Soda-lime-silica float glass batch mixture

Abstract: A glass batch mixture for producing a soda-lime-silica glass is claimed containing oxides of iron comprising a source of silicon, a source of silicon, a source of sodium, a source of calcium and wuestite.

Description: The present invention relates to a soda-lime-silica float glass batch mixture for producing soda-lime-silica glass. Such batch mixture according to the prior art includes sand, limestone, dolomite, soda ash, salt cake or gypsum, niter, iron oxide, carbon and depending on the type of glass, selenium, cobalt oxide, titanium oxide and/or a nickel compound. The batch mixtures are used for example, to produce special glasses for use in vehicles which have low levels of direct solar heat transmission (DSHT) and ultraviolet transmission (UVT). These glasses aim to reduce the problems caused by excessive heating within the vehicle on sunny days, and to protect the interior furnishings of the car from the degradation caused by ultraviolet radiation. Glasses having good infrared absorption properties are usually produced by reducing iron present in the glass to the ferrous state or by adding copper. Such materials give glasses a blue color. The materials added to achieve good ultraviolet radiation absorption are Fe 3+ , Ce, Ti or V. The quantities of such materials which are added to provide the desired level of absorption tend to color the glass yellow. Accordingly, if both good UV and good IR absorption are required in the same glass, the color of such glass is, almost inevitably, either green or blue. When the color of the glasses is defined by the CIELAB system, such commercial glasses, in 4 mm thickness and having greater than 60% light transmission, are found to be either very green (-a*>8) or very blue (-b*>7), neither of which are currently desirable from an aesthetic viewpoint. Attempts have been made to produce grey or bronze-colored vehicle glazings having good protection against both IR and UV radiation, but such glasses still tend to have a greenish yellow tinge. We have identified a requirement for a range of glasses having a neutral tint and a visible light transmittance (Illuminant A) of at least 70 percent such that, in the CIELAB system, the glasses have color co-ordinates lying in the ranges a* from -7 to +1, b* from -5 to +7.5. The term "neutral tint" is hereinafter used to describe glasses having such color coordinates. The carbon included in the batch mixture according to the prior art for producing soda-lime-silica glass is a very deleterious element in neutral tint glasses e.g. grey and bronze glasses but is required to raise the ferrous values of the glasses where employing rouge as the batch iron source. It is therefore an object of the invention to provide a soda-lime-silica glass batch mixture in which the carbon content is reduced. This object is solved by the features of claim 1. The inventive batch mixture may be used for an IR and UV absorbing soda-lime-silica glass of neutral tint (as herein defined) and in a 4 mm thickness, a visible light transmission of at least 70%, a direct solar heat transmission at least 12 percentage points below the visible light transmission, a UV transmission not greater than 55%, a dominant wavelength less than 560 nm and a color purity not greater than 6, preferably not more than 5 and most preferably no more than 3. Most, preferably the direct solar heat transmission is at least 20 percentage points lower than the visible light transmission. The composition comprises a soda-lime-silica base glass and a total iron content, expressed as Fe 2 O 3 in the range of from 0.3 to 0.7% by weight. The glass is tinted to a neutral color by the inclusion of 0.5 to 10 parts per million (ppm) of Se, from about 3 to 25 ppm of Co 3 O 4 , and a ferrous iron content to provide a ratio of ferrous iron to total iron in the range of 21 to 34, preferably 25 to 31 (i.e., percent of total iron as ferrous iron (Fe 2+ ) of 21% to 34%, preferably 25% to 31%). As is well known, the iron content in glasses is usually present in both the Fe 2 O 3 (ferric) and FeO (ferrous) forms. As is conventional, the total amount of iron present in a glass is expressed herein as Fe 2 O 3 , regardless of the form actually present. NiO and TiO 2 may be added to the glass, in ranges of 0 to 50 ppm NiO and 0 to 1.5 weight percent TiO 2 . Thus, it has been determined that amounts of NiO and TiO 2 , in the above ranges can produce beneficial