Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1994-04-15
2003-10-07
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S418000, C106S417000, C106S472000, C106S482000, C106S483000, C106S459000, C106S450000, C106S453000
Reexamination Certificate
active
06630018
ABSTRACT:
The invention relates to platelet-shaped pigments having high luster and high covering power or high transparency.
Covering power and luster can, in the case of platelet-shape pigments, often be achieved simultaneously to a satisfactory degree only with difficulty. Thus, mica platelets which are coated, for example, with one or more ma thin metal oxide layers are distinguished by interference colors and high luster, but at the same time also, as a result of the transparent substrate, by a high transparency and thus a comparatively low covering power. It is true that the covering power can be improved by using colored metal oxide layers composed, for example, of chromium oxide or iron oxide, but even the covering power of pigments of this type often do not meet all requirements. The covering power can be further increased if a rough substrate surface is present or if relatively rough metal oxide layers are deposited. The increased number of scattering centers increases the covering power but decreases the luster.
Depending on the refractive index of the metal oxide used and on the desired interference color, the thickness of the metal oxide layers is typically between 50 and 250 nm. Since reflections occur at the phase boundary of materials having different optical refractive indices, interference effects which depend on the angle of observation and the optical densities of the metal oxide layers are observed on parallel alignment of the platelet-like pigments in the reflected light; the corresponding complementary colour is seen in the transmitted light with non-absorbing materials.
A condition for the occurrence of interference effects of this type is a smooth substrate surface.
Until now, natural materials such as, for example, mica have mainly been used as platelet-shaped substrates for the production of interference pigments. Since this is a naturally occurring material, the surface of substrates of this type is not ideally smooth, but has irregularities such as, for example, steps, as a result of which the quality of the resulting interference pigments is limited.
A further disadvantage of natural materials, such as, for example, mica, is contamination by coloring foreign ions, for example iron ions, which have an adverse effect on the color purity of the final product.
Furthermore, it has been proposed to use platelet-shaped metal substrates instead of transparent mica, which leads to pigments having very high covering power. On the other hand, this simultaneously results in a very hard, metallic luster, which is often aesthetically not completely satisfactory. In addition, metal pigments do not have a deep action since the incident light is immediately reflected by the surface.
Thin glass sheets obtained by rolling a glass melt followed by milling have been proposed as the synthetic material. It is true that interference pigments based on such materials show color effects which are superior to conventional mica-based pigments. However, their disadvantage is that the glass sheets have a very high average thickness of about 10-15 &mgr;m and a very broad distribution of thicknesses (typically of between 4 and 20 &mgr;m), while 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 treated with a jet of air at temperatures of 480-500° C., resulting in the formation of bubbles having small wall thicknesses; the bubbles are then comminuted, giving platelet-shaped alkali metal silicate substrates having a thickness of less than 3 &mgr;m. However, the process is complicated and the distribution of thicknesses of the platelets obtained is relatively broad.
EP 0,240,952 proposed a continuous belt process for the preparation of various platelet-shaped materials, also including silica. In this process, a thin liquid film of defined thickness of a precursor of the platelet-like material is applied to a smooth belt via a system of rollers; the film is dried and peeled off from the belt, thus forming platelet-shaped particles. The particles are then, if desired, ignited and, if desired, milled, followed by classification.
Precursor materials used are organometallic: compounds (alkoxides), such as, for example, tetraethyl orthosilicate. The film is polymerized by drying and is scraped off the belt using a scraper, small platelets being obtained; these are then ignited at temperatures of 500° C. to convert them into the corresponding metal oxide. A metal oxide sol which is optionally dispersed in methanol and which is applied as a film, dried and ignited in an analogous manner is additionally used as a precursor.
However, a disadvantage is the use of very expensive precursor materials and in particular the increased requirements of workplace safety which are necessary when organometallic compounds are used. In order to achieve complete chemical conversion of the precursor into the desired film material, vigorous heating of the film and of the belt material are usually necessary. Apart from the resulting substantial thermal stress on the belt material, the high amount of energy used and the limitation of the process rate also have a very disadvantageous effect on the economy of the process.
The use of aqueous oxide or hydroxide sols described in EP 0,236,952 is also problematical, since the films formed are not homogeneous but are composed of irregularly sized particles. This requires a treatment at very high temperatures in order to give the material the necessary homogeneity, precision of shape and strength.
U.S. Pat. No. 3,138,475 describes a continuous belt process for the preparation of platelet- or leaf-shaped oxides or hydrated oxides of metals from groups IV and V and the iron group of the periodic table. In this process, first, if desired, a release layer comprising, for example, a silicone coating, is applied to a continuous belt in order to facilitate the later peeling-off of the metal oxide film. A liquid film comprising a solution of a hydrolyzable compound of the metal to be converted into the desired oxide is then applied, the film is dried and then peeled off by means of a shaking device. It is true that this publication mentions that it is also possible to prepare SiO
2
platelets by this process, but the process is described only very generally and no concrete example is given.
JP 64-9803 describes a process for the preparation of a platelet-shaped metal oxide containing dispersed fine particles of a second metal oxide of higher refractive index on a continuous belt. The products obtained by this process, which are used as light protection filters in cosmetics, consist, for example, of a matrix of silica in which fine particles of titanium dioxide are dispersed. However, this product shows no interference colors, since the titanium dioxide particles acting as scattering centers are not uniformly distributed on a smooth surface.
JP 2-32,170 discloses a pigment which consists of a base material, for example a titanium dioxide-mica pigment, in which colloidal metal particles (silver) are sputtered onto a first interference layer of titanium dioxide. As a second interference layer, titanium dioxide is then in turn applied as a covering layer. This pigment has the disadvantage that white cannot be produced, since the metal particles absorb and thus give the product a dark color. In addition, production is very expensive, since four different layers have to be applied using different processes.
A pigment of similar construction is disclosed in EP-A-0,484,108. Titanium is applied by sputtering to a titanium dioxide-mica pigment as base material. A part of the titanium reduces the titanium dioxide of the base material to suboxides, which act as light-absorbing regions on the pigment surface. The remaining unchanged titanium particles impart metallic lustre to the pigment.
Apart from the high production costs, only dark pigments of low transparency can be prepared by this process. In addition, interference colors cannot be produced.
The object of the present invention is to make ava
Bauer Gerd
Brenner Günther
Kniess Helge-Bettina
Osterried Karl
Schmidt Christoph
Koslow C. Melissa
Merck Patent Gesellschaft mit Beschränkter Haftung
Millen White Zelano & Branigan P.C.
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