Pressure-sensitive adhesive composition

Abstract

Provided is a pressure-sensitive adhesive composition having excellent flex resistance in a low-temperature region.　The pressure-sensitive adhesive composition includes: a (meth)acrylic acid ester including an ester moiety which has one or more branching points on the first to fourth atoms in the ester moiety from the oxygen atom in the ester bond and which has, on the branching points, two or more hydrocarbon-group-containing chains in each of which the number of chain-forming atoms is 4 or greater; and / or a polymer of the (meth)acrylic acid ester. It is preferable that the difference between the number of atoms forming the main chain of the ester moiety and the number of atoms forming the side chains among the hydrocarbon-group-containing chains, [(number of atoms forming the main chain)-(number of atoms forming the side chains)], be two or greater.

Description

Pressure-sensitive adhesive composition

[0001] The present invention relates to a pressure-sensitive adhesive composition.

[0002] In recent years, image display devices such as liquid crystal displays (LCDs) and organic light-emitting diode (OLED) displays, as well as touch panels incorporating touch sensors into these image display devices, have become widely used in various fields. Such image display devices and touch panels have a configuration in which various optical components, such as a polarizing film, a retardation film, an optical compensation film, a touch sensor film, and a cover film, are laminated on an image display panel. Pressure-sensitive adhesive sheets having a pressure-sensitive adhesive layer are used to bond these optical components. For example, optical pressure-sensitive adhesive sheets are used to bond various optical components in image display devices (see, for example, Patent Documents 1 to 3).

[0003] Meanwhile, development of repeatedly foldable display panels is progressing, for example, for smartphones and tablet terminals. Specifically, foldable display panels can be repeatedly deformed between a bent shape and a flat, non-bent shape. In such foldable display panels, each element in the laminated structure is made to be repeatedly foldable, and a thin optical adhesive sheet is used to bond such elements. Optical adhesive sheets for flexible devices such as foldable display panels are described, for example, in Patent Document 4 listed below.

[0004] JP 2003-238915 A JP 2003-342542 A JP 2004-231723 A JP 2018-111754 A

[0005] Adhesive sheets for flexible devices used in low temperature regions are required to have high flexibility even in low temperature regions.

[0006] The present invention has been made to solve these problems, and an object of the present invention is to provide a pressure-sensitive adhesive composition that has excellent flex resistance in a low temperature range.

[0007] The present invention provides a pressure-sensitive adhesive composition comprising a (meth)acrylic acid ester having an ester moiety in which a branch point is located on the 1st to 4th atom in the ester moiety from the oxygen atom in the ester bond, and the branch point has two or more hydrocarbon group-containing chains, each of which has four or more atoms constituting the chain, and / or a polymer thereof.

[0008] The difference between the number of atoms constituting the main chain of the ester moiety and the number of atoms constituting the side chain of the hydrocarbon group-containing chain [number of atoms constituting the main chain - number of atoms constituting the side chain] is preferably 2 or more.

[0009] In the ester moiety, the atoms constituting the main chain are preferably only carbon atoms or carbon atoms and oxygen atoms.

[0010] At least one of the hydrocarbon group-containing chains is preferably an aliphatic hydrocarbon group.

[0011] The proportion of the (meth)acrylic acid ester is preferably 15 mass % or more based on the total amount of polymerizable components in the pressure-sensitive adhesive composition.

[0012] The shear storage modulus (G') at -20°C is preferably 70 kPa or less.

[0013] The pressure-sensitive adhesive composition of the present invention has excellent flex resistance in a low temperature range.

[0014] 1 is a schematic diagram (cross-sectional view) illustrating one embodiment of a pressure-sensitive adhesive sheet of the present invention.

[0015] [Adhesive composition] The adhesive composition of the present invention contains at least a (meth)acrylic acid ester having an ester moiety in which a branch point is located on the 1st to 4th atom in the ester moiety from the oxygen atom in the ester bond, and in which the branch point has two or more hydrocarbon group-containing chains, each of which has four or more atoms constituting the chain, and / or a polymer thereof. In this specification, the (meth)acrylic acid ester may be referred to as "(meth)acrylic acid ester (A)."

[0016] The (meth)acrylic acid ester (A) has a branched ester moiety. In this specification, "branched" refers to a structure in which the ester moiety has a branch and the branched chains are not bonded to each other to form a ring. For example, a (meth)acrylic acid ester having only one alicyclic hydrocarbon group, such as cyclohexyl (meth)acrylate, is not included in the (meth)acrylic acid ester having a "branched" ester moiety.

[0017] The (meth)acrylic acid ester (A) functions as a polymerizable component constituting a polymer (preferably a base polymer) contained in a pressure-sensitive adhesive layer formed using the pressure-sensitive adhesive composition of the present invention. The pressure-sensitive adhesive composition of the present invention contains one or more selected from the group consisting of a (meth)acrylic acid ester (A), a polymer (preferably a base polymer) of a monomer composition containing the (meth)acrylic acid ester (A), and a partial polymer of the monomer composition. The polymer of the (meth)acrylic acid ester (A) includes both the polymer and the partial polymer.

[0018] The base polymer is not particularly limited, but examples thereof include acrylic polymers in acrylic pressure-sensitive adhesive compositions, rubber polymers in rubber pressure-sensitive adhesive compositions (such as natural rubber pressure-sensitive adhesive compositions and synthetic rubber pressure-sensitive adhesive compositions), silicone polymers in silicone pressure-sensitive adhesive compositions, polyester polymers in polyester pressure-sensitive adhesive compositions, urethane polymers in urethane pressure-sensitive adhesive compositions, polyamide polymers in polyamide pressure-sensitive adhesive compositions, epoxy polymers in epoxy pressure-sensitive adhesive compositions, vinyl alkyl ether polymers in vinyl alkyl ether pressure-sensitive adhesive compositions, fluorine polymers in fluorine-based pressure-sensitive adhesive compositions, etc. Only one type of base polymer may be used, or two or more types may be used.

[0019] The base polymer is an adhesive component that imparts adhesiveness to the adhesive layer. The base polymer is preferably an acrylic polymer. The acrylic polymer is a (co)polymer of polymerizable components containing 50% by mass or more of a (meth)acrylic acid ester. "(Meth)acrylic" means acrylic and / or methacrylic.

