COATED GLASS OR GLASS CERAMIC SUBSTRATE, COATING COMPRISING CLOSED PORES, AND METHOD FOR COATING A SUBSTRATE

Abstract

Coated glass or glass ceramic substrates having high temperature resistance, high strength, and a low coefficient of thermal expansion. The coating includes pores, is fluid-tight and suitable for coating a temperature-resistant, high-strength glass or glass ceramic substrate with a low coefficient of thermal expansion, and to a method for producing such a coated substrate.

Claims

1. A method for coating a glass or glass ceramic substrate, comprising the steps of: preparing a suspension comprising a glass powder and an agent that decomposes upon temperature increase to form a volatile substance; applying the suspension to the glass or glass ceramic substrate so that at least part of the substrate is covered by the suspension; fixing the suspension applied on the substrate at a first temperature between 0° C. and 300° C. to form a substrate with a fixed suspension; annealing the substrate with the fixed suspension at a second temperature between 450° C. and 900° C. so that the agent decomposes to form the volatile substance and to cause formation of a coated substrate with a coating that has closed pores; and cooling the coated substrate to room temperature.

2. The method of claim 1, wherein the coating is a barrier against passage of fluids.

3. The method of claim 2, wherein the closed pores have a size between 0.1 μm and 30 μm.

4. The method of claim 1, wherein the substrate has a coefficient of thermal expansion of less than 5*10.sup.−6/K in a range from 20° C. to 700° C.

5. The method of claim 1, wherein the second temperature is between 450° C. and 750° C.

6. The method of claim 1, wherein the step of applying the suspension comprises applying the suspension to the substrate in a laterally patterned form of a predetermined pattern.

7. The method of claim 6, wherein the laterally patterned form comprises at least one portion of the substrate that remains free of the suspension.

8. The method of claim 1, further comprising, after the applying step and before the annealing step, the step of applying a non-adhering cover sheet the suspension.

9. The method of claim 8, wherein the non-adhering cover sheet remains in place during the fixing and annealing steps.

10. The method of claim 1, wherein the applying step comprises a process selected from a group consisting of printing, ink-jet printing, offset printing, pad printing, screen printing, rolling, flooding, dipping, spraying, doctor blading, and spin coating.

11. The method of claim 1, further comprising the step of visually inspecting the coating.

12. The method of claim 11, wherein the step of visually inspecting the coating comprises: applying a drop of a liquid onto an area of the coating of the substrate; allowing the liquid to act for a duration of 15 seconds; wiping off residual moisture of the liquid using a dry cloth; turning over the substrate so that the coating is disposed on the side of the substrate facing away from an inspecting person; and visually verifying whether a color change is recognizable in the area or in a region adjacent to the area.

13. The method of claim 12, wherein the visually verifying is performed in daylight of standard illuminant D65 or under lighting from a source selected from a group consisting of an incandescent lamp, a compact fluorescent lamp, a fluorescent lamp, and a light-emitting diode, the light source having illuminance of at least 500 lx at a distance of less than 600 mm from the area, and at a viewing angle between 5° and 90°.

14. The method of claim 12, wherein the fluid is selected from a group consisting of water, alcohol, and glass cleaning agent.

15. The method of claim 1, wherein the coating comprises at least 95 wt. % of inorganic constituents and has a thickness between 1.5 μm and 50 μm.

16. The method of claim 1, wherein the coating is temperature resistant for temperatures greater than 400° C.

17. The method of claim 1, wherein the substrate is a sheet-like substrate having a thickness between at least 1 mm and at most 10 mm.

18. The method of claim 1, wherein the coating comprises a material selected from a group consisting of a colorant agent, an effect agent, a color pigment, an effect pigment, and any combinations thereof.

19. The method of claim 1, wherein the coating is opaque to electromagnetic radiation in a wavelength range from 380 nm to 780 nm.

