Weather-resistant pearlescent pigments, process for the production and use thereof

Abstract

The present invention relates to weather-resistant pearlescent pigment, comprising a coated or uncoated platelet-shaped substrate which is selected from the group consisting of synthetic mica platelets, glass platelets, SiO.sub.2 platelets, Al.sub.2O.sub.3 platelets, synthetic boehmite platelets, BiOCl platelets and mixtures thereof, a chromophoric coating with at least one highly refractive metal oxide, and a top layer, wherein the top layer comprises the following layer sequence:
a) a first layer made of tin oxide and/or tin hydroxide and/or hydrated tin oxide, b) a second cerium-containing layer which comprises cerium oxide and/or cerium hydroxide and/or hydrated cerium oxide, c) an organic-chemical coating applied to the cerium-containing layer which contains oligomeric silanes or consists of them, wherein the oligomeric silanes have one or more amino groups and the oligomeric silanes are chemically bonded with the cerium-containing layer. The invention further relates to a process for the production and use of these pearlescent pigments.

Claims

1. A weather-resistant pearlescent pigment, comprising: a coated or uncoated platelet-shaped substrate which is selected from the group consisting of synthetic mica platelets, glass platelets, SiO.sub.2 platelets, Al.sub.2O.sub.3 platelets, synthetic boehmite platelets, BiOCl platelets and mixtures thereof, a chromophoric coating comprising at least one highly refractive metal oxide, and a top layer, wherein the top layer comprises the following layer sequence: a) a first layer comprising tin oxide and/or tin hydroxide and/or hydrated tin oxide, b) a second cerium-containing layer consisting of cerium oxide and/or cerium hydroxide and/or hydrated cerium oxide, c) an organic-chemical coating applied to the cerium-containing layer which comprises one or more oligomeric silane(s), wherein the oligomeric silane(s) have one or more amino groups and the oligomeric silane(s) are chemically bonded with the cerium-containing layer.

2. The weather-resistant pearlescent pigment according to claim 1, wherein, the substrate is selected from the group consisting of synthetic mica platelets, glass platelets and mixtures thereof.

3. The weather-resistant pearlescent pigment according to claim 1, wherein, the proportion of tin oxide, tin hydroxide and/or hydrated tin oxide, calculated as SnO.sub.2, in the top layer lies in a range of from 0.4 to 4.0 wt.-%, relative to the total weight of the pearlescent pigment.

4. The weather-resistant pearlescent pigment according to claim 1, wherein, the weight ratio of tin oxide, tin hydroxide and/or hydrated tin oxide, calculated as SnO.sub.2, to Ce, calculated as elemental Ce, in the top layer lies in a range of from 2 to 10.

5. The weather-resistant pearlescent pigment according to claim 1, wherein, the proportion by weight of Ce, calculated as elemental Ce, in the top layer lies in a range of from 0.05 to 1.5 wt %, relative to the total weight of the pearlescent pigment.

6. The weather-resistant pearlescent pigment according to claim 1, wherein, the proportion by weight of C (carbon proportion), lies in a range of from 0.03 to 0.5 wt.-%, relative to the total weight of the pearlescent pigment.

7. The weather-resistant pearlescent pigment according to claim 1, wherein, the top layer consists of layers a), b) and c).

8. The weather-resistant pearlescent pigment according to claim 1, wherein, the reaction products of the oligomeric silanes independently of one another have one or more alkyl groups of from 1 to 18 C atoms.

9. The weather-resistant pearlescent pigment according to claim 1, wherein, the proportion of the top layer on all of the pearlescent pigment lies in a range of from 1.0 to 5.0 wt.-%, relative to the total weight of the pearlescent pigment.

10. The weather-resistant pearlescent pigment according to claim 1, wherein, the coating of the pearlescent pigment with all of the top layer takes place in aqueous environment.

11. The weather-resistant pearlescent pigment according to claim 1, wherein, the substrate is coated with one or more chromophoric, highly refractive metal oxide layer(s) which is selected from the group consisting of TiO.sub.2, Fe.sub.2O.sub.3, TiFe.sub.2O.sub.5, Fe.sub.2Ti.sub.3O.sub.9, FeTiO.sub.3 and mixtures thereof.

12. The weather-resistant pearlescent pigment according to claim 11, wherein, the chromophoric, highly refractive coating comprises TiO.sub.2 in the rutile structure.

