WO2014045978A1 - Molten salt for use in toughening glass, production method of toughened glass, and life extension method of molten salt for use in toughening glass - Google Patents

Description

Molten salt for glass strengthening, method for producing tempered glass, and method for extending the life of molten salt for glass strengthening

The present invention relates to a molten salt for strengthening glass, a method for producing tempered glass, and a method for extending the life of a molten salt for strengthening glass.

Glass that has been chemically strengthened by ion exchange or the like (hereinafter also referred to as chemically tempered glass) is used for a cover glass of a display device such as a digital camera, a mobile phone, and a PDA (Personal Digital Assistants) and a glass substrate of the display. Yes. Although glass has a high theoretical strength, the strength is greatly reduced by scratching. Chemically tempered glass is suitable for these applications because it has higher mechanical strength than unstrengthened glass and prevents damage to the glass surface.

Chemical strengthening treatment by ion exchange compresses the glass surface by substituting metal ions with a small ionic radius (for example, Na ions) and metal ions with a larger ionic radius (for example, K ions) contained in the glass. This is a process for generating a stress layer and improving the strength of the glass.

When Na ₂ O is contained in the glass composition, the glass is immersed in a molten salt (inorganic potassium salt) containing K ions, and Na ions in the glass and K ions in the molten salt are ion-exchanged. As the molten salt, an inorganic potassium salt that is in a molten state at the strengthening treatment temperature is used, and potassium nitrate is often used among them.

As one of evaluation methods for chemically strengthened glass, surface compressive stress (CS) can be cited. The reason why the molten salt for chemical strengthening mainly composed of potassium nitrate can give the highest CS value to the glass after the chemical strengthening treatment is to use a molten salt that has not been subjected to ion exchange (new molten salt). Limited to time, in practice, the obtained CS value gradually decreases depending on the accumulated glass processing area.

It is known that the factor that lowers the CS value is due to the fact that potassium nitrate molten salt is diluted by Na ions eluted from the glass by ion exchange, and there is a correlation between Na ion concentration and CS value reduction. Therefore, a method of exchanging all or a part of the molten salt with a new molten salt when a CS value higher than a certain value cannot be obtained can be considered. However, in these methods, the replacement frequency of the molten salt is increased, and there is a concern that the cost may be increased and the processing efficiency may be reduced due to downtime at the time of replacement.

Therefore, a method for extending the service life (life) of the molten salt is being studied. For example, Non-Patent Document 1 discloses a method for reducing the influence of Na ions by adding silica to potassium nitrate molten salt in advance.

Nagaoka Gijutsu Kagaku Daigaku Kenkyu Hokoku (1982), 4, 1-4.

However, the effect of the addition of silica described in Non-Patent Document 1 is that 0.2% of sodium nitrate with respect to potassium nitrate is mixed in a very small amount of 0.2 ppm, that is, Na ion is 500 ppm with respect to 0.1% silica dope. In this case, there is only a description that the life of the molten salt is prolonged, and no mention is made of the case where the amount of Na ions is large.

Therefore, an object of the present invention is to provide a molten salt having a sufficiently extended service life with respect to a molten salt containing potassium nitrate used for chemical strengthening of glass.

As a result of earnest study, the present inventors have chemically strengthened one or more of potassium carbonate and potassium phosphate so that the molten salt containing potassium nitrate contains at least one of carbonate anion and phosphate anion. By adding in advance before the treatment, it was found that the service life of the molten salt can be extended, and the present invention has been completed.

That is, the present invention is as follows.
<1>
A molten salt for strengthening glass used to form a compressive stress layer on the glass surface by ion exchange,
A molten salt for glass strengthening containing potassium nitrate and further containing at least one of a carbonate anion and a phosphate anion.
<2>
The molten salt for glass reinforcement according to the above <1>, which contains potassium nitrate, carbonate anion and phosphate anion.
<3>
The melting for glass strengthening according to <1> or <2>, wherein the carbonate anion is an anion species of potassium carbonate, and the phosphate anion is at least one anion species of potassium orthophosphate and potassium pyrophosphate. salt.
<4>
The molten salt for glass strengthening according to <3>, wherein the content of the potassium carbonate is 3.5 to 24 mol% with respect to the potassium nitrate.
<5>
The molten salt for glass reinforcement according to the above <3> or <4>, wherein the content of the potassium orthophosphate is 0.8 to 13.5 mol% with respect to the potassium nitrate.
<6>
The molten salt for glass reinforcement according to any one of the above <3> to <5>, wherein the content of the potassium pyrophosphate is 3.5 to 9.0 mol% with respect to the potassium nitrate.
<7>
A method for producing tempered glass, comprising a step of forming a compressive stress layer on the glass surface using the molten salt for strengthening glass according to any one of the above items <1> to <6>.
<8>
A method for extending the lifetime of a molten salt for strengthening glass used for forming a compressive stress layer on a glass surface by ion exchange,
A method for extending the life of a molten salt for strengthening glass, comprising mixing at least one of potassium carbonate and potassium phosphate so that at least one of carbonate anion and phosphate anion is contained in the molten salt before strengthening treatment containing potassium nitrate.

According to the molten salt according to the present invention, the decrease in surface compressive stress (CS) caused by the increase in the concentration of Na ions eluted from the glass is suppressed or alleviated, and the service life of the molten salt can be extended. As a result, the replacement frequency of the molten salt decreases, and the cost of chemical strengthening treatment can be reduced and the throughput can be improved.

