Cellulose nanofiber-reinforced waterborne raw lacquer coatings with enhanced toughness, adhesion, and aging resistance

Raw lacquer (RL) is a durable bio-based coating, but its industrial application is restricted by high viscosity, poor processability, intrinsic brittleness, limited adhesion, and aging-induced cracking. In this study, a cellulose nanofiber (CNF)-reinforced waterborne raw lacquer (WRL-C) coating was developed through surfactant-assisted waterborne modification and ultrasonic dispersion to simultaneously improve processability, mechanical performance, and durability. An optimal CNF content of 0.2 wt% was identified. At this loading, the WRL-C coating exhibited accelerated drying, increased pencil hardness from 3 H to 4 H, improved surface compactness, the highest gloss, and a maximum water contact angle of 93°. Compared with pristine RL, WRL-C0.2 showed a 96.45% increase in toughness and a 79.9% increase in elongation at break while maintaining comparable tensile strength, effectively overcoming the conventional strength–toughness trade-off of flexible-chain-modified lacquer systems. The coating/wood adhesion was also significantly enhanced, with the maximum pull-off strength reaching 3.67 MPa. After UV–condensation aging, pristine RL exhibited severe cracking, whereas WRL-C0.2 retained a continuous and nearly unchanged surface morphology. Chemical characterization showed that CNF did not completely prevent aging-induced oxidation of the lacquer matrix. Instead, the CNF network and associated hydrogen-bonding/polar interactions stabilized the coating structure, redistributed aging-induced stresses, and suppressed crack initiation and propagation. This work provides a sustainable and spray-processable strategy for fabricating high-performance waterborne raw lacquer coatings for wood protection and coating applications.