Study of curative interactions in cis‐1,4‐polyisoprene. XI. Network maturing reactions in the cis‐1,4‐polyisoprene–sulfur–zinc dimethyldithiocarbamate and cis‐1,4‐polyisoprene–sulfur–zinc dimethyldithiocarbamate–ZnO systems

Abstract

AbstractThe network maturing reactions of the inefficient cis‐1,4‐polyisoprene (IR)–sulfur–zinc dimethyldithiocarbamate (ZDMC) and IR–sulfur–ZDMC–ZnO vulcanization systems were discussed. The Moore–Trego efficiency E was exceptionally high and increased, for example, from 13.0 at 132.4°C to 49.9 at 170.0°C in the IR–sulfur–ZDMC mixture. Calculations showed that ∼ 60% of the elemental sulfur was combined in the network in the form of cyclic sulfides and/or vicinal crosslinks at 148.2°C. The inefficient use of sulfur at the initial cure stages may be attributed to the insolubility of ZDMC in rubber. The increase in E at the later stages of vulcanization was attributed to a decrease in the total number of polysulfidic crosslinks, as the number of mono‐ and/or disulfidic crosslinks remained constant. The destruction of polysulfidic crosslinks was enhanced in the absence of ZnO, due to the catastrophic attack of thiophilic species such as Me2NH. The Me2NH resulted from the rapid decomposition of dimethyldithiocarbamic acid, which formed as a by‐product in the initial crosslink reactions. No evidence was found for the participation of ZDMC in crosslink desulfuration. ZDMC is not a vulcanizing agent.