CA1054755A - Rutile-containing lustrous pigments - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Mica flake-based lustrous pigments having a plurality of TiO2 and SnO2 layers thereon are provided herein. The pigment has a metal oxide layer comprising the sequence of rutile/SnO2/rutile. The innermost layer is rutile and the thickness thereof is a maximum of 25 nm. Such pigments have im-proved properties, including light and weathering fastness.

Description

This invention relates~to lustrous mica-flake pigments coated with TiO2 and SnO2.
A whole series of mica flake-based lustrous pigments coated with metal oxides has been known, e.g. from U. S. Patents 3,087,828; 3,087,829; and 3,711,308; German Patent Specification

2,009,566 and German published Specification No. 2,214,545.
However, for special purposes, the light fastness and the weathering resistance of all of these pigments leaves something to be desired due primarily to the fact that in `
these pigments the TiO2 is present in the anatase modification which is evidently favored by the mica substrate.
There are also known mica flake-based lustrous pigments which contain both TiO2 and SnO2. The tin oxide is either present as a surface coating on a TiO2 coating or as a mixed precipitate with TiO2. See, e.g., German Patent Specifi-cation No. 1,467,468. X-ray investigation of such pigments shows that the TiO2, in spite of its tin content, is present in the anatase modification. According to puhlished German Specification No. 2,214,545, pigments are preferred in which the metal oxide layer consists preponderantly of rutile TiO2 plus a small amount of tin oxide, with the concentration of the tin oxide being greater in the regions closest to the mica. The SnO2 conte~t of the metal oxide layer is preferably 0.5 to 5 wt. % but in no case to more than 20 wt. %. However, it has been found that these pigments contain TiO2 which is not completely and not reproducibly in the rutile form but instead is very frequently present at least partially in the anatase form, although it is stated in published German Patent Specification No. 2,214,545 that the TiO2 was present "substantially in rutile form."

Thus, it has been known for some time that, on the one hand, SnO2 acts as catalyst for the conversion of anatase into rutile and that, on the other hand, mica favors the formation of the anatase modification of the TiO2. Never-theless, a satisfactory solution to the problem of producingrutile-containing pigments does not exist.
It is an object of one aspect of this invention to provide lustrous pigments based on mica flakes coated with TiO2 having improved properties in which the TiO2 is present substantially completely and reliably as rutile, even if the pigments have a comparatively thick layer of TiO2 exhibiting interference colors.
It is an object of another aspect of this invention to pro-vide such pigments which, compared to comparable pigments in which the anatase modification is present, display considerable technical advantages in use, including better light and weather-ing fastness.
It is an object of still another aspect to provide processes for the production and use o, such pigments.
According to one aspect of this invention, mica flake-based pigments are provided having improved properties.
Thus by an aspect of this invention, an improvement is pro-vided in a lustrous mica flake pigment wherein the mica flakes are coated with a plurality of metal oxide layers consisting essentially of alternating layers of TiO2 and SnO2 in which each rutile layer is roentogenographically free from the anatase form, the improvement wherein the metal oxide layers comprise the sequence rutile/AnO2/rutile and the innermost layer is rutile and wherein the thickness of the innermost rutile layer is a maximum of 25 nm.

By one variant, the outermost metal oxide layer is rutile.
By another variant the collective SnO2 content of the metal oxide layers is 13.1 to 37.5 wt. %.

~ _3_ By yet another variant, at least one of the rutile/SnO2/
rutile layers contains A12O3 in a collective amount sufficient to prevent crack formation when tin dioxide hydrate layers of high water content are calcined and to promote the formation of smooth tin dioxide layers and the quantitative deposition of the precipitates on the mica surface.
By a variation thereof, each layer contains aluminum oxide, in an amount collectively of up to 10 wt. %.
By another variant, the SnO2 content of the pigment is

