Glass sheet capable of having controlled warping through chemical strengthening

Abstract

A float glass sheet having a boron- and lithium-free glass composition comprising the following in weight percentage, expressed with respect to the total weight of glass: 65SiO.sub.278% 5Na.sub.2O20% 1K.sub.2O<8% 1Al.sub.2O.sub.3<6% 2CaO<10% 0MgO8%; K.sub.2O/(K.sub.2O+Na.sub.2O) ratio which is from 0.1 to 0.7; wherein the glass sheet has: $0.01<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.3.$ The glass sheet may be a chemically-temperable soda-silica type glass composition suitable for mass production that shows reduced or controlled increased warping effect.

Claims

1. A float glass sheet comprising the following in weight percentage, expressed with respect to the total weight of glass: 65SiO.sub.278%; 5Na.sub.2O20%; 1K.sub.2O<8%; 1Al.sub.2O.sub.3<6%; 2CaO<10%; 0MgO8%; and K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.1 to 0.7, wherein the glass sheet has: $0.01<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.3,$ and wherein the glass sheet is boron- and lithium-free.

2. A float glass sheet according to claim 1, wherein the composition comprises total iron (expressed in the form of Fe.sub.2O.sub.3) in a content ranging from 0.002 to 1.7% by weight.

3. A float glass sheet according to claim 2, wherein the composition comprises total iron (expressed in the form of Fe.sub.2O.sub.3) in a content ranging from 0.002 to 0.06% by weight.

4. A float glass sheet according to claim 3, wherein the composition comprises total iron (expressed in the form of Fe.sub.2O.sub.3) in a content ranging from 0.002 to 0.02% by weight.

5. A float glass sheet according to claim 1, wherein the composition comprises an alumina content such that 1Al.sub.2O.sub.34 wt %.

6. A float glass sheet according to claim 5, wherein the composition comprises an alumina content such that : 2Al.sub.2O.sub.33 wt %.

7. A float glass sheet according to claim 1, wherein the composition comprises : 5CaO<10 wt %.

8. A float glass sheet according to claim 1, wherein the composition comprises : 1K.sub.2O<6 wt %.

9. A float glass sheet according to claim 1, wherein the composition comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.1 to 0.5.

10. A float glass sheet according to claim 1, wherein the composition comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.2 to 0.5.

11. A float glass sheet according to claim 1 wherein the composition comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.2 to 0.4.

12. A float glass sheet according to claim 1, wherein the glass sheet has: $\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}1.$

13. A float glass sheet according to claim 1, wherein R (CS.sub.465 C./CS.sub.420 C.) is from 0.76 to 0.92.

14. A float glass sheet according to claim 1, wherein the glass sheet has a thickness of less than 1.5 mm.

15. A float glass sheet according to claim 1, wherein the glass sheet has a thickness of less than 0.7 mm.

16. A float glass sheet according to claim 1, wherein the float glass sheet is chemically tempered.

17. An electronic device comprising the float glass sheet according to claim 16.

18. An electronic device comprising the float glass sheet according to claim 1.

19. A float glass sheet comprising the following in weight percentage, expressed with respect to the total weight of glass: 65SiO.sub.278%; 5Na.sub.2O20%; 1K.sub.2O<8%; 3Al.sub.2O.sub.3<6%; 2CaO<10%; 0MgO8%; and K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.1 to 0.7, wherein the glass sheet has: $0.02<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.3,$ and wherein the glass sheet is boron- and lithium-free.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

(2) FIG. 1 is an illustration of warping of a glass sheet; and

(3) FIG. 2 shows evolution of warp vs. Na.sub.2O.sub.air/Na.sub.2O.sub.tin ratio on a glass sample.

DETAILED DESCRIPTION

(4) Other features and advantages of the invention will be made clearer from reading the following description of preferred embodiments given by way of simple illustrative and non-restrictive examples.

(5) The glass sheet of the invention is made of a soda-silica glass composition/matrix, comprising SiO.sub.2 and Na.sub.2O as the main components and further comprising MgO, Al.sub.2O.sub.3, etc and optionally CaO, K.sub.2O etc.

(6) The glass sheet of the invention is able to be chemically tempered or, in other words, ion-exchangeable/able to undergo an ion-exchange, with reduced or even no warping effect or alternatively, with increased warping to design a shape.

(7) The glass sheet of the invention is a float glass sheet. The term float glass sheet is understood to mean a glass sheet formed by the float process, which consists in pouring the molten glass onto a bath of molten tin, under reducing conditions. A float glass sheet comprises, in a known way, a tin face, that is to say a face enriched in tin in the body of the glass close to the surface of the sheet. The term enrichment in tin is understood to mean an increase in the concentration of tin with respect to the composition of the glass at the core, which may or may not be substantially zero (devoid of tin). Therefore, a float glass sheet can be easily distinguished from sheets obtained by other glassmaking processes, in particular by the tin oxide content which may be measured, for example, by electronic microprobe to a depth of 10 microns. In many cases and as illustration, this content lies between 1 and 5 wt %, integrated over the first 10 microns starting from the surface.

