Fluorine-free radiative cooling paints and reproducible assessment

Passive daytime radiative cooling (PDRC) achieves sub-ambient temperature without the need for external energy input by simultaneously suppressing incoming sunlight and radiating heat to the cold universe. PDRC paints are highly desirable due to their advantages of being metal-free, having a simple single-layer structure, providing good conformality, and allowing for facile large-area manufacturing. However, many PDRC materials face challenges such as the need for improved solar reflectance and thermal radiation, reliance on fluorine substances, and inconsistent assessment protocols. These challenges can lead to potential environmental issues and inconsistent or erroneous data. To address these limitations, we develop a hydrophobic, fluorine-free PDRC paint, based on a Y₂O₃-poly(methylphenysiloxane) (PMPS) filler-binder composite. The Y₂O₃ fillers possess a large energy gap of 5.6 eV and a high refractive index above 1.9, resulting in an absolute solar reflectance of 96.1 % and mid-infrared emissivity of 0.97. The proposed reproducible assessment protocols, for the first time, enable the measurement of cooling performance using commercially available Stevenson screens without any involvement of homemade components. These protocols demonstrate that the fluorine-free PDRC paint realizes a sub-ambient cooling temperature of up to 4.8 °C, with an average of 2.4 °C under direct sunlight with a maximum solar irradiance of 902 W m⁻² during continuous 24-hour measurements. This work not only presents the design and realization strategies for environmentally friendly, fluorine-free PDRC paints but also emphasizes the importance of establishing reproducible assessment methods, which are critical for mitigating measurement deviations and facilitating credible comparisons among numerous PDRC studies.