Bis(diisopropyl)thiophosphoryl sulfides in vulcanization reactions

A synergistic combination of bis(diisopropyl)thiophosphoryl disulfide, dimorpholyl disulfide, and sulfur is used to produce an efficient vulcanizing system for a range of rubbers. This produces vulcanizates with the exceptional thermal and thermal-oxidative stability characteristic of sulfur donor vulcanizates and the rapid and extensive crosslinking reaction normally associated with a conventional sulfur–accelerator combination. A pronounced induction period is noted. The crosslinks initially produced in cis-1,4-polyisoprene rubbers are predominantly polysulfide but reduce to mono- and disulfides at optimum and extended cures. The crosslinks of the ethylene–propylene terpolymer and the styrene–butadiene vulcanizates are initially mainly disulfide but are rapidly reduced to monosulfides at the high curing temperatures (180°C) used. A comparison with vulcanization systems based on tetramethylthiuram disulfide and bis(diisopropyl)thiophosphoryl tri-and tetrasulfides as sulfur donors and with a conventional cyclohexylbenthazyl-2-sulfenamide–sulfur combination, respectively, shows it to have distinct advantages.