JP2504061B2 - Ultraviolet curable paint and its curing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an ultraviolet curable coating material and a curing method thereof.

(Conventional technology) Conventionally, solvent-based and water-soluble acrylic / melamine resin coatings and acrylic / urethane resin coatings have been used as coating materials for vehicles that require a high degree of aesthetics and a wide range of excellent coating film performance. Many are used. In addition, in recent years, the following (a),
The photocurable paint and the radiation curable paint shown in (b) are also known.

(A) JP-A-57-115464: An acrylic / aminoplast coating composition in which photopolymerization curing is followed by heat curing.

(B) JP-A-54-17967: Based on an ethylene type polymer having a (meth) acrylic acid ester and a hydroxyl group, a vinyl polymer and a photoinitiator, and a reaction product of the same with a polyisocyanate compound. Radiation curable paint included.

(Problems to be Solved by the Invention) The above-mentioned acrylic / melamine resin coating material and acrylic / urethane resin coating material have unsatisfactory points such as not always having sufficient image clarity and being difficult to obtain a high film thickness. As one of the improvement methods for obtaining a high film thickness, a method of increasing the paint heating residue at the time of coating can be considered, but for that reason, it is not possible to use a general means such as decreasing the molecular weight of the resin component to reduce the viscosity. However, the physical properties of the coating film, chemical resistance, weather resistance, and coating workability are deteriorated, which is not preferable.

Further, although the photocurable coating composition (a) and the radiation curable coating composition (b) are easy to obtain a high film thickness and have excellent image clarity,
Since the strain during curing is large, the adhesion to the substrate is poor, and also
There is a defect that the curing is insufficient at the portion of the surface of the article to which the ultraviolet ray irradiation is insufficient and the weather resistance cannot always be satisfied.

Therefore, the object of the present invention is to obtain a high film thickness by a single coating operation, to obtain a sufficiently cured coating film even when the irradiation of ultraviolet rays is non-uniform, and to improve image clarity, adhesion, weather resistance, etc. It is to provide an excellent high-quality ultraviolet curable coating material.

(Means for Solving the Problems) As a result of intensive studies to achieve the above-mentioned object, the present inventors have found that an ultraviolet-curable specific polyfunctional (meth) acrylate is used together with an acrylate polymer having a hydroxyl group. According to a coating material in which a specific amount of a specific melamine resin or urea resin is used in combination, and a light stabilizer and a photopolymerization initiator are further added thereto, a high film thickness can be obtained by a single coating operation, and ultraviolet rays can be obtained. Even if the irradiation is non-uniform, it is possible to obtain a sufficiently cured coating film, and it is possible to highly satisfy all of the various characteristics of this coating film such as image clarity, adhesion and weather resistance. As a result, I came to complete the present invention.

That is, the present invention includes: A) a UV-curable polyfunctional (meth) acrylate having two or more (meth) acryloyl groups in one molecule and having a number average molecular weight of 190 to 2,000; and B) a lactone-modified hydroxyl value. A polyhydric alcohol mono (meth) acrylate polymer of 10 to 100 and a C) melamine resin or urea resin, wherein the A component is 10 to 80% by weight and the B and C components are 90 to 100% by weight in the total amount of these three components. 20% by weight
And the C component is contained in a ratio of 1/9 to 4/6 with respect to the B component, and D) a light stabilizer and E) a photopolymerization initiator are contained as essential components other than these three components. It is a UV curable coating composition.

The present invention provides this UV-curable coating material: A) a UV-curable polyfunctional (meth) acrylate having two or more (meth) acryloyl groups in one molecule and a number average molecular weight of 190 to 2,000; and B) a lactone. A modified polyhydric alcohol mono (meth) acrylate polymer having a hydroxyl value of 10 to 100 and C) a melamine resin or a urea resin, wherein A component is 10 to 80% by weight in the total amount of these three components, B, 90 to 20% by weight of C component
And the C component is contained in a ratio of 1/9 to 4/6 with respect to the B component, and D) a light stabilizer and E) a photopolymerization initiator are contained as essential components other than these three components. 90-160 ℃ after irradiating with ultraviolet rays
It is cured by heating after 5 to 20 minutes.

In the present invention, the term “(meth) acryloyl group” means an acryloyl group and / or a methacryloyl group,
(Meth) acrylate means acrylate and / or methacrylate, and (meth) acrylic acid means acrylic acid and / or methacrylic acid.

