Particulate filler, preparation and use thereof

Abstract

The present invention relates to a particulate filler which has a coating on support particles in each case surrounding the latter, which comprises a titanium dioxide doped with niobium and at least one further element, to a process for the preparation of a particulate filler of this type and to the use thereof, in particular as varistor filler having nonlinear electrical properties in coating compositions and moulding compounds.

Claims

1. Particulate filler which consists of support particles and a coating in each case surrounding the support particles, characterised in that the support particles comprise or in each case consist of at least one aluminium compound or silicon compound and the coating comprises a titanium dioxide doped with niobium and at least one further element.

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

3. Particulate filler according to claim 2, characterised in that the support particles consist of aluminium oxide, silicon dioxide, mullite, fly ash, kaolinite, pumice stone or perlite.

4. Particulate filler according to claim 1, a characterised in that the support particles are in flake form, are spherical or have an isotropically irregular shape.

5. Particulate filler according to claim 1, characterised in that it has a density in the range from 1.5 to 4.5 g/cm.sup.3.

6. Particulate filler according to claim 1, characterised in that, besides niobium, the titanium dioxide is doped with at least one further element selected from the group consisting of Mn, Cr and Ce.

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

8. Particulate filler according to claim 1, characterised in that it has an average particle size in the range from 1 to 150 μm.

9. Particulate filler according to claim 1, characterised in that the coating is in granular form on the support particles.

10. Process for the preparation of a particulate filler according to claim 1, characterised in that support particles which comprise or in each case consist of at least one aluminium compound 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 aqueous suspension at a pH which is suitable in each case, and in that the support particles provided with the coating are subsequently dried and calcined, during which the coating is converted into a titanium dioxide in granular form which is doped with niobium and at least one other element.

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

12. Process according to claim 10, characterised in that the support particles consist of aluminium oxide, silicon dioxide, mullite, fly ash, kaolinite, pumice stone or perlite.

13. A molding or coating composition pigmented by a particulate filler according to claim 1.

14. The molding or coating composition according to claim 13, characterised in that the coating composition or moulding compound comprises or consists of silicones, EPDM, polyurethanes, polyethylenes, epoxides, phenolic resins or a ceramic material.

15. The molding or coating composition according to claim 13, characterised in that the particulate filler is present in the coating composition or moulding compound with a pigment volume concentration in the range 3-33% by vol., based on the volume of the coating composition or moulding compound.

16. The molding or coating composition according to claim 13, characterised in that the particulate filler has nonlinear electrical properties in the coating composition or moulding compound.

Description

[0063] The particulate fillers in accordance with the present invention have nonlinear electrical properties, i.e. varistor properties, in the coating composition or moulding compound of the application medium. Due to the possibility of selecting various support particles which, besides different shape and size, may also have different densities, and due to the possibility of variation in the type and amount of the doping of the coating on the respective support particles, both the electrical properties and also the density of the particulate fillers can be matched optimally to the respective requirements of the application medium. All starting materials here are readily available and the preparation of the particulate fillers by means of an uncomplicated coating process can be carried out without major technical effort. The particulate fillers according to the invention can be employed in direct-current and alternating-current applications and achieve high switching field strengths of >3000 kV/m. In the ground state, the electrical conductivity of the particulate fillers according to the invention substantially corresponds to the electrical conductivity of the insulating materials surrounding them, meaning that unintended generation of an electrical conductivity under non-stress conditions does not have to be expected. On switching through in the stress case, by contrast, a change in the measured current density in the coating composition or moulding compound over several orders of magnitude is possible. The increased current density here stands for the changed nonlinear conductivity of the composite.

[0064] FIG. 1: shows the diagrammatic circuit of the measurement arrangement (1b, bottom) with voltage source (DC), integrated voltage measurement (U), pico-ammeter (A) and test specimen, and the test specimen with the relevant dimensions (1a, top).

