Adhesive sheet

Abstract

To provide an adhesive sheet that exhibits excellent adhesion to adherends under high-temperature and high-humidity conditions and suppresses excessive elongation when bent.SOLUTION: An adhesive sheet 1 is used for bonding components of a foldable optical member. The adhesive sheet 1 comprises an adhesive layer including a structure where an acrylic polymer is crosslinked with an aromatic polyisocyanate, where the adhesive layer has a storage modulus at 60°C of 40-100 kPa when measured at a frequency of 1 Hz, a gel fraction of 20-65%, and a thickness of 5-50 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a pressure-sensitive adhesive sheet. [Background technology]

[0002] Adhesive sheets used in image display devices and portable electronic devices such as mobile phones, smartphones, digital cameras, liquid crystal displays (LCDs), and organic light-emitting diode (OLED) displays require a variety of performance characteristics, including high adhesive strength.

[0003] In recent years, organic electroluminescence (EL) panels using bendable substrates (flexible substrates) such as resin films have been put to practical use, and foldable flexible displays have been proposed. In flexible displays, not only are display panels such as organic EL panels bendable, but the components are also bendable, and these components are bonded together via adhesive sheets. In foldable displays, the display is repeatedly bent at the same location. At the bent point, compressive stress is applied to the inside and tensile stress to the outside, causing distortion at the bent point and its surroundings, which raises concerns about creases or marks at the bent point or peeling of the adhesive layer.

[0004] For example, Patent Documents 1 to 7 disclose pressure-sensitive adhesive sheets that are intended to have bending resistance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-111754 [Patent Document 2] Japanese Patent Application Publication No. 2019-108498 [Patent Document 3] Japanese Patent Publication No. 2020-111734 [Patent Document 4] Japanese Patent Publication No. 2020-139034 [Patent Document 5] Japanese Patent Publication No. 2020-164575 [Patent Document 6] Japanese Patent Application Publication No. 2018-27996 [Patent Document 7] Japanese Patent Application Publication No. 2019-218513 Summary of the Invention [Problem to be solved by the invention]

[0006] Low-elasticity polymers are often used as adhesives for adhesive sheets with excellent flex resistance used in foldable optical components such as foldable displays. However, adhesive sheets using low-elasticity polymers have the problem of poor adhesion to adherends in high-temperature, high-humidity environments. Furthermore, such adhesive sheets are designed so that when the foldable optical component is bent, the adhesive sheet follows the bending of the optical component, but there is a problem that pressure is applied to the adhesive sheet when bent, causing excessive stretching. Excessive stretching of the adhesive sheet can cause the problem of glue overflow.

[0007] The present invention was conceived under these circumstances, and its purpose is to provide a pressure-sensitive adhesive sheet that has excellent adhesion to an adherend in a high-temperature, high-humidity environment and that is inhibited from excessively stretching when bent. [Means for solving the problem]

[0008] As a result of extensive research to achieve the above object, the present inventors have found that a specific pressure-sensitive adhesive sheet exhibits excellent adhesion to an adherend in a high-temperature, high-humidity environment and is inhibited from excessive elongation when bent. The present invention was completed based on these findings.

[0009] That is, the present invention is a pressure-sensitive adhesive sheet used for bonding components of a foldable optical member together, Provided is a pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer that includes a structure in which an acrylic polymer is crosslinked with an aromatic polyisocyanate, has a storage modulus of 40 to 100 kPa at 60°C when measured at a frequency of 1 Hz, a gel fraction of 20 to 65%, and a thickness of 5 to 50 μm.

[0010] The content of the acrylic polymer in the pressure-sensitive adhesive layer is preferably 90% by mass or more.

[0011] The acrylic polymer preferably contains, as monomer components, an alkyl (meth)acrylate and (meth)acrylic acid.

[0012] The (meth)acrylic acid alkyl ester is preferably a (meth)acrylic acid alkyl ester having an alkyl group having 2 to 6 carbon atoms.

