# Patent

## Title
Phosphate optical glass

## Abstract

The object of the present invention is to provide an optical glass which has optical constants such as a refractive index of from 1.45 to 1.65 and an Abbe number of at least 65 and which is yet excellent in chemical durability. A phosphate optical glass comprising, as glass components by mass %, from 73 to 85 of P2O5, from 14 to 26 of Al2O3, from 1 to 12 of K2O+Li2O, from 0 to 12 of SiO2, from 0 to 12 of B2O3, from 0 to 12 of Na2O, from 0 to 12 of Y2O3, from 0 to 12 of La2O3, from 0 to 12 of MgO+CaO+SrO+BaO, from 0 to 10 of ZrO2+TiO2+Gd2O3, from 0 to 10 of Ta2O5+GeO2+Ga2O3+Nb2O5+WO3+TeO2, from 0 to 2 of Sb2O3, and from 0 to 12 of ZnO+PbO.

## Background
TECHNICAL FIELD
The present invention relates to a phosphate optical glass which is precision press moldable and which is excellent particularly in chemical durability.
BACKGROUND ART
Heretofore, as an optical glass having high dispersibility, a phosphate optical glass containing P2O5 as the basic component has been known. However, the conventional phosphate optical glass has had a problem that it is poor in durability, particularly in chemical durability, and in a polishing step or washing step of the optical glass or during its use as a lens for a long period of time, weathering and erosion are likely to occur on its surface. Especially in the case of an optical glass to be subjected to precision press molding, the question of chemical durability becomes important, since it contains alkali metal/alkaline earth metal elements so that it can be easily molded.
As a proposal to improve the chemical durability of such phosphate optical glass, a composition containing a specific amount of Al2O3 has been proposed in Patent Document 1, but it is still not satisfactory from the point of chemical durability. Further, as a phosphate optical glass for precision press molding, a composition is disclosed also in e.g. Patent Document 2, but it is to cover a low dispersibility range of an Abbe number being from 23 to 42 and is not one to cover a high dispersibility range of an Abbe number being at least 65.
Patent Document 1: JP-A-57-118045 (detailed description of the invention)
Patent Document 2: JP-A-2003-238197 (detailed description of the invention)
DISCLOSURE OF THE INVENTION
Object to be Accomplished by the Invention
It is an object of the present invention to provide an optical glass which has optical constants such as a refractive index of from 1.45 to 1.65 and an Abbe number of at least 65 and which is yet excellent in chemical durability.
Means to Accomplish the Object
The present inventors have conducted an extensive research to accomplish the above object and as a result have arrived at the present invention. Namely, the present invention provides the following:

    (1) A phosphate optical glass comprising, as glass components by mass %, from 73 to 85 of P2O5, from 14 to 26 of Al2O3, from 1 to 12 of K2O+Li2O, from 0 to 12 of SiO2, from 0 to 12 of B2O3, from 0 to 12 of Na2O, from 0 to 12 of Y2O3, from 0 to 12 of La2O3, from 0 to 12 of MgO+CaO+SrO+BaO, from 0 to 10 of ZrO2+TiO2+Gd2O3, from 0 to 10 of Ta2O5+GeO2+Ga2O3+Nb2O5+WO3+TeO2, from 0 to 2 of Sb2O3, and from 0 to 12 of ZnO+PbO.
    (2) The phosphate glass according to the above (1), which comprises, as glass components by mass %, from 74 to 80 of P2O5, from 15 to 25 of Al2O3 and from 1 to 8 of K2O+Li2O.
    (3) The phosphate glass according to the above (1) or (2), which contains from 1 to 4 mass % of Li2O.
    (4) The phosphate glass according to any one of the above (1) to (3), which has a refractive index (nd) of from 1.45 to 1.65 and an Abbe number (νd) of from 65 to 80.
    (5) The phosphate glass according to any one of the above (1) to (4), which has a glass transition point (Tg) of at most 600° C.


Effects of the Invention
The phosphate optical glass of the present invention has optical constants such as a refractive index being from 1.45 to 1.65 and an Abbe number being at least 65 and yet is excellent in chemical durability, which has not been accomplished by a conventional product.
Further, it is an optical glass which is readily meltable and scarcely devitrified and has a low glass transition point, thus being suitable for press molding. Further, it does not require expensive Ga2O3, GeO2 and Ta2O5 components as essential components, and thus, it is an optical glass advantageous also from the aspect of raw material costs.


