Glass sheet capable of having controlled warping through chemical strengthening

Abstract

The invention relates to 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% 0K.sub.2O<5% 1Al.sub.2O.sub.3<6% 0CaO<4.5% 4MgO12% a (MgO/(MgO+CaO)) ratio0.5 characterized in that the glass sheet has: (I). The invention corresponds to an easy chemically-temperable soda-silica type glass composition, which is more suited for mass production than aluminosilicate glass, and therefore is available at low cost, and with a base glass/matrix composition that is close to or very similar to compositions already used in existing mass production, and finally which shows reduced or controlled increased warping effect.

Claims

1. 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.2 78%; 5Na.sub.2O20; 0K.sub.2O<5%; 1Al.sub.2O.sub.3 <6%; 0CaO<4.5%; 4MgO12%,and having a (MgO/(MgO+CaO)) ratio0.5 wherein the glass sheet has: $0.01<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.3.$

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 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 0.06% by weight.

4. 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 0.02% by weight.

5. A float glass sheet according to claim 1, wherein the composition comprises: 1Al.sub.2O.sub.3<5 wt %.

6. A float glass sheet according to claim 1, wherein the composition comprises: 1Al.sub.2O.sub.3<4 wt %.

7. A float glass sheet according to claim 1, wherein the composition comprises: 1Al.sub.2O.sub.3<3 wt %.

8. A float glass sheet according to claim 1, wherein the composition comprises: 2<Al.sub.2O.sub.3<6 wt %.

9. A float glass sheet according to claim 1, wherein the composition comprises: 2<Al.sub.2O.sub.3<4 wt %.

10. A float glass sheet according to claim 1, wherein the composition comprises: 1.0CaO<4.5% wt %.

11. A float glass sheet according to claim 1, wherein the composition comprises: 0.5[MgO/(MgO+CaO)]<1.

12. A float glass sheet according to claim 1, wherein the composition comprises: 0.75[MgO/(MgO+CaO)]<1.

13. A float glass sheet according to claim 1, wherein the composition comprises: 0.88[MgO/(MgO+CaO)]<1.

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

15. A float glass sheet according to claim 1, wherein the glass sheet has a thickness of 0.8 mm or less.

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

17. A float glass sheet according to claim 1, wherein the float glass sheet has been chemically tempered and has a surface compressive stress of at least 600 MPa and Depth of exchanged layer of at least 15 m.

18. An electronic device, comprising the float glass sheet of claim 1.

19. 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%; 0K.sub.2O<5%; 1Al.sub.2O.sub.3<6%; 0.6CaO<4.5%; 4MgO12%, and having a (MgO/(MgO+CaO)) ratio0.5, wherein the glass sheet has: $0.01<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.3.$

20. 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%; 0K.sub.2O<2%; 1Al.sub.2O.sub.3<6%; 0CaO<2%; 4MgO12%; and having 0.75(MgO/(MgO+CaO))1, wherein the glass sheet has: $0.01<.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.3.$

Description

EXAMPLES

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

(2) 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.

(3) Composition of comparative example 1 corresponds to a classical low-iron soda-lime (SL) glass according to the state of the art and composition of comparative example 2 corresponds to a commercially available alumino-silicate (AS) glass.

(4) Compositions of examples 1-10 correspond to compositions according to the invention.

(5) Two glass samples from each composition from examples were then treated with a dealkalization substance: samples were pre-heated at 200 C. inside an electric furnace. They were then heated-up in another electrical furnace up to 450 C. The heating-up step took 40 min. A thermocouple placed inside the furnace allowed checking the temperature of the sample before and after the dealkalinization treatment.

(6) TABLE-US-00001 TABLE 1 Comp. Comp. ex. 1 ex. 2 Wt % (SL) (AS) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 SiO.sub.2 72 60.9 72.1 71.3 70.6 70.2 68.4 69.4 70.5 74.0 73.9 73.1 Al.sub.2O.sub.3 1.3 12.8 2.0 2.3 3.0 2.9 4.9 3.8 3.1 1.1 1.2 1.1 MgO 4.5 6.7 7.5 6.7 9.1 9.2 8.8 8.3 7.2 8.9 8.9 6.9 CaO 7.9 0.1 3.0 4.3 0.6 0.6 1.0 1.1 3.4 0.3 0.4 3.6 Na.sub.2O 13.9 12.2 15.0 13.6 16.3 15.2 16.5 16.0 13.8 15.3 15.3 14.9 K.sub.2O 0 5.9 0 1.4 0 1.2 0.02 0.95 1.7 0.2 0.2 0.2 Fe.sub.2O.sub.3 0.01 0 0.01 0.01 0.01 0.01 0.01 0.01 0.09 0.09 1.10 1.10 BaO 0 0.2 0 0 0 0 0 0 0 0 0 0 SO.sub.3 0.36 0 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 SrO 0 0.2 0 0 0 0 0 0 0 0 0 0 ZrO.sub.2 0 1.0 0 0 0 0 0 0 0 0 0 0

(7) In order to insure the same thermal history for all samples, the samples were maintained at 450 C. during 20 minutes. The dealkalization treatment consisted in injecting SO.sub.2 (10% SO.sub.2+90% dried air) at 20 l/h with humidified air at 2 l/h (bubbling of dried air inside demi-water at 25 C.) selectively on a glass face 1 (then called the treated face 1) for a given time (t=0 (reference), 3, 10, 17 or 20 minutes), leading to different levels of soda depletion. The dealkalization step is done so that this step happens during but at the end of the maintain at 450 C. (see for example, FIG. 2 for temperature profile for 10 minutes of dealkalization treatment), in order to avoid any relaxation effect and retro diffusion of soda from the bulk towards the surface.

