CaCl2-induced interfacial deposition for the preparation of high-strength and flame-retardant plywood using geopolymer-based adhesive

Abstract Widely used formaldehyde-based resins for modern wood-based panels are toxic to humans and the environment. Geopolymer-based adhesives (GA) are abundant environmentally inorganic materials with wide applications in engineering fields. However, the modification and design of GA with excellent properties for practical applications remain a major challenge. Here, calcium chloride (CaCl2) solution was used to coat the wood surface, which quickly polymerized in situ with the geopolymer to form calcified geopolymer during hot-pressing, leading to the construction of a robust geopolymer/wood bonding interface. The formation mechanism of the reinforced interface between wood and geopolymer was characterized, and the wet shear strength and flammability were evaluated based on the binding application of plywood. The calcified geopolymer can achieve the dual effects of blocking the penetration of lye and promoting in situ reinforcement during hot-pressing, which effectively eliminates the impact of the weak interface layer of the alkali/wood and reorganizes the microstructure of the geopolymer/wood bonding interface. The wet shear strength of plywood with a 2.0 mol/L CaCl2 solution loading was increased from 0.45 MPa to 0.87 MPa, an increase of 93% compared with the pristine samples. The inherent flame retardancy of geopolymers contributed to a 42.6% reduction in the first peak of the heat release rate and a decrease of 28.9% in the total heat release compared with industrial urea-formaldehyde resin. This resulting GA, which was prepared by a facile and green method with markedly increased wet shear strength and inherent flammability, may have potential applications in functional wood-based composites and be used as a replacement for formaldehyde-based adhesives.