JP7610176B2 - Method for producing alkaline earth aluminosilicate glass - Google Patents

Description

The present invention relates to a method for producing alkaline earth aluminosilicate glass, and in particular to a method for producing alkaline earth aluminosilicate glass for use as substrates for liquid crystal displays and organic EL displays.

Non-alkali glass sheets, that is, non-alkali alkaline earth aluminosilicate glass sheets, are used as substrates for liquid crystal displays and the like.

In recent years, attempts have been made to increase the productivity of glass plates by allowing the presence of alkali metal oxides (Li ₂ O, Na ₂ O, K ₂ O) in amounts that do not adversely affect the devices, for use in substrates of liquid crystal displays and organic EL displays.

JP 2006-265001 A JP 2012-148959 A

For example, Patent Document 1 discloses that the glass composition contains 0.03 mass % or more and less than 0.1 mass % of an alkali metal oxide to increase the basicity of the glass, thereby improving the solubility of _SO3 in the molten liquid during melting, thereby enhancing the bubble suppressing effect of _SO3 as a fining agent and suppressing the generation of reboil bubbles after the fining reaction.

Since most of the alkali salts, such as alkali carbonates, melt or react at the beginning of the glass batch vitrification reaction, when melting a glass batch containing an alkali salt, the melt formed at the beginning of the melting process contains a lot of alkali components. Therefore, the melt formed at the beginning of the vitrification reaction of the glass batch containing an alkali salt becomes too basic and takes in an excessive amount of SO _3. After that, poorly soluble components such as SiO ₂ and Al ₂ O _3, which have low basicity, melt or react with the initial melt. At that time, the initial melt rich in alkali metals has a high basicity, and the difference in basicity between the initial melt rich in SiO ₂ and Al ₂ O ₃ and the melt formed thereafter is large, so the sulfur components reboil and cause bubble defects.

Patent Document 2 discloses that the fining effect is enhanced by using alumina containing Na ₂ O, which is an impurity, in the range of 0.1 to 0.6 mass % as a raw material for introducing Al ₂ O ₃ .

However, since Na ₂ O contained in this alumina remains near the alumina even after the glass batch is mixed, it is difficult to contribute to the reaction with the silica raw material. Therefore, even if the alumina described in Patent Document 2 is introduced, the effect of suppressing melt separation due to undissolved silica is insufficient, and further, the suppression of reboil is also insufficient.

The present invention was made in consideration of the above circumstances, and its technical objective is to devise a method for producing alkaline earth aluminosilicate glass that is less likely to cause undissolved silica or molten separation during the melting of the glass batch, and is less likely to cause reboiling.

As a result of intensive research, the present inventors have found that the above technical problems can be solved by using an alkali feldspar raw material as a raw material for introducing an alkali metal oxide of an alkaline earth aluminosilicate glass, and propose this as the present invention. That is, the method for producing an alkaline earth aluminosilicate glass of the present invention is a method for producing an alkaline earth aluminosilicate glass by mixing raw materials to prepare a glass batch, and then melting, fining, and forming the obtained glass batch to produce an alkaline earth aluminosilicate glass, characterized in that the content of the alkali metal oxide in the alkaline earth aluminosilicate glass is 0.01 to 1 mass %, and an alkali feldspar raw material is used as a raw material for introducing an alkali metal oxide. Here, the "alkali feldspar raw material" refers to feldspar or feldspar containing an alkali component such as orthoclase (KAlSi ₃ O ₈ ) or albite (NaAlSi ₃ O ₈ ), a solid solution of feldspar or feldspar containing at least a part of an alkali component, or a mixture thereof. Note that the "alkali feldspar raw material" may contain impurities such as quartz and anorthite in addition to containing an alkali component.

In alkaline earth aluminosilicate glass used for display substrates, alkali salts such as alkali carbonates, alkali nitrates, alkali sulfates, and alkali halides have traditionally been used as the raw material for introducing alkali metal oxides. Since the decomposition and reaction temperatures of many alkali salts are lower than the melting points of the alkali feldspar raw materials, they form a melt earlier than other raw materials during the vitrification reaction of the glass batch and react with the components of the glass batch. The melt containing the alkali components has a high basicity. When an alkali salt is introduced, a melt containing the alkali components is formed at the beginning of the batch reaction. The basicity of the melt is then homogenized by the melting of the low basicity components and the convection and stirring of the melt. When an alkali salt is introduced, the sulfur solubility of the melt formed at the beginning of the melting is excessively high, and as the vitrification reaction progresses, the sulfur solubility decreases, making the melt more likely to reboil. As described above, the alkali components contained as impurities in the alumina raw material are sparingly soluble in alumina, so the alkali components in the alumina rarely come into contact with other raw materials, particularly silica raw materials, and are unlikely to contribute to preventing melting and separation due to undissolved silica.

