IL29802A - Chemical tempering treatment of vitreous and vitrocrystalline bodies - Google Patents

Description

CHEMICAL TEMPERING TRBATMEBT OF VITREOUS AHD VITRO-CRYSTAlilNE BODIES '8'3 This invention relates to a process for tempering vitreous and vitro-crystalline bodies.

It is known that metal ions can be caused to diffuse into glass from a contacting medium in exchange for other ions from the glass.

If the nature of the ions entering the glass and the temperature conditions obtaining during the ion exchanges are appropriately chosen the ion exchange leads to the production or increase of compressive stresses in exterior layers of the glass. In other words the glass is chemically tempered.

Two types of process for chemically tempering glass are known per se. In one type of process, (referred to hereinafter as the "high temperature type") , an ion exchange takes place at a temperature sufficiently high for stress relaxation to occur in the glass and the ions entering the glass are such as to confer a lower coefficient of thermal expansion on the external gl ss layers.

In the other type of process (referred to hereinafter as "the low temperature type") ions in exterior layers of the glass are replaced by larger ions and the ion exchange is effected while the exterior glass layers are at a temperature below the annealing 132 point ( corresponding to a viscosity of 10 poises), so that stress relaxation will not occur, or will occur only to a minor extent.

It has been found, according to the present invention, that chemical tempering processes of the high and low temperature _ .. , , . i.e. the increase in strength of the body is types are more effective/if the temperature of the body decreases, greater within the temperature range which is viable for the type of chemical tempering process involved, during the ion exchange period.

Accordingly the present invention includes any process of strengthening a body of glass by a chemical tempering process of the high temperature type as herein defined, wherein during the period over which the ion exchange between the glass and the contacting medium takes place, the temperature of the glass is caused to decrease within a range of temperatures sufficiently high for stress relaxation to occur in the glass.

The invention also includes any process of strengthening a body of glass by a chemical tempering process of the low temperature type herein def ned, wherein during the period over which the ion exchange between the glass and the contacting medium takes place, the temperature of the glass is caused to decrease within a range of temperatures below the annealing point of the glass.

The invention is particularly well suited for use in tempering glass made fran ordinary inexpensive constituents such as silica, soda, lime and feldspar. The ion exchange may be an exchange of alkali metal ions, e.g. it may involve the substitution of sodium ions in the glass by lithium ions in the case of a high temperature process, and by potassium ions in the case of a low temperature process.

It is an advantage to commence the ion exchange at a temperature high in the permissible temperature range because the higher the temperature the higher is the coefficient of diffusion of given ions into the glass. The effects on the glass, in terms of its mechanical properties, are however better if the temperature is decreased during the ion exchange process than if the initial temperature is maintained throughout the process. The reason for the improvement is not known with certainty. The performance of the ion exchange at a temperature which is kept high in the permissible range throughout the process appears to lead to an undue build up in the concentra-tion of the substituting ions in a surface layer of the glass between 1 and 30 microns in thickness. Theee ions are found in sites normally not occupied by the alkali metal ions of the unsubstituted glass. If the temperature is sufficiently decreased during the ion exchange period, this excessive concentration is avoided or reduced and this appears to contribute to the improved results. The performance of a tempering process according to the invention increases the tensile strength of the glass, and the increase in strength is greater than can be obtained by keeping the temperature constant at the average value of the temperatures prevailing during the tempering. The tensile strength imparted by the tempering treatment also tends to be retained for longer periods when the tempering is performed according to -the invention.

It had also been found that in many cases the surface of the tempered glass had an enhanced chemical resistance to weathering.

When performing a high temperature type tempering process, the initial temperature during the ion exchange period is preferably at least equal to 1.05 Tg where Tg is the temperature 132 at which the viscosity of the glass is 10 poises. For a low temperature type process the initial temperature is preferably at least 0.6 Tg.

Preferably the temperature of the glass surface during the ion exchange is reduced by at least 0°C. The temperature may drop considerably more than that, e.g. by more than 100°C. , say by 150°C or even 200°C.

