Method of depositing tin oxide and titanium oxide coatings on flat glass and the resulting coated glass

TITLE

0002METHOD FOR DEPOSITING TIN OXIDE AND TITANIUM OXIDE COATINGS ON FLAT GLASS AND THE RESULTING COATED GLASS

BACKGROUND OF THE INVENTION

00041. Field of the Invention

0005This invention relates to a process for depositing titanium oxide and tin oxide coatings on a flat glass substrate, and the resulting coated glass. More particularly, this invention

0006relates to a chemical vapour deposition process for producing titanium oxide and tin oxide coatings on flat glass using a coating precursor gas mixture comprising the corresponding metal tetrachloride and an organic oxidant

00072. Summary of Related Art

0008Titanium oxide and tin oxide coatings have been proposed for use on glass containers, for example bottles, to improve the mechanical strength of the containers. It has also been proposed to use both titanium oxide and tin oxide coatings on flat glass to modify the characteristics of the glass for architectural use; titanium oxide coatings deposited under

0009vacuum (by reactive sputtering) are used as components of sputtered multi-layer infra red reflecting coatings, while tin oxide coatings are used, not only as layers of multi-layer sputtered coatings, but also deposited pyrolytically with a dopant as infra red reflecting and/or electroconducting coatings.

0010GB patent specification 1 115 342 describes a process for producing glass containers with good inherent strength and good abrasion resistance by spraying the containers, while still hot from the manufacturing process, with a solution or dispersion of stannic chloride (that is, tin tetrachloride) in an organic liquid, isopropyl alcohol being preferred. A small amount of titanium chloride may be incorporated as a modifier. The liquid solution is fed

0011to atomisers, which may be of the pressure jet variety, located on either side of a tunnel

0012over a conveyor for hot glass bottles to produce a 'mist of liquid reagent' so that a layer of liquid is formed on all the external surfaces of the bottles where it reacts to form a layer of tin oxide.

0013GB patent specification 1 187 784 describes an improvement of the process described

0014in GB patent specification 1 115 342 and apparently more suitable for incorporation into a process for the automatic manufacture of glassware without interfering with the normal running of such process and without requiring additional supervision. The specification

0015proposes to treat glass containers, at high temperature, with a liquid solution of an organic

0016compound of tin "which compound has properties such that upon application of heat it decomposes into two materials, one of which is an organic compound of tin of high

0017decomposition temperature which reacts with the glass surface to produce a diffusion layer

0018of tin oxide within the glass surface, while the other is a volatile compound of tin such that a

0019substantial proportion of vapour of said compound is produced, and subjecting the

0020containers to a heat treatment such that a reaction is caused to occur between the glass at at

0021least the surfaces of the containers and the tin compounds". The material used for treating

0022the glass containers may be provided by reacting tin tetrachloride with organic substances containing carbonyl groups of moderate activity e.g. organic esters of ethyl, n-propyl, isopropyl, n-butyl and isobutyl alcohols with acetic, propionic and butyric acids. The

0023resulting solution may be sprayed, in the presence of ambient atmosphere, on to the hot

0024containers e.g. in the form of a fine mist after they leave the forming machine and before they enter the annealing lehr. GB patent specification 1 187 783 describes an analogous process to that described in 1 187784 in which an organic compound of titanium is sprayed on to the hot glass containers in place of the organic compound of tin. The organic titanium compound may be produced, in an analogous manner to the organic compound of tin, by reacting titanium tetrachloride with an organic ester e.g. n-butyl acetate. Again, the resulting solution is sprayed onto the glass in the ambient atmosphere on the container

0025production line.

0026It has also been proposed to use tin tetrachloride, applied either as a liquid spray or, more recently, in gaseous form, to apply a tin oxide coating to hot flat glass to form an electroconductive, infra-red reflecting coating on the hot glass surface; water is used to hydrolyse the tin tetrachloride and as a source of oxygen for formation of the tin oxide.

0027Processes involving use of the reactants in gaseous form (also called CVD or chemical vapour deposition processes) have certain advantages over spray processes for coating flat glass, especially when the reactants can be premixed before application to the glass. Unfortunately, tin tetrachloride reacts readily with water so that previous proposals to use tin tetrachloride and water vapour in gaseous form have usually involved supplying the gases separately to the glass surface and mixing them while in contact with the glass.

0028GB patent specification 2044 137 A relates to such a process in which discrete laminar streams of each reactant are formed and projected on to a hot glass substrate by bringing the streams together in reciprocal tangential contact over the glass. Titanium tetrachloride may be used as one of the gaseous reactants, in place of the tin tetrachloride, in order to form a titanium oxide coating. The patent also suggests supplying hydrogen to one of the gas streams to attenuate the violent reaction between the tin tetrachloride and the water vapour. This may be done either by direct addition of gaseous hydrogen, or by the addition of methanol, which is said to react in situ to produce the desired gaseous hydrogen.

0029GB patent specification 2 026454B describes a process in which a coating chamber is

0030positioned over a hot float glass ribbon as it advances from the float bath and successive

0031gaseous streams of (1) preheated nitrogen carrier gas, (2) tin tetrachloride entrained in preheated nitrogen and (3) air, water vapour and hydrofluoric acid are introduced into the coating chamber so they flow along the glass substrate surface being coated as a

0032substantially turbulence free layer. The patent specifies the concentration of water vapour

0033and tin tetrachloride in the gaseous medium over the glass.

