TWI462883B - Manufacture of glass plates - Google Patents

Description

Glass plate manufacturing method

The present invention relates to a method of manufacturing a glass sheet.

In a glass substrate used for a flat panel display such as a liquid crystal display or a plasma display (hereinafter referred to as "FPD"), a thin glass plate having a thickness of, for example, 0.5 to 0.7 mm is used. The glass substrate for FPD has a size of, for example, 300 × 400 mm in the first generation, but has a size of 2,850 × 3050 mm in the 10th generation.

In order to manufacture such a glass substrate for FPD of a larger size after the eighth generation, an overflow down draw method is most often used. The overflow down-draw method includes a step of forming a sheet glass under the molded body by overflowing the molten glass from the upper portion of the molded body in the forming furnace, and a step of slowly cooling the sheet glass in the quenching furnace. The quenching furnace is used to slowly cool the sheet glass by reducing the internal strain or heat shrinkage of the sheet glass by introducing the sheet glass into a desired thickness between the pair of rolls. Thereafter, the sheet glass is cut into a specific size and stored as a glass plate on another glass plate and stored. Or transport the glass plate to the next step.

A glass plate manufactured by such molding is used as a glass substrate of a liquid crystal display in which a semiconductor element is formed on a glass surface, but in order not to deteriorate the characteristics of the semiconductor element formed on the surface of the glass due to the glass composition of the glass substrate, It is preferred to use a glass plate which does not contain an alkali metal component at all or even contains a small amount.

However, if bubbles are present in the glass plate, the defects are caused. Therefore, the glass plate in which the bubbles are present is not suitable as a glass substrate for a flat panel display. Therefore, it is required that the bubbles do not remain in the glass plate. In particular, in glass substrates for liquid crystal displays or glass substrates for organic EL (Electro Luminescent) displays, the requirements for bubbles are strict.

However, in order to suppress the deterioration of the characteristics of the semiconductor element, the glass plate does not contain an alkali metal component, or even if it contains a small amount, the glass plate has a problem of high temperature viscosity compared with a glass plate containing a large amount of alkali metal such as soda lime glass. Higher, the bubbles are difficult to detach from the molten glass in the manufacturing.

From the standpoint of reducing environmental load, it is required to limit the use of As ₂ O _{3 which} is previously used with higher toxicity. Therefore, in recent years, in place of As ₂ O ₃ , SnO ₂ or Fe ₂ O _{3 having} a poorer clarification function than As ₂ O ₃ has been used as a clarifying agent. Since SnO ₂ or Fe ₂ O _{3 is} a cause of devitrification or coloration of the glass, it is not possible to add a large amount to the glass in order to secure a clarifying function equivalent to As ₂ O ₃ . Therefore, bubbles become more likely to remain in the glass plate as the final product.

In order to improve the defoaming effect, a technique for producing a glass substrate in which the temperature of the alkali-free glass produced by the vitrification reaction at 1300 to 1500 ° C is raised to, for example, 1650 ° C, is disclosed. The β-OH value of the molten glass is 0.485/mm or more (Patent Document 1).

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2005-97090

Here, for example, in the glass composition which does not contain an alkali metal component or contains a small amount, the degree of melting of SO ₂ which can be melted in the molten glass is small, and therefore, once the bubble of SO ₂ is generated, As a defect of the bubble in the glass plate as a final product, it becomes easy to remain.

However, in the technique described in Patent Document 1, there is a problem that the generation of SO ₂ bubbles after the clarification step cannot be sufficiently suppressed.

Accordingly, it is an object of the present invention to provide a method for producing a glass sheet which can efficiently reduce bubbles remaining in a glass sheet when manufacturing a glass sheet.

The first aspect of the present invention is a method for producing a glass plate for producing a glass plate. The method includes a melting step of melting a glass raw material containing SnO ₂ as a fining agent by at least electric heating to prepare a molten glass, and a clarification step comprising: a defoaming treatment, which heats the temperature of the molten glass to 1630 ° C or more, thereby forming bubbles in the molten glass to perform defoaming; and absorbing treatment, after the defoaming treatment, the molten glass is allowed to be 2 ° C / min or more in a temperature range of 1600 ° C to 1500 ° C The cooling rate is lowered to absorb bubbles in the molten glass, and a molding step is performed to form the molten glass after the clarification step into a sheet glass.

At this time, the content of SnO _{2 in} the produced glass plate is preferably 0.01 to 0.5% by mass. Further, it is preferred to use 0.01 to 0.5% by mass of SnO ₂ and 0.01 to 0.1% by mass of Fe ₂ O ₃ .

According to a second aspect of the present invention, in a method of producing a glass sheet according to the first aspect of the present invention, in the absorbing treatment, a temperature drop rate of the molten glass in a temperature range of 1500 ° C or lower is higher than the above 1600 ° C to 1500 ° C The temperature drop in the temperature range is faster.

According to a third aspect of the present invention, in the method of producing a glass sheet according to the first or second aspect of the present invention, in the temperature range of 1500 ° C or lower, the molten glass may be a platinum or a platinum alloy through which the molten glass is circulated. The flow rate of the molten glass is adjusted in the tube. In the above absorption treatment, the temperature drop rate of the molten glass in a temperature range of 1500 ° C or lower is slower than the temperature drop rate in the temperature range of 1600 ° C to 1500 ° C.

A method of producing a glass sheet according to any one of the first to third aspects of the present invention, wherein the forming step comprises forming a sheet glass from the molten glass by an overflow down-draw method.

A method of producing a glass sheet according to any one of the first to fourth aspects of the present invention, wherein the viscosity of the molten glass at 1630 ° C is 130 to 350 poise.

A method of producing a glass sheet according to any one of the first to fifth aspects of the present invention, wherein the clarification step is carried out in a platinum or platinum alloy tube through which the molten glass flows; The temperature rise of the molten glass is controlled by at least two different regions extending along the length of the platinum or platinum alloy tube The current flows in the domain.

A method of producing a glass sheet according to any one of the first to sixth aspects of the present invention, wherein the glass sheet has a R' ₂ O content of 0 to 2.0% by mass (R' ₂ O is the total of the components contained in Li ₂ O, Na ₂ O, and K ₂ O).

A method of producing a glass sheet according to any one of the first to seventh aspects of the present invention, wherein the glass sheet is used for a glass substrate for a flat panel display.

According to a ninth aspect of the invention, the method for producing a glass sheet according to any one of the first to eighth aspect of the invention, wherein the clarifying step and the forming step comprise uniformly stirring the components of the molten glass a stirring step of supplying the molten glass to the clarification step at a temperature higher than a temperature at which the melting of the molten glass is started in the melting step, and in the clarifying step, at a temperature higher than the temperature after the defoaming treatment The molten glass is supplied to the stirring step at a lower temperature, and the molten glass is supplied to the molten glass at a temperature at which the viscosity η (poise) of the molten glass becomes log η = 4.3 to 5.7 in the forming step, and is formed into a sheet glass. .

The method for producing a glass sheet of the above aspect can efficiently reduce bubbles remaining in the glass sheet.

Hereinafter, a method of producing the glass sheet of the present embodiment will be described.

(Overall summary of the manufacturing method of glass plate)

Fig. 1 is a view showing the steps of a method for producing a glass sheet of the embodiment.

