Enhancing the Consistency of Porous Polymer Passive Daytime Radiative Cooling Paint in Outdoor Applications
Materials Research Society, BOSTON, Massachusetts(2023)
Dongpyo Hong, Ok Sung Jeon, Se Hun Lee, Yong Joon Lee, Insung Lee, Ye Rim Kim, Yunju La, Yeong-pyo Jeon, Young Joon Yoo, and Sang Yoon Park
Abstract
Coating the building envelope with passive daytime radiative cooling (PDRC) paint has garnered enormous attention as an energy- and carbon-free alternative cooling technology. Achieving this goal requires the spontaneous formation of a suitable optical structure during the solvent drying process of the paint that effectively reflects sunlight and enables thermal emission through the atmospheric window (8-13 µm). In addition to their cooling abilities, PDRC coatings must also possess affordability, durability, and compatibility with diverse surfaces and climates for practical use.
Recently, porous polymer coating (PPC) has gained significant attention due to its numerous advantages, including cooling performance, cost-effectiveness, flexibility, and scalability. However, achieving consistent performance over a wide range of outdoor conditions is a challenge due to the morphology of PPC formed via the evaporation-induced phase separation method (EIPS), which is highly dependent on evaporation environment parameters such as temperature and humidity. Despite the advantages of PPC, there is a lack of systematic studies on the consistency of PDRC performance under varying environmental conditions of EIPS, and potential solutions to this issue have not been addressed.
In this study, we demonstrate the humidity vulnerability of PPC during the drying process and propose a simple strategy to mitigate this issue. Specifically, we found that the solar reflectance of PPC rapidly decreases with increasing humidity from 30% RH, causing the PPC to completely lose its PDRC ability at 45% RH and even become a solar-heating material at higher humidity levels. However, by adding a trace amount of fumed silica into PPC, we were able to maintain its PDRC performance up to 60% RH, resulting in a 1050% enhancement of areal coverage in the United States. This study highlights a crucial consistency issue for PPC-based PDRC paint that has been previously overlooked and offers engineering guidance to address this fundamental challenge for the development of reliable PDRC paint for industrial applications.