WO2025008392A1 - Coated substrate, paste, and method for producing said paste - Google Patents

Abstract

The invention relates to a coated substrate made of glass or a glass ceramic, wherein the coating has pigment particles embedded in a glass matrix, wherein the coating has an average layer thickness of 2 to 5 µm, wherein the coating has a transmittance (formula (I)) of at most 2%, wherein the pigment particles have an average particle size d₅₀ of less than 1.0 µm, wherein the polydispersity index PI of the particle size distribution of the pigment particles is at most 2.0.

Description

Coated substrate, paste and method for producing the paste

Description

The invention relates to a coated substrate made of glass or a glass ceramic, as well as a paste for producing such a coated substrate and a corresponding method for producing the paste.

Coated substrates made of glass or glass ceramic and in particular transparent substrates made of glass or glass ceramic with a coating in the form of a decoration are known in the prior art in a variety of designs.

For example, such coated substrates made of glass and glass ceramic are used as fireplace viewing panels and create a separation between the combustion chamber of the fireplace stove and the surrounding area. A coating in the form of an opaque decoration is often applied to an edge area of the substrate and serves to conceal the transition areas between the fireplace viewing panel and the stove door in which the fireplace viewing panel is mounted.

The use as a fireplace viewing panel places high demands on the decoration and substrate. When the fireplace is in operation, the fireplace viewing panel is sometimes exposed to high temperatures of well over 100 °C. Heating the substrate and decoration in this way usually causes a non-negligible thermal expansion of the substrate and decoration, which can lead to the decoration detaching from the substrate if the extent of the expansion varies. Glass-ceramic substrates are often used as fireplace viewing panels, with a thermal expansion coefficient in the range of 0 to 2 x 10' ⁷ K' ¹ in a temperature range of 20-700 °C. In another application scenario, coated glass substrates are also used in vehicle construction in the form of viewing windows, for example. Here, too, the glass substrates are only provided with a decorative coating in certain areas, with the coating usually serving to cover mounting areas of the window or wires running in the window or electronic components located behind the window. The substrates used in such applications are often made of borosilicate glass or soda-lime glass and have thermal expansion coefficients in the range of 10' ⁶ K ^-1 to 10' ⁵ K ^-1 .

Coated glass substrates are also used as viewing windows for oven doors, although in this case substrates made of borosilicate glass or soda-lime glass are also often used.

In the application scenarios described above for decorations to conceal certain areas, the basic requirement for the decoration is to be completely opaque, i.e. not to be seen through. In order to produce such opaque layers on transparent substrates, enamel systems with a high degree of pigmentation and a layer thickness of at least 5 pm are often used. An enamel usually consists of a melted glass frit and pigment(s), whereby a paste comprising the glass frit, the pigment and a binder is applied to the substrate and then fired to produce the enamel. During the firing process, the glass frit melts and forms a glass matrix that is bonded to the substrate and in which the pigment is embedded.

At typical process temperatures for firing such a decorative layer of 600-950°C, a glass frit is required that melts well in this range. Such glass frits usually have a coefficient of thermal expansion (CTE) of at least 4 x 10' ⁶ K' ¹ or more. Pigments that are stable in such a firing process are usually oxides, especially above all non-ferrous metal spinels. Such spinels have a CTE of around 10 x 10' ⁶ K' ¹ . The resulting CTE of the decorative layer is then around 6 x 10' ⁶ K ^-1 and, at the above-mentioned layer thicknesses of more than 4 pm, creates a tension between the substrate and the decorative layer, which can lead to a reduction in the mechanical strength of the component.

In contrast, the present invention is based on the object of specifying a coated substrate and a paste for producing such a coated substrate, which overcomes the above-mentioned disadvantages of the prior art and at the same time provides a decoration that is opaque and at the same time resistant to thermal stress.

In a first aspect, the invention relates to a coated substrate made of glass or a glass ceramic, wherein the coating has pigment particles embedded in a glass matrix, wherein the coating has an average layer thickness of 2 to 5 pm, preferably 2 to 4 pm, particularly preferably 2 to 3.5 pm, wherein the coating has a transmittance Tvis of at most 2%, preferably at most 1%, particularly preferably at most 0.5%, very particularly preferably at most 0.3%, wherein the pigment particles have an average particle size dso of less than 1.0 pm, preferably less than 0.8 pm, particularly preferably less than 0.6 pm, wherein the polydispersity index PI of the particle size distribution of the pigment particles is at most 2.0, preferably at most 1.5, particularly preferably at most 1.

A particle size dso of less than 1.0 pm, in particular less than 0.8 pm, and especially less than 0.6 pm of the pigment particles in conjunction with a polydispersity index of maximum 2 results in a particularly high opacity even with a low layer thickness. A low layer thickness in turn has the advantage that the risk of mechanical failure of the coating due to different thermal expansion coefficients of the coating and substrate is reduced. The opacity of a coating is understood to be the reciprocal of the transmission.

The specification of the particle size dso as less than 1.0 pm describes that when looking at all the pigment particles in the coating, 50% of the particles have a particle size of less than 1.0 pm. The particle size is usually measured using laser diffraction, with the diameter determined in this way indicating the diameter of a spherical pigment that has the same scattering behavior as the determined pigment. Because the particles are usually not perfectly round, for example in the case of a pigment particle with a determined diameter of 1.0 pm, the dimensions of the pigment particle in different spatial directions can deviate from this value. In this case, one also speaks of a volume-equivalent sphere diameter.

