GB2206878A

GB2206878A - Pyrolytically coated sheet glass and process of manufacfuring same

Info

Publication number: GB2206878A
Application number: GB08716388A
Authority: GB
Inventors: Andre Hecq; Jean-Francois Thomas
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1987-07-11
Filing date: 1987-07-11
Publication date: 1989-01-18
Anticipated expiration: 2007-07-11
Also published as: IT1219388B; ATA172588A; SE465921B; DE3823089A1; SE8802578D0; LU87246A1; FR2617833B1; CH675416A5; GB8716388D0; AT398753B; GB2206878B; FR2617833A1; IT8867576A0; JPS6433033A; SE8802578L; BE1002216A4; ES2006972A6; NL8801713A

Description

2206878 QG16 RANSES 590 (0213G) 1 PYROLYTICALLY COATED SHEET GLASS AND

PROCESS OF MANUFACTURING SAME The present invention relates to a product comprising a sheet of pyrolytically coated glass, and includes a process of manufacturing a pyrolytically coated sheet glass product.

Pyrolytically coated glass as such is well known and Is used for a wide variety of -purposes. As examples. transparent pyrolytic coatings are often applied to sheet glass to be used for glazing purposes to modify its electromagnetic radiation transmitting properties for example to increase reflectivity of solar radiation or to reduce emissivity of long wavelength (above 3 micrometres) infra-red radiation., electrically conductive pyrolytic coatings may be applied to glass. inter alia as parts of electrical circuits for control and other purposes and as resistance heating elements; and conductive pyrolytic coatings may also be applied to glass objects of various sorts to prevent the build up of a static electric charge.

The presence of sodium Ions tends to promote haze in the coated product. and this is particularly disadvantageous for transparent products to be used for glazing purposes.

In order to overcome this problem of haze In pyrolytically formed coatings. a variety of solutions has been proposed. One is to avoid the problem altogether by using glass of a special low-alkali composition. This may be justified for special products, but it adds appreciably to the cost of the glass. A further suggestion has been to apply a sodiumimpervious coating - of silica to ordinary soda-lime giass. but this is also rather expensive. It is also known to subject glass to an acidleaching treatment to dealkalise surface layers of the glass. This it less expensive. Thus for example. British Patent specification No 705 934 (Pittsburgh Plate Glass company) proposes applying a transparent electrically conductive coating to a i i 1 11< 2 sheet of glass whose surface is low in sodium. and states that that surface may be provided by a subbing layer of silica or by treatment with an acid or a base exchange agent. Various treatment apart from coating with silica are exemplified: coating the glass with a kaolin slip and heating to 52CC.. treating with sulphur dioxide at 650C; heating to 615C and spraying with 25% aqueous HCl; boiling in water for 6 hours; boiling In a 4% aqueous cuprous chloride solution; heating to 627C and coating with a mixture of 1800g ethyl silicate. 1800g ethanol. 180g water and 18g of 1% Hel; and boiling in various other acid and salt solutions. Such treatments can be effective in reducing haze which may develop at the coating/glass interface during pyrolytic formation of the coating.

However a large part of the benefit of such acid-leaching or base exchange treatments is lost when a pyrolytic coating is subsequently to be applied to the glass. For a useful coating yield from a pyrolytic coating process. the glass must often be at a temperature above 400C and sometimes as high as 6000C. It will be borne in mind that the coating process itself must take some time. and also that much glass Is highly susceptible to thermal shock so it may not be heated or cooled too rapidly. The high temperature dwell time i glass in the course of a pyrolytic coating treatment is thus appreciable. At such high temperatures, ionic diffusion within the glass lattice bdComes quite rapid: alkali metal ions would tend to migrate towards the surfaces of the treated glass to achieve equilibrium of the ionic population throughout the thickness of the glass. with the result that the benefit of any such treatment would be markedly reduced. Indeed. in British Patent Specification No 705 934. the effect of such a coating process is simulated by heating the treated glass to 6270C for a few minutes and cooling the glass. The glass is then Imersed in water. at an unstated temperature. for two hours, and it is found that some 1 to 4 mg sodium oxide is leached from the glass per square metre surface area. The prior art product does not necessarily still have a surface which is dealkalised in such a way as to resist any further treatment which may be performed.

It is known that the presence of sodium ions in the glass. for example as occurs in ordinary soda-lime glass even after an acid-leaching treatment. sometimes makes a pyrolytic coating insufficiently adherent. and furthermore that the aging properties of the coated product are not as good as they might be. especially when the coated product is submitted to further severe conditions such as a high temperature reducing atmosphere or a glow discharge technique.

i 1 i 1 1 i i 1 i 3 It will be apparent that it is desirable for such coatings to have high mechanical and chemical stability against aging and any further manufacturing steps which may be taken. so that the particular property conferred by the. coating applied will last as long as possible. coating stability is particularly important in relatively expensive products. such as products in which a further coating is superimposed on the pyrolytically formed coating. By way of example. it is known to make various electrical circuit components by applying appropriate coating layers to a glass sheet.

