Thermo-Oxidative Ageing Mechanisms in Polypropylene–Xylite Composites: A Multiscale Analysis of Surface, Structural, and Mechanical Degradation

This study investigates the accelerated ageing behaviour of isotactic polypropylene (iPP) composites containing 10 and 25 wt.% xylite using a multiscale approach combining water sorption, contact angle and surface free energy measurements, DSC, WAXS, FTIR, SEM, and mechanical testing. Xylite incorporation significantly modified the surface and structural response of iPP during ageing. Although the composites initially exhibited increased hydrophobicity and a lower polar contribution to the surface free energy, exposure for up to 1000 h progressively altered their wetting behaviour. Xylite-containing composites also showed higher long-term water uptake than neat iPP, particularly at 25 wt.% filler content, indicating increased contributions from filler accessibility, interfacial pathways, and ageing-induced microstructural changes. DSC and WAXS revealed a complex evolution of the semicrystalline iPP matrix, indicating that oxidative degradation occurs concurrently with crystalline reorganization. These structural changes were reflected in the mechanical response, with xylite-containing composites showing greater retention of mechanical stability during prolonged ageing than neat iPP. SEM observations further confirmed progressive surface and interfacial deterioration. Overall, the results demonstrate that xylite modifies the ageing pathway of iPP through coupled surface, structural, moisture-related, and interfacial processes. The multiscale analysis highlights filler content as a key factor controlling the balance between structural reorganization and ageing-induced damage in iPP–xylite composites.