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.