Laminated coating film and coated article

Abstract

A multilayer coating film includes a lower coating film 5 containing a colorant, and an upper coating film 6. If the upper coating film 6 has a refractive index n1 of greater than or equal to 1.2 and less than or equal to 1.5, the ratio n1/n2 of the refractive index n1 of this upper coating film 6 to the refractive index n2 of the lower coating film 5 is set to be greater than or equal to 0.67 and less than or equal to 1. If the upper coating film has a refractive index n1 of greater than or equal to 1.6 and less than or equal to 3.0, the refractive index ratio n1/n2 is set to be greater than 1 and less than or equal to 1.67.

Claims

1. A multilayer coating film comprising: a lower coating film configured to be provided on a target article, the lower coating film containing a colorant and a second constituent; and an upper coating film stacked on the lower coating film, the upper coating film containing a first constituent, wherein the upper coating film is stacked directly on the lower coating film, the first constituent contains a resin as a main ingredient, the resin containing nanoparticles that have a higher refractive index than the resin, the first constituent has a refractive index n1 of greater than or equal to 1.6, the second constituent has a refractive index n2 of less than or equal to 1.4, and a ratio n1/n2 of the refractive index n1 of the first constituent to the refractive index n2 of the second constituent is greater than 1 and less than or equal to 1.67.

2. A coated article, wherein the multilayer coating film of claim 1 is provided on the target article.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view illustrating an exemplary multilayer coating film according to the present invention.

(2) FIG. 2 is a graph showing the relation between the refractive index n1 of a clear coating film and the reflectance at the surface of the clear coating film.

(3) FIG. 3 is a graph showing the relation between the ratio of the refractive index n1 of the clear coating film to the refractive index n2 of a base coating film and the reflectance at the interface between the clear and base coating films.

(4) FIG. 4 exemplifies the perpendicular reflectance and transmittance of light on the multilayer coating film in the situation where the clear coating film is set to have a low refractive index.

(5) FIG. 5 exemplifies the perpendicular reflectance and transmittance of light on the multilayer coating film in the situation where the clear coating film is set to have a high refractive index.

DESCRIPTION OF EMBODIMENTS

(6) Embodiments of the present invention will now be described with reference to the drawings. The following preferred embodiments are set forth merely for the purposes of examples in nature, and are not intended to limit the scope, applications, and use of the invention.

(7) In the multilayer coating film illustrated in FIG. 1, the reference numeral 1 denotes a target article made of steel (e.g., a body outer plate of an automobile). An undercoating film 2 made of an epoxide-based cationic electrodeposition paint is formed on the surface of this target article 1. An intermediate coating film 3 having undercoat concealability to increase light resistance, chipping resistance, and coloring performance is superimposed on this undercoating film 2. Then, a top coating film 4 is superimposed on the intermediate coating film 3.

(8) The top coating film 4 includes a base coating film (lower coating film) 5, and a clear coating film (upper coating film) 6 superimposed on the base coating film 5. The base coating film 5 includes a pigment 7, a bright material 8, and other suitable materials that function as a colorant and are dispersed into a resin, and provides coloring performance and design properties to the multilayer coating film. The clear coating film 6 provides weather resistance and wear resistance to the multilayer coating film. In the example shown in FIG. 1, the clear coating film 6 contains nanoparticles 9 for adjusting its refractive index.

(9) The present invention is characterized in that the refractive index n1 of the clear coating film 6 and the refractive index n2 of the base coating film 5 are optimized. The refractive indices n1 and n2 are each the refractive index of a constituent of an associated one of the coating films except the colorant. The feature of the present invention will now be specifically described.

(10) <Relation Between Refractive Index n1 of Clear Coating Film and Surface Reflectance/Interface Reflectance>

(11) The surface reflectance R of a light beam incident perpendicularly on a material having a refractive index N1 through a material having a refractive index NO has been known to be given by the following formula.
R=((N0?N1)/(N0+N1)).sup.2

(12) The surface reflectance (perpendicular reflectance) of light incident perpendicularly on the clear coating film 6 having the refractive index n1 through air is calculated by the above formula, and is shown in Table 1, where the refractive index of air is 1. As shown in FIG. 2, as the refractive index n1 increases, the surface reflectance increases.

