JP2023119468A - hard coating agent - Google Patents

Abstract

To provide a hard coating agent capable of forming a coating layer which excels in low curling, hardness and bendability.SOLUTION: A hard coating agent according to one embodiment includes urethane (meth)acrylate, which is a reaction product between polyol including pentaerythritol (meth)acrylate with a hydroxyl value of 80-160 mgKOH/g and polyisocyanate including xylylene diisocyanate.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a hard coating agent, and more particularly to a hard coating agent containing urethane (meth)acrylate, which can be cured by irradiation with active energy rays, and a cured product obtained therefrom.

It is known to use a resin composition that can be cured by irradiation with an active energy ray as a hard coating agent and apply it to various substrates to form a coating film with high hardness. For example, Patent Document 1 discloses a urethane (meth)acrylate composition obtained by reacting a (meth)acrylic acid adduct of pentaerythritol with a polyisocyanate compound, a fluorine-containing (meth)acrylate compound, and polymerization inhibition. Actinic ray-curable resin composition containing an agent, it is described that a polymerization inhibitor is contained in a larger amount than usual, whereby a cured coating film having excellent antifouling performance and scratch resistance can be obtained. Are listed.

When using an active energy ray-curable resin composition containing urethane (meth)acrylate as a hard coating agent, there is a problem that curing shrinkage of the coating film occurs and the cured coating film tends to curl. On the other hand, in recent years, portable electronic devices with foldable displays, that is, foldable displays, are becoming popular, and there is a demand for flexibility that prevents streaks from forming even when a plastic film on which a cured coating film is formed is bent.

In Patent Document 2, pentaerythritol (meth)acrylate having a hydroxyl value of 200 mgKOH/g or more is reacted with a polyvalent isocyanate for the purpose of forming a cured coating film that is resistant to curling and has excellent hardness and flexibility. and a composition [II] obtained by reacting dipentaerythritol (meth)acrylate having a hydroxyl value of 40 mgKOH/g or more with a polyvalent isocyanate. .

JP 2019-104908 A JP 2018-178093 A

As described above, hard coating agents are required to have low curling properties, high hardness, and high flexibility. However, in general, the low curling property and the hardness are contradictory performances. Moreover, even if the low curling property and the hardness are excellent, the flexibility may be inferior, and it is not easy to satisfy the low curling property, the hardness and the flexibility at the same time.

Note that Patent Document 1 specifically describes the use of urethane acrylate obtained by reacting isophorone diisocyanate with pentaerythritol acrylate having a hydroxyl value of 120 mgKOH/g. However, it has been found that the use of the urethane acrylate is insufficient in terms of flexibility.

In view of the above points, an object of the embodiments of the present invention is to provide a hard coating agent capable of forming a coating film excellent in low curling property, hardness and flexibility.

The inventors of the present invention have made intensive studies in view of the above problems, and found that the problems can be solved by using a urethane acrylate that is a reaction product of pentaerythritol acrylate having a specific hydroxyl value and xylylene diisocyanate. .

The present invention includes embodiments shown below.
[1] A hard coating agent containing urethane (meth)acrylate which is a reaction product of a polyol containing pentaerythritol (meth)acrylate having a hydroxyl value of 80 to 160 mgKOH/g and a polyisocyanate containing xylylene diisocyanate.
[2] The hard coating agent according to [1], which contains 90% by mass or more of the pentaerythritol (meth)acrylate in 100% by mass of the polyol.
[3] The hard coating agent according to [1] or [2], which contains 90% by mass or more of the xylylene diisocyanate in 100% by mass of the polyisocyanate.
[4] The hard coating agent according to any one of [1] to [3], which is for foldable displays.
[5] A cured product obtained by curing the hard coating agent according to any one of [1] to [4].
[6] An article containing the cured product of [5].

The hard coating agent according to the embodiment of the present invention can form a coating film excellent in low curling property, hardness and flexibility.

