COATING FLAT GLASS BY CHEMICAL VAPOR DEPOSITION

Abstract: In a method and apparatus for coating flat glass by chemical vapour deposition, a stream of inert gas is induced to flow over the surface, adjacent the glass, of diverting means used to divert a flow of used coating gas away from the glass surface being coated. The stream of inert gas avoids build-up of deposits from the used coating gas on the said surface of the diverting means, and permits operation for extended periods without the need to stop and remove such deposits.

Description: The invention relates to the coating of flat glass and in particular to a method of coating flat glass by deposition of a coating from a coating gas and to an apparatus suitable for use in operating the method. In the coating process described in EP 0213045, which may be used for applying a silica containing coating onto glass, reactant gases are directed through gas supply ducts downwardly towards a horizontally advancing hot glass substrate in a coating chamber defined between walls constituted by vertical plates on the upstream and downstream (with reference to the direction of glass advance) sides of the gas supply ducts. Used gases are extracted upwardly through upstream and downstream exhaust channels, the upstream wall of the upstream channel being defined by the wall forming the upstream wall of the coating chamber and the downstream wall of the downstream channel being defined by the wall forming the downstream wall of the coating chamber. Extraction of used coating gas through these exhaust channels results in external gases being sucked in under the lower extremities of the plates forming the vertical walls of the coating chamber. No provision is made for smooth flow of the external gases under the chamber walls so the gases mix with the coating gas in the coating chamber, with consequent risk of disrupting the uniformity of that flow and the resultant coating. It is known to coat flat glass by chemical vapour deposition, and UK patent specification 1 507 996 describes a method and apparatus in which a coating gas is directed on to the surface of a hot ribbon of glass to be coated from a distributor extending across that surface transverse to the direction of movement of the ribbon, so that the gas flows substantially parallel to the glass under laminar flow conditions and uniformly across the width of the surface. A coating is deposited on the hot glass surface and the used coating gas is directed upwardly away from the glass surface along an exhaust channel formed between upstream and downstream guide walls. An extractor duct is provided in the region of the upper end of the exhaust channel to collect used coating gas issuing therefrom, but is not connected to the exhaust channel. When the method and apparatus described in UK patent specification 1 507 996 is used for the deposition of a reflecting silicon coating on a ribbon of hot glass, it is found that, after a period of operation, a silicon containing deposit builds up on the undersurface of the downstream toe, that is, the surface, opposed and adjacent the glass, of a guide member which lies in the path of the coating gas over the glass, limiting the coating zone at its downstream end and diverting used gas away from the glass. This deposit eventually touches the glass and damages the coating formed on the glass, and the coating process has to be interrupted while the apparatus is removed from over the glass and cleaned to remove the deposit. It has been found that build-up of the deposit can be reduced or avoided, and satisfactory coating maintained over significantly longer periods, by deliberately passing a stream of inert gas over the said surface adjacent the glass. According to the present invention, there is provided a method of coating flat glass by chemical vapour deposition wherein a coating gas is caused to flow over a hot glass surface in a coating zone and used gas is directed away from the glass at the downstream end of the coating zone along an exhaust channel by diverting means including a surface adjacent the glass surface and a surface defining the downstream wall of the exhaust channel which is continuous with said surface adjacent the glass characterised in that a stream of inert gas is induced over said surface of the diverting means to inhibit formation of a deposit from the used gas on said surface by suction applied to the exhaust channel and the diverting means is shaped for smooth flow of the inert gas over said surfaces of the diverting means without mixing the inert gas and the used coating gas in the coating zone. The flow of inert gas reduces or prevents the formation of a deposit from the used gas which would otherwise occur on the said surface adjacent the glass surface. The volume flow rate of the inert gas over the said surface is preferably at least 50% of the volume flow rate of used coating gas directed away from the glass at the downstream end of the coating zone. The expression 'inert gas' is used herein to refer to a gas which does not itself deposit a coating on the glass and does not undergo any reaction detrimental to the coating process with the used coating gas. The stream of inert gas may be composed of the atmosphere surrounding the coating station at which the coating is applied. When the coating is applied to a ribbon of float glass as it is advanced over the float bath, the stream of inert gas used may convenienty be composed of the protective atmosphere of the float bath. The flow of inert gas reduces or prevents the formation of a deposit from the used gas which would otherwise occur on the said surface adjacent the glass surface. The method of the invention is useful