WO2011078258A1 - Method for vacuum-degassing molten glass and process for producing glass product - Google Patents

Abstract

Disclosed is a method in which optimal vacuum-degassing conditions are used when a molten glass is vacuum-degassed using a chloride-based clarificant. The method for vacuum-degassing a molten glass comprises passing the molten glass through a vacuum-degassing tank, the inside of which is kept in a vacuum state, to thereby vacuum-degas the molten glass. The method is characterized in that the molten glass is an alkali-free glass, and that the internal pressure of the vacuum-degassing tank during the vacuum degassing is kept not higher than the bubble growth initiation pressure for the molten glass, P_bg (mmHg), but not lower than the reboiling pressure for the molten glass, P_rb (mmHg).

Description

Vacuum degassing method for molten glass and glass product manufacturing method

The present invention relates to a vacuum degassing method for molten glass and a glass product manufacturing method using the vacuum degassing method.

2. Description of the Related Art Conventionally, in order to improve the quality of a molded glass product, a clarification process for removing bubbles generated in the molten glass before the molten glass in which the raw material is melted in a melting furnace is molded by a molding apparatus has been used.
In this fining step, a fining agent is added to the raw material in advance, and the molten glass obtained by melting the raw material is stored and maintained at a predetermined temperature for a certain period of time. A method for removing the surface by floating is known. In addition, the molten glass is introduced into the reduced-pressure atmosphere, and bubbles in the molten glass flow that continuously flows under this reduced-pressure atmosphere are greatly grown to lift and remove bubbles contained in the molten glass, A vacuum degassing method for discharging from a vacuum atmosphere is known.
In order to efficiently remove bubbles from the molten glass, it is preferable to carry out the above two methods in combination, that is, to carry out the vacuum degassing method using the molten glass to which a clarifier is added.

As glass refining agents, oxide refining agents such as As ₂ O ₃ , Sb ₂ O ₃ and SnO ₂ , sulfate refining agents such as CaSO ₄ and BaSO ₄ , and alkali metal chlorides such as NaCl There are clarifiers. Among these, sulfate-type fining agents, in the case of alkali-free glass with low basicity, have a low solubility of SO ₄ ²⁻ , and thus the effect of removing bubbles from the molten glass is insufficient.
In addition, As ₂ O ₃ and Sb ₂ O ₃ , particularly As ₂ O ₃ , have a large environmental load, and thus their use is required to be suppressed.
In addition, SnO ₂ has a high oxygen releasing temperature of 1500 ° C. or higher, and it may be difficult to effectively use it as a fining agent.
Alkali metal chloride is a clarifier that cannot be used because an alkali metal is contained in the alkali-free glass when a sufficient amount is added for clarification.

As a result of investigating the possibility as a clarifier of alkali-free glass, the present inventors have found that a compound containing chlorine exhibits an excellent effect as a clarifier combined with vacuum degassing. Examples of such chloride fining agents include BaCl ₂ , SrCl ₂ , CaCl ₂ , MgCl ₂ , AlCl ₃ and NH ₄ Cl.

The conditions such as pressure and temperature in the vacuum degassing tank when performing vacuum degassing are shown in Patent Documents 1 and 2 and the like.

International Publication WO2008 / 029649 International Publication WO2008 / 093580

When the present inventors use a chloride-based clarifier such as NH ₄ Cl or SrCl ₂ as a clarifier for an alkali-free glass, the ratio of chloride contained in the glass composition affects the preferred vacuum degassing conditions. I found out.
The present invention is based on the above findings, and an object of the present invention is to provide a non-alkali glass defoaming method that provides optimum defoaming conditions when defoaming using a chloride clarifier. And

In order to achieve the above object, the present invention is a method for degassing molten glass by flowing molten glass into a vacuum degassing tank whose inside is maintained in a reduced pressure state,
The molten glass is alkali-free glass,
The pressure in the vacuum degassing tank at the time of carrying out the vacuum defoaming is equal to or lower than the bubble growth starting pressure P _bg (mmHg) of the molten glass represented by the following formula (1) or formula (2), and the following formula (3) providing vacuum degassing method for molten glass, characterized in that to hold in the above reboil pressure P _rb of molten glass (mmHg) represented.
P _bg = (2.6082 × T ₂ −3538.2) × [β-OH] + (− 1.2102 × T ₂ +2612.2) × [Cl] −80.3 (1)
P _bg = (− 0.2462 × T ₂ +111.7) × [β-OH] + (1.9714 × T ₂ −1730.6) × [Cl] −187.3 (2)
・ P _rb = 0.8325 × P _bg -59.5 (3)
(In the above formula, T ₂ represents the temperature (° C.) at which the viscosity of the molten glass is 10 ² dPa · s, [β-OH] represents the β-OH value (mm ⁻¹ ) of the alkali-free glass, [ Cl] represents the chlorine content (mass%) in the alkali-free glass.When [Cl] is 0.12 mass% or more, P _bg is represented by the formula (1), and [Cl] is 0.00. In the case of less than 12% by mass, P _bg is represented by the formula (2).)
The [β-OH] is preferably 0.15 to 0.6 mm ⁻¹ , and the [Cl] is preferably 0.03 to 0.3% by mass.
The T ₂ is preferably 1500 to 1750 ° C.
The present invention also includes a glass melting step for producing a molten glass by melting a glass raw material, a vacuum defoaming step by the above-described vacuum degassing method for molten glass, and a glass product for molding the vacuum degassed molten glass. The manufacturing method of the glass product which has a formation process and has these processes in this order is provided.
The alkali-free glass in the present invention is a glass that does not contain an alkali metal, that is, a glass that does not substantially contain an alkali metal, except that it is inevitably mixed as an impurity.

According to the vacuum degassing method of the present invention, vacuum degassing can be carried out under optimum conditions when performing vacuum degassing of alkali-free glass using a chloride clarifier. As a result, bubbles and foreign substances in the molten glass after the vacuum degassing treatment are reduced, and a high-performance and high-quality glass with few defects can be produced.
Since the reduced pressure defoaming method of the present invention uses a chloride clarifier, the human body and the global environment are not adversely affected. In addition, glass products manufactured using the vacuum degassing method of the present invention do not require any special precautions regarding the suppression of bubbles in handling at manufacturing plants and processing plants, resulting in problems in recycling of glass products. Absent.

