The role of lignin in enhancing adhesion performance and reducing formaldehyde emissions of phenol–resorcinol–formaldehyde resin adhesives

Phenol–resorcinol–formaldehyde (PRF) resins are widely used as adhesives in laminated wood products due to their excellent strength and durability. However, when cured under cold-press conditions, PRF resins encounter several challenges, including the presence of unreacted free phenol in the resin solution, which remains unreacted with formaldehyde at ambient temperature. Excessive free phenol not only reduces the resin's reactivity but also poses environmental and health risks. To address these issues, this study examines the partial substitution of phenol with lignin, a structurally similar yet more sustainable alternative, to reduce dependence on fossil-based materials while enhancing resin performance. The results indicate that replacing 10–30 % of phenol with lignin improves resin properties, including increased viscosity, enhanced reactivity, greater cohesion strength, and a higher degree of methylene linkages, which contribute to stronger adhesive bonds. In addition, laminated veneer lumber (LVL) bonded with lignin-modified PRF resins exhibited superior adhesion performance and lower formaldehyde emissions compared to conventional PRF resins.