Adhesive sheet and flexible image display device

Abstract

Provided is an adhesive sheet having excellent adhesive force while having good flexibility and restorability.　An adhesive sheet having a storage shear modulus at -20°C [G'(-20°C)] of 700 kPa or less, wherein the adhesive sheet is formed from an adhesive composition [I] containing an acrylic polymer (A) and a radical polymerizable compound (B); the acrylic polymer contains a structural unit derived from an alkyl (meth)acrylate (a1) in which the alkyl group has 5-20 carbon atoms and a structural unit derived from a hydroxyl group-containing (meth)acrylate (a2); and the radical polymerizable compound contains a di(meth)acrylate (B1) having a C2-4 alkylene group.

Description

Adhesive sheet and flexible image display device

[0001] The present invention relates to an adhesive sheet, particularly an adhesive sheet that can be suitably used for bonding components of curved image display devices and bendable flexible image display devices, and a flexible image display device using the adhesive sheet.

[0002] In recent years, curved image display devices and foldable flexible image display devices using organic light-emitting diodes (OLEDs) and quantum dots (QDs) have been developed and are becoming widely commercialized. Such image display devices have a laminated structure in which multiple component sheets, such as a cover lens, a circular polarizer, a touch film sensor, and a light-emitting element, are bonded together with a transparent adhesive sheet, and each laminated structure can be considered as a laminated sheet formed by stacking component sheets and adhesive sheets.

[0003] Foldable flexible image display devices have various problems caused by interlayer stress when folded. For example, layers may peel off when folded (delamination: the phenomenon in which layers peel off is called "delamination"). Therefore, a laminated sheet that does not peel off when folded is required.

[0004] There is also a need for a laminate sheet that quickly restores to a flat state when the screen is unfolded from a folded state without any residual effects from being left 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, which can cause cracks and eventually break, so there is a need for a laminate sheet that is durable against repeated folding operations at low temperatures, which are particularly severe conditions.

[0005] Regarding foldable flexible image display devices, for example, Patent Document 1 discloses a pressure-sensitive adhesive composition for foldable displays, including a thermosetting resin and a crosslinking agent, wherein the thermosetting resin contains units derived from a compound containing at least one N or O and at least one unshared electron pair in the molecule, and the thermosetting resin has a glass transition temperature of −70°C or lower. It also discloses a pressure-sensitive adhesive composition for foldable displays, a pressure-sensitive adhesive film using the same, and a foldable display including the same. Specifically, the pressure-sensitive adhesive composition for foldable displays includes a thermosetting resin copolymerized with carbitol acrylate, ethylhexyl acrylate, and acrylic acid, and a composition containing an epoxy-based crosslinking agent or an isocyanate-based crosslinking agent. Patent Documents 2 and 3 also disclose pressure-sensitive adhesives that aim to improve durability and step-following ability by focusing on distortion and distortion recovery when shear force is applied.

[0006] JP 2021-500445 A JP 2020-196903 A JP 2020-143284 A

[0007] However, the adhesive film containing a large amount of carbitol acrylate disclosed in Patent Document 1 has a small storage modulus at low temperatures, so it can reduce stress caused by folding, but on the other hand, it has a problem that it has low adhesive strength and is prone to delamination with the component sheet when folded, particularly at low temperatures.In addition, because carbitol acrylate easily relieves internal stress by internal rotation around the ether bond, it also has a problem that fold marks are difficult to remove when folding is performed.

[0008] Furthermore, although Patent Documents 2 and 3 aim to improve durability and step-following ability, they are not related to pressure-sensitive adhesive sheets used for bonding components of flexible image display devices, and do not take into consideration specific problems such as delamination and creases caused by a high elastic modulus when folding is performed in a low-temperature environment, in particular. Patent Documents 2 and 3 do not solve these problems.

[0009] Therefore, the present invention relates to an adhesive sheet having a low storage shear modulus at low temperatures, which is formed from an adhesive composition containing an acrylic polymer, and which has the restoring property (also referred to as "strain recovery") to return to a flat state when folded, while having improved adhesive strength so that delamination does not occur when folded, particularly an adhesive sheet used for bonding components of a flexible image display device, and further provides a flexible image display device using the same.

[0010] However, the present inventors have conducted extensive research in light of these circumstances and have found that a pressure-sensitive adhesive sheet having a storage shear modulus at -20°C [G'(-20°C)] of not more than a predetermined value, the pressure-sensitive adhesive sheet being formed from a pressure-sensitive adhesive composition containing an acrylic polymer and a radically polymerizable compound, using an acrylic polymer containing a relatively long-chain alkyl group and a hydroxyl group as the acrylic polymer, and further using in combination a di(meth)acrylate having an alkylene group with a relatively short chain length, can have good flexibility and recovery properties while still improving adhesive strength.

[0011] That is, the present invention has the following aspects. [1] A pressure-sensitive adhesive sheet having a storage shear modulus at -20°C [G'(-20°C)] of 700 kPa or less, the pressure-sensitive adhesive sheet being formed from a pressure-sensitive adhesive composition [I] containing an acrylic polymer (A) and a radically polymerizable compound (B), the acrylic polymer (A) comprising a structural moiety derived from an alkyl(meth)acrylate (a1) having an alkyl group of 5 to 20 carbon atoms and a structural moiety derived from a hydroxyl group-containing (meth)acrylate (a2), and the radically polymerizable compound (B) comprising a di(meth)acrylate (B1) having an alkylene group of 2 to 4 carbon atoms. [2] The pressure-sensitive adhesive sheet according to [1], having a storage shear modulus at -20°C [G'(-20°C)] of 500 kPa or less. [3] The pressure-sensitive adhesive sheet according to [1] or [2], wherein the alkyl(meth)acrylate (a1) is a linear alkyl(meth)acrylate. [4] The pressure-sensitive adhesive sheet according to any one of [1] to [3], wherein the weight-average molecular weight of the acrylic polymer (A) is 600,000 to 1,500,000. [5] The pressure-sensitive adhesive sheet according to any one of [1] to [4], wherein the glass transition temperature (Tg) of the acrylic polymer (A), defined at the maximum point of the loss tangent (tan δ) as measured by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, is −50°C or higher and −10°C or lower. [6] The pressure-sensitive adhesive sheet according to any one of [1] to [4], wherein the glass transition temperature (Tg) of the acrylic polymer (A), defined at the maximum point of the loss tangent (tan δ) as measured by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, is −50°C or higher and lower than −25°C. [7] The pressure-sensitive adhesive sheet according to any one of [1] to [6], wherein the di(meth)acrylate (B1) is a di(meth)acrylate having a linear alkylene group. [8] The pressure-sensitive adhesive sheet according to any one of [1] to [7], wherein the content of the radically polymerizable compound (B) is 0.1 to 10 parts by mass relative to 100 parts by mass of the acrylic polymer (A). [9] The pressure-sensitive adhesive sheet according to any one of [1] to [8], wherein the pressure-sensitive adhesive sheet has a gel fraction of 30 to 95% by mass.

[10] The pressure-sensitive adhesive sheet according to any one of [1] to [8], wherein the gel fraction of the pressure-sensitive adhesive sheet is 30 to 65% by mass.

[11] The pressure-sensitive adhesive sheet according to any one of [1] to

[10] , which is used for bonding components of a flexible image display device.

[12] A flexible image display device having the pressure-sensitive adhesive sheet according to any one of [1] to

[11] .

[0012] The adhesive sheet of the present invention is an adhesive sheet having a low storage shear modulus at low temperatures, and is formed from an adhesive composition containing a specific alkyl group- and hydroxyl group-containing acrylic polymer and a specific di(meth)acrylate. Therefore, the adhesive sheet has good flexibility and resilience while also having improved adhesive strength, and can be particularly suitably used as an adhesive sheet for use in flexible image display devices.

[0013] The present invention will be described in detail below. In the present invention, the term "film" includes the term "sheet," and the term "sheet" includes the term "film." Furthermore, when a "panel" is used, such as an image display panel or a protective panel, it includes a plate, a sheet, and a film.

[0014] In the present invention, 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 means "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.

[0015] In the present invention, the term "main component" refers to a component that significantly affects the properties of the target object. The content of the component is typically 30% by mass or more, preferably 35% by mass or more, and more preferably 50% by mass or more in the target object. Furthermore, the component is often the component that occupies the largest mass ratio in the target object. It is expected that the component occupies 50% by mass or more, particularly 55% by mass or more, of which 60% by mass or more, of which 70% by mass or more, of which 80% by mass or more, and of which 90% by mass or more (including 100% by mass) are considered to be the components that occupy 50% by mass or more, particularly 55% by mass or more, of which 60% by mass or more, of which 70% by mass or more, of which 80% by mass or more, and of which 90% by mass or more (including 100% by mass). In the present invention, "(meth)acrylic" encompasses "acrylic" and "methacrylic," "(meth)acrylate" encompasses "acrylate" and "methacrylate," and "(meth)acryloyl" encompasses "acryloyl" and "methacryloyl." Furthermore, "acrylic polymer" encompasses a polymer containing a monomer unit derived from (meth)acrylate, and includes (meth)acrylic copolymers.

[0016] <<Present Adhesive Sheet>> A pressure-sensitive adhesive sheet according to one embodiment of the present invention (also referred to as "the present adhesive sheet") is a pressure-sensitive adhesive sheet formed from a pressure-sensitive adhesive composition [I] containing an acrylic polymer (A) and a radically polymerizable compound (B), and is particularly useful as a pressure-sensitive adhesive sheet used for bonding components of flexible image display devices.

[0017] <<Pressure-Sensitive Adhesive Composition [I]>> The pressure-sensitive adhesive composition [I] contains an acrylic polymer (A) and a radically polymerizable compound (B), and preferably contains the acrylic polymer (A) as a main component.

[0018] <Acrylic Polymer (A)> The acrylic polymer (A) used in the present pressure-sensitive adhesive sheet is an acrylic polymer containing a structural moiety derived from an alkyl(meth)acrylate (a1) having an alkyl group with 5 to 20 carbon atoms and a structural moiety derived from a hydroxyl group-containing (meth)acrylate (a2). Preferably, the acrylic polymer (A) is obtained by copolymerizing the alkyl(meth)acrylate (a1) having an alkyl group with 5 to 20 carbon atoms and the hydroxyl group-containing (meth)acrylate (a2) as copolymerization components constituting the acrylic polymer (A). Alternatively, the acrylic polymer (A) may be obtained by copolymerizing the alkyl(meth)acrylate (a1) having an alkyl group with 5 to 20 carbon atoms and the hydroxyl group-containing (meth)acrylate (a2) with another monomer component (a3) ​​as the copolymerization component.

