GB2265367A - Highly refractive ophthalmic and optical glass - Google Patents

Description

1 2265367 Hiqhly refractive, o-phthalmic and optical qlass The invention

relates to highly refractive glasses, particularly for the production of lenses for optical and/or ophthalmic purposes.

The optical effect of glass lenses, for example, for spectacles, is determined essentially by the radii of curvature of the surfaces forming the boundary of the glass lenses, and by the refractive index of the glass. As the dioptre number increases (from approx. 4 dpt upwards), the edge and middle zones of the lenses must be made relatively thick so that their weight may become unpleasant for the spectacle wearer. Remedy is provided, for example, by the development of glasses with a low density and a relatively high refractive index (also known as index of refraction (nd)), which permit the production of thinner and lighter glasses with an equally high focal power. Glasses with a refractive index (nd) of > 1.77 are particularly suitable and the industrial standard for said purposes.

Light glasses with a high refractive index (nd) often do not have a sufficiently high Abbe coefficient (0d), however. An Abbe coefficient that is as high as possible plays a particularly advantageous part in removing image defects in lenses or optical systems. Particularly for lenses of spectacles, it is advantageous to use glasses with Abbe coefficients of > 34.

A whole series of glasses have now become known for use for ophthalmic or optical purposes, which meet wholly or partially the requirements in respect of refractive index (nd) and Abbe coefficient (75d) 2 JP-A 83-229 949 describes glasses made of the S'02 (B203) CaO-Nb20. system, which have a refractive index (nd) oE 1.77 to 1.81. Although the glasses according to JP-A 83229 949 thus exhibit extremely well a refractive index preferred for spectacle lenses, the chemical stability of the glasses according to said JP-A leaves much to be desired. This is attributable, inter alia, to the very high proportions of B20. and the comparatively small proportions of T'02 in the glass composition.

In JP-A 89-7012, glasses are described for spectacle lenses likewise made of the S'02(B203)-CaO-Nb2O, system, which have a refractive index (nd) of between 1.790 and 1.813 and an Abbe coefficient of between 32.2 and 37.0, but where the predominant number of glasses melted by way of example according to said document has an Abbe coefficient (Od) of < 34. The glasses according to JP-A 89-7012 are relatively rich in Cao and/or MgO (sum of CaO + MgO = 16 to 420-. by wt.) and free from CS20, whilst they contain relatively little La20, (up to a maximum of 14s by wt.). In the illustrative glasses according to said document, only a maximum of 10-. by wt. La203 is melted at the same time. As a result, however, it becomes difficult to achieve in all cases the high Abbe coefficients required. Moreover, the glasses of the JP-A in question frequently contain a particularly high Nb2o, content (between 1 and 350-. by wt., but according to the examples also often between 28 and 339.- by wt.), where said content may be suitable for helping to achieve the high refractive indices required and desired but is not sufficient to produce the required Abbe coefficients. This is also caused in particular by the very high potential T'02 contents in the glasses of JP-A 89-7012 of up to 19'1 by wt., increasing T'02 contents markedly reducing the Abbe coefficient of the glass. Likewise, the comparatively high proportions of T'02 and Nb2o, lead to a relatively low devitrification stability of the 3 glasses, i.e., the glasses of JP-A- 89-7012 are more susceptible to crystallisation than comparable glasses with lower T'02 and Nb2C). contents.

The remarks of the preceding paragraph also apply essentially to the glasses disclosed in DE-Patent 34 20 306, where the glasses described have an MgO + CaO content of more than 20.40-h by wt., an La203 proportion of a maximum of 6.8-1; by wt. and are likewise free from CS20.

Glasses with a refractive index (nd) of between 1.62 and 1.85 and an Abbe coefficient (19d) of 35 to 65 are described in JP-A 85-221 338. In the cited glasses made of the B203-S'02 system, extremely high proportions of '203 of up to 5006 by wt. are possible, but a minimum quantity of l-. by wt. must always be contained in the glass composition. Moreover, in the glasses of JP-A 85-221 338, the oxides are substituted in large quantities, in some cases, by fluorides, so that relatively high fluoride proportions of up to 20% by wt. result. Moreover, the glasses concerned must necessarily always have a minimum Y203 content, resulting in comparatively high prices for the glass batch. On the whole, however, according to JP-A 85-221 338, a large number of glass batches can be produced on the basis of the claimed composition ranges so that, as arises merely on account of the large range of properties, for example, very many of the glasses that can be obtained have a refractive index that is too small and no longer satisfies industrial requirements.

