JP2004217513A - Optical glass, preform for press molding and optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide optical glass which is free of lead and fluorine, and which has a low glass transition temperature permitting press-molding with a mold formed of stainless steel and high weather resistance. <P>SOLUTION: The optical glass contains, by mol %, 25-44% of P<SB>2</SB>O<SB>5</SB>, 10-40% of the total of Li<SB>2</SB>O, Na<SB>2</SB>O and K<SB>2</SB>O, 5-40% of ZnO, 1-35% of BaO and at least one component selected from Nb<SB>2</SB>O<SB>5</SB>, Bi<SB>2</SB>O<SB>3</SB>and WO<SB>3</SB>, having a glass transition temperature (Tg) of 370°C or lower and being free of lead and fluorine. Or, the optical glass is free of lead and fluorine, has a mass loss ratio of less than 0.25% when the optical glass is immersed in pure water of 100°C for 60 min, and has a glass transition temperature (Tg) of 370°C or lower. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to an optical glass, a preform for press molding and a method for producing the same, and an optical element and a method for producing the same. More specifically, the present invention relates to an optical glass which does not contain lead or fluorine, has a low transition temperature capable of being press-formed with a stainless steel forming die, and has high weather resistance, and a press-forming mold made of the optical glass. The present invention relates to a reform and a method for producing the same, and an optical element made of the optical glass and a method for producing an optical element by press-molding the preform.

  In general, precision press molding of glass is known as a method for producing an optical functional surface such as an aspherical lens by press molding, and it is difficult to process by grinding and polishing, or a glass having an optical functional surface that is extremely troublesome. An optical element can be provided with high productivity and low cost. In order to obtain a press-formed product by such precision press-forming, a glass having a low glass transition temperature (Tg) is advantageous from the viewpoint of lowering the press-forming temperature to prevent breakage and deterioration of the press-forming die. .

  Therefore, among optical glasses, glass having a relatively low glass transition temperature of 500 to 600 ° C. is used as precision press-molding glass. In addition to using glass with a low transition temperature and using a high heat-resistant mold material such as ceramics for the press mold, a release film is provided on the molding surface to prevent fusion between the mold and the glass. Press molding is performed in a non-oxidizing atmosphere in order to prevent deterioration due to oxidation of the mold.

  By the way, optical elements produced by precision press molding are required to have higher shape accuracy with respect to the shape including the optical function surface. For example, in an imaging optical system for a mobile device, the number of lenses tends to be reduced for miniaturization. Therefore, good correction of aberrations and the like must be performed with a small number of lenses, and a specially shaped lens is required, and the shape accuracy of the lens must be further improved. Further, with the increase in the number of pixels of the image sensor, a higher level of performance required for the optical system is required.

  In order to produce such an optical element by precision press molding, a more accurate press mold is necessary, and cutting and electric discharge machining are suitable for producing such a mold. However, although ceramic-based mold materials conventionally used are excellent as press-molding mold materials, there is a problem that cutting is difficult.

  On the other hand, a metal material such as stainless steel used for injection molding of a plastic lens can be used as a material which can be cut and which can be molded with higher precision. However, stainless steel cannot withstand the high temperature of 500 ° C. or higher required for press forming of glass, and when a stainless steel mold is repeatedly used at a high temperature of 400 ° C. or higher, the mold surface becomes rough and becomes brittle. It is not suitable as a mold for precision press molding.

  Therefore, in order to use a stainless steel press mold, glass that can be press-molded at a temperature lower than 400 ° C. is required.

  Until now, it was not intended to use a stainless steel press mold, but attempts to significantly lower the transition temperature by incorporating lead, and to lower the glass by introducing fluorine, Attempts have been made to convert. However, while lead is an effective component in lowering the glass transition temperature, it is now a substance that should be discouraged because it has a serious impact on the environment. In addition, fluorine exhibits high volatility at high temperatures, so when molding a preform for press molding from molten glass, it causes striae due to volatilization, making it difficult to produce a preform stably. At this time, volatiles from the glass adhere to the press mold, which causes the shape accuracy of the press-formed surface to be reduced or deteriorated. Therefore, a glass that does not contain lead or fluorine is desired, and an optical glass that does not contain such lead or fluorine has been proposed (for example, see Patent Documents 1 and 2).

  The invention described in Japanese Patent Application Laid-Open Publication No. H11-157,086 is to impart low-temperature softening properties to glass while solving the problems caused by the introduction of lead and fluorine. However, in the glass described in this publication, it is difficult to reduce the sagging temperature of the glass to 400 ° C. or lower because the amount of the alkali metal oxide which is a component for lowering the glass transition temperature is insufficient. . Therefore, when press-molding the glass described in this publication, a stainless steel mold cannot be used.

On the other hand, the glass described in Patent Literature 2 has a P ₂ O ₅ content of 48 to 58% by weight, so that the thermal stability of the glass, that is, the devitrification resistance, and the weather resistance also decrease. Resulting in. The devitrification resistance is a property necessary to prevent the transparency of the glass from being impaired when the preform for press molding is formed from the molten glass. One of the advantages of glass compared to plastic is that it has high weather resistance. However, if the weather resistance is reduced as described above, the superiority of glass over plastic materials is impaired. Will be.

JP-A-11-139845 JP-A-2000-72474

  The present invention, under such circumstances, contains no lead or fluorine, has a low transition temperature that can be press-molded with a stainless steel mold, and has an optical glass with high weather resistance, from the optical glass. It is an object of the present invention to provide a preform for press molding and a method for producing the same, and an optical element made of the optical glass and a method for producing an optical element by press molding the preform.

  The present inventors have conducted intensive studies in order to achieve the above object, and as a result, an optical glass having a specific composition and a glass transition temperature not higher than a certain value and containing no lead and fluorine, or lead and fluorine. Not containing, the weight loss rate and the glass transition temperature when immersed in pure water, the optical glass of a certain value or less, respectively, found that the object can be achieved, based on this finding to complete the present invention. Reached.

