Low melting, durable phosphate glasses
BACKGROUND OF THE INVENTION 
     Glass compositions which combine good chemical durability with the capability of being formed at low temperatures, i.e., the compositions have low transition temperatures (Tg), are potentially useful for a host of applications including low temperature sealing glasses, matrices for glass-polymer alloys, and press molding optically finished lenses. To illustrate: 
     U.S. application Ser. No. 07/704,864, filed May 23, 1991 by George H. Beall, James E. Dickinson, Jr., and Robert M. Morena under the title ZINC PHOSPHATE LOW TEMPERATURE GLASSES and assigned to the same assignee as the present application, now U.S. Pat. No. 5,122,484 is directed with particularity to low melting, lead-free glass compositions suitable as sealing frits in television picture tube applications. Those glasses are essentially free of PbO and consist essentially, expressed in terms of weight percent on the oxide basis, of: 
     
         ______________________________________
P.sub.2 O.sub.5
           38-50    SnO.sub.2    0-10
Al.sub.2 O.sub.3
           0-5      MoO.sub.3    0-10
ZnO        28-42    WO.sub.3     0-10
Li.sub.2 O 0.75-5   MoO.sub.3 + WO.sub.3
                                 2-15
Na.sub.2 O  2-10    Cl           0-8 (analyzed)
K.sub.2 O   2-10    SnO.sub.2 + MoO.sub.3 +
                                 2-25
Li.sub.2 O + Na.sub.2 O +
            5-25    WO.sub.3 + Cl
K.sub.2 O
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     U.S. Pat. No. 5,043,369 (Bahn et al.) broadly discloses the production of melt blends or alloys composed of a glass and an organic polymer. The disclosed articles comprise a melt mixture of an inorganic glass and an organic polymer, the working temperature of the glass being compatible with that of the polymer. That patent referred to U.S. Pat. No. 4,920,081 (Beall et al.) and U.S. Pat. No. 4,940,677 (Beall et al.) as describing glass compositions having working temperatures and transition temperatures suitable for use in preparing melt blends or alloys. 
     U.S. Pat. No. 4,920,081 recites glass compositions consisting essentially, expressed in terms of mole percent on the oxide basis, of: 
     
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P.sub.2 O.sub.5
             44-58     Li.sub.2 O    0-30
Al.sub.2 O.sub.3
              0-7      Na.sub.2 O + Li.sub.2 O
                                    10-45
B.sub.2 O.sub.3
              0-10     Cu.sub.2 O    0-20
Al.sub.2 O.sub.3 + B.sub.2 O.sub.3
              4-10     Li.sub.2 O + Cu.sub.2 O
                                    10-30
Na.sub.2 O   10-30
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     with, optionally, up to 8% total of at least one member of the group MgO, CaO, SrO, BaO, MnO, and ZnO, up to 8% ZrO 2 , up to 3% SiO 2 , and up to 5% F. 
     U.S. Pat. No. 4,940,677 reports glass compositions consisting essentially, expressed in terms of mole percent on the oxide basis, of 10-35% R 2  O, wherein R 2  O consists of at least two alkali metal oxides in the indicated proportions selected from the group 0-25% Li 2  O, 0-25% Na 2  O, and 0-25% K 2  O, 23-55% ZnO, 28-40% P 2  O 5 , and up to 35% total of the following optional ingredients in the indicated proportions selected from the group 0-6% Al 2  O 3 , 0-8% B 2  O 3 , 0-8% Al 2  O 3  +B 2  O 3 , 0-15% Cu 2  O, 0-5% F, 0-35% PbO, 0-35% SnO, 0-35% PbO+SnO, 0-5% ZrO 2 , 0-4% SiO 2 , and 0-15% MgO+CaO+-SrO+BaO+MnO, consisting of 0-10% MgO, 0-10% CaO, 0-10% SrO, 0-12% BaO, and 0-10% MnO. 
     U.S. Pat. No. 5,021,366 (Aitken) provides glass compositions which were designed specifically for use in press molding optically finished lenses, but which are also suitable for use in preparing melt blends of glass and organic polymer. That patent records glass compositions consisting essentially, expressed in terms of mole percent on the oxide basis, of: 
     