0034European patent specifications 0 365 239B 1 and 0 376240B 1 describe a method and apparatus for depositing a tin oxide coating on a hot glass ribbon. A first gaseous stream of

0035tin tetrachloride in preheated dry air is caused to flow along the surface of the hot ribbon of glass advancing beneath a coating chamber, a second turbulent stream of hydrofluoric acid and steam introduced into the coating chambers at right angles to the plane of the glass and

0036direction of flow of the first gaseous stream, and the combined first and second gas streams

0037drawn through the coating chamber over the glass under turbulent flow conditions. The method and apparatus may also be used to apply a coating of titanium oxide using titanium

0038tetrachloride in place of the tin tetrachloride.

0039US patent 4590 096 describes a process in which a coating solution comprising a

0040substantially solvent free mixture of an organotin chloride and a reactive organic fluorine compound soluble in or miscible with the organotin chloride is introduced to a preheated

0041carrier gas stream which contains sufficient water vapour that the relative humidity of the

0042gas stream at 18^βC is about 6% to about 100%. The resulting gas stream is passed over a

0043hot glass surface to deposit a fluorine doped tin oxide coating on the hot glass. A wide range of organotin compounds may be used, and the possibility of using tin tetrachloride is mentioned. Similarly, a wide range of organic fluorine compounds, including oxygen containing compounds, for example trifluoroacetic acid and ethyltrifluoroacetate, may be used. Some of the fluorine-containing dopants have limited solubilities in the organotin

0044compounds used, and an optional solubiliser may be used to increase the solubility of the fluorine dopant on the organotin compound; acetic anhydride, ethyl acetate, hexane, methyl isobutyl ketone and butyraldehyde are listed as non-limiting examples of the solubilisers that may be used. However, the US patent, in common with the other patents utilising chemical

0045vapour deposition methods to deposit a metal oxide from a gaseous metal tetrachloride, utilises water vapour as the source of oxygen.

0046US patent 4751 149 Vijaykumar et al is concerned with deposition of zinc oxide coatings by chemical vapour deposition at low temperature (60^δ to 350°C, preferably 100⁹ to 200°C) on heat sensitive photoconductor substrates, and proposes to deposit the zinc oxide coatings from an organozinc compound and an oxidant, which may be an oxygen containing organic compound e.g. an ester, and an inert carrier gas. Although the patent is

0047not entirely clear, it apparently proposes to introduce separate streams of the organozinc compound and oxidant into the deposition chamber, and certainly there is no proposal to pre-mix these components together before delivery to the coating chamber.

0048It would be advantageous to provide a method for depositing tin or titanium oxide

0049coatings by a CVD process applied to hot flat glass using a premixture of the corresponding metal tetrachloride as a low cost reactant and a source of oxygen without premature reaction between the metal tetrachloride and oxygen source (previously water) resulting in formation of metal oxide in the coating equipment with consequent problems and

0050inefficiency. It would be particularly advantageous if the method allowed for deposition of the coating at high rates, enabling a required coating thickness to be deposited on a moving glass ribbon during the glass production process.

SUM ARY OF THE INVENTION

0052In accordance with the present invention, there is provided a chemical vapour deposition process for laying down a tin oxide or titanium oxide coating on a hot glass

0053substrate using a precursor gas mixture containing the corresponding metal tetrachloride

0054and an organic source of oxygen, without the requirement for inclusion of water vapour and the consequent risk of premature reaction.

0055The present invention provides a process for depositing a tin oxide or titanium oxide

0056coating on hot flat glass comprising the steps of

0057(a) preparing a precursor gas mixture containing the corresponding metal tetrachloride and an organic oxygen containing compound as a source of oxygen for formation of the

0058metal oxide,

0059(b) maintaining said precursor gas mixture at a temperature below the temperature at which the metal tetrachloride reacts to form the metal oxide while delivering the

0060mixture to a coating chamber opening on to the hot glass,

0061(c) introducing the precursor gas mixture into the coating chamber whereby the mixture is

0062heated to cause deposition of the corresponding metal oxide incorporating oxygen from the organic compound on to the hot glass surface.

0063Surprisingly, a wide range of oxygen-containing organic compounds may be used as the

0064source of oxygen, without requiring the presence of water vapour or gaseous oxygen,

0065including compounds normally considered reducing agents rather than oxidising agents, for example, alcohols. However, the preferred organic compounds are carbonyl compounds, especially esters; and particularly good results have been obtained using esters having an alkyl group with a β hydrogen. The alkyl group with a β hydrogen will normally contain two to ten carbon atoms.

0066It is preferred to use organic compounds, especially esters, containing from two to ten carbon atoms, since larger molecules tend to be less volatile and hence less convenient for

0067use in the CVD process of the present invention.

0068Particularly preferred esters for use in the practice of the present invention include ethyl formate, ethyl acetate, ethyl propionate, isopropyl formate, isopropyl acetate, n-butyl acetate and t-butyl acetate.

