GB2102453A

GB2102453A - Protecting silver and copper films

Info

Publication number: GB2102453A
Application number: GB08121785A
Authority: GB
Inventors: Albert Servais; Jean Desart
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1981-07-15
Filing date: 1981-07-15
Publication date: 1983-02-02
Also published as: BE893807A; FR2509744A1

Abstract

When producing an article comprising a substrate bearing a thin film of copper or silver treated by a substituted azole, in order to improve the protection afforded, the metallic film is wetted and is then contacted by an aqueous solution of substituted azole e.g. by spraying while the metallic film is still wet. The use of 5-aminotetrazole is preferred.

Description

SPECIFICATION Protecting silver and copper films The present invention relates to a method of producing an article comprising a substrate bearing a thin film of copper or silver treated by a substituted azole.

There are several articles in widespread use which comprise a thin metallic film deposited on a substrate. As examples may be cited mirrors which have a rear reflective coating where the substrate is transparent or a front reflective coating whether the substrate is transparent or not. In some such mirrors the reflective coating is formed by an opaque copper film, while in other mirrors a rear reflective coating of silver is covered by a copper film. The use of thin transparent metallic films on glazing material for selective reflection especially of infra-red radiation is also common. Where such a metallic film is used for example for one of these purposes it requires to be of a uniformly high optical quality.

Unfortunately, when copper or silver is exposed to the atmosphere its surface becomes degraded by oxidation or attack by atmospheric pollutants so that this high optical quality is lost.

For this reason many attempts have been made to protect such a metallic film, and in particular, British Patent Specification No. 1 074 076 Pittsburgh Plate Glass Company) proposes a protective treatment using an amino-triazole, an aminotetrazole, an aminoindazole or indazole.

This patent teaches various methods of applying the azole, in particular by applying azole powder or spraying an azole solution usually an alcoholic solution onto a washed and dried film-bearing substrate. The present invention is based on the discovery that by applying the azole in a particular way, better protection is given to the metallic film.

According to the present invention therefore, there is provided a method of producing an article comprising a substrate bearing a thin film of copper or silver treated by a substituted azole characterised in that such metallic film is wetted with water and then contacted by an aqueous solution of substituted azole while the metallic film is still wet.

It has been found that applying the substituted azole in aqueous solution to the metallic film while the film is wetted with water gives a better protection. It will of course be appreciated that the water used for such wetting need not be pure water. Such water may contain minor proportions of other substances. For example in some preferred embodiments of the invention, the water contains a surfactant to promote wetting of the metallic film. Of course any such substance added to the water used for wetting must be compatible with the substituted azole used.

A possible explanation of the beneficial effect afforded by the present invention is that if an aqueous solution is substituted azole is supplied to a water-wet metallic film, the applied solution will immediately diffuse the film of water on the metal so that a highly uniform coverage can be achieved. This cannot occur in the known prior art processes: when spraying a solution of substituted azole onto a dry metallic film, the substituted azole is in some measure deposited spot-wise on the metallic film. Similar problems arise when a substituted azole powder is applied to a dry metallised surface.

Heating the azole solution permits solutions of higher concentrations to be used. Preferably, said solution immediately prior to its contact with the metallic film, is at a higher temperature than that film.

The metallic film is preferably contacted by said solution while the film is at a temperature in the range 300 to 600 inclusive. This is particularly advantageous in allowing solutions containing greater concentrations of azole to be used. The solubility of azoles is temperature dependent so it is advantageous that the film of water on the metallic film should be warm in order to reduce the risk of local depositions of azole as a result of supersaturation of the azole solution on the metallised surface of the article. Even a temperature of 300 is sufficient to allow greater concentrations of azole to be used. Of course the film should not be so hot that it dries prior to the application of the azole solution, so there is a preferred maximum temperature of 600C.

Preferably, said metallic film is wetted with heated water to bring its temperature within said range. In this way, the heating and wetting steps can be performed simultaneously thus avoiding the risks of drying a pre-applied film of water in a subsequent heating step and of cooling a prewarmed metallic film by applying cold water.

The solution may be applied to the metallic film in any convenient manner, for example by spraying or dipping. Spraying is found most convenient.

The substituted azole to be used in the performance of the invention is preferably selected from the group consisting of: aminosubstituted tetrazoles and triazoles, indazoles and amino-substituted indazoles. Very good results have been found when the substituted azole is selected from: 5-aminotetrazoles; 3-amino-1,2,4triazole; 7-aminoindazole; indazole, as is preferred. Of these, the first two named, especially 5-aminotetrazole, give excellent protection to the metallic film.

The substrate is preferably made of glass, and the invention is particularly useful in embodiments in which said substrate is a clear glass sheet and said metallic film comprises an opaque reflective copper film to form a mirror.

The method of the invention is not only beneficial for protecting the main surface of a metallic film, but also for protecting any exposed edge of the film. Thus the invention can with advantage be applied during the formation of a new edge of the substrate. In particular, when the metallised substrate is a mirror, the edges of the metallic film can be protected by applying the invention during cutting, grinding or polishing of the edges of the mirror.

