Adhesive sheet, flexible image display member, and flexible image display device

Abstract

Provided is an adhesive sheet in which interlayer stress can be reduced when the sheet is folded, especially in a low temperature state, which exhibits excellent durability for suppressing cracks in a member sheet or flexible member, and which exhibits excellent recoverability such that the sheet rapidly recovers to a flat state after being folded.　This adhesive sheet is formed from an adhesive composition containing a (meth)acrylic acid ester copolymer and a crosslinking agent. The (meth)acrylic acid ester copolymer contains, as constituent components of the copolymer, (A) a branched chain or straight chain alkyl (meth)acrylate ester monomer having 1-20 carbon atoms, (B) a monomer which has an alkylene glycol group in the molecule and also has a (meth)acryloyl group, and (C) a nitrogen-containing vinyl monomer. The adhesive sheet has a storage shear modulus at -30ºC (G'(-30ºC)), as determined by means of dynamic viscoelasticity measurements in shearing mode at a frequency of 1 Hz, of 250 kPa or less, and has a strain recovery rate (400%, 1 minute) of 70% or more.

Description

Adhesive sheet, flexible image display member, and flexible image display device

[0001] The present invention relates to an adhesive sheet that can be suitably used for curved image displays, bendable flexible image displays, etc., and to a flexible image display member and flexible image display device that use the adhesive sheet.

[0002] In recent years, flexible image display devices using organic light-emitting diodes (OLEDs) and quantum dots (QDs) have been developed and are becoming widely commercialized. Flexible image display devices include bendable devices with a curved image display surface, foldable devices that can be repeatedly bent, rollable devices that can be rolled up, and stretchable devices that can be stretched and contracted. Such image display devices have a laminated structure in which multiple component sheets, such as a cover lens, a circular polarizing plate, a touch film sensor, and a light-emitting element, are bonded together with a transparent adhesive sheet, and each of these laminated structures can be considered as a laminated sheet in which a component sheet and an adhesive sheet are stacked together.

[0003] Bendable flexible display devices have various issues due to interlayer stress when folded. For example, there is a need for a laminate sheet that quickly restores to a flat state when the screen is unfolded without any residual effects from being in a bent state. Furthermore, repeated folding operations can cause stress to be applied to the component sheet that is the adherend of the pressure-sensitive adhesive sheet, leading to cracks and eventually breakage. Therefore, there is a need for a laminate sheet that is durable against repeated folding operations, particularly under harsh conditions such as low temperatures.

[0004] Regarding the pressure-sensitive adhesive sheet for such a foldable flexible display device, for example, Patent Document 1 discloses that the storage modulus of a pressure-sensitive adhesive composition containing an acrylic polymer and a crosslinking agent after curing is 60×10 at a temperature of −20° C., a temperature of 25° C., and a temperature of 200° C. 4 ~95 x 10 4 Pa, 8 x 10 4 ~11 x 10 4 Pa, 2 x 10 4 ~5 x 10 4A pressure-sensitive adhesive composition for a foldable display is disclosed, which is characterized by satisfying the Pa range.

[0005] Furthermore, Patent Document 2 discloses a pressure-sensitive adhesive composition for foldable displays, which comprises a (meth)acrylic copolymer containing a structural unit derived from a monomer having a hydroxyl group and a structural unit derived from a (meth)acrylic acid alkyl ester monomer, and a crosslinking agent, and when a pressure-sensitive adhesive layer is formed, the storage modulus at -20°C is in the range of 0.05 MPa or more and 0.5 MPa or less, the ratio of the storage modulus at -20°C to the storage modulus at 100°C is 15.0 or less, and the gel fraction is 50 mass% or more.

[0006] Patent No. 6845952 JP 2021-91772 A

[0007] The adhesive sheet for a foldable flexible display device made of an acrylic polymer and a crosslinking agent described in Patent Document 1 has high adhesive strength and recovery rate, which reduces peeling when folded and allows the sheet to quickly recover to a flat state when opened from a folded state. However, because the storage modulus at low temperatures is high, repeated folding operations are likely to put stress on the component sheet that is the adherend to the adhesive sheet, making it prone to breakage.

[0008] Furthermore, the adhesive sheet for a foldable flexible display device made of a (meth)acrylic polymer and a crosslinking agent described in Patent Document 2 has a low storage modulus at low temperatures, so stress on the component sheet due to repeated folding operations is reduced, but there is no mention of recovery.

[0009] Therefore, the present invention provides an adhesive sheet that can reduce interlayer stress when folded, particularly when folded at low temperatures, has excellent durability (also referred to as "low-temperature flexural durability") that prevents cracking of the component sheet or flexible component, and has excellent recovery properties (also referred to as "strain recovery") that allow it to quickly return to a flat state when folded, as well as a flexible image display component and a flexible image display device that use the same.

[0010] In order to solve the above problems, the pressure-sensitive adhesive sheet of the present invention has the following configuration.

[0011] [1] A pressure-sensitive adhesive sheet formed from a pressure-sensitive adhesive composition comprising a (meth)acrylic acid ester copolymer and a crosslinking agent, wherein the (meth)acrylic acid ester copolymer comprises, as components constituting the copolymer, (A) a branched or linear alkyl (meth)acrylic acid ester monomer having 1 to 20 carbon atoms, (B) a monomer having an alkylene glycol group and a (meth)acryloyl group in the molecule, and (C) a nitrogen-containing vinyl monomer, wherein the pressure-sensitive adhesive sheet has a storage shear modulus (G'(-30°C)) at -30°C, obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, of 250 kPa or less and a strain recovery rate (400%, 1 minute) of 70% or more. [2] The pressure-sensitive adhesive sheet according to [1], wherein the monomer component (A) is a linear alkyl (meth)acrylic acid ester monomer having 8 to 12 carbon atoms. [3] The pressure-sensitive adhesive sheet according to [1] or [2], wherein the (meth)acrylic acid ester copolymer further comprises (D) a hydroxyl group-containing monomer and / or a carboxy group-containing monomer. [4] The pressure-sensitive adhesive sheet according to any one of [1] to [3], wherein the pressure-sensitive adhesive composition comprises 20 to 90 parts by mass of the crosslinking agent per 100 parts by mass of the (meth)acrylic acid ester copolymer. [5] The pressure-sensitive adhesive sheet according to any one of [1] to [4], wherein the pressure-sensitive adhesive composition comprises 30 to 90 parts by mass of the crosslinking agent per 100 parts by mass of the (meth)acrylic acid ester copolymer. [6] The pressure-sensitive adhesive sheet according to any one of [1] to [5], wherein the crosslinking agent is a (meth)acrylate having an average functionality of 1.0 or more and less than 2.0. [7] The pressure-sensitive adhesive sheet according to any one of [1] to [6], wherein the crosslinking agent comprises a monofunctional urethane (meth)acrylate. [8] The pressure-sensitive adhesive sheet according to any one of [1] to [7], wherein the monomer component (B) is an alkoxypolyalkylene glycol (meth)acrylate. [9] The pressure-sensitive adhesive sheet according to [8], wherein the alkoxypolyalkylene glycol (meth)acrylate is contained in an amount of 2 to 4 mass % of all monomer components constituting the (meth)acrylic acid ester copolymer.

[10] The pressure-sensitive adhesive sheet according to any one of [1] to [9], wherein the pressure-sensitive adhesive composition contains a photopolymerization initiator.

[11] A flexible image display device member having a configuration in which two flexible members are bonded together via the adhesive sheet according to any one of [1] to

[10] .

[12] A flexible image display device having the flexible image display device member according to

[11] .

[0012] The pressure-sensitive adhesive sheet of the present invention has a low storage modulus at a low temperature such as -30°C and is excellent in strain recovery, and can be suitably used particularly for flexible image display devices.

[0013] The present invention will be described in detail below, but the present invention is not limited to the following embodiments.

