DE102005018615B4 - Transparent conductive pigments - Google Patents

Abstract

Transparent, platelet-shaped, electrically conductive pigments comprising platelet-shaped substrates coated with a conductive layer, characterized in that 1) the platelet-shaped substrates are selected from the group consisting of platelet-shaped TiO 2, synthetic or natural mica, layer silicates, glass platelets, platelet-shaped SiO 2 and / or platelet-shaped Al2O3, 2) the conductive layer comprises one or more doped metal oxides selected from the group consisting of tin oxide, zinc oxide, indium oxide and / or titanium oxide, 3) the number-weighted average grain area F50 of the transparent, platelet-shaped, electrically conductive pigments is greater than or equal to 150 μm 2 and 4) the grain area indicates the value of the major surface area with the longest axis of the pigment platelets and by measuring under the microscope and counting the measured particles visually or automatically by means of a video camera and a suitable automatic image analysis software is determined, wherein at least 1,000 particles are measured.

Description

The present invention relates to transparent, platelet-shaped, electrically conductive pigments comprising platelet-shaped substrates coated with a conductive layer, wherein the number-weighted average grain area F _{50 of} the transparent conductive pigments is greater than or equal to 150 μm ² , processes for the preparation of the pigments and their use.

State of the art

Electrically conductive pigments are used today in various applications, such. For example, for antistatic coatings, antistatic floor coverings, antistatic equipment explosion-proof rooms or electrically conductive primers for painting plastics. Common is the use of carbon black or graphite to increase the conductivity. These substances have the disadvantage that they have no transparency and lead to darkening of the materials thus offset.

Furthermore, electrically conductive pigments based on transparent platelet-shaped substrates, especially based on thin mica platelets, are known. These pigments consist of mica coated with (Sn, Sb) O ₂ or with a layer sequence of TiO ₂ / SiO ₂ / (Sn, Sb) O ₂ (eg ^Minatec® 31 CM or 30 CM from Merck KGaA) , Such pigments are z. In the patents DE 38 42 330 . DE 42 37 990 . EP 0 139 557 . EP 0 359 569 and EP 0 743 654 described. In the EP 0 139 557 A1 describes conductive pigments whose platelet-shaped substrates have a major axis of 0.1-100 microns, a major axis to minor axis ratio of 1-30 and a ratio of minor axis to thickness of ≥ 5. A certain grain area is not required for these pigments.

All of these pigments are distinguished from the main customary conductive pigment carbon black in that they have a light color and a higher transparency. Therefore, these pigments have significant advantages over the carbon black compared to the carbon black because they give the stylist crucial additional latitude in the color of the application system (paints, plastics, inks). Thus, color and electrical conductivity can be combined in a very good manner, while the use of soot is always limited to dark colors. Overall, however, the demand for particularly transparent and light conductive pigments has not been met.

task

The highest demands in terms of transparency exist in the application of conductive pigments in clearcoat films, printing inks and transparent or translucent plastic articles, eg. As slides, containers, cards or discs. For these applications, it is not sufficient for the conductive pigment to be only slightly colored or gray, because in the more transparent media the light scattering of the pigments is becoming increasingly noticeable. The outer conductive coating of the pigment particles consists of metal oxides whose refractive index is higher than that of the conventional binders in paints, coatings or plastics. Therefore, the pigments embedded in a polymer matrix act as scattering centers for light, causing turbidity of the layers containing the pigments. This occurs even when the pigment particles themselves are highly transparent. The turbidity is the more significant the more transparent the medium itself is. Particularly sensitive are clear plastic films and clearcoat films. Are these materials with pigments from the prior art, for. B. Minatec ^® 31 CM, these appear in the review highly cloudy and in the supervision of white to light gray. By lowering the pigment concentration, the turbidity can be reduced, but this is accompanied by a disproportionate decrease in conductivity and is therefore not a viable alternative for the user. There is therefore an urgent need for electrically conductive pigments with improved transparency in the application medium while maintaining a good conductivity.

