WO2005102949A2

WO2005102949A2 - Optical glass

Info

Publication number: WO2005102949A2
Application number: PCT/JP2005/008095
Authority: WO
Inventors: Susumu Uehara
Original assignee: Kabushiki Kaisha Ohara
Priority date: 2004-04-26
Filing date: 2005-04-21
Publication date: 2005-11-03
Also published as: WO2005102949A3

Abstract

The optical glass of the present invention has optical constants of a refractive index (nd) within a range from 1.74 to 1.80 and an Abbe number (ν d) within a range from 47 to 51, contains, as essential components, SiO2, B2O3. La2O3, Gd2O3, ZrO2, Ta2O5, ZnO and Li2O, is substantially free of Pb, As and F, and contains ZrO2 in an amount of 6 mass % or less. The optical glass has ratio of ZnO/(ZrO2 + Ta2O5) within a range from 0.45 to 1.5 and glass transition temperature (Tg) within a range from 530 °C to 630 °C.

Description

DESCRIPTION

Optical Glass

Technical Field

This invention relates to an optical glass and, more particularly, to an optical glass having a low glass transition temperature (Tg) and high refractive index and low dispersion characteristics, and excellent chemical durability, particularly phosphate resistance, and being suitable for precision press molding.

Prior Art

There are spherical lenses and aspherical lenses as lenses used for constituting an optical system. Many spherical lenses are produced by lapping and polishing glass pressings obtained by reheat press molding glass materials. On the other hand, aspherical lenses are mainly produced by precision press molding, i.e., the method according to which lens preforms which have been softened by heating are press molded with a mold having a high precision molding surface and the shape of the high precision molding surface of the mold is transferred to the lens preforms.

In obtaining glass moldings such as aspherical lenses by precision press molding, it is necessary to press lens preforms which have been softened by heating in a high temperature environment for transferring the shape of the high precision molding surface of the mold to the lens preforms and, therefore, the mold used for such precision press molding is subjected to a high temperature and, moreover, a high pressing force is applied to the mold. Hence, in heating and softening the lens preforms and press molding the lens preforms, the molding surface of the mold tends to be oxidized or eroded, or a release film provided on the molding surface tends to be damaged with the result that the high precision molding surface of the mold cannot be maintained or the mold itself tends to be damaged. In such a case, the mold must be replaced and, as a result, frequency of replacement of the mold increases and production of products at a low cost in a large^' scale thereby cannot be achieved. Accordingly, glasses used for precision press molding are desired to have the lowest possible glass transition temperature (Tg) from the standpoint of preventing such damage to the mold, maintaining the high precision molding surface of the mold for a long period of time and enabling precision press molding at a low pressing force.

In conducting precision press molding, the glass of a lens preform needs to have a mirror surface or a surface close to a mirror surface. A lens preform generally is either produced directly from molten glass by the dripping method or produced by lapping and polishing glass pieces. The dripping method is more generally employed in view of advantages in the cost and number of processing steps. The lens preform produced by the dripping method is called gob or glass gob. It is necessary for such gob or glass gob to remove dust and dirt from the surface thereof by a cleaning process before it is subjected to precision press molding. Optical glasses used for precision press molding, however, have drawbacks that their chemical durability is generally so poor that the surface of the lens preforms made of these optical glasses is tarnished during some processing such as cleaning with resulting difficulty in maintaining a mirror surface or a surface which is close to a mirror surface. This particularly poses a problem in the cleaning process and phosphate resistance in chemical durability is an important property required for optical glasses used for precision press molding.

For these reasons, from the point of view of utility for optical design, there has been a strong demand for an optical glass having high refractive index and low dispersion characteristics, a low glass transition temperature (Tg) and excellent phosphate resistance. There has particularly been a strong demand for a high refractive index and low dispersion optical glass having optical constants of refractive index (nd) within a range from 1.74 to 1.80 and Abbe number ( d) within a range from 47 to 51.

Since a high refractive index and low dispersion optical glass is very useful in the optical design, various glasses of this type have for a long time been proposed.

Japanese Patent Application Laid-open Publications No. 2002-249337 and No. 2003-201143 disclose optical glasses having a low glass transition temperature (Tg). Optical glasses among specifically disclosed optical glasses of these publications having optical constants of the above described ranges, however, have a ratio in mol % of ZnO/(ZrO₂ + Ta2θs) which is outside of a range from 0.9 to 2.5 and, for this reason, have an insufficient phosphate resistance. Japanese Patent Application Laid-open Publications No. Hei

8-259257, No.2002- 12443 and Sho 60-221338 disclose optical glasses having a low glass transition temperature (Tg). Optical glasses among specifically disclosed optical glasses of these publications having optical constants of the above described ranges, however, have a ratio in mass % of ZnO/(Zrθ2 + Ta2θs) which is outside of a range from 0.45 to 1.5 and, for this reason, have an insufficient phosphate resistance.

Japanese Patent Application Laid-open Publication No. Hei 8-217484 contains LU2O3 which is very expensive as an essential component and, therefore, increases the cost of manufacture to a significantly high degree and hence this optical glass is impractical. Besides, optical glasses among specifically disclosed optical glasses of this publication having optical constants of the above described ranges do not contain alkali and ZnO components which are effective for producing a glass of a low glass transition temperature (Tg) and, accordingly, this optical glass of this publication has the drawback that its glass transition temperature (Tg) is high.

Japanese Patent Application Laid-open Publication No. Sho 55-3329 discloses an optical glass which contains, as an essential component, Snθ2 which becomes a metal tin when the melting atmosphere becomes a reducing state and erodes the melting apparatus by alloying with platinum used in the melting apparatus resulting in occurrence of leakage of glass and, therefore, is impractical. Besides, optical glasses among specifically disclosed optical glasses of this publication having optical constants of the above described ranges do not contain, or contain only a little amount of alkali and ZnO components which are effective for producing a glass of a low glass transition temperature (Tg) and, accordingly, this optical glass has the drawback that its glass transition temperature (Tg) is high.

