Physico-chemical aspects of polyethylene processing in an open mixer. Part 17. Effect of oxygen availability

The gas in contact with polyethylene has considerable impact on its oxidation. The rate of oxidation product formation is mostly larger with oxygen blanketing than in air. Similarly, the rate in air is larger than that under nitrogen blanketing. Moreover, the relative effect of the surrounding gas is depending heavily on the particular oxidation product considered. The effect on the alcohol concentration on passing from air to pure oxygen is the same as that on the hydroperoxide concentration. It is only under pure nitrogen that alcohol formation is relatively more affected than hydroperoxide formation. The overall carbonyl groups as well as the ketones show the expected ranking, i.e. faster rate in pure oxygen than in air and faster rate in air than under pure nitrogen. However, carboxylic acids are formed much faster in oxygen than in air. For the acids the results in air and under nitrogen are significantly closer in the initial stages of processing than the results obtained under pure oxygen. This is different for γ-lactones for which formation is faster in oxygen than in air where it is faster than under nitrogen. With trans-vinylene groups the situation is opposite to that observed for carboxylic acids: the rate of formation is close for the experiments performed under air and under oxygen and significantly faster than under nitrogen. The results for hydroperoxides, alcohols and ketones are easily interpreted taking into account the kinetics developed in previous work. Fitting the data to the heterogeneous kinetics shows the effect of the oxygen concentration on this kinetics. It is especially unexpected with respect to its impact on the initiation rate. It is discussed taking into account various possibilities. The only one that is compatible with all the data envisages chain initiation resulting from interaction of oxygen with strained polymer molecules.