Very low sparkle glass sheet

Abstract

The present invention relates to a glass sheet a glass composition comprising, in a content expressed in percentages of the total weight of the glass: SiO2 55% - 85%, preferably 55% - 78%; Al2O3 0 - 5%, preferably 0 - 4%; B2O3 0 - 20%, preferably 0 - 18%; Na2O 0 - 25%, preferably 5% - 20%; CaO 0 - 20%, preferably 2% – 12%; MgO 0 - 15%, preferably 0 - 12%; K2O 0 - 20%, preferably 0 - 10%; and BaO 0 - 20%, preferably 0 - 5% Said glass sheet comprises at least one etched surface having a surface roughness defined, when measured on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm, by: 0.020 ≤ Ra ≤ 0.400 microns, and 7 ≤ RSm ≤ 30 microns. Said glass sheet having the following optical properties when measured from said etched surface : - a sparkling value of 1.50% to 8.00%, measured with a display resolution of 264 PPI, and - a diffusion value of from 2% to 50%.

Description

[0001] Very low sparkle glass sheet

[0002] 1. Field of the Invention

[0003] The present invention relates to a glass sheet which is particularly suitable for high resolution display applications as cover glass. In particular, the invention relates to a glass sheet which demonstrates very low sparkling combined with reasonable anti-glare optical properties. Moreover, the glass sheet of the invention also provides a "soft touch".

[0004] 2. Solutions of the Prior Art

[0005] Texturing a glass surface is widely used in the display industry to provide specific optical properties. This texturing can be produced by several known methods like (i) removal of material from the smooth glass surface by chemical-etching or sandblasting or laser ablation (ii) the application on the smooth surface of a rough coating by, for example, spraying, polymer web-coating or dip-coating.

[0006] With texturing, it is recognized that a compromise between glare reduction of the surface and the degradation of transmission / resolution properties of the glass must be reached. Moreover, with recent increases in the brightness and high-resolution of displays on the market, there is also an increasing demand to provide an anti-glare / anti-sparkle solution for cover glass sheets while keeping / reaching a pleasant smooth touch sensation (often called a satin, silk or soft touch).

[0007] Indeed, sparkling can be caused by the interaction between two structured layers being the regular matrix of pixels composing the display and the randomly structured surface of the anti-glare cover glass. Sparkling is known to increase with the increasing resolution of the new displays being characterized by higher density of pixel and smaller and smaller size. The morphology finetuning (size, height, shape, distribution, ...) of the microstructures of the anti-glare glass is the key to get the right balance between a good anti-glare level (the highest diffusion of reflected light possible) and the lowest sparkling effect for high resolution displays. It is well recognized in the art that it is very challenging to obtain at the same time both properties. Typically, current glass sheet for display applications provide either a low level of sparkling but a poor level of anti-glare being very low level of light diffusion, or a good level of anti-glare being higher diffusion level but with an important level of sparkling. Therefore, there is still a need to provide cover glass sheet that demonstrates very low sparkling while maintaining low to medium diffusion. In particular there is still a need to provide such glass sheet that are cost effective for mass production market

[0008] 3. The summary of the present invention

[0009] The present invention relates to a glass sheet having a glass composition comprising, in a content expressed in percentages of the total weight of the glass:

[0010] SiO255% - 85%, preferably 55% - 78%;

[0011] AI2O3 0 - 5%, preferably 0 - 4%;

[0012] B2O30 - 20%, preferably 0 - 18%;

[0013] Na2O 0 - 25%, preferably 5% - 20%;

[0014] CaO 0 - 20%, preferably 2% - 12%;

[0015] MgO 0 - 15%, preferably 0 - 12%;

[0016] K2O 0 - 20%, preferably 0 - 10%; and

[0017] BaO 0 - 20%, preferably 0 - 5%;

[0018] Said glass sheet comprises at least one etched surface having a surface roughness defined, when measured on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm, by : 0.020 < Ra < 0.400 microns, and 7 < RSm < 30 microns.

[0019] Said glass sheet has the following optical properties when measured from said etched surface : a sparkling value of 1.50% to 8.00%, preferably of 1.75% to 7.00%, more preferably of 2.00% to 5.00% measured with a display resolution of 264 PPI, and a diffusion value of from 2% to 50%, preferably a diffusion value comprised between >2.5%, preferably >3.0%, more preferably >4.0% to <40%, preferably <35%, preferably <30%, more preferably <25%.