[0020] The monomer composition may be composed of only the (meth)acrylic acid ester (A), or may be a mixture of polymerizable components containing two or more polymerizable components including the (meth)acrylic acid ester (A). The partially polymerized product is a composition in which one or more polymerizable components of the polymerizable components of the monomer composition are partially polymerized.

[0021] The (meth)acrylic acid ester (A) is a methacrylic acid ester having an oxygen atom (—C(═O)) in an ester bond. １ ) -O ２ "O" in "-" ２ ") in the ester moiety. That is, the carbon atoms in the ester moiety that are one to four from the oxygen atom in the ester bond are tertiary or quaternary carbon atoms (preferably tertiary carbon atoms).

[0022] Two or more hydrocarbon group-containing chains, each having four or more atoms constituting the chain, are bonded to the branch point. In this specification, the chain having the largest number of atoms constituting the chain in the ester moiety is referred to as the "main chain," and the chain bonded to an atom constituting the main chain and not included in the main chain is sometimes referred to as a "side chain." The hydrocarbon group-containing chain bonded to the branch point is sometimes referred to as a "branched chain." That is, the (meth)acrylic acid ester (A) has two or more branched chains at the branch point of the ester moiety, and at least two of the branched chains each have four or more atoms constituting the chain. One of the branched chains is a part of the main chain, and the other is a side chain. The main chain is composed of a chain extending from the oxygen atom in the ester bond to the branch point and one branched chain. The number of branched chains bonded to the branch point is two or more, as described above, and preferably two.

[0023] The number of atoms constituting at least two of the branched chains is 4 or more, more preferably 5 or more, and even more preferably 6 or more. The number of atoms is, for example, 12 or less, and preferably 10 or less. The number of atoms constituting the at least two chains may be the same or different.

[0024] In this specification, the "number of atoms constituting the chain" refers to the number of atoms constituting the chain only, and if the chain is a hydrocarbon group, for example, it is the total number of carbon atoms in the hydrocarbon group, or if the chain is a hydrocarbon group having an ether bond, it is the total number of carbon atoms and oxygen atoms in the hydrocarbon group having an ether bond. Such a configuration can easily achieve a low shear storage modulus at -20°C.

[0025] The number (total number) of atoms constituting the chain of the ester moiety of the (meth)acrylic acid ester (A) is 9 or more, preferably 12 or more, and more preferably 14 or more. The number (total number) of atoms constituting the chain of the ester moiety of the (meth)acrylic acid ester (A) is preferably 24 or less, more preferably 22 or less, and even more preferably 20 or less. Such a configuration makes it easier to achieve a low shear storage modulus at −20° C.

[0026] In the ester moiety of the (meth)acrylic acid ester (A), the difference between the number of atoms constituting the main chain and the number of atoms constituting the branched chains corresponding to the side chains [number of atoms constituting the main chain - number of atoms constituting the side chains] is preferably 2 or more, more preferably 3 or more, and even more preferably 4 or more. Moreover, the difference is preferably 10 or less, more preferably 8 or less, and even more preferably 6 or less. Such a configuration makes it easier to achieve a low shear storage modulus at -20°C.

[0027] The atoms constituting the branched chain are preferably only carbon atoms or carbon atoms and oxygen atoms. The branched chain is preferably a hydrocarbon group optionally having an alkoxy group, more preferably a hydrocarbon group, and even more preferably an aliphatic hydrocarbon group.

[0028] In the ester moiety of the (meth)acrylic acid ester (A), the atoms constituting the main chain are preferably only carbon atoms or carbon atoms and oxygen atoms. The main chain is preferably a hydrocarbon group optionally having an alkoxy group, more preferably a hydrocarbon group, and even more preferably an aliphatic hydrocarbon group.

[0029] The (meth)acrylic acid ester (A) preferably has one or more (preferably two or more) side chains each consisting of only carbon atoms or a combination of carbon atoms and oxygen atoms. The one or more (preferably two or more) side chains are preferably hydrocarbon groups optionally having an alkoxy group, more preferably hydrocarbon groups, and even more preferably aliphatic hydrocarbon groups.

[0030] Examples of the (meth)acrylic acid ester (A) include branched alkyl (meth)acrylates, alkoxyalkyl (meth)acrylates, and polyoxyalkylene (meth)acrylates.

[0031] Examples of the alkyl (meth)acrylate include 2-hexyldecyl (meth)acrylate.

[0032] The (meth)acrylic acid ester (A) is preferably a (meth)acrylic acid alkyl ester having a branched alkyl group, more preferably a (meth)acrylic acid alkyl ester having a branched alkyl group having 9 to 24 carbon atoms, and even more preferably 2-hexyldecyl (meth)acrylate (2HDA). For example, 2-hexyldecyl (meth)acrylate has an oxygen atom (the above-mentioned "O") in the ester bond. ２ "), and on the branch point, there is a branched chain which is a hydrocarbon group having 8 atoms constituting the chain and a branched chain which is a hydrocarbon group having 6 atoms constituting the chain, and the number of atoms constituting the main chain is 10, the number of atoms constituting the side chain is 6, and [number of atoms constituting the main chain - number of atoms constituting the side chain] is 4.

[0033] From the viewpoint of more easily achieving a low shear storage modulus at −20° C., the proportion of the (meth)acrylic acid ester (A) relative to the total amount (100% by mass) of polymerizable components in the pressure-sensitive adhesive composition is preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and may be 30% by mass or more, 35% by mass or more, 40% by mass or more, or 45% by mass or more, and particularly preferably 50% by mass or more. The proportion is preferably 99% by mass or less, more preferably 98% by mass or less, even more preferably 97% by mass or less, and may be 96% by mass or less, 95% by mass or less, 93% by mass or less, or 90% by mass or less.

[0034] In the pressure-sensitive adhesive composition of the present invention, together with the (meth)acrylic acid ester (A), other (meth)acrylic acid alkyl esters other than the (meth)acrylic acid ester (A), such as a (meth)acrylic acid alkyl ester having a branched alkyl group with 8 or less carbon atoms (preferably 4 to 8 carbon atoms) or a (meth)acrylic acid alkyl ester having a linear alkyl group with 9 to 24 carbon atoms, may be used in combination. Only one type of such other (meth)acrylic acid alkyl ester may be used, or two or more types may be used.

[0035] Examples of the other (meth)acrylic acid alkyl esters include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, Examples thereof include decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate (i.e., lauryl (meth)acrylate), isotridecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, cetyl (meth)acrylate, heptadecyl (meth)acrylate, hexadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, and nonadecyl (meth)acrylate.

