High-speed processing, which normally is associated with high cooling rates, is a general trend in the industry today. For semicrystalline materials, this means solidification at comparatively lower temperatures. For a polymorphic material like PP this may result into a partial „flip“ into other, less stable and less favorable modifications like the mesomorphic phase. Overall, the ageing process of PP is determined by the nature of the polymer, the type and conditions of conversion and the temperature level of ageing. The cumulative effects of crystallization behavior of the polymer and cooling history determine the crystallinity and superstructure of the formed article, which in turn determines the ageing behavior. On the temperature scale, two or (for an impact copolymer) three clear transitions related to the mobilization of parts of the material can be identified [2], which clearly limits the applicability of accelerated tests at elevated temperatures to predict long-term behavior.
Special attention must also be paid to the morphology changes in multiphase materials. The results gathered for the PP impact copolymer studied here are only indicative of the possible property changes resulting from improved interactions between the matrix phase and the dispersed elastomer. Finding physically meaningful explanations and models for the processes occurring in physical ageing and post-crystallization of PP will, however, require some additional investigations