A Kinetic Model for Accelerated Sulfur Vulcanization for Natural Rubber
Event:156th Technical Meeting (INCOMPLETE) Location: Orlando, FL Date: September 21, 1999 Author: Prasenjeet Ghosh, Santhoji Katare, Priyan Patkar, and James Caruthers Paper Number: 7
A fundamental kinetic model has been developed for the sulfur vulcanization of natural rubber with a 2-4-morpholinothio-benzothiazole sulfenamide (MBS) accelerator. The model was developed using population balance methods that explicitly acknowledge the polysulfidic nature of the crosslinks and various reactive intermediates. This approach allows for a more faithful incorporation of the underlying molecular mechanisms that control the vulcanization process. For the MBS vulcanization system we need a total of nine rate constants: two to describe the formation of the sulfur-accelerator intermediate, four to describe the primary cure reaction including scorch delay, two to describe the regeneration and degradation of the accelerator, and one final constant to describe the long-time reversion of the crosslinks. The rate constants can be efficiently and robustly determined from isothermal oscillating disk rheometer cure data at three different sulfur/accelerator concentrations. The kinetic model can quantitatively predict the complete cure response including induction, curing and the reversion for a complete range of compositions; moreover, the population balance model can be readily extended to also include the retarder chemistry. The model not only predicts the evolution of the crosslink density, but also the concentration of all the intermediates of different polysulfidic lengths - important information that is difficult to measure experimentally. The level and detail of information obtained from the model can be used effectively to understand the cure and post-cure behavior of an accelerated vulcanization system and to determine the optimal amounts of accelerator, sulfur, activator and other ingredients needed to design the formulation for a particular engineering application.