Adhesive composition, adhesive, adhesive sheet, laminate, method for producing same, flexible display, and display device

Abstract

An adhesive composition comprising: a monofunctional urethane (meth)acrylate that is the reaction product of a polyether monool and a monoisocyanate having a (meth)acryloyloxy group and has a weight-average molecular weight of 10,000 to 40,000; and a bifunctional urethane (meth)acrylate that is the reaction product of a polyether polyol having a hydroxyl group-equivalent molecular weight of 3,500 or less, a polyisocyanate compound, and a hydroxyl group-containing (meth)acrylate compound, and has a weight-average molecular weight of 10,000 to 50,000, wherein the total content of the monofunctional urethane (meth)acrylate and the bifunctional urethane (meth)acrylate relative to 100% by mass of the adhesive composition is more than 45% by mass.

Description

Adhesive Composition, Adhesive, Adhesive Sheet, Laminate, Method for Producing the Same, Flexible Display, and Display Device

[0001] The present invention relates to an adhesive composition, an adhesive, an adhesive sheet, a laminate, a method for producing the same, a flexible display, and a display device, and particularly to an adhesive composition capable of obtaining an adhesive having small residual strain, excellent adhesiveness, and excellent optical properties (transparency), an adhesive which is a cured product of the adhesive composition, an adhesive sheet containing the adhesive, a laminate having an adhesive layer containing the adhesive and a method for producing the same, a flexible display including the laminate, and a display device including an adhesive layer containing a cured product of the adhesive composition.

[0002] In a display device in which a protective plate is laminated via an adhesive layer on a display device, when the adhesive layer is cured, the stress generated by the shrinkage of the adhesive layer may affect the display device. When stress is applied to the display device, the display forming material in the display device may be affected by the stress, and the uniformity of the display may be impaired. For example, in the case of a liquid crystal display device using liquid crystal as a display forming material, the arrangement of the liquid crystal enclosed in the display device may be disturbed by external stress and may be visually recognized as display unevenness such as color unevenness.

[0003] The adhesive layer between the display device and the protective plate is required to have a low elastic modulus, good flexibility, and a low curing shrinkage rate in order to prevent the above-described display unevenness.

[0004] On the other hand, for example, Patent Document 1 discloses that an adhesive layer formed using a composition containing a specific polyfunctional oligomer and a specific monofunctional oligomer has a low elastic modulus after curing, a low curing shrinkage rate, and is difficult to peel off.

[0005] Japanese Patent Application Laid-Open No. 2014-156566

[0006] Although the adhesive layer described in Patent Document 1 has a low elastic modulus after curing, a low curing shrinkage rate, and is difficult to peel off, further improvement in adhesiveness and optical properties (transparency) has been desired.

[0007] The present invention has been made in view of the above circumstances, and aims to provide an adhesive composition that yields an adhesive with low residual strain, excellent tackiness, and excellent optical properties (transparency); an adhesive which is a cured product of the adhesive composition; an adhesive sheet containing the adhesive; a laminate having an adhesive layer containing the adhesive; a method for manufacturing the same; a flexible display comprising the laminate; and a display device comprising an adhesive layer containing a cured product of the adhesive composition.

[0008] The present invention is based on the discovery that by using an adhesive composition containing predetermined amounts of a specific monofunctional urethane (meth)acrylate and a specific difunctional urethane (meth)acrylate, an adhesive can be obtained that exhibits low residual strain, excellent tackiness, and superior optical properties (transparency).

[0009] The present invention provides the following means: [1] An adhesive composition comprising a monofunctional urethane (meth)acrylate and a difunctional urethane (meth)acrylate, wherein the monofunctional urethane (meth)acrylate is a reaction product of a polyether monool and a monoisocyanate having a (meth)acryloyloxy group, the monofunctional urethane (meth)acrylate has a weight-average molecular weight of 10,000 to 40,000, the difunctional urethane (meth)acrylate is a reaction product of a polyether polyol, a polyisocyanate compound and a hydroxyl group-containing (meth)acrylate compound, the difunctional urethane (meth)acrylate has a weight-average molecular weight of 10,000 to 50,000, the molecular weight calculated from the hydroxyl value of the polyether polyol is 3,500 or less, and the total content of the monofunctional urethane (meth)acrylate and the difunctional urethane (meth)acrylate relative to 100% by mass of the adhesive composition is greater than 45% by mass. [2] The adhesive composition according to [1], further comprising a (meth)acrylic monomer compound having a molecular weight of 1,000 or less, wherein the content of the (meth)acrylic monomer compound is 1 to 50% by mass with respect to 100% by mass of the adhesive composition. [3] The adhesive composition according to [1] or [2], further comprising a tackifier, wherein the content of the tackifier is 1 to 30% by mass with respect to 100% by mass of the adhesive composition. [4] The adhesive composition according to any one of [1] to [3], wherein the urethane bond content per 100 number average molecular weight of the urethane acrylate calculated by the following formula (1) of the bifunctional urethane (meth)acrylate is 0.070 mol or more. Urethane bond content [mol] = {Number average molecular weight of the difunctional urethane (meth)acrylate ÷ (Molecular weight of the polyether polyol + Molecular weight of the polyisocyanate compound) × 2 + 2} / Number average molecular weight of the difunctional urethane (meth)acrylate × 100 ... Formula (1) [5] The adhesive composition according to any one of [1] to [4] above, wherein the content of the monofunctional urethane (meth)acrylate is 10 to 1000 parts by mass per 100 parts by mass of the difunctional urethane (meth)acrylate.[6] An adhesive which is a cured product of the adhesive composition described in any of [1] to [5] above. [7] An adhesive sheet which contains the adhesive described in [6] above in at least a portion of a base sheet. [8] A laminate having a flexible member and an adhesive layer present on the flexible member which contains the adhesive described in [6] above. [9] The laminate according to [8] above, wherein the flexible member is at least one selected from the group consisting of a surface protection panel, an optical film, a touch panel, and a display panel body.

[10] A flexible display comprising the laminate according to [8] or [9] above.

[11] A display device in which an adhesive layer containing a cured product of the adhesive composition described in any of [1] to [5] above is sandwiched between a transparent surface material and a display device.

[12] A method for manufacturing a laminate, comprising sandwiching an uncured layer containing the adhesive composition described in any of [1] to [5] above between a pair of surface materials and curing the uncured layer.

[13] A method for manufacturing a laminate, comprising: sandwiching an uncured layer containing the adhesive composition described in any of [1] to [5] above between a pair of facing materials in a first reduced-pressure atmosphere, forming a laminate precursor in which the uncured layer is sealed by a seal portion provided around the uncured layer, and curing the uncured layer in a second atmosphere having a pressure higher than the first reduced-pressure atmosphere.

[14] A method for manufacturing a laminate according to

[12] or

[13] above, wherein one of the pair of facing materials is a transparent facing material and the other of the pair of facing materials is a display device.

[0010] By using the adhesive composition of this embodiment, it is possible to provide an adhesive with low residual strain, excellent adhesiveness, and excellent optical properties (transparency), an adhesive sheet containing the adhesive, a laminate having an adhesive layer containing the adhesive, a method for manufacturing the same, a flexible display comprising the laminate, and a display device comprising an adhesive layer containing a cured product of the adhesive composition.

[0011] The definitions and meanings of terms and notations used in this specification are as follows: "(meth)acrylic" is a general term for acrylic and methacrylic. Similarly, "(meth)acrylate" is a general term for acrylate and methacrylate, and "(meth)acryloyloxy" is a general term for acryloyloxy and methacryloyloxy. Numerical ranges expressed using "~" mean that the numbers before and after "~" are the lower and upper limits. For numerical ranges (e.g., ranges of content, etc.), the lower and upper limits described in steps may be combined independently. The lower and upper limits of numerical ranges may be replaced with the numbers described in the examples. In this specification, "monofunctional" in "monofunctional urethane (meth)acrylate" means "the number of (meth)acryloyloxy groups is one," and "difunctional" in "difunctional urethane (meth)acrylate" means "the number of (meth)acryloyloxy groups is two." The weight-average molecular weight (Mw) and number-average molecular weight (Mn) are polystyrene-equivalent molecular weights determined by gel permeation chromatography (GPC) based on a calibration curve created using standard polystyrene samples. Specifically, they are determined by the method described in the examples. The glass transition temperature (Tg) is defined as the maximum peak temperature of the loss tangent tanδ in dynamic viscoelasticity measurement for the polymer or cured product. Specifically, it is determined by measuring the dynamic viscoelasticity of the polymer or cured product in the range of -80°C to 100°C at a frequency of 1 Hz and a shear strain of 1% using an Anton Paar rheometer MCR702e. The hydroxyl value is a value obtained by measurement in accordance with JIS K 1557:2007. The molecular weight converted from the hydroxyl value is the value calculated by applying the hydroxyl value to the formula "{56100 / (hydroxyl value)} × {number of hydroxyl groups in the initiator)". The viscosity is the value measured at 25°C using an E-type viscometer. Specifically, it is determined by using a VICOMETER RE-85U E-type viscometer manufactured by Toki Sangyo Co., Ltd., with a No. 1 or No. 3 cone rotor, and measuring at a rotation speed of 0.5 to 50 rpm. "Sheet" includes film and tape, and these are not particularly distinguished.

