Raman monitoring of the sol-gel process on OTES/TEOS hybrid sols for the protection of historical glasses

In the frame of a wider research work focused on the study of Medieval Potash-Lime-Silica (PLS) glass, several hybrid sols have been studied as water repellent protective coating for historical windows. The sol gel process currently is one of the most used methods to study new chemical products for the conservation of the Cultural Heritage. For historical windows the sols should achieve the protection from the most diffused weathering agents (SOx, NOx, CO2…) dissolved in the environmental water that acts like a trigger for the glass alteration mechanisms. Currently the protection of historical windows is achieved by the installation of protective glazing on the external surface of the windows or by the application of resins. In the first case the slabs strongly reduce the transmission of light, darkening the glasses colors and having a bad impact on the appearance of the monuments [Pallot-Frossard et al., 2005]. The acrylic resins have, as main drawbacks, the physical and chemical incompatibility with the inorganic substrate, the occurrence of yellowing phenomena due to the photo-oxidation and the thermal instability since their Tg range between 15 and 40°C [Chiantore & Lazzari, 2000]. In this work TEOS (Tetraethylorthosilicate) based sols -in isopropanol as solvent and using HCl as catalyst- have been added with several functionalized Si-alkoxides in different proportions to achieve a good surface water repellency. Three compositions are particularly effective: 80%TEOS-20%OTES (Octyltriethoxysilane), 95% TEOS-5% HDTMS (Hexamethyltrimethoxysilane) and 75%TEOS-20%OTES-5%HDTMS. The sol gel process was followed by Raman spectroscopy to study the evolution of some peaks suitable to monitor the hydrolysis and the condensation reaction and the results of the investigation on the 80%TEOS-20%OTES composition are reported here. Since the isopropanol and ethanol (reaction by-product) Raman features are particularly intense, to obtain strong alkoxide Raman features the spectra were acquired on a sol containing an higher Si concentration (2M) with respect to those originally used as protective coatings (0.5 M) and working at pH~5, instead pH~2, to slow down the reaction kinetics and then better follow the changes of the Raman features. The intensity decrease of the TEOS features at 652, 930 and 1090 cm-1 indicates the progress of the hydrolysis reaction. In particular, the symmetric stretching modes of the Si-O bonds of both alkoxides at ≈ 650 cm-1 completely disappear after 10 minutes, indicating that the reaction was completed [Marino et al., 2005]. The evolution of the Si-O anti-symmetric stretching peak at ≈795 cm-1 can be used to follow the condensation reactions. This mode initially is found to increase in intensity with the number of hydrolyzed alkoxide molecules. Then its intensity decreases since the condensation gives the formation of siloxanic -Si-O-Si- groups [Gnyba et al., 2003]. This is confirmed by the intensity of the 1047 cm-1 peak, attributed to the asymmetric stretching motions of the Si-O-Si bonds, which increases over the reaction time. The process was followed for 48 hours: the typical large silica band appears between 250 - 450 cm-1, together with changes -in intensity and position- in the high frequency region concerning the stretching motions of the C-H groups. The Raman monitoring of the sol-gel process to follow the hydrolysis and condensation reactions proved to be a fast and easy way to understand the changes occurred in the system over the reaction time. REFERENCES [1] Pallot-Frossard I., BernardiA., Van GriekenR., RollekeS., Verità M., RivistadellaStazionesperimentale del Vetro 2005; 35: 75-83 [2] Chiantore O., Lazzari M. Polymer 2000; 42: 17-27. [3] Marino I. G., Lottici P. P., Bersani D., Raschella R., Lorenzi A., Montenero A. Journal of Non-Crystalline Solids 2005; 351: 495-498. [4] Gnyba M., Jędrzejewska-Szczerska M., Keranen M., Suhonen, Proceedings J., XVII IMEKO World Congress, June 22 – 27, 2003, Dubrovnik, Croatia, 237.