The ion exchange may take place between the glass and a contacting molten medium which constitutes a coating carried by the glass or which constitutes a bath or baths in which the glass body is immersed. When using a bath of the medium, the glass body can be pre-heated before contact with the bath to avoid or reduce thermal shock and to reduce the processing time. A coating of medium can of course also be applied to a pre-heated glass body. A molten medium can e.g. be composed of a potassium salt, e.g., potassium nitrate, a lithium salt, e„g. , lithium chloride, or a mixture of substances including a potassium or lithium salt.

When a chemical tempering bath is used, an article to be tempered can be moved progressively through the bath and a temperature gradient can be maintained in the bath so that the required temperature decrease occurs as the article advances. The temperature gradient can be easily maintained by providing a succession of immersion heaters in the bath, at different temperatures. Moreover by appropriate choice of the number, spacing and relative dimensions of the heaters along the course followed by the glass body, the surface temperature curve at any part of the body during the ion exchange can be given any predetermined shape. The speed of movement of the body through the bath can also vary. When performing the invention in this manner, each of successive surface portions of the body reckoning from its leading to its trailing end, is exposed to a temperature which decreases during the ion exchange.

The use of a chemical tempering bath with a temperature gradient which decreases in the direction of movement of the article through the bath is of particular importance for temperr-ing a drawn glass ribbon as it moves continuously away from the drawing zone at which the glass is at a high temperature. In a Colbum-type glass drawing machine , -the drawn glass ribbon passes over a bending roller and then along a lehr in which the glass is progressively cooled. Tempering by ion exchange according to the invention may take place in such a lehr. Thus, a tempering bath may be provided in the lehr and the ribbon of glass can be guided through this bath so that tempering occurs at a temperature which decreases towards the exit end of the bath.

Tempering according to the invention may occur in each of two or more bath.3 in succession, the successive baths being at successively lower temperatures. When tempering drawn glass, the glass may be cut between successive bath treatments, before the tempering has advanced so far as to make clean cutting difficult.

It has been found that the discovery on which the invention is based is useful also in the chemical tempering of a body of vitfco-crystalline material of whatever degree of crystallisation. A vitro-crystalline body can be chemically tempered by substituting ions in external layers of the body, at a temperature sufficiently high for stress relaxation to occur, by ions whioh confer a lower coefficient of thermal expansion on said external layers, or by substituting ions in external layers of the body by larger ions while the said layers are at a temperature below the annealing point of the vitreous phase or phases of the body. The information given hereinbefore relating to the use of the invention in the chemical tempering of glass is generally applicable to the use of the invention in chemically tempering vitro-crystalline material.

The invention includes apparatus for use in performing a process as hereinbefore defined and comprising at least one tank for holding a bath of medium containing ions for diffusing into a said body, means for conveying a body along a path extending into and out of said, tank, and means for maintaining a tempera-ture gradient in a said bath, the temperature decreasing along - X said path in the direction of movement of said conveying means.

The conveying means may comprise an endless conveyor carrying racks or other supports for articles to be tempered or it may comprise rollers suitably arranged for displacing and guiding a continuous ribbon of glass into and out of the tank or tanks.

Certain embodiments of the invention will now be described with reference to the accompanying diagrammatic drawings comprising Pigs. 1 and 2 which are sectional elevations of two different apparatus. EXAMPLE The apparatus shown in Pig. 1 includes a tank 1, a preheating compartment 2 and a cooling compartment 3· T e tank 1, comprising bottom wall and" end walls 5 and 6 contains a bath 7 of molten potassium salt. Within the tank there are heat exchangers 8 which are located near the bottom and in which streams of fluid are circulated to maintain the heat exchangers at temperatures which decrease from the exchanger at the left hand end to the exchanger at the right hand end of the tank.

The preheating compartment 2 , which is defined in part by lateral walls 9 , 10 is equipped with heating means (not shown) to provide for slight or considerable preheating of articles as they pass through this compartment. The cooling compartment 3 which is defined in part by lateral walls 11> 12 is equipped with coolers (not shown) to provide for slow or rapid cooling of the articles after they have passed through the tank 1.

