JP2015187081A - Manufacturing method of glass plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a glass plate capable of efficiently reducing bubbles remaining in a glass plate in manufacturing a glass plate by a down-draw method.SOLUTION: A manufacturing method of a glass plate includes a melting step for melting glass-making feedstock to which SnOis added as a clarifier to produce molten glass, a clarifying step, and a molding step for molding the molten glass after the clarifying step by a down-draw method to manufacture a glass plate. The clarifying step includes defoaming treatment for removing bubbles generated in the molten glass by heating the molten glass to 1,630°C or higher and absorbing treatment for making the molten glass absorb the bubbles remaining in the molten glass by lowering the temperature of the molten glass at a rate of 2°C/min. or more in the range from 1,600°C to 1,500°C after the defoaming treatment.

Description

  The present invention relates to a method for producing a glass plate.

  As a glass substrate used for a flat panel display (hereinafter referred to as “FPD”) such as a liquid crystal display or a plasma display, a thin glass plate having a thickness of, for example, 0.5 to 0.7 mm is used. For example, the FPD glass substrate has a size of 300 × 400 mm in the first generation, but has a size of 2850 × 3050 mm in the tenth generation.

  The overflow downdraw method is most often used to manufacture such a large glass substrate for FPD of the eighth generation or later. The overflow downdraw method includes a step of forming a sheet glass below the formed body by causing the molten glass to overflow from the upper part of the formed body in a forming furnace, and a step of gradually cooling the sheet glass in a slow cooling furnace. The slow cooling furnace draws the sheet glass between the pair of rollers and stretches the sheet glass to a desired thickness, and then slowly cools the sheet glass so as to reduce internal distortion and thermal shrinkage of the sheet glass. Thereafter, the plate-like glass is cut into a predetermined size to form a glass plate, which is laminated and stored on another glass plate. Or a glass plate is conveyed by the following process.

  The glass plate manufactured by such molding is used for a glass substrate of a liquid crystal display in which a semiconductor element is formed on the glass surface, but the characteristics of the semiconductor element formed on the glass surface are not deteriorated by the glass composition of the glass substrate. As described above, a glass plate containing no alkali metal component or containing a small amount of the alkali metal component is preferably used.

By the way, since a bubble will be a cause of a display defect if a bubble exists in a glass plate, the glass plate in which a bubble exists is not suitable as a glass substrate for flat panel displays. For this reason, it is calculated | required that a bubble does not remain | survive in a glass plate. In particular, the requirement for bubbles is severe in glass substrates for liquid crystal displays and glass substrates for organic EL displays.
However, in order to suppress the deterioration of the characteristics of the semiconductor element, a glass plate that does not contain an alkali metal component or that contains a small amount is a glass plate that contains a large amount of alkali metal such as soda lime glass. There is a problem that the high-temperature viscosity is higher than that of the glass, and bubbles are difficult to escape from the molten glass being produced.
From the viewpoint of reducing the environmental load, it is required to limit the use of highly toxic As ₂ O ₃ which has been conventionally used. In recent years the, SnO ₂ and Fe ₂ O ₃ which refining capabilities compared to As ₂ O ₃ in place of As ₂ O ₃ is less is used as a fining agent. Since SnO ₂ and Fe ₂ O ₃ cause devitrification and coloring of the glass, it cannot be added in a large amount to the glass in order to ensure the refining function equivalent to As ₂ O ₃ . For this reason, bubbles remain more easily on the glass plate as the final product.

  On the other hand, in order to improve the defoaming effect in the method of producing a glass substrate in which a non-alkali glass in which a vitrification reaction occurs at 1300 to 1500 ° C. is defoamed by raising the temperature to 1650 ° C. Has been proposed (Patent Document 1).

JP-A-2005-97090

Here, for example, in a glass composition that does not contain an alkali metal component or is contained in a small amount, the solubility of SO _{2 that} can be dissolved in the molten glass is small, so once SO ₂ bubbles are generated It tends to remain as a defect of bubbles in the glass plate as the final product.
However, the technique described in Patent Document 1 has a problem that the generation of SO ₂ bubbles after the clarification step cannot be sufficiently suppressed.

  Then, this invention aims at providing the manufacturing method of the glass plate which can reduce the bubble which remain | survives in a glass plate efficiently, when manufacturing a glass plate.

1st aspect of this invention is a manufacturing method of the glass plate which manufactures a glass plate. The method is
A melting step of melting a glass raw material containing SnO ₂ as a fining agent by at least electric heating to produce a molten glass;
The temperature of the molten glass is raised to 1630 ° C. or more to generate bubbles in the molten glass and defoaming, and after the defoaming treatment, the molten glass is heated from 1600 ° C. to 1500 ° C. A clarification step including an absorption treatment in which the molten glass absorbs bubbles in the molten glass by lowering the temperature at a temperature decreasing rate of 2 ° C./min or more in a temperature range of
A molding step of molding the molten glass after the clarification step into a sheet glass.
In this case, the content of SnO ₂ glass plates produced is preferably 0.01 to 0.5 wt%. Further, it is preferable to use a combination of 0.01 to 0.5 mass% of SnO ₂ and 0.01 to 0.1 mass% of Fe ₂ O _3.

According to a second aspect of the present invention, in the absorption treatment, the temperature decrease rate in the temperature range where the molten glass is 1500 ° C. or less is faster than the temperature decrease rate in the temperature range of 1600 ° C. to 1500 ° C. It is a manufacturing method of the glass plate of 1 aspect.
According to a third aspect of the present invention, in the temperature range where the molten glass is 1500 ° C. or less, the flow rate of the molten glass can be adjusted in a platinum or platinum alloy tube through which the molten glass flows. It is the manufacturing method of the glass plate of the 1st or 2nd aspect of this invention whose temperature-fall rate in the temperature range whose molten glass is 1500 degrees C or less is slower than the temperature-fall rate in the said 1600 degreeC to 1500 degreeC temperature range.
A fourth aspect of the present invention is the glass plate manufacturing method according to any one of the first to third aspects of the present invention, wherein the forming step forms a sheet glass from the molten glass by an overflow down draw method. is there.
A fifth aspect of the present invention is the method for producing a glass plate according to any one of the first to fourth aspects of the present invention, wherein the molten glass at 1630 ° C. has a viscosity of 130 to 350 poise.

In a sixth aspect of the present invention, the clarification step is performed in platinum or a platinum alloy tube through which the molten glass flows.
The temperature rise of the molten glass in the refining step is performed by controlling currents that are separately supplied to at least two different regions extending in the length direction of the platinum or platinum alloy tube. The method for producing a glass plate according to any of the above aspects.

In a seventh aspect of the present invention, the glass plate has an R ′ ₂ O content of 0 to 2.0% by mass (R ′ ₂ O is Li ₂ O, Na ₂ O and K ₂ O). The total of the components to be contained), and a method for producing a glass plate according to any one of the first to sixth aspects of the present invention.
The 8th aspect of this invention is a manufacturing method of the glass plate in any one of the 1st-7th aspect of this invention in which the said glass plate is used for the glass substrate for flat panel displays.

  A ninth aspect of the present invention includes an agitation step of homogeneously stirring the components of the molten glass between the clarification step and the molding step, and in the melting step, the temperature at the start of melting of the molten glass is set. The molten glass is supplied to the clarification step at a temperature higher than that, and in the clarification step, the molten glass is supplied to the stirring step at a temperature lower than the temperature after the defoaming treatment. With respect to the viscosity η (poise) of the molten glass, the molten glass is supplied at a temperature of log η = 4.3 to 5.7, and is formed into a sheet glass, according to the first to eighth aspects of the present invention. It is a manufacturing method of any glass plate.

  The manufacturing method of the glass plate of the said form can reduce the foam which remain | survives in a glass plate efficiently.

