Vulcanising agents

Accelerators are chemical compounds that iacrease the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanising agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiasoles are aU. classified as accelerators. By far, the most widely used are the thiasoles, represented by mercaptobensothiasole (MBT) and bensothiasyl disulfide (MBTS). 

The principal commercial uses of sulfur monochloride are in the manufacture of lubricant additives and vulcanising agents for mbber (147,154,155) (see Lubrication AND lubricants Rubber chemicals). The preparation of additives for wear and load-bearing improvement of lubricating oils is generally carried out in two steps and the technology is described in numerous patents (155) (see Sulfurization and sulfchlorination). 

The first type includes vulcanising agents, such as sulphur, selenium and sulphur monochloride, for diene rubbers formaldehyde for phenolics diisocyanates for reaction with hydrogen atoms in polyesters and polyethers and polyamines in fluoroelastomers and epoxide resins. Perhaps the most well-known cross-linking initiators are peroxides, which initiate a double-bond... 

Accelerated sulphur systems also require the use of an activator comprising a metal oxide, usually zinc oxide, and a fatty acid, commonly stearic acid. For some purposes, for example where a high degree of transparency is required, the activator may be a fatty acid salt such as zinc stearate. Thus a basic curing system has four components sulphur vulcanising agent, accelerator (sometimes combinations of accelerators), metal oxide and fatty acid. In addition, in order to improve the resistance to scorching, a prevulcanisation inhibitor such as A -cyclohexylthiophthalimide may be incorporated without adverse effects on either cure rate or physical properties. 

Health hazards now associated with what was one of the most common vulcanising agents for the rubber (ethylenethiourea) have caused problems because of the difficulties of finding an acceptable alternative. 

Other mild oxidising agents which abstract the terminal hydrogen atoms and thus facilitate disulphide formation may be used as vulcanising agents. They include benzoyl peroxide, p-nitrosobenzene and p-quinone dioxime. 

Another approach has been adopted by the Du Pont Company with Adiprene C. This is a urethane-type polymer with unsaturated groups in the polymer. Because of the unsaturation the polymer may be vulcanised with sulphur, the standard vulcanising agent of the rubber industry. This is a clear-cut example of a product being modified to suit the processor rather than that of a processor adapting himself to meet new products. Whereas Adiprene C has poor tensile strength when unfilled, the use of carbon black leads to appreciable reinforcement (as is the case with SBR and to some extent natural rubber. 

At Goodyear laser-desorption MS has been used for direct analysis of rubber additives (e.g. antioxidants, antiozonants, vulcanising agents, processing oils, silica fillers, etc.), in situ at the surface of an elastomeric vulcanisate [74,75]. 

A vulcanising agent particularly for silicone rubber and fluoroelastomers it has been used as a non-sulphur vulcanising agent for natural rubber. It is also a catalyst in emulsion polymerisation. Beta Rays... 

A non-sulphur vulcanising agent for natural rubber and some types of synthetic rubber. One trade name is Dicup. /V,A/-Dicyclohexyl-2-benzothiazole sulphenamide... 

Also known as vulcanite and (mainly in the USA) hard rubber . The hard, horn-like product obtained when natural rubber and some synthetic rubbers such as nitrile (NBR) are vulcanised with a high proportion of sulphur or organic nonsulphur vulcanising agent. Butyl rubber and polysulphide rubber do not form ebonites. Ebullioscopy... 

Also known as hexamine, hexa or HMT, a once-popular aldehyde-amine type of accelerator for natural rubber, now used as a vulcanising agent for polyacrylate rubbers. 

Lead monoxide, PbO, formerly used as an inorganic accelerator but now mainly used as a vulcanising agent in some polychloroprene... 

Elastomers (natural and synthetic) employ as vulcanising agent a quantity (2.5 phr on average) of elemental sulphur. The use of any other substance (TMT, peroxides, quinones, etc.) is termed non-sulphur vulcanisation. 

A non-sulphur vulcanising agent of particular application in butyl rubber compounds, the activator being red lead. 

A non-metallic element with chemical properties similar to those of sulphur. It has been used as a compounding ingredient in low sulphur compounds to resist reversion and improve ageing. It is classed as a secondary vulcanising agent. 

A procedure to prevent premature vulcanisation of very fast-curing mbber compounds, doughs or solutions. One part of the batch contains the vulcanising agent and the other contains the accelerator requisite quantities of the two parts are mixed together immediately prior to use. 

Any substance which will bring about vulcanisation. Elemental sulphur remains a widely used vulcanising agent, but sulphur-containing substances (such as thiuram disulphides) have important uses. Organic peroxides (benzoyl and dicumyl) and nitroso derivatives (e.g., benzoquinone dioxime) are used in vulcanising certain synthetic elastomers. 

W. Hofmann, Vulcanisation and Vulcanising Agents, Maclaren and Sons Ltd., London, Palmerton Publishing Co., Inc., New York, 1967. 

There is wide variety of vulcanisation agents and methods available for crosslinking rubber materials including peroxide, radiation, urethane, amine-boranes, and sulfur compounds [20]. Because of its superior mechanical and elastic properties, ease in use, and low cost, sulfur vulcanisation is the most widely used. Although vulcanisation with sulfur alone is not practical compared to the accelerated sulfur vulcanisation in terms of the slower cure rate and inferior physical properties of the end products, many fundamental aspects can be learned from such a simply formulated vulcanisation system. The use of sulfur alone to cure NR is typically inefficient, i.e., requiring 45-55 sulfur atoms per crosslink [21], and tends to produce a large portion of intramolecular (cyclic) crosslinks. However, such ineffective crosslink structures are of interest in the understanding of complex nature of vulcanisation reactions. 

Silicone elastomer-based rubber compounds are prepared in conventional apparatuses (closed agitators, roll mills, etc.) and consist of the following ingredients elastomer, active fillers, vulcanising agent, stabiliser, pigment additives. 

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