Between the preheating compartment 2 and the tank 1 and between the cooling compartment 3 and the tank 1 there are gates 13 which open and close as required by the movement of articles to be tempered as will presently be referred to. Further gates 13 at the top of the preheating and cooling compartments separate these compartments from an upper compartment l -where thin sheets 15 of glass are loaded onto racks 16 preparatory to tempering and ' subsequently removed after tempering. The racks 16 are suspended from an endless conveyor 17 which travels in an anticlockwise direction in the aspect of the figure.

A typical operation of the apparatus will now be described. Glass sheets 15* 2 mm in thickness, are loaded onto the racks 16 for the time being in compartment 14. By the rotation of the conveyor 17 the glass sheets are carried through the preheating comparlment 2 in which they are heated to a temperature of 500°C. The sheets are subsequently carried into the bath 7 of molten potassium nitrate, and along the bath to its right hand end where the sheets are lifted out of the bath and carried into the cooling compartment. The sheets remain in the bath for fteen hours, during which the temperature of the sheets is lowered from 500°C to 350°C. This decrease in temperature is determined by the temperature of the successive heat exchangers 8, the first of which ( the exchanger at the left hand end) is kept at 500°C, and the temperature of the others of which decrease in stages by 37.5°C During the immersion of the sheets in the bath, sodium ions in external layers of the glass become replaced by potassium ions and this ion exchange leads to the production of compressive stresses in the external glass layers. During the time the sheets are at the high temperature, the ion exchange progresses rapidly. Some reaction resulting in build up of the potassium ion concentration in exterior layers of the glass sheet may take place in this high temperature region, but any tendency for this build up to occur diminishes as the temperature decreases. In the actual process described, a considerable part of the ion exchange occurs at the lower temperatures.

The tempered sheets are slowly cooled in cooling compartanent 3 and are subsequently removed from the racks 16 after again entering the top compartment 14.

A similar treatment may be performed on vitro-crystalliiie sheets, obtained from a batoh having the following composition in percentages by weight : Si02 2.

A1203 31.2% Na20 10.4% K20 6.2% CaO 1.8% As 0 0.7% EXAMPLE 2 Referring now to Pig..2: this figure shows part of a glass drawing-machine comprising a drawing chamber 18 in which a ribbon of glass is drawn upwardly from a supply of molten glass and subsequently passed along a horizontal cooling chamber 20. A tank 21 is installed in this horizontal chamber.

The drawing chamber 18 comprises spaced walls 23, 4 and at the top of this chamber there is a bending roller 25 over which the glass ribbon travels to the horizontal chamber 20. The horizontal chamber, which is defined in part by bottom wall 26 and roof 27, contains horizontal rollers 28 for conveying the glass ribbon. The tank 21, which is defined in part by bottom wall 29 and end walls 30, 31 holds a bath 32 consisting of a mixture of sodium and lithium salts. Within the tank there are heat exchangers 33, located near the bottom 29. Fluid streams at different temperatures circulate in the successive heat exchangers to maintain a temperature gradient in the bath.

Two rollers 34 are placed within the tank and the glass ribbon passes under these rollers. The glass ribbon is thus caused to travel through the molten salt bath.

A typical operation of the installation according to Fig. 2 is as follows: The speed of the glass ribbon and the dimensions of the tank 21 are such that each part of the ribbon remains immersed in the tank 21 for ten minutes. The left hand heat exchanger is kept at 800°C and the temperatures of the succeeding heat exchangers drop by 50°C from one exchanger to the next so that the temperature of the bath near the exit end of the tank is 600°C, which is above the annealing point of the glass. While any given portion of the glass ribbon is in the bath, lithium ions enter the surface layers of glass and replace sodium ions and this ion exchange takes place while the temperature of the glass decreases as above described. After leaving the tank, the glass ribbon is bent over a roller 28 and then continues its movement horizontally along the chamber 20. In the subsequent portion of the chamber the ribbon continues to be cooled and downstream from the chemical tempering bath the ribbon is washed with water to remove remaining traces of salt, and dried with hot air. The washing and drying stations are not shown.