The formation of such a further coating layer can cause a subjacent pyrolytic coating layer to be subjected to quite extreme conditions: for example the formation of such a layer may require a high temperature glow discharge technique. or it may require to be rigorously cleaned beforehand. The pyrolytic coating must also, for certain purposes, be able to withstand subjection to aggressive atmospheres during its working life. for example when used as an electrode in a plasma discharge display.

surprisingly, we have now discovered that it is possible to provide a product comprising pyrolytically coated sheet glass in which Improved mechanical and chemical stability is afforded to the coating by dealkalisisation of the glass, and it is an object of the present invention to provide such a product.

According to the present invention. there is provided a product comprising a sheet of pyrolytically coated glass, characterised in that such glass contains sodium and has been dealkalised so that the glass. in its pyrolytically coated state, has a surface layer 1 lim In thickness which is depleted in sodium by an amount of at least 5 milligrams Na per square metre as compared with an immediately subjacent layer of the same thickness.

The- sodium Ion concentration at various depths in the surface layer of the 'glass can be analysed in known manner by a proton bombardment technique which results In the conversion of 23 Na to 20 Ne with the' evolution of an alpha particle. By monitoring proton and resonance energies and alpha particle emission it is possible to derive the sodium ion concentration. at various depths beneath the surface with a resolution of 15nm. and the results can be plotted on a graph to give a stepped line showing the mean sodium Ion concentration of successive thin strata against the depth of those strata beneath the surface. From the information thus derived, it is then a simple calculation to determine the difference in sodium ion content as between a surface layer 1 micrometre in thickness and the immediately subjacent layer of the same thickness. It will of course be appreciated that the said sodium 11 4 depletion in the surface layer of the glass is not uniformly distributed through the thickness of that layer. in general, the lowest sodium ion concentration will be at the surface of the glass. and this concentration will rise asymptotically towards the maximum value which may be attained at a depth o f a few hundred nanometres. For example. the glass at a depth of 50Onm may have substantially the same sodium ion content as the glass at greater depths. In such a case. the entire required sodium ion depletion of at least 5 milligrams per square metre will be afforded by the.depletion in that first 50Onm of glass thickness.

The stepped line graph of sodium ion concentration against depth beneath the surface of the glass can be smoothed out to give a notional concentration at any particular depth. It is convenient for some purposes to refer to the sodium ion concentration at any given depth in terms of a percentage of the sodium content of the glass before any dealkalising treatment. in fact for most practical purposes, it can be assumed that the sodium content at depths greater than 1 micrometre will be substantially unaffected by the dealkalising treatments in view and may thus he referred to as a sodium ion concentration of 100%, which. for a typical soda-lime glass. will correspond to a sodium content of 12 to 14% (or thereabouts) calculated as Na 2 0 by weight of the glass.

A product according to the invention has various advantages due to the presence of relatively low proportions of sodium ions at the surface of the glass. As a result. for a given coating technique applied to glass of a given base composition. there is a tendency for the coating to be more adherent. and for the aging properties of the coated product to be better than they would otherwise be. even when the coating is submitted to severe conditions such as a high temperature reducing atmosphere or a glow discharge technique as referred to above. The presence of a reduced proportion of sodium ions at the glass surface also tends to promote a reduction in haze in the coated product, and this Is particularly advantageous for transparent products to be used for glazing purposes. In many cases. it Is found that for a given weight of coating material deposited on the surface of the glass. when the coating is applied to dealkalised glass in accordance with the invention. the coating is thinner. for example up to about 10% thinner. than when applied to ordinary soda-lime glass. The coating is therefore denser. and this promotes mechanical and chemical resistance. it Is also found that the coating tends to Include a lesser quantity of sodium ions. and this also promotes good aging properties for the coating.

f 1 c The benefits afforded by the invention are_ greater as the depletion of sodium ions at the surface of the glass Is increased. It is accordingly preferred that said surface layer is depleted in sodium ions by an amount of 2 at least 10 mglm For similar reasons. it is advantageous that the sodium Ion concentration of the glass at Its surface is less than 80%, and preferably less than 50%. of the sodium Ion concentration at a depth of 1 micrometre.

