White or colored magnetic pigment

Abstract

Magnetic color pigment (1) with a pigment core (2) consisting of at least one magnetic particle (9) and a color layer (5) surrounding the pigment core (2) with a plurality of color particles (6), characterized in that the color particles (6) have a particle size between 100 nm and 600 nm and form a porous structure, and the thickness of the color layer (5) is at least 500 nm.

Description

[0001] The invention relates to a magnetic color pigment, i.e., a white or colored pigment, with a pigment core composed of at least one magnetic particle and a color layer surrounding the pigment core containing a plurality of color particles or chromophores. The invention also relates to a method for producing such a magnetic color pigment. The magnetic color pigment can be deliberately white, i.e., have adjustable colors.

[0002] Due to their properties, magnetic materials have a virtually unlimited number of possible applications. Predicting when a material is magnetic is difficult. A necessary, although not necessarily sufficient, condition is that the material's atomic building blocks contain unpaired electron spins. This applies to elements such as iron, cobalt, nickel, or rare earth metals such as samarium or dysprosium. Solids composed of these elements in pure form or as oxides, ferrites, or alloys (with various elements from the periodic table) can possess magnetic properties so pronounced that they are suitable for use in functional materials. In this case, the materials are ferromagnets or ferrimagnets (paramagnets, diamagnets, and antiferromagnets generally do not have the desired properties).

[0003] The physical properties of these elements mean that, in their pure form, as alloys with other elements, or as oxides (and ferrites), they always exhibit a dark color—usually black, gray, or brown. This dark color makes the materials unattractive for a variety of applications where magnetism or electromagnetic properties resulting from the magnetic atoms, such as interaction phenomena such as inductive properties, would be advantageous, but the dark color is disruptive. A dark color may be undesirable for aesthetic reasons or because other physical color effects cannot be realized due to the "black body."

[0004] For this reason, it would be desirable to have a color pigment, i.e., a particle system made up of interconnected, different particles, that exhibits magnetic properties but is overall less dark or not dark at all—for example, neither brown, nor gray, and / or black, but rather light, ideally white. If such a magnetic white pigment were available, it would provide an attractive base color (for example, paper, building walls, sanitary tiles, textiles, clothing, dental materials, thermoplastics, and the like are often white). Furthermore, a white body, and thus also the white pigment, could be easily modified to appear colorful with a precisely matched other color (unlike black, white does not "absorb" color, but rather provides an ideal, color-independent background for any color).

[0005] However, to date, no approaches are known that clearly demonstrate a simple route to a magnetic white and thus color pigment. For example, US Pat. No. 6,110,633 A describes a complex interference multilayer film to produce a magnetic white pigment. Another attempt is described in US Pat. No. 2010,140,916 A1. The current state of research is described, for example, in the article "High Reflective Efficiency and Durability of Multilayered Core Shell Composite Particles with Controlled Shell Vectors" by Kim S.-D. et al., which appeared in "Dyes and Pigments" 2018, 153, 53-60.

[0006] DE 10 2006 054 173 A1 discloses a process for producing magnetic silica particles, the process comprising the following steps: a) providing a liquid reaction mixture containing a liquid, aqueous reaction medium, magnetic particles suspended in the mixture and at least one compound serving as a silica source, b) adding a pH modifier, and c) spray drying, wherein an organic pore former is added before step c).

[0007] DE 10 2011 055 433 A1 discloses a process for producing particles with a highly porous surface coating, the process comprising the following steps: i) providing a nano- or microparticulate material or providing a mixture of nano- and microparticulate materials, and coating the surface(s) with compounds containing functional groups, ii) introducing a spacer group by reaction with the functional groups from step i), iii) if necessary.Activating the surface(s) with functional groups that coordinate with a metal ion via an electron donor function or form a dative / coordinative bond, unless the spacer group in step ii) already contains such a group, iv) contacting the particles with a solution of a metal cation, v) carrying out a washing step to remove unbound metal ions, vi) contacting the particles with a solution of an organic ligand molecule with electron donor function, vii) carrying out a washing step to remove unbound ligand molecules, and viii) repeating steps iv) - vii) until a coating of the desired thickness is achieved on the particle surface.

