NON-METALLIC PIGMENTS HAVING METAL PROPERTIES

Abstract

The present invention relates to non-metallic interference pigments, in particular laminar interference pigments, which comprise a thin high-refractive layer and an outermost layer that contains crystalline carbon in the form of graphite and/or graphene. The invention also relates to a method for producing such pigments and the use of the thus produced pigments.

Claims

1. Non-metallic interference pigments based on a flake-form non-metallic support, where the pigments have a particle size having a volume-weighted d.sub.90 value of <25 μm, the support has a coating comprising one or more successive layers of a colourless material having a refractive index n of n≧1.8 with a geometrical overall layer thickness of at most 70 nm, and where the pigments have 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.

2. Interference pigments according to claim 1, characterised in that they have a volume-weighted d.sub.50 value of less than 15 μm.

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

4. Interference pigments according to claim 1, characterised in that the outermost, crystalline carbon-containing layer has a geometrical layer thickness in the range from 0.5 to 5 nm.

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

6. 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, TiO.sub.2 flakes, glass flakes, borosilicate flakes, SiO.sub.2 flakes, Al.sub.2O.sub.3 flakes, boron nitride flakes, or mixtures of two or more thereof.

7. Interference pigments according to claim 1, characterised in that the layer or layers of a colourless material having a refractive index n in the range n≧1.8 is/are arranged between the support and the crystalline carbon-containing outermost layer and the pigments have no further layers.

8. Interference pigments according to claim 1, where the colourless material having a refractive index n in the range n≧1.8 is selected from the group consisting of titanium dioxide, titanium dioxide hydrate, zirconium dioxide, zirconium dioxide hydrate, tin oxide, tin oxide hydrate, zinc oxide, zinc oxide hydrate and/or mixed phases thereof.

9. Interference pigments according to claim 8, characterised in that the layer comprising materials 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 specific powder resistance of less than 1×10.sup.6 ohm*cm.

11. Interference pigments according to claim 10, characterised in that the specific powder resistance is less than 100 ohm*cm.

12. Process for the preparation of non-metallic interference pigments according to claim 1, in which flake-form support particles which have been coated with one or more successive layers of a colourless material having a refractive index n of n≧1.8 with a geometrical overall layer thickness of at most 70 nm and have a particle size having a volume-weighted d.sub.90 value of <25 μm 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.

13. Process according to claim 12, characterised in that the outermost, crystalline carbon-containing layer is applied with a geometrical layer thickness in the range from 0.5 to 5 nm.

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

15. Process according to claim 12, characterised in that the flake-form support particles are kept in motion in the reactor.

16. A paint, coating, printing ink, coating composition, security application, plastic, ceramic material, glass, paper, film, heat protection composition, floorcovering, laser marking composition, dry preparation or pigment preparation comprising a non-metallic interference pigment according to claim 1.

Description

[0102] 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

[0103] FIG. 2: shows the percolation curve for coating layers comprising a pigment in accordance with Example 1

[0104] FIG. 3: shows the percolation curve for coating layers comprising a pigment in accordance with Example 3B

[0105] The present invention is intended to be explained below with reference to examples, but not restricted thereto.

EXAMPLES

Example 1

[0106] 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.

[0107] 500 g of an interference pigment having a silver interference colour (Iriodin® 111 Rutile Fine Satin, particle size 1-15 μm, d.sub.90 10.5 μm, d.sub.50 6 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. powder T (reaction time) resistance C content by weight [min] [ohm * cm] [%] ΔL value 0 >10.sup.9 0.0 22.5 30 9 × 10.sup.5 0.5 4.6 60 358.3 0.9 3.3 90 321.5 1.2 2.6 120  30.0 2.2 1.0

[0108] The pigments obtained exhibit a silver interference colour, high metallic lustre and a strongly increasing hiding power with increasing carbon content. Even at a carbon content of only 0.5% by weight, silver-coloured interference pigments having high hiding power and good electrical conductivity are obtained.

