Patent ID: 12234177

DETAILED DESCRIPTION

FIG.1is a schematic view, not drawn to scale, of a glass or glass ceramic substrate1with a coating2that includes closed pores. The coating is designed as a barrier against the passage of fluids.

The coating2may be applied over the entire surface of the substrate1, or else—as schematically shown inFIG.1—only over part of the substrate. It is in particular possible for the coating2to be applied to the substrate1in the form of a predetermined pattern, for example in order to apply a lettering or a logo to the glass or glass ceramic substrate.

FIG.2is a schematic sectional view, not drawn to scale, of a glass or glass ceramic substrate1. A surface10which preferably faces away from the operating person during intended use of the glass or glass ceramic substrate1is partly covered by a coating2. This coating2includes closed pores3. For the sake of clarity, these pores3have not all been designated.

Here, the pores3are 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.

FIG.3is a schematic sectional view, not drawn to scale, through a further exemplary embodiment in which the surface10of the glass or glass ceramic substrate1has a porous coating2that includes pores30with an anisotropic cross-sectional shape. For example, the pores30have an elliptical cross section. Pores with such a shape may be obtained, for example, when using rice starch as the blowing agent.

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.

Pore sizePore former(μm)Pore shapeCaCO35-30roundishSodium hydrogen5-30roundishphosphatesRice starch0.1-5elongatedPotato starch10-15ovoid potato-shapedWheat starch2-10grain-shaped

Furthermore, the coatings schematically illustrated inFIGS.2and3may also comprise, in addition to the closed pores3,30, 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 coating2in 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 coating2down to the upper surface10of the substrate1.

FIGS.3aand3bschematically illustrate embodiments in which the coating deposited on the glass1includes two pores,32and33, respectively. In both cases, these are closed pores.FIG.3ashows an embodiment with largely spherical pores32. Such pores may be obtained by using calcium carbonate as a blowing agent, for example. In contrast, the pores33shown inFIG.3bhave 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.

FIGS.4a-4dshow several photographs which are light microscopic images of coatings2according 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, inFIG.4a, was fired at approximately 750° C., and the next sample on the left, inFIG.4b, was fired at approximately 720° C. The two samples on the right, inFIGS.4cand4d, show coatings which were printed using a 100 mesh screen. The sample shown half right, inFIG.4c, was fired at approximately 750° C., the samples shown far right, inFIG.4d, was fired at approximately 720° C.

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.

FIG.5shows a micrograph of a section through a substrate1to which a coating2was applied that includes closed samples3. Sodium hydrogen phosphate was added to the glass2as a blowing agent, in an amount of 10 vol %.

FIGS.6ato6dshow further images of a coating2including closed pores3. The coating was obtained by mixing glass1with calcium carbonate. For the samples ofFIGS.6aand6b,5 vol % of calcium carbonate was added to the glass1, for the samples ofFIGS.6cand6d10 vol % of calcium carbonate. The samples ofFIGS.6aand6cwere fired at approximately 750° C., the samples ofFIGS.6band6dat 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.

FIG.7shows transmittance profiles of several glass ceramic substrates in the wavelength range from approximately 300 nm to 5000 nm. Curve4shows 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.

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

Curve5shows the case of a substrate which in its non-coated state exhibits a transmittance similar to that of curve4, and for which a cobalt-iron spinel with nanoscale particle size was used as a pigment. No blowing agents were added. Curve6represents 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.

Curve7represents a coating which, instead of the pigment used for the coating of curve6, comprises Co—Mn—Fe—Cr spinel pigment (d50˜0.5 μm), with an otherwise unchanged composition. For curve8, chromium-iron-nickel black spinel (d50˜1-2.5 μm) was used as the pigment, with an otherwise unchanged composition.

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 curves6to8. So, transmittance of the coated substrate is further reduced by the pores, and hence opacity is increased. Opacity represents the reciprocal of transmittance.

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)).

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.

FIG.8shows 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.

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.

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.

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°.

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

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.

FIG.9schematically shows a possible configuration of a cooking oven door. Here, the outer pane100has a porous coating3on one side thereof. The non-coated side of the substrate faces outwards. The intermediate pane101and the inner pane102of the oven door are coated with a coating9on one side thereof. Coating9may include transparent conductive oxides, for example.

