Preparation and Performance of Transparent Thermal Insulation Coatings Based on Hollow Nano-TiO2 Spheres

Hollow nano-TiO2 spheres (HNTSs) with shell coating ratios of 28.62%, 29.33%, 30.72%, 36.31%, and 38.17% were prepared by a sol–gel method. The novelty of this work lies in the use of the shell coating ratio as a tunable design parameter to balance visible-light transmittance and thermal insulation in waterborne polyurethane (WPU) coatings on glass. HNTS with a coating ratio of 30.72% provided the most balanced performance and was selected for composite optimization. The HNTSs were incorporated into WPU at loadings of 1, 2.5, and 5 wt%, and the resulting coatings were applied to glass substrates. At 5 wt% HNTS, the coating retained 72.19% visible-light transmittance and reduced the temperature rise rate to 1.94 °C/min, approximately 43% lower than that of blank glass (3.42 °C/min). These values compare favorably with previous reports of transparent TiO2-based and hollow-particle insulating coatings while requiring only a relatively low HNTS loading of 5 wt%. The low-refractive-index air encapsulated in the HNTS cavities lowers the effective refractive index of the composite, weakening visible-light scattering while maintaining transparency. In addition, the enclosed air cavities suppress solid-state heat conduction, and the TiO2 shells contribute to near-infrared scattering and attenuation. These combined effects demonstrate a practical design strategy for transparent thermal insulation in glass coatings.