Rubber is considered as elastic substance, which is obtained from the exudations of certain tropical plants (natural rubber) or derived from petroleum and natural gas. Rubber also termed as elastomer, a type of polymer. In 1770 Joseph Priestley coined the term rubber.
Rubber is divided into two groups based on its origin
Natural rubber – used in racing car tires, bus tires, truck tires
Synthetic rubber – examples butadiene rubber, styrene butadiene rubber, neoprene
Physical & Chemical properties of rubber
In relaxed state, rubber is in the form of long, coiled-up chains. By stretching of rubber the all chains will come very close as result, kinetic energy exerted in the form of heat. In chain elongation process entropy and temperatures required during this process are increases. When chain in relaxed state both entropy and temperatures decreases.
Relaxation of a stretched rubber band is thus driven by a decrease in entropy and temperature, and the force experienced is a result of the cooling of the material being converted to potential energy. The material undergoes adiabatic cooling during contraction.
Vulcanization of rubber creates disulfide bonds between chains. The result is that the chains tighten more quickly for a given strain, thereby increasing the elastic force constant and making rubber harder and less extensible.
Like plastic, rubber is also a type of polymer, made of subunits called monomers. In rubber, the monomer is isoprene. As the latex dries, the isoprene molecules mass together and one isoprene molecule attacks a carbon-carbon double bond of a neighboring molecule. One of the double bonds breaks and the electrons repositioned to form a bond between the two isoprene molecules.
The process continues until long strands of many isoprene molecules linked like a chain. This long chain of strands is called as polyisoprene polymer. As the drying continues, the polyisoprene strands stick together by forming electrostatic bonds. The attraction between these strands holds the rubber fibers together and allows them to stretch and to recover.
Synthetic rubber production
Emulsion polymerization is the widely used method to produce synthetic rubber.
Emulsion polymerization is a type of radical polymerization process that usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water.
In emulsion polymerization, the monomers are emulsified in water with a suitable soap and a water-soluble free-radical catalyst is added to induce polymerization. After polymerization has reached the desired level, the reaction is stopped by adding a radical inhibitor. About 10 per cent of synthetic elastomer produced through emulsion technique. The rest is coagulated with acidified brine, washed, dried, and pressed into 35-kg bales.