3 to 25 wt. ~.
By a further variant, the rutile content of the pigment is 7 to 70 wt. %.
By yet another variant, the metal oxide coating consists of three layers, wherein the innermost layer consists of rutile, the middle layer consists of SnO2 and the outermost metal oxide layer consists of rutile, wherein the SnO2 content of the pig-ment is 2 to 25 wt. ~ and of the metal oxides is 13.1 to 37.5 wt. ~ and the rutile content of the pigment is 7 to 70 wt.
and of the metal oxides is 86.9 to 62.5 wt. ~.
By a still further variant, the collective SnO2 content of the metal oxide layers is at least 5 wt. %, wherein the SnO2 layer contains aluminum oxide in an amount of up to 10 wt. %
and wherein the outermost metal oxide layer is rutile of a thickness of at least 20 nm.
Such pigments have hitherto not been described. The arrangement of the metal oxide layers according to aspects of this -3a-invention is critical to the improved properties of the novel pigments. Surprisingly, the first layer, i.e., the TiO2 layer directly in contact with the surface of the mica flakes must have a very small thickness, i.e., no more than 25 nm., in order to provide satisfactory rutile pigments. In general, the layer thickness is from 0.1 to 25 nm. However, even thinner layers which are too thin to be measured with accuracy, are sufficient to provide pigments with satisfactory rutile layers. Admittedly, the TiO2 layer directly in contact with the surface of the mica is still subject to the anatase-inducing influence of the mica, but evidently provides an acceptable and uniform covering of the mica surface which is a prerequisite for a uniform and acceptable SnO2 coating.
According to X-ray analysis, upon calcining, the TiO2 layer is converted substantially completely into rutile.
To the titanium hydroxide-coated mica flakes is then applied an intermediate layer of SnO2, preferably of a thick-ness of 2 to 25 nm. This SnO2 layer, in an especially preferred embodiment, consists solely of SnO2 or, optionally, like the TiO2 layers, can contain certain amounts of A12O3 or a hydrated form thereof, e.g., up to 10%, preferably up to 7~, of the total metal oxide content of the SnO2 layer.
The total amount of SnO2 in the rutile/TiO2/rutile layers is preferably 5 wt. % or more, preferably 8 to 30 wt. %
of the total metal oxide content of the alternating rutile/
SnO2 layers. SnO2 contents of up to 90 wt. ~ are technically possible but usually are uneconomical.
Over the SnO2 layer is then provided a further layer of TiO2, which in the final pigment, like the first TiO2 layer is substantially completely in the for~ of rutile. miS~ or a like rutile layer i further intermediate metc~l oxide layers are provided, is preferably the top or covering layer. m e thickness of this layer depends solely upon the desired interference color of the pigment. As a rule, it varies from 20 to 200 nm. Hcwever, this range is not limiting since only the color is affected by the thickness. m us, in principle, any amDunt or layer thick-ness can be deposited which is conventional for TiO2-coated mica pigments, m~ny of which are commercially available.
According to an aspect of this invention a plurality of tin oxide layers, alternating between TiO2 layers, can be applied to the mica flakes so that a multiple sandwich structure results. In this case, the interfer-ence color of the pigment is determined by theltotal construction of the metal oxide layers. The amounts of oxide, i.e., the layer thickness, neces-sary therefor are determined by the refractive index of the oxides and can be calculated according to known rules~ It is essential, however, that the bottcm layer in contact with the mica flake be a thin TiO2 layer. As the top or covering layer either a rutile or an SnO2 layer can be emçloyed but a rutile layer is preferred. As stated a bove, all or any of the individual oxide layers can have an A12O3 content.
In a process aspect, this invention provides an improvement in a process for the production of a TiO2 coated mica flake pigment comprising a layer on the mica flakes of TiO2 in the rutile form only over a layer of SnO2 wherein tin hydroxide and titanium hydroxide are precipitated succes- -sively onto the mica flakes from a tin salt solution and a titanium salt solu-tion, respectively, and the thus-coated flakes are then washed, dried and calcined, the improvement ~hich comprises: coating the mica flakes first with a thin layer of titanium hydroxide of up to 25 nm. thickness; precipitating the layer of tin hydroxide onto the thus-coated mica flakes from a tin II
salt solutioni in the presence of an oxidizing agent; and then depositing a fur~her layer of titanium hydroxide onto the coated mica flakes.
., ~/
~ - 5 -By a variant thereof, the tin hydroxide is precipitated in the presence of KC103 as the oxidation agent.
By another variant, at least tin hydroxide layer is formed in the r presence of a water-soluble aluminium salt.
By a further -~ariant, the titanium and tin hydroxide layers are formed successively without isolation of the intermediate products.
By yet another variant, the final metal oxide layer deposited on the mica flakes is titanium hydroxide.
By a still further variant, at least the tin hydroxide layer is formed in the presence of a water-soluble aluminum salt, and wherein the titanium and tin hydroxide layers are for~med successively without is~lation of the intermediate products.
By another variant, the tin hydroxide is precipitated in the presence of ~C103 as the oxidation agent.