[0020] The at least one etched surface of the glass sheet of the present invention, has a surface roughness preferably defined, when measured on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm, by: 0.025 < Ra < 0.300 microns, more preferably 0.035 < Ra < 0.250 microns.

[0021] The at least one etched surface of the glass sheet of the present invention, has a surface roughness preferably defined, when measured on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm, by: 10 < Rsm < 28 microns, more preferably : 15 < Rsm < 25 microns.

[0022] In a preferred embodiment, the at least one etched surface of the glass sheet of the present invention, has a surface roughness defined, when measured on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm, by : 0.10 < Rz < 2.00 microns; preferably, by : 0.12 < Rz < 1.50 microns.

[0023] In a preferred embodiment, the glass sheet of the present invention has a clarity of from 30% to 100%, preferably from 45% to 100%, more preferably comprised between 60% and 100% when measured from said etched surface and / or a haze value of from 1% to 50%, preferably from 2% to 40%, more preferably, from 3% to 35%.

[0024] Preferably the glass composition of the glass sheet of the present invention will comprise AI2O3 in a content expressed in percentages of the total weight of the glass of 0 - 4wt%; preferably of 0 - 3wt%, more preferably of 0 - 2wt% and / or CaO in a content expressed in percentages of the total weight of the glass of 3wt% - 10wt%, preferably of 5wt% - 10wt%, more preferably from 6wt% - 10wt%.

[0025] The glass composition of the glass sheet of the present invention will preferably further comprises a total iron (expressed in terms of FejOa) content ranging from 0.002 to 0.06 wt%; preferably, ranging from 0.002 to 0.04 wt%, more preferably, ranging from 0.002 to 0.02 wt%, most preferably ranging from 0.002 to 0.015 wt%. The glass composition of the glass sheet of the present invention will preferably further comprises chromium in a content such as : 0.0001% < CrjOa < 0.06%, expressed in percentages of the total weight of glass; preferably, in a content such as : 0.002% < CrjOa < 0.06%.

[0026] Typically, the glass sheet has a glass thickness of from 0.1 to 6 mm, preferably from 0.1 to 2.2 mm, more preferably from 0.5mm to 1.5mm. In a preferred embodiment, the glass sheet is a heat strengthened glass, a thermally toughened safety glass, or a chemically tempered glass, preferably a chemically tempered glass. The glass sheet can coated with at least one antireflection layer, preferably the glass sheet is coated with said antireflection layer on the same glass face as the etched surface. Typically, the antireflection layer is 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. The present invention further relates to a display device that comprise the glass sheet of the present invention.

[0027] 4. Brief description of the drawings

[0028] Figures 1, 2 and 3 show pictures of optical microscopy of glass sheets according to the invention.

[0029] 5. Detailed description of the Invention

[0030] The objective of the invention is to remedy the cited disadvantages and resolving the technical problem, i.e. to provide a glass sheet which very low sparkle and at the same time allowing to consider some acceptable and controlled diffusion of reflected light, that is responsible of the anti-glare effect.

[0031] The objective of the present invention is indeed to find the best comprise between sparkling and diffusion, in particular for high resolution display application. Sparkling is to be avoided since it causes grainy appearance and disturbs colour rendering and image display to the user, in particular with high resolution displays. The anti-glare treatment is to reduce the negative effect of incident light on the glass sheet by diffusing the light reflections. However, it is recognised in the art that decreasing sparkling has a negative effect on diffusion. Similarly, it has been recognised that increasing diffusion has a negative impact on sparkling.

[0032] Another objective of the invention in at least one of its embodiments is to provide a glass sheet which shows very low sparkle, very low to medium diffusion, combined with a "soft touch".

[0033] Another objective of the invention in at least one of its embodiments is to provide a glass sheet which shows very low sparkle, very low to medium diffusion and which is chemically or thermally temperable.

[0034] 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.

[0035] The objective of the present invention is therefore to provide glass sheets that demonstrate the best compromise between sparkling and diffusion. It has been surprisingly found that maintaining relatively medium structural features on the etched surface of the soda-lime-silicate glass sheet allows to achieve excellent low sparkling with low to medium diffusion for good anti-glare, good haze and excellent clarity optical properties.

[0036] The present invention relates to a glass sheet comprising at least one etched surface having a surface roughness defined, when measured on an evaluation length of 4mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm, by:

[0037] 0.020 < Ra < 0.40 microns,

[0038] 7 < Rsm < 30 microns, said glass sheet having the following optical properties when measured from said etched surface

[0039] - a diffusion value of from 2% to 50%; and

[0040] - a sparkling value of 1.50% to 8.00%, measured with a display resolution of 264 PPI.

