Hybrid sol-gel based coatings for the protection of historical window glass

Medieval glass is commonly attacked by atmospheric pollutants conveyed by water, triggering the corrosion process. Current conservation strategies aim to maintain window glass in its original context, and so it is necessary to protect it from further degradation. Sol-gel technology is very effective for the preparation of protective films, using Si-alkoxide precursors chemically similar to the substrate. The present work discusses water-repellent hybrid sol-gel coatings made from tetra-ethyl-ortho-silicate with different quantities of Si-alkoxides, functionalized with various alkyl groups. The coatings were deposited using the dip-coating technique and characterized by UV-VIS and FT-IR spectroscopy. Static and dynamic contact angle measurements showed an overall homogeneity of the coatings and indicated improved water-repellency when functionalized by long alkyl chains. The coatings with best performance in terms of transparency and contact angle, and with the lowest organic content, were selected and applied to medieval-like glass samples. Colorimetric characterization was carried out using a spectrophotometric scanner before and after application of the coatings and no significant color changes were found. The electron microscopy images revealed relatively even, crack-free coatings, especially with higher organic contents. Contact angle and colorimetric measurements were repeated after accelerated ageing by exposure to UV light and an SO2-saturated atmosphere. A significant decrease in the contact angle was observed only for the samples treated at the highest SO2 concentration. The silica-based materials did not give reaction by-products, were water-repellent, compatible with the substrate, colorless, transparent, and stable under exposure to chemicals and light, thereby satisfying the main requirements for the conservation of Cultural Heritage.