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Li.sub.2 O   5-10      Al.sub.2 O.sub.3
                                   0-5
Na.sub.2 O   5-15      CeO.sub.2   0-2
K.sub.2 O    0-6       P.sub.2 O.sub.5
                                   30-36
Li.sub.2 O + Na.sub.2 O + K.sub.2 O
             15-25     SnO         0-20
ZnO          10-33     PbO         0-20
CaO + SrO + BaO
             12-25     Sb.sub.2 O.sub.3
                                   0-12
CaO          0-20      Bi.sub.2 O.sub.3
                                   0-6
SrO           0-20     SnO + PbO + 0-20
BaO          0-20      Sb.sub.2 O.sub.3 + Bi.sub.2 O.sub.3
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     The lenses molded from those compositions exhibited refractive indices of about 1.605 and linear coefficients of thermal expansion (25°-300° C.) between 145-170× 1O -7  /°C. 
     U.S. application Ser. No. 07/696,173, filed May 6, 1991 by me under the title HIGH INDEX FLUORINE-FREE PHOSPHATE GLASSES and assigned,,to the same assignee as the present application, now U.S. Pat. No. 5,153,151 is likewise directed principally to glass compositions designed for use in press molding optically finished lenses, but wherein refractive indices between 1.65-1.8 can be obtained. The glasses, having compositions expressed below in terms of mole percent on the oxide basis, are also noted as being operable in preparing glass-plastic alloys. 
     
         ______________________________________
P.sub.2 O.sub.5
              24-36    Al.sub.2 O.sub.3
                                     0-5
ZnO           0-45     SrO           0-20
Li.sub.2 O    0-15     BaO           0-20
Na.sub.2 O    0-20     CaO + SrO + BaO
                                     0-25
K.sub.2 O     0-10     Sb.sub.2 O.sub.3
                                     0-61
Ag.sub.2 O    0-25     Bi.sub.2 O.sub.3
                                     0-10
Tl.sub.2 O    0-25     Sb.sub.2 O.sub.3 + Bi.sub.2 O.sub.3
                                     0-61
Li.sub.2 O + Na.sub.2 O +
              15-30    Sb.sub.2 O.sub.3 + Bi.sub.2 O.sub.3
                                     7-76
K.sub.2 O + Ag.sub.2 O + Tl.sub.2 O
                       Ag.sub.2 O + Tl.sub.2 O
PbO           0-20     B.sub.2 O.sub.3
                                     0-10
CuO           0-5      Al.sub.2 O.sub.3 + B.sub.2 O.sub.3
                                     0-10
CaO           0-20     SnO           0-5
CeO.sub.2     0-2
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     with the following provisos: 
     (1) in the absence of Sb 2  O 3  and/or Bi 2  O 3 , the total Ag 2  O and Tl 2  O will range 11-25; 
     (2) in the absence of Ag 2  O and/or Tl 2  O, the total Sb 2  O 3  +Bi 2  O 3  will range 7-61; 
     (3) when present in the absence of Bi 2  O 3  and/or Ag 2  O and/or Tl 2  O, Sb 2  O 3  will range 10-61, and, when present in the absence of Sb 2  O 3  and/or Ag 2  O and/or Tl 2  O, Bi 2  O 3  will range 7-10; 
     (4) when present in the absence of Bi 2  O 3 , Sb 2  O 3 , and Tl 2  O, Ag 2  O will range 13-25; and 
     (5) when present in the absence of Ag 2  O, Bi 2  O 3 , and Sb 2  O 3 , Tl 2  O will range 11-25. 
     The glasses exhibited annealing points between 300°-340° C., linear coefficients of thermal expansion (25°-300° C.) between 135-180×10 -7  /°C., and weight losses after exposure to boiling water for six hours no greater than 0.33%. 
     Further laboratory research has led to the discovery of transparent glasses having compositions within closely-circumscribed limits which demonstrate very low annealing points, i.e., less than 325° C. with some compositions having annealing points as low as about 290° C., improved chemical durability and resistance to weathering, and indices of refraction between 1.57-1.66, which indices are compatible with certain transparent organic polymers, such as polycarbonate (n d  =1.586), polysulfone (n d  =1.633)and polyethersulfone (n d  =1.650), thereby recommending their utility in preparing transparent glass-plastic melt blends and composites. 
     SUMMARY OF THE INVENTION 
     The inventive glasses have compositions within the basic R 2  O-Ag 2  O-Tl 2  O-ZnO-P 2  O 5  system; i.e., the compositions thereof are an improvement upon the glasses disclosed in U.S. Pat. No. 4,940,677, supra, wherein Ag 2  O and/or Tl 2  O are substituted for a portion of the R 2  O, i.e., the Li 2  O, Na 2  O, and K 2  O. The Ag 2  O and Tl 2  O serve three functions relevant to the present inventive glasses; viz., they lower the annealing point of the glass, they raise the index of refraction of the glass, and they improve the chemical durability of the glass. 
     The inventive glasses consist essentially, expressed in terms of mole percent on the oxide basis, of 
     
