1. Field of the Invention

The present invention relates to a glass sheet which is able to be chemically tempered/strengthened. In particular, the present invention relates to a glass sheet which is able to be easily chemically tempered/strengthened and which is inexpensive and easy to produce.

Chemically strengthened glass sheets are finding increasing applications in specialized glazing jobs where a mechanical resistance is required/mandatory, in a monolithic or laminated form, like transportation (i.e. aeronautical, automotive), building/architecture and display industries. Amongst such applications, the display industry has become in the several past years a huge market on demand for chemically strengthened transparent glass sheets as protective/cover glass, viewing window or (touch)screen for numerous electronic devices like mobile phones, smartphones, TV, computers, digital cameras, etc. Indeed, as many of these devices are portable, the glass used is mechanically solicited a lot and it is therefore highly desirable that it is able to tolerate impact and/or damage, such as scratches or impact, during use and transport. Chemical strengthening is even more of great importance in the domain of displays because such a domain requires glass sheets of low thickness (as low as less than 1 mm) and because chemical strengthening is known as the process of choice to mechanically reinforce (ultra-)thin glass sheets. For weight reasons, it is also advantageous to use thin glass sheets as cover glass for solar, thermal or photovoltaic devices.

2. Solutions of the Prior Art

The chemical strengthening of a glass article is a heat induced ion- exchange, involving replacement of smaller alkali sodium ions in the surface layer of glass by larger ions, for example alkali potassium ions. Increased surface compression stress occurs in the glass as the larger ions "wedge" into the small sites formerly occupied by the sodium ions. Such a chemical treatment is generally carried out by immerging the glass in an ion-exchange molten bath containing one or more molten salt(s) of the larger ions, with a precise control of temperature and time. The rupture strength of a glass article which has been so treated is thus increased by a value approximately equal to the surface compressive stress generated.

Nevertheless, a damage capable of affecting the surface of a chemically strengthened glass during its use leads to a decrease in this strengthening effect and can even annihilate it if the damage is such that the layer under compression is penetrated. In consequence, depending on the use intended for the chemically strengthened glass, focus is made on achieving a high value of surface compressive stress (or "CS") and/or a high value of thickness of the layer under compression (which is associated with the parameter called the "depth of layer" or "DoL", namely the depth reached by the ions introduced) which ideally at least equal to the depth of the largest possible defect/damage that the glass may undergo during its use or that might be present before reinforcement. The combination of these two parameters are generally considered to define appropriately the quality of the resulting mechanical strength. Indeed, for a given defect size, it is possible to estimate the "reinforcement" of a chemically tempered glass based on its compressive stress and depth of layer. In the article "Review - Ion exchange for glass strengthening" by Rene Gy and published in Master Science and Engineering B (149, 2008, 159-165), a good approximation of this reinforcement (or ¾ _nforcement ), is given by the following formula: where σ ₃ is the surface compressive stress (CS) , x _c is the depth of exchanged layer (DoL) and c is the defect depth. In order to get reinforcement, the first criterion is to have a DoL deeper than the maximal defect depth. Then, both CS and DoL should be optimized to get the highest possible reinforcement. In most applications, the highest possible reinforcement is wanted, and more particularly, a reinforcement higher than 300Mpa, or better higher than 400MPa, 500MPa and most preferably, above 600Mpa, is desired.

Moreover, in the display domain, when using a "piece-by-piece process" to produce chemically strengthened glass sheets (cutting to final size is carried out before tempering treatment), a high value of DoL (preferably higher than 10 microns and very preferably higher than 12 microns or even better higher than 15 microns) is searched for edge strength, while when using a "sheet process" (cutting to final size is carried out after tempering treatment), "central tension" (defined as (CS*DoL)/(glass thickness - 2*DoL)) must be kept low.

It is also known that the two strengthening parameters also depend significantly, for a given glass composition, on the conditions of temperature and time of the ion exchange process. Thus, the thickness of the layer under compression increases with the temperature and with the duration of the ion-exchange according to the known diffusion laws. But the higher the temperature, the more rapidly the stresses induced by the ion exchange relax. Likewise, extending the treatment for a too long period allows giving the stresses the necessary time to relax and thus results in a less degree of toughening. The conditions to be chosen for the process therefore reside generally in a compromise between the optimum temperature and the minimum duration, to optimize process cost.