[0019] <Alkyl(meth)acrylate (a1) in which the alkyl group has 5 to 20 carbon atoms> Examples of the alkyl(meth)acrylate (a1) in which the alkyl group has 5 to 20 carbon atoms include linear alkyl(meth)acrylates such as n-pentyl(meth)acrylate, n-hexyl(meth)acrylate, n-heptyl(meth)acrylate, n-octyl(meth)acrylate, n-nonyl(meth)acrylate, and n-decyl(meth)acrylate; branched alkyl(meth)acrylates such as isopentyl(meth)acrylate, neopentyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, isooctyl(meth)acrylate, isononyl(meth)acrylate, and isodecyl(meth)acrylate; and alicyclic (meth)acrylates such as cyclohexyl(meth)acrylate and t-butylcyclohexyl(meth)acrylate. These may be used alone or in combination of two or more.

[0020] Among these, linear alkyl (meth)acrylates or branched alkyl (meth)acrylates are preferred from the viewpoint of adhesiveness, and linear or branched alkyl (meth)acrylates having an alkyl group with 6 to 18 carbon atoms, more preferably 6 to 16, and even more preferably 8 to 12 carbon atoms are preferred.

[0021] Among these, linear alkyl (meth)acrylates are preferred from the viewpoint of adhesion and recovery, and in particular, linear alkyl (meth)acrylates having an alkyl group with 6 to 18 carbon atoms, more preferably 6 to 16, and even more preferably 8 to 12 carbon atoms are preferred from the viewpoint of suppressing an increase in storage shear modulus (G') at low temperatures and improving flexibility, such as n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, n-nonyl (meth)acrylate, and decyl (meth)acrylate. Of these, n-octyl (meth)acrylate, n-nonyl (meth)acrylate, and n-decyl (meth)acrylate are preferred, with n-octyl (meth)acrylate being particularly preferred.

[0022] Acrylates are particularly preferred from the viewpoint of suppressing an increase in storage shear modulus (G') at low temperatures and improving flexibility.

[0023] In the present pressure-sensitive adhesive sheet, the alkyl (meth)acrylate (a1) having 5 to 20 carbon atoms in the alkyl group preferably accounts for 50 to 95 mass % of all copolymerization components constituting the acrylic polymer (A) in order to suppress an increase in the storage shear modulus (G') at low temperatures, more preferably 60 to 90 mass %, and particularly preferably 70 to 85 mass %. When the proportion of the alkyl (meth)acrylate (a1) is equal to or greater than the lower limit, an increase in the storage shear modulus (G') at low temperatures can be suppressed, and when it is equal to or less than the upper limit, it is preferred in terms of achieving compatibility with other physical properties such as adhesiveness.

[0024] <Hydroxyl Group-Containing (Meth)acrylate (a2)> Examples of the hydroxyl group-containing (meth)acrylate (a2) include hydroxy(meth)acrylates such as 2-hydroxyethyl(meth)acrylate, 4-hydroxybutyl(meth)acrylate, 5-hydroxypentyl(meth)acrylate, 6-hydroxyhexyl(meth)acrylate, and 8-hydroxyoctyl(meth)acrylate; caprolactone-modified hydroxy(meth)acrylates such as caprolactone-modified 2-hydroxyethyl(meth)acrylate; diethylene glycol(meth)acrylate; polyethylene glycol (meth)acrylate; Examples of such hydroxyl group-containing (meth)acrylates include oxyalkylene-modified (meth)acrylates such as ethylene glycol (meth)acrylate, primary hydroxyl group-containing (meth)acrylates such as 2-acryloyloxyethyl-2-hydroxyethyl phthalate, secondary hydroxyl group-containing (meth)acrylates such as 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, and 3-chloro-2-hydroxypropyl (meth)acrylate, and tertiary hydroxyl group-containing (meth)acrylates such as 2,2-dimethyl 2-hydroxyethyl (meth)acrylate. These may be used alone or in combination of two or more.

[0025] Among the hydroxyl group-containing (meth)acrylates (a2), in terms of reducing the storage shear modulus (G') at low temperatures, primary hydroxyl group-containing (meth)acrylates, such as 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxypropyl (meth)acrylate, are preferred, and particularly hydroxyl group-containing (meth)acrylates having a hydroxyalkyl group having 1 to 10 carbon atoms, more preferably 1 to 6, and even more preferably 2 to 4 carbon atoms, such as 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are preferred, with 2-hydroxyethyl (meth)acrylate being particularly preferred.

[0026] From the viewpoint of adhesive strength, the content of the hydroxyl group-containing (meth)acrylate (a2) is preferably 5 to 50 mass %, more preferably 10 to 40 mass %, and particularly preferably 15 to 30 mass %, based on the total copolymerization components of the acrylic polymer (A). When the content of the hydroxyl group-containing (meth)acrylate (a2) is equal to or greater than the lower limit, high adhesiveness can be obtained, and when it is equal to or less than the upper limit, an increase in the storage shear modulus (G') at low temperatures can be suppressed, which is preferable.

[0027] In the present pressure-sensitive adhesive sheet, a monomer component (a3) ​​(excluding the above-mentioned components (a1) and (a2)) copolymerizable with the alkyl (meth)acrylate (a1) and / or hydroxyl group-containing (meth)acrylate (a2) in which the alkyl group has 5 to 20 carbon atoms can also be used in combination. Examples of such monomer component (a3) ​​include ethylenically unsaturated group monomers having a functional group other than a hydroxyl group, alkyl (meth)acrylates containing an alkyl group having 1 to 4 carbon atoms or more than 20 carbon atoms, and other copolymerizable monomers. These can be used alone or in combination of two or more types.

[0028] Examples of the ethylenically unsaturated group monomer having a functional group other than a hydroxyl group (hereinafter sometimes referred to as "functional group-containing ethylenically unsaturated monomer") include a functional group-containing monomer having a nitrogen atom, a carboxy group-containing monomer, an acetoacetyl group-containing monomer, a glycidyl group-containing monomer, etc. Among these, from the viewpoint of imparting cohesive strength and a crosslinking promoting effect, a functional group-containing monomer having a nitrogen atom is preferred, an amino group-containing monomer, an amide group-containing monomer, or an isocyanate group-containing monomer is more preferred, and an amino group-containing monomer is even more preferred.

[0029] Examples of the amino group-containing monomer as the nitrogen atom-containing functional group-containing monomer include primary amino group-containing (meth)acrylates such as aminomethyl (meth)acrylate and aminoethyl (meth)acrylate; secondary amino group-containing (meth)acrylates such as t-butylaminoethyl (meth)acrylate and t-butylaminopropyl (meth)acrylate; and tertiary amino group-containing (meth)acrylates such as ethylaminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, diethylaminopropyl (meth)acrylate, and dimethylaminopropylacrylamide.

[0030] Examples of the amide group-containing monomer include (meth)acrylamide; N-alkyl(meth)acrylamides such as N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-propyl(meth)acrylamide, N-n-butyl(meth)acrylamide, diacetone(meth)acrylamide, and N,N'-methylenebis(meth)acrylamide; N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, N,N-ethylmethylacrylamide, and N,N-diallyl(meth)acrylamide; hydroxyalkyl(meth)acrylamides such as N-hydroxymethyl(meth)acrylamide and N-hydroxyethyl(meth)acrylamide; and alkoxyalkyl(meth)acrylamides such as N-methoxymethyl(meth)acrylamide and N-(n-butoxymethyl)(meth)acrylamide.

[0031] Examples of the isocyanate group-containing monomer include 2-(meth)acryloyloxyethyl isocyanate and alkylene oxide adducts thereof, etc. The isocyanate group may be protected with a blocking agent such as methyl ethyl ketone oxime, 3,5-dimethylpyrazole, 1,2,4-triazole, or diethyl malonate.

[0032] Examples of the carboxy group-containing monomer include (meth)acrylic acid, carboxyethyl (meth)acrylate, 2-(meth)acryloyloxyethyl hexahydrophthalic acid, 2-(meth)acryloyloxypropyl hexahydrophthalic 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, monomethyl maleate, and monomethyl itaconate.

[0033] Examples of the acetoacetyl group-containing monomer include 2-(acetoacetoxy)ethyl (meth)acrylate and allyl acetoacetate.

[0034] Examples of the glycidyl group-containing monomer include glycidyl (meth)acrylate and allyl glycidyl (meth)acrylate.

[0035] These functional group-containing ethylenically unsaturated monomers may be used alone or in combination of two or more. From the viewpoint of reducing the decrease in adhesiveness due to bleed-out, the upper limit of the content of the functional group-containing ethylenically unsaturated monomer is preferably 30% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less, based on the total copolymerization components of the acrylic polymer (A). The lower limit is usually 0% by mass.

[0036] Examples of the alkyl (meth)acrylate containing an alkyl group having 1 to 4 carbon atoms or more than 20 carbon atoms include linear alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, and icosyl (meth)acrylate; and branched alkyl (meth)acrylates such as isopropyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl (meth)acrylate, t-butyl (meth)acrylate, and isoicosyl (meth)acrylate. These may be used alone or in combination of two or more.

[0037] Furthermore, in the case where the alkyl (meth)acrylate containing the alkyl group having 1 to 4 carbon atoms or more than 20 carbon atoms is contained, the upper limit of the content is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, based on the total copolymerization components of the acrylic polymer (A), from the viewpoint of maintaining recovery properties. The lower limit is usually 0% by mass.

