PIGMENTS WITH LIGHT PEARL CONTAINING TITANATE
DESCRIPTION OF? INVENTION The present invention relates to very thin pearlescent pigments based on lamellar-like titanium dioxide, coated with metal oxide. The pigments with pearlescent luster (with luster similar to that of a pearl) containing iron oxide have been described in several instances and have been used successfully for many years. Both pigments have been described in which the iron oxide is precipitated together with the titanium dioxide on the substrate, as pigments in which the precipitations are carried out in succession. During subsequent calcination, the iron oxide diffuses into the TiO2 layer and pseudobroo is formed (Fß2TiOs). Mica is used exclusively as a substrate. U.S. Patent No. 3,087,828 reports that gold-colored mica pigments, which assume a reddish tint upon calcining, are obtained by depositing a layer of Fß2? 3 on a layer of TIO2. U.S. Patent No. 3,874,890 describes a process for the preparation of pearlescent pigments with gold color, in which a pigment
REF: 26377 of mica coated with Ti02 and / or Zr? 2, is first coated with iron hydroxide (II), which is subsequently oxidized to Fe2? 3. US Patent No. 4,744,832 discloses a pearl-luster pigment based on lamella-like substrates, in particular mica, coated with metal oxides, wherein the metal oxide layer comprises both titanium and iron and the pigment has a layered construction multiple, consisting of a layer of pseudobrookite and an iron oxide layer, after a first layer of TiO2 in the rutile form. Mica pigments are widely used in the printing and coating industry, in cosmetics and in plastics processing. These are distinguished by interference colors and a high luster; however, the mica pigments are not suitable for the formation of extremely thin layers, since the mica, as a substrate for the metallic oxide layers of the pigment, already has a thickness of 200 to 1200 nm. Another disadvantage is that the thickness of the mica lamellae within a certain fraction, which is determined by the size of the lamella, sometimes varies significantly around an average value. In addition, mica is a mineral of natural origin that is contaminated by foreign ions. In addition, processing steps are necessary that are very expensive industrially and consume time. These include, above all, crushing and classification (granulometry). The pearlescent pigments which are based on thick mica lamellae and coated with metal oxides, have a significant dispersion value due to the thickness of the edges, especially at fine particle size distributions, below 20 μm. Thin glass slides have been proposed which are obtained by grinding molten glass with the subsequent grinding, as a substitute for mica. Interference pigments based on such materials, in fact show color effects that are superior to those of conventional mica-based pigments. However, a disadvantage is that the glass lamellae have a very large average thickness, of about 10 to 15 μm and a very wide thickness distribution (typically between 4 and 20 μm), while the thickness of the interference pigments is typically is not greater than 3 μm. European Patent EP 0,384,596 describes a process in which hydrated alkali metal silicate is charged with an air jet at temperatures of 400 to 500 ° C, forming bubbles with a low wall thickness. Then, the bubbles are pulverized and lamellar-like alkali metal silicate substrates having a thickness of less than 3 μm are obtained. However, the process is expensive and the thickness distribution of the resulting lamellae is relatively broad. The German Patent DE 1No. 136,042 discloses a continuous band process for the preparation of lamellar-like, spangle-like oxides or oxides, of metals of groups IV and V and of the iron group of the Periodic Table of the Elements. In this process, if appropriate, first a separation layer, for example, of silicone varnish, is applied to a continuous band to facilitate the subsequent detachment of the metal oxide layer. Then a liquid film of a solution of a hydrolyzable compound of the metal to be transformed into the desired oxide is applied, and the film is dried and subsequently peeled off with a vibrating device. The thickness of the layer and the resulting lamellae is established as 0.2 to 2 μm, without giving specific examples of this. European Patents EP 0,240,952 and EP 0,236,952 have proposed a web process for the preparation of various lamellar-like materials, including silicon dioxide, aluminum oxide and titanium dioxide. In this process, a thin liquid film of defined thickness from a precursor of the lamella-like material is applied to a smooth strip by a roller system; the film dries and falls off the band, forming lamella-like