[0008] The object of the present invention can therefore be seen in providing a magnetic color pigment which can be produced in a simple manner and which has improved reflection properties and can thus take on brighter colors, i.e. a brighter chromatic color and / or a brighter white, than the color pigments known so far.

[0009] This problem is solved by the subject matter of the independent patent claims. Advantageous embodiments emerge from the dependent patent claims, the description, and the figures.

[0010] One aspect relates to a magnetic color pigment with a pigment core consisting of at least one magnetic particle and a color layer surrounding the pigment core and comprising a plurality of color particles. The plurality of color particles adhere to one another and also to the at least one magnetic particle. The color particles form a layer surrounding the pigment core. The magnetic color pigment is characterized in that the color particles have a particle size between 100 nm and 600 nm and form a porous structure, and the thickness of the color layer is at least 500 nm. Preferably, the color layer, as well as the further layers introduced below, completely surrounds the core or further inner layers. Any material that exhibits corresponding magnetic properties can be used for the pigment core.

[0011] In the context of the present disclosure, white is also considered a color; thus, the magnetic color pigment can be either a magnetic "color pigment," for example, a blue pigment, or a magnetic "white pigment," which, for example, reflects the entire visible spectrum. The "lighter colors" described above can include white via total internal reflection as well as white (or white impression) via light emission. Furthermore, the "lighter colors" can also include the combination of color and white, which can be generated in particular via the various physical effects mentioned. For example, a white can be generated via total internal reflection at the color layer, and at the same time a color such as green can be generated by light emission from a luminescent coordination compound, as described in more detail below.The luminescent coordination compound can be present in the color layer as well as in an outer layer described in more detail below.

[0012] Iron or iron oxide particles, for example carbonyl iron or iron oxide, can preferably be selected as magnetic particles. The magnetic particles can also be or comprise magnetite particles. Carbonyl iron particles can be highly pure, spherical iron particles with a size of a few µm, iron oxide particles, or magnetite particles, for example, particles produced from a wet-chemical oxidative precipitation synthesis with a size of at least 10 nm and / or at most 10 µm. Synthesis routes to particles of this type are generally known, and the particles of this type can also be purchased commercially. The magnetic particles can therefore have a size of at least 10, preferably at least 30 nm. Alternatively or additionally, they can have a maximum size of 10 µm, preferably 5 or 3 µm.

[0013] According to the invention, the particles of the color layer form a porous structure. The color particles are thus arranged in the form of a porous structure. Assuming spherical color particles, the degree of space filling of the porous structure can be between 30 and the maximum space filling for the densest sphere packing, i.e., 74%. The degree of space filling is preferably below 65%, particularly preferably below 60%. The color particles are preferably made at least partially, i.e., partially or entirely, of a material with a refractive index of at least 2, in particular at least 2.2, or preferably 2.5, for example, titanium dioxide (preferably rutile). The refractive index can also be at least 2.5.

[0014] The loose structure of the color particles, also referred to as satellite particles, which are present in a porous and thus not sintered arrangement, enables the diffuse scattering effect required for a bright, particularly white, color impression. This is because aggregated particles or particles connected continuously in a closed shell, or a homogeneous, non-particulate shell, or a dense or smooth layer, even with a high refractive index, do not have sufficient scattering effect. Only through the loose arrangement ("association") of the satellite particles in the shell around the magnetic particle(s) can the scattering effect be created with the desired strength, and thus, in particular, a white color appearance. This can be achieved by the approach described below of spray-dried coating of the magnetic particle(s) with the satellite particles with a high refractive index.

