GB2131792A - Vitreous material bearing a multi-layer coating and method and apparatus for forming such coating - Google Patents

Abstract

A hot vitreous substrate e.g. a freshly formed ribbon of glass 1 is provided with a thin subbing coating 6 and one or more overcoating layers 8 by conveying the ribbon through a succession of coating stations 5, 7. In the first-coating station 5, the subbing coating 6 is formed by contacting the substrate 1 with a cloud of underlayer precursor material formed by an ultrasonic generator e.g. a resonant cavity pulverisator 11. And upper coating layers such as 8 is formed by directing a stream of precursor material to contact the substrate 1. Heaters 12 are optionally provided in the cloud chamber 5 and subbing layer precursor reaction products are aspirated through ducting 21. <IMAGE>

Description

SPECIFICATION Vitreous material bearing a multi-layer coating and method and apparatus for forming such coating The present invention relates to a process of and apparatus for forming a coating on a hot vitreous substrate by chemical reaction and/or decomposition of coating precursor material, by conveying the hot substrate through a succession of coating zones to form a said coating which comprises an underlayer and at least one thicker upper layer. The invention includes vitreous material coated by such a method.

This invention is principally, but not exclusively concerned with bilayer coatings in which the upper layer is a metal oxide layer.

For example British Patent specification No.

1,049,922 (Compagnie de Saint Gobain) describes in Example 2 a process in which a glass tube is conveyed vertically and successively through two compartments with a liquid seal between them, the compartments being supplied respectively with aerosols containing SnCI4 and chromyl chloride for the deposition of a coating having a stannic oxide underlayer and a chromic oxide upper layer. These two layers are applied so that by varying their respective thicknesses the resistivity of the coating may be altered.

The present invention on the other hand is principally concerned with the optical properites of multi-layer coatings.

There is a demand for glass products, in particular for flat glass, which bears a coating of high optical quality which modifies the radiation transmitting characteristics of the product but causes little or no diffusion of transmitted light.

Any significant light diffusion in a transparent product is apparent as haze.

Haze can arise because of defects in the coating. Such defects broadly fall into three categories. Those at the coating/substrate interface which may be caused by spurious deposits of coating precursor decomposition products or reaction of those products with the substrate, those which are internal of the coating and may be considered as being due to a nonuniform crystal structure within the coating, and those at the coating surface which may be caused by environmental contamination downstream of the coating station.

It has been found that the quality of many coatings can be improved by providing an underlayer beneath a thicker main coating layer.

BFG Glassgroup's British Patent specification No. GB 2 033 374A relates particularly to the formation of low-haze bi-layer oxide coatings, and teaches a process in which a tin oxide coating is formed on a hot glass substrate by chemical reaction and/or decomposition of tin halide supplied in the vapour phase, the substrate being conveyed through two successive coating zones, in the first of which it is contacted with an acetylacetonate of alkylate of titanium, nickel or zinc to cause deposition of a metal oxide underlayer on the substrate, and in the second of which such underlayer is contacted by a gaseous, tin halide containing medium to form a thicker tin oxide upper layer.

When using such a process, it is suggested, tin oxide coatings of 700 nm thickness or even more can be produced with very low haze, less than 2% of transmitted light being diffused.

One particular reason for providing such an underlayer is to shield the vitreous substrate from the larger quantity of reaction products arising when the main coating layer is formed. Because the main coating layer is thicker it will contain more coating material and this implies that the quantity of waste products formed in the coating reaction will be greater for the main layer than for the underlayer. For example in the case where a tin oxide coating layer is formed from a tin halide (for example a tin chloride) the hydrogen halide and/or free halogen will be generated as a reaction product and this could attack the vitreous substrate in the absence of an underlayer. It is desirable for the underlayer to be thin especially where the reactions forming the underlayer give rise to active reaction products.For example the underlayer may be formed using coating precursor material which has the same composition as that used for forming a thicker main coating layer. In some circumstances it is also possible for the underlayer to have an effect on the uniformity and/or adherence of the main coating layer. For this reason, it is desirable that the underlayer formed should have a uniform structure.

It is an object of the present invention to provide a method and apparatus for forming multilayer coatings on hot vitreous substrates in which the coating comprises a thin, uniform undercoating layer.

According to the present invention, there is provided a process of forming a coating on a hot vitreous substrate by chemical reaction and/or decomposition of coating precursor material by conveying the hot substrate through a succession of coating zones to form a said coating which comprises an underlayer and at least one thicker upper coating layer, characterised in that, in the first coating zone, the substrate is contacted by an ultrasonically generated aerosol cloud of underlayer precursor material to form such underlayer, and in that in the or a later coating zone, the substrate is contacted by a stream of fluid medium comprising upper coating layer precursor material.

