Improvement of radiative cooling paints by surface functionalization of filler particles

Radiative cooling is a sustainable path toward the reduction of energy used for cooling. Exterior paints remain the most accessible format of radiative cooling. To reduce costs, filler particles have been incorporated into paints along with titanium dioxide (TiO₂) pigment without contributing to their cooling capabilities. This work investigated the surface functionalization of calcium carbonate (CaCO₃), barium sulfate (BaSO₄), and silica (SiO₂) filler particles with methyltrimethoxysilane (MTMS) for improving paint cooling performance. Functionalized fillers improved paint solar reflectance, but this effect became minimal when the fillers were incorporated along with TiO₂ pigment. MTMS-modified CaCO₃ and BaSO₄ fillers, incorporated alone or along with TiO₂, enhanced paint infrared emission in the atmospheric transparency window, while the functionalization of SiO₂ fillers had little impact on the emission. The MTMS functionalization also improved the thermal emissivity of BaSO₄ and CaCO₃ paints with TiO₂ pigment but reduced that of SiO₂. Outdoor testing demonstrated that the use of MTMS-modified BaSO₄ fillers led to an additional reduction of paint temperature by 2.3°C during the daytime. Therefore, the functionalization of BaSO₄ filler particles is a simple and inexpensive method for improving the performance of radiative cooling paints without requiring significantly altering industrial paint production processes.