[0036] Furthermore, the other (meth)acrylic acid alkyl esters mentioned above include (meth)acrylic acid alkyl esters having an alicyclic alkyl group. Examples of (meth)acrylic acid alkyl esters having an alicyclic alkyl group include (meth)acrylic acid cycloalkyl esters, (meth)acrylic acid alkyl esters having a bicyclic aliphatic hydrocarbon ring, and (meth)acrylic acid alkyl esters having a tricyclic or higher aliphatic hydrocarbon ring. Examples of (meth)acrylic acid cycloalkyl esters include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate. Examples of (meth)acrylic acid alkyl esters having a bicyclic aliphatic hydrocarbon ring include isobornyl (meth)acrylate. Examples of (meth)acrylic acid alkyl esters having three or more aliphatic hydrocarbon rings include dicyclopentanyl (meth)acrylate, dicyclopentanyloxyethyl (meth)acrylate, tricyclopentanyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate.

[0037] The other (meth)acrylic acid alkyl esters may have one or more alkoxy groups. Examples of the (meth)acrylic acid alkyl esters having an alkoxy group include 2-methoxymethyl (meth)acrylate, 2-methoxyethyl (meth)acrylate, 2-methoxybutyl (meth)acrylate, ethyl carbitol (meth)acrylate (ethoxyethoxyethyl (meth)acrylate), and 2-ethylhexyl carbitol (meth)acrylate.

[0038] From the viewpoint of properly exhibiting basic properties such as adhesiveness in the pressure-sensitive adhesive sheet, the total proportion of the (meth)acrylic acid ester (A) and the other (meth)acrylic acid alkyl esters is preferably 55% by mass or more, more preferably 60% by mass or more, and even more preferably 80% by mass or more, relative to the total amount (100% by mass) of polymerizable components in the pressure-sensitive adhesive composition. The total proportion is preferably 99% by mass or less, more preferably 98% by mass or less, even more preferably 97% by mass or less, and may be 96% by mass or less, 95% by mass or less, 93% by mass or less, or 90% by mass or less.

[0039] The polymerizable component may contain a copolymerizable monomer copolymerizable with the (meth)acrylic acid ester (A). Examples of the copolymerizable monomer include a monomer having a polar group. Examples of the polar group-containing monomer include a hydroxy group-containing monomer, a carboxy group-containing monomer, and a monomer having a nitrogen atom-containing ring. The polar group-containing monomer is useful for modifying the acrylic polymer, such as by introducing crosslinking points into the acrylic polymer and ensuring the cohesive strength of the acrylic polymer. The copolymerizable monomer may be used alone or in combination of two or more.

[0040] Examples of hydroxy group-containing monomers include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, and 12-hydroxylauryl (meth)acrylate.

[0041] The polar group-containing monomer is preferably a hydroxy group-containing monomer, more preferably a hydroxyalkyl (meth)acrylate, and from the viewpoint of more easily achieving a low shear storage modulus at −20° C., even more preferably a hydroxyalkyl (meth)acrylate in which the alkyl group has 3 or more carbon atoms (for example, 3 to 8), and particularly preferably 4-hydroxybutyl acrylate (4HBA).

[0042] The proportion of the hydroxy group-containing monomer is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, relative to the total amount (100% by mass) of polymerizable components in the PSA composition, from the viewpoints of introducing a crosslinked structure into the acrylic polymer and ensuring cohesive strength in the PSA layer. From the viewpoint of realizing a low shear storage modulus at −20° C., the proportion is preferably 50% by mass or less, and may be 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, or 15% by mass or less.

[0043] Examples of carboxy group-containing monomers include acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, and isocrotonic acid.

[0044] The proportion of the carboxyl group-containing monomer is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, relative to the total amount (100% by mass) of polymerizable components in the pressure-sensitive adhesive composition, from the viewpoints of introducing a crosslinked structure into the acrylic polymer, ensuring the cohesive strength of the pressure-sensitive adhesive layer, and ensuring the adhesive strength of the pressure-sensitive adhesive layer to the adherend. From the viewpoints of adjusting the glass transition temperature of the acrylic polymer and avoiding the risk of corrosion of the adherend by acid, the proportion is preferably 3% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less.

[0045] Examples of monomers having a nitrogen atom-containing ring include N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole, N-(meth)acryloyl-2-pyrrolidone, N-(meth)acryloylpiperidine, N-(meth)acryloylpyrrolidine, N-vinylmorpholine, N-vinyl-3-morpholinone, N-vinyl-2-caprolactam, N-vinyl-1,3-oxazin-2-one, N-vinyl-3,5-morpholinedione, N-vinylpyrazole, N-vinylisoxazole, N-vinylthiazole, and N-vinylisothiazole.

[0046] The proportion of the monomer having a nitrogen atom-containing ring relative to the total amount (100% by mass) of polymerizable components in the pressure-sensitive adhesive composition is, for example, 0.5% by mass or more, 1% by mass or more, or even 1.5% by mass or more. From the viewpoint of more easily achieving a low shear storage modulus at −20° C., the proportion is preferably 10% by mass or less, more preferably 7% by mass or less, even more preferably 5% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0.5% by mass or less, and the pressure-sensitive adhesive composition may be substantially free of the monomer having a nitrogen atom-containing ring. In the pressure-sensitive adhesive composition, the phrase “substantially free of” a monomer having a nitrogen atom-containing ring means that the monomer having a nitrogen atom-containing ring is not actively blended in, except in cases where the monomer having a nitrogen atom-containing ring is inevitably mixed in.

[0047] The polymerizable component may contain other copolymerizable monomers. Examples of the other copolymerizable monomers include acid anhydride monomers, sulfonic acid group-containing monomers, phosphoric acid group-containing monomers, epoxy group-containing monomers, cyano group-containing monomers, and aromatic vinyl compounds. One or more of the other copolymerizable monomers may be used.

[0048] The pressure-sensitive adhesive composition may contain a polymerization initiator such as a photopolymerization initiator. The photopolymerization initiator may be used alone or in combination of two or more kinds.

[0049] Examples of the photopolymerization initiator include a radical photopolymerization initiator, a cationic photopolymerization initiator, and an anionic photopolymerization initiator.

