JP2016216330A - Method for producing chemical strengthened glass - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a chemical strengthened glass in which the glass is not eroded by an inorganic salt after ion exchange, and recessed defects are reduced.SOLUTION: Provide is a method for producing a chemical strengthened glass comprising a process where an inorganic salt including potassium nitrate is contacted with a glass to ion-exchange Na in the glass and K in the inorganic salt, the inorganic salt includes at least one kind of salt selected from the group consisting of KCO,NaCO,KHCO,NaHCO,KOH and NaOH, and, after the ion exchange step, the step of holding for 3 min or more at a temperature lower than the chemical strengthening temperature T1 in the ion exchange step and at 315°C or more or (at the melting point Tm under the equilibrium vapor pressure of the inorganic salt-20°C or more) in the ion exchange step.SELECTED DRAWING: Figure 1

Description

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

  In a flat panel display device such as a digital camera, a mobile phone, or a personal digital assistant (PDA), a thin plate-like cover glass that has a wider area than the image display portion in order to enhance the protection and aesthetics of the display. Is placed in front of the display. Although glass has a high theoretical strength, the strength is greatly reduced due to scratches. Therefore, a chemically strengthened glass with a compressive stress layer formed on the glass surface by ion exchange or the like is used for the cover glass that requires strength. Yes.

  With the demand for weight reduction and thinning of the flat panel display device, it is required to make the cover glass itself thin. Accordingly, the cover glass is required to have further strength on both the surface and the end surface in order to satisfy the purpose.

Therefore, Patent Document 1 discloses a method for producing chemically strengthened glass, which includes a step of ion-exchange of Na in glass and K in the inorganic salt by bringing a glass containing sodium into contact with an inorganic salt containing potassium nitrate. Have been described. The inorganic salt is at least selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , K ₃ PO ₄ , Na ₃ PO ₄ , K ₂ SO ₄ , Na ₂ SO ₄ , KOH and NaOH. It contains a kind of salt and includes a step of washing the glass after the ion exchange, a step of acid-treating the glass after the washing, and a step of alkali-treating the glass after the acid treatment.

International Publication No. 2015/008763

  In the method described in Patent Document 1, it is possible to prevent the strength of the glass from being lowered by washing after ion exchange, acid treatment, and alkali treatment. However, when ion exchange is performed with a specific inorganic salt as in the method described in Patent Document 1, the inorganic salt after ion exchange locally erodes the glass and may cause a dent-like defect in the glass. It turned out to be. Since such a dent-like defect impairs the aesthetics of the glass, it is required to reduce the dent-like defect.

  An object of this invention is to provide the method of manufacturing the chemically strengthened glass which reduced the dent-like fault, without glass eroding by the inorganic salt after ion exchange.

  The inventors of the present invention perform chemical strengthening treatment by ion exchange using an inorganic salt containing a specific salt, and then perform a process of holding for a certain period of time in a temperature range within a certain range, thereby reducing the concave defects. The present inventors have found that few chemically strengthened glasses can be obtained and completed the present invention.

That is, the present invention is as follows.
<1> A method for producing chemically strengthened glass comprising a step of ion-exchanging Na in the glass and K in the inorganic salt by bringing a glass containing sodium into contact with an inorganic salt containing potassium nitrate,
The inorganic salt includes at least one salt selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , KOH and NaOH,
A method for producing chemically tempered glass, comprising a step of holding at a temperature lower than the chemical strengthening temperature T1 ° C. in the ion exchanging step at a temperature of 315 ° C. or higher for 3 minutes or longer after the ion exchanging step.
<2> A method for producing chemically strengthened glass comprising a step of ion-exchanging Na in the glass and K in the inorganic salt by bringing a glass containing sodium into contact with an inorganic salt containing potassium nitrate,
The inorganic salt includes at least one salt selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , KOH and NaOH,
After the ion exchange step, a step of holding for 3 minutes or more at a temperature lower than the chemical strengthening temperature T1 ° C. in the ion exchange step and (melting point Tm−20 under equilibrium vapor pressure of the inorganic salt) ° C. or more. The manufacturing method of the chemically strengthened glass characterized by including.
<3> A step of washing the glass after the holding step,
The method for producing chemically tempered glass according to <1> or <2>, including a step of acid-treating the glass after the step of washing.
<4> The method for producing chemically tempered glass according to <3>, including a step of alkali-treating the glass after the acid-treating step.
<5> Any one of <1> to <4>, wherein a difference in mass of the glass immediately after the ion exchange step and after the holding step is 0.15 g or more per 2500 mm ^{2 of} glass. Of manufacturing chemically strengthened glass.