It is process drawing of the manufacturing method of the glass plate of this embodiment. It is a figure which shows typically the apparatus which performs a melting process-a cutting process among the manufacturing methods of the glass plate of this embodiment. It is a figure which mainly shows the apparatus structure which performs the clarification process of this embodiment. It is a figure which mainly shows the apparatus structure which performs the shaping | molding process and cutting process of this embodiment. It is a figure explaining an example of the temperature history from the melting process of this embodiment to a forming process. Is a diagram showing a measurement result of the content of SO ₂ contained in the pores in the glass which reproduces bubbles remaining in the glass plate. It is a figure which shows the relationship between a bubble level when a glass plate is produced with the experimental furnace which simulated the temperature history of the molten glass shown in FIG. 5, and a temperature fall rate. It is a figure which shows the relationship between the bubble level which exists in a glass plate when a glass plate is manufactured using the apparatus which manufactures a glass plate, and a temperature fall rate.

  Hereinafter, the manufacturing method of the glass plate of this embodiment is demonstrated.

(Overall overview of glass plate manufacturing method)
FIG. 1 is a process diagram of a method for producing a glass plate of the present embodiment.
The glass plate manufacturing method includes a melting step (ST1), a refining step (ST2), a homogenizing step (ST3), a supplying step (ST4), a forming step (ST5), and a slow cooling step (ST6). And a cutting step (ST7). In addition, a plurality of glass plates that have a grinding process, a polishing process, a cleaning process, an inspection process, a packing process, and the like and are stacked in the packing process are conveyed to a supplier.

  FIG. 2 is a diagram schematically showing a glass substrate manufacturing apparatus that performs the melting step (ST1) to the cutting step (ST7). As shown in FIG. 2, the apparatus mainly includes a melting apparatus 200, a forming apparatus 300, and a cutting apparatus 400. The melting apparatus 200 mainly has a melting tank 201, a clarification tank 202, a stirring tank 203, and glass supply pipes 204, 205, and 206. In addition, since the glass supply pipes 204 and 205 are a pipe | tube which flows molten glass MG so that it may mention later, since it has a clarification function, it is also a clarification tank substantially. Hereinafter, the glass supply pipe 204 is referred to as a first clarification tank 204, the clarification tank 202 is referred to as a second clarification tank 202, and the glass supply pipe 205 is referred to as a third clarification tank 205. In addition, the main parts of the first clarification tank 204, the third clarification tank 205, the glass supply pipe 206, the second clarification tank 202, and the stirring tank 203 that connect the respective tanks from the melting tank 201 to the molding apparatus 300 are made of platinum. Alternatively, it is composed of a platinum alloy tube. The first clarification tank 204 and the third clarification tank 205 have a cylindrical shape or a bowl shape.

In the melting step (ST1), a glass raw material to which SnO ₂ is added as a clarifier and supplied into the melting tank 201, that is, a glass raw material containing SnO ₂ as a clarifier is melted by energization heating using at least an electrode. Thus, a molten glass is obtained. Further, in addition to the electric heating using the electrodes, a glass raw material may be melted using a flame (not shown) to obtain a molten glass MG. When performing melting of the glass raw material using electric heating and flame, specifically, the glass raw material is supplied while being dispersed on the liquid surface of the molten glass MG using a raw material charging device (not shown). The glass raw material is heated and melted gradually by a gas phase heated to a high temperature by a flame and melted in the molten glass MG. Molten glass MG is heated by energization heating. In addition, you may bubble with oxygen gas in a molten glass between a melting process or a melting process and a refining process. In addition, it is preferable not to perform bubbling in the initial stage of a melting process. This is because, in the initial stage of the melting process (for example, the temperature of the molten glass is less than 1540 ° C.), when the molten glass MG is energized and heated in the melting tank 201, the electric resistance of the members such as bricks constituting the melting tank 201 is larger. This is because, since the electric resistance of the glass is larger, it becomes easier for a current to flow through a member such as a brick, and it becomes difficult to energize and heat the molten glass MG using electrodes.

The clarification step (ST2) is performed in at least the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205. In the clarification process, when the molten glass MG in the first clarification tank 204 is heated, bubbles containing gas components such as O ₂ , CO ₂ or SO ₂ contained in the molten glass MG are clarifiers. It grows by absorbing O ₂ generated by a certain SnO ₂ reduction reaction, floats on the liquid surface of the molten glass MG, and is released. In the clarification step, the internal pressure of the gas component in the foam is lowered due to the temperature drop of the molten glass MG, and SnO obtained by the reduction reaction of SnO ₂ undergoes an oxidation reaction due to the temperature drop of the molten glass MG. Thus, gas components such as O ₂ in the foam remaining in the molten glass MG are reabsorbed in the molten glass MG, and the foam disappears. The oxidation reaction and reduction reaction by the fining agent are performed by adjusting the temperature of the molten glass MG. The temperature of the molten glass MG is adjusted by adjusting the temperatures of the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205. The temperature of each clarification tank is adjusted by direct energization heating in which electricity is supplied to the tube itself, or by using heaters arranged around the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205. Indirect heating for heating, indirect cooling with an air-cooling or water-cooling cooler, air blowing to the first clarification tank 204, the second clarification tank 202, the third clarification tank 205, or any heating such as water spraying, It is performed by a cooling method or a combination of these methods. Moreover, in FIG. 2, the tank which performs clarification is divided into three parts, the 1st clarification tank 204, the 2nd clarification tank 202, and the 3rd clarification tank 205, but it may naturally be further subdivided.
In the adjustment of the temperature of the molten glass MG of the present embodiment, direct current heating, which is one of the methods described above, is used. Specifically, between a metal flange (not shown) provided in the first clarification tank 204 that supplies the molten glass MG to the second clarification tank 202 and a metal flange (not shown) provided in the second clarification tank 202. (See arrow in FIG. 3), a metal flange (not shown) provided in the second clarification tank 202, and a second clarification tank 202 downstream of the molten glass MG with respect to the metal flange. The temperature of the molten glass MG is adjusted by passing an electric current between the metal flange (not shown) (arrow in FIG. 3). In the present embodiment, the first clarification tank 204 and the second clarification tank 202 are energized by supplying different constant currents to the first area between the metal flanges and the second area between the metal flanges. Although the temperature of the molten glass MG is adjusted by heating, this energization heating is not limited to the temperature adjustment by the energization heating of two regions, or the energization heating of one region or three or more regions It is also possible to adjust the temperature of the molten glass MG by conducting electrical heating.
In the homogenization step (ST3), the glass component is homogenized by stirring the molten glass MG in the stirring tank 203 supplied through the third clarification tank 205 using the stirrer 203a. Two or more stirring tanks 203 may be provided.
In the supply step (ST4), molten glass is supplied to the forming apparatus 300 through the glass supply pipe 206.

In the molding apparatus 300, a molding process (ST5) and a slow cooling process (ST6) are performed.
In the forming step (ST5), the molten glass MG is formed into a sheet glass G, and a flow of the sheet glass G is created. In this embodiment, an overflow down draw method using a molded body 310 described later is used. In the slow cooling step (ST6), the plate-like glass G that is formed and flows is cooled so that internal distortion does not occur.
In the cutting step (ST7), the cutting device 400 cuts the plate glass G supplied from the forming device 300 into a predetermined length, thereby obtaining a glass plate. The cut glass plate is further cut into a predetermined size to produce a target size glass plate. Thereafter, the end face of the glass is ground and polished, and the glass plate is cleaned. Further, after the presence or absence of defects such as bubbles is inspected, the glass plate that has passed the inspection is packed as a final product.

(Clarification process)
FIG. 3 is a diagram mainly showing a device configuration for performing the refining process. The clarification step includes a defoaming step and an absorption step. In the defoaming step, the temperature of the molten glass MG is raised to 1630 ° C. or higher, oxygen is released to SnO ₂ as a refining agent, and this oxygen is taken into the existing bubbles B of the molten glass MG. Increase the bubble diameter. Thereby, due to the synergistic effect of the expansion of the bubble diameter due to the internal pressure increase of the gas component in the bubble B due to the temperature increase of the molten glass MG and the decrease in the viscosity of the molten glass MG due to the temperature increase of the molten glass MG, The rising speed of the bubble B is increased, and defoaming is promoted.
In the absorption treatment, by reducing the temperature of the molten glass MG contrary to the defoaming treatment, the oxygen in the bubbles B in the molten glass MG is absorbed by the molten glass MG again, and the temperature of the molten glass MG is decreased. Due to the synergistic effect of reducing the internal pressure of the gas component in the bubbles B, the bubble diameter is reduced and the bubbles B disappear in the molten glass MG. In the absorption step, the temperature of the molten glass MG is lowered in a temperature range of 1600 ° C. to 1500 ° C. at a temperature lowering rate of 2 ° C./min or more.