The manufacturing method of the glass plate mainly includes a melting step (ST1), a clarification step (ST2), a homogenization step (ST3), a supply step (ST4), a forming step (ST5), a slow cooling step (ST6), and a cutting step (ST7). ). In addition to this, a grinding step, a grinding step, a washing step, an inspection step, a packing step, and the like are included, and a plurality of glass sheets stacked in the packing step are transported to the ordering party.

Fig. 2 is a view schematically showing a glass substrate manufacturing apparatus that performs a melting step (ST1) to a cutting step (ST7). As shown in FIG. 2, the apparatus mainly includes a melting device 200, a forming device 300, and a cutting device 400. The melting device 200 mainly includes a melting tank 201, a clarification tank 202, a stirring tank 203, and glass supply pipes 204, 205, and 206. Further, since the glass supply pipes 204 and 205 are pipes which flow the molten glass MG and have a clarifying function as described below, they are substantially clarification grooves. 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. Further, after the melting tank 201 is connected, 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 between the respective grooves of the molding apparatus 300 include platinum or platinum. Alloy tube. The first clarification tank 204 and the third clarification tank 205 are formed in a cylindrical shape or a groove shape.

In the melting step (ST1), the addition of SnO ₂ as a fining agent and fed to a glass material within the melting vessel 201, i.e., glass raw material containing SnO ₂ as a fining agent at least by the use of electric heating and melting of the electrode, whereby A molten glass is obtained. Further, in addition to the electric heating by the electrodes, the glass frit may be melted by a flame (not shown) to obtain the molten glass MG. In the case of performing the melting of the glass raw material by the electric heating and the flame, specifically, the glass raw material is dispersed and supplied to the liquid surface of the molten glass MG by a raw material input device (not shown). The glass raw material is heated and slowly melted by a gas phase which becomes a high temperature in the flame, and is melted in the molten glass MG. The molten glass MG is heated by electric heating. Further, in the melting step, or between the melting step and the clarification step, foaming with oxygen may be performed in the molten glass. Further, it is preferred that no foaming is performed at the beginning of the melting step. This is because, in the initial stage of the melting step (for example, the temperature at which the molten glass is less than 1540 ° C), when the molten glass MG is electrically heated in the melting tank 201, the resistance of the member such as the brick constituting the melting tank 201, the resistance of the glass Since it is larger, the electric current easily flows into a member such as a brick, and it is difficult to heat the electric power to the molten glass MG by the electrode.

The clarification step (ST2) is performed at least in the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205. In the clarification step, by heating the molten glass MG in the first clarification tank 204, the bubbles contained in the molten glass MG containing gas components such as O ₂ , CO ₂ or SO ₂ are absorbed by SnO as a clarifying agent. reduction of O ₂ to generate the ₂ grown and float to the liquid surface of the molten glass MG and release. Further, in the clarification step, the internal pressure of the gas component in the bubble is lowered due to the decrease in the temperature of the molten glass MG, and the SnO obtained by the reduction reaction of SnO ₂ is generated due to the decrease in the temperature of the molten glass MG. By the oxidation reaction, the gas component such as O ₂ remaining in the bubbles in the molten glass MG is reabsorbed into the molten glass MG, and the bubbles disappear. The oxidation reaction and the reduction reaction using the clarifying agent are carried out 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 of the clarification tanks is adjusted by any one of the following heating and cooling methods, or a combination of the methods: direct electric heating to energize the tube itself, or use in the first clarification tank 204 and the second clarification tank 202. The heaters around the third clarification tank 205 are heated by heating between the respective tanks, and further cooled by air-cooled and water-cooled coolers to the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205. Blowing, or water spray, etc. In addition, in FIG. 2, the tank which clarified is divided into the three parts of the 1st clarification tank 204, the 2nd clarification tank 202, and the 3rd clarification tank 205, and it can carry out further refinement.

In the adjustment of the temperature of the molten glass MG of the present embodiment, direct electric heating as one of the above methods is employed. Specifically, the metal flange (not shown) provided in the first clarification tank 204 for supplying the molten glass MG to the second clarification tank 202 is made of metal (not shown) provided in the second clarification tank 202. An electric current (arrow in FIG. 3) flows between the flanges, and a metal flange (not shown) provided in the second clarification tank 202 is disposed downstream of the molten glass MG with respect to the metal flange. An electric current (arrow in FIG. 3) flows between the metal flanges (not shown) in the second clarification tank 202 on the side, thereby adjusting the temperature of the molten glass MG. In the present embodiment, the first clarification tank 204 and the second clarification tank 202 are electrically heated by flowing a fixed current between the first region between the metal flanges and the second region between the metal flanges. Therefore, the temperature of the molten glass MG is adjusted, but the electric heating is not limited to the temperature adjustment by the electric heating of the two regions, and the electric heating may be performed in one region or the electric heating may be performed in three or more regions. From The temperature of the molten glass MG is adjusted.

In the homogenization step (ST3), the molten glass MG in the stirring tank 203 supplied through the third clarification tank 205 is stirred by the agitator 203a, whereby the glass component is homogenized. Two or more stirring tanks 203 may be provided.

In the supply step (ST4), the molten glass is supplied to the molding apparatus 300 through the glass supply tube 206.

In the molding apparatus 300, a molding step (ST5) and a slow cooling step (ST6) are performed.

In the molding step (ST5), the molten glass MG is formed into a sheet glass G, and the flow direction of the sheet glass G is produced. In the present embodiment, an overflow down-draw method using the molded body 310 described below is used. In the slow cooling step (ST6), the sheet glass G which has been formed to flow is cooled so as not to generate internal strain.

In the cutting step (ST7), in the cutting device 400, the sheet glass G supplied from the forming device 300 is cut into a specific length, thereby obtaining a glass plate. The cut glass sheet is then cut to a specific size to produce a glass sheet of a target size. Thereafter, the end surface of the glass is ground, polished, and cleaned with a glass plate. Further, after checking for defects such as bubbles, the glass plate of the inspected product is packaged as a final product.

(clarification step)

Fig. 3 is a view mainly showing the configuration of a device for performing a clarification step. The clarification step includes a defoaming step and an absorbing step. In the defoaming step, the molten glass MG is heated to 1630 ° C or higher, and the SnO ₂ as a clarifying agent releases oxygen, and the oxygen is absorbed into the existing bubble B of the molten glass MG to expand the bubble diameter of the existing bubble B. . As a result, the increase in the bubble diameter caused by the increase in the internal pressure of the gas component in the bubble B due to the increase in the temperature of the molten glass MG causes a decrease in the viscosity of the molten glass MG due to the temperature rise of the molten glass MG. The synergistic effect is that the floating speed of the bubble B is increased to promote defoaming.

In the absorption treatment, the oxygen in the bubble B in the molten glass MG is again absorbed into the molten glass MG by lowering the temperature of the molten glass MG by the opposite of the defoaming treatment, and the temperature by the molten glass MG is lowered. On the other hand, the synergistic effect of lowering the internal pressure of the gas component in the bubble B reduces the bubble diameter and causes the bubble B to disappear in the molten glass MG. Further, in the absorption step, the molten glass MG is cooled at a temperature drop rate of 2 ° C / min or more in a temperature range of 1600 ° C to 1500 ° C.

