Transparent hybrid films for stone preventive conservation

Conservation of historical buildings is an important issue. The environmental conditions seriously affect the monumental stones. The protection of the cultural heritage buildings and monuments by surface treatment with polymers is a common practice due to their ability to form a protective layer on the monumental surface as well as to control the transport of different fluids from the surface to monument interior. [1,2] Moreover, in the last few years, nanomaterials have been frequently applied for restoration and conservation of artworks. It is worth noting that inorganic nano-oxides (e.g. silica and titania) improve the performances of materials used in conservation fields. [3] In particular, the application of these coatings on stone has been investigated for providing surface protection and self-cleaning properties. [4] Three different kinds of stone, commonly used in monuments, were selected for the present study (Angera, Botticino and Carrara). From each stone, small blocks were obtained and polished with commercial grade diamond abrasive disks. The stones were treated with commercial water-repellent protective agents and then exposed to a typical polluted urban environment, placing them in a site where traditionally aerosol particulate matter (Milan University Campus). Among the several commercially available resins, SILRES® BS16 and ALPHA® SI30 were adopted in the present work since they are soluble in aqueous medium, representing an eco-friendly solution (low toxicity for human health and environment). Moreover, the performance of superhydrophobic nanoparticles mixed with the polymer deposited onto the stone surfaces was evaluated in order to have a comparison with what obtained by using only water-repellent protective agent. The bare and coated samples were characterized by several techniques, such as XRD, SEM-EDS, FTIR, TGA, IC, CIE-Lab colorimetric analyses and contact angle measurements. In order to investigate the stability of the coatings, accelerated aging tests by UV irradiation (500W, 250-315 nm) were carried out. Colorimetric measurements (CIELab) were performed to verify the color modification (yellowing) of the protective film due to solar exposition. Aiming at evaluating the soluble salts formation [5] into the stones and to test the protective action of the applied coatings, the three substrates were exposed to air in a typical polluted urban environment (Milan University Campus). Results, obtained after prolonged exposure, show the effectiveness of the coating layers in reducing salts formation.