N,N '-Pentamethylenethiuram disulfide and N,N '-pentamethylenethiuram hexasulfide accelerated sulfur vulcanization. IV. Vulcanization of polyisopreneand 2,3-dimethyl-2-butene in the presence of ZnO

Polyisoprene and model compound, 2,3-dimethyl-2-butene, were vulcanized wit h N,N'-dipentamethylenethiuram disulfide (CPTD), CPTD/sulfur and N,N'-dipen tamethylenethiuram hexasulfide (CPTP6) in the presence of ZnO and zinc (bis penta-methylenedithiocarbamate) (ZPD), and residual extractable curatives a nd reaction intermediates analyzed by HPLC at various stages of the reactio n. CPTD and ZnO do not react at vulcanization temperatures and, as in zinc- free systems, the reaction is initiated by accelerator polysulfides that le ad to the formation of thiuram terminated pendent groups. Pentamethylenedit hiocarbamic acid, liberated in the process, is trapped by ZnO as ZPD. Cross linking is more rapid than in the absence of ZnO. ZPD catalyzes the crossli nking of thiuram pendent groups, and high crosslink densities are achieved because the ZPD catalyzed crosslinking of thiuram crosslink pendent groups is more efficient than when thiuram pendent groups crosslink, mainly via a reaction with thiol pendent groups, as is suggested in the absence of zinc. When free sulfur is available in the system, ZPD acts as an accelerator in its own right. Thiuram pendent groups were not detected in ZPD/sulfur form ulations with model compounds, but their formation or the formation of thio l pendent groups, either of which may form via sulfurated ZPD, cannot be ex cluded. ZPD promoted the rapid crosslinking of such pendent groups. Overall , the reaction mechanism with zinc-containing formulations is similar to th ose for the corresponding TMTD systems. (C) 2000 John Wiley & Sons, Inc.