Effect of thermo-oxidative aging during use phase on mechanical and thermal properties of polypropylene used in automotive applications

Abstract Semicrystalline polypropylene (PP) undergoes changes in its mechanical and thermal properties over its lifecycle, primarily due to thermo-oxidative aging during the use phase and its exposure to environmental influences. This study examines the aging behavior and its effect on mechanical and thermal properties of PP, mainly used in the automotive industry, after accelerated thermal aging at temperatures of 120 °C, 135 °C, and 150 °C for one and three weeks. Thermo-oxidative aging, based on oxygen diffusion into the amorphous areas of the polymer, results in the damage of polymer chains near the surface due to oxidation. Since the degradation of PP is a heterogeneous process starting at the surface, we developed a new sampling method to analyze different aging phenomena across the sample’s cross-section. Using a microtome, we analyzed samples in differential scanning calorimetry (DSC) with a reproducible thickness at specific and defined depths. The reaction of oxygen with the radicals from polymer chains can be measured by changes in mechanical properties, a decrease in the melting temperature, and a shift in the size of the crystallites towards smaller structures. However, the presence of functional groups indicating degradation (carbonyl groups) could not be verified in infrared spectroscopy.