WO1998053010A1

WO1998053010A1 - Metal oxide coated titanium dioxide lamellas

Info

Publication number: WO1998053010A1
Application number: PCT/EP1997/002650
Authority: WO
Inventors: Stephanie Andes; Gerd Bauer; Günter BRENNER; Dieter BRÜCKNER; Michael Schmelz; Andrea Heyland; Matthias Kuntz; Karl Osterried; Gerhard Pfaff
Original assignee: Merck Patent Gmbh
Priority date: 1996-05-09
Filing date: 1997-05-23
Publication date: 1998-11-26
Also published as: DE19618568A1

Abstract

The invention relates to a pearly lustre pigment, consisting of a titanium oxide lamella-shaped core and one or several coatings of different metal oxides or metal oxide hydrates, which can be obtained by hardening an aqueous solution of a titanium compound that can be thermally hydrolyzed on a continuous strip, by separating the layer thus arising, by coating the titanium dioxide lamellas thus obtained with one or several metal oxide or metal oxide hydrates, e.g. Fe2O3, Fe3O4, FeOOH or Cr2O3 after or without intermediate drying in a wet process and by separating, drying and optionally calcinating the material thus obtained.

Description

Metal oxide coated titanium dioxide platelets

The invention relates to very thin pearlescent pigments based on platelet-shaped titanium dioxide coated with metal oxide or coated with metal oxide hydrate.

Pearlescent pigments based on Giimmer, which have further metal oxide layers on a titanium dioxide layer, are known. In US 3087 828 and 3 087 829 aluminum oxide, zirconium oxide, zinc oxide and tin oxide are mentioned as colorless oxides for a second metal oxide layer and iron oxide, nickel oxide, cobalt oxide, copper oxide and chromium oxide are mentioned as oxides which have their own color. The deposition of a second metal oxide on the titanium dioxide hydrate layer leads to a clear stabilization with regard to the light sensitivity of this layer. Titanium dioxide pigments containing iron oxide in particular have been used successfully for many years.

In US 30 87 828 it is described that by depositing an Fe ₂ 0 ₃ layer on a TiO ₂ layer, gold-colored mica pigments are obtained which take on a reddish color when annealed.

US Pat. No. 3,874,890 describes a process for the production of gold-colored pearlescent pigments, in which a mica pigment coated with Ti0 ₂ and / or Zr0 _{2 is first} coated with iron (II) hydroxide, which is then oxidized to Fe ₂ 0 ₃ .

US 4744832 describes a pearlescent pigment based on platelet-shaped substrates coated with metal oxides, in particular mica, the metal oxide layer containing both titanium and iron and the pigment having a multilayer structure, with a first

Layer of Ti0 ₂ in the rutile form is followed by a layer of pseudobrookite and an iron oxide layer. Mica pigments are widely used in the printing and coatings industry, in cosmetics and in plastics processing. They are characterized by interference colors and a high gloss. However, mica pigments are not suitable for the formation of extremely thin layers because mica as a substrate for the metal oxide layers of the pigment already has a thickness of 200 to 1200 nm. A further disadvantage is that the thickness of the mica platelets fluctuates significantly around an average value within a certain fraction, which is determined by the platelet size. In addition, mica is a naturally occurring mineral that is contaminated by foreign ions. Furthermore, technically very complex and time-consuming preparation steps are necessary. Above all, this includes grinding and classifying.

Pearlescent pigments based on thick mica platelets and coated with metal oxides have a clear scattering component due to the thickness of the edge, especially with finer grain size distributions below 20 μm.

Thin glass plates, which are obtained by rolling a glass melt with subsequent grinding, have been proposed as a replacement for mica. Interference pigments based on such materials have color effects which are superior to those of conventional mica-based pigments. However, it is disadvantageous that the glass flakes have a very large average thickness of approximately 10-15 μm and a very wide distribution of thicknesses (typically between 4 and 20 μm), while the thickness of interference pigments is typically not greater than 3 μm.

