Peroxide vulcanization: correlations between rheometric cure curves and peroxide decompositions kinetic.

In the paper, a combined numerical-experimental approach for the prediction of the activity of different peroxides at different vulcanization temperatures for EPM-EPDM rubber curing is proposed. Several experimental tests were conducted on the same recipe of an EPDM elastomer used in industry for medium and high voltage isolation cables, changing both peroxides used and controlled vulcanization temperature. In particular, test were conducted in a range temperature varying between 160 and 200°C, with a 20°C step, using different peroxides, changing their concentration and considering also a mixture of three of them. A huge amount of rheometric cure curves were obtained (one for each temperature, peroxide used and concentration) and quantitatively compared one each other, to have an insight into the most effective temperature and peroxide to use during such kind of vulcanization, in terms of both vulcanization velocity and maximum torque reached. In order to interpret theoretically experimental results obtained, a peroxide decomposition kinetic model is presented, considering the complex set of reactions that characterize peroxide crosslinking, namely homolytic cleavage of peroxide, hydrogen abstraction, polymer radicals formation, polymer radicals coupling, cross-link formation and polymer scission. Starting from the above -most probable- kinetic scheme, a system of non-linear differential equations with few variables is obtained and solved numerically, after suitable variable substitutions, by means of a Runge-Kutta algorithm. Crosslinking density is thus evaluated numerically and compared to rheometer experimental curve, which may provide a reliable indication of the average cured rubber crosslinking, once suitably scaled to unitary values. By means of the numerical approach proposed, an estimation of the kinetic constants per single reaction, once known kinetic decomposition laws, may be evaluated and hence the activity of each peroxide may be estimated using a few experimental data at only two different temperatures.