Patent Application
20250075080
The present invention relates to the production of flake-form, crystalline particles having an aurivillius structure, to the use thereof as effect pigments or as substrates of effect pigments, furthermore to effect pigments which contain coated or uncoated flake-form, crystalline particles having an aurivillius structure as substrates, and to the use thereof.
Crystals of the so-called aurivillius phases grow in flake form owing to the particular layer structure. Some of these compounds additionally have a high refractive index n. The most important representative of this class of substances is bismuth titanate Bi4Ti3O12. Further examples are Bi5Ti3FeO15 and Bi2WO5.
The aurivillius phase denotes a form of perovskite of the generic formula (Bi2O2)(An−1BnO3n+1), where A denotes a large, twelve-fold-coordinated cation and B denotes a small, six-fold-coordinated cation. The structure of aurivillius phases is basically formed by alternating layers of [Bi2O2]2+ and pseudo-perovskite blocks, where the perovskite layers have n octahedrally coordinated layers.
Bi4Ti3O12 and other members of the aurivillius series have been of interest in materials research for some years owing to their particular electrical properties, since they are ferroelectric and piezoelectric. They are consequently in principle suitable for the production of electronic components, such as capacitors or sensors. In addition, they have catalytic properties.
The preferred route for the production of BIT particles starts from bismuth oxide, Bi2O3, and titanium dioxide, TiO2. These two starting compounds are typically mixed with salts, such as halides or sulfates of alkali metals, e.g. KCl, NaCl, KBr, NaBr, Na2SO4, K2SO4 or mixtures thereof and heated at temperatures of 800-950° C. for several hours.
According to the prior art, on use of the chloride salt matrix small BIT flakes having a diameter of up to about 15 μm form, while on use of sulfates very small flakes and/or particles of undefined shape are obtained (<3 μm). However, such particles are too small and, with respect to the shape, not suitable for generating interesting optical effects.
However, the advantage of the chloride matrix for relatively large particles is countered by a high chemical load of the production plants, since alkali metal chlorides have a high evaporation rate at the high process temperatures that are necessary. The chloride vapours have a very strongly corrosive action on the production plant, meaning that economic operation is highly doubtful. In addition, the reproducibility in relation to the size and thickness of the crystal flakes is unsatisfactory according to the prior art.
There is therefore a need for a production process which does not have the disadvantages of the prior art, and in particular enables the production of relatively large particles without an excessive load on the production plant.
Transparent flake-form materials having a high refractive index n have already been utilised for some time as effect pigments, e.g. basic lead carbonate (n=2.0) and bismuth oxychloride (n=2.15). Whereas basic lead carbonate is no longer used owing to its toxicity, bismuth oxychloride is widely used, in particular as cosmetic colourant.
However, it is desirable to have further effect pigments available.
The object of the invention is to provide a process for the production of crystalline particles having an aurivillius structure and novel effect pigments having flake-form substrates.
This object is achieved in accordance with the invention by a process as described and claimed below, in particular having the features of Claims 1-11, and by effect pigments having the features of Claims 12-16.
In particular, the present application relates to a process for the preparation of crystalline compounds having an aurivillius structure, in which the starting materials, preferably a bismuth oxide and a titanium oxide, and optionally a further oxide, in particular a transition-metal oxide, particularly preferably an iron oxide, are mixed with a salt mixture comprising NaCl and Na2SO4 and preferably one or more assistants to give a homogenised salt matrix/oxide mixture and are heated to 800-1200° C.
In the process according to the invention, Bi203, TiO2 and optionally a further oxide, in particular a transition-metal oxide, particularly preferably an iron oxide, are preferably mixed with a salt mixture comprising NaCl and Na2SO4 and preferably one or more assistants to give a homogenised salt matrix/oxide mixture and heated at 800-1200° C.
Surprisingly, it has been found that the process in accordance with the present application enables large bismuth titanate flakes to be obtained. The starting materials are reacted in a salt matrix mixture of NaCl and Na2SO4. The mass ratio of NaCl to Na2SO4 is preferably 55-70:45-30, particularly preferably 55-65:45-35. A eutectic mixture of NaCl and Na2SO4 is particularly preferably employed.
