New protective coatings for ancient glass: silica thin-films from perhydropolysilazane

The aim of the work is to synthesise a new silica coating to protect ancient glass from weathering. The coating is prepared starting from an unusual precursor (Perhydropolysilazane e 20%, NL120A-20, Clariant) that allows to achieve high-quality thin-films of silica at room temperature. The obtained films are uncoloured, even in absence of strong heat-treatment. Perhydropolysilazane (PHPS) is a polymer of [eSiH2eNHeSiH2e]n units. When deposited on a typical microscope slide, it reacts with atmospheric moisture, the SieH and SieNH bonds are hydrolysed to SieO bonds and the typical structure of silica is produced. The conversion to silica is completed in about 66 h when operating at room temperature, using vapours of a 15 mol L1 ammonia solution. A quicker method is the application of a weak heat-treatment (45 C), achievable using as heater a common tungsten filament lamp. With this approach, the conversion to silica films is completed in 2 h. The coated glass samples have been studied by XPS and OM techniques