Relation between activation energy and induction in rubber sulfur vulcanization: An experimental study

The article presents the possibility to use the so-called frequency factors in order to select the optimal temperature to mix rubber blend recipes before vulcanization without inducing premature crosslinking. As secondary result of the research, it provides a very simple approach to evaluate induction assuming a first order kinetic approach. It has been frequently observed that the activation energy in a first order rubber vulcanization scheme appears to be function of the particular activators used and that activation energy links to the so-called waiting time (and induction) by means of the frequency factor through an exponential relationship. A total of 212 rheometer curves are experimentally obtained and suitably postprocessed, consisting of pure natural rubber (NR), polybutadiene (PB), and 50–50 or 70%–30% NR-PR blends in presence of different concentrations of sulfur and accelerants (diphenyl guanidine [DPG] and N-t-butylbenzothiazole-sulfenamide [TBBS]). Data reduction is carried out to have an insight into the most suitable temperature to utilize in the different cases in order to: (a) Obtain the best mixed blends without risking premature vulcanization and (b) design the induction phase without over vulcanize the blend and/or delaying curing because the time required is too long. Waiting time and induction are evaluated by means of a well-established exponential law requiring the knowledge of the activation energy (assuming for the sake of simplicity that crosslinking occurs following a first order reaction kinetic law) and a constant called “frequency factor”. Frequency factor, considering all reactions monomolecular for the sake of simplicity, is assumed of the order of magnitude of 10−13–10−14 s. All experimental data carried out are critically compared and a wide explanation of the expected induction times and most suitable temperatures to use in the mixing phase is provided in the different cases considered. The article provides a meaningful insight into the importance of the link existing among activation energy, vulcanization recipe (with particular regard to concentration of accelerants, blend between two rubbers with very different activation energies and interaction between two accelerants) and mixing temperature to adopt before curing.