Method for producing chemically strengthened glass, and chemically strengthened glass

Abstract

Provided is a method for producing chemically strengthened glass, the method making it possible to produce chemically strengthened glass having higher drop strength.　A method for producing chemically strengthened glass includes immersing glass for chemical strengthening in a molten salt to produce chemically strengthened glass, wherein the molten salt contains sulfate ions, and the sulfate ion content in the molten salt is 1.35 mass% or greater relative to the total mass of the molten salt.

Description

Chemically strengthened glass manufacturing method, chemically strengthened glass

[0001] The present invention relates to a method for producing chemically strengthened glass. More specifically, the present invention relates to a method for producing chemically strengthened glass by immersing glass for chemical strengthening in a molten salt to produce chemically strengthened glass. The present invention also relates to chemically strengthened glass.

[0002] In recent years, cover glass has been used for the purpose of protecting and enhancing the aesthetic appearance of display devices such as mobile phones, smartphones, and tablet terminals. Cover glass for these applications is required to have excellent strength to prevent breakage due to impact, etc.

[0003] Conventionally, a method for increasing the surface strength of glass by chemically strengthening the glass by immersing the glass in a potassium nitrate molten salt or the like has been known. For example, Patent Document 1 discloses lithium aluminosilicate glass in which a relatively large surface compressive stress layer and compressive stress layer depth are obtained by two-stage chemical strengthening. The document describes that lithium aluminosilicate glass can be subjected to a two-stage chemical strengthening treatment using a sodium salt in the first stage and a potassium salt in the second stage, thereby increasing both the surface stress and the stress layer depth while suppressing the tensile stress generated inside the chemically strengthened glass.

[0004] Special Publication No. 2013-520388

[0005] In recent years, there has been a demand for further improvement in the strength of cover glasses. Cover glasses of mobile terminals and the like can break due to deformation when dropped, etc., and therefore, there is a demand for cover glasses that will not break even when dropped from a higher position (having higher drop strength). The present inventors have studied the cover glass described in Patent Document 1 and found that there is room for improvement in terms of drop strength.

[0006] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing chemically strengthened glass that can produce chemically strengthened glass having higher drop strength. Another object of the present invention is to provide chemically strengthened glass.

[0007] As a result of extensive research into the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by adjusting the content of sulfate ions in the molten salt, and have thus completed the present invention.

[0008] That is, the inventors have found that the above-mentioned problems can be solved by the following configurations. [1] A method for producing chemically strengthened glass, comprising immersing glass for chemical strengthening in a molten salt to produce chemically strengthened glass, wherein the molten salt contains sulfate ions, and the content of sulfate ions in the molten salt is 1.35 mass% or more relative to the total mass of the molten salt. [2] The method for producing chemically strengthened glass according to [1], wherein the molten salt contains a sulfate and one or more salts selected from the group consisting of nitrates, nitrites, sulfites, carbonates, phosphates, and halide salts. [3] The method for producing chemically strengthened glass according to [2], wherein the molten salt contains the nitrate and the sulfate. [4] The method for producing chemically strengthened glass according to [2] or [3], wherein the sulfate contains at least one salt selected from the group consisting of lithium sulfate, sodium sulfate, and potassium sulfate. [5] The method for producing chemically strengthened glass according to any one of [1] to [4], wherein the molten salt is substantially free of solid-state salts. [6] The method for producing chemically strengthened glass according to any one of [1] to [5], wherein the content of sulfate ions in the molten salt is 5.00 to 24.00 mass% relative to the total mass of the molten salt. [7] The value of the content of sulfate ions in the molten salt is C ＳＯ４ The method for producing chemically strengthened glass according to any one of [1] to [6], wherein the following formula (1) is satisfied when the temperature of the molten salt is T: 0.0215T-4.0124≦C ＳＯ４ ≦ 0.0720T−8.4000 In formula (1), the unit of T is ° C. The value of T is 200 or more. ＳＯ４ The unit is mass%, and C ＳＯ４The value of is the content of sulfate ions relative to the total mass of the molten salt. [8] The method for producing chemically strengthened glass according to any one of [1] to [7], wherein, when one element whose concentration in the molten salt increases the most before and after immersion of the glass for chemical strengthening in the molten salt is a specific alkali element, the content of the specific alkali element in the molten salt is 1 to 50,000 ppm by mass relative to the total mass of the molten salt. [9] The method for producing chemically strengthened glass according to any one of [1] to [8], wherein the molten salt contains two or more salts selected from the group consisting of lithium salts, sodium salts, and potassium salts.

[10] The method for producing chemically strengthened glass according to any one of [1] to [9], wherein the molten salt contains two or more salts selected from the group consisting of lithium nitrate, sodium nitrate, and potassium nitrate, and the molten salt further contains one or more salts selected from the group consisting of sodium sulfate and potassium sulfate.

[11] The method for producing chemically strengthened glass according to any one of [1] to

[10] , wherein the molten salt contains sodium nitrate and the ratio of the potassium nitrate content to the sodium nitrate content is 0.00 to 24.00 by mass.

[12] The method for producing chemically strengthened glass according to any one of [1] to

[11] , wherein the molten salt contains sodium nitrate and the ratio of the potassium nitrate content to the sodium nitrate content is 0.25 to 9.00 by mass.

[13] The method for producing chemically strengthened glass according to any one of [1] to

[12] , wherein the molten salt contains sodium nitrate and the ratio of the potassium nitrate content to the sodium nitrate content is 0.42 to 2.34 by mass.

[14] The method for producing chemically strengthened glass according to any one of [1] to

[13] , wherein the glass for chemical strengthening contains silicon, aluminum, and lithium.

[15] A method for producing chemically strengthened glass, further comprising: cooling the molten salt after carrying out the method for producing chemically strengthened glass according to

[14] to precipitate a specific metal salt containing lithium while maintaining the molten state of the molten salt; recovering the specific metal salt from the molten molten salt to obtain a regenerated molten salt; and immersing glass for chemical strengthening in the regenerated molten salt to perform a chemical strengthening treatment.

[16] The method for producing chemically strengthened glass according to

[15] , wherein, when obtaining the regenerated molten salt, a sulfate is added to the molten salt after recovering the specific metal salt.

[17] A method for producing chemically strengthened glass, wherein the regenerated molten salt after carrying out the method for producing chemically strengthened glass according to

[15] or

[16] is subjected to the molten salt regeneration treatment and the chemical strengthening treatment alternately.

[18] Chemically strengthened glass, wherein the value of the hydrogen concentration at a depth of 0.5 μm from the outermost surface of the chemically strengthened glass is C. Ｈ (0.5), and the hydrogen concentration at a depth of 0.98 μm is C Ｈ (0.98), R calculated by the following formula (H1) Ｈ A chemically strengthened glass having a formula (H1) R Ｈ = C Ｈ (0.5) / C Ｈ (0.98) In formula (H1), C Ｈ (0.5) and C Ｈ (0.98) units are 10 ２０ atoms / cm ３

[19] Chemically strengthened glass, wherein the hydrogen concentration at a depth of 0.6 μm from the outermost surface of the chemically strengthened glass is C Ｈ (0.6), and the hydrogen concentration at a depth of 0.8 μm is C Ｈ (0.8), S calculated by the following formula (H2) Ｈ Chemically strengthened glass having a structural formula (H2) of -0.40 or more. Ｈ = (C Ｈ (0.8)-C Ｈ (0.6)) / 0.2 In formula (H2), C Ｈ (0.8) and C Ｈ (0.6) units are 10 ２０ atoms / cm ３ is.

[0009] According to the present invention, there is provided a method for producing chemically strengthened glass, which allows chemically strengthened glass having higher drop strength to be obtained. Furthermore, according to the present invention, there is also provided chemically strengthened glass.

[0010] The method for producing chemically strengthened glass of the present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be modified as desired within the scope of the present invention. In this specification, the glass composition is expressed in mole percentage based on oxides, and mole % may be simply written as %. In addition, the symbol "to" indicating a numerical range is used to mean that the numerical values ​​written before and after it are included as the lower and upper limits.

[0011] In the glass composition, "substantially not containing" means that the glass does not contain any components except for unavoidable impurities contained in raw materials, etc., that is, the glass is not intentionally contained. Specifically, the content of components other than those described as the glass composition is preferably less than 0.1 mol%, more preferably 0.08 mol% or less, and even more preferably 0.05 mol% or less.

[0012] In this specification, "chemically strengthened glass" refers to glass after chemical strengthening treatment, and "glass for chemical strengthening" refers to glass before chemical strengthening treatment.

[0013] In this specification, unless otherwise specified, examples of salts contained in the molten salt refer to salts used as raw materials when preparing the molten salt. Furthermore, in this specification, unless otherwise specified, the contents and ratios of components in the molten salt refer to the contents and ratios of the components used as raw materials when preparing the molten salt.