- 5 a -The coating of the mica flakes can be accomplished according to conventional methods. The mica to be coated is normally slurried in deionized water and brought to an elevated temperature, e.g., 40 to 90 C. The first thin TiO2 layer is then deposited by precipitation of titanium hydroxide or a hydrate thereof from titanium salt solutions, e.g., from titanyl sulfate or titanium chloride. The reaction conditions are described in the literature, e.g., in German Patent Specifi-cation No. 2,009,566. This first TiO2 layer should be as thin as possible in order to provide a pigment which contains roentgenographically pure rutile. The amount of titanium salt necessary for the selected mica surface can be calculated in the usual way. The coating operation is stopped when the desired layer thickness, up to a maximum of 25 nm., is reached.
Next, the tin hydroxide is deposited on the thus-coated mica. Contrary to the statements in the previously published literature, the coating of the mica flakes coated with TiO2, with the formation of lustrous pigments, is accomplished best not with Sn IV salt solutions but instead with Sn II salt solutions in the presence of an oxidation agent.

It is evidently important for this process that the oxidation occurs only slowly and during the precipitation. Under these conditions, one obtains, surprisingly, smooth tin dioxide hydrate layers with a previously unachievable uniformity.
It has been found, surprisingly, that for the achieve-ment of satisfactory layers, the presence in the precipitating bath of a small amount of a soluble aluminum salt from which A12O3 is subsequently formed, is desirable. Although the influence of the aluminum ions and/or of the aluminum oxide or its hydrate cannot be completely explained, three phenomena are especially worth mentioning. Surprisingly, the co-precipita-tion of aluminum hydroxide with the Ti or Sn hydroxide prevents crack formation which frequently appears when tin dioxide hydrate layers of high water content are calcined. Also, the addition of the aluminum salts appears to promote the formation of smooth tin dioxide layers and the quantitative deposition of the precipitates on the mica surface.
For the deposition of the tin oxide layer, the suspension containing mica flakes coated with a TiO2 hydrate is adjusted by the addition of an acid, e.g., hydrochloric acid, to a p~
value of 0.5 to 3.0, preferably 1.0 to 2.5. The addition of the tin salt solution is then conducted in such a manner that the precipitated Tio2 hydrate is deposited immediately and quantitatively as it forms. Thus, per unit time, there can be supplied to the mica slurry only such amount of the salt solutions to the reaction which the surface to be coated can take up per unit time. This is conveniently achleved by adding an aqueous solution of the tin ~054755 salt slowly to the warmed suspension of the mica flakes. The acidity of the aqueous solution of the tin salt is adjusted in the usual manner, e.g., with hydrochloric acid, to a molarity of acid of between 0.1 and 5.
As soluble tin salts, especially preferred are tin II
chloride and tin II sulfate. However, the anion is not critical.
The concentration of the tin II salt solution can be varied over a wide range but generally for reasons of expediency, is preferably from 0.1 to 3 moles per liter.
The tin salt solution optionally and preferably contains a water-soluble aluminum salt, e.g., aluminum chloride, acetate, nitrate and sulfate, as well as alkali metal aluminum mixed salts, e.g., potassium, sodium and ammonium aluminum sulfate.
Preferably, 0.01 to 2 moles Al per mole Sn is present, more preferably 0.1 ta 1 mole. It will be apparent that the aluminum salt can be added concurrently as a separate salt solution. If aluminum salts are added to the mica flake slurry, both metal ions should, in any case, be present simultaneously in the solution during the precipitation. Surprisingly, the desired effects are also achieved even in the presence of only trace amounts of Al ions. This is all the more surprising since only a very small part of the A1 ions present in the case of the precipitation or formation of the hydrolysis products is incorporated into the pigment. Whereas the tin ions are deposited quantitatively on the mica flakes, a quite