It will be understood that the tempering effects can be varied by adjusting the temperatures of the heat exchangers.

EXAMPLE 3 As has already been referred to, the ion exchange in a process according to the invention may take place between a glass body and a quantity of medium -carried as a coating on the glass. The following is an example of this procedure: A sheet of soda-lime glass was pre-heated to 490°C and quickly immersed in and withdrawn from a bath of potassium nitrate at the same temperature. As a result of the dipping* the withdrawn sheet was entirely covered by a thin film of the potassium salt. On withdrawal from the bath, the sheet was cooled slowly from 9 °C to 350°C over a period of ten hours in a cooling chamber. The glass sheet was then rinsed and dried. As a result of the treatment, compressive stresses were set up in the surface layers of the glass.

In a comparative process, a glass sheet of the same com-position as that treated in Example ■£ was subjected to the same treatment except that the temperature of the sheet was kept at 90°C for the whole ten hour period. The compressive stresses set up in the surface layers of the glass were less than the compressive stresses produced in Example 2. Moreover the sheet treated according to Example 2 was less susceptible to chemical attack by water than -the sheet which was tempered under the constant temperature conditions.

Claims (1)

1. WHAT WE CLAIM IS l 1· A process for strengthening a body of glass or vitroorystalline material by ion exchange of alkali metal ions between the body and a contacting medium, the medium containing, alkali metal ions which confer a lower coefficient of thermal expansion to layers of the body wherein they replace ions of such layers , or containin alkali metal ions larger than ions in tho surface layers of the body in which they replace such ions, characterised in that during at least part of the period of said ion exchange· the temperature of the temperature at which the viscosity of the body is 10 Jt poises) when said ions from the medium oonfer a lower coefficient of thermal expansion to the external layers of the body, or being at least 0,6 Tg when said dons from the medium are larger than the ions of the body* 2. A process according to Claim 1, wherein the glass is a soda-lime glass* 3. * A process according to Claim 2, wherein the ion exchange involves the substitution of sodium ions in the glass by potassium or lithium ions* 4. ¾· A process according to Claim 3* wherein the said medium is composed of a molten potassium or lithium salt or a mixture of molten substances including a potassium or lithium salt* 5. * A process according to any preceding claim, wherein the glass body is passed through a quantity of the medium containing the ions which diffuse into the glass in exchange for other ions, and wherein the temperature of the medium decreases along the path of movement of the body* 29802 * 6. A process according to Claim 5, wherein the body of glass is a ribbon of glass which is passed progressively through the medium* 7. · A process according to Claim 6, wherein the body o glass is a ribbon of glass which passes coftHnuously into the medium from the drawing zone of a glass drawing machine 8. A process according to any of Claims 1 to 7» wherein the contacting medium is carried as a coating on the glass during the ion exchange. 9. A process, according to any preceding claim, wherein the glass body, is pre-heated and then brought into contact with the said medium, the latter being at the temperature at which ion exchange is to commence. 10. A process of strengthenin a body of vitreous or vitrocrystalline material substantially as herein described. 11» Apparatus for use in strengthening bodies of vitreous in a said bath, the temperature decreasing along said path in the direction of movement of said conveying means. 12. Apparatus according to Claim 11, wherein said means for maintaining a temperature ,gradient in a. said bath comprises at least one eat exchanger through which a fluid can be circulated. 13. Apparatus, according to Claim 11 or 12, wherein said conveying means comprises at least one support en which an article to be treated can be placed and which is mounted on.a driven carrier..... .. l4. Apparatus according to Claim 11 or 12» wherein said conveying means comprises rollers suitably arranged for displacing and//] guidin a continuous ribbon o glass into and out of said tank o tanks* 16. . Apparatus for use in strengthening bodies of vitreous or vitrocrystalline material by a chemical temperin process, substantially as hereiii described. 1,7· A body of vitreous or vitrocrystalline material which has been strengthened by a process according to any of Claims 1 to 10.