20. The method of claim 1, wherein the coating has a τ.sub.vis (in-line transmittance) with a value of less than 20% in a wavelength range of visible light.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0203] In the drawings:

[0204] FIGS. 1 to 3 are schematic views, not drawn to scale, of a glass or glass ceramic substrate with a coating including closed pores;

[0205] FIGS. 4a to 4d, 5, 6a to 6d show micrographs of coatings that include closed pores;

[0206] FIG. 7 shows transmittance profiles of one non-coated and several coated glass or glass ceramic substrates;

[0207] FIG. 8 shows results of strength tests on differently coated substrates;

[0208] FIG. 9 is a schematic view of the configuration of a cooking oven door;

[0209] FIG. 10 is a schematic view of the measurement setup for determining the surface temperature of the outer oven door pane;

[0210] FIG. 11 is a graph of measured temperature profiles of the maximum temperature of the outer oven door pane of various exemplary embodiments which differ with regard to the porosity of the coating, at an operating temperature of the oven of 450° C.; and

[0211] FIG. 12 shows the averaged values of the temperature profiles shown in FIG. 11.

DETAILED DESCRIPTION

[0212] FIG. 1 is a schematic view, not drawn to scale, of a glass or glass ceramic substrate 1 with a coating 2 that includes closed pores. The coating is designed as a barrier against the passage of fluids.

[0213] The coating 2 may be applied over the entire surface of the substrate 1, or else—as schematically shown in FIG. 1—only over part of the substrate. It is in particular possible for the coating 2 to be applied to the substrate 1 in the form of a predetermined pattern, for example in order to apply a lettering or a logo to the glass or glass ceramic substrate.

[0214] FIG. 2 is a schematic sectional view, not drawn to scale, of a glass or glass ceramic substrate 1. A surface 10 which preferably faces away from the operating person during intended use of the glass or glass ceramic substrate 1 is partly covered by a coating 2. This coating 2 includes closed pores 3. For the sake of clarity, these pores 3 have not all been designated.

[0215] Here, the pores 3 are schematically illustrated as circles or spherical sections. The pores may also be of different size and shapes, i.e., more generally, need not be spherical, not restricted to the example schematically illustrated here.

[0216] FIG. 3 is a schematic sectional view, not drawn to scale, through a further exemplary embodiment in which the surface 10 of the glass or glass ceramic substrate 1 has a porous coating 2 that includes pores 30 with an anisotropic cross-sectional shape. For example, the pores 30 have an elliptical cross section. Pores with such a shape may be obtained, for example, when using rice starch as the blowing agent.

[0217] The pores may be of different size and shapes, i.e., more generally, are not limited by the example schematically illustrated here and need not be spherical.

TABLE-US-00019 Pore size Pore former (μm) Pore shape CaCO.sub.3 5-30 roundish Sodium hydrogen 5-30 roundish phosphates Rice starch 0.1-5   elongated Potato starch 10-15  ovoid potato- shaped Wheat starch 2-10 grain-shaped

[0218] Furthermore, the coatings schematically illustrated in FIGS. 2 and 3 may also comprise, in addition to the closed pores 3, 30, pores located at the interface of the layer, that is to say in the form of a downward indentation of the coating. However, such pores that are open to one side have no detrimental impact on the coating 2 in terms of being effective as a barrier against the passage of fluids. Rather, what is important is that there are no continuous pores extending from the surface of the coating 2 down to the upper surface 10 of the substrate 1.

[0219] FIGS. 3a and 3b schematically illustrate embodiments in which the coating deposited on the glass 1 includes two pores, 32 and 33, respectively. In both cases, these are closed pores. FIG. 3a shows an embodiment with largely spherical pores 32. Such pores may be obtained by using calcium carbonate as a blowing agent, for example. In contrast, the pores 33 shown in FIG. 3b have an elliptical cross-sectional shape and hence an anisotropic structure. Pores with such a shape may be obtained by using rice starch as a blowing agent, for example.

[0220] FIGS. 4a-4d show several photographs which are light microscopic images of coatings 2 according to embodiments of the disclosure, which were using screens of different sizes and fired at different temperatures. The two coatings on the left were each applied using a 77 mesh screen. The sample shown far left, in FIG. 4a, was fired at approximately 750° C., and the next sample on the left, in FIG. 4b, was fired at approximately 720° C. The two samples on the right, in FIGS. 4c and 4d, show coatings which were printed using a 100 mesh screen. The sample shown half right, in FIG. 4c, was fired at approximately 750° C., the samples shown far right, in FIG. 4d, was fired at approximately 720° C.

[0221] What is particularly evident is the impact of temperature on the formation of the pores: while the samples fired at approximately 750° C. tend to have fewer, but larger pores, the samples fired at approx. 720° C. include more pores, with smaller dimensions.