[0014] <Method for producing chemically strengthened glass> The method for producing chemically strengthened glass of the present invention involves immersing glass for chemical strengthening in molten salt to produce chemically strengthened glass, wherein the molten salt contains sulfate ions in an amount of 1.35 mass% or more relative to the total mass of the molten salt. According to the method for producing chemically strengthened glass of the present invention, chemically strengthened glass with higher drop strength can be obtained. Although the mechanism is not entirely clear, the inventors speculate as follows: When chemically strengthening glass is immersed in molten salt to perform chemical strengthening, exchange of ions contained in the glass for chemical strengthening with ions contained in the molten salt proceeds. Here, sulfate ions have a strong effect of attracting components contained in the glass for chemical strengthening, and it is believed that the activity of ions exchanged from the chemically strengthened glass is likely to be reduced by sulfate ions. This is believed to promote ion exchange during chemical strengthening. As a result, it is believed that the method for producing chemically strengthened glass of the present invention can produce chemically strengthened glass with higher drop strength.

[0015] Hereinafter, the chemically strengthened glass and the molten salt used in the method for producing chemically strengthened glass of the present invention will be described in detail.

[0016] [Chemically strengthened glass] The chemically strengthened glass used in the method for producing chemically strengthened glass of the present invention is not particularly limited, but preferably contains lithium, and more preferably contains silicon, aluminum, and lithium. Hereinafter, a preferred embodiment of the chemically strengthened glass will be described. Hereinafter, a preferred composition of the chemically strengthened glass (hereinafter also referred to as "mother glass composition") will be described.

[0017] The mother glass composition preferably contains Li (lithium), and is preferably an aluminosilicate glass containing Li, Si (silicon), and Al (aluminum). More specifically, the mother glass composition, expressed in mole percent on an oxide basis, is: SiO ２ 52 to 75% Al ２ O ３ 8 to 20%, Li ２ It is preferable that the mother glass contains 5 to 16% of O. A preferable mother glass composition will be explained below. ２The content expressed as mole percentage based on oxides is expressed as "[SiO ２ ]" may be written as follows.

[0018] SiO ２ is a component that constitutes the skeleton of glass. It is also a component that increases chemical durability and reduces the occurrence of cracks when the glass surface is scratched. ２ The content of SiO is preferably 52% or more, more preferably 55% or more, and particularly preferably 60% or more. ２ The content is preferably 75% or less, more preferably 72% or less, even more preferably 70% or less, and particularly preferably 68% or less.

[0019] Al ２ O ３ is an effective component from the viewpoint of improving the ion exchange performance during chemical strengthening and increasing the surface compressive stress after strengthening. ２ O ３ The content of Al is preferably 8% or more, more preferably 9% or more, even more preferably 10% or more, particularly preferably 11% or more, and typically 12% or more. ２ O ３ If the Al content is too high, crystals tend to grow during melting, which tends to cause devitrification defects and reduce yield. In addition, the viscosity of the glass increases, which reduces melting properties. ２ O ３ The content is preferably 20% or less, more preferably 19% or less, and further preferably 18% or less.

[0020] SiO ２ and Al ２ O ３ These are all components that stabilize the structure of the glass, and in order to reduce brittleness, the total content is preferably 65% ​​or more, more preferably 70% or more, and even more preferably 75% or more.

[0021] Li ２ O is a component that forms surface compressive stress by ion exchange and improves the melting property of the glass. ２By containing O, it is possible to introduce surface compressive stress by ion-exchanging lithium ions on the glass surface with sodium ions, and then ion-exchanging the sodium ions with potassium ions. ２ The content of O is preferably 5% or more, more preferably 7% or more, even more preferably 9% or more, particularly preferably 10% or more, and most preferably 11% or more. ２ If the O content is too high, the crystal growth rate during glass molding increases, and the yield may decrease significantly due to devitrification defects. ２ The O content is preferably 20% or less, more preferably 16% or less, even more preferably 14% or less, and particularly preferably 12% or less.

[0022] Na ２ O and K ２ Although not essential, O is a component that improves the meltability of the glass and reduces the crystal growth rate of the glass, and is preferably contained in a total amount of 2% or more in order to improve the ion exchange performance, and is preferably contained in a total amount of 10% or less, preferably 9% or less, more preferably 8% or less, even more preferably 7% or less, and particularly preferably 5% or less.

[0023] Na ２ O is a component that forms a surface compressive stress layer in a chemical strengthening treatment using a potassium salt, and is also a component that can improve the melting property of glass. ２ The O content is preferably 1% or more, more preferably 2% or more, even more preferably 3% or more, and particularly preferably 4% or more. On the other hand, from the viewpoint of avoiding a decrease in surface compressive stress (CS) in the strengthening treatment with a sodium salt and from the viewpoint of increasing the internal compressive stress, the O content is preferably 8% or less, more preferably 7% or less, even more preferably 6% or less, and particularly preferably 5% or less.

[0024] K ２ O may be contained for the purpose of improving ion exchange performance. ２When O is contained, the content is preferably 0.1% or more, more preferably 0.15% or more, particularly preferably 0.2% or more. In order to further prevent devitrification, the content is preferably 0.5% or more, more preferably 1.2% or more. On the other hand, since a large amount of K may cause brittleness or a decrease in surface stress due to back-exchange during strengthening, the content is preferably 5% or less, more preferably 3% or less.

[0025] Li ２ O content, Na ２ The content of O and K ２ The total content of O, R, is preferably 5% or more, more preferably 8% or more, even more preferably 10% or more, and particularly preferably 12% or more. The total content of O, R, is preferably 25% or less, and more preferably 20% or less.

[0026] Li for the above R ２ The ratio of the content of O ([Li ２ O] / ([Li ２ O] + [Na ２ O] + [K ２ O]), hereinafter referred to as “Li ２ O / R ２ From the viewpoint of further improving the chemical strengthening properties against compressive stress in the deep layer portion, Li is more preferably 0.52 or more, and even more preferably 0.55 or more. ２ O / R ２ From the viewpoint of further enhancing chemical resistance, O is more preferably 0.80 or less, further preferably 0.78 or less, and particularly preferably 0.75 or less.

[0027] Na for the above R ２ The ratio of the content of O ([Na ２ O] / ([Li ２ O] + [Na ２ O] + [K ２ O]), hereinafter referred to as “Na ２ O / R ２ From the viewpoint of further improving the chemical strengthening properties against compressive stress in the deep layer, Na is preferably 0.05 or more, more preferably 0.08 or more, and even more preferably 0.10 or more. ２ O / R ２From the viewpoint of further enhancing chemical resistance, O is preferably 0.60 or less, more preferably 0.50 or less, even more preferably 0.40 or less, and particularly preferably 0.30 or less.

[0028] K for the above R ２ The ratio of the content of O ([K ２ O] / ([Li ２ O] + [Na ２ O] + [K ２ O]), hereinafter referred to as “K ２ O / R ２ From the viewpoint of further improving the electrical resistance of the glass, K is preferably 0.05 or more, more preferably 0.08 or more, and even more preferably 0.10 or more. ２ O / R ２ From the viewpoint of further enhancing the chemical strengthening properties against compressive stress near the surface, O is preferably 0.50 or less, more preferably 0.40 or less, even more preferably 0.30 or less, and particularly preferably 0.20 or less.

[0029] Also, Li ２ O / R ２ O and Na ２ O / R ２ O and K ２ O / R ２ From the viewpoint of suppressing an increase in the devitrification temperature, the product with O is preferably 0.005 or more, more preferably 0.008 or more, and even more preferably 0.010 or more. Moreover, from the viewpoint of improving chemical resistance, the product is preferably 0.030 or less, and more preferably 0.028 or less.

[0030] Al relative to the above R ２ O ３ The ratio of the content of ([Al ２ O ３ ] / ([Li ２ O] + [Na ２ O] + [K ２ O]), hereinafter referred to as “Al ２ O ３ / R ２ Al is preferably 0.20 or more, more preferably 0.30 or more, even more preferably 0.40 or more, and still more preferably 0.50 or more. ２ O ３ / R ２O is preferably 0.90 or less, more preferably 0.88 or less, and even more preferably 0.85 or less.

[0031] MgO may be contained to reduce viscosity during dissolution, etc. The MgO content is preferably 0.5% or more, more preferably 1% or more, even more preferably 2% or more, and particularly preferably 3% or more. On the other hand, if the MgO content is too high, it may be difficult to increase the compressive stress layer during chemical strengthening treatment. The MgO content is preferably 15% or less, more preferably 10% or less, even more preferably 8% or less, and particularly preferably 6% or less.

[0032] ZrO ２ Although it is not necessary to contain ZrO, it is preferable to contain ZrO from the viewpoint of increasing the surface compressive stress of the chemically strengthened glass. ２ The content of ZrO is preferably 0.1% or more, more preferably 0.15% or more, even more preferably 0.2% or more, particularly preferably 0.25% or more, and typically 0.3% or more. ２ If the content of ZrO is too high, devitrification defects are likely to occur, and it may be difficult to increase the compressive stress value during chemical strengthening treatment. ２ The content is preferably 2% or less, more preferably 1.5% or less, further preferably 1% or less, and particularly preferably 0.8% or less.

[0033] Y ２ O ３ The content of Y is preferably 0.1% or more, more preferably 0.2% or more, even more preferably 0.5% or more, and particularly preferably 1% or more. On the other hand, if the content is too high, it may be difficult to increase the compressive stress layer during chemical strengthening treatment. ２ O ３ The content is preferably 5% or less, more preferably 3% or less, even more preferably 2% or less, and particularly preferably 1.5% or less.