The first clarification tank 204, the second clarification tank 202, and the third clarification tank 205 are means for defoaming the molten glass MG and absorbing the bubbles B by applying the above-described temperature history to the molten glass MG. Therefore, there is a temperature adjustment function capable of heating and cooling the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205 to a 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 any one of the following methods or a combination of the above methods: direct energization heating for energizing each clarification tank itself, or utilizing The clarification tanks of the heaters (not shown) disposed around the respective tanks are heated to each other, and further cooled by air-cooled or water-cooled coolers, and blown to each of the clarification tanks, water spray, or the like.

According to Figure 3, the clarification is explained in more detail.

The liquid molten glass MG which is melted in the melting tank 201 and contains a large number of bubbles B formed by the decomposition reaction of the glass raw material is introduced into the first clarification tank 204.

In the first clarification tank 204, the molten glass MG is heated to 1630 ° C or higher by heating of the platinum or platinum alloy tube as the main body of the first clarification tank 204, and a large amount of the clarifying agent is promoted by a reduction reaction. Oxygen is released into the molten glass MG. The existing bubble B in the molten glass MG is enlarged by the bubble diameter which is caused by the increase in the pressure of the gas component in the bubble B due to the temperature rise of the molten glass MG, and the oxygen diffusion released by the reduction reaction of the clarifying agent And entering the bubble B, the bubble diameter of the existing bubble B is expanded by this synergistic effect. Further, the first clarification tank 204 has a smaller cross section than the second clarification tank 202, and the upper open space different from the second clarification tank 202 does not have a gas phase environment space. Therefore, in other words, the first clarification tank 204 is melted. Since the glass MG is filled and flows through the entire inner cross section of the first clarification tank 204, the temperature of the molten glass MG can be efficiently increased as compared with the second clarification tank 202. In other words, in the first clarification tank 204, the temperature of the molten glass MG is raised to 1630 ° C or higher, and the temperature of the molten glass MG is raised to 1630 ° C or higher in the second clarification tank 202, whereby the second clarification tank 202 can be lowered. Since the heating temperature is high, it is preferable from the viewpoint of suppressing volatilization or melt loss of the platinum alloy constituting the second clarification tank 202.

Then, the molten glass MG is introduced into the second clarification tank 202.

The second clarification tank 202 is different from the first clarification tank 204 in that the upper open space inside the second clarification tank 202 is an environmental space in the gas phase, and the bubbles B in the molten glass MG can be floated to the liquid surface of the molten glass MG and released. To the outside of the molten glass MG.

In the second clarification tank 202, the molten glass MG is maintained at a high temperature of 1630 ° C or higher by heating of the platinum or platinum alloy tube as the main body of the second clarification tank 202, and the bubble B in the molten glass MG is second. The clarification tank 202 floats upward to break the surface of the molten glass MG, whereby the molten glass MG is defoamed. In particular, when the molten glass MG is heated to 1630 ° C or higher (for example, 1630 to 1700 ° C), the SnO ₂ accelerates the reduction reaction. In this case, for example, in the case of producing a glass plate for a flat panel display such as a liquid crystal display, the viscosity of the glass becomes suitable for the floating and defoaming viscosity (200 to 800 poise) of the bubble B due to the increase in the temperature of the molten glass MG.

Here, the gas component which is broken and released in the upper opening space above the second clarification tank 202 is released from the gas discharge port (not shown) to the outside of the second clarification tank 202. In the second clarification tank 202, the molten glass MG which removes the bubble B having a large diameter and which has a large floating speed by the floating and defoaming of the bubble B is introduced into the third clarification tank 205.

In the present embodiment, for example, as shown in FIG. 3, the second clarification tank 202 to the third clarification tank 205 may be controlled by two along the length direction of the platinum or platinum alloy tube constituting the main body. The temperature of the molten glass MG is increased by the current flowing in each of the different regions. Further, the temperature of the molten glass MG may be raised by controlling a current flowing in each of three or more different regions extending in the longitudinal direction of the platinum or platinum alloy tube constituting the body of the clarification tank.

As described above, the temperature rise of the molten glass MG is preferably controlled by controlling the current flowing through at least two regions different from each other in the clarification tank, and the defoaming treatment efficiency is preferably performed.

In the third clarification tank 205, the molten glass MG is cooled by cooling the platinum or platinum alloy tube as the main body of the third clarification tank 205 or by suppressing the heating of the third clarification tank 205. Since the temperature of the molten glass MG is lowered by the cooling, the floating and defoaming of the bubbles B are not performed, and the pressure of the gas component in the small remaining bubbles B is lowered, and the bubble diameter is gradually reduced. Further, when the temperature of the molten glass MG is 1600 ° C or lower, one part of SnO obtained by the reduction reaction of SnO ₂ in the defoaming treatment absorbs oxygen, and is restored to SnO ₂ . Therefore, oxygen which is a gas component in the bubble B is reabsorbed into the molten glass MG, and the bubble B becomes smaller and smaller, and is absorbed into the molten glass MG and eventually disappears. At this time, the molten glass MG is cooled at a temperature of 1600 ° C to 1500 ° C at an average temperature of 2 ° C / min or more, more preferably 2.5 ° C / min or more. Further, since the third clarification tank 205 has a smaller cross section than the second clarification tank 202, the molten glass MG can be cooled more efficiently than the second clarification tank 202. In other words, it is preferable that the temperature of the molten glass MG is cooled in the third clarification tank 205 to accelerate the temperature drop rate as compared with the temperature at which the molten glass MG is cooled in the second clarification tank 202.

In the example shown in FIG. 3, the clarification tank for performing the clarification step is divided into three portions of the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205. Of course, the clarification tank can be further refined. The refining and clarifying tank can perform temperature adjustment of the molten glass MG in more detail. In particular, it is advantageous to refine the clarification tank in such a manner that it is easy to adjust the temperature when changing the type or the amount of melting of the molten glass MG.

Further, in the above description, in order to simplify, the molten glass MG is heated to 1630 ° C in the first clarification tank 204, and the molten glass is melted in the second clarification tank 202. In the third clarification tank 205, the molten glass MG is immersed in the third clarification tank 205, and the bubble B is absorbed by the cooling of the molten glass MG, and the function of each clarification tank is described. However, the function may not be described. Fully functionalized for each clarification tank. The portion to the middle of the longitudinal direction of the second clarification tank 202 may be configured to increase the temperature of the molten glass MG, or may be used to make the molten glass halfway between the longitudinal direction of the second clarification tank 202 and the third clarification tank 205. The MG's cooling begins with a part of the way.

In the present embodiment, 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 that the molten glass MG does not contact is measured, and temperature control is performed. Manage the heating rate and cooling rate of the molten glass MG. The surface temperature of the first clarification tank 204, the second clarification tank 202, and the third clarification tank 205 can be calculated in advance by the computer simulation using the flow rate and temperature conditions of the molten glass MG supplied to the clarification tank. The relationship between the average temperature of the molten glass MG flowing through the groove 204, the second clarification tank 202, and the third clarification tank 205 (the average value of the temperature of the molten glass MG having a temperature distribution in the clarification tank). Therefore, the temperature increase rate and the temperature decrease rate can be calculated from the measured surface temperature on the outer side of the clarification tank, and the temperature increase rate and the temperature decrease rate can be managed. Further, the flow rate of the molten glass MG can be calculated from the volume of each device and the amount of molten glass MG per unit time flowing into the molding apparatus 300. Further, the temperature of the molten glass MG can be calculated from the viscosity and thermal conductivity of the glass.

