GB2338954A

GB2338954A - Lead-free optical glasses

Info

Publication number: GB2338954A
Application number: GB9914153A
Authority: GB
Inventors: Uwe Kolberg; Monika Gierke; Magdalena Winkler-Trudewig; Alwin Weitzel
Original assignee: Carl Zeiss AG
Current assignee: Carl Zeiss AG
Priority date: 1998-06-29
Filing date: 1999-06-18
Publication date: 2000-01-12
Anticipated expiration: 2019-06-18
Also published as: CN1121987C; FR2780395A1; JP4213301B2; GB9914153D0; JP2000053441A; GB2338954B; CN1241543A; FR2780395B1; DE19828992C1; MY119592A

Description

1 2338954 Lead-free optical glasses The invention relates to lead-free

optical glasses having refractive indices nd of between 1.50 and 1.56 and Abbe numbers Vd of between 50 and 64. These glasses belong to the barium light flint (BaLF) barium crown (BaC), crown (C), crown flint (CF) and boron crown (BC) types of optical glass.

Since the glass components PbO and AS203 have been discussed in public in recent years as being 1 0 environmental pollutants, the manufacturers of optical instruments also demand Pbo-free and preferably also AS203-free glasses having the respective optical properties.

It is also an aim to omit PbO in the production of lightweight glass components, i.e.- glasses of low density.

Simple replacement of the lead oxide by one or more constituents is generally not a successful way of reproducing the desired optical and technical properties affected by Pbo. Instead, new developments or substantial modifications to the glass composition are necessary.

The patent literature has already revealed some specifications which describe lead-free glasses having optical values in the above range. However, these glasses have a wide variety of disadvantages.

German Patent Specification DE 973 350 describes glasses having a refractive index which varies within a wide range and a relatively high dispersion, based on the refractive index. These glasses contain only up to 5% by weight of alkaline earth metal oxides, furthermore, besides Si02 and/or B203 and F, up to 30% of alkali metal oxides and at least one component from the group consisting of A1203, Ti021 Sb203, AS203 and PbO, the sum of these five oxides and the alkali metal oxides being between 35 and 69.85% by weight. Besides the possible presence of Pbo and AS203 in amounts of up to 55 and up to 5% by weight respectively, these glasses have the disadvantage that 2 the high refractive index is achieved by using expensive components or components which result in a yellowish coloration. Another disadvantage is the sometimes fairly high content of alkali metal oxides, which results in poor chemical resistance of the glasses.

DE 2621741 discloses glasses comprising the Si02 - R20 - ZnO - TiO2 system which, owing to their content of coloured components, are used as glasses for sharp-cut absorption filters.

JP 60-54249 B2 discloses optical glasses containing 10-20% by weight of CaO and no BaO. CaO is needed here in -order to achieve high Abbe numbers. However, Cao has the disadvantage, in particular compared to BaO, of not increasing the refractive index so much.

GB 804,451 and GB 812,576 also disclose Baofree optical glasses. Their refractive indices nd of at least 1.56 and 1.54 respectively can only be achieved by using components which are relatively expensive, have high melting points or cause yellowish discolorations. For example, the glasses of the former specification contain up to 20% by weight of Ti02. The glasses of the latter specification, which have a relatively low Si02 content, contain up to 20% by weight of Zr02 and up to 9% by weight of Ti02, the sum of Ti02 and Zr02 being up to 20% by weight.

on the other hand, JP 8-34633 A discloses Sn02containing glasses which contain only small amounts of Ti02. Ti02 here serves as solarization protection and does not have a significant effect on the nonoptical properties. These glasses will not have good chemical resistance.

DE 196 50 692 Al describes lead-free crown glasses containing both Ti02 and BaO only as optional constituents in a maximum amount of < 1% by weight and at most 3% by weight respectively.

DE 196 09 735 Al describes lead-free crown flint glasses in which ZnO, CaO and BaO are only - 3 optional components. B203 is likewise not essential and must remain restricted to less than 5% by weight.

