CN109803938B

CN109803938B - Method for producing chemically strengthened glass

Info

Publication number: CN109803938B
Application number: CN201780060568.4A
Authority: CN
Inventors: 鹿岛出; 藤原祐辅; 世良洋一
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2016-09-30
Filing date: 2017-09-26
Publication date: 2022-07-12
Anticipated expiration: 2037-09-26
Also published as: KR102436191B1; TW201817689A; CN109803938A; TWI728189B; JPWO2018062141A1; JP6919658B2; KR20190065255A; WO2018062141A1

Abstract

The invention provides a method for producing chemically strengthened glass having high surface strength, which does not weaken the strength of the glass even if a chemical strengthening treatment is performed at a high temperature for a long time, and which exhibits a deep compressive stress layer (DOC). The invention relates to a method for manufacturing chemically strengthened glass, which comprises the following steps: a chemical strengthening step in which the glass is ion-exchanged by contacting the glass with an inorganic salt having a hydrogen ion index (pH) of 7.5 to 10.5 and containing at least one of sodium nitrate and potassium nitrate when prepared into a 10 mass% aqueous solution; and an acid treatment step of bringing the glass after the chemical strengthening step into contact with an acidic solution having a hydrogen ion index (pH) of less than 7.0 to perform acid treatment.

Description

Method for producing chemically strengthened glass

Technical Field

The present invention relates to a method for producing chemically strengthened glass.

Background

In flat panel display devices such as Digital cameras, cellular phones, and personal Digital assistants (pdas), a thin plate-like cover glass is disposed on the front surface of a display so as to form an area larger than an image display portion in order to protect the display and improve the appearance. Although glass has high theoretical strength, strength is greatly reduced by damage, and therefore, chemically strengthened glass in which a compressive stress layer is formed on the surface of glass by ion exchange or the like is used for cover glass that requires strength.

With the demand for weight reduction and thickness reduction of flat panel display devices, cover glass itself is required to be thin. Therefore, the cover glass is required to have a further strength on the surface in order to satisfy the purpose.

As one of methods for improving the strength of glass, patent document 1 discloses a method in which chemical strengthening is performed using an inorganic salt containing a specific salt, and then acid treatment and alkali treatment are performed.

Documents of the prior art

Patent document

Patent document 1: international publication No. 2015/008763

Disclosure of Invention

However, with the method described in patent document 1, if chemical strengthening is performed at a high temperature for a long time for the purpose of increasing the depth of the compressive stress layer (defined as the depth at which the compressive stress value becomes zero, hereinafter also simply referred to as doc (depth of compression)) to obtain high strength, there is a problem as follows: the strength of the glass is lowered as a side effect, and the temperature condition and time for chemical strengthening are limited.

In addition, conventionally, the surface strength has been improved by performing a polishing treatment after a chemical strengthening treatment, but there is a possibility that the surface of the glass is damaged by polishing, and the surface strength is rather lowered. Further, the warpage of the glass may increase due to polishing.

Accordingly, the present invention provides a method for producing a chemically strengthened glass having a deep DOC without limiting the temperature condition and time of chemical strengthening, and having a high surface strength without weakening the strength of the glass even when a chemical strengthening treatment is performed at a high temperature for a long time.

As a result of intensive studies, the present inventors have found that a chemically strengthened glass exhibiting a deep DOC even when subjected to a chemical strengthening treatment at a high temperature for a long period of time without being limited in temperature conditions and time for chemical strengthening can be obtained by performing a chemical strengthening step of bringing the pH of a salt used for chemical strengthening into a predetermined range and an acid treatment step of subjecting a glass after the chemical strengthening step to an acid treatment, and have completed the present invention.

Namely, the present invention is as follows.

1. A method for producing chemically strengthened glass, comprising the steps of:

a chemical strengthening step in which the glass is ion-exchanged by contacting the glass with an inorganic salt having a hydrogen ion index (pH) of 7.5 to 10.5 and containing at least one of sodium nitrate and potassium nitrate when prepared into a 10 mass% aqueous solution; and

and an acid treatment step of bringing the glass after the chemical strengthening step into contact with an acidic solution having a hydrogen ion index (pH) of less than 7.0 to perform acid treatment.

2. The method for producing chemically strengthened glass according to claim 1, further comprising: and an alkali treatment step of bringing the glass after the acid treatment step into contact with an alkaline solution having a hydrogen ion index (pH) of more than 7.0 to perform alkali treatment.

3. The method for producing a chemically strengthened glass according to claim 1 or 2, wherein the chemical strengthening step is a step of bringing the glass into contact with the inorganic salt at 400 ℃ or higher for 2 hours or longer to perform ion exchange.

4. The method for producing a chemically strengthened glass according to any one of claims 1 to 3, wherein the glass after the chemical strengthening step has a compressive stress layer having a depth of 35 μm or more.

5. The method for producing chemically strengthened glass according to any one of the above 1 to 4, wherein the glass after the chemical strengthening step has a surface strength F (N) of not less than 1000 Xt relative to the plate thickness t (mm) of the glass plate, which is measured under the following conditions in a ball and ring test (ボールオンリング test test)²。

Ball ring test conditions:

a glass plate having a plate thickness t (mm) was placed on a stainless steel ring having a diameter of 30mm and a radius of curvature of a contact portion of 2.5mm, a ball made of steel having a diameter of 10mm was brought into contact with the glass plate, the ball was lowered at a lowering speed of 1mm/min to apply a load to the center of the ring, the breaking load (unit N) at the time of breaking the glass plate was defined as BOR strength, and the average value of 20 measurements of the BOR strength was defined as surface strength F (N). Wherein data obtained when the starting point of breakage of the glass plate is 2mm or more away from the load point of the sphere is excluded from data for calculating the average value.

