CN1876585B - Special float glass and its manufacturing method - Google Patents

Abstract

The present invention describes a process for reducing the number of surface defects in the production of float glass, in which a stream of thermally protective gas is passed over the free surface of the molten metal of the float pool from the direction of the glass ribbon towards the side walls of the float chamber and contains The gas flow of evaporated impurities is removed through the openings in the side wall regions of the floating chamber.

Description

Special float glass and its manufacturing method

technical field

The present invention relates to a float glass with a reduced number of surface defects (top spots) and a process for its manufacture, the specialty glass having a transition temperature (Tg) above 600°C at a viscosity η of 10 ¹³ dPas.

Background technique

In the flotation process, which has been known and widely used for decades, a ribbon of liquid glass is continuously injected over a pool of liquid metal, usually a tin or tin alloy pool. As it passes along the pool, the strip is shaped and cooled enough to retain its shape when it is removed from the pool. Because tin baths can be easily oxidized, float baths are operated in closed chambers (float chambers) into which a protective atmosphere at a pressure slightly above atmospheric pressure is introduced, which includes nitrogen with some percentage of hydrogen . However, it is practically impossible to completely exclude oxygen from the atmosphere above the liquid tin and glass ribbon and from the liquid tin itself.

Oxygen, for example, can enter the float as an impurity to nitrogen and hydrogen, can enter the float through leaks in seals located on the sides of the float, can enter the float through outlet seals and with the liquid glass itself. Oxygen contained in the atmosphere participates in the interaction reaction with hydrogen to form water, reacts with liquid tin, increases the degree of oxygen, and reacts with the glass itself. Liquid tin can absorb oxygen through the interaction with the atmosphere, glass and ceramic floating pool tiles.

If oxygen or sulfur is present in the floating bath, tin sublimates in the form of SnO or SnS above about 850°C. The vapor pressure of SnO and SnS is 10 times higher than that of tin (at 1000°C). Sulfur is provided by the glass itself. The SnO or SnS sublimated in this way condenses or deposits in a region with a relatively low temperature, for example, in the top plate area of the floating pool, to form a top plate deposition product. In addition, SnO or SnS is reduced by hydrogen to form metallic tin, which drips on the ribbon to generate defects. Sometimes SnO and SnS may also drop.

Sublimed SnO can also be reduced in ambient atmosphere, so very fine droplets can settle on the glass ribbon. Defects, also known as top spots, consist primarily of tin particles, sometimes tin oxide or tin sulfide particles, located on or adhered to the surface, but also microscopic small irregularities on the glass surface, known as "impact craters", It is formed when tin particles impinge on soft glass surfaces.

Increased evaporation of pool constituents occurs especially in the case of special float glass, which is produced and floats at much higher temperatures than standard soda-lime glass (window glass), which leads to an increased number of surface defects.

Examples of measures aimed at avoiding these disadvantages include internal devices arranged in the lower part of the ceiling of the floating chamber and attempting to prevent the small tin droplets formed from dripping onto the glass ribbon. An internal device of this type is described, for example, in DE 40 21 223 C2.

Protective gas is usually sent to the float chamber through the top. Some of this gas exits the float chamber at the end of the float chamber (exit area), since the glass ribbon also leaves the float chamber from here, and imperfect sealing is unavoidable. There is therefore a flow tendency of the protective atmosphere in the float chamber in this direction, ie in the direction of the cooler parts of the float chamber.

However, full gas flow to lower temperatures should be avoided, as impurities dissolved in the ambient gas may condense as the gas temperature drops, contaminating the top side of the glass. As a further measure, the hot ambient air is thus sucked out of the hot zone by means of known vents, thereby preventing a total flow in the direction of the outlet end. A process of this type has been described, for example, in DE-B1 596 586 and in the equivalent US 3 356 476 A. However, controlled influence on the flow of ambient air through the suction ventilation is only possible to a very limited extent because of the limited range. The reason is the lack of momentum transferred to the gas molecules. During this process, a very large amount of gas is extracted from the side wall of the floating chamber in the hot zone, cooled to below 100°C to condense out gaseous impurities, and then reintroduced into the top plate of the floating chamber. Cooling and recirculating such large volumes of gas (from 50% to over 80% of the total gas supplied) results in high energy costs, and blowing such large volumes of cold gas has an adverse effect on the thermal balance of the float chamber, especially in the manufacture of thin Glass can cause problems. Known measures generally give satisfactory results for standard soda-lime glass (window glass), since the Tg of about 520° C. is low, and the resulting low floating pool temperature and relatively low demands on surface quality . However, the requirements for the absence of surface defects in special float glasses are significantly higher, especially for thin glasses that are used for display applications, especially for TFT displays (TFT = Thin Film Transistor), so there is a need for a reduction in the number of straps The pressing need for surface defects in specialty float glass and its manufacturing process.

