Particulate fillers, production and uses thereof

Abstract

The present invention relates to particulate fillers comprising a coating on support particles surrounding each of the support particles and containing titanium dioxide doped with niobium and at least one further element, a process for producing particulate fillers of the type described above and their uses, particularly as varistor fillers having non-linear electrical properties in coating compositions and molding compounds.

Description

[Technical field]

[0001] The present invention relates to a particulate filler having on its support particles a coating comprising titanium dioxide doped with niobium and at least one further element, in each case surrounding the support particles, to a process for the preparation of a particulate filler of this type, and to its use as a varistor filler with non-linear electrical properties, especially in coating compositions and in moulding compounds. [Background technology]

[0002] Coating compositions or molding compounds, consisting for example of silicone, EPDM (ethylene-propylene-diene monomer), polyurethane, polyethylene, epoxide, phenolic resins or ceramic materials and containing certain functional fillers, are employed to control the electric fields occurring therein in various application forms, for example in high voltage technology. The aim of the use of these functional fillers is also the controlled electric field control or overvoltage protection in the application medium. The functional fillers employed for this purpose are also known as varistors ("variable" + "resistance"). They are characterized in that their electrical conductivity in the application medium depends on the applied electric field strength, i.e. occurs in a non-linear manner. Such fillers are particularly suitable for use in protecting application media and materials used therein in that high, constant or variable electric field strengths are generated.

[0003] Typical areas of use here are for field control in high voltage cables where the main insulation has been removed and where large electric field strength gradients arise at the interfaces of electrically conductive and insulating components (electrical stresses), e.g. in muffs or also at the ends, and their interconnectedness, or alternatively for overvoltage protection against transient loads such as switching voltages, lightning damage or discharges. Further areas of use also reside in the encapsulating compositions' control of electric fields and contacting circuits in high power electronics.

[0004] Application media filled with varistor materials, for example polymers, exhibit both resistive and capacitive field control properties. These materials are therefore suitable for use in DC and AC voltage applications and are able to absorb transient events (pulses). The reason for this is that the voltage drop between the high voltage supply (e.g. cable) and ground (e.g. outer conductor) through the varistor-filled application medium is well controlled. Due to the non-linear electrical properties (conductivity) of the varistor material, the electric field is homogenized (resistive field control) and at the same time, due to the invariance of the dielectric properties of the application medium, which is increased due to its presence together with the low losses (tan δ) of the varistor material, the field strength distribution is broadened (capacitive field control).

[0005] The non-linear electrical properties of varistor materials were already exploited several decades ago for the protection of electrical systems, for example in the form of sintered ceramics, which included SiC or ZnO as varistor materials and were employed for the protection of telephone systems. Other varistor materials have also been described, often in the form of particles of various shapes and sizes, for example titanium carbide, titanium suboxide (TiO) or conductive copper particles or carbon black particles.

[0006] Thus, for example, WO 97 / 26693 A1 discloses the incorporation of milled zinc oxide particles in concentrations of 25-50% into polymer compositions of various compositions as microvaristors in order to effect a shift towards higher switching field strengths in the application medium and thus reduce the probability of electrical breakdown. However, the materials produced therein still have a relatively high electrical conductivity.

[0007] EP2020009 B1 describes voltage protection means for electronic components and circuits comprising microvaristor particles of a variety of possible materials, such as doped zinc oxide, tin oxide, silicon carbide, or strontium titanate, arranged in a defined manner in monolayers or in chains. DE 102010052888 A1 discloses varistor particles having an electrically conductive layer consisting of antimony-doped tin oxide in a defined composition on a flake-form substrate, such as mica.

[0008] The varistor materials used in the prior art have various disadvantages. Thus, for example, SiC microparticles are extremely hard, making the grinding of the material more difficult in the application medium and processing process and leading to high abrasiveness of the particles. In addition, micronized SiC particles tend to oxidize on their surface, which adversely affects their electrical properties.

[0009] Zinc oxide is 5.5 to 6.5 g / cm depending on the degree of compaction achieved by the sintering process. 3 Since the density of the varistor particles is significantly greater than that of the application medium, these varistor particles have a strong tendency to settle even during the preparation of the mixture with the application medium, making the mixing of a uniform and durable material more difficult. The irregular composition of the application medium correspondingly leads to irregularities in its electrical properties. In general, there is therefore a continuing need for fillers with declared non-linear electrical properties and positive material properties and varistor properties that can be mechanically introduced easily and uniformly into various application media without exhibiting settling behavior. Summary of the Invention

[0010] The object of the present invention therefore consists in providing a filler having varistor properties which can be easily and in a stable mixture introduced into a wide variety of application media, which can be controlled in its composition so as to be optimally adapted to the respective requirements of the application media, which exhibits reliable non-linear electrical behavior, and which can be employed both in direct current and in alternating current applications.

[0011] A further object of the invention consists in providing a process for the preparation of this type of filler. In addition, an additional object of the present invention consists in pointing out the use of this type of filler. The object of the invention is achieved by a particulate filler consisting of support particles and in each case a coating surrounding the support particles, where the support particles comprise or in each case consist of at least one aluminum or silicon compound, and the coating comprises titanium dioxide doped with niobium and at least one further element.

