JPH05209038A - Ultraviolet-curable urethane acrylate oligomer - Google Patents

Abstract

PURPOSE:To obtain a urethane oligomer having a high rate of ultraviolet curing and excellent film properties. CONSTITUTION:The objective oligomer is obtained by reacting a urethane prepolymer containing an isocyanate group and an isocyanurate ring with a hydroxylated acrylic monomer containing at least 30mol% 4-hydroxybutyl (meth) acrylate; and a coating composition is prepared from this oligomer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultraviolet curable polyurethane resin. More specifically, it relates to a urethane acrylate oligomer having a low resin viscosity, a high ultraviolet curing speed, and excellent physical properties such as heat resistance.

[0002]

2. Description of the Related Art A UV-curable resin composition comprises an acrylic oligomer, an acrylic monomer and a photopolymerization initiator as its main components. The oligomer is the main factor that determines the physical properties and solution properties of the cured product. Representative oligomers include urethane acrylate, epoxy acrylate, and polyester acrylate.

In particular, urethane acrylates are synthesized which are characterized by flexibility, toughness, weather resistance, chemical resistance, glass transition point, etc. by changing the types and ratios of polyol component, polyisocyanate component and hydroxyl group-containing acrylic monomer. Therefore, it is used for many purposes such as woodworking, vinyl chloride flooring, plastic molding coating agents, and optical fiber coating agents.

The coating agent itself using such urethane acrylate is disclosed in JP-B-54-2223.
No. 8 and Japanese Patent Publication No. 57-34853.
However, the hydroxyl group-containing acrylates disclosed therein are only 2-hydroxyethyl acrylate and 2-hydroxypropyl acrylate, and only 2-hydroxyethyl acrylate is specifically used in the examples. Is. And, none of these can be used as a coating agent unless a solvent is used.

On the other hand, Japanese Patent Application Laid-Open No. 1-161010 also describes a coating agent using urethane acrylate, which relates to an electron beam curable resin composition and is intended to be an ultraviolet curable resin. In addition to being different from the present invention, only 2-hydroxyethyl acrylate is used exclusively in the examples, and it is essential to use an acrylic monomer as the reactive diluent.

Also, Japanese Patent Laid-Open No. 1-185314 relates to a photocurable resin similarly to the present invention, and it is described that urethane acrylate is used, but in the examples, only 2-hydroxyethyl acrylate is used. However, a reaction diluent such as an acrylic monomer is used for this.

[0007]

As described above, since the conventional urethane acrylate oligomer has many urethane bonds, the viscosity of the oligomer itself becomes high, so that more diluent (acrylic monomer, acrylic monomer, Therefore, there is a problem in terms of solvent-free and resource saving.

Further, the ultraviolet curing rate and the physical properties of the cured coating film, that is, flexibility, heat resistance and cold heat repeatability, have not been sufficiently satisfied. The object of the present invention is to have a low viscosity of the oligomer itself, a fast ultraviolet curing rate, excellent flexibility, heat resistance, cold repeatability, scratch resistance, and other coating film physical properties, and thickness unevenness without using a diluent. An object of the present invention is to provide a urethane acrylate oligomer having excellent coating film forming performance in which wrinkle unevenness hardly occurs.

[0009]

Means for Solving the Problems The present inventors have made various studies in view of the present situation, and as a result, obtained by reacting a urethane prepolymer having an isocyanate group with a hydroxyl group-containing acrylic monomer and / or a methacrylic monomer. Urethane acrylate oligomer having an isocyanurate ring as the urethane prepolymer and 4-hydroxybutyl acrylate and / or 4-hydroxybutyl methacrylate as the hydroxyl group-containing acrylic monomer and / or methacrylic monomer Compared with conventional urethane acrylate oligomers, acrylic oligomers have much lower viscosity of the oligomers, faster UV curing speed, and superior coating properties such as flexibility, heat resistance, cold heat repeatability, and scratch resistance. Found that the obtained and achieved the object of the present invention.

The present invention will be described in detail below. The urethane acrylate oligomer used in the present invention is a urethane acrylate oligomer obtained by reacting a urethane prepolymer having an isocyanate group with a hydroxyl group-containing (meth) acrylic monomer, and a urethane prepolymer having an isocyanurate ring is used. In addition, 4-hydroxybutyl (meth) acrylate is used as the hydroxyl group-containing (meth) acrylic monomer, and an oligomer having a low viscosity of 600 poise or less can be obtained.

