Stable, chemically-strengthenable lithium aluminosilicate glasses
BACKGROUND OF THE INVENTION 
     That the strength of alkali metal silicate glass articles can be significantly enhanced through an ion exchange reaction, termed chemical strengthening, is well known to the glass art. The process contemplates contacting such glass articles with an external source of cations having larger ionic diameters than the alkali metal ions present in the glass composition. Most frequently, the larger-diameter cations are other alkali metal ions. The contact is made at elevated temperatures, but below the strain point of the glass, so that an ion exchange reaction takes place wherein the smaller alkali metal ions pass out of the glass and the larger cations move into the sites of the glass surface previously occupied by the smaller ions. Because the exchange reaction is conducted at temperatures below the strain point of the glass, viscous flow in the glass surface is inhibited. Consequently, the larger cations are, in essence, stuffed or crowded into the sites formerly filled by the small alkali metal ions, which phenomenon causes the development of compressive stresses in the surface of the glass article. It is this surface compression layer which imparts improved mechanical strength to the glass article. 
     That alkali metal aluminosilicate glass articles are especially adaptable to chemical strengthening has been disclosed in U.S. Pat. No. 3,790,430 and experience has indicated that lithium aluminosilicate glasses are notably outstanding in this regard. Nevertheless, the high viscosity and the extreme tendency to devitrify at elevated temperatures, particularly in the vicinity of the softening point, exhibited by the simple lithium aluminosilicate glasses have limited their utility to applications which do not require reheating in their shaping. hence, such processes as, for example, the sagging of already-formed ophthalmic lens blanks or the re-drawing of glass cane and tubing, are not suited to those glasses. 
     The most widespread use of chemical strengthening has been seen in the manufacture of ophthalmic lens. The food and Drug Administration (FDA) has decreed that all glass ophthalmic lenses must survive the drop of a steel ball having a diameter of 0.625&#34; from a height of 50&#34;. The common ophthalmic lenses of commerce, having a thickness of about 2 mm, are highly susceptible to breakage in that test unless such are subjected to thermal tempering or chemical strengthening, with the latter providing greater strength improvements than obtained via thermal tempering. 
     The density of glasses employed in ophthalmic lenses is considerably greater than that of plastic utilized in ophthalmic lens fabrication. Accordingly, the capability of achieving very high mechanical strengths in a glass suitable for the production of ophthalmic lenses would permit the use of thinner cross sections which, in turn, would result in a lens of lighter weight. 
     OBJECTIVES OF THE INVENTION 
     The principal objectives of the instant invention are five: 
     (1) to provide lithium aluminosilicate glasses which are resistant to gross devitrification at elevated temperatures, especially at temperatures in the vicinity of the softening point of the glass; 
     (2) to provide such glasses which also demonstrate excellent resistance to surface wrinkling, haze, devitrification, spalling, and cracking when maintained for several hours at elevated temperatures, i.e., above the annealing point of the glass but below the softening point thereof; 
     (3) to provide glass compositions having base compositions within the Li 2  0-Al 2  O 3  -SiO 2  system, but modified to exhibit some softening or fluxing effect when compared to the simple Li 2  O-Al 2  O 3  -SiO 2  ternary; 
     (4) to provide glasses displaying very high chemical strengthening capability, as evidenced by modulus of rupture values in excess of 60,000 psi; and 
     (5) to provide glasses which will demonstrate the characteristics set forth in (1)-(4) above and will be suitable for ophthalmic lenses; i.e., the index of refraction manifested by the glasses can be corrected to 1.523, or higher values where necessary, and the chemical durability of the glasses is acceptable for ophthalmic applications. 
