CN1121991C - Thermal dimensional stability treatment of vitreous sheet material by contacting with a molten alkali matel salt - Google Patents

Abstract

A sheet of vitreous material for use as a substrate is produced by contacting the sheet with a molten alkali metal salt prior to any firing step or other heat treatment associated with the application of coating layers. The so-treated sheet has improved thermal dimensional stability, such that wide ranges of heating and cooling applied by the heat treatment do not create an unacceptable amount of shrinkage of the vitreous material.

Description

Thermal Dimensional Stabilization of Glassy Materials Contacted with Molten Alkali Metal Salts

The present invention relates to processing vitreous sheet material such as glass and sheets formed therefrom. In particular, the invention relates to a process for the preparation of vitreous sheets with improved dimensional stability.

Since vitreous materials such as glass are amorphous, they undergo dimensional changes, such as area reduction, when heated and cooled. Dimensional reduction (also known as densification or shrinkage) is inconsequential for many applications. However, there is an increasing demand for glasses with improved dimensional stability, such as glass sheets used as substrates in electronic applications. In these applications, even a small amount of shrinkage is unsatisfactory because the glass sheet is required to be precisely matched to the other components.

Various proposals have been made to obtain the temperature history of the glass in order to reduce its tendency to shrink when post-processing the glass. An effective method is to pre-shrink the glass, thereby densifying the glass to reduce shrinkage of the glass during the required heat treatment.

Annealing is a known densification method, however, it requires prolonged treatment at high temperature, consumes a lot of thermal energy, and results in unstable quality. The degree of densification varies with annealing temperature, time and cooling rate. Another challenge is the loss of glass viscosity at high temperatures, which can warp the glass sheet or create surface defects during annealing.

Another solution is to choose a glass of a specialty composition with a high strain point, on which shrinkage is negligible when processed.

A particular problem arises from firing glass substrates at about 500°C, i.e. above the annealing point of soda-lime glass, so that untreated soda-lime glass sheets have smaller linear dimensions after cooling, e.g. About 600 microns/meter smaller than the starting sheet.

It is an object of the present invention to provide a simple and reliable method of improving the thermal stability of vitreous materials so that they can be subjected to heat treatment with reduced dimensional shrinkage.

Chemical quenching treatments of vitreous materials are known. Its purpose is to increase the mechanical and thermomechanical resistance of vitreous materials such as glass and microcrystalline materials. There are two types of chemical quenching. According to one method, the ion exchange is performed at a temperature high enough that the glass undergoes stress relief, while the ions entering the glass lower the coefficient of thermal expansion of the surface layers of the glass. According to another method, the ions already present in the surface layer of the glass are replaced by large ions and the ion exchange is carried out at a temperature below the annealing point of the glass so that any significant amount of stress relief does not occur.

GB 1276186 discloses a chemical quenching treatment of a glass body, including allowing ions of at least one substance to enter the surface layer of the glass body, followed by application of a coating such as oxides of titanium, silicon, aluminum, chromium or other metals, titanium nitride, or silicon, titanium , tantalum or other metal carbides. Chemical quenching increases the adhesion of the coating. Examples of the quenching chemicals in the above patents include potassium nitrate, sodium nitrate, lithium carbonate and lithium chloride.

We have now found that in order to reduce the amount of shrinkage of the vitreous sheet during firing or other heat treatment forming part of the step of applying a coating to the sheet, it is advantageous to bring the sheet into contact with a molten alkali metal salt prior to said heat treatment of the sheet. efficient.

According to the invention, there is provided a method for the production of a vitreous sheet for use as a substrate, characterized in that said sheet is combined with The molten alkali metal salt is contacted, wherein the temperature of the molten alkali metal salt is maintained at 400-500° C. and kept uniform with a maximum deviation of 2° C.

The contact time between the vitreous material and the salt is generally from 15 minutes to 72 hours, preferably from 4 to 8 hours. For potassium and other large ions, the temperature of the molten salt should be lower than the annealing point of the glassy material, and the melting temperature is generally 400-500°C. Uniformity of temperature of the molten salt is important throughout the period of tanking and soaking. The maximum deviation of molten salt temperature should be 2°C. Molten salt mixing to maintain uniformity can be achieved by passing carbon dioxide through the salt, for example as taught in earlier patent GB-A-1274733.

The method is carried out so that the alkali metal ions of the molten salt diffuse into the skin of the board and the ions of the skin diffuse into the molten salt, whereby the ion exchange generates or increases the compressive stress of the skin.

It has been unexpectedly found that, in addition to other effects of the vitreous sheet, chemical quenching improves the dimensional stability of the vitreous sheet such that extensive heating and cooling due to heat treatment does not produce undesired amounts of shrinkage. It is totally unexpected that after heat treatment, such as firing at a temperature above the annealing point of the vitreous material (e.g. 580°C), all stresses induced by ion exchange in the surface layer of the sheet disappear, and the glass strengthened by chemical quenching The strengthening effect of the quality material will be lost.