[0034] The composition of the chemically strengthened glass (mother glass composition) to be subjected to chemical strengthening is preferably the composition described above. The method for obtaining a chemically strengthened glass having a mother glass composition is not particularly limited, and known methods can be applied. For example, to obtain a glass of the above composition, glass raw materials are appropriately blended, heated and melted in a glass melting furnace, and then homogenized by bubbling, stirring, adding a clarifier, etc., and formed into a glass plate of a predetermined thickness and slowly cooled. Alternatively, the glass may be formed into a plate by forming it into a block, slowly cooling it, and then cutting it.

[0035] Examples of methods for forming into a plate include the float method, press method, fusion method, and down-draw method. In particular, when producing a large glass plate, the float method is preferred. In addition, continuous forming methods other than the float method, such as the fusion method and down-draw method, are also preferred.

[0036] The chemically strengthened glass may also be glass-ceramics. When the chemically strengthened glass is glass-ceramics, it is preferably glass-ceramics containing one or more crystals selected from the group consisting of lithium silicate crystals, lithium aluminosilicate crystals, and lithium phosphate crystals. As the lithium silicate crystals, lithium metasilicate crystals, lithium disilicate crystals, etc. are preferred. As the lithium phosphate crystals, lithium orthophosphate crystals, etc. are preferred. As the lithium aluminosilicate crystals, β-spodumene crystals, petalite crystals, etc. are preferred.

[0037] The crystallization rate of the crystallized glass is preferably 10% or more in terms of improving mechanical strength, more preferably 15% or more, even more preferably 20% or more, and particularly preferably 25% or more. Furthermore, in order to increase transparency, it is preferably 90% or less, more preferably 70% or less, more preferably 60% or less, and even more preferably 50% or less. A small crystallization rate is also advantageous in that it is easy to heat and bend. The crystallization rate can be calculated from X-ray diffraction intensity using the Rietveld method. The Rietveld method is described in "Crystal Analysis Handbook," edited by the Editorial Committee of the Crystallographic Society of Japan (Kyoritsu Shuppan, 1999, pp. 492-499).

[0038] The average particle size of the precipitated crystals of the crystallized glass is preferably 300 nm or less, more preferably 200 nm or less, even more preferably 150 nm or less, and particularly preferably 100 nm or less, in order to improve transparency. The average particle size of the precipitated crystals can be determined from a transmission electron microscope (TEM) image. It can also be estimated from a scanning electron microscope (SEM) image.

[0039] The Young's modulus of the glass for chemical strengthening is preferably 80 GPa or more, more preferably 83 MPa or more. ＩＣ ) is 0.70 MPa m １／２ More than 0.75 MPa m １／２ More preferably, 0.80 MPa m １／２ The above is more preferable. ＩＣ is 2.00 MPa m １／２ In most cases, it is 1.80 MPa m １／２ The following is preferred. ＩＣ " is measured with reference to the DCDC method [Reference: M. Y. He, M. R. Turner and A. G. Evans, Acta Metall. Mater. 43 (1995) 3453.].

[0040] [Molten Salt] The molten salt used in the method for producing chemically strengthened glass of the present invention contains sulfate ions in an amount of 1.35% by mass or more relative to the total mass of the molten salt. The sulfate ion content in the molten salt is preferably 5.00% by mass or more, more preferably 6.00% by mass or more, and even more preferably 7.00% by mass or more relative to the total mass of the molten salt. The sulfate ion content in the molten salt is preferably 24.00% by mass or less, more preferably 20.00% by mass or less, even more preferably 17.00% by mass or less, and particularly preferably 15.00% by mass or less, relative to the total mass of the molten salt.

[0041] The content of sulfate ions in the molten salt is C ＳＯ４ When the temperature of the molten salt is T, it is also preferable that the following formula (1) is satisfied: 0.0215T-4.0124≦C ＳＯ４≦ 0.0720T−8.4000 In formula (1), the unit of T is ° C. The value of T is 200 or more. ＳＯ４ The unit is mass%, and C ＳＯ４ The value of is the content of sulfate ions relative to the total mass of the molten salt.

[0042] The content of sulfate ions in the molten salt is C ＳＯ４ It is more preferable that the following formula (2) be satisfied when the temperature of the molten salt is T: 0.0300T-5.6174≦C ＳＯ４ ≦ 0.0660T−7.7000 Furthermore, the content of sulfate ions in the molten salt is C ＳＯ４ It is more preferable that the following formula (3) be satisfied when the temperature of the molten salt is T: 0.0429T-8.0249≦C ＳＯ４ ≦0.0600T−7.0000 In the formulas (2) and (3), the unit of T is ° C. The value of T is 200 or more. In the formulas (2) and (3), C ＳＯ４ The unit is mass%, and C ＳＯ４ The value of is the content of sulfate ions relative to the total mass of the molten salt.

[0043] The sulfate ion content in the molten salt is analyzed by barium chloride turbidimetry. Specifically, Kyoritsu Chemical Laboratory's DPR Reagent for Water Quality Meters (DPR-SO4) and Kyoritsu Chemical Laboratory's Digital Pack Test DPM2-SO4 are used. Note that if the salts used in preparing the molten salt are known, the sulfate ion content may be calculated from the mass ratio of those salts.

[0044] The molten salt typically contains a salt consisting of an anion and a cation. Examples of anions contained in the molten salt include, in addition to sulfate ions, one or more anions selected from the group consisting of nitrate ions, nitrite ions, sulfite ions, carbonate ions, phosphate ions, and halide ions. That is, the molten salt preferably contains a sulfate and one or more salts selected from the group consisting of nitrates, nitrites, sulfites, carbonates, phosphates, and halide salts. Furthermore, the molten salt preferably contains a nitrate and a sulfate. For example, "nitrate" refers to a salt containing a nitrate ion as an anion.

[0045] Examples of cations contained in the molten salt include one or more selected from the group consisting of lithium (Li) ions, sodium (Na) ions, and potassium (K) ions, and two or more selected from the group consisting of lithium ions, sodium ions, and potassium ions are preferred. That is, the molten salt preferably contains one or more salts selected from the group consisting of lithium salts, sodium salts, and potassium salts, and more preferably contains two or more salts selected from the group consisting of lithium salts, sodium salts, and potassium salts. Note that, for example, "lithium salt" refers to a salt containing lithium ions as the cation.

[0046] Specific salts contained in the molten salt include lithium nitrate (LiNO ３ ), sodium nitrate (NaNO ３ ), potassium nitrate (KNO ３ ), lithium sulfate (Li ２ SO ４ ), sodium sulfate (Na ２ SO ４ ), and potassium sulfate (K ２ SO ４) is a salt selected from the group consisting of. Among these, it is preferable that the molten salt contains two or more salts selected from the group consisting of lithium nitrate, sodium nitrate, and potassium nitrate, and further contains one or more salts selected from the group consisting of sodium sulfate and potassium sulfate. Furthermore, it is preferable that the sulfate salt contains at least one selected from the group consisting of lithium sulfate, sodium sulfate, and potassium sulfate. One preferred embodiment of the molten salt is a molten salt containing sodium nitrate, potassium nitrate, and sodium sulfate.

[0047] Furthermore, when the molten salt contains sodium nitrate, the ratio of the potassium nitrate content to the sodium nitrate content (potassium nitrate / sodium nitrate) is, in mass ratio, preferably 0.00 to 24.00, more preferably 0.00 to 9.00, even more preferably 0.25 to 4.00, and particularly preferably 0.42 to 2.34.

[0048] The molten salt may contain lithium atoms (lithium salt). When the molten salt contains lithium atoms, the content of lithium atoms is preferably 1 mass ppm or more, more preferably 10 mass ppm or more, and even more preferably 100 mass ppm or more, relative to the total mass of the molten salt. When the molten salt contains lithium atoms, the content of lithium atoms is preferably 50,000 mass ppm or less, more preferably 20,000 mass ppm or less, and even more preferably 10,000 mass ppm, relative to the total mass of the molten salt. The method for producing chemically strengthened glass of the present invention is useful in that it can efficiently produce chemically strengthened glass even when the molten salt contains eluted ions contained in the chemically strengthened glass (for example, lithium ions in a molten salt containing sodium nitrate and sodium ions in a molten salt containing potassium nitrate).

[0049] Specific examples of the composition of the molten salt include a first molten salt containing potassium nitrate, sodium nitrate, lithium nitrate, and sodium sulfate. In the first molten salt, the potassium nitrate content is preferably 0.0 to 96.0 mass%, more preferably 20.0 to 80.0 mass%, and even more preferably 30.0 to 70.0 mass%. In the first molten salt, the sodium nitrate content is preferably 4.0 to 100.0 mass%, more preferably 20.0 to 80.0 mass%, and even more preferably 30.0 to 70.0 mass%. In the first molten salt, the lithium nitrate content is preferably 0.0 to 10.0 mass%, and more preferably 0.01 to 5.0 mass%. In the first molten salt, the sodium sulfate content is preferably 10.0 to 30.0 mass%, and more preferably 10.0 to 20.0 mass%.

[0050] It is also preferable that the molten salt used in the method for producing chemically strengthened glass of the present invention is substantially free of solid-state salt. The term "molten salt is substantially free of solid-state salt" refers to a state in which no precipitates are observed when the molten salt is visually observed. When the molten salt does not contain solid-state salt, solid-state salt is less likely to adhere to the surface of the chemically strengthened glass when the chemically strengthened glass is pulled up from the molten salt, which is preferable because defects in appearance are suppressed.

