Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
1999-09-29
2001-02-13
Bell, Mark L. (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S079000, C501S901000
Reexamination Certificate
active
06187702
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to ophthalmic and optical glasses having a low specific gravity and a refractive index (nd) higher than 1.88.
In the past, glasses having a low refractive index (nd) of about 1.52 have been widely used for ophthalmic lenses. In a case where positive power or negative power lenses having a high diopter value are made of a glass having such a low refractive index, the thickness of the lenses in the edge or in the center increases with resulting increase in the weight of the lenses and also increase in chromatic aberration which produces iris in the edge of the lenses. These lenses are disadvantageous to the wearer of the spectacles both in use and appearance. For reducing the thickness of lenses having a high diopter value, glasses having a high refractive index (nd) of about 1.9 may be used. As one of such glasses having a high refractive index, Japanese Patent Application Laid-open Publication No. Sho 52-155614 discloses an optical glass of a B
2
O
3
—La
2
O
3
—Gd
2
O
3
—WO
3
—Zro
2
and/or Ta
2
O
5
system. This glass, however, contains a relatively large total amount of Gd
2
O
3
, WO
3
and Ta
2
O
5
resulting in a high specific gravity and, therefore, this glass cannot sufficiently reduce the weight of the lenses. Moreover, this glass tends to be colored and has not sufficient resistance to devitrification. Japanese Patent Application Laid-open Publication No. Sho 48-23809 discloses a high refractive optical glass in which TiO
2
, ZrO
2
and Nb
2
O
5
are added to a B
2
O
3
—La
2
O
3
—Gd
2
O
3
—Ta
2
O
5
—ZnO system glass. This glass, however, contains a relatively large amount of La
2
O
3
resulting in a high specific gravity and, moreover, is not sufficient in acid-proof property and resistance to devitrification. Japanese Patent Application Laid-open Publication No. Sho 53-4023 discloses a high refractive optical glass in which HfO
2
is introduced into a B
2
O
3
—La
2
O
3
system glass. This glass also has a high specific gravity because it contains a large amount of La
2
O
3
and HfO
2
. Besides, since HfO
2
is a very expensive material, this glass is not suited for a large scale production.
Japanese Patent Application Laid-open Publication No. Hei 6-56462, Japanese Patent Application Laid-open Publication No. Hei 6-87628 and International Application Laid-open Publication WO98/32706 also disclose low density and high refractive glasses but these glasses do not have a sufficient stability against devitrification and therefore are not suited for a large scale production.
Japanese Patent Application Laid-open Publication No. Sho 62-100449 discloses a g lass of a B
2
O
3
—La
2
O
3
—ZnO—Li
2
O—Sb
2
O
3
system. This glass, however, has a low refractive index and, besides, since it contains a large amount of Sb
2
O
3
, the Sb
2
O
3
ingredient evaporates in melting of the glass and, as a result, it is difficult to obtain a homogeneous glass.
It is, therefore, an object of the present invention to provide ophthalmic and optical glasses which have eliminated the above described disadvantages of the prior art glasses and have a low specific gravity, a high refractive index and sufficient homogeneity a nd resistance to devitrification.
SUMMARY OF THE INVENTION
For achieving the object of the invention, repeated studies and experiments made by the inventor of the present invention have resulted in the finding, which has led to the present invention, that, by selecting a composition within specific content ranges of a SiO
2
—B
2
O
2
—La
2
O
3
—Gd
2
O
3
—TiO
2
—Nb
2
O
5
—WO
3
—ZnO—CaO—R
2
) (R
2
O being one or more oxides selected from the group consisting of Li
2
O, Na
2
O and K
2
O) system glass which has never been disclosed in the above described prior art glasses, glasses which, while maintaining a high refractive index, a low specific gravity and excellent chemical durability, have remarkably improved resistance to devitrification and excellent homogeneity and very suitable for a large scale production.
