Interfacial Mechanisms and Performance of Injection‐Molded Plant Fiber/Polypropylene Composites for Automotive Applications

ABSTRACT In response to the automotive industry's demand for comparatively lightweight and sustainable materials, plant fiber (straw, hemp, wood fiber) reinforced polypropylene (PP) composites were prepared by injection molding process, and the relationship between interface micro‐mechanism and comprehensive performance was systematically explored. The fiber‐matrix interface bonding strength was improved by NaOH treatment and maleic anhydride grafted polypropylene (MAPP) combined modification, and the nonlinear effects of different fiber types (7.5–30 wt%) on material properties were analyzed. The results show that wood fiber composites perform best at 30 wt% filling, with a flexural strength of 52.37 MPa, an initial thermal degradation temperature increased by 15°C, and a hardness increase of 75%; however, the presence of high‐rigidity fibers results in a substantial decrease in impact strength (3.05 kJ/m ² ), representing a 67.1% reduction relative to PP. In contrast, hemp fiber exhibits excellent impact toughness (6.26 kJ/m ² ) due to its higher aspect ratio, while straw fiber has a significant advantage in friction performance (friction coefficient reduced by 23%). Microscopic analysis shows that the interface is more uniform when the fiber content is ≤ 15 wt%, and fiber agglomeration and defects are prone to occur when the fiber content is ≥ 25 wt%. Thermal analysis results show that the heterogeneous nucleation effect of the fiber increases the crystallization temperature by 4.8°C, but excessive filling reduces the degree of crystallinity (by 30%). This study delivers a theoretical basis for applying plant fiber reinforced composites (PFRCs) in automotive structural parts by regulating fiber content and interface modification strategies.