U.S. patent application number 09/757657 was filed with the patent office on 2001-05-17 for multilayer interference pigments.
This patent application is currently assigned to Merck Patent Gesellschaft mit Beschrankter Haftung. Invention is credited to Andes, Stephanie, Bauer, Gerd, Brenner, Gunter, Bruckner, Dieter, Heyland, Andrea, Kuntz, Matthias, Osterried, Karl, Pfaff, Gerhard, Schmelz, Michael.
Application Number | 20010001174 09/757657 |
Document ID | / |
Family ID | 22864531 |
Filed Date | 2001-05-17 |
United States Patent
Application |
20010001174 |
Kind Code |
A1 |
Andes, Stephanie ; et
al. |
May 17, 2001 |
Multilayer interference pigments
Abstract
Multilayer interference pigment consisting of plateletlike
titanium dioxide as carrier material, coated with alternating
layers of metal oxides of low and high refractive index, the
difference in the refractive indices being at least 0.1, which is
obtainable by solidification and hydrolysis of an aqueous solution
of a thermally hydrolysable titanium compound on a continuous belt,
detachment of the resulting coat, coating of the resulting titanium
dioxide platelets, with or without drying in between, by a wet
method with, alternately, a metal oxide hydrate of high refractive
index and a metal oxide hydrate of low refractive index by
hydrolysis of the corresponding, water-soluble metal compounds,
separation, drying and, if desired, calcining of the material
obtained.
Inventors: |
Andes, Stephanie; (Maintal,
DE) ; Bauer, Gerd; (Kleinostheim, DE) ;
Brenner, Gunter; (Griesheim, DE) ; Bruckner,
Dieter; (Darmstadt, DE) ; Schmelz, Michael;
(Kriftel, DE) ; Heyland, Andrea; (Ober-Kainsbach,
DE) ; Kuntz, Matthias; (Seeheim, DE) ;
Osterried, Karl; (Dieburg, DE) ; Pfaff, Gerhard;
(Munster, DE) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
Arlington Courthouse Plaza I
Suite 1400
2200 Clarendon Boulevard
Arlington
VA
22201
US
|
Assignee: |
Merck Patent Gesellschaft mit
Beschrankter Haftung
Darmstadt
DE
|
Family ID: |
22864531 |
Appl. No.: |
09/757657 |
Filed: |
January 11, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09757657 |
Jan 11, 2001 |
|
|
|
09230260 |
Mar 9, 1999 |
|
|
|
09230260 |
Mar 9, 1999 |
|
|
|
PCT/EP97/02652 |
May 23, 1997 |
|
|
|
Current U.S.
Class: |
106/415 ;
106/436; 106/437; 106/438; 106/439; 106/441 |
Current CPC
Class: |
C09C 2200/1004 20130101;
C09C 1/0051 20130101; C09C 1/0018 20130101; C09C 2220/20 20130101;
C09C 2200/303 20130101; C09C 2200/302 20130101; C09C 2220/106
20130101; C09C 2220/10 20130101; C09C 2200/301 20130101 |
Class at
Publication: |
106/415 ;
106/436; 106/437; 106/438; 106/439; 106/441 |
International
Class: |
C09C 001/36 |
Claims
1. Multilayer interference pigment consisting of plateletlike
titanium dioxide as carrier material, coated with alternating
layers of metal oxides of low and high refractive index, the
difference in the refractive indices being at least 0.1, which is
obtainable by solidification and hydrolysis of an aqueous solution
of a thermally hydrolysable titanium compound on a continuous belt,
detachment of the resulting coat, coating of the resulting titanium
dioxide platelets, with or without drying in between, by a wet
method with, alternately, a metal oxide hydrate of high refractive
index and a metal oxide hydrate of low refractive index by
hydrolysis of the corresponding, water-soluble metal compounds,
separation, drying and, if desired, calcining of the material
obtained.
2. Interference pigment according to claim 1, characterized in that
the oxide of high refractive index is TiO.sub.2, ZrO.sub.2,
Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, Cr.sub.2O.sub.3, ZnO or a mixture
of these oxides or an iron titanate, an iron oxide hydrate, a
titanium suboxide or a mixture or mixed phase of these
compounds.