[0014] <Meaning of Terms, etc.> In the present invention, the term "sheet" conceptually encompasses sheets, films, and tapes. In this specification, "(meth)acrylic" encompasses "acrylic" and "methacrylic," and "(meth)acrylate" encompasses "acrylate" and "methacrylate." Furthermore, when the term "panel" is used, such as an image display panel or a protective panel, it encompasses plates, sheets, and films.

[0015] In this specification, when it is stated that "X to Y" (X and Y are any numbers), it means "X or more and Y or less" unless otherwise specified, and also includes the meaning of "preferably greater than X" or "preferably smaller than Y". Furthermore, when it is stated that "X or more" (X is any number), it means "preferably greater than X" unless otherwise specified, and when it is stated that "Y or less" (Y is any number), it also means "preferably smaller than Y" unless otherwise specified. Furthermore, "X and / or Y (X and Y are any configurations)" means at least one of X and Y, and can mean three possibilities: X only, Y only, or X and Y.

[0016] <<Adhesive Sheet>> A pressure-sensitive adhesive sheet according to an embodiment of the present invention (hereinafter also referred to as "the present pressure-sensitive adhesive sheet") is formed from a pressure-sensitive adhesive composition (hereinafter also referred to as "the present pressure-sensitive adhesive composition") containing a (meth)acrylic acid ester copolymer and a crosslinking agent.

[0017] <(Meth)acrylic Acid Ester Copolymer> In order to adjust the viscoelasticity and recovery rate within predetermined ranges, the (meth)acrylic acid ester copolymer preferably contains, as monomer components constituting the copolymer, (A) a branched or linear alkyl (meth)acrylic acid ester monomer having 1 to 20 carbon atoms, (B) a monomer having an alkylene glycol group and a (meth)acryloyl group in the molecule, and (C) a nitrogen-containing vinyl monomer.

[0018] <Monomer Component (A)> The (A) branched or linear alkyl(meth)acrylate monomer having 1 to 20 carbon atoms refers to a branched or linear alkyl(meth)acrylate monomer having an alkyl group with 1 to 20 carbon atoms, and examples thereof include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate (n-butyl (meth)acrylate), pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, and propyl (meth)acrylate. Examples of linear alkyl (meth)acrylate esters include octyl acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate.

[0019] Other examples include branched alkyl (meth)acrylates such as isopropyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, isopentyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylate, isodecyl (meth)acrylate, isomistyryl (meth)acrylate, isostearyl (meth)acrylate, 2-propylheptyl (meth)acrylate, isoundecyl (meth)acrylate, isododecyl (meth)acrylate, isotridecyl (meth)acrylate, isopentadecyl (meth)acrylate, isohexadecyl (meth)acrylate, and isoheptadecyl (meth)acrylate. These can be used alone or in combination of two or more.

[0020] Furthermore, as the (A) branched or linear alkyl (meth)acrylate monomer having 1 to 20 carbon atoms, linear alkyl (meth)acrylate monomers having 8 to 12 carbon atoms are particularly preferred in order to adjust the viscoelasticity and recovery rate within predetermined ranges, and among these, it is preferred that any one or more alkyl (meth)acrylates selected from octyl (meth)acrylate (n-octyl (meth)acrylate) and 2-ethylhexyl (meth)acrylate be the main component of all monomer components constituting the (meth)acrylate ester copolymer. Note that the term "main component" here refers to the component that accounts for the largest mass ratio among all monomer components constituting the (meth)acrylate ester copolymer, and specifically refers to a monomer component that accounts for 50% by mass or more of all monomer components, more preferably 55% by mass or more, and even more preferably 60% by mass or more of all monomer components.

[0021] <Monomer Component (B)> Examples of the monomer (B) having an alkylene glycol group and a (meth)acryloyl group in the molecule include branched or linear alkylene glycols, dialkylene glycols, trialkylene glycols, and polyalkylene glycol (meth)acrylates having 1 to 20 carbon atoms. From the viewpoint of improving adhesive strength, the alkylene group preferably has 1 to 4 carbon atoms, and particularly preferably 2 to 3 carbon atoms. From the viewpoint of recovery, the number of repeating units (n) of the alkylene group is preferably 5 to 15, more preferably 7 to 13, and particularly preferably 9 to 11. Furthermore, from the viewpoint of recovery, a linear alkylene group is preferable. Furthermore, from the viewpoint of suppressing an increase in the storage shear modulus (G') at low temperatures and improving flexibility, acrylates are particularly preferable.

[0022] Other examples include alkoxyalkylene glycol (meth)acrylates, alkoxydialkylene glycol (meth)acrylates, alkoxytrialkylene glycol (meth)acrylates, and alkoxypolyalkylene glycol (meth)acrylates, into which a functional group consisting of an alkyl group having 1 to 18 carbon atoms and an alkylene glycol group having 1 to 4 carbon atoms has been introduced, and phenoxyalkylene glycol (meth)acrylates, phenoxydialkylene glycol (meth)acrylates, phenoxytrialkylene glycol (meth)acrylates, and phenoxypolyalkylene glycol (meth)acrylates, into which a functional group consisting of a phenoxy group and an alkylene glycol group having 1 to 4 carbon atoms has been introduced. These can be used alone or in combination of two or more types.

[0023] Among these, alkoxypolyalkylene glycol (meth)acrylates such as methoxypolyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate, and methoxypolyneopentyl glycol (meth)acrylate are preferred, and among these, one or more selected from the group consisting of alkoxypolyethylene glycol (meth)acrylate and alkoxypolypropylene glycol (meth)acrylate are preferred because they have a low glass transition temperature (Tg) and are easily available, and particularly one or more selected from the group consisting of methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate, and ethoxypolypropylene glycol (meth)acrylate are preferred.

[0024] <Monomer Component (C)> Examples of the nitrogen-containing vinyl monomer (C) include (meth)acrylamide, N-tert-butylacrylamide, N-vinylpyrrolidone, N,N-dimethylacrylamide, N,N-ethylacrylamide, N,N-dimethylaminopropylacrylamide, N-methylol(meth)acrylamide, N-methylolpropane(meth)acrylamide, N-methoxymethyl(meth)acrylamide, N-butoxymethyl(meth)acrylamide, diacetone(meth)acrylamide, maleic acid amide, maleimide, N-isopropylacrylamide, N-phenylacrylamide, dimethylaminopropylacrylamide, N-vinylcaprolactam, acryloylmorpholine, dimethylaminoethyl acrylate, and acryloylpiperidine. Other examples include aminoalkyl (meth)acrylates such as aminomethyl (meth)acrylate, aminoethyl (meth)acrylate, aminopropyl (meth)acrylate, and aminoisopropyl (meth)acrylate, as well as amino group-containing (meth)acrylate monomers such as N-alkylaminoalkyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, and N,N-dimethylaminopropyl (meth)acrylate. These can be used alone or in combination of two or more.

[0025] (Other Monomers) Examples of other monomer components copolymerizable with (A) to (C) include hydroxyl group-containing monomers (excluding alkylene glycol (meth)acrylates as (B)), carboxyl group-containing monomers, epoxy group-containing monomers, and other copolymerizable monomers. Among these, the use of (D) a hydroxyl group-containing (meth)acrylate monomer and / or a carboxyl group-containing (meth)acrylate monomer is particularly preferred from the viewpoint of improving the adhesive strength of the pressure-sensitive adhesive sheet to the adherend. These can be used alone or in combination of two or more.

[0026] (Hydroxyl Group-Containing Monomer) Examples of the hydroxyl group-containing monomer include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-1-methylethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, glycerin mono(meth)acrylate, polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polyethylene glycol polypropylene glycol mono(meth)acrylate, polyethylene glycol polybutylene glycol mono(meth)acrylate, polypropylene glycol polybutylene glycol mono(meth)acrylate, hydroxyphenyl (meth)acrylate, etc. Among these, the use of one or more hydroxyl group-containing (meth)acrylates selected from the group consisting of 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate is particularly preferred from the viewpoint of improving the adhesive strength of the pressure-sensitive adhesive sheet to the adherend. These may be used alone or in combination of two or more.

