Spray-Coated Macroporous CaCO3–PVA Structures for Radiative Cooling

Current dielectric composites require coating normally thicker than 400 μm to produce the radiative cooling effect, compared to 50–100 μm for standard paints. In this study, CaCO3–PVA composite coatings with thicknesses below 300 μm are produced, forming macro-porous yet smooth structures. Controlled ball milling of CaCO3 produces uniformly distributed micro- and nanosized particles with optimized sizes for solar radiation backscattering. To produce a favorable material structure, optimized particle sizes are spray-coated to construct the composites. The composite has macro pores and micro particles of CaCO3 with PVA as a stable binding and coating agent. Optical and scanning electron microscopy confirms uniform particle dispersion and macropore formation across coating thicknesses. The coatings exhibit strong thickness-dependent reflectivity, achieving over 95% solar reflectivity for 290 μm thickness, while maintaining emissivity of ∼90% in the 8–13 μm atmospheric window. Raman spectroscopy verifies that no significant chemical or structural changes to CaCO3 occur during fabrication. Based on measured spectral, indoor solar simulator tests, environmental and typical solar irradiation data for hot climate conditions, modeling predicts an average subambient temperature reduction of approximately 3–6 °C. These results demonstrate that CaCO3–PVA composites provide a cost-effective, scalable, and NIR-absorption-free alternative to conventional white cooling paints, with strong potential for practical, large-scale passive cooling applications.