3. Interference pigment according to claims 1 and 2, characterized
in that the metal oxide of low refractive index is SiO.sub.2,
Al.sub.2O.sub.3, AlOOH, B.sub.2O.sub.3 or a mixture thereof, it
being possible if desired for alkali metal oxides and alkaline
earth metal oxides to be present as additional constituents.
4. Process for the preparation of the novel pigment according to
claims 1 to 3, characterized in that an aqueous solution of a
thermally hydrolysable titanium compound is applied as a thin film
to a continuous belt, the liquid film is solidified by drying,
during the course of which the titanium dioxide is developed from
the solution by means of a chemical reaction, the resulting layer
is subsequently detached from the belt and washed, the titanium
dioxide platelets obtained, with or without drying in between, are
suspended in water and coated with, alternately, a metal oxide
hydrate of high refractive index and a metal oxide hydrate of low
refractive index, by addition and hydrolysis of the corresponding,
water-soluble metal compounds, and the coated titanium dioxide
platelets are separated out from the aqueous suspension, dried and,
if desired, calcined.
5. Process according to claim 4, characterized in that the oxide of
high refractive index is TiO.sub.2, ZrO.sub.2, Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, Cr.sub.2O.sub.3, ZnO or a mixture of these oxides
or an iron titanate, an iron oxide hydrate, a titanium suboxide or
a mixture or mixed phase of these compounds.
6. Process according to claim 4, characterized in that the metal
oxide of low refractive index is SiO.sub.2, Al.sub.2O.sub.3, AlOOH,
B.sub.2O.sub.3 or a mixture thereof, it being possible if desired
for alkali metal oxides and alkaline earth metal oxides to be
present as additional constituents.
7. Process according to at least one of claims 4 to 6,
characterized in that the metal oxides are applied in a
fluidized-bed reactor by CVD following intermediate drying of the
material to be coated.
8. Use of the pigments according to claims 1 to 3 for pigmenting
paints, printing inks, plastics, cosmetics, agricultural films and
glazes for ceramics and glass.
9. Use according to claim 8, characterized in that the pigments are
employed as mixtures of commercially available pigments.
10. Paints, printing inks, plastics, cosmetics, agricultural films,
ceramics and glass which are pigmented with a pigment according to
claims 1 to 3.
Description
1. The invention relates to multilayer interference pigments with
plateletlike titanium dioxide as substrate.
2. Multilayer pigments of low transparency are known. The metal
oxide layers are prepared either in a wet process, by precipitating
the metal oxide hydrates from a metal salt solution onto a carrier
material, or by vapour deposition or sputtering under reduced
pressure. In general, the vapour deposition processes are too
complex and costly for mass production of pigments. Thus U.S. Pat.
No. 4,434,010 describes a multilayer interference pigment
consisting of a central layer of a reflecting material (aluminium)
and alternating layers of two transparent, dielectric materials of
high and low refractive index, for example titanium dioxide and
silicon dioxide, either side of the central aluminium layer. This
pigment is employed for the printing of securities.
3. JP H7-759 (Kokoku) describes a multilayer interference pigment
with a metallic lustre. It consists of a substrate coated with
alternating layers of titanium dioxide and silicon dioxide. The
substrate is formed from flakes of aluminium, gold or silver or
from platelets of mica and glass which are coated with metals.
Accordingly, it is a typical metal-effect pigment. This pigment is
of high opacity. For applications where a high level of
transparency of the pigmented material is required, for example for
agricultural films, the pigment is unsuitable. Furthermore, it has
the disadvantage that the depth effect typical of interference
pigments is not produced since, owing to the total reflection of
the light at the metal layer which forms the core, a number of
pigment particles are unable to enter into interaction. The
interference effect therefore remains limited to the coats located
on the metal layer.
4. Mica is the substrate employed most frequently for the
production of interference pigments.
5. Mica pigments are used widely in the printing and coating
industries, in cosmetics and in polymer processing. They are
distinguished by interference colours and a high lustre. For the
formation of extremely thin coats, however, mica pigments are not
suitable, since the mica itself, as a substrate for the metal oxide
coats of the pigment, has a thickness of from 200 to 1200 nm. A
further disadvantage is that the thickness of the mica platelets in
some cases varies markedly about a mean value. Moreover, mica is a
naturally occurring mineral which is contaminated by foreign ions.