The polydispersity index PI of the particle size distribution in the sense of the invention is understood to be the decimal logarithm of the quotient of dgo and di o values of the distribution and is calculated as follows:

PI = log (dgo/dio)

In this case, the polydispersity index provides information about how much the particle sizes of the pigment particles fluctuate around a mean value. The closer the value of the polydispersity index approaches the value 0, the more homogeneous the particle size distribution is. A homogeneous particle size distribution also enables a more homogeneous distribution of the pigment particles in the coating, so that a high pigment density in the layer can be achieved with less pigment and therefore a lower layer thickness. This in turn causes the coating to be highly opaque. With a value of the polydispersity index PI of the particle size distribution of the pigment particles of a maximum of 2.0, preferably a maximum of 1.5, particularly preferably a maximum of 1, a particularly homogeneous and therefore opaque layer is achieved.

The specified value of the transmittance Tvis refers to a wavelength of 700 nm of light incident on the coating and can be determined, for example, using a clamp photometer.

The substrate can be any form of glass or glass ceramic. In particular, borosilicate glass can be used. Such a borosilicate glass can, for example, have a composition with the following components, each given in % by weight on an oxide basis:

SiO2 60 to 85, particularly preferably up to 82

B2O3 7 to 26

AI2O3 0 to 12, preferably greater than 0 to 11, particularly preferably 0 to 7

Ü2O 0 to 1

Na2Ü 0.5 to 6

K2O 0 to 3

MgO 0 to 6

CaO 0 to 5

SrO 0 to 4

ZnO 0 to 3

ZrÜ2 0 to 3.

Furthermore, other components that are usually used in glass production can also be included, such as refining agents. These are usually included in the glass in a content of no more than 2% by weight. For the following compositions, deviations from 100% in the total weight fraction may occur due to rounding errors in the analysis.

An exemplary glass is given in the following composition range in wt.% on an oxide basis:

SiO ₂ 75 - 85

B2O3 10-15

AI2O3 1-3

Na ₂ O 2-5

K2O 0-1

NaCI less than 0.5

An exemplary composition of a glass in this composition range in wt.% on an oxide basis is given as follows:

SiO ₂ 80.8

B2O3 12.7

AI2O3 2.4

Na ₂ O 3.5

K2O 0.6

NaCI 0.1

Another glass is given in the following composition range in wt.% on oxide basis:

SiO ₂ 73-83

B2O3 8-12

AI2O3 1-4

Na ₂ O 2-4

K2O 1-3 MgO 1 -3

CaO 1 -3

Another exemplary composition of a glass in this composition range in wt.% on an oxide basis is given as follows:

SiO ₂ 78.1

B2O3 9.8

AI2O3 2.5

Na ₂ O 2.8

K2O 2.5

MgO 1.8

CaO 2.5

Such glasses in the composition range mentioned above, in particular with the concrete compositions mentioned above as examples, are advantageous because they not only have thermal expansion coefficients, which can advantageously be between 2 * 10' ⁶ /K and 6 * 10 ^-6 /K, depending on the exact composition, but also because they can have sufficient mechanical resistance, for example also to surface loads.

Preferably, however, the substrate is a glass ceramic, in particular a lithium aluminum silicate (LAS) glass ceramic. Such glass ceramics are known in many different designs in the prior art and are characterized by very low thermal expansion, with the corresponding thermal expansion coefficient of such a LAS glass ceramic usually being in a range from 0 to 2 x 10' ⁷ K ^-1 . In principle, the substrate can be either a transparent or a colored substrate. However, transparent substrates are particularly preferred. By way of example, such a substrate may have a composition comprising the following components, each given in % by weight on an oxide basis:

SiO ₂ 65 - 69

AI2O3 18 - 24

Li ₂ O 3.0 - 4.2

Na ₂ O 0 - 0.8

K2O 0 - 0.5

MgO 0 - 1 ,5

BaO 0.5 - 1.2

ZnO 1.0 - 2.0

TiO ₂ 1.8 - 3.0

ZrO ₂ 1.5 - 2.5

The refining of a green glass to produce a glass ceramic according to the composition described above can be carried out, for example, with AS2O3 or _SnO2 . Accordingly, an exemplary substrate can further contain 0.5 - 1.5 wt.% AS2O3 or 0.05 - 0.5 wt.% _SnO2 .

The substrate is preferably disk-shaped, with a thickness of 2 mm to 10 mm, preferably 2.5 mm to 6 mm, particularly preferably 4 mm. In the context of the present disclosure, a disk is generally understood to mean a plate-shaped molded body. A glass pane (which can be coated or uncoated) is a pane comprising or made of glass. A molded body is plate-shaped if its spatial dimensions in one spatial direction of a Cartesian coordinate system are at least one order of magnitude smaller than the spatial dimensions in the two other spatial directions of the Cartesian coordinate system perpendicular to the first spatial direction. In other words, the thickness of the molded body is at least one order of magnitude smaller than its length and width. The two main surfaces or main surfaces of the disk, i.e. those whose size is determined by length and width are also referred to simply as sides in the context of the present disclosure. In principle, the substrate can also be a curved disk.