  According to the method for producing chemically tempered glass of the present invention, chemically tempered glass having few dent-like defects and excellent in aesthetics can be obtained.

It is a graph showing the relationship between the quantity of the salt adhering to glass, and holding time in the process to hold | maintain.

Hereinafter, the present invention will be described in detail, but the present invention is not limited to the following embodiments, and can be arbitrarily modified without departing from the gist of the present invention.
<Method for producing chemically strengthened glass>
The method for producing a chemically strengthened glass according to the present embodiment includes a step of ion-exchanging Na in the glass and K in the inorganic salt by bringing a glass containing sodium into contact with an inorganic salt containing potassium nitrate. A method for producing chemically tempered glass, wherein the inorganic salt contains at least one salt selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , KOH and NaOH, and the ion exchange After the step of performing, the step of holding for 3 minutes or more at a temperature of 315 ° C. or higher which is lower than the chemical strengthening temperature T1 ° C. in the step of ion exchange is characterized.

In another method for producing chemically strengthened glass according to this embodiment, Na in the glass and K in the inorganic salt are ion-exchanged by bringing the glass containing sodium into contact with the inorganic salt containing potassium nitrate. A method for producing chemically strengthened glass comprising a step, wherein the inorganic salt includes at least one salt selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , KOH and NaOH, After the step of ion exchange, including a step of holding for 3 minutes or more at a temperature lower than the chemical strengthening temperature T1 ° C. in the ion exchange step and (melting point Tm−20 under equilibrium vapor pressure of the inorganic salt) ° C. or higher. It is characterized by.

(Glass composition)
The glass used in the present invention only needs to contain sodium, and glass having various compositions can be used as long as it has a composition that can be strengthened by molding and chemical strengthening treatment. Specific examples include aluminosilicate glass, soda lime glass, borosilicate glass, lead glass, alkali barium glass, and aluminoborosilicate glass. Among these, since the aluminosilicate glass and the aluminoborosilicate glass are likely to have a dent-like defect due to conventional chemical strengthening, the effect of the present invention becomes more remarkable.

  The method for producing the glass is not particularly limited, and a desired glass raw material is charged into a continuous melting furnace, and the glass raw material is preferably heated and melted at 1500 to 1600 ° C., clarified, and then supplied to a molding apparatus. It can be manufactured by forming into a plate shape and slowly cooling.

  Various methods can be employed for forming the glass. For example, various forming methods such as a down draw method (for example, an overflow down draw method, a slot down method and a redraw method), a float method, a roll-out method, and a press method can be employed.

  The thickness of the glass is not particularly limited, but is usually preferably 5 mm or less and more preferably 3 mm or less in order to effectively perform the chemical strengthening treatment.

  In producing chemically tempered glass, the larger the size of glass (area of the main surface), the more likely the dent-like defects are generated. Therefore, the size of the glass is not particularly limited, but is preferably a glass plate of 150 mm × 100 mm or more in that the effect of the present invention becomes more remarkable.

  Moreover, the shape of the glass used by this invention is not specifically limited. For example, various shapes of glass such as a flat plate shape having a uniform plate thickness, a shape having a curved surface on at least one of the front surface and the back surface, and a three-dimensional shape having a bent portion can be employed.

