THERMAL DIMENSIONAL STABILITY TREATMENT OF VITREOUS SHEET MATERIAL BY CONTACTING WITH A MOLTEN ALKALI METAL SALT

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 DIMENδlONAL δTABILITY TREATMENT OF VlTREOUδ δHEET MATERIAL BY CONTACTING WITH A MOLTEN ALKALI METAL δALT The present invention relates to treatment of a sheet of a vitreous material such as glass and to the sheet so-formed It is especially concerned with the production of sheets of vitreous material having improved dimensional stability Because glass and like vitreous materials are non-crystalline treating them by means involving heating and cooling tends to result in dimensional changes such as reduction in area The reduction in the dimensions also known as compaction or shrinkage is for many purposes insignificant There are however increasing demands for glass with improved dimensioned stability for example for glass sheets to be used as substrates m electronic applications In these applications even very small shrinkages may be unacceptable because of the need for precise alignment of the glass sheet with other components Various proposals have been made in the past to give glass a thermal history which renders it less prone to shrinkage during subsequent treatment Effectively the glass is subjected to a pre-shπnkage, the glass being thereby densified so that the propensity to shrink during a desired heat treatment is lessened Annealing is a known densification process but it involves prolonged treatment at elevated temperatures requiring substantial heat energy input, and the results tend to be of inconsistent quality The extent of the densification varies with the annealing temperature time and cooling rate Another difficulty is that the glass viscosity is reduced at the elevated temperature such that a glass sheet may become warped or acquire surface defects during the annealing process An alternative solution consists of selecting a special composition glass with a very high strain point which when processed undergoes negligible shrinkage A particular problem arises with glass substrate sheets which are subjected to firing steps at temperatures above 500°C I e higher than the annealing point for soda-lime glass, such that an untreated soda-lime glass sheet after cooling has smaller linear dimensions for example some 600 um/m smaller, than the starting sheet. It is an object of the present invention to provide a simple and reliable way of improving the thermal stability of vitreous material such that it will withstand thermal treatment with a reduced degree of dimensional shrinkage. Chemical tempering treatment of a vitreous material is well known. Its purpose is to impart an increased mechanical and thermomechanical resistance to vitreous materials such as glass and vitrocrystalline materials. The chemical tempering can be of two types. According to one type of process ion exchange is performed at a temperature which is sufficiently high to cause stress relaxation to occur in the glass, while the ions entering the glass are such as to impart a lower co-efficient of thermal expansion to the glass surface layers. According to the other type of process, ions already in the glass surface layers are replaced by larger ions and the ion exchange is effected at a temperature below the glass annealing point so that stress relaxation does not occur to any substantial degree. GB patent 1276186 teaches chemical tempering treatment of a vitreous body by the entry of ions of at least one substance into the superficial layers of the body, followed by the application of a coating layer such as an oxide of titanium, silicon, aluminium, chromium or other metals, titanium nitride, or a carbide of silicon, titanium, tantalum or other metals. The chemical tempering increases the adherence of the coating layer. Examples of chemicals in the said patent for effecting the tempering include potassium nitrate, sodium nitrate, lithium carbonate and lithium chloride. We have now found that in order to provide for a reduced extent of shrinkage of a vitreous sheet upon firing or other thermal treatment which forms part of a step to apply a coating to the sheet it is highly effective to contact the sheet with a molten alkali metal salt prior to subjecting the sheet to the thermal treatment. According to the invention there is provided a method for producing a sheet of vitreous material for use as a substrate, characterised in that prior to any firing step or other heat treatment associated with the application of coating layers to the said sheet it is contacted with a molten alkali metal salt to improve its thermal dimensional stability. The method is conducted such that alkali metal ions from the molten salt diffuse into superficial layers of the said sheet and ions from the said superficial layers diffuse into the molten salt, the resulting ion exchange producing or increasing compressive stresses in the superficial layers. Unexpectedly it has been found that in addition to its other effects on the vitreous sheet the chemical tempering improves its dimensional stability, such that wide ranges of heating and cooling applied by heat treatment do not create an unacceptable amount of shrinkage. This is totally unexpected given that after a heat treatment such as firing at a higher temperature than the annealing point of the vitreous material, for example 580°C. all the stresses induced by the ion exchange in the superficial layers of the sheet will have disappeared and the vitreous material will have lost its strengthening from chemical tempering. The resulting sheet also has a better uniformity of dimensional stability than sheets treated solely by annealing. The invention thus provides a sheet of vitreous material which is suitable for use in applications which demand precision in the sheet ' s sizing. The reasons for the effectiveness of this chemical tempering on thermal dimensional stability are not fully understood. It appears to result in