A “curved disc” is understood to mean both curved discs and discs that are otherwise rounded. Furthermore, such a curved disc can have both a constant radius of curvature and a profile with locally variable curvature. A disc with one or more angularly curved sections would also be understood as a “curved disc” in the above sense.

The coating can in particular be designed as a decoration that is not applied to the entire surface of the substrate, but is only deposited on partial areas of the substrate surface. Such a decoration can be composed of one or more coated areas. According to one embodiment, it can be provided that the surface coverage of the substrate with the coating is at least 10% and at most 80%, preferably at least 15% and at most 65% of the entire surface of the side of the substrate on which the coating is applied.

In particular, the coating can be a surrounding decoration in the edge area of the substrate. Furthermore, a pattern can also be applied to the substrate over the entire surface. The entirety of the decorations on the substrate is understood as a "coating". In principle, there is no limitation as to which side of the substrate the coating is arranged on. In particular, it is also possible in principle for a coating to be applied to both sides of the substrate, whereby the coating on the top of the substrate does not have to be identical to the coating on the bottom of the substrate. In the context of the present disclosure, a pigment is understood to mean a particle-based color body. The pigment according to the present disclosure is also advantageously designed to be temperature-stable, and is preferably a ceramic color body. In the context of the present disclosure, a color body (or pigment) is generally understood to mean that the color body consists of particles, which can also be referred to here as pigment particles. If it is therefore stated in the context of the present disclosure that a coating comprises a pigment, this is understood to mean that the coating comprises particles of a specific pigment or color body, i.e. particles with the composition of the pigment or color body.

wobei V_P das Volumen des Partikels und A_P seine Oberfläche bezeichnet. According to a further embodiment, a uniform distribution of the pigment particles in the decoration is supported by the pigment particles having a sphericity MJ of at least 0.6, preferably at least 0.7, particularly preferably at least 0.8. The sphericity MJ is a parameter for how spherical the particle is. The sphericity MJ of a particle is preferably calculated as the ratio of the surface of a sphere of the same volume to the surface of the particle:

where V _P is the volume of the particle and A _P is its surface area.

Exemplary sphericity values M-J for different particle types are:

ball: 1 ,0

Drop, bubble, round grain: 0.7 - 1.0

Angular grain: 0.45 - 0.6

Needle-shaped particle: 0.2 - 0.45

Platelet-shaped particle: 0.06 - 0.16 Particles with a highly fractured surface: 10 ^-8 — 10' ⁴

High sphericities M-J in the sense of the present invention are achieved when M-J has a value of at least 0.6. The more spherical the pigment particles in the coating are, the more densely the pigment particles can be packed in the layer. This in turn means a high opacity with a small amount of pigment particles.

In particular, the interplay of a low polydispersity index of less than 2.0, preferably about 1.0, and a high sphericity of at least 0.6, preferably at least 0.8, has a particularly advantageous effect on the opacity of the coating at low layer thicknesses. In this case, a spherical, preferably polyhedral shape of the pigment particles in conjunction with a uniform size distribution of the pigment particles can achieve a very regular and dense arrangement of the pigment particles in the coating. In this way, even very thin layers can provide an opacity that corresponds to the claimed limit values, while the thermal resistance of the coating is improved by the low thickness of the layer.

The combination of a small diameter Dso of the pigment particles and a low polydispersity index with high sphericity MJ of the pigment particles also results in a very large specific surface area of the pigment particles in the coating of 35 to 55 m ² /g, which also has a beneficial effect on the scattering properties of the layer and thus on the opacity. A further effect of the described high sphericity and low polydispersity index of the pigment particles is that the pigment particles in the coating form a very fine porous structure. In this way, in particular, different CTE of the coating and substrate can be compensated, since stresses in the coating resulting from the different thermal expansion are at least can be partially compensated. Accordingly, the described design of the pigment particles also directly supports the mechanical and thermal strength of the coating.

According to a further embodiment, the homogeneity of the distribution of the pigments in the coating is also shown in that the coating does not have any pigment agglomerates that have a diameter of more than 1.0 pm, preferably more than 0.9 pm, particularly preferably more than 0.8 pm, very particularly preferably more than 0.7 pm. An agglomerate is understood to be a collection of pigments in which the pigments are in direct contact, so that there is an area between the pigments in which there is no binding agent. The maximum diameter is understood to be the maximum extent of the agglomerate in any spatial direction.

The previously described properties of the pigment particles in the decorative layer, in particular the uniform distribution of the particles and the high sphericity of the particles, are also reflected in other properties of the decorative layer, in particular with regard to its surface. According to a further embodiment, it is further provided that the coating has a waviness Wt of less than 0.3 pm. The waviness Wt corresponds to the vertical distance from a lowest point of the coating surface relative to the substrate to a highest point of the coating surface relative to the substrate. The waviness provides direct information about the fluctuation in the layer thickness, which in turn correlates with the homogeneity of the distribution of the pigments. The waviness of the coating surface can be determined in accordance with DIN EN ISO 4287.

Furthermore, the uniform distribution of the pigment particles in the decorative layer is also reflected in a very low roughness Ra of the decorative layer according to DIN EN ISO 4287, which according to a further embodiment is less than 0.4 pm, preferably less than 0.35 pm, particularly preferably less than 0.3 pm. A low waviness has the advantage that the optical impression of the coating and in particular the transmission is evenly distributed across the coating. Furthermore, a low waviness of the surface also has a positive effect on the abrasion resistance and generally the mechanical durability of the coating.