Although it does not specifically limit as a composition of the chemically strengthened glass of this invention, For example, the following glass compositions are mentioned.
(I) 50% to 80% of SiO ₂ , ₂ to 25% of Al ₂ O ₃ , 0 to 10% of Li ₂ O, 0 to 18% of Na ₂ O, K ₂ O with a composition expressed in mol% Is represented by a glass (ii) mol% containing 0-10% MgO, 0-15% MgO, 0-5% CaO and 0-5% ZrO ₂ , SiO ₂ 50-74%, Al ₂ O ₃ and 1-10% 6-14% of Na ₂ O, _{K 2} O 3-11% of MgO 2 to 15% of CaO Less than six% and ZrO ₂ to contain 0-5% The total content of SiO ₂ and Al ₂ O ₃ is 75% or less, the total content of Na ₂ O and K ₂ O is 12 to 25%, and the total content of MgO and CaO is 7 to 15%. a composition which is displayed at a certain glass (iii) mol%, a SiO ₂ 68 to 80%, the _Al 2 _{O 3} 4 to 10%, a ₂ O and 5 to 15%, _{K 2} O 0 to 1%, the MgO 4 to 15% and ZrO ₂ is composition displaying a glass (iv) mole% containing 0 to 1%, a SiO ₂ 67 75%, the _Al 2 _{O 3} 0 to 4%, a Na ₂ O 7 to 15%, the _{K 2} O 1 to 9%, the MgO having 6 to 14% and the ZrO ₂ contained 0 to 1.5% The total content of SiO ₂ and Al ₂ O ₃ is 71 to 75%, the total content of Na ₂ O and K ₂ O is 12 to 20%, and when CaO is contained, the content is 1% the composition displaying a glass (v)% by weight is less than the SiO ₂ 65 to 75%, the _Al 2 _{O 3} 0.1 to 5%, the MgO 1 to 6%, a CaO containing 1% to 15% , _{composition Na} 2 O + _{K 2} O is displayed in a glass (vi) mass% 10 to 18 percent, a SiO ₂ 65 to 7 %, The _Al 2 _{O 3 3.4~8.6%,} the MgO from 3.3 to 6%, the CaO 6.5 to 9%, 13 to 16% of Na ₂ O, the _{K 2} O 0 to 1 %, TiO _{2 0} to 0.2%, Fe ₂ O ₃ 0.01 to 0.15%, SO ₃ 0.02 to 0.4%, (Na ₂ O + K ₂ O) / Al ₂ The composition expressed in terms of glass (vii) mass% in which O ₃ is 1.8 to 5.0 is SiO ₂ 60 to 72%, Al ₂ O ₃ 1 to 10%, MgO 5 to 12%, CaO 0.1 to 5%, Na ₂ O 13 to 19%, K ₂ O 0 to 5%, RO / (RO + R ₂ O) is 0.20 or more and 0.42 or less (in the formula, RO Is an alkaline earth metal oxide, and R ₂ O is an alkali metal oxide. ) Glass.

(Inorganic salt)
The chemically strengthened glass according to the present invention has an ion-exchanged compressive stress layer on the glass surface. In the ion exchange method, the surface of glass is ion exchanged to form a surface layer in which compressive stress remains. Specifically, alkali metal ions (typically Li ions, Na ions) having a small ion radius on the surface of the glass plate by ion exchange at a temperature below the glass transition point are converted to alkali ions (typically Is substituted for Na ions or K ions for Li ions and K ions for Na ions. Thereby, compressive stress remains on the surface of the glass, and the strength of the glass is improved.

In the production method of the present invention, the chemical strengthening is an inorganic salt containing potassium nitrate (KNO ₃ ), and further from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , KOH and NaOH. It is carried out by bringing the glass containing sodium described above into contact with an inorganic salt containing at least one selected salt.
The melting point Tm of potassium nitrate under the equilibrium vapor pressure is 330 ° C., and has a melting point below the strain point (usually 500 to 600 ° C.) of the glass to be chemically strengthened. The salt (hereinafter also referred to as “flux”) has a property of cutting a glass network typified by a Si—O—Si bond. Since the temperature at which the chemical strengthening treatment is performed is as high as several hundred degrees Celsius, the covalent bond between Si-O of the glass is appropriately broken at that temperature, and the density reduction treatment described later easily proceeds.

  The degree of breaking the covalent bond varies depending on the chemical composition treatment conditions such as the glass composition, the type of salt (flux) used, the temperature and time for the chemical strengthening treatment, but the four covalent bonds extending from Si. Of these, it is considered preferable to select conditions that are sufficient to break one or two bonds.

  A high-density compressive stress layer is formed by ion exchange between Na ions on the glass surface and K ions in the inorganic salt. Examples of the method of bringing the glass into contact with the inorganic salt include a method of applying a paste-like inorganic salt, a method of spraying an aqueous solution of an inorganic salt onto the glass, and a method of immersing the glass in a salt bath of a molten salt heated to a melting point or higher. Although possible, among these, the method of immersing in molten salt is desirable.

The amount of flux added is preferably 0.1 mol% or more, more preferably 0.5 mol% or more, more preferably 1 mol% or more, and particularly preferably 2 mol% or more from the viewpoint of controlling the surface hydrogen concentration. Further, from the viewpoint of productivity, the saturation solubility or less of each salt is preferable. Addition in excess may lead to glass corrosion. For example, when K ₂ CO ₃ is used as the flux, it is preferably 24 mol% or less, more preferably 12 mol% or less, and particularly preferably 8 mol% or less.

  In addition to potassium nitrate and a flux, the inorganic salt may contain other chemical species as long as the effects of the present invention are not impaired. For example, alkali salts such as sodium chloride, potassium chloride, sodium borate, potassium borate, etc. Examples include chlorides and alkali borates. These may be added alone or in combination of two or more.