Furthermore, technically highly complex and time-consuming
processing steps are required including, in particular, grinding
and classifying.
6. Pearl lustre pigments based on thick mica platelets and coated
with metal oxides have, owing to the thickness of the edge, a
marked scatter fraction, especially in the case of relatively fine
particle-size distributions below 20 .mu.m.
7. As a substitute for mica it has been proposed to use thin glass
flakes which are obtained by rolling of a glass melt with
subsequent grinding. Indeed, interference pigments based on such
materials exhibit colour effects superior to those of conventional,
mica-based pigments. Disadvantageous, however, is that the glass
flakes have a very large mean thickness of about 10-15 .mu.m and a
very broad thickness distribution (typically between 4 and 20
.mu.m), whereas the thickness of interference pigments is typically
not more than 3 .mu.m.
8. EP 0,384,596 describes a process in which hydrated alkali metal
silicate is subjected at temperatures of 480-500.degree. C. to the
action of an air jet, forming bubbles with thin walls; the bubbles
are subsequently comminuted to give plateletlike alkali metal
silicate substrates with a thickness of less than 3 .mu.m. However,
the process is complex and the thickness distribution of the
resulting platelets is relatively broad.
9. DE 11 36 042 describes a continuous belt method of preparing
plateletlike or glitterlike oxides or oxide hydrates of metals of
groups IV and V and of the iron group of the Periodic Table. In
this method, a release layer comprising, for example, a silicone
coating is first of all applied, if desired, to a continuous belt
in order to facilitate the subsequent detachment of the metal oxide
layer. Then a liquid film is applied which comprises a solution of
a hydrolysable compound of the metal which is to be converted into
the desired oxide, and the film is dried and subsequently detached
using a vibration device. The coat thickness of the platelets
obtained is given as being from 0.2 to 2 .mu.m, although no
concrete examples of this are cited.
10. EP 0 240 952 and EP 0 236 952 propose a continuous belt method
of preparing different plateletlike materials, including silicon
dioxide, aluminium oxide and titanium dioxide. In this method, a
thin liquid film of defined thickness of a precursor of the
plateletlike material is applied, via a roller system, to a smooth
belt; the film is dried and detached from the belt, forming
plateletlike particles. The particles are subsequently, if desired,
calcined, ground and classified.
11. The thickness of the platelets obtained in accordance with the
method described in EP 0 240 952 is relatively well defined, since
the film is applied very uniformly, for example to the continuous
belt via a roller system. The layer thickness of the platelets is
given in the examples as being from 0.3 to 3.0 .mu.m. According to
Example 1, a first roller is wetted with the precursor used by
immersing this roller partially into a stock container which is
filled with the precursor. The film is transferred from this roller
to a second, co-rotating roller which is in very close contact with
the first roller. Finally, the film is rolled off from the second
roller onto the continuous belt.
12. Disadvantages, however, are the use of very expensive precursor
materials and, in particular, the increased requirements in terms
of workplace safety which must be applied when organometallic
compounds are used. The complete chemical conversion of the
precursor into the desired coating material requires, in general,
high heating of the film and of the belt material. In addition to
the considerable thermal stress which this places on the belt
material, the high energy consumption and the restriction on the
process speed are also highly disadvantageous for the economy of
the method.
13. Wo 93/08 237 describes plateletlike pigments consisting of a
plateletlike matrix comprising silicon dioxide, which may contain
soluble or insoluble colourants and which is coated with one or
more reflecting layers of metal oxides or metals. The plateletlike
matrix is prepared by solidification and hydrolysis of water glass
in a continuous belt.
14. DE 12 73 098 describes the preparation of a mother-of-pearl
pigment by vapour deposition of ZnS, MgF.sub.2, ZnO, CaF.sub.2 and
TiO.sub.2 films on to a continuous belt. This process however, like
the process described in U.S. Pat. No. 4,879,140 in which
plateletlike pigments with Si and SiO.sub.2 coats are obtained by
plasma deposition from SiH.sub.4 and SiCl.sub.4, is associated with
very high expenditure on apparatus.
15. The object of the invention is to provide an essentially
transparent interference pigment having strong interference colours
and/or a strong angular dependency of the interference colours.
Furthermore, the object of the invention is to provide a pigment
which consists only of optically functional layers and is therefore
extremely thin.
