Tempered glass substrate having reduced iridescence

Abstract

A process for the manufacture of a heat strengthened glass substrate, includes the application of a temporary layer including a polymer on a glass substrate including a glass sheet, then the application to the glass substrate coated with the temporary layer of a treatment for the heat strengthening of the glass including heating, leading to the removal of the temporary layer, and then cooling by blowing of air through nozzles. The glass substrate thus obtained exhibits a reduced level of iridescences.

Claims

1. A process for the manufacture of a heat strengthened glass substrate, comprising: applying a temporary layer comprising a polymer on a glass substrate comprising a glass sheet, then applying to the glass substrate coated with the temporary layer a treatment for the heat strengthening of the glass comprising heating, leading to a removal of the temporary layer, and then, after said heating and removing the temporary layer, cooling the glass substrate by blowing of air through nozzles, wherein a thickness of the temporary layer is selected such that a normal emissivity of the substrate coated with the temporary layer is greater than a normal emissivity of the substrate before application of the temporary layer and over 60% of a surface of the glass substrate initially covered by the temporary layer exhibits a mean retardation of less than 50 nm after the glass substrate is cooled.

2. The process as claimed in claim 1, wherein the heating is carried out at a temperature of greater than 550° C.

3. The process as claimed in claim 1, wherein the glass substrate exhibits, before application of the temporary layer, a normal emissivity of less than 10%.

4. The process as claimed in claim 1, wherein the temporary layer has a thickness of between 1 and 100 micrometers.

5. The process as claimed in claim 1, wherein the glass substrate comprises a functional coating, the temporary layer being applied on the functional coating.

6. The process as claimed in claim 5, wherein the functional coating is of the low-e type or of the solar control type.

7. The process as claimed in claim 5, wherein the functional coating is deposited by cathode sputtering assisted by a magnetic field and wherein the temporary layer is directly in contact with the functional coating.

8. The process as claimed in claim 5, wherein the functional coating comprises an upper layer chosen from nitrides, oxides or oxinitrides of titanium and/or of zirconium and/or of hafnium.

9. The process as claimed in claim 1, wherein the functional coating comprises at least one metal layer.

10. The process as claimed in claim 9, wherein the functional coating comprises a stack of thin layers comprising an alternation of x metal layers based on silver or on a metal alloy containing silver and of (x+1) antireflective coatings, each antireflective coating comprising at least one dielectric layer, each metal layer being positioned between two antireflective coatings, x being greater than or equal to 1.

11. The process as claimed in claim 1, wherein the cooling produces a surface stress of the glass of greater than 40 MPa.

12. The process as claimed in claim 11, wherein the cooling is a heat tempering producing a surface stress of greater than 90 MPa.

13. The process as claimed in claim 1, wherein the polymer is a polymer of a (meth)acrylate.

14. The process as claimed in claim 13, further comprising: preparing a liquid composition comprising (meth)acrylate compounds chosen from monomers, oligomers, prepolymers or polymers comprising at least one (meth)acrylate functional group, applying the liquid composition on the glass substrate, then solidifying, by polymerization and/or crosslinking, the composition, so as to form the temporary layer.

15. The process as claimed in claim 14, wherein the liquid composition comprises less than 20% by weight of solvent and has a viscosity of between 0.05 and 5 Pa.Math.s at its application.

16. The process as claimed in claim 1, further comprising cutting up the glass substrate between the application of the temporary layer and the heating.

17. The process as claimed in claim 1, wherein over 70% of the surface of the glass substrate initially covered by the temporary layer exhibits a mean retardation of less than 50 nm after the glass substrate is cooled.

18. The process as claimed in claim 17, wherein at least over 80% of a surface of the glass substrate initially covered by the temporary layer exhibits a mean retardation of less than 50 nm after the glass substrate is cooled.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1a and 1b show two substrates observed with a circular polariscope and corresponding to, respectively, examples 1 and 2;

(2) FIGS. 2a and 2b show two substrates observed with a circular polariscope and corresponding to, respectively, examples 3 and 4.