[0051] The molten salt used in the method for producing chemically strengthened glass of the present invention may be a regenerated molten salt obtained by performing a molten salt regeneration treatment described below. The regenerated molten salt may be a regenerated molten salt obtained by repeating the molten salt regeneration treatment two or more times, as described below.

[0052] [Molten Salt Regeneration Treatment] When the method for producing chemically strengthened glass (chemical strengthening treatment) of the present invention is carried out, components (e.g., lithium ions) contained in the chemically strengthened glass are exchanged with ions in the molten salt. When the chemical strengthening treatment is carried out, components (e.g., lithium ions and sodium ions) in the chemically strengthened glass are exchanged with ions contained in the molten salt, and a layer having compressive stress is formed due to the difference between the ionic radius of the ions before the exchange and the ionic radius of the exchanged ions. In the molten salt after the chemical strengthening treatment, the content of components (e.g., lithium ions) contained in the chemically strengthened glass increases. If the content of components contained in the chemically strengthened glass increases in the molten salt, the ion exchange becomes less likely to occur, and the chemical strengthening treatment may not proceed sufficiently. Therefore, in the method for producing chemically strengthened glass of the present invention, the molten salt after the method for producing chemically strengthened glass of the present invention may be cooled to precipitate a specific metal salt containing a specific metal component while maintaining the molten state of the molten salt, and the specific metal salt is recovered from the molten molten salt to obtain a regenerated molten salt. The chemically strengthened glass may then be immersed in the regenerated molten salt and subjected to the chemical strengthening treatment. In this specification, the term "specific metal component" refers to one or more metals selected from the group consisting of lithium, sodium, and potassium, and the specific metal component preferably includes lithium. Furthermore, in this specification, the term "specific metal salt" refers to one or more metal salts selected from the group consisting of lithium salts, sodium salts, and potassium salts, and the specific metal salt preferably includes lithium salt. The molten salt regeneration process will now be described.

[0053] In the molten salt regeneration process, the molten salt obtained after the method for producing chemically strengthened glass of the present invention is cooled to precipitate a specific metal salt containing a specific metal component while maintaining the molten state of the molten salt, and the specific metal salt is recovered from the molten molten salt to obtain a regenerated molten salt. That is, the molten salt regeneration process includes a precipitation step in which the molten salt is cooled to precipitate a specific metal salt containing a specific metal component while maintaining the molten state of the molten salt, and a recovery step in which the precipitated specific metal salt is recovered from the molten molten salt.

[0054] The precipitation step and the recovery step will be described below.

[0055] (Precipitation step) In the molten salt regeneration process, a precipitation step is carried out in which the molten salt is cooled to precipitate a specific metal salt containing a specific metal component while maintaining the molten salt in a molten state. By the precipitation step, a solid specific metal salt is precipitated in the molten salt.

[0056] The molten salt used in the precipitation step is obtained by carrying out a method for producing chemically strengthened glass, and is the same as the molten salt used in the method for producing chemically strengthened glass of the present invention, except that the ratio of cation content is changed by ion exchange. Below, differences from the molten salt used in the method for producing chemically strengthened glass of the present invention will be described. Then, a method for performing the precipitation step will be described in detail.

[0057] The molten salt subjected to the precipitation step contains a specific metal component. The specific metal component is preferably dissolved in the molten salt. The specific metal component preferably contains lithium. That is, the specific metal salt precipitated in the precipitation step preferably contains lithium. When the specific metal component contains lithium, the molten salt preferably contains 1,000 ppm by mass or more of lithium atoms, more preferably 2,000 ppm by mass or more, and even more preferably 3,000 ppm by mass or more, relative to the total mass of the molten salt. Furthermore, the molten salt preferably contains 30,000 ppm by mass or less of lithium atoms, more preferably 20,000 ppm by mass or less, and even more preferably 5,000 ppm by mass or less, relative to the total mass of the molten salt.

[0058] Specific examples of the composition of the molten salt include a first molten salt containing potassium nitrate, sodium nitrate, lithium nitrate, and sodium sulfate. In the first molten salt, the potassium nitrate content is preferably 10.0 to 96.0 mass%, more preferably 30.0 to 70.0 mass%. In the first molten salt, the sodium nitrate content is preferably 4.0 to 90.0 mass%, more preferably 30.0 to 70.0 mass%. In the first molten salt, the lithium nitrate content is preferably 1.0 to 50.0 mass%, more preferably 1.5 to 15.0 mass%. In the first molten salt, the sodium sulfate content is preferably 3.0 to 30.0 mass%, more preferably 4.0 to 20.0 atomic%.

[0059] In the precipitation step, the molten salt is subjected to a cooling treatment to precipitate a specific metal salt containing a specific metal component while maintaining the molten state of the molten salt. The method for cooling the molten salt in the precipitation step is not particularly limited, and known methods can be applied. Furthermore, the molten salt may be cooled entirely, or only a portion of the molten salt may be cooled.

[0060] In this case, it is preferable to cool the molten salt without adding an additive to the molten salt. The additive is a substance that reacts with the specific metal component to precipitate the specific metal salt, and specific examples thereof include the salts contained in the molten salt described above.

[0061] Examples of methods for cooling the entire molten salt include a method of bringing the molten salt or a container containing the molten salt into contact with a fluid having a temperature lower than that of the molten salt. More specifically, examples of the fluid include air, and a method of air-cooling the molten salt or a container containing the molten salt is included. When air-cooling the molten salt or a container containing the molten salt, air may be circulated in the vicinity. Another method for cooling the molten salt includes a method of circulating a cooling medium through the container containing the molten salt. In this case, the container containing the molten salt preferably includes a pipe through which the cooling medium circulates. In this method, the cooling medium is preferably heat-exchanged in a heat exchanger connected to the pipe, and the cooling medium is circulated between the container containing the molten salt and the heat exchanger.

[0062] When the entire molten salt is cooled, a specific metal salt containing a specific metal component precipitates from the molten salt. The specific metal salt may be dispersed in the molten salt and precipitated, or may be precipitated on the wall surface of a container containing the molten salt, or on the surface of a jig, which will be described later.

[0063] Furthermore, when cooling the molten salt, a portion of the molten salt may be cooled. As a method for cooling a portion of the molten salt, a jig having a cooling part is used, and the cooling part is brought into contact with the molten salt. The jig having the cooling part can freely control the temperature, and when the molten salt is cooled, the molten salt precipitates on the surface of the cooling part, etc.

[0064] In the present invention, the cooling of the molten salt is carried out while maintaining the molten state of the molten salt. Furthermore, it is preferable that the molten salt is cooled to a predetermined temperature and then maintained at that temperature for the holding time described below. The cooling temperature is not particularly limited as long as the molten state of the molten salt is maintained and the specific metal salt precipitates; however, the temperature at which the molten salt is cooled is preferably 150°C or higher, more preferably 200°C or higher, and even more preferably 230°C or higher. The temperature at which the molten salt is cooled is preferably 500°C or lower, more preferably 400°C or lower, and even more preferably 350°C or lower. The temperature difference between the temperature of the molten salt before cooling and the temperature of the molten salt after cooling is preferably 50°C or higher, more preferably 100°C or higher. The temperature difference is preferably 250°C or lower, more preferably 180°C or lower. The holding time is not particularly limited as long as the specific metal salt precipitates; however, it may be, for example, 5 minutes or longer, preferably 10 minutes or longer, and more preferably 30 minutes or longer. The retention time is, for example, 24 hours or less, preferably 12 hours or less, and more preferably 6 hours or less. The temperature of the molten salt before cooling is preferably 370°C or more, and more preferably 390°C or more. The temperature of the molten salt after cooling is preferably 360°C or less, more preferably 330°C or less, and even more preferably 300°C or less. The temperatures of the molten salt before cooling and the molten salt after cooling can be appropriately adjusted depending on the composition of the molten salt, etc.

[0065] In the precipitation step, the molten salt is preferably cooled while generating convection in the molten salt. Examples of a method for generating convection in the molten salt include stirring the molten salt with a stirring blade. By cooling the molten salt while generating convection, the molten salt can be cooled uniformly.

[0066] Examples of an apparatus for carrying out the precipitation step include an apparatus having a container for containing molten salt, a temperature measuring means immersed in the molten salt and measuring the temperature of the molten salt, a heating means for heating the container containing the molten salt and the contained molten salt, and a stirring blade for stirring the molten salt. The container for containing the molten salt is preferably made of a material that does not dissolve in the molten salt. Examples of materials that do not dissolve in the molten salt include metal materials such as stainless steel, ceramic materials such as quartz glass and alumina, and heat-resistant resin materials such as tetrafluoroethylene (PTFE). The temperature measuring means is not particularly limited, but examples include known thermocouples. The heating means is not particularly limited, but examples include heaters with heating wires. In the above apparatus, for example, the molten salt can be cooled by controlling the output of the heating means while monitoring the temperature with the temperature measuring means so that the heat input supplied from the heating means is less than the heat dissipation from the molten salt and the container containing the molten salt. Furthermore, the temperature of the molten salt can be maintained constant by controlling the output of the heating means so that the heat dissipation and heat input are equal. The above-mentioned device may be equipped with a jig or the like used in the recovery step described below.