(Manufacture of molten salt)
The molten salt can be produced by a known process. That is, it can be obtained by preparing a molten potassium nitrate salt and then adding a flux to the molten potassium nitrate salt. As another method, it can be obtained by mixing potassium nitrate and a flux and then melting a mixed salt of the potassium nitrate and the flux.

The molten salt used in the production method of the present invention preferably has a Na concentration of 500 ppm by weight or more, more preferably 1000 ppm by weight or more. It is more preferable that the Na concentration in the molten salt is 500 ppm by weight or more because the low-density layer is easily deepened by the acid treatment step described later. There is no restriction | limiting in particular as an upper limit of Na density | concentration, It is permissible until a desired surface compressive stress (CS) is obtained.
In addition, the molten salt which performed the chemical strengthening process once or more contains the sodium eluted from glass. Therefore, if the Na concentration is already within the above range, glass-derived sodium may be used as it is as the Na source, or when the Na concentration is less than the above range, or when a chemically strengthened unused molten salt is used. Can be adjusted by adding an inorganic sodium salt such as sodium nitrate.

(Ion-exchange process)
Next, chemical strengthening treatment is performed using the prepared molten salt. The chemical strengthening treatment is performed by immersing the glass in the molten salt and ion-exchanging (substituting) Na ions in the glass with K ions in the molten salt. By this ion exchange, the composition of the glass surface can be changed to form a compressive stress layer in which the glass surface has a high density. Since compressive stress is generated by increasing the density of the glass surface, the glass can be strengthened.

  In practice, the density of chemically strengthened glass gradually increases from the outer edge of the intermediate layer (bulk) in the center of the glass toward the surface of the compressive stress layer. There is no clear boundary between which the density changes rapidly. Here, the intermediate layer is a layer present in the center of the glass and sandwiched between the compressive stress layers. Unlike the compressive stress layer, this intermediate layer is a layer that is not ion-exchanged.

Specifically, the chemical strengthening treatment (ion exchange step) in the present invention can be performed by the following procedure.
First, the glass is preheated, and the above-described molten salt is adjusted to a temperature at which chemical strengthening is performed. Next, the preheated glass is immersed in the molten salt for a predetermined time, and then the glass is pulled up from the molten salt and allowed to cool. In addition, it is preferable to perform shape processing according to a use, for example, mechanical processing, such as a cutting | disconnection, an end surface processing, and a drilling process, before a chemical strengthening process.

The preheating temperature of the glass depends on the temperature immersed in the molten salt, but is generally preferably 100 ° C. or higher.
The chemical strengthening temperature is preferably not higher than the strain point (usually 500 to 600 ° C.) of the glass to be tempered, and more preferably not higher than (strain point −50) ° C. In order to obtain a higher compressive stress layer depth, the chemical strengthening temperature is preferably 350 ° C. or higher, and more preferably 400 ° C. or higher.

  The immersion time of the glass in the molten salt is preferably 1 minute to 10 hours, more preferably 5 minutes to 8 hours, and even more preferably 10 minutes to 4 hours. In such a range, a chemically strengthened glass excellent in the balance between strength and depth of the compressive stress layer can be obtained, which is preferable.

(Holding process)
In the present invention, after immersing the glass in the molten salt in the ion exchange step, the glass is pulled up from the molten salt and kept at a high temperature for a certain time or more. If the molten salt is pulled up from the molten salt and allowed to cool immediately, the molten salt adhering to the glass surface is solidified to become a solid, and the solid chemically reacts with the glass surface to cause a dent-like defect on the glass surface. It is done. The molten salt can be removed from the glass by pulling it up from the molten salt and holding it at a high temperature for a certain time or longer, so that the solidified inorganic salt can be prevented from reacting with the glass surface. As a result, it is possible to obtain a chemically strengthened glass with few dent-like defects.

The holding temperature in the holding step is 315 ° C. or higher because the molten salt is difficult to solidify rapidly, and even when the molten salt is solidified, it is difficult to form particles having a size that forms a concave defect on the glass surface. Preferably, 330 degreeC or more is more preferable, and 350 degreeC or more is further more preferable. Furthermore, when the melting point under equilibrium vapor pressure of the inorganic salt brought into contact with glass is Tm, the holding temperature is preferably (Tm−20) ° C. or higher, more preferably (Tm−5) ° C. or higher, and (Tm + 10) ° C. or higher. Is more preferable.
The upper limit of the temperature is not particularly limited, but if it is too high, stress relaxation occurs and the glass strength tends to decrease. Therefore, if the holding temperature in the holding step is T1 ° C., the chemical strengthening temperature in the ion exchange step is The strengthening temperature is preferably less than T1 ° C, more preferably less than (T1-20) ° C, and even more preferably less than (T1-40) ° C.