It is known from JP 10-085648A, JP 09-295832A and JP 09-295833A to convert existing surface defects (top spots) into volatile halides by decomposing ammonium halides at a temperature of 400-900°C, or by wet chemical means These surface defects are dissolved using an oxidizing acid treatment, or etched away by an HF solution. However, this is still clearly not enough to produce a perfect surface, because in JP 09-295833 A a polishing operation is also carried out after the acid treatment, to remove pits which appear on the glass surface. Further processing of this type is expensive and entails high costs for environmental protection measures to ensure that the chromium ions, HF etc. used in the oxidation do not enter the environment.

Contents of the invention

It was therefore an object of the present invention to find a special float glass, in particular for display applications, which has a reduced number of surface defects when it comes out of the float chamber, and to find a process for its manufacture.

The flat glass produced by the float process according to the invention has a transition temperature Tg of at least 600° C. at a viscosity η of 10 ¹³ dPas, has a thickness of less than 1 mm, and has up to three surface defects per square meter having a size exceeding 50 μm (top spot). It is especially suitable for the production of TFT (Thin Film Transistor) screens. Since heat treatment is used as part of producing the screen, it is advantageous to use a glass with a relatively high transition temperature to facilitate the stabilization of the glass. Preference is therefore given to flat glass with a transition temperature Tg between 600°C and 850°C. Also, it is advantageous to use the thinnest glass possible to reduce mass. Glass with a thickness between 200 μm and 900 μm is therefore preferred. If the thickness is smaller than this range, it will be difficult to handle this ultra-thin glass, especially the large size exceeding 1×1 m ² . The number of surface defects (top spots) and their size are important for the quality of the glass, especially for TFT screen applications. Glasses with at most two surface defects are therefore preferred. In addition, it is preferable if the surface defect is not larger than 35 μm, especially not larger than 20 μm. Since the apex is usually circular, a dimension designation of 50 or 35 or 20 μm refers to a circular defect with a diameter of that size. In the case of an indicated defect having an oblate or similar shape, the dimension index refers to the largest diameter of the defect. It is pointed out only for the sake of clarity that the float glass according to the invention with high surface quality is float glass coming out of the floatation plant, ie without subsequent processing steps such as grinding, polishing or chemical treatment of the surface.

The way the new method operates involves blowing a stream of hot gas from the glass ribbon in the direction towards the side walls of the chamber to provide a controlled momentum to the ambient furnace atmosphere in this region and thereby create a targeted flow over the free surface of the chamber . The result is that steam escaping from the floating tank is immediately absorbed and discharged through openings in the side walls of the floating chamber. Condensation may therefore not occur on the cooler areas of the float chamber. Additionally, any oxygen that enters the floating chamber through an imperfectly sealed area also has less impact because it is absorbed by the targeted flow of protective gas and exhausted before it can react extensively with the floating tank.

The protective gas used is preferably a mixture of nitrogen and hydrogen, which is also used for inerting the floating chamber. It is advantageous to supply the protective gas in the edge region of the glass ribbon, but it is also possible to supply the protective gas in the upper center of the glass ribbon. The protective gas flow needs to be hot so as not to cause undesired cooling of the glass ribbon or pool. The protective gas flow is provided at a temperature typically between 400°C and 1200°C. It is preferred that the temperature of the gas corresponds approximately to the temperature of the floating tank at the point of introduction of the gas; deviations from this temperature of up to 100° C. do not cause any damage.

It is preferred that the gas flow passing over the floating pond is passed in a laminar flow over the floating pond, as this prevents the occurrence of reverse flow or mixing which may be caused by turbulent flow.