According to a further embodiment, it is further provided that the coating contains 7.5 to 50 vol.%, preferably 15 to 40 vol.%, particularly preferably 25 to 38 vol.% pigment particles. In this range, a particularly good compromise is achieved between mechanical hardness of the layer and low transmission. The proportion of pigment particles in the coating can also depend on the substrate. Thus, the coating for a substrate made of glass, for example borosilicate glass, particularly preferably has a proportion of 20 to 26 vol.% pigment particles in the coating, while the coating for a substrate made of glass ceramic particularly preferably has a proportion of 32 to 38 vol.% pigment particles in the coating.

When using a glass-ceramic substrate, a high impact resistance of the coating can be achieved. When using a substrate made of glass, in particular borosilicate glass, a high bending tensile strength of up to 60 MPa can also be achieved, which represents an improvement of more than 50% compared to coatings for borosilicate glasses known in the prior art. The bending tensile strength is preferably determined using a double ring method in accordance with DIN 1288-5. The respective area of the coating subjected to the measurement is preferably coated over the entire surface, so that uncoated areas of the substrate had essentially no influence on the measurement results. According to a further embodiment, it is further provided that further coatings different from the coating are arranged on the substrate. These further coatings can in particular be functional layers, such as layers with a high reflectivity in the infrared spectral range. The sequence and arrangement of the coatings is initially not restricted. It can thus be provided that the further coatings are arranged on the same side of the substrate as the previously described coating, or alternatively on the opposite side. Furthermore, further coatings can also be arranged on both sides of the substrate. It is also possible for the further coatings to overlap completely or only partially with the described coating. It is also fundamentally possible for a further coating to be arranged above and/or below the described coating, so that a stack of coatings of any configuration is created.

In a further aspect, the invention relates to a paste for producing a coated substrate, as described above, comprising a glass-based binder, a pigment and a carrier medium, the pigment particles having a particle size dso of less than 1.0 pm, preferably less than 0.8 pm, particularly preferably less than 0.6 pm. In the context of the present disclosure, a paste is understood to mean in particular a so-called decorative or color paste. To produce a coated substrate according to the invention, the paste can be applied to the areas of the substrate to be printed using a suitable printing process, in particular screen printing, and then fired. If the substrate of the end product is a glass ceramic, the paste can be fired both together with the ceramization of the substrate, i.e. the conversion of the green glass to a glass ceramic (also referred to as primary firing), and subsequently, i.e. after the ceramization of the substrate (also referred to as secondary firing). The glass-based binder can in particular be a glass frit, i.e. finely ground glass powder with a grain size of a maximum of 5 pm, preferably between 0.5 pm and 1.5 pm, which melts on the substrate during the firing of the paste and in this way forms a glass matrix in which the pigment particles are embedded. A layer created in this way is also referred to as enamel. An example of a glass frit can contain the following components in % by weight on an oxide basis: SiO ₂ 10 to 70

B2O3 10 to 26

AI2O3 more than 0 to 9.

The specific design of the glass frit and in particular its chemical

The composition can be tailored to the substrate to be printed on. Particular attention can be paid to keeping the difference between the CTE of the glass frit and the CTE of the substrate as small as possible.

In general, it is also possible for the coating to comprise, in addition to a glass frit, another glass-based binder that does not consist of a glass frit or is not derived from it, for example a sol-gel-based binder. In this case, the binder of the coating is formed as a mixture of the different binders.

The pigment particles can be, for example, metal oxides such as spinels, in particular non-ferrous metal spinels, or hematites. For example, the pigment can contain cobalt oxide (CoO), chromium oxide (Cr2O3), manganese oxide (MnO) and iron oxide (Fe ₂ O3), with the following ranges being preferred in terms of weight percent on an oxide basis: Co 20 - 35, Cr 15 - 25, Mn 20 - 35,

Fe 20 - 30. The following compositions can be used as black pigments:

(Fe, Mn)3O4

(Cr,Fe)(Fe,Co) ₂ O ₄

CuCr ₂ O4

(Ni,Fe)(Fe,Cr) ₂ O ₄

(Fe,Mn) ₂ O3

CrFe ₂ Ü4

Solvents with a vapor pressure of less than 10 bar, in particular less than 5 bar and especially less than 1 bar, are preferably used as the carrier medium or dispersion medium for screen-printable coating solutions. These can be, for example, combinations of water, n-butanol, diethylene glycol monoethyl ether, tripropylene glycol monomethyl ether, terpineol, n-butyl acetate. In order to be able to set the desired viscosity, appropriate organic and inorganic additives are used. Organic additives can be, for example, hydroxyethyl cellulose and/or hydroxypropyl cellulose and/or xanthan gum and/or polyvinyl alcohol and/or polyethylene alcohol and/or polyethylene glycol, block copolymers and/or triblock copolymers and/or tree resins and/or polyacrylates and/or polymethacrylates. In general, commercially available screen printing media based on e.g. glycol or terpineol are suitable, but others are also suitable.

According to a further embodiment, it is further provided that the paste contains the following proportions in % by weight:

Solid (consisting of binder and pigment): 35 - 60 Carrier medium 40 - 65 (particularly preferred 45 - 62.5) This composition is particularly preferred when a pseudoplastic carrier medium with viscosities between 2 and 3.5 Pas at 50/s is used.