[0041] The inventors have found that it is possible to obtain an anti-sparkle glass sheet while mitigating that antiglare effect with pleasant smooth touch feeling, by considering a glass surface with a specific finetuned roughness together with specific optical properties. It has been found that such optical properties can be obtained even for glass sheet of low thickness and / or used in combination with high resolution displays.

[0042] 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. 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.

[0043] According to the invention, the glass sheet comprises at least one etched surface. While indeed both surfaces of the glass sheet can be etched, it is preferred for regular display application and processing that only one of the two surfaces of the glass sheet, is etched. By "etched surface", it is meant a surface which has been attacked by a mechanical or chemical way, removing a certain amount of glass material and giving a specific surface texture / roughness. We talk about chemically-etched glass when material removal occurs by chemical reactions / attack (i.e. acid etching). We talk about mechanically- etched glass when material removal occurs by mechanical reactions / attack (i.e. sandblasting). In a preferred embodiment of the present invention, the glass sheet is chemically-etched glass, more preferably is acid chemically-etched glass. According to the invention, said at least one etched surface may be etched advantageously over substantially the entire glass surface, that-is-to-say over at least 90% of the glass surface.

[0044] The etched surface of a glass sheet is usually characterized by its surface texture or roughness, and in particular, by the Ra, Rz and Rsm values (expressed as microns) defined in the standard ISO 4287- 1997. The texture / roughness is a consequence of the existence of surface irregularities / patterns. These irregularities consist of bumps called "peaks" and cavities called "valleys". On a section perpendicular to the etched surface, the peaks and valleys are distributed on either side of a "center line" (algebraic average) also called "mean line". In a profile and for a measurement along a fixed length (called "evaluation length") :

[0045] - Ra (amplitude value) corresponds to the average difference of texture, that is, means the arithmetic average of absolute values of differences between the peaks and valleys. Ra measures the distance between this average and the " line" and gives an indication of the height of the patterns on the etched surface;

[0046] - Rz (amplitude value) corresponds to the sum of height of the largest profile peak height , Rp, and the largest profile valley depth, Rv, within the evaluation length.

[0047] - Rsm (spacing value, sometimes also called Sm) is the average distance between two successive passages of the profile through the "mean line"; and this gives the average distance between the "peaks" and therefore the average value of the widths of the patterns.

[0048] The roughness values according to the invention may be measured with a profilometer using 2D profiles (according to ISO4287 standard). Alternatively, one can use the technique of 3D profilometry (according to ISO 25178 standard) but isolating a 2D profile which then gives access to the parameters defined in the ISO4287 standard.

[0049] According to the invention, the roughness values are measured with a Gaussian filter, which is a filter of long wavelengths, also called profile filter Ac. It is used for separating the components of roughness / texture from components of undulation of the profile.

[0050] The evaluation length L according to the invention is the length of the profile used to evaluate the roughness. Base length, I is the part of the evaluation length used to identify irregularities characterizing the profile to assess. The evaluation length L is divided / cut into n base lengths I which depend on the profile irregularities. The base length I corresponds to the "cut-off" wavelength (or limit wavelength) of the Gaussian filter (I = c). Typically, the evaluation length is of at least five times the base length. In roughness measurements, a short wavelength filter (profile filter Xs) is also commonly used to eliminate the effects of very short wavelengths which are background noise.

[0051] The surface roughness of the etched surface of glass sheet of the invention is such as : 0.020 < Ra < 0.400 microns. Preferably, the surface roughness of the etched surface of the invention is such as : 0.025 < Ra < 0.300 microns, and more preferably, such as 0.035 < Ra < 0.250 microns.

[0052] The surface roughness of the etched surface of the invention is such as : 7 < Rsm < 30 microns. Preferably, the surface roughness of the etched surface of the invention is such as : 10 < Rsm < 28 microns, and more preferably, such as : 15 < Rsm < 25 microns.

[0053] According to another advantageous embodiment of the invention, the surface roughness of the etched surface of the invention is such as : 0.10 < Rz < 2.00 microns, preferably as : 0.12 < Rz < 1.50 microns.

[0054] Such a limited ranges of Ra and Rsm and preferably Rz roughness values provide glass sheets of the invention with an excellent anti-sparkling effect together with a low to medium diffusion level for a lower haze value.

