Wire coat compounds, natural rubber

Heat ageing in the presence of air is often used to categorise rubber compounds. Natural rubber and high c/5-polyisoprenes are not particularly resistant to ageing temperatures above 70 ""C. For some applications, however, polyisoprene compounds may well exceed this temperature during their normal service life and often under dynamic conditions. This would be the case for compounds used in tyre carcass and for wire coat compounds where ingress of air or oxygen is limited. 

Insoluble Sulfur. In natural rubber compounds, insoluble sulfur is used for adhesion to brass-coated wire, a necessary component in steel-belted radial tires. The adhesion of rubber to the brass-plated steel cord during vulcanization improves with high sulfur levels ( 3.5%). Ordinary rhombic sulfur blooms at this dose level. Crystals of sulfur on the surface to be bonded destroy building tack and lead to premature failure of the tire. Rubber mixtures containing insoluble sulfur must be kept cool (<100°C) or the amorphous polymeric form converts to rhombic crystals. 

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