We have already referred to the fact that a product according to this invention tends to contain in its coating a lesser quantity of sodium ions than does a coating containing the same weight of coating material deposited on ordinary soda-lime glass. When forming a coating on glass of a given surface composition from given starting materials. it is usual that the thicker the coating, the longer must be the time for which the glass is subjected to elevated temperatures and thus the greater is the opportunity for sodium ions to migrate out into the coating. It is therefore appropriate to refer in general terms to the sodium content of a given coating volume as opposed to a given coating area. By way of example. tin oxide coatings formed pyrolytically on ordinary. untreated soda-lime glass typically contain about 20 milligrams sodium ions per cubic centimetre of coating volume. In preferred embodiments of the present invention, the pyrolytic coating does not contain sodium ions in an amount in excess of 10mg/cm 3 of the coating. Such a low content of sodium Ions implies that the coating will have improved resistance to chemical attack, and it reduces the risk that sodium Ions will migrate from or through the pyrolytic coating into a subsequently applied coating with possibly damaging effects on that overcoating.

The invention is of particular value in embodiments in which ' such pyrolytic coating Is overcoated with at least one further coating layer.

Dealkalised soda-lime glass is of particular commercial benefit.

it will be appreciated that any dealkalising treatment applied to sodium containing glass will" result in the formation of a corresponding sodium salt on the surface of the glass and that this must be removed. for example by washing. so that the coating can be applied to a clean glass surface. It Is surprising that the results of any dealkalising treatment are not completely lost when the glass is reheated after dealkalising and washing to the temperatures required for the pyrolytic formation of a coating. Typical pyrolytic coating techniques known per se are performed at temperatures In the rangeL 500C to 60TOC, and at such temperatures it would have been expected that the tendency would be for rapid ionic migration so that 6 equilibrium in the sodium Ion population would be reached very quickly. Nevertheless. we -have found that a depletion in the sodium Ion population of glass can be retained even when following temperature schedules such as are commonly used for the pyrolytic coating of. for example. glass sheets.

We believe that the manufacture of the new and advantageous dealkalised and pyrolytically coated sheet glass product defined above is attributable at least in part to a new process which we have discovered. Accordingly. it is another object of the present invention to provide a process of manufacturing a pyrolytically coated sheet glass product which combines a dealkalising treatment with a pyrolytic coating treatment and affords improved mechanical and chemical stability to the coating.

According to the present invention. there is also provided a process of manufacturing a pyrolytically coated sheet glass product. characterised in that the glass is pyrolytically coated. and in that before such coating. the glass is subjected to a dealkalising treatment by exposing It to an acidic atmosphere in such manner that in the resulting dealkalised and coated sheet. a surface layer of the glass 1 micrometre in thickness is depleted in sodium by an amount of at least 5 milligrams Na + per square metre as compared with an immediately subjacent layer of the same thickness.

Such a process affords improved mechanical and chemical stability to the coating.

In the most preferred embodiments of the invention. in said dealkalising treatment. the glass is exposed to an atmosphere containing an acid gas while its temperature is in excess of 2000C and remains exposed to such atmosphere for a period of time during which. or at least at the end of which, its temperature is below 35CC, whereafter the dealkalised glass is washed and reheated and is contacted while at a temperature of at least 40CC with coating precursor material which reacts pyrolytically on contact with the glass and forms an adherent pyrolytic coating thereon.

It is surprising that such a combination of treatments does promote coating quality. we have noted that the surface layer of the glass which is relatively poor in alkali Ions at the end of a conventional dealkalising treatment may only be a few hundred nanometres In thickness: at a depth of 50Onm. the composition of the glass may be substantially unaffected by the dealkalising treatment. Many conventional pyrolytic coating processes require that the glass should be at a temperature of up to 600C when contacted by the coating precursor material in order that the coating reactions should proceed in a satisfactory manner. it would normally be 4 1 1 v i 7 expected that reheating the glass to a temperature such as is required by such pyrolytic coating processes would raise the speed of migration of the ions within the glass to such an extent that ionic equilibrium throughout the glass would be reached rather rapidly, thus removing the effect of the dealkalising treatment. especially when one considers how small a proportion of the glass Is affected by that treatment. But be that as It may. we have found that the adoption of the invention does lead to the formation of coatings of high mechanical and chemical stability. and in particular it promotes the resistance to deterioration of such a coating. as compared with a like coating applied to untreated glass. when the coated glass Is subjected to a further manufacturing step such as another high temperature coating process. Furthermore, a reduction In the appearance of haze In such a coating is promoted.

Such a process is particularly useful for forming a product according to the invention as hereinbefore defined.

It will of course be appreciated that for high coating quality the surface to be coated must be clean. so the alkali metal salts which will he formed on the surface of the glass during the dealkalising treatment must be washed off. Water is the'ost suitable washing liquid.

