WO2009011792A1

WO2009011792A1 - A method and apparatus for forming a glass sheet

Info

Publication number: WO2009011792A1
Application number: PCT/US2008/008514
Authority: WO
Inventors: Chunhong C He; Qiao Li; Steven M Milillo
Original assignee: Corning Incorporated
Priority date: 2007-07-19
Filing date: 2008-07-09
Publication date: 2009-01-22
Also published as: JP2010533640A; KR20100063031A; JP5615702B2; CN101795985B; CN101795985A; KR101476480B1; TW200922885A; TWI398414B

Abstract

An apparatus for forming a glass sheet is disclosed, the apparatus comprising a forming body for supporting a glass melt flow, the forming body having substantially vertical forming surfaces and inclined forming surfaces. The height of the vertical forming surfaces and the angle between the inclined forming surfaces are selected so as to minimize the amount of material comprising the forming body that is dissolved into the glass melt and subsequently re-crystallized on the forming body.

Description

A METHOD AND APPARATUS FOR FORMING A GLASS SHEET

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0001] The invention is directed to an apparatus for forming glass sheets, and more particularly to a forming body for forming glass sheets that produces a reduced amount of solid inclusions in the glass.

TECHNICAL BACKGROUND

[0002] The fusion process is one of the basic techniques used in the glass making art to produce sheet glass. Compared to other processes known in the art, e.g., the float and slot draw processes, the fusion process produces glass sheets whose surfaces have superior flatness and smoothness. As a result, the fusion process has become of particular importance in the production of the glass substrates used in the manufacture of liquid crystal displays (LCDs). [0003] The fusion process, specifically, the overflow downdraw fusion process, is the subject of commonly assigned U.S. Patents Nos. 3,338,696 and 3,682,609, to Stuart M. Dockerty. As described therein, a glass melt is supplied to a trough formed in a refractory body known as an "isopipe".

[0004] In an exemplary fusion downdraw process as described in the Dockerty patent, once steady state operation has been achieved, the glass melt overflows the top of the trough on both sides so as to form two half sheets of glass that flow downward and then inward along the outer surfaces of the isopipe. The two sheets meet at the bottom or root of the isopipe, where they fuse together into a single glass sheet. The single sheet is then fed to drawing equipment which controls the thickness of the sheet by the rate at which the sheet is drawn away from the root. The drawing equipment is located sufficiently downstream of the root so that the single sheet has cooled before coming into contact with the equipment.

[0005] The outer surfaces of the final glass sheet do not contact any part of the outside surface of the isopipe during any part of the process. Rather, these surfaces see only the ambient atmosphere. The inner surfaces of the two half sheets which form the final sheet do contact the isopipe, but those inner surfaces fuse together at the root of the isopipe and are thus buried in the body of the final sheet. In this way, the superior properties of the outer surfaces of the final sheet are achieved.

[0006] An isopipe used in the fusion process is subjected to high temperatures and substantial mechanical loads as the glass melt flows into its trough and over its outer surfaces. To be able to withstand these demanding conditions, the isopipe is typically and preferably made from an isostatically pressed block of a refractory material (hence the name "iso-pipe"). In particular, the isopipe is preferably made from an isostatically pressed zircon refractory, i.e., a refractory composed primarily of ZrO₂ and SiO₂ For example, the isopipe can be made of a zircon refractory in which ZrO₂ and SiO₂ together comprise at least 95 wt. % of the material, with the theoretical composition of the material being ZrO₂*SiO₂ or, equivalently, ZrSiO₄. [0007] A source of losses in the manufacture of sheet glass for use as LCD substrates is the presence of zircon crystal inclusions (referred to herein as "secondary zircon crystals" or "secondary zircon defects") in the glass as a result of the glass' passage into and over the zircon isopipe used in the manufacturing process. The problem of secondary zircon crystals becomes more pronounced with devitrification-sensitive glasses which need to be formed at higher temperatures.