The uniformity of the dimensional stability of the produced plate is better than that of the plate only annealed. The invention thus provides vitreous panels which are suitable for applications in which the dimensions of the panels are critical.

The reason why chemical quenching is effective for thermal dimensional stability is not fully understood. Apparently this is partly due to the good thermal contact between the molten salt and the sheet surface, and partly due to the ion exchange that takes place between the salt and the sheet.

Although the invention is generally applicable to soda-lime glasses and has been described primarily for this type of glass, the method of the invention can be applied to other types of glasses such as borosilicates, aluminosilicates and microcrystalline materials if desired .

The choice of chemical quenching salt depends on many factors, such as the specific composition of the vitreous material. Molten salts of metals or metal mixtures selected from lithium and potassium are generally suitable. Molten potassium nitrate is particularly suitable for soda lime glass. The contact between the sheet and the molten salt is conveniently accomplished by immersing the sheet in the salt in a suitable container.

The temperature profile of the vitreous material should be carefully adjusted through the different stages of the method of the invention. The salt needs to be kept molten at high temperature. The temperature of the vitreous material is preferably raised slowly to the temperature of the molten salt, more preferably the rate of temperature rise is less than 15°C/min. To achieve the required quenching, the immersion in the molten salt should be continued for a period of time to ensure that the temperature of the vitreous material rises and remains at the molten salt temperature.

When the vitreous material is taken out of the molten salt, it should be cooled slowly from the salt temperature, preferably to room temperature, preferably at a cooling rate of less than 15°C/min. The slow cooling rate is presumed to induce a small amount of shrinkage in order to avoid breaking the structure of the vitreous material. In some cases, the cooling is best performed in two steps with different cooling rates for each step.

The invention is further described with reference to the following non-limiting examples. The glass plate sample used was cut to a standard size of 350 x 270 mm. The plate thickness of Examples 1-5 was 1.1 mm, and the plate thickness of Examples 6 and 7 was 3 mm. The samples of Examples 1-5 were placed in a heating chamber at 200°C until removed before being processed as follows.

Accurately measure various samples with a length of 350mm before treatment and at various stages of treatment, and all length measurements are carried out at room temperature of 20°C.

Example 1

Place 12 common soda-lime float glass samples at 200°C on the carrier, put them into the preheating chamber, and heat them uniformly to 460°C in this chamber, the heating time is 1.5 hours, and the heating rate is about 3°C /minute. The glass and carrier were then placed in a tank containing molten potassium nitrate at a temperature of 460°C and left in the tank for 4 hours for ion exchange. The glass and support were then placed in a drain chamber at 460°C for 30 minutes, during which time most of the molten potassium nitrate was drained from the tank containing the glass and support and collected for further use.

The glass samples were then placed in a cooling chamber and cooled slowly, first at a rate of about 1°C/minute for 2 hours, then at a rate of about 2°C/minute for 1 hour and 15 minutes.

After the above-mentioned treatment, the dimensions of various plate samples nominally 350 mm were accurately measured again at 20°C, and compared with the numbers measured before treatment, in order to determine the amount of shrinkage caused by the treatment. The results are shown in the appended table, where the figures are the average of all 12 samples.

Example 2

A batch of 10 ordinary soda-lime float glass samples at 200°C was placed on the carrier, and the treatment was roughly carried out as described in Example 1, but the difference was that the carrier was directly placed in the tank of molten potassium nitrate, and the A step of preheating to a tank temperature of 460°C is included. This heats the glass to tank temperature more rapidly, with an estimated temperature rise rate of about 50°C/minute.

After finishing the treatment, the dimensions of various plate samples nominally 350mm were accurately measured again at 20°C and compared with the numbers measured before treatment to show the amount of shrinkage. The averaged results are shown in the attached table.

Example 3

The 7 plate samples treated according to Example 1 were subjected to treatment (A), which included heating the sample from room temperature of 20°C to 460°C at a heating rate of 5°C/min, holding the temperature for 30 minutes, and then slowly cooling to room temperature. The sample size, nominally 350 mm, is again accurately measured and compared with the figure measured before treatment (A). The average shrinkage is shown in the table.

Example 4

The 5 plate samples treated according to Example 1 were subjected to treatment (B), comprising heating the sample from a room temperature of 20° C. to 460° C. at a heating rate of 3° C./min, i.e. lower than that of treatment (A), Hold for 30 minutes, then cool slowly to room temperature. The sample size, nominally 350 mm, was accurately measured again at 20° C. and compared with the figure measured before treatment (B). The average shrinkage is shown in the table.

Example 5

In addition, 20 control panel samples of ordinary soda-lime float glass were subjected to a simple heat treatment, except that they were not exposed to potassium nitrate or other chemical quenching agents. The treatment consisted of heating the sample from 200°C to 460°C at a heating rate of 3°C/min, holding for 30 minutes and cooling slowly to 20°C. Shrinkage was again measured and the average numbers shown in the table are two times higher than those obtained for chemically quenched and heat treated samples.

