[0001] This application claims the benefit of priority under 35. U.S.C. § 1 19 of U. S. Provisional Application Serial No. 61/557521 filed on November 9, 201 1 the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0002] The invention relates generally to a method and an apparatus for forming glass ribbon, and more particularly to drawing a flat, thin glass ribbon from a glass preform.

TECHNICAL BACKGROUND

[0003] There is an increasing demand for flat glass ribbons, especially precision flat glass ribbons of high surface quality and consistent thickness, which are made of a glass-based material or a glass-ceramic. Flat panel displays incorporating such flat ribbons have received a great deal of attention. Much of the attention has centered on small units such as those used in laptop computers and very large units, such as flat panel displays, i.e. televisions. However, consideration is now being given to flexible displays, and the need for very thin, flexible glass substrates has become apparent.

[0004] Two methods commonly used in manufacturing display substrates are the float process and the fusion process. Both of these processes require a refractory glass melter to deliver a stream of molten glass-forming material to a ribbon-forming device. In the case of high strain-point glass compositions, a relatively large high-temperature glass melter is needed to deliver a high-quality stream of molten glass-forming material to the ribbon- forming device. This is because high strain-point glasses have high fusion temperatures, typically in excess of 1700°C.

[0005] In the float process, a stream of molten glass-forming material is discharged from a melting furnace into a float furnace that contains a liquid metal medium. Typically, the metal is tin. The atmosphere in the float furnace is controlled to prevent oxidation of the tin. The molten glass floats and spreads out on the liquid tin in the form of a flat, continuous ribbon. The ribbon of glass is conveyed into an annealing lehr or cooling tunnel, where it is cooled at a controlled rate to ambient temperature. The cooled glass has a flat, smooth surface that may, in some instances, require further finishing by processes such as grinding and polishing. [0006] However, it is very difficult to form glasses having high strain points in an enclosure containing molten tin. This is because tin has high vapor pressures at temperatures in excess of 1050 to 1100°C. At the high forming temperatures required for high strain-point glasses, the molten tin will vaporize inside the float furnace and subsequently condense in colder parts of the furnace. In some cases, the condensation may be sufficiently high to create what is referred to as "tin rain," a situation where tin rains on the glass and is incorporated on the glass surface.

[0007] In the fusion process, a glass-forming melt flows into a refractory trough and then overflows in a controlled manner from either side of the trough. A key advantage of this process is that the surface of the glass ribbon that is ultimately formed does not come in contact with any refractory material or other forming equipment. Another benefit of the process is that it yields a very flat and uniformly thick ribbon of glass. As a result, no secondary processing is needed to obtain a smooth, flat, and uniform ribbon of glass for display applications. However, the method suffers from not being able to process glasses having high strain points due to the high temperatures required, since such temperatures greatly accelerate deterioration of the glass forming components, and there is potential for increased contamination of the glass melt. Typically, it is desirable to form the glass at viscosities in the range of 10 ⁵ to 10 ⁶ poise to obtain optimum flatness and uniform thickness.

[0008] Unfortunately, neither the fusion draw process nor the float glass process is effective in producing flat, very thin ribbon from a glass composition having a high strain point, for example a strain point that may exceed 900°C.

SUMMARY

[0009] The ability to produce a thin flexible glass substrate is of interest for roll-to-roll processing of flexible electronics and displays. As used herein, roll-to-roll refers to the supply of a glass ribbon from a first, or source roll to a second or take-up roll, wherein processing of the ribbon occurs as the glass ribbon travels from the source roll to the take-up roll. Current processes for fabricating thin glass sheet, such as slot draw, fusion forming and float, have limitations in producing thin glass ribbons, such as glass ribbons having a thickness as thin or thinner than about 200 μιη, for example 50 to 100 micron. Redraw or down drawing a glass sheet preform enables the fabrication of glass ribbon having a thickness less than 100 μιη with good geometrical and strength attributes. Even though redraw is not a new forming process and has been used for fiber, tubing and other glass products, the capability of drawing flat glass ribbon is a unique art to achieve a substrate thin enough and flexible enough that it can be spooled and used in a roll-to-roll process. The roll-to-roll process enables coatings to be printed onto the thin substrate with existing industry technology. For example, thin film electronics may be deposited onto the moving glass ribbon in a roll-to-roll process.