[0222] FIG. 5 shows a micrograph of a section through a substrate 1 to which a coating 2 was applied that includes closed samples 3. Sodium hydrogen phosphate was added to the glass 2 as a blowing agent, in an amount of 10 vol %.

[0223] FIGS. 6a to 6d show further images of a coating 2 including closed pores 3. The coating was obtained by mixing glass 1 with calcium carbonate. For the samples of FIGS. 6a and 6b, 5 vol % of calcium carbonate was added to the glass 1, for the samples of FIGS. 6c and 6d 10 vol % of calcium carbonate. The samples of FIGS. 6a and 6c were fired at approximately 750° C., the samples of FIGS. 6b and 6d at approximately 720° C. What is clearly recognizable is the influence of the amount of blowing agent which leads to a significant increase in the number of bubbles.

[0224] FIG. 7 shows transmittance profiles of several glass ceramic substrates in the wavelength range from approximately 300 nm to 5000 nm. Curve 4 shows the transmittance profile for a non-coated glass ceramic substrate. In the visible spectral range, i.e. from approx. 380 nm to approx. 780 nm, transmittance is high, so the substrate can therefore be described as being transparent in this range. Thus, structures located below such a substrate would be visible to a user.

[0225] This changes when a coating according to embodiments of the disclosure is applied.

[0226] Curve 5 shows the case of a substrate which in its non-coated state exhibits a transmittance similar to that of curve 4, and for which a cobalt-iron spinel with nanoscale particle size was used as a pigment. No blowing agents were added. Curve 6 represents a coating which, in addition to the nanoscale cobalt-iron spinel (15 vol %), furthermore comprises 20 vol % of sodium dihydrogen phosphate as a blowing agent.

[0227] Curve 7 represents a coating which, instead of the pigment used for the coating of curve 6, comprises Co—Mn—Fe—Cr spinel pigment (d50˜0.5 μm), with an otherwise unchanged composition. For curve 8, chromium-iron-nickel black spinel (d50˜1-2.5 μm) was used as the pigment, with an otherwise unchanged composition.

[0228] It can be seen that in particular the substrates provided with coatings according to embodiments of the present disclosure exhibit transmittance profiles in which a very good covering effect is achieved in the visible. This is illustrated by curves 6 to 8. So, transmittance of the coated substrate is further reduced by the pores, and hence opacity is increased. Opacity represents the reciprocal of transmittance.

[0229] In optics, absorbance A or optical density is the opacity O formulated as decadic logarithm in line with human perception and thus a measure of the attenuation of radiation (e.g. light) after having passed through a medium (Wikipedia https://de.wikipedia.org/wiki/Extinktion (Optik)).

[0230] Here, in-line transmittance is represented (in contrast to total transmittance). When measuring total transmittance, the entire light that is scattered forward is captured on a detector, whereas for in-line transmittance only the forward directed light is captured on the detector (given an opening angle of normally 5° of the measuring devices, also the scattered light exiting at this small angle). The difference between total and in-line transmittances gives a measure of scattering. In the present case, with regard to the layer, scattering is in particular caused by the pigment particles of the layer and the pores.

[0231] FIG. 8 shows how the strength, as determined by what is known as a ball drop test, changes for a coated substrate depending on the composition of the coating.

[0232] If the coating comprises only a glass or a glass together with a pigment, only very low strength values are obtained in the ball drop test. These are layers which do not include closed pores and therefore do not represent layers according to the invention.

[0233] In contrast, if a coating is produced by applying a suspension which comprises a blowing agent in addition to glass or glass and pigment, layers are formed in accordance with embodiments of the present disclosure which include closed pores. A substrate coated in this way exhibits significantly higher strength than if no blowing agent is used.

[0234] Visual inspection of the coating according to embodiments of the present disclosure is performed by the following steps: applying a liquid onto a surface area of the coating of the substrate; allowing the liquid to act for a duration of 15 seconds; wiping off residual moisture of the liquid using a dry cloth; turning over the substrate so that the coating is disposed on the side of the substrate facing away from the inspecting person; and verifying, by visual inspection, whether a color change is recognizable in the area or in an area adjacent to the area, wherein

a) the visual inspection is performed in daylight according to standard illuminant D65 or under lighting of an incandescent lamp, compact fluorescent lamp, fluorescent lamp, or light-emitting diode;
b) illuminance is at least 500 lx at a distance of less than 600 mm from the coating, i.e. from the inspected area; and
c) the viewing angle of the inspecting person is between 5° and 90°, preferably at least 30°.