In the method for producing chemically strengthened glass of the present invention, the glass is chemically strengthened by using an inorganic salt having a pH within a predetermined range, and OH in the inorganic salt is utilized^-The Si-O-Si bond of the glass is appropriately cut, and a low-density layer in which the surface layer of the compressive stress layer is modified is formed on the surface of the glass. Then, the low-density layer can be uniformly removed by performing the acid treatment, and the surface strength of the glass can be effectively and remarkably improved without performing the polishing treatment.

Therefore, according to the method for producing chemically strengthened glass of the present invention, chemically strengthened glass having a deep DOC and high surface strength can be easily obtained without limiting the temperature condition and time for chemical strengthening, even when chemical strengthening treatment is performed at a high temperature for a long time.

Drawings

FIGS. 1(a) to (d) are schematic views showing steps for producing a chemically strengthened glass of the present invention.

Fig. 2 is a diagram for explaining a method of the ball and ring test.

Fig. 3A is an AFM image of a glass surface with surface grinding flaws, and fig. 3B is an AFM image of a glass surface without surface grinding flaws.

Fig. 4A is a view showing a state where white fog is not generated in the glass surface, and fig. 4B is a view showing a state where white fog is generated in the glass surface.

FIG. 5A shows the stress distribution of the chemically strengthened glass obtained in examples 1 and 3 and comparative example 1, FIG. 5B shows the stress distribution of the chemically strengthened glass obtained in examples 7 and 8 and comparative example 6, and FIG. 5C shows the stress distribution of the chemically strengthened glass obtained in examples 10 and 11 and comparative example 11.

Fig. 6A and 6B show results obtained by evaluating the surface strength of the chemically strengthened glass obtained in examples 1 and 5 and comparative examples 1, 4, and 5.

Fig. 7A and 7B show the results of evaluation of the surface strength of the chemically strengthened glass obtained in examples 7 and 8 and comparative example 6.

Fig. 8A and 8B show the results of evaluation of the surface strength of the chemically strengthened glass obtained in examples 10 and 11 and comparative example 11.

Detailed Description

The present invention will be described in detail below, but the present invention is not limited to the following embodiments, and can be arbitrarily modified and implemented within a range not departing from the gist of the present invention.

In the present specification, "mass%" and "weight%", and "mass ppm" and "weight ppm" have the same meanings, respectively. In addition, when only "ppm" is described, it means "ppm by weight".

In the present specification, "to" indicating a numerical range is used in a meaning including numerical values described before and after the numerical range as a lower limit value and an upper limit value, and unless otherwise specified, "to" is used in the present specification in the same meaning.

< method for producing chemically strengthened glass >

One embodiment of a method for producing the chemically strengthened glass of the present invention (hereinafter, also referred to as the method of the present invention) will be described below, but the present invention is not limited thereto. Unless otherwise specified, the glass composition is in terms of mole percent based on oxides.

(chemical strengthening step)

The chemical strengthening step in the method of the present invention is a step of: the method comprises bringing glass into contact with an inorganic salt having a hydrogen ion index (pH) of 7.5 to 10.5 and containing at least one of sodium nitrate and potassium nitrate when the glass is prepared into a 10 mass% aqueous solution, and forming a compressive stress layer on the surface of the glass by ion-exchanging Na in the glass with K in the inorganic salt, and further forming a low-density layer in which the surface layer of the compressive stress layer is modified to have a low density.

The hydrogen ion index (pH) of the inorganic salt is 7.5 or more, preferably 8.0 or more, and more preferably 8.5 or more when the inorganic salt is prepared into a 10 mass% aqueous solution. The hydrogen ion index (pH) when prepared as a 10 mass% aqueous solution is 10.5 or less, preferably 10.0 or less, and more preferably 9.5 or less.

By setting the pH of the inorganic salt to the above range, the inorganic salt can be usedOH in the salt^-The Si-O-Si bond of the glass is appropriately cut, and a low-density layer in which the surface layer of the compressive stress layer is modified is formed on the surface of the glass. The pH of the inorganic salt can be measured at 25 ℃ using a portable pH meter such as D-71S manufactured by horiba.

The inorganic salt preferably contains KNO₂、NaNO₂、K₂CO₃、Na₂CO₃、KHCO₃、NaHCO₃At least one salt selected from KOH and NaOH, and the pH of the inorganic salt can be adjusted appropriately by the content of the above-mentioned salt.

The inorganic salt contains at least one of sodium nitrate and potassium nitrate. By containing at least one of sodium nitrate and potassium nitrate as the inorganic salt, the glass is in a molten state at a strain point or lower, and the handling is easy in a general temperature range when the chemical strengthening treatment is performed. By containing sodium nitrate as the inorganic salt, a chemically strengthened glass having a large DOC and a CTlimit value or less can be obtained. It should be noted that the CTlimit value is known to be empirically-38.7 Xln (t) +48.2[ MPa ]. Here, t represents the thickness of the glass in mm.

The content of sodium nitrate in the inorganic salt is preferably 1% by mass or more, and more preferably 5% by mass or more. Here, the content of sodium nitrate in the inorganic salt refers to the sodium concentration of a liquid-phase salt in which the inorganic salt is in a liquid state. The upper limit of the content of sodium nitrate in the inorganic salt is not particularly limited.