FIG. 1 is a graph showing the relationship between the Na ion concentration in a molten salt and the CS value when chemical strengthening treatment is performed without adding another inorganic potassium salt to the potassium nitrate molten salt. FIG. 2 is a graph showing the relationship between the amount of Na added to potassium nitrate and the obtained CS value when chemical strengthening treatment is performed by adding potassium orthophosphate to the molten potassium nitrate salt. FIG. 3 is a graph showing the relationship between the amount of potassium orthophosphate added to the molten salt and the life ratio of the molten salt. FIG. 4 is a graph showing the relationship between the amount of Na added to potassium nitrate and the obtained CS value when chemical strengthening treatment is performed by adding potassium carbonate to a molten potassium nitrate salt. FIG. 5 is a graph showing the relationship between the amount of potassium carbonate added to the molten salt and the life ratio of the molten salt. FIG. 6 is a graph showing the relationship between the amount of Na added to potassium nitrate and the obtained CS value when chemical strengthening treatment is performed by adding potassium pyrophosphate to a molten potassium nitrate salt. FIG. 7 is a graph showing the relationship between the amount of potassium pyrophosphate added to the molten salt and the life ratio of the molten salt. FIG. 8 is a graph showing the relationship between the total ion exchange ability of anion species of potassium orthophosphate, potassium pyrophosphate and potassium carbonate added to the molten salt and the life ratio of the molten salt. FIG. 9 is a graph showing the relationship between the amount of Na added to potassium nitrate and the obtained CS value when chemical strengthening treatment is performed by adding silica to molten potassium nitrate. FIG. 10 is a graph showing the relationship between the amount of silica added to the molten salt and the life ratio of the molten salt.

Hereinafter, the present invention will be described in detail.
In the present specification, “mass%” and “wt%” are synonymous.

<Molten salt>
The molten salt for glass strengthening of the present invention (hereinafter also referred to as the molten salt of the present invention) contains an inorganic potassium salt. The inorganic potassium salt preferably has a melting point below the strain point (usually 500 to 600 ° C.) of the glass to be chemically strengthened. In the present invention, it contains potassium nitrate (melting point 330 ° C.) as a main component. If potassium nitrate is a main component, it is preferable because it is in a molten state below the strain point of glass and is easy to handle in the operating temperature range. Here, the main component means containing 50% by mass or more.

The molten salt of the present invention further contains at least one of a carbonate anion and a phosphate anion in addition to potassium nitrate as a main component. Thereby, the service life of molten salt can be extended compared with the case where a carbonate anion and a phosphate anion are not included.

If the same molten salt is continuously used in the chemical strengthening treatment, the chemical strengthening is performed with a molten salt that has not been subjected to the chemical strengthening treatment (hereinafter also referred to as “a molten salt in an initial state” or “a new molten salt”). In comparison, the CS value that can be imparted to the glass gradually decreases according to the cumulative glass treatment area.
In the present invention, the molten salt before the chemical strengthening treatment is mixed with one or more of potassium carbonate and potassium phosphate in advance so as to include at least one of the carbonate anion and the phosphate anion, and the chemical strengthening treatment is performed. Thus, at the initial stage of elution of sodium, sodium is trapped by the carbonate anion and phosphate anion previously present in the molten salt, and the sodium ion concentration in the vicinity of the glass surface decreases. Furthermore, since the sodium salt of a carbonate anion and a phosphate anion will precipitate when the saturation solubility is exceeded, an increase in the sodium ion concentration in the molten salt is suppressed. It is considered that the life of the molten salt can be extended by such an action.

In order to quantitatively compare and evaluate the life extension ability of potassium carbonate and potassium phosphate to be added, it is considered as follows. For example, in the case of potassium orthophosphate, the anion that constitutes potassium orthophosphate is PO ₄ ³⁻ , which is a trivalent anion. The larger the valence of the anion, the stronger the force of attracting the cation and the easier it is to hold the cation.
Potassium orthophosphate has three monovalent cations K ^+, but from the acid dissociation constants of phosphoric acid (pKa1 = 1.83, pka2 = 6.43, pka3 = 11.146), Na ions in molten salt Assuming that one of the three potassiums is exchanged, the potential Na—K ion exchange capacity per mole of potassium orthophosphate is
Potential Na—K ion exchange capacity = (valence of anion species) × (exchangeable K ion amount) = 3 × 1 = 3
Can be evaluated. From this, the total ion exchange capacity of potassium orthophosphate added to the molten salt is
Total ion exchange capacity = (potassium orthophosphate added amount) × (potential Na—K ion exchange capacity)
It can ask for.
The higher the total ion exchange capacity, the higher the probability that Na ions in the molten salt ion-exchanged with glass will be ion-exchanged with the K ions of the added inorganic potassium salt, thereby inhibiting ion exchange by sodium in the molten salt. It is thought to contribute to extending the life of molten salt.

In the present invention, the following indices are used to quantitatively evaluate the service life (life) of the molten salt. First, the desired CS value is defined as a CS value of 90% or more when the CS value obtained from the molten salt composed of potassium nitrate in the initial state is 100%. When the desired CS value cannot be obtained by the chemical strengthening treatment, that is, when the CS value decreases by 10% or more with respect to the desired CS value, the Na ion concentration in the molten salt is determined as the service life of the molten salt. It is defined as
In addition, the lifetime of molten salt can be evaluated as follows. First, a predetermined amount of sodium nitrate is intentionally added to the molten salt as a Na ion source in order to create a pseudo state after repeated chemical strengthening treatments. Then, the glass is chemically strengthened with the molten salt to which the Na ion source has been added, and when the CS value of the glass after the treatment falls below the desired CS value, the Na ion concentration is calculated from the amount of sodium nitrate added. Can be used as an indicator of the lifetime of the molten salt.

