JP2865732B2 - Metallic finishing method for aluminum wheels - Google Patents

Description

The present invention relates to a disk-type wheel (hereinafter, referred to as an aluminum alloy casting) as a tire mounting member for automobiles and the like.
(May be abbreviated as "aluminum wheel") in a metallic finish.

2. Description of the Related Art Conventionally and its problems Conventionally, automobile wheels are mainly made of steel. However, in recent years, aluminum wheels are being replaced with aluminum wheels having excellent weight reduction, corrosion resistance and design. The aluminum wheel is coated with a metallic finish in order to enhance aesthetics and weather resistance.

The metallic coating has a scaly metallic pigment contained in the coating, and externally incident light is reflected on the metallic pigment and glitters brilliantly. It has a unique appearance with excellent aesthetic appeal, and is often applied to the outer panels of automobiles and motorcycles in addition to aluminum wheels.

As a metallic finishing method of an aluminum wheel, for example, a metallic paint containing a metallic pigment is applied, cured by heating, further coated with a clear paint for forming a transparent coating film, and then heated and cured again to form a two-coat two-bake. The two-coat, one-bake method (2C1B), in which both paints are applied in the above order and the two coats are simultaneously cured by one heating, is often used, but has many problems. Therefore, a solution is strongly desired.

That is, the aluminum wheel is subjected to a lot of work in order to improve the design, and therefore, there are many sharp and sharp portions (hereinafter may be abbreviated as "edge portions"), and the aluminum wheels are coated. The coating material melts and flows at the time of heating and curing, making it difficult to form a hardened coating film having a sufficient thickness on the edge portion, causing many rusts and the like, and further reducing the finished appearance. Because both upper paints are mainly aluminum-acrylic thermosetting type and relatively hard,
If pebbles or gravel collides with the surface during traveling, peeling or cracking is likely to occur. The applied metallic paint or the like is sucked into the small holes on the surface of the aluminum wheel, and the gloss and sharpness are reduced, pinholes are easily generated, and the metallic feeling is insufficient. Poor adhesion between metallic coating and aluminum wheel. Metallic pigments have defects such as non-uniform orientation and easy occurrence of metallic unevenness.

Means for Solving the Problems An object of the present invention is to eliminate the above-described various defects in metallic coating of an aluminum wheel, and the feature thereof is a three-coat method. That is, a metallic coating film was formed on the layer and a hard coating film was formed on the outermost layer. As a result, the above object of the present invention has been achieved.

That is, the present invention provides a particle size of 5% per 100 parts by weight of a thermosetting or thermoplastic resin forming a coating film having an elongation of 30% (20 ° C.) or more and a glass transition temperature of −25 to less than 60 ° C. A liquid paint (A) containing 5 to 35 parts by weight of at least one selected from transparent fine powder silica and alumina of 50 to 50 mμ is applied, and the elongation is 3% (20 ° C.) or more.
A liquid paint (B) containing a thermosetting resin and a metallic pigment as main components for forming a coating film having a glass transition temperature of −25 to 130 ° C. is applied, and (C) the elongation is 3 to less than 30% (20%). C), and a method for applying a transparent thermosetting acrylic resin paint (C) for forming a coating film having a glass transition temperature of 60 to 130 ° C.

The feature of the present invention resides in that the aluminum wheel is overcoated with the specific paints (A), (B) and (C). As a result, the metallic pigment in which the metallic pigment is uniformly oriented is excellent, and the gloss is bright. Since a metallic coating film having good properties can be formed, and the coating properties on the edge portion and the like are excellent, the occurrence of yarn rust from the edge portion, which has been a problem, can be improved. Further, since the coating film made of the coating material (A) has remarkably excellent flexibility and adhesion, it is also effective for chipping resistance and the like.

That is, since the coating film made of the coating material (A) has the above-mentioned specific physical properties, the flexibility and the adhesion are remarkably excellent. Cracks and peeling can be prevented by being absorbed in the coating film, and no generation of thread rust is observed. In addition, since a specific fine powder is also compounded, the viscosity of the applied coating material (A) is very high, so that the edge portion can be coated thickly. Even when baked at a high temperature, especially the coating material (A) applied to the edge portion has almost no heat flow, so that the coating property on the edge portion is excellent. When the fine powder is not blended, the coating composition has low viscosity and thixotropy, so that it tends to heat flow in the baking step, and almost no coating on the edge portion can be expected.

