Physico-chemical aspects of polyethylene processing in an open mixer 3. Experimental kinetics of functional group formation

The work is an attempt to better understand polyethylene processing in air. This requires first of all good knowledge of the functional groups formed on processing in air. Secondly, it requires the precise rate laws for the formation of these functional groups. To this avail, the experimental kinetics of the main functional groups has been determined. It is compared to literature results. For every functional group the data are fitted to elementary mathematical relations. The best fits found are compared to corresponding expressions from the literature. The comparison shows in part significant differences that are discussed in detail. The formation of some functional groups shows considerable changes with an increase in processing temperature. This is particularly true for the total hydroxyl group concentration (alcohols and hydroperoxides) which is increasing linearly with processing time from a temperature of 170 degrees C on. For lower temperatures (150 and 160 degrees C), the initial linear increase, comparable to that at higher temperature, is followed by a second linear increase corresponding to a considerably reduced rate. For elucidation of the oxidation processes, the results will need to be confronted with the possible laws deduced from formal homogeneous kinetics applied to polymer melts. This will be the object of forthcoming work. (C) 2000 Elsevier Science Ltd. All rights reserved.