FIG. 1 is a cross-sectional view showing a configuration example of a vacuum degassing apparatus used in the vacuum degassing method of the present invention. FIG. 2 is a graph plotting the relationship between T ₂ and the coefficient a in equation (4). FIG. 3 is a graph plotting the relationship between T ₂ and the coefficient b in Equation (4). FIG. 4 is a graph plotting the relationship between T ₂ and the coefficient c in equation (5). FIG. 5 is a graph plotting the relationship between T ₂ and the coefficient d in equation (5). FIG. 6 is a graph plotting the relationship between the reboil pressure P _rb and the bubble growth start pressure P _bg . FIG. 7 shows the experimental value (mmHg) of the bubble growth start pressure P _bg and the bubble growth start when the chlorine content (% by mass) is 0.12% by mass or more for the alkali-free glass compositions A, B, and C. The graph which plotted the relationship with the calculated value (mmHg) of the pressure _Pbg . FIG. 8 shows the experimental value (mmHg) of the bubble growth start pressure P _bg and the bubble growth start when the chlorine content (% by mass) is less than 0.12 mass% for the alkali-free glass compositions A, B, and C. The graph which plotted the relationship with the calculated value (mmHg) of the pressure _Pbg .

Hereinafter, the vacuum degassing method of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing a configuration example of a vacuum degassing apparatus used in the vacuum degassing method of the present invention. In the vacuum degassing apparatus 1 shown in FIG. 1, a cylindrical vacuum degassing tank 12 is housed and disposed in the vacuum housing 11 so that its long axis is oriented in the horizontal direction. A rising pipe 13 oriented in the vertical direction is attached to the lower surface of one end of the vacuum degassing tank 12, and a lowering pipe 14 is attached to the lower surface of the other end. A part of the ascending pipe 13 and the descending pipe 14 is located in the decompression housing 11.
The ascending pipe 13 communicates with the vacuum degassing tank 12 and introduces the molten glass G from the melting tank 20 into the vacuum degassing tank 12. The downcomer 14 communicates with the vacuum degassing tank 12 and guides the molten glass G after the vacuum degassing to the next processing tank (not shown). In the decompression housing 11, a heat insulating material 15 such as a heat insulating brick is provided around the decompression defoaming tank 12, the ascending pipe 13 and the descending pipe 14 to insulate these.

In the vacuum degassing apparatus 1 shown in FIG. 1, the vacuum degassing tank 12, the rising pipe 13, and the descending pipe 14 are made of a material excellent in heat resistance and corrosion resistance against molten glass because they are conduits for molten glass. ing. For example, it is made of platinum, platinum alloy, or reinforced platinum obtained by dispersing a metal oxide in platinum or platinum alloy. Further, it may be made of a ceramic nonmetallic inorganic material, that is, a dense refractory. Further, a dense refractory material with platinum or a platinum alloy lined may be used.

In the vacuum degassing method of the present invention, the molten glass G supplied from the melting tank 20 is passed through the vacuum degassing tank 12 depressurized to a predetermined degree of vacuum to perform vacuum degassing. It is preferable that the molten glass G is continuously supplied to and discharged from the vacuum degassing tank 12. The flow rate of the molten glass is preferably 1 to 200 tons / day from the viewpoint of productivity.
In order to prevent a temperature difference from the molten glass G supplied from the melting tank 20, the vacuum degassing tank 12 has an internal temperature range of 1200 ° C. to 1600 ° C., particularly 1350 ° C. to 1550 ° C. It is preferable to be heated.

The molten glass G used in the vacuum degassing method of the present invention is an alkali-free glass, and the following chloride-based clarifier is added to a glass raw material for producing the alkali-free glass. Specific examples of the chloride fining agent include at least one selected from the group consisting of BaCl ₂ , SrCl ₂ , CaCl ₂ , MgCl ₂ , AlCl ₃ and NH ₄ Cl. Among these, alkaline earth chlorides such as BaCl ₂ , SrCl ₂ , CaCl ₂ and MgCl ₂ , AlCl ₃ and NH ₄ Cl are usually present as hydrated salts. Therefore, the chloride fining agent added in the production of the alkali-free glass of the present invention includes, among these, BaCl ₂ .2H ₂ O, SrCl ₂ .6H ₂ O, and NH ₄ Cl is preferred.
The content of chlorine in the alkali-free glass (hereinafter sometimes referred to as [Cl] in the present specification) is preferably 0.03 to 0.3% by mass. [Cl] is more preferably 0.05 to 0.25% by mass. When [Cl] is less than 0.03% by mass, the clarification effect of the alkali-free glass may be insufficient.
In addition, although the use of an alkali metal chloride is also considered as a chloride-based fining agent, adding an amount sufficient for clarification of molten glass will cause the alkali metal to be contained in the alkali-free glass. A fining agent consisting of chlorides is not suitable.

When decompression defoaming is performed, the air in the decompression housing 11 is exhausted from the outside by a vacuum decompression means (not shown) such as a vacuum pump through a suction opening 16 provided at a predetermined location of the decompression housing 11. . Thereby, the air in the decompression defoaming tank 12 accommodated in the decompression housing 11 is indirectly exhausted, and the inside of the decompression defoaming tank 12 is decompressed to a predetermined pressure. In the vacuum degassing method of the present invention, the pressure in the vacuum degassing tank 12 is maintained below the bubble growth starting pressure P _bg (mmHg) represented by the following formula (1) or (2).
P _bg = (2.6082 × T ₂ −3538.2) × [β-OH] + (− 1.2102 × T ₂ +2612.2) × [Cl] −80.3 (1)
P _bg = (− 0.2462 × T ₂ +111.7) × [β−OH] + (1.9714 × T ₂ −1730.6) × [Cl] −187.3 (2)
Here, [Cl] in the formulas (1) and (2) represents the chlorine content (mass%) in the alkali-free glass. When [Cl] is 0.12% by mass or more, the bubble growth starting pressure P _bg is represented by the above formula (1). On the other hand, when [Cl] is less than 0.12% by mass, P _bg is represented by the formula (2).

