Pigments

Details Pigments Pigments

2002-11-14

2004-11-16

Koslow, C. Melissa (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

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

C106S434000, C106S435000, C106S446000, C106S447000, C106S454000, C106S475000

Reexamination Certificate

active

06818051

ABSTRACT:

The invention relates to pigments, especially interference pigments, which are characterized by a three-dimensional periodic arrangement of monodisperse spheres in the nanometer range.
Natural precious opals comprise monodisperse, regularly arranged silica gel spherules with diameters of 150-400 nm. The colour play of these opals comes about by Bragg-like scattering of the incident light at the lattice planes of the spherules, with their crystalline arrangement.
There has been no lack of attempts to synthesize white and black opals for jewellery purposes, using waterglass or silicone esters as starting material.
U.S. Pat. No. 4,703,020 describes a process for producing a decorative material comprising amorphous silica spherules in three-dimensional, arrangement with zirconium oxide or zirconium hydroxide in the interstices between the spherules. The spherules have a diameter of 150-400 nm. Production takes place in two stages. In a first stage, silica spherules are caused to sediment from an, aqueous suspension. The resulting composition is then dried in air and subsequently calcined at 800° C.
In a second stage, the calcined material is introduced into the solution of a zirconium alkoxide, with the alkoxide penetrating into the interstices between the spheres and with zirconium oxide being precipitated by hydrolysis. This material is then calcined at 1000-1300° C.
In comparison to a natural opal, the material obtained possesses zirconium dioxide in the voids between the individual spheres.
The material is given mechanical stability firstly by inclusion of the zirconium oxide and also, very substantially, by the calcining, which, as is known, modifies the physical and chemical structure of the material. The production process described here has the disadvantage that the material has to be calcined a number of times at high temperatures. It is, consequently, a very energy-consuming process.
It was an object of the present invention to avoid the abovementioned disadvantages. The intention was in particular to provide a particulate material which exhibits opalescent effects similar to those of natural opal while having sufficient mechanical stability. A further object of the invention was to provide a preparation process for such a material that allows defined particles suitable for use as pigments to be obtained with optimized energy consumption.
The present invention therefore first provides particles with an opalescent effect, which have a particle size in the range from 5 &mgr;m to 5000 &mgr;m, the particles comprising monodisperse spheres having a diameter of 50 nm-2 &mgr;m with a standard deviation of less than 5% in a three-dimensional, regularly ordered structure which is closely packed in terms of domains and is mechanically stabilized by physical or chemical modification.
The present invention further provides a process for preparing particles, where, in one step
a) monodisperse spheres having a diameter of 50 nm to 2 &mgr;m with a standard deviation of less than 5% are suspended in a liquid medium,
b) the suspension is applied to a surface, and
c) the liquid medium is removed.
The invention further provides for the use of the particles of the invention as pigments especially in paints, varnishes, printing inks, plastics, ceramic materials, glasses and cosmetic formulations. For this purpose they may also be used together with customary commercial pigments, examples being organic and inorganic absorption pigments, metal effect pigments and LCP pigments. Furthermore, the particles of the invention are also suitable for producing pigment preparations and for producing dry preparations, such as granules, for example.
With the particles of the invention, the three-dimensionally packed and regularly ordered structure is particularly important for achieving the opalescent effect. Normally, this three-dimensionally packed and regularly ordered structure is not a perfect order extending over the entire particle. For achieving the desired colour effects, it is sufficient for the particles of the invention to have individual domains within which there is a uniform arrangement. For the explanation of this structure, reference may be made in particular to
FIGS. 1 and 2
, which show precisely the presence of ordered domains of monodisperse spheres and yet at the same time show that in this case there is by no means close, let alone very close, spherical packing for the particle as a whole.
Also important for the particles of the invention with opalescent effect is the physical or chemical modification for mechanical stabilization, since without such stabilization the particles would not be fixed in their three-dimensional form. Nevertheless, the modification cannot be made by just any mode of mechanical stabilization, since otherwise the opalescent effect, as already explained above, would be impaired. The difference in refractive index between the spheres and the material in the interstices critically influences the optical properties of the particles. Therefore, preference is given to those physical or chemical modifications which make it possible to maintain an appropriate difference in refractive index. In principle, the effects according to the invention may be observed with differences in refractive index in the range from about 0.01 to about 2. The optimum difference in refractive index for opalescent pigments is in the range from about 0.1 to 0.6, although much smaller or even larger differences in refractive index are suitable for exhibiting opalescent effects. With small differences in refractive index, such as from about 0.01 to about 0.02, the particles-are substantially transparent and therefore show particularly pronounced opalescent effects owing to the many effective reflection planes. Because of the transparency, however, the intensity of these effects is weak.
The monodisperse spheres used, including any coatings that may be present, possess a diameter in the range from 50 nm to 2 &mgr;m, preference being given to the use of spheres having a diameter of 150-1500 nm. Particular preference is given to using spheres in the range from 200-500 nm, since with particles in this order of magnitude the reflections of different wavelengths of visible light differ distinctly from one another and, accordingly, the opalescence occurs to a particularly pronounced extent in a variety of colours. In one variant of the present invention, however, it is also preferred to use multiples of this preferred particle size, which then result in reflections corresponding to the higher orders, and thus in a broad play of colour.
The monodisperse spheres may comprise almost any materials which are sufficiently transparent for the wavelengths of the desired light reflections, so that this light is able to penetrate several sphere diameters deep into the particle. Preferred spheres comprise metal chalcogenides, preferably metal oxides or metal pnictides, preferably nitrides or phosphides. Metals in the sense of these terms are all elements which may occur as an electropositive partner in comparison to the counterions, such as the classic metals of the transition groups and the main group metals of main groups one and two, but also including all elements of main group three and also silicon, germanium, tin, lead, phosphorous, arsenic, antimony and bismuth. The preferred metal chalcogenides and metal pnictides include, in particular, silicon dioxide, aluminum oxide, titanium dioxide, zirconium dioxide, gallium nitride, boron nitride and aluminum nitride and also silicon nitride and phosphorous nitride.
Starting material for the production of the particles of the invention preferably comprises monodisperse spheres of silicon dioxide which are obtainable, for example, by the process described in U.S. Pat. No. 4,911,903. The spheres are produced by hydrolytic polycondensation of tetraalkoxysilanes in an aqueous-ammoniacal medium, a sol of primary particles being produced first of all and then the SiO
2
particles obtained being brought to the desired particle size by continuous, controlled additio

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