Patent Application
20060288736
The present invention relates to a float bath and a float forming method suitable for float-forming a glass whose viscosity becomes 104 poise at a temperature (hereinafter this temperature is referred to as forming temperature) higher than that of soda lime silica glass.
Heretofore, glass sheets produced by float-forming soda lime silica glass in a molten state, has been widely used for window glasses of e.g. buildings and automobiles, glass substrates for STN liquid crystal displays and the like.
The method of float-forming soda lime silica glass in a molten state, was invented by an employee of Pilkington plc of United Kingdom in 1952, industrialized in 1959 and licensed for glass sheet manufacturers over the world thereafter. As a result, the number of facilities (float bath) for producing soda lime silica glass sheets by float-forming became at least 150 in 1996, and float-forming is a main production method for soda lime silica glass sheets presently (refer to Non-Patent Document 1).
A float bath is a huge bath with molten tin, and an upper space above the molten tin (a space covered by a roof) is separated into an upper space and a lower space by a roof-brick layer, and many heaters (usually heaters made of SiC) are disposed through many holes formed on the roof-brick layer. These heaters are connected to e.g. a busbar disposed in the upper space above the roof-brick layer by electric wires via aluminum straps, whereby e.g. an atmosphere above the molten tin is heated lo by heat generated by heat-generating portions of the heaters projecting in the lower space under the roof brick layer.
Basic specifications of such a float bath such as the structure, basic dimensions, component dimensions and the like, have been specified in the specification document presented by the licenser to each glass sheet manufacturer at a time of licensing, and are used without change. In other words, these specifications are common over the world.
The reason why the basic specifications of float bath are thus common over the world, is as follows.
Namely, cost for float glass manufacturing equipment including a float bath is huge, and a production stop due to a trouble after start of production causes a huge loss. Accordingly, basic specifications expected to ensure successful production, are not usually changed, and improvements of production efficiency and product quality are attempted by e.g. adjustment of facility operating conditions within a possible range. As a result, the basic specifications have not been changed and have become common over the world.
By the way, for a glass substrate of TFT liquid crystal display (TFT-LCD), a non-alkali glass having a forming temperature higher by at least 100° C. than that of soda lime silica glass, is employed, and a fusion method was initially used for producing glass sheets for the glass substrate. However, increase of the size of glass substrates is required thereafter, and production of non-alkali glass sheets by a float method more suitable for such a requirement, has brought into practice.
Non-Patent Document 1: “Glass engineering handbook” edited-by Masayuki Yamane etc. first edition, published by Asakura Shoten, Jul. 5, 1999, pages 358-362
Problems to be Solved by the Invention
However, various problems arise when a non-alkali glass having a forming temperature higher by at least 100° C. than that of soda lime silica glass, is formed into a glass sheet by using a float method or a float bath established for soda lime silica glass.
Such problems includes rise of the temperature of an atmosphere of the above-mentioned upper space (hereinafter, it may be simply referred to as upper space) to be described later.
In the upper space, e.g. electric wiring components such as busbars, electric wires and straps, and power supply portions of heaters to be connected with the straps, and the like are present.
Among these electric wiring components, aluminum straps directly connected to the power supply portions of heaters having a highest temperature due to heat conduction from heat-generating portions, has the highest temperature.
These straps may be damaged by such a high temperature, which causes to prevent power supply to a heater to which the strap is attached, and prevents sufficient heating. When such a damage occurs, it is unlikely that such a trouble occurs to only one strap but it is likely that the trouble occurs to many straps at the same time in most cases, and in such a case, heating is substantially prevented and production has to be stopped.
In order to prevent such a stop of production due to damage of straps, an atmospheric temperature T (hereinafter referred to as T) in the upper space is usually controlled so as not to exceed 300° C. Here, the reason why direct control of strap temperature is not conducted, is because the measurement is not easy and the number of heaters, namely, the number of straps are extremely large.
The 300° C. as the control upper limit of T is established as a temperature guaranteeing no occurrence of strap damage for a long period such as 10 years based on actual performances and experiences obtained by long time application of float method to soda lime silica glass.
By the way, if a glass having higher forming temperature (hereinafter, it may be referred to as high viscosity glass) than soda lime silica glass, is formed by float method, T becomes higher as compared with a case of forming soda lime silica glass by float method.
