Oxygen uptake measurements to identify the cause of unexpected differences between accelerated and outdoor weathering

Abstract

For unstabilised PE the rate of oxygen uptake and CO and CO2 formation, the increase in carbonyl absorbance and the deterioration of the elongation at break are highest for two different accelerated weathering tests and less high in outdoor weathering tests in Florida and in the south of France. These rates are lowest in degradation tests in the Netherlands. The degradation rate of unstabilised PE in an accelerated test depends on the temperature. At 40–50°C (Suntest) degradation is faster than at 29°C (Weather-Ometer). However, in outdoor weathering tests this is not the case. Although the mean average temperature in Florida is higher than in the south of France, the degradation rates of unstabilised PE are about the same in both cases, while for the stabilised PE the degradation rate in the south of France is even faster than in Florida. In the case of unstabilised PE the outdoor weathering test (Florida, the south of France and the Netherlands) and the accelerated degradation test at the highest temperature (30–40°C) yield comparable relationships between the formation of carbonyl groups or the deterioration of mechanical properties and the oxygen uptake. In stabilised PE, more oxygen is required during weathering in the Netherlands than during accelerated weathering (Suntest) to achieve a comparable carbonyl absorbance or decline in mechanical properties. The results are explained by assuming oxygen uptake via a charge transfer complex (CTC) initiation mechanism and by `normal oxidation'. In unstabilised PE `normal oxidation' is the most important oxygen uptake mechanism, which explains the good correlation between the results of an accelerated test (Suntest) and outdoor weathering. In the stabilised PE the oxygen uptake is due mainly to the CTC initiation mechanism, which explains the poor correlation between outdoor weathering in the Netherlands and the accelerated test (Suntest).