EP 0,384,596 describes a process in which hydrated alkali silicate is subjected to an air jet at temperatures of 480-500 ° C., whereby bubbles with thin walls form; the bubbles are then crushed and you get platelet-shaped

Alkaline silicate substrates with a thickness of less than 3 μm. However, the process is complex and the thickness distribution of the platelets obtained is relatively broad. DE 11 36 042 describes a continuous belt process for the production of platelet-like or flake-like oxides or oxide hydrates of metals of the IV and V groups and of the iron group of the periodic table. In this case, if necessary, a separating layer of, for example, silicone varnish is first applied to a continuous belt in order to later detach the

Lighten metal oxide layer. Then a liquid film is applied from a solution of a hydrolyzable compound of the metal to be converted into the desired oxide, the film is dried and then detached with a vibrating device. The layer thickness of the platelets obtained is given as 0.2 to 2 μm, without giving specific examples.

EP 0.240.952 and EP 0.236.952 have proposed a continuous belt process for the production of various platelet-shaped materials, including silicon dioxide, aluminum oxide and titanium dioxide. A thin liquid film of a defined thickness of a precursor of the platelet-shaped material is applied to a smooth belt using a roller system; the film is dried and peeled off the tape, forming platelet-shaped particles. The particles are then optionally annealed, ground and classified.

The thickness of the platelets obtained by the process described in EP 0 240 952 is relatively well defined, since the film is applied very uniformly, for example, to the continuous belt using a roller system. The layer thickness of the platelets is given in the examples as 0.3 to 3.0 μm. According to Example 1, a first roll is wetted with the precursor used by partially immersing this roll in a storage container filled with the precursor. The film is transferred from this reel to a second reel rotating in the same direction, which is in very close contact with the first. Finally, the film is unwound from the second roll onto the continuous belt. However, the use of very expensive precursor materials and, in particular, the increased demands on job security that have to be made when using organometallic compounds are disadvantageous. The complete chemical conversion of the precursor into the desired layer material generally requires a strong heating of the film and the strip material. In addition to the considerable thermal load on the strip material, the high energy consumption and the limitation of the process speed have a very disadvantageous effect on the economy of the process.

WO 93/08 237 describes platelet-shaped pigments, consisting of a platelet-shaped matrix of silicon dioxide, which can contain soluble or insoluble colorants and which is coated with one or more reflective layers of metal oxides or metals. The platelet-shaped matrix is produced by solidifying water glass on an endless belt.

DE 1 273 098 describes the production of a mother-of-pearl pigment by vapor deposition of ZnS, MgF2, ZnO, CaF2 and TiO2 films on an endless belt. However, this process, like the process described in US Pat. No. 4,879,140, in which platelet-shaped pigments with Si and SiO 2 layers are obtained by plasma deposition from SiH4 and SiCl4, is associated with a very high outlay on equipment.

Despite numerous attempts, it has not yet been possible to develop an economical process for the production of very thin platelet-shaped titanium dioxide pigments with a layer thickness of less than 500 nm.

The object of the invention is to provide high-gloss titanium dioxide-containing pearlescent pigments with a layer thickness of less than 500 nm and a layer thickness tolerance of less than 10%. This object is achieved according to the invention by a pearlescent pigment with a multilayer structure, a layer of another metal oxide or metal oxide hydrate being followed by a core of platelet-shaped titanium dioxide, obtainable by solidifying an aqueous solution of a thermally hydrolyzable titanium compound on an endless belt, detaching the resulting layer, coating of the titanium dioxide platelets obtained without intermediate drying in the wet process with another metal oxide, separation, drying and optionally calcining the material obtained.

The other metal oxide or metal oxide hydrate that is applied to the titanium dioxide is Fe ₂ 0 ₃ , Fe ₃ 0 ₄ , FeOOH, Cr ₂ 0 ₃ , CuO, Ce ₂ 0 ₃₎ Al ₂ 0 ₃ , Si0 ₂ , B.VO ₄ , NiTi0 ₃ , CoTi0 ₃ and antimony-doped, fluorine-doped or indium-doped tin oxide.

