Demethylation technique has been used to enhance lignin reactivity for preparation of phenolic resins. However, the demethylation efficiency and the demethylated lignin (DL) reactivity were still unsatisfactory. To improve the demethylation efficiency, alkali lignin was demethylated under different mild conditions using sodium sulfite as a catalyst. Lignin and DL were characterized by ¹H-NMR (nuclear magnetic resonance) and Fourier transform infrared (FT-IR) spectroscopy to determine the demethylation mechanism. With the demethylation of lignin, the methoxyl group content decreased from 1.93 m mol/g to 1.09 m mol/g, and the phenolic hydroxyl group content increased from 0.56 m mol/g to 0.82 m mol/g. These results revealed that methoxyl groups were attacked by SO₃²⁻, and some methoxyl groups were converted to phenolic hydroxyl groups by a nucleophilic substitution reaction, generating DL with high reactivity. The chemical properties of lignin-based phenolic resins were studied by ¹³C-NMR and FT-IR spectroscopy, and their physical properties were also investigated. The results indicated that lignin-based phenolic resins exhibited faster curing rate and shorter gel time. In addition, the bonding strength increased from 0.92 MPa to 1.07 MPa, and the formaldehyde emission decreased from 0.58 mg/L to 0.22 mg/L after lignin demethylated at the optimum condition.

Condensed tannins from larch bark extracts are a natural renewable and eco-friendly material and are potential substitutes for phenolic petrochemicals. However, the wide application of tannin is restricted by its low reactivity. Therefore, the goal of this study was to enhance the reactivity of larch tannin by depolymerization and determine optimal reaction conditions. The structures of larch tannin and depolymerized larch tannin were characterized by Fourier transform infrared (FT-IR) spectroscopy, solid phase ¹³C-NMR and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. The thermal stability of larch tannin before and after depolymerization was evaluated by thermogravimetric analysis (TGA). The results indicated that the monomeric units of larch tannin were mainly composed of catechin/epicatechin, gallocatechin/epigallocatechin, catechin–gallocatechin esters, and stilbene glucosides. The presence of a catechin gallate dimer that had lost both gallic acid residues and a hydroxy group and a small amount of fisetinidin units were also observed. Additionally, a series of peaks corresponding to oligomers of larch tannin of up to 11 repeating units were observed from the MALDI-TOF MS data. Depolymerization treatment, especially using 2-mercaptoethanol as a nucleophilic reagent, was found to be beneficial to the enhancement of thermal stability. The optimization of the depolymerization reaction allowed the reaction to be completed in two hours, at 60 °C, using 2-mercaptoethanol as a nucleophile and 0.1 mol L⁻¹ HCl. Many compounds of molecular weight less than 600 Da, mainly dimers and monomers, were obtained under these reaction conditions.