The polyfunctional (meth) acrylate as the component A of the present invention is a UV-curable compound having two or more (meth) acryloyl groups in one molecule and a number average molecular weight of 190 to 2,000, which is different from this compound. If the number of (meth) acryloyl groups in one molecule is less than 2, the curability will be insufficient, and if the number average molecular weight exceeds 2,000, improvement in aesthetics (vividness) will not be seen and curing will occur. If the number average molecular weight is less than 190, the flexibility of the coating film is deteriorated, and thus both are not suitable for the present invention.

The polyfunctional (meth) acrylate according to the present invention includes polyvalent esters of polyhydric alcohols and (meth) acrylic acid, as well as various (meth) acrylate compounds described below. One or two or more of them are selected and used.

Specific examples of the polyvalent ester include 1,3-butanediol di (meth) acrylate, diethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, hydroxypivalic acid ester neopentyl glycol di (meth) acrylate, and tripentadiene di (meth) acrylate. Methylolpropane tri (meth) acrylate, pentaerythritol tri (meth) acrylate, dipentaerythritol hexa (meth) acrylate and the like can be mentioned.

Further, examples of the (meth) acrylate compound other than the polyvalent ester include a polyester acrylate composed of n moles of adipic acid and 2 moles of (n + 1) moles of hexanediol (meth) acrylic acid, an alicyclic epoxy compound. Epoxy (meth) acrylate esterified with (meth) acrylic acid, polyurethane (meth) acrylate obtained by reacting hexamethylene diisocyanate with (meth) acrylate having a hydroxyl group (for example, hydrotroxyethyl acrylate), methylol melamine And a melamine (meth) acrylate obtained by reacting hydroxyethyl (meth) acrylate with the methylol group.

Among these polyfunctional (meth) acrylates, those having an acryloyl group in the molecule give better curability results than those having a methacryloyl group, and are therefore particularly preferably used in the present invention. Further, it is most preferable to use a polyfunctional acrylate having 3 or more acryloyl groups in one molecule alone or to use a polyfunctional acrylate having 2 acryloyl groups in one molecule as a mixture. is there.

The lactone-modified hydroxyl value of the B component of the present invention is
As the polyhydric alcohol mono (meth) acrylate polymer of 10 to 100, hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, polyethylene glycol mono (meth) acrylate, polypropylene glycol mono (meth) acrylate, neopentyl glycol Homopolymers or copolymers of mono (meth) acrylate, glycerin mono (meth) acrylate and other hydroxyl group-containing monomers modified with lactone, or monomers copolymerizable with the above monomers, such as methyl (meth) acrylate, ethyl ( (Meth) acrylate,
Examples thereof include copolymers with one kind or two or more kinds of propyl (meth) acrylate, butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, styrene and the like.

Among these polymers, lactone-modified 2-hydroxyethyl (meth) acrylate and / or 2-hydroxypropyl (meth) acrylate is used as the hydroxyl group-containing monomer, and the hydroxyl group-containing monomer and the above-mentioned copolymerizable monomer are used. Copolymers with and are particularly preferably used.

The lactone-modified acrylic copolymer is produced by ring-opening polymerization of ε-caprolactone with a hydroxyl group-containing monomer or copolymer. The amount of ε-caprolactone is preferably 1 to 5 mol with respect to 1 mol of the hydroxyl group. In the subsequent heat-curing method described below, the lactone-modified one is remarkably excellent in weather resistance, moisture resistance and water resistance as compared with a non-reacted one.
Even if the modified amount of ε-caprolactone exceeds 5 mol, the curability does not change and the hardness decreases.

The reason why the hydroxyl value of component B is limited to 10 to 100 is that if the hydroxyl value is less than 10, curability may not be satisfied, and if the hydroxyl value exceeds 100, boiling resistance of the coating film This is because there is a fear that various properties such as resistance, moisture resistance, and weather resistance may deteriorate. The molecular weight of the B component is not particularly limited, but it is usually preferable that the number average molecular weight is about 2,000 to 20,000.

As the melamine resin or urea resin which is the C component of the present invention, a melamine resin or urea resin which is usually used for paints is used. These include partially methylated and / or butylated methylol melamine or urea resins.

In the present invention, the above A, B, as the proportion of the C component used,
In the total amount of these three components, the A component is 10 to 80% by weight, preferably 30 to 70% by weight, and the B and C components are 90 to 20% by weight, preferably 70% by weight.
~ 30% by weight, and C component is 1/9 to 4 / with respect to B component
6, preferably a ratio of 2/8 to 3/7.

