Liberating Phenolic Hydroxyl of Lignin via Deep Eutectic Solvent for Wood Bioadhesives

Formaldehyde-free wood bioadhesives represent a critical advancement in eliminating hazardous emissions. This study develops a green approach using deep eutectic solvents (DES) to treat lignin, selectively liberating phenolic hydroxyl groups and yielding active lignin. Results showed that the total phenol content in lignin was increased by 108% after the treatment in DES at 130 °C. ReaxFF molecular dynamics simulations confirmed the liberation of phenolic hydroxyls in lignin. The resultant active lignin-based bioadhesive demonstrated an excellent wet shear strength of 1.92 MPa, surpassing the Chinese national standard GB/T 9846–2015 (≥0.7 MPa) by 174%. The phenolic–OH in lignin created reactive surface moieties that subsequently reacted with poly(vinyl alcohol) (PVA), glycerol triglycidyl ether, and poly(ether imide) (PEI) to construct the bioadhesive’s spatial network. The covalent bond formation played a key role in improving the bioadhesives’ performance, while high active lignin in bioadhesives improved their flame resistance. Finally, this work confirmed the role of DES in liberating phenolic hydroxyl groups in lignin and contributed to the development of green lignin-based formaldehyde-free bioadhesives.