Surprisingly, it has been found that pigments according to the present invention fulfill this requirement profile. The present invention therefore relates to transparent, platelet-like, electrically conductive pigments comprising platelet-shaped substrates coated with a conductive layer, wherein 1) the platelet-shaped substrates are selected from the group consisting of platelet-shaped TiO ₂ , synthetic or natural mica, layer silicates, glass platelets, platelet-shaped SiO ₂ and / or platelet-shaped Al ₂ O ₃ , 2) the conductive layer comprises one or more doped metal oxides selected from the group consisting of tin oxide, zinc oxide, indium oxide and / or titanium oxide, 3) the number-weighted average grain area F _{50 of} the transparent, platelet-shaped , electrically conductive pigments is greater than or equal to 150 μm ² and 4) the grain area is the value for the size of the main area with the longest axis indicates the pigment platelet and is determined by measuring under the microscope and counting the measured particles visually or automatically performed by means of a video camera and a suitable automatic image analysis software, wherein at least 1000 are measured. The present invention furthermore relates to a process for the preparation of the pigments of the invention comprising the coating of platelet-shaped substrates with a conductive layer, wherein before and / or after coating with a conductive layer, the proportion of platelet-shaped substrates or transparent conductive pigments having a grain area of less than 150 microns ^{2 is} reduced by sedimentation, decantation, air classification and / or screening such that the average number-weighted grain area F _{50 of} the transparent conductive pigments is greater than or equal to 150 microns ² . The use of the pigments according to the invention in paints, lacquers, printing inks, plastics, floor coverings, films, formulations, ceramic materials, glasses, paper, for laser marking, in thermal protection, in dry preparations or in pigment preparations is likewise an object of the present invention.

Electrically conductive pigments according to the present invention have the advantage that they show improved transparency of the conductive material at higher conductivity compared to those from the prior art already at the same use concentration. The reduced according to the present invention fines of the transparent, platelet-shaped, electrically conductive pigments contribute apparently less than proportional to the conductivity in the application, but disproportionately to the light scattering. The gain in conductivity allows a reduction in the use concentration of the pigment and thus a further improvement in the transparency (lower turbidity) in the respective application.

The transparent, platelet-shaped, electrically conductive pigments according to the invention are based on platelet-shaped substrates which are coated with a conductive layer. The platelet-shaped substrates are selected from the group consisting of platelet-shaped TiO ₂ , synthetic or natural mica, phyllosilicates, glass platelets, platelet-shaped SiO ₂ and / or platelet-shaped Al ₂ O ₃ . The platelet-shaped substrates are preferably mica, platelet-shaped SiO ₂ or glass platelets.

The electrically conductive layer comprises one or more doped metal oxides selected from the group consisting of tin oxide, zinc oxide, indium oxide, and / or titanium oxide. Preferably, the metal oxide is tin oxide, indium oxide and / or zinc oxide. The said metal oxides are doped in the conductive layer, wherein the doping with gallium, aluminum, indium, thallium, germanium, tin, phosphorus, arsenic, antimony, selenium, tellurium and / or fluorine can take place. In this case, individual of the stated dopants but also combinations thereof can be present in the conductive layer. Preferably, aluminum, indium, tellurium, fluorine and / or antimony are used for doping the metal oxides. The proportion of dopants in the conductive layer may be 0.1 to 30 wt .-%, preferably it is in the range of 2 to 15 wt .-%. In a particularly preferred embodiment, antimony-doped tin oxide, tin oxide doped with antimony and tellurium, indium oxide doped with tin, aluminum-doped zinc oxide or fluorine-doped tin oxide are used as the conductive layer, antimony-doped tin oxide being particularly preferred. The tin to antimony ratio in this preferred combination may be 4: 1 to 100: 1, preferably the ratio is 7: 1 to 50: 1. Lower antimony contents negatively affect the conductivity, whereas higher antimony contents lower the transparency of the pigments of the invention.

The proportion of the conductive layer, based on the platelet-shaped substrate, may be 25 to 120 wt .-%, preferably, the proportion is from 50 to 75 wt .-%.

An essential feature of the transparent, platelet-shaped, electrically conductive pigments according to the invention is the number-weighted mean grain area F _{50 of} the transparent, platelet-shaped, electrically conductive pigments which should be greater than or equal to 150 μm ² , the grain area having the value for the size of the main area with the longest Indicates the axis of the pigment platelets and is determined by measuring under the microscope and counting the measured particles visually or automatically with the aid of a video camera and a suitable automatic image analysis software, wherein at least 1000 particles are measured. Preferably, the number-weighted mean grain area F _{50 of} the transparent, platelet-shaped, electrically conductive pigments is greater than or equal to 200 μm ² . The transparent, conductive pigments of the invention are thin platelets having an average thickness of 2 microns or less, preferably 0.3-0.6 microns and a form factor of at least 50, preferably more than 100. For the function of the transparent, platelet-shaped, electrically conductive pigments in the Dielectric application media, the grain area is a very significant size. Larger grain area platelets are better suited for the formation of electrically conductive paths in dielectric media than smaller platelets because the same Path length less large platelets must contact than in the case of smaller platelets. Because the dielectric medium surrounding the pigments acts as an insulator and impedes the charge transfer from pigment particles to pigment particles, a conduction path of many small conductive particles is less conductive than an equally sized path of fewer larger particles.

