Development of an Eco-Friendly, Cost-Effective, and Anti-Mildew Soybean-Based Adhesive with Multiple Cross-Linking Networks

With increasing concerns over environmental sustainability and living conditions, developing formaldehyde-free and environmentally friendly wood adhesives is of great significance. Soybean-based adhesives have become widely adopted as sustainable alternatives to petroleum-derived adhesives in the wood industry. However, their poor bonding strength, weak water resistance, and tendency to mildew severely limit their large-scale industrial application. To address these issues, this study adopted a novel cocrosslinking strategy to prepare a soybean adhesive with multiple cross-linking networks by incorporating soybean flour (SF), polyamidoamine-epichlorohydrin (PAE) resin, citric acid, and vanillin. In this system, soy protein and PAE served as the adhesive matrix, while vanillin and citric acid played a bridging role to effectively enhance both application performance and antifungal properties. The plywood test results demonstrated that the incorporation of these plant-derived components significantly improved the bonding performance, achieving maximum dry and wet shear strengths of 2.47 and 1.28 MPa, respectively, far exceeding the Class II plywood requirements (GB/T 9846–2015). Owing to the establishment of a dense cross-linked network, the modified adhesive exhibited a 184.4% increase in wet strength and a 17.1% improvement in flame retardancy, outperforming conventional soybean-based adhesives. In addition, molecular simulation (DFT) results indicated that these plant-derived components possessed natural antimicrobial activity, giving the adhesive excellent antifungal properties comparable to commercial antifungal agents. This work provides a new formaldehyde-free adhesive system for the industrial production of eco-friendly wood composites, with broad application prospects in furniture manufacturing and building materials.