[0015] The porous structure thus offers the advantage of disproportionately enhanced surface effects, so that even with a comparatively thin ink layer, the light components absorbed by the pigment core are drastically reduced, thus creating the desired bright magnetic color pigment. Such a thin ink layer can also be produced cost-effectively on a large industrial scale using the spray-drying process described below. The porous structure and the spray-drying process described below also enable the incorporation of different color particles into the ink layer, thus creating magnetic colored pigments in specific, predefined colors and white pigments with white as the color.

[0016] A further aspect, which can be combined with the other aspects or also realized independently of them, relates to a magnetic color pigment with a pigment core made of at least one magnetic particle and a color layer surrounding the pigment core, in particular made of one or more light-emitting components, for example an inorganic phosphor and / or a luminescent coordination compound, for example a metal-organic framework (MOF).

[0017] According to the invention, the color particles have a particle size between 100 nm and 600 nm, preferably between 150 nm and 350 nm. Especially with particle sizes between 150 nm and 350 nm, advantageous half-wavelength effects, so-called λ / 2 effects, occur in the visible spectrum. In particular, the color particles have a D50 value of essentially 280 nm, i.e., 280 nm with a predetermined maximum deviation, which can be, for example, 10%, 5%, or 2%. This has the advantage that particles of this size have the highest scattering effect for visible light, but at the same time do not absorb visible light, which would have the negative effect of darkening.

[0018] In a particularly preferred embodiment, the pores of the porous structure, i.e., the pores of the individual color pigment, which are not to be confused with the pores between several color pigments in an accumulation of color pigments, have a size of more than 50 nm. The size of the pores is preferably less than 500 nm. The pores of the porous structure are therefore macropores according to the IUPAC definition. This has the advantage that the scattering effect on the color layer is particularly pronounced.

[0019] According to the invention, the thickness of the color layer is at least 500 nm. In particular, the thickness of the color layer can be at least four particle layers or at least 1 µm. The thickness of the color layer is preferably no more than 3 µm, in particular no more than 2 µm. This has the advantage that the color pigment is particularly small, which in the present context results in favorable light refraction effects. The minimum thickness ensures that no or only a negligible proportion of incident light is absorbed by the pigment core.

[0020] In a particularly advantageous embodiment, the color pigment has a size between 1 µm and 20 µm, in particular between 2 µm and 10 µm. This has the advantage that with larger amounts of the color pigments, the white impression is enhanced and the color pigment can be easily further modified with inorganic or organic colored molecules and / or pigments in a simple manner known to those skilled in the art, thus allowing virtually any color to be achieved in a particularly simple manner.

[0021] In another advantageous embodiment, it is provided that the magnetic particle or particles, i.e. the pigment core, is larger than the respective color particles. In particular, the pigment core or the magnetic particle(s) can be at least a factor of 2 larger, preferably at least a factor of 5 or 10 larger than the respective color particles. Preferably, the pigment core or the magnetic particle(s) also has a higher density than the color layer or the color particles. The difference in density and / or size promotes thermophoretically driven self-assembly in the form that the magnetic particle(s) are present in the pigment core and the white, smaller color particles are arranged as satellites around the larger pigment core. This also simply promotes the white or light impression of the color pigment.

[0022] In a further particularly advantageous embodiment, the color particles partly contain a first material with a higher refractive index, for example titanium dioxide, or are partly formed by the first material, and partly contain a second further material with a lower refractive index than the first material or are partly formed by the second material. Color particle mixtures made up of color particles of different types are therefore possible and even advantageous. The scattering effect can be enhanced even further, particularly when the different color particles are of a similar size, for example come from the same size range described above. For this purpose, the highly refractive particles can be individually spaced apart from one another at a distance which is greater than the distance between the particles when only one type of color particle is used.For example, the high-refractive index color particles such as titanium dioxide can be mixed with colloidal silica particles in a 50:50 ratio.

[0023] In another advantageous embodiment, the particles at least partially, i.e., partially or entirely, contain titanium dioxide or are formed from titanium dioxide. The titanium dioxide has, in particular, the "rutile" crystallization form, preferably entirely the rutile crystallization form. This ensures particularly good light scattering, since titanium dioxide, especially in the rutile crystallization form, has a particularly high refractive index.