consid-erable part of the Al ions remains behind in the precipitation solution. This residual ionic Al can be recovered by back titration, e.g., in amounts of up to 96% of the Al salt used, from the aqueous phase after removal of the mica flakes. This applies similarly to the addition of aluminum ions to the titanium salt solutions. Methods for such additions are per se known. The A12O3 content of the Tio2 layers is not critical. However, not only the salt solution for the precipitation of the first thin TiO2 layer but also those employed for th~ deposition of subsequent layers or any one thereof, preferably contain small amounts of water-soluble aluminum salts which co-precipitate to provide A12O3 in the final metal oxide layer.
If Al ions are present during the formation of the SnO2 layer, the addition of more Al salts is not required to supply A12O3 to subsequent oxide coatings because of residual Al ions present in the precipitation solution. These Al salts can be added to the titanium and tin salt solutions prior to the precipitation step or can be added separately and concurrently therewith to the mica flake slurry.
The deposition of the hydrated tin oxide is carried out in the presence of an oxidation agent, which can be present in the salt solution or added separately and concurrently therewith to the slurry. Oxidation agents which can be employed are those which oxidize Sn (II) ions into Sn (IV) ions, e.g., air, oxygen, H2O2 and hypochlorites. Especially useful are hydrogen peroxide and chlorates, especially potassium chlorate. The oxidation agent is expediently added in amounts about chemically equivalent to the tin salt solution employed. However, the concentrations and amounts are not critical and, if desired, atmospheric oxygen for the oxidation can be additionally introduced by the stirring of the slurry. Therefore, the oxidation agent can also be added in an excess or in a deficient amount. The optimum conditions in each case easily be ascertained by routine experiments. Expediently, the aqueous solution of the oxidation agent is added separately from but simul- ~
taneously with the metal salt solution. It is normally preferable to avDid the introduction, by the oxidation agent, of further foreign ions, which may adversely influence the reaction.
It is expedient to maintain the pH value as constant as possible during the whole period of precipitation. This can be accomplished by the slow addition of, e.g., aqueous alkali metal hydroxide or ammDnium hydroxide, preferably dilute aqueous sodium hydroxide solution or gaseous ammDnia, as needed.
The production of the rutile pigments according to the process of aspects of this invention and their quality depends in great part upon the special coating process by which the tin hydroxide is deposited. Hitherto, the coating with uniform SnO2 layers and thus the dependable production of pure rutile pigments was not possible.
For pigments with only 3 metal oxide layers, the subsequent color-imparting TiO2 layer is dep~sited in the desired layer thickness according to conventional methods, preferably after an interval of 10 to 60 minutes after completion of the tin salt addition, in order to ensure complete hy-drDlysis of the tin salts. For details of the reaction conditions, see German Patent Specification No. 2,009,566. m e deposition takes place in the same way as the deposition of the first TiO2 layer, e.g., employing titanium salt solutions at pH values of fro~ 0.5 to 5.0 and at temperatures from about 50 to 100C., the only difference being that the thickness of coated products, although, in principle, a separation, washing and conceiva-bly also calcination of the prcducts is possible after each deposition.
However, such isolations generally are omitted for economic reasons. me deposition of the various alternating layers is conducted in the same manner, with the final covering layer being either TiO2 or SnO2 but preferably the former.

~05475S

the layer can and preferably does exceed 25 nm. so as to produce a pigment with an interference color. Upon reaching the desired layer thickness, the coating operation is discontinued. During the coating, the color scale known from conventional lustrous pigments is passed through with increasing layer thickness, as illustrated below.