[0038] Examples of the other copolymerizable monomers include aromatic (meth)acrylates such as phenyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenyl diethylene glycol (meth)acrylate, phenoxy polyethylene glycol (meth)acrylate, phenoxy polyethylene glycol-polypropylene glycol-(meth)acrylate, and nonylphenol ethylene oxide adduct (meth)acrylate; 4-acryloyloxybenzophenone, 4-acryloyloxyethoxybenzophenone, 4-acryloyloxy-4'-methoxybenzophenone, 4-acryloyloxyethoxy-4'-methoxybenzophenone, 4-acryloyloxy-4'-bromobenzophenone, 4-acryloyloxyethoxy-4'-bromobenzophenone, 4-methacryloyloxybenzophenone, and 4-methacryloyloxyethoxybenzophenone. Examples of vinyl monomers include (meth)acrylates having a benzophenone structure such as benzophenone, 4-methacryloyloxy-4'-methoxybenzophenone, 4-methacryloyloxyethoxy-4'-methoxybenzophenone, 4-methacryloyloxy-4'-bromobenzophenone, 4-methacryloyloxyethoxy-4'-bromobenzophenone, and mixtures thereof, acrylonitrile, methacrylonitrile, styrene, α-methylstyrene, vinyl stearate, vinyl propionate, vinyl acetate, vinyl chloride, vinylidene chloride, alkyl vinyl ethers, vinyl toluene, vinylpyridine, vinylpyrrolidone, itaconate dialkyl esters, fumarate dialkyl esters, allyl alcohol, acrylic chloride, methyl vinyl ketone, N-acrylamidomethyltrimethylammonium chloride, allyltrimethylammonium chloride, and dimethylallyl vinyl ketone. These may be used alone or in combination of two or more.

[0039] Among these, the inclusion of a nitrogen-containing vinyl monomer such as vinylpyridine or vinylpyrrolidone tends to form hydrogen bonds with the hydroxyl group-containing (meth)acrylate (a2), thereby increasing the adhesive strength and cohesiveness of the pressure-sensitive adhesive sheet. When the other copolymerizable monomer is included, the upper limit is preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less, based on the total copolymerization components of the acrylic polymer (A), from the viewpoint of improving flexibility and stress relaxation properties. The lower limit is usually 0% by mass.

[0040] The acrylic polymer (A) can be obtained by copolymerizing the various monomer components according to a conventionally known polymerization method, such as solution radical polymerization, suspension polymerization, bulk polymerization, or emulsion polymerization.

[0041] The acrylic polymer (A) may have a photoactive moiety, such as a polymerizable carbon-carbon double bond group, introduced into its side chain, thereby increasing the crosslinking efficiency of the pressure-sensitive adhesive composition [I] and enabling the pressure-sensitive adhesive composition [I] to be crosslinked in a shorter time, thereby increasing productivity.

[0042] Examples of a method for introducing a polymerizable carbon double bond group into the side chain of the acrylic polymer (A) include a method in which a copolymer containing the above-mentioned hydroxyl group-containing (meth)acrylate (a2) or a functional group-containing ethylenically unsaturated monomer is prepared, and then a compound having a polymerizable carbon double bond group and a functional group reactive with the functional group is subjected to a condensation or addition reaction while maintaining the activity of the polymerizable carbon double bond group.

[0043] Examples of combinations of these functional groups include an epoxy group (glycidyl group) and a carboxy group, an amino group and a carboxy group, an amino group and an isocyanate group, an epoxy group (glycidyl group) and an amino group, a hydroxyl group and an epoxy group, and a hydroxyl group and an isocyanate group. Among these combinations of functional groups, a combination of a hydroxyl group and an isocyanate group is preferred because of the ease of reaction control. Of these, a combination in which the copolymer has a hydroxyl group and the compound has an isocyanate group is preferred.

[0044] Examples of the isocyanate compound having a polymerizable carbon-carbon double bond group include the above-mentioned 2-(meth)acryloyloxyethyl isocyanate and alkylene oxide adducts thereof.

[0045] From the viewpoint of improving adhesiveness and stress relaxation properties, the content of the compound having a functional group reactive with the functional group and a polymerizable carbon-carbon double bond group is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, even more preferably 1 part by mass or less, and particularly preferably 0.1 parts by mass or less, relative to 100 parts by mass of the acrylic polymer (A). The lower limit is usually 0 part by mass.

[0046] The glass transition temperature (Tg) of the acrylic polymer (A), which is defined as the maximum point of the loss tangent (tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, is preferably −10° C. or lower, more preferably −20° C. or lower, even more preferably less than −25° C., particularly preferably −27° C. or lower, and most preferably −30° C. or lower, in order to suppress an increase in the storage shear modulus (G′) at low temperatures. Note that, in view of concerns about adhesive overflow and the like, the lower limit of the glass transition temperature (Tg) is usually −50° C., preferably −45° C.

[0047] In this pressure-sensitive adhesive sheet, the glass transition temperature (Tg) of the acrylic polymer (A) is determined by reading the temperature at which the loss tangent (loss shear modulus G" / storage shear modulus G'=tan δ) becomes maximum when dynamic viscoelasticity is measured using a dynamic viscoelasticity measuring device in a shear mode at a frequency of 1 Hz. For example, the acrylic polymer (A) is molded into a cylindrical body with a diameter of 8 mm (height of 1.0 mm), and the loss tangent (tan δ) of this can be measured using a viscoelasticity measuring device (manufactured by T.A. Instruments, product name "DHR 2") under the following measurement conditions.

[0048] (Measurement conditions) Measurement jig: Φ8 mm parallel plate Distortion: 0.1% Frequency: 1 Hz Measurement temperature: -60 to 100°C Heating rate: 5°C / min

[0049] The theoretical Tg of the acrylic polymer (A) is preferably −52° C. or lower, more preferably −54° C. or lower, and even more preferably −56° C. or lower, from the viewpoint of suppressing an increase in the storage shear modulus (G′) at low temperatures. In addition, due to concerns about adhesive overflow and the like, the lower limit of the theoretical Tg of the acrylic polymer (A) is usually −70° C., preferably −65° C.

[0050] The theoretical Tg of the acrylic polymer (A) means a value calculated by the Fox formula from the glass transition temperature and composition ratio of the polymer obtained from the homopolymer of each component of the copolymer. The Fox formula is a calculated value obtained by the following formula, and can be obtained using the values ​​described in Polymer Handbook [Polymer Handbook, J. Brandrup, Interscience, 1989]. (Formula) 1 / (273+Tg)=Σ(Wi / (273+Tgi)) [where Wi is the weight fraction of monomer i, and Tgi is the Tg (°C) of the homopolymer of monomer i.]

[0051] The weight-average molecular weight (Mw) of the acrylic polymer (A) is preferably 600,000 or more, more preferably 700,000 or more, and even more preferably 800,000 or more, from the viewpoint of obtaining a pressure-sensitive adhesive composition [I] with high cohesive strength. The upper limit of the weight-average molecular weight (Mw) of the acrylic polymer (A) is preferably 1,500,000 or less, more preferably 1,200,000 or less, and even more preferably 1,100,000 or less, from the viewpoint of handleability and uniform stirrability.

[0052] 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 acrylic polymer (A) 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

[0053] <Radical Polymerizable Compound (B)> The pressure-sensitive adhesive composition [I] contains a radically polymerizable compound (B) in addition to the acrylic polymer (A). This allows the pressure-sensitive adhesive composition [I] to form a crosslinked structure, which can impart cohesive strength and high recovery upon bending to the pressure-sensitive adhesive layer (pressure-sensitive adhesive sheet). The pressure-sensitive adhesive layer has appropriate cohesive strength, which can prevent glue from leaking out when wound into a roll and maintain good adhesiveness. Furthermore, the high recovery upon bending can improve fold marks and prevent delamination at bent portions.

[0054] It is important that the radical polymerizable compound (B) contains a di(meth)acrylate (B1) having an alkylene group having 2 to 4 carbon atoms (hereinafter, may be abbreviated as "di(meth)acrylate (B1)"), which allows the adhesive to have good flexibility and recovery properties while further improving adhesiveness.

[0055] As the di(meth)acrylate (B1), either a di(meth)acrylate having a linear alkylene group or a di(meth)acrylate having a branched alkylene group can be used, but from the viewpoint of recovery, a di(meth)acrylate having a linear alkylene group is preferred. Specific examples of such di(meth)acrylates having a linear alkylene group include ethanediol di(meth)acrylate, propanediol di(meth)acrylate, and butanediol di(meth)acrylate. Among these, butanediol di(meth)acrylate is preferred from the viewpoints of adhesion, versatility, recovery, and low storage shear modulus (G') at low temperatures. Furthermore, from the viewpoint of suppressing an increase in storage shear modulus (G') at low temperatures and improving flexibility, acrylates are particularly preferred. These can be used alone or in combination of two or more.

[0056] The radical polymerizable compound (B) may also be used in combination with a radical polymerizable compound (B2) other than the di(meth)acrylate (B1). Examples of the radical polymerizable compound (B2) include, other than the di(meth)acrylate (B1), di(meth)acrylates having an alkylene group having 1 or 5 or more carbon atoms, and (meth)acrylic monomers and (meth)acrylic oligomers having two or more functional groups. These may be used alone or in combination of two or more.

[0057] Examples of the di(meth)acrylate having an alkylene group having 1 carbon atom or 5 or more carbon atoms include methanediol di(meth)acrylate, pentanediol di(meth)acrylate, hexadiol di(meth)acrylate, heptanediol di(meth)acrylate, octanediol di(meth)acrylate, nonanediol di(meth)acrylate, decanediol di(meth)acrylate, undecanediol di(meth)acrylate, and dodecanediol di(meth)acrylate.

[0058] Examples of the (meth)acrylic monomer having two or more functional groups include glycerin di(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerin glycidyl ether di(meth)acrylate, tricyclodecane dimethacrylate, tricyclodecane dimethanol di(meth)acrylate, bisphenol A polyethoxy di(meth)acrylate, bisphenol A polypropoxy di(meth)acrylate, bisphenol F polyethoxy di(meth)acrylate, ethylene glycol dimeth ... Ethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, trimethylolpropane trioxyethyl (meth)acrylate, ε-caprolactone-modified tris(2-hydroxyethyl) isocyanurate tri(meth)acrylate, pentaerythritol tri(meth)acrylate, propoxylated pentaerythritol tri(meth)acrylate, ethoxylated pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, propoxylated pentaerythritol tri(meth)acrylate, Ethoxylated pentaerythritol tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, tris(acryloxyethyl)isocyanurate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol penta(meth)acrylate acrylate, tripentaerythritol hexa(meth)acrylate, tripentaerythritol penta(meth)acrylate, hydroxypivalic acid neopentyl glycol di(meth)acrylate, di(meth)acrylate of hydroxypivalic acid neopentyl glycol adduct ε-caprolactone, trimethylolpropane tri(meth)acrylate, trimethylolpropane polyethoxytri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, and the like.