When the molten salt of the present invention contains potassium carbonate, the content thereof is preferably 3.5 mol% to 24 mol% (5.0 wt% to 30 wt%) with respect to potassium nitrate in the molten salt. 0 mol% to 24 mol% (10.9 mass% to 30 mass%) is more preferable, and 16.0 mol% to 24 mol% (21.5 mass% to 30 mass%) is particularly preferable.
When the amount of potassium carbonate added to potassium nitrate is 30% by mass or less, there is no fear of an increase in the amount of solid phase in the molten salt due to potassium carbonate having a high melting point, and handling is good. In addition, there is no risk of temperature unevenness during the ion exchange treatment, and the entire glass can be ion exchanged uniformly.

Examples of potassium phosphate include potassium orthophosphate (K ₃ PO ₄ ), potassium pyrophosphate (K ₄ P ₂ O ₇ ), and potassium metaphosphate. The potassium orthophosphate is suitable for the total ion exchange capacity of the added anionic species. It is preferable from the point of life extension efficiency.

When potassium orthophosphate is added as potassium phosphate, the potassium orthophosphate may be hydrated or dehydrated. Further, the content of potassium orthophosphate in the molten salt is preferably 0.8 mol% to 13.5 mol% (1.5 mass% to 25 mass%), preferably 1.5 mol% to 13. 5 mol% (3.0 wt% to 25 wt%) is more preferable, 3.0 mol% to 13.5 mol% (6.0 wt% to 25 wt%) is further preferable, and 6.0 mol% to 13.5 mol% (11.5 mass% to 25 mass%) is particularly preferred. If the lower limit is within this range, it is preferable because the molten salt life that provides a desired CS value can be extended more than twice.

In addition, potassium orthophosphate has a high melting point (> 1000 ° C.), and the amount dissolved in potassium nitrate is very small in the temperature range (<500 ° C.) used for chemical strengthening. For this reason, if an excessive amount is added, a precipitate is deposited on the bottom of the container, and the handling of the molten salt becomes worse. Therefore, if it is 25 mass% or less with respect to potassium nitrate, the ratio of the solid phase of potassium orthophosphate can be suppressed, the liquid phase volume which can be used for chemical strengthening can be secured sufficiently, and the potassium phosphate precipitate in the molten salt can be obtained. There is no risk of contacting the glass and inducing corrosion of the glass surface, which is preferable.

When potassium pyrophosphate is added as potassium phosphate, it is preferably 3.5 mol% to 9.0 mol% (10.5 mass% to 25 mass%) based on potassium nitrate in the molten salt. More preferably, it is 5 mol% or more (21.0 mass% or more). If the lower limit is within this range, it is preferable because the molten salt life that provides a desired CS value can be extended more than twice.
Similarly to potassium orthophosphate, potassium pyrophosphate may corrode the glass surface when the powder comes into contact with the glass for chemical strengthening due to addition of an excessive amount, so the upper limit of the content of potassium pyrophosphate is 25% by mass. It is preferable to do.

In addition, you may use together the said potassium carbonate and potassium phosphate. In that case, the addition amount of each of potassium carbonate and potassium phosphate may be any combination as long as the above ranges are satisfied.

The molten salt of the present invention may contain other chemical species in addition to potassium nitrate, potassium carbonate and potassium phosphate as long as the effects of the present invention are not impaired. For example, sodium sulfate, potassium sulfate, sodium chloride, chloride Examples thereof include alkali sulfates such as potassium and alkali chlorides. These multiple types may be used in combination.

<Method for producing molten salt>
The molten salt of the present invention can be produced by the steps shown below.
Step 1: Preparation of potassium nitrate molten salt Step 2: Addition of other inorganic potassium salt to molten salt

(Process 1)
In step 1, molten salt is prepared by putting potassium nitrate into a container and heating and melting to a temperature equal to or higher than the melting point. Since potassium nitrate has a melting point of 330 ° C. and a boiling point of 500 ° C., it melts at a temperature within that range. In particular, the melting temperature is preferably 350 to 470 ° C. from the viewpoint of the balance between the surface compressive stress and the stress layer depth that can be applied to the glass, and the strengthening time.

As the container for melting potassium nitrate, metal, quartz, ceramics, or the like can be used. Among these, a metal material is desirable from the viewpoint of durability, and a stainless steel (SUS) material is desirable from the viewpoint of corrosion resistance.

(Process 2)
In step 2, the potassium nitrate molten salt prepared in step 1 is added with an inorganic potassium salt other than potassium nitrate, such as potassium carbonate and potassium phosphate, and the whole is uniformly mixed with a stirring blade while maintaining the temperature within a certain range. Mix to become. When potassium carbonate and potassium phosphate are used in combination, the order of addition is not limited, and either may be added first or may be added simultaneously. The temperature is preferably not less than the melting point of potassium nitrate, that is, not less than 330 ° C., more preferably 350 to 500 ° C. The stirring time is preferably 1 minute to 10 hours, more preferably 10 minutes to 2 hours. Then, it is allowed to stand until the precipitate is precipitated. This precipitate contains potassium carbonate and potassium phosphate exceeding the saturation solubility, and sodium salt of carbonate anion and sodium salt of phosphate anion.
Thus, the molten salt of this invention can be manufactured.

<Chemical strengthening treatment>
Next, a chemical strengthening treatment method using the molten salt of the present invention will be described.
The chemical strengthening treatment is performed by immersing glass in a molten salt and replacing metal ions in the glass with metal ions having a large ion radius in the molten salt. The glass can be strengthened by changing the composition of the glass surface by this ion exchange and generating a compressive stress in the glass surface layer.
The chemical strengthening treatment in the present invention can be performed by the following steps following the above-described molten salt production method (step 1, step 2).

Process 3: Glass chemical strengthening treatment process 4: Disposal of molten salt

(Process 3)
In step 3, the glass is preheated, and the molten salt prepared in steps 1 and 2 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 to glass.

The preheating temperature of glass depends on the temperature immersed in the molten salt, but is generally preferably 100 ° C. or higher.

