METHOD FOR VARIABLE ADJUSTMENT OF THE ELECTRICAL INSULATING PROPERTIES OF VARISTOR-CONTAINING MATERIALS

Description

[0001] The present invention relates to a method for variably adjusting the electrical insulating properties of varistor-containing composite materials by means of defined filler mixtures, to the use of such filler mixtures, and to composite materials with resistive and capacitive field control properties containing such filler mixtures.

[0002] Varistor fillers are used to manufacture field-controlling insulating composites based on polymers. Particulate fillers containing or consisting of silicon carbide or zinc oxide have long been used as varistor fillers in polymers. However, particles of titanium carbide, titanium suboxide (preferably TiO), or even electrically conductive particles of copper or carbon black are now also being incorporated into various polymer systems to create field-controlling properties.

[0003] Various systems in the form of coatings or molding compounds are used as polymer matrices, consisting, for example, of silicones, EPDM (ethylene-propylene-diene monomer), polyurethanes, polyethylenes, epoxies or phenolic resins.

[0004] The aim of using functional fillers is to achieve controlled field control or overvoltage protection in the application medium.

[0005] The functional fillers used for this purpose are also called varistors ("variable" + "resistor"). They are characterized by the fact that their electrical conductivity in the application medium depends on the applied field strength, thus occurring nonlinearly.

[0006] Such fillers are particularly suitable for use in application media and for protecting the materials used therein, in which high constant or variable field strengths occur.

[0007] Typical applications include field control on high-voltage cables, where the main insulation is removed to connect them, for example, in joints or terminations, resulting in large field strength gradients (electrical stress) at the interfaces between electrically conductive and insulating components, or surge protection against transient loads such as switching voltages, lightning strikes, or discharges. Other applications include field control of potting compounds and contacts in high-power electronics circuits.

[0008] Polymeric application media filled with varistor materials exhibit both resistive and capacitive field control properties. These materials are therefore suitable for use in both DC and AC applications and can also absorb transient phenomena (pulses). This is caused by a well-controlled voltage drop between the high-voltage source (e.g., a conductor in a cable) and the ground (e.g., the outer conductor) across the varistor-filled application medium. Due to the nonlinear electrical properties of the varistor material (conductivity) and its slightly increased dielectric constant combined with low loss (tan δ), the electric field is evened out (resistive and capacitive field control).

[0009] The varistor fillers known today are characterized by the fact that they exhibit a specific characteristic curve shape (E / J diagram or E / p diagram) in the application medium, either deliberately (ZnO, SnO 2 , TiO 2 ) or randomly (SiC), depending on the type of synthesis. This characteristic curve can only be shifted to a limited extent by changing the concentration of this filler in the application medium, since this would often also shift several other parameters. Therefore, if the electrical conductivity of a composite material is to be changed by changing the concentration of the respective varistor filler, the switching point of the composite material also changes. For example, if the specific resistance of the composite material (p) decreases with increasing filler content (percolation effect), the switching point simultaneously shifts undesirably to lower field strengths (E), i.e., the electrical strength in the sense of an "insulation strength" of the composite material decreases.

[0010] A high filler content of varistor fillers in polymers, as with other fillers in general, can lead to problems with the mechanical strength of such composite materials. Such highly filled polymer materials become brittle and can no longer adequately withstand mechanical pressure. It would therefore be advantageous to be able to limit the filler content of polymer materials in general, and varistor fillers in particular, to a tolerable level without compromising the mechanical strength of the corresponding composite materials or under-designing the material's conductivity. Furthermore, it would be advantageous to be able to variably adjust the resistivity of composite materials in relation to the specific requirements of the application medium.In addition, it is necessary that the corresponding fillers are homogeneously distributed in the polymer matrix so that no local differences in the conductivity of the material occur in the application medium.

[0011] With regard to an optimal homogeneous distribution of varistor fillers in the polymer matrix, the current patent applicant has succeeded in developing a microvaristor filler that, due to its material composition and resulting low density, exhibits an exceptionally low tendency to settle in the application medium and also possesses very good nonlinear electrical properties. The composition, production, and use of this microvaristor filler are described in patent application WO 2021 / 105319 A1 by the same patent applicant. The contents of this patent application are incorporated herein by reference.

[0012] However, the microvaristor fillers described in WO 2021 / 105319 A1 also have an E / p diagram in a dielectric polymer matrix containing them, in which the specific resistance of the composite material can be reduced with increasing weight fraction of microvaristor filler in the polymer matrix, but at the same time a leakage current flows already at a lower field strength, which leads to a lower electrical strength of the composite material.