To lower the cost of the chemical strengthening (limiting duration and/or temperature to reach searched values of compressive stress and DoL), a lot of glass compositions which are "easily chemically temperable" (meaning that they especially favour ion exchange) have been proposed (merely described or already on the market) but they generally have various drawbacks.

Many of them comprise ingredients originating from expensive raw materials and/or considerably modifying the physical properties of the glass (molten or final). Some of the chemically temperable glass compositions known contain, for example, significant contents of lithium and/or boron. However, lithium has the disadvantage of increasing the density of the glass while boron has the disadvantage to cause sometimes formation of ream by its evaporation and furnace wall/refractories corrosion. Moreover, both have the additional and significant drawback to greatly increase final glass price, due to high price of their corresponding raw materials.

Aluminosilicate-type glass compositions, such as for example those described in US Patent Application US2012/0196110 Al, the GORILLA® glass product from Corning or the DragonTrail® glass product from Asahi Glass Co., are also known to be very efficient for chemical tempering. However, they have a lot of drawbacks. Their high temperature properties make them very difficult to produce (viscosity, fining ability, forming, refractories corrosion). Their cost is relatively high due to expensiveness of some raw materials to use (i.e. alumina) and due to the high temperatures required for their production (high content of energy/fuel). Contrary to aluminosilicate glass compositions, soda-lime-silica glass compositions are generally not considered as good candidates for easy chemically temperable compositions, even if they are by far less expensive.

Finally, it is known that it is quite difficult to modify, even slightly, a glass composition, because :

- a glass production line, and in particular a float line, represents considerable investment and it is not easily repairable if the composition causes, for example, damages to the refractories; and

- the transition time while changing from a composition to another is one parameter which is of high importance when producing glass, because if long, the production cost of the final glass is drastically negatively impacted.

Accordingly, there is a demand of the market in the display domain in particular for a chemically-temperable soda-lime-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.

In this context, international patent application WO2015150207A1 proposes a glass composition with a very low alumina content, comprising the following in weight percentage: 65≤ SiO ₂ ≤ 78 %; 5≤ Na ₂ O≤ 20 %; 0≤ K ₂ O < 5 %; 1≤ Al ₂ O ₃ < 4 %; 0≤ CaO < 4.5 %; 4≤ MgO≤ 12 %; as well as a (CaO/MgO) ratio which is less than 1. Unfortunately, such a glass composition does not allow to reach sufficient reinforcement factor for reasonable treatment time, even if it answers at least partially to technical problems posed in the state of the art in terms of chemical strengthening (DOL) and processing properties.

3. Objectives of the Invention

The objective of the invention in particular is to remedy the cited disadvantages and resolving the technical problem, i.e. to provide a glass composition which is easily chemically temperable or, in other words, more favourable to ion exchange than conventional soda-lime-silica glass compositions.

Another objective of the invention in at least one of its embodiments is to provide a glass composition which is easily chemically temperable and which allows reaching high values for compressive stress (CS). In particular, an objective of the invention in such a context is to provide a glass composition which is easily chemically temperable and which allows obtaining CS values comparable to state of the art alumino-silicate glass, higher than 900Mpa.

Another objective of the invention in at least one of its embodiments is to provide a glass composition which is easily chemically temperable and which allows reaching strengthening parameters appropriate for a "piece-by-piece" process used to produce cover glass for display devices (edge strength obtained typically by DoL > 10-15 microns). Another objective of the invention in at least one of its embodiments is to provide a glass composition which is easily chemically temperable which allows obtaining high DoL, while keeping high compressive stress values that result in a high reinforcement value. In particular, an objective of the invention in such a context is to provide a glass composition which allows obtaining a reinforcement higher than 500 Mpa or better, higher 600Mpa.

