Coated colouring spinel pigments, process for manufacturing them and their use.

The invention relates to coated spinel color pigments, the core of which consists of a colored spinel and the coating of which is made of a silicate material with a glassy structure. The invention is also directed to the production of the coated spinel color pigments and their use for the production of sharply contoured ceramic decorations.

Color pigments, which are used to color and decorate porcelain. Stoneware and other ceramic products burned in the presence of glass frits above the softening point of the glass frit must have a high temperature, oxidation and glaze resistance. Decorative colors, which consist essentially of a color pigment and a glass frit, are used in particular to produce decorations with more or less fine patterns.

Color bodies of the spinel type have long been used for coloring glazes and as a component of decorative colors due to their often brilliant colors. Colored bodies crystallizing in the spinel type, examples include Thenards blue (CoAl₂O₄) and copper chromite (CuCr₂O₄), are mostly very color-intensive pigments. Color pigments of the spinel type are generally largely stable in terms of their color in the presence of an aggressive glass flow, but the so-called bleeding effect is disadvantageous. Due to the interaction of the pigment with the glass flow, the pigment is dissolved to a greater or lesser extent, ie components of the pigment migrate into the glass flow surrounding it and color it. This "bleeding" prevents the production of contoured decors. This effect is particularly noticeable with Co-containing spinel CoAl₂O₄.

In order to avoid the problem of bleeding and / or other instabilities during the stoving, such color bodies have already been enclosed in transparent crystals of glaze-stable substances - see DE-AS 23 12 535. This document also states that by heating a mixture of zirconium oxide , Silicon dioxide and finely divided Thenards blue, a light blue inclusion pigment is accessible. The inclusion of spinel color bodies in glaze-stable, colorless zirconium silicate crystals is possible, but only very weak inclusion pigments are obtained in this way.

The object of the invention is therefore to show coated spinel color pigments which have a higher color intensity than the colored spinels enclosed in zirconium silicate crystals. Compared to the non-coated spinel color pigments, the new pigments should not bleed out or at least significantly less bleed when baked, and thus allow the production of decorations with improved contour definition.

The coated spinel color pigments according to the invention are characterized by a core made of a colored spinel and a glassy layer at least partially enveloping this core made of a silicate material which essentially consists of silicon dioxide and one or more oxides from the series of alkali metal, alkaline earth metal and / or earth metal oxides and may additionally contain ions originating from the coated spinel.

The core of the new color pigments is preferably 45 to 60% by weight, the shell 55 to 40% by weight. In principle, pigments are also available outside of these limits, but products with a higher proportion of shell become paler and products with a higher proportion of core show less resistance to bleeding. The coated spinel color pigments according to the invention are a fine powder. The grain size (D50 value) of the pigments is generally above about 1 μm and below 20 μm; in practice, however, pigments with a D50 value below 10 μm, in particular below 5 μm, are preferred.

The coated spinel color pigments contain as a core a colored spinel of the general formula XY₂O₄ or X₂ZO₄, where X is a divalent element from the series Mn, Fe, Co, Ni, Cu, Zn, Mg, Y is a trivalent metal from the series Al, Cr, Fe, Mn and Z for Ti. Two different X-metals and / or two different Y-metals can also be contained in a spinel. Examples include CoAl₂O₄, (Co, Zn) Al₂O₄, Co (Al, Cr) ₂O₄, CoCr₂O₄, Co₂TiO₄, CuCr₂O₄, NiFe₂O₄, (Fe, Zn) Fe₂O₄, (Fe, Mn) (Fe, Mn) ₂O₄. The cobalt-containing spinels are particularly preferred because bleeding is particularly troublesome here. With spinels with a lower and / or less visually disturbing tendency to bleed than the Co-containing spinels, decorations with acceptable contour sharpness can optionally also be obtained without the coating according to the invention.

Pigments with a spinel core that is as completely encased as possible have a high resistance to bleeding. As the degree of sheathing decreases, bleeding resistance understandably decreases. Through intensive grinding of the initially largely coated spinel color pigments, fractures are created through which the color-giving spinel can bleed out during the firing of the ceramic decor. When the spinel color pigments coated according to the invention are produced by the sintering process according to the invention, spinels with a grain size are preferably used such that gentle grinding of the sintered product is sufficient to obtain the desired pigment fineness.

The covering consists of a silicate material and has a glassy, ​​ie X-ray amorphous structure. The components of the coating are essentially SiO₂ and one or more oxides from the series of alkali, alkaline earth and / or earth metal oxides. In addition, ions from the coated spinel may have penetrated into the silicate layer thereof during the production of the color pigments according to the invention. The term "essentially" is understood to mean that at least 95% by weight of the coating layer consist of the aforementioned components, but up to 5% by weight of other conventional glass components can be incorporated in the silicate coating. The coating generally contains one or more oxides from the series Li₂O, Na₂O, K₂O, MgO, CaO and Al₂O₃ as basic oxides, alkali oxides and here in particular K₂O being preferred. The silicate layer enveloping the spinel core is preferably a very SiO₂-rich glass which has the desired high softening point and thus a high resistance to the glass flow. Ingredients that are known to lower the softening temperature of glasses, such as lead oxide and boric acid, are not or only in a small amount contained in the silicate coating.

