Thermooxidative degradation of polyolefins in the solid state .1. Experimental kinetics of functional group formation

The knowledge accumulated so far on thermal oxidation of polyolefins does not yield a model of the processes that is sufficiently accurate to account for a majority of the experimental data accumulated with unstabilized as well as with stabilized polymers. The present paper is concerned with some aspects of thermal oxidation of polyolefins, especially polyethylene, in the solid state. In this respect, the kinetics of thermal oxidation of polyethylene reported in the literature are tested with numerous experimental data generated in our laboratory. It is found that the experimental and formal kinetics proposed so far do not account for our results. Thus, instead of quadratic or biquadratic formation of carbonyl groups with oxidation time as reported in the literature, the rate law seems more complicated. It is found that there is an exponential type increase in the early stages of the oxidation. This is followed by a linear increase in later stages. The discrepancy with the literature results is attributed on the one hand to the fact that the conclusions were based on a very limited amount of experimental results. It is attributed on the other hand to the use of fitting to power laws. The possible pitfalls associated with this procedure are discussed in detail. (C) 1996 Elsevier Science Limited.