Abstract:
Optical glasses having high refractive indexes and low specific gravities are provided for use as both distant and near vision spectacle lenses. The glasses consist essentially of low molecular weight cationic oxides, alkali metal oxides and divalent oxides to promote the crystallization stability of the glass. They have good chemical resistance, chemical hardenability and a thermal expansion coefficient between 93 and 99×10 -7  /° C. in the range of 20°-300° C. Optical lenses can be continuously produced due to the high crystallization stability of the glasses and the resultant spectacle lenses have high dioptric values even with thinner edge or center thicknesses.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to high refractive index optical glasses having a low specific gravity for utilization as both distant vision and near vision spectacle lenses having refractive indexes nd=1.79-1.81, dispersions vd=31-29 and specific gravities s=3.4-3.7. 
     The first lightweight high refractive index glasses utilized as spectacle lenses became known in 1973. These glasses had a refractive index nd=1.70, a dispersion vd=30 and a specific gravity s=3.0. 
     The object of the present invention is to produce high refractive index glasses which allow an optician to produce spectacle lenses of high dioptric values having thinner edge thicknesses (negative lenses) and/or smaller center thicknesses (positive lenses) than those previously used. The shallow curvature of such lenses increases the optically effective field of vision and improves the cosmetic effects of the spectacle in which they are used. The specific gravity for high refractive index glasses must be maintained as low as possible if the glasses are to benefit the spectacle-wearer, e.g., where a spectacle glass has a refractive index nd=1.79 and a specific gravity of approximately 3.3 then a 46 mm diameter lens will be no heavier than a lens made of a glass having a lower refractive index nd=1.70, a dispersion vd=30 and a specific gravity s=3.0. Where the glass has a refractive index nd=1.79 and a specific gravity of 3.7, then the weight of a 46 mm diameter lens increases by approximately 10-14 percent (approximately 2-3 g) according to the dioptric value thereof. 
     Accordingly, the object of the invention is more particularly to produce optical glasses having the following properties: 
     (a) nd=1.79-1.81 
     (b) vd=31-29 
     (c) s=3.4-3.7 
     The optical glasses must also fulfill the following important conditions in order to be suitable for use as spectacle lenses: 
     (a) they must have high crystallization stability to allow continuous production processes; 
     (b) they must have good chemical resistance; 
     (c) they must have chemical hardenability to withstand the falling ball tests of ASTM and DIN 4646; and 
     (d) in order to use the glass not only for distant vision spectacle lenses but also for near vision spectacle lenses they must be capable of fusing with conventional, distant vision commercial crown glasses (nd=1.523) and light, higher refractive index glasses (nd=1.70) so that the expansion coefficient must consequently lie between 93 and 99×10 -7  /°C. in the range of 20°-300° C. and the viscosity related to temperature must be adjusted within the melting range to the viscosity of normal commercial distant vision glasses. 
     Known optical glasses having an nd=1.79-1.81 do not fulfill the aforementioned requirements, as illustrated by the following examples: 
     
         ______________________________________                               α × 10.sup.7 /°                               C.Type          nd        vd     s    (20°-300° C.)______________________________________Heavy Flint GlassesSF 6          1.8051    25.43  5.18 90SF 11         1.7847    25.76  4.74 69LaF and LaSF GlassesLaF 9         1.795     28.38  4.96 81LaSF 8        1.80741   31.61  4.87 70LaSF N 3      1.8080    40.75  4.68 72LaSF 11       1.80166   44.26  4.62 70LaSF 30       1.80200   46.47  4.90 71______________________________________ 
    
     Other optical glasses are disclosed by German Pat. No. 1,260,712 having refractive indexes nd=1.78-1.81 with specific gravities ≦4.0 which, as a result of their strong tendency to crystallize, are not suitable for continuous mass production purposes (e.g., glasses for spectacle lenses and large volume crucible melting). The crystallization tendency is determined by the magnitude of the crystallization growth speed and the crystallization temperature range in relation to the viscosity. 
     Such known glasses having a refractive index nd&gt;1.75 contain 14-46 percent by weight of alkaline earth metals. Their ZnO content is 10-20 percent by weight, their TiO 2  content is 10-25 percent by weight and their SiO 2  content is 12-20 percent by weight. The alkali metal oxide content for glasses of nd&gt;1.79 must not exceed 3 percent by weight and such glasses are not chemically hardenable. 
     The cooling of these glass melts, according to the above noted German patent description, must be undertaken &#34;very rapidly, in order to avoid the yellow-brown coloration&#34;. The melts must be cast with a viscosity of &lt;100 poises since the strong tendency to crystallize renders continuous and automatic production impossible. The high ZnO content of 10-20 percent by weight in conjunction with a 10-25 percent content by weight of TiO 2  is unfavorable in this respect. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide high refractive index optical glasses which have a low specific gravity and can be utilized as both distant and near vision spectacle lenses. 
     Another object of the present invention is to provide high refractive index optical glasses which have high crystallization stability in order to permit continuous production thereof, good chemical resistance, satisfactory chemical hardenability and the capability of being fused with conventional distant vision glasses and light, higher refractive index glasses. 
     An additional object of the present invention is to provide optical glasses which can be utilized to produce spectacle lenses having high dioptric values with thinner edge thickness or center thickness lenses than are conventionally used. 
     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 optical glasses for use as spectacle lenses which have a refractive index nd=1.79-1.81, a dispersion vd=31-29 and a specific gravity s=3.4-3.7, the glasses consisting essentially of: 
     
