Studies on a New Binary Accelerator System for the Sulfur Vulcanization of Styrene—Butadiene Rubber

Abstract

Abstract The chemical reactions involved when a binary accelerator system is used in rubber vulcanization varies with the systems used. The exact mechanism of these reactions is not fully understood even now. Accelerators in which sulfur is combined as S—S, C—S—C, or S—N are generally inactive at low vulcanization temperatures because of the high thermal stability of their sulfur bonds. But sulfur-containing nucleophiles such as thiourea or its derivatives enable accelerators like TMTD or CBS to operate at lower vulcanization temperature, indicating a nucleophilic reaction mechanism in these vulcanization reactions. But, no conclusive evidence is given for such a postulate. In the present study, 1-phenyl-2,4-dithiobiuret and 1,5-diphenyl-2,4-dithiobiuret, which are more nucleophilic than thiourea but which vary in their nucleophilic reactivity, were tried as secondary accelerators in binary systems containing TMTD or CBS in sulfur vulcanization of SBR. Both the above dithiobiurets were found to reduce the optimum cure time considerably compared to the reference mixes. 1-phenyl-2,4-dithiobiuret, which is more nucleophilic than 1,5-diphenyl-2,4-dithiobiuret, is found to reduce the optimum cure time more, showing a nucleophilic reaction mechanism in the systems under review. In both cases, the optimum dosage of the secondary accelerator required has been established. The vulcanizates obtained with these new systems showed definite increase in tensile properties and good retention of these properties after aging. In the evaluation of other physical properties, these vulcanizates gave promising results. In order to correlate these physical properties to the type of chemical crosslinks formed, chemical characterization of the vulcanizates was also carried out. Also, to study the effect of these nucleophiles on the temperature of vulcanization, these mixes were cured and evaluated at two different temperatures, viz., 120°C and 150°C. To overcome the lower green strength and gum tensile strength in SBR, incorporation of fine reinforcing fillers and the use of special combinations of accelerators is required. In this context, the study was extended to evaluate the effect of fillers—carbon black, precipitated silica, and china clay—on the cure characteristics and vulcanizate properties.