After the defoaming treatment, the temperature of the molten glass MG is lowered in a temperature range of 1600 ° C to 1500 ° C at a temperature decreasing rate of 2 ° C / min or more, so as to remain in the glass plate as a final product as described below. Inside The number of bubbles per unit mass is reduced. Here, the bubble B means a bubble having a volume equal to or larger than the volume of a predetermined bubble, for example, a volume of a bubble having a diameter of 20 μm.

Further, the faster the temperature drop rate is, the lower the number of bubbles remaining in the glass sheet can be reduced, but the reduction effect becomes smaller as the temperature drop rate increases. The above cooling rate is preferably 3 ° C / min or more. Further, the upper limit of the above-described cooling rate is not particularly set. However, in the case of manufacturing a glass plate industrially, the upper limit is 50 ° C / min for the following reasons.

In other words, when the temperature drop rate of the molten glass MG is too fast, the oxygen in the bubble B of the molten glass MG is prevented from being reabsorbed to the molten glass MG, and as a result, the bubble B itself in the molten glass MG may not be reduced. Moreover, the thermal conductivity of the glass is as small as 20 to 50 W/(m.K) even at high temperatures. Therefore, the rapid cooling of the molten glass MG can only be performed from the third one as long as no special method is adopted. When the outer side of the clarification tank 205 is cooled, 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 maintained at a high temperature. In other words, in the third clarification tank 205, the temperature difference between the outer surface portion and the center portion of the molten glass MG is increased. In this case, there is a problem that crystals are precipitated from the molten glass MG of the outer surface portion. In the third clarification tank 205, when the molten glass MG is stirred while the temperature difference between the surface portion and the center portion of the molten glass MG is increased, the glass having a large temperature difference is mixed. In addition to the generation of the bubble B, the composition of the glass tends to hinder the homogeneity. Moreover, in order to accelerate the cooling rate of the molten glass MG, it is necessary to increase the number from the third clarification tank 205. Since the heat is dissipated, it is necessary to reduce the thickness of the support member such as the support brick supporting the body of the platinum or platinum alloy tube of the third clarification tank 205. However, only the thickness of the support member is thinned, and the strength of the device is lowered. Therefore, in the case of manufacturing a glass plate industrially, it is not appropriate to accelerate the temperature drop rate of the molten glass MG only by causing the above problems.

From the above, it is understood that the upper limit of the temperature drop rate of the molten glass MG from 1600 ° C to 1500 ° C is preferably 50 ° C / min, more preferably 35 ° C / min. That is, in the present embodiment, the temperature drop rate is preferably 2 ° C / min to 50 ° C / min, more preferably 2.5 ° C / min to 50 ° C / min, and still more preferably 3 ° C / min ~ 35 ° C / min. .

(forming step)

Fig. 4 is a view mainly showing the configuration of a device for performing a forming step and a cutting step. The forming apparatus 300 includes a forming furnace 340 and a quenching furnace 350.

The forming furnace 340 and the quenching furnace 350 are formed by surrounding a furnace wall (not shown) including a refractory such as refractory brick. The forming furnace 340 is disposed vertically above the quenching furnace 350. The molded body 310, the environmental spacing member 320, the cooling roller 330, and the conveying rollers 350a to 350d are provided in the internal space of the furnace surrounded by the furnace walls of the forming furnace 340 and the quenching furnace 350.

In the molded body 310, the molten glass MG that has flowed in from the melting device 200 through the glass supply pipe 206 shown in FIG. 2 is formed into a sheet glass G. The molten glass supplied to the molded body 310 becomes a temperature at which the viscosity η (poise) becomes log η = 4.3 to 5.7. The temperature of the molten glass MG differs depending on the type of glass, but is, for example, 1200 to 1300 ° C in the case of glass for liquid crystal display. Thereby, the flow direction of the plate glass G under the vertical direction is produced in the molding apparatus 300. to make The body 310 is an elongated structure including a refractory brick or the like, and as shown in Fig. 4, the cross section is formed in a wedge shape. A supply groove 312 serving as a flow path for guiding the molten glass is provided on the upper portion of the molded body 310. The supply groove 312 is connected to the glass supply pipe 206 in a supply port provided in the molding apparatus 300, and the molten glass MG that has flowed in 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 walls on both sides of the formed body 310. The molten glass flowing through the side wall merges at the lower end portion 313 of the molded body 310 shown in FIG. 4 to form one sheet glass G.

(glass composition)

The glass plate produced by the method for producing a glass plate of the present embodiment is preferably used for a glass substrate for a flat panel display. For example, it has substantially no component of Li ₂ O, Na ₂ O, and K ₂ O, or contains at least one of Li ₂ O, Na ₂ O, and K ₂ O, Li ₂ O, Na. _When the total content of the components contained in ₂ O and K ₂ O is also 2% by mass or less, it is preferable in terms of the effect of the present embodiment. The glass composition is preferably exemplified below.

(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 (where R is selected from At least one of Mg, Ca, Sr, and Ba, and a total of components contained in the RO-based MgO, CaO, SrO, and BaO, and (i) R' ₂ O: more than 0.1% by mass and 2.0% by mass Hereinafter, (wherein R' is selected from at least one of Li, Na, and K, and R' ₂ O is a total of components contained in Li ₂ O, Na ₂ O, and K ₂ O), (j) The metal oxide of at least one selected from the group consisting of SnO ₂ , Fe ₂ O ₃ and cerium oxide is 0.05 to 1.5% by mass in total.

Further, the compositions of the above (i) and (j) are not essential, but may have the composition of (i) and (j). In the above glass, substantially no As ₂ O ₃ and PbO are contained, and SnO ₂ is contained. Further, from the viewpoint of environmental problems, it is preferred that substantially no Sb ₂ O ₃ is contained.

Further, the content of R' ₂ O of (i) may be 0% by mass.

In addition to the above components, the glass plate of the present embodiment may contain various other oxides in order to adjust various physical properties, melting, clarification, and molding properties of the glass. Examples of such other oxides are not limited to the following, but examples thereof include 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 .

Further, in the present embodiment, since SnO ₂ is a component which tends to devitrify the glass, the content thereof is preferably 0.01 to 0.5% by mass in order to improve the clarity and prevent devitrification. More preferably, it is 0.05 to 0.3% by mass, and further preferably 0.1 to 0.3% by mass. Fe ₂ O ₃ is a component that enhances infrared absorption of glass, and defoaming is promoted by containing Fe ₂ O ₃ . However, Fe ₂ O ₃ is a component that lowers the transmittance of glass. Therefore, when the content of Fe ₂ O ₃ is too large, it becomes unsuitable for a glass substrate for a display. As seen from the above, the case of Fe ₂ O ₃ contained in the above metal oxides, and to improve the clarification of suppressing decrease of the transmittance of the glass viewpoint of the Fe ₂ O ₃ content of preferably 0.01 to 0.1 The mass % is more preferably 0.01 to 0.08 mass%. Further, from the viewpoint of improving the clarity and completing the defoaming step in a shorter period of time and suppressing the generation of the SO ₂ bubbles in the absorption step, it is preferred to combine 0.01 to 0.5% by mass of SnO ₂ and 0.01 to 0.1% by mass. Used for Fe ₂ O ₃ .