The invention the subject of the present application may with advantage be combined with the -invention described and claimed in our copending application No 86 29 042 filed 4th December 1986. Accordingly, in preferred embodiments of the process according to this Invention. the glass Is dealkalised in stages. in one stage the glass being dealkalised by exposing it to contact by acid gas of a dealkalising medium for a period of at least 1 minute while the glass is above 40CC, and in a subsequent stage the thus dealkalised glass being further dealkalised by exposing it to contact by acid gas of a dealkalising medium for at least 3 minutes while the temperature of the glass Is at least 50C below the temperature or the minimum temperature of the glass during said one stage, and is between 400C and 250C.

such a process can readily be performed to result in an intermediate product in which. before any pyrolytic coating treatment. over at least a portion of thp surface of the glass, the depth at which the sodium Ion concentration is 90% of the maximum sodium ion concentration of the glass is at least twice the depth at which the sodium ion concentration is 50% of said maximum concentration. and the sodium ion concentration at a depth of SOnm is not more than 50% of said maximum concentration.

In fact. it has been found that with previously known dealkalising 1 a processes. the sodium ion concentration increases with depth in almost linear fashion from an assumed zero sodium ion concentration at the surface until the 90% sodium ion concentration depth is reached. whereafter the plotted line moves up to 100% sodium ion concentration asymptotically. In fact values for the 50% sodium ion concentration depths of prior art dealkalised glasses. of 0.51 to 0.54 times the 90% sodium ion concentration depth are typical and for such known glasses, the shapes of the plots of sodium Ion concentration against depth are all substantially similar.

It will be appreciated that the resulting dealkalised state of the glass surface is unstable in that there will be a tendency for sodium ions to migrate from within the de pth of the glass towards the surface in order to re-establish an ionic population distribution there which Is close to that of ionic equilibrium throughout the mass of glass. There are various factors which will govern the time -taken for such equilibrium to be substantially re-established. and among the most important of these are the temperature of the glass and the extent to which the sodium ion concentration has been depleted in the surface layers of the glass. it will be appreciated that a given extent of surface dealkalisation can be expressed in terms of the depth at which the sodium ion concentration has a value of. for example. 50%. Because previously known dealkalised glasses have similar ion population distributions, as evidenced by the similar shapes of their plots of sodium ion concentration against depth. the surface dealkalisation benefits of known dealkalised glasses having a given 50% sodium Ion concentration depth will he lost over a similar period of time. provided of course that those known glasses are stored or treated under similar conditions.

Glass which is dealkalised in accordance with such preferred emboditents of the present invention retains the benefits of dealkalisation for a longer period of time than does previously known dealkalised sheet glass of the same base composition which has been dealkalised to the same 50% sodium Ion concentration depth and maintained under similar conditions. This better retention of the benefits of dealkalisation is attributed to the greater depth to which the glass is depleted in alkali metal ions. For a given 50% sodium Ion concentration depth. the 90% sodium ion concentration depth. at which there will clearly be a 10% sodium ion depletion. is greater than has hitherto been achieved. This in turn leads to an increase in the mean length of the ion migration path required for the glass to return to a condition in which. close to its surface. there is a given alkali metal ion population distribution close to equilibrium.

1 1 1 1 i 1 1 1 j i i i i i 9 v Furthermore. because there is a greater distance between the 50% and 90% sodium ion concentration depths. the mean ion population gradient between those depths will be lower in sheet glass according to this preferred embodiment of the invention than in previously known dealkalised glass. and because of this lower gradient. the tendency towards Ionic migration will itself be reduced. Thus. not only will the mean migration path be longer. but also the mean migration speed will be lower.

The temperature of the glass is an important factor in the dealkalising treatment. At temperatures in excess of 65CC. the glass may be subject to such attack by the acid gas that its optical quality and finish can easily be Impaired. If the optical quality of the product Is of importance It is therefore desirable to dealkalise at lower temperature. and If particularly high optical quality is required, the glass should not be exposed to the acid gas If its temperature is above 50CC.

The extent of dealkalisation will also depend inter alia on the temperature of the glass when it is exposed to the acid gas. High temperature promotes rapid removal of ions from the surface of the glass. but equally it promotes rapid migration of ions from the interior of the glass to its surface layers as the alkali metal ion population of the glass seeks equilibrium. At lower temperatures, Ion migration within the glass is slowed down. and accordingly. the alkali metal ions from the interior of the glass do not move so rapidly into the. surface layers of the glass. In the most preferred embodiments of the invention. the glass is exposed to said acidic atmosphere for a period of time at the end of which the temperature of the glass is below 3.00C. This Is found to promote dealkalisation of the surfaces of the glass because at such temperatures. alkali metal ions removed from such surface layers are not readily or too rapidly replaced by ions migrating from the interior of the glass.

In some preferred embodiments of the invention. the glass is exposed to said acidic atmosphere only while the temperature of the glass Is below 350C. Effecting a dealkalising treatment at such temperatures permits a highly satisfactory reduction in the alkali metal ion content in surface layers of the glass. and is of particular economic benefit in processes in which the glass is reheated for such dealkalisation. In such embodiments, it is preferred that the glass is exposed to said acidic atmosphere only while the temperature of the glass is below 30CC. The adoption of this feature allows yet further savings in the fuel required for such reheating.

