CA1302663C - Nacreous pigments - Google Patents

Abstract

Abstract Platelet-shaped coloured pigments in which a platelet-shaped substrate is provided with a compact coating containing iron(II) oxide, which coating has a high gloss and, depending on the layer, thickness, shows the interference colour of thin platelets are described.

Description

Merck Patent Gese~lschaft mit beschrankter Haftur19 i3~26~3 6100 D a r m s t a d t Nacreous pigments The Application relates to platelet-shaped coloured pigments in which a platelet-shaPed substrate is covered with a layer containing iron oxide.
Coloured platelet-shaped pigments containing iron oxide are known per se. In particular, layers of Fe203, if appropriate together with other metal oxides, such as, for example, TiO2, are applied to pldtelet-shaped sub-strates, in particular mica. However, it is furthermore also known from German Offenlegungsschrift 2,313,331 that layers containing iron(II) oxide can be deposited, uni-form layers of magnetite on mica or on mica coated with metal oxides being produced by precipitation from iron-(II) salt solutions in the presence of an oxidizing agent.
As explained in the third paragraph on page 7 of German Offenlegungsschrift 2,313,331, these are rough layers which show no interference colours and therefore also have no nacreous lustre. Only if mica/metal oxide interference pigments are used as a substrate for a magnetite coating can this interference colour still shine through in the I case of very thin magnetite layers, but the magnetite fl layer itself is not capable of producing interference colours of thin platelets.
i The same applies to the black pigments described in Example 8 of European Patent Application 0,077,959, which are prepared analogously to the process of German Offenlegungsschrift 2,313,331, except that the precipita-tion of the magnetite is carried out in the presence of an alkaline earth metal salt with the aim of improved heat stability.
Although these known magnetite pigments can read-ily be used as platelet-shaped black pigments for many purposes, they cannot be referred to as nacreous pigments in the actual sense because of the lack of interference ~?, 13~ 3 ability and a lack of lustre.
There was therefore the object of discovering platelet-shaped pigments which contain iron oxide and, in addition to the body colour, additionally have a high gloss and can also display the interference colour of thin platelets, depending on the thickness of the layer.
These pigments should also be suitable for applications both in technology and in cosmetics.
This object has been achieved by the present invention. In particular, it has been found that uniform smooth layers which contain iron(lI) oxide, have a high lustre and impart to the pigment an attractive powder colour, and if appropriate can produce the interference colours of thin platelets, can be produced on platelet-shaped substrates.
The Application therefore relates to platelet-shaped coloured pigments in which a platelet-shaped substrate is covered with a layer containing iron oxide, which are characterized in that the layer contains iron(II) oxide and is formed as a compact lustrous layer.
It has been found, surpris;ngly, that these layers which are capable of interference and contain iron oxide can be prepared both by wet ch-emistry, by direct precipitation of magnetite from iron(II) salt solutions in the presence of an oxidizing agent, and from ' Fe203-coated pigments by heating the pigments in the presence of a reducing gas. An essential feature of the j preparation by wet chemistry is that, in contrast to the known processes, the oxidizing agent is not taken in the suspension of the substrate to be coated, but both the iron(II) salt solution and the solution of the oxidizing agent are added simultaneously to the suspension.
The Application therefore also relates to a pro-cess for the preparation of coloured platelet-shaped pig=
ments containing iron oxide, which is characterized in that, to produce lustrous layers which can exhibit the interference colours of thin platelets, either an iron-(II) salt solution and the solution of an oxidizing agent are simultaneously added to an aqueous suspension of a 13~ ;3 platelet-shaped substrate at a largely constdnt tempera-ture and largely constant pH and the pigment is separated off and, if appropriate, washed and dried, or a platelet-shaped substrate coated with a smooth homogeneous layer of Fe203 in a manner which is known per se is exposed to a reducing atmosphere at a temperature above 100C.
The Application furthermore relates to the use of these pigments for pigmenting lacquers, paints, plastics and cosmetics.
In the context of the present invention, platelet-shaped pigments which contain iron(II) oxide and have a high gloss and the interference colours of thin platelets are rendered accessible for the first time. A critical factor in this is that the layer containing iron oxide on the new pigments consists of very much smaller and more densely packed primary particles than that of the known pigments. As can be seen from electron scan micro-scopy (ESM) photographs, the primary magnetite particles in the known pigments, for example those prepared accord-20 ing to German Offenlegungsschrift 2,313,331 or European Offenlegungsschr;ft 0,077,959, are in the form of rela-tively coarse crystals of about 0.5 ~m diameter distribu-ted irregularly and with relatively large free intermed-iate spaces on the surface of the substrate.
In the pigments according to the invention, on the other hand, the primary particles are significantly smaller and, above all, also lie in closely packed dis-j tribution on the substrate.
The size of the primary particles is substan-tially less than 0.3 ~m and, depending on the preparation process used, even in some cases significantly below 0.1 ~m. However, it is critical that in each case the packing of the particles is so dense that the intermed-iate spaces between adjacent primary particles are as a rule smaller than the particle diameters.
The process for the preparation of these advan-tageous new pigments is also new. In this process, either a magnetite layer can be precipitated directly onto a suitable platelet-shaped substrate by wet chemis-13~6ti3 try, or the platelet-shaped substrate can first be coated with iron~III) oxide, which is then reduced to a l3yer containing iron(lI) oxide.
Possible platelet-shaped substrates per se are all the platelet-shaped materials which are stable under the coating conditions, such as, for examPle, mica, glass platelets, metal platelets, graphite and other platelet-shaped materials.
Mica, such as, for example, muscovite or phlogo-pite, is preferably used. However, materials whichalready have a coating of metal oxide can also be used as the platelet-shaped substrate. In particular, mica platelets with one or more coatings of, for examPle, TiO2~ ZrO2, SnO2~ Cr23, ~iOCl, Fe203, Al203, SiO2, ZnO
or mixtures of these metal oxides can be employed as the platelet-shaped substrate.
The size of these platelet-shaped substrates is i not critical per se'and particles of a size suitable for the intended use can therefore be employed. As a rule, the substrate is employed in particle sizes of about 1 ~ to 200 ~m, in particular about 5 to 100 ~m. The thick-¦ ness of the particles is as a rule about 0.1 to 5 ~m, in particular about 0.5 ~m.
The starting materials used as substrates are known or can be prepared by known processes. Mica particles of the desired order of size can be obtained by grinding of mica and subsequent grading. Materials coated with metal oxide, in particular mica platelets coated with metal oxide, are both commercially available, for example from E. Merck, Darmstadt, as Iriodin(R) nac-reous pigments, and able to be prepared by known processes.
Such processes are described, for example, in the follow-ing patents and patent applications:
US 3,087,828, US 3,087,829, DE 1,959,998, DE 2,009,566, DE 2,214,545, DE 2,244,298, DE 2,313,331, DE 2,522,572, DE 3,137,808, DE 3,137,809, DE 3,151,343, DE 3,151,354, DE 3,151,355, DE 3,211,602 and DE 3,235,017.