In addition, the R' ₂ O of the above (i) is a component which is eluted from the glass to deteriorate the characteristics of the TFT, and the coefficient of thermal expansion of the glass is increased to break the substrate during the heat treatment. In the case of a glass substrate or a glass substrate for an organic EL display, it is preferably not substantially contained. However, since the glass contains a predetermined amount of the above-mentioned components, the thermal expansion of the glass can be suppressed within a fixed range without deteriorating the characteristics of the TFT, and the alkalinity of the glass can be increased to change the valence. Oxidation of the metal becomes easy and clarification is exhibited. Further, R' ₂ O lowers the specific resistance of the glass and improves the meltability. Therefore, the content of R' ₂ O is preferably 0 to 2.0% by mass, more preferably 0.1% by mass or more and 1.0% by mass or less, and still more preferably 0.2 to 0.5% by mass. Further, it is preferable to contain K ₂ O which is most difficult to be eluted from the glass and which deteriorates the characteristics of the TFT, which does not contain Li ₂ O or Na ₂ O. The content of K ₂ O is preferably 0 to 2.0% by mass, more preferably 0.1 to 1.0% by mass, still more preferably 0.2 to 0.5% by mass.

In order to obtain characteristics of a glass substrate which is preferably used for a liquid crystal display or an organic EL display, the glass plate of the present embodiment has a higher viscosity in a clarification temperature of the molten glass MG than a glass plate containing a large amount of alkali or the like. Therefore, the bubble floating speed is likely to be slow in the defoaming process. In particular, it is used for low temperature polycrystalline germanium. The glass substrate of the TFT requires a higher strain point, and therefore, the viscosity in the clarification temperature of the molten glass MG is further increased. Therefore, for example, in the case of producing a glass having a strain point of 680 ° C or higher, particularly a glass having a strain point of 690 ° C or higher, the floating speed of the bubbles in the defoaming treatment is likely to be further slowed down. When the glass plate of the present embodiment is used for a glass substrate such as a liquid crystal display or an organic EL display, for example, the viscosity at 1630 ° C is preferably 130 to 350 poise. Moreover, the glass viscosity of the glass constituting the glass substrate is 10 ^2.5 dP. The present invention is more preferably in the range of 1550 ° C to 1680 ° C, and the effect of the present invention is remarkable in the range of 1570 ° C to 1680 ° C. The present invention is in the range of 1590 ° C to 1680 ° C. The effect becomes more significant.

(temperature history of molten glass)

Fig. 5 is a view showing an example of the temperature history of the melting step to the forming step in the embodiment.

In order to obtain a target chemical composition, the glass raw materials used in the production of the glass sheet of the present embodiment are weighed and preferably mixed to obtain a glass raw material. At this time, a specific amount of SnO ₂ was added as a clarifying agent to the glass raw material. The glass raw material containing SnO ₂ thus produced is placed in the melting tank 201 and melted by at least electric heating to produce molten glass MG. The glass raw material charged into the melting tank 201 is decomposed when it reaches the decomposition temperature of its components, and becomes a molten glass MG by a vitrification reaction. The molten glass MG gradually increases in temperature while flowing through the melting tank 201, and advances toward the first clarification tank 204 (glass supply pipe 204) from the vicinity of the bottom of the melting tank 201.

Therefore, in the melting tank 201, the temperature of the molten glass MG has a gently rising temperature history from the temperature T1 at the time when the glass raw material is charged to the temperature T3 at the time point of entering the first clarifying tank 204 (glass supply pipe 204). . Furthermore, in FIG. 5, T1 < T2 < T3, but it may be T2 = T3 or T2 > T3, and at least T1 < T3. Further, the temperature (T3) of the molten glass in the inlet of the first clarification tank 204 immediately after the melting step is, for example, 1580 ° C, and is in the range of 1560 to 1620 ° C.

Platinum or platinum alloy tube of the first clarification tank 204 by a current which is fixed between the metal flange (not shown) of the first clarification tank 204 and the metal flange of the second clarification tank 202 (not shown) The electric heating is performed, and the second clarification is performed by the current flowing between the metal flange (not shown) of the second clarification tank 202 and the other metal flange of the second clarification tank 202 (not shown). The platinum or the platinum alloy of the groove 202 is electrically heated, whereby the molten glass MG entering the first clarification tank 204 is rapidly heated from the temperature T3 to a temperature T4 at which the SnO ₂ releases oxygen (for example, 1630 ° C or more, more preferably 1630 to 1700 ° C, more preferably 1640 to 1680 ° C), and further, the molten glass MG entering the second clarification tank 202 is maintained at a temperature T4 to a temperature T5 substantially equal to the temperature T4. That is, the temperature T3 < temperature T4. Further, the temperature adjustment in the temperature T3 to the temperature T5 is a method in which the respective clarification tanks are electrically heated by the present embodiment, but the method is not limited thereto. For example, the temperature adjustment may be performed by heating between heaters (not shown) disposed around the respective clarification tanks.

At this time, the reduction reaction of SnO ₂ as a fining agent is promoted by heating the molten glass MG to 1630 ° C or higher. Thereby, a large amount of oxygen is released into the molten glass MG. The existing bubble B in the molten glass MG is caused by an increase in the pressure of the gas component in the bubble B due to an increase in the temperature of the molten glass MG, and is released by the reduction reaction of the clarifying agent. Oxygen diffuses and enters the bubble B, and the bubble diameter is enlarged by the synergistic effect.

The bubble B after the bubble diameter is enlarged follows the Stokes law, and the floating speed of the bubble B is increased, thereby promoting the floating and breaking of the bubble B.

In the second clarification tank 202, the molten glass MG is also maintained at a high temperature of 1630 ° C or higher. Therefore, the bubbles B in the molten glass MG float to the surface of the molten glass MG and are broken on the surface of the liquid. The defoaming of the molten glass MG is performed.

The defoaming treatment is carried out in FIG. 5, and the temperature of the molten glass MG is raised from the temperature T3 to the temperature T4, and then maintained at a temperature T5 substantially the same as the temperature T4. In Fig. 5, T4 and T5 are substantially the same, but may be T4 < T5 or T4 > T5.

Further, although the example in which the temperature of the molten glass MG reaches the temperature T4 in the first clarification tank 204 has been described, the inside of the second clarification tank 202 may be used.

Moreover, it is preferable that the first highest temperature of the molten glass when the molten glass MG flows through the first clarification tank 204 is equal to or higher than the second highest temperature of the molten glass MG when flowing through the second clarification tank 202. . Therefore, when the molten glass moves from the first clarification tank 204 to the second clarification tank 202, the temperature of the molten glass is relatively high and is maintained at a temperature higher than the temperature at which the clarifying agent is subjected to the reduction reaction. Therefore, the second clarification tank 202 is not required to be used. Further heating the molten glass heat. Therefore, the heating temperature of the second clarification tank 202 can be suppressed lower than before. Therefore, it is possible to suppress the volatilization of platinum from the second clarification tank 202 containing platinum or a platinum alloy, and to produce a defect in which impurities such as platinum crystals adhering to the inner wall surface of the clarification tube are mixed with the molten glass MG due to volatilization of platinum. That is, a glass plate which is less defective in the above impurities. When the molten glass MG flows through the first clarification tank 204, the temperature of the molten glass MG preferably reaches the first maximum temperature. In this case, the heating temperature of the second clarification tank 202 is lower than the case where the molten glass reaches the first highest temperature and the second highest temperature at the connection position between the first clarification tank 204 and the second clarification tank 202. It is possible to more easily suppress the volatilization of platinum from the second clarification tank 202 containing platinum or a platinum alloy.