for coating flat glass by chemical vapour deposition. The expression "chemical vapour deposition" is used herein to refer to processes in which the coating is formed by reaction of one or more gas phase sources. For example, the coating gas may be a gas, such as a metal carbonyl, which pyrolyses to deposit a coating on the glass, or a mixture of gases which react together, for example stannic chloride vapour and water vapour, to deposit a coating on the glass. The coating gas may contain a non-reactive component, for example nitrogen, as a carrier. The method has been found to be especially useful when using a silane-containing gas to apply a coating to hot glass. Silane containing gases may be used to deposit a reflecting silicon coating as described in UK patents GB 1 507 465 and GB 1 507 996, or a coating containing silicon and oxygen and having a high light transmission, useful as a barrier coating to prevent migration of alkali metal ions from the glass. The formation of certain such barrier coatings is described in published UK specification GB 2 163 146A. According to a further aspect of the invention, there is provided an apparatus for coating flat glass by deposition of a coating from a coating gas comprising a support for the glass to be coated, means for advancing the glass over said support, a gas supply duct extending transversely of the path of travel of the glass arranged to direct a coating gas onto the hot glass surface in a coating zone, diverting means including a surface adjacent the glass surface for diverting used gas away from the glass at the downstream end of the coating zone, an exhaust channel for flow of used coating gas away from the glass surface, and suction means for collecting used gas from the exhaust channel characterised in that the suction means is connected to the exhaust channel and the said surface of the diverting means is spaced from the path of travel of the glass for induction of a stream of inert gas over said surface by operation of the suction means, and the diverting means includes a surface defining the downstream wall of the exhaust channel continuous with said surface adjacent the glass and shaped for smooth flow of the inert gas thereover without mixing of the inert gas and the used coating gas in the coating zone. The surface adjacent the glass is preferably substantially parallel to the glass surface, and is preferably spaced at least 5 mm from the path of travel of the surface of the glass. This facilitates a flow of outside atmosphere between the glass surface and said surface, the atmosphere serving as the inert gas flowing over said surface and inhibiting the formation of a deposit from the used coating gas on said surface. In a preferred embodiment of the invention, the apparatus comprises a centre block and first and second side members disposed adjacent said centre block defining a substantially U-shaped guide channel for gas from the gas supply duct between the first side member and the upstream side wall of the centre block, between the bottom wall of the centre block and the path of the glass, and between the downstream side wall of the centre block and the second side member, the part of the guide channel between the downstream side wall of the centre block and the second side member constituting the exhaust channel. The invention is illustrated but not limited by the following description with reference to the accompanying drawings in which:- Figure 1 is a vertical section through a float glass manufacturing apparatus showing, in outline (and not to scale), the position of a coating apparatus for operation of the invention extending transversely of the path of travel of a ribbon of glass. Figure 2 is an enlarged vertical section through the coating apparatus shown in Figure 1. Figure 3 is an elevation of part of the coating apparatus of Figures 1 and 2 in the direction of arrow III in Figure 2 with the graphite block 37 and extractor 60 removed. Figure 4 is a front elevation of extractor 60 in the direction of arrow IV in Figure 2 with the horizontal guide members 56,57 of the extractor 60 removed. Figure 5 is a plan view of a blockage clearing device incorporated in extractor 60. Figure 6 is a section on the line VI-VI in Figure 5. Figure 1 illustrates molten glass 1 being delivered onto a bath of molten tin or tin alloy 2 contained in a tank structure 3. A ribbon of glass 4, narrower than the bath of the molten metal, is formed on the bath where it is supported by the molten tin. The ribbon is advanced along the bath and withdrawn from the bath over driven traction rollers 5 mounted beyond the outlet 6 of the bath. A protective atmosphere, for example 95% nitrogen and 5% hydrogen, is maintained at a plenum in the headspace 7 over the molten metal bath, being supplied through ducts 8 extending downwardly through the roof 9 and connected to a common header 11. A temperature gradient is maintained down the bath from a temperature of about 1000°C at the inlet end 12 of the bath to a temperature in the range from 570°C to 650°C at the outlet 6 where the ribbon is discharged from the bath. A gas distributor, generally designated 14, for supplying coating gas to the surface of the glass ribbon is located transversely of the path of travel of the ribbon towards the outlet end of the bath. The distributor thus extends across the upper surface of the ribbon of glass and transversely of the path of travel of the glass. As illustrated in Figure 2, the gas distributor 14 comprises an inverted U-section channel member 21 having side walls 22,23 and a top wall 24. The channel within member 21 is divided by a vertical partition 25 welded to the top wall 24 of member 21. A