Glass manufacturing – Processes – Glass preform treating
Reexamination Certificate
2000-08-02
2002-04-23
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes
Glass preform treating
C065S030100, C065S030130, C065S030140, C065S117000, C065S374130
Reexamination Certificate
active
06374640
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for compacting or shrinking flat glass panes, especially display glasses, in which on at least one panel at least one flat glass pane is subjected to a heat treatment in a furnace at a temperature ranging from 300° to 900° C.
2. Description of the Related Art
Structurally, glass is amorphous, this amorphous structure not being fixed but dependent on the prior thermal history. The amorphous structure can also change after the manufacturing process, if the glass product is subjected to a thermal stress. Associated with each change in the amorphous structure is a change in the density to higher or lower values, measured at room temperature. These structural changes and the corresponding changes in the density can be calculated at least approximately using known physical laws from the temperature-time curve of the thermal stress (George W. Scherer: Relaxation in Glass and Composites, John Wiley & Sons, Inc. (New York, Chichester, Brisbane, Toronto, Singapore), 1986, Library of Congress Catalog Card Number: 85-17871).
From the JP Abstract 08-301628 A, a thermal conduction furnace is known, in which a glass plate, which is to be heated, lies on a heating block, which is provided with an aluminum layer in order to ensure good thermal conductivity. Above the glass plate, a further heating block is disposed movably and pressed onto the glass plate. Since the heating elements are integrated into this heating block, the danger exists that, due to the different heat outputs, a different and, with that, harmful input of heat into the glass plate, which is to be treated, takes place.
The DE 3 422 347 A1 describes a method for leveling thin glasses, for which a stack of glass plates is formed, which lies on at least one layer of paper. Paper layers have also been provided between the individual glass plates. The lower layer of paper lies on a flat supporting plate, which may consist of graphite, ceramic, glass or metal. The coefficient of expansion of the supporting plate should vary in the same way as that of the thin glass, which is to be leveled, so that thick glass plates of the same material are preferred.
Aside from such leveling methods, which are carried out above the upper cooling point, there are so-called compaction methods.
During a heat treatment following the actual manufacturing process at temperatures ranging from the lower to the upper cooling point (these are the temperatures, at which the viscosity of the glass is 10
14.5
dPas and 10
13
dPas respectively), there is generally shrinkage of the material, which is strong at high temperatures and weak at low temperatures. It follows from the physical laws cited that the weak shrinkage at a low temperature is even weaker, if the glass previously was compacted at a higher temperature, preferably between the lower and the upper cooling points.
Flat glass, intended for the production of flat displays, generally must be compacted, so that there is no further significant shrinkage during the different temperature stresses in the course of the further manufacturing process of the display pane. If there were such further shrinkage, the different structures, applied in layers during the manufacture of the flat display, would no longer be aligned as desired.
Liquid crystal display screens (LCD's) constitute a significant proportion of the flat displays. The usual process temperatures during their manufacture are between 200° C. and 400° C. The maximum permissible shrinkage of the glass substrate during the manufacturing process depends on the technology employed. For the thin film transistor (TFT) based on amorphous silicon, the shrinkage must not exceed 10 ppm (T. Yukawa, K. Taruta, Y. Shigeno, Y. Ugai, S. Matsumoto, S. Aoki (1991): Recent progress of liquid crystal display devices, In: Science and Technology of new glasses. Eds.: S. Sakka & N. Soga, pages 71-82, Tokyo, 1991).
Flat plasma displays are also widespread. Their manufacturing process comprises, for instance, the mounting of electrodes, cross members, phosphorus and dielectric layers usually at temperatures ranging from 450° C., and 600° C. The shrinkage of the thin glasses, used as substrate, may not exceed 20 ppm during this process.
Immediately after the glass pane is produced, for example, by a drawing or floating process, the glass generally is not yet compacted sufficiently, so that a further temperature treatment (post-annealing) must be carried out.
For example, the shrinkage of an alkali-free glass, typical of glasses used for display applications (such as AF 45 of the Deutsche Spezialglas AG, Griinenplan) during a subsequent annealing for 1 hour at 450° C. is about 50 ppm if the glass has not previously been compacted. This shrinkage can be reduced to values less than 12 ppm by an appropriate temperature treatment. In the case of a glass with a low cooling point, (such as glass D263 of the Deutsche Spezialglas AG, Grünenplan), the shrinkage directly after the manufacturing process is even more than 300 ppm during annealing at 450° C. for 1 hour. This value can be reduced to less than 20 ppm by a suitable post-annealing.
The post-annealing is carried out in a batch or continuous furnace. For economic reasons, the glass panes, generally 10 to 20 panes with a thickness of the order of 1 mm, are combined into stacks. These stacks are placed onto a supporting panel and sometimes weighed down with a covering panel, for which purpose quartz plates, for example, are used.
The tendency of the stacked glass panels to adhere at higher temperatures, such as those between the lower and upper cooling point, creates difficulties. In order to avoid this adhesion of the panels, layers of inorganic powders are introduced as release agent (U.S. Pat. No. 5,073,181) between the glass panels. It is a disadvantage that the powder can affect the optical quality of the surface of the panel, if a certain particle size is exceeded.
Furthermore, the need to assure the best possible temperature homogeneity within the whole of the stack during the annealing creates difficulties. Any temperature inhomogeneity from pane to pane (that is, a vertical temperature inhomogeneity in the stack) means that, depending on the temperature program, the different panes pass through different temperature histories and, with that, have different compactions.
For a single pane, a vertical temperature gradient within the stack generally does not present a problem, since the height of the pane usually is small in relation to the height of the stack. It is different for a lateral temperature inhomogeneity. The latter means that, depending on the temperature program, the different sections of a pane pass through different temperature histories and, with that, have different compactions. For the individual pane, however, a lateral temperature inhomogeneity also means that, at the end of the annealing process, an internal stress develops in the pane, the relaxation of which during subsequent temperature stresses in turn can lead to local changes in volume. If there is a temperature gradient in the pane in the plane of the pane during the annealing, then this leads to a mismatching of the different sections of the pane during the temperature equalization at the end of the annealing process. This mismatching is compensated for by a mutual distortion of the different sections of the glass. If these tensions are relaxed during a subsequent temperature treatment, the different sections of the glass can expand or contract.
Such inhomogeneous volume expansion or shrinkage effects are a major problem for the display manufacturer, because the latter cannot compensate for them by an appropriate dimension of the masks during the subsequent coating processes.
The existence of a certain temperature inhomogeneity is unavoidable. During the heating, which necessarily is a part of the annealing process, heat must flow into the stack; during the cooling, which is also necessarily part of the annealing process, heat must flow out of the stack
Buergel Wolfgang
Fotheringham Ulrich
Hoelzel Eva
Ostendarp Heinrich
Sprenger Dirk
Colaianni Michael
Schott Glas
Striker Michael J.
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