SYNTHESIS OF SILICA PROTECTIVE FILMS BY TIN-CATALIZED SOL-GEL PROCESS

DESCRIPTION TECHNICAL FIELD

The invention concerns the development of a new synthetic path to obtain highly pure glassy silica thin films operating at low temperatures and in a neutral environment. Such films are used in the protection of vitreous or metallic works of art from weathering.

BACKGROUND ART

Planning the protection of artistic objects, implies the whole comprehension of chemical and physical phenomena involved in weathering. The intervention must be able to stop or slow down weathering and has to preserve features allowing the full appreciation of the work of art.

A lot of methods were developed for the protection of objects made of glass. The use of isothermal glasses is one of the most recent [A. Corallini, V. Bertuzzi, Il restauro delle vetrate, Cardini Editore, 1994]. External protective glasses can assure protection, but have to be installed avoiding the condensation of vapour on the glasses surfaces. On the other hand they alter the aspect of the work of art and don't belong to the context.

Organic protective coatings are also employed. They can be polymers (acrylic, vinylic or polyurethanic), epossidic resins or polyesters. [R. Newton, S. Davison, Conservation of Glass,

Butterworth-Heinemann, 1996]. In the long run the action of solar light deteriorates these coatings and the work of art change of color, become opaque, its constituents crystallize and separate. The object protected in this way can thus be damaged or altered.

Most recently new organically-modified silicon-based coatings have been set up [M. PiIz, H. Roemich, J. Sol-Gel Sci. Technol. 8 (1997) 1071-1075; The Getty Conservation Institute Newsletter, Vol. 16, n ⁰ 1, Spring 2001]. However, the presence of an organic component doesn't assure the reliability in time of the coating.

In literature thin films of glassy silica can be obtained by the sol-gel transformation of acid solutions of silicon alcoxides. The density increase of the coatings is usually performed by heat treatments at temperatures between 500 and 600 ⁰ C. [CJ. Brinker, G.W. Scherrer, Sol-Gel Science - The Physics and Chemistry of Sol-Gel Processing, Academic Press, Inc. Harcourt Brace & Co. Publishers, 1990]. This heat treatment is not applicable on historical art works because it would cause the irreversible degradation of the ancient glass. It is then necessary to develop a protections suitable for the application in conservation of works of art. One of the proposing authors has already patented a protection methodology of silicate materials, involving the deposition of silica thin films by sol-gel process in Brønsted acid environment, without the need of high temperature treatments [R. Bertoncello, S. Coronaro, L. Armelao, A. Glisenti, E. Tondello -

Method for protecting manufactured silica and silicate art goods by coating with a vitreous silica film - Ital. IT 1301744 Bl 7 JuI 2000, 31 pp. CODEN: ITXXBY. CLASS: ICM: C03C. APPLICATION: IT 1998-MI1393 18 Jun 1998. DOCUMENT TYPE: Patent CA Section: 57 (Ceramics)]. Such method is valid for glass objects with a good ratio between alkaline and alkaline earth elements, but is less suitable for glasses with large alkaline to alkaline earths ratios and for glasses sensitive to acid environment (i.e. lead-rich glasses).

In order to overcome this problem the same author quoted above, found a new method for the protection of silicate materials by the deposition of silica thin films obtained using Lewis-type acid catalysts. The density increase of the coatings is performed at temperatures between 10 and 40 ⁰ C for 10- 15 days.

The need, but also the possibility, of the use of such catalysts emerges from the observation of phenomena that take place when a lead-rich glass is dipped in sol-gel acid solutions. Lead-rich glasses, in acid environment, undergo a strong lisciviation, due to the high mobility of lead ions in glass matrices [H. Scholze, Glass: Nature, Structure and properties, Springer-Verlag, USA (1990); L. Milanese, R. Bertoncello, A. Bouquillon, J.-C. Dran, J. Salomon, B. Mille, Leaching of lead silicate glasses in acid environment: compositional and structural changes, Applied Physics A: Materials Science and Processing 79 (2004) 193-198].