BEST MODE FOR CARRYING OUT THE INVENTION
The phosphate optical glass of the present invention (hereinafter referred to as the present glass) is characterized in that it comprises at least (1) P2O5, (2) Al2O3 and (3) Li2O and/or K2O. In this specification, “%” means “mass %” unless otherwise specified.
In the present glass, P2O5 is the main component to constitute the optical glass (the glass-constituting oxide) and contained in an amount of from 73 to 85%. If P2O5 is less than 73%, the desired optical constants tend to be hardly obtainable. On the other hand, if P2O5 exceeds 85%, the chemical durability tends to deteriorate. With respect to the upper limit, P2O5 is preferably at most 80%, more preferably at most 78%. In the present glass, P2O5 is preferably at least 74%, whereby the balance of the optical constants and the chemical durability will be good. More preferably, P2O5 is at least 75%, and particularly preferably, P2O5 is at least 76%.
In the present glass, Al2O3 is an essential component from the viewpoint of the chemical durability and is contained in an amount of from 14 to 26%. If Al2O3 is less than 14%, no adequate chemical durability tends to be obtainable. On the other hand, if Al2O3 exceeds 26%, the melting property tends to be low, and the desired optical constants may not be obtainable. In the present glass, with respect to the lower limit, Al2O3 is preferably at least 15%, and more preferably Al2O3 is at least 16%. Likewise, in the present glass, with respect to the upper limit, Al2O3 is preferably at most 25%, more preferably at most 23%.
In the present glass, Li2O and/or K2O is an essential component to lower the glass transition temperature to a temperature suitable for press molding and is contained in an amount of from 1 to 12%, preferably from 1 to 8%. If Li2O and/or K2O is less than 1%, the glass transition temperature tends to be high, and the press moldability tends to be low. On the other hand, if Li2O and/or K2O exceeds 12%, the chemical durability tends to deteriorate, and the desired optical constants may not be obtainable. In the present glass, Li2O is preferably contained in an amount of from 1 to 4% with a view to improvement of the low temperature moldability and the chemical durability. In the present glass, in a case where Li2O is not contained and only K2O is contained, K2O is preferably contained in an amount of from 1 to 6% with a view to improvement of the low temperature moldability and the chemical durability.
In the present glass, a part of Li2O and/or K2O may be replaced by Na2O. However, if the amount of Na2O becomes large, the chemical durability will decrease. Accordingly, the Na2O content is preferably lower as far as possible.
In the present glass, SiO2 is not an essential component, but is effective for improving the chemical durability while the low dispersibility (a high Abbe number) is maintained. However, if the amount is large, the desired optical characteristics tend to be hardly obtainable, and at the same time, the glass transition temperature tends to increase, and consequently, the molding temperature during the press molding tends to increase, and the productivity, etc. tend to deteriorate. Accordingly, in the present glass, SiO2 is added preferably from 0 to 12%, more preferably from 0 to 7%. In the present glass, when SiO2 is added, the lower limit is preferably at least 1%, more preferably at least 3%, particularly preferably at least 5%.
In the present glass, B2O3 is not an essential component, but is effective for maintaining the low dispersibility. On the other hand, if the amount becomes large, the desired optical characteristics tend to be hardly obtainable. Accordingly, when B2O3 is added, it is preferably from 0 to 12%, more preferably from 0 to 9%, particularly preferably from 0 to 6%. In the present glass, when B2O3 is added, the lower limit is preferably at least 2%, more preferably at least 3%, particularly preferably at least 4%.
In the present glass, Y2O3 is not an essential component, but it is capable of increasing the refractive index while maintaining the low dispersibility, and it is further effective for improving the chemical durability. However, if the amount is too large, the melting temperature rises, and the melting property decreases. Accordingly, when Y2O3 is added, it is preferably from 0 to 12%, more preferably from 0 to 7%. In the present glass, when Y2O3 is added, the lower limit is preferably at least 2%, more preferably at least 4%, particularly preferably at least 6%.
In the present invention, La2O3 has the same effects as Y2O3. Accordingly, when La2O3 is added, it is preferably from 0 to 12%, more preferably from 0 to 7%. In the present glass, when La2O3 is added, the lower limit is preferably at least 2%, more preferably at least 4%, particularly preferably at least 6%.
In the present glass, at least one member of MgO, CaO, SrO and BaO is not an essential component, but if it is added, the refractive index can be improved. However, if the amount is too large, the desired optical constants tend to be hardly obtainable. Accordingly, the amount to be added in the present invention is preferably from 0 to 12%, more preferably from 0 to 11%, and its amount is particularly preferably from 0 to 8%. In the present glass, when at least one member of MgO, CaO, SrO and BaO is added, the lower limit is preferably at least 1%, more preferably at least 2%, particularly preferably at least 3%.
In the present glass, at least one member selected from the group consisting of ZrO2, TiO2 and Gd2O3 is not an essential component, but if it is added, the chemical durability or the devitrification resistance can be improved. On the other hand, if the amount is too large, the Abbe number will decrease. Accordingly, the amount is preferably from 0 to 10%, more preferably from 0 to 5%. In the present glass, when at least one member of ZrO2, TiO2 and Gd2O3 is added, the lower limit is preferably at least 1%, more preferably at least 2%, particularly preferably at least 3%.