(8) The samples were then removed from the heating furnace, washed and analyzed: one sample was used for XRF measurements of composition of the glass bulk and Na.sub.2O amount on the treated glass face 1; the other one has undergone chemical tempering.

(9) Chemical Tempering

(10) Chemical tempering #1: Some samples prepared in above section were chemically tempered at the same time and in the same conditions. The samples of different compositions were placed in a cassette, preheated and then dipped in a molten KNO.sub.3 (>99%) bath at 420 C. for 220 minutes.

(11) Chemical tempering #2: Some samples prepared in above section were chemically tempered at the same time and in the same conditions. The samples of different compositions were placed in a cassette, preheated and then dipped in a molten KNO.sub.3 (>99%) bath at 430 C. for 240 minutes.

(12) After the ion exchange, the samples were cooled down and washed. Subsequently the surface compressive stress (CS) and the depth of exchanged layer (DoL) were measured via photoelasticimetry.

(13) Table 2 summarizes, for chemical tempering #1, the average value of CS and DoL from treated face 1, for random samples of each of examples 1-4 and 7-10 according to the invention and Comparative examples 1-2.

(14) TABLE-US-00002 TABLE 2 Comp. Comp. chemical ex. 1 ex. 2 tempering #1 (SL) (AS) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Surface 861 884 785 803 820 818 808 645 646 693 compressive stress (MPa) Depth of 6.7 36.1 13.3 11.5 16.6 18.1 13.9 20.3 18.6 12.6 exchanged layer (m)

(15) Those results show that combining, in a soda-silica glass matrix, a low alumina and CaO content as well as a (MgO/(MgO+CaO)) ratio which is higher than 0.5 allows to significantly improve the depth of exchanged layer, while keeping a high surface compressive stress and thus, to increase the glass reinforcement.

(16) Moreover, DOL values of compositions according to the invention are well appropriate for a piece-by-piece process used to produce cover glass for display devices (preferably higher than 10 microns and very preferably higher than 12 microns or even better higher than 15 microns).

(17) Table 3 summarizes, for chemical tempering #2 and for the treated face 1, Na.sub.2O amount as well as the average value of CS and DoL for random samples of examples 5-6 according to the invention and comparative example 1, depending on the duration of dealkalization treatment. Table 3 also shows the same values/parameters obtained for the other face 2 (non-treated by a dealkalization treatment), as such (case 1) or alternatively, while simulating a tin face coming from a float process (case 2).

(18) TABLE-US-00003 TABLE 3 chemical tempering #2 Comp. ex. 1 (SL) Ex. 5 Ex. 6 Dealk. duration (min) 0 3 10 20 0 3 10 20 0 3 10 20 Treated face 1 Na.sub.2O on treated 13.54 12.97 12.74 12.51 16.24 15.66 14.70 14.46 15.49 15.38 13.51 14.07 face (%) Surface compressive 662 617 578 592 785 771 730 715 695 714 637 678 stress (MPa) Depth of exchanged 11.9 11.8 11.6 11.7 25.4 25.3 25.6 25.8 30.9 29.5 30.0 29.4 layer (m) Case 1: Non-treated face 2 Surface compressive 662 662 662 662 785 785 785 785 695 695 695 695 stress (MPa) Depth of exchanged 11.9 11.9 11.9 11.9 25.4 25.4 25.4 25.4 30.9 30.9 30.9 30.9 layer (m) Computed warpage 0.000 0.019 0.038 0.031 0.000 0.014 0.041 0.049 0.000 0.010 0.074 0.046 (%) Case 2: Non-treated face 2 with float simulation (tin face) Na.sub.2O on non-treated 13.1 13.1 13.1 13.1 15.8 15.8 15.8 15.8 15.0 15.0 15.0 15.0 face (%) $.Math.\frac{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{air}}}{{\left({\mathrm{Na}}_{2}O\right)}_{\mathrm{tin}}}-1.03.Math.$ 0 0.04 0.06 0.08 0.00 0.04 0.10 0.11 0 0.01 0.13 0.09 Surface compressive 722 722 722 722 856 856 856 856 758 758 758 758 stress (MPa) Depth of exchanged 10 10 10 10 21 21 21 21 26 26 26 26 layer (m) Computed warpage 0.022 0.003 0.016 0.009 0.050 0.036 0.009 0.001 0.051 0.041 0.023 0.005 (%)

(19) Firstly, one can observe that the dealkalization treatment (treated face 1) allows to tune the chemical tempering performance of the treated face, enabling a controlled warpage. The dealkalinization treatment decreases a little bit CS but while keeping high performances on the DOL. Nevertheless, the CS levels obtained even with the dealkalization treatment with composition of the invention remains globally higher than the soda-lime reference (comparative example 1).

