WO2013030848A1

WO2013030848A1 - Glass composition for strengthened cover glass

Info

Publication number: WO2013030848A1
Application number: PCT/IN2012/000185
Authority: WO
Inventors: Jeetendra Sehgal
Original assignee: Sterlite Technologies Ltd
Priority date: 2011-08-26
Filing date: 2012-03-19
Publication date: 2013-03-07
Also published as: KR20140057474A; JP2014527015A; CN103534216A; AU2011101310A4

Abstract

A cover glass and a glass composition, wherein said glass comprises: SiO₂ from 70.5 to 77.0 mole%; A1₂O₃from 8.5 to 12.5 mole%; Na₂O from 10.6 to 14.6 mole%; K₂O from 1.1 to 5.1 mole%; B₂O₃ from 0 to 3 mole%; MgO from 2.8 to 5.8 mole% and CaO from 0 to 3.7 mole%; wherein inclusion of silica from 70.5 to 77.0 mole% decreases the overall density of the cover glass; wherein the density of glass is in the range from 2.3 to 2.45 g/cm³. The glass is ion exchangeable at low temperature to a depth of at least 10μm.

Description

GLASS COMPOSITION FOR STRENGTHENED COVER GLASS

FIELD

The present disclosure relates to a glass composition and a cover glass.

Particularly the present disclosure relates to a glass composition and a cover glass, wherein the cover glass is scratch resistant and is featured with high silica content, reduced density, reduced brittleness and high strength.

BACKGROUND

Devices such as the mobiles or cell-phones, palm top computers, watches, laptops, portable gaming devices with displays, notebooks, televisions, displays in vehicles, touch panels screens and other electronic devices, etc., have become ubiquitous in devices, wherein at least a glass cover plate is included to protect the device (particularly the display part therein). The cover plate is generally transparent to allow the user to view a display.

The glass cover plates used in such devices are prone to breaking and/or damage due to various reasons such as accidental dropping, improper cleaning and also as the usage of the device increases. It is desirable to have cover glasses designed to survive the high levels of ill-treatment or accidental dropping that may occur due to contact or impact of the cover glasses with sharp objects.

Additionally, the glass cover plates must also exhibit high strength and at the same time must be scratch resistant if the device with the cover glass is frequently contacted or touched such as in a touch screen display.

Typically, the glass covers are manufactured by one of the sheet glass manufacturing processes, namely, the float glass process, the down-draw fusion process, slot draw process, press-formable (molding) process, roller press process and the like.

In particular, the down-draw fusion process [also known as the down-draw process] is capable of producing a precision fire-polished surface that requires no additional modification such as grinding or polishing prior to use. The specifications of US patents 3,338,696 and 3,682,609 disclose fusion downdraw processes which include allowing flow of molten glass over the edges or weirs of a forming wedge, referred to as isopipe. The molten glass flows over converging forming surfaces of the isopipe and the separate flows reunite at the apex or root where the two converging forming surfaces meet to form a glass sheet. Thus, in the fusion process the glass which has been in contact with the forming surfaces is located in the inner portion of the glass sheet and the exterior surfaces of the glass sheet are contact free. Pulling rolls positioned downstream of isopipe root capture edge portions of the glass sheet so formed to control the rate at which the glass sheet leaves the isopipe and thus aids in controlling the thickness of the finished sheet. As the glass sheet descends from the root of the isopipe past the pulling rolls, it cools to form a solid elastic glass sheet, which may be processed further.

In order to achieve a good quality cover glass using the fusion technique, various material properties of the glass in the molten stage must be controlled within specified limits during the down draw process or in any other sheet manufacturing process.

One such material property is the viscosity of the glass. It is desired to maintain the viscosity of the glass at the location where it leaves the isopipe at a value greater than about 10⁵ poise, which if not maintained the glass sheet flatness and thickness across its width becomes difficult to control, which may result in a glass sheet being not suitable for display applications.

