A fundamental study of lignin reactions with formaldehyde and glyoxal†

Abstract

This study investigates the synthesis and characterization of lignin-formaldehyde (LF) and fully biobased lignin-glyoxal (LG) resins as alternatives to the phenol-formaldehyde (PF) adhesive currently used in the manufacturing of plywood and oriented strand boards (OSB). In this process, phenol was entirely replaced by a commercially available kraft-softwood lignin, while formaldehyde was substituted with glyoxal (a biobased dialdehyde) in the LG resins. Additionally, lignin monomers were used as model compounds to better understand the behavior of lignin in LF and LG resins. The reactions of phenol, lignin monomers, and commercial lignin with formaldehyde and glyoxal were investigated through Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy in both solution and solid states. The results confirmed the successful integration of formaldehyde and glyoxal into phenolic structures, leading to the formation of methylene and glyoxylene linkages during resin synthesis. This process created a robust three-dimensional network, as evidenced by 2D ¹³C–¹³C correlation solid-state NMR spectra and FT-IR analyses, which are crucial in studying the structural properties of the cured thermoset solid resins that are insoluble in NMR solvents. These findings highlight the innovative potential of lignin as a renewable alternative to petroleum-based phenol and emphasize the practical applications of a fully biobased lignin-glyoxal adhesive in the production of greener, more sustainable, and formaldehyde-free plywood and OSB wood panels commonly used in building construction.