Chemical vapor deposition of tin oxide on float glass in the tin bath

Abstract: A method and apparatus are disclosed for producing a low emissivity metal oxide film on a glass surface within a nonoxidizing atmosphere inside a float bath.

Description: The present invention relates generally to the art of producing infrared-reflecting coated glass products, and more particularly to non-iridescent, high transmittance, low emissivity, infrared-reflecting coated glass products. Transparent infrared-reflecting films such as tin oxide may be deposited on a substrate such as glass by a variety of methods, including the application of thermally decomposable compounds to a heated surface. Useful methods for forming transparent infrared reflecting tin oxide films are taught in US-A-3,107,177 to Saunders et al, US-A-3,677,814 to Gillery, and US-A-4,263,335 to Wagner et al. Tin oxide films are especially effective infrared reflectors at thicknesses of about 100 to 800 nm (1000 to 8000 Angstroms). However, if the thickness is not sufficiently uniform, the films tend to display a multiplicity of interference color effects commonly referred to as iridescence. Such interference effects render the coated glass aesthetically unacceptable for most architectural applications. Iridescence is not observed in thinner films, however, these films have insufficient infrared reflectance to be practically useful. Likewise, iridescence is not observed in thicker films; however, these films tend to be hazy, have relatively low transmittance, and are difficult to make uniformly. Therefore, various methods to mask interference effects have been developed. US-A-3,681,042 to Edwards et al discloses coating a surface of float glass by vaporizing a coating material, entraining the vapor in a stream of hot carrier gas, and directing the gas-borne coating material to the glass surface to be coated, which surface is at a coating-receptive temperature. US-A-3,710,074 to Stewart discloses an electrically heated multiple glazed window unit having an electroconductive coating on an enclosed surface and a selective reflecting film having an absolute infrared reflectance of at least 0.17 to improve the heat insulating character of the unit and reduce the visible iridescence of the conductive film. US-A-3,850,679 to Sopko et al discloses depositing a metal oxide coating on a hot glass surface by applying a mature of carrier air, vaporized solvent and vaporized metal-containing coating reactant to the hot glass surface through a nozzle at a Reynolds number exceeding 2500 with the nozzle-to-glass spacing at least 1.25 times the characteristic dimension of the nozzle. US-A-3,852,098 to Bloss et al discloses coating a glass substrate with a metal-containing film by heating the glass and contacting the glass with a gaseous mixture from 50 to 100 percent saturated with the vapor of a reactive metal compound at its temperature immediately before contacting the glass. The gaseous mixture is then heated by the glass to a sufficient temperature to cause the metal compound to react thereby depositing the film. US-A-4,206,252 to Gordon describes transparent glass windows having a first coating of infrared reflective material displaying iridescence which is markedly reduced by provision of a layer of continuously varying refractive index between the glass and the coating. The invention also encompasses processes for making such windows. US-A-4,294,193 to Gordon describes a vapor coating apparatus for producing the coated glass described above wherein a layer between the glass and the infrared reflective coating has a refractive index which increases continuously from the glass to the coating. The apparatus is described as suitable for use in making coatings of gradually changing compositions from gaseous reactants in general. US-A-4,325,988 to Wagner discloses a method and apparatus for producing a film on a substrate surface from a cloud of dust-sized particles of a coating reactant, preferably using a jet mill. US-A-4,344,986 to Henery discloses a method for depositing a coating from a powder coating reactant wherein turbulence is created in the carrier gas stream. US-A-4,377,613 to Gordon discloses transparent window structures comprising a glass sheet bearing a coating of infrared reflective material wherein the observance of iridescence is reduced by provision of a very thin coating system beneath the infrared reflective coating which reflects and refracts light to interfere with the observation or iridescence. US-A-4,401,695 to Sopko discloses a method and apparatus for depositing a coating from a gaseous stream of powder coating reactant, wherein the carrier gas is supplied at a high volume rate and low pressure. US-A-4,144,362 to Larkin discloses a method of producing a stannic oxide coating on a heated glass article using finely divided liquid monobutyltin trichloride wherein unpyrolyzed reactant is recovered for subsequent reuse. US-A-4,187,366; 4,206,252 and 4,308,316 to Gordon disclose transparent glass window structures comprising a glass sheet bearing a first coating of infrared reflective material, wherein the observance of iridescence resulting from the first coating is reduced by a second coating of particular refractive