[0055] It is recognised in the art that decreasing sparkling has a negative effect on diffusion. Similarly, it has been recognised that increasing diffusion has a negative impact on Sparkling. It has been surprisingly found that the best comprise between Sparkling and Diffusion for highly commercial relevant cover glass sheet is the following:

[0056] The glass sheet of the present invention as a sparkling value of 1.50% - 8.00%, when measured from said etched surface. Preferably the glass sheet of the present invention will demonstrate a sparkling value of 1.75% to 7.00%, more preferably of 2.00% to 5.00%. Sparkling is herein measured by an SMS- 1000 instrument with an iPad screen (pixel resolution of 264PPI) using the Sparkling Image Difference method in accordance with the system manufacturer recommendations, Display-Messtechnik & Systeme (Germany) with a lens with a 50mm focal length and a 5.6mm aperture, and with no distance and no optical bonding between the glass sheet and the display. The iPad is typically an iPad 4 with Retina display (pixel resolution of 264PPI) displaying a full green background image (RGB color 0, 255, 0). The lens with the 50mm focal length and a 5.6mm aperture has typically a pixel ratio of 2.47. "Sparkle" refers to small bright spots (approximately at the pixel-level size scale) that appear in the instant texture of an image of a display screen through an anti-glare glass surface and which gives to the transmitted image a grainy appearance. The "sparkling effect" is thus an optical interaction between two surface areas : the regular display pixel matrix (light source) and the anti-glare glass surface with less regular microstructures. It appears as a random fluctuation in intensity on a display (involving refraction, diffraction, diffusion phenomena) as the viewer's head moves from side-to-side.

[0057] For sparkle measurement, the level of sparkle, the intensity modulations caused by the pixel matrix of the display (regular modulations) have to be separated from the random intensity modulations that are perceived as sparkle. A sample of a glass sheet of is applied to a display screen with a specific pitch of the pixel matrix (Display resolution of 264 PPI) and an image of that combination is taken with the camera of the SMS-1000. For the measurement herein, the glass sheet has a thickness of 1mm. During the measurement of sparkle usually only the green subpixels of the display are activated because under that condition the human eye is very sensitive with respect to detection of small features. Exposure is set on the SMS-1000 instrument without the sample and placed at the highest setting without over exposing the image. The pixel ratio (ratio of pixels on the target to camera pixels) is set and the reference is set to 0. The sample is placed over the target and the first data point is captured. The sample is again placed over the target and the second data point is captured. The recorded image is numerically low-pass filtered to account for the limited angular resolution of the human eye and to separate the display pixel modulation from the sparkle. The level of sparkle is evaluated as the standard deviation of the gray-level distribution of the filtered image divided by the mean value (similar to the speckle contrast).

[0058] To provide the anti-glare benefit, the glass sheet of the present invention demonstrate a diffusion value comprised between the minimal values of >2.0%, preferably >2.5%, more preferably from >3.0% and even more preferably >4.0% and the maximum values of <50%, preferably <40%, preferably <35%, preferably <30%, more preferably <25%, when measured from said etched surface. The higher the diffusion of the glass sheet, the better is its anti-glare optical property.

[0059] The diffuse light reflection, also called scattering or diffusion, of a glass surface represents the relative intensity of reflected light scattered by the surface. It corresponds to the portion of light that is reflected in all directions other than the specular direction. It will be herein referred to as 'diffusion'. Non-roughened glass surfaces are commonly assessed to have a diffusion value close to 0%. More roughening, higher the reflection scattering and hence higher becomes the diffusion value. Diffusion values are measured with a SMS-1000 equipment from DM&S (Display-Messtechnik & Systeme) in accordance with the system manufacturer procedure, with an angle of incident light of 6°, a distance of 260mm between the glass and the lens, and a lens with a 16mm focal length and a 5.6mm aperture. The SMS-1000 equipment from DM&S is run in accordance with the Reflectance Distribution Function method from the system manufacturer procedure, typically with the isotropic linear light source integrated in the SMS-1000 equipment.

