BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present invention relates to a decorative coating film that is formed on a surface of a resinous base and particularly to a decorative coating film having excellent weather resistance. 2. Description of Related Art

[0002] In a vehicle such as an automobile, a radar device such as a millimeter-wave radar is mounted at the center of the front of the vehicle to measure the distance to an obstacle ahead of the vehicle or the distance to the preceding vehicle.

Electromagnetic waves such as millimeter waves from the radar device are directed forward through the front grill and vehicle-manufacturer emblem and reflected by an object such as a preceding vehicle or an obstacle ahead, and the reflected waves return to the radar device through the front grill.

[0003] Thus, it is preferred that a material or paint which causes little radio-wave transmission loss and can provide a good desired appearance is used for the portions of the front grill, emblem, etc. which are placed on a beam path of the radar device. Therefore, typically, a decorative coating film is formed on a surface of a resinous base.

[0004] On the other hand, a silver coating has high visible light reflectance and an excellent infrared light shielding property and thus is used for various applications.

Further, due to its excellent electromagnetic wave shielding property, for example, the silver coating can protect electronic apparatuses, which malfunction due to electromagnetic waves, from external electromagnetic waves or can suppress radiation of electromagnetic waves generated from electronic apparatuses. Therefore, the silver coating may be used as an electromagnetic wave shielding coating.

[0005] For example, Japanese Patent Application Publication No. 2004-263290 (JP 2004-263290 A) discloses a silver alloy film for electromagnetic wave shielding containing 0.01 at% to 10 at% of bismuth (Bi) and/or antimony (Sb). On this silver alloy film for electromagnetic wave shielding, a transparent dielectric coating is formed, and even if the silver alloy film is directly exposed to air through a defect such as a pinhole or a scratch formed on the coating, aggregation of silver is not likely to occur.

[0006] However, when silver is used on a surface of a resinous base, such as the emblem, that is placed on a path of electromagnetic waves of a radar device in order to enhance the design, for example, when a silver coating is coated on the resinous base as disclosed in JP 2004-263290 A, it is difficult for electromagnetic waves such as millimeter waves radiated from a radar device to pass through the resinous base. A configuration can be conceived from the above finding in which, for example, silver fine particles and a binder resin for binding the silver fine particles are coated on the substrate surface as a decorative coating film.

[0007] In this case, the silver fine particles in the decorative coating film are not directly exposed to air, but the decorative coating film containing the silver fine particles is discolored over time. Even the use of fine particles of a silver alloy to which silver and bismuth are added cannot sufficiently suppress the discoloration of the decorative coating film having a highly luminous appearance. SUMMARY OF THE INVENTION

[0008] According to the invention, it is possible to provide a decorative coating film that is formed on a surface of a resinous base placed on a path of electromagnetic waves of a radar device, in which the discoloration of the decorative coating film can be sufficiently suppressed even when fine particles of a silver alloy are used and bound to each other through a binder resin.

[0009] To that end, as a result of thorough investigation, the present inventors have found that a decorative coating film is discolored by surface plasmon resonance absorption on fine particles of silver or a common silver alloy and a surface of a binder resin for binding the fine particles. That is, the present inventors have thought that, as illustrated in FIG. 7A, when the decorative coating film is irradiated with light, not only the fine particles of silver or a silver alloy but also the binder resin for binding the fine particles vibrate due to the light energy, free electrons inside the fine particles of silver or a silver alloy move, and the fine particles of silver or a silver alloy are likely to be polarized.

[0010] In this way, as illustrated in FIG. 7B, surface electromagnetic waves called surface plasmon polariton are generated on fine particles of silver or a silver alloy and a surface of a binder resin and absorb light in a specific wavelength range. Accordingly, the energy of, in particular, the fine particles of the silver or the silver alloy is likely to be amplified (surface plasmon resonance absorption).

[0011] As a result, the present inventors have newly found that the amplified energy is likely to affect a material forming the peripheries of the fine particles of the silver or the silver alloy and causes the discoloration of the decorative coating film. Accordingly, the present inventors have thought that it is important to select a combination of fine particles of silver or a silver alloy and a binder resin where surface plasmon resonance absorption is not likely to occur and have also thought that the hue of the decorative coating film contributes to surface plasmon resonance absorption as the above-described combination.