[0012] [Adhesive Composition] The adhesive composition of this embodiment (hereinafter referred to as "this embodiment") comprises a monofunctional urethane (meth)acrylate and a difunctional urethane (meth)acrylate, and optionally further comprises other components such as a (meth)acrylic monomer compound, a tackifier, and a photopolymerization initiator. An adhesive composition containing such components can be obtained that has low residual strain (stress can be relieved), excellent impact resistance, and excellent bending durability (long-term reliability). The adhesive composition of this embodiment contains a monofunctional urethane (meth)acrylate and a difunctional urethane (meth)acrylate, and in particular, by containing a monofunctional urethane (meth)acrylate, it is possible to form "physical crosslinking by urethane bonds" rather than "chemical crosslinking," thereby improving flexibility, and consequently reducing residual strain and improving bending durability.

[0013] (Monofunctional Urethane (Meth)acrylate) The monofunctional urethane (meth)acrylate component of the adhesive composition of this embodiment is a reaction product of a polyether monool and a monoisocyanate having a (meth)acryloyloxy group. These may be used individually or in combination of two or more.

[0014] <Polyether Monool> There are no particular restrictions on the polyether monool; for example, polyoxyethylene monool, polyoxypropylene monool, and other polyoxyalkylene monools are examples. These may be used individually or in combination of two or more. Among these, polyoxypropylene monool is preferred from the viewpoint of improving adhesive strength by reducing the water absorption of the resulting adhesive. Polyether monool can be obtained, for example, by a known method of ring-opening addition polymerization of a monoepoxide using an alcohol having one hydroxyl group in one molecule or a compound obtained by adding an alkylene oxide to an alcohol as an initiator, or by a known method of ring-opening addition polymerization of a monoepoxide to a hydroxyl group of a monocarboxylic acid.

[0015] There are no particular restrictions on the hydroxyl value of the polyether monool, but it is preferably 1.6 to 56.1 mg KOH / g, and more preferably 3.7 to 14.0 mg KOH / g.

[0016] There are no particular restrictions on the molecular weight calculated from the hydroxyl value of the polyether monool, but from the viewpoint of improving the flexibility of the resulting adhesive, it is preferably 2,000 to 40,000, more preferably 3,000 to 30,000, and most preferably 4,000 to 20,000.

[0017] <Monoisocyanates having a (meth)acryloyloxy group> There are no particular restrictions on monoisocyanates having a (meth)acryloyloxy group, that is, compounds having a (meth)acryloyloxy group and one isocyanate group in one molecule, but compounds in which a (meth)acryloyloxy group is bonded to a hydrocarbon skeleton having one isocyanate group are preferred. There are no particular restrictions on the hydrocarbon skeleton, but an aliphatic hydrocarbon group is preferred and may contain an etheric oxygen atom. There are no particular restrictions on the number of carbon atoms in the aliphatic hydrocarbon group, but it is preferably 8 or less, more preferably 2 to 6, and particularly preferably 2 to 4. There is usually one (meth)acryloyloxy group in one molecule of monoisocyanate.

[0018] There are no particular limitations on specific examples of monoisocyanates having a (meth)acryloyloxy group. Examples include compounds having one (meth)acryloyloxy group per molecule, such as isocyanate methyl (meth)acrylate, 2-isocyanate ethyl (meth)acrylate, and 1,1-(bisacryloyloxymethyl)ethyl isocyanate. There are no particular limitations on commercially available monoisocyanates having a (meth)acryloyloxy group. Examples include "Karenz (registered trademark; hereinafter omitted) AOI" (2-isocyanate ethyl acrylate) and "Karenz MOI" (2-isocyanate ethyl methacrylate).

[0019] <Reaction Products> Monofunctional urethane (meth)acrylate is a reaction product obtained by urethane reaction between a polyether monool and a monoisocyanate. The urethane reaction can be carried out by known methods, and is usually carried out by mixing the polyether monool and monoisocyanate and using a urethane catalyst in an atmosphere of nitrogen gas or an inert gas.

[0020] <<Urethane Catalyst>> There are no particular restrictions on the urethane catalyst, and examples include organotin compounds such as dibutyltin(IV) dilaurate, dioctyltin(IV) dilaurate, dibutyltin(IV) dioctoate, and tin(II) 2-ethylhexanoate; iron compounds such as iron(III) acetylacetonate and iron(III) chloride; lead compounds such as lead(II) 2-ethylhexanoate; bismuth compounds such as bismuth(III) 2-ethylhexanoate; and tertiary amines such as triethylamine and triethylenediamine. These may be used individually or in combination of two or more. Among these, organotin compounds, lead(II) 2-ethylhexanoate, and bismuth(III) 2-ethylhexanoate are preferred.

[0021] There are no particular restrictions on the amount of urethane catalyst used, but it is preferably 0.001 to 1 part by mass, more preferably 0.002 to 0.5 parts by mass, and especially preferably 0.005 to 0.1 parts by mass, per 100 parts by mass of the polyether monool reactant. There are no particular restrictions on the reaction temperature of the urethane reaction, but it is preferably 20 to 100°C, more preferably 30 to 90°C, and especially preferably 40 to 80°C.

[0022] Furthermore, from the viewpoint of preventing oxidation and inhibiting polymerization reactions during storage, an antioxidant or polymerization inhibitor may be added to the monofunctional urethane (meth)acrylate obtained by the urethane reaction, within a range that does not affect the physical properties and effects of the adhesive composition (for example, an amount of 0.2 parts by mass or less per 100 parts by mass of polyether monool). There are no particular restrictions on the antioxidant or polymerization inhibitor, and examples include hindered phenols such as 2,6-di-tert-butyl-p-cresol; hydroquinones such as 2,5-di-tert-butylhydroquinone and p-methoxyphenol; and so on. These may be used individually or in combination of two or more.

[0023] <Physical Properties of Monofunctional Urethane (Meth)acrylate> The weight-average molecular weight (Mw) of the monofunctional urethane (meth)acrylate is not particularly limited as long as it is between 10,000 and 40,000, but from the viewpoint of improving the flexibility of the resulting adhesive (reducing residual strain), it is preferably between 10,000 and 36,000, more preferably between 10,000 and 32,000, and particularly preferably between 10,000 and 30,000.

[0024] There are no particular restrictions on the number-average molecular weight (Mn) of the monofunctional urethane (meth)acrylate, but from the viewpoint of improving the flexibility (reducing residual strain) and adhesive strength of the resulting adhesive, it is preferably 6,000 to 30,000, more preferably 7,000 to 27,000, and particularly preferably 8,000 to 24,000.

[0025] There are no particular restrictions on the molecular weight distribution (Mw / Mn) of the monofunctional urethane (meth)acrylate, but from the viewpoint of reducing the viscosity of the urethane (meth)acrylate, it is preferably 1.00 to 3.00, more preferably 1.10 to 1.50, and particularly preferably 1.20 to 1.30.

[0026] There are no particular restrictions on the glass transition temperature of the monofunctional urethane (meth)acrylate, but from the viewpoint of good adhesiveness and bending durability of the adhesive, it is preferably -80 to -30°C, more preferably -80 to -40°C, and most preferably -80 to -50°C.

[0027] There are no particular restrictions on the viscosity (at 25°C) of the monofunctional urethane (meth)acrylate, but from the viewpoint of ease of handling of the adhesive composition and the adhesive, it is preferably 0.1 to 30 Pa·s, more preferably 1 to 20 Pa·s, and particularly preferably 2 to 10 Pa·s.

[0028] Furthermore, if the adhesive composition contains two or more monofunctional urethane (meth)acrylates, it is preferable that each of the monofunctional urethane (meth)acrylates is within the numerical range of the weight-average molecular weight, number-average molecular weight, molecular weight distribution, glass transition temperature, and viscosity described above.

[0029] The monofunctional urethane (meth)acrylate, which is the urethane reaction product described above, has a polyoxyalkylene group such as a polyoxypropylene group and one (meth)acryloyloxy group per molecule. Because the monofunctional urethane (meth)acrylate has such a structure, the cured product of the adhesive composition readily forms a uniform crosslinking network, resulting in an adhesive with low residual strain and excellent bending durability and impact resistance.

[0030] There are no particular restrictions on the content of monofunctional urethane (meth)acrylate in the adhesive composition, but from the viewpoint of good adhesiveness and bending durability of the adhesive, it is preferably 6 to 100 parts by mass, more preferably 8 to 80 parts by mass, and particularly preferably 10 to 60 parts by mass per 100 parts by mass of the adhesive composition.

[0031] (Bifunctional Urethane (Meth)acrylate) The bifunctional urethane (meth)acrylate component of the adhesive composition of this embodiment is a reaction product of a polyether polyol, a polyisocyanate compound, and a hydroxyl group-containing (meth)acrylate compound. These may be used individually or in combination of two or more. Because the bifunctional urethane (meth)acrylate is a reaction product of a polyether polyol and a polyisocyanate compound, its compatibility with monofunctional urethane (meth)acrylate is improved, resulting in improved optical properties (transparency).