The hard coating agent according to the present embodiment contains a polyol containing pentaerythritol (meth)acrylate having a hydroxyl value of 80 to 160 mgKOH/g and a urethane (meth)acrylate which is a reaction product of a polyisocyanate containing xylylene diisocyanate. .

As used herein, (meth)acrylate represents acrylate and/or methacrylate. (Meth)acrylic acid represents acrylic acid and/or methacrylic acid. (Meth)acryloyl represents acryloyl and/or methacryloyl.

[Polyol]
In this embodiment, pentaerythritol (meth)acrylate having a hydroxyl value of 80 to 160 mgKOH/g is used as the polyol. If the hydroxyl value is less than 80 mgKOH/g, it becomes difficult to ensure low curling properties of the coating film. When the hydroxyl value exceeds 160 mgKOH/g, it becomes difficult to achieve both low curling property and hardness of the coating film. The hydroxyl value of pentaerythritol (meth)acrylate is preferably 100 mgKOH/g or more, more preferably 110 mgKOH/g or more, and preferably 150 mgKOH/g or less, more preferably 140 mgKOH/g or less. be.

As used herein, the hydroxyl value is measured according to JIS K0070-1992. Specifically, a sample (pentaerythritol (meth)acrylate) is dissolved in acetic anhydride/pyridine (15 parts by weight/85 parts by weight). Then, after reacting at 90° C. for 1.5 hours, add a small amount of water, react for another 10 minutes, and then cool to room temperature. Then, phenolphthalein is added as an indicator, and the hydroxyl value is determined by titration with a 1 mol/L potassium hydroxide (KOH) ethanol solution.

Pentaerythritol (meth)acrylate is obtained by reacting pentaerythritol and (meth)acrylic acid, in which the hydrogen atoms of the hydroxyl groups of pentaerythritol are substituted with (meth)acryloyl groups. A hydroxyl value is a value corresponding to the number of substituted hydroxyl groups. Pentaerythritol and (meth)acrylic acid can be reacted by a known method. As a result, pentaerythritol mono (meth) acrylate with one (meth) acrylic acid added to pentaerythritol, pentaerythritol di (meth) acrylate with two additions, pentaerythritol tri (meth) acrylate with three additions, Four-addition pentaerythritol tetra(meth)acrylates may be formed, usually mixtures comprising two or more of these (meth)acrylates.

In one embodiment, the pentaerythritol (meth)acrylate preferably contains 35 to 55% by mass, more preferably 40 to 50% by mass, of pentaerythritol tetra(meth)acrylate. That is, the content of pentaerythritol tetra(meth)acrylate in 100% by mass of the (meth)acrylate mixture is preferably 35 to 55% by mass, more preferably 40 to 50% by mass. Also, the content of pentaerythritol di(meth)acrylate is preferably 15% by mass or less, more preferably 3 to 12% by mass. The content of pentaerythritol tri(meth)acrylate may be, for example, 35 to 55% by mass, or may be 40 to 50% by mass. The content of pentaerythritol mono(meth)acrylate may be, for example, 5% by mass or less, or may be 0 to 3% by mass.

In this embodiment, the polyol may be composed only of pentaerythritol (meth)acrylate having a hydroxyl value of 80 to 160 mgKOH/g, but may contain other polyols as long as the effect is not impaired. In a preferred embodiment, the polyol contains 90% by mass or more of the pentaerythritol (meth)acrylate. That is, it is preferable that 90% by mass or more of pentaerythritol (meth)acrylate is contained in 100% by mass of polyol.

Other polyols are not particularly limited, and include, for example, dipentaerythritol (meth)acrylate, glycerin (meth)acrylate, trimethylolpropane (meth)acrylate, ditrimethylolpropane (meth)acrylate, and the like.

