N,N′‐Pentamethylenethiuram disulfide and N,N′‐pentamethylenethiuram hexasulfide accelerated sulfur vulcanization. III. Vulcanization of polyisoprene and 2,3‐dimethyl‐2‐butene in the absence of ZnO

Abstract

Polyisoprene and model compound, 2,3-dimethyl-2-butene, were vulcanized with N,NЈ-dipentamethylenethiuram disulfide (CPTD), CPTD/sulfur and N,NЈ-dipentamethylenethiuram hexasulfide (CPTP6) in the absence of ZnO and residual extractable curatives and reaction intermediates analyzed by HPLC at various stages of the reaction. Accelerator polysulfides, required for the formation of accelerator-terminated polysulfidic pendent groups, form rapidly, or are present from the outset in the case of CPTP6. Model compounds confirm the formation of thiuram-terminated polysulfidic pendent groups as intermediates in the vulcanization process. Removal of pentamethylenedithiocarbamic acid (Hpmtc) from the system during heating delays the onset of vulcanization and leads to very low crosslink densities. Rubbers heated under vacuum can subsequently be crosslinked by the addition of zinc stearate, pointing to the presence in the compound of thiuram-terminated pendent groups. However, such pendent groups do not readily crosslink on their own, and hydrogen-terminated polysulfidic pendent groups, formed by the reaction of sulfurated Hpmtc with the polymer, are suggested to be involved in the crosslink formation. N,NЈ-Pentamethylenethiurea forms in the vulcanizate, but is not as product of crosslinking reactions, rather of CPTD degradation. The data are discussed with respect to mechanisms proposed in the literature for crosslinking, and it is concluded that the data support recently formulated mechanisms in which crosslinking involves reaction between thiuram and thiol-terminated pendent groups, with Hmptc playing and essential role in the overall process.