In formulas (1) and (2), T ₂ represents a temperature (° C.) at which the viscosity of the molten glass is 10 ² dPa · s, and can be measured using a high-temperature rotational viscometer.
The viscosity of 10 ² dPa · s is a reference viscosity indicating that the viscosity of the molten glass is sufficiently low. Therefore, the temperature T _{2 at} which the viscosity of the molten glass is 10 ² dPa · s is the reference temperature of the molten glass, and is 1500 to 1750 ° C. in the case of the alkali-free glass for a TFT liquid crystal display substrate.
It is preferable that T ₂ is 1500 to 1720 ° C. because the rising speed of bubbles in the molten glass is increased and the defoaming performance at the time of degassing under reduced pressure is excellent. T ₂ is more preferably 1560 to 1700 ° C., and further preferably 1590 to 1680 ° C.

In the formulas (1) and (2), [β-OH] represents the β-OH value (mm ⁻¹ ) of the alkali-free glass. The β-OH value is used as an indicator of the amount of water in the glass. The β-OH value was measured by using a Fourier transform infrared spectrophotometer (FT-IR) to measure the transmittance of a non-alkali glass test piece obtained by molding molten glass after degassing under reduced pressure into a plate shape. Can be obtained.
Β-OH value = (1 / X) log ₁₀ (T ₁ / T ₂ )
-X: Glass wall thickness (mm)
T ₁ : Transmittance (%) at a reference wave number of 4000 cm ⁻¹
T ₂ : Minimum transmittance (%) in the vicinity of hydroxyl absorption wave number 3570 cm ⁻¹
The β-OH value of the alkali-free glass is preferably 0.15 to 0.6 mm ⁻¹ . The β-OH value of the alkali-free glass is governed by the water content in the raw material, the water vapor concentration in the melting tank, the combustion method (oxygen combustion, air combustion), and the like. A method for adjusting β-OH by the water vapor concentration in the dissolution tank will be described later. The β-OH value is particularly preferably 0.2 to 0.55 mm ⁻¹ . As the β-OH value, the β-OH value after vitrification is generally used.

In the present specification, the bubble growth starting pressure P _bg is defined as follows.
When the vacuum degassing tank 12 is depressurized under a constant temperature condition, the volume of bubbles (bubble diameter) in the molten glass in the vacuum degassing tank 12 increases according to Boyle's law. However, when the pressure in the vacuum degassing tank 12 is reduced to a certain pressure, the volume of bubbles in the molten glass (the diameter of the bubbles) suddenly increases outside Boyle's law. This pressure is referred to as a bubble growth starting pressure P _bg . When the pressure in the vacuum degassing tank 12 is kept below the bubble growth starting pressure P _bg (mmHg), bubbles contained in the molten glass can be sufficiently grown in the vacuum degassing tank 12. As a result, bubbles in the molten glass can be efficiently removed.

In the present invention, the bubble growth starting pressure P _bg can be determined by the following procedure.
In order to reproduce the situation in the vacuum degassing vessel 12, a quartz glass crucible containing a non-alkali glass cullet is placed in a vacuum vacuum vessel. The crucible is heated to a predetermined temperature (for example, 1300 ° C. or 1400 ° C.) to melt the alkali-free glass. After the alkali-free glass is completely melted, the diameter of the bubbles in the molten glass is observed while reducing the pressure in the vacuum vacuum container. In order to observe the diameter of the bubbles in the molten glass, for example, the bubbles in the molten glass may be photographed using a CCD camera from a viewing window provided in the vacuum decompression container. It should be noted that the number of bubble samples for measuring the bubble diameter is 20 or more.
As the pressure in the vacuum decompression vessel is lowered, the diameter of bubbles in the molten glass increases according to Boyle's law. However, when the inside of the vacuum decompression container is depressurized to a certain pressure, the diameter of bubbles in the molten glass deviates from Boyle's law and increases rapidly. The degree of decompression in the vacuum decompression vessel at this time is defined as a bubble growth starting pressure P _bg .

As a result of intensive investigations on the relationship between the bubble growth starting pressure P _bg and various parameters related to the vacuum degassing of the alkali-free glass, the inventors of the present application have found that the temperature at which the viscosity of the molten glass becomes 10 ² dPa · s. (T ₂ ), the β-OH value of the alkali-free glass ([β-OH]), and the chlorine content ([Cl]) in the alkali-free glass affect the bubble growth starting pressure P _bg I found. Based on this finding, the relationship between the bubble growth starting pressure P _bg and T ₂ , [β-OH], and [Cl] was experimentally derived. The above formulas (1) and (2) It is. The procedure for deriving equations (1) and (2) will be described more specifically.

For alkali-free glasses A to C having different T _2, the β-OH value [β-OH] of the alkali-free glass or the chlorine content [Cl] in the alkali-free glass is different, and other composition values are the same alkali-free. Glass was prepared, and the bubble growth starting pressure P _bg was determined by the above procedure. Here, NH ₄ Cl was used as the chloride clarifier. As will be described in detail later, when the alkali-free glass is melted in the melting tank 20, the β-OH value of the alkali-free glass can be adjusted by adjusting the ratio of oxygen and air mixed with the fuel. The compositions of alkali-free glasses A to C and T ₂ are as follows. The composition of the following alkali-free glass is expressed in mass% in terms of the following oxide.
(Non-alkali glass A)
SiO ₂ : 59.5% by mass,
Al ₂ O ₃ : 17.7% by mass,
B ₂ O ₃ : 7.9% by mass,
MgO: 3.2% by mass,
CaO: 3.7% by mass,
SrO: 7.9% by mass,
BaO: 0.1% by mass.
(T ₂ : 1660 ° C.)
(Non-alkali glass B)
SiO ₂ : 59.4% by mass,
Al ₂ O ₃ : 16.9% by mass,
B ₂ O ₃ : 8.6% by mass,
MgO: 4.0% by mass,
CaO: 5.4% by mass,
SrO: 5.7% by mass,
BaO: 0.0 mass%.
(T ₂ : 1617 ° C)
(Non-alkali glass C)
SiO ₂ : 59.5% by mass,
Al ₂ O ₃ : 17.0% by mass,
B ₂ O ₃ : 8.0% by mass,
MgO: 4.7% by mass,
CaO: 6.0% by mass,
SrO: 4.8% by mass,
BaO: 0.0 mass%.
(T ₂ : 1597 ° C)

In each of the alkali-free glasses A to C, when the chlorine content [Cl] in the alkali-free glass is 0.12% by mass or more, the bubble growth starting pressure P _bg , the β-OH value of the alkali-free glass ( It has been found that the relationship represented by the following formula (4) holds for the [β-OH]) and the chlorine content ([Cl]) in the alkali-free glass.
P _bg = a × [β-OH] + b × [Cl] −80.3 (4)
Equation (4) for each alkali-free glass A to C is as follows.
(Non-alkali glass A)
_{· P bg = 800.6 × [β} -OH] + 660.1 × [Cl] -80.3 ......... (4-A)
(Non-alkali glass B)
・ P _bg = 650.0 × [β-OH] + 664.9 × [Cl] −80.3 (4-B)
(Non-alkali glass C)
_{· P bg = 646.9 × [β} -OH] + 672.8 × [Cl] -80.3 ......... (4-C)

FIG. 2 is a graph plotting the relationship between T ₂ and the coefficient a of Equation (4) based on the results obtained above. FIG. 3 is a graph plotting the relationship between T ₂ and the coefficient b of Equation (4) based on the results obtained above.
The equation (1) is obtained from the regression line shown in FIGS.