Another objective of the invention in at least one of its embodiments is to provide a glass composition which is easily chemically temperable and easy to produce, in particular on an existing line of production of classical soda-lime-silica glass. In particular, an objective of the invention in such a context is to provide a glass composition which is easily chemically temperable and which does not require long transition time when passing from the production of the classical soda-lime-silica composition to the temperable composition (and vice-versa). Still in such a context, an objective of the invention in such a context is to provide a glass composition which is easily chemically temperable and which does not require to use raw materials, techniques and/or industrial installations which are different from those employed for classical soda-lime-silica glass ordinary produced (or, in other words, compatible with classical float process). More particularly, an objective of the invention in at least one of its embodiments is to provide a glass composition which is easily chemically temperable and with targeted properties (lower viscosity, lower working point temperature, melting point < 1550-1500°C, sulfates fining ability, low refractories corrosion, appropriate devitrification temperature), thereby avoiding known drawbacks of alumino-silicate composition and making composition compatible with existing tools for production of soda-lime glass. Finally, another objective of the invention is to provide a solution to the disadvantages to the prior art that is simple, quick and, above all, economical.

4. Outline of the Invention The invention relates to a glass sheet having glass composition comprising the following in weight percentage, expressed with respect to the total weight of glass :

60≤ SiO ₂ ≤ 78 %

5≤ Na ₂ O≤ 20 %

0.9 < K ₂ O≤ 12 %

4.9≤ Al ₂ O ₃ ≤ 8 %

0.4 < CaO < 2 %

4 < MgO≤ 12 %

Hence, the invention rests on a novel and inventive approach, since it enables a solution to be found for the disadvantages of prior art, in particular the disadvantages of aluminosilicate glasses while keeping, at least partially, their advantages. The inventors have indeed found that it is possible to obtain an easily chemically temperable glass sheet, in particular deep enough DOL and high reinforcement factor, and which is unexpensive and easy to mass produce by combining in a soda-silica glass matrix, a mid-alumina content, a very low CaO content, a specific range of MgO and the mandatory presence of K ₂ O in significant amount.

Throughout the present text, when a range is indicated, the extremities are included. In addition, all the integral and subdomain values in the numerical range are expressly included as if explicitly written. Also throughout the present text, the values of content as percentages are values by weight (also mentionned as wt%), expressed with respect to the total weight of the glass.

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

The glass sheet of the invention is able to be chemically tempered or, in other words, ion-exchangeable/able to undergo an ion-exchange.

The glass sheet of the invention may be a glass sheet obtained by the float process, a drawing process, a rolling process or any other process known to manufacture a glass sheet starting from a molten glass composition. According to a preferred embodiment, the glass sheet 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.

The glass sheet according to the invention may have varied and relatively large sizes. It can, for example, have sizes ranging up to 3.21 m ^χ 6 m or 3.21 m x 5.50 m or 3.21 m χ 5.10 m or 3.21 m χ 4.50 m ("PLF" glass sheet) or also, for example, 3.21 m χ 2.55 m or 3.21 m χ 2.25 m ("DLF" glass sheet). The glass sheet according to the invention may have a thickness of from 0.1 to 25 mm. Advantageously, in the case of display applications, the glass sheet according to the invention has preferably a thickness of from 0.1 to 6 mm. More preferably, in the case of display applications and for reasons of weight, the thickness of the glass sheet according to the invention is of from 0.1 to 2.2 mm.

According to an embodiment of the invention, the composition of the 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 ₂ O ₃ content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production. For example, B ₂ O ₃ content in the composition of the glass sheet of the invention is less than 0.01 or even better less than 0.005 wt%.

According to an embodiment of the invention, the composition of the 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 ₂ O content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production. For example, Li ₂ O content in the composition of the glass sheet of the invention is less than 0.01 wt% or even better less than 0.005 wt%.