The chemical strengthening temperature is preferably the strain point (usually 500 to 600 ° C.) or less of the glass to be tempered, and 350 ° C. or more is particularly preferable in order to obtain a higher compressive stress layer depth (Depth of Layer: DOL).

The immersion time of the glass in the molten salt is preferably 10 minutes to 12 hours, more preferably 30 minutes to 10 hours. If it exists in this range, the chemically strengthened glass excellent in the balance of an intensity | strength and the depth of a compressive-stress layer can be obtained.

(Process 4)
When step 3 is repeated, the ion exchange between the molten salt and the glass increases the Na ion concentration in the molten salt, so that the ion exchange capacity of the molten salt decreases as the glass treatment area increases. A desired CS value cannot be obtained. Therefore, in Step 4, by measuring the Na ion concentration in the molten salt or the surface compressive stress (CS) value after chemical strengthening, the molten salt can be continuously used for chemical strengthening treatment, Determine whether to discard.

<Glass>
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 soda lime glass, aluminosilicate glass, borosilicate glass, lead glass, alkali barium glass, and aluminoborosilicate glass.

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 heated and melted preferably 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.

It should be noted that 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.

Although it does not specifically limit as a composition of the glass for chemical strengthening of this invention, For example, the following glass compositions are mentioned.
(I) a composition that is displayed in mol%, the SiO ₂ 50 ~ 80%, the _{Al 2 O 3 2 ~ 25%} , the Li ₂ O 0 ~ 10%, a _{Na 2 O 0 ~ 18%,} K 2 O Is represented by a glass (ii) mol% containing 0-10%, MgO 0-15%, CaO 0-5% and ZrO ₂ 0-5%, SiO ₂ 50-74%, Al ₂ O ₃ 1-10%, Na ₂ O 6-14%, K ₂ O 3-11%, MgO 2-15%, CaO 0-6% and ZrO ₂ 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 ~ 80%, the _{Al 2 O 3 4 ~ 10%} , The a ₂ O 5 ~ 15%, the _{K 2} O 0 to 1%, the MgO 4 ~ 15% and ZrO ₂ is composition displaying a glass (iv) mole% containing 0 to 1%, a SiO ₂ 67 -75%, Al ₂ O ₃ 0-4%, Na ₂ O 7-15%, K ₂ O 1-9%, MgO 6-14% and ZrO ₂ 0-1.5% 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% Glass that is less than

Glass may be polished before chemical strengthening treatment as necessary. Examples of the polishing method include a method of polishing with a polishing pad while supplying a polishing slurry. As the polishing slurry, a polishing slurry containing an abrasive and water can be used. As the abrasive, cerium oxide (ceria) and silica are preferable.

If the glass is polished, the polished glass is cleaned with a cleaning solution. The washing liquid is preferably a neutral detergent and water, and more preferably washed with water after washing with a neutral detergent. A commercially available neutral detergent can be used.

The glass substrate cleaned by the cleaning process is finally cleaned with a cleaning solution. Examples of the cleaning liquid include water, ethanol, and isopropanol. Of these, water is preferred.

After the final cleaning, the glass is dried. The drying conditions may be selected in consideration of the cleaning solution used in the cleaning process, the characteristics of the glass, and the like.

Examples of the present invention will be specifically described below, but the present invention is not limited to these.

(Glass composition)
Two types of glass, soda lime glass and aluminosilicate glass, were used as the chemically strengthened glass.
Soda lime glass (composition expressed in mol%): SiO ₂ 72.0%, Al ₂ O ₃ 1.1%, Na ₂ O ₃ 12.6%, K ₂ O 0.2%, MgO 5.5% , CaO 8.6%
Aluminosilicate glass (composition expressed in mol%): SiO ₂ 64.4%, Al ₂ O ₃ 8.0%, Na ₂ O ₃ 12.5%, K ₂ O 4.0%, MgO 10.5% , CaO 0.1%, SrO 0.1%, BaO 0.1%, ZrO ₂ 2.5%

(Evaluation of glass)
Glass was evaluated by measuring surface compressive stress (CS) and compressive stress layer depth (DOL). CS and DOL were determined by measuring the difference in refractive index between the glass surface and the inside using a surface stress meter (FSM-6000LE manufactured by Orihara Seisakusho).

[Example 1: Addition of potassium orthophosphate]
(Examples 1A-1 to 1A-6)
Example 1A-1
250 g of potassium nitrate was added to a SUS cup and heated to 430 ° C. with a mantle heater to prepare a molten salt. 30.4 g of potassium orthophosphate trihydrate (K ₃ PO ₄ content with respect to potassium nitrate: 4.4 mol%) was added to the molten salt thus prepared, and the mixture was stirred for 2 hours using a stirring motor and four propeller blades. Let stand for 2 hours. Thereafter, the soda lime glass was preheated to 100 ° C. and immersed in a molten salt at 430 ° C. for 4 hours for chemical strengthening treatment. Thereafter, the glass was washed with ion exchange water at 100 ° C. and dried at 60 ° C. for 2 hours. CS and DOL of the glass after the chemical strengthening treatment were measured.

Example 1A-2
0.93 g of sodium nitrate was added to the molten salt subjected to the chemical strengthening treatment in Example 1A-1, stirred for 2 hours using a stirring motor and four propeller blades, and allowed to stand for 2 hours. Thereafter, the soda lime glass was preheated to 100 ° C. and immersed in a molten salt at 430 ° C. for 4 hours for chemical strengthening treatment. The glass was washed with ion exchange water at 100 ° C. and dried at 60 ° C. for 2 hours. Thereafter, CS and DOL were measured respectively.