On the other hand, when the chlorine content in the alkali-free glass is less than 0.12% by mass, the relationship represented by the following formula (5) is established for P _bg , [β-OH], and [Cl]. .
P _bg = c × [β-OH] + d × [Cl] -187.3 (5)
Formula (5) for each alkali-free glass A to C is as follows.
(Non-alkali glass A)
P _bg = 713.6 × [β-OH] + 1530.0 × [Cl] -187.3 (5-A)
(Non-alkali glass B)
P _bg = 721.7 × [β-OH] + 1494.6 × [Cl] -187.3 (5-B)
(Non-alkali glass C)
P _bg = 729.8 × [β-OH] + 1392.1 × [Cl] -187.3 (5-C)

FIG. 4 is a graph plotting the relationship between T ₂ and the coefficient c of Equation (5) based on the results obtained above. FIG. 5 is a graph plotting the relationship between T ₂ and the coefficient d of Equation (5) based on the results obtained above.
The equation (2) is obtained from the regression line shown in FIGS.

As described above, in the vacuum degassing method of the present invention, the pressure in the vacuum degassing tank 12 is maintained below the bubble growth starting pressure P _bg (mmHg) represented by the above formula (1). When the pressure in the tank 12 is extremely low, reboiling may occur in the molten glass flowing in the vacuum degassing tank 12.
For this reason, in the vacuum degassing method of the present invention, the pressure in the vacuum degassing tank 12 is maintained at or above the reboiling pressure P _rb (mmHg) of the molten glass represented by the following formula (3).
・ P _rb = 0.8325 × P _bg -59.5 (3)

In this specification, reboiled pressure P _rb are defined as follows.
In order to sufficiently grow the bubbles contained in the molten glass, it is preferable to reduce the pressure in the vacuum degassing tank 12 as much as possible. However, when the pressure in the vacuum degassing tank 12 is extremely low, bubbles may be generated at the glass interface in contact with the molten defoaming tank 12 made of platinum, a platinum alloy, or a dense refractory. This phenomenon is called the reboiling (reboil), the pressure in the vacuum degassing vessel 12 at this time that reboil pressure P _rb.
Incidentally, reboil pressure P _rb can be obtained by the following procedure.
In order to reproduce the situation in the vacuum degassing vessel 12, a quartz glass crucible containing a non-alkali glass cullet is placed in a vacuum vacuum vessel. The crucible is heated to a predetermined temperature (for example, 1300 ° C. or 1400 ° C.) to melt the alkali-free glass. A test piece made of a material constituting the vacuum defoaming tank, more precisely, a material constituting the glass contact surface of the vacuum defoaming tank (platinum, platinum alloy, or dense refractory) is put in the molten glass. Immerse. In this state, the inside of the vacuum decompression vessel is gradually decompressed, and the generation of bubbles at the glass interface of the test piece is observed. The decompression degree of the vacuum decompression container when bubbles occur in the glass interface and reboiling pressure P _rb.

The present inventors have found that a reboiled pressure P _rb, various parameters and a result of extensive studies on the relationship associated with the vacuum degassing of the alkali-free glass, and reboiled pressure P _rb, bubble growth starting and pressure P _bg, of I found out that there was a specific relationship between them.
The above formula (3) can be specified by plotting the relationship between the reboil pressure P _rb and the bubble growth starting pressure P _bg . FIG. 6 is a graph plotting the relationship between the reboil pressure P _rb and the bubble growth start pressure P _bg .
In FIG. 6, a platinum-rhodium alloy (platinum 90% by mass, rhodium 10% by mass) was used as a test piece immersed in molten glass. Further, alkali-free glasses A to C were used as the molten glass. The temperature of the molten glass was 1400 ° C.

In the vacuum degassing method of the present invention, the pressure in the vacuum degassing tank 12 is maintained at or above the reboiling pressure P _rb (mmHg). When holding the pressure in the vacuum degassing vessel 12 to reboil pressure P _rb (mmHg) or higher, reboiled in the molten glass flowing in the vacuum degassing vessel 12 is prevented from being generated. As a result, the number of bubbles remaining in the molten glass after the vacuum defoaming treatment is reduced, and a high-functional and high-quality glass with few bubbles can be produced.

That is, in the vacuum degassing method of the present invention, the pressure in the vacuum degassing tank 12 is equal to or lower than the bubble growth starting pressure P _bg (mmHg) represented by the above formulas (1) and (2), and the above formula (3). By maintaining the reboil pressure P _rb (mmHg) or higher represented by the formula, bubbles contained in the molten glass can be sufficiently grown in the vacuum degassing vessel 12, and the bubbles in the molten glass are efficiently removed. On the other hand, reboiling is prevented from occurring in the molten glass flowing in the vacuum degassing vessel 12. As a result, bubbles remaining in the molten glass after the vacuum defoaming treatment are extremely reduced, and a high-performance and high-quality glass with extremely few bubbles can be produced.