[0012] A first aspect of the invention relates to a decorative coating film which is formed on a surface of a resinous base placed on a path of electromagnetic waves of a radar device. The decorative coating film includes: fine particles of silver or a silver alloy that are dispersed in the decorative coating film; and a light-transmissive binder resin that binds the fine particles of the silver or the silver alloy. In the CIE 1976 (L*a*b*) color space, a chromaticness index a* of the decorative coating film is in a range of -1.5 to 1.5 and a chromaticness index b* of the decorative coating film is in a range of -1.6 to 3.6.

[0013] The decorative coating film includes at least: fine particles of the silver or the silver alloy that are dispersed in the decorative coating film; and a light-transmissive binder resin that binds the fine particles of the silver or the silver alloy. As a result, the decorative coating film has a metallic luster in appearance and has an electromagnetic wave transmitting property (electric insulating property). [0014] In the CIE 1976 (L*a*b*) color space, as chromaticness indices a* and b* of the decorative coating film approach 0, the color of the decorative coating film becomes to be close to an achromatic color. When the color of the decorative coating film is in a range of an achromatic color to a predetermined color, that is, when a chromaticness index a* of the decorative coating film is in a range of -1.5 to 1.5 and a chromaticness index b* of the decorative coating film is in a range of -1.6 to 3.6, surface plasmon resonance absorption is suppressed (light energy absorption is suppressed). Therefore, the energy applied to peripheral materials of the fine particles of the silver or the silver alloy by chronological light irradiation is suppressed, and changes in the tone of the decorative coating film can be suppressed.

[0015] That is, when the chromaticness index a* is less than -1.5, the color of the decorative coating film becomes to be close to green, and when the chromaticness index a* is more than 1.5, the color of the decorative coating film becomes to be close to red. In other words, the fine particles of the silver or the silver alloy are likely to absorb the light energy through the binder resin by surface plasmon resonance absorption.

[0016] On the other hand, when the chromaticness index b* is less than -1.6, the color of the decorative coating film becomes to be close to blue, and when the chromaticness index b* is more than 3.6, the color of the decorative coating film becomes to be close to yellow. That is, the fine particles of the silver or the silver alloy is likely to absorb the light energy through the binder resin by surface plasmon resonance absorption.

[0017] In this configuration, the silver alloy may consist of an alloy of silver and nickel. When an alloy of silver and nickel is used among silver alloys, as compared to other alloys, surface plasmon resonance absorption is likely to be suppressed in a specific wavelength of light, and the chromaticness indices a* and b* of the decorative coating film are likely to be within the above-described ranges.

[0018] The fine particles of the silver or the silver alloy may have an average particle diameter of 2 to 200 nm. It is known that, when the average particle diameter of the fine particles of the silver or the silver alloy is more than 200 nm, the fine particles of the silver or the silver alloy are likely to cause diffused reflection, which is likely to decrease the silver luster. As a result, the preferable range of the average particle diameter of the silver or the silver alloy is defined to be 200 nm or less. In addition, when the average particle diameter of the fine particles of the silver alloy is less than 2 nm, light incident on the decorative coating film is not likely to be reflected.

[0019] Further, the silver or the silver alloy may have a crystallite diameter in a range of 2 to 98 nm. When the crystalline diameter is less than 2 nm, light incident on the decorative coating film is not likely to be reflected.

[0020] According to the invention, even when fine particles of silver or a silver alloy are used in a decorative coating film that is formed on a surface of a resinous base placed on a path of electromagnetic waves of a radar device, the discoloration of the decorative coating film can be sufficiently suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic diagram illustrating a decorative coating film according to an embodiment of the invention;

FIG. 2 is a schematic diagram illustrating a configuration of the decorative coating film illustrated in FIG. 1 ;

FIG. 3 is a schematic diagram illustrating a relationship between the front grill (resinous base) and emblem (placed on the surface of the front grill) at the front of a vehicle and a radar device that is located behind the resinous base in the vehicle;

FIG. 4 is a schematic diagram illustrating a relationship between the front grill (resinous base) and emblem (placed on the surface of the front grill) at the front of a vehicle and a radar device that is located behind the resinous base in the vehicle;

FIG. 5 is a diagram illustrating a relationship between chromaticness indices a* and b* of decorative coating films according to Examples 1 to 4 and Comparative Examples 1 and 2;