[Polyisocyanate]
In this embodiment, xylylene diisocyanate (XDI) is used as the polyisocyanate. By using a urethane (meth)acrylate obtained by reacting xylylene diisocyanate with pentaerythritol (meth)acrylate having a specific hydroxyl value, all of the hardness, low curling property and flexibility of the coating film are satisfied. can be done.

In this embodiment, the polyisocyanate may consist of only xylylene diisocyanate, but may contain other polyisocyanates as long as the effect is not impaired. In a preferred embodiment, the polyisocyanate contains 90% by mass or more of xylylene diisocyanate. That is, it is preferable that 90% by mass or more of xylylene diisocyanate is contained in 100% by mass of polyisocyanate.

Other polyisocyanates are not particularly limited, and examples include tolylene diisocyanate, diphenylmethane diisocyanate, polyphenylmethane polyisocyanate, phenylene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, lysine diisocyanate, hydrogenated diphenylmethane diisocyanate, Hydrogenated xylylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, or trimer or polymer compounds of these polyisocyanates, allophanate-type polyisocyanates, biuret-type polyisocyanates, and reaction products of these polyisocyanates and polyols. It's okay.

[Urethane (meth)acrylate]
Urethane (meth)acrylate is obtained by reacting the above polyol and polyisocyanate. Specifically, it is obtained by reacting the hydroxyl group of pentaerythritol (meth)acrylate and the isocyanate group of xylylene diisocyanate. be done.

The reaction between the polyol and the polyisocyanate can be performed by a known method and is not particularly limited. From the viewpoint of the hardness and safety of the coating film, the charging ratio of both is the molar ratio ([OH]/[NCO]) of the hydroxyl group (OH) of the polyol and the isocyanate group (NCO) of the polyisocyanate. 00 to 1.50, more preferably 1.01 to 1.40.

When reacting the polyol and the polyisocyanate, a polymerization inhibitor that inhibits polymerization of the (meth)acryloyl group may be added. Examples of polymerization inhibitors include p-benzoquinone, naphthoquinone, tolquinone, 2,5-diphenyl-p-benzoquinone, hydroquinone, 2,5-di-t-butylhydroquinone, methylhydroquinone, mono-t-butylhydroquinone, and hydroquinone. monomethyl ether, 2,6-di-t-butylcresol, pt-butylcatechol and the like.

When reacting polyol and polyisocyanate, a catalyst may be added to accelerate the reaction. Examples of catalysts include dibutyltin dilaurate, dibutyltin diacetate, tin octylate, zinc hexanoate, cobalt naphthenate, triethylamine, triethylenediamine, bismuth nitrate, bismuth bromide, 1,8-diazabicyclo[5,4,0 ] undecene and the like.

Also, in the reaction, organic solvents such as ethyl acetate, butyl acetate, methyl ethyl ketone, and toluene may be used.

[Hard coating agent]
The hard coating agent according to the present embodiment contains the above urethane (meth)acrylate, and is an active energy ray-curable resin composition that can be cured by being irradiated with an active energy ray.

The hard coating agent may be composed only of the above urethane (meth)acrylate, but preferably further contains a photopolymerization initiator. Photopolymerization initiators include, for example, 1-hydroxycyclohexylphenyl ketone, 2-hydroxy-2-methylpropiophenone, benzyldimethylketal, 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropio phenone, benzoin isopropyl ether and the like. These can be used either singly or in combination of two or more.

The content of the photopolymerization initiator is not particularly limited. It may be up to 5 parts by mass.

The hard coating agent may contain an organic solvent, if necessary, in order to adjust the viscosity during application. Examples of organic solvents include alcohols such as methanol, ethanol, propanol, n-butanol and isobutanol; ketones such as acetone, methyl isobutyl ketone, methyl ethyl ketone and cyclohexanone; cellosolves such as ethyl cellosolve; Aromatics, glycol ethers such as propylene glycol monomethyl ether, acetic acid esters such as methyl acetate and ethyl acetate, and diacetone alcohol. These organic solvents may be used either singly or in combination of two or more.