Further, baking (or drying) the coating film of the paint (A)
It is particularly preferable to apply the coating material (B) without coating, in order to orient the metallic pigment of the coating material (B) parallel and uniformly to the coating surface and to improve the metallic appearance.

The surface of the aluminum wheel has many minute irregularities called casting surface, which causes pinholes and the like on the painted surface, which causes a decrease in smoothness and sharpness. By coating (A), generation of pinholes and the like could be prevented, and all defects based on them could be eliminated.

 Next, the method of the present invention will be described in more detail.

Aluminum wheels: Aluminum alloy casting cars,
This is a wheel used for motorcycles and the like, and is an object to be coated by the coating method intended by the present invention. Shot blasting and cutting are often performed. Chrome-based, if necessary
Chemical conversion treatment (surface treatment) such as non-chrome type is performed.

Coating (A): Particle size of 5 to 50 mμ per 100 parts by weight of thermosetting or thermoplastic resin forming a coating film having an elongation of 30% (20 ° C.) or more and a glass transition temperature of −25 to 60 ° C. Is a paint containing 5 to 35 parts by weight of at least one selected from the group consisting of transparent fine powder silica and alumina. The paint is applied to the aluminum wheel prior to paint (B) described later.

The thermosetting resin and the thermoplastic resin used in the coating material (A) need to have the elongation and the glass transition temperature (Tg) of the cured coating film composed of only the resin within the above ranges. That is, at 20 ° C., the elongation is 30% or more, preferably 50 to 600%, more preferably 100 to 400%.
The Tg is in the range of −25 to less than 60 ° C., preferably −20 to 55 ° C., and more preferably −10 to 50 ° C. Elongation and Tg
However, the coating film contained within this range has good flexibility and adhesion, and hardly generates thread rust, and can prevent cracks, peeling, and the like from being generated on the coating film even when small stones collide with the coating film. However, even if the temperature changes from high temperature to low temperature and from high humidity to low humidity, little adhesion deterioration is observed. Therefore, when the elongation rate deviates from the above range or when Tg exceeds 60 ° C., adhesion deterioration occurs.On the other hand, when Tg is lower than −25 ° C., the coating film of the final product is too soft and is damaged by pebbles or the like. It is easy to stick and has a practical problem.

First, a thermosetting resin is a composition that undergoes a three-dimensional cross-linking and curing reaction at room temperature or by heating, and is mainly composed of a base resin and a curing agent.
Must be within the above ranges. Further, the thermoplastic resin is a resin which does not involve a crosslinking and curing reaction and is plasticized when heated, and it is necessary that the elongation and Tg of a coating film formed by an appropriate means are included in the above range.

As the thermosetting resin, those already known per se can be used, and examples thereof include a mixture of a base resin capable of forming a coating film having the above physical properties and a curing agent. Specifically, acrylic resin, polyester resin, urethane resin, silicon resin, fluorine resin, butadiene resin, etc. as the base resin, melamine resin, urea resin as the curing agent (all include methylolated products, alkyl etherified products), Polyisocyanate compounds (including blocked products), epoxy resins and the like can be used. Among them, an acrylic resin excellent in weather resistance is preferable.

Adjustment of physical property values within the above range by thermosetting resin,
It is easily carried out by appropriately selecting the composition of the two components, the amount of the crosslinkable functional group, the compounding ratio, the curing method and the like. In addition, a self-crosslinking resin containing Nn butoxymethylacrylamide or the like is also useful as the thermosetting resin.

Known thermoplastic resins can also be used as long as the physical properties of the formed coating film are within the above ranges. Specifically, vinyl acetate / ethylene copolymers, linear saturated polyester resins, thermoplastic polyurethane elastomers (polyester polyols, polyhydric alcohols and the like obtained by increasing the molecular weight with a diisocyanate compound), styrene-butadiene copolymers, polybutadiene Acrylonitrile-butadiene copolymer, butyl rubber, acrylic resin, and modified polyolefin obtained by graft-polymerizing an unsaturated dicarboxylic acid (including an anhydride) to a polyolefin. The adjustment of the physical properties of the thermoplastic resin can be easily performed by appropriately selecting the composition, ratio, molecular weight, and the like of the resin.