The urethane acrylate oligomer of the present invention has a number average molecular weight of 500 to 10,000, preferably 80.
It is 0 to 8000. When the number average molecular weight is 500 or less,
The cured film has insufficient flexibility. On the other hand, when the number average molecular weight is 10,000 or more, the viscosity of the oligomer is too high, which causes a problem in coating. The number average molecular weight of the urethane acrylate oligomer is determined by GPC analysis and the molecular weight calibration curve of standard polystyrene.

The urethane acrylate oligomer preferably contains at least two (meth) acrylic groups in one molecule on average. If it is less than 2, the physical properties such as solvent resistance and scratch resistance are poor. The method for producing the urethane acrylate oligomer used in the present invention is appropriately selected from known production methods in consideration of the types of raw materials used. For example, if necessary, a solvent or an acrylic monomer which is usually inert to an isocyanate group is used, and if necessary, an ordinary urethanization catalyst is used at 10 to 100 ° C., preferably 20 ° C.
It is carried out in the temperature range of 90 ° C.

The urethane prepolymer having an isocyanate group and an isocyanurate ring used in the present invention comprises a polyol and an excess of an organic di (poly) isocyanate or a urethane prepolymer having an isocyanate group at a terminal thereof and a cyclic trimerization catalyst such as N, N. ′, N ″ -tris (dimethylaminoalkyl) hexahydrotriazine,
Trimerization reaction in the presence of tertiary amines such as 2,4,6-tris (dimethylaminomethyl) phenol, carboxylates such as potassium acetate and lead 2-ethylhexanoate, or quaternary ammonium salts of organic acids. Is carried out and the reaction is stopped when the isocyanurate ring content reaches a desired value.

In order to obtain a urethane prepolymer having an isocyanurate ring, an organic diisocyanate compound /
Polyol = 5/0 to 5/1 (NCO / OH equivalent ratio) is preferable. It is necessary to use prepolymers containing isocyanurate rings. If it does not contain an isocyanurate ring, the low viscosity oligomer defined in the present invention cannot be obtained.

Examples of the organic diisocyanate compound which is a raw material of the urethane prepolymer of the present invention include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 1,3-xylylene diisocyanate and 1,4-tolylene diisocyanate.
Xylylene diisocyanate, diphenylmethane-4,
4'-diisocyanate, diphenylmethane-2,4 '
-Diisocyanate, 3-methyldiphenylmethane diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-
4,4'-diisocyanate, dicyclohexylmethane-2,4'-diisocyanate, ω, ω'-diisocyanatodimethylcyclohexane, lysine diisocyanate and the like can be mentioned.

The polyol compound having at least two hydroxyl groups usually has a molecular weight of 50 to 200.
Known polyol compounds generally used for the production of polyurethane resins, such as low molecular weight glycols, low molecular weight polyols, polyether polyols, polyester polyols, polybutadiene polyols, hydrogenated polybutadiene polyols, silicon-containing polyols, A phosphorus-containing polyol or the like can be used.

As the low molecular weight glycol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol,
1,3-butanediol, 1,4-butanediol, pentaethylene glycol, hexamethylene glycol,
Neopentyl glycol, 2-ethyl-1,3-hexanediol, bisphenol A, etc. can be used.

As the low molecular weight polyoli, trimethylolpropane, glycerin pentaerythritol, erythritol, sorbitol and the like can be used. Examples of the polyether polyol include polymerization products or copolymers of ethylene oxide, propylene oxide, tetrahydrofuran and the like. Further, polyethers obtained by condensation of the low-molecular glycol, or mixed ethers,
Furthermore, products obtained by addition-polymerizing ethylene oxide and propylene oxide to these polyethers and the above-mentioned low molecular weight glycol can also be used.