To prevent extensive mixing of vapors emerging from the floating pond with the ambient atmosphere of the floating pond, these vapors should be removed by the gas stream as soon as possible after they are formed. For this purpose, it is advantageous to have the gas flow pass over the floating pond over the shortest distance possible. For this purpose, protective gas is blown through nozzles directed towards the side walls of the floating tank at a distance of 3 to 30 cm above the edge of the glass ribbon. The nozzles may be oriented such that the flow of protective gas contacts the surface of the floating pond tangentially. The term nozzle is to be understood not only as a nozzle in the narrow sense, but as any form of outlet opening from which a gas flow can emerge. These openings can be holes, polygonal, slot-shaped, oblate or similar openings. Nozzles which are already present in the flotation chamber, eg for washing windows or camera lenses, can also be used for the purpose of the invention if they can be oriented in a suitable manner. However, it is also possible to direct the protective gas flow at a small distance from the surface above the surface of the floating pool, and at least the distance between the protective gas flow and the surface of the floating pool should preferably not be greater than 20 cm, because otherwise the gas may form a glass band directly above the Sn surface. reverse direction.

The velocity of the protective gas flow over the floating pond may be relatively low. Usually, depending on the structure of the floating pool, a speed of at least 0.1 ms ^-1 is used. Speeds greater than 1 ms ^-1 are preferred. The gas flow should have the above velocity at every position on the free surface of the floating pool. The formation of dead flow spaces should be avoided as much as possible.

Mixing of the protective gas stream containing substances evaporated from the surface of the floating pool with the remainder of the protective atmosphere located in the floating pool should be avoided because otherwise the impurities will spread throughout the floating pool and the glass surface will be contaminated. Thus, the protective gas flow containing evaporated substances is removed from the floating chamber in the region of the side walls immediately after it passes over the floating tank, advantageously through the openings in the side walls. These openings can be arranged in the side walls especially for this purpose, but those slots already present in the side walls, for example the slots through which the top roller shaft is led, can also be used for this purpose. Due to the superatmospheric pressure present in the floating chamber, the gas flow can be withdrawn from the opening without additional means. However, it is also possible to cause the gas flow to be drawn from the opening by a fan or other suitable means, such as a jet pump. In addition, the gas flow can also be taken off from the side wall region via a pipe with suction openings, which can advantageously be arranged at the side wall of the floating chamber parallel to the surface of the floating tank. It is advantageous if the protective gas flow is taken off close to the surface of the buoyant gas flow in the region of the side walls. The outlet or suction opening should advantageously be arranged no higher than 30 cm above the surface of the floating pool. In order to achieve a predetermined removal of the protective gas flow, there should be a relatively large number of removal openings. However, it is also possible to use openings in the form of wide slot suction openings.

Best results are obtained if the surface of the pool is purged with a protective gas along the entire length of the pool. However, because the higher the temperature, the more intense the evaporation of the destructive float components, for economical reasons, it is usually only carried out with a protective gas flow for those areas of the float where the molten metal temperature of the float is higher than about 800°C purify.

The protective gas stream can pass over the free surface of the floating pool from a large number of individual nozzles, but of course wide slot nozzles can also be used. It is only necessary to ensure that a directed flow of protective gas from one region develops in the direction of the side walls of the pontoon chamber. It is advantageous for the supply line of the protective gas to be made of quartz glass, since this material has sufficient strength even at high temperatures and does not lead to Sn or any damaging impurities in the glass. However, it is also possible to use supply lines made of other suitable materials such as high temperature resistant stainless steel, Monel (up to 400°C), Hastelloy (up to 1090°C), Al ₂ O ₃ , ZrO ₂ etc. .

In the simplest possible solution, the supply line for the protective gas flow enters the float chamber from the outside through the chamber side walls (side seals). However, it is also possible to introduce the supply lines through the top of the float chamber, resulting in longer lines in the hot float chamber. However, this has the advantage of being equal to the temperature of the ambient air in the thermal chamber.

Description of drawings

The present invention is further explained with reference to the following figures, each figure is:

Figure 1 shows a partial sectional view through the floating chamber with protective gas flow and extraction openings,

Figures 2 and 2a show in section and plan a partial view of the floatation chamber with a flow of protective gas fed through the top drum opening,

Figure 3 shows a sectional view through the floatation chamber, where the top drum opening is used both as a protective gas supply opening and as a protective gas discharge opening,

Figures 4 and 4a show a partial view through a floatation chamber similar to that shown in Figure 1, where the protective gas is fed through a number of nozzles arranged on the tube.