When the content of sodium nitrate in the inorganic salt is 1 mass% or more, the glass is in a molten state at a strain point or less of the glass, and the handling is easy in a general temperature range when the chemical strengthening treatment is performed. The content of sodium nitrate in the inorganic salt is determined by appropriately adjusting the content so as to obtain a desired surface compressive stress value (CS, in MPa).

The inorganic salt may contain other chemical species in addition to sodium nitrate or potassium nitrate within a range not impairing the effects of the present invention, and examples thereof include alkali metal chloride salts such as sodium chloride, potassium chloride, sodium borate and potassium borate, and alkali metal borate salts. These may be added alone or in combination of two or more.

The inorganic salt contains KNO₂In the case of KNO in an inorganic salt₂The content of (b) is preferably 0.2% by mass or more, more preferably 0.4% by mass or more, and further preferably 0.6% by mass or more. Further, it is preferably 10.0% by mass or less, more preferably 8.0% by mass or less, and further preferably 6.0% by mass or less. By making KNO₂The content of (b) is in the above range, and the pH of the inorganic salt in the case of preparing a 10 mass% aqueous solution can be 7.5 to 10.5.

As a method of bringing the glass into contact with the inorganic salt, a method of applying a paste-like inorganic salt, a method of spraying an aqueous solution of an inorganic salt to the glass, a method of immersing the glass in a salt bath of a molten salt heated to a melting point or higher, and the like can be mentioned, and among them, a method of immersing in a molten salt is preferable.

The glass used in the method of the present invention may contain sodium, and various glasses may be used as long as they have a composition that enables molding and strengthening by chemical strengthening treatment. Specific examples thereof include aluminosilicate glass, soda-lime glass, borosilicate glass, lead glass, alkali barium glass, and aluminoborosilicate glass.

The glass can be produced by the following method, without any particular limitation: a desired glass raw material is charged into a continuous melting furnace, the glass raw material is heated and melted preferably at 1500 to 1600 ℃, and after the glass raw material is clarified, the glass raw material is supplied to a molding device, and then the molten glass is molded into a plate shape and slowly cooled.

Note that various methods can be used for forming the glass. For example, various molding methods such as a down-draw method (e.g., an overflow down-draw method, a flow-hole down-draw method, a redraw method, etc.), a float method, a roll-pressing method, and a pressing method can be used.

The thickness of the glass is not particularly limited, but is preferably 3mm or less, more preferably 2mm or less, and further preferably 1mm or less, for the purpose of efficiently performing the chemical strengthening treatment.

The shape of the glass used in the method of the present invention is not particularly limited. For example, glass having various shapes such as a flat plate shape having a uniform plate thickness, a shape having a curved surface on at least one of the front and back surfaces, and a three-dimensional shape having a curved portion can be used.

Specific examples of the composition of the glass used in the method of the present invention include the following compositions of glass.

(i) 56 to 72% of SiO in terms of mole percentage based on oxide ₂5 to 18% of Al₂O₃0 to 15% of B₂O₃And 0.1 to 10% of P₂O₅And Na₂O and K₂Glass containing 3 to 30% of total O.

(ii) Contains 55.5 to 80% of SiO in terms of mole percentage based on oxide₂12 to 20% of Al₂O₃8-25% of Na₂O, 2.5% or more of P₂O₅And 1% or more of alkaline earth metal RO (RO is MgO + CaO + SrO + BaO).

(iii) Contains 57 to 76.5% of SiO in terms of mole percentage based on oxide₂12 to 18% of Al₂O₃8 to 25% of Na₂O, 2.5-10% of P₂O₅And 1% or more of an alkaline earth metal RO.

(iv) 56 to 72% of SiO in terms of mole percentage based on oxide₂8 to 20% of Al₂O₃3-20% of B₂O₃8 to 25% of Na₂O, 0-5% of K₂O, 0-15% of MgO, 0-15% of CaO, 0-15% of SrO₂0-15% of BaO and 0-8% of ZrO₂The glass of (2).

(v) Contains 50 to 80% of SiO in terms of mole percentage based on oxide ₂2 to 25% of Al₂O₃0 to 10% of Li₂O, 0-18% of Na₂O, 0 to 10% of K₂O, 0-15% of MgO, 0-5% of CaO and 0-5% of ZrO₂The glass of (2).

(vi) Contains 50 to 74% of SiO in terms of mole percentage based on oxide ₂1 to 10% of Al₂O₃6 to 14% of Na₂O, 3-11% of K₂O, 2-15% of MgO and 0-6% of CaOAnd 0 to 5% of ZrO₂And SiO₂With Al₂O₃Has a total content of Na of 75% or less₂O and K₂A glass having a total content of O of 12 to 25% and a total content of MgO and CaO of 7 to 15%.

(vii) Contains 68-80% of SiO in terms of mole percentage based on oxide ₂4 to 10% of Al₂O₃5 to 15% of Na₂O, 0 to 1% of K₂O, 4-15% of MgO and 0-1% of ZrO₂The glass of (2).

(viii) Contains 67 to 75% of SiO in terms of mole percentage based on oxide ₂0 to 4% of Al₂O₃7-15% of Na₂O, 1-9% of K₂O, 6 to 14% of MgO and 0 to 1.5% of ZrO₂And SiO₂With Al₂O₃The total content of (A) is 71-75%, Na₂O and K₂Glass containing 12 to 20% of total O and less than 1% of CaO in the case of containing the same.