The lower limit of the holding temperature is preferably (T1-120) ° C. or higher, more preferably (T1-105) ° C. or higher, and (T1-85) ° C. or higher for the same reason in relation to the chemical strengthening temperature T1. Further preferred.
The absolute value of the upper limit of the holding temperature is preferably less than 435 ° C., more preferably less than 415 ° C., and even more preferably less than 395 ° C. for the same reason.
The holding temperature may be a constant temperature throughout the holding step, or may change in temperature.

The holding time in the holding step is appropriately adjusted according to the chemical strengthening temperature T1 ° C. and the holding temperature. For example, the higher the holding temperature, the shorter the time required to remove the molten salt from the glass, and the lower the holding temperature, the longer the time. Also, the appropriate holding time varies depending on the type of glass material and how the glass is pulled up.
Therefore, although it is difficult to determine a preferable holding time strictly, when the holding temperature is 350 ° C. or higher, the holding time is preferably 3 minutes or longer, and more preferably 15 minutes or longer. When the holding temperature is 315 ° C. or higher and lower than 350 ° C., the holding time is preferably 30 minutes or longer, and more preferably 60 minutes or longer.
The upper limit of the holding time is not particularly limited, but when the holding temperature is 350 ° C. or higher, the holding time is preferably 2 hours or less from the viewpoint of preventing stress relaxation and reducing the glass strength. Less than the time is more preferable. When the holding temperature is 315 ° C. or higher and lower than 350 ° C., it is preferably 4 hours or shorter, more preferably 2 hours or shorter from the viewpoint of preventing stress relaxation and reducing the glass strength. .

Although preferable holding temperature and holding time differ depending on the glass material and the glass size, the difference (W1-W2) between the glass mass W1 immediately after the ion exchange step (immediately after being pulled up from the molten salt) and the glass mass W2 after the holding step. ) Is not less than 0.15 g (0.15 g / 2500 mm ² ) per 2500 mm ^{2 of} glass, since the molten salt is removed from the glass, and a chemically strengthened glass with very few indentations can be obtained. The value of (W1-W2) corresponds to the mass difference between the inorganic salts attached to the glass immediately after the ion exchange step and after the holding step.
The value of (W1-W2) is more preferably 0.20 g / 2500 mm ² or more, 0.25 g / 2500 mm ² or more is more preferable. In addition, since the mass of the inorganic salt adhering to the glass is included in W1 and W2, the value of (W1-W2) can be rephrased as the mass of the inorganic salt removed during the step of holding the glass. .

The direction of the glass plate used in the step of pulling up and holding the glass immersed in the molten salt in the ion exchange step is not particularly limited.
For example, after immersion, a. The two sides of the glass plate may be raised parallel to the ground, the other two sides perpendicular to the ground, and the glass plate surface perpendicular to the ground, b. The two sides of the glass plate may be parallel to the ground, the other two sides may be inclined with respect to the ground, and the glass plate surface may be tilted and pulled up with the surface held diagonally with respect to the ground, and c. All four sides of the glass plate are slanted with respect to the ground, with one vertex of the glass plate at the bottom and diagonally with the glass plate surface held perpendicular to the ground. You may pull it up.
A. By doing so, the liquid reservoir in the lower part of the glass immediately after pulling can be made uniform. B. Even if it is a case where a several glass plate is chemically strengthened at once, it can prevent that glass plates adhere. In addition, c. By doing so, the total amount of the liquid pool can be reduced, and the amount of molten salt to be reused can be increased.

  The atmosphere in the holding step is not particularly limited. For example, it is preferable to hold in the air from the viewpoint of improving productivity.

(Step 1 for cleaning (first cleaning step))
In the manufacturing method according to the present invention, the glass is cooled after the holding step, and then the glass is washed. In the first cleaning step, the glass is cleaned using industrial water, ion exchange water, or the like. Use the treated water if necessary. Of these, ion-exchanged water is preferred.
The washing conditions vary depending on the washing solution used, but when ion-exchanged water is used, washing at 0 to 100 ° C. is preferable from the viewpoint of completely removing the attached salt.
In the first cleaning step, various methods such as a method of immersing chemically tempered glass in a water bath containing ion-exchanged water, a method of exposing the glass surface to running water, a method of spraying a cleaning liquid toward the glass surface by a shower, etc. Can be used.

