Abstract:
A glass having a high refractive index, preferably &gt;1.77, and a low specific gravity, preferable ≦3.8, is provided for utilization as spectacle lenses. The glass consists essentially of 80 mol percent or more of light cationic oxides with the remainder being cationic oxides having a greater molecular weight. It has a high crystallization stability in a viscosity range &gt;200 poises and an expansion coefficient α×10 7  between 20°-300° C. of ≦100. Sufficiently light in weight to be well-suited for use as spectacle lenses, the glass can be continuously produced, as it has no pronounced tendency toward crystallization. The resulting corrective optical lenses have relatively high dioptric values, even with small center and edge thicknesses.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to glasses having a high refractive index and a low specific gravity for use as spectacle lenses. 
     In order to replace the hitherto lightest-weight and highest refractive index glasses (nd=1.70, s=3.0) used as spectacle lenses by an even higher refractive index glass (e.g., nd&gt;1.79), so as to produce spectacle lenses having higher dioptric values with even smaller edge thicknesses (for negative lenses) and/or even smaller center thicknesses (for positive lenses) than previously done, thereby enabling further enlargement of the optically effective field of vision while also improving cosmetic appearances, the specific gravity must be approximately 3.3 if the weight of the glass lens is to remain approximately the same. In a glass with a refractive index of nd=1.79 and a specific gravity of approximately 3.7, the eventual glass weight increase is approximately 12 percent for negative and positive lenses as compared to a glass having a refractive index of nd=1.70 and a specific gravity of 3.0. 
     Optical glasses with refractive indexes nd of 1.76-1.80 and specific gravities s of ≦4.5 are known, but as a result of their weight are not suited for use as glasses for spectacle lenses, even though such glasses can be produced by continuous mass-production means. 
     Examples of these unsuitable glasses are heavy flint glasses as follows: 
     
         ______________________________________SF 6    nd = 1.8051  vd = 25.43  s = 5.18SF 11   nd = 1.7847  vd = 25.76  s = 4.74SF 14   nd = 1.7612  vd = 26.53  s = 4.54______________________________________ 
    
     The known LaF glasses with a refractive index nd of &gt;1.77 but a lower dispersion than heavy flint glasses have specific gravities of &gt;4.0. Examples of the known LaF and LaSF glasses are as follows: 
     
         ______________________________________LaF 25    nd = 1.7843                vd = 41.30                          s = 4.45LaF 22    nd = 1.7818                vd = 37.1 s = 4.21Laf 9     nd = 1.7950                vd = 28.39                          s = 4.96LaSF N 3  nd = 1.8080                vd = 40.75                          s = 4.68LaSF 8    nd = 1.80741                vd = 31.61                          s = 4.87______________________________________ 
    