Detailed ways

Figure 1 schematically depicts a partial sectional view through the float chamber. The glass ribbon whose edge 1 is visible floats on the tin pool in the direction towards the viewer. The protective gas is blown in through the supply line 6, which passes through the side wall 4 of the floating chamber in the direction indicated by the arrow, and the protective gas comes out through the opening at the hook-shaped end of the supply line and in the direction of the outlet 2 in the The direction indicated by the arrow flows over the free surface of the tin pool 3 . It can be seen from the figure that the protective gas flows through the tin pool at a small distance above the tin pool 3 and then immediately leaves the floating pool through the side wall 4 .

Figures 2 and 2a show a similar structure, with the difference that the supply line 6 for the protective gas passes through the opening for the passage of the shaft of the top roller 5 already present in any case on the side wall 4 of the buoyancy chamber , of course the opening is sealed with respect to the outside. The protective gas flowing in the direction indicated by the arrow 7 is taken out through the opening 2 arranged in the side wall.

FIG. 3 shows an embodiment similar to that shown in FIG. 2 , except that the gas discharge line 2 is also formed by a tube which, similar to the supply line 6 , is arranged in the side wall 4 provided to the top. in the opening of the drum. The tube 2 is provided with openings through which the protective gas which has been supplied and flows in the direction indicated by the arrow 7 is drawn off directly above the tin bath 3 .

In FIG. 4 , the pipe for supplying the protective gas is developed with a nozzle pipe 8 which runs parallel to the edge of the glass ribbon 1 and is provided with a plurality of individual nozzles. A nozzle tube of this type makes it possible to pass a wide gas cloud over the free surface of the tin bath 3 in a simple manner. The gas flow is taken off through the opening 3 . As an alternative to the nozzle tube, wide-slot nozzles can of course also be used. It is also advantageous to arrange devices like nozzle pipes or wide-slot nozzles through which the protective gas is drawn inside the side wall 4 of the float chamber, because in this case the protective gas can flow uniformly across the tin bath 3 above the free surface 3, instead of flowing to the opening 2 in a star shape as shown in Figure 4. Wide-slotted nozzles or pipes with openings arranged inside the side wall 4 of the floating chamber are advantageous for the discharge of the protective gas flowing over the floating tank, since in this case the required passage through the side wall 4 for the discharge There are fewer gaps for the gas, however the protective gas is withdrawn very uniformly.

Claims (12)

1. float glass is characterized in that:

10 ¹³At least 600 ℃ transition temperature Tg under the viscosity η of dPas,

Less than the thickness of 1mm,

When from floating apparatus, coming out, every square metre at the most three sizes surpass the surface imperfection of 50 μ m.

2. float glass according to claim 1 is characterized in that, it has 600 ℃ to 850 ℃ transition temperature Tg.

3. float glass according to claim 1 and 2 is characterized in that its thickness is from 0.2mm to 0.9mm.

4. float glass according to claim 1 and 2 is characterized in that, it have every square metre at the most three sizes surpass the surface imperfection of 35 μ m.

5. float glass according to claim 1 and 2 is characterized in that, it has at the most the surface imperfection that three sizes surpass 20 μ m.

6. float glass according to claim 1 and 2 is characterized in that, it has two surface imperfection at the most.

7. make the method for the described float glass of claim 1 by floating process; it is characterized in that; 3 to 30cm are blown into the protective gas stream that is preheating to from 400 ℃ to 1200 ℃ temperature heat above the glass ribbon edge; and protective gas stream is from the top of glass ribbon towards the free surface of the deposite metal in the floating pond of direction process of floating chamber's sidewall; and the gas stream that contains the impurity that comes from deposite metal evaporation is removed by the opening of floating chamber's sidewall areas, makes this float glass have the surface imperfection that quantity reduces.

8. method according to claim 7 is characterized in that, uses laminar gas flow.

9. according to claim 7 or 8 described methods, it is characterized in that gas stream has the speed of at least 0.1 meter per second above the free surface of floating pond.

10. process according to claim 9 is characterized in that gas stream has the speed that is higher than 1 meter per second.

11., it is characterized in that gas stream is through floating pool area top according to claim 7 or 8 described processes, the deposite metal is in and is higher than under 800 ℃ of temperature in the floating pond.

12., it is characterized in that gas stream is with above the deposite metal of distance through floating pond that reaches 20cm according to claim 7 or 8 described processes.

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