0069The method of the present invention is generally practised in connection with the formation of a continuous glass ribbon substrate, for example during a float glass production process. However, the method of the present invention may be employed in coating other flat glass substrates either on-line or off-line.

0070The present invention involves the preparation of a precursor gas mixture which includes tin or titanium tetrachloride and an organic oxygen containing compound; a carrier gas or diluent, for example, nitrogen, air or helium, will normally also be included in the gas mixture. Since thermal decomposition of the organic oxygen containing compound may initiate the metal oxide deposition reaction at a high rate, it is desirable that the precursor mixture be kept at a temperature below the thermal decomposition temperature of the organic oxygen compound to prevent prereaction of the gaseous mixture with formation of the metal oxide.

0071The gaseous mixture is maintained at a temperature below that at which it reacts to form the metal oxide, and delivered to a location near a flat glass substrate to be coated, the substrate being at a temperature above said reaction temperature (and above the decomposition temperature of the organic oxygen compound in the precursor gas mixture).

0072The precursor gas mixture is thereafter introduced into the vapour space directly over the substrate. The heat from the substrate raises the temperature of the precursor gas above

0073the thermal decomposition temperature of the organic oxygen compound. The organic

0074oxygen compound then decomposes with reaction with the metal tetrachloride producing a

0075metal dioxide coating on the substrate.

0076The present invention permits the production of tin and titanium oxide coatings

0077deposited on the hot glass at a high deposition rate e.g. over 13θA/second and, in preferred

0078embodiments, over 25θA per second.

0079The deposition rate is dependent upon the particular organic oxygen containing compound used, and the concentrations of both the organic oxygen containing compound and the metal chloride, as well as the temperature of the glass. For any particular

0080combination of compounds, the optimum concentrations (and in particular the optimum proportion of organic oxygen containing compound to metal tetrachloride) and flow rates

0081for rapid coating deposition may be determined by simple trial. However, it will be

0082appreciated that the use of higher concentrations of reactants and high gas flow rates is

0083likely to result in a less efficient overall conversion of the reactants into coating, so that the optimum condition for commercial operation may differ from the conditions which provide the highest deposition rates.

0084Preferably, the organic oxygen containing compound will be at a concentration by

0085volume of about 0.5, especially 1, to 5 times the concentration by volume of the metal chloride. It will commonly be used in an amount of at least 30% by weight of the weight of

0086metal chloride. The method of the invention permits the production, at high rates, of titanium oxide and tin oxide coatings on hot flat glass substrates on line during the glass production process. The titanium oxide coatings may be produced with a high refractive index (at least 2.4) permitting the achievement of desired optical effects, especially when used in

0087combination with other coating layers. The tin oxide coatings may be doped, for example with fluorine, by incorporating an appropriate precursor for the dopant into the precursor

0088gas mixture, increasing the electrical conductivity and infra red reflectivity of the coatings, and hence their utility as electrical conducting coatings and/or low emissivity coatings in

0089architectural glazing and other applications.

BRIEF DESCRIPTION OF THE DRAWINGS

0091The above, as well as other advantages of the present invention, will become readily

0092apparent to those skilled in the art from the following detailed description of preferred

0093embodiments when considered in the light of the accompanying drawings in which:

0094Fig 1 is a schematic view of a vertical section of an apparatus for practising a float glass process which includes gas distributors suitably positioned to enable the practising of the

0095method of the present invention.

0096Fig 2 is broken sectional view of an article coated according to this invention; and

0097Fig 3 is an enlarged schematic end view of a gas distributor beam suitable for use in

0098practising the present invention.

0099Fig 4 is an enlarged schematic end view of an alternative gas distributor beam which may be used in practising the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

0101Referring now more particularly to the drawings, there is illustrated generally at 10 in

0102Fig 1 a float glass installation utilized as a means for practising the method of the present invention. The float glass apparatus more particularly comprises a canal section 12 along which molten glass 14 is delivered from a melting furnace (not shown), to a float bath section 16 wherein a continuous glass ribbon 18 is formed in accordance with the well known float process. The glass ribbon 18 advances from the bath section 16 through an adjacent annealing lehr 20 and a cooling section 22. The continuous glass ribbon 18 serves as the substrate upon which the metal oxide coating is deposited in accordance with the present invention.

0103The float section 16 includes a bottom section 24 within which a bath of molten tin 26 is contained, a roof 28, opposite sidewalls 30, and end walls 32. The roof 28, side walls 30, and end walls 32 together define an enclosure 34 in which a non-oxidizing atmosphere is maintained to prevent oxidation of the molten tin.

0104Additionally, gas distributor beams 64, 66 and 68 are located in the bath section 16. The gas distributor beams 64 and 66 in the bath section may be employed to apply additional coatings onto the substrate prior to applying the tin or titanium oxide coating by the method of the present invention. The additional coatings may include silicon and silica.