The hard coating agent may also include other urethane (meth)acrylates, ethylenically unsaturated monomers other than urethane (meth)acrylates, acrylic resins, surface conditioners, and leveling agents as long as the effects of the present embodiment are not impaired. , polymerization inhibitors, fillers, dyes, pigments, oils, plasticizers, waxes, desiccants, dispersants, wetting agents, gelling agents, stabilizers, antifoaming agents, surfactants, thixotropic agents, antioxidants agents, flame retardants, antistatic agents, matting agents, cross-linking agents, silica, zirconium compounds, preservatives, and the like.

The hard coating agent can be used as a curable composition that is applied to various substrates to form a coating film. After the hard coating agent is applied to the substrate, it can be cured by irradiating it with active energy rays. When the hard coating agent contains an organic solvent, it may be cured by irradiating active energy rays after drying to remove the organic solvent after coating.

The substrate to which the hard coating agent is applied is not particularly limited, and examples thereof include plastics made of polyolefin resins, polyester resins, polycarbonate resins, acrylonitrile-butadiene-styrene copolymers (ABS), polystyrene resins, and the like. Base materials, composite material base materials obtained by mixing glass fibers or inorganic substances with these resins, metal base materials, glass base materials, etc., may be mentioned, and laminated base materials obtained by laminating a plurality of layers made of these various materials may also be used. The shape of these substrates may be, for example, a film, a sheet, or a three-dimensionally molded one.

Examples of the method of applying the hard coating agent include wet coating methods such as spray, gravure, dipping, roll, spin, and screen printing.

Examples of active energy rays used for curing the hard coating agent applied on the substrate include light rays such as far ultraviolet rays, ultraviolet rays, near ultraviolet rays, and infrared rays; electromagnetic waves such as X rays and γ rays; radiation, proton radiation, neutron radiation, and the like. Among these, curing by ultraviolet irradiation is preferable. In that case, for example, using a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a carbon arc lamp, a metal halide lamp, a xenon lamp, an LED lamp, or the like, ultraviolet rays of 30 to 3,000 mJ/cm ² may be irradiated for curing. .

The hardness of the hard coating agent after curing (that is, the hardness of the cured product) is preferably, for example, 350 N/mm ² or more, more preferably 380 N/mm ² or more in Martens hardness. Although the upper limit of Martens hardness is not particularly limited, it is usually 500 N/mm ² or less.

The method for measuring Martens hardness is as follows. The hard coating agent was applied on the PET film so that the film thickness in the dry state was about 6 μm, and after drying, a high-pressure mercury lamp (80 W / cm × 1 lamp) was used, and the integrated illuminance was 600 mJ / cm. ² to obtain a cured coating film. The indentation hardness (load) of the obtained coating film was measured with a microhardness meter (“ENT-1100a” manufactured by Elionix Co., Ltd., test load 0.3 mN, triangular pyramid indenter edge interval 115 degrees). Martens hardness (HM) is obtained from the formula HM = F / As (h). is the divided value.

The hard coating agent according to the present embodiment can be applied to various substrates and cured to form a cured product (cured coating film), and various articles containing such cured products can be provided. can be done. Such articles include, for example, various electrical and electronic devices such as displays and touch panels, and may be optical films and the like incorporated in the electrical and electronic devices. Preferred articles include portable electronic devices such as smart phones with foldable displays.

The present invention will be described in more detail below based on Examples and Comparative Examples, but the present invention is not limited thereto.

Synthesis examples of Acrylates 1 to 6 used in Examples and Comparative Examples are shown below.

