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  Optically detectable, floatable arsenic- and antimony-free, glazable lithium-aluminosilicate glassUSPTO Application #: 20070259767Title: Optically detectable, floatable arsenic- and antimony-free, glazable lithium-aluminosilicate glass Abstract: An optically detectable, floatable arsenic- and antimony-free, glazable lithium-aluminosilicate glass that can be prestressed and the glass ceramic converted therefrom are described. The glass or the glass ceramic has a composition (in % by weight based on oxide) of essentially SiO2 55-69, Al2O3 19-25, Li2O 3.2-5, Na2O 0-1.5, K2O 0-1.5, MgO 0-2.2, CaO 0-2.0, SrO 0-2.0, BaO 0-2.5, ZnO 0-<1.5, TiO2 1-3, ZrO2 1-2.5, SnO2 0.1-<1, Σ TiO2+ZrO2+SnO2 2.5-5, P2O5 0-3, Nd2O3 0.01-0.6, CoO 0-0.005, F 0-1, B2O3 0-2. (end of abstract) Agent: Millen, White, Zelano & Branigan, P.C. - Arlington, VA, US Inventors: Friedrich Siebers, Hans-Werner Beudt, Bernd Rudinger, Gerhard Lautenschlager, Klaus Schneider, Michael Jacquorie, Wolfgang Schmidbauer USPTO Applicaton #: 20070259767 - Class: 501059000 (USPTO) Related Patent Categories: Compositions: Ceramic, Ceramic Compositions, Glass Compositions, Compositions Containing Glass Other Than Those Wherein Glass Is A Bonding Agent, Or Glass Batch Forming Compositions, Silica Containing, 40 Percent - 90 Percent By Weight Silica, And Halogen Or Nitrogen, Fluorine, , The Patent Description & Claims data below is from USPTO Patent Application 20070259767. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The invention relates to an optically detectable, floatable arsenic- and antimony-free, glazable lithium-aluminum silicate glass that can be prestressed and the glass ceramic that is converted therefrom. [0002] Because of their excellent thermal properties, panes made of glass ceramic or their precursor glass have multiple uses, i.a., for stove tops, oven doors, fireplace doors and for fire protection glazing. For many applications of glasses, the panes are prestressed thermally or chemically for the purpose of increasing the strength as safety glasses or for operator protection. [0003] To achieve a high optical quality, these glasses are produced according to the float process. Transparent panes then can virtually no longer be distinguished visually from window glass (lime-sodium glass). Also, the floating allows the production of flat glasses with larger dimensions than other shaping processes, since during floating, band widths of over 2 m to about 5 m are common. Such a glass is described in, e.g., DE 100 17 01 C2 and the corresponding U.S. Pat. No. 6,846,760 B2. [0004] If such glasses or glass ceramics are recycled together with normal flat glass, in the glassworks that produce common sodium-lime glass, a large proportion of the problems that they experience are associated with the cullets that are delivered to them, since glass ceramics and their precursor glasses dissolve only very slowly in the melting conditions that prevail in the sodium-lime melting tanks and, when they accumulate in large amounts, impair the function of the melting tank and the shaping. [0005] Glass ceramics and their precursor glasses, which are to be produced according to the float process, have to be refined free of arsenic and antimony. Under the effect of the reducing conditions during floating, namely the above-mentioned refining agents are reduced right on the glass surface and form disruptive and visually obvious metallic deposits. The removal of the latter for the application of disruptive and toxicologically harmful deposits by grinding and polishing is disadvantageous for economic reasons. In addition, the use of As.sub.2O.sub.3 and Sb.sub.2O.sub.3 is also disadvantageous from safety and environmental aspects, since in the recovery of raw materials and preparation and because of the evaporation in the melt, as well as in post-processing processes and in recycling and dumping of waste, special precautionary measures must be taken. [0006] In addition to the very expensive underpressure refining by purely physical means, the arsenic- and antimony-free refining is usually carried out chemically and preferably with use of tin compounds. This tin refining has the drawback, however, that in particular in glazing, a disruptive SnTi color complex occurs, which absorbs in the short- to middle-wave portion of the visible light. This color complex causes disruption in the floated starting glass only in the case of high quality requirements, but it considerably intensifies during glazing and results in a clearly observable yellow-brown coloring. [0007] For economic and environmental-protection political reasons, increasing importance will be given to glass recycling in the future. To ensure as economical recycling of old cullets as possible, optical processes that separate the cullets based on their different absortion bands are increasingly used. In this case, e.g., the cullets are transported in an assembly line