1 In other preferred embodiments of the invention. the temperature of the glass is between 40CC and 500C during part of the time for which it is exposed to said acidic atmosphere. Operating at such temperatures during part---ofthe dealkalising treatment allows rapid extraction of alkali metal Ions from the glass. There is however a penalty to be paid in increased fuel Consumption in cases in which the glass has to be reheated for the treatment, and so the adoption of this feature is of particular benefit when applied to glass which Is still hot as a result of some other manufacturing operation.

It is preferred that the glass should be allowed to cool over a temperature range of at least 60C while it is substantially continuously exposed to the acidic atmosphere. AS the glass cools. ion migration within the glass slows down. and accordingly. the alkali metal ions from the Interior of the glass do not move so rapidly into the surface layers of the glass. Because those surface layers are exposed to the acidic atmosphere during such cooling. alkali metal Ions continue to be removed from the surface layers of the glass so that they remain dealkalised.

Advantageously. the temperature of the glass is below 350C for at least 20% of the time during which that glass is exposed to said acidic atmosphere. This also helps to reduce repopulation of the dealkalised surface layers of the glass by alkali metal ions migrating from the interior of that glass.

Preferably. said acidic atmosphere is caused continuously to circulate in contact with said glass. in order to promote uniformity of the dealkalising treatment.

In preferred embodiments of the invention. the glass is dealkalised batchwise.

There are a number of acid gases which could be used in a method according to this invention. Among such gases may be cited HCl. However the use of hydrochloric acid gas will in general present 'serious handling problems and also give rise to severe erosion of the treatment chamber. and it is preferred that said acid gas comprises sulphur trioxide. It will be appreciated that sulphur trioxide Itself is not easy to handle. but it does have the advantage that it can be generated in situ. Preferably. said sulphur trioxide is introduced into said atmosphere by passing sulphur dioxide over an oxidation promoting catalyst under oxidising conditions. Sulphur dioxide is relatively less noxious than the trioxide. Vanadium pentoxide is a very suitable catalyst for promoting oxidation of sulphur dioxide..and its use for that purpose Is preferred.

i 1 i i f i i 11 In fact the reactions which appear to take place during oxidation Of the sulphur dioxide are v 2 0 5 + so 2 v 2 0 4 + so 3. and 2(V 2 0 4) + 0 2 2(V 2 0 5).

It will be apparent that for continuous operation without replenishment of the catalyst the second reaction must proceed as fast as the first. The speed of the second reaction is promoted when the reaction takes place at elevated temperature in an excess of oxygen.

Advantageously therefore. sulphur dioxide is passed over a said oxidation promoting catalyst so that oxidation takes place at a temperature of at least 400C. This promotes oxidati on of the sulphur dioxide. and Indeed enables 90% or more of the sulphur dioxide to be converted to sulphur trioxide. Furthermore, it is preferred that sulphur dioxide is passed over a said oxidation promoting catalyst in admixture with an excess of air. the air being present in an amount at least three times (and preferably at least five times) that which is stoichiometrically necessary for the complete oxidation of the sulphur dioxide. The use of such an excess of air as carrier gas helps not only to promote oxidation, but also to give a better and more uniform distribution of the sulphur dioxide in the atmosphere in which the dealkalising treatment takes place.

When glass is attacked by sulphur trioxide, a thin film of sodium sulphate, sulphate bloom, will form on the surface of the glass. If the reaction with the glass is too strong. this can lead to irregular surface treatment. thus giving rise to surface defects in the glass. Furthermore. the sulphate bloom will itself form a barrier against further reaction between the sulphur trioxide and the glass.

Advantageously. said acid gas includes an organic fluorine compound which will decompose to release fluorine ions at the temperature of the glass in the region where It Is introduced. This has been found to inhibit the formation of sulphate bloom.

Many pyrolytic coating processes known per se may be incorporated in the performance of this invention: such processes may make use of reactants inthe vapour phase. or they may be processes in which droplets of coating precursor solution are sprayed onto the glass. The Invention Is suitable for manufacturing glass products bearing pyrolytic coatings of a variety of materials. The invention is particularly suitable for forming high quality durable coatings of metal oxide, and it is accordingly preferred that said coating precursor material reacts to form a metal oxide coating. and i 1 i 12 advantageously. said coating precursor material reacts to form a coating comprising tin oxide. it has been found that the adoption of the present invention gives particular benefits when applied for the manufacture of tin oxide coated products. Tin oxide coatings are often used as a base coating layer in transparent electronic components comprising a plurality of coating layers. The resulting coating is durable even when subjected to such severe conditions as may be encountered in a subsequent coating treatment. Furthermore. it is found that a process according to the Invention tends to promote a reduction in haze in a tin oxide coating which Is especially important in coatings applied to form transparent electronic devices and in coated glass products to be used for glazing purposes.