Preferred embodiments of the invention will now be described by way of Example and with reference to the accompanying schematic drawing which represents part of an installation for performing a process according to the invention.

Example 1 Polishing A glass sheet is polished in the conventional way, for example using an abrasive based on iron oxide and is then washed or rinsed to remove all traces of the abrasive.

Metallisation The polished surface of the glass sheet is pretreated in known manner with an acidic solution of tin chloride (SnCl2). The sheet is rinsed with demineralised water before and after this pretreatment. The later rinsihg is to remove any remaining tin salts.

The pretreated surface is then subjected to a second activation by applying a dilute solution of silver nitrate followed by a further rinsing with demineralised water.

The tin- and silver-activated surface is then subjected to a metallising step in which a thin film of copper is deposited thereon. For this purpose use may be made of a known copper-containing solution such as is conventionally used in known metallising techniques. This coppering step is preceded and followed by rinses with demineralised water. The preceding rinse, which may be the same rinse as that following the silver-activation step, leaves the substrate wet so as to reduce or avoid the appearance of defects in the copper coating such as blooms and other marks. The following rinse is intended to inhibit the formation of undesired precipitates on the newly formed metallic film.

Protection In order to protect the metallic film, it was, in accordance with the invention, wetted by spraying with water and then contacted with an aqueous solution of a substituted azole while it was still wet. In this example, the azole solution was an aqueous solution containing between 10 and 40 g 5-aminotetrazole (monohydrate) per litre of water which was sprayed onto the copper film. The water sprayed on to wet the metallic film was at 800C and heated the metallic film so that its surface temperature was 400C when it was contacted by the azole solution. Immediately prior to contact with the wet metallic film, the azole solution was at a temperature of 600C.

The solution was allowed to react on the copper film for two to three minutes in free air whereafter it was rinsed off, care being taken to avoid supersaturate precipitation of the azole, to leave the copper film protected.

In one variant of this Example, the wetness of the copper film immediately prior to application of the azole was due to the rinsing which took place after formation of that copper film. Of course this rinsing step can be performed using hot water.

An installation for performing a particularly convenient series production process is illustrated in the accompanying drawing. Sheets of glass 1 are polished and washed and then placed horizontally on a conveyor 2 by which they are conveyed beneath a succession of spraying stations. The first of these (not shown) sprays the sheets 1 with demineralised water. At the next spraying station 3, they are sprayed with an acidic tin chloride solution whereafter they are again sprayed with demineralised water for spraying station 4. The next spraying station 5 is arranged to spray the sheets 1 with a dilute silver nitrate solution. Remaining silver salts are washed off by spraying with demineralised water at the next spraying station 6 whereafter the sheets 1 are sprayed with a copper-containing solution at spraying station 7 to produce a copper film of the required thickness.At spraying station 8, the sheets are again sprayed with demineralised water, but here the water is heated so that the upper metallised surfaces of the sheets 1 bear a film of water and are at a temperature of about 400C when they pass beneath the next spraying station 9 where they are sprayed with an aqueous solution of the desired substituted azole. A final wash is given the sheets by spraying with water at spraying station 10 whereafter the sheets are allowed to dry.

A suitable conveying speed is 1.5 m/minute, and the concentrations of the various solutions sprayed, the amounts sprayed and the spacing between each coating station and its succeeding rinsing station can be adjusted to permit build-up of the desired coating thicknesses. The 5 a minotetrazole solution specifically referred to above may be sprayed for example at a rate of 0.7L/min. It is suitable to spray the sheets using bat-wing jets. Glass sheets up to 1.8 metres or more in breadth can readily be treated in this way.

If the protected metallic film on the sheet is opaque, it may be given some degree of protection against scratches by a coating of paint such as is conventionally used for protecting mirror surfaces.

It will be appreciated that the protection afforded to a metallised surface in accordance with the invention will be most effective if the substituted azole solution is applied to a freshly formed film of metal, but that a degree of protection can also be afforded against tarnishing of an older metal film.

Claims

1. A method of producing an article comprising a substrate bearing a thin film of copper or silver treated by a substituted azole characterised in that such metallic film is wetted with water and then contacted by an aqueous solution of substituted azole while the metallic film is still wet.

2. A method according to claim 1, wherein said solution, immediately prior to its contact with the metallic film, is at a higher temperature than that film.

3. A method according to claim 1 or 2, wherein the metallic film is contacted by said solution while the film is at a temperature in the range 300C to 600C inclusive.

4. A method according to claim 3, wherein said metallic film is wetted with heated water to bring its temperature within said range.

5. A method according to any preceding claim, wherein the solution is sprayed onto the metallic film.

6. A method according to any preceding claim, wherein the substituted azole is selected from: 5aminotetrazole; 2-amino- 1 ,2,4-triazole; 7aminoindazole indazole.

7. A method according to any preceding claim, wherein said substrate is of glass.

8. A method according to claim 7, wherein said substrate is a clear glass sheet and said metallic film comprises an opaque reflective copper film to form a mirror.

9. A method according to claim 1 and substantially as hereinbefore described.

10. An article produced by a method according to any preceding claim.