Vulcanization of rubber : The process of heating natural rubber with sulphur to improve its properties is called vulcanization. Vulcanization was defined by Charles Goodyear.
Although natural rubber is thermoplastic substance in which there are no cross link between the polymer chain and it on vulcanization set into a given shape which is retained.
The vulcanization process performed initially was slow. Now a days, some additives such as zinc oxide etc. are needed to speed up the rate of vulcanization. In the mean while vulcanization, sulphur cross links are formed (figure) the double bonds in the rubber molecule acts as reactive sites. The allylic -CH2, alpha to double bond is also very active. In the mean while vulcanization, sulphur forms cross links at these active sites. As a product, rubber finds stiffened and intermolecular movement of rubber springs is prevented resulting in physical character of rubber. The extent of stiffness of vulcanized rubber depend upon the amount of sulphur added. For example about 5% sulphur is used for making tyre rubber while 30% of the sulphur is used for making battery case rubber.
In a polymer, the chains are normally tangled up with one another. When the rubber is expanded, the chains straighten out to some point. The chains may not slip past one another due to the polysulphide bridges. Thus, rubber may be expended only to a limited point. When the tension is replaced, the chains try to coil up again and the rubber resumes its original shape.
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