Multilayered interference pigments

0001The invention relates to multilayer interference pigments consisting of a transparent carrier material which is coated with alternating layers of metal oxides with low and high refractive index.

0002Multilayer interference pigments with alternating layers of materials with high and low refractive indices are known. They differ in terms of the carrier material and the material for the individual layers, as well as the manufacturing process. The layers are produced either by precipitation using the wet process or by vapor deposition or sputtering in a vacuum. The layers applied to the carrier or a “release layer” are all optically active and contribute to the formation of the interference colors. The carrier materials are only optically active in exceptional cases.

0003<patcit id="pcit0001" dnum="US4434010A"><text>US 4,434,010</text></patcit> describes a multilayer interference pigment consisting of a central layer of a reflective material (aluminum) and alternating layers of two transparent, dielectric materials with high and low refractive indices, for example titanium dioxide and silicon dioxide on both sides of the central aluminum layer. This pigment is used e.g. B. used for printing securities.

0004<patcit id="pcit0002" dnum="JPH7759A"><text>JP H7-759</text></patcit> (Kokoku) describes a multilayer interference pigment with a metallic sheen. It consists of a substrate coated with alternating layers of titanium dioxide and silicon dioxide. The substrate is formed from aluminum, gold or silver plates or mica and glass plates coated with metals.

0005<patcit id="pcit0003" dnum="JPH7246366B"><text>JP H7-246 366</text></patcit> describes a multilayer interference pigment with alternating layers of a high refractive index material and a low refractive index material. Glass is used as the substrate and silicon dioxide and titanium dioxide for the coating. The individual layers have an optical thickness that is an integral multiple of a quarter of the wavelength at which interference is to be expected. This results in a layer thickness of 44.4 nm with a refractive index of 2.7 for the titanium dioxide layers and a layer thickness of 80 nm with a refractive index of 1.5 for the silicon dioxide layers.

0006In the case of the pigments described above, the silicon dioxide layers act as optically active layers. This optically active effect begins with a layer thickness of approximately 20 nm. Silicon dioxide layers are also used as diffusion barriers for conductive pigments. So describes<patcit id="pcit0004" dnum="EP0373575A"><text>EP 0 373 575</text></patcit> Conductive platelet-shaped pigments, which consist of a platelet-shaped metal oxide or of a platelet-shaped material coated with a metal oxide and a conductive layer, a silicon dioxide layer being arranged between the metal oxide layer and the conductive layer, which prevents ions from being formed during the calcination from the substrate or the metal oxide layer diffuse the conductive layer, because otherwise the conductivity of the antimony-doped tin oxide layer will be reduced. The thickness of the silicon dioxide intermediate layer is between 8 and 30 nm at concentrations of 5 to 20% by weight of SiO₂ based on the substrate.

0007In <patcit id="pcit0005" dnum="FR2352867"><text>FR 2 352 867</text></patcit> describes a mica pigment which has a thin layer of a colorless oxide selected from titanium dioxide or aluminum oxide on its surface and then a layer of a mixture of α-FeOOH and δ-FeOOH, the colorless oxide having a layer thickness of 2 up to 21 nm is applied.

0008Surprisingly, it was found that the quality of commercially available interference pigments can be significantly improved with regard to their mechanical properties if at least one intermediate layer made of another metal oxide which has no optical activity is arranged in the metal oxide layers with a high refractive index.

0009Suitable metal oxides have in common that, despite their small thickness, they are still obtained as a layer even after the pigment has been calcined, since they form mixed phases with the surrounding highly refractive metal oxides only with difficulty. They also have a significantly lower refractive index than the metal oxides forming the optical layer.

0010The invention thus relates to multilayer interference pigments according to claim 1.

0011To produce the pigments according to the invention, the transparent carrier material is suspended in water and alternately coated with a metal oxide hydrate with a high refractive index and a metal oxide hydrate with a low refractive index by adding and hydrolysing the corresponding water-soluble metal compounds, the pH value necessary for the precipitation of the respective metal oxide hydrate being adjusted and kept constant by simultaneous addition of acid or base, and then the coated carrier material is separated from the aqueous suspension, dried and optionally calcined.

