Urea-Formaldehyde Resin Systems Modified with Partially Pyrolyzed and Delignified Lignin: A Review

Urea-formaldehyde UF resins are typical adhesives employed in the production of wood-derived products. Application of such fossil fuelbased resins that emit formaldehyde emissions endangers the environmental and human health conditions profoundly. Bio-based lignin polymer can be utilized as a replacement additive in UF resins because it has inherent adhesive and water repellent nature. When lignin is added into the structure of the resin, the C-N bonds are reduced through the creation of new bonds within regions containing methylene bonds. Raw lignin cannot be directly added to UF resins; it must undergo pretreatment due to its poor binding capacity and amorphous nature. Therefore, partial pyrolysis of lignin in the low temperature zone under inert atmosphere was suggested in this study. This reaction creates a longer chain structure by partial C-O bond fragmentation; thus lignin is altered. This method is likely to reduce the formaldehyde emissions, but the same chain structure will increase the water repellency and reduce the moisture content of lignin. Literature research reveals that lignin additives have great effects on resin viscosity, free formaldehyde content, adhesion quality and panel performance.