HK1027337B - Method for precompressing flat glass - Google Patents

Description

The invention relates to a process for pre-compacting flat glass panes, particularly display glass panes, whereby at least one plate of flat glass is subjected to a heat treatment in a furnace in the range of 300°C to 900°C.

The structure of glass is amorphous, and this amorphous structure is not fixed but depends on the thermal history of the glass. It can change even after manufacture if the glass product is subjected to thermal stress. With any change in the amorphous structure, a change in density measured at room temperature is associated with a change to higher or lower values. These structural changes and corresponding changes in density can be calculated at least approximately by applying known physical laws from the temperature-time curve of thermal stress (George W. Scherer: Relaxation in Glass and Composites, John Wiley & Sons, Inc. (New York, Chichester, Brisbane, Toronto, Singapore), 1986.

A heat conduction furnace is known from the summary of JP-A-08-301628 in which a glass plate to be heated is placed on a heating block which is coated with an aluminium layer to ensure good thermal conductivity. Above the glass plate there is another heating block which is mobility-oriented and pressed on the glass plate.

DE-A1-3 422 347 describes a process for planning thin panes of glass in which a stack of glass sheets is formed on at least one paper layer. Paper interlayer is also provided between the individual glass sheets. The bottom paper layer is on a flat support plate which may be made of graphite, ceramic, glass or metal. The expansion coefficient of the support plate should be the same as that of the glass to be planned, so that thick glass sheets of the same material are preferred.

In addition to such planning procedures, which are performed above the upper cooling point, there are so-called pre-compression procedures.

The temperature treatment following the actual manufacturing process, in the temperature range between the lower and upper cooling point (the temperatures at which the viscosity of the glass is 1014.5 dPa and 1013 dPa respectively), generally results in a strong, low-temperature compression of the material.

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The maximum permitted compression of the glass substrate during the manufacturing process depends on the technology used. In the case of the thin-film transistor (TFT) technology based on amorphous silicon, the compression may 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 science. Eds.: S. Soga & S. Sakka, pp. 71-82, Tokyo, 1991).

Flat plasma displays are also widely used, the manufacturing process involving, inter alia, the application of electrodes, rods, phosphorus and dielectric layers, usually in the temperature range between 450°C and 600°C. The density of the thin glass used as substrate during these processes may not exceed 20 ppm.

Immediately after the glass is made, for example by a pulling or floating process, the glass is generally not sufficiently pre-compacted and a further temperature treatment (night tempering) must be added.

For example, the shrinkage of an alkaline-free glass typical for display applications (e.g. AF 45 of Deutsche Spezialglas AG, Green Plan) at a subsequent temperature of 1 hour at 450°C is about 50 ppm if the glass has not been pre-compacted.

In the case of glass with a lower cooling point (e.g. glass D263 by Deutsche Spezialglas AG, Green Plan), the shrinkage is even more than 300 ppm immediately after the manufacturing process at 450°C for 1 hour.

The night tempering is carried out in a charger or in a blast furnace. For economic reasons, the glass panes, usually 10-20 panes (thickness 1 mm), are stacked in piles. These piles are placed on a support plate and sometimes charged with a cover plate, for which, for example, quartz plates are used.

The problem is that stacked glass panes tend to stick at higher temperatures, such as between the lower and upper cooling points, and to prevent this, layers of inorganic powder are inserted between the panes as a separating agent (US Patent 5,073,181).

The need to ensure the greatest possible temperature homogeneity throughout the whole pile during the heating process is also a problem: temperature inhomogeneity from plate to plate (i.e. vertical temperature inhomogeneity in the pile) means that, depending on the temperature programme, the different plates undergo different temperature ranges and thus have different pre-compression.

For a single sheet, a vertical temperature gradient in the pile is generally not a problem, since the height of the sheet is small in relation to the height of the pile. This is different for a lateral temperature inhomogeneity. It means that, depending on the temperature program, the different sections of a sheet go through different temperature layers and thus have different pre-compressures. A lateral temperature inhomogeneity for the individual sheet also means that at the end of the tempering process an internal tension builds up in the sheet, the relaxation of which at subsequent temperature loads can lead to local volumes changes on the one hand.

Such inhomogeneous volume expansion or shrinkage effects are particularly problematic for the display manufacturer because it cannot compensate for them by appropriate mask sizing in subsequent coating processes.