     Further high refractive index optical glasses, e.g., as disclosed by German Patent Specification No. 12 60 712, having refractive indexes nd of 1.75-1.80 and specific gravities ≦4.0 are known, but they are not suitable for continuous production of solid glasses (e.g., glasses for spectacle lenses) and crucible melting due to their pronounced tendency towards crystallization. This crystallization tendency is determined by the magnitude of the crystal growth speed and the crystallization temperature range in relation to viscosity. 
     Glasses having a refractive index &gt;1.75 consist of 14-46 percent by weight of alkaline earth metal oxides, including a heavy BaO content ≦10 percent by weight. The ZnO content is 10-20 percent by weight and the TiO 2  -content is 10-25 percent by weight. Such glasses contain 12-20 percent by weight of SiO 2  and 8-20 percent by weight of B 2  O 3  as glass components, the sum of glass forming components being &lt;31 percent by weight. The color of such high titanium-bearing glasses must be influenced by rapid cooling in such a manner that the glass has a strongly marked titanium color (yellow-brown). The glasses must be cast at a viscosity &lt;100 poises because their strong tendency towards crystallization precludes tank production and automatic processing at higher viscosities, e.g. between 200 to 600 poises. The ZnO content of 10-20 percent by weight in conjunction with the high TiO 2  -content of 10-25 percent by weight is unfavorable for these glasses. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a glass having a high refractive index and a low specific gravity which can be utilized in spectacle lenses. 
     Another object of the present invention is to provide a glass which has high crystallization stability within a given viscosity range in order that it may be continuously produced, good chemical resistance and a thermal expansion coefficient α×10 7  between 20°-300° C.≦100. 
     An additional object of the present invention is to provide a glass which can be used to produce spectacle lenses having high dioptric values with smaller edge or center thicknesses than lenses which are currently known. 
     A more particular object of the present invention is to form a series of new glasses to be used in spectacle manufacture which have refractive indexes nd of 1.77-1.81 and specific gravities s of ≦3.8 (particularly those within the 3.4-3.8 range), additionally having the following characteristics in order to be technically viable from a manufacturing and practical viewpoint: 
     (a) high crystallization stability in a viscosity range of &gt;200 poises to allow continuous production; 
     (b) a thermal expansion coefficient α×10 7  between 20°-300° C. of ≦100; and 
     (c) good chemical resistance. 
     Upon study of the specification and appended claims, further objects, features and advantages of the present invention will become more fully apparent to those skilled in the art to which this invention pertains. 
     SUMMARY OF THE INVENTION 
     Briefly, the above and other objects, features and advantages of the present invention are attained in one aspect thereof by providing a glass for spectacle lenses having a refractive index nd of 1.77-1.81 and a specific gravity s of 3.4-3.8 which consists essentially of: 
     (a) more than 80 mol percent of mono and polyvalent cationic oxides having a molecular weight ≦100, namely 
     
         ______________________________________SiO.sub.2    22-30 mol percentB.sub.2 O.sub.3        17-23 mol percentSiO.sub.2 + B.sub.2 O.sub.3        40-49 mol percentAlkali metal oxides        0- 8 mol percentCaO          12-20 mol percentMgO + CaO    12-22 mol percentTiO.sub.2    17-24 mol percent; and correspondingly______________________________________ 
    
     (b) less than 20 mol percent of cationic oxides having a molecular weight &gt;100, namely 
     
         ______________________________________La.sub.2 O.sub.3          4-8 mol percentP.sub.2 O.sub.5          0-1 mol percentBaO            0-5 mol percentSrO            3-10 mol percentBaO + SrO      5-10 mol percentPbO            0-2 mol percentAl.sub.2 O.sub.3          0-1 mol percentZrO.sub.2      0-2 mol percentNb.sub.2 O.sub.5          0-1.5 mol percentTa.sub.2 O.sub.5          0-1.5 mol percent                           0-2WO.sub.3       0-0.5 mol percent______________________________________ 
    
     the glass having no pronounced tendency towards crystallization as determined by the magnitude of the crystal growth speed and the crystallization temperature range in relation to viscosity. 
    
    
     DETAILED DESCRIPTION 
     The new glasses according to the invention consist mainly (&gt;80 mol percent) of light cationic oxides (having molecular weights of &lt;100), whereby polyvalent components such as La 2  O 3  (&lt;8 mol percent) and eventually Y 2  O 3  cannot be excluded. Y 2  O 3  reduces the specific gravity in relation to La 2  O 3 . La 2  O 3  may be replaced extensively by Y 2  O 3 , but only in small proportions of Y 2  O 3  where this is combined with a reduction in crystallization tendencies as defined hereinabove. 
     The porportion of glass-forming components SiO 2  +B 2  O 3  (+P 2  O 5  +GeO 2 ) is 30-40 percent by weight, the SiO 2  content being greater than the B 2  O 3  content. 
     The alkali metal content of such glasses is restricted within the following limits: 
     
         ______________________________________Li.sub.2 O 0-5Na.sub.2 O 0-5          0-5 percent by weightK.sub.2 O  0- 5______________________________________ 
    
     so that the required expansion coefficient α×10 7  can be adjusted between 20°-300° C. to ≦100. 
     The sum of alkaline earth metal components should be 19-25 percent by weight, the BaO content being no higher than 8 percent by weight, the BaO+SrO content being &lt;15 percent by weight, and the MgO+CaO content being 7-15 percent by weight. 
     The glasses formed in accordance with the present invention consequently consist essentially of: 
     (a) more than 80 mol percent of mono to polyvalent cationic oxides with molecular weights ≦100, namely 
     
