Transparent, electrically semiconducting interference TiOx pigments with high color strength

Abstract

The present invention relates to transparent, electrically semiconducting interference pigments having high color strength, and in particular to flake-form interference pigments which comprise an oxygen-deficient layer of TiO.sub.2-x, to a process for the preparation of such pigments, and to the use of the pigments prepared in this way.

Claims

1. A transparent, electrically semiconducting flake-form interference pigment comprising a layer of TiO.sub.2-x, where 0.001≦x<0.05.

2. An interference pigment according to claim 1, wherein the layer of TiO.sub.2-x is located on a transparent, flake-form support.

3. An interference pigment according to claim 2, wherein the layer of TiO.sub.2-x consists of two or three part-layers, where the part-layers are in each case separated from one another by an interlayer comprising a transparent material having a refractive index n, where n is <1.8 and the interlayer in each case has a geometrical layer thickness d≦15 nm, and where a system comprising two part-layers with one interlayer or comprising three part-layers with two interlayers with in each case an outer, final part-layer of TiO.sub.2-x is formed on the flake-form support.

4. An interference pigment according to claim 3, wherein 0.001≦x<0.05 in all part-layers of TiO.sub.2-x.

5. An interference pigment according to claim 3, wherein 0.001≦x<0.05 only in the outer, final part-layer of TiO.sub.2-x.

6. An interference pigment according to claim 3, wherein the interlayer consists of SiO.sub.2, Al.sub.2O.sub.3, silicon oxide hydrate, aluminium oxide hydrate, MgF.sub.2, or of mixtures of two or more thereof.

7. An interference pigment according to claim 2, wherein an additional layer of SnO.sub.2 is located between the transparent, flake-form support and the layer of TiO.sub.2-x where the layer of TiO.sub.2-x is located directly on the layer of SnO.sub.2.

8. An interference pigment according to claim 2, wherein the flake-form support is natural or synthetic mica flakes, kaolin, sericite or talc flakes, glass flakes, borosilicate flakes, Al.sub.2O.sub.3 flakes or mixtures of two or more thereof.

9. An interference pigment according to claim 1, wherein it consists of a flake-form support and a layer of TiO.sub.2-x surrounding the support.

10. An interference pigment according to claim 9, wherein the flake-form support is natural or synthetic mica flakes, Kaolin, sericite or talc flakes, glass flakes, borosilicate flakes, AhO.sub.3 flakes or mixtures of two or more thereof.

11. An interference pigment according to claim 9, wherein an additional layer of SnO.sub.2 is located between the flake-form support and the layer of TiO.sub.2, where the layer of TiO.sub.2-x is located directly on the layer of SnO.sub.2.

12. An interference pigment according to claim 1, wherein the layer of TiO.sub.2-x is doped with 0.1 to 3 mol % of Sn.

13. A process for the preparation of an interference pigment according to claim 1, wherein a transparent, flake-form interference pigment which consists of TiO.sub.2, or which consists of a coated transparent, flake-form support which has a layer of TiO.sub.2 on an outer surface, is thermally treated in a gas phase with addition of a reducing gas over a period in the range from 5 to 60 minutes, where the TiO.sub.2 is converted into TiO.sub.2-x and 0.001≦x<0.05.

14. A process according to claim 13, wherein the thermal treatment is carried out in a gas mixture which has a proportion of the reducing gas of 0.05 to 10% by vol.

15. A process according to claim 14, wherein the proportion of reducing gas in the gas mixture is 0.05 to <5% by vol. at a temperature of 800° C. and 5 to 10% by vol. at a temperature of 400° C.

16. A process according to claim 14, wherein the reducing gas is hydrogen, ammonia or a C.sub.1-C.sub.4-hydrocarbon compound, and the gas mixture furthermore comprises nitrogen or argon.

17. A process according to claim 16, wherein the hydrocarbon compound is methane, ethylene or propanone.

18. A process according to claim 13, wherein the thermal treatment is carried out at a temperature in the range from 400° C. to 800° C.

19. A method of using the transparent, electrically semiducting, flake-form interference pigments according to claim 1 comprising including said pigments according to claim 1 in paints, coatings, printing inks, plastics, sensors, security applications, floorcoverings, textiles, films, ceramic materials, glasses, paper, for laser marking, heat protection, or photosemiconductivity, in preparations.

20. A method according to claim 19, wherein the interference pigments are employed as a mixture with organic and/or inorganic colorants and/or electrically conducting materials and/or non-electrically conducting effect pigments.

21. A method according to claim 19, wherein the interference pigments are employed in security products which are subjected to the influence of an electromagnetic field.