0105In operation, the molten glass 14 flows along the canal 36 beneath a regulating tweel 38 and downwardly onto the surface of the tin bath 26 in controlled amounts. On the tin

0106bath the molten glass spreads laterally under the influences of gravity and surface tension, as well as certain mechanical influences, and it is advanced across the bath to form the ribbon 18. The ribbon is removed over lift out rolls 40 and is thereafter conveyed through the annealing lehr 20 and the cooling section 22 on aligned rolls 42. The application of the coating of the present invention may take place in the float bath section 16, or further along the production line, for example in the gap between the float bath and the annealing lehr, or in the annealing lehr. A suitable non-oxidizing atmosphere, generally nitrogen or a mixture of nitrogen and

0107hydrogen in which nitrogen predominates, is maintained in the bath enclosure 34 to prevent oxidation of the tin bath. The atmosphere gas is admitted through conduits 44 operably coupled to a distribution manifold 46. The non-oxidizing gas is introduced at a rate

0108sufficient to compensate for normal losses and maintain a slight positive pressure, on the order of about 0.001 to about 0.01 atmosphere above ambient atmospheric pressure, so as to prevent infiltration of outside atmosphere. Heat for maintaining the desired temperature regime in the tin bath 26 and the enclosure 34 is provided by radiant heaters 48 within the

0109enclosure. The atmosphere within the lehr 20 is typically atmospheric air, while the cooling section 22 is not enclosed and the glass ribbon is open to the ambient atmosphere. Ambient air may be directed against the glass ribbon as by fans 50 in the cooling section.

0110Heaters (not shown) may also be provided within the annealing lehr for causing the

0111temperature of the glass ribbon to be gradually reduced in accordance with a predetermined regime as it is conveyed therethrough.

0112Fig 1 illustrates the use of gas distributor beams 64, 66 and 68 positioned in the float

0113bath 16 to deposit the various coatings on the glass ribbon substrate. The gas distributor beam is one form of reactor that can be employed in practising the process of the present invention.

0114A convenient configuration for the distributor beams suitable for supplying the

0115precursor materials in accordance with the invention is shown generally schematically at Fig 3. An inverted generally channel-shaped framework 70 formed by spaced inner and outer walls 72 and 74 defines enclosed cavities 76 and 78. A suitable heat exchange medium is

0116circulated through the enclosed cavities 76, 78 in order to maintain the distributor beams at

0117a desired temperature. The precursor gas mixture is supplied through a fluid cooled supply conduit 80. The supply conduit 80 extends along the distributor beam and admits the gas through drop lines 82 spaced along the supply conduit. The supply conduit 80 leads to a delivery chamber 84 within a header 86 carried by the framework. Precursor gases admitted through the drop lines 82 are discharged from the delivery chamber 84 through a passageway 88 toward a coating chamber defining a vapour space opening on to the glass where they flow along the surface of the glass 18 in the direction of the arrows in Fig 3.

0118Baffle plates 90 may be provided within the delivery chamber 84 for equalizing the flow of precursor materials across the distributor beam to assure that the materials are discharged against the glass 18 in a smooth, laminar, uniform flow entirely across the distributor beam. Spent precursor materials are collected and removed through exhaust chambers 92 along the sides of the distributor beam.

0119Various forms of distributor beams used for chemical vapour deposition are suitable for

0120the present method and are known in the prior art.

0121One such an alternative distributor beam configuration is illustrated schematically in Figure 4 of the drawings. Using this distributor, which is generally designated 100 (and more fully described in European patent EP 0 305 102B), the precursor gas mixture is introduced through a gas supply duct 101 where it is cooled by cooling fluid circulated through ducts 102 and 103. Gas supply duct 101 opens through an elongated aperture 104 into a gas flow restrictor 105.

0122Gas flow restrictor 105 is of the kind more fully described in UK patent specifications

0123GB 1 507 996, and comprises a plurality of metal strips longitudinally crimped in the form of a sine wave and vertically mounted in abutting relationship with one another extending along the length of the distributor. Adjacent crimped metal strips are arranged "out of phase" to define a plurality of vertical channels between them. These vertical channels are of small cross-sectional area relative to the cross-sectional area of gas supply duct 101, so that the gas is released from the gas flow restrictor 105 at substantially constant pressure along the length of the distributor.

0124The coating gas is released from the gas flow restrictor into the inlet side 107 of a substantially U-shaped guide channel generally designated 106 comprising inlet leg 107, coating chamber 108 which opens onto the hot glass substrate 110 to be coated, and

0125exhaust leg 109, whereby used coating gas is withdrawn from the glass. The rounded

0126corners of the blocks defining the coating channel promote a uniform laminar flow of coating parallel to the glass surface across the glass surface to be coated.

0127The following examples (in which gas volumes are expressed under standard

0128conditions, i.e. one atmosphere pressure and ambient temperature, unless other stated) are presented for the purpose of further illustrating and disclosing the present invention, and are not to be construed as a limitation on the invention:

0129Examples 1 to 5

0130In this series of Examples, a bi-directional coating reactor of the type shown in Fig 3 was employed in the laboratory to deposit a titanium oxide coating.

0131In Examples 1, 2 and 3, the glass was heated on a conveyor furnace to simulate the

0132coating reaction conditions of a float glass process in order to test the method of the present

0133invention. The furnace utilized in-line rollers to convey a glass substrate through a heating zone prior to practising the method of the present invention. In Example 1, the glass substrate was float glass which had been initially provided with a silica coating. The silica

0134coating was deposited on the float glass through a known chemical vapour deposition process utilizing a precursor of monosilane in an oxygen enriched atmosphere. The silica deposition forms no part of the present invention.