Another significant factor is liquidus viscosity of the glass substrate. It is desired to have lower liquidus temperatures and hence higher liquidus viscosities for the down draw process or any other manufacturing process. The lower liquidus temperature and higher liquidus viscosity result in the glass being resistant to crystallization during the forming process. Since the forming is started at glass viscosities from about 10⁴ to about 10⁵ poise, it is desired that the glass exhibits a liquidus viscosity of greater than about 10⁵ poise.

Further, the substrate glass is desired to exhibit high durability to chemical treatments during the TFT manufacturing and long term exposure to environmental conditions in service.

Still further, as the cover glass is to be used in portable devices such as laptops, clocks, mobiles, which are light-weight devices. This implies that the substrate glass which forms an important part should also have a relative low weight, which in-turn implies that the substrate glass itself should be light weight. Furthermore, light weight means that the thickness of the glass is less, and for portable device applications the thickness is less than about 1.1 mm, preferably less than about 0.7 mm and most preferably less than about 0.5 mm. It should be possible to manufacture glasses with the desired thickness and it is also desirable to have glass composition that are suitable for manufacture of such glasses, wherein the glasses may be melt drawn into thin sheets of desired thickness.

Additionally, it is desired that the substrate glass should exhibit a low coefficient of thermal expansion (CTE), typically, in the range of from about 72 x 10^"7/°C to about 76 x 10^"7/°C to be compatible with processes for Soda Lime glass which has a CTE close to 80 x 10^"7/°C.

Further, screens or glasses that are used for touch sensitive screens of electronic devices are typically chemically-strengthened by the ion-exchange method. The ion chemical strengthened ion-exchanging of the glasses provides extra strength to the glasses. The ion exchange step is carried out after the glass sheets are formed by chemically treating the heated glass sheets with a heated solution of ions having larger ionic radius than ions that are present in the glass surface, thereby replacing the smaller ions with larger ions. Typically the smaller ions are those of sodium and are replaced with larger ions such as those of potassium.

It is known that glasses such as "the soda lime" glasses are compatible with large-scale sheet glass manufacturing via float process but cannot be formed by methods particularly the down-draw process as the viscosities of the soda glasses are too low owing to their high liquidus temperatures. It is desired to have glasses that could be formed using any of the above mentioned processes for forming the glass sheets.

Also, the density of commercial soda lime glass is close to 2.5 g/cc, which makes the glass heavier for the application of the cover glass for displays.

The above described attributes or properties are impacted by the cover glass composition. OBJECTS

Some of the objects of the present disclosure are described herein below: It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.

Another object of the present disclosure is to provide a cover glass and a glass composition for the cover glass which meets almost all the above described attributes and yet its density is less than 2.45 g/cc.

Still another object of the present disclosure is to provide a cover glass and a glass composition compatible with the down draw process or the float glass process or slot draw process or press-formable (molding) process or roller press process.

Yet another object of the present disclosure is to provide a cover glass and a glass composition having higher viscosity.

A further object of the present disclosure is to provide a cover glass and a glass composition, wherein the liquidus viscosity of the glass is higher.

Still further object of the present disclosure is to provide a cover glass which is light-weight.

Another object of the present disclosure is to provide a cover glass with coefficient of thermal expansion in the range from 72 x 10^"7/°C to 76 x 10^"7/°C.

Another object of the present disclosure is to have a cover glass composition for use in mobiles or cell-phones, televisions, palm top computers, watches, laptops, portable gaming devices with displays, notebooks, displays in vehicles, touch panels screens and other electronic devices.

Another object of the present disclosure is to have a glass composition wherein the glass formed therefrom is ion-exchangeable.

Another object of the present disclosure is to have a glass composition wherein the glass could be formed by any of the glass forming processes including the down-draw process, the float glass process, the slot draw process, the press-formable process, the roller press process etc.