part from the excellent thermal contact between a molten salt and the surface of the sheet and in part because of ion exchange between the salt and the sheet. Although the invention is typically applicable to a conventional soda lime glass and the present description relates primarily to this type, the method of the invention can be applied if required to other types of glass, for example borosilicate, aluminosilicate and vitrocrystalline material. The choice of chemical tempering salt depends on factors such as the specific constituents of the vitreous material. In general a molten salt of a metal or metals selected from lithium and potassium is suitable. Molten potassium nitrate is particularly suitable for soda-lime glass. Contact between the sheet and the molten salt is conveniently achieved by immersing the sheet in the salt in a suitable vessel. The temperature profile of the vitreous material should be carefully regulated through the stages of the method of the invention. The salt is necessarily at a high temperature to maintain it in a molten state. The temperature of the vitreous material is preferably raised slowly towards the temperature of the molten salt, most preferably at a rate of less than 15°C/minute. Soaking in the molten salt for an extended period to achieve the required tempering ensures that the temperature of the vitreous material rises to, and is held at, the molten salt temperature. Contact between the vitreous material and the salt is typically maintained for a period of 15 minutes to 72 hours, preferably 4 to 8 hours. For potassium and other large ions the molten salt should be at a temperature below the annealing point of the vitreous material, which temperature is typically in the range 400 to 500°C. Uniformity of temperature of the molten salt is important, both throughout the tank in which it is held and throughout the soaking period. The molten salt temperature should preferably have a maximum deviation of 2°C. Mixing of the molten salt to maintain the uniform conditions can conveniently be achieved by blowing carbon dioxide through the salt, for example as taught in our earlier patent GB-A- 1274733. Upon removal from the molten salt, the vitreous material should be cooled slowly from the salt temperature, preferably at a rate of less than 15°C/minute. Slow rates of cooling appear to be desirable to avoid structural damage to the vitreous material and to encourage a low level of shrinkage. In some instances it is advantageous to conduct the cooling in two stages, each stage having a different rate of cooling. The invention is further described with reference to the following non-limiting examples. These employed sample sheets of glass, cut to nominal dimensions of 350 x 270 mm. The sheets for examples 1 to 5 had a thickness of 1.1 mm and the sheets for examples 6 and 7 had a thickness of 3 mm. The samples for examples 1 to 5 were held in a warmed chamber at 200°C until they were withdrawn for treatment as described below. The 350 mm length dimension was accurately measured for each sample before treatment and at various stages of the process, all the length measurements being made at an ambient temperature of 20°C. Example 1 Twelve sample sheets of ordinary soda lime float glass at 200°C were placed in a carrier and introduced into a pre-heating chamber in which they were steadily heated to 460°C over a period of 1 hour 30 minutes, representing a heating rate of about 3°C/minute. The glass and carrier were then placed in a tank of molten potassium nitrate at a temperature of 460°C and held therein for a period of four hours for the ion exchange to proceed. The glass and carrier were then placed in a drainage chamber at 460°C for 30 minutes, during which period most of the molten potassium nitrate removed from the tank with the glass and carrier drained off and was collected for reuse. The glass samples were then placed in a cooling chamber in which they were slowly cooled, first for a period of 2 hours at a rate of about l°C/minute and next for a period of 1 hour 15 minutes at a rate of about 2°C/minute. After the treatment procedure described above the nominal 350 mm dimension of each of the sample sheets was again accurately measured at 20°C and compared with the figure obtained before the treatment so as to determine the extent of shrinkage resulting from the treatment. The result is shown in the accompanying table as an average figure for all twelve samples. Example 2 A batch of ten sample sheets of ordinary soda lime float glass at 200°C was placed in a carrier and treated generally as described in Example 1, but with the difference that the carrier was placed directly into the tank of molten potassium nitrate, omitting the step of pre-heating to the 460°C tank temperature. The glass was thereby heated more quickly to the tank temperature, it being estimated that the rate of temperature increase was about 50°C/minute. After the treatment procedure was completed the nominal 350 mm dimension of each of the sample sheets was again accurately measured at 20°C and compared with the figure obtained before the treatment to show the extent of shrinkage. The average result is shown in the accompanying table. Example 3 Seven of the sample sheets treated according to Example 1 were subjected to a Treatment (A) which comprised heating from the ambient temperature of 20°C at a rate of 5 min to 460°C. holding at that temperature for 30 minutes and then slowly cooling to the ambient temperature. The nominal 350 mm dimensions were again accurately measured and compared with the figure obtained before Treatment (A) . The average shrinkage is shown in the table. Example 4 Five of the sample sheets treated according to Example 1 were subjected to a Treatment (B) which comprised heating from the ambient temperature of 20°C at 3 min to 460°C, i.e. a slower rate than in Treatment (A), holding at that temperature for 30 minutes and then slowly cooling to the ambient