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Li.sub.2 O   0-10      CaO + SrO +  0-20
                       BaO(RO)
Na.sub.2 O   0-10      ZnO         26-50
Li.sub.2 O + Na.sub.2 O
             1-20      RO + ZnO    26-50
K.sub.2 O    0-8       SnO         0-5
Li.sub.2 O + Na.sub.2 O +
             5-25      Sb.sub.2 O.sub.3
                                   0-8
K.sub.2 O(R.sub.2 O)
Ag.sub.2 O   0-13      Al.sub.2 O.sub.3
                                   0.25-5
Tl.sub.2 O   0-12      P.sub.2 O.sub.5
                                   30-36
Ag.sub.2 O + Tl.sub.2 O
             1-14      B.sub.2 O.sub.3
                                   0-5
R.sub.2 O + Ag.sub.2 O + Tl.sub.2 O
             15-30     CuO         0-5
CaO          0-20      Bi.sub.2 O.sub.3
                                   0-3
BaO          0-10      CeO.sub.2   0-2
SrO          0-10      PbO          0-10
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     with the proviso that, in the presence of SnO and/or Bi 2  O 3 , Ag 2  O will be essentially absent. 
     The inclusion of at least 26% ZnO has been found to be essential in maintaining the annealing points of these glasses at sufficiently low levels, preferably below 300° C., in order that the glasses will be capable of being processed with the aforementioned organic polymers. 
     The presence of the alkaline earth oxides raises the refractive index of the glass without increasing the dispersion therein and elevating the annealing point thereof. Their presence also frequently improves the chemical durability of the glass. Nevertheless, excessive levels increase the susceptibility of the glass to devitrification. 
     The inclusion of lead in a glass composition is well known in the art for its effect in raising the refractive index thereof, but its presence also has the adverse effect of rendering the glass susceptible to discoloration through reduction, particularly when blended with polymers or molded with metals. Furthermore, the addition of lead sharply raises the coefficient of thermal expansion of the glass which requires compensation through addition of the alkaline earth metal oxides and/or Sb 2  O 3  and/or SnO and/or Al 2  O 3  and/or B 2  O 3  Accordingly, although PbO can be tolerated in the inventive compositions, it is much preferred that levels thereof be held below 5%, if present at all. 
     Al 2  O, B 2  O 3 , and SnO exert a favorable effect upon the chemical durability of the inventive glasses. Nevertheless, the amounts of Al 2  O 3  and/or B 2  O 3  must be held at low concentrations to forestall elevating the annealing point to unacceptable values. High levels of SnO can lead to difficulties in melting; this oxide cannot be used in conjunction with Ag 2  O. 
     Sb 2  O 3  and Bi 2  O 3  raise the refractive index of the inventive glasses. Sb 2  O 3  is the more preferred because it is less subject to reducing conditions and is more readily soluble in the molten base glass than Bi 2  O 3 . 
     CeO 2  may be included to render the glass resistant to such radiations as X-radiations. 
     Whereas it is not mathematically possible to convert composition intervals expressed in terms of mole percent to exact composition ranges expressed in terms of weight percent, the following values reflect approximations of the compositions of the inventive glasses in terms of weight percent: 
     