[0059] Examples of the (meth)acrylic oligomer having two or more functional groups include polyfunctional (meth)acrylic oligomers such as polyester (meth)acrylate oligomers, epoxy (meth)acrylate oligomers, urethane (meth)acrylate oligomers, polyether (meth)acrylate oligomers, etc. Among these, urethane (meth)acrylate oligomers are preferred from the viewpoint of imparting appropriate toughness to the cured product.

[0060] The content of the radical polymerizable compound (B) is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 1 part by mass or more, relative to 100 parts by mass of the acrylic polymer (A) from the viewpoint of imparting shape stability to the PSA sheet and durability when formed into a laminate sheet. The upper limit is preferably 10 parts by mass or less, more preferably 7 parts by mass or less, particularly preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less, from the viewpoint of reducing the storage shear modulus (G') at low temperatures.

[0061] It is also important to use the di(meth)acrylate (B1) as the radical polymerizable compound (B), and the content of the di(meth)acrylate (B1) is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, even more preferably 0.7 parts by mass or more, and particularly preferably 1 part by mass or more, relative to 100 parts by mass of the acrylic polymer (A). The upper limit is preferably 10 parts by mass or less, more preferably 7 parts by mass or less, particularly preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less, in order to maintain high adhesiveness. It is preferable to use the di(meth)acrylate (B1) as the main component of the radical polymerizable compound (B), and in particular, it is preferable to use only the di(meth)acrylate (B1) as the radical polymerizable compound (B).

[0062] In addition to the radical polymerizable compound (B), a thermal crosslinking agent can be used in combination to further improve crosslink density and long-term reliability. Examples of such thermal crosslinking agents include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, aziridine-based crosslinking agents, melamine-based crosslinking agents, aldehyde-based crosslinking agents, amine-based crosslinking agents, and metal chelate-based crosslinking agents. Among these, it is preferable to use an isocyanate-based crosslinking agent because of its excellent reactivity with the acrylic polymer (A).

[0063] <Photopolymerization initiator (C)> The present pressure-sensitive adhesive sheet preferably further contains a photopolymerization initiator (C) in addition to the acrylic polymer (A) and the radically polymerizable compound (B). The photopolymerization initiator (C) may be any compound that generates radicals when exposed to active energy rays.

[0064] The photopolymerization initiator (C) is roughly classified into two types depending on the radical generation mechanism: a cleavage-type photopolymerization initiator that can generate radicals by cleaving and decomposing the single bond of the initiator itself, and a hydrogen abstraction-type photopolymerization initiator that can transfer hydrogen from the hydrogen donor by forming an exciplex between the excited initiator and the hydrogen donor in the system.

[0065] The photopolymerization initiator (C) may be either a cleavage-type photopolymerization initiator or a hydrogen-abstraction-type photopolymerization initiator, and either one may be used alone or a mixture of the two may be used. Furthermore, one or more types of each may be used in combination.

[0066] In the present pressure-sensitive adhesive sheet, it is preferable to use a hydrogen abstraction photopolymerization initiator, since the acrylic polymer (A) itself does not require a functional group such as a polymerizable carbon-carbon double bond group and crosslinking can be efficiently carried out.

[0067] Examples of the cleavage-type photopolymerization initiator include 2,2-dimethoxy-1,2-diphenylethan-1-one, 1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, 1-(4-(2-hydroxyethoxy)phenyl)-2-hydroxy-2-methyl-1-propan-1-one, 2-hydroxy-1-[4-{4-(2-hydroxy-2-methyl-propionyl)benzyl}phenyl]-2-methyl-propan-1-one, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone), methyl phenylglyoxylate, 2-benzyl-2-dimethylamino- Examples of such compounds include 1-(4-morpholinophenyl)butan-1-one, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, 2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, (2,4,6-trimethylbenzoyl)ethoxyphenylphosphine oxide, bis(2,6-dimethoxybenzoyl)2,4,4-trimethylpentylphosphine oxide, and derivatives thereof.

[0068] Examples of the hydrogen abstraction photopolymerization initiator include benzophenone, 4-methylbenzophenone, 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-t-butylanthraquinone, 2-aminoanthraquinone, and derivatives thereof. Among these, benzophenone-based hydrogen abstraction photopolymerization initiators are preferred, with 4-methylbenzophenone and 2,4,6-trimethylbenzophenone being more preferred.

[0069] The content of the photopolymerization initiator (C) is usually 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, and more preferably 1 to 3 parts by mass, relative to 100 parts by mass of the acrylic polymer (A). When the content is equal to or greater than the lower limit, poor curing tends to be prevented, while when the content is equal to or less than the upper limit, a decrease in solution stability such as precipitation from the pressure-sensitive adhesive composition [I] tends to be easily suppressed, and problems such as embrittlement and coloration tend to be easily suppressed.

[0070] <Other Components> The pressure-sensitive adhesive composition [I] may contain, as "other components", various additives such as plasticizers, silane coupling agents, ultraviolet absorbers, rust inhibitors, tackifying resins, antioxidants, light stabilizers, metal deactivators, antiaging agents, moisture absorbers, rust inhibitors, and inorganic particles, as needed, to the extent that the effects of the present invention are not impaired. Furthermore, the pressure-sensitive adhesive composition [I] may contain, as needed, reaction catalysts such as tertiary amine compounds, quaternary ammonium compounds, and tin laurate compounds. These may be used alone or in combination of two or more.

[0071] (Plasticizer) A plasticizer is a material that improves processability and flexibility by softening a resin with a high elastic modulus. Examples of the plasticizer include monofunctional (meth)acrylic oligomers such as polyester (meth)acrylate, urethane (meth)acrylate, and polyether (meth)acrylate. Among them, urethane (meth)acrylate oligomers are preferred from the viewpoint of imparting appropriate toughness to the cured product.

[0072] (Silane coupling agent) Silane coupling agent is an organic silicon compound, which contains reactive functional group and one or more alkoxy group bonded with silicon atom in its structure.As the reactive functional group, for example, epoxy group, (meth)acryloyl group, mercapto group, hydroxyl group, carboxyl group, amino group, amide group, isocyanate group can be listed, among these, epoxy group and mercapto group are preferred from the viewpoint of balance of durability.

[0073] The alkoxy group bonded to the silicon atom preferably contains an alkoxy group having 1 to 8 carbon atoms, from the viewpoint of durability and storage stability, and is particularly preferably a methoxy group or an ethoxy group. The silane coupling agent may also contain an organic substituent other than the reactive functional group and the alkoxy group bonded to the silicon atom, such as an alkyl group or a phenyl group.

[0074] Examples of the silane coupling agent used in the present pressure-sensitive adhesive sheet include monomeric epoxy group-containing silane coupling agents, which are silane compounds such as 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; and silane coupling agents in which a portion of the silane compound is hydrolyzed and condensed, or in which the silane compound is polymerized with methyltriethoxysilane, ethyltriethoxysilane, methyl ... Oligomeric epoxy group-containing silane coupling agents which are silane compounds obtained by co-condensation of alkyl group-containing silane compounds such as ethoxysilane, methyltrimethoxysilane, and ethyltrimethoxysilane; monomeric mercapto group-containing silane coupling agents which are silane compounds such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, γ-mercaptopropyldimethoxymethylsilane, and 3-mercaptopropylmethyldimethoxysilane; and silane coupling agents which are silane compounds obtained by hydrolysis and condensation polymerization of a part of the silane compounds or by silanization. oligomeric mercapto group-containing silane coupling agents, which are silane compounds obtained by co-condensation of a silane compound with an alkyl group-containing silane compound such as methyltriethoxysilane, ethyltriethoxysilane, methyltrimethoxysilane, or ethyltrimethoxysilane; (meth)acryloyl group-containing silane coupling agents, such as 3-acryloxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, or 3-acryloxypropyltrimethoxysilane; silane coupling agents: amino group-containing silane coupling agents such as N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethyl-butylidene)propylamine, and N-phenyl-3-aminopropyltrimethoxysilane; isocyanate group-containing silane coupling agents such as 3-isocyanatepropyltriethoxysilane;Examples of suitable silane coupling agents include vinyl group-containing silane coupling agents such as vinyltrimethoxysilane and vinyltriethoxysilane. These may be used alone or in combination of two or more.

[0075] Among these, epoxy group-containing silane coupling agents and mercapto group-containing silane coupling agents are preferably used because of their excellent durability, and epoxy group-containing silane coupling agents are particularly preferred.

[0076] The content of the silane coupling agent is preferably 0.005 to 10 parts by mass, particularly preferably 0.01 to 5 parts by mass, and even more preferably 0.05 to 1 part by mass, relative to 100 parts by mass of the acrylic polymer (A). When the content is within the above range, durability tends to be improved.

[0077] (Ultraviolet Absorber) Examples of the ultraviolet absorber include benzophenone-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, triazine-based ultraviolet absorbers, salicylic acid-based ultraviolet absorbers, cyanoacrylate-based ultraviolet absorbers, benzoxazine-based ultraviolet absorbers, etc. These ultraviolet absorbers can be used alone or in combination of two or more.

[0078] The content of the ultraviolet absorber is preferably 0.01 to 20 parts by mass, particularly preferably 0.1 to 15 parts by mass, and even more preferably 0.5 to 10 parts by mass, relative to 100 parts by mass of the acrylic polymer (A). When the content is equal to or greater than the lower limit, lightfastness reliability tends to improve, and when the content is equal to or less than the upper limit, yellowing resistance tends to improve.

[0079] (Rust inhibitor) As the rust inhibitor, for example, triazoles, benzotriazoles, etc. are preferred, which can prevent corrosion of optical components. The content of the rust inhibitor is preferably 0.01 to 5 parts by mass, and more preferably 0.1 to 3 parts by mass, per 100 parts by mass of the acrylic polymer (A).

[0080] The content of the other components is preferably 5 parts by mass or less, particularly preferably 1 part by mass or less, and further preferably 0.5 parts by mass or less, per 100 parts by mass of the acrylic polymer (A). If the content is too high, the compatibility with the acrylic polymer (A) tends to decrease, and durability tends to decrease.

