THERMALLY HARDENED ISOTROPIC GLASS

Abstract

A process for manufacturing a heat strengthened glass, includes a heat treatment applied to a thermally tempered glass. Moreover, a heat strengthened glass sheet in accordance with standard EN1863-1:2011 has a surface stress of greater than 30 MPa, an edge compressive stress of greater than 30 MPa, a mean optical retardation of less than 40 nm.

Claims

1. A process for manufacturing a heat strengthened glass, referred to as final heat strengthened glass, comprising a heat treatment referred to as a post-heat treatment, applied to a thermally tempered glass, referred to as intermediate glass, leading to the final heat strengthened glass.

2. The process as claimed in claim 1, wherein the final heat strengthened glass has a surface stress lower than a surface stress of the intermediate glass.

3. The process as claimed in claim 2, wherein the final heat strengthened glass has a surface stress within the range from 30 to 60 MPa.

4. The process as claimed in claim 1, wherein the intermediate glass has a surface stress within the range from 35 to 90 MPa.

5. The process as claimed in claim 1, wherein the post-heat treatment is carried out at a temperature above a minimum temperature, said minimum temperature being 250 C.

6. The process as claimed in claim 5, wherein the post-heat treatment is carried out at a temperature below a maximum temperature, said maximum temperature being 550 C.

7. The process as claimed in claim 6, wherein the post-heat treatment comprises heating for at least one hour between the minimum temperature and the maximum temperature.

8. The process as claimed in claim 1, wherein the glass is a sheet having a thickness within the range from 5 to 13 mm.

9. The process as claimed in claim 1, wherein the post-heat treatment is carried out in a drying oven.

10. The process as claimed in claim 1, wherein the intermediate glass was produced by thermal tempering of a glass, referred to as primary glass, comprising heating said primary glass to at least 580 C. followed by air jet impingement cooling.

11. The process as claimed in claim 10, wherein the primary glass is conveyed by conveying rollers during the heating and/or the cooling.

12. The process as claimed in claim 11, wherein the conveying rollers are wrapped with braids or strips made of polymer material.

13. The process as claimed in claim 1, wherein the intermediate glass is a soda-lime-silica glass.

14. A heat strengthened glass sheet in accordance with standard EN1863-1:2011 having a surface stress of greater than 30 MPa, an edge compressive stress of greater than 30 MPa, a mean optical retardation of less than 40 nm.

15. The sheet as claimed in claim 14, wherein the surface stress is between 32 and 55 MPa.

16. The sheet as claimed in claim 14, wherein the edge compressive stress is less than 45 MPa.

17. The sheet as claimed in claim 14, wherein a thickness thereof is within the range from 5 to 13 mm.

18. The sheet as claimed in claim 14, wherein the sheet is made of soda-lime-silica glass.

19. The process as claimed in claim 5, wherein the minimum temperature is 280 C.

20. The process as claimed in claim 6, wherein the maximum temperature is 480 C.

Description

[0030] FIG. 1 is a photo showing an air blowing nozzle 1 for administering a thermal tempering cooling, positioned between two rollers 2 and 3 for conveying glass sheets, said rollers being equipped with braids 4 made of Kevlar. It is these braids that come into contact with the glass in order to convey it.

[0031] FIG. 2 shows a photo of a sheet of thermally tempered glass (of intermediate glass type) displaying the quench mark in a quite pronounced manner and which the invention makes it possible to reduce or even to make disappear. It is estimated that the grid pattern marks are the result of a temperature inhomogeneity of the glass due to the contact thereof with the Kevlar braids shown in FIG. 1. This photo is an image of the glass taken using a circular polariscope in which the glass was placed, the image having been recorded by an optical sensor.

[0032] FIG. 3 shows the quench marks of glazings produced according to the examples.

EXAMPLES

[0033] Heat strengthened glass sheets having a thickness of 8 mm and 10 mm were produced, the main faces of which had dimensions of 1100360 mm. Some of these sheets were then subjected to a post-heat treatment. Next, the following measurements were carried out: [0034] surface stress; [0035] permanent edge compressive stress; [0036] mean optical retardation; [0037] compliance with standard EN1863-1:2011 (including the 4 point bending strength according to standard EN1288-3 and the fragmentation).

[0038] Table 1 collates the conditions for carrying out the various tests and the results.

TABLE-US-00001 TABLE 1 Mean optical Surface Edge Example Post-heat retardation stress stress no. Thickness treatment (nm) (MPa) (MPa) 1 (comp) 8 mm 46.8 52.4 52.8 2 8 mm 350 C., 4 h 42.7 50.9 38.7 3 (comp) 10 mm 57.3 60.8 51 4 10 mm 450 C., 4 h 30.8 33.6 31.9

[0039] The glass from example 2 corresponds to that of example 1 except that it has additionally undergone a post-heat treatment. The glass from example 4 corresponds to that of example 3 except that it has additionally undergone a post-heat treatment. It is seen that the post-heat treatment results in a reduction in the mean optical retardation. The glasses from all these examples were in accordance with standard EN1863-1:2011.

[0040] Images were furthermore produced representing the quench mark of each of these glazings from their mapping of optical retardations. These images were produced using a circular polariscope in which the glass was placed, the image having been recorded by an optical sensor. These images are visible in FIG. 3. It is seen that the post-heat treatment (ex 2 and ex 4) has indeed reduced the quench mark compared to the same glazings that have not undergone the post-heat treatment (ex 1 to be compared with ex 2; ex 3 to be compared with ex 4).