High coverage metallic effect pigments

Abstract

The present invention relates to a method for producing metallic effect pigments based on aluminum platelets provided with a metal oxide coating, comprising the following steps: (a) feeding the aluminum platelets into an organic solvent to form a corresponding dispersion and adding at least one metal oxide precursor compound to dissolve the metal oxide precursor compound, and (b) decomposing the metal oxide precursor compound in the organic solvent to form a metal oxide coating on the aluminum platelets. Furthermore, the present invention relates to metallic effect pigments obtainable by the method according to the invention and to the use of the metallic effect pigments according to the invention.

Description

[Technical field]

[0001] The present invention relates to a process for producing metallic effect pigments based on aluminium flakes provided with a metal oxide coating, to the metallic effect pigments obtainable by the process of the invention and to the use of the metallic effect pigments of the invention. [Background technology]

[0002] Colored, light-stable metallic effect pigments are produced according to the prior art by coating thin platelet-shaped substrates of metals such as aluminum with metal oxides, such as Fe2O3 and TiO2. To achieve high hiding power, the total thickness of the metallic effect pigments must be very thin. The thinner the metallic effect pigment, the higher its hiding power, since a larger area can be covered for a given mass of pigment deposited. Therefore, the substrates usually used are very thin metallic platelets, i.e. flakes. These thin metallic flakes are designated as pigments in themselves.

[0003] This type of aluminum substrate, i.e. aluminum pigments, is produced industrially in two different ways. Relatively thin aluminum flakes can be obtained, for example, by a cost-effective, low-impact wet grinding process. In the course of grinding, metal beads release rounded aluminum particles. Depending on the size of these particles, the grinding time and the metal beads employed, thin aluminum flakes are obtained. With a particularly low-impact grinding process, it is possible to achieve median substrate thicknesses in the range of 60 nm to 100 nm on an industrial scale for relatively large flake diameters. However, as a result of the cold deformation during grinding, the aluminum flakes always have a certain thickness variation and surface roughness. That is, the aluminum flakes are not completely flat. This thickness variation and surface roughness, on the contrary, have the advantage that the resulting aluminum flakes do not come into extensive contact during drying and can therefore be dried relatively easily without agglomeration. However, as the thickness of the substrate decreases and the specific surface area increases, non-agglomerated drying becomes increasingly difficult. And as the grinding factor increases, i.e., as more thin aluminum flakes are released, the production-related thickness variations become disadvantageous. Generally, the aluminum flakes have cracks and scattered wear edges, which form scattering centers and further impair the optical properties of the pigment. These wear edges occur more and more in particular in pigments with substantial diameters in the range of 20 μm and above. It is therefore not possible to obtain arbitrarily thin aluminum flakes of large diameters by wet grinding. For example, Patent Document 1 describes aluminum flakes obtained by wet grinding, with an average thickness of 32 nm, but with a thickness of d 50 Aluminum flakes with diameters smaller than 10 μm have been described. However, larger diameters 50 For aluminum flakes having a thickness of d, this kind of average thickness could not be experimentally demonstrated without the cracks and scattered wear edges mentioned above. 50 indicates the average particle size of the aluminum flakes below which 50% of the aluminum flakes are smaller than that particular value.

[0004] To obtain thinner aluminum flakes, more complex, expensive and energy-intensive manufacturing methods are employed. According to the prior art, thinner aluminum flakes are industrially produced by physical vapor deposition (abbreviated PVD). In this method, a polymer film is usually first coated with a removable release coating and a nanometer-thick aluminum layer is applied by vacuum deposition. The advantage of this method compared to wet milling is the formation of a very smooth, mirror-like metal surface. The almost complete metal layer can be detached from the film and then powdered to obtain the desired flakes. Therefore, aluminum flakes produced by PVD are, as a process-specific result, significantly thinner and smoother than aluminum flakes obtained from wet milling processes. The smaller thickness of the aluminum flakes produced by PVD makes it possible to obtain metallic effect pigments with a much higher hiding power than when using aluminum flakes from wet milling. Usually, aluminum flakes produced by PVD have a thickness in the range of 5 nm to 50 nm. However, the smooth surface has the disadvantage of a strong tendency to agglomerate. The contact area is so large that the tendency to agglomerate when dry is much more pronounced than for aluminum flakes from wet milling. Aluminum flakes produced by PVD are sometimes called vacuum metallized pigments (abbreviated VMP).