13. The weather-resistant pearlescent pigment according to claim 12, wherein, the proportion of TiO.sub.2 in the rutile structure lies in a range of from 30 to 80 wt.-%, relative to the total weight of the pearlescent pigment.

14. The weather-resistant pearlescent pigment according to claim 12, wherein, the chromophoric, highly refractive coating with TiO.sub.2 in the rutile structure has an average thickness which lies in a range of from 80 to 280 nm.

15. The weather-resistant pearlescent pigment according to claim 1, wherein, the weather-resistant pearlescent pigment has a cumulative frequency distribution based on a volume-averaged size distribution function with the values D.sub.10, D.sub.50, D.sub.90, with a span D of from 0.7-1.4, wherein the span D is calculated according to Formula (I):
D=(D.sub.90D.sub.10)/D.sub.50(I).

16. The weather-resistant pearlescent pigment according to claim 15, wherein, the pearlescent pigment has a span D in a range of from 0.75-1.3.

17. A process for the production of weather-resistant pearlescent pigment according to claim 1, wherein, the process comprises the following steps: a) optionally classifying coated or uncoated platelet-shaped substrates, obtaining substrates which preferably have the values D.sub.10, D.sub.50, D.sub.90 from a cumulative frequency distribution of a volume-averaged size distribution function with a span D in a range of from 0.7-1.4, b) suspending coated or uncoated platelet-shaped substrates, optionally from step a), in aqueous solution, and coating the coated or uncoated platelet-shaped substrates with one or more highly refractive metal oxide(s), obtaining pearlescent pigments which are optionally calcined, c) coating the pearlescent pigments coated in step b) in aqueous solution with tin oxide, tin hydroxide and/or hydrated tin oxide by hydrolysis of a tin salt or of a hydrolyzable tin metalorganic compound in a pH range of from 1.5 to 3.0, d) coating the pearlescent pigments coated in step c) in aqueous solution with cerium oxide, cerium hydroxide and/or hydrated cerium oxide by hydrolysis of a cerium salt or of a hydrolyzable cerium metalorganic compound, e) coating the pearlescent pigments coated in step d) in aqueous solution with one or more oligomeric silane(s), f) separating off the pearlescent pigments coated in step e), optionally washing with demineralized water, and g) optionally drying the pearlescent pigments of step f).

18. The process of using the weather-resistant pearlescent pigment of claim 1 for the pigmentation of coatings, printing inks, plastics and cosmetics.

19. The weather-resistant pearlescent pigment according to claim 1, wherein the organic-chemical coating consists of one or more oligomeric silane(s), wherein the oligomeric silane(s) have one or more amino groups and the oligomeric silane(s) are chemically bonded with the cerium-containing layer.

20. The weather-resistant pearlescent pigment according to claim 11, wherein, the chromophoric, highly refractive coating consists of TiO.sub.2 in the rutile structure.

21. The process according to claim 17, wherein the pearlescent pigments of step f) are optionally dried at a temperature from 80 C. to 160 C.

22. A weather-resistant pearlescent pigment, comprising: a coated or uncoated platelet-shaped substrate which is selected from the group consisting of synthetic mica platelets, glass platelets, SiO.sub.2 platelets, Al.sub.2O.sub.3 platelets, synthetic boehmite platelets, BiOCl platelets and mixtures thereof, a chromophoric coating comprising at least one highly refractive metal oxide, and a top layer, wherein the top layer comprises the following layer sequence: a) a first layer comprising tin oxide and/or tin hydroxide and/or hydrated tin oxide, b) a second cerium-containing layer consisting of cerium oxide and/or cerium hydroxide and/or hydrated cerium oxide, c) an organic-chemical coating applied to the cerium-containing layer which comprises one or more oligomeric silane(s), wherein the oligomeric silane(s) have one or more amino groups and the oligomeric silane(s) are chemically bonded with the cerium-containing layer, and wherein the proportion of tin oxide, tin hydroxide, and/or hydrated tin oxide, calculated as SnO.sub.2, in the first layer of the top layer lies in a range of from 0.4 to 4.0 wt.-%, relative to the total weight of the pearlescent pigment.

23. The weather-resistant pearlescent pigment according to claim 22, wherein the proportion of tin oxide, tin hydroxide, and/or hydrated tin oxide, calculated as SnO.sub.2, in the first layer of the top layer lies in a range of from 1.0 to 3.0 wt.-%, relative to the total weight of the pearlescent pigment.