[Acrylate 1]
(Pentaerythritol acrylate with a hydroxyl value of 120 mgKOH/g)
Into a four-necked flask equipped with a thermometer, a stirrer and a water-cooled condenser, 1151 parts by mass (16.0 mol) of acrylic acid, 604 parts by mass (4.44 mol) of pentaerythritol (manufactured by Koei Chemical Industry Co., Ltd.), 43.9 parts by mass of p-toluenesulfonic acid, 2.1 parts by mass of hydroquinone monomethyl ether, and 552 parts by mass of toluene were added and mixed. After that, under reduced pressure, air was blown into the mixture while maintaining the reaction temperature at about 100° C., and the mixture was reacted until 83% of all hydroxyl groups in pentaerythritol were esterified. The reaction was carried out while removing condensed water. After completion of the reaction, 353 parts by mass of toluene was added. A neutralization treatment is carried out by adding a 20% by mass aqueous solution of sodium hydroxide in an amount corresponding to 1.4 times the molar amount of the acid content of the reaction solution to which toluene has been added, with stirring, thereby removing excess acrylic acid. and p-toluenesulfonic acid were removed. Thereafter, the organic layer was separated and washed with water by adding 10 parts by mass of water to 100 parts by mass of the organic layer while stirring. After that, the organic layer was separated again and heated under reduced pressure to distill off the toluene. Acrylate 1 obtained was 1,082 parts by mass and had a hydroxyl value of 120 mgKOH/g.

In Acrylate 1, the content ratio of each component with respect to the total of components (A1) to (A4) below was as follows.
(A1) pentaerythritol monoacrylate 0% by mass
(A2) Pentaerythritol diacrylate 6% by mass
(A3) pentaerythritol triacrylate 49% by mass
(A4) Pentaerythritol tetraacrylate 45% by mass

The content ratio of each component in the acrylate was determined by high performance liquid chromatography (HPLC, "Prominence-iLC2050C" (manufactured by Shimadzu Corporation)) analysis. Specifically, column ("Inertsil ODS-2", inner diameter 4.6 mm × length 250 mm (manufactured by GL Sciences Inc.), column temperature 40 ° C., mobile phase methanol / water = 20/80 to 100/0 gradient elution method.

[Acrylate 2]
(Pentaerythritol acrylate with a hydroxyl value of 110 mgKOH/g)
Pentaerythritol acrylate synthesized by the same method as Acrylate 1, but different lots, had a hydroxyl value of 110 mgKOH/g, and contained the following components (A1) to (A4) in the following ratios:
(A1) pentaerythritol monoacrylate 0% by mass
(A2) Pentaerythritol diacrylate 6% by mass
(A3) pentaerythritol triacrylate 44% by mass
(A4) Pentaerythritol tetraacrylate 50% by mass

[Acrylate 3]
(Pentaerythritol acrylate with a hydroxyl value of 139 mgKOH/g)
Pentaerythritol acrylate synthesized by the same method as Acrylate 1, but with a different lot, had a hydroxyl value of 139 mgKOH/g, and contained the following components (A1) to (A4) in the following ratios:
(A1) pentaerythritol monoacrylate 0% by mass
(A2) Pentaerythritol diacrylate 11% by mass
(A3) pentaerythritol triacrylate 47% by mass
(A4) Pentaerythritol tetraacrylate 42% by mass

[Acrylate 4]
(Pentaerythritol acrylate with a hydroxyl value of 78 mgKOH/g)
1151 parts by mass (16.0 mol) of acrylic acid, 544 parts by mass (4.0 mol) of pentaerythritol (manufactured by Koei Chemical Industry Co., Ltd.), 42.4 parts by mass of p-toluenesulfonic acid, 2.0 parts by mass of hydroquinone monomethyl ether, and 532 parts by mass of toluene were added and mixed. After that, under reduced pressure, air was blown into the mixture while maintaining the reaction temperature at about 100° C., and the mixture was reacted until 89% of all hydroxyl groups in pentaerythritol were esterified. The reaction was carried out while removing condensed water. After completion of the reaction, 341 parts by mass of toluene was added. A neutralization treatment is carried out by adding a 20% by mass aqueous solution of sodium hydroxide in an amount corresponding to 1.4 times the molar amount of the acid content of the reaction solution to which toluene has been added, with stirring, thereby removing excess acrylic acid. and p-toluenesulfonic acid were removed. Thereafter, the organic layer was separated and washed with water by adding 10 parts by mass of water to 100 parts by mass of the organic layer while stirring. After that, the organic layer was separated again and heated under reduced pressure to distill off the toluene. Acrylate 4 obtained was 1222 parts by mass and had a hydroxyl value of 78 mgKOH/g.

