A Fluorine-Free Superhydrophobic Chitosan/ZnO−Silane−PDMS Hybrid Coating for Self-Cleaning Leather

Abstract

Environmental concerns over per- and polyfluoroalkyl (PFAS)-based superhydrophobic coatings drive the need for simple, eco-friendly, and durable alternatives. Furthermore, traditional methods for imparting superhydrophobicity to leather suffer from relatively short-term stability or complex, multistep treatment procedures. Inspired by the lotus leaf, where wettability is dictated by both chemical composition and surface topography, this study presents a facile strategy for engineering superhydrophobic leather using chitosan (CS)-coated ZnO nanoparticles (NPs). Pristine ZnO NPs were first prepared via the hydrothermal synthesis method. The as-prepared ZnO NPs were then coated with CS through an ex situ precipitation in CS solution, followed by functionalization with 3-glycidyloxypropyltrimethoxysilane (GPTMS) via epoxide ring opening. The nanostructures were spray-deposited onto the leather and subsequently sealed with polydimethylsiloxane (PDMS) to produce a superhydrophobic surface. A series of characterizations were conducted. X-ray diffraction analysis confirmed the wurtzite crystal structure of the ZnO NPs, while ATR-FTIR spectroscopy verified the chemical functionality of both the NPs and the leather coating. Scanning electron microscopy imaging revealed that the ZnO NPs possess a hexagonal cone-shaped structure, while the coated leather surface exhibited hierarchical micro-/nanoscale protrusions. By lowering the surface energy, the PDMS topcoat enabled the (GPTMS-CS/ZnO)-coated leather to achieve a water contact angle of 156.9° ± 2.7° and a sliding angle of 5.0° ± 0.6°. Consequently, water droplets readily rolled off the surface, effectively carrying away the contaminants. Furthermore, the leather retained high hydrophobicity even after 5 cycles of abrasion, static water immersion, UV light exposure, and tape-peel adhesion tests. This strategy offers a simple, environmentally safe approach for fabricating durable, self-cleaning superhydrophobic leather surfaces with great potential for performance-driven applications.