24. A weather-resistant pearlescent pigment, consisting of: a coated or uncoated platelet-shaped substrate which is selected from the group consisting of synthetic mica platelets, glass platelets, SiO.sub.2 platelets, Al.sub.2O.sub.3 platelets, synthetic boehmite platelets, BiOCl platelets and mixtures thereof, a chromophoric coating comprising at least one highly refractive metal oxide, and a top layer, wherein the top layer consists of the following layer sequence: a) a first layer comprising tin oxide and/or tin hydroxide and/or hydrated tin oxide, b) a second cerium-containing layer consisting of cerium oxide and/or cerium hydroxide and/or hydrated cerium oxide, c) an organic-chemical coating applied to the cerium-containing layer which comprises one or more oligomeric silane(s), wherein the oligomeric silane(s) have one or more amino groups and the oligomeric silane(s) are chemically bonded with the cerium-containing layer.

Description

EXAMPLES

(1) The following examples are intended to explain the invention in more detail, without, however, limiting it. All percentages are to be understood as wt.-%.

I Production of the Pigments

Example 1

(2) 100 g commercially available blue pearlescent pigment based on TiO.sub.2-coated synthetic mica with the fineness 10-40 m (Symic C261, from Eckart) was suspended in 900 g water. The dispersion was then heated to 70 C. and at a pH of 2.2 a solution of 5.17 g SnCl.sub.45H.sub.2O in 45 ml dilute hydrochloric acid was metered in at a rate of 2 ml/min, wherein the pH was kept constant by simultaneous dropwise addition of 20% sodium hydroxide solution. Stirring then continued for a further 20 min and the pH was increased to 4.2 with sodium hydroxide solution. A solution consisting of 2.17 g Ce(NO.sub.3).sub.36 H.sub.2O dissolved in 50 ml demineralized water was then metered in. At the same time, the pH was kept constant by dropwise addition of a 10% NaOH solution. After all of the solution had been added, stirring continued for 1 h after which the pH was adjusted to 10 with dilute sodium hydroxide solution. 5.7 g Dynasylan 1146 diluted with 24.3 g demineralized water was then added to the suspension followed by stirring for 180 min, then the suspension was filtered off and the filter cake subsequently washed with demineralized water. The filter cake was dried under vacuum at 95 C.

(3) The pigment had an SnO.sub.2 content of 2.2 and a Ce content of 0.7 wt.-% (corresponding to 0.82 wt.-% Ce.sub.2O.sub.3), relative to the total weight of the pigment.

(4) The pigment had the following particle size distribution (MALVERN Mastersizer MS 2000): D.sub.10=11.2 m, D.sub.50=22.5 m, D.sub.90=39.2 m. The C content was 0.2 wt.-%.

Example 2

(5) 100 g commercially available red pearlescent pigment based on TiO.sub.2-coated synthetic mica with the fineness 10-40 m (Symic C241, from Eckart) was suspended in 900 g water. The dispersion was then heated to 70 C. and at a pH of 2.2 a solution of 5.64 g SnCl.sub.4*5H.sub.2O in 45 ml dilute hydrochloric acid was metered in at a rate of 2 ml/min, wherein the pH was kept constant by simultaneous dropwise addition of 20% sodium hydroxide solution. Stirring then continued for a further 20 min and the pH was increased to 4.2 with sodium hydroxide solution. A solution consisting of 2.17 g Ce(NO.sub.3).sub.36H.sub.2O dissolved in 50 ml demineralized water was then metered in. At the same time, the pH was kept constant by dropwise addition of a 10% NaOH solution. After all of the solution had been added, stirring continued for 1 h after which the pH was adjusted to 10 with dilute sodium hydroxide solution, 5.7 g Hydrosil 2627 diluted with 24.3 g demineralized water was then added to the suspension followed by stirring for 180 min, then the suspension was filtered off and the filter cake subsequently washed with demineralized water. The filter cake was dried under vacuum at 95 C.

(6) The pigment had an SnO.sub.2 content of 2.4 and a Ce content of 0.7 wt.-% (corresponding to 0.82 wt.-% Ce.sub.2O.sub.3), relative to the total weight of the pigment.

(7) The pigment had the following particle size distribution (MALVERN Mastersizer MS 2000): D.sub.10=10.6 m, D.sub.50=22.3 m, D.sub.90=40.4 m. The C content was 0.1 wt.-%.