[0010] A glass ribbon having no warp, wrinkle or other visible distortion, excluding the very edges of the ribbon, can be achieved by controlling the viscosity of the drawn glass and maintaining a specific thermal profile within the draw furnace. This can be expanded by employing a second furnace that thermally conditions the drawn shape. Distortion can be further reduced by applying a low draw tension to the glass ribbon. The drawn glass ribbon may be applied with a protective coating to maintain its strength attribute and enables the glass ribbon to be spooled.

[0011] Accordingly, in one embodiment, a method for making a glass ribbon is disclosed comprising heating a glass preform in a draw furnace to form a glass ribbon, the glass preform comprising a central portion, a pair of opposing edge portions and a thickness of the glass preform is greater than 200 μιη, but preferably less than 1.5 mm, the heating comprising heating the glass preform such that a temperature of the central portion of the glass ribbon is greater than a temperature of the edge portions of the glass ribbon within a visco-elastic region of the glass ribbon. The glass ribbon is drawn to a predetermined thickness such that a central portion of the glass ribbon is less than 200 μιη and thermally treated in a thermal conditioning furnace at a temperature greater than an annealing temperature of the glass ribbon but less than a softening point of the glass ribbon. A first coating may be applied to the glass ribbon, after which the glass ribbon may be wound onto a take-up spool, wherein a bend radius of the wound glass ribbon is less than about 10 cm.

[0012] The draw furnace may comprise edge heating elements positioned orthogonal to side heating elements. The method may further comprise heating the glass preform in a preheat furnace prior to the heating in the draw furnace.

[0013] Drawing the glass ribbon preferably comprises contacting the glass ribbon with a tractor assembly that draws the glass ribbon downward between two counter-rotating belts. Preferably, a central portion of the glass ribbon has a thickness less than about 200 μηι. Edge portions of the glass ribbon may also be less than 200 μιη.

[0014] A strain point of the glass preform is preferably greater than about 600°C, and in some embodiments the strain point is greater than about 900°C.

[0015] A protective coating may be applied to the glass ribbon before the contacting by the tractor assembly such that the coating is positioned between the glass ribbon and the counter- rotating belts. For example, the protective coating may be applied as a solid film sourced from a roll of coating material that is unwound from the source roll and applied to the glass ribbon. Once formed, the glass ribbon may be rolled onto a spool.

[0016] In another embodiment, an apparatus for drawing glass ribbon is described comprising a draw furnace configured to heat a solid glass preform, the draw furnace comprising a first plurality of heating elements arrayed horizontally along a direction of a width of the draw furnace and a second plurality of heating elements arrayed horizontally in a direction orthogonal to the width of the draw furnace.

[0017] The apparatus may further comprise a thermal conditioning furnace comprising a plurality of heating elements arrayed vertically along a length of the thermal conditioning furnace. Opposing counter-rotating belts rotatably mounted below the draw furnace and preferably configured to extend and retract toward and away from the glass ribbon, respectively, are designed to engage the glass ribbon and apply a downward force to the glass ribbon. The apparatus preferably comprises a first coating applicator for applying a film material between the glass ribbon and the counter-rotating belts. In some embodiment the apparatus comprises a second coating applicator positioned downstream of the counter- rotating belts

[0018] The first plurality of heating elements are preferably separately controlled (i.e. a temperature of the heating elements is separately controlled. The second plurality of heating elements may also be separately controlled.

[0019] The apparatus may further comprise a preheating furnace positioned upstream of the draw furnace.

[0020] The invention will be understood more easily and other objects, characteristics, details and advantages thereof will become more clearly apparent in the course of the following explanatory description, which is given, without in any way implying a limitation, with reference to the attached Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a side cross sectional view of an apparatus for redrawing glass sheets into thinner glass sheets;

[0022] FIG. 2 is a lateral cross sectional view of a draw furnace, including optional preheating furnace and a thermal conditioning furnace, used in the apparatus of FIG. 1 ;

[0023] FIG. 3 is a top down cross sectional view of a draw furnace of FIG. 2 showing horizontally arrayed heating elements that comprise separately-controlled heating zones;

[0024] FIG. 4 is a perspective view of an individual heating element for the draw furnace of FIG. 2;

[0025] FIG. 5 is a top down cross sectional view of a thermal conditioning furnace of FIG. 2 comprising vertically arrayed heating elements that comprise separately-controlled heating zones.