FIG. 6 is a diagram illustrating a relationship between wavelengths of light incident on decorative coating films, in which fine particles of silver or a silver alloy according to Example 1 and Comparative Examples 1 and 2 were used, and reflectance values of the decorative coating films;

FIG. 7A is a diagram illustrating states of fine particles of a silver alloy until being polarized by light; and

FIG. 7B is a diagram illustrating surface plasmon resonance absorption. DETAILED DESCRIPTION OF EMBODIMENTS

[0022] Hereinafter, embodiments of the invention will be described with reference to the drawings. FIG. 1 is a schematic diagram illustrating a decorative coating film according to an embodiment of the invention. FIG. 2 is a schematic diagram illustrating a configuration of the decorative coating film illustrated in FIG. 1. FIG. 3 is a schematic diagram illustrating a relationship between the front grill (resinous base) and emblem (placed on the surface of the front grill) at the front of a vehicle and a radar device that is located behind the resinous base in the vehicle. FIG 4 is a schematic diagram illustrating a relationship between the front grill (resinous base) and emblem (placed on a surface of the front grill) at the front of a vehicle and a radar device that is located behind the resinous base in the vehicle.

[0023] A decorative coating film 1 illustrated in FIG. 1 forms a part of an emblem mounted on a surface of a resinous base 20 which is a front grill. As illustrated in FIG. 3, a radar device D installed at the front of a vehicle body A is placed behind the front grill. As illustrated in FIG. 4, millimeter- waves (millimeter- waves LI) that are emitted from the radar device D are directed forward through the front grill and the emblem 10 mounted on the front grill and reflected by an object such as a preceding vehicle or an obstacle ahead. The reflected waves (millimeter- waves L2) return to the radar device D through the emblem 10 and the front grill. In this way, the emblem 10 is formed on the surface of the resinous base 20 that is placed on a path of electromagnetic waves of the radar device. [0024] Since the decorative coating film 1 is formed on the surface of the resinous base 20 (front grill) placed on a path of electromagnetic waves of a radar device, the decorative coating film 1 has a metallic luster in appearance and has an electromagnetic wave transmitting property (electric insulating property).

[0025] Specifically, as illustrated in FIG. 1 , a colorless transparent resin coating layer 2 is laminated on the decorative coating film 1 in a visual recognition direction (X direction) to form the emblem 10. An adhesive seal or the like is bonded to the decorative coating film 1. Alternatively, the adhesive seal may be bonded to the resinous base 20. As illustrated in FIG. 2, the decorative coating film 1 includes at least: fine particles la of silver or a silver alloy that are dispersed in the decorative coating film; and a light-transmissive binder resin lb that binds the fine particles la of silver or the silver alloy. It is more preferable that the decorative coating film 1 further include a dispersant (protective agent) lc in order to improve the dispersibility of the fine particles la of silver or the silver alloy.

[0026] In this way, in the decorative coating film 1, the fine particles of silver or the silver alloy are discontinuously dispersed in the layer. Since the distance between the fine particles of silver or the silver alloy is extremely short, the particles densely aggregate. Accordingly, the particles provide a metallic luster when viewed by human eyes, and when electromagnetic waves pass through individual nanoparticles, millimeter wave loss is extremely small. As a result, the decorative coating film 1 has a metallic luster in appearance and has an electric insulating property.

[0027] The term "millimeter waves" described in this specification refers to electromagnetic waves in a frequency band of approximately 30 GHz to 300 GHz among electromagnetic waves, and electromagnetic waves with a frequency of approximately 76 GHz can be considered as typical examples. In addition, the term "decorative coating film" described in this specification refers to a component included in the above-described vehicle-manufacturer emblem or a decoration unique to the vehicle. The emblem or the like which is formed of the decorative coating film or includes the decorative coating film as a part thereof is formed on the surface of the front grill which is the resinous base. [0028] In the CIE 1976 (L*a*b*) color space (JIS Z 8729), a chromaticness index a* of the decorative coating film 1 according to the embodiment is in a range of -1.5 to 1.5 and a chromaticness index b* of the decorative coating film according to the embodiment is in a range of -1.6 to 3.6.