Example 3

(8) 100 g of the green-colored pearlescent pigment obtained from comparison example 7 was suspended in 900 g water. The dispersion was then heated to 70 C. and at a pH of 2.2 a solution of 5.64 g SnCl.sub.4*5H.sub.2O in 45 ml dilute hydrochloric acid was metered in at a rate of 2 ml/min, wherein the pH was kept constant by simultaneous dropwise addition of 20% sodium hydroxide solution. Stirring then continued for a further 20 min and the pH was increased to 4.2 with sodium hydroxide solution, A solution consisting of 2.17 g Ce(NO.sub.3).sub.36H.sub.2O dissolved in 50 ml demineralized water was then metered in. At the same time, the pH was kept constant by dropwise addition of a 10% NaOH solution. After all of the solution had been added, stirring continued for 1 h after which the pH was adjusted to 10 with dilute sodium hydroxide solution. 5.7 g Hydrosil 2907 diluted with 24.3 g demineralized water was then added to the suspension followed by stirring for 180 min, then the suspension was filtered off and the filter cake subsequently washed with demineralized water. The filter cake was dried under vacuum at 95 C.

(9) The pigment had SnO.sub.2 content of 2.4 and a Ce content of 0.7 wt.-% (corresponding to 0.82 wt.-% Ce.sub.2O.sub.3), relative to the total weight of the pigment.

(10) The pigment had the following particle size distribution (MALVERN Mastersizer MS 2000): D.sub.10=8.7 m, D.sub.50=18.8 m, D.sub.90=35.9 m. The C content was 0.1 wt.-%.

Comparison Example 1

(11) 100 g commercially available blue pearlescent pigment based on TiO.sub.2-coated mica with the fineness 5-25 m (PHOENIX PX 2261, from Eckart) was suspended in 900 g water. The dispersion was then heated to 70 C. and at a pH of 2.4 a solution of 2.30 g SnCl.sub.45H.sub.2O in 45 ml dilute hydrochloric acid was metered in at a rate of 2 ml/min, wherein the pH was kept constant by simultaneous dropwise addition of 20% sodium hydroxide solution. After subsequently stirring for 30 min, the solid was filtered off, washed with water and dried at 120 C.

(12) The dry, coated pigment was suspended in 300 ml isopropanol and brought to boiling temperature. Accompanied by stirring, first 2.0 g H.sub.2O and then, within one hour, a solution of 2.17 g Ce(NO.sub.3).sub.36H.sub.2O in 100 g isopropanol were added. A solution of 0.45 g ethylene diamine in 8 g H.sub.2O was then added. Over a period of 2 h, 14.6 g tetraethoxysilane and 20 g isopropanol were then introduced continuously with a dosing pump (Ismatec). The suspension was then allowed to continue reacting for 6 h further. 0.4 g Dynasylan AMEO and 1.3 g Dynasylan 9116 were then added and cooling was allowed to take place slowly. The mixture was stirred at room temperature overnight and filtered off by suction the next day. The pigment filter cake was then dried under vacuum at 100 C. for 6 hours.

(13) The pigment had an SnO.sub.2 content of 1.0%, a cerium content of 0.7 wt.-%, a C content of 0.9 wt.-% and an SiO.sub.2 content of 4.2 wt.-%.

Comparison Example 2

(14) 100 g commercially available blue pearlescent pigment based on TiO.sub.2-coated synthetic mica with the fineness 10-40 m (Symic C261, from Eckart) was suspended in 900 g water. The dispersion was then heated to 70 C. and at a pH of 2.4 a solution of 2.30 g SnCl.sub.45 H.sub.2O in 45 ml dilute hydrochloric acid was metered in at a rate of 2 ml/min, wherein the pH was kept constant by simultaneous dropwise addition of 20% sodium hydroxide solution. After subsequently stirring for 30 min, the solid was filtered off, washed with water and dried at 120 C.

