Dynamic solutions for urban cooling: integrating thermochromism and photoluminescence in dynamic retroreflective skins for heat mitigation

In this study, we explore innovative coatings for building applications to mitigate surface overheating and the associated Urban Heat Island phenomenon. We examined the interaction among different types of cool materials, starting with a base coating comprising highly reflective paint (HR), further enhanced with thermochromic (TH) and photoluminescent pigments (PH). These materials were applied onto a PVC substrate and subsequently overlaid with a retroreflective (RR) layer containing fine-grained (FG) and medium-grained (MG) glass microspheres at varying weight percentages (25, 40, and 50 wt%). A combination of optical and thermal analysis techniques was used to assess the interactions between the layered materials and the impact of RR coatings on key performance metrics. Results highlight the significant influence of glass microsphere size: MG microspheres achieved total solar reflectance values exceeding 70 %, while FTIR analysis showed stable thermal emittance around 50 %, with a notable 20 % reduction in the 11–19 μm range for MG samples. Surface roughness measurements further revealed smoother surfaces in MG-coated samples compared to FG ones. Decay time simulations under low and high irradiance conditions for PH samples indicated prolonged photoluminescent persistence in FG-based systems, especially under low irradiation. Overall, while MG microspheres offer superior solar reflectance, FG microspheres demonstrated more balanced performance, making them particularly promising for UHI mitigation strategies in urban coatings.