The 1st clarification tank 204, the 2nd clarification tank 202, and the 3rd clarification tank 205 are apparatuses which degas | foam the molten glass MG and absorb the bubble B by giving the temperature history mentioned above to the molten glass MG. . For this reason, it has the temperature control function which can heat and cool the 1st clarification tank 204, the 2nd clarification tank 202, and the 3rd clarification tank 205 to the target temperature.
The temperature adjustment of each of the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205 is performed by direct energization heating for energizing each clarification tank itself, or indirectly by a clarification tank by a heater (not shown) arranged around each tank. Heating, air cooling, indirect cooling with a water-cooled cooler, air blowing to each clarification tank, water spraying, or the like, or a combination of these methods is performed.

The clarification will be described in more detail with reference to FIG.
Liquid molten glass MG containing a large amount of bubbles B melted in melting tank 201 and generated by the decomposition reaction of the glass raw material is introduced into first clarification tank 204.
In the first clarification tank 204, the molten glass MG is heated to 1630 ° C. or more by heating the platinum or platinum alloy tube which is the main body of the first clarification tank 204, and the reduction reaction of the clarifier is promoted, so that a large amount of oxygen Is released into the molten glass MG. In the existing bubble B in the molten glass MG, the oxygen released by the reductive reaction of the clarifier is due to the expansion of the bubble diameter due to the pressure increase effect of the gas component in the bubble B caused by the temperature increase of the molten glass MG. The diffusion and entry into the bubbles B overlap, and this synergistic effect increases the bubble diameter of the existing bubbles B. The first clarification tank 204 has a smaller pipe cross section than the second clarification tank 202, and unlike the second clarification tank 202, the upper open space does not have a gas phase atmosphere space. In the 1 clarification tank 204, since the molten glass MG is filled and flows in the entire inner cross section of the first clarification tank 204, the temperature of the molten glass MG can be increased efficiently as compared with the second clarification tank 202. That is, rather than raising the temperature of the molten glass MG to 1630 ° C. or higher in the second clarification tank 202, it is second to raise the temperature of the molten glass MG to 1630 ° C. or higher in the first clarification tank 204. Since the heating temperature of the clarification tank 202 can be lowered, it is preferable from the viewpoint of suppressing volatilization and melting of the platinum alloy constituting the second clarification tank 202.

Subsequently, the molten glass MG is introduced into the second clarification tank 202.
Unlike the first clarification tank 204, the second clarification tank 202 has an upper open space inside the second clarification tank 202 as a gas-phase atmosphere space, and bubbles B in the molten glass MG float on the liquid surface of the molten glass MG. Thus, it can be discharged out of the molten glass MG.
In the second clarification tank 202, the molten glass MG is continuously maintained at a high temperature of 1630 ° C. or higher by heating the platinum or platinum alloy tube which is the main body of the second clarification tank 202, and the bubbles B in the molten glass MG The molten glass MG is degassed by rising upward from the tank 202 and breaking bubbles on the liquid surface of the molten glass MG. In particular, when the molten glass MG is heated to 1630 ° C. or higher (for example, 1630 to 1700 ° C.), SnO ₂ accelerates the reduction reaction. At this time, for example, when producing a glass plate for a flat panel display such as a liquid crystal display, the viscosity of the glass is a viscosity suitable for the rising and defoaming of bubbles B due to a rise in the temperature of the molten glass MG (200 to 800 poise). It has become.
Here, the gas components broken and released in the upper open space above the second clarification tank 202 are discharged out of the second clarification tank 202 from a gas discharge port (not shown). In the second clarification tank 202, the molten glass MG from which the large bubbles B having a high floating speed are removed by the rising and defoaming of the bubbles B is introduced into the third clarification tank 205.
In the present embodiment, for example, as shown in FIG. 3, in the second clarification tank 202 to the third clarification tank 205, they are separately supplied to two different regions extending in the length direction of the platinum or platinum alloy tube constituting the main body. The temperature of the molten glass MG may be increased by controlling the current. Moreover, the temperature rise of the molten glass MG may be performed by controlling the electric current separately supplied to the 3 or more different area | region extended in the length direction of the platinum or platinum alloy pipe | tube which comprises the main body of a clarification tank.
As described above, it is preferable that the temperature rise of the molten glass MG is performed by controlling the currents separately supplied to at least two regions of the clarification tank in terms of efficiently performing the defoaming process.

In the third clarification tank 205, the molten glass MG is cooled by cooling the platinum or platinum alloy tube which is the main body of the third clarification tank 205, or by suppressing the degree of heating of the third clarification tank 205. Since the temperature of the molten glass MG is lowered by this cooling, the bubbles B are not floated and defoamed, the pressure of the gas component in the remaining small bubbles B is lowered, and the bubble diameter is gradually reduced. Furthermore, when the temperature of the molten glass MG is 1600 ° C. or less, a part of SnO obtained by the reduction reaction of SnO _{2 in} the defoaming process absorbs oxygen and tries to return to SnO ₂ . For this reason, oxygen which is a gas component in the bubbles B is reabsorbed in the molten glass MG, and the bubbles B become smaller and absorbed in the molten glass MG and finally disappear. At this time, the molten glass MG is cooled in the temperature range of 1600 ° C. to 1500 ° C. at an average rate of 2 ° C./min or more, more preferably at an average rate of 2.5 ° C./min or more. In addition, since the 3rd clarification tank 205 has a smaller cross section than the 2nd clarification tank 202, compared with the 2nd clarification tank 202, the molten glass MG can be cooled efficiently. That is, it is more preferable to cool the temperature of the molten glass MG in the third clarification tank 205 than to cool the temperature of the molten glass MG in the second clarification tank 202 from the viewpoint of increasing the rate of temperature decrease.

In the example shown in FIG. 3, the clarification tank that performs the clarification step is divided into three parts, a first clarification tank 204, a second clarification tank 202, and a third clarification tank 205, but the clarification tank is further subdivided. Of course it is good. If the clarification tank is subdivided, the temperature of the molten glass MG can be adjusted more finely. In particular, subdividing the clarification tank is advantageous in that the temperature can be easily adjusted when changing the type or melting amount of the molten glass MG.
Further, in the above description, for simplification, in the first clarification tank 204, the molten glass MG is heated to 1630 ° C., and in the second clarification tank 202, the bubble B of the molten glass MG is floated and defoamed, In the 3rd clarification tank 205, although the molten glass MG demonstrated the function for every clarification tank so that bubble B might be absorbed by the temperature fall of the molten glass MG, the function was not completely divided for every clarification tank. May be. A portion of the second clarification tank 202 up to the middle of the length direction may be configured to raise the temperature of the molten glass MG. Between the middle of the second clarification tank 202 in the length direction and the third clarification tank 205, the molten glass It can also be configured to be a part for starting the temperature decrease of MG.
In this embodiment, by controlling the temperature by measuring the surface temperature of the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205, that is, the surface temperature outside the clarification tank where the molten glass MG does not contact. The temperature increase rate and temperature decrease rate of the molten glass MG can be managed. The surface temperature of the first clarification tank 204, the second clarification tank 202 and the third clarification tank 205, and the average temperature of the molten glass MG flowing in the first clarification tank 204, the second clarification tank 202 and the third clarification tank 205 ( The average temperature of the molten glass MG having a temperature distribution in the clarification tank) can be calculated in advance by computer simulation using the flow rate and temperature conditions of the molten glass MG supplied to the clarification tank. . For this reason, it is possible to manage the temperature increase rate and the temperature decrease rate by calculating the temperature increase rate and the temperature decrease rate from the measured surface temperature outside the clarification tank using the above relationship. The flow rate of the molten glass MG can be calculated from the volume of each device and the amount of the molten glass MG per unit time flowing into the forming device 300. Further, the temperature of the molten glass MG can be calculated from the viscosity and thermal conductivity of the glass.