22. A security product containing interference pigments according to claim 1.

Description

EXAMPLES

Examples 1-3

(1) 100 g of ground and classified mica (10-50 μm, d.sub.90 25 μm) are suspended in 1900 ml of demineralised water. 100 ml of a solution of 0.75 g of concentrated HCl and 2.2 g of SnCl.sub.4 in water are slowly added to the suspension in an acidic medium at 75° C. with stirring. The pH is kept constant by simultaneous addition of sodium hydroxide solution. The mixture is subsequently stirred at 75° C. for a further 30 min., then coated with TiO.sub.2 at pH 1.6 by slow addition of an aqueous TiCl.sub.4 solution (400 g/l of TiCl.sub.4) while keeping the pH constant using 32% sodium hydroxide solution. The coating is terminated when the following colour end points have been reached:

Example 1: Gold

Example 2: Red

Example 3: Blue

(2) The reaction mixture is subsequently cooled to room temperature with stirring and neutralised. The pigments obtained are filtered off via a suction filter, washed with water and dried at 140° C.

(3) The dried pigments are subjected to a thermal treatment under the conditions shown in Table 1.

Example 4

(4) Preparation of a Pigment Having an Optically Inactive SiO.sub.2 Interlayer:

(5) 100 g of ground and classified mica (10-50 μm, d.sub.90 25 μm) are suspended in 1900 ml of demineralised water. 100 ml of a solution of 0.75 g of concentrated HCl and 2.2 g of SnCl.sub.4 in water are slowly added to the suspension in an acidic medium at 75° C. with stirring. The pH is kept constant by simultaneous addition of sodium hydroxide solution. The mixture is subsequently stirred at 75° C. for a further 30 min., then coated with TiO.sub.2 at pH 1.6 by slow addition of an aqueous TiCl.sub.4 solution (400 g/l of TiCl.sub.4) while keeping the pH constant using 32% sodium hydroxide solution. The addition is terminated at the 2nd order green colour end point, the mixture is stirred for a further 30 min., and the pH is adjusted to 7.5 using sodium hydroxide solution.

(6) An approx. 12% sodium water-glass solution in water is subsequently metered in at pH 7.5 until a total of 2.5 g of SiO.sub.2 have been added. The mixture is then stirred for a further 15 min., a pH of 1.6 is established again using hydrochloric acid, and the addition of TiCl.sub.4 solution is continued until the 3rd order green colour end point has been reached. The reaction mixture is subsequently cooled to room temperature with stirring and neutralised. The pigment obtained is filtered off via a suction filter, washed with water and dried at 140° C. The SiO.sub.2 interlayer is located at about ⅔ of the geometrical TiO.sub.2 layer thickness. The green interference pigment obtained is subjected to a thermal reaction in accordance with the conditions shown in Table 1.

(7) TABLE-US-00001 TABLE 1 Thermal treatment under reducing conditions Pigment Example from Ex. Atmosphere Temperature Duration  5 (comp.) 1 Air 750° C.  30 min.  6 (comp.) 2 Air 750° C.  30 min.  7 (comp.) 3 Air 750° C.  30 min.  8 (comp.) 4 Air 750° C.  30 min.  9 (inv.) 1 N.sub.2/H.sub.2(5% of H.sub.2) 500° C.  30 min. 10(inv.) 2 N.sub.2/H.sub.2(5% of H.sub.2) 500° C.  30 min. 11(inv.) 3 N.sub.2/H.sub.2(5% of H.sub.2) 500° C.  30 min. 12(inv.) 4 N.sub.2/H.sub.2(5% of H.sub.2) 500° C.  30 min. 13(inv.) 2 N.sub.2/H.sub.2(5% of H.sub.2) 500° C.  10 min. 14(inv.) 2 N.sub.2/H.sub.2(5% of H.sub.2) 450° C. 30 min 15(inv.) 2 N.sub.2/H.sub.2(5% of H.sub.2) 550° C. 30 min 16(inv.) 3 N.sub.2/H.sub.2(5% of H.sub.2) 450° C. 30 min 17(inv.) 2 N.sub.2/2% of C.sub.2H.sub.4 500° C. 30 min 18(inv.) 2 N.sub.2/2% of C.sub.2H.sub.4 550° C. 60 min 19(inv.) 3 N.sub.2/2% of C.sub.2H.sub.4 500° C. 60 min 20(inv.) 2 N.sub.2/0.5% of acetone 500° C. 30 min 21(inv.) 2 N.sub.2/H.sub.2(1% of H.sub.2) 750° C. 30 min 22(inv.) 3 N.sub.2/H.sub.2(1% of H.sub.2) 750° C. 30 min 23(comp.) 3 N.sub.2 750° C. 30 min

Example 24

(8) Testing of the Electrical Properties in a Coating Film:

(9) The pigments obtained after the thermal treatment in accordance with Table 1 are dispersed in NC lacquer (12% of collodium/butyl acrylate in a solvent mixture). PET films are coated with the coating preparation. The concentration of the pigments in the dry coating layer is 48.1% by weight, the layer thickness of the coating layer is 50 μm. After drying of the coating layers, the surface resistance is measured at a measurement voltage of 1000 V with the aid of a spring-tongue electrode (1 cm electrode separation, length 10 cm). The results are shown in Table 2. A comparative coating film without conducting pigment exhibits a specific resistance of >10.sup.12 ohm.