That is, the present invention
(1) by _mol%, P 2 _{O 5} to _{25~44%, Li 2 O, 10~40} % in a total amount of Na ₂ O and _{K 2} O, the ZnO 5 to 40%, a BaO 1 to 35 %, Further containing at least one component selected from Nb ₂ O ₅ , Bi ₂ O ₃ and WO ₃ , having a glass transition temperature (Tg) of 370 ° C. or less and containing no lead and fluorine. Optical glass (referred to as optical glass I),
(2) In terms of mol%, Li ₂ O is 5 to 30%, Na ₂ O is 0 to 25%, K ₂ O is 0 to 15%, and the total amount of Nb ₂ O ₅ , Bi ₂ O ₃ and WO ₃ is in 0.1 to _15% B 2 _{O 3} of 0% _La 2 _{O 3} 0 to 5% of _Gd 2 _{O 3} 0 to _5%, include Y 2 _{O 3} 0 to 5%, P The optical glass according to the above (1), wherein the total content of ₂ O ₅ , ZnO, BaO and each of the components is 96% or more,
(3) The composition does not contain lead and fluorine, has a weight loss of less than 0.25% when immersed in pure water at 100 ° C. for 60 minutes, and has a glass transition temperature (Tg) of 370 ° C. or less. Characteristic optical glass (referred to as optical glass II),
(4) The optical glass according to any one of the above (1) to (3), wherein the refractive index (nd) is 1.52 to 1.7 and the Abbe number (νd) is 42 to 70.
(5) A preform for press molding, comprising the optical glass according to any one of the above (1) to (4),
(6) A preform for press molding comprising the optical glass according to any one of the above (1) to (4), the weight being equal to the predetermined weight while the predetermined weight of the molten glass is in a softened state. A method for producing a preform for press molding, characterized by molding
(7) An optical element comprising the optical glass according to any one of the above (1) to (4), and (8) the preform for press molding according to the above (5) or the above (6). Heating the preform for press molding produced by the production method described in the above, a method for producing an optical element characterized by press molding,
Is provided.

According to the present invention, it is possible to provide an optical glass that does not contain lead or fluorine, has a low transition temperature that can be press-formed with a stainless steel forming die, and has high weather resistance.
Further, according to the present invention, a press-molding press for producing an optical element having high transition resistance and having a low transition temperature which can be press-formed with a stainless steel forming die and made of the above-mentioned glass by press-forming. A reform and a method for manufacturing the same can be provided.
Furthermore, according to the present invention, an optical element made of the glass and having a low transition temperature that can be press-molded with a stainless steel mold and having high weather resistance and an optical element obtained by press-molding the preform are used. A method of manufacturing can be provided.

<Optical glass>
First, the optical glass of the present invention will be described.
The glass transition temperature (Tg) is a factor that determines the temperature at which the glass is heated and softened and pressed. By lowering the glass transition temperature, it is possible to lower the heating temperature of the preform for press molding and the press mold during press molding. If the temperature of the press mold exposed during press molding can be reduced to less than 400 ° C. by further lowering the glass transition temperature, a stainless steel press mold excellent in workability can be used. When the stainless steel is exposed to a high temperature of 400 ° C. or more, the surface becomes rough or the brittleness advances, so that it cannot be used as a press mold.

  The transition temperature of glass that can be pressed by a stainless steel press mold is 370 ° C. or less. Further, in order for the mold to withstand mass production, an optical glass having a glass transition temperature of 360 ° C. or lower is preferable, a glass transition temperature of 350 ° C. or lower is more preferable, and a glass transition temperature of 340 ° C. or lower is more preferable.

The optical glass of the present invention does not contain lead and fluorine, that is, does not use lead or fluorine raw materials. Because of its toxicity, lead is a raw material that should not be used in consideration of its impact on the environment. Furthermore, when the optical glass containing lead is heated and softened and press-molded in a non-oxidizing atmosphere such as nitrogen gas, the lead oxide in the glass is reduced and the glass surface becomes cloudy or adheres to the press mold. The problem arises. On the other hand, fluorine has a high volatility, so it will volatilize when forming a preform for press molding from molten glass and generate striae and an altered layer in the glass, or will volatilize during press molding to form a press mold. This causes problems such as adhesion. Therefore, the above problem can be solved by eliminating lead and fluorine from glass.
Further, the optical glass of the present invention is a glass having excellent durability using water resistance as an index.

The optical glass of the present invention has two embodiments, optical glass I and optical glass II, each of which will be described below.
[Optical glass I]
The optical glass I is represented by mol%, ₂₅ to 44% of P ₂ O ₅ , 10 to 40% of Li ₂ O, Na ₂ O and K ₂ O in total, 5 to 40% of ZnO, and 1 of BaO. To 35%, further containing at least one component selected from Nb ₂ O ₅ , Bi ₂ O ₃ and WO ₃ , having a glass transition temperature (Tg) of 370 ° C. or lower, preferably 360 ° C. or lower, An optical glass characterized by not containing lead and fluorine. However, the content of P ₂ O ₅ is desirably 46% or less in terms of weight%, and the total content of Li ₂ O, Na ₂ O and K ₂ O is desirably 10% or more in terms of weight%. Hereinafter, the contents are indicated by mol% unless otherwise specified.

This optical glass I is selected from Nb ₂ O ₅ , Bi ₂ O ₃ and WO ₃ based on P ₂ O ₅ —R ₂ O (R = Li, Na, K) —ZnO—BaO based glass. While introducing at least one component, taking a wide range of BaO content and reducing the content of Li ₂ O, Na ₂ O, and K ₂ O, the glass transition temperature is lowered, This is a significant improvement in stability and weather resistance.
When Bi ₂ O ₃ is not contained in the optical glass I, the amount of TiO ₂ introduced is preferably less than 2%, more preferably 1.5% or less.

In the optical glass I, 5 to 30% of Li ₂ O, 0~25% of Na ₂ O, _0~15% of _{K 2} O, the _{Nb 2 O 5, Bi 2 O} 3 and WO ₃ in a total amount of 0. 1 to _15% B 2 _{O 3} of 0% _La 2 _{O 3} 0 to 5% of _Gd 2 _{O 3} 0 to _5%, include _{Y 2 O 3 0~5%, P} 2 O 5 , ZnO, BaO and a glass having a total amount of the above components of 96% or more are preferable.

A more preferred _range, the P 2 _{O 5} 25~38%, the Li ₂ O 7~25%, 0~20% of Na ₂ O, 0% to _{K 2} O _(where, Li 2 O, Na ₂ 20-40% in a total amount of O and _{K 2} O), _7~30% of ZnO, 5 to _20% of _BaO, B 2 _{O 3} less than 8%, _La 2 _{O 3} and 0 to 3%, Gd ₂ O _{3 0-3%} the Y 2 _{O 3} 0-3%, the _Al 2 _{O 3} 0-2%, comprises TiO ₂ less than 0-2%, the total content of the respective components is not less than 98% Range.