(ix) Contains 65 to 75% of SiO in terms of mass% based on the oxide₂0.1 to 5% of Al₂O₃1-6% of MgO, 1-15% of CaO and Na₂O+K₂And O is 10-18% of glass.

(x) Contains 60 to 72% of SiO in terms of mass% based on the oxide ₂1 to 10% of Al₂O₃5 to 12% of MgO, 0.1 to 5% of CaO, 13 to 19% of Na₂O and 0 to 5% of K₂O, and RO/(RO + R)₂O) is 0.20 to 0.42 (wherein RO represents an alkaline earth metal oxide, R₂O represents an alkali metal oxide).

The chemical strengthening treatment is performed by immersing glass in a molten salt of an inorganic salt in a molten salt bath to replace metal ions (Na ions) in the glass with metal ions (K ions) having a large ionic radius in the molten salt. By this ion exchange, the composition of the glass surface can be changed, and a compressive stress layer 20 having a high density on the glass surface can be formed [ fig. 1(a) to (b) ]. Since the surface of the glass is densified to generate a compressive stress, the glass can be strengthened.

In the chemical strengthening step in the method of the present invention, in the chemical strengthening, the chemical strengthening treatment is performed using an inorganic salt having a hydrogen ion index (pH) of 7.5 to 10.5 and containing at least one of sodium nitrate and potassium nitrate when the inorganic salt is prepared into a 10 mass% aqueous solution, whereby OH in the inorganic salt is utilized^-The Si-O-Si bonds of the glass are appropriately cut to form a low-density layer 10 in which the surface layer of the compressive stress layer is modified to have a low density [ FIGS. 1(b) to (c) ]]。

In reality, since the density of the chemically strengthened glass gradually increases from the outer edge of the interlayer 30 (main body) existing at the center of the glass toward the surface of the compressive stress layer, there is no clear boundary at which the density rapidly changes between the interlayer 30 and the compressive stress layer 20. Here, the intermediate layer is a layer present in the center of the glass and sandwiched by the compressive stress layer. The intermediate layer is a layer that is not ion-exchanged, unlike the compressive stress layer.

The chemical strengthening step can be specifically performed as follows. In the chemical strengthening step, the glass is preheated and the molten salt is adjusted to the treatment temperature for chemical strengthening. Next, the preheated glass is immersed in the molten salt for a predetermined time, and then the glass is pulled out from the molten salt and cooled. Before the chemical strengthening treatment, the glass is preferably subjected to shape processing, for example, mechanical processing such as cutting, edge processing, and hole forming, depending on the application.

The preheating temperature of the glass depends on the temperature of immersion in the molten salt, and is preferably 100 ℃ or higher.

From the viewpoint of obtaining a chemically strengthened glass having a deep DOC, the temperature for performing chemical strengthening is preferably 400 ℃ or higher, more preferably 450 ℃ or higher, and still more preferably 470 ℃ or higher. The upper limit of the temperature for chemical strengthening is not particularly limited, but typically, the strain point of the glass to be strengthened (usually 500 to 600 ℃ C.) is preferably not higher.

The time for immersing the glass in the molten salt depends on the chemical strengthening temperature, but is preferably 2 hours or more, more preferably 4 hours or more, and even more preferably 8 hours or more, from the viewpoint of obtaining a chemically strengthened glass having a deep DOC. The upper limit is not particularly limited, but is usually 48 hours or less, and if 24 hours or less, it is more preferable from the viewpoint of productivity.

From the viewpoint of imparting sufficient strength to the glass, the Depth (DOC) of the compressive stress layer formed on the surface layer of the glass after the chemical strengthening step is preferably 35 μm or more, more preferably 45 μm or more, and still more preferably 55 μm or more.

The compression stress value of the chemically strengthened glass produced by the method of the present invention is preferably 100MPa or more, more preferably 200MPa or more, and still more preferably 300MPa or more. The upper limit is not particularly limited, but is typically 1200MPa or less.

The depth of the compressive stress layer can be measured using an EPMA (electron probe micro analyzer) or a surface stress meter (for example, FSM-6000 manufactured by FABRICATION).

Since the low-density layer is removed by the acid treatment step described later, the thicker the low-density layer is, the easier the removal of the glass surface is. Therefore, the thickness of the low-density layer is preferably 10nm or more, more preferably 20nm or more, from the viewpoint of the amount of removal of the glass surface. The thickness of the low-density layer can be controlled by the sodium concentration in the molten salt in the chemical strengthening step, the temperature, the time, or the like.

The low-density layer can be further removed by removing the low-density layer by an acid treatment step and then performing an alkali treatment.

From the viewpoint of removability of the glass surface, the density of the low-density layer is preferably lower than the density of a region (bulk) deeper than the ion-exchanged compressive stress layer.

The thickness of the low-density layer is determined from the period (. DELTA.theta.) measured by X-ray-reflectance (XRR). The density of the low-density layer was determined from the critical angle (θ c) measured by XRR. The formation of the low-density layer and the thickness of the layer can be confirmed simply by observing the cross section of the glass with a Scanning Electron Microscope (SEM).

In the chemical strengthening step, in combination with the chemical strengthening treatment using an inorganic salt having a hydrogen ion index (pH) of 7.5 to 10.5 and containing at least one of sodium nitrate and potassium nitrate when prepared into a 10 mass% aqueous solution, the chemical strengthening treatment and the chemical strengthening treatment step in which at least one of the conditions of the composition of the inorganic salt, the hydrogen ion index, the temperature of the chemical strengthening and the time of the chemical strengthening is changed may be performed a plurality of times before and after the chemical strengthening treatment step.