However, in the present invention, by using an alkali feldspar raw material as the raw material for introducing the alkali metal oxide, it is possible to suppress melt separation during the melting of the glass batch, reduce local differences in the amount of dissolved sulfur in the molten liquid, and suppress reboiling.

The reason for using an alkali feldspar raw material as the raw material for introducing an alkali metal oxide will be described in detail below. The melting temperature of the alkali feldspar raw material is lower than that of SiO ₂ and Al ₂ O ₃ , which are poorly soluble raw materials, and higher than the melting temperature and reaction temperature of the alkali salt. Therefore, by using an alkali feldspar raw material, the formation of an alkali-containing melt can be delayed compared to the case where an alkali salt is used. If the alkali content of the melt formed at the beginning is reduced, the basicity of the melt formed at the beginning is reduced, so that the increase in the amount of sulfur dissolved in the melt formed at the beginning can be suppressed. In addition, since the melt of the alkali feldspar raw material becomes a mixed melt of an alkali metal oxide, SiO ₂ , and Al ₂ O _3, the composition is closer to that of an alkaline earth aluminosilicate glass than that of an alkali salt or an alkali metal oxide formed when an alkali salt is used, and the homogeneity of the melt can be improved. Furthermore, since the amount of silica raw material used, which is a poorly soluble raw material, can be reduced, melt separation due to undissolved silica can be suppressed. From the above, when the alkali metal oxide is introduced from the alkali feldspar raw material, it is possible to form a homogeneous melt while suppressing an increase in the basicity of the melt formed in the initial stage, and therefore it is possible to significantly suppress reboiling.

As described above, the alkali feldspar raw material contains an alkali component that deteriorates the display characteristics. For example, when albite is used as the alkali feldspar raw material, about 12 mass% of Na ₂ O in the albite is introduced into the glass composition. When orthoclase is used as the alkali feldspar raw material, about 17 mass% of K ₂ O in the orthoclase is introduced into the glass composition. However, when a silica raw material or an alumina raw material is used in addition to the alkali feldspar raw material, the content of alkali metal oxide in the alkaline earth aluminosilicate glass can be restricted to 0.01 to 1 mass%. According to the inventor's research, if the content of alkali metal oxide in the alkaline earth aluminosilicate glass is 1 mass% or less, it is acceptable because it does not deteriorate the display characteristics much.

In the method for producing alkaline earth aluminosilicate glass of the present invention, the alkaline earth aluminosilicate glass preferably has a sulfur content of 0.1 to 100 ppm by mass in terms of _SO3 .

In the method for producing alkaline earth aluminosilicate glass of the present invention, the alkaline earth aluminosilicate glass preferably has a B ₂ O ₃ content of 15 mass % or less.

In addition, in the method for producing alkaline earth aluminosilicate glass of the present invention, it is preferable that the total amount of alkaline earth metal oxides (MgO + CaO + SrO + BaO) in the alkaline earth aluminosilicate glass is 1 to 40 mass %.

In addition, the method for producing alkaline earth aluminosilicate glass of the present invention preferably uses the alkaline earth aluminosilicate glass as a display substrate.

The method for producing alkaline earth aluminosilicate glass of the present invention is a method for producing alkaline earth aluminosilicate glass by blending raw materials to prepare a glass batch, and then melting, fining, and forming the obtained glass batch to produce alkaline earth aluminosilicate glass, characterized in that the content of alkali metal oxide in the alkaline earth aluminosilicate glass is 0.01 to 1 mass %, and an alkali feldspar raw material is used as the raw material for introducing the alkali metal oxide. The method for producing alkaline earth aluminosilicate glass of the present invention is described in detail below.

First, the raw materials that are the source of each component are prepared and mixed to produce a glass batch so that the desired glass composition and glass characteristics are achieved. If necessary, glass cullet may be used as a raw material. Glass cullet is glass waste generated during the glass manufacturing process, etc.