Thus, after the defoaming treatment, the temperature of the molten glass MG is lowered to a temperature range of 1600 ° C. to 1500 ° C. at a temperature lowering rate of 2 ° C./min or more, as will be described later. This is to reduce the number of bubbles per unit mass remaining inside. The bubble B here refers to a bubble having a volume equal to or greater than a preset volume of the bubble, for example, a volume of a bubble having a diameter of 20 μm.
In addition, although the said temperature fall rate can shorten the number of bubbles which remain | survive in a glass plate, so that this reduction effect becomes small with the raise of the said temperature fall rate. The temperature lowering rate is preferably 3 ° C./min or more. In addition, although the upper limit of the said temperature fall rate is not specifically provided, when manufacturing a glass plate industrially, 50 degreeC / min becomes an upper limit for the following reasons.

That is, if the temperature drop rate of the molten glass MG becomes too fast, the phenomenon that oxygen in the bubbles B of the molten glass MG is reabsorbed by the molten glass MG is inhibited, and as a result, the bubbles B themselves in the molten glass MG do not decrease. there is a possibility. Moreover, since the thermal conductivity of the glass is as small as about 20 to 50 W / (m · K) even at high temperatures, the rapid cooling of the molten glass MG is performed from the outside of the third clarification tank 205 unless special measures are taken. Since only the cooling rate is increased, only the molten glass MG near the outer surface of the third clarification tank 205 is cooled, and the molten glass MG at the center of the third clarification tank 205 is kept at a high temperature. Is done. That is, in the 3rd clarification tank 205, a temperature difference will become large between the outer surface part and center part of molten glass MG. In this case, there arises a problem that crystals are precipitated from the molten glass MG on the outer surface portion. Further, in the third clarification tank 205, when the molten glass MG is stirred in a state where the temperature difference of the molten glass MG is large between the outer surface portion and the center portion of the molten glass MG, the glass having a large temperature difference is mixed. Therefore, in addition to the generation of bubbles B, the homogeneity of the glass tends to be hindered. Moreover, in order to increase the temperature drop rate of the molten glass MG, the heat radiation from the third clarification tank 205 must be increased, so that the support of a backup brick or the like that supports the main body of the platinum or platinum alloy tube of the third clarification tank 205 is required. The thickness of the member must be reduced. However, the strength of the equipment is reduced by the thickness of the support member. For this reason, when manufacturing a glass plate industrially, making the temperature drop rate of the molten glass MG unnecessarily high causes only the problems as described above and is not appropriate.
From the above, the upper limit of the cooling rate from 1600 ° C. to 1500 ° C. of the molten glass MG is preferably 50 ° C./min, and more preferably 35 ° C./min. That is, in the present embodiment, the temperature lowering rate is preferably 2 ° C./min to 50 ° C./min, more preferably 2.5 ° C./min to 50 ° C./min, and 3 ° C./min to More preferably, it is 35 ° C./min.

(Molding process)
FIG. 4 is a diagram mainly showing an apparatus configuration for performing the molding process and the cutting process. The molding apparatus 300 includes a molding furnace 340 and a slow cooling furnace 350.
The forming furnace 340 and the slow cooling furnace 350 are configured to be surrounded by a furnace wall (not shown) made of a refractory material such as a refractory brick. The forming furnace 340 is provided vertically above the slow cooling furnace 350. In a furnace internal space surrounded by the furnace walls of the forming furnace 340 and the slow cooling furnace 350, a formed body 310, an atmosphere partition member 320, a cooling roller 330, and conveying rollers 350a to 350d are provided.
The molded body 310 forms the molten glass MG flowing from the melting device 200 through the glass supply pipe 206 shown in FIG. The molten glass supplied to the molded body 310 has a temperature at which log η = 4.3 to 5.7 with respect to the viscosity η (poise). Although the temperature of this molten glass MG changes with kinds of glass, if it is glass for liquid crystal displays, it is 1200-1300 degreeC, for example. Thereby, in the shaping | molding apparatus 300, the flow of the sheet glass G of the vertically downward direction is made. The molded body 310 is a long and narrow structure formed of firebrick or the like, and has a wedge-shaped cross section as shown in FIG. A supply groove 312 serving as a flow path for guiding the molten glass is provided in the upper part of the molded body 310. The supply groove 312 is connected to the glass supply pipe 206 at a supply port provided in the molding apparatus 300, and the molten glass MG flowing through the glass supply pipe 206 flows along the supply groove 312.
The molten glass overflowing from the supply groove 312 flows down along the vertical wall surface and the inclined wall surface of the side wall on both sides of the molded body 310. The molten glass that has flowed through the side walls merges at the lower end 313 of the molded body 310 shown in FIG.

(Glass composition)
The glass plate manufactured by the manufacturing method of the glass plate of this embodiment is used suitably for the glass substrate for flat panel displays. For example, any component of Li ₂ O, Na ₂ O, and K ₂ O is not substantially contained, or at least one component of Li ₂ O, Na ₂ O, and K ₂ O is present Even if it is contained, the total content of the components contained in Li ₂ O, Na ₂ O, and K ₂ O has a glass composition of 2% by mass or less, so that the effect of the present embodiment is efficient. It is preferable in that it performs well. The glass composition is preferably exemplified as follows.
(A) SiO ₂ : 50 to 70% by mass,
(B) B ₂ O ₃ : 5 to 18% by mass,
(C) Al ₂ O ₃ : 10 to 25% by mass,
(D) MgO: 0 to 10% by mass,
(E) CaO: 0 to 20% by mass,
(F) SrO: 0 to 20% by mass,
(G) BaO: 0 to 10% by mass,
(H) RO: 5 to 20% by mass (wherein R is at least one selected from Mg, Ca, Sr and Ba, and RO is the total of components contained in MgO, CaO, SrO and BaO),
(I) R ′ ₂ O: more than 0.1% by mass and 2.0% by mass or less (where R ′ is at least one selected from Li, Na and K, and R ′ ₂ O is Li ₂ O, Na ₂ O and the sum of the components contained in K ₂ O),
_{(J) SnO 2, Fe 2} O 3 and 0.05 to 1.5 wt% of at least one metal oxide in total selected from and cerium oxide.
The compositions (i) and (j) are not essential, but the compositions (i) and (j) can be included. The glass is substantially free of As ₂ O ₃ and PbO and contains SnO ₂ . From the viewpoint of environmental problems, it is preferable that Sb ₂ O ₃ is not substantially contained.
Moreover, the content of R ′ ₂ O in (i) may be 0% by mass.

In addition to the components described above, the glass plate of this embodiment may contain various other oxides to adjust various physical properties, melting, fining, and forming properties of the glass. Examples of such other oxides include, but are not limited to, TiO ₂ , MnO, ZnO, Nb ₂ O ₅ , MoO ₃ , Ta ₂ O ₅ , WO ₃ , Y ₂ O ₃ , and La _2. O ₃ is mentioned.
In the present embodiment, since SnO ₂ is a component for devitrified glass, in order not to cause devitrification while improving the clarity, the content of 0.01 to 0.5 mass% It is preferable that it is 0.05-0.3 mass%, and it is more preferable that it is 0.1-0.3 mass%. Fe ₂ O ₃ is a component that enhances infrared absorption of glass, and defoaming can be promoted by including Fe ₂ O ₃ . However, Fe ₂ O ₃ is a component that lowers the transmittance of glass. Therefore, if the content of Fe ₂ O ₃ is too large, the unsuitable for a glass substrate for a display. From the above, when containing Fe ₂ O ₃ in the metal oxide, the Fe ₂ O _3, from the viewpoint of suppressing the decrease in transmittance of the glass while improving the clarity, 0.01 is the amount The content is preferably 0.1% by mass, and more preferably 0.01 to 0.08% by mass. Further, from the viewpoint of improving the clarity and completing the defoaming process in a short time, and also suppressing the generation of SO ₂ foam in the absorption process, 0.01 to 0.5% by mass of SnO ₂ and 0.01 to It is preferable to use in combination with 0.1% by mass of Fe ₂ O ₃ .