Example 1A-3
0.92 g of sodium nitrate was further added to the molten salt subjected to the chemical strengthening treatment in Example 1A-2 (the total amount of sodium nitrate was 1.85 g), and the mixture was stirred with a stirring motor and four propeller blades. Stir for hours and let stand for 2 hours. Thereafter, the soda lime glass was preheated to 100 ° C. and immersed in a molten salt at 430 ° C. for 4 hours for chemical strengthening treatment. The glass was washed with ion exchange water at 100 ° C. and dried at 60 ° C. for 2 hours. Thereafter, CS and DOL were measured respectively.

Example 1A-4
0.94 g of sodium nitrate was further added to the molten salt subjected to the chemical strengthening treatment in Example 1A-3 (the total amount of sodium nitrate added was 2.79 g), and the mixture was stirred using a stirring motor and four propeller blades. Stir for hours and let stand for 2 hours. The soda lime glass was preheated to 100 ° C. and immersed in a molten salt at 430 ° C. for 4 hours for chemical strengthening treatment. The glass was washed with ion exchange water at 100 ° C. and dried at 60 ° C. for 2 hours. Thereafter, CS and DOL were measured respectively.

(Example 1A-5)
1.93 g of sodium nitrate was further added to the molten salt subjected to the chemical strengthening treatment in Example 1A-4 (total amount of sodium nitrate added was 4.72 g), and the mixture was stirred using a stirring motor and four propeller blades. Stir for hours and let stand for 2 hours. The soda lime glass was preheated to 100 ° C. and immersed in a molten salt at 430 ° C. for 4 hours for chemical strengthening treatment. The glass was washed with ion exchange water at 100 ° C. and dried at 60 ° C. for 2 hours. Thereafter, CS and DOL were measured respectively.

(Example 1A-6)
Further, 4.87 g of sodium nitrate was added to the molten salt subjected to the chemical strengthening treatment in Example 1A-5 (the total amount of sodium nitrate was 9.59 g), and the mixture was stirred using a stirring motor and four propeller blades. Stir for hours and let stand for 2 hours. The soda lime glass was preheated to 100 ° C. and immersed in a molten salt at 430 ° C. for 4 hours for chemical strengthening treatment. The glass was washed with ion exchange water at 100 ° C. and dried at 60 ° C. for 2 hours. Thereafter, CS and DOL were measured respectively.

(Examples 1B-1 to 1B-6)
Except that the glass was changed to an aluminosilicate glass, a molten salt was prepared and subjected to a chemical strengthening treatment in the same manner as in Examples 1A-1 to 1A-6, and CS and DOL were measured.

[Comparative Example 1: Potassium nitrate only]
(Comparative Examples 1A-1 to 1A-6)
A chemical strengthening treatment was performed in the same manner as in Examples 1A-1 to 1A-6 except that potassium orthophosphate trihydrate was not used and a molten salt containing only potassium nitrate was used, and CS and DOL were measured. (Comparative Examples 1B-1 to 1B-6)
A chemical strengthening treatment was carried out in the same manner as in Examples 1B-1 to 1B-6 except that potassium orthophosphate trihydrate was not used and a molten salt containing only potassium nitrate was used, and CS and DOL were measured.

[Example 2: Addition of potassium orthophosphate]
(Examples 2A-1 to 2A-6)
A chemical strengthening treatment was performed in the same manner as in Examples 1A-1 to 1A-6, except that the amount of potassium orthophosphate trihydrate added was 6.7 g (K ₃ PO ₄ content with respect to potassium nitrate: 1 mol%). , DOL was measured respectively.
(Examples 2B-1 to 2B-6)
A chemical strengthening treatment was performed in the same manner as in Examples 1B-1 to 1B-6, except that the amount of potassium orthophosphate trihydrate added was 6.7 g (K ₃ PO ₄ content with respect to potassium nitrate: 1 mol%). , DOL was measured respectively.

[Example 3: Addition of potassium orthophosphate]
(Examples 3A-1 to 3A-6)
A chemical strengthening treatment was carried out in the same manner as in Examples 1A-1 to 1A-6 except that the amount of potassium orthophosphate trihydrate added was 43.2 g (K ₃ PO ₄ content with respect to potassium nitrate: 6 mol%). , DOL was measured respectively.
(Examples 3B-1 to 3B-6)
A chemical strengthening treatment was performed in the same manner as in Examples 1B-1 to 1B-6 except that the amount of potassium orthophosphate trihydrate added was 43.2 g (K ₃ PO ₄ content with respect to potassium nitrate: 6 mol%). , DOL was measured respectively.

Table 1 shows the measurement results of Examples 1 to 3, and Table 2 shows the measurement results of Comparative Example 1.

[Example 4: Addition of potassium carbonate]
Example 4A-1
250 g of potassium nitrate was added to a SUS cup and heated to 430 ° C. with a mantle heater to prepare a molten salt. To the molten salt thus prepared, 29.7 g of potassium carbonate (K ₂ CO ₃ content with respect to potassium nitrate: 8 mol%) was added, stirred for 10 hours using a stirring motor and four propeller blades, and allowed to stand for 2 hours. Thereafter, the soda lime glass was preheated to 100 ° C. and immersed in a molten salt at 430 ° C. for 4 hours for chemical strengthening treatment. Thereafter, the glass was washed with ion exchange water at 100 ° C. and dried at 60 ° C. for 2 hours. CS and DOL of the glass after the chemical strengthening treatment were measured.

(Examples 4A-2 to 4A-6)
In the same manner as in Examples 1A-2 to 1A-6, sodium nitrate was sequentially added to the molten salt subjected to chemical strengthening treatment, chemical strengthening treatment was performed, and CS and DOL were measured.

(Examples 4B-1 to 4B-6)
Except that the glass was changed to an aluminosilicate glass, a molten salt was prepared and subjected to a chemical strengthening treatment in the same manner as in Examples 4A-1 to 4A-6, and CS and DOL were measured.