[0235] Liquids that may be used include water, oil, alcohol, and/or glass cleaning agent.

[0236] The aforementioned visual inspection in particular includes the examination of whether a water mark and/or a water stain is noticeable from the side of the substrate opposite the coated side. Here, a layer is described as very good if after the test no color change on the front side nor on the rear side is revealed. A layer is described as good if after the test no color change on the front side and exhibits a wipeable border on the rear side is revealed.

[0237] FIG. 9 schematically shows a possible configuration of a cooking oven door. Here, the outer pane 100 has a porous coating 3 on one side thereof. The non-coated side of the substrate faces outwards. The intermediate pane 101 and the inner pane 102 of the oven door are coated with a coating 9 on one side thereof. Coating 9 may include transparent conductive oxides, for example.

[0238] FIG. 10 schematically shows a measurement setup for determining the surface temperatures of a coated glass sheet under laboratory conditions. In this case, a laboratory oven 12 is heated to a temperature of 450° C. The oven has an opening with a diameter of 3 cm. The glass pane 1 with the coating 2 to be measured is placed at a distance of 2.5 cm from this opening with the coating 2 facing the opening of the oven. The surface temperature of the coated glass sheet 1 is determined using a pyrometer 13 (impac, IE 120/82L), with the focal point adjusted to the outer surface of the decorated sheet. The pyrometer 13 is arranged behind a glass substrate 14 and at a distance of 50 cm from the glass sheet 1 to be measured.

[0239] FIGS. 11 and 12 show the temperature profile on the outer surface of several coated substrates as a function of operating time. Here, the oven was heated to a temperature of 450° C., and subsequently the surface temperature of the coated glass sheet was determined as a function of operating time using the measurement device shown in FIG. 10.

[0240] FIG. 11 shows the maximum temperatures measured in this way, as a function of operating time of the oven. FIG. 12 shows a fit of the temperature profiles shown in FIG. 11, obtained by averaging.

[0241] Curves 15, 16, and 17 correspond to temperature profiles of comparative examples in which the coating has IR-reflecting pigments but is not porous. Curves 18 to 21 can be associated with temperature profiles of exemplary embodiments in which the coating includes closed pores and IR-reflecting pigments.

[0242] The comparison examples and the exemplary embodiments are characterized in more detail in the table below. The examples comprise a soda-lime glass as the substrate, glass 1 from the table was used as glass frit or glass flux. Firing was performed in the laboratory oven at 680° C. for 15 minutes, while the samples were supported horizontally.

TABLE-US-00020 Blowing Type of T.sub.max (° C.) Optical density L*a*b* (SCE; Pigment agent blowing after 1 h (glass side (coated side Gloss Sclerometer Sidolin test Curve (vol %) (vol %) agent at 450° C. facing upwards) facing upwards) (60°) (10N) (porosity) 16 20 0 N/A 47.6 2.6 34.63/0.45/−3.71 18.5 ok very good 17 17.5 0 N/A 46.9 2.1 36.04/0.44/−4.09 55.9 ok very good 21 17.5 20 CaCO.sub.3 43.8 2.2 34.63/0.45/−3.71 4.0 ok good 20 17.5 10 CaCO.sub.3 43.2 2.6 31.58/0.49/−3.56 17.5 ok good 19 17.5 20 rice starch 45.7 2.1 21.38/0.6/−1.75 41.0 ok very good 18 17.5 10 rice starch 45.4 2.6 18.56/0.65/−1.95 50.5 ok very good
Characterization of the samples shown in FIGS. 11 and 12

[0243] The coatings of comparative examples 16 to 18 were produced without using blowing agents. Examples 19 to 21, by contrast, are porous coatings. For producing these coatings, blowing agents were used as listed in the table above, and therefore the coatings obtained in this way include closed pores. All of the temperature profiles shown in FIGS. 11 and 12 were obtained using the measurement setup shown in FIG. 10. The respective coating compositions were applied to the substrate by screen printing using a 77T mesh.

