Physico-chemical aspects of polyethylene processing in an open mixer. Part 16: Mechanisms and kinetics of ketone formation at low temperature

Ketones are formed essentially on various reactions of polyethylene hydroperoxide. In the low temperature range (150-160 °C), some reactions are the same as in the high temperature range (170-200 °C) of the experiments. However, there is much more complexity at low temperature than at high temperature. The experimental kinetics reveals three significantly different processes compared to only two at high temperature. Ketone formation according to a constant rate results from the cage reaction between a peroxy radical and a hydroperoxide group during the chain propagation reaction. One of the reactions envisaged for the ketones formed according to the rate increasing with processing time in the initial stages is similar from the chemical point of view to the reaction yielding ketones according to the constant rate. The reaction proceeds between the hydroperoxides accumulated in an elementary volume and the peroxy radicals responsible for an additional oxidation of this volume. It cannot be accounted for by formal homogeneous kinetics that accounts well for the constant rate but by the heterogeneous kinetics taking into account repeated oxidation of elementary volumes. The second possibility of ketone formation according to the rate increasing with time is based on bimolecular hydroperoxide decomposition involving mainly associated hydroperoxides. The kinetic treatment for this process combines the heterogeneous kinetics of oxidation volume overlapping with monomolecular decomposition of associated hydroperoxides. The rate constant deduced from the data is in agreement with literature values as well as with the values deduced previously from the experiments with PE melts.