[0008] Zircon which results in the zircon crystals that are found in the finished glass sheets has its origin at the upper portions of the zircon isopipe. In particular, these defects ultimately arise as a result of zirconia (i.e., ZrO₂ and/or Zr⁺⁴ + 20²) dissolving into the glass melt at the temperatures and viscosities that exist in the isopipe's trough and along the upper walls (weirs) on the outside of the isopipe. The temperature of the glass is higher and its viscosity is lower at these portions of the isopipe as compared to the isopipe's lower portions since as the glass travels down the isopipe, it cools and becomes more viscous.

[0009] The solubility and diffusivity of zirconia in a glass melt is a function of the glass' temperature and viscosity (i.e., as the temperature of the glass decreases and the viscosity increases, less zirconia can be held in solution and the rate of diffusion decreases). As the glass nears the bottom (root) of the isopipe, it may become supersaturated with zirconia. As a result, zircon crystals (i.e., secondary zircon crystals) nucleate and grow on the bottom portion (e.g. root) of the zircon isopipe. Eventually these crystals grow long enough to break off into the glass flow and become defects at or near the fusion line of the sheet. BRIEF SUMMARY OF THE INVENTION

[0010] In one embodiment an apparatus for forming a glass sheet is disclosed comprising a forming body comprising inclined forming surfaces, substantially vertical forming surfaces intersecting with the inclined forming surfaces and wherein an angle between the inclined forming surfaces is less than 42 degrees.

[0011] In another embodiment, a method for forming a glass sheet is disclosed comprising providing a glass melt, flowing the glass melt over a forming body comprising a crystalline refractory material, the forming body further comprising inclined forming surfaces and substantially vertical forming surfaces intersecting with the inclined forming surfaces, and wherein a maximum height of the vertical forming surfaces and an angle between the inclined forming surfaces are chosen to minimize the concentration of dissolved refractory material in the glass melt.

[0012] It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate an exemplary embodiment of the invention and, together with the description, serve to explain the principles and operations of the invention.

BRIEF SUMMARY OF THE DRAWINGS

[0013] FIG. 1 is a side view of a forming body for forming a glass sheet in accordance with en embodiment of the present invention.

[0014] FIG. 2 is a cross sectional view of the forming body of FIG. 1 showing the angle between the inclined forming surfaces.

[0015] FIG. 3 is a graphical depiction of the effect on the temperature profile at the forming surfaces (moving up or down the forming surfaces) as the angle between the inclined forming surfaces and the height of the vertical forming surfaces is reduced

[0016] FIG. 4 is a graphical depiction of the concentration of isopipe material dissolved in the glass melt in wt. % minus the saturation concentration of the material in wt. % as a function of the distance from the forming surface of the isopipe for two isopipe root angles. [0017] FIG. 5 is another embodiment of a forming body in accordance with the present invention comprising multiple pairs of inclined forming surfaces.

[0018] FIG. 6 is a plot showing the concentration of isopipe material dissolved in a glass melt in excess of the saturation concentration as a function of distance from the isopipe.

DETAILED DESCRIPTION

[0019] In a fusion downdraw process for making a glass sheet in accordance with the present invention, glass forming precursors (batch) are melted in a furnace to form a molten raw material, or glass melt, which is thereafter flowed over a forming body to form the glass sheet. Generally, such forming bodies include upper forming surfaces, and inclined forming surfaces intersecting with the upper forming surfaces. The inclined forming surfaces converge at the bottom or root of the forming body. The upper forming surfaces are typically substantially vertical and parallel.

[0020] The design of the forming body, or isopipe, must take into consideration a number of competing interests. Molten raw material is introduced into a trough in the forming body bounded at its sides by dams (weirs). The molten raw material must be introduced to the forming body at a viscosity low enough, that is, at a high enough temperature, to produce an even flow of glass melt over the tops of the weirs. The molten raw material then flows down the exterior forming surfaces of the forming body, including the inclined forming surfaces, to the bottom of the body. The angle between the inclined forming surfaces must not be so large that gravity will cause the glass to separate from the isopipe.