Glasses for ophthalmic and/or optical purposes are also known from EP-A 227 269, which glasses have a refractive index (nd) of between 1.78 and 1. 82, an Abbe coefficient (6d) of > 31 and originate from the S'02-,3203CaO-Nb2o.La20. system. Glasses of EP-A 227 269 have alkaline earth oxide contents of between 16 and 33; by wt. These 4 relatively high alkaline earth oxide contents, lead, however, to relatively poor devitrification behaviour in glasses of the type described. In addition, the high contents of divalent glass components may hinder the diffusion of alkali oxides, as is known for the potassium/sodium exchange (Rauschenbach, Richter in 11Silikattechnik11, 33 (1982), pages 70 -72). The glasses of EP-A 227 269 also have a chemical resistance which is still in need of improvement. The chemical resistance which, according to said document, is determined by the weight loss of a sample after 10 minutes' contact with 10% HCl, is up to 13 mg per dM2 in the case of the illustrative glasses. Stringent requirements, customary nowadays, in respect of the chemical resistance of the glasses of < 2 mg per drr under considerably more aggressive test conditions are fulfilled either inadequately by the glasses or not at all.

In view of the disadvantages of the glasses known from the prior art, the object of the invention is to provide glasses with refractive indices (nd) of > 1.77 and Abbe coefficients (0d) of > 34, where the glasses should at the same time have good resistance to crystallisation and improved resistance to chemical attacks. In addition, there should be the possibility of hardening the glass chemically and the batch price should also be relatively favourable.

According to the present invention, there is provided a highly refractive ophthalmic and optical glass with a refractive index (nd) of more than 1. 77, an Abbe coefficient (15d) of more than 34, with good chemical resistance and very good crystallisation resistance, having the following composition in % by wt.:

S:102 Ge02 2' 'S102+Ge02 B203 P205 L'20 Na20 K20 CS20 7-M20 Mgo CaO Sro BaO ZnO PbO EMO A120:3 La203 B'203 Gd203 1'M203 T'02 zr02 Ta20. Nb205 W03 F S03 7 35 0 7 0 3 0 0 0 - 5 0.1 - 5 0.1 - 5 0 - 5 5 is 0 - 7 0 - 7 0 7 0 5 is 0 5 25 0 2 0 5 25 13 0 10 0 5 25 0 5 0 3 0 1 and, if necessary, one or more refining agents, where M20 represents the alkali metal oxides, MO represents the alkaline earth metal oxides, Pbo and ZnO, and M203 represents La20., Gd203 and B'203.

6 Preferably, the glass has the following composition in % by wt.:

S'02 13203 P205 L120 EK20+Na20 CS20 EL'20+K.O+CS20+Na20 Mgo CaO SrO ZnO MgO+CaO+ZnO+SrO A1203 La203 T'02 Zr02 Nb205 F S03 26 2 - 5 1 0 1.8 0 31 5 1 0.5 - 2 2.3 0 2 8 is 0 3 0 6 8 15 0 1 17 24 10 2 7 14 20 0 1.5 0 1 and, if necessary, one or more refining agents.

The glasses according to the invention are characterised in particular by fulfilling the nowadays customary requirements in respect of the refractive indices of > 1.77, by sufficiently high Abbe coefficients of > 34 (corresponding to low dispersion), by good resistance to crystallisation, excellent resistance to chemical attacks in the generally preferred presence of CS20 and good chemical hardenability, for example, in the ion exchange processes.

Preferably, the glass has a weight loss (removal) of less than 2 mg/dm2 in the combined acid/base test.

7 The glass system in the glass according to the invention is based on the components S'02, alkali oxides, Cao, La20. and T'02, where S'02 acts as network former. The other components listed act as glass-changing components, as a result of whose incorporation modified bonding conditions and groupings are produced in the network structure, which in turn result in corresponding changes in the physical and chemical properties of the glasses.

