Physico-chemical aspects of polyethylene processing in an open mixer: Part 11: heterogeneous kinetics of hydroxyl group formation

Abstract

The work proposes a new model for polyethylene melt oxidation. It is aimed at understanding the experimental kinetics for polyethylene oxidation products. All the models envisaged previously and based on homogenous kinetics do not give a reasonable account of hydroperoxide and alcohol group formation. It is even so on superimposing geminate recombination of free radicals onto homogeneous kinetics. It is the mandatory assumption of geminate recombination of the chain propagating radicals that suggests an entirely heterogeneous oxidation model for polymer melts. It involves oxidation restricted to small portions of the melt, the oxidation volumes or domains in which clusters of oxidation products are formed. These elementary volumes contain the oxidation products resulting from the action of one pair of free radicals until their geminate recombination. At the beginning of the processing experiments the elementary oxidation volumes are mainly isolated. With advancing processing time, the new elementary oxidation volumes are formed increasingly in parts of the melt already oxidized previously. As a result, there is possibility of reaction between the new oxidation products and those present before in the elementary volumes. The kinetics of overlapping oxidation, i.e. of repeated oxidation of the same elementary oxidation volumes is straightforward. There results a simple mathematical model for the oxidation processes in polyethylene melts. This model accounts easily for the experimental kinetics of hydroperoxide and alcohol group formation.