Hydrophobic silanes-modified nano-SiO₂ reinforced polyurethane nanocoatings with superior scratch resistance, gloss retention, and metal adhesion

Highly transparent coatings with scratch-resistant properties have garnered significant attention in both scientific research and practical applications. In this study, a high-gloss, scratch-resistant, and hydrophobic polyurethane (PU) nanocomposite topcoat was fabricated. To achieve enhanced hydrophobicity, nanosilica (NS) particles were surface-functionalized using two cost-effective silane coupling agents (i.e., octyltriethoxysilane (OTES) and vinyltriethoxysilane (VTES)) with varying silane concentrations (1%, 5%, 10%, and 20% of stoichiometric content) to maximize grafting efficiency. The surface-modified nanoparticles were characterized using FTIR, TGA, and XRD. The water contact angle (WCA) measurements reveal a clear hydrophobicity trend where VTES-modified NS (1% optimal concentration, 135°) exhibits superior water-repellency compared to OTES-modified (5% optimal concentration, 107°) and unmodified NS (52°), demonstrating the effectiveness of vinyl-group functionalization in enhancing surface hydrophobicity. Both pristine and silanized nanoparticles were incorporated into the PU topcoat at 1, 3, and 5wt%, and their effects on gloss, mechanical properties (hardness and scratch resistance), and steel substrate adhesion were evaluated. Notably, the scratch resistance of the PU coating improved by 25% with 5wt% OTES-modified NS and 37.5% with 1 wt% VTES-modified NS, compared to coatings with unmodified NS. WCA measurements show that the pure PU surface has a contact angle of 34°, which decreases to 5° with NS, increases to 91° with OTES-modified NS, and reaches 111° with VTES-modified NS, highlighting the progressive enhancement of hydrophobicity. Nano-scratch testing further revealed that the modified coatings delayed crack initiation, enhancing metal surface durability. This approach also demonstrates scalability for industrial applications, as evidenced by the successful fabrication of automotive topcoat prototypes.