Sulfur vulcanisation sulfurating agents

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). 

Deformulation of vulcanised rubbers and rubber compounds at Dunlop (1988) is given in Scheme 2.3. Schnecko and Angerer [72] have reviewed the effectiveness of NMR, MS, TG and DSC for the analysis of rubber and rubber compounds containing curing agents, fillers, accelerators and other additives. PyGC has been widely used for the analysis of elastomers, e.g. in the determination of the vulcanisation mode (peroxide or sulfur) of natural rubbers. 

Thermoset polymers (sometimes called network polymers) can be formed from either monomers or low MW macromers that have a functionality of three or more (only one of the reagents requires this), or a pre-formed polymer by extensive crosslinking (also called curing or vulcanisation this latter term is only applied when sulfur is the vulcanising or crosslinking agent.) The crosslinks involve the formation of chemical bonds — covalent (e.g., carbon-carbon bonds) or ionic bonds. 

Sulfur has a very important role in the chemical industry. The vast majority of sulfur is used to produce perhaps the most important industrial chemical, sulfuric acid. Sulfur is also used to vulcanise rubber, a process which makes the rubber harder and increases its elasticity. Relatively small amounts are used in the manufacture of matches, fireworks and fungicides, as a sterilising agent and in medicines. 

Schotman and co-workers tentatively assigned the new Raman peaks at 1625 and 1592 cm 1 observed during sulfur vulcanisation of squalene, to the formation of conjugated dienes and trienes, respectively [70]. When vulcanisation was carried out in the presence of l,3-di(citraconimidomethyl)benzene, this resulted in a reduced intensity of these two new peaks, corroborating that conjugated dienes and trienes, formed as a result of reversion, react with the diimide. Obviously, the diimide is not an anti-reversion agent in the sense that it prevents reversion, but it is in the sense that it repairs crosslinks when reversion has occurred. 

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. 

Sometimes curing agents, antioxidants and accelerators can interact, creating new toxic chemicals e.g., when guanidine accelerators are used during the vulcanisation of rubber with sulfur with phenylene diamine-based antioxidants, aromatic amines and isothiocyanates can be produced, both of which are suspected carcinogenic agents [11]. 

Barnes and co-workers [21] describe a HPLC-MS method for the determination of sulfur containing vulcanising agents in acidic acetonitrile extracts of foodstnffs which have been in contact with crosslinked rubbers. The detection limit was between 0.005 and 0.043 mg/kg for 2-mercaptobenzothiazole and benzothiazole. No trace of these two compounds and other vulcanising agents was found in any retail foodstuffs. 

When a rubber is subject to vulcanisation, with sulfur or any other curing agent, it is converted into a thermoset system that is lightly crosslinked, with a molecular weight (Mw) between each crosslink of approximately 10,000 Da. This leaves the majority of the rubber chain free and, as a consequence, the glass transition temperature of the material is below ambient temperature. This gives rubber its... 

Some of these chemical agents (e.g., the amine/thiol mixtures) were used in the chemical probe rubber chemistry research work being undertaken at TARRC in the 1950s and 1960s to establish the chemical nature and processes involved during the sulfur vulcanisation of diene rubbers, such as NR. 

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