Abstract:
An alkali-alumina-silica-zirconia glass capable of being chemically strengthened by exchanging sodium ions in the glass with potassium ions from an external source below the strain point of the glass has its ion exchange rate increased by incorporating 2 to 20 percent P 2  O 5  into the initial glass composition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 605,108, filed Aug. 15, 1975 now U.S. Pat. No. 4,015,045, which was a continuation-in-part of application Ser. No. 432,006, filed Jan. 9, 1974, now abandoned. 
    
    
     This application is also related in subject matter to applicant&#39;s copending application Ser. No. 706,661, filed on even date herewith, entitled CHEMICALLY STRENGTHENED OPAL GLASS. 
     BACKGROUND OF THE INVENTION 
     This invention relates to glass compositions and articles made therefrom which are essentially transparent to visible light and are capable of being ion exchange strengthened at an exceptionally high rate of speed. Moreover, the novel glasses of the present invention are rendered suitable for large scale production and subsequent use in automotive, aircraft, architectural, tableware and other applications by virtue of their low melting temperatures and high degree of resistance to chemical attack. 
     Ion exchange strengthening (or &#34;chemical tempering&#34;) of glass involves an exchange of ions near the surface of the glass article with ions from an external source, typically a molten inorganic salt bath, the object being the generation of a zone near the surface of the glass which is in a state of compression relative to the interior portions of the glass. There are two types of ion exchange strengthening which differ substantially in theory and operation. The first type of ion exchange treatment is carried out above the strain point of the glass and has as its object the alteration of the glass composition at the surface so as to lower the thermal coefficient of expansion in the surface layer. As the glass is cooled, a compressive stress develops at the surface due to the expansion differential. This approach was taught by Hood and Stookey in U.S. Pat. No. 2,779,136. The second type of ion exchange strengthening is characterized by treatment below the strain point of the glass, wherein surface compression is generated by substituting large ions from an external source (e.g., a molten salt bath) for smaller ions in the glass. Typically, the substitution is of sodium or potassium for lithium in the glass, or of potassium for sodium in the glass. The below-the-strain-point technique was first taught by Weber in U.S. Pat. No. 3,218,220. 
     Of the two types of ion exchange strengthening, the second (below the strain point) type is preferred for large-scale commercial use. This is because maintaining the glass below its strain temperature avoids causing undesirable distortion defects in the glass. Furthermore, since it is costly to include lithium in a glass as a batch ingredient, and because greater strengthening can generally be achieved, it is desirable that sodium, rather than lithium, be the ion in the glass which is replaced. In that case, the larger ion which enters the glass is most advantageously potassium. Hence, this invention is directed specifically to the improvement of ion exchange strengthening processes which involve replacing sodium with potassium below the strain point of the glass. 
     Conventional soda-lime-silica flat glass compositions can be strengthened by ion exchange, but the great length of time required to produce a significant compression layer depth is incompatible with many high volume commercial operations. For this reason, special glass compositions have been developed which have greatly enhanced ion exchange properties, chief among which are the Al 2  O 3  and/or ZrO 2  containing glasses disclosed by Mochel in U.S. Pat. Nos. 3,485,702; 3,752,729; and 3,790,430. Variations of these alumina or zirconia containing glasses may be seen in many U.S. patents including the following (some of which contain lithium): 
     U.s. pat. No. 3,357,876 -- Rinehart 
     U.s. pat. No. 3,433,611 -- Saunders et al. 
     U.s. pat. No. 3,481,726 -- Fischer et al. 
     U.s. pat. No. 3,485,647 -- Harrington 
     U.s. pat. No. 3,498,773 -- Grubb et al. 
     U.s. pat. No. 3,772,135 -- Hara et al. 
     U.s. pat. No. 3,778,335 -- Boyd 
     U.s. pat. No. 3,844,754 -- Grubb et al. 
     While these specially adapted ion exchange glass compositions of the prior art greatly reduce the amount of time required for ion exchange treatment compared to conventional soda-lime-silica glass, the commercial use of lithium-free ion exchange glasses remains limited to low volume specialty items because treatment times are still impractically long for many applications. Moreover, many of the prior art compositions have melting temperatures considerably higher than soda-lime-silica glass and thus are not readily adapted for use in existing melting and forming facilities. Thus, it would be highly desirable to have glass compositions available which could be more rapidly strengthened by exchange treatment with potassium and at the same time be possessed of melting temperatures more in line with ordinary soda-lime-silica glass. Other factors such as transparency, chemical durability, and the cost of raw materials also must be taken into consideration. 
     Substantial progress toward the above-noted goals was attained by the glass compositions disclosed in parent application Serial No. 605,108, the disclosure of which is hereby incorporated by reference. The present invention represents an even greater improvement over those glasses, particularly in regard to the speed with which a deep compression layer can be created in the glasses. 
     SUMMARY OF THE INVENTION 
     It has now been found that an alkali-alumina-zirconia-silica ion exchange glass of the type disclosed in parent application Ser. No. 605,108 can be adapted to include P 2  O 5 , the result being a significantly enhanced propensity for exchange with potassium ions. At least ninety percent by weight of the glass consists of SiO 2 , Na 2  O, K 2  O, Al 2  O 3 , ZrO 2 , R 2  O 5 , ZnO, MgO and TiO 2 , and the glass is essentially lithium-free. By carefully selecting the amounts of these constituents, it has been found possible to obtain the ion exchange advantages and at the same time produce a glass that is very transparent, low temperature melting, and resistant to chemical erosion. 
    