[0013] Document US 4 726 991 A discloses a method according to the preamble of claim 1.

[0014] The object of the present invention is therefore to provide a method by means of which the insulating properties of varistor-containing composite materials based on dielectric polymers can be variably adjusted according to application requirements by means of a small number and concentration of fillers, wherein the fillers are homogeneously distributed in the polymer matrix and wherein the electrical strength of the composite materials is largely or completely retained.

[0015] A further object of the present invention is to provide suitable fillers which are suitable for variably adjusting the electrical insulating properties of varistor-containing composite materials.

[0016] Furthermore, an additional object of the invention is to provide composite materials with resistive and capacitive field control properties, the electrical insulation properties of which can be variably adjusted according to the requirements of the respective application medium by means of a small number and concentration of fillers.

[0017] The object of the present invention is achieved by a method according to claim 1.

[0018] It was surprisingly found that the electrical insulating properties of varistor-containing composite materials can be adjusted as needed if the composite material contains, in addition to a particulate microvaristor filler (A), another particulate filler (B) whose electrical conductivity is either greater than the electrical conductivity of the microvaristor filler but also in the semiconductive range, or whose electrical conductivity is lower than that of the microvaristor filler. If one or the other particulate fillers (B) are used in a mixture with a particulate microvaristor filler (A) in a dielectric polymer matrix, the specific electrical resistance of the respective composite material can be adjusted as needed without having to increase or decrease the total filler percentage compared to using the microvaristor filler alone.Thus, the electrical strength of the composite material remains almost unaffected, since the switching point of the composite material hardly changes compared to the use of the microvaristor filler alone (i.e. a leakage current only flows above a certain field strength).

[0019] In order to achieve a homogeneous distribution of the microvaristor filler in the polymer matrix, it is advantageous if the microvaristor filler has a comparatively low density, the difference between which and the density of the polymer material is as small as possible in order to minimize or avoid settling of the microvaristor filler during the production of the composite material.

[0020] Known microvaristor fillers made of doped zinc oxide exhibit an extraordinarily high density of 5.5 to 6.5 g / cm³, depending on the degree of densification achieved by the sintering process used in their production. Since the density of these microvaristor particles is significantly greater than the density of the polymeric application medium, these varistor fillers have a strong tendency to settle during the preparation of the mixtures with the polymeric application medium, which makes homogeneous and permanent mixing of the materials difficult. The irregular composition of the application medium accordingly leads to irregularities in its electrical properties. For this reason, microvaristor fillers containing zinc oxide are unsuitable for use in the process according to the invention.

[0021] In contrast, the microvaristor fillers described in the aforementioned patent application WO 2021 / 105319 A1 by the same patent applicant have proven particularly suitable for the process according to the invention because they have a low density in the range of 1.5 to 4.5 g / cm³ with an average particle size in the range of 1 to 150 µm and can thus be very well distributed homogeneously in a polymer matrix. Uniform nonlinear electrical properties can thus be achieved homogeneously in the corresponding composite materials.

[0022] The microvaristor fillers mentioned according to WO 2021 / 105319 A1 are particulate fillers which consist of carrier particles and a coating enveloping the carrier particles, wherein the carrier particles contain or each consist of at least one aluminum compound or silicon compound and the coating contains a titanium dioxide doped with niobium and at least one other element.

[0023] Particularly suitable aluminum compounds include aluminum oxide (Al 2 O 3 ) or natural and synthetic aluminosilicates of various compositions. A particularly suitable silicon compound is silicon dioxide (SiO 2 ), which can be used in crystalline and, particularly preferably, amorphous form, for example, as diatomaceous earth.

[0024] Materials consisting of or containing aluminosilicates include mullite, fly ash, kaolinite, pumice, or perlite. Mullite and fly ash are particularly preferred.

[0025] In addition, carrier particles consisting of a mixture of an aluminosilicate with aluminum oxide and / or silicon dioxide are suitable.

[0026] The carrier particles for the particulate microvaristor fillers A preferably used according to the invention can have various shapes. With regard to the subsequent application media and their specific requirements, platelet-shaped or spherical carrier particles, or even carrier particles with an isotropic, irregular shape, are particularly suitable. The shape of the carrier particles is retained even after coating with the doped titanium dioxide.

[0027] The density of the particulate microvaristor fillers A used is in the range from 1.5 to 4.5 g / cm 3 , preferably in the range from 1.5 to 3.0 g / cm 3 , and in particular in the range from 1.5 to 2.5 g / cm 3 . For the microvaristor fillers described in WO 2021 / 105319 A1, it is composed of the average of the densities of the carrier particles and the coating and can be adapted to the requirements of the application medium by selecting the appropriate material, in particular the carrier particles.