[0027] (Carboxy Group-Containing Monomer) Examples of the carboxy group-containing monomer 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)acryloyloxy Examples of suitable acrylates include ethylsuccinic acid, 2-(meth)acryloyloxypropylsuccinic acid, crotonic acid, fumaric acid, maleic acid, itaconic acid, citraconic acid, monomethyl maleate, monoethyl maleate, monooctyl maleate, monomethyl itaconate, monoethyl itaconate, monobutyl itaconate, monooctyl itaconate, monomethyl fumarate, monoethyl fumarate, monobutyl fumarate, monooctyl fumarate, and monoethyl citraconic acid. Among these, acrylates other than (meth)acrylic acid, particularly 2-(meth)acryloyloxyethylsuccinic acid and 2-(meth)acryloyloxypropylsuccinic acid, are particularly preferred from the viewpoint of improving the adhesive strength of the PSA sheet to the adherend. These acrylates may be used alone or in combination of two or more.

[0028] Examples of the epoxy group-containing monomer include glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate glycidyl ether, etc. These may be used alone or in combination of two or more.

[0029] Examples of the other copolymerizable monomers include acid anhydride group-containing monomers such as maleic anhydride and itaconic anhydride, heterocyclic basic monomers such as vinylpyridine and vinylcarbazole, macromonomers, etc. These may be used alone or in combination of two or more.

[0030] Furthermore, a polyfunctional (meth)acrylate may be used in combination as a monomer component constituting the (meth)acrylic acid ester copolymer. As the polyfunctional (meth)acrylate, a bifunctional (meth)acrylate is preferred, and a bifunctional urethane (meth)acrylate is particularly preferred, from the viewpoint of making it easier to adjust the loss shear modulus (G" (23°C)) of the PSA sheet and facilitating the formation of an appropriate crosslinked network for improving recovery properties.

[0031] Examples of the bifunctional (meth)acrylate include 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, neopentyl glycol adipate di(meth)acrylate, hydroxypivalic acid neopentyl glycol di(meth)acrylate, dicyclopentanyl di(meth)acrylate, caprolactone-modified dicyclopentenyl di(meth)acrylate, ethylene oxide-modified phosphate di(meth)acrylate, di(acryloxyethyl)isocyanurate, and allylated cyclohexyl di(meth)acrylate.

[0032] The bifunctional urethane (meth)acrylate is a urethane (meth)acrylate having two (meth)acryloyloxy groups (CHR=C(=O)O-, where R is a hydrogen atom or a methyl group) and a urethane group (-NHC(=O)O-). The bifunctional urethane acrylate usually has a polyurethane chain, which is a polycondensation reaction product of a diol and a diisocyanate, and (meth)acryloyl groups bonded to both ends of the polyurethane chain.

[0033] Examples of diols that can be used as raw materials for the bifunctional urethane (meth)acrylate include polycarbonate diols, polyester diols, polyether diols, and polycaprolactone diols. These can be used alone or in combination of two or more.

[0034] Examples of diisocyanates that can be used as a raw material for the bifunctional urethane acrylate include tolylene diisocyanate, xylylene diisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, tetramethylxylylene diisocyanate, isophorone diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate, hydrogenated diphenylmethane diisocyanate, m-phenylene diisocyanate, biphenylene diisocyanate, tetramethylene diisocyanate, and hexamethylene diisocyanate. These can be used alone or in combination of two or more.

[0035] Examples of monofunctional acrylic monomers having a hydroxyl group that can be used as raw materials for the bifunctional urethane acrylate include 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, and 1,4-cyclohexanedimethanol mono(meth)acrylate. These can be used alone or in combination of two or more.

[0036] Of all the monomer components constituting the (meth)acrylic acid ester copolymer, the content of the (A) branched or linear alkyl (meth)acrylic acid ester monomer having 1 to 20 carbon atoms is preferably 60 to 90 mass%, more preferably 65 to 85 mass%, and most preferably 70 to 80 mass%, from the viewpoint of adjusting the viscoelasticity within a predetermined range.

[0037] Furthermore, from the viewpoint of suppressing an increase in the storage shear modulus (G') at low temperatures and improving flexibility, the content of the (B) monomer having an alkylene glycol group and a (meth)acryloyl group in the molecule of all the monomer components constituting the (meth)acrylic acid ester copolymer is preferably 1 to 5 mass%, more preferably 1.5 to 4.5 mass%, and most preferably 2 to 4 mass%. In particular, the content of the alkoxy polyalkylene glycol (meth)acrylate in all the monomer components constituting the (meth)acrylic acid ester copolymer is preferably 2 to 4 mass%.

[0038] Furthermore, the content of the nitrogen-containing vinyl monomer (C) in all the monomer components constituting the (meth)acrylic acid ester copolymer is preferably 0.1 to 5 mass%, more preferably 0.5 to 4 mass%, and most preferably 1 to 3 mass%, from the viewpoint of improving strain recovery.

[0039] Furthermore, from the viewpoint of improving the adhesive strength of the pressure-sensitive adhesive sheet to the adherend, the content of the other monomers is preferably 8 to 30 mass %, more preferably 10 to 25 mass %, and most preferably 12 to 20 mass %.

[0040] The content of the polyfunctional (meth)acrylate is preferably 0 to 2 mass %, more preferably 0.25 to 1.5 mass %, and most preferably 0.5 to 1 mass %, from the viewpoint of making it easier to adjust the loss shear modulus (G" (23°C)) of the PSA sheet and to make it easier to form an appropriate crosslinked network for improving recovery properties.

[0041] <Crosslinking Agent> In addition to the (meth)acrylic acid ester copolymer, the present pressure-sensitive adhesive composition contains a crosslinking agent in order to reduce the storage modulus at low temperatures and adjust the viscoelasticity to the desired level. Examples of crosslinking agents used in the present pressure-sensitive adhesive composition include (meth)acrylate-based crosslinking agents, isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, melamine-based crosslinking agents, carbodiimide-based crosslinking agents, hydrazine-based crosslinking agents, amine-based crosslinking agents, peroxide-based crosslinking agents, metal chelate-based crosslinking agents, metal alkoxide-based crosslinking agents, and metal salt-based crosslinking agents. These crosslinking agents can be used alone or in combination of two or more.

[0042] In view of aging-freeness, ease of adjusting the degree of crosslinking, etc., it is preferable to use a photocrosslinking agent, which is a compound that has the property of curing upon irradiation with light, among crosslinking agents, for the present pressure-sensitive adhesive composition, thereby forming a crosslinked structure with the (meth)acrylic acid ester copolymer. Note that "forming a crosslinked structure" includes not only cases where polymer chains are crosslinked via chemical bonds, but also cases where (pseudo) crosslinking is achieved by non-covalent bonds due to interactions such as hydrogen bonds within or between polymer chains, electrostatic interactions, and van der Waals forces. The crosslinked structure also includes cases where crosslinking agents are crosslinked via chemical bonds, and cases where pseudo crosslinking is achieved by entanglement of polymer chains with each other or between polymer chains and the crosslinking agent.

[0043] The photocrosslinking agent is preferably a compound having an ethylenically unsaturated group in the molecule, from the viewpoint of easily forming a crosslinked structure with the (meth)acrylic acid ester copolymer, and is particularly preferably a (meth)acrylate. In particular, when formed into a homopolymer, i.e., when polymerized alone to form a polymer, the glass transition temperature is preferably -30°C or lower, and (meth)acrylates of -35°C or lower are more preferred. The lower limit of the glass transition temperature is typically -80°C. By using a photocrosslinking agent with a glass transition temperature within this range, the glass transition temperature of the (meth)acrylic acid ester copolymer can be set relatively low. This allows the pressure-sensitive adhesive sheet to exhibit particularly excellent effects of ensuring adhesiveness while imparting flexibility sufficient to withstand buckling during bending deformation and providing bending durability. Examples of the (meth)acrylate include polyfunctional (meth)acrylates. Monofunctional (meth)acrylates are also suitable, and monofunctional urethane (meth)acrylates are even more preferred.

