Polypropylene (hereinafter PP), as compared with other plastic materials, has lower specific gravity of 0.91, offers higher impact resistance and easier molding, and is less expensive. For these reasons it is now in wide use in the manufacture of various automobile parts. When PP is for exterior use, such as for automobile bumpers or motorcycles fenders, it is rather a common practice to apply paint for cosmetic purpose.
However, due to its high crystallinity and low polarity, PP has some problems, particularly as to its ability to retain paint on its surface. The purpose of this study is to improve its ability to retain paint on its surface. To achieve this, the study was concentrated on its surface pretreatment as well as on the improvement of PP itself. Further it was seeked to develop a paint which is highly durable and which, at the same time, is flexible enough to be applied to PP automobile exterior parts.