[0024] In a particularly advantageous embodiment, a metallic mirror layer, i.e. a metal mirror layer or metal mirror layer, is arranged between the color layer and the pigment core. In particular, this mirror layer can be made of or with a metal that reflects visible light, for example silver and / or aluminum and / or gold and / or copper and / or nickel and / or chromium. Silver has proven particularly advantageous here. This has the advantage that visible light that may have passed through the color layer is not absorbed by the pigment core, but is reflected by the metal mirror layer. This also increases the brightness of the color pigment.

[0025] In a particularly advantageous embodiment, a primer layer, a so-called primer layer, can also be placed between the mirror layer and the pigment core.

[0026] Primer, can be applied. This primer layer can consist of or contain titanium dioxide. This has the advantage that the metal mirror layer adheres particularly well to the pigment core. Furthermore, due to the material-typical roughness of a titanium dioxide coating on a pigment core with or made of a magnetic particle, diffuse light reflection on the metal mirror layer is promoted, which further promotes a bright color impression.

[0027] In a further particularly advantageous embodiment, the color layer is arranged between the pigment core and an outer layer. The outer layer in particular contains one or more light-emitting components or consists of one or more light-emitting components. Such a light-emitting component is, for example, an inorganic phosphor based on oxides or semiconductors, as used in common lighting devices, and / or a complex or coordination compound composed of cationic metal centers and organic ligands, for example a so-called metal-organic framework (MOF), in particular a luminescent MOF. In this way, it is also possible to provide the color pigment with a coating, for example with an ormocer varnish or other varnish, in order to increase its resistance to oxidation or mechanical influences.A metal-organic framework (MOF) or another compound that luminesces white, for example, can be applied or integrated onto, under, or within this coating. This can optionally further enhance the brilliance of the white color and, in the case of colorful pigments, increase the brightness. MOFs also enable additional functions for new applications in the field of switches and sensors that rely on changes in luminescence under the influence of chemical and / or physical changes (see, for example, patent application DE 10 2015 109 637).

[0028] The composite nature of such a composite color pigment allows for the targeted implementation of components that enhance the color or even carry properties independent of the color layer. The white-emitting phosphors offer the advantage of enhancing the white impression or brightness in the optical appearance of the color pigment, even with moderate emission intensity. Furthermore, the spherical emission during the luminescence process ensures angle independence, which is beneficial for the color impression. The fact that MOFs can in principle be combined with Fe3O4-based magnetic particles, albeit without a white impression and without white light emission, is evident from the aforementioned German patent application DE 10 2015 109 637.

[0029] The use of strong white emitters, i.e. luminescent substances that emit (intense) white light when stimulated, in one of the layers of the color pigment also allows further functions to be realized in the composite color pigment. While the described weak intensity merely enhances brightness, strong white emission can be used for switchable materials where the emission can be switched on and off, thus making the white character visible. This also allows particularly preferred properties to be created, for example combining a specific color with white in such a way that the color is visible under visible light and white is visible under UV radiation. The switchability is stimulus-dependent, i.e. on the excitation, typically light with higher energy, but also dependent on other stimuli of a chemical or physical nature.This opens up further application possibilities for the color pigment, which can be used in optical switches and sensors, as well as in switchable materials where the emission can be switched on and off, thus also making the white character visible. This switchability can also be used for complex security features.