An even more preferred range is that P ₂ O ₅ is 25 to 38%, Li ₂ O is 7 to 25%, Na ₂ O is 0 to 20%, and K ₂ O is 0 to 10% (however, Li ₂ O, Na ₂ 20-40% in a total amount of ₂ O and _{K 2} O), a ZnO 7 to 30% 5 to _20% of _BaO, B 2 _{O 3} less than 8%, the _La 2 _{O 3} 0 to 3% Gd ₂ O _{3 0-3%} the Y 2 _{O 3} 0-3%, the _Al 2 _{O 3} 0 to 2% of TiO ₂ comprising from 0 to 1.5%, the total content of each component is 98% This is the above range.

Further, preferred compositions over the entire optical glass I are as follows (i) to (iii).
(I) The total content of Nb ₂ O ₅ , Bi ₂ O ₃ and WO ₃ is 1 to 15%, more preferably 2 to 12%, and still more preferably 1 to 8%.
(Ii) Li ₂ O, Na ₂ O and K ₂ O having a total content of 22 to 35%.
(Iii) Those containing at least one of Na ₂ O and K ₂ O.
In addition, those satisfying both (i) and (ii), those satisfying both (ii) and (iii), those satisfying both (iii) and (i), those satisfying (i), (ii) and (iii) Are more preferably satisfied.

Further, a preferable composition over the entire composition range is P ₂ O ₅ , Li ₂ O, Na ₂ O, K ₂ O, ZnO, BaO, Nb ₂ O ₅ , Bi ₂ O ₃ , WO ₃ , B ₂ O ₃ , La The total content of ₂ O ₃ , Gd ₂ O ₃ , Y ₂ O ₃ , Al ₂ O ₃ , and TiO ₂ is 99% or more, and more preferably, the total content is 100%. _{P 2 O 5, Li 2 O} , Na 2 O, K 2 O, ZnO, BaO, Nb 2 O 5, Bi 2 O 3, WO 3, the total content of B 2 _{O 3} is further not less than 99% Preferably, 100% is particularly preferred. However, fining agents such as Sb ₂ O ₃ are not included in the total content. Sb ₂ O ₃ is preferred as the fining agent. As ₂ O ₃ is not recommended because it is toxic and adversely affects the press mold.
In addition, radioactive substances such as uranium and thorium, and harmful substances such as cadmium should be eliminated.

  When imparting near-infrared absorbing properties to the optical glass I, a small amount of CuO can be introduced, but from the viewpoint of obtaining a glass without coloring, it is preferable not to introduce a coloring component such as CuO.

Next, the reason for limiting the composition range of each component in the optical glass I will be described.
P ₂ O ₅ is a main component constituting the network structure of glass, and is a component essential for stable operation and glass formation. If the content is less than 25%, the thermal stability of the glass is reduced, and the weather resistance is also reduced. On the other hand, if it exceeds 44%, the viscosity of the glass melt becomes high, and it becomes difficult to introduce Nb ₂ O ₅ , Bi ₂ O _3, or WO ₃ , so that hot preform molding may not be possible. Therefore, the introduction amount is 25 to 44%. More preferably, it is in the range of 25 to 38%. Note that the content of P ₂ O ₅ is desirably 46% by weight or less.

In the optical glass I, at least one kind of alkali metal oxide R ₂ O (R = Li, Na, K) of Li ₂ O, Na ₂ O, and K ₂ O is used to lower the glass transition temperature, melting point, and softening temperature. It is necessary to contain it. The introduction of alkali metal oxides also has the effect of lowering the liquidus viscosity (viscosity of the glass at the liquidus temperature), and imparts the processing characteristics required for precision press molding glass and press preform molding to glass. Can be. If the total content of Li ₂ O, Na ₂ O, and K ₂ O is less than 10%, the glass transition temperature increases, and the viscosity of the glass melt at the hot preform molding temperature increases. Difficulty in molding. On the other hand, if the total of these contents exceeds 40%, the weather resistance and the thermal stability of the glass become low. Therefore, the total content of the optical glass I is set to 10 to 40%. It is more preferably in the range of 20 to 40%, and still more preferably in the range of 22 to 35%. Note that the total content of Li ₂ O, Na ₂ O, and K ₂ O is desirably 10% by weight or more.

Among the alkali metal oxides, Li ₂ O has a large effect of lowering the glass transition temperature, and thus it is preferable to introduce Li ₂ O as the alkali metal oxide. When Li ₂ O is introduced, it is desirable that the content be 5% or more. However, if it is more than 30%, the glass tends to be devitrified and the liquidus temperature may be increased. It is preferred to be in the range of 5 to 30%. A particularly preferred range is from 7 to 25%.

Further, in order to reduce the tendency of devitrification when only Li ₂ O is contained as the alkali metal oxide R ₂ O (R is Li, Na, K), at least one of Na ₂ O and K ₂ O is contained. It is preferable that both are contained.
The amount of Na ₂ O introduced is preferably 0 to 25%, and the amount of K ₂ O introduced is preferably 0 to 15%. The reason is that if Na ₂ O exceeds 25%, the durability and stability of the glass deteriorate, and if K ₂ O exceeds 15%, it becomes difficult to lower the melting point of the glass. A more preferred range is in Na ₂ O is 0 to 20%, _{K 2} O is 0 to 10%.

  ZnO is a modifying component of glass and is used to adjust various properties of glass. In particular, it greatly contributes to lowering the melting point of glass, but if its content exceeds 40%, the thermal stability of the glass decreases, or the liquidus viscosity of the glass increases, making it suitable for precision press molding glass. The processed characteristics are impaired. However, if the content is less than 5%, low softening properties and high durability of the glass may not be obtained. Therefore, the content is set to 5 to 40%. More preferably, it is in the range of 7 to 30%.

BaO is also a modifying component of glass, and is used to adjust various properties of glass. It also has the effect of increasing the weather resistance of the glass. If the content exceeds 35%, low softening properties cannot be obtained, and the liquidus temperature may increase. On the other hand, if the content is less than 1%, required durability and weather resistance cannot be obtained, and the glass is extremely lost. The amount of introduction is set to 1 to 35% because it is easily transparent. More preferably, it is in the range of 5 to 20%.
Further, at least one selected from MgO, CaO and SrO may be introduced in a total amount of 0 to 8%.

