A powder coating has been drawing much attention as a coating fit for the recent preference for pollution-free and resource-saving production, since it can form a coating film without using an organic solvent. Thus, the amount used of the power coating has been on a marked increase, and the field of its use has been rapidly widening.
Such powder coating is generally made from a powder material containing a film-forming resin as a main component, and typically applied to a coating object by an electrostatic powder coating method. The electrostatic powder coating method comprises charging a powder coating by a corona charge method or a frictional electrification method, spraying said powder coating on an earthed coating object to allow electrostatic adhesion of the powder coating, and heating-melting same to form a coating film.
In the above-mentioned electrostatic powder coating method, it is required to uniformly adhere the charged powder coating to the entire coating object, so that a coating film having a uniform thickness can be formed thereon. When the object has complicated non-planar parts (e.g., concave part and convex part), such as a box or an object having a folded part, however, the powder coating does not penetrate into the non-planar part easily, so that the amount of the powder coating adhered to the non-planar surface tends to be less than that adhered to other planar surfaces. Therefore, a coating film formed on the entire object cannot have a uniform thickness, because of the smaller film thickness of the non-planar part as compared to that of other parts. In particular, when a powder coating is charged by a corona charge method, Faraday cage effect prevents the powder coating from adhering to the non-planar surfaces, so that the thickness of the coating film formed on the object tends to be non-uniform. On the other hand, when the powder coating is charged by a frictional electrification method, the transfer efficiency and throwing power of the powder coating can be improved more than when a corona charge method is used, though only with regard to certain coating objects, such as net type objects and miniaturized parts having complicated shapes. When the coating object has a three dimensional structure having more planar parts such as corrugated sheet and box, the powder coating does not adhere to the non-planar surface easily, and the coating film tends to have non-uniform film thickness, as in the case of the corona charge method.
It is therefore an object of the present invention to provide a powder coating capable of forming a coating film having a uniform thickness even on a coating object having complicated shape inclusive of non-planar parts, a method for coating same and a coating film obtained therefrom.