Synergistic Improvement of Mechanical, Creep, and Dimensional Stability in Ultra-Highly Filled Wood Fiber/Polyethylene Composites Using Multimodal Alloy Matrices

Enhancing the mechanical properties and dimensional stability of ultra-highly filled wood-plastic composites (UH-WPCs) presents significant challenges. This study developed UH-WPCs with 70–90 wt% loading using six binary alloy matrices with multimodal and unimodal distributions. FTIR spectroscopy and thermogravimetric analysis revealed an average MAPE esterification rate of 11.9% at 80 wt% loading. Density, morphology, and dynamic mechanical analysis revealed that multimodal high molecular weight alloys significantly improved uniformity and interfacial adhesion compared to unimodal alloys. This increased tensile, flexural, and impact strengths by 30.1%, 22.7%, and 61.8%, respectively, while reducing thermal expansion, creep, and water absorption by 14.0%, 17.1%, and 13.1%. The low molecular weight fraction of multimodal HDPE facilitated miscibility with MAPE, promoted esterification, and minimized fiber damage, while the high molecular weight fraction enhanced composite integrity and cohesiveness. Notably, chain entanglement within the alloy was more critical than esterification rate in improving the mechanical and dimensional stability of UH-WPCs.