Effect of Wood Flour Incorporation on the Morphological, Thermal, and Mechanical Behavior of Recycled HDPE /Multilayer Plastic Blends

The recycling of multilayer plastic (MLP) waste is challenging due to the limited miscibility of its polymeric components. This study developed wood–plastic‐multilayer plastic composites (WPMCs) from recovered bimodal high‐density polyethylene (HDPE), post‐consumer MLP waste, and treated medium‐density fiberboard (MDF) flour. Formulations containing 5–20 wt% MLP and 10 wt% MDF were produced by extrusion and injection molding and characterized for morphology, thermal behavior, melt flow, mechanical properties, and water absorption. FTIR identified polyethylene, polyethylene terephthalate, polyamide, and other constituents on MLP surfaces, while SEM revealed heterogeneous domains and localized interfacial discontinuities. WPMCs showed lower melt flow indices than neat HDPE and significant differences in melting and crystallization enthalpies, but no significant changes in transition temperatures or apparent HDPE crystallinity. At 20 wt% MLP, Young's modulus, yield strength, and resilience increased by 6.4%, 18.2%, and 27.8%, respectively, relative to neat HDPE, although the modulus increase was not statistically significant. Conversely, ultimate tensile strength and Charpy impact resistance decreased by 25.0% and 45.9%, respectively. Water absorption increased with MDF incorporation. These findings suggest that WPMCs offer a potential route for waste valorization in secondary applications prioritizing stiffness and yield strength over ductility and impact resistance.