The transparent, platelet-shaped, electrically conductive pigments according to the invention are characterized by their grain area. This refers to the value for the size of the main surface of the platelets, ie the surface with the longest axis. The platelets themselves can be approximately circular or oblong, the shape more or less regular. As a rule, the plates are somewhat oblong with an average ratio of the major axis to the minor axis (sufficiency factor) of approximately 1.2. The particle size distribution of the pigments according to the invention can vary within wide ranges, depending on the requirements of the applications. For use in paints tighter particle size distributions are required than for example for use in plastic articles. For varnish applications, offset or gravure printing inks, the number-weighted grain area F ₉₅ should not be more than 1500 μm ² , preferably not more than 1200 μm ² , ie a maximum of 5% of the particles have a grain area of 1500 μm ² , preferably 1200 μm ² or more. In contrast, in plastic applications a number-weighted grain area F ₉₅ of 7500 μm ² at a number-weighted average grain area F ₅₀ of 1200 μm ^{2 is} still acceptable. In general, a narrow particle size distribution of the pigment in terms of application properties is low, the value for F ₉₅ should preferably be 5 to 7 times the value for F _50th However, the proportion of fine grain in the pigment is important for the transparency of the medium containing the transparent, platelet-shaped, electrically conductive pigment.

It has proved to be particularly advantageous if the number-weighted proportion of platelets with a particle area of less than 80 μm ^{2 is} less than or equal to 33%, based on the transparent, platelet-shaped, electrically conductive pigments. The number-weighted proportion of platelets having a particle area of less than 80 μm ² is preferably less than 25%. Particularly advantageous results in terms of conductivity and transparency of the respective applications are achieved when the number-weighted proportion of platelets with a particle size of less than 40 microns ^{2 is} less than or equal to 15%, preferably less than or equal to 10%, based on the transparent platelet-shaped , electrically conductive pigments. As a result of the abovementioned reduction of the fines, the light scattering and thus the haze in the applications is reduced, so that systems which appear to be particularly transparent are obtained, but at the same time have a conductivity which corresponds to the requirements.

In a further embodiment, one or more layers comprising metal oxides, metal oxide hydrates, metal suboxides, metal fluorides, metal nitrides, metal oxynitrides or mixtures of these materials can be located above and / or below the conductive layer. By applying these additional layers, the color properties of the pigments can be adapted to the requirements of the users, in particular if the additional layers are located below the conductive layer. The application of additional layers above the conductive layer allows the conductivity in the applications containing these pigments to be adapted to the requirements. The application of additional layers in the pigments of the invention thus allows the targeted control of the color impression and the conductivity in the respective applications. The metal oxide, metal oxide hydrate, metal suboxide, metal fluoride, metal nitride, metal oxynitride layers or mixtures thereof may be low (refractive index <1.8) or high refractive index (refractive index ≥ 1.8). Suitable metal oxides and metal oxide hydrates are all common metal oxides or metal oxide hydrates to be applied as layers, such as. Example, alumina, alumina hydrate, silica, Siliziumoxidhydrat, iron oxide, tin oxide, cerium oxide, zinc oxide, zirconium oxide, chromium oxide, titanium oxide, in particular titanium dioxide, titanium oxide and mixed phases thereof, such as. Ilmenite or pseudobrookite. As metal suboxides, for example, the titanium suboxides can be used. As a metal fluoride, for example, magnesium fluoride is suitable. As metal nitrides or metal oxynitrides, for example, the nitrides or oxynitrides of the metals titanium, zirconium and / or tantalum can be used. Preference is given to applying metal oxide, metal fluoride and / or metal oxide hydrate layers and very particularly preferably metal oxide and / or metal oxide hydrate layers above and / or below the conductive layer. Furthermore, multi-layer constructions of high-refractive and low-refractive index metal oxide, metal oxide hydrate or metal fluoride layers can also be present, alternating preferably high and low refractive index layers.

