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ABSTRACT
The chemistry of accelerated sulfur vulcanization is reviewed and a fundamental kinetic model for the vulcanization process is developed. The vulcanization of natural rubber by the benzothiazolesulfenamide class of accelerators is studied, where 2-(morpholinothio) benzothiazole (MBS) has been chosen as the representative accelerator. The reaction mechanisms that have been proposed for the different steps in vulcanization chemistry are critically evaluated with the objective of developing a holistic description of the governing chemistry, where the mechanisms are consistent for all reaction steps in the vulcanization process. A fundamental kinetic model has been developed for accelerated sulfur vulcanization, using population balance methods that explicitly acknowledge the polysulfidic nature of the crosslinks and various reactive intermediates. The kinetic model can accurately describe the complete cure response including the scorch delay, curing and the reversion for a wide range of compositions, using a single set of rate constants. In addition, the concentration profiles of ail the reaction intermediates as a function of polysulfidic lengths are predicted. This detailed information obtained from the population balance model is used to critically examine various mechanisms that have been proposed to describe accelerated sulfur vulcanization. The population balance model provides a quantitative framework for explicitly incorporating mechanistically reasonable chemistry of the vulcanization process.
I. INTRODUCTION
The discovery of sulfur vulcanization of rubber by Goodyear and independently by Hancock more than 150 years ago was the genesis of one of the important classes of engineering materials. A vigorous research effort continues today to develop a fundamental understanding of this complex chemical process. Although the initial work by Goodyear and Hancock was concerned with the addition of sulfur to natural rubber, various accelerators, activators and retarders have been subsequently employed in order to better control the various aspects of the rate and extent of the vulcanization process. More recently, mixed accelerator systems like thiuram-sulfenamides have been developed to allow additional control over the vulcanization process. It is not surprising that the cure chemistry is complicated for these complex mixtures of elastomer, sulfur, accelerators, activators, and retarders. Notwithstanding the considerable research in studying the vulcanization process, a fundamental understanding is far from complete as clearly stated by two pioneers in the field:
"Perhaps nowhere in chemistry is there encountered a field which even...