0135In accordance with the present invention, a titanium oxide coating was deposited on the silica coated substrate. The substrate was at a temperature of a 1170^SF/630°C and the substrate line speed was at 300 inches/8 metres per minute.

0136To deposit the titanium oxide, a precursor gas mixture was developed comprising titanium tetrachloride, ethyl acetate, oxygen, and helium. Helium was included in the precursor mixture as a carrier for the reactants. The precursor mixture was prepared by simultaneously introducing all four gas streams through a manifold system. An in line static mixer was used to ensure a homogeneous precursor mixture. The volume percent composition of the precursor mixture was 0.7% titanium tetrachloride, 17.2% ethyl acetate, 7.2% oxygen, and 74.9% helium, with the flow rates for the components at the manifold being as shown in the accompanying Table 1.

0137The temperature of the precursor mixture was kept above 300°F/150^QC in order to prevent the adduct reaction of titanium tetrachloride and ethyl acetate. The precursor temperature was also kept below the 950^QF - 1130^QF (510°C - 610^QC) thermal decomposition temperature range of ethyl acetate in order to prevent the mixture from

0138prereacting.

0139The precursor mixture was introduced into the reactor just above the moving substrate. The temperature at the precursor tower was 250^QF/120^eC. The temperature at the reactor

0140face was 350°F/175°C. The higher substrate temperature initiated the thermal decomposition of the ethyl acetate which then resulted in the deposition of the titanium oxide. The resulting coated glass was allowed to cool in air and the coating analysed. It was found to be titanium oxide with a carbon content of 2.5-3.5 atomic percent The thickness of the titanium oxide coating was measured 49θA and the thickness and growth rate (150 A per second) are shown in Table 1. The optical properties of the resulting product included an observed Dluminant C transmittance (10° observer) of 62.3% and an observed Illuminant C reflectivity of 35.6%. The extinction coefficient was 0.008 at 550 nm, and the refractive index of the titanium oxide coating was 2.44.

0141In Examples 2 and 3 the coating procedure set out in Example 1 was repeated, except that in Example 2 ethyl formate was used as the organic source of oxygen, and in Example 3 isopropanol was used as the organic source of oxygen and uncoated glass (in place of the silicon oxide coated glass of Examples 1 and 2) was used as the substrate. The gas flow rates used and, in the case of Example 2, the thickness and growth rate of the titanium oxide coating produced are shown in Table 1. In Example 3, the isopropanol burned in the

0142reactor leaving only paniculate titanium oxide on the glass, the corresponding deposition rate therefore being quoted as OA/second.

0143The procedure for Examples 4 and 5 was as used in the previous Examples (the reactor temperature and the substrate being identical to Example 1), except that the substrate was

0144static and not dynamic. The static sample was positioned under the reactor for 10 seconds. Under static conditions, the residence time of the substrate under the reactor is increased from the dynamic conditions by a factor of five.

0145In Example 4, methyl acetate was used as the organic source of oxygen, and in

0146Example 5 t-butyl acetate was used; in each case a titanium oxide coating was produced. The gas flow rates, resulting titanium oxide coating thickness and coating growth rates are as shown in Table 1. The relatively slow growth rate achieved using methyl acetate is discussed hereinafter.

0147Example 6

0148A float glass process was used in producing a continuous glass ribbon having a

0149thickness of 0.125 inches/3 mm at a line speed of 434 inches/11 metres per minute. The glass temperature was at 1140°F/615°C at the desired point of application in the float bath section of a titanium oxide coating using a coating reactor similar to that shown in Figure 3.

0150The temperature at the precursor tower was 400°F/205°C and the temperature at the

0151reactor face was 500°F/260^QC. Prior to practising the method of the present invention, a silica coating was deposited on the glass substrate in the float bath section at a thickness of about 339 A. The same chemical vapour deposition process as described in Example 1 was

0152used to deposit the silica coating. The silica deposition forms no part of the present

0153invention.

0154The precursor gas mixture was developed comprising titanium tetrachloride and ethyl

0155acetate in a helium carrier gas. Oxygen was not used in the precursor as result of earlier

0156Examples indicated that the coating reaction was not sensitive to the oxygen concentration. The precursor mixture was prepared by simultaneously introducing the three components through a manifold system. The volume percent composition of the precursor mixture was

01570.6% titanium tetrachloride, 1.8% ethyl acetate, and 97.5% helium. The flow rates for the

0158components were 480.0 1/m of helium, 3.0 1 m of titanium tetrachloride, 9.21 m of ethyl acetate. The total flow rate for the precursor mixture was 492.2 1 m.

0159The resulting titanium oxide coating was 684 A thick. The carbon content of the

0160coating was less than 2 atomic percent. The growth rate of the coating was 309 A per

0161second. Example 7

0162The same procedure carried out Example 6 was utilized in this Example. The substrate

0163comprised coatings of silicon and then silica over the glass substrate. The coatings were deposited by a known chemical vapour deposition process in the float bath section. The silicon coating was deposited by CVD from monosilane with a non-oxidizing carrier gas.