After the chemical strengthening step, the glass is cleaned with industrial water, ion-exchanged water, or the like. Among them, ion-exchanged water is preferable. The cleaning conditions vary depending on the cleaning liquid used, but when ion-exchanged water is used, it is preferable to completely remove the adhering salts if the cleaning is performed at 0 to 100 ℃.

(acid treatment Process)

In the acid treatment step, the glass cleaned after the chemical strengthening step is further subjected to acid treatment. The acid treatment of the glass is carried out by contacting the glass with an acidic solution having a hydrogen ion index (pH) of less than 7.0.

The solution used for the acid treatment is not particularly limited as long as it is acidic, and the acid used may be either weak or strong as long as the pH is less than 7.0. Specifically, acids such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, acetic acid, oxalic acid, carbonic acid, and citric acid are preferable. These acids may be used alone or in combination of two or more.

The temperature at which the acid treatment is carried out varies depending on the kind, concentration and time of the acid used, and is preferably 100 ℃ or lower. In addition, from the viewpoint of easy removal of the low-density layer, it is preferably not less than 20 ℃. The time for performing the acid treatment varies depending on the kind, concentration and temperature of the acid used, but is preferably 10 seconds to 5 hours, more preferably 1 minute to 2 hours, from the viewpoint of productivity.

The concentration of the solution to be subjected to the acid treatment varies depending on the type, time, and temperature of the acid used, but the concentration is preferably a concentration at which corrosion of the container is less likely to occur, and more specifically, is preferably 0.1 to 20% by mass.

Specific examples of the conditions for the acid treatment include a condition in which the glass after the chemical strengthening step is brought into contact with a 0.1 to 10 mass% nitric acid aqueous solution preferably at 35 to 75 ℃ for 1 to 15 minutes.

By the acid treatment, the lowering of the density of the glass surface is accelerated, and the surface layer in which part or all of the low-density layer is removed is exposed [ fig. 1(c) and (d) ]. This makes it possible to obtain a chemically strengthened glass having a significantly improved surface strength. Further, by removing the low-density layer, damage existing on the glass surface is also removed at the same time, and it is considered that this also contributes to improvement of strength.

(alkali treatment Process)

In the process of the present invention, an acid treatment may be followed by an alkali treatment. By performing the alkali treatment, the amount of removal of the low-density layer can be increased compared to the case of only the acid treatment, and the surface strength can be further improved.

The solution used for the alkali treatment is not particularly limited as long as it is alkaline, and a weak base or a strong base may be used as long as the pH exceeds 7.0. Specifically, a base such as sodium hydroxide, potassium carbonate or sodium carbonate is preferable. These bases may be used alone or in combination of two or more.

The temperature for the alkali treatment varies depending on the kind, concentration and time of the alkali used, but is preferably 0 to 100 ℃, more preferably 10 to 80 ℃, and particularly preferably 20 to 60 ℃. This temperature range is preferable because there is no concern of glass corrosion.

The time for the alkali treatment varies depending on the kind, concentration and temperature of the alkali to be used, but is preferably 10 seconds to 5 hours, more preferably 1 minute to 2 hours, from the viewpoint of productivity. The concentration of the solution subjected to the alkali treatment varies depending on the kind, time and temperature of the alkali used, but is preferably 0.1 to 20% by mass from the viewpoint of removability of the glass surface.

Specific examples of the conditions for the alkali treatment include conditions for contacting the glass after the acid treatment step with a 0.1 to 10 mass% aqueous sodium hydroxide solution preferably at 35 to 75 ℃ for 1 to 15 minutes.

By the alkali treatment, a surface layer in which the low-density layer is further removed is exposed as compared with the glass after the acid treatment. This makes it possible to obtain a chemically strengthened glass having further improved surface strength. In addition, the scratches present on the glass surface are also further removed, and this also contributes to further improvement in the surface strength.

It is preferable to have a cleaning step between the acid treatment step and the alkali treatment step or after the alkali treatment step is completed, the same as the cleaning step after the chemical strengthening step.

The amount of the low-density layer to be removed depends on the acid treatment step and the conditions of at least one of the acid treatment step and the alkali treatment step. Fig. 1(d) shows a state where the low-density layer 10 is completely removed, but a part of the low-density layer 10 may be removed and a part may remain. From the viewpoint of improving the strength, the effect can be obtained even if the low-density layer is not entirely removed.

< chemically strengthened glass >

The surface strength of the chemically strengthened glass produced by the method of the present invention can be evaluated by the following ball and ring test.

(ball and Ring test)

The evaluation was made from BOR strength F (N) measured by a Ball On Ring (BOR) test in which a glass plate was placed on a stainless steel Ring having a diameter of 30mm and a contact portion with a rounded corner having a radius of curvature of 2.5mm, and a Ball made of steel having a diameter of 10mm was loaded on the center of the Ring under a static load in a state where the Ball was in contact with the glass plate.

The chemically strengthened glass produced by the present invention preferably satisfies F.gtoreq.1000 Xt²More preferably F.gtoreq.1200 Xt²[ wherein F is BOR strength (N) measured by a ball and ring test, and t is a plate thickness (mm) of a glass plate]. When the BOR strength F (N) is in this range, excellent strength is exhibited even when the sheet is made thin.

Fig. 2 shows a schematic diagram for explaining the ball and ring test. In the Ball On Ring (BOR) test, the glass plate 1 was pressed with a pressing jig 2 (hardened steel, 10mm in diameter, mirror finished) made of SUS304 in a state where the glass plate 1 was horizontally placed, and the strength of the glass plate 1 was measured.

