Modified geopolymer-based wood adhesive using waterborne polyurethane

Geopolymer binders show great potential in the application of eco-friendly wood composite adhesives. The applicability of organic-inorganic hybrid-reinforced geopolymer composites as plywood binder was investigated. In this study, a geopolymer-based wood adhesive was fabricated by mixing a matrix-geopolymeric slurry; a toughening agent, waterborne polyurethane; and a silane coupling agent, that served as the covalent “bridge” between the waterborne polyurethane with a geopolymer matrix. The results showed that the waterborne polyurethane exhibited excellent compatibility with the geopolymer and served as a flexibilizer, which transformed the matrix from a microfractured structure to a denser morphology. Moreover, the shear strength of bonded plywood and the morphology of the fracture surface after the tensile measurement were measured. The resulting geopolymer/wood interface was well bonded, and the interfacial bonding strength was higher than the wood strength matrix after modification. The introduction of waterborne polyurethane and silane coupling agent improved the water resistance of the composites and increased the wet shear strength of plywood from 0 MPa to 0.35 MPa. Notably, a weak wood/alkali interface was formed under alkaline conditions due to the strong diffusion of alkali metal ions between the interfaces.