[0060] The glass sheet of the present invention will demonstrate very low sparking while being able to maintaining acceptable anti-glare via a range of diffusion values from very low to medium diffusion :

[0061] • Very low diffusion (VLD): 1 < Diffusion (%) < 10

[0062] • Low diffusion (LD): 10 < Diffusion (%) < 30

[0063] • Medium diffusion (MD): 30 < Diffusion (%) < 60

[0064] The optical properties of the glass sheet can be further characterized by

[0065] • the direct total light transmission (or specular light transmission) ;

[0066] • the diffuse light transmission, measured through (i) the "haze" and (ii) the "clarity" : the "haze" corresponds to the diffuse transmittance at wide angles scattering while the "clarity" corresponds to the diffuse transmittance at narrow angles scattering ; and

[0067] • the gloss, characterizing, for example, the brightness or shine of a surface, and more particularly corresponding to the specular reflectance of a surface relative to a standard (such as, for example, a certified black glass standard) in accordance with ASTM standard D523 at a specific angle, and it is expressed in SGU (standard gloss units).

[0068] The term "diffuse" used for the light transmission is the proportion of light which, when passing through the glass, is deflected from the incident beam by dispersion of more than 2.5°. The term "diffuse" used for the light reflection is the proportion of light which, by reflection at the glass / air interface, is deflected from the specularly reflected beam by dispersion of more than 2.5°. Haze and clarity can be measured in accordance to ASTM standard D1003 with illuminant C.

[0069] In a preferred embodiment, the glass sheet of the invention has one or more of the following further optical properties when measured from said etched surface. According to an advantageous embodiment of the invention, the glass sheet has a clarity of from 30% to 100%, preferably from 45% to 100%, more preferably comprised between 60% and 100%. Typically, the haze of the glass sheet of the present invention will range from 1% to 50%, preferably of from 2% to 40%, more preferably of from 3% to 35%.

[0070] The glass sheet according to the invention preferably has a light transmission TLD4 of at least 85 %, preferably at least 90%. To quantify the glass transmission in the visible range, we define light transmission (TL) calculated between the wavelengths of 380 and 780 nm according to the IS09050 standard and measured with the D65 illuminant (TLD) such as defined by ISO / CIE 10526 standard by considering the standard colorimetric observer CIE 1931 as defined by the ISO / CIE 10527 standard. As used herein, the light transmission is measured according to said standards and given for a thickness of 4 mm (TLD4) under a solid viewing angle of 2 °.

[0071] COMPOSITION

[0072] The glass sheet of the present invention has a the following composition comprising, in a content expressed in percentages of the total weight of the glass. According to a preferred and more preferred embodiment of the invention, the glass sheet of the present invention has the following composition comprising, in a content expressed in percentages of the total weight of the glass:

[0073] Soda-lime-type base glass compositions have the advantages to be inexpensive even if it is less mechanically resistant as such. In a preferred embodiment, the glass composition comprises AI2O3in a content expressed in percentages of the total weight of the glass of 0 - 4wt%, preferably 0 - 3wt%, preferably of 0 - 2wt%, more preferably 0 - lwt% and / or in a preferred embodiment, the glass composition comprises CaO in a content expressed in percentages of the total weight of the glass of 3wt% - 10wt%, preferably of 5wt% - 10wt%, more preferably from 6wt% - 10wt%.

[0074] Ideally, 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, B2O3 content in the composition of the glass sheet reaches only level of an impurity unavoidably included in the production. For example, B2O3 content in the composition of the glass sheet of the invention is less than <0.01wt% or even better less than <0.005 wt%. According to an advantageous embodiment of the invention, combinable with previous embodiments on base glass composition, the glass sheet has a composition comprising a total iron (expressed in terms of FejOa) content ranging from 0.002 to 0.06 wt%. A total iron (expressed in the form of FejOa) 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 distortion 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, starting materials and also purification of these. Preferably, the composition comprises a total iron (expressed in the form of FejOa) content ranging from 0.002 to 0.04 wt%. More preferably, the composition comprises a total iron (expressed in the form of FejOa) 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 FejOa) content ranging from 0.002 to 0.015 wt%.

[0075] According to another embodiment of the invention, in combination with previous embodiments on FejOa content, the glass has a composition comprising chromium in a content such as : 0.0001% < CrjOa < 0.06%, expressed in percentages of the total weight of glass. Preferably, the glass has a composition comprising chromium in a content such as : 0.002% < CrjOa < 0.06%. This chromium content allows getting a glass with a higher IR transmission and it is thus advantageous when using the glass sheet in a touch panel using optical IR touch technologies like, for example, the Planar Scatter Detection (PSD) or Frustrated Total Internal Reflection (FTIR) (or any other technology requiring high transmission of IR radiation) in order to detect the position of one or more objects (for example, a finger or a stylus) on a surface of the glass sheet.

