Elucidating pathways of polypropylene chain cleavage and stabilization for multiple loop mechanical recycling

Mechanical recycling is a decisive contribution for a sustainable circular economy of polypropylene (PP) but faces the challenge of processing-induced and use-phase accumulated molecular damage, exacerbated by multiple loop recycling. In this contribution, the molecular origin of degradation of PP under up to 30 repetitive reprocessing loops is analyzed together with the influence of stabilization, and processability data linked to changes of the entire molecular weight distribution. By comparison of experiments with simple Monte Carlo simulations, a coupled mechanism of random scission and shear-induced cleavage can be inferred, while stabilizing additives effectively protect the material from progressing along this combined degradation pathway.