0012The invention also relates to the use of the pigments according to the invention for pigmenting lacquers, printing inks, plastics, glazes for ceramics and glasses and cosmetics.

0013For this purpose, they can be used as mixtures with commercially available pigments, for example inorganic or organic absorption pigments, metallic effect pigments and LCP pigments.

0014The carrier material is e.g. B. mica, another layered silicate, glass platelet or platelet-shaped silicon dioxide, which after the in<patcit id="pcit0006" dnum="WO9308237A"><text>WO 93/08237</text></patcit> described method is produced on an endless belt by solidification and hydrolysis of a water glass solution.

0015The metal oxide with a high refractive index is an oxide or mixtures of oxides with or without absorbent properties, such as TiO₂, ZrO₂, Fe₂O₃, Fe₃O₄, Cr₂O₃ or ZnO.

0016The low refractive index metal oxide is e.g. B. SiO₂, Al₂O₃, AIOOH, B₂O₃ or a mixture thereof and can also have absorbent or non-absorbent properties. The oxide layer with a low refractive index can optionally contain alkali and alkaline earth oxides as constituents.

0017The metal oxide layers are preferably applied wet-chemically, it being possible to use the wet-chemical coating processes developed for the production of pearlescent pigments; such processes are described, for example, in<patcit id="pcit0007" dnum="DE1467468"><text>DE 14 67 468</text></patcit>, <patcit id="pcit0008" dnum="DE1959988"><text>DE 19 59 988</text></patcit>. <patcit id="pcit0009" dnum="DE2009566"><text>DE 20 09 566</text></patcit>, <patcit id="pcit0010" dnum="DE2214545"><text>DE 22 14 545</text></patcit>, <patcit id="pcit0011" dnum="DE2215191"><text>DE 22 15 191</text></patcit>, <patcit id="pcit0012" dnum="DE2244298"><text>DE 22 44 298</text></patcit>, <patcit id="pcit0013" dnum="DE2313331"><text>DE 23 13 331</text></patcit>, <patcit id="pcit0014" dnum="DE2522572"><text>DE 25 22 572</text></patcit>, <patcit id="pcit0015" dnum="DE3137808"><text>DE 31 37 808</text></patcit>, <patcit id="pcit0016" dnum="DE3137809"><text>DE 31 37 809</text></patcit>, <patcit id="pcit0017" dnum="DE3151343"><text>DE 31 51 343</text></patcit>, <patcit id="pcit0018" dnum="DE3151354"><text>DE 31 51 354</text></patcit>, <patcit id="pcit0019" dnum="DE3151355"><text>DE 31 51 355</text></patcit>, <patcit id="pcit0020" dnum="DE3211602"><text>DE 32 11 602</text></patcit>, <patcit id="pcit0021" dnum="DE3235017"><text>DE 32 35 017</text></patcit> or also in other patent documents and other publications.

0018For the coating, the substrate particles are suspended in water and one or more hydrolyzable metal salts are added at a pH value suitable for the hydrolysis, which is chosen so that the metal oxides or metal oxide hydrates are deposited directly on the particles without causing co-precipitation . The pH value is usually kept constant by simultaneously adding a base or alkali. The pigments are then separated off, washed and dried and optionally annealed, the annealing temperature being able to be optimized with regard to the coating in each case. If desired, the pigments can be separated off after application of individual coatings, dried and optionally annealed, in order then to be resuspended to precipitate the further layers. The latter method is usually uneconomical.

0019Titanium dioxide is preferably used as the metal oxide with a high refractive index and silicon dioxide as the metal oxide with a low refractive index.

0020For the application of the titanium dioxide layers that is in the <patcit id="pcit0022" dnum="US3553001A"><text>US 3,553,001</text></patcit> described method preferred.