The spinel color pigments according to the invention can be produced in a simple manner by using a homogeneous powder mixture of the colored spinel, silicon dioxide and at least one mineralizer from the series of alkali metal, alkaline earth metal and / or earth metal halides at 900 to 1300 ^OC sinters for 0.5 to 5 hours and, if necessary, grinds the sintered product wet or dry. The atomic ratio of silicon to alkali metal in the silicate layer of coated spinel color pigments obtained using alkali chloride as the mineralizer is in the range from about 6 to 1 to 60 to 1, preferably about 20 to 30 to 1. Only a small part of the alkali ion present in the process is incorporated into the silicate network; the greater part can be washed out of the pigment with water.

The spinel used and preferably also the other constituents are used in finely divided form - D50 value 0.1 to 20 μm, preferably 0.5 to 10 μm. The spinel to be coated particularly preferably has a D50 value of 0.5 to 2 μm; with a suitable selection of the coating components and management of the sintering process, after careful grinding of the sintered product, coated pigments with a D50 value of around 2 to 5 μm are obtained.

The product to be sintered generally consists of 25 to 60% by weight of the colored spinel, 25 to 60% by weight of silicon dioxide and 5 to 40% by weight of mineralizer. The powder constituents are preferably used in such an amount that pigments result, the spinel core of which makes up 45 to 60% by weight. The halides, preferably chlorides, of the alkali metals, alkaline earth metals and / or earth metals can be used as mineralizers.

A particularly preferred powder mixture consists of 33 to 45 wt .-% silicon dioxide, 13 to 23 wt .-% potassium chloride, and 42 to 50 wt .-% colored spinel, especially CoAl₂O₄. During the sintering process in the presence of atmospheric oxygen and / or moisture, if halides are used as the mineralizer, the halogen is completely or partially split off, so that the resulting metal oxide can become part of the silicate coating.

The person skilled in the art will determine the optimum sintering temperature and sintering time as well as the heating rate for reaching the sintering temperature by means of orientation tests. If necessary, the sintered material is gently ground and can also be subjected to a known wet aftertreatment in order to remove soluble mineralizer components from the coated pigment.

In contrast to the previously known inclusion pigments, the coated spinel color pigments according to the invention are very color-intensive and have a high resistance to bleeding. They can therefore be used in an excellent way for the production of color-intensive, crisp ceramic decorations. Depending on the selection of the spinel that forms the core, blue, green, brown, gray or black coated spinel color pigments with high bleeding resistance are easily obtained.

In Figure 1, a decor is shown using a coated CoAl₂O₄ color pigment according to the invention, in Figure 2 the same decor using the pure CoAl₂O₄ pigment. The pigment was pasted in a screen printing oil; the order was made by screen printing on a transparent glaze (39.9% PbO; 0.5% Na₂O; 1.3% K₂O; 0.2% MgO; 4.7% CaO; 7.1% Al₂O₃; 39.7 % SiO₂; 6.6% B₂O₃ - each wt .-%) provided test plate. Firing was carried out in the so-called oblique firing, since the bleeding behavior of a pigment can be perceived most clearly when positioned obliquely during the fire. The comparison of the two decors shows the superiority of the pigments according to the invention.

The coating of pigment surfaces by applying a protective layer tailored to the intended use has long been known per se. DE-OS 36 43 247 describes, for example, a process for improving the application properties of a bismuth vanadate / molybdate pigment, a layer of an alkali metal silicate or alkaline earth metal silicate being applied to the pigment and then covered with a wax emulsion. However, the silicate layer of this coated pigment is not a glassy layer. Knowing the bleeding properties of the spinels in the presence of glass flows, it was completely surprising that the bleeding behavior could be significantly improved by enveloping the colored spinel with a material of the composition according to the invention. The coated spinel color pigments according to the invention can therefore be produced very well for producing contour-sharp decors using conventional printing processes or using decals. The pigment can be used as part of a decorative color or in conventional organic printing media.

Examples 1 to 6

Powder mixtures of the compositions given in the table were after homogenization at 1000 ^OC sintered for 2 hours. After the sintering process, grinding was carried out for 5 minutes. The properties of the decor containing coated spinel color pigments produced according to the invention and, as described above, applied and baked in the oblique bake with regard to color intensity and sharpness of contour follow from the table.<tables id="tabl0001" num="0001"><img file="EP0455933A2_D0001.tif" /></tables>