GB2320023A

GB2320023A - Lead-free crown glass

Info

Publication number: GB2320023A
Application number: GB9720753A
Authority: GB
Inventors: Marc Clement; Volkmar Geiler
Original assignee: Carl Zeiss AG
Current assignee: Carl Zeiss AG
Priority date: 1996-12-06
Filing date: 1997-10-01
Publication date: 1998-06-10
Anticipated expiration: 2017-10-01
Also published as: GB9720753D0; DE19650692A1; CN1269754C; GB2320023B; FR2756821B1; CN1186051A; DE19650692C2; FR2756821A1; JPH10167753A; MY133265A; JP4412747B2

Abstract

Lead-free crown glass having a Refractive Index of between 1.49 and 1.54 and AbbÚ coefficient of between 55 and 62 and having the following composition (in % by weight based on oxides): SiO 2 60 - 75, B 2 O 3 0 - 8; Na 2 O 0 - 12, K 2 O 0 - 25 with Na 2 O + K 2 O 12 - 25, GaO 0 - <5, ZnO 3 - 15, TiO 2 0 - <1 and ZrO 2 0 - 3.

Description

2320023 Lead-free crown glass The invention relates to lead-free crown

glass which have refractive indices nd of between 1.49 and 1.54 and Abbe coefficients Yd of between 55 and 62.

Since, in recent years, the glass components PbO and AS203 have come to be widely considered (and to public discussion) as pollutants of the environment, there is also a trend towards using glass which is free of Pbo, and also free of AS2031 in optical instruments. Therefore, such glass having the specific optical properties, for example with respect to refractive index, Abbe coefficient and also transmission, should be available on the market.

Reproducing all the desired optical and also glass (-technical) properties affected by the lead oxide by simple replacement of the Pbo by one or more other constituents is as a rule unsuccessful. Rather, new developments in the glass composition are necessary in most cases. Specifications in which lead-free crown flint glass or crown glass is described are already to be found in the patent literature. However, this glass still has extremely wide-ranging disadvantages.

German Patent Specification DE 973,350 describes glass which can be leadfree but always contains fluorine in order to lower the refractive index. The same applies to the glass from JP 1-133,956 A. As regards the environmental protection idea already mentioned, the use of this component should strictly be avoided as well, since, under some circumstances, harmful emissions of fluorine can occur during the smelting process.

The glass described in British Patent Specification GB 2,029,401 B contains up to 20k by weight of Cao. However, the introduction of CaO, in particular in relatively large quantities, lowers the viscosity at high temperatures, but increases it at low temperatures; that is to say the viscosity/temperature curve becomes steeper, the addition of CaO thus making the glass shorter.

European Patent Specification EP 0,151,346 BI mentions crown glass which requires high B203 contents for reducing the melting temperatures and high T'02 contents for reaching the refractive index and for adjusting the UV edge and which must contain AS203. T'02 considerably reduces the UV transmission. However, T'02, in particular in relatively large proportions, can lead to undesirable coloration of the glass. The disadvantages resulting from the high T'02 content also exist in the case of glass from German Auslegeschrift DE-AS 1,050,965, which contains up to 8% by weight of T'02- As in the glass from EP 0,151,346 B1, A1203 is also a necessary constituent described in the crown glass in JP 4-40,301 B2 and JP 470,262 B2 (up to 10 and 15 or 140-k by weight respectively). Such glass will show a is heightened tendency to devitrification.

JP 6-107,425 A describes glass whose optical properties vary over a wide range and which necessarily contains BaO (up to 45% by weight) and Nb20. (up to 25% by weight). The use of the last-mentioned, expensive component enormously increases the price of the mixture. The low SiO, contents of the glass are compensated by relatively high proportions of B203. which allowsthe conclusion that the chemical resistance of the glass is not very high.

It is the object of the invention to provide lead-free crown glass having a refractive index nd Of between 1.49 and 1.54 and an Abbe coefficient Vd Of between 55 and 62, which has a high light transmission in the wavelength range between 380 nm and 700 nm and is readily fusible and processable and can be produced costeffectively.

Claims

This object is achieved by the glass described in Patent Claim 1.

The glass belongs to an S'02-M20-ZnO glass system in which S'02 functions as a network former. S'02 'S present in proportions from 60 to 75% by weight. It can, however, be replaced by up to 8% by weight of B.03. As a result, the melting temperatures are reduced. In the case of higher proportions of B203. the chemical resistance would be reduced and segregations would occur in the glass.

To lower the melting temperatures, the glass contains Na20 (0 - 12C1; by weight) and/or K.0 (0 - 25-0k by weight), the total of these two components being at least 1226 by weight, in order to reduce the melting tempera tures sufficiently, and being at most 25k by weight, so as not to cause the high chemical resistance of the glass to deteriorate. Furthermore, the glass can contain up to
2.1k by weight each of Li20 and CS20, but even then the total of the alkali metal oxides should not exceed 25% by weight (Na20 + K20 + L'20 + CS20 12 - 25% by weight).