0021An aqueous titanium salt solution is slowly added to a suspension of the material to be coated heated to about 50-100 ° C., in particular 70-80 ° C., and a largely constant pH is obtained by simultaneously metering in a base, such as, for example, aqueous ammonia solution or aqueous alkali metal hydroxide solution -Value of about 0.5 - 5, in particular about 1.5 - 2.5 observed. As soon as the desired layer thickness of the TiO₂Precipitation is reached, the addition of the titanium salt solution and the base is stopped.

0022This method, also known as the titration method, is characterized in that an excess of titanium salt is avoided. This is achieved by adding only the amount of hydrolysis per unit of time that is necessary for uniform coating with the hydrated TiO₂ is required and how can be taken up by the available surface of the particles to be coated per unit of time. Therefore, there are no hydrated titanium dioxide particles that are not deposited on the surface to be coated.

0023The following procedure must be used to apply the silicon dioxide layers: A sodium silicate solution is metered into a suspension of the material to be coated, which is heated to approximately 50-100 ° C., in particular 70-80 ° C. By simultaneously adding 10% hydrochloric acid, the pH is kept constant at 1.5 to 10, preferably 6.5 to 8.5. After adding the water glass solution, stirring is continued for a further 30 min.

0024It is also possible to change the powder color of the pigment by applying further layers, such as colored metal oxides or Berlin blue, compounds of the transition metals such as Fe, Cu, Ni, Co, Cr or organic compounds such as dyes or colored lacquers.

0025It is also possible to subject the finished pigment to a post-coating or post-treatment which further increases the light, weather and chemical stability, or to facilitate handling of the pigment, in particular incorporation into different media. Post-coatings or post-treatments include, for example, those in the<patcit id="pcit0023" dnum="DE2215191C"><text>DE-PS 22 15 191</text></patcit>, <patcit id="pcit0024" dnum="DE3151354A"><text>DE-OS 31 51 354</text></patcit>, <patcit id="pcit0025" dnum="DE3235017A"><text>DE-OS 32 35 017</text></patcit> or <patcit id="pcit0026" dnum="DE3334598A"><text>DE-OS 33 34 598</text></patcit> described methods in question.

0026The additional substances applied make up only about 0.1 to 5% by weight, preferably about 0.5 to 3% by weight, of the total pigment.

0027An additional coating with complex salt pigments, in particular cyanoferrate complexes, such as for example Berliner Blau and Turnbulls Blau, as described in US Pat <patcit id="pcit0027" dnum="EP01412173A"><text>EP 0 1412 173</text></patcit> and <patcit id="pcit0028" dnum="DE2313332"><text>DE 23 13 332</text></patcit> is described.

0028The pigment according to the invention can also be used with organic dyes and in particular with phthalocyanine or metal phthalocyanine and / or indanthrene dyes <patcit id="pcit0029" dnum="DE4009567"><text>DE 4009 567</text></patcit> be coated. For this purpose, a suspension of the pigment is prepared in a solution of the dye and this is then brought together with a solvent in which the dye is sparingly soluble or insoluble.

0029The thickness of the intermediate layers of low-index metal oxides within a high-index metal oxide layer is 1 to 20 nm, preferably 2 to 10 nm. Within this range, a low-index metal oxide layer, for example silicon dioxide, is optically inactive, which is an essential feature of the present invention is.

0030The thickness of the layers of metal oxides with a high refractive index is between 20 and 350 nm, preferably between 40 and 260 nm. Since the intermediate layers made of low-index metal oxides greatly increase the mechanical stability of the layers made of high-index metal oxides, thicker layers with sufficient stability can also be produced. In practice, however, only layer thicknesses of up to 260 nm are used, which is the case with a titanium dioxide mica pigment green III. Order corresponds.