(8) The float glass sheet according to the invention may have varied and relatively large sizes. It can, for example, have sizes ranging up to 3.21 m6 m or 3.21 m5.50 m or 3.21 m5.10 m or 3.21 m4.50 m (PLF glass sheet) or also, for example, 3.21 m2.55 m or 3.21 m2.25 m (DLF glass sheet).

(9) The thickness of the float glass sheet is not particularly limited. In order to effectively perform chemical strengthening treatment described below, the thickness of the glass sheet is usually preferably 5 mm or less, more preferably 3 mm or less, more preferably 1.5 mm or less, and particularly preferably 0.8 mm or less (for example, less than 0.7 mm or less than 0.55 mm or even less than 0.35 mm). The problem of warpage after chemical strengthening is likely to occur when the thickness of the glass sheet is less than 3 mm, and typically, less than 1.5 mm.

(10) According to the present invention, the float glass sheet has:

(11) $0.01<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.3$

(12) The value of 1.03 subtracted from the ratio

(13) $\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}$
allows eliminating contribution from reference (glass sheet not treated for warpage control). The defined term in absolute value allows covering both decrease of warpage or controlled increase of warpage.

(14) To obtain the specific Na.sub.2O ratio between air and tin faces in the glass sheet of the invention, a dealkalization treatment is implemented, and the difference between the degree of dealkalization between that in one face thereof and that in the other face thereof is set to be within a specific range. As a result, it is possible to control the exchange rate of ions in a face versus the opposite one, and it is possible to achieve a balance in the degree of behaviour of chemical strengthening between one face and the other one. For this reason, in the glass sheet of the invention, it is possible to control the warpage (reduce/avoid the warpage or alternatively increase the warpage) of the strengthened glass sheet, without conducting grinding or polishing treatment before strengthening.

(15) The amount of Na.sub.2O in the air face, namely (Na.sub.2O).sub.air, means the Na.sub.2O amount in the bulk of the glass near the extreme surface of the air face. The amount of Na.sub.2O in the tin face, namely (Na.sub.2O).sub.tin, means the Na.sub.2O amount in the bulk of the glass near the extreme surface of the tin face. According to the invention, the Na.sub.2O amount on each face (tin or air) is measured by an X-ray fluorescence (XRF) spectrometer using NaK rays. In present text, the amount of Na.sub.2O was determined by using a calibration curve method build with International glass reference samples. As the measurement apparatus, S4 Explorer manufactured by Bruker is exemplified with following measurement parameters: Output: Rh 30 kV-100 mA Filter: No Mask: 34 mm Colimator: 0.46 Analyzing crystal: XS55 Detector: FC Element Rays: NaK Peak angle (2/deg.): 25,017 Peak measurement time period (seconds): 30 B. G. 1 (2/deg.): NA B. G. 1 measurement time period (seconds): 0 B. G. 2 (20/deg.): NA B. G. 2 measurement time period (seconds): 0 PHA FC: 37-174.

(16) If the float glass sheet according to the invention is covered by a coating or a layer, the amount of Na.sub.2O is determined while excluding the coating/layer itself, taking into account the glass only.

(17) Preferably, the float glass sheet has:

(18) $0.03<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.1.5$

(19) More preferably, the float glass sheet has:

(20) $0.05<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.1.2$

(21) Even more preferably, the float glass sheet has:

(22) $0.1<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.0.9$

(23) In a very preferred manner, the float glass sheet has:

(24) $0.2<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.0.45$

(25) According to a preferred embodiment of the invention, the float glass sheet has:

(26) $\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}1.01.$
According to this embodiment, preferably, the float glass sheet has:

(27) 0$\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}1.$
More preferably, the float glass sheet has:

(28) $\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}0.99\mathrm{or}\mathrm{even}\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}0.97.$
The most preferably, the float glass sheet has:

(29) $\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}0.95\mathrm{or}\mathrm{even},\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}0.93.$
Such embodiments are advantageous as they allow decreasing more and more warpage through chemical strengthening and thereby keeping as much as possible flatness of the glass sheet. Some can also lead to negative warpage (or antiwarpage), which is desirable in some applications.

(30) According to the invention, the composition of the float glass sheet is boron-free. This means that boron is not intentionally added in the glass batch/raw materials and that, if it is present, B.sub.2O.sub.3 content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production. For example, B.sub.2O.sub.3 content in the composition of the float glass sheet of the invention is less than 0.01 or even better less than 0.005 wt %.

(31) According to the invention, the composition of the float glass sheet is lithium-free. This means that lithium is not intentionally added in the glass batch/raw materials and that, if it is present, Li.sub.2O content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production. For example, Li.sub.2O content in the composition of the float glass sheet of the invention is less than 0.01 wt % or even better less than 0.005 wt %.

