Metal oxide coated titanium dioxide lamellas

Details Metal oxide coated titanium dioxide lamellas Metal oxide coated titanium dioxide lamellas

1999-03-09

2001-05-29

Bell, Mark L. (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

Pigment, filler, or aggregate compositions, e.g., stone,...

C106S415000, C106S437000, C106S438000, C106S439000, C106S441000, C106S442000, C106S446000

Reexamination Certificate

active

06238472

ABSTRACT:

The invention relates to very thin pearl lustre pigments based on metal oxide-coated or metal oxide hydrate-coated platelet shaped titanium dioxide.
Pearl lustre pigments based on mica, which have further metal oxide layers on a titanium dioxide layer, are known. U.S. Pat. Nos. 3,087,828 and 3,087,829 mention aluminium oxide, zirconium oxide, zinc oxide and tin oxide as colourless oxides for a second metal oxide layer and, as oxides which have an intrinsic colour, iron oxide, nickel oxide, cobalt oxide, copper oxide and chromium oxide. The deposition of a second metal oxide on the titanium dioxide hydrate layer leads to a marked stabilization with respect to the photosensitivity of this layer. Iron oxide-containing titanium dioxide pigments, in particular, have been used successfully for many years.
It is described in U.S. Pat. No. 3,087,828 that by deposition of an Fe
2
O
3
—layer on a TiO
2
layer gold-coloured mica pigments are obtained which assume a reddish colour shade on calcining.
In U.S. Pat. No. 3,874,890, a process for the preparation of gold-coloured pearl lustre pigments is described in which a mica pigment coated with TiO
2
and/or ZrO
2
is coated first with iron (II) hydroxide, which is then oxidized to Fe
2
O
3
.
U.S. Pat. No. 4,744,832 describes a pearl lustre pigment based on plateletlike substrates coated with metal oxides, in particular mica, the metal oxide layer containing both titanium and iron and the pigment having a multilayer structure, a layer of pseudobrookite and an iron oxide layer following on a first layer of TiO
2
in the rutile form.
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 layers, however, mica pigments are not suitable, since the mica itself, as substrate for the metal oxide layers of the pigment, has a thickness of from 200 to 1200 nm. A further disadvantage is that the thickness of the mica platelets within a certain fraction defined by the platelet size in some cases varies markedly about a mean value. Moreover, mica is a naturally occurring mineral which is contaminated by foreign ions. Moreover, technically highly complex and time-consuming processing steps are required, including, in particular, grinding and classifying.
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 &mgr;m.
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. Disadvantages, however, are that the glass flakes have a very large average thickness of about 10-15 &mgr;m and a very broad thickness distribution (typically between 4 and 20 &mgr;m), whereas the thickness of interference pigments is typically not more than 3 &mgr;m.
EP 0,384,596 describes a process in which hydrated alkali metal silicate is subjected at temperatures of 480-500° C. to the action of an air jet, forming bubbles with thin walls; the bubbles are subsequently comminuted to give platelet shaped alkali metal silicate substrates with a thickness of less than 3 &mgr;m. However, the process is complex and the thickness distribution of the resulting platelets is relatively broad.
DE 11 36 042 describes a continuous belt method of preparing platelet shaped 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 layer thickness of the platelets obtained is given as being 0.2 to 2 &mgr;m, although no concrete examples of this are cited.
EP 0,240,952 and EP 0,236,952 propose a continuous belt method of preparing different platelet shaped materials, including silicon dioxide, aluminium oxide and titanium dioxide. In this method, a thin liquid film of defined thickness of a precursor of the platelet shaped material is applied, via a roller system, to a smooth belt; the film is dried and detached from the belt, forming platelet shaped particles. The particles are subsequently, if desired, calcined, ground and classified.
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, via a roller system, to the continuous belt, for example. The layer thickness of the platelets is given in the examples as being 0.3 to 3.0 &mgr;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 tranferred 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.
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 layer material requires, in general, strong 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 highly disadvantageous for the economy of the method.
WO 93/08 237 describes plateletlike pigments consisting of a platelet shaped matrix comprising silicon dioxide, which may contain soluble or insoluble colourants and which is covered with one or more reflecting layers of metal oxides or metals. The platelet shaped matrix is prepared by solidification of waterglass on a continuous belt.
DE 1 273 098 describes the preparation of a mother-of-pearl pigment by vapour deposition of ZnS, MgF
2
, ZnO, CaF
2
and TiO
2
films onto a continuous belt. This process, however, like the process described in U.S. Pat. No. 4,879,140 in which platelet shaped pigments with Si and SiO
2
layers are obtained by plasma deposition from SiH
4
and SiCl
4
, is associated with very high expenditure on apparatus.
Despite numerous attempts, it has not hitherto been possible to develop any economic process for preparing very thin platelet shaped titanium dioxide pigments having a layer thickness of less than 500 nm.
The object of the invention is to provide highly lustrous pearl lustre titanium dioxide-containing pigments having a layer thickness of less than 500 nm and a layer-thickness tolerance of less than 10%.
This object is achieved in accordance with the invention by a pearl lustre pigment having a multilayer structure, where, on a core of platelet shaped titanium dioxide, there follows a layer of another metal oxide or metal oxide hydrate, obtainable by solidifying an aqueous solution of a thermally hydrolysable titanium compound on a continuous belt, detaching the resulting layer, coating the resulting titanium dioxide platelets, without drying in between, with another metal oxide by a wet method, separating, drying and, if desired, calcining the material obtained.
The other metal oxide or metal oxide hydrate which is applied to the titanium dioxide is Fe
2
O
3
, Fe
3
O
4
, FeOOH, Cr
2
O
3
, CuO, Ce
2
O
3
, Al
2
O
3
, SiO
2
, BiVO
4
, NiTiO
3
, CoTiO
3
and also antimony-doped, fluorine-doped or indium-doped tin oxide.
In a particular embodiment of the novel pigment, on

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