Particularly preferred are layer packages of a high and a low refractive index layer, it being possible for one or more of these layer packages to be applied above and / or below the conductive layer. The order of the high- and low-index layers can be adapted to the substrate in order to include this in the multi-layer structure. Conductive pigments containing these Layers can show an angle-dependent change of the color (color flop) due to interference with a suitable selection of the materials and the respective proportions.

The outer layer of the above single or multiple layers in a preferred embodiment is a high refractive index metal oxide. This outer layer may additionally be part of a layer package on the above-mentioned layer packages or on high-index substrates and z. Example of TiO ₂ , titanium suboxides, Fe ₂ O ₃ , SnO ₂ , ZnO, ZrO ₂ , Ce ₂ O ₃ , CoO, Co ₃ O ₄ , V ₂ O ₅ , Cr ₂ O ₃ and / or mixed phases thereof, such as Example ilmenite or pseudobrookite exist. TiO ₂ is particularly preferred.

The proportion and thus the thickness of the individual layers, in particular of the metal oxide layers, is generally not critical. Overall, however, the layer thicknesses of the single or multiple additional layers are to be adjusted such that the transparency of the conductive pigments according to the invention is not substantially reduced.

Particularly preferred pigments of the present invention include mica coated with an antimony doped tin oxide layer, mica coated with a titanium oxide layer, a silicon oxide layer and an antimony doped tin oxide layer or mica doped with an antimony doped tin oxide layer and a metal oxide layer , in particular a titanium oxide layer, is coated.

Likewise provided by the present invention are processes for the preparation of the pigments of the invention, comprising the coating of platelet-shaped substrates with a conductive layer, characterized in that before and / or after coating with a conductive layer, the proportion of platelet-shaped substrates or transparent, platelet-shaped, electrically conductive pigments with a number-weighted grain area of less than 150 microns ² by sedimentation, decantation, air classification and / or screening is reduced such that the number-weighted mean grain area F _{50 of} the transparent, platelet-shaped, electrically conductive pigments is greater than or equal to 150 microns ² . The proportion of platelet-shaped substrates or transparent, platelet-shaped, electrically conductive pigments having a particle area of less than 200 μm ^{2 is} preferably reduced such that the number-weighted mean particle area F _{50 of} the transparent, platelet-shaped, electrically conductive pigments is greater than or equal to 200 μm ² . This can be achieved, for example, by reducing the proportion of pigments having a grain area of less than 80 μm ² to less than 33%, preferably to less than 25%, based on the transparent, platelet-shaped, electrically conductive pigments. In addition, in the process according to the invention, the proportion of pigments having a particle size of less than 40 μm ^{2 can be} reduced to less than 15%, preferably to less than 10%, based on the transparent, platelet-shaped, electrically conductive pigments. This reduction of the fines is crucial for the improved properties of the pigments of the invention and can be carried out either during the grinding and classification of the platelet-shaped substrate or at the end of the production process on the crude product. The separation of the fines is carried out by sedimentation, decantation, air classification and / or screening. In the case of sedimentation, for example ground mica can already be sedimented several times during the milling and classification process. The classification of ground mica by air classification is possible, but the mica must be dried in this process variant. The thus classified, fines depleted mica is then provided with a conductive layer. The depletion of the fine fraction can also take place from the conductive pigment after coating with a conductive layer, for. B. by multiple sedimentation of the crude product and separation of the slower sedimenting smaller particles by decantation. Finally, the classification of the dried and calcined pigment by air classification or sieving is possible. The control of the reduction of fines is done by measuring under the microscope and counting the measured particles. This can be carried out visually, possibly simplified by comparisons of the samples against counted standards or automatically by means of a video camera and a suitable automatic image analysis software. Such automatic evaluation systems for particle size analysis are known to the person skilled in the art and commercially available. For a statistically verified particle size analysis at least 1000, preferably 2000 particles or more should be measured.

The coating of the platelet-shaped substrates with a conductive layer takes place by the coating methods known from the prior art, preferably wet-chemically, the substrates being suspended in water and admixed with one or more hydrolyzable metal salts at a pH suitable for the hydrolysis, the metal oxides , Metal hydroxides or metal oxide are precipitated on the platelets and the pigments are separated, dried and calcined.

