Early‐Stage Degradation and Stabilization of Polypropylene Under Simulated Mechanical Recycling: Assessment and Practical Indicators

ABSTRACT The durability and recyclability of polypropylene (PP) require sensitive indicators of early‐stage degradation. In this study, we simulated the closed‐loop mechanical recycling of PP via repeated oven aging, grinding, and remolding using two model systems: non‐stabilized PP (PP‐NS) and antioxidant‐stabilized PP (PP‐S). We quantified stabilizer consumption and examined the relationship between additive depletion, structural changes, and mechanical performance. Among the analytical methods investigated, the total luminous intensity from chemiluminescence (CL‐TLI) and molecular weight from high‐temperature gel permeation chromatography effectively tracked early degradation before the loss of physical properties. In contrast, the carbonyl index from Fourier transform infrared spectroscopy and thermal properties were less sensitive for detecting early‐stage degradation. The oxidation induction time (OIT), measured by differential scanning calorimetry (DSC‐OIT) or chemiluminescence (CL‐OIT), also showed promise as a simple method to estimate residual antioxidant levels. These findings indicate that the CL‐TLI, CL‐OIT, and DSC‐OIT are particularly useful for evaluating oxidative degradation. These analytical methods may also serve as practical tools for recyclability screening and for guiding restabilization strategies in closed‐loop recycling systems with available initial material information.