    
     THE DRAWINGS 
     FIG. 1 is a plot of compression layer depth versus ion exchange treatment time for a preferred example of the present invention and for a commercially available ion exchange glass at two different temperatures; 
     FIG. 2 is a plot of center tension versus exchange time for two preferred examples of the present invention and a commercially available ion exchange glass; and 
     FIG. 3 is a plot of viscosity versus temperature for a preferred example of the present invention, a commercially available ion exchange glass, a typical float glass composition, and a typical sheet glass composition. 
    
    
     DETAILED DESCRIPTION 
     In its broadest aspects, the novel glass composition of the present invention includes the following: 
     
         ______________________________________            Percent by Weight,Component        Oxide basis______________________________________SiO.sub.2        34 - 61Na.sub.2 O       12 - 20K.sub.2 O        2 - 9(Na.sub.2 O + K.sub.2 O)            17 - 23Al.sub.2 O.sub.3  2 - 15ZrO.sub.2         0 - 10(Al.sub.2 O.sub.3 + ZrO.sub.2)             9 - 23P.sub.2 O.sub.5   2 - 20ZnO              0 - 8MgO              0 - 6TiO.sub.2        0 - 5(ZnO + MgO + TiO.sub.2)            2 - 9______________________________________ 
    
     The glass includes no more than trace amounts of Li 2  O, i.e., no more than about 0.1 percent by weight. The composition may also include small amounts (no more than about 10 percent) of other conventional fluxing ingredients such as BaO, PbO, and B 2  O 3 . 
     When about 10 percent or more P 2  O 5  is included in the glass, it is necessary to include at least 2 percent ZnO in the glass. Moreover, it is preferred in that case that the (ZnO + MgO + TiO 2 ) component consist almost entirely of ZnO. Thus, for high phosphate concentrations (10 to 20 percent) the preferred composition is as follows: 
     
         ______________________________________            Percent by Weight,Component        Oxide Basis______________________________________SiO.sub.2        34 - 52Na.sub.2 O       12 - 20K.sub.2 O        2 - 9(Na.sub.2 O + K.sub.2 O)            20 - 23Al.sub.2 O.sub.3  8 - 15ZrO.sub.2        0 - 9(Al.sub.2 O.sub.3 + ZrO.sub.2)            10 - 23P.sub.2 O.sub.5  10 - 20ZnO              2 - 8______________________________________ 
    
     At lower P 2  O 5  levels (2 to 10 percent) ZnO is not an essential ingredient, but in order to obtain practical melting temperatures, the combined amount of ZnO + MgO + TiO 2  should fall within the range of about 2 to 9 percent. In that case, the preferred composition may be stated as: 
     
         ______________________________________            Percent by Weight,Component        Oxide Basis______________________________________SiO.sub.2        44 - 61Na.sub.2 O       12 - 18K.sub.2 O        3 - 9(Na.sub.2 O + K.sub.2 O)            17 - 23Al.sub.2 O.sub.3  2 - 15ZrO.sub.2         2 - 10(Al.sub.2 O.sub.3 + ZrO.sub.2)             9 - 20P.sub.2 O.sub.5   2 - 10ZnO              0 - 5MgO              0 - 6TiO.sub.2        0 - 5(ZnO + MgO + TiO.sub.2)            2 - 9______________________________________ 
    
     SiO 2  is the primary glass former and is the largest single component of these glasses. Minimizing the SiO 2  content is desirable so as to lower the melting temperature, but too little SiO 2  can adversely affect durability of the glass. Na 2  O is present as a fluxing agent and provides the sites for subsequent ion exchange. Although an excess of Na 2  O can reduce the resistance of the glass to chemical attack, at least the minimum amount prescribed should be present in the glass to maintain the melting temperature of the glass at a practical level and to provide the glass with an appreciable ion exchange propensity. K 2  O is also desirably present in the glass as a fluxing agent. At higher concentrations, K 2  O can lower the strength obtained by ion exchange with potassium, but it has been found that in small amounts K 2  O actually tends to improve exchange of sodium by potassium. 
     The ingredients primarily responsible for practical below-the-strain-point ion exchange strengthening are Al 2  O 3  and ZrO 2 . The inclusion of excess Al 2  O 3  in this type of glass, however, raises its melting temperature substantially and lowers its chemical durability. ZrO 2  does not have these drawbacks, but it has been found that the inclusion of more than a few percent of ZrO 2  in these glasses leads to devitrification which can adversely affect forming operations. However, Al 2  O 3  and a limited amount of ZrO 2  tend to make a glass clearer. The aforementioned application Ser. No. 605,108 discloses glass compositions which successfully include both Al 2  O 3  and ZrO 2  in such a way as to avoid the drawbacks of both. Although such glasses are excellent ion exchange glasses and represent major improvements, upper limits on their Al 2  O 3  and ZrO 2  contents acts as restraints on the ion exchange rates obtainable. The inclusion of P 2  O 5  in the glasses here has been found to extend the ion exchange properties of that type of glass beyond previous limits. In particular, the P 2  O 5  increases the speed with which a given depth of compression layer can be attained. The P 2  O 5  also advantageously raises the strain point and has a beneficial effect on the melting temperature, but an excess may adversely affect chemical durability and homogeneity. However, a high degree of chemical durability can be maintained at the upper end of the P 2  O 5  range (i.e., about 15 to 20 percent) if the glass includes no more than about 13 percent Al 2  O 3 . Furthermore, larger amounts of P 2  O 5  than necessary are undesirable from a batch cost standpoint. 
     One or more of ZnO, MgO, and TiO 2  are present in the glass to aid melting. However, at a P 2  O 5  content of about 10 percent or more, the presence of ZnO is essential in order to avoid hazinesses caused by light scattering in the glass. Moreover, at the 10 percent-plus level of P 2  O 5 , only small amounts of MgO or TiO 2  (preferably none) should be present if the glass is to be clear. 
     As discussed above, the glasses of the present invention are intended to be lithium-free. However, trace amounts of Li 2  O (up to about 0.1 percent by weight) may be tolerated. 
     Minor amounts of colorants may be added to the compositions in order to produce tinted glass. Many such colorants are known to those of skill in the art and include, for example: CuO, CoO, Cr 2  O 3 , Fe 2  O 3 , and MnO. 
     A major object of the present invention is the production of ion exchange glasses which have melting temperatures similar to that of conventional soda-lime-silica glass. Melting temperature, for the purposes of this application, is taken as the temperature at which the glass has a viscosity of 100 poises. Since commerical flat glass made by the float process or the sheet updraw process usually exhibit a viscosity of 100 poises at a temperature below about 2800° F. (1538° C.), that temperature is considered herein as being the upper limit for a desirable melting temperature. 
     The accompanying examples illustrate the effect each of the constituents has on the properties of the glass, and include examples within as well as outside the scope of the present invention. The example glasses were melted from laboratory batch materials including: &#34;Supersil&#34; sand, zircon sand (ZrSiO 4 ), aluminum metaphosphate, reagent grade ZnO, MgO, Na 2  CO 3 , K 2  CO 3 , and Al(OH) 3 , and commercial grade TiO 2 . The following batch composition, for Example 586, is typical: 
     