[0012] The objects of the invention are achieved by a process in which support particles, which also comprise or in each case consist of at least one aluminium or silicon compound, are provided with a coating comprising at least one titanium compound, at least one niobium compound and at least one compound of a further element in an aqueous suspension which is in each case at a suitable pH, and the support particles provided with the coating are subsequently dried and calcined, during which the coating is converted into titanium dioxide in finely divided form doped with niobium and at least one further element.

[0013] In addition, the objects of the present invention are achieved by the use of particulate fillers of this type for pigmentation of coating compositions and of molding compounds, whereby the coating compositions and molding compounds thus produced are provided with non-linear electrical properties.

[0014] The present invention therefore relates to particulate fillers having a coating on certain support particles that affects the electrically semiconductive or electrically conductive behavior of the filler in the application medium. It has been found to be advantageous to prepare fillers with the desired varistor properties from materials of various compositions. In the case of this type of procedure, the material properties of the varistor filler, such as hardness, density, size, shape and ultimately the semiconductive or conductive properties, can be better controlled, and often also more specifically adjusted, than can be done in the case of particulate fillers of uniform composition.

[0015] Thus, materials which have proven advantageous for the support particles of the filler according to the invention are, inter alia, those which contain, or in each case consist of, at least one aluminum or silicon compound. A particularly suitable aluminum compound which comes into consideration here is aluminum oxide (Al 2 O 3 ) or aluminosilicates of various compositions, both natural and synthetic. A particularly suitable silicon compound is silicon dioxide (SiO 2 ), which can be employed in crystalline form and particularly preferably in amorphous form, for example as diatomaceous earth.

[0016] Materials consisting of or containing aluminosilicates are, for example, mullite, fly ash, kaolinite, pumice or perlite. All these materials are highly suitable as support materials for the particulate fillers of the present invention because, due to their natural composition or due to the specific processing process, they have a very low density and advantageous size and shape and can be well coated. In some cases, they are easily and cheaply available as natural materials or waste products from large-scale industrial processes. Mullite and fly ash may be specifically and preferably employed.

[0017] The support particles for the particulate filler according to the invention can have various shapes. Taking into account the subsequent application media and their specific requirements, flake-shaped or spherical support particles are particularly preferred, or alternatively also support particles having an isotropically irregular shape. Spherical support particles are taken to mean those in the shape of solid spheres, in the form of hollow spheres or in the form of spheroids which do not have an ideal geometrical spherical shape and can be formed as solid bodies or alternatively as hollow bodies. Due to the nature of the materials mentioned in the present invention, geometrically ideal spheres can only be produced in very rare cases, i.e. spheroidal shapes predominate for technical reasons.

[0018] In accordance with the present invention, the term "isotropically irregular" applies to all granular particles in which no preferential axis can be optically identified, but which have approximately equal or similar diameters and lateral surfaces in all spatial directions. Such particles are also commonly referred to as granules.

[0019] According to the invention, the support particles in flake form are advantageously made of pure aluminum oxide (Al 2 O 3 ) or in the form of aluminum oxide flakes which, besides aluminum oxide, contain a foreign fraction of further metal oxides in a proportion of 0.1 to 10% by weight, preferably 0.1 to 5% by weight, based on the weight of the support particle. These are oxides or oxide hydrates of Ti, Sn, Si, Ce, Ca, Zn, In and / or Mg. Al 2 O 3 The use of support particles is preferred, which is Al 2 O 3 In addition, 0.1 to 5 wt. % of TiO 2 Also includes.

[0020] This type of flake-form Al 2 O3 The supported particles can be obtained, for example, by the process disclosed in EP 763573 A2. Corresponding products of suitable particle size are also commercially available. The support particles in flake form are particularly suitable as starting material for particulate fillers intended to be employed according to the invention in thin layers on substrates or in thin components, for example on insulators, on bushings, on electrical conductors, or on internal fittings in electrical machines, such as transformers, generators and motors, in resins or in paint layers. Because the particulate fillers retain the flake shape of the starting support particles, they are able to align sufficiently laterally in the thin layers of such applications and thus form the requisite conductive tracks when required.

[0021] Aluminosilicate (general formula Al x S y O z The carrier particles consisting of or containing (x, y and z are variables) are, inter alia, the above-mentioned materials, which are available in large quantities at low cost as natural decomposition products or as waste products of large-scale industrial processes or as synthetic products. The corresponding products are already employed as aggregates, for example, in the construction industry. Fly ash and mullite are particularly preferably used as starting materials for the carrier particles of the filler according to the invention, since they are already available in the desired form, frequently and preferably as spherical hollow bodies, and in the desired particle size. All other materials mentioned can, if necessary, be brought to the required particle size by conventional grinding processes.

