Wholly bio-based hyperbranched waterborne polyurethane coatings reinforced with surface-engineered cellulose nanofibers for sustainable wood protection

Bio-based waterborne polyurethane (BWPU) coatings have drawn attention as renewable alternatives to petroleum-based wood finishes for Larix gmelinii , yet their mechanical strength, hardness, and barrier properties remain insufficient for demanding applications. In this work, cellulose nanofibers (CNFs) extracted from Pennisetum flaccidum were first modified through acetylation, sulfonation, and silanization, and then covalently incorporated into a hyperbranched castor oil-based BWPU as macromolecular cross-linkers and chain extenders. The functional groups grafted onto the CNF surfaces governed the cross-linking density, hydrogen-bond strength, and degree of microphase separation within the cured films, thereby establishing a direct relationship between surface chemistry and coating performance. Among the three modified variants, silanized CNFs (Si-CNFs) constructed a hydrophobic association network that raised the storage modulus from 1315 to 2873 MPa, increased T 10% to 310 °C, and reduced the fractional free volume from 54.5% to 20.2%. The resulting Si-CNF coating achieved a tensile strength of 33.5 MPa and a pendulum hardness of 0.819, and exhibited the lowest water absorption and water vapor transmission rate among all tested formulations, outperforming both a commercial WPU and a wood wax oil benchmark. After 30 days under soil burial conditions, the coating lost approximately 80% of its initial mass. These results demonstrate that surface-engineered CNFs, when covalently integrated into a bio-based polyurethane network, can simultaneously reinforce, toughen, and hydrophobize the coating in a single processing step, providing a practical pathway toward sustainable, high-performance wood protection.