DE102007007577A1 - Method for avoiding a surface coating and/or crater-like hole defects during the production of ceramic floated lithium-aluminum silicate flat glass comprises producing a glass melt, refining, floating the refined melt and ceramicizing - Google Patents

Abstract

Method for avoiding a surface coating and/or crater-like hole defects during the production of ceramic floated lithium-aluminum silicate flat glass comprises producing a glass melt, refining the melt, floating the refined melt on a surface made from liquid metal in a protective gas atmosphere and ceramicizing. The flat glass contains (in wt.%) 3-5 lithium oxide, 18-25 aluminum oxide and 55-70 silicon dioxide as the main components. The hydrogen content in the protective gas is delimited to a maximum of 7 vol.%. The oxygen partial pressure in the floating glass melt is adjusted using a metal. An independent claim is also included for a ceramic flat glass obtained by the above method.

Description

The The invention relates to a method for avoiding surface defects in the case of flat float glass or ceramised flat glass, a optionally ceramised flat glass and its use.

One possibility for the production of flat glass ceramics consists of floating glass, which is ceramized in a further process step. Such a float method for glass is generally known and for example in the US 2,911,759 described. In doing so, an endless glass ribbon having desired dimensions in width and thickness is prepared by continuously pouring molten glass onto the surface of a bath of molten metal. On the surface of this molten metal now floating on the floating glass, such as oil on water. The temperatures in and especially over the molten metal have a hotter and a colder area, wherein the molten glass is applied in the hot area and slowly lifted and discharged in the colder area in the solidified state. Due to the speed of the glass application and the glass discharge from the tub, it is possible to adjust the thickness of the respective glass ribbon. One commonly used metal for this process is molten tin. However, under the usually prevailing at floating temperatures above 600 ° C, especially 700 ° C to 1250 ° C, the hot tin is highly sensitive to oxidation, so that the float process is carried out under an atmosphere of forming gas, which is about 10 vol .-% H ₂ and 90 vol .-% N ₂ contains. In this way, the oxidation of the liquid tin to SnO _{2 is} avoided, which would lead to the destruction of the molten metal.

For example, in the DE 102 09 742 A1 describes a process for the production of float glass, in which the oxygen concentration over the molten metal is to be adjusted so that it does not exceed the saturation solubility of oxygen in the metal bath, which prevails at the cold end, that is at the point of discharge of the metal bath.

In the US-A-4,081,262 In order to avoid the defects described above, it is proposed to arrange a curtain of reducing gas which protects the exposed surface of the tin bath from oxidation. Furthermore, in the US-A-3,356,476 described to remove surface defects of the glass by removing a part of the atmosphere present above the bath and passed over the bath after a cooling process. In this process, the gas during cooling, impurities are removed, for example, when flowing through a bed of silica gel.

It has now shown that when floating glasses, which are ceramized in retrospect, especially of glasses which have been refined with tin on which the molten metal remote glass side, d. H. on the side, which with the atmosphere in contact, form hole defects, which evidently by evaporation of glass components in the ambient atmosphere of the glass ribbon arise.

Such crater-shaped holes can easily reach a diameter and / or a depth of a few microns and are usually less than 1 micron. In addition, it has been found that a whitish deposit of powdered tin oxide is often deposited on the surface of the glass. If such glasses are further processed into glass ceramics, property changes occur during the ceramization in the material, as a result of which the ceramics are destroyed when they are finished. The use of arsenic-refined glass is not suitable for float processes since it is known that arsenic-refined glasses, under the conditions encountered in flooding, tend to have brownish, dark discolorations known as lead or arsenic levels. Such unwanted discoloration under the reducing conditions in the production of flat glass by flooding is for example in the US 4,015,966 described.

This Holes on the top of the glass facing away from the melt forming defect is from the well-known, classical flower effect (Fringling or top service distortion) on the glass bottom arises. This arises z. B. with soda lime glasses (window glass) on the glass bottom when lifting off the float bath by flames, whereby tin droplets get caught on the glass.

