Long-term effect of weather in Dfb climate subtype on properties of hydrophobic coatings on sandstone

Abstract

Hydrophobic poly(alkyl siloxane) coatings (pure and containing 0.5 wt% of nano-ZnO) were applied by spraying on sandstone. Coated (S/P and S/PZnO) and uncoated (S) samples were characterized and exposed to weather for 545 days in Ostrava, Czech Republic, an industrial city having persistently high concentrations of particulate matter in air (PM10 ≈ 20±10 μg/m3) and belonging to the Dfb climate subtype. Despite the more distinct final colour change of S/P and S/PZnO front faces, the differences between front and back faces were significantly smaller (ΔE*F–B values of ∼3.7 and ∼3.1 for S/P and S/PZnO, respectively) compared to S (ΔE*F–B ≈ 7.7). Correlations of colour changes with meteorological data revealed effect of relative air humidity, total precipitation, and concentration of particulate matter. Static contact angle (initial values of 137° and 142° for S/P and S/PZnO, respectively) remained >120° during the experiment, and the coatings effectively prevented the falling rainwater to soak into the sandstone. The high hydrophobicity was not accompanied by closure of pores. Despite a decrease in Zn content, no algae was observed on S/PZnO during the experiment. On S and S/P samples, the algae was found after 473 days.