DETAILED DESCRIPTION

[0026] In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of the present invention. Finally, wherever applicable, like reference numerals refer to like elements.

[0027] Embodiments of the present invention comprise, inter alia, providing a glass preform, heating the glass preform in a furnace, forming a gob and drawing the preform into a glass ribbon. By forming a gob what is meant is heating a glass preform to at least its softening point, whereupon a thickened portion of the preform, the gob, pulls away from the body of the preform, drawing with it a broad flow of glass. The softening point is generally regarded as the temperature at which glass will deform under its own weight, a viscosity of approximately 10 ^7'6 poise. [0028] In one embodiment of the invention, a glass preform is formed by conventional glass forming techniques. Such techniques include chemical vapor deposition and casting methods, including the use of sol gels. Chemical vapor deposition (CVD) techniques are well known in the optical fiber arts, and include outside vapor deposition (OVD), vapor phase deposition (VAD), and modified chemical vapor deposition (MCVD), to name a few. Both OVD and VAD entail hydrolyzing glass precursor chemicals in a flame to form a soot, and depositing the soot onto a target to form a porous glass soot preform. The porous soot preform may then be cleaned, dehydrated and consolidated by first heating the preform in the presence of a cleansing gas, such as a chlorine-containing gas, after which the preform is further heated to a temperature sufficient to cause the soot particles to consolidate into a clear, solid glass preform. However, it should be noted that the available glass deposition methods are not limited to the examples presented above.

[0029] In contrast to OVD or VAD, casting of a glass preform may include the mixing of organic glass precursors to form a greenware preform. The greenware preform is dried by heating and/or exposure to a suitable cleansing gas such as a chlorine-containing gas, then heated to consolidate the greenware preform into a clear, solid glass preform. Alternative methods of casting a glass preform include melting glass (e.g. cullet or glass soot) in a suitable crucible and thereafter pouring the molten glass into an appropriate mold to form the desired preform shape. Both casting methods are well known in the art, and will not be described further. As with the methods of depositing glass described previously, it should be noted that casting methods are not limited to the examples presented herein.

[0030] In other embodiments, a glass preform can be formed by other conventional glass forming techniques such as the previously described fusion or float process. Again, such processes are well known and will not be described further.

[0031] FIG. 1 shows an exemplary apparatus, generally designated by reference numeral 10, for drawing glass ribbon 12 from a glass preform 14 in accordance with an embodiment of the present invention. Glass preform 14 is a glass sheet, seen edge-on in FIG. 1. Glass preform 14 may be greater than 200 μιη, such as greater than 0.5 mm, greater than 0.7 mm, greater than 1.0 mm or greater than 1.2 mm. However, glass preform 14 is typically, though required to be, less than 1.5 mm. Apparatus 10 according to the present embodiment comprises a downfeed assembly 16 for holding and moving glass preform 14, a draw furnace 18, an optional thermal conditioning furnace 20, an optional preheat furnace 22, a first coating applicator 24, a tractor 26, an optional second coating applicator 28, and a take-up spooling device 30. Typically, apparatus 10 is capable of producing thin glass ribbons from a preform in an about 3 : 1 ratio according to a width of the preform. That is, due to necking- down of the glass ribbon as it is drawn from the glass preform, the glass ribbon typically has a total width that is approximately one third the total width of the glass preform, comprising edge portions and a central portion disposed therebetween.

[0032] Glass preform 14 may be provided by any of the techniques described above, or by any other known glass making techniques. Preferably, glass preform 14 is rectangular in shape, having generally parallel opposing sides and a width that is greater than the thickness. The glass of the preform is preferable substantially transparent at visible wavelengths, having a transmittance of at least about 95% over the wavelength range from about 390 nm to about 750 nm. For preforms that may be formed in other shapes, such as cylindrical, the preform may be shaped, such as by grinding, into a generally rectangular shape. Although glass ribbon 12 may be drawn from glass preforms having a strain point similar to conventional glasses used in float or fusion processes, e.g. between about 600°C and 700°C, glass having much higher strain points, such as strain points greater than about 700°C, 800°C or even greater than about 900°C, may be drawn. For example, pure fused silica having a strain point of about 1956°C may be drawn into a glass ribbon using the apparatus and methods of the present invention.

[0033] Glass preform 14 is typically suspended from preform downfeed assembly 16, which comprises clamp 32 for clamping onto and securely holding glass preform 14.

