Effect of natural filler and compatibilization on HDPE-based walnut shell composites

Abstract

This study investigates the preparation, mechanical performance, and environmental degradation behavior of high-density polyethylene (HDPE) composites reinforced with walnut shell powder (WSP). Composites containing different WSP loadings were fabricated by melt blending and compression molding, while maleic anhydride-grafted polyethylene (PE-g-MA) was used as a compatibilizer to improve interfacial adhesion between the hydrophobic HDPE matrix and the hydrophilic lignocellulosic filler. The effects of filler loading and compatibilization on melt flow behavior, tensile properties, soil-burial response, FTIR, and surface morphology were systematically evaluated. The results showed that WSP incorporation increased composite stiffness, whereas PE-g-MA significantly improved tensile strength and filler dispersion through enhanced matrix–filler interactions. Soil burial tests revealed progressive moisture absorption and early-stage environmentally induced deterioration associated mainly with the lignocellulosic phase. FTIR and optical microscopy analyses confirmed chemical and morphological changes after soil exposure, while the HDPE matrix remained relatively stable during the investigated period. These findings demonstrate the potential of walnut shell powder as a sustainable reinforcement for HDPE-based wood–plastic composites and lightweight non-structural applications. Compatibilized composites exhibited improved interfacial adhesion and more homogeneous particle distribution, indicating that agricultural waste fillers can contribute to sustainable polymer composite development with reduced dependence on conventional mineral reinforcements.