In FIG. 2, a glass plate 1 as a sample was horizontally placed on a receiving jig 3 made of SUS304 (diameter: 30mm, curvature R of contact portion: 2.5mm, contact portion: hardened steel, mirror finished). A pressing jig 2 for pressing the glass plate 1 is provided above the glass plate 1. In the present embodiment, the central region of the glass plate 1 is pressurized from above the glass plate 1.

The test conditions are as follows.

Lowering speed of the pressing jig 2: 1.0(mm/min)

At this time, the breaking load (unit N) when the glass sheet is broken is referred to as the BOR strength, and the average value of 20 measurements of the BOR strength is referred to as the surface strength f (N). When the starting point of breakage of the glass plate is 2mm or more away from the load point of the sphere, the breakage is excluded from the data for calculating the average value.

The Depth (DOC) of the compressive stress layer of the chemically strengthened glass produced by the method of the present invention is preferably 35 μm or more, more preferably 45 μm or more, and still more preferably 55 μm or more.

Since the thickness of the low-density layer removed by the acid treatment step and the alkali treatment step is about 1000nm as in the example even if it is greater than about 10nm as described above, the Depth (DOC) of the compressive stress layer formed in the chemical strengthening step is substantially the same as the Depth (DOC) after the acid treatment step and the alkali treatment step.

The chemically strengthened glass produced by the method of the present invention has a surface compressive stress value (CS) of preferably 100MPa or more, more preferably 200MPa or more, and still more preferably 300MPa or more. The upper limit is not particularly limited, but is typically 1200MPa or less.

The compressive stress value can be measured using EPMA (Electron Probe Micro Analyzer), a surface stress meter (for example, FSM-6000 manufactured by TOYOBO Co., Ltd.), or the like. The compressive stress value can be calculated using the stress distribution calculation method disclosed in japanese patent laid-open No. 2016-142600.

The internal tensile stress (CT) of the chemically strengthened glass produced by the method of the present invention is preferably 72MPa or less, more preferably 62MPa or less, and still more preferably 52MPa or less. The lower limit is not particularly limited, and is typically 20MPa or more. The stress distribution was measured, and the stress distribution was integrated by thickness to obtain a CT value.

In addition, the CTlimit value is known to be empirically-38.7 Xln (t) +48.2[ MPa ]. Here, t represents the thickness of the glass in mm.

The chemically strengthened glass produced by the method of the present invention may be produced by performing a polishing step of polishing the surface of the glass before the chemical strengthening step. Here, the polishing in the present invention means smoothing by cutting the glass surface with abrasive grains.

The presence or absence of polishing scratches caused by the polishing step can be determined by surface observation with an AFM (Atomic Force Microscope), and when there are no 2 or more scratches having a length of 5 μm or more and a width of 0.1 μm or more in a 10 μm × 5 μm region, it can be said that there are no polishing scratches on the surface. Fig. 3A shows a state with surface grinding flaws, and fig. 3B shows a state without surface grinding flaws.

The chemically strengthened glass produced by the production method of the present invention has a surface roughness Ra of preferably 0.2nm or more, more preferably 0.25nm or more, in a measurement range of 10 μm × 5 μm as measured by AFM surface observation. Further, it is preferably 1.5nm or less, more preferably 1.2nm or less. The surface roughness of a conventional unpolished chemically strengthened glass plate is usually 0.15nm or more and less than 0.2 nm.

Examples

The present invention will be specifically described below by way of examples and comparative examples, but the present invention is not limited to these examples and comparative examples.

[ production of chemically strengthened glass ]

After the chemical strengthening step was performed under the following conditions, the acid treatment step, the alkali treatment step, and the polishing step were performed in this order to produce a chemically strengthened glass. The presence or absence of each step in each example and comparative example is shown in tables 1 and 2.

(chemical strengthening step)

Materials of inorganic salts were added to a SUS cup so as to have the compositions and pH shown in tables 1 and 2, and heating was performed by a jacketed resistance heater until the temperature became the temperature shown in tables 1 and 2, to prepare molten salts. Aluminosilicate glasses A to C having a thickness of 50mm × 50mm in a plan view and shown in tables 1 and 2 were prepared, preheated to 200 to 400 ℃, subjected to an ion exchange treatment under the conditions shown in tables 1 and 2, and then cooled to around room temperature, thereby performing a chemical strengthening step. The obtained chemically strengthened glass is washed with water and subjected to the next step. In addition to the compositions shown in tables 1 and 2, KNO was added to the compositions of the inorganic salts₃The total is 100 mass%. The pH of the inorganic salt was measured at 25 ℃ by a portable pH meter D-71S manufactured by horiba, to obtain a 10 mass% aqueous solution.

(acid treatment Process)

A 6 mass% nitric acid aqueous solution was prepared in a beaker, and the temperature was adjusted to 40 ℃ using a water bath. The glass obtained in the chemical strengthening step was immersed in a nitric acid aqueous solution prepared for 120 seconds to be subjected to acid treatment, then washed with pure water several times, and then dried by air blowing. The glass thus obtained was subjected to the next step.

(alkali treatment Process)

A4.0 wt% aqueous solution of sodium hydroxide was prepared in a beaker, and the temperature was adjusted to 40 ℃ using a water bath. The glass obtained in the acid treatment step was immersed in the adjusted aqueous solution of sodium hydroxide for 120 seconds to be subjected to alkali treatment, then washed with pure water several times, and then dried by air blowing.

