# Patent

## Title
Optical glass

## Abstract

An optical glass has a composition including: 40 to 65 mass % of P2O5, and 0 to less than 5 mass % of Li2O+Na2O+K2O. In the optical glass, (MgO+CaO)/(Li2O+Na2O+K2O) which is a ratio of the total content expressed in mass % of MgO and CaO to the total content expressed in mass % of Li2O, Na2O, and K2O, is in a range from 6 to 38.

## Background
This application is based on Japanese Patent Application No. 2009-124083 filed on May 22, 2009, in Japanese Patent Office, the entire content of which is hereby incorporated by reference.
TECHNICAL FIELD
The present invention relates to optical glass, and more particularly relates to phosphate optical glass with low dispersion.
BACKGROUND
Phosphate optical glass containing P2O5 as its main component is valuable optical glass which is used as a material for various optical elements because it exhibits low dispersion as its characteristics. On the other hand, since P2O5 is soluble in water, phosphate optical glass has a relatively low weather resistance, which has been a problem. Therefore, attempts to enhance the weather resistance of phosphate optical glass have been made. For example, JP-A No. 63-11544 and U.S. Pat. No. 7,598,189 disclose phosphate optical glass with improved weather resistance prepared by determining the ratios of composition to be prescribed ranges.
In recent years, an increasing number of equipments, such as portable equipments and onboard equipments, have been used in a severe environment. Therefore, phosphate optical glass has been required to have higher weather resistance. However, it is still difficult to say that phosphate optical glass disclosed in JP-A No. 63-11544 and U.S. Pat. No. 7,598,189 has sufficient weather resistance, and it has been a problem that the optical glass hardly meet the above demand.
SUMMARY
The present invention has been achieved, in view of such the above technical problem, to provide optical glass with excellent weather resistance and low dispersion.
An optical glass relating to the prevent invention has a composition comprising: 40-65 mass % of P2O5: and 1-5 mass % of Li2O+Na2O+K2O, where 5 mass % is excluded. In the optical glass, (MgO+CaO)/(Li2O+Na2O+K2O) which is a ratio of a total content expressed in mass % of MgO and CaO to a total content expressed in mass % of Li2O, Na2O, and K2O, is in a range from 6 to 38.
Objects, features and advantages according to the present invention will become more apparent upon reading of the following detailed description.