The optical glass I contains at least one component selected from oxides composed of Nb ₂ O ₅ , Bi ₂ O ₃ and WO ₃ . These components dramatically improve the durability of the glass and greatly contribute to improving the stability of the glass. However, if too many of these components are introduced, low softening properties of the glass cannot be obtained, the glass tends to be colored, and the devitrification resistance also deteriorates. The total content of the above three components is preferably in the range of 0.1 to 15%, more preferably in the range of 1 to 12%, further preferably in the range of 1 to 8%, and still more preferably in the range of 2 to 8%. The content of _{Nb 2 O 5, Bi 2 O} 3, WO 3, Nb 2 O 5 is 0 to 10%, _Bi 2 _{O 3} is 0-10%, WO ₃ is preferably a range of 0%. Among them, those in which Nb ₂ O ₅ and Bi ₂ O ₃ are introduced and WO ₃ is not introduced, and those in which Nb ₂ O ₅ or Bi ₂ O ₃ is introduced and WO ₃ is not introduced are more preferable.

B ₂ O ₃ is a component that has an effect of improving the weather resistance of the glass by adding a small amount thereof and reducing the optical characteristics of the glass. However, if the content exceeds 10%, the transition temperature of the glass is significantly increased, and the durability is greatly reduced. Therefore, the content is preferably set to 0 to 10%. More preferably, it is less than 8%.

La ₂ O ₃ , Gd ₂ O ₃ , and Y ₂ O ₃ have the effect of improving the weather resistance of the glass and are components that can be arbitrarily introduced. However, if the content exceeds 5%, it becomes difficult to obtain a desired low softening property, and the liquidus viscosity tends to increase. Therefore, the content of each of La ₂ O ₃ , Gd ₂ O ₃ , and Y ₂ O ₃ is preferably set to 0 to 5%. More preferably, it is 0 to 3%. The total content of La ₂ O ₃ , Gd ₂ O ₃ , and Y ₂ O ₃ is preferably 0 to 5%, more preferably 0 to 3%, and more preferably 0 to 2% over the entire optical glass I. Is more preferable.
The content of other optional components is 0 to 2% for Al ₂ O _{3 and} 0 to 2% for TiO _2. However, from the viewpoint of further improving the above-mentioned properties, neither Al ₂ O ₃ nor TiO ₂ is introduced. Is preferred.

  Furthermore, when imparting a near-infrared light absorption function, an appropriate amount of CuO may be introduced, but from the viewpoint of avoiding coloring of the glass, it is preferable not to introduce CuO. Further, in order to avoid coloring of the glass, introduction of Cr, V, Fe, and the like should be avoided. Other preferable examples of not introducing are Ag, Au, Te, Se, Hf, Tl, and Ni.

[Optical glass II]
Optical glass II does not contain lead and fluorine, has a weight loss of less than 0.25% when immersed in pure water, and has a glass transition temperature (Tg) of 370 ° C. or less. Optical glass. The optical glass II, like the optical glass I, does not contain lead and fluorine and has a glass transition temperature (Tg) of 370 ° C. or lower.

  The method of measuring the weight loss (Dw) of the mass when immersed in pure water is based on the 1999 edition of the Japan Optical Glass Industrial Association Standards, "Measurement Method for Chemical Durability of Optical Glass (Powder Method) 06". In this method, a glass having a new broken surface is crushed in a mortar, and a powder having a size of 425 μm, which passes through a standard mesh sieve (JIS Z 8801) of 600 μm and is kept at 425 μm, is collected from powder having passed through an auxiliary mesh sieve of 710 μm. Three times the specific gravity of the glass is placed in a 50 ml beaker, 15 ml of 99.5% by volume methyl alcohol is added, and the glass fines are removed by a gradient method. After this washing is repeated 5 times, the resultant is dried in an air bath at 120 to 130 ° C. for 60 minutes and stored in a silica gel desiccator.

A sample prepared in this way was collected in a 177-210 μm elution basket having a cylindrical platinum standard sieve having a diameter of 15 mm and a height of 60 mm corresponding to a specific gravity gram of glass, shaken, and then placed in a weighing bottle with a lid. Weigh precisely. The mass of the sample obtained at this time is defined as M1. Next, 80 ml of pure water having a pH of 6.5 to 7.5 is put into the washed and dried round-bottomed flask, attached with a cooling tube, and kept in a heating device for 10 minutes. The elution basket containing the sample is gently inserted into the round bottom flask, treated in a heating device for 60 minutes, and then removed. Next, 80 ml of 99.5% by volume of methyl alcohol is put into a 100 ml beaker, and a basket is immersed in the beaker for washing. After repeating the immersion and washing using methyl alcohol three times, the sample is placed in a weighing bottle and dried in an air bath at 120 to 130 ° C. for 60 minutes. The dried sample is transferred to a silica gel desiccator, allowed to cool for 60 minutes, and precisely weighed with the lid closed. The mass of the sample obtained at this time is defined as M2. As the heating device, a boiling water bath having a depth capable of completely accommodating the spherical portion having a diameter of 60 mm of the round bottom flask is used. The temperature of the water in the water bath can be maintained at 99 ° C. or higher at a horizontal position of 20 mm from the bottom of the round bottom flask.
The above weight loss rate is represented by (M1−M2) / M1, but it is desirable to repeat the above operation twice in order to obtain an accurate weight loss rate.

  When the weight loss rate thus obtained is expressed in%, it is less than 0.25% for the optical glass II. In the optical glass II, a more preferable weight loss rate is less than 0.10%, and a further preferable weight loss rate is 0.05% or less. The above weight loss rate indicates the degree of elution of glass into water, and indicates white burn resistance. Glass with a large weight loss rate is liable to cause white burns due to the washing process and moisture in the air. When the surface of the optical element is burnt, the transparency of the optical element is impaired, and the optical element cannot be used. In addition, the surface of the preform is exposed to the air from the time the preform for press molding is formed until it is subjected to precision press molding. At this time, unless the glass has a weight loss rate of less than 0.25% and is excellent in white burn resistance, white burn occurs on the preform surface. When precision press molding is performed using a preform in which white burn has occurred, an altered layer due to white burn remains on the surface of the press-formed product. Precision press molding is a press molding method that does not apply grinding or polishing to the optically functional surface of the molded product (for example, the aspherical surface of an aspherical lens). I do not want to remove it by polishing. It is also conceivable that white burn of the preform adversely affects the molding surface of the press mold during press molding. Therefore, the weight loss rate needs to be suppressed to less than 0.25%.

  One of the features of this optical glass II is that the glass transition temperature is extremely low. If the weather resistance may be low, a plastic that can be molded at a low temperature may be used. Therefore, an optical glass used for an optical element made of glass cannot show an advantage over plastic unless it has high weather resistance. An optical glass exhibiting the above-mentioned weight loss rate can obtain much better weather resistance than plastic, and is very useful as a glass material for an optical element that performs a sufficient function in various situations.

