N,N′-Pentamethylenethiuram disulfide and N,N′-pentamethylenethiuram hexasulfide accelerated sulfur vulcanization. IV. Vulcanization of polyisoprene and 2,3-dimethyl-2-butene in the presence of ZNO

Polyisoprene and model compound, 2,3‐dimethyl‐2‐butene, were vulcanized with N,N′‐dipentamethylenethiuram disulfide (CPTD), CPTD/sulfur and N,N′‐dipentamethylenethiuram hexasulfide (CPTP6) in the presence of ZnO and zinc (bispentamethylenedithiocarbamate) (ZPD), and residual extractable curatives and reaction intermediates analyzed by HPLC at various stages of the reaction. CPTD and ZnO do not react at vulcanization temperatures and, as in zinc‐free systems, the reaction is initiated by accelerator polysulfides that lead to the formation of thiuram terminated pendent groups. Pentamethylenedithiocarbamic acid, liberated in the process, is trapped by ZnO as ZPD. Crosslinking is more rapid than in the absence of ZnO. ZPD catalyzes the crosslinking 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 formulations with model compounds, but their formation or the formation of thiol pendent groups, either of which may form via sulfurated ZPD, cannot be excluded. ZPD promoted the rapid crosslinking of such pendent groups. Overall, the reaction mechanism with zinc‐containing formulations is similar to those for the corresponding TMTD systems. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1112–1120, 2000