[0032] <Polyether Polyols> There are no particular restrictions on polyether polyols as long as the molecular weight calculated from the hydroxyl value is 3500 or less. However, polyols having two or more hydroxyl groups in one molecule and having polyoxyalkylene groups (including polyoxypropylene groups) are preferred. More preferably, polyether polyols (i.e., polyetherdiols) have two functional groups derived from hydroxyl groups (number of hydroxyl groups) and are obtained from the viewpoint of obtaining a straight-chain, unbranched, bifunctional urethane (meth)acrylate and improving the flexibility and tackiness of the resulting adhesive. Polyether polyols can be obtained, for example, by ring-opening polymerization of a compound having a cyclic ether structure to an initiator having two or more active hydrogens in one molecule. Commercial products can also be used. There are no particular restrictions on commercially available polyether polyols, but for example, "Preminol (registered trademark; hereinafter omitted) S 4013F", "Preminol S 4318F", "Preminol S 3011", "Preminol 5001F", "Preminol 7001K", "Preminol 7012", "Preminol S 4011", "Preminol S 4015", "Preminol S 3025", "Preminol S 6420" (all manufactured by AGC Inc.); "ACCLAIM (registered trademark; hereinafter omitted) 4200", "ACCLAIM 8200", "ACCLAIM 2220N", "ACCLAIM 3300N", "ACCLAIM 4220N", "ACCLAIM 6300", "Acclaim Examples include the "2200" and "ACCLAIM 6320N" (both manufactured by Covestro).

[0033] The polyoxyalkylene group preferably contains a polyoxypropylene group. The polyoxyalkylene group may further contain linear or branched oxyalkylene groups having 1 to 14 carbon atoms, but linear is preferred. There is no particular limit to the number of carbon atoms in the oxyalkylene group, but it is preferably 2 to 4. The polyoxyalkylene group may consist of one type alone or two or more types. As an oxyalkylene group other than the polyoxypropylene group, the oxyethylene group is preferred. There is no particular limit to the content of the polyoxypropylene group in the polyoxyalkylene group, but from the viewpoint of good impact resistance of the adhesive, it is preferably 50% by mass or more, more preferably 60 to 100% by mass, even more preferably 80 to 100% by mass, and particularly preferably 100% by mass. In this embodiment, the content of the polyoxypropylene group in the polyoxyalkylene group is a value based on the amount of the raw material that constitutes the polyoxyalkylene group in the raw material for the synthesis of the polyether polyol.

[0034] There are no particular limitations on the compounds having a cyclic ether structure, and examples include ethylene oxide, propylene oxide, 1,2-butylene oxide, 2,3-butylene oxide, methyl glycidyl ether, butyl glycidyl ether, 2-ethylhexyl glycidyl ether, lauryl glycidyl ether, hexyl glycidyl ether, and tetrahydrofuran. Propylene oxide can constitute a polyoxypropylene group.

[0035] There are no particular restrictions on the active hydrogen group in the initiator; examples include hydroxyl groups, carboxyl groups, and amino groups having a hydrogen atom bonded to a nitrogen atom. These may be used individually or in combination of two or more. Among these, hydroxyl groups are preferred, and alcoholic hydroxyl groups are more preferred. There are no particular limitations on initiators having two or more active hydrogens in one molecule, and examples include ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, butylene glycol, 1,4-butanediol, 1,6-hexanediol, triethylene glycol, tripropylene glycol, polyoxyalkylenediol (e.g., polyethylene glycol, polypropylene glycol, etc.), glycerin, trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol, pentaerythritol, diglycerin, dipentaerythritol, sorbitol, sucrose, polyoxyalkylene polyol (e.g., polyoxyethylene polyol, polyoxypropylene polyol, etc.), polyols such as triethanolamine; bisphenols such as bisphenol A, bisphenol F, bisphenol AD; dihydroxybenzenes such as catechol, resorcinol, hydroquinone; and amines such as methylamine, ethylamine, propylamine, butylamine, ethylenediamine, and diethylenetriamine. These may be used individually or in combination of two or more. Among these, polyols are preferred.

[0036] There are no particular restrictions on the ring-opening polymerization, and it can be carried out by using known catalysts such as alkali catalysts such as potassium hydroxide, transition metal compound-porphyrin complex catalysts such as complexes obtained by reacting organoaluminum compounds with porphyrin, complex metal cyanide complex catalysts (for example, zinc hexacyanocobaltate complex with tert-butanol as a ligand), and catalysts containing phosphazene compounds.

[0037] There are no particular restrictions on the hydroxyl value of the polyether polyol, but it is preferably 30 to 230 mg KOH / g, and more preferably 45 to 75 mg KOH / g.

[0038] The molecular weight of the polyether polyol, calculated from its hydroxyl value, is not particularly limited as long as it is 3,500 or less. However, from the viewpoint of improving the tackiness of the adhesive obtained by increasing the urethane bond content of the bifunctional urethane (meth)acrylate, it is preferably 500 to 3,500, more preferably 1,000 to 3,000, and particularly preferably 1,500 to 2,500.

[0039] <Polyisocyanate Compounds> There are no particular restrictions on the polyisocyanate compounds, and examples include aliphatic polyisocyanate compounds and alicyclic polyisocyanate compounds. These may be used individually or in combination of two or more. Among these, a bifunctional aliphatic polyisocyanate compound (i.e., an aliphatic diisocyanate compound) having two functional groups derived from isocyanate is preferred from the viewpoint of obtaining a straight-chain, unbranched, bifunctional urethane (meth)acrylate and improving the flexibility and tackiness of the adhesive. There are no particular limitations on specific examples of polyisocyanate compounds, and examples include hexamethylene diisocyanate (HDI), tetramethylene diisocyanate, pentamethylene diisocyanate, heptamethylene diisocyanate, octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate, 2,2,4- or 2,4,4-trimethylhexamethylene diisocyanate, hydrogenated xylylene diisocyanate (H6XDI), 1,4-cyclohexane diisocyanate, 4,4'-dicyclohexylmethane diisocyanate (H12MDI), isophorone diisocyanate (IPDI), 2,5-norbornane diisocyanate, 2,6-norbornane diisocyanate, etc. These may be used individually or in combination of two or more. Among these, hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) are preferred from the viewpoint of improving the transparency of the resulting adhesive.

[0040] <Hydroxygroup-containing (meth)acrylate compounds> There are no particular restrictions on the hydroxygroup-containing (meth)acrylate compounds, but from the viewpoint of obtaining a straight-chain, unbranched, bifunctional urethane (meth)acrylate and improving the flexibility and tackiness of the adhesive, hydroxygroup-containing (meth)acrylate compounds containing one hydroxyl group in the molecule are preferred. There are no particular restrictions on the hydroxyl group-containing (meth)acrylate compounds, and examples include 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 5-hydroxypentyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, caprolactone-modified 2-hydroxyethyl (meth)acrylate, diethylene glycol mono (meth)acrylate, polyethylene glycol mono (meth)acrylate, 2-acryloyloxyethyl-2-hydroxyethyl phthalic acid, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-chloro-2-hydroxypropyl (meth)acrylate, and 2,2-dimethyl-2-hydroxyethyl (meth)acrylate. These may be used individually or in combination of two or more. Among these, 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxypropyl (meth)acrylate are preferred from the viewpoint of good curability of the adhesive composition and bending durability of the adhesive, and 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are more preferred.

[0041] <Reaction Products> Bifunctional urethane (meth)acrylate is a reaction product obtained by urethane-forming a polyether polyol, a polyisocyanate compound, and a hydroxyl group-containing (meth)acrylate compound. The urethane-forming reaction can be carried out by known methods, and is usually carried out by mixing the polyether polyol, the polyisocyanate compound, and the hydroxyl group-containing (meth)acrylate compound, and using a urethane-forming catalyst in an atmosphere of nitrogen gas or an inert gas.

[0042] <<Urethane-forming catalyst>>The urethane-forming catalyst is not particularly limited. For example, organotin compounds such as dibutyltin(IV) dilaurate, dioctyltin(IV) dilaurate, dibutyltin(IV) dioctoate, tin(II) 2-ethylhexanoate; iron compounds such as iron(III) acetylacetonate, iron(III) chloride; lead compounds such as lead(II) 2-ethylhexanoate; bismuth compounds such as bismuth(III) 2-ethylhexanoate; and tertiary amines such as triethylamine, triethylenediamine can be mentioned. These may be used alone or in combination of two or more. Among these, organotin compounds, lead(II) 2-ethylhexanoate, and bismuth(III) 2-ethylhexanoate are preferred.

[0043] The amount of the urethane-forming catalyst used is not particularly limited, but is preferably 0.001 to 1 part by mass, more preferably 0.002 to 0.5 part by mass, and particularly preferably 0.005 to 0.1 part by mass with respect to 100 parts by mass of the polyether polyol as the reactant. The reaction temperature of the urethane-forming reaction is not particularly limited, but is preferably 20 to 100°C, more preferably 30 to 90°C, and particularly preferably 40 to 80°C.

[0044] In addition, from the viewpoints of antioxidant protection and inhibition of polymerization reaction during storage, the bifunctional urethane (meth)acrylate obtained by the urethane-forming reaction may be added with an antioxidant such as hydroquinones such as 2,5-di-tert-butylhydroquinone within a range that does not affect the physical properties and effects of the adhesive composition (for example, an addition amount of 0.2 part by mass or less with respect to 100 parts by mass of the polyether polyol). The antioxidant or polymerization inhibitor is not particularly limited. For example, hindered phenols such as 2,6-di-tert-butyl-p-cresol; hydroquinones such as 2,5-di-tert-butylhydroquinone, p-methoxyphenol; etc. can be mentioned. These may be used alone or in combination of two or more.

[0045] <Physical properties of bifunctional urethane (meth)acrylate> The weight average molecular weight (Mw) of the bifunctional urethane (meth)acrylate is not particularly limited as long as it is 10,000 to 50,000. However, from the viewpoint of obtaining an adhesive with high adhesiveness, it is preferably 12,000 to 48,000, more preferably 14,000 to 46,000, and particularly preferably 15,000 to 45,000.