[0244] The coatings of all examples 15 to 21 contain IR-reflecting pigments, so that these coatings exhibit good IR reflectivity. This manifests in particular in the fact that for all examples the measured temperature of the outer pane was less than 50° C., for an oven operating time of 60 minutes at 450° C. What becomes evident from this is that the IR reflectivity of the coating can be significantly enhanced through the porosity thereof. For samples 18 to 21, lower temperatures were measured than for the comparative samples 15 to 17 with a dense coating. The temperature difference measured after 60 minutes of operation between the dense sample 15 and the porous sample 20 is more than 4° C. It is assumed that the pores within the coating represent structures which additionally scatter the IR radiation.

[0245] The impact of this positive effect on the maximum surface temperature of the pane seems to be dependent on the shape of the pores. The blowing agent used in samples 18 and 19 was rice starch, while CaCO.sub.3 was used in samples 20 and 21. When rice starch is used as a blowing agent, anisotropic pores with an ellipsoidal cross section will preferably be formed, while the use of CaCO.sub.3 as a blowing agent leads to largely spherical pores (cf. FIGS. 2 and 3).

[0246] FIG. 12 shows that for the coated glasses 20 and 21 which have pores of spherical or largely spherical shape, the isolation effect is higher than for the coated glasses 18 and 19 that have a coating with ellipsoidal or rice-shaped pores.

[0247] FIG. 12 moreover shows that the percentage of blowing agent in the paste has an impact on the IR reflection of the corresponding coating. Samples 20 and 21 only differ in their content of blowing agent. While the amount of blowing agent in the paste for producing coating 21 is 20 vol %, the corresponding paste for producing coating 20 contains only 10 vol % of CaCO.sub.3 as the blowing agent. And, sample 20 exhibits a better isolation effect than sample 21, so that under comparable conditions and after an operating time of 180 minutes the maximum temperature of sample 20 is lower than the maximum temperature of sample 21 by 0.8° C.

[0248] An excessive amount of blowing agents in the paste results in a formation of so many pores that they in part combine so that open pores are created. An indication of open pores and an uneven surface associated therewith. It is assumed here that closed pores promote IR reflectivity.

[0249] Another way of increasing IR reflectivity of the coating is to increase layer thickness, for example by repeatedly applying the corresponding paste or suspension to the substrate. This becomes evident from the table below. Here, the samples were applied onto the substrate by screen printing using a 77T mesh, dried and optionally printed a second time using a 77T mesh before the coating was fired while being supported horizontally in the laboratory oven for 15 minutes at 680° C. The table indicates the number of printing processes (single or double print) and the maximum temperature determined with the measurement setup shown in FIG. 10 on the outer surface of the pane after 60 minutes of operation of the oven at a temperature of 450° C.

TABLE-US-00021 Percentage T.sub.max (° C.) Percentage of blowing after 1 h Optical density L*a*b* (SCE; Sample Number of of pigment agent Blowing heating (glass side (coated side Gloss Sclerometer Sidolin test ID prints [vol %] [vol %] agent used at 450° C. facing upwards) facing upwards) (60°) (10N) (porosity) 21 1 17.5 20 CaCO.sub.3 43.8 2.2 34.63/0.45/−3.71 4.0 ok good  21a 2 17.5 20 CaCO.sub.3 41.3 3.2 36.04/0.44/−4.09 1.4 ok good 19 1 17.5 20 rice starch 45.7 2.1 21.38/0.6/−1.75 41.0 ok very good  19a 2 17.5 20 rice starch 45.2 4.2 23.49/0.42/−2.15 35.4 ok very good 18 1 17.5 10 rice starch 45.4 2.6 18.56/0.65/−1.95 50.5 ok very good  18a 2 17.5 10 rice starch 44.9 4.7 19.86/0.40/−2.38 48.9 ok very good

LIST OF REFERENCE NUMERALS

[0250] 1 Glass or glass ceramic substrate [0251] 2 Coating including closed pores [0252] 3, 30 Closed pores [0253] 4 Transmittance profile of non-coated substrate [0254] 5 Transmittance profile of substrate not coated according to the invention [0255] 6,7,8 Transmittance profiles of substrates coated according to embodiments of the disclosure [0256] 9 Coating comprising conductive oxides [0257] 10 Surface of substrate 1 [0258] 12 Laboratory oven [0259] 13 Pyrometer [0260] 15, 16, 17 Temperature profiles of comparative examples with dense coatings [0261] 18, 19, 20, 21 Temperature profiles of exemplary embodiments with porous coatings [0262] 100 Outer oven door pane [0263] 101 Intermediate oven door pane [0264] 102 Inner oven door pane