Examples of polyester polyols include polyester polyols obtained by dehydration condensation reaction of low-molecular glycols and dibasic acids, and lactone polyols obtained by ring-opening polymerization of lactones such as ε-caprolactone in the presence of the aforementioned low-molecular glycols. Can be mentioned. next,
The thus obtained urethane prepolymer having an isocyanurate ring and an isocyanate group and 4-hydroxybutyl acrylate and / or 4-hydroxybutyl methacrylate are mixed in such a ratio that hydroxyl group equivalent number / isocyanate equivalent number ≧ 0.5. The reaction is carried out at a temperature of 30 to 90 ° C.

In the reaction, it is desirable to use a polymerization inhibitor such as ortho-nitrotoluene, hydroquinone, hydroquinone monomethyl ether, copper chloride in the range of 50 to 5000 ppm in order to control the thermal polymerization reaction of the acrylic group. It is essential in the present invention to use 4-hydroxybutyl acrylate and / or 4-hydroxybutyl methacrylate as the hydroxyl group-containing acrylic monomer and / or methacrylic monomer. An oligomer having the features of the present invention cannot be obtained with less or more carbon chains. However, this particular (meta)
As long as the acrylate is used, it is also possible to mix and use other hydroxyl group-containing (meth) acrylates. In that case, 4-hydroxybutyl acrylate and / or 4-hydroxybutyl methacrylate is used in an amount of 30 mol% or more, preferably 3 mol% or more of the total hydroxyl group-containing monomer.
It is necessary to use 5 mol% or more, particularly preferably 40 mol% or more.

When the proportion of 4-hydroxybutyl (meth) acrylate is less than 30% by weight, the resin having a low viscosity and excellent coating properties which are the effects of the present invention cannot be obtained. Other hydroxyl group-containing (meth) acrylic monomers include 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, and 2
-Hydroxyethyl methacrylate, 4-hydroxycyclohexyl acrylate, caprolactone-modified hydroxyethyl acrylate, 5-hydroxycyclooctyl acrylate, 2-hydroxy-3-phenyloxypropyl acrylate, pentaerythritol triacrylate and the like can be used.

The UV-curable urethane acrylate oligomer obtained by the present invention has a remarkably low viscosity of 600 poise or less as compared with the conventional urethane acrylate oligomer, and therefore, it is used as a UV-curable paint or adhesive. In that case, it is characterized in that a good coating film can be obtained without using a solvent or a reactive diluent, but if necessary, it may be mixed with a small amount of a reactive diluent or another UV curable resin. It does not prevent you from using it.

Examples of the reactive diluent include tetrahydrofurfuryl (meth) acrylate, 2-ethylhexyl (meth) acrylate, methoxyethyl (meth) acrylate, N-vinylpyrrolidone, phenyl (meth) acrylate and 2-hydroxyethyl (meth). Acrylate, diethylene glycol di (meth) acrylate, tetraoxyethylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, pentaerythritol hexa (meth) acrylate, trimethylolpropane di (meth) acrylate, pentaerythritol tetra (meth) ) Acrylate and the like can be mentioned.

Other UV curable resins include epoxy acrylate oligomers, polyester acrylate oligomers, urethane acrylate oligomers and the like. The photopolymerization initiator to be added to the UV-curable urethane acrylate obtained in the present invention is not particularly limited, but for example, benzophenone, acetophenone, benzoin, benzoin isobutyl ether, benzoin isopropyl ether, benzoin ethyl ether, 4 , 4'-bisdimethylaminobenzophenone, 2
-Hydroxy-2-methyl-1-phenylpropane-1
-One, benzyl methyl ketanol, 2-chlorothioxanthone, 2,4-dimethylthioxanthone, 1-hydroxycyclohexyl phenyl ketone and the like can be used.

In the case of coating with a low viscosity such as spray coating, it is possible to dilute with a solvent such as acetone, methyl ethyl ketone, methyl isobutyl ketone, dioxane toluene, cyclohexanone, tetrahydrofuran, ethyl acetate, butyl acetate. The urethane acrylate oligomer or the composition containing the urethane acrylate oligomer obtained in the present invention may be used by adding a dye, a pigment, a metal powder or the like as a colorant.

When used as a coating material, wood, metal, concrete, asbestos, paper,
It can be applied to plastics and the like. As the irradiation light source for curing, a low pressure mercury lamp, a high pressure mercury lamp, an ultrahigh pressure mercury lamp, a metal halide lamp, or the like can be used.
The urethane acrylate of the present invention can of course be cured using an electron beam.

