Sulphur donors

A further comparison of closely related compounds is provided by the related chlorides, Ph2(Cl)Sn(S2CNR2), where R is Et211 or CgHn212. These have structures similar to 22b but with reduced asymmetry in Sn—S (244.5 and 271.6 pm for R = Et 244.0 and 

7 pm for the cyclohexyl). The Sn—Cl bonds are the same at 243.9 pm and the Cl and the longer S occupy axial sites in a very distorted trigonal bipyramid (CISnS angles of 158° and 154°, respectively). The f-Bu analogues of (22a) and of the monochloride show similar features. There is thus a subtle interplay between substituent size and geometrical features, especially in the asymmetry of the SnS2 coordination. 

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A term apphed to vulcanisation systems in which sulphur or a sulphur donor is used very efficiently for crosslinking the rubber. EV systems produce vulcanisates with crosslinks that are mainly monosulphidic, which are thermally and mechanically stable. 

NR can be crosslinked by the use of sulphur, sulphur donor systems, peroxides, isocyanate cures and radiation, although the use of sulphur is the most common method. 

Polybutadiene based compounds can be cured by sulphur, sulphur donor systems and peroxides. Less sulphur and a higher level of accelerators are required when compared to NR. The cure of polybutadiene by peroxides is highly efficient in that a large number of crosslinks are produced per free radical, the resultant highly crosslinked rubber exhibiting high resilience this factor is utilised in the manufacture of superballs . 

Nitrile rubber can be cured by sulphur, sulphur donor systems and peroxides. However, the solubility of sulphur in nitrile rubber is much lower than in NR, and a magnesium carbonate coated grade (sulphur MC) is normally used this is added as early in the mixing cycle as possible. Less sulphur and more accelerator than is commonly used for curing natural rubber is required. A cadmium oxide/magnesium oxide cure system gives improved heat resistance, but the use of cadmium, a heavy metal, will increasingly be restricted. 

Newer polyacrylate rubbers can be cured with certain amines and are more responsive to a broad range of curative systems, e.g., alkali metal stearate/sulphur or sulphur donor, methyl zimate and ammonium adipate. 

In low or sulphurless systems, thiurams containing more than one sulphur atom act as sulphur donors. They also confer low tendency to reversion, and good heat stability. 

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