Japanese Patent Application Laid-open Publications No. Hei 8-26765, No.2000- 16831, No. 2003-252647, No. Hei 6-305769 and No. 2003-267748 disclose optical glasses having a low glass transition temperature (Tg). Optical glasses specifically disclosed in these publications, however, do not have optical constants of the above described ranges and therefore fail to satisfy the above described requirement for the optical design.

Optical glasses which are specifically disclosed in Japanese Patent Application Laid-open Publications No. Sho 59- 195553, Sho 56- 160340, Sho 52- 155615, Sho 52- 14607, 2002- 128539 and Sho 53-4023 have a high glass transition temperature (Tg) and therefore are not suitable for precision press molding.

It is, therefore, an object of the present invention to provide an optical glass which has comprehensively eliminated the above described drawbacks of the prior art optical glasses and has the above described optical constants, a low glass transition temperature (Tg) and an excellent phosphate resistance and therefore is suitable for precision press molding.

Disclosure of the Invention For achieving the above described object of the invention, in the first aspect of the invention, there is provided an optical glass having optical constants of a refractive index (nd) of 1.74 or over and an Abbe number ( d) of 47 or over, being substantially free of Pb, As and F, comprising rθ2 in an amount of 6 mass % or less, having a ratio of ZnO/(Zrθ2 + Ta₂Oδ) within a range from 0.45 to 1.5, and having phosphate resistance (ISO9689) of Class 3, 2 or 1.

In the second aspect of the invention, there is provided an optical glass as defined in the first aspect having a glass transition temperature (Tg) of 630°C or below.

In the third aspect of the invention, there is provided an optical glass as defined in the first or second aspect wherein logarithm log η of viscosity (dPa ^• s) in liquidus temperature is within a range from 0.4 to 2.0.

In the fourth aspect of the invention, there is provided an optical glass having optical constants of a refractive index (nd) within a range from 1.74 to 1.80 and an Abbe number ( v d) within a range from 47 to 51, comprising SiO₂, B2O3, La2θ3, Gd2θ3, Zrθ2, Ta2θδ, ZnO and Li2θ as essential components, being substantially free of Pb, As and F, comprising Zrθ2 in an amount of 6 mass % or less, having a ratio of ZnO/(Zrθ2 + Ta2θβ) within a range from 0.45 - 1.5 and having a glass transition temperature (Tg) within a range from 530°C to 630°C .

In the fifth aspect of the invention, there is provided an optical glass comprising, in mass %, SiO₂ 4.5 - less than 6%

Gd2θ3 more than 25% and 35% or less ZrO₂ 0.5 - 6% Ta₂O₅ 0.5 - 10% ZnO 0.5 - 15% and Li₂O 0.1 - 2.5%, and

Yb₂O₃ 0 - 10% and/or GeO₂ 0 - 5% and/or TiO₂ 0 - 5% and/or Nb₂O₅ 0 - 5% and/or WO 3 0 - 5% and/or RO 0 - 10% where RO is one or more oxides selected from a group consisting of CaO, SrO and BaO and/or Sb₂O₃ 0 - 1%.

In the sixth aspect of the invention, there is provided an optical glass as defined in the fifth aspect having a ratio in mass % of ZnO/(ZrO₂ + Ta₂O₅) within a range from 0.45 to 0.98.

In the seventh aspect of the invention there is provided an optical glass as defined in any of the first to sixth aspects having a ratio in mass % of Siθ2/B₂θ3 of 0.18 or over.

In the eighth aspect of the invention, there is provided an optical glass as defined in any of the first to seventh aspects having a total amount of Siθ2 + B2O3 within a range from 23 mass % to 35 mass %.

In the ninth aspect of the invention, there is provided an optical glass as defined in any of the first to eighth aspects having a total amount of Y2O3 + La₂O3 + Gd2θ3 + Yb3θ3 within a range from 48 mass % to 58 mass %.

In the tenth aspect of the invention, there is provided an optical glass having optical constants of a refractive index (nd) within a range from 1.74 to 1.80 and an Abbe number ( v d) within a range from 47 to 51, comprising SiO₂, B2O3, La2θ3, Gd₂O3, Zrθ2, Ta2θ5, ZnO and Li₂O as essential components, being substantially free of Pb, As and F, comprising Zrθ2 in an amount of 6.5 mol % or less, having a ratio in mol % of ZnO/(Zrθ2 + Ta2θs) within a range from 0.9 - 2.5 and having a glass transition temperature (Tg) within a range from 530°C to 630°C . In the eleventh aspect of the invention, there is provided an optical glass comprising, in mol %, SiO₂ more than 10% and 13% or less B2O3 more than 40% and 60% or less La₂O₃ 5 - 15% Gd₂O₃ 8 - 15% ZrO₂ 0.2 - 6.5% Ta₂O₅ 0.1 - 5% ZnO 0.5 - 18% and

and Yb₂O₃ 0 - 5% and/or GeO₂ 0 - 3% and/or TiO₂ 0 - 3% and/or Nb₂O₅ 0 - less than 3% and/or WO 3 0 - 3% and/or RO 0 - 10% where RO is one or more oxides selected from a group consisting of CaO, SrO and BaO and/or Sb O₃ 0 - 1%.

In the twelfth aspect of the invention, there is provided an optical glass as defined in the tenth or eleventh aspect, having a ratio in mol % of ZnO/(Zrθ2 + Ta2θs) within a range from 0.9 to 2.4.

In the thirteenth aspect of the invention, there is provided an optical glass as defined in any of the tenth to twelfth aspects having a ratio in mol % of Siθ2/B2θ3 of 0.20 or over.

In the fourteenth aspect of the invention, there is provided an optical glass as defined in any of the tenth to thirteenth asapects having a total amount of Siθ2 + B2O3 of more than 50 mol % and 65 mol % or less.

In the fifteenth aspect of the invention, there is provided an optical glass as defined in any of the tenth to fourteenth aspects having a total amount of Y2O3 + La2θ3 + Gd2θ3 + YbβOβ within a range from 16 mol % to 23 mol %.

In the sixteenth aspect of the invention, there is provided a lens preform made of an optical glass as defined in any of the first to fifteenth aspects of the invention.

