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Natural Rubber

Rubber is a natural polymer and possesses elastic properties. It is obtained from a rubber tree. When the bark of the tree is cut, a sticky white liquid, latex, oozes out. It is a suspension of rubber particles in water.

Natural rubber is a linear polymer of 2-methyl-1, 3-butadiene (isoprene). It is also called as cis-1, 4-poly isoprene. On an average it contains 5000 isoprene units. All the double bonds in natural rubber are Cis. Rubber is a water proof material.

The cis-poly isoprene molecule consists of various chains held by weak van der Waals interactions and has a coiled structure. Thus it can be stretched like a spring and exhibits elastic properties. Gutta-percha (getah means gum and percha means tree) is a naturally occurring isomer of rubber in which all the double bonds are trans. Like rubber, gutta percha is exuded by certain trees. It is harder and more brittle than rubber.

Vulcanization of rubber

The natural as well as synthetic rubbers are very soft and sticky. These are hardened by a process known as vulcanization. Charle’s Good year found that when rubber is heated with sulphur, it becomes hard and more flexible. He called the heating of rubber with sulphur as vulcanization. (the name is coined after Vulcan, the Roman god of fire).

Heating rubber with sulphur causes cross-linking of polymer chains through disulphide bonds. Thus, the individual chains which were entangled together in the rubber now get together into a giant molecule through vulacanisation. As the polymer has double bonds, the chains have bends and kinks that prevent them from forming a tightly packed crystalline polymer. When rubber is stretched, the chains straighten out along the direction of pull. Cross-linking prevents the polymer from being torn when it is stretched. When the stress is removed it springs back to original shape and size.

The physical properties of rubber can be controlled by regulating the amount of sulphur used in the vulcanization process.

For example

(i) Rubber made with 1-3% sulphur is soft and stretchy and is used to make rubber bands.

(ii) Rubber made with 3-10% sulphur is more rigid and is used to manufacture tyres for automobiles, etc.