ELECTRICALLY CONDUCTIVE, COLORED INTERFERENCE PIGMENTS

Abstract

The present invention relates to electrically conductive, coloured interference pigments, in particular flake-form interference pigments, which have an outermost layer which comprises crystalline carbon in the form of graphite and/or graphene, to a process for the preparation of such pigments, and to the use of the pigments prepared in this way.

Claims

1. Electrically conductive, coloured interference pigments based on a flake-form support, where the support has a coating comprising one or more layers and where the outermost layer furthest remote from the support consists of at least 95% by weight, based on the weight of this layer, of carbon and comprises crystalline carbon in the form of graphite and/or graphene.

2. Interference pigments according to claim 1, characterised in that the outermost, crystalline carbon-containing layer consists of at least 98% by weight of carbon.

3. Interference pigments according to claim 1, characterised in that the outermost, crystalline carbon-containing layer has a geometrical layer thickness in the range from 1 to 5 nm.

4. Interference pigments according to claim 1, characterised in that the proportion of the outermost, crystalline charge-carrier layer, based on the weight of the pigment, is 0.5 to 5% by weight.

5. Interference pigments according to claim 1, characterised in that the flake-form support is natural or synthetic mica flakes, kaolin, sericite or talc flakes, BiOCl flakes, iron oxide flakes, TiO.sub.2 flakes, glass flakes, borosilicate flakes, SiO.sub.2 flakes, Al.sub.2O.sub.3 flakes, boron nitride flakes, metal flakes, or mixtures of two or more thereof.

6. Interference pigments according to claim 1, characterised in that they have one or more dielectric layers between the flake-form support and the outermost, crystalline carbon-containing layer, where at least one of these dielectric layers is composed of a material having a refractive index n in the range n≧1.8.

7. Interference pigments according to claim 6, characterised in that the geometrical thickness of at least one dielectric layer comprising a material having a refractive index n in the range n≧1.8 is greater than 70 nm.

8. Interference pigments according to claim 6, where the dielectric layer consists of a material having a refractive index n in the range n≧1.8 consists of titanium dioxide, titanium dioxide hydrate, zirconium dioxide, zirconium dioxide hydrate, tin oxide, tin oxide hydrate, zinc oxide, zinc oxide hydrate, iron(II) oxide, iron(III) oxide, geothite and/or mixed phases thereof.

9. Interference pigments according to claim 8, characterised in that the dielectric layer comprising a material having a refractive index n in the range n≧1.8 consists of titanium dioxide and/or titanium dioxide hydrate.

10. Interference pigments according to claim 1, characterised in that they have a particle size in the range from 1 to 250 μm, where interference pigments having a particle size of less than 10 μm are present with a percentage proportion by volume of at most 30% in a pigment powder consisting of the interference pigments

11. Interference pigments according to claim 1, characterised in that they have a specific powder resistance of less than 1×10.sup.6 ohm*cm.

12. Interference pigments according to claim 11, characterised in that the specific powder resistance is less than 1×10.sup.4 ohm*cm.

13. Process for the preparation of electrically conductive, coloured interference pigments according to claim 1, in which flake-form support particles, which may have been coated with one or more layers, are coated with an outermost layer which consists of at least 95% by weight, based on the weight of this layer, of carbon and comprises crystalline carbon in the form of graphite and/or graphene, in a reactor in a stream of carrier gas with feed of a gaseous, carbon-containing compound by pyrolytic decomposition of the carbon-containing compound.

14. Process according to claim 13, characterised in that the outermost, crystalline carbon-containing layer is applied with a geometrical layer thickness in the range from 1 to 5 nm.

15. Process according to claim 13, characterised in that the gaseous, carbon-containing compound employed is acetone or 2-methyl-3-butyn-2-ol.

16. Process according to claim 13, characterised in that the flake-form support particles are kept in motion in the reactor.

17. Paints, coatings, printing inks, coating compositions, security applications, plastics, ceramic materials, glasses, paper, films, in heat protection, in floorcoverings, for laser marking, in dry preparations or pigment preparations containing an electrically conductive, coloured interference pigment according to claim 1.

