JP6544043B2 - Method of producing chemically strengthened glass - Google Patents

Description

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

  In flat panel display devices such as digital cameras, mobile phones or personal digital assistants (PDA), a thin plate-like cover glass so as to have a larger area than an image display portion in order to enhance display protection and aesthetics. It has been done to place it on the front of the display. Although glass has high theoretical strength, its strength is greatly reduced by scratching, so chemically tempered glass in which a compressive stress layer is formed on the glass surface by ion exchange or the like is used as the cover glass for which strength is required. There is.

  With the demand for lighter and thinner flat panel display devices, it is also required to make the cover glass thinner. Therefore, the cover glass is required to have further strength at the surface and the end face in order to satisfy the purpose.

Therefore, Patent Document 1 discloses a method for producing a chemically strengthened glass including a step of ion-exchanging Na in glass with K in the inorganic salt by bringing the glass containing sodium into contact with the 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. The method further comprises the steps of: washing the glass after the ion exchange, acid-treating the glass after the washing, and alkali-treating the glass after the acid treatment.

International Publication No. 2015/008763

  In the method described in Patent Document 1, the decrease in strength of the glass can be suppressed by the washing after the ion exchange, the acid treatment, and the 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 may locally corrode the glass and cause a dent-like defect in the glass. It turned out to be. Since such a dent-like defect impairs the appearance of the glass, it is required to reduce the dent-like defect.

  An object of the present invention is to provide a method for producing a chemically strengthened glass in which the concave defects are reduced without the glass being corroded by inorganic salts 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 hold the process in a fixed temperature range for a fixed period of time or longer. The inventors have found that less chemically strengthened glass can be obtained and complete the present invention.

That is, the present invention is as follows.
<1> A method for producing a chemically strengthened glass, comprising the step of ion exchanging Na in the glass with K in the inorganic salt by bringing the glass containing sodium into contact with the inorganic salt containing potassium nitrate,
The inorganic salt comprises 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 a chemically strengthened glass, comprising the step of maintaining the temperature of at least 315 ° C., which is lower than the chemical strengthening temperature T1 ° C. in the step of ion exchange, for 3 minutes or more after the step of ion exchange.
<2> A method for producing a chemically strengthened glass, comprising the step of ion exchanging Na in the glass with K in the inorganic salt by bringing the glass containing sodium into contact with the inorganic salt containing potassium nitrate,
The inorganic salt comprises 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, the step of holding at a temperature of (melting point Tm-20 of the inorganic salt under the equilibrium vapor pressure of the inorganic salt) ° C. or more for 3 minutes or more lower than the chemical strengthening temperature T1 ° C. in the ion exchange step A method of producing a chemically strengthened glass comprising:
<3> a step of washing the glass after the step of holding,
The manufacturing method of the chemically strengthened glass as described in said <1> or <2> including the process of acid-treating glass after the process to wash | clean.
The manufacturing method of the chemically strengthened glass as described in said <3> including the process of carrying out the alkali treatment of the glass after the process to carry out the <4> said acid treatment.
<5> The mass difference between the glass immediately after the step of ion exchange and after the holding step is any one of the above <1> to <4>, which is 0.15 g or more per 2500 mm ^{2 of} glass. Method of chemically strengthened glass.

  According to the method for producing a chemically strengthened glass of the present invention, it is possible to obtain a chemically strengthened glass which is excellent in an aesthetic appearance and in which the defects in the concave shape are small.

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 and implemented without departing from the scope of the present invention.
<Method of producing chemically strengthened glass>
The method for producing a chemically strengthened glass according to the present embodiment includes the step of ion-exchanging Na in the glass and K in the inorganic salt by bringing the glass containing sodium into contact with the inorganic salt containing potassium nitrate. A method for producing a chemically strengthened glass, comprising: 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, wherein the inorganic salt comprises After the step of performing the ion exchange, the method further comprising the step of holding at a temperature of 315.degree. C. or higher, which is lower than the chemical strengthening temperature T.sub.1.degree. C. in the ion exchange step, for 3 minutes or more.