Examples of the optical glass II include P ₂ O ₅ , ZnO, and BaO, and at least one component selected from Li ₂ O, Na ₂ O, and K ₂ O, Nb ₂ O ₅ , Bi ₂ O _3, and WO ₃ Preferred are those containing at least one component selected from the group consisting of: a component obtained by adding B ₂ O and Sb ₂ O ₃ to the aforementioned components; and the sum of the contents of P ₂ O ₅ , ZnO, BaO and the above components Those having a value of 98 mol% or more are more preferable, those having a total of 99% or more are more preferable, and those having a total of 100% are particularly preferable. When Bi ₂ O ₃ is contained in the optical glass II, the amount of TiO ₂ introduced is preferably set to less than 0 to 2%, more preferably 0 to 1.5%.
Further, those having the same composition as the optical glass I are also preferable.

Also, As ₂ O ₃ is not recommended because it has toxicity and adverse effects on the press mold. In addition, radioactive substances such as uranium and thorium, and harmful substances such as cadmium should be eliminated. When imparting near-infrared absorption properties to the optical glass II, a small amount of CuO can be introduced, but from the viewpoint of obtaining a glass without coloring, coloring components such as Cr and V including CuO are not introduced. Is preferred. Further, it is preferable not to introduce Ag, Au, Te, Se, Hf, Tl, Ni and the like.

[Characteristics common to optical glasses I and II]
Next, characteristics common to the optical glasses I and II will be described.
[Liquid phase temperature and liquid phase viscosity]
The liquidus temperature of the optical glass of the present invention is preferably 900 ° C. or lower, more preferably 850 ° C. or lower, further preferably 820 ° C. or lower, and even more preferably 800 ° C. or lower. Since the liquidus temperature is low as described above, when forming a glass molded body from molten glass, it is easy to prevent devitrification of the glass.
When a predetermined amount of a molten glass lump is taken out of the molten glass and hot forming is performed to form a preform for press molding while the lump is in a softened state, restrictions are imposed on the high-temperature working viscosity of the glass. In this respect, the viscosity of the glass at the liquidus temperature is preferably 4 dPa · s or more, more preferably 5 dPa · s or more. Further, the viscosity is preferably 100 dPa · s or less, and more preferably 50 dPa · s or less. When the viscosity of the glass at the liquidus temperature is less than 4 dPa · s, it is difficult to control the amount of the molten glass when flowing or dripping the molten glass from a pipe or a nozzle, and it is difficult to form a preform for press molding having a predetermined weight. It may not be possible. In particular, it becomes extremely difficult to drop a predetermined amount of molten glass droplet from the nozzle. On the other hand, when the viscosity exceeds 100 dPa · s, it becomes difficult to separate a predetermined amount of molten glass lump from the molten glass flow without using a cutting blade. When the temperature is increased to lower the viscosity of the glass and to separate the molten glass flow, there is a problem in that the components of the glass are likely to evaporate during forming, so that surface striae are likely to occur.
Therefore, a glass exhibiting a viscosity of 5 to 50 dPa · s at the liquidus temperature is more preferred, and a glass exhibiting a viscosity of 10 to 50 dPa · s is particularly preferred.

[Chemical durability]
Next, the chemical durability of the optical glass of the present invention will be described. Chemical durability can be quantitatively evaluated by the degree of light scattering caused by haze on a polished surface of an optical glass placed in a predetermined environment.

  The method for measuring the degree of the light scattering may be based on the "Method for measuring the chemical durability of optical glass (surface method) 07" of the Japan Optical Glass Industrial Association. First, a sample made of glass to be measured and having two surfaces parallel to each other optically polished is prepared. If it is not possible to obtain a small preform or an optical element having a size sufficient for performing the above evaluation, a sample made of glass having the same composition may be prepared. Here, optical polishing refers to a polished state in which the optical functional surface of an optical element such as a lens is finished to a surface average roughness Ra. Specifically, a polishing state in which the surface average roughness Ra is sufficiently smaller than the wavelength on the short wavelength side of the visible light region, for example, 1/10 or less, may be used as a guide. In addition, a sample in a clean state is used. Such a sample is held for one week in a thermo-hygrostat (atmosphere is air) maintained at a temperature of 65 ° C. and a relative humidity of 90%, for example. The air is preferably clean, for example cleaner than class 1000, preferably cleaner than class 100. Next, when white light (C light source or standard light C) is irradiated from the vertical direction onto the optically polished surface of the sample after holding for one week, the incident light intensity of white light and the transmitted light intensity transmitted through the sample are measured. Then, the ratio of the scattered light intensity to the transmitted light intensity (scattered light intensity / transmitted light intensity) is obtained by setting the scattered light intensity to a value obtained by subtracting the transmitted light intensity from the incident light intensity. If the above ratio is small, the degree of fogging is small, indicating that the glass has high chemical durability.

  The ratio of the scattered light intensity to the transmitted light intensity (scattered light intensity / transmitted light intensity) thus measured preferably shows a value of 0.08 or less, more preferably 0.03 or less. This value is called a haze value when expressed in%. Therefore, those having a haze value of 8% or less are preferable, and those having a haze value of 3% or less are more preferable.

  Glass having a ratio of the scattered light intensity to the transmitted light intensity larger than the above range may be eroded by various chemical components such as water droplets and water vapor attached to the glass and gas in a use environment, or a reactant may be formed on the glass surface. It is a glass that has a high rate of formation, that is, has low chemical durability. When such a glass is used as an optical element, foreign matter is generated on the surface of the optical glass element due to erosion of the glass or a product on the glass surface, and optical characteristics such as transmittance may be reduced. Such glasses are not preferred as optical glass compositions. On the other hand, if the glass has the chemical durability in the above range, a practical and highly reliable optical element can be produced.

In particular, in a pickup lens for reading information from an optical disk such as a compact disk or a DVD, and for reading and writing information, the reflection of light incident on the optical disk after passing through the pickup lens is picked up by the same pickup lens. Lead to the detector. Therefore, the light will pass through the pickup lens at least twice. Therefore, even if the foreign matter or fogging is slight on the lens surface, a large loss of an optical signal is caused. In particular, in order to increase the recording density, light having a shorter wavelength tends to be used. However, since the output of such a light source is still at a low level, it is strongly desired to minimize the loss in the optical system. . The same applies to the case of a collimator lens used for recording and reproduction of an optical disk.
Since the optical glass of the present invention has excellent durability and weather resistance as described above, it is particularly suitable as a lens material to be disposed in an optical path where light reciprocates, as in the above-described pickup lens.