(15) The dry, coated pigment was suspended in 300 ml isopropanol and brought to boiling temperature. Accompanied by stirring, first 2.0 g H.sub.2O and then, within one hour, a solution of 2.17 g Ce(NO.sub.3).sub.36H.sub.2O in 100 g isopropanol were added. A solution of 0.45 g ethylene diamine in 8 g H.sub.2O was then added. Over a period of 2 h, 14.6 g tetraethoxysilane and 20 g isopropanol were then introduced continuously with a dosing pump (lsmatec). The suspension was then allowed to continue reacting for 6 h further. 0.4 g Dynasylan AMEO and 1.3 g Dynasylan 9116 were then added and cooling was allowed to take place slowly. The mixture was stirred at room temperature overnight and filtered off by suction the next day. The pigment filter cake was then dried under vacuum at 100 C. for 6 hours.

(16) The pigment had a theoretical SnO.sub.2 content of 1.0%, a cerium content of 0.7%, a C content of 0.9 wt.-% and an SiO.sub.2 content of 4.2 wt.-%.

Comparison Example 3

(17) 100 g commercially available blue pearlescent pigment based on TiO.sub.2-coated synthetic mica with the fineness 10-40 m (Symic C261, from Eckart) was suspended in 900 g water. The dispersion was then heated to 70 C. and at a pH of 2.4 a solution of 2.30 g SnCl.sub.45 H.sub.2O in 45 nil dilute hydrochloric acid was metered in at a rate of 2 ml/min, wherein the pH was kept constant by simultaneous dropwise addition of 20% sodium hydroxide solution. The pH was then increased to 7.5 with 5 wt.-% NaOH solution and stirring was carried out for 15 min. A water glass solution (15 g water glass solution, 3.9 wt.-% SiO.sub.2, mixed with 20.7 g demineralized water) was then introduced slowly into the suspension and the pH was kept constant at pH 7.5. Stirring was then continued for a further 20 min and the pH was reduced to 4.2 with dilute hydrochloric acid. A solution consisting of 2.17 g Ce(NO.sub.3).sub.36H.sub.2O dissolved in 50 ml demineralized water was then metered in. At the same time, the pH was kept constant by dropwise addition of a 10% NaOH solution. After all of the solution had been added, stirring continued for 1 h after which the pH was adjusted to 10 with dilute sodium hydroxide solution. 0.4 g Dynasylan AMEO and 1.3 g Dynasylan 9116 were then added into the suspension followed by stirring for 280 min, then the suspension was filtered off and the filter cake subsequently washed with demineralized water. The filter cake was dried under vacuum at 95 C.

(18) The pigment had a Ce content of 0.7 wt.-%, relative to the total weight of the pigment.

(19) The pigment had the following particle size distribution (MALVERN Mastersizer MS 2000): D.sub.10=11.5 m, D.sub.50=22.9 m, D.sub.90=39.6 m.

Comparison Example 4

(20) 100 g commercially available blue pearlescent pigment based on TiO.sub.2-coated synthetic mica with the fineness 10-40 m (Symic C261, from Eckart) was suspended in 900 g water. The dispersion was then heated to 70 C. and at a pH of 2.4 a solution of 2.30 g SnCl.sub.45 H.sub.2O in 45 ml dilute hydrochloric acid was metered in at a rate of 2 ml/min, wherein the pH was kept constant by simultaneous dropwise addition of 20% sodium hydroxide solution. Stirring was then continued for a further 20 min and the pH was reduced to 4.2 with dilute hydrochloric acid. A solution consisting of 2.17 g Ce(NO.sub.3).sub.36H.sub.2O dissolved in 50 ml demineralized water was then metered in. At the same time, the pH was kept constant by dropwise addition of a 10% NaOH solution. After all of the solution had been added, stirring continued for 1 h after which the pH was adjusted to 10 with dilute sodium hydroxide solution. 0.4 g Dynasylan AMEO and 1.3 g Dynasylan 9116 were then added to the suspension followed by stirring for 280 min, then the suspension was filtered off and the filter cake subsequently washed with demineralized water. The filter cake was dried under vacuum at 95 C.

(21) The pigment had a Ce content of 0.7 wt.-%, relative to the total weight of the pigment.

(22) The pigment had the following particle size distribution (MALVERN Mastersizer MS 2000): D.sub.10=11.2 m, D.sub.50=22.5 m, D.sub.90=38.9 m.