         ______________________________________Oxides            Weight Percent______________________________________SiO.sub.2         25-35TiO.sub.2         10-15                        28-35Nb.sub.2 O.sub.5  18-24ZrO.sub.2         3-9SrO               0-7                        12-20BaO               7-20B.sub.2 O.sub.3   0-7Li.sub.20         5-8Na.sub.2 O        0-3        5-9K.sub.2 O         0-2MgO               0-2                        0-5CaO               0-5ZnO               0-4PbO               0-3Al.sub.2 O.sub.3  0-2La.sub.2 O.sub.3  0-5Ta.sub.2 O.sub.5  0-4Gd.sub.2 O.sub.3, Y.sub.2 O.sub.3,Bi.sub.2 O.sub.3, WO.sub.3             0-3; andrefining agent    0.2-1,______________________________________ 
    
     the optical glasses further having a high crystallization stability in order to permit continuous production of optical lenses. 
    
    
     DETAILED DESCRIPTION 
     The glasses formed in accordance with the present invention lie within the above-indicated composition range. Glasses of the following composition range are presently preferred: 
     
         ______________________________________Oxides            Percent by Weight______________________________________SiO.sub.2         26-28TiO.sub.2         10-14                      30-33Nb.sub.2 O.sub.5  18-23ZrO.sub.2         3-6SrO               5-7                      16-18BaO               9-12B.sub.2 O.sub.3   3-5Li.sub.2 O        6-8                      7-8Na.sub.2 O        1-2CaO               0-3ZnO               0-2PbO               0-2Al.sub.2 O.sub.3  0-1La.sub.2 O.sub.3  0-4Ta.sub.2 O.sub.5  0-2As.sub.2 O.sub.3  0.1-1Gd.sub.2 O.sub.3, Y.sub.2 O.sub.3,Bi.sub.2 O.sub.3 and WO.sub.3             0-2______________________________________ 
    
     The glasses formed in accordance with the present invention thus consist extensively of the following: low molecular weight cationic oxides such as SiO 2  +B 2  O 3  ; alkali metal oxides--preferably Li 2  O+Na 2  O for ion exchange in the hardening process; ZrO 2  +TiO 2  +Nb 2  O 5  to adjust the optical position; and divalent oxides in small quantities such as ZnO, PbO, MgO, CaO for crystallization stability and adaption of the thermal expansion coefficient. 
     The heavier oxides such as SrO and BaO (12-20 percent by weight) are particularly important to crystallization stability in glasses having a TiO 2  content of 10-15 percent and an Nb 2  O 5  content of 18-24 percent by weight. 
     La 2  O 3 , Y 2  O 3 , Bi 2  O 3 , Gd 2  O 3  and WO 3  contribute slightly to the reduction of crystallization when present in quantites of &lt;3 percent by weight, but they tend to increase the specific gravity and are preferably avoided or used only in small amounts for economic reasons. The use of GeO 2  and P 2  O 5  instead of SiO 2  in small amounts of &lt;3 percent by weight is permissible once a marked stabilization of crystallization has been achieved. Refining of the melts is achieved with known refining agents such as As 2  O 3  and/or Sb 2  O 3  and/or M-fluorides of &lt;1 percent by weight. 
     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. 
     In the appended tables, specific examples of glasses formed in accordance with the composition claimed in the invention are listed: Table 1a shows the composition of 10 typical glasses in percent by weight; Table 1b shows the properties of glasses listed in Table 1a; and Table 2 lists the same glasses as Table 1a but expressed in mol percent. 
     