Then, since the molten glass MG entering the third clarification tank 205 from the second clarification tank 202 absorbs the remaining bubbles B, it is cooled from the temperature T5 to the temperature T7 via the temperature T6 (for example, 1600 ° C) (suitable for the stirring step) The temperature varies depending on the type of glass nitrate and the type of stirring device, for example, 1500 ° C).

When the temperature of the molten glass MG is lowered, the floating and defoaming of the bubbles B are not generated, and the pressure of the gas component remaining in the small bubbles in the molten glass MG is also lowered, and the bubble diameter is gradually reduced. Further, when the temperature of the molten glass MG is 1600 ° C or lower, oxygen is partially absorbed by one of SnO (which is obtained by reduction of SnO ₂ ), and is restored to SnO ₂ . Therefore, the oxygen in the remaining bubbles B in the molten glass MG is reabsorbed into the molten glass MG, and the vesicles are further reduced. The molten glass MG absorbs the vesicles, and the vesicles eventually disappear.

The treatment of absorbing O ₂ as a gas component in the bubble B by the oxidation reaction of SnO is an absorption treatment, and is performed while decreasing from the temperature T5 to the temperature T7 via the temperature T6. In Fig. 5, the temperature drop rate of temperature T5~T6 is faster than the temperature drop rate of T6~T7, but the temperature drop rate of temperature T5~T6 can be slower than the temperature drop rate of temperature T6~T7, and can be equal. At least during the 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 drop rate of 2 ° C / min or more. However, the molten glass MG is increased at a higher temperature of the cooling rate during the state, and the following early suppress the diffusion of SO ₂ of the aspect of the terms to be absorbed within the bubble B ₂ SO reduction, the temperature is preferably The cooling rate of T5~T6 is faster than the temperature drop of temperature T6~T7.

Further, by making the temperature drop rate of the temperature T6 to T7 slower than the temperature drop rate of the temperature T5 to T6, the SO ₂ absorbed into the bubble B can be reduced, and the molten glass MG flowing into the stirring tank 203 can be made. 3 The temperature difference between the outer side surface portion and the center portion in the clarification tank 205 (glass supply tube 205) becomes small.

Further, in terms of improvement in productivity of the glass sheet and reduction in equipment cost, in the absorption treatment, it is preferred that the molten glass MG is 1500 ° C or lower (specifically, from 1500 ° C to supply to the forming step). The temperature range in the range of the molten glass temperature, for example, 1500 ° C to 1300 ° C, is faster than the temperature drop in the temperature range of 1600 ° C to 1500 ° C. Further, in the case of performing temperature control of the molten glass MG, it is preferable to provide a flow rate adjusting device for adjusting the amount of the molten glass MG supplied to the molding step.

Further, the SO ₂ absorbed into the bubble B can be reduced, and the amount of the molten glass MG supplied to the forming step can be adjusted by the temperature management of the molten glass MG in the glass supply pipe 206. In the treatment, it is preferred that the molten glass MG has a cooling rate ratio in a temperature range of 1500 ° C or lower (specifically, a range from 1500 ° C to a molten glass temperature supplied to the forming step, for example, 1500 ° C to 1300 ° C). The temperature drop in the temperature range of 1600 ° C to 1500 ° C is slower. Thereby, it is not necessary to process the glass supply pipe 206 into a special shape, or to provide a flow rate adjustment device other than the glass supply pipe 206, and the amount of the molten glass MG which flows into a shaping|molding process can be adjusted easily. Further, the temperature difference between the outer surface portion and the center portion of the glass supply tube 206 flowing into the molten glass MG of the molding step can be made small.

After the above-described absorption treatment or during the absorption treatment, the molten glass MG enters the stirring tank 203. The agitation vessel 203 reduces the composition unevenness in the molten glass MG to homogenize the molten glass MG. Further, in the stirring tank 203, the above-described absorption treatment may be continued. Thereafter, the molten glass MG is cooled until it becomes a temperature T8 suitable for molding in the molding step, for example, 1200 to 1300 °C.

As described above, between the clarification step and the forming step, a stirring step of uniformly stirring the components of the molten glass MG is included. The term between the clarification step and the forming step means that the timing of the start of the stirring step is between the timing at which the clarification step starts and the timing at which the forming step starts. The stirring step can be initiated halfway through the clarification step or after the clarification step. Further, in Fig. 1, the clarification step (ST2) and the homogenization step (ST3) are expressed in the order of the start timing. In the melting step, the molten glass MG is supplied to the clarification step at a temperature T3 higher than the temperature T1 at the start of melting of the molten glass MG. In the clarification step, the molten glass is cooled at a temperature lower than the temperature T7 The MG is supplied to the stirring step. In the stirring step, the molten glass MG is supplied to the forming step at a temperature at which the viscosity η (poise) becomes log η = 4.3 to 5.7. In the molding step, the molten glass MG is formed into a sheet glass in a state where the temperature of the molten glass MG is, for example, 1200 to 1300 °C. Further, the liquid phase viscosity of the glass plate is preferably log η = 4 or more, and the liquidus temperature of the glass plate is preferably 1050 ° C to 1270 ° C. By setting such a liquid phase viscosity and a liquidus temperature, an overflow down-draw method can be applied as a forming method.

As described above, in the bubble absorption treatment performed after the defoaming treatment, the molten glass MG is cooled at a temperature drop rate of 2 ° C / min or more in a temperature range of 1600 ° C to 1500 ° C. This is done for the reasons described below.

While the molten glass MG is heated from the temperature T3 to the temperature T4 and reaches the temperature T5, the molten glass MG is heated to a temperature of 1600 to 1630 ° C or higher which is reduced by the release of oxygen from the SnO ₂ , and therefore, in addition to the promotion of the molten glass MG other than bubbles B absorbing oxygen SnO ₂ release, but we also promote to a high temperature and dissolved in O within the molten glass MG _2, CO _2, diffusion SO _2, the dissolved in O within the molten glass MG _2, CO _2, SO ₂ is also absorbed in the bubble B. Further, the degree of melting of the gas component in the molten glass MG varies depending on the glass component. However, in the case of SO ₂ , the degree of melting in the glass having a large content of the alkali metal component is high, but the alkali metal is not contained. The component or the glass plate used for the glass substrate for liquid crystal displays of this embodiment containing a small amount is melt|dissolved in the molten glass MG, and the melting degree is low. In the glass plate used for the glass substrate for liquid crystal display, the S (sulfur) component is not artificially added as a glass raw material, but the impurity in the raw material or the combustion gas used in the melting tank 201 (natural gas, urban Among gases, propane gas, and the like, it is contained in a trace amount as an impurity. Therefore, the S component contained as the impurities is oxidized to become SO ₂ and diffused into the bubbles B contained in the molten glass MG. Since SO _{2 is} hard to be reabsorbed, it remains as bubbles B. This phenomenon occurs very significantly compared to the previous use of As ₂ O ₃ as a fining agent.