SUMMARY

These and other objects of the present disclosure are to a great extent dealt within the disclosure. In accordance with present disclosure there is provided a glass comprising S1O2 from 70.5 mole % to 77.0 mole %; A1₂0₃ from 8.5 mole % to 12.5 mole %; Na₂0 from 10.6 mole % to 14.6 mole %; K₂0 from 1.1 mole % to 5.1 mole %; B₂0₃ from t 0 mole % to 3 mole %; MgO from 2.8 mole % to 5.8 mole %, CaO from 0 mole % to 3.7 mole % and at least one refining agent from 0 mol% to 0.8mol%, said refining agent being selected from the group consisting ofTi0₂, As₂0₃, Sb₂03 _, metal halide and sodium sulphate.

Typically, the inclusion of silica from 70.5 mole % to 77.0 mole % decreases the overall density of the cover glass. Typically, the density of glass is in the range from 2.3 to 2.45 g/cm³ wherein the glass is down-drawable using the fusion drawing process.

In accordance with one embodiment the glass satisfies at least one of the following conditions:

- the sum of mol% of B₂0₃, Na₂0, K₂0, MgO and CaO ranges from 14.5 mol% to 21.7 mol%;

- the sum of mol% of Na₂0 and K₂0 ranges from 1 1.7 mol% to 16.9 mol%;

- the sum of mol% of MgO and CaO ranges from 2.8 mol% to 6.5 mol%;

- the sum of mol% of Si0₂ and A1₂0₃ ranges from 78.1 mol% to 85.5 mol%;

- the sum of A1₂0₃ and B₂0₃ ranges from 8.1 mol% to 12.5 mol%;

- the ratio of R0/A1₂0₃ ranges from 0 to 0.7, where R represents one of Mg or Ca;

- the ratio of A₂0/A1₂0₃ ranges from 0 to 1.8, where A represents one of Na or K; and

- the ratio of B₂0₃/A1₂0₃ ranges from 0 to 0.36.

In accordance with the present disclosure the glass exhibits a coefficient of thermal expansion in the range from 72 x 10^"7/°C to 76 x 10^"7/°C, a logarithm of liquidus viscosity in the range from 6 to 7.4, a Young's modulus in the range from 72 GPa to 76 GPa, an annealing temperature in the range from 500 °C to 700 °C.

In accordance with one embodiment of the present disclosure the glass is down-drawable or slot drawn or press-formable in a mold or drawable through roller press or drawable through float process.

In one embodiment of the present disclosure the glass is capable of being chemically strengthened by ion exchange and exhibits a composition which can be formed by any of the forming processes known in the prior art including the down-draw process, the float glass process, the slot draw process, the press-formable process, the roller press process etc.

In accordance with the present disclosure the glass may be ion exchanged to a depth of 10 μηι or more.

Ih accordance with another embodiment the glass exhibits a compressive stress of at least 100 MPa, a depth of layer of at least 10 μηι and a thickness of at least 0.3 mm.

In accordance with another aspect of the present disclosure there is provided a glass cover plate prepared from the glass of the present disclosure.

In accordance with still another aspect of the present disclosure there is provided an electronic device comprising a glass cover plate, wherein said glass cover plate is prepared from the glass of the present disclosure.

DETAILED DESCRIPTION

The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein

The present disclosure is concerned with a glass composition and cover glass. The inventors of the present disclosure particularly developed a cover glass which is scratch resistant and is featured with high silica content, reduced density, reduced brittleness and high strength. The glass cover can be used in devices such as mobiles or cell-phones, televisions, palm top computers, watches, laptops, portable gaming devices with displays, notebooks, displays in vehicles, touch panels screens and other electronic devices.

In accordance with the present disclosure, the glass contains following major components: Silica (Si0₂), Alumina (AI2O3), Boron trioxide (B2O3), alkaline earth oxides (AEO) such as MgO and CaO, alkali metal oxides such as Na₂0 and ₂0.