[0028] The carrier particles of the particulate microvaristor filler according to WO 2021 / 105319 A1 are each coated with a doped titanium dioxide doped with niobium and at least one other element. The coating envelops each carrier particle individually and is present separately on it, thus not forming a coherent phase around multiple carrier particles.

[0029] The additional element is at least one element selected from the group consisting of Mn, Cr, Ce, V, Co, Fe, Zn, Sn, Y, Zr, Ta, Ca, Sr, and Ba. Mn, Cr, and Ce are preferably used as doping elements.

[0030] Doping is particularly preferred in the following combinations: Nb, Mn; Nb, Mn, Cr; Nb, Mn, Ce; Nb, Cr; Nb, Cr, Ce; Nb, Ce; or Nb, Mn, Cr, Ce, with the combinations Nb, Mn, Ce and Nb, Cr being particularly preferred. The doping elements are present in cationic form in the TiO 2 crystal lattice or at the grain boundaries of the titanium dioxide granules.

[0031] The doping in titanium dioxide is present in an amount of 0.01 to 5 atomic percent, based on the total number of Ti atoms and doping element atoms. The proportion of doping elements (total doping elements) is preferably between 0.02 and 2 atomic percent.

[0032] Preferably, the coating on the carrier particles of the microvaristor filler consists of multiply doped titanium dioxide, which is present in the form of granules on the surface of the carrier particles and forms a largely closed, granular layer there.

[0033] The coating on the carrier particle has a geometric thickness in the range of 100 to 5000 nm, preferably 150 to 4000 nm, and in particular 200 to 2000 nm. The geometric layer thickness of the coating can be easily determined using conventional measurement methods (e.g., SEM) based on cross-sectional images (Ar ion beam) of individual particles of the particulate filler.

[0034] The average particle size d 50 of the particulate microvaristor filler is in the range of 1 to 150 µm, preferably 2 to 100 µm, and especially 5-50 µm. Due to the origin of the aforementioned isotropic carrier materials, the particle sizes of the individual filler particles can vary within a relatively wide range, so that often only the volume-related average particle size d 50 seems reasonable. Even when using platelet-shaped carrier particles, there is usually a certain range of variation in the particle size of the individual particles.

[0035] The particle size of the particulate microvaristor filler and, if applicable, the carrier particles is preferably determined using a laser diffraction method, which is generally accepted and has the advantage of also being able to determine the particle size distribution of the particles. For the microvaristor fillers A used here and their carrier particles according to WO 2021 / 105319 A1, the particle sizes were determined using a Malvern Mastersizer 3000, APA 300 (product of Malvern Instruments, Ltd., UK).

[0036] Further details regarding the properties and the manufacturing process of the microvaristor filler A preferably used according to the invention can be found in the aforementioned patent application WO 2021 / 105319 A1, the disclosure content of which is hereby incorporated in its entirety.

[0037] In addition to the particulate microvaristor filler A, another particulate filler, designated here as B, is added to the dielectric polymer material according to the present invention. The particulate filler B can have either a higher electrical conductivity or, alternatively, a lower electrical conductivity than the particulate microvaristor filler A.

[0038] In a first embodiment, the particulate filler B has a lower electrical conductivity than the particulate microvaristor filler A. In this case, the specific powder resistance of the particulate filler B is in the range of ≥10 14< Ohm*cm (at a measuring voltage of 100 V).

[0039] Such particulate fillers are commonly referred to as dielectric, but can still lead to a current flow when an electric field with a high electric field strength is applied to the matrix material containing them.

[0040] Ceramic materials, such as those made of aluminosilicates, have proven particularly suitable for use in the present invention. In particular, ceramic microspheres from 3M, designated "3M™ Ceramic Microspheres," are used in the present invention. These microspheres have a density of approximately 2.4 g / cm³. This density is in the range of the density of the particulate microvaristor filler A, so that a mixture of both fillers can be well distributed in the polymer matrix, and the fillers do not settle even upon solidification of the resulting composite materials.

[0041] Also suitable as particulate filler B in the first embodiment are fillers whose material structure is almost analogous to the above-described particulate microvaristor filler A, but whose electrical conductivity in the coating is set lower than that of the particulate microvaristor filler A via an extremely low doping of the titanium dioxide layer with Nb and at least one further doping element, or which have no doping at all in the TiO 2 layer and thus have a specific powder resistance in the range of ≥10 14 < Ohm*cm. The density of these particles essentially corresponds to the density of the particulate microvaristor filler A.

