Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
1999-08-13
2001-07-17
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C528S272000, C528S481000, C528S491000, C528S503000, C524S700000, C524S730000
Reexamination Certificate
active
06261690
ABSTRACT:
The present invention relates to homogeneously colored, spherical polyester particles having particle sizes <50 &mgr;m, which form a continuous coating at temperatures <200° C., to a process for their preparation and to their use as powder coatings.
Powder coatings consist in general of a film-forming polymer which may be crosslinkable, of additives such as, for example, flow improvers or devolatizing auxiliaries, and, in the case of colored powder coatings, of pigments and, if desired, fillers.
Powder coatings are traditionally prepared by subjecting the abovementioned components to intensive mixing in an extruder at a temperature above the softening temperature of the film-forming polymer but below the crosslinking temperature and then, by means of a milling process, bringing the resulting extrudate to a mean particle size of from about 40 to 70 &mgr;m. The milling process leads to powders of irregular structure, which means that powders having a mean particle size of markedly less than 30 &mgr;m can no longer be processed by the electrostatic spray techniques customary in the processing of powder coatings. For example, EP-A-0 459 048 mentions that powder coating compositions having a particle size of less than 15 &mgr;m cannot be processed by the electrostatic spray technique.
The milled powders used in the prior art have a mean particle diameter of from about 40 to 70 &mgr;m and lead typically to a coat thickness of from 40 to 70 &mgr;m. The milling technology produces, in particular, a very broad particle size distribution. In addition, a broadening of this distribution is observed with increasing fineness of the powders.
The breadth of a particle size distribution is characterized using not only the parameter d50, for which just 50% of the particles are greater than or smaller than the value d50, but also two further parameters: d10 designates the particle size for which 10% of the particles are smaller than this limit value. Correspondingly, d90 designates the particle size to which 90% of the particles are finer than the value d90. The breadth of a particle size distribution generally characterized by forming a quotient which is referred to as the span and is calculated in accordance with the following formula: span=d90−d10/d50. The relationship is thus: the smaller the span the narrower the particle size distribution. A powder comprising spheres identical in size would have a span of 0. For milled powders of the prior art, with a mean particle size d50 of 50 &mgr;m, a span of 3-4 is typically obtained.
On the basis of economic considerations (lower material consumption) but also because of technical advantages (greater flexibility of the coating) a relatively low coat thickness is desirable for powder coatings. A relatively low coat thickness can be realized only by reducing the particle size of the powder. Another critical factor is that the powders have a very narrow particle size distribution, since otherwise there are difficulties in processing, especially with a high fines content.
There has therefore been no lack of attempts in the past to obtain a reduction in the particle size of powder coatings by means of new technologies without incurring the abovementioned disadvantages in powder processability. The aim is, in general, to prepare particles with a near-ideal spherical form, since such powders exhibit substantially more favorable flow behaviour than the irregular milled powders. It has been attempted, for example, to prepare near-spherical particles by spraying polymer melts. The results presented in WO 92/00342 indicate, however, that this leads only to moderate success. The particles obtained by this technique, although having a smoother surface than milled powders, are still far removed from the ideal structure of a sphere.
Another method which has been investigated for the preparation of spherical particles the spraying of polymers from a supercritical solution, as described, for example, in EP-A-0 661 091 or EP-A-0 792 999. This method too has substantial disadvantages. For example, in the cited applications it is stated that, owing to the sudden evaporation of the supercritical “solvent”, a powder is obtained which has a porous structure. If these powders are employed to prepared films there is—in comparison with nonporous powders—an increased occurrence of bubble formation and thus of defects in the coating, since the porous structure means that a large amount of gas is trapped in the powder and must be removed in the course of the process of film formation. The use of supercritical solvents, moreover, is technically complex since, for example, it requires operation under high pressures.
A method of producing spherical particles which differs in its principle is the production of a dispersion. Physical laws mean that in a dispersion the perfect spherical form is the preferred geometry of the particles obtained. Under appropriate conditions it is possible to prepare spherical particles having a very narrow particle size distribution.
There has therefore been no lack of attempts in the past to obtain polymer particles which can be used as binders in coating systems, preferably in high-solids liquid coating systems, by preparing them in dispersion (Keith Barett, Dispersion Polymerization in Organic Media, John Wiley & Sons, London, 1975). GB-1 373 531, for example, describes the preparation of stable dispersions of polycondensation polymers, such as polyesters.
The possibility of using the polymer particles from nonaqueous dispersion processes based in particular on polyesters, as a powder coating is addressed in DE-C-21 52515. Here, an existing polymer is brought into dispersion at a temperature <200° C. and coloration is obtained by adding pigments, preferably after the dispersion has been cooled to below the “solidification point” of the polymer particles. The resulting particles are described as substantially spherical “aggregates” of primary polymer particles, having a particle size of from 0.05 to 20 &mgr;m, and pigment articles. The aggregates, described as secondary particles, have a particle size of from 10 to 90 &mgr;m or from 100 to 300 &mgr;m and are obtained by spraying the dispersion. In the process described, pigments are added at room temperature or only slightly elevated temperature, which means that the pigment particles are attached only loosely to the polymer particles; experience has shown this to lead to problems in connection with the processing of the powder, since separation of the pigments from the polymer binder takes place. The possibility of adding pigments at a relatively high temperature prior to solidification of the binder is described as difficult and not preferable, because there may be a change in the particle size.
In addition, no methods are indicated of how powder coating systems can be prepared which crosslink at the desired low temperatures of between 120 and 200° C. The crosslinking systems mentioned all have a crosslinking temperature which is above the temperature required for dispersing.
The use, as described in DE-C-21 52 515, of a polymer which has already been condensed to high molecular weights as a starting product for dispersion preparation, moreover, has the following disadvantages: the already considerable viscosity of the polymers, which in the case of the commercially used polymers is in the range from 3000 to 20,000 mPas (at 200° C.) makes it difficult to achieve good division of the melt and to obtain a homogeneous particle size distribution.
The object of the present invention, consequently, is to provide homogeneously colored, spherical polyester particles, having a very low particle size and a narrow particle size distribution, with which there is no separation of the pigments from the polymeric binder in the course of powder processing, and which can be processed and, if desired, crosslinked even at low temperatures to form a continuous coating and are therefore suitable for use as powder coatings.
The present invention achieves this object and provides homogeneously colo
Blatter Karsten
Simon Peter
Acquah Samuel A.
Aventis REsearch & Technology GmbH & Co. KG
Frommer & Lawrence & Haug LLP
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