The role of dimethyldithiocarbamic acid in accelerated sulfur vulcanization

Polyisoprene/tetramethylthiuram disulfide (TMTD)/sulfur compounds were vulc anized under a variety of conditions. TMTD does not decompose to tetramethy lthiourea (TMTU) at vulcanization temperatures as has been suggested, neith er is it formed as an integral part of the crosslinking process. Instead, i t results from the attack of dimethylamine, released on decomposition of di methyldithiocarbamic acid (Hdmtc), on TMTD. It is demonstrated that the for mation of TMTU in vulcanizates may be overlooked, as it is readily lost in the work-up for HPLC analysis. Hdmtc is shown to play an essential role in the crosslinking process in polyisoprene/TMTD/sulfur formulations, and its removal from the system during vulcanization severely impedes crosslinking. Polysulfidic thiuram-terminated pendent groups are formed, in part, by the interaction of tetramethylthiuram polysulfides with the polymer chain, but largely by an exchange between Hdmtc and polysulfidic thiol pendent groups . The latter are formed when sulfurated Hdmtc reacts with the polymer chain . Crosslinking of thiuram-terminated pendent groups is slow, and :in the ab sence of ZnO crosslinking results from reaction between polysulfidic thiura m pendent groups and thiols. Crosslinking is delayed until the bulk of the accelerator is bound to the polymer chain, at which point the concentration of free thiuram groups, :in the form of Hdmtc, is low, and exchanges betwe en newly formed thiol pendent groups and Hdmtc is less frequent, permitting crosslinking of thiuram pendent groups with these newly formed thiol pende nt groups. Data to support the proposed reaction mechanism is presented. Hd mtc on its own accelerates sulfur vulcanization and acts as a catalyst for the reaction, being regenerated in the crosslinking process. (C) 1999 John Wiley & Sons, Inc.