index and thickness providing at least two interfaces forming means to reflect and refract light to interfere with the observance of iridescence. US-A-3,674,453 discloses the preparation of float glass having an oxidized metal modified surface. The metal oxide is dissolved in the surface stratum. The hot glass is contacted with the metal compound in vapor form under oxidizing conditions. The oxidizing conditions are regulated that no reaction between the oxidizing gas and the metal compound vapor occurs before that metal compound vapor reaches the glass surface. The glass is treated within a float glass chamber supported on a bath of molten metal. From EP-A-186 481 a method for chemical vapor deposition of fluorine-doped tinoxide coatings is known which uses a liquid coating composition which is vaporized into a wet carrier gas. The preferred carrier gas is air. EP-A-305 102 which is a document under Article 54(3) EPC, refers to coating float glass within the chamber containing the glass ribbon supported on a bath of molten metal by chemical vapor deposition. The headspace of the float glass chamber contains a protective atmosphere comprising nitrogen and hydrogen. The built-up of the deposit is reduced by passing a stream of inert gas over the coated surface adjacent the glass. It is the object of the invention to provide a method for depositing a uniform tin oxide film onto the surface of a float glass ribbon, which film having low surface resistance and hence low emissivity. This object is attained by a method for depositing a low emissivity tin oxide film onto a continuous glass substrate conveyed through a coating station by: a. supporting a glass substrate on a molten metal bath; b. maintaining a nonoxidizing atmosphere over the molten metal bath and glass substrate; c. contacting said glass substrate in a coating station on the surface which is not supported on said molten metal bath with a mixture of air as carrier gas and a tin-containing coating reactant in vapor form while maintaining the surface being coated free from a reducing atmosphere; and d. thermally reacting said coating reactant to provide said glass with a tin oxide film and e. removing unreacted coating reactant, undeposited reaction product and reaction by-products and carrier gas from the coating site, The process of the invention can be carried out by using an apparatus for depositing a metal oxide film on the top surface of a float glass ribbon, the bottom surface of which is supported by a bath of molten metal in a nonoxidizing atmosphere comprising: a. means for vaporizing a coating reactant; b. means for delivering said vaporized coating reactant to said top surface, said means comprising a nozzle having a slot-shaped orifice substantially as long as the parallel dimension of the substrate surface to be coated; c. means for maintaining said means for delivering at a uniform temperature below the decomposition temperature of said coating reactant; and d. exhaust means adjacent said nozzle and extending substantially completely along the length of said nozzle, both upstream and downstream, capable of removing substantially all unreacted coating reactant, undeposited reaction product, and reaction by-products and carrier gas. By coating the glass on the bath, a higher glass surface temperature provides a tin oxide coating composition with a lower resistance and therefore lower emissivity at given coating thickness. BRIEF DESCRIPTION OF THE DRAWING Figure 1 illustrates the position of a coater in the float glass bath in accordance with the present invention. Figure 2 is an enlarged view of the coater of Figure 1. Figure 3 is a flow schematic of the carrier gas, coating reactant, vaporizer and heat exchange system which feeds the coater of Figure 2. Figure 4 is a chromaticity diagram with the x and y chromaticity coordinates measured on the corresponding x and y axes. The wavelengths of observed colors are marked about the periphery. Point C marks the coordinates for illuminant C in accordance with the Commission Internationale de L'Eclairage (CIE). The spiral shaped curve is a plot of the chromaticity coordinates of tin oxide films at increasing film thicknesses. Points A and B mark the thicknesses corresponding with the preferred coating thickness range of the present invention. The present invention provides a method for depositing a relatively thick, non-iridescent, infrared-reflective tin oxide film onto a float glass surface while the glass is still supported in the tin bath in a nonoxidizing atmosphere. By coating the glass on the bath, a higher glass surface temperature provides a tin oxide coating with lower resistance and therefore lower emissivity at a given coating thickness. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1, a glass substrate, preferably clear soda-lime-silica-glass, in the form of a continuous float glass ribbon is conveyed in a horizontal position through a coating station while the glass is supported on molten metal, preferably tin, in a float bath in a nonoxidizing atmosphere, preferably nitrogen. The coating apparatus illustrated in Figure 2 is located above the glass ribbon at a point where the glass surface temperature is preferably in the range of 621 to 677°C (1150 to 1250°F), most preferably 649 to 677°C (1200 to 1250°F). The coating apparatus directs a gaseous stream using air as carrier gas, and a coating reactant, preferably butyltin trichloride, into contact with the hot glass surface, whereupon the coating reactant thermally decomposes to form a tin oxide film. The coating apparatus comprises a narrow chamber with a coating reactant inlet end, and an outlet end substantially as long as the width of the glass area to be coated. The chamber is supplied with a mixture of carrier gas and coating reactant vapor. The coating reactant is vaporized before it enters the chamber in order to save the space that would be required to place vaporizing means inside the chamber. The chamber is preferably tapered from a cylindrical-shaped inlet end to a narrow slot-shaped outlet end or nozzle which directs the vaporized coating reactant gaseous mixture to the glass surface to be coated. Suitable nozzles are described in detail in US-A-3,850,679 to Sopko et al; 3,888,649 and 3,942,469 to Simhan. In a most preferred embodiment, a distributor is placed between the chamber and the nozzle to promote uniform distribution of the coating reactant vapor along the length of the nozzle. A preferred distributor is a structural element, positioned over the outlet end of the chamber, having a plurality of evenly spaced apertures through which the vapor passes into the nozzle. Preferably, the individual jets of coating reactant vapor and carrier gas are diffused before the mixture exits from the nozzle. Diffusion may be accomplished by means of diffuser elements in the inlet end of the nozzle, similar in configuration to the baffles shown in the powder coater of U.S.-A-4,344,986 to Henery. Preferred coating reactants for chemical vapor deposition of a low emissivity coating in the float bath in accordance with the present invention are organometallic compounds, preferably organotin compounds. Many organometallic compounds which exist in solid form at ambient temperature may be employed in solution for vaporization and chemical vapor deposition. A variety of aliphatic and olefinic hydrocarbons and halocarbons are suitable as solvents in carrying out the methods disclosed herein. Single component solvent systems, particularly a solvent system employing methylene chloride, are effectively employed in the present invention. Solvent systems employing two or more solvents are also found to be particularly useful. Some representative solvents which may be employed in carrying out the present invention are: methylene bromide; carbon tetrachloride; carbon tetrabromide; chloroform; bromoform; 1,1,1-trichloroethane; perchlorethylene; 1,1,1-trichloroethane; dichloroiodomethane; 1,1,2-tribromoethane; trichloroethylene; tribromoethylene; trichloromonofluoroethane; hexachloroethane; 1,1,1,2-tetrachloro-2-fluoroethane; 1,1,2-trichloro-1,2-difluoroethane; tetrafluorobromethane; hexachlorobutadiene; tetrachloroethane; etc. and mixtures thereof. Other solvents may also be employed, particularly as mixtures of one or more organic polar solvents, such as an alcohol containing 1 to 4 carbon atoms and one hydroxyl group and one or more aromatic non-polar compounds, such as benzene, toluene or xylene. The volatility of these materials makes their use less preferred than the use of the above preferred halogenated hydrocarbons and halocarbons, but they have particular economic utility. A solution of a reactive organometallic salt in an organic solvent may be directed to a vaporizing chamber. The vaporizing chamber is constructed so as to provide a heating element which heats the space surrounding the element to a temperature sufficient to vaporize the coating solution within that space, rather than vaporizing the liquid only in contact with the heating element itself. A carrier gas is directed across and away from the heater to intercept the coating composition to mix with it, enhancing its rate of vaporization, and to carry the vapors through the heater to the substrate to be coated. Vapors of the solvent and organometallic coating reactant are directed from the vaporizer to the coater shown in the drawing. Some preferred organometallic compounds in accordance with the present invention are liquid at ambient temperature, and may be employed without the use of solvents. A particularly preferred organometallic compound is monobutyltin trichloride, a colorless liquid, characterized by an atmospheric boiling point of (420°F) 221°C, a partial pressure of 0.1 bar (0.1 atmosphere) of 154.4°C (310°F), heat of vaporization of 60.69 kJ (14.5 kilocalories) and entropy of vaporization of 123. J/°K (29.4 Clausius) per mole. Monobutyltin trichloride is preferably vaporized by contact with hot air, preferably maintained at a temperature below about 204°C (400°F) to avoid decomposition, typically about 196°C (385°F). Suitable vaporizers are described in detail in US-A-3,970,037 to Sopko and 4,297,971 to Henery. In a preferred embodiment of the present invention, a fraction of the total volume of heated carrier gas is mixed with the monobutyltin trichloride in a vaporizer comprising a coil of tubing immersed in hot oil. The heavily saturated mixture of coating reactant vapor in carrier gas is then