[Example 5: Addition of potassium carbonate]
(Examples 5A-1 to 5A-6)
Chemical strengthening treatment was performed in the same manner as in Examples 4A-1 to 4A-6, except that the amount of potassium carbonate added was 18.0 g (K ₂ CO ₃ content with respect to potassium nitrate: 5 mol%), and CS and DOL were measured. did.
(Examples 5B-1 to 5B-6)
Chemical strengthening treatment was carried out in the same manner as in Examples 4B-1 to 4B-6, except that the amount of potassium carbonate added was 18.0 g (K ₂ CO ₃ content with respect to potassium nitrate: 5 mol%), and CS and DOL were measured. did.

[Example 6: Potassium carbonate addition]
(Examples 6A-1 to 6A-6)
Chemical strengthening treatment was carried out in the same manner as in Examples 4A-1 to 4A-6, except that the amount of potassium carbonate added was 55.6 g (K ₂ CO ₃ content with respect to potassium nitrate: 14 mol%), and CS and DOL were measured respectively. did.
(Examples 6B-1 to 6B-6)
Chemical strengthening treatment was carried out in the same manner as in Examples 4B-1 to 4B-6, except that the amount of potassium carbonate added was 55.6 g (K ₂ CO ₃ content with respect to potassium nitrate: 14 mol%), and CS and DOL were measured respectively. did.

Table 3 shows the measurement results of Examples 4 to 6.

[Example 7: Addition of potassium pyrophosphate]
(Examples 7A-1 to 7A-6)
(Example 7A-1)
250 g of potassium nitrate was added to a SUS cup and heated to 430 ° C. with a mantle heater to prepare a molten salt. 30.4 g of potassium pyrophosphate trihydrate (K ₄ P ₂ O ₇ content relative to potassium nitrate: 4.4 mol%) was added to the molten salt thus prepared, and the mixture was stirred for 2 hours using a stirring motor and four propeller blades. And left to stand for 2 hours. Thereafter, the soda lime glass was preheated to 100 ° C. and immersed in a molten salt at 430 ° C. for 4 hours for chemical strengthening treatment. Thereafter, the glass was washed with ion exchange water at 100 ° C. and dried at 60 ° C. for 2 hours. CS and DOL of the glass after the chemical strengthening treatment were measured.

(Examples 7A-2 to 7A-6)
In the same manner as in Examples 1A-2 to 1A-6, sodium nitrate was sequentially added to the molten salt subjected to chemical strengthening treatment, chemical strengthening treatment was performed, and CS and DOL were measured.

(Examples 7B-1 to 7B-6)
Except that the glass was changed to an aluminosilicate glass, a molten salt was prepared and subjected to a chemical strengthening treatment in the same manner as in Examples 7A-1 to 7A-6, and CS and DOL were measured.

[Example 8: Potassium pyrophosphate added]
(Examples 8A-1 to 8A-6)
A chemical strengthening treatment was performed in the same manner as in Examples 7A-1 to 7A-6 except that the amount of potassium pyrophosphate trihydrate added was 6.7 g (K ₄ P ₂ O ₇ content with respect to potassium nitrate: 1 mol%). , CS and DOL were measured respectively.
(Examples 8B-1 to 8B-6)
A chemical strengthening treatment was performed in the same manner as in Examples 7B-1 to 7B-6 except that the amount of potassium pyrophosphate trihydrate added was 6.7 g (K ₄ P ₂ O ₇ content with respect to potassium nitrate: 1 mol%). , CS and DOL were measured respectively.

[Example 9: Addition of potassium pyrophosphate]
(Examples 9A-1 to 9A-6)
A chemical strengthening treatment was performed in the same manner as in Examples 7A-1 to 7A-6 except that the amount of potassium pyrophosphate trihydrate added was 43.2 g (K ₄ P ₂ O ₇ content relative to potassium nitrate: 6 mol%). , CS and DOL were measured respectively.
(Examples 9B-1 to 9B-6)
A chemical strengthening treatment was carried out in the same manner as in Examples 7B-1 to 7B-6 except that the amount of potassium pyrophosphate trihydrate added was 43.2 g (K ₄ P ₂ O ₇ content with respect to potassium nitrate: 6 mol%). , CS and DOL were measured respectively.

Table 4 shows the measurement results of Examples 7 to 9.

(Evaluation of life of molten salt)
Based on the result of Comparative Example 1, when the chemical strengthening treatment was performed without adding other inorganic potassium salt to the potassium nitrate molten salt, the Na ion concentration in the molten salt (NaNO ₃ was A (mol), KNO ₃ was The relationship between A / (A + B when B (mol) and CS) and CS is summarized in FIG. From the results of FIG. 1, it can be seen that the CS of the chemically strengthened glass was uniformly higher in the aluminosilicate glass (Comparative Example 1B) than in the soda lime glass (Comparative Example 1A). Therefore, in order to demonstrate the effect of adding an inorganic potassium salt to a molten salt, further discussion will be made using the results obtained with an aluminosilicate glass.
In Comparative Example 1B, since the CS value given by the molten salt in the initial state (Comparative Example 1B-1) was 864 MPa, the CS value (800 MPa), which was about 10% lower than this, was used as the reference for the deteriorated state. It was. That is, the Na ion content in the molten salt (added amount (mol%) with respect to sodium nitrate) at the CS value of 800 MPa was evaluated as an index of the lifetime of the molten salt.

(Effect of potassium orthophosphate added to molten salt)
Based on the results of Example 1B, Example 2B, and Example 3B, the amount of Na added to potassium nitrate (KNO ₃ ) (mass ppm) in the case of performing chemical strengthening treatment by adding potassium orthophosphate to the molten potassium nitrate salt 2 and the relationship with the obtained CS are summarized in FIG. For comparison, only potassium nitrate, that is, the relationship when no inorganic potassium salt is added is also shown (Comparative Example 1B).

