PRODUCTION OF INFRA RED REFLECTING COATINGS ON GLASS

Abstract: A process for applying an infra red reflecting coating on hot glass by directing onto a glass surface above 660°C a tin compound in the form of a finely divided solid which decom­ poses under the conditions of application to deposit a fluorine-­ containing tin oxide on the glass. The use of a high glass tem­ perature above 660°C gives a product with increased infra red reflectivity and light transmission. The glass may be in the form of a ribbon e.g. of rolled glass, which is hot from the forming process.

Description: PRODUCTION OF INFRA RED REFLECTING COATINGS ON GLASS The invention relates to a process for the productLon of infra red reflecting fluorinecontaining tin oxide coatings on glass. It is known that fluorine-containing tin oxide coatings of appropriate thickness, for example 100 nm to 1000 nm, on flat glass reflect infra red radiation, i.e. hava a low emissivity, whilst transmitting light in the visible part of the spectrum, and that this property makes them useful in energy efficient architectural glazing. It is also known that such coatings may be produced by projecting a tin compound in fine powder form in tne presence of a source of fluorine onto the surface of flat glass at a temperature in the range 400 to 6500C. The coatings produced in this way are generally colourless in appearance although, in common with other thin films, they give rise to inteference colours in reflection. The interference colours depend on the thickness of tile coating, so that variations in thickness across a coating lead to an observable variation in the inteference colour seen. The visual impact of the interferance colour, and the variations in such colour resulting from variations in thickness of the coating, are generally less the greater the mean thickness of the coating. However, it will be appreciated that, since the coatings absorb visible light, the light transmission of the coated glass is reduced as the thickness of the coating is increased. It has now been found that the light transmission of fluorine-containing tin oxide coatings of given thickness produced as described above may be increased, and the infra red reflectivity of the coatings increased, by operating at higher glass temperatures. According to the present invention, there is provided a process for the production of an infra red reflecting coating on glass by directing on to a hot glass surface a tin compound in the form of a finely divided solid which decomposes under the conditions of application to deposit a fluorine-containing tin oxide coating on the glass wherein the tin compound is directed onto the hot glass surface at a glass temperature above 660 C. It has been found that, in general, the higher the glass temperature above 660"C the higher the light transmission of a coating of given thickness. Preferably the coating is applied to a continuous ribbon of glass, for example, rolled glass which is hot from the forming process. If the glass is stretched after the coating process, the coating is liable to crack. Although such cracks may not seriously affect the heat reflecting performance of the coated glass, they are not aesthetically pleasing and are preferably avoided. It is therefore preferred to apply the coating to the ribbon of glass after any stretching of the ribbon is complete. Thus the coating is preferably applied to the glass at a temperature below 750"C; it is especially preferred to coat the glass at a temperature in the range 6800 to 720"C. The tin compound used should be a compound that decomposes under the conditions of application to form fluorine-containing tin oxide. Examples of tin compounds which may be used are alkyl tin oxides, for example butyl tin oxides, and alkyl tin halides of the formula Rx Sn Hal4¯ where each R is an alkyl group, preferably containing 1-6 carbon atoms, x is 1, 2 or 3 and Hal represents halogen. When x=l or 2, the halogen atoms may be the same or different. It is preferred to use non-hygroscopic compounds which are available in dry solid form to provide a free-flowing finely divided solid source of tin. Examples of preferred tin compounds include dibutyl tin oxide and dibutyl tin difluoride. Alternatively, an inorganic tin compound may be used, for an example an ammonium hexahalostannate of formula (NH4)2 Sn Hal6 wherein a minor atomic proportion of the halogen present is fluorine, and the balance is chlorine. Such compounds may be prepared by forming an aqueous solution from sources of ammonium ions- stannic ions, rluoride ions and chloride ions in appropriate proportions and evaporating, under vacuum, to dryness Stannic chloride may be used as a source of stannic ions and chloride ions, and ammonium chloride and ammonium fluoride may be used as sources of ammonium ions and halide ions. Such hexanalostannates are preferably used in the presence of water vapour. It is believed that the use of very fine particles of the tin compound results in more uniform coatings, and it is generally preferred to use the tin compound in the form of particles with a maximum particle size below 100 microns, and especially below 20 microns. If desired a mixture of tin compounds may be used. By using a mixture of a tin compound containing fluorine and a fluorine-free tin compound, it is possible to regulate the fluorine content of the coating produced. If the tin compound used does not contain fluorine or contains insufficient fluorine to provide the desired fluorine content in the coating. a separate source of fluorine may be used. It is preferred to use a source, for example hydrogen fluoride gas, which does not result in the formation of by-products which contaminate the coating. The oxygen required for the formation of the tin oxide may be provided by the tin compound or compounds used, but it is usually necessary or at least desirable to provide a separate source of oxygen. This may be done by, for example using air or oxygen as the carrier gas for the solid tin compound. Other carrier gases may be used if deslred provided care is taken to ensure that sufficient oxygen is present to ensure formation of tin oxide on thermal decomposi ion of the tin compound applied to the hot glass. The invention also includes glass having an infra red reflecting fluorine-containing tin oxide coating produced by a process in accordance with the invention. The invention is illustrated but not limited by the following Examples In the examples, the values quoted for light transmission are for transmission of light from a C.I.E. Illuminant C source. The vaiues of emissivity