Multisource Lignin from Acidified Tetrahydrofurfuryl Alcohol Extraction for Phenolic Resin Adhesives

Although lignin shows potential as an industrial raw material for sustainable development, its structure complexity significantly limits its industrial application. This study comparatively investigated the structural differences among lignins isolated from walnut shells, poplar, pine, and corn stalk to elucidate the impact of lignin structure and substitution rates on the physicochemical properties of synthetic phenolic resin. The results showed that a higher content of hydroxyl groups and active hydrogen (by 1H NMR) was observed in lignin extracted with tetrahydrofurfuryl alcohol. Moreover, lignin with lower molecular weight exhibited great potential in replacing phenol to synthesize phenolic resin. When the phenol substitution ratio was 50%, the lignin-based phenolic resin had a bonding strength of 1.40 MPa, which is much higher than the national standard (0.7 MPa). The performance analysis (bonding strength, free formaldehyde, and viscosity) revealed that phenolic resins prepared from phenylated lignin exhibited significantly enhanced bonding strength. Both pine and corn stover lignin proved suitable for phenolic resin preparation, providing a theoretical basis for subsequent research.