[0067] (Recovery Step) In the molten salt regeneration process, a recovery step is carried out in which the specific metal salt precipitated in the precipitation step is recovered from the molten molten salt. In the recovery step, the solid specific metal salt is separated from the liquid molten salt, and a specific metal salt containing a specific metal component is recovered. When the specific metal salt is recovered, the specific metal component (preferably including lithium) in the molten salt is reduced. The method for carrying out the recovery step is not particularly limited, and known methods can be applied. Specific methods for carrying out the recovery step are described below, but the recovery step is not limited to the following method, and various methods can be applied.

[0068] First Embodiment of the Recovery Step One aspect of the recovery step is a first embodiment in which, when recovering a specific metal salt from a molten molten salt, a first jig having a mesh-like mesh portion is used, the mesh portion is pulled out of the molten salt, and the precipitated specific metal salt is recovered from the molten molten salt. The mesh portion of the first jig recovers the specific metal salt precipitated as a solid and separates it from the liquid molten salt. The first jig is preferably placed in a container containing the molten salt and immersed in the molten salt before performing the above-mentioned precipitation step. By placing the first jig in a container containing the molten salt before performing the precipitation step, it is easy to recover the specific metal salt by pulling up the first jig after performing the precipitation step to precipitate the specific metal salt. The first jig may have a handle portion for pulling up the mesh portion from the molten salt. It is also preferable that the handle portion is directly or indirectly connected to the mesh portion and protrudes above the liquid surface of the molten salt, allowing it to be pulled up without coming into contact with the molten salt. The first jig may be immersed in molten salt after the precipitation step to recover the specific metal salt. The first jig is preferably made of a material that does not dissolve in the molten salt. Examples of materials that do not dissolve in the molten salt include metal materials such as stainless steel, ceramic materials such as quartz glass and alumina, and heat-resistant resin materials such as tetrafluoroethylene (PTFE). The mesh portion of the first jig is not particularly limited as long as it can recover the specific metal salt and can be selected appropriately depending on the size of the precipitated particles of the specific metal salt. For example, the mesh opening of the mesh portion is preferably 0.05 to 0.2 mm. It is also preferable that the mesh portion of the first jig has a shape that conforms to the container that contains the molten salt.

[0069] -Second embodiment of the recovery process- Another aspect of the recovery process is a second embodiment in which, when cooling a molten salt containing a specific metal component and precipitating a specific metal salt containing the specific metal component while maintaining the molten state of the molten salt, the specific metal salt is precipitated on the surface of the salt precipitation portion of a second jig having a salt precipitation portion, and the salt precipitation portion is lifted from the molten salt to recover the specific metal salt from the molten molten salt. When the second jig is lifted from the molten salt to precipitate the specific metal salt in the salt precipitation portion of the second jig, the precipitated specific metal salt as a solid is separated from the liquid molten salt, and the specific metal salt containing the specific metal component is recovered. When the precipitation process is performed, the second jig is placed in a container containing the molten salt and immersed in the molten salt. The salt precipitation portion of the second jig is not particularly limited, but preferably has a large surface area. The salt deposition portion may have any known structure. Examples include a structure in which plate-like structures are arranged at predetermined intervals in a direction perpendicular to their main surfaces and connected to one another, and a structure in which a single plate is folded into a bellows shape. The plate-like structure or plate may have holes. The plate-like structure or plate is preferably a metal plate. That is, the plate-like structure or plate is preferably a punched metal. Alternatively, the plate-like structure or plate may be subjected to a surface roughening treatment. The plate-like structure or plate may be a mesh made of wire. The mesh opening is preferably 0.5 to 2.0 mm. The presence of fine irregularities on the jig surface is preferred, as this promotes salt deposition. The second jig may have a handle for lifting the mesh portion from the molten salt. It is also preferred that the handle be directly or indirectly connected to the mesh portion and protrude above the liquid surface of the molten salt, allowing it to be lifted without coming into contact with the molten salt. The second jig is preferably made of a material that does not dissolve in the molten salt. Examples of the preferred material are the same as those of the first jig.

[0070] -Third embodiment of the recovery process- Another embodiment of the recovery process involves cooling a molten salt containing a specific metal component and precipitating a specific metal salt containing the specific metal component while maintaining the molten salt in a molten state. This involves using a third jig with a temperature-controllable cooling section to cool the molten salt by changing the temperature of the cooling section, precipitating the specific metal salt on the surface of the cooling section, and then lifting the cooling section from the molten salt to recover the specific metal salt from the molten salt. When the molten salt is cooled by the cooling section of the third jig, the temperature of the molten salt around the cooling section is more likely to drop, making it easier for the specific metal salt to precipitate on the surface of the cooling section. When the third jig with the precipitated specific metal salt is lifted from the molten salt, the precipitated specific metal salt as a solid is separated from the liquid molten salt, and the specific metal salt containing the specific metal component is recovered. The cooling section of the third jig is not particularly limited, but examples include a structure having a pipe inside through which a refrigerant can flow. The refrigerant is not particularly limited, and any known refrigerant can be used. The cooling method of the refrigerant is not particularly limited. The cooling unit may be configured by connecting a mesh or the like made of a metal plate or wire to a pipe through which the refrigerant can flow. When the cooling unit has a metal plate or the above-mentioned mesh, the specific metal salt tends to precipitate efficiently in that part. The third jig is preferably made of a material that does not dissolve in the molten salt. Examples of preferred materials are the same as those of the first jig.

[0071] Fourth Embodiment of the Recovery Process Another aspect of the recovery process involves recovering a specific metal salt from molten molten salt. This recovery process includes a fourth embodiment, in which a molten salt container containing the molten salt is used, a circulation path connected to the molten salt container and through which molten salt discharged from the molten salt container returns to the molten salt container, and a filter disposed along the circulation path capable of separating solids from the molten salt. The molten salt containing the precipitated specific metal salt is introduced into the circulation path, the specific metal salt is separated from the molten salt by the filter, and the specific metal salt is recovered from the filter. The filter separates the specific metal salt from the molten salt and allows the specific metal salt to be recovered from the filter. In one embodiment of the fourth embodiment, the circulation path connected to the molten salt container includes a circulation means for circulating the molten salt, and the molten salt is pumped from one end of the circulation path to the other end of the circulation path. When the filter is installed in this circulation path, the precipitated specific metal salt is captured by the filter. The specific metal salt is recovered by discharging some or all of the molten salt from the circulation path and removing the filter from the circulation path. The circulation means is not particularly limited, and known means can be used. Examples of the filter include a sintered metal filter, a ceramic filter, and a stainless steel mesh. The pore size of the filter can be adjusted appropriately depending on the specific metal salt precipitated. The filter and circulation path are preferably made of a material that does not dissolve in the molten salt. Examples of preferred materials are the same as those of the first jig.

[0072] Furthermore, the specific metal salt recovered in any of the first to fourth embodiments may be heated at a temperature higher than the melting point of the molten salt used to precipitate the specific metal salt. The temperature is preferably higher than the temperature at which the specific metal salt is precipitated and lower than the temperature of the molten salt before the precipitation step. The temperature is preferably 10 to 150°C higher than the temperature at which the specific metal salt is precipitated. Heating within the preferred temperature range allows the components of the molten salt precipitated in association with the specific metal salt to be dissolved and removed from the specific metal salt, thereby increasing the content of the specific metal component contained in the specific metal salt.

[0073] As described above, the specific metal salt recovered in the recovery step contains a specific metal component. As described above, the specific metal component preferably contains lithium. Below, a representative preferred embodiment in which lithium is included as the specific metal component will be described. When the specific metal salt contains lithium, the specific metal salt is preferably a sulfate containing lithium, and more preferably a sulfate containing lithium and sodium. The specific metal salt that is a sulfate containing lithium preferably contains crystals with a crystal structure of space group P31c, in which a diffraction peak appears in the range of 30.2 to 30.5° in an X-ray diffraction chart measured with Cu Kα radiation. The X-ray diffraction chart is measured by the so-called focusing method (also known as the θ / 2θ method). The angle range is the 2θ range.

[0074] Note that, when the recovery step is performed, the specific metal salt may contain components other than the specific metal component. In such cases, the molten salt regeneration process may include a step of adjusting the composition of the molten salt by adding components contained in the recovered specific metal salt to the molten salt after the recovery step. That is, the molten salt regeneration method of the present invention may further adjust the composition of the molten salt after recovering the specific metal component from the chemically strengthened molten salt. When adjusting the composition of the molten salt, it is preferable to add a sulfate. It is also preferable to add a component containing a sodium salt, and it is more preferable to add a component containing sodium sulfate. Furthermore, components other than sulfate (more preferably sodium sulfate) may be added depending on the components recovered as the specific metal salt. Examples of such components include one or more components selected from the group consisting of sodium nitrate, potassium nitrate, and potassium sulfate. Furthermore, the ratio of sodium sulfate and the above components can be adjusted depending on the components recovered as the specific metal salt to maintain a substantially constant composition in the molten salt.

[0075] As described above, the molten salt regeneration process reduces the content of a specific metal component (lithium) in the molten salt, while also maintaining a predetermined composition by further including a step of adjusting the components of the molten salt. Therefore, the molten salt regeneration process allows the molten salt to be repeatedly used without replacing the molten salt used for chemical strengthening. That is, a chemically strengthened glass may be immersed in the regenerated molten salt obtained by the molten salt regeneration process, followed by a chemical strengthening process. The molten salt regeneration process and the chemical strengthening process may be alternately performed on the regenerated molten salt.