Downfeed assembly 16 is capable of moving the glass preform in parallel vertical directions, either upward or downward (along z axis 34), via motor 36 coupled to screw 38 by a nut (not shown). As used herein, the X, Y and Z axes represent three orthogonal axes. Other suitable drive arrangements as may be known in the art capable of providing precise control of downfeed speed may, however, be substituted. Downfeed assembly 16 is also capable of moving the preform in a direction orthogonal to the z axis (i.e. in the x-y plane) so that the glass preform may be positioned appropriately within draw furnace 18. For example, glass preform 14 is preferably centered within the draw furnace to ensure even heating of the preform. [0034] Once suspended from downfeed assembly 16, glass preform 14 is lowered into a hot zone of draw furnace 18 by the downfeed assembly, whereupon a lower portion of glass preform 14 is heated to at least its softening point. For example, the draw furnace may be heated to a temperature of at least about 1075°C to "gob" the preform. The practice of gobbing allows the glass to attenuate its width under its own weight and is done at slightly higher temperature than the actually drawing operation temperature. Draw furnace 18 may be a resistance furnace, wherein heat is derived by flowing a current through resistance heating elements; an induction furnace, wherein heat is derived by inducing a current flow in a microwave susceptor; or any other heating method capable of heating the furnace to a temperature of at least the softening point of the glass preform. For example, the furnace could be a gas furnace wherein a gas fuel is burned to form a flame. Preferably, the furnace is capable of heating the glass to a temperature of at least about 900°C; more preferably at least about 1500°C; and most preferably to at least about 2200°C. Once the preform has been "gobbed", the draw furnace temperature may be reduced.

[0035] In the embodiment of draw furnace 18 shown in FIG. 2 the draw furnace is of the resistance type comprising a pair of opposing side plates 42 and a pair of opposing end plates 44 arranged in a rectangular shape defining a hollow interior space 46. Draw furnace 18 comprises multiple horizontally-arrayed heating zones, labeled Zones 2 - 5 in FIG. 2, each heating zone comprising one or more heating elements 48 arranged laterally along the two major sides of the glass ribbon. Preferably, the heating elements of each heating zone are controlled independently from the heating elements of other heating zones, and in some embodiments, independently of other heating elements within the same heating zone. Each heating element 48 is preferably shaped as a bar to ensure sufficient current carrying capacity, and may be formed from, for example, moly-disilicide. To assist with inducing an even thermal gradient and to reduce hot spots generated from close proximity of the glass preform with the heating elements, side plates 42 and end plates 44 may be formed from a suitable high temperature heat conducting material such as silicon carbide (e.g. Hexoloy®) and are positioned between the glass preform and the heating elements. The side plates diffuse the heat provided by the heating elements and provide a more even thermal profile within draw furnace 18 at each heating zone. The end plates 44 of the rectangular draw furnace are of similar construction to side plates 42 and are heated as separate heating zones (labeled as zone 6 in FIG. 3) from zones 2 - 5. Separate heating elements 50 within the end zones (zone 6) provide the ability to draw the sheet edge at a different viscosity than the center section of the glass ribbon by heating the edges of the glass ribbon to a different temperature than the center section of the glass ribbon. End heating elements 50 may be of the same design as side heating elements 48 but arranged in an orientation generally orthogonal to the orientation of side heating elements 48, and positioned to heat the edges of the glass ribbon rather than the lateral (major) surfaces of the glass ribbon. Preferably, the edges of the glass preform are heated to a temperature greater than an interior, central portion of the glass preform. End plates 44 separate end heating elements 50 from glass ribbon 12. Side plates 42 and end plates 44 mitigate warp and other flatness distortions the drawn glass ribbon may experience due to thermal variation and applied drawing stress across the preform.