For this reason, for example, in the US-A-6,065,309 proposed instead of a tin bath to use a bath of molten precious metals such as gold, silver and copper or their alloys as a float. However, such amounts of molten noble metals are so costly that this technique is not economically viable for normal applications.

The The invention now has for its object a method for producing floated To provide glass-ceramics, which overcomes the problems described above become.

This The aim is achieved by the measures defined in the claims and features achieved.

In the context of the invention, extensive investigations have shown that the aforementioned problems can be avoided by limiting the hydrogen content of the inert gas atmosphere to a maximum of 7% by volume and / or by selectively altering the oxygen activity or the partial pressure of oxygen in the glass itself in that the mixture is admixed with at least one metal or one metal oxide whose reaction enthalpy for the oxidation is ΔG (Me / Me _x O _y )> -575.8 + 0.21 · T [K], or at least one metal - or metal oxide is added whose reaction enthalpy for the oxidation ΔG (Me / Me _x O _y ) <-575.8 + 0.21 · T [K]. According to the invention, it has been found that the crater-shaped holes are caused by redox reactions of tin and tin oxide.

Of the Content of hydrogen in the atmosphere above the Float glass is a maximum of 7% by volume, with a maximum of 6% by volume are preferred. Particularly preferred is a maximum hydrogen content of 5% by volume, with 3% by volume being more preferred. Most notably preference is given to an atmosphere which is at most 1% by volume Contains hydrogen.

Especially Preferably, these previously mentioned low levels of hydrogen prevail the glass melt and in particular in the area of the Spout Lip and in the Exit area of the float bath. The area of the Spout Lip up to a maximum of 1/3 of the float bath, preferably up to 1/4, wherein up to 1/5 of the float bath is particularly preferred. The exit area is preferably the area of the last bay, especially at least the area of half the last bays. Typical Bay lengths are about 3 m.

about The float baths preferably prevails overpressure by Max. 0.2 mbar (20 Pa), in particular max. 0.15 mbar (15 Pa), where excess pressures of max. 0.1 mbar (10 Pa) especially are preferred. Very particular preference is given to overpressures of less than 0.07 mbar (<7 Pa) and in particular less than 0.05 mbar (<5 Pa).

In In another preferred embodiment, the atmosphere is the one above the glass dominates that atmosphere separated, with the exposed, that is not covered with glass Surface of the molten tin comes into contact. Latter contains expediently hydrogen and although preferably in an amount of at least 10% by volume. A Such separation is readily for example by means of separating Components and / or accessible by means of corresponding gas nozzles, which separate the respective surfaces from each other Provide flow.

The adjustment according to the invention of the oxygen activity in the glass takes place by adding metals or oxides thereof whose formation enthalpy ΔG ⁰ at the float temperatures is either below (first group) or above (second group) that of Sn / SnO ₂ . Such metals / metal oxide systems can be found, for example, in a Richardson-Ellingham diagram. Of course, the adjustment of the oxygen activity according to the invention presupposes that the oxygen activity of the glass or of the melt has previously been determined. For this common tools and procedures are described in Baucke ( FGK Baucke: Electrochemical cells for on-line measuring of oxygen fugacities in glass-forming melts. Glastechn. Ber. 61 (1988), 87 ). These are of course polyvalent metals, ie metals with at least two different, ie at least two positive oxidation states or cationic oxidation states.

Preferably, the free enthalpy of formation .DELTA.G is at least 10 kJ / mol, preferably at least 20, in particular at least 30 kJ / mol above or below the aforementioned system. The added redox systems particularly preferably have an enthalpy of formation which, at the float temperatures, is at least 50, in particular at least 70, and very particularly at least 100 kJ / mol above or below that of Sn / SnO ₂ . Accordingly, metal / metal oxides preferably to be added have a free enthalpy of formation of at least -575.8 + 0.21 × T [K], where T is the temperature prevailing in the float in K. Preferred temperatures of the floating according to the invention are 600-1250 ° C, wherein the hot temperature range of the float bath greater than 1000 ° C is particularly preferred. Further preferred minimum values are -560, in particular -555, with -550 + 0.21 * T [K] being particularly preferred. Further preferred minimum values are 50, in particular 70 kJ / mol, and in particular 100 kJ / mol, above the aforementioned minimum value.

preferred Metals / metal oxides of this first group are in particular Cu, Ni, As, Bi, and optionally Au, Ag and Pt, with Ni and As being particularly are preferred.