(32) According to the invention, the composition of the float glass sheet comprises: 1Al.sub.2O.sub.3<6 wt %. Preferably, the composition of the float glass sheet comprises: 1Al.sub.2O.sub.3<5 wt % or even: 1Al.sub.2O.sub.3<4 wt %. More preferably, the composition of the float glass sheet comprises: 1Al.sub.2O.sub.33 wt %. Alternatively, the composition of the float glass sheet comprises: 2<Al.sub.2O.sub.3<6 wt %. Preferably, the composition of the float glass sheet comprises: 2<Al.sub.2O.sub.3<5 wt % or even: 2<Al.sub.2O.sub.3<4 wt %. More preferably, the composition of the float glass sheet comprises: 2<Al.sub.2O.sub.33 wt %. Advantageously and alternatively also, 3Al.sub.2O.sub.3<6 wt %. Preferably, the composition of the float glass sheet comprises: 3Al.sub.2O.sub.3<5 wt % or even: 3Al.sub.2O.sub.3<4 wt %. Alternatively, the composition of the float glass sheet comprises: 4Al.sub.2O.sub.3<6 wt % or even 4Al.sub.2O.sub.3<5 wt %.

(33) According to the invention, the composition of the glass sheet comprises: 2CaO<10 wt %. Preferably, the composition of the glass sheet comprises: 3CaO<10 wt % and more preferably, 4CaO<10 wt %. In a very particularly preferred embodiment, the composition of the glass sheet comprises: 5CaO<10 wt %. In the most preferred embodiment, the composition of the glass sheet comprises: 6CaO<10 wt %.

(34) According to the invention, the composition of the glass sheet comprises: 0MgO8 wt %. Preferably, the composition of the glass sheet comprises: 0MgO7 wt % and more preferably, 0MgO6 wt %. In the most preferred embodiment, the composition of the glass sheet comprises: 0MgO<5 wt %.

(35) According to the invention, the composition of the glass sheet comprises: 1K.sub.2O<8 wt %. Preferably, the composition of the glass sheet comprises: 1K.sub.2O<7 wt % and more preferably, 1K.sub.2O<6 wt %. In a very particularly preferred embodiment, the composition of the glass sheet comprises: 1K.sub.2O<5 wt %. Alternatively, the composition of the glass sheet comprises: 2K.sub.2O6 wt %, or even better 3K.sub.2O6 wt %. In the most preferred embodiment, the composition of the glass sheet comprises: 2K.sub.2O4 wt %.

(36) According to the invention, the composition of the glass sheet comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.1 to 0.7. Preferably, the composition of the glass sheet comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.1 to 0.6. More preferably, the composition of the glass sheet comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.2 to 0.6. Alternatively, the composition of the glass sheet comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.1 to 0.5 In a very particularly preferred embodiment, the composition of the glass sheet comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.2 to 0.5. In a most preferred embodiment of the invention, the composition comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio of from 0.2 to 0.4.

(37) According to an embodiment of the invention, the composition comprises total iron (expressed in the form of Fe.sub.2O.sub.3) in a content ranging from 0.002 to 1.7% by weight. Preferably, the composition of the invention comprises a total iron (expressed in terms of Fe.sub.2O.sub.3) content ranging from 0.002 to 0.6 wt % and, more preferably, ranging from 0.002 to 0.2 wt %.

(38) In a very preferred embodiment, the composition of the invention comprises a total iron (expressed in terms of Fe.sub.2O.sub.3) content ranging from 0.002 to 0.06 wt %. A total iron (expressed in the form of Fe.sub.2O.sub.3) content of less than or equal to 0.06 wt % makes it possible to obtain a glass sheet with almost no visible coloration and allowing a high degree of flexibility in aesthetic designs (for example, getting no color variation when white silk printing of some glass elements of smartphones). The minimum value makes it possible not to be excessively damaging to the cost of the glass as such, low iron values often require expensive, very pure, raw materials and also purification of these. Preferably, the composition comprises a total iron (expressed in the form of Fe.sub.2O.sub.3) content ranging from 0.002 to 0.04 wt %. More preferably, the composition comprises a total iron (expressed in the form of Fe.sub.2O.sub.3) content ranging from 0.002 to 0.02 wt %. In the most preferred embodiment, the composition comprises a total iron (expressed in the form of Fe.sub.2O.sub.3) content ranging from 0.002 to 0.015 wt %.

(39) Alternatively, the composition may comprise total iron as follows: 0.06<Fe.sub.2O.sub.31.7% by weight. More preferably, the composition may comprise total iron as follows: 0.06<Fe.sub.2O.sub.30.6% by weight, and the most preferably, 0.06<Fe.sub.2O.sub.30.2% by weight.