[0036] Most glass forming operations require isothermal conditions, but when redrawing a wide flat ribbon it is preferable to have non-isothermal conditions. An isothermal condition across the glass preform coupled with a pulling force applied to the attenuated glass ribbon may produce a non-uniform draw tension across the width of the preform. Under isothermal conditions the draw tension will be greater in the center of the preform than at the edges and therefore the glass in the center of the preform will be drawing faster than the edges. The resulting differential draw tension across the width of the preform can create warp and thickness variations in the glass ribbon. For example, curling at the edges of the ribbon can manifest. Heating the edges of the glass preform, particularly at the root of the preform, to a temperature higher than the center of the preform mitigates the draw tension effect and produces a flatter drawn ribbon. As used herein the preform root refers to the point where the glass preform transitions from an elastic solid to a viscous liquid, generally characterized by a width reduction. More simply put, the root denotes the region where the glass preform ends and the glass ribbon begins. The ability to have temperature control between the glass preform center and the preform edges, particularly at the preform root, enables drawing of glass ribbon that is flat and substantially free of warp. However, the glass ribbon may exhibit some slight thickness variation on the extreme edges. Consequently, removal of the edge portions may be needed.

[0037] In one embodiment, individual side heating elements 48 may be arranged linearly across the width of the furnace, i.e. in a single row. Preferably, the individual heating elements 48 are separately controlled, for example by a controller (not shown) such that a temperature of the individual heating elements may be separately adjusted. The use of individual heating elements, the temperature of which are separately controllable, provides greater flexibility to the drawing process by facilitating the application of a specific spatial temperature profile to the glass preform, and particularly to the glass ribbon across a width of the glass ribbon, thereby reducing temperature related defects such as warping of the glass ribbon due to an uneven temperature profile across the width of the ribbon. In the instance where multiple and separately controlled heating elements are employed, the controller may also control the temperature of the individual heating elements to adjust a temperature profile that is applied to the glass preform.

[0038] As shown in FIG. 3, each draw furnace heating element 48 and 50 may be formed as a "U" shaped element that extends in a longitudinal direction L consistent with the longitudinal direction of draw furnace 18. That is, preferably at least a portion of each individual heating element 48 and/or 50 extends vertically along a length of the draw furnace in a draw direction. Thermocouples 52 may be inserted into draw furnace 18 at locations suitable for monitoring temperatures within the draw furnace.

[0039] In addition to draw furnace 18 a preheat furnace 22, as shown in FIG. 4, may be positioned above draw furnace 18. Draw furnace 18 may be idled at about 500°C between drawing cycles to extend the life of the draw furnace heating elements and to reduce the time needed to heat the furnace to the draw temperature. Immersing the glass preform into a draw furnace at 500°C is generally not a problem if the thermal expansion of the glass is sufficiently low. However, for high CTE glasses, or if the glass is an ion-exchangeable glass, care must be taken when loading the glass preform into the draw furnace. To prevent thermal shock to the glass preform, or to the draw furnace components, optional preheat furnace 22 may be used to preheat the glass preform to a suitable temperature prior to glass preform 14 entering draw furnace 18. Preheat furnace 22 comprises one or more heating elements 53 arranged across a width of preheat furnace 22, which heating elements may be resistive heating elements. For example, heating elements 53 may be wire heating elements, such as coiled wire heating elements. A suitable heating element material may comprise tungsten or nichrome. [0040] Apparatus 10 may comprise an optional thermal conditioning furnace 20, an embodiment of which is shown in FIGS. 2 and 5, that may be positioned underneath draw furnace 18 relative to a draw direction. Preferably, thermal conditioning furnace 20 has multiple heating zones arrayed vertically along the conditioning furnace in direction L, the multiple heating zones comprising a plurality of heating elements 54 arranged vertically down the length of the conditioning furnace. Thermal conditioning furnace 20 typically has a reduced temperature capability compared to draw furnace 18, and may, for example, be fitted with resistance wire heating elements 54. That is, heating elements 54 preferably have less current carrying capability than draw furnace heating elements 48 and/or 50. Thermal conditioning furnace 20 is preferably rectangular in shape with independently controlled side and end heating elements. It should be noted that thermal conditioning furnace 20 is not an anneal furnace since temperatures in the upper heating zones of the thermal conditioning furnace are intended to be operated above the anneal point of the glass ribbon. Thermal conditioning furnace 20 is used to control the shape of the glass ribbon to reduce distortions in the glass ribbon. In addition, thermal conditioning furnace 20 helps reduce some of the internal stress induced into the glass ribbon by quenching the glass ribbon. Thermal conditioning furnace 20 may be directly coupled to draw furnace 18, as shown in FIG. 1, or thermal conditioning furnace 20 may be separate and spaced apart from draw furnace 18.