That is, when the content of the A component in the total amount of the three components is less than 10% by weight, it is difficult to obtain the clearness of the cured coating film and the high film thickness by one coating operation. The curing shrinkage strain becomes large, resulting in poor adhesion and poor weather resistance. Further, if the C component is less than 1/9 of the B component, uniform curability becomes insufficient, and if it exceeds 4/6, the flexibility and water resistance of the coating film deteriorate.

In the present invention, in addition to the above A, B, and C components, a light stabilizer that is a D component and a photopolymerization initiator that is an E component are
Used as an essential ingredient. The light stabilizer is for improving the weather resistance of the coating film, and the photopolymerization initiator is for absorbing ultraviolet rays to start the polymerization reaction.

As the light stabilizer, dissolved or uniformly dispersed in the coating film,
A compound that does not cause curing inhibition during UV curing and does not cause yellowing of the coating film is selected. The compound includes a UV absorber and an antioxidant. Examples of the ultraviolet absorber include benzophenone or a derivative thereof, phenyl salicylate or a derivative thereof, benzotriazole or a derivative thereof, and among these, a benzotriazole-based ultraviolet absorber is particularly preferable. Further, as the antioxidant, a hindered amine-based antioxidant is particularly preferably used.

The light stabilizer can be used alone or in combination of two or more of the above-mentioned UV absorbers and antioxidants, and the amount thereof is generally 0.5 per 100 parts by weight of the total amount of the components A, B and C.
Approximately 5 parts by weight is acceptable.

Further, as the photopolymerization initiator, benzoin or its derivative having absorption in the ultraviolet region of 260 to 450 nm, benzophenone or its dielectric, acetophenone or its derivative, Michler's ketone, benzyl or its derivative, tetraalkylthiuram monosulfide, thioxanes Among these, it is particularly preferable to use acetophenone or a derivative thereof.

The photopolymerization initiator may be one kind or a mixture of two or more kinds, and the amount thereof is generally about 0.5 to 5 parts by weight per 100 parts by weight of the total amount of the above-mentioned A, B and C components. Is acceptable.

The UV-curable coating composition of the present invention is obtained by uniformly mixing the above components A to E. At that time, a diluent solvent is used in such a proportion that the heating residue is up to 40% by weight for the purpose of adjusting the viscosity. Also, known additives such as sensitizers for photopolymerization reactions such as amine compounds, urea compounds, and sulfur compounds that are commonly used for UV-curable coatings, repellents, fluidity modifiers, etc. , An organic peroxide for uniformly performing polymerization and curing, a reaction accelerator for promoting polymerization, and a pigment,
You may make it include paint etc. suitably.

The method for coating an ultraviolet-curable coating material of the present invention is a method in which a bake-curable colored coating material is applied and cured in advance on an object to be coated, and a marking tape or the like is attached on the coating material if necessary. The composition may be spray coated or electrostatically coated, and then irradiated with ultraviolet rays to be cured. In this case, if necessary, preheating for removing the solvent may be performed before the irradiation with ultraviolet rays.

After irradiation with ultraviolet rays, additional heating is performed at 90 to 160 ° C for 5 to 20 minutes. The post-heating method in which heating is performed after irradiation with ultraviolet rays is significantly superior in appearance of the coating film to the pre-heating method in which heating is performed before irradiation with ultraviolet rays.

The ultraviolet irradiation device used in the present invention is preferably one that can irradiate the surface of a three-dimensional object to be coated as uniformly as possible. As the ultraviolet ray source, a high pressure mercury lamp, a metal halide lamp, or the like can be used.

(Function) The polyfunctional (meth) acrylate as the component A used in the present invention contributes to the curability by ultraviolet irradiation and contributes to the reduction of the diluting solvent and contributes to increase the film thickness by one coating operation, As it covers unevenness of the fabric, you can get a high quality aesthetic.

The polyhydric alcohol mono (meth) acrylate as the B component and the melamine resin or urea resin as the C component used in the present invention are important for supplementing the curability when the ultraviolet irradiation is non-uniform, and the coating film By significantly reducing the shrinkage strain during curing, the adhesion to the base material and the weather resistance of the coating film are made good, and at the same time, the coating surface between the marking tape and the paint shrinks, and there are no problems such as cracks. It also contributes greatly to the finish.

Then, in the present invention, the crosslinking density of the coating film can be appropriately adjusted by changing the mixing ratio of the above A, B, and C components,
This makes it possible to easily obtain performances such as gasoline resistance and abrasion resistance according to the application of the paint.