And from each relationship of FIG. 2, Na ion content (addition amount (mol%) with respect to sodium nitrate) in molten salt when CS value was set to 800 Mpa was calculated by linear approximation, respectively. Furthermore, from the Na ion content, the life ratio was calculated by setting the case where no inorganic potassium salt other than potassium nitrate was added to the molten salt (Comparative Example 1B-1) as the standard of the molten salt life (1.00 times).
Further, the total ion exchange capacity corresponding to the added amount of potassium orthophosphate was calculated by the method described above. Since potassium orthophosphate has an anion valence of 3, assuming that the number of exchangeable potassium ions is 1, the potential Na—K ion exchange capacity is 3.
Total ion exchange capacity = (potassium orthophosphate added amount) × (potential Na—K ion exchange capacity)

The results are shown in Table 5.
Further, FIG. 3 shows the relationship between the amount of potassium orthophosphate (mol%) added to potassium nitrate and the life ratio of the molten salt (ratio when the life of the molten salt containing only potassium nitrate is 1.00 times).

From FIG. 2, when potassium orthophosphate was not added to the molten salt, the decrease in CS of the tempered glass was remarkable as the amount of Na in the molten salt increased (Comparative Example 1B), whereas potassium orthophosphate was melted. Pre-addition to the salt resulted in a very slow decrease in CS (Examples 1B, 2B and 3B). Further, FIG. 3 shows that the effect of extending the life of the molten salt increases with an increase in the amount of potassium orthophosphate added.
From the above, it has been found that the addition of potassium orthophosphate has a significant effect on extending the life of the molten salt.

(Effect of adding potassium carbonate to molten salt)
Based on the results of Example 4B, Example 5B, and Example 6B, the amount of Na added (in ppm by mass) to potassium nitrate (KNO ₃ ) when chemical strengthening treatment was performed by adding potassium carbonate to the molten potassium nitrate salt. The relationship with the obtained CS is summarized in FIG. For comparison, only potassium nitrate, that is, the relationship when no inorganic potassium salt is added is also shown (Comparative Example 1B).

In the same manner as described above, the Na ion content in the molten salt when the CS value was 800 MPa was calculated from each relationship in FIG. 4 by linear approximation, and the life ratio was calculated from the Na ion content.
Further, the total ion exchange capacity corresponding to the added amount of potassium carbonate was calculated by the method described above. Since potassium carbonate has an anion valence of 2, assuming that the number of exchangeable potassium ions is 1, the potential Na—K ion exchange capacity is 2.
Total ion exchange capacity = (potassium carbonate addition amount) x (potential Na-K ion exchange capacity)

The results are shown in Table 6.
Further, FIG. 5 shows the relationship between the amount (mol%) of potassium carbonate added to potassium nitrate (KNO ₃ ) and the life ratio of the molten salt (ratio when the life of the molten salt containing only potassium nitrate is 1.00 times).

From FIG. 4, when potassium carbonate was not added to the molten salt, the CS of the tempered glass decreased significantly as the amount of Na in the molten salt increased (Comparative Example 1B), whereas potassium carbonate was used as the molten salt. In advance, the decrease in CS was moderated (Examples 4B, 5B, and 6B). Moreover, FIG. 5 shows that the lifetime extension effect of molten salt increases with the increase in the amount of potassium carbonate added.
From the above, it was found that the addition of potassium carbonate has a significant effect on extending the life of the molten salt.

(Effect of potassium pyrophosphate added to molten salt)
Based on the results of Example 4B, Example 5B, and Example 6B, the amount of Na added to potassium nitrate (KNO ₃ ) (mass ppm) in the case of performing chemical strengthening treatment by adding potassium pyrophosphate to potassium nitrate molten salt FIG. 6 summarizes the relationship with the obtained CS. For comparison, only potassium nitrate, that is, the relationship when no inorganic potassium salt is added is also shown (Comparative Example 1B).

In the same manner as described above, the Na ion content in the molten salt when the CS value was 800 MPa was calculated from the relationships in FIG. 6 by linear approximation, and the life ratio was calculated from the Na ion content.
Further, the total ion exchange capacity corresponding to the added amount of potassium pyrophosphate was calculated by the method described above. Since potassium pyrophosphate has an anion valence of 4, assuming that the number of exchangeable potassium ions is 1, the potential Na—K ion exchange capacity is 4.
Total ion exchange capacity = (potassium pyrophosphate addition amount) x (potential Na-K ion exchange capacity)

The results are shown in Table 7.
Furthermore, FIG. 7 shows the relationship between the addition amount (mol%) of potassium pyrophosphate to potassium nitrate (KNO ₃ ) and the life ratio of the molten salt (ratio when the life of the molten salt containing only potassium nitrate is 1.00 times). .

From FIG. 6, when potassium pyrophosphate was not added to the molten salt, the decrease in CS of the tempered glass was remarkable as the amount of Na in the molten salt increased (Comparative Example 1B). Addition to the molten salt in advance moderated the decrease in CS (Examples 7B, 8B and 9B). In addition, it can be seen from FIG. 7 that the effect of extending the life of the molten salt increases as the amount of potassium pyrophosphate added increases.
From the above, it has been found that the addition of potassium pyrophosphate has a significant effect on extending the life of the molten salt.

Summarizing the above, the total ion exchange capacity of each inorganic potassium salt (anion species) of potassium phosphate or potassium carbonate added to the molten salt (the valence of the added anion species × added amount (mol%)) and the molten salt FIG. 8 shows the relationship with the life ratio (the ratio when the life of the molten salt containing only potassium nitrate is 1.00 times).
From FIG. 8, the lifetime can be extended by adding these inorganic potassium salts to the molten glass reinforcing glass mainly composed of potassium nitrate. Among these, it can be seen that the effect of extending the life due to potassium orthophosphate is remarkable.