The method for producing a chemically strengthened glass according to another embodiment ion-exchanges Na in the glass with K in the inorganic salt by bringing the glass containing sodium into contact with the inorganic salt containing potassium nitrate. A method for producing a chemically strengthened glass, comprising the step of: said inorganic salt comprising 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 the step of holding at a temperature of (melting point Tm-20 of the inorganic salt under the equilibrium vapor pressure of the inorganic salt) ° C. or more for 3 minutes or more lower than the chemical strengthening temperature T1 ° C. in the ion exchange step It is characterized by

(Glass composition)
The glass used in the present invention only needs to contain sodium, and various compositions can be used as long as it has a composition capable of being strengthened by molding and chemical strengthening treatment. Specifically, for example, aluminosilicate glass, soda lime glass, borosilicate glass, lead glass, alkali barium glass, aluminoborosilicate glass and the like can be mentioned. Among these, since the aluminosilicate glass and the aluminoborosilicate glass are likely to have a dent-like defect due to conventional chemical strengthening, the effects of the present invention become more remarkable.

  The method for producing glass is not particularly limited, and a desired glass raw material is introduced 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 forming apparatus to supply molten glass. It can be manufactured by forming into a plate and annealing.

  In addition, various methods are employable for shaping | molding of glass. For example, various forming methods such as a downdraw method (e.g., an overflow downdraw method, a slot down method, a redraw method, etc.), 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 preferably 5 mm or less, more preferably 3 mm or less, in order to effectively perform the chemical strengthening treatment.

  In the production of a chemically strengthened glass, the larger the size of the glass (the area of the main surface), the more likely a dent-like defect occurs. Therefore, the size of the glass is not particularly limited, but a glass plate of 150 mm × 100 mm or more is preferable in that the effect of the present invention is more remarkable.

  Further, the shape of the glass used in the present invention is not particularly limited. For example, glass of various shapes such as a flat plate shape having a uniform 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 bending portion or the like can be employed.

Although it does not specifically limit as a composition of the chemical strengthening glass of this invention, For example, the composition of the following glass is 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, and K ₂ O in the composition represented by mol% Composition containing 0-10% MgO, 0-15% MgO, 0-5% CaO and 0-5% ZrO ₂ (ii) mole% expressed as SiO ₂ , 50-74% SiO ₂ , Al 1 to 10% of ₂ O ₃ , 6 to 14% of Na ₂ O, 3 to 11% of K ₂ O, ₂ to 15% of MgO, 0 to 6% of CaO and 0 to 5% of ZrO ₂ , 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 it contains CaO, the content is 1% The composition represented by the glass (v) mass% which is less than contains 65 to 75% of SiO ₂ , 0.1 to 5% of Al ₂ O ₃ , 1 to 6% of MgO, and 1 to 15% of CaO , _{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 represented by glass (vii) mass% whose O ₃ is 1.8 to 5.0 is 60 to 72% of SiO ₂ , 1 to 10% of Al ₂ O ₃ , 5 to 12% of MgO, CaO Containing 0.1 to 5% of Na ₂ O, 13 to 19% of Na ₂ O, and 0 to 5% of K ₂ O, and 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. ) Is a 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 the glass is ion exchanged to form a surface layer in which compressive stress remains. Specifically, an alkali metal ion (typically, Li ion, Na ion) having a small ion radius on the surface of a glass plate by ion exchange at a temperature below the glass transition temperature (typically, an alkali ion having a larger ion radius (typically Is a Na ion or a K ion for Li ion and a K ion for Na ion). 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 contacting the above-mentioned sodium-containing glass with an inorganic salt containing at least one salt selected.
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. In addition, the above-mentioned salt (hereinafter sometimes referred to as "fluxing agent") has a property of cutting a glass network represented by a Si-O-Si bond. Since the temperature at which the chemical strengthening treatment is performed is as high as several hundred degrees C., the covalent bond between Si-O of the glass is appropriately broken at that temperature, and the density reduction treatment described later tends to proceed.

  The degree of breaking the covalent bond varies depending on the glass composition, the type of salt (flux) used, the temperature at which the chemical strengthening treatment is performed, and the chemical strengthening treatment conditions such as time, but four covalent bonds extending from Si Among them, it is considered preferable to select conditions under which one or two bonds are broken.

  Ion exchange between Na ions on the glass surface and K ions in the inorganic salt forms a high-density compressive stress layer. As a method of contacting the glass with the inorganic salt, a method of applying a paste-like inorganic salt, a method of spraying an aqueous solution of the inorganic salt onto the glass, a method of immersing the glass in a salt bath of molten salt heated to the melting point or more, etc. Among these, the method of immersing in molten salt is desirable.

