Enhancement of second life cycle potential of polypropylene exposed to combined thermo-oxidative aging during an automotive use phase and thermo-mechanical aging in mechanical recycling processing

Abstract Plastic components undergo changes during the use and recycling phases, altering their material properties. Two relevant aging processes are identified for polypropylene (PP), a commonly used polymer in the automotive industry. During the use phase, thermo-oxidative aging occurs, while the production and recycling phase is dominated by thermo-mechanical aging. Thermo-oxidative aging typically causes changes starting at the surface. Mechanical recycling can partially undo these aging effects. During the mechanical recycling process involving grinding, extrusion, and finishing processes like injection molding, thermo-mechanical aging results in further property changes. This study examines potential options for enhancing the quality of PP after mechanical recycling. To evaluate the aging behavior, two life cycles were simulated, consisting of a combination of use phase and mechanical recycling. The use phase was simulated by accelerated thermal aging at 135 °C for three weeks. The study explores two approaches to improve the quality of recycled PP for a second life cycle. Firstly, the altered polymer chains near the surface were removed through friction grinding. Secondly, post-stabilization scenarios were performed during the recycling process. While the post-stabilization showed a clear effect on the aging resistance of the recyclates, the effects of the frictional grinding were not evident.