[0076] According to a preferred embodiment, the glass sheet of the invention is a float glass sheet. The term "float glass sheet" is understood to mean a glass sheet formed by the float process, which consists in pouring the molten glass onto a bath of molten tin, under reducing conditions. A float glass sheet comprises, in a known way, a "tin face", that is to say a face enriched in tin in the body of the glass close to the surface of the sheet. The term "enrichment in tin" is understood to mean an increase in the concentration of tin with respect to the composition of the glass at the core, which may or may not be substantially zero (devoid of tin). Therefore, a float glass sheet can be easily distinguished from sheets obtained by other glassmaking processes, in particular by the tin oxide content which may be measured, for example, by electronic microprobe to a depth of ~ 10 microns. 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, even more preferably from 0.5mm to 1.5m.

[0077] The glass sheet according to the invention can advantageously be a prestressed glass. By prestressed glass, it is meant herein a heat strengthened glass, a thermally toughened safety glass, or a chemically tempered glass. Preferably, the glass sheet of the present invention is a chemically tempered glass.

[0078] Heat strengthened glass is heat treated using a method of controlled heating and cooling which places the outer glass surfaces under compression and the inner glass surface under tension. This heat treatment method delivers a glass with a bending strength greater than annealed glass but less than thermally toughened safety glass. Thermally toughened safety glass is heat treated using a method of controlled heating and cooling which puts the outer glass surface under compression and the inner glass surface under tension. Such stresses cause the glass, when impacted, to break into small granular particles instead of splintering into jagged shards. The granular particles are less likely to injure occupants or damage objects. The chemical tempering of a glass article is a heat induced ionexchange, 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.

[0079] According to the applications, intended use and / or properties desired, various layer(s) / treatment(s) can be deposited / done on the glass sheet of the invention, on same face as the etched surface according to the invention and / or on the opposite face.

[0080] 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 SnOz:F, SnO2:Sb or ITO (indium tin oxide), ZnO:AI or also ZnO:Ga. Advantageously, according to this embodiment, the glass sheet is coated with said transparent and electrically conducting thin layer on the glass face opposite to the etched surface. According to another embodiment of the invention, the glass sheet is coated with at least one antireflection layer. Advantageously, according to this embodiment, the glass sheet is coated with said antireflection layer on the same glass face as the etched surface. This embodiment is advantageous in the case of use of the glass sheet of the invention as front cover 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.

[0081] According to still another embodiment, the glass sheet has at least one anti-fingerprint layer / treatment so as to reduce or prevent fingerprints from registering. Advantageously, according to this embodiment, the glass sheet has said anti-fingerprint layer / treatment on the same glass face as the etched surface. This embodiment is also advantageous in the case of use of the glass sheet of the invention as front cover of a touchscreen. Such a layer / treatment can be combined with a transparent and electrically conducting thin layer deposited on the opposite face. Such a layer / treatment can be combined with an antireflection layer deposited on the same face.

[0082] According to still another embodiment of the invention, the glass sheet has an antibacterial layer / treatment. Advantageously, according to this embodiment, the glass sheet has said antibacterial layer / treatment on the same glass face as the etched surface. For example, such an antibacterial treatment could be a diffusion of silver ions in the bulk of the glass sheet close to the external surface.

[0083] Moreover, the glass sheet according to the invention shows excellent mechanical properties. In particular, it shows an excellent resistance to abrasion.

[0084] Finally, the invention also relates to a display device comprising a glass sheet according to the invention. All previously described embodiments for the glass sheet also apply to the invention of display device.

[0085] 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

[0086] Comparative Examples 1-2 : soda-lime (SL) etched "LST" (or "LSTouch" or "low sparkling touch") glass from AGC Glass Europe, sold mainly for display applicationse.

[0087] Comparative Examples 3-4 : soda-lime (SL) etched "VRD" (or "Verre a Reflexion Diffuse") glass from AGC Glass Europe, sold mainly for display applications.

[0088] Example 5-7 (according to the invention) - on soda-lime-type composition (SL)

[0089] For each example 5-7, a sheet of extra-clear glass of 1.0 mm thickness (10 cm x 10 cm) was washed with an aqueous detergent and dried. A tape was applied on one side of the glass in order to protect it during the etching process. Then, the glass was dipped in 200 mL of an acid-etching solution at 20- 25°C during a period of time t. Finally the glass was removed and immediately washed with an aqueous detergent.