temperature. The nominal 350 mm dimension was again accurately measured at 20°C and compared with the figure obtained before Treatment (B). The average shrinkage is shown in the table. Example 5 By way of comparison 20 sample sheets of ordinary soda lime float glass were subjected to a simple thermal treatment, excluding any contact with potassium nitrate or other chemical tempering agent. The treatment comprised heating the samples from 200°C to 460°C at a rate of 37minute, holding the temperature at that level for 30 minutes and slowly cooling to 20°C. The shrinkage was again measured and the average figure, shown in the table, was more than double that measured for the samples which had been subjected to a chemical tempering and heat treatment. The sample sheets prepared according to Example 3 were especially advantageous in that the amount of shrinkage caused by high temperature treatment was less than 100 μm/m. Examples 6 & 7 Examples 6 & 7 were conducted in a typical industrial unit for chemical tempering of glass in molten potassium nitrate. The unit included a pre-heating chamber, a tank containing the molten potassium nitrate, a drainage chamber and a cooling chamber and then a washing station. The samples employed for example 6 were of ordinary soda lime float glass whereas those for example 7 were of extra-clear glass (having a very low iron content). The samples (20 for example 6. 10 for example 7) were placed in carriers and put into a pre-heating chamber where their temperature was raised at a rate of about 10°C/minute to 400°C. i.e. approaching the working temperature of the molten potassium nitrate. The carrier was then placed in the tank of molten potassium nitrate and left to soak for a period of eight hours. A push-rod gradually advanced the carrier through the molten salt tank to the drainage chamber over the said period. The temperature of the molten potassium nitrate was maintained at within 2° of 465°C throughout the period. Continuous bubbling of gaseous carbon dioxide through the tank, which was employed to promote ion exchange, effected gentle agitation and mixing of the salt and helped to maintain a uniform temperature throughout the volume of the molten salt. Upon removal from the tank the samples were left to drain for a few minutes and then cooled in two stages, first to 220°C at a rate of 12°C/minute and then to the ambient temperature of 20°C at a rate of 5°C/minute or less. The cooled samples were then washed, dried and measured. The average shrinkage is shown in the Table. There was considerable uniformity between the shrinkage results for different samples in the respective examples, in ail cases within 10% of each other. This is a much better uniformity than is obtained by annealing procedures. The high shrinkage obtained after the chemical tempering ensures negligible dimensional changes when the glass is subjected to a subsequent heat treatment. Table ± 30 μm/m Treatment of a sheet of a vitreous material according to the present invention is especially useful in the production of display panels as described in the copending patent application n Q 97 11815 filed in France by Thomson Tubes Electroniques under the title "Procede de realisation d'un panneau de visualisation comportant une dalle a stabilite dimensionelle amelioree" [Process for production of a display panel comprising a sheet of improved dimensional stability] on September 23, 1997.

Claims: CLAIMS I. A method for producing a sheet of vitreous material for use as a substrate, characterised in that prior to any firing step or other heat treatment associated with the application of coating layers to the said sheet it is contacted with a molten alkali metal salt to improve its thermal dimensional stability. 2. A method as claimed in claim 1, in which alkali metal ions from the molten salt diffuse into superficial layers of the said sheet and ions from the said superficial layers diffuse into the molten salt, the resulting ion exchange producing or increasing compressive stresses in the superficial layers. 3. A method as claimed in claim 1 or claim 2, in which the alkali metal is potassium. 4. A method as claimed in claim 3. in which the molten salt is potassium nitrate. 5. A method as claimed in any preceding claim, in which the contact with the molten salt is maintained for a period of 15 minutes to 72 hours. 6. A method as claimed in claim 5, in which the contact with the molten salt is maintained for a period of 4 to 8 hours. 7. A method as claimed in any preceding claim, in which the molten salt is maintained at a temperature in the range 400 to 500°C. 8. A method as claimed in any preceding claim, in which the molten salt is maintained at a uniform temperature with a maximum deviation of 2°C. 9 A method as claimed in any preceding claim, in which the vitreous material is slowly raised towards the temperature of the molten salt. 10. A method as claimed in claim 9, in which the temperature of the vitreous material is raised at a rate of less than 15°C/minute. II. A method as claimed in any preceding claim, in which after contact with the molten salt the vitreous material is slowly cooled to ambient temperature. 12. A method as claimed in claim 11, in which the vitreous material is cooled at a rate of less than 15°C/minute. 13. A method as claimed in claim 11 or claim 12. in which the vitreous material is cooled in two stages, each stage having a different rate of cooling. 14. A sheet of vitreous material prepared by a method as claimed in any preceding claim. 15. A sheet of vitreous material as claimed in claim 14. in which the amount of shrinkage caused by high temperature treatment is less than 100 μm/m.

Inventors: BAUDIN POL, PALIN NELLO, POELS JEAN-PIERRE, BAUDIN, POL, PALIN, NELLO, POELS, JEAN-PIERRE
Applicants: BAUDIN POL, GLAVERBEL, PALIN NELLO, POELS JEAN PIERRE, BAUDIN, POL, PALIN, NELLO, POELS, JEAN-PIERRE
Application year: 1998
Main classification: C03C21
IPC codes: C03C21