[0076] The molten salt regeneration treatment may be performed only once or may be performed two or more times. As will be described later, it is also preferable to analyze the components of the molten salt after the molten salt regeneration treatment, and if the components satisfy the above-mentioned predetermined criteria, subject the molten salt to a chemical strengthening treatment, or if the components do not satisfy the above-mentioned predetermined criteria, perform the molten salt regeneration treatment again.

[0077] The chemical strengthening method for immersing the chemically strengthened glass in molten salt in the method for producing chemically strengthened glass of the present invention is not particularly limited, and known methods can be applied. The conditions for the chemical strengthening treatment can be appropriately adjusted depending on the chemically strengthened glass and molten salt used. For example, the chemical strengthening treatment can be performed by immersing the chemically strengthened glass in molten salt heated to 360 to 600°C for 0.1 to 500 hours. The heating temperature of the molten salt is preferably 375°C or higher and 500°C or lower. The immersion time of the glass plate in the molten salt is preferably 0.3 hours or longer and 200 hours or shorter. The chemical strengthening treatment may be performed only once, or multiple times (multi-stage strengthening) under two or more different conditions. The chemical strengthening treatment may be performed in one stage, but two or more stages are also preferred. That is, the chemical strengthening treatment may use two or more molten salts with different compositions.

[0078] The chemical strengthening treatment may be repeatedly performed using molten salt. That is, the chemical strengthening treatment may be performed by immersing another chemically strengthened glass in the molten salt in which the chemically strengthened glass has been immersed at least once. When repeatedly performing the chemical strengthening treatment, it is also preferable to analyze the components of the molten salt after the chemical strengthening treatment. If the predetermined criteria are met, the molten salt is subjected to the chemical strengthening treatment again. If the predetermined criteria are not met, the molten salt regeneration treatment is performed. The predetermined criteria include the content of components (e.g., lithium ions) contained in the specific metal salt recovered in the molten salt regeneration treatment when analyzed. When analyzing the lithium ion content as the predetermined criterion, the predetermined criterion is preferably set to a lithium ion content of a predetermined value or less relative to the total mass of the molten salt. For example, the predetermined value is preferably 200 to 50,000 ppm by mass, more preferably 2,000 to 20,000 ppm by mass, and even more preferably 3,000 to 10,000 ppm by mass, relative to the total mass of the molten salt.

[0079] In addition, when two or more molten salts having different compositions are used in the chemical strengthening treatment, it is also preferable to confirm whether each of them satisfies the above-mentioned predetermined criteria and decide whether to subject it to the chemical strengthening treatment again or to perform the molten salt regeneration treatment.

[0080] In the method for producing chemically strengthened glass of the present invention, when the above-mentioned chemical strengthening treatment and the molten salt regeneration treatment are alternately performed, the number of times of alternation may be at least once, and may be repeated two or more times. The molten salt regeneration treatment makes it possible to use the molten salt semi-permanently without replacing it.

[0081] Further, in the method for producing chemically strengthened glass of the present invention, chemically strengthened glass may be produced using the specific metal salt recovered in the recovery step or a specific metal salt treated with the specific metal salt as a raw material, and the chemically strengthened glass may be subjected to the method for producing chemically strengthened glass of the present invention. That is, it is also preferable to produce chemically strengthened glass using the specific metal salt recovered in the molten salt regeneration treatment as a raw material, and then subject the chemically strengthened glass to the chemical strengthening treatment. When chemically strengthened glass is produced using the specific metal salt recovered in the molten salt regeneration treatment as a raw material, the components transferred to the molten salt by the chemical strengthening treatment can be effectively utilized, which is preferable.

[0082] <Chemically strengthened glass> The chemically strengthened glass of the present invention may be embodied in a first embodiment or a second embodiment. The embodiments will be described below.

[0083] [First embodiment] In a first embodiment of the chemically strengthened glass of the present invention, the value of the hydrogen concentration at a depth of 0.5 μm from the outermost surface of the chemically strengthened glass is C Ｈ (0.5), and the hydrogen concentration at a depth of 0.98 μm is C Ｈ (0.98), R calculated by the following formula (H1) Ｈ is 2.00 or less. Ｈ = C Ｈ (0.5) / C Ｈ (0.98) In formula (H1), C Ｈ (0.5) and C Ｈ (0.98) units are 10 ２０ atoms / cm ３ That is, in the first embodiment of the chemically strengthened glass of the present invention, the hydrogen concentration at a depth of 0.5 μm is 2.00 times or less than the hydrogen concentration at a depth of 0.98 μm. The hydrogen concentration at a predetermined depth from the outermost surface of the chemically strengthened glass can be measured by secondary ion mass spectrometry (SIMS). In addition, since SIMS can perform analysis while cutting the surface by ion irradiation, it is possible to measure the hydrogen concentration at a predetermined depth.

[0084] In the present invention, the hydrogen concentration is measured by SIMS using an IMS-7f (manufactured by CAMECA). The measurement conditions for SIMS are as follows: Cs ＋ The acceleration voltage is 15.0 kV. For secondary ion detection, the detection area is set to 30 μmφ, and secondary ions with negative polarity are detected. A neutralization gun is used. In order to ensure measurement accuracy, it is preferable to keep the inside of the device as highly vacuum as possible. The sputtering rate of primary ions is measured in advance, and the sputtering time is converted into depth. In the present invention, the hydrogen concentration is determined by measuring the hydrogen concentration in a standard sample with a known hydrogen concentration. １ H － / ３０ Si － The depth profile of the intensity ratio and the chemically strengthened glass being measured. １ H － / ３０ Si － The intensity ratio is determined by comparing it with the depth profile.

[0085] The hydrogen concentration in the standard sample is determined by the following procedure. First, a portion of the glass plate to be measured is cut out. Next, a region of 50 μm or more is removed from the surface of the cut glass plate by polishing or chemical etching. The removal process is performed on both sides. In other words, the thickness of the removed portion on both sides is 100 μm or more. This glass plate after the removal process is used as the standard sample. Infrared spectroscopy (IR) is performed on the obtained standard sample, and the 3,550 cm －１ The absorbance height A of the peak top near ３５５０ and 4,000 cm －１ The absorbance height A ４０００ Next, the thickness d (cm) of the standard sample is measured using a thickness measuring instrument such as a micrometer. ２ Infrared practical absorption coefficient ε of O ｐｒａｃｔ [L / (mol cm)] was set to 75, and the hydrogen concentration (H ２ Calculate the hydrogen concentration of the standard sample = (A ３５５０-A ４０００ ) / (ε ｐｒａｃｔ ・d)...Formula (II) Reference A) S. Ilievski et al., Glastech. Ber. Glass Sci. Technol., 73 (2000) 39.

[0086] A first embodiment of the chemically strengthened glass of the present invention is obtained by the method for producing chemically strengthened glass of the present invention. As described above, in the method for producing chemically strengthened glass of the present invention, the molten salt used for chemical strengthening has a sulfate ion content of a predetermined amount or more. Sulfate ions are thought to have a strong effect of attracting hydrogen ions contained in the molten salt, and are thought to suppress the introduction of hydrogen ions (hydrogen) into the glass for chemical strengthening. As a result, it is presumed that chemically strengthened glass that satisfies the requirements regarding hydrogen concentration specified in the first embodiment of the chemically strengthened glass of the present invention can be obtained.

[0087] In the above formula (H1), C Ｈ The value of (0.5) is often 0.30 or more, preferably 0.40 or more, and more preferably 0.50 or more. The value is often 1.50 or less, preferably 1.20 or less, and more preferably 1.00 or less. In the above formula (H1), C Ｈ The value of (0.98) is often 0.10 or more, preferably 0.20 or more, and more preferably 0.50 or more. The value is often 1.20 or less, preferably 1.00 or less, and more preferably 0.80 or less.

[0088] R calculated by the above formula (H1) Ｈ The value of R is often 0.10 or more, preferably 0.50 or more, and more preferably 0.80 or more. Ｈ In the first embodiment of the chemically strengthened glass, the value is 2.00 or less, preferably 1.50 or less, more preferably 1.20 or less.

[0089] [Second embodiment] In a second embodiment of the chemically strengthened glass of the present invention, the hydrogen concentration at a depth of 0.6 μm from the outermost surface of the chemically strengthened glass is C Ｈ (0.6), and the hydrogen concentration at a depth of 0.8 μm is C Ｈ(0.8), S calculated by the following formula (H2) Ｈ Chemically strengthened glass having a structural formula (H2) of -0.40 or more. Ｈ = (C Ｈ (0.8)-C Ｈ (0.6)) / 0.2 In formula (H2), C Ｈ (0.8) and C Ｈ (0.6) units are 10 ２０ atoms / cm ３ That is, it can be said that the second embodiment of the chemically strengthened glass of the present invention has a gradual change in hydrogen concentration in the depth direction. Ｈ The value of is usually 0.00 or less, and preferably -0.10 or less. Ｈ The value of is −0.40 or more, preferably −0.35 or more, and more preferably −0.30 or more.

[0090] In the second embodiment of the chemically strengthened glass of the present invention, the hydrogen concentration at a predetermined depth from the outermost surface of the chemically strengthened glass can be measured by secondary ion mass spectrometry (SIMS). Details of the SIMS measurement method are as described in the first embodiment, so further description will be omitted.

