Nitrile rubber–bentonite composites: a thermal degradation study

The aim of the present work was to study the kinetics of the thermal degradation mechanisms of NBR–bentonite composites by means of three methods: E2 Function, Coast-Redfern, and Invariant Kinetics Parameters (IKP). Composites with 10, 20, and 30 phr of pristine bentonite and of treated bentonite with octadecylamine were prepared. Thermogravimetric analysis was followed at different heating rates (5, 7, 10, and 13 °C/min) under N2 atmosphere. The kinetic parameters values calculated according to the E2 Function reflect a decrease in the activation energy with the increase in the filler content, being this effect more pronounced for the treated filler, as an indicative of a lower thermal stability. With respect to the IKP method, the activation energy of the different composites lies between 262 and 293 kJ/mol. The distribution of probabilities associated to the 18 kinetic functions indicate that all the studied systems present the kinetic model of nucleation and nuclei growth as the most probable degradation mechanism, S3 with 24% of probability and S1, S2, and S4 with 12%; followed by the interphase reaction model with the S6, S7, and S8 functions with a probability ranging between 13 and 10%. In addition, we can conclude that both types of bentonites employed in this study promote an increase in the probability of the diffusion mechanism, being this fact an indicative of the formation of small molecules that diffuse in the interphase and give rise to degradation mechanisms corresponding to the reaction order kinetic model. Finally, we can say that the results of the thermal study can be corroborated by the rheological behavior and the morphology of the NBR–bentonite composites.