     SUMMARY OF THE INVENTION 
     Laboratory investigations have indicated that glasses in the simple Li 2  O-Al 2  O 3  -SiO 2  system exhibit maxima in chemical strengthening capability where the molar ratio Al 2  O 3  :Li 2  O is about 1. Unfortunately, however, such glasses are highly prone to devitrify at temperatures in the vicinity of the softening point of the glass. This extreme propensity to devitrify has been utilized to produce a large variety of glass-ceramic articles containing beta-quartz solid solution and/or beta-spodumene solid solution as the predominant crystal phase. The compositions therefor include such nucleation agents as TiO 2 , ZrO 2 , and SnO 2  which, upon heat treatment, cause internal nucleation and crystal growth resulting in strong, low expansion, fine-grained, highly crystalline materials. It is quite apparent that the production of thermally stable glasses precludes the presence of any ingredient which would initiate nucleation. 
     Nevertheless, even in the absence of recognized nucleating agents, glasses in the Li 2  O-Al 2  O 3  -SiO 2  system evidence a strong proclivity toward surface devitrification at elevated temperatures leading to surface wrinkling, cracking, haze, etc. the crux of the instant invention resides in the partial molar substitution of PbO for Li 2  O in the glass compositions such that the mole ratio Al 2  O 3  :Li 2  O+PbO ranges about 0.95-1.05. Such a substitution has been found to be much more efficacious in accomplishing the above-stated objectives than the incorporation into the base Li 2  O-Al 2  O 3  -SiO 2  compositions of such components as alkali metal oxides, alkaline earth metal oxides, fluxing glass formers such as B 2  O 3  and P 2  O 5 , and mechanical fluxes such as fluoride. 
     In general, the melting and forming behavior, as well as the physical properties and the capability for chemical strengthening, are more favorable in those glasses wherein the Li 2  O-Al 2  O 3  -SiO 2  stoichiometry varies over the range of about Li 2  O·Al 2  O 3  ·4-6SiO 2 . Although glasses containing higher SiO 2  contents can be formed, the ion exchange strengthening is much more efficient with higher concentrations of Li 2  O. On a molar basis with the stoichiometry of Li 2  O·Al 2  O 3  ·4-6SiO 2 , the most useful substitutions of PbO for Li 2  O vary between about 0.5-1.5 moles. 
     Accordingly, glasses operable in the present invention consist essentially, in weight percent on the oxide basis as calculated from the batch, of: 
     
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       SiO.sub.2     59-71%                                               
       Al.sub.2 O.sub.3                                                   
                    20-28                                                 
       Li.sub.2 O   4.5-7.5                                               
       PbO          3-9                                                   
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     Whereas minor amounts of fluoride and compatible metal oxides such as the other alkali metal oxides, the alkaline earth metal oxides, B 2  O 3 , and P 2  O 5  can be tolerated, the most preferred compositions will consist essentially solely of the four above-recited constituents in the indicated ranges with the total of all additions not exceeding about 5%. The fluoride content will be held below 1%. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Table I records several glass compositions, expressed in terms of parts by weight on the oxide basis as calculated from the batch, illustrating the parameters of the instant invention. Because the sum of the individual components totals or approximates 100, for all practical purposes the values reported may be deemed to reflect weight percent. The actual batch ingredients may comprise any materials, either the oxide or other compound, which, when melted in combination with the other constituents, will be converted into the desired oxide in the proper proportions. As 2  O 3  was included in the batch in its customary capacity as a fining agent. Table IA lists the batch components in terms of moles to better illustrate the inventive substitution of PbO for Li 2  O on a molar basis. It will be observed from Table IA that the mole ratio Al 2  O 3  :Li 2  O+substitutions for Li 2  O, if any, ranges slightly above unity. Inasmuch as it is not known with which cations(s) the fluoride is combined, it is merely tabulated in terms of AlF 3 , the batch ingredient employed to incorporate fluoride into the glass. 