(grinding step)

Cerium oxide having an average particle diameter (d50) of 1 μm was dispersed in water to prepare a polishing slurry, and the obtained slurry was used to polish both surfaces of a plate glass by a nonwoven fabric polishing pad having a hardness (shore a hardness) of 74 under a pressure of 0.1kPa to a total of about 6 μm.

< evaluation method >

Various evaluations in the present example were performed by the following analytical methods.

(amount of surface removal)

The thickness of the amount of glass removed was determined by measuring the weight before AND after the reagent treatment (acid treatment AND alkali treatment) with an electronic balance for analysis (HR-202i, AND), AND converting the thickness using the following formula.

(thickness of removal amount per one side) [ (weight before treatment) — (weight after treatment) ]/(glass specific gravity)/treated area/2

At this time, the glass specific gravities of the glass materials (glass a, glass B, and glass C) were calculated as follows using these values.

Glass A: 2.42 (g/cm)³)

And (3) glass B: 2.48 (g/cm)³)

And (3) glass C: 2.39 (g/cm)³)

(surface Strength)

The glass face strength was measured by the ball and ring test. Fig. 2 shows a diagram for explaining the ball and ring test used in the present invention. The glass plate 1 (aluminosilicate glass a in the following examples) was horizontally placed, and the glass plate 1 was pressed by using a pressing jig 2 (quenched steel, 10mm in diameter, mirror finished) made of SUS304, and the strength of the glass plate 1 was measured.

In FIG. 2, a glass plate 1 as a sample was horizontally placed on a receiving jig 3 made of SUS304 (diameter: 30mm, curvature R of contact portion: 2.5mm, contact portion: hardened steel, mirror finished). A pressing jig 2 for pressing the glass plate 1 is provided above the glass plate 1.

The central region of the glass plate 1 was pressed from above the glass plate 1 obtained in the examples and comparative examples. The test conditions were as follows.

Lowering speed of the pressing jig 2: 1.0(mm/min)

At this time, the breaking load (unit N) when the glass is broken is referred to as the BOR strength, and the average value of 20 measurements of the BOR strength is referred to as the surface strength f (N). When the starting point of breakage of the glass plate is 2mm or more away from the load point of the sphere (pressing jig), the breakage is excluded from the data for calculating the average value.

The surface strength f (n) depends on the plate thickness t (mm) of the glass plate, and is therefore compared by normalizing (referencing) the plate thickness t (mm) of the glass plate. A (unit N/mm) is a value normalized (normalized) by the plate thickness t (mm) of the glass plate²). The value of a is represented by the formula: a ═ F/t²And (4) calculating.

(depth of surface compressive stress layer)

The surface compressive stress value (CS) and the depth of the compressive stress layer (DOC in μm) were measured using a surface stress meter (FSM-6000) manufactured by Seikagaku Kogyo Co. The compressive stress value (CS) and the depth of the compressive stress layer (DOC) are calculated using a stress distribution calculation method disclosed in japanese patent laid-open No. 2016-142600.

(tensile stress)

The tensile stress value (CT, unit MPa) is calculated by measuring the stress distribution using the stress distribution calculation method disclosed in japanese patent laid-open No. 2016-142600 and integrating the stress distribution by thickness.

(grinding damage)

The presence or absence of the grinding flaw was identified by surface observation using AFM. When no 2 or more scratches having a length of 5 μm or more and a width of 0.1 μm or more were present in the 10 μm × 5 μm region, the surface was not scratched.

(quality of appearance)

The appearance was observed under a high-brightness light source under a condition of an illuminance of 100000Lux, and the quality of the appearance was evaluated by the following evaluation criteria. Fig. 4A is a view showing a state where white fog is not generated in the glass surface, and fig. 4B is a view showing a state where white fog is generated in the glass surface.

O: no white haze was generated in the glass surface.

X: white fog was generated in the glass surface.

The results obtained are shown in tables 1 and 2 and FIGS. 5 to 8.

[ Table 1]

[ Table 2]

As shown in Table 1, chemically strengthened glasses of examples 1 to 12 were obtained by the production method of the present invention, which comprises the steps of: a chemical strengthening step in which glass is brought into contact with an inorganic salt having a pH of 7.5 to 10.5 and containing at least one of sodium nitrate and potassium nitrate to perform ion exchange; and an acid treatment step of bringing the glass after the chemical strengthening step into contact with an acidic solution having a pH of less than 7 to perform acid treatment. The chemically strengthened glasses of examples 1 to 12 have higher surface strength, deeper depth of compressive stress layer (DOC) and higher surface compressive stress value (CS), and do not generate white haze in the glass surface, compared to the chemically strengthened glasses obtained in comparative examples 1 to 11, even when subjected to chemical strengthening treatment at high temperature for a long time, and are excellent in appearance quality.

The chemically strengthened glasses of comparative examples 1, 2, 4, 6, 7, 10 and 11, in which the glasses were not subjected to acid treatment after the chemical strengthening step in which the glasses were brought into contact with an inorganic salt having a pH of 7.5 to 10.5 and containing at least one of sodium nitrate and potassium nitrate to perform ion exchange, were lower in surface strength than the chemically strengthened glasses obtained in the examples. In addition, the chemically strengthened glasses of comparative examples 1, 2, 4, 8 and 9 produced white haze in the glass surface.