In accordance with the present disclosure silica (Si0₂) behaves as the basic glass former, wherein the former (silica in this case) forms the basic skeleton or network. Formers alone may form glass however the melting point in some cases (particularly when the former is silica) will be so high so as to make it impractical to commercially melt the glasses. In accordance with the present disclosure the glass has high silica content in the range from 70.5 mol % to 77 mol %. The high silica content reduces the overall density of the cover glass as the density of the silica is 2.2 g/cm .

The glass in accordance with the present disclosure mainly contains various oxides, the concentrations of which are expressed in mole percentage. In accordance with the present disclosure the glass comprises A1₂0₃ (mole percentage) in the range from 8.5 % to 12.5 %, wherein A1₂0₃ enhances viscosity of the glass.

In accordance with the present disclosure the glass comprises B₂0₃ in the range from 0 to 3 mole percentage.

In accordance with the present disclosure alkali metal oxides reduces the melting temperatures of the glass and also achieve low liquidus temperatures. The alkali metal oxides incorporated in the glass of the present disclosure include Na₂0 and K₂0. In particular, in accordance with the present disclosure Na₂0 is used to permit ion exchange in order to manufacture considerably improved glass strength. In accordance with the present disclosure Na₂0 is provided in the range from 10.6 to 14.6 mole percent. In accordance with the present disclosure the glass is provided with potassium oxide (K₂0) in the range from 1.1 to 5.1 mole percent, wherein ₂0 achieve low liquidus temperatures along with decreased glass viscosity.

In accordance with the present disclosure the provision of addition of the alkaline earth oxides such as MgO and CaO results in modification of the glass network formed by the glass formers so as to reduce the melting temperature and thereby improve other glass forming processes, e.g., refining. In accordance with the present disclosure the MgO and CaO are utilized as the modifiers in the ranges from 2.8 to 5.8 mole percent and 0 to 4.5 mole percent respectively.

The inventors of the present disclosure also developed a glass containing titania (Ti0₂) as an additional ingredient to increase the refractive index as well as the intrinsic strength of the glass. Ti0₂ is used in an amount of 0 mol% to 0.7 mol%.

Exemplary compositions of the glass of the present disclosure are listed in Table 1.

Table 1

Table 1 continued ...

Table 1 (continued ...)

Table 1 (continued ...)

Table 1 (continued ...)

Tablel (continued ...)

Fe₂0₃ 0.07 0.1 0.02

Sn0₂ 0.53 0.5 0

Ti0₂ 0 0.6 0

PROPERTY

Density (g/cc) 2.425 2.425 2.425 2.425 2.425 2.425 2.425 2.425

CTE (xlO^A-7/degC) 80 80 80 80 80 80 80 80

Liquidus Viscosity

(Logn) Poise 6.6 6.6 6.6 6.6 6.6 6.6 6.6 6.6

Strain Point (deg C) 555 555 555 555 555 555 555 555

Young's Modulus

(GPA) 75 75 75 75 75 75 75 75

Annealing

Temperature (deg C) 634 634 634 634 634 634 634 634

In accordance with the present disclosure the sum of mol% of B₂0₃, Na₂0, ₂0, MgO and CaO ranges from 14.5 mol% to 21.7 mol%, wherein the oxides B2O₃, Na₂0, K₂0, MgO and CaO serve as fluxes, the fluxes being used to tailor the melting temperatures suitable for continuous manufacturing process. To achieve the melting temperatures less than 1650°C, the sum of mol% of B₂03, Na₂0, K₂0, MgO and CaO should be in the range from 14.5 mol% to 21.7 moI%.

In accordance with the present disclosure the sum of moI% of Na₂0 and K₂0 ranges from 11.7 mol% to 16.9 mol%. Alkali metal oxides aids in achieving low liquidus temperatures and low melting temperatures of the glass. Na₂0 is used to enable ion exchange, wherein Na₂0 is provided in the concentration ranging from 10.6 mol% to 14.6 mol%. Potassium oxide (K₂0) is included to obtain low liquidus temperatures, wherein it is known that K₂0 decreases the viscosity of the glass even more than Na₂0. In accordance with the present disclosure the concentration of K₂0 ranges from 1.1 mol % to 5.1 mol %.

