Development and characterization of retro-reflective colored tiles for advanced building skins

RR materials, able to reflect the solar radiation mainly toward the incident direction, are a strategy to mitigate the Urban Heat Island (UHI) phenomenon, by reducing the heat trapped inside the urban canyon. This paper introduces new ceramic tiles for outdoor applications properly treated to get RR optic properties. Traditional colored tiles were covered with a UV resistant, transparent paint containing RR micro spheres for exterior applications, through an ad-hoc continuous tape casting process, developed for this specific application. The goal is to produce RR tiles through industrial continuous process in order to ensure the penetration of such innovative building materials in the market. Three types of microspheres were tested: i) clear barium titanate glass microspheres; ii) hemispherically aluminum coated barium titanate glass microspheres; iii) hemispherically aluminum coated barium titanate glass microspheres with an additional fluoropolymer coating. The obtained tiles were investigated with a spectrophotometric analysis, a study of the spatial distribution of the reflected radiation, a colorimetric analysis to evaluate changes in tiles' original color. Reflectance is increased by clear barium titanate glass microspheres, while is almost halved by aluminum-coated spheres. All the three types of microspheres provide strong RR behavior for incident light directions from 0 to 60 with respect to the surface normal. Nevertheless, the aluminum-coated microspheres change completely the tiles' original color, making the application of the aluminum RR tiles very limited. Clear barium microspheres instead improve the optic properties of the original tiles, with negligible effects on the color.