Vulcanization of rubber

Dr. Dmitri Kopeliovich

Vulcanization of rubber is a process of improvement of the rubber elasticity and strength by heating it in the presence of sulfur, which results in three-dimensional cross-linking of the chain rubber molecules (polyisoprene) bonded to each other by sulfur atoms.

There are two general types of rubber:

• Natural rubber is produced from latex - milky emulsion drawn from the rubber tree (Hevea brasiliensis) or some other plants. When coagulated latex transforms into soft, plastic and sticky substance (crude rubber), which is then vulcanized (cured). Natural rubber is composed of polyisoprene molecules.
• Synthetic rubbers are Elastomers, elastic properties of which are similar to those of the natural rubber. Synthetic rubber may have chemical composition similar to the natural rubber (synthetic polyisoprene). Other kinds of synthetic rubbers are: polybutadiene, poly(styrene-butadiene-styrene), polychloroprene (Neoprene), polyisobutylene (Butyl rubber), silicone.

Vulcanization process was invented by Charles Goodyear in 1839.

Vulcanization technique comprises the following principal stages:

• Mixing of crude rubber with about 5-30% of sulfur (cross-linking agent) and other additives such as:
  
  • activator (commonly zinc oxide or stearic acid),
  • accelerator (guanidines, thiazoles, dithiocarbamates, xanthates, thiurams) ,
  • coagulants (acetic acid, calcium chloride),
  • anti-oxidants (amines, phenolics, phosphites),
  • color pigments,
  • surfacants,
  • softeners (oils),
  • ant-foaming agents,
  • anti-tack agents (Rosin derivates, coumarone-indene resins, aliphatic petroleum resins, alkyl-modified phenol-formaldehyde resins).

Slow cross-linking starts at this stage. It is neccessary to avoid active vulcanization during mixing, which may cause cracks formation at the molding stage.

• Molding (shaping) the rubber mixture. The rubber must be shaped prior to heating stage since cross-linking makes shaping impossible.
• Heating the mixture to 250-400ºF (120-200ºC). Increased temperature speeds up the vulcanization process resulting in fast and complete cross-linking. C-S bonds replace C-H bonds linking chain polyisoprene molecules. Each link is formed by one to seven sulfur atoms.

Density of cross-links must be controlled in order to prevent formation of brittle rubber.

Properties of rubber improved by vulcanization:

• tensile strength;
• elasticity;
• hardness;
• tear strength;
• abrasion resistance;
• resistance to Solvents.

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