[0050] Examples of the radical photopolymerization initiator include an acylphosphine oxide photopolymerization initiator, a benzoin ether photopolymerization initiator, an acetophenone photopolymerization initiator, an α-ketol photopolymerization initiator, an aromatic sulfonyl chloride photopolymerization initiator, a photoactive oxime photopolymerization initiator, a benzoin photopolymerization initiator, a benzyl photopolymerization initiator, a benzophenone photopolymerization initiator, a ketal photopolymerization initiator, and a thioxanthone photopolymerization initiator.

[0051] Examples of acylphosphine oxide photopolymerization initiators include bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-di-n-butoxyphenylphosphine oxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide. Examples of benzoin ether photopolymerization initiators include benzoin methyl ether, benzoin ethyl ether, benzoin propyl ether, benzoin isopropyl ether, benzoin isobutyl ether, and 2,2-dimethoxy-1,2-diphenylethan-1-one. Examples of acetophenone photopolymerization initiators include 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexyl phenyl ketone, 4-phenoxydichloroacetophenone, and 4-(t-butyl)dichloroacetophenone. Examples of α-ketol photopolymerization initiators include 2-methyl-2-hydroxypropiophenone and 1-[4-(2-hydroxyethyl)phenyl]-2-methylpropan-1-one. Examples of aromatic sulfonyl chloride photopolymerization initiators include 2-naphthalenesulfonyl chloride. Examples of photoactive oxime photopolymerization initiators include 1-phenyl-1,1-propanedione-2-(o-ethoxycarbonyl)-oxime. Examples of benzoin photopolymerization initiators include benzoin. Examples of benzyl photopolymerization initiators include benzyl. Examples of benzophenone photopolymerization initiators include benzophenone, benzoylbenzoic acid, 3,3'-dimethyl-4-methoxybenzophenone, and polyvinylbenzophenone. Examples of ketal photopolymerization initiators include benzyl dimethyl ketal. Examples of thioxanthone photopolymerization initiators include thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, isopropylthioxanthone, 2,4-diisopropylthioxanthone, and dodecylthioxanthone.

[0052] The photopolymerization initiator is preferably at least one selected from the group consisting of bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, 2,2-dimethoxy-1,2-diphenylethan-1-one, and 1-hydroxycyclohexyl phenyl ketone.

[0053] The amount of the photopolymerization initiator used (when multiple photopolymerization initiators are used, the total amount of the multiple photopolymerization initiators) is, relative to 100 parts by mass of the total amount of the polymerizable components, for example, 0.01 parts by mass or more, preferably 0.03 parts by mass or more, more preferably 0.05 parts by mass or more, and for example, 1 part by mass or less, preferably 0.5 parts by mass or less, more preferably 0.3 parts by mass or less, and even more preferably 0.2 parts by mass or less.

[0054] The pressure-sensitive adhesive composition may contain a crosslinking agent. Use of the crosslinking agent results in the base polymer having a crosslinked structure. Examples of methods for introducing a crosslinked structure into the base polymer include the following first and second methods. In the first method, a base polymer having a functional group reactive with the crosslinking agent and the crosslinking agent are blended into the pressure-sensitive adhesive composition, and the base polymer and the crosslinking agent are reacted in the pressure-sensitive adhesive layer. In the second method, a polyfunctional compound (crosslinking agent) such as a polyfunctional monomer is added together with a polymerizable component, and a base polymer having a branched structure (crosslinked structure) introduced into the polymer chain is formed by polymerization of the polymerizable component. These methods may be used in combination.

[0055] Examples of the crosslinking agent used in the first method include compounds that react with the polar groups (such as hydroxyl and carboxyl groups) in the polar group-containing monomers. Examples of such crosslinking agents include isocyanate crosslinking agents, peroxide crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, and metal chelate crosslinking agents. Only one type of crosslinking agent may be used, or two or more types may be used.

[0056] In the first method, the amount of crosslinking agent is, from the viewpoint of ensuring the cohesive strength of the adhesive layer, preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, even more preferably 0.05 parts by mass or more, per 100 parts by mass of the total amount of polymerizable components (or the total amount of base polymers), and is preferably 3 parts by mass or less, more preferably 1 part by mass or less, even more preferably 0.5 parts by mass or less.

[0057] In the second method, the monofunctional monomer and the polyfunctional compound, such as the polyfunctional monomer for introducing a crosslinked structure, may be polymerized in one step or in multiple steps. In the multi-step polymerization method, the monofunctional monomer is first polymerized (preliminary polymerization), thereby preparing a prepolymer composition containing a partial polymer (a mixture of a low-polymerization polymer and an unreacted monomer). Next, a polyfunctional compound is added as a crosslinking agent to the prepolymer composition, and then the partial polymer and the polyfunctional compound are polymerized (main polymerization).

[0058] Examples of polyfunctional compounds include polyfunctional monomers and polyfunctional oligomers containing two or more ethylenically unsaturated double bonds in one molecule, and polyfunctional (meth)acrylates.

[0059] Examples of the polyfunctional (meth)acrylate include difunctional (meth)acrylate, trifunctional (meth)acrylate, and tetrafunctional or higher polyfunctional (meth)acrylate.

[0060] Examples of bifunctional (meth)acrylates include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, glycerin di(meth)acrylate, ethoxylated bisphenol A diacrylate (BPAEODE), and neopentyl glycol di(meth)acrylate.

[0061] Examples of trifunctional (meth)acrylates include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and tris(acryloyloxyethyl)isocyanurate.

[0062] Examples of tetrafunctional or higher polyfunctional (meth)acrylates include ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol monohydroxypenta(meth)acrylate, alkyl-modified dipentaerythritol penta(meth)acrylate, and dipentaerythritol hexa(meth)acrylate.

[0063] Examples of polyfunctional oligomers include urethane (meth)acrylate oligomers, polyester (meth)acrylate oligomers, polyether (meth)acrylate oligomers, polyol (meth)acrylate oligomers, epoxy (meth)acrylate oligomers, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate.

[0064] The polyfunctional compound used as the crosslinking agent in the second method may be used alone or in combination of two or more kinds. As the polyfunctional compound, at least one selected from the group consisting of 1,6-hexanediol diacrylate (HDDA) and dipentaerythritol hexaacrylate (DPHA) is preferably used.