Comparison Example 5

(23) 100 g commercially available red pearlescent pigment based on TiO.sub.2-coated synthetic mica with the fineness 10-40 m (Symic C241, from Eckart) was suspended in 900 g water. The dispersion was then heated to 70 C. and at a pH of 2.4 a solution of 2.48 g SnCl.sub.45 H.sub.2O in 45 ml dilute hydrochloric acid was metered in at a rate of 2 ml/min, wherein the pH was kept constant by simultaneous dropwise addition of 20% sodium hydroxide solution. Stirring then continued for a further 20 min. After this, the pH was subsequently adjusted to 10 with dilute sodium hydroxide solution. 5.7 g Hydrosil 2627 diluted with 24.3 g demineralized water was then added to the suspension followed by stirring for 180 min, then the suspension was filtered off and the filter cake subsequently washed with demineralized water. The filter cake was dried under vacuum at 95 C.

(24) The pigment had the following particle size distribution (MALVERN Mastersizer MS 2000): D.sub.10=10.9 m, D.sub.50=21.8 m, D.sub.90=38.6 m. The C content was 0.1 wt.-%.

Comparison Example 6

(25) 100 g of the pigment from comparison example 7 was suspended in 850 ml demineralized water and heated to 85 C. with vigorous stirring. The pH was reduced to 4.2 with dilute hydrochloric acid. A solution consisting of 0.93 g Ce(NO.sub.3).sub.36 H.sub.2O dissolved in 40 ml demineralized water was then metered in. At the same time, the pH was kept constant by dropwise addition of a 10% NaOH solution. After all of the solution had been added, stirring continued for 1 h after which the pH was adjusted to 10 with dilute sodium hydroxide solution. 5.7 g Dynasylan 1146 diluted with 24.3 g demineralized water was then added to the suspension followed by stirring for 180 min, then the suspension was filtered off and the filter cake subsequently washed with demineralized water. The filter cake was dried under vacuum at 95 C. The pigment had a theoretical Ce content of 0.3 wt.-%, relative to the total weight of the pigment.

(26) The pigment had the following particle size distribution (MALVERN Mastersizer MS 2000): D.sub.10=8.2 m, D.sub.50=18.2 m, D.sub.90=35.3 m. The C content was 0.2 wt.-%.

Comparison Example 7

(27) 100 g synthetic mica platelets FM1040 from Jhejan, China with the particle size distribution according to a MALVERN Mastersizer MS 2000: D.sub.10=11.4 m, D.sub.50=21.8 m, D.sub.90=40.0 m, was suspended in 850 ml demineralized water and heated to 80 C. accompanied by stirring. The pH was reduced to 1.9 with dilute hydrochloric acid. A layer was then formed by adding a solution consisting of 3 g SnCl.sub.45 H.sub.2O (in 10 ml conc. HCl plus 50 ml demineralized water), with simultaneous meteringin of a 10% NaOH. The pH was then reduced to pH 1.6 with dilute HCl and then a solution of 950 ml TiCl.sub.4 (200 g TiO.sub.2/I demineralized water) was metered into the suspension in parallel with 10% strength aqueous sodium hydroxide solution. After the coating had ended stirring was carried out for 1 h before filtering off. The filter cake was dried at 90 C. in a vacuum drying cabinet for 12 h and then calcined. An intensively green interfering pearlescent pigment was obtained (3.sup.rd order interference).

Comparison Example 8

(28) Red interference pearlescent pigment based on synthetic mica platelets Symic C241 from Eckart GmbH.

II Characterization of the Pigments

(29) IIa Particle Size Measurement

(30) The size distribution curve of the platelet-shaped synthetic substrates and of the pearlescent pigments was determined with a device from Malvern (device: MALVERN Mastersizer 2000) according to the manufacturer's instructions. For this, approx. 0.1 g of the corresponding substrate or pigment as aqueous suspension, without addition of dispersion auxiliaries, was introduced by means of a Pasteur pipette into the sample preparation chamber of the measuring device, accompanied by constant stirring, and measured several times. The resultant average values were formed from the individual measurement results. The scattered light signals were evaluated according to the Fraunhofer method.

(31) By the average size D.sub.50 is meant within the framework of this invention the D.sub.50 value of the cumulative frequency distribution of the volume-averaged size distribution function, as obtained by laser diffraction methods. The D.sub.50 value indicates that 50% of the non-metallic platelet-shaped synthetic substrates or pigments have a diameter which is equal to or smaller than the value indicated, for example 20 m. Correspondingly, the D.sub.90 value indicates that 90% of the substrates or pigments have a diameter which is equal to or smaller than the respective value. Furthermore, the D.sub.10 value indicates that 10% of the substrates or pigments have a diameter which is equal to or smaller than the respective value.

