Abstract:
A green tinted, ultraviolet absorbing glass is disclosed having a standard soda-lime-silica base glass composition and a colorant portion consisting essentially of: 
     
       ______________________________________ 
     
     CeO 2        Less than 0.5 weight %Total iron      Greater than 0.85 weight %(as Fe 2  O 3 )FeO/total iron  Less than 0.275.______________________________________ 
     The glass reduces the amount of costly cerium required to yield low ultraviolet transmittance, viz., no greater than 31 percent (300 to 390 nanometers) at a reference thickness of 3.9 millimeters.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to soda-lime-silica glass particularly suitable for controlling transmittance of solar radiation in window glazing applications. The glass may be generally described as being green tinted, and is designed to have low heat transmittance and particularly enhanced absorption in the ultraviolet wavelength range. This is desirable for the sake of reducing the rate at which the sun&#39;s rays deteriorate plastics and fabrics in applications such as automobiles. A particular objective of the invention is to permit glass of this type to be made at a lower cost by reducing the amount of costly ingredients required. 
     Soda-lime-silica flat glass may be essentially characterized by the following composition on a weight percentage basis of the total glass: 
     
         ______________________________________  SiO.sub.2         68-75%  Na.sub.2 O        10-20  CaO    5-15  MgO   0-5  Al.sub.2 O.sub.3        0-5  K.sub.2 O        0-5______________________________________ 
    
     Other minor ingredients, including melting and refining aids such as SO 3 , may also appear in the glass composition. Small amounts of K 2  O, BaO or B 2  O 3  and other minor constituents have also sometimes been included in flat glass and may be considered optional. To this base glass are added the coloring constituents that produce the transmittance properties of the glass. The primary colorant in the category of glasses relevant to the present invention is iron, which is usually present in both the Fe 2  O 3  and FeO 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. A typical green tinted automotive glass has about 0.5 percent by weight total iron, with the ratio of FeO to total iron being about 0.25. 
     Larger amounts of iron have been used in some specialty glasses intended to lower the total solar energy transmittance. However, merely increasing the amount of total iron does not reduce the ultraviolet transmittance to the extent desired even when the amount of iron present is sufficient to lower the luminous (visible light) transmittance to the minimum acceptable for automotive applications under conventional commercial melting conditions. The colorant composition and transmittance properties for examples of two commercial products of this high iron, dark green tinted type along with an example of the conventional, light green tinted glass described above are set forth below: 
     
         ______________________________________     Light Green              Dark Green Dark Green     Example A              Example B  Example C______________________________________Total Iron (wt. %)       0.550      0.805      0.720FeO/Tot. Iron       0.251      0.293      0.270LT.sub.A (%)       79.5       70.9       71.6TSUV (%)    48.1       36.9       36.5TSIR (%)    36.7       22.9       29.2TSET (%)    56.8       44.6       48.8______________________________________ 
    
     The transmittance data in the table above and throughout are based on a glass thickness of 3.9 millimeters (0.154 inch). Luminous transmittance (LT A ) is measured using C.I.E. standard illuminant &#34;A&#34; over the wavelength range 380 to 770 nanometers. Total solar ultraviolet transmittance (TSUV) is measured over the wavelength range 300 to 390 nanometers. Total solar infrared transmittance (TSIR) is measured over the wavelength range 800 to 2100 nanometers. Total solar energy transmittance (TSET) represents a computed value based on measured transmittances from 300 to 2100 nanometers at 50 nanometer intervals. 
     Recently, a goal has been established to limit ultraviolet transmittance to no more than 31 percent in some automotive glass. At the same time, it is a requirement that glass in vision areas of automobiles have an LT A  of at least 70 percent. Simply increasing the amount of iron to lower the ultraviolet transmittance would impermissibly lower the luminous transmittance as well, so an alternative approach is needed. The use of cerium oxide in glass to reduce ultraviolet transmittance is disclosed in U.S. Pat. No. 2,860,059, and the following two examples are of commercial products that take this approach: 
     
