THIURAM SULFIDE

Thiuram Sulfides. These compounds, (8) and (9), are an important class of accelerator. Thiurams are produced by the oxidation of sodium dithiocarbamates. The di- and polysulfides can donate one or more atoms of sulfur from their molecular stmcture for vulcanization. The use of these compounds at relatively high levels with litde or no elemental sulfur provides articles with improved heat resistance. The short-chain (methyl and ethyl) thiurams and dithiocarbamates ate priced 2/kg. Producers have introduced ultra-accelerators based on longer-chain and branched-chain amines that are less volatile and less toxic. This development is also motivated by a desire to rninirnize airborne nitrosamines. 

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). 

As these things came to light, the rubber processing industry was the obvious first choice to remove the nitroso compounds from the raw material pallet since substitutes were available in order to eliminate one of the components in the material itself. On the other hand, the nitrous oxides, which are adsorbed to the active surfaces of the fillers or present in the air we breathe as combustion gases from vehicle engines, cannot be eliminated. These particular substances can be reduced by optimized combustion and catalysts, but not avoided completely. Substances that form nitrosable amines include, among others, di-n-alkyl dithiocarba-mates, di-n-aUcyl thiurame sulfides, A,V -dialkyl benzothiazyl sulfenamides and benzothiazyl sulfene morpholide. 

In the vulcanization of polydienes with sulfur, sulfur bonds are formed [see reaction (23-33)]. In the uncatalyzed reaction, approximately one cross-linking bond is formed for every 50 sulfur atoms added. In catalyzed reactions, (with ZnO, pyridine derivatives, thiuram sulfides, etc.), this number decreases to 1.6. Natural rubber can also be vulcanized cold, however, with S2CI2 or MgO. Basically, all reactions are suitable for cross-linking if the attacking agent is at least bifunctional ... 

Vulcanization by sulfur alone is of little help as it is too slow. The vulcanizing systems currently used contain accelerators like thiuram sulfides (R2N-CS-S-S-CS-NR2), zinc dithiocarbamates [(R2N-CS-S)2Zn], thiourea (H2N-CS-NH2), or zinc benzothiazoles,... 

Previously, accelerators such as thiurams, thiazoles and carbamates were used, but their use has declined because of problems with skin allergies. This issue is discussed in detail by Estlander et al. [6]. In addition to the problems of contact dermatitis, it may be necessary to consider the formation of N-nitrosamines by accelerators. Dithiocarbamates and thiuram sulfides have the potential to decompose to give N-nitrosamine precursors [3]. N-nitrosamines are believed to be carcinogenic, although conclusive evidence for human carcinogenicity is scant, as discussed recently by Loadman [7]. 

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