16. This object is achieved in accordance with the invention by a
multilayer interference pigment consisting of plateletlike titanium
dioxide as carrier material, coated with alternating layers of
metal oxides of low and high refractive index, the difference in
the refractive indices being at least 0.1, which is obtainable by
solidification and hydrolysis of an aqueous solution of a thermally
hydrolysable titanium compound on a continuous belt, detachment of
the resulting coat, coating of the resulting titanium dioxide
platelets, with or without drying in between, by a wet method with,
alternately, a metal oxide hydrate of high refractive index and a
metal oxide hydrate of low refractive index by hydrolysis of the
corresponding, water-soluble metal compounds, separation, drying
and, if desired, calcining of the material obtained.
17. This object is additionally achieved in accordance with the
invention by a process for preparing the novel pigment, in
which
18. an aqueous solution of a thermally hydrolysable titanium
compound is applied as a thin film to a continuous belt,
19. the liquid film is solidified by drying, during the course of
which the titanium dioxide is developed from the solution by means
of a chemical reaction,
20. the resulting layer is subsequently detached from the belt and
washed,
21. the titanium dioxide platelets obtained, with or without drying
in between, are suspended in water and coated with, alternately, a
metal oxide hydrate of high refractive index and a metal oxide
hydrate of low refractive index, by addition and hydrolysis of the
corresponding, water-soluble metal compounds, and
22. the coated titanium dioxide platelets are separated out from
the aqueous suspension, dried and, if desired, calcined.
23. The invention additionally relates to the use of the novel
pigments for pigmenting paints, printing inks, plastics, cosmetics
and glazes for ceramics and glass and for producing agricultural
films.
24. For this purpose they can be employed as mixtures with
commercially available pigments, for example inorganic and organic
absorption pigments, metal-effect pigments and LCP pigments.
25. The novel pigments are based on plateletlike titanium dioxide.
These platelets have a thickness of between 10 nm and 500 nm,
preferably between 40 and 150 nm. The extent in the two other
dimensions is between 2 and 200 .mu.m and, in particular, between 5
and 50 .mu.m.
26. The metal oxide of high refractive index can be an oxide or
mixture of oxides with or without absorbent properties, for example
TiO.sub.2, ZrO.sub.2, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4,
Cr.sub.2O.sub.3 or ZnO, or a compound of high refractive index such
as, for example, iron titanates, iron oxide hydrates and titanium
suboxides, or mixtures and/or mixed phases of these compounds with
one another or with other metal oxides.
27. The metal oxide of low refractive index is SiO.sub.2,
A1.sub.2O.sub.3, AlOOH, B.sub.2O.sub.3 or a mixture thereof and can
likewise have absorbent or nonabsorbent properties. If desired, the
oxide layer of low refractive index may contain alkali metal oxides
and alkaline earth metal oxides as constituents.
28. The thickness of the layers of the metal oxides of high and low
refractive index is critical for the optical properties of the
pigment. Since a product with strong interference colours is
desired, the thicknesses of the layers must be adjusted relative to
one another. If n is the refractive index of a layer and d its
thickness, the colour which appears in a thin layer is the product
of n and d, i.e. the optical thickness. The colours of such a film,
as produced with normal incidence of light in reflected light,
result from an intensification of the light of wavelength
.lambda.=(4/2N-1).multidot.nd and by attenuation of light of
wavelength .lambda.=(2/N) .multidot.nd, where N is a positive
integer. The variation in colour which takes place as the thickness
of the film increases results from the intensification or,
respectively, attenuation of particular wavelengths of the light by
interference. For example, a 115 nm thick film of titanium dioxide
of refractive index 1.94 has an optical thickness of
115.times.1.94=223 nm, and light of wavelength 2.times.223 nm=446
nm (blue) is attenuated in the course of reflection, with the
result that the reflected light is yellow. In the case of
multilayer pigments, the interference colour is determined by the
intensification of specific wavelengths and, if two or more layers
in a multilayer pigment possess the same optical thickness, the
colour of the reflected light becomes more intense and full as the
number of layers increases. Moreover, by a suitable choice of the
layer thicknesses it is possible to achieve a particularly marked
variation of colour in dependency on the viewing angle. A
pronounced colour flop develops, which may be desirable for the
pigments according to the invention. The thickness of the
individual metal oxide layers, independently of their refractive
index, is therefore from 20 to 500 nm, preferably from 50 to 300
nm.