(13) TABLE-US-00001 TABLE 1 Refractive Index n1 Perpendicular Reflectance 1 0 1.1 0.23 1.2 0.83 1.3 1.7 1.4 2.78 1.5 4 1.6 5.33 1.7 6.72 1.8 8.16 1.9 9.63 2 11.11 2.1 12.59 2.2 14.06 2.3 15.52 2.4 16.96 2.5 18.37 2.6 19.75 2.7 21.11 2.8 22.44 2.9 23.73 3 25

(14) The reflectance (perpendicular reflectance) of light incident perpendicularly on the clear coating film 6 having the refractive index n1 through air at the interface between the base coating film 5 and the clear coating film 6 is shown in Table 2, where the refractive index of air is 1, and the refractive index n2 of the base coating film 5 is 1.5. As shown in FIG. 3, if the refractive index ratio n1/n2 is 1, the interface reflectance is zero. As the ratio n1/n2 decreases from one, the interface reflectance increases. As the ratio n1/n2 increases from one, the interface reflectance increases.

(15) TABLE-US-00002 TABLE 2 Ratio Critical Refractive Index n1 n1/n2 Perpendicular Reflectance Angle (?) 1 0.67 4 None 1.1 0.73 2.36 None 1.2 0.8 1.22 None 1.3 0.87 0.5 None 1.4 0.93 0.12 None 1.5 1 0 90 1.6 1.07 0.1 69.6 1.7 1.13 0.36 61.9 1.8 1.2 0.76 56.4 1.9 1.27 1.25 52.1 2 1.33 1.81 48.6 2.1 1.4 2.43 45.6 2.2 1.47 3.08 43 2.3 1.53 3.74 40.7 2.4 1.6 4.42 38.7 2.5 1.67 5.1 36.9 2.6 1.73 5.78 35.2 2.7 1.8 6.44 33.7 2.8 1.87 7.09 32.4 2.9 1.93 7.72 31.1 3 2 8.33 30

(16) The interface reflectance is calculated by the following formula.
Interface Reflectance=(Perpendicular Transmittance of Light Transmitted Through Air To Clear Coating Film 6)?(Perpendicular Reflectance at Interface between Base Coating Film 5 and Clear Coating Film 6)

(17) FIG. 4 exemplifies the perpendicular reflectance and transmittance of light on the multilayer coating film, where the refractive index n1 of the clear coating film 6 is equal to 1.3. Table 1 shows that the perpendicular reflectance of light at the surface of the clear coating film 6 having a refractive index n1 of 1.3 is 1.7%. Since the refractive index n2 of the base coating film 5 is equal to 1.5, the perpendicular reflectance at the interface between the base coating film 5 and the clear coating film 6, which is calculated by the relational expression between the refractive index and the reflectance, is 0.51%.

(18) Thus, since 1.7% of all light components from a light source are reflected off the surface of the clear coating film 6, the transmittance, i.e., the ratio of some of the light components entering the clear coating film 6 to the incident light components, is 98.3%. Since 0.51% of transmitted light components corresponding to 98% of the light components are reflected off the interface between the base coating film 5 and the clear coating film 6, the interface reflectance is obtained as follows:
98.3%?0.51?0.01?0.50%

(19) FIG. 5 exemplifies the perpendicular reflectance and transmittance of light on the multilayer coating film, where the refractive index n1 of the clear coating film 6 is equal to 1.8. Table 1 shows that the perpendicular reflectance of light at the surface of the clear coating film 6 having a refractive index n1 of 1.8 is 8.2%. Since the refractive index n2 of the base coating film 5 is equal to 1.5, the perpendicular reflectance at the interface between the base coating film 5 and the clear coating film 6 is 0.83%.

(20) Thus, since 8.2% of all light components from a light source are reflected off the surface of the clear coating film 6, the transmittance, i.e., the ratio of some of the light components entering the clear coating film 6 to the incident light components, is 91.8%. Since 0.83% of transmitted light components corresponding to 91.8% of the light components are reflected off the interface between the base coating film 5 and the clear coating film 6, the interface reflectance is obtained as follows:
91.8%?0.83?0.01?0.76%

First Embodiment

(21) In this embodiment, the refractive index n1 of a clear coating film 6 is set to be greater than or equal to 1.2 and less than or equal to 1.5, and the ratio n1/n2 of this refractive index n1 to the refractive index n2 of a base coating film 5 is set to be greater than or equal to 0.67 and less than or equal to 1.