`- s Depending on the preparation process and the sub-strate used, the layer containing iron(II) oxide can have various compositions. Thus, it is possible to apply a magnetite coating by wet chemistry. In contrast to the known processes, however, compact lustrous layers which also exhibit the interference colour of thin platelets if the layer thickness is adequate are obtained by the process according to the invention.
If platelets coated with meta~ oxide are employed as the substrate, mixed phases can develop at the magnet-ite/metal oxide phase boundary. These mixed phases arealso to be understood as magnetite layers in the context of the invention.
The layer containing iron(II) oxide can, however, also be present as a substantially pure layer of iron(II) oxide (wustite phase; Feo 90-o 95) or as a mixed oxide with other metal oxides. Examples of such mixed oxides which may be mentioned are iron aluminate (FeAlz04), chromite (FeCrzO4), iron orthosilicate (FeSiO4) and, 20 in particular, also ilmenite (FeTiO3).
Both wustite and the mixed oxides with other metals are obtained, in particular, by reducing layers containing iron(III) oxide with a reducing gas at ele-vated temperature. Here also, other mixed phases may 25 develop, especially at the phase interfaces, depending on the substrate used. These are also included in thedefinition, according to the invention, of layers con-taining iron(II) oxide.
For coating with magnetite by wet chemistry, the substrates are suspended in water and an iron(II) salt solution and an oxidizing agent are added at a suitable temperature and suitable pH. The suitable temperature range is about O to 100C; the reaction is preferably carried out at about 50 to 100C. The pH of the suspen-sion should be greater than 7; a pH between 8 and 11 ispreferably established.
The addition of the iron(lI) salt, such as, for example, ammonium iron(II) sulfate, iron(II) halides or, 13(~26~3 in particular, ironlII) sulfate, is carried out so that the iron oxide hydrate precipitated is deposited immedi-ately on the substrate and no secondary precipitation occurs in the suspension. The pH is kept constant as far as possible during the precipitation~ This is most advantageously effected by simultaneous addition of a base, such as, for example, NaOH, KOH or ammonia; however, it is also possible to use a suitable buffer system.
The precipitation of the iron oxide is carried out in the presence of an oxidizing agent, preferably a nitrate or a chlorate, which as far as possible is employed in the stoichiometric amountr that is to say in the case of a nitrate 1 mol of the nitrate is used for a maximum of 12 mol of iron(II) ions. It ;s critical that the oxidizing agent is not taken in the suspension, as is the case in the processes of the prior art, but is metered into the suspension in the required stoichiometric amount at the same time as the iron(II) salt solution.
Surprisingly, very smooth dense layers of Fe304 which, in contrast to those of the prior art, produce the inter-ference colours of thin platelets are deposited. The ESM
photographs clearly show that the magnetite layer con-sists of finely crystalline densely packed crystals with a size in the range from about 0.1 to 0.3 ~m.
Depending on the desired effect, the layer contain-ing iron oxide can be up to about 500 nm thick, preferably 0.1 to 250 nm thick. As a rule Fe304 contents which, based on the substrate, make up about 0.1 to 200% by weight, in particular 5 to 100% by weight, are achieved.
Depending on the thickness of the magnetite layer, inter-ference colours are achieved which pass from silver via gold, red, violet and blue to green and finally to inter-ference colours of a higher order as the layer thickness increases.
When the desired interference colour is reached, coating is interrupted and the coated substrate is as a rule separated off from the reaction mixture, washed with water and dried. To avoid undesirable oxidation, drying can be carried out, if appropriate, in an inert gas 13~ 3 atmosphere, such as, for example, nitrogen, or even by admixing a reducing gas, such as, for examp~e, hydrogen.