[0065] FIG. 2: shows the characteristic DC field strength/current density curve (E/J) of a filler in accordance with Example 1 in RTV-2 silicone with pigment mass concentrations of 15, 20, 25 and 30%. The percolation threshold is marked (Examples 5-1 to 5-4).

[0066] FIG. 3: shows the characteristic DC field strength/current density curve (E/J) of the filler in accordance with Example 2 in the pigment mass concentrations 25 and 50.8% in RTV-2 silicone (Examples 5-6 and 5-7).

[0067] FIG. 4: shows the characteristic DC field strength/current density curve (E/J) of the filler in accordance with Example 3 in the pigment mass concentrations 25 and 37% in RTV-2 silicone (Examples 5-8 and 5-9).

[0068] FIG. 5: shows the characteristic DC field strength/current density curve (E/J) of a filler in accordance with Example 4 in RTV-2 silicone with pigment mass concentrations of 15, 20, 25, 30 and 35%. The percolation threshold is between 15 and 30% PMC (Examples 5-10 to 5-14).

[0069] FIG. 6: shows the percolation curves of the fillers in accordance with Examples 1 and 4 in RTV-2 silicone as a function of the pigment volume concentration PVC. The specific conductivity of the samples at a field strength of 20 kV/m is plotted.

[0070] FIG. 7: shows the characteristic DC field strength/current density curve (E/J) of the filler in accordance with Example 1 in the pigment mass concentrations 25 and 35% in LSR silicone (Examples 6-1 and 6-2).

[0071] FIG. 8: shows the characteristic DC field strength/current density curve (E/J) of the filler in accordance with Example 4 in the pigment mass concentrations 20 and 35% in epoxide (Examples 7-1 and 7-2).

[0072] FIG. 9: shows the characteristic DC field strength/current density curve (E/J) of the filler in accordance with Example 1 in HD-PE in the concentrations 30 and 40% by weight (Examples 8-1 and 8-2).

[0073] FIG. 10: shows the characteristic DC field strength/current density curve (E/J) of the test specimens in accordance with Examples 5-10 to 5-18 (Table 3) in RTV-2 silicone. Test specimens 5-10, 5-15 and 5-17 have a PMC of 25%, test specimens %-14, 5-16 and 5-18 have a PVC of about 27%.

[0074] FIG. 11: shows the change in impedance Z, permittivity ϵ′ and loss factor tan δ(×100) at an alternating voltage of 50 Hz of the filler in accordance with Example 1 in HDPE with various filler concentrations (0 to 40% by weight).

[0075] FIG. 12: shows a) a diagrammatic representation of the particulate fillers in accordance with the present invention with dielectric support particle (1) and a granular, nonlinear electrically conductive coating (2) and b) a diagrammatic representation of a polymer matrix (3) filled with the fillers according to the invention.

[0076] The invention is intended to be described below with reference to examples, but is not intended to be restricted thereto.

EXAMPLES

[0077] Preparation of Particulate Fillers:

Example 1

[0078] 100 g of aluminium oxide flakes (BET 3.3 m.sup.2/g, density 3.75 g/cm.sup.3, particle size 5-40 μm) are suspended in about 2 I of deionised water. A solution of 700 g of titanium oxychloride (400 g/I), 3.3 g of niobium pentachloride in 24 ml of hydrochloric acid (37%) and 0.8 g of manganese sulfate monohydrate is added dropwise to the suspension at 75° C. with stirring in an acidic medium. The pH of 2 is kept constant by simultaneous regulated metered addition of sodium hydroxide solution. When all the solution has been added, the mixture is stirred at 75° C. for a further 15 min, before a solution of 13.5 g of cerium chloride heptahydrate in 150 ml of deionised water is added uniformly and with metered addition of sodium hydroxide solution at a constant pH of 7. The mixture is subsequently cooled to room temperature with stirring, and the reaction mixture is neutralised again. The pigment obtained is filtered off via a suction filter, washed with water, dried at 140° C. and calcined at 1100° C. for 120 min., giving a pale, ochre-coloured pigment powder. The pigment particles obtained have aluminium oxide flakes (particle size <15 μm) as support particles and a coating comprising niobium-, manganese- and cerium-doped titanium dioxide adherently precipitated onto the latter.