         ______________________________________SiO.sub.2    22-30 mol percentB.sub.2 O.sub.3        17-23 mol percentSiO.sub.2 + B.sub.2 O.sub.3        40-49 mol percentAlkali metal oxides        0- 8 mol percentCaO          12-20 mol percentMgO + CaO    12-22 mol percentTiO.sub.2    17-24 mol percent; and correspondingly______________________________________ 
    
     (b) less than 20 mol percent of cationic oxides having a molecular weight of &gt;100, namely 
     
         ______________________________________La.sub.2 O.sub.3          4-8 mol percentP.sub.2 O.sub.5          0-1 mol percentBaO            0-5 mol percentSrO            3-10 mol percentBaO + SrO      5-10 mol percentPbO            0-2 mol percentAl.sub.2 O.sub.3          0-1 mol percentZrO.sub.2      0-2 mol percentNb.sub.2 O.sub.5          0-1.5 mol percentTa.sub.2 O.sub.5          0-1.5 mol percent                           0-2WO.sub.3       0-0.5 mol percent______________________________________ 
    
     The glasses made in accordance with the present invention consist (in percent by weight) essentially of: 
     
         ______________________________________SiO.sub.2      15-20 weight percentB.sub.2 O.sub.3          10-20 weight percentCaO            8-15 weight percentSiO.sub.2 + B.sub.2 O.sub.3          30-40 weight percentLa.sub.2 O.sub.3          16-26 weight percentTiO.sub.2      15-25 weight percentMgO            0-3 weight percent -SrO                         1.0-10 weight percent                               8-15BaO            0-8 weight percentZnO            0-4 weight percentPbO            0-2 weight percentAlkaline earth metaloxides         20-25 weight percentAl.sub.2 O.sub.3          0-2 weight percentZrO.sub.2      0-3 weight percentNb.sub.2 O.sub.5          0-5 weight percentTa.sub.2 O.sub.5          0-2 weight percentAlkali metal oxides          0-5 weight percentP.sub.2 O.sub.5          0-3 weight percentGeO.sub.2      0-3 weight percent______________________________________ 
    
     The sum of divalent components should be 20-29 percent by weight, the CaO+MgO content not being more than 15 percent by weight, since the crystallization tendency will otherwise increase considerably. 
     ZnO may be added to adjust the expansion coefficient up to 4 percent by weight; it is only with higher ZnO contents that the crystallization and segregation tendency increases. 
     P 2  O 5  or GeO 2  can be used in smaller proportions, providing that a maximum crystallization stabilization is achieved. 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following examples, therefore, are to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever. Unless otherwise indicated, all pressures and temperatures are ambient and all parts and percentages are by weight. 
     The tables give specific examples of glasses formed according to the composition range suggested by the invention. Table 1 shows the compositions of 12 typical glasses as a percentage by weight and Table 2 shows the same glasses listed in molar percentages. 
     