In the seventeenth aspect of the invention, there is provided an optical element formed of an optical glass as defined in any of the first to the fifteenth aspect of the invention.

Best Mode for Carrying Out the Invention

Description will be made about components which the optical glass of the present invention can comprise. Unless otherwise described, the composition ratio of each component will be expressed in mass %.

Siθ2 is an indispensable component which is very effective for increasing viscosity of the glass and improving resistance to devitrification and phosphate resistance of the glass. If, however, the amount of this component is less than 4.5%, these effects cannot be achieved sufficiently whereas if the amount of this component is 6% or over, the glass transition temperature (Tg) rises and the melting property of the glass is deteriorated. Therefore, the lower limit of the amount of this component should preferably be 4.5%, more preferably more than 4.5% and, most preferably be 4.7% and the upper limit of the amount of this component should be less than 6%, more preferably 5.9% and, most preferably be 5.8%. Siθ2 can be incorporated in the glass by using, e.g., Siθ2 as a raw material.

In the optical glass of the present invention which is a lanthanum glass, B2O3 is an indispensable component as a glass forming oxide. If, however, the amount of this component is less tan 20%, resistance to devitrification becomes insufficient whereas if the amount of this component exceeds 30%, phosphate resistance is deteriorated. Therefore, the lower limit of the amount of this component should preferably be 20%, more preferably more than 20% and, most preferably be 22% and the upper limit of this component should preferably be 30%, more preferably 29% and, most preferably be 28%. B2O3 can be incorporated in the glass by using, e.g., H3BO3 or B2O3 as a raw material.. .

Y2O3 is effective for increasing refractive index and lowering dispersion. If, however, the amount of this component exceeds 2%, resistance to devitrifcation is sharply deteriorated. Therefore, the upper limit of this amount should preferably be 2%, more preferably 1% and, most preferably, this component should not be added. Y2O3 can be incorporated in the glass by using, e.g., Y2O3 as a raw material.

La2U3 is an indispensable component which is effective for increasing refractive index and lowering dispersion. If, however, the amount of this component is less than 15%, it is difficult to maintain the optical constants within the above described values whereas if the amount of this component is 35% or over, resistance to devitrification is deteriorated. Therefore, the lower limit of the amount of this component should preferably be 15%, more preferably 16% and, most preferably be 18% and the upper limit of the amount of this component should preferably be less than 35%, more preferably 33% and, most preferably be 30%. La2θ3 can be incorporated in the glass by using, e.g., La2θ3, lanthanum nitrate or its hydrate as a raw material.

Gd2θ3 is an indispensable component which is very effective for increasing refractive index and lowering dispersion and also for improving resistance to devitrification by having this component coexist with La₂O3. If, however, the amount of this component is 25% or less, these effects cannot be achieved sufficiently whereas if the amount of this component exceeds 35%, resistance to devitrification decreases rather than increases. Therefore, lower limit of the amount of this component should preferably be more than 25%, more preferably 25.1 and, most preferably be 25.2% and the upper limit of the amount of this component should preferably be 35%, more preferably 34% and, most preferably be 32%. Gd2θ3 can be incorporated in the glass by using, e.g., Gd2θ3 as a raw material.

Yb2θ3 is effective for increasing refractive index and lowering dispersion. If, however, the amount of this component exceeds 10%, resistance to devitrification is deteriorated. Therefore the upper limit of the amount of this component should preferably be 10%, more preferably 9% and, most preferably be 7%. Yb2θ3 can be incorporated in the glass by using, e.g., Yb2θ3 as a raw material.

GeO₂ is effective for increasing refractive index and improving resistance to devitrification. Since, however, this component is very expensive, the upper limit of the amount of this component should preferably be 5%,- more preferably less than 2% and, most preferably, this component should not be added. GeO₂ can be incorporated in the glass by using, e.g., Geθ2 as a raw material.

Tiθ2 is effective for adjusting optical constants and improving resistance to devitrification. If, however, the amount of this component exceeds 5%, resistance to devitrification decreases rather than increases. Therefore the upper limit of the amount of this component should preferably be 5%, more preferably 1% and, most preferably be 0.5%. Tiθ2 can be incorporated in the glass by using, e.g., Tiθ2 as a raw material.

Zrθ2 is an indispensable component which is very effective for adjusting optical constants, improving resistance to devitrification and improving phosphate resistance. If, however, the amount of this component is insufficient, these effects cannot be achieved sufficiently whereas if the amount of this component is excessive, resistance to devitrification is deteriorated and it becomes difficult to maintain the glass transition temperature (Tg) at a desired low temperature. Therefore, lower limit of the amount of this component should preferably be 0.5%, more preferably 1% and, most preferably be 2% and the upper limit of the amount of this component should preferably be 6%, more preferably 5.9% and, most preferably be 5.8%. Zrθ2 can be incorporated in the glass by using, e.g., Zrθ2 as a raw material. Nb2θδ is effective for increasing refractive index and improving phosphate resistance and resistance to devitrification. If, however, the amount of this component exceeds 5%, resistance to devitrification decreases rather than increases. Therefore, the upper limit of the amount of this component should preferably be 5%, more preferably less than 1% and, most preferably, this component should not be added. Nb2θs can be incorporated in the glass by using, e.g., Nb2θs as a raw material.

Ta2θδ is an indispensable component which is very effective for increasing refractive index and, improving phosphate resistance and resistance to devitrification. If, however, the amount of this component is less than 0.5%, these effects cannot be achieved sufficiently whereas if the amount of this component exceeds 10%, it becomes difficult to maintain the above described optical constants. Therefore, lower limit of the amount of this component should preferably be 0.5%, more preferably 1% and, most preferably be 2% and the upper limit of the amount of this component should preferably be 10%, more preferably 8% and, most preferably be 6%. Ta2θs can be incorporated in the glass by using, e.g., Ta2θδ as a raw material.