0031The number and position of the intermediate layers depends on the total layer thickness of the metal oxide layer with a high refractive index. The intermediate layers are preferably arranged in such a way that the layer thickness of the metal oxide partial layers with a high refractive index corresponds to the optical thickness or an integral multiple of this optical thickness, which is necessary for the respective interference color. For example, a green III. Order the intermediate layers so that one layer is green 1st order and a second layer green 2nd order. This statement applies to pure interference colors. In another case, the person skilled in the art can determine the optimal position of the intermediate layers without inventive step. For example, you get a yellow green III. Order if you put an intermediate layer on Gold II. Order compared to a pure Green III. Order, in which the intermediate layer is arranged on a second-order green.

example

0032100 g of potash mica (10-60 µm) are suspended in 2 l of fully deionized water. The suspension, heated to 75 ° C., is adjusted to pH 1.8 with dilute hydrochloric acid and first by adding 3.3 ml / min of SnCl₄ Solution (from 2.2 g SnCl₄ and 0.75 g conc. Hydrochloric acid in 100 ml of deionized water) with SnO₂ coated. The pH is kept constant with 32% sodium hydroxide solution.

0033The mixture is left to stir for 15 min and then with TiO under the same pH / T conditions₂ coated by adding 1.5 ml / min TiCl₄Solution (400 g TiCl₄/ l) and keeping the pH constant with 32% sodium hydroxide solution. After the second-order color end point of green has been reached, the coating is interrupted, the mixture is stirred for a further 15 minutes and, after the pH has been adjusted to 8.0, the mixture is stirred for a further 10 minutes with dilute sodium hydroxide solution (within about 15 minutes).

0034Now the coating is done with SiO₂ by adding 3 ml / min sodium water glass solution (from 7.3 g sodium water glass with 27% SiO₂ and 80 ml of fully deionized water) without pH compensation. After completion, the mixture is left to stir for 15 minutes, adjusted again to 1.8 with dilute hydrochloric acid (within about 10 minutes) and by adding TiCl₄Solution a second TiO₂-Layer, as described above, applied. After reaching the comparison end point green III. Order is broken off, stirring is continued for 15 minutes and then the pigment is filtered off, washed, dried and calcined at 850 ° C. for 30 minutes.

0035The pigment obtained has an intense green interference color. The division of the TiO₂-Layers is as follows:<tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" rowsep="0"><colspec colnum="1" colname="col1" colwidth="26mm" /><colspec colnum="2" colname="col2" colwidth="21mm" /><tbody><row><entry>1. layer</entry><entry>approx. 170 nm</entry></row><row><entry>2nd layer</entry><entry>approx. 85 nm</entry></row><row rowsep="1"><entry>Overall layer</entry><entry>approx. 260 nm.</entry></row></tbody></tgroup></table></tables>

0036The thickness of the SiO₂Intermediate layer is approx. 5 nm.

Comparative example

0037100 g of potassium mica (10-60 µm) are suspended in 2 l of fully demineralized water. The suspension, heated to 75 ° C., is adjusted to pH 1.8 with dilute hydrochloric acid and first by adding 3.3 ml / min of SnCl₄Solution (from 2.2 g SnCl₄ and 0.75 g conc. Hydrochloric acid in 100 ml of deionized water) with SnO₂ coated. The pH is kept constant with 32% sodium hydroxide solution.

0038The mixture is left to stir for 15 min and then with TiO under the same pH / T conditions₂ coated by adding 1.5 ml / min TiCl₄Solution (400 g TiCl₄/ l) and keeping the pH constant with 32% sodium hydroxide solution. After reaching the color end point green III. Allocation is terminated, stirring is continued for 15 minutes and then the pigment is filtered off, washed, dried and annealed at 850 ° C. for 30 minutes. The thickness of the TiO₂Layer is approximately 255 nm. The pigment obtained is significantly weaker in the interference color strength than that produced according to the invention. The platelets show severe cracks and flaking in the TiO₂-Layer. Coloristics of the pigments obtained (Hunter - L, ab) using nitrocellulose lacquer cards