     Example 1 lists a glass of approximately Li 2  O.Al 2  O 3 .5Sio 2  stoichiometry and Examples 2-7 illustrate operable PbO-containing compositions covering stoichiometries over the range Li 2  O·Al 2  O 3  ·4-6SiO 2 . Examples 8-10 report B 2  O 3  additions to PbO-containing glasses and Examples 11-16 recite additions other than PbO to the base Li 2  O-Al 2  O 3  -SiO 2  compositions. It will be noted that Na 2  O and CaO were substituted for part of the Li 2  O, B 2  O 3  and P 2  O 5  were substituted on a cationic basis for part of the SiO 2 , and F was merely included as an overall addition. 
     The batch materials were compounded, ballmilled together to aid in achieving a homogeneous melt, and then deposited into platinum crucibles. A lid was placed upon each crucible and the covered crubibles moved to a furnace operating at 1650° C. After melting overnight (about 16 hours), cane of about 0.25&#34; was hand-drawn from the melts and the remainder of the molten batch poured into slabs having dimensions of about 6&#34;×6&#34;×0.5&#34;. The cane samples were cooled in the ambient air and the slabs were immediately transferred to an annealer operating at about 550° C. 
     
                       TABLE I                                                     
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        1      2      3    4    5    6    7    8                          
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SiO.sub.2                                                                 
       68.5    61.4   59.4 66.5 64.6 70.4 68.7 59.1                       
Al.sub.2 O.sub.3                                                          
       24.0    27.4   26.6 23.8 23.1 21.0 20.5 27.8                       
Li.sub.2 O                                                                
       6.8     7.1    6.4  6.2  5.6  5.5  4.9  7.2                        
As.sub.2 O.sub.3                                                          
       0.6     0.3    0.3  0.3  0.3  0.3  0.3  0.3                        
PbO    --      3.8    7.4  3.3  6.4  2.9  5.7  3.9                        
B.sub.2 O.sub.3                                                           
       --      --     --   --   --   --   --   1.8                        
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       9       10     11   12   13   14   15   16                         
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SiO.sub.2                                                                 
       64.5    68.7   64.6 63.5 67.6 67.7 67.9 63.9                       
Al.sub.2 O.sub.3                                                          
       24.0    21.2   24.6 24.2 23.7 23.8 22.8 22.8                       
Li.sub.2 O                                                                
       6.2     5.5    7.0  6.9  5.4  5.4  6.8  6.9                        
As.sub.2 O.sub.3                                                          
       0.3     0.3    0.6  0.6  0.6  0.6  0.6  0.6                        
PbO    3.3     2.9    --   --   --   --   --   --                         
B.sub.2 O.sub.3                                                           
       1.6     1.4    3.3  1.6  --   --   --   3.2                        
Na.sub.2 O                                                                
       --      --     --   --   2.8  --   --   --                         
CaO    --      --     --   --   --   2.5  --   --                         
P.sub.2 O.sub.5                                                           
       --      --     --   3.3  --   --   --   --                         
AlF.sub.3                                                                 
       --      --     --   --   --   --   2.5  2.6                        
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                       TABLE IA                                                    
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        1      2      3    4    5    6    7    8                          
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SiO.sub.2                                                                 
       37.5    30.0   30.0 3.75 37.5 45.0 45.0 28.5                       
Al.sub.2 O.sub.3                                                          
       7.75    7.90   7.90 7.90 7.90 7.90 7.90 7.90                       
Li.sub.2 O                                                                
       7.50    7.00   6.50 7.00 6.50 7.00 6.50 7.00                       
As.sub.2 O.sub.3                                                          
       0.10    0.045  0.045                                               
                           0.05 0.05 0.055                                
                                          0.055                           
                                               0.045                      
PbO    --      0.5    1.0  0.5  1.0  0.5  1.0  0.5                        
B.sub.2 O.sub.3                                                           
       --      --     --   --   --   --   --   0.75                       
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       9       10     11   12   13   14   15   16                         
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SiO.sub.2                                                                 
       36.0    43.5   34.5 34.5 37.5 37.5 37.5 34.5                       
Al.sub.2 O.sub.3                                                          
       7.90    7.90   7.75 7.75 7.75 7.75 7.25 7.25                       
Li.sub.2 O                                                                
       7.00    7.00   7.50 7.50 6.00 6.00 7.50 7.50                       
As.sub.2 O.sub.3                                                          
       0.050   0.055  0.10 0.10 0.10 0.10 0.10 0.10                       
PbO    0.5     0.5    --   --   --   --   --   --                         
B.sub.2 O.sub.3                                                           
       0.75    0.75   1.50 0.75 --   --   --   --                         