quoted are those obtained by applying the formula Emissivity, E = EMI6.1 where e# = spectral remittance and B (# ,T) = black body spectral energy distribution at 3000K. The emissivity, E, calculated as above, is related to the infra red reflectivity at the coating such that, in general, the lower the emissivity, the higher the intra red reflectivity. example 1 Dibutyl tin di fluoride was mixed with 2% by weight of "Cab-o-Sii" (an amorphous silica available in commerce from Cabot Carbon Limited of Ellesere Port, Englana) a; E n g 1 a n a) a S a milling additive and flow promoter, and the composition milled in a pin mill to a maximum particle size of less than 100 microns. The milled solid was distributed in a stream of air as carrier gas and directed through a slot 6 mm wide onto 6 mm thick hot glass advancing at 5 metres/minute past the slot. The air was supplied at a rate of 0.04 m3 at 45 kPa/cm length of slot/minute, and the milled solid at a rate of 4 gram/cm length of slot/minute. The thickness of the coating, the light transmission of the coated glass and the emissivity of the coated glass were measured. The results obtained at gLass temperature of 600 C ana 695"C are set out below: Temp. Thickness of Transmission Emissivity C coating, nm % 600 1080 59.1 0.24 695 1040 66.7 0.17 The coating obtained at 695"C is thinner than the coating obtained at 6000C, but the difference in thickness is not sufficient to account for the substantial increase in light transmission trom 59.1% to 66.7% observed on increasing the coating temperature. Moreover tne einissivity ot the higher temperature coating is tower indicating an increase in the infra red reflectivity of the coating. Example 2 The coating process was repeated at a reduced powder supply rate. The air and powder supply rates were 0.04 m at 45 kpa/cm length of slot/ainute and 2.5 grams/cm length of slot/minute respectively. The properties of the coated product obtained were measured and the results obtained set out below: Temp. Thickness at Iransmission Emissivity C coating, nm % 600 750 66.9 .28 695 740 71.2 .16 Again, there i3 a marginal reduction in thickness on increasing the temperature from 6000C to 650C. but it is not sufficient to account for the signiticant increase in light transmission from 66.9% to 71.2%. The increased light transmission is accompaniod by a reduction in emissivity l.e. an increase in the infra red reflectivity of the coating. Example 3 Dimethyltin difluoride was mixed with 2% ny weight of "Cab-o-Sil" as a milling additive and flow promoter, and the composition milled in a jet mill to a maximum particle size of less than 100 microns. The milled solid was distributed in a stream of air as carrier gas and directed through a slot 6 mm wide onto 6 mm thick hot glass advancing at 5 metres/minute past the slot. The air was supplied at a rate of 0.04 m3 at 45 kPa/cm length of slot/minute, and the milled solid at a rate of 6 grams/cm length of slot/minute. The properties of the coated glass were measured and the results obtained at glass temperature of 600 C and 7050C are set out below. Temp. Thickness of Transmission Emissivity OC coating, nm 600 450 74.7 0.35 705 440 77.d 0.20 The coating produced at 7050C is marginally thinner than the coating obtained at 6000C, but the difference in thickness is insufficient to account for the increase in light transmission from 74.7e to 77.8w. Tne increase in light transmission is accompanied by a substantial increase in infra red reflectivity as indicated by the reduction in emissivity from 0.35 to 0.20. Example 4 A 1:1 mixture by weight of dibutyl tin oxide and dibutyl tin difluoride was mixed with 2% by weight of VCab-o-Sil" as a milling additive and flow promoter, and the composition milled in a pin mili to a particle size of less than 100 microns. The milled solid was distributed in a stream of air as carrrier gas and directed through a slot 6 m wide and 1 mete long onto 6 mm thick hot float glass advancing at 3 etres/minute past the slot. The air was supplied at the rate of 5 m3/minute and the milled solid at a rate of 300 grams/minute. The properties of the coated glass were measured and tne result obtained at different temperatures (two at each temperatures) set out below. In each case, the thickness of the coating was 700 nm. Temp. Transmission Emissivity OC 600 67.1, 67.1 0.30, 0.31 675 68.0, 68.0 0.27, 0.27 725 69.4, 69.7 0.22, 0.21 The infra red reflectivity (as evidenced by the measured emissivity) and light transmission both increase as the temperature is raised. The rate of increase is greater above 6750C, with a tall in average emissivity of 0.035 between 600 C and 675 C, compared to a fall of average emissivity of 0.55 between 675 C and 7250C. Similarly, the average light transmission increases from 67.1* to 68.0% between 600 C and 6750C, but from 68.0* to 69.7% between 6750C and 725 C. Example 5 Dibutyl tin difluoride was mixed with 2% by weight of "Ca-o " as a milling auditive and flow promoter, and the composition milled in a Alpine Kolioplex Laboratory Mill 160z mill to a maximum particle size of less than POO microns. The milled solid was distributed at a rate of 300 gm/minute in a stream of 4.7 m3 airdminute as carrier gas and directed through a slot 1.3 m long and 6 mm wide onto a ribbon of 4 am rolled glass hot from the forming process. The powder was directed onto the glass at a glass temperature of 700"C; the lehr speed of the glass ribbon was 526 m/hour. An infra red reflecting fluorine containing tin oxide coating was formed on the glad surface. The thickness of the coating was measured and found to be 420 nm. The light transmission of the coated glass was 77.3% and the emissivity 0.20, indicating good infra red reflectivity.

Claims: Claims I. A process for the production of an infra red reflecting coating on giass by directing onto a hot glass surface a tin compound in the form of a finely divided solid which decomposes under the conditions of application to deposit a fluorine-containing tin oxide coating on the glass characterised in that the tin compound is directed onto the hot glass surface at a glass temperature above 6600C 2. A process according to claim 1 characterised in that the glass temperature is in the range 6800C to 7200C. 3. A process according to claim 1 or claim 2 characterised in that the solid tin compound is directed onto a ribbon of rolled glass which is hot from the forming process.

Inventors: HARGREAVES, EDWARD, SIMPSON, ANDREW FRASER
Applicants: PILKINGTON BROTHERS P.L.C.
Application year: 1985
Main classification: C03C17
IPC codes: C03C17, C23C20