Solar reflective properties of porcelain tiles for UHI mitigation: effect of highly reflective frits in the engobe's formulation

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The 2030 Agenda for Sustainable Development charts the course for transforming the world socially, economically, and environmentally by 2030. Cities represent a critical issue in this transformation: the population growth and the intense anthropogenic activity make them centers of strong environmental impact. For instance, cities have become Urban Heat Islands (UHIs) due to the significant increase in temperatures caused by pollution and heat produced by human activities. The urgency to make cities sustainable from an environmental point of view urges the search for innovative solutions to counter this phenomenon. One of those solutions is reflective tiles, as they absorb less heat and stay cooler than traditional materials and are considered an effective mitigation technique against the UHI phenomenon. This study investigated the obtaining of solar reflective ceramic tiles through the modification of the engobe's formulation. To achieve this goal, a glass-ceramic frit with high solar reflectance (SR) property (0.95) was incorporated into the formulation of a commercially available ceramic engobe. To verify whether the modified engobe can be used to prepare solar reflective cool color ceramic tiles, the tiles were decorated in quadrichromy by ink-jet printing according to the four-color CMYK color model. The Gray Scale (GS) printing system was used to investigate five different printing intensities. The results suggest that when a glass-ceramic frit with a high intrinsic SR value is incorporated into the formulation of an industrial engobe, it can raise its SR value, passing from 0.69 to 0.75. However, the productive process used to obtain ceramic tiles limits the SR values due to crystallization. It was also found that ink-jet decoration negatively affects the solar reflective property of the tiles prepared with the new engobe due to its coverage by the inks and their mineralogical composition. Thus, the results presented in this work evidence that changing the engobe's formulation is not enough to obtain ceramic tiles with high solar reflective properties.