Metal / metal oxide systems having a free formation enthalpy below the aforesaid value have a ΔG ⁰ of at most -580 + 0.21 × T [K], with at most -585 and -590, especially -600 + 0.21 × T [K] being most preferred are. Further particularly preferred values are 30, in particular 50, preferably 70 kJ / mol below the abovementioned maximum limit of 150.

preferred Metals / metal oxides of the second group are in particular Fe, Zn, Cr, Mn, Ti and V.

The metals added according to the invention or metal oxides of the first and the second group are preferably present in a concentration of 5 ppm-50,000 ppm. Preferred minimum amounts are 10 or 20 ppm, in particular 50 ppm. Preferred maximum amounts are 30,000 ppm, in particular 20,000 ppm, with 15,000 ppm, in particular 10,000 ppm being particularly preferred. In many cases, even maximum levels of less than 100 ppm and less than 90 ppm and even less than 80 ppm and less than 75 ppm have been found to be sufficient. According to the invention, the addition of elements or their oxides of the first group, ie those which avoid the reduction of SnO ₂ to SnO or oxidize or form SnO ₂ to SnO ₂ , is particularly preferred. Appropriately, such metals or metal oxides are added, which do not form a network with the other glass components or polymerize with these not to glass backbone. Ie. Preferred additives according to the invention are so-called network converters or else intermediate oxides with an increased ionic character.

The inventive method is possible in principle with all glass compositions. However, preference is given to glasses which consist essentially of SiO ₂ , Al ₂ O ₃ , alkaline earth oxides, in particular CaO and / or MgO, and, if appropriate, B ₂ O ₃ . Preferred glasses have no alkali oxides.

typical Float materials are easily liquefiable metals such as Tin and tin alloys with a specific density larger than that of the respective glass.

Aluminosilicate glasses which are preferred according to the invention contain at least 55% by weight of SiO 2, at least 60% by weight and in particular at least 64% by weight being particularly preferred. The maximum limit of SiO ₂ is 70% by weight, in particular less than 68% by weight, with at most 67% by weight being very particularly preferred. B ₂ O ₃ is present in the glass according to the invention in an amount of 0% by weight, in particular at least 0.05% by weight, with minimum contents of 0.1% by weight being particularly preferred. The maximum levels of B ₂ O ₃ are 2% by weight in the glass according to the invention, with 1.5% by weight being preferred.

Al ₂ O ₃ is in the glass according to the invention in an amount of at least 18 wt .-%, in particular at least 19 wt .-% and preferably at least 20 wt .-%, wherein the maximum amount 25 wt .-%, in particular 24.5 wt % and more preferably 24% by weight.

Li ₂ O is contained in the glass of the invention in an amount of 3 to at most 5 wt .-%, with minimum amounts of 3.25 wt .-% and in particular 3.5 wt .-% are preferred. The maximum amount of Li ₂ O is at most 5 wt .-%, especially a maximum of 4.8 wt .-%, with maximum limits of not more than 4.75 wt .-% and in particular 4.6 wt .-% are particularly preferred. The content of Na ₂ O and K ₂ O in the glass according to the invention is 0 to a maximum of 3% by weight, with a maximum limit of not more than 1.5% by weight, preferably not more than 1% by weight being preferred for Na ₂ O. For K ₂ O, the preferred maximum amount is 1.5% by weight, in particular 1% by weight. The sum of the alkali oxides Na ₂ O and K ₂ O is 0 wt .-% and at most 3 wt .-% in the glass according to the invention, with minimum amounts of 0.25 wt .-%, in particular 0.5 wt .-% and maximum amounts of 3 wt .-%, in particular 2.75 wt .-% and preferably at most 2.5 wt .-% are still suitable.