Description

[0095] FIG. 1: shows a diagram for the characterisation of carbon with the aid of Raman spectroscopy by the method of A. C. Ferrari and J. Robertson, University of Cambridge

[0096] FIG. 2: shows a diagram for illustrating the relationship of specific powder resistance and ΔL for a pigment in accordance with Example 1

[0097] FIG. 3: shows shows a diagram for illustrating the relationship of specific surface resistance of a coating layer and ΔL for a pigment in accordance with Example 3

[0098] The present invention is intended to be explained below with reference to examples, but not restricted thereto.

EXAMPLES

Example 1

[0099] In all inventive examples, a fluidised-bed apparatus which comprises a vertical, cylindrical reaction space which is fitted with a distributor plate at the lower end and with a filter system for retaining the pigments at the upper end, is used as reactor. The fluidised-bed apparatus can be heated and is provided with a vibration device. The pigments are fluidised by the flow of a carrier gas against the pigment bed and, if necessary, additionally by the use of the vibration device. The reaction temperature is in each case in the range from 500° C. to 700° C. The volatile carbon-containing compound (carbon precursor) is fed to the reactor in a mixture with the carrier gas. The reaction time is between 30 and 120 minutes.

[0100] 500 g of an interference pigment having a red interference colour (Iriodin® 7215 Ultra Red, particle size 10-60 μm, d.sub.50 about 25 μm, volume weighted, TiO.sub.2 on mica, Merck KGaA, Germany) are initially introduced in the fluidised-bed apparatus. The carrier gas nitrogen is passed through a gas wash bottle containing acetone and thus saturated with acetone. The gaseous nitrogen/acetone mixture is passed into the fluidised-bed apparatus, and the pigments are fluidised by the stream of carrier gas and, if necessary, by switching on the vibration device. When the reaction temperature in the range from 500° C. to 700° C. has been reached, the pigments obtained are removed in several batches after reaction times of 30, 60, 90 and 120 minutes and investigated for their carbon content, the specific powder resistance and the ΔL value. The result is shown in Table 1:

TABLE-US-00001 TABLE 1 spec. T (reaction time) powder resistance C content by weight [min] [ohm * cm] [%] ΔL value 0 .sup. >10.sup.9   0.0 50.9 30 1132.5  0.6 23.4 60 815.7 0.8 22.8 90 185.8 1.0 19.0 120  54.5 1.3 14.7

[0101] All samples of pigments in accordance with Example 1 (reaction time of 30, 60, 90 and 120 min.) investigated by means of Raman spectroscopy confirm the presence of nanocrystalline carbon in the outermost layer. The pigments obtained exhibit a red interference colour, high lustre and an increasing hiding power with increasing carbon content.

Example 2

[0102] 500 g of an interference pigment having a blue interference colour (Iriodin® 7225 Ultra Blue, particle size 10-60 μm, TiO.sub.2 on mica, Merck KGaA, Germany) are initially introduced in the fluidised-bed apparatus. The carrier gas nitrogen is passed through a gas wash bottle containing 2-methyl-3-butyn-2-ol which is heated to a temperature in the range from 30 to 90° C. and thus enriched with the latter. The gaseous nitrogen/2-methyl-3-butyn-2-ol mixture is passed into the fluidised-bed apparatus, and the pigments are fluidised by the stream of carrier gas and, if necessary, by switching on the vibration device. The reaction temperature is set to 500° C. to 700° C. Pigment samples are taken before commencement of the reaction and after 120 minutes and investigated for their carbon content, the specific powder resistance and the ΔL value. The result is shown in Table 2.

TABLE-US-00002 TABLE 2 spec. T (reaction time) powder resistance C content by weight [min] [ohm * cm] [%] ΔL value 0 >10.sup.9 0.0 61.3 120  62.4 2.9 14.7

[0103] The samples of pigments in accordance with Example 2 investigated by means of Raman spectroscopy (reaction time 120 min.) confirm the presence of nanocrystalline carbon in the outermost layer.

[0104] The pigments obtained exhibit a blue interference colour, high lustre and a comparatively high hiding power with semitransparency present.

