The effect of silica nano-additive on the water repellency of waterborne polyurethane coatings

Abstract

Waterborne polyurethane (WBPU) coatings represent eco-friendly alternatives to solvent-based systems due to their low volatile organic compounds (VOCs) emissions and reduced toxicity. However, their inherent hydrophilicity limits water repellence, thereby restricting their use in demanding environments. In this research, pristine calcium carbonate (CaCO₃), sodium stearate-modified CaCO₃ (SS-CaCO₃), and a mixture of SS-CaCO₃ with silica nanoparticles (SiO₂NPs) improved the hydrophobicity of WBPU. Four formulations, namely, pristine WBPU (P1), WBPU+CaCO₃ (P2), WBPU + SS-CaCO₃ (P3), and P3 + SiO₂NPs (P4), were prepared and applied onto mild steel substrates. The composite formation was confirmed using Fourier-transform infrared (FTIR) spectroscopy, with the intensity of the O-H band in P4 being lower. According to the X-ray diffraction (XRD) study, the dispersion of the SiO₂NPs in P4 was improved, which reduced the crystallite size to 22.38 nm. The scanning electron microscope (SEM) images indicated that the surface roughness of P4 increased with the enrichment of SiO₂NPs on the surface of the coating. The spectroscopic evidence of the energy-dispersive X-ray (EDX) spectroscopy revealed the corresponding elemental distribution and a higher proportion of oxygen. The measurements of water contact angle (WCA) showed that P4 had the best hydrophobicity (105.08 ± 0.74°), which can be explained by the fact that P4 possessed the best surface roughness produced by the nanoparticles. These results indicate that the presence of SS-CaCO₃ and SiO₂NPs enhances the performance and water repellence of WBPU, which further improves its prospects as an environmentally friendly coating.