Alternatively, the paste contains the following proportions in % by weight:

Solids (consisting of binder and pigment): 60-80 Carrier medium 20 - 40. (particularly preferred 25-33) wt% This composition is particularly preferred when a Newtonian screen printing medium with viscosities between <1 Pas between 50/s and 300/s is used.

The above-mentioned ranges for the proportions of solid and carrier medium in the paste were chosen with the aim of setting a preferred paste viscosity of 2500-4000 mPas. Since the paste viscosity in turn depends on the properties of the carrier medium used, the weight ratios of the composition are preferably adapted to the carrier medium used.

In both cases, the solid consists of 30-70% by weight of binder (frit) and 70-30% by weight of pigment and thus determines the final layer composition.

Analogous to the above description of the coated substrate, with regard to the paste, a preferred embodiment provides that the polydispersity index PI of the particle size distribution of the pigment particles is a maximum of 2.0, preferably a maximum of 1.5, particularly preferably a maximum of 1.

Also analogous to the above description of the coated substrate, according to a further embodiment it is further provided that the pigment particles have a sphericity MJ of at least 0.6, preferably at least 0.7, particularly preferably at least 0.8. The application of the paste to a substrate for producing the previously described coated substrate is improved according to a further embodiment in that the paste has a viscosity of 2000 to 8000 mPas, preferably 2500 to 6000 mPas, very particularly preferably 3000 to 4500 mPas at a shear rate of 200/s. The viscosity of the paste can be determined, for example, using a plate viscometer. With the viscosity values described, the paste can be applied particularly well to substrates, in particular to glass panes, on a large scale using conventional application methods. The paste can advantageously be applied to the substrate using a printing method such as screen printing. Other printing methods such as screen printing, pad printing, jet printing or inkjet printing can also be used here.

In a further aspect, the invention relates to a method for producing a paste as described above. The method comprises the following steps:

• Production of a starting powder from a precursor solution, wherein the particles contained in the starting powder have a particle size of at most 20 nm,

• Calcination of the starting powder to remove possible volatile residues and allow crystallites to grow

• Drying of the calcined starting powder,

• Mixing the dried powder with the binder and the carrier medium to produce the paste.

The precursor solution is preferably a metal-containing nitrate, sulfate, carbonate or chloride solution, whereby the precursor solution preferably contains Mn and/or Fe and/or Co and/or Ni and/or Cu and/or Cr. The starting media for pigment production are salts of the metal types desired for pigment production, which are dissolved in water, whereby the optical properties of the pigment can be influenced by the selection of the metal types.

Alternatively, the precursor solution can also be a solvent-based system, for example on a sol-gel basis.

Different approaches can be used to produce the starting powder. For example, (co)precipitation of the precursor solution, but also processes such as flame hydrolysis, thermal spray drying, gas condensation, laser ablation, plasma spray methods (CVS processes), sol-gel methods, hydrothermal methods, combustion or similar can be used. The particles produced are initially in the very fine nanometer range and have a grain size of around 2-20 nm.

When using a precipitation reaction, the grain shape and grain fraction in the starting powder are highly dependent on the precipitation conditions. By choosing the precipitation medium (carbonate precipitation, hydroxide precipitation, oxalate precipitation) of a nitrate or chloride solution, for example, a wide range of different starting powders can be produced. Powders of different qualities and starting properties (e.g. special surfaces) can also be achieved by using different drying methods for the filter cake produced from the precipitation (simple drying in air, freeze drying, azeotropic distillation). During precipitation, a large number of other parameters (pH value, stirrer speeds, temperature, precipitation volume, etc.) also influence the starting powder produced and its physical and chemical properties. Ideally, the particles of the starting powder are only connected to one another by weak bridges in the form of sinter necks. According to a preferred embodiment, the particles of the dried powder have a sphericity MJ of at least 0.6.

According to a particularly preferred embodiment, the starting powder is produced by means of a hot gas process, in particular with the aid of a pulsation reactor. The use of a pulsation reactor to produce the starting powder has the advantage that the properties of the particles of the starting powder can be adjusted very precisely by selecting the process parameters of the pulsation reactor accordingly. At the same time, the second process step of calcining the starting powder can also be carried out directly using the pulsation reactor.

The calcination serves to remove volatile residues (carbonates, sulfates, etc.) that remain in the particles of the starting powder, for example after precipitation of the precursor solution. At the same time, the calcination serves to fully form the crystal phases of the particles. The calcination can also be carried out in a chamber furnace, rotary kiln or roller kiln at temperatures between 800°C and 1400°C, in particular 900°C and 1200°C, particularly preferably 900°C and 1100°C, in particular 900 to 1000°C, or 950 to 1100°C for a period of 0.5h to 12h, preferably 0.5 to 10h, in particular 2h to 8h, particularly preferably 3h to 6h. Furthermore, the calcination can also be carried out over a period of 0.5 to 8 hours, particularly preferably 0.5 to 5 hours.

