Nanotechnology applied to glass surface protection. Claudia Altavilla Dipartimento di Scienze Chimiche Università di Catania, viale A. Doria 6, 95125 Catania, Italy e-mail:
[email protected] Generally, we are accustomed to consider glass an inert and durable material with high chemical stability. In addition to its apparent inertia, glass has a lot advantageous properties, such as its transparency or the ability to take any color as the result of the addition of small amounts of transition metals. In fact it has been extensively used, thanks to its unique mechanical and chemical–physical properties, from ancient until modern times. Before describing the structure of glass it is important to define the term “glass”. The definition given by the American Society for Testing and Material is : “Glass, is an inorganic product of fusion which has cooled to a rigid condition without crystallizing”. In particular we are concerned with oxide glasses based on silica: the alkali-alkali earth-silica family of glasses. The first theory of the structure of glass to become widely accepted was that of Zachariasen (1932), called the random network theory . This theory explains many characteristics of glassy state such as: a) The optical isotropy of glasses results from the random atomic arrangement. b) In a random network no two atoms occupy exactly identical sites and so the lack of abrupt change of state can be understood. c) The composition of glass is not stoichiometric ( it is a mixture). d) There are no crystal cleavage planes in glass. It is possible summarize these characteristics of glassy state in the absence of “long range order” which is a fundamental properties of a crystalline structure. As consequence of the random network theory, glass-forming oxide has been classified as network former, network modifiers and intermediates. Network formers are those that be found in the vitreous state as pure substances (SiO2, GeO2, B2O3...). The network modifiers disrupt the continuity of the network changing the chemical and physical properties and drop the working temperature. This category includes the alkali metal oxide (Na2O, K2O) and alkaline earth oxides (CaO, MgO) Intermediates are oxides which can either enter in the network as a network former and occupy interstitial holes as network modifiers, but are unable to form glasses themselves. Included in this class is alumina, titania and zirconia ( Al2O3, TiO2, ZrO2) that play an important role in stabilizing the network. In the archaeological glasses, the principal network former is silica (SiO2) with the alkalis (soda Na2O and potassia, K2O) and alkaline earth ( lime, CaO and magnesia, MgO) as the network modifiers.
Figure 1 Schematic representation of the vitreous SiO2 modified by Na2O showing bridging and not bridging oxygen sites according to the random network theory
While glass is generally a stable and strong material, when in contact either with liquid and vapour water it is vulnerable to decay. The durability of glass may be defined as its resistance to attack by water, aqueous acid or base solutions, steam or atmospheric agents. The particular case of attack of glass by water combined with atmospheric gases (SO2, CO2, NO2….) is termed weathering. It is possible to distinguish glass decay phenomena into bulk deterioration and surface deterioration: Devitrification is a natural process that occurs on siliceous material, by which glassy substances change their structure into that of crystalline solids. On the contrary, the decay of glassy surfaces due to weathering problems is often the effect of leaching and corrosion processes. According to leaching process, which occurs when glass is exposed to acid solutions, the alkaline ions, coming from the glass, exchange and inter-diffuse with protons ions of water. Through this mechanism a silica gel layer with a low concentration of alkaline ions forms on glass surface. ≡S i-O-Na + H2O → ≡Si-OH + Na + + OH During corrosion, which occurs when glass is exposed to basic solutions, the Si-O- bonds of glass network break owing to the attach of OH- ions to the silicon sites and part of the material is released into solution. Because of this mechanism all glasses show low durability when exposed to alkaline solutions . ≡Si-O-Si≡ + OH - → ≡ Si-OH + ≡ Si-O ≡ Si-O- + H2O → ≡ Si- OH + OH – ≡ Si-O-Si ≡ + H2O → 2 [≡ Si-OH] So, water plays a fundamental role in surface degradation of glassy materials in particular when artefacts are exposed to external conditions. The real challenge is learning to modify glass surfaces in order to avoid environmental interactions, that can compromise their performance and longevity. By controlling chemical contact between water and glass surface it can be supposed that surface alteration phenomena be greatly limited. The main point of this communication is to optimize the deposition of chemisorbed organic films, with a thickness of some angstroms or nanometers. The coating would increase glass- water contact angle, making glass surface hydrophobic, and also
have an excellent chemical resistance, in order to assure a very long durability. Moreover, nanometric thickness of the film assures important features: i. a nanostructured film does not influence bulk properties of the material but modifies only the surface characteristics such as its wetting and reactivity toward atmospheric pollutants; ii. it does not modify the colour of the objects: in fact nano-films show light absorbance absolutely negligible; iii. in the visible field of wavelengths, diffraction phenomena are completely avoided and iridescence, typical of thicker coatings, is not observed. There are different routes for the preparation of organic nanometric films but the most promising method to obtain enduring chemisorbed coatings is the Self Assembled Monolayer (SAM) technique. Self assembled monolayers are ordered molecular assemblies that are formed spontaneously by the adsorption of a surfactant with a specific affinity of its endgroup to a substrate.