Pyrolytically coated glass manufactured by a process as herein defined may be considered as a final product in its own right. subject to any desired cutting to size and framing. or it may be considered as an intermediate product which is to be subjected to some further manufacturing step: as an example. some preferred embodiments of the invention provide that said glass and said conductive coating are transparent. and that a further coating layer is formed over the conductive coating.

The invention extends to pyrolytically coated glass being the product of a process as herein defined.

The present inventiod will now be described in greater detail with reference to the accompanying diagrammatic drawings in which:

Figure 1 shows a dealkalising chamber for dealkalising glass sheets. and Figure 2 shows a coating chamber for coating glass sheets.

Figure 1 illustrates an apparatus for batch-wise dealkalisation of sheets of glass. In Figure 1. pre-cut sheets of glass 1 are held by tongs 2 mounted on supports 3 in a tteatment chamber 4. The chamber 4 Is provided with a plurality of radiant heaters 5 for temperature control and with conduits 6 for the introduction of an acidic gas such as SO 2 or SO 3

Figure 2 illustrates a coating apparatus for applying a coating to sheets of glass. In Figure 2. sheets of glass 1 are held by tongs 2 mounted for movement along a support 3 in the form of a rail. for conveyance through a coating tunnel 7 past a spraying zone containing one or more nozzles 8 for spraying coating material onto the sheets as they pass. A plurality of static nozzles as shown may be used, they being arranged to give uniform coverage of the sheets as they pass. or one or more vertically reciprocating nozzles may be used. Such coating apparatus is known per se and need not be further specifically described here.

1 i i i 1 1 1 1 1 f i i 1 1 1 1 13 EYLE 1 In a specific practical example. the sheets of glass were Introduced into the chamber 4 and heated to 220C when 50 2 or SO 3 was Introduced. The temperature of the sheets was increased to 285C and was maintained at that temperature for about 70 minutes whereafter the glass sheets were allowed to cool to 220C at which stage they were removed from the acidic atmosphere in the treatment chamber. The sheets were allowed to cool further and are then washed thoroughly to remove alkali metal sulphate deposits.

To case handling of the acid gas. each acid gas discharge duct preferably comprises a catalyst for the in situ oxidation of sulphur dioxide. Furthermore, each such duct preferably comprises heating means so that the temperature therein can be maintained at a value of at least 400C to promote such oxidation.

After washing. the sheets were provided with a pyrolytically formed coating. for example as described with reference to Figure 2.

In a specific example, glass sheets were conveyed through the tunnel 7 at a speed of 60cm/min for coating with a fluorine-doped Sno 2 coating 240= thick and having a resistivity of 19 ohms per square by spraying with a solution in dimethylformamide of SnCl 4 SH 2 0 and trifluoroacetic acid. In a second example, the coating was formed In the same way but to a thickness of 40Orim and with a resistivity of 12 ohms per square.

Heating means (not shown) was arranged In the tunnel 7 upstream of the coating station shown and was arranged to'raise the temperature of the sheets from room temperature to about 580C in the course of about 10 minutes. The temperature In the atmosphere In front of the nozzles 8 was 460C. while the glass entered the spraying zone at a temperature of 580C.

The coated sheets then continued along the tunnel 7 and were allowed to cool to room temperature over the course of about 10 minutes.

Such processgs resulted in coatings having a highly "uniform structure which had especially good mechanical and chemical stability.

The thus dealkalised and pyrolytically coated glass was then subjected to two tests, a haze test and a lixiviation test, and the results were compared with those given by a specimen of glass of the same composition which was coated In the same way. but- which had not previously been dealkalised.

The haze test consisted of subjecting the coated glass to a cyclic temperature variation of 450C to WC and back to 450C. 24 cycles per day, In an atmosphere having a relative humidity of 99%. The ordinary coated soda- 14 lime glass exhibited iridescence after 2 to 3 days. The specimen of coated dealkalised glass did not exhibit iridescence before more than 6 days had passed.

in the lixiviation test. the specimens of coated glass were immersed for 30 minutes In water at a temperature of WC, and the water was subsequently analysed for its sodium content. From the ordinary coated glass. it was found that more than 5mg sodium was extracted from the glass per square metre surface area. From the dealkalised coated glass, less than 2.7mg sodium was extracted per square metre surface area.

The alkali metal ion population distribution in surface layers of the resulting dealkalised and coated glass product were measured by a known technique in which the glass surface is bombarded with protons. The sodium ion concentration at various depths is converted to a percentage of the maximum sodium ion concentration in the glass, and this Is in effect. the concentration at depths greater than 1 micrometre beneath the surface of the glass. From this it is simple to calculate the sodium ion depletion in the surface layer 1 micrometre thick. It was f ound to be 13 mg Na + per square metre.