[0044] The polyfunctional (meth)acrylate may be a mixture containing a small amount of a monofunctional (meth)acrylate as a by-product, and the monofunctional (meth)acrylate may be a mixture containing a small amount of a polyfunctional (meth)acrylate.

[0045] Furthermore, the crosslinking agent is preferably a (meth)acrylate having an average functionality of 1.0 or more and less than 2.0. When the average functionality is equal to or greater than the lower limit, a suitable crosslinked structure can be formed to maintain the sheet shape. When the average functionality is less than the upper limit, the crosslinking density does not become too high, making it easy to control the storage modulus low, resulting in a pressure-sensitive adhesive sheet with excellent adhesion to various component sheets and flexibility. From this perspective, the average functionality of the crosslinking agent is preferably 1.05 to 1.8, more preferably 1.1 to 1.6. Note that, in this specification, the average functionality refers to the average number of (meth)acryloyl groups present in one molecule of the crosslinking agent. Note that, in this application, "monofunctional" refers to an average functionality of 1.0 to 1.2.

[0046] Examples of methods for adjusting the average number of functional groups of the crosslinking agent to fall within the above range include a method of using, as the crosslinking agent, a (meth)acrylate oligomer having an average number of added (meth)acryloyl groups of 1.0 or more and less than 2.0, and a method of using, as the crosslinking agent, a mixture of a polyfunctional (meth)acrylate and a monofunctional (meth)acrylate.

[0047] Furthermore, the crosslinking agent is preferably a (meth)acrylate having an alkylene glycol skeleton. The crosslinking agent having the above structure enhances the affinity with the monomer component (B) in the (meth)acrylic acid ester copolymer, thereby improving the compatibility of the pressure-sensitive adhesive composition and facilitating control of the storage modulus at low temperatures, which is preferable in that it is easy to obtain a pressure-sensitive adhesive sheet with excellent low-temperature flexural durability. Examples of the glycol skeleton include a polyethylene glycol skeleton, a polypropylene glycol skeleton, a polytetramethylene glycol skeleton, and a polyhexamethylene glycol skeleton. Among these, a polyethylene glycol skeleton and / or a polypropylene glycol skeleton are particularly preferred.

[0048] Examples of the polyfunctional (meth)acrylate include (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,2-ethylene glycol di(meth)acrylate, and 1,6-hexanediol. Examples of the ester compound include ester compounds of polyhydric alcohols and (meth)acrylic acid, such as ol di(meth)acrylate, 1,12-dodecanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and tetramethylolmethane tri(meth)acrylate; allyl (meth)acrylate; vinyl (meth)acrylate; divinylbenzene; epoxy acrylate; polyester acrylate; urethane acrylate; butyl di(meth)acrylate; and hexyl di(meth)acrylate.

[0049] The monofunctional urethane (meth)acrylate can be obtained by reacting a polyhydric alcohol, a polyisocyanate, and a (meth)acrylate having a hydroxy group.

[0050] Examples of the polyhydric alcohol include poly C2-C6 alkenylene glycols such as polybutadiene glycol, hydrogenated polybutadiene glycol, polyisoprene glycol, and hydrogenated polyisoprene glycol, and hydrogenated poly C2-C6 alkenylene glycols; alkylene glycols having 1 to 10 carbon atoms such as neopentyl glycol, 3-methyl-1,5-pentanediol, ethylene glycol, propylene glycol, 1,4-butanediol, and 1,6-hexanediol, and poly C2-C10 alkylene glycols in which one or more selected from these alkylene glycols are condensed via an ether bond.

[0051] Examples of the polyisocyanate include isophorone diisocyanate, hexamethylene diisocyanate, tolylene diisocyanate, xylene diisocyanate, diphenylmethane-4,4'-diisocyanate, and dicyclopentanyl diisocyanate.

[0052] Examples of the (meth)acrylate having a hydroxy group include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, dimethylolcyclohexyl mono(meth)acrylate, and hydroxycaprolactone (meth)acrylate.

[0053] Among the monofunctional urethane (meth)acrylates, from the viewpoint of low glass transition temperature and improved strain recovery, monofunctional urethane acrylates having a polypropylene glycol skeleton are preferred, and in particular, monofunctional urethane acrylates represented by the following formula 1 are particularly preferred.

[0054] (wherein, in formula 1, R1 represents hydrogen or a methyl group, X represents a urethane bond, R2, R3, and R4 each represent an alkyl group, and n is an integer of 2 or greater.)

[0055] The weight average molecular weight (Mw) of the crosslinking agent is preferably 1,000 to 100,000, more preferably 3,000 to 50,000, and even more preferably 5,000 to 30,000, from the viewpoint of obtaining a pressure-sensitive adhesive composition with high cohesive strength.

[0056] In this pressure-sensitive adhesive sheet, the weight-average molecular weight (Mw) can be determined, for example, as follows. (Method for measuring weight-average molecular weight) 4 mg of crosslinking agent is dissolved in 12 mL of tetrahydrofuran (THF) to prepare a measurement sample, and the weight-average molecular weight (Mw) can be determined by measuring the molecular weight distribution curve under the following conditions using a gel permeation chromatography (GPC) analyzer (Tosoh Corporation, HLC-8320GPC). Guard column: TSKguardcolumnHXL Separation column: TSKgelGMHXL (4 columns) Temperature: 40°C Injection volume: 100 μL Polystyrene equivalent Solvent: THF Flow rate: 1.0 mL / min

[0057] The content of the crosslinking agent is preferably in the range of 1 to 100 parts by mass, more preferably in the range of 10 to 95 parts by mass, even more preferably in the range of 15 to 95 parts by mass, even more preferably in the range of 30 to 90 parts by mass, and most preferably in the range of 50 to 90 parts by mass, relative to 100 parts by mass of the (meth)acrylic acid ester copolymer. A content of 55 to 80 parts by mass, and even more preferably in the range of 60 to 75 parts by mass, is preferred. By including the crosslinking agent in such a proportion, a good balance between adhesive strength and flexural durability can be achieved. Furthermore, from the viewpoint of obtaining a pressure-sensitive adhesive sheet with excellent strain recovery, the content of the crosslinking agent is preferably in the range of 20 to 90 parts by mass, more preferably in the range of 22 to 88 parts by mass, and even more preferably in the range of 25 to 85 parts by mass, relative to 100 parts by mass of the (meth)acrylic acid ester copolymer, in addition to the preferred range of the crosslinking agent content.

[0058] <Photopolymerization initiator> The pressure-sensitive adhesive composition preferably further contains a photopolymerization initiator. Preferred examples of the photopolymerization initiator include compounds that generate active radical species when irradiated with light such as ultraviolet light or visible light, more specifically, light with a wavelength of 200 to 780 nm.

[0059] The photopolymerization initiator may be either a cleavage type or a hydrogen abstraction type. However, when a hydrogen abstraction type photopolymerization initiator is used, a hydrogen abstraction reaction occurs from the (meth)acrylic acid ester copolymer, and not only the crosslinking agent but also the (meth)acrylic acid ester copolymer is incorporated into the crosslinked structure, thereby forming a crosslinked structure with many crosslinking points, which is preferable.

[0060] Examples of the hydrogen abstraction type photopolymerization initiator include benzophenone, 4-methyl-benzophenone, 2,4,6-trimethylbenzophenone, 4-phenylbenzophenone, 3,3'-dimethyl-4-methoxybenzophenone, 4-(meth)acryloyloxybenzophenone, methyl 2-benzoylbenzoate, methyl benzoylformate, bis(2-phenyl-2-oxoacetate)oxybisethylene, 4-(1,3-acryloyl-1,4,7,10,13-pentaoxotridecyl)benzophenone, thioxanthone, 2-chlorothioxanthone, 3-methylthioxanthone, 2,4-dimethylthioxanthone, 2-methylanthraquinone, 2-ethylanthraquinone, 2-tert-butylanthraquinone, 2-aminoanthraquinone, and derivatives thereof.