Thickness of 2nd Interference Tio2 Layer in nm. Color about 40 silver about 55 yellow about 85 red about 95 violet about 120 blue about 150 green With still thicker layers, colors of higher order appear. In the case of pigments with more than 3 layers, the Ti- and Sn-containing layers continue to be deposited alternatingly. In each case, reaction conditions which are known in the art can be employed.
All precipitations are so carried out that, in each case, temperature and pH value are kept as constant as possible during the whole coating operation. Furthermore, the addition of the reaction components is at a rate such that hydrated titanium or tin oxide is deposited immediately and quantitatively on the mica flakes as it is precipitated. Thus, per unit time, only such an amount of the reactants is supplied to the reaction which the surface of the mica to be coated can take up per unit time. Preferably, the various depositions are carried out successively, without isolation of the resulting intermediate m e coated mica flakes are separated in the usual ~ay from the slurry, are thoroughly washed with water and then dried. me coated mica flakes are then subjected to a conventional calcination process, e.g., for 10 minutes up to 4 hours at temperatures of up to 1100, preferably from 600 to 1000. After this treatment, one obtains rutile pigments, the x-ray analysis of ~hich no longer shows anatase lines.
me ne~ pigments according to aspects of the invention can be pro-duced from mica flakes conventionally employed for this purpose. me m~st frequently employed mica is muscovite. As a rule, the mica flakes have a diameter of 5 to 200 microns and a t~ickness of 0.1 to 5 microns, preferably 0.5 micron.
Depending on the desired color shade, the ne~ pigments contain metal oxide layers on the mica flakes in a total thickness of about 20 to akout 200 nm., corresp~nding to a metal oxide content for t~e coated flakes of 10 to 90 wt. %. As a rule, pigments ~ith a metal oxide content sub-stantially lower than 5 wt. % do not possefis the desired lustrous quality.
An accurate determination of the aluminum oxide content of the metal oxide ~-layer is for all practical purposes not possible since aluminum is also con-tained in the mica itself. m e precise A12O3 content in the metal oxide layer is not critical. Instead, it is the presence of Al ions during the precipi-tation of the hydrolysis products formed from the tin salts ~hich appears essential to the improved results achieved. From the values obtained in the case of back titration, it can, however, be deduced that the proportion of A12O3 in the SnO2 layer of the final pigment varies fr~m traces, e.g., 0.001 wt. %, to about 10~, with 7% ordinarily not being exceeded.

1(~54755 Typical examples of the pi~ments of aspects of this invention have the following composition:

~ica Flake Pi~ments ~etal Oxide Coating 25 to 90 wt. % mica 0 to 10 wt. % A1203 7 to 70 wt. % TiO 60 to 95 wt. % TiO2 ~rutileJ2 3 to 25 wt. % SnO2 5 to 30 wt. % SnO2 Hkwever, depending on the selected thickness of the respective metal oxide layers, pigments of aspects of this invention can also be obtained whose analyses vary considerably from the above values.
~m e pigments of aspects of this invention can be employed in the same m~nner as hl~wn mica flake pigments. m ey are especially suited for those fields of use where light fastness is of primary importance, e.g., motor car lacquers, and in cosmetics, where definite and very differentiated color nuances are desired. m ey are also well suited for the coloring of synthetic resins. The concentration thereof in the final compositions varies, depending upon the field of use, over a wide range, e.g., between 0.1 and 80~. _ They can also be employed as starting materials for the production of other lustrous pigments. For example, the novel pigments can advantageously be coated in per se known manners with one or more further layers, e.g, with silicates, iron oxides, chromium oxides, A12O3, zirconium dioxide, as well as dyestuffs and/or colored lakes, especially Al colored lakes.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative.

Example_l 60 g. of muscovite (diameter of 20 - 40~ ) are suspended in 1.5 liters of completely deionized water. The pH
of the suspension is brought to 2.2 with dilute hydrochloric acid. The suspension is then heated to 75 C. To the heated slurry is slowly added with stirring 30 ml. of a solution of 150 g. TiC14 and 50 g. HCl per liter (solution I), simultaneously with a 15% aqueous sodium hydroxide solution at a rate which maintains the pH constantly at 2.2. 10 minutes after the addition is completed, 300 ml. of a tin chloride solution (19.1 g. SnC12.2H2O per liter of 5% hydrochloric acid, solution II) is slowly added to the slurry with stirring, simultaneously with 300 ml. of a potassium chlorate solution 25 (4.3 g. RC103 per liter) and a 15~ aqueous sodium hydroxide solution so as to keep the pH value constantly at 2.2.
1/2 hour after the addition is complete, about 270 ml. of solutionl is again added while keepinq the pH value constant. The pigment acquires a strong, silvery luster.
The coating operation is discontinued and the suspension is further stirred for about 1 hour at 75 C. Then the pigment is filtered off, washed free of salts with completely deionized water, dried and calcined at 950 C. for half an hour. The pigment displays a strong, silvery luster. X-ray analysis shows that the TiO2 is present in rutile form only. The pigment consists of 72.4% mica, 23.0% TiO2 and 4.6% tin dioxide.
Example 2 The following solutions are employed:
Solution I: A solution of titanium tetrachloride in hydrochloric acid (15% TiC14 and 4% HCl);
Solution II: A solution of 38.2 g. SnC12.2H20 in 1000 ml.
5% hydrochloric acid; and Solution III: A solution of 8.3 g. KC103 in 1000 ml. water.
60 g. of muscovite (diameter 10 - 40~ ) are suspended in 2 liters completely deionized water and the pH value of the suspension adjusted to 2.2 with Solution I. The suspension is heated to 70 C. While stirring,30 ml. of Solution I are slowly added simultaneously with a 15% aqueous sodium hydroxide solution at a rate which maintains the pH value of the slurry at a constant 2.2. After 1/4 hour after completion of the addition of Solution I, during which time stirring is continued, Solution II and Solution III are slowly run in simultaneously at about the same rate, while maintaining the pH value constant at 2.2 with 15% aqueous sodium hydroxide solution. 1/2 hour after completion of the addition, Solution I is again added in the same manner as the first coating operation. During this coating step, the mica flakes exhibit 105475~

