EP1786736A1

EP1786736A1 - Method and device for producing flat glass according to a float method

Info

Publication number: EP1786736A1
Application number: EP05783940A
Authority: EP
Inventors: Carsten Schumacher; Armin Vogl; Frank Klette; Christian Kunert; Bernhard Langner; Andreas Morstein; Andreas Roters
Original assignee: Schott AG
Current assignee: Schott AG
Priority date: 2004-09-18
Filing date: 2005-08-26
Publication date: 2007-05-23
Also published as: JP2008513323A; TW200628415A; KR20070050089A; WO2006029695A1; TW200619155A; WO2006029765A1; EP1805111A1; US20070261443A1; TWI379816B; JP2008513322A; KR20070064324A; US20090181230A1; TWI380955B

Abstract

The invention relates to a method for reducing surface defects during production of a float glass with a transformation temperature Tg equal to or greater than 600 °C consisting in removing impurities from the surface of a glass strip in a float chamber by a molten metal flowing over the glass strip in a float bath. A device for carrying out the inventive method and a float glass whose transformation temperature is equal to or greater than 600 °C and which has maximum 3 surface defects (Top Specs) whose size is greater than 35 ηm per m2 at the exit from the float chamber are also disclosed.

Description

Process and apparatus for the production of flat glass by the float process

The invention relates to a method and apparatus for producing flat glass having a transformation temperature of at least 600 ⁰ C according to the float method in which molten glass in a float chamber on a bath of molten metal in the form of an endless belt moves forward, the glass ribbon cooled and is solidified and the solidified glass ribbon is lifted from the bath.

Due to the high temperatures prevailing in the float chamber of a float glass plant, it is unavoidable that both the molten glass and the molten metal (typically tin or tin alloys) vaporize components which precipitate at cooler locations in the float chamber. By means of internals in the upper part of the float chamber and by suitable guidance of the process gas, it is largely prevented that such condensed constituents can pass from there to the glass ribbon and form there a precipitate called "Top Specs". The glass thus produced has sufficient freedom from particles on its surface for many applications.

But there are also applications for which a glass, as it comes from a float chamber, has insufficient surface cleanliness. This applies in particular to high-melting glasses, for example aluminosilicate glasses and borosilicate glasses, in particular for display applications. For these cases, the glass had previously been cleaned after its production, generally only after processing after cutting to the final format, which is expensive and causes high costs. Thus, it is known from US 3,284,181 to etch the underside of the glass ribbon, which has been associated with the tin bath, with an HF solution to remove the glass layer contaminated with the diffused tin ions.

This idea was taken up in JP 92 95 833 to remove small foreign particles from the top of the glass ribbon. Besides the use of hydrofluoric acid, it is also possible to use an aqueous acid solution containing 2-valent chromium ions. However, after this acid treatment, a polishing of the glass ribbon is still required. Also, according to JP 92 95 832, an etching of the top of the glass ribbon with an acid solution containing chromium ²⁺ ions takes place. Another etching method is described in JP 1008 5684 A. Here, an ammonium halide is decomposed on the highly heated glass ribbon and the impurities on the top of the glass ribbon volatilize in the form of easily evaporating halides.

All of these solutions to remove tin contaminants from the glass top take place as aftertreatment steps. They are, in particular by the required treatment and disposal of the etching solutions and reaction products, consuming and expensive.

It is therefore an object to find a method and an apparatus for producing glass by the float method in which cleaning of the surface of the glass sheet still takes place in the float chamber, i. in which a largely free of impurities on the glass top glass ribbon leaves the float chamber.

This object is achieved by the method according to claim 1. as well as the device according to claim 9 solved. Another The invention relates to a float glass with a high surface quality according to claim 16.