[0046] The number average molecular weight (Mn) of the bifunctional urethane (meth)acrylate is not particularly limited. However, from the viewpoint of obtaining an adhesive with high adhesiveness, it is preferably 6,000 to 30,000, more preferably 7,000 to 25,000, and particularly preferably 8,000 to 20,000.

[0047] The molecular weight distribution (Mw / Mn) of the bifunctional urethane (meth)acrylate is not particularly limited. However, from the viewpoint of improving flexibility, it is preferably 1.00 to 3.00, more preferably 1.30 to 2.70, and particularly preferably 1.60 to 2.40.

[0048] The urethane bond content contained per 100 of the number average molecular weight of the urethane acrylate calculated by the following formula (1) of the bifunctional urethane (meth)acrylate is not particularly limited. However, from the viewpoint of improving the adhesiveness of the obtained adhesive, it is preferably 0.070 mol or more, more preferably 0.080 to 0.300 mol, and particularly preferably 0.090 to 0.200 mol. Urethane bond content [mol] = {Number average molecular weight of bifunctional urethane (meth)acrylate ÷ (Molecular weight of polyether polyol + Molecular weight of polyisocyanate compound) × 2 + 2} / Number average molecular weight of bifunctional urethane (meth)acrylate × 100... Formula (1)

[0049] The glass transition temperature of the bifunctional urethane (meth)acrylate is not particularly limited. However, from the viewpoints of good adhesiveness and bending durability of the adhesive, it is preferably -80 to 0 °C, more preferably -80 to -20 °C, and particularly preferably -80 to -30 °C.

[0050] There are no particular restrictions on the viscosity (at 25°C) of the bifunctional urethane (meth)acrylate, but from the viewpoint of ease of handling of the adhesive composition and the adhesive, it is preferably 10 to 10,000 Pa·s, more preferably 100 to 1,000 Pa·s, and particularly preferably 300 to 500 Pa·s.

[0051] Furthermore, if two or more types of difunctional urethane (meth)acrylate are included in the adhesive composition, it is preferable that each of the difunctional urethane (meth)acrylates is within the numerical range of the weight-average molecular weight, number-average molecular weight, molecular weight distribution, glass transition temperature, and viscosity described above.

[0052] The difunctional urethane (meth)acrylate, which is the urethane reaction product described above, has a polyoxyalkylene group such as a polyoxypropylene group and two (meth)acryloyloxy groups in one molecule. Because the difunctional urethane (meth)acrylate has such a structure, the cured product of the adhesive composition readily forms a uniform crosslinking network, resulting in an adhesive with low residual strain and excellent bending durability and impact resistance.

[0053] There are no particular restrictions on the content of the difunctional urethane (meth)acrylate relative to 100% by mass of the adhesive composition, but from the viewpoint of good adhesiveness and bending durability of the adhesive, it is preferably 6 to 100% by mass, more preferably 8 to 80% by mass, and particularly preferably 10 to 60% by mass.

[0054] The total content of monofunctional urethane (meth)acrylate and difunctional urethane (meth)acrylate per 100% by mass of the adhesive composition is not particularly limited as long as it is greater than 45% by mass. However, from the viewpoint of obtaining an adhesive with low residual strain, excellent impact resistance, and excellent bending durability, it is preferably 46 to 90% by mass, more preferably 48 to 80% by mass, and particularly preferably 50 to 70% by mass.

[0055] There are no particular restrictions on the content of monofunctional urethane (meth)acrylate, but in order to further reduce residual strain, it is preferably 10 to 1000 parts by mass, more preferably 150 to 350 parts by mass, and particularly preferably 200 to 300 parts by mass per 100 parts by mass of bifunctional urethane (meth)acrylate.

[0056] ((meth)acrylic monomer compounds) There are no particular restrictions on (meth)acrylic monomer compounds, and examples include hydroxyl group-containing (meth)acrylate monomers, alkyl (meth)acrylate monomers, and ethylenically unsaturated monomers. These may be used individually or in combination of two or more. Among these, alkyl (meth)acrylate monomers are preferably used from the viewpoint of good adhesiveness, handling and availability of the adhesive. (Meth)acrylic monomer compounds also include monofunctional (meth)acrylic monomers, difunctional (meth)acrylic monomers, and trifunctional or more (meth)acrylic monomers.

[0057] The molecular weight of the (meth)acrylic monomer compound is preferably 1,000 or less. More preferably, the molecular weight of the (meth)acrylic monomer compound is 50 to 800, even more preferably 100 to 400, and particularly preferably 120 to 300, from the viewpoint of improving the flexibility of the resulting adhesive.

[0058] There are no particular restrictions on the content of the (meth)acrylic monomer compound, but from the viewpoint of improving the flexibility and adhesive strength of the resulting adhesive, it is preferably 1 to 50% by mass, more preferably 10 to 45% by mass, and particularly preferably 20 to 40% by mass, based on 100% by mass of the adhesive composition.

[0059] There are no particular restrictions on the hydroxyl group-containing (meth)acrylate compounds, and examples include 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 5-hydroxypentyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, caprolactone-modified 2-hydroxyethyl (meth)acrylate, diethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, 2-acryloyloxyethyl-2-hydroxyethyl phthalic acid, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-chloro-2-hydroxypropyl (meth)acrylate, and 2,2-dimethyl-2-hydroxyethyl (meth)acrylate. These may be used individually or in combination of two or more. Among these, 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxypropyl (meth)acrylate are preferred from the viewpoint of good curability of the adhesive composition and bending durability of the adhesive, and 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are more preferred.

[0060] There are no particular restrictions on the alkyl (meth)acrylate monomer. Examples include compounds in which an alkyl group having 1 to 14 carbon atoms is bonded to a (meth)acryloyloxy group, such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate, isobutyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, isobornyl (meth)acrylate, hexyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, and lauryl acrylate. These may be used individually or in combination of two or more. Among these, from the viewpoint of obtaining an adhesive with excellent flexibility, it is preferable to include alkyl (meth)acrylate monomers in which the alkyl group has 5 to 14 carbon atoms, such as hexyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, and lauryl acrylate.

[0061] There are no particular restrictions on the ethylenically unsaturated monomers, and examples include carboxyl group-containing monomers, amino group-containing monomers, amide group-containing monomers, acetoacetyl group-containing monomers, glycidyl group-containing monomers, aromatic (meth)acrylate monomers, and other vinyl monomers. These may be used individually or in combination of two or more.

[0062] There are no particular restrictions on the carboxyl group-containing monomers, and examples include (meth)acrylic acid, 2-(meth)acryloyloxyethylhexahydrophthalic acid, 2-(meth)acryloyloxypropylhexahydrophthalic acid, 2-(meth)acryloyloxyethyl phthalic acid, 2-(meth)acryloyloxypropyl phthalic acid, 2-(meth)acryloyloxyethyl maleic acid, 2-(meth)acryloyloxypropyl maleic acid, 2-(meth)acryloyloxyethyl succinic acid, 2-(meth)acryloyloxypropyl succinic acid, crotonic acid, fumaric acid, maleic acid, itaconic acid, etc. These may be used individually or in combination of two or more.

[0063] There are no particular restrictions on the amino group-containing monomers, and examples include aminomethyl (meth)acrylate, aminoethyl (meth)acrylate, tert-butylaminoethyl (meth)acrylate, tert-butylaminopropyl (meth)acrylate, ethylaminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, and diethylaminopropyl (meth)acrylate. These may be used individually or in combination of two or more.

[0064] There are no particular restrictions on the amide group-containing monomers, and examples include (meth)acryloylmorpholine (ACMO), (meth)acrylamide; N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-propyl(meth)acrylamide, N-n-butyl(meth)acrylamide, diacetone(meth)acrylamide, N,N'-methylenebis(meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, N,N-ethylmethylacrylamide, N,N-diallyl(meth)acrylamide, N-hydroxymethyl(meth)acrylamide, N-hydroxyethyl(meth)acrylamide, N-methoxymethyl(meth)acrylamide, N-(n-butoxymethyl)(meth)acrylamide, vinylpyrrolidone, etc. These may be used individually or in combination of two or more.

[0065] There are no particular restrictions on the acetoacetyl group-containing monomer, and examples include 2-(acetoacetoxy)ethyl (meth)acrylate and allyl acetoacetate. These may be used individually or in combination of two or more.

[0066] There are no particular restrictions on the glycidyl group-containing monomer; for example, glycidyl (meth)acrylate and allyl glycidyl (meth)acrylate can be used. These may be used individually or in combination of two or more.

[0067] There are no particular restrictions on the aromatic (meth)acrylate monomers, and examples include phenyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenyldiethylene glycol (meth)acrylate, phenoxypolyethylene glycol (meth)acrylate, phenoxypolyethylene glycol-polypropylene glycol-(meth)acrylate, nonylphenol ethylene oxide adduct (meth)acrylate, etc. These may be used individually or in combination of two or more.

[0068] Other vinyl monomers are not particularly limited and include, for example, (meth)acrylonitrile, styrene, α-methylstyrene, vinyl stearate, vinyl chloride, vinylidene chloride, alkyl vinyl ether, vinyltoluene, vinylpyridine, dialkyl itaconate, dialkyl fumarate, allyl alcohol, acrylic chloride, methyl vinyl ketone, N-acrylamidomethyltrimethylammonium chloride, allyltrimethylammonium chloride, dimethylallyl vinyl ketone, etc. These may be used individually or in combination of two or more.