[0029] In the CIE 1976 (L*a*b*) color space, as the chromaticness indices a* and b* approach 0, the color of the decorative coating film becomes to be close to an achromatic color. By controlling the color of the decorative coating film 1 to be within a range from an achromatic color to a predetermined color in the CIE 1976 color space (JIS Z 8729) (the chromaticness index a* is in a range of -1.5 to 1.5 and the chromaticness index b* is in a range of -1.6 to 3.6), surface plasmon resonance absorption is suppressed in the decorative coating film 1 including the fine particles la of the silver alloy, the binder resin lb, and the dispersant (protective agent) lc. Therefore, the energy applied to peripheral materials of the fine particles of silver or the silver alloy by chronological light irradiation is suppressed, and changes in the tone of the decorative coating film can be suppressed.

[0030] That is, when the chromaticness index a* is less than -1.5, the color of the decorative coating film becomes to be close to green, and when the chromaticness index a* is more than 1.5, the color of the decorative coating film becomes to be close to red. In other words, within the above-described ranges, the fine particles of the silver alloy are likely to absorb the light energy through the binder resin by surface plasmon resonance absorption.

[0031] On the other hand, when the chromaticness index b* is less than -1.6, the color of the decorative coating film becomes to be close to blue. When the chromaticness index b* is more than 3.6, the color of the decorative coating film becomes to be close to yellow. That is, in this case, similarly, the fine particles of the silver alloy are likely to absorb the light energy through the binder resin by surface plasmon resonance absorption.

[0032] In order to control the chromaticness indices a* and b* to be within the above-described ranges, it is preferable that the silver alloy be an alloy of silver and nickel. When the alloy of silver and nickel is used among silver alloys, as compared to other silver alloys, surface plasmon resonance absorption is likely to be suppressed in a specific wavelength of light, and the chromaticness indices a* and b* of the decorative coating film are likely to be within the above-described ranges.

[0033] It is preferable that the silver alloy contain 1 mass% too 30 mass% of nickel with respect to silver. When the silver alloy contains less than 1 mass% of nickel with respect to silver, a ratio of silver to nickel in the silver alloy is high. Therefore, surface plasmon resonance absorption is accelerated, and the decorative coating film may be discolored (the tone thereof changes). On the other hand, when the silver alloy contains more than 30 mass% of nickel with respect to silver, the luminance of the decorative coating film may decrease.

[0034] The term "fine particles" of the silver alloy described in the embodiment refers to "nanoparticles". "Nanoparticles" refers to particles having an average particle diameter on the nano scale. Examples of a method of measuring a particle diameter of nanoparticles include a method including: extracting metal particles from a predetermined range of a SEM image or a TEM image of the fine particles of the silver alloy; and obtaining an average value of particle diameters thereof as an average particle diameter.

[0035] In particular, since the fine particles of the silver alloy is nanometer size, in this embodiment, light is likely to be absorbed by a phenomenon called localized surface plasmon resonance absorption. Even in this state, the absorption of the light energy is suppressed by the fine particles of the silver alloy having the above-described nickel composition ratio. Therefore, even when the fine particles of the silver alloy having the above-described size are used, the color tone change of the decorative coating film can be suppressed.

[0036] An average particle diameter of the fine particles of the silver-nickel alloy may be 2 nm to 200 nm. When the average particle diameter of the fine particles of the silver alloy is more than 200 nm, the fine particles of the silver alloy are likely to cause diffused reflection, which is likely to decrease the silver luster. In addition, when the average particle diameter of the fine particles of the silver alloy is less than 2 nm, light incident on the decorative coating film is not likely to be reflected. [0037] Further, a crystalline diameter of the silver alloy is preferably in a range of 2 nm to 98 nm. When the crystalline diameter is less than 2 nm, light incident on the decorative coating film is not likely to be reflected. On the other hand, when the crystalline diameter is more than 98 nm, electromagnetic waves are not likely to pass through the decorative coating film.

[0038] The fine particles of the silver alloy can be prepared, for example, by pouring a reducing agent into an ion solution in which silver and nickel for alloying silver are present in the ionic state. The fine particles obtained using this preparation method are particles having a particle size on the nano scale.

