In the case of coating metal parts such as vehicle outer panels that require weather resistance, a laminated coating film structure is typically employed in which, after forming a lower layer coating film (undercoating film) by electrodeposition coating for the purpose of preventing rust, an intermediate coating film having substrate hiding ability is formed thereon followed by laminating an upper layer coating film (top coating film) thereon. Although the intermediate coating film is provided to enhance light resistance, chipping resistance and coloring, from the viewpoints of conservation of resources, reduction of production steps, reduction of costs and the like, attempts have been made to directly laminate the upper layer coating film on the lower layer coating film without providing the intermediate coating film.
For example, Patent Document 1 (International Publication WO 96/33814) describes the coating of a first color base coat having substrate hiding ability onto a cationic electrodeposited coating film, coating a second color base coat having transparency on the coated film thereof without substantially curing, and heat-curing both color base coats followed by coating with a clear coating. It is described that this is carried out for the purpose of imparting a function of an intermediate coating film to the first color base coat, and that the pigment content is increased so that the light transmission rate of the coating film within a wavelength range of 400 to 700 nm is 5% or less. In addition, it is also described that the pigment content of the second color base coat is lower than the pigment content of the first color base coat. This is carried out to enable the color of the first color base coat to be able to be recognized through the coating film of the second color base coat due to the transparency thereof.
In the case of obtaining vivid coloring with a laminated coating film structure, an intermediate coating film d having good light reflection using a white pigment has conventionally been provided as shown in FIG. 1. Furthermore, FIG. 1 is a cross-sectional view schematically showing a laminated coating film structure having a conventional intermediate coating film in FIG. 1. In FIG. 1, a indicates a first layer coating film (base 1), b a second layer coating film (base 2), c a clear coating film, and e an electrodeposited coating film. With respect to this structure, in the case of laminating the first layer coating film a directly onto the electrodeposited coating film e without providing an intermediate coating as shown in FIG. 2, there is the problem of it being difficult to obtain vivid coloring. This is because the electrodeposited coating film e ends up absorbing light. Although it is possible to enhance light reflection of the electrodeposited coating film e, in this case, general-purpose electrodeposited coating films are unable to be used, thereby resulting in increased costs. Furthermore, FIG. 2 is a schematic diagram for explaining the case of enhancing pigment concentration of the first layer coating film in a laminated coating film structure not having an intermediate coating film.
Thus, it is necessary to impart substrate hiding ability to the first base coating film a as described in the above-mentioned Patent Document 1 in order to obtain vivid coloring without providing an intermediate coating film. As means for accomplishing this, the addition of an iridescent material (metallic pigment) such as aluminum flakes to the first base coating film a has been considered. However, when an iridescent material is incorporated in the first layer coating film a, hiding of the first layer coating material becomes inadequate at a typical thickness of about 10 um even if the pigment concentration in the second layer coating film b is increased to the maximum coatable concentration. Accordingly, it is difficult to obtain vivid coloring by incorporating an iridescent material in the first base coating film a. Note “um” stands for micro-meter unless otherwise specifically indicated.
In addition, there are additional problems required to be solved in order to obtain vivid coloring. This is readily understood by considering the mechanism by which pigment renders color. For example, in the case or red color, color is rendered as a result of light contacting the pigment, light or short and medium wavelengths being selectively reflected while light of long wavelengths is transmitted, after which the transmitted light is reflected at the surface of other pigment. This selective absorption of light by pigment does not allow the obtaining of adequate selective absorption effects in the case of selective absorption by a single pigment. Adequate selective absorption is only obtained by light passing through the pigment numerous times. A vivid red color can be obtained as a result of this selective absorption. If the pigment content of the coating film is excessively low, light of short and medium wavelengths is not adequately absorbed. As a result, instead of light of long wavelengths only, light of short and medium wavelengths is also reflected. A vivid red color is no longer obtained as a result of this light of short and medium wavelengths being reflected together with light of long wavelengths.
In contrast, enhancing the pigment concentration of a coating film is effective to a certain degree for improving substrate hiding ability and selective absorption and reflection of light. However, there are limitations on the extent to which vivid coloring can be obtained by enhancing the pigment concentration of a coating film. This is due to the reasons indicated below. Since the strength of a coating film is obtained by hardening a pigment with a resin, increasing the pigment concentration results in a corresponding decrease in the amount of resin. Consequently, if vivid coloring is attempted to be obtained by enhancing the pigment concentration of a coating film, it becomes difficult to ensure required coating film strength. Moreover, since coating film viscosity increases as the concentration of pigment increases, spray coating also becomes difficult. In addition, the use of dyes, which are coloring materials that dissolve in coating materials, has been considered instead of pigment. However, although dyes have satisfactory selective absorption properties, they have the problem of being susceptible to fading caused by ultraviolet light.