Development of Hydrophobic Barrier Coatings by Silica-Functionalized Cellulose Nanocrystals

A hydrophobic barrier coating was fabricated on paper substrates using cellulose nanocrystals (CNCs), hydrophobic silica nanoparticles (SiO₂), and methyltrimethoxysilane (MTMS). The coating was applied via a rod-coating process followed by thermal curing, resulting in the formation of a crosslinked hybrid CNC–silica network on the paper surface. Surface morphology and chemical structure were systematically characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), confirming uniform coating coverage and the development of micro/nanostructured surface features. The coated paper exhibited a significant increase in water contact angle from ∼72° (base paper) to ∼134°, indicating strong hydrophobicity. Barrier performance improved with a ∼33% reduction in water vapor transmission rate (WVTR), reaching ∼382 g/m²·day. Mechanical properties were also enhanced, with the highest dry tensile strength of 45.62 N·m/g in the machine direction (MD) and 20.12 N·m/g in the cross direction (CD) and the highest wet tensile strength of 5.15 N·m/g in the MD direction and 2.52 N·m/g in the CD direction. Overall, these results correspond to increases of up to ∼8.8% in dry tensile strength and ∼64.9% in wet tensile strength compared to the base paper and improved mechanical flexibility of up to 402 double folds (df). Together, these enhancements demonstrate a scalable, environmentally benign approach for engineering durable hydrophobic surfaces on cellulosic substrates, highlighting the potential of CNC-silica hybrid coatings for advanced paper-based barrier applications.