When T almost exceeds 300° C., usually a volume flow rate Vg (hereinafter referred to as Vg) of atmospheric gas (typically a mixed gas of nitrogen and hydrogen) is increased. Here, the atmospheric gas is introduced through a hole provided in e.g. an upper face of the roof casing into the upper space, and cools e.g. electric wiring components, and introduced through holes of the roof brick layer into the lower space to prevent oxidation of molten tin.
Increase of such a Vg not only causes a vicious cycle that decrease of heating by heaters → increase of heater power to compensate the decrease → another increase of T → increase of Vg, but also increases a risk of generation or increase of tin defects in a glass ribbon (top speck).
Recently, the size of glass substrates for TFT-LCD has been increased, and demand for high quality has been increased. However, the above-mentioned increase of top speck reduces production efficiency, in particular, production efficiency of large-sized above-mentioned substrates.
Further, requirements to properties of glasses to be employed for the substrates, have become high level and a glass satisfying such requirements has been developed. However, the forming temperature of such a glass usually becomes higher. Namely, T becomes higher.
As a result, at a time. of float-forming a glass for TFT-LCD substrates, it has become required to make T at most 300° C. without generating or increasing top speck due to increase of Vg.
It is an object of the present invention to provide a float bath and a float-forming method capable of solving such problems.
Means for Solving the Problems
The present invention provides a float bath comprising a bottom filled with molten tin and a roof for covering the bottom, wherein a space in the roof is divided into an upper space and a lower space by a roof brick layer, and a heater is disposed through a hole formed in the roof brick layer, characterized in that the roof brick layer has a thickness of at least 320 mm.
Further, the present invention provides the float bath, wherein a gap between an inner surface of the hole formed in the roof brick layer and the heater in the hole, that is averaged in a circumferential direction, is at most 20 mm.
Further, the present invention provides the float bath, wherein the heater has an outer diameter of from 23 mm to 50 mm at least at a heat-generating portion in the lower space.
Further, the present invention provides a float forming method for forming a glass whose viscosity becomes 104 poise at a temperature of at least 1,100° C., by a float process, characterized in that the method comprises continuously pouring the glass in a molten state from one end of the float bath as defined in any one of claims 1 to 3 onto the molten tin, forming the glass into a glass ribbon on the molten tin, and continuously drawing out the glass ribbon from the other end of the float bath.
Further, the present invention provides the float forming method, wherein the glass ribbon is continuously drawn out at a speed of from 1 to 200 tons/day.
The present inventors have achieved the present invention in the following way.
A non-alkali glass AN635 (trade name manufactured by Asahi Glass Company, Limited; forming temperature=1,210° C.) has been used for a glass of TFT-LCD for a long time, and as the nbh-alkali glass satisfying the above-mentioned high-level requirements to glass properties, AN100 (trade name manufactured by Asahi Glass Company, Limited; forming temperature=1,2680° C.) has been developed.
However, it has turned out that if AN100 is float-formed by using a float bath dedicated for float-forming AN635, load to a unit area of heaters becomes too large and it is difficult to carry out production for a long time.
To cope with this problem, the diameter of heat-generating portion of heaters disposed in the lower space under the roof brick layer, is increased from conventional 20 mm to 25 mm to reduce the load of heaters, and as a result, T is successfully decreased to 320° C. by increasing Vg within a range that risk of increasing top speck does not significantly increase. In order to further ensure long time production of AN100 using the float bath, the present inventors have conducted various measurements to the float bath, and from the result of measurements, they constructed the following calculation model.
The calculation model is a heat balance model of the upper space 20.
Incoming heat is a heat transfer Qin from the lower space 21.
Outgoing heat are a heat radiation to the outside Qout1 from a portion (hereinafter referred to as wall face portion) of the roof casing in contact with the upper space 20, and a heat quantity Qout2 consumed to increase the temperature of atmospheric gas supplied to the upper space 20, and a formula Qin=Qout1+Qout2 is satisfied.