DESCRIPTION OF THE PREFERRED EMBODIMENT
Hereafter, although an embodiment of the present invention will be described in details, the present invention is not limited to this embodiment.
The inventor of the prevent invention, as a result of earnest and extensive study, has found that the ratio (MgO+CaO)/(Li2O+Na2O+K2O) which is a ratio of a total content expressed in mass % of MgO and CaO to a total content expressed in mass % of Li2O, Na2O, and K2O in glass components, can be an indicator of obtaining glass with excellent weather resistance. It has been found that MgO and CaO are greatly effective components to enhance the weather resistance of phosphate optical glass. On the other hand, it has also been found that Li2O, Na2O and K2O worsen the weather resistance, but they are essential components to stabilize phosphate optical glass. As a result of a further study based on these matters, the inventor has found that phosphate optical glass with an excellent weather resistance and low dispersion can be obtained by employing a composition including 40 to 65 mass % of P2O5 and 1 to less than 5 mass % of the sum of Li2O, Na2O and K2O and by determining (MgO+CaO)/(Li2O+Na2O+K2O) which is a ratio of a total content expressed in mass % of MgO and CaO to a total content expressed in mass % of Li2O, Na2O, and K2O to be in the range from 6 to 38.
The reason why the content of each component of the optical glass relating to the present invention is determined as stated above, will be explained below.
P2O5 is a main and essential component in view of forming glass. When its content is 40 mass % or more, the optical glass keeps aimed optical constants. When the content is 65 mass % or less, the weather resistance is enhanced. Therefore, the content of P2O5 is determined to be in the range from 40 to 65 mass %. Especially, the range from 41 to 62 mass % is more preferable.
Al2O3 is an effective component to enhance the weather resistance. When its content is 1 mass % or more, it exhibits the effect to further enhance the weather resistance. When the content is 10 mass % or less, the glass is hardly devitrified. Therefore, it is preferable that the content of Al2O3 is in the range from 1 to 10 mass %. Especially, the range from 3 to 8 mass % is more preferable.
B2O3 has an effect to lower the dispersion and is a component contributing to stabilization of the glass. When its content is 1 mass % or more, such the effect can be satisfactory obtained. When the content is 20 mass % or less, deterioration of the weather resistance can be avoided. Therefore, it is preferable that the content of B2O3 is in the range from 1 to 20 mass %. Especially, the range from 5 to 15 mass % is more preferable.
Li2O, Na2O and K2O are alkali metal oxide components, and each component eases the vitrification and contributes to stabilization of the glass. When the total content of Li2O, Na2O and K2O is 1 mass % or more, such the effect can be satisfactory obtained. When the total content is less than 5 mass %, deterioration of the weather resistance can be avoided. Therefore the total content of Li2O, Na2O and K2O is determined to be in the range from 1 to less than 5 mass %. It is more preferable that the total content is in the range from 1 to 4.6 mass %. In these components, LiO2 has the highest effect for the stabilization of the glass, and its adverse influence to the weather resistance is relatively small. From the viewpoint of the influence to the glass stabilization and weather resistance, it is preferable that the content of Li2O is in the range from 1 to less than 5 mass %, and it is more preferable that the range is 1 to 4.6 mass %. Na2O and K2O are optional components. It is preferable that the content of Na2O is in the range from 0 to 3 mass % (where the range includes 0), and that the content of K2O is in the range from 0 to 3 mass % (where the range includes 0).
CaO is a greatly effective component to enhance the weather resistance. CaO is a main component of tooth enamel, which is the same as P2O5. Therefore, CaO can form calcium phosphate Ca3(PO4)2 with transparency, colorlessness, and water-insolubility. When the content of CaO is 6 mass % or more, the effect to enhance the weather resistance can be sufficiently obtained. When the content is 38 mass % or less, the glass is hardly devitrified. Therefore, it is preferable that the content of CaO is in the range from 6 to 38 mass %, and it is more preferable that the content is in the range from more than 18 to 34 mass %.
MgO has an effect to enhance the weather resistance by the same extent to CaO. However, being compared with CaO, MgO is inferior to CaO in washability. Under an ultrasonic cleaning in pure water, it can easily make latent scratch. Therefore, MgO is provided as an optional component. It is preferable that the content of MgO is in the range from 0 to 25 mass % (where the range includes 0), and it is more preferable that the range is from 0 to 20 mass % (where the range includes 0).
As described above, the ratio (MgO+CaO)/(Li2O+Na2O+K2O) which is the ratio of the total content expressed in mass % of MgO and CaO to the total content expressed in mass % of Li2O, Na2O, and K2O, is defined to be in the range from 6 to 38 weigh %, in the viewpoint of a balance between the weather resistance and stabilization of the glass. It is more preferable that the range is from 8 to 30 mass %.
SrO is an optional component. Though SrO has an effect to stabilize the glass, the excessive content can deteriorate the washability significantly. Therefore, it is preferable that the content of SrO is in the range from 0 to 10 mass % (where the range includes 0).
BaO is an optional component. Though BaO is greatly effective to stabilize the glass, the excessive content can significantly deteriorate the weather resistance. Therefore, it is preferable that the content of BaO is in the range from 0 to 12 mass % (where the range includes 0).
ZnO is an optional component. Though ZnO is greatly effective to increase refractive index of the glass, the excessive content can deteriorate the weather resistance. Therefore, it is preferable that the content of ZnO is in the range from 0 to 14 mass % (where the range includes 0).
Sb2O3 is an optional component. Sb2O3 can be employed effectively as a clarifying agent when the glass is melted. Further, it can also be used as a decolorizing agent. It is preferable that the content of Sb2O3 is in the range from 0 to 0.5 mass % (where the range includes 0). When there is no need to use Sb2O3 as a clarifying agent or decolorizing agent, it is preferable that the glass does not contain Sb2O3.
It is preferable that the optical glass does not contain components other than those described above (for example, SiO2, La2O3, Y2O3, TiO2, CuO, Ta2O5, PbO, As2O3, and a fluorine component). However, it is allowed that the optical glass contains these components as much as they do not affect the properties of the optical glass relating to the present invention. In this case, it is preferable that the total content of P2O5, B2O3, Li2O, Na2O, K2O, CaO, MgO, SrO, BaO, ZnO, and Sb2O3 is 95 mass % or more, and it is more preferable that the total content is 98 mass % or more.
Especially in the above components, it is preferable that the content of SiO2 is in the range from 0 to 0.1 mass % (where the range includes 0), and it is more preferable that the optical glass does not contain SiO2. It avoids the problem that the excessive content of SiO2 can easily causes striae.
It is further preferable that the optical glass does not contain PbO and As2O3. Thereby, an influence of the optical glass on the environment can be controlled to be the minimum. Further, the optical glass does not preferably contain a fluorine component as a glass component, because a fluorine component requires a special processing apparatus for maintaining a proper work environment at the time of producing the optical glass.
Since the phosphate optical glass relating to the present invention contains the above components in the predetermined proportions, the optical glass exhibits an excellent weather resistance and a characteristic to have optical constants of the refractive index (nd) in the range from 1.54 to 1.62, and the Abbe number (νd) in the range from 65 to 70.
EXAMPLES
Examples which are provided to verify the effect of the present invention will be described below, but the scope of the present invention is not limited to the examples.
Fifteen kinds of optical glass (Examples 1 to 15) each having a composition with the component ratios satisfying those of the above-described embodiment, and two kinds of optical glass (Comparative Examples 1 and 2) each having a composition with component ratios outside those of the above-described embodiment were prepared according to the procedure described below. The glass compositions of Examples 1 through 15 are listed in Tables 1 through 3 and those of Comparative Examples 1 and 2 are listed in Table 4. The Comparative Example 1 was prepared by replication of Example 2 described in JP-A No. 63-11544 and Comparative Example 2 was prepared by replication of Example 3 described in U.S. Pat. No. 7,598,189.