Example 2

[0079] 100 g of spherical aluminium oxide particles (BET 1.4 m.sup.2/g, density 2.85 g/cm.sup.3, particle size d.sub.5-d.sub.95=14 μm−45 μm) are suspended in about 2 I of deionised water. A solution of 577 g of titanium oxychloride (400 g/I), 5.04 g of niobium pentachloride in 24 ml of hydrochloric acid (37%) and 0.16 g of manganese sulfate monohydrate is added dropwise to the suspension at 90° C. with stirring in an acidic medium. The pH of 2 is kept constant by simultaneous regulated metered addition of sodium hydroxide solution. When all the solution has been added, the mixture is stirred at 90° C. for a further 15 min, before a solution of 7.6 g of cerium chloride heptahydrate in 150 ml of deionised water is added uniformly and with metered addition of sodium hydroxide solution at a constant pH of 7. The mixture is subsequently cooled to room temperature with stirring, and the reaction mixture is neutralised again. The pigment obtained is filtered off via a suction filter, washed with water, dried at 140° C. and calcined at 1100° C. for 120 min., giving an ochre-coloured pigment powder. The pigment particles obtained have hollow aluminium oxide spheres (particle size <70 μm) as support particles and a coating comprising niobium-, manganese- and cerium-doped titanium dioxide adherently precipitated onto the latter.

Example 3

[0080] 100 g of spherical aluminosilicate particles (BET 1.4 m.sup.2/g, density 0.87 g/cm.sup.3, particle size d.sub.5-d.sub.95=5 μm−63 μm) are suspended in about 2 I of deionised water. A solution of 577 g of titanium oxychloride (400 g/I), 5.04 g of niobium pentachloride in 24 ml of hydrochloric acid (37%) and 0.16 g of manganese sulfate monohydrate is added dropwise to the suspension at 90° C. with stirring in an acidic medium. The pH of 2 is kept constant by simultaneous regulated metered addition of sodium hydroxide solution. When all the solution has been added, the mixture is stirred at 90° C. for a further 15 min, before a solution of 7.6 g of cerium chloride heptahydrate in 150 ml of deionised water is added uniformly and with metered addition of sodium hydroxide solution at a constant pH of 7. The mixture is subsequently cooled to room temperature with stirring, and the reaction mixture is neutralised again. The pigment obtained is filtered off via a suction filter, washed with water, dried at 140° C. and calcined at 1100° C. for 120 min., giving an ochre-coloured pigment powder. The pigment particles obtained have hollow aluminosilicate spheres (particle size<100 μm) as support particles and a coating comprising niobium-, manganese- and cerium-doped titanium dioxide adherently precipitated onto the latter.

Example 4

[0081] The filler is prepared analogously to Example 3, with the change that, besides 2.5 g of niobium pentachloride (as 12.5% solution in HCl), only 0.35 g of chromium trichloride is added in 598 g of titanium oxychloride solution. A virtually white pigment powder is obtained. The pigment particles obtained have hollow aluminosilicate spheres (particle size<100 μm) as support particles and a coating comprising niobium- and chromium-doped titanium dioxide adherently precipitated onto the latter.

[0082] Table 1 shows the composition of the particulate fillers according to the invention from Examples 1 to 4. The doping of the TiO.sub.2 coating is indicated in atom-% in the titanium dioxide.