The salt mixture preferably comprises an assistant, preferably in an amount of 1-10 mol %, in particular of 1-6 mol %, e.g. 1-2 mol % or 5 mol %, based on the amount of TiO2 employed. Preferred assistants are Na2CO3, K2CO3, CaCO3, CaO, CaCl2, Na4P2O7 and KBr, particularly preferably Na2CO3, Na4P2O7 and KBr. A large part of the chlorides in these mixtures has been replaced by sodium sulfate. This significantly lowers the vapour pressure of the chlorides. This can easily be recognised from the amount of NaCl precipitated on the crucible lid.
In addition, an unexpectedly large reduction in the vapour pressure of the NaCl by the admixing of the Na2SO4 can be observed. A greater corrosion-reducing effect by the Na2SO4 fraction can consequently be assumed than would be expected from the pure amount fraction.
The reaction can basically take place at temperatures of 800-1200° C., preferably of 700-1100° C., particularly preferably of 800-1050° C. The reactants are usually heated at these temperatures for several hours, preferably 2-10 h.
In a preferred embodiment, the process according to the invention comprises the following steps:
1. Heating of the starting materials with the salt mixture and the optional assistants from room temperature to a temperature of 800-1200° C., preferably of 700-1100° C., particularly preferably of 800-1050° C., preferably over the course of 3-4 h.
2. Holding of the reaction mixture at the temperature indicated in step 1, preferably for 2-6 h.
3. Cooling of the reaction mixture to room temperature, preferably over the course of >7 h, e.g. by natural cooling.
As starting materials, the corresponding oxides on which the desired products are based are mixed with the salt matrix mixture and the assistants and thermally treated as described. For example, Bi203 and TiO2 are employed for the preparation of Bi4Ti3O12 and Bi203, TiO2 and Fe2O3 are used for the preparation of Bi5Ti3FeO15.
The preparation is preferably carried out in suitable ceramic crucibles, e.g. made from Al2O3. After the heating and cooling, the particles obtained can be detached from the crucible, e.g. with water, filtered off, washed with water and dried.
In a variant of the process according to the invention, moulded pieces having a high oxide content of Bi2O3 and TiO2, preferably 7-50% by weight, particularly preferably 7-14% by weight, are pressed from a homogenised salt matrix/oxide mixture. Preferably, these are then calcined resting on corundum beads in ceramic crucibles. Due to the high product content, the pressed pieces retain their shape and the hot salts do not touch the crucible.
This process variant avoids the problems of the production processes known to date, such as the low stability of available crucible materials to the hot salt melt containing bismuth oxide. Bi2O3 penetrates through virtually all oxides known for crucibles and very greatly shortens their service life by causing fissures and cracks. In addition, bismuth is a known “platinum poison”, which means that using platinum crucibles also cannot come into consideration for commercial production.
The comments made here for Bi4Ti3O12 also apply in a comparable manner to other compounds of the aurivillius type (see Dalton Trans., 2015, 44, 20568) having the general formula [Bi2O2]2+[An−1BnO3n+1]2−, where A denotes a large, twelve-fold-coordinated cation and B denotes a small, six-fold-coordinated cation. These compounds are built up from alternating layers of a perovskite-like block having n layers (n=1−∞) and B cations coordinated octahedrally by oxide and layers of (Bi2O2)2+ with oxide in a quadratic planar arrangement with Bi3+ directed alternately upward and downward.
The invention furthermore relates to effect pigments which comprise, as substrates, coated or uncoated flake-form, crystalline particles having an aurivillius structure, in particular in which the flake-form, crystalline particles having an aurivillius structure are produced by a process as described above and below.
Above and below, the term “substrate(s)” stands, unless indicated otherwise, for the flake-form, crystalline particles having an aurivillius structure.
Preferred substrates are titanates, in particular Bi4Ti3O12 or Bi5Ti3FeO15, in particular Bi4Ti3O12.
The flake-form particles of the invention can be obtainable and used with various optical properties, from matt to glossy and from transparent to opaque.