[0061] The content of the photopolymerization initiator is preferably 0.01 to 5 parts by mass, more preferably 0.03 to 4 parts by mass, and even more preferably 0.05 to 3 parts by mass, relative to 100 parts by mass of the (meth)acrylic acid ester copolymer. If the content of the photopolymerization initiator is within this range, a good crosslinking reaction will proceed.

[0062] <Other Components> The pressure-sensitive adhesive composition may contain other components in addition to the (meth)acrylic acid ester copolymer, crosslinking agent, and photopolymerization initiator. The "other components" are not particularly limited. Examples include a rust inhibitor and a silane coupling agent, which will be described below.

[0063] The anti-rust agent is preferably, for example, a triazole, a benzotriazole, or the like, and can prevent corrosion of the transparent electrodes on the touch panel. The content of the anti-rust agent in the pressure-sensitive adhesive composition (100% by mass) is preferably 0.01 to 5% by mass, and more preferably 0.1% by mass or more and 3% by mass or less.

[0064] Examples of the silane coupling agent include a silane coupling agent containing a glycidyl group, a silane coupling agent having a (meth)acrylic group or a vinyl group, etc. By including these, when the pressure-sensitive adhesive sheet is used to form a laminate, the adhesion to the component sheet or flexible component can be improved, and foaming phenomenon under a humid and hot environment can be suppressed.

[0065] The silane coupling agent is preferably contained in the pressure-sensitive adhesive composition (100% by mass) in an amount of 0.01 to 3% by mass, and more preferably in an amount of 0.1% to 1% by mass. Depending on the adherend, the silane coupling agent may be effective even at a content of 0.01% by mass, while adjusting the content to 3% by mass or less can suppress foaming due to dealcoholization.

[0066] The PSA composition may also contain, as other components, one or a combination of two or more additives such as curing accelerators, fillers, coupling agents, UV absorbers, UV stabilizers, antioxidants, stabilizers, pigments, etc. The amounts of these additives are typically preferably selected so as not to adversely affect the curing of the PSA sheet or the physical properties of the PSA sheet.

[0067] <Configuration of the Present Pressure-Sensitive Adhesive Sheet> The present pressure-sensitive adhesive sheet may be a single-layer sheet consisting of only an adhesive layer (also referred to as the "present pressure-sensitive adhesive layer") formed from the present pressure-sensitive adhesive composition, or a multilayer sheet comprising the present pressure-sensitive adhesive layer and other layers. When the present pressure-sensitive adhesive sheet is a multilayer sheet comprising other layers, it is preferable that the present pressure-sensitive adhesive layer has the greatest thickness of all the layers constituting the present pressure-sensitive adhesive sheet. Furthermore, in order to obtain the effects of the present invention, the thickness of the present pressure-sensitive adhesive layer preferably accounts for 10 to 90% of the total thickness of the present pressure-sensitive adhesive sheet, and more preferably accounts for 20% to 80%, and even more preferably 30% to 70%.

[0068] <Physical Properties of the Present Pressure-Sensitive Adhesive Sheet> The present pressure-sensitive adhesive sheet can have the following physical properties.

[0069] (Storage Shear Modulus) The pressure-sensitive adhesive sheet has a storage shear modulus at -30°C (G'(-30°C)), obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, of 250 kPa or less, preferably 200 kPa or less, more preferably 180 kPa or less, and particularly preferably 150 kPa or less. From the viewpoint of shape maintenance, the lower limit of the storage shear modulus (G'(-30°C)) of the pressure-sensitive adhesive sheet is preferably 10 kPa or more.

[0070] When the storage shear modulus (G'(-30°C)) of the present adhesive sheet is within the above range, for example, when the present adhesive sheet is adhered to a component sheet to form a laminated sheet or a flexible image display device component, the interlayer stress when the laminated sheet or flexible image display device component is bent can be reduced, particularly at low to high temperatures, and cracking of the component sheet or flexible component can be suppressed.

[0071] (Maximum point of loss tangent (tan δ) and glass transition temperature (Tg)) The maximum point of the loss tangent (tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz for the present pressure-sensitive adhesive sheet is preferably −40° C. or lower. The maximum point of the loss tangent (tan δ) can be interpreted as the glass transition temperature (Tg), and having the glass transition temperature (Tg) in the above range makes it easier to adjust the storage shear modulus (G'(−30° C.)) of the present pressure-sensitive adhesive sheet to 250 kPa or lower.

[0072] The "glass transition temperature" refers to the temperature at which the peak of the main dispersion of the loss tangent (tan δ) appears. Therefore, when only one maximum point of the loss tangent (tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz is observed, in other words, when the tan δ curve exhibits a unimodal shape, it can be considered that the glass transition temperature (Tg) is single.

[0073] The "maximum point" of the loss tangent (tan δ) means the point having the maximum value in a predetermined range or the entire range among the peak values ​​in the tan δ curve, that is, the inflection points where the tan δ curve changes from positive (+) to negative (-) when differentiated.

[0074] The elastic modulus (storage shear modulus) G', viscous modulus (loss shear modulus) G" and tan δ = G" / G' at various temperatures can be measured using a strain rheometer.

[0075] The storage shear modulus (G'), loss shear modulus (G"), and loss tangent (tan δ) can be adjusted to fall within the above ranges by adjusting the types, blend amounts, and weight average molecular weights of the components of the present PSA composition constituting the present PSA sheet (for example, the monomer components and crosslinking agents constituting the (meth)acrylic acid ester copolymer), and by further adjusting the gel fraction, etc. of the PSA sheet. However, this method is not limited to this.

[0076] (Strain recovery) The strain recovery rate (400%, 1 minute) of the present pressure-sensitive adhesive sheet, calculated by the method described in the Examples, is 70% or more, preferably 75% or more, more preferably 80% or more, and particularly preferably 82% or more. The upper limit of the strain recovery rate (400%, 1 minute) of the present pressure-sensitive adhesive sheet is preferably 99% or less from the viewpoint of the pressure-sensitive adhesive.

[0077] When the strain recovery rate (400%, 1 minute) of the present adhesive sheet is within the above range, for example, when the present adhesive sheet is attached to a component sheet to form a laminated sheet or a flexible image display component, even when a folding operation is performed at low temperatures, the adhesive sheet has excellent resilience and is not affected by being placed in a bent state.

[0078] The strain recovery rate (400%, 1 minute) can be adjusted to fall within the above range by adjusting the types, amounts, and weight average molecular weights of the components of the present pressure-sensitive adhesive composition that constitutes the present pressure-sensitive adhesive sheet (for example, the monomer components and crosslinking agents that constitute the (meth)acrylic acid ester copolymer), and by adjusting the gel fraction, etc. of the pressure-sensitive adhesive sheet, although this is not limited to this method.

[0079] (Gel fraction) The present pressure-sensitive adhesive sheet preferably has a gel fraction of 45% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more, as calculated by the method described in the examples. When the gel fraction of the present pressure-sensitive adhesive sheet is equal to or greater than the lower limit, the shape can be sufficiently maintained. On the other hand, the gel fraction of the present pressure-sensitive adhesive sheet is preferably equal to or less than 90% by mass, more preferably equal to or less than 85% by mass, and even more preferably equal to or less than 80% by mass. When the gel fraction of the present pressure-sensitive adhesive sheet is equal to or less than the upper limit, the adhesive strength can be increased.

[0080] (Thickness) The thickness of the present pressure-sensitive adhesive sheet is not particularly limited, and if the thickness is 5 μm or more, it is easy to handle, and if the thickness is 1000 μm or less, it can contribute to making the laminate thinner. Therefore, the thickness of the present pressure-sensitive adhesive sheet is preferably 5 μm or more, more preferably 8 μm or more, and particularly preferably 10 μm or more. On the other hand, the upper limit is preferably 1000 μm or less, more preferably 500 μm or less, and particularly preferably 250 μm or less.

[0081] <Preferred Uses of the Present Pressure-Sensitive Adhesive Sheet> The present pressure-sensitive adhesive sheet is preferably used for laminating components constituting display members (also referred to as "display members"), particularly flexible members for displays used in producing displays, and is particularly preferably used as an adhesive component for flexible displays used in producing flexible displays. The flexible members that can be used are the same as those described below.

