Abstract: In this work, enzymatic lignin was demethylated to improve its hydroxyl content, so as to increase the degree of epoxidation of lignin and ethy-lene glycol diglycidyl ether.The epoxy graft polymer of lignin (EPDL) with high epoxy value was synthesized by the reaction of lignin with ethy-lene glycol diglycidyl ether (EGDE) in alkaline condition, and which was used as a crosslinking agent to prepare high-performance green soy protein-based adhesive. Demethylated lignin (DL) and EPDL were characterized by 31P-NMR and FTIR. Hydroxyl content of lignin before and after demethylation, epoxy value and chemical structure of epoxy graft copolymer were discussed. The effect of soybean protein adhesives modified by EPDL on the wet bonding strength was investigated. The results showed that the content of hydroxyl group in lignin was increased by demethylation. Microwave heating significantly shortened the demethylation time of lignin and increased the hydroxyl content by 29% compared with the original lignin (L). More epoxy groups were grafted onto lignin and the epoxy value reached 0.350 mol/100 g by demethylation, which increased the cross-linking reaction activity between EPDL and SPI molecule. The water-resistant chemical structure could be formed by the crosslinking reaction of EPDL with hydroxyl group and amino group of soybean protein, which could improve the heat resistance and bonding strength of soy protein-based adhesive. When the amount of EPDL was 7%, the wet bonding strength of EPDL-SPI adhesive reached to the maximum value of 1.14 MPa, that far outstrips the requirements of class Ⅱ plywood specified in GB/T9846-2015 national standard.
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2021, Vol. 35 