EXAMPLES 1 AND 2

(3) Two glass sheets with a thickness of 6 mm and with dimensions of 800 mm×600 mm, of Saint Gobain Glass CoolLite 154 II trademark, are taken. These two sheets exhibited, on one of their main faces, a functional coating of the solar control type made of a stack of thin layers successively comprising, from the glass, an alternation of two silver layers and of three antireflective coatings, each antireflective coating comprising several dielectric layers, including one made of Si.sub.3N.sub.4 and one made of ZnO, so that each silver layer is positioned between two antireflective coatings. The total thickness of this functional coating is between 150 and 200 nm.

(4) On one of the sheets, a temporary layer is applied directly on the functional coating in the following way:

(5) A liquid composition was prepared with mixtures of oligomers and of monomers, having at least one acrylate functional group sold by Sartomer: CN9276: tetrafunctional aliphatic urethane-acrylate oligomer, SR351: trimethylolpropane triacrylate, trifunctional acrylate monomer, SR833S: tricyclodecanedimethanol diacrylate, difunctional acrylate monomer.

(6) The presence of the urethane-acrylate oligomer makes it possible to adjust the hardness and flexibility properties of the temporary protective layer. The temporary protective layer is subsequently cured by crosslinking with UV radiation. Irgacure 500, sold by BASF, as polymerization initiator, is added to the liquid composition. The acrylates and the initiator were present in the liquid composition in the following proportions, given as parts by weight:

(7) TABLE-US-00001 TABLE 1 Acrylate oligomer CN9276 60 Trifunctional acrylate SR351 20 Difunctional acrylate SR833S 20 Initiator Irgacure 500 5

(8) The liquid composition had a viscosity at 25° C. of 1.08 Pa.Math.s and was applied on the glass substrate by roll-to-roll coating. A thickness of between 10 and 20 μm is obtained using speeds for the applicator roll of between 15 and 25 m/min. The temporary layer is cured by UV radiation provided by a mercury lamp with a power of 120 W. Under these conditions, the polymerization of the mixture of monomers and of oligomers is obtained within the thickness range from 10 to 20 μm.

(9) The emissivity of the glass substrate without a polymer layer (example 1, comparative) was 2.52%. The emissivity of the glass substrate with the polymer layer (example 2, according to the invention) was 56.9%. The emissivity is the normal emissivity (in the perpendicular direction) measured by the standard EN12898 of January 2001.

(10) Heat strengthening of these two sheets is carried out by applying heating at 600° C. to them, followed by rapid cooling by blowing of air by nozzles which blow over the two main faces of the two sheets. The surface compression is subsequently measured at 60 MPa.

(11) These substrates are subsequently observed with a circular polariscope (the substrates are placed between two polarizing filters). FIG. 1 shows these two substrates. In a), the substrate which had not received the temporary layer (example 1) exhibits a strong white mark in the central region.

(12) The arithmetic mean of the retardations of these two sheets was measured by the method described in the paper by M. Illguth, M. Schuler and O. Bucak, 2015, “The effect of optical anisotropies on building glass façades and its measurement method”, Frontiers of Architectural Research, 4 pp. 119-126. A measurement per mm.sup.2 was carried out and then the arithmetic mean was calculated. The retardations were measured with circular polarized light. The results are collated in Table 2.

(13) TABLE-US-00002 TABLE 2 Mean % of surface area with retardants retardation <50 nm Ex 1 (without polymer layer) 56.6 50% Ex 2 (with polymer layer) 33.2 80%

EXAMPLES 3 AND 4

(14) The procedure as for Examples 1 and 2 is used, except that the heat strengthening is more intense because of faster cooling and leads to a surface compression of 110 MPa and that the observation is simply carried out in reflection with the naked eye. FIG. 2 shows these two substrates. In a), the substrate had not received the temporary layer and, in b), the substrate had received the temporary layer. The substrate which had not received the temporary layer shows much more marked iridescences.