In general, the flake-form particles have a thickness between 0.02 and 5 μm, in particular between 0.05 and 4.5 μm, preferably between 0.05 and 2 μm, particularly preferably between 0.05 and 1 μm, very particularly preferably between 0.1 and 0.5 μm.
The size of the flake-form particles is not crucial per se and can be matched to the respective application. The particle size is usually 0.5-50 μm, preferably 1-35 μm, particularly preferably 2-25 μm. It is preferably also possible to employ substrate mixtures consisting of flakes having different particle sizes.
The particle sizes are determined by means of laser diffraction on the powder or on pigment suspensions using commercially available instruments known to the person skilled in the art (e.g. from Malvern, Horiba).
The flake-form particles preferably have an aspect ratio (diameter/thickness ratio) of 5-120, in particular of 10-70 and very particularly preferably of 25-50.
The flake-form particles may be coated on one or more sides with one or more transparent, semitransparent and/or opaque layers comprising metal oxides, metal oxide hydrates, metal suboxides, metals, metal fluorides, metal nitrides, metal oxynitrides or mixtures of these materials. The substrate is preferably surrounded by these layers. The metal oxide, metal oxide hydrate, metal suboxide, metal, metal fluoride, metal nitride, metal oxynitride layers or the mixtures thereof can be of low refractive index (refractive index <1.8) or high refractive index (refractive index ≥1.8, preferably ≥2.0.). Suitable metal oxides and metal oxide hydrates are all metal oxides or metal oxide hydrates known to the person skilled in the art, such as, for example, aluminium oxide, aluminium oxide hydrate, silicon oxide, silicon oxide hydrate, iron oxide, tin oxide, cerium oxide, zinc oxide, zirconium oxide, chromium oxide, titanium oxide, in particular titanium dioxide, in the rutile or anatase modification, titanium oxide hydrate and mixtures thereof, such as, for example, ilmenite or pseudobrookite. Metal suboxides which can be employed are, for example, the titanium suboxides. Suitable metals are, for example, chromium, aluminium, nickel, silver, gold, titanium, copper or alloys, a suitable metal fluoride is, for example, magnesium fluoride. Metal nitrides or metal oxynitrides which can be employed are, for example, the nitrides or oxynitrides of the metals titanium, zirconium and/or tantalum. Metal oxide, metal, metal fluoride and/or metal oxide hydrate layers are preferably applied to the substrate and metal oxide and/or metal oxide hydrate layers are very particularly preferably applied.
Particular preference is given to oxides and/or oxide hydrates of aluminium, silicon, iron, tin or titanium, in particular titanium dioxide, in the rutile or anatase modification, preferably in the rutile modification, and mixtures of these compounds. In order to convert titanium dioxide into the rutile modification, a tin dioxide layer is usually applied beneath a titanium dioxide layer. The effect pigments according to the invention may thus also contain a tin dioxide between substrate and outer coating in order to convert the titanium dioxide present in the outer coating that is essential to the invention into the rutile modification. Furthermore, multilayered systems comprising high- and low-refractive-index metal oxide, metal oxide hydrate, metal or metal fluoride layers may also be present, with high- and low-refractive-index layers preferably alternating. Particular preference is given to layer packages comprising a high-refractive-index layer (refractive index ≥2.0) and a low-refractive-index layer (refractive index ≤1.8), where one or more of these layer packages may be applied to the substrate. The sequence of the high- and low-refractive-index layers can be adapted to the substrate in order to include the substrate in the multilayered system. In a further embodiment, the metal oxide, metal oxide hydrate, metal suboxide, metal, metal fluoride, metal nitride, metal oxynitride layers may be mixed or doped with colourants or other elements. Suitable colourants or other elements are, for example, organic or inorganic coloured pigments, such as coloured metal oxides, e.g. magnetite, chromium oxide or coloured pigments such as, for example, Berlin Blue, ultramarine, bismuth vanadate, Thenard's Blue, or alternatively organic coloured pigments, such as, for example, indigo, azo pigments, phthalocyanines or also Carmine Red, or elements such as, for example, yttrium or antimony. Effect pigments comprising these layers exhibit great colour variety in relation to their mass tone and may in many cases exhibit an angle-dependent change in the colour (colour flop) due to interference.
