Mechanisms and Properties for Sustainable Fiberboard Recycling via Simultaneous Utilization of Residual Urea‐Formaldehyde Resin and Lignin

ABSTRACT A substantial amount of discarded fiberboard is ground for reuse globally. However, the presence of cured urea‐formaldehyde resin within the fibers adversely affects the mechanical properties of the recycled fiberboards. This study proposes a novel approach that simultaneously utilizes cured adhesive and intrinsic lignin to produce high‐performance recycled fiberboard. Under acidic conditions, cured urea‐formaldehyde resin partially hydrolyzes, meanwhile a portion of lignin is oxidatively depolymerized by sodium chlorite. During hot pressing, activated lignin softens, penetrates, and undergoes condensation, while activated resin participates in polycondensation. The recycled fiberboards exhibit excellent mechanical properties, which may be attributed to fiber entanglements within the board and the formation of new hydrogen and chemical bonds between lignin‐fibers, and UF‐fibers. The boards achieve an internal bonding strength of ~1.98 MPa, a modulus of rupture (MOR) of ~65.6 MPa, and a modulus of elasticity (MOE) of ~6678 MPa. This simple co‐utilization strategy reduces petrochemical adhesive use and enhances material recyclability, offering a promising route for high‐performance, sustainable recycled fiberboards. Such a closed‐loop recycling strategy may guide the future recycling of wood‐based composites.