The amount of the 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. In addition, the saturated solubility of each salt or less is preferable from the viewpoint of productivity. Excessive addition may lead to corrosion of the glass. For example, in the case of using _K 2 CO ₃ as a flux it is preferably less 24 mol%, more preferably less 12 mol%, particularly preferably not more than 8 mol%.

  The inorganic salt may contain other chemical species in addition to potassium nitrate and flux as long as the effects of the present invention are not impaired. For example, alkalis such as sodium chloride, potassium chloride, sodium borate and potassium borate Chloride salts and alkali borates can be mentioned. These may be added alone or in combination of two or more.

(Production of molten salt)
The molten salt can be produced by known processes. That is, it can be obtained by preparing potassium nitrate molten salt and then adding a flux to the potassium nitrate molten salt. Alternatively, it can be obtained by mixing potassium nitrate and a flux, and then melting the mixed salt of 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. When the Na concentration in the molten salt is 500 ppm by weight or more, the low density layer is more easily deepened by the acid treatment step described later, which is further preferable. There is no particular limitation on the upper limit of the Na concentration, which is acceptable until the desired surface compressive stress (CS) is obtained.
The molten salt subjected to the chemical strengthening treatment once or more contains sodium dissolved from the 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 if the Na concentration is below the above range or if a chemically strengthened unused molten salt is used. Can be adjusted by adding an inorganic sodium salt such as sodium nitrate.

(Step of ion exchange)
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 is densified. The densification of the glass surface generates a compressive stress, so that the glass can be strengthened.

  In practice, the density of the chemically strengthened glass is gradually increased from the outer edge of the intermediate layer (bulk) existing at the center of the glass toward the surface of the compressive stress layer, so that the intermediate layer and the compressive stress layer In the meantime, there is no clear boundary where the density changes rapidly. Here, the intermediate layer means a layer which exists in the center of the glass and is sandwiched by the compressive stress layer. This intermediate layer is a layer which is not ion exchanged unlike the compressive stress layer.

Specifically, the chemical strengthening treatment (step of ion exchange) in the present invention can be performed according to 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 out of 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 cutting, end surface processing, and drilling, to glass before chemical strengthening treatment.

The preheating temperature of the glass depends on the temperature at which it is immersed in the molten salt, but in general, the temperature is preferably 100 ° C. or higher.
The chemical strengthening temperature is preferably the strain point (generally 500 to 600 ° C.) or less of the glass to be tempered, and more preferably (strain point −50) ° C. or less. Further, in order to obtain a higher compressive stress layer depth, the chemical strengthening temperature is preferably 350 ° C. or more, and more preferably 400 ° C. or more.

  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 still more preferably 10 minutes to 4 hours. In such a range, a chemically strengthened glass excellent in the balance between the strength and the depth of the compressive stress layer can be obtained, which is preferable.

(Step of holding)
In the present invention, after immersing the glass in the molten salt in the step of ion exchange, the glass is pulled out from the molten salt, and is kept as it is at a high temperature for a certain time or more. It is thought that when the molten salt is pulled up and allowed to cool immediately, the molten salt adhering to the glass surface solidifies to become a solid, and the solid chemically reacts with the glass surface to cause a concave defect on the glass surface. Be The molten salt can be removed from the glass by passing through the step of pulling up from the molten salt and maintaining the temperature as it is for a certain period of time or more, 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 concave defects.

The holding temperature in the holding step is that the molten salt does not solidify rapidly, and even if the molten salt solidifies, it is difficult to form particles of such a size as to form a dent-like defect on the glass surface. Preferably, 330 ° C. or more is more preferable, and 350 ° C. or more is more preferable. Further, when the melting point of the inorganic salt to be brought into contact with the glass under the equilibrium vapor pressure is Tm, the holding temperature is preferably (Tm-20) ° C or higher, more preferably (Tm-5) ° C or higher, (Tm + 10) ° C or higher Is more preferred.
The upper temperature limit is not particularly limited, but if it is too high, stress relaxation occurs and the glass strength tends to decrease. Therefore, assuming that 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 still 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. More preferable.
The absolute value of the upper limit of the holding temperature is preferably less than 435 ° C., more preferably less than 415 ° C., and still more preferably less than 395 ° C., for the same reason.
The holding temperature may be a constant temperature throughout the holding step, or may be changed.