[0005] As mentioned above, colored, light-stable metal effect pigments are typically obtained by coating with metal oxides. The provision of the colored metal oxide coating on the substrate can be achieved in the gas phase, for example by chemical vapor deposition (abbreviated CVD), or by precipitation in aqueous processes, for example by hydrolysis of the corresponding metal halides. The color of the pigment is caused by the interaction of absorption, transmission and interference with the thickness and refractive index of the metal oxide coating, which in particular determine the color of the metal effect pigment.

[0006] Direct CVD coating provides an oxide coating on only one of the two aluminum surfaces, resulting in only one of the two aluminum surfaces having color. In order to uniformly coat all surfaces with metal oxides, fluidized bed processes have been developed and are described, for example, in US Pat. Nos. 5,233,933, 5,233,933 and 5,233,933.

[0007] In these methods, the substrate is coated with iron oxide, usually in a fluidized bed or swirl layer, using iron pentacarbonyl as precursor and oxygen as oxidant, and optionally with a further transparent oxide. Using CVD methods, it is possible to produce highly saturated metal effect pigments, even with iron oxide layers as thin as 20-35 nm thick. These metal effect pigments are commercially available, for example, as Paliocrom® 2850 (orange) and Paliocrom® 2050 (gold). The disadvantage of CVD coating by fluidized bed methods is that the target substrate must be fully fluidized (mobile) in the gas phase. If it is not fully fluidized, agglomerates will form during the coating process. The agglomerates impair hiding power and reduce the proportion of useful particle size, ultimately resulting in a lower yield.

[0008] Thin aluminum flakes produced by the above-mentioned methods using PVD have a pronounced tendency to agglomerate and are extremely sensitive to shear, making them unusable industrially. Very thin and almost perfectly smooth aluminum flakes cannot be fully fluidized in a fluidized bed, or can be fully fluidized only with great difficulty. Therefore, aluminum flakes produced by wet milling are the only substrates industrially used for coating with metal oxides by CVD processes. As already mentioned, aluminum flakes from wet milling are significantly thicker compared to thin aluminum flakes produced by PVD, as a process-specific result. Thus, in patent document 3, pigments with a thickness in the range of about 100 nm to 200 nm are identified, which are relatively easy and inexpensive to produce by wet milling. To date, there has been no description of a fluidized bed CVD coating based on thin aluminum flakes produced by PVD.

[0009] One possibility for coating thin aluminum flakes produced by PVD with high (refractive index) metal oxides is described in patent application WO 2005 / 023966. Here, thin aluminum flakes are used with a thickness preferably in the range of 5 nm to 30 nm. The pigments are not coated in a fluidized bed, but instead by hydrolysis of a salt solution. The overall structure is a three-layer structure, comprising a layer of high refractive index metal oxide, a low refractive index intermediate layer, for example of SiO2, and an outer layer composed of a low refractive index material. Patent application WO 2005 / 023966, which describes the production of printing inks, also mentions the possibility of using thin aluminum flakes produced by PVD, as well as pigments obtained by grinding with a ball mill. In this case, the high refractive index metal oxide layer is applied by hydrolysis in water, and a phosphorus-containing additive is also required.

[0010] As also described in US Pat. No. 5,999,563, the application of metal oxide layers in aqueous media has many disadvantages. The pH required for precipitation is in the range in which a reaction between aluminum and water can occur. This reaction is fatal, especially in the case of thin aluminum flakes, due to their large specific surface area. The reaction reproducibility is poor, since the pigment is attacked. It is therefore necessary to use dilute solutions, which also require a large number of operating steps.

[0011] As described in patent document 7, application of high refractive index metal oxide layers by wet chemical oxidation in organic solvents is also possible, but the amount of water in the solvent is still between 3% and 60% by weight. A mixed layer is then formed between the substrate and the high refractive index metal oxide layer. Patent documents 8 and 2 describe alternative methods that make the coating an anhydrous process by carrying it out in a fluidized bed reactor. However, as mentioned above, in this case the disadvantage arises that the thin aluminum flakes produced by PVD, as a result of the combination of a high specific surface area and low surface roughness, have a very pronounced tendency to agglomerate during drying and therefore cannot be used for this type of CVD coating. These flakes can therefore be fluidized very poorly, even if they can be fluidized. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] European Patent No. 2102294 [Patent Document 2] European Patent Application Publication No. 0033457 [Patent Document 3] DE 4223384 [Patent Document 4] European Patent Application Publication No. 0562329 [Patent Document 5] International Publication No. 2015 / 014484 [Patent Document 6] International Publication No. 2009 / 083176 [Patent Document 7] International Publication No. 2005 / 049739 [Patent Document 8] International Publication No. 2013 / 175339 Summary of the Invention [Problem to be solved by the invention]

