Crosslinking reactions of a model aminated lignin compound as a platform for building thermosetting polymers for lignin-based bio adhesives

Abstract

As millions of tons of kraft lignin are being wasted, a potential application is to use its crosslinking reactions to build thermosetting bio adhesives. However, the crosslinking reactions between lignin molecules are not fully understood. The present study aims to elucidate the crosslinking reactions of the model lignin compound guaiacylglycerol-β-guaiacyl ether (GGE) via one-step hydroxymethylation/ amination with formaldehyde and diethylenetriamine (DETAM), or one-step glyoxylation/ amination with glyoxal and DETAM via liquid NMR techniques such as ¹H NMR, ¹³C NMR, 2D ¹H–¹³C, and ¹H–¹⁵N HSQC NMR. Specifically, the 2D ¹H–¹³C HSQC NMR spectra confirm the presence of –CH₂–NH– with a chemical shift of ¹H 2.6–3.6/¹³C 40–60 ppm, and the formation of methylene linkages via the crosslinking reaction. Also, the 2D ¹H–¹⁵N HSQC NMR spectra clearly detect the formation of amide and imine bonds at ¹H 7.8/¹⁵N 110 and ¹H 8.07/¹⁵N 121.5 ppm from the crosslinked GGE.