To achieve the said optical properties, the glass contains
3 - 15.015 by weight of ZnO. In order to keep the ZnO content low in the case of the desired data, the glass can also contain Ti02 (0 - <1% by weight) and ZrO, (0 - 3% by weight). Higher Ti02 contents lead to trans mission losses in the UV range, and higher Zr02 contents raise the melting temperatures. Preferably, the total of T'02 and Zr02 in the glass is at least 0. 1.1k by weight. It is particularly preferred for the glass to contain at least 0. 1% by weight of TiO.. This is because T'02 has a more pronounced effect on the Abbe coefficient than ZnO, and an unduly high ZnO content would reduce the devitrification stability.

For a modification of the optical position, up to 3% by weight of Nb20., up to 3% by weight of Ta20., and up to 3% by weight of W03 can also be added to the gl ass according to the invention, but this entails a higher cost of the mixture owing to the high price of these components.

Furthermore, the glass can contain up to less than 5% by weight of Cao. This component advantageously improves the chemical resistance and the processability.

With higher proportions, however, there is a risk of the optical properties no longer being achieved.

In addition, the glass can also contain up to 3k by weight each of BaO, SrO and MgO, without the melting and processing properties being substantially changed.
4 - However, MgO at least in relatively high proportions, diminishes the devitrification stability of the glass, so that this component is omitted in most cases.

Within the range claimed in the main claim, there are two separate preferred glass composition regions which, with their refractive indices and Abbe co efficients, each replace a commercially available lead containing glass type.

on the one hand, this is the high-CaO and high Zno composition region (in % by weight based on oxides) with S'02 65 70, B203 2 6, Na20 3 - 8, K20 9 - 14, CaO 1 - 4, Ti02 0 <', ZnO 7 11 and Zr02 0 '.

on the other hand, this is the high-K20 and low ZnO composition region (in % by weight based on oxides) is with S'02 66 - 70, B203 2 - 5, Na20
5 - 8, K20 15 - 18 with Na20 + K20 s25, CaO 0 - 1, T'02 0 - <', ZnO 4 - 7 and Zr02 0 - 1.

For fining the glass, fining agents known per se can be added to the mixture. If no As.03 is used but, for example, sulphates, chlorides, Sb203 or Ce02 are used instead, which is possible without reducing the glass quality, the lead-free glass according to the invention is additionally free of arsenic.

The lead-free optical glass according to the invention, which represents a further group of crown glass having the given refractive indices and Abbe coefficients, is distinguished by high transmission in the wavelength range between 380 nm and 700 nm. The glass is readily fusible and processable and can be produced cost-effectively; it shows high devitrification stability and good chemical resistance.

Examples:

Six examples of glass according to the invention were smelted from conventional raw materials. Their compositions (in % by weight based on oxides) and their refractive indices nd and their Abbe coefficients Yd are listed in the table.

Table Compositions (in % by weight based on oxides) of glass according to the invention:

1 2 3 4 5 6 5i02 67.89 68.59 66.26 69.02 68.35 67.71 B203 3.21 3.53 3.73 4.49 4.59 4.56 Na20 6.96 6.40 6.17 4.59 4.38 4.86 Y'20 16.08 16.15 12.87 10.71 10.14 10.23 ZnO 5.45 5.04 8.16 8.30 9.40 9.61 TiO2 0.32 0.14 0.32 0.32 0.40 0.45 ZrO, 0.10 0.14 - - - - CaO - - 2.49 2.58 2.74 2.59 Ild 1.5115 1.5103 1.5213 1.5189 1.5213 1.5220 Yd 59.43 60.17 59.16 60.16 59.72 59.47 For highlighting the high transmission, the spectral pure transmission ratio at the wavelength of X = 400 nm and a layer thickness of d = 25 mm may be mentioned: Ti (400 nm, 25 mm) is 0. 991 (Example 2) or 0.995 (Example 6).
6. Crown glass according to at least one of Claims 1 to 5, characterized in that, apart from unavoidable contaminations it is free of arsenic oxide.

s25 0 1 4 7 0 <1 0 1 agents in the customary to at least one of Claims it additionally contains:

0 2 0 2 s25 0 0 0 0 0 3 3 3 3 3

6 CLAIMS Lead-free crown glass having a refractive index nd of between 1.49 and 1. 54 and an Abbe coefficient Vd Of between 55 and 62, characterized by the following composition (in % by weight based on oxides):

S'02 60 - 75 B203 Na20 K20 with Na20 + K20 CaO ZnO T'02 Zr02 is and, if appropriate, fining quantities.

0 0 0 12 0 3 8 12 <s is 0 <1 0 3 agents in the customary Crown glass according to Claim 1, characterized in that the total of T'02 + Zr02 is at least0.12k by weight.

3. Crown glass according to Claim 1 or 2, charac- terized by the following composition (in t by weight based on oxides):

S'02 B203 Na.0 K20 CaO ZnO TiO2 ZrO, and, if appropriate, quantities.

4. Crown glass according to Claim 1 or 2, charac terized by the following composition (in 1; by weight based on oxides): S'02 B203 Na.0 K20 70 2 6 3 8 9 14 1 4 7 11 0 <1 0 1 fining agents in the customary 66 - 70 2 - 5 S 8 is - 18 with Na20 + K20 CaO ZnO Ti02 Zr02 and, if appropriate, fining quantities.

5. Crown glass according 1 to 4, characterized in that L'20 CS20 with Na20 + K20 + L'20 + CS20 Mgo SrO BaO Nb205 Ta20.