Vulcanization

Vulcanization is a chemical- technical process developed in 1839 by Charles Goodyear, is made resistant to the rubber under the influence of time, temperature and pressure to atmospheric and chemical agents and to mechanical stress.

Method

For vulcanizing a rubber compound composed of crude rubber, sulfur or sulfur-donating substances such as sulfur monochloride ( S2Cl2 ), catalysts (to increase the rate of reaction, for example, 2-mercaptobenzothiazole and tetramethylthiuram disulfide, or zinc oxide, and fatty acids ) were heated and fillers. Vulcanized is nowadays usually 1.8 to 2.5% sulfur, and a temperature of 120 to 160 ° C. The long-chain rubber molecules are cross-linked by sulfur bridges. Thereby move the plastic properties of the rubber or rubber mix is lost, the pulp is transferred by means of the method of curing the plastic to an elastic state.

The resultant of this process has rubber compared to the starting material permanently elastic properties, returns under mechanical stress each back to its original position back, has a higher tensile strength, elongation and resistance to aging and weathering.

The elasticity of the rubber material is dependent on the number of sulfur bridges. The more sulfur bridges are, the harder the rubber. The number of sulfur bridges in turn is dependent on the added amount of sulfur, and the duration of the vulcanization.

With aging of the rubber, the sulfur bridges are replaced by oxygen bridges; the rubber becomes brittle and porous.

In addition to the sulfur vulcanization of conventional cross-linking of rubbers is frequently also by means of peroxides, metal oxides, or high-energy radiation. Since vulcanization with sulfur requires the presence of double bonds, are especially with rubbers which do not contain double bonds (e.g., EPM, EVA ), that method is used. Metal oxides are, inter alia, in the crosslinking of chloroprene rubber used. Compounds such as dibutyl accelerate the vulcanization. The course of vulcanization can be a test or measuring instrument, the vulcameter or even a simple rheometer measured and graphically documented.

The vulcanization of neoprene or chloroprene rubber (CR rubber) is performed using metal oxides (especially MgO and ZnO, sometimes PbO). In addition, due to other processing factors (mainly scorch "or Scorch, this being the premature crosslinking of rubbers due to the influence of heat), the selection of promoters regulated by other parameters. The most commonly used accelerators are problematic when CR rubbers are cured. Is as important for accelerator ethylene thiourea (ETU ) are used. This substance is indeed an excellent and proven catalyst for polychloroprene, but it is classified as toxic to reproduction. The European rubber industry, the research project Safe Rubber started to develop a safer alternative to the use of ETU.

Styrene - butadiene rubber ( SBR) and nitrile rubber (NBR ) can also be cured by purely thermal ( Thermovulkanisation ).

Vulcanization accelerators

Dithiocarbamates, thiuram disulfides and mono - sulfide, xanthates

Mercaptobenzothiazole and aldehydeamine condensation products

Dibenzothiazyl, diarylguanidines as diphenyl guanidine, Arylbiguanide such as o- tolyl biguanide, urotropin, aldehyde- and most condensation products of aliphatic aldehydes and aromatic amines

Triarylguanidine, Diarylthioharnstoffe and the condensation products of formaldehyde and aromatic amines