In addition, the chemically strengthened glass of comparative example 5, in which the glass was brought into contact with an inorganic salt having a pH of 7.5 to 10.5 and containing at least one of sodium nitrate and potassium nitrate to perform ion exchange and then subjected to polishing treatment without acid treatment, had a slightly higher surface strength than the other comparative examples. However, polishing scratches were observed on the glass surface, and the surface strength was lower than that of the chemically strengthened glass obtained in examples.

Further, the chemically strengthened glass of comparative example 3, which was subjected to the acid treatment and the alkali treatment after the chemical strengthening step using the inorganic salt having a pH of less than 7.5, the chemically strengthened glass of comparative example 8, which was subjected to the acid treatment after the chemical strengthening step using the inorganic salt having a pH of more than 10.5, and the chemically strengthened glass of comparative example 9, which was subjected to the acid treatment and the alkali treatment after the chemical strengthening step using the inorganic salt having a pH of more than 10.5, had lower surface strength and generated white haze in the glass surface, as compared with the chemically strengthened glass obtained in examples.

As shown in fig. 5A, the stress distributions of the chemically strengthened glasses obtained in example 1 and comparative example 1 were substantially uniform. Further, as shown in fig. 5B, the stress distributions of the chemically strengthened glasses obtained in examples 7 and 8 and comparative example 6 were substantially uniform. Further, as shown in fig. 5C, the stress distributions of the chemically strengthened glasses obtained in examples 10 and 11 and comparative example 11 were substantially uniform.

Fig. 6A and 6B show the results of evaluating the surface strength of the chemically strengthened glass obtained in examples 1 and 5 and comparative examples 1, 4, and 5. As shown in fig. 6A and 6B, the surface strength of the chemically strengthened glass obtained in examples 1 and 5 was significantly improved as compared with comparative examples 1 and 4 in which the acid treatment was not performed after the chemical strengthening step and comparative example 5 in which the acid treatment was not performed after the chemical strengthening step and the polishing treatment was performed.

Fig. 7A and 7B show the results of evaluating the surface strength of the chemically strengthened glass obtained in examples 7 and 8 and comparative example 6. As shown in fig. 7A and 7B, the surface strength of the chemically strengthened glass obtained in examples 7 and 8 was significantly improved as compared with comparative example 6 in which the acid treatment was not performed after the chemical strengthening step.

Fig. 8A and 8B show the results of evaluation of the surface strength of the chemically strengthened glass obtained in examples 10 and 11 and comparative example 11. As shown in fig. 8A and 8B, the surface strength of the chemically strengthened glass obtained in examples 10 and 11 was significantly improved as compared with comparative example 11 in which the acid treatment was not performed after the chemical strengthening step.

According to these results, by performing the acid treatment after the chemical strengthening step using the inorganic salt having a pH of 7.5 to 10.5 and containing at least one of sodium nitrate and potassium nitrate, it is possible to obtain a chemically strengthened glass exhibiting a deep DOC without weakening the strength of the glass even when the chemical strengthening treatment is performed at a high temperature for a long time, and having a high surface strength.

The present invention has been described in detail with reference to the specific embodiments, but it is apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. The present application is based on Japanese patent application laid out at 30.9.2016 (Japanese patent application No. 2016-193972), the entirety of which is incorporated herein by reference. Additionally, all references cited herein are incorporated in their entirety into this specification.

Description of the symbols

10 low density layer

20 layer of compressive stress

30 middle layer

1 glass plate

2 pressing jig

3 bearing clamp

Claims

a chemical strengthening step in which the glass is brought into contact with an inorganic salt having a hydrogen ion index when made into a 10 mass% aqueous solution, that is, a pH of 7.5 to 10.0, and containing at least one of sodium nitrate and potassium nitrate, and ion-exchanged; and

and an acid treatment step of bringing the glass after the chemical strengthening step into contact with an acidic solution having a hydrogen ion index, that is, a pH of less than 7.0 to perform acid treatment.

2. The method for producing chemically strengthened glass according to claim 1, further comprising: and an alkali treatment step of bringing the glass after the acid treatment step into contact with an alkaline solution having a hydrogen ion index, i.e., a pH of more than 7.0 to perform alkali treatment.

3. The method for producing chemically strengthened glass according to claim 1 or 2, wherein the chemical strengthening step is a step of bringing the glass into contact with the inorganic salt at 400 ℃ or higher for 2 hours or longer to perform ion exchange.

4. The method for producing a chemically strengthened glass according to claim 1 or 2, wherein the glass after the chemical strengthening step has a compressive stress layer having a depth of 35 μm or more.

5. The method for producing chemically strengthened glass according to claim 1 or 2, wherein the glass after the chemical strengthening step has a surface strength F measured by a ball and ring test under the following conditions, which is F.gtoreq.1000 x t relative to the thickness t of the glass sheet²The unit of the surface strength F is N, the unit of the plate thickness t is mm,

ball ring test conditions:

a glass plate having a plate thickness t in mm is placed on a stainless steel ring having a diameter of 30mm and a radius of curvature of a contact portion of 2.5mm, a ball made of steel having a diameter of 10mm is brought into contact with the glass plate, the ball is lowered at a lowering speed of 1mm/min to apply a load to the center of the ring, a breaking load when the glass plate is broken is defined as a BOR strength, and an average value of 20 measurements of the BOR strength is defined as a surface strength F, wherein data obtained when a load point 2mm or more apart from the ball at a breakage starting point of the glass plate is excluded from data for calculating the average value, wherein the plate thickness t is in mm, the breaking load is in N, and the surface strength F is in N.