[0069] (Other Components) Depending on the ease of handling and its application, the adhesive composition may contain other components in addition to monofunctional urethane (meth)acrylate, difunctional urethane (meth)acrylate, and any (meth)acrylic monomer compound. Examples of other components include polymerization initiators; other monomer components other than monofunctional urethane (meth)acrylate and difunctional urethane (meth)acrylate and (meth)acrylic monomer compounds (e.g., trifunctional or more (meth)urethane (meth)acrylate, active energy ray curable monomers); colorants such as pigments and dyes; silane coupling agents; tackifiers; antioxidants; light stabilizers; metal deactivators; rust inhibitors; anti-aging agents; hygroscopic agents; hydrolysis inhibitors; defoamers; fillers; and the like. Organic solvents may also be included.

[0070] Other components in the adhesive composition may be included in amounts that do not impair the effects of the present invention. The content of the organic solvent in the adhesive composition is not particularly limited as long as it does not hinder the effects of the present invention, but from the viewpoint of uniform mixability and coating properties of the adhesive composition, it is preferably 50% by mass or less, more preferably 10 to 50% by mass, and particularly preferably 10 to 40% by mass. If the adhesive composition contains an organic solvent, it is preferable to remove the organic solvent by volatilization during or after curing.

[0071] <Polymerization Initiator> A radical polymerization initiator is preferred as the polymerization initiator. The radical polymerization initiator may be a photopolymerization initiator or a thermal polymerization initiator, and known ones can be used. The polymerization initiator may be added as a separate component from the adhesive composition when manufacturing the adhesive. From the viewpoint of ease of control of the polymerization reaction, a photopolymerization initiator that can be used by irradiation with ultraviolet light at a wavelength of 380 nm or less is preferred, and a thermal polymerization initiator that can be used by heating in the range of 50 to 120°C is preferred.

[0072] There are no particular restrictions on the photopolymerization initiator, and examples include 1-hydroxycyclohexylphenyl ketone, 2-(2-oxo-2-phenylacetoxyethoxy)ethyl oxyphenyl acetate, 2-(2-hydroxyethoxy)ethyl oxyphenyl acetate, 2-hydroxy-2-methylpropiophenone, diethoxyacetophenone, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropanone, 1-(4-dodecylphenyl)-2-hydroxy-2-methylpropanone, 4-(2-hydroxyethoxy)-phenyl(2-hydroxy-2-propyl)ketone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin- Examples include n-butyl ether, benzoin phenyl ether, benzyl dimethyl ketal, benzophenone, benzoyl benzoic acid, methyl benzoyl benzoate, 4-phenylbenzophenone-4-methoxybenzophenone, thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, isopropylthioxanthone, 2,4-dichlorothioxanthone, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, 2,4,6-trimethylbenzoyl diphenylphosphine oxide, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide), methylphenylglyoxylate, benzyl, and camphorquinone. These may be used individually or in combination of two or more. Among these, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide is preferred from the viewpoint of solubility and maximum absorption wavelength.

[0073] There are no particular restrictions on the thermal polymerization initiator, and examples include azo compounds; organic peroxides such as hydroperoxides, dialkyl peroxides, peroxyesters, diacyl peroxides, peroxydicarbonates, peroxyketals, and ketone peroxides. Specifically, azobisisobutyronitrile, benzoyl peroxide, tert-butylperoxy-2-ethylhexanoate, 2,5-dimethyl-2,5-di(2-ethylhexanoyl)peroxyhexane, and tert-butylperoxybenzoate. Examples include tert-butyl peroxide, cumene hydroperoxide, dicumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-dibutylperoxyhexane, 2,4-dichlorobenzoyl peroxide, 1,4-di(2-t-butylperoxyisopropyl)benzene, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, methyl ethyl ketone peroxide, and 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate. These may be used individually or in combination of two or more.

[0074] It is preferable that each component in the adhesive composition be uniformly mixed, and the mixing order is not limited. Heat treatment may be performed after mixing each component. Each component may be mixed in advance, or it may be mixed immediately before curing. For example, a polymerization initiator may be added immediately before curing to a premixture in which all components except the polymerization initiator have been mixed in advance.

[0075] There are no particular restrictions on the content of the polymerization initiator in the adhesive composition, but from the viewpoint of an appropriate polymerization rate, it is preferably 0.001 to 20% by mass, more preferably 0.01 to 10% by mass, and particularly preferably 0.1 to 7% by mass, based on 100% by mass of the adhesive composition.

[0076] <Tackifier> There are no particular restrictions on the tackifier, and examples include rosin ester compounds, terpene resins, alicyclic saturated hydrocarbon resins, styrene resins, acrylic resins, etc. These may be used individually or in combination of two or more. Among these, rosin ester compounds (ultra-pale rosin derivative Pine Crystal KE-311, manufactured by Arakawa Chemical Industries, Ltd.) are preferred from the viewpoint of transparency of the adhesive.

[0077] There are no particular restrictions on the content of the tackifier in the adhesive composition, but from the viewpoint of improving adhesive strength, it is preferably 1 to 30% by mass, more preferably 4 to 20% by mass, and particularly preferably 7 to 15% by mass, based on 100% by mass of the adhesive composition.

[0078] [Adhesive] The adhesive of this embodiment is a cured product of the adhesive composition of this embodiment described above, and can exhibit excellent adhesive strength and flexibility. The adhesive of this embodiment can be suitably used for display applications, but can also be used for applications other than displays. There are no particular limitations on applications other than display applications for the adhesive, and examples include flexible printed circuit board (FPC) applications, coating or sealing material applications for printed circuit boards, backgrind tape applications, bonding sheet applications, coverlay applications, interlayer insulating film applications, 3D printer resin applications, shock-absorbing sheet applications, etc. The adhesive of this embodiment has low residual strain when dynamic shear strain is applied, excellent adhesiveness, and excellent optical properties (transparency). Furthermore, the adhesive of this embodiment has high shock absorption capacity and excellent impact resistance.

[0079] There are no particular restrictions on the glass transition temperature of the adhesive in this embodiment, but from the viewpoint of having excellent bending durability, it is preferably -70 to 0°C, more preferably -65 to -20°C, and most preferably -60 to -30°C.

[0080] The adhesive composition can be cured using known methods such as irradiation with active energy rays (e.g., ultraviolet light or electron beams) or heating.

[0081] When obtaining an adhesive by irradiating an adhesive composition with light, the light source can be appropriately set according to the light absorption capacity of the photopolymerization initiator and the active energy ray curable monomer used. For example, ultraviolet light-emitting diodes (LEDs), low-pressure mercury lamps, high-pressure mercury lamps, mercury xenon lamps, metal halide lamps, tungsten lamps, arc lamps, excimer lamps, excimer lasers, semiconductor lasers, YAG lasers, laser systems combining lasers and nonlinear optical crystals, and high-frequency induced ultraviolet generators can be used as light sources. There are no particular restrictions on the integrated light quantity, but it is preferably 0.01 to 50 J / cm². ２ It is approximately [amount]. There are no particular restrictions on the cumulative light intensity in the case of ultraviolet irradiation, but it is preferably 0.01 to 10 J / cm². ２ More preferably 0.1 to 5 mJ / cm² ２ Furthermore, from the viewpoint of further stabilizing the physical properties of the adhesive, a heat treatment may be applied after light irradiation. There are no particular restrictions on the heating temperature, but it is preferably around 40 to 200°C. There are no particular restrictions on the heating time, but it is preferably around 1 minute to 15 hours. Alternatively, the physical properties of the adhesive can also be stabilized by letting it stand at room temperature (around 15 to 25°C) for 1 to 48 hours.

[0082] When obtaining an adhesive by heating an adhesive composition using a thermal polymerization initiator, there are no particular restrictions on the heating temperature, but it is preferably around 40 to 250°C, and there are no particular restrictions on the heating time, but it is preferably around 5 minutes to 24 hours. It is preferable to shorten the heating time when the heating temperature is high, and to lengthen the heating time when the heating temperature is low.

[0083] [Adhesive Sheet] The adhesive sheet of this embodiment contains the adhesive of this embodiment described above in at least a portion of the base sheet. The adhesive layer and adhesive sheet of this embodiment are flexible and exhibit excellent bending durability, showing no change in appearance even after being repeatedly folded at a bending radius of 1.5 mm at least 200,000 times.

[0084] Adhesive sheets can be manufactured, for example, by the following methods. First, an adhesive composition (diluted with an organic solvent as needed) is applied to one or both sides of a base sheet and then dried. Alternatively, the adhesive composition may be melted by heating and extruded onto the base sheet using a T-die or the like. If necessary, a release sheet may be bonded to the adhesive composition on the base sheet. Next, after forming an adhesive layer on the base sheet, the adhesive composition on the base sheet is cured using the curing method described above to obtain an adhesive sheet having an adhesive layer.

[0085] The adhesive sheet can also be a double-sided adhesive sheet without a base sheet (base sheet-less), where an adhesive layer is formed on the side of the release sheet, and another release sheet is attached to the opposite side of the adhesive layer. When using the adhesive sheet, the release sheet is peeled off from the adhesive layer and attached to the adherend.

[0086] There are no particular restrictions on the base sheet, and examples include synthetic resin sheets (e.g., polyester such as polyethylene terephthalate (PET), polyimide, etc.); metal foils such as aluminum, copper, and iron; paper such as fine paper and glassine paper; woven fabrics containing glass fibers, natural fibers, synthetic fibers, etc.; nonwoven fabrics; and so on. The base sheet may be a single layer or a multi-layered body containing two or more of the same or different types.