(40) According to a particularly preferred embodiment, the composition of the float glass sheet of the invention comprises the following in weight percentage, expressed with respect to the total weight of glass:

(41) 65SiO.sub.278%

(42) 8Na.sub.2O15%

(43) 1K.sub.2O<6%

(44) 1Al.sub.2O.sub.3<3%

(45) 4CaO<10%

(46) 0MgO6%;

(47) a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio which is ranging from 0.1 to 0.5.

(48) According to this last embodiment, the composition of the float glass sheet of the invention more preferably comprises:

(49) 65SiO.sub.278%

(50) 8Na.sub.2O15%

(51) 2K.sub.2O<6%

(52) 1Al.sub.2O.sub.3<3%

(53) 6CaO<10%

(54) 0MgO6%;

(55) a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio which is ranging from 0.2 to 0.5.

(56) According to another preferred embodiment, the composition of the float glass sheet of the invention more preferably comprises:

(57) 65SiO.sub.278%

(58) 8Na.sub.2O15%

(59) 2K.sub.2O<4%

(60) 1Al.sub.2O.sub.3<3%

(61) 6CaO<10%

(62) 0MgO5%;

(63) a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio which is ranging from 0.2 to 0.4.

(64) According to another embodiment, the composition of the float glass sheet comprises ZnO in a content lower than 0.1 wt % Preferably, the composition of the glass sheet is free of ZnO. This means that the element zinc is not intentionally added in the glass batch/raw materials and that, if it is present, ZnO content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production.

(65) According to another embodiment, the composition of the float glass sheet comprises ZrO.sub.2 in a content lower than 0.1 wt %. Preferably, the composition of the glass sheet is free of ZrO.sub.2. This means that the element zirconium is not intentionally added in the glass batch/raw materials and that, if it is present, ZrO.sub.2 content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production.

(66) According to still another embodiment, the composition of the float glass sheet comprises BaO in a content lower than 0.1 wt %. Preferably, the composition of the glass sheet is free of BaO. This means that the element barium is not intentionally added in the glass batch/raw materials and that, if it is present, BaO content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production.

(67) According to still another embodiment, the composition of the float glass sheet comprises SrO in a content lower than 0.1 wt %. Preferably, the composition of the glass sheet is free of SrO. This means that the element strontium is not intentionally added in the glass batch/raw materials and that, if it is present, SrO content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production.

(68) According to still another embodiment, the composition of the float glass sheet comprises bulk SnO.sub.2 in a content lower than 0.1 wt % (bulk content excluding SnO.sub.2 in the tin face of a float glass sheet). Preferably, the composition of the glass sheet is free of bulk SnO.sub.2. This means that the element tin is not intentionally added in the glass batch/raw materials and that, if it is present, bulk SnO.sub.2 content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production.

(69) According to an embodiment of the invention, the composition comprises coloring components other than iron, chromium and cobalt oxides in a total content which is less than 0.005 wt %. Such an embodiment allows to control color and thus to provide a glass sheet which is neutral as mainly requested for display applications. More preferably, the composition of the invention comprises coloring components other than iron, chromium and cobalt oxides in a total content which is less than 0.003 wt %.

(70) Advantageously, the composition of the invention may further comprise chromium and/or cobalt oxides in a total content which is between 0.001 and 0.025 wt %. This means that the composition may comprise only chromium, only cobalt or both. Such a specific composition makes the glass especially suitable for touch technology based on IR transmission.

(71) According to one embodiment of the invention, the float glass sheet is coated with at least one transparent and electrically conducting thin layer. A transparent and conducting thin layer according to the invention can, for example, be a layer based on SnO.sub.2:F, SnO.sub.2:Sb or ITO (indium tin oxide), ZnO:Al or also ZnO:Ga.

(72) According to another advantageous embodiment of the invention, the float glass sheet is coated with at least one antireflection layer. This embodiment is obviously advantageous in the case of use of the glass sheet of the invention as front face of a screen. An antireflection layer according to the invention can, for example, be a layer based on porous silica having a low refractive index or it can be composed of several layers (stack), in particular a stack of layers of dielectric material alternating layers having low and high refractive indexes and terminating in a layer having a low refractive index.

(73) According to another embodiment, the float glass sheet is coated with at least one anti-fingerprint layer or has been treated so as to reduce or prevent fingerprints from registering. This embodiment is also advantageous in the case of use of the glass sheet of the invention as front face of a touchscreen. Such a layer or such a treatment can be combined with a transparent and electrically conducting thin layer deposited on the opposite face. Such a layer can be combined with an antireflection layer deposited on the same face, the anti-fingerprint layer being on the outside of the stack and thus covering the antireflection layer.

(74) According to still another embodiment, the float glass sheet is coated with at least one layer or has been treated so as to reduce or prevent glaring and/or sparkling. This embodiment is of course advantageous in the case of use of the glass sheet of the invention as front face of a display device. Such an anti-glare or anti-sparkling treatment is for example an acid-etching producing a specific roughness of the treated face of the glass sheet.