According to the invention, it is, in principle, possible to use only S'02 in a quantity of between 20 and 35'k by wt. as network former. A quantity of up to 7-0. by wt. of S'02 may be replaced within the framework of the invention by the homologue Geo, it being preferred, however, to dispense with the substitution by the related Ge02 since the substitution represents a marked increase in the cost of the glass.

Moreover, in the case of highly refractive ophthalmic and optical glass of the invention, P20, in a quantity of up to 31 by wt. and/or B203 in a quantity of up to 7; by wt. may also be suitable as network former. Of the two alternative network formers, however, B.03 is preferred since P20, in relatively high concentrations may easily lead to unwanted opaqueness of the glasses. It is advisable, therefore, if P20, is not dispensed with entirely, to use said network former only up to a quantity of one percent by weight. In contrast, in a preferred embodiment of the glass, an essential content of at least 2; by wt. of B203 is desired because the melting behaviour is improved by low B203 contents.

Advantageously, however, a proportion of 50-. by wt. Of B203 should not be exceeded because otherwise there is a risk that the desired chemical resistances will no longer be fulfilled.

8 The M20 (the alkali metal oxides) are contained in the glass of the invention in a quantity of between 0.1 and 5% by wt., inter alia in order to reduce the melting points to a more favourable range. Surprisingly, it has emerged that, in the glass system according to the invention, CS20 displays the best effect, which is why it is essential.

In a preferred embodiment, the alkali metal oxides Li.0, K20 and CS20 are contained in a total quantity of 2.3 to 55% by wt. in the glass batch. If the total quantity of 2.30k. by wt. is not reached, the desired melting point can be adjusted only with added difficulty, but if the total quantity of 5'-. by wt. is exceeded, there is again the risk that the chemical resistance will leave much to be desired and the optical properties may be reached only with added difficulty.

Apart from the essential CS20. of which the preferred proportion is between 0.5 and 2-. by wt., as a result of which a reduction in the melting point is obtained without increasing the density of the glass too greatly and allowing the prices of the glass batch to rise too greatly, of the other alkali metal oxides the use of L'20 in a quantity of between 1.8 and 50-. by wt. is preferred. L'20 is preferred to Na20 or K20 because the other alkali metal oxides would increase the density of the ophthalmic glass too greatly and a proportion of Li.0 is advantageous for chemical hardening. An insufficient weight saving for various applications would result from an increase in density. In view of the above remarks, it is particularly preferred to limit the maximum content of Na20/K20 to a maximum of i'-. by wt., and in particular Na20 should be dispensed with altogether.

The divalent metal oxides MgO, CaO, SrO, BaO, ZnO and PbO are contained in the highly refractive glass of the 9 invention in a total quantity of at least 5% by wt. in order to increase the chemical resistance of the glass. The upper limit for the proportion of the divalent metal oxides listed is 15-h by wt. because there is a risk with increased use, particularly of alkaline earth metal oxides, that the stability of the glass will be reduced because the susceptibility to devitrification increases.

Amongst the divalent metal oxides in question, the use of CaO is essential for improving the chemical resistance of the glass, proportions of 8 to 150-h by wt. being preferred.

Up to 5'-. by wt. of CaO may be exchanged for MgO, but the susceptibility of the glass to devitrification increases greatly. Similarly, replacing up to 7% by wt. of CaO by SrO, BaO or ZnO is possible, or also up to 5% by wt. of CaO by PbO, where only up to a maximum of 10% by wt. of Cao may be exchanged for mixtures of the other divalent oxides. Moreover, each of the divalent oxides may participate only up to its individual upper limit in the mixture intended for the exchange. The potential replacement according to the invention of CaO by SrO and/or BaO adversely affects the density of the glass, however, so that the upper limits according to the invention result. A similar situation applies to the glass components ZnO and PbO, for which Cao may be exchanged in the above-mentioned ranges.

In a preferred glass composition, the use of PbO and BaO is therefore dispensed with altogether, and the divalent oxides MgO, SrO and ZnO are used up to a maximum of 2% by wt., 3-05 by wt., and 6% by wt. respectively, where a total of up to 7% by wt. of CaO may be exchanged for a mixture of the three oxides, in which each of the three oxides may be present up to its individual upper limit.

