Physico-chemical aspects of polyethylene processing in an open mixer. Part 18: Mechanisms and kinetics of trans-vinylene group formation

Numerous reactions can yield trans-vinylene groups on polyethylene oxidation. The first problem on data interpretation consists in the separation of the experimental data into components corresponding to well-defined mechanisms. This is achieved by fitting the experimental data into an equation comprising a linear and a parabolic term. The linear term corresponds to trans-vinylene formation at a constant rate from the beginning of the experiments. It can be attributed to trans-vinylene formation on direct decomposition of polyethylene peroxy radicals. The second term accounts for trans-vinylene group formation on cage reactions of various free radicals resulting from hydroperoxide decomposition.The first mechanism can be interpreted by formal homogeneous kinetics. Formation of trans-vinylene groups according to the second mechanism can be accounted for by the heterogeneous kinetics. It proceeds in parallel with the formation of alcohols and ketones. However, reaction of the double bonds with various reactive species in the oxidizing polymer melt does not only lead to a limiting value of the concentration in the advanced stages of polyethylene processing, but also affects the accuracy of the calculations already in the early stages.