through a light barrier, whereby the light wave frequency that is emitted or absorbed by the respective cullet is detected, and the cullet is blown in general pneumatically, depending on the detected frequency, into the corresponding collecting tank. [0008] The object of the invention consists in further developing a glass ceramic or its precursor glass, which are in the composition range of the above-mentioned DE 100 177 01 C2 or U.S. Pat. No. 6,846,760, such that they can be detected in a cullet sorting unit by optical processes, and the yellow-brown coloring that is based on the formation of the Sn/Ti complex can be reduced or completely suppressed. [0009] This object is achieved by the glass or the glass ceramic described in Claim 1. [0010] The optically detectable, floatable arsenic- and antimony-free, glazable glass according to the invention that can be prestressed and the glass ceramic converted therefrom have a composition (in % by weight based on oxide) of TABLE-US-00001 SiO.sub.2 55-69 Al.sub.2O.sub.3 19-25 Li.sub.2O 3.2-5 Na.sub.2O 0-1.5 K.sub.2O 0-1.5 MgO 0-2.2 CaO 2.0 SrO 2.0 BaO 0-2.5 ZnO 0-<1.5 TiO.sub.2 1-3 ZrO.sub.2 1-2.5 SnO.sub.2 0.1-<1 .SIGMA. TiO.sub.2 + ZrO.sub.2 + SnO.sub.2 2.5-5 P.sub.2O.sub.5 0-3 Nd.sub.2O.sub.3 0.01-0.6 CoO 0-0.005 F 0-1 B.sub.2O.sub.3 0-2 [0011] In the case of floated, prestressable glass ceramic according to the invention or in the case of flat glass that can be converted into a transparent, colorless glass ceramic with high-quartz mixed crystals or keatite-mixed crystals as a main-crystal phase, the disruptive inherent color that is based on Sn/Ti color complexes is reduced by additions of compounds of Nd in contents of 100 to 6,000 ppm. In this information, the Nd content is converted into an oxide base (Nd.sub.2O.sub.3), whereby the type of Nd additive in the batch is not limited to the indicated oxide, but rather any Nd compounds can be added. [0012] Studies have shown that the Nd behaves to a large extent inertly as a labeling and staining agent in the float process. The selection of Nd is advantageous, since the Nd with its stable valence as a trivalent ion cannot be reduced by the reducing action of the float atmosphere that consists of forming gas or liquid Sn. Such a reduction is usually associated with surface defects during floating. [0013] Additions of CO in a total amount of up to 50 ppm (converted to CoO) to the Nd additive are advantageously to set the color point of the floated flat glass or the transparent glass ceramic produced therefrom more precisely in the direction of the achromatic point. The Nd additive by itself does not exactly shift the color point in the direction of the achromatic point, so that this slight correction may be advantageous. In these low contents, the above-mentioned additives have proven to be non-disruptive in the float process. In addition to CO, other staining agents, such as e.g., Ni, V, Cr, Mn, Cu, Ce, Se or rare earth ions, can also be used in addition in small contents to set the color shade. [0014] The additions of Nd have the advantage that this element in addition also readily counteracts the coloring by Fe/Ti complexes, as known in the art from U.S. Pat. No. 4,093,468, which produce a similar color to the Sn/Ti complex. [0015] The color point that is measured in the CIE color system or in the lab color system is shifted by Nd quite well in the direction of the achromatic point. In addition, the Nd as a coloring ion from the 4f group of the periodic table has a great number of characteristic absorption bands that make possible a clear labeling. In the conversion of the floated flat glass into the transparent glass ceramic, these absorption lines are changed only slightly, while the absorption bands, e.g., the Co and Ni, are noticeably changed. This is substantiated in that the coloring ions of the 3d-element group of the periodic table with their absorption bands are influenced more greatly by the crystal field environment. The change in the crystal field environment takes place in the glazing in that Co and Ni are incorporated in the high-quartz mixed crystal. [0016] The oxides Li.sub.2O, Al.sub.2O.sub.3 and SiO.sub.2 are necessary components within the indicated limits for the chemical prestressability of the floated flat glass and for the conversion into the glass ceramics with high-quartz and/or keatite-mixed crystal phases. Li.sub.2O contents of over 5% by weight result in an accidental devitrification in the production process. As additional components, MgO, ZnO and P.sub.2O.sub.5 can be incorporated in the crystal phases. The ZnO content is limited because of the problem of forming glass defects during floating. The MgO content is limited to a maximum of 2.2% by weight, preferably to 0.1 to 2.0% by weight, since otherwise it unacceptably increases the expansion coefficients of the glass ceramic. To avoid high viscosities of the glass and the tendency toward undesirable crystallization of mullite, the Al.sub.2O.sub.2 