subject: Special points of new powder coating [print this page] Special points of new powder coating Special points of new powder coating
The powder coating for use in a coating method which includes the steps of mixing and applying two or more powder coatings to a substrate, the powder coating comprising a resin not having thermosetting properties at a temperature of 160 C. or less by itself, and being heat-cured at a temperature of 160 C. or less when used in combination with other powder coatings. The powder coating composition comprising two or more powder coatings, wherein each of the powder coatings comprises a resin not having thermosetting properties at a temperature of 160 C. or less by itself, and being heat-cured at a temperature of 160 C. or less to form a coating film when used in combination with other powder coatings.
The present invention relates to a powder coating for use in a coating method comprising mixing and applying to a substrate two or more powder coatings, a powder coating composition containing the same, and a coating method therewith.
2. Discussion of the Related Art
Conventionally, powder coatings have been prepared by mixing resins, curing agents, additives, and the like, melt-kneading the mixture, followed by cooling, pulverization, and classification. In order to uniformly disperse the above additives, it has been necessary to carry out melt-kneading at a temperature of from 60 to 160 C.
However, when a curing agent which can be low-temperature cured in the above-mentioned temperature range is used, the curing of the resins initiates during melt-kneading, and therefore a powder coating cannot be prepared. In addition, when a curing agent having a high curing temperature is used in order to avoid the curing of the resins during melt-kneading, the stoving temperature is also elevated, so that a new plant investment is necessitated, and such defects as high overall running costs are increased.
On the other hand, when the kneading temperature is lowered by using a curing agent having a low curing temperature, it has been necessary to lower the fusing temperature of the resins contained in the powder coating in order that the resins can be sufficiently kneaded even at a low temperature. However, since the resin having a low fusing temperature also has a low glass transition temperature, it has poor storage stability, so that there arise defects in the quality guarantee of the resulting powder coating. In addition, although a method of mixing a curing agent with the powder coating by dry-blending has been known, the resulting powder coating is liable to have unevenness in the cured product, thereby causing the defects in lowering the coating properties. Therefore, various improvements therefor have been studied, but a powder coating with satisfactory performance has not yet been obtained (Japanese Patent Laid-Open Nos. 57-31966 and 59-24762).
An object of the present invention is to provide a powder coating for use in mixing and applying powder coatings and a powder coating composition which can be low-temperature cured and maintains sufficient storage stability.
Another object of the present invention is to provide a coating method using the powder coatings and the powder coating composition.
The powder coating of the present invention is concerned with a powder coating for use in a coating method which includes the steps of mixing and applying two or more powder coatings to a substrate, the powder coating comprising a resin not having thermosetting properties at a temperature of 160 C. or less by itself, and being heat-cured at a temperature of 160 C. or less when used in combination with other powder coatings.
The resin usable in the powder coating of the present invention may be any conventionally known resin without limitation, wherein the fusing temperature of the resin is 160 C. or less, preferably 90 to 140 C., and the fusing temperature must be adjusted to a temperature not higher than the curing temperature of the mixed powder coatings.
In the present invention, the fusing temperature refers to a softening temperature, i.e. a temperature at which the resin liquefies. The fusing temperature of the resin can be adjusted in a manner such as modification of the chemical structure or modification of the molecular weight distribution of the resin.
In addition, it is desired that the glass transition temperature of the resin usable in the present invention is 40 C. or more, preferably 50 C. or more from the aspect of storage stability, and that the glass transition temperature is 90 C. or less, preferably 80 C. or less from the aspect of the fusing temperature of the powder coating.
In the present invention, in the case where two or more powder coatings are mixed and applied by suitably selecting a combination of resins respectively used for two or more powder coatings, each of which does not have thermosetting properties alone at a temperature of 160 C. or less, and preparing the powder coating according to the following embodiments, the powder coatings are fused and mixed to be heat-cured at a temperature of 160 C. or less, to form a coating film.
Incidentally, the phrase "does not have thermosetting properties" refers to the fact that an exothermic peak corresponding to thermosetting cannot be detected when measured by using a differential scanning calorimeter ("DSC210," manufactured by Seiko Instruments), at a heating rate of 10 C./minute. In addition, the term "resin" used in the present specification refers to a resin which is a polymeric substance having a number-molecular weight of 500 or more, preferably 1000 or more, and having an ability to form a film by heat-fusing.