[0039] In addition, by changing the content of each metal contained in the ion solution, the composition ratio of silver and nickel in the alloy can be adjusted. By adjusting the content of the reducing agent, the stirring time, or the heating temperature during stirring after pouring the reducing agent into the ionic solution in which silver and nickel are ionized, the average particle diameter of the particles of the silver alloy and the crystalline diameter of the silver alloy can be adjusted.

[0040] The resin coating layer 2 and the binder resin lb are polymeric resins having light permeability, and examples of the polymeric resins include acrylic resins, polycarbonate resins, polyethylene terephthalate resins, epoxy resins, and polystyrene resins.

[0041] In addition, during the addition of the dispersant (protective agent) lc, it is preferable that the dispersant (protective agent) 1 c be a resin having high adhesiveness to the fine particles la of the silver alloy and high affinity to the binder resin lb. When one of the above described exemplary resins is selected as the binder resin, it is preferable that the resin have a carbonyl group. For example, when an acrylic resin is selected as the binder resin lb, it is preferable that an acrylic resin having a carbonyl group be selected as the dispersant (protective agent) lc.

[0042] In this way, by the dispersant (protective agent) having a carbonyl group, the adhesiveness to the fine particles la of the silver alloy can be improved. Further, by selecting the same resin as the binder resin lb, the affinity of the dispersant to the binder resin lb can be improved.

[0043] The content of the fine particles 1 a of the silver alloy with respect to the total mass of the decorative coating film 1 is preferably 83 mass% to 99 mass%. When the content of the fine particles la of the silver alloy is less than 83 mass%, metal luster obtained by the fine particles la of the silver alloy may not be not sufficient. When the content of the fine particles la of the silver alloy is more than 99 mass%, the binding of the fine particles to the substrate through the binder resin lb may not be sufficient.

[0044] Hereinafter, the invention will be described using Examples.

<Example 1 : Ag-Ni Fine Particles> 220 g of silver nitrate and 16 g of nickel nitrate were mixed. This mixture was added to 597 g of amino alcohol (reducing agent), followed by heating and mixing at 60°C for 120 minutes to precipitate silver alloy particles. This solution was ultrafiltrated at room temperature for 3 hours (average particle diameter of fine particles: 35 nm, crystalline diameter of silver alloy: 30 nm, ratio of nickel to silver: 5.1 mass%).

[0045] Next, as a compounding agent, compounding agent 1 was prepared which was obtained by mixing 40 g of propylene glycol monoethyl ether, 8.86 g of styrene, 8.27 g of ethylhexyl acrylate, 15 g of lauryl methacrylate, 34.8 g of 2-hydroxyethyl methacrylate, 3.07 g of methacrylic acid, 30 g of acid phosphoxyhexa monomethacrylate, 43 g of a polymerization initiator of propylene glycol monoethyl ether, and 0.3 g of tertiary butyl peroctoate. 0.38 g of DISPERBY 190 (manufactured by BYK-Chemie Japan K. .), 0.23 g of EPOCROS WS-300 (manufactured by Nippon Shokubai Co., Ltd.), 0.09 g of BYK-330 (manufactured by BYK-Chemie Japan K.K.), and 150 g of l-ethoxy-2-propanol were mixed with 0.465 g of compounding agent 1 to prepare a resin. The resin prepared as above was mixed with the silver alloy particles as a binder resin. Next, the obtained mixture was coated with a spin coater, followed by heating at 80°C for 30 minutes to form a decorative coating film.

[0046] <Example 2: Ag-Ni Fine Particles> A decorative coating film was formed with the same method as that of Example 1. A different point of Example 2 from Example 1 is the composition of fine particles of a silver alloy. Specifically, 220 g of silver nitrate and 64 g of nickel nitrate were mixed. This mixture was added to 597 g of amino alcohol (reducing agent), followed by heating and mixing at 60°C for 120 minutes to precipitate silver alloy particles. This solution was ultrafiltrated at room temperature for 3 hours (average particle diameter of fine particles: 25 nm, crystalline diameter of silver alloy: 20 nm, ratio of nickel to silver: 20.4 mass%).