Qout1, is represented by the following formula using an external temperature Ta, an area Aw of the wall face portion, and a general heat transfer coefficient hc:
Qout1=hcAw(T−Ta)
wherein hc is determined by Qout1, measured values of Ta and T, and Aw. Here, Qout1 can be obtained from Ta, Aw, an external face temperature Tw of the wall face portion and a heat transfer coefficient hw by using a relational formula Qout1=hwAw(Tw−Ta)
Qout2 is represented by the following formula using T, Ta, and a volume flow rate Vg, a density ρg and a specific is heat Cg of atmospheric gas:
Qout2=VgρgCg(T−Ta)
Qin is represented by the following formula using an atmospheric temperature T of the upper space 20, an atmospheric temperature T1 of the lower space 21, and an area Ar, a general heat transfer coefficient hr and thickness t of the roof brick layer 16:
Qin=hrAr(T1−T)
By representing hr=k/t using a coefficient k, the above formula becomes:
Qin=(k/t)Ar(T1−T)
k is determined by actually measured values of T1 and T, Qin, t and Ar, and Qin is obtained as a sum of Qout1 and Qout2. Namely, from a formula Qin=Qout1+Qout2, the following formula is satisfied:
From Formula (1), a relational formula between t and T can be obtained.
Since T=320° C. when t=292 mm (the above-mentioned basic specification common over the world), the relational formula (1) of t and T can be represented by the following formula provided that mm and ° C. is used for the units of the thickness and the temperature respectively:
(k/292)Ar(T1−320)=(hcAw+VgρgCg) (320−Ta) (2)
On the other hand, when the thickness is t and the atmospheric temperature is T1, Formula (1) is applicable, and by dividing left and right sides of Formula (1) by those of Formula (2) respectively, the following formula is obtained:
[(k/292) Ar(T1−320)]/[(k/t)Ar(T1−T)]=[(hcAw+VgρgCg) (320-Ta)]/[(hcAw+VgρgCg) (T−Ta)]
This formula is organized into:
(t/292)[(T1−320)/(T1−T)]=(320−Ta)/(T−Ta)
and the above formula is further organized into the following formula:
t/292=[(T1−T)/(T1−320)][(320−Ta)/(T−Ta)]
From
According to the present invention, it becomes possible to float-form such a high-viscosity glass that extremely shortens the equipment lifetime or significantly increases a risk of generating or increasing top speck when the glass is float-formed by using a conventional float bath, without increasing such a risk.
Further, it becomes possible to suppress the flow rate of an atmospheric gas for lowering an upper space atmospheric temperature, and thus it become possible to fundamentally suppress generation of top speck at a time of float-forming such a high-viscosity glass that intermittently produces top speck although the glass has been generating no significant top speck heretofor.
From now, embodiments according to the present invention are described in detail with reference to drawings. Here, the present invention is not limited to these embodiments.
The maximum width of the molten tin 11 in the float bath 10 depends on the size of the float bath 10 and typically from 1 to 10 m.
The roof 14 has a roof casing 19 made of a steel and hanged from an upper structure (not shown) such as a beam of a building where the float bath is installed, a side wall 15 made of a keep-warm brick and being a lining for a lower part of the roof casing 19, and a Bide seal 13 made of a steel and having a box shape and disposed on an edge portion of the bottom 12.
A space in the roof 14 is separated into an upper space 20 and a lower space 21 by a roof brick layer 16.
The roof brick layer 16 as a construction that substantially rectangular solid-shaped combined brick blocks called PBA are mounted on a grid-shaped framework constructed by assembling many support tiles (not shown) made of sillimanite and rail tiles (not shown) disposed on the support tiles so as to be perpendicular to the support tiles. The support tiles are hanged from e.g. a ceiling portion of the roof casing 19 by a member (not shown) called hanger. Namely, the roof brick layer 16 is retained horizontally at a desired height above the molten tin by the hanger.
Here, a side surface of the roof brick layer 16 contact with an upper portion of a side surface of the side wall 15, and the upper face of the roof brick layer 16 is set so as to be approximately the same height as the upper face of the side wall 15.
Then, holes 17 are formed in the roof brick layer 16 so as to install the heaters 18 through the holes.
The PBA30 is a combined brick block formed by assembling a heat-insulating ceramics plate 30a, a keep-warm brick 30b for low temperature, a keep-warm brick 30c for high temperature and a sillimanite brick 30d, made of e.g. Insulating Board Hemisul (trade name) manufactured is by Tokyo Materials Co., Ltd., and tightened by a hanger (another hanger different from the above-mentioned hanger for hanging the roof brick layer 16 from e.g. a ceiling portion of the roof casing 19), not shown. Irregular-shaped portions in the left and right sides of the sillimanite brick 30d are portions to be mounted on support tiles.
