Optimization of ultrasound-assisted lignin depolymerization using response surface methodology for low-formaldehyde emission wood adhesives

The heterogeneity, high molecular weight, and low content of functional groups have limited the high-value utilization of lignin to prepare high-performance resins. To overcome this obstacle, ultrasound-assisted lignin depolymerization was investigated as a gentle, efficient, and inexpensive method to improve the functionality and homogeneity of lignin. The response surface methodology combined with the Box–Behnken design was employed to optimize the yield and molecular weight of oligomeric lignin by varying three parameters: sonication time, lignin mass fraction, and solvent ratio. The predicted optimal yield (33 %) and molecular weight (M_n = 700) under optimal conditions (3 h, 14.3 % lignin, and MeOH/H₂O = 3:1) were almost the same as those obtained experimentally (M_n = 710), yield of 33.8 %, proving the validity of the model. The depolymerized lignin (DL) was directly substituted for phenol to prepare depolymerized lignin phenol-formaldehyde resins (DLPF). The rheological and adhesive properties of these resins were compared with pure PF resin and lignin phenol-formaldehyde (LPF) resins. An enhanced bonding strength, but lower curing temperature and formaldehyde emission, were observed. This study presents a cost-effective and efficient method to prepare depolymerized lignin as the precursors of high-performance resins.