         ______________________________________          Example D                  Example E______________________________________CeO.sub.2 (wt. %)            1.06      0.70Total Iron (wt. %)            0.780     0.858FeO/Tot. Iron    0.290     0.282LT.sub.A (%)     71.4      70.4TSUV (%)         27.8      28.3TSIR (%)         22.8      20.6TSET (%)         44.6      42.9______________________________________ 
    
     These glasses exhibit the desired combination of low ultraviolet transmittance and high luminous transmittance, but the high cost of cerium sources substantially increases the cost of making these glasses. It would be desirable if these objectives could be met without incurring such high raw material costs. High cerium content also produces an undesirable solarization effect, that is, the glass tends to darken upon exposure to ultraviolet radiation. For these reasons, it would be desirable to lower the amount of cerium required in this type of glass. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide green tinted glass that has luminous transmittance of at least 70 percent and ultraviolet transmittance of no more than 31 percent (both transmittances at a reference thickness of 3.9 millimeters), without requiring the use of as much costly cerium oxide as in prior compositions of this type. These properties may be achieved by the present invention with less than 0.5 percent by weight CeO 2  in the glass, preferably less than 0.4 percent by weight CeO 2 . The optimized embodiments of the present invention contain less than 0.35 percent by weight CeO 2 . Compared to competing glasses, the glasses of the present invention are characterized by larger amounts of total iron and smaller proportions of the iron in the ferrous state. The total iron (expressed as Fe 2  O 3 ) is greater than 0.85 percent by weight of the total glass composition, and the ratio of FeO/total iron (ferrous iron expressed as FeO divided by the total iron expressed as Fe 2  O 3 ) is less than 0.275. 
    
    
      DETAILED DESCRIPTION 
     The base glass composition is not critical to the present invention and may consist of any conventional soda-lime-silica flat glass composition, which may be characterized by the ranges set forth above. Preferably, the base glass is one known by those of skill in the art to be producible in a continuous melting furnace and formed into a flat sheet by the float process. A specific example of a glass of the present invention can be seen in Example 1. 
     
         ______________________________________SiO.sub.2        71.88  percent by weightNa.sub.2 O       13.47CaO              8.91MgO              3.66Al.sub.2 O.sub.3 0.27K.sub.2 O        0.08SO.sub.3         0.15Fe.sub.2 O.sub.3 (total)            0.898CeO.sub.2        0.28______________________________________ 
    
     Additionally, traces of impurities may be present without significant effect on the glass. Melting and fining aids such as SO 3  are useful during production of the glass, but their residual amounts in the glass can vary and have no significant effect on the properties of the glass product. K 2  O is present in this example as an impurity, and its presence is not required, although it serves a function in the glass essentially equivalent to that of Na 2  O. the ratio of ferrous iron (expressed as FeO) to total iron (expressed as Fe 2  O 3 ) for Example 1 was 0.245. The batch mixture of raw materials from which the glass of Example 1 was melted was as follows: 
     
         ______________________________________Sand              1000   parts by weightSoda ash          324Limestone         84Dolomite          242Salt cake         5Rouge             12.32Cerium carbonate  5.75(hydrate)Coal              1.55______________________________________ 
    
     The glass of Example 1 exhibited the following transmittance properties at a thickness of 3.9 millimeters: 
     
         ______________________________________   LT.sub.A         70.8%   TSUV  29.6%   TSIR  22.1%   TSET  43.7%______________________________________ 
    
