GB2328937A

GB2328937A - Lead-free optical barium flint glass

Info

Publication number: GB2328937A
Application number: GB9818277A
Authority: GB
Inventors: Uwe Kolberg; Alwin Weitzel; Magdalena Winkler-Trudewig; Monica Geirke
Original assignee: Carl Zeiss AG
Current assignee: Carl Zeiss AG
Priority date: 1997-09-03
Filing date: 1998-08-24
Publication date: 1999-03-10
Anticipated expiration: 2018-08-24
Also published as: GB9818277D0; JP4213264B2; MY124577A; DE19738428C1; FR2767814B1; JPH11171586A; CN1216822C; GB2328937B; FR2767814A1; CN1214324A

Abstract

Lead-free optical barium flint glasses having refractive indices nd of between 1.55 and 1.66 and Abbe numbers vd of between 39 and 52, have the following composition (in % by weight based on oxide): SiO2 40 - 60; B2O3 6.5 - 12; BaO 10 - 35; CaO 0 - 1; ZnO 0 - 2; TiO2 5 - 12; Al2O3 0.5 - 3.5; Na2O 3 - 7.5; K2O 3 - 8; with Na2O + K2O # 15; The glasses may contain as refining agents 0 - 0.5% Sb2O3 and/or 0 - 0.1% F'. The glasses have a positive anomalous to normal partial dispersion in the blue range of the spectrum.

Description

1 Load-from optical ba=ium flint glass 2328937 The invention relates to a

lead-free optical barium flint glass which has a refractive index nd of between 1. 55 and 1. 66 and an Abbe number Yd of between 39 and 52.

In contrast to a customary classification of commercial glasses, according to which barium flint glasses have Abbe numbers of between 43 and 50, the term barium flint glasses will in the present application also cover glasses having higher or lower Abbe values, up to the aforementioned limits.

Since questions have publicly been raised in the last few years as to whether the glass components PbO and AS203 are environmentally safe, some manufacturers. of optical instruments are changing over to using only glasses which are free of PbO and also A5203. Glasses of this type, with the respective optical properties, ought therefore to be available on the market.

It is also desirable to eliminate PbO in the production of lightweight glass components, that is to say low-density glasses.

In general, it is not possible to reproduce the desired optical and technical properties of glass which are influenced by PbO merely by replacing the lead oxide with one or more constituents. Instead, it is necessary to develop new glass compositions, or at least make extensive changes to them.

The Patent literature already contains numerous documents in which lead-free glasses with optical values in these and closely related ranges are described. However, these glasse.s have a wide variety of disadvantages.

2 - For example, JP 60-221338 A describes glasses, sometimes with relatively high refractive indices, which necessarily contain Y203, La203, B203 and Li20 while, for example, Si02 is only an optional constituent. Owing to the Y203, and La203 contents, which may be up to 20%by weight or up to 52% by weight, respectively, the glasses are expensive and uneconomical for a continuous manufacturing process. On account of the sometimes very high level of B203 (UP to 50% by weight), as well as the presence of Li20, the glasses will exhibit poor chemical stability, which both makes them more difficult to process further and limits their use. Li20 reduces the crystallization stability of the glasses. In addition, the glasses described in JP 3-5341 A, sometimes with considerably higher Abbe numbers, require Li20 and also relatively high levels of La203.

In addition, the glasses in Patents US 2,433,883 and US 2,523,266 necessarily contain Li20. They furthermore contain ZrO2 (4 to 15% by weight and 2 to 15% by weight, respectively) which also makes the glasses more susceptible to crystallization. The latter point is also true as regards the glasses in US 2,523,265 with from 6 to 8% by weight of Zr02.

Similar points as for La203 and Y203 can be made for Nb205. This oxide increases the overall price and therefore the productions costs enormously. It is a necessary constituent of the glasses described in Patent DE 1 496 524 and in JP 6-107425 A and JP 52-45612 A. The former glasses have rather high refractive indices (at least 1.65). In the latter two glasses, Nb205 is needed in order to moderate the extremely poor effect of the sometimes very high alkali metal oxide content (up to 30% by weight and 40% by weight, respectively) on the chemical stability and crystallization stability.