[0041] Similar to draw furnace 18, thermal conditioning furnace 20 preferably includes heat dissipating plates 39 positioned along a width of the thermal conditioning furnace between the heating elements and the drawn glass ribbon. Thermal conditioning furnace may also include heat dissipating plates 41 positioned between the end heating elements and the edges of the glass ribbon. Heat dissipating plates 19 and 23 may be formed from Hexoloy® or similar material.

[0042] The addition of thermal conditioning furnace 20 under the redraw furnace further enables the ability to draw flat glass ribbon. Whereas the top zone in the conditioning furnace is at a temperature just below the softening point of the glass but above the anneal point of the glass, it is still hot enough to allow viscoelastic deformation of the glass. The edges of the drawn glass ribbon are cooling faster than the center region as the ribbon traverses through the upper zones of the thermal conditioning furnace. With the draw tractor applying a draw tension uniformly across the more ridged portions of the glass ribbon (not to be confused with the lower viscosity region of the preform as mentioned above) the glass will flatten out as the edges quench. The center zone of the conditioning furnace is at a temperature greater than the anneal temperature of the glass of the glass ribbon and the lower zone is at a temperature at approximately the strain point of the glass. This allows any residual stress in the glass ribbon to be reduced or eliminated since the glass ribbon is so thin that the time during which the ribbon is within these zones is sufficient. This, in turn, enables the glass to be spooled without breakage and subsequently used in a roll-to-roll process.

[0043] After the drawing process has been stabilized, a polymer coating 56 is preferably applied to the ribbon above draw tractor 26. Coating 56 protects the glass ribbon from contact with downstream draw components to maintain an acceptable optical quality of the surface and prevent surface damage to the glass that may reduce the strength of the glass ribbon. In addition to, or alternatively to the coating application, a system to apply a tape to the ribbon edges may be performed during drawing. The taped edges of the glass ribbon can enable roll-to-roll processing from the spool by providing a location where the ribbon can be handled. That is, the roll-to-roll handling equipment is able to grip the tape with a mechanical delivery system. Alternatively, the edge taping application may be performed off-line if the polymer coating is to be removed before the tape is applied. Thus, the equipment and process is capable of applying full or partial- width protective coatings to the glass ribbon surface, and the coatings can be permanently or temporarily bonded to the glass.

[0044] Accordingly, FIG. 1 shows a first coating applicator 24 for applying protective polymer coating 56 to glass ribbon 12. Protective polymer coating 56 may be supplied from at least one supply roll 58 and applied to glass ribbon 12 by an applicator 60. For example, applicator 60 may comprise rollers for pressing a coating film to the glass ribbon. Preferably, the applied protective polymer coating 56 is applied to both major surfaces of the glass ribbon. Protective polymer coating 56 provides mechanical protection to the quality area of glass ribbon 12 and prevents direct contact between the glass ribbon and tractor 26. As used herein, the term quality area refers to that portion of the glass ribbon that is eventually sold and used in the manufacture, in direct comparison with the edge portions of the glass ribbon that are typically removed from the glass ribbon and may be used as cullet in further glass forming processes. Thus, the quality area is the central portion of the glass ribbon. [0045] Once drawing has been initiated the glass preform is lowered at a precise feed rate into the furnace dictated by the downfeed motor speed and the temperature profile established within draw furnace 18. For example, a typical downfeed rate for the glass preform is on the order of 10 to 12 mm/min. As previously noted, the draw furnace is lowered to a suitable draw temperature from the starting gobbing temperature (e.g. about 1075°C) once the gob has been dropped and the glass ribbon engaged by tractor 26. The main parameter drivers for the redraw process are: the downfeed rate, the furnace temperature that controls the viscosity of the glass and the pull rate, also known as the draw speed. The draw viscosity is normally in the range between about 10 ⁶ poise to about 10 ⁷ poise. A suitable draw tension, as measured by a load cell located in the downfeed system, satisfactory draw tension can be in the range from about 2 to 3 pounds. A difficulty in drawing a sheet-shaped glass ribbon from a glass preform is that the preform may have, for example, a thickness of about 0.70 mm thick but about 300 mm wide, which equates to an approximately 1 :400 aspect ratio or greater. Thus, to maintain an appropriate flatness of the drawn ribbon requires careful monitoring of the glass preform downfeed rate, the draw furnace temperature and the draw rate applied by tractor 26.