29. The number and thickness of the layers is dependent on the
desired effect. Normally, the desired effects are achieved if the
5-layer system TiO.sub.2/SiO.sub.2/TiO.sub.2/SiO.sub.2/TiO.sub.2 is
built up and if the thicknesses of the individual layers are
matched optically to one another. When using optically relatively
thin TiO.sub.2 layers (layer thickness <100 nm) it is possible,
for example, to produce interference pigments with a blue colour
which, with a substantially smaller TiO.sub.2 content, are stronger
in colour and more transparent than pure TiO.sub.2-mica pigments.
The saving in terms of TiO.sub.2 is up to 80% by weight.
30. By means of the precipitation of thick SiO.sub.2 layers (layer
thickness>100 nm), pigments having a strongly pronounced angular
dependency of the interference colour are obtained.
31. By precipitating further SiO.sub.2 and TiO.sub.2 layers it is
also possible to obtain higher systems, the number of layers then
being limited by the economics of the pigment.
32. Since, in contrast to mica, plateletlike titanium dioxide as
substrate is an optically functional layer, covering the substrate
with, for example, two layers of the abovementioned structure gives
an interference system comprising 5 thin layers of sharply defined
thicknesses. The reflection or transmission spectrum of such a
pigment exhibits finer and more precisely matchable structures than
the spectrum of a corresponding pigment based on a substrate with a
broad thickness distribution, such as mica.
33. Even with extremely thin TiO.sub.2 layers (layer thickness: 40
nm), these pigments exhibit strong interference colours. The
angular dependency of the interference colour is also particularly
pronounced. This extreme colour flop is not observed with
conventional metal oxide-mica pigments.
34. The novel pigments are prepared in a two-stage process. In the
first stage, plateletlike titanium dioxide particles are prepared
with the aid of a continuous belt.
35. First of all, the belt method will be explained with reference
to FIG. 1.
36. The continuous belt 1, which is guided via a roller system 2,
passes through an applicator unit 3 in which it is coated with a
thin film of an aqueous solution of a thermally hydrolysable
titanium compound. Preference is given to the use of an aqueous
titanium tetrachloride solution. The concentration of the titanium
salt in these solutions is from 7 to 30% by weight, preferably from
8 to 15% by weight. Suitable applicator units which can be employed
are roller applicators and also flow-type units. The belt speed is
between 2 and 400 m/min, preferably 5-200 m/min.
37. In order to achieve uniform wetting of the plastics belt it is
expedient to add a commercially available wetting agent to the
coating solution or to activate the surface of the belt by flame
treatment, corona treatment or ionization.
38. The coated belt passes subsequently through a drying section 4
in which the coat is dried at temperatures between 30 and
200.degree. C. As dryers it is possible, for example, to employ
commercially available infrared, circulating-air jet and UV
dryers.
39. After passing through the drying section the belt is passed
through the detachment baths 5 containing an appropriate detachment
medium, for example deionized water, where the dried layer is
removed from the belt. The detachment procedure is supported by
additional devices, for example jets, brushes or ultrasound.
40. In a subsequent dryer 6, the belt is dried before being coated
again.
41. The continuous belt should be made from a chemically and
thermally resistant plastic in order to ensure an adequate service
life and high drying temperatures. Suitable materials for the belt
include polyethylene terephthalate (PET) or other polyesters and
polyacrylates.
42. The film width is typically between a number of centimeters and
two or more meters. The thickness is between 10 .mu.m and a number
of millimeters, these two parameters being optimized in respect of
the particular requirements.
43. Further details of continuous belt methods are known from U.S.
Pat. No. 3,138,475, EP 0 240 952 and WO 93/08 237.
44. In a second stage, the titanium dioxide platelets detached from
the belt are coated, without being dried beforehand, with, in
alternation, a metal oxide hydrate of low refractive index and a
metal oxide hydrate of high refractive index.
