The thermo-oxidative degradation of metallocene polyethylenes. Part 1: Long-term thermal oxidation in the solid state

The long-term thermo-oxidative degradation of different metallocene polyethylenes (mPEs) was investigated in the solid state (oven ageing in air at 90 °C). The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content. Two Phillips-type HDPEs were also studied for comparison. The rate of oxidation was assessed by carbonyl index measurements. Initial vinyl unsaturation and short chain branch contents were correlated with induction times. The hydroperoxide concentrations of the aged materials and the levels of metal catalyst residues were also evaluated. Outstanding oxidative stability was exhibited for all the mPEs in the solid state relative to the Phillips HDPEs. This was believed to be due to the low concentration of both adventitious metal catalyst residues and initial vinyl unsaturation. Degree of short chain branching also appeared to influence the oxidative stability of the mPEs. Besides inherent properties, the density/crystallinity appeared to be the main factor influencing their oxidative stability. In the case of the Phillips HDPEs, the high level of vinyl unsaturation together with chromium catalyst residues may contribute to their poor oxidative stability.