A proportion of 15 to 25% by wt. of La20. is essential for the ophthalmic glasses according to the invention. If this range is not reached or exceeded, the high refractive indices of > 1.77 and the required Abbe coefficients of > 34 cannot be achieved. In the same way as the essential constituent CaO, La203 may also be exchanged within certain limits for glass components with a similar effect such as B'203 or Gd20, On the whole, corresponding weight proportions of La203 may be replaced by up to 2% by wt. of B'203 and/or 5 by wt. of Gd20.. In the overall exchange, however, it should be borne in mind that such an exchange has an adverse effect on the price of the glass. In addition, the use of D'203 may lead to an undesirable yellow colour of the glasses, for which reason the upper limit of exchange for B'203 is also to be set relatively low at only 2-. by wt.

In a preferred embodiment, B'203 and Gd203 are dispensed with altogether and exclusively La203 is used in a quantity of between 17 and 24% by wt.

Two other essential components for the glass according to the invention are Ti02 and Nb20. In the stated ranges of between 5 and 130-. by wt. (T'02) and 10 to 25% by wt. (Nb205), both play an important part in the required increase in the refractive index.

Other likewise refractive index-increasing components which may be added to the glass according to the invention, if required, are Zr02 (up to 1OPj by wt.), and TaA, and WO:, (up to 5% by wt. in each case). The use of said components, particularly Zr02. is desired because T'02 not only increases the refractive index greatly but also the dispersion, so that in the case of T'02 contents of > 13% by wt., there would be a risk that the required Abbe coefficients of > 34 may not be reached.

11 In a particularly preferred glass composition, therefore, the upper limit of the T'02 content is 10% by wt., whilst Zr02 becomes an essential component which is melted at the same time in the glass batch in a proportion of between 2 and 7% by wt. In this case, the glass is preferably free from W03 or Ta20.. since the latter component adversely af f ects the price of the glass and W03 may lead to a colour cast.

Another reason for the desired simultaneous use of Zr02 and T'02 is to be seen in the fact that both components have a positive effect on the whole on the chemical properties of the glass. Zr02 proportions of up to 10% by wt. increase the resistance of the glass to alkaline attacks, whilst T'02 proportions of up to 13'-h by wt. increase the resistance to acid attack on the glass.

In the highly refractive ophthalmic and optical glass according to the invention, up to 59k by wt, preferably up to 2-. by wt, of A1203 may also be contained, as a result of which the chemical resistance of the glass may be improved. It is preferred, however, to add to the glass a maximum of up to one percent by weight of A1203 since the devitrification stability of the glass falls as the A1203 content rises, but particularly in the case of quantities above 2-. by wt.

Moreover, up to 3; by wt. of F and up to 10-h by wt. of S03 may be contained in the glass according to the invention. In a preferred embodiment, the upper limit for the fluorine content is 1.5% by wt. and the S03 content is a maximum of 1% by wt. Advantageously, an S03 proportion is dispensed with altogether. Both F and S03 may originate from additives which are used as refining agents. Contents that are higher than the upper limits listed may lead to signs of opaqueness, particularly in the case of F.

12 If necessary, one or more refining agents known according to the prior art may be added to the glass in quantities of up to one percent by weight, as are customary in the production of ophthalmic and optical glasses. Known refining agents are arsenic and antimony oxides and compounds, and halides and sulphates.

In order to produce the glass according to the invention, the raw materials (oxides, carbonates, nitrates, fluorides etc.) are mixed thoroughly, melted at 1,400 OC, refined and homogenised. The glasses are poured into preheated moulds at approx. 1,300 OC and cooled.

The following table contains five glasses melted by way of example. In the table, all the proportions of the glass constituents are given in weight percent. In example 5, oxides were replaced partially by fluorides. F2-0 in this example represents the proportion of oxygen atoms that were replaced by fluorine. By stating F2-0. is possible to standardise the examples to 100% if fluorine is used. In the concrete case, the conversion of the fluorine proportion to F2-0 'S carried out by means of the following ratio:

F2-0: F2 = 22: 38.