It has been found that a liquid applied to the surface of the glass ribbon, the composition of which largely corresponds to that of the float bath, absorbs particles located on the glass ribbon. By the concept that the composition of the liquid largely corresponds to that of the float bath, it is understood that impurities, e.g. Metals such as Cn, An, Ag, Pb, Bi can be present as long as they do not interfere with the operation of the float bath. If this liquid does not enter the float bath in large quantities, impurities of up to 10% by weight in the liquid can be tolerated. This liquid is therefore referred to hereinafter as cleaning liquid for the sake of simplicity. The impurities are literally rinsed off or taken up by the cleaning fluid. Float bath material is preferably used as the cleaning liquid, since then no separate storage tanks are required for the bath liquid. But it is also possible to use fresh, not used or cleaned Floatbadmaterial.

US Pat. No. 3,798,016 discloses a method for surface modification of a glass ribbon located on a float bath, in which lead ions are electrolytically diffused from a lead melt on the glass ribbon at high temperature and using an electrical current in the surface of the glass ribbon connected as cathode , This process produces a heat reflection glass with a gray-bronze hue. As a result of the high temperature generated by the electric current and the low boiling point of lead, a portion of the lead evaporates and recondensed in the direction seen behind the diffusion zone on the there colder glass ribbon. This lead is going through molten lead, which is held stationary by means of a copper bar, detached again. The method is limited to the removal of lead and is obviously not suitable for the removal of top specs, since about 30 years after the publication of this document, the top specs are removed by means of the cited elaborate etching process.

To avoid unwanted cooling of the glass by the cleaning liquid, it should have about the temperature of the float bath at this point. These are generally temperatures between 400 and 900 ⁰ C. The cleaning liquid accumulates with the particles absorbed by the glass ribbon particles. It is therefore expedient for the cleaning liquid located on the glass ribbon to be renewed regularly as a function of the degree of soiling. It is particularly advantageous if the cleaning liquid is continuously supplied to the surface and, after the overflow of the glass ribbon, of course also removed continuously from the glass ribbon by sucking it off or allowing it to flow into the float bath. The cleaning liquid can be abandoned in the middle of the glass ribbon and removed at one side edge or both side edges, but it is also possible to give the cleaning liquid on a side edge, to run across the glass ribbon and remove on the other side again. The supply of the cleaning liquid is advantageously carried out with a suitable pump. The removal of the impurities from the surface of the glass ribbon takes place before lifting the glass ribbon from the surface of the float bath, ie at a point where the glass ribbon already largely solidifies, that is solidified so far that the cleaning liquid on it no longer cause deformation can. If the cleaning liquid supplied to the glass ribbon at a location where it is already rigid, you can, for example by means of a roller, press the side of the tape on which the cleaning liquid is removed, something down in the direction of the float bath. This creates a slope that creates a voluntary flow while facilitating the removal of the cleaning fluid. Behind the roll, the glass ribbon then returns to its original shape. When the cleaning liquid is dispensed in the middle of the belt, it is expedient to press both edges downwards.

So that the cleaning liquid can not extend too far along the glass ribbon, it is advantageous if the propagation of the cleaning liquid in and / or against the running direction of the glass ribbon is restricted. This can be done by mutual lightly pressing down the edges on both sides, so that forms a trough-shaped depression in which the cleaning liquid is held. However, this solution is only possible with relatively thick glass bands, since only these are rigid enough. For thin glass bands, this solution is not possible because only the edges are pressed down in these, but the center of the glass band remains essentially unchanged.

Universally applicable is a transversely to the running direction of the glass ribbon usable bar-shaped or squeegee-like fluid limiter. It must be placed so close to the glass surface that the cleaning liquid is held back by it. The term "transverse" should not only be understood to mean an angle of 90 ° to the direction of travel, but also that the fluid limiter can be arranged at a different angle to the direction of travel. In general, the angle should not be greater than 45 degrees, otherwise the cleaning device occupies a disproportionate space in the float chamber and the fluid limiter is very long. However, a small angle of up to about 15 degrees may be advantageous, since it promotes a flow of cleaning liquid on the glass ribbon towards the edge. If an angle is used, it is useful to adjust the angle to the speed of the glass ribbon, which can be done by a few simple experiments.

