PVDF films with optimized porous structure using non-solvent induced phase separation for passive radiative cooling

Abstract

Today, as personal thermal management gains importance for both increased comfort and energy saving advantages, body cooling through the clothing system by using natural energies has come to the fore. In this study, as the first step of a cooling garment design project, radiative cooling films having porous structures with tailored optical structures to enhance UV-VIS-NIR reflectivity were produced from polyvinylidene fluoride (PVDF), via non-solvent-induced phase separation method using two solvents and water. The porous structure of PVDF films was optimized by differing polymer (5 and 10wt. %), water concentrations (3-10%) and drying conditions. Besides SEM analyses to determine the pore characteristics, cooling performances were evaluated by a hotplate apparatus combined with a halogen lamb as a sun simulator. Indirect emissivity calculations were carried out from the IR camera images of the films heated at the same temperature with a fabric whose temperature was measured with sensors. According to SEM images, PVDF films having 10 wt.% polymer and 3% water concentrations had the most porous structure. The emissivity of the optimized PVDF film with a hierarchically porous structure was enhanced to 96.6% with a temperature decrease of 1.06°C under a solar irradiance of 600 W·m-2 indicating a good potential for a further cooling structure as a nanocomposite and coated fabric forms.