In addition, since R ′ ₂ O of (i) is a component that may be eluted from the glass to deteriorate the characteristics of the TFT, and increase the thermal expansion coefficient of the glass to damage the substrate during the heat treatment. When applied as a glass substrate for a liquid crystal display or a glass substrate for an organic EL display, it is preferably substantially not contained. However, by deliberately containing the above-mentioned components in the glass, the basicity of the glass is increased while the thermal expansion of the glass is suppressed within a certain range without deteriorating the characteristics of the TFT, and the valence is increased. It is possible to facilitate the oxidation of the fluctuating metal and exhibit clarity. Further, R ′ ₂ O can lower the electrical specific resistance of the glass and improve the meltability. Therefore, the content of R ′ ₂ O is preferably 0 to 2.0% by mass, more preferably more than 0.1% by mass and 1.0% by mass or less, and more preferably 0.2 to 0.5%. More preferred is mass%. Incidentally, Li 2 _O, without Na 2 _O is allowed to contain, in the component, it is preferable to most glass eluted from contain hardly K _{2 O} to produce a deterioration of the characteristics of the TFT. The content of K ₂ O is preferably 0 to 2.0% by mass, more preferably 0.1 to 1.0% by mass, and further preferably 0.2 to 0.5% by mass.

In order to obtain the characteristics such that the glass plate of the present embodiment is suitably used as a glass substrate used in a liquid crystal display, an organic EL display or the like, the viscosity at the refining temperature of the molten glass MG contains a large amount of alkali. Since it becomes high compared with a glass plate etc., a bubble floating speed tends to become slow in a defoaming process. In particular, since the glass substrate used for the low-temperature polysilicon / TFT is required to have a high strain point, the viscosity at the refining temperature of the molten glass MG is further increased. For this reason, for example, when producing a glass having a strain point of 680 ° C. or higher, particularly a strain point of 690 ° C. or higher, the bubble rising speed tends to be further slowed in the defoaming treatment. When the glass plate of this embodiment is a glass substrate used for a liquid crystal display, an organic EL display, etc., it is preferable that the viscosity in 1630 degreeC is 130-350 poise, for example. The glass constituting the glass substrate preferably has a glass temperature of 1550 ° C. to 1680 ° C. when the glass viscosity is 10 ^2.5 dP · s, and is in the range of 1570 ° C. to 1680 ° C. The effect of the present invention becomes remarkable, and the effect of the present invention becomes more remarkable when it is in the range of 1590 ° C to 1680 ° C.

(Temperature history of molten glass)
FIG. 5 is a diagram illustrating an example of a temperature history from the melting process to the molding process in the present embodiment.
The glass raw material used for manufacture of the glass plate of this embodiment measures various raw materials so that it may become a target chemical composition, and mixes well, and a glass raw material is made. At that time, SnO ₂ is added to the glass raw material in a predetermined amount as a fining agent. The glass raw material containing SnO ₂ produced in this way is put into the melting tank 201 and melted at least by energization heating, whereby a molten glass MG is produced. The glass raw material thrown into the melting tank 201 is decomposed when the decomposition temperature of the component is reached, and becomes a molten glass MG by vitrification reaction. While the molten glass MG flows through the melting tank 201, it gradually proceeds to the first clarification tank 204 (glass supply pipe 204) from near the bottom of the melting tank 201 while gradually raising the temperature.
For this reason, in the melting tank 201, the temperature at which the temperature of the molten glass MG rises gently from the temperature T1 when the glass raw material is charged to the temperature T3 when entering the first clarification tank 204 (glass supply pipe 204). Have a history. In FIG. 5, T1 <T2 <T3, but T2 = T3 or T2> T3 may be used, and at least T1 <T3. In addition, the temperature (T3) of the molten glass in the 1st clarification tank 204 entrance immediately after a melting process is 1580 degreeC, for example, and is the range of 1560-1620 degreeC.

By passing a constant current between a metal flange (not shown) of the first clarification tank 204 and a metal flange (not shown) of the second clarification tank 202, the platinum or platinum alloy tube of the first clarification tank 204 is energized and heated. Furthermore, a constant current is passed between a metal flange (not shown) of the second clarification tank 202 and another metal flange (not shown) of the second clarification tank 202 to energize platinum or a platinum alloy in the second clarification tank 202. By heating, the molten glass MG that has entered the first clarification tank 204 is heated to a temperature T4 at which SnO ₂ releases oxygen from the temperature T3 (for example, 1630 ° C. or higher, more preferably 1630 to 1700 ° C., 1640 To 1680 ° C.), and the molten glass MG entering the second clarification tank 202 is further heated to a temperature T4. Substantially maintained at the same temperature T5 and Luo temperature T4. That is, temperature T3 <temperature T4. In addition, in this embodiment, although the temperature control in temperature T3-temperature T5 uses the system which energizes and heats each clarification tank, it is not limited to this system. For example, the temperature adjustment may be performed using indirect heating by a heater (not shown) arranged around each clarification tank.

At this time, the molten glass MG is heated to 1630 ° C. or more, whereby the reduction reaction of SnO ₂ that is a fining agent is promoted. Thereby, a large amount of oxygen is released into the molten glass MG. In the existing bubble B in the molten glass MG, oxygen released by the reductive reaction of the clarifier is added to the expansion of the bubble diameter due to the effect of increasing the pressure of the gas component in the bubble B caused by the temperature increase of the molten glass MG. The diffusion of the bubbles into the bubbles B overlaps, and this synergistic effect increases the bubble diameter.
The bubble B having an enlarged bubble diameter has a faster rising speed of the bubble B according to Stokes' law, and the rising and breaking of the bubble B are promoted.
In the second clarification tank 202, the molten glass MG is continuously maintained at a high temperature of 1630 ° C. or higher, so that the bubbles B in the molten glass MG float on the liquid surface of the molten glass MG and break the bubbles on the liquid surface. Thereby, defoaming of molten glass MG is performed.

In FIG. 5, the defoaming process is performed during a period in which the temperature of the molten glass MG rises from the temperature T3 to the temperature T4 and is maintained at the temperature T5 that is substantially the same as the temperature T4. In FIG. 5, T4 and T5 are substantially the same, but T4 <T5 may be sufficient and T4> T5 may be sufficient.
In addition, although the temperature of the molten glass MG reached the temperature T4 has been described with reference to the example of the first clarification tank 204, it may be in the second clarification tank 202.
The first highest temperature of the molten glass when the molten glass MG flows through the first clarification tank 204 is equal to or more than the second highest temperature of the molten glass MG when flowing through the second clarification tank 202. High is preferred. Thereby, when the molten glass moves from the first clarification tank 204 to the second clarification tank 202, the temperature of the molten glass is sufficiently high and maintained above the temperature at which the reductive reaction of the clarifier occurs. 202 does not require heating for further raising the temperature of the molten glass. For this reason, the heating temperature of the 2nd clarification tank 202 can be restrained lower than before. Therefore, the volatilization of platinum is suppressed from the second clarification tank 202 made of platinum or a platinum alloy, and foreign matters such as platinum crystals adhering to the inner wall surface in the clarification tube due to the volatilization of platinum are mixed into the molten glass MG. A glass plate with few defects, that is, defects caused by the foreign matter can be manufactured. It is preferable that the temperature of the molten glass MG reaches the first maximum temperature while the molten glass MG flows through the first clarification tank 204. In this case, the heating temperature of the second clarification tank 202 is lower than when the molten glass reaches the first maximum temperature and the second maximum temperature at the connection position between the first clarification tank 204 and the second clarification tank 202. Therefore, volatilization of platinum can be more easily suppressed from the second clarification tank 202 composed of platinum or a platinum alloy.