0039The NC lacquer cards are produced in such a way that 0.9 g pigment is carefully dispersed in 54.5 g NC lacquer and after degassing the mixture is applied to cardboard with a black or white background on one side using a doctor blade. The wet film thickness is 500 µm. After drying, the coloristic data are measured under the specified geometries.<tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="10"><colspec colnum="1" colname="col1" colwidth="31mm" /><colspec colnum="2" colname="col2" colwidth="13mm" /><colspec colnum="3" colname="col3" colwidth="13mm" /><colspec colnum="4" colname="col4" colwidth="13mm" /><colspec colnum="5" colname="col5" colwidth="14mm" /><colspec colnum="6" colname="col6" colwidth="14mm" /><colspec colnum="7" colname="col7" colwidth="14mm" /><colspec colnum="8" colname="col8" colwidth="13mm" /><colspec colnum="9" colname="col9" colwidth="13mm" /><colspec colnum="10" colname="col10" colwidth="13mm" /><thead valign="top"><row><entry namest="col1" nameend="col10" align="center">geometry</entry></row><row><entry rowsep="0" /><entry namest="col2" nameend="col4" align="left">45 ° / 0 ° Black card</entry><entry namest="col5" nameend="col7" align="left">22.5 ° / 22.5 ° Black card</entry><entry namest="col8" nameend="col10" align="left">45 ° / 0 ° Black card</entry></row><row><entry /><entry>L</entry><entry>a</entry><entry>b</entry><entry>L</entry><entry>a</entry><entry>b</entry><entry>L</entry><entry>a</entry><entry>b</entry></row></thead><tbody><row rowsep="0"><entry>example</entry><entry>28,6</entry><entry>-6,4</entry><entry>-4,0</entry><entry>63,5</entry><entry>-28.3</entry><entry>+5,7</entry><entry>83,0</entry><entry align="char" char="," charoff="42">+11,5</entry><entry align="char" char="," charoff="30">-0,8</entry></row><row><entry>Comparative example</entry><entry>28,4</entry><entry>-5,3</entry><entry>-3,7</entry><entry>55,9</entry><entry>-21.3</entry><entry>+5,9</entry><entry>85,4</entry><entry align="char" char="," charoff="42">+7,9</entry><entry align="char" char="," charoff="30">+0,8</entry></row></tbody></tgroup></table></tables>

0040The advantages of the pigment according to the invention compared to the comparative example are readily apparent:<ul id="ul0001" list-style="none"><li>Color strength, represented by the negative a-value gloss (22.5 ° / 22.5 °): 28.3 vs. 21.3</li><li>Brightness, represented by the C value gloss (22.5 ° / 22.5 °): 63.5 vs. 55.9</li><li>Gloss, represented by the "gloss number"<maths id="math0001"><math display="block"><mfenced><mfrac><mmultiscripts><mrow><mn>22</mn><mo>,</mo><mn>5</mn><mo>⁢</mo><mi mathvariant="normal">°</mi><mo>/</mo><mn>22</mn><mo>,</mo><mn>5</mn><mo>⁢</mo><mi mathvariant="normal">°</mi><mo>-</mo><mmultiscripts><mrow><mn>45</mn><mo>⁢</mo><mi mathvariant="normal">°</mi><mo>/</mo><mn>0</mn><mo>⁢</mo><mi mathvariant="normal">°</mi></mrow><mprescripts /><none /><mi mathvariant="normal">L</mi></mmultiscripts></mrow><mprescripts /><none /><mi mathvariant="normal">L</mi></mmultiscripts><mmultiscripts><mrow><mn>22</mn><mo>,</mo><mn>5</mn><mo>⁢</mo><mi mathvariant="normal">°</mi><mo>/</mo><mn>22</mn><mo>,</mo><mn>5</mn><mo>⁢</mo><mi mathvariant="normal">°</mi></mrow><mprescripts /><none /><mi mathvariant="normal">L</mi></mmultiscripts></mfrac></mfenced><mo>:</mo></math><img file="EP0882099B2_D0001.tif" /></maths>55.0 vs. 49.2</li></ul>