In a special embodiment of the pigment according to the invention, a third layer of a further metal oxide or metal oxide hydrate is present on the second layer made of another metal oxide or metal oxide hydrate. This further metal oxide or metal oxide hydrate is aluminum oxide or aluminum oxide hydrate, silicon dioxide or silicon dioxide hydrate, Fe ₂ 0 ₃ , Fe ₃ θ ₄ , FeOOH, Zr0 _2l Cr ₂ 0 ₃ and antimony-doped, fluorine-doped or indium-doped tin oxide.

The aqueous solution of a thermally hydrolyzable titanium compound for the production of the titanium dioxide flakes on the endless belt is preferably an aqueous titanium tetrachloride solution.

The concentration of the titanium salt in these solutions is 7 to 30% by weight, preferably 8 to 15% by weight.

Furthermore, this object is achieved according to the invention by a method for producing the pigments according to the invention by an aqueous solution of a thermally hydrolyzable titanium compound is applied as a thin film to an endless belt,

the liquid film is solidified by drying and the titanium dioxide is developed from the precursor by a chemical reaction,

the resulting layer is then removed from the belt and washed,

the titanium dioxide platelets obtained are suspended in water after or without intermediate drying and are coated with another metal oxide and optionally with further metal oxides,

- The coated titanium dioxide platelets are separated from the aqueous suspension, dried and optionally calcined.

After intermediate drying, the coating of the titanium dioxide platelets with metal oxides or metal oxide hydrates can also be e.g. in a fluidized bed reactor by gas phase coating, e.g. the methods proposed in EP 0.045.851 and EP 0.106.235 for the production of pearlescent pigments can be used accordingly.

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

Mixtures with commercially available pigments, for example inorganic and organic absorption pigments, metallic effect pigments and LCP pigments, can also be used for this purpose. The pigments according to the invention are based on platelet-shaped titanium dioxide. These platelets have a thickness between 10 nm and 500 nm, preferably between 40 and 150 nm. The extent in the other two dimensions is between 2 and 200 μm and in particular between 5 and 50 μm.

The layer of another metal oxide which is applied to the titanium dioxide platelets has a thickness of 5 to 300 nm, preferably between 5 and 150 nm.

The pigments according to the invention are produced in a three-stage process. In the first stage, platelet-shaped titanium dioxide particles are produced using an endless belt.

First of all, the belt process will be explained with reference to FIG. 1.

The endless belt 1, which is guided over a roller system 2, passes through an application unit 3, where it is coated with a thin film of the precursor. Roller applicators and flowers can be used as suitable applicators. The belt speed is between 2 and 400 m / min, preferably 5-200 m / min.

In order to achieve uniform wetting of the plastic tape, it is advisable to add a commercially available wetting agent to the coating solution or to activate the tape surface by flame treatment, corona treatment or ionization.

The coated strip then runs through a drying section 4 in which the layer is dried at temperatures between 30 and 200 ° C. For example, commercially available infrared, circulating air and UV dryers can be used as dryers. After passing the drying section, the tape is passed through the release baths 5 with a suitable release medium, for example deionized water, where the dried layer is removed from the tape. The detachment process is supported by additional devices, for example nozzles, brushes or ultrasound.

The tape is dried in a post-dryer 6 before it is re-coated.

The endless belt should be made of a chemically and thermally resistant plastic to ensure a sufficient service life and high drying temperatures. Materials such as polyethylene terephthalate (PET) or other polyesters and polyacrylates are suitable for this.

The film width is typically between a few centimeters up to several meters. The thickness is between 10 μm and a few mm, whereby these two parameters are optimized with regard to the respective requirements.

Further details on continuous belt processes are known from US 3 138475, EP 0 240 952 and WO 93/08237.

The titanium dioxide flakes detached from the tape are coated in a second process step without prior drying with another metal oxide by a known method.

When coating with hematite (Fe ₂ 0 ₃ ), iron (III) salts can be used as described, for example, in US Pat. No. 3,987,828 and US Pat. No. 3,087,829, and iron (II) - salt, as described in US Pat. No. 3,874,890, the initially formed coating of iron (II) hydroxide being oxidized to the iron (III) oxide hydrate. The preferred starting point is iron (III) salts. For this purpose, an aqueous suspension of the titanium dioxide platelets at a temperature of 60 to 90 ° C and an iron (III) chloride solution is metered in at a pH of 2.5 to 4.5. The pH value is kept constant by simultaneous dosing of 32% sodium hydroxide solution. After working up and drying at 110 ° C., a red pigment is obtained.