Drying is as a rule carried out at temperatures of about 80 to 120C, in particular in an N2/H2 atmosphere, but higher temperatures of up to 800C can also be used, additional sintering of the magnetite layer occurring.
In some cases it is advantageous additionally to provide a covering layer on the new pigments. Layers of colourless oxides, for example titanium dioxide, zircon-ium dioxide, aluminium oxide, antimony oxide, zinc oxide,silicon dioxide, magnesium oxide or tin dioxide, each of which can be applied by itself or as a mixture, are ad-vantageously used for this in a known manner. Such a covering layer can be applied by customary methods to pigments which have already been dried, or more easily still before separation of the pigments from the precipi-tation so~ution. The covering layer is in general thinner than the layers applied according to the inven-tion. A layer of aluminium oxide hydrate or alumin;um oxide usually has the effect of additional stabilization, and in particular both in respect of mechanical proper-t;es and in respect of resistance to weathering. The layer thickness here is not particularly critical, since aluminium oxide hydrates and aluminium oxides have a relatively low refractive index. The methods for appli-cation of such layers are known and are described, for example, in German Offenlegungsschrift 1,467,468. The precipitation of the magnetite can also be carried out in the presence of an alkaline earth metal salt in accord-ance with the method of European Offenlegungsschrift77,959, to improve the heat stability of the pigments.
Alternatively to the process by wet chemistry, a coating, according to the invention, containino irontII) oxide can, however, also be achieved by reduction of a previously applied layer containing Fe203. Possible starting materials here are all the abovementioned sub-strates. These platelet-shaped materials can be coated with iron oxide or iron oxide hydrate in a known manner.
Such processes are described, for example, in U.S. Patent 13~26~3 Specification 3,0S7,828, U.S. Patent Specificatir,n 3,087,929, German Offenlegungsschrift 1,959,998, German Offenlegungsschrift 2,244,298, German 0ffenlegungsschrift 2,313,331, German Offenlegungsschrift 2,723,871, German Offenlegungsschrift 3,030,056 and German Offenlegungs-schrift 3,237,264. Mica-based pigments coated with iron oxide are also commercially available. The pigments marketed by E. Merck, Darmstadt, with the tradename Iriodin(R) 400, 500, 502, 504, 520, 522, 524 and 530 and the Mearl-Russet, Cloisonne-Russet, Bronze and Copper grades marketed by Mearl, USA, may be mentioned in parti-cular.
Either the substrates coated with iron oxide or iron oxide hydrate by one of the known processes or the commercially obtainable pigments are then exposed to a reducing atmosphere at an elevated temperature above 100C. Temperatures of about Z00 to 1,000C, preferably 400 to 800C, are used in particular here. Possible reducing agents are in principle all the reducing gases.
Examples which may be mentioned are hydrogen, carbon monoxide, methane and ammonia, hydrogen preferably being employed. These gases can be employed in the pure form, or diluted with an inert gas, such as, for example, nitrogen, argon, helium or steam. Mixtures which contain about 20 to 60% of the reducing gas are preferably used.
The conversion of Fe203 into iron(II) oxide, magnetite or mixed phases of iron(II) oxide with other metal oxides takes place at a rate depending on the tem-perature and the nature of the reducing gas or gas mix-ture. The thickness of the Fe203-containing layer to be reduced, and whether the entire layer is to be conver-ted or only a thicker or thinner covering layer, are also critical for the duration of the reaction. The period can thus be varied within a very wide framework. However, the optimum reduction time can in all cases be determined by a few orientating experiments. As a rule, periods of about 0.25 to 2 hours are appropriate. The reduction can in principle be carried out in any oven to be charged Z6~;~

