Strong, Durable and Eco-Friendly Plywood Bonded with Chemically-Upcycled Wasted Disposable Masks

In the post-COVID-19 era, the effective upcycle of wasted disposable medical masks (DMMs) is crucial to minimize negative environmental impacts. This study focuses on the preparation of a strong, durable and ecofriendly three-ply plywood by bonding veneers with chemically-upcycled polypropylene filter (PPF) from DMMs. Alkaline pretreatment was used to enhance the surface polarity and wettability of PPF. Acacia tannin pretreatment cross-lined the phenolic hydroxyl group of acacia tannin with wood constituents and acted as a coupling agent to connect PPF and wood. Electrochemical activation and hot-pressing were applied sequentially ("two-steps") and simultaneously ("one-step"). Both methods improved mechanical properties compared to the untreated plywood (CK). The "one-step" method showed the highest mechanical properties (bonding strength: 1.2 MPa, modulus of rupture: 65 MPa, and modulus of elasticity: 6.3 GPa). Electrochemical activation magnified acacia tannin's coupling effect, creating continuous and uniform bond lines among wood-PPF interface. The "one-step" plywood withstood significant deformation in hygro-and thermal-environments, with less strain distribution due to simultaneous electrochemical activation and hot pressing. The formaldehyde emission from both "two-steps" and "one-step" plywoods was as low as 0.07 mg/L, significantly lower than that prepared by the conventional resins. This strong, durable and eco-friendly plywood provides an effective approach to upcycle DMMs and manufacture formaldehyde-free wood-based panels of high-value.