[0042] In a second embodiment, semiconductive particulate materials having a specific powder resistance in the range of 10 8< to 10 12< Ohm*cm are used as particulate fillers B, wherein the specified values ​​refer to a measuring voltage of 100 V. Materials having a specific powder resistance of 10 10< to 10 12< Ohm*cm are particularly preferably used.

[0043] For use as particulate fillers B according to the present invention, materials having a core-shell structure and a density of < 5 g / cm 3 , in particular < 4 g / cm 3 , have proven particularly preferred. Due to different core and shell materials, the density of such particles can be easily adjusted within the stated range, so that here too, a mixture of particulate microvaristor filler A and semiconductive particulate filler B leads neither to a significant demixing of the two filler components nor to significant settling of these fillers in the polymer matrix. This makes it possible to maintain stable electrical properties, in this case insulating properties, in the application medium across the entire cross-section or longitudinal section of the composite material.

[0044] Advantageously, particulate fillers B are used according to the invention which have an electrically semiconductive, firmly adhering coating consisting of a doped metal oxide on a core of mica or talc particles. The doped metal oxide is preferably a tin oxide doped with antimony. The proportion of the doping element antimony in the tin oxide coating must be so low that the resulting coated particles are not electrically conductive, but semiconductive. This is achieved when the molar ratio of tin to antimony in the coating is only in the range of 99.99:0.01 to 97:3 and when, if necessary, the doped tin oxide layer on the core particle is covered on its surface by another, dielectric, metal oxide layer. The semiconductive particles preferably have a particle size of less than 15 µm.

[0045] Suitable semiconductive particulate materials of the described structure are commercially available and are offered, for example, by Merck KGaA, Germany, under the name Iriotec ®< 7510 or Iriotec ®< 7550, the latter being preferably used in the present invention.

[0046] However, fillers which are also suitable as particulate filler B are those which are materially analogous to the particulate microvaristor filler A described above, but whose electrical conductivity in the coating is set higher than that of the particulate microvaristor filler A via a higher doping of the titanium dioxide layer with Nb and at least one further doping element, and which thus have a specific powder resistance in the range of 10 8< to 10 12< Ohm*cm. Electrically conductive particulate materials are not suitable for use in the present invention, even if they consist of mica particles which have a coating of electrically conductive, antimony-doped tin dioxide (with a proportion of > 3% antimony).

[0047] The electrical properties of the particulate fillers B are characterized by their specific powder resistance. To measure the specific resistance of a pigment powder, an acrylic tube with an inner diameter of 2 cm is filled with a small amount of the respective pigment powder (approximately 0.5 to 1 g) and pressed against a metal electrode using a 10 kg weight with a metal stamp. The specific resistance p is determined from the layer thickness L of the compressed powder according to the following equation: ρ = R * π * d / 2 2 / L Ohm * cm

[0048] Here, R represents the actually measured electrical resistance at a measuring voltage of 100 V and d the diameter of the pigment column. If a differently dimensioned tube is to be used for the measurement of the specific powder resistance, the values ​​for d and L in the formula must be adjusted accordingly.

[0049] According to the method of the invention, the particulate microvaristor filler A and the particulate filler B are added together to one or more defined mass units of a dielectric polymer material.

[0050] According to the invention, the dielectric polymer material is silicone, polyurethane, polyethylene, epoxide, phenolic resin, or EPDM. Such dielectric polymers are commonly used as polymer matrices for varistor-containing composite materials with nonlinear electrical properties.

[0051] The mass unit of the dielectric polymer material is understood here as the mass of dielectric polymer material required for the respective application, determined in the mass units gram, kilogram, ton, etc.

[0052] A predefined total mass of fillers A+B is added to the respective mass unit of the dielectric polymer material, whereby the total mass A+B is expressed here in weight percent and is related to the total weight of the respective mass unit of the dielectric polymer material including the total mass A+B of the particulate fillers A+B.

[0053] The total mass A+B thus represents the mass A+B corresponding to a specific percentage weight of fillers in the respective mass unit of the dielectric polymer material provided with fillers. According to the invention, this percentage lies in the range of 5 to 35 wt.%, preferably in the range of 15 to 30 wt.%, in each case based on the total weight of the mass unit of the dielectric polymer material.

[0054] For example, if a total mass of 50 g of a mixture of a particulate microvaristor filler A and a particulate filler B is added to a mass of 150 g of a dielectric polymer material, the total mass A+B, expressed in weight percent, is 25 wt.%.

[0055] Within the total mass A+B, the mass ratio A:B can, according to the invention, be in the range of 1:99 to 99:1. This internal mass ratio of the particulate fillers is determined in each case by the electrically insulating properties to be achieved in the final composite material.