In Acrylate 4, the content ratio of each component with respect to the total of components (A1) to (A4) below was as follows.
(A1) pentaerythritol monoacrylate 0% by mass
(A2) Pentaerythritol diacrylate 1% by mass
(A3) Pentaerythritol triacrylate 39% by mass
(A4) Pentaerythritol tetraacrylate 60% by mass

[Acrylate 5]
(Pentaerythritol acrylate with a hydroxyl value of 190 mgKOH/g)
Into a four-necked flask equipped with a thermometer, a stirrer and a water-cooled condenser, 1151 parts by mass (16.0 mol) of acrylic acid, 604 parts by mass (4.44 mol) of pentaerythritol (manufactured by Koei Chemical Industry Co., Ltd.), 43.9 parts by mass of p-toluenesulfonic acid, 2.1 parts by mass of hydroquinone monomethyl ether, and 552 parts by mass of toluene were added and mixed. Then, under reduced pressure, air was blown into the mixture while maintaining the reaction temperature at about 100° C., and the reaction was continued until 75% of all hydroxyl groups in pentaerythritol were esterified. The reaction was carried out while removing condensed water. After completion of the reaction, 353 parts by mass of toluene was added. A neutralization treatment is carried out by adding a 20% by mass aqueous sodium hydroxide solution in an amount corresponding to 1.1 times the molar amount of the acid content of the reaction solution to which toluene has been added, with stirring, thereby removing excess acrylic acid. and p-toluenesulfonic acid were removed. Thereafter, the organic layer was separated and washed with water by adding 10 parts by mass of water to 100 parts by mass of the organic layer while stirring. After that, the organic layer was separated again and heated under reduced pressure to distill off the toluene. Acrylate 5 obtained was 837 parts by mass and had a hydroxyl value of 190 mgKOH/g.

In Acrylate 5, the content ratio of each component with respect to the total of components (A1) to (A4) below was as follows.
(A1) pentaerythritol monoacrylate 0% by mass
(A2) Pentaerythritol diacrylate 23% by mass
(A3) pentaerythritol triacrylate 45% by mass
(A4) Pentaerythritol tetraacrylate 32% by mass

[Acrylate 6]
(Dipentaerythritol acrylate with a hydroxyl value of 50 mgKOH/g)
3166 parts by mass (43.9 mol) of acrylic acid, 1526 parts by mass (6.0 mol) of dipentaerythritol, and 187.7 parts by mass of p-toluenesulfonic acid were placed in a four-necked flask equipped with a thermometer, a stirrer and a water-cooled condenser. parts, 9.4 parts by mass of hydroquinone monomethyl ether, and 939 parts by mass of toluene were added and mixed. Then, under reduced pressure, air was blown into the mixture while maintaining the reaction temperature at about 100° C., and the reaction was continued until 88% of all hydroxyl groups in dipentaerythritol were esterified. The reaction was carried out while removing condensed water. After completion of the reaction, a neutralization treatment was carried out by adding 1.2 times the molar amount of the corresponding 25% by weight aqueous sodium hydroxide solution with stirring, thereby removing excess acrylic acid and p-toluenesulfonic acid. Thereafter, the organic layer was separated and washed with water by adding 25 parts by mass of water to 100 parts by mass of the organic layer while stirring. After that, the organic layer was separated again and heated under reduced pressure to distill off the toluene. Acrylate 6 obtained was 3894 parts by mass and had a hydroxyl value of 50 mgKOH/g.

