Characterization of Lignin-Phenol-Formaldehyde Resins Synthesized with Different Lignin Sources

The growing demand for sustainable adhesives has increased interest in lignin as a renewable alternative to phenol in PF resins. In this study, lignins from hardwood, softwood, and bamboo were evaluated as partial replacements for phenol in plywood and paper panel applications. Softwood lignin exhibited the highest molecular weight and guaiacyl-rich structure, hardwood lignin contained more syringyl units, and bamboo lignin presented notable p-hydroxyphenyl content with minimal ash and sulfur. Incorporating up to 50% lignin increased resin viscosity and markedly shortened gel time from 136–152 s for neat PF resins to 58–139 s, depending on lignin type and resin formulation. Among the tested lignins, softwood lignin produced the strongest adhesives, whereas bamboo lignin resulted in the lowest formaldehyde emission (<0.6 mg/L). These findings demonstrate the feasibility of tailoring LPF resin performance by selecting lignin type.