The coating with magnetite (Fe ₃ θ ₄ ) is carried out by hydrolysis of an iron (II) salt solution, for example iron (II) sulfate, at a pH of 8.0 in the presence of potassium nitrate. The exact precipitation examples are described in EP 0 659 843. A black pigment is obtained after working up and drying at 100 ° C.

A coating of the titanium dioxide platelets with yellow FeO (OH) in the goethite modification is also preferred. An aqueous FeS0 ₄ solution is metered into a suspension of the titanium dioxide platelets in a nitrogen atmosphere at 70 ° C. Then the pH is adjusted to 4 with a 10% Na ₂ CO ₃ solution and oxygen is introduced into the suspension. After working up and drying at 110 ° C., a yellow pigment is obtained. With regard to the exact precipitation conditions, reference is made to EP 0 659 843.

For better adhesion of the iron oxide layers on the titanium dioxide platelets, it is advisable to apply a tin oxide layer first. An SnCU solution mixed with hydrochloric acid is added to the suspension of the titanium dioxide platelets, which is adjusted to a pH value of 1.8, and the pH value is kept constant by simultaneous dosing of 32% sodium hydroxide solution. After the tin dioxide hydrate layer has been noticed, the coating with an iron oxide hydrate can then be connected immediately.

Another metal oxide that is preferentially struck on the titanium dioxide platelets is chromium oxide. The deposition can easily be effected by the thermal hydrolysis which occurs when ammonia evaporates from an aqueous solution of a hexamine-chromium (III) derivative, or by thermal hydrolysis of a chromium salt solution which is buffered with borax. The coating with chromium oxide is described in US 3087828 and 3,087,829.

The pigments according to the invention do not have to be calcined in every case. Drying at temperatures of 110 ° C is sufficient for certain applications. If the pigment is calcined, temperatures between 400 ° C and 1000 ° C are set, the preferred range being between 400 ° C and 700 ° C.

The pigment of the invention can additionally be coated with sparingly soluble, firmly adhering inorganic or organic colorants. Colored lacquers and in particular aluminum colored lacquers are preferably used. For this purpose, an aluminum hydroxide layer is struck, which is then varnished with a colored varnish. The process is described in more detail in DE 24 29 762 and DE 29 28 287.

An additional coating with complex salt pigments, in particular cyanoferrate complexes, such as for example Berliner Blau and Turnbulls Blau, as described in EP 0 141 173 and DE 23 13 332, is also preferred.

The pigment according to the invention can also be coated with organic dyes and in particular with phthalocyanine or metal phthalocyanine and / or indanthrene dyes according to DE 40 09 567. 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.

Metal chalcogenides or metal chalcogenide hydrates and carbon black can also be used for an additional coating. It is also possible to subject the pigments to a post-coating or post-treatment which further increases the light, weather and chemical stability, or to facilitate the handling of the pigment, in particular the incorporation into different media. The following layers are, for example, those described in DE-PS 22 15 191, DE-

OS 31 51 354, DE-OS 3235017 or DE-OS 3334598 methods described in question. Owing to the very good properties of the pigments according to the invention even without these additional measures, these substances which may still be applied make up only about 0 to 5, in particular about 0 to 3% by weight of the total pigment.

With regard to the thickness, the pigments according to the invention represent the ideal state which can be attained as far as possible with pearlescent pigments, since they consist only of optically functional layers and an otherwise customary carrier material, such as mica or glass plates, which does not contribute to the optical effect, is missing. Due to the thickness of the mica, mica pigments are up to 25 times thicker with the same thickness of the functional layers. With regard to technical applications, this results in intrinsic advantages that no other conventional pearlescent pigment can achieve. For example, lacquer layers can be made thinner and the necessary amount of pigment can be reduced because the pigments are more optically active due to the lack of the “filler” carrier material.