[0091] A second embodiment of the chemically strengthened glass of the present invention is obtained by the method for producing chemically strengthened glass of the present invention. As described above, in the method for producing chemically strengthened glass of the present invention, the molten salt used for chemical strengthening has a sulfate ion content of a predetermined amount or more. As described above, it is thought that chemical strengthening using a molten salt containing sulfate ions suppresses the introduction of hydrogen ions (hydrogen) into the chemically strengthened glass. As a result, it is presumed that chemically strengthened glass that satisfies the requirements regarding the hydrogen concentration specified in the second embodiment of the chemically strengthened glass of the present invention can be obtained.

[0092] In the above formula (H2), C Ｈ The value of (0.8) is often 0.10 or more, preferably 0.20 or more, and more preferably 0.50 or more. The value is often 1.00 or less, preferably 0.90 or less, and more preferably 0.80 or less. In the above formula (H2), C ＨThe value of (0.6) is often 0.10 or more, preferably 0.20 or more, and more preferably 0.50 or more. The value is often 1.20 or less, preferably 1.00 or less, and more preferably 0.80 or less.

[0093] [Compressive stress] The chemically strengthened glass of the present invention (first and second embodiments, the same applies hereinafter) often has a compressive stress layer on the surface side where compressive stress acts. Preferred parameters related to the compressive stress will be described below.

[0094] In this specification, the term "stress profile" refers to a pattern that expresses compressive stress values ​​with the depth from the glass surface as a variable. A negative compressive stress value means tensile stress. In this specification, the "stress profile" can be measured using a method that combines an optical waveguide surface stress meter and a scattered light photoelastic stress meter. The optical waveguide surface stress meter can accurately measure stress near the glass surface in a short time. An example of an optical waveguide surface stress meter is the FSM-6000 manufactured by Orihara Seisakusho Co., Ltd. On the other hand, a method that uses a scattered light photoelastic stress meter can measure stress inside the glass. An example of a scattered light photoelastic stress meter is the SLP-2000 manufactured by Orihara Seisakusho Co., Ltd. In this specification, two types of measuring devices, an optical waveguide surface stress meter and a scattered light photoelastic stress meter, are used in combination to perform accurate stress measurements. In addition, in this specification, the compressive stress layer depth is the depth at which the compressive stress value becomes zero.

[0095] The compressive stress (CS) of the outermost surface of the chemically strengthened glass of the present invention ０ ) is preferably 50 MPa or more, more preferably 100 MPa or more, and even more preferably 150 MPa or more. ０ In many cases, the CS of the chemically strengthened glass of the present invention is 1,500 MPa or less, preferably 1,000 MPa or less, more preferably 800 MPa or less, and even more preferably 600 MPa or less. ０ is measured by an optical waveguide surface stress meter.

[0096] The compressive stress (CS) at a depth of 50 μm of the chemically strengthened glass of the present invention ５０) is preferably 0 MPa or more, more preferably 30 MPa or more, and even more preferably 50 MPa or more, in that when another object collides with the chemically strengthened glass of the present invention, cracks are less likely to occur even with a larger impact. ５０ is often 600 MPa or less, preferably 500 MPa or less, more preferably 400 MPa or less, even more preferably 350 MPa or less, and may be 200 MPa or less. The compressive stress at each depth can be calculated from the stress profile obtained by the above-mentioned method.

[0097] The depth of compressive stress layer (DOC) of the chemically strengthened glass of the present invention is preferably 30 μm or more, more preferably 50 μm or more, and still more preferably 80 μm or more. The DOC of the chemically strengthened glass of the present invention is preferably 300 μm or less, more preferably 250 μm or less, and still more preferably 200 μm or less.

[0098] [Tensile stress] The chemically strengthened glass of the present invention often has a compressive stress layer on its surface, and in this case, a tensile stress that balances this acts inside the chemically strengthened glass. Preferred parameters related to the tensile stress will be described below.

[0099] The maximum value of the tensile stress of the chemically strengthened glass of the present invention (CT Ｍａｘ ) is preferably 30 MPa or more, more preferably 40 MPa or more, and even more preferably 100 MPa or more. Ｍａｘ is often 300 MPa or less, preferably 250 MPa or less, and more preferably 220 MPa or less. Ｍａｘ is determined from the stress profile and usually acts at the mid-thickness position.

[0100] The average value of the tensile stress of the chemically strengthened glass of the present invention (CT ａｖｅ ) is preferably 30 MPa or more, more preferably 50 MPa or more, and even more preferably 95 MPa or more. ａｖｅis often 200 MPa or less, and preferably 150 MPa or less. The average value of tensile stress is obtained by dividing the integral value of tensile stress in the thickness direction in the depth region showing the tensile stress in the stress profile by the length of the tensile stress portion.

[0101] The integrated tensile stress (ICT) of the chemically strengthened glass of the present invention is often 70,000 Pa m or less, preferably 60,000 Pa m or less, and more preferably 55,000 Pa m or less. The lower limit of ICT is not particularly limited, but is often 10,000 Pa m or more, preferably 20,000 Pa m or more. ICT is determined by integrating the tensile stress in the depth region showing the tensile stress from the stress profile.

[0102] [Thickness] The thickness of the chemically strengthened glass of the present invention (first and second embodiments) can be adjusted appropriately depending on the application, but is often 0.1 mm or more, preferably 0.2 mm or more, more preferably 0.3 mm or more, and even more preferably 0.4 mm or more. The thickness of the chemically strengthened glass of the present invention is often 2.0 mm or less, preferably 1.5 mm or less, more preferably 1.2 mm or less, and even more preferably 1.0 mm or less.

[0103] [Composition] The chemically strengthened glass of the present invention (first embodiment and second embodiment) is obtained by chemically strengthening a plate glass (glass for chemical strengthening) before chemical strengthening. The glass composition of the glass for chemical strengthening is the same as the composition at the center position of the plate thickness of the chemically strengthened glass. That is, the preferred composition at the center position of the plate thickness of the chemically strengthened glass is the same as the preferred composition of the glass for chemical strengthening. The preferred composition of the glass for chemical strengthening is the same as the embodiment described in the section on the manufacturing method of chemically strengthened glass of the present invention, so description will be omitted.

[0104] [Uses] The chemically strengthened glass of the present invention (first and second embodiments) exhibits high drop strength and can be applied to various applications. For example, the chemically strengthened glass of the present invention is useful as a cover glass. The cover glass can also be suitably used for purposes such as surface protection of displays and solar cell modules. In particular, the chemically strengthened glass of the present invention is useful as a cover glass used for mobile devices such as mobile phones, smartphones, personal digital assistants (PDAs), and tablet terminals. Furthermore, it is useful as a cover glass for non-portable display devices such as televisions (TVs), personal computers (PCs), and touch panels, as a cover glass provided on the surface of solar cell modules, elevator walls, walls (full-surface displays) of buildings such as houses and buildings, building materials such as window glass, and tabletops, interiors of automobiles and airplanes. It is also useful as a cover glass for the above-mentioned articles. Furthermore, it can be applied to applications such as housings having curved shapes by bending and bending forming.

[0105] The present invention will be described in further detail below based on examples. The materials, amounts used, ratios, treatment contents, and treatment procedures shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the examples shown below. In the examples, the surface compressive stress of chemically strengthened glass was measured using a scattered light photoelastic stress meter (SLP). The SLP used was an SLP-2000 manufactured by Orihara Seisakusho Co., Ltd. Below, Examples 2, 4, 6 to 11, 13, 15, and 18 to 20 are examples, and Examples 1, 3, 5, 12, 14, 16, and 17 are comparative examples.

[0106] Example 1 First, a molten salt having the following composition was prepared: potassium nitrate: 50.0% by mass, lithium nitrate: 2.5% by mass, and sodium nitrate: 47.5% by mass.

[0107] Chemically strengthened glass having the following composition was immersed in the prepared molten salt to perform chemical strengthening treatment, thereby obtaining chemically strengthened glass. The composition of the chemically strengthened glass is expressed in mole percent based on oxides: SiO２ : 66.2% Al ２ O ３ :11.2% MgO:3.1% CaO:0.2% TiO ２ : 0.1% ZrO ２ : 1.3% Y ２ O ３ : 0.5% Li ２ O: 10.4% Na ２ O: 5.6% K ２ O: 1.5% In Example 1, the chemically strengthened glass having the above composition was subjected to a chemical strengthening treatment under the following conditions. The surface compressive stress of the obtained chemically strengthened glass was measured by the above-mentioned method, and the compressive stress value CS50 at a depth of 50 μm from the surface was measured. The results are shown in the table below. Chemical strengthening temperature: 410°C Chemical strengthening time: 120 minutes

[0108] <Examples 2 to 16> Chemical strengthening treatment was performed in the same manner as in Example 1, except that the molten salt used for chemical strengthening was changed to a molten salt having the composition shown in the table below. In addition, the surface compressive stress of the obtained chemically strengthened glass was measured by the method described above in the same manner as in Example 1. The results are shown in the table below.