(32) IIb Determination of the Average Thickness of the Platelet-Shaped Synthetic Substrates

(33) To determine the average thickness of the non-metallic platelet-shaped synthetic substrates, the substrates or the pigments were incorporated to a level of 10 wt.-% in an Autoclear Plus HS 2K clear coat from Sikkens, with a brush and applied to a film using a spiral doctor blade (26 m wet film thickness) and dried. After drying for 24 h, cross-section polishes of these doctor-blade drawdowns were prepared and measured by scanning electron microscopy. At least 100 pigment particles were measured here in order to obtain informative statistics.

(34) IIc Determination of the Cerium Oxide Content

(35) The cerium oxide contents of the pigments were determined by means of X-ray fluorescence analysis (XRFA).

(36) For this, the pigment was incorporated into a lithium tetraborate glass tablet, fixed in solid sample measuring vessels and measured therefrom. The Advantix ARL device from Thermo Scientific was used as measuring device.

III Weather-Resistance of the Pigments

(37) A Condensation Water Test

(38) A few pigment samples were incorporated into a waterborne coating system and the test applications produced by spray painting. The base coat was overcoated with a 1K clear coat customary in the trade and then stoved. These applications were tested according to DIN 50 017 (condensation waterconstant climates). The adhesion was tested by means of cross cutting according to DIN EN ISO 2409 immediately after completion of the test in comparison with the unloaded sample. Here, Gt 0 means no change and Gt 5 a very significant change.

(39) The swelling behavior was assessed visually immediately after condensation water loading according to DIN 53230. Here, the value 0 means no change and the value 5 a very significant change.

(40) Finally the DOI (distinctness of image) was assessed visually. It is influenced, i.a., also by the respective substrate and can change substantially as a result of the swelling processes due to the retention of water.

(41) TABLE-US-00001 TABLE 1 Condensation water results Gloss Gloss Loss 20 20 of Immediate before after gloss cross Swelling Sample CW test CW test in % DOI cutting visual Example 1 88.7 85.6 3.5% 80.6 0 1 Example 2 88.6 88.7 0% 82.3 1 0 Comparison 90.0 79.1 12.1% 85.4 0 2 example 1 Comparison 88.5 55.4 37.4% 76.8 0 4 example 2 Comparison 88.5 21.3 75.9% 80.1 4 4 example 3 Comparison 88.3 68.2 22.8% 79.8 2 3 example 4

(42) The examples according to the invention had an optimal condensation water resistance. Comparison examples 1 and 2 likewise exhibited very good cross cutting results. However, comparison example 2 exhibited an extremely strong swelling behavior, which can presumably be explained by the additional SiO.sub.2 layer. The negative, swelling influence of the SiO.sub.2 layer was additionally confirmed by comparison examples 3 and 4.

(43) Comparison examples 3 and 4 furthermore exhibited a significantly poorer adhesion (cross cutting) which can be explained i.a. by the use of monomeric silane systems. In comparison example 4, in which all coatings took place in aqueous environment, the binding of the silanes, in particular of the hydrophobic alkysilane, to the pigment surface was presumably not optimal.

(44) As the hydrolysis and condensation rates of different monomeric silanes can thus differ significantly (by up to a factor of 850), as described in Hydrolysis and Condensation of organosilanesEU 10-002/MS/fk/September 97, with the use of two different silanes, as takes place in comparison examples 1 to 4 and 6, it is very probable that the aminosilane hydrolyzes and/or condenses significantly earlier and can thus fix on the pigment surface first. The degree of cross-linking of the already converted aminosilane is thus also different and the surface modification can be very inhomogeneous. Only much later does condensation occur, and thus precipitation of the alkylsilane which in this way can cover some of the introduced cross-linking groups from the aminosilane, and these are no longer available for binding to the coating system. A coating with alkylsilane, both because of the pre-coating with already converted aminosilane and also because of steric hindrance due to the alkyl group, might also not be able to bind optimally to the coated pigment surface, which could explain the more pronounced swelling of the coating layer and thus less favorable intercoat adhesion.

(45) The high DOI value of comparison example 1 can be explained by means of the starting material used. In this example only an interference pigment Blue based on natural mica was used. This is significantly thinner than comparable pigments based on synthetic mica, whereby the distinctness of image is positively influenced.