[0022] Amorphous silicon oxide particles are also available on a large industrial scale, for example as diatomaceous earth. The isotropic shape (regular or irregular) of the materials described herein is 、The particles are carried over into the shape of the particulate fillers which originate from them. The latter are preferably employed in molding compounds, which are employed in voluminous forms, such as for example for the production of cable muffs, insulators, bushings or cable terminals. These products are frequently produced by injection molding processes, in some cases directly at the site of use. It is therefore of great importance that the volume polymers employed in the present invention (silicones, epoxides, polyethylene, polyurethane, EPDM, etc.) have rheological properties suitable for carrying out the injection molding process, despite the addition of particulate fillers. In this specification, flake-shaped particles may adversely affect the flow properties of the injection molding compounds, which means that isotropic particles, which do not adversely affect or only to a small extent affect the flow properties of the pigmented volume polymers, are preferred for these areas of application. In addition, the very low density of some of these support materials has a particularly advantageous effect on the sedimentation tendency of the particulate fillers in the application medium.

[0023] According to the invention, the density of the particulate filler according to the invention is between 1.5 and 4.5 g / cm 3 and preferably in the range of 1.5 to 3.0 g / cm 3 and in particular from 1.5 to 2.5 g / cm 3 It is composed of the average density of the support particles and the coating and can in each case be adapted to the requirements in the application medium through the selection of the corresponding materials, especially the support particles.

[0024] The support particles of the particulate filler according to the invention are surrounded by a coating comprising in each case doped titanium dioxide doped with niobium and at least one further element, the coating here surrounding each support particle individually and being in separate form in its surrounding, i.e. not forming a coherent phase around a plurality of support particles.

[0025] According to the invention, the further element is at least one element selected from the group consisting of Mn, Cr and Ce. Preferably the doping is a combination of: Nb, Mn; Nb, Mn, Cr; Nb, Mn, Ce; Nb, Cr; Nb, Cr, Ce Nb, Ce; or Nb, Mn, Cr, Ce, where the combinations Nb,Mn,Ce and Nb,Cr are specifically preferred. The doping elements are TiO 2 It is located in cationic form in the crystal lattice or at the grain boundaries of titanium dioxide granules.

[0026] The doping in titanium dioxide is in each case present in an amount of 0.01 to 5 atomic % of the element, based on the sum of the number of Ti atoms and doping atoms. The proportion of the doping element (total of the doping elements) is preferably a content of 0.02 to 2 atomic %. While the niobium determines the electrical conductivity of the filler in the application medium, and the manganese or chromium determines the steepness of the nonlinear characteristic of the electrical conductivity, additional doping with cerium can result in a shift of the nonlinear electrical conductivity of the application medium to higher electric field strengths.

[0027] The coating on the support particles of the filler according to the present invention preferably comprises multi-doped titanium dioxide in the form of fine particles on the surface of the support particles where it forms a substantially continuous fine-grained layer. The coating on the support particles in either case has a geometric thickness in the range of 100 to 5000 nm, preferably in the range of 150 to 4000 nm, and especially in the range of 200 to 2000 nm. The geometric layer thickness of the coating can be easily determined using conventional measurement methods with reference to tomographic images (Ar ion beam) of individual particles of the particulate filler.

[0028] The average particle size d of the particulate filler according to the invention 50 is in the range of 1 to 150 μm, preferably in the range of 2 to 100 μm, and in particular in the range of 5 to 50 μm. Due to the origin of the above-mentioned support materials of isotropic shape, the particle size of the individual filler particles can vary in a relatively wide range, and often the average particle size with respect to volume d 50 Only citations of are meant to be deemed reasonable herein. Some tolerance for variation in particle size of individual particles generally also exists in the use of flake shaped support particles.

[0029] The particle size of the particulate filler and, where appropriate, of the support particles is preferably determined via a laser diffraction method, which is generally known and has the advantage that it is also possible to determine the particle size distribution of the particles. For the filler and its support particles according to the invention, the particle size was determined using a Malvern Mastersizer 3000, APA 300 (product from Malvern Instruments, Ltd., UK).

[0030] The present invention also relates to a process for the preparation of particulate fillers, in which support particles comprising or in each case consisting of at least one aluminium or silicon compound as described above are provided with a coating comprising at least one titanium compound, at least one niobium compound and at least one compound of a further element, in each case in aqueous suspension at a suitable pH.

[0031] Generally, compounds of niobium and other doping elements are employed in the coating process in water-soluble or acid-soluble compounds, in this dissolved form and in an amount selected corresponding to the degree of doping. It should be noted here that successive settings of several different pH values ​​are necessary under some circumstances for the precipitation of oxide hydrates of the corresponding elements on the surface of the support particles. The usual pH values ​​are in the acidic to neutral range, from pH 2 to pH 7. When all the precursor compounds have been deposited on the surface of the support particles in the coating, the coated support particles are subsequently dried and calcined. At a calcination temperature ranging from 850 to 1300° C., fine granules of titanium dioxide doped with niobium and at least one further element are formed on the surface of the support particles.

[0032] Corresponding to the details already given above, the support particles employed in the process according to the invention preferably have a flake morphology, a spherical or an isotropically irregular shape and comprise or in any case consist of aluminium oxide, silicon dioxide or aluminosilicate.

[0033] The support particles employed preferably consist of aluminium oxide, silicon dioxide, mullite, fly ash, kaolinite, pumice or perlite, and very particularly preferably consist of mullite or fly ash.