According to the invention, the composition of the glass sheet comprises : 0.4 < CaO < 2 wt%. Preferably, the composition of the glass sheet comprises : CaO≥ 0.6 wt% and more preferably, CaO≥ 0.7 wt% In a particularly preferred embodiment, the composition of the glass sheet comprises : CaO≥ 0.8 wt%. Preferably also, the composition of the glass sheet comprises: CaO≤ 1.8 wt%, or more preferably, CaO≤ 1.5 wt%, or even, CaO≤ 1.2 wt%. The minimum values of the preferred ranges in CaO are chosen so as to limit as much as possible the transition time which become very long for very low CaO amounts to reach. The maximum values of the preferred ranges in CaO are chosen in order to limit the negative impact of the low field intensity of CaO on the interdiffusion of potassium and sodium during chemical strengthening. To avoid any doubt herein, each embodiment concerning lower limits in CaO is of course combinable independently with any possible embodiment concerning higher limits in CaO. In a very preferred embodiment, the composition of the glass sheet comprises : 0.6≤ CaO≤ 1.2 wt% or even, 0.8≤ CaO≤ 1.2 wt%. According to the invention, the composition of the glass sheet comprises : 4.9≤ Al ₂ O ₃ ≤ 8 wt%. Preferably, the composition of the glass sheet comprises : Al ₂ O ₃ ≥ 5 wt% and in a more preferred manner, Al ₂ O ₃ ≥ 5.2 wt% or even better, Al ₂ O ₃ ≥ 5.5 wt%. Preferably also, the composition of the glass sheet comprises: Al ₂ O ₃ ≤ 7.8 wt% and in a more and more preferred manner, Al ₂ O ₃ ≤ 7.5 wt%; 7 wt%; 6.5 wt%; 6.2 wt%; 6wt%. These minimum values make it possible to ensure sufficient compressive stress and reinforcement factor. Theses maximum values allow to limit as much as possible negative impact on viscosity. To avoid any doubt herein, each embodiment concerning lower limits in Al ₂ O ₃ is of course combinable independently with any possible embodiment concerning higher limits in CaO.. In a very preferred mode, the composition of the glass sheet comprises: 5≤ Al ₂ O ₃ ≤ 6.5 % or even better, 5.2≤ Al ₂ O ₃ ≤ 6.5 %. In a particularly preferred mode, the composition of the glass sheet comprises: 5.2≤ Al ₂ O ₃ ≤ 5.9 %.

According to the invention, the composition of the glass sheet comprises : 5≤ Na ₂ O≤ 20 wt%. Preferably, the composition of the glass sheet comprises : Na ₂ O≥ 7 wt% and in a more preferred manner, Na ₂ O≥ 9 wt% or even better, Na ₂ O≥ 10 wt%. %. In a very preferred mode, the composition of the glass sheet comprises: Na ₂ O≥ 12 wt%. These minimum values allow to ensure sufficiently low viscosity. Preferably also, the composition of the glass sheet comprises: Na ₂ O≤ 19 wt%, or even better, Na ₂ O≤ 18 wt%, in order to avoid as much as possible corrosion of furnace refractories and also to limit as much as possible negative impact on CS. To avoid any doubt herein, each embodiment concerning lower limits in Na ₂ O is of course combinable independently with any possible embodiment concerning higher limits in Na ₂ O.

According to the invention, the composition of the glass sheet comprises : 4 < MgO ≤ 12 wt%. Preferably, the composition of the glass sheet comprises : MgO≥ 5 wt% and in a more and more preferred manner, MgO≥ 6 wt%; 7 wt%; 7.5 wt%; 8 wt%; 8.5 wt%; 9 wt%. Preferably also, the composition of the glass sheet comprises : MgO ≤ 11 wt% or better, MgO ≤ 10 wt%. These minimum values allow to get sufficient CS while ensuring a viscosity not too high. The maximum values make it possible to limit as much as possible negative impact on DOL (by limiting the field intensity impact of MgO on alkali diffusion) while also ensuring not to increase too much devitrification temperature. To avoid any doubt herein, each embodiment concerning lower limits in MgO is of course combinable independently with any possible embodiment concerning higher limits in MgO. In a particularly preferred mode, the composition of the glass sheet comprises : 7.5≤ MgO≤ 11 wt%.