The presence of a certain temperature inhomogeneity is inevitable. During the heating necessarily contained in the temperature process, heat must flow into the pile; while the cooling necessarily contained in the temperature process, heat must flow out of the pile. Both processes also involve an internal heat flow in the pile, which requires an internal temperature gradient as a driving force. For geometrical reasons (a typical value for the pile height is 2 cm, where the lateral dimensions vary to the order of 1x1m), it is preferable to allow the heat flow to descend over the plate.

A large lateral temperature difference would have two unfavourable effects (different pre-compression and voltage effect), whereas a vertical temperature difference has only one unfavourable effect, namely the different pre-compression. It is therefore desirable to have a temperature distribution as homogeneous as possible in the lateral direction.

The technical implementation of a temperature homogeneity test depends on the temperature range in question. For night temperature tests for display lenses, these are typically 500°C to 700°C (the lower and upper cooling points of D 263 are 529°C and 557°C respectively; the lower and upper cooling points of AF 45 are 627°C and 663°C respectively, and 300°C to 900°C for special lenses).

For high temperature homogeneity requirements, air converters are used, in which air is heated to the desired furnace temperature and turned in the furnace.

In the case of heating or cooling, the air is made slightly hotter or cooler than the outside of the heap to generate a driving force for a heat flow into or out of the heap.

The heating of the glass panes is not desirable for several reasons: first, the ventilation involves additional costs; second, the ventilation releases the parts of the separating agent from the glass panes' spaces and may introduce unwanted dirt into the spaces; this is particularly undesirable for display panes, which are usually processed in clean rooms; and, accordingly, a circulation-free tempering is desirable.

US-A-5597.395 is known for a precompressing process in which the glass panes are simultaneously subjected to a gas pressure at a temperature in an oven.

The purpose of the invention is to provide a method for pre-compressing flat glass panes which is simpler and therefore more cost-effective and which ensures a high temperature homogeneity in the glass.

This is achieved by a process of the type described at the outset, characterized by heat treatment in a radiation furnace, where the flat glass pane is placed on at least one ceramic plate with a thermal conductivity at least five times greater than that of the glass pane to be treated.

The heat treatment temperature or temperature range shall be chosen according to the glass values of the glass plate to be treated, preferably between the lower and upper cooling points.

The use of a radiation furnace is cheaper than a circulating furnace because the additional costs of ventilation etc. are eliminated.

Furthermore, when separating materials are used, they do not release the glass from the glass panes.

Radiant furnaces have not been used in the temperature range 300°C to 900°C in general, because in this temperature range the heat transport by radiation is generally not sufficient to overcome the design-related inefficiencies in the furnace, such as different performance of apparently identical heating elements, uneven insulation, etc., and to produce a high temperature homogeneity in the furnace space.

In addition, glass has a poor thermal conductivity, typically 1 W/mK, which further increases the occurrence of temperature inhomogeneities in the glass.

It has been shown that these disadvantages of a radiation furnace can be compensated by placing at least one glass pane on at least one ceramic plate with a thermal conductivity at least five times greater than that of the glass pane to be treated in the temperature range in which the heat treatment is carried out.

The advantage of such ceramic panels is that they absorb the heat flow and distribute it simultaneously over the large area of the glass pane, thus balancing temperature differences in the plane of the pane.

The advantages of the quartz sheets used so far have not been achieved because the thermal conductivity of this material is only about that of the glass panes to be treated.

Preferably, a stack of flat glass panes is placed on the ceramic plate and heat treated.Temperature homogeneity in the glass can be further improved by arranging the glass plate or stack of glass panes between at least two such ceramic plates.

The preferred use of porous ceramic panels is that such panels cannot absorb foreign materials such as detergent residues or the like which may adversely affect the surface of the glass panels to be treated during the heat treatment.

Ceramic tiles preferably made of SiC or containing SiC are used, such as tiles made of nitrite-bound SiC and silicon-filtered SiC, the latter being particularly preferred because of its porosity.

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However, there is no evidence of the state of the art that these ceramic materials are also suitable for use in the pre-compression of flat glass panes, because other properties of the ceramic material are of primary importance.