various interference colors, depending on the amount of Solution I added. Upon reaching the blue interference color (800 ml. of Solution I), the coating operation is stopped. The suspension is then stirred for 1 hour. The pigment is filtered off, washed with water, dried and calcined for 30 minutes at 950 C. Roentgenographic investigation shows that the TiO2 layer has only the rutile form. The pigment exhibits very good luster and luminescent color power. It contains 50.4% mica, 43.1~ TiO2 and 6.5% SnO2.
Example 3 60 g. of muscovite with a flake diameter of about 10 - 50 nm. are suspended in 1 liter completely deionized water. The suspension is adjusted to pH 2.2 with dilute hydrochloric acid. After heating to 75 C., titanium hydroxide is first deposited on the flakes employing 30 ml. of a solution of 150 g. of TiC14 in 1000 ml. of 5% hydrochloric acid in the manner described above, keeping the pH value at a constant 2.2 with 15% aqueous sodium hydroxide solution. Subsequently, a layer of tin hydroxide is so deposited on the mica flakes by 20 adding 300 ml. amounts of 10~ hydrochloric acid containing tin II chloride at a concentration shown in Table I simultaneously with a like volume of KC103 solution of equivalent concentration as shown in Table I at the same flow in rate,again maintaining the pH value at a constant 2.2 by the simultaneous addition of 15~ aqueous sodium hydroxide solution. 20 minutes after the addition is completed, the flakes are then coated with titanium hydroxide in the same manner as the first coating step employing 270 ml. of a solution of 150 g. TiC14 in 1000 ml.
of 5% hydrochloric acid,again maintaining the pH value at a 105475$

constant 2.2 by the simultaneous addition of 15~ aqueous sodium hydroxide solution.
After conclusion of the coating operation, the coated mica flakes are filtered off, washed with completely deionized water, dried at 100 C. and subsequently calcined at 950 C.
for 30 minutes. The pigments exhibit a strong silvery luster.
In all cases, the TiO2 is present solely in rutile form (Roentgenographic detection). Table I is a summary of the coating conditions and the composition of the pigments thus-obtained.

TABLE I

Experi- Concentra- Concentra- Composition of the Oxide Layers ment tion of tion of SnCl KClO Inner Middle Outer Solu~ion Solu~ion Layer Layer Layer ZU~C ~ in g/l. ~ ~i~

A 19.1 4.3 8 17 75 20B 28.6 6.0 8 23 69 C 38.2 7 ~ 64 Example 4 Analogously to Example 3, muscovite is coated with 3 successive layers of titanium hydroxide, tin hydroxide and titanium hydroxide, in a series of experiments in which the pH
is maintained at other constant values during the tin hydroxide precipitation. The coating conditions, insofar as they vary from those of Example 3, are summarized in Table II. In all cases, there is obtained a strongly glossy silver pigment, which contains TiO2 solely in rutile form.