The A1203-free glasses from SU 975617 and the glasses proposed in JP 517176 A require Sro. They contain this component in amounts'of up to 20%. by weight and up to 14% by weight, respectively. The Zr02-COntaining glasses from US 5,300,467 also contain up to 20% by weight. of SrO. Since the ef f ect of SrO on the optical properties corresponds approximately to that of BaO and CaO, while the latter components can be incorporated into the glass by means of considerably less expensive raw materials, it is advantageous for economical production if SrO can be eliminated.

JP 56-73646 A describes glasses which have high BaO contents and in which Ti02 is only an optional constituent, at at most 3% by weight. At least when free of PbO, these glasses do not achieve optical status with a high refractive index and lo w Abbe number. Furthermore, their chemical stabilities will not be good.

In the glasses in JP 1-133956 A, the A1203 concentration is very high. This makes the glasses difficult to melt, in particular if the only optional component B203 is not present.

The glasses in DE-B 1918350 do not contain alkali metal oxides, or only contain them in small amounts. The poor meltability which would result from this Is compensated for by a fluorine content of from 2% by weight to 14.5% by weight. With these high fluorine contents, it is difficult and expensive to clean the waste gases, which contain HF and SIF4, in the case of production on an industrial scale.

The object of the invention is to provide a lead-free barium flint glass having a refractive index nd Of between 1.55 and 1.66 and an Abbe number vd of between 39 and 52, which has good melting and, processing properties, is economical to produce and has very high - 4 chemical stability. The latter property is important both for further processing of the glasses and for their application possibilities.

Claims

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

The glass contains 40 - 60% by weight of the glass forming compound Si02. At lower levels, the acid resistance of the glass would be reduced, and at higher levels the meltability would be impaired.

In order to improve the acid resistance, theglass contains at least 0.5% by weight of A1203 and at least 5% by weight of Ti02. However, the maximum contents of these oxides, 3.5% by weight of A1203, (preferably 3% by weight) and 12% by weight, should not be exceeded since otherwise the meltability would again be impaired. Ti02 serves not only to improve the chemical stability, but also to achieve high refractive indices and for fine adjustment of the Abbe number, in particular at high refractive indices.

Because of their parallel effect on acid resistance and meltability, in a preferred embodiment of the invention, provision is made to limit the sum of the said oxides (Si02 + A1203 + Ti02) to from 47 to 72% by weight.

The glass contains from 3 to 7.5% by weight of Na20 and f rom, 3 to 8% by weight of K20, with the proviso that the sum of Na20 and K20 should not exceed 15% by weight.

These alkali metal oxides serve to improve meltability.

At lower contents,. the melting temperatures are not reduced sufficiently, and economical manufacture therefore becomes more difficul t At higher contents, the coefficient of thermal expansion is increased, the transformation temperature is lowered and chemical stability is impaired.

The glass furthermore contains from 6.5 to 12% by weight of B203. With this component., the meltability is likewise improved, but in this case no negative effects on expansion need to be taken into account. However, at excessively high levels, the chemical stability, in.particular the alkali resistambe, is reduced. It is preferable to limit the maximum content to 11% by weight.

" Restricting the ratio B203/(si02 + Ti02 + A1203) to at most 0.20, which represents a particularly preferred embodiment of the invention, also entails, depending on the content of the oxides appearing in the denominator, a limitation of the B203 content. Those glasses which fulfil this condition exhibit very good alkali resistance. As the ratio increases, this resistance decreases, so that above a value of 0.2, the best alkali resistance class, class 1, would no longer be achieved.