The glasses of JP 52-45616 A likewise contain high proportions of the relatively expensive and high- melting component Zr02 (2-20% by weight), which reduces the crystallization stability of these glasses. the same applies to the glasses of US 2,523,265, which contain 2-15% by weight of Zr02; they need this component to improve the chemical resistance since they contain only from 33 to 55% by weight of SiO2.

The chemical resistance of the glasses of US 5,523,265 (45-60% by weight of Si02) and JP 62-13293 B2 (23-61% by weight) will co rrespondingly not be high.

An earlier, as yet unpublished German application by the applicant also describes lead-free optical barium crown glasses and barium light flint glasses and also barium flint glasses containing a maximum of 60% by weight Of Si02 DE 198 27 568.4 discloses glasses containing 45-60% by weight Of Si02. The fact that they nevertheless have good chemical resistance is due to the presence of Zr02 and Ti02 DE 197 38 428 C1 discloses glasses containing 40-60% by weight Of Si02 and 0.5-3.5% by weight of A1203. The latter component serves to improve the acid resistance.

The object of the invention is to provide lead free optical glasses having a refractive index rid of between 1.50 and 1.56 and an Abbe number Vd of between and 64 which have good melting and processing properties and good chemical resistance and good crystallization stability.

This object is achieved by the glasses

Claims

described in Claim 1.

The glasses contain from 62 to 75% by weight of Si02. At higher contents, the melting properties would be impaired, while at lower contents, the acid resistance would be reduced. In addition, a high 4 content Of Si02 facilitates a low refractive index.

Preference is given to an Si02 content-of greater than 63% by weight.

In order to improve not only the acid resistance, but the chemical resistance overall, the glasses contain from I to 7% by weight, preferably at least 4% by weight, of Ti02. Ti02 increases the refractive index and reduces the Abbe number.

In order to improve the chemical resistance further and to vary the refractive index and Abbe number, the glasses may contain up to 1% by weight of each of Zr02, Nb205 and Ta205. However, since Nb205 and Ta205 are relatively expensive components, these two components are preferably omitted.

The glasses contain from
2 to 10% by weight of Na20 (preferably from
3.5 to 9% by weight) and from
4.
5 to 8% by weight of K20. Na20 and K20 serve to improve the melting properties. At higher contents, the coefficient of thermal expansion, the glass transition temperature and the acid resistance are adversely affected. At lower contents, the high melting temperatures make economical production more difficult.

The glasses furthermore contain B203 in an amount of from 5 to 9.5% by weight. This component improves the melting properties without any adverse effects, i.e. an increase, on the coefficients of expansion. However, excessive proportions of B203 impair the chemical resistance, in particular the alkali resistance. In a preferred embodiment, the B203 content is at least 8% by weight.

The glasses contain from 4.5 to 21% by weight of Bao in order to achieve an Abbe number in the stated range. At a higher content, the devitrification tendency would also increase. In particular for glasses having low Abbe numbers or for glasses of low density, a restriction in the BaO content to a maximum of 7% by weight may also be preferred.

For stabilization against crystallization, the glasses contain at least 0.05% by weight of ZnO and/or CaO. The content of CaO is between 0 and 0.5% by weight, while the content of Zno is preferably between 0 and less than 14% by weight; in particular if a low density is desired, it is a maximum of 2% by weight. In these quantities, ZnO enables the refractive index to be increased while the Abbe numbers remain in the moderate region.

The total amount of BaO, ZnO and CaO is at most.25% by weight, since otherwise the refractive index and Abbe number would be too high. In addition, the chemical resistance would be impaired. Said total amount is preferably at most 23.5% by weight.

The glasses may contain up to 0.5% by weight of F_. This is particularly preferred in order to achieve specific refractive indices in the region of crown glasses and boron crown glasses. F_ is added, for example, as KF or KHF2. As is generally known, the high evaporation tendency of F--containing compounds means that a certain excess of F-, based on the desired content, is usually added.

In order to improve the glass quality, one or more refining agents can be added to the batch in conventional amounts in order to refine the glass. The glass then has a particularly good internal quality with respect to freedom from bubbles and streaks.

