Physico-chemical aspects of polyethylene processing in an open mixer: 8. Various reactions of polyethylene hydroperoxide in the melt

Abstract

Hydroperoxides generated on polyethylene processing in air show complex behavior. The maximum hydroperoxide concentration reached in the experiments decreases significantly with increasing processing temperature. In the temperature range 150–160 °C, the maximum of the hydroperoxide concentration is determined by preferential reaction of the peroxy radicals with the hydroperoxides accumulating in the melt. Abstraction of the tertiary hydrogen atom of the secondary hydroperoxide by a peroxy radical yields a new hydroperoxide group. Simultaneously the original hydroperoxide is transformed into an α-alkyl-hydroperoxy radical. Monomolecular decomposition of the last yields a ketone and a hydroxyl radical continuing the chain reaction. Thus, the hydroperoxide group destroyed is replaced by another one so that the concentration remains practically constant for some time, especially at 150 °C. In the temperature range 170–200 °C, the maximum of the hydroperoxide concentration is determined by bimolecular decomposition of hydroperoxide groups sufficiently close. This is a consequence of initial non-homogeneous distribution of the hydroperoxides in the melt. It is caused by “clusters” of hydroperoxide groups resulting from the reactions initiated by a pair of primary radicals before geminate termination. A concerted reaction involving two hydroperoxide groups as well as the tertiary hydrogen atom of one of the hydroperoxide groups accounts for the data. The mechanisms and kinetics of the reactions are discussed.