In order to reach the desired Abbe number range, the glass contains from 10 to 35% by weight of BaO. At lower levels, an Abbe number in the barium flint glass range is not achieved, and at higher levels the susceptibility to devitrification increases. In order to improve the crystallization stability, the glass may contain up to 1% by weight of CaO and up to 2% by weight of- ZnO. The presence of at least 0.1% by weight of CaO and/or ZnO, in particular at least 0.1% by weight of CaO, is preferred. If the aforementioned maximum levels of CaO and ZnO are exceeded, it will be more difficult to reach high refractive indices together with high Abbe numbers.

In a prior German application, n ot yet published at the date of the present application, by the applicant company (official file reference P 19 703 391.1) leadfree optical glasses are described which sometimes - 6 overlap with the glasses described in the present application, in terms of their refractive indices and Abbe numbers, and also have similar compositions. In comparison with those glasses, the use of Zr02 can be entirely eliminated in the glasses described here: owing to the fact that the glasses are free of Zr02, they can be melted at lower temperatures and can be produced more economically. The glasses also have better meltability. owing to the higher Na20 content.

Within the composition range claimed in the main claim for glasses with the aforementioned refractive indices and Abbe numbers, there are two preferred composition ranges, one of which gives glasses with high refractive indices and the other of which gives glasses with low refractive indices:

0 Glasses with high BaO contents and rather 1OW Si02 contents, which have the following composition (in % by weight based on oxide):

Si02 41 - 48; B203 6.5 - 8; BaO 28 - 35; CaO 0.5 -1; ZnO 1 - 2; Ti02 5 12; A1203 0.5 - 1.5; Na20 3 - 5; K20 3 - 5; with Na20 + K20 6.5 - 8; These glasses have ref ractive indices nd of between 1.60 and 1.66 and Abbe numbers Vd of between 39 and 52.

The relatively Si02-rich glasses with low BaO contents, which have the following composition (in % by weight based on oxide):

Si02 46 - 59; B203 6.5 - 8.5; BaO 10 - 28 (preferred - <28); CaO 0. 1 - 1; ZnO 0 - 2; Ti02 6. 5 - 12 (preferred 7 - 12); A1203 0. 5 - 2; Na20 3 - 7 -5; K20 3 - 7.5; with Na20 + K20 t 6.5 These glasses have refractive indices nd Of between 1.56 and 1.61 and Abbe numbers Yd Of between 39 and 52.

Furthermore, within the composition range of the main claim, there is a group of barium flint glasses with particularly balanced composition, whose optical data nd, Yd vary i n a narrow range and which have very good chemical stabilities and very good meltability and processability. Their composition range (in % by weight based on oxide) is as follows:

Si02 43 - 57; B203 7 11; BaO 11 - 26; CaO 0.1 - 0.5; ZnO 0 - 1.5; Ti02 7.5 11; A1203 0.8 - 3; Na20 3.5 - 7; K20 3.5 - 7.5; with Na20 + K20 5 14; These glasses have refractive indices na of between 1.57 and 1.61 and Abbe numbers Vd of between 43 and 48.

In order to improve the quality of the glass, one or more refining agents, known per se, may be added to the mixture in the usual amounts in order to refine the glass. The glass will then have particularly good internal glass quality in terms of being tree of bubbles and veins..

If As203 is not used as the refining agent, but for examp le Sb203 is used instead of it, which can be done without losses in terms of the quality of the glass, then the lead-free glass according to the invention will also be free of arsenic.

In a further preferred embodiment of the invention, the glass contains the following refining agents: at least 0.05% by weight of Sb203+ F- with from 0 to 0.5% by weight of Sb203 and from 0 to 0.1% by weight of F- by means of which an excellent internal glass quality is achieved. F- is, for example, added in the form of KF.

The glasses according to the invention have a positive anomalous to normal partial dispersion in the blue range of the spectrum.

Explanation:

The capacity for partial dispersion, the so-called relative partial dispersion, in the blue part of the spectrum is given by the expression n,, -nF nF- n, The "normal line" corresponds by definition in the blue range of the spectrum to the equation Pg. F = 0. 6438 - 0. 001682 Vd.

is Glasses whose partial dispersion lies on this line are referred to as "normal glasses".