TABLE ~I
Composition of the Experi- Coating with Tin Hydroxide Oxide Layers ment pH Concen- Amount Concen- Inner Middle Outer tration of tration Layer Layer Layer of SSoclltion KC103 (% TiO2) (~ SnO2) (%TiO2) Solution (ml.) Solution 2g2~C11~2- (g/l.) D 2.219.1 300 4.3 8 17 75 E 1.819.1 300 4.3 8 17 75 F 1.519.1 300 4.3 8 17 75 15 G 1.019.1 300 4.3 8 17 75 Example 5 Analogously to Example 3, in a series of experiments pigments are prepared in which the two TiO2 layers vary in layer thickness. The same solutions are employed as in 20 Example 3 but the volume of the solutions used for the two coating steps is varied. Also, the tin hydroxide coating step is conducted at a pH value of 1.5. In all cases, silver pigments are obtained with very good gloss and which contain the TiO2 solely in rutile form.
Table III is a summary of the experimental conditions employed and the composition of the pigments obtained.

T.~BL~ III
~ri- 1st Coating Coating 2nd Coating ~osition of the ment with Titanium with Tin with Titanium Oxide Layers Hydroxide Hydroxide Hydroxide _ _ pH Amount ~ pH Amount Inner l1iddle O~ter of of Layer Layer Layer TiC14 TiC14 Solu- Solu- (%TiO2) (~SnO2)(%TiO2) tion, ml. tion,ml. _ H 2.2 60 1.5 2.2 240 1717 66 I 2.2 30 1.5 2.2 270 8 17 75 ¦ K 2.2 15 1.5 2.2 285 4 17 79 Example 6 Analogously to Experiment A described in Example 3, muscovite is coated with 3 successive layers of titanium hydroxide, tin hydroxide and titanium hydroxide, except that instead of the KC103 solution, there is used as oxidation agent a chemically equivalent amount of 5% hydrogen peroxide. The pigment obtained exhibits an excellentluster, a silvery color and contains the TiO2 solely in rutile form. The analysis data agree completely with those of the product according to Experiment A.
Example 7 5.0 kg. of muscovite with a particle diameter of 10 to 40~ are suspended in 100 liters of completely deionized water. The pH is adjusted to 2.1 with dilute hydrochloric acid.
The suspension is heated to 75 C. and with stirring there is added 2.5 liters of an aqueous solution of 150 g. TiC14 and 40 g. HCl per liter, over a period of 20 minutes,simultaneous with a 30~ aqueous sodium hydroxide solution at a rate which maintains the pH value at a cons~ant 2.1. Subsequently, a solution of 162 g. KC103 in 2.5 liters of water is added and a solution of 717 g. SnC12.2H2O in 2.86 liters concentrated hydrochloric acid in 21.8 liters water is passed in at a rate of about 10 liters/hour, concurrently with 30% aqueous sodium hydroxide solution at a rate which keeps the pH value at a constant 2.1. The slurry is allowed to stand for about 30 minutes and the final titanium hydroxide coating is applied, employing a solution of 150 g. TiC14 and 50 g. HCl per liter, again keeping the pH value at 2.1 with 30% aqueous sodium hydroxide solution. During the coating, the pigment acquires a strong silver gloss, whereupon the coating operation is stopped. The slurry is filtered as usual, the mica flakes washed and dried and subsequently calcined at 960 C. for 35 minutes. Roentgenographic investigation reveals that the TiO2 is present solely in the rutile form. The product contains 68% mica, 26% Tio2 and 6% SnO2.
Example 8 The following solutions are employed:
Solution I: A hydrochloric acid-TiC14 solution containing 10% TiC14, 5% HCl and 10 g. AlC13.6H2O per liter;
Solution II: A hydrochloric acid-tin (II) solution containing 200 g. SnC12.2H2O, 50 g. HCl and 50 g. AlC13.6H2O per liter; and Solution III: A KC103 solution containing 40 g. KC103 per liter.

60 g. of muscovite with a diameter of 10 to 40~ are suspended in completely deionized water and the suspension is adjusted with Solution I to a pH value of 2Ø
After heating to 75 C., 40 ml. of Solution I are slowly added simultaneously with a 15% aqueous sodium hydroxide solution so as to keep the pH value at a constant 2Ø
15 minutes after addition is complete, 60 ml. of Solution II
is added slowly simultaneously with 60 ml. of Solution III and with 15% aqueous sodium hydroxide solution at a rate which keeps the pH value at a constant 2Ø 30 minutes after the addition of the tin solution is complete, in order to permit the hydrolysis to go to completion, the coating operation is continued in the same manner according to the following scheme, alternately employing titanium tetrachloride and tin chloride solutions, at a constant pH 2:

Oxide Layer Produced Solution I Solution II Solution III

Tio 320 ml. ~
SnO2 _ 60 ml. 60 ml.
TiO2 320 ml. _ SnO2 _ 60 ml. 60 ml.
Tio2 320 ml. _ SnO2 _ 60 ml. 60 ml.
TiO2 320 ml. _ SnO2 60 ml. 60 ml.