[0056] The starting point for a specific adaptation of the electrically insulating properties of varistor-containing composite materials is in any case a composite material which contains a predefined mass of a dielectric polymer material and an equally defined mass (expressed in wt%) of the particulate microvaristor filler A.

[0057] After the respective mixture has been transferred into a cylindrical test specimen mold and cured, the specimen is transferred to a suitable device (explained later), and an E / p characteristic curve of the material is created based on the measurement results. This shows the specific resistance of the test specimen p as a function of the field strength E of the applied electric field.

[0058] Using this characteristic curve, the specific initial resistance of the test specimen can be determined at a very low field strength of the applied electric field as well as the switching point, which indicates the field strength at which a leakage current begins to flow in the test specimen.

[0059] In the event that a higher specific initial resistance is required for a specific application than can be obtained by using the particulate microvaristor filler A alone, a particulate filler B is added according to the invention, which has a lower electrical conductivity than the particulate microvaristor filler A. The specific powder resistance of such a particulate filler B is in the range of ≥10 14< Ohm*cm, as described above.

[0060] The total mass A+B corresponds to the mass A used to determine the initial characteristic curve. A certain mass fraction of the originally used mass of A is thus replaced by the same mass of B, so that the total mass of particulate fillers in the polymer matrix does not change compared to the initial measurement. By conducting series tests with an increasing mass fraction of particulate filler B in the total mass A+B while maintaining the same mass A+B, the desired specific initial resistance of the respective test specimens can now be increased. The switching point lies in the same field strength range for all test specimens containing a mixture of the particulate fillers A and B in any ratio in the range from 1:99 to 99:1.The switching strength of the test specimens is therefore maintained when adding a particulate filler B with a lower electrical conductivity than that of the particulate microvaristor filler A, compared to using the particulate microvaristor filler A alone in the same dielectric polymer material and with a constant mass fraction of particulate fillers that determine the electrical insulation properties of the respective composite material.

[0061] If the E / p characteristics of the different test specimens are represented in a single diagram, each of the mass units of the dielectric polymer material containing the particulate fillers A and B (here in the test specimen) has an E / p characteristic that lies spatially between an E / p characteristic of a mass unit of the same polymer material containing only the particulate microvaristor filler A and the E / p characteristic of a mass unit of the same polymer material containing only the particulate filler B.

[0062] A corresponding diagram is shown in Figure 3 shown.

[0063] If a higher electrical conductivity and thus a lower specific resistance is required in an application medium than can be achieved with the sole use of microvaristor filler A, a semiconductive particulate filler B is added to the respective mass unit of the dielectric polymer material in addition to the particulate microvaristor filler A, in an amount increasing with each sample, analogous to the previous description. The semiconductive particulate filler B here has a specific powder resistance in the range of 10 8 < to 10 12 < ohm*cm, as previously described.

[0064] By creating and comparing the E / p characteristics of various test specimens, each with different mass fractions of A and B in the total mass A+B, which remains constant in all tests, a composition of a varistor-containing composite material can be found that exactly meets the respective requirements of the application medium. A corresponding diagram is shown in Figure 4 shown.

[0065] Since the internal ratio A:B in the total mass A+B can be individually controlled as needed, the method according to the invention allows the specific electrical resistance of a varistor-containing composite material to be adjusted very precisely via a mass ratio A:B, which produces the desired electrically insulating properties in the application medium, analogous to the correspondingly composed test specimen. With only a few particulate fillers and a small number of tests, electrically insulating properties can be adjusted in varistor-containing composite materials according to the invention, which exhibit an optimal electrical resistance for the respective application medium without compromising the electrical strength of the material.

[0066] The present invention also relates to the use of a filler mixture for variably adjusting the electrical insulating properties of varistor-containing composite materials, wherein the filler mixture consists of a predefined total mass A+B of a particulate microvaristor filler A and a further particulate filler B, wherein the particulate filler B either has a lower electrical conductivity than the particulate microvaristor filler A or wherein the particulate filler B is a semiconductive particulate material with a higher electrical conductivity than the particulate microvaristor filler A, and wherein a mass ratio A:B in the range of 1:99 to 99:1 in the total mass A+B is present,wherein the filler mixture is added to one or more mass units of a dielectric polymer material and the mass ratio A:B in each of the mass units is set to be the same or different from each other.,

[0067] Details regarding the material composition and the specific powder resistances of the particulate fillers A and B have already been described previously.