. ~
with reducing gas. In order to be able to carry out the process continuously, a rotating tube oven is preferably employed The nature of the layer containing iron(lI) oxide produced can also be influenced by the temperature.
Thus, pure Fe203 layers are predominantly converted into magnetite at a relatively low temperature, such as, for example, about 400-500C, whereas the wustite phase is formed at high temperatures of about 700-900C.
In the thermal treatment with a reducing gas the quality of the layer produced by reduction is pri-marily determined by the quality of the original Fe203-containing layer. Since it is possible to prepare very finely crystalline homogeneous, densely packed Fe203-containing coatings, it is in this way possible to obtain coatings of the same quality containing iron(II) oxide.
Thus, for example, reduction of the commercially avail-able Iriodin(R~ pigments of the abovementioned 500 series gives pigments which, when examined by RSM, reveal that the pr;mary particles in the layer are only about 0.1 ~m and below in size. The reduct;on can also be carr;ed out w;th non-annealed products containing Fe203. The layers conta;ning iron(II) oxide which are obtained by reduction of compact Fe203 layers are therefore particularly lustrous and stable.
The new p;gments represent a substantial enrich-ment of the art. The powder colour which ranges from dark to black, and the interference colour which can be produced as desired, result in extremely interesting effects which can be utilized for various applications, and in the case in particular of substrates which them-selves already have an interference colour, this can be intensified and varied, for example by an iron(II) oxide/
titanium dioxide mixed phase (iLmenite) or a magnetite layer. Fields of application result both in cosmetics,-where the pigments according to the invention are used,for example, in powders, ointments, emulsions, grease sticks and other agents in concentrations of as a rule between 0.1 and 80%, and in industry, for example for pigmenting paints, lacquers or plastics. The advantage 13~2~6;3 of ihe pigments according to the invention when used in cosmetics is on the one hand that magnetite, for examPle, is permitted as a cosmetics pigment, and on the other hand that both an outstanding colour gloss and a black 5 body colour can be delivered with a single pigment.
There are moreover also further fields of use for platelet-shaped iron oxides with the crystal structure of magnetite where the combination of their electromag-netic properties and their shape is utilized. The mag-10 netic interactions lead to a very pronounced parallel orientation of the individual particles in coating materials. In comparison with conventional iron oxides, very much higher packing densities can consequently be achieved, which manifests itself, for example, in an 15 increased anti-corrosive effect, in a good shielding from electromagnetic interference fields and in a high con-i ductivity.
The ease of alignment of platelet-shaped magnetic ! particles in magnetic fields and their different light-20 scatterir1g ability, depending on the orientation of the I platelets relative to an incident beam of light, can be J utilized for magneto-optical displays. Utilization of the Faraday effect opens up use for magneto-optical , memories to the platelet-shaped iron oxides according to i 25 the invention.
Example 1 A solution of 6ûO g of FeS04 . 7 H20 in 2,000 ml of water acidified with S0 ml of concentrated sulphuric acid and a solution of 150 9 of KN03 in 2,000 ml of water 30 are metered simultaneously into a suspension of 100 9 of potash mica of diameter 5 to S0 ~m in 2,500 ml of water at 80C and a pH of 8 in the course of one hour, with vigorous stirring, the pH being kept constant by addition of 15% strength sodium hydroxide solution. The pigment ; 35 with a blue-black lustre and a magnetite coating is then filtered off, washed and dried at 100C for 3 hours.
Example 2 The procedure followed is analogous to Example 1, but a solution of 350 9 of FeS04 . 7 H20 and a solution 1, - 13~ 3 ot 90 9 of ~NO3 dre metered in. A black-gold pigment ~ith a magnetite coating is obtained.
Example 3 The procedure followed is analogous to Example 1, but a solution of 450 9 of FeS04 . 7 H20 and a solution of 112 9 of KNC)3 are metered in. A black-red pigment with a magnetite coating is obtained.
Example 4 The procedure followed is analogous to Example 1, but a solution of 700 g of FeSO4 . 7 H20 and a solution of 175 g of KNO3 are metered in. A black-green pigment with a magnetite coating is obtained.
Example 5 The procedure followed is analogous to Example 1, but a mica/TGiO2 pigment with a red interference colour, corresponding to Example 4 of German Patent Specification 1,467,468, is emPloyed as the starting material and is coated using 250 9 of FeSO4 . 7 H2û and 50 g of KN03 to give a black-green pigment.
Example 6 50 g of a m;ca/Fez03 pigment which has a red-brown lustre and an iron oxide content of 43% by we;ght (prepared according to Example 1 b of German Offenlegungs-; schrift 2,313,331) are reacted with a 1 : 1 mixture of 2S nitrogen and,hydrogen at a flow rate of 100 l per hour ina 50 cm long flow tube at 800C for 0.5 hour. A
platelet-shaped product which has an anthracite lustre and shows the characteristic lines of FeO and mica in the Debye-Scherrer diagram immediately recorded is obtained.
Example 7 The procedure followed is analogous to Example 6, but instead of the calcined Fez03 mica pigment, the precursor which has been merely dried is employed. At a reduction temperature of 500C, a product which has a black lustre and a red interference colour and shows the characteristic lines of Fe304 and mica in the Debye-Scherrer diagram is obtained.