A second Flüssigkeitssbegrenzer can be seen in the direction of the glass ribbon behind the first Flüssigkeitssbegrenzer, if there is a risk that the cleaning liquid expands undesirably far in the strip running direction. A second fluid limiter is often not required, especially if the cleaning device is in the spatial region of the Abhebestelle because the Abhebewinkel, i. the resulting slope of the glass ribbon, which limits the spread of the cleaning liquid.

The bar-shaped fluid limiter can be made of a variety of materials, wherein it is important that they do not dissolve in the liquid or deform or melt at high temperatures of about 600 to 1200 ⁰ C. If necessary, the beam must be cooled. A suitable material is that which is wetted by the respective liquid, as this the liquid is well retained. Depending on the temperature, tungsten, SiC and customary ceramic materials, which may also be porous, are suitable as material for this purpose. Depending on the viscosity of the liquid, bars of wettable material may have a distance of up to 6 mm, preferably of <1 to 3 mm, from the glass surface. They are as robust and economically viable to choose.

Also suitable is a strip of graphite, possibly with a metal frame for easier installation. Since graphite is not wetted by the cleaning liquid, a graphite strip must have a Distance to the glass, which is well below the tin equilibrium thickness of about 5 to 6 mm. Grinding on the glass should be avoided, but is tolerable at short notice. A graphite strip has the advantage that it is inexpensive to produce and easy to work and behaves uncritically with respect to glass and liquid contact.

Disadvantages, on the other hand, are the low mechanical strength of the graphite and the need for a protective gas atmosphere at high temperatures, which, however, is automatically present within the float plant.

Furthermore, a beam is suitable, which is designed with gas passages in the direction of the glass surface, wherein the gas passages may be holes or slots or have the shape of an open-pore material. By passing gas through the openings creates a levitation effect, the beam floats above the glass and can not touch the surface. This has the advantage that the distance of the beam to the glass ribbon can be kept very low without the risk of contact between glass and beams. However, it is complicated that gas is constantly consumed to produce the levitation effect and that the gas must be heated so that the glass ribbon is not damaged. The gas used in the float glass plant inert gas can be used, which must be passed only by means of a corresponding fan in the beam. An additional heating is in this case only slightly or not at all required.

Furthermore, a beam is also suitable in which suitable magnetic fields are generated, which are set in such a way that a force is generated which pushes the cleaning liquid away from the beam. By a suitable arrangement of the magnetic fields, the Cleaning liquid additionally be imposed a lateral speed, so that the cleaning liquid additionally receives a flow in the direction of the glass edge. Such a beam is very reliable as a liquid barrier and is absolutely contact-free to the glass ribbon running under it. Various forms of magnetic fields can be used, eg static magnetic fields whose strength and direction does not change and which operate on the principle of the eddy current brake, moving magnetic fields, as used in a linear motor, or high frequency alternating fields with frequencies above 250 Hz.

Furthermore, one can provide a beam with gas outlet openings to blow away the cleaning liquid from the beam. The flow rate for the gas should be greater than 1 ms ^{' 1} , preferably greater than 5 m -s ^"1 , in particular greater than 10 ms ^{" 1} , in order to push the cleaning fluid away from the beam. But it should not be so large that the cleaning liquid is blown away drop-shaped. However, this method requires relatively large amounts of preheated gas. Again, circulated Floatbadatmosphäre can be used as gas. Instead of using inert gas, such a beam can also be charged with air or oxygen. The oxygen reacts with the float bath atmosphere and creates a preheated gas curtain. However, damage to the glass surface can occur due to the resulting flame curtain with careless process control.