The holding time in the holding step is appropriately adjusted by the chemical strengthening temperature T1 ° C. and the holding temperature. For example, the higher the holding temperature, the shorter the time taken to remove the molten salt from the glass, and the lower the holding temperature, the longer the time. The appropriate holding time also differs depending on the type of glass material and the method of pulling up the glass.
Therefore, although it is difficult to determine the preferred holding time strictly, when the holding temperature is 350 ° C. or more, the holding time is preferably 3 minutes or more, and more preferably 15 minutes or more. When the holding temperature is 315 ° C. or more and less than 350 ° C., the holding time is preferably 30 minutes or more, and more preferably 60 minutes or more.
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, in order to prevent stress relaxation and reduction in glass strength. Less than time is more preferred. When the holding temperature is 315 ° C. or more and less than 350 ° C., it is preferably 4 hours or less, more preferably 2 hours or less, from the viewpoint of preventing stress relaxation and reduction in glass strength. .

The preferred holding temperature and holding time differ depending on the glass material and glass size, but the difference between the glass mass W1 immediately after the step of ion exchange (immediately after pulling up from the molten salt) and the glass mass W2 after the holding step (W1-W2 It is preferable that the molten salt is removed from the glass at 0.15 g (0.15 g / 2500 mm ² ) or more per 2500 mm ^{2 of} the glass, whereby a chemically strengthened glass with very few concave defects can be obtained. The value of (W1-W2) corresponds to the mass difference of the inorganic salt attached to the glass immediately after the step of ion exchange and after the holding step.
The value of (W1-W2) is more preferably 0.20 g / 2500 mm ² or more, and still more preferably 0.25 g / 2500 mm ² or more. Since W1 and W2 contain the mass of the inorganic salt attached to the glass, the value of (W1-W2) can be translated into the mass of the inorganic salt removed during the step of holding the glass. .

The orientation of the glass plate is not particularly limited in the step of pulling up and holding the glass immersed in the molten salt in the step of ion exchange.
For example, after immersion a. The two sides of the glass plate may be pulled parallel to the ground, the other two sides perpendicular to the ground, and the surface of the glass plate perpendicular to the ground, b. With the two sides of the glass plate parallel to the ground and the other two sides inclined with respect to the ground, the glass plate surface may be pulled up with the surface held obliquely with respect to the ground, and c. Diagonal with all four sides of the glass plate being oblique to the ground, with the top of the glass plate at the bottom and the glass plate surface held perpendicular to the ground You may
A. By setting it, it is possible to make the liquid pool in the lower part of the glass just after pulling up uniform. In addition, b. Even if chemical strengthening treatment is performed on a plurality of glass plates at once, it is possible to prevent the glass plates from sticking to each other. In addition, c. The total amount of liquid pool can be reduced by setting the amount of molten salt to be reused.

  The atmosphere in the holding step is not particularly limited. For example, it is preferable to hold | maintain in air | atmosphere from the point of productivity improvement.

(Step 1 of washing (first washing 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 treated water as needed. Among them, ion exchange water is preferred.
Although the washing conditions differ depending on the washing liquid used, when ion exchange water is used, washing at 0 to 100 ° C. is preferable from the viewpoint of completely removing attached salts.
In the first cleaning step, various methods such as a method of immersing the chemically strengthened glass in a water tank containing ion exchange 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, Can be used.

(Step of 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 carried out 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 by H. That is, the surface layer of the compressive stress layer is altered on the glass surface, specifically, it is further reduced in density, and it will further have a low density layer. The solution is not particularly limited as long as it is acidic. 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 preferable. These acids may be used alone or in combination of two or more.

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.
The time for acid treatment varies depending on the type, concentration and temperature of the acid used, but 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 to be subjected to the acid treatment varies depending on the type, time, and temperature of the acid used, but is preferably a concentration with little concern for container corrosion, specifically, 0.1 wt% to 20 wt%.

  The low density layer is removed by alkali treatment described later, so the thicker the low density layer, the easier it is to remove the glass surface. Accordingly, the thickness of the low density layer is preferably 5 nm or more, more preferably 20 nm 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 flux concentration, sodium concentration, temperature, time, etc. in the chemical strengthening step.