TABLE-US-00001 TABLE 1 wt.-% of wt.-% at-% at-% at-% at-% Example Support support of TiO.sub.2 of Nb of Mn of Cr of Ce 1 Al2O3 flakes 50 50 0.5 0.05 — 1.2 2 Al2O3 hollow spheres 44 56 1 0.05 — 1.1 3 Aluminosilicate 40 60 1 0.05 — 1.1 hollow spheres 4 Aluminosilicate 40 60 0.05 — 0.05 — hollow spheres

[0083] Table 2 shows the density and size distribution of the particulate fillers in accordance with Examples 1 to 4.

TABLE-US-00002 TABLE 2 Example Density [g/cm.sup.3] d.sub.5 [μm] d.sub.50 [μm] d.sub.90 [μm] 1 3.94 7.9 18.5 32.5 2 2.87 2.4 6.6 59.9 3 1.61 7.6 30.8 54.2 4 1.50 7.1 30.7 53.3

Example 5

[0084] Production of Test Specimens from Room-Temperature-Crosslinking Silicone

[0085] The amounts indicated in Table 3 of core/shell particles from Examples 1 to 4 and the comparative materials α-SiC (Alfa Aesar Art. #40155, density 3.51 g/cm.sup.3) and the ZnO microvaristor powder (ABB/Switzerland, density 5.22 g/cm.sup.3) are roughly premixed in a can with the respective proportions of component A of a commercial room-temperature-crosslinking silicone resin RTV-2 (manufacturer's material data: A:B=9:1, viscosity of the mixture 3500 mPa*s at 23° C., Shore A hardness 45°) and homogenised for at least 2 minutes in a vacuum SpeedMixer (Hauschild) at a reduced pressure of 4 mbar and 1600 revolutions per minute. The respective amounts of component B of the same RTV2 silicone resin are subsequently added, the components are again roughly premixed and homogenised in the vacuum SpeedMixer for at least 1 minute at 4 mbar and 1600 revolutions per minute. The viscous material is then poured rapidly, while observing the pot life, into a mould, which determines the geometrical dimensions of the test specimen. The silicone resin is cured in the mould for at least 30 minutes at 70° C. After the mould has cooled, the mould is opened and the test specimen is removed and stored under dust-free conditions. The layer thicknesses of the crosslinked test specimens are between 500 μm and 600 μm and are determined at various points for each of the circular test specimens (diameter 60 mm) as the average of ten measurements using an eddy-current layer thickness measuring instrument (Fischer Dualscope FMP30 with FD10 sensor in accordance with Din EN ISO 2360). The test specimens for Examples 5-17 and 5-18 can only be produced with difficulty, even in small amounts, since sedimentation already commences during preparation in the SpeedMixer can, leading to uneven distribution of the fillers in the test specimen.

TABLE-US-00003 TABLE 3 RTV-2 silicone test specimens: Filler: RTV-2 (A): Filler from RTV-2 (B) Degree of filling Example Example (m/m/m) PMC %/PVC % 5-1 1 3.8 g:19.1 g:2.1 g 15%/4.4% 5-2 1 5.0 g:18.0 g:2.0 g 20%/6.1% 5-3 1 6.3 g:16.9 g:1.9 g 25%/8.0% 5-4 1 7.5 g:15.8 g:1.7 g 30%/10.0% 5-5 1 8.8 g:14.6 g:1.6 g 35%/12.3% 5-6 2 6.3 g:16.9 g:1.9 g 25%/10.6% 5-7 2 12.7 g:11.1 g:1.2 g 58%/26.8% 5-8 3 6.3 g:16.9 g:1.9 g 25%/17.4% 5-9 3 9.2 g:14.3 g:1.6 g 36.6%/26.8% 5-10 4 3.8 g:19.1 g:2.1 g 15%/10.7% 5-11 4 5.0 g:18 g:2.0 g 20%/14.5% 5-12 4 6.3 g:16.9 g:1.9 g 25%/18.5% 5-13 4 7.5 g:15.8 g:1.7 g 30%/22.5% 5-14 4 8.8 g:14.6 g:1.6 g 35%/26.0% 5-15 SiC 6.3 g:15.0 g:3.8 g 25%/8.8% 5-16 SiC 13.9 g:10.0 g:1.1 g 53.5%/26.8% 5-17 ZnO 6.3 g:16.9 g:1.9 g 25%/6.1% 5-18 ZnO 16.3 g:7.9 g:0.9 g 65.1%/26.8%