According to the invention, the composition of the glass sheet comprises : 0.9 < K ₂ O≤ 12 wt%. Preferably, the composition of the glass sheet comprises : K ₂ O≥ 1 wt% and in a more and more preferred manner, K ₂ O≥ 1.1 wt%; 1.2 wt%; 1.5 wt%. Preferably also, the composition of the glass sheet comprises: K ₂ O≤ 11 wt% and in a more and more preferred manner, K ₂ O≤ 10 wt%;≤ 9 wt%;≤ 8 wt%;≤ 7 wt%;≤ 6 wt%;≤ 5 wt%;≤ 4 wt%;≤ 3 wt%. The minimum values of the preferred ranges in K ₂ O allow to improve the chemical strengthening performances and also to decrease viscosity. The maximum values allow to decrease negative impact on Tg and also decrease price of the final glass sheet. To avoid any doubt herein, each embodiment concerning lower limits in K ₂ O is of course combinable independently with any possible embodiment concerning higher limits in K ₂ O. In a particularly preferred mode, the composition of the glass sheet comprises : 0.9 < K ₂ O≤ 6 wt% and even better, 0.9 < K ₂ O≤ 3 wt%.

According to an embodiment of the invention, the composition of the glass sheet comprises: 0.7≤ [MgO/(MgO+CaO)] < 1. Preferably, the composition of the glass sheet comprises: [MgO/(MgO+CaO)] ≥ 0.75 and in a more and more preferred manner, [MgO/(MgO+CaO)]≥ 0.8;≥ 0.88;≥ 0.89. Preferably also, the composition of the glass sheet comprises: [MgO/(MgO+CaO)] < 0.98 and in a more and more preferred manner, [MgO/(MgO+CaO)] < 0.95; 0.92; 0.90. The minimum values of the preferred ranges are chosen so as to guarantee sufficient chemical strengthening performances. The maximum values of the preferred ranges are chosen to limit the transition time required to reach such a ratio [MgO/(MgO+CaO)], while keeping at an acceptable level other properties/performances. To avoid any doubt herein, each embodiment concerning lower limits in ratio [MgO/(MgO+CaO)] is of course combinable independently with any possible embodiment concerning higher limits in ratio [MgO/(MgO+CaO)],

According to another embodiment of the invention, the composition of the glass sheet comprises: 0.05 ≤ [K ₂ O/(K ₂ O+Na ₂ O)] ≤ 0.7. Preferably, the composition of the glass sheet comprises: [K ₂ O/(K ₂ O+Na ₂ O)]≥ 0.06 and in a more and more preferred manner, [K ₂ O/(K ₂ O+Na ₂ O)]≥ 0.07; 0.08; 0.09; 0.1. Preferably also, the composition of the glass sheet comprises: [K ₂ O/(K ₂ O+Na ₂ O)]≤ 0.6 and in a more and more preferred manner, [K ₂ O/(K ₂ O+Na ₂ O)]≤ 0.5;≤ 0.4;≤ 0.3. Such preferred ranges ensure to optimize the so-called "mixed-alkali effect" in order to reach sufficient potassium-sodium interdiffusion. To avoid any doubt herein, each embodiment concerning lower limits in ratio [K ₂ O/(K ₂ O+Na ₂ O)] is of course combinable independently with any possible embodiment concerning higher limits in ratio [K ₂ O/(K ₂ O+Na ₂ O)].

According to an embodiment of the invention, the composition comprises total iron (expressed in the form of Fe ₂ O ₃ ) in a content ranging from 0.002 to 1.7 wt%. Preferably, the composition of the invention comprises a total iron (expressed in terms of Fe ₂ O ₃ ) content ranging from 0.002 to 0.6 wt% and, more preferably, ranging from 0.002 to 0.2 wt% or even from 0.002 to 0.1 wt%. In a very preferred embodiment, the composition of the invention comprises a total iron (expressed in terms of Fe ₂ O ₃ ) content ranging from 0.002 to 0.06 wt%. A total iron (expressed in the form of Fe ₂ O 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 ₂ O ₃ ) content ranging from 0.002 to 0.04 wt%. More preferably, the composition comprises a total iron (expressed in the form of Fe ₂ O 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 F _e2 O ₃ ) content ranging from 0.002 to 0.015 wt%, or even from 0.002 to 0.012 wt%.