(46) B WOM Test (Based on Red or Green Pigments with Synthetic Mica as Substrate)

(47) The pigment samples were incorporated into a waterborne coating system and the test applications were produced by spray painting. The base coat was overcoated with a clear coat customary in the trade and then stoved. The accelerated weathering test took place according to SAE 2527 in a Q-Sun Xe 3 HS (Q-Lab) Xenon test device. The determination of the E* values and grayscale classification took place in each case relative to the corresponding unloaded sample.

(48) TABLE-US-00002 Loss of gloss 20 Color change E* after 4000 h after 4000 h Example 2 18% 1.42 Comparison example 8 43% 3.45 Comparison example 6 16% 2.14

(49) The reduction in gloss and the change in color were most strongly pronounced in comparison example 8. Comparison example 6 (without SnO.sub.2 layer) exhibited good gloss retention but the change in color was simply insufficient due to the higher photoactivity.

(50) C UV Resistance on Doctor-Blade Drawdowns

(51) This test was carried out according to the UV rapid test described in EP 0 870 730 to determine the photochemical UV activity of TiO.sub.2 pigments.

(52) For this, 1.0 g of the pearlescent pigment was dispersed in 9.0 g of a melamine-containing coating rich in double bonds. Doctor-blade drawdowns were prepared on carded paper and dried at room temperature. The doctor-blade drawdowns were divided and in each case one of the two sections was stored in the dark as an unloaded comparison sample. The samples were then irradiated in a QUV device from Q-Panel for 150 min with UV-containing light (UVA-340 lamp, irradiation intensity 1.0 W/m.sup.2/nm). Immediately after completion of the test, color values of the loaded test pieces were ascertained relative to the respective reference sample using a CM-508i colorimeter from Minolta. The resulting E* values, calculated according to the Hunter-L*a*b*-Formula, are shown in Table 2.

(53) In the test, essentially a gray-blue discoloration of the TiO.sub.2 layer of the pearlescent pigment in the doctor-blade drawdowns is observed because of Ti(III) centers formed under the influence of UV light. The condition for this is that the electron hole has physically left the TiO.sub.2 andfor example due to reaction with olefinic double bonds of the binder cannot immediately recombine again with the remaining electron. As a melamine-containing coating layer significantly slows down the diffusion of water (vapor) and oxygen onto the pigment surface, there is a significant delay in reoxidation of the titanium(III) centers with the result that the graying can be measured and the E value can be used as a measure for the UV resistance of the pigments. A higher E* numerical value of the loaded sample relative to the unloaded reference sample thus means a lower UV resistance of the examined pigment.

(54) TABLE-US-00003 TABLE 2 UV doctor-blade test results Sample Composition E* Example 1 Symic C261/SnO.sub.2/Ce(OH).sub.3/olig. silane 1.0 Example 2 Symic C241/SnO.sub.2/Ce(OH).sub.3/olig. silane 1.6 Example 3 Interference Green/SnO.sub.2/Ce(OH).sub.3/olig. 3.2 silane Comparison example 5 Symic C241/SnO.sub.2 21.4 Comparison example 6 Interference Green/Ce(OH).sub.3/olig. silane 10.4 Comparison example 7 Interference Green 28.3 Comparison example 8 Symic C241 (Interference Red) 24.0

(55) All the comparison examples exhibited a much greater color change (E*) following corresponding exposure to light. The additional SnO.sub.2 intermediate layer of the pigments according to the invention acts as an additional barrier layer vis--vis moisture, whereby, in conjunction with the cerium hydroxide layer, radical formation on the surface of the pigments is inhibited and thus the photoactivity is significantly reduced. If the pigments are coated only with the individual components, the stabilizing effect is still visible but is significantly less than in the combination according to the invention.

(56) D Color-Shade Constancy and Optical Properties Compared with the Starting Material

(57) Any color-shade changes occurring were examined visually on the basis of doctor-blade drawdowns of the respective pigments in a conventional nitrocellulose coating (Dr. Renger Erco bronzing mixed varnish 2615e; from Morton, pigmentation of 6 wt.-%, relative to the total weight of the wet varnish) on black-white opacity charts (Byko-Chart 2853, Byk Gardner). It was ascertained here that with the pearlescent pigments according to the invention no difference can be visually detected with respect to the starting material. In contrast, doctor-blade drawdowns of the pigments of comparison examples 1-3 exhibited scattered so-called specks in relation to the starting material used in the comparison examples.