Hygrothermal aging and mechanical performance of reprocessed polypropylene/olive stone flour composites

The increasing demand for sustainable materials has driven significant interest in natural fiber–reinforced polymer composites as environmentally friendly alternatives to conventional systems. However, challenges such as weak interfacial adhesion, limited compatibility between hydrophobic polymer matrices and hydrophilic reinforcements, and potential degradation after recycling remain critical issues affecting their long-term performance. This study investigates the effect of hygrothermal aging on the mechanical and morphological behavior of polypropylene (PP) composites reinforced with 50 wt.% olive stone flour (OSF), before and after injection molding reprocessing. The composites were compatibilized with 5 wt.% maleic anhydride-grafted polypropylene (MAPP) to enhance interfacial bonding. Water absorption behavior was analyzed to determine diffusion kinetics and assess its influence on mechanical properties. Results reveal that water uptake follows a Fickian diffusion mechanism for both virgin and reprocessed materials. Hygrothermal aging leads to a reduction in tensile strength and stiffness; however, the reprocessed composites exhibit a lower degradation rate compared to virgin materials. Interestingly, reprocessed samples show improved impact strength and elongation at break, which is attributed to the plasticizing effect of absorbed moisture. Scanning Electron Microscopy (SEM) observations confirm that interfacial adhesion between the PP matrix and OSF particles remains relatively stable after reprocessing and aging, supporting the mechanical results. Overall, the findings demonstrate that reprocessed PP/OSF composites can maintain satisfactory mechanical performance under hygrothermal conditions, highlighting their potential as sustainable materials for applications subjected to combined humidity and temperature exposure.