[0034] The latter, due to the isotropic particle shape and the presence of at least partially isotropic hollow bodies, have an advantageous low density which predetermines the fillers derived therefrom, in particular for use in bulk polymers. The density of these support materials is <2 g / cm 3The low density of facilitates a particularly good matching of the overall density of the particulate filler according to the invention to the density of the polymeric application medium, so that separation of the composite during pot life or during cross-linking in the application case can be avoided. This means that settling of the filler, together with the resulting disadvantages such as gradient formation in the application medium and the resulting non-uniform electrical properties of the workpiece, can be easily avoided, which can only be achieved with excessive equipment complexity in the case of denser fillers.

[0035] Details of the doping elements and their advantageous combinations have already been described herein above. To this extent, reference is made to the scope of this specification. This also applies to all details regarding the shape, size and density of the support particles and the geometric thickness and composition of the coating.

[0036] The present invention also relates to the use of particulate fillers according to the above description for pigmenting coating compositions and molding compounds, in particular with the objective of providing these coating compositions and molding compounds with non-linear electrical properties, i.e. acting as varistor fillers. The corresponding unfilled coating compositions or molding compounds generally comprise or consist of silicone, EPDM, polyurethane, polyethylene, epoxide, phenolic resins or ceramic materials as major constituents. Depending on the application medium and the required electrical properties, the particulate fillers according to the invention are present in the coating composition or in the molding compound in a pigment volume concentration in the range of 3 to 33% by volume, preferably 3 to 25% by volume, and in particular 4 to 20% by volume, based on the (solid) volume of the coating composition or of the molding compound.

[0037] Although the specification of the mass proportion of the filler in the application medium (pigment mass concentration PMC) can be easily performed, for better comparability of different systems, the pigment mass concentration PMC is frequently specified in practice. Fillers have different densities and therefore occupy different volumes in the same weight of application medium introduced. Particularly in the case of the penetrating particle systems described herein, the particle-particle contact, the particle size and their size distribution, and the particle geometry essentially determine the electrical properties of the resulting composite material. The effect of a particular varistor filler is therefore best described by a comparison of the pigment volume concentrations.

[0038] The pigment mass concentration PMC and the pigment volume concentration PVC are defined as follows (the volatile components are not taken into account in both cases):

number

[0039] The particulate fillers according to the invention have non-linear electrical properties, i.e. varistor properties, in the coating compositions or molding compounds of the application medium. Due to the possibility of selecting different support particles, which may have different shapes and sizes as well as different viscosities, and due to the possibility of variation in the type and amount of doping of the coating on each support particle, both the electrical properties and also the density of the particulate fillers can be optimally adapted to the respective requirements of the application medium. All starting materials herein are readily available, and the preparation of the particulate fillers by means of uncomplicated coating processes can be carried out without great technical effort. The particulate fillers according to the invention can be employed in applications in direct and alternating current and can achieve high switching field strengths of >3000 kV / m. In the ground state, the electrical conductivity of the particulate fillers according to the invention corresponds substantially to that of the insulating material surrounding them, which means that no unintended generation of electrical conductivity under unloaded conditions has to be expected. In contrast, for switching throughout a load event, variations in measured current density in a coating composition or molding compound over several orders of magnitude are possible, where the increased current density represents the nonlinear conductivity of the varied composite. [Brief description of the drawings]

[0040] [Figure 1] FIG. 1 shows a schematic circuit of the measurement arrangement (1b, bottom) with a voltage supply (DC), an integrated voltage measurement (U), a picoammeter (A) and a test specimen, and the test specimen (1a, top) with the relevant dimensions. [Diagram 2] Figure 2 shows the characteristic DC field strength / current density curves (E / J) of the filler according to Example 1 in RTV-2 silicone with pigment mass concentrations of 15, 20, 25 and 30%. The percolation thresholds are marked (Examples 5-1 to 5-4). [Diagram 3] FIG. 3 shows the characteristic DC field strength / current density curves (E / J) for fillers with pigment mass concentrations of 25 and 50.8% in RTV-2 silicone according to Example 2 (Examples 5-6 and 5-7). [Figure 4] FIG. 4 shows the characteristic DC field strength / current density curves (E / J) for 25 and 37% pigment mass concentration fillers in RTV-2 silicone according to Example 3 (Examples 5-8 and 5-9). [Diagram 5] Figure 5 shows characteristic DC field strength / current density curves (E / J) for fillers in RTV-2 silicone with pigment mass concentrations of 15, 20, 25, 30 and 35% according to Example 4. The percolation threshold is between 15 and 30% PMC (Examples 5-10 to 5-14). [Figure 6] 6 shows the permeation curves as a function of the pigment volume concentration PVC of the filler in RTV-2 silicone according to Examples 1 and 4. The specific conductivity of the samples at a field strength of 20 kV / m is plotted. [Figure 7] FIG. 7 shows the characteristic DC electric field strength / current density curves (E / J) of fillers with 25 and 35% pigment mass concentrations in LSR silicone according to Example 1 (Examples 6-1 and 6-2). [Figure 8] FIG. 8 shows the characteristic DC field strength / current density curves (E / J) for fillers with 20 and 35% pigment mass concentration in the epoxide according to Example 4 (Examples 7-1 and 7-2). [Figure 9] FIG. 9 shows the characteristic DC electric field intensity / current density curves (E / J) of the filler in HD-PE according to Example 1 at concentrations of 30 and 40 wt % (Examples 8-1 and 8-2). [Figure 10] 10 shows the characteristic DC electric field strength / current density curves (E / J) of specimens in RTV-2 silicone according to Examples 5-10 through 5-18 (Table 3). Specimens 5-10, 5-15 and 5-17 have 25% PMC, and specimens 5-14, 5-16 and 5-18 have about 27% PVC. [Figure 11] FIG. 11 shows the variation in impedance Z, dielectric constant ε′, and loss factor tan δ(×100) at 50 Hz AC voltage for fillers in HDPE with various filler concentrations (0-40 wt %) according to Example 1. [Figure 12]FIG. 12 shows a) a schematic representation of a particulate filler according to the present invention with dielectric support particles (1) and a fine-grained, non-linear, electrically conductive coating (2), and b) a schematic representation of a polymer matrix filled with a filler according to the present invention (3).

