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This paper describes research methodologies for the investigation of the mechanism of vulcanization and discusses the reactivity of thiuram and dithiocarbamate chemicals. The combined knowledge is subsequently applied to thoroughly review the mechanism and chemistry of both thiuram- and dithiocarbamate-accelerated sulfur vulcanization. Integration of the original mechanistic ideas from the 1960s and the results obtained in the past three decades now have led to a more balanced appraisal of events during vulcanization. Questions have been answered, solutions for old problems are proposed, and remaining fields of endeavor are identified.

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SYNOPSISThe rate of formation of tetramethylthiuram polysulfides (TMTP), that play an important role in vulcanization, was studied. After a short induction period (< 30 s), tetramethylthiuram disulfide (TMTD) and TMTD-sulfur mixes, heated to 130-150°C in the absence of rubber, rapidly form a series of TMTPs. The concentrations of TMTPs of lower sulfur rank increase most rapidly, indicating that sulfur atoms are added to the accelerator sequentially. The incorporation of sulfur molecules to give TMTPs, which subsequently desulfurate, does not occur. Equilibrium concentrations of the various TMTPs are achieved in about 2 min. Little tetramethylthiourea is formed below 200°C. Tetramethylthiuram monosulfide (TMTM) is stable, but TMTM-sulfur mixes form TMTPs. A mechanism is proposed to account for the large amount of TMTM formed on heating TMTD in the absence of sulfur and the correspondingly higher TMTP concentrations in the presence of sulfur. 0 1995 John Wiley & Sons, Inc.

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