(75) According to the applications and/or properties desired, other layer(s)/treatment(s) can be deposited/done on one and/or the other face of the float glass sheet according to the invention.

(76) The glass sheet of the invention is obtained by a float method. In the float method, a glass sheet is manufactured using a melting furnace in which a raw material of glass is melted, a float bath in which molten glass is floated on a molten metal (tin) to form a glass ribbon, and an annealing furnace in which the glass ribbon is annealed. Hereinafter, in the method description, the term glass sheet may be used as a generic term indicating the glass sheet and/or the glass ribbon.

(77) In an exemplified method of preparing the float glass sheet of the invention, at least the air face of the glass sheet (or glass ribbon) is subjected to a dealkalization treatment, thereby removing alkaline components, and thus, reaching the specific ratio according to the invention. For example, the dealkalization method may advantageously be a method of treating the glass with a substance capable of ion exchange reaction(s) with alkaline components in the glass. As a substance capable of ion exchange reaction(s) with alkaline components in the glass, examples include molecules having fluorine atoms in the structure thereof, sulphur-based compounds, acid, or nitride. The substance capable of ion exchange reaction(s) with alkaline components in the glass may be for example in the form of gas, liquid, . . . or any other suitable form (available form(s) depend(s) amongst others of the substance itself).

(78) Examples of substance containing molecules having fluorine atoms in the structure thereof include hydrogen fluoride (HF), freon (for example, chlorofluorocarbon, fluorocarbon, hydrochlorofluorocarbon, hydrofluorocarbon, halon and the like), hydrofluoric acid, fluorine (simple substance), trifluoroacetic acid, carbon tetrafluoride, silicon tetrafluoride, phosphorus pentafluoride, phosphorus trifluoride, boron trifluoride, nitrogen trifluoride, chlorine trifluoride, and the like.

(79) Examples of sulphur-based compounds include sulfurous acid, sulfuric acid, peroxomonosulfuric acid, thiosulfuric acid, dithionous acid, disulfuric acid, peroxodisulfuric acid, polythionic acid, hydrogen sulfide, sulfur dioxide, and the like.

(80) Examples of an acid include hydrochloric acid, carbonic acid, boric acid, lactic acid, and the like.

(81) Examples of a nitride include nitric acid, nitric monoxide, nitrogen dioxide, nitrous oxide, and the like.

(82) The method for application of the substance capable of ion exchange reaction(s) with alkaline components in the glass may be chosen depending on the form of the substance and any other suitable and desired parameter.

(83) In the float process in which glass is formed on a molten metal (tin) bath, the substance capable of ion exchange reaction(s) with alkaline components in the glass may be supplied to the glass sheet being conveyed on the molten metal bath from the side not in contact with the metal surface, thereby treating the top face of the glass sheet/ribbon (air face). In the annealing zone subsequent to the molten metal (tin) bath, the glass sheet is conveyed by roller conveying. Here, the annealing zone includes not only the inside of the annealing furnace but also a portion where the glass sheet is conveyed from the molten metal bath to the annealing furnace in the float bath. In the annealing zone, the substance capable of ion exchange reaction(s) with alkaline components in the glass may be supplied from the face that was not in contact with the molten metal (air face) and/or the opposite face (tin face).

(84) The invention also relates to the use of the chemically tempered float glass sheet according to the invention in an electronic device.

EXAMPLES

(85) Powder raw materials were mixed together and placed in melting crucibles, according to the compositions specified in the following tables 1-4. The raw material mix was then heated up in an electrical furnace to a temperature allowing complete melting of the raw material.

(86) In a first series (examples 1.x), the base molar composition was kept constant, and the proportion between Na.sub.2O and K.sub.2O was varied in the range of the invention while keeping constant the molar fraction of alkali (Na.sub.2O+K.sub.2O13.3 mol %) over the total composition. Example 1.1 is a comparative example, similar to state-of-the-art soda-lime float glass composition, with a classical K.sub.2O/(K.sub.2O+Na.sub.2O) ratio.

(87) In a second series (examples 2.x.x), the molar composition in SiO.sub.2 and Al.sub.2O.sub.3 was kept constant. The molar fraction of total alkali (Na.sub.2O+K.sub.2O13.3 mol %) and total alkali-earth (MgO+CaO15 mol %) were as well kept constant, but the proportion between K.sub.2O and Na.sub.2O and between MgO and CaO were varied. As replacement of CaO by MgO is known to improve DoL, 3 levels of CaO/MgO (sub-series 2.1.x, 2.2.x and 2.3.x) were tested to show the beneficial impact of an improved K.sub.2O/(K.sub.2O+Na.sub.2O) ratio over a large range of compositions. Examples 2.1.1, 2.2.1, 2.3.1 are comparative examples, for each sub-series, with a classical K.sub.2O/(K.sub.2O+Na.sub.2O) ratio.