[0041] The present invention is intended to be described with reference to the examples set forth below, but is not intended to be limited thereto.

[0042] example: Preparation of particulate filler: Example 1: 100g Aluminum Oxide Flakes (BET 3.3m 2 / g, density 3.75g / cm 3 , particle size 5-40 μm) was suspended in about 2 liters of deionized water. A solution of 700 g of titanium oxychloride (400 g / l), 3.3 g of niobium pentachloride and 0.8 g of manganese(II) sulfate monohydrate in 24 ml of hydrochloric acid (37%) was added dropwise to the suspension with stirring in acidic medium at 75 °C. The pH was kept constant at 2 by simultaneous regulated metered addition of sodium hydroxide solution. When the addition of all the solutions was completed, a solution of 13.5 g of cerium chloride heptahydrate in 150 ml of deionized water was added uniformly and the mixture was stirred for a further 15 minutes at 75 °C before adding sodium hydroxide solution with a constant pH of 7 by metered addition. The mixture was subsequently cooled to room temperature with stirring and the reaction mixture was neutralized again. The resulting pigment was filtered through a suction filter, washed with water, dried at 140°C and calcined at 1100°C for 120 minutes to obtain a pale ochre coloured pigment powder. The resulting pigment particles have aluminium oxide flakes (particle size <15 μm) as support particles and a coating comprising niobium-doped, manganese-doped and cerium-doped titanium dioxide adherently precipitated thereon.

[0043] Example 2: 100 g of spherical aluminum oxide particles (BET1.4m 2 / g, density 2.85g / cm 3 , particle size d 5 -d 95 =14μm-45μm) was suspended in about 2 l of deionized water. A solution of 577 g of titanium oxychloride (400 g / l), 5.04 g of niobium pentachloride and 0.16 g of manganese(II) sulfate monohydrate in 24 ml of hydrochloric acid (37%) was added dropwise to the suspension with stirring in acidic medium at 90 °C. The pH was kept constant at 2 by simultaneous controlled and metered addition of sodium hydroxide solution. When the addition of all the solutions was completed, the mixture was stirred for a further 15 min at 90 °C before a solution of 7.6 g of cerium chloride heptahydrate in 150 ml of deionized water was added uniformly and with metered addition of sodium hydroxide solution at a constant pH of 7. The mixture was subsequently cooled to room temperature with stirring and the reaction mixture was neutralized again. The resulting pigment was filtered through a suction filter, washed with water, dried at 140°C, and calcined at 1100°C for 120 minutes to obtain an ochre colored pigment powder. The resulting pigment particles have hollow aluminum oxide flake spheres (particle size <70 μm) as support particles and a coating comprising niobium-doped, manganese-doped, and cerium-doped titanium dioxide adherently precipitated thereon.

[0044] Example 3: 100g of spherical aluminosilicate particles (BET1.4m 2 / g, density 0.87g / cm 3 , particle size d 5 -d 95=5μm-63μm) was suspended in about 2 l of deionized water. A solution of 577 g of titanium oxychloride (400 g / l), 5.04 g of niobium pentachloride in 24 ml of hydrochloric acid (37%) and 0.16 g of manganese(II) sulfate monohydrate was added dropwise to the suspension with stirring in acidic medium at 90 °C. The pH was kept constant at 2 by simultaneous controlled and metered addition of sodium hydroxide solution. When the addition of all the solutions was completed, the mixture was stirred for a further 15 min at 90 °C before a solution of 7.6 g of cerium chloride heptahydrate in 150 ml of deionized water was added uniformly and with metered addition of sodium hydroxide solution at a constant pH of 7. The mixture was subsequently cooled to room temperature with stirring and the reaction mixture was neutralized again. The resulting pigment was filtered through a suction filter, washed with water, dried at 140°C, and calcined at 1100°C for 120 minutes to obtain an ochre colored pigment powder. The resulting pigment particles have hollow aluminosilicate spheres (particle size <100 μm) as support particles and a coating comprising niobium-doped, manganese-doped, and cerium-doped titanium dioxide adherently precipitated thereon.