The layers of metal oxides, hydroxide and/or oxide hydrates are preferably applied by wet-chemical methods, it being possible to use the wet-chemical coating methods that were developed for the production of effect pigments, which lead to surrounding of the substrate.
The coating of the flake-form particles with one or more metal oxides can be carried out, for example, as described in WO 93/08237 (wet-chemical coating) or DE-A 196 14 637 (CVD process) or also in further patent documents and other publications known to the person skilled in the art.
The thickness of the individual layers on the substrate is, as familiar to the person skilled in the art, essential for the optical properties of the pigment. The thickness of the metal oxide, metal oxide hydrate, metal suboxide, metal, metal fluoride, metal nitride, metal oxynitride layers or mixtures thereof is usually 10 to 1000 nm, preferably 15 to 800 nm, in particular 20 to 600 nm. Layer thicknesses of 20 to 200 nm are particularly suitable. The thickness of the metal layers is preferably 4 to 50 nm.
The effect pigments of the invention may have organic and/or inorganic coatings as stabilisation layers on the substrate or on the outer metal oxide layer, as described, for example, in EP 1 661 952, DE 43 05 280.
The effect pigments according to the invention can be used in a very wide variety of ways, e.g. in products selected from paints, coatings, automobile paints, powder coatings, printing inks, security printing inks, plastics, ceramic materials, glasses, paper, paper coatings, toners for electrophotographic printing processes, seed, greenhouse sheeting, tarpaulins, absorbers for the laser marking of paper and plastics, absorbers in the laser welding of plastics, cosmetic formulations, in pigment pastes, pigment preparations and dry preparations. Products of this type are prepared in a manner as is known to the person skilled in the art in this field.
The effect pigments according to the invention can preferably be used, for example, in lipsticks, lip gloss, rouge, eyeliner, eye shadows, (volume) mascara, nail varnishes, day creams, night creams, body lotions, cleansing milk, body powders, sticks, hair gels, hair masks, hair dyes, hair rinses, hair shampoos, shower gels, shower preparations, shower oils, bath additives, sun protection, pre-sun and after-sun preparations, tanning lotions, tanning sprays, rouge, make-ups, care creams, lotions, soaps, bath salts, toothpaste, face masks, compact powders, loose powders and gels, etc. Products of this type are prepared in a manner as is known to the person skilled in the art in this field.
It goes without saying that, for the various applications, the effect pigments according to the invention can also advantageously be used in a blend with other effect pigments, such as, for example, pearlescent pigments, interference pigments, goniochromatic pigments, BiOCl flakes, multilayered pigments, metal pigments, organic dyes, organic coloured pigments and other pigments, such as, for example, transparent and opaque white, coloured and black pigments, and with flake-form iron oxides, holographic pigments, LCPs (liquid crystal polymers) and conventional transparent, coloured and black lustre pigments based on metal oxide-coated mica and SiO2 flakes, etc. The effect pigments according to the invention can be mixed in any ratio with commercially available (effect) pigments.
Even without further comments, it will be assumed that a person skilled in the art will be able to utilise the above description in the broadest scope. The preferred embodiments and examples should therefore merely be regarded as descriptive disclosure which is absolutely not limiting in any way. All starting components are either commercially available or can be synthesised by known methods. The amounts indicated in the description and in the examples for the constituents in the compositions always add up to a total of 100%.
The following abbreviations are used below: RT=room temperature, DI water=deionised water
0.8 g of Bi2O3, 0.2 g of TiO2, 0.01 g of Na2CO3 and 8.4 g/5.5 g of NaCl/Na2SO4 (eutectic mixture) are weighed out in a can (60 ml), mixed in a Speedmixer at 1000 rpm for 1 min and subsequently ground 3×5s in in a blade mill (SEVERIN KM3868; 140 W).
15 g of the ground mixture are weighed out in an aluminium oxide crucible. The crucible is covered with an aluminium oxide lid and heated and cooled in an oven (Heraeus Thermicon P) in accordance with the following program:
After cooling to room temperature, the crucible with the salt melt is stirred for about 2 h in 11 of deionised water at 50° C. and 200 rpm. The contents (detached from the crucible) are filtered, washed with 11 of deionised water and dried at 110° C. for 12 h. Yield: 0.8 g (78%).
