Lead-free optical glasses

Details Lead-free optical glasses Lead-free optical glasses

2000-05-08

2001-12-25

Sample, David R. (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S079000, C501S901000, C501S903000

Reexamination Certificate

active

06333288

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to essentially lead-free optical glasses, and more particularly to essentially lead-free optical glasses having refractive indexes n
d
of from about 1.65 to about 1.87 and Abbe numbers &ngr;
d
of from about 27 to about 43.
BACKGROUND OF THE INVENTION
Because the glass components PbO and As
2
O
3
have entered public debate as environmental pollutants, there is a demand by the manufacturers of optical equipment for 10 PbO-free and preferably also As
2
O
3
-free glasses having the optical properties substantially the same as glasses having these oxides.
Simple replacement of lead oxide by one or more constituents generally does not succeed in reproducing the desired optical and glass-engineering properties affected by PbO. Instead, new developments or extensive changes in the glass composition are necessary.
The patent literature reveals some specifications which have already described lead-free glasses having optical values from the stated range and having similar compositions. However, the glasses have a wide variety of disadvantages.
Both the glasses of the unexamined Japanese publication JP 62-100 449 A, having refractive indexes n
d
of from 1.64 to 1.88, and those of JP 60-22 133 8 A, having refractive indexes n
d
of from 1.62 to 1.85, contain Li
2
O as a required constituent, contents of up to 15 and 12% by weight respectively being possible. Li
2
O-containing glasses of this type are very susceptible to crystallization. The same also applies to the glasses of JP 63-170 247 A, which contain at least 5 mol-% of Li
2
O.
The glasses of the first-mentioned specification also contain from 2 to 20% by weight of Sb
2
O
3
. Use of this component in such amounts makes increased safety precautions necessary during production.
DE-A 35 34 575 Al relates to glasses having refractive indexes n
d
≦1.59 for eyeglass lenses having color contrast amplification. The glasses, which vary greatly in their composition, contain various coloring components in a base glass containing up to 33% by weight of Nd
2
O
3
.
The glasses of JP 53-90 312 A have relatively low refractive indexes of n
d
≦1.67. They contain at least 3% by weight of the glass former Al
2
O
3
in order to counter the tendency toward crystallization caused by the relatively high content of alkaline earth metal oxides, but this has a disadvantageous effect on the melting properties and the “length” of the glass. The latter also applies to the Al
2
O
3
- and F-containing glasses of JP 1-133956 A having low refractive indexes. The glasses of JP 62-126 33 A, which likewise have low refractive indexes, contain expensive Cs
2
O as a required component and have high proportions of SiO
2
of from 30 to 75 mol-%.
German Patent Specification DE 32 01 344 C2 describes high-refractive index glasses having a refractive index n
d
≦1.79 in which BaO is only an optional component and which contain high proportions of at least 13% by weight of the expensive component La2O
3
, which promotes crystallization at these rather high contents.
JP 52-25812 A and JP 58-46459 B2 describe flint glasses from the SiO
2
—Nb
2
O
5
—TiO
2
—R
2
O(—RO) system, where some of the Nb
2
O
5
can be replaced by Ta
2
O
5
and some of the SiO
2
can be replaced by B
2
O
3
. The B
2
O
3
content should not exceed 10% by weight and 5% by weight respectively (based on all the glass) owing to crystallization phenomena.
SUMMARY OF THE INVENTION
As used herein, the term “essentially component-free” means a material having an extremely low level of the referenced component, such as, for example, an unavoidable impurity.
A feature of the invention is to provide lead-free, an essentially desirably lead-free, optical glass having a refractive index n
d
of from about 1.65 to about 1.87 and an Abbe number &ngr;
d
of from about 27 to about 43 which have good melting and processing properties and are inexpensive to produce.