In the case where SnO _{2 is} used as the glass composition of the clarifying agent, the longer the holding time in the high temperature of the molten glass MG, the more the SO _{2 is} promoted to diffuse into the existing bubbles B in the molten glass MG. This is considered to be because the diffusion rate in the molten glass MG of SO ₂ is increased at a high temperature, and it is easy to enter the bubble B.

In addition, when the temperature of the molten glass MG is maintained at a high temperature of 1630 ° C or higher for a long period of time, the molten glass MG is excessively reduced, and when the molten glass MG is cooled, the following SO ₂ bubbles are likely to be generated. On the other hand, if the time kept at 1630 ° C or more is too short, defoaming in the defoaming step becomes insufficient. Therefore, the time for maintaining the temperature of the molten glass MG at 1630 ° C or higher is preferably 15 minutes to 90 minutes, more preferably 30 minutes to 60 minutes.

Thereafter, when the temperature of the molten glass MG is lowered from the temperature T5 to the temperature T7, the SnO obtained by the reduction of SnO ₂ is oxidized by absorbing oxygen by the oxidation reaction. Therefore, O ₂ present in the bubble B remaining in the molten glass MG is absorbed by SnO. However, the diffusion of SO ₂ or CO ₂ in the molten glass MG into the existing bubble B is maintained. Therefore, the concentration of SO ₂ and CO ₂ in the gas component in the bubble B during the period from the temperature T5 to the temperature T7 is higher than the concentration of the SO ₂ and CO _{2 in} the period from the temperature T3 to the temperature T5. In particular, in the molten glass MG used in the present embodiment, since the composition of the alkali metal is small, the degree of melting in the molten glass MG of SO ₂ is small. Therefore, when SO ₂ is absorbed as a gas by the bubble B, the SO ₂ is hardly absorbed into the molten glass MG in the absorption process.

As described above, during the period from the temperature T5 to the temperature T7, the O _{2 in the} bubble B is absorbed by the SnO by the oxidation reaction of SnO, but the diffusion of the SO ₂ and the CO ₂ into the existing bubble B is maintained. When the period is 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. Therefore, during the absorption process from the temperature T5 to the temperature T7, the molten glass MG is cooled at a temperature drop rate of 2 ° C/min or more in a temperature range of 1600 ° C to 1500 ° C, whereby the glass plate can be suppressed as described below. The number of bubbles.

Fig. 6 is a graph showing the measurement results of the content of SO ₂ contained in the pores after the bubble B in the glass is reproduced, and shows the dependence on the temperature of the glass and the content of SO _{2 in} the temperature maintenance time. The size of the black dot in Fig. 6 indicates the size of the bubble B and indicates the content of SO ₂ .

The glass has the same glass composition as the above-described glass substrate for liquid crystal display having a small content of an alkali metal, and contains SnO ₂ as a clarifying agent. The measurement result of FIG. 6 is specifically a result of using a glass substrate for a liquid crystal display having the following glass composition.

SiO ₂ : 60% by mass

Al ₂ O ₃ : 19.5% by mass

B ₂ O ₃ : 10% by mass

CaO: 5.3% by mass

SrO: 5 mass%

SnO ₂ : 0.2% by mass

The glass plate having the glass composition is formed into a plate-shaped glass plate and the glass plate is manually opened, and the glass plate after the opening is sandwiched by a glass plate composed of the same kind of glass in an oxygen environment. The hole filled with O ₂ is reproduced as the bubble B. The glass plate having the pores was heat-treated at various temperatures and temperature maintenance times of 1200 ° C or higher, and the content of SO _{2 in} the pores 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.

As can be seen from Fig. 6, SO ₂ is contained in the pores filled with O ₂ at a temperature of approximately 1500 ° C or higher. In particular, it is understood that the higher the temperature and the longer the temperature maintenance time, the more the content of SO ₂ increases. It means that the diffusion of SO ₂ dissolved in the glass in a molten state is promoted by high temperature and absorbed into the pores.

Therefore, in the absorption treatment of the molten glass MG after the defoaming treatment, it is preferred to rapidly cool down to less than 1500 ° C. In the present embodiment, the molten glass MG is in the temperature range of 1600 ° C to 1500 ° C at 2 ° C / min. The above cooling rate is cooled.

Fig. 7 is a graph showing the results of measurement of the relationship between the bubble level and the temperature drop rate when the glass plate was produced in the experimental furnace simulating the temperature history of the molten glass MG shown in Fig. 5. The cooling rate is an average speed in a temperature range of 1600 ° C to 1500 ° C. The glass plate produced had the same glass composition as the glass substrate for liquid crystal display having a small content of alkali metal, and SnO _{2 was} used as a clarifying agent. Specifically, a glass substrate for a liquid crystal display having the same glass composition as that of FIG. 6 was used.

It can be seen that if the cooling rate is less than 2 ° C / min, the bubble level rises sharply. In addition, the bubble level indicates the number of bubbles per unit glass mass at a temperature drop rate of 10 ° C /min, and the number of bubbles is deteriorated to what extent. For example, the bubble level of 3.0 indicates the number of bubbles which is three times the number of bubbles when the temperature drop rate is set to 10 ° C /min.

As can be seen from Fig. 7, in order to lower the bubble level, the temperature drop rate may be 2 ° C / min or more.

(Example)

Fig. 8 is a view showing the results of measurement of the relationship between the number of bubbles existing in the glass plate and the temperature drop rate when the glass plate is produced by the apparatus for producing a glass plate. After the melting step, the clarification step, and the stirring step, the glass substrate is produced by an overflow down-draw method. At this time, the temperature history of the molten glass MG adopts the history shown in FIG. 5 in addition to the temperature drop rate. The so-called cooling rate is an average speed in a temperature range of 1600 ° C to 1500 ° C.

The glass plate produced had the same glass composition as the glass substrate for liquid crystal display having a small content of alkali metal, and SnO _{2 was} used as a clarifying agent. Specifically, a glass substrate for a liquid crystal display having the same glass composition as that of FIG. 6 was used. The bubble level shown in Fig. 8 indicates how much the bubble is deteriorated by the number of bubbles per unit mass when the temperature drop rate is set to 8.4 ° C /min. For example, the bubble level 5.0 indicates the number of bubbles which are five times the number of bubbles when the temperature drop rate is 8.4 ° C /min. The bubble level of the cooling rate of 7.9 ° C / min is 1.1, the bubble level of the cooling rate of 4.9 ° C / min is 1.6, the bubble level of the cooling rate of 4.2 ° C / min is 1.8, and the bubble level of the cooling rate of 3.0 ° C / min is 1.8. On the other hand, the bubble level of the cooling rate of 1.8 ° C / min is 3.0, the bubble level of the cooling rate of 0.5 ° C / min is 83, and the number of bubbles when the cooling rate is set to 8.4 ° C / min is more than 3 times. bubble.

As can be seen from Fig. 8, when the temperature drop rate is less than 2 ° C / minute, the number of bubbles rises sharply. Therefore, it is understood that when the molten glass MG is cooled in a temperature range of 1600 ° C to 1500 ° C at a temperature drop rate of 2 ° C / min or more, preferably 2.5 ° C / min or more, the number of bubbles is lowered. As can be seen from Fig. 8, for example, it is more effective in reducing the number of bubbles in the case where the temperature drop rate is from 3 ° C / min to 8 ° C / min.