Na.sub.2 O                                                                
       --      --     --   --   1.50 --   --   --                         
CaO    --      --     --   --   --   1.50 --   --                         
P.sub.2 O.sub.5                                                           
       --      --     --   0.75 --   --   --   --                         
AlF.sub.3                                                                 
       --      --     --   --   --   --   1.00 1.00                       
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 Table II reports physical property data obtained primarily utilizing cane
 samples as test specimens. Determinations of the refractive index
 (n.sub.D), softening point (Soft.), annealing point (Ann.), and the
 coefficient of thermal expansion (Exp.) of each glass were conducted
 employing test procedures conventional in the glass art. The softening and
 annealing points are recited in terms of °C. and the coefficients
 of thermal expansion over the range 0°-300° C. are recorded
 in terms of ×10.sup.-7 /°C.
 
    
     The reported sag test provides a simple means for observing the behavior of a glass when subjected to elevated temperatures. As recorded in Table II, cane samples were suspended on sillimanite forms spaced apart to yield a three-inch span. The specimens were thereafter heated at 400° C./hour to 800° C., maintained at that level for four hours, and then cooled in the ambient air. After cooling, the samples were inspected visually and the amount of sag or thermal deformation measured in millimeters. 
     Chemical strengthening determinations were undertaken by immersing several four-inch long cane samples into a bath of molten NaNO 3  operating at 400° C. After about two hours, the samples were removed from the bath and allowed to cool. The adhering salt was removed therefrom with tap water, the samples were dried, and modulus of rupture values (psi) measured on abraded samples utilizing techniques standard in the glass art. The figures tabulated reflect average values of several measurements. 
     The chemical durability of the glasses was determined via an immersion of test samples for 30 minutes in a bath of aqueous 3N HCl solution operating at room temperature (20°-25° C.). None of the test samples evidenced any attack by that solution, thereby indicating the utility of the glasses on the score for ophthalmic applications. 
     
                                           TABLE II                                
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       1    2    3    4    5    6    7    8                               
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PHYSICAL                                                                  
Soft.  Devit.                                                             
            910°                                                   
                 897°                                              
                      947°                                         
                           944°                                    
                                986°                               
                                     970°                          
                                          --                              
Ann.   715°                                                        
            688°                                                   
                 674°                                              
                      705°                                         
                           695°                                    
                                718°                               
                                     712°                          
                                          --                              
Exp.   47   58   58   --   45   --   37   --                              
n.sub.D                                                                   
       1.510                                                              
            1.528                                                         
                 1.534                                                    
                      --   1.520                                          
                                --   1.514                                
                                          --                              
SAG TEST                                                                  
Appearance                                                                
       OK   OK   OK   OK   OK   OK   OK   Wrin-                           
                                          kled                            
Sag    3    7    12   6    10   4    6    17                              
STRENGTH                                                                  
M.O.R. 84,900                                                             
            91,000                                                        
                 83,100                                                   
                      82,400                                              
                           76,000                                         
                                75,000                                    
                                     67,900                               
                                          85,100                          
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       9    10   11   12   13   14   15   16                              
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PHYSICAL                                                                  
Soft.  --   942°                                                   
                 845°                                              
                      910°                                         
                           955°                                    
                                935°                               
                                     903°                          