This feature can be achieved by the glasses described as lead-free optical glasses having a refractive index n
d
of from 1.65 to 1.87 and an Abbe number &ngr;
d
of from 27 to 43, and including the following composition (in % by weight, based on oxide): SiO
2
10-25, B
2
O
3
>10-25, Na
2
O 2.5-10, K
2
O 0-3, CaO 0.5-5, BaO 15-30, ZnO 5-15, TiO
2
6-15, ZrO
2
0.5-8, Y
2
O
3
0.1-2, La
2
O
3
0.5-8, Nb
2
O
5
6-12, with La
2
O
3
+Nb
2
O
5
≦17, Ta
2
O
5
0-1, Gd
2
O
3
0-1and optionally a fining agent.
The glasses can include the glass formers SiO
2
(about 10-about 25% by weight) and B
2
O
3
(>about 10-about 25% by weight) in balanced proportions. This produces both good melting properties of the glasses, which generally improve with increasing B
2
O
3
content and generally worsen with increasing SiO
2
content, and good chemical resistance, which would generally worsen at an excessive B
2
O
3
content. The B
2
O
3
content also can contribute to the good devitrification stability. It has been found that the devitrification tendency would generally increase excessively if said minimum B
2
O
3
content were reduced. The B
2
O
3
content is preferably at most about 20% by weight. Particular preference is given to a B
2
O
3
content of from about 14 to about 19% by weight. The SiO
2
content is preferably at least about 15% by weight and particularly preferably at most about 20% by weight.
Desirably, the glasses include about 15-about 30% by weight of BaO. This proportion of BaO is required in order to achieve the desired Abbe number region. The BaO content is preferably at least about 18% by weight. Particular preference is given to a content of at least about 25% by weight. Especially due to the balanced proportions of B
2
O
3
and SiO
2
in relation to the BaO content, the melting properties of the glasses can be particularly good. In addition to BaO, the glasses can include about 0.5-about 5% by weight, preferably about 0.5-about 3% by weight, particularly preferably about 0.5-about 2.5% by weight, of CaO. The presence of CaO in addition to BaO can improve the crystallization stability of the glasses, as does the presence of ZnO, of which the glass can include about 5-about 15% by weight, preferably about 5-about 12% by weight, for stabilization and in order to set the high refractive index. Particular preference is given to a ZnO content of from about 7 to about 11% by weight.
The glasses can include both TiO
2
(about 6-about 15% by weight, particularly preferably about 7-about 12% by weight) and ZrO
2
(about 0.5-about 8% by weight, preferably about 1-about 5% by weight, particularly preferably at least about 2.5% by weight). These two components can improve the chemical resistance, in particular the alkali resistance. At higher contents, the crystallization stability could be greatly reduced.
It is particularly preferred for the glasses to include ZrO
2
and TiO
2
in a certain ratio, more precisely in a ZrO
2
/TiO
2
weight ratio of from about 0.06 to about 0.85. This ratio can make it easier to achieve a high refractive index at the same time as a low Abbe number. Desirably, it is satisfied with ZrO
2
in an amount of from about 1 to about 5% by weight.
Desirably, the glasses include about 0.5-about 8% by weight, particularly preferably about 0.5-about 2% by weight, of La
2
O
3
and about 6-about 12% by weight, preferably about 6-about 10% by weight, particularly preferably about 7-about 9.5% by weight, of Nb
2
O
5
. These ranges facilitate the desired optical position, in particular low Abbe numbers at the same time as high refractive indexes. In addition, both components can increase the transmission of the glasses. A further increase in the content could result in deviating n
d
and &ngr;
d
values and an unnecessary increase in the cost of the batch. A sum (La
2
O
3
+Nb
2
O
5
)≦about 17% by weight, preferably≦about 14% by weight, should therefore not be exceeded, particular preference being given to (La
2
O
3
+Nb
2
O
5
)≦about 11% by weight.
In order to vary and precisely set the low Abbe number at the same time as a high refractive index, desirably the

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