Using an ultrasonically generated aerosol cloud precursor material for forming a said underlayer affords considerable advantages. Such a cloud is formed of micro-droplets which have a narrow size range distribution. This promotes the reliable formation of an underlayer which is thin and uniform.

When spraying fluid through an atomising nozzle, the characteristics of the spray formed are dependent to a considerable extent on the rate of flow through the nozzle. When an ultrasonic pulverisator is used for aerosol generation however, the characteristics of the aerosol cloud formed are substantially independent of the rate of flow of carrier gas used. Thus, since it is only necessary to form a thin underlayer, and indeed since a thin underlayer is desirable, smaller quantities of carrier material can be used than when applying the precursor material in some other way such as by spraying through one or more atomising nozzles.This implies a reduced requirement for fluid material at the first coating zone and this in turn contributes to economy and furthermore means that, generally speaking, the substrate will be cooled less as a result of the undercoating process so that a subsequent coating layer can be formed more easily. A further point to be noted is that where an aerosol cloud chamber defines the first coating zone, this can occupy a shorter length of the substrate path than other types of coating station for applying a coating of similar thickness, for example a vapour coating station. This is clearly advantageous in circumstances in which the space available for applying the coating is limited. In fact this is often the case. Many flat glass production lines were laid down before the demand for coated or multilayer coated glass reached its present level.For reasons of heat economy it is often desirable to apply coatings of the type with which the present invention is concerned to a freshly formed ribbon of hot glass between the exit from a flat glass forming station and the entrance to an annealing lehr, especially in large scale production. Many such installations were originally laid out without consideration of the present demand for coated glass and hence space between the glass forming station and the lehr is limited.

A further advantage of reducing the quantity of fluid material at the first coating zone which is enabled by the present invention is that there is a reduced requirement for aspirating spent or partially spent material from the undercoating zone so that when aspirating means is present (as is preferred) such means can be smaller than would be the case where large quantities of precursor material to be used at that first coating zone.

When the underlayer has been formed, a thicker upper coating layer can easily be applied by directing a said stream of coating precursor material onto the under-layer to form a coating of high optical quality. Advantageously, the aerosol cloud of said undercoating precursor material is formed using a resonant cavity pulverisator.

Precursor material for forming the or a said upper layer may be applied in the vapour phase or in the liquid phase, that is, in solution, according to which is most convenient for the purposes particularly in view. These are both very convenient ways of applying sufficient quantities of coating precursor material to form a said thicker upper layer. In general however, it is preferred that the or at least one said upper coating layer is applied by directing a stream of droplets comprising upper coating layer precursor material into contact with the undercoated substrate.

Relatively thick layers are more easily applied in this way.

The invention is especially applicable in cases where there is a single said upper layer, and is principally, but not exclusively, concerned with the formation of a said coating wherein the or each upper layer comprises a metal oxide, preferable tin oxide.

A said underlayer may be formed using an organic compound of titanium, zirconium, tin, zinc, magnesium or nickel to leave an underlayer of a respective metal oxide. The use of organic compounds as coating precursor materials is preferred because they do not give rise to corrosive reaction products.

Preferably, said underlayer is formed by contacting the hot substrate with an acetylacetonate or alkylate of titanium, nickel or zinc to cause deposition of a metal oxide underlayer.

The acetylacetonates are particularly advantageous because they lead to the formation of an underlayer composed of a large number of very small crystals which is a condition helpful to the formation of a superimposed tin oxide coating having a high proportion of favourably oriented crystals. The acetylacetonates will yield underlayers of favourable crystallographic characteristics for promoting preferential growth of the subsequently formed tin oxide crystals in a direction normal to the substrate. The acetylacetonates have the additional advantage that they are not susceptible to hydrolysation and can be supplied to the substrate in a moist environment without giving rise to premature decomposition.

An alternative to using an acetylacetonate of the selected metal for the underlayer precursor is to employ an alkylate. The alkylates also yield underlayers composed of a large number of small crystals. When an alkylate is used the environment in the coating zone should be dry. Examples of alkylates which can be used include the ethylates, propylates and butylates of titanium and of nickel.

The or a said metal oxide upper layer is preferably formed by contacting the still hot undercoated substrate with a fluid medium comprising a metal halide, preferably a metal chloride. Metal halides, particularly chlorides can be readily pyrolysed to give rise to an oxide coating layer.

Advantageously, the aerosol cloud is formed in or projected into a cloud chamber which is open to the path of the substrate and which is heated. The use of such a chamber confines the aerosol cloud to the desired region and reduced heat loss from the substrate during formation of the underlayer thus facilitating the later coating operation or operations.