Figure 2. On the left, scheme of self assembled monolayer. On the right, computer simulation of a glass with an organo-functional silane grafted to the surface.
In this study we used some organosilane molecular precursors, in particular alkylsilanes (OTS) and fluoroalkylsilanes (FAS) to prepare water-repellent surfaces. These molecules assure good coating and high hydrophobic properties according to literature and registered patents. The SAM protective nano-coatings are able to change the glass wetting properties, without changing appearance of the handicraft to the naked eye.
Figure 3 Wetting properties of clean glass, OTS on glass and FAS on glass
The wetting properties of the FAS and OTS layers were estimated by using contact angle measurements. The topography of the surface was investigated by Atomic Force Microscopy (AFM) while the surface chemical compositions were obtained by X-Ray Photoelectron Spectroscopy (XPS). Chemical resistance of the prepared SAMs was also evaluated by following the variations of surface parameters vs time after weekly immersion cycles of samples in pH controlled solutions, UV radiations and heating stresses.
The most relevant results were: Thermal and UV treatment induced an improvements of hydrophobic properties of the SAMs. Colour measurements acquired before and after every artificially aging protocol did not show any chromatic alteration of the investigated systems. Contact angle measurements have indicated that in neutral-acid environment all the investigated SAMs show very high stability. The base treatments induced more evident degradation especially in the case of FAS coatings. References M. Pollard, Archaeological Chemistry, RCS Paperbacks, the Royal Society of Chemistry 1996, ISBN 0-85404-523-6, pag. 149-195 Marco Verità; Alessandro M. Renier. Metodi analitici applicati allo studio di vetri antichi, CnrProgetto Finalizzato Beni Culturali, Science of Art, Edizione Progetto Padova, Bressanone, Marzo 2001, pag. 259-268. K. Cummings; W. A. Lanford; M. Feldmann. Weathering of glass in moist and polluted air, Nuclear Instruments and Methods in Physics Research, B 136-138 (1998) 858-862. U. Drewello; R. Weibmann; S. Rolleko; E. Muller; S. Wuertz; F. Fekrsanati; C. Troll; R. Drewello. Biogenic surface layers on historical window glass and the effect of excimer laser cleaning, J. Cult. Heritage 1 (2000) S 161-S 171. A.Y. Fadeev; McCarthy. Langmuir, 16 (2000) 7268-7274. H. Brunner; U. Mayer; T. Vallant; H. Hoffman; T. Leitner; R. Resch; G. Fridbacher. Journal of Physical Chemistry, 102 (1998) 7190-7197. A.G. Lambert; D.J. Neivant;R.A. Mc Aloney. Langmuir, 16 (2000) 8377-8328. F.Schreiber. Structure and growth of self-assembling monolayers, Progress in Surface Science, 65 (2000) 151-256. B.S. Hong; J.H. Han; S.T. Kim; Y.J. Cho; M. S. Park; T. Dolukhanyan; C. Sung. Endurable waterrepellent glass for automobiles, Thin Solid Films 351 (1999) 247-278. K. Kamitani; t. Teranishi. Development of Water-Repellent Glass Improved Water-Sliding Property and durability, Journal of sol-gel science and technology 26 (2003) 823-825. U.S. Patent No. 5,415,927. C. Altavilla, E. Ciliberto: “Decay characterization of glassy pigments: an XPS investigation of smalt paint layers” Applied Physics. A. 79, 309 2004 C. Altavilla, E. Ciliberto,C. Buccheri, Nanotecnologie e conservazione: tecniche Self-assebled monolayer nella protezione delle superfici vetrose. p.160,LO STATO DELL’ARTE Atti del II° CONGRESSO NAZIONALE IGIIC LO STATO DELL’ARTE , AltriLIBRI 5 - ISBN 88-8724394-8 Palazzo Reale , Genova.27-29 settembre 2004