WO3 is effective for adjusting optical constants and improving resistance to devitrification. If, however, the amount of this component exceeds 5%, resistance to devitrification and transmittance in the short wavelength region of the visible ray region are deteriorated. Therefore, the upper limit of the amount of this component should preferably be 5%, more preferably less than 0.5% and, most preferably, this component should not be added. WO3 can be incorporated in the glass by using, e.g., WO3 as a raw material.

ZnO is an indispensable component which is effective for lowering the glass transition temperature (Tg) . If, however, the amount of this component is less than 0.5%, this effect cannot be achieved sufficiently whereas if the amount of this component exceeds 15%, resistance to devitrification is deteriorated. Therefore, lower limit of the amount of this component should preferably be 0.5%, more preferably 1% and, most preferably be 2% and the upper limit of the amount of this component should preferably be 15%, more preferably 13% and, most preferably be 10%. ZnO can be incorporated in the glass by using, e.g., ZnO as a raw material.

RO (one or more oxides selected from the group consisting of CaO, SrO and BaO) is effective for adjusting optical constants. If, however, the amount of this component exceeds 10%, resistance to devitrification is deteriorated. Therefore, the upper limit of the amount of this component should preferably be 10%, more preferably 8% and, most preferably be 4.5%. RO can be incorporated in the glass by using, e.g., CaO, SrO or BaO, or its carbonate, nitrate or hydroxide as a raw material. More preferably, this component should not be substantially added. Li2θ is an indispensable component which is effective for lowering the glass transition temperature (Tg) substantially and facilitating melting of mixed glass materials. If, however, the amount of this component is less than 0.1%, these effects cannot be achieved sufficiently whereas if the amount of this component exceeds 2.5%, resistance to devitrification is sharply deteriorated. Therefore, the lower limit of the amount of this component should preferably be 0.1%, more preferably 0.2% and, most preferably be more than 0.5% and the upper limit of the amount of this component should preferably be 2.5%, more preferably 2.3% and, most preferably be less than 2%. Li2θ can be incorporated in the glass by using, e.g., Li2θ, Li2CO3, LiOH or LiNOβ as a raw material.

Sb2θ3 may be optionally added for defoaming during melting of the glass. If the amount of this component is excessive, transmittance in the short-wave region of the visible ray region is deteriraoted. Therefore, the upper limit of the amount of this component should preferably be 1%, more preferably 0.5% and, most preferably, be 0.2%.

The above described raw materials used in the respective components of the glass have been cited for illustrative purpose only and raw materials which can be used for the glass of the present invention are not limited to the above described oxides etc. but can be selected from known materials in accordance with various modifications of manufacturing conditions for manufacturing the glass.

The inventor of the present invention has found that, by adjusting the ratio of amounts of Siθ2 to B2O3 to a predetermined range, phosphate resistance of the glass is improved. More specifically, the ratio in mass % of Siθ2 B2θ3 should preferably be 0.16 or. over, more preferably 0.17 or over and, most preferably be 0.18 or over. In the present invention, the phosphate resistance can be significantly improved by adjusting the ratio of the amounts of SiO₂ to B2O3 as well as the ratio of ZnO/(ZrO₂ + Ta₂O₅) to predetermined ranges. The inventor of the present invention has found that an optical glass having an excellent phosphate resistance can be obtained in optical glasses having the above described optical constants by adjusting the ratio of the amount in mass % of ZnO to the total amount of Zrθ2 and Ta₂Oδ, i.e., ZnO/(ZrO₂ + Ta2θδ) to a value within a predetermined range. While these three components are effective for improving resistance to devitrification, they impart high dispersion characteristic to the glass and, accordingly, the total amount of these components is restricted for maintaining the optical constants within the above described ranges.

In the present invention, the lower limit of ZnO/(Zrθ2 + Ta2θδ) should preferably be 0.45, more preferably 0.5 and, most preferably be 0.6 and the upper limit of this ratio should preferably be 1.5, more preferably 1.10 and, most preferably be 0.98.

The inventor of the present invention has also found that an optical glass having an excellent phosphate resistance can be obtained in optical glasses having the above described optical constants by adjusting the total amount of Siθ2 and B2O3, i.e., Siθ2 + B2O3 and/or adjusting the total amount of Y2O3, La2θ3, Gd2θ3 and Yb2θ3, i.e., Y2O3 + La2θ3 + Gd2θ3 + Yb2θ3 to values in predetermined ranges. More specifically, the inventor has found that an optical glass having an excellent phosphate resistance can be obtained by adjusting Siθ2 + B2O3 to a value within a range from 23 mass % to 35 mass % and/or Y2O3 + La₂O₃ + Gd₂O₃ + Yb₂O₃ to a value within a range from 48 mass % to 58 mass %. Each of these total amounts can achieve the above described effect to some degree and, when these total amounts are both satisfied, this effect is achieved to a particularly significant degree. In the present invention, therefore, the lower limit of Siθ2 + B2O3 should preferably be 23 mass %, more preferably 25 mass % and, most preferably be 28 mass % and the upper limit of Siθ2 + B2O3 should preferably be 35 mass %, more preferably 33 mass % and, most preferably be 32 mass %. The lower limit of Y2O3 + La2θ3 + Gd2θ3 + Yb₂O3 should preferably be 48 mass %, more preferably 50 mass % and, most preferably be 51 mass % and the upper limit of Y2O3 + La2θ3 + Gd2θ₃ + Yb₂O3 should preferably be 58 mass %, more preferably 56 mass % and, most preferably be 55 mass %.

The inventor has also found that a particularly excellent phosphate resistance can be achieved by simultaneously adjusting the values of SiO₂/B2θ₃, ZnO/(ZrO₂ + Ta₂O₅), SiO₂ + B2O3 and Y2O3 + La2θ3 + Gd2θ₃ + Yb2θ3 to values within the above described ranges.

