Effect of water and hydrogen sulfide in zinc dimethyldithiocarbamate accelerated sulfur vulcanization

Abstract

AbstractRubber and the model compound 2,3‐dimethyl‐2‐butene (TME) are vulcanized with zinc dimethyldithiocarbamate [Zn2(dmtc)4] accelerated sulfur formulations. When heated in dry nitrogen, Zn2(dmtc)4 is stable at vulcanization temperatures. However, it shows a mass increase when heated in moist nitrogen, indicating strong coordination with water; in a nitrogen/H2S atmosphere rapid degradation to dimethyldithiocarbamic acid (Hdmtc) and ZnS occurs. Model compound studies show that crosslinked sulfides are essentially bis(alkenyl) and confirm the absence of accelerator terminated pendant groups in the vulcanizates, while the ease with which rubber vulcanizates crystallize on cooling in a density column also suggests that pendant groups are largely absent. However, the rates of crystallization, measured as the time for the crystallization process to go to 50% completion, are slower in lightly crosslinked gels than in peroxide cures of similar crosslink density, particularly in the vulcanizates cured in a vacuum; this is interpreted as an indication that some residual pendant groups are present in Zn2(dmtc)4 vulcanizates. Water promotes the rate of crosslink formation in both rubber and TME systems, and it is suggested that the strong coordination of water with zinc in Zn2(dmtc)4 promotes its reactivity. The H2S liberated in the vulcanization process promotes decomposition of Zn2(dmtc)4 to Hdmtc, and this reaction makes an important contribution to the amount of Hdmtc that is formed in situ. The importance of Hdmtc as an accelerator and its role in providing alternative routes to crosslink formation in Zn2(dmtc)4 accelerated sulfur vulcanization are discussed. It is suggested that water, which is liberated when Hdmtc reacts with ZnO to form Zn2(dmtc)4, activates newly formed Zn2(dmtc)4 molecules; and this accounts for the beneficial influence of ZnO in Zn2(dmtc)4 formulations. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1516–1531, 2002