(88) The third series (examples 3.x) is similar to the series 2.2.x, but with a higher content of Al.sub.2O.sub.3. Example 3.1 is a comparative example, with a classical K.sub.2O/(K.sub.2O+Na.sub.2O).

(89) In a fourth series (examples 4.x), the base molar composition was kept constant, and the proportion between Na.sub.2O and K.sub.2O was varied in the range of the invention while keeping constant the molar fraction of alkali (Na.sub.2O+K.sub.2O13.3 mol %) over the total composition. Two glass tints were prepared, characterized by their levels of iron: 0.2% wt % of Fe.sub.2O.sub.3 (light green glass), and 0.45 wt % of Fe.sub.2O.sub.3 (green glass). In this series, examples 4.1 and 4.4 are comparative examples, similar to state of the art float compositions, with a classical K.sub.2O/(K.sub.2O+Na.sub.2O) ratio.

(90) TABLE-US-00001 TABLE 1 EX1.1 Wt % (comp) EX1.2 EX 1.3 EX 1.4 SiO.sub.2 71.9 71.7 71.3 70.6 Al.sub.2O.sub.3 1.1 1.1 1.2 1.2 CaO 8.1 7.8 7.1 6.7 MgO 4.0 4.0 4.2 4.3 Na.sub.2O 14.3 13.3 11.3 9.4 K.sub.2O 0.2 1.7 4.6 7.4 Fe.sub.2O.sub.3 0.038 0.034 0.031 0.030 K.sub.2O/(Na.sub.2O + K.sub.2O) 0.01 0.11 0.29 0.44

(91) TABLE-US-00002 TABLE 2 EX EX EX 2.1.1 EX EX 2.2.1 EX EX 2.3.1 EX EX Wt % (comp) 2.1.2 2.1.3 (comp) 2.2.2 2.2.3 (comp) 2.3.2 2.3.3 SiO.sub.2 71.8 71.6 70.9 72.0 71.7 71.0 73.0 72.7 71.4 Al.sub.2O.sub.3 1.1 1.1 1.2 1.1 1.2 1.2 1.1 1.2 1.2 CaO 8.1 7.8 6.5 7.2 6.9 5.7 3.4 3.3 2.8 MgO 4.1 4.2 4.2 4.7 4.8 4.8 7.1 7.2 7.3 Na.sub.2O 14.3 13.3 9.3 14.4 13.4 9.4 14.8 13.7 9.6 K.sub.2O 0.2 1.7 7.6 0.2 1.7 7.6 0.2 1.7 7.6 Fe.sub.2O.sub.3 0.034 0.003 0.027 0.035 0.032 0.028 0.034 0.033 0.028 K.sub.2O/(Na.sub.2O + K.sub.2O) 0.01 0.11 0.45 0.01 0.11 0.45 0.01 0.11 0.44

(92) TABLE-US-00003 TABLE 3 EX3.1 Wt % (comp) EX3.2 EX3.3 SiO.sub.2 70.4 69.9 68.7 Al.sub.2O.sub.3 3.1 3.1 3.2 CaO 7.3 6.9 5.9 MgO 4.8 4.7 4.8 Na.sub.2O 14.2 13.4 9.4 K.sub.2O 0.2 1.7 7.6 Fe.sub.2O.sub.3 0.0 0.0 0.0 K.sub.2O/(Na.sub.2O + K.sub.2O) 0.01 0.11 0.45

(93) TABLE-US-00004 TABLE 4 EX4.1 EX4.4 Wt % (comp) EX4.2 EX4.3 (comp) EX4.5 EX4.6 SiO.sub.2 72.1 71.4 71.0 71.9 71.2 70.6 Na.sub.2O 14.2 11.2 9.3 14.3 11.2 9.3 K.sub.2O 0.1 4.8 7.9 0.1 4.8 7.9 Al.sub.2O.sub.3 1.1 1.1 1.1 1.1 1.1 1.1 CaO 8.1 7.1 6.4 8.0 7.1 6.5 MgO 4.2 4.2 4.2 4.1 4.2 4.2 Fe.sub.2O.sub.3 0.21 0.19 0.18 0.48 0.45 0.42 K.sub.2O/(Na.sub.2O + K.sub.2O) 0.01 0.30 0.46 0.01 0.30 0.46

(94) After the melting and the homogenization of the composition, the glass was cast in several small samples of 40*40 mm and annealed in an annealing furnace. Subsequently, the samples were polished up to a surface state similar to floated glass (mirror polishing). Several samples were produced for each composition, in order to allow to perform different tempering treatment for each composition.

(95) Chemical Tempering

(96) The samples prepared in above section were chemically tempered under two different tempering conditions, and for each of them the different samples were treated at the same time and in the same conditions. The samples of different compositions were placed in a cassette, preheated and then dippen in a molten KNO.sub.3 (>99%) bath. After the ion exchange, the samples were cooled down and washed.