In the present glass, at least one member selected from the group consisting of Ta2O5, GeO2, Ga2O3, Nb2O5, TeO2 and WO3, is not an essential component, but when it is added, the refractive index will increase. However, if the amount is too large, the Abbe number will decrease. Accordingly, the amount is preferably from 0 to 10%, more preferably from 0 to 5%. Further, raw materials for such components are expensive. Accordingly, if the material costs are important, it is preferred to minimize their amount. In the present glass, when at least one member selected from the group consisting of Ta2O5, GeO2, Ga2O3, Nb2O5, TeO2 and WO3 is added, the lower limit is preferably at least 1%, more preferably at least 2%, particularly preferably at least 3%.
In the present glass, Sb2O3 is not an essential component, but may be added as a fining agent at the time of melting the glass. Its amount is preferably at most 2%, more preferably at most 1%. In the present glass, when Sb2O3 is added, the lower limit is preferably at least 0.01%, more preferably at least 0.1%, particularly preferably at least 0.2%.
In the present glass, addition of ZnO and/or PbO is effective to lower the melting temperature. The amount of ZnO and/or Pbo is preferably from 0 to 12%, more preferably from 0 to 7%. However, when the influence to the environment, etc. are taken into consideration, it is preferred not to substantially contain them. In the present glass, when ZnO and/or PbO is added, the lower limit is preferably at least 1%, more preferably at least 3%, particularly preferably at least 5%.
Further, in the present glass, each element of Ag, Cu or Fe is preferably not substantially contained, since its ions will cause coloration.
As an optical characteristic of the present glass, the refractive index (nd) is preferably from 1.45 to 1.65. The refractive index (nd) is further preferably at least 1.50, and particularly preferably, the refractive index (nd) is at least 1.53. The Abbe number (νd) of the present glass is preferably from 65 to 80. The Abbe number (νd) is more preferably at least 67, and particularly preferably, the Abbe number (νd) is at least 70. The glass transition point (Tg) of the present glass is preferably at most 600° C., and more preferably, the glass transition point is at most 550° C. Namely, as optical characteristics of the present glass, the refractive index (nd) is from 1.45 to 1.65, preferably from 1.50 to 1.55, and the Abbe number (νd) is from 65 to 80, preferably from 68 to 72.
The process for producing the present glass is not particularly limited, and it may be produced, for example, by weighing and mixing raw materials to be used for a usual optical glass, such as oxides, hydroxides, carbonates, nitrates, etc., putting them into a platinum crucible, melting them at a temperature of from about 1,200 to 1,300° C. for from 2 to 6 hours, followed by fining and stirring, and then casting the melt into a mold preheated to about 500° C., followed by annealing.
EXAMPLES
Now, the present invention will be described in further detail with reference to Examples, etc. However, it should be understood that the present invention is by no means restricted to the following Examples. Here, Examples 1 to 3 are Comparative Examples to the present invention, and Examples 4 to 13 are working Examples of the present invention.
Chemical Compositions/Sample Preparation Method
Raw materials were weighed to bring the chemical compositions (%) as shown in Tables 1 and 2. With respect to raw materials for each glass, H3PO4 was used for P2O5, KNO3 was used for K2O, Li2CO3 was used for Li2O, CaCO3 was used for CaO, H3BO3 was used for B2O3, and an oxide was used for Al2O3. The weighed raw materials were dry-mixed for 30 minutes by a mixer, put into a platinum crucible having an internal capacity of about 300 cm3 and melted at 1,200° C. to 1,300° C. for from 2 to 6 hours, followed by fining and stirring and then, casted into a rectangular mold of 15 mm×50 mm preheated to about 500° C. and then annealed at a rate of about 1° C./min to obtain a sample.
Evaluation Method
The refractive index (nd) was obtained as a refractive index to helium d-line, and the Abbe number (νd) was obtained from νd=(nd−1)/(nF−nC), wherein nF and nC were refractive indices to hydrogen F-line and hydrogen C-line, respectively. Here, the refractive index was measured by a refractometer (trade name: KRP-2, manufactured by Kalnew optical Industrial Co., Ltd.). The glass transition point (Tg) was measured by subjecting each glass powder obtained, to a differential thermal analysis (DTA) by means of a thermal analyzer (trade name: EXSTAR6000TG/DTA, manufactured by Seiko Instruments Inc.). As a measuring condition, the temperature was raised at 10° C./min.
For the chemical durability, the sample was held under a high temperature high humidity condition for a long time, and the chemical durability was evaluated by the change in transmittance as between before and after the holding. Specifically, a flat plate-shaped sample having a size of 15 mm×15 mm×10 mm having both surfaces of 15 mm×15 mm made to be mirror surfaces, was held at a temperature of 60° C. under a relative humidity of 90% for 336 hours, and the transmittances before and after the holding were measured by a spectrophotometer (trade name: U-3500, manufactured by Hitachi, Ltd.), whereupon it was calculated as the absolute value |Δ(%)| of change in transmittance Δ (%)=transmittance (after holding under the high temperature high humidity condition)−transmittance (before holding under the high temperature high humidity condition). The smaller the change in transmittance, the higher the chemical durability. In each of Examples 1 to 3, an average value of three times is shown, in each of Examples 4 to 7, an average value of two times is shown, in each of Examples 8 to 11, a value measured once is shown, and in each of Examples 12 and 13, an average value of two times is shown.
With respect to the melting property, etc. of glass, as a result of visual observation at the time of preparing a sample as described above, it was confirmed that in Examples 4 to 13, there was no problem with respect to the melting property, and the obtained glass samples were free from bubbles and stria.