horizontal member 26 is welded to the base of side wall 23 and extends inwardly and outwardly therefrom; the inwardly extending section of member 26 forms, with partition 25, side wall 23 and upper wall 24, a rectangular duct 27. Horizontal members 28,29 extend inwardly from side wall 22 and vertical partition 25 respectively and define between them an elongated aperture 30. A second smaller inverted U-section channel member 31 is centrally located between side wall 22 and partition 25, with its lower edges welded to horizontal members 28,29. The two inverted U-section channel members 21,31 define between them, together with horizontal members 28 and 29, a U-section duct 32 for the passage of a cooling liquid, and rectangular duct 27 serves as a return duct for the cooling liquid. The interior face of U-section channel member 31, together with the bevelled ends of horizontal members 28,29 defines a coating gas supply duct 33 which opens through elongated aperture 30 into a gas flow restrictor 34. The gas flow restrictor 34 is of the kind described in UK patent specification GB 1 507 996 and, for further details and illustrations, reference should be made to that specification. It 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 (that is across the width of the path of the glass to be coated). Adjacent crimped metal strips are arranged "out of phase" with each other 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 the gas supply duct 33, so that when coating gas is supplied under pressure to the duct 33, the pressure drop along the duct 33 is small compared to the pressure drop through the vertical channels, and the gas flow restrictor 34 provides release of coating gas from the gas supply duct 33 at substantially uniform pressure along the length of the duct. Upstream, centre and downstream, shaped graphite blocks 35,36 and 37 respectively extend across the width of the glass to be coated. Upstream graphite block 35 is secured by bolts (not shown) to horizontal member 28, while the centre graphite block 36 is secured by bolts (not shown) to horizontal members 26 and 29. Downstream graphite block 37 is similarly secured by bolts 48 passing through distance pieces 40 (Figures 2,3) spaced across the width of the distributor to spaced steel blocks 38 welded to the outer face of side member 23 of the U-section channel member 21. An upper series of spaced steel blocks 39 is welded to the outer face of side member 23 of U-section channel member 21 above the lower steel blocks 38. A fourth shaped graphite block 41 stands on the back upper edge of centre block 36 and is secured by bolts 49 to the lower steel blocks 38. A fifth shaped graphite block 42 stands on the top of block 41 and is secured by bolts 50 to the upper steel blocks 39. The graphite blocks 41 and 42 extend across the width of the glass to be coated. The graphite blocks 35 and 37 constitute first and second side members which together with the centre block define a substantially U-shaped guide channel 47a, 47b, 47c for coating gas flowing from the gas supply duct 33 through the gas flow restrictor 34, between the graphite block 35 and the upstream side wall of the centre block, between the bottom wall of the centre block and the path of the glass, and between the downstream side wall of the centre block and the graphite block 37. The part 47b of the U-shaped guide channel defined between the bottom wall of the centre block and the path of the glass constitutes the coating zone. The graphite block 37 has a surface 43 opposed to and adjacent the glass surface on which a deposit from said used coating gas would tend to form, in the absence of the inert gas flow provided by the present invention. The block 37 has a convex curved surface 45 which is continuous with surface 43 and defines the lower part of the downstream wall of the exhaust channel. The shaped graphite blocks 41 and 42 define, with the downstream block 37, a continuation 44 of the exhaust channel. An extractor generally designated 60 is mounted downstream of the gas distributor to extract gas from the exhaust channel. The distributor is also provided with graphite end blocks, in contact with the lateral faces of shaped graphite blocks 35, 36, 37, 41 and 42, which define the lateral edges of channels 47a, 47b, 47c and 44, preventing escape of coating gas at the ends of the distributor. The extractor 60 (see Figures 2 and 4) comprises a square hollow section steel member 61 closed at its ends 62,63 with a slot 64 cut in its front face and terminating short of the ends. Horizontal guide members 56,57 with vertical flanges 58,59 abutting the vertical faces of carbon blocks 42,37 are welded on to the front face of member 61 above and below the slot to provide a gas flow path 55 forming a further continuation of the exhaust channel to the slot 64 in member 61. End plates in contact with the vertical edges of guide members 56 and 57 define the lateral edges of flow path 55 preventing escape of used gas from the edges of the distributor. A rectangular hollow section member 65 (Figure 2) with a slot 54 cut in its front face and terminating level with ends of slot 64 is mounted centrally in member 61 by means of steel rods 66. Horizontal plates 67 and 68 above and below the slots 64,54 are welded to the inner front face of member 61 and the outer front face of member 65. A horizontal plate 69 is welded to the outer rear face of member 65 and the inner rear face of member 61. The rectangular hollow section member 65 together with plates 68,69 defines the upper side of a lower channel 70 for cooling water flow through the member 61. The member 65 with plates 67,69 defines the lower side of an upper channel 71 