Next, the fine powdered silica and alumina used for the paint (A) are for imparting thixotropic properties to the paint (A), and have a particle size of 5 to 50 mμ (0.005 to 0.05 μ), preferably 5 to 23 mμ. (0.005 to 0.023μ) is a fine powder containing 90% by weight or more. Here, silica is called fine silica or white carbon, and alumina is also called aluminum oxide, and these are all known as extenders for paints. When the particle size is smaller than 5 mμ, the viscosity and thixotropic properties are too high, and the production and coating workability of the paint are significantly reduced.On the other hand, when the particle size is larger than 50 mμ, the thixotropic properties are reduced and the coverage of the edge portion is insufficient. Are not preferred.

The amount of the fine powder is 5 to 35 parts by weight, preferably 15 to 25 parts by weight, per 100 parts by weight (solid content) of the thermosetting and / or thermoplastic resin,
When the amount is less than 5 parts by weight, the coating property of the edge portion is not sufficient, and when the amount is more than 35 parts by weight, the coating workability is deteriorated, and neither is preferable.

The coating material (A) contains the above-mentioned thermosetting resin and / or thermoplastic resin and at least one selected from finely divided silica and alumina as main components, and these are dissolved or dispersed in an organic solvent to form the coating material. Can be

The coating method of the coating material (A) is not particularly limited, but spray coating methods such as air spray, airless spray, and electrostatic coating are particularly preferable because of excellent coating workability and coating efficiency. It is suitable to adjust the fraction content to 15 to 50% by weight, preferably 20 to 40% by weight.

Prior to the coating of the paint (A), the aluminum wheel may be preliminarily coated with a normal surface treatment, an undercoat or an intermediate coat as necessary.

Paint (B): elongation percentage is 3% or more (20 ° C), Tg is -25 or more
A paint containing a thermosetting resin at 130 ° C. and a metallic pigment, which is applied to the coating surface of the paint (A).

The thermosetting resin used in the coating material (B) has a cured film composed of only the resin having an elongation and a glass transition temperature (Tg) within the above ranges, that is, an elongation at 20 ° C. of 3% or more, preferably 3-30%, more preferably 5-20%,
The Tg must be in the range of -25 to 130C, preferably 60 to 130C, more preferably 60 to 110C.

The resin is composed of a base resin and a curing agent, and specifically, those exemplified in the above-mentioned coating material (A) are suitable. The adjustment of elongation and Tg is performed by adjusting the composition of the two components, the amount of the crosslinkable functional group. ,
It is easily performed by appropriately selecting the compounding ratio, the curing method, and the like. Further, as a thermosetting resin, N-n-
Self-crosslinking resins such as butoxymethylacrylamide are also included.

In the paint (B), when the elongation of the coating film is less than 3%, the chipping resistance and the adhesion are poor, and the Tg is -25 ° C.
The lower the value, the lower the pencil hardness of the final product, and the higher the Tg
If the temperature is higher than 130 ° C., the chipping resistance and adhesion are insufficient, which is not preferable.

Examples of the metallic pigment used in the paint (B) include aluminum flake, copper bronze flake, mica-like iron oxide, mica flake, mica-like iron oxide coated with a metal oxide, and mica flake coated with a metal oxide. Can be mentioned.

The coating material (B) is obtained by dissolving or dispersing the above-mentioned thermosetting resin and metallic pigment in an organic solvent, and may further contain a coloring pigment, an extender pigment and the like as necessary.

Paint (C): elongation percentage is less than 3-30% (20 ° C), Tg is 60
A transparent thermosetting acrylic resin paint having a film thickness of about 130 ° C., which is applied to the coating surface of the paint (B).

Specifically, it has the above physical properties, and has a finished appearance (sharpness, smoothness, gloss, etc.), weather resistance (gloss holding properties, color retention, chalk resistance, etc.), chemical resistance, water resistance. Nature, moisture resistance,
A thermosetting acrylic resin-based paint known per se, which forms a coating film having excellent curability, can be used in any form such as an organic solution type, a non-aqueous dispersion type, or a high solid type. For example, one selected from a hydroxyl group and a glycidyl group
Amino resin, urea resin, polyisocyanate as a curing agent for acrylic resin having more than one kind of functional groups (number average molecular weight is about 10,000-100,000, hydroxyl value is suitable for 15-100, acid value is 0-15) The paint of the above-mentioned form which uses one or more kinds selected from a compound, a blocked polyisocyanate compound and the like can be mentioned.