[0013] The present invention therefore aims to provide a method for producing high-hitting metallic effect pigments based on thin aluminum flakes with a metal oxide coating, which overcomes the above-mentioned disadvantages known in the prior art for coating thin aluminum flakes, as occurs, for example, in CVD-based fluidized bed processes or wet chemical precipitation processes. [Means for solving the problem]

[0014] This object is achieved by the embodiments of the invention that are characterized in the claims.

[0015] In particular, the present invention relates to a method for producing metallic effect pigments based on aluminium flakes provided with a metal oxide coating, comprising the steps of: The aluminum flakes have a thickness of 5 nm to 90 nm and are coated with a metal oxide coating; The metal oxide coating is composed of one or more metal oxides having a thickness of 5 nm to 150 nm and has a refractive index of at least 1.9; There is no additional coating covering the aluminum flake between the surface of the aluminum flake and the metal oxide coating; The process is as follows: (a) introducing aluminum flakes into an organic solvent to form a corresponding dispersion and adding at least one metal oxide precursor compound to dissolve the metal oxide precursor compound; (b) decomposing the metal oxide precursor compound in an organic solvent to form a metal oxide coating on the aluminum flakes; The present invention provides a method comprising:

[0016] The method of the invention makes it possible to provide high-hitting metal effect pigments based on thin aluminum flakes with a metal oxide coating, the coating of the aluminum flakes according to the invention being carried out in an organic solvent. The relatively simple and inexpensive coating method allows time and costs to be saved while at the same time overcoming the known disadvantages of fluidized bed and wet chemical precipitation processes. In particular, the thin aluminum flakes do not agglomerate and are not attacked by solvents. Given the small thickness of the aluminum flakes used, a larger area can be covered for a given mass of pigment placed, so that a much higher hiding power can be achieved compared to comparable competing products in the prior art. The metal effect pigments obtainable by the method of the invention therefore have a hiding power in varnish systems with a coloring level of only 3% to 4%. The metal effect pigments of the invention are suitable for a variety of applications, such as printing inks, especially as part of digital printing processes (inkjet), coating systems or cosmetics, where they can be used as such in paste form.

[0017] In the following the method for producing metallic effect pigments based on coated aluminum flakes according to the invention is explained in more detail.

[0018] In step (a) of the method of the present invention, aluminum flakes are introduced into an organic solvent to form a corresponding dispersion. Also in step (a), at least one metal oxide precursor compound is added to dissolve the metal oxide precursor compound. Here, the order of introducing the aluminum flakes and adding the at least one metal oxide precursor compound is not important. That is, the aluminum flakes may be introduced after adding the at least one metal oxide precursor compound.

[0019] The aluminum flakes introduced in step (a) may be passivated, i.e. may have a coating of a native oxide layer, the thickness of which is typically between 3 nm and 5 nm, which for the purposes of the present invention is included in the thickness of the aluminum flakes defined below.

[0020] According to the invention, the aluminum flakes have a thickness in the range of 5 nm to 90 nm, preferably in the range of 5 nm to 50 nm, more preferably in the range of 5 nm to 30 nm. By using thin aluminum flakes, a particularly high hiding power can be achieved. The thickness of the aluminum flakes is the average thickness, which is understood as the numerical average of all the measured thicknesses. The average thickness, also called the mean thickness, is determined by measurements based on transmission electron microscope (TEM) images. Here, the average thickness is the average of at least 200 measurements on different aluminum flakes.

[0021] The aluminum flakes produced in step (a) typically have a thickness variation (Δh) of at most 30%. This thickness variation is preferably at most 25%, for example at most 20%, more preferably at most 15%, even more preferably at most 10%. The thickness variation is determined by the thickness SPAN (t SPAN The thickness SPAN is calculated from the width of the thickness distribution, also called the thickness SPAN, according to the following formula: t SPAN =(t 90 -t 10 ) / t 50

[0022] The indices in the above formula represent each value in the cumulative distribution curve. Therefore, t 10 is that 10% of the aluminum flakes have 10 The thickness value is smaller than the t value, while 90% of the aluminum flakes are 10 It means having a thickness equal to or greater than the thickness value.