The strong acidity of such solutions causes the ionic migration of lead from the substrate to the film implying the formation of films so many thick, iridescent and non protective. On the other hand, considering that metallic ions in aqueous solution show Lewis acidic characteristics, Pb2+ ions were employed in the catalysis of the sol-gel process.

On the basis of what stated above, the catalytic activities of other Lewis acid systems were investigated. The study of the catalytic activity of tin in many industrial processes [SJ. Blunden, The Industrial Uses of Tin Chemicals; Royal Society of Chemistry, London (1985)] and, in particular, in the vulcanization of RTV (Room Temperature Vulcanization) silicone rubbers [F. W. Van der Weij, The Action of Tin Compounds in Condensation-type RTV Silicone Rubbers, Makromol. Chem. 181, (1980) 2541-2548], process with many analogies with sol-gel process, shows that the catalytic activity of this element derives from its Lewis-type acid features.

Tin-based catalysts used in such applications were in most cases organometallic species, but in this work inorganic tin salts were employed in order to avoid the inclusion of organic fragments in the films to be synthesized.

DISCLOSURE OF INVENTION

A methodology for the preparation of a sol-gel precursors' solution using a Lewis acid as a catalyst, instead of hydrochloric acid, has been set up. With this solution it is possible to deposit silica thin films in neutral environment.

Sn(IV) has been chosen for this purpose, because in aqueous solution it gives complete hydrolysis to Sn(IV), which is a really strong Lewis acid. Sn(IV) aqueous solution undergo spontaneous solution- sol-gel transition, in which a tin (IV) oxide gel is produced by hydrolysis and condensation reactions. Such solutions have been used to prepare sol-gel precursors' solutions with different Sn/TEOS molar ratios and thus different pH values.

Table 1: Features of the prepared precursors' solutions

The prepared solutions has been stirred for two hours and then kept at 2 ⁰ C. With these four solutions several deposition tests were performed on soda-lime glasses, using the following dipping parameters:

Table 2: Dipping parameters used in the deposition operations

The four solutions show different gelation times. The gelation, anyway, occurs in each case because it was possible to obtain the deposition of silica thin films from each solution. In the following table are reported the results of the deposition tests:

" " No deposit ; "*" Dishomogeneous Deposit ; "x" Homogeneous Deposit ; "•" Solution Solidification Table 3: Scheme of the obtained results with solutions A, B, C and D at different aging times

With solution C, if it is kept at 25 ⁰ C, it is possible to obtain homogeneous coatings after an aging time of 21 days.

On the basis of such results it is possible to adapt the synthesis parameters according to the needs of aging time and pH values. Characterization of the coatings:

Macroscopic and microscopic optical analysis X-ray Photoelectron Spectroscopy (XPS) Secondary Ion Mass Spectrometry (SIMS)

Optical Analysis:

The obtained films, at first sight, are homogeneous and transparent (Figure 1) The microscope analysis of the films show that the surface of the coatings don't have great defects. A planarization effect of the coated surfaces is observed (Figure 2 and 3)

XPS and SIMS Analysis:

Composition and thickness of the films were determined by XPS and SIMS analysis. The XPS analysis showed that tin and chlorine in the films are below the detection limit of the instrument. A depth profile of the elemental concentration is reported in figure 4. With the SEVIS analysis it was possible to determine the thickness of the films: around 210 run. In this case signals related to tin and chlorine can also be seen. (Figure 5).

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 : Picture of a soda lime glass covered with a silica film, obtained from solution B.

Figure 2: Microscopy picture of the soda-lime glass surface. Some defects are visible on the surface, (reflected light bright field, 5x)

Figure 3: Microscopy picture of the film deposited on the same glass in figure 2. The film is homogeneous and the defects are no longer visible, (reflected light bright field, 5x)

Figure 4: Depth profile carried out with XPS analysis

Figure 5: Depth profile carried out with SIMS analysis