In the present invention, raw materials containing an alkali feldspar raw material are mixed, and an alkali feldspar raw material is used as all or part of the alkali-introducing raw material, and preferably the majority of the alkali content in the glass composition is introduced by the alkali feldspar raw material, and more preferably 80 mass % or more of the alkali content in the glass composition is introduced by the alkali feldspar raw material. A melt of an alkali aluminosilicate is formed by dissolving the alkali feldspar raw material. The melt of an alkali aluminosilicate contains poorly soluble SiO ₂ and Al ₂ O ₃ in a molten state, so that the homogeneity of the melt can be improved. On the other hand, when an alkali salt is used as the alkali-introducing raw material, an initial melt of high basicity containing an alkali is formed. After that, the difference in basicity with a low basicity component such as silica (SiO ₂ ) dissolved in the melt becomes large, and reboiling is easily caused.

When comparing the use of alkali feldspar raw material as the alkali introduction raw material with the use of alkali salts, the initial melt contains fewer alkaline components and has a lower basicity when the alkali feldspar raw material is used. The alkali feldspar raw material that melts afterwards has a higher basicity than silica. Therefore, the difference in basicity between the initial melt and the alkali feldspar raw material components becomes smaller, making it less likely that reboiling will occur.

Of the raw materials, the silica feedstock is poorly soluble, while the alkaline earth oxide feedstock, boron oxide feedstock, and alkali metal oxide feedstock are easily soluble. Alkaline earth oxides have a low melting point and high density, so they melt and liquidize in the early stages of the melting process and tend to sink. On the other hand, silica has a high melting point and low density, so it tends to float relatively easily. Therefore, if an alkali feldspar feedstock is used as the alkali metal oxide feedstock, the amount of undissolved silica can be reduced, and the degree of melt separation can be reduced.

In the method for producing alkaline earth aluminosilicate glass of the present invention, it is _preferable to prepare a glass _batch by blending and mixing raw materials so that the glass composition contains, in mass % calculated as the oxides below, 50-70% _SiO2 , 10-25% Al2O3, 0-15% B2O3, _0.01-1 % _Li2O + _Na2O + _K2O , 0-8% _MgO , 3-10% CaO, and 1-40% MgO+CaO+SrO+BaO. The reasons for limiting the glass composition as above are as follows. In the explanation of the content range of each component, % indicates mass %.

SiO ₂ is a component that forms the skeleton of glass. The content of SiO ₂ is preferably 50 to 70%, 54 to 68%, 56 to 66%, and particularly 58 to 64%. If the content of SiO ₂ is too low, the density becomes too high and the acid resistance is easily reduced. On the other hand, if the content of SiO ₂ is too high, the high-temperature viscosity becomes high, the melting property is easily reduced, and devitrified crystals such as cristobalite are easily precipitated, and the liquidus temperature is easily increased.

Al ₂ O ₃ is a component that forms the skeleton of glass, increases the strain point and Young's modulus, and suppresses phase separation. The content of Al ₂ O ₃ is preferably 10 to 25%, particularly 15 to 22%. If the content of _{Al 2} O ₃ is too small, the strain point and Young's modulus tend to decrease, and the glass tends to undergo phase separation. On the other hand, if the content of Al ₂ O ₃ is too large, devitrified crystals such as mullite and anorthite tend to precipitate, and the liquidus temperature tends to increase.

B ₂ O ₃ is a component that enhances melting property and devitrification resistance. The content of B ₂ O ₃ is preferably 0 to 15%, 0.1 to 6%, 0.3 to 3%, and particularly 0.5 to 2.3%. If the content of _{B 2} O ₃ is too small, the melting property and devitrification resistance are likely to decrease, and the resistance to hydrofluoric acid-based chemicals is likely to decrease. On the other hand, if the content of B ₂ O ₃ is too large, the Young's modulus and strain point are likely to decrease.

Alkali metal oxides (Li ₂ O, Na ₂ O, K ₂ O) are essential components for lowering the melting temperature and molding temperature, and for reducing melt separation due to undissolved silica. However, if the content of alkali metal oxides is too high, it will have a negative effect on the performance of the display. Therefore, the content of Li ₂ O + Na ₂ O + K ₂ O (total amount of Li ₂ O, Na ₂ O and K ₂ O) is preferably 0.01 to 1%, 0.02 to 0.75%, 0.03 to 0.5%, and particularly 0.04 to 0.5%. In the present invention, the alkali metal oxide is mainly introduced from the alkali feldspar raw material, but the alkali metal oxide may be introduced from a raw material other than the alkali feldspar raw material. For example, the alkali metal oxide may be introduced from a raw material such as a lithium salt, a sodium salt, or a potassium salt, or the alkali metal oxide may be introduced as an impurity of another raw material.

