Fluorine-free superhydrophobic coatings on cotton fabric and kraft paper via chitosan-tannic acid interfacial assembly

Developing durable fluorine-free superhydrophobic coatings on porous, hydrophilic cellulose substrates is challenging because their fibrous structures promote capillary wetting and can hinder coating retention. Here, a bio-derived coating composed of chitosan (CS), tannic acid (TA), octadecylamine (ODA), and octadecanethiol (ODT) was deposited onto cotton fabric and kraft paper by spray coating. The CS/TA-rich phase provided a fiber-contacting scaffold, whereas the C18 alkyl chains supplied low surface energy and promoted the formation of an alkyl-enriched outer interface. FTIR and XPS confirmed the incorporation of the principal coating components and were consistent with changes in hydrogen-bonding environments and surface enrichment by long alkyl chains. SEM revealed hierarchical micro/nanostructures on the fibers while preserving the porous fibrous morphology of both substrates. Coating assembly is attributed primarily to hydrogen bonding, alkyl-chain association, deposition on irregular fiber surfaces, and physical interlocking. The modified cotton fabric and kraft paper exhibited static water contact angles of 153 ± 1.2° and 154 ± 1.1°, respectively, together with repellency toward coffee, methylene blue solution, orange juice, milk, and artificial blood. The coated cotton fabric achieved an ultraviolet protection factor of 316, while the coated cotton fabric and kraft paper showed antibacterial rates of 99.3–100% against Escherichia coli and Staphylococcus aureus and diffusion-based antifungal activity against Penicillium and Aspergillus niger . Water repellency was retained after tape-peeling cycles, accelerated ultraviolet exposure, short-term exposure to pH 2 solution, controlled sandpaper abrasion, and ambient indoor storage. This work presents a particle-free, fluorine-free coating strategy that uses a single formulation to impart multifunctional surface protection to cellulose substrates with distinct fibrous architectures, without a separate curing treatment.