Performance Degradation Mechanism of Hemp Fiber-Reinforced Polypropylene Composites Under Accelerated Aging

In the context of increasing resource scarcity and environmental concerns, the development of green composite materials is essential for promoting sustainability in the automotive industry. However, poor interfacial compatibility between plant fibers and polypropylene (PP), as well as the performance deterioration under complex environmental aging conditions, severely limits their engineering applications. In this study, a synergistic interfacial modification strategy combining alkali treatment of hemp fibers (HFs) with polypropylene grafted maleic anhydride (PP-g-MAH) was employed to enhance fiber–matrix interaction. Hemp fiber-reinforced polypropylene composites (HFRPs) with varying fiber contents (7.5–30 wt%) were fabricated via injection molding. Accelerated aging tests were conducted on the compatibilized HFRPs for up to 2400 h under ultraviolet–thermal–moisture coupled conditions, in accordance with the SAE J2527 standard. The evolution of surface color, mechanical properties, chemical structure, and microstructure was systematically characterized. After aging, surface whitening of the composites was observed. Tensile strength and impact strength decreased by 9.57–22.12% and 38.68–46.03%, respectively, while flexural strength remained relatively stable due to the supporting effect of the fiber skeleton. The aging of compatibilized HFRPs follows an outside-in progressive degradation mechanism, characterized by a stepwise cascade of surface oxidation, crack propagation, moisture ingress, interfacial degradation, and mechanical performance deterioration. These findings offer valuable insights into the long-term durability of natural fiber-reinforced thermoplastic composites and provide theoretical and practical guidance for their structural design and application in demanding service environments.