  From the viewpoint of glass surface removability, 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.

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

(Step 2 of washing (second washing step))
After completion of the acid treatment step, it is preferable to have the same washing step as the first washing step.

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

The temperature at which the alkali treatment is performed varies depending on the type, concentration, and time of the base used, but 0 to 100 ° C. is preferable, 10 to 80 ° C. is more preferable, and 20 to 60 ° C. is particularly preferable. If it is this temperature range, there is no possibility of corrosion of glass, and it is preferable.
The time for performing the alkali treatment varies depending on the type, concentration, and temperature of the base used, but 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 subjected to the alkali treatment varies depending on the type of the base used, time, and temperature, but is preferably 0.1% by weight to 20% by weight from the viewpoint of glass surface removability.

  By the above-mentioned alkali treatment, a part or all of the low density layer in which H penetrates is removed, whereby a chemically strengthened glass having an improved surface strength can be obtained. Furthermore, it is thought that this point also contributes to the improvement of the strength, since the scratch existing on the glass surface is simultaneously removed by removing the low density layer.

(Step 3 to wash (third washing step))
It is preferable to have the washing | cleaning process similar to a 1st washing | cleaning process after completion | finish of the process to carry out alkali treatment.

<Chemically strengthened glass>
The chemically strengthened glass obtained by the manufacturing method of the present invention is a chemically strengthened glass having a compressive stress layer formed by an ion exchange method on the surface layer, and can reduce the surface concave defect.

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

  EXAMPLES Although an Example is given to the following and this invention is concretely demonstrated to it, this invention is not limited to these.

<Evaluation method>
(Evaluation of glass: dented defect)
With respect to the chemically strengthened glass, a portion located at the lowermost part of the glass which is a liquid pool in the step of holding from the chemical strengthening step was observed by a laser microscope. In addition, a dent-shaped defect observes 20 sheets of chemically strengthened glass obtained on the same conditions, and calculates | requires the ratio of the glass which has ten or more dents 1 micrometer or more in diameter.
In Examples 2, 6, and 9 and Comparative Example 3, the portion where the glass at the side portion of the glass was gripped was observed, which is a portion other than the portion located at the lowermost portion of the glass.

Example 1
(Chemical strengthening process)
In a stainless steel (SUS) tank, potassium nitrate, potassium carbonate and sodium nitrate were mixed and added so as to be 6 mol% of potassium carbonate and 10000 ppm by weight of sodium, and heated to 450 ° C. by 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.degree. An aluminosilicate glass A (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 to perform ion exchange treatment. At this time, the two sides of the glass plate were immersed parallel to the ground, the other two sides perpendicular to the ground, and the glass plate surface 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%

(Step of holding)
The glass obtained above was pulled straight up and kept at 350 ° C. for 15 minutes under the atmosphere. After that, it was naturally cooled to room temperature.
After confirming that the glass temperature had dropped to room temperature, it was immersed in pure water at 60 ° C. for 30 minutes, then immersed in pure water at ordinary temperature for 10 minutes, and then left in the air to naturally dry. 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. Then, after washing with pure water several times, it was 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 subjected to the acid treatment step and the washing step was immersed in the prepared aqueous potassium hydroxide solution for 120 seconds to perform alkali treatment. Then, after washing with pure water several times, it was dried by air blow.
From the above, the chemically strengthened glass of Example 1 was obtained.

Example 2
Chemically strengthened glass was manufactured in the same manner as in Example 1 except that the holding time in the holding step was 60 minutes.
Example 3
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, with the two sides of the glass plate parallel to the ground and the other two sides inclined to the ground, the molten salt is in a state where the glass plate surface is inclined to the ground Chemically tempered glass was produced in the same manner as in Example 1 except that the glass was immersed in the above and the glass was directly pulled upward and held.
Example 5
Chemically strengthened glass was produced in the same manner as in Example 4 except that the holding time in the step of holding was 30 minutes.
Example 6
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
Chemically strengthened glass was produced in the same manner as in Example 4 except that the holding temperature in the step of holding was 330 ° C. and the holding time was 60 minutes.
Example 8
Chemically strengthened glass was produced in the same manner as in Example 4 except that the holding temperature in the step of holding was 315 ° C. and the holding time was 60 minutes.

