
(g) Thiokol Polysulphide Rubbers: -
These rubbers are resistant to gasoline, fuel oil, lubricating oils and solvents. These are cold resistant but not heat resistant.
Application: -
They are used for printer blankets, cable coverings and coating for balloon fabrics, etc.
(h) Hypalon: -
This is used for making covers for conveyor belts, gaskets in the ozone generators, etc.
(i) Adduct Rubbers: -
They show extraordinary performance when used as removal white side walls for tyres, radiator coolant hose, impregnant for air-ship fabric, and oil resistant foam for a railway journal boxes.