Example 6

[0086] Production of Test Specimens in LSR

[0087] The liquid silicone rubber LSR samples are produced analogously to Example 5, but are cured in a polypropylene casting mould in a hot press at 120° C. for 20 min. For LSR, the mixing ratio of components A and B is 1:1 (see Table 4).

TABLE-US-00004 TABLE 4 LSR silicone test specimens: Filler from Filler: LSR (A):LSR (B) Degree of filling Example Example (m/m/m) PMC %/PVC % 6-1 1 4.0 g:8.0 g:8.0 g 20%/15.0% 6-2 1 7.0 g:6.5 g:6.5 g 35%/27.5%

Example 7

[0088] Production of Epoxy Resin Test Specimens

[0089] The amounts of fillers from Example 4 indicated in Table 5 are initially introduced in a can with the said proportions of a binder consisting of in each case 4% of benzyl alcohol, 76% of ARALDITE DBF BD and 20% of ARADUR HY 2966, roughly premixed and homogenised successively in a vacuum SpeedMixer (Hauschild) at a reduced pressure of 4 mbar for 2 minutes at 1000 revolutions per minute, 2 minutes at 1800 revolutions per minute and 30 seconds at 800 revolutions per minute. The mixed or homogenised epoxy resin is then poured rapidly into a Teflon casting chamber and cured at 60° C. for about 1 hour. After the sample has cooled, the epoxide plate is removed and the circular test specimen is drilled out. The layer thickness is determined using a dial gauge.

TABLE-US-00005 TABLE 5 Epoxide test specimens Filler from Composition Degree of filling Example Example filler: binder PMC %/PVC % 7-1 4 20 g:80 g 20%/16.2% 7-2 4 25 g:75 g 35%/29.4%

Example 8

[0090] Production of HD-PE Test Specimens

[0091] The filler prepared in Example 1 is used to produce a 40% master match in an HDPE (Purell GA 7760) in a DSE Leistritz Micro 27 twin-screw extruder. This is diluted to 30% with further HDPE. 5% and 2% dilutions are produced from a 10% masterbatch. The extrudates are quenched and granulated. 60 mm*90 mm plates with a layer thickness of 1 mm are produced from the granules in an injection-moulding machine, from which test specimens having a diameter of 50 mm are produced using a core drill.

TABLE-US-00006 TABLE 6 HD-PE test specimens Degree of filling Example Filler Composition wt % 8-1 1 Ex.1: HDPE 400 g:600 g 40% 8-2 1 Ex.8-1: HDPE 750 g:250 g 30% 8-3 1 Ex.1: HDPE 100 g:900 g 10% 8-4 1 Ex.8-3: HDPE 500 g:500 g  5% 8-5 1 Ex.8-3: HDPE 200 g:800 g  2%

[0092] Measurement of the electrical properties of the test specimens:

[0093] The nonlinear conductive character of the test specimens arises from a current/voltage measurement which is intended to show the following deviation from ohmic behaviour:

U˜I.sup.α  (1)

[0094] The deviation is described by the nonlinearity exponent α, which has the value 1 in the case of an ohmic resistance.

[0095] The current/voltage characteristic of the varistor filler/polymeric test specimen produced is measured using a Heinzinger 10 kV DC voltage source (PNChp 10000-20 ump) and a Kethley pico-ammeter (6514 system electrometer) on a ring electrode in accordance with DIN EN 61340-2-3.

[0096] The diagrammatic structure of the measurement apparatus and the test specimen dimensions to be observed are shown by FIG. 1.