Further, it has SiO ₂ : 60% by mass, Al ₂ O ₃ : 19.5% by mass, B ₂ O ₃ : 10% by mass, CaO: 5.3% by mass, SrO: 5% by mass, SnO ₂ : 0.15% by mass, Fe _The number of bubbles in the glass plate of ₂ O ₃ : 0.05% by mass was substantially reduced by a small amount, but substantially the same result was obtained. Further, it has SiO ₂ : 61% by mass, Al ₂ O ₃ : 19.5% by mass, B ₂ O ₃ : 10% by mass, CaO: 9% by mass, SnO ₂ : 0.3% by mass, and R ₂ O (R-based Li, In the production of a 0.2% by mass glass plate (strain point: 700 ° C), the same results as described above were also obtained in the total composition contained in the glass plate among Na and K.

As described above, according to the present embodiment, the number of SO ₂ bubbles in the molten glass can be reduced, and therefore, the bubbles which become the cavitation core due to the rotation of the stirring blade in the stirring step can be reduced, and as a result, the glass can be lowered. The number of bubbles in the board. This effect is more remarkable in the method of producing a glass substrate having a small glass composition of BaO or SrO.

More specifically, MgO, CaO, SrO, and BaO contained as a glass composition are often added as a carbonate to a raw material, and MgO is the lowest in the decomposition temperature, and is changed in the order of CaO, SrO, and BaO. high. That is, the higher the decomposition temperature, the higher the temperature at which CO ₂ is released. From the above, it is also known that if the molten glass MG is cooled after the defoaming treatment, the higher the decomposition temperature, the higher the temperature starts to absorb CO ₂ . For example, BaO begins to absorb CO ₂ at around 1300 °C.

However, in the production of a glass substrate having a small content of BaO or SrO which is a glass composition which starts to absorb CO ₂ in a relatively high temperature region, the absorption of CO ₂ after the temperature of the molten glass MG is lowered, that is, the molten glass MG Start after the viscosity becomes higher. Here, CO ₂ rapidly diffuses into the molten glass MG when the viscosity of the molten glass MG is low. Therefore, in the method for producing a glass substrate in which the absorption of CO ₂ is started after the viscosity of the molten glass MG is increased (after the temperature is lowered), the CO ₂ is likely to remain as bubbles in the molten glass MG.

According to the present embodiment, if the SO ₂ which is present as a gas component of the bubble in the molten glass can be reduced, even if it is a glass plate which is likely to remain CO ₂ as described above, the core which becomes cavitation can be suppressed. The generation of bubbles, as a result, also reduces the number of bubbles in the glass sheet as the final product. From the above, it is understood that the present embodiment is suitable for the production of a glass substrate having a BaO content of 0 to 1.0% by mass, and further suitable for a method for producing a glass substrate which does not substantially contain BaO. Further, this embodiment is suitable for the production of a glass substrate having a content of SrO of 0 to 3.0% by mass, and is further suitable for a method for producing a glass substrate which does not substantially contain SrO.

In the above, the method of manufacturing the glass substrate of the present invention has been described in detail. However, the present invention is not limited to the above-described embodiments, and various modifications and changes can be made without departing from the spirit and scope of the invention.

200‧‧‧melting device

201‧‧‧melting tank

202‧‧‧Clarification tank (2nd clarification tank)

203‧‧‧Stirring tank

203a‧‧‧Agitator

204‧‧‧Glass supply pipe (1st clarification tank)

205‧‧‧Glass supply pipe (3rd clarification tank)

206‧‧‧Glass supply tube

300‧‧‧Forming device

310‧‧‧Formed body

312‧‧‧ supply trench

313‧‧‧ bottom end

320‧‧‧Environmental spacers

330‧‧‧Cooling roller

350a~350d‧‧‧Transport roller

340‧‧‧Forming furnace

350‧‧‧Xu cold furnace

400‧‧‧ cutting device

Fig. 1 is a view showing the steps of a method for producing a glass sheet of the embodiment.

Fig. 2 is a view schematically showing an apparatus for performing a melting step to a cutting step in the method for producing a glass sheet of the embodiment.

Fig. 3 is a view mainly showing the configuration of a device for performing a clarification step in the embodiment.

Fig. 4 is a view mainly showing the configuration of a device for performing a forming step and a cutting step in the embodiment.

Fig. 5 is a view showing an example of a temperature history from the melting step to the forming step in the embodiment.

Fig. 6 is a view showing the measurement results of the content of SO ₂ contained in the pores in the glass after the bubbles remaining in the glass plate are reproduced.

Fig. 7 is a graph showing the relationship between the bubble level and the temperature drop rate when a glass plate is produced in a laboratory furnace simulating the temperature history of the molten glass shown in Fig. 5.

Fig. 8 is a view showing the relationship between the bubble level and the temperature drop rate which are present in the glass plate when the glass plate is manufactured by the apparatus for manufacturing a glass plate.

Claims (8)

A method for producing a glass sheet, characterized in that it is a glass plate, and includes a melting step of melting a glass material containing SnO ₂ as a fining agent by at least electric heating to form a molten glass; and a clarifying step, The present invention includes a defoaming treatment for raising the temperature of the molten glass to 1630 ° C or higher, thereby generating bubbles in the molten glass and performing defoaming, and an absorption treatment, after the defoaming treatment, the molten glass Cooling at a temperature drop rate of 1600 ° C to 1500 ° C at a temperature drop rate of 2 ° C / min or more, thereby absorbing bubbles in the molten glass into the molten glass; and a forming step of melting the melt after the clarification step The glass is formed into a sheet glass. The method for producing a glass sheet according to claim 1, wherein in the absorbing treatment, the cooling rate of the molten glass in a temperature range of 1500 ° C or lower is faster than a temperature lowering rate in the temperature range of 1600 ° C to 1500 ° C. The method for producing a glass sheet according to claim 1 or 2, wherein the forming step comprises forming the sheet glass from the molten glass by an overflow down-draw method. The method for producing a glass sheet according to claim 1 or 2, wherein the viscosity of the molten glass in the above 1630 ° C is 130 to 350 poise. The method for producing a glass plate according to claim 1 or 2, wherein the clarifying step is carried out in a platinum or platinum alloy tube through which the molten glass flows, and the temperature rise of the molten glass in the clarifying step is controlled by The current flows through at least two different regions extending in the longitudinal direction of the platinum or platinum alloy tube. The method for producing a glass plate according to claim 1 or 2, wherein the content of R' ₂ O of the glass plate is 0 to 2.0% by mass (R' ₂ O is among Li ₂ O, Na ₂ O and K ₂ O) The total of the ingredients contained). The method for producing a glass sheet according to claim 1 or 2, wherein the glass sheet is used for a glass substrate for a flat panel display. The method for producing a glass plate according to claim 1 or 2, wherein a stirring step of uniformly stirring the components of the molten glass is included between the clarifying step and the forming step; and melting in the molten glass is performed in the melting step The molten glass is supplied to the clarification step at a temperature higher at the beginning; in the clarification step, the molten glass is supplied to the stirring step at a temperature lower than a temperature after the defoaming treatment; In the step, the molten glass is supplied to the molten glass so that the viscosity η (poise) of the molten glass becomes a log η = 4.3 to 5.7, and is formed into a sheet glass.