         ______________________________________                WeightIngredient           (grams)______________________________________&#34;Supersil&#34; sand      317.1Sodium carbonate (anhydrous)                230.9Aluminum hydrate [Al(OH).sub.3 ]                 73.6Potassium carbonate   33.1Zircon sand (ZrSiO.sub.4)                 50.8Aluminum metaphosphate                143.8Zinc oxide            37.5______________________________________ 
    
     The batch ingredients were added incrementally over a period of approximately 3 hours to a 4 inch (10 centimeter) diameter platinum crucible held at about 1450° C. (higher where required). Each melt was stirred during the batch additions and for an additional period of about 15 hours during which the melt was maintained at 1450° C. Thereafter, each melt was held at 1450° C. for about one more hour in a quiescent state, after which a number of rods were drawn from the melt and the remainder of the melt was cast onto a plate. 
     The depth of the compression layer and the strength obtained after two different ion exchange treatments in molten KNO 3  are reported for many of the examples. Compression layer depth was measured optically on a ground cross-sectional slice sawed from a plate of the ion-exchange glass. Each slice was placed between cross-polaroids under a microscope, where the compression zone could be observed. Strength is reported as modulus of rupture (M.O.R.) obtained by the abraded rod technique. Rods nominally 3 millimeters in diameter drawn from the glass melt were cut into approximately 2.5 inch (6.5 centimeter) lengths and heated at about 650° C. for half an hour, then cooled to room temperature overnight to anneal the rods. After ion exchange treatment, the rods are subjected to tumble abrasion by being placed in a 16 oz. jar containing 240 grit silicon carbide. The jar is rotated horizontally about its main axis at a speed of about 167 revolutions per minute for 30 minutes. Each abraded rod is then placed singly on a 1.5 inch (3.8 centimeter) support span and stressed to fracture with a 3/4 inch (1.9 centimeter) wide load member. The load at fracture is recorded, and M.O.R. is calculated by the following formula: 
     
         M.O.R. = 8WL/πab.sup.2 
    
     where W is the load at fracture, L is the difference between the load and support spans, a is the larger diameter of the rod, and b is the smaller diameter of the rod (assuming that the rod is not perfectly circular in cross-section). Each reported M.O.R. value represents the average of at least ten rods. 
     The annealing points given in the examples were determined by a differential thermal analysis technique. Melting temperature, defined for the purposes of this application as the temperature at which the viscosity of the glass is 100 poises, and forming temperature, defined as the temperature at which the viscosity of the glass is 10,000 poises, were both determined by a standard rotating spindle viscometer technique. Where any of the measurements were duplicated, the values shown in the tables represent averages. 
     The examples use terms such as &#34;clear,&#34; &#34;hazy,&#34; and &#34;cloudy&#34; to describe the appearance of the glasses. More precisely, they represent the degree of light scattering observed in each glass by the unaided eye, that is, the sharpness with which an image may be perceived through the glass. Because such a quality is difficult to measure quantitatively, the somewhat subjective terms &#34;clear,&#34; &#34;hazy,&#34; and &#34;cloudy&#34; are necessarily relied on. &#34;Clear&#34; is intended to mean that the glass appears to be fully transparent (although it may be colored). &#34;Hazy&#34; means that some bluish or whitish irridescence or interference with visibility is detected, particularly at oblique angles, but that images are still at least fairly recognizable through the glass. Glasses which are &#34;cloudy&#34; are difficult to see through and possess a definite whitish cast from all angles. The reported appearances are based on observations of rods and/or plates of each glass with thicknesses of about 1/8 inch (3 millimeters) to about 1/4 inch (6 millimeters). It should be noted that the degree of light scattering is independent of light transmittance. Thus, a colored (or &#34;tinted&#34;) piece of glass which absorbs a substantial portion of the light striking it may be substantially free from light scattering and would thus be considered &#34;clear&#34;. 
     