The use of ceramic materials has the advantage that a lateral temperature difference in the furnace (for example between two opposite side walls) is only noticeably reduced in the furnace, especially when the heat transfer between the radiating surfaces in the furnace is small at the relevant temperature.

It has been shown that the use of high-conductivity ceramics can reduce the lateral temperature gradient in the stack by more than half if the height of the glass stack and the thickness of both ceramic plates are adjusted.

Preferably ceramic tiles of such thickness are used that the ratio of the total thickness of the ceramic tiles to the glass pile height is at least 1/λ·40·W/ ((mK), where λ is the thermal conductivity of the ceramic material in the heat treatment temperature range.

This is illustrated by the following example.

The ceramic plates are placed in the oven in such a way that one side of the ceramic plate is parallel to the side walls of the oven and both exchanging radiation. The widths of the ceramic plates are insulated against the oven. The average temperature in the oven room is 500°C. The side walls of the oven, like the ceramic plates, have an evaporation of Δ. A net heat flow flows through the rigid sides of the ceramic plates. The heat flow density is approximately 100 W/m2K.

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Preferably, the glass pane or the glass pane stack is placed between at least two plates of silicon-infused SiC (SiSiC). If several ceramic plates are to be used between the glass pane and the furnace wall, the silicon-infused SiC plate is preferably facing the glass pane because the silicon-infused silicon carbide has the advantage of not having porosity, which can deposit detergent residues, metallic dust and the like, which can react with the adjacent glass surface during tempering and render it unusable.

Preferably, the silicon-filtered SiC sheets are finely ground to produce a roughness Rtm ≤ 10 μm, preferably ≤ 1 μm, to avoid optical effects from the forced slip of the glass on the silicon-filtered silicon carbide during pre-compaction during tempering and further slip caused by the different coefficients of thermal expansion.

The use of a separator, as is known from US-A-5,073,181, has a lasting effect on the temperature homogeneity in the stack. The vertical thermal conductivity is a weighted mean of the thermal conductivity of the glass and that of the powder - typical value 0.1 W/mK. If the powder layers are 1/10th the thickness of the glass sheet, for example, the weighted mean is 0.5 W/mK. The powder thus increases the temperature homogeneity in the stack. To further improve the temperature homogeneity, it has been surprisingly found that the glass sheet stacks can be tempered without further separators if the samples are subjected to a chemical treatment.

The final surface treatment consists of a rinse with distilled water, preferably in a cask rinse. The distilled rinse is preferably subjected to a particle filtration up to 1 μm diameter. The specific resistance of the distilled water is > 1 MO, e.g. 18 MO. The different types of distilled water are also included. The distilled water must be kept in a constant state of equilibrium between the temperature of the dry air and the temperature of the dry air, so that no significant pressure is required to control the flow of the liquid.

The method of the invention is explained in more detail below by means of examples.

The Commission shall adopt implementing acts laying down the rules for the application of this Regulation.

The temperature was measured in 540*420*6 mm3 SiSiC plates produced by the slitting process with subsequent Si infiltration.

The surface was ground with 200 grains of SiSiC on a metal grinding disc until a surface roughness of maximum 5 μm was achieved at a plane of 60 μm above the diagonal. The plates did not show any ground bubbles larger than 10 μm after grinding. After the grinding process, the plates were stored for 24 h in a 6 molar HCl solution and then washed.

Chemical treatment Type of glass AF45

To temper the AF45 type glass without a separator, the glass was made to a size of 320*320 mm from 1.1 mm thickness. The glass was held in a stainless steel vessel with Teflon strips on the sides and bottom for the treatment process. The distance was chosen so that the glass was not under tension. The glass was treated in a cleaning system, the first basin of which was filled with a bath with a pH of 12. The glass was left in the basin at 50°C for 5 minutes. The glass was then placed with the derb in a basin of water with the lead distilled and there for about 3 minutes, measured below 1 μS, so that the glass could be transported in the bag. The glass was placed in a bath with a pH of 2°C and kept in a bag at 50°C for 5 minutes.

The basket was then placed back in a sink with distilled water and washed again to a conductivity of 1 μS. Finally, it was purified in a purified water produced by reverse osmosis and electro-ionization to a conductivity of 0.5 μS. A 10 μm particulate filter is implemented in this sink.