As a 1st example of the alkali free glass applied in the vacuum degassing method of this invention, the composition of the alkali free glass containing the following components by the mass% display of the following oxide conversion is mentioned preferably.
SiO ₂ : 50 to 66% by mass,
Al ₂ O ₃ : 10.5 to 24% by mass,
B ₂ O ₃ : 0 to 12% by mass,
MgO: 0 to 8% by mass,
CaO: 0 to 14.5% by mass,
SrO: 0 to 24% by mass,
BaO: 0 to 13.5% by mass,
MgO + CaO + SrO + BaO: 9 to 29.5% by mass.
As a 2nd example of the alkali free glass applied in the vacuum degassing method of this invention, the composition of the alkali free glass containing the following components by the mass% display of the following oxide conversion is mentioned more preferably.
SiO ₂ : 58 to 66% by mass,
Al ₂ O ₃ : 15-22% by mass,
B ₂ O ₃ : 0 to 12% by mass,
MgO: 0 to 8% by mass,
CaO: 0 to 9% by mass,
SrO: 3 to 12.5% by mass,
BaO: 0-2% by mass,
MgO + CaO + SrO + BaO: 9 to 18% by mass.

The reason for limiting the range of each component of the above alkali-free glass composition will be described below.
If the SiO ₂ content exceeds 66%, the solubility of the glass decreases and the glass tends to devitrify. Preferably it is 64% or less, More preferably, it is 62% or less. If it is less than 50%, the specific gravity increases, the strain point decreases, the thermal expansion coefficient increases, and the chemical resistance decreases. Preferably it is 58% or more, further 58.5% or more, more preferably 59% or more.

Al ₂ O ₃ is a component that suppresses the phase separation of the glass and increases the strain point, and is essential. If it exceeds 24%, devitrification tends to occur, and chemical resistance decreases. Preferably it is 22% or less, further 20% or less, more preferably 18% or less. If it is less than 10.5%, the glass tends to undergo phase separation or the strain point decreases. Preferably it is 15% or more, further 15.5% or more, more preferably 16% or more.

B ₂ O ₃ is not essential, but is a component that reduces the specific gravity, increases the solubility of the glass, and makes it difficult to devitrify. If it exceeds 22%, the strain point is lowered, the chemical resistance is lowered, or the volatilization at the time of melting the glass becomes remarkable, so that the inhomogeneity of the glass is increased. Preferably it is 12% or less, More preferably, it is 9% or less. If it is less than 5%, the specific gravity increases, the solubility of the glass decreases, and devitrification easily occurs, so 5% or more is desirable, preferably 6% or more, more preferably 7% or more.

MgO is not essential, but is a component that reduces the specific gravity and improves the solubility of the glass. If it exceeds 8%, the glass tends to undergo phase separation, devitrification tends to occur, or chemical resistance decreases. Preferably it is 6% or less, More preferably, it is 5% or less. When it contains MgO, it is preferable to make it contain 1% or more. In particular, it is preferable to contain 3% or more in order to reduce the specific gravity while maintaining the solubility.

CaO is not essential, but can be contained up to 14.5% in order to increase the solubility of the glass and make it difficult to devitrify. If it exceeds 14.5%, the specific gravity increases, the coefficient of thermal expansion increases, and devitrification tends to occur. Preferably it is 9% or less, further 8% or less, more preferably 7% or less. When CaO is contained, it is preferable to contain 2% or more. More preferably, it is 3.5% or more.

SrO is a component that suppresses the phase separation of glass and makes it difficult to devitrify. If it exceeds 24%, the specific gravity increases, the coefficient of thermal expansion increases, and devitrification tends to occur. Preferably it is 12.5% or less, further 10.5% or less, more preferably 8.5% or less. Considering the addition of chloride as a fining agent, there is no concern about deliquescence and it is easy to remain in the glass when the raw material is melted. Therefore, it is preferable to use SrCl ₂ · 6H ₂ O or BaCl ₂ · 2H ₂ O. In order to give a degree of freedom to the addition amount of Cl, it is preferable to contain 3% or more of SrO as the glass component. If SrO is less than 3%, the amount of Cl added is restricted, which is not preferable. Preferably it is 4% or more, More preferably, it is 4.5% or more.

BaO can be contained up to 13.5% in order to suppress phase separation of the glass and make it difficult to devitrify. If it exceeds 13.5%, the specific gravity increases and the thermal expansion coefficient becomes large. Preferably it is 2% or less, further 1% or less, more preferably 0.1% or less. In particular, when importance is attached to the weight reduction of the glass substrate, it is preferably not contained substantially.

It is preferable that As ₂ O ₃ and Sb ₂ O ₃ are not contained except for those inevitably mixed as impurities or the like, that is, not substantially contained. Incidentally, within a range not contrary to the object of the present invention, because of such solubility enhancing, may contain ZrO ₂ other trace ingredients up to 5% by weight in total.

According to the vacuum defoaming method of the present invention, for example, the hardly-soluble alkali-free glass D (described below) containing more SiO ₂ and Al ₂ O ₃ than the alkali-free glasses A to C described above. Also for the oxide-based mass% display), it is possible to carry out vacuum degassing under optimum conditions, and the effect of reducing the generation of bubbles and foreign matters can be obtained.
(Non-alkali glass D)
SiO ₂ : 62% by mass,
Al ₂ O ₃ : 20% by weight,
B ₂ O ₃ : 0% by mass,
MgO: 4.7% by mass,
CaO: 4.4% by mass,
SrO: 8.1% by mass,
ZrO ₂ : 0.9% by mass.
(T ₂ : 1690 ° C.)

In the vacuum degassing method of the present invention, a clarifier other than the chlorinated clarifier may be used in combination. In this case, specific examples of other fining agents that can be used in combination include SO ₃ , F, SnO _{2, and the} like. These other fining agents can be contained in the alkali-free glass in an amount of 2% by mass or less, preferably 1% by mass or less, more preferably 0.5% by mass or less.

In the vacuum degassing method of the present invention, it may be necessary to adjust [β-OH], that is, the moisture content of the molten glass. The amount of water in the molten glass can be adjusted by changing the composition of the gas mixed with the fuel when the fuel is burned, that is, the ratio of oxygen and air mixed with the fuel.

The dimension of each component of the vacuum degassing apparatus used in the vacuum degassing method of the present invention can be appropriately selected as necessary. The dimensions of the vacuum degassing tank can be appropriately selected according to the vacuum degassing apparatus to be used regardless of whether the vacuum degassing tank is made of platinum, platinum alloy, or dense refractory. In the case of the vacuum degassing tank 12 shown in FIG. 1, the specific example of the dimension is as follows.
Horizontal length: 1-20m
Inner diameter: 0.2-3m (circular cross section)
When the vacuum degassing tank 12 is made of platinum or a platinum alloy, the wall thickness is preferably 0.5 to 4 mm.