[0021] On the other hand, the molten raw material leaving the bottom or root of the forming body must have a viscosity high enough - at a low enough temperature — to allow the molten raw material to be drawn successfully, yet not so low that the viscosity of the molten raw material falls below the liquidus viscosity of the molten raw material, which can cause the glass melt to crystallize.

[0022] Futhermore, the forming body should be capable of withstanding the stress of transient heating conditions (e.g. heating up or cooling down) without damage to the forming body, and resist sagging or creep of the forming body due to the mass of the forming body and the extended periods of time operating at high temperatures.

[0023] If the glass melt overflowing the forming body remains at a high temperature for too long a time as it descends the forming surfaces, the material comprising the forming body may dissolve, then re-crystallize at a colder portion of the forming body, such as the root. Crystals may grow to the extent that they break off and become entrained in the glass flow, resulting in a defect in the finished glass product. The present invention seeks to reduce the amount of crystal re-growth by limiting the amount of forming body material that dissolves into the glass melt. [0024] Shown in FIGS. 1 and 2 is a forming body or isopipe 10 used in the manufacture of glass sheets in accordance with an embodiment of the present invention. Isopipe 10 comprises trough 12 for receiving the glass melt from a supply (not shown) through inlet 14, weirs 16, 18 bounding trough 12, vertical forming surfaces 20, 22 and inclined forming surfaces 24, 26. Vertical forming surfaces 20, 22 intersect inclined forming surfaces 24, 26 along transition lines, or breaks, 28, 30, respectively. Inclined forming surfaces 24, 26 are angularly displaced by angle α, and intersect at the bottom or root 32 of the isopipe. Vertical forming surfaces 20, 22 are preferably substantially parallel. Isopipe 10 has a length L , and an overall height H. The vertical distance between root 32 and breaks 28, 30 is h, while the distance between breaks and the tops of weirs 16, 18 is h' where h' varies between a maximum value h'_max and a minimum value h'_mm. Thus, H varies between-a maximum H_max and a minimum H_mιn. [0025] Glass melt 34 is supplied to isopipe 10 through inlet 14, and the glass melt thereafter overflows isopipe 10 at the tops of weirs 16, 18, e.g. at the top of the vertical forming surfaces 20, 22, and flows down forming surfaces 20, 22 and 24, 26 as two distinct flows. The two glass flows re-unite or fuse at root 32 to form glass sheet 36 that is drawn downward by pulling equipment, represented by pulling rolls 38. Isopipe 10 is typically comprised of a ceramic refractory material, such as zircon or alumina. Isopipe 10 is preferably housed in an enclosure 40 including heating elements 42 arranged vertically within the enclosure for controlling a temperature of the glass melt on the forming surfaces of the isopipe.

[0026] As can be seen from FIG. 2, vertical forming surfaces 20, 22 are generally parallel to internal muffle walls 44, 46 that are heated by heating elements 42. Generally, the temperature of the glass melt flowing down vertical forming surfaces 20, 22 is substantially constant. On the other hand, inclined forming surfaces 24, 26 are inclined and are exposed to the cooler temperatures below the isopipe. That is, the inclined or converging forming surfaces have a horizontal component to their orientation as well as a vertical component. Consequently, the glass melt cools as it descends the inclined forming surfaces. The resulting profile of temperature as a function of location on the isopipe looks like curve 50 of FIG. 3, showing a generally constant temperature over the vertical forming surfaces (from the top of the weirs to the break - section 50a), and a generally linear decrease in temperature moving down the isopipe from the break to the root - section 50b.

[0027] It has been found that prolonged time at high temperature results in the material comprising the isopipe to dissolve into the glass melt. Consequently, a reduction in the amount of isopipe material that is dissolved into the glass melt, and therefore available to precipitate out of the flow of the glass melt can be reduced by reducing the height of the vertical forming surfaces.

[0028] Without wishing to be held to any one particular theory, it is thought that decreasing the distance on the vertical forming surfaces over which the glass melt must travel reduces the time the glass melt is exposed to the high temperatures existing at the top of the isopipe. Thus, there is less time for the isopipe material to dissolve into the glass melt, and consequently less dissolved isopipe material available to precipitate out at the cooler regions of the isopipe, e.g. proximate root 32.