[0027]

The reason why the urethane acrylate oligomer of the present invention is much lower in viscosity than the conventional urethane acrylate oligomer, the UV curing speed is fast, and the physical properties of the coating film are excellent is not clear, but 2-hydroxyethyl (meth) Compared to acrylate and 2-hydroxypropyl (meth) acrylate, the distance between the hydroxyl group and the (meth) acrylic group is longer by 2 carbon atoms, which makes the (meth) acrylic group easier to move and the cross-linking reaction to occur easily. This is probably because Further, if the distance between the hydroxyl group and the (meth) acrylic group becomes longer than this, the elastic modulus of the crosslinked film becomes too low. In addition, the solubility and compatibility of the urethane oligomer with other oligomers, diluted monomers, and solvents deteriorate.

[0028]

EXAMPLES Next, the present invention will be described in more detail with reference to Examples and Test Examples, but the present invention is not limited to the following Examples and Test Examples unless it exceeds the gist. Resin Synthesis Example 1 Isophorone diisocyanate 44 was placed in a four-necked flask equipped with a thermometer, a stirrer, a dropping funnel, and a cooling tube with a drying tube.
Charge 4 g, polytetramethylene ether glycol (number average molecular weight 650) 650 g, and urethanization catalyst dibutyltin dilaurate 0.1 g, and the reaction solution temperature is 60 ° C.
And stirred for 2 hours to obtain a urethane prepolymer containing no isocyanurate ring. Subsequently, the reaction liquid temperature was lowered to 40 ° C., and dibutyltin dilaurate 0.6 was added.
g, polymerization inhibitor (hydroquinone monomethyl) 0.4 g
Was added and 2.4 mol of 4-hydroxybutyl acrylate (molecular weight 144) was added dropwise over about 30 minutes to carry out the reaction. Then, the temperature of the reaction solution was raised to 60 ° C., and an isocyanate of 2250 cm ⁻¹ in infrared absorption spectrum was obtained. The reaction was carried out until the absorption of the groups disappeared (about 5 hours) to obtain a urethane acrylate oligomer (number average molecular weight 1400: hereinafter referred to as urethane acrylate 1).

Resin Synthesis Examples 2 to 5 504 g of hexamethylene diisocyanate and 8.1 g of 1,3-butanediol were charged in the same reactor as in Resin Synthesis Example 1 and the urethanization reaction was carried out at 70 ° C. for 3 hours. Is lowered to 50 ° C and Dabco is used as a trimerization catalyst.
0.5 g of 70% 2-ethylhexanol solution of TMR (trade name of Air Product Co., Ltd.) was added and the reaction was carried out at 50 ° C. for 4 hours. The isocyanate content at that time is 3
It was 9.7%. Phosphate ester A as a reaction terminator
The reaction was stopped by adding 0.4 g of a 10% ethyl acetate solution of P-1 (trade name of Daihachi Chemical Co., Ltd.).

Next, thin film distillation was performed to remove unreacted hexamethylene diisocyanate from the reaction solution. The hexamethylene diisocyanate monomer content is 0.5 wt% or less and the isocyanate content is 21.0 under the conditions of 160 ° C. and a vacuum degree of 0.6 to 0.8 mmHg.
% Urethane prepolymer (isocyanurate-modified hexamethylene diisocyanate) was obtained.

Next, to 200 g of this modified isocyanurate of hexamethylene diisocyanate, 0.1 g of the urethane-forming catalyst dibutyltin dilaurate and 0.1 g of the polymerization inhibitor hydroquinone monomethyl ether were added, and the reaction liquid temperature was kept at 40 ° C. 1.2 mol of each of the various hydroxyacrylates shown was added dropwise over 30 minutes, the temperature was raised to 60 ° C. after completion of the reaction, and the reaction was carried out until the isocyanate groups disappeared, to obtain urethane acrylate 2 to urethane acrylate 5 shown in Table 1.

