Citric Acid-Mediated Lignocellulosic Fiber Bonding for Sustainable Materials: Multiscale Insights into Molecular Cross-Linking and Physical Interlocking

Lignocellulose-based materials are promising sustainable alternatives to petroleum-derived plastics but face challenges of complex pretreatments, poor moisture resistance, and reliance on synthetic resins. This study proposes a fully biobased and organic-solvent-free strategy to fabricate recyclable lignocellulose-based materials using wood fibers from industrial residues and citric acid (CA) as a multifunctional modifier. Multiscale characterization, ranging from molecular analyses to microstructural observations, revealed a distinctive synergistic bonding mechanism in CA-mediated fiber consolidation that integrates chemical cross-linking and physical consolidation. In this process, CA reorganized the fiber cell-wall architecture without removing lignin or disrupting cellulose crystallinity, which enhanced polysaccharide accessibility and promoted interfiber esterification. At the same time, CA induced lignin softening, migration, and recuring, forming physical interlocks at the fiber interfaces and further strengthening the overall adhesion. The resulting material exhibited an internal bonding strength of 0.85 MPa and a flexural strength of 31.1 MPa, along with excellent dimensional stability and mildew resistance lasting over 10 days under 95% relative humidity. Moreover, the reversibility of ester cross-linking enabled recyclability and reprocessability. Life cycle assessment indicated a 38.8% reduction in global warming potential compared with petroleum-based alternatives, confirming the sustainability of this CA-mediated approach.