Since the term "phosphate resistance" is used in the present specification assuming a case where a lens preform, i.e., gob is cleaned before precision press molding or a case where a lens made by precision press molding is cleaned, this term designates degree of tarnished state when the glass has been exposed to a chemical or the like for a certain period of time. Phosphate resistance is measured in accordance with ISO

Testing Method "Phosphate resistance" (ISO9689:1990)E) and evaluated by PR value of phosphate resistance. More specifically, as a test specimen, a glass specimen of 30mm X 30mm X 2mm polished in its six surfaces is put, by hanging it with a platinum line in a O.Olmol/L sodium polyphosphate aqueous solution heated to 50°C and treated for a predetermined period of time (15 minutes, 1 hour, 4 hours and 16 hours). After the treatment, reduction in the mass of the specimen is weighed and time required for eroding the glass layer having thickness of 0.1 μ m is calculated in accordance with the following formula. In this calculation, a value which is obtained by minimum time required for mass reduction of at least lmg per 1 specimen is used. According to the standard of evaluation by this method, Class 1 indicates that time required for eroding the glass layer of 0.1 μ m is longer than 240 minutes, Class 2 indicates that this time is longer than 60 minutes up to 240 minutes, Class 3 indicates that this time is 60 minutes - 15 minutes and Class 4 indicates that this time is less than 15 minutes. to.i = to ^• d ^• S/((m₁ - m₂) ^• 100) to.i ^: time (minutes) required for eroding the glass layer of O. l μ m to ^: treating time (minutes) d : specific gravity S : surface area (cm²) of the specimen mi — m2 5 reduction of mass (mg) of the specimen

Phosphate resistance required for the optical glass of the present invention preferably is Class 3, more preferably Class 2 and, most preferably Class 1. AI2O3 is effective for improving phosphate resistance. If, however, the amount of this component exceeds 3%, resistance to devitrification is sharply deteriorated. Therefore, the upper limit of this component should preferably be 3%, more preferably 1% and, most preferably, this component should not be added.

The glass may comprise LU2O3, Hf2θ3, Snθ2, Ga2θ3, Bi2θ3 and BeO. Since LU2O3, Hf2θ3 and Ga₂O3 are expensive materials, use of these components increases the manufacturing cost and it is not practical to use these components in commercial production. As to Snθ2, there is likelihood that, when glass materials are melted in a platinum crucible or a melting furnace which is formed with platinum in a portion which comes into contact with molten glass, tin of Snθ2 is alloyed with platinum and heat resisting property of the alloyed portion is deteriorated with resulting making of a hole in the alloyed portion and leakage of the molten glass from the hole. Bi₂O3 and BeO have the problem that these components adversely affect the environment and therefore impose a heavy burden to the environment. Accordingly, the upper limit of the amount of each of these components should preferably be less than 0.1%, more preferably 0.05% and, most preferably these components should not be added at all.

Description will now be made about components which the optical glass of the present invention should not comprise. Fluorine causes occurrence of striae in the production of a gob for a lens preform and therefore makes it difficult to produce a gob. Fluorine therefore should not be added to the optical glass of the present invention. A lead compound not only has the problem that it tends to be fused with the mold during precision press molding, has the problem that steps must be taken for protecting the environment not only in production of the glass but also in cold processing such as polishing and waste of the glass and therefore it imposes a heavy burden to the environment. The lead compound therefore should not be added to the optical glass of the present invention.

AS2O3, cadmium and thorium adversely affect the environment and therefore impose a heavy burden to the environment. These components therefore should not be added to the optical glass of the present invention.

P2θ tends to deteriorate resistance to devitrification when it is added to the glass and, therefore, it is not preferable to add P2θ to the optical glass of the present invention.

As to Teθ2, there is likelihood that, when glass materials are melted in a platinum crucible or a melting furnace which is formed with platinum in a portion which comes into contact with molten glass, tin of Teθ2 is alloyed with platinum and heat resisting property of the alloyed portion is deteriorated with resulting making of a hole in the alloyed portion and leakage of the molten glass from the hole. Teθ2 therefore should not be added to the optical glass of the present invention.

The optical glass of the present invention should preferably not comprise coloring components such as N, Cr, Mn, Fe, Co, Νi, Cu, Mo, Eu, Νd, Sm, Tb, Dy and Er. That is to say, these coloring components should not be intentionally added except for a case where these components are mixed as impurities.

Since the glass composition of the present invention is express in mass %, it cannot be directly expressed in mol %. A composition expressed in mol % of respective oxides existing in the glass composition satisfying the properties required by the present invention generally assumes the following values^ SiO₂ more than 10% and 13% or less B₂O₃ more than 40% and 60% . or less

Gd₂O₃ 8 - - 15% ZrO₂ 0.2 - 6.5% Ta₂O₅ 0.1 - 5% ZnO 0.5 - - 18% and

and

Yb₂O₃ 0 - 5% and/or GeO₂ 0 - - 3% and/or TiO₂ 0 - 3% and/or Νb₂Oδ 0 - less than 3% and/or WO₃ 0 - 3% and/or RO 0 - 10% where RO is one or more oxides selected from a group consisting of CaO, SrO and BaO and/or Sb₂O₃ 0 - 1%. In the optical glass of the present invention, SiO₂ is an indispensable component which is very effective for increasing viscosity of the glass and improving resistance to devitrification and phosphate resistance of the glass. The upper limit of the amount of this component should preferably be 13 mol %, more preferably less than 13 mol % and, most preferably be 12.5 mol % and the lower limit of the amount of this component should be more than 10 mol %, more preferably 10.1 mol % and, most preferably be 10.5 mol %.

In the optical glass of the present invention, B2O3 is an indispensable component as a glass forming oxide. The upper limit of the amount of this component should preferably be 60 mol %, more preferably 56 mol % and, most preferably be 53 mol % and the lower limit of this component should preferably be more than 40 mol %, more preferably 41 mol % and, most preferably be more than 45 mol %..

In the optical glass of the present invention, Y2O3 is effective for increasing refractive index and lowering dispersion. The upper limit of this amount should preferably be 1 mol %, more preferably 0.5 mol % and, most preferably, this component should not be added. In the optical glass of the present invention, La2θ3 is effective for increasing refractive index and lowering dispersion. The upper limit of the amount of this component should preferably be 15 mol %, more preferably 14 mol % and, most preferably be 13 mol % and the lower limit of the amount of this component should preferably be 5 mol %, more preferably more than 6 mol % and, most preferably be 7 mol %.