The following examples are intended to illustrate the invention without limiting it.

example 1

A circumferential belt made of polyethylene terephthalate (width: 0.3 m, speed: 20 m / min) is coated with a 20% titanium tetrachloride solution in counter-rotation on an application roller. The coating solution contains 0.3% by weight of surfactant (DISPERSE-AYD W-28, manufacturer: DANIEL PRODUCTS COMPANY). The aqueous film on the belt is dried in a drying section by exposure to hot air at 70 ° C. and the layer formed is detached from the belt in a detachment basin filled with deionized water. The platelet-shaped titanium dioxide particles are filtered and washed with deionized water. The silvery, shiny platelets have a layer thickness of 100 ± 10 nm. To coat them with another metal oxide, they are redispersed in deionized water.

2 l of the dispersion (dry matter content: 15 g TiO 2) are heated to 75 ° C. and adjusted to a pH of 1.8 with dilute hydrochloric acid.

4.3 g of SnCl4 • 5 H2O are dissolved in 29 ml of HCl, made up to 290 ml with distilled water and stirred for 10 min.

The Snd4 solution is added to the TiO 2 suspension at 3 ml / min and the pH 1.8 is kept constant with 32% NaOH solution. After the SnO 2 coating, the suspension is stirred for 15 minutes at constant temperature and pH.

The pH is then adjusted to 3.0 with 32% NaOH solution and an aqueous 8% iron (III) chloride solution is metered in at a rate of 3 ml / min, the pH being increased by simultaneous addition of the NaOH Solution is kept constant. The FeCI ₃ addition is continued until the desired interference color is reached. The pigment obtained is filtered off, washed with deionized water and dried at 110 ° C. for 12 hours. The pigment has the desired interference color and a red body color.

Claims

claims

1. Colored pearlescent pigment, consisting of a core of platelet-shaped titanium dioxide and one or more layers of other metal oxides or metal oxide hydrates, obtainable by solidifying an aqueous solution of a thermally hydrolyzable titanium compound on an endless belt, detaching the resulting layer, coating the titanium dioxide platelets obtained after or without intermediate drying in the wet process with one or more other metal oxides or metal oxide hydrates, separation,

Drying and optionally calcination of the material obtained.

2. Pearlescent pigment according to claim 1, characterized in that the other metal oxide is Fe ₂ 0 ₃ , Fe ₃ θ ₄ , FeOOH or Cr ₂ 0 ₃ .

3. A process for the preparation of the pearlescent pigment according to claims 1 to 2, characterized in that

an aqueous solution of a thermally hydrolyzable titanium compound is applied as a thin film to an endless belt,

the liquid film is solidified by drying and the titanium dioxide is developed from the solution by a chemical reaction,

the resulting layer is then removed from the belt and washed,

the titanium dioxide platelets obtained are suspended in water after or without intermediate drying and coated with one or more metal oxides, the coated titanium dioxide platelets are separated from the aqueous suspension, dried and optionally calcined.

4. The method according to claim 3, characterized in that the aqueous solution of a thermally hydrolyzable titanium compound is an aqueous titanium tetrachloride solution.

5. Process according to claims 3 and 4, characterized in that Fe ₂ 0 ₃ , Fe ₃ θ ₄ , FeOOH and Cr ₂ 0 ₃ are used as other metal oxides.

6. The method according to at least one of claims 3 to 5, characterized in that the pigment is dried at temperatures of 110 to 150 ° C.

7. The method according to at least one of claims 3 to 6, characterized in that the pigment is calcined at temperatures from 400 to 700 ° C.

8. The method according to at least one of claims 3 to 7, characterized in that the other metal oxide or further metal oxides are applied in a fluidized bed reactor by CVD to the titanium dioxide platelets.

9. Use of the pigments according to claims 1 and 2 for pigmenting paints, printing inks, plastics, cosmetics and glazes for ceramics and glasses.

10. Use according to claim 9, characterized in that the

Pigments can be used as mixtures with commercially available pigments.

11. Lacquers, printing inks, plastics and cosmetics which are pigmented with a pigment according to claims 1 and 2.