(97) Two types of treatments were applied on the different glass compositions. The first one was carried out at 420 C. during an immersion time of 220 minutes (so called low temperature). The second one was carried out at 465 C. during 480 minutes (so called high temperature). Subsequently the surface compressive stress (CS) and the depth of exchanged layer (DoL) were measured via photoelasticimetry. The following tables 5-7 summarize the average value of CS and DoL for 3 random samples of each composition and each treatment.

(98) TABLE-US-00005 TABLE 5 EX1.1 (comp) EX1.2 EX1.3 EX 1.4 CS.sub.465 C. (MPa) 501 523 481 427 DOL.sub.465 C. (m) 21.4 25.4 31.6 37.9 CS.sub.420 C. (MPa) 747 706 588 495 DOL.sub.420 C. (m) 8.6 9.7 13.0 16.1

(99) TABLE-US-00006 TABLE 6 EX EX EX 2.1.1 EX EX 2.2.1 EX EX 2.3.1 EX EX (comp) 2.1.2 2.1.3 (comp) 2.2.2 2.2.3 (comp) 2.3.2 2.3.3 CS.sub.465 C. (MPa) 502 541 417 511 527 438 537 468 425 DOL.sub.465 C. (m) 20.8 22.5 37.7 21.7 27.4 38.8 31.6 34.9 51.2 CS.sub.420 C. (MPa) 740 687 462 749 697 471 743 695 465 DOL.sub.420 C. (m) 8.4 9.3 16.0 9.0 10.2 16.7 13.0 14.3 23.9

(100) TABLE-US-00007 TABLE 6 EX3.1 (comp) EX3.2 EX3.3 CS.sub.465 C. (MPa) 579 585 450 DOL.sub.465 C. (m) 18.9 25.5 41.3 CS.sub.420 C. (MPa) 827 722 501 DOL.sub.420 C. (m) 8.2 9.8 18.8

(101) TABLE-US-00008 TABLE 7 EX4.1 EX4.4 (comp) EX4.2 EX4.3 (comp) EX4.5 EX4.6 CS.sub.465 C. (MPa) 516 512 476 517 526 489 DOL.sub.465 C. (m) 22.1 29.6 38.6 21.9 29.6 36.4 CS.sub.420 C. (MPa) 811 671 525 809 662 533 DOL.sub.420 C. (m) 7.9 10.6 14.7 7.7 10.6 14.5

(102) Based on the measured values of the chemical tempering properties (CS and DoL), the ratio R between the high temperature and low temperature compressive stresses can be computed: R=CS.sub.465 C./CS.sub.420 C.. This R ratio is an image of the surface compressive stress preservation at high temperature. A value of R close to 1 means that the glass tends to limit stress relaxation at high temperature, and that low and high temperature treatment finally yields the same level of compressive stress. On the other hand if the R ratio is small, it means that the glass submitted to a high temperature treatment tends to relax the generated stresses to a large extent.

(103) The gain in DoL (G factor) can also be computed for each composition according to the invention by using the corresponding comparative sample: G=(DoL.sub.sampleDoL.sub.comparative)/DoL.sub.comparative. This G factor as to be as high as possible in order to improve the resistance of the glass pieces versus mechanical solicitations.

(104) The R ratios and G factors for the different examples are summarized in the following tables 8-11.

(105) TABLE-US-00009 TABLE 8 EX1.1 (comp) EX1.2 EX 1.3 EX 1.4 K.sub.2O/(Na.sub.2O + K.sub.2O) 0.01 0.11 0.29 0.44 R (CS.sub.465 C./CS.sub.420 C.) 0.67 0.74 0.82 0.86 G.sub.465 C. (DoL improvement) 0% 19% 48% 77% G.sub.420 C. (DoL improvement) 0% 13% 51% 86%

(106) TABLE-US-00010 TABLE 9 EX EX EX 2.1.1 EX EX 2.2.1 EX EX 2.3.1 EX EX (comp) 2.1.2 2.1.3 (comp) 2.2.2 2.2.3 (comp) 2.3.2 2.3.3 K.sub.2O/(Na.sub.2O + K.sub.2O) 0.01 0.11 0.45 0.01 0.11 0.45 0.01 0.11 0.44 R (CS.sub.465 C./CS.sub.420 C.) 0.68 0.79 0.90 0.68 0.76 0.93 0.72 0.67 0.91 G.sub.465 C. (DoL improvement) 0% 8% 81% 0% 26% 79% 0% 10% 62% G.sub.420 C. (DolLimprovement) 0% 10% 90% 0% 13% 86% 0% 10% 84%