In the case of glasses which have a dispersive 20 behaviour that differs from "normal glasses", the ordinal dif f erence A P9.F by which the relevant P9.F - Vd point is shifted from the "normal line", is specified:

It is customary to categorize the glasses roughly in two groups. depending on whether P9. F is "above" (positive anomalous partial dispersion: A P9.F = POS.) or "below" (negative anomalous partial dispersion: A P9.F = neg.) the "normal line".

Glasses with anomalous partial dispersions are important for remedying residual colour effects, the so-called secondary spectrum.

The glass composition range according to the invention provides a further group of lead-free optical glasses having the aforementioned optical properties. With these glasses, the disadvantages of the prior art are - 9 overcome, without it being necessary to accept other disadvantages.

The glasses have high chemical stability and high crystallization stability. The latter point makes it possible to produce the glasses in fairly large melting systems, for example in an optical melting area. The glasses are thus easy to melt and process and can be produced economically.

The fact that the glasses are free of PbO is not only important because of the environmental protection considerations which have been mentioned, but also has a positive effect since the density of the glasses is reduced and the transformation temperature of the glasses is increased. A further advantage is that the glasses are also free of AS203 in a preferred embodiment.

Examples:

Eight examples of glasses according to the invention in the preferred composition range were melted from the usual raw materials.

Table 2 gives the respective composition (in % by weight based on oxide), refractive index rid, Abbe number vd, partial dispersion in the blue range of the spectrum Pg.p and anomaly of this partial dispersion A Pg. r, density p[g/CM3), coefficient of thermal expansion CL201300 [10- 6/K] and transformation temperature Tg[C] of the glasses.

For examples 6 to 8, further glass properties were determined, these being given in Table 3 and, inter alia, documenting the high crystallization stability (one measure of this is the liquidus temperature of the crystals) and the good chemical stability: the glasses belong to acid resistance class 1, specifically the acid resistance is 1.0. They also belong to alkali resistance class 1. In this case, the alkali resistance improves with decreasing weight ratio B203/(SiO2 + Ti02 + A1203) from an acid resistance of 1.3 at a ratio of 0.19 to an acid resistance of 1.0 at 0.12.

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 Sb203 was added, and the constituents were then thoroughly mixed. The glass mixture was melted at about 1340 - 13900C in a continuous melting system, then refined and thoroughly homogenized. The casting temperature was 1180 - 12300C.

is Table 1 shows an example of a melt.

Table 1 Example of a melt for glass calculated to 100 kg Oxide % by weight Raw material Weighed amount [kg] Si02 47.0 Si02 47.02 B203 6.9 H3B03 12.26 BaO 28.0 BaC03 36.58 Ti02 7.0 Ti02 7.04 ZnO 2.0 ZnO 2.00 CaO 1.0 CaC03 1.77 A1203 1.0 Al(OW3 1.51 K20 3.5 K2C03 4.41 KN03 1.07 Na20 3.3 NaHC03 8.80 Sb203 0.3 Sb203 0.31 The properties of the glass obtained in this way are given in Table 2, Example 3.