content is limited to a maximum of 25% by weight, preferably 24% by weight. The SiO.sub.2 content is to be at most 69% by weight, preferably 68% by weight, since this component greatly increases the viscosity of the glass. Thus, for the melting down of glasses and with respect to the temperature stress of the float part in the shaping, higher contents of SiO.sub.2 are disadvantageous. The addition of the alkalis Na.sub.2O, K.sub.2O, the alkaline-earths CaO, SrO, BaO, as well as F and B.sub.2O.sub.3 improves the meltability and the devitrification behavior of the glass during floating. The contents are limited, however, since these components essentially remain in the glass ceramic residual glass phase, and increase the thermal expansion in an unacceptable way, by which the temperature resistance of the glass ceramic deteriorates. Also, higher contents can adversely affect the crystallization behavior in the conversion of the floated flat glass into the glass ceramic. The sum of the alkalis Na.sub.2O+K.sub.2O is preferably to be 0.1 to 2% by weight, preferably 0.2 to 2% by weight, in particular 0.4 to 1.5% by weight. The addition of P.sub.2O.sub.5 can be up to 3% by weight and is preferably limited to 2% by weight. The addition of P.sub.2O.sub.5 is advantageous for the devitrification resistance during floating, but higher contents have a disadvantageous effect on the acid resistance. The contents of the nucleating components TiO.sub.2, ZrO.sub.2, and SnO.sub.2 can be controlled within relatively narrow limits. On one side, minimum contents of 2.5% by weight, preferably at least 3% by weight, are necessary overall to produce nuclei in high density during the nucleation so that transparent glass ceramics can be produced after the high-quartz mixed crystals grow. By the high nuclear density, the mean crystallite size of the high-quartz mixed crystals remains limited to values of <100 nm, by which a disruptive light scattering is avoided. Higher nucleating agent contents than 5% by weight result, however, under the time/temperature conditions of the floating even in disruptive surface crystals in contact between glass and tin bath. A nucleating agent content of at most 4.5% by weight is preferred. In any case, a minimum content of TiO.sub.2 of 1% by weight is necessary for an effective nucleation. The TiO.sub.2 content is to be at most 3.0% by weight, preferably up to 2.6% by weight, since this component is involved in the formation of the Fe/Ti and Sn/Ti color complexes that are disruptive for the inherent colors. The contents of Nd are necessary to achieve the purpose according to the invention of a reduction of the inherent colors of the floated flat glasses and the transparent glass ceramics that are produced therefrom by staining. In addition, they are used to label clearly the flat glasses according to the invention and the glass ceramics that are produced therefrom and to improve the recycling capability. Additions of Co allow the color site to be placed more precisely in the vicinity of the achromatic point. [0017] The production of disruptive surface defects during floating of LAS glasses is avoided in a way that is known in the art by the limiting of the contents of Pt to less than 300 ppb, Rh to less than 30 ppb, and ZnO to less than 1.5% by weight, as well as SnO.sub.2 to less than 1% by weight. If the glass contains more than 300 ppb of Pt or more than 30 ppb of Rh in dissolved form, metallic excretions of Pt or Rh particles can be formed by the reducing conditions of the float atmosphere near the glass surface. The latter act as nuclei for large to up to 150 .mu.m high-quartz mixed crystals and thus produce a disruptive surface crystallization. These noble metals, which are used in float units, in particular as electrodes, lining, stirrers, transport pipes, slide valves, etc., in the melt or float part are therefore avoided to a great extent in units for producing the flat glass according to the invention and are replaced by ceramic materials, or the construction is designed such that the above-mentioned contents are not exceeded. [0018] The ZnO content is limited to 1.5% by weight, preferably to at most 1% by weight. It has been shown that under the reducing conditions of the floating, the zinc is partially reduced in the surface of the glass and thus evaporated into the float atmosphere because of the higher vapor pressure of Zn.sup.o compared to the Zn.sup.2+. In addition to the evaporation that is undesirable for the operation of the float unit and the separation of the Zn at colder locations, the uneven distribution of the Zn in the floated flat glass is disadvantageous. The Zn is depleted on the top side of the floated flat glass, which is exposed to the float atmosphere, compared to the bottom side of the floated flat glass, which is in contact with the Sn bath. This unequal distribution of the Zn content results in different behaviors of the top and bottom sides of the floated flat glass, e.g., in