0164The silica coating was then deposited onto the silicon coating through the use of the same procedure as described in Example 1.

0165The precursor for the titanium oxide coating included titanium tetrachloride and ethyl acetate in a helium carrier gas. The volume percent composition of the precursor was 0.5%

0166titanium tetrachloride, 1.9% ethyl acetate, and 97.6% helium. The corresponding flow rates for the components were 480.01/m of helium, 2.4 1 m of titanium tetrachloride, 9.2 1/m of ethyl acetate. The total flow rate for the precursor mixture was 491.6 1/m.

0167The resulting coated article 52 is illustrated in Fig 2. The glass substrate 54 is depicted

0168with a stack of multiple coatings 56. The coatings comprise a layer of silicon 58, a layer of silica 60, then a titanium oxide coating 62 on top of the article. The titanium oxide coating

0169on the resulting article had a thickness of 836 A. The optical properties of the resulting

0170coating stack included an observed Illuminant C transmittance of 13.1% and an observed

0171Dluminant C reflectivity of 82.5%. The growth rate of the titanium oxide coating was 378 A per second. TABLE 1

0172Example Flow rates (litres/minute)

0173Titanium Organic oxygen Oxygen Helium Thickness Growth rate tetrachloride compound A A/sec

01740.2 4.8 ethyl acetate 2.0 20.9 490 150

01750.5 1.6 ethyl formate 6.0 17.4 800 250 0

01760.45 1.5 isopropanol 4.0 15.45 0

01770.5 1.2 methyl formate 6.0 17.4 <100 <10

01780.5 0.5 t-butyl acetate 6.0 16.5 1300 130

0179Examples 8 - 13

0180In this series of Examples, a static coater was used in the laboratory to apply a tin oxide

0181coating on to a float glass substrate carrying a colour suppressing silicon oxide layer produced as described in European patent EP 0 275 662B.

0182The float glass to be coated was supported on a nickel block in a reactor vessel and the

0183block heated from below by electric heating elements to provide a glass temperature of

01841085°F/585°C. A flat graphite plate was mounted approximately 0.4 inches/10 mm above the glass and parallel thereto to provide a gas flow path 0.4 inches/10 mm deep between the glass surface bearing the silicon oxide layer and the plate.

0185A precursor gas mixture containing tin tetrachloride and an organic source of oxygen, in air and a small proportion of additional nitrogen as carrier gas, was delivered through a

0186gas line maintained at a temperature of 435°F ± 25°F 225°C ± 15^δC and provided with a fish tail nozzle opening on to the gas flow path over the hot glass in a general direction

0187parallel to the glass surface. The total carrier gas flow rate was 13 m³/hour. The flow rates of the tin tetrachloride, and the nature and flow rates of the organic compound used, were

0188as shown in the accompanying Table 2. In Examples 9 and 11, small amounts of 40% hydrogen fluoride were incorporated in the precursor gas mixture to dope the resulting tin

0189oxide coating with fluorine, as shown in the Table.

0190The gas flow containing the reactant gases was applied for approximately 8 seconds, and the coating apparatus and coated glass then allowed to cool under a flow of air at

0191345°F/225°C. On dismantling the coating apparatus, the delivery gas line, nozzle and plate

0192defining the gas flow path over the glass found to be free, in each case, from deposit, indicating an absence of undesirable prereaction. In each case, the glass had a tin oxide

0193coating applied over the silicon oxide, the thickness of the coating varying with distance from the fishtail nozzle. The maximum thickness and corresponding growth rate for each precursor gas mixture used is shown in Table 2. The emissivity, resistivity and haze of the samples producing using hydrogen fluoride to incorporate a fluorine dopant (Examples 9

0194and 11) were measured and the results reported in Table 2.

0195This series of Examples shows that an organic source of oxygen can be used as part of a premixed precursor gas mixture comprising tin tetrachloride to deposit a tin oxide coating

0196without significant undesirable prereaction detrimentally affecting the coating process e.g.

0197by deposition of tin oxide in the gas supply ducts. Moreover, if desired, a source of dopant, such as hydrogen fluoride, may be incorporated in the gaseous premixture to reduce the emissivity and resistivity of the coating while continuing to avoid significant detrimental prereaction.

0198Example 14

0199In this Example, a coating distributor as illustrated schematically in Figure 4 was used

0200in a float bath to apply a coating of tin oxide by a method in accordance with the invention.

0201The ribbon speed was approximately 233 inches per minute/350 minutes per hour and the

0202glass thickness was 0.05 inches/1.2 mm. The glass temperature was approximately

02031170°F/630°C. The temperature of the gas supply duct 101 which served as a primary gas

0204mixing chamber was maintained at 300°F/150⁹C and the 'static' waffle gas distributor 105

0205was approximately 645°F/340°C. The tin tetrachloride and butyl acetate vapours were

0206delivered by bubbling nitrogen through the liquids maintained at 175°F/80°C in bubblers

0207and, hence, through separate heated conduits to gas supply duct 101. The vapours mixed at the primary chamber, passed through the waffle pack gas distributor, and then under laminar TABLE 2

0208ample Precursor Gas Mixture Max tin Max growth Emissivity Resistivity H oxide rate ohm/cm thick 0ness A/second

A

0210SnCl₄ Organic Oxygen Source 40% HF

0211Flow Rate Flow Rate

0212(ml/min) Compound Flow (ml/ Rate min) (ml/min)

02138 12 ethyl acetate 10 2750 344

02149 12 ethyl acetate 10 1 2680 335 0.25 5.3 x 10- 0.