45. The metal oxide layers are preferably applied by a wet-chemical
process which may be one of the wet-chemical coating techniques
developed for the preparation of pearl lustre pigments; techniques
of this kind are described, for example, in DE 14 67 468, DE 19 59
988, DE 20 09 566, DE 22 14 545, DE 22 15 191, DE 22 44 298, DE 23
13 331, DE 25 22 572, DE 31 37 808, DE 31 37 809, DE 31 51 343, DE
31 51 354, DE 31 51 355, DE 32 11 602, DE 32 35 017 or else in
further patent documents and other publications.
46. In the case of wet coating, the substrate particles are
suspended in water, and one or more hydrolysable metal salts are
added at a pH which is suitable for the hydrolysis and is chosen so
that the metal oxides and/or metal oxide hydrates are precipitated
directly onto the platelets without any instances of secondary
precipitation. The pH is usually kept constant by simultaneous
metered addition of a base. The pigments are then separated off,
washed and dried and, if desired, calcined, it being possible to
optimize the calcining temperature with respect to the particular
coating present. If desired, the pigments to which individual
coatings have been applied can be separated off, dried and, if
desired, calcined before being resuspended in order to apply the
further layers by precipitation.
47. Furthermore, coating can also be carried out in a fluidized-bed
reactor by gas-phase coating, in which context it is possible, for
example, to employ correspondingly the techniques proposed for the
preparation of peal lustre pigments in EP 0 045 851 and EP 0 106
235.
48. The metal oxide of high refractive index used is preferably
titanium dioxide, and the metal oxide of low refractive index
preferably used is silicon dioxide.
49. For the application of the titanium dioxide layers the process
described in U.S. Pat. No. 3,553,001 is preferred.
50. An aqueous titanium salt solution is added slowly to a
suspension, heated to about 50-100.degree. C., in particular
70-80.degree. C., of the material to be coated, and a substantially
constant pH of about 0.5-5, in particular about 1.5-2.5, is
maintained by simultaneous metered addition of a base, for example
aqueous ammonia solution or aqueous alkali metal hydroxide
solution. As soon as the desired layer thickness of the TiO.sub.2
precipitation has been reached, the addition of the titanium salt
solution and of the base is stopped.
51. This method, which is also called the titration method, is
notable for the fact that it avoids an excess of titanium salt.
This is achieved by supplying to the hydrolysis only that quantity
per unit time which is necessary for uniform coating with the
hydrated TiO.sub.2 and which can be adsorbed per unit time by the
available surface area of the particles to be coated. There is
therefore no production of hydrated titanium dioxide particles not
precipitated on the surface to be coated.
52. For the application of the silicon dioxide layers, the
following process is to be employed: a sodium silicate solution is
metered into a suspension, heated to about 50-100.degree. C., in
particular 70-80.degree. C., of the material to be coated. The pH
is held constant at 7.5 by simultaneous addition of 10%
hydrochloric acid. As soon as the desired layer thickness of the
SiO.sub.2 precipitation has been reached, the addition of the
silicate solution is stopped. Stirring is carried out subsequently
for 30 minutes.
53. the wet-chemical production of two or more interference layers
of high and low refractive index with precisely defined thicknesses
and a smooth surface on finely divided plateletlike substrates has
not been disclosed to date. It should be noted that it is necessary
first of all to apply a metal oxide of low refractive index to the
transparent carrier material.
54. It is additionally possible to subject the finished pigment to
an aftercoating or aftertreatment process which further increases
the stability to light, weather and chemicals, or which facilitates
the handling of the pigment, especially its incorporation into
different media. Suitable aftercoating and aftertreatment processes
are those described, for example, in DE-C 22 15 191, DE-A 31 51
354, DE-A 32 35 017 or DE-A 33 34 598.
55. The substances applied additionally make up only from about 0.1
to 5% by weight, preferably from about 0.5 to 3% by weight, of the
overall pigment.
56. In addition, the novel pigment can also be coated with firmly
adhering inorganic or organic colourants of low solubility.
Preference is given to the use of colour lakes and, in particular,
of aluminium colour lakes. For this purpose a layer of aluminium
hydroxide is applied by precipitation and in a second step is laked
with a colour lake. The process is described in more detail in DE
24 29 762 and DE 29 28 287.
57. Preference is also given to an additional coating with complex
salt pigments, especially cyanoferrate complexes, for example
Prussian blue and Turnbull's blue, as is described in EP 0 141 173
and DE 23 13 332.