                                           TABLE I__________________________________________________________________________EXAMPLE NO.:           411 492 493 515 514 501 516 523 534 545COMPOSITION (% by weight): SiO.sub.2             54.5                      52.5                          49.5                              47.5                                  46.5                                      44.5                                          52.5                                              39.5                                                  36.5                                                      34.5 Na.sub.2 O            18.0                      18.0                          18.0                              18.0                                  18.0                                      18.0                                          18.0                                              18.0                                                  18.0                                                      18.0 Al.sub.2 O.sub.3      10.0                      10.0                          10.0                              10.0                                  10.0                                      10.0                                          10.0                                              10.0                                                  10.0                                                      10.0 MgO                   5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 K.sub.2 O             3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 ZrO.sub.2             4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 P.sub.2 O.sub.5       --  2.0 5.0 7.0 8.0 10.0                                          12.0                                              15.0                                                  18.0                                                      20.0 TiO.sub.2             5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0APPEARANCE:            Clear                      Clear                          Clear                              Clear                                  Clear                                      Clear-                                          Hazy                                              Cloudy                                                  Very                                                      Almost                                      slight      Cloudy                                                      Opaque                                      hazeCOMPRESSION LAYER DEPTH (microns)after 1 hr. at 1000° F (538° C):                  54  57  60  70  65  75  85  100 115 130M.O.R. (Kg/cm.sup.2)after 1 hr. at 1000° F (538° C):                  4700                      4100                          3800                              5000                                  5100                                      5200                                          5400                                              5700                                                  5100                                                      --after 4 hrs. at 900° F (482° C):                  5600                      5300                          5400                              5300                                  5600                                      6000                                          6600                                              6600                                                  6100                                                      --ANNEALING POINT (° C):                  573 566 550 558 562 582 604 621 621 648MELTING TEMPERATURE (° C):                  1416                      1417                          1398                              1390                                  1379                                      1372                                          --  1342                                                  1312                                                      1298FORMING TEMPERATURE (° C):                      1042                          1028                              1019                                  1016                                      1016                                          --  1001                                                  991 989__________________________________________________________________________ 
    
     Table I shows the effect of progressively substituting P 2  O 5  for SiO 2  in a glass of the type disclosed in the parent application, beginning with Example 411, which was a preferred example therein. At 10 percent P 2  O 5  a slight haze begins to appear in the glass, which increases to an almost opaque white appearance at 20 percent P 2  O 5 . However, in Examples 492, 493, 515, and 514 (and marginally in 501) sufficient P 2  O 5  was included to significantly increase the compression layer depth attained after a one hour exchange treatment, and yet those glasses were found to be clear. Additionally, those examples exhibit very favorable melting temperatures, due in large measure to the presence of MgO and TiO 2 . 
     Variations of the same system, with P 2  O 5  limited to 10 percent or less, are shown in Table II. As noted, Table II includes glasses which were clear as well as others which were not for the sake of comparison. Example 551 is of particular interest in that, despite the fact that it is a clear glass, it is not a preferred embodiment because of an excessively high melting temperature, due to the absence of MgO and TiO 2 . 
     
                                           TABLE II__________________________________________________________________________EXAMPLE NO.:   526 533 544 550 551 554 555 558 560 565 566Composition(% by weight): SiO.sub.2     46.0              48.0                  48.0                      44.0                          52.5                              50.0                                  52.5                                      54.5                                          49.5                                              54.5                                                  50.5 Na.sub.2 O    18.0              18.0                  18.0                      18.0                          18.0                              18.0                                  18.0                                      18.0                                          18.0                                              18.0                                                  18.0 Al.sub.2 O.sub.3          12.0              12.0                  12.0                      10.0                          12.0                              10.0                                  10.0                                      8.0 10.0                                              4.5 14.0 MgO           4.5 4.5 2.5 5.5 --  4.5 5.0 4.5 5.0 5.5 4.5 K.sub.2 O     3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 ZrO.sub.2     4.5 4.5 4.5 4.5 4.5 4.5 4.5 2.0 4.0 4.5 10.0 P.sub.2 O.sub.5          10.0              10.0                  10.0                      10.0                          10.0                              10.0                                  7.0 10.0                                          10.0                                              10.0                                                  1.0 TiO.sub.2     2.0 --  2.0 5.0 --  --  --  --  --  --  --Appearance:    Clear-              Clear                  Clear                      Hazy                          Clear                              Clear                                  Clear                                      Clear                                          Clear                                              Clear                                                  Hazy          Smoky               Slight  Slight                              Haze    HazeCompression Layer Depth(microns)after 1 hr. at 1000° F (538° C):              90  95              90  90  90M.O.R. (kg/cm.sup.2)after 1 hr. at 1000° F (538° C):          5800              5800                  5600                      --  3700                              4300                                  4400                                      2400                                          4300                                              2500after 4 hrs. at 900° F (482° C):          6000              5800                  5600                      4900                          4700                              5500                                  5200                                      3900                                          5800                                              3800                                                  5400Annealing Point (° C):              617 625         598 571     589 566Melting Temperature (° C):              1474        1543                              1468                                  1471                                      1487    1422Forming Temperature (° C):              1091        1119                              1081                                  1079                                      1073    1029EXAMPLE NO.:   666 711 721 723 724 745 753 754 792 795 813 814Composition(% by weight): SiO.sub.2     59.0              44.5                  47.5                      51.5                          50.5                              47.5                                  47.5                                      45.5                                          44.5                                              54.5                                                  56.0                                                      61.0 Na.sub.2 O    18.0              18.0                  12.0                      18.0                          18.0                              18.0                                  18.0                                      18.0                                          18.0                                              18.0                                                  16.0                                                      17.0 Al.sub.2 O.sub.3          10.0              15.0                  12.0                      10.0                          10.0                              10.0                                  12.0                                      14.0                                          14.0                                              10.0                                                  2.0 4.5 MgO           --  5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 K.sub.2 O     3.0 3.0 9.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 -- ZrO.sub.2     --  4.5 4.5 4.5 4.5 4.5 4.5 4.5 5.5 4.5 10.0                                                      4.5 P.sub.2 O.sub.5          10.0              10.0                  10.0                      8.0 9.0 10.0                                  10.0                                      10.0                                          9.0 5.0 8.0 8.0 TiO.sub.2                         2.0         0.5Appearance:    Hazy              Clear                  Clear                      Clear                          Clear                              Clear                                  Clear                                      Clear                                          Clear                                              Clear                                                  Clear                                                      Clear              Smoky                  Smoky                      Smoky   Slight  Slight                              Haze    HazeCompression Layer Depth(microns)after 1 hr. at 1000° F (538° C):          120M.O.R. (kg/cm.sup.2)after 1 hr. at 1000° F (538° C):              6900    4500                          4600    6300            3300                                                      --after 4 hrs. at 900° F (482° C):              8200    6000                          6000    6500            4600                                                      3500Annealing Point (° C):          541Melting Temperature (° C):          1574            1474                    1462                                                      1537Forming Temperature (° C):          1024            1085                    1098                                                      1114__________________________________________________________________________ 
    