Example 2

[0109] 500 g of the interference pigment from Example 1 (Iriodin® 111 Rutile Fine

[0110] Satin) are initially introduced in the fluidised-bed apparatus. The carrier gas nitrogen is passed through a gas wash bottle containing acetone or 2methyl-3-butyn-2-ol which is heated to a temperature in the range from 30 to 90° C. and thus in each case enriched with the latter. The gaseous nitrogen/acetone or 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 or after 40 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 T spec. C content (reaction time) powder resist by weight [min] C compound [ohm * cm] [%] ΔL value 0 >10.sup.9 0.0 22.5 120 Acetone  30 2.2 1.0 40 2Methyl3butynol 604 3.0 1.8

[0111] As can be seen from Table 2, silver-coloured interference pigments having a metallic lustre, high hiding power and good electrical conductivity are obtainable by pyrolytic decomposition of various carbon-containing compounds in the process according to the invention.

[0112] The pigments obtained in Examples 1 and 2 are characterised by analysis of the carbon contents, measurement of the colouristic properties on paint test charts, SEM photographs, thermal differential analysis, Raman spectroscopy and measurements of the electrical resistance. The evaluation of the carbon content gives values between 0.5 and 3% by weight, based on the weight of the pigments. The SEM photographs show continuous carbon layers having a thickness of 1-3 nm, which corresponds to about 3-9 graphite layers, 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 paint test charts. The lightness contrast ΔL is calculated from the L values of the black/white paint test charts. It is found that the lightness contrast ΔL correlates with the carbon content of the pigments.

Example 3

[0113] In each case, 1 kg of a silver interference pigment (A: Iriodin® 119 Polar White, particle size 5-25 μm, d.sub.90 19 μm, d.sub.50 10 μm, volume-weighted, TiO.sub.2 on mica, and B: Xirallic® T61-10 WNT Micro Silver, particle size≦20 μm, TiO.sub.2 on aluminium dioxide, are coated with a crystalline carbon-containing layer by the process described in Example 1 at reaction times of 120 minutes in each case. The results are shown in Table 3.

TABLE-US-00003 TABLE 3 spec. powder T (reaction time) resistance C content by weight [min] [ohm * cm] [%] ΔL value A  0 >10.sup.9 0.0 36.6 120  61.3 1.4 2.4 B  0 >10.sup.9 0.0 30.4 120  29.9 1.2 5.7

[0114] In each case, silver-coloured pigments are obtained which have a high metallic lustre, a high hiding power and high electrical conductivity.

Example 4

Use Example Surface Coating

[0115] A pigment prepared in accordance with Example 1 (Iriodin® 111 Rutile Fine Satin having a carbon content of 2.2% by weight) is dispersed in various parts by weight 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 varies between 8 and 60%, the layer thickness of the coating layer is in each case 40 μm. The coating layers obtained are in each case opaque and exhibit an attractive metallic-silver colour.

[0116] After drying of the coating layers, the respective surface resistance of the coating layer is measured with the aid of a spring tongue electrode (electrode separation 1 cm, length 10 cm) at a field strength of 100 V/cm, and specific surface resistances, based on a square area, are calculated. In this way, the percolation curve for the pigment is drawn up. The results are shown in FIG. 2. The specific surface resistance of 1.4 Kohm obtained at a pigment weight concentration (PWC) of 60% represents an extraordinarily low value which is comparable with the electrical conductivity of graphite particles. Even the electrical resistances obtained at a PCW of 38% are sufficient for the antistatic finishing of coatings in practice. The pigments according to the invention are thus eminently suitable for use in antistatic and dissipative coatings.

Example 5

Use Example Surface Coating

[0117] Example 4 is repeated with the modification that, instead of the pigment according to Example 1, a pigment according to Example 3B (Xirallic® T6110 WNT Micro Silver having a carbon content of 1.2% by weight) is employed.

[0118] The percolation curve obtained is shown in FIG. 3. The specific surface resistance of 2.2 Kohm obtained at a pigment weight concentration of 60% is likewise comparable with the specific surface resistance of coating layers which can be achieved with graphite particles and represents an excellent value. The coating layers obtained each have a silver-coloured metal lustre and have a high hiding power.

