Lead-free optical heavy flint glasses

Details Lead-free optical heavy flint glasses Lead-free optical heavy flint glasses

2002-07-10

2004-03-23

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S077000, C501S063000, C501S072000

Reexamination Certificate

active

06709998

ABSTRACT:

The invention relates to lead-free optical glasses which have refractive indices n
d
of between 1.65 and 1.80 and Abbe numbers &ngr;
d
of between 21 and 33. These glasses belong to the optical glass type consisting of the heavy flint glasses (HF).
Since, in recent years, the glass components PbO and As
2
O
3
have been considered to be environmentally polluting in public discussions, the manufacturers of optical apparatuses also require PbO-free and preferably also As
2
O
3
-free glasses having the respective optical properties.
It is desirable to dispense with PbO also for the production of light glass parts, i.e. of glasses having a low density.
It is as a rule not possible to reproduce the desired optical and technical glass properties influenced by PbO by simply replacing lead oxide by one or more components. Instead, new developments or wide-ranging changes in the glass composition are necessary.
The patent literature already includes some publications in which lead-free glasses having optical values from said range are described. However, these glasses have a very wide range of disadvantages.
DE 32 164 51 A describes optical lightweight glasses having a refractive index n
d
of >1.70 and an Abbe number &ngr;
d
of ≧22 and a density &rgr; of ≦3.5 g/cm
3
. These glasses contain up to 3% by weight of B
2
O
3
, a component which is aggressive towards Pt. If such glasses are melted in Pt crucibles or if they come into contact with other Pt components, which would improve the homogeneity and low bubble count of the glasses, they have a higher level of Pt impurities, with the result that their transmittance is adversely affected.
U.S. Pat. No. 3,589,918 describes an optical glass for lens systems comprising the glass system SiO
2
—K
2
O—TiO
2
—Sb
2
O
3
. The high Sb
2
O
3
contents of up to 45% by weight in this glass make the glass susceptible to separation and heavy and adversely affect its transmittance so that it is not suitable for modern applications in optics.
JP 53-16718 A discloses glasses having high contents of divalent oxides (MO=MgO+CaO+SrO+BaO+ZnO 15-50% by weight) and relatively low contents of TiO
2
(1-25% by weight). These glasses have Abbe numbers of between 30 and 45. Owing to the high MO content, their stability to crystallization is low.
JP 52-25812 A discloses TiO
2
- and Nb
2
O
5
-glasses whose compositions vary over a wide range. According to the examples, the glasses have very high (25-45% by weight) or very low (5% by weight) of Nb
2
O
5
contents. The same applies to the MO content (21 and 30% by weight and 0-5% by weight, respectively). These glasses having TiO
2
contents of up to 50% by weight are also not sufficiently stable to crystallization for economical continuous production.
It is an object of the invention to provide lead-free optical glasses having a refractive index n
d
of between 1.65 and 1.80 and an Abbe number &ngr;
d
of between 21 and 33, which possess good melting and processing properties and have good chemical resistance, good stability to crystallization and a low density.
This object is achieved by glasses described in Patent claim 1.
The good fusibility meltability of the glasses is achieved by the balanced proportions of fluxes (Na
2
O, K
2
O) to glass formers (SiO
2
+optionally B
2
O
3
, Al
2
O
3
) in relation to the poorly melting highly refractive components (MO (BaO, CaO+optionally SrO, MgO), TiO
2
, Nb
2
O
5
, WO
3
+optionally ZrO
2
)
The glasses contain 27 to 40% by weight of the main glass former SiO2. In the case of higher proportions, the desired high refractive index would not be reached and the fusibility would deteriorate; in the case of lower proportions, the stability to crystallization and the chemical resistance would be reduced. An SiO
2
content of at least 29% by weight is preferred, particularly preferably of at least 31% by weight of SiO
2
. A content of not more than 36% by weight is particularly preferred.
For further stabilization, the glasses may contain up to 6% by weight of Al
2
O
3
, preferably up to <3% by weight of Al
2
O
3
, and up to <0.5% by weight of B
2
O
3
. Higher proportions of glass formers would reduce the fusibility. Preferably, Al
2
O
3
is dispensed with. It is a major advantage that the B
2
O
3
content can remain limited to said low proportions, since the aggressiveness of the glass melt is thus reduced, so that glasses containing extremely small amounts of Pt impurities and hence having very high transmittances can be produced in Pt components.
In order to achieve the desired optical position of a heavy flint glass, relatively high proportions of highly refractive components are required. The proportion of the glass formers and of fluxes having a low refractive index (Na
2
O, K
2
O) is therefore limited. Preferably 69.5% by weight of SiO
2
+Al
2
O
3
+B
2
O
3
+Na
2
O +K
2
O are not exceeded, and very particularly preferably the limit of this sum is max. 60.5% by weight.
In addition to the glass formers, the glasses contain a proportion of fluxes which is sufficient for good fusibility. Thus, they contain at least 8% by weight and not more than 28% by weight of Na
2
O+K
2
O, and in particular 7-18% by weight of Na
2
O and 1-10% by weight of K
2
O. A flux content of 12-26% by weight with 9-16% by weight of Na
2
O and 3-10% by weight of K
2
O is preferred, and at least 14% by weight of Na
2
O+K
2
O with 10-15% by weight of Na
2
O and 4-9% by weight of K
2
O are particularly preferred. A sum of Na
2
O and K
2
O of not more than 21% by weight is very particularly preferred.
The glasses contain the following highly refractive components:
They contain 1.5-<15% by weight of alkaline earth metal oxides, preferably 3.5-14, especially ≦11, particularly preferably ≦10, % by weight.
Specifically:
1-10% by weight of BaO, preferably 3-10, particularly preferably 3-8, % by weight
0.5-5% by weight of CaO, preferably 0.5-3% by weight
0-3% by weight of MgO, preferably 0-<2% by weight, preferably Mgo-free
0-3% by weight of Sro, preferably 0-<2% by weight, preferably SrO-free
The proportion of alkaline earth metal oxide is limited to said maximum content since a further increase would be possible only by reducing the glass former and flux content and would lead to crystallization effects, particularly since the further components which increase the refractive index are comparatively good nucleating agents. Said minimum contents of the alkaline earth metal oxides are necessary in order to establish the high refractive index and to stabilize the chemical resistance.
The glasses contain 21-37% by weight of TiO
2
, preferably 23-35, particularly preferably 26-33, % by weight.
The glasses furthermore contain 5-17% by weight of Nb
2
O
5
, preferably >5, especially 7-15, particularly preferably ≦12, % by weight.
These two components form the basis of the high refractive index at the desired Abbe number. An increase in the TiO
2
content would reduce the Abbe number excessively and also excessively increase the tendency to crystallization. An increase in the Nb
2
O
5
content would increase the Abbe number to a very excessive extent and slightly reduce the refractive index.
For stabilization to crystallization, the glasses may contain up to 7% by weight of ZrO
2,
preferably <5% by weight. Preferably, the ZrO
2
content replaces a corresponding part of the TiO
2
content, so that preferably the maximum sum of TiO
2
+ZrO
2
is 37% by weight, in particular 35% by weight.
A further increase of ZrO
2
would in turn lead to an increase in the tendency to crystallization; furthermore the optical position would be undesirably shifted.
By parallel use of different nucleating agents and crystal formers, namely TiO
2
in addition to Nb
2
O
5
and optionally ZrO
2
, the formation of defined crystals is impeded and it is possible to achieve the desired exceptional refractive index by means of high proportions of these components without adding lead.
An important component is WO
3
. It is present in an am

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