Example 3

[0105] In each case 1 kg of the red interference pigment in accordance with Example 1 are coated at various reaction times with a crystalline carbon-containing layer by the process described in Example 1. Pigments having a carbon content between 0.5 and 2.3% by weight, based on the weight of the interference pigment employed (without carbon layer), are obtained.

[0106] The carbon layers are characterised by SEM photographs, elemental analysis, thermal differential analysis and Raman spectroscopy. In the SEM photographs, continuous carbon layers having a thickness of 1-3 nm, which corresponds to about 3-9 layers of graphite, are evident on the surface of the pigments. The Raman spectra show the presence of essentially graphitic carbon. In addition, the specific powder resistances of the pigments and the colouristic values are determined from black/white coating test charts. The dependence of the specific powder resistance on the lightness contrast ΔL of the pigments is shown in FIG. 2. The figure shows the presence of a specific powder resistance in the range <1×10.sup.6 ohm*cm, preferably <1×10.sup.4 ohm*cm, at ΔL values of the pigments in the range from 10 to 30.

Example 4

Use Example:

[0107] The pigments in accordance with Example 3 are dispersed in NC lacquer (12% of collodium/butyl acrylate in a solvent mixture). PET films are coated with the respective coating preparation. The pigment weight concentration (PWC) of the pigments in the dry coating layer is 42%, the layer thickness of the coating layer is 40 μm. After drying of the coating layers, the respective surface resistance of the coating layer is measured at a field strength of 100 V/cm with the aid of a spring-tongue electrode (1 cm electrode separation, length 10 cm).

[0108] The results are shown in FIG. 3. The results show that both optically attractive and also electrically conductive coatings are obtained with the pigments according to the invention. By increasing the pigment weight concentration, an electrical conductivity in the antistatic range (<10.sup.9 ohm) can also be achieved, even with the least conductive pigment, while the PWC of the very highly conductive pigments can also be reduced further for this aim.

[0109] Coating layers of the same colour which can be obtained by blending Iriodin® 7215 Ultra Red with carbon black would, by contrast, have no electrical conductivity at all.

Comparative Example 1

[0110] Mixtures of the red interference pigment from Example 1 (Iriodin® 7215 Ultra Red) with carbon black (Printex L from Orion Engineered Carbons, Inc.) are prepared in such a way that the mixture in each case has the carbon content shown in Table 3. In each case, the specific powder resistance of the mixtures obtained is measured. In addition, black/white coating test charts are provided with a coating which, apart from binder and solvent, only comprises the corresponding pigment mixture. The ΔL values of the dried coatings are determined as described above.

TABLE-US-00003 TABLE 3 spec. powder resistance Carbon black content by weight [ohm * cm] [%] ΔL value >10.sup.9 0.0 50.9 >10.sup.9 0.5 12.5 >10.sup.9 1.0 9.8 >10.sup.9 1.5 6.3 >10.sup.9 2.0 5.0 >10.sup.9 2.5 4.3

[0111] The desired specific powder resistance of less than 1×10.sup.6 ohm*cm cannot be achieved with the carbon black/interference pigment mixtures. In addition, the blends exhibit a ΔL value in the coating application which is very greatly reduced even with the smallest added amount of carbon black and which cannot be established specifically via the added amount of carbon black.

Comparative Example 2

[0112] Carbon-containing coatings on effect pigments, inter alia on interference pigments, which are obtained by decomposition of polymeric coatings on the surface of interference pigments are known from the prior art. As comparison, therefore, 50 g of Iriodin® 7215 Ultra Red (see Example 1) are mixed intimately with 10 g of furfuryl alcohol. The polymerisation of furfuryl alcohol is subsequently initiated by addition of hydrochloric acid. An interference pigment having an outermost layer of polyfurfuryl alcohol is obtained. The pigment powder is subsequently pyrolysed under inert gas at a temperature in the range from 500° C. to 800° C. The dark pigment obtained has a proportion by weight of carbon of 5.8% and a specific powder resistance of 6.3×10.sup.6 ohm*cm. The ΔL value determined from the coatings of black/white coating test charts is 5.3.