[0068] If serial tests are conducted on test specimens to determine the optimal material and percentage composition of the particulate fillers as described above, the predefined total mass A+B will be the same in each mass unit of the dielectric polymer material. Only the A:B ratio is selected differently in each test specimen and thus in each mass unit of the dielectric polymer material. Once the optimal composition of the test specimen has been found in this way, a varistor-containing composite material can be produced according to its material composition. This composite material contains a dielectric polymer material and a filler mixture of components A and B as described above.In this composite material, which is intended for use as an electrically insulating material with nonlinear electrical properties, both the total mass A+B and the ratio A:B are always the same, regardless of how many mass units of the dielectric polymer material must be individually mixed with components A and B to provide a suitable mass of varistor-containing composite material for the respective application. The total mass of the particulate fillers A+B, expressed as a percentage by weight, is in each case in the range of 5 to 35 wt.%, preferably in the range of 15 to 30 wt.%, in each case based on the total weight of the respective mass unit of the dielectric polymer material including the total mass A+B of the particulate fillers A and B.

[0069] It goes without saying that the materials already described above for the dielectric polymer materials as well as for the particulate microvaristor filler A and the particulate fillers B are used here. A detailed description is therefore not necessary here.

[0070] The present invention also relates to a varistor-containing composite material with resistive and capacitive field control properties, comprising mass units of a dielectric polymer material, as well as a particulate microvaristor filler A and a further particulate filler B in a predefined total mass A+B in each mass unit of the dielectric polymer material, wherein the particulate filler B has a lower electrical conductivity than the particulate microvaristor filler A or wherein the particulate filler B is a semiconductive particulate material with a higher conductivity than the particulate microvaristor filler A, and wherein the same mass ratio A:B is present in each of the mass units of the polymer material containing the particulate fillers A+B and the mass ratio A:B is in the range from 1:99 to 99:1.

[0071] In this varistor-containing composite material, the total mass A+B in each of the mass units is in the range of 5 to 35 wt.%, based on the total weight of the mass unit of the dielectric polymer material including the total mass A+B of the particulate fillers A and B.

[0072] According to the invention, the dielectric polymer material in this composite material is silicone, polyurethane, polyethylene, epoxies, phenolic resins or EPDM and has resistive or capacitive field-controlling properties.

[0073] The type of particulate fillers A and B, as well as the determination of the optimal percentage composition of the filler mixture A+B, are determined based on a few series tests in test specimens as described above. Based on the composition thus determined, a varistor-containing composite material with resistive and capacitive field control properties is produced, which is optimally adapted to the respective application conditions and can be used in various application media.Potential applications include those where varistor-containing materials are typically used, such as field control on high-voltage cables, where the main insulation is removed to connect them, for example, in joints or terminations, and large field strength gradients build up at the interfaces between electrically conductive and insulating components (electrical stress), or surge protection against transient loads such as switching voltages, lightning strikes, or discharges. Other areas of application include the field control of potting compounds and contacts in power electronics circuits. Applications for both direct current and alternating current are possible.

[0074] The present invention provides varistor-containing composite materials optimized for the respective application, as well as a method by which such an optimized composition of the composite materials can be determined. Using a small number of fillers and the dielectric polymer materials commonly used for such composite materials, varistor-containing composite materials with a homogeneous composition can be obtained that exhibit electrically insulating and simultaneously resistive or capacitive field-controlling properties and high electrical strength. Figure 1 :shows the schematic circuit of the measuring arrangement (1b, below) with voltage source (DC), integrated voltage measurement (U), pico-ammeter (A) and test specimen, as well as the cylindrical test specimen with the relevant dimensions (1a, above) for determining the electrical measurement results for the creation of an E / p characteristic curve. Figure 2 : shows the characteristic DC field strength-resistance curve (E / p) of a filler according to Examples 5-10 to 5-14 of WO 2021 / 105319 A1 in RTV-2 silicone with pigment mass concentrations of 15, 20, 25, 30, and 35 wt.%. The switching points are shaded. Figure 3 : shows the characteristic DC field strength-resistance curve (E / p) of a filler according to Example 1 of the present invention in RTV-2 silicone with various ratios of fillers A:B. The areas for the switching points are shaded. Figure 4 :shows the characteristic DC field strength-specific resistance curve (E / p) of a filler according to Example 2 of the present invention in RTV-2 silicone with different ratios of fillers A:B.