[0090] The soda-lime type (SL) composition of the glass was as follows, in weight percentages:

[0091] SiO272.8%

[0092] AI2O30.9%

[0093] Na2O 13.8%

[0094] CaO 8.0%

[0095] MgO 4.48%

[0096] K2O 0.02%

[0097] Example 5 (on SL) : Acid-etching aqueous solution was composed of:

[0098] • (NH4)HF24.75 mol %

[0099] • KHF21.5 mol %

[0100] • Citric acid 0.5 mol%

[0101] • Glycerol 0.5 mol %

[0102] The glass sample was removed after 12 seconds.

[0103] Example 6 (on SL) : Acid-etching aqueous solution was composed of:

[0104] • (NH4)HF27.0 mol%

[0105] • KHF21.2 mol%

[0106] • HF 1.0 mol%

[0107] • TWEEN 60 0.035mol% The glass sample was removed after 40 seconds. After rinsing and drying of the glass sheet is dipped in a second acid-etching aqueous solution composed of:

[0108] • HF 25 mol%

[0109] • H2SO410 mol%

[0110] • HCI 8 mol%

[0111] • (NH4)HF20.4 mol%

[0112] • Xanthan gum 0.15mol%

[0113] The glass sample was removed after 90 seconds.

[0114] Example 7 (on SL) : Acid-etching aqueous solution was composed of:

[0115] • (NH4)HF27.0 mol%

[0116] • KHF21.8 mol%

[0117] • HF 0.5 mol%

[0118] The glass sample was removed after 30 seconds.

[0119] After rinsing and drying of the glass sheet is dipped in a second acid-etching aqueous solution composed of:

[0120] • HF 20 mol%

[0121] • H2SO410 mol%

[0122] • (NH4)HF21.0mol%

[0123] • Glycerol 0.8 mol%

[0124] The glass sample was removed after 120 seconds.

[0125] Texture and optical properties

[0126] Each of the glass sheets from Examples 1 to 7 were analyzed in terms of texture / surface roughness and optical properties.

[0127] Surface roughness measurements were performed using a 3D optical profiler Sensofar Model S neox 090 with a Nikon 50x lens, using the "SensoSCAN S neox 7.7" software, on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm. Roughness values are calculated from the profile measured according to ISO4287 standard. The sample is first cleaned with detergent and dried. It is then placed under the microscope and after conventional settings, the profile of a 2D acquisition is then initiated. Optical microscopy pictures have also been taken for several samples with a Leica DM2700 M device with a x50 magnification lens. Diffusion and sparkling measurement were measured by an SMS-1000 instrument from the manufacturer Display-Messtechnik & Systeme (Germany). The sparkling is evaluated with an iPad screen (pixel resolution of 264PPI), a lens with a 50mm focal length and a 5.6mm aperture, and with no distance and no optical bonding between glass article and display, using the Sparkling Image Difference method in accordance with the system manufacturer Display-Messtechnik & Systeme (Germany). Diffusion is measured in accordance with the system manufacturer procedure, with an angle of incident light of 6°, a distance of 260mm between the glass and the lens, and a lens with a 16mm focal length and a 5.6mm aperture. The iPad is the iPad 4 with Retina display (pixel resolution of 264PPI) displaying a full green background image (RGB color 0, 255, 0) and the lens has a pixel ratio of 2.47.

[0128] In order to emphasize the benefit of the present invention, the sparkling (S) measured can also be expressed as an "anti-sparkling (AS)" characteristic through the formula: AS= 1-(S / Sref) wherein AS and S are respectively the anti-sparkling and sparkling properties of the sample considered and Sref is the measured sparkling of a reference sample having a very high sparkling value. The chosen reference sample is the VRD140 being a soda-lime (SL) etched "VRD" (or "Verre a Reflexion Diffuse"), commercially available product of AGC Glass Europe and having a sparkling of 26.6%.

[0129] Haze and clarity measurements were performed according to ASTM standard D1003 with a BYK Haze- gard i device, using the illuminant C. Gloss measurements were performed according to ASTM standard D523 at a specific angle of 60° with a BYK micro-TRI-gloss device.

[0130] The roughness parameters obtained and results for optical properties of Examples are given in Table below. Observations

[0131] The sparking values of comparative examples 3 to 6 have been measured at a thickness of 1.9mm or 2.9mm. Therefore, as well known by persons skilled in the art, sparkling values measured for glass sheet samples with higher thickness will demonstrate even worse sparkling values if sparking was to be measured at the required thickness of 1mm by a factor of about 20 to 30%. All others values of diffusion, haze and clarity do not change.