TABLE 1







Ex. 1
Ex. 2
Ex. 3





















P2O5
71.0
73.0
74.0



B2O3
2.0
2.0
5.0



Al2O3
14.0
11.0
12.0



MgO
—
2.0
—



CaO
—
—
—



LiO2
—
—
—



K2O
13.0
12.0
9.0



Refractive
1.51
1.51
1.52



index



Abbe number
71.0
70.5
70.6



Tg/° C.
—
—
—



Δ/%
0.37
0.26
0.26























TABLE 2







Ex. 4
Ex. 5
Ex. 6
Ex. 7
Ex. 8





















P2O5
77.0
77.4
77.5
77.4
77.0


B2O3
—
3.5
2.3
4.7
4.6


Al2O3
17.0
15.4
16.3
14.5
14.5


MgO
—
—
—
—
—


CaO
4.7
1.9
1.9
1.9
1.9


LiO2
1.2
1.8
2.0
1.5
2.0


K2O
—
—
—
—
—


Refractive
1.53
1.53
1.53
1.53
1.53


index


Abbe number
69.8
70.4
69.8
70.8
70.5


Tg/° C.
—
562
554
575
535


Δ/%
0.01
0.00
0.00
0.01
0.01






















TABLE 3







Ex. 9
Ex. 10
Ex. 11
Ex. 12
Ex. 13





















P2O5
77.4
77.8
77.4
74.6
75.0


B2O3
4.7
4.7
4.7
3.4
4.5


Al2O3
15.0
15.0
15.0
14.8
14.1


MgO
—
—
—
—
—


CaO
0.9
—
—
1.8
1.8


LiO2
2.0
2.5
3.0
—
—


K2O
—
—
—
5.3
4.6


Refractive
1.53
1.53
1.53
1.52
1.52


index


Abbe number
70.8
70.8
70.8
70.5
70.7


Tg/° C.
564
551
535
620
614


Δ/%
0.01
0.01
0.01
0.02
0.02









INDUSTRIAL APPLICABILITY
The phosphate optical glass of the present invention is an optical glass which has low dispersibility and a desired value of refractive index and which is excellent in chemical durability and yet is excellent in press moldability. Thus, it is useful as an optical glass for precision press molding.
The entire disclosure of Japanese Patent Application No. 2005-311293 filed on Oct. 26, 2005 including specification, claims and summary is incorporated herein by reference in its entirety.

## Claims

1. A phosphate optical glass comprising, as glass components by mass %, from 1 to 4 mass % of Li2O, 73 to 85 of P2O5, from 14 to 26 of Al2O3, from 1 to 12 of K2O+Li2O, from 0 to 12 of SiO2, from 0 to 12 of B2O3, from 0 to 12 of Na2O, from 0 to 12 of Y2O3, from 0 to 12 of La2O3, from 0 to 12 of MgO+CaO+SrO+BaO, from 0 to 10 of ZrO2+TiO2+Gd2O3, from 0 to 10 of Ta2O5+GeO2+Ga2O3+Nb2O5+WO3+TeO2, from 0 to 2 of Sb2O3, and from 0 to 12 of ZnO+PbO and which has a glass transition point (Tg) of at most 600° C.

2. The phosphate glass according to claim 1, which comprises, as glass components by mass %, from 74 to 80 of P2O5, from 15 to 25 of Al2O3 and from 1 to 8 of K2O+Li2O.

3. The phosphate glass according to claim 1, which has a refractive index (nd) of from 1.45 to 1.65 and an Abbe number (νd) of from 65 to 80.

4. The phosphate glass according to claim 1, which has refractive index (nd) of from 1.50 to 1.55.

5. The phosphate glass according to claim 1, which has an Abbe number (νd) of from 68 to 72.