[Refractive index (nd) and Abbe number (νd)]
Next, the optical characteristics of the optical glass will be described.
In order to provide the glass with good weather resistance and to provide a glass excellent in low-temperature moldability while realizing the extremely low glass transition temperature as described above, the refractive index (nd) of the optical glass I and the optical glass II is important. ) Is desirably 1.52 to 1.7, and the Abbe number (νd) is desirably 42 to 70.
Further, a region where the refractive index (nd) is 1.52 to 1.7 and the Abbe number (νd) is 42 to 70 is a region where the refractive index (nd) and the Abbe number (νd) of the commonly used optical glass are used. Occupies the middle position. With the miniaturization of optical devices and mobile devices, there is a demand for an optical system in which the number of lenses is reduced as much as possible. In such a case, an optical glass having an optical constant in the above range is particularly useful.

[Use of optical glass]
The optical glass of the present invention is used in addition to lens elements such as aspherical lenses, micro lenses, pickup lenses, and collimator lenses, diffraction gratings, lenses with diffraction gratings, prisms, and the like, and is also used as a material for various optical elements described in detail below. However, it is particularly suitable as a material for a preform for press molding, and further suitable as a preform for precision press molding.
The preform for press molding is a glass molded body which is subjected to press molding in a heated and softened state, and is made of glass having a weight corresponding to the weight of the press molded article, and has a shape suitable for press molding (for example, , A spheroid, a spheroid, a shape obtained by flattening a spheroid in the direction of the rotation axis, etc.).
Precision press molding is a method in which glass in a heated and softened state is pressed and molded by a press mold, and the molding surface of the press mold is precisely transferred to the glass. The optical function surface of the optical element formed by this method is formed with high precision, and does not require any mechanical processing. In addition, the optical function surface is used to function as an optical element that refracts, reflects, or diffracts light, such as an aspheric surface of an aspheric lens or a surface on which a diffraction grating is formed. This is the aspect that is needed.

<Preform for press molding and manufacturing method thereof>
The preform for press molding of the present invention comprises the above optical glass I or optical glass II. Therefore, the preform has the properties of the optical glass of the present invention, and the optical element manufactured using the preform also has the properties of the optical glass.
Since this preform for press molding is made of the optical glass of the present invention, it has no defects such as devitrified portions and striae, and is excellent in weather resistance and chemical durability. Further, since the glass transition temperature is as low as 370 ° C. or less, preferably 360 ° C. or less, it is particularly suitable as a preform for precision press molding. As described above, even for glass for precision press molding, since the glass transition temperature is extremely low, press molding is possible in a temperature range of 400 ° C. or less, and usually precision press molding of SiC or cemented carbide is used. It is possible to use not only the press mold used but also a stainless steel press mold. Further, even when a press mold made of SiC or cemented carbide is used, the press temperature can be set low, so that the load on the press mold is reduced, and as a result, the life of the press mold can be extended. .

  The preform radius Rp (equivalent to the radius of a sphere in the case of a spherical preform) when the preform for press molding is placed in the press mold and viewed from the pressing direction is the molding surface of the press mold. (If the molding surface is an aspheric surface, it is equivalent to the radius of curvature when the portion having the largest curvature is approximated by a spherical surface), the distance between the preform and the molding surface of the press mold is larger. Creates a closed space. If press molding is performed in this state, the gas confined in the closed space will not escape, and the molded product will have a poor shape due to the formation of depressions on the surface of the press molded product. Although such a problem is called a defect due to a gas trap, according to the preform for press molding of the present invention, the viscosity of the glass at the time of pressing can be reduced without applying a large thermal load to the press mold. Therefore, a gas trap is less likely to be generated, and in addition to the press forming satisfying Rp ≦ Rm, the press forming satisfying Rp> Rm can be favorably performed.

According to the preform for press molding of the present invention, since an optical glass having excellent weather resistance and chemical durability is used, a deteriorated layer such as white burn occurs on the preform surface before being subjected to press molding. It is not necessary to perform a process for removing the deteriorated layer in press molding.
Therefore, good press molding can be performed, and defects caused by the altered layer hardly occur in the press molded product. Therefore, it is particularly suitable as a preform for precision press molding.
The preform for press molding is a glass molded body having a weight equal to that of a press molded product as described above. The preform is formed into an appropriate shape according to the shape of the press-formed product, and is heated and subjected to press-forming so as to have a viscosity that allows press-forming.

The surface of the preform for press molding, particularly the surface of the preform for precision press molding, may be provided with a film having a release function for preventing fusion of the glass with the molding surface of the press molding die, A film having a lubricating function for facilitating the spread of the glass in the press mold during molding may be provided, or a film having both functions may be provided.
The preform preferably has a smooth and clean surface. For this reason, it is preferable that a predetermined amount of molten glass lump separated from the molten glass flow is formed in a floating state under wind pressure. Further, a preform having a free surface is preferred. Further, it is desirable that there is no cut mark called a shear mark. Shear marks are generated when the flowing molten glass is cut by a cutting blade. If the shear mark remains even at the stage of being formed into a precision press-formed product, that portion becomes a defect. Therefore, it is preferable to remove the shear mark from the preform stage.

  The method for producing a preform of the present invention is called a hot forming method. In the hot forming method, a predetermined weight of molten glass is formed to produce a preform having the above weight. For example, the molten glass flowing out of the outflow pipe is separated by a predetermined weight, and a preform is formed while the separated molten glass lump is in a softened state.

  When separating the molten glass by a predetermined weight, it is desirable not to use a cutting blade in order to prevent generation of shear marks. As a separation method without using a cutting blade, a method of dropping molten glass from an outflow pipe, or supporting a tip of a molten glass flow flowing out of an outflow pipe, and supporting the above at a timing at which a predetermined weight of molten glass lump can be separated. remove. Alternatively, the tip of the molten glass flow is received by the receiving die, and the receiving die descends at the above timing faster than the outflow speed of the molten glass flow. By doing so, the glass can be separated at the constricted portion formed between the tip end side of the molten glass flow and the outflow pipe side, and a molten glass lump having a predetermined weight can be obtained.

  Subsequently, while the obtained molten glass lump is in a softened state, it is formed into a shape suitable for being subjected to press molding. It is preferable that the molding is performed while applying a wind pressure to float the molten glass lump or the glass lump in the middle of molding. By forming the glass in a floating state, a preform having a smooth surface and a clean surface can be formed.