     Table III sets forth numerous attempts to produce a clear ion exchange glass containing more than 10 percent P 2  O 5 . ZnO was not present, and apparently as a result, none of the attempts successfully produced a clear glass. 
     
                                           TABLE III__________________________________________________________________________EXAMPLE NO.:           527 536 557 608 612 649 657 662 671COMPOSITION (% by weight): SiO.sub.2             44.5                      41.5                          46.5                              50.0                                  44.5                                      49.5                                          44.5                                              54.0                                                  48.0 Na.sub.2 O            18.0                      18.0                          18.0                              18.0                                  18.0                                      18.0                                          18.0                                              18.0                                                  18.0 Al.sub.2 O.sub.3      10.0                      10.0                          10.0                              4.5 10.0                                      10.0                                          15.0                                              10.0                                                  10.0 MgO                       5.0     5.0 5.0 K.sub.2 O             3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 ZrO.sub.2             4.5 4.5 4.5 4.5 4.5 4.5 4.5     6.0 P.sub.2 O.sub.5       15.0                      18.0                          18.0                              15.0                                  15.0                                      15.0                                          15.0                                              15.0                                                  15.0 TiO.sub.2             5.0APPEARANCE:            Cloudy                      Cloudy                          Hazy                              Cloudy                                  Cloudy                                      Hazy                                          Slight                                              Hazy                                                  Cloudy                                          hazeCOMPRESSION LAYER DEPTH (microns)after 1 hr. at 1000° F (538° C)                  115 135 130     125 130 170 125M.0.R. (kg/cm.sup.2)after 1 hr. at 1000° F (538° C):                  3900                      5300    4400                                  5500after 4 hrs. at 900° F (482° C):                      6100    5300                                  6100    5300ANNEALING POINT (° C):                      688 670     642         614MELTING TEMPERATURE (° C):                      1411                          1519        1536    1568FORMING TEMPERATURE (° C):                      1069                          1123        1121    1125EXAMPLE NO.:           673 674 675 676 677 687 722 755COMPOSITION (% by weight): SiO.sub.2             47.5                      46.0                          49.0                              42.5                                  40.5                                      47.0                                          42.5                                              39.5 Na.sub.2 O            18.0                      18.0                          18.0                              18.0                                  18.0                                      18.0                                          18.0                                              18.0 Al.sub.2 O.sub.3      12.0                      10.0                          15.0                              10.0                                  10.0                                      10.0                                          10.0                                              15.0 MgO                               7.0 9.0     5.0 5.0 K.sub.2 O             3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 ZrO.sub.2             4.5 8.0     4.5 4.5 7.0 4.5 4.5 P.sub.2 O.sub.5       15.0                      15.0                          15.0                              15.0                                  15.0                                      15.0                                          15.0                                              15.0 TiO.sub.2                                     2.0APPEARANCE:            Hazy                      Slight-                          Hazy                              Cloudy                                  Cloudy                                      Hazy                                          Cloudy                                              Cloudy                      ly                      HazyCOMPRESSION LAYER DEPTH (microns)after 1 hr. at 1000° F (538° C):M.O.R. (kg/cm.sup.2)after 1 hr. at 1000° F (538° C):                      5200                5700after 4 hrs. at 900° F (482° C):                      5500                6000ANNEALING POINT (° C):MELTING TEMPERATURE (° C):                  1546    1593FORMING TEMPERATURE (° C):                  1130    1135__________________________________________________________________________ 
    
     In Table IV the effect of ZnO on clarity in P 2  O 5  -containing ion exchange glasses is demonstrated. Not only did the ZnO enable clarity to be maintained at P 2  O 5  levels as high as 20 percent, but also, as seen in the examples, the melting temperature in most cases was held below 1550° C. The degree of success varied, however, as can be seen in the data. One exceptional, unexplained problem was encountered in Examples 658, 688, and 689. There, despite initial clarity, the glasses developed chalky, white deposits on portions of some of the samples several weeks after being made. 
     