(Process for acid treatment)
In the manufacturing method according to the present invention, the step of acid-treating the glass is performed after the first cleaning step.
The acid treatment of the glass is performed by immersing the chemically strengthened glass in an acidic solution, whereby Na and / or K on the surface of the chemically strengthened glass can be replaced with H. That is, the surface layer of the compressive stress layer is altered on the glass surface, specifically, the density is reduced, and the solution having a low density layer is not particularly limited as long as it is acidic, and may be less than pH 7. The acid used may be a weak acid or a strong acid. Specifically, acids such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, acetic acid, oxalic acid, carbonic acid and citric acid are preferred. These acids may be used alone or in combination.

The temperature at which the acid treatment is performed varies depending on the type, concentration, and time of the acid used, but is preferably 100 ° C. or less.
Although the time for performing the acid treatment varies depending on the type, concentration and temperature of the acid used, 10 seconds to 5 hours is preferable from the viewpoint of productivity, and 1 minute to 2 hours is more preferable.
The concentration of the solution for acid treatment varies depending on the type, time, and temperature of the acid used, but is preferably a concentration with little concern about container corrosion, and specifically 0.1 wt% to 20 wt%.

  Since the low density layer is removed by an alkali treatment described later, the glass surface is more easily removed as the low density layer is thicker. Therefore, the thickness of the low density layer is preferably 5 nm or more, and more preferably 20 nm or more from the viewpoint of the glass surface removal amount. The thickness of the low density layer can be controlled by the flux concentration, sodium concentration, temperature, time, etc. in the chemical strengthening step.

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

The thickness of the low-density layer can be obtained from the period (Δθ) measured by the X-ray reflectivity method (X-ray-Reflectometry: XRR).
The density of the low density layer can be determined from the critical angle (θc) measured by XRR.
In addition, it is also possible to confirm the formation of the low density layer and the thickness of the layer by simply observing a cross section of the glass with a scanning electron microscope (SEM).

(Step 2 for cleaning (second cleaning step))
It is preferable to have the same cleaning step as the first cleaning step after the acid treatment step.

(Process of alkali treatment)
In the production method of the present invention, an alkali treatment step is performed after the acid treatment step and the second cleaning step.
The alkali treatment is performed by immersing the chemically strengthened glass in a basic solution, whereby a part or all of the low density layer can be removed.
The solution is not particularly limited as long as it is basic, and may have a pH exceeding 7, and a weak base or a strong base may be used. Specifically, bases such as sodium hydroxide, potassium hydroxide, potassium carbonate and sodium carbonate are preferred. These bases may be used alone or in combination.

Although the temperature which performs an alkali treatment changes also with the kind of base used, a density | concentration, and time, 0-100 degreeC is preferable, 10-80 degreeC is more preferable, and 20-60 degreeC is especially preferable. If it is this temperature range, there is no possibility that glass will corrode and it is preferable.
Although the time for performing the alkali treatment varies depending on the type, concentration and temperature of the base used, it is preferably 10 seconds to 5 hours from the viewpoint of productivity, and more preferably 1 minute to 2 hours.
The concentration of the solution used for the alkali treatment varies depending on the type of base used, the time, and the temperature, but is preferably 0.1% by weight to 20% by weight from the viewpoint of glass surface removability.

  By the alkali treatment, a part or all of the low density layer into which H has invaded is removed, whereby a chemically strengthened glass having improved surface strength can be obtained. Further, since the scratches existing on the glass surface are also removed at the same time by removing the low density layer, this point is also considered to contribute to the strength improvement.

(Step 3 for cleaning (third cleaning step))
It is preferable to have the same cleaning step as the first cleaning step after the alkali treatment step.

<Chemical tempered glass>
The chemically tempered glass obtained by the production method of the present invention is a chemically tempered glass having a compressive stress layer formed on the surface layer by an ion exchange method, and can reduce surface-dented defects.

(Dimpled defects on the glass surface)
The presence or absence of a dent-like defect on the surface of the obtained chemically strengthened glass can be confirmed by observation with a laser microscope or by visual observation in a dark room. The glass pulled up from the molten salt that has been subjected to the chemical strengthening treatment has a liquid pool in the lower portion in the holding step and the portion that holds the glass. Therefore, it is confirmed whether there is a dent-like defect near the liquid reservoir.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these.

<Evaluation method>
(Evaluation of glass: dent-like defects)
With respect to the chemically strengthened glass, a portion located at the lowermost part of the glass that becomes a liquid pool in the process of holding from the chemical strengthening process was observed with a laser microscope. The dent-like defect is obtained by observing 20 chemically strengthened glasses obtained under the same conditions, and determining the ratio of glass having 10 or more dents having a diameter of 1 μm or more.
In Examples 2, 6, and 9 and Comparative Example 3, the portion where the glass on the side of the glass, which is a portion that becomes a liquid pool other than the portion located at the bottom of the glass, was also observed.