13~26~3 ExamPle 8 66 9 of a pigment which has a coating of 6 9 of Fe203 in the non-annealed form on 60 9 of an annealed TiO2/mica pigment with a blue interference colour are reduced at 800C in the same flow tube as in Example 6.
A pigment which has a deep dark-blue metallic lustre and shows the characteristic lines of ilmenite (FeTiO3)2, TiO2 and mica in the Debye-Scherrer diagram is obtained.

Claims (9)

1. Platelet-shaped coloured pigments in which a platelet-shaped substrate is covered with a layer containing iron oxide, characterized in that the layer contains iron(II) oxide and is formed as a compact lustrous layer.

2. Pigment according to claim 1, characterized in that the iron(II) oxide is present as a mixed oxide with the oxide of a di-, tri- or tetravalent metal.

3. Pigment according to claim 1, characterized in that the thickness of the layer is such that interference colours of thin platelets can appear.

4. Pigment according to claim 2, characterized in that the thickness of the layer is such that interference colours of thin platelets can appear.

5. Pigment according to one of claims 1 to 4, characterized in that the primary particles in the layer containing iron(II) oxide essentially do not exceed a size of 0.3 µm.

6. Pigment according to one of claims 1 to 4, characterized in that the primary particles in the layer containing iron(II) oxide are so densely packed that the distance between adjacent particles is as a rule smaller than their diameter.

7. Pigment according to one of claims 1 to 4, characterized in that the primary particles in the layer containing iron(II) oxide essentially do not exceed a size of 0.3 µm and are so densely packed that the distance between adjacent particles is as a rule smaller than their diameter.

8. A process for the preparation of platelet-shaped coloured pigments according to any one of claims 1 to 4, characterized in that (i) an iron(II) salt solution and a solution of an oxidizing agent are added simultaneously to an aqueous suspension of a platelet-shaped substrate at a substantially constant temperature and a substantially constant pH, the pigment is separated off and, if required, washed and dried, or (ii) a platelet-shaped substrate is coated with a smooth homogeneous layer of Fe2O3 and is exposed to a reducing atmosphere at a temperature above 100°C.

9. A method of pigmenting a lacquer, paint, plastic or cosmetic which comprises incorporating a platelet-shaped pigment according to any of claims 1 to 4 in the said lacquer, paint, plastic or cosmetic.