Alkali lignin depolymerization under eco-friendly and cost-effective NaOH/urea aqueous solution for fast curing bio-based phenolic resin

It is still a challenge to use lignin without any pretreatment due to its high molecular weight, strong steric hindrance, and low reactivity. To prepare value-added low molecular weight aromatic compounds for preparation of lignin-based phenolic resins, alkali lignin (AL) was depolymerized under low cost and environmental-friendly NaOH/urea aqueous solution at low temperature in this paper. The molecular structure changes of the depolymerized AL (DAL) products were characterized by MALDI-TOF-MS, H-1 NMR, and FT-IR spectrophotometers. Low molecular weight aromatic compounds mainly composed of phenyl-propane trimers were prepared after depolymerization. The active proton content at C5 position increased from 0.03 mmol/g to 0.26 mmol/g, and the phenolic hydroxyl group content increased from 0.07 mmol/g to 0.12 mmol/g. Moreover, a depolymerization mechanism was proposed to explicate the depolymerization process. AL and DAL products were used for the preparation of lignin-based phenolic resins. The chemical properties of lignin-based phenolic resins were studied by liquid-state C-13 NMR and FT-IR spectroscopy, and its physical properties were also investigated. The results indicated that the depolymerized alkali lignin-phenol-formaldehyde (DALPF) resins exhibited enhanced properties, such as faster curing rate, lower formaldehyde emission, and higher bonding strength, compared to alkali lignin-phenol-formaldehyde (ALPF) resins.