[0081] The pressure-sensitive adhesive composition [I] is prepared by mixing predetermined amounts of the acrylic polymer (A) and the radically polymerizable compound (B), preferably further a photopolymerization initiator (C), and, if necessary, other components such as a silane coupling agent, an ultraviolet absorber, and a rust inhibitor. The pressure-sensitive adhesive composition [I] thus obtained is used for pressure-sensitive adhesive sheets, particularly pressure-sensitive adhesive sheets used for bonding components of flexible image display devices.

[0082] <Configuration> 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 pressure-sensitive adhesive composition [I], or a multi-layer sheet in which a plurality of the present pressure-sensitive adhesive layers are laminated. Alternatively, the present pressure-sensitive adhesive sheet may be a multi-layer sheet in which the present pressure-sensitive adhesive layer and an adhesive layer other than the present pressure-sensitive adhesive layer are laminated.

[0083] <Physical Properties of the Present Pressure-Sensitive Adhesive Sheet> The present pressure-sensitive adhesive sheet can have the following physical properties.

[0084] (Storage Shear Modulus) The pressure-sensitive adhesive sheet preferably has a storage shear modulus [G'(-40°C)] at -40°C, as determined by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, of 50,000 kPa or less. When the pressure-sensitive adhesive sheet has a storage shear modulus [G'(-40°C)] within the above range, for example, when the pressure-sensitive adhesive sheet is attached to a component sheet to form a laminated sheet or a flexible image display device component, the interlayer stress during bending of the laminated sheet or flexible image display device component can be reduced, particularly in a low-temperature environment, and delamination or cracking of the component sheet or flexible component can be suppressed. From this viewpoint, the pressure-sensitive adhesive sheet preferably has a storage shear modulus at -40°C [G'(-40°C)], as determined by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, of 40,000 kPa or less, more preferably 30,000 kPa or less, even more preferably 10,000 kPa or less, particularly preferably 9,000 kPa or less, and most preferably 8,000 kPa or less. The lower limit of the storage shear modulus [G'(-40°C)] of the pressure-sensitive adhesive sheet is preferably 100 kPa or more, in view of the balance with the storage shear modulus at high temperatures.

[0085] The pressure-sensitive adhesive sheet has a storage shear modulus [G'(-20°C)] at -20°C, as determined by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, of 700 kPa or less. Of these, it is preferably 600 kPa or less, more preferably 500 kPa or less, even more preferably 400 kPa or less, and even more preferably 300 kPa or less, and especially preferably 200 kPa or less. The lower limit of the storage shear modulus [G'(-20°C)] of the pressure-sensitive adhesive sheet is preferably 50 kPa or more, from the viewpoints of preventing adhesive extrusion and maintaining the shape of the pressure-sensitive adhesive sheet.

[0086] When the storage shear modulus [G' (-20°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 delamination and cracking of the component sheet or flexible component can be suppressed.

[0087] From the viewpoint of obtaining high adhesiveness, the storage shear modulus [G'(25°C)] of the pressure-sensitive adhesive sheet at 25°C, as determined by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, is preferably 100 kPa or less, more preferably 50 kPa or less, even more preferably 40 kPa or less, and especially preferably 30 kPa or less. The lower limit of the storage shear modulus [G'(25°C)] of the pressure-sensitive adhesive sheet is preferably 5 kPa or more, from the viewpoint of preventing adhesive extrusion and maintaining the shape of the pressure-sensitive adhesive sheet.

[0088] From the viewpoint of obtaining high adhesiveness, the storage shear modulus [G'(80°C)] of the pressure-sensitive adhesive sheet at 80°C, as determined by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, is preferably 100 kPa or less, more preferably 50 kPa or less, even more preferably 30 kPa or less, and especially preferably 20 kPa or less. The lower limit of the storage shear modulus [G'(80°C)] of the pressure-sensitive adhesive sheet is preferably 1 kPa or more, from the viewpoint of preventing adhesive extrusion and maintaining the shape of the pressure-sensitive adhesive sheet.

[0089] (Loss shear modulus) The loss shear modulus [G" (23°C)] of the pressure-sensitive adhesive sheet at 23°C, obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, is preferably 8 kPa or more, more preferably 10 kPa or more, and particularly preferably 12 kPa or more. On the other hand, the upper limit of the loss shear modulus [G" (23°C)] is preferably 400 kPa or less, from the viewpoint of reducing stress during bending.

[0090] When the loss shear modulus [G" (23°C)] of the pressure-sensitive adhesive sheet is within the above range, the adhesive strength of the pressure-sensitive adhesive sheet can be further increased.

[0091] (Maximum of loss tangent (tan δ) and glass transition temperature (Tg)) The maximum of the loss tangent (tan δ) of the pressure-sensitive adhesive sheet, obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, is preferably −25° C. or lower, and more preferably −30° C. or lower. The lower limit is typically −50° C. The maximum of the loss tangent (tan δ) can be interpreted as the glass transition temperature (Tg), and having a glass transition temperature (Tg) within the above range makes it easy to adjust the storage shear modulus [G'(−20° C.)] of the pressure-sensitive adhesive sheet to 700 kPa or lower, particularly 500 kPa or lower.

[0092] When only one inflection point is observed in the loss tangent (tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, in other words, when the tan δ curve exhibits a single peak, it can be considered that the glass transition temperature (Tg) is single.

[0093] 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.

[0094] 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.

[0095] The storage shear modulus (G'), loss shear modulus (G"), and loss tangent (tan δ) can be adjusted to within the above-mentioned ranges by adjusting the types and weight average molecular weights of the components of the pressure-sensitive adhesive composition [I] constituting the pressure-sensitive adhesive sheet (for example, the acrylic polymer (A) and the radically polymerizable compound (B)), and by further adjusting the gel fraction of the pressure-sensitive adhesive sheet. However, this method is not limited to this.

[0096] (Resilience) The resilience of this pressure-sensitive adhesive sheet can be measured by applying a shear strain equivalent to 7 times the thickness at 25°C, maintaining this for 10 minutes, and then reading the strain value (residual strain value) 10 minutes after removing the stress. The resilience can be calculated using the following formula: Resilience (%) = [(700 - residual strain value) / 700] x 100

[0097] If the pressure-sensitive adhesive sheet has such resilience, it can be made into a pressure-sensitive adhesive sheet with excellent resilience, in which no crease marks due to being placed in a bent state remain even when the pressure-sensitive adhesive sheet is attached to a component sheet and folded at low or high temperatures. From this perspective, it is preferable that the resilience calculated from the residual strain value 10 minutes after applying a shear strain equivalent to 7 times the thickness at 25°C and maintaining it for 10 minutes, and then removing the stress, is 20% or more, 40% or more, particularly 50% or more, and even 70% or more. Since a higher resilience is preferable, the upper limit is 100%.

[0098] (Gel Fraction) The gel fraction of the present pressure-sensitive adhesive sheet is preferably 30 to 95% by mass, more preferably 50 to 90% by mass, even more preferably 55 to 85% by mass, and particularly preferably 60 to 85% by mass. 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, and when it is equal to or less than the upper limit, the adhesive strength can be increased. Furthermore, from the viewpoint of improving recovery, the gel fraction of the present pressure-sensitive adhesive sheet is preferably 30 to 65% by mass, and more preferably 35 to 60% by mass. The gel fraction is an indicator of the degree of crosslinking (degree of cure) and can be measured under the measurement conditions described in the Examples below.

[0099] (Total Light Transmittance, Haze) The total light transmittance of the present pressure-sensitive adhesive sheet is preferably 85% or more, more preferably 88% or more, and even more preferably 90% or more.

[0100] Furthermore, the pressure-sensitive adhesive sheet preferably has a haze of 1.0% or less, more preferably 0.8% or less, and particularly preferably 0.5% or less. Since the pressure-sensitive adhesive sheet has a haze of 1.0% or less, it can be used for image display devices. In order to keep the haze of the pressure-sensitive adhesive sheet within the above range, it is preferable that the pressure-sensitive adhesive sheet does not contain particles such as organic particles.

[0101] <Thickness> The thickness of the present pressure-sensitive adhesive sheet is not particularly limited, and if the thickness is 10 μm or more, the handling property is good, and if the thickness is 1000 μm or less, it can contribute to the thinning of the present pressure-sensitive adhesive sheet. Therefore, the thickness of the present pressure-sensitive adhesive sheet is preferably 10 μm or more, more preferably 15 μm or more, particularly 20 μm or more, and even more preferably 25 μm or more. On the other hand, the upper limit is preferably 1000 μm or less, more preferably 500 μm or less, particularly 250 μm or less, even more preferably 100 μm or less, and especially more preferably 50 μm or less.

[0102] <Preferred uses of the present pressure-sensitive adhesive sheet> The present pressure-sensitive adhesive sheet is used for laminating components constituting display members (also referred to as "display members"), in particular flexible members for displays used in producing displays, and is used as an adhesive component for flexible displays used in producing flexible displays. Note that the same flexible members as those described below can be used.

[0103] <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.

[0104] In producing the present pressure-sensitive adhesive sheet, a pressure-sensitive adhesive composition [I] for forming the present pressure-sensitive adhesive sheet is prepared, which contains the acrylic polymer (A), the radically polymerizable compound (B), and further the photopolymerization initiator (C), and, if necessary, other components, etc.; the pressure-sensitive adhesive composition [I] is formed into a sheet, cured by crosslinking, i.e., polymerization reaction, and then appropriately processed as necessary, thereby producing the present pressure-sensitive adhesive sheet.

[0105] Furthermore, in producing the present pressure-sensitive adhesive sheet, the pressure-sensitive adhesive composition [I] for forming the present pressure-sensitive adhesive sheet is prepared in the same manner as described above, and then coated onto a substrate sheet or a flexible substrate, and the pressure-sensitive adhesive composition [I] is cured to form the present pressure-sensitive adhesive sheet, although the method is not limited to this.

[0106] When preparing the pressure-sensitive adhesive composition [I] for forming a pressure-sensitive adhesive sheet, 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.

[0107] The method for forming the pressure-sensitive adhesive composition [I] into a sheet can be a known method, such as a wet lamination method, a dry lamination method, an extrusion casting method using a T-die, an extrusion lamination method, a calendar method, an inflation method, an injection molding method, a liquid injection curing method, etc. Among these, when producing a sheet, the wet lamination method, the extrusion casting method, and the extrusion lamination method are preferred.