[Comparative Example 2: Addition of silica powder]
(Comparative Examples 2A-1 to 2A-4)
(Comparative Example 2A-1)
250 g of potassium nitrate was added to a SUS cup and heated to 430 ° C. with a mantle heater to prepare a molten salt. 0.2 g of silica powder (SiO ₂ content with respect to potassium nitrate: 0.1% by mass) was added to the molten salt thus prepared, and the mixture was stirred for 2 hours using a stirring motor and four propeller blades, and allowed to stand for 2 hours. Thereafter, the soda lime glass was preheated to 100 ° C. and immersed in a molten salt at 430 ° C. for 4 hours for chemical strengthening treatment. Thereafter, the glass was washed with ion exchange water at 100 ° C. and dried at 60 ° C. for 2 hours. CS and DOL of the glass after the chemical strengthening treatment were measured.

(Comparative Examples 2A-2 to 2A-4)
In the same manner as in Examples 7A-2 to 7A-4, sodium nitrate was sequentially added to the molten salt subjected to chemical strengthening treatment, chemical strengthening treatment was performed, and CS and DOL were measured.

(Comparative Examples 2B-1 to 2B-4)
A molten salt was prepared, subjected to chemical strengthening treatment, and CS and DOL were measured in the same manner as in Comparative Examples 2A-1 to 2A-4 except that the glass was changed to aluminosilicate glass.

[Comparative Example 3: Addition of silica powder]
(Comparative Examples 3A-1 to 3A-4)
A chemical strengthening treatment was performed in the same manner as in Comparative Examples 2A-1 to 2A-4 except that the amount of silica powder added was 25.1 g (SiO ₂ content with respect to potassium nitrate: 10% by mass), and CS and DOL were measured respectively. .

(Comparative Examples 3B-1 to 3B-4)
A chemical strengthening treatment was performed in the same manner as in Comparative Examples 2B-1 to 2B-4 except that the amount of silica powder added was 25.1 g (SiO ₂ content with respect to potassium nitrate: 10% by mass), and CS and DOL were measured respectively. .

Table 8 shows the measurement results of Comparative Examples 2A, 2B, 3A, and 3B.

(Effect of silica addition to molten salt)
Based on the results of Comparative Examples 2B and 3B, the relationship between the amount of Na added (mass ppm) to potassium nitrate (KNO ₃ ) and the obtained CS when silica was added to the potassium nitrate molten salt for chemical strengthening treatment. Is summarized in FIG. For comparison, only potassium nitrate, that is, the relationship when no inorganic potassium salt is added is also shown (Comparative Example 1B).

And from each relationship of FIG. 9, the Na ion content (addition amount (mol%) with respect to sodium nitrate) in molten salt when CS value is set to 800 MPa was calculated by linear approximation. Further, from the Na ion content, the life ratio was calculated by setting the case where no inorganic potassium salt other than potassium nitrate was added to the molten salt (Comparative Example 1B-1) as the standard of the molten salt life (1.00 times).

The above results are shown in Table 9.
Furthermore, the relationship between the addition amount of silica (addition amount of silica SiO ₂ (mass%) with respect to potassium nitrate KNO ₃ ) and the life ratio of molten salt (ratio when the life of molten salt containing only potassium nitrate is 1.00 times) As shown in FIG.

From FIG. 9, when inorganic salt is not added to the molten salt, as the amount of Na in the molten salt increases, the CS of the tempered glass decreases significantly (Comparative Example 1B), and silica is added to the molten salt in advance. However, the decrease in CS was significant (Comparative Examples 2B and 3B). Further, FIG. 10 shows that even if the amount of silica added increases, the life extension effect of the molten salt can hardly be confirmed.
From the above, it has been found that the addition of silica has no effect on extending the life of the molten salt.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on a Japanese patent application filed on September 18, 2012 (Japanese Patent Application No. 2012-205039), the contents of which are incorporated herein by reference.

The molten salt according to the present invention can be repeatedly applied to the chemical strengthening treatment of glass by lowering the replacement frequency of the molten salt while keeping the CS value applied to the glass at the same level as before. As a result, it is possible to reduce the cost and improve the throughput in the glass chemical strengthening treatment.

Claims (8)

A molten salt for strengthening glass used to form a compressive stress layer on the glass surface by ion exchange,
A molten salt for glass strengthening containing potassium nitrate and further containing at least one of a carbonate anion and a phosphate anion. The molten salt for glass reinforcement according to claim 1, comprising potassium nitrate, carbonate anion and phosphate anion. The molten salt for glass reinforcement according to claim 1 or 2, wherein the carbonate anion is an anion species of potassium carbonate, and the phosphate anion is at least one anion species of potassium orthophosphate and potassium pyrophosphate. The molten salt for glass reinforcement according to claim 3, wherein the content of the potassium carbonate is 3.5 to 24 mol% with respect to the potassium nitrate. The molten salt for glass reinforcement according to claim 3 or 4, wherein a content of the potassium orthophosphate is 0.8 to 13.5 mol% with respect to the potassium nitrate. The molten salt for glass reinforcement according to any one of claims 3 to 5, wherein the content of the potassium pyrophosphate is 3.5 to 9.0 mol% with respect to the potassium nitrate. A method for producing tempered glass, comprising a step of forming a compressive stress layer on a glass surface using the molten salt for strengthening glass according to any one of claims 1 to 6. A method for extending the lifetime of a molten salt for strengthening glass used for forming a compressive stress layer on a glass surface by ion exchange,
A method for extending the life of a molten salt for strengthening glass, comprising mixing at least one of potassium carbonate and potassium phosphate so that at least one of carbonate anion and phosphate anion is contained in the molten salt before strengthening treatment containing potassium nitrate.

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