Assessment of Avrami, Ozawa and Avrami-Ozawa equations for determination of EVA crosslinking kinetics from DSC measurements

The copolymer ethylene-vinyl acetate (EVA) was crosslinked with different amounts (1, 2 and 4 wt%) of dicumyl peroxide (DCP) in a differential scanning calorimeter (DSC), under non-isothermal conditions. Crosslinking degree as a function of temperature (or time) was calculated from the crosslinking enthalpies. The feasibility of Avrami, Ozawa, Avrami-Ozawa and Flynn-Wall-Ozawa (FWO) models for the determination of EVA crosslinking kinetic parameters was evaluated. All the models fitted the experimental data very well, providing EVA kinetic parameters such as reaction rate (k), half-life time (t1/2) and activation energy (Ea), as well as inferring the type of process dimensionalities and crosslinking mechanism. The Avrami exponents (n) were found to remain practically constant (2.08-2.32) during the course of the reaction and with varying DCP content, suggesting that the growth of crosslinks is sporadic and spherical, and occurs from nuclei. Systematic changes in Ea (FWO analysis), Ozawa exponent (m) and F(T) parameter (Avrami-Ozawa model) values showed that the increase in bulk viscosity during the reaction alters the EVA solid-state process mechanism by lowering radical diffusion and hindering the crosslink growth. On the basis of the results, we can conclude that the Avrami, Ozawa, Avrami-Ozawa and Flynn-Wall-Ozawa (FWO) models successfully describe the EVA crosslinking kinetics.