021510 12 butyl acetate 13.4 - 3460 432 - -

021611 12 butyl acetate 13.4 1.3 2880 360 0.25 6.9 x 10"* 0.

021712 6 isopropyl 120 - 2284 262 - - alcohol

021813 17 trifluoracetic 16.2 - 2840 335 - - acid

0219flow conditions through U-shaped guide channel 106 comprising coating chamber 108 opening on to the hot glass ribbon.

0220The flow rates used were sufficient to obtain tin tetrachloride : butyl acetate molar

0221ratios of between 1: 1 and 1 : 5. The trial was carried out for 5 hours. On dismantling the coater it was discovered that the cooled surfaces and the associated conduits were over 90% free of deposits, thus showing that tin tetrachloride and butyl acetate used for producing a tin dioxide coating on glass can be premixed with one another without

0222substantial prereaction. A thin tin oxide coating was obtained on the glass ribbon.

0223It will be appreciated that various changes and modifications can be made from the

0224specific details of the invention as incorporated in the foregoing Examples without departing from the spirit and scope thereof as defined in the appended claims. In its

0225essential details, the invention is a continuous chemical vapor deposition process for laying down tin oxide and titanium oxide coatings onto a glass substrate at high deposition rates through the use of the corresponding metal tetrachloride and an organic compound used as

0226a source of oxygen in a preformed precursor gas mixture.

0227The metal tetrachlorides are preferred sources of the respective metals because of the availability and cost of the raw material.

0228It has been found, especially when depositing titanium oxide coatings from titanium

0229tetrachloride, that, in order to form the metal oxide at the optimum deposition rates, it is desirable to use an organic oxygen containing compound which is an ester, particularly an

0230ester in which the group derived from the alcohol is an alkyl group with a β hydrogen.

0231Additionally, the decomposition temperature of the ester should not be greater than the

0232reaction temperature of the coating precursor gas mixture at the desired point of

0233application. Esters utilized in the precursor gas mixture that have a β hydrogen and appropriate decomposition temperatures will deposit the coatings at high deposition rates. The preferred group of esters used in practicing the present invention includes the group consisting of ethyl formate, ethyl acetate, ethyl propionate, isopropyl formate, isopropyl

0234acetate, n-butyl acetate, and t-butyl acetate.

0235In general an ester decomposes in a continuous fashion over a given temperature range. In the present invention, the thermal decomposition temperature of the ester is defined as the temperature at which the unimolecular decomposition rate constant of the ester is

02360.01/sec. The unimolecular decomposition rate constants of common esters such as ethyl acetate and t-butyl acetate are well known and can be found in the chemical literature. For ethyl acetate and t-butyl acetate, the thermal decomposition temperatures using the above

0237definition are 935 and 650° Fahrenheit (500°C and 344°C), respectively. One skilled in the

0238art will recognize that the choice of ester and specific deposition temperature employed will determine the optimum coating growth rate. Reaction temperatures below the defined thermal decomposition temperature, but within the decomposition range of the selected

0239ester, will result in lower coating growth rates.

0240In accordance with the present invention, the alkyl group of an ester used in the coating precursor gas mixture may be a carbon compound having a range of 2-10 carbon atoms.

0241The lower limit of the range is dictated by the β hydrogen requirement on the alkyl group.

0242The upper limit is to avoid flammability and volatility issues that arise when the alkyl group

0243contains more than ten carbon atoms.

0244In practising the method of the present invention, a manifold may be used to connect and regulate the individual gas streams to formulate the coating precursor gas mixture. A

0245common delivery line may be used to deliver the precursor gas mixture from the manifold to the gas beam distributor. An in line static mixer may be used in the delivery line to ensure a

0246homogeneous gas mixture. Additionally, the baffles in the gas distributor beam, illustrated in Fig. 3, or a gas flow restrictor as described with reference to Figure 4, may provide further mixing of the precursor gas at the reactor stage.

0247In many of the Examples, oxygen was included in the coating precursor gas mixture.

0248However, the deposition rate of the metal oxide coating was not sensitive to the oxygen concentrations, and no oxygen gas was used in Examples 6 or 7 showing the inclusion of oxygen to be unnecessary

0249The concentration of the reactive components of the coating precursor gas mixture may

0250be selected to obtain the optimum coating growth rate. The concentration of metal tetrachloride is generally 0.1 to 5.0 percent by volume in the precursor gas mixture. The concentration of metal tetrachloride is based upon the amount of metal needed to provide the desired coating thickness in the available residence time. Thus the metal tetrachloride

0251concentration is adjusted according to process variables, such as the line speed of the ribbon

0252in a float glass process.