[0075] The invention will be described below using examples, but is not limited to these. Examples: Production of particulate fillers: Production of a particulate microvaristor filler A:

[0076] 100 g of spherical aluminosilicate particles (BET 0.50 m 2 / g density 2.45 g / cm 3 , particle size d 5 -d 95 = 1.2 µm - 17 µm ) are suspended in approximately 2 L of deionized water. A solution of 599 g of titanium oxychloride (400 g / l), 0.38 g of niobium pentachloride, and 0.24 g of potassium chromium sulfate dodecahydrate is added dropwise to the suspension at 75°C under acidic conditions while stirring. The pH is kept constant at 2 by simultaneous, controlled addition of sodium hydroxide solution. After adding the entire amount of the solution, the mixture is stirred for a further 15 minutes at 75°C. The mixture is then cooled to room temperature while stirring, and the reaction mixture is adjusted to a pH of 5. The resulting pigment is filtered through a suction filter, washed with water, dried at 110°C, and calcined at 850°C for 120 minutes. An ochre-colored pigment powder is obtained.The resulting pigment particles comprise aluminosilicate hollow spheres (particle size <40 µm) as carrier particles and a coating of titanium dioxide doped with niobium and manganese that is firmly deposited on them. Production of a semiconductive particulate filler B with increased electrical conductivity compared to the microvaristor filler A:

[0077] In a 5L stirred tank, 100g of mica with a maximum particle size of < 15 µm is suspended in 2L deionized water. At 75°C, 590g of a hydrochloric acid solution containing 121g SnCl 4 and 0.273g SbCl 3 is added over a period of 2 hours while stirring. The pH is kept constant at pH 1.6 by simultaneously adding sodium hydroxide solution. After the addition is complete, 10.6g of a 40% w / w hydrochloric acid titanium oxychloride solution are added at pH 2 and stirred for half an hour at 75°C. The pH is then adjusted to 4 with sodium hydroxide solution, and the suspension is cooled to room temperature. The pigment is filtered off, washed with water until free of salt, dried, and calcined at 750°C for 30 minutes. 171g of pigment is obtained as a white, glossy powder. The antimony content in the tin oxide layer is 0.25 mol%, based on the sum of Sn+Sb. The pigment has a specific powder resistivity of approximately 10 12 Ohm*cm (measurement voltage 100 V). Example 1: Production of test specimens with room temperature curing silicone as polymer component

[0078] The particulate microvaristor filler A obtained as above is incorporated into a room-temperature-curing silicone resin to produce a test specimen with a pigment mass concentration of 25%. Further test specimens are also produced, each with a total mass concentration of 25%. These additional test specimens contain, in addition to the microvaristor filler A, another filler B that has a lower electrical conductivity than the microvaristor filler A (3M™< Ceramic Microspheres W-210 from 3M, particle size d 10 -d 90 2-12 µm). The percentage of A is adjusted between 25 and 90%, and the percentage of B is adjusted in the opposite direction between 75 and 10%, each based on the total mass of A+B.

[0079] The particulate fillers are roughly premixed in a can with the respective proportions of component 1 of a commercial, room-temperature-curing RTV-2 silicone resin (manufacturer's material data: Comp. 1: Comp. 2 = 9:1, viscosity of the mixture 3500 mPa*s at 23°C, Shore A hardness 45°) and homogenized in a vacuum speed mixer (Hauschild) at a pressure reduced to 4 mbar and 1600 revolutions per minute for at least 2 minutes. Subsequently, the respective amounts of component 2 of the same RTV-2 silicone resin are added, the components are roughly premixed again, and homogenized in the vacuum speed mixer for at least 1 minute at 4 mbar and 1600 revolutions per minute. The viscous mass is then quickly poured into a mold that specifies the geometric dimensions of the test specimen, observing the pot life. The silicone resin is cured in the mold for at least 30 minutes at 70°C.After the mold has cooled, the mold is opened, the test specimen is removed, and stored in a dust-free environment. The layer thicknesses of the cross-linked test specimens are between 500 µm and 600 µm and are determined for each test specimen (circular base, 60 mm diameter) as the average of ten measurements taken at various locations using an eddy current coating thickness gauge (Fischer Dualscope FMP30 with FD10 sensor according to DIN EN ISO 2360).

[0080] (Note: Typically, the two components of commercial room temperature curing RTV-2 silicone resins are referred to by the manufacturers as A and B. To avoid confusion with the particulate filler components of the present invention, the silicone resin components are referred to herein as Component 1 and Component 2.)