(20) Next to that, as a result of CS modification due to the dealkalinization treatment, the mechanical constraints induced by the chemical tempering on the treated face 1 will evolve comparing to the other face 2. While evolving, one could play on this equilibrium of mechanical constraints in order to obtain the desired warpage or suppress it. This is illustrated in Table 3 by evaluation of the mechanical constraints and induced warpage on examples 5-6 and comparative example 1. The warpage was computed mainly based on CS, DOL and sample geometrical, dimensions for a square glass sheet (0.7 mm thickness, 44 cm), as being the elevation of the middle of the side versus the centre of the sample while convex face is down (d/L, see FIG. 1). A negative value of warpage means that treated face is concave, while a positive value means that the treated face is convex.

(21) Case 1 in table 3 shows the evolution of the warpage when face 1 is treated by a dealkalization and face 2 is as such (untreated). Such a case mimics an industrial case in which the glass sheet is produced by the float process but wherein the tin face has been polished (face 2) and the air face has been treated by a dealkalization treatment (face 1), prior to tempering. One can observe the variation of the warpage while applying different time of dealkalinization treatment, thereby modifying the Na.sub.2O available content, on the treated face 1 versus the non-treated face 2.

(22) Case 2 in table 3 shows the evolution of the warpage when face 1 is treated by a dealkalization and face 2 is untreated but corresponds to a simulated tin face coming from a float process. Such a case mimics an industrial case in which the glass sheet is produced by the float process (face 2 is tin face) and the air face has been treated by a dealkalization treatment (face 1), prior to tempering. In reference conditions, the ratio Na.sub.2O air/tin is 1.03. Also, the CS level is classically 9% higher on tin face than on air face and DOL is 17% lower on tin face than on air face. This allows for each composition kind to establish a reference tin face. From the results, one can clearly observe the presence of a significant warpage for examples 5-6 and comparative example 1 while no warpage treatment is applied (duration=0). Next, one can also observe that the initial warpage can be controlled/modified/suppressed by controlling the amount of Na.sub.2O in the air face versus in the tin face, in the range claimed.

(23) Other Properties

(24) The following properties were evaluated 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)) for compositions of examples 1-4 according to the invention as well as of Comparative examples 1-2: Glass melt density evaluated at 1200 and 1400 C.; Viscosity through the Melting point temperature T2; Working point temperature T4; Devitrification temperature T0; Coefficient of thermal expansion (CET);

(25) Moreover, refractories corrosion behaviour was evaluated according to the known Dunkl corrosion test (during 36 h at 1550 C.), given in percentage corresponding to the loss of material at the metal line.

(26) In a general manner:

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

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

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

(30) The loss of material at the metal line during corrosion test is preferably less than 13%, more preferably less than 11%, the most preferably less than 9%.

(31) CET value (in 10.sup.6/K) is preferably at most 9.6 and more preferably at most 9.4.

(32) Table 4 summarizes these properties for examples 1-4 and 7-10 according to the invention and Comparative examples 1-2.

(33) 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:

(34) their melting point temperature T2 being lower than 1500 C. and which are comparable to a classical soda lime glass (Comparative ex. 1) and significantly lower compared to an aluminosilicate glass (Comparative ex. 2); their working point temperature T4 which is lower than 1100 C. and which are comparable to a classical soda lime glass (Comparative ex. 1) and lower compared to an aluminosilicate glass (Comparative ex. 2); their devitrification temperature T0 are suitable because lower than working point temperature T4; their glass density which is very close to soda lime and aluminosilicate glasses (Comparative ex. 1-2), thereby avoiding/limiting density defects during composition change (transition); their good results in term of refractory corrosion, better than a classical soda lime glass (Comparative ex. 1).

(35) Moreover, the compositions according to present invention have coefficients of thermal expansion (CET) which reach in a known manner appropriate values for a subsequent chemical tempering (limiting differentiated cooling deformation phenomenon). More specifically, the compositions according to present invention show better (lower) values for CET than aluminosilicate glass and thus are less sensitive to differentiated cooling issues than AS glass.

(36) TABLE-US-00004 TABLE 4 Comp Comp ex. 1 ex. 2 (SL) (AS) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Glass at 2.37 2.32 2.34 2.35 2.33 2.33 2.35 2.33 2.33 2.34 melt 1200 C. density at 2.34 2.32 2.32 2.33 2.32 2.32 2.33 2.31 2.31 2.32 1400 C. Melting point T2 1463 1601 1484 1492 1485 1493 1499 1485 1487 1478 ( C.) Working point T4 1037 1176 1050 1055 1048 1053 1058 1047 1050 1044 ( C.) Devitrification 994 951 958 968 989 1028 986 989 990 932 temperature T0 ( C.) CET @210 C. (10.sup.6/ 9.15 9.68 9.07 9.17 9.33 9.37 9.23 8.90 8.93 9.10 K) Dunkl test 12.10 7.69 5.53 5.29 36 h/1550 C. (%)

(37) 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.