[0065] When a polyfunctional monomer is used as the polyfunctional compound, the amount of the polyfunctional monomer is, from the viewpoint of ensuring the cohesive strength of the pressure-sensitive adhesive layer, preferably 0.01 parts by mass or more, more preferably 0.03 parts by mass or more, and even more preferably 0.05 parts by mass or more, per 100 parts by mass of the total amount of the monofunctional monomers. From the viewpoint of ensuring the flexible deformability of the pressure-sensitive adhesive layer, the amount of the polyfunctional monomer is, from the viewpoint of ensuring the flexible deformability of the pressure-sensitive adhesive layer, preferably 3 parts by mass or less, more preferably 2 parts by mass or less, and even more preferably 1 part by mass or less, per 100 parts by mass of the total amount of the monofunctional monomers.

[0066] When a polyfunctional oligomer is used as the polyfunctional compound, the amount of the polyfunctional oligomer in the polymerizable component is, from the viewpoint of ensuring the cohesive strength of the pressure-sensitive adhesive layer, preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 0.7 parts by mass or more per 100 parts by mass of the total amount of the monofunctional monomers. From the viewpoint of ensuring the flexible deformability of the pressure-sensitive adhesive layer, the amount of the polyfunctional oligomer is, from the viewpoint of ensuring the flexible deformability of the pressure-sensitive adhesive layer, preferably 8 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less per 100 parts by mass of the total amount of the monofunctional monomers.

[0067] In the polymerization, a chain transfer agent may be used for the purpose of molecular weight adjustment, etc. That is, the PSA composition may contain a chain transfer agent. Examples of the chain transfer agent include α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, 2,3-dimercapto-1-propanol, and α-methylstyrene dimer. One type of chain transfer agent may be used alone, or two or more types may be used.

[0068] The pressure-sensitive adhesive composition may contain a silane coupling agent. The content of the silane coupling agent is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, per 100 parts by mass of the total amount of polymerizable components (or the total amount of base polymers). The content is preferably 5 parts by mass or less, more preferably 3 parts by mass or less.

[0069] The pressure-sensitive adhesive composition may contain other components in addition to the above-mentioned components, as long as the effects of the present invention are not impaired. Examples of the other components include curing agents, curing catalysts, crosslinking accelerators, tackifying resins (rosin derivatives, polyterpene resins, petroleum resins, oil-soluble phenols, etc.), antioxidants, fillers (metal powders, organic fillers, inorganic fillers, etc.), colorants (pigments, dyes, etc.), antioxidants, plasticizers, softeners, surfactants, antistatic agents, surface lubricants, leveling agents, light stabilizers, UV absorbers, polymerization inhibitors, rust inhibitors, granular materials, foil-like materials, flame retardants, and ion trapping agents. Each of the other components may be used alone or in combination of two or more.

[0070] The pressure-sensitive adhesive composition is preferably solventless. That is, the pressure-sensitive adhesive composition preferably does not contain or substantially does not contain an organic solvent. The organic solvent is not particularly limited as long as it is an organic compound used as a solvent, and examples thereof include hydrocarbon solvents such as cyclohexane, hexane, and heptane; aromatic solvents such as toluene and xylene; ester solvents such as ethyl acetate and methyl acetate; ketone solvents such as acetone and methyl ethyl ketone; and alcohol solvents such as methanol, ethanol, butanol, and isopropyl alcohol. The organic solvent may be a mixed solvent containing two or more organic solvents.

[0071] In the PSA composition, "substantially free of" organic solvent means that the organic solvent is not actively blended in, except in cases where the organic solvent is inevitably mixed in. Specifically, a PSA composition having an organic solvent content of 1.0% by mass or less (preferably 0.5% by mass or less, and more preferably 0.2% by mass or less) relative to the total amount of the PSA composition (total mass, 100% by mass) can be said to be substantially free of organic solvent.

[0072] The pressure-sensitive adhesive composition can be prepared by a known or conventional method, for example, by mixing an additive, if necessary, with a mixture of polymerizable components or a partially polymerized product thereof.

[0073] The pressure-sensitive adhesive composition of the present invention preferably has a shear storage modulus (G') at -20°C of 70 kPa or less, more preferably 60 kPa or less, even more preferably 55 kPa or less, and particularly preferably 50 kPa or less. When the shear storage modulus is 70 kPa or less, the pressure-sensitive adhesive sheet has high flexible deformation properties in the low temperature range and excellent flex resistance in the low temperature range. Furthermore, from the viewpoint of ensuring the cohesive strength of the pressure-sensitive adhesive sheet in the low temperature range, the shear storage modulus (G') at -20°C is preferably 30 kPa or more, more preferably 35 kPa or more, and even more preferably 40 kPa or more.

[0074] From the viewpoint of ensuring a high degree of flexible deformability that enables the pressure-sensitive adhesive composition of the present invention to conform to an adherend when the adherend deforms, the shear storage modulus (G') at 25°C is preferably 40 kPa or less, more preferably 35 kPa or less, and more preferably 30 kPa or less. The shear storage modulus (G') at 25°C is preferably 4 kPa or more, more preferably 8 kPa or more, and even more preferably 10 kPa or more.

[0075] The shear storage modulus (G') at -20°C and the shear storage modulus (G') at 25°C can be obtained by measuring the PSA layer formed from the PSA composition. The shear storage modulus (G') at -20°C and the shear storage modulus (G') at 25°C can be adjusted by adjusting the type of base polymer constituting the PSA composition, the monomer composition, the molecular weight of the base polymer, the type and content of additives such as crosslinking agents, etc.

[0076] The pressure-sensitive adhesive composition of the present invention may have any form, such as a solvent-based, emulsion-based, hot-melt-based, or solventless type. Solvent-free pressure-sensitive adhesive compositions do not require a step of volatilizing and removing the solvent from a coating film of the composition during the process of producing a pressure-sensitive adhesive layer from the composition. Therefore, a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition of the present invention is suitable for reducing environmental impact.

[0077] [Pressure-sensitive adhesive layer] A pressure-sensitive adhesive layer can be formed using the pressure-sensitive adhesive composition. A pressure-sensitive adhesive layer formed using the pressure-sensitive adhesive composition of the present invention may be referred to as the "pressure-sensitive adhesive layer of the present invention." The pressure-sensitive adhesive layer can be produced, for example, by applying the pressure-sensitive adhesive composition to a release-treated surface of a release liner or a substrate to form a pressure-sensitive adhesive composition layer, and then solidifying the pressure-sensitive adhesive composition layer by removing the solvent through heating or by polymerization through irradiation with radiation to form a base polymer.

