Natural Rubber

 Natural rubber is an excellent example of a natural polymer and an elastomer in particular. Elastomers are substances that can be readily stretched. They retract rapidly to their original form when released. Natural rubber is also called plantation rubber.

Preparation

Latex, the white milky liquid obtained by making a cut in the rubber tree contains 30%-40% of rubber as a colloidal solution in water. This is coagulated (changed from fluid to solid state or clotting) with acetic or formic acid and can then be squeezed, rolled, milled and vulcanized.

Structure of Natural Rubber

Natural rubber is a linear polymer of an unsaturated hydrocarbon called isoprene (2-methyl butadiene). There may be as many as 11,000 to 20,000 isoprene units in a polymer chain of natural rubber.

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Properties of Natural Rubber

·           Crude rubber is a tough and an elastic solid. It becomes soft and sticky as the temperature rises.

·           Its specific gravity is 0.915.

·           The most important property of natural rubber is its elasticity. When stretched, it expands and attains its original state, when released. This is due to its coil-like structure. The molecules straighten out when stretched and when released, they coil up again. Therefore applying a stress can easily deform rubber. Note that when this stress is removed, it retains its original shape.

·           Raw natural rubber has elasticity over a narrow range of temperature from 10 to 60 degrees centigrade. Because of this, articles made of raw natural rubber don't work well in hot weather.

·           Raw natural rubber has low tensile strength and abrasion resistant.

·           It absorbs large quantities of water.

·           It is insoluble in water, alcohol, acetone, dilute acids and alkalis.

·           It is soluble in ether, carbon disulphide, carbon tetrachloride, petrol and turpentine.

·           Pure rubber is a transparent, amorphous solid, which on stretching or prolonged cooling becomes crystalline.

Vulcanisation of Rubber

Charles Goodyear discovered the process of vulcanization in 1893 to modify the properties of natural rubber. Vulcanisation is the addition of right amount of sulphur to natural rubber to impart high elasticity, tensile strength and resistance to abrasion.

Hot Vulcanisation

Raw dry rubber is heated with sulphur (5%-8% based on the requirement), zinc oxide (a filler, 5%) and accelerator (0.5% -1%) at 400-440K for about half an hour. As the sulphur quantity increases, the rubber becomes tougher. 50% sulphur gives ebonite (vulcanite). An accelerator containing nitrogen, sulphur or both is used to increase the reaction rate and for vulcanisation to occur at room temperatures.

sulphur gives ebonite

Vulcanisation of Rubber - Cross-links between Molecular Chains

The double bonds in natural rubber permit formation of sulphur bridges between different chains. These cross-links are responsible for removing the tackiness of untreated rubber.

Comparison between raw natural rubber and vulcanised natural rubber

 Raw Natural Rubber

 Vulcanized Natural Rubber

 Soft and sticky

 Comparatively hard and non-sticky

 Low tensile strength and not very strong

 High tensile strength and very strong

 Low elasticity

 High elasticity

 Can be used over a narrow range of temperature from 10 to 60 degrees centigrade

 Can be used over a wide range of temperature from -40 to 100 degrees centigrade

 Low abrasion resistance

 High abrasion resistance

 Absorbs a large amount of water

 Absorbs a small amount of water

 Soluble in solvents like ether, carbon disu p hide, carbon tetrachlo ride, petrol and turpentine

 Insoluble in all the usual solvents

 

Hardening of Natural Rubber

Rubber can be hardened by adding carbon black as a filler (solid substance added for strength and to reduce the cost) to it during the vulcanization process. This increases the strength and abrasion resistance of natural rubber. Carbon black when mixed with rubber in rubber tyres, makes them more durable and cuts the cost. The vulcanised and hardened rubber is used to make tyres and tubes of automobiles and conveyor belts for industrial use.

Uses of Vulcanised Rubber

vulcanised natural rubber is used for making:

·           Rubber bands, football bladders, gloves and rubber tubes.

·           Tyres and tubes of automobiles and conveyor belts for industrial use after hardening.