Conventionally, resins such as polyethylene, polyamide, epoxy resin, and polyvinyl chloride are used as resins for powder coating to protect metal substrates or the like from rust, solvents, etc. Recently, in particular, the use of organic solvent-based coating materials has been avoided from the viewpoint of environmental load, and non-solvent-based coating has replaced such solvent-based coating at an accelerated pace. Even water-based coating cannot eliminate the use of organic solvents completely, and has difficulties in coating workability. In addition, water-based coating still has a problem of effluent treatment. In contrast, powder coating does not require the use of any solvents, and in addition, it is expected in terms of, for example, an increase in production rate and a cost reduction.
Among the powder coating materials, polyamide powder coating materials are highly valued because of their excellent impact resistance. The polyamide powder coating materials have, however, insufficient adhesion to substrates, and the surface of the metal needs to be subjected to pretreatment such as primer coating before coating. In addition, they are disadvantageous in that the hardness of the resulting coating films is lower.
On the other hand, polyvinyl acetal powder coating materials have been developed as powder coating materials capable of forming coating films having good adhesion to substrates, high impact resistance, and high hardness. For example, JP 63 (1988)-193970 A (the scope of claims for patent) proposes an epoxy resin powder coating material containing 100 parts by weight of an epoxy resin, which is a thermosetting resin, 5 to 20 parts by weight of polyvinyl butyral or polyvinyl formal, and further a curing agent and a filler. JP 2000-281966 A (the scope of claims for patent) proposes a powder coating material composed of a hydrophobic resin (such as polyvinyl butyral) and a water-absorbent resin (such as polyacrylic acid salt).
However, the polyvinyl acetal powder coating materials as mentioned above do not necessarily have good smoothness when the substrate temperatures are low. They also may cause coating defects such as pinholes. Therefore, the substrates are heated after the coating, if necessary, but the heating conditions such as long time heating and high temperature heating accelerate the deterioration of the resin, which may yellow the coating films. Furthermore, many of the polyvinyl acetal powder coating materials having low melt flow rates (MFR) do not improve their smoothness due to pinholes or the like even if they are subjected to post-heating or applied at high temperatures.
Some of the substrates have complex final shapes, and such substrates are bent for assembly after the coating in some cases. In such cases, if the coating films have poor flexibility, they may crack or peel off in the bent portions when the substrates are bent.