[0081] The pigment mass concentration PMK is defined as follows (the volatile components are not shown), but is given here as a percentage: PMK = m Füllstoff m Füllstoff + m Bindemittel m= mass Example 2:

[0082] Test specimens are prepared as described in Example 1 using room-temperature-curing silicone as the polymer component. However, in addition to the particulate microvaristor filler A, they also contain the semiconductive particulate filler B as specified above in the manufacturing instructions. The pigment mass concentration is 25% in each case. The percentage of A is adjusted between 25 and 75%, and the percentage of B is adjusted between 75 and 25%, each based on the total mass of A+B. Measurement of the test specimens with regard to their electrical properties:

[0083] The current-voltage characteristics of the produced varistor filler / polymer test specimens are measured with a Heinzinger 10 kV DC voltage source (PNChp 10000-20 ump) and a Kethley pico ammeter (6514 System Electrometer) on a ring electrode according to DIN EN 61340-2-3.

[0084] The schematic structure of the measuring device and the dimensions of the test specimen to be observed are shown Figure 1 .

[0085] To normalize the results, the Figure 1 named dimensions of the sample and the electrode, according to formulas (2-4) the electric field strength E and the current density J are calculated: E = V / h J = I / A A = d 1 + g 2 * π / 4 V = voltage in volts (V) I = current in amperes (A) A = effective electrode area (m 2< ) h = electrode distance (sample thickness): -0.5 mm d 1 -d 4 = electrode diameter (see Figure 1) d 1 = Diameter of central electrode: 25 mm g = Distance between ring electrode and central electrode: 2.5 mm

[0086] The specific resistances p of the test specimens are obtained from the equation p = E / J.

[0087] The current measurements are carried out with a step-like voltage ramp at room temperature and relative humidity between 20% and 30%.

[0088] The corresponding diagrams of all determined E / p characteristics are shown in the Figures 3 (Example 1) and 4 (Example 2).

[0089] For comparison, Figure 2 the diagram of E / p characteristics is shown, which comprise test specimens made of silicone resins containing only a particulate microvaristor filler A in increasing pigment mass concentration (according to Examples 5-10 to 5-14 of WO 2021 / 105319 A1).

Claims

1. Process for the variable setting of the electrical insulation properties of varistor-containing composite materials, where one or more mass unit(s) of a dielectric polymer material are provided and in each case a particulate microvaristor filler A and a further particulate filler B in a predefined total mass A+B are added to the mass units, where the particulate filler B either has a lower electrical conductivity than the particulate microvaristor filler A or where the particulate filler B is a semiconductive particulate material having a higher electrical conductivity than that of the particulate microvaristor filler A, and where a mass ratio A:B in the range from 1:99 to 99:1 is in each case set differently in different mass units of the polymer material in the total mass A+B, characterised in that the total mass A+B is in the range from 5 to 35% by weight, based on the total weight of a mass unit of the dielectric polymer material including the total mass A+B of the particulate fillers A and B.

2. Process according to Claim 1, characterised in that the total mass A+B, expressed in % by weight, is the same in each of the mass units of the polymer material, in each case based on the total weight of a mass unit of the dielectric polymer material including the total mass A+B of the particulate fillers A and B.

3. Process according to Claim 1 or 2, characterised in that the dielectric polymer material comprises silicones, polyurethanes, polyethylenes, epoxides, phenolic resins or EPDM.

4. Process according to one or more of Claims 1 to 3, characterised in that the particulate microvaristor filler A is a filler which consists of support particles and a coating surrounding each of the support particles, where the support particles comprise or in each case consist of at least one aluminium compound or one silicon compound, and where the coating comprises a titanium dioxide doped with niobium and at least one further element.

5. Process according to Claim 4, characterised in that the support particles comprise or in each case consist of aluminium oxide, silicon dioxide or an aluminosilicate.

6. Process according to Claim 4 or 5, characterised in that the support particles consist of aluminium oxide, silicon dioxide, mullite, fly ash, kaolinite, pumice stone or perlite.

7. Process according to one or more of Claims 4 to 6, characterised in that the titanium dioxide is doped, besides niobium, with at least one further element selected from the group consisting of Mn, Cr, Ce, V, Co, Fe, Zn, Sn, Y, Zr, Ta, Ca, Sr and Ba.

8. Process according to Claim 7, characterised in that the titanium dioxide is doped, besides niobium, with at least one further element selected from the group consisting of Mn, Cr and Ce.

9. Process according to one or more of Claims 1 to 8, characterised in that the particulate microvaristor filler A has an average particle size in the range from 1 to 150 µm.

10. Process according to one or more of Claims 1 to 9, characterised in that the particulate filler B is a semiconductive particulate material which has a higher electrical conductivity than the particulate microvaristor filler A and has a specific powder resistance in the range from 108 to 1012 ohm*cm.

11. Process according to one or more of Claims 1 to 9, characterised in that the particulate filler B has a lower electrical conductivity than the particulate microvaristor filler A and has a specific powder resistance in the range ≥ 1014 ohm*cm.