SEM photomicrographs show the flake-form structure. The aurivillius crystal structure is confirmed by x-ray structural analysis.
2.5 g of Bi2O3 nanopowder 90-120 (Sigma-Aldrich 637017-250G), 0.64 g of TiO2 1002 (KRONOS), 0.04 g of Na2CO3 (Merck 1.06392.1000), 7.2 g of NaCl (Sigma Aldrich S5886-5 kg) and 4.7 g of Na2SO4 (Merck 1.06645.2500) are mixed in a Speedmixer at 1000 rpm for 1 min. and homogenised 3 times for 5 sec. in a blade mill (SEVERIN KM3868; 140 W). In each case, 2.2 g of the mixture are pressed in a hydraulic press at a pressure of 1.9 t/cm2 to give seven pressed discs having a diameter of 1.3 cm. The bottom of an aluminium oxide crucible (GTS CCM 0070-AC AL99-G; 153 ml) is covered with aluminium oxide beads and the seven pressed discs are distributed thereon. The crucible is closed with an aluminium oxide lid, calcined at 1000° C. for 4 h and allowed to cool slowly to below 50° in the oven over a period of about 7 h. The crucible is subsequently treated in warm deionised water at 50° C. with gentle stirring until the salt matrix has completely dissolved (about 2 hours). The finely divided Bi4Ti3O12 is then filtered off, washed with deionised water and dried at 110° C. for about 12 h.
About 2.9 g of Bi4Ti3O12 having a mother-of-pearl-like lustre are obtained.
75 g of bismuth titanate flakes (Bi4Ti3O12) are suspended in 7501 of deionised water and heated to 75° C. with stirring at 500 rpm. The suspension is adjusted to pH1.8 by addition of HCl (5%) and stirred for a further 15 min.
An aqueous solution of SnCl4 (50%) and HCl (37%) is metered into the suspension until the desired proportion of TiO2 has been reached, with the pH being kept at pH 1.8 by addition of dilute sodium hydroxide solution, and the mixture is subsequently stirred for a further 15 min.
An aqueous solution of TiCl4 (25%) and HCl (25%) is metered into the suspension until the desired proportion of TiO2 has been reached, with the pH being kept at pH 1.8 by addition of dilute sodium hydroxide solution.
The pH of the suspension is adjusted to pH 5.0, the pigment is subsequently filtered off, washed, dried at 110° C. for 12 h and calcined at 850° C. for 30 min.
Coated pigments with SnO2 and various proportions of TiO2 based on the bismuth titanate flakes, are prepared as indicated in Table 1.
The layer structure of the pigments (SnO2/TiO2 monolayer) is evident in SEM photomicrographs.
75 g of bismuth titanate flakes (Bi4Ti3O12) are suspended in 7501 of deionised water and heated to 75° C. with stirring at 500 rpm. The suspension is adjusted to pH1.8 by addition of HCl (5%) and stirred for a further 15 min.
An aqueous solution of TiCl4 (10%) is metered into the suspension until the desired proportion of TiO2 has been reached, with the pH being kept at pH 1.8 by addition of dilute sodium hydroxide solution, and the mixture is subsequently stirred for a further 15 min.
Fe2O3 Coating:
The pH of the suspension is adjusted to pH 2.8, an aqueous solution of FeCl3 (3.5%) is subsequently metered in until the desired proportion of Fe2O3 has been reached, with the pH being kept at pH 2.8 by addition of dilute sodium hydroxide solution.
The pH of the suspension is adjusted to pH 5.0, the pigment is subsequently filtered off, washed, dried at 110° C. for 12 h and calcined at 850° C. for 30 min and at 50° C. for 30 min.
Coated pigments having various proportions of TiO2 and Fe2O3, based on the bismuth titanate flakes, are prepared as indicated in Table 2.
The layer structure of the pigments (TiO2/Fe2O3 bilayer) is evident in SEM photomicrographs.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21171454.8 | Apr 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/061291 | 4/28/2022 | WO |