                                          837°                     
                                     Devit                                
Ann.   --   690°                                                   
                 632°                                              
                      670°                                         
                           716°                                    
                                716°                               
                                     645°                          
                                          603°                     
Exp.   --   --   55   40   46   46   48   57                              
n.sub.D                                                                   
       --   1.510                                                         
                 --   --   --   --   --   --                              
SAG TEST                                                                  
Appearance                                                                
       Minor                                                              
            Slight                                                        
                 Wrin-                                                    
                      Wrin-                                               
                           Slight                                         
                                OK   Hazy,                                
                                          Wrin-                           
       Wrin-                                                              
            Wrin-                                                         
                 kled kled Wrin-     Spall-                               
                                          kled,                           
       kles kles           kles      ing  Spall-                          
                                          ing                             
Sag    15   11   Soft-                                                    
                      15   5    6    2    3                               
                 ened                                                     
STRENGTH                                                                  
M.O.R. 78,600                                                             
            68,300                                                        
                 68,300                                                   
                      83,500                                              
                           68,100                                         
                                73,100                                    
                                     67,300                               
                                          61,200                          
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     It is believed apparent from an inspection of the physical property, sag test, and strength data reported in Table II that the molar substitution of PbO for Li 2  O accomplishes the objectives of the present invention. For example, reference glass Example 1, approximating the Li 2  O·Al 2  O 3  ·5SiO 2   stoichiometry, devitrifies in the range of the softening point, but exhibits high refractoriness in the sag test and a great capability for chemical strengthening. Moreover, its refractive index is too low for ophthalmic use. The PbO-containing variations of that composition, viz., Examples 2-7, do not devitrify in the softening range, demonstrate some fluxing action (the softening point of Example 1 would be higher than 990° C. if it could be measured), maintain good surface quality during the sag test, manifest excellent abraded strength, and increased refractive indices. Examples 2 and 3, having lower SiO 2  contents, are especially noteworthy, in view of the inherent strong tendency of comparable ternary Li 2  O-Al 2  O 3  -SiO 2  glasses to devitrify. Those glasses successfully resist surface-induced devitrification and/or distortion, display excellent strength after exposure to an ion exchange reaction, and bracket the required refractive index (1.523) for ophthalmic applications. Such glasses would be very suitable for the production of sagged lens blanks from which thin, light weight, high strength, ophthalmic lenses could be fabricated. Such glass would also be operable in the production of sagged-to-shape non-prescription eyeglasses, although the refractive index correction would not be demanded for that application. In both instances, however, the PbO content would be of value in decreasing unwanted ultraviolet transmission through the glass. 
     In general, the B 2  O 3  -containing glasses (Examples 8-12 and 16) manifest considerable fluxing relative to the sample Li 2  O-Al 2  O 3  -SiO 2  ternary glass, but assume an unacceptable degree of surface wrinkling during high temperature heat treatment. The addition of B 2  O 3  alone reduces the abraded strength of the glass considerably, but, when present in concert with P 2  O 5  (Example 12) or PbO (Example 8-10), yields excellent strengths. Finally, B 2  O 3  acts to reduce the refractive index of the glass. Example 10, which reflects a high silica glass, viz., a Li 2  O·Al 2  O 3  ·6SiO 2  stoichiometry, containing PbO and B 2  O 3 , seems to imply that a slightly higher SiO 2  or, more preferably, PbO level might completely eliminate surface wrinkling. Such a combination could be optimized to provide efficient fluxing for viscous Li 2  O-Al 2  O 3  -SiO 2  glasses. 
     The fluoride-containing glasses (Examples 15 and 16) demonstrate excessive surface crystallization and are unacceptable because of the development of haze, spalling, and cracking therein. The Na 2  O-containing glass (Example 13) undergoes wrinkling at elevated temperatures and the CaO-containing glass (Example 14) has a steep viscosity curve which adversely affects its forming capabilities. And, of course, these latter two glasses exhibit refractive indices below the 1.523 required for ophthalmic applications.