[0030] A further aspect relates to a paint - for example a wall paint or a car paint or a decorative paint - or paper, or ceramic, or an elastomer - for example silicone such as magnetic sealing lips in silicone seals in showers - or a thermoplastic or a fluid - in particular a fluid with capsules, for example for color-changing properties in magnetically switchable paints - or a tracer - in particular a magnetic tracer in white powders such as medicines or drugs - or a display device - in particular a display device with magnetic ink or a magnetochromic display device - or a dental material or an adhesive - in particular a polymeric and / or elastomeric adhesive - or a polymer or a thermally crosslinkable monomer or an electromagnetic shield - such as a conductive material on an insulator, preferably in paper and textiles,for example in lightweight construction applications, or an optical switch - in particular a microscopic optical switch - or a sensor - in particular a sensor dependent on temperature and / or pressure and / or chemical substances and / or pH value - or a security feature with one or more of the described color pigments.

[0031] This has the advantage that the magnetic properties of the color pigments can be exploited, for example, to actuate and / or detect and / or inductively heat the material modified with the color pigments. Thus, in the various applications mentioned, the combination of a white or tunable colored color pigment with magnetic properties, i.e. the activation of the color pigments by electromagnetic fields, can be exploited. The focus can be on magnetic actuation via the magnetic core and magnetically inductive heating via the magnetic core, but also on the combination of core and conductive metallic shell, as well as on electromagnetic shielding via the combination of the magnetic core and the conductive shell. The white or bright color impression can be modified and enhanced by the emission of white light.The color pigments can be used as additives in paper, ceramics, polymers, especially elastomers, textiles such as technical textiles and clothing, as well as other matrices where a white color is important. Paper can also be enhanced with electromagnetic properties and then used, for example, as magnetically actuated paper or for shielding against electromagnetic fields. The use of color pigments in textiles also opens up the field of application for sensory textiles in the field of bodywear. When used as magnetic pigments for anti-counterfeiting protection, they can be read as magnetic markers or tracers by suitable detectors. They can also be used as magnetically movable white or colored pigments, for example in capsules for color changes.

[0032] In crosslinking reactions (bonding and / or debonding), in polymer and / or elastomeric adhesives, and the like, the magnetic color pigments can be used through inductive heating. In magnetochromic applications, the magnetic color pigments can be used, for example, for electromagnetically controllable white pigments or magnetophoretic displays or indicators.

[0033] A further aspect relates to a method for producing magnetic color pigments with a pigment core composed of at least one magnetic particle and with a color layer surrounding the pigment core, which color layer comprises a plurality of color particles forming a porous structure with a particle size between 100 nm and 600 nm and a thickness of at least 500 nm. The method comprises the process steps of first producing a first suspension with the color particles in a first solvent and the process step of second producing a second suspension with the magnetic particles in a second solvent. The first and second solvents can be identical, for example, water.

[0034] As a further process step, the particles of the color particles of the first suspension are arranged around the magnetic particles of the second suspension by means of spray drying, in which the suspensions are brought together and sprayed by means of a nozzle, for example a two-fluid nozzle, into a drying chamber in which the solvents of the suspensions are evaporated.

[0035] This has the advantage that the described color pigments with the advantageous properties explained above are produced in a simple and robust manner.

[0036] In a particularly advantageous embodiment, the drying chamber is heated to at least 100°C, in particular at least 130°C, for example 140°C. Spray drying is particularly effective in this temperature regime.

[0037] Furthermore, advantageous embodiments and advantages of the method correspond to the embodiments and advantages described for the magnetic color pigment.

[0038] One aspect also relates to a magnetic color pigment produced by the described method, comprising a pigment core composed of at least one magnetic particle and a color layer surrounding the pigment core, the thickness of which is at least 500 nm and which comprises a plurality of color particles forming a porous structure and having a particle size between 100 nm and 600 nm. The advantageous embodiments and advantages described above also apply to this.

[0039] The features and feature combinations mentioned above in the description, as well as the features and feature combinations mentioned below in the description of the figures and / or shown alone in the figures, can be used not only in the respective combination specified, but also in other combinations without departing from the scope of the invention. Thus, embodiments are also to be considered encompassed and disclosed by the invention that are not explicitly shown and explained in the figures, but which emerge and can be produced by separate feature combinations from the explained embodiments. Embodiments and feature combinations are also to be considered disclosed that therefore do not have all the features of an originally formulated independent or dependent claim.Furthermore, embodiments and combinations of features are to be regarded as disclosed, in particular by the embodiments set out above, which go beyond or deviate from the combinations of features set out in the reliances of the claims.