[0046] In an alternative embodiment, a polymeric coating may be applied, for example, by drawing the glass ribbon through a liquid coating bath. Alternatively, a liquid coating may be sprayed onto the surface of the glass. The liquid coating may be applied to one or more surfaces of the glass. The liquid coating may thereafter be cured by a suitable curing apparatus as is required for the coating type. For example, the curing apparatus may be an oven for coating which is cured by heating (thermal cure), or the curing apparatus may cure the coating by exposing the coating to ultraviolet light (photocure). The coatings or coatings applied to the glass ribbon can be permanently or temporarily bonded to the glass.

[0047] Once the gob has dropped through and below thermal conditioning furnace 20 it is pulled down, typically by hand, through the coating applicator and placed into draw tractor assembly 26. Draw tractor 26 comprises two counter-rotating belts 62 driven by a plurality of drive wheels 64 such that the motion between them is downward. Counter-rotating belts 62 are movable inward, toward the glass ribbon, or outward, away from the glass ribbon. When the belts are brought together, glass ribbon 12 is pinched between the belts. The belts may be opened and closed by pneumatically driven actuators, and the pinch force applied by the belts when closed can be adjusted. For a thin glass ribbon having a central portion with a thickness (e.g. average thickness) equal to or less than about 200 μηι, such as equal to or less than about 150 μηι, equal to or less than about 100 μηι and in some instances equal to or less than about 50 μιη, the pinch force is maintained at a minimal amount so as not to crush the glass ribbon between the belts, but enough to allow the belts to open and close smoothly. The tractor unit speed is servo-controlled and set to a speed sufficient to maintain a slight tension on the glass ribbon. A load cell on the clamping device (not shown) provides measurement feedback to the controller so that draw tension applied to the preform can be controlled. A typical draw speed or pull rate is about 0.30 m/min.

[0048] Belts 62 draw glass ribbon 12 downward from glass preform 14. Belts 62 are preferably formed from a high temperature-resistant resilient material. It has been found that a softer, more resilient belt performs better than a hard belt surface. Preferably, belts 62 extend across the entire width of the glass ribbon, and may, for example, be wider than the glass ribbon. A coating as previously described may be positioned between the glass ribbon and the belts, such as by applying the coating to the glass ribbon, to protect the major surfaces of the glass ribbon from direct contact with the belts.

[0049] Alternatively, tractor 26 may comprises a plurality of narrow belts that can be independently vertically positioned and wherein each belt can be run open or closed. For example, six belts may be used, where three belts are positioned on one side of the glass ribbon and the other three belts are positioned on the other side of the glass ribbon. This tractor system enables drawing the glass ribbon from only its edges or only the center or a combination of these. Such an arrangement of tractor belts allows the application of a substantially uniform drawing force to be exerted across a much wider ribbon, e.g. glass ribbons having widths up to 500 mm. The previously described "single" tractor is limited to ribbon widths less than about 150 mm.

[0050] As glass ribbon 12 is drawn downward by tractor 26 the glass ribbon thickness attenuates until the central portion of the glass ribbon reaches a predetermined thickness. That is, the ribbon will typically include a central portion and thickened edge portions. The edge portions may be removed, wherein the central portions can be further processed. The thickness of the glass ribbon, and particularly a thickness of the central portion, is a factor of, inter alia, the speed at which the ribbon is drawn from the preform, the speed at which the preform is fed into draw furnace 18 (the downfeed rate), and the temperature of the draw furnace. The upper limit for the thickness of a glass ribbon that may be drawn is generally determined by the thickness of the preform. Preferably, the maximum thickness of the drawn glass ribbon is less than about 1.5 mm, less than about 1.0 mm, and more preferably less than about 0.7 mm, but typically greater than 200 μιη. Glass ribbon drawn in accordance with embodiments of the present invention may be drawn such that a thickness of the central portion of the ribbon is equal to or less than about 200 μιη, equal to or less than about 150 μιη, equal to or less than about 100 μιη, or equal to or less than about 50 μιη. A thickness of the edge portions of the glass ribbon may also be drawn to be less than 200 μιη.