Next, the molten glass MG that has advanced from the second clarification tank 202 to the third clarification tank 205 absorbs the remaining bubbles B, and therefore, from temperature T5 to temperature T6 (for example, 1600 ° C.), the temperature T7 (stirring) It is a temperature suitable for the process, and it is cooled to, for example, 1500 ° C. although it differs depending on the glass glass type and the type of the stirring device.
As the temperature of the molten glass MG decreases, the bubbles B do not float and defoam, and the pressure of the gas components in the small bubbles remaining on the molten glass MG also decreases, and the bubble diameter becomes smaller. Further, when the temperature of the molten glass MG becomes 1600 ° C. or less, a part of SnO (obtained by reduction of SnO ₂ ) absorbs oxygen and tries to return to SnO ₂ . For this reason, the oxygen in the bubble B which remain | survives in the molten glass MG is reabsorbed in the molten glass MG, and a small bubble becomes still smaller. The small bubbles are absorbed by the molten glass MG, and the small bubbles eventually disappear.
The treatment for absorbing O ₂ , which is a gas component in the bubbles B, by the oxidation reaction of SnO is an absorption treatment, and is performed during a period in which the temperature decreases from temperature T5 to temperature T7. In FIG. 5, the temperature decrease rate of the temperatures T5 to T6 is faster than the temperature decrease rate of the temperatures T6 to T7, but the temperature decrease rate of the temperatures T5 to T6 may be slower than the temperature decrease rate of the temperatures T6 to T7. , May be equivalent. At least during this absorption treatment, the temperature of the molten glass MG may be lowered in a temperature range of 1600 ° C. to 1500 ° C. at a temperature lowering rate of 2 ° C./min or more. However, the temperature T5 is reduced in that the temperature drop rate when the molten glass MG is in a higher temperature state is increased, the diffusion of SO ₂ described later is suppressed early, and the SO ₂ taken into the bubbles B is reduced. It is preferable that the temperature decrease rate of T6 is faster than the temperature decrease rate of temperatures T6 to T7.
In addition, the third temperature of the molten glass MG that flows into the stirring vessel 203 is decreased while the temperature decrease rate of the temperatures T6 to T7 is slower than the temperature decrease rate of the temperatures T5 to T6, thereby reducing SO ₂ taken into the bubbles B. The temperature difference between the outer surface portion and the center portion in the clarification tank 205 (glass supply tube 205) can be reduced.
In addition, from the point of improvement of productivity of a glass plate and reduction of equipment costs, in the absorption process, the molten glass MG is 1500 ° C. or less (specifically, from 1500 ° C. to the molten glass temperature when supplied to the forming process). It is preferable that the temperature decrease rate in a temperature range that is a range, for example, 1500 ° C. to 1300 ° C. is faster than the temperature decrease rate in a temperature range of 1600 ° C. to 1500 ° C. In addition, when performing temperature control of such molten glass MG, it is preferable to provide the flow volume adjustment apparatus which adjusts the quantity of molten glass MG supplied to a formation process.
In the absorption process, the amount of molten glass MG supplied to the molding process can be adjusted by controlling the temperature of the molten glass MG in the glass supply pipe 206 while reducing SO ₂ taken into the bubbles B. The temperature drop rate in the temperature range where the molten glass MG is 1500 ° C. or lower (specifically, the range from 1500 ° C. to the molten glass temperature when supplied to the forming process, for example, 1500 ° C. to 1300 ° C.) is 1600 It is preferable that it is slower than the rate of temperature decrease in the temperature range from 1C to 1500C. This makes it easy to adjust the amount of molten glass MG flowing into the forming step without processing the glass supply tube 206 into a special shape or providing a flow rate adjusting device other than the glass supply tube 206. Moreover, the temperature difference between the outer surface portion and the center portion in the glass supply pipe 206 of the molten glass MG flowing into the molding process can be reduced.

  After the absorption treatment or during the absorption treatment, the molten glass MG enters the stirring vessel 203. The stirring tank 203 homogenizes the molten glass MG by reducing the composition unevenness in the molten glass MG. In the stirring tank 203, the absorption process may be performed continuously. Thereafter, the molten glass MG is cooled to a temperature T8 suitable for molding in the molding step, for example, 1200 to 1300 ° C.

  As above-mentioned, the stirring process which stirs uniformly the component of molten glass MG is included between a refining process and a formation process. Between the clarification step and the molding step means that the timing at which the stirring step is started is between the timing at which the clarification step is started and the timing at which the molding step is started. The stirring step may be started in the middle of the clarification step, or may be started after the clarification step. In FIG. 1, the clarification step (ST2) and the homogenization step (ST3) are shown in the order of the timing of starting. In the melting step, the molten glass MG is supplied to the clarification step at a temperature T3 that is higher than the temperature T1 at the start of melting the molten glass MG. In the clarification process, the molten glass MG is supplied to the stirring process at a temperature lower than the temperature T7. In the stirring step, molten glass MG is supplied to the forming step at a temperature at which log η = 4.3 to 5.7 with respect to the viscosity η (poise). In the forming step, the temperature of the molten glass MG is, for example, 1200 to 1300 ° C., and the molten glass MG is formed into a sheet glass. In addition, it is preferable that the liquid phase viscosity of a glass plate is log (eta) = 4 or more, and it is preferable that the liquid phase temperature of a glass plate is 1050 to 1270 degreeC. By setting such a liquid phase viscosity and a liquid phase temperature, an overflow down draw method can be applied as a molding method.

As described above, in the foam absorption process performed after the defoaming process, the molten glass MG is cooled at a temperature decreasing rate of 2 ° C./min or more in the temperature range of 1600 ° C. to 1500 ° C. This is done for the reasons described below.
Since the temperature of the molten glass MG is increased from the temperature T3 to the temperature T4 and the temperature reaches the temperature T5, the molten glass MG is heated to 1600 to 1630 ° C. or higher, which is a temperature at which SnO ₂ releases oxygen and is reduced. In addition to promoting the incorporation of oxygen released by SnO ₂ into the bubbles B in the molten glass MG, the diffusion of O ₂ , CO ₂ and SO ₂ dissolved in the molten glass MG at a high temperature is promoted. O ₂ , CO ₂ and SO ₂ dissolved in the molten glass MG are also taken into the bubbles B. Although the solubility of the gas component in the molten glass MG varies depending on the glass component, in the case of SO ₂ , the glass having a high content of the alkali metal component has a relatively high solubility, but does not contain an alkali metal component, The glass plate used for the glass substrate for a liquid crystal display as in the present embodiment, which is contained in a small amount, has a low solubility that can be dissolved in the molten glass MG. In a glass plate used for a glass substrate for a liquid crystal display, an S (sulfur) component is not added artificially as a glass raw material. However, a combustion gas (natural gas, used as an impurity in the raw material or in the melting tank 201 is used. City gas, propane gas, etc.) are contained in trace amounts as impurities. For this reason, the S component contained as these impurities is oxidized to SO ₂ and diffuses into the bubbles B contained in the molten glass MG. Since SO ₂ is difficult to be reabsorbed, it remains as foam B. This phenomenon appears significantly more markedly than when conventional As ₂ O ₃ was used as a fining agent.
In the case of a glass composition using SnO ₂ as a fining agent, the longer the holding time of the molten glass MG at a high temperature, the more the diffusion of SO ₂ into the existing bubbles B in the molten glass MG is promoted. This is presumably because the diffusion rate of SO _{2 in} the molten glass MG increased as the temperature became high, and the bubble B easily entered.
If the temperature of the molten glass MG is kept at a high temperature of 1630 ° C. or higher, the molten glass MG is excessively reduced, and the following SO ₂ bubbles are likely to be generated when the temperature of the molten glass MG is lowered. Become. On the other hand, if the time for maintaining the temperature at 1630 ° C. or higher is too short, defoaming in the defoaming step becomes insufficient. For this reason, it is preferable that the time which hold | maintains the temperature of molten glass MG at 1630 degreeC or more is 15 minutes-90 minutes, and it is more preferable that it is 30 minutes-60 minutes.