                                           TABLE IV__________________________________________________________________________EXAMPLE NO.:      586 642 643 645 646 647 648 650 651 652  653Composition (% by weight): SiO.sub.2        44.5                 45.5                     46.5                         44.5                             43.5                                 43.5                                     43.5                                         42.5                                             47.5                                                 45.5 43.5 Na.sub.2 O       18.0                 17.0                     18.0                         10.0                             18.0                                 17.0                                     17.0                                         18.0                                             18.0                                                 18.0 18.0 A1.sub.2 O.sub.3 10.0                 10.0                     8.0 10.0                             10.0                                 10.0                                     10.0                                         12.0                                             10.0                                                 10.0 8.0 MgO                                  2.0 K.sub.2 O        3.0 3.0 3.0 11.0                             2.0 3.0 3.0 3.0 3.0 3.0  3.0 ZrO.sub.2        4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5  4.5 P.sub.2 O.sub.5  15.0                 15.0                     15.0                         15.0                             15.0                                 15.0                                     15.0                                         15.0                                             15.0                                                 15.0 15.0 ZnO              5.0 5.0 5.0 5.0 7.0 5.0 7.0 5.0 2.0 4.0  8.0Appearance:       Clear                 Clear                     Hazy                         Hazy                             Clear                                 Hazy                                     Clear                                         Clear                                             Hazy                                                 Hazy ClearCompression Layer Depth (microns)after 1 hr at 1000° F (538° C):             135 125 115 150         120 145 130 135M.O.R. (kg/cm.sup.2)after 1 hr. at 1000° F (538° C):             4800                 5100                     5000    4800    5100                                         5400after 4 hrs. at 900° F (482° C):             5600                    6100                                         6300         5000Annealing Point (° C):             674Melting Temperature (° C):             1466    1404    1423Forming Temperature (° C):             1082EXAMPLE NO.:      654 655 656 658 659 660 664 678 686 688  689Composition (% by weight): SiO.sub.2        45.0                 45.0                     45.0                         49.0                             44.5                                 39.5                                     36.5                                         46.0                                             42.5                                                 48.0 47.0 Na.sub.2 O       18.0                 18.0                     18.0                         18.0                             18.0                                 18.0                                     18.0                                         18.0                                             18.0                                                 18.0 18.0 A1.sub.2 O.sub.3 10.0                 8.0 12.0                         10.0                             8.0 10.0                                     10.0                                         10.0                                             10.0                                                 10.0 10.0 K.sub.2 O        3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0  3.0 ZrO.sub.2        4.5 4.5 4.5 --  6.5 4.5 4.5 --  4.5 -- P.sub.2 O.sub.5  15.0                 15.0                     15.0                         15.0                             15.0                                 20.0                                     20.0                                         15.0                                             15.0                                                 15.0 15.0 ZnO              4.5 6.5 2.5 5.0 5.0 5.0 8.0 8.0 7.0 6.0  7.0Appearance:       Clear                 Clear                     Clear                         Clear*                             Hazy                                 Hazy                                     Hazy                                         slight-                                             Clear                                                 Cloudy*                                                      Clear*                                         ly                                         HazyCompression Layer Depth (microns)after 1 hr at 100° F (538° C):             140 115 140 145 130 180M.O.R. (kg/cm.sup.2)after 1 hr. at 1000° F (538° C):                     5000                             4600after 4 hrs. at 900° F (482° C):                     5700                         4700    5000Melting Temperature (° C):                     1524Forming Temperature (° C):                     1118EXAMPLE NO.:      690 692 693 694 705 706 712 713 714 719  720Composition (% by weight): SiO.sub.2        49.5                 47.5                     44.5                         39.5                             43.0                                 51.5                                     52.5                                         39.5                                             40.5                                                 45.5 42.5 Na.sub.2 O       18.0                 18.0                     16.0                         18.0                             18.0                                 18.0                                     12.0                                         12.0                                             12.0                                                 14.0 14.0 A1.sub.2 O.sub.3 10.0                 10.0                     10.0                         15.0                             10.0                                 10.0                                     12.0                                         15.0                                             12.0                                                 12.0 12.0 K.sub.2 O        3.0 3.0 3.0 3.0 3.0 3.0 9.0 9.0 9.0 9.0  7.0 ZrO.sub.2        4.5 4.5 4.5 4.5 6.0 4.5 4.5 4.5 4.5 4.5  4.5 P.sub.2 O.sub.5  10.0                 12.5                     15.0                         15.0                             15.0                                 8.0 15.0                                         15.0                                             15.0                                                 10.0 15.0 ZnO              5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 7.0 5.0  5.0 TiO.sub.2                2.0Appearance:       Clear                 Clear                     Hazy                         Clear                             Clear                                 Clear                                     Clear                                         Clear                                             Clear                                                 Clear                                                      ClearCompression Layer Depth (microns)after 1 hr. at 1000° F (538° C):             100 105     150     90  195 185 160after 4 hrs. at 900° F (482° C):             90  105             90  235 215 205M.O.R. (kg/cm.sup.2)after 1 hr. at 1000° F (538° C):             4300                 4700    4600                             4900                                 4300                                     4400        4400after 4 hrs. at 900° F (482° C):             5100                 5400    6500                             5800                                 5900                                     4000        5300 5200EXAMPLE NO.:      725 729 730 733 735 736 737 738 739 742  743Composition (% by weight): SiO.sub.2        47.5                 42.5                     44.5                         40.5                             42.5                                 39.0                                     37.0                                         46.5                                             39.0                                                 44.5 40.5 Na.sub.2 O       12.0                 18.0                     16.0                         14.0                             14.0                                 14.0                                     14.0                                         14.0                                             14.0                                                 12.0 12.0 A1.sub.2 O.sub.3 12.0                 10.0                     10.0                         14.0                             10.0                                 14.0                                     14.0                                         12.0                                             15.0                                                 10.0 14.0 K.sub.2 O        9.0 3.0 5.0 7.0 7.0 7.0 7.0 7.0 7.0 9.0  9.0 ZrO.sub.2        4.5 4.5 4.5 4.5 4.5 6.0 6.0 4.5 6.0 4.5  4.5 P.sub.2 O.sub.5  10.0                 15.0                     15.0                         15.0                             15.0                                 15.0                                     15.0                                         11.0                                             15.0                                                 15.0 15.0 ZnO              5.0 5.0 5.0 5.0 7.0 5.0 7.0 5.0 4.0 5.0  5.0 TiO.sub.2            2.0Appearance:       Clear                 Cloudy                     Clear                         Clear                             Clear                                 Clear                                     Clear                                         Clear                                             Clear                                                 Hazy ClearM.O.R. (kg/cm.sup.2)after 1 hr. at 1000° F (538° C):             4000                6000    --  5800     5200after 4 hrs. at 900° F (482° C):             4300    6000        5600    5400                                             6400     --EXAMPLE NO.:      756 757 761 781 783 793 796 803 812 815  819Composition (% by weight): SiO.sub.2        44.5                 36.5                     34.5                         37.1                             38.5                                 44.5                                     54.5                                         40.5                                             45.5                                                 59.0 49.5 Na.sub.2 O       18.0                 18.0                     18.0                         17.4                             18.0                                 18.0                                     18.0                                         18.0                                             20.0                                                 16.0 13.0 A1.sub.2 O.sub.3 12.0                 18.0                     15.0                         17.0                             14.0                                 4.5 10.0                                         10.0                                             10.0                                                 4.5  10.0 MgO              2.0 --  --  1.9 2.0 K.sub.2 O        3.0 3.0 3.0 2.9 3.0 3.0 3.0 3.0 --  --   3.0 ZrO.sub.2        4.5 4.5 4.5 4.3 4.5 10.0                                     4.5 8.5 4.5 4.5  4.5 P.sub.2 O.sub.5  12.0                 15.0                     20.0                         14.5                             15.0                                 15.0                                     5.0 15.0                                             15.0                                                 11.0 15.0 ZnO              4.0 5.0 5.0 4.8 5.0 5.0 5.0 5.0 5.0 5.0  5.0Appearance:       Clear                 Clear                     Clear                         Clear                             Hazy                                 Hazy                                     Clear                                         Clear                                             Clear                                                 Slight                                                      Hazy                                                 HazeM.O.R. (kg/cm.sup.2)after 1 hr. at 1000° F (538° C):             5500                 5500                     6500                         6800                             5900    3000                                         6000    --after 4 hrs. at 900° F (482° C):             6300                 6800                     5700                         7000                             --      4500                                         6600                                             5100                                                 2300__________________________________________________________________________ .sup.* White deposits on surface 
    