[0045] Example 4: A filler was prepared similarly to Example 3 with the modification that besides 2.5 g of niobium pentachloride (as a 12.5% ​​HCl solution), only 0.35 g of chromium trichloride was added to 598 g of titanium oxychloride solution. It can be said that a substantially white pigment powder was obtained. The obtained pigment particles have hollow aluminosilicate spheres (particle size <100 μm) as carrier particles and a coating comprising niobium-doped and chromium-doped titanium dioxide adherently precipitated thereon. Table 1 shows the compositions of the particulate fillers according to the invention in Examples 1 to 4. 2 The doping of the coating is given as atomic % in titanium dioxide.

[0046] [Table 1]

[0047] Table 2 shows the density and size distribution of the particulate fillers according to Examples 1-4. [Table 2]

[0048] Example 5: Production of test specimens from room temperature crosslinkable silicones The core / shell particles from Examples 1-4 in the amounts shown in Table 3 and the comparative material α-SiC (Alfa Aesar Art. #40155, density 3.51 g / cm 3 ) and ZnO microvaristor powder (ABB / Switzerland, density 5.22g / cm 3 ) was roughly premixed in a metal container with the respective proportions of component A of the commercially available room temperature crosslinking silicone resin RTV-2 (manufacturer's material data: A:B=9:1, mixture viscosity 3500 mPa*s at 23°C, Shore A hardness 45°) and homogenized in a vacuum SpeedMixer (Hauschild) at 4 mbar vacuum and 1600 rpm for at least 2 minutes. The respective amounts of component B of the same RTV2 silicone resin were subsequently added, the components were roughly premixed again and homogenized in a vacuum SpeedMixer at 4 mbar and 1600 rpm for at least 1 minute. The viscous material was then poured into a mold that quickly determined the geometric dimensions of the specimens while observing the pot life. The silicone resin was cured in the mold at 70°C for at least 30 minutes. After the mold had cooled, it was opened and the specimens were removed and stored under dust-free conditions. The layer thickness of the crosslinked test specimens was determined using an eddy current layer thickness measuring instrument (Fischer Dualscope FMP30 with FD10 sensor according to DIN EN ISO 2360) as the average of ten measurements for each of the annular test specimens (diameter 60 mm) at various points between 500 μm and 600 μm. Test specimens such as 5-17 and 5-18 can only be produced with difficulty even in small quantities, since sedimentation begins in the SpeedMixer vessel already during preparation, leading to an uneven distribution of the filler in the test specimens.

[0049] [Table 3]

[0050] Example 6: Production of test specimens during LSR Liquid silicone rubber LSR samples were produced similar to Example 5, but cured in a hot press at 120° C. for 20 minutes in a polypropylene mold. For LSR, the mixing ratio of components A and B is 1:1 (see Table 4).

[0051] [Table 4]

[0052] Example 7: Production of epoxy resin test specimens The filler from Example 4 in the amount shown in Table 5 was first introduced into a metal container with the binder in the indicated proportions, which in each case consisted of 4% benzyl alcohol, 76% ARALDITE DBF BD, and 20% ARADUR HY2966, roughly premixed and homogenized in a vacuum SpeedMixer (Hauschild) successively at a reduced pressure of 4 mbar for 2 minutes at 1000 rpm, 2 minutes at 1800 rpm, and 30 seconds at 800 rpm. The mixed or homogenized epoxy resin was then rapidly poured into a Teflon® casting chamber and cured at 60° C. for about 1 hour. After the samples had cooled, the epoxide plate was removed and annular test pieces were drilled out. The layer thickness was determined using a dial gauge.

[0053] [Table 5]

[0054] Example 8: Production of HD-PE test specimens The filler prepared in Example 1 was used to produce a 40% masterbatch in HDPE (pure 111 GA 7760) in a DSE Leistritz Micro 27 twin screw extruder. This was diluted to 30% with additional HDPE. 5% and 2% dilutions were produced from the 10% masterbatch. The extrudate was quenched and granulated. 60mm*90mm plates with a layer thickness of 1mm were produced from the granules in an injection moulding machine, and then test specimens with a diameter of 50mm were produced using a core drill.

[0055] [Table 6] Measurement of electrical properties of test specimens: The non-linear conductive character of the specimen emerges from current / voltage measurements which are intended to show deviations from ohmic behavior: U~I α (1) The deviation is described by the nonlinearity index α, which has the value 1 in the case of an ohmic resistor.

[0056] The current / voltage characteristics of the produced varistor filler / polymer specimens were measured on ring electrodes according to DIN EN61340-2-3 using a Heinzinger 10 kV DC voltage source (PNChp 10000-20 ump) and a Kethley pico-ammeter (6514 series electrometer). A schematic measurement setup and the dimensions of the specimen to be observed are shown in FIG. To normalize the results, the electric field strength E and current density J were calculated according to equations (2-4) for the sample and electrode dimensions given in FIG. 1: E=V / h (2) J=I / A (3) A=(d 1 +g) 2 *π / 4 (4) α = ln(J 2 / J 1 ) / ln(E 2 / E1 ) (5) V = voltage in volts (V) I = current intensity in amperes (A) A = effective electrode area (m 2 ) h = electrode separation (sample thickness): ~0.5 mm d 1 -d 4 = electrode diameter (see Figure 1) d 1 = Center electrode diameter: 25mm g = separation of annular electrode from central electrode: 2.5 mm α=nonlinearity index J 1 , J 2 = Current density at two measurement points E 1 , E 2 = Electric field strength at two measurement points

[0057] Current measurements were performed using a step shaped voltage ramp at room temperature and relative atmospheric humidity between 20% and 30%. The nonlinearity appears as the slope of the curve in a plot of the EJ exponent, which follows the double logarithmic equation (5). Generally, only the fraction greater than 5000 kV / m is considered for the calculation of the nonlinearity exponent α. Impedance measurements were performed on the PE specimens from Examples 8-1 to 8-5 in a Novocontrol GmbH Alpha-A broadband dielectric spectrometer at 25° C. with PHECOS temperature control between 0.01 Hz and 100 MHz.