In Acrylate 6, the content ratio of each component with respect to the total of components (A1) to (A2) below was as follows.
(A1) Dipentaerythritol pentaacrylate 47% by mass
(A2) Dipentaerythritol hexaacrylate 53% by mass

[Example 1]
A flask was charged with 188.19 g (1.0 mol) of xylylene diisocyanate (manufactured by Mitsui Chemicals, Inc.) as a polyisocyanate, 0.61 g of hydroquinone monomethyl ether as a polymerization inhibitor, and pentaerythritol having a hydroxyl value of 120 mgKOH/g as a polyol. After charging 1028.68 g (2.2 mol in terms of hydroxyl group) of acrylate (Acrylate 1), the urethane of Example 1 was prepared by reacting at 50 to 80°C until the amount of free isocyanate was 0.1% or less. Acrylate was obtained.

[Examples 2 to 5 and Comparative Examples 1 to 16]
The amounts (g) of polyol, polyisocyanate and polymerization inhibitor to be charged were changed as shown in Tables 1 to 4 below, and the other urethanes of Examples 2 to 5 and Comparative Examples 1 to 16 were prepared in the same manner as in Example 1. synthesized acrylates.

Hard coating agents were prepared using the urethane acrylates of Examples 1 to 5 and Comparative Examples 1 to 16 obtained above, and the Martens hardness, low curling property, hardness/low curling index, and flexibility were measured and evaluated. . In Comparative Example 17, a hard coating agent was prepared using dipentaerythritol acrylate instead of urethane acrylate, and measured and evaluated in the same manner. The results are shown in Tables 1-4. The measurement and evaluation methods are as follows.

[Martens hardness]
Urethane acrylate/toluene/"Omnirad 184" (manufactured by IGM Resins B.V.) were mixed at a ratio of 60 parts by mass/40 parts by mass/1.8 parts by mass to prepare a hard coating agent. The hard coating agent was applied to a 100 μm-thick PET film (Cosmo Shine A4360, manufactured by Toyobo Co., Ltd.) so that the film thickness in a dry state would be about 6 μm, and dried at 80°C. After that, in a nitrogen atmosphere, a high-pressure mercury lamp (80 W/cm×1 lamp) was used to irradiate ultraviolet rays at an integrated illuminance of 600 mJ/cm ² to obtain a cured film. For the resulting film, the indentation hardness (load) was obtained with a microhardness tester (Elionix Co., Ltd. "ENT-1100a", test load 0.3 mN, triangular pyramid indenter edge interval 115 degrees), and Martens hardness ( HM) were obtained. A higher numerical value indicates a higher hardness of the coating film.

[Low curl]
Urethane acrylate/"Omnirad 184" (manufactured by IGM Resins B.V.) were mixed at a ratio of 100 parts by mass/3 parts by mass to prepare a hard coating agent. The hard coating agent was applied to a 100 μm-thick PET film (“Cosmo Shine A4360” manufactured by Toyobo Co., Ltd.) so that the film thickness in a dry state would be about 10 μm. After that, in a nitrogen atmosphere, a high-pressure mercury lamp (80 W/cm×1 lamp) was used to irradiate with an integrated illuminance of 600 mJ/cm ² to obtain a cured film. After cutting the resulting film into a size of 6 cm×6 cm, it was placed on a horizontal surface and one corner was fixed. Then, the floating (mm) of the three corners from the horizontal surface was measured, the average value was calculated, and the low curling property was indicated as an index with the value of Comparative Example 17 being 100. The smaller the numerical value, the less the warpage and the better the low curl property.

[Hardness/low curl index]
The value obtained by dividing the Martens hardness by the low-curl index (“Martens hardness”/“low-curl index”) was defined as an index (hardness/low-curl index) showing the effect of both hardness and low-curl properties. The larger the value, the better the effect of achieving both hardness and low curling property.