<Example 1>
(Chemical strengthening process)
Potassium nitrate, potassium carbonate, and sodium nitrate were mixed and added to a stainless steel (SUS) tank so as to be 6 mol% potassium carbonate and 10,000 ppm by weight sodium, and heated to 450 ° C. with a heater to prepare a molten salt. At this time, the melting point of the inorganic salt (molten salt) under the equilibrium vapor pressure was about 335 ° C. An aluminosilicate glass A having a size of 244 mm × 172 mm × 0.56 mm was prepared, preheated to 200 to 400 ° C., and then immersed in a molten salt at 435 ° C. for 2 hours for ion exchange treatment. At this time, the glass plate was immersed so that the two sides of the glass plate were parallel to the ground, the remaining two sides were perpendicular to the ground, and the glass plate surface was perpendicular to the ground.
Aluminosilicate glass A composition (in mol%): SiO ₂ 64.4%, Al ₂ O ₃ 8.0%, Na ₂ O 12.5%, K ₂ O 4.0%, MgO 10.5%, CaO 0.1%, SrO 0.1%, BaO 0.1%, ZrO ₂ 0.5%

(Holding process)
The glass obtained above was pulled straight up and held at 350 ° C. for 15 minutes in the atmosphere. Then, it naturally left to cool to room temperature.
After confirming that the glass temperature had dropped to room temperature, the glass was immersed in pure water at 60 ° C. for 30 minutes, then immersed in pure water at room temperature for 10 minutes, and then allowed to stand in the air for natural drying. The glass thus obtained was subjected to the next step.

(Acid treatment process, washing process)
6% by weight of nitric acid (HNO ₃ ; manufactured by Otsuka Chemical Co., Ltd.) was prepared in a beaker, and the temperature was adjusted to 40 ° C. using a water bath. The glass obtained in the chemical strengthening step was immersed in the prepared nitric acid for 120 seconds to perform acid treatment. Thereafter, it was washed several times with pure water and then dried by air blow.

(Alkali treatment process, washing process)
A 4.0 wt% aqueous sodium hydroxide solution was prepared in a beaker, and the temperature was adjusted to 40 ° C. using a water bath. The glass that had undergone the acid treatment step and the washing step was immersed in the prepared aqueous potassium hydroxide solution for 120 seconds to perform alkali treatment. Thereafter, it was washed several times with pure water and then dried by air blow.
From the above, chemically strengthened glass of Example 1 was obtained.

<Example 2>
A chemically strengthened glass was produced in the same manner as in Example 1 except that the holding time in the holding step was 60 minutes.
<Example 3>
A chemically strengthened glass was produced in the same manner as in Example 1 except that the holding time in the holding step was 3 minutes.

<Example 4>
In the chemical strengthening process, the molten salt is in a state where the two sides of the glass plate are parallel to the ground, the other two sides are inclined with respect to the ground, and the glass plate surface is inclined with respect to the ground. A chemically tempered glass was produced in the same manner as in Example 1 except that the glass was directly immersed in and subjected to the process of pulling the glass straight upward and holding it.
<Example 5>
A chemically strengthened glass was produced in the same manner as in Example 4 except that the holding time in the holding step was 30 minutes.
<Example 6>
A chemically strengthened glass was produced in the same manner as in Example 4 except that the holding time in the holding step was 60 minutes.
<Example 7>
A chemically strengthened glass was produced in the same manner as in Example 4 except that the holding temperature in the holding step was 330 ° C. and the holding time was 60 minutes.
<Example 8>
A chemically strengthened glass was produced in the same manner as in Example 4 except that the holding temperature in the holding step was 315 ° C. and the holding time was 60 minutes.

<Example 9>
In the chemical strengthening process, all four sides of the glass plate are slanted with respect to the ground, and the glass plate surface is held perpendicular to the ground so that one vertex of the glass plate is at the bottom. A chemically strengthened glass was produced in the same manner as in Example 1 except that it was immersed in a molten salt in the state and subjected to a step of pulling and holding the glass diagonally as it was.

<Comparative Example 1>
A chemically strengthened glass was produced in the same manner as in Example 1 except that the holding temperature in the holding step was 300 ° C.
<Comparative example 2>
A chemically strengthened glass was produced in the same manner as in Example 1 except that the holding temperature in the holding step was 300 ° C. and the holding time was 60 minutes.

