Improving Properties of Wood-plastic Composites with High-density Reinforcements under Water Immersion

Effects of high-density agricultural by-product fillers, coconut shell flour (CSF), and palm kernel shell flour (PKSF) were investigated relative to the physical and mechanical properties after water immersion of wood-plastic composites (WPCs), compared to conventional rubberwood flour (RWF). The WPCs were fabricated with filler contents of 30, 40, and 50 wt% and subjected to long-term water immersion for 8 weeks. Morphological analyses revealed that CSF and PKSF particles exhibited denser and more angular structures than RWF, thereby enhancing filler-matrix interfacial compatibility. Mechanical testing showed that modulus of rupture decreased with increasing filler content while modulus of elasticity increased, and RWF composites yielded the highest initial strength. Under prolonged water exposure, CSF-filled composites exhibited the lowest degradation in flexural strength, screw withdrawal resistance, hardness, and surface quality, which can be attributed to CSF’s high lignin content and structural compactness. Further, RWF-filled composites showed the most considerable deterioration. Water absorption and thickness swelling were lowest in CSF composites, confirming superior dimensional stability. These findings underscore the potential of CSF and PKSF as sustainable reinforcements to improve WPC durability in moist environments, supporting circular material utilization from agricultural residues.