FIG.10schematically shows a measurement setup for determining the surface temperatures of a coated glass sheet under laboratory conditions. In this case, a laboratory oven12is heated to a temperature of 450° C. The oven has an opening with a diameter of 3 cm. The glass pane1with the coating2to be measured is placed at a distance of 2.5 cm from this opening with the coating2facing the opening of the oven. The surface temperature of the coated glass sheet1is determined using a pyrometer13(impac, IE 120/82L), with the focal point adjusted to the outer surface of the decorated sheet. The pyrometer13is arranged behind a glass substrate14and at a distance of 50 cm from the glass sheet1to be measured.

FIGS.11and12show 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 inFIG.10.

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

Curves15,16, and17correspond to temperature profiles of comparative examples in which the coating has IR-reflecting pigments but is not porous. Curves18to21can be associated with temperature profiles of exemplary embodiments in which the coating includes closed pores and IR-reflecting pigments.

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, glass1from 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.

BlowingType ofTmax(° C.)Optical densityL*a*b* (SCE;Pigmentagentblowingafter 1 h(glass side(coated sideGlossSclerometerSidolin testCurve(vol %)(vol %)agentat 450° C.facing upwards)facing upwards)(60°)(10N)(porosity)16200N/A47.62.634.63/0.45/−3.7118.5okvery good1717.50N/A46.92.136.04/0.44/−4.0955.9okvery good2117.520CaCO343.82.234.63/0.45/−3.714.0okgood2017.510CaCO343.22.631.58/0.49/−3.5617.5okgood1917.520rice starch45.72.121.38/0.6/−1.7541.0okvery good1817.510rice starch45.42.618.56/0.65/−1.9550.5okvery good
Characterization of the samples shown inFIGS.11and12

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 inFIGS.11and12were obtained using the measurement setup shown inFIG.10. The respective coating compositions were applied to the substrate by screen printing using a 77T mesh.

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 samples18to21, lower temperatures were measured than for the comparative samples15to17with a dense coating. The temperature difference measured after 60 minutes of operation between the dense sample15and the porous sample20is more than 4° C. It is assumed that the pores within the coating represent structures which additionally scatter the IR radiation.

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 samples18and19was rice starch, while CaCO3was used in samples20and21. When rice starch is used as a blowing agent, anisotropic pores with an ellipsoidal cross section will preferably be formed, while the use of CaCO3as a blowing agent leads to largely spherical pores (cf.FIGS.2and3).

FIG.12shows that for the coated glasses20and21which have pores of spherical or largely spherical shape, the isolation effect is higher than for the coated glasses18and19that have a coating with ellipsoidal or rice-shaped pores.

FIG.12moreover shows that the percentage of blowing agent in the paste has an impact on the IR reflection of the corresponding coating. Samples20and21only differ in their content of blowing agent. While the amount of blowing agent in the paste for producing coating21is 20 vol %, the corresponding paste for producing coating20contains only 10 vol % of CaCO3as the blowing agent. And, sample20exhibits a better isolation effect than sample21, so that under comparable conditions and after an operating time of 180 minutes the maximum temperature of sample20is lower than the maximum temperature of sample21by 0.8° C.

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.

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 inFIG.10on the outer surface of the pane after 60 minutes of operation of the oven at a temperature of 450° C.

PercentageTmax(° C.)Percentageof blowingafter 1 hOptical densityL*a*b* (SCE;SampleNumber ofof pigmentagentBlowingheating(glass side(coated sideGlossSclerometerSidolin testIDprints[vol %][vol %]agent usedat 450° C.facing upwards)facing upwards)(60°)(10N)(porosity)21117.520CaCO343.82.234.63/0.45/−3.714.0okgood21a217.520CaCO341.33.236.04/0.44/−4.091.4okgood19117.520rice starch45.72.121.38/0.6/−1.7541.0okvery good19a217.520rice starch45.24.223.49/0.42/−2.1535.4okvery good18117.510rice starch45.42.618.56/0.65/−1.9550.5okvery good18a217.510rice starch44.94.719.86/0.40/−2.3848.9okvery good

LIST OF REFERENCE NUMERALS

1Glass or glass ceramic substrate2Coating including closed pores3,30Closed pores4Transmittance profile of non-coated substrate5Transmittance profile of substrate not coated according to the invention6,7,8Transmittance profiles of substrates coated according to embodiments of the disclosure9Coating comprising conductive oxides10Surface of substrate112Laboratory oven13Pyrometer15,16,17Temperature profiles of comparative examples with dense coatings18,19,20,21Temperature profiles of exemplary embodiments with porous coatings100Outer oven door pane101Intermediate oven door pane102Inner oven door pane