[0040] Embodiments of the invention are explained in more detail below with reference to schematic drawings. In the drawings: Fig. 1 a schematic structure of an exemplary color particle; Fig. 2 shows a schematic flow of a method for producing exemplary magnetic color pigments; and Fig. 3 two representations of a magnetization curve of an exemplary magnetic color pigment.

[0041] In the figures, identical or functionally identical elements are provided with the same reference symbols.

[0042] In part A of Fig. 1 shows an exemplary magnetic color pigment. The color pigment 1 has a pigment core 2 consisting of at least one magnetic particle 9, around which, in the present embodiment, a primer layer 3 consisting of titanium dioxide is arranged. A metallic mirror layer 4 is arranged around the primer layer 3, which, like the base layer 3, completely surrounds the pigment core 2 and the primer layer 3. The metallic mirror layer can be made of silver, for example.

[0043] A color layer 5 containing a multitude of color particles 6 is arranged around the pigment core 2 and, in this case, also around the two additional layers 3, 4. In the example shown, the color particles 6 also consist of titanium dioxide. The color particles 6 form a porous structure with corresponding pores 7, which promote multiple reflections on the respective color particles 6, i.e., in this case, titanium dioxide surfaces, thus enabling diffuse light reflection with an intensity previously unknown for such pigments.

[0044] In the example shown, it can also be seen that the primer layer 2 and the metallic mirror layer 3 are sintered layers and are therefore significantly thinner than the color particles 6 and the magnetic particle 9. In the example shown, the magnetic particle 9 and thus the pigment core 2 are at least three times as large as the color particles 6.

[0045] In part B, the color pigment 1 known from part A is additionally provided with an outer layer 8, which can be designed, for example, as a lacquer, in order to envelop the particles 6 and protect them from any mechanical damage or oxidative attack.

[0046] Alternatively or in addition to the outer layer 8, the previously white color pigment 1 can also be provided with any desired colors, for example by dye infiltration into the porous color layer 5 or a further spray-drying process with a color particle dispersion, in which colored color particles are deposited around the pigment core 2 and the further layers 3, 4 in addition to the white color particles 6.

[0047] In Fig. Figure 2 shows an exemplary embodiment of a self-assembly synthesis principle for the step of assembling the color particles as a satellite particle shell around the pigment core, based on the principle of thermophoresis, which is achieved by means of a spray-drying process. This requires a first suspension 10 and a second suspension 11.

[0048] To create the first suspension 10, commercially available white color particles can be used, for example, nanostructured titanium dioxide produced by flame pyrolysis or other means. Alternatively, suitable white color particles can also be produced in-house, based on an emulsion paint from common paint manufacturers, such as those available at hardware stores. To do this, the emulsion paint must first be diluted several times with the solvents ethanol and water in a typical mass ratio of 1:3, and then the inorganic color pigments are filtered out by centrifugation. These filtered-out color pigments can then be washed with concentrated acid, for example, hydrochloric acid, to remove by-products such as calcium carbonate. This yields inorganic, white micro- and submicroparticles.These or similar white color particles are dispersed after neutralization, i.e. washing with water, for example in water as the first solvent 15 to form the first suspension 10.

[0049] To obtain the second suspension 11, the magnetic particles 9 are dispersed, for example, in water as the second solvent 15' and then wet-chemically modified, for example, initially with a sol-gel titanium dioxide coating to produce the primer layer 3. A thin layer of a metal having reflective properties for the visible light spectrum, preferably silver, gold, copper, or chromium, can then be deposited.