(22) The situation where the refractive index n1 of the clear coating film 6 is greater than or equal to 1.2 and less than or equal to 1.5 means that, as is clear from Table 1, the surface reflectance (perpendicular reflectance) of light on the clear coating film 6 is less than or equal to 4%, and the transmittance of light, i.e., the ratio of light components entering the clear coating film 6 to incident light components, is relatively high. The situation where the refractive index ratio n1/n2 is greater than or equal to 0.67 and less than or equal to 1 means that the perpendicular reflectance at the interface between the base coating film 5 and the clear coating film 6 is less than or equal to 4%, and the transmittance of light, i.e., the ratio of light components entering the base coating film 5 to incident light components, is relatively high. Furthermore, the situation where the refractive index ratio n1/n2 is less than or equal to 1 means that the refractive index n1 of the clear coating film 6 is lower than the refractive index n2 of the base coating film 5, i.e., that light is not totally reflected off the interface between the base coating film 5 and the clear coating film 6.

(23) Thus, according to this embodiment, the transmittance of light, i.e., the ratio of light components entering the base coating film 5 to incident light components, increases. As a result, the probability of contact between incident light and a colorant (a pigment 7 and a bright material 8) increases, and a brighter color is provided. Furthermore, since the reflectance at the interface between the base coating film 5 and the clear coating film 6 is low, irregular reflections of light at this interface are reduced. In addition, since light is not totally reflected off the interface between the upper and lower coating films, lightness does not vary significantly among the angles at which a viewer sees the coating films.

First Example

(24) Table 3 shows a configuration for a top coating film of a first example.

(25) TABLE-US-00003 TABLE 3 First Example Solid Content Mass % Base Coating Film Resin: Acrylic Resin 95 Pigment: Carbon Black 5 Clear Coating Film Resin: Clear Acrylic Resin 60 Nanoparticles: SiO.sub.2 40

(26) In Table 3, an acrylic resin (having an acid value of 20 mgKOH/g, a hydroxyl value of 75 mgKOH/g, a number average molecular weight of 5000, and a solid content of 60 mass %) manufactured by NIPPONPAINT Co., Ltd., was used as an acrylic resin forming a base coating film 5. Mitsubishi carbon black #2650 manufactured by Mitsubishi Chemical Industries, Ltd., was used as a pigment. The refractive index n2 of the base coating film 5 was 1.5.

(27) An acrylic resin manufactured by NIPPONPAINT Co., Ltd. was used as a clear acrylic resin forming a clear coating film 6. To adjust the refractive index n1 of the clear coating film 6 to a lower level, SiO.sub.2 nanoparticles were added to the clear acrylic resin. Nano Tek Slurry (having a SiO.sub.2 solid content of 20 mass %) manufactured by C. I. Kasei Co., Ltd., was used as SiO.sub.2 nanoparticles. The refractive index n1 of the clear coating film 6 was 1.4.

Second Example

(28) Table 4 shows a configuration for a top coating film of a second example.

(29) TABLE-US-00004 TABLE 4 Second Example Solid Content Mass % Base Coating Film Resin: Acrylic Resin 80 Pigment: Carbon Black 10 Nanoparticles: SiO.sub.2 10 Clear Coating Film Resin: Clear Acrylic Resin 60 Nanoparticles: SiO.sub.2 40

(30) Unlike the first example, SiO.sub.2 nanoparticles (Nano Tek Slurry (having a SiO.sub.2 solid content of 20 mass %) manufactured by C. I. Kasei Co., Ltd.) were added to a base coating film 5 so that the refractive index n2 of the base coating film 5 was equal to 1.4.

Comparative Example

(31) Table 5 shows a configuration for a top coating film of a comparative example.

(32) TABLE-US-00005 TABLE 5 Solid Comparative Example Content Mass % Base Coating Film Resin: Acrylic Resin 95 Pigment: Carbon Black 5 Clear Coating Film Resin: Clear Acrylic Resin 100

(33) Unlike the first example, a clear coating film did not contain SiO.sub.2 nanoparticles, and was made of only a clear acrylic resin manufactured by NIPPONPAINT Co., Ltd., and its refractive index was equal to 1.5.