Further, the light stabilizer as the component D used in the present invention contributes to the improvement of the weather resistance of the cured coating film. Among these light stabilizers, the ultraviolet absorber is the component of the ultraviolet curable coating composition and its curing. Although it was generally considered to be unfavorable from the viewpoint of properties, in the present invention, even if such an ultraviolet absorber is used, curing of the coating film is not hindered.

As described above, the UV-curable coating composition of the present invention in which the photopolymerization initiator as the E component is further added to the A to D components having the above-described actions, is excellent in the film thickness and the curability, and has a high degree. It exhibits excellent action curing by giving a cured coating film which has excellent aesthetics and excellent weather resistance and has various other properties.

(Effects of the Invention) The ultraviolet-effect coating composition of the present invention is a coating film which can obtain a high film thickness in a single coating operation by the action of each of the above-mentioned components and which is sufficiently cured even when the ultraviolet irradiation is uneven. Since it is possible to obtain the above-mentioned coating film and satisfy various characteristics such as image clarity, adhesion, and weather resistance to hardness, it is particularly required to have excellent appearance and weather resistance. Or plastics), and is also useful for applications to metals other than steel plates for which the same performance as described above is desired.

(Example) Next, the present invention will be described in more detail with reference to Examples and Comparative Examples. In addition, "part" in an example shows a weight part.

Examples 1 to 5 Five kinds of ultraviolet curable coating materials (clear coating materials) of the present invention were prepared by uniformly mixing the components A to E shown in Table 1 in the compounding parts shown in the same table.

Comparative Examples 1 to 4 Four types of comparative UV-curable paints (clear paints) were prepared by uniformly mixing components A to E shown in Table 1 in the compounding parts shown in the same table.

Comparative Example 5 Commercially available trade name Aqua No. 7100 clear (water-soluble acrylic melamine resin transparent coating; manufactured by NOF CORPORATION); heating residue
40% by weight) was used as a paint for comparison.

Note that * 1 to * 14 in the remarks for the components in Table 1 indicate the following.

A1 (* 1) Coronate HK50EX (polymer of hexamethylene diisocyanate manufactured by Nippon Polyurethane Industry Co., Ltd .; isocyanate group content 10.2%) 1,008 parts, 2-hydroxypropyl acrylate 348 parts, hydroquinone 0.2 parts and triethylamine 0.1 part, Heating residue of polyurethane acrylate with a number average molecular weight of 1,800, obtained by reacting at 80 ° C for 10 hours 6
3% by weight solution.

A2 (* 2) Araldite XB-3084 (hydrogenated epoxy resin manufactured by Ciba Geigy Ltd .: epoxy equivalent 227) 450 parts, acrylic acid 14
4 parts of hydroquinone, 2 parts of triethylamine and 200 parts of xylene were reacted at 150 ° C. for 5 hours to obtain an epoxy acrylate having a number average molecular weight of 800 and a heating residue of 75% by weight.
Solution of.

A3 (* 3) Trimethylolpropane triacrylate (number average molecular weight 300) A4 (* 4) Polyethylene glycol (400 mol) diacrylate (trade name PEG400DA manufactured by NOF Corporation; number average molecular weight 52)
0) B1 (* 5) 10 parts of styrene, 13 parts of methyl methacrylate, 50 parts of butyl methacrylate, 25 parts of 2-hydroxyethyl methacrylate, 2 parts of methacrylic acid, 2 parts of tertiary butyl peroxybenzoate and 62 parts of xylene. After mixing and reacting at 140 ° C. for 6 hours while stirring, 66 parts of ε-caprolactone was mixed and allowed to react for 7 hours.
%, A polymer solution having a polymer hydroxyl value of 65.

B2 (* 6) 10 parts of styrene, 13 parts of methyl methacrylate, 50 parts of butyl methacrylate, 90 parts of caprolactone-modified 2-hydroxyethyl methacrylate (Placcel FM-4, trade name of Daicel Chemical Industries, Ltd.), 2 parts of methacrylic acid , Third
A polymer solution having a heating residue of 70% and a polymer hydroxyl value of 52, which was obtained by sequentially mixing 2 parts of primary butyl peroxybenzoate and 70 parts of xylene and reacting at 140 ° C. for 6 hours while stirring.