(107) TABLE-US-00011 TABLE 10 EX3.1 (comp) EX3.2 EX3.3 K.sub.2O/(Na.sub.2O + K.sub.2O) 0.01 0.11 0.45 R (CS.sub.465 C./CS.sub.420 C.) 0.70 0.81 0.90 G.sub.465 C. .sub.(DoL improvement) 0% 35% 118% G.sub.420 C. (DoL improvement) 0% 19% 128%

(108) TABLE-US-00012 TABLE 11 EX4.1 EX4.4 (comp) EX4.2 EX4.3 (comp) EX4.5 EX4.6 K.sub.2O/(Na.sub.2O + K.sub.2O) 0.01 0.30 0.46 0.01 0.30 0.46 R (CS.sub.465 C./CS.sub.420 C.) 0.64 0.76 0.91 0.64 0.79 0.92 G.sub.465 C. (Dol improvement) 0% 34% 86% 0% 37% 87% G.sub.420 C. (Dol improvement) 0% 34% 75% 0% 35% 66%

(109) From the above tables 8-11, the beneficial effect of the composition of the invention is highlighted: by increasing the K.sub.2O/(K.sub.2O+Na.sub.2O) ratio while keeping the rest of the composition stable on a molar point of view, the G factors (420 C. and 465 C.) of the composition increases significantly, meaning that the composition according to the invention allows faster ion exchange at the two tested temperatures. Moreover, this surprising effect is also observed and similar for the different glass tints, i.e. for the two different iron levels in series of samples 4.x (Table 11).

(110) Similarly, the R ratio increases with higher values of K.sub.2O/(K.sub.2O+Na.sub.2O), highlighting the effect of stress conservation for high temperature treatment. In this set of experiments, the comparative examples present a R ratio around 0.65-0.7, meaning that increasing the treatment temperature from 420 C. to 465 C. will reduce the surface compressive stress by 30-35%. On the other side, examples according to the invention present a R ratio up to 0.92, meaning that the higher temperature treatment only reduces the compressive stress by 10% or less with respect to low temperature treatment.

(111) By this way, interesting combinations of DoL (up to 50 m) and CS (kept higher than 400 MPa) can be obtain with the composition according to the invention, by applying higher temperature treatments.

(112) Warping Behaviour

(113) FIG. 2 shows evolution of warp (on 0.7 mm thickness sheet and for 200 mm sample length) vs. Na.sub.2O.sub.air/Na.sub.2O.sub.tin ratio for (i) classical soda-lime float glass from the state-of-the-art (plain line) and (ii) classical alumino-silicate float glass from the state-of-the-art (in particular, a DragonTrail glass from Asahi Glass Co.) (dashed line). In view of this figure, the behaviour from the composition of the invention will be intermediate between both shown trendlines.

(114) Other Properties

(115) The following properties were evaluated for the series of examples 4.x on the basis of glass composition using Fluegel model (Glass Technol.: Europ. J. Glass Sci. Technol. A 48 (1): 13-30 (2007); and Journal of the American Ceramic Society 90 (8): 2622 (2007)): Glass melt density evaluated at 1200 and 1400 C.; Viscosity through the Melting point temperature T2; Working point temperature T4; Devitrification temperature T0;

(116) Table 12 summarizes obtained results.

(117) In a general manner:

(118) The melting point temperature T2 is preferably at most 1550 C., more preferably at most 1520 C., the most preferably at most 1500 C.

(119) The Working point temperature T4 is preferably at most 1130 C., more preferably at most 1100 C., the most preferably at most 1070 C.

(120) The devitrification temperature T0 is preferably at most T4, more preferably at most T4-20 C., the most preferably at most T4-40 C.

(121) The compositions according to present invention are suitable for forming by a float process and while using existing furnace tools for production of soda lime glass because of: their melting point temperature T2 being lower than 1500 C. and which are close to the one of classical soda lime glass (Comparative ex.1.1 and 2.1). their working point temperature T4 which is lower than 1100 C. and which are close to a classical soda lime glass (Comparative ex.1.1 and 2.1). their devitrification temperature T0 are suitable because lower than working point temperature T4; their glass density which is very close to soda lime glasses (Comparative EX4.1 and 4.4), thereby avoiding/limiting density defects during composition change (transition).

(122) TABLE-US-00013 TABLE 12 EX4.1 EX4.4 (comp) EX4.2 EX4.3 (comp) EX4.5 EX4.6 Glass melt density (1200 C.) 2.37 2.35 2.34 2.36 2.35 2.34 Glass melt density (1400 C.) 2.33 2.32 2.31 2.33 2.32 2.32 Melting point T2 ( C.) 1450 1480 1500 1449 1479 1498 Working point T4 ( C.) 1028 1041 1049 1027 1040 1047 Devitrification temperature T0 ( C.) 993 988 975 989 988 980

(123) Finally, compositions according to the invention allow to get sulfate fining ability during their manufacture/melting, thanks to an adequate solubility of sulfate and suitable high-temperature viscosity.