[0108] Furthermore, the pressure-sensitive adhesive composition [I] can be irradiated with active energy rays to cure it, thereby producing a cured product. In addition to the irradiation of active energy rays, further curing can also be achieved by heating. In particular, the pressure-sensitive adhesive composition [I] can be molded into a molded product, for example, a sheet, and then irradiated with active energy rays to produce the pressure-sensitive adhesive sheet. In addition to the irradiation of active energy rays, further curing can also be achieved by heating.

[0109] 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 (C) and polymerize the monomer components. When a hydrogen abstraction photopolymerization initiator is used as the photopolymerization initiator (C), a hydrogen abstraction reaction also occurs from the acrylic polymer (A), and the acrylic polymer (A) is incorporated into the crosslinked structure, forming a crosslinked structure with many crosslinking points. Therefore, it is preferable that the pressure-sensitive adhesive sheet is cured using a hydrogen abstraction photopolymerization initiator.

[0110] In another embodiment of the method for producing the pressure-sensitive adhesive sheet, the pressure-sensitive adhesive composition [I] can be dissolved in an appropriate solvent and coated using various coating methods. When a coating method is used, the pressure-sensitive adhesive sheet can be obtained by curing with heat, in addition to curing by irradiation with active energy rays as described above. In the case of coating, the thickness of the pressure-sensitive adhesive sheet can be adjusted by the coating thickness and the solids concentration of the coating solution.

[0111] For example, the pressure-sensitive adhesive composition [I] can be dissolved in a solvent, coated on a release film, dried, and cured by active energy ray irradiation to form the present pressure-sensitive adhesive sheet. Furthermore, a release film may be laminated, if necessary. In this case, the pressure-sensitive adhesive composition [I] may be coated on a release film, dried, cured by active energy ray irradiation, and a release film may be laminated thereon, or the pressure-sensitive adhesive sheet may be formed by coating on a release film, drying, laminating a release film, and then curing by active energy ray irradiation.

[0112] The solvent is not particularly limited as long as it dissolves the pressure-sensitive adhesive composition [I], and examples thereof include ester solvents such as methyl acetate, ethyl acetate, methyl acetoacetate, and ethyl acetoacetate; ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; aromatic solvents such as toluene and xylene; and alcohol solvents such as methanol, ethanol, and propyl alcohol. These can be used alone or in combination of two or more. Among them, ethyl acetate, acetone, methyl ethyl ketone, and toluene are preferred in terms of solubility, drying properties, cost, and the like, and ethyl acetate is particularly preferred.

[0113] The content of the solvent is preferably 600 parts by mass or less, more preferably 500 parts by mass or less, even more preferably 400 parts by mass or less, and particularly preferably 300 parts by mass or less, relative to 100 parts by mass of the acrylic polymer (A) in terms of drying property. On the other hand, it is preferably 1 part by mass or more, more preferably 50 parts by mass or more, even more preferably 100 parts by mass or more, and particularly preferably 150 parts by mass or more. Coating can be performed by a conventional method such as roll coating, die coating, gravure coating, comma coating, screen printing, or bar coating.

[0114] The solvent content in the pressure-sensitive adhesive composition [I] after drying is preferably 1% by mass or less, more preferably 0.5% by mass or less, particularly preferably 0.1% by mass or less, and most preferably 0% by mass.

[0115] The drying temperature is usually 40 to 150° C., more preferably 45 to 140° C., even more preferably 50 to 130° C., and particularly preferably 55 to 120° C. When the temperature is within this range, the solvent can be removed efficiently and relatively safely while suppressing thermal deformation of the release film.

[0116] The drying time is usually 1 to 30 minutes, more preferably 3 to 25 minutes, and even more preferably 5 to 20 minutes. When the drying time is within this range, the solvent can be removed efficiently and sufficiently.

[0117] Drying methods include, for example, drying with a dryer, drying with a heated roll, drying by blowing hot air onto the film, etc. Among these, using a dryer is preferred because it allows for uniform and easy drying. These methods can be used alone or in combination of two or more.

[0118] Examples of the active energy rays in the active energy ray irradiation include light rays such as far ultraviolet rays, ultraviolet rays, near ultraviolet rays, infrared rays, and visible light rays, as well as ionizing radiation such as X-rays, α-rays, β-rays, γ-rays, electron beams, proton beams, and neutron beams. Among these, ultraviolet rays are preferred from the viewpoints of suppressing damage to optical device components and controlling reactions. Furthermore, curing by ultraviolet irradiation is advantageous from the viewpoints of curing speed, ease of availability of an irradiation device, cost, and the like.

[0119] Examples of light sources for ultraviolet irradiation include high-pressure mercury lamps, ultra-high-pressure mercury lamps, carbon arc lamps, metal halide lamps, xenon lamps, chemical lamps, electrodeless discharge lamps, and LEDs, all of which emit light in the wavelength range of 150 to 450 nm. Of these, it is preferable to use a high-pressure mercury lamp.

[0120] The amount of active energy ray irradiation (cumulative light amount) is 30 to 3000 mJ / cm from the viewpoint of curing. 2 is preferred, and more preferably 100 to 2000 mJ / cm 2 , more preferably 300 to 1500 mJ / cm 2 After the irradiation of the active energy rays, the degree of cure can be increased by heating, if necessary.

[0121] A release film may be provided on at least one surface of the pressure-sensitive adhesive sheet obtained above to prevent blocking and adhesion of foreign matter.

[0122] As the release film, any known release film can be used as appropriate. Examples of the material for the release film include a polyester film, a polyolefin film, a polycarbonate film, a polystyrene film, an acrylic film, a triacetyl cellulose film, a fluororesin film, or the like, which has been subjected to a release treatment by coating with a silicone resin, and release paper, etc.

[0123] The thickness of the release film is not particularly limited. Among them, from the viewpoint of processability and handling, for example, it is preferably 10 to 250 μm, more preferably 25 to 200 μm, and even more preferably 35 to 190 μm.

[0124] If necessary, the sheet may be embossed or may have various irregularities (such as a cone, pyramid, or hemisphere).In addition, the sheet may be subjected to various surface treatments such as corona treatment, plasma treatment, and primer treatment in order to improve adhesion to various component sheets.

[0125] 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 (the present pressure-sensitive adhesive sheet) made of the pressure-sensitive adhesive composition [I].

[0126] <<Present Laminate Sheet>> A laminate sheet according to one embodiment of the present invention (hereinafter, sometimes referred to as "the present laminate sheet") is a sheet comprising the present pressure-sensitive adhesive sheet and other layers. Of the layers constituting the present laminate sheet, the thickness of the present pressure-sensitive adhesive sheet preferably accounts for 10 to 90% of the total thickness of the present laminate sheet, more preferably 20% to 80%, and even more preferably 30% to 70%. Furthermore, the present laminate sheet is preferably one in which a component sheet is provided on at least one side of the present pressure-sensitive adhesive sheet, or one in which the present pressure-sensitive adhesive sheet is provided on at least one side of a component sheet.

[0127] 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.

[0128] <Component Sheets> Component sheets constituting the present laminate sheet, i.e., component sheets attached to the present pressure-sensitive adhesive sheet (including the "first component sheet" and / or the "second component sheet"), can include, for example, resin sheets containing at least one resin selected from the group consisting of polyester resin, cycloolefin resin, triacetyl cellulose resin, polymethyl methacrylate resin, epoxy resin, polyimide resin, aramid resin, and polyurethane resin as a primary component, or glass such as thin-film glass. Here, thin-film glass refers to glass having the thickness of the component sheets listed above. In particular, the 25°C tensile strength (ASTM D882) of resin sheets primarily composed of cycloolefin resin is low, at 40 to 60 MPa at a thickness of 100 μm. Laminate sheets using component sheets with such low tensile strength are prone to cracking when bent, and it has been difficult to eliminate this cracking using conventional techniques.

[0129] The component sheet may be any conventionally known material, and is not particularly limited, but examples thereof include the following: PET film "Mitsubishi Chemical Corporation, S100, thickness 50 μm" (tensile strength: 73 MPa) PEN film "Teijin Limited, FS205S, thickness 50 μm" (tensile strength: 193 MPa) PI film "KOLON Corporation, C_50, thickness 53 μm" (tensile strength: 204 MPa)

[0130] The "main component" refers to the component that occupies the largest mass ratio among the resin components that make up the component sheet, and specifically, it is a component that occupies 50 mass% or more of the component sheet or the resin composition that forms the component sheet, and it is even more preferable that it occupies 55 mass% or more, and even more preferably 60 mass% or more of that.

[0131] 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 member sheet and the second member sheet 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 member sheet has a touch input function. When the laminate sheet has the above-mentioned second member sheet, the second member sheet may also have a touch input function.

[0132] (Tensile strength at 25°C) Furthermore, with regard to the first component sheet, the tensile strength at 25°C measured in accordance with ASTM D882 (also referred to as "25°C tensile strength (ASTM D882)") is preferably 10 to 900 MPa, more preferably 15 MPa or more and 800 MPa or less, and even more preferably 20 MPa or more and 700 MPa or less. If the 25°C tensile strength (ASTM D882) of the first component sheet is in the above range, it is preferable because it is less likely to crack when bent.

[0133] When the laminate sheet has the second component sheet described above, the second component sheet preferably has a tensile strength at 25°C measured in accordance with ASTM D882 of 10 to 900 MPa, more preferably 15 MPa to 800 MPa, and even more preferably 20 MPa to 700 MPa. If the 25°C tensile strength (ASTM D882) of the second component sheet is in the above range, it is preferable because it is less likely to crack when bent.

[0134] In particular, it is preferable that both the first member sheet and the second member sheet have a tensile strength of 10 to 900 MPa at 25°C measured in accordance with ASTM D882. The first member sheet and the second member sheet may be made of the same material or different materials.

[0135] Examples of the member sheets with high tensile strength (including the first member sheet and the second member sheet) include polyimide films and polyethylene naphthalate (PEN) films, and the tensile strengths of these are generally 900 MPa or less. The lower limit is usually 50 MPa. On the other hand, examples of member sheets with slightly lower tensile strength include polyethylene terephthalate (PET) films, triacetyl cellulose (TAC) films, cycloolefin polymer (COP) films, and the like, and the tensile strengths of these are generally 10 MPa or more. The upper limit is usually 200 MPa. Even if the present laminate sheet includes member sheets made of such materials with slightly lower tensile strength, defects such as cracking can be suppressed by the action of the present pressure-sensitive adhesive sheet.