[0058] Figure 2, Examples 5-1 to 5-4: The flake-form varistor filler from Example 1 shows a clear non-linear electrical conductivity. At field strengths between 1000 kV / m and 20000 kV / m, this non-linear character (slope in FIG. 2) increases in a clearly visible manner, and the silicone composite reaches a non-linearity index α of 2.3 to 3.7 from about 5000 kV / m. In Examples 5-1 to 5-4, the pigment mass concentration PMC of the varistor filler increases in 5% steps from 15% to 30%. The percolation threshold is evident as a clear separation between the curves of Examples 5-2 and 5-3. In the considered field strength range, the curve extends over a current density of about 10 to the power of 2.5, where the basic conductivity depends on the varistor content in the composite. Samples with a pigment volume concentration PVC of 26.8% (58.5% PMC) can no longer be produced with the flake-form varistor filler. At such high loadings, the flake-form fillers tend to form rheologically unfavorable "houses of cards" structures in the polymer matrix.

[0059] Figure 3, Examples 5-6 and 5-7: The spherical varistor filler from Example 2 exhibits an even more pronounced nonlinear electrical conductivity with a nonlinearity index α of 4.4, approximately two orders of magnitude greater than the current density, from about 5000 kV / m to a loading level of 25% PMC. A higher loading level with a pigment volume concentration of 26.8%, which corresponds to a pigment mass concentration of 50.8% for this filler, leads to a flatter curve course, i.e. a somewhat lower nonlinearity index α of 3, but a correspondingly higher conductivity (the entire curve is shifted to higher current densities).

[0060] Figure 4, Examples 5-8 and 5-9 The spherical varistor filler from Example 3 already exhibits a pronounced nonlinear electrical conductivity with a nonlinearity index α of 4.4 over the entire measurement range and over four orders of magnitude increase in current density at a loading level of 25% PMC. A higher loading level of 26.8% pigment volume concentration leads to a flatter curve course, i.e. a somewhat lower nonlinearity index α of 1.4, and thus to a higher conductivity.

[0061] Figure 5, Examples 5-10 to 5-14 The spherical varistor fillers from Example 4 exhibit a clear non-linear electrical conductivity. The switching point (indicated by an arrow ↑ in FIG. 5) at which the non-linear characteristics of the exemplary composites begin shifts to higher filling levels and lower electric field strengths. In Example 5-10 with a filling level of 15% PMC, this switching point is at about 6000 kV / m. The switching point occurs already at 4000 kV / m in the case of Example 5-11 with 20% PMC and at about 2000 kV / m in the case of Example 5-12 with 25% PMC. In the case of Example 5-13 with 30% PMC, the curvature range is recorded just at the beginning of the measurement at 1000 kV / m, and in the case of Example 5-14 with 35% PMC, it is below the measurement limit. The curve of the material from Example 5-11 is steepest just before the percolation threshold with a non-linearity index α of 4.5. Again, the conductivity of the material increases with increasing filling degree, shifting the example curves to higher current densities.

[0062] Figure 6: The penetration effect was even more pronounced in the case of composites with the low density spherical material from Example 4 than with the high density flake shaped material from Example 1 (Figure 6). For the same weight introduced (PMC 15%-35% for the composite in Example 1 and in the RTV-2 silicone in Example 4), these varistor fillers achieved a significantly higher volumetric loading (PVC in RTV-2 silicone: 4.4%-12.3% in Example 1 and 10.7%-26.8% in Example 4).

[0063] Figures 7-9: The materials produced in Examples 1-4 can be converted into insulating materials that control electric fields in a wide variety of polymer matrices. This is evident from the nonlinear electrically conductive characteristics of these composites. The curves shown in Figure 7 show this characteristic of the varistor filler from Example 1 in a more highly crosslinked LSR silicone (Examples 6-1 and 6-2), and Figure 8 shows the nonlinear electrical conductivity curves of the varistor filler from Example 4 in an epoxide composite (Examples 7-1 and 7-2). The electrically conductive behavior emerges similar to the composites shown so far. Besides these two component systems, Figure 9 shows the nonlinear electrical properties of composites of HD-PE produced directly by extrusion of particles (Example 8-1) or from a masterbatch (Example 8-2).

[0064] Figure 10: Figure 10 illustrates a comparison between the varistor filler according to the invention (Example 4 in the formulations from Examples 5-10 and 5-14 here) and other field control materials. For this purpose, silicon carbide (Examples 5-15 / 5-16) and zinc oxide microvaristors (Examples 5-17 / 5-18) are selected. For better comparability, the formulations were prepared once and in each case the pigment mass concentration corresponds to 25% PMC (Examples 5-10, 5-15 and 5-17), and further formulations were prepared with higher concentrations than the same volume of filling of 26.8% PVC (Examples 5-14, 5-16 and 5-18).

