Fluid sprinkling – spraying – and diffusing – Electrostatic type – With fluid entrainment
Reexamination Certificate
1998-08-14
2001-03-20
Kashnikow, Andres (Department: 3752)
Fluid sprinkling, spraying, and diffusing
Electrostatic type
With fluid entrainment
C239S690000, C239S692000, C239S706000, C239S708000, C239S427300, C239S432000
Reexamination Certificate
active
06202945
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for electrostatic powder coating.
DESCRIPTION OF RELATED ART
In recent years, many efforts have been made on an international level to prevent the deterioration of the global environment, because changes of the global environmental generate problems. In the field of coating technology, paint containing organic solvent generates various problems, for example, public pollution due to organic solvent spilled by coating work, environmental pollution due to volatile organic compound (VOC), and malodor. To resolve these problems, high-solid paint, aqueous paint, and methods of powder coating have been developed. The powder coating, in particular, is free from the problems of public pollution and hazards due to organic solvents, because it uses powder paint containing no organic solvent, so that it is useful to resolve the above problems. Other advantages of the powder coating are ease of thick coating and automated operation.
The electrostatic powder coating is one of the methods of powder coating. It uses powder paint positively or negatively charged to form a coating film on the earthed subject of coating by blowing the powder paint.
In powder coating, however, many powder paints of different hues are required, thus necessitating a vast storage area for the powder paints.
Also, it is difficult to have a number of electrostatic powder coating guns corresponding to the number of the hues of the powder paints. For this reason, in case of forming plural coating films of plural hues, an exchange of a powder paint of one hue for another powder paint of another hue must be frequently performed to blow them from one electrostatic powder coating gun. However, this exchange of the powder paints takes long time, so that actual operating time is extremely reduced.
Also, in case of forming plural coating films of plural hues, the amounts of powder paints of different hues are hardly equal to each other. Therefore, a great amount of powder paints remains unused. Disposal of the remaining powder paints results in increased cost. Storage instead of the disposal of the remaining powder paints results in deteriorated chargeability of the powder paints; this in turn decreases coating efficiency and reduces productivity. Furthermore, coating efficiency reduction necessitates the recovery of the powder paints, which cannot be adhered to the subject of coating, for recycled use. However, the recycled powder paint is problematic in that it is inferior to fresh paints in terms of coating performance, so that it is incapable of forming a uniform coating film.
Regarding powder paint production, a binder resin, a hardener, other additives, etc., and a prepared pigment are first mixed by using a mechanical mixer, then kneaded in a molten state. After being cooled, the mixture is milled to a given particle size to yield a powder paint for testing. Test coating is conducted by using the powder paint to form a coating film. If the hue of the coating film is not the desired one, the powder paint for testing is supplemented with another pigment to obtain another powder paint for testing. This process must be repeated until the desired hue is obtained. Also, when problems due to heat hysteresis must be avoided, another powder paint for testing must be produced from a new binder resin, hardener, other additives, etc., and a newly prepared pigment. In short, the preparation of a powder paint of a desired hue takes a great deal of labor and time.
To resolve these problems, a conventional method has been proposed in which a powder paint of a desired hue is prepared by mixing plural kinds of powder paints of different hues (National Publication No. H4-504431 for International Application).
When the conventional method is used for the preparation of a powder paint of a desired hue, however, a uniform hue cannot be obtained, if the mean diameter of the starting particles is greater than 10 &mgr;m. That is, the diameter of particles constituting powder paint has a significant influence on the obtainment of a uniform hue.
In the conventional method described above, the preparation of a powder paint of a desired hue is performed by simply mixing plural kinds of powder paints of different hues. It should be noted, however, that different kinds of powder paints differ from each other in terms of physical properties such as fluidity and chargeability. Because such different kinds of powder paints of different physical properties are difficult to uniformly mix together, the mixed powder paint is difficult to be uniformly charged. For this reason, in the conventional method, the diameter of the starting particles must be decreased to improve the uniformity of the powder paint, and the starting particles must be granulated after the mixing.
Also, powder paints often undergo chargeability reduction during a period of several days from the production date, even when they are sufficiently chargeable at the time of production. This can result in decreased coating efficiency and hence hamper the obtainment of a uniform coating film.
The present invention is directed to provide a method and apparatus for electrostatic powder coating capable of resolving the above-described problems.
SUMMARY OF THE INVENTION
The electrostatic powder coating method of the present invention comprises the steps of mixing plural kinds of powder paints of different hues without melting for preparation of a powder paint of a desired hue, mixing a fluidity improver, which possesses a charge control function, into the powder paints simultaneous with said mixing step without melting, charging the mixed powder paint containing the fluidity improver, and coating a subject of electrostatic powder coating by the charged powder paint.
According to the method of the present invention, a powder paint of a desired hue is prepared by mixing plural kinds of powder paints without melting and granulation. In this operation, the plural kinds of powder paints can be uniformly mixed with each other via the fluidity improver possessing a charge control function, by mixing the fluidity improver into the powder paints simultaneously with the mixing step without melting. Therefore, it is possible to reduce or eliminate the charge amount difference between the plural kinds of powder paints, when the mixed powder paint containing the fluidity improver is charged. That is, the powder paint of a desired hue can be uniformly charged. By performing the electrostatic powder coating with the uniformly charged powder paint, a uniformly coating film of a uniform hue can be formed. Thereby, a powder paint of a uniform hue can be obtained from plural kinds of powder paints without melting and granulating, and the influence of the diameter of particles constituting the powder paint can be reduced.
The fluidity improver improves the fluidity of the powder paint by inhibiting the direct contact of the particles constituting the powder paint with each other. The fluidity improver can be constituted of fine particles smaller than the particles constituting the powder paint. By coating each of the fine particles constituting the fluidity improver with a substance that positively or negatively charges the powder paint, the fluidity improver can obtain a charge control function.
The ratio of the fluidity improver to the plural kinds of powder paints is preferably 0.05 to 1% by weight, more preferably 0.1 to 0.5% by weight. If the ratio is lower than 0.05% by weight, the fluidity improving effect is insufficient. If the ratio exceeds 1% by weight, free particles are increased, so that the surrounding environment is polluted, the charge amount is decreased, and the strength of adhesion between the coating film and the subject of coating is reduced.
In the method of the present invention, it is preferable that the mixing of the powder paints and fluidity improver, the charging of the mixed powder paint, blowing of the charged powder paint to the subject of coating are continuously performed, with the powder paints and the fl
Maruta Masayuki
Sato Yukiya
Yasuda Shin-ichiro
Birch & Stewart Kolasch & Birch, LLP
Evans Robin O.
Kao Corporation
Kashnikow Andres
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