The product is worked up as in Example 1 and calcined at 900 C. for 40 minutes. The pigment exhibits a strong green interference color with good gloss. P~oentgenographic investigation reveals the Tio2 is solely in rutile form.
The pigment contains 40~ mica, 35~ rutile, 21 SnO2 and 3~ Al203 (determined by back titration).

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a lustrous mica flake pigment wherein the mica flakes are coated with a plurality of metal oxide layers consisting essentially of al-ternating layers of TiO2 and SnO2 in which each rutile layer is roentogeno-graphically free from the anatase form, the improvement wherein the metal oxide layers comprise the sequence rutile/SnO2/rutile and the innermost layer is rutile and wherein the thickness of the innermost rutile layer is a maxi-mum of 25 nm.

2. A pigment according to claim 1 wherein the outermost metal oxide layer is rutile.

3. A pigment according to claim 1 wherein the collective SnO2 con-tent of the metal oxide layers is 13.1 to 37.5 wt. %.

4. A pigment according to claim 1 wherein at least one of the rutile/SnO2/rutile layers contains Al2O3 in a collective amount sufficient to prevent crack formation when the dioxide hydrate layers of high water con-tent are calcined and to promote the formation of smooth tin dioxide layers and the quantitative deposition of the precipitates on the mica surface.

5. A pigment according to claim 4 wherein each layer contains aluminum oxide, in an amount collectively of up to 10 wt. %.

6. A pigment according to claim 1 wherein the SnO2 content of the pigment is 3 to 25 wt. %.

7. A pigment according to claim 1 wherein the rutile content of the pigment is 7 to 70 wt. %.

8. A mica flake pigment according to claim 1 wherein the metal oxide coating consists of three layers, wherein the innermost layer consists of rutile, the middle layer consists of SnO2 and the outermost metal oxide layer consists of rutile, wherein the SnO2 content of the pigment is 3 to 25 wt. % and of the metal oxides is 13.1 to 37.5 wt. % and the rutile con-tent of the pigment is 7 to 70 wt. % and of the metal oxides is 86.9 to 62.5 wt. %.

9. A mica flake pigment according to claim 1 wherein the collective SnO2 content of the metal oxide layers is at least 5 wt. %, wherein the SnO2 layer contains aluminum oxide in an amount of up to 10 wt. % and where-in the outermost metal oxide layer is rutile of a thickness of at least 20 nm.

10. In a process for the production of a TiO2 coated mica flake pigment comprising a layer on the mica flakes of TiO2 in the rutile form only over a layer of SnO2, wherein tin hydroxide and titanium hydroxide are precipitated successively onto the mica flakes from a tin salt solution and a titanium salt solution, respectively, and the thus-coated flakes are then washed, dried and calcined, the improvement which comprises: coating the mica flakes first with a thin layer of titanium hydroxide of up to 25 nm.
thickness; precipitating the layer of tin hydroxide onto the thus-coated mica flakes from a tin II salt solution; in the presence of an oxidizing agent;
and then depositing a further layer of titanium hydroxide onto the coated mica flakes.

11. A process according to claim 10 wherein the tin hydroxide is precipitated in the presence of KClO3 as the oxidation agent.

12. A process according to claim 10 wherein at least tin hydroxide layer is formed in the presence of a water-soluble aluminum salt.

13. A process according to claim 10 wherein the titanium and tin hydroxide layers are formed successively without isolation of the intermedi-ate products.

14. A process according to claim 10 wherein the final metal oxide layer deposited on the mica flakes is titanium hydroxide.

15. A process according to claim 10 wherein at least the tin hy-droxide layer is formed in the presence of a water-soluble aluminum salt , and wherein the titanium and tin hydroxide layers are formed successively without isolation of the intermediate products.

16. A process according to claim 15 wherein the tin hydroxide is precipitated in the presence of KClO3 as the oxidation agent.