The decompression housing 11 is made of metal, for example, stainless steel, and has a shape and size that can accommodate a decompression deaeration tank.
Regardless of whether the riser pipe 13 and the downfall pipe 14 are made of platinum, a platinum alloy, or a dense refractory, they can be appropriately selected according to the vacuum degassing apparatus to be used. For example, the dimensions of the ascending pipe 13 and the descending pipe 14 can be configured as follows.
・ Inner diameter: 0.05 to 0.8m
・ Length: 0.2-6m
When the ascending pipe 13 and the descending pipe 14 are made of platinum or a platinum alloy, the thickness is preferably 0.4 to 5 mm.
The method for producing a glass product of the present invention comprises a glass melting step in which a glass raw material is melted to produce a molten glass, a vacuum degassing step by the vacuum degassing method for the molten glass, and a vacuum glass defoamed molten glass. It has each process with the glass product molding process to form in this order.
The glass melting step described above can employ, for example, a conventionally known glass melting method. For example, a predetermined glass is prepared by heating a glass material mixed and mixed according to the type of glass to about 1400 ° C. or higher. This is a process of melting the raw material. The glass raw material used is not particularly limited as long as it is a raw material adapted to the alkali-free glass to be produced. For example, cinnabar, boric acid, limestone, aluminum oxide, strontium carbonate, magnesium oxide, and other raw materials that become known glass components are used. The glass raw material prepared so that it may become the composition of the target alkali-free glass product can be used. A predetermined amount of the above-described chloride fining agent employed in the present invention is added to the glass raw material. Moreover, the glass product shaping | molding process can also employ | adopt the conventionally well-known molding method of glass products. When manufacturing a glass plate as a glass product, for example, various forming methods such as a float plate glass forming method, a roll-out forming method, and a fusion forming method can be used.

Hereinafter, based on an Example, this invention is demonstrated more concretely. However, the present invention is not limited to this.
Example 1
In this example, alkali-free glass A, which is known in advance that [β-OH] is 0.29 mm ⁻¹ , is used. NH ₄ Cl is added as a chloride clarifier. NH ₄ Cl is added in such an amount that the mass% of chlorine with respect to the total mass after vitrification becomes 0.20 mass%.
From the above formula (1), the bubble growth starting pressure P _bg is 270 mmHg.
From the P _bg obtained here and the above equation (3), the reboil pressure P _rb is 165 mmHg.

Pt 90% / Rh 10% platinum alloy with 45mm width, 7mm depth and 1mm thickness is placed in a quartz glass container of width 50mm x depth 10mm x height 50mm. Put. Thereafter, the quartz glass container is placed in an electric furnace and heated to 1400 ° C. to melt the alkali-free glass A.
Next, the atmospheric pressure of the electric furnace is reduced to a predetermined pressure, and the amount of bubbles in the molten glass is observed. The amount of bubbles in the molten glass is confirmed by visual observation from a viewing window provided on the side surface of the electric furnace.

Atmospheric pressure in an electric furnace, or 165mmHg is reboiled pressure P _rb, and, when held to 270mmHg below a bubble growth starting pressure P _bg, it is confirmed the amount of bubbles remaining in the molten glass is very low. On the other hand, when holding the atmospheric pressure in an electric furnace to less than 165mmHg is reboiled pressure P _rb, for significant increase in the amount of bubbles generated from a platinum plate in the molten glass is observed, the amount of bubbles remaining in the molten glass are very It is confirmed that there are many.

(Example 2)
In this example, the same procedure as in Example 1 was performed except that NH ₄ Cl was added to the alkali-free glass A in such an amount that the mass% of chlorine with respect to the total mass after vitrification was 0.07 mass%. To do.
From the above formula (2), the bubble growth starting pressure P _bg is 127 mmHg.
From P _bg obtained here and the above equation (3), the reboil pressure P _rb is 46 mmHg.
Atmospheric pressure in an electric furnace, or 46mmHg is reboiled pressure P _rb, and, when held to 127mmHg below a bubble growth starting pressure P _bg, it is confirmed the amount of bubbles remaining in the molten glass is very low. On the other hand, when holding the atmospheric pressure in an electric furnace to below 46mmHg is reboiled pressure P _rb, for significant increase in the amount of bubbles generated from a platinum plate in the molten glass is observed, the amount of bubbles remaining in the molten glass are very It is confirmed that there are many.

(Example 3)
In this example, alkali-free glass B, which is known in advance that [β-OH] is 0.29 mm ⁻¹ , is used. NH ₄ Cl is added as a chloride clarifier. NH ₄ Cl is added in such an amount that the mass% of chlorine with respect to the total mass after vitrification becomes 0.20 mass%.
From the above formula (1), the bubble growth starting pressure P _bg is 248 mmHg.
From the P _bg obtained here and the above equation (3), the reboil pressure P _rb is 147 mmHg.

Put platinum alloy of Pt90% / Rh10% of width 45mm, depth 7mm, thickness 1mm into a quartz glass container of width 50mm x depth 10mm x height 50mm. On top of that, 50g of block alkali-free glass B Put. Thereafter, the quartz glass container is placed in an electric furnace and heated to 1400 ° C. to melt the alkali-free glass B.
Next, the atmospheric pressure of the electric furnace is reduced to a predetermined pressure, and the amount of bubbles in the molten glass is observed. The amount of bubbles in the molten glass is confirmed by visual observation from a viewing window provided on the side surface of the electric furnace.

Atmospheric pressure in an electric furnace, or 147mmHg is reboiled pressure P _rb, and, when held to 248mmHg below a bubble growth starting pressure P _bg, it is confirmed the amount of bubbles remaining in the molten glass is very low. On the other hand, when holding the atmospheric pressure in an electric furnace to less than 147mmHg is reboiled pressure P _rb, for significant increase in the amount of bubbles generated from a platinum plate in the molten glass is observed, the amount of bubbles remaining in the molten glass are very It is confirmed that there are many.