The content of CaO and SrO in the glass according to the invention is in each case independently from 0 to a maximum of 2 wt .-%. A preferred minimum amount of SrO and CaO in the glass according to the invention is in each case independently of one another 0.1% by weight, the preferred maximum amount of SrO and CaO being in each case independently 2% by weight and preferably 1.5% by weight. TiO ₂ and ZrO ₂ are contained in the glass according to the invention in amounts of 0 to a maximum of 3% by weight and 1 to a maximum of 2.5% by weight. Where TiO _{2 is} a minimum amount of 0.5 wt .-%, preferably of 0.8 wt .-% and a maximum amount of 2.75 wt .-%, in particular 2.5 wt .-% is preferred. For ZrO 2, a minimum amount of 1.5% by weight, but especially of 1.5% by weight is preferred and a maximum amount of 2.25% by weight, in particular of 2% by weight. SnO ₂ is contained in the glass according to the invention in amounts of 0.1 to at most 1 wt .-%, wherein a minimum amount of 0.15 wt .-%, preferably of 0.18 wt .-% and a maximum amount of 0.9 wt .-%, in particular 0.8 wt .-% is preferred. The sum of the oxides TiO ₂ , ZrO ₂ and SnO ₂ is in the glass according to the invention 2.5 wt .-% and at most 5 wt .-%, with minimum quantities of 3 wt .-%, in particular 3.25 wt .-% and maximum amounts of 4.9 wt .-%, in particular 4.8 wt .-% are preferred.

The glass according to the invention contains a content of MgO and / or BaO, the content of MgO advantageously being 0-2.2% by weight and BaO independently being 0-2.5% by weight. Preferred is a minimum amount of 0.05 wt .-%, in particular a minimum amount of 0.1 wt .-%, and a maximum amount of 2 wt .-%. Furthermore, the glass according to the invention contains a content of ZnO and / or P ₂ O ₅ , wherein expediently the content of ZnO is 0 to <1.5% by weight and P ₂ O _{5 is} independently 0-3% by weight , Preferred is a minimum amount of ZnO of 0.05 wt .-%, in particular a minimum amount of 0.1 wt .-%, and a maximum amount of 1 wt .-%, in particular 0.75 wt .-%. At P ₂ O ₅ is a minimum amount of 0.5 wt .-%, in particular a minimum amount of 1 Wt .-%, and a maximum amount of 2.5 wt .-%, in particular 2 wt .-% preferred. The glass of the invention contains Nd ₂ O ₃ in a content of 0.01 to 0.6 wt .-%, with 0.05 wt .-% being preferred as a minimum amount and 0.1 wt .-% are particularly preferred. As the maximum amount, 0.55 wt% is preferable and 0.5 wt% is particularly preferable.

It has been found that, although the glass according to the invention is very stable against solarization under UV irradiation, its solarization stability is due to low levels of PdO, PtO ₃ , PtO ₂ , PtO, RhO ₂ , Rh ₂ O ₃ , IrO ₂ and / or Ir ₂ O ₃ can be further increased. In this case, sum amounts of the above oxides in an amount of at most 0.1 wt .-%, vorzugswei se maximum 0.01 wt .-%, in particular at most 0.001 wt .-% have proven to be particularly suitable. The minimum content for these purposes is usually 0.01 ppm, with at least 0.05 ppm and in particular at least 0.1 ppm being preferred.

Although the glass of the present invention may contain small amounts of PbO and Sb ₂ O ₃ for increasing chemical resistance and processability, they are preferably free thereof. If iron is present, it is converted by the oxidizing conditions during the melt in its oxidation state 3 ⁺ and thus causes no discoloration in the visible wavelength range more.