particles. Subsequently, the particles are calcined and, if appropriate, crushed and subjected to classification (granulometry). The thickness of the lamellae obtained by the process described in European Patent EP 0,240,952 is relatively well defined, since the film, for example, is applied very uniformly to the web through a roller system. The thickness of the lamella layer is set as 0.3 to 3.0 μm in the examples. In accordance with Example 1, a first roller is wetted with the precursor used by partially immersing this roller in a container tank containing the precursor. The film is transferred from this roller to a second rotating roller in the same direction, which is in very close contact with the first. Finally, the film is applied from the second roller to the web. However, the disadvantages are the use of very expensive precursor materials and, in particular, the higher safety requirements in the workplace, which must be imposed when using organometallic compounds. The complete chemical transformation of the precursor into the desired material of the layer, as a general rule, requires strong heating of the film and the web material. In addition to the considerable thermal stress of the material of the band that occurs in this case, the high energy consumption and the limitation of the speed of the process are also very adverse effects on the profitability of the process. WO 93/08237 discloses lamellar-like pigments comprising a lamella-like matrix of silicon dioxide, which may comprise soluble or insoluble coloring agents and is coated with one or more reflecting layers of metal oxides or metals. The lamella-like matrix is prepared by solidifying water crystal
(sodium silicate) on a continuous band. German Patent DE 1,273,098 describes the preparation of a pearlescent pigment by vapor deposition of films of ZnS, MgF2, ZnO,
CaF2 and TIO2 on a continuous band. However, in a manner similar to the process described in US Pat. No. 4,879,140, in which lamellar-like pigments with Si and SiO2 layers are obtained by plasma phase deposition from SiH4 and
YES, this process is associated with very high equipment costs. Despite numerous attempts, it has not yet been possible to develop an economical process for the preparation of very thin lamella-like titanium dioxide pigments having a coating thickness of less than 500 nm. The object of the present invention is to provide a pearlescent pigment containing titanium, with high luster, having a coating thickness of less than 500 nm and a layer thickness tolerance of less than 10%. This object is achieved, in accordance with the present invention, by a single layer or multilayer pearlescent pigment comprising iron titanate and, if appropriate, titanium oxide and / or iron oxide, which is obtained by the solidification of an aqueous solution of a thermally hydrolysable titanium compound on a continuous strip, the detachment of the formed layer, the coating of the resultant titanium dioxide lamellae, without intermediate drying, with iron oxide in a wet process , and the drying and calcination of the resulting material in an oxidizing or reducing gaseous atmosphere, at a temperature of not less than 700 ° C. The iron titanate comprises either pseudobrookite (Fe2Ti? 5) or ilmenite (FeTi03). The aqueous solution of a thermally hydrolysable titanium compound for the preparation of titanium dioxide lamellae in the continuous web is preferably an aqueous solution of titanium tetrachloride. The concentration of the titanium salt in this solution is from 7 to 30% by weight, preferably from 8 to 15% by weight. The pigments according to the present invention are based on lamellar-like titanium dioxide particles. These lamellae have a thickness between 10 and 500 nm, preferably between 40 and 150 nm. The values of the other two dimensions are between 2 and 200 μm and, in particular, between 5 and 50 μm. The composition and layers constructed of the pigment according to the present invention depend on the thickness of the titanium dioxide lamellae used as the substrate, the layer thickness of the iron oxide and titanium oxide applied, and the calcination conditions. The iron oxide layer and the titanium dioxide layer are preferably applied to the titanium dioxide lamellae by the known wet chemistry processes. In addition, the titanium dioxide lamellae can also be coated by gas phase coating in a fluidized bed reactor, the use of the processes proposed in European Patent EP 0,045,851 and EP 0,106,235 being possible for the preparation of pearlescent pigments. In a first embodiment, the pigment comprises only iron titanate. In a second embodiment, the pigment has two layers, the core being formed of titanium dioxide similar to lamellae and the