During the treatment, the basket was moved up and down continuously by 6 cm in all the basins, ultrasound was used in all the basins except the sinks, the same treatment was also carried out for 1.9 mm glass, and drying was carried out at 200°C for 20 minutes.

Chemical treatment Glass type D263

The glass was treated in an automated cleaning system in a clean room, where the basket was taken from a storage tank and transported to the first basin.

The bath had a pH of 12, and the glass was left in the basin at 60°C for 5 minutes. The basket was then placed in a sink with distilled water, left there for 5 minutes, and then transported to the acid basin. The pH was set to 2, and the glass was left in the basin at 60°C for 5 minutes.

The final treatment was carried out in a cascade basin (3 times, each 5 minutes) until a measured conductivity of 0.05 μS was reached. The basin incorporated a 5 μm particle filter. The temperature in the distilled water in the basin and cascade basin was 60°C. In the final basin the basket was slowly moved upwards with a lifting mechanism of 1 cm per second. The drying was carried out in a dry web module with a high-performance clean room temperature of 100 °C for 8 minutes.

The discs were then packed in the clean room and transported shockproof.

Temperament

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The test chemical is used to determine the concentration of the test chemical in the test medium.

For the measurement of the compression, the discs were fitted with a grid of engraved marking crosses at a distance of 100 mm before the subsequent temperature load. The distances of all adjacent markers were measured with a coordinate measuring machine before and after the subsequent temperature load. The temperature load for D 263 was 1h, 450°C, heating and cooling rate 5K/min. The temperature load for AF 45 was 1h, 590°C, heating and cooling rate 6K/min. Three discs (one from the middle, the second top and the second bottom) from each stack were fed into the compression measurement.

The results were as follows:

Tab.:

Δl/l [ppm] für D 263 und AF 45 Gläser nach einer zusätzlichen Temperaturbehandlung, gemessen für je 3 Glassheiben.
Glas-Typ	Scheiben Nr.
	1	2	3
D 263	-1 ± 3	1 ± 2	3 ± 3
AF 45	0 ± 3	2 ± 5	1 ± 4

Claims (13)

1. Method for precompressing flat glass panes, especially display glasses, in which at least one flat glass pane on at least one panel is subjected to a heat treatment in a furnace in the range from 300° to 900°C, characterized in that the heat treatment is carried out in a radiation furnace, in which the flat glass pane is disposed on at least one ceramic panel with a thermal conductivity which, in the heat- treatment temperature range, is at least 5 times as great as that of the glass pane which is to be treated.
2. Method according to Claim 1, characterized in that a stack of flat glass panes is arranged between at least two ceramic panels.
3. Method according to Claim 1 or 2, characterized in that pore-free ceramic panels are used.
4. Method according to one of Claims 1 to 3, characterized in that ceramic panels which consist of SiC or contain SiC are used.
5. Method according to one of Claims 1 to 4, characterized in that ceramic panels are used having a thickness which is such that the ratio of the total thickness of the ceramic panels to the height of the glass stack is at least 1/λ/·40W/(mK), λ being the thermal conductivity of the ceramic material in the heat-treatment temperature range.
6. Method according to one of Claims 1 to 5, characterized in that at least two panels of silicon-infiltrated SiC are used.
7. Method according to Claim 6, characterized in that the silicon-infiltrated SiC panels face the glass stack.
8. Method according to one of Claims 1 to 7, characterized in that when silicon infiltrated SiC panels are used, the latter are subjected to a precision grinding in order to produce a roughness R_tm ≤ 10 µm, preferably ≤ 1 µm, before they are brought together with the stack of glass panes.
9. Method according to one of Claims 1 to 8, characterized in that the glass panes are subjected to a chemical treatment before they are assembled into a stack.
10. Method according to Claim 9, characterized in that the chemical treatment comprises the following steps:

    - immersion in an alkaline solution having a pH 10,

    - treatment with distilled water,

    - treatment with an acidic medium having a pH < 4, the acidic medium possibly containing surfactants,

    - rinsing with distilled water,

    - drying the glass panes.
11. Method according to Claim 10, characterized in that the distilled rinsing water is subjected to a particle filtration down to 1 µm.
12. Method according to Claim 10 or 11, characterized in that the resistivity of the distilled rinsing water is > 1 Mohm.
13. Method according to one of Claims 10 to 12, characterized in that the temperatures of the liquids used are between 40°C and 80°C.