Self-crosslinking of acetone-fractionated and glyoxalated hardwood kraft lignin as bio-adhesives for wood bonding

The research on hardwood lignin is quite limited for lignin-based adhesives due to its heterogeneity and chemical complexity compared with those of softwood and non-wood lignin. Herein, a novel approach of producing entirely bio-based and nontoxic wood adhesives by self-crosslinking of industrial hardwood kraft lignin (HKL) is presented to replace formaldehyde-based resin adhesives. The key issues of converting HKL to bio-wood adhesive are their low reactivity for crosslinking capacity and high heterogeneity. For this, HKL extracted from industrial black liquor was acetone fractionated to reduce its heterogeneity, resulting in acetone-soluble HKL (AS-HKL) and acetone-insoluble HKL (AI-HKL), which were then glyoxylated to facilitate crosslinking and form a three-dimensional network structure. Lignin samples were analyzed by gel permeation chromatography (GPC), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), 31P nuclear magnetic resonance (31P NMR), solid state 13C cross-polarization/magic angle spinning (13C CP/MAS) NMR, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The adhesive strength of lignin-based adhesives was evaluated by determining the tensile shear strength of plywood. In this current research the chemical structure changes especially the crosslinked structure and adhesion performance were investigated. Results showed that the self-crosslinking was successfully formed by ether bonds formation between glyoxalated lignin. Also, the self-crosslinked acetone insoluble hardwood kraft lignin fractionation (AI-HKL) showed the best adhesion performance. FTIR, XPS and 13C CP/MAS NMR spectra revealed new peaks and ether bonds in the glyoxylated lignin. Therefore, these results indicate a successful self-crosslinking of the hardwood kraft lignin after acetone fractionation and glyoxalation for adhesives via network formation. The glyoxalated AI-HKL (AI-HKL–GLY) exhibited the highest viscosity and provided the best plywood adhesion strength (0.8 MPa) among the adhesives, and showed a good potential as wood adhesives. Thus, the results suggest that the self-crosslinking of hardwood kraft lignin after a proper fractionation and glyoxalation is a promising approach of developing lignin-based bio-adhesives for wood bonding.