With respect to the single cured coating film of the coating material (C), the elongation percentage is 3 to 30.
%, Preferably 5-20%, more preferably 5-15%
% And a Tg of 60 to 130 ° C., preferably 60 to 130 ° C.
It is important to be at ~ 110 ° C.

If the elongation percentage of the coating film of the coating (C) is less than 3%, the flexibility is poor and cracks easily occur. If it exceeds 30%, the coating film hardness is not sufficient. If the film becomes soft, and if the temperature is higher than 120 ° C., the flexibility decreases, and neither is preferable.

The elongation (tensile elongation at break) of the coating films of paints (A), (B) and (C) was measured using a universal tensile tester equipped with a thermostat (Shimadzu Corporation, Autograph SD type) with a length of 20 mm. The values measured at a tensile speed of 20 mm / min at + 20 ° C for a 5 mm wide sample. The samples used for these measurements are:
The paint was applied to a tin plate having a thickness of 60 μm based on the formed coating film, baked at 140 ° C. for 30 minutes, and then isolated by a mercury amalgam method.

The glass transition temperature is DAYNAMIC VISCOELASTOMETE
R MODEL VIBRON DDV-II-EA (TOYO BACDWIN CO.Lt
Measured using d). The sample was prepared in the same manner as used for the measurement of the elongation.

Painting of paints (A), (B) and (C): First, paint (A) is applied to an aluminum wheel. As the coating method, for example, a spray coating of air spray coating, air spray coating, or electrostatic coating is preferable. The viscosity of the paint (A) at the time of painting can be arbitrarily selected depending on the painting method.
The viscosity when applied to an aluminum wheel is preferably 150 to 800 poise, particularly preferably 200 to 500 poise (all at 20 ° C.). The coating amount of the paint (A) is not particularly limited,
Based on the flat part of the aluminum wheel, 5-40
μ, especially 10-25 μ, is suitable.

After coating the coating material (A), when coating the coating material (B), the coating film of the coating material (A) is in an uncured state (an organic solvent in the coating film is about 3% by weight or more, particularly 5 to 30% by weight). Is preferred), but curing may be performed. Curing can be performed at room temperature or by heating, but a temperature of 60 to 140 ° C. is suitable for heating.

Next, the paint (B) is applied to the cured or uncured coating film surface of the paint (A). The coating is preferably selected from the methods exemplified above. The viscosity of the paint (B) at the time of coating can be arbitrarily selected based on the coating method, but the viscosity when applied to the surface to be coated is 10 to 200 poise, particularly 30 to 100 poise (both at 20 ° C.). Is preferred. The coating amount of the coating material (B) is not particularly limited, but is preferably 10 to 50 μm, particularly 15 to 30 μm for a cured coating on the flat portion of the surface to be coated.

After applying the paint (B), when applying the paint (C), the coating film of the paint (B) is in an uncured state (about 3% by weight or more, especially 5 to 30% by weight of an organic solvent in the coating film). Is preferred), but curing may be performed. Heating to a temperature of 60 to 140 ° C. is suitable for curing.

Further, the coating (C) is applied to the cured or uncured coating film surface of the coating (B). The coating is preferably selected from the methods exemplified above. The viscosity of the paint (C) at the time of coating can be arbitrarily selected based on the coating method, but the viscosity when applied to the surface to be coated is 2 to 40 poise, particularly 5 to 20 poise (all at 20 ° C.). Is preferred. The coating amount of the coating material (C) is not particularly limited, but 10 to 50 μm, particularly 20 to 40 μm is suitable for a cured coating on a flat portion of the surface to be coated.

After coating the coatings (A), (B) and (C) as described above, the coating is heated to cure only the coating of the coating (C) or the coatings of the coatings (B) and (A). Heating temperature is 120
A range of -200 ° C is preferred.

In the present invention, as a method of curing the paint films (A), (B) and (C), for example, (I) a three-coat three-bake method in which each paint is cured, (II) A three-coat two-bake method of curing by two heatings and (III) a three-coat one-bake method of simultaneously curing a plurality of coating films by one heating after all the paints are applied, of which (III) )) Is preferred.

