Demethylated lignin-Fe³⁺ chelation synergistically constructs a triple-network wood adhesive with soybean protein

This study developed a high-performance bio-based soy protein/lignin glue adhesive. The directional demethylation modification of alkali lignin from corn cob (MAL) catalyzed by sodium sulfite significantly improved its phenolic hydroxyl content and reactivity. Furthermore, FeCl₃·6H₂O was introduced to construct a "hydrogen bond-covalent bond-ionic bond" triple synergistic cross-linking network: MAL phenolic hydroxyl groups formed multiple hydrogen bonds with amino/carboxyl groups of soybean protein isolate (SPI), MAL quinone groups underwent Schiff base covalent cross-linking with SPI amino groups under oxidation conditions, and Fe³⁺ formed metal chelate coordination bonds with phenolic hydroxyl/amino/carboxyl groups. This synergistic strategy comprehensively improved the performance of the adhesive: with appropriate viscosity, the dry/wet-state shear strength increased by 81.52 % and 103.45 % compared to the pure SPI system, respectively, meeting the Chinese National Standard GB/T 9846-2015 for interior plywood application (Type II). Meanwhile, the glue exhibits significant water resistance (moisture absorption reduced by 32.07 %, water contact angle increased by 60.90 %), excellent hot stability (maximum decomposition temperature increased by 29.87 °C), and durable anti-mold performance (anti-mold period extended by 7 days). This study provides an innovative chemical modification-metal chelation synergistic enhancement paradigm for the high-value utilization of lignin and the design of environmentally friendly adhesives, and subsequent research will focus on industrial adaptation optimization.