In a simplest embodiment of the process according to the invention, a layer of a doped metal oxide, metal hydroxide or metal oxide hydrate is applied to the platelet-shaped substrate from suitable precursors. In this case, the precursors for the metal oxide and the doping can be either separately, preferably continuously, or mixed with one another, ie jointly present in a solution. Suitable precursors are the corresponding halides, nitrates, sulfates, phosphates or oxalates, preferably the corresponding halides are used. Such methods are for. B. described in DE 42 37 990 . DE 38 42 330 . EP 0 139 557 , the disclosure of which is hereby incorporated by reference. The optimization of the application conditions lies in the area of expert know-how. Usually, in the case of wet coating, the substrates are suspended in water and admixed with one or more hydrolyzable metal salts at a pH suitable for the hydrolysis, which is chosen such that the metal oxides, metal hydroxides or metal oxide hydrates are precipitated directly on the platelets without it comes to Nebenfällungen. The pH is usually kept constant by simultaneous addition of a base or acid. The pigments are separated after application of the conductive layer, dried and usually calcined in air or under inert gas or reducing conditions, usually at temperatures of 300 to 900 ° C, preferably at temperatures of 650 to 850 ° C.

In a further embodiment of the method according to the invention, one or more layers comprising metal oxides, metal oxide hydrates, metal suboxides, metal fluorides, metal nitrides, metal oxynitrides or mixtures of these materials are applied before and / or after the coating of the platelet-shaped substrates with a conductive layer. The application of single or multiple layers can be carried out by means of sol-gel method, CVD and / or PVD method. Preferably, a coating with these materials is wet-chemically.

If desired, after the conductive layer has been applied, the pigments can be separated, dried and optionally calcined, in order then to be re-suspended for the precipitation of further additional layers. In an alternative embodiment, all the desired layers can first be precipitated and subsequently calcined in their entirety.

The pigments according to the invention are suitable for a wide range of applications because of their advantageous properties. The invention therefore also relates to the use of the pigments according to the invention in paints, coatings, printing inks, plastics, in security applications, floor coverings, films, formulations, ceramic materials, glasses, paper, for laser marking, in thermal protection, in dry preparations or in pigment preparations.

In the case of formulations, the pigments of the invention are particularly suitable for formulations which are to have a conductivity, for. B. conductive pastes. Of course, the pigments according to the invention in the formulations can also be combined with any type of raw materials and excipients. These include u. a. Oils, fats, waxes, film formers, preservatives and general application properties determining auxiliaries, such. As thickeners and rheological additives such as bentonites, hectorites, silica, Ca silicates, gelatin and / or surface-active aids, etc.

When using the pigments in paints and paints all known in the art application areas are possible, such. As powder coatings, automotive coatings, printing inks for gravure, offset, screen or flexographic printing and coatings in outdoor applications. For the production of printing inks a variety of binders, in particular water-soluble types, suitable, for. Based on acrylates, methacrylates, polyesters, polyurethanes, nitrocellulose, ethylcellulose, polyamide, polyvinyl butyrate, phenolic resins, maleic resins, starch or polyvinyl alcohol. The paints may be water- or solvent-based paints, the selection of the paint components is subject to the general knowledge of the skilled person.

In addition, the pigments according to the invention can be used in particular for the production of conductive films and plastics, such. As for conductive films and discs, plastic containers and moldings for all known in the art applications that require conductivity. Suitable plastics are all common plastics for the incorporation of the conductive pigments of the invention, for. As thermosets or thermoplastics. The description of the applications and the usable plastics, processing methods and additives can be found z. In RD 472005 or in R. Glausch, M. Kieser, R. Maisch, G. Pfaff, J. Weitzel, pearlescent pigments, Curt R. Vincentz Verlag, 1996, 83 et seq., The disclosure of which is hereby incorporated by reference.

The pigments of the invention are also suitable for use in blends with organic dyes, pigments and / or other conductive materials, such. As soot, transparent and covering white, colored and black pigments and with platelet-shaped iron oxides, organic pigments, holographic pigments, LCPs (Liquid Crystal Polymers) and conventional transparent, colored and black luster pigments based on metal oxide-coated platelets based on mica, metal, glass, Al ₂ O ₃ , Fe ₂ O ₃ , SiO ₂ etc. The pigments according to the invention can be mixed in any ratio with commercially available pigments and fillers.

As fillers z. Natural and synthetic mica, nylon powder, pure or filled melamine resins, talc, glasses, kaolin, oxides or hydroxides of aluminum, magnesium, calcium, zinc, biocl, barium sulfate, calcium sulfate, calcium carbonate, magnesium carbonate, carbon, as well as physical or chemical combinations to name these substances.