     One of the ways to control the ratio of ferrous to total iron is by selecting the amount of coal or other reducing agent included in the batch mixture. Instead of coal, various other sources of carbon are known to serve as reducing agents during melting of glass. Another means of control is by way of the ratio of air to fuel in the melting furnace. A higher air to fuel ratio produces conditions in the melting furnace that are more oxidizing, which, in turn, leads to a lower ferrous to total iron ratio. Specifying the oxidizing control measures depends upon the particular operating characteristics of a particular melting furnace. In Example 1, oxidizing conditions during melting were exceptionally high, so the amount of coal included in the batch was larger than would b desired under other conditions. In some cases it may be desirable to include no coal or other reducing agent in order to attain the oxidizing conditions required for the present invention. 
     High melting temperatures also tend to have a reducing effect on the glass, as can be seen in the relatively high ferrous to total iron ratios of Examples D and E of the prior art, which indicates that those glasses probably required exceptionally high temperature melting conditions. Because of the low infrared transmittance of the category of dark green tinted, high iron containing glasses to which the present invention pertains, relatively high melting temperatures are to be expected. As a result, making the conditions more oxidizing in accordance with the present invention may yield ferrous to total iron ratios that are relatively low for dark green glass, but not substantially different from those for lightly tinted or clear glass. Therefore, the requirement of a ferrous to total iron ratio less than 0.26 in the present invention is relatively low for this type of glass. 
     Examples 2 through 7 are additional embodiments of the present invention. Only the colorant portion of the compositions are set forth because the base glass portions vary only slightly. 
     
         ______________________________________  Exam- Exam-   Exam-   Exam- Exam- Exam-  ple 2 ple 3   ple 4   ple 5 ple 6 ple 7______________________________________LT.sub.A 70.5    70.3    70.4  70.9  70.2  71.0TSUV     28.4    26.6    26.4  31.0  29.9  30.2TSIR     23.9    23.3    25.1  20.9  20.1  21.5TSET     44.3    43.9    44.9  43.4  42.6  43.7Fe.sub.2 O.sub.3 (total)    0.957   0.973   1.132 0.882 0.907 0.896FeO/Fe.sub.2 O.sub.3    0.222   0.222   0.161 0.261 0.254 0.243CeO.sub.2    0.27    0.28    &lt;0.01 0.29  0.31  0.31______________________________________ 
    
     Minimizing the amount of cerium oxide used is best for meeting the objectives of reducing the cost of the glass and avoiding solarization, and Example 4 is an embodiment within the scope of the invention in which no cerium was used. However, the very low ferrous to total iron ratio required when no cerium is used may be difficult to attain in some melting furnaces. Therefore, it is preferred that a small amount of cerium be used to yield the desired reduction in ultraviolet transmittance without requiring an unduly low ferrous to total iron ratio. Additionally, cerium oxide is itself an oxidizing agent whose presence assists in attaining the required ferrous to total iron ratio. Accordingly, the preferred embodiments disclosed above include 0.20 to 0.35 percent by weight CeO 2  although some commercially operating furnaces may require up to 0.4 percent or more. 
     The ultraviolet transmittance herein has been reported with reference to the wavelength range 300 to 390 nanometers. Others may use the range 300 to 400 nanometers for measuring ultraviolet. The goal of a maximum ultraviolet transmittance of 31 percent for the present invention would be approximately equivalent to 38 percent if the range 300 to 400 nanometers is used. 
     Cerium is sometimes included in glass to &#34;decolorize&#34; the glass. But the glass of the present invention has a distinctly green color. The color may be a matter of taste, and the specific color characteristics need not be considered critical to the present invention, but the glasses that have been made in accordance with the invention have been characterized by excitation purity greater than 1 percent, usually from 2 to 4 percent, and dominant wavelength from 495 to 535 nanometers. 
     The total solar energy transmittance (TSET) of the glass of the present invention is relatively low, thereby significantly reducing the amount of thermal energy that passes through a window glazed with the glass. Although not critical to the invention, the TSET of the glasses of the present invention are generally lower than 45 percent. 
     The invention has been described with reference to specific embodiments, but it should be understood that variations and modifications that are known to those of skill in the art may be resorted to within the scope of the invention as defined by the claims that follow.