upper layer formed of iron titanate. In a third embodiment, the pigment has a three layer construction, a titanium dioxide core similar to lamellae, followed by a layer of iron titanate and finally a top layer of iron oxide. In a particular embodiment, this pigment has an additional iron titanate layer and a titanium dioxide layer as the top layer. Furthermore, this objective is achieved, according to the present invention, by a process for the preparation of a pigment according to the invention, the pigment comprising only iron titanate, process in which - an aqueous solution of a thermally hydrolysable titanium compound is applied as a thin film to a continuous band, the thickness of the film being adjusted in such a way that the stoichiometric ratio between the iron and the titanium necessary for the formation of pseudobrookite or ilmenite, the liquid film is solidified by drying, during which titanium dioxide is formed in the solution by a chemical reaction, - subsequently, the formed layer is detached from the band and washed, the dioxide lamellae The resulting titanium are suspended in water, after intermediate drying or without it, and are coated with iron oxide, where the layer thickness is adjusted in such a way as to obtain the stoichiometric ratio between the iron and the titanium necessary for the formation of pseudobrookite or ilmenite, and the coated particles are separated from the aqueous suspension and dried, and calcined in an atmo oxidizing or reducing at a temperature not lower than 500 ° C. In addition, this objective is achieved in accordance with the present invention, by a process for the preparation of a pigment according to the invention, wherein the pigment is formed from a core of lamellar-like titanium dioxide and a top layer of iron titanate, in which an aqueous solution of a thermally hydrolysable titanium compound is applied as a thin film to a continuous band, the thickness of the film being adjusted in such a way that an excess of titanium dioxide is present, for the formation of both ilmenite and pseudobrookite, - the liquid film is solidified by drying, during which titanium dioxide is formed in the. solution by a chemical reaction, subsequently, the formed layer is detached from the band and washed, - the resulting titanium dioxide particles are suspended in water and, after intermediate drying or without it, are coated with iron oxide, being adjust the layer thickness in such a way that an excess of titanium dioxide is present, for the formation of both ilmenite and pseudobrookite, and the coated particles are separated from the aqueous suspension and dried, and calcined in an oxidizing or reducing atmosphere at a temperature not lower than 500 ° C.
In addition, this objective is achieved, according to the present invention, by a process for the preparation of a pigment according to the invention, wherein the pigment has three layers, these being a core of titanium dioxide similar to lamellae, followed by by a layer of iron titanate and finally a top layer of iron oxide, a process in which an aqueous solution of a thermally hydrolysable titanium compound is applied as a thin film to a continuous band, the thickness of the film being adjusted so that a layer thickness of the TiO2 layer is obtained, of at least 40 nm, - the liquid film is solidified by drying, during which the titanium dioxide is formed from the precursor by a chemical reaction, subsequently, the formed layer is detached from the strip and washed, - the resulting titanium dioxide particles are suspended in water and, after intermediate drying or without it, they are coated with iron oxide, where the precipitation of the iron oxide is carried out in such a way that a layer thickness of at least 20 nm is obtained after drying, and the coated particles are separated from the aqueous suspension and dried, and calcined in an oxidizing or reducing atmosphere at a temperature of not less than 500 ° C. In a particular embodiment, the pigment has an additional layer of iron titanate and an upper layer of titanium dioxide, in this embodiment, a layer of titanium dioxide is also precipitated on the iron oxide layer, the precipitation of titanium dioxide in such a way that a layer thickness of at least 40 nm is obtained after drying. The present invention also relates to the use of the pigments according to the present invention, for pigmented coatings, inks for printing, plastics, cosmetics and glazing for ceramics and glass. Mixtures with commercially available pigments can also be used, for example absorption pigments, metallic effect pigments and organic and inorganic LCP-type pigments. The pigments according to the present invention are prepared in a multi-step process.