     Table V shows examples of clear, colored glasses within the scope of the present invention. 
     
                       TABLE V______________________________________Example No.:    772     773     774   776   777   787Composition(% byweight): SiO.sub.2    44.2    44.45   44.2  44.0  44.0  43.5 Na.sub.2 O    18.0    18.0    18.0  18.0  18.0  18.0 A1.sub.2 0.sub.3    10.0    10.0    10.0  10.0  10.0  10.0 MnO.sub.2                     0.5 K.sub.2 O    3.0     3.0     3.0   3.0   3.0   3.0 ZrO.sub.2    4.5     4.5     4.5   4.5   4.5   4.5 P.sub.2 O.sub.5    15.0    15.0    15.0  15.0  15.0  15.0 ZnO     5.0     5.0     5.0   5.0   5.0   5.0 CuO     0.3 CoO             0.05 Cr.sub.2 O.sub.3        0.3 Fe.sub.2 O.sub.3                    0.5   1.0Appearance:    Clear   Clear   Clear Clear Clear Clear    Blue    Blue    Green Purple                                Green GreenM.O.R.(kg/cm.sup.2)after 1 hr. at1000° F(538° C):    5100    4600    4700after 4 hrs. at900° F(482° C):    6300    6300    6300______________________________________ 
    
     Which of these examples may be considered the most preferred depends upon the particular application for which the glass is intended and the importance placed upon each of the various parameters. One very clear composition which combines a number of desirable properties and appears to be suitable for conventional flat glass applications is Example No. 586. The relative merits of such a composition may be seen by comparing it with a standard, commercially available ion exchange glass sold by Corning Glass Works under the name &#34;Chemcor.&#34; 
     
         ______________________________________                      COMMERCIAL         EXAMPLE      ION-EXCHANGECOMPOSITION   586          GLASS______________________________________SiO.sub.2     44.5         61.91Na.sub.2 O    18.0         12.58K.sub.2 O     3.0          3.45A1.sub.2 O.sub.3         10.0         17.43ZrO.sub.2     4.5          0.02P.sub.2 O.sub.5         15.0         --ZnO           5.0          --CaO           --           0.15MgO           --           3.46TiO.sub.2     --           0.63Melting temp., ° C.:         1466         1750    (estimated)Annealing point, ° C.:          674         627M.O.R. (kg/cm.sup.2)(KNO.sub.3, 4 hrs. 482° C.):         4800         4300(KNO.sub.3, 4 hrs. 482° C.):         5600         6600______________________________________ 
    
     The melting temperature of the commercial ion exchange glass could not be measured directly, but was estimated by extrapolating obtainable viscosity data. 
     Additional differences between Example 586 and the commercial ion exchange glass are illustrated in the drawings. In FIG. 1, the compression layer depth attained after treatment in molten KNO 3  at two temperatures is plotted against time for both glasses. It is readily apparent that the 586 glass develops a compression layer at a much faster rate than the commercial glass. 
     FIG. 2 compares the center tension developed in the glasses in terms of optical retardation as measured by the quartz wedge technique. All the glasses were tested on 0.084 inch (6 millimeter) thick samples exchanged for one hour at 1000° F. (538° C.) in molten KNO 3 . Center tension is a measure of the stresses created in the interior of the glass in counteraction to the compression stresses in the surface portions, and is directly related to the ability of the treated glass to fragment into small particles upon fracture. The plots in the figure show that the Example 586 glass also has a significantly more rapid buildup of center tension than the commercial ion exchange glass. FIG. 2 also includes a plot for the glass of Example 694, another embodiment, and it is shown to be even faster in developing center tension. 
     FIG. 3, shows temperature-viscosity curves for Example 586 and the commercial ion exchange glass as well as for ordinary float glass and sheet glass. The float glass and sheet glass have the following compositions (percent by weight, oxide basis): 
     