[0023] The thickness variation is finally obtained as a percentage value from the thickness SPAN. Δh=t SPAN ×100%

[0024] In one preferred embodiment of the present invention, the aluminum flakes are vacuum metallized pigments (VMPs). In this case, the aluminum flakes are thin aluminum flakes produced by physical vapor deposition, which exhibit very little thickness variation and are therefore much smoother than wet-milled aluminum flakes. Metal effect pigments based on VMPs not only have high hiding power but also excel in other color data. VMPs are commercially available from various sources (Decomet® from Schlenk Metallic Pigments GmbH, METALURE® from Eckart GmbH, Metasheen® from BASF SE).

[0025] As far as the size of the aluminum flakes is concerned, the present invention is not subject to any further limitations. The size of the aluminum flakes is typically within the range of d 50 As already mentioned, d 50 indicates the average particle size of the aluminum flakes below which 50% of the aluminum flakes are smaller. 50 is preferably 1 μm to 100 μm, for example, 1 μm to 50 μm, 1 μm to 25 μm, 1 μm to 10 μm, or 1 μm to 3 μm, but is not limited thereto. 50 is determined by laser scattering based measurements.

[0026] The ratio of particle size to thickness of the aluminum flakes, also called aspect ratio, is likewise not subject to further limitations herein: this ratio can lie in the ranges of 50-5000, 100-2000, or 200-1000, but is not limited thereto.

[0027] The aluminum flakes may also be present in a dispersed form prior to step (a). For this purpose, as described below, an organic solvent is used which is the same as or different from the organic solvent used in step (a), preferably different. The aluminum flakes may also be present as an isopropanol dispersion prior to step (a), but this is not limited thereto. This solvent does not substantially affect the coating operation in step (b).

[0028] The aluminum flakes, which may already be in dispersed form, are introduced into an organic solvent in step (a) of the method of the present invention. When selecting an organic solvent in step (a), it is necessary to ensure that at least one metal oxide precursor compound, which must be added together, has sufficient solubility in the organic solvent. Furthermore, the boiling point of the organic solvent used must be higher than the temperature required in the organic solvent for the decomposition of at least one metal oxide precursor compound in step (b).

[0029] As organic solvent in step (a), it is possible to use, for example, 1-methoxy-2-propanol, 2-isopropoxyethanol, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, triethylene glycol, tetraethylene glycol, polyethylene glycol 400, propylene carbonate, N,N-dimethylacetamide and dimethylsulfoxide, or mixtures thereof. Particularly advantageous in this context is 1-methoxy-2-propanol, also simply called methoxypropanol. 1-Methoxy-2-propanol has good solubility and also a relatively high boiling point.

[0030] The organic solvent may contain a small amount of water, and according to the present invention, the amount of water is preferably 1 part by weight or less per 100 parts by weight of the organic solvent.

[0031] The dispersion of the aluminum flakes introduced into the organic solvent can be achieved, for example, by mechanical action, such as stirring. Corresponding measures are known to the person skilled in the art.

[0032] Furthermore, in step (a) of the method of the present invention, at least one metal oxide precursor compound is added to dissolve the metal oxide precursor compound. As mentioned above, the addition of the at least one metal oxide precursor compound can also be carried out initially, i.e. before the introduction of the aluminum flakes.

[0033] At least one metal oxide precursor compound added in step (a) functions as a precursor of the metal oxide film formed in step (b) by decomposition of the metal oxide precursor compound. This film is a high refractive index film composed of one or more metal oxides. Therefore, the metal oxide precursor compound contains each metal of the metal oxide film to be formed. As already mentioned, its solubility in the organic solvent used must be sufficient.

[0034] A high refractive index coating in the context of the present invention refers to a metal oxide coating having a refractive index of at least 1.9.

[0035] In one preferred embodiment of the present invention, the metal atom of the at least one metal oxide precursor compound is iron, copper, or zinc. The metal oxide film may be composed of, for example, Fe2O3, CuO, ZnO, or mixtures thereof. These metal oxides have in common the characteristic of a high refractive index. Metal oxide precursor compounds that may be considered in this context include, but are not limited to, nitrates, acetates, acetylacetonates, malonates, alkoxides, oxalates, and oximates of iron, copper, or zinc, respectively. In contrast to the corresponding halides, oxohalides, and pseudohalides, these compounds have the advantage that they can be decomposed at relatively low temperatures in step (b) to form a metal oxide film. Furthermore, the nitrates, acetates, acetylacetonates, malonates, alkoxides, oxalates, and oximates of iron, copper, or zinc, respectively, may have a higher solubility in the relevant organic solvents compared to the corresponding halides, oxohalides, and pseudohalides.