The pigments according to the invention are furthermore suitable for the preparation of flowable pigment preparations and dry preparations comprising one or more pigments according to the invention, binders and optionally one or more additives. Dry preparations are also to be understood as preparations which contain 0 to 8% by weight, preferably 2 to 8% by weight, in particular 3 to 6% by weight, of water and / or of a solvent or solvent mixture. The dry preparations are preferably in the form of pellets, granules, chips, sausages or briquettes and have particle sizes of 0.2-80 mm.

embodiment

The following examples are intended to illustrate the invention without, however, limiting it.

Example 1: Production variant A for samples 1 and 2

a) Classification of the substrates:

Raw mica is finely ground in a Koller and classified by means of a decanter into different fractions, which differ in grain size and grain spectrum. The fractions are listed in Table 1 with their particle size, characterized by the number-weighted mean grain area F ₅₀ and the fines fraction based on platelet-shaped particles below 80 μm ² and below 40 μm ² in%, measured on the particle number.

b) Coating the Substrates with Conductive Layer:

100 g of mica according to Table 1 are suspended in 1900 ml of deionized water and adjusted to pH 2.1 with hydrochloric acid. At 75 ° C., a mixture of 191.8 g of an aqueous SnCl ₄ solution (50% by weight), 56 ml of HCl (37% by weight), 20.4 g of an aqueous SbCl ₃ solution (35% by weight) is added with stirring. %) added continuously to the suspension over 7 hours. By simultaneous controlled metered addition of sodium hydroxide solution, the pH is kept constant. After addition of the total amount of 290 ml of the solution is stirred for a further 30 min at 75 ° C, then cooled with stirring to room temperature and the reaction mixture adjusted to pH 3. The resulting pigment is filtered through a suction filter, washed with water, dried at 140 ° C and calcined at 800 ° C for 30 min. This gives 158 g of a light gray pigment powder. The ratio Sn: Sb in the coating is about 92: 8.

Example 2 Production Variant B for Samples 1 and 2

a) Coating of the Substrates with Conductive Layer:

Ground mica with a high fines content is coated analogously to Example 1b) with conductive antimony tin oxide.

b) Classification of Conductive Pigments:

21 kg of the crude pigment thus obtained are classified by air classification. 3 kg of fines, preferably fines with a grain area below 80 microns ² , are separated. There are obtained 18 kg of pigment according to Table 1.

Comparative Examples (Samples 3 and 4):

As in Example 1, Samples 3 and 4 are prepared by appropriate coating with conductive antimony tin oxide.

Testing the conductivity in the paint film:

The samples according to Table 1 are dispersed in NC lacquer (6% collodion and 6% butyl acrylate-vinylisobutyl ether copolymer in a solvent mixture). With the paint preparation glass plates are coated. A glass plate is coated with varnish without pigment. The concentration of the conductive pigments in the dry lacquer layer is 48 wt .-%, the layer thickness 40 microns. After drying the lacquer layers, the bleeder resistance according to DIN 53482 is measured with the aid of a spring tongue electrode. The results are shown in Table 1. The comparison resist film (Sample 5) without conductive pigment has a resistance of> 10 ⁷ kilo-ohms.

Measurement of turbidity:

The pigments according to Table 1 are dispersed in NC lacquer (6% collodium and 6% butyl acrylate-vinylisobutyl ether copolymer in a solvent mixture). The paint preparation is coated with a squeegee PET films (thickness 100 microns). The layer thickness of the dry layer is 60 microns, the concentration of the pigments in the paint is 14 wt .-%, based on the binder. To determine the turbidity, the coated films are measured in a UV-VIS IR spectrometer (type: Perkin Elmer Lambda 900) with integrating sphere (150 mm diameter). The total transmission (diffuse + directional transmission) of the samples and the diffuse transmission are measured. The proportion of diffuse transmission in the total transmission is a measure of the turbidity of the samples. To compare the samples, a turbidity factor (Haze factor, H) is determined, defined as the percentage of diffuse transmission (T _d ) on the total transmission (T _g ) according to the formula H [%] = (T _d / T _g ) x 100.

The transmission is measured in the spectral range of 400-700 nm.