[0082] <Method for producing the present pressure-sensitive adhesive sheet> Next, a method for producing the present pressure-sensitive adhesive sheet will be described. However, the following description is an example of a method for producing the present pressure-sensitive adhesive sheet, and the present pressure-sensitive adhesive sheet is not limited to sheets produced by this method.

[0083] The present pressure-sensitive adhesive sheet can be produced by preparing the present pressure-sensitive adhesive composition containing a (meth)acrylic acid ester copolymer (including a partial polymer obtained by polymerizing the monomer components constituting the copolymer) or a mixture of the monomer components constituting the copolymer, a crosslinking agent, a photopolymerization initiator, other components, etc., forming the present pressure-sensitive adhesive composition into a sheet, polymerizing (meaning "crosslinking" is included, the same applies hereinafter) the (meth)acrylic acid ester copolymer and / or the crosslinking agent to cure it, and then processing it appropriately as necessary. In this way, the present pressure-sensitive adhesive sheet has a polymer (meaning "crosslinked product" is included, the same applies hereinafter) of the (meth)acrylic acid ester copolymer, i.e., a cured product.

[0084] When preparing the present pressure-sensitive adhesive composition, the raw materials may be kneaded using a temperature-controllable kneader (for example, a single-screw extruder, a twin-screw extruder, a planetary mixer, a twin-screw mixer, a pressure kneader, etc.) When mixing the various raw materials, various additives such as silane coupling agents and antioxidants may be blended together with the resin in advance and then fed to the kneader, or all of the materials may be melt-mixed in advance and then fed, or a masterbatch in which only the additives are concentrated in the resin may be prepared and then fed.

[0085] The pressure-sensitive adhesive composition can be formed into a sheet by any known method, such as wet lamination, dry lamination, extrusion casting using a T-die, extrusion lamination, calendaring, inflation, injection molding, and liquid injection curing. Of these, when producing a sheet, wet lamination, extrusion casting, and extrusion lamination are preferred.

[0086] To impart curability to the pressure-sensitive adhesive sheet, it is preferable to polymerize, or in other words, crosslink, the pressure-sensitive adhesive composition using a crosslinking agent and / or polymerization initiator, as described above. When the pressure-sensitive adhesive composition contains a photopolymerization initiator, the pressure-sensitive adhesive composition can be polymerized and cured by irradiating it with heat and / or active energy rays. For example, the pressure-sensitive adhesive sheet can be produced by irradiating a molded product, such as a sheet, of the pressure-sensitive adhesive composition with heat and / or active energy rays. Examples of the active energy rays to be irradiated include ionizing radiation such as α-rays, β-rays, γ-rays, neutron beams, and electron beams, ultraviolet rays, and visible light. Among these, ultraviolet rays are preferred from the viewpoints of suppressing damage to optical device components and controlling reactions.

[0087] Furthermore, the irradiation energy, irradiation time, irradiation method, etc. of the active energy rays are not particularly limited, as long as they can activate the photopolymerization initiator and polymerize the (meth)acrylic acid ester copolymer and / or the crosslinking agent. When a hydrogen abstraction photopolymerization initiator is used as the photopolymerization initiator, a hydrogen abstraction reaction also occurs from the (meth)acrylic acid ester copolymer, and not only the crosslinking agent but also the (meth)acrylic acid ester copolymer is incorporated into the crosslinked structure, forming a crosslinked structure with many crosslinking points. Therefore, it is even more preferable to produce the pressure-sensitive adhesive sheet by curing using a hydrogen abstraction photopolymerization initiator.

[0088] Another example of a method for producing the present PSA sheet that is different from the above is a method in which the present PSA composition is prepared in the same manner as above, and then coated onto a member having a release-treated surface, such as a release film, and the PSA composition is cured to form a PSA layer (which includes the term "PSA sheet"), but is not limited to these methods.

[0089] When the pressure-sensitive adhesive composition is coated as described above, the pressure-sensitive adhesive composition may be dissolved in an appropriate solvent, if necessary.

[0090] The method for coating the present pressure-sensitive adhesive composition is not particularly limited, and any common coating method can be used, such as roll coating, die coating, gravure coating, comma coating, and screen printing.

[0091] When such a coating method is used, the present pressure-sensitive adhesive sheet can be obtained by heat curing in addition to the above-mentioned curing by irradiation with active energy rays. In the case of coating, the thickness of the present pressure-sensitive adhesive sheet can be adjusted by the coating thickness and the solids concentration of the coating liquid.

[0092] To prevent blocking and adhesion of foreign matter, a protective film having a release layer laminated thereon can be provided on at least one side of the pressure-sensitive adhesive sheet. If necessary, embossing or various unevenness (conical, pyramidal, hemispherical, etc.) may be performed. Furthermore, in order to improve adhesion to various component sheets, various surface treatments such as corona treatment, plasma treatment, and primer treatment may be performed on the surface.

[0093] <<Present Laminate Sheet>> A laminate sheet according to one example of an embodiment of the present invention (hereinafter also referred to as "present laminate sheet") comprises the present pressure-sensitive adhesive sheet and a component sheet provided on at least one side thereof.

[0094] The present laminate sheet is preferably a laminate sheet having a configuration in which, for example, a component sheet (hereinafter sometimes referred to as the "first component sheet"), the present pressure-sensitive adhesive sheet, and another component sheet (hereinafter sometimes referred to as the "second component sheet") are laminated in this order. The present laminate sheet can be produced by adhering the present pressure-sensitive adhesive sheet to the first component sheet and / or the second component sheet. However, the present laminate sheet is not limited to this manufacturing method. The first component sheet and the second component sheet may be the same or different.

[0095] <Component Sheet> The component sheet constituting the present laminate sheet, i.e., the component sheet attached to the present pressure-sensitive adhesive sheet (including the "first component sheet" and the "second component sheet"), may be, for example, a resin sheet containing, as a main component, one or more resins selected from the group consisting of cycloolefin resin, triacetyl cellulose resin, polymethyl methacrylate resin, epoxy resin, polyimide resin, and polyurethane resin, or glass such as thin film glass. Here, thin film glass refers to glass having the thickness of the component sheets listed above.

[0096] The "main component" mentioned above means a component that occupies the largest mass ratio among the resin components that make up the component sheet, and specifically, a component that occupies 50 mass% or more of the component sheet or the resin composition that forms the component sheet, and more preferably 55 mass% or more, and even more preferably 60 mass% or more of that.

[0097] Although it depends on the configuration of the flexible image display device and the position of the pressure-sensitive adhesive sheet, examples of the first and second component sheets include a cover lens, a polarizing plate, a retardation film, a barrier film, a touch sensor film, a light-emitting element, etc. In particular, considering the configuration of the image display, it is preferable that the first component sheet has a touch input function. When the laminate sheet has the above-mentioned second component sheet, the second component sheet may also have a touch input function.

[0098] <Thickness of the Present Laminate Sheet> The thickness of the present laminate sheet is not particularly limited. For example, when used in an image display device, the present laminate sheet is in sheet form. If the thickness is 0.01 mm or more, it has good handleability, and if the thickness is 1 mm or less, it can contribute to making the laminate thinner. Therefore, the thickness of the present laminate sheet is preferably 0.01 mm or more, more preferably 0.03 mm or more, and particularly preferably 0.05 mm or more. On the other hand, the upper limit is preferably 1 mm or less, more preferably 0.7 mm or less, and particularly preferably 0.5 mm or less. The present pressure-sensitive adhesive sheet can also be provided as a pressure-sensitive adhesive sheet with a release film by laminating a release film on one or both sides of the pressure-sensitive adhesive layer made of the present pressure-sensitive adhesive composition.

[0099] <Method for manufacturing the present laminate sheet> Next, a method for manufacturing the present laminate sheet will be described. However, the following description is an example of a method for manufacturing the present laminate sheet, and the present laminate sheet is not limited to sheets manufactured by this manufacturing method.