Thereafter, when the temperature of the molten glass MG is lowered from the temperature T5 to the temperature T7, SnO obtained by the reduction of SnO ₂ attempts to oxidize by absorbing oxygen through an oxidation reaction. Therefore, O _{2 in the} bubbles B remaining in the molten glass MG is absorbed by SnO. However, the diffusion of SO ₂ and CO _{2 in} the molten glass MG into the existing bubbles B is still maintained. For this reason, the gas components in the bubbles B during the period from the temperature T5 to the temperature T7 have higher SO ₂ and CO ₂ concentrations than during the period from the temperature T3 to the temperature T5. In particular, the molten glass MG used in the present embodiment has a low alkali metal content, so that the solubility of SO _{2 in} the molten glass MG is small. Therefore, when the SO ₂ is captured once bubbles B as a gas, the SO ₂ is less likely to be absorbed in the molten glass MG in the absorption process.

As described above, in the period from the temperature T5 to the temperature T7, O ₂ in the bubbles B is absorbed by SnO by the oxidation reaction of SnO, but diffusion of SO ₂ and CO ₂ into the existing bubbles B is still maintained. Therefore, by making this period short, the diffusion of SO ₂ and CO ₂ into the existing bubbles B can be reduced, and the growth of the bubbles B can be suppressed. For this reason, during the period of the absorption treatment from the temperature T5 to the temperature T7, the molten glass MG is cooled at a rate of temperature decrease of 2 ° C./min or more in the temperature range of 1600 ° C. to 1500 ° C. The number of bubbles can be suppressed.

FIG. 6 is a diagram showing the measurement result of the content of SO ₂ contained in the pores reproducing the bubbles B in the glass, and shows the dependence of the SO ₂ content on the glass temperature condition and the temperature maintenance time. Show. The size of the black circle in FIG. 6 indicates the size of the bubble B, and the content of SO ₂ .
The glass has the same glass composition as the above-described glass substrate for a liquid crystal display with a low alkali metal content, and contains SnO ₂ as a fining agent. The measurement result of FIG. 6 is a result using the glass substrate for liquid crystal displays which has the following glass compositions specifically ,.
SiO ₂ : 60% by mass
Al ₂ O ₃ : 19.5% by mass
B ₂ O ₃ : 10% by mass
CaO: 5.3 mass%
SrO: 5% by mass
SnO ₂ : 0.2% by mass
Artificially pierced the molten glass of the glass composition on a glass plate and formed in a plate-shape, in an oxygen atmosphere glass plate perforated from both sides by interposing a glass plate of the glass composition of the same type, O ₂ The holes filled with were reproduced as bubbles B. The glass plate having the holes was heat-treated at various temperatures of 1200 ° C. or higher and the temperature maintaining time, and the SO ₂ content in the holes was measured by gas analysis. Since the glass plate is heated to 1200 ° C. or higher, the glass plate is in a molten state, and the bubbles B remaining in the molten glass can be reproduced.
According to FIG. 6, it can be seen that SO ₂ is contained in the pores filled with O ₂ at a temperature of about 1500 ° C. or higher. In particular, it can be seen that the SO ₂ content increases as the temperature increases and as the temperature maintenance time increases. This means that the diffusion of SO ₂ dissolved in the molten glass is promoted by the high temperature and taken into the holes.
Therefore, it is preferable that the molten glass MG is quickly cooled to less than 1500 ° C. in the absorption treatment after the defoaming treatment. In this embodiment, the molten glass MG is 2 ° C./2° C. in the temperature range of 1600 ° C. to 1500 ° C. The temperature is lowered at a rate of more than a minute.

FIG. 7 is a diagram showing a measurement result showing a relationship between a bubble level generated when a glass plate is produced in an experimental furnace simulating the temperature history of the molten glass MG shown in FIG. 5 and a cooling rate. The temperature decreasing rate is an average rate in a temperature range of 1600 ° C to 1500 ° C. The produced glass plate had the same glass composition as the glass substrate for liquid crystal displays with a low alkali metal content, and SnO ₂ was used as a fining agent. Specifically, a glass substrate for a liquid crystal display having the same glass composition as in FIG. 6 was used.
It can be seen that the bubble level rises sharply when the cooling rate is less than 2 ° C./min. In addition, a bubble level represents how much the number of bubbles deteriorates on the basis of the number of bubbles per unit glass mass when the temperature decrease rate is 10 ° C./min. For example, a foam level of 3.0 means a number of bubbles that is three times the number of bubbles when the cooling rate is 10 ° C./min.
According to FIG. 7, it can be seen that the temperature lowering rate should be 2 ° C./min or higher in order to lower the bubble level.

(Example)
FIG. 8 is a diagram showing measurement results showing the relationship between the number of bubbles present in the glass plate and the temperature lowering rate when the glass plate is manufactured using an apparatus for manufacturing the glass plate. After passing through the melting step, the clarification step, and the stirring step, a glass substrate was produced by the overflow down draw method. At this time, the temperature history of the molten glass MG took the history shown in FIG. The temperature decrease rate is an average rate in a temperature range of 1600 ° C to 1500 ° C.
The produced glass plate had the same glass composition as the glass substrate for liquid crystal displays with a low alkali metal content, and SnO ₂ was used as a fining agent. Specifically, a glass substrate for a liquid crystal display having the same glass composition as in FIG. 6 was used. The bubble level shown in FIG. 8 represents how much the number of bubbles deteriorates on the basis of the number of bubbles per unit mass when the cooling rate is 8.4 ° C./min. For example, a bubble level of 5.0 means that the number of bubbles is 5 times the number of bubbles when the cooling rate is 8.4 ° C./min. The bubble level with a temperature drop rate of 7.9 ° C./min is 1.1, the bubble level with a temperature drop rate of 4.9 ° C./min is 1.6, and the bubble level with a temperature drop rate of 4.2 ° C./min Was 1.8, and the bubble level at a cooling rate of 3.0 ° C./min was 1.8. On the other hand, when the cooling rate is 1.8 ° C./min, the bubble level is 3.0, the cooling rate is 0.5 ° C./min, the bubble level is 83, and the cooling rate is 8.4 ° C./min. The number of bubbles was more than 3 times the number of bubbles.
According to FIG. 8, it can be seen that the number of bubbles rapidly increases when the temperature lowering rate is less than 2 ° C./min. Therefore, it can be seen that the number of bubbles is reduced when the temperature of the molten glass MG is lowered at a temperature lowering rate of 2 ° C./min or more, preferably 2.5 ° C./min or more in the temperature range of 1600 ° C. to 1500 ° C. From FIG. 8, it can be seen that, for example, the temperature drop rate is more effective in reducing the number of bubbles at 3 ° C./min to 8 ° C./min.
Incidentally, SiO _2: 60 wt%, Al ₂ O _3: 19.5 wt%, B ₂ O _3: 10 wt%, CaO: 5.3 wt%, SrO: 5 wt%, SnO _2: 0.15 mass %, Fe ₂ O ₃ : 0.05% by mass, although the number of bubbles was reduced by a small amount as a whole, almost the same result was obtained. Further, _SiO 2: 61 _{wt%, Al} 2 O 3: 19.5 _{wt%, B 2} O 3: 10 wt%, CaO: 9 _mass%, SnO 2: 0.3 _{wt%, R} 2 O _(R is , Li, Na, K, all components contained in the glass plate): In the production of a glass plate (strain point 700 ° C.) having 0.2% by mass, the same result as above was obtained.

