Aqueous lignin adhesive for high-performance wood bonding

Developing formaldehyde-free and high-performance wood adhesives is imperative for sustainable wood utilization. To overcome performance trade-offs in lignin adhesives, we developed a formaldehyde-free aqueous adhesive formulation via reactive cysteine-functionalized lignin, achieving high performance with an unprecedentedly low lignin loading (20 wt %), which is sprayable and injectable and exhibits excellent fluidity (1.96 mPa·s, 10³ s^-1). It undergoes self-curing via forming amide bond, achieving robust bonding performance under simple hot-pressing conditions. The solvent-free lignin adhesive for plywood outperforms commercial resins, demonstrating exceptional dry strength (4.0 MPa), high wet-strength retention (83.7%), and superior durability against extreme temperatures (-196° to 190°C), humid heat aging, and solvents. Cysteine substitution enhances fracture toughness eightfold (reaching 11.06 kJ/m²) by forming dual covalent/noncovalent networks that facilitate the polarity-enabled adhesion and crosslinking-induced toughening. This work establishes an environmentally friendly and scalable strategy for constructing full bio-based adhesives with exceptional processability and durability, providing a paradigm for lignin valorization within lignin-first biorefinery frameworks.