In accordance with the present disclosure the B₂0₃ serves as flux that is a component added to reduce the melting temperatures. In accordance with the present disclosure the sum of mol% of MgO and CaO ranges from 2.8 mol% to 6.5 mol%. MgO is the most effective flux in case when the total alkali metal oxide concentration exceeds that of AI2O3. If the concentration of MgO is low formation of forsterite (Mg2Si04) takes place, whereas at high concentration of MgO, the glass may have melting temperature within the desired limit however, the liquidus temperature may become too high, thereby the glass liquidus viscosity may become too low. Addition of at least one of CaO or B2O3 reduces the liquidus temperature of the MgO containing compositions.

In accordance with the present disclosure the sum of mol% of Si0₂ and AI2O3 ranges from 78.1 mol% to 85.5 mol%. Presence of AI2O3 ensures enhancement of the glass viscosity. Higher concentration of AI2O3 may lead to very high glass viscosity along with very high liquidus temperatures, which are controlled by providing total concentration of alkali (sum of Na20 and K2O) greater than the concentration of AI2O3. In accordance with the present disclosure the Si0₂ content is high to reduce the overall density of the glass preferably to a value less than 2.42 g/cm³. The increase in melting temperature of glass due to presence of high Si0₂ content is also compensated by provision of Na₂0 and K₂0.

In accordance with the present disclosure the sum of AI2O3 and B2O3 ranges from 8.1 mol% to 12.5 mol%.

In accordance with the present disclosure the ratio of RO/AI2O3 ranges from 0 to 0.7, where R represents one of Mg or Ca.

In accordance with the present disclosure the ratio of A2O/AI2O3 ranges from 0 to 1.8, where A represents one of Na or K.

In accordance with the present disclosure the ratio of B2O3/AI2O3 ranges from 0 to 0.36.

In accordance with the present disclosure there is also provided a glass which contains AS2O3 as an additional ingredient in the range from 0 to 0.7 mole %, wherein AS2O3 acts as a fining agent and aids in removal of bubbles or gaseous inclusions from the molten glass.

Further, in accordance with the present disclosure there is also provided a glass which contains Sb₂0₃ as an additional ingredient in the range from 0 to 0.8 mole %, wherein Sb₂03 also acts as a fining agent and aids in the removal of bubbles or gaseous inclusions from the molten glass.

In addition to the above mentioned refining agents, metal halides and sodium sulfate can also be used as a refining agent.

In accordance with the present disclosure the glass has high liquidus viscosity and is lightweight. The glass has a logarithm of liquidus viscosity in the range from 6 to 7.4.

In accordance with the present disclosure the glass exhibit coefficient of thermal expansion in the range from 72 x 10^"7/°C to 76 x 10^"7/°C.

In accordance with the present disclosure an advantage of the glass is that it exhibit higher viscosity, typically of the order of 10⁶ poise.

In accordance with the present disclosure the glass is down-drawable or slot drawable.

In accordance with the present disclosure the glass has a density of less than 2.45 g/cm3, preferably in the range from 2.3 to 2.45 g/cm³.

In accordance with the present disclosure the glass has a Young's Modulus in the range from 72 GPa to 76 GPa.

In accordance with the present disclosure the glass has an annealing temperature in the range from 500 °C to 700 °C.

In accordance with the present disclosure the glass is used for manufacturing cover plate for electronic devices and glass plate.

In accordance with the present disclosure the glass has a thickness ranging from 0.3 mm to 2 mm, preferably in the range from 0.3 mm to 1.1 mm and most preferably in the range from 0.4 mm to 0.5 mm. In accordance with the present disclosure the glass is capable of being chemically strengthened by ion exchange and exhibits a composition which can be formed by any of the forming processes known in the prior art including the down-draw process, the float glass process, the slot draw process, the press-formable process, the roller press process etc. In accordance with the present disclosure the ion-exchange is a process wherein the glass is strengthened by ion exchange processes, wherein the glass sheet formed is treated chemically with salt bath comprising KNO3 for a time period of 5 hours or more and at a temperature of 400 °C or more.