<Optical element and manufacturing method thereof>
Next, an optical element and its manufacturing method will be described. The optical element of the present invention is made of optical glass I or optical glass II. According to the present invention, since the glass constituting the optical element is the optical glass I or the optical glass II, the glass has the characteristics of each optical glass, and has the above-mentioned optical constant (refractive index (nd) 1.52 to 1). 0.7, Abbe number (νd) 42 to 70), and an optical element capable of maintaining high reliability for a long period of time by utilizing excellent weather resistance and chemical durability can be provided.

Examples of the optical element of the present invention include various lenses such as a spherical lens, an aspherical lens, a micro lens, and a rod lens, a diffraction grating, a lens with a diffraction grating, a lens array, a prism, and a filter. It is desirable that the optical element is obtained by heating, softening, and precision press-molding a preform.
Note that this optical element may be provided with an optical thin film such as an antireflection film, a total reflection film, a partial reflection film, or a film having spectral characteristics, if necessary.

  Next, in the method for producing an optical element of the present invention, the preform or the preform for press molding produced by the method for producing a preform for press molding is heated, softened, and press-molded to obtain an optical element. To manufacture. In the present invention, it is preferable to heat and soften the preform for press molding and to perform precision press molding.

According to the present invention, the required optical constant (refractive index (nd) is 1.52 to 1.7, Abbe number (νd) is 42 to 70) can be manufactured with high productivity. Furthermore, since a preform having excellent weather resistance is used, an optical element that can maintain high reliability for a long time can be manufactured.
The optical element manufactured by the manufacturing method of the present invention is the same as the above-described optical element.

In precision press molding, a heated and softened preform is press-molded using a press mold having a molding surface of a predetermined shape, and the shape of the molding surface is precisely transferred to glass. The shape of the press-formed product is the shape of the final product or a shape very similar to the shape of the final product, and the surface accuracy of the surface on which the molding surface of the press-molding die is transferred is equal to the surface accuracy of the final product. Therefore, a non-spherical surface of an aspherical lens or a surface on which a grating of a diffraction grating is formed is formed in a press molding die, and precision press molding is performed. A surface on which a spherical surface or a lattice is formed can be formed. In addition, by precisely transferring the molding surface of the press mold to the optical function surface of the optical element, not only the surface where the aspherical surface and the grating are formed, the optical function surface can be highly productive without using machining. Can be formed under In precision press molding, an optical element can be manufactured only by press molding without machining, or all or a part of the optical function surface can be press-molded, and machining such as lens centering is easy. Only parts can be machined.
Note that in precision press molding, in order to reduce damage due to oxidation of the press mold, it is desirable to perform press molding in a non-oxidizing gas atmosphere, for example, in a nitrogen gas atmosphere, or in a mixed gas atmosphere of nitrogen gas and hydrogen gas. .

  In the method for manufacturing an optical element of the present invention, the glass transition temperature of the preform for press molding is extremely low, so that the heating temperature of the press mold during press molding can be set to less than 400 ° C. Therefore, stainless steel, which causes surface roughness and becomes brittle at a high temperature exceeding 400 ° C., can be used as a press-molding die material. Stainless steel is a material that can be cut as well as ground and can be subjected to electrical discharge machining, so it is possible to accurately mold molding surfaces with complex shapes and aspheric surfaces of aspherical lenses. Easy to process. Therefore, by using a stainless steel press mold for press molding in the method of manufacturing an optical element of the present invention, an optical element having a more complicated shape, or a lens having a thicker wall than the effective diameter can be easily and precisely formed. It can be produced by press molding. For example, a holographic optical element, a micro Fresnel lens, a prism having a spherical or aspherical light entrance / exit surface instead of a flat surface, an anamorphic lens, a micro lens array, and the like can be easily manufactured. In particular, a mold for press-molding a lens having a large wall thickness compared to the effective diameter, which has conventionally been difficult by grinding, can be manufactured with high precision by cutting or electric discharge machining. The mold for pressing such lenses must engrave the mold at a large angle to its surface. Therefore, the grinding process is not only practically difficult but also takes time and costs and is not practical.

Therefore, the preform for press molding, the optical element and the method for producing the same according to the present invention are preferably applied to a lens having an effective diameter of 1.0 to 4.0 mm and a wall thickness of 0.8 to 3.0 mm, The application to a lens having an effective diameter of 1.3 to 3.8 mm and a thickness of 1.0 to 2.8 mm is more preferable, and an effective diameter of 1.3 to 3.5 mm and a thickness of 1.0 to 1.0. Further application to a 2.5 mm lens is more preferred. In particular, application to an aspherical lens is desirable.
Further, application to a lens having a numerical aperture (NA) of 0.6 or more, more preferably 0.65 or more, further preferably 0.75 or more, still more preferably 0.80 or more, and particularly preferably 0.85 or more. Is desirable.

A lens having the above effective diameter and thickness, or a lens having the above numerical aperture, is used for recording and reading information on an optical recording medium such as an optical recording medium (optical disk) or a magneto-optical recording medium. Ideal for pickup lenses. Further, since the optical element is made of the optical glass of the present invention, it has a high transmittance in a wavelength range from visible light to near-infrared light, and performs writing / reading of information using light in a wavelength range of 400 to 850 nm. Ideal for pickup lenses. Particularly effective wavelength ranges include 400 to 455 nm, 770 to 840 nm, and the like. It is suitable for a pickup lens or a collimator lens when a light emitting element or a semiconductor laser that outputs light in the above wavelength range is used.
Further, since it is made of an optical glass having excellent durability and weather resistance, it is particularly suitable as a lens to be disposed on an optical path where light reciprocates, such as a pickup lens.

As the stainless steel used as a press forming die, for example, a steel containing 12% by weight or more of chromium, a martensitic stainless steel, a ferritic stainless steel, an austenitic stainless steel, and a plastic molding die are used. Stainless steel that can be shown.
A known material such as a press mold, a release film, etc. and pressing conditions can be appropriately selected and used. The obtained optical element is appropriately subjected to an annealing treatment, or an optical thin film such as an anti-reflection film, a reflection film having a wavelength selection function, a total reflection film, or the like is formed as needed to be put to practical use.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.
Example 1
By using oxides, carbonates, sulfates, nitrates, phosphates, hydroxides, and the like corresponding to the respective glass components, a predetermined ratio is obtained so that glasses having the compositions shown in Tables 1, 3, and 5 can be obtained. Was weighed and mixed well to form a blended batch, which was placed in a platinum crucible and melted in air for 2-4 hours while stirring at 1100-1300 ° C. After melting, the glass melt (molten glass) is poured into a 40 × 70 × 15 mm carbon mold, allowed to cool to the glass transition temperature, and immediately placed in an annealing furnace, and annealed at about the glass transition temperature for about 1 hour. And allowed to cool to room temperature in a furnace. In the obtained optical glass, no crystal observable with a microscope was precipitated.

