Thermooxidative degradation of polyolefins in the solid state .5. Kinetics of functional group formation in PE-HD and PE-LLD

The experimental kinetics of oxidation product formation on thermal oxidation of PE-LLD and PE-HD do not correspond to the rate laws reported in the literature. Instead of a quadratic or biquadratic formation of carbonyl groups with oxidation time, the rate law seems to be more complicated. There is an exponential-type increase in the early stages of the oxidation process, followed by a linear increase in the later stages. As previously seen with PE-LD, the discrepancy with the results in the literature is attributed on the one hand to the fact that the conclusions were based on a very limited amount of experimental results, and on the other hand, to the use of fitting to power laws. The heterogeneous oxidation model and the corresponding kinetics developed for PE-LD also explain the formation of free hydroperoxides, associated hydroperoxides and carbonyl groups in PE-LLD and PE-HD. Hence, they seem to be valid for polyethylene in general. The oxidation spreading rates are affected by polyethylene density as well as by the chemical nature of the catalyst residues. There is usually some similarity between PE-LLDs, although they are different from PE-LDs. There is also some similarity between different Ziegler-type PE-HDs. However, they behave differently from both Phillips-type PE-HDs and PE-LLDs. (C) 1996 Elsevier Science Limited