B3 (* 7) 16 parts of styrene, 22 parts of methyl methacrylate, 50 parts of butyl methacrylate, 10 parts of 2-hydroxyethyl methacrylate, 2 parts of methacrylic acid, 2 parts of tertiary butyl peroxybenzoate and 54 parts of xylene are sequentially mixed. After reacting at 140 ° C. for 6 hours with stirring, 26 parts of ε-caprolactone was mixed and allowed to react for 7 hours.
%, Polymer solution having a polymer hydroxyl value of 34.

B4 (* 8) 12 parts of styrene, 20 parts of methyl methacrylate, 50 parts of butyl methacrylate, 16 parts of 2-hydroxyethyl methacrylate, 2 parts of methacrylic acid, 2 parts of tertiary butyl peroxybenzoate and 41 parts of xylene are sequentially mixed. , Heating residue obtained by reacting at 140 ° C for 6 hours with stirring
%, A polymer solution having a polymer hydroxyl value of 69.

C1 (* 9) Melamine resin (trade name Super Bekkamin L-116-70 manufactured by Dainippon Ink and Chemicals, Inc.) C2 (* 10) Urea resin (trade name Bekkamin manufactured by Dainippon Ink and Chemicals, Inc.) P-138) D1 (* 11) Hindered amine type antioxidant (Product name Sanol LS-292 manufactured by Sankyo Co., Ltd.) D2 (* 12) Benzotriazosol type ultraviolet absorber (trade name manufactured by Ciba Geigy Co., Ltd.) Tinuvin 900) E1 (* 13) Acetophenone photoinitiator (Ciba Geigy Co., Ltd.)
Product name Irgacure 184) E2 (* 14) Acetophenone-based photopolymerization initiator (Product name Darocurer 1173 manufactured by Merck Ltd.) Using the coating materials of Examples 1 to 5 and Comparative Examples 1 to 4 described above, A coating film test piece was prepared by the following method, and the following characteristic evaluation was performed using this test piece. The results were as shown in Table 2 below.

<Preparation of coating film test piece> As a coating object, a motorcycle fuel tank with a total displacement of 498 cc and a capacity of 17 liters (hereinafter referred to as coating object A), or 70 mm in length and 15 in width
Steel plate SPCC-SB with a thickness of 0 mm and a thickness of 1 mm (hereinafter referred to as the coated object B)
First, these products are treated with zinc phosphate, and then Aqua No. 7100 black (a water-soluble acrylic resin paint manufactured by NOF CORPORATION) is electrostatically coated to a dry film thickness of 30 μm. Heat dried at 150 ° C. for 3 minutes.

Then, a marking tape (manufactured by Sumitomo Three M Co., Ltd.) is attached to a part of the coated surface, or is not attached to each of the coating materials of Examples 1 to 5 and Comparative Examples 1 to 4. Was spray-coated or electrostatically coated so that the dry thickness would be a predetermined thickness, and set at room temperature for 10 minutes.

After that, in the case of using the coating materials of Examples 1, 3 to 5 and Comparative Examples 1 to 4, a far infrared heater (manufactured by Nippon Insulators Co., Ltd.,
After heating the coated object at 70 ° C for 5 minutes with Infrastein, a high-pressure mercury lamp (high-cure lamp manufactured by Nippon Battery Co., Ltd .; length 20 cm, 3 bulbs, total 4.8 k)
W) is uniformly irradiated on the surface of the coating object for about 2 seconds from a distance of about 20 cm, and after this UV irradiation, it is further heated for 10 minutes by the far infrared heater so that the temperature of the coating object becomes 130 ° C. A test piece was prepared.

In the case of using the paint of Example 2, a metal halide lamp (manufactured by Nippon Battery Co., Ltd .;
A coating film test piece was prepared by performing the curing treatment in the same manner as described above, except that irradiation was performed for 3 seconds using a length of 20 cm, 3 balls, and a total of 4.8 kW). On the other hand, in the case where the coating material of Comparative Example 5 was used, a coating film test piece was prepared by heating with a far-infrared heater so that the temperature of the object to be coated was 150 ° C. for 30 minutes.

<Characteristics evaluation> § Maximum film thickness Using the object B to be coated and spray coating or electrostatic coating under the condition that no marking tape is attached, one coating operation (1
The maximum film thickness at which a normal coating film can be obtained by (double coating) is expressed in μm. The normal coating film refers to a coating film in which no abnormalities such as sagging, flowing, and foaming are observed during coating and subsequent curing treatment, and the maximum film thickness of 60 μm or more was accepted.

§ Vividness test For a coating film test piece similar to the case of gloss retention rate, it was measured with a morphological sharpness gloss meter PGD-IV manufactured by Tokyo Koden Co., Ltd. using a scale showing the degree of aesthetics of the finish. , 0.9 and above were considered good.