[0132] Optical microscopy images for example 5, example 6 and example 7, given at Figures 1, 2 and 3, respectively, show the morphologies / geometrical structures that can be obtained while reaching roughness parameters and optical properties according to the invention.

[0133] Results in table illustrates that glass sheets according to the invention show the best comprise between the focus on excellent anti-sparkle effect as requested by the cover glass sheet market with reasonable, acceptable and controlled low to medium diffusion - compared to existing commercialized glass sheets. The glass sheets according to the invention presented in examples 5 to 7 offer a lower level of sparkling than comparative examples 1 to 4, resulting in a better rendering of color and display image, with good diffusion resulting in a very acceptable anti-glare effect. Each sample according to the invention displays a touch feeling which is softer glass sheets from comparative examples.

Claims

CLAIMS1. A glass sheet having a glass composition comprising, in a content expressed in percentages of the total weight of the glass:SiO255% - 85%, preferably 55% - 78%;AI2O3 0 - 5%, preferably 0 - 4%;B2O30 - 20%, preferably 0 - 18%;Na2O 0 - 25%, preferably 5% - 20%;CaO 0 - 20%, preferably 2% - 12%;MgO 0 - 15%, preferably 0 - 12%;K2O 0 - 20%, preferably 0 - 10%; andBaO 0 - 20%, preferably 0 - 5% said glass sheet comprising at least one etched surface having a surface roughness defined, when measured on an evaluation length of 4 mm and with a Gaussian filter of which the cutoff wavelength is 0.8 mm, by:0.020 < Ra < 0.400 microns,7 < RSm < 30 microns, said glass sheet having the following optical properties when measured from said etched surface : a sparkling value of 1.50% to 8.00%, measured with a display resolution of 264 PPI, and a diffusion value of from 2% to 50%.

2. A glass sheet according to any one of the preceding claims having a sparkling value of 1.75% to 7.00%, preferably of 2.00% to 5.00%.

3. A glass sheet according to any one of the preceding claims having a diffusion value comprised between the minimal values of >2.5%, preferably >3.0%, more preferably >3.5%, even more preferably >4.0% to the maximal values of <40%, preferably <35%, preferably <30%, more preferably <25%.

4. Glass sheet according to any one of the preceding claims wherein the at least one etched surface having a surface roughness defined, when measured on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm, by: 0.025 < Ra < 0.300 microns, preferably 0.035 < Ra < 0.250 microns.

5. A glass sheet according to any one of the preceding claims wherein the at least one etched surface having a surface roughness defined, when measured on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm, by: 7 < Rsm < 28 microns, preferably : 10 < Rsm < 25 microns.

6. A glass sheet according to any one of the preceding claims wherein the at least one etched surface having a surface roughness defined, when measured on an evaluation length of 4 mm and with a Gaussian filter of which the cut-off wavelength is 0.8 mm, by: 0.10 < Rz < 2.00 microns; preferably, by : 0.12 < Rz < 1.50 microns.

7. A glass sheet according to any one of the preceding claims wherein the glass sheet has a clarity of from 30% to 100%, preferably from 45% to 100%, more preferably comprised between 60% and 100% when measured from said etched surface.

8. A glass sheet according to any one of the preceding claims wherein the glass sheet has a haze value of from 1% to 50%, preferably from 2% to 40%, more preferably, from 3% to 35%.

9. A glass sheet according to any one of the preceding claims wherein the glass composition comprises AI2O3 in a content expressed in percentages of the total weight of the glass of 0 - 4wt%; preferably of 0 - 3wt%, more preferably of 0 - 2wt%.

10. A glass sheet according to any one of the preceding claims wherein the glass composition comprises CaO in a content expressed in percentages of the total weight of the glass of 3wt% - 10wt%, preferably of 5wt% - 10wt%, more preferably from 6wt% - 10wt%.

11. Glass sheet according to preceding claims, characterized in that it is a heat strengthened glass, a thermally toughened safety glass, or a chemically tempered glass, preferably chemically tempered glass.

12. A Glass sheet according to any one of the preceding claims wherein the glass sheet is coated with at least one antireflection layer, preferably the glass sheet is coated with said antireflection layer on the same glass face as the etched surface.

13. A glass sheet according to claim 12 wherein the antireflection layer is 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.

14. A glass sheet according to any one of the preceding claims, having a glass thickness of from0.1 to 6 mm, preferably, from 0.1 to 2.2 mm, more preferably from 0.5mm to 1.5mm.

15. Display device comprising a glass sheet according to any one of the preceding claims.

Images