[0087] There are no particular restrictions on the release sheet; for example, it can be a synthetic resin sheet, paper, cloth, nonwoven fabric, etc., similar in material to the base sheet mentioned above, that has been treated with a release agent (e.g., silicone treatment). These may be used individually or in combination of two or more types.

[0088] There are no particular restrictions on the application method of the adhesive composition, and examples include roll coating, die coating, gravure coating, comma coating, and screen printing. These may be used individually or in combination of two or more types.

[0089] There are no particular restrictions on the thickness of the adhesive layer, but from the viewpoint of good adhesion and impact resistance, and from the viewpoint of influence on the dimensions of the adherend, it is preferably 10 to 1000 μm, more preferably 15 to 250 μm, and most preferably 20 to 100 μm.

[0090] There are no particular restrictions on the adhesive content in the adhesive layer, but it is preferably 70 to 100% by mass, more preferably 80 to 100% by mass, particularly preferably 90 to 100% by mass, and most preferably 100% by mass, relative to 100% by mass of the adhesive layer.

[0091] [Laminate] There are no particular limitations on the laminate using the adhesive of this embodiment, and examples include laminates for flexible display applications and laminates for non-flexible display applications. A laminate for flexible display applications has a flexible member and an adhesive layer containing the adhesive of this embodiment described above, which is present on the flexible member. On the other hand, a laminate for non-flexible display applications has a pair of surface materials and an adhesive layer containing the adhesive of this embodiment described above, which is formed between the pair of surface materials. Here, examples of the pair of surface materials include a combination in which one is a transparent surface material such as a glass plate and the other is a display device.

[0092] <Laminate for Flexible Display Applications> The laminate for flexible display applications comprises a flexible member and an adhesive layer present on the flexible member, which includes the adhesive of this embodiment described above. The flexible member is attached to a substrate by the adhesive layer, forming the laminate. The adhesive of this embodiment has low residual strain, excellent impact resistance, and excellent bending durability, making it suitable for use in the production of laminates with flexible members. The laminate of this embodiment is also flexible.

[0093] There are no particular limitations on the flexible members, and examples include members that constitute a flexible display panel, specifically, surface protection panels, optical films, touch panels, and display panel bodies. The adhesive of this embodiment is also suitable as an optically clear adhesive (OCA) or the like applied to such flexible members.

[0094] There are no particular restrictions on the surface protection panel; for example, thin cover glass or cover film can be used. These may be used individually or in combination of two or more types. The optical film has optical functions and is not particularly limited; for example, polarizing film, phase difference film, optical filter, anti-reflective film, near-infrared cut film, or electromagnetic wave shielding film can be used individually or in combination of two or more types. The touch panel has a configuration in which a touch sensor is mounted on a thin glass or plastic substrate. There are no particular restrictions on the display panel body; for example, an organic EL display panel can be used. These may be used individually or in combination of two or more types.

[0095] [Flexible Display] The flexible display of this embodiment comprises the laminate for flexible display applications described above. As described above, the laminate for flexible display applications is flexible and suitable for flexible displays, and in particular suitable for foldable displays.

[0096] [Method for Manufacturing Laminates for Display Devices] The method for manufacturing a laminate for display devices according to this embodiment includes the steps of sandwiching an uncured layer containing the adhesive composition of this embodiment between a pair of surface materials and curing the uncured layer. These steps can be carried out using known methods as appropriate. There are no particular restrictions on the surface materials, but it is preferable that at least one of the pair of surface materials is a transparent surface material, because when curing the uncured layer, it can be cured by irradiating it with light through the transparent surface material.

[0097] When it is desirable that no air bubbles remain in the cured adhesive composition, such as when laminating a display device and a transparent surface material such as a protective plate, it is preferable to use a method in which the adhesive composition is sealed between a pair of surface materials under reduced pressure and cured in an atmosphere of higher pressure (reduced pressure sealing-pressure curing method). Specifically, it is preferable to manufacture a laminate by a method including a first step of forming a laminate precursor in which an uncured layer containing the adhesive composition is sandwiched between a pair of surface materials under a first reduced pressure atmosphere and the uncured layer is sealed by a seal portion provided around the uncured layer, and a second step of curing the uncured layer under a second atmosphere of higher pressure than the first reduced pressure atmosphere. Such reduced pressure sealing-pressure curing methods are known, and for example, the methods described in International Publication No. 2009 / 016943 and International Publication No. 2011 / 158840 can be used.

[0098] For example, the process can be suitably carried out under conditions where the pressure in the first reduced-pressure atmosphere is 100 Pa or less, and the second atmosphere is an atmospheric pressure atmosphere. There are no particular restrictions on the thickness of the cured layer (adhesive layer between the pair of surface materials), but it is preferably 0.03 to 2 mm, more preferably 0.1 to 0.8 mm. The seal portion may be formed using a double-sided adhesive type sealant, as described in International Publication No. 2009 / 016943, or the seal portion may be formed by applying a photocurable resin on a light-transmitting double-sided adhesive type sealant. The photocurable resin of the seal portion can be cured simultaneously with the curing of the uncured layer containing the adhesive composition. As described in International Publication No. 2011 / 158840, the seal portion may be formed using a photocurable resin composition for forming seal portions that has a higher viscosity than the adhesive composition (for example, 500 to 3000 Pa·s at 25°C). The sealing portion may be cured simultaneously with the curing of the uncured layer containing the adhesive composition, or it may be partially cured before curing the uncured layer containing the adhesive composition, and then further cured simultaneously with the curing of the uncured layer.

[0099] In the manufacturing method of a laminate for display device applications of this embodiment, one of a pair of surface materials is a transparent surface material, and the other is a display device. The transparent surface material is a surface material (also called a transparent surface material) that has light transmittance. There are no particular restrictions on the transparent surface material, and examples include glass plates and transparent resins. Among these, glass is preferred from the viewpoint of weather resistance, low birefringence, and high flatness accuracy. There are no particular restrictions on the display device, and examples include liquid crystal display devices, EL display devices, plasma display devices, and electronic ink type display devices. The display device has a structure in which a pair of surface materials, at least one of which is a transparent surface material, are bonded together, and the transparent surface material side is arranged to be in contact with the adhesive layer. In this case, in some display devices, an optical film such as a polarizing plate or a phase difference plate may be installed on the outermost layer side of the transparent surface material that is in contact with the adhesive layer. In this case, the adhesive layer bonds the optical film on the display device to the surface material.

[0100] [Display Device] In this embodiment, an adhesive layer containing a cured product of the adhesive composition of this embodiment is sandwiched between a transparent surface material and a display device. Examples of display devices include flexible smartphones and flat smartphones. When a display device is manufactured using the adhesive composition of this embodiment and the above-described vacuum sealing-pressure curing method, even relatively large-area display devices can be manufactured without generating air bubbles in the adhesive layer. Even if air bubbles remain in the uncured layer sealed under reduced pressure, the volume of the air bubbles decreases and they easily disappear due to the pressure applied to the uncured layer under the high-pressure atmosphere before curing. The adhesive composition satisfies the requirement of low viscosity (for example, 0.05 to 50 Pa·s at 25°C) suitable for the vacuum sealing-pressure curing method, while simultaneously achieving a low curing shrinkage rate and a low elastic modulus of the adhesive (cured product) after curing. Therefore, a laminate, preferably a display device, can be obtained in which stress generation due to the curing of the adhesive layer is well suppressed and air bubbles in the adhesive layer have well disappeared.

[0101] The present invention will be specifically described below based on examples. The present invention is not limited to the following examples, and various modifications are possible without departing from the spirit of the invention.

[0102] [Synthesis of Monofunctional and Difunctional Urethane Acrylates] Various monofunctional and difunctional urethane acrylates for use in the manufacture of adhesive compositions were synthesized. Details of the various raw materials used are as follows: ・AOI: 2-Isocyanate ethyl acrylate; "Karenz AOI", manufactured by Resonac Co., Ltd. ・HDI: Hexamethylene diisocyanate ・IPDI: Isophorone diisocyanate ・HEA: 2-Hydroxyethyl acrylate ・ACMO: Acryloyl morpholine, KJ Chemicals Co., Ltd. ・LA: Lauryl acrylate, manufactured by Arkema, trade name: SR-335 ・Omnirad 819: Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, IGM Resins B. V. Rosin ester compound manufactured by Arakawa Chemical Industries, Ltd.: Ultra-pale rosin derivative Pine Crystal KE-311, manufactured by Arakawa Chemical Industries, Ltd.

[0103] (Production Example 1) Synthesis of Polyether Monool (1) 0.2 g of zinc hexacyanocobaltate-tert-butyl alcohol complex (hereinafter referred to as "DMC-TBA"), a complex metal cyanide catalyst, and 59 g of n-butanol, an initiator, were charged into a pressure reactor equipped with a stirrer and a nitrogen inlet tube. Under a nitrogen atmosphere at 130°C, 3941 g of propylene oxide (hereinafter also referred to as "PO") was added at a constant rate over 7 hours. After confirming that the decrease in the internal pressure of the pressure reactor had stopped, 4000 g of the product was withdrawn. The main component of the product, excluding by-products and metals derived from the catalyst, was polyoxypropylene monool (polyether monool (1)) with a hydroxyl value of 11.2 mg KOH / g (molecular weight calculated from the hydroxyl value: 5,000) and an average number of hydroxyl groups of 1.03. Furthermore, the value obtained by measurement in accordance with JIS K 1557:2007 was defined as the "hydroxyl value," and the value calculated by applying the obtained hydroxyl value to the formula "{56100 / (hydroxyl value)} × {number of hydroxyl groups in the initiator)" was defined as the "molecular weight converted from the hydroxyl value."