When the coating of the paint (A) is omitted and the coating is performed using only the paints (B) and (C), the finished appearance (for example, metallic feeling, gloss, sharpness, etc.), water resistance, and the like are inferior. In addition, impact resistance, edge portion corrosion resistance, physical properties, and the like also decrease.

Next, examples and comparative examples according to the present invention will be described. All parts and percentages are in principle based on weight.

I. Preparation of sample (1) Coating object Aluminum alloy casting AC4T T-6 (100 × 150 ×
8mm) aluminum wheels that have been cut and then treated with Bonderite BT3753 (Nippon Parker Co., Ltd.).

(2) Paints Paints (A), (B) and (C) were produced by mixing and dispersing the compositions and amounts shown in Table 1. The compounding amount indicates the weight ratio of the solid content.

As an organic solvent for mixing and dispersing these components, in the paint (A), a mixed system consisting of 10% of n-butanol, 40% of toluene, 40% of xylene and 10% of Swazole 1500 (trade name of Cosmo Oil Co., Ltd.) (B) is a mixed system consisting of 30% n-butanol, 30% xylene, 20% Swazole 1000 (trade name of Cosmo Oil Co., Ltd.) and 20% butyl cellosolve, and 30% n-butanol in paint (C).
A mixed solvent consisting of 50% xylene and 1500% and 20% swazole was used, respectively.

(* 1) Substrate resin (K-1): Self-crosslinkable acrylic resin having Nn-butoxymethylacrylamide as a crosslinkable functional group component (trade name of Mitsubishi Rayon Co., Ltd .: Dianal SE-5482) [Heat Curable Resin] (K-2): Urethane-modified polyester (phthalic acid
Polyester composed of adipic acid / trimethylolpropane / neopentyl glycol, obtained by increasing the molecular weight with hexamethylene diisocyanate) [Thermosetting resin] (K-3): styrene / methyl methacrylate / n-
Copolymer of butyl acrylate, 2-ethylhexyl acrylate, lauryl methacrylate, and hydroxyethyl methacrylate (hydroxyl value 60) (K-4): Acrydic 44-590 (trade name, manufactured by Dainippon Ink and Chemicals, OH function) (Acrylic resin) (K-5): Dianal HR-517 (trade name of Mitsubishi Rayon Co., Ltd., acrylic resin) (* 2) Curing agent (H-1): Block polyisocyanate compound (manufactured by Takeda Pharmaceutical Co., Ltd.) Name, Takenate B-870-N).

(H-2): Butylated melamine resin (manufactured by Mitsui Toatsu Chemicals, Uban 20SE-60).

(H-3): 88% Cymel 370 (Mitsui Toatsu Chemical Co., Ltd.)
(* 3) Fine powder (P-1): Aerosil # 380 trade name, manufactured by Nippon Aerosil Co., Ltd., fine silica powder, average particle size 7 mμ.

(P-2): Aluminum oxide C Fine powder of aluminum oxide manufactured by Nippon Aerosil Co., Ltd. Particle size 20mμ.

(* 4) Metallic pigment (M-1): 10 parts of aluminum paste A (a product of Toyo Aluminum Co., Ltd., aluminum paste # 4919) and 20 parts of aluminum paste B (a product of Toyo Aluminum Co., Ltd., aluminum paste # 55-519) 20 Mixture consisting of parts.

(* 5) Carbon black (Columbia Carbon Co., Ltd.)
Products, Neo Spectra Beads AG) (* 6) Elongation of coating film (tensile elongation at break): Universal tensile tester with thermostat (Shimadzu Autograph S
-D type) at + 20 ° C for 20mm long and 5mm wide sample
At a tensile speed of 20 mm / min.
The sample used for these measurements was prepared by applying the paint to a tin plate so as to have a thickness of 60 μm based on the formed coating film.
After baking at 30 ° C for 30 minutes, it was isolated by the mercury amalgam method.

(* 7) Glass transition temperature: DAYNAMIC VISCOELASTOMETER MODEL VIBRON DDV-II E
The measurement was performed using a type A (TOYO BACDWIN CO. Ltd.). The sample was prepared in the same manner as used for the measurement of the elongation.

(* 8) Viscosity of coating liquid Each paint is spray-painted on a tin plate (the coating is preferably performed under the most practical conditions possible), and the applied paint is collected in a glass bottle and then cone-and-plate type. The measurement was carried out by reading the viscosity at a shear rate of 10 sec ^{-1 with} a viscometer.