[0036] Typically, the metal atom of at least one metal oxide precursor compound may be present in a complexed form, such as, but not limited to, a urea complex or a urea derivative complex.

[0037] When forming a Fe2O3 coating in step (b), the metal oxide precursor compound used can be, for example, [Fe(H2N(CO)NH2)6](NO3)3. When forming a CuO coating in step (b), the metal oxide precursor compound used can be, for example, [Cu(H2N(CO)NH2)4](NO3)2. When forming a ZnO coating in step (b), the metal oxide precursor compound used can be, for example, [Zn(H2N(CO)NH2)4](NO3)2 or [Zn(H2N(CO)NH2)4(H2O)2](NO3)2. In all these cases, the metal atom is complexed with a plurality of urea molecules (urea ligands) and, optionally, with water molecules (aqua ligands).

[0038] The preparation of the relevant metal oxide precursor compounds is known to those skilled in the art. Thus, the complexes [Fe(H2N(CO)NH2)6](NO3)3 and [Cu(H2N(CO)NH2)4](NO3)2 can be prepared, for example, by reaction of the corresponding nitrates Fe(NO3)3·(H2O)9 and Cu(NO3)2·(H2O)3 with urea. Similarly, for the complexes [Zn(H2N(CO)NH2)4](NO3)2 or [Zn(H2N(CO)NH2)4(H2O)2](NO3)2.

[0039] Instead of a single metal oxide precursor compound, it is also possible to add two or more different metal oxide precursor compounds in step (a). This allows the formation of mixed metal oxide layers to be achieved in step (b). In principle, the formation of alternating layers is also conceivable, but according to the invention it is preferred to add only one metal oxide precursor compound in step (a), since this allows a more effective control of the refractive index and therefore also of the color of the metallic effect pigment.

[0040] The color of the metallic effect pigments is strongly determined not only by the refractive index of the metal oxide coating but also by the thickness of the metal oxide coating. The thickness of the metal oxide coating can be adjusted by the loading amount of the relevant metal oxide precursor compound in step (a). Naturally, for a given loading amount of aluminum flakes, the thickness of the metal oxide coating increases as the loading amount of the relevant metal oxide precursor compound increases. Here, the loading amount of the metal oxide precursor compound is adjusted in such a way that a metal oxide coating having a thickness of 5 nm to 150 nm, for example 10 nm to 100 nm or 20 nm to 75 nm, is formed in step (b).

[0041] In step (b) of the process of the present invention, a metal oxide precursor compound is decomposed in an organic solvent to form a metal oxide coating on the aluminum flakes.

[0042] To achieve the decomposition of the relevant metal oxide precursor compounds, the organic solvent dispersion of the aluminum flakes and the added metal oxide precursor compounds is heated to the temperature required for decomposition to occur. The decomposition is therefore a thermal decomposition. As mentioned above, the boiling point of the organic solvent must be higher than the temperature required for the decomposition of at least one metal oxide precursor compound in the relevant organic solvent. As a result of the thermal decomposition, the respective metal oxides are deposited on the aluminum flakes and are optionally passivated. As a result, a metal oxide coating is formed that covers the aluminum flakes. This coating may still initially contain residual organic matter from the relevant metal oxide precursor compounds.

[0043] After step (b), the metal effect pigments obtained are in paste form and can be used as is for the corresponding purpose or adapted to different purposes by exchanging or changing the solvent without drying. However, an alternative option is to perform spray drying, in which the metal effect pigments are not used as is but instead stored in solid form. A further option is, for example, calcination in diphenyl ether, which decomposes residual organic matter originating from the relevant metal oxide precursor compounds.

[0044] According to the present invention, there is no further coating covering the aluminum flakes between the surface of the aluminum flakes and the metal oxide coating covering the surface, which is composed of one or more metal oxides having a thickness of 5 nm to 150 nm and has a refractive index of at least 1.9.

[0045] However, if necessary, an optional layer can be formed on the metal oxide film. If present, this layer functions as an external protective layer and necessarily has a refractive index less than 1.8. The optional layer can be composed of, for example, SiO2. Modification of the metal oxide film or the optional external protective layer formed thereon with silane is also possible.