diluted with additional heated carrier gas in the chamber en route to the nozzle which delivers the coating reactant to the glass surface. Preferably, monobutyltin trichloride is doped with a fluorine-containing compound to enhance the conductivity of the tin oxide film formed therefrom. A preferred dopant is trifluoroacetic acid, preferably in the range of 1 to 10 percent, most preferably about 5 percent by weight. In order to minimize the possibility of contamination of the deposited film by unreacted or undeposited coating reactant or reaction by-products, the coating apparatus of the present invention comprises integral exhaust means. Adjacent the nozzle along substantially its entire length is an aperture maintained at a negative pressure to provide exhaust means for removing unreacted coating reactant, undeposited reaction product and reaction by-products from the coating site so that neither the freshly coated surface nor the approaching surface to be coated becomes contaminated. Since the chemical vapor deposition method of the present invention does not depend on diffusion of the coating reactant vapor through the normal boundary layer, it is not limited to coating reactants with high entropies of vaporization as disclosed in US-A-3,852,098 to Bloss et al. Tin oxide infrared reflecting films formed in accordance with the process of the present invention have a resistivity less than about 40 ohms per square, preferably 25 ohms per square or less, and low emissivity, preferably less than 0.2. The thickness of the film is chosen to correspond with a minimum in the luminous reflectance curve which plots luminous reflectance as a function of film thickness. The preferred thickness of a tin oxide film deposited on float glass in the bath in accordance with the present invention is in the range of 250 to 350 nm (2500 to 3500 Angstroms), most preferably about 320 nm (3200 Angstroms). A tin oxide film of this thickness exhibits a third order blue interference color as shown in Figure 4, and an emissivity as low as 0.15 when produced in accordance with the present invention. The advantages of depositing a tin oxide coating in the float bath are not only the decreased resistance and lower emissivity exhibited by the film, but also the improved coating uniformity which results from temperature uniformity of the substrate provided by contact of the glass with the molten tin in the bath, and reduced reflected distortion which results from higher substrate coating temperatures available without additionally heating the glass. Referring to Figure 3, coating reactant in recirculating pump system 10 is preheated to about 177°C (350°F). Carrier air is preheated in an air heater 20 also to about 177°C (350°F). The carrier air is supplied at 566.5 standard dm 3 (20 cubic feet) per minute per (30 inches) 76.2 centimeters of slot length at a (3/16th inch) 4.8 millimeter slot width. The carrier air velocity is preferably about (950-1150 feet) about 290 to 351 meters per minute. The carrier air picks up coating reactant from supply 30 until the air is essentially saturated. The carrier air/coating reactant vapor mixture travels to the coating vaporizer 40 which is also maintained at a temperature of about about 177°C (350°F) where the coating reactant becomes completely vaporized. The vaporized coating reactant and carrier air mixture travels through heated transport lines 50 to the coating vapor distributor cavities 5 shown in Figure 2. The vaporized coating reactant is distributed through orifices 6 into the vapor cavity 7 from which it is directed through the nozzle 8 to the glass surface as shown in Figure 1. After the coating reactant thermally reacts with the hot glass surface to form a tin oxide film thereon, the carrier air, unreacted coating reactant vapor and any decomposition by-products are immediately exhausted in a controlled manner through exhaust orifices 9 of the vacuum platens 10. The transport lines, distributor cavities, vapor cavity, exhaust orifices and vacuum platens are all maintained at a constant temperature of about (350°F) about 177°C by means of a circulating oil heat transfer system 11. Negative pressure sufficient to exhaust the carrier air, unreacted coating reactant and reaction by-products is supplied through exhaust means. The upstream exhaust orifice is preferably about 22.2 mm (7/8 inch) wide and the downstream orifice about 25,4 mm (1 inch). Both run the full length of the coater nozzle. At the coating reactant and carrier gas flow rates of the following examples, the pressure drops measured in Pa (inches of water) are preferably about 1071 (4.3) for the upstream and about 922 Pa (3.7) for the downstream exhaust orifices. The distance between the coater nozzle and the glass is preferably in the range of about 9.5 to 19.1 millimeters (0.375 to 0.75 an inch), most preferably about 1.3 centimeters (around 0.5 inch) as in the following examples. The bath atmosphere is preferably pure nitrogen at a slight positive pressure, preferably in the range of 12,5 to 17,5 Pa (1.27 to 1.78 mm H 2 O), most preferably about 14,7 Pa (1.5 mm H 2 O). The carrier air/vaporized coating reactant mixture as in the following examples preferably comprises about 18 cm 3 of reactant