CS260146B1 - A method for heat treating glass articles for ion exchange reinforcement - Google Patents

Abstract

Glass products intended for strengthening by ion exchange are heat treated by maintaining them at a temperature in the transformation interval of the glass from which they are made, namely at a temperature in the dilatometric transformation temperature range of the said glass ςΗ C for a period of at least 40 minutes. In this way, a reduction in the fictitious temperature is achieved, which results in a slowdown, or even complete elimination of the relaxation of stresses arising during ion exchange. The resulting strengthening compressive stress and thus the achieved strength of the glass products are thereby increased. In addition, the processing time in the strengthening bath is shortened and the necessity of using special expensive and difficult-to-melt high-viscosity glass is possibly eliminated. The heat treatment also affects the refractive index of the surface layer in such a way that a greater difference in refractive indices is achieved between the surface and the interior of the glass

Description

The invention relates to a method of heat treatment of glass articles. products to be strengthened by ion exchange by maintaining them at temperatures in the transformation interval of the respective glass.

The essence of the heat treatment of glass products to be strengthened by ion exchange is to heat them to a temperature such that they do not crack upon contact with the molten bath in which the ion exchange itself takes place. The predominantly strengthened thin-walled products are placed in a preheating furnace having a temperature close to a bath temperature of + 20 ° C and usually left in it for 20 minutes. The ion exchange strengthening is most often carried out at temperatures below the lower cooling temperature of the treated glass by exchanging sodium ions Na in the glass surface with larger potassium ions K ⁺ from the bath of molten potassium nitrate KNO 4 so as to create a strengthening pressure stress in the glass surface layer. The size of the resulting reinforcement is determined by the amount of compressive stress created in the surface layer. .

In conventional soda-lime-silica glasses, the reinforced layer of the required thickness is formed relatively slowly in the reinforcing bath. The methods used to strengthen the ion exchange at temperatures that practically do not relax the generated pressure stress, i.e., 100 ° C or lower * than the dilatometric transformation point of the processed glass, are unbearably long, tens of hours.

Another group of consolidation methods therefore perform consolidation of conventional soda-lime silicate glasses at an elevated temperature close to the lower cooling temperature, respectively. to dilatometric transformations of the treated glass. This results in the formation of a sufficiently thicker layer in a shorter time, but the disadvantage is that during the ion exchange, the resulting stress is relaxed.

In order not to prevent this adverse effect, special glasses with a content of 5 to 25% by weight are used for strengthening purposes. alumina and AlgO4, respectively. zirconia Zr0 ₂ having a higher viscosity than conventional sodnoyápenatokřemičitó glass. The disadvantage of these glasses is that they are more difficult to melt and more expensive than conventional soda-lime silicate glasses.

These disadvantages can be eliminated when the method of heat treatment of the glass articles to be strengthened by ion exchange according to the invention is based on the fact that the articles are kept at a temperature in the transforming interval of the glass from which they are made. a temperature corresponding to a dilatometric transformation temperature of said glass of at least 40 minutes. In practice, the heat treatment time does not normally exceed 24 hours. In this way, a reduction of the fictional temperature is achieved, which has the effect of slowing down or completely eliminating the relaxation of the stresses generated during the ion exchange.

The fictitious (structural) temperature is the temperature that corresponds to the instantaneous structural state of the glass and on which, in addition to the actual temperature, its properties depend, for example, density, viscosity, refractive index, stress relaxation time, etc.

By using the process according to the invention, the resulting compressive stress is increased and thus the strength of the ion-exchange strengthened glass articles is increased without the need to process the articles in the strengthening bath for tens of hours or to form them from special expensive and difficult to melt glass of high viscosity. At the same time, the specific path difference of polarized light in the surface layer is increased and the difference in refractive index between the surface and the interior of the glass product is increased.

The following examples illustrate the invention.

Example 1

Plates of flat soda-lime-silica sheet glass

- 3 dí diellatometric transformation temperature of 525 ° C, 100x100 has a surface area of 2.8 mm, and heated in an oven at 500 ° C for 24 hours, then transferred to a 470 ° C molten potassium nitrate bath containing 470 ° C containing not more than 0.001% CaO, and left for 3 hours.

A thickness of 24 to 26 nm is formed on the surface of the plates

with a compressive stress of 450 MPa, the average flexural strength of the plates being 520 MPa. The specific pathway difference of polarized light on the surface is 11,250 nm / cm, the maximum refractive index difference between the surface and the inside of the glass is nn = 620.10 ”.

The same plates treated before being transferred to the molten potassium nitrate bath KNO in the usual manner, i.e. preheated in an oven at 460 to 480 ° C for 20 to 30 minutes and then further processed as in the above example, have a layer on the surface thicknesses 24 to 26 µm with a compressive stress of only 300 MPa. The specific pathway difference of polarized light on the surface is 7,500 nm / cm, the maximum refractive index difference between the surface and the inside of the glass / kn = 390.10 ”\

Example 2

Plates of flat soda-lime-silica sheet glass having a dielectric transformation temperature of 525 ° C, 100 x 100 mm area and 1.3 mm thickness are placed in an oven at 520 ° C for 30 minutes, then the sample oven temperature The samples are transferred to a 470 ° C molten potassium nitrate bath containing not more than 0.001% CaO and kept in the bath for 3 hours at a rate of 2-3 ° C / min.

A 29 µm thick layer is formed on the surface of the plates with a compressive stress of 370 MPa, the average flexural strength of the plates being 660 MPa. The specific distance difference of the polarized light on the surface is 9 250 nm / cm, the maximum difference of refractive indices between the surface and the inside of the glass Δη ⁼ 520.10 \

The same plates, processed before transferring to a bath of 260146

The 4-potassium nitrate KNO 2 in conventional manner, i.e. preheating in an oven at 460 to 480 ° C for 20 to 30 minutes and then further processed as in the above example, has a 29 / wn thick film on the surface. with a compressive stress of 300 MPa, the average bending strength of the plates is 520 MPa. The specific path difference of polarized light on the surface is 7 500 nm / cm, the maximum difference in refractive indices between the surface and the interior of the glass Zt n = 460.10 ”^,

Example 3

Plates of flat soda-lime-silica glass thickness

1.2 mm with a dilatometric transformation temperature of 520 ° C are heated in an oven at 472 ° C for 2 500 hours; alternatively, they can then be cooled to 20 ° C at a rate of 5 to 10 ° C. 10 ° C / rain> preheat in an oven at 450 to 480 ° C for 20 to 30 minutes before placing in the bath. The samples are then placed in a bath of molten potassium nitrate KNO @ a temperature of 480 ° ^C, containing less than 0.001% CaO, for 3 h. The size of the resulting reinforcing continuous voltage is 680 MPa.

The same plates, treated prior to transfer to the molten potassium nitrate bath KNO, in the usual manner, i.e. preheating in an oven at 460-480 ° C for 20-30 minutes, and then treated as in the above example, exhibit a strengthening compressive stress 360 MPa.

The method of strengthening the glass products according to the invention is intended primarily for thin-walled or more complex shaped articles made of conventional soda-lime silicate glass, on which extraordinary strength requirements are imposed. resistance to sudden changes in temperature. Further, due to its influence on the refractive index of the surface layer, it can be used, for example, in the production of optoelectronic elements.

Claims (1)

OF THE INVENTION Method of heat treatment of glass articles for strengthening by ion exchange, characterized by the fact that the products are maintained at a temperature in the transition interval of the glass of which they are made, and at a temperature corresponding dilatometric transformation temperature of said glass ° ^C for at least 40 minutes.