[0078] The pressure-sensitive adhesive layer may be a pressure-sensitive adhesive layer that is cured by irradiation with radiation (a radiation-curable pressure-sensitive adhesive layer), or may be a pressure-sensitive adhesive layer that is not cured by irradiation with radiation (a radiation-non-curable pressure-sensitive adhesive layer). Examples of the radiation include electron beams, ultraviolet rays, α rays, β rays, γ rays, and X-rays.

[0079] The pressure-sensitive adhesive layer of the present invention is preferably a sheet-shaped pressure-sensitive adhesive (solvent-free pressure-sensitive adhesive sheet) formed from a solvent-free pressure-sensitive adhesive composition. Therefore, the pressure-sensitive adhesive layer (solvent-free pressure-sensitive adhesive composition) of the present invention preferably contains at least a photopolymerizable polymer as a base polymer. The photopolymerizable polymer is a polymer formed by a polymerization method in which a polymerization reaction of a polymerizable component is promoted by irradiation with radiation such as ultraviolet light.

[0080] The PSA composition may be applied (coated) using a known coating method, such as a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, bar coater, knife coater, spray coater, comma coater, or direct coater.

[0081] The thickness of the pressure-sensitive adhesive layer of the present invention is preferably 5 μm or more, more preferably 15 μm or more, and even more preferably 20 μm or more from the viewpoint of ensuring sufficient adhesion to an adherend. The thickness of the pressure-sensitive adhesive layer of the present invention is preferably 300 μm or less, more preferably 200 μm or less, even more preferably 100 μm or less, and even more preferably 70 μm or less from the viewpoint of thinning the device in which it is used.

[0082] [Adhesive Sheet] An adhesive sheet can be obtained using the adhesive layer of the present invention. An adhesive sheet comprising the adhesive layer of the present invention may be referred to as the "adhesive sheet of the present invention." The adhesive sheet may be a double-sided adhesive sheet in which both sides are adhesive layer surfaces, or a single-sided adhesive sheet in which only one side is an adhesive layer surface. Among these, a double-sided adhesive sheet is preferred from the viewpoint of bonding two members together. In this specification, the term "adhesive sheet" also includes a tape-like material, i.e., an "adhesive tape." In addition, in this specification, the surface of the adhesive layer may be referred to as the "adhesive surface."

[0083] The PSA sheet may be a so-called "substrate-less" PSA sheet that does not have a substrate (substrate layer) (hereinafter, sometimes referred to as a "substrate-less PSA sheet"), or may be a PSA sheet that has a substrate (hereinafter, sometimes referred to as a "substrate-attached PSA sheet"). Examples of the substrate-less PSA sheet include a double-sided PSA sheet consisting solely of the PSA layer of the present invention, and a double-sided PSA sheet consisting of the PSA layer of the present invention and a PSA layer other than the PSA layer of the present invention (sometimes referred to as an "other PSA layer"). On the other hand, examples of substrate-attached PSA sheets include PSA sheets having the PSA layer of the present invention on at least one side of a substrate. Among these, substrate-less PSA sheets (substrate-less double-sided PSA sheets) are preferred, and substrate-less double-sided PSA sheets consisting solely of the PSA layer of the present invention are more preferred. PSA sheets having PSA layers on both sides of a substrate (substrate-attached double-sided PSA sheets) are also preferred. The PSA layers on both sides of the substrate-attached double-sided PSA sheet may both be the PSA layer of the present invention, or one may be the PSA layer of the present invention and the other may be another PSA layer. The "substrate (substrate layer)" does not include a release liner that is peeled off when the pressure-sensitive adhesive sheet is used (applied).

[0084] The substrate is an element that functions as a support for the pressure-sensitive adhesive layer in the pressure-sensitive adhesive sheet. Examples of the substrate include a plastic substrate (particularly a plastic film). The substrate may be a single layer or a laminate of the same or different substrates.

[0085] The substrate is not particularly limited, and examples thereof include various optical films such as plastic films, antireflection (AR) films, antiglare (AG) films, polarizing plates, and retardation plates. Examples of materials for the plastic films include polyester resins such as polyethylene terephthalate (PET), acrylic resins such as polymethyl methacrylate (PMMA), polycarbonate, triacetyl cellulose (TAC), polysulfone, polyarylate, polyimide, polyvinyl chloride, polyvinyl acetate, polyethylene, polypropylene, ethylene-propylene copolymers, cyclic olefin polymers such as "ARTON" (cyclic olefin polymer, manufactured by JSR Corporation) and "ZEONOR" (cyclic olefin polymer, manufactured by Zeon Corporation), and fluorine-based polymers. These plastic materials may be used alone or in combination of two or more.

[0086] The surface of the substrate on which the pressure-sensitive adhesive layer is provided may be subjected to surface treatments such as physical treatments such as corona discharge treatment, plasma treatment, sand matting treatment, ozone exposure treatment, flame exposure treatment, high-voltage shock exposure treatment, and ionizing radiation treatment; chemical treatments such as chromic acid treatment; and adhesion-enhancing treatments using a coating agent (primer), for the purpose of improving adhesion and retention with the pressure-sensitive adhesive layer. The surface treatment for improving adhesion is preferably applied to the entire surface of the substrate on which the pressure-sensitive adhesive layer is provided.

[0087] The pressure-sensitive adhesive sheet may have a release liner on its adhesive surface until use. The release liner protects the adhesive surface that contacts the pressure-sensitive adhesive layer until use, and is peeled off when the pressure-sensitive adhesive layer is used. When the pressure-sensitive adhesive sheet is a double-sided pressure-sensitive adhesive sheet, each adhesive surface may be protected by two release liners, or may be protected by a single release liner with release surfaces on both sides and wound into a roll. The release liner is used as a protective material for the pressure-sensitive adhesive layer, and is peeled off when the pressure-sensitive adhesive sheet is attached to the adherend. When the pressure-sensitive adhesive sheet is a substrate-less pressure-sensitive adhesive sheet, the release liner also serves as a support for the pressure-sensitive adhesive layer. The release liner is not necessarily provided.

