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

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 approximate to 20 +/- 10 mu g/m(3)) 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 (Delta EF-B* values of similar to 3.7 and similar to 3.1 for S/P and S/PZnO, respectively) compared to S (Delta EF-B* approximate to 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 degrees and 142 degrees for S/P and S/PZnO, respectively) remained >120 degrees 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.