In fact these tests were performed on the uncoated face of the glass. but it Is assumed that the results would be very little different for the coated face, since. apart from any extra cooling of the coated face during the actual formation of the coating. both faces of the sheet are subjected to substantially the same temperature schedule.

The sodium ion content of the coating formed was measured. and It was found to be 12 mglcm 3 ELE 2 In a variant of Example 1. the glass sheets were subjected to a different coating treatment. in which the sheets were reheated to a temperature of 52CC and were contacted with a first gas stream containing tin tetrachloride vapour entrained in nitrogen at 450C and with a second gas stream comprising air, water vapour and hydrofluoric acid also at 4506C to form a fluorine-doped tin oxide coating and the sheets were then cooled. The total heating and cooling time was about as given In Example 1.

After cooling. the coated sheets were tested as before, and the results given were very similar to those set out in Example 1.

In a variant of this example. a fluorine containing gas. namely difluoroethane or tetrafluoromethane. was mixed with the sulphur dioxide introduced through the discharge ducts 6 in an amount of 10% by volume 1 i 1 J i 1 i i i i i i 1 i is 1 so 2 Each of these gases decomposed to liberate fluorine Ions which tended to reduce the formation of sulphate bloom on the faces of the sheets.

EXAMPLE 3 - In a variant of the dealkalising treatment described in Example 1. the glass sheets were introduced into the treatment chamber 4 and were heated to a maximum temperature of 400C and SO 3 was introduced Into the chamber when the glass reached that temperature. The sheets were maintained at that temperature of 400C for 5 minutes when their temperature was allowed to drop. reaching a temperature of 3000C after a further 10 minutes. and they were removed from the acidic atmosphere within the treatment chamber after a total of 20 minutes in contact with that atmosphere when their temperature was 250C. This procedure also gave very good results.

After coating as described in Example 1. and after cooling. dealkalised sheets were tested as before. and the results were as follows:

Na + depletion in surface lomg/m 2 Haze test more than 5 days Lixiviation test about 2.6mg/m 2 Sodium ion content of coating 15mg/cm 3 EXAMPLE 4

2mm thick sheets of drawn soda-lime glass having an alkali metal content of 12 to 14% calculated as weight per cent sodium oxide in the glass were introduced into the chambe,r 4 which was heated to a temperature of 490C. A mixture of sulphur dioxide and a stoichlometric excess of air was circulated to the acid gas inlet openings. The glass was allowed to cool at a rate of WC/min, and when the glass had cooled to 37CC, the rate at which the sulphur dioxide was fed to the chamber 1 was increased to between 40 and 50 L/h in between 1000 and 200OL/h air. The glass was further allowed to cool to 320C when the rate of Introduction of sulphur dioxide was Increased to between 70 and 80 L/h sulphur dioxide In an excess of air.' Each Inlet duct contained vanadium pentoxide as catalyst to promote oxidation of the sulphur dioxide. The ducts were heated to a temperature in excess of 400C so that at each duct In excess of 90% of the sulphur dioxide was oxidised. The sheets were exposed to the acidic atmosphere in the chamber for a period in excess of ten minutes.

The thus dealkalised glass was then cooled, washed and tested as before, prior to coating. and the results are given In the following Table 4a.

i k 16 TABLE 4a

Na + depletion in surface Concentration at depth 25run Depth at 50% concentration Depth at 80% concentration Depth at 90% concentration 90% depth:50% depth ratio 90% depth:80% depth ratio Uncoated glass 29m9/m 2 20% 78nm, 145= 25Onm 3.20 1.72 Haze test 17 days Lixiviation test 0.3mg/m 2 The sheets were then coated by a method as described in Example 1. and again tested as before. The results are given in the following Table 4b. TABLE 4b Na + depletion in surface Haze test Lixiviation test Sodium ion content of coating EXAMPLE 5 Coated. glass (Example 4) 17mg/m 2 10 days lmglm 2 6Mg/cm 3 In a variant of the dealkalising treatment of Example 1. the sheets were maintained at a temperature of 20CC and were in contact with the acidic atmosphere for 90 minutes. This also gave satisfactory dealkalisation, as shown by the results given in the following Table for the glass which was coated as described in Example 1.

Coated glass Na + depletion in surface Haze test Lixiviation test sodium ion content of coating 1; i:

