Reactivity Optimization of Prevulcanization Inhibitors

Abstract

Abstract A series of phthalimide-based prevulcanization inhibitors has been used to probe the relationship between MBT reactivity and inhibitor performance efficiency in NR accelerated with TBBS. Based on these results, there appears to be a reactivity window for best performance. For the phthalimide series, it occurs with secondary alkyl mercaptans. Inhibitors with MBT reactivity below the optimum are less effective because they do not compete with the accelerator for the autocatalyst. The activity also falls off at higher reactivities. There are two explanations for this behavior. First, as proposed by others, the very reactive inhibitors may not completely survive mixing and precuring, so that some of the inhibitor is lost to unproductive side reactions. Over and above this effect, the performance declines because the MBT is trapped less securely when the BtSSR is derived from a more reactive mercaptan. Although other leaving groups were not evaluated in this work, it is clear that reactivity optimization can be achieved with any leaving group, L, by proper selection of -SR. For example, in the work of Morita and Sullivan with thioketal type prevulcanization inhibitors, it was demonstrated that primary alkyl and aromatic mercaptans were required to achieve optimum inhibitor activity.