[0088] Examples of the substrate for the release liner include polyethylene films, polypropylene films, polybutene films, polybutadiene films, polymethylpentene films, polyvinyl chloride films, vinyl chloride copolymer films, polyethylene terephthalate films, polyethylene naphthalate films, polybutylene terephthalate films, polyurethane films, ethylene vinyl acetate films, ionomer resin films, ethylene-(meth)acrylic acid copolymer films, ethylene-(meth)acrylic acid ester copolymer films, polystyrene films, polycarbonate films, polyimide films, and fluororesin films. Crosslinked films of these may also be used. Laminated films of these may also be used.

[0089] The release surface of the release liner (particularly the surface that comes into contact with the pressure-sensitive adhesive layer) is preferably subjected to a release treatment. Examples of release agents used for the release treatment include alkyd-based, silicone-based, fluorine-based, unsaturated polyester-based, polyolefin-based, and wax-based release agents.

[0090] The thickness of the release liner is not particularly limited, but is, for example, about 20 to 150 μm.

[0091] Fig. 1 is a cross-sectional schematic diagram showing one embodiment of a pressure-sensitive adhesive sheet of the present invention. The pressure-sensitive adhesive sheet 1 shown in Fig. 1 is a substrate-less double-sided pressure-sensitive adhesive sheet consisting of a single pressure-sensitive adhesive layer 2 that is the pressure-sensitive adhesive layer of the present invention, and release liners 3 and 4 are provided on both pressure-sensitive adhesive surfaces, respectively.

[0092] [Uses] The pressure-sensitive adhesive composition of the present invention, the pressure-sensitive adhesive layer of the present invention, and the pressure-sensitive adhesive sheet of the present invention are preferably used for optical applications, i.e., for applications in which they are bonded to optical members. More specifically, they are used, for example, for applications in which optical members are bonded (for bonding optical members) and for manufacturing products (optical products) in which the optical members are used. The pressure-sensitive adhesive composition of the present invention, the pressure-sensitive adhesive layer of the present invention, and the pressure-sensitive adhesive sheet of the present invention are used for optical applications, and are therefore highly reliable.

[0093] The pressure-sensitive adhesive layer of the present invention and the pressure-sensitive adhesive sheet of the present invention are used, for example, in optical components of electrical and electronic devices, when attaching (mounting) various members or components to predetermined locations (e.g., housings, front panels, window portions, etc.). The term "electrical and electronic devices" refers to devices that fall into at least one of electrical and electronic devices. Examples of the electrical and electronic devices include image display devices such as liquid crystal displays, organic / inorganic electroluminescence displays, and plasma displays, as well as portable electronic devices. Examples of the image display devices include image display devices in portable electronic devices, in-vehicle displays, and digital signage (electronic signboards and electronic bulletin boards). The image display devices may be of a form (structure) such as a so-called "rigid type" or a so-called "flexible type," or may be of a form (structure) that can be bent or folded, such as a so-called "foldable type" or "rollable type."

[0094] Examples of the portable electronic devices include mobile phones, smartphones, tablet computers, notebook computers, various wearable devices (for example, wristwear devices worn on the wrist like a wristwatch, modular devices worn on a part of the body with a clip or strap, eyewear devices including eyeglasses (monocular and binocular, including head-mounted devices), clothing devices attached to shirts, socks, hats, etc. as accessories, earwear devices attached to the ears like earphones), digital cameras, digital video cameras, audio equipment (portable music players, IC recorders, etc.), calculators (calculators, etc.), portable game devices, electronic dictionaries, electronic organizers, e-books, in-car information devices, portable radios, portable televisions, portable printers, portable scanners, portable modems, etc. In this specification, the term "portable" does not simply mean that the device is portable, but rather means that the device has a level of portability that allows an individual (average adult) to carry it relatively easily.

[0095] The pressure-sensitive adhesive composition of the present invention, the pressure-sensitive adhesive layer of the present invention, and the pressure-sensitive adhesive sheet of the present invention have excellent flex resistance in low-temperature environments. Therefore, even when used in a location where repeated bending occurs in a low-temperature environment, creases or marks are unlikely to form at the bent location, and the pressure-sensitive adhesive sheet is unlikely to peel off. For this reason, the pressure-sensitive adhesive sheet of the present invention is preferably used in a location where it is repeatedly folded. Therefore, the pressure-sensitive adhesive composition of the present invention, the pressure-sensitive adhesive layer of the present invention, and the pressure-sensitive adhesive sheet of the present invention are preferably used for bonding components (particularly between components) in electrical and electronic devices that are used while being folded, such as electrical and electronic devices having a foldable image display device (flexible display) (particularly a foldable image display device (foldable display)).

[0096] The above-described embodiments have been described to facilitate understanding of the present invention, and are not intended to limit the present invention.

[0097] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Note that the number of parts (parts by mass) shown in Table 1 all indicates the number of parts of each component.

[0098] Example 1 (Preparation of Prepolymer Composition) In a flask, 0.05 parts by mass of 2,2-dimethoxy-1,2-diphenylethan-1-one (trade name "Omnirad 651", manufactured by IGM Resins) as a first photopolymerization initiator and 0.05 parts by mass of 1-hydroxycyclohexylphenyl ketone (trade name "Omnirad 184", manufactured by IGM Resins) as a second photopolymerization initiator were added to a monomer mixture containing 54 parts by mass of 2-hexyldecyl acrylate (2HDA), 35 parts by mass of lauryl acrylate (LA), and 11 parts by mass of 4-hydroxybutyl acrylate (4HBA). The mixture was then irradiated with ultraviolet light in a nitrogen atmosphere to polymerize a portion of the polymerizable components in the mixture, thereby obtaining a first prepolymer composition (containing a polymerizable component that had not undergone a polymerization reaction) with a polymerization rate of approximately 10%.

[0099] (Preparation of Pressure-Sensitive Adhesive Composition) 100 parts by mass of the prepolymer composition, 0.06 parts by mass of dipentaerythritol hexaacrylate (DPHA) as a crosslinking agent, and 0.3 parts by mass of a silane coupling agent (trade name "KBM-403", manufactured by Shin-Etsu Chemical Co., Ltd.) were mixed to prepare a pressure-sensitive adhesive composition.

[0100] (Formation of Pressure-Sensitive Adhesive Layer) A long first release liner was run in the length direction of the release liner, and the coating process, first lamination process, photocuring process, peeling process, heating process, and second lamination process were carried out in sequence (roll-to-roll process). The first release liner was a 75 μm thick polyethylene terephthalate (PET) film with one side treated with silicone release. The running speed of the first release liner was 4.0 m / min.