[0097] In order to standardise the results, the electric field strength E and the current density J are calculated in accordance with formulae (2-4) with the sample and electrode dimensions given in FIG. 1:

E=V/h  (2)

J=I/A  (3)

A=(d.sub.1+g).sup.2*π/4  (4)

α=In(J.sub.2/J.sub.1)/In(E.sub.2/E.sub.1)  (5) [0098] V=voltage in volts (V) [0099] I=current strength in amperes (A) [0100] A=effective electrode area (m.sup.2) [0101] h=electrode separation (sample thickness): ˜0.5 mm [0102] d.sub.1-d.sub.4=electrode diameters (see FIG. 1) [0103] d.sub.1=diameter of central electrode: 25 mm [0104] g=separation of ring electrode from central electrode: 2.5 mm [0105] α=nonlinearity exponent [0106] J.sub.1, J.sub.2=current density at 2 points of the measurement [0107] E.sub.1, E.sub.2=electric field strength at 2 points of the measurement

[0108] The measurements of the current carried out using a step-shaped voltage ramp at room temperature and relative atmospheric humidity between 20% and 30%.

[0109] The nonlinearity arises as the slope of the curves in a double-logarithmic plot of the E-J characteristic in accordance with equation (5). In general, only the fraction greater than 5000 kV/m is taken into account for calculation of the nonlinearity exponent α.

[0110] Impedance measurements are carried out at 25° C. in a Novocontrol GmbH Alpha-A broadband dielectric spectrometer with PHECOS temperature control between 0.01 Hz and 100 MHz on the PE test specimens from Example 8-1 to 8-5.

[0111] FIG. 2, Examples 5-1 to 5-4:

[0112] The flake-form varistor filler from Example 1 shows clear nonlinear electrical conductivity. At field strengths between 1000 kV/m and 20000 kV/m, this nonlinear character (slope in FIG. 2) increases in a clearly visible manner and the silicone composites reach a nonlinearity exponent 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 fillers increases from 15% to 30% in 5% steps. The percolation threshold is evident as a clear separation between the curves of Example 5-2 and Example 5-3. In the field-strength range considered, the curves extend over a current density of about two and a half powers of ten, where the basic conductivity is dependent on the varistor content in the composite. A sample having the pigment volume concentration PVC 26.8% (58.5% PMC) can no longer be produced with the flake-form varistor filler. At such high degrees of filling, flake-form fillers tend to form rheologically unfavourable so-called “house of cards” structures within the polymer matrix.

[0113] FIG. 3, Examples 5-6 and 5-7:

[0114] The spherical varistor filler from Example 2 exhibits an even more pronounced nonlinear electrical conductivity at a degree of filling of 25% PMC from about 5000 kV/m, with a nonlinearity exponent α of 4.4 over about 2 orders of magnitude of the current density. High degrees of filling with a pigment volume concentration of 26.8%, which corresponds to a pigment mass concentration of 50.8% for this filler, lead to a flatter curve course, i.e. a somewhat lower nonlinearity exponent α of 3, but correspondingly higher conductivity (the entire curve is shifted to higher current density)

[0115] FIG. 4, Examples 5-8 and 5-9

[0116] The spherical varistor filler from Example 3 already exhibits pronounced nonlinear electrical conductivity at a degree of filling of 25% PMC over the entire measurement range over four orders of magnitude of the current density with a nonlinearity exponent α von 4.4. High degrees of filling with a pigment volume concentration of 26.8% lead to a flatter curve course, i.e. a somewhat lower nonlinearity exponent α of 1.4 and higher conductivity.