0253The concentration of the organic oxygen compound in the coating precursor gas

0254mixture is generally one to five times the concentration of the metal tetrachloride, being selected within this range based upon the deposition temperature. When using an ester, lower deposition temperatures will result in slower ester decomposition rates and therefore,

0255will require greater concentrations of the ester to react with the metal tetrachloride. In

0256Examples 6 and 7, the optimum concentration of the ethyl acetate in the precursor gas mixture is 1 to 3 times the concentration of the titanium tetrachloride. Concentrations above or below the optimum range will produce metal oxide coatings at lower coating

0257growth rates. The temperature of the precursor gas mixture is critical for control of the reaction, in particular to avoid undesirable pre-reaction or adduct formation resulting in formation of an involatile product in the precursor lines. In one preferred embodiment, especially applicable

0258when using an ester, the temperature is maintained above 300°F/150°C in the precursor gas lines. The precursor gas mixture is also preferably kept below the thermal decomposition temperature of the organic oxygen compound to prevent prereaction of the mixture.

0259The present inventive process utilizes the heat from the substrate to initialize the coating reaction. In on-line situations, such as the float glass process, the substrate is formed at extremely high temperatures. Therefore, the method of the present invention may be applied at a point in the float glass process where the substrate temperature is

0260lowered but is still above the temperature at which the coating is formed (and preferably

0261after the glass ribbon has substantially finished stretching i.e. below 1380°F/750°C). Offline applications of the present invention will require heating the substrate to a temperature above the decomposition temperature of the ester.

0262In practising the method of the present invention in the float glass process, the preferred point of application is in the float bath section. The temperature range at the point of application for the coating is usually about 1100°-1320^oF/590°-715^oC. The

0263temperature is an important operating parameter because it influences the concentration of

0264the organic compound utilized in the precursor gas mixture. The temperatures of the substrate in the float bath section are relatively stable and therefore exhibit little variation at the point of application. In examples 6 and 7 using ethyl acetate, the preferred substrate

0265temperature range is 1100°-1250^oF/590^oC-680°C. The heat from the substrate raises the temperature of the precursor gas mixture above

0266the temperature required for coating formation (and when an ester is used as the organic compound above thermal decomposition temperature of the ester). The metal deposition reaction may be initiated by the decomposition of the organic oxygen compound. When

0267titanium tetrachloride is used in combination with an ester having an alkyl group with a β

0268hydrogen, the titanium oxide coating then forms on the substrate at decomposition rates that are ten times higher than known coating methods. In the on-line application with a

0269float glass ribbon process, the ribbon passes under the gas distributor beam at a relatively

0270fast rate. The metal oxide coating is deposited onto the float glass ribbon as the ribbon passes under the coater.

0271The inventors propose the following theory regarding the chemical reaction that may

0272take place when using an ester having an alkyl group with a β hydrogen. However, the

0273inventors do not wish to limit the invention to just this possible explanation, and therefore offer it merely as an aid to understanding the results of the present inventive process.

0274The inventors propose that as the ester decomposes, the carbon-hydrogen bond on one

0275of the β hydrogens breaks and the hydrogen transfers to the carbonyl group eliminating an

0276alkene and forming a caboxylic acid. The hydrolysis reaction simultaneously takes place between the carboxylic acid and the metal tetrachloride leading to the formation of the

0277metal oxide coating on the substrate.

0278In general, the resulting article produced in accordance with the present invention

0279comprises a substrate having a titanium oxide or tin oxide coating. The coating may be applied directly to the substrate or as a layer in a plurality of coatings on a substrate. The

0280rate of deposition of the metal oxide coating is effected by the decomposition rate of the organic oxygen compound. At constant reaction temperatures different organic oxygen compounds will provide different coating growth rates because of the difference in the

0281decomposition temperatures. Therefore, the desired metal oxide coating growth rate for a

0282given system is selected by matching a specific organic oxygen compound to the precursor gas mixture temperature and the substrate temperature at the point of application.

0283The deposition rate of the titanium oxide coating in the present invention may be ten

0284times greater than rates in known deposition methods. The present inventive process

0285permits deposition rates over 13θA per second with some deposition rates measured well over 30θA per second. The higher deposition rates for titanium oxide yield a coating with a refractive index greater than 2.4.

0286A further advantage of the invention, in addition to the high coating rates achievable, is

0287that it employes low cost metal precursor compounds and, especially when the precursor gas mixture is directed over the substrate under the preferred laminar flow conditions, it enables high conversion efficiency (of the metal tetrachloride) to be achieved.

0288In the present invention, the resulting oxide coating contains little residual carbon from the decomposition of the organic oxygen compound, especially when an ester is used.

0289Carbon is an undesirable byproduct of the coating reaction because high levels of carbon in

0290deposition coatings create absorption problems with the coating. The concern in using an organic oxygen compound in the coating precursor gas mixture is that decomposition will

0291result in levels of carbon that adversely affect the absoφtion properties of the finished glass.

0292The carbon content in the coatings produced from the method of the present invention

0293showed less than four atomic percent of carbon where measured. This low level of carbon will not significantly affect absoφtion properties of the coating. It is to be understood that the forms of the invention herewith shown and described are to be taken as illustrative embodiments only of the same, and that various changes in the shape, size and arrangement of parts, as well as various procedural changes, may be resorted to without departing from the spirit of the invention.