[0051] The thickness of the glass ribbon may be measured as part of the drawing process, and the result of such a measurement may be used to control, for example, the downfeed rate of the preform and/or the draw rate of tractor 26. The glass ribbon thickness may be measured by a suitable measuring device, such as a laser micrometer, indicated by reference numeral 66 in FIG. 1. Such devices are commercially readily available. An error signal is developed by measuring device 66 based on a predetermined set point for glass ribbon thickness which has been input into a controller. The error signal is relayed to the controller (not shown). The controller may be, for example, a computer. The controller may then adjust downfeed rate, edge roller rotational speed and/or torque, or furnace temperature, or a combination thereof according to pre-determined instructions (such as a computer program), to reduce the error signal from measuring device 66 and therefore correct the glass ribbon thickness.

[0052] In some embodiments, an optional second coating applicator 28 may be employed. As shown in FIG. 1, second coating applicator assembly 28 for applying protective film 68 to glass ribbon 12 may be included. Protective film is supplied from at least one supply roll 70 and applied to glass ribbon 12 by an applicator roll 72. Preferably, the applied protective film 68 is applied to both major surfaces of the glass ribbon.

[0053] The final step is spooling the coated ribbon. The manner of recovering the drawn glass ribbon from the drawing operation depends in part upon the thickness of the glass ribbon, and more particularly to the thickened edge portions. For example, if the glass ribbon thickened edge portions have a thickness equal to or less than about several hundreds of microns, the glass ribbon may be rolled onto bulk spool 31, shown in FIG. 1, using a motorized spooler. The spooler has a tension control that allows the glass to be wrapped securely onto the spool with minimal tension. The spools could be stored for long periods of time and from bending the glass around the spool diameter a surface tension is induced on the ribbon therefore having a lower spooling tension on the glass will reduce breakage. Bulk spool 31 may then be used as a source spool in a subsequent roll-to-roll process wherein the glass ribbon is moved from the source spool to a subsequent take-up spool and intermediate processing occurs to the glass ribbon as it travels from the source spool to the take-up spool. The intermediate processing can include, for example, finishing (e.g. removal) of the glass ribbon edges, deposition of one or more thin film layers, or any other process than may be used to add value to the glass ribbon in the furtherance of providing a finished product. For roll-to-roll processing several spools of ribbon can be spliced together to provide longer lengths of ribbon.

[0054] Alternatively, individual panels of predetermined size may be cut from the ribbon at a later time if desired. Glass ribbon having a thickness such that the glass ribbon would break during an attempt to roll the ribbon, for example greater than about a millimeter in thickness, must be cut into individual panels of a predetermined size or sizes during the drawing process. Cutting of the glass ribbon into individual panels may be accomplished by any conventional method known in the art, including scoring and breaking, or laser cutting of the glass ribbon.

Example

[0055] As described above, careful management of the temperature profile is need to overcome the differential draw tension experienced by the glass preform as the glass preform is drawn into a glass ribbon. To that end, the draw furnace and the thermal conditioning furnace are segregated into zones, wherein temperatures within the zones can be individually manipulated. In reference to FIGS. 2 and 3 for example, an exemplary temperature profile for drawing a glass ribbon from a glass sheet preform can be established so that the zones for the draw and thermal conditioning furnaces are heated in accordance with Table 1.

Table 1

Temperature (°C) Zone

1038 2 1039 3

1038 4

1039 5

1050 6

880 8

883 9

785 10

785 11

663 12

667 13

Preform feed rate 12 mm/min.

Load cell 2.8 pounds

Draw speed 0.3 m/min.

[0056] The glass preform according to the present example may have a width in a range from about 280 mm to about 325 mm. The length of the glass sheet is consistent with the amount of glass intended to be drawn and the physical capability of the draw apparatus. The glass preform has a strain point in a range from about 600°C to about 1956°C, for example, from about 600°C to about 1000°C, from about 600°C to about 900°C or from about 600°C to about 800°C. In other embodiments the glass preform has a strain point in a range from about 700°C to about 1956°C, from about 800°C to about 1956°C, from about 900°C to about 1956°C or from about 1000°C to about 1956°C. A thickness of the preform may be in a range, for example, from about 0.70 to about 1.5 mm.

[0057] Preferably, the glass preform is driven downward by downfeed assembly 16 at a feed rate in a range from about 10 mm/min to about 12 mm/min. A draw speed applied by tractor assembly 26 is in a range from about 0.2 to about 0.4 meters/min. For example, the draw speed applied by tractor 26 may be about 0.3 meters/min, resulting in a draw tension at the centerline of the glass ribbon of about 2.8 pounds.

[0058] It will be apparent to those skilled in the art that various other modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.