§ Cross-cut test A coating film test piece similar to the case of the gloss retention rate was measured according to JIS K 5400 6.15 on the scale for evaluating the adhesion to the substrate, and when it was 100, it was regarded as good.

§ Tape property test The dry thickness is 70 μm using the objects A and B to which the marking tape is attached (however, in Comparative Example 4 only 40
(μm) of the coating film test piece electrostatically coated to check that there is no abnormality such as cracking or shrinkage in the coating film on the surface of the object to which the tape is attached. It was good.

§ Weather resistance test A coating film test piece similar to that used in the tape property test was fixed on a test stand tilted 30 degrees from the horizontal in the south of the coast of Okinawa and exposed for 2 years. The case where there was no change in color such as color change or cracks on the coated surface and there was no significant decrease in gloss was regarded as good.

As is clear from the results shown in Table 2 above, Examples 1 to 5 using the ultraviolet-curable coating material of the present invention show satisfactory results.

On the other hand, in Comparative Example 5 which is a water-soluble acrylic / melamine resin transparent paint, the image clarity and the maximum film thickness were unsatisfactory. Further, in Comparative Example 1 in which the hydroxyl group of the component B of the present invention was not lactone-modified, one year after the weather resistance test, cracking occurred on the entire surface.

In Comparative Example 2 containing less B and C components of the present invention, the adhesiveness, tape property and weather resistance were poor.

Further, in Comparative Example 3 in which the amount of the component A of the present invention was small, the image clarity and the maximum film thickness were unsatisfactory. In Comparative Example 4 containing no D component of the present invention, cracking occurred on the entire surface after one year in the weather resistance test.

(Effects of the Invention) Thus, according to the present invention, a coating film having a high film thickness can be obtained by a single coating operation, and a coating film that is sufficiently cured even when the ultraviolet irradiation is insufficient can be obtained. It is possible to obtain an ultraviolet curable coating material having excellent image clarity, adhesion, weather resistance and the like.

It goes without saying that various changes and modifications can be made to the present invention without departing from the broad spirit and scope of the present invention.

Claims (7)

(57) [Claims] 1. A) a UV-curable polyfunctional (meth) acrylate having two or more (meth) acryloyl groups in one molecule and a number average molecular weight of 190 to 2,000, and B) a lactone-modified hydroxyl value. 10 to 100 polyhydric alcohol mono (meth) acrylate polymer and C) melamine resin or urea resin, wherein A component is 10 to 80% by weight in the total amount of these three components,
The B and C components account for 90 to 20% by weight, and the C component is contained in a ratio of 1/9 to 4/6 with respect to the B component, and as an essential component other than these three components, D) light stabilizer And E) a UV-curable coating material comprising a photopolymerization initiator. 2. The ultraviolet curable coating composition according to claim 1, wherein the component A is a polyfunctional (meth) acrylate having three or more (meth) acryloyl groups in one molecule. 3. A copolymer of a lactone-modified 2-hydroxyethyl (meth) acrylate and / or 2-hydroxypropyl (meth) acrylate having a hydroxyl group and a monomer copolymerizable therewith. The ultraviolet-curable coating material according to claim 1 or 2. 4. The ultraviolet curable coating composition according to claim 1, wherein the C component is a partially allyl etherified methylol melamine resin or urea resin. 5. The ultraviolet curable coating composition according to claim 1, wherein component D is a hindered amine antioxidant and / or a benzotriazole ultraviolet absorber. 6. The ultraviolet curable coating composition according to claim 1, wherein the E component is acetophenone or a derivative thereof. 7. A) a UV-curable polyfunctional (meth) acrylate having two or more (meth) acryloyl groups in one molecule and a number average molecular weight of 190 to 2,000, and B) a lactone-modified hydroxyl value. 10 to 100 polyhydric alcohol mono (meth) acrylate polymer and C) melamine resin or urea resin, wherein A component is 10 to 80% by weight in the total amount of these three components,
The B and C components account for 90 to 20% by weight, and the C component is contained in a ratio of 1/9 to 4/6 with respect to the B component, and as an essential component other than these three components, D) light stabilizer And E) A UV-curable coating material containing a photopolymerization initiator is applied to a film thickness of 20 to 150 μm, and after irradiating with UV light, 90 to 160 ° C.
A method for curing an ultraviolet curable coating material, characterized in that after-heating is performed for 5 to 20 minutes.