Fluoropolymer Based Radiative Cooler Having High Durability for External Environment

Global warming is accelerating as fossil fuel consumption increases. The global temperature is rising every year due to global warming. According to this environmental issue, exposure of heat stress can reduce people’s satisfaction with life in outdoor activities especially in hot summer. Also, it can decrease the efficiency of machine ability. Therefore, a lot of electricity is used for cooling purpose to relieve the heat in the city. In Korea, household electricity consumption is concentrated in August. Also, according to the current trend, the number of days with heat waves is expected to increase further. The increase in electricity consumption leads to an increase in the use of fossil fuels, which is a vicious cycle that causes global warming worse. Passive daytime Radiative cooling (PDRC) can lower the temperature without consuming energy. By reflecting sunlight (0.3-2.5μm) and radiating the heat to the space (0K) through the atmosphere’s window (8-13μm), it can cool the ambient temperature below the radiative cooler. When it is set on rooftop, the building inside temperature goes down. PDRC is new-coming energy saving technology. In this study, we fabricated radiative cooling material in the form of solution and we can obtain white film caused by diffused reflection of nanoparticles. It shows white surface without metal layer so it doesn’t cause glare. Fluoropolymer is fluorocarbon-based polymer and it has a high resistance to solvents. Especially, Ethylene Tetra fluoro Ethylene(ETFE) has good chemical resistance among various fluoropolymers. Also, ETFE material has high thermal resistance and its operating temperature is around 520K. In other words, this material can remain at extreme environment. Due to the nature of PDRC installed outside, exposure to weather conditions is inevitable. When acid rain falls in hot summer, normal radiative cooling material which is made of ceramic particle and polymer binder can be degraded by acid and high-temperature, thereafter its cooling performance decrease and durability also downgraded. In this work, ETFE particle is used to make white monolayer with PUA (polyurethane acrylate) for radiative cooling and inside can be cooled easily by coating it thinly on substrate like building outside. ETFE radiative cooler coated on glass substrate shows solar reflectivity of 92.5% and emissivity of 91.7% across atmosphere’s window and its calculated cooling power is 95.6W/m2. Because ETFE material which is one of fluoropolymer has good high-temperature resistance and high-chemical resistance, we propose PDRC which can operate at high-temperature and chemical environment. ETFE particle cannot solve with organic solvent and also cannot melt at room temperature. Soomin Son et al. fabricated PVDF film using PUA with UV hardening. We dissolved PUA to ethanol and combined white ETFE particle into solution with film forming agent. We fabricated heating chamber which continues to radiate heat by halogen lamp. With 20W lamp, the temperature inside heating chamber can increase up to maximum 400K. By putting ETFE radiative cooling material coated on aluminum on heating chamber, it decreases 24K contrast to ambient temperature when outdoor measurement with heating chamber. By chemical resistance test, we dipped ETFE radiative cooler into sulfuric acid which is same PH range of acid rain during one day with white commercial paint sample (e.g., titanium dioxide-based paint) and zirconia based radiative cooler. Commercial paint sample was yellowed, and also zirconia based radiative cooler was cracked and its film came off. But the film of ETFE radiative cooler was remained and its optical properties was also remained within a small range of error. In this way, ETFE radiative cooling coating hosts promising future in building cooling and it might be also found wide applications in outdoor or devices which of efficiency is sensitive to its temperature.