Studies on diphenylguanidine–accelerated sulfur vulcanization of styrene butadiene rubber in the presence and absence of dicumylperoxide

Diphenylguanidine (DPG) raises the rate of decomposition of dicumylperoxide (DCP) and the crosslinking maxima due to DCP is lowered to some extent by DPG. When the molar proportion of DPG–S is approximately 1:1, no additive results of crosslink formation (as reported for NR) are observed. Zinc oxide and stearic acid increase the rate of crosslinking as well as the crosslinking maxima. In the present study it appears most probable that DPG-accelerated sulfuration of SBR is an ionic process. At a constant level of DCP and sulfur crosslink density increases when the amount of DPG is increased; a constant level of DCP and DPG crosslinking increases with rising sulfur concentration. An increase in the concentration of DPG or sulfur leads to greater formation of the complex as well as a change in its composition. The effect of DPG is more pronounced, for by reducing the number of sulfur atoms more sulfur is available for crosslinking. Sulfur absorbs little SO₂ or H₂S; if it is already saturated, there is no perceptible effect. DPG does absorb H₂S or SO₂ and the rate and maxima of crosslinks is increased. The effect of SO₂ is higher because of the higher K_a values of H₂SO₃ and consequently higher concentration of HSO₃^?. Reversion is a free-radical process inhibited by free DPG present in the system. In the presence of zinc oxide and stearic acid the reaction follows a polar mechanism as well as a radical mechanism.