Recycling of Polymer Composites: New Insights Into Structure–Radical Relationships and Their Impact on Material Properties and Stability

This study reports an investigation of polypropylene (PP) bio‐composites reinforced with waste black cherry wood subjected to six consecutive mechanical recycling cycles, with a particular focus on the fundamental relationship between radical chemistry, crystalline structure, and macroscopic material properties. A comprehensive structure–radical–property relationship is established, demonstrating how recycling‐induced radical formation and subsequent reactions govern polymorphic transformations in the PP matrix and, in turn, determine the mechanical and thermal performance of the composites. Using X‐ray diffraction, electron paramagnetic resonance spectroscopy, mechanical and impact testing, and thermogravimetric analysis, we show that repeated recycling leads to pronounced structural homogenization of the PP matrix and inhibits the β → α polymorphic transition. This effect is directly correlated with the evolution and stabilization of radical species generated during reprocessing. As a result, the β‐phase content in the PP matrix increases by approximately 140% after six recycling cycles compared to the initial cycle, accompanied by a 30% enhancement in impact strength and a 57% increase in relative elongation at break. Thermal analysis further reveals the formation of a thermostabilizing char structure at elevated temperatures, contributing to enhanced thermal stability of the recycled composites. These findings provide fundamental insight into radical‐mediated structural stabilization mechanisms in recycled polymer composites and demonstrate how controlled recycling can be leveraged as a tool to tailor crystalline morphology and properties. The results advance the molecular‐level understanding of degradation‐structure–property relationships in recycled polyolefin‐based composites and offer a framework for the rational design of high‐performance materials from secondary resources.