The results are shown in Table 1. They show that only the coatings with the pigments according to the invention show good conductivity and low haze and thus improved transparency. Table 1: sample F ₅₀ [μm ² ] C _particles <80 μm ² [%] C _particles <40 μm ² [%] Conductivity [kΩ] Turbidity factor H [%] Remarks 1 195 21 13 4 30 invention 2 230 14 8.5 10 22 invention 3 36 74 54 100 41 comparison 4 60 52 39 74 56 comparison 5 - - - > 10 ⁷ 7.6 No pigment

Claims (14)

Transparent, platelet-shaped, electrically conductive pigments comprising platelet-shaped substrates coated with a conductive layer, characterized in that 1) the platelet-shaped substrates are selected from the group consisting of platelet-shaped TiO ₂ , synthetic or natural mica, layer silicates, glass platelets, platelet-shaped SiO ₂ and / or platelet-shaped Al ₂ O ₃ , 2) the conductive layer comprises one or more doped metal oxides selected from the group consisting of tin oxide, zinc oxide, indium oxide and / or titanium oxide, 3) the number-weighted average grain area F _{50 of} the transparent, platelet-shaped, electrically conductive pigments is greater than or equal to 150 μm ² ; and 4) the grain area indicates the size of the major area of the longest axis of the pigment platelets, and visually or automatically by microscopy and counting of the measured particles amera and a suitable automatic image analysis software is determined, with at least 1000 particles are measured. Transparent conductive pigments according to claim 1, characterized in that the number-weighted average grain area F _{50 of} the transparent conductive pigments is greater than or equal to 200 μm ² . Transparent conductive pigments according to claim 1 or 2, characterized in that the number-weighted proportion of pigments having a particle size of less than 80 μm ^{2 is} less than or equal to 33%, based on the transparent conductive pigments. Transparent conductive pigments according to one of claims 1 to 3, characterized in that the number-weighted proportion of pigments having a particle size of less than 40 μm ^{2 is} less than or equal to 15% by weight, based on the transparent conductive pigments. Transparent conductive pigments according to one of claims 1 to 4, characterized in that the platelet-shaped substrates are selected from the group consisting of platelet-shaped, synthetic or natural mica, glass platelets and / or platelet-shaped SiO ₂ . Transparent conductive pigments according to any one of claims 1 to 5, characterized in that the metal oxide of the conductive layer is tin oxide, zinc oxide and / or indium oxide. Transparent conductive pigments according to any one of claims 1 to 6, characterized in that the conductive layer of metal oxides doped with gallium, aluminum, indium, thallium, germanium, tin, phosphorus, arsenic, antimony, selenium, tellurium and / or fluorine. Transparent conductive pigments according to one of Claims 1 to 7, characterized in that one or more layers comprising metal oxides, metal oxide hydrates, metal suboxides, metal fluorides, metal nitrides, metal oxynitrides or mixtures of these materials are located above and / or below the conductive layer. A process for the preparation of transparent conductive pigments according to claim 1 comprising the coating of platelet-shaped substrates with a conductive layer, characterized in that before and / or after the coating with a conductive layer, the proportion of platelet-shaped substrates or transparent conductive pigments having a grain area of less is reduced as 150 microns ² by means of sedimentation, decantation, air classification and / or screening such that the number-weighted mean grain area F _{50 of} the transparent conductive pigments is greater than or equal to 150 microns ² . A method according to claim 9, characterized in that the coating of the platelet-shaped substrates with a conductive layer is wet-chemical, wherein the substrates are suspended in water and treated with one or more hydrolyzable metal salts at a suitable pH for the hydrolysis, the metal oxides, metal hydroxides or metal oxide hydrates are precipitated on the platelets and the pigments are separated off, dried and calcined. Method according to one of claims 9 to 10, characterized in that before and / or after the coating of the platelet-shaped substrates with a conductive layer, one or more layers comprising metal oxides, metal oxide hydrates, metal suboxides, metal fluorides, metal nitrides, metal oxynitrides or mixtures of these materials are applied. A method according to claim 11, characterized in that the application of the single or multiple layers by means of sol-gel method, CVD and / or PVD method is carried out. Use of transparent conductive pigments according to Claim 1 in paints, lacquers, printing inks, plastics, in security applications, floor coverings, films, formulations, ceramic materials, glasses, paper, for laser marking, in thermal protection, in dry preparations or in pigment preparations. Use according to claim 13, characterized in that the pigments are mixed with organic, inorganic colorants and / or conductive materials.