Effects of natural aging on the properties of a cool surface exposed in different Brazilian environments

•Paint exposed for 2y in São Paulo, Pirassununga, São Sebastião and Belém, Brazil.•The lowest value of the reflectance was 0.49.•Highest surface roughness increase, caused by thicker biofilms, was 1.80.•Natural aging studies needed in Brazil for improving durability of “cool” surfaces. Solar reflectance and thermal emittance are fundamental parameters in the evaluation of a building's thermal and energetic performance, particularly important for “cool” surfaces. However, there is little information on typical values of these parameters in Brazil. We determined values of reflectance, thermal emittance, surface roughness and color difference, for a white high-reflectance paint exposed for two years in 4 cities in the States of São Paulo (SP) and Pará (PA), south-eastern and north Brazil, respectively. After 2 years, painted panels exposed in Pirassununga, SP, showed significantly lower reflectance and increased color difference compared with the original paint and the panels exposed at other sites. This appeared to be related to the presence of a biofilm, in addition to particles of the red clay characteristic of this area. This was likely also to be the explanation of the increased surface roughness determined in Pirassununga samples. Emissivity of the paint changed very little at any of the exposure sites over 2 years, but Solar Reflectance Index, calculated from the average values of reflectance and emittance in the four cities, was reduced from between 105 and 107 to between 94 (in São Sebastião, SP) and 77 (in Pirassununga, SP) after two years. Macroscopically, panels from Pirassununga and Belém, were the most discolored and the greatest presence of biofilm was demonstrated by digital and scanning electron microscopy. Such natural aging studies are important for the production of standard tests and determination of control strategies to increase durability of cool surfaces in Brazil.