[0029] At the same time, merely decreasing the height of the vertical forming surfaces can lead to a reduction in the overall height of the isopipe. A reduction in overall height is undesirable because the isopipe becomes more prone to sag. Thus, there is a desire to reduce the height of the vertical forming surfaces without reducing the overall height H of the isopipe itself. This can be accomplished by reducing the angle α between the inclined forming surfaces. Assuming a constant weir-to-weir width W of the isopipe, angle α becomes smaller as the height h' of the vertical forming surfaces becomes smaller to maintain the overall height H of the isopipe. [0030] Reducing the angle between the inclined forming surfaces also has the added benefit of reducing the overall mass of the isopipe, leading to reduced sag, and less thermally-induced stress during temperature transients (e.g. heat up and cool down).

[0031] Referring to FIG. 2, reducing angle α has the effect of moving inclined forming surfaces 24, 26 inward to the position designated as 24', 26'. Consequently, breaks 28, 30 move upward to the position designated as 28', 30', respectively, h' decreases and excess isopipe material is eliminated. This can be visualized in the context of temperature with the aid of FIG. 3 and curve 52. As indicated, although the start and finish temperatures for curve 52 are the same as for curve 50, the overall temperature profile represented by curve 52 is flattened (more linear), and the temperature at the newly-defined break is cooler. That is, the temperature profile between the top of the weirs (e.g. the top of the vertical forming surfaces) - section 52a - decreases in temperature more rapidly than that for section 50a, and the portion of curve 50 between the break and the root (section 52b) changes temperature less than its counterpart 50b. Preferably, angle α is less than 42 degrees. Preferably, α is less than about 35 degrees, preferably less than 30 degrees.

[0032] FIG. 4 shows modeled data for an isopipe having a length of about 295 cm and an overall maximum height H_max of 100 cm at the inlet end of the isopipe. The flow over the isopipe was assumed to be about 1500 pounds/hour. Curve 54 represents the isopipe with an angle α of 42 degrees, a top-of-the-weir temperature of 1246°C a break temperature of 1238°C and a root temperature of 1181.5°C, while curve 56 represents the same isopipe with an angle α between the inclined forming surfaces of 30 degrees, a top-or-the-weir temperature of 1242⁰C, a break temperature of 1238°C and a root temperature of 1181.5°C. The propensity for crystal growth is represented by the vertical axis depicting the saturation concentration of zirconia in the glass melt (Cs) subtracted from the amount of zirconia dissolved in the glass melt (C). FIG. 4 shows that when the angle at the root was reduced from 42 degrees to 30 degrees, the predicted propensity for the formation of re-crystalization of ziron from dissolved zirconia decreased. For example, at a distance from a forming surface of the isopipe of 20 μm, the concentration of zirconia C minus the saturation concentration for zirconia Cs at a given temperature for an angle of 42 degrees is about 0.05 wt. %, whereas for an angle of 30 degrees the concentration difference is about 0.046 wt.%. It should also be apparent from FIG. 4 that the peak amount of dissolved zirconia has moved inward, closer to the forming surface(s) of the isopipe as the angle between the inclined forming surfaces decreased from 42 degrees to 30 degrees. This implies that crystal growth is retarded nearer the surface of the isopipe for the smaller angle. Thus, crystals growth beyond a certain length can be retarded, with less likelihood that a crystal will break off and become entrained in the glass melt flow.

[0033] The table below gives approximate values for α and h'_max for an exemplary isopipe having a maximum overall height H_max of about 97.8 cm and a width W of about 28 cm. The length of the isopipe can exceed 254 cm.

