Synergistically Enhanced Soy Protein Adhesives via Epoxidized Lignin for High-Strength, Flame-Retardant, and Cost-Friendly Wood Bonding

This study adopts a simple, efficient, and environmentally benign method for the preparation of epoxidized lignin while simultaneously reporting a plant protein-based formaldehyde-free wood panel adhesive. Epoxidized lignin (EPL) was synthesized via a one-step epoxidation reaction of alkali lignin with a polyepoxide compound. The resulting EPL was subsequently employed as a reinforcing phase to formulate an environmentally friendly wood adhesive through compounding with a soy protein isolate (SPI). The wet shear strengths of the modified adhesives reached 0.94 and 1.45 MPa, which were 59.32 and 145.76%, respectively, higher than that of the pure SPI adhesive (0.59 MPa). The introduction of EPL endows the adhesives with antimold and flame-retardant properties and simultaneously improves the hydrophobicity and thermal stability. Compared with commercially available adhesives, the developed adhesives exhibit superior dry shear strengths, excellent water resistance, and more eco-friendly and cost-effective characteristics. This study therefore provides innovative insights and approaches for the development of high-performance biomass-based wood adhesives.