The Flüssigkeitssbegrenzer does not have to consist of a straight bar, pipe, a bar or the like but can also be curved or designed arrow-shaped. However, the bow and arrow shape should always be arranged on the belt so that no "dead" spaces can form in which the cleaning fluid can accumulate without being replaced by fresh cleaning fluid. As already stated above, the cleaning liquid should be renewed regularly or preferably continuously supplied to the glass ribbon. The removal of the supplied cleaning liquid can be carried out by generally customary methods. Thus, the cleaning liquid supplied on one side of the glass ribbon by means of a pump on the other side can also be sucked off with a pump. It is also possible to rinse the cleaning liquid from the glass ribbon electromagnetically with a device operating on the principle of the linear motor. Since the cleaning liquid (largely) has the composition of the float bath, it is particularly easy to flush the liquid into the float bath. If the mechanical nature of the glass ribbon allows it, one side of the glass ribbon may be pressed into the float bath by means of a suitable device so that the top edge of the glass ribbon (including the border) is below the liquid level of the float bath. In this case, no suction device or the like is required, since the supplied cleaning liquid can flow away from the surface of the glass ribbon without aids and flow into the bath. As a device for depressing the glass ribbon can serve in the edge region, in particular on the border of the glass ribbon rolling role, but it is also possible to use a shoe, since the border is discarded anyway. You can also use, for example, a gas-loaded body, which pushes down the glass ribbon by the levitation effect. Of course, it is also possible to press the edge of the glass on both sides down.

The amount of cleaning fluid applied to the glass ribbon depends on the number of particles on the glass ribbon (ie, the desired cleaning effect) and may be in one wide range, with the width of the glass ribbon to be cleaned is taken into account. The extension of the cleaning liquid on the glass ribbon in the longitudinal direction is preferably 1 to 100 cm, in particular 1 to 10 cm. The layer thickness of the cleaning liquid should be on the glass ribbon about 1 to 30 mm, preferably 3 to 6 mm. However, it is dependent on the surface tension and weight of the cleaning liquid at the respective temperature. It is important to ensure that the glass ribbon is not deformed too much by the weight of the cleaning liquid, as this may occur in the forward, still soft part of the glass ribbon undesirable tensile forces that can deform the still soft glass ribbon.

The cleaning fluid can be passed very conveniently between two fluid limiters. This is particularly useful when the layer thickness of the cleaning liquid to be kept high above the glass ribbon. Since the liquid would expand at high film thickness without limiting device far on the glass ribbon, one can reduce the consumption of cleaning fluid and thus the energy consumption for the pump by a multi-sided limitation. In principle, one comes with a limitation, but if necessary, several boundaries can be arranged one behind the other, to securely hold any liquid not detected by a boundary. In the case of two boundaries arranged one behind the other, the distance between the two can in principle be arbitrary, but of course the spatial conditions in the float chamber must be taken into account. Therefore, the distance of the limiter should preferably be within the specified longitudinal extent of the cleaning liquid. The two fluid limiters can work according to the same principle of operation. To exclude influences of the fluid limiter with each other, but can Also operating according to different principles fluid limiter can be used, for example, a limiter in which a magnetic field is generated and a limiter from which a gas stream emerges.

The invention further provides an alkali-free float glass with a transformation temperature Tg of at least 600 ⁰ C at a viscosity η of 10 ¹³ dPas with an exceptionally high surface quality, being understood float glass as the float glass, as it comes from the float, ie without chemical or mechanical aftertreatment such as etching, grinding, polishing and the like.