It is preferred that gases emanating from said first coating zone are aspirated from the path of the substrate at least at a region which is located between that coating zone and the next successive coating zone.

Large scale production is facilitated and heat economies are afforded when the method of the invention is performed for coating a still hot continuous ribbon of flat glass in the course of its conveyance from a flat glass forming station, for example as it moves from a float tank to an annealing lehr.

The invention includes apparatus for forming a coating on a hot vitreous substrate by chemical reaction and/or decomposition of coating precursor material comprising means for conveying the hot substrate through a succession of coating stations of which the first coating station is adapted to supply coating precursor material at a lower rate than the or at least one other station so that said coating is formed, in coating zones defined by said coating stations, with an underlayer and at least one thicker upper coating layer, characterised in that said first coating station comprises means for delivering coating precursor material including an ultrasonic pulverisator for forming a cloud of such precursor material and a cloud chamber which is open to the path of conveyance of said substrate, and in that the or at least one layer coating station includes means for directing a stream of fluid medium comprising upper coating layer precursor material into contact with an undercoated substrate on said path.

Such apparatus preferably includes one or more of the following optional features: (i) said ultrasonic pulverisator is a resonant cavity pulverisator; (ii) the or at least one later coating station includes means for directing a stream of droplets comprising upper coating layer precursor material into contact with an undercoated substrate on said path: (iii) there are provided only two said coating stations; (iv) said cloud chamber incorporates one or more heaters; (v) means is provided for aspirating gases emanating from said cloud chamber at least at a region which is located between that cloud chamber and the next successive coating station; (vi) said coating stations are located between a flat glass forming station and an annealing lehr for annealing a continuous ribbon of flat glass formed therein; (vii) said flat glass forming station comprises a float tank.

Preferred embodiments of the present invention will now be described in greater detail with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a side view of an embodiment of apparatus for performing the present invention; Figure 2 is a view along the line Il-Il of Figure 1,and Figure 3 is a sectional view of an aerosol generator for use in performing the present invention.

In Figure 1, a freshly formed ribbon of hot glass 1 is conveyed on conveyor rolls 2 from a glassforming machine 3 to an annealing lehr 4 via a first coating station 5 where an underlayer 6 is applied to the glass ribbon 1 , and one or more further coating stations generally indicated at 7 where at least one thicker upper coating layer 8 is applied on top of the underlayer 6.

The space between the glass-forming machine 3 which may for example of a float tank and the annealing lehr 4 may if desired be closed by a refractory sole wall 9, and refractory side walls and a roof (not shown).

As also shown in Figure 2, the first coating station 5 comprises a cloud chamber 10 open to the path of the glass ribbon 1, and an ultrasonic pulverisator 11 for forming an aerosol cloud.

Optional heaters 12 are provided within the cloud chamber 10.

As shown in Figure 2, the ultrasonic pulverisator 11 is located in a side passage 1 3 leading to the cloud chamber 10 so that it is not directly heated by the hot glass ribbon 1 or by the heaters 12 when present, and the aerosol generated thereby is passed into the cloud chamber 10 by means of a fan 14 which may for example impart a uniform helicoidal swirling movement to the aerosol cloud.

As shown in Figure 1, the first coating station 5 is surrounded by an aspirating hood 21 for drawing off gases emanating from that coating station. Unused coating precursor material drawn off in this way can be recycled if desired.

As shown in Figure 3, such an ultrasonic pulverisator 11 may consist of a main body portion 1 5 and a cup portion 1 6 fastened thereto by struts 1 7. The main body portion 1 5 defines a central passageway 1 8 for feeding with compressed gas, surrounded by a coaxial passageway 19 for feeding by the liquid to be atomised. That liquid is fed from the outer passageway 19 to the central passageway 1 8 via a plurality of branch passages 20 so that it is projected against the cup portion 1 6 of the pulverisator whence it rebounds along its path and is formed into an aerosol.

Such a pulverisator is available from Giesler S.A., F-i8260 Acheres, France under the trade name SONICORE.

It will be appreciated that such an apparatus may be used for coating hot sheets of glass or indeed of other vitreous material if desired.

The further coating station or stations indicated at 7 in Figure 1 may for example be one or more spray coating stations as is well known in the art, for example from British Patent Application No.

82 12 670 in the name of Glaverbel.

In a specific practical example, a float glass ribbon 1 travelling from a float tank 3 at 4.5 m/minute has been provided with an underlayer 30 nm thick of titanium dioxide. The glass ribbon was at about 6000C when it entered the first coating station 5, and it was 2.5 metres in width.

Titanium acetylacetonate as coating precursor was fed to the pulverisator 11 at a rate of 3.7 kg/hour. The compressed gas used for forming the aerosol was air. The titanium acetylacetonate was uniformly dispersed inside the cloud chamber 10 and on contact with the hot glass it reacted to form a uniform adherent coating layer of titanium dioxide.