[0100] The present laminate sheet may be produced by preparing the present pressure-sensitive adhesive composition in the same manner as in the manufacturing method of the present pressure-sensitive adhesive sheet, and then applying the present pressure-sensitive adhesive composition to, for example, a first member sheet and / or a second member sheet, followed by curing. In this case, the methods for preparing the present pressure-sensitive adhesive composition, the coating method, and the curing method of the present pressure-sensitive adhesive composition are the same as in the manufacturing method of the present pressure-sensitive adhesive sheet.

[0101] Alternatively, the present laminated sheet may be produced by laminating a pre-produced present pressure-sensitive adhesive sheet to the first member sheet and / or the second member sheet.

[0102] In order to improve adhesiveness, the surfaces of the pressure-sensitive adhesive sheet, the first member sheet and the second member sheet may be subjected to various surface treatments such as corona treatment, plasma treatment and primer treatment.

[0103] When the present laminated sheet is configured such that a component sheet is laminated on only one side of the present adhesive sheet, a protective film having a release layer laminated thereon can also be provided on the side of the present adhesive sheet on which the component sheet is not laminated.

[0104] <The flexible image display device member> A flexible image display device member according to one embodiment of the present invention (hereinafter also referred to as "the flexible image display device member") is a flexible image display device member having a configuration in which two flexible members are bonded together via the adhesive sheet.

[0105] Of the components of the flexible image display device member, the adhesive sheet has been described above, and the components other than the adhesive sheet will be described below.

[0106] (Flexible Member) Examples of the flexible member constituting the flexible image display device member include flexible members for displays such as flexible displays such as organic electroluminescence (EL) displays, cover lenses (cover films), polarizing plates, polarizers, retardation films, barrier films, viewing angle compensation films, brightness enhancement films, contrast enhancement films, diffusion films, semi-transmitting reflective films, electrode films, transparent conductive films, metal mesh films, and touch sensor films. Any one of these may be used alone or in combination. Examples include a combination of a flexible display with another flexible member, or a combination of a cover lens with another flexible member.

[0107] The term "flexible member" means a member that can be bent, particularly a member that can be repeatedly bent, and is particularly preferably a member that can be fixed into a curved shape with a bending radius of 25 mm or more, and is particularly preferably a member that can withstand repeated bending at a bending radius of less than 25 mm, more preferably less than 3 mm.

[0108] In the above-described configuration, the main component of the flexible member may be, for example, a cycloolefin resin, a triacetyl cellulose resin, a polymethyl methacrylate resin, a polyurethane, an epoxy resin, a polyimide resin, or glass. The main component may be one of these resins, or two or more of these resins. The term "main component" as used herein refers to the component that accounts for the largest mass ratio among the components constituting the flexible member. Specifically, the main component accounts for 50% or more by mass of the resin composition forming the flexible member, preferably 55% or more by mass, and particularly preferably 60% or more by mass. The flexible member may also be made of thin-film glass.

[0109] <Method for manufacturing the flexible image display device member of the present invention> The method for manufacturing the flexible image display device member of the present invention is not particularly limited, and as described above, the flexible image display device member may be formed by applying the present pressure-sensitive adhesive composition onto a flexible member, or the flexible image display device member may be formed into a sheet shape using the present pressure-sensitive adhesive composition in advance, and then laminated to a flexible member.

[0110] <Present Image Display Device> An image display device according to an example embodiment of the present invention (hereinafter also referred to as "the present image display device") is an image display device incorporating the present laminate sheet or the present flexible image display device member. For example, by laminating the present laminate sheet on other image display device components, a flexible image display device including the present laminate sheet can be formed.

[0111] The term "flexible image display device" refers to an image display device that can be repeatedly bent without leaving any trace of bending, can quickly recover to its original state when released from the bending, and can display images without distortion even when bent. More specifically, an example of such a display device is an image display device made of a member that can be curved and fixed to a bending radius of 25 mm or more, particularly a member that can withstand repeated bending at a bending radius of less than 25 mm, more preferably less than 3 mm.

[0112] One of the features of this laminated sheet is that it can prevent delamination and cracking of the laminated sheet even when folded in a low-temperature environment, and it has good recovery properties, so it can be used to manufacture image display devices with excellent flexibility.

[0113] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. In the examples, "parts" and "%" are based on mass.

[0114] First, the pressure-sensitive adhesive compositions containing a (meth)acrylic acid ester copolymer, a crosslinking agent, etc. used in the examples and comparative examples will be described in detail.

[0115] (1) (Meth)acrylic acid ester copolymers (I to IV) Table 1 shows the compositions (mass proportions of monomer components) of the (meth)acrylic acid ester copolymers (I to IV) used in the examples and comparative examples.

[0116]

[0117] (2) Crosslinking Agent As a crosslinking agent, a propylene glycol skeleton-containing monofunctional urethane acrylate (weight average molecular weight (Mw): about 10,000, manufactured by AGC, "PEM-X264"), which is a photocrosslinking agent, was used.

[0118] (3) Photopolymerization Initiator As the photopolymerization initiator, a mixture of 4-methylbenzophenone and 2,4,6-trimethylbenzophenone (Esacure TZT, manufactured by IGM), which is a hydrogen abstraction type photopolymerization initiator, was used.

[0119] (4) Silane Coupling Agent 3-glycidoxypropyltrimethoxysilane was used as the silane coupling agent.

[0120] (5) Ethyl acetate Ethyl acetate was used as a solvent.

[0121] (6) Preparation of Pressure-Sensitive Adhesive Compositions The (meth)acrylic acid ester copolymers (I to IV) shown in Table 1 were mixed with the crosslinking agent, photopolymerization initiator, and silane coupling agent as shown in Table 2, and ethyl acetate was added to give a solids concentration of 35% to prepare pressure-sensitive adhesive compositions of the Examples and Comparative Examples.

[0122]

[0123] <Preparation of Pressure-Sensitive Adhesive Sheets> In Examples 1 to 4 and Comparative Examples 1 to 4, pressure-sensitive adhesive sheets were obtained as follows. Raw materials were blended in the mass ratios shown in Table 2 and mixed uniformly to prepare pressure-sensitive adhesive compositions. Subsequently, the composition was applied with an applicator to a 100 μm-thick release film (PET film manufactured by Mitsubishi Chemical Corporation) that had been treated with silicone release agents so that the thickness after solvent drying would be 50 μm. After application, the composition was placed in a dryer heated to 90°C and held there for 10 minutes to volatilize the solvent contained in the pressure-sensitive adhesive composition.

[0124] Thereafter, a 75 μm thick release film (PET film manufactured by Mitsubishi Chemical Corporation) that had been treated with silicone release was laminated on the surface of the pressure-sensitive adhesive composition from which the solvent had been dried, and the pressure-sensitive adhesive composition was irradiated with ultraviolet light using a high-pressure mercury lamp through the release film so that the integrated light intensity at a wavelength of 365 nm was the irradiation amount shown in Table 2, thereby obtaining a pressure-sensitive adhesive sheet with release films laminated on both the front and back sides of the pressure-sensitive adhesive sheet.

[0125] <Measurement / Evaluation of Pressure-Sensitive Adhesive Sheet> The pressure-sensitive adhesive sheets obtained in the examples and comparative examples were measured and evaluated as follows.

[0126] <Dynamic Viscoelasticity> The release films were removed from the pressure-sensitive adhesive sheets with release films produced in the Examples and Comparative Examples, and multiple layers of pressure-sensitive adhesive sheets were laminated together to form a laminate with a thickness of 0.8 mm. A cylinder with a diameter of 10 mm (height of 1.0 mm) was punched out from the resulting laminate, and this was used as a measurement sample. Dynamic viscoelasticity measurements were performed on this measurement sample using a viscoelasticity measuring device (manufactured by T.A. Instruments, product name "DHR 20") and a φ8 mm parallel plate in shear mode at a frequency of 1 Hz and a strain of 0.1%, and the storage shear modulus (G'), loss shear modulus (G"), and loss tangent (tan δ) at each temperature were obtained.

