Physico-chemical aspects of polyethylene processing in an open mixer. Part 22: Mechanisms and kinetics of vinyl and vinylidene group consumption

The reactions of vinyl and vinylidene groups in oxidizing polyethylene melts are partly unexpected. The main possibilities of consumption that can be envisaged are reactions with peroxy radicals, molecular oxygen, hydroperoxides and peracids. These different reactions can all contribute to some extent to the removal of vinyl and vinylidene groups. However, the dominant reactions are quite specific for the two unsaturated groups and the temperature range. Consumption of vinylidene groups results mainly from reaction with peroxy radicals and with hydroperoxides. It decreases significantly in the high temperature range in which the hydroperoxides do not accumulate. Reaction with hydroperoxides seems also to be the dominant reaction removing vinyl groups in polyethylene melts at low temperature. The reaction with peroxy radicals seems negligible in the whole temperature range of the experiments. The increasing consumption rates in the high temperature range are attributed to dimerisation involving two vinyl groups. The same reaction is thought to account for molecular enlargement in polyethylene types with significant amounts of vinyl groups. In this respect it complements macro-alkyl radical addition to vinyl groups. The contributions of the two mechanisms to molecular enlargement are discussed.