Second Embodiment

(34) In this embodiment, the refractive index n1 of a clear coating film 6 is set to be greater than or equal to 1.6 and less than or equal to 3.0, and the ratio n1/n2 of this refractive index n1 to the refractive index n2 of a base coating film 5 is set to be greater than 1 and less than or equal to 1.67.

(35) The situation where the refractive index n1 of the clear coating film 6 is greater than or equal to 1.6 and less than or equal to 3.0 means that, as is clear from Table 1, the surface reflectance (perpendicular reflectance) at the clear coating film 6 is greater than or equal to 5%, and the proportion of light entering the base coating film 5 through the clear coating film 6 decreases. The situation where the refractive index ratio n1/n2 is greater than 1 and less than or equal to 1.67 means that the difference in refractive index between these coating films is small. The situation where the refractive index ratio n1/n2 is greater than 1 means that when light is incident on the base coating film 5 through the clear coating film 6, light is totally reflected off the interface between these coating films.

(36) Thus, according to this embodiment, lightness varies among the angles at which a viewer sees the coating films (i.e., high FF is provided), thereby allowing a coating color to have an appearance of depth. In addition, since light is strongly reflected off the surface, a darker muted coating color is obtained.

Third Example

(37) Table 6 shows a configuration for a top coating film of a third example.

(38) TABLE-US-00006 TABLE 6 Third Example Solid Content Mass % Base Coating Film Resin: Acrylic Resin 95 Pigment: Carbon Black 5 Clear Coating Film Resin: Clear Acrylic Resin 60 Nanoparticles: ZrO.sub.2 40

(39) A base coating film 5 was identical to that of the first example, and its refractive index n2 was 1.5. Unlike the first example, a clear coating film 6 contained not SiO.sub.2 nanoparticles but ZrO.sub.2 nanoparticles so as to adjust its refractive index n1 to a higher level. A ZrO.sub.2 nanoparticle dispersion (having a ZrO.sub.2 solid content of 20 mass %) manufactured by Sumitomo Osaka Cement Co., Ltd., was used as ZrO.sub.2 nanoparticles. The refractive index n1 of the clear coating film 6 was 1.8.

Fourth Example

(40) Table 7 shows a configuration for a top coating film of a fourth example.

(41) TABLE-US-00007 TABLE 7 Fourth Example Solid Content Mass % Base Coating Film Resin: Acrylic Resin 80 Pigment: Carbon Black 10 Nanoparticles: SiO.sub.2 10 Clear Coating Film Resin: Clear Acrylic Resin 80 Nanoparticles: ZrO.sub.2 20

(42) Unlike the third example, SiO.sub.2 nanoparticles (Nano Tek Slurry (having a SiO.sub.2 solid content of 20 mass %) manufactured by C. I. Kasei Co., Ltd.) were added to a base coating film 5 so that the refractive index n2 of the base coating film 5 was equal to 1.4.

(43) <Others>

(44) The base coating film 5 of each of the embodiments is a single layer. However, the base coating film 5 may be structured to include two upper and lower subfilms, and the refractive index n2 of the upper base coating subfilm may be set just like an associated one of the first and second embodiments.

(45) The present invention may be applied also to a situation where a transparent color base coating film is used as the base coating film 5 of the top coating film 4, and the intermediate coating film 3 is made of color paint for intermediate coating. In this case, the refractive index n1 of the transparent color base coating film 5 serving as an upper coating film and the refractive index n2 of the color intermediate coating film 3 serving as a lower coating film are set just like each of the first and second embodiments.

(46) In each of the embodiments, the intermediate coating film is provided between the undercoating film and the top coating film. However, the present invention may be applied also to a multilayer coating film that does not include an intermediate coating film but includes an undercoating film and a base coating film superimposed directly on the undercoating film.

(47) Speaking of a colorant, a dye may be used instead of or in combination with the pigment.

DESCRIPTION OF REFERENCE CHARACTERS

(48) 1 Target Article 2 Undercoating Film 3 Intermediate Coating Film 4 Top Coating Film 5 Base Coating Film (Lower Coating Film) 6 Clear Coating Film (Upper Coating Film) 7 Pigment (Colorant) 8 Bright Material (Colorant) 9 Nanoparticle