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

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. Highlights Polyacrylamide graft modification improved the adhesive performance of alkali lignin. Flexible polyacrylamide side chains enabled efficient energy dissipation. TGA regulated adhesive viscosity and promoted epoxy-mediated network formation. Dry shear strength increased from 0.21 to 2.88 MPa after modification. The adhesive balanced bonding strength, fracture resistance, and water resistance without formaldehyde.