Example 9
In the chemical strengthening process, all four sides of the glass plate were oblique to the ground, and one top of the glass plate was at the bottom, and the surface of the glass plate was held perpendicular to the ground Chemically strengthened glass was manufactured in the same manner as in Example 1 except that it was immersed in molten salt in the state, and subjected to the step of pulling up and holding the glass diagonally as it was.

Comparative Example 1
Chemically strengthened glass was produced in the same manner as in Example 1 except that the holding temperature in the step of holding 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 step of holding 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 step of holding was 300 ° C. and the holding time was 60 minutes.

Evaluation of the pitted defect was performed on the chemically strengthened glass thus obtained. Table 1 shows the conditions of the step of holding each chemically strengthened glass and the result of the dent defect.
In Table 1, in the “direction” of holding, “a” means that the two sides of the glass plate are parallel to the ground, the remaining two sides are perpendicular to the ground, and the surface of the glass plate is on the ground It means the state of holding vertically. "B" means a state where the two sides of the glass plate are parallel to the ground, the other two sides are oblique to the ground, and the surface of the glass plate is held obliquely to the ground. "C" means that all four sides of the glass plate are oblique to the ground, one vertex of the glass plate is directed to the lowermost, and the surface of the glass plate is held perpendicular to the ground Means
Moreover, the "lower part" of the recess-like defect means a portion located at the lowermost part in the holding step and being a liquid pool, and the "gripping part" is a glass in the chemical strengthening step and the holding step. It means the part holding the plate, and it is a small amount of liquid accumulation, though not as low as the lowermost part.

The chemical strengthening process is performed in the same manner as in Example 1 except that the size of the glass material is 50 mm × 50 mm × 0.56 mm (area of the main surface 2500 mm ² × thickness 0.56 mm), and the mass of the glass plate before the chemical strengthening process The mass of the salt adhering to the glass plate was determined by determining the difference between the mass of the glass plate immediately after the chemical strengthening step and before being subjected to the holding step (holding time 0 seconds). Similarly, from the mass of the glass plate after holding 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 salt adhering to each glass plate The mass of was determined. The results are shown in FIG.
From this, when the holding time is 3 minutes or more, the mass difference between the inorganic salt attached to the glass immediately after the chemical strengthening step (the step of ion exchange) 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 step of holding the glass, if the mass difference of the inorganic salt attached to the glass is 0.15 g / 2500 mm ² or more, it is melted from the glass It has been found that the salt can be sufficiently removed and a chemically strengthened glass with very few pitting defects can be obtained. The thickness of the glass is small enough to be ignored.
By carrying out the same test while 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 a chemically strengthened glass with very few pitting defects on the glass surface. The chemically strengthened glass according to the present invention can be suitably used as a cover glass for a display such as a mobile phone, a digital camera or a touch panel display because of its excellent appearance.

Claims (5)

A method for producing a chemically strengthened glass, comprising the step of ion-exchanging Na in the glass and K in the inorganic salt by bringing the glass containing sodium into contact with an inorganic salt containing potassium nitrate,
The inorganic salt comprises 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 a chemically strengthened glass, comprising the step of maintaining the temperature of at least 315 ° C., which is lower than the chemical strengthening temperature T1 ° C. in the step of ion exchange, for 3 minutes or more after the step of ion exchange. A method for producing a chemically strengthened glass, comprising the step of ion-exchanging Na in the glass and K in the inorganic salt by bringing the glass containing sodium into contact with an inorganic salt containing potassium nitrate,
The inorganic salt comprises 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, the step of holding at a temperature of (melting point Tm-20 of the inorganic salt under the equilibrium vapor pressure of the inorganic salt) ° C. or more for 3 minutes or more lower than the chemical strengthening temperature T1 ° C. in the ion exchange step A method of producing a chemically strengthened glass comprising: 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 process to wash | clean.   The method for producing a chemically strengthened glass according to claim 3, comprising the step of alkali treating the glass after the step of acid treatment. The mass difference of the inorganic salt attached to the glass immediately after the step of ion exchange and after the holding step is 0.15 g or more per 2500 mm ² of the area of the main surface of the glass. The manufacturing method of chemically strengthened glass as described in 1 or 2.

Images