[0046] In a further aspect, the present invention relates to a metallic effect pigment based on aluminium flakes provided with a metal oxide coating, obtainable by the process of the present invention as described above, The aluminum flakes have a thickness of 5 nm to 90 nm and are coated with a metal oxide coating; The metal oxide coating is composed of one or more metal oxides having a thickness of 5 nm to 150 nm and has a refractive index of at least 1.9; and It concerns metallic effect pigments in which there is no further coating covering the aluminium flakes between the surface of the aluminium flakes and the metal oxide coating.

[0047] The metallic effect pigments of the present invention are subject to analogous procedures as described above with respect to the method for producing the metallic effect pigments of the present invention.

[0048] At a color depth of 3%, the metallic effect pigments of the invention have a color difference dE110° of less than 1.5. At a color depth of 4%, the metallic effect pigments of the invention can also have a color difference dE110° of less than 1.0. The color difference dE110°, which is a measure of the hiding power of metallic effect pigments, is determined according to the DIN 55987 standard.

[0049] Finally, the present invention relates to the use of the metal effect pigments of the present invention for printing inks, inkjet applications, coating systems, raw coloring of plastics or cosmetics.As mentioned above, the metal effect pigments of the present invention, which are characterized by high hiding power, can often be used as is in paste form without prior drying.The metal effect pigments of the present invention have proven to be particularly advantageous especially for digital printing processes (inkjet).

[0050] The present invention can provide high-hiding metallic effect pigments based on thin aluminum flakes with a metal oxide coating that can be produced in a simple and inexpensive manner. In particular, the present invention overcomes the disadvantages known in the prior art for coating thin aluminum flakes, as occurs, for example, in CVD-based fluidized bed processes or wet chemical precipitation processes. [Brief description of the drawings]

[0051] [Figure 1] 1 is a graph plotting the colour difference dE110° of metallic effect pigments of the present invention and also of prior art metallic effect pigments as a function of colour depth (%). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLES

[0052] The present invention is further illustrated by the following non-limiting examples.

[0053] Example 1 Aluminum flake powder with Fe2O3 coating Synthesis of metal oxide precursor compound [Fe(H2N(CO)NH2)6](NO3)3: 30.062 g (0.5005 mol) of urea was dissolved in 700 ml of ethanol with gentle heating (40 °C). Subsequently, a solution of 31.11 g (0.077 mol) of Fe(NO3)3·(H2O)9 in 175 ml of ethanol was added. The solution was stirred for 2 h, at which point the solid [Fe(H2N(CO)NH2)6](NO3)3 formed was isolated by filtration, washed with ethanol and dried in a drying cabinet at 50 °C for 12 h. The yield was 33.8 g (73%).

[0054] Coating of thin aluminum flakes by thermal decomposition of [Fe(H2N(CO)NH2)6](NO3)3 in solution: The thickness is 25 nm, d 50 11 g of an isopropanol dispersion of aluminum flakes containing 1.1 g of aluminum flakes (VMP, Decomet® from Schlenk Metallic Pigments GmbH) with a diameter of 21 μm was dispersed in 1 liter of 1-methoxy-2-propanol, 10 g of [Fe(H2N(CO)NH2)6](NO3)3 was dissolved therein, and the dispersion was heated to boiling (120°C) under reflux for 2 hours.

[0055] In two further steps, in each case following cooling, 10 g of [Fe(H2N(CO)NH2)6](NO3)3 was added and in each case the dispersion was heated to boiling under reflux for 2 hours, thus using a total of 30 g of metal oxide precursor compound.

[0056] After cooling, the sample thus obtained was filtered, the solid product was washed with isopropanol and dispersed in isopropanol without drying, the suspension was then spray-dried to finally obtain a golden powder.

[0057] Example 2 Suspension of aluminum flakes with Fe2O3 coating Synthesis of metal oxide precursor compound [Fe(H2N(CO)NH2)6](NO3)3: The synthesis was carried out in the same manner as in Example 1.