As described above, according to the present embodiment, since the number of SO ₂ bubbles in the molten glass can be reduced, the bubbles that are the core of cavitation generated by the rotation of the stirring blade in the stirring step can also be reduced. As a result, the glass plate The number of bubbles inside can be reduced. This effect becomes more prominent in the method for producing a glass substrate having a low BaO or SrO content as a glass composition.
More specifically, MgO, CaO, SrO, and BaO contained as a glass composition are often added to the raw material as carbonates, and the decomposition temperature is lowest for MgO and higher in the order of CaO, SrO, and BaO. Become. That is, the higher the decomposition temperature, the higher the temperature at which CO ₂ begins to be released. As apparent from the above, when the molten glass MG falls after the defoaming treatment, the higher the decomposition temperature, the higher the CO ₂ begins to be absorbed. For example, BaO begins to absorb CO ₂ near 1300 ° C.
However, in the production of a glass substrate with a low BaO or SrO content in which CO ₂ absorption begins in a relatively high temperature range as a glass composition, CO ₂ absorption occurs after the temperature of the molten glass MG decreases, that is, melted. It begins after the viscosity of glass MG is increased. Here, CO ₂ diffuses faster in the molten glass MG when the viscosity of the molten glass MG is lower. Therefore, in the method for producing a glass substrate in which absorption of CO ₂ starts after the viscosity of the molten glass MG increases (after the temperature decreases), CO ₂ tends to remain in the molten glass MG as bubbles.
If SO ₂ existing as a gas component of bubbles in the molten glass can be reduced as in this embodiment, generation of bubbles serving as the core of cavitation is produced even in the production of a glass plate in which CO ₂ tends to remain as described above. As a result, the number of bubbles in the glass plate as the final product can be reduced. From the above, this embodiment is suitable for the production of a glass substrate having a BaO content of 0 to 1.0% by mass, and is further suitable for a method for producing a glass substrate that does not substantially contain BaO. In addition, this embodiment is suitable for producing a glass substrate having a SrO content of 0 to 3.0% by mass, and is further suitable for a method for producing a glass substrate that does not substantially contain SrO.

  As mentioned above, although the manufacturing method of the glass substrate of this invention was demonstrated in detail, this invention is not limited to the said embodiment, In the range which does not deviate from the main point of this invention, you may make various improvement and a change. Of course.

200 melting apparatus 201 melting tank 202 clarification tank (second clarification tank)
203 Stirring tank 203a Stirrer 204 Glass supply pipe (first clarification tank)
205 Glass supply pipe (third clarification tank)
206 Glass supply pipe 300 Molding device 310 Molded body 312 Supply groove 313 Lower end 320 Atmosphere partition member 330 Cooling rollers 350a to 350d Conveying roller 340 Molding furnace 350 Slow cooling furnace 400 Cutting device

One aspect of the present invention is a glass plate manufacturing method for manufacturing a glass plate. The method includes a melting step for producing a molten glass containing a fining agent, a defoaming treatment for raising the temperature of the molten glass and defoaming the molten glass by a reduction reaction of the fining agent, and the defoaming treatment. And a clarification step including an absorption treatment for lowering the temperature of the molten glass and absorbing bubbles contained in the molten glass by an oxidation reaction of the clarifier.
In the absorption treatment, the temperature drop rate of the molten glass is adjusted so as to prevent the oxidizing gas from being taken into the bubbles of the molten glass due to the diffusion of the oxidizing gas of carbon or sulfur component contained in the molten glass. .

At that time, the temperature drop rate of the molten glass is adjusted at least in a temperature range in which the oxidizing gas is taken into the bubbles contained in the molten glass due to diffusion of the oxidizing gas of carbon or sulfur component contained in the molten glass. It is preferable that

  The clarification step is performed in at least a clarification tank and a glass supply pipe connected to a downstream end of the clarification tank in the flow direction of the molten glass, and diffusion of the oxidizing gas in the glass supply pipe Thus, it is preferable to adjust the temperature drop rate of the molten glass so as to prevent the oxidizing gas from being taken into the bubbles of the molten glass.
Moreover, it is preferable that the said clarification process is performed with a platinum tube or a platinum alloy tube, and the temperature of the said molten glass is adjusted by the direct current heating of the said platinum tube or the said platinum alloy tube.

The average temperature drop rate of the molten glass at the initial stage including the start of the absorption treatment is preferably higher than the average temperature drop rate of the molten glass from the initial stage to the end.
It is preferable to start homogenization of the glass component of the molten glass after the absorption treatment.
The fining agent preferably contains at least one of tin oxide and iron oxide.
The glass plate has a content of R ′ ₂ O of 0 to 2.0% by mass (R ′ ₂ O is a total of components contained in Li ₂ O, Na ₂ O and K ₂ O). preferable.
It is preferable that the sulfur component in the molten glass is a component derived from a glass raw material.
It is preferable that the melting step is performed using heating of a combustion gas by a flame, and the combustion gas contains a sulfur component.
The glass plate is preferably a glass substrate for liquid crystal display or a glass substrate for organic EL display.
The strain point of the glass plate is preferably 680 ° C. or higher.

  In the manufacturing method of the said glass plate containing each preferable form mentioned above, it is preferable that the temperature fall rate of the said molten glass adjusts the temperature of the said molten glass in the temperature range of 1500 degreeC or more. Moreover, it is preferable to melt | dissolve the glass raw material using an electrical heating and a flame in the said melting process. The temperature of the molten glass at the entrance of the first clarification tank where the clarification process is performed immediately after the melting process is preferably in the range of 1560 to 1620 ° C. Moreover, it is preferable to heat up the said molten glass which entered the 1st clarification tank in which the said clarification process is performed to 1640-1680 degreeC. The glass plate preferably includes at least one of MgO, CaO, SrO, and BaO, and at least one of the MgO, CaO, SrO, and BaO is preferably added to the glass raw material as a carbonate. Moreover, it is preferable that content of SrO in the said glass plate is 0-3.0 mass%.

Claims (8)

A glass plate manufacturing method for manufacturing a glass plate,
A melting step of melting a glass raw material containing SnO ₂ as a fining agent by at least electric heating to produce a molten glass;
The temperature of the molten glass is raised to 1630 ° C. or more to generate bubbles in the molten glass and defoaming, and after the defoaming treatment, the molten glass is heated from 1600 ° C. to 1500 ° C. A clarification step including an absorption treatment in which the molten glass absorbs bubbles in the molten glass by lowering the temperature at a temperature decreasing rate of 2 ° C./min or more in a temperature range of
And a forming step of forming the molten glass after the refining step into a plate-like glass.   2. The method for producing a glass sheet according to claim 1, wherein, in the absorption treatment, a temperature decrease rate in a temperature range where the molten glass is 1500 ° C. or less is faster than a temperature decrease rate in the temperature range of 1600 ° C. to 1500 ° C. 2.   The said shaping | molding process is a manufacturing method of the glass plate of Claim 1 or 2 which forms plate glass from the said molten glass by the overflow down draw method.   The manufacturing method of the glass plate of any one of Claims 1-3 whose viscosity of the said molten glass in the said 1630 degreeC is 130-350 poise. The clarification step is performed in a platinum or platinum alloy tube through which the molten glass flows.
The temperature rise of the molten glass in the clarification step is performed by controlling currents separately supplied to at least two different regions extending in the length direction of the platinum or platinum alloy tube. The manufacturing method of the glass plate of Claim 1. The glass plate has an R ′ ₂ O content of 0 to 2.0% by mass (R ′ ₂ O is the total of components contained in Li ₂ O, Na ₂ O and K ₂ O), The manufacturing method of the glass plate of any one of 1-5.   The said glass plate is a manufacturing method of the glass plate of any one of Claims 1-6 used for the glass substrate for flat panel displays. Between the clarification step and the forming step, including a stirring step of stirring the components of the molten glass homogeneously,
In the melting step, the molten glass is supplied to the clarification step at a temperature higher than the temperature at the start of melting of the molten glass,
In the clarification step, the molten glass is supplied to the stirring step at a temperature lower than the temperature after the defoaming treatment,
In the forming step, the molten glass is supplied at a temperature of log η = 4.3 to 5.7 with respect to the viscosity η (poise) of the molten glass, and is formed into a sheet glass. The manufacturing method of the glass plate of any one.

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