Tin Oxide Coating

In our prior application, Ser. No. 431,705, now U.S. Pat. No. 3,880,633 was disclosed a method of coating a glass sheet with a thin film of SnO.sub.x, in a continuous process, the glass moving continuously in a bath of molten material, and a spray containing a tin compound being applied in successive transverse passes of the glass. The glass moves in a bath of molten material which may be tin, but may also be a salt, capable of sustaining the glass or partially sustaining the glass. In the latter case further support is supplied for the glass, so that its upper surface remains exposed, while at least its under surface is immersed. The contents of Ser. No. 431,705 are incorporated herein by reference.

It is known to coat glass with SnO.sub.x, and it is also known that if glass is coated with SnO.sub.x, the SnO.sub.x being transparent and electrically conducting, the coating will serve to reflect infra-red energy while passing visible light, so that heating energy applied internally of a heated enclosure, employing one or more panes of such coated glass, is conserved internally of the enclosure since radiation of infra-red energy outwardly of the structure through the glass is reduced. It is an object of the present invention to improve such coatings in such manner as to increase infra-red reflectivity of the SnO.sub.x coating, while maintaining good transmission in the visible spectrum. The increase is a function of the reduced resistivity of the film, which is about 8-17 ohms per square, transmissibility of visible light being about 80%. Conventional coatings have resistivities of about 50-90 ohms per square and transmissibilities of about 70-78%. As .OMEGA./sq. is reduced transmissibility of visible light is not radically reduced but a large increase of infra-red reflectivity occurs.

Glass coated with SnO.sub.x having a resistivity of about 8-17 ohms per square and having reflectivity to infra-red energy of about 82%, coupled with substantially 80% transmissivity of visible rays, the coating being formed by an intermittent spraying process while supplying heat from a liquid bath involving spraying a non-aqueous solution of phenyl hydrazene hydrochloride, ammonium bifluoride, tin chloride (anhydrous), carried out at a substantially constant temperature.

A molten bath is preheated to 600.degree.F. A glass plate is heated to 500.degree.F and moves along the bath. The bath is heated so that the exposed surface of the glass is at about 950.degree.F. Spraying is accomplished by means of an air atomizing nozzle of an air spray, at 40 PSI for about 1 1/2 minutes, at 50 cc per minute. The following solution is employed: 250. ml methanol 0.7 gm phenyl hydrazene hydrochloride 3.0 gm ammonium bifluoride (NH.sub.4 FHF) 70. gm SnCl.sub.2 (anhydrous)

The solution should be stirred for about five minutes before spraying, to clear the solution.

The solvent must not contain water, but methanol is employed as a solvent only because it is the most economical solvent available. Other non-aqueous solvents may be employed. The phenyl hydrazene hydrochloride may be omitted, but its presence has been found to reduce haze in the film. The proportions specified are selected on an economic and not a chemical basis, i.e., the smallest amount is employed which is satisfactory in producing a clear film. NH.sub.4 FHF is a dopant, which can decrease film resistivity by a large factor, as much as 300%. The quantity of NH.sub.4 FHF employed is selected on an economic basis, i.e., as little is used as is consistent with desired resistivity values.

The coating formed is about .3-.6 microns thick. The coated glass has reflectivity for infra-red of about 82%. The best prior art SnO.sub.x coated glass with high transmission of the visible spectrum has an infra-red reflectivity of about 50%.

It is crucial, in the process of the invention, in order to achieve the desired characteristics of low resistivity, high reflectivity to infra-red radiation, and high transmissivity of the visible spectrum in a thin film coating, to supply heat to the glass continuously and uniformly and at uniform temperature. This cannot be accomplished by placing the glass on a heated surface, nor in an oven, but only by means of a heated liquid bath in which the glass is supported and which constitutes a large capacity heat reservoir to which the bottom surface of the glass is exposed. It is required that spraying be accomplished intermittently, and very slowly, so that the temperature of the exposed surface of the glass can recover between sprays, and so that the glass is not cooled appreciably by the sprays. It is important that the sprayed mixture be anhydrous, and that environmental water be avoided, and that all supply lines, spray heads and the like be carefully flushed with methanol before spraying commences. Presence of water increases resistivity and produces spots in the film.

After the coating process is completed the glass sheet is slowly cooled, to anneal the glass, according to conventional techniques.

The present system lends itself to coating a sheet of glass with SnO.sub.x, as the glass is produced by a continuous process glass making machine, as in our Ser. No. 431,705 now U.S. Pat. No. 3,880,633 which is incorporated herein by reference. Glass is produced, in such a continuous process, in long sheets about ten feet wide. Since the glass is hot when produced, its heat is employed to maintain hot the float tanks in which the coating process is carried out. The process of coating may, accordingly, be carried out as part of a continuous glass making process. The process may, however, also be carried out on pre-fabricated glass. In the latter case, the glass may be pre-heated to 950.degree.F in a conveyor furnace, and transferred to a molten liquid tank, the liquid being heated to a temperature which maintains the exposed surface of the glass at about 950.degree.F. In the first method the waste heat of the glass making process is employed to heat the glass, which represents an economic advantage.

Glass coated in accordance with the present method has particular advantage as window glass, for vehicles, dwellings, large buildings, hot houses, and the like. It also has application to ovens having viewing windows, and to solar heating systems, where it is desirable to avoid heat loss via infra-red radiation.

While the preferred fluoride is NH.sub.4 FHF, it is feasible to employ SnF.sub.2, i.e., a fluoride containing only F and Sn. The function of the NH.sub.4 FHF is to provide fluorine plus a volatile remainder.

The material in the float tanks may be liquid tin or a molten salt capable of being maintained at the required temperature.