Activation of methoxy group of hardwood kraft lignin via cross-coupling and crosslinking for lignin-based wood adhesives

Abstract

The methoxy (–OCH₃) group of hardwood kraft lignin (HKL) exhibits poor reactivity and limits its valorization due to its steric hindrance. Herein, we report an efficient activation of the –OCH₃ group in a cross-coupling reaction, enabling its crosslinking for lignin-based wood adhesives. Specifically, the –OCH₃ groups are replaced with an activated nucleophile, CH₂–SiMe₃, which is then successfully used for the subsequent crosslinking with either epichlorohydrin (ECH) or glyoxal (GLY) as crosslinkers. The new peaks in the FT-IR, ¹³C NMR, and 2D HSQC NMR spectra, and a decrease in methoxy group confirm the activation. Furthermore, the activated HKL exhibits a decrease in molecular weight and glass transition temperature. The successful crosslinking with ECH or GLY is demonstrated by an exothermic reaction by differential scanning calorimetry (DSC) and ether and acetal linkages formation by solid-state ¹³C NMR, respectively. Both unactivated and activated HKL-based adhesives are cured at hot-pressing temperatures for plywood. The activated HKL-based adhesives significantly improve adhesion strength, reaching a maximum value (2.61 MPa) compared to the unactivated HKL (2.18 MPa), exceeding the Korean standard requirement (0.60 MPa). These results suggest that methoxy group activation via cross-coupling effectively promotes HKL crosslinking for lignin-based wood adhesives.