Photo-Oxidative Stability of Recycled Polypropylene: Effect of a Repair Additive on Degradation and Mechanical Retention

The increasing use of recycled polypropylene (rPP) in technical and outdoor applications requires strategies to limit photo-oxidative degradation while maintaining adequate performance after reprocessing. In this work, the photo-oxidative stability of rPP films was investigated under accelerated weathering conditions, focusing on the effect of a commercially available additive, Nexamite® R201 (NEX), previously shown to partially restore PP molecular weight after reprocessing. Films of rPP and rPP containing 5 wt.% NEX were produced by cast extrusion and exposed to cyclic UVA irradiation and water condensation in a QUV chamber, and the evolution of the functional and structural degradation of the materials was monitored as a function of aging time. Spectroscopical analyses showed progressive oxidation in both systems, with carbonyl growth starting after an induction period of about 200 h. A faster increase in the carbonyl index was observed for rPP containing NEX, indicating that the additive does not improve chemical oxidative resistance under the adopted conditions. However, NEX significantly enhanced the retention of mechanical properties during aging, with higher elongation and stress at break compared with unmodified rPP, thus delaying embrittlement. Overall, the results show that the investigated additive effectively mitigates the loss of mechanical integrity during photo-aging, likely as a consequence of the macromolecular restructuring induced during reprocessing.