Calcination can cause the powder particles to cake together, depending on the method used, so that a subsequent de-agglomeration step may be necessary, which can be achieved in particular by a grinding process. Grinding can be carried out dry or wet, whereby in the case of wet grinding, alcohols, liquid hydrocarbons such as heptanes or water-based media can be used. The grinding times can be up to 24 hours, but should be chosen so that It must be ensured that no abrasion can occur from the grinding media or the grinding drum lining, as this can lead to contamination of the particles with Al2O3 or ZrÜ2, for example, depending on the material of the grinding media/grinding drum. For dry grinding, resonance mixers, sieve mills, jet mills, tumblers or a roller bank can be used, for example, while wet grinding can be carried out using an annular gap mill, disk mill, attritor, basket mill or agitator cone mill.

The mixture can again be dried in air at low temperatures. For example, the grinding suspension can be dried by spray drying. Spray drying preferably produces soft agglomerates, which can be easily crushed in the three-roll mill in the subsequent paste production. Drying can also be carried out using a drying cabinet, freeze dryer or rotary evaporator. The particles of the powder after drying preferably have an average particle size dso of less than 1 pm.

To produce the paste used to manufacture the coated substrate, the dried powder is mixed with the binder and the carrier medium of the paste.

The paste is preferably designed such that the glass frit encompassed by the paste has a linear thermal expansion coefficient of between at least 2*10' ⁶ /K and at most 10*10' ⁶ /K, preferably between at least 3*10' ⁶ /K and at most 6*10' ⁶ /K. In this way, it is possible to obtain coatings when using the paste which enable the glass pane to be sufficiently strong, particularly in the case of preferably opaque coatings. Thermal expansion coefficients in the range between at least 3*10' ⁶ /K and at most 6*10' ⁶ /K are particularly advantageous here. Screen printing is preferably used as the printing method for applying the paste to the substrate. A screen thickness of 54-64, 54-70, 68-64, 68-70, 71-55, 77-48, 77-55, 100-40, 110-34W, 110-35Y, 120-35W, 130-34Y, 140-31, 140-31 W or 165-27Y is preferably used to achieve the layer thicknesses described. A screen thickness of 100-40, 110-34W or 140-31 is particularly preferred. This corresponds to a theoretical ink volume of the screen used of 10 to 30 cm ³ /m ² .

After the paste has been printed onto the substrate, the layer created in this way is fired to produce the coating. The carrier medium essentially evaporates in the process, so that the composition of the coating essentially results from the proportions of solids in the paste used. With the composition ranges of the paste described above, coatings are produced which consist of 30-70% by weight of binder, i.e. the melted glass frit, and 70-30% by weight of pigment. This is equivalent to proportions of 85 and 50% by volume of binder and 15 - 50% by volume of pigment in the coating. A binder content in the coating of between 80 and 60% by volume is preferred, particularly preferably between 75 and 60% by volume, which corresponds to preferred proportions of pigment in the coating of 20 - 40% by volume, particularly preferably 25 and 40% by volume.

In this way, coatings can be obtained which have sufficient covering power (or opacity) while at the same time having sufficient scratch resistance and adhesion strength of the coating - particularly for the thicknesses of the coating disclosed here.

In the following, preferred embodiments of the invention are explained in more detail with reference to the drawings. Figure 1 Detailed images of exemplary pigment particles for producing a coating in comparison to pigment particles from the prior art,

Figure 2 is a sectional view of an exemplary coated substrate compared to a coated substrate according to the prior art,

Figure 3 is a flow chart of an exemplary process for producing a paste,

Figure 4 Detailed images of the particles of an exemplary starting powder,

Figure 5 Detailed images of the particles after calcination at 800°C for 4h, and

Figure 6 Detailed images of the particles after calcination at 1000°C for 4h.,

In the following, identical or functionally similar features are identified with the same reference symbols.

Figure 1 shows detailed images of exemplary pigment particles 100 for producing a coating in comparison to a pigment from the prior art. Figure 1 a) shows pigment particles as used according to the invention to produce an exemplary coated substrate, while Figure 1 b) shows pigment particles 102 as used in comparable coated substrates according to the prior art. The images in Figure 1 were taken using a scanning electron microscope, with the scale of the two images being identical. As can be clearly seen in the comparison of Figures 1 a) and 1 b), the geometries of the exemplary pigment particles in Figure 1 a) differ greatly from the geometry of the pigment particles used to date in Figure 1 b). While the pigment particles 102 used to date have a very irregular shape with sharp edges and very different ratios of length, width and thickness of the particle for each particle, the exemplary pigment particles 100 in Figure 1 a) are much more homogeneous in their shape and generally have a polyhedral shape that largely resembles a sphere. Accordingly, the pigment particles 100 preferably have a sphericity, i.e. a computationally determined "sphericity" MJ of at least 0.6. A sphericity MJ of 1 would describe a perfect sphere. Such a particle shape is particularly advantageous for the scattering cross section of the particles in the coating and thus for the opacity of the coating on the substrate.

In contrast, the pigment particles 102 according to the prior art have a significantly lower sphericity, which is usually less than 0.5. Overall, the particles 102 have a shard shape

Another significant difference between the pigment particles 100 of Figure 1 a) and the pigment particles 102 of Figure 1 b) is the size distribution of the pigment particles 100 and 102, respectively. The size distribution of the pigment particles 100 in Figure 1 a) is very homogeneous, with few outliers, such as the comparatively large particles 104. At the same time, in the embodiment shown, the pigment particles 100 have an average particle size dso of approximately 0.3 pm. In contrast, the pigment particles 102 shown in Figure 1 b) have an average particle size dso of more than 0.6 pm, with the size varying greatly from particle to particle and the particle size being difficult to determine in some cases due to agglomeration of the particles. The pigment particles 100, as shown in Figure 1 a), can be produced, for example, by a process as described below with reference to Figure 3.