for the return flow of cooling water. Inlet and outlet ducts 72,73 open into channels 70,71 respectively. Rectangular hollow section member 65 constitutes a water-cooled gas extraction duct 74 across the width of the coating apparatus and is connected by exhaust ducts 75,76 (Figure 4) to a low pressure vacuum pump. As shown in Figure 4, the slot 64 in member 61 is blanked off by spaced plates 77 over the front of the channel. The blanking plates 77 are of reduced length towards the centre of the slot in order to ensure, so far as possible, a uniform suction effect along the length of the extractor when suction is applied through exhaust ducts 75,76. Horizontal steel strips 78,79 are welded to the front face of member 61 above and below the blanking plates 77 and carry slotted plates 80 with slots 81 (not all numbered) over the openings between blanking plates 77 in slot 64. The slots 81 provide a gas flow path from the exhaust channel between the carbon blocks and horizontal guide members 56,57 through slot 64, into the channel 82 defined between horizontal plates 67,68 within member 61 and thence into extraction duct 74 in rectangular hollow section member 65. In use the slots 81 in slotted plates 80 tend to become partially blocked with solid deposited from the used coating gas. To clear such deposits, the slots are provided with pins 83 which are mounted for reciprocation in the slots. Each of the pins 83 is mounted in a block 84 (see Figures 2, 5 and 6) and extends through the corresponding slot 81. The blocks 84 are joined by upper and lower horizontal steel strips 85,86 embedded in slots in the blocks 84. The strips 85,86 are located between the inner front face of rectangular section 65 (see Figure 2) and a support structure 87 comprising upper and lower horizontal plates 88,89 spaced by vertical rods 90 within the rectangular section 65. Horizontal members 85,86 are connected to handles 91,92 extending beyond the ends of section 65 (Figure 4). Reciprocation of the strips 85, 86 within member 65 by means of handles 91,92 moves the pins 83 to and fro within the slots 81 in plates 83 removing any deposits from the sides of the slots and preventing any blockage that might otherwise occur. The extractor 60 connected to the low pressure vacuum pump (not shown) constitutes suction means for withdrawing gas along the exhaust channel, and, through the use of blanking plates 77 of varying length, is adapted to provide a uniform suction effect across the width of the exhaust channel. Each of the graphite blocks 35 and 37, like the centre block 36, is spaced from the path of travel of a glass ribbon over the float bath. The optimum gap between surface 43 of the bottom of the block 37 and the path of travel of the glass surface will vary with the coating gas used and the coating conditions, but will usually be at least 5 mm to allow a flow of bath atmosphere to be drawn between surface 43 and the glass surface. In the apparatus shown in Figure 2, the second side member constituted by graphite block 37 terminates short of the path of travel of the surface of the glass by a distance which is about equal to the height of U-shaped guide channel 47b in the coating zone between the bottom wall of the centre block and the path of travel of the surface of the glass. Thus, the volume flow rate of the inert gas constituted by the bath atmosphere over the surface 43, between surface 43 and the glass surface, and into the exhaust channel is similar to the volume flow rate of used coating gas into the exhaust channel. In use, the rate of supply of coating gas is adjusted to provide laminar flow of the coating gas parallel to the glass surface under the centre block 36. At the same time, the height of the block 37 is selected, together with the suction applied by the extractor 60 to the exhaust channel, to withdraw a stream of bath atmosphere between the lower surface 43 of block 37 and the glass ribbon 4. The curvature of surface 45, which is continuous with surface 43 of block 37, is similar to and the approximate mirror image of the curvature of the downstream wall of the centre block 36, and provides a smooth flow of bath atmosphere thereover without mixing of the used coating gas and the bath atmosphere in the coating zone or in the lower part of the exhaust channel, which might disrupt the uniform laminar flow of coating gas parallel to the glass surface under the centre block 36, and hence affect the uniformity of the coating. The slots 81 for exhaust of the used coating gas in extractor 60 are spaced so as to ensure that substantially uniform suction is applied to the exhaust channel across the width of the glass ribbon being coated, so maintaining uniform coating across the width of the glass. When necessary, handles 91 and 92 are reciprocated to move pins 83 to and fro in slots 81 and prevent deposits from the coating gas building up and blocking the slots. The invention permits the coating process to be used for extended periods without the need for continued interruptions to clean deposits off the surface of the diverting means which directs the used coating gas away from the glass surface. For example, the apparatus described was used to coat 6 mm float glass with a reflecting silicon coating for a continuous period of 17 hours without any need to stop the process to remove deposits of silicon from the apparatus. The expression "used coating gas" is used herein to refer to a gas which has been used to coat the glass. It does not imply that all reactive components in the gas have necessarily been used and, indeed, the used coating gas removed through the exhaust channel may contain significant amounts of unreacted coating components.