                                           TABLE 1a__________________________________________________________________________Composition in Weight PercentExampleOxide1  2  3  4  5  6  7  8  9  10__________________________________________________________________________SiO.sub.228.40   25.80      27.60         27.71            28.49               28.80                  28.84                     27.60                        27.68                           27.60B.sub.2 O.sub.33.20   5.76      4.10         4.12            3.91               3.12                  4.13                     4.10                        4.11                           4.10Li.sub.2 O.sub.36.35   6.57      6.20         7.23            6.37               7.25                  6.25                     6.20                        6.22                           6.20Na.sub.2 O1.10   1.43      1.10         1.10            1.10               1.11                  1.11                     1.10                        1.60                           1.10MgO  -- 0.65      -- -- -- -- -- -- -- --ZnO  -- -- 2.00         -- -- -- -- -- -- --CaO  -- 1.63      2.00         -- -- -- 2.92                     2.90                        3.41                           2.90SrO  6.50   5.81      6.30         6.33            6.52               6.34                  6.35                     6.30                        6.32                           6.30BaO  10.80   9.71      10.50         10.54            10.83               10.57                  10.58                     10.50                        10.53                           10.50Gd.sub.2 O.sub.3-- 1.20      -- -- -- -- -- -- -- --La.sub.2 O.sub.33.2   3.83      2.10         2.11            3.21               1.11                  2.12                     -- 2.11                           2.10Y.sub.2 O.sub.3-- -- -- -- -- -- -- 2.10                        -- --ZrO.sub.26.25   4.54      3.90         7.13            6.27               8.16                  5.14                     5.10                        5.12                           5.10TiO.sub.212.30   11.94      11.00         11.04            13.34               11.08                  14.11                     11.00                        14.04                           11.00Nb.sub.2 O.sub.521.70   20.93      22.90         22.49            19.76               22.16                  18.25                     22.90                        18.66                           22.90As.sub.2 O.sub.30.20   0.20      0.20         0.20            0.20               0.20                  0.20                     0.20                        0.20                           0.20__________________________________________________________________________ 
    
     
                                           Table 1b__________________________________________________________________________ Properties of the Glasses in Table 1A__________________________________________________________________________Example  1      2      3      4      5__________________________________________________________________________nd       1.8010 1.80008                  1.80159                         1.80021                                1.81017vd       30.0   30.5   30.2   30.1   29.42s        3.547  3.526  3.547  3.519  3.546α × 10.sup.7 /° C. 20-300° C.    92     98.3   94.7   96     95.5 Tg ° C.    561    524    539    544    549 EW ° C.    679           652    655    669Cryst. DTA-TestRising Temp. KGmax ° C.    910    882    871    890    880 Poises  500    300    500    440    390Chem. hardening15h at ° C.    520    480           500tempering μ/cm    5670   5960          2640layer thick- ness in μ    45-50  45            40-45melting stress  581    102    93     96in μ/cm      compression                  compression                         compression                                tension__________________________________________________________________________Example  6      7      8      9      10__________________________________________________________________________nd       1.8003 1.79860                  1.80009                         1.80166                                1.8017vd       30.1   30.1   30.5   30.0   30.5s        3.509  3.482  3.521  3.497  3.530α × 10.sup.7 /°C. 20-300° C.    96     95.5   94.6   98     94 Tg °C.    547    551    543    534    548 EW °C.    664           661    656    661Cryst. DTA-TestRising Temp. KGmax °C.    895 weak           890    911    869    891 Poises  280    ca 500 118    ca 360 430Chem. hardening15h at °C.    480    500           500    480tempering μ/cm    7420   4020          5320   5980layer thick- ness in μ    60     45            45     35melting stress    41     97     23     418    33in μ/cm    compression           compression                  tension                         compression                                tension__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________Composition in Mol PercentExampleOxide1  2  3  4  5  6  7  8  9  10__________________________________________________________________________SiO.sub.240.09   35.68      38.41         38.50            39.58               39.68                  38.61                     38.28                        37.18                           38.37B.sub.2 O.sub.33.90   6.88      4.93         4.94            4.69               3.71                  4.77                     4.91                        4.77                           4.92Li.sub.2 O.sub.318.02   18.27      17.35         20.20            17.60               20.09                  16.83                     17.29                        16.80                           17.33Na.sub.2 O1.51   1.92      1.48         1.48            1.48               1.48                  1.44                     1.48                        2.08                           1.48MgO  -- 1.34      -- -- -- -- -- -- -- --ZnO  -- -- 2.06         -- -- -- -- -- -- --CaO  -- 2.42      2.98         -- -- -- 4.19                     4.31                        4.91                           4.32SrO  5.32   4.66      5.08         5.10            5.25               5.07                  4.93                     5.08                        4.92                           5.08BaO  5.97   5.26      5.73         5.74            5.90               5.71                  5.55                     5.71                        5.54                           5.72La.sub.2 O.sub.30.83   0.98      0.54         0.54            0.82               0.28                  0.52                     -- 0.52                           0.54Gd.sub.2 O.sub.3-- 0.49      -- -- -- -- -- -- -- --Y.sub.2 O.sub.3-- -- -- -- -- -- -- 0.77                        -- --ZrO.sub.24.30   3.06      2.65         4.83            4.25               5.49                  3.36                     3.45                        3.35                           3.46TiO.sub.213.06   12.42      11.51         11.53            13.94               11.48                  14.20                     11.47                        14.18                           11.50Nb.sub.2 O.sub.56.92   6.54      7.20         7.06            6.21               6.93                  5.52                     7.18                        5.67                           7.20As.sub.2 O.sub.30.08   0.08      0.08         0.08            0.08               0.08                  0.08                     0.08                        0.08                           0.08__________________________________________________________________________ 
    
