Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1999-03-29
2001-06-05
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S494000, C106S495000, C106S496000, C106S497000, C106S498000, C106S499000
Reexamination Certificate
active
06241813
ABSTRACT:
The invention relates to a novel process for granulating organic colour pigments in the presence of aqueous or alcoholic media at low pressure, and to the dust-free pigment granules which can be prepared by this process.
BACKGROUND
Organic pigments consist of very fine particles, of low solubility in customary solvents, whose dimensions can lie within the range from submicroscopic to about 100 &mgr;m. For practical use, organic pigments having approximate particle sizes of from 0.01 to 0.1 &mgr;m for transparent forms and from 0.1 to 10 &mgr;m for opacifying forms have proved most suitable.
The physical properties of the pigment particles are very important to their use. For instance, very small particles possess an often relatively low light fastness and fastness to weathering and a strong propensity to agglomeration. Very coarse particles, on the other hand, give rise to undesirably low colour intensities and duller shades. In the case of the physical properties, however, particle size distribution and agglomeration play a key part, especially with respect to the dispersibility of the pigments [cf. Farbe und Lack 82/1, 7-14 (1976)].
It is therefore of critical importance for pigments to have a very narrow particle size distribution, which can usually be achieved by reprecipitation, recrystallization or heat treatment in a polar solvent, at atmospheric or superatmospheric pressure or under a high shear force (U.S. Pat. No. 4,879,380). However, all such pigments, irrespective of the narrow particle size distribution, still have the great disadvantage of producing dust. Consequently, when they are used, expensive measures (for example of a workplace safety, ecological or quality assurance nature) are necessary and valuable material is lost.
A very large number of methods have therefore already been investigated for converting the pigments into a low-dust or even dust-free form. It has however been found that improvements in the dusting behaviour can be obtained in the case of the known methods only, among other disadvantages, at the expense of the physical properties of the pigment particles, and especially at the expense of the dispersibility. Consequently, the known methods described below are unable to satisfy fully the long-held wish for dust-free organic pigments which continue to have good physical properties.
Known compacting methods, such as compression moulding (tableting or briquetting), granulation in mix granulators and granulating discs (Aufbereitungs-Technik 12 (1975)) and in formers (Chem.-Ing.-Tech. 49/5, 374-380 (1977)), roll granulation (DE-A 27 23 221) or pressure granulation (Powder Technology 74, 1-6 (1993)) always lead, with pure organic pigments, to highly agglomerated products having performance properties worse than those of the powder. A common feature of these methods is that the pigment particles collide with one another with a relatively high force.
Pigments for use in plastics can be incorporated into polymer concentrates. The pigment is employed as a dry, dusting powder. In this context, high shear forces and temperatures are required to disperse the pigment particles thoroughly, and the physical properties and colour properties are changed. The resulting polymer grains must in turn be mixed intimately with uncoloured polymer grains for the end use, again under high shear force, since it is necessary to effect homogeneous distribution of the pigment particles together with the completely surrounding polymer. Moreover, the concentrate polymer must be compatible with the other polymer, which is why, for a single pigment, a range of two or more products is required for different plastics applications.
Pigments can also be applied to the surface of externally softened polymer granules to give spherical particles (U.S. Pat. No. 4,310,483). However, the size of such particles is difficult to control, and the fraction having the desired diameter has to be isolated by sieving. It is said that the granulating auxiliary can be used in amounts of 2-50% by weight (preferably 5-30% by weight), although it has been found that good dispersibility can only be achieved with amounts of at least 15-20% by weight. An additive which can be used in addition to the polymer granules is a wax-like binder whose melting point is typically from 49 to 88° C. (U.S. Pat. No. 5,455,288). In the latter case, however, the pigment content is at an unsatisfactorily low level of from 5 to 50%. In both cases, collision forces are principally at work in the case of low shear forces, and the presence of more than 10% by weight of a substance of low melting point is disadvantageous from the performance standpoint.
Pigments can also be embedded in resins. This is done by first preparing a dispersion of the pigment in an inert solvent (for example water) and a solution of the resin in an appropriate solvent and then mixing the two, and precipitating the resin from the solution, either directly in the course of mixing or else later, the pigment being enveloped by the precipitating resin. innumerable publications have proposed, as the resin, almost all substances known to have a certain resinous character, including rosin. Various processes in accordance with this principle are known, for example acid/base precipitations (CS 216 590; IN 156 867; DE-A 33 27 562) and one- or two-phase solvent granulations (U.S. Pat. No. 4,055,439; U.S. Pat. No. 4,208,370). The not entirely satisfactory dispersibility of such resin-embedded pigments can be improved by the use of special resin mixtures coupled with a very high shear force (U.S. Pat. No. 4,116,924; U.S. Pat. No. 4,168,180). Nevertheless, the preparation of the pigment dispersion in any case requires intensive milling, especially when the inert solvent used is an aqueous medium, in which case the pigment, however, is comminuted in an undesirable manner. Instead of resins it is also possible to use surfactants (EP 403 917): in this case, although dispersion is made easier, the product is not obtained in a dust-free form but rather in a powder form.
In the case of the acid/base precipitations, the control of neutralization is a further problem which cannot be solved with complete satisfaction by the method described in DE 33 27 562. When precipitating with acid, in fact, the resin does not precipitate in a completely neutral form, which in many cases causes problems for high-grade applications, such as coating operations or the mass colouring of plastics. In the case of solvent granulations, on the other hand, large amounts of solvents are required which, disadvantageously, have to be recovered from usually aqueous mixtures. Therefore, the use of acetic or propionic anhydride as solvent has been proposed (EP 069 617), giving rise to aqueous solutions which can purportedly be used in the chemical industry but which, for lack of demand, have to be disposed of at considerable cost.
It is also known that colorants can be converted into a low-dust flowable form by spray-drying or in a fluidized bed (EP 039 841; EP 670 352). The additives used therein, however, are completely unusable in the case of pigments that are to be used in high-grade applications such as the mass colouring of plastics or automotive finishing operations. Moreover, in the case of spray-drying or in a fluidized bed it is hardly possible to prepare homogeneous granules having a particle size of more than a few 100 &mgr;m (cf. e.g. Chemie-Technik 2116, 72-78 (1992); Arch. Pharm. Chemi, Sci. Ed. 1978/6, 189-201). Furthermore, it is not possible to fluidize all powders in a fluidized bed (Powder Technology 57, 127-133 (1989)), so that it is in no way possible to make generalized use of this method.
An improved variant of fluidized bed granulation, especially for pigments, is also known (U.S. Pat. No. 4,264,552) where the particle size distribution of these granules is very broad and the great majority of the particles (about half by weight) are smaller than 500 &mgr;m. Furthermore, these granules still have an excessive propensity to produce dust. In Example 2 the use is discl
Balliello Paolo
Brücker Horst Olaf
Bell Mark L.
Ciba Specialty Chemicals Corporation
Crichton David R.
DiVerdi Michael J.
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