The float glass has a maximum of three surface defects (top specs) with a size of more than 50 μm per m ² . Preference is given to an alkali-free float glass having a transformation temperature Tg of at least 600 ⁰ C at a viscosity of η 10 ¹³ dPas and a thickness of less than 1, 5 mm. It is particularly suitable for the production of TFT (Thin Film Transistor) screens. Since thermal processes are used during screen production, it is advantageous to use glasses with a higher transformation temperature for greater glass stability. Preference is therefore given to a glass having a transformation temperature Tg of 650 to 78O ⁰ C, in particular 700 to 73O ⁰ C. Such glasses are preferably alkali-free borosilicate glasses or aluminosilicate glasses for TFT applications. Furthermore, it is advantageous for the purpose of weight saving to have the thinnest possible glass. Preference is therefore given to glasses with a thickness of 0.2 to 0.9 mm. The number of surface defects (top specs) and their size is important for the quality of the glass, especially in the application TFT screen. It is therefore preferred if the surface defects are not greater than 35 .mu.m, in particular not greater than 20 microns. Because the top specs Typically round, the 50, 35, or 20 micron specification refers to a circular defect of such diameter. In the case of oval or similarly shaped surface defects, the dimension refers to the greatest extent of the defect.

In the figure, the invention will be further explained. It shows

1 is a schematic plan view of the glass ribbon with cleaning device with lateral supply of the cleaning liquid,

2 shows a cross section through FIG. 1, seen from the boundary,

FIG. 3 schematically shows a plan view of the glass band, in which the cleaning liquid is applied centrally, FIG.

Fig. 4 is a cross-section through Fig. 3, viewed from the boundary.

In Figures 1 and 2, a section of a float system in plan view and cross section is shown schematically. The glass ribbon 1 ₁ which carries from the previous drawing operation braids 2 and 2 'at both edges, moves in the direction of arrow 3 on the consisting of tin or a tin alloy float 4 through the pipe 5 is cleaning liquid, in this case, molten tin , abandoned on the glass ribbon 1 and flows in the direction of arrow 6 on the opposite side of the glass ribbon. Here, the cleaning liquid is sucked through the suction tube 7 and removed from the glass. The flow of the cleaning liquid in the direction of the suction tube 7 is assisted by the fact that with the aid of the pressure roller 8, a pressure on the border 2 'is exerted, so that the Glass strip 1 a slope in the direction of the suction tube 7 receives. So that the cleaning liquid does not spread too far on the glass ribbon 1, a liquid barrier 9 is provided. The liquid barrier 9 consists of a beam, for example tungsten, arranged above the glass band, which is held above the borders 2 and 2 'at a distance of <1 mm. Thanks to its wettability with the cleaning fluid, it reliably retains the metallic cleaning fluid. The pressure roller 8 may be made of metal, but is preferred for graphite. It is usually not driven and only serves to exert pressure on the side edge of the glass ribbon. Since the cleaning device is installed at a position in the float chamber to which the glass ribbon is hardly plastically deformable, takes place by the pressure roller 8 and no permanent deformation of the glass.

Figs. 3 and 4 show another embodiment of the cleaning device. Here, the cleaning liquid is centered by a feeder 10, which has many small nozzles similar to a sprinkler possibly also a slot die, placed on the glass ribbon and flows, as shown by the arrows, to the two edges of the glass ribbon 1. This is supported Flow characterized in that with the aid of the pressure rollers 12 and 13, a slightly convex surface of the glass ribbon is produced, which causes a slope for the cleaning liquid in the direction of the side edges. In the illustration shown, the side edges of the glass ribbon 1, the borders 2 and 2 ', so deeply pressed into the float bath 4 that its upper edge is at the same level or below the bath level of the float bath 4. The cleaning liquid supplied by the supply device 10, which has the same composition as the float bath 4, can therefore easily run into the float bath without further aids. The curvature of the glass ribbon shown in Fig. 4 is not shown to scale. In practice, the border is only slightly thicker than the glass ribbon and therefore the side edges (borders) must only be pressed down accordingly, so that they end below the bath level of the float bath 4. A Flüssigkeitsbegrenzer 9 ensures here too that a spread of the cleaning liquid against the ^' strip running direction can not be done.

With the invention, it has become possible for the first time to produce a glass with such a quality in the float chamber that it can be used even in demanding fields of application without major cleaning steps.