II Example The above aluminum wheel and NT cutter spare blade L-300
(Nippon Transfer Paper Co., Ltd., trade name, blade angle 20 ° C)
Paints (A), (B) and (C) in the combinations shown in the table
Was spray-coated and heated at 150 ° C. for 30 minutes to simultaneously cure each coating. The results of the performance test of the coating film are also shown in the table.

(* 9) The film thickness was measured for the formed (cured) coating film on the flat part.

(* 10) Metallic unevenness is the result of visual judgment.

：: The metallic pigment is parallel and uniformly oriented to the coating surface, and no metallic unevenness is observed.

 Δ: Metallic unevenness was clearly observed.

(* 11) The sharpness is a result determined by visual observation.

 A fluorescent lamp or the like was projected on the painted surface, and the degree of sharpness was judged.

 ：: Fluorescent light is clearly reflected.

 Δ: The fluorescent light is blurred.

(* 12) Adhesion The initial adhesion is measured with a sharp blade on the coating so as to reach the surface to be coated with 100 × 2 mm goban eyes, and adheres to the surface with an adhesive cellophane tape. Was quickly removed, and the number of the remaining paint film was examined.

The secondary adhesion was measured by immersing the coated plate in warm water at 40 ° C. ± 2 ° C. for 120 hours, taking out the coated plate, and performing the same test as the above-mentioned initial adhesion one hour later.

(* 13) Edge coverage (copper sulfate method): Paints (A), (B) and (C) directed toward the edge of the L-300 blade for NT cutter. Spray-coating is performed under the conditions shown in Table 2 and baking and curing are performed under the same conditions as described above. After this was immersed in a solution of crystalline copper sulfate / concentrated hydrochloric acid / water = 20/10/70 (weight ratio) for 30 seconds, the copper deposition state of the sharp edge portion was visually observed, 10-40.
Observe with a magnifying glass or stereo microscope. If the edge portion is exposed or partially exposed or only covered with an extreme coating, copper deposition is recognized, and it is determined that the coating is inappropriate.

 ：: No precipitation of copper was observed.

△: Copper slightly precipitated (* 14) Chipping resistance Stepping stone tester (Suga Tester Co., Ltd., JA-400 type) was used. A coated test plate is vertically mounted on the sample holder of the tester, and 50 g of crushed stone of No. 7 is jetted with a pressure gauge of the tester at an air pressure of ² kg / cm ² to collide the crushed stone at right angles to the test plate.
The degree of peeling at that time was good (O) to poor (X).
It was judged at the stage.

(* 15) Yarn rust resistance: Two diagonal cuts of the coating film are cut by a cutter to reach the substrate, placed in a salt water spray tester according to JIS Z 2371 for 24 hours, and then deionized. After washing the painted surface with water, keep the temperature and humidity box (temperature 40 ± 2 ° C, humidity 85 ± 2)
%) For 240 hours as one cycle,
The state of yarn rust occurrence was examined using the creep width (one side) from the cross cut portion and the edge portion after the cycle was performed.

 ：: Creep width of 2 mm or less Δ: Creep width of 2 to 4 mm ×: Creep width of 4 mm or more (* 16) Paint (A-4): Paint obtained by removing fine powder from paint (A-1).

(* 17) Paint (A-5): (K-3) in the paint (A-3) is replaced with (C-2)
Paint substituted for (K-5) used in the above.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁶ , DB name) B05D 5/06 B05D 7/14 B05D 7/24 303 B60B 3/04

Claims (1)

(57) [Claims] An elongation percentage is not less than 30% (20 ° C.) and a glass transition temperature is −25 to less than 60 ° C. A particle size of 5 to 100 parts by weight per 100 parts by weight of a thermosetting or thermoplastic resin. A liquid paint (A) containing 5 to 35 parts by weight of at least one selected from transparent fine powder silica and alumina having a particle size of 50 μm is applied, and then has an elongation of 3% (20 ° C.) or more and a glass transition. A liquid paint (B) containing a thermosetting resin and a metallic pigment as main components for forming a coating film having a temperature of -25 to 130 ° C. is applied, and the elongation is 3 to less than 30% (20 ° C.); A method for applying a metallic finish to an aluminum wheel, comprising applying a transparent thermosetting acrylic resin paint (C) for forming a coating film having a temperature of 60 to 130 ° C.