         ______________________________________
Li.sub.2 O 0-3.4      CaO + SrO + 0-25.3
Na.sub.2 O 0-10.6     BaO(RO)
Li.sub.2 O + Na.sub.2 O
           0.2-14     ZnO         13.5-43.3
K.sub.2 O  0-8.4      RO + ZnO    14.0-48.4
Li.sub.2 O + Na.sub.2 O +
           1-18.8     SnO         0-8.3
K.sub.2 O(R.sub.2 O)  Sb.sub.2 O.sub.3
                                  0-22.2
Ag.sub.2 O 0-28.1     Al.sub.2 O.sub.3
                                  0.2-5.7
Tl.sub.2 O 0-39.6     P.sub.2 O.sub.5
                                  27.1-55.8
Ag.sub.2 O + Tl.sub.2 O
           1.9-39.6   B.sub.2 O.sub.3
                                  0-4.0
R.sub.2 O +
           6.8-48.1   CuO         0-4.5
Ag.sub.2 O + Tl.sub.2 O)
                      Bi.sub.2 O.sub.3
                                  0-14.1
CaO        0-12.8     CeO.sub.2   0-3.9
BaO        0-15.8     PbO         0-21.7
SrO        0-11.2
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     with the proviso that, in the presence of SnO and/or Bi 2  O 3 , Ag 2  O will be essentially absent. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Table I reports a group of glass compositions melted on a laboratory scale and recited in terms of mole percent on the oxide basis illustrating the inventive glasses. Table IA records the same group of compositions expressed in terms of parts by weight on the oxide basis. Because the sum of the individual constituents listed in Table IA totals or closely approximates 100, for all practical purposes the tabulated values may be considered to represent weight percent. The actual batch ingredients can comprise any materials, either oxides or other compounds, which, when melted in combination with the other components, will be converted into the desired oxide in the proper proportions. For example, zinc orthophosphate may be employed as a source of ZnO and P 2  O 5  and LiPO 3  and NaPO 3  can comprise the sources of Li 2  O and Na 2  O, respectively. 
     The batch ingredients were compounded, tumble mixed together to assist in securing a homogeneous melt, and thereafter charged into platinum crucibles. After placing lids thereon, the crucibles were introduced into a furnace operating at about 1000° C. and the batches melted for about 3 hours. The melts were then poured into steel molds to form glass slabs having dimensions of about 6&#34;×4&#34;×0.5&#34; (˜15.2×10×1.3 cm) which were transferred immediately to an annealer operating at 300°-325° C. 
     (Whereas the above description reflects melting on a laboratory scale only, it must be appreciated that large scale melts thereof can be carried out in commercial melting units. Hence, it is only necessary that the batch materials be melted at a temperature and for a time sufficient to obtain a homogeneous melt.) 
     
                       TABLE I
______________________________________
(Mole Percent)
______________________________________
        1         2      3        4    5
______________________________________
Li.sub.2 O
        7.0       3.5    3.5      7.0  7.0
Na.sub.2 O
        8.0       4.0    4.0      8.0  4.0
K.sub.2 O
        1.25      2.5    2.5      --   0.5
Ag.sub.2 O
        --        --     --       --   4.5
Tl.sub.2 O
        3.75      10.0   10.0     5.0  --
CaO     11.25     --     12.0     11.25
                                       11.25
BaO     3.75      --     6.0      3.75 3.75
ZnO     28.6      45.0   26.0     27.0 29.6
SnO     1.0       --     1.0      --   --
Al.sub.2 O.sub.3
        0.4       2.0    2.0      0.5  0.4
Bi.sub.2 O.sub.3
        --        --     --       2.5  --
P.sub.2 O.sub.5
        35.0      33.0   33.0     35.0 35.0
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        6         7      8        9    10
______________________________________
Li.sub.2 O
        2.8       2.8    7.0      --   --
Na.sub.2 O
        3.2       3.2    8.0      --   --
K.sub.2 O
        2.0       2.0    5.0      --   --
Ag.sub.2 O
        12.8      12.0   --       20.0 --
Tl.sub.2 O
        --        --     --       --   20.0
CaO     --        --     22.5     --   --
ZnO     44.6      45.0   22.5     45.0 45.0
Al.sub.2 O.sub.3
        2.0       2.0    --       --   --
P.sub.2 O.sub.5
        32.7      33.0   35.0     35.0 35.0
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                       TABLE IA
______________________________________
(Parts by Weight)
1             2      3         4    5
______________________________________
Li.sub.2 O
        1.9       0.8    0.8     1.7  2.0
Na.sub.2 O
        4.5       1.9    1.8     4.0  4.8
K.sub.2 O
        1.1       1.8    1.7     --   0.5
Ag.sub.2 O
        --        --     --      --   10.0
Tl.sub.2 O
        14.3      31.7   31.3    17.1 --
CaO     5.7       --     5.0     5.1  6.1
BaO     5.2       --     6.8     4.6  5.5
ZnO     21.0      27.4   15.6    17.7 23.1
SnO     1.4       --     1.1     --   --
Al.sub.2 O.sub.3
        0.4       1.5    1.5     0.4  0.4
Bi.sub.2 O.sub.3
        --        --     --      9.4  --
P.sub.2 O.sub.5
        44.7      35.0   34.5    40.0 47.7
______________________________________
(Concluded)
        6         7      8       9    10
______________________________________
Li.sub.2 O
        0.7       0.7    2.3     --   --
Na.sub.2 O
        1.7       1.7    5.4     --   --
K.sub.2 O
        1.6       1.6    5.1     --   --
Ag.sub.2 O
        24.9      23.6   --      35.0 --
Tl.sub.2 O
        --        --     --      --   49.6
CaO     --        --     13.6    --   --
ZnO     30.5      31.0   19.8    27.6 21.4
Al.sub.2 O.sub.3
        1.7       1.7    --      --   --
P.sub.2 O.sub.5
        39.0      39.7   53.7    37.4 29.0
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     Table II records the softening point (S.P.), annealing point (A.P.), and strain point (St.P.) in terms of °C., the linear coefficient of thermal expansion (Exp.) over the temperature interval of 25°-300° C. expressed in terms of X10 -7  /°C., the refractive index (n d3 ), and the density (Den.) expressed in terms of grams/ determined in accordance with measuring techniques conventional in the glass art. Table II also lists the weight loss (W.L.) exhibited by the glasses after an immersion for six hours in a bath of boiling deionized water, expressed in terms of percent, and a qualitative appraisal of the weathering resistance of the glasses (Weath.) based upon the visual appearance thereof after an exposure in a humidity cabinet for 500 hours at 60° C. and 90% relative humidity. Legends for the weathering resistance character of the glasses include: nc=no change in appearance; xl=extremely light frosted appearance; vl=very light frosted appearance; lt=light frosted appearance; 1-vl=light to very light frosted appearance; and hf=heavy frosted appearance. A weight loss in excess of 0.1% is deemed to constitute unsatisfactory chemical durability, with losses less than 0.05% being greatly preferred; thus, a chemical durability considerably improved over the glasses of Ser. No. 07/696,173 supra. The most preferred glasses will display no frosting or haze. Where haze can be observed only when the glass is viewed at a small angle (exemplified by legends xl and vl), however, the glasses will perform satisfactorily in most applications. 
     
