Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Patent
1999-08-13
2000-10-17
Cain, Edward J.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
524878, C08L 3300
Patent
active
061333441
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to colored, pulverulent coating compositions comprising spherical particles having a mean particle size <40 .mu.m and a monomodal particle size distribution with a span (d90-d10/d50) of .ltoreq.2.5. The novel coating compositions can be melted at temperatures <200.degree. C. to form a continuous coating and are particularly suitable for use as powder coatings.
Powder coatings consist in general of a film-forming polymer which is crosslinkable if desired, of additives, for example flow improvers or devolatilization auxiliaries, and, in the case of colored powder coatings, of one or more pigments or dyes and fillers.
Powder coatings are traditionally prepared by mixing the abovementioned components thoroughly in an extruder at a temperature which is above the softening temperature of the film-forming polymer but below the crosslinking temperature and then bringing the resulting extrudate to a particle size of from about 40 to 70 .mu.m by means of a milling process.
A key disadvantage of powder coatings in comparison with liquid coating systems is the fact that it is necessary to establish the color of the powder coating during the preparation of the extrudate, prior to the milling process, by appropriate metering of color-imparting components. Altering the shade in the finished powder by blending different-colored powders has to date not been possible because, owing to the particle size of the powders from the milling processes employed to date, the human eye is still able to distinguish, in the finished coating, the individual colors employed for the blend, and thus of the coating gives a nonuniform impression (D.A. Bate, "The Science of Powder Coatings" page 17, SITA, UK 1990 ISBN 0 9477798005).
For liquid coatings, on the other hand, it is usual to prepare paints in various base colors and to establish the shade required at the time by blending these base colors. This procedure has the advantage that it is only necessary to stock the base colors. The logistical complexity is therefore much less than for conventional powder coating systems, where each shade must be prepared specifically and where it is impossible to correct color deviations caused, for example, by fluctuations in the quality of the raw materials employed.
For this reason there has been no lack of attempts already, in the past, to overcome these disadvantages of powder coatings. For example, EP-B-0 459 048 describes a process for preparing colored powder coatings by blending ultrafine particles having a size <20 .mu.m which are obtained by extensive milling processes. In the case of the coating compositions described here it is necessary for essentially all of the particles to have a particle size of 20 .mu.m, in particular <15 .mu.m. The disadvantage of the coating compositions disclosed is, in particular, that particles having a size of less than 15 .mu. can no longer be processed by the electrostatic spray process. The state of affairs is essentially due to the irregular structure of the particles obtained in milling processes. The colored particles mentioned above must therefore be agglomerated, in an additional process step prior to their application as a powder coating, to form larger particles having a diameter of about 30 .mu.m or more. This additional process step entails considerable expense. In particular, establishing the color of a powder coating by simple mixing of two or more colored powders, without aftertreatment, is not possible in accordance with this process either.
The average particle diameter of a powder, and the breadth of the particle size distribution, are characterized using not only the parameter d50, for which precisely 50% of the particles are smaller or larger than the value d50, but also two other 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. distributio
REFERENCES:
Patent Abstract of Japan No. 07188586 dated Jul. 25, 1995.
Patent Abstract of Japan No. 08143788 dated Apr. 6, 1996.
Blatter Karsten
Simon Peter
Aventis Research & Technologies GmbH & Co. KG
Cain Edward J.
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