Preparation of Natural Rubber
The latex obtained from rubber trees is a suspension of small rubber globules in water and has the appearance of milk. Rubber is obtained by coagulation of the latex by adding dilute acid. The latex consists of 30-3 per cent of the rubber hydrocarbon. The coagulated latex is washed, dried and passed through rollers to get sheets of natural rubber. Natural rubber is sticky, thermoplastic, and elastic over only short range of temperature and swells readily in organic solvents. The average molecular mass of natural rubber molecules is about 1,000,000.
Vulcanization of Rubber:
The properties of natural rubber can be modified and improved by the process of vulcanization. Vulcanization is carried out by heating rubber with 5-10 per cent of sulphur at a temperature of 125-1400C for 2-4 hours. During vulcanization, sulphur cross-links are formed between hydrocarbon chains. Due to the addition of sulphur, double bonds in the rubber decrease. There is loss of one double bond per sulphur atom added.
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Vulcanized rubber has the following properties:
1. It has greater tensile strength.
2. It is more elastic than natural rubber. Cross-linkages help in preventing the slippage of chains on application of stress. Cross-links also help to regain the shape after the removal of the stress.
3. IF 30-50 per cent sulphur is used, a hard organic solvents than untreated rubber. Thus, vulcanization increases the resistance of rubber to organic solvents.
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Uses of Rubber:
1. Rubber is used for making gloves, rubber bands and tubes.
2. Hardened rubber, obtained by adding carbon black to the rubber is used for making tyres, tubes and conveyor belts.