Preparation of high-performance phenolic resin adhesives using demethylated lignin as substitutes for phenol.

Traditional phenolic resin adhesives involve the use of petrochemical-based phenol, raising environmental and health concerns. In this study, lignin was demethylated to substitute for phenol and prepare a high-lignin-content adhesive with perfect shear strength performance. The hydroxyl content of demethylated lignin can reach up to 6.54 mmol/g, which provides more active site quantities for promoting the reaction synthesis. At the same time, its uniform molecular weight contributed to the stability of the adhesive. Demethylated lignin was used to replace up to 75 % of phenol in the preparation of phenolic resin adhesive, which exhibited excellent mechanical properties, with a maximum shear strength of 2.46 MPa, even better than that of the pure phenolic resin adhesive (2.36 MPa). Moreover, under low- or high-temperature conditions, the maximum shear strengths reached 2.29 and 2.81 MPa, respectively. The contact angle test indicates that the hydrophilicity of the demethylated lignin phenolic resin adhesive is weakened, thereby enhancing its durability and resistance to erosion in humid environments. Under 100 % relative humidity, the shear strength remained at 1.54 MPa. This work provides theoretical guidance for the preparation of high-quality phenolic resin with high lignin content and offers a feasible approach for the high-value utilization of lignin.