[0058] Coating of thin aluminum flakes by thermal decomposition of [Fe(H2N(CO)NH2)6](NO3)3 in solution: The thickness is 25 nm, d 50 1 g of an isopropanol dispersion of aluminum flakes containing 0.24 g of aluminum flakes (VMP, Decomet® from Schlenk Metallic Pigments GmbH) with a diameter of 21 μm was dispersed in 100 ml of 1-methoxy-2-propanol and 0.5 ml of water. 0.5 g of [Fe(H2N(CO)NH2)6](NO3)3 was dissolved therein, and the dispersion was heated to boiling under reflux (120° C.) for 2 hours. After cooling, 0.5 g of [Fe(H2N(CO)NH2)6](NO3)3 was further added, and the dispersion was heated to boiling under reflux again for 2 hours.

[0059] After cooling the dispersion, the solvent was slowly removed by filtration, while avoiding complete drying of the obtained sample. The sample was then dispersed in 100 ml of fresh 1-methoxy-2-propanol and 0.5 ml of water. 0.5 g of [Fe(H2N(CO)NH2)6](NO3)3 was added again and the dispersion was heated to boiling under reflux for 2 hours. Finally, after cooling the dispersion, 0.5 g of [Fe(H2N(CO)NH2)6](NO3)3 was further added and the dispersion was heated to boiling under reflux again for 2 hours.

[0060] The above steps were repeated once in total, so that a total of 3.0 g of metal oxide precursor compound was used.

[0061] After cooling, the sample thus obtained was filtered and the solid product was washed with isopropanol and ethyl acetate and then redispersed in ethyl acetate without drying, finally obtaining a gold suspension.

[0062] Example 3 Aluminum flake powder with CuO coating Synthesis of metal oxide precursor compound [Cu(H2N(CO)NH2)4](NO3)2: 2.7 g (0.045 mol) urea was dissolved in 100 ml butanol with gentle heating (40 °C). After the solution was cooled to room temperature, 2.42 g (0.01 mol) Cu(NO3)2·(H2O)3 was added. The solution was stirred for 2 h, at which point the solid [Cu(H2N(CO)NH2)4](NO3)2 formed was isolated by filtration, washed with butanol and acetone, and dried in a drying cabinet at 50 °C for 16 h. The yield was 3.176 g (74.3%).

[0063] Coating of thin aluminum flakes by thermal decomposition of [Cu(H2N(CO)NH2)4](NO3)2 in solution: The thickness is 25 nm, d 50 1 g of an isopropanol dispersion of aluminum flakes (VMP, Decomet® from Schlenk Metallic Pigments GmbH) containing 0.24 g of 21 μm aluminum flakes was dispersed in 100 ml of 1-methoxy-2-propanol, in which various amounts of [Cu(H2N(CO)NH2)4](NO3)2 were dissolved in several steps, in each step the dispersion was heated to boiling (120 °C) under reflux for 2 h.

[0064] In the first two steps, 0.2 g of [Cu(H2N(CO)NH2)4](NO3)2 was added, while in the next two steps, 0.3 g of [Cu(H2N(CO)NH2)4](NO3)2 was added. At that point, the samples obtained in between were slowly separated by filtration, without drying in the process, and redispersed in 100 ml of 1-methoxy-2-propanol. Then, in two further steps, 0.5 g of [Cu(H2N(CO)NH2)4](NO3)2 was added.

[0065] The sample obtained in the meantime was again separated by filtration, without drying in the process, and again redispersed in 100 ml of 1-methoxy-2-propanol. In two further steps, 0.5 g of [Cu(H2N(CO)NH2)4](NO3)2 was added. Thus, a total of 3.0 g of metal oxide precursor compound was used.

[0066] The sample thus obtained was then separated by filtration without drying in the process, dispersed in 50 ml of diphenyl ether, and heated to boiling (258°C) for 5 minutes. After cooling, the sample was separated by filtration, washed with ethanol, acetone, and diethyl ether, and dried at 50°C for 16 hours, finally obtaining a pale gold powder.

[0067] Color data review The metallic effect pigments of Examples 1 to 3 were examined in more detail with regard to their colour data and compared with metallic effect pigments of the prior art.

[0068] The color data were determined by measurements on the corresponding drawdown cards. For this purpose, knife-coated drawdowns of each metallic effect pigment at various color intensities in a solvent-based nitrocellulose / polycyclohexanone / polyacrylic varnish with a solids content of 10% were produced on TQC black / white DIN A5 cards using a 38 μm wire doctor on a Zehntner automatic film coater, where the color in each case refers to the proportion of pigment in the overall varnish mixture. All percentages here are to be understood as % by weight.

