PURPOSE: To improve chipping resistance and to obtain a coating film excellent in appearance, hardness and flatting property by controlling value of Tg, value of PWC and surface tension while using a same basic resin and a same hardener in applying two kinds of intermediate coats each containing a polyester resin and a melamine resin hardener as essential components on a precoated steel sheet by electrodeposition.
CONSTITUTION: The steel sheet after precoated by electrodeposition is coated with two kinds of the intermediate coats each containing the polyester resin and the melamine resin hardener as the essential components in wet-on-wet. In this case, though the two kinds of the intermediate coats practically use the same melamine resin hardener and the same ratio of polyester resin/ melamine resin, Tg of the initial coating material 1 is regulated to 30-60°C lower than Tg of the next by controlling the kind of polybasic acid, polyhydric alcohol of raw materials of the polyester resin. Surface tension of the coating material 2 is regulated to ≤1 by using a surface controlling agent and difference of PWC value of the coating material 2 is regulated to ≤10% on the basis of the coating material 2. Thus, the coating sheet remarkably excellent in appearance and chipping resistance is obtained.
FUJIMOTO KUNIHIKO
YOSHIDA AKIKAGE
TOYOTA MOTOR CORP