         ______________________________________     Float Glass Sheet Glass______________________________________SiO.sub.2   73.2%         73.0%Na.sub.2 O  13.7          13.3K.sub.2 O   0.03          0.2CaO         8.9           8.2MgO         3.9           3.7A1.sub.2 O.sub.3       0.1           1.2SO.sub.3    0.2           0.2Fe.sub.2 O.sub.3       0.1           0.1______________________________________ 
    
     The plot for glass 586 advantageously lies very close to the plots for sheet and float glass, but in contrast it is readily apparent from the figure that the commercial ion exchange glass is much more difficult to melt than sheet or float glass. 
     The advantages of rapid compression layer development are considerable. Strength of a particular treated glass as measured by M.O.R. values is by itself seldom sufficient to indicate that a glass article will actually have significantly improved strength when put into service. This is because a high degree of compression can sometimes be concentrated in a very thin layer near the surface of the glass, which, when abraded or scratched in use, can lose its strengthening effect. Therefore, the most demanding requirement for many applications is the need to generate a compression layer deep enough to withstand the physical abuses to which the surface of the glass may be subjected. In this light, it is interesting to compare the treatment times required to obtain a given compression layer thickness as follows: 
     
         ______________________________________     Compression  Treatment   AbradedGlass     Layer Thickness                  In KNO.sub.3                              Rod M.O.R.______________________________________Example 586     150 microns  1.2 hrs. at 538° C                              4800 kg/cm.sup.2Example 586     150 microns    2 hrs. at 510° C                              5500 kg/cm.sup.2Commercial I-X     150 microns  7.3 hrs. at 538° C                              5000 kg/cm.sup.2Commercial I-X     150 microns  9.6 hrs. at 510 ° C                              5800 kg/cm.sup.2______________________________________ 
    
     It can be seen that for a fixed compression layer depth, M.O.R. values for the Example 586 glass were almost the same as for the commercial ion exchange glass, yet the compression layer in 586 was developed in a fraction of the time required by the commercial glass. The minimum compression layer depth required will depend upon the particular end use of the glass, but theoretically the compression layer need extend only beyond the deepest penetration of surface defects in order for the glass to be strengthened. This has been estimated to be about five microns in the case of a glass plate subjected to reasonably careful handling. 
     It should be apparent that the increase in glass strength attained by ion exchange will vary by degree, ranging from a de minimis effect to a many-fold increase. For many applications, however, it is preferred that the strengthening be at least as great as that achieved by thermal tempering of glass. Thermal usually at least doubles the M.O.R. of float glass, or increases the M.O.R. to a value in the area of about 1700 kg/cm 2  or more. The degree to which glass strength may be enhanced by ion exchange is illustrated in the following comparison of abraded rod M.O.R. values: 
     
         ______________________________________   No Ion   After Ion Exchange in KNO.sub.3   Exchange 1 hr. 538° C                        4 hrs. 493° C______________________________________Example 586:     550 kg/cm.sup.2                4800 kg/cm.sup.2                            5600 kg/cm.sup.2Float glass:     460 kg/cm.sup.2                 690 kg/cm.sup.2                             820 kg/cm.sup.2______________________________________ 
    
     The chemical durability of the glasses of Example 586 was compared with that of commercial float glass having the composition set forth above. The tests were run on glass discs about 2.5 inches (6.35 centimeters) in diameter formed by casting the glass onto a plate. Without being ion exchanged, the discs were immersed in boiling 2 percent by weight sulfuric acid for 30 minutes, after which they were removed from the acid and measured for weight loss. Other discs were subjected to boiling 0.1 Normal NaOH for 30 minutes, with the following results: 
     
         ______________________________________       Example     Float       586         Glass______________________________________Weight loss in acid         0.003         0.002 (milligrams/cm.sup.2)Weight loss in alkali         0.004         0.043 (milligrams/cm.sup.2)______________________________________ 
    
     These results show the novel compositions to have chemical durability comparable to, or even better than, float glass, rendering them suitable for use in a wide variety of applications. 
     Radiant energy transmittance of Example 586 may be compared with float glass as follows for 1/4 inch (6 millimeter) thickness: 
     
         ______________________________________       Example    Float       586        Glass______________________________________Luminous      90.1%        89%Ultraviolet   67.1%        70%Infrared      88.9%        68%Total Solar Energy         88.8%        78%Dominant wavelength         574 mμ    499 mμ______________________________________ 
    
     The details of the process which the ion exchange treatment is carried out are well known in the art and need not be elaborated upon in great length here. In general, it can be said that the type of ion exchange treatment to which this invention pertains is limited to temperatures below the strain point of the glass, but it should be understood that temperatures at, or slightly above, the strain point are permissible as long as the glass does not remain at such a temperature for sufficient length of time to cause appreciable relaxation of the induced compression at the surface of the glass. In other words, the exchange bath temperature may be slightly above the strain point if the treatment time is short. It is preferred that the highest temperature possible be used in order to shorten treatment time; in other cases it is desirable to ion-exchange more slowly to obtain greater strength. Typically, temperatures on the order of 400° to 600° C. are employed. In addition to KNO 3 , other molten salts of potassium may constitute the exchange bath, such as the chloride, fluoride, or phosphate salts. Other methods of contacting the glass with the exchange medium besids immersing in a molten salt bath have been proposed. These include spraying and coating with a carrier, which may be adaptable to the ion exchange treatment of the glass compositions disclosed herein. 
     Other modifications and variations as are known in the art may be resorted to without departing from the spirit and scope of the invention as defined by the appended claims.