affects on color purity and UV absorption, respectively, without deleteriously influencing the properties of our glass. For the purpose of the present specification, references to visible light transmission are to light transmission (LT) measured using CIE Illuminant A; UVT or ultraviolet radiation transmission is an integrated term representing the area under the transmission versus wavelength curve for wavelengths between 300 and 400 nm; and references to direct solar heat transmission (DSHT) are references to solar heat transmission integrated over the wavelength range 350 to 2100 nm according to the relative solar spectral distribution Parry Moon for air mass 2. The use of wuestite as the iron source instead of rouge greatly increases the ferrous value of a glass. The use of a raw material with a higher natural ferrous value allows better control of higher ferrous values in glasses such as the neutral tint glasses of this invention. The batch materials are conveniently melted together in a conventional glass making furnace, to form a neutral tinted infrared energy and ultraviolet radiation absorbing glass composition, which thereafter may be continuously cast onto the molten metal bath in a float glass process. The composition of soda-lime-silica flat glasses suitable for use in accordance with the present invention typically have the following weight percentage constituents: SiO 2 65 - 80% Na 2 O 10 - 20 CaO 5 - 15 MgO 0 - 10 Al 2 O 3 0 - 5 K 2 O 0 - 5 BaO 0 - 5 B 2 O 3 0 - 5 Other minor ingredients, including melting and refining aids such as SO 3 may also appear in the glass composition. The coloring constituents set forth above are added to this base glass. The glass is essentially free of colorants other than iron, cobalt, and selenium, and optionally nickel and titanium, other than any trace amounts of oxides that may be present as impurities. The glass may be melted and refined in a conventional tank-type melting furnace and formed into flat glass sheets of varying thicknesses by the float method in which the molten glass is supported on a pool of molten metal, usually tin, as it assumes a ribbon shape and is cooled. The glass compositions are particularly suited for the production of infrared energy and ultraviolet radiation absorbing glass for automotive and architectural glazings. Thus, glass sheets of this composition may be heat strengthened or tempered, or alternately annealed and laminated together through an interposed transparent resinous layer, for example composed of polyvinyl butyral, and employed, for example, as a windshield. Generally, the glass sheets for windshield use are of a thickness in the range of from about 1.7 mm to about 2.5 mm, while those tempered and used as sidelights or backlights are in the range of about 3 mm to about 5 mm thick. Unless otherwise noted, the term percent (%) as used herein means percent (%) by weight. Wavelength dispersive X-ray fluorescence was used to determine the weight percents of TiO 2 and total iron expressed as Fe 2 O 3 . Percent reduction of total iron was determined by first measuring the radiant transmission of a sample at a wavelength of 1060 nanometers, using a spectrophotometer. The 1060 nm transmission value was then used to calculate optical density, using the following formula: Optical density = Log 10 T 0 T (T 0 = 100 minus estimated loss from reflection = 92; T = transmission at 1060 nm). The optical density was then used to calculate the percent reduction: percent reduction = (110) x (optical density) (Glass thickness in mm) x (wt% total Fe 2 O 3 ) The concentrations of each of the three essential colorant constituents depend upon the optical properties desired for the glass and are interrelated to each other. Iron is added, typically as Fe 2 O 3 , and is partially reduced to FeO. The total amount of iron in the batch is critical, and must equal from 0.3 percent to about 0.7 percent by weight, expressed as Fe 2 O 3 . Likewise, the degree of reduction is critical and must equal between 21% and 34%. If the iron is more highly reduced than the critical amount, or if a higher total amount of iron is employed, the glass will become too dark and the Illuminant A visible light transmittance will drop below about 70 percent. Additionally, the glass batch melting process will become increasingly difficult as the increased amount of FeO prevents the penetration of heat to the interior of the melt. If the iron is less reduced than the critical amount, or -if a lower total amount of iron is employed, then the direct solar heat transmittance for a desired thickness glass can rise to an unacceptable level, i.e., above about 58%. From about 3 to about 25 ppm cobalt oxide is added, typically as Co 3 O 4 , along with about 0. 