[0032]

[Table 1]

Example 1 and Comparative Examples 1 to 4 The urethane acrylate oligomers obtained in the resin synthesis examples were measured for their viscosity, UV curability, and
The cured product was tested for gel fraction and film physical properties. The results are shown in Tables 2 and 3. (1) Viscosity The viscosity was measured at 25 ° C. with an E-type rotational viscometer (manufactured by Tokyo Keiki). (2) UV-Curable (UV-Curable) Preparation of Paint Based on 100 parts of the synthesized urethane acrylate, 25 parts of methyl ethyl ketone, 3 parts of benzophenone, and 2-hydroxy-2-methyl-1-phenylpropan-1-one 1
The parts were heated and mixed at 60 ° C. to prepare a paint.

Curing conditions: Coated on a glass plate with a 6 mil applicator and heated to 80 ° C.
After drying for 2 minutes, it is irradiated with ultraviolet rays. UV irradiation is 80W
/ Cm high pressure mercury lamp, the distance between the lamp and the object to be coated is 15
The irradiation time was set to cm by adjusting the conveyor speed. Evaluation of curability The curability was evaluated by rubbing the cured coating film 10 times with gauze soaked with acetone and observing the appearance. The curability was indicated by the number of seconds required for UV irradiation until the coating film did not change. (3) Gel Fraction About 1 g of the coating film cured by ultraviolet irradiation was immersed in a tetrahydrofuran solvent at room temperature for 48 hours, and the gel fraction was measured by the weight of the coating film before and after the immersion. (4) Tensile physical properties of cured film (physical properties of film) Film forming conditions After coating on a glass plate with a 6 mil applicator, the film was irradiated with ultraviolet rays and then treated at 80 ° C. for 30 minutes and then at a temperature of 20.
The physical properties of a sample kept at ℃ and humidity of 60% for 24 hours or more were measured.

Ultraviolet irradiation conditions 80 W / cm high pressure mercury lamp, lamp distance 15 cm,
The irradiation time was 3.5 seconds. Tensile physical property measurement conditions Sample shape: width 10 mm strip type, membrane pressure 50 to 60 μm Tensile speed: 5 mm / min Incidentally, tensile elastic modulus, breaking elongation and breaking strength are JIS K7.
It measured according to 113.

[0036]

[Table 2]

[0037]

[Table 3]

Example 2 and Comparative Examples 5 to 6 (1) Preparation of coating material 3 parts by weight of benzophenone as a photopolymerization initiator was added to 100 parts by weight of the urethane acrylate oligomer shown in Table 4.
1 part of -hydroxy-2-methyl-phenylpropan-1-one was heated and mixed at 60 ° C to prepare a coating liquid without using a solvent reactive diluent. (2) Film forming conditions A coating liquid heated to 60 ° C. was applied to a PET film on a 15 cm wide and 20 cm long area using a bar coater (# 28), and the paintability of this was visually observed. Then, it was irradiated with ultraviolet rays to cure the coating film, and the film thickness distribution was measured with an electromagnetic film thickness meter. The results are shown in Table 4.

[0039]

[Table 4]

From the results shown in Table 4, since the urethane acrylate oligomer using 4-HBA has a low viscosity, it is possible to form a normal coating film having a narrow film thickness distribution without adding a solvent-reactive diluent. I understand. When industrially coating a highly viscous paint, problems are likely to occur in terms of uneven film thickness, uneven coating, control of film thickness, and high-speed coating. The present invention solves these problems by reducing the viscosity of urethane acrylate.

Solvents and reactive diluents (monomers)
Although there is also a method of lowering the viscosity by adding, it is not preferable to use a solvent because it goes against resource and energy saving and causes air pollution. Since the reactive diluent has a low molecular weight and a high volatility, it has a problem of skin irritation and a problem of deterioration of coating film physical properties, and it is a compound whose use amount should be reduced as much as possible.

Resin Synthesis Examples 6 to 9 Hexamethylene diisocyanate (504 g) and 1,3-butanediol (8.1 g) were charged in a four-necked flask equipped with a thermometer, a stirrer, a dropping funnel, and a cooling tube with a dryer.
Carry out urethanization reaction at ℃ for 3 hours, lower the temperature of the reaction solution to 50 ℃, and use Dabco TMR as a trimerization catalyst.
0.5 g of 70% 2-ethylhexanol solution (trade name of Air Product Co., Ltd.) was added and the reaction was carried out at 50 ° C. for 4 hours. The isocyanate content at that time was 39.7.
%Met. Phosphate ester AP-I as a reaction terminator
10% ethyl acetate solution of (trade name of Daihachi Chemical Co., Ltd.)
The reaction was stopped by adding 4 g.

