Synthesis of Biobased Novolac Phenol–Formaldehyde Wood Adhesives from Biorefinery-Derived Lignocellulosic Biomass

Lignocellulosic biomass is a sustainable alternative to petroleum-derived chemicals to develop biobased wood adhesives, which motivates integrated biorefineries to effectively convert biomass feedstock into desirable chemicals. Herein, lignin recovered from kraft biorefinery (L-KB) and two bio-oils, prepared from laboratory-scale solvent liquefaction of lignin (BO-SL/L) and fast pyrolysis of pinewood (BO-FP/PW), respectively, have been used to substitute 50% (w/w) of phenol in a novolac phenol–formaldehyde (NPF) resin system. The molecular structures of the L-KB, BO-SL/L, and BO-FP/PW were characterized via FTIR, 13C–1H HSQC 2D-NMR, GCMS, and carbohydrate analysis. Characterization results revealed the presence of functional moieties derived from lignin and polysaccharides. Further, the obtained resin adhesive structures were examined by FTIR and 1H NMR spectroscopy, which confirmed the formation of methylene bridges during the resin preparation. Subsequently, to understand the curing behavior of each of the NPF resins with hexamethylenetetramine (HMTA) curing agent, DSC analysis was performed, which helped to optimize the bonding process. The resulting bonding strength of each resin adhesive, measured by gluing two pieces of wood, indicated significantly different adhesion ability due to structural differences, which was analyzed by two-way ANOVA, followed by Tukey's post hoc test. The 50-NPF-L-KB adhesive demonstrated a tensile shear strength of 3.46 ± 0.55 MPa, higher than the values of other tested adhesives. This indicates that the lignin derived from kraft biorefinery is a potential substitute for phenol in the NPF resin system for use in wood adhesive applications.