Rubber sulfur vulcanization – a long way walked but still a lot of questions to be answered

Since the accidental discovery of vulcanization by Charles Goodyear, there has been continued progress toward the improvement of the process. Even though almost two centuries of improvements have greatly modified the rubber formulations increasing their complexity, the recipes still use sulfur as vulcanizing agent. Many other curing systems have been discovered, but, so far, none of them has been able to establish the overall balance of desirable properties that is achieved by modern sulfur-based systems. There are several major problems scientists have to deal with when studying the mechanism of rubber vulcanization, i.e. many simultaneous reactions, accelerators interactions, handling of the samples during study, etc. Moreover, the low concentration of chemical crosslinks, the insolubility of the polymer three-dimensional network after vulcanization and the complexity of discriminating between different types of crosslink's, makes its structural analysis very thorny. Although significant improvements have been made in the field of physical analysis, most of the present knowledge comes from the use of model systems. Advantages of this low molecular weight model approach are the solubility of the reaction products and the use of standard separation and spectroscopic techniques. Our group has been working in the use of squalene as model molecule of Natural Rubber in vulcanization studies. Interesting results in the role of ZnO, the chemistry occurring during the scorch time, or the synergetic effect related with the use of a mixture of accelerators have been obtained. The aim of this paper is to give an overview of the progress achieved in vulcanization chemistry in the last 10 years and to show some of the questions that are still under discussion.