Claims: A method of coating flat glass by chemical vapour deposition wherein a coating gas is caused to flow over a hot glass surface in a coating zone and used gas is directed away from the glass at the downstream end of the coating zone along an exhaust channel by diverting means including a surface adjacent the glass surface and a surface defining the downstream wall of the exhaust channel which is continuous with said surface adjacent the glass characterised in that a stream of inert gas is induced over said surface of the diverting means to inhibit formation of a deposit from the used gas on said surface by suction applied to the exhaust channel and the diverting means is shaped for smooth flow of the inert gas over said surfaces of the diverting means without mixing the inert gas and the used coating gas in the coating zone. A method according to claim 1, wherein said stream of inert gas is composed of the atmosphere surrounding the coating station at which the coating is applied. A method according to claim 2, wherein the coating is applied to a ribbon of float glass as it is advanced over the float bath and the stream of inert gas is composed of the protective atmosphere of the float bath. A method according to any of the preceding claims, wherein the reactant gas comprises a silane. A method according to claim 4, wherein a reflecting silicon coating is deposited on the glass. A method according to claim 4, wherein a coating comprising silicon and oxygen and having a high light transmission useful as a barrier coating to prevent migration of alkali metal ions from the glass surface is deposited on the glass. An apparatus for coating flat glass by deposition of a coating from a coating gas comprising a support (2) for the glass to be coated, means (15) for advancing the glass over said support, a gas supply duct (33) extending transversely of the path of travel of the glass (4) arranged to direct a coating gas onto the hot glass surface in a coating zone (47b), diverting means (37) including a surface (43) adjacent the glass surface for diverting used gas away from the glass at the downstream end of the coating zone, an exhaust channel (47c) for flow of used coating gas away from the glass surface, and suction means (60) for collecting used gas from the exhaust channel characterised in that the suction means (60) is connected to the exhaust channel (47c) and the said surface (43) of the diverting means is spaced from the path of travel of the glass (4) for induction of a stream of inert gas over said surface by operation of the suction means (60), and the diverting means (37) includes a surface (45) defining the downstream wall of the exhaust channel continuous with said surface (43) adjacent the glass and shaped for smooth flow of the inert gas thereover without mixing of the inert gas and the used coating gas in the coating zone. An apparatus according to claim 7, wherein the said surface (43) adjacent the glass is spaced at least 5 mm from the path of travel of the surface of the glass. An apparatus according to claim 7 or claim 8, comprising a centre block (36) and first (35) and second (37) side members disposed adjacent said centre block defining a substantially U-shaped guide channel (47a, 47b, 47c) for gas from the gas supply duct between the first side member (35) and the upstream side wall of the centre block (36), between the bottom wall of the centre block (36) and the path of the glass (4), and between the downstream side wall of the centre block (36) and the second side member (37), the part (47c) of the guide channel between the downstream side wall of the centre block and the second side member constituting the exhaust channel. An apparatus according to claim 9, wherein the surface of the second side member (37) adjacent the glass is spaced from the path of travel of the surface of the glass by a distance about equal to the height of the U-shaped guide channel (47b) between the bottom wall of the centre block (36) and the path of travel of the surface of the glass. An apparatus according to claim 10, wherein said distance is at least 5 mm.

Inventors: BERRY RONALD FRANK, BERRY, RONALD FRANK
Applicants: PILKINGTON PLC
Application year: 1988
Main classification: C03C17
IPC codes: C23C16, C03C17