Table 2 Glass compositions (in % by weight based on oxide) and essential properties of the 5 glasses B203 BaO CaO ZnO A1203 Ti02 Na20 K:O Sb203 nd Vd Pg. F Apg. F p[g/CM3] CL20/300 (10-6/K1 Tg[OCI 1 57.0 7.0 10.0 0.1 1.0 12.0 6.1 6.5 0.3 1.59106 43.11 0.5741 0.0028 2.74 7.81 586 2 58.5 8.0 10.0 0.1 1.5 7.0 7.1 7.5 0.3 1.56290 50.29 0.5596 0.0004 2.71 8.30 576 3 47.0 6.9 28.0 1.0 2.0 1.0 7.0 3.3 3.5 0.3 1.60887 47.82 0.5643 0.0010 3.15 7.94 607 4 42.0 6.9 28.0 1.0 2.0 1.0 12.0 3.3 3.5 0.3 1.64668 40.94 0.5781 0.0032 3.23 8.11 610 42.0 6.9 35.0 1.0 2.0 1.0 5.0 3.3 3.5 0.3 1.61464 49.91 0.5600 0.0002 3.32 8.80 593 6 55.7 7.6 11.6 0.3 1.3 1.0 9.0 6.5 6.9 0.3 1.58283 46.65 0.5663 0.0010 2.78 8.30 568 7 50.1 8.5 16.4 o.3 1.7 10.7 6.2 5.8 0.3 1.60585 43.69 0.5731 0.0028 2.88 8.32 577 8 44.6 10.7 25.6 0.1 2.8 8.1 4.0 3.8 0.3 1.60822 46.50 0.5678 0.0022 3.04 7.82 594 Table 3 Further properties of the glasses No. 6 to No. 8 6 7 8 Acid 1.0 1.0 1.0 resistance Alkali 1.0 1.2 1.3 resistance T (Liquidus) 840/1.47105 995/1463 1025/800 min [OC/dPas] rj400 run/25 mm 0.887 0.84 0.794 In this table:

Acid resistance:

Alkali resistance:

T (Liquidus) 60 min:

Ti400run/25 =n:

According to ISO 8424 According to ISO draft 10629 Liquidus temperature of the most meltable phase after a measurement time of 60 min and associated viscosity pure spectral transmittance at the wavelength 1 = 400 = and a sample thickness d = 25.

MM 1 13- CLAIMS 1. lead-free optical barium flint glass having a refractive index nd Of between 1.55 and 1.66 and an Abbe numberWof between 39 and 52. characterized by the following composition (in % by weight based on oxide):

si02 B203 BaO CaO ZnO Ti02 A1203 Na20 K20 40-60 6.5-12 10-35 0-1 0-2 5-12 03-3.5 3-7.5 3-8 with Na20 + K20:! 15, and optionally at least one refining agent.
2. Barium flint glass according to Claim 1, having a refractive index nd Of between 1.60 and 1.66, characterized by the following composition (in % by weight based on oxide):

Si02 41-48 3203 65-8 BaO 28-35 CaO 05-1 ZnO 1-2 Ti02 5-12 A1203 Na20 0.5-1.5 3-5 K20 3-5 with Na20+ K20 65-8, and optionally at least one refining agent.
3. Barium flint glass according to Claim 1, having a refractive index nd Of between 1.56 and 1.61, characterized by the following composition (in % by weight based on oxide):

Si02 46-59 B203 BaO CaO ZnO Ti02 A1203 Na20 K20 6.5-8.5 10-28 0.1 -1 0-2 6.5-12 03-2 3-7.5 3-7.5 with Na20 + K20 2: 6.5, and optionally at least one refining agent.
4. Barium flint glass according to Claim 1, having a refractive index nd Of between 1.57 and 1.61 and an Abbe number W of between 43 and 48, characterized by the following composition (in % by weight based on oxide): Si02 B203 43-57 7-11 -is- BaO Cao ZnO Ti02 A1203 11-26 0.1-0.5 0-1.5 75-11 0-8-3 Na20 3-5-7 K20 15-7.5 with Na20 + K20:5 140 and optionally at least one refining agent.
5. Barium flint glass according to at least one of Claims 1 to 3, characterized in that the sum of Sid2, TiO2, and A1203 is between 47 and 72% by weight.
6. Barium flint glass according to at least one of Claims 1, 4 and 5, characterized in that the weight ratio B203 SiO2+ TiO2+AI203 is at most 0.20.
7. Barium flint glass according to at least one of Claims 1 to 6, characterized in that, as refining agent(s), it contains (in % by weight):

Sb203 0-0.5 F- 0-0.1 with F- + Sb203.10.05 1
8. Barium flint glass according to at least one of Claims 1 to 7, characterized in that, apart from unavoidable impurities, it is free of arsenic oxide.