[0013] The present invention also provides an electric / electronic device comprising the above-mentioned pressure-sensitive adhesive sheet, wherein both adhesive surfaces of the pressure-sensitive adhesive sheet bond and fix components provided in the foldable optical component to each other. [Effects of the Invention]

[0014] The pressure-sensitive adhesive sheet of the present invention has excellent adhesion to an adherend under high-temperature, high-humidity environments and is suppressed from excessive elongation when bent. Therefore, when used to bond components of a foldable optical component together under high-temperature, high-humidity environments, the pressure-sensitive adhesive sheet is less likely to peel from the adherend and adhesive extrusion is less likely to occur when the foldable optical component is bent. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view of a pressure-sensitive adhesive sheet according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] [Adhesive sheet] A pressure-sensitive adhesive sheet according to one embodiment of the present invention comprises a pressure-sensitive adhesive layer having a structure in which an acrylic polymer is crosslinked with an aromatic polyisocyanate, a storage modulus of 40 to 100 kPa at 60°C measured at a frequency of 1 Hz, a gel fraction of 20 to 65%, and a thickness of 5 to 50 μm. In this specification, the pressure-sensitive adhesive layer may also be referred to as the "pressure-sensitive adhesive layer of the present invention."

[0017] The pressure-sensitive adhesive sheet of the present invention comprises the pressure-sensitive adhesive layer of the present invention. That is, the pressure-sensitive adhesive sheet of the present invention is a so-called "substrate-less type" pressure-sensitive adhesive sheet that does not have a substrate (substrate layer). In this specification, a "substrate-less type" pressure-sensitive adhesive sheet may also be referred to as a "substrate-less pressure-sensitive adhesive sheet" to distinguish it from a "substrate-attached pressure-sensitive adhesive sheet" that is a type of pressure-sensitive adhesive sheet that has a substrate. The "substrate (substrate layer)" mentioned above refers to a support, and is the part that is attached to the adherend together with the pressure-sensitive adhesive layer when the pressure-sensitive adhesive sheet is used (attached) to the adherend. The release liner that is peeled off when the pressure-sensitive adhesive sheet is used (attached) is not included in the substrate.

[0018] Figure 1 is a cross-sectional schematic diagram showing one embodiment of a pressure-sensitive adhesive sheet of the present invention. Pressure-sensitive adhesive sheet 1 shown in Figure 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, with release liner 3 and release liner 4 provided on both adhesive surfaces, respectively.

[0019] (Adhesive Layer of the Present Invention) The pressure-sensitive adhesive sheet of the present invention may have only one pressure-sensitive adhesive layer of the present invention, or may have two or more pressure-sensitive adhesive layers of the present invention. When the pressure-sensitive adhesive sheet of the present invention has two or more pressure-sensitive adhesive layers of the present invention, the multiple pressure-sensitive adhesive layers of the present invention may be the same pressure-sensitive adhesive layer, or may be pressure-sensitive adhesive layers with different compositions, thicknesses, physical properties, etc.

[0020] The pressure-sensitive adhesive layer of the present invention has a storage modulus of 40 to 100 kPa, preferably 50 to 90 kPa, and more preferably 60 to 80 kPa, at 60°C when measured at a frequency of 1 Hz. Having a storage modulus of 40 kPa or more provides excellent adhesion to an adherend in a high-temperature, high-humidity environment. Furthermore, excessive elongation upon bending is suppressed. Having a storage modulus of 100 kPa or less provides excellent flex resistance, with little breakage or marking at the bent position or peeling of the pressure-sensitive adhesive sheet occurring even when repeatedly bent in a high-temperature, high-humidity environment.

[0021] In this specification, the term "storage modulus" refers to the dynamic shear storage modulus (G'), which can be measured, for example, using a viscoelasticity tester in shear mode at a temperature range of -50 to 150°C and a heating rate of 5°C / min while applying a shear strain at a frequency of 1 Hz, and can be obtained as the storage modulus at 60°C.

[0022] The pressure-sensitive adhesive layer of the present invention has a gel fraction (mass proportion of solvent-insoluble components) of 20 to 65%, preferably 25 to 55%, and more preferably 30 to 50%. A gel fraction of 20% or more suppresses excessive elongation when bent. A gel fraction of 65% or less provides an appropriate cohesive strength for the pressure-sensitive adhesive layer, resulting in excellent adhesion to the adherend in high-temperature, high-humidity environments. The gel fraction can be controlled, for example, by the monomer composition and weight-average molecular weight of the acrylic polymer, the amount of crosslinking agent used (added), etc.

[0023] Specifically, the gel fraction is a value calculated by the following "Method for Measuring Gel Fraction," for example. (Method for measuring gel fraction) Approximately 0.2 g of the adhesive layer was sampled from the adhesive sheet, wrapped in a porous tetrafluoroethylene sheet (trade name "NTF1122", manufactured by Nitto Denko Corporation) with an average pore size of 0.2 μm, and then tied with kite string. The mass at this time was measured and this mass was defined as the pre-immersion mass. The pre-immersion mass was the total mass of the mass (