MgO is a component that reduces high-temperature viscosity and increases melting properties, and among alkaline earth metal oxides, it is a component that significantly increases Young's modulus. The MgO content is preferably 0-8%, 1-7%, 2-6%, and particularly 2.5-6%. If the MgO content is too low, the melting properties and Young's modulus are likely to decrease. On the other hand, if the MgO content is too high, the devitrification resistance and strain point are likely to decrease.

CaO is a component that reduces high-temperature viscosity and significantly improves melting properties without lowering the strain point. In addition, among alkaline earth metal oxides, it is a component that reduces raw material costs because the raw materials used are relatively inexpensive. The CaO content is preferably 3 to 10%, 4 to 10%, and particularly 5 to 9%. If the CaO content is too low, it becomes difficult to achieve the above effects. On the other hand, if the CaO content is too high, the glass is prone to devitrification and the thermal expansion coefficient is likely to increase.

SrO and BaO are components that increase devitrification resistance, but also promote melt separation. They also reduce high-temperature viscosity and increase meltability without lowering the strain point, and suppress the rise in liquidus temperature. The SrO content is preferably 0-8%, 0.1-7%, and particularly 0.5-6%. The BaO content is preferably 0-20%, 0.1-18%, 1-17%, 3-16%, and particularly 5-15%. If the SrO content is too high, strontium silicate devitrification crystals are more likely to precipitate, and devitrification resistance is more likely to decrease. If the BaO content is too high, the density becomes too high and the meltability is more likely to decrease. In addition, devitrification crystals containing BaO are more likely to precipitate, and the liquidus temperature is more likely to increase.

Alkaline earth oxides are components that reduce high-temperature viscosity. The total amount of alkaline earth oxides (MgO+CaO+SrO+BaO) is preferably 1-40%, 5-30%, and particularly 10-25%. If the content of MgO+CaO+SrO+BaO is too high, the glass is prone to devitrification. On the other hand, if the content of MgO+CaO+SrO+BaO is too low, the amount of energy consumed to melt the glass batch increases, and the manufacturing costs of the glass become too high.

_SnO2 is a component that acts as a clarifier, and its content is preferably 0 to 1%, 0.1 to 0.5%, particularly 0.2 to 0.4%. If the _SnO2 content is too high, devitrified crystals are likely to precipitate and the liquidus temperature is likely to rise.

_ZrO2 is a component that easily causes devitrification. Therefore, the content of _ZrO2 is preferably less than 0.5%, more preferably less than 0.2%, and it is particularly preferable that _ZrO2 is not contained except for an unavoidable amount of impurity.

SO ₃ is a component that causes foam defects due to reboiling of SO ₂ gas. If the content of SO ₃ is too high, foam defects due to reboiling of SO ₂ gas are likely to occur. The content of SO ₃ can be reduced by using a raw material with a low sulfur component, but this causes an increase in production costs. Therefore, the content of SO ₃ is preferably 0.1 to 100 mass ppm, 0.5 to 50 mass ppm, and particularly 1 to 10 mass ppm.

In addition to the above components, other components may be added, such as ZnO, P ₂ O ₅ , F, Cl, and Mo. From the viewpoint of accurately enjoying the effects of the present invention, the content of the other components other than the above components is preferably 10% or less in total, and particularly preferably 5% or less.

In the alkaline earth aluminosilicate glass according to the present invention, the strain point is preferably 650°C or higher, 700°C or higher, and particularly preferably 730 to 850°C. If the strain point is too low, the glass plate is likely to undergo thermal shrinkage during heat treatment in the display manufacturing process. On the other hand, if the strain point is too high, the manufacturing costs of the glass plate are likely to rise. The "strain point" refers to a value measured based on the method of ASTM C336.