[0104] (Production Example 2) Synthesis of Polyether Monool (2) Except for changing the amount of PO added, the product was manufactured in the same manner as in Production Example 1 to obtain polyoxypropylene monool (polyether monool (2)) with a hydroxyl value of 5.60 mg KOH / g (molecular weight calculated from the hydroxyl value: 10,000) and an average number of hydroxyl groups of 1.03.

[0105] (Production Example 3) Synthesis of Polyether Monool (3) Except for changing the amount of PO added, the product was manufactured in the same manner as in Production Example 1 to obtain polyoxypropylene monool (monool (3)) with a hydroxyl value of 3.74 mg KOH / g (molecular weight calculated from hydroxyl value: 15,000) and an average number of hydroxyl groups of 1.03.

[0106] (Production Example 4) Synthesis of Polyether Polyol (1) In a pressure reactor equipped with a stirrer and a nitrogen inlet tube, 12 g of potassium hydroxide, which is the catalyst, and 680 g of exenol-420P (manufactured by AGC Inc., polyoxypropylene glycol, (molecular weight calculated from hydroxyl value: 400), which is the initiator) were charged, and PO (4890 g) was reacted for 5 hours under a nitrogen atmosphere at 110°C. After that, a neutralizing agent was added, and dehydration treatment was carried out at 90°C for 2 hours. The product was withdrawn from the pressure reactor to obtain a polyoxyalkylene polyol (polyether polyol (1)) with a hydroxyl value of 56.5 mg KOH / g (molecular weight calculated from hydroxyl value: 2,000).

[0107] (Production Example 5) Except for using exenol-1020 (manufactured by AGC Inc., polyoxypropylene glycol, (molecular weight calculated from hydroxyl value: 1000)) as the synthesis initiator for polyether polyol (2) and changing the amount of PO added, the product was manufactured in the same manner as in Production Example 4 to obtain a polyoxypropylene polyol (polyether polyol (2)) with a hydroxyl value of 28.3 mg KOH / g (molecular weight calculated from hydroxyl value: 4,000).

[0108] (Production Example 6) Synthesis of Polyether Polyol (3) 0.2 g of zinc hexacyanocobaltate-tert-butyl alcohol complex, which is the catalyst, and 400 g of exenol-1020 (manufactured by AGC Inc., polyoxypropylene glycol, molecular weight calculated from hydroxyl value: 1000), which is the initiator, were charged into a pressure reactor equipped with a stirrer and a nitrogen inlet tube. 7200 g of PO was added at a constant rate over 7 hours in a nitrogen atmosphere at 130°C. After confirming that the internal pressure of the pressure reactor had stopped decreasing, the product was withdrawn to obtain a polyoxyalkylene polyol (Polyol (3)) with a hydroxyl value of 6.4 mg KOH / g (molecular weight calculated from hydroxyl value: 18,000).

[0109] (Synthesis Examples 1-3) Synthesis of Monofunctional Urethane Acrylates (A-1) to (A-3) In a flask equipped with an internal thermometer, stirrer, and condenser, 97.1 parts by mass of polyoxypropylene monool (polyether monool (1)) prepared in Production Example 1 and 2.9 parts by mass of 2-acryloyloxyethyl isocyanate (manufactured by Resonaq, catalog number Karenz AOI), which is a (meth)acrylate having isocyanate groups (1.0 equivalent relative to the hydroxyl groups in polyether monool (1)) were reacted in the presence of 0.008 parts by mass of urethane reaction catalyst (dioctyl tin (IV) dilaurate) until the remaining isocyanate groups (NCO content) became 0% by mass (reaction temperature 60-70°C) to obtain monofunctional urethane acrylate (A-1). Furthermore, GPC measurements revealed that the number-average molecular weight (Mn) was 8,800, the weight-average molecular weight (Mw) was 11,000, and the molecular weight distribution (Mw / Mn) was 1.25. Monofunctional urethane acrylates (A-2) to (A-3) were synthesized similarly using the component ratios shown in Table 1.

[0110] (Synthesis Examples 4-7) Synthesis of bifunctional urethane acrylates (B-1) to (B-4) 10.1 parts by mass of HDI as a polyvalent isocyanate component and 85.3 parts by mass of polyoxypropylene polyol (polyether polyol (1)) produced in Production Example 4 as a polyol component were added to a flask equipped with an internal thermometer, stirrer, and condenser. After heating to 80°C, 0.013 parts by mass of tin catalyst was added as a reaction catalyst and the reaction was carried out. Three hours after catalyst addition, 4.6 parts by mass of 2-hydroxyethyl acrylate as a hydroxyl group-containing (meth)acrylate compound, 0.026 parts by mass of tin catalyst as a reaction catalyst, and 0.05 parts by mass of 2,6-di-tert-butyl cresol as a polymerization inhibitor were added and reacted at 80°C. The reaction was then terminated when the residual isocyanate group (NCO content) became 0.1% by mass or less, yielding a composition containing urethane (meth)acrylate (B-1) (number average molecular weight (Mn): 8,600, weight average molecular weight (Mw): 15,000, molecular weight distribution (Mw / Mn): 1.74, urethane bond content: 0.115 mol). Urethane (meth)acrylates (B-2) to (B-4) were similarly synthesized in the component ratios shown in Table 2.

[0111] (Synthesis Examples 8-11) Synthesis of bifunctional urethane acrylates (B-5) to (B-8) The bifunctional urethane acrylates (B-5) to (B-8) were synthesized in the same manner as in Synthesis Example 4, except that IPDI was used instead of HDI as the polyvalent isocyanate component, with the component ratios shown in Table 2.

[0112] (Synthesis Example 12) The bifunctional urethane acrylate (B-9) was synthesized using the same method as in Synthesis Example 8, except that the polyoxypropylene polyol (polyether polyol (2)) produced in Production Example 5 was used as the polyol component of the bifunctional urethane acrylate (B-9), with the component ratios shown in Table 2.

[0113] (Synthesis Example 13) Except for using the polyoxypropylene polyol (polyether polyol (3)) produced in Production Example 6 as the polyol component of the synthesized bifunctional urethane acrylate (B-10), the bifunctional urethane acrylate (B-10) was synthesized in the same manner as in Synthesis Example 8, with the component ratios shown in Table 2.

[0114] (Synthesis Example 14) Difunctional urethane acrylate (B-11) was synthesized using the same method as in Synthesis Example 8, except that Kuraray Polyol P-2010 (manufactured by Kuraray Co., Ltd., a polyester polyol with a molecular weight of 2,000) was used as the polyol component of the difunctional urethane acrylate (B-11), with the component ratios shown in Table 2.

[0115] (Synthesis Example 15) Difunctional urethane acrylate (B-12) was synthesized using the same method as in Synthesis Example 8, except that Kuraray Polyol C-2050 (manufactured by Kuraray Co., Ltd., a polycarbonate polyol with a molecular weight of 2,000) was used as the polyol component, with the component ratios shown in Table 2.

[0116] (Weight-average molecular weight (Mw), number-average molecular weight (Mn)) The monofunctional urethane acrylates (A-1) to (A-3) and bifunctional urethane acrylates (B-1) to (B-12) obtained in Synthesis Examples 1 to 15 were measured by gel permeation chromatography (GPC) under the following measurement conditions. Tables 1 and 2 show the results with two significant figures. <Measurement Conditions> ・Equipment used: "HLC-8320GPC", manufactured by Tosoh Techno System Co., Ltd. ・Columns used: The following two types of columns connected in series: "TSKgel (registered trademark) GMHXL", manufactured by Tosoh Corporation, 3 columns "TSKgel (registered trademark) G2000HXL", manufactured by Tosoh Corporation, 1 column ・Column temperature: 40℃ ・Detector: Differential refractive index (RI) detector ・Eluent: Tetrahydrofuran ・Flow rate: 0.8 mL / min ・Sample concentration: 0.5% by mass ・Sample injection volume: 100 μL ・Standard sample: Polystyrene

[0117] <Method for Calculating Urethane Group Concentration> The urethane bond content listed in Table 2 represents the number of urethane bonds contained per 100 units of the number-average molecular weight of the urethane acrylate, and was calculated using the following formula (1): Urethane bond content [mol] = {Number-average molecular weight of the bifunctional urethane acrylate ÷ (Molecular weight of polyether polyol + Molecular weight of polyisocyanate compound) × 2 + 2} / Number-average molecular weight of the bifunctional urethane acrylate × 100 ... Formula (1)

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[0119]

[0120] [Preparation of Adhesive Compositions] As shown in Tables 3 to 8, monofunctional urethane acrylate, bifunctional urethane acrylate, (meth)acrylic monomer compound, photopolymerization initiator, and tackifier were mixed to prepare the adhesive compositions of Examples 1 to 51.

[0121] [Evaluation of the properties of the adhesive compositions] Tables 3 to 8 show the evaluation results for the adhesive compositions of Examples 1 to 51, including tackiness (adhesion strength), optical properties (transparency), residual strain, bending durability (repeated bending test), and impact resistance. Examples 9 to 51 are examples, and Examples 1 to 8 are comparative examples. Details of each evaluation item are as follows.

