Natural rubber requires vulcanization .Give reasons .With appropriate reactions explain how the drawbacks are overcome.
The natural rubber has the following properties :
1) Its plasticity is greater than elasticity. It cannot sustain stress .Thus when stretched to a great extent ,it undergoes deformation permanently .
2) It has large water absorption tendency ,which makes it weak .
3) It has very low tensile strength .
4) Due to large percentage of unsaturation in its structure , it is easily attacked by various reagents such as HNO3 ,conc . H2SO4 ,organic matter ,air ,oxygen ,ozone ,etc . and as a result gets gradually disintegrated.
5) It possesses high percentage of tackiness (property of developing stickiness on surface ) which makes difficult to store the rubber stocks .
6) Durability and abrasion resistance of natural rubber is very low .
Thus in order to improve the undesirable properties of natural rubber, it requires vulcanization .
In vulcanization of rubber,crude rubber is mixed with vulcanizing agent like sulphur . Mixture is heated toabout 110-140 C ,where sulphur chemically combines with rubber
.During vulcanization double bonds present in rubber structure break and ‘ S’ gets added to it forming cross links .
Formation of cross links between C-atoms restrict intermolecular movement ,which gives stiffness (hardness) to rubber and decreases elasticity .The extent of hardness in vulcanised rubber depends on the amount of sulphur chemically added to rubber .
The properties of rubber are changed considerably by vulcanization ,vulcanized rubber are of high molecular weights and less unsaturated than the raw rubber .
The quantity of sulphur used varies upto 45 % ,soft rubber contains 0.5-5 % of sulphur ,hard rubber contains more than 30 % ,tyre rubber contains 3-5 % and battery case rubber contains 30 % of sulphur ..