[0050] For the titanium dioxide coating, for example, 4g of carbonyl iron particles can be dispersed in 95g of ethanol and 1g of deionized water (DI water) for 10 minutes using an ultrasonic wand. While stirring, for example, using a KPG stirrer, a tetrabutyl orthotitanate (TBOT) solution containing 2g of TBOT and 8g of ethanol is then slowly and continuously added over a period of, for example, 15 minutes in the described variant. This reaction solution can then be vigorously stirred for three hours at approximately room temperature (20°C or 20°C + / -5°C). The particles can then be magnetically separated, washed several times, for example four times, with ethanol, and finally dried, preferably for approximately four hours at 60°C. The "approximately" specification here and below can refer to a maximum deviation of + / - 10%, + / - 5%, or + / - 2%.

[0051] In the example shown, the mirror layer 4 is also applied. An exemplary possibility in the form of a silver coating is described below. For example, the pigment cores, preferably the pigment cores 9 coated with a primer layer of titanium dioxide described in the previous paragraph, for example 0.3g, can be added together with 0.28g of silver nitrate to 140g of deionized water. 0.06g of sodium hydroxide and 0.40g of a 25% ammonia solution are then added to the resulting dark gray suspension. The dispersion is stirred for 25 minutes. After adding the reducing solution, namely 10g of deionized water with 0.45g glucose and 0.04g trisodium citrate, the reaction solution is stirred for 15 minutes at room temperature. The particles are then magnetically separated, washed four times with ethanol, and then dried for approximately four hours at 60°C. The resulting pigments have a gray color with a metallic luster.

[0052] The magnetic particles 9 coated with silver and / or titanium dioxide, or not coated at all, form the second suspension 11 with water.

[0053] Subsequently, the particles of the suspension 1 are arranged around the coated particles 2 of the second suspension 11 by spray drying in order to obtain the desired core-satellite particle system, the magnetic color pigment 1.

[0054] For this purpose, the first suspension 10 and the second suspension 11 are combined to form a further suspension 12, which is sprayed into a drying chamber 13 using a nozzle, for example a two-fluid nozzle. The suspension 12 is atomized into fine droplets 14, which enter the hot drying chamber 13, which is heated to, for example, 140°C. Over a time course Δt, the solvent, in this case the water in the droplets 14, evaporates, and as a result, the droplets 14 become smaller and smaller. Accordingly, the particles 6, 9 are exposed to thermophoresis. The difference in density and size benefits the thermophoretically driven self-assembly of the color pigments 1 in that the smaller white color particles 6 are present as satellites around the magnetic particle 9 or the pigment core 2.Preferably, the magnetic particle 9 or the coated or uncoated pigment core 2 is at least a factor of 2 or 5, preferably 10, larger than the color particles.

[0055] After the particles or droplets 14 or color pigments 1 have passed through the hot drying chamber 13, this process produces a powder in which the magnetic particle-pigment core@titanium dioxide-primer layer@silver-mirror layer particles are surrounded by the color layer 5, which is composed of the small, here white, color particles 6, thus forming the desired core-satellite system. This thus constitutes the respective {magnetic particle-pigment core@titanium dioxide-primer layer@silver-mirror layer}-core-{titanium dioxide-satellite particle}-color layer pigments. Therefore, three shells or layers are applied around the magnetic particle 9: first, a sol-gel layer 3, then a metal-mirror layer 4, and finally, the nanostructured, particle-based color layer 5.

[0056] In the figure, in addition to the drying chamber 13, the droplets 14 with the magnetic particles 9 and the color particles 6 are shown symbolically for the process taking place in the drying chamber 13 over the time course Δt with their increasing drying and the resulting formation of the color pigment 1.

[0057] In Fig. Figure 3 shows the magnetization curve of an exemplary resulting magnetic color pigment compared to other known systems. Here, the magnetization in emu / g is plotted against the magnetic field strength in oersteds (Oe). Part B is an enlargement of a section of part A.

[0058] The short dashed curve is the magnetization curve of carbonyl iron, the solid magnetization curve is the magnetization curve of the coated pigment cores, i.e. an iron pigment core with a titanium dioxide primer layer and a silver mirror layer, and the long dashed curve is the magnetization curve of the color pigments.