[0122] <Adhesion (Adhesion Strength)> An adhesive composition was applied to a PET sheet (thickness 38 μm) using a doctor blade and cured by ultraviolet irradiation to form an adhesive layer approximately 70 μm thick. The adhesive layer was attached to a PET sheet (thickness 30 μm) to create a laminate test specimen (width 25 mm, length 100 mm, thickness 138 nm). An adhesion test was performed on the test specimen under the following test conditions. <<Test Conditions>> ・Equipment used: RTG-1310 ・Tensile speed: 300 mm / min ・Tensile angle: 180 degrees <<Evaluation Criteria>> A: Adhesion strength is 4 N / 25 mm or more. B: Adhesion strength is 3 N / 25 mm or more and less than 4 N / 25 mm. C: Adhesion strength is less than 3 N / 25 mm. An evaluation of A indicates that the adhesiveness of the adhesive layer is good.

[0123] <Optical Properties (Transparency)> An adhesive composition was applied to a PET sheet (38 μm thick) using a doctor blade and cured by ultraviolet irradiation to form an adhesive layer approximately 70 μm thick. The adhesive layer was attached to a release PET sheet (30 μm thick) to create a laminate test specimen (50 mm wide, 50 mm long, 138 nm thick). The test specimen was subjected to an optical properties (transparency) test based on JIS K7136:2000 under the following conditions. <<Test Conditions>> ・Equipment used: Tensilon Universal Material Tester RTG-1310 <<Evaluation Criteria>> A: Total light transmittance is 90% or more. B: Total light transmittance is 85% or more and less than 90%. C: Total light transmittance is less than 85%. If the evaluation is A or B, it can be said that the optical properties (transparency) of the adhesive layer are good. A rating of A indicates that the optical properties (transparency) of the adhesive layer are particularly good.

[0124] <Residual Strain> The adhesive composition was sandwiched in a 0.2 mm wide gap between a soda-lime glass stage and a measuring spindle ("Disposable Plate D-PP20 / AL / S07", manufactured by Anton Paar). Under a nitrogen gas atmosphere at 35°C, a mercury xenon lamp ("SpotCure® SP-9", manufactured by Ushio Inc.; illuminance 100 mW / cm) was installed below the stage. ２ The adhesive sample was cured by irradiating it with ultraviolet light for 5 minutes using a rheometer (Physica MCR301, Anton Paar). The spindle position was automatically adjusted to prevent stress from being generated in the direction normal to the spindle during the curing of the adhesive composition. While irradiating with ultraviolet light, a 2% dynamic shear strain was applied for 30 minutes using a rheometer (Physica MCR301, Anton Paar). After removing the strain, the residual strain was measured 30 minutes later. The residual strain was evaluated using the following evaluation criteria, with the strain before the application of dynamic shear strain being set to 0% (reference). <<Evaluation Criteria>> A: 0.1% or less B: Greater than 0.1% and 2.0% or less C: Greater than 2.0% If the evaluation is A or B, it can be said that the adhesive has excellent shape recovery even after the external force on the adhesive is removed. Furthermore, if the evaluation is A, it is particularly suitable as an adhesive for flexible displays.

[0125] <Bending Durability (Repeated Bending Test)> An adhesive composition was applied to a PET release sheet using a doctor blade and cured by ultraviolet irradiation to form an adhesive layer approximately 25 μm thick. The adhesive layer was peeled off the PET release sheet, and a polyimide film (50 μm thick) was attached to one side, and a corona-treated PET sheet (50 μm thick) was attached to the other side to prepare a laminate test specimen (50 mm wide, 100 mm long, 125 nm thick). Repeated bending tests were performed on the test specimen under the following test conditions. <<Test Conditions>> ・Equipment used: Planar material unloaded U-shaped stretch tester "DLDM111LH", manufactured by Yuasa System Equipment Co., Ltd. ・Test temperature: 25℃ ・Inner surface of bending: Polyimide film side ・Bending radius: 1.5 mm ・Bending speed: 60 times / min The bending durability was evaluated according to the following evaluation criteria, along with the evaluation of adhesiveness, by visual observation of the appearance of the test specimen during repeated bending tests (whitening and cracking of the adhesive layer, peeling and lifting of the laminate, etc.). <<Evaluation Criteria>> A: More than 200,000 bending cycles without change in appearance B: More than 100,000 bending cycles without change in appearance and less than 200,000 C: Less than 100,000 bending cycles without change in appearance If the evaluation is A or B, it can be said that the adhesiveness and bending durability of the adhesive layer are good. In addition, if the evaluation is A, it is particularly suitable as an adhesive for flexible displays.

[0126] <Impact Resistance> An adhesive composition was applied to a PET release sheet using a doctor blade and cured by ultraviolet irradiation to form an adhesive layer approximately 25 μm thick. The adhesive layer was peeled off the PET release sheet, and polyimide film (50 μm thick) was attached to both sides of the adhesive layer to create a laminate (adhesive layer thickness 25 μm). The laminate was placed on pressure-sensitive paper set on a steel plate, and an impact test was conducted by dropping a stainless steel ball (5 g) from a height of 5 cm above the laminate.

[0127] The impact marks of the spheres recorded on the pressure-sensitive paper after the impact test were visually inspected, and the impact resistance was evaluated according to the following evaluation criteria. <<Evaluation Criteria>> A: No impact marks B: One impact mark C: Two or more impact marks An A or B rating indicates good impact resistance. Furthermore, an A rating indicates that it is particularly suitable as an adhesive for flat smartphones.

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[0134] Tables 3 to 8 show that, according to the adhesive compositions of Examples 9 to 51, in which the weight-average molecular weight of the monofunctional urethane (meth)acrylate, which is the reaction product of a polyether monool and a monoisocyanate having a (meth)acryloyloxy group, is 10,000 to 40,000, and the weight-average molecular weight of the difunctional urethane (meth)acrylate, which is the reaction product of a polyether polyol with a molecular weight calculated from its hydroxyl value of 3,500 or less, a polyisocyanate compound, and a hydroxyl group-containing (meth)acrylate compound, is 10,000 to 50,000, and the total content of the monofunctional urethane (meth)acrylate and the difunctional urethane (meth)acrylate per 100% by mass of the adhesive composition is more than 45% by mass, an adhesive can be obtained with low residual strain, excellent tackiness, and excellent optical properties (transparency).

[0135] The adhesive composition of this embodiment can be used for flexible display applications, flat smartphone applications, and the like.

Claims

1. An adhesive composition comprising a monofunctional urethane (meth)acrylate and a difunctional urethane (meth)acrylate, wherein the monofunctional urethane (meth)acrylate is a reaction product of a polyether monool and a monoisocyanate having a (meth)acryloyloxy group, the monofunctional urethane (meth)acrylate has a weight-average molecular weight of 10,000 to 40,000, the difunctional urethane (meth)acrylate is a reaction product of a polyether polyol, a polyisocyanate compound and a hydroxyl group-containing (meth)acrylate compound, the difunctional urethane (meth)acrylate has a weight-average molecular weight of 10,000 to 50,000, the molecular weight calculated from the hydroxyl value of the polyether polyol is 3,500 or less, and the total content of the monofunctional urethane (meth)acrylate and the difunctional urethane (meth)acrylate relative to 100% by mass of the adhesive composition is greater than 45% by mass.

2. The adhesive composition according to claim 1, further comprising a (meth)acrylic monomer compound having a molecular weight of 1,000 or less, wherein the content of the (meth)acrylic monomer compound is 1 to 50% by mass with respect to 100% by mass of the adhesive composition.

3. The adhesive composition according to claim 1, further comprising a tackifier, wherein the content of the tackifier is 1 to 30% by mass based on 100% by mass of the adhesive composition.

4. The adhesive composition according to claim 1, wherein the urethane bond content per 100 units of the number average molecular weight of the urethane acrylate, calculated by the following formula (1) for the difunctional urethane (meth)acrylate, is 0.070 mol or more. Urethane bond content [mol] = {Number average molecular weight of the difunctional urethane (meth)acrylate ÷ (Molecular weight of the polyether polyol + Molecular weight of the polyisocyanate compound) × 2 + 2} / Number average molecular weight of the difunctional urethane (meth)acrylate × 100 ... Formula (1) 5. The adhesive composition according to claim 1, wherein the content of the monofunctional urethane (meth)acrylate is 10 to 1000 parts by mass per 100 parts by mass of the difunctional urethane (meth)acrylate.

6. An adhesive which is a cured product of the adhesive composition according to any one of claims 1 to 5.

7. An adhesive sheet comprising a base sheet containing the adhesive described in claim 6 in at least a portion thereof.

8. A laminate comprising a flexible member and an adhesive layer present on the flexible member, the adhesive layer containing the adhesive described in claim 6.

9. The laminate according to claim 8, wherein the flexible member is at least one selected from the group consisting of a surface protection panel, an optical film, a touch panel, and a display panel body.

10. A flexible display comprising the laminate described in claim 8.

11. A display device in which an adhesive layer containing a cured product of the adhesive composition described in any one of claims 1 to 5 is sandwiched between a transparent surface material and a display device.

12. A method for manufacturing a laminate, comprising sandwiching an uncured layer containing the adhesive composition described in any one of claims 1 to 5 between a pair of facing materials and curing the uncured layer.

13. A method for manufacturing a laminate, comprising: sandwiching an uncured layer containing the adhesive composition described in any one of claims 1 to 5 between a pair of facing materials in a first reduced-pressure atmosphere, forming a laminate precursor in which the uncured layer is sealed by a sealing portion provided around the uncured layer, and curing the uncured layer in a second atmosphere having a pressure higher than the first reduced-pressure atmosphere.

14. The method for manufacturing a laminate according to claim 12, wherein one of the pair of facing materials is a transparent facing material and the other of the pair of facing materials is a display device.

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