Polyacrylamide Graft Modification of Alkali Lignin for Formaldehyde‐Free Wood Adhesives With Enhanced Bonding Strength, Fracture Resistance, and Water Resistance

ABSTRACT Developing formaldehyde‐free lignin‐based wood adhesives with balanced bonding strength, fracture resistance, and water resistance remains a considerable challenge. Herein, we propose a synergistic modification strategy integrating energy dissipation by grafted flexible polyacrylamide chains, epoxy‐mediated network formation, and wood interfacial reinforcement. Polyacrylamide‐grafted alkali lignin (PAMAL) was prepared through H 2 O 2 /CaCl 2 ‐initiated free‐radical graft polymerization of acrylamide in the presence of alkali lignin. The introduced flexible polyacrylamide side chains were designed to enhance chain entanglement, hydrogen‐bonding interactions, and energy‐dissipation capability. Subsequently, the epoxy crosslinker triglycidylamine (TGA) was incorporated to regulate adhesive viscosity, improve coating and penetration into wood, and promote epoxy‐mediated network formation during curing. Consequently, the optimized 2PAMAL/TGA adhesive exhibited a dry shear strength of 2.88 MPa, compared with 0.21 MPa for the unmodified lignin adhesive, while the bonded‐joint debonding work increased from 0.010 to 1.096 J. In addition, the moisture absorption decreased by 83.2%, the residual rate increased by 331.5%, and the wet shear strength reached 1.26 MPa after immersion in water at 63°C for 3 h when hot‐pressed at 190°C. These improvements are attributed to the synergistic effects of flexible‐chain energy dissipation, enhanced wood interfacial wetting/penetration, and epoxy‐mediated network stabilization.