                       TABLE II
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1            2        3        4      5
______________________________________
S.P.    414      390      442    414    399
A.P.    308      297      338    311    302
St.P.   288      277      319    289    --
Exp.    153      147.7    143.6  151.9  148.1
Den.    3.498    4.037    4.045  3.824  3.409
n.sub.d 1.601    1.631    1.646  1.639  1.604
W.L.    &lt;0.01    0.01     &lt;0.01  &lt;0.01  0.01
Weath.  xl       nc       nc     lt     1-vl
______________________________________
(Concluded)
        6        7        8      9      10
______________________________________
S.P.    374      --       --     --     --
A.P.    290      299      328    --     --
St.P.   273      287      307    --     --
Exp.    156      131      148    --     --
Den.    3.889    3.790    --     --     --
n.sub.d 1.634    1.622    1.564  1.669  1.687
W.L.    &lt;0.01    0.01     0.02   &gt;0.1   &gt;0.1
Weath.  1-vl     nc       lt     hf     hf
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     As can be observed, Examples 8-10 having compositions outside of the prescribed ranges are subject to significant weathering. The refractive index of Example 8 is also below the minimum desired; conversely, the refractive indices of Examples 9 and 10 are too high for the desired application. 
     Based upon an overall appraisal of the chemical and physical properties demonstrated by the inventive glasses, taken in conjunction with their melting and forming characteristics, the preferred glasses consist essentially, expressed in terms of mole percent on the oxide basis, of: 
     
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Li.sub.2 O  2-8     CaO + SrO + BaO(RO)
                                      0-15
Na.sub.2 O  2-12    ZnO              26-46
Li.sub.2 O + Na.sub.2 O
            5-20    RO + ZnO         26-48
K.sub.2 O   0-6     SnO              0-3
Li.sub.2 O + Na.sub.2 O +
            5-20    Sb.sub.2 O.sub.3 0-8
K.sub.2 O(R.sub.2 O)
Ag.sub.2 O  0-13    Al.sub.2 O.sub.3 0.5-3
Tl.sub.2 O  0-12    P.sub.2 O.sub.5  30-34
Ag.sub.2 O + Tl.sub.2 O
            1-14    B.sub.2 O.sub.3  0-3
R.sub.2 O + Ag.sub.2 O +
            15-25   CuO              0-5
Tl.sub.2 O
CaO         0-15    Bi.sub.2 O.sub.3 0-3
BaO         0-8     CeO.sub.2        0-2
SrO         0-8     PbO              0-5
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     with the proviso that in the presence of SnO and/or Bi 2  O 3 , Ag 2  O will be essentially absent. 
     The composition providing the most preferred combination of properties and working behavior is Example 2. That glass not only functions well when being press molded into lenses, but also exhibits excellent forming capability when blended with organic polymers to produce glass-polymer alloys and composites, and is particularly well-suited for blending with polysulfone to make a transparent glass/polymer alloy or composite because of the closeness of their respective indices of refraction.