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

60≤ SiO ₂ ≤ 78 %

5≤ Na ₂ O≤ 20 %

0.9 < K ₂ O≤ 6 %

4.9≤ Al ₂ O ₃ ≤ 7 %

0.4 < CaO < 1.5 %

6≤ MgO≤ 12 %.

According to this last embodiment, the composition of the glass sheet of the invention more preferably comprises the following in weight percentage, expressed with respect to the total weight of glass :

60≤ SiO ₂ ≤ 78 %

5≤ Na ₂ O≤ 20 %

0.9 < K ₂ O≤ 3 %

5≤ Al ₂ O ₃ ≤ 6.5 %

0.6≤ CaO < 1.2 %

7.5≤ MgO≤ 11 %.

According to another embodiment, the composition of the 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.

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

According to still another embodiment, the composition of the glass sheet comprises BaO in a content lower than 0.1 wt%, even lower than 0.05 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.

According to still another embodiment, the composition of the glass sheet comprises SrO in a content lower than 0.1 wt%, even lower than 0.05 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.

According to still another embodiment, the composition of the glass sheet comprises bulk SnO ₂ in a content lower than 0.1 wt% (bulk content excluding SnO ₂ in the "tin face" of a float glass sheet). Preferably, the composition of the glass sheet is free of bulk SnO 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 ₂ content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production.

According to a preferred embodiment of the invention, the composition comprises coloring components other than iron, chromium, cobalt and erbium 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%.

Advantageously, the composition of the invention may further comprise chromium and/or cobalt oxides. According to a preferred embodiment, when the composition of the invention comprises a total iron (expressed in terms of Fe ₂ O ₃ ) content ranging from 0.002 to 0.06 wt%, the composition may further 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.

Advantageously, the composition may comprise an additional element or combination of elements allowing to approach or reach neutrality (0;0 coordinates in a*,b* diagram). Examples of such elements are erbium, erbium-cobalt, selenium- cobalt as described in patent application n°15 172 778.1, or colloidal gold. According to one embodiment of the invention, the 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 ₂ :F, SnO ₂ :Sb or ITO (indium tin oxide), ZnO:Al or also ZnO:Ga. According to another advantageous embodiment of the invention, the 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. According to another embodiment, the 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.

According to still another embodiment, the 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. 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 glass sheet according to the invention.

The invention also relates to a glass sheet according to the invention which is chemically tempered. All previously described embodiments and preferred composition ranges also apply to the invention of chemically tempered glass sheet.

Finally, the invention also relates to the use of the chemically tempered glass sheet according to the invention : in an electronic device; in automotive; in an aeronautic glazing (interior/exterior); in a building. Embodiments of the invention will now be further described, by way of examples only, together with some comparative examples, not in accordance with the invention. The following examples are provided for illustrative purposes, and are not intended to limit the scope of this invention.

Examples

Examples

Powder raw materials were mixed together and placed in melting crucibles, according 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 1

Comparative Comparative Comparative Ex. 4

ex. 1 (SL) ex. 2 (AS) ex. 3 (AS) (Invention)

Si0 ₂ 72 60.9 68 66.5

A1 ₂ 0 ₃ 1.3 12.8 4.4 5.8

CaO 7.9 0.1 3.7 0.9

MgO 4.5 6.7 7.7 9.5

Na _z O 13.9 12.2 14.9 15.7 κ ₂ ο 0 5.9 1.0 1.1

Fe ₂ 0 ₃ 0.01 0.02 0 0.01

BaO 0 0.2 0 0

S0 ₃ 0.36 0 0.36 0.36

SrO 0 0.2 0 0

ZrO _z 0 1.0 0 0

After the melting and the homogenization of the composition, the glass was cast in several small samples of 40*40mm 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. Composition of comparative example 1 corresponds to a classical soda- lime (SL) glass according to the state of the art, composition of comparative example 2 corresponds to a common alumino-silicate (AS) glass, and composition of comparative example 3 corresponds to another alumino-silicate glass recently commercially available and announced as an improved glass composition for chemical strengthening (it corresponds to the Glanova™ glass from NSG-Pilkington Group). Composition of example 4 correspond to a composition according to the invention.