[0047] <Example 3 : Ag-Ni Fine Particles> A decorative coating film was formed with the same method as that of Example 1. A different point of Example 3 from Example 1 is that a compounding agent as a binder resin was prepared by mixing 3.16 g of Plameez WY main agent (manufactured by Origin Electrico Co., Ltd.), 0.72 g of Plameez WY curing agent (manufactured by Origin Electrico Co., Ltd.), 0.03 g of BYK-330 (manufactured by BYK-Chemie Japan K.K.), and 13.97 g of l-ethoxy-2-propanol, and this compounding agent was mixed with the silver alloy particles as the binder resin.

[0048] <Example 4: Ag Fine Particles> A decorative coating film was formed with the same method as that of Example 1. Different points of Example 4 from Example 1 are that silver fine particles were prepared without adding nickel nitrate, the binder resin was changed, and the heating conditions after coating was changed. Specifically, first, 220 g of silver nitrate was added to 597 g of amino alcohol (reducing agent), followed by heating and mixing at 60°C for 120 minutes to precipitate silver metal particles, and this solution was ultrafiltrated at room temperature for 3 hours. Second, a compounding agent was prepared as a paint by mixing 3.16 g of Plameez WY main agent (manufactured by Origin Electrico Co., Ltd.), 0.72 g of Plameez WY curing agent (manufactured by Origin Electrico Co., Ltd.), 0.03 g of BYK-330 (manufactured by BYK-Chemie Japan K.K.), and 13.97 g of l-ethoxy-2-propanol, and this compounding agent was mixed with the silver alloy particles as a binder resin. Third, after coating, the mixture was heated at 120°C for 30 minutes to form a decorative coating film.

[0049] <Comparative Example 1 : Ag Fine Particles> A decorative coating film was formed with the same method as that of Example 1. A different point of Comparative Example 1 from Example 1 is that silver fine particles were prepared without adding nickel nitrate. Specifically, 220 g of silver nitrate was added to 597 g of amino alcohol (reducing agent), followed by heating and mixing at 60°C for 120 minutes to precipitate silver metal particles, and this solution was ultrafiltrated at room temperature for 3 hours.

[0050] Comparative Example 2: Ag Fine Particles> A decorative coating film was formed with the same method as that of Example 1. Different points of Comparative Example 2 from Example 1 are that silver fine particles were prepared without adding nickel nitrate, and the composition of the binder resin was changed. Specifically, first, 220 g of silver nitrate was added to 597 g of amino alcohol (reducing agent), followed by heating and mixing at 60°C for 120 minutes to precipitate silver metal particles, and this solution was ultrafiltrated at room temperature for 3 hours. Second, a compounding agent was prepared as a paint by mixing 3.16 g of Plameez WY main agent (manufactured by Origin Electrico Co., Ltd.), 0.72 g of Plameez WY curing agent (manufactured by Origin Electrico Co., Ltd.), 0.03 g of BYK-330 (manufactured by BYK-Chemie Japan K.K.), and 13.97 g of l-ethoxy-2-propanol, and this compounding agent was mixed with the silver alloy particles as a binder resin.

[0051] [Weather Resistance Test (Xenon Test)] A weather resistance test (xenon test) was performed on the decorative coating films according to Examples 1 to 4 and Comparative Examples 1 and 2 (100 W - 125 MJ/m ² ). Next, after the weather resistance test, in the CIE 1976 (L*a*b*) color space (JIS Z 8729), the lightness L* and the chromaticness indices a* and b* of the decorative coating films of Examples 1 to 4 and Comparative Examples 1 and 2 were measured using a color difference meter (manufactured by MURAKAMI COLOR RESEARCH LABORATORY CMS-35SP), and a color difference ΔΕ was calculated based on these values. The results are shown in FIG. 5 and Table 1. FIG. 5 is a diagram illustrating a relationship between chromaticness indices a* and b* of decorative coating films according to Examples 1 to 4 and Comparative Examples 1 and 2. [Table 1]

[0052] [Measurement of Reflectance] Before the weather resistance test, the decorative coating films according to Examples 1 and Comparative Examples 1 and 2 were irradiated with light, and the reflectance of the decorative coating films at each wavelength was measured from the spectra of the decorative coating films. FIG. 6 is a diagram illustrating a relationship between wavelengths of light incident on decorative coating films, in which fine particles of silver or a silver alloy according to Example 1 and Comparative Examples 1 and 2 were used, and reflectance values of the decorative coating films.