[0059] Especially in part B it is evident that the magnetization is above 5 emu / g and significantly above 10 emu / g, in this case around 13 emu / g, so that a sufficiently strong magnetization is present for a large number of applications.

[0060] Further functionalization of the pigments can be achieved by solvothermal conversion of the color pigments obtained via the processes described above (preferably {iron oxide pigment core@titanium dioxide primer layer} color pigments) with white-emitting substances such as inorganic phosphors or luminescent coordination compounds. For this purpose, the spray-dried pigments are treated with white-emitting substances (preferably europium and terbium-doped IFP-1) with anhydrous hexane in a Duran ® -glass ampoule. After dispersing the reactants in an ultrasonic bath, the reaction mixture in the glass ampoule is heated to 90 °C using a corundum tube furnace. The resulting functionalized, luminescent iron oxide MOF color pigments are separated from the solvent under an inert gas atmosphere and dried in a vacuum.

Claims

[1] Magnetic color pigment (1) with a pigment core (2) made of at least one magnetic particle (9) and a color layer (5) surrounding the pigment core (2) with a plurality of color particles (6), characterized by that the color particles (6) have a particle size between 100 nm and 600 nm and form a porous structure, and a thickness of the color layer (5) is at least 500 nm. [2] Colour pigment (1) according to claim 1, characterized by that the color particles (6) have a particle size between 150 nm and 350 nm. [3] Colour pigment (1) according to one of the preceding claims, characterized by that the pores (7) of the porous structure have a size of more than 50nm. [4] Colour pigment (1) according to one of the preceding claims, characterized by that the thickness of the color layer (5) is at least 1µm. [5] Colour pigment (1) according to one of the preceding claims, characterized by that the color pigment (1) has a size between 1 and 20µm. [6] Colour pigment (1) according to one of the preceding claims, characterized by that the magnetic particle (9) is larger than the color particles (6). [7] Colour pigment (1) according to one of the preceding claims, characterized by that the color particles (6) partly contain a first material with a higher refractive index or are formed by the first material and partly contain a second further material with a lower refractive index compared to the first material or are formed by the second material. [8] Colour pigment (1) according to one of the preceding claims, characterized by that the color particles (6) at least partially contain titanium dioxide or are formed from titanium dioxide. [9] Colour pigment (1) according to one of the preceding claims, characterized by that a metallic mirror layer (4) is arranged between the color layer (5) and the pigment core (2). [10] Colour pigment (1) according to claim 9, characterized by that a primer layer (3) is arranged between the mirror layer (4) and the pigment core (2). [11] Colour pigment (1) according to one of the preceding claims, characterized by that the color layer (5) is arranged between the pigment core (2) and an outer layer (8). [12] Paint or paper or ceramic or elastomer or thermoplastic or fluid or tracer or display device or dental material or adhesive or polymer or thermally crosslinkable monomer or electromagnetic shielding or optical switch or sensor or banknote or security feature with color pigments (1) according to any one of the preceding claims. [13] Method for producing magnetic colour pigments (1) with a pigment core (2) made of at least one magnetic particle and with a colour layer (5) surrounding the pigment core (2), which has a multiplicity of colour particles (6) forming a porous structure with a particle size between 100 nm and 600 nm and a thickness of at least 500 nm, with the method steps: - producing a first suspension (10) with the color particles (6) in a first solvent (15); - producing a second suspension (11) with the magnetic particles (9) in a second solvent (15'); characterized by a - Arranging the color particles (6) of the first suspension (10) around the magnetic particles (9) of the second suspension (11) by means of spray drying, in which the suspensions (10, 11) are brought together and sprayed by means of a nozzle into a drying chamber (13) in which the solvents (15, 15') of the suspensions (10, 11) are evaporated. [14] Method according to claim 13, characterized by that the drying chamber (13) is heated to at least 100°C.

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