Chemical tempering - classical cycle

A first part of the 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 (15min) and then dipped in a molten KNO ₃ (>99%) bath at 420°C for 220 minutes. After the ion exchange, the samples were cooled down (15min) and washed. Subsequently the surface compressive stress (CS) and the depth of exchanged layer (DoL) were measured via photoelasticimetry. The table 3 summarize the average value of CS and DoL for ten random samples of composition of example 4 according to the invention and compositions of Comparative examples 1-3.

Table 3

Chemical tempering - temperature impact and reinforcement

The remaining samples of each composition were grouped in 4 different sets and chemically tempered in four different cycles of 220 minutes, at 400°C, 420°C, 440°C and 460°C. Each set of samples of different compositions was placed in a cassette, preheated (2h) and then dipped in a molten KNO ₃ (>99.5%) bath for 220 minutes at the desired temperature. After the ion exchange, the samples were cooled down (2h) and washed. Subsequently the surface compressive stress (CS) and the depth of exchanged layer (DoL) were measured via photoelasticimetry. The following table summarizes the average value of CS and DoL for each treatment temperature and for ten random samples of composition 4 according to the invention and composition of Comparative examples 1-3. In table 4 below, the average reinforcement, ¾ _nforcement for each type of sample and treatment is also computed (as described above), for a theoretical defect size of 10 /xm. Results are also shown in Figure 1.

Table 4

Those results show that combining a mid-content in Al ₂ O ₃ a very low CaO content, high enough MgO and K ₂ O contents allows to significantly improve the compressive stress (reached range 700-1000 Mpa), while keeping a sufficient depth of exchanged layer (reached range —12-32 μιτι) and thus, to increase the glass reinforcement, especially to values higher than 500 or even 600MPa (reached range 500-610 Mpa).

Moreover, the reached DoL value of the composition according to the invention is well appropriate for a "piece-by-piece" process used to produce cover glass for display devices. It is indeed higher than 12 microns for each temperature treatment.

In addition, the combinaison of CS and DoL achievable by the composition according to the invention is well appropriate for a "sheet by sheet" type process, as they allow to keep central tension (CT = (CS*DoL)/(glass thickness - 2*DoL) ) below 25MPa for 0.7mm and even 0.55mm thick glass, hence ensuring cuttability after chemical tempering.

Other properties

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 TO ;

Coefficient of thermal expansion (CET) ;

In a general manner:

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

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

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

CET value is preferably at most 9.6 and more preferably at most 9.5.

Comparative Comparative Comparative Ex.4

ex. 1 ex. 2 ex. 3 (invention) Glass melt 2.37 2.32 2.36 2.35 density (1200°C)

Glass melt 2.34 2.32 2.34 2.34 density (1400°C)

Melting point T2 1463 1602 1486 1498

(°C)

Working point T4 1037 1124 1053 1061

(°C)

Devitrification

temperature TO 994 951 994 1036

(°C)

CET @210°C 9.15 9.68 9.49 9.50

(10 ^"6 /K)

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 comparable to a classical soda lime glass (Comparative ex.1) and a recently improved aluminosilicate glass (Comparative ex.3), and significantly lower compared to a common aluminosilicate glass (Comparative ex.2);

- their working point temperature T4 which is lower than 1100°C and which are lower compared to a common aluminosilicate glass (Comparative ex.2) and, comparable to a classical soda lime glass (Comparative ex.1) and a recently improved aluminosilicate glass (Comparative ex.3);

- their devitrification temperature TO are suitable because lower than working point temperature T4;

- their glass density which is very close to soda lime and aluminosilicate glasses (Comparative ex. 1-3), thereby avoiding/limiting density defects during composition change (transition) ; 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 a common aluminosilicate glass and thus are less sensitive to differentiated cooling issues.

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.