Soybean protein wood adhesive with enhanced water resistance and low environmental impact through lignin-protein hybridization

Soybean meal, a by-product of soybean oil production, exhibits poor water resistance and is highly susceptible to mold, which are significant limitations hindering its large-scale industrial application. Inspired by the phenolic-amine chemistry, a fully bio-based soy protein wood adhesive was engineered to achieve excellent water resistance and environmental benefits. The process starts with oxidation of alkaline lignin using sodium periodate (NaIO 4 ) to generate aldehyde/quinone functionality on the expense of some aliphatic/phenolic hydroxyl groups, for promoting its subsequent interaction with soy protein to yield target adhesive by Schiff-base formation and Michael addition reaction. Importantly, residual NaIO 4 was meticulously removed to minimize its impact on the bonded substrate, the cross-linked network, and the environment, as it does not participate in the construction of the cross-linked architecture. The results showed that the dry and wet shear strength of SM-OL1.5 increased by 51.2 % and 80.8 % to 1.83 MPa and 0.94 MPa compared with SM adhesives, surpassing the China National Standard (GB/T 9846–2015, ≥0.7 MPa). Interestingly, the developed adhesive formulation exhibited mildew resistance, which can be attributed to the inherent feature of lignin to disrupt cell membranes and alter permeability, leading to cell lysis. Furthermore, the hybridized adhesive showcased minimal environmental impact, low volatile organic compound (VOC) emissions, and full compliance with global sustainability initiatives, making it a promising candidate for eco-friendly wood adhesive applications. • A totally bio-sourced soybean protein-oxidized lignin wood adhesive was designed. • Oxidized lignin served as the cross-linker with a low addition (less than 1.6 %). • The resultant wood adhesive shown a desired bonding performance and water resistance. • The as-obtained adhesive has low environmental impact which was verified by LCA. • The soybean protein-oxidized lignin adhesive features low VOCs emission.