Figure 2 a) shows a sectional view of an exemplary substrate 106 with a coating 110 in comparison to a substrate 108 with a coating 112 according to the prior art, which is shown in Figure 2 b). The substrates 106 and 108 are made of the same material. Here too, Figures 2 a) and 2 b) have the same scale, so that a direct comparison of the coatings is simplified. The thickness d of the coating 110 of the substrate 106 indicated in Figure 2 a) is 3.5 pm in the embodiment shown.

When comparing Figures 2 a) and b), it is clearly evident that in the coating 110 of Figure 2 a), the pigment particles 100 are distributed much more homogeneously than the pigment particles 102 in the coating 112 of Figure 2 b). The homogeneity of the particle size distribution is also much better in the coating 110 of Figure 2 a) than in the coating 112 of Figure 2 b). Accordingly, the coating 110 of Figure 2 a) has a significantly improved opacity compared to the coating 112 with the same layer thickness.

A further effect of the homogeneous particle size distribution and the uniform distribution of the pigment particles in the coating 110 of Figure 2 a) can be seen in the comparison of the course of the respective surfaces 114 and 116 of the coatings 110 and 112. While the coating 112 has a strongly wavy surface 116, the surface 114 of the coating 110 is significantly smoother. The reason for this can already be seen in Figure 2 b) in the fact that local agglomerations 118 of pigment particles 102 occur in the coating 112, with relatively large pigment particles 102 being present in these agglomerations. At the same time, there are areas 120 in the coating 112 in which the density of the pigment particles 102 is clearly reduced compared to the rest of the coating.

Figure 3 shows a flow chart 200 of an exemplary method for producing a paste as used to produce the coating of Figure 2a.

In a first process step 202, a precursor solution is first prepared. For this purpose, salts of the starting materials required for pigment production are preferably dissolved so that the precursor solution is a metal-containing nitrate, sulfate, carbonate or chloride solution, with the precursor solution preferably containing Mn and/or Fe and/or Co and/or Ni and/or Cu and/or Cr. The selection of the metals contained significantly influences the optical properties of the pigment and thus of the coating produced. Alternatively, the precursor solution can also be a solvent-based system, for example on a sol-gel basis.

In a subsequent process step 204, a starting powder is produced from the precursor solution. The precursor solution can in particular be an aqueous salt solution in the form of nitrates, carbonates and/or sulfates, and/or the precursor solution can be produced on the basis of a solvent-based system, for example as a sol-gel. In this case, a pulsation reactor known in the prior art can be used, for example, to produce the pigment particles. The particle size can be controlled by a suitable choice of the reaction time in the pulsation reactor.

The particles contained in the starting powder, which were obtained, for example, from the reactor process and usually initially have a few fine crystallites with sizes in the low nanometer range (about 2 - 20 nm), are then subjected to calcination in process step 206. Such calcination is preferably carried out at a temperature of 800 to 1400°C, particularly preferably at 900 to 1200°C, very particularly preferably at 900 to 1100°C, in particular 900 to 1000°C, or 950 to 1100°C for a period of 1 to 12 hours, preferably 0.5 to 10 hours, particularly preferably 2 to 8 hours, very particularly preferably 3 to 6 hours. Furthermore, the calcination can also take place over a period of 0.5 to 8 hours, particularly preferably 0.5 to 5 hours. The calcination serves to remove volatile residues (carbonates, sulfates, etc.) from the starting powder and to fully form the crystal phases.

After the calcination of the pigment particles, deagglomeration of the pigment particles is provided in process step 208. Deagglomeration is usually necessary because the particles of the starting powder can cake together as a result of calcination, so that the particle size after calcination usually does not yet meet the respective requirements. Grinding processes, in particular dry grinding or wet grinding, can be used for deagglomeration.

Subsequently, in process step 210, the powder de-agglomerated according to step 208 is dried. Different drying methods can be used for this, such as spray drying or freeze drying. Furthermore, a drying cabinet or rotary evaporator can also be used.

A pigment produced in this way can, for example, contain cobalt oxide (CoO), chromium oxide (Cr2O3), manganese oxide (MnO) and iron oxide (Fe2Os), with the following ranges being preferred in terms of weight percent on an oxide basis: Co 20 - 35, Cr 15 - 25, Mn 20 - 35,

Fe 20 - 30. The pigment produced in the form of the dried powder is mixed in a final process step 212 with a glass-based binder and a carrier medium (also referred to as dispersion medium) to produce the paste.

Figures 4 to 6 show detailed images of the particles during the production of the pigment at different stages of the process. The figures each show two images at different magnifications, with a scale indicated for each figure at the bottom right edge of the figure.

Figure 4 shows detailed images of the particles of an example starting powder, such as can be produced by precipitation of a precursor solution. The particles are a CoMnFeCr spinel. As can be clearly seen in the images shown, the individual particles are still agglomerated into large clumps and do not yet have a recognizable crystalline structure. This is particularly clearly visible in Figure 4 b) in the area of the agglomerate shown in the right half of the image, in which some of the particles have flaked off the agglomerate.