[0127] (Measurement conditions) Adhesive jig: φ8 mm parallel plate, Distortion: 0.1%, Frequency: 1 Hz, Measurement temperature: -50 to 100°C, Heating rate: 5°C / min

[0128] <Gel Fraction> The release film was removed from the pressure-sensitive adhesive sheets with release films prepared in the Examples and Comparative Examples, and multiple layers of pressure-sensitive adhesive sheets were laminated to form a 0.8 mm thick laminate. The resulting laminate was used as a measurement sample. This measurement sample was immersed in ethyl acetate for 24 hours, then dried at 70°C for 4.5 hours, and the mass fraction of the remaining gel component was determined and used as the gel fraction. The result was the average of the measured values ​​of two samples.

[0129] <Adhesive strength> One release film was peeled off from the pressure-sensitive adhesive sheets with release films prepared in the Examples and Comparative Examples, and the adhesive surface was exposed. The adhesive surface was irradiated with an integrated light intensity of 2 J / cm at a wavelength of 365 nm using a high-pressure mercury lamp. 2 A 50 μm thick polyester film (PET film manufactured by Mitsubishi Chemical Corporation) that had been UV-irradiated to the desired thickness was roll-laminated with a hand roller as a backing film. This was cut into 10 mm wide strips, the remaining release film was peeled off, and the exposed adhesive surface was roll-laminated onto glass with a hand roller. The sample was then autoclaved (60 ° C, gauge pressure 0.2 MPa, 20 minutes) for finish lamination to prepare a sample for adhesive strength measurement consisting of glass / adhesive sheet / polyester film. The polyester film and adhesive sheet were peeled from the glass at a peel angle of 180 ° and a peel rate of 300 mm / min in an environment of 23 ° C and 50% RH, and the peel strength (N / cm) at the interface between the glass and the adhesive sheet was measured. The results were the average of the measured values ​​for three samples.

[0130] <Strain recovery> The release film was removed from the pressure-sensitive adhesive sheet with release film prepared in the Examples and Comparative Examples, and multiple layers of pressure-sensitive adhesive sheets were laminated to obtain a 0.8 mm thick laminate. A cylinder with a diameter of 10 mm (height 1.0 mm) was punched out from the obtained laminate, and this was used as a measurement sample. The measurement sample was strained to 400% at 25 ° C. using a viscoelasticity measuring device (manufactured by T.A. Instruments, product name "DHR 20") and a φ8 mm parallel plate, and held for 10 minutes at a strain (%) according to the following formula: Strain (%) = Radius (r) × Twist angle (θ) / Thickness (l) × 100. The strain was then released, and the resulting strain recovery was measured. The strain recovery rate (400%, 1 minute) was calculated using the following formula from the strain recovery value 1 minute after the strain was released. Strain recovery rate (400%, 1 minute) = (strain recovery value after 1 minute (%) / 400 (%))

[0131] <Low-Temperature Flexural Durability> The release film was removed from the pressure-sensitive adhesive sheets with release film prepared in the Examples and Comparative Examples, and a 23 μm-thick cyclic olefin polymer (COP) film and a 50 μm-thick transparent polyimide film (CPI) were roll-laminated using a hand roller. The sheets were then autoclaved (60°C, gauge pressure 0.2 MPa, 20 minutes) for finish lamination to produce a three-layer laminate sheet consisting of COP / pressure-sensitive adhesive sheet / CPI. The resulting laminate was cut into 40 mm-wide strips to serve as measurement samples. A bending test was performed on the measurement samples using a bending environment tester (manufactured by Yuasa System Co., Ltd., product name "ETS with CHAMBER CL09 type-D01") with the COP side folded inward under the conditions of a temperature of -20°C, a bending R=2, a bending speed of 60 r / min, and a number of bending times of 100,000.

[0132] The measurement samples after the bending test were evaluated as follows: ◯ (Good): No cracks were observed in the film at the bent portion of the measurement sample after the bending test, confirming excellent bending durability at low temperatures. × (Poor): Cracks were observed in the film at the bent portion of the measurement sample after the bending test, confirming poor bending durability at low temperatures.

[0133] The results obtained by the measurement and evaluation of the pressure-sensitive adhesive sheet are shown in Table 3.

[0134]

[0135] The adhesive sheet of the example is formed from an adhesive composition containing a (meth)acrylic acid ester copolymer of a specific composition and a crosslinking agent, and therefore has both a low storage modulus at low temperatures and strain recovery properties.It has excellent durability that prevents the component sheet from cracking when repeatedly folded at low temperatures, and excellent recovery that quickly restores to a flat state after folding operations, making it suitable for use as an adhesive sheet for flexible image display devices.

[0136] On the other hand, the pressure-sensitive adhesive sheets of the comparative examples did not satisfy either or both of the requirements of low storage modulus and strain recovery at low temperatures, and cracks occurred in the component sheets when repeatedly folded at low temperatures, making them inferior as pressure-sensitive adhesive sheets for flexible image display devices.

[0137] Although the above examples show specific embodiments of the present invention, the examples are merely illustrative and should not be construed as limiting. Various modifications that are obvious to those skilled in the art are intended to fall within the scope of the present invention.

[0138] According to the present invention, it is possible to obtain a pressure-sensitive adhesive sheet for flexible image display devices that has a low storage modulus at low temperatures such as -30°C, and therefore has excellent durability (also referred to as "low-temperature flexural durability") that prevents delamination between layers when folded at low temperatures, and has excellent recovery properties (also referred to as "strain recovery") that allow the sheet to quickly return to a flat state after a folding operation. Therefore, the pressure-sensitive adhesive sheet obtained is useful as a pressure-sensitive adhesive sheet for various flexible image display devices such as bendable, foldable, rollable, and stretchable, and is particularly suitable as a pressure-sensitive adhesive sheet for foldable image display devices that are subject to repeated folding.

Claims

1. A pressure-sensitive adhesive sheet formed from a pressure-sensitive adhesive composition containing a (meth)acrylic acid ester copolymer and a crosslinking agent, The (meth)acrylic acid ester copolymer contains, as components constituting the copolymer, (A) a branched or linear alkyl (meth)acrylic acid ester monomer having 1 to 20 carbon atoms, (B) a monomer having an alkylene glycol group and a (meth)acryloyl group in the molecule, and (C) a nitrogen-containing vinyl monomer; A pressure-sensitive adhesive sheet having a storage shear modulus (G'(-30°C)) at -30°C obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz of 250 kPa or less, and a strain recovery rate (400%, 1 minute) of 70% or more.

2. 2. The pressure-sensitive adhesive sheet according to claim 1, wherein the monomer component (A) is a linear alkyl(meth)acrylate monomer having 8 to 12 carbon atoms.

3. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the (meth)acrylic acid ester copolymer further comprises (D) a hydroxyl group-containing monomer and / or a carboxyl group-containing monomer.

4. 3. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive composition contains 20 to 90 parts by mass of the crosslinking agent per 100 parts by mass of the (meth)acrylic acid ester copolymer.

5. 3. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive composition contains 30 to 90 parts by mass of the crosslinking agent per 100 parts by mass of the (meth)acrylic acid ester copolymer.

6. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the crosslinking agent is a (meth)acrylate having an average functionality of 1.0 or more and less than 2.

0.

7. The pressure-sensitive adhesive sheet according to claim 1 , wherein the crosslinking agent comprises a monofunctional urethane (meth)acrylate.

8. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the monomer component (B) is an alkoxypolyalkylene glycol (meth)acrylate.

9. The pressure-sensitive adhesive sheet according to claim 8, wherein the alkoxypolyalkylene glycol (meth)acrylate is contained in an amount of 2 to 4 mass % of all monomer components constituting the (meth)acrylic acid ester copolymer.

10. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the pressure-sensitive adhesive composition contains a photopolymerization initiator.

11. A flexible image display device member having a configuration in which two flexible members are bonded together via the adhesive sheet according to claim 1 or 2.

12. A flexible image display device comprising the flexible image display device member according to claim 11.