[0065] The curve of the varistor filler from Example 5-15 intersects with the curve of silicon carbide from Example 5-16. Depending on the concentration, it is also feasible to create a lower conductivity in the insulating material than with silicon carbide, as shown by a comparison of the curves from Examples 5-10 (varistor filler) and 5-15 (silicon carbide). On the other hand, from the curves of Examples 5-17 and 5-18 filled with zinc oxide microvaristors, it is clear that this material is difficult to control. It only exhibits low nonlinearity at low loadings (Example 5-17), whereas it is clearly hyperconductive in the case of high loadings (Example 5-18). The switching point into the region of nonlinear conductivity is then at very low electric field strengths (<<100 kV / m).

[0066] Figure 11: The dielectric measurements on an impedance spectroscopy also allow the properties of the varistor fillers according to the invention to be determined in an AC-current environment. Figure 11 shows the impedance (AC resistance) Z at 50 Hz, and the dielectric constant ε' and the loss factor tan δ of the composites containing the varistor filler from Example 1 and the HD-PE in the formulations from Examples 8-1 to 8-5. The dielectric constant ε' increases only slowly with the filler percentage (as an average of the polyethylene and doped varistor filler percentages). On the contrary, the impedance, but also the loss factor, suggests a classical percolation curve shape with different phase stages from a filler percentage of about 25%. The dielectric constant and the loss factor are still very small. For pure HDPE, the literature gives values ​​of about 2.4 for the dielectric constant and 2*10 for the loss factor. -4 However, the loss factor measured for unfilled HDPE is already 4*10 -2 and is exceeded by a factor of 10 only by a loading of 37% by weight of varistor filler.

Claims

1. 1. A particulate filler consisting of support particles and in each case a coating surrounding the support particles, characterized in that the support particles comprise or in each case consist of at least one aluminium or silicon compound and that the coating comprises titanium dioxide doped with niobium and at least one further element, wherein in addition to niobium the titanium dioxide is doped with at least one further element selected from the group consisting of Mn, Cr and Ce.

2. 2. Particulate filler according to claim 1, characterized in that the support particles comprise or in each case consist of aluminium oxide, silicon dioxide or aluminosilicate.

3. 3. The particulate filler of claim 2, wherein the support particles are comprised of aluminum oxide, silicon dioxide, mullite, fly ash, kaolinite, pumice, or perlite.

4. Particulate filler according to any one of claims 1 to 3, characterized in that the support particles are in flake form, spherical or have an isotropically irregular shape.

5. 1.5 to 4.5 g / cm 3 5. Particulate filler according to claim 1, characterized in that it has a density ranging from 0.1 to 1000000.

6. A particulate filler according to any one of claims 1 to 5, characterized in that the titanium dioxide is doped with a combination selected from (a) Nb, Mn, (b) Nb, Mn, Cr, (c) Nb, Mn, Ce, (d) Nb, Cr, (e) Nb, Cr, Ce, (f) Nb, Ce, or (g) Nb, Mn, Cr, Ce.

7. 7. Particulate filler according to claim 1, characterized in that the doping is present in the titanium dioxide in an amount of 0.01 to 5 atomic %.

8. Particulate filler according to any one of claims 1 to 7, characterized in that the average particle size is in the range from 1 to 150 µm.

9. Particulate filler according to any one of claims 1 to 8, characterized in that the coating is in granular form on the support particles.

10. 10. A process for the preparation of particulate fillers according to any one of claims 1 to 9, characterized in that support particles comprising or in each case consisting of at least one aluminium or silicon compound are provided with a coating comprising, in an aqueous suspension, in each case at a suitable pH, at least one titanium compound, at least one niobium compound and at least one compound of a further element selected from the group consisting of Mn, Cr and Ce, and that the support particles provided with the coating are subsequently dried and calcined, during which the coating is converted into titanium dioxide in finely divided form doped with niobium and at least one other element selected from the group consisting of Mn, Cr and Ce.

11. 11. The process according to claim 10, characterized in that the support particles are in the form of flakes, are spherical or have an isotropically irregular shape and comprise or in each case consist of aluminium oxide, silicon dioxide or aluminosilicate.

12. 12. The process according to claim 10 or 11, characterized in that the support particles consist of aluminium oxide, silicon dioxide, mullite, fly ash, kaolinite, pumice or perlite.

13. Use of the particulate filler according to any one of claims 1 to 9 for pigmenting coating compositions and moulding compounds.

14. 14. Use according to claim 13, characterized in that the coating composition or moulding compound comprises or consists of silicone, EPDM, polyurethane, polyethylene, epoxide, phenolic resins or ceramic materials.

15. 15. Use according to claim 13 or 14, characterized in that the particulate filler is present in the coating composition or in the molding compound in a pigment volume concentration in the range from 3 to 33% by volume, based on the volume of the coating composition or of the molding compound.

16. Use according to any one of claims 13 to 15, characterised in that the particulate filler has non-linear electrical properties in the coating composition or in the moulding compound.

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