[0069] Except for the 60° gloss measurements, the colorimetric data was measured using a Byk-mac i instrument manufactured by Byk. The colorimetric data was measured on the white and black sides of a DIN A5 card to determine the hiding power, expressed as color difference dE110°. The 60° gloss measurements were performed using a Byk micro-TRI-gloss instrument manufactured by Byk. The results are summarized in Table 1 below.

[0070] [Table 1]

[0071] The values ​​contained in Table 1 for the color difference dE110° of the metal effect pigments of Examples 1 to 3 according to the invention are plotted in Figure 1 as a function of the degree of coloration (%). Also shown in Figure 1 are the values ​​of the color difference dE110° of metal effect pigments of the prior art, determined by the same measurement method. As is evident from the comparison of these values, the metal effect pigments of the invention have a significantly higher hiding power, as can be seen from the lower values ​​of the color difference dE110°. Thus, with a coloration degree of only 3%, a color difference dE110° of less than 1.5 is obtained. With a coloration degree of 4%, the color difference dE110° is indeed significantly lower than 1.0. In the case of the metal effect pigments of the prior art, on the contrary, the coloration degree necessary to achieve this is about twice as high.

Claims

1. 1. A method for producing metallic effect pigments based on aluminium flakes provided with a metal oxide coating, comprising the steps of: the aluminum flakes have a thickness of 5 nm to 90 nm and are coated with the metal oxide coating; The metal oxide coating has a thickness of 5 nm to 150 nm. ２ O ３ , CuO, ZnO, or a mixture thereof, and has a refractive index of at least 1.9; there is no additional coating between the surface of the aluminum flake and the metal oxide coating, The following steps: (a) introducing the aluminum flakes into an organic solvent to form a corresponding dispersion and adding at least one metal oxide precursor compound to dissolve the metal oxide precursor compound; (b) decomposing the metal oxide precursor compound in the organic solvent to form the metal oxide coating on the aluminum flakes; Including, the at least one metal oxide precursor compound is selected from the group consisting of iron, copper, or zinc nitrates, acetates, acetylacetonates, malonates, alkoxides, oxalates, and oximates; method.

2. 10. The method of claim 1, wherein the aluminum flakes have a thickness variation (Δh) of up to 30%.

3. 3. The method of claim 1 or 2, wherein the aluminum flake is a vacuum metallized pigment.

4. 4. The method of any one of claims 1 to 3, wherein the organic solvent is selected from the group consisting of 1-methoxy-2-propanol, 2-isopropoxyethanol, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, triethylene glycol, tetraethylene glycol, polyethylene glycol 400, propylene carbonate, N,N-dimethylacetamide, and dimethylsulfoxide, or a mixture thereof.

5. The method of claim 4, wherein the organic solvent is 1-methoxy-2-propanol.

6. 6. The method according to claim 1, wherein the aluminium flakes are also present in dispersed form prior to step (a), and for this purpose an organic solvent is used which is the same as or different from the organic solvent used in step (a).

7. 7. The method of claim 6, wherein the aluminum flakes are present as an isopropanol dispersion prior to step (a).

8. The method according to any one of claims 1 to 7, wherein the metal atom of the at least one metal oxide precursor compound is present in a complexed form.

9. The method of claim 8, wherein the complexed form is a urea complex or a urea derivative complex.

10. The at least one metal oxide precursor compound is [Fe(H ２ N(CO)NH ２ ) ６ ](NO ３ ) ３ , [Cu(H ２ N(CO)NH ２ ) ４ ](NO ３ ) ２ , and [Zn(H ２ N(CO)NH ２ ) ４ ](NO ３ ) ２ The method according to any one of claims 1 to 9, wherein the compound is selected from the group consisting of:

11. Metallic effect pigments based on aluminium flakes provided with a metal oxide coating, obtained by the method according to any one of claims 1 to 10, the aluminum flakes have a thickness of 5 nm to 90 nm and are coated with the metal oxide coating; The metal oxide coating has a thickness of 5 nm to 150 nm. ２ O ３ , CuO, ZnO, or mixtures thereof, and having a refractive index of at least 1.9; and A metallic effect pigment, wherein there is no further coating covering the aluminium flakes between the surface of the aluminium flakes and the metal oxide coating.

12. 12. Metallic effect pigments according to claim 11, having a colour difference dE110° of less than 1.5 at a colour level of 3%.

13. 13. Use of the metallic effect pigments according to claim 11 or 12 for printing inks, inkjet applications, coating systems, mass colouring of plastics, or cosmetics.

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