Rubber

Monomers

Important conjugated dienes used in synthetic rubbers include isoprene (2-methyl-1,3-butadiene), 1,3-butadiene, and chloroprene (2-chloro-1,3-butadiene).

Condition for the reaction leading to formation of rubber

• Synthetic elastomers are produced on an industrial scale in either solution or emulsion polymerization methods
  • In solution polymerization, the monomer or monomers are dissolved in a hydrocarbon solvent, usually hexane or cyclohexane, and polymerized, using an organometallic catalyst such as butyllithium. Polymers made in solution generally have more linear molecules (that is, less branching of side chains from the main polymer chain), and they also have a narrower distribution of molecular weight (that is, greater length) and flow more easily. In addition, the placement of the monomer units in the polymer molecule can be controlled more precisely when polymerization is conducted in solution.

• • In emulsion polymerization, the monomer (or monomers) is emulsified in water with a suitable soap (e.g., sodium stearate) employed as a surfactant,  and a water-soluble free-radical catalyst (e.g., potassium persulfate, peroxides, a redox system) is added to induce polymerization. When emulsion polymerization of SBR is carried out “hot” (i.e., at 50 °C, or 120 °F), the polymer molecules are more branched. When polymerization is carried out “cold” (i.e., at 5 °C, or 40 °F), they are more linear and generally higher in molecular weight—features that improve the rolling resistance and wear resistance of tires. In some cases polymerization is continued in order to give products of such high molecular weight that they would normally be intractable. In these cases about 30 percent of a heavy is added before coagulation to yield “oil-extended” elastomers with superior wear resistance.

Vulcanization of rubber

This is heating rubber with sulphur

• to make more resistant to heat and cold,
• give it  higher tensile strength and resistance to swelling and abrasion,
• increase its  elasticity over a greater range of temperatures.

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Compiled by Dr. Bbosa Science + 256 778 633 682