Example 4
In this example, the same procedure as in Example 3 was performed except that NH ₄ Cl was added to the alkali-free glass B in such an amount that the mass% of chlorine with respect to the total mass after vitrification was 0.07 mass%. To do.
From the above formula (2), the bubble growth starting pressure P _bg is 125 mmHg.
From P _bg obtained here and the above equation (3), the reboil pressure P _rb is 45 mmHg.
Atmospheric pressure in an electric furnace, or 45mmHg is reboiled pressure P _rb, and, when held to 125mmHg below a bubble growth starting pressure P _bg, it is confirmed the amount of bubbles remaining in the molten glass is very low. On the other hand, when holding the atmospheric pressure in an electric furnace to below 45mmHg is reboiled pressure P _rb, for significant increase in the amount of bubbles generated from a platinum plate in the molten glass is observed, the amount of bubbles remaining in the molten glass are very It is confirmed that there are many.

(Example 5)
In this example, alkali-free glass C, which is known in advance that [β-OH] is 0.29 mm ⁻¹ , is used. NH ₄ Cl is added as a chloride clarifier. NH ₄ Cl is added in such an amount that the mass% of chlorine with respect to the total mass after vitrification becomes 0.20 mass%.
From the above formula (1), the bubble growth starting pressure P _bg is 237 mmHg.
From The obtained P _bg and the formula (3), reboiled pressure P _rb becomes 138MmHg.

Put platinum alloy of Pt90% / Rh10% of width 45mm, depth 7mm, thickness 1mm in a quartz glass container of width 50mm x depth 10mm x height 50mm. On top of that, 50g of block alkali-free glass C Put. Thereafter, the quartz glass container is placed in an electric furnace and heated to 1400 ° C. to melt the alkali-free glass C.
Next, the atmospheric pressure of the electric furnace is reduced to a predetermined pressure, and the amount of bubbles in the molten glass is observed. The amount of bubbles in the molten glass is confirmed by visual observation from a viewing window provided on the side surface of the electric furnace.

Atmospheric pressure in an electric furnace, or 138mmHg is reboiled pressure P _rb, and, when held to 237mmHg below a bubble growth starting pressure P _bg, it is confirmed the amount of bubbles remaining in the molten glass is very low. On the other hand, when holding the atmospheric pressure in an electric furnace to less than 138mmHg is reboiled pressure P _rb, for significant increase in the amount of bubbles generated from a platinum plate in the molten glass is observed, the amount of bubbles remaining in the molten glass are very It is confirmed that there are many.

(Example 6)
In this example, the same procedure as in Example 5 was performed except that NH ₄ Cl was added to the alkali-free glass C in such an amount that the mass% of chlorine with respect to the total mass after vitrification was 0.07 mass%. To do.
From the above formula (2), the bubble growth starting pressure P _bg is 123 mmHg.
From P _bg obtained here and the above equation (3), the reboil pressure P _rb is 43 mmHg.
Atmospheric pressure in an electric furnace, or 43mmHg is reboiled pressure P _rb, and, when held to 123mmHg below a bubble growth starting pressure P _bg, it is confirmed the amount of bubbles remaining in the molten glass is very low. On the other hand, when holding the atmospheric pressure in an electric furnace to below 43mmHg is reboiled pressure P _rb, for significant increase in the amount of bubbles generated from a platinum plate in the molten glass is observed, the amount of bubbles remaining in the molten glass are very It is confirmed that there are many.

(Example 7)
In this example, an alkali-free glass D that is known in advance that T ₂ is 1690 ° C. and [β-OH] is 0.29 mm ⁻¹ is used. NH ₄ Cl is added as a chloride clarifier. NH ₄ Cl is added in such an amount that the mass% of chlorine with respect to the total mass after vitrification becomes 0.20 mass%.
From the above formula (1), the bubble growth starting pressure P _bg is 285 mmHg.
From the P _bg obtained here and the above equation (3), the reboil pressure P _rb is 178 mmHg.

Pt 90% / Rh 10% platinum alloy of width 45mm, depth 7mm, thickness 1mm is put in a quartz glass container of width 50mm x depth 10mm x height 50mm. Put. Thereafter, the quartz glass container is put in an electric furnace and heated to 1475 ° C. to melt the alkali-free glass D.
Next, the atmospheric pressure of the electric furnace is reduced to a predetermined pressure, and the amount of bubbles in the molten glass is observed. The amount of bubbles in the molten glass is confirmed by visual observation from a viewing window provided on the side surface of the electric furnace.

Atmospheric pressure in an electric furnace, or 178mmHg is reboiled pressure P _rb, and, when held to 285mmHg below a bubble growth starting pressure P _bg, it is confirmed the amount of bubbles remaining in the molten glass is very low. On the other hand, when holding the atmospheric pressure in an electric furnace to less than 178mmHg is reboiled pressure P _rb, for significant increase in the amount of bubbles generated from a platinum plate in the molten glass is observed, the amount of bubbles remaining in the molten glass are very It is confirmed that there are many.

(Example 8)
In this example, the same procedure as in Example 3 was performed except that NH ₄ Cl was added to the alkali-free glass D in such an amount that the mass% of chlorine with respect to the total mass after vitrification was 0.07 mass%. To do.
From the above formula (2), the bubble growth starting pressure P _bg is 129 mmHg.
From the P _bg obtained here and the above formula (3), the reboil pressure P _rb is 48 mmHg.
Atmospheric pressure in an electric furnace, or 48mmHg is reboiled pressure P _rb, and, when held to 129mmHg below a bubble growth starting pressure P _bg, it is confirmed the amount of bubbles remaining in the molten glass is very low. On the other hand, when holding the atmospheric pressure in an electric furnace to below 48mmHg is reboiled pressure P _rb, for significant increase in the amount of bubbles generated from a platinum plate in the molten glass is observed, the amount of bubbles remaining in the molten glass are very It is confirmed that there are many.

Example 9
Here, Tables 1 to 5 and FIGS. 7 to 8 show experimental results showing the effectiveness of the formulas (1) and (2) for the alkali-free glasses A, B, and C described above. In the experiment, the bubble growth initiation pressure P _bg was obtained by changing the β-OH value (mm ⁻¹ ) and the chlorine content (mass%) for each glass composition. Each glass is melted at a predetermined temperature in an electric furnace having an observation window by putting about 50 g of molten glass in a quartz cell. After the glass is melted, the pressure is reduced to a desired pressure at a constant speed, and then the change of the bubble diameter is photographed with a CCD camera and recorded under a constant pressure. After the experiment was completed, the change in the bubble diameter was analyzed to determine the bubble growth start pressure P _bg .