Holes 17 for installing the heaters through the holes, are formed so as to perforate through the PBA30. t is a distance between the upper face of the ceramics plate 30a and the lower face of the sillimanite brick 30d, namely, the thickness of PBA30 which correspond to the thickness of roof brick layer 16.
Heretofore, t has been 292 mm commonly over the world as described above, but in the present invention, t is set to be at least 320 mm. By thus constructing, it becomes possible to float-forming a high-viscosity glass such as a non-alkali glass for TFT-LCD without significantly increasing flow rate of atmospheric gas (N2+H2). t is preferably at least 340 mm, more preferably at least 360 mm. Here, t is typically at most 500 mm. Further, the heat-generating portion 18C of a heater 18 in the lower space, preferably has a heater outer diameter of from 23 mm to 50 mm, more preferably from 23 mm to 30 mm, particularly preferably about 25 mm.
Back to
The heaters 18 are usually made of SiC, and three heaters are grouped and lower ends of these heaters are connected each other by a connecting member 25 to constitute a unit. Here, the heat-generating portion 18C of each heater 18 has a substantially cylindrical shape having an outer diameter of 25 mm.
As shown in
A gap g between an inner face of the hole 17 of the 10 roof brick layer 16 and a heater 18 (corresponding to non-heat-generating portion 18B) present in the form 17, that is averaged in a circumferential direction, is typically at least 0.5 mm (more preferably at least 1 mm) and at most 20 mm (more preferably at most 10 mm), and is the portion of the heater 18 where g averaged in a circumferential direction is at least 0.5 mm and at most 20 mm, is preferably at least 80% of the length (=t) of the hole 17, more preferably 100%.
Returning again to
T is, as described above, set to a proven controlled 25 temperature, namely 300° C. or lower, considering that prevention of equipment trouble due to a damage of the strap 24 as the highest priority. Flow rate of the atmospheric gas used in this case can be set so that the flow rate does not significantly increase top speck.
Here, atmospheric gas supplied to the upper space 20 flows into the lower space 21 through e.g. each gap g between a hole 17 and a heater 18 (a non-heat-generating portion 18B of
In the float forming method of the present invention, a glass having a forming temperature of at least 1,100° C. is float-formed by employing such a float bath.
Namely, from a known spout lip (not shown, it is present for example in the back side of
The glass ribbon drawn out by the lift out roller, is gradually cooled in a layer (gradual cooling furnace) and then, cut into a desired shape to be a glass sheet.
By employing the float-forming method of the present invention, it becomes possible to float-form a high-viscosity glass without particularly increasing the number of top speck-and without increasing a risk of causing such a case that production has to be stopped even for a short time.
The present invention is not limited to the above-mentioned embodiments, and can be made appropriate modifications and improvements, and the bottom, the roof, the roof brick layer, the upper space, the lower space, the heater, the atmospheric gas, the temperature, the pulling out amount, and the material, the shape, the dimension, the shape, the number, the disposed position and the thickness of members of float bath and the like exemplified in the above-mentioned embodiments, are optional within a range departing from the object of the present invention.
Further, the high-viscosity glass is not limited to a glass for TFT-LCD substrate, but it can, for example, be a glass for a plasma display panel substrate.
Further, the float bath of the present invention may be used for float-forming not only a high-viscosity glass but also a soda lime glass.
AN100 is float-formed by employing a float bath (thickness t of roof brick layer: 394 mm; a gap gAV between an inner face of a hole of roof brick layer and a heater inserted in the hole, that is averaged in a circumferential direction: 9 mm) of the present invention.
When the volume flow rate Vg of the atmospheric gas is set to be 95% of Vg by which T became 320° C. at a time of float forming AN100 in the float bath (t: 292 mm, gAV; 9 mm) employed for float-forming the above AN635, T became 270° C. (black square in
According to the present invention, it becomes possible to float form a high-viscosity glass which tends to significantly shorten equipment lifetime and tends to generate or increase top speck when the glass is float-formed by using a conventional float bath, without increasing such a risk.
The entire disclosures of Japanese Patent Application No. 2003-428974 filed on Dec. 25, 2003 and Japanese Patent Application No. 2004-165758 filed on Jun. 3, 2004 including specifications, claims, drawings and summaries are incorporated herein by reference in their entireties.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-428974 | Dec 2003 | JP | national |
| 2004-165758 | Jun 2004 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP04/19117 | Dec 2004 | US |
| Child | 11474434 | Jun 2006 | US |