in 708 standard dm 3 (25 cubic feet) per minute of carrier gas per foot of nozzle length, and is preferably supplied at a pressure of about 0,276 bar (4 pounds per square inch) in laminar flow. The glass line speeds may vary over a wide range, for 2.5 millimeter glass, e.g. a range of about 0.76 to 7.9 meters (30 to 310 inches) per minute. The present invention will be further understood from the descriptions of specific examples which follow: Comparative EXAMPLE I To illustrate the effect of hydrogen in the bath atmosphere on the resistivity of a tin oxide film produced in accordance with the present invention, films of approximately the same thickness, about 260 nm (2600 Angstroms), were produced in various bath atmospheres. A 3.3 millimeter thick soda-lime-silica float glass ribbon is coated while supported on molten tin in the float bath in the normal bath atmosphere of nitrogen containing about 7% hydrogen. The upper surface of the glass ribbon is contacted at a temperature of about 664°C (1227°F) with carrier air essentially saturated with vaporized monobutyltin trichloride maintained at a temperature of about 177°C (350°F) by means of a circulating oil heat exchange system. A tin oxide film deposited at a thickness of about 260 nm (2600 Angstroms) has a resistivity of 900 ohms per square. Comparative EXAMPLE II A float glass ribbon is coated as in the previous example except that the amount of hydrogen in the float bath atmosphere is reduced to 2%. At a tin oxide coating thickness of 260 nm (2600 Angstroms), the resistivity of the tin oxide film is 125 ohms per square. EXAMPLE I A float glass ribbon is coated as in the previous examples with a tin oxide film except that the float bath atmosphere is pure nitrogen. A tin oxide film deposited at a thickness of 260 nm (2600 Angstroms) has a resistivity of 25 ohms per square. At a resistivity of 25 ohms per square, the emissivity of the tin oxide film is 0.20. EXAMPLE II A 3.3 millimeter thick soda-lime-silica float glass ribbon is coated while supported on molten tin in a float bath in an atmosphere of pure nitrogen. The upper surface of the ribbon is contacted at a temperature of about 664°C (1227°F) with air essentially saturated with vaporized monobutyltin trichloride which is maintained at a temperature of about 177°C (350°F) by means of a circulating oil heat exchange system. Exhaust means adjacent to the coater nozzle remove air, unreacted monobutyltin trichloride and any by-products of thermal decomposition from the coating station without contaminating the surrounding nitrogen atmosphere. A tin oxide film is deposited at a thickness of 320 nm (3200 Angstroms). The film has a surface resistivity of about 20 ohms per square. The coating has a third order blue-green color and 16 percent luminous reflectance. The luminous transmittance of the coated glass is 72 percent and the emissivity is 0.17. The above examples are offered to illustrate the present invention, and show the importance of maintaining the surface being coated free from a reducing atmosphere. With the apparatus used in the above examples, it is preferred to eliminate hydrogen from the bath atmosphere. However, various coating reactants, coater designs, process parameters and so on are within the scope of the present invention. While a hydrogen-free bath atmosphere may be preferred, the coating apparatus may also be modified to exclude hydrogen in the bath atmosphere from only the glass surface at which the reaction of the coating reactant is taking place, for example by means of a nitrogen purge about the perimeter of the coater.

Claims: A method for depositing a low emissivity tin oxide film onto a continuous glass substrate conveyed through a coating station by: a. supporting a glass substrate on a molten metal bath; b. maintaining a nonoxidizing atmosphere over the molten metal bath and glass substrate; c. contacting said glass substrate in a coating station on the surface which is not supported on said molten metal bath with a mixture of air as carrier gas and a tin-containing coating reactant in vapor form while maintaining the surface being coated free from a reducing atmosphere; and d. thermally reacting said coating reactant to provide said glass with a tin oxide film and e. removing unreacted coating reactant, undeposited reaction product and reaction by-products and carrier gas from the coating site. A method according to claim 1, wherein said molten metal bath comprises tin. A method according to claim 2, wherein said molten metal bath consists essentially of tin. A method according to claim 2, wherein said atmosphere over the molten metal bath consists essentially of nitrogen. A method according to claim 1, wherein an organotin compound is used as reactant. A method according to claim 5, wherein said organotin compound is monobutyltin trichloride. A method according to claim 1, wherein the temperature of the glass surface is 621° to 677°C (1150° to 1250°F). A method according to claim 7, wherein the temperature of the glass surface is 635° to 663°C (1175° to 1225°F).

Inventors: HENERY VERN ALLAN, HENERY, VERN ALLAN
Applicants: PPG INDUSTRIES INC, PPG INDUSTRIES, INC.
Application year: 1988
Main classification: C03C17
IPC codes: C23C16, C03C17