CHEMICALLY TEMPERABLE GLASS SHEET

Abstract

The invention relates to a glass sheet having a boron-, strontium- and lithium-free glass composition comprising the following in weight percentage, expressed with respect to the total weight of glass: 65≦SiO.sub.2≦78%; 8≦Na.sub.2O≦15%; 1≦K.sub.2O<6%; 1≦Al.sub.2O.sub.3<6%; 2≦CaO<10%; 0≦MgO≦6%; as well as a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio which is ranging from 0.1 and 0.7. The invention corresponds to an easy chemically-temperable soda-lime-silica type glass composition, which is more suited for use in electronic devices applications.

Claims

1. A glass sheet having a boron-, strontium- and lithium-free glass composition, comprising the following in weight percentage, expressed with respect to the total weight of glass: 65≦SiO.sub.2≦78%; 5≦Na.sub.2O≦20%; 1≦K.sub.2O<8%; 1≦Al.sub.2O.sub.3<6%; 2≦CaO<10%; 0≦MgO≦8%; and K.sub.2O/(K.sub.2O+Na.sub.2O) ratio which is from 0.1 to 0.7.

2. The 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. The 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 0.6% by weight.

4. The 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 0.2% by weight.

5. The glass sheet according to claim 1, wherein the composition comprises total iron as follows: 0.06<Fe.sub.2O.sub.3≦1.7% by weight.

6. The glass sheet according to claim 5, wherein the composition comprises total iron as follows: 0.06<Fe.sub.2O.sub.3≦0.6% by weight.

7. The glass sheet according to claim 6, wherein the composition comprises total iron as follows: 0.06<Fe.sub.2O.sub.3≦0.2% by weight.

8. The glass sheet according to claim 7, wherein the composition comprises a low alumina content such that: 1≦Al.sub.2O.sub.3≦4 wt %.

9. The glass sheet according to claim 8, wherein the composition comprises: 2≦Al.sub.2O.sub.3≦3 wt %.

10. The glass sheet according to claim 1, wherein the composition comprises: 5≦CaO<10 wt %.

11. The glass sheet according to claim 1, wherein the composition comprises: 1≦K.sub.2O<6 wt %.

12. The glass sheet according to claim 1, wherein the composition comprises: 3≦K.sub.2O≦6 wt %.

13. The glass sheet according to claim 1, wherein the composition comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio ranging from 0.1 to 0.5.

14. The glass sheet according to claim 1, wherein the composition comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio ranging from 0.2 to 0.5.

15. The glass sheet according to claim 1, wherein the composition comprises a K.sub.2O/(K.sub.2O+Na.sub.2O) ratio ranging from 0.2 to 0.4.

Description

EXAMPLES

[0091] Powder raw materials were mixed together and placed in melting crucibles, according to the compositions specified in the following table 1. The raw material mix was then heated up in an electrical furnace to a temperature allowing complete melting of the raw material.

TABLE-US-00001 TABLE 1 Wt % Ex1.1 Ex1.2 Ex1.3 Ex2.1 Ex2.2 Ex2.3 SiO2 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 + 0.01 0.30 0.46 0.01 0.30 0.46 K.sub.2O)

[0092] In those samples, 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.2O˜13.3 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 in the series of examples 1.x (˜light green glass), and ˜0.45 wt % of Fe.sub.2O.sub.3 in the series 2.x (˜green glass). For each series, the first example (x.1) is a comparative sample, similar to state of the art float composition.

[0093] 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.

Chemical Tempering

[0094] 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 dipped in a molten KNO.sub.3 (>99%) bath. After the ion exchange, the samples were cooled down and washed.

[0095] 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. Table 2 summarizes the average value of CS and DoL for each composition and each treatment.

TABLE-US-00002 TABLE 2 Ex1.1 Ex1.2 Ex1.3 Ex2.1 Ex2.2 Ex2.3 CS.sub.465° C. (MPa) 516 512 476 517 526 489 DOL.sub.465° C. (μm) 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. (μm) 7.9 10.6 14.7 7.7 10.6 14.5

[0096] 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.

[0097] 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.sample—DoL.sub.comparative)/DoL.sub.comparative. This G factor has to be as high as possible in order to improve the resistance of the glass pieces versus mechanical solicitations. The R ratios and G factors for the different compositions are summarized in the following table.

TABLE-US-00003 TABLE 3 Ex1.1 Ex1.2 Ex1.3 Ex2.1 Ex2.2 Ex2.3 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 0% 34% 86% 0% 37% 87% improvement) G.sub.420° C. (Dol 0% 34% 75% 0% 35% 66% improvement)

[0098] From table 3 and FIG. 1, the beneficial effect 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. This surprising effect is observed and similar for the different glass tints, i.e. for the two different iron levels.

[0099] 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 treatement. In this set of experiment, the comparative samples present a R ratio around ˜0.65, meaning that increasing the treatment temperature from 420° C. to 465° C. will reduce the surface compressive stress by ˜35%. On the other side, samples according to the invention present a R ratio up to 0.92, meaning that the higher temperature treatment only reduces the compressive stress by less than 10% with respect to low temperature treatment.

[0100] By this way, interesting combinations of DoL (close to 40 μm) and CS (kept well higher than 400 MPa) can be obtain with the composition according to the invention, by applying higher temperature treatments.

Other Properties

[0101] 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)): [0102] Glass melt density evaluated at 1200 and 1400° C.; [0103] Viscosity through the “Melting point temperature T2”; [0104] “Working point temperature T4”; [0105] Devitrification temperature T0;

[0106] Table 4 summarizes obtained results.

[0107] In a general manner:

[0108] The melting point temperature T2 is preferably at most 1550° C., more preferably at most 1520° C., the most preferably at most 1500° C.

[0109] The Working point temperature T4 is preferably at most 1130° C., more preferably at most 1100° C., the most preferably at most 1070° C.

[0110] The devitrification temperature TO is preferably at most T4, more preferably at most T4-20° C., the most preferably at most T4-40° C.

TABLE-US-00004 TABLE 4 Ex1.1 Ex1.2 Ex1.3 Ex2.1 Ex2.2 Ex2.3 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 993 988 975 989 988 980 T0 (° C.)

[0111] 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: [0112] 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). [0113] 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). [0114] their devitrification temperature T0 are suitable because lower than working point temperature T4; [0115] their glass density which is very close to soda lime glasses (Comparative ex.1.1 and 2.1), thereby avoiding/limiting density defects during composition change (transition);

[0116] 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.