[0053] (Result 1) As illustrated in FIG. 5 and Table 1, when the chromaticness index a* of the decorative coating film was in a range of -1.5 to 1.5 and the chromaticness index b* of the decorative coating film was in a range of -1.6 to 3.6 as in the case of Examples 1 to 4, the color difference ΔΕ was 3.2 or less. In the decorative coating films of Comparative Examples 1 and 2 in which the chromaticness indices a* and b* were out of the above range, the color difference ΔΕ was more than 10. Further, as illustrated in FIG. 6, in the decorative coating films of Comparative Examples 1 and 2, the reflectance significantly changed depending on the change in wavelength as compared to Example 1.

[0054] Accordingly, it is considered that, when the silver fine particles of Comparative Examples 1 and 2 were irradiated with light, light in a specific wavelength was absorbed and thus the energy of the silver fine particles was amplified (surface plasmon resonance absorption). It is also considered that, by using, as in the case of Examples 1 to 4, the fine particles of the alloy of silver and nickel or the silver fine particles in combination with the binder resin whose curing was accelerated, the chromaticness indices a* and b* of the decorative coating film was likely to be in the above-described range, surface plasmon resonance absorption was suppressed, and the energy applied to peripheral materials of the fine particles of silver or the silver alloy by chronological light irradiation was suppressed, and changes in the tone of the decorative coating film was able to be suppressed.

[0055] <Example 5> A decorative coating film was formed with the same method as that of Example 1. A different point of Example 5 from Example 1 is that the heating temperature, the mixing time, and the concentration of the dispersant during the mixing of silver nitrate, nickel nitrate, amino alcohol, and the dispersant were changed such that the average particle diameter of the fine particles of the silver alloy was 200 nm. Metal particles were extracted from a predetermined region of a TEM image, and an average value of particle diameters thereof were measured as an average particle diameter of the fine particles of the silver alloy.

[0056] <Comparative Example 3: Ag Fine Particles> A decorative coating film was formed with the same method as that of Example 5. A different point of Comparative Example 3 from Example 5 is that the heating temperature, the mixing time, and the concentration of the dispersant during the mixing of silver nitrate, nickel nitrate, amino alcohol, and the dispersant were changed such that the average particle diameter of the fine particles of the silver alloy was 500 nm.

[0057] (Result 2) When the decorative coating films of Example 5 and

Comparative Example 3 were observed, the result was as follows. In the decorative coating film of Comparative Example 3 (in which the average particle diameter of the fine particles of the silver alloy was more than 200 nm), the fine particles of the silver alloy caused diffused reflection, and metal gloss was likely to decrease as compared to the decorative coating film of Example 5. In addition, the average particle diameter is preferably to be 2 nm or more in consideration of the results of the crystalline diameter described below.

[0058] <Example 6> A decorative coating film was formed with the same method as that of Example 5. A different point of Example 6 from Example 5 is that the heating temperature, the mixing time, and the concentration of the dispersant during the mixing of silver nitrate, nickel nitrate, amino alcohol, and the dispersant were changed such that the particle diameter of the silver alloy was in a range of 2 nm to 98 nm (specifically, 2 nm, 36 nm, and 98 nm). The crystalline diameter of the silver alloy was measured by X-ray diffraction analysis defined in JIS H 7805.

[0059] <Comparative Example 4> A decorative coating film was formed with the same method as that of Example 5. A different point of Comparative Example 4 from Example 5 is that the heating temperature and the mixing time of silver nitrate, nickel nitrate, and amino alcohol were changed such that the crystalline diameter of the silver alloy was less than 2 nm or more than 98 nm (specifically, 1 nm and 99 nm).

[0060] (Result 3) When the decorative coating films of Example 6 and Comparative Example 4 were observed, the result was as follows. In Comparative Example 4, when the crystalline diameter was less than 2 nm, light incident on the decorative coating film is not likely to be reflected. On the other hand, in Comparative Example 4, when the crystalline diameter is more than 98 nm, electromagnetic waves were not likely to pass through the decorative coating film. The decorative coating film according to Example 6 had a metallic luster and an excellent electromagnetic wave transmitting property.

[0061] Hereinabove, the embodiments of the invention have been described with reference to the embodiment of the invention. However, a specific configuration is not limited to the embodiments, and design changes and the like which are made within a range not departing from the scope of the invention are included in the invention.