i i 1 i 1 1 i lomglm 2 5 days about 3mg/m 14mg/cm 3 2 k - 17

Claims

A product comprising a sheet of pyrolytically coated glass. characterised in that such glass contains sodium and has been dealkalised so that the glass, in its pyrolytically coated state, has a surface layer 1 pm in thickness which is depleted in sodium by an amount of at least 5 milligrams Na + per square metre as compared with an immediately subjacent layer of the same thickness.
2. A product according to claim 1, wherein said surface layer is depleted in sodium Ions by an amount of at least 10 mg/m 2.
3. A product according to claim 1 or 2. wherein the sodium ion concentration of the glass at its surface is less than 80% of the sodium ion concentration at a depth of 1 micrometre.
4. A product according to claim 3. wherein the sodium ion concentration of the glass at its surface is less than 50% of the sodium ion concentration at a depth of 1 micrometre.
5. A product according to any preceding claim. wherein said pyrolytic coating does not contain sodium ions in an amount in excess of 10mg/cm 3 of the coating.
6. A product according to any preceding claim. wherein such pyrolytic coating is overcoated with at least one further coating layer.
7. A product according to any preceding claim, wherein such glass is dealkalised soda-lime glass.
8. A process of manufacturing a pyrolytically coated sheet glass product. characterised in that the glass Is pyrolytically coated, and in that before such coating. the glass is subjected to a dealkalising treatment by exposing it to an acidic atmosphere In such manner that In the resulting dealkalised and coated sheet; a surface layer of the glass 1 micrometre in thickness Is depleted in sodium by an amount of at least 5 milligrams Na per square metre as compared with an immediately subjacent layer of the same thickness.
9. A process according to claim 8. wherein. in said dealkalising treatment, the glass is exposed to an atmosphere' containing an acid gas while its temperature is in excess of 200C and remains exposed to such atmosphere for a period of time during which, or at least at the end of which. its temperature is below 350C, whereafter the dealkalised glass is washed and reheated and is contacted while at a temperature of at least 400C with coating precursor material which reacts pyrolytically on contact with the glass and forms an adherent pyrolytic coating thereon. - i i 1 i 18
10. A process according to claim 8 or 9. wherein the glass is dealkalised in stages. in one stage the glass being dealkalised by exposing it to contact by acid gas of a dealkalising medium for a period of at least 1. minute while the glass is above 40CC, and in a subsequent stage the thus dea lkalised glass being further dealkalised by exposing it to contact by acid gas of a dealkalising medium for at least 3 minutes while the temperature of he glass is at least 50C below the temperature or the minimum temperature of the glass during said one stage. and is between 4000C and 2500C.
11. A process according to any of claims 8 to 10, wherein the glass is exposed to said acidic atmosphere for a period of time at the end of which the temperature of the glass is below 30CC.
12. A process according to any of claims 8 to 11. wherein the glass is exposed to said acidic atmosphere only while the temperature of the glass is below 3500C.
13. A process according to claim 12. wherein the glass is exposed to said acidic atmosphere only while the temperature of the glass is below 30CC.
14. A process according to any of claims 8 to 11, wherein the temperature of the glass is between 4000C and 500C during part of the time for which it is exposed to said acidic atmosphere.
15. A process according to any of claims 8 to 14. wherein the glass is allowed to cool over a temperature range of at least WC while it Is substantially continuously exposed to the acidic atmosphere.
16. A process according to any of claims 8 to 15, wherein the temperature of the glass is below 350C for at least 20% of the time during which that glass is exposed to said acidic atmosphere.
17. A process according to any of claims 8 to 16. wherein said acidic atmosphere is caused continuously to circulate in contact with said glass.
18. A process according to any of claims 8 to 17. wherein said acidic atmosphere comprises sulphur trioxide.
19. A process according to claim 18. wherein said sulphur trioxide Is introduced Into said atmosphere by passing sulphur dioxide over an oxidation promoting catalyst under oxidising conditions.
20. A process according to claim 19. wherein vanadium pentoxide is used as said catalyst.
21. A process according to claim 19 or 20. wherein sulphur dioxide is passed over a said oxidation promoting catalyst so that oxidation takes place at a temperature of at least 400C.

1 1 1 1 1 1 i 1 1 1 1 1 1 1 1 i 1 19 -
22. A process according to any of claims 18 to 21. wherein sulphur dioxide is passed over a said oxidation promoting catalyst In admixture with an excess of air. the air being present in an amount at least three times that which is stoichiometrically necessary for the complete oxidation of the sulphur dioxide. -
23. A process according to any of claims 18 to 22. wherein said acid gas i ncludes an organic fluorine compound which will decompose to release fluorine ions.
24. A process according to any of claims 8 to 23, wherein said glass is pyrolytically coated with a conductive metal oxide coating.
25. A process according to claim 24. wherein said coating precursor material reacts to form a tin oxide coating.
26. A process according to claim 24 or 25. wherein said glass and said conductive coating are transparent. and a further coating layer is formed over the conductive coating.
27. Pyrolytically coated glass being the product of a process according to any of claims 8 to 26.

- o 0 o - A Published 19813 at The Pater,' C)ffire. State House. 66 71 High lklborn, London, WC 1R 4TP. Airther copies maybe obtained from The Patent Office, S-Jes Brarch. St Mary Cray, Orpington, 2,ent. BW 3RDT-rinted by Multple. -.: cecl,--jcries ltd, St Maiy Cray, Kent. Con. 1,8-, i i 1