Tables 1 to 5 show the glass temperature at the time of bubble observation, β-OH value (mm ⁻¹ ), chlorine content (mass%), experimental value of bubble growth starting pressure P _bg (mmHg), and bubble growth start. The calculated value (mmHg) of the pressure P _{bg according to} the formula (1) or the formula (2) is described. Tables 1 to 3 show the respective results for 122 examples of the alkali-free glass compositions A, B, and C having a chlorine content (% by mass) of 0.12% by mass or more. Tables 4 to 5 show the results of 41 cases with non-alkali glass compositions A, B, and C and a chlorine content (mass%) of less than 0.12 mass%.

FIG. 7 shows the experimental value (mmHg) of the bubble growth starting pressure P _bg and the bubble growth when the chlorine content (% by mass) is 0.12% by mass or more for the alkali-free glass compositions A, B, and C. The correlation of 122 points | pieces of the calculated value (mmHg) of the starting pressure _Pbg is shown. FIG. 8 shows the experimental value (mmHg) of the bubble growth starting pressure P _bg and the bubble growth when the chlorine content (% by mass) is less than 0.12% by mass for the alkali-free glass compositions A, B, and C. The correlation of 41 points | pieces of the calculated value (mmHg) of the starting pressure _Pbg is shown.

The result of FIG. 7 shows that the slope of the regression equation is 0.92 and the square of the correlation coefficient is 0.82, and Equation (1) can estimate the experimental value well. The result of FIG. 8 shows that the slope of the regression equation is 0.97 and the square of the correlation coefficient is 0.89, and Equation (2) is able to estimate the experimental value well. From these, it can be seen that the equations (1) and (2) are effective in estimating the bubble growth starting pressure P _bg . Plotting the data of the example for the alkali-free glass D on the graphs of FIGS. 7 and 8 gives the same result. Tables 1 to 5 show glass temperatures when bubbles are observed, but the glass temperature at which bubbles are generated is not defined in the formulas (1) and (2) of the present invention. This is considered to be because the clarification according to the present invention is a clarification mainly involving water, and water has a small temperature dependency of solubility.

According to the vacuum degassing method for molten glass of the present invention, the pressure in the vacuum degassing tank is maintained at a bubble growth starting pressure P _bg or lower and a reboyl pressure P _rb (mmHg) or higher. Bubbles contained in the molten glass can be sufficiently grown, and the bubbles in the molten glass can be efficiently removed, while reboiling is prevented from occurring in the molten glass flowing through the vacuum degassing tank. Bubbles remaining in the molten glass after the foam treatment are extremely reduced, and a high-performance and high-quality glass with extremely few bubbles can be produced. Thus, it is possible to obtain molten glass and glass products that are extremely excellent in foam quality, and in particular, it is possible to produce an alkali-free glass substrate for FPD, which is useful.
The entire contents of the specification, claims, drawings, and abstract of Japanese Patent Application No. 2009-294230 filed on Dec. 25, 2009 are incorporated herein as the disclosure of the present invention. .

DESCRIPTION OF SYMBOLS 1: Depressurization degassing apparatus 11: Decompression housing 12: Depressurization degassing tank 13: Ascending pipe 14: Downcomer pipe 15: Heat insulating material 16: Suction opening 20: Dissolution tank

Claims (7)

1. A method of degassing molten glass by flowing molten glass into a vacuum degassing tank whose inside is maintained in a reduced pressure state,
   The molten glass is alkali-free glass,
   The pressure in the vacuum degassing tank at the time of carrying out the vacuum defoaming is equal to or lower than the bubble growth starting pressure P _bg (mmHg) of the molten glass represented by the following formula (1) or formula (2), and the following formula (3) A reduced pressure defoaming method for molten glass, characterized in that the molten glass is maintained at a reboiling pressure P _rb (mmHg) or higher.
   P _bg = (2.6082 × T ₂ −3538.2) × [β-OH] + (− 1.2102 × T ₂ +2612.2) × [Cl] −80.3 (1)
   P _bg = (− 0.2462 × T ₂ +111.7) × [β-OH] + (1.9714 × T ₂ −1730.6) × [Cl] −187.3 (2)
   P _rb = 0.8325 × P _bg -59.5 (3)
   (Wherein T ₂ represents the temperature (° C.) at which the viscosity of the molten glass is 10 ² dPa · s, [β-OH] represents the β-OH value (mm ⁻¹ ) of the alkali-free glass, and [Cl ] Represents the content (mass%) of chlorine in the alkali-free glass.When [Cl] is 0.12 mass% or more, P _bg is represented by the formula (1), and [Cl] is 0.12 In the case of less than% by mass, P _bg is represented by the formula (2).)
2. The method for degassing a molten glass according to claim ¹ , wherein the [β-OH] is 0.15 to 0.6 mm ^-1 .
3. The method for degassing a molten glass according to claim 1 or 2, wherein the [Cl] is 0.03 to 0.3 mass%.
4. The method for degassing a molten glass according to any one of claims 1 to 3, wherein the T ₂ is 1500 to 1750 ° C.
5. The method for degassing a molten glass according to any one of claims 1 to 4, wherein the alkali-free glass contains the following components in terms of mass% in terms of the following oxide.
   SiO ₂ : 50 to 66% by mass,
   Al ₂ O ₃ : 10.5 to 24% by mass,
   B ₂ O ₃ : 0 to 12% by mass,
   MgO: 0 to 8% by mass,
   CaO: 0 to 14.5% by mass,
   SrO: 0 to 24% by mass,
   BaO: 0 to 13.5% by mass,
   MgO + CaO + SrO + BaO: 9 to 29.5% by mass.
6. The method for degassing a molten glass according to any one of claims 1 to 4, wherein the alkali-free glass contains the following components in terms of mass% in terms of the following oxide.
   SiO ₂ : 58 to 66% by mass,
   Al ₂ O ₃ : 15-22% by mass,
   B ₂ O ₃ : 0 to 12% by mass,
   MgO: 0 to 8% by mass,
   CaO: 0 to 9% by mass,
   SrO: 3 to 12.5% by mass,
   BaO: 0 to 2% by mass,
   MgO + CaO + SrO + BaO: 9 to 18% by mass.
7. A glass melting step for producing a molten glass by melting a glass raw material, a vacuum degassing step by the vacuum degassing method for molten glass according to any one of claims 1 to 6, and a vacuum glass defoamed molten glass A glass product forming step for forming a glass product, and a glass product manufacturing method having these steps in this order.

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