[0109] <Evaluation> The drop strength of the obtained chemically strengthened glass of each example was evaluated using the following procedure. The chemically strengthened glass of each example was fitted into a structure whose mass and rigidity were adjusted to the size of a commonly used smartphone, to create a pseudo-smartphone. The pseudo-smartphone was allowed to fall freely onto #180 SiC sandpaper, with the side on which the chemically strengthened glass was placed facing the ground, while varying the height. The drop height was first 5 cm. If the chemically strengthened glass did not break after being dropped from a height of 5 cm, it was dropped from a height of 10 cm. Furthermore, if the chemically strengthened glass did not break after being dropped from a height of 10 cm, it was dropped from a height of 15 cm. In this way, if the glass did not break after being dropped, it was dropped from a position 5 cm higher than the previous drop height, and this process was repeated until the chemically strengthened glass broke. The height at which the chemically strengthened glass first broke in the above procedure was taken as the drop height. The drop height was measured for each of the 19 pieces of tempered glass, and the arithmetic average drop height is shown in the table below as the drop height. Note that a larger drop height value corresponds to the chemically strengthened glass not breaking even when dropped from a higher position, i.e., the higher the drop strength.

[0110] <Results> The compositions of the molten salts used in Examples 1 to 16, the compressive stress values ​​of the obtained chemically strengthened glasses, and the evaluation results are shown in Table 1. ３ / LiNO ３ The column " indicates the mass ratio of the potassium nitrate content to the lithium nitrate content in the molten salt.

[0111]

[0112] From the results shown in Table 1, it is clear that the results are consistent between examples using molten salts with similar compositions ("KNO ３ / LiNO ３") it was confirmed that when the content of sulfate ions in the molten salt is equal to or greater than a predetermined amount, chemically strengthened glass having higher drop strength can be obtained. More specifically, from the comparison between Example 1 and Example 2, Example 3 and Example 4, Example 5 and Examples 6 to 10, Example 12 and Example 11, Example 14 and Example 13, and Example 16 and Example 15, it was confirmed that when molten salts having similar lithium nitrate contents are compared, when the content of sulfate ions in the molten salt is equal to or greater than a predetermined amount, chemically strengthened glass having higher drop strength can be obtained. Note that in Example 10, solid salt was contained in the molten salt, and the solid salt may have adhered to the obtained chemically strengthened glass, resulting in defects in appearance.

[0113] Example 17 First, a molten salt having the following composition was prepared: potassium nitrate: 47.6% by mass, lithium nitrate: 4.8% by mass, and sodium nitrate: 47.6% by mass.

[0114] Chemically strengthened glass having the following composition was immersed in the prepared molten salt to perform chemical strengthening treatment, thereby obtaining chemically strengthened glass. The composition of the chemically strengthened glass is expressed in mole percent based on oxides: SiO ２ : 66.2% Al ２ O ３ :11.2% MgO:3.1% CaO:0.2% TiO ２ : 0.1% ZrO ２ : 1.3% Y ２ O ３ : 0.5% Li ２ O: 10.4% Na ２ O: 5.6% K ２ In Example 17, the chemically strengthened glass having the above composition was subjected to a chemical strengthening treatment under the following conditions. The hydrogen concentration at each depth from the surface of the obtained chemically strengthened glass was measured by the above-mentioned method, and the R calculated by the above-mentioned formula (H1) was Ｈ and the value of S calculated by the above formula (H2) Ｈ The results are shown in Table 2 below. Chemical strengthening temperature: 410°C Chemical strengthening time: 120 minutes

[0115] <Examples 18 to 20> Chemical strengthening treatment was carried out in the same manner as in Example 17, except that the molten salt used for chemical strengthening was changed to a molten salt having the composition shown in Table 2 below. In addition, the hydrogen concentration of the obtained chemically strengthened glass was measured by the method described above in the same manner as in Example 17, and R Ｈ and S Ｈ The values ​​were calculated and the results are shown in the table below.

[0116] <Results> The compositions of the molten salts used in Examples 17 to 20 and the R of the obtained chemically strengthened glasses were Ｈ and S Ｈ The values ​​are shown in Table 2.

[0117]

[0118] From the results shown in Table 2, it was confirmed that when the content of sulfate ions in the molten salt is a predetermined amount or more, chemically strengthened glass having higher drop strength can be obtained. More specifically, from the comparison between Example 17 and Example 18, and the comparison between Example 17 and Example 20, it was confirmed that when molten salts having similar lithium nitrate contents are compared, when the content of sulfate ions in the molten salt is a predetermined amount or more, chemically strengthened glass having higher drop strength can be obtained. Furthermore, in the chemically strengthened glasses of Examples 18, 19, and 20, the R calculated by the above-mentioned formula (H1) Ｈ It was confirmed that the value of S calculated by the above formula (H2) was 2.00 or less. Ｈ It was confirmed that the value of was −0.40 or more.

[0119] The entire contents of the specification, claims and abstract of Japanese Patent Application No. 2024-043187, filed on March 19, 2024, are hereby incorporated by reference as the disclosure of the present invention.

Claims

1. A method for producing chemically strengthened glass, comprising immersing glass for chemical strengthening in molten salt to produce chemically strengthened glass, wherein the molten salt contains sulfate ions, and the content of sulfate ions in the molten salt is 1.35 mass% or more relative to the total mass of the molten salt.

2. The method for producing chemically strengthened glass according to claim 1, wherein the molten salt comprises a sulfate and at least one selected from the group consisting of a nitrate, a nitrite, a sulfite, a carbonate, a phosphate, and a halide salt.

3. The method for producing chemically strengthened glass according to claim 2, wherein the molten salt contains the nitrate and the sulfate.

4. The method for producing chemically strengthened glass according to claim 2, wherein the sulfate comprises at least one selected from the group consisting of lithium sulfate, sodium sulfate, and potassium sulfate.

5. The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein the molten salt is substantially free of solid-state salt.

6. The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein the content of sulfate ions in the molten salt is 5.00 to 24.00 mass% based on the total mass of the molten salt.

7. The content of sulfate ions in the molten salt is C ＳＯ４ The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein the following formula (1) is satisfied, where T is a temperature of the molten salt: 0.0215T-4.0124≦C ＳＯ４ ≦ 0.0720T−8.4000 In formula (1), the unit of T is ° C. The value of T is 200 or more. ＳＯ４ The unit is mass%, and C ＳＯ４ The value of is the content of sulfate ions relative to the total mass of the molten salt.

8. The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein, when a specific alkali element is defined as one element whose concentration in the molten salt increases the most before and after immersion of the glass for chemical strengthening in the molten salt, the content of the specific alkali element in the molten salt is 1 to 50,000 ppm by mass relative to the total mass of the molten salt.

9. The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein the molten salt contains two or more salts selected from the group consisting of lithium salts, sodium salts, and potassium salts.

10. The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein the molten salt contains two or more salts selected from the group consisting of lithium nitrate, sodium nitrate, and potassium nitrate, and the molten salt further contains one or more salts selected from the group consisting of sodium sulfate and potassium sulfate.

11. The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein the molten salt contains sodium nitrate, and the ratio of the potassium nitrate content to the sodium nitrate content is 0.00 to 24.00 by mass.

12. The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein the molten salt contains sodium nitrate, and the ratio of the potassium nitrate content to the sodium nitrate content is 0.25 to 9.00 by mass.

13. The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein the molten salt contains sodium nitrate, and the ratio of the potassium nitrate content to the sodium nitrate content is 0.42 to 2.34 by mass.

14. The method for producing chemically strengthened glass according to any one of claims 1 to 4, wherein the glass for chemical strengthening contains silicon, aluminum, and lithium.

15. A method for producing chemically strengthened glass, further comprising: cooling the molten salt after carrying out the method for producing chemically strengthened glass described in claim 14 to precipitate a specific metal salt containing lithium while maintaining the molten state of the molten salt; recovering the specific metal salt from the molten molten salt to obtain a regenerated molten salt; and immersing glass for chemical strengthening in the regenerated molten salt to perform a chemical strengthening treatment.

16. The method for producing chemically strengthened glass according to claim 15, wherein, when obtaining the regenerated molten salt, a sulfate is added to the molten salt after recovering the specific metal salt.

17. A method for producing chemically strengthened glass, comprising alternately subjecting the regenerated molten salt, after carrying out the method for producing chemically strengthened glass according to claim 15, to the molten salt regeneration treatment and the chemical strengthening treatment.

18. Chemically strengthened glass, wherein the hydrogen concentration at a depth of 0.5 μm from the outermost surface of the chemically strengthened glass is C Ｈ (0.5), and the hydrogen concentration at a depth of 0.98 μm is C Ｈ (0.98), R calculated by the following formula (H1) Ｈ A chemically strengthened glass having a formula (H1) R Ｈ = C Ｈ (0.5) / C Ｈ (0.98) In formula (H1), C Ｈ (0.5) and C Ｈ (0.98) units are 10 ２０ atoms / cm ３ is.

19. Chemically strengthened glass, wherein the hydrogen concentration at a depth of 0.6 μm from the outermost surface of the chemically strengthened glass is C Ｈ (0.6), and the hydrogen concentration at a depth of 0.8 μm is C Ｈ (0.8), S calculated by the following formula (H2) Ｈ Chemically strengthened glass having a structural formula (H2) of -0.40 or more. Ｈ = (C Ｈ (0.8)-C Ｈ (0.6)) / 0.2 In formula (H2), C Ｈ (0.8) and C Ｈ (0.6) units are 10 ２０ atoms / cm ３ is.

Images