subject: Powder Coating Machine [print this page] The present invention provides a combination of powder coatings usable in a powder coating method for forming a coating film having visually homogeneous hue by mixing two or more powder coatings of which each color is different, whereby a coating film having visually homogeneous hue is obtained by applying onto a substrate to be coated two or more powder coatings. In the present specification, the coating film having visually homogeneous hue means that the hue of the formed coating film is homogeneous to such an extent that the colors of individual powder coatings blended therein cannot be discerned by human eyes.
The powder coating of the present invention may be prepared by melt-kneading each of the above components using an extruder. After cooling the resulting kneaded mixture, it is subjected to physical pulverization using pulverizing devices, such as hammer mills and jet mills, and then the pulverized product is classified using classifiers, such as air classifiers and micron-classifiers, to give powder coatings of desired average particle sizes. The powder coating of the present invention preferably is a powder having an average particle size of from 1 to 50 m, more preferably from 5 to 30 m. Further, from the viewpoint of easy handling, the average particle size is more preferably from 10 to 20 m. The average particle size of the powder is preferably 50 m or less, from the aspect of having a sufficiently thin coating film, and the average particle size is preferably 1 m or more, from the aspect of preventing the agglomeration of the particles, thereby making it possible to achieve good homogeneous blending. In the powder coating of the present invention, a visually homogeneous coating film can be easily prepared even when the particle size of the coating to be applied has a size exceeding 10 m, as in the case of having a size of 10 to 20 m as mentioned above.
In the present invention, an example of a method for measuring triboelectric charges of each of powder coating is as follows. First, a coating powder is applied using the coating machine and the substrate to be coated under working conditions but without curing. After air blowing-off of the powder from the substrate, a difference in voltage owing to the efflux of electric charges from the substrate and the amount of the powder coatings removed are measured. Alternatively, as a simple method, the triboelectric charge is measured by a blow-off method when each of the powder coatings is blended with various powder coatings having particle sizes larger than that of the powder coating to be tested.