If the ref ining agent used is not AS203, but instead, for example, Sb203, preferably in an amount of 0.5% by weight, which is possible without losses regarding the glass quality, the lead-free glasses according to the invention are in addition free from arsenic.

In a preferred embodiment, the total amount of Sb203 and F-, which likewise has a refining function, is between 0.05 and 0.8% by weight. In this way, an excellent internal glass quality is achieved.

Within the composition range according to the invention, there is a particularly preferred composition range for glasses having refractive indices rid of between 1.53 and 1.56 and Abbe numbers Vd Of between 50 and 55. It is (in % by weight, based on oxide): SiO2 62-65; B203 8-9.5; BaO 4.5-7; ZnO 0-2; Cao 0-0.5 where ZnO + CaO: 0.05; Ti02 4-7; Na20
6.5-9.5; K20 6-8; ZrO2 0-1; F- 0-0.5.

Glasses in this preferred range can likewise be produced, if desired, using conventional refining agent in conventional amounts or the abovementioned preferred variants.

The glasses according to the invention have negative anomalous partial dispersion in the blue region of the spectrum.

For explanation:

The partial dispersion capacity, known as the relative partial dispersion, in the blue region of the spectrum is given by the expression P91F = n. nF n. - n, The "standard straight line" is defined As 20 obeying, in the blue region of the spectrum, the equation P9,F = 0.6438 - 0.001682 0 VdGlasses whose partial dispersion is on this straight line are referred to as nstandard glasses".

Forglasses whose dispersion differs from that of "standard glasses", the ordinal difference A P,F, by which the relevant P5,F - Vd point is shifted with respect to the "standard straight line", is given:

A rough classification of these glasses having anomalous partial dispersion into two groups is usual, depending on whether P9,F is "above" (positive partial dispersion: A P9,F POS.) or "below" (negative partial dispersion: A P9,F neg.) the"standard straight line".

Glasses having anomalous partial dispersions are of importance for elimination of residual colour defects, known as the secondary spectrum.
7. A lead-free optical glass according to at least one of claims I to 6, having a maximum density p of 2.9 g/cm, an acid resistance SR of 2.3 or better, an alkali resistance AR of 3.3 or better and a glass transition temperature Tg of at least 55(rC.

i I

7 Examples:

Five examples of glasses according to the invention are produced from conventional raw materials.

Table 2 shows the respective composition (in % by weight based on oxide), the refractive index nd, the Abbe number Vd, the partial dispersion in the blue region of the spectrum Pg,F, and the anomaly of this partial dispersion A P9,F, the density p [g/cm'], the coefficient of thermal expansion a2O/300 [10"'IK] and the 10 glass transition temperature T. [OC] of the glasses.

The glasses according to the invention were produced as follows: the raw materials for the oxides, preferably carbonates and nitrates, were weighed out.

The refining agent(s) was added, and the batch was then mixed well. The glass batch was melted at about 1380 14000C in a continuous melter, then refined and homogenized well. The casting temperature was 1270 12900C.

Table 1 shows an illustrative melt. 20 Table 1 Illustrative melt for 100 kg of calculated glass oxide % by weight Raw material Amount [kgl Si02 63.5 Si02 63.52 B203
8.8 H3BO3 15.64 BaO 4.5 BaC03 5.88 Ti02 7.0 Ti02 7.04 CaO 0.2 CaC03 0.35 K20 7.6 KN03 2.15 K2CO3
9.71 Na20 8.1 Na2CO3 13.87 Sb203 0.3 Sb203 0.30 The properties of the resultant glass are shown in Table 2, Example 1.

8 Table 2 Glass compositions (in % by weight, based on oxide) and major properties:

1 2 3 4 5 Si02 63.5 74.5 75.0 63.5 63.5 B203 8.8 5.0 9.5 5.0 8.8 BaO 4.5 4.5 5.0 20.0 5.6 CaO 0.2 0.2 0.2 0.2 0.14 ZnO - - - 0.5 - Zr02 - 0.3 F_ 0.5 - - - Ti02 7.0 5.0 1.0 1.2 5.0 Na20 8.1 4.0 4.0 4.0 9.1 K20 7.6 6.0 5.0 5.0 7.6 Sb203 0.3 0.3 0.3 0.3 0.3 nd 1.55347 1.52498 1.50919 1.54069 1.54810 Vd 50.74 54.79 63.10 59.22 53.87 Pg, F 0.5578 0.'5506 0.5334 0.5430 0.5523 A Pg, F -0.0006 -0.0.010 -0.0043 -0.0012 -0.0009 p [g/CM3] 2.61 2.50 2.48 2.80' 2.62 a20/300 (10-6/K] 8.02 5.94 5.59 7.40 8.67 Tg [OCI 578 576 596 578 565 Further properties were determined for glass No. 5:

Its acid resistance in accordance with ISO 8424 (SR) is 1.0; its alkali resistance in accordance with ISO draft 10629 (AR) is 2.0, and its internal spectral transmission factor Ti 400 rm/25 vo at wavelength 400 nm. and a sample thickness d = 25 mm is 0.955.

The glass composition range according to the 15 invention offers a further group of optical glasses of the BaLF, BaC, CF, C and BC type having said optical data which are lead-free and, in a preferred embodiment, also AS203-free. The lead freedom of the glasses is advantageous not only owing to the environmental protection considerations mentioned, but - 9 also has a positive effect on their density and their glass transition temperatures.

The glasses have the following advantages: they have high chemical resistance, especially an acid resistance of SR 2.3 or better, and an alkali resistance of AR 3.3 or better. The high chemical resistance of the glasses is important for their further treatment, such as grinding and polishing. The' glasses have high crystallization stability. This enables the glasses to be produced in relatively large melters, for example in an optical tank. A measure of a crystallization stability which is adequate for this type of production is the viscosity at the upper devitrification limit: for continuous production, it should be: 1000 dPas. This is satisfied by the glasses according to the invention. The glasses are easy to melt and process. They have relatively high glass transition temperatures Tg of at least 5500C. Their 3 density p is quite low at a maximum of 2.9 g/cm. The transmission of the glasses in the visible region of the spectrum is high. The glasses have a negative partial dispersion in the blue region of the spectrum.

CLAIMS 1. A lead-free optical glass having a refractive index nd of between 1.50 and 1.56 and an Abbe number Vd of between 50 and 64, characterized by the following composition (in % by weight, based on oxide):

si% 62 - 75 B203 5 - 9.5 BaO 4.5 - 21 ZnO 0 - < 14 CaO 0 - 0.5 where ZnO + CaO _> 0.05 where BaO + ZnO + CaO < 25 TiO2 1 - 7 Na2O 2 - 10 K20 4.5 - 8 Zr02 0 - 1 Nb2O5 0 - I Ta205 0 - 1 F 0 - 0.5 and optionally one or more refining agents.

2. A lead-free optical glass according to claim 1, characterized by the following composition (in % by weight, based on oxide):

SiO2 62 - 75 B203 5 - 9.5 BaO 4.5 - 21 ZnO 0 - 2 CaO 0 - 0.5 11 where ZnO + CaO > 0. 05 TiO2 1 7 Na2O 3.5 9.5 K20 4.5 8 ZrO2 0 1 F 0 - 0.5 and optionally one or more refining agents.

3. A lead-free optical glass according to claim 1 or 2, characterized by the following composition (in % by weight, based on oxide) with a refractive index ndof between 1.53 and 1.56 and an Abbe numberVdof between 50 and 55:

SiO2 62 65 B203 8 - 9.5 BaO 4.5 - 7 ZnO 0 - 2 CaO 0 - 0.5 where ZnO + CaO _> 0.05 U02 4 - 7 Na2O 6.5 - 9.5 K20 6 - 8 ZrO2 0 - 1 F 0 - 0.5 and optionally one or more refining agents.

4. A lead-free optical glass according to at least one of claims 1 to 3, characterized containing 0-0. 5 % by weight of Sb203- 12 5. A lead-free optical glass according to at least one of claims I to 4, wherein the total amount of F and Sb03 is 0. 05-0.8 % by weight.

6. A lead-free optical glass according to at least one of claims I to 5, which is free from arsenic oxide, apart from unavoidable impurities.