RUBBERS: Characteristics of Rubbers

RUBBERS

According to American Society for Testing Materials (ASTM), rubber (elastomer) is a polymeric material, which at room temperature, can be stretched to at least twice its original length and will quickly return to its original length when the stretching force is withdrawn.

Plastics are also polymeric material, but rubbers are different from them on account of this property of stretching to this extent and returning to original length. Rubbers are both ‘‘natural’’ and “synthetic”. Natural rubber oozes out as a milky liquid if a cut is made on the stem of a rubber tree.

Until World War II, almost all rubber used was natural rubber. During that war, synthetic elastomers were developed due to the scarcity of natural rubber. Synthetic rubbers have better properties than natural rubber and are widely used now.

Natural rubber is soft and has poor resistance to abrasion. Its properties can be improved by ‘‘vulcanizing’’. Vulcanization consists of heating 100 parts of natural rubber with 1 to 5 parts of sulphur by weight. Vulcanization improves the modulus of elasticity, tensile strength and resistance to oxidation of natural rubber. It also hardens the rubber and makes it useful for industrial application.

Useful temperature range of natural rubber is 10–60°C, the same for vulcanized rubber is – 40 to 100°C.

The tensile strength improves from 70 kg/cm2 to 700 kg/cm2. Vulcanized natural rubber is used in manufacture of tyres, tubes, rubber soles of shoes and gaskets. Apart from sulphur, other additives are also added to rubbers to improve their properties or to develop a specific property. Automobiles tyres contain about 15–30% by volume of carbon black.

Characteristics of Rubbers

The rubbers are noncrystalline. They are poor conductors of heat. They are non conductors of electricity.

They have relatively low softening temperatures. They have high resistance to oily, chemical and corrosive atmospheres. However they show signs of ageing showing hardness and cracks and a lowering of properties. They have good vibration damping characteristics.

Synthetic Rubbers: Main synthetic rubbers used in industry are described below briefly:

(1) Neoprene: It is the first commercial synthetic rubber which was developed in 1930. Generally its properties are similar to that of natural rubber, but in compression particularly in elevated temperature conditions, it is superior to natural rubber. However it has excellent weathering and heat resistance, good oil resistance, excellent flame resistance but its dielectric strength is lower than that of natural rubber.

Its main uses are for manufacturing heavy duty conveyer belting, V-belts, hoses and gaskets.

(2) Butyl rubber: It is also similar to natural rubber. It however is not costly. It shows high resistance to abrasion, tearing and flexing. It has low gas and air permeability. It have good weathering resistance and good chemical resistance. It has very good dielectric strength.

Their main application is for cable insulation, high pressure steam hoses, machinery mounting pads and suspension bushes.

(3) Nitrile rubber: Main property is excellent oil resistance whether oil is vegetable or mineral oil. Typical application is for manufacture of gasoline, chemical and oil hoses, o-rings, seals, and sole of shoes.

(4) Isoprene rubber: It is more or less like natural rubber. But its good electrical properties and low moisture absorption make it a very good insulation material.

(5) Silicone-rubber: It has excellent resistance to high and low temperatures, although its mechanical strength is low. Silicone elastomers are one of the most stable elastomers and have high resistance to oils and chemicals.

Typical applications are for seals, gaskets, o-rings, insulation for wires and cables and tubing for food and medical uses.

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