Optimized material composition to improve the physical and mechanical properties of extruded wood–plastic composites (WPCs)

Wood–plastic composite (WPC) is an environmentally progress way of combining recycled plastics and wood flour. The composite typically consists of four major elements: wood flour, thermoplastic plastics, coupling agent, and lubricant. The physical and mechanical properties of WPCs highly depend on the material formulation, and the optimal material composition is an essential topic of current research. This study investigated the effects of changing material compositions on the physical and mechanical properties of WPCs. The studied WPCs were extruded molding WPCs manufactured using recycled polypropylene (RPP) plastics and wood flour. The study evaluated four parameters: (1) wood flour particle size; (2) coupling agent dosage; (3) lubricant content; and (4) the mass ratio of wood and RPPs. The results showed that using finer wood flour (smaller than 125 μm) can improve the tensile and flexural strength of WPCs, and reduce the swelling due to water adsorption. The optimal concentration of the coupling agent (Maleic Anhydride Polypropylene) and lubricant (Zinc stearate) in WPCs were both 3%. Adding the proper amount of coupling agent can improve the mechanical properties and significantly reduce the swelling, but over-dosing the lubricant significantly increased swelling and reduced all the mechanical properties of the WPCs. Maintaining wood content at 50% or less produced the best mechanical properties, and wood content above approximately 50% resulted in reduction of all physical and mechanical properties of WPCs. The study demonstrated the relationship between moisture adsorption to thickness swelling. Reducing thickness swelling from water adsorption, or reducing the exposure of wood fibers to atmosphere, may also improve all the mechanical performances of WPCs.