The glass according to the invention can be clarified in the usual way, although a refining with fluorine, chlorine, arsenic, antimony and / or tin is preferred. Particularly preferred is a combined refining with at least two of the aforementioned refining agents, if appropriate also together with further refining agents. For fluorine, a content of 0 ppm to 100,000 ppm (1 wt%) is preferable. For chlorine, a content of 5 ppm to 500,000 ppm (5% by weight) is preferred. In the others mentioned, a content of refining agent equal to that of SnO _{2 is} preferred.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• US 2911759 [0002]
• DE 10209742 A1 [0003]
• - US 4081262 A [0004]
• - US 3356476 A [0004]
• - US 4015966 [0006]
• - US 6065309 A [0008]

Cited non-patent literature

• - FGK Baucke: Electrochemical cells for on-line measuring of oxygen fugacities in glass-forming melts. Glastechn. Ber. 61 (1988), 87 [0016]

Claims (12)

A method for avoiding a surface covering and / or crater-shaped hole defects in the production of ceramised, floated lithium aluminum silicate flat glass, comprising producing a glass melt, refining the melt and floating the refined melt on a surface of liquid metal under a protective gas atmosphere and subsequent ceramization of the Glass, characterized in that the flat glass contains as main constituents: Li ₂ O: 3-5 wt% Al ₂ O ₃ : 18-25 wt% SiO ₂ : 55-70 wt% and that i) a hydrogen content in the protective gas to a maximum 7% by volume and / or ii) determines the partial pressure of oxygen in the floating glass melt and a) selectively adjusts it by means of a metal whose reaction enthalpy for the oxidation ΔG ⁰ (Me / Me _x O _y )> -575.8 + 0.21 × T [K] is or b) selectively adjusted by means of a metal whose reaction enthalpy for the oxidation ΔG ⁰ (Me / Me _x O _y ) <-575.8 + 0.21 · T [K] amount t. A method according to claim 1, characterized in that the lithium-aluminosilicate glass SiO ₂ 55-69 Al ₂ O ₃ 19-25 Li ₂ O 3.2 to 5 Na ₂ O 0-1.5 K ₂ O 0-1.5 MgO 0-2,2 CaO 0-2.0 SrO 0-2.0 BaO 0-2.5 ZnO 0- <1.5 TiO ₂ 0-3 ZrO ₂ 1-2,5 SnO ₂ 0.1 <1 TiO ₂ + ZrO ₂ + SnO ₂ 2,5-5 P ₂ O ₅ 0-3 F 0-1 B ₂ O ₃ 0-2, Nd ₂ O ₃ 0.01-0.6 in% by weight based on oxide and optionally additives of coloring oxides, such as. B. Fe ₂ O ₃ , CoO, NiO, V ₂ O ₅ , Nd ₂ O ₃ , CeO ₂ , Cr ₂ O ₃ , in amounts up to 1 wt .-%. Method according to claim 1, characterized in that that the hydrogen content of the inert gas atmosphere over the glass is at most 1 vol .-%. Process according to claims 1 and 2, characterized characterized in that the hydrogen content is especially in the range of Spout lip and exits maximum 3 vol .-% is. Method according to one of the preceding claims, characterized in that a metal is added to the glass whose reaction enthalpy ΔG (Me / Me _x O _y ) is at least -560 + 0.21 · T [K]. Method according to claim 5, characterized in that the metal is Au, Ag, Pt, Cu, Ni, As and / or Bi. Method according to one of claims 1 to 4, characterized in that the added metal has a reaction enthalpy ΔG ⁰ (Me / Me _x O _y ) of at most -580 + 0.21 · T [K]. Method according to claim 7, characterized in that the metal is Fe, Zn, Cr, Mn, Ti and / or V. Method according to one of claims 5 to 8, characterized in that the metal is added in a concentration of 5 ppm to 5 × 10 ⁴ ppm. Method according to one of the preceding claims, characterized in that the melt with As, Sb and / or Sn is purified. Ceramised flat glass available after the method according to any one of claims 1 to 9. Use of a flat glass-ceramic according to claim 10 as a fire-resistant glass, fireplace visor, oven slice, especially for pyrolysis stoves, covers of lights high energy, cooking surface or as a cover plate in microwave ovens.