TABLE 1






Example 1
Example 2
Example 3
Example 4
Example 5





















P2O5
(mass %)
48.0
50.0
57.5
61.0
60.0


Al2O3
(mass %)
4.0
5.0
5.5
7.0
6.5


B2O3
(mass %)
11.0
12.0
10.0
10.0
9.8


Li2O
(mass %)
3.0
3.0
3.0
2.8
2.8


Na2O
(mass %)



0.2



K2O
(mass %)







CaO
(mass %)
34.0
30.0
24.0
19.0
20.9


MgO
(mass %)







SrO
(mass %)







BaO
(mass %)







ZnO
(mass %)







Sb2O3
(mass %)







Total
(mass %)
100.0
100.0
100.0
100.0
100.0


Li2O + Na2O + K2O
(mass %)
3.0
3.0
3.0
3.0
2.8


MgO + CaO
(mass %)
34.0
30.0
24.0
19.0
20.9


(MgO + CaO)/

11.33
10.00
8.00
6.30
7.46












(Li2O + Na2O + K2O)







nd
1.5975
1.5892
1.5740
1.5597
1.5653


νd
65.6
66.7
67.5
68.2
68.0


Weather resistance
A
A
A
A
A





















TABLE 2






Example 6
Example 7
Example 8
Example 9
Example 10





















P2O5
(mass %)
48.0
47.8
47.0
40.8
47.2


Al2O3
(mass %)
4.2
4.5
4.2
4.4
3.9


B2O3
(mass %)
10.8
10.8
11.8
17.8
11.8


Li2O
(mass %)
2.0
1.9
2.7
3.0
2.7


Na2O
(mass %)







K2O
(mass %)


0.3




CaO
(mass %)
23.5
24.0
24.9
20.0
18.6


MgO
(mass %)



14.0
15.7


SrO
(mass %)


9.1




BaO
(mass %)
11.5






ZnO
(mass %)

11.0





Sb2O3
(mass %)




0.1


Total
(mass %)
100.0
100.0
100.0
100.0
100.0


Li2O + Na2O + K2O
(mass %)
2.0
1.9
3.0
3.0
2.7


MgO + CaO
(mass %)
23.5
24.0
24.9
34.0
34.3












(MgO + CaO)/
11.75
12.63
8.30
11.33
12.70


(Li2O + Na2O + K2O)







nd
1.5963
1.5916
1.5936
1.5836
1.5857


νd
66.8
65.2
66.2
65.4
66.2


Weather resistance
A
A
A
A
A





















TABLE 3






Example 11
Example 12
Example 13
Example 14
Example 15





















P2O5
(mass %)
48.0
48.0
47.9
47.9
61.2


Al2O3
(mass %)
6.1
4.2
4.7
3.4
6.8


B2O3
(mass %)
8.9
10.8
10.5
11.7
10.2


Li2O
(mass %)
4.1
2.0
1.0
2.9
2.8


Na2O
(mass %)
0.3






K2O
(mass %)
0.2






CaO
(mass %)
13.0
7.9
11.0
29.7
6.9


MgO
(mass %)
14.9
17.8
19.0
2.9
10.6


SrO
(mass %)



0.7



BaO
(mass %)
4.5
9.3

0.8
1.1


ZnO
(mass %)


5.9

0.4


Sb2O3
(mass %)







Total
(mass %)
100.0
100.0
100.0
100.0
100.0


Li2O + Na2O + K2O
(mass %)
4.6
2.0
1.0
2.9
2.8


MgO + CaO
(mass %)
27.9
25.7
30.0
32.6
17.5












(MgO + CaO)/
6.07
12.85
30.00
11.24
6.25


(Li2O + Na2O + K2O)







nd
1.5878
1.5767
1.5732
1.6010
1.5475


νd
67.4
67.4
67.6
65.9
69.0


Weather resistance
A
A
A
A
A


















TABLE 4






Comparative
Comparative



Example 1
Example 2


















P2O5
(mass %)
65.0
52.90


Al2O3
(mass %)
8.0
4.68


B2O3
(mass %)
2.0
11.98


Li2O
(mass %)
5.0
3.43


Na2O
(mass %)

