The effect of the solvent on the cross-link density of SiO2 coatings
Introduction
Wet-chemical coating techniques are used to produce thin layers on glass [1], e.g. computer monitor tubes and television tubes. Anti-reflection and chrominance coatings improve the optical performance of the tubes [2], [3]. Antistatic coatings are especially useful for monitor tubes but also interesting for television applications [3], [4]. As the coatings are applied to the outside of the tubes, high demands on scratch resistance and chemical inertness are imposed. These demands can be fulfilled by using the sol-gel chemistry of tetraethylorthosilicate (TEOS). Coating liquids from TEOS, acidified water, and ethanol can be prepared and spincoated on the tubes followed by a heat treatment. This heat treatment is limited to 160°C because the coatings are applied onto the tube after assembly of screen and cone, which limits the maximum temperature.
With increasing screen sizes, spin-coating a uniform layer becomes an increasingly difficult task. By changing the coating composition uniform coatings can still be made, but the scratch resistance often decreases. The scratch resistance of the silica sol-gel coating is related to the cross-link density. It was previously observed that the specific surface area of autoclave dried gels reduces markedly if water vapour is present during drying, which correlates with a higher cross-link density [5]. We therefore decided to study the effect of water content in the coating liquid on the cross-link density. The effects of NH3 vapour and HF on the condensation enhancement are also known for other sol-gel systems [6] and their effect on coating liquids used for tube coatings is reported. Finally, results on the effect of the type of alcohol, which also influences the number of Si–OH groups at different stages of coating formation, are given and discussed.
Measuring the cross-link density in a coating which is about 100 nm-thick is a difficult task. It is not possible to remove an already cured coating from a substrate and if it were possible a large area would be needed for analysis. Therefore we had to perform our measurements on powders which were made under the same circumstances as used for coating preparation. Peeters et al. [7] have tested this technique for thicker coatings which can more easily be collected as a powder and found strong agreement between the cross-link density of the bulk material and the coating. The technique used to quantify the cross-link density is 29Si solid state magic angle spinning NMR, which allows exploration of the chemical environment of the 29Si atom.
Section snippets
Sample preparation
The hydrolysis mixtures consisted of TEOS, ethanol and water with a weight ratio of 2:1:1. The water fraction is acidified with HCl to a concentration of 0.175 mol/l. After a hydrolysis time of 30 min the samples were diluted with the appropriate solvent in a 1:6 weight ratio (hydrolysis mixture versus solvent). Within 2 h the coating liquids were used for the various experiments. Apart from the effect of the solvent, the effect of F− on the cross-link density was investigated, hydrogen
Results
In Fig. 1NMR spectra are given for a powder prepared from a hydrolysis mixture diluted with ethanol and a powder prepared from the same hydrolysis mixture diluted with n-propanol. The assignment of the observed resonances is indicated [10]. The powder obtained from the ethanol containing coating liquid is more condensed, a value for AFQ of 3.29 is found compared with a value of 3.04 for the propanol containing system. This is also directly visible from the spectra. For the ethanol system only a
Discussion
For hydrolysis mixtures diluted with ethanol, the last liquid to evaporate before a dry film is obtained, is water. This water rich system causes Q species with only hydroxy groups left on the remaining sites, which easily condense. If n-propanol is used as solvent, no water is left during gelation because an azeotropic mixture containing 30% water evaporates from the system. The last liquid to evaporate from the system is n-propanol. This n-propanol rich system causes re-esterification leading
Conclusions
The degree of cross-linking of TEOS based sol-gel coatings is influenced by the ratio of hydroxy and alkoxy groups on the Si atoms. More hydroxy groups lead to a higher degree of cross-linking. This can be achieved by creating coating liquids which have water and not an alcohol as the final component to evaporate. By using a bulky alcohol re-esterification can be totally prevented, which is also a route towards hydroxy-rich systems leading to high degrees of cross-linking. Well known
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