Modification of nanolignin by deep eutectic solvent to improve the properties of phenol-formaldehyde resin

To improve the reactive sites of lignin nanoparticles for use in phenol-formaldehyde resin (PF), a deep eutectic solvent (DES) composed of Choline Chloride–ZnCl₂ (ChCl-ZnCl₂) was employed to pretreat nanolignin. The modified and unmodified lignin nanoparticles were used to replace high proportions of phenol (50%, 60%, and 70%) to prepare the lignin-phenol-formaldehyde (LPF) adhesive. FTIR spectroscopy was used to characterize the changes in the functional groups of nanolignin during DES treatment. The changes in glass transition temperature (T_g) and the changes in curing temperature of the LPF resin were determined by Differential Scanning Calorimetry (DSC). The incorporation of modified nanolignin into the PF resin significantly increased the solid content, viscosity and accelerated the resin gel time. The phenolic hydroxyl content of DES-treated nanolignin increased and its methoxy content decreased moving away from the optimized experimental conditions. DSC curves show that DES-treated nanolignin has lower T_g value (81 °C) than unmodified nanolignin and virgin lignin (84 °C; 92 °C), respectively. The LPF resin with DES-treated nanolignin showed a lower peak temperature compared to those prepared using unmodified nanolignin. The shear strength of the LPF adhesive prepared by DES-modified nanolignin was higher than that of the control samples. When the substitution degree of modified nanolignin for phenol reached 70%, the bond strength of the plywood prepared by the DES pretreated nanolignin was 2.3 MPa and the free formaldehyde content was 2.9 mg/100 g, thus meeting the requirements of international standards. Based on the findings of this research, a high proportion of phenol can be substituted by nanolignin in LPF resins without changing their properties when nanolignins are treated by DES.