In the alkaline earth aluminosilicate glass according to the present invention, the temperature at 10 ^2.5 dPa·s is preferably 1530 to 1680°C, more preferably 1550 to 1650°C, and particularly preferably 1580 to 1630°C. If the temperature at 10 ^2.5 dPa·s is too low, it becomes difficult to control the deformation of the glass sheet during the forming of the glass sheet. On the other hand, if the temperature at 10 ^2.5 dPa·s is too high, the melting property decreases, and the manufacturing cost of the glass sheet tends to increase. The "temperature at 10 ^2.5 dPa·s" can be measured by the well-known platinum ball pulling method.

Next, after the blending process, the resulting glass batch is charged into a melting furnace. The glass batch is usually charged into the melting furnace continuously using a raw material feeder such as a screw charger, but may be charged intermittently.

The glass batch that is placed in the melting furnace is heated by the combustion atmosphere of a burner or electrodes installed inside the melting furnace, and becomes molten liquid.

The resulting melt then undergoes a clarification process, a stirring process, and a feeding process, after which it is gradually cooled in order to be fed into the molding device.

The molten liquid is then fed to a forming device and formed into a plate with a specified thickness and surface quality, and then cut to a specified size to become a glass product (glass plate). Forming methods that can be used include the overflow downdraw method and the float method. In particular, the overflow downdraw method is preferred because it can produce unpolished glass plates with a smooth surface.

The glass plate produced in this manner is suitable for use as a substrate for liquid crystal displays, organic electroluminescence displays, etc.

The present invention will be described below based on examples. Note that the following examples are merely illustrative. The present invention is not limited to the following examples in any way.

Table 1 shows samples No. 1 and 2.

Each sample was prepared as follows. The raw materials were mixed to obtain the glass composition shown in the table, and a glass batch equivalent to 100 g of glass was prepared. The raw materials shown in the table were used as the raw materials for introducing the alkali metal oxide. "(Alkali metal derived from alkali feldspar raw material)/(Alkali metal in glass)" is the ratio of the mass % of the alkali metal oxide derived from the alkali feldspar raw material to the mass % of the alkali metal oxide in the glass. The raw materials other than the raw materials for introducing the alkaline earth metal oxide were exactly the same. The obtained glass batch was placed in a platinum alloy crucible having an approximately truncated cone shape, held at 1200°C for 1 hour, heated to 1600°C, melted for 2 hours, and then quenched. The glass was then peeled off from the crucible to obtain a glass sample having an approximately truncated cone shape with a diameter of 55 mm on the top surface, a diameter of 25 mm on the bottom surface, and a height of 25 mm, and the composition of the cross section 5 mm and 21 mm from the top surface was analyzed by XRF. The "silica top/bottom difference,""alkaline earth oxide top/bottom difference," and "SO3 _top /bottom difference" are values obtained by subtracting the analysis value at 5 mm from the top from the analysis value at 21 mm from the top. The bubble density was determined by immersing the glass between 5 mm and 21 mm from the top in a refractive index matching liquid and counting the number of bubbles.

As can be seen from the table, when the alkali metal oxide was introduced from the alkali feldspar raw material, the absolute values of the silica top/bottom difference, the alkaline earth oxide top/bottom difference, and the _SO3 top/bottom difference were all smaller, and the bubble number density was smaller, than when the alkali carbonate was used.

Claims (5)

A method for producing alkaline earth aluminosilicate glass, which comprises blending raw materials to prepare a glass batch, and then melting, fining, and forming the resulting glass batch to produce alkaline earth aluminosilicate glass, in which the content of alkali metal oxide in the alkaline earth aluminosilicate glass is 0.01 to 1 mass %, and an alkali feldspar raw material is used as the raw material for introducing the alkali metal oxide. 2. The method for producing alkaline earth aluminosilicate glass according to claim 1, wherein the alkaline earth aluminosilicate glass has a BaO content of 3 to 16 mass %. 3. The method for producing alkaline earth aluminosilicate glass according to claim 2 , wherein the alkaline earth aluminosilicate glass has a sulfur content of 0.1 to 100 ppm by mass as _SO3 . The method for producing alkaline earth aluminosilicate glass according to claim 2 or 3 , wherein the alkaline earth aluminosilicate glass has a B ₂ O ₃ content of 15 mass % or less. 5. The method for producing alkaline earth aluminosilicate glass according to claim 2 , wherein the total amount of alkaline earth metal oxides (MgO+CaO+SrO+BaO) in the alkaline earth aluminosilicate glass is 5 to 40 mass %.