[Flexibility]
Urethane acrylate/"Omnirad 184" (manufactured by IGM Resins B.V.) were mixed at a ratio of 100 parts by mass/3 parts by mass to prepare a hard coating agent. The hard coating agent was applied to a 100 μm-thick PET film (“Cosmo Shine A4360” manufactured by Toyobo Co., Ltd.) so that the film thickness in a dry state would be about 6 μm. After that, in a nitrogen atmosphere, a high-pressure mercury lamp (80 W/cm×1 lamp) was used to irradiate with an integrated illuminance of 600 mJ/cm ² to obtain a cured film. The film obtained by curing the coating film was measured according to JIS K5600-5-1:1999, and the flexibility was measured and evaluated according to the following criteria.
A: The mandrel diameter is 4 mm and no streaks are formed.
B: Streaks are formed when the mandrel diameter is 5 to 7 mm.
C: Streaks formed when the mandrel diameter was 8 mm or more.

The results are shown in Tables 1-4. Comparative Example 17, which is a control using dipentaerythritol acrylate, had high hardness, but was inferior in low curling properties and flexibility. On the other hand, in Examples 1 to 5 using urethane acrylate, which is a reaction product of pentaerythritol acrylate and xylylene diisocyanate having a specific hydroxyl value, low curling property was remarkably improved while maintaining high hardness. It was excellent in both hardness and low curling property, and was also excellent in flexibility.

On the other hand, in Comparative Example 1 using urethane acrylate, which is a reaction product of pentaerythritol acrylate with a small hydroxyl value and xylylene diisocyanate, the effect of compatibility between hardness and low curling property is inferior to Examples 1 to 5, and bending It was also inferior in sex. Comparative Example 3 using urethane acrylate, which is a reaction product of pentaerythritol acrylate having a large hydroxyl value and xylylene diisocyanate, was inferior to Examples 1 to 5 in terms of both hardness and low curling properties.

When a polyisocyanate other than xylylene diisocyanate is used as the polyisocyanate constituting the urethane acrylate, pentaerythritol acrylate having a specific hydroxyl value is used as the polyol, as shown in Comparative Examples 2, 5, 9, and 15. , the effect of coexisting hardness and low curling property was inferior to those of Examples 1 to 5, and the flexibility was also inferior.

When pentaerythritol acrylate having a large hydroxyl value is used as the polyol constituting the urethane acrylate, and this is combined with a polyisocyanate other than xylylene diisocyanate, as shown in Comparative Examples 4, 6, 8, and 10, hardness and low curling The evaluation of at least one of the compatibility effect of flexibility and flexibility was inferior. Further, when dipentaerythritol acrylate was used as the polyol constituting the urethane acrylate, as shown in Comparative Examples 11 to 14 and 16, low curling property and flexibility were inferior.

It should be noted that the various numerical ranges described in the specification can be arbitrarily combined with their upper and lower limits, and all combinations thereof are described in this specification as preferred numerical ranges. Further, the description of the numerical range "X to Y" means X or more and Y or less.

Although several embodiments of the invention have been described above, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and modifications can be made without departing from the scope of the invention. These embodiments, their omissions, replacements, modifications, etc., are included in the invention described in the scope of claims and equivalents thereof, as well as being included in the scope and gist of the invention.

Claims (6)

A hard coating agent containing urethane (meth)acrylate which is a reaction product of a polyol containing pentaerythritol (meth)acrylate having a hydroxyl value of 80 to 160 mgKOH/g and a polyisocyanate containing xylylene diisocyanate. 2. The hard coating agent according to claim 1, wherein 90% by mass or more of said pentaerythritol (meth)acrylate is contained in 100% by mass of said polyol. 3. The hard coating agent according to claim 1, wherein 90% by mass or more of said xylylene diisocyanate is contained in 100% by mass of said polyisocyanate. The hard coating agent according to any one of claims 1 to 3, which is for a foldable display. A cured product obtained by curing the hard coating agent according to any one of claims 1 to 4. An article comprising the cured product of claim 5.