In the first stage, the substrate is prepared in the form of lamellae of titanium dioxide with the aid of a continuous band. In a second step, iron oxide is precipitated on the substrate, also a layer of titanium dioxide being precipitated on the iron oxide layer, if appropriate. In a conclusion stage, the pigment is calcined under oxidizing or reducing conditions, resulting in the final layer structure of the pigment. The first web process will be explained with the help of Figure 1. The web 1, which is guided over a roller system 2, passes through an applicator 3, where it is coated with a thin film of the precursor. Roller applicators and flow units can be used as suitable applicators. The speed of the band is from 2 to 400 m / minute, preferably from 5 to 200 m / minute. In order to achieve a uniform soaking of the plastic strip, it is necessary to add a commercially available wetting agent to the coating solution or activate the surface of the strip by flaming, corona treatment or ionization. Subsequently, the coated band runs through a drying zone 4, where the layer is dried at a temperature between 30 and 200 ° C. They can be used as dryers, for example, infrared dryers, environmental air jet dryers and UV dryers.
After passing through the drying zone, the web is guided through the release baths 5, with a suitable release medium, for example completely desalinated water, where the dried layer is removed from the web. In this case, the detachment operation is aided by additional devices, for example jets, brushes or ultrasound. The band is dried in a subsequent dryer 6 before renewing the coating. The band continues. It must be made of a chemically stable and heat-resistant plastic, in order to ensure an adequate useful life and high drying temperatures. For this, materials such as polyethylene terephthalate (PET) or other polyesters and polyacrylates are suitable. The width of the film is typically between a few centimeters to several meters. The thickness is from 10 μm to a few mm, these two parameters are optimized with respect to the particular requirements. Additional details of the web processes can be obtained in US Patents 3,138,475;
EP 0,240,952 and WO 93/08237. The lamellae of titanium dioxide detached from the band are coated with iron oxide by the known processes in a second stage of the process, without prior intermediate drying. The raw materials used herein may be iron salts (III), as described, for example, in US Pat. Nos. 3,187,828 and 3,087,829; and iron salts (III) such as those described in US Pat. No. 3,874,890, wherein the initially formed iron hydroxide (II) coating is oxidized to hydrated iron (III) oxide. Preferably iron salts (III) are used as raw material. For this, one. Iron chloride solution (III) is mixed with an aqueous suspension of the titanium dioxide lamellae at a temperature of 60 to 90 ° C and at a pH of 2.5 to 4.5. The pH is kept constant by mixing simultaneously with a 32% sodium hydroxide solution. If appropriate, a layer of hydrated titanium dioxide is also precipitated onto the hydrated iron oxide layer by known processes. The processes described in US Pat. No. 3,553,001 are preferably used. In this process, an aqueous solution of titanium salt is slowly added to a suspension, heated to a temperature of about 50 to 100 ° C, preferably 70 to 80 ° C, of the titanium dioxide flakes coated with anhydrous oxide. Hydrated iron and the pH is maintained constant at a value of about 0.5 to 5, preferably about 1.5 to 2.5, while simultaneously adding a base, such as for example an aqueous solution of ammonia or an aqueous solution of an alkali metal hydroxide. As soon as the desired thickness of the TiO2 precipitate layer is reached, the addition of the titanium salt solution is suspended. This process, which is also called the titration process, has the characteristic that an excess of titanium salt is avoided. This is achieved by a process in which only a quantity, per unit of time, is added to the hydrolysis that is necessary for a uniform coating with the hydrated TIO2 and such that it can be absorbed, per unit of time, by the available surface. of the particles to be coated. Therefore, particles of hydrated titanium dioxide do not form which do not precipitate on the surface to be coated. The amount of titanium salt added per minute, in this case, is of the order of about 0.01 to 2 