In accordance with the present disclosure the glass may be ion exchanged to a depth of 10 μπι or more.

Furthermore, the glass prepared in accordance with the present disclosure exhibits a compressive stress of at least 100 MPa, a depth of layer of at least 10 μηι and a thickness of at least 0.3 mm when the ion exchange process is carried out in the manner described herein above. The compressive stress is the stress caused by substitution during the chemical strengthening of an alkali metal ion contained in the glass sheet surface layer by an alkali metal ion having larger ionic radius.

In one embodiment potassium ions are substituted for sodium ions in the surface layers and the glass sheet is observed to exhibit a compressive stress of at least 100 MPa.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression "at least" or "at least one" suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Claims

We claim:

1. A glass comprising:

a. Si0₂ from 70.5 mol% to 77 mol%;

b. A1₂0₃ from 8.5 mol% to 12.5 mol%;

c. B2O3 from 0 mo% to 3 mo%;

d. Na₂0 from 10.6 mol% to 14.6 mol%;

e. K₂0 from 1.1 mol% to 5.1 mol%;

f. MgO from 2.8 mol% to 5.8 mol%;

g. CaO from 0 mol% to 3.7 mol%; and

h. at least one refining agent from 0 mol% to 0.8mol%, said refining agent being selected from the group consisting of Ti0₂, AS2O3, Sb₂0_{3 ,} metal halide and sodium sulphate.

2. The glass as claimed in claim 1, wherein the glass satisfies at least one of the following conditions :

- mol % of Si0₂ is varied in the range from 70.5 mol % to 77 mol % so that the density of the glass is less than 2.45 g/cm³, and preferably in the range from 2.3 to 2.45 g/cm³;

- the sum of mol% of B2O₃, Na₂0, K₂0, MgO and CaO ranges from 14.5 mol% to 21.7 mol%;

- the sum of mol% of Na₂0 and K₂O ranges from 1 1.7 mol% to 16.9 mol%;

- the sum of moI% of MgO and CaO ranges from 2.8 mol% to 6.5 mol%;

- the sum of mol% of Si0₂ and AI₂O₃ ranges from 78.1 mol% to 85.5 mol%;

- the sum of AI₂O₃ and B₂0₃ ranges from 8.1 mol% to 12.5 mol%;

- the ratio ofRO/Al₂0₃ ranges from 0 to 0.7, where R represents one of Mg or Ca;

the ratio of A₂O/AI₂O₃ ranges from 0 to 1.8, where A represents one of Na or ; and

- the ratio of B2O3/AI2O3 ranges from 0 to 0.36.

3. The glass as claimed in claim 1, wherein said glass posses at least one of the following properties: a coefficient of thermal expansion in the range from 72 x 10^"7/°C to 76 x 10^"7/°C; a logarithm of liquidus viscosity in the range from 6 to 7.4;

a Young's Modulus in the range from 72 GPa to 76 GPa; and

an annealing temperature in the range from 500 °C to 700 °C;

4. The glass as claimed in claim 1, wherein the glass has a thickness ranging from 0.3 mm to 2 mm, preferably from 0.3 mm to 1.1 mm.

5. The glass as claimed in claim 1, wherein the glass is down-drawable or slot drawn or press-formable in a mold or drawable through roller press or drawable through float process.

6. The glass as claimed in claim 1, wherein the glass is ion exchanged and typically when ion exchanged, has a surface compressive stress of at least 100 MPa or a surface compressive layer of thickness of at least 210 μηι.

7. A glass cover plate prepared from the glass as claimed in claim 1.

8. An electronic device comprising a glass cover plate, wherein said glass cover plate is prepared from the glass as claimed in claim 1.