the post-processing processes, such as coating and prestressing, and it is also disadvantageous during glazing. In interaction with other polyvalent glass components, the Zn can result in disruptive surface defects, such as, e.g., a crystal band. This crystal band is produced in interaction with the reducing influence of the float atmosphere on polyvalent glass components, here in particular the partial reduction of Ti.sup.4+ to Ti.sup.3+. Since both Zn and Ti are involved in the production of the crystal band, it has been shown that their contents preferably are to meet the condition in % by weight: 3.2.times.ZnO+TiO.sub.2.ltoreq.4.3. The ZnO content also intensifies the formation of pellets that consist of metallic Sn, or an Sn/Zn alloy in the glass on the float top side of the glass. It is therefore advisable to keep the starting value of the ZnO in the glass small from the outset. [0019] The SnO.sub.2 content in the glass is to be 0.1 to less than 1% by weight, preferably 0.2 to 0.6% by weight. SnO.sub.2 is necessary for the refining of the comparatively high-melting glasses. The limitation of the SnO.sub.2 content to less than 1% by weight helps to improve the devitrification resistance of the glass melts. Higher contents of SnO.sub.2 can result in that in the area of the shaping, i.e., in viscosities in the processing temperature of the glass of 10.sup.4 dPas, undesirable Sn-containing crystal phases are produced. The upper devitrification limit (OEG) is preferably to be below the processing temperature V.sub.A. Also, by higher SnO.sub.2 contents, the corrosive attack of the glass melts on internals that consist of Pt or Pt/Rh is intensified and can increase their contents via the critical boundary values. Another glass defect caused by higher SnO.sub.2 contents is the formation of pellets ("hole defects") that consist of metallic Sn in glass on the float top side, which is exposed to the reducing float atmosphere. These pellets are about 100 nm in size and can be partially removed during cooling or cleaning, but leave behind more spherical holes in the glass surface, which cause problems when the glass is put to use. [0020] The glasses according to the invention are refined without using the refining agents arsenic oxide and/or antimony oxide that are common for glasses from the Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2 system and are thus technically free of these components that are disadvantageous under safety and environmental protection aspects. If these components should be present as contaminants, their content has to be limited to less than about 400 ppm. At higher contents, the above-mentioned refining agents are reduced under the action of the reducing conditions during floating, namely directly below the surface, and form disruptive and visually obvious deposits. The removal of the latter for the use of disruptive and toxicologically harmful deposits by grinding and polishing is disadvantageous for economic reasons. [0021] In addition to the tin compounds that are used for refining, in addition still other chemical refining agents, such as sulfate, chloride and fluoride compounds, can be used if necessary. [0022] In the case of especially high requirements of the bubble quality, it may be necessary to combine chemical refining and physical refining processes. The combination of the refining agent SnO.sub.2 with a high-temperature refining <1700.degree. C. has proven especially advantageous to achieve low numbers of bubbles of less than 10 bubbles/kg of glass (relative to the bubble sizes above 0.1 mm) with comparatively low SnO.sub.2 contents. This holds true since SnO.sub.2 cleaves the oxygen that is required for refining at comparatively high temperatures. [0023] A glass or a glass composition that essentially has the composition below (in % by weight based on oxide) is especially suitable: [0024] SiO.sub.2 60-68, Al.sub.2O.sub.3 19-24, Li.sub.2O 3.5-4.5, Na.sub.2O 0.2-1, K.sub.2O 0-0.8, .SIGMA.Na.sub.2O+K.sub.2O 0.4-1.5, MgO 0.1-2, CaO 0-1.5, in particular 0-1, SrO 0-1.5, in particular 0-1, BaO 0-2.5, ZnO 0-1, TiO.sub.2 1-2.6, ZrO.sub.2 1.2-2.2, SnO.sub.2 0.2-0.6, .SIGMA. TiO.sub.2+ZrO.sub.2+SnO.sub.2 3-4.5, P.sub.2O.sub.5 0-2, B.sub.2O.sub.3 0-2, in particular 0-1 Nd.sub.2O.sub.3 0.025-0.46, CoO 0-0.003. [0025] The water content of the glasses according to the invention depends on the selection of the raw materials of the batch and on the process conditions in the melt, usually between 0.015 and 0.06 mol/l. This corresponds to .beta..sub.OH values of 0.16 to 0.64 mm.sup.-1. Continue reading... Full patent description for Optically detectable, floatable arsenic- and antimony-free, glazable lithium-aluminosilicate glass Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Optically detectable, floatable arsenic- and antimony-free, glazable lithium-aluminosilicate glass patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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