                                           TABLE 1__________________________________________________________________________Composition in percent by weightOxide  1   2   3   4   5   6   7   8   9   10  11  12__________________________________________________________________________SiO.sub.2  15.0      17.4          16.3              17.0                  18.6                      18.8                          19.4                              18.6                                  18.0                                      19.7                                          19.5                                              18.7B.sub.2 O.sub.3  16.0      17.4          17.3              17.0                  15.6                      15.8                          15.6                              15.6                                  15.2                                      16.0                                          15.6                                              15.7SiO.sub.2 + B.sub.2 O.sub.3  31.0      34.8          33.6              34.0                  34.2                      34.6                          35.0                              34.2                                  33.2                                      35.7                                          35.1                                              34.4P.sub.2 O.sub.5  --  --  --  1.0 --  --  --  --  --  --  --  --Li.sub.2 O  --  --  --  --  --  0.8 --  --  2.4 --  --  --Na.sub.2 O  --  --  --  --  --  0.3 --  --  0.9 0.1 --  2.0K.sub.2 O  --  --  --  --      0.2 --  --  0.5 --  --  --Alkali metaloxides --  --  --  --  --  1.3 --  --  3.8 0.3 --  2.0MgO    --  --  --  --  1.9 1.0 1.0 --  --  --  1.0 --CaO    11.5      11.8          11.7              12.4                  8.8 12.4                          8.8 8.8 8.5 8.5 8.8 9.9MgO + CaO  11.5      11.8          11.7              12.4                  10.7                      13.4                          9.8 8.8 8.5 8.5 9.8 9.9BaO    8.0 3.0 3.1 1.0 2.0 --  2.0 2.0 2.8 --  2.0 2.0SrO    5.0 8.2 8.1 8.0 9.8 7.9 9.8 9.8 9.5 8.0 9.8 8.0BaO + SrO  13.0      11.2          11.2              9.0 11.8                      7.9 11.8                              11.8                                  12.3                                      9.0 11.8                                              10.0ZnO    --  --  --  --  --  --  --  --  --  4.0 --  --PbO    --  --  --  --  --  --  2.0 --  --  --  --  --Al.sub.2 O.sub.3  1.0 --  --  --  0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2La.sub.2 O.sub.3  26.3      24.6          26.0              22.4                  23.4                      19.1                          23.4                              23.4                                  16.1                                      19.5                                          22.8                                              23.2Y.sub.2 O.sub. 3  --  --  --  1.0 --  1.0 --  --  --  1.0 --  --Bi.sub.2 O.sub.3  --  --  --  --  --  --  --  --  --  --  --  --ZrO.sub.2  --  --  --  --  --  2.0 --  --  --  2.0 --  --TiO.sub.2  17.0      17.4          17.3              17.0                  16.6                      16.8                          16.6                              16.5                                  22.9                                      17.1                                          16.6                                              17.8Ta.sub.2 O.sub.5  --  --  --  --  --  --  --  --  --  --  0.5 1.0Nb.sub.2 O.sub.5  --  --  --  3.0 2.9 3.0 3.0 2.9 2.8 3.0 3.0 1.5WO.sub.3  --  --  --  --  --  0.5 --  --  --  0.5 --  --As.sub.2 O.sub.3  0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 --NaCl   --  --  --  --  --  --  --  --  --  0.2 --  --nd     1.7992      1.7892          1.7958              1.7928                  1.7927                      1.7846                          1.7893                              1.7986                                  1.8115                                      1.7737                                          1.7889                                              1.7861vd     34.1      34.4          34.3              33.7                  33.6                      33.7                          33.8                              33.1                                  30.4                                      33.2                                          33.6                                              33.4s      3.78      3.67          3.71              3.60                  3.67                      3.54                          3.66                              3.74                                  3.54                                      3.56                                          3.66__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________Composition in mol percentOxide  1   2   3   4   5   6   7   8   9   10  11  12__________________________________________________________________________SiO.sub.2  22.9      25.3          24.1              24.9                  27.2                      26.0                          28.6                              28.1                                  24.4                                      28.5                                          28.7                                              27.3B.sub.2 O.sub.3  21.2      21.9          22.1              21.5                  19.7                      18.9                          19.8                              20.4                                  17.8                                      19.9                                          19.8                                              19.8SiO.sub.2 + B.sub.2 O.sub.3  44.1      47.2          46.2              46.4                  46.9                      44.9                          48.4                              48.5                                  42.2                                      48.4                                          48.5                                              47.1P.sub.2 O.sub.5  --  --  --   0.6                  --  --  --  --  --  --  --  --Li.sub.2 O  --  --  --  --  --  2.2 --  --  6.5 --  --  --Na.sub.2 O  --  --  --  --  --  0.4 --  --  1.2 0.4 --  2.8K.sub.2 O  --  --  --  --      0.2 --  --  0.4 --  --  --Alkali metaloxides --  --  --  --  --  2.8 --  --  8.1 0.4 --  2.8MgO    --  --  --  --  4.1 2.1 2.2 --  --  --  2.2 --CaO    18.8      18.4          18.6              19.4                  13.8                      18.4                          13.9                              14.3                                  12.3                                      13.2                                          13.9                                              15.5MgO + CaO  18.8      18.4          18.6              19.4                  17.9                      20.5                          16.1                              14.3                                  12.3                                      13.2                                          16.1                                              15.5BaO    4.8  1.7           1.8               0.6                  1.1 --  1.2 1.2 1.5 --  1.1 1.1SrO    4.4  6.9           6.9               6.8                  8.3 6.3 8.4 8.6 7.4 6.7 8.4 6.8BaO + SrO  9.2  8.6           8.7               7.4                  9.4 6.3 9.6 9.8 8.9 6.7 9.5 7.9ZnO    --  --  --  --  --  --  --  --  --  4.3 --  --PbO    --  --  --  --  --  --  --  0.8 --  --  --  --Al.sub.2 O.sub.3  0.9 --  --  --  0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2La.sub.2 O.sub.3  7.4  6.6           7.1               6.0                  6.3 4.9 6.3 6.5 4.0 5.2 6.7 6.2Y.sub.2 O.sub.3  --  --  --   0.4                  --  --  --  0.4 --  --Bi.sub.2 O.sub.3  --  --  --  --  --  --  --  --  --  --  --  --ZrO.sub.2  --  --  --  --  --  1.3 --  --  --  1.4 --  --TiO.sub.2  19.5      19.1          19.3              18.7                  18.2                      17.5                          18.4                              18.8                                  23.3                                      18.6                                          18.4                                              19.6Ta.sub.2 O.sub.5  --  --  --  --  --  --  --  --  --  --  0.1 0.2Nb.sub.2 O.sub.5  --  --  --   1.0                  1.0 0.9 0.9 1.0 0.9 0.9 0.9 0.5WO.sub.3  --  --  --  --  --  0.2 --  --  --  0.2 --  --As.sub.2 O.sub.3  0.1  0.1           0.1               0.1                  0.1 0.1                      0.1 0.1 0.1 0.1 0.1 --NaCl   --  --  --  --  --  --  --  --  --  --  --  --__________________________________________________________________________ 
    
