Thiuram-accelerated sulfur vulcanization. II. The formation of crosslink precursors

Abstract

2,3-Dibutyl-2-butene (TME) was used as a model for polyisoprene in studying the formation of pendent groups that act as precursors to crosslink formation during tetramethylthiuram disulfide (TMTD)-accelerated sulfur vulcanization. TME was heated at 130°C with TMTD, TMTD/sulfur, and TMTD/sulfur/ZnO in sealed tubes for various times, after which the mixture was analyzed by HPLC. Tetramethylthiuram monosulfide (TMTM) and tetramethylthiuram polysulfides (TMTP) formed rapidly. The rate at which these bound to the network could not be accounted for in terms of a free-radical mechanism, and a concerted mechanism was proposed. Dimethyldithiocarbamic acid (Hdmtc), a product of pendent group formation, is stable at vulcanization temperatures and does not decompose to dimethylamine. Instead, it exchanges with thiuram pendent groups. The resultant thiol pendent groups are unreactive and their formation delays the onset of crosslinking. After a few minutes at 130°C, there appears to be no further increase in the number of pendent groups, but this is due to the thiol groups being undetectable in the HPLC analyzer. © 1996 John Wiley & Sons, Inc.