According to the invention, there are provided ophthalmic and optical glasses having the following composition expressed in weight percent on the oxide basis:
SiO
2
4.5-9.5%
B
2
O
3
10-19.5%
Al
2
O
3
0-3%
GeO
2
0-5%
La
2
O
3
18-less than 23%
Gd
2
O
3
0.3-less than 2%
TiO
2
5.5-13.5%
ZrO
2
0-7.5%
Nb
2
O
5
15-21%
Ta
2
O
5
0-1.5%
WO
3
0.3-5%
with the proviso that (Ta
2
O
5
+WO
3
)/B
2
O
3
is 0.5 or below in the ratio in terms of weight
ZnO
2.1-10%
MgO
0-5%
CaO
7-12.5%
SrO
0-5%
BaO
0-4%
Li
2
O + Na
2
O + K
2
O
≧0.1%
in which Li
2
O
0-1%
Na
2
O
0-1%
K
2
O
0-1%
As
2
O
3
0-0.5%
Sb
2
O
3
0-0.5%
and having a specific gravity of 4.1 or below.
In one aspect of the invention, there are provided ophthalmic and optical glasses having the following composition in weight percent:
SiO
2
4.5-9.5%
B
2
O
3
10-19.5%
Al
2
O
3
0-3%
GeO
2
0-5%
La
2
O
3
18-less than 23%
Gd
2
O
3
0.3-less than 2%
TiO
2
5.5-13.5%
ZrO
2
0-7.5%
Nb
2
O
5
15.5-less than 20.5%
Ta
2
O
5
0-1.5%
WO
3
0.3-5%
with the proviso that (Ta
2
O
5
+WO
3
)/B
2
O
3
is 0.5 or below in the ratio in terms of weight
ZnO
2.1-10%
MgO
0-5%
CaO
7-12.5%
SrO
0-5%
BaO
0-4%
Li
2
O + Na
2
O + K
2
O
≧0.1%
in which Li
2
O
0-1%
Na
2
O
0-1%
K
2
O
0-1%
As
2
O
3
0-0.5%
Sb
2
O
3
0-0.5%
and having a specific gravity of 4.1 or below.
In another aspect of the invention, there are provided ophthalmic and optical glasses having the following composition in weight percent:
SiO
2
4.5-9.5%
B
2
O
3
10-19.5%
Al
2
O
3
0-3%
GeO
2
0-5%
La
2
O
3
18-less than 23%
Gd
2
O
3
0.3-less than 2%
TiO
2
5.5-13.5%
ZrO
2
0-7.5%
Nb
2
O
5
15.5-20%
Ta
2
O
5
0-1.5%
WO
3
0.3-5%
with the proviso that (Ta
2
O
5
+WO
3
)/B
2
O
3
is 0.5 or below in the ratio in terms of weight
ZnO
2.1-10%
MgO
0-5%
CaO
7-12.5%
SrO
0-5%
BaO
0-4%
Li
2
O + Na
2
O + K
2
O
≧0.1%
in which Li
2
O
0-1%
Na
2
O
0-1%
K
2
O
0-1%
As
2
O
3
0-0.5%
Sb
2
O
3
0-0.5%
and having a specific gravity of 4.1 or below.
In another aspect of the invention, there are provided ophthalmic and optical glasses having any of the above described compositions and having optical constants of a refractive index (nd) exceeding 1.88 and up to 1.93 and an Abbe number (&ngr;d) within a range from 28 to 31.
In still another aspect of the invention, there are provided ophthalmic and optical glasses having any of the above described compositions and having optical constants of a refractive index (nd) exceeding 1.88 and up to 1.93 and an Abbe number (&ngr;d) within a range from 28 to less than 30.4.
DETAILED DESCRIPTION OF THE INVENTION
Reasons for restricting the content ranges of the respective ingredients of the ophthalmic and optical glasses according to the present invention will now be described.
In the glasses of the present invention, the B
2
O
3
ingredient is an important ingredient which has been found to afford capability of introducing La
2
O
3
, Nb
2
O
5
and TiO
2
which impart a high refractivity to the glass while maintaining sufficient resistance to devitrification. If, however, the amount of this ingredient is below 10%, tendency to devitrification increases significantly with the result that the glass becomes sharply instable. If the amount of this ingredient exceeds 19.5%, a high refractivity which is a target of the present invention cannot be achieved.
The SiO
2
ingredient is required in the amount of 4.5% or more for maintaining chemical durability of the glass and optimum viscosity during forming. If, however, the amount of this ingredient exceeds 9.5%, an unmelted residue is produced and, as a result, a homogeneous glass cannot be obtained.
The CaO ingredient is effective in reducing the specific gravity of the glass and improving chemical durability of the glass and, for these p
Bell Mark L.
Hedman, Gibson & Costigan, P.C.
Kabushiki Kaisha Ohara
Sample David
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