Enhancing interfacial adhesion through coupling agent incorporation in plywood/plastic waste composite materials

The objective this study was to fabricate 5-ply boards using virgin and recycled high-density polyethylene (vHDPE and rHDPE) films as formaldehyde-free adhesives, respectively. To enhance the interfacial adhesion between the wood veneer and HDPE film, various loadings (0, 3, 6, 8, and 11 wt%) of polyethylene grafted maleic anhydride (PE-g-MA) were applied as a coupling agent. The physical and mechanical performance of the resulting plywood boards was evaluated using standard test methods, with a UF resin-bonded plywood used as a control for comparison. All materials used were for commercial-scale plywood production. Fourier transform infrared spectroscopy (FT-IR) results provided valuable information about chemical interactions that occurred during the coupling of poplar fibers with HDPE using PE-g-MA. The experimental results showed that the mechanical and physical properties of the plywood boards improved as the amount of coupling agent increased from 0 to 11 wt%. PE-g-MA played a crucial role in the properties of plywood boards made with rHDPE and vHDPE by enhancing the interfacial bonding ability between the HDPE and adjacent veneers, leading to good stress propagation and improved mechanical properties. The properties of samples bonded with vHDPE were significantly more improved than those made with rHDPE, likely due to the presence of color pigments, additives, fillers, and impurities in the rHDPE, which made the coupling agent less effective. In addition, the results demonstrated that the samples made with rHDPE and vHDPE were superior to the plywood bonded with UF resin (control), and the optimal content of the coupling agent was found to be 8 and 11 wt%. These findings suggest that incorporating recycled HDPE films and using coupling agents, such as PE-g-MA, can produce high-quality plywood composite materials with improved mechanical and physical properties, providing an eco-friendly alternative to traditional UF resin-bonded plywood.