x 10 mol of titanium salt per square meter of surface to be coated. In a third stage of the process, the titanium dioxide flakes coated with hydrous iron oxide and, if appropriate, additionally with titanium oxide hydrate, are calcined at a temperature of 500 to 950 ° C, preferably 800 to 900 ° C. , under oxidizing or reducing conditions, after being separated from the suspension, washed and dried. Under these conditions, the iron diffuses into the layer of TÍO2, forming pseudobrookita in the presence of oxygen; and ilmenite being formed in the presence of a reducing agent, for example hydrogen. With a suitable selection of the layer thickness of the iron oxide and titanium dioxide, a pigment is obtained which exclusively comprises iron titanate. This is the case if the stoichiometric relationship between iron and titanium necessary for the formation of pseudobrookite or ilmenite is established. On the other hand, if the iron oxide content is below the stoichiometric ratio, a pigment having a core of titanium dioxide similar to lamellae and a top layer of iron titanate is obtained. If you want a 3-layer construction, Ti? 2 / iron titanate / F? 2? 3 or a 5-layer construction, Ti? 2 / iron titanate / F? 2? 3 / iron titanate / Ti? 2 of the compliance pigment with the present invention, both the layer of titanium dioxide lamellae and the layer of hydrated iron oxide precipitated thereon as the layer of titanium dioxide hydrate also precipitated thereon, if appropriate, they should have a minimum thickness. A layer thickness of the titanium dioxide hydrate layer or the titanium dioxide lamellae, from 40 to 200 nm and, in particular, from 40 to 150 nm, is preferred. However, in particular, the layer thickness of the precipitated Fß2? 3 layer is also essential for the construction of 3 or 5 layers. This must be large enough, in each case, so that, after calcining and forming an intermediate layer of pseudobrookite or ilmenite, which is carried out during this operation, a layer of pure Fe2? 3 is also present when less about 15 nm and preferably layers with a thickness of about 15 to about 50 nm, in particular those of about 20 to about 40 nm. Additional information on the preparation of a multilayer pigment can be found in U.S. Patent No. US 4,744,832. In addition, it is possible to subject the pigments to a subsequent coating or to a subsequent treatment which additionally increase the stability to light, to the weather and to chemical products, or which facilitate the handling of pigment, in particular the incorporation to various media. The possible subsequent coating or after-treatment processes are, for example, those described in German Patents DE-C / A 2,215,191; DE-C / A 3,151,354; DE-C / A 3,235,017 or DE-OS 3,334,598. Due to the fact that the properties of the pigments according to the present invention are already very good without these additional measures, when these substances are applied, if necessary, they will constitute only from about 0 to 5% in particular from 0 to 3% by weight of the total pigment. The pigment according to the present invention can also be additionally coated with small amounts of soluble, firmly adherent, organic or inorganic coloring agents. Preferably colored lacquers and in particular colored aluminum lacquers are used. For this, a layer of aluminum hydroxide is precipitated and lacquered with a colored lacquer in a second step. The process is described in greater detail in German Patents DE 2,429,762 and DE 2,928,287. Additional coatings with complex salt pigments, in particular cyanoferrate complexes, such as for example Prussian blue or Turnbull blue, are also preferred, such as those described in EP 0,141,173 and DE 2,313,332. The pigment according to the present invention can also be coated with organic dyes and in particular with phthalocyanine or metal phthalocyanine and / or indanthrene dyes, in accordance with German Patent DE 4,009,567. For this, a suspension of the pigment is prepared in a solution of the dye and subsequently mixed with a solvent, in which the dye is sparingly soluble or insoluble. They can also be used for additional coatings, metal chalcogens or metal hydrocarboxes and carbon black. The pigment according to the present invention represents the maximum ideal state that can be achieved in pigments with pearlescent luster, with respect to thickness, since it