Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Patent
1999-08-13
2000-11-21
Acquah, Samuel A.
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
528272, 528302, 528307, 528308, 5283086, 528481, 528488, 528490, 528491, 528503, 525437, B32B 1502, C08F 600
Patent
active
061500216
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to spherical polyester particles having particle sizes <50 .mu.m, which can be crosslinked if desired at temperatures <200.degree. C., to a process for their preparation and to their use as powder coatings.
Transparent powder coatings consist in general of a film-forming polymer which may be crosslinkable, and of additives such as, for example, flow improvers or devolatilizing auxiliaries.
Powder coatings are traditionally prepared by subjecting the abovementioned components to intense mixing in an extruder at a temperature above the softening point 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 .mu.m. The milling process leads to powders of irregular structure, which means that powders having a mean particle size of markedly less than 30 .mu.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 .mu.m cannot be processed by the electrostatic spray method.
The milled powders used in the prior art have a mean particle diameter of from about 40 to 70 .mu.m and lead typically to a coating thickness of from 40 to 70 .mu.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 value. Correspondingly, d90 designates the particle size for which 90% of the particles are finer than the value d90. To characterize the breadth of a particle size distribution it is usual to form 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 with a mean particle size d50 of 50 .mu.m, a span of 3-4 is typically obtained.
On the basis of economic considerations (lower material consumption) but also of technical advantages (greater flexibility of the coating) a relatively low coat thickness is desirable for powder coatings.
There has therefore been no lack of attempts in the past to obtain a reduction in the particle size by means of new technologies without incurring the abovementioned disadvantages in processability. The aim is, in general, to prepare particles with a near-ideal spherical form, since such powders exhibit substantially more favorable flow behavior 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 is 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. When 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 powd
REFERENCES:
patent: 4687811 (1987-08-01), Sasaki et al.
patent: 5736621 (1998-04-01), Simon et al.
Blatter Karsten
Simon Peter
Acquah Samuel A.
Aventis Research & Technologies GmbH & Co. KG
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