     Falling Ball Test 
     The falling ball test is standarized according to ASTM and DIN specifications as follows: 
     
         ______________________________________       Fall    Ball       Fall       Height  Weight    Loading______________________________________ASTM          127 cm    15.8 p    1.75 cmkpDIN 4646      130 cm    43.8 p    5.80 cmkpadditional test         &#34;-Russian standard&#34;                   different 173.1 p  variable______________________________________ 
    
     A standard support is used in all cases. In the event of glasses not breaking by all three balls, the fall height is increased to destruction with the &#34;Russian standard.&#34; The fall loading in cmkp is taken as a measure of the impact-resistance for assessment purposes. 
     
                       Table 1, Example 10______________________________________Melting Example                       Kg amounts weighed in   Percent   Starting  for a 120 kg computedOxides  by Weight Materials quantity of glass______________________________________SiO.sub.2   27.6      SiO.sub.2 33.17B.sub.2 O.sub.3   4.1       H.sub.3 BO.sub.3                       8.71Li.sub.2 O   6.2       Li.sub.2 CO.sub.3                       1.85Na.sub.2 O   1.1       Na.sub.2 CO.sub.3                       2.26CaO     2.9       CaCO.sub.3                       5.30SrO     6.3       Sr(NO.sub.3).sub.2                       15.59BaO     10.52     BaCO.sub.3                       16.05La.sub.2 O.sub.3   2.1       La.sub.2 O.sub.3                       2.53ZrO.sub.2   5.1       ZrO.sub.2 6.14TiO.sub.2   16.0      TiO.sub.2 13.23Nb.sub.2 O.sub.5   22.9      Nb.sub.2 O.sub.5                       27.50______________________________________ 
    
     The batch is carefully mixed and placed in a platinum crucible or ceramic bath at 1260° C. Refining is carried out at approximately 1300°-1320° C. At approximately 970° C. (≧500 poises), the batch is drained and immediately pressed by automatic means into spectacle lens blanks. 
     The glass has the following physical-chemical properties: 
     
         ______________________________________nd                     1.8017vd                     30.5s                      3.53α × 10.sup.7 /°C.                  94(20°-300° C.)α × 10.sup.7 /°C.                  80(-30° to +70° C.)Tg °C.          548EW °C.          661______________________________________ 
    
     
         ______________________________________Chemical Resistance______________________________________Acid resistance, class f                ≦2Climatic change, resistance                2Discoloration tendency                0______________________________________ 
    
     
                       Table 1, Example 8______________________________________ Melting Example 2                       Kg amounts weighed in   Percent   Starting  for a 120 kg computedOxides  by Weight Materials quantity of glass______________________________________SiO.sub.2   27.6      SiO.sub.2 33.17B.sub.2 O.sub.3   4.1       H.sub.3 BO.sub.3                       8.71Li.sub.2 O   6.2       Li.sub.2 CO.sub.3                       18.48Na.sub.2 O   1.1       Na.sub.2 CO.sub.3                       2.26CaO     2.9       CaCO.sub.3                       6.30SrO     6.3       Sr(NO.sub.3).sub.2                       15.597BaO     10.5      BaCO.sub.3                       16.05La.sub.2 O.sub.3   2.1       La.sub.2 O.sub.3                       2.53ZrO.sub.2   5.1       ZrO.sub.2 6.14TiO.sub.2   11.0      TiO.sub.2 13.23Nb.sub.2 O.sub.5   22.9      Nb.sub.2 O.sub.5                       27.50As.sub.2 O.sub.3   0.2       As.sub.2 O.sub.3                       0.24______________________________________ 
    
     After weighing in, the starting materials are mixed in a commercial mechanical mixer. The mixture is melted down at about 1200° C. in a melting trough, is refined at about 1280° C. and then stirred until it reaches about 1200° C. At about 1160° C. the molten mass is drained through a platinum pipe and is processed into bars; at lower drainage temperatures (and proportionally higher viscosity) it is possible to manufacture blobs and automatically processed die-formed parts. 
     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.