How does the substrate material affect the optic performance of retro-reflective coatings? An experimental investigation

In highly populated cities the Urban Heat Island (UHI) phenomenon affects in a negative way the urban environment and outdoor thermal comfort. Many efforts have been made in order to counteract UHI effects, especially regarding cool materials for urban surfaces. Retro-reflectivity is a promising surface capability, which has attracted the interest of researchers for building application. This work aims at studying the impact of the substrate material on the optic performance of retro-reflective (RR) coatings. Three types of substrate material were investigated: smooth pine wood panels; rough plywood panels and smooth acetate sheets. The RR coating samples were made by firstly adding a high reflective white paint onto the substrate material and a homogeneous RR glass beads layer on the top. As a reference case, also diffusive samples, without RR beads, were developed. Samples have been tested through a spectrophotometric and an angular reflectivity analysis. To explore the angular reflectance, an ad-hoc experimental facility was used. Results show that, despite a lower global reflectance of the RR samples with respect to the diffusive ones, the glass beads coating provides a good retro-reflective capability to all the diffusive samples. Additionally, the choice of a rougher substrate leads to a maximum RR capability of up to 16%, with respect to the smooth substrate samples. Future developments may involve the optimum design of RR coatings, in terms of their optical performance by varying the substrate roughness, the glass beads density and dimension.