1.78


K2O
(mass %)

2.70


CaO
(mass %)

7.07


MgO
(mass %)

8.09


SrO
(mass %)

2.97


BaO
(mass %)
15.0
4.40


La2O3
(mass %)
5.0



Total
(mass %)
100.0
100.00


Li2O + Na2O + K2O
(mass %)
5.00
7.91


MgO + CaO
(mass %)
0.0
15.16









(MgO + CaO)/(Li2O + Na2O + K2O)
0.00
1.92


nd
1.5550
1.5606


νd
68.0
68.5


Weather resistance
B
B









At first, employing glass raw materials which are generally used, for example, an oxide material, carbonate material, nitrate material, phosphate material, and hydroxide material, the raw materials of glass were prepared so as to achieve the aimed glass composition shown in Tables 1 to 4, and were sufficiently mixed as powder to prepare a mother material. The mother material was charged into a fusing furnace heated at 900° C.-1,300° C., then, it was melted, clarified, and mixed to be homogeneous. After that, the resulting material was cast in a cast die made of iron heated in advance, and was gradually cooled to the room temperature (25° C.) at a cooling rate of −50° C./hour. Thereby respective samples were prepared.
With respect to each sample, refractive index (nd) which is a refractive index at Helium d-line (wavelength of 587.56 nm) and Abbe number (νd) were measured, and the weather resistance was evaluated. The refractive index nd and the Abbe number νd were obtained by measuring each of the prepared example with measuring equipment “KPR-200” manufactured by Kalnew Optical Industries Co., Ltd. The weather resistance was evaluated by the following process. After each of the prepared samples was polished to have a mirror-finished surface, each sample was held in a thermo-hygrostat chamber with cleanliness at temperature of 60° C. and humidity of 95% for 168 hours. Then, each sample was taken out from the chamber and the surface of each sample was observed by a microscope (×40) to evaluate the weather resistance. When the condition of the surface appeared to be unchanged and each sample was judged to have an excellent weather resistance, the evaluation result was represented by the mark “A”. When something unusual (generation of cloudiness and deposit of abnormal materials) appeared on the surface and each sample was judged to have a problem in the weather resistance, the evaluation result was represented by the mark “B”.
The results are also shown in Tables 1 to 4. It has been verified that Comparative examples 1 and 2 which were optical glass having the composition with component ratios outside those of the above embodiments, showed a problem in the weather resistance, and that, on the other hand, Examples 1 to 15 which were optical glass having the composition with the component ratios satisfying the above embodiment, showed an excellent weather resistance.
Namely, optical glass of each of the examples has a composition comprising: 40-65 mass % of P2O5: and 1-5 mass % of Li2O+Na2O+K2O, where 5 mass % is excluded. In the optical glass, (MgO+CaO)/(Li2O+Na2O+K2O) which is a ratio of a total content expressed in mass % of MgO and CaO to a total content expressed in mass % of Li2O, Na2O, and K2O, is in a predetermined range, where MgO and CaO are components which are effective in enhancement of the weather resistance, and Li2O, Na2O, and K2O are components which worsen the weather resistance but are essential to stabilizing glass. Therefore, phosphate optical glass with excellent weather resistance and low dispersion can be obtained.
Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein.

## Claims

1. An optical glass having a composition comprising:
P2O5: 41-62 mass %, and
Li2O+Na2O+K2O: 1-5 mass %, where 5 mass % is excluded, wherein (MgO+CaO)/(Li2O+Na2O+K2O) which is a ratio of a total content expressed in mass % of MgO and CaO to a total content expressed in mass % of Li2O, Na2O, and K2O is in a range from 6 to 38.

2. The optical glass of claim 1, wherein the composition further comprising:
Al2O3: 1-10 mass %, B2O3: 1-20 mass %, Li2O: 1-5 mass %, where 5 mass % is excluded, and CaO: 6-38 mass %.

3. The optical glass of claim 2, wherein the composition further comprising:
Na2O: 0-3 mass %, K2O: 0-3 mass %, MgO: 0-25 mass %, SrO: 0-10 mass %, BaO: 0-12 mass %, ZnO: 0-14 mass %, and Sb2O3: 0-0.5 mass %.

4. The optical glass of claim 2, wherein the composition further comprising:
SiO2: 0-0.1 mass %.

5. The optical glass of claim 1, wherein the optical glass does not contain PbO, As2O3, and a fluorine component.

6. The optical glass of claim 1, wherein the optical glass has a refractive index from 1.54 to 1.62, and an Abbe number from 65 to 70.

7. The optical glass of claim 1, wherein the optical glass comprises Li2O, Na2O, K2O, MgO and CaO.