5 to about 10 ppm selenium. The proper selenium and cobalt content provides an aesthetically pleasing, neutral tint, somewhat gray color to the glass. Preferred compositions, include a soda-lime-silica base glass and colorants consisting essentially of 0.45 to 0.65 total iron (as Fe 2 O 3 ), with a ratio of ferrous iron to total iron of 25 to 31, 1 to 5 ppm Se, 8 to 20 ppm Co 3 O 4 to 35 ppm NiO and 0 to 1 weight percent TiO 2 . The following examples illustrate glass compositions produced with the invention that are readily formed into glass articles or glazings such as automobile windshields. The compositions absorb both infrared and ultraviolet rays and have an Illuminant A visible light transmission of at least about 70% and a direct solar heat transmission at least 12 percentage points less than the visible light transmission. The examples, except examples 1 and 11 which are for comparison purposes only, use the batch mixture of the invention. In the examples, all parts and percentages are by weight and: (a) Fe 2 O 3 , FeO and TiO 2 are expressed in percent; Se, Co 3 O 4 and NiO are expressed in parts per million; (b) total iron is expressed as if all iron present were present as ferric oxide; and (c) the FeO content is calculated from the equation %FeO = %Fe 2+ 100 x Fe 2 O 3 x 143.7 159.7 Fe 2 O 3 = percentage total iron, expressed as Fe 2 O 3 in the glass (143.7 being the molecular weight of 2 x FeO and 159.7 being the molecular weight of Fe 2 O 3 ). The transmittance data in the Table below and throughout are based on a nominal glass thickness of 4mm. The base glass composition for example 7, which is essentially the same for all of the examples, was as follows: Component Weight Percent of Total Glass Si0 2 73.91 Na 2 O 14.04 CaO 7.86 MgO 3.47 S0 3 0.20 A1 2 O 3 0.16 K 2 O 0.039 The batch mixture for example 7, which is likewise similar for all of the examples except for the colorants, was: Constituent Parts by Weight Sand 154 Soda Ash 50 Gypsum 1 Limestone 11 Dolomite 33 Wuestite 1.02 C 3 O 4 0.0011 Selenium 0.0014 Carbon 0.027 The composition can be manufactured into flat glass products using commercial manufacturing processes, in particular the float process. A sheet of glass that has been formed by the float process is characterized by measurable amounts of tin oxide that migrated into surface portions of the glass on at least one side. Typically a piece of float-forming glass has an SnO 2 concentration of at least 0.05% by weight in the first few microns below the surface that was in contact with the tin. Glass made by the float process typically ranges from about 2 millimeters to 10 millimeters in thickness. Another characteristic of most float glass is the presence of traces of melting and refining aids such as sulfur, analyzed in the glass as SO 3 , or fluorine or chlorine. Small amounts of these melting and refining aids (usually less than 0.3% by weight) may be present in the glass compositions of the present invention without effect on the properties.

Claims: A glass batch mixture for producing a soda-lime-silica glass containing oxides of iron comprising a source of silicon, a source of sodium, a source of calcium and wuestite. Glass batch mixture according to claim 1 comprising sand, soda ash, dolomite, limestone, a sulfate selected from the group consisting of salt cake and gypsum and wuestite. Use of the glass batch mixture according to claim 1 or 2 to produce a soda-lime silica glass containing oxides of iron, the wuestite being at least a partial source of the iron oxides in the resulting glass. Process for producing a soda-lime-silica glass containing oxides of iron including admixing, heating and melting a soda-lime-silica glass batch mixture comprising including in said batch mixture wuestite as at least a partial source of the iron oxides in the resulting glass.

Inventors: BAKER RODNEY G, FYLES KENNETH MELVIN, HIGBY PAIGE L, MCPHAIL HELEN LOUISE, PENROD BRET E, BAKER, RODNEY G., FYLES, KENNETH MELVIN, HIGBY, PAIGE L., MCPHAIL, HELEN LOUISE, PENROD, BRET E.
Applicants: LIBBEY OWENS FORD CO, PILKINGTON PLC, LIBBEY-OWENS-FORD CO.
Application year: 1995
Main classification: C03C3
IPC codes: C03C1, C03C3