[0117] FIG. 5, Examples 5-10 to 5-14

[0118] The spherical varistor filler from Example 4 exhibits clear nonlinear electrical conductivity. The switching point (denoted by an arrow in FIG. 5), at which the nonlinear character of the example composites sets in, moves to lower electric field strengths with higher degree of filling. In Example 5-10 with a degree of filling of 15% PMC, this switching point is at about 6000 kV/m. The switching point already occurs 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 apparently just recorded at the beginning of the measurement with 1000 kV/m and in the case of Example 5-14 with 35% PMC, it is below this measurement limit. The curve of the material from Example 5-11 is steepest just before the percolation threshold with a nonlinearity exponent α of 4.5. Here too, the conductivity of the materials increases with increasing degree of filling and the curves of the examples shift to higher current densities.

[0119] FIG. 6:

[0120] The percolation effect is much more pronounced in the case of composites with the spherical materials of low density from Example 4 than in the case of those with flake-form materials of high density from Example 1 (FIG. 6). For the same weight introduced (PMC 15%-35% for composites with Example 1 and Example 4 in RTV-2 silicone), these varistor fillers achieve significantly higher volume filling (PVC in RTV-2 silicone: 4.4%-12.3% with Example 1 and 10.7%-26.8% with Example 4).

[0121] FIGS. 7 to 9:

[0122] The materials produced in Examples 1 to 4 can be converted into field-controlling insulating materials in a very wide variety of polymer matrices. This is evident from the nonlinear electrically conductive character of these composites. The curves shown in FIG. 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 FIG. 8 shows curves with nonlinear electrical conductivity of the varistor filler from Example 4 in an epoxide composite (Examples 7-1 and 7-2). The electrically conductive behaviour arises analogously to the composites shown so far. Besides these two-component systems, FIG. 9 shows the nonlinear electrical properties of a composite with an HD-PE produced by extrusion with the particles directly (Example 8-1) or from a masterbatch (Example 8-2).

[0123] FIG. 10:

[0124] FIG. 10 draws the comparison between the varistor fillers according to the invention (here Example 4 in the formulations from Example 5-10 and 5-14) and other field-controlling materials. To this end, a silicon carbide (Example 5-15/5-16) and a zinc oxide microvaristor (Example 5-17/5-18) is selected. For better comparability, on the one hand a formulation is prepared which in each case corresponds to a pigment mass concentration of 25% PMC (Examples 5-10, 5-15 and 5-17) and a further formulation with a high concentration in the same volume filling of 26.8% PVC (Example 5-14, 5-16 and 5-18).

[0125] The curve of the varistor filler from Example 5-15 crosses over the curve of silicon carbide from Example 5-16. Depending on the concentration, however, it is also possible to establish lower conductivities in the insulating material than with silicon carbide, as shown by the comparison of the curves from Examples 5-10 (varistor filler) and 5-15 (silicon carbide). By contrast, it becomes clear with the curve of Examples 5-17 and 5-18 filled with zinc oxide microvaristor that this material is difficult to control. Either it exhibits only low nonlinearity with low filling (Example 5-17) or it is clearly too conductive in the case of high filling (Example 5-18). The switching point to the nonlinearly conductive region is then at very low electric field strengths (<<100 kV/m).

[0126] FIG. 11:

[0127] The dielectric measurements on an impedance spectrometer also allow the properties of the varistor fillers according to the invention to be determined in an alternating-current environment. FIG. 11 shows the impedance (alternating current resistance) Z and the permittivity ϵ′ and the loss factor tan δ of the composites comprising the varistor filler from Example 1 and an HD-PE, in the formulations from Examples 8-1 to 8-5, at 50 Hz. The permittivity ϵ′ increases only slowly with the filler proportion (as average of the proportions of polyethylene and the doped varistor filler). By contrast, the impedance, but also the loss factor, suggest the shape of a classical percolation curve with its step from a filler proportion of about 25%. Permittivity and loss factor are still very small. For pure HDPE, the literature indicates a value of about 2.4 for the permittivity and of 2*10.sup.−4 for the loss factor. However, the measured loss factor for the unfilled HDPE is already 4*10.sup.−2 and is only exceeded by a factor of ten at a degree of filling of 37% by weight of varistor filler.