Robust and Environmentally-Friendly Nanocomposite for Daytime Radiative Cooling

Radiative cooling technologies exchange thermal energy with deep space and require no moving parts or energy input. Radiative cooling solutions have been implemented via a variety of innovative approaches within scientific literature. They rely on achieving high solar reflectance and high thermal emittance to obtain a surface temperature below their local ambient temperature, which in consequence can combat climate change and promote passive daytime cooling. Nonetheless, many of the proposed radiative cooling solutions face various challenges in implementing them into real world applications. High-performance ultra-white pigments have produced the first single-layer and metal-free full daytime sub-ambient cooling materials in the form of solvent-based ultra-white radiative cooling Acrylic-paints. However, these radiative cooling Acrylic-paints rely on solvent processing during fabrication, which produces volatile organic compounds (VOCs) that have serious negative impacts on human health and on the environment. The rising concerns of utilizing VOCs have introduced persisting environmental and health regulations across many industries, including the paint manufacturing industry. Moreover, elastomeric coatings are popular in water-proofing and in cool-roof applications, which are also known for their enhanced mechanical robustness. However, commercial white elastomeric coatings contain TiO2 as their white pigment, where no daytime cooling has been reported for elastomeric coatings within the scientific literature. To address these challenges, in this dissertation we have utilized these ultra-white pigments to develop water-based radiative cooling paints that demonstrate full daytime sub-ambient cooling similar to the mentioned solvent-based radiative cooling Acrylic-paints. These paints demonstrate full daytime sub-ambient cooling of 2.7°C, 2.6°C, 2.5°C, utilizing the ultra-white pigments to produce high solar reflectance of 95.4%, 93.7%, and 96.1%, and strong sky window emissivity of 0.932, 0.924, and 0.825, for BaSO4, CaCO3, and hBN paints, respectively. Their hydrophobic performance yield high water contact angles of 118°, 139.9°, and 136.7°, and they achieved the low-VOC classification by recording 26 g/L, 18 g/L, and 30 g/L of VOC content for BaSO4, CaCO3, and hBN paints, respectively. These VOC levels are lower than many commercial water-based paints with VOC content of up to 200 g/L. Furthermore, we have utilized these ultra-white pigments to fabricate environmentally-friendly ultra-white elastomeric coatings (or ultra-white elastomers), with daytime sub-ambient cooling similar to the mentioned solvent-based radiative cooling Acrylic-paints. These ultra-white elastomers recorded total solar reflectance of 95.2%, 94.4%, 94.4%, along with strong sky window emissivity of 0.956 ± 0.001, which recorded average temperatures of 1.9 °C, 2.1 °C, and 1.3 °C below ambient for BaSO4 CaCO3, and hBN ultra-white elastomeric coatings, respectively. These elastomers demonstrated >100% elongation rates, and elastic moduli of 975 psi ± 88 psi, 18.4 ± 0.04 psi, 439 psi ± 13 psi, along with excellent hydrophobicity featuring water contact angles of 137°, 154°, and 144° for BaSO4, CaCO3, and hBN elastomeric coatings, respectively. The cooling performance of our environmentally-friendly ultra-white elastomeric coatings surpasses commercial elastomeric coatings. In summary, this dissertation attempts to advance the development of radiative cooling materials by providing environmentally-friendly and high-performance alternatives to the solvent-based ultra-white radiative cooling Acrylic-paints and to the existing TiO2-based commercial elastomeric coatings. While radiative cooling paints and coatings can provide passive daytime cooling in a scalable approach, producing them with environmental sustainability help accelerate their commercialization process where they can replace the conventional TiO2-based commercial white paints and coating. As a result, these materials add the value of combating the global heating effect and climate change, while also providing additional energy cost savings in producing passive cooling.