Claims

claims

A method for producing flat glass having a transformation temperature of at least 600 ⁰ C by the float method in which molten glass on a molten metal bath in the form of an endless belt moves forward, the glass ribbon is cooled and solidified and the solidified glass ribbon of the bath is lifted off,

characterized,

that before lifting off the already largely solidified glass ribbon, the surface of the glass ribbon is cleaned in its essential width with a cleaning liquid whose composition substantially corresponds to that of the bath.

2. The method according to claim 1, characterized in that the cleaning liquid is renewed regularly.

3. The method according to claim 1 or 2, characterized in that the cleaning liquid is continuously supplied to the surface of the glass ribbon.

4. The method according to one or more of claims 1 to 3, characterized in that the propagation of the cleaning liquid in and / or against the direction of the glass ribbon is limited.

5. The method according to claim 4, characterized in that the boundary by at least one transverse to the direction Fluid limiter is arranged directly above the glass ribbon.

6. The method according to claim 5, characterized in that the Flüssigkeitssbegrenzer is arranged at an angle of 0 to 45 degrees, in particular from 0 to 15 degrees to the direction of the glass ribbon.

7. The method according to claim 6, characterized in that a Flüssigkeitsbegrenzer is used from a liquid wettable by the material.

8. The method according to claim 5 or 6, characterized in that as liquid limiter, a beam is used by magnetic fields are generated, through which the cleaning liquid is pressed directed away from the beam.

9. An apparatus for producing float glass having a transformation temperature of at least 600 ⁰ C in ribbon form on a float in a float chamber of molten metal, means for feeding liquid glass on one side of the float chamber, means for cooling the glass and means for removal the solidified glass ribbon on the other side of the float chamber

marked by

a supply device (5, 10) for a cleaning liquid, the composition of which largely corresponds to the composition of the float bath, to which on the float bath located, largely solidified glass ribbon (1) and a Discharge device (7) for removing the used cleaning fluid from the glass ribbon (1) located on the float bath.

Device according to claim 9, characterized in that in the region of the discharge means (7) a device e.g. Roll (8,12,13) is arranged, with which the edge of the glass ribbon is pressed down to improve the flow guidance.

11. The device according to claim 9 or 10, characterized in that in the region of the supply and discharge means of the cleaning liquid to prevent an excessively large expansion of the cleaning liquid a bar-shaped over the entire width of the glass ribbon extending Flüssigkeitssbegrenzer (9) at a small distance from the glass ribbon is arranged.

12. The device according to claim 11, characterized in that the distance between liquid limiter (9) and Glas¬ band (1) is 1 to 5 mm.

13. The apparatus of claim 11 or 12, characterized in that the Flüssigkeitssbegrenzer (9) consists of a wettable by the cleaning liquid material.

14. Device according to claim 11 or 12, characterized in that magnetic fields can be generated in the liquid limiter (9).

15. The apparatus of claim 11 or 12, characterized in that the Flüssigkeitssbegrenzer (9) consists of a beam of porous material or with holes is provided so that a gas cushion between glass ribbon and Flüssigkeitssbegrenzer can be generated.

16. After the float process produced alkali-free flat glass having a transformation temperature Tg of at least 600 ⁰ C at a viscosity η of 10 ¹³ dPas with a maximum of three surface defects (Top Specs) with a size of more than 50 microns per m ² at exit from the float chamber.

17. Flat glass according to claim 16, characterized in that it has a maximum of three surface defects with a size of more than 35 microns per m ² .

18. Flat glass according to claim 16, characterized in that it has a maximum of three surface defects with a size of more than 20 microns.

19. Flat glass according to at least one of claims 16 to 18, characterized in that it has a maximum of 2 surface defects per m ² .

20. Flat glass according to at least one of claims 16 to 19, characterized in that it has a transformation temperature Tg of 650 to 78O ⁰ C, in particular from 700 to 730 ° C at a viscosity η of 10 ¹³ dPas.

21. Flat glass according to at least one of claims 16-20, characterized in that it has a thickness of less than 1, 5 mm, in particular 0.2 to 0.9 mm.