Next, thin film distillation was performed to remove unreacted hexamethylene diisocyanate from this reaction solution. The hexamethylene diisocyanate monomer content is 0.5 wt% or less and the isocyanate content is 21.0 under the conditions of 160 ° C. and a vacuum degree of 0.6 to 0.8 mmHg.
% Of hexamethylene diisocyanate was obtained.

Next, to 200 g of a modified isocyanurate of hexamethylene diisocyanate, 0.1 g of a urethane-forming catalyst dibutyltin dilaurate and 0.1 g of a polymerization inhibitor hydroquinone monomethyl ether were added, and the reaction liquid temperature was kept at 40 ° C., as shown in Table 5. 1.2 mol of various hydroxy acrylates are added dropwise in 30 minutes, and after completion of the addition 60
The reaction was carried out by raising the temperature to 0 ° C. until the isocyanate groups disappeared, and urethane acrylate oligomers 6 to 9 shown in Table 5 were obtained.
Got

[0045]

[Table 5]

Examples 3 to 6 and Comparative Examples 7 to 9 (1) Preparation of coating material 3 parts of benzophenone as a photopolymerizable initiator and 100 parts of urethane acrylate shown in Table 7 were used as a photopolymerizable initiator.
1 part of 2-methyl-phenylpropan-1-one at 60 ° C
And mixed by heating to prepare a paint without adding a solvent or a reactive diluent. (2) Film-forming conditions (for evaluation of coating film appearance) PET film A coating solution heated to 1260 ° C was applied to an area of 15 cm in width and 20 cm in length using a bar coater (# 28), and the coating property was visually observed. I observed it. Then, this was irradiated with ultraviolet rays to cure the coating film, and the film thickness distribution was measured by the same method as in Example 4. (3) Film-forming conditions (for measurement of physical properties of cured coating film) A glass plate was coated with a 6-mil applicator at 80 ° C.
After being dried with hot air for 2 minutes, it was irradiated with ultraviolet rays. UV irradiation is 8
Using a 0 W / cm high pressure mercury lamp, the lamp distance was 15 cm, and the irradiation time was 3.5 seconds. (4) Tensile physical property measurement conditions Sample shape: width 10 mm strip type, film thickness 50 to 60 μm Tensile speed: 5 mm / min Incidentally, the measuring methods of various physical properties are the same as in Example 1. The results are shown in Table 6.

[0047]

[Table 6]

[0048]

As can be seen from the results of Examples and Comparative Examples, the urethane acrylate using 4-hydroxybutyl (meth) acrylate has a low viscosity and excellent coatability,
In addition, it is excellent in ultraviolet curability, and the cured coating film has a high gel fraction and appropriate flexibility, so that it has elongation and high strength.

If only a low viscosity is to be achieved, a hydroxy acrylate such as Placcel FA-1 can be used.
It is also possible to use (lactone-modified hydroxyethyl acrylate), but as can be seen from Comparative Example 9, such urethane acrylate has a low viscosity and a good coating property, but has a low elastic modulus and a low breaking strength. Only a film can be obtained.

Claims (4)

[Claims] 1. A urethane acrylate oligomer obtained by reacting a urethane prepolymer having an isocyanate group with a hydroxyl group-containing acrylic monomer and / or a methacrylic monomer, wherein a urethane prepolymer having an isocyanurate ring is used, and An ultraviolet-curable urethane acrylate oligomer, wherein 4-hydroxybutyl acrylate and / or 4-hydroxybutyl methacrylate is used as the hydroxyl group-containing acrylic monomer and / or methacrylic monomer. 2. The ultraviolet-curable urethane acrylate oligomer according to claim 1, which has a viscosity measured at 25 ° C. of 600 poise or less. 3. The ultraviolet-curable urethane acrylate oligomer according to claim 1, wherein 4-hydroxybutyl acrylate and / or 4-hydroxybutyl methacrylate accounts for 30 mol% or more of the total hydroxyl group-containing acrylic monomer and / or methacrylic monomer. 4. A coating composition comprising the UV-curable urethane acrylate oligomer according to claim 1 and a photopolymerization initiator as essential components.