The agglomerates shown in Figure 4 also contain some volatile residues (carbonates, sulfates, etc.) that come from the precursor solution. These residues are removed during the calcination of the particles. In addition, the calcination stimulates the growth of the crystal phases of the pigment particles. Figures 5 and 6 show detailed images of particles after calcination for different parameters of the calcination process.

In the example shown in Figure 5, the particles were calcined at about 800°C for a period of 4 hours. In particular, the detailed image shows Figure 4 b) clearly shows that the surface of the agglomerate shown has changed compared to the representation in Figure 4 in that the previously comparatively smooth surface appears increasingly porous. Individual particles are already clearly visible in the surface of the agglomerate, whereas this was not yet the case with the agglomerates of particles shown in Figure 4.

In the example shown in Figure 6, the effect of calcination is even clearer. Calcination was carried out at a temperature of 1000°C over a period of 4 hours, which led to a much more pronounced crystal structure of the pigment particles clustered together in the agglomerates shown compared to Figure 5. Grinding the agglomerates thus obtained and subsequent drying of the pigment particles thus obtained then leads to the pigments shown at the beginning in Figure 1 a).

The following examples show the respective optical properties for different layer compositions with different pigments on different substrates.

The following pigment compositions were used for pigment production (data in % by weight):

The pigments were each obtained by a precipitation reaction of a nitrate solution and subsequent calcination at a temperature between 900°C and 1100°C for a period of 3 to 5 hours and subsequent grinding. For a layer composition with a weight ratio of 45 wt.% binder (i.e. a glass frit) and 55 wt.% pigment, the following properties were determined for the different pigments when coating a transparent substrate made of glass ceramic (CERAN® Cleartrans) with a thickness of 4 mm and firing the coating during the ceramization of the substrate (so-called primary firing) at a temperature of about 920 °C:

Pigment 1 :

Pigment 2:

Pigment 3:

Pigment 3:

Pigment 4:

The values L* a* and b* refer to the CIELab color system. The pasting ratio indicates the ratio of solids (pigment and binder) to carrier medium of the paste used to produce the coating.

When using a glass substrate made of BOROFLOAT® 33 and a coating composition of 65% by weight binder and 35% by weight pigment, the following coating parameters were determined after firing the coating at 680 °C:

The optical density refers to the decimal logarithm of the opacity.

Claims

patent claims 1. Coated substrate made of glass or a glass ceramic, wherein the coating has pigment particles embedded in a glass matrix, wherein the coating has an average layer thickness of 2 to 5 pm, wherein the coating has a transmittance Tvis of at most 2%, wherein the pigment particles have an average particle size dso of less than 1.0 pm, wherein the polydispersity index PI of the particle size distribution of the pigment particles is at most 2.0. 2. Coated substrate according to claim 1, characterized in that the pigment particles have a sphericity M-J of at least 0.6. 3. Coated substrate according to claim 1 or 2, characterized in that the coating has a waviness Wt of less than 0.3 pm. 4. Coated substrate according to one of the preceding claims, characterized in that the coating contains 7.5 to 50 vol.% pigment particles. 5. Coated substrate according to one of the preceding claims, characterized in that the coating does not contain pigment agglomerates which have a maximum diameter of more than 1.0 pm. 6. Coated substrate according to one of the preceding claims, characterized in that further coatings different from the coating are arranged on the substrate. 7. Paste for producing a coated substrate according to one of the preceding claims, comprising a glass-based binder, a pigment and a carrier medium, wherein the pigment particles have a particle size d50 of less than 1.0 pm. 8. Paste according to claim 7, characterized in that the paste contains the following proportions in % by weight: Solids (consisting of binder and pigment) 35 - 60 (preferably 37.5 - 55), Carrier medium 40 - 65 (preferably 45 - 62.5). 9. Paste according to claim 7, characterized in that the paste contains the following proportions in % by weight: Solids (consisting of binder and pigment) 60 - 80 (preferably 57 - 75), Carrier medium 20 - 40 (preferably 25 - 33). 10. Paste according to one of claims 7 to 9, characterized in that the polydispersion index PI of the particle size distribution of the pigment particles is a maximum of 2.0. 11. Paste according to one of claims 7 to 10, characterized in that the pigment particles have a sphericity M-J of at least 0.6. 12. Paste according to one of claims 7 to 11, characterized in that the paste has a viscosity of 2000 to 8000 mPas at a shear rate of 200/s. 13. A process for producing a paste according to any one of claims 7 to 12, comprising the steps of • Production of a starting powder from a precursor solution, wherein the particles contained in the starting powder have a particle size of at most 20 nm, • Calcination of the starting powder, • Drying of the calcined starting powder, • Mixing the dried powder with the binder and the carrier medium to produce the paste. 14. The method according to claim 13, wherein the precursor solution is a metal-containing nitrate, sulfate, carbonate or chloride solution, wherein the precursor solution preferably contains Mn and/or Fe and/or Co and/or Ni and/or Cu and/or Cr. 15. The method according to claim 13 or 14, wherein the particles of the dried powder have a sphericity M-J of at least 0.6. 16. The method according to one of claims 13 to 15, wherein the production of the starting powder takes place by means of a hot gas process, in particular with the aid of a pulsation reactor.