Example 6

Use Example Printing Ink

[0119] For the preparation of various printing inks, pigments from various examples described above are in each case introduced into an unpigmented, solvent-containing printing ink for gravure printing (NC TOP OPV 00, solids content 28%, Siegwerk Druckfarben AG & Co. KGaA). After adjustment of the viscosity using a solvent, the printing inks are printed over the entire surface of coated papers having a white, black and dark-blue base colour by gravure printing using an anilox roll having a line screen of 60 lines/cm. The pigment weight concentration in the dry layer is in each case 62%. The prints are assessed visually for appearance, print quality and hiding power. Criterion for the hiding power is the invisibility of the original paper colour after printing. In addition, the surface resistances of the print layers are determined using the spring tongue electrode and are in all cases at values in the range from 10 to 100 Kohm. The visual results are summarised in Table 4.

TABLE-US-00004 TABLE 4 Pigment from Example Substrate colour Visual impression Ex. 1, 2.2% of C blue very good Ex. 1, 2.2% of C white very good Ex. 1, 2.2% of C black very good Ex. 3A, 1.4% of C blue very good Ex. 3A, 1.4% of C white very good Ex. 3A, 1.4% of C black very good Ex. 3B, 1.2% of C blue very good Ex. 3B, 1.2% of C white very good Ex. 3B, 1.2% of C black very good

[0120] All pigments according to the invention give rise to print images having a lustrous, silver-coloured appearance with a smooth surface and a print image with a homogeneous appearance. The original paper colour is completely hidden in all cases and the prints exhibit low electrical surface resistances. The examples show that the pigments according to the invention are eminently suitable for the production of silver-coloured, dissipative packaging.

Comparative Example 1

[0121] Mixtures of the silver-coloured interference pigment from Example 1 (Iriodin® 111 Rutil Fine Satin) 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 5. In each case, the specific powder resistance of the mixtures obtained is measured.

TABLE-US-00005 TABLE 5 spec. powder resistance Carbon black content by weight [ohm * cm] [%] >10.sup.9 0.0 >10.sup.9 0.5 >10.sup.9 1.0 >10.sup.9 1.5 >10.sup.9 2.0 >10.sup.9 2.5

[0122] The desired specific powder resistance of less than 1×10.sup.6 ohm*cm cannot be achieved with the carbon black/interference pigment mixtures.

Comparative Example 2

[0123] A silver-coloured interference pigment (Iriodin® 100 Silver Pearl, particle size 10-60 μm, d.sub.90 50 μm, d.sub.50 25 μm, TiO.sub.2 on mica, Merck KGaA) is coated with carbon as described in Example 1. The reaction time is varied in the range from 30 to 120 minutes. The results are shown in Table 6.

TABLE-US-00006 TABLE 6 spec. powder T (reaction time) resistance C content by weight [min] [ohm * cm] [%] ΔL value 0   >10.sup.9 0.0 45.9 30 253690 0.5 26.4 60  1439.4 0.8 18.6 120   71 1.3 11.9

[0124] The results show that opaque pigments cannot be obtained with pigment particles which are not in the size range according to the invention, not even by increasing the carbon content.

Comparative Example 3

[0125] Silver-coloured interference pigments (A: Xirallic® Crystal Silver d.sub.90 35 μm, d.sub.50 19 μm, TiO.sub.2 on aluminium oxide, and B: Ronastar® Noble Sparks, D.sub.90 170 μm, products from Merck KGaA) are coated with carbon in accordance with Example 1. Pigments having the properties shown in Table 7 are obtained.

TABLE-US-00007 TABLE 7 spec. powder T (reaction time) resistance C content by weight [min] [ohm * cm] [%] ΔL value A  0 >10.sup.7 0.0 45 120  22.9 0.9 16.9 B  0 >10.sup.9 0.0 68.5 120 270.7 0.6 45.8

[0126] The pigment obtained in accordance with Comparative Example 3A is silver-coloured and semitransparent, i.e. does not have the requisite hiding power in order to be able to simulate a metal-like appearance of a coating. The pigment obtained in Comparative Example 3B is a silver-coloured pigment having a low hiding power and a strong sparkle effect.