Synthesis of acetone-fractionated hardwood kraft lignin-based adhesive crosslinked with epichlorohydrin

Studies on lignin have garnered great interest for increasing the value of lignin-added products, especially lignin-based adhesives. However, even with the advantage of abundant aromatic materials in lignin, its aromaticity and heterogeneity require its fractionation with various solvents. This study used hardwood kraft lignin (HKL) extracted from black liquor and was acetone-fractionated to produce acetone-soluble HKL (AS-HKL) and acetone-insoluble HKL (AI-HKL). The lignins were crosslinked with different amounts of epichlorohydrin (ECH; 5%, 10%, and 15%) for the synthesis of lignin-based adhesives with a three-dimensional network. The crosslinked HKLs were characterized with gel permeation chromatography (GPC), Fourier transform infrared (FT-IR), nuclear magnetic resonance (¹³C-NMR), and differential scanning calorimetry (DSC) to investigate their molecular weight, chemical structure, and thermal curing behavior. AI-HKL exhibited higher molecular weight than that of AS-HKL. As expected, the FT-IR and ¹³C-NMR spectra showed successful crosslinking of HKL by ECH with the intensity reduction of − OH, increased C–O stretching, and new peak appearance. From the DSC results, the exothermic peak temperature for crosslinking was 100 °C–130 °C. HKL and AI-HKL plywoods showed good tensile shear strength with minimal variations when they were crosslinked with 10 and 15% ECH. These results suggest the potential of high molecular weight HKL as lignin-based adhesives for wood bonding.