Example 25

(10) Testing of the Coloristic Properties:

(11) Samples of the pigments in accordance with Table 1 are dispersed in NC lacquer in accordance with Example 24 (1.7% by weight of pigment in the lacquer). The lacquer is then applied to black/white cardboard with a wet layer thickness of 500 μm and dried. The dried layer has a thickness of 40 μm and a pigment mass concentration (PMC) of 12.3%. The cards are then measured in reflection using a spectrophotometer (ETA-Optik from Steag Optik) at the following angles:

(12) 45°/90° over black and white and 75°/95° over black, where the angle 90° represents the perpendicular to the plane of the card.

(13) The L*, a*, b* values are then determined from the raw data of the measurements. The L* value over white is a measure of the mass tone of the pigment. The values are likewise shown in Table 2.

(14) TABLE-US-00002 TABLE 2 Resistances and colorimetric values of the pigments L*45°/90° Example Colour L*a*b*(b) (w) Resistance  5 (comp.) gold 152.0/6.8/76.0 85.8 >1 Tohm  6 (comp.) red 123.7/56/−7.2 85.6 >1 Tohm  7 (comp.) blue 104.4/−11.9/−85.9 88.6 >1 Tohm  8 (comp.) green 113.1/−49.8/37.6 83.7 >1 Tohm  9 (inv.) gold 150.2/6.1/74.3 82.9 14 Mohm 10 (inv.) red 123.5/53.9/−4.7 80.8 18 Mohm 11 (inv.) blue 102.7/−9.6/−86.8 79.5 20 Mohm 12 (inv.) green 111.9/−47.8/85.9 81.9 45 Mohm 13 (inv.) red 123.2/55.5/−5.5 83.7 33 Mohm 14 (inv.) red 124.3/54.0/−4.8 80.0 27 Mohm 16 (inv.) blue 102.7/−10.9/−87.0 78.9 50 Mohm 17 (inv.) red 122.8/55.1/−5.0 83.3 22 Mohm 21 (inv.) red 123.3/53.7/−4.5 81.0 14 Mohm 22 (inv.) blue 102.3/−3.2/−89.6 80.0 33 Mohm 23 (comp.) purple 92.0/31.4/−91.6 87.1 >1 Tohm

Example 26: Comparison

(15) Strongly Reduced Pigment having a Dark Mass Tone:

(16) The pigment from Example 3 (blue) is calcined at 900° C. under forming gas (5% of H.sub.2) for 45 minutes, giving a pigment having a dark purple mass tone. Paint films and paint cards of the pigment are produced and measured in accordance with Examples 24 and 25. The resistance of the film is 9.8 Mohm, the L* value over white is 32. The low L* value indicates the strong mass tone and the high hiding power of the pigment. The electrical resistance of the paint film is only insignificantly lower than the resistances of the paint films comprising the transparent interference pigments according to the invention. For antistatic-dissipative coatings, all resistances are sufficiently low.

Example 27

(17) Determination of the Oxygen Deficit of the Pigments:

(18) The change in mass of pigment samples is determined with the aid of differential thermoanalysis, in which the pigment samples are heated on a balance under air from room temperature to 1000° C. at 10° C./min. The pigments lose their residual moisture up to 300° C. From 400° C., the reduced pigments increase in mass due to re-oxidation. The oxygen deficit is calculated from the increase in mass from 400° C.

(19) The following pigments are investigated:

(20) Example 7 (comp.) increase: −0.1%

(21) Example 11 (inv.) increase: 0.3%

(22) Example 16 (inv.) increase: 0.12%

(23) Example 26 (comp.) increase: 0.66%

(24) The pigments comprise about 48% by weight of TiO.sub.2-x. Based on the TiO.sub.2-x content, the increase in weight is 0.625% in Example 11, 0.25% in Example 16 and 1.33% in Example 26. This gives rise to the following compositions:

(25) Example 7: TiO.sub.2

(26) Example 11: TiO.sub.1.96

(27) Example 16: TiO.sub.1.98

(28) Example 26: TiO.sub.1.93