Construction of a multi-scale microporous structure in EVA matrix toward passive cooling roofs

Global warming has brought tremendous attention to developing passive cooling materials. Due to the high performance of solar reflection, heat insulation and infrared radiation, constructing porous structures in polymeric matrix has emerged as a promising technique to fabricate passive cooling materials. This work herein demonstrated a multi-scale microporous structure in ethylene-vinyl acetate copolymer (EVA) matrix via chemical foaming and chemical etching. It was found that pore size and shape depended upon the addition and removal of inorganic particles CaCO₃. In chemical foaming system, by reflecting 76.9% NIR light and transmitting heat through the atmosphere in the long-wave infrared, the cellular EVA/CaCO₃ composites (40% porosity, 13.84 μm avg. pore size)-covered device exhibited a final temperature of 31.6 °C after solar simulation for 1 h with the intensity of ∼594 W/m². To further enhance the cooling effect, the CaCO₃ particles in cellular EVA/CaCO₃ composites were removed via 20% HCl solution etching to minimize the pore structure. Higher heat insulation with thermal conductivity of 0.148 Wm⁻¹K⁻¹ accompanied by 65.5% NIR irradiance and 0.84 thermal emittance made the excellent cooling property with a final temperature of 30.6 °C (lower than all EVA/CaCO₃ composites-covered device) realize after chemical etching. Furthermore, the prepared porous EVA materials possessed good tensile properties and are of potential value in the field of passively cooled roofs.