comprises only optically functional layers and customary materials, such as for example, mica or lamellae, are absent. of glass, which do not contribute to the optical effect. As a result of the thickness of the mica, the mica particles have a thickness that is greater by a factor of up to 25 for the same thickness of the functional layers. As a result of this, intrinsic advantages are obtained with respect to industrial applications that can not be achieved with any other pigment with conventional pearl luster. For example, coatings can be applied in thinner layers and the amount of pigment needed can be reduced, since the pigments are more optically active due to the absence of "filler" vehicle materials. The Examples described below are intended to illustrate the present invention, without limiting it. EXAMPLE 1 15 g of TiO2 lamellae (layer thickness of approximately 60 nm) were suspended in 2000 ml of completely deionized water (CDS water) and the suspension was heated to 75 ° C. The pH of the suspension was adjusted to 3.0 with 10% hydrochloric acid. Then 610 g of an aqueous solution of 10% FeCl 3 were added at 75 ° C over the course of 4 hours, keeping the pH constant by the simultaneous addition of a 32% sodium hydroxide solution. To complete the precipitation, the mixture was subsequently stirred at 75 ° C for an additional 45 minutes. Subsequently, the mixture was allowed to cool to room temperature and the resulting red-brown pigment was filtered, washed with CDS water until free of salts and dried at 100 ° C. Finally, it was calcined at 850 ° C for 45 minutes. A pigment with copper-red pearl luster of pseudobrookite with a golden interference color was obtained. EXAMPLE 2 38 g of TiO2 slides were suspended in 2000 ml of CDS water and the suspension was heated to 75 ° C. The pH of the suspension was adjusted to 3.0 with 10% hydrochloric acid. 770 g of an aqueous solution of 10% FeCl 3 were added at 75 ° C over the course of 5 hours, while keeping the pH constant by the simultaneous addition of a 32% sodium hydroxide solution. To complete the precipitation, the mixture was subsequently stirred at 75 ° C for an additional 45 minutes. Subsequently, the mixture was allowed to cool to room temperature and the resulting pigment was filtered, washed with CDS water until free of salts and dried at 100 ° C. Finally, it was calcined at 850 ° C for 45 minutes. A gold-colored pearl luster pigment was obtained, which comprises 25% titanium dioxide and 75% pseudobrookite. EXAMPLE 3 20 g of the dried pigment prepared in Example 2 were calcined in a tubular oven under a gaseous atmosphere (N2 / H2 = 95/5) at 750 ° C for 3 hours.
A lustrous blue-black pigment with an ilmenite structure was obtained. EXAMPLE 4 The desiccated cake prepared in Example 2 was calcined in air at a temperature of not less than 700 ° C. A red-brown pearl-luster pigment was obtained, which has a copper-colored interference color and comprises a core of titanium dioxide, a layer of pseudobrookite and an outer layer of hematite. EXAMPLE 5 30 g of TiO2 lamellae were suspended in
2000 ml of CDS water and the suspension was heated to 75 ° C. The pH of the suspension was adjusted to 3.0 with 10% hydrochloric acid. 815 g of an aqueous solution of 10% FeCl 3 were added at 75 ° C over the course of 5 hours, while keeping the pH constant by the simultaneous addition of a 32% sodium hydroxide solution. To complete the precipitation, the mixture was subsequently stirred at 75 ° C for an additional 30 minutes. Then, 120 ml of an aqueous solution of TÍCI4 (400 g of TÍCI4 / I) was added in an oven, in a period of 60 minutes. The pH was kept constant at 2.2 with a 32% NaOH solution throughout the addition. When the addition was complete, the mixture was subsequently stirred at 75 ° C for 30 minutes, in order to complete the precipitation. Subsequently, the mixture was allowed to cool to room temperature and the resulting pigment was filtered, washed with CDS water until free of salts and dried at 110 ° C. Finally, it was calcined at 850 ° C for 30 minutes. A gold-colored pearlescent pigment was obtained, which has a reddish interference color and comprises a core of titanium dioxide, a layer of pseudobrookite and an outer layer of titanium dioxide. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as an antecedent, what is contained in the following is claimed as property.