[0034] In another embodiment illustrated in FIG. 5, forming body (isopipe) 60 is shown in cross section comprising trough 62, weirs 64, 66, a first pair of inclined forming surfaces 68, 70 and a second pair of inclined forming surfaces 72, 74. The second pair of inclined forming surfaces 72, 74 intersect at root 76 at the bottom of the isopipe. In accordance with the embodiment shown in FIG. 4, the first pair of inclined forming surfaces 68, 70 form an angle β therebetween. Angle β is preferably less than 42 degrees. Second forming surfaces 72, 74 intersect first forming surfaces 68, 70. Second forming surfaces 72, 74 form angle θ therebetween. Isopipe 60 may include vertical forming surfaces 78, 80 intersecting with the first pair of inclined forming surfaces 68, 70.

[0035] Angle θ is preferably less than angle β. Angle β may be for example, less than 42 degrees, preferably less than 35 degrees, more preferably less than 25 degrees, preferably less than 20 degrees. Advantageously, the embodiment of FIG. 4 a smaller angle between the inclined forming surfaces adjacent the root can be achieved in a given vertical distance. For example, for a given forming body height (distance between the root and the top of the weirs) and a given forming body maximum width (distance from the outside of one weir to the outside of the opposing weir), a smaller angle between the inclined forming surfaces adjacent the root can be achieved than when only one pair of inclined forming surfaces are used. [0036] FIG. 6 shows modeled data indicating exemplary limits for the concentration of dissolved isopipe material in the glass melt flowing over the forming surfaces of an isopipe, and in particular for an isopipe comprising zircon taken at the mid-point along the length of the isopipe. A glass melt flow of 1500 pounds/hr. was assumed, with a temperature range (top to bottom) between about 1243°C and 1125°C. The plot illustrates three zones divided by two curves 82, 84.

[0037] Operating the isopipe under conditions that result in zone I zirconia concentrations (within curve 82) suggests safe operation, with little opportunity for crystal breakage. That is, as can be done for FIG. 4, the X-axis can be interpreted as the maximum length of crystal growth on the isopipe for a given concentration of dissolved isopipe material exceeding the saturation concentration. By way of illustration, at a C-Cs concentration of zero, curve 82 suggests a maximum crystal length of 70 μm. Beyond 70 microns there is insufficient dissolved isopipe material to feed crystal growth. At 70 microns and below, crystal breakage has been found to be less troublesome.

[0038] Operation under zone III conditions suggests undesirable performance. Operating in zone HI (outside curve 84) provides a sufficient concentration of dissolved isopipe material that crystal growth can exceed a safe length - the crystals can easily break and become entrained in the glass melt. Finally, operation within zone II (between curves 82 and 84) suggests performance intermediate between that of zones I and III.

[0039] It should be emphasized that the above-described embodiments of the present invention, particularly any "preferred" embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.

Claims

What is claimed is:

1. An apparatus for forming a glass sheet comprising: a forming body comprising: a first pair of inclined forming surfaces; substantially vertical forming surfaces intersecting the first pair of inclined forming surfaces; and wherein an angle between the first pair of inclined forming surfaces is less than 42 degrees.

2. The apparatus according to claim 1 wherein a maximum height h'_max of the vertical forming surfaces is less than about 60 cm.

3. The apparatus according to claim 1 wherein a length of the forming body is at least about 265 cm.

4. The apparatus according to claim 1 further comprising a second pair of inclined forming surfaces intersecting the first pair of inclined forming surfaces.

5. The apparatus according to claim 4 wherein the second pair of inclined forming surfaces intersect at a bottom of the isopipe.

6. The apparatus according to claim 4 wherein an angle between the second pair of inclined forming surfaces is less than 42 degrees.

7. A method of forming a glass sheet comprising: providing a glass melt; flowing the glass melt over a forming body comprising a crystalline refractory material, the forming body further comprising inclined forming surfaces and substantially vertical forming surfaces intersecting with the inclined forming surfaces; and wherein a maximum height of the vertical forming surfaces and an angle between the inclined forming surfaces are chosen to minimize the concentration of dissolved refractory material in the glass melt.

8. The method according to claim 7 wherein a maximum height of the vertical forming surfaces is less than about 60 cm.

9. The method according to claim 7 wherein the angle between the inclined forming surfaces is less than about 42 degrees.