The outstanding advantage of the plate sample prepared according to Example 3 is that the shrinkage caused by the high temperature treatment is less than 100 microns/meter.

Examples 6 and 7

Examples 6 and 7 were carried out in molten potassium nitrate in a typical commercial plant for chemically tempering glass. The unit consists of a preheating chamber, a tank containing molten potassium nitrate, a draining chamber, a cooling chamber and a washing chamber. The samples used in Example 6 were ordinary soda-lime float glass, whereas in Example 7 an extra clear glass (very low iron content) was used.

The samples (20 pieces in Example 6 and 10 pieces in Example 7) were placed on the carrier and placed in a preheating chamber, in which the temperature of the sample was raised to 400° C., and the rate of temperature rise was about 10 °C/min, which is close to the working temperature of molten potassium nitrate. The carrier was then placed in a tank of molten potassium nitrate and soaked for 8 hours. During said period, the push rod gradually advances the carrier through the molten salt tank and into the drain chamber. The temperature of the molten potassium nitrate was maintained within 2° of 465°C throughout the process.

Carbon dioxide gas is continuously passed into the tank to facilitate ion exchange, to provide gentle agitation, to mix the salt and to maintain a uniform temperature throughout the molten salt.

When removing the specimen from the jar, drain for a few minutes and then cool in two steps, first at 12°C/min to 220°C, then at 5°C/min or less to 220°C Room temperature 20°C. Wash, dry and measure the cooled samples.

The average shrinkage is shown in the table. The shrinkage results of different samples in each embodiment are quite consistent, all within 10% of each other. This is better than the consistency obtained by annealing. The high shrinkage obtained after chemical quenching ensures negligible dimensional changes during subsequent heat treatments of the glass.

surface Example Chemical quenching with preheating Chemical quenching without preheating Heat treatment A Heat treatment B Average Shrinkage of Specimen [X] (µm/m) 1 x 246[12] 2 x 147[10] 3 x x 96[7] 4 x x 125[5] 5 x 283[20] 6 x 470 ^* [20] 7 x 440 ^* [10] *±30μm/m

Glassy sheets treated according to the invention are particularly useful in the production of display panels, as described in co-pending patent application n° 9711 815, filed September 23, 1997, by Thomson Tubes Electroniques, France, entitled "Procede de realization d'unpanneau de visualization comportant une dalle a stabilitedimensionelle amelioree" (Method for the preparation of display panels containing panels with improved dimensional stability).

Claims (21)

1. improve the method for the dimensional stability of the glassiness sheet material that is used as substrate, it is characterized in that: carry out any fire step or with apply other relevant thermal treatments of coating to described sheet material before described sheet material is contacted with molten alkali matel salt, wherein, the temperature of described molten alkali matel salt remains on 400-500 ℃, and keep evenly, maximum deviation is 2 ℃.

2. according to the process of claim 1 wherein that the alkalimetal ion of melting salt is diffused in the top layer of described sheet material, and the ion diffusion on described top layer is in melting salt, and the ion conversion of carrying out thus generates or improved the stress on top layer.

3. according to the process of claim 1 wherein that basic metal is potassium.

4. according to the method for claim 2, wherein basic metal is potassium.

5. according to the method for claim 3, wherein melting salt is a saltpetre.

6. according to the method for claim 4, wherein melting salt is a saltpetre

7. according to the method for above-mentioned each claim, wherein be 15 minutes to 72 hours the duration of contact with melting salt.

8. according to the method for claim 7, wherein be 4-8 hour the duration of contact with melting salt.

9. according to each method among the claim 1-6, wherein make vitreous material slowly rise to the temperature of melting salt.

10. according to the method for claim 7, wherein make vitreous material slowly rise to the temperature of melting salt.

11. method according to Claim 8 wherein makes vitreous material slowly rise to the temperature of melting salt.

12. according to the method for claim 9, wherein the temperature rise rate of vitreous material is less than 15 ℃/minute.

13., wherein, vitreous material is slowly cooled to room temperature with after melting salt contacts according to each method among the claim 1-6.

14., wherein, vitreous material is slowly cooled to room temperature with after melting salt contacts according to the method for claim 7.

15. method according to Claim 8 wherein with after melting salt contacts, slowly cools to room temperature with vitreous material.

16., wherein, vitreous material is slowly cooled to room temperature with after melting salt contacts according to the method for claim 9.

17. according to the method for claim 13, wherein with speed cooled glass material less than 15 ℃/minute.

18. according to the method for claim 13, cooled glass material in two steps wherein, the rate of cooling difference in per step.

19. according to the method for claim 14, cooled glass material in two steps wherein, the rate of cooling difference in per step.

20. glassiness sheet material according to the preparation of the method for above-mentioned each claim.

21. according to the glassiness sheet material of claim 20, wherein the shrinkage that causes through pyroprocessing is less than 100 microns/meter.