Antioxidant ‘crossover effect’ in oven ageing of polypropylene

Abstract Thermal oxidative stabilities of polypropylene formulations are frequently assessed by ageing samples in circulating-air ovens. A 150°C test temperature is almost an accepted standard throughout the polymer industry although it often bears little relation to the actual service temperatures seen during the life of the polypropylene which are frequently below 100°C. Phenolic antioxidants are necessary for the extended service life of polypropylene. When the antioxidants are compared over a range of evaluation temperatures, their relative performance can change quite markedly. As evaluation temperatures are decreased and approach actual use temperature, the chemical kinetics of stabilization and the influencing physical characteristics of the antioxidant appear to change causing a reversal in the relative effectiveness between two high molecular weight antioxidants. This has been defined as the ‘crossover effect’. This understanding should allow for improved predictions of polymer service life.