[0136] <Physical Properties of the Present Laminate Sheet> The present laminate sheet can have the following physical properties.

[0137] (Adhesive Strength) The adhesive strength of the present laminate sheet to a component sheet (peel angle 180°: peel rate 300 mm / min: temperature 23°C) is preferably 1 to 30 N / cm, more preferably 2 to 20 N / cm, and even more preferably 3 to 10 N / cm. The adhesive strength of the present laminate sheet to a component sheet (peel angle 180°: peel rate 300 mm / min: temperature 60°C) is preferably 0.5 to 30 N / cm, more preferably 1 to 20 N / cm, and even more preferably 1.5 to 10 N / cm. Within these ranges, sufficient adhesiveness is achieved, making the sheet suitable for use as an adhesive sheet for flexible image display devices. The adhesive strength can be measured under the measurement conditions described in the Examples below.

[0138] In order to improve and further enhance the adhesive strength of the present pressure-sensitive adhesive sheet, it is important to use, as the radically polymerizable compound (B), a bifunctional (meth)acrylate monomer having an alkylene group of a certain length or less, specifically, a di(meth)acrylate (B1) containing an alkylene group having 2 to 4 carbon atoms, and further it is important to incorporate into the acrylic polymer (A) a structural moiety derived from an alkyl (meth)acrylate (a1) having an alkyl group having 5 to 20 carbon atoms.

[0139] A bifunctional (meth)acrylate having an alkylene group of a certain length or less forms a crosslinked structure with the acrylic polymer (A) in the presence of a photopolymerization initiator. By selecting the (meth)acrylate, a small network of crosslinked structures is formed. This makes it difficult for the polymers to slip through each other when conducting adhesion tests and recovery tests, increasing the loss shear modulus G" and loss tangent tanδ, thereby enabling the adhesiveness to be further improved while maintaining flexibility and recovery.

[0140] Furthermore, in measuring the adhesive strength, the peeling mode when peeling the pressure-sensitive adhesive sheet from the component sheet is preferably interfacial peeling. Image display device components used in image display devices, particularly bendable flexible image display devices, are often expensive, and if a failure occurs during the component lamination process, reworkability is required, allowing for peeling without leaving any adhesive residue. Furthermore, to prevent scratches on the image display screen, a surface protection film may be laminated on the surface of the cover window. In such cases, it is necessary to prevent the adhesive layer from remaining on the surface of the cover window when peeling the adhesive layer of the surface protection film, a so-called "sad separation" from occurring. By using interfacial peeling as the peeling mode, a pressure-sensitive adhesive sheet with excellent reworkability that does not contaminate products can be obtained. Making a pressure-sensitive adhesive sheet flexible and having excellent flexibility reduces cohesive strength, and the peeling mode when a peel force is applied tends to be cohesive failure. However, by using a bifunctional (meth)acrylate monomer having an alkylene group of a certain length or less as the radical polymerizable compound (B), specifically a di(meth)acrylate (B1) containing an alkylene group having 2 to 4 carbon atoms, a pressure-sensitive adhesive sheet that is flexible yet has excellent cohesive strength can be obtained. The peeling mode of the acrylic pressure-sensitive adhesive can be determined, for example, by visually observing the adherend after the adhesive strength measurement test and checking for the presence or absence of adhesive residue. More specifically, the presence of adhesive residue that can be visually confirmed can be determined to be cohesive failure.

[0141] (Dynamic Bending Durability) In a dynamic bending reliability test (dynamic bending durability) of the present laminate sheet, in a cycle evaluation of U-shaped bending under settings of a radius of curvature R = 1.5 mm, 60 rpm (1 Hz), and -20°C, the number of bending cycles at which defects (delamination, breakage, buckling, flow) at the bent portion do not occur is preferably 100,000 or more, more preferably 200,000 or more. The dynamic bending durability test can be measured under the measurement conditions described in the Examples below.

[0142] <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 a sheet form, and 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 present laminate sheet 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.

[0143] <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.

[0144] The present laminate sheet may be produced by preparing a pressure-sensitive adhesive composition [I] in the same manner as in the production method of the present pressure-sensitive adhesive sheet, and then applying the pressure-sensitive adhesive composition [I] to, for example, a first member sheet and / or a second member sheet, followed by curing the composition to form a pressure-sensitive adhesive sheet. In this case, the method for preparing the pressure-sensitive adhesive composition [I], the coating method, the method for curing the pressure-sensitive adhesive composition [I], etc. are the same as in the production method of the present pressure-sensitive adhesive sheet.

[0145] 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.

[0146] 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.

[0147] 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.

[0148] <<The flexible image display device member>> A flexible image display device member according to one embodiment of the present invention (hereinafter, sometimes 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.

[0149] Of the components of the flexible image display device member, the adhesive sheet is as described above, and the components other than the adhesive sheet will be described below.

[0150] (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.

[0151] 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 radius of curvature of 25 mm or more, and is particularly preferably a member that can withstand repeated bending at a radius of curvature of less than 25 mm, more preferably less than 3 mm.

[0152] In the above-described configuration, the main component of the flexible member may be a resin sheet or glass. Examples of materials for such a resin sheet include polyester resin, cycloolefin resin, triacetyl cellulose resin, polymethyl methacrylate resin, polyurethane resin, epoxy resin, polyimide resin, and aramid resin. These may be one type of resin or two or more types of resin. Among these, a resin sheet containing at least one resin selected from the group consisting of polyester resin, cycloolefin resin, triacetyl cellulose resin, polymethyl methacrylate resin, epoxy resin, polyimide resin, aramid resin, and polyurethane resin as the main component is preferred. Here, the term "main component" refers to the component that accounts for the largest mass ratio among the components constituting the flexible member. Specifically, the component accounts for 50% by mass or more of the resin composition (resin sheet) forming the flexible member, preferably 55% by mass or more, and particularly preferably 60% by mass or more. The flexible member may also be made of thin-film glass.

[0153] In the above-described configuration, it is particularly preferable that one of the two flexible members, i.e., the first flexible member, has a tensile strength at 25°C measured in accordance with ASTM D882 of 10 to 900 MPa, more preferably 15 MPa or more and 800 MPa or less, and even more preferably 20 MPa or more and 700 MPa or less. If the 25°C tensile strength (ASTM D882) of one flexible member is within the above-described range, it is preferable because it is less likely to crack when bent.

[0154] The other flexible member, i.e., the second flexible member, preferably has a tensile strength at 25°C measured in accordance with ASTM D882 of 10 to 900 MPa, more preferably 15 MPa or more and 800 MPa or less, and even more preferably 20 MPa or more and 700 MPa or less. If the 25°C tensile strength (ASTM D882) of the other flexible member is within the above range, it is preferable because it is less likely to crack when bent.

[0155] Examples of flexible members with high tensile strength include polyimide films, polyester films, and aramid films, which generally have a tensile strength of 900 MPa or less. On the other hand, examples of flexible member sheets with slightly lower tensile strength include triacetyl cellulose (TAC) films and cycloolefin polymer (COP) films, which generally have a tensile strength of 10 MPa or more. Even if the flexible image display device member includes a flexible member made of such a material with a slightly lower tensile strength, the adhesive sheet can suppress defects such as cracking.

[0156] <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 adhesive composition [I] may be applied to a flexible member to form an adhesive sheet, or an adhesive sheet may be formed in advance using the adhesive composition [I], and then the adhesive sheet may be laminated to a flexible member.

[0157] <<The Present Flexible Image Display Device>> A flexible image display device according to an embodiment of the present invention (hereinafter, may be referred to as the "present flexible image display device") is an image display device incorporating the present laminate sheet or the present flexible image display device member. For example, the present laminate sheet can be laminated on other image display device components to form the present flexible image display device including the present laminate sheet.

[0158] The term "flexible image display device" refers to an image display device that can be repeatedly bent without leaving any traces of bending, can quickly return 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 curvature radius of 25 mm or more, particularly a member that can withstand repeated bending with a curvature radius of less than 25 mm, more preferably less than 3 mm.

[0159] One of the features of this laminated sheet is that it has excellent adhesiveness, can prevent delamination and cracking of the laminated sheet, and has good recovery properties, making it possible to manufacture flexible image display devices with excellent flexibility.

[0160] The present invention will be described in more detail below with reference to examples, but 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 by mass.

[0161] <Raw Materials> First, the acrylic polymers and pressure-sensitive adhesive compositions prepared in the Examples, Comparative Examples, and Reference Examples will be described in detail.

[0162] <Acrylic Polymer> Acrylic polymers (1) to (3) having copolymer component compositions as shown in Table 1 were prepared.

[0163]

[0164] <Radical Polymerizable Compound> The following radical polymerizable compounds were prepared: (B1-1): 1,4-butanediol diacrylate (BDDA) (B'-1): 1,10-decanediol diacrylate (DDDA)

[0165] <Photopolymerization initiator> Esacure TZT (manufactured by IGM, a mixture of 4-methylbenzophenone and 2,4,6-trimethylbenzophenone (hydrogen abstraction type))

[0166] [Examples 1 to 3, Comparative Examples 1 to 3, Reference Example 1] According to the formulations shown in Table 2, an acrylic polymer, a radical polymerizable compound, a photopolymerization initiator, and ethyl acetate as a solvent were uniformly mixed to obtain pressure-sensitive adhesive composition solutions (solid concentration: 33%).

[0167]

[0168] The pressure-sensitive adhesive composition solution was coated onto a release film (manufactured by Mitsubishi Chemical Corporation, silicone release-treated polyester film, thickness 100 μm) so as to have a thickness after drying as shown in Table 2. After coating, the film was placed in a dryer heated to 90°C for 7 minutes to evaporate and dry the solvent contained in the pressure-sensitive adhesive composition. Furthermore, a release film (manufactured by Mitsubishi Chemical Corporation, silicone release-treated polyester film, thickness 75 μm) was laminated on the surface of the pressure-sensitive adhesive composition from which the solvent had been dried to form a laminate, and the pressure-sensitive adhesive composition was irradiated with ultraviolet light through the release film using a high-pressure mercury lamp (see Table 2 for each irradiation dose) to obtain a pressure-sensitive adhesive sheet laminate (pressure-sensitive adhesive sheet with release film). The obtained pressure-sensitive adhesive sheet laminate was evaluated as follows.