[0101] In the coating step, the PSA composition was applied to the release-treated surface of a first release liner using a roll coater to form a coating film. In the first lamination step, the release-treated surface of a second release liner, one side of which had been treated with a silicone release agent, was laminated to the coating film on the first release liner. The second release liner was a long PET film (75 μm thick) with one side treated with a silicone release agent.

[0102] In the photo-curing step, the coating film between the release liners was irradiated with ultraviolet (UV) rays to photo-cure the coating film, thereby forming a 50 μm thick pressure-sensitive adhesive layer. In the UV irradiation, a black light was used as the irradiation light source, and the irradiation intensity was about 6.5 mW / cm. ２ The cumulative irradiation light amount was 1200 mJ / cm ２ It was decided.

[0103] In the peeling step, the second release liner was peeled from the PSA layer on the first release liner. In the heating step, first heating and second heating were carried out sequentially in a hot air heating oven to dry the PSA layer on the first release liner. In the first heating, the heating temperature (first heating temperature) was 120°C. In the second heating, the heating temperature (second heating temperature) was 90°C.

[0104] In the second lamination step, the release-treated surface of a third release liner, one side of which had been treated with a silicone release agent, was laminated to the pressure-sensitive adhesive layer on the first release liner. The third release liner was a long PET film (75 μm thick) with one side treated with a silicone release agent.

[0105] In this manner, a pressure-sensitive adhesive sheet of Example 1 with double-sided release liners was prepared.

[0106] Examples 2 to 6, Comparative Examples 1 and 2 PSA layers and PSA sheets of each example were produced in the same manner as in Example 1, except that in the preparation of the prepolymer composition, the monomer composition was changed as shown in Table 1. In Table 1, "2EHA" represents 2-ethylhexyl acrylate, "NOAA" represents n-octyl acrylate, "CBA" represents ethyl carbitol acrylate (ethoxyethoxyethyl acrylate), and "2EHCA" represents 2-ethylhexyl carbitol acrylate.

[0107] <Evaluation> The pressure-sensitive adhesive sheets of the Examples and Comparative Examples were evaluated as follows, and the evaluation results are shown in the table below.

[0108] (Shear Storage Modulus) The required number of measurement samples were prepared for each pressure-sensitive adhesive sheet. Specifically, multiple pieces of pressure-sensitive adhesive sheet cut from the pressure-sensitive adhesive sheet were first laminated together to prepare a sample sheet approximately 1.5 mm thick. Next, this sheet was punched out to obtain cylindrical pellets (7.9 mm in diameter) as measurement samples. The measurement samples were then fixed to a 7.9 mm diameter parallel plate jig using a dynamic viscoelasticity measuring device (product name "Discovery Hybrid Reometer-2 (DHR-2)", manufactured by TA Instruments) and subjected to dynamic viscoelasticity measurement. In this measurement, the measurement mode was shear mode, the measurement temperature range was -50°C to 150°C, the heating rate was 5°C / min, and the frequency was 1 Hz. The shear storage modulus at predetermined temperatures (-20°C, 25°C) was read from the measurement results.

[0109]

[0110] As shown in Table 1, the PSA sheets of the Examples were evaluated as having a low shear storage modulus at -20°C and excellent flex resistance in the low temperature range. On the other hand, when 2EHA was used instead of 2HDA (Comparative Example), the PSA sheets were evaluated as having a high shear storage modulus at -20°C and poor flex resistance in the low temperature range.

[0111] Variations of the invention according to the present disclosure are described below. [Appendix 1] A pressure-sensitive adhesive composition comprising a (meth)acrylic acid ester having an ester moiety, the ester moiety having a branch point on the 1st to 4th atom in the ester moiety from the oxygen atom in the ester bond, and the branch point having two or more hydrocarbon group-containing chains, each having four or more atoms constituting the chain, on the branch point. [Appendix 2] The pressure-sensitive adhesive composition according to Appendices 1, wherein the difference between the number of atoms constituting the main chain of the ester moiety and the number of atoms constituting a side chain in the hydrocarbon group-containing chain [number of atoms constituting the main chain - number of atoms constituting the side chain] is two or more. [Appendix 3] The pressure-sensitive adhesive composition according to Appendices 1 or 2, wherein the atoms constituting the main chain in the ester moiety are only carbon atoms, or carbon atoms and oxygen atoms. [Appendix 4] The pressure-sensitive adhesive composition according to any one of Appendices 1 to 3, wherein at least one of the hydrocarbon group-containing chains is an aliphatic hydrocarbon group. [Appendix 5] The pressure-sensitive adhesive composition according to any one of Appendices 1 to 4, wherein a proportion of the (meth)acrylic acid ester is 15 mass% or more relative to the total amount of polymerizable components in the pressure-sensitive adhesive composition. [Appendix 6] The pressure-sensitive adhesive composition according to Appendices 1 to 5, wherein a shear storage modulus (G') at -20°C is 70 kPa or less.

[0112] 1 adhesive sheet 2 adhesive layer 3, 4 release liner

Claims

1. A pressure-sensitive adhesive composition comprising a (meth)acrylic acid ester and / or a polymer thereof, which has an ester moiety with a branch point on the 1st to 4th atom in the ester moiety from the oxygen atom in the ester bond, and which has two or more hydrocarbon group-containing chains on the branch point, each chain having four or more atoms.

2. The pressure-sensitive adhesive composition according to claim 1, wherein the difference between the number of atoms constituting the main chain of the ester moiety and the number of atoms constituting the side chain of the hydrocarbon group-containing chain [number of atoms constituting the main chain - number of atoms constituting the side chain] is 2 or more.

3. The pressure-sensitive adhesive composition according to claim 1 or 2, wherein the atoms constituting the main chain of the ester moiety are carbon atoms only, or carbon atoms and oxygen atoms.

4. The pressure-sensitive adhesive composition according to claim 1 or 2, wherein at least one of the hydrocarbon group-containing chains is an aliphatic hydrocarbon group.

5. The pressure-sensitive adhesive composition according to claim 1 or 2, wherein the proportion of the (meth)acrylic acid ester is 15 mass % or more based on the total amount of polymerizable components in the pressure-sensitive adhesive composition.

6. The pressure-sensitive adhesive composition according to claim 1 or 2, which has a shear storage modulus (G') at -20°C of 70 kPa or less.

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