Low Sulfur Compounding

Abstract

Abstract Since the beginning of the rubber industry, sulfur dosages used for soft rubber have declined from 20 to 2 or 3 per cent customary today. The decline is still under way, and present sulfur ratios are high in comparison with what is actually required to produce satisfactory commercial vulcanization—fractions of 1 per cent. Vulcanization with extremely low sulfur ratios, the use of selenium and tellurium as auxiliary vulcanizing materials, and the proper type of acceleration required to “cure in” all the sulfur and produce satisfactory commercial results, are discussed. Comparative data, covering many physical attributes, are shown for pure gum and gas black stocks compounded with 3 per cent sulfur on the one hand, and 0.3 and 1 per cent (gas black stock) on the other. Advantages of very low sulfur compounds are less tendency to scorch, better physical properties, better aging, higher resilience and lower power losses, less discoloration, greater resistance to hot solvents, greater abrasion resistance, and less flex-cracking on aging; disadvantages are higher set at low temperatures and reduced capacity to adhere to metals. It seems probable that, in the future, compounding will be based largely on the use of very low sulfur ratios.