[0169] <Gel Fraction> The release film was removed from each of the pressure-sensitive adhesive sheet laminates produced in the Examples, Comparative Examples, and Reference Examples, and multiple layers of pressure-sensitive adhesive sheets were laminated to form a 1.0 mm thick laminate. An 8 mm diameter cylinder was then punched out, and this was used as a sample. This was wrapped in a 200-mesh SUS wire mesh and immersed in ethyl acetate adjusted to 23°C for 72 hours. Thereafter, the sample was dried at 75°C for 4.5 hours, and the mass of the pressure-sensitive adhesive before and after immersion in ethyl acetate was measured, and the difference between the two masses was taken as the mass of the insoluble pressure-sensitive adhesive remaining in the wire mesh. The mass percentage of the insoluble pressure-sensitive adhesive remaining in the wire mesh relative to the mass of the pressure-sensitive adhesive before immersion in ethyl acetate was calculated as the gel fraction (%).

[0170] <Adhesive Strength> From each of the adhesive sheet laminates produced in the Examples, Comparative Examples, and Reference Examples, one release film was removed, and a polyethylene terephthalate film (Diafoil "S100", manufactured by Mitsubishi Chemical Corporation, thickness 50 μm) was roll-laminated using a hand roller as a backing film to the adhesive surface of the adhesive sheet laminate. This was cut into 10 mm wide x 150 mm long strips, and the remaining release film was peeled off, and the exposed adhesive surface was roll-laminated using a hand roller to a transparent polyimide film (main component: transparent polyimide, "C_50" manufactured by KOLON Corporation, hereinafter referred to as "CPI film") that had been previously laminated to a stainless steel plate, to produce a laminate sheet consisting of CPI film / adhesive sheet / backing film, and this laminate sheet was left to stand at room temperature (23°C) for 24 hours to age, and a sample for adhesive strength measurement was produced.

[0171] In an environment of 23 ° C or 60 ° C, the backing film was peeled at an angle of 180 ° to the CPI film at a peeling rate of 300 mm / min, and the tensile strength was measured with a load cell to measure the 180 ° peel strength (N / 10 mm) of the pressure-sensitive adhesive sheet to the CPI film, which was taken as the adhesive strength (23 ° C or 60 ° C). When the pressure-sensitive adhesive sheets prepared in the examples were peeled from the CPI film, the peeling mode was interfacial peeling. In addition, those in which the peeling mode was cohesive failure are indicated in the table with "*".

[0172] <Viscoelasticity Measurement> The dynamic viscoelasticity of the pressure-sensitive adhesive sheet was measured, and from the results, the maximum temperature of the loss tangent (tan δ) (glass transition temperature: Tg) and the storage shear modulus (G') at -40°C, -20°C, 25°C and 80°C were read.

[0173] [Loss tangent (tan δ), storage shear modulus (G')] The release film was removed from each of the pressure-sensitive adhesive sheet laminates produced in the Examples, Comparative Examples, and Reference Examples, and multiple layers of pressure-sensitive adhesive sheets were laminated to form a 1.0 mm thick laminate. From the resulting pressure-sensitive adhesive sheet (adhesive layer) laminate, a cylindrical object with a diameter of 8 mm (height of 1.0 mm) was punched out, and this was used as a sample. The temperature dispersion of dynamic viscoelasticity of this sample was measured using a viscoelasticity measuring device (manufactured by T.A. Instruments, product name "DHR 2") under the following measurement conditions. From the obtained temperature dispersion data of dynamic viscoelasticity, the peak temperature of the loss tangent (tan δ) (glass transition temperature (Tg)), the storage shear modulus at -40°C [G'(-40°C)], the storage shear modulus at -20°C [G'(-20°C)], the storage shear modulus at 25°C [G'(25°C)], and the storage shear modulus at 80°C [G'(80°C)] were read.

[0174] (Measurement conditions) Measurement jig: Φ8 mm parallel plate Distortion: 0.1% Frequency: 1 Hz Measurement temperature: -60 to 100°C Heating rate: 5°C / min

[0175] <Resilience> The release film was removed from each of the pressure-sensitive adhesive sheet laminates produced in the Examples, Comparative Examples, and Reference Examples, and multiple layers of pressure-sensitive adhesive sheets were laminated to form a 1.0 mm thick laminate. A cylindrical object with a diameter of 8 mm (height of 1.0 mm) was punched out from the resulting pressure-sensitive adhesive sheet (adhesive layer) laminate, and this was used as a sample. The recovery rate of each sample was measured under the following measurement conditions using a viscoelasticity measuring device (manufactured by T.A. Instruments, product name "DHR 2") to evaluate recovery. That is, a shear strain equivalent to 7 times the thickness was applied at 25°C and maintained for 10 minutes, and then the residual strain value was read 10 minutes after the stress was removed to measure recovery. Recovery was calculated using the following formula: Recovery (%) = [(700 - residual strain value) / 700] x 100

[0176] The results obtained by the above measurements and evaluations are shown in Table 3.

[0177]

[0178] From the above evaluation results, the pressure-sensitive adhesive sheets in the Examples have a low storage shear modulus at low temperatures, and by incorporating a bifunctional (meth)acrylate monomer having a relatively short-chain alkylene group into the acrylic polymer (A) as the pressure-sensitive adhesive composition, the pressure-sensitive adhesive sheet has even better adhesive strength. This is because the bifunctional (meth)acrylate monomer bonds the side chains of the acrylic polymer (A), making it difficult for the polymers to slip apart, thereby increasing the loss shear modulus G". It is also presumed that the use of a crosslinking agent with a relatively short chain length reduces the size of the network structure associated with crosslinking, making it even more difficult for the polymers to slip apart, thereby achieving high adhesive strength. Therefore, it can be seen that flexible image display devices using this pressure-sensitive adhesive sheet have resilience and flexibility, and furthermore, due to their excellent adhesive strength, delamination is suppressed.

[0179] Specifically, Comparative Example 1 does not contain a bifunctional (meth)acrylate monomer having an alkylene group with a relatively short chain length, whereas Example 1 contains such a bifunctional (meth)acrylate monomer, and as a result, the adhesive strength of Example 1 is improved compared to Comparative Example 1. Similarly, the adhesive strength of Example 2 is improved compared to Comparative Example 2. This shows that the adhesive sheets of the Examples have resilience and also have superior adhesive strength, and that flexible image display devices using the adhesive sheets of the Examples have resilience and flexibility, as well as superior adhesive strength.

[0180] Furthermore, while Example 3 contains a bifunctional (meth)acrylate monomer having a relatively short alkylene group, Comparative Example 3 does not contain such a (meth)acrylate monomer, resulting in a pressure-sensitive adhesive sheet with a high storage shear modulus, particularly at low temperatures. Therefore, when used to bond flexible image display device components, it is difficult to resolve issues such as delamination and creases that occur during folding. Furthermore, in Comparative Example 3, during adhesive strength measurement, peeling was not possible at the interface between the pressure-sensitive adhesive sheet and the CPI film, resulting in cohesive failure of the pressure-sensitive adhesive sheet, making it impossible to accurately measure the adhesive strength at the CPI interface. This demonstrates that reworkability is improved in the examples in which the peel mode is interfacial peeling.

[0181] Furthermore, in Reference Example 1, the pressure-sensitive adhesive composition contains an acrylic polymer (A) and a bifunctional (meth)acrylate monomer having an alkylene group with a relatively long chain length. Therefore, although the pressure-sensitive adhesive composition has recovery properties, it has lower adhesive strength than the examples.

[0182] 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.

[0183] The pressure-sensitive adhesive sheet of the present invention has excellent flexibility and recovery properties, and also has excellent adhesive strength, and is therefore useful as a pressure-sensitive adhesive sheet for obtaining 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 bending.

Claims

1. A pressure-sensitive adhesive sheet having a storage shear modulus at −20° C. [G′(−20° C.)] of 700 kPa or less, the pressure-sensitive adhesive sheet is a pressure-sensitive adhesive sheet formed from a pressure-sensitive adhesive composition [I] containing an acrylic polymer (A) and a radical-polymerizable compound (B), the acrylic polymer (A) contains a structural moiety derived from an alkyl (meth)acrylate (a1) having an alkyl group with 5 to 20 carbon atoms and a structural moiety derived from a hydroxyl group-containing (meth)acrylate (a2), The pressure-sensitive adhesive sheet, wherein the radically polymerizable compound (B) comprises a di(meth)acrylate (B1) having an alkylene group having 2 to 4 carbon atoms.

2. 2. The pressure-sensitive adhesive sheet according to claim 1, which has a storage shear modulus at −20° C. [G′(−20° C.)] of 500 kPa or less.

3. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the alkyl(meth)acrylate (a1) is a linear alkyl(meth)acrylate.

4. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the acrylic polymer (A) has a weight average molecular weight of 600,000 to 1,500,000.

5. 3. The pressure-sensitive adhesive sheet according to claim 1, wherein the acrylic polymer (A) has a glass transition temperature (Tg) of -50°C or higher and -10°C or lower, the glass transition temperature (Tg) being defined as the maximum point of the loss tangent (tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz.

6. 3. The pressure-sensitive adhesive sheet according to claim 1, wherein the acrylic polymer (A) has a glass transition temperature (Tg) defined at the maximum point of loss tangent (tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz, of -50°C or higher and lower than -25°C.

7. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the di(meth)acrylate (B1) is a di(meth)acrylate having a linear alkylene group.

8. 3. The pressure-sensitive adhesive sheet according to claim 1, wherein the content of the radically polymerizable compound (B) is 0.1 to 10 parts by mass per 100 parts by mass of the acrylic polymer (A).

9. 3. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive sheet has a gel fraction of 30 to 95 mass %.

10. 3. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive sheet has a gel fraction of 30 to 65 mass %.

11. The pressure-sensitive adhesive sheet according to claim 1 or 2, which is used for bonding components of a flexible image display device.

12. A flexible image display device comprising the pressure-sensitive adhesive sheet according to claim 1 or 2.