In the case of example 5, therefore, this leads to a proportion of 0.5996 by wt. of oxygen atoms replaced by fluorine atoms. The line F therefore gives only the proportion of fluorine that corresponds to F2-0. whilst the contents of all the components in one column up to and including line F.-0 give the sum of 100%.

The measurements of the refractive index (nd) and the Abbe coefficient (15d) were carried out in accordance with the customary methods. In order to determine (nd) I the yellow line beo S87 nm of He was used.

13 The chemical resistance (given in the table as removal in mg per dm') is determined by the weight loss (removal in mg per dm') by means of a combined acid/base test. To this end, the glass to be examined is treated for 15 minutes in 50% by wt. of KOH at 90 OC, then treated for 15 minutes in 0.05 N HN03 at 60 OC and for another IS minutes in 0.05 N HNO, at ambient temperature. The glass is rinsed, dried and the weight loss obtained by the treatment is determined in mg/dm'.

It can be derived from the examples from the table that while fulfilling the required values for the refractive index and the Abbe coefficient, the glasses according to the invention display excellent chemical resistance in the combined acid/base test, which is evident from the removal of less than 2 mg/dm' or, in the case of two examples (no. I and no. 3), even less than 1.0 mg/dM2.

14 Table (Details in -. by wt.) 1 2 3 4 5 S'02 29.21 28.83 28.23 26.10 28.86 B203 2.30 2.40 2.50 3.80 2.29 P205 - - - 1.00 - A1203 0.30 0.30 0.70 0.50 0.30 L120 4.00 4.10 4.00 2.00 3.98 K20 0.05 - - 0.50 - CS20 0.50 0.50 0.50 2.00 0.50 Mgo 0.20 0.50 1.50 2.00 0.20 CaO 14.01 14.01 13.01 10.00 13.93 ZnO - - - - - SrO - - - 3.00 - La203 21.11 21.22 21.22 17.80 21.00 T'02 8.80 8.61 8.71 8.00 8.86 Zr02 5.30 5.31 5.21 7.00 5.27 Nb205 14.21 14.21 14.41 16.30 14.23 F2-0 - - - - 0.59 F 1.02 F, MO 14.21 14.51 14.51 15.00 14.13 E M20 4.55 4.60 4.50 4.50 4.48 nd 1.7936 1.7936 1.7971 1.7882 1.785 vd 34.95 35.19 35.0 36.8 35.2 removal <1.0 1.3 <1.0 1.9 1.3 [mg/dm21 is

Claims (5)

1. Claims

   A highly refractive ophthalmic and optical glass with a refractive index (nd) of more than 1. 77, an Abbe coefficient (0d) of more than 34, with good chemical resistance and very good crystallisation resistance, having the following composition in % by wt.:

   S'02 Ge02 2S'02+Ge02 B203 P205 L'20 Na2C) K20 CS20 2' 'MP Mgo CaO SrO BaO ZnO PbO EMO A1203 La203 B'203 Gd203 r-M203 T'02 Zr02 Ta20. Nb205 W03 35 0 7 35 0 7 0 3 0 0 0 0.1 - 0.1 - 0 - - 5 5 is 7 7 0 0 0 7 0 5 15 0 5 25 0
2. 2 0 5 - 25 5 - 13 0 - 10 0 5 25 0 5 16 F S03 0 - 3 0 - 1 and, if necessary, one or more refining agents, where M20 represents the alkali metal oxides, MO represents the alkaline earth metal oxides, Pbo and ZnO, and M203 represents La203, Gd.03 and B'20 2. A glass according to claim 1, having the following composition in % by wt.

   S'02 B203 P205 L'20 EK20+Na20 CS20 EL'20+K20+CS20+Na20 Mgo CaO SrO ZnO MgO+CaO+ZnO+SrO A1203 La203 T'02 Zr02 Nb205 F S03 26 2 0 1.8 0 0.5 - 2 2.3 - 5 0 8 0 2 14 36 5 1 5 1 0 6 8 15 0 1 17 24 5 10 7 20 0 1.5 0 1 and, if necessary, one or more refining agents.
3. 3. A glass according to claim 1 or 2, having a weight loss (removal) of less than 2 mg/dm in the combined acid/base test.

   Q 17
4. 4. A glass according to claim 1, substantially as hereinbefore described in any one of Examples 1 to
5. 5.