Process for preparing a composition of pearlescent pigments

p0001The invention relates to a method for preparing a pearlescent pigment composition (preparation) as a precursor for a printing ink containing pearlescent pigment.

p0002Pearlescent pigment is usually dust or powder commercially. For processing in the printing ink the pearlescent pigment is mixed with a binder solution, and optionally usual additives pressure and the mixture adjusted to the application in the printing process for the required viscosity. To achieve the typical pearlescent soft luster effect in the printing ink a uniform distribution and orientation of the pearlescent pigment particles in the binder is required. In principle, high shear forces should be avoided at any processing of pearlescent pigments. Thus, the pearlescent pigment must be pre-treated for several hours before the actual dispersing process for manufacturing the printing ink solvent (so-called. "Swamps"). This process is time-consuming and therefore costly. Despite the pre-treatment by "swamps" and stirring the desired uniform dispersion can not always be achieved and thus not fully satisfy the printing.

p0003Object of the invention is therefore to provide pearlescent pigment for printing inks in a manner that thereby the actual production of the ink is greatly facilitated and simplified, and in particular the expected print quality, that is, an excellent pearlescent effect is achieved.

p0004This object is achieved in that the pearlescent pigment is prepared by the method defined in claim 1 to give a particulate precursor of the printing ink.

p0005It has been found, surprisingly, that takes place by a pre-wetting the pearlescent pigment particles with that of binder which is necessary or desirable for the product to be processed ink, a considerably more rapid and more uniform dispersion of the pigment particles in the taken into consideration in the actual manufacture of the printing ink binder solution , In addition to a time saving and associated flexible for the user, this results in a significant improvement of the intended pearlescence in pressure. Surprisingly, this is because, given the presence of the pearlescent pigment preparation in coarse particulate form, a more intensive mixing process is expected to achieve a sufficiently uniform dispersion. Decisive for the pre-wetting the pigment particles with the corresponding binder appears to be the remains surprisingly obtained despite placing the paste into a compact particulate. Obviously the occurring pressure or compression of the paste even leads to a particularly intimate pre-wetting the pigment particles with the binder.

p0006As binders for pre-wetting the pearlescent pigment all normally be used for printing inks in question binders or binder mixtures which, insofar as they are the same or at least compatible with the respective binders used system of the ink. Thus binder may on the basis of cellulose, polyacrylate, polymethacrylate, alkyd, polyester, polyphenol, urea, melamine, polyterpene, polyvinyl, polyvinyl chloride, polyvinyl pyrrolidone resins, polystyrenes, polyolefins, indene-coumarone , hydrocarbon, ketone, aldehyde, aromatic-formaldehyde resins, carbamic acid, sulfonamide, epoxy resins, polyurethanes and / or natural oils or derivatives of said substances. A particularly good redispersibility of the pearlescent pigment preparation of the invention has been shown, when cellulose or cellulose compounds are used as binders. Cellulose or cellulose compounds cause visible to an intensive pre-wetting the pearlescent pigment particles and thus give an ink precursor which can turn into a compatible therewith binder solution rapidly dispersed in the manufacture of ink. This is especially true since apparently the one hand, the binding of the other hand, assist the formation of the paste it to the particles of the pearlescent pigment of the pre-wetting with cellulose compounds from which by compacting the preparation is produced in particulate form.

p0007The group of cellulose or cellulose compounds which are particularly for the novel process include cellulose nitrate, alkyl cellulose, Hydroxicellulose, Hydroxialkylcellulose and cellulose acetate butyrate.

p0008The amount of binder used is not critical and is in the range of 0.5 to 45 wt .-%, preferably 2 to 15 wt .-%.

p0009However, of importance is the set by adding solvents for the binder viscosity of the mixture prior to forming the preparation particles by compacting the particulate, as if you have high solvent content of the mixture tend to the pearlescent pigments strongly for the separation of the binder solution while at a to high viscosity, the wetting incomplete and the formation of particles is considerably more difficult.

p0010The conversion of the pearlescent pigment paste into a compact particle shape can be produced by various, familiar to the expert method, namely by tabletting, briquetting, pelletizing, granulating or spray granulation. In all these cases, a pressure is exerted on the paste in a more or less viscous state that supports the above-described pre-wetting of the pigment particles with the binder. Accordingly, the particles are in the form of tablets, briquettes, pellets, granules, balls, sausages or other appropriate coarse-grained form. Particularly preferred as the compaction operation is a pressing or extrusion.

p0011If pressing the mixture or paste made of pearlescent pigment and binder solution through a perforated plate, the batch is done either by means of pistons or continuously through an extruder, particles may be formed in sausage form. By appropriately adjusting the viscosity of the mixture can be ensured that the length of the particles is automatically regulated by breaking or tearing off on emergence from the perforated plate. However, the separation of the exiting from the perforated plate paste strands into particles may also in a known manner by mechanical action, eg., by circular severing knife, done. The pelletizing or granulation takes place in a known manner on pelleting plates or in granulating and usually leads to spherical preparation particles. The particles initially produced by a perforated plate by means of pressing can also be subsequently reformed in the way of pelleting or granulating. A tableting or briquetting is done by pressing the paste in appropriate molds.

p0012By subsequently taking place to the forms of the preparation particles drying the solvent contained in the binding agent is substantially removed, so that the preparation particles on the one hand exhibit a sufficient mechanical resistance in order to maintain the handling and transportation their shape, on the other hand with the contact decompose rapidly with a binder solution at the slightest mechanical action (agitation) and disperse by.

p0013When the binder after removal of the solvent is not sufficiently viscous to allow compacting the paste to particulate form, may be thought by drying to remove the solvent from the paste before the compaction. If necessary, can be reduced by increasing the temperature of the paste the viscosity for compaction.

p0014The type of drying the particles produced by compacting or paste and thereby made to drive off the solvent is substantially dependent on the type of solvent used. Since most of this candidate solvent, in particular in connection with the cellulose as a binder, below the softening point of the binder is volatile, the drying can be performed at elevated temperature. It can be thought of in the case of drying the particles from to select the temperature close to the softening point of the binder, to thereby cause an onset at least in a surface layer of the particles or sintering of the pre-wet the binder coated pearlescent pigment particles. In this way, the dimensional stability of the particles is improved preparation for the handling and the transport time.

p0015The present invention also relates to pearlescent pigment based on mica, such. As muscovite, biotite or synthetic mica, with a TiO₂- And / or Fe₂O₃Coating. Optionally, in addition, further metal oxide layers of the series B, Al, Si, Cr, Sn, Zn, Zr or Ce be contained. It is also possible to use pearlescent pigment with a coating of titanium suboxides, titanium oxynitrides and nitrides or mixtures thereof. In addition to the metal oxide may also be present an occupancy of coloring pigments or dyes on the pearlescent pigment. Moreover, such a coloristic aspect by mixing the pearlescent pigments with coloring substances when pasting realized.

p0016As part of the process of the invention also mixtures of pearlescent and metallic pigments in the desired preparation can be prepared, as it has been shown that a certain proportion of metal pigments does not impair the effect of pre-wetting with the binder. However, the proportion of metal pigments, should not be more than 20% by weight of pearlescent pigment fraction. When metal pigment are any in effect pigment sector common metals such as aluminum, copper, zinc, tin and their alloys. Aluminum and gold bronze pigments are preferred for this purpose.