For each optical glass, the refractive index (nd), Abbe number (νd), glass transition temperature (Tg), liquidus temperature (LT) and chemical durability of the glass were measured as follows. The results are shown in Tables 2, 4 and 6 together with Dw.
(1) Refractive index (nd) and Abbe number (νd)
Measurement was performed on the optical glass obtained at a slow cooling rate of -30 ° C / h.
(2) Glass transition temperature (Tg)
The measurement was performed using a thermomechanical analyzer of Rigaku Denki Co., Ltd. at a heating rate of 4 ° C./min.
(3) Liquidus temperature (LT)
Put the glass in a 50 ml platinum crucible, cover it with a lid, keep it in a devitrification test furnace with a temperature gradient of 400 to 1100 ° C for 2 hours, and after cooling, crystal when observing the inside of the glass with a 100 × microscope. Was determined from the presence or absence of
(4) Chemical durability (intensity of scattered light / intensity of transmitted light)
The measurement was performed using a fully automatic haze meter (TC-HIIIDPK) manufactured by Tokyo Denshoku. The optical conditions are standard light C of JIS K7105 integrating sphere method, and the light source is a halogen lamp 12V, 50W, 2000H. The value measured here is called a haze value when expressed in%.

Example 2
The optical glass obtained in Example 1 was formed into a spherical preform having a diameter of 2 to 30 mm using the hot preform floating forming apparatus shown in FIG. The preform is formed so that its weight precisely matches the weight of the press-formed product. FIG. 1 is a schematic cross-sectional view of a preform molding apparatus, in which reference numeral 21 denotes molten glass, 22 denotes glass drops, 23 denotes a preform mold, and 24 denotes a temperature control unit.
Next, the preform is placed between a stainless steel lower mold 2 and a stainless steel upper mold 1 having a spherical or aspherical shape in a precision press molding apparatus shown in a schematic sectional view of FIG. Then, the inside of the quartz tube 11 was heated by energizing the heater 12 with the inside of the quartz tube 11 as a nitrogen atmosphere. The temperature inside the mold was set to 395 ° C., and while maintaining the same temperature, the push rod 13 was lowered to push the upper mold 1 to press-mold the preform 4 in the mold. The molding pressure was 8 MPa and the molding time was 30 seconds. After the pressing, the molding pressure is reduced, and the pressed glass molded article is gradually cooled to a temperature 30 ° C. lower than the glass transition temperature while being kept in contact with the lower mold 2 and the upper mold 1, It was rapidly cooled to room temperature to obtain an aspheric lens. The aspheric lens was then removed from the mold. The obtained aspherical lens was an extremely accurate optical lens. In FIG. 2, reference numeral 3 denotes a guide type (body type), 9 denotes a support bar, 10 denotes a support base, and 14 denotes a thermocouple.

Example 3
A pattern having a shape obtained by inverting the diffraction grating pattern of the diffraction grating was formed by cutting on the forming surface of a stainless steel forming die, and a press mold for forming a diffraction grating having a lower die and an upper die was prepared.
Then, using the preform used in Example 2, a diffraction grating was press-molded under the same conditions as in Example 2. In this way, a good diffraction grating could be obtained.

Example 4
A press mold for forming a Fresnel lens having a lower mold and an upper mold was prepared by forming a pattern having a shape inverted from the Fresnel lens pattern on the molding surface of a stainless steel mold by cutting.
Then, using the preform used in Example 2, a Fresnel lens was press-molded under the same conditions as in Example 2. Thus, a good Fresnel lens could be obtained.

Example 5
A pattern having a shape obtained by inverting the pattern of the diffraction grating was formed on a molding surface of a stainless steel mold by cutting, and a press mold having a lower mold and an upper mold was prepared.
Then, using the preform used in Example 2, a lens with a diffraction grating was press-molded under the same conditions as in Example 2. Note that the lens may be a spherical lens or an aspherical lens. Thus, a good lens with a diffraction grating could be obtained.

Since the optical glass of the present invention has a low glass transition temperature, it can be suitably used for press molding using a stainless steel mold.

It is an outline sectional view of the preform molding device used in the example. It is a schematic sectional drawing of the precision press-molding apparatus used in the Example.

Explanation of reference numerals

1 upper die 2 lower die 3 guide type (body type)
Reference Signs List 4 preform 9 support rod 10 support base 11 quartz tube 12 heater 13 push rod 14 thermocouple 21 molten glass 22 glass droplet 23 preform mold 24 temperature control unit

Claims (8)

By _mol%, P 2 _{O 5} and from 25 to _44%, Li 2 O, 10 to 40% Na ₂ O and _{K 2} O in a total amount, 5-40% of ZnO, 1 to 35% of BaO, further , Nb ₂ O ₅ , Bi ₂ O ₃ and WO ₃ , characterized in that they contain at least one kind of component, have a glass transition temperature (Tg) of 370 ° C. or lower, and do not contain lead and fluorine. Optical glass. By mol%, 5-30% of Li ₂ O, 0 to 25% of Na ₂ O, 0 to _15% of _{K 2} O, the _{Nb 2 O 5, Bi 2 O} 3 and WO ₃ in a total amount of 0. 1 to _15% B 2 _{O 3} of 0% _La 2 _{O 3} 0 to 5% of _Gd 2 _{O 3} 0 to _5%, include _{Y 2 O 3 0~5%, P} 2 O 5 The optical glass according to claim 1, wherein the total content of ZnO, BaO and each of the components is 96% or more. It does not contain lead and fluorine, has a weight loss of less than 0.25% when immersed in pure water at 100 ° C. for 60 minutes, and has a glass transition temperature (Tg) of 370 ° C. or less. Optical glass. The optical glass according to any one of claims 1 to 3, wherein a refractive index (nd) is 1.52 to 1.7, and an Abbe number (νd) is 42 to 70. A preform for press molding, comprising the optical glass according to claim 1. While the molten glass of a predetermined weight is in a softened state, a preform for press molding comprising the optical glass according to any one of claims 1 to 4 and having a weight equal to the predetermined weight is formed. For producing press-formed preforms. An optical element comprising the optical glass according to claim 1. A method for producing an optical element, comprising heating and press-molding a preform for press molding according to claim 5 or a preform for press molding produced by the production method according to claim 6.

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