58. The novel pigment can also be coated with organic dyes and, in
particular, with phthalocyanine or metal phthalocyanine and/or
indanthrene dyes in accordance with DE 40 09 567. To this end a
suspension of the pigment in a solution of the dye is prepared and
this solution is then combined with a solvent in which the dye is
of low or zero solubility.
59. Furthermore, metal chalcogenides or metal chalcogenide hydrates
and carbon black can also be employed for an additional
coating.
60. The examples which follow are intended to illustrate the
invention in more detail without limiting it.
EXAMPLE 1
61. A circulating belt of polyethylene terephthalate (width: 0.3 m,
speed: 20 m/min) is coated with a 20% titanium tetrachloride
solution by means of a counterrotating applicator roll. The coating
solution contains 0.3% by weight of surfactant (DISPERSE-AYD W-28,
manufacturer: DANIEL PRODUCTS COMPANY). The aqueous film on the
belt is dried in a drying section by subjecting it to hot air at
70.degree. C., and the layer formed is detached from the belt in a
detachment basin filled with deionized water. The titanium dioxide
platelets are filtered and washed with deionized water. The
platelets have a silvery lustre and a coat thickness of 100.+-.10
nm.
62. For further coating, they are either redispersed in deionized
water or dried at 110.degree. C.
EXAMPLE 2
63. 5-layer system of
TiO.sub.2/SiO.sub.2/TiO.sub.2/SiO.sub.2/TiO.sub.2
64. (1) SiO.sub.2 layer:
65. 50 g of TiO.sub.2 flakes having a yellow interference colour
(particle size<20 .mu.m) are suspended in 1.5 l of deionized
water and the suspension is heated to 75.degree. C. 270 ml of a
sodium silicate solution (125 g of SiO.sub.2/l) are metered into
this suspension at 75.degree. C. over the course of 90 minutes.
During this addition the pH is held constant at 7.5 with 10%
hydrochloric acid. When addition is over, stirring is carried out
at 75.degree. C. for 30 minutes in order to complete the
precipitation.
66. (2) TiO.sub.2 layer:
67. The pH of the suspension is lowered to 2.2 with 10%
hydrochloric acid, and 590 ml of an aqueous TiCl.sub.4 solution
(400 g of TiCl.sub.4/l) are metered in over the course of 3 h.
Throughout the addition the pH is held constant at 2.2 with 32%
NaOH solution. When addition is over, stirring is carried out at
75.degree. C. for a further 30 minutes in order to complete the
precipitation.
68. The mixture is then allowed to cool to room temperature and the
pigment obtained is filtered off, washed salt-free with deionized
water and dried at 110.degree. C. The pigment is then calcined at
700.degree. C. for 30 minutes. The pigment thus obtained exhibits a
brilliant, golden interference colour which is substantially more
intense than that of the starting material.
EXAMPLE 3
69. 5-layer system of
TiO.sub.2/SiO.sub.2/TiO.sub.2/SiO.sub.2/TiO.sub.2 with a blue
interference colour
70. (1) Sio.sub.2 layer:
71. 40 g of TiO.sub.2 flakes having a silver interference colour
are suspended in 1.5 l of deionized water and the suspension is
heated to 75.degree. C. 580 ml of a sodium silicate solution (125 g
of SiO.sub.2/l) are metered into this suspension at 75.degree. C.
over the course of 180 minutes. During this addition the pH is held
constant at 7.5 with 10% hydrochloric acid. When addition is over,
stirring is carried out at 75.degree. C. for 30 minutes in order to
complete the precipitation.
72. (2) TiO.sub.2 layer:
73. The pH of the suspension is lowered to 2.2 with 10%
hydrochloric acid, and 470 ml of an aqueous TiC'.sub.4 solution
(400 g of TiCl.sub.4/l) are metered in over the course of 120 min.
Throughout the addition the pH is held constant at 2.2 with 32%
NaOH solution. When addition is over, stirring is carried out at
75.degree. C. for a further 30 minutes in order to complete the
precipitation.
74. The mixture is then allowed to cool to room temperature and the
pigment obtained is filtered off, washed salt-free with deionized
water and dried at 110.degree. C. The pigment is then calcined at
700.degree. C. for 30 minutes.
75. The pigment thus obtained exhibits a deep-blue interference
colour.
* * * * *