<Comparative Example 3>
A chemically strengthened glass was produced in the same manner as in Example 4 except that the holding temperature in the holding step was 300 ° C. and the holding time was 60 minutes.

The chemically tempered glass thus obtained was evaluated for dent defects. Table 1 shows the process conditions held by each chemically strengthened glass and the results of the concave defects.
In Table 1, in the “direction” of holding, “a” means that two sides of the glass plate are parallel to the ground, the other two sides are perpendicular to the ground, and the surface of the glass plate is on the ground. It means a state in which it is held vertically. “B” means a state in which the two sides of the glass plate are parallel to the ground, the remaining two sides are oblique to the ground, and the glass plate surface is held oblique to the ground. “C” means a state in which all four sides of the glass plate are inclined with respect to the ground, one vertex of the glass plate faces downward, and the surface of the glass plate is held perpendicular to the ground. Means.
In addition, the “lower part” of the dent-like defect means a part that is located at the lowermost part in the holding process and has become a liquid pool, and the “gripping part” means glass in the chemical strengthening process and the holding process. It means the part that holds the plate, and although it is not as low as the bottom, it is a small pool of liquid.

Except that the size of the glass material was 50 mm × 50 mm × 0.56 mm (main surface area 2500 mm ² × thickness 0.56 mm), the chemical strengthening step was performed in the same manner as in Example 1, and the mass of the glass plate before the chemical strengthening step And the mass of the salt adhering to the glass plate was calculated | required by calculating | requiring the difference with the mass of the glass plate immediately before using the process to hold | maintain immediately after a chemical strengthening process (holding time 0 second). Similarly, the salt adhering to each glass plate from the mass of the glass plate after being held at 300 ° C. for 30 seconds, 1 minute, 2 minutes, 4 minutes, 6 minutes, 8 minutes and 10 minutes after the chemical strengthening step. The mass of was determined. The results are shown in FIG.
Accordingly, when the holding time is 3 minutes or more, the mass difference between the inorganic salt adhering to the glass immediately after the chemical strengthening step (ion exchange step) and after the holding step is 0.15 g / 2500 mm ² or more. Since this mass difference can be regarded as the mass of the inorganic salt removed during the process of holding the glass, if the mass difference of the inorganic salt adhering to the glass is 0.15 g / 2500 mm ² or more, it is melted from the glass. It has been found that a chemically strengthened glass can be obtained in which the salt is sufficiently removed and the dent-like defects are very few. The glass thickness is assumed to be negligibly small.
By performing the same test by changing the glass material, glass size, holding temperature, etc., it is possible to predict in advance the appropriate holding temperature and holding time for each glass material and size.

  According to the present invention, it is possible to easily obtain chemically tempered glass with very few indentations on the glass surface. Since the chemically strengthened glass according to the present invention is excellent in aesthetics, it can be suitably used for a cover glass for a display such as a mobile phone, a digital camera, or a touch panel display.

Claims (5)

A method for producing chemically strengthened glass comprising a step of ion-exchanging Na in the glass and K in the inorganic salt by bringing a glass containing sodium into contact with an inorganic salt containing potassium nitrate,
The inorganic salt includes at least one salt selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , KOH and NaOH,
A method for producing chemically tempered glass, comprising a step of holding for 3 minutes or more at a temperature lower than the chemical strengthening temperature T1 ° C. in the ion exchanging step and at 315 ° C. or higher after the ion exchanging step. A method for producing chemically strengthened glass comprising a step of ion-exchanging Na in the glass and K in the inorganic salt by bringing a glass containing sodium into contact with an inorganic salt containing potassium nitrate,
The inorganic salt includes at least one salt selected from the group consisting of K ₂ CO ₃ , Na ₂ CO ₃ , KHCO ₃ , NaHCO ₃ , KOH and NaOH,
After the ion exchange step, a step of holding for 3 minutes or more at a temperature lower than the chemical strengthening temperature T1 ° C. in the ion exchange step and (melting point Tm−20 under equilibrium vapor pressure of the inorganic salt) ° C. or more. The manufacturing method of the chemically strengthened glass characterized by including. Washing the glass after the holding step;
The manufacturing method of the chemically strengthened glass of Claim 1 or 2 including the process of acid-treating glass after the said process to wash | clean.   The manufacturing method of the chemically strengthened glass of Claim 3 including the process of alkali-treating glass after the said acid-treating process. The mass difference between the inorganic salt adhering to the glass immediately after the ion exchange step and after the holding step is 0.15 g or more per 2500 mm ² of the main surface area of the glass. A method for producing the chemically strengthened glass according to claim 1.

Images