     Melting Example 
     Composition: 
     
         ______________________________________                       Kg amounts weighed in  Percent    Starting  for a 90 kg computedOxides by Weight  Materials quantity of glass______________________________________SiO.sub.2  18.7       SiO.sub.2 16.926B.sub.2 O.sub.3  15.7       H.sub.3 BO.sub.3                       25.296Li.sub.2 0  0.8        Li.sub.2 CO.sub.3                       1.769Na.sub.2 O  0.3        Na.sub.2 CO.sub.3                       0.455K.sub.2 O  0.2        K.sub.2 CO.sub.3                       0.267MgO    1.0        MgCO.sub.3                       2.292CaO    12.4       CaCO.sub.3                       20.059SrO    7.9        Sr(NO.sub.3).sub.2                       14.457Al.sub.2 O.sub.3  0.2        A10(OH)   0.237La.sub.2 O.sub.3  19.1       La.sub.2 O.sub.3                       17.184Y.sub.2 O.sub.3  1.0        Y.sub.2 O.sub.3                       0.889TiO.sub.2  16.8       TiO.sub.2 15.168ZrO.sub.2  2.0        ZrO.sub.2 1.798Nb.sub.2 O.sub.5  3.0        Nb.sub.2 O.sub.5                       2.668NaCl   0.2        NaCl      0.178WO.sub.3  0.5        WO.sub.3  0.445As.sub.2 O.sub.3  0.2        As.sub.2 O.sub.3                       0.178______________________________________ 
    
     After weighing in, the starting materials are mixed in a commercial mechanical mixer. The mixture is melted down, for example, in a Pt-crucible at 1200°-1220° C., refined at 1280° C. for 4 hours and then stirred until the temperature drops to about 1060° C. The resultant melt is drained through a Pt-pipe and is processed into a bar or a blob. 
     The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants or operating conditions of this invention for those used in the preceding examples. 
     From the foregoing description, one skilled in the art to which this invention pertains can easily ascertain the essential characteristics thereof and, without departing from the spirit and scope of the present invention, can make various changes and modifications to adapt it to various usages and conditions.