After the formation of the underlayer, a thicker upper coating layer was formed in a further coating station in a manner known per se. In a particular example, a coating of fluorine doped tin oxide was depositied by spraying an aqueous solution of tin chloride (SnCI2.2H2O) containing small quantities of ammonium bifluoride (NH4.HF2) onto the underlayer on the ribbon of hot glass.

This solution was sprayed, using air as carrier gas, from a nozzle located 25 cm above the path of the ribbon and pointing in the direction of advance along that path with the nozzle axis inclined downwardly at 30O to the horizontal. The nozzle was displaced to and fro across the ribbon path at a frequency set to build up a tin oxide coating 700 nm thick by pyrolysis.

Claims (22)

1. A process of forming a coating on a hot vitreous substrate by chemical reaction and/or decomposition of coating precursor material by conveying the hot substrate through a succession of coating zones to form a said coating which comprises an underlayer and at least one thicker upper coating layer, characterised in that, in the first coating zone, the substrate is contacted by an ultrasonically generated aerosol cloud of underlayer precursor material to form such underlayer and in that in the or a later coating zone, the substrate is contacted by a stream of fluid medium comprising upper coating layer precursor material.

2. A process according to Claim 1 , wherein the aerosol cloud of said underlayer precursor material is formed using a resonant cavity pulverisator.

3. A process according to Claim 1 or 2, wherein the or at least one said upper coating layer is applied by directing a stream of droplets comprising upper coating layer precursor material into contact with the undercoated substrate.

4. A process according to any preceding claim, wherein there is a single said upper layer.

5. A process according to any preceding claim, wherein the or each upper layer comprises a metal oxide, preferably tin oxide.

6. A process according to Claim 5, wherein said underlayer is formed by contacting the hot substrate with an acetylacetonate or alkylate or titanium, nickel or zinc to cause deposition of a metal oxide undercoating.

7. A process according to Claim 5 or 6 wherein the or a said metal oxide upper layer is formed by contacting the still hot undercoated substrate with a fluid medium comprising a metal halide, preferably a metal chloride.

8. A process according to any preceding claim, wherein the aerosol cloud is formed in or projected into a cloud chamber which is open to the path of the substrate and which is heated.

9. A process according to any preceding claim, wherein gases emanating from said first coating zone are aspirated from the path of the substrate at least at a region which is located between that coating zone and the next successive coating zone.

10. A process according to any preceding claim, performed for coating a still hot continuous ribbon of flat glass in the course of its conveyance from a flat glass forming station.

1 A process according to Claim 10, wherein the vitreous substrate is coated as it moves from a float tank to an annealing lehr.

12. Apparatus for forming a coating on a hot vitreous substrate by chemical reaction and/or decomposition of coating precursor material comprising means for conveying the hot substrate through a succession of coating stations of which the first coating station is adapted to supply coating precursor material at a lower rate than the or at least one other station so that said coating is formed, in coating zones defined by said coating stations, with an underlayer and at least one thicker upper coating layer, characterised in that the said first coating station comprises means for delivering coating precursor material including an ultrasonic pulverisator for forming a cloud of such precursor material, and a cloud chamber which is open to the path of conveyance of said substrate, and in that the or at least one later coating station includes means for directing a stream of fluid medium comprising upper coating layer precursor material into contact with an undercoated substrate on said path.

13. Apparatus according to Claim 12, wherein said ultrasonic pulverisator is a resonant cavity pulverisator.

14. Apparatus according to Claim 12 or 13, wherein the or at least one later coating station includes means for directing a stream of droplets comprising upper coating layer precursor material into contact with an undercoated substrate on said path.

1 5. Apparatus according to any of Claims 12 to 14, wherein there are provided only two said coating stations.

1 6. Apparatus according to any of Claims 12 to 15, wherein said cloud chamber incorporates one or more heaters.

1 7. Apparatus according to any of Claims 12 to 16, wherein means is provided for aspirating gases emanating from said cloud chamber at least at a region which is located between that cloud chamber and the next successive coating station.

1 8. Apparatus according to any of Claims 12 to 17, wherein said coating stations are located between a flat glass forming station and an annealing lehr for annealing a continuous ribbon of flat glass formed therein.

19. Apparatus according to Claim 18, wherein said flat glass forming station comprises a float tank.

20. Apparatus according to any of Claims 12 to 17, and substantially as herein described with reference to the accompanying drawings.

21. A process according to any of Claims 1 to 11 and substantially as herein described with reference to the accompanying drawings.

22. Vitreous material coated by a method according to any of Claims 1 to 11 and 21.