High-strength, lightweight, co-extruded wood flour-polyvinyl chloride/lumber composites: Effects of wood content in shell layer on mechanical properties, creep resistance, and dimensional stability

The defects of creep deformation and brittle fracture, which coexist in wood plastic composites (WPCs), significantly restrict WPCs as structural engineering materials. This study aims to advance WPCs via structural design and co-extrusion technology, for the development of a lightweight, high-strength, tough, creep-resistant, and water-resistant composite, for structural engineering applications. Novel wood-plastic/lumber composites (WPLCs) were fabricated with high-strength lightweight laminated veneer lumber (LVL) as a core and water-resistant wood-polyvinyl chloride composites (WPVCs) as the shell layer using multi-phase co-extrusion technology. The effect of wood content in the shell layer on flexural properties, impact strength, low-velocity impact behavior, creep resistance, and dimensional stability of the WPLCs were investigated and compared with those of LVL and WPVCs. Increasing wood content within the scope of the experiment resulted in increased flexural strength and modulus in both WPVCs and WPLCs. The flexural modulus and impact strength of WPLC containing 60 phr wood in shell layer (WPLC60) were significantly increased by 112.8% and 303.6%, respectively, compared with that of the corresponding WPVC60 control. Factoring in composite density, the comparison between WPLCs and WPVCs showed specific flexural modulus and impact strength of WPLC60 as 284% and 704% of the values of WPVC60, respectively. The WPLCs exhibited higher impact resistance and bending stiffness and lower absorbed energy and permanent indentation than those of LVL. The creep strain of the WPLCs decreased with increasing wood content in the shell layers. Specifically, the creep strain of WPLC60 was 85.8% and 28.9% of the values of LVL and WPVC60, respectively. The WPLCs almost kept their original appearance except for discoloration after nine cycles of hot water immersion. The water absorption and thickness swelling of the WPLCs were below 1.0% and considerably lower than those of the LVL and pure PVC co-extruded samples. The experimental results indicated that the novel co-extruded wood-plastic/lumber composites effectively achieved excellent performance suitable for structural engineering applications.