In the optical glass of the present invention, Gd2θ3 is effective for increasing refractive index and lowering dispersion and also for improving resistance to devitrification by having this component coexist with La2θ3. The upper limit of the amount of this component should preferably be 15 mol %, more preferably 14 mol % and, most preferably be 13 mol % and the lower limit of the amount of this component should preferably be 8 mol %, more preferably more than 8 mol % and, most preferably be 8.5 mol %.

In the optical glass of the present invention, Yb2θ3 is effective for increasing refractive index and lowering dispersion.

The upper limit of the amount of this component should preferably be 5 mol %, more preferably 4 mol % and, most preferably be 3 mol %.

In the optical glass of the present invention, Geθ2 is effective for increasing refractive index and improving resistance to devitrification. The upper limit of the amount of this component should preferably be 3 mol %, more preferably 2 mol % and, most preferably, this component should not be added.

In the optical glass of the present invention, Tiθ2 is effective for adjusting optical constants and improving resistance to devitrification. The upper limit of the amount of this component should preferably be 3 mol %, more preferably 1 mol % and, most preferably be 0.5 mol %.

In the optical glass of the present invention, Zrθ2 is effective for adjusting optical constants, improving resistance to devitrification and improving phosphate resistance. The upper limit of the amount of this component should preferably be 6.5 mol %, more preferably 6.4 mol % and, most preferably be 6.3 mol % and the lower limit of the amount of this component should preferably be 0.2 mol %, more preferably 1 mol % and, most preferably be 2 mol %.

In the optical glass of the present invention, Nb2θδ is effective for increasing refractive index and improving phosphate resistance and resistance to devitrification., The upper limit of the amount of this component should preferably be less than 3 mol %, more preferably 2 mol % and, most preferably, this component should not be added.

In the optical glass of the present invention, Ta2θ is effective for increasing refractive index and, improving phosphate resistance and resistance to devitrification. The upper limit of the amount of this component should preferably be 5 mol %, more preferably 4 mol % and, most preferably be 3 mol % and the lower limit of the amount of this component should preferably be 0.1 mol %, more preferably 0.2 mol % and, most preferably be 0.5 mol %.

In the optical glass of the present invention, WO3 is effective for adjusting optical constants and improving resistance to devitrification. The upper limit of the amount of this component should preferably be 3 mol %, more preferably less than 1 mol % and, most preferably, this component should not be added.

In the optical glass of the present invention, ZnO is effective for lowering the glass transition temperature (Tg) . The upper limit of the amount of this component should preferably be 18 mol %, more preferably 16 mol % and, most preferably be 14 mol % and the lower limit of the amount of this component should preferably be 0.5 mol %, more preferably 1 mol % and, most preferably be 2 mol %.

In the optical glass of the present invention, RO (one or more oxides selected from the group consisting of CaO, SrO and BaO) is effective for adjusting optical constants. The upper limit of the amount of this component should preferably be 10 mol %, more preferably 8 mol % and, most preferably be 5 mol %.

In the optical glass of the present invention, Li2θ is effective for lowering the glass transition temperature (Tg) substantially and facilitating melting of mixed glass materials. The upper limit of the amount of this component should preferably be 10 mol %, more preferably 9 mol % and, most preferably be more than 8 mol % and the upper limit of the amount of this component should preferably be 0.5 mol %, more preferably 1 mol % and, most preferably be 2 mol %.

In the optical glass of the present invention, Sb₂O3 is effective for defoaming during melting of the glass. The upper limit of the amount of this component should preferably be 1 mol %, more preferably 0.5 mol % and, most preferably, be 0.2 mol %.

As to the values of S1O2/B2O3, ZnO/(ZrO₂ + Ta O₅), S1O2 + B2O3 and Y2O3 + La2U3 + Gd2θ3 + Yb2θ3, these values cannot be expressed directly in mol % but these values generally assume the following values.

In the present invention, the lower limit of Siθ2 B2θ3 should preferably be 0.18, more preferably 0.19 and, most preferably 0.20.

The lower limit of ZnO/(Zrθ2 + Ta2θs) should preferably be 0.9, more preferably be 1.2 and, most preferably be 1.5 and the upper limit thereof should be 2.50, more preferably 2.45 and, most preferably be 2.40. The lower limit of Siθ2 + B2O3 should preferably be more than 50 mol 5, more preferably 52 mol % and, most preferably be 55 mol % and the upper limit thereof should preferably be 65 mol %, more preferably 63 mol % and, most preferably be 60 mol %.

The lower limit of Y2O3 + La2θ3 + Gd2θ3 + Yb2θ3 should preferably be 16 mol %, more preferably 17 mol % and, most preferably be 18 mol % and the upper limit of Y2O3 + La₂O3 + Gd₂O3

+ Yb2θ3 should preferably be 23 mol %, more preferably 22 mol % and, most preferably be 21 mol %.

In the same manner as in the case of expression in mass %, a particularly excellent phosphate resistance can be achieved by simultaneously adjusting the values of Siθ2/B2θ3, ZnO/(Zrθ2 + Ta₂O₅), S1O2 + B2O3 and Y2O3 + La₂O₃ + Gd₂O₃ + Yb₂O₃ to values within the above described ranges.

Description will now be made about the properties of the optical glass of the present invention.

As described above, the optical glass of the present invention should preferably have, from the standpoint of utility in the optical design, optical constants of a refractive index (nd) within a range from 1.74 to 1.80 and an Abbe number ( d) within a range from 47 to 51, more preferably a refractive index (nd) within a range from 1.75 to less than 1.80 and an Abbe number ( d) within a range from 47 to 51, further more preferably a refractive index (nd) within a range from 1.75 to 1.79 and an Abbe number ( d) within a range from 47 to 50 and, most preferably, a refractive index (nd) within a range from more than 1.75 up to 1.79 and an Abbe number ( d) within a range from 47 to less than 50.

In the optical glass of the present invention, an excessively low Tg deteriorates chemical durability and hence resistance to devitrification. An excessively high Tg tends to cause, as described previously, deterioration in the mold in conducting precision press molding. In the optical glass of the present invention, therefore, the lower limit of Tg should preferably be 530°C , more preferably 550°C and, most preferably, be 560°C and the upper limit of Tg should preferably be 630°C , more preferably lower than 630°C and, most preferably be 620°C .

In the optical glass of the present invention, for realizing a stable production by the manufacturing method to be described later, it is important to maintain liquidus temperature of the glass below 1150°C . A particularly preferable liquidus temperature is 1100°C or below because, at this liquidus temperature, the range of visocisty which enables a stable production is broadened and the melting temperature of the glass is lowered and energy consumption thereby can be reduced. The liquidus temperature means the lowest temperature at which no crystal is observed when crushed glass specimen is put on a platinum plate, held in a furnace with temperature graduations for 30 minutes and thereafter is taken out of the furnace and presence or absence of crystals in the softened glass is observed with a microscope.

As described previously, the optical glass of the present invention can be used as a preform for press molding or, alternatively, molten glass can be directly pressed. In a case where it is used as a preform, the method for manufacturing the preform and the manner of precision press molding are not particularly limited but known manufacturing method and known precision press molding method can be used. As a method for manufacturing a preform, a preform can be made in a manner as described in the gob forming method disclosed in Japanese Patent

Application Laid-open Publication No. Hei 8-319124 or a preform can be made directly from molten glass as described in the manufacturing method and apparatus of an optical glass disclosed in Japanese Patent Application Laid-open Publication No. Hei

8-73229. A preform can also be made by cold processing a strip material. In a case where a preform is made by dripping molten glass by using the optical glass of the present invention, if viscosity of the molten glass is too low, striae tends to occur in the preform whereas if viscosity is too high, cutting of glass by weight and surface tension of dripping glass becomes difficult.

Accordingly, for producing a high-quality preform stably, logarithm log η of viscosity (dPa ^• s) should preferably be within a range from 0.4 to 2.0, more preferably within a range from 0.5 to 1.8 and, most preferably be within a range from 0.6 to 1.6.

Although the method of precision press molding a preform is not limited, a method as disclosed in Japanese Patent Application Laid-open Publication No. Sho 62-41180, Method for Forming an Optical Element, may for example be used.

Examples

Examples of the present invention will now be described, though the present invention in no way is limited by these examples.

Tables 1 to 7 show compositions of Example No. 1 to No. 38 of the optical glass of the present invention together with their refractive index (nd), Abbe number ( v d), glass transition temperature (Tg), yield point (At) and phosphate resistance. In the tables, composition of the respective components are expressed in mass %.

Table 1

Table 3

Table 4

Table 5

Table 7

Table 8 shows compositions of the glasses of Comparative Example No. A to No. C together with their refractive index (nd), Abbe number ( v d), glass transition temperature (Tg), yield point (At) and phosphate resistance. Table 8

For manufacturing the glasses of Example No. 1 to No. 38, ordinary raw materials for an optical glass including oxides, carbonates and nitrates were weighed and mixed so as to realize the composition ratio of the respective examples shown in Tables 1 to 7. The raw materials were put in a platinum crucible and melted at a temperature within a range from 1000°C to 1300°C for three to five hours depending upon the melting property of the composition. After refining and stirring the melt for homogenization, the melt was cast into a mold and annealed to provide the glasses.

Refractive index (nd) and Abbe number ( d) of the glasses were measured with respect to glasses which were obtained by setting the rate of lowering of annealing temperature at — 25°C /Hr.

Glass transition temperature (Tg) of the glasses was measured in accordance with the Japan Optical Glass Industry

Standard JOGIS08-²⁰⁰³ "Measuring Method of Thermal Expansion of Optical Glass". A specimen having length of 50mm and diameter of 4mm was used as a test specimen.

Yield point (At) was measured in the same manner as in measuring glass transition temperature (Tg), and a temperature at which stretching of the glass ceased and shrinking of the glass started was adopted as yield point.

In the manner as described previously, phosphate resistance was measured in accordance with ISO Testing Method "Phosphate resistance" (ISO9689: 1990)E). More specifically, as a test specimen, a glass specimen of 30mm X 30mm X 2mm polished in its six surfaces was put, by hanging it with a platinum line, in a O.Olmol/L sodium polyphosphate aqueous solution heated to 50°C and treated for a predetermined period of time (15 minutes, 1 hour, 4 hours and 16 hours). After the treatment, reduction in the mass of the specimen was weighed and time required for eroding the glass layer having thickness of 0.1 . m was calculated in accordance with the following formula. In this calculation, a value which was obtained by minimum time required for mass reduction of at least lmg per 1 specimen was used. According to the standard of evaluation by this method, Class 1 indicates that time required for eroding the glass layer of OΛ μ m is longer than 240 minutes, Class 2 indicates that this time is longer than 60 minutes up to 240 minutes, Class 3 indicates that this time is 60 minutes - 15 minutes and Class 4 indicates that this time is less than 15 minutes. to.i = to ^• d ^• S/((mι - m₂) ^• 100) to.i ^: time (minutes) required for eroding the glass layer of 0.1 μ m to ^: treating time (minutes) d : specific gravity S : surface area (cm²) of the specimen mι- m2 >^' reduction of mass (mg) of the specimen

As shown in Tables 1 to 7, the optical glasses of Example No. 1 to No. 38 all have the optical constants (refractive index (nd) and Abbe number ( d) of the above described ranges and their glass transition temperature (Tg) is within a range from 530°C to 630°C and, therefore, they are suitable for precision press molding. They have also excellent phosphate resistance and, therefore, have excellent chemical durability.

On the other hand, the specimens of Comparative Example No. A to No. C shown in Table 8 were manufactured under the same conditions as the examples of the present invention were manufactured and the manufactured glasses were evaluated by the same evaluation methods as used for evaluating the examples of the present invention. In Comparative Example No. A and No. C, the ratio in mass % of ZnO/(Zrθ2 + Ta2θ ) was outside of the range from 0.45 to less than 0.98 and the value of Y2O3 + La2θ3 + Gd₂O3 + Yb₂O3 was outside of the range from 48 mass % to 58 mass % and the evaluation of phosphate resistance was Class 4. Thus, neither Comparative No. A nor No. C satisfied the property required by the present invention. In Comparative Example No. B, the ratio in mass % of ZnO/(Zrθ2 + Ta2θ ) was outside of the range from 0.45 to less than 0.98 and the evaluation of phosphate resistance was Class 4. Thus, Comparative Example No.B did not satisfy the property required by the present invention.

Industrial Applicability As described above, the optical glass of the present invention which is of a SiO₂ - B2O3 - La₂O₃ - Gd₂O₃ - ZrO₂ - Ta₂O₅ - ZnO - Li2θ glass is free of Pb, As and F and has optical constants of a refractive index (nd) within a range from 1.74 to 1.80 and an Abbe number ( v d) within a range from 47 to 51 and glass transition temperature (Tg) within a range from 530°C to 630°C and hence is suitable for precision press molding and has sufficient industrial utility.

Moreover, since the optical glass of the present invention has excellent phosphate resistance, in a case where a lens preform, i.e., gob, is cleaned before precision press molding, or in a case where a lens made by precision press molding is cleaned, there is no likelihood of occurrence of tarnish and, therefore, the optical glass can be very easily treated.

The examples of the present invention have been described for illustrative purpose only and various modifications can be made without departing from the spirit and scope of the present invention.

Claims

1. An optical glass having optical constants of a refractive index (nd) of 1.74 or over and an Abbe number ( d) of 47 or over, being substantially free of Pb, As and F, comprising Zrθ2 in an amount of 6 mass % or less, having a ratio of ZnO/(ZrU2 + Ta2θ ) within a range from 0.45 to 1.5, and having phosphate resistance (ISO9689) of Class 3, 2 or 1.

2. An optical glass as defined in claim 1 having a glass transition temperature (Tg) of 630°C or below.

3. An optical glass as defined in claim 1 or 2 wherein logarithm log η of viscosity (dPa ^• s) in liquidus temperature is within a range from 0.4 to 2.0.

4. An optical glass having optical constants of a refractive index (nd) within a range from 1.74 to 1.80 and an Abbe number f v d) within a range from 47 to 51, comprising Siθ2, B2O3, La2θ3, Gd₂O3, Zrθ2, Ta2θ , ZnO and Li₂O as essential components, being substantially free of Pb, As and F, comprising ZrO₂ in an amount of 6 mass % or less, having a ratio of ZnO/(Zrθ2 + Ta2θ ) within a range from 0.45 - 1.5 and having a glass transition temperature (Tg) within a range from 530°C to 630°C .

5. An optical glass comprising, in mass %,

La₂O₃ 15 - less than 35% Gd2θ3 more than 25% and 35% or less ZrO₂ 0.5 - 6% Ta₂O₅ 0.5 - 10% ZnO 0.5 - 15% and

and

Yb₂O₃ 0 - 10% and/or GeO₂ 0 - 5% and/or TiO₂ 0 - 5% and/or Nb₂O₅ 0 - 5% and/or WO₃ 0 - 5% and/or RO 0 - 10% where RO is one or more oxides selected from a group consisting of CaO, SrO and BaO and/or Sb₂O₃ 0 - 1%.

6. An optical glass as defined in claim 5 having a ratio in mass % of ZnO/(ZrO₂ + Ta₂O₅) within a range from 0.45 to 0.98.

7. An optical glass as defined in any of claims 1 — 6 having a ratio in mass % of Siθ2 B2θ3 of 0.18 or over.

8. An optical glass as defined in any of claims 1 — 7 having a total amount of Siθ2 + B2O3 within a range from 23 mass % to 35 mass %.

9. An optical glass as defined in any of claims 1 — 8 having a total amount of Y2O3 + La2θ3 + Gd2θ3 + YbβOs within a range from 48 mass % to 58 mass %.

10. An optical glass having optical constants of a refractive index (nd) within a range from 1.74 to 1.80 and an Abbe number ( v d) within a range from 47 to 51, comprising Siθ2, B2O3, La2θ3, Gd2θ3, Zrθ2, Ta2θ , ZnO and Li2θ as essential components, being substantially free of Pb, As and F, comprising Zrθ2 in an amount of 6.5 mol % or less, having a ratio in mol % of ZnO/(Zrθ2 + Ta2θ ) within a range from 0.9 — 2.5 and having a glass transition temperature (Tg) within a range from 530°C to 630°C .

11. An optical glass comprising, in mol %, SiO₂ more than 10% and 13% or less B2O3 more than 40% and 60% or less La₂O₃ 5 - 15% Gd₂O₃ 8 - 15% ZrO₂ 0.2 - 6.5% Ta₂O₅ 0.1 - 5% ZnO 0.5 - 18% and Li₂O 0.5 - 10%, and Y₂O₃ 0 - 1% and/or Yb₂O₃ 0 - 5% and/or GeO₂ 0 - 3% and/or TiO₂ 0 - 3% and/or Nb2θδ 0 - less than 3% and/or WO 3 0 - 3% and/or RO 0 - 10% where RO is one or more oxides selected from a group consisting of CaO, SrO and BaO and/or Sb₂O₃ 0 - 1%.

12. An optical glass as defined in claim 10 or 11 having a ratio in mol % of ZnO/(ZrO + Ta₂O₅) within a range from 0.9 to 2.4.

13. An optical glass as defined in any of claims 10 - 12 having a ratio in mol % of Siθ2/B2θ3 of 0.20 or over.

14. An optical glass as defined in any of claims 10 — 13 having a total amount of Siθ2 + B2O3 of more than 50 mol % and 65 mol % or less.

15. An optical glass as defined in any of claims 10 - 14 having a total amount of Y2O3 + La2θ3 + Gd2θ3 + YbsOβ within a range from 16 mol % to 23 mol %.

16. A lens preform made of an optical glass as defined in any of claims 1 - 15.

17. An optical element formed of an optical glass as defined in any of claims 1 - 15.