Synergistic effect of silica aerogel and titanium dioxide in porous polyurethane composite coating with enhanced passive radiative cooling performance

Passive daytime radiative cooling is a promising technology that can mitigate the increasing issues of global warming and urban heat island effects. In this work, a series of porous polyurethane (PPU) composite coatings containing silica aerogel (SA) and/or titanium dioxide (T) were prepared though a wet phase inversion method. The effects of the filler types on relevant properties of composite coatings, such as pore microstructure, radiative cooling, mechanical, and hydrophobic performances, were systematically investigated. The typical PPU-SA10@T10 composite coating showed an interleaving porous structure, and the incorporated SA and T fillers were uniformly stacked in the pore cavities. Taking advantages of the strong light-back scattering and molecular oscillation functions, the solar reflectance, thermal emittance in the atmospheric transparency window, and the net cooling power of the PPU-SA10@T10 coating reached 0.917, 0.973, and 87.8 W/m2, respectively, which results in a sub-ambient cooling of 7.8 °C at midday under a solar irradiance of ∼640 W/m2. Moreover, the PPU-SA10@T10 coating possessed well balanced mechanical properties and superior hydrophobicity (water contact angle was 121.7°). The present work provides a viable way for developing radiative cooling coating with desirable mechanical properties and water-repellency.