For the formation of a coating film on the bodies of vehicles such as automobiles, motorcycles, container vehicles and the like, a method is widely used which comprises applying a thermosetting coating composition to the vehicle body to be coated, and thereafter curing the applied coating composition by heating. This method can form a coating film having excellent properties in terms of adhesion, finish quality, weatherability, etc.
The above-mentioned coating film formation method is now required to reduce energy consumption and increase productivity. For example, the heat-curing step on an automobile body coating line is usually conducted at about 140° C. for about 40 minutes; when the conveyor runs at a speed of 3 m/min, the step requires a space for the drying furnace line about 120 m in length. Therefore, in order to save space and energy, shortening of the heating time is demanded in the heat-curing step.
To shorten the heating time in the formation of a coating film, Japanese Unexamined Patent Publication No. 1989-11169 discloses a method of coating a substrate with an ultraviolet-curable and thermosetting coating composition, and then carrying out ultraviolet irradiation followed by heating at about 90° C. to about 160° C. for about 5 to about 20 minutes to form a cured coating film. However, this coating film formation method is disadvantageous in that ultraviolet irradiation promotes the curing of a coating film surface alone, thus preventing sufficient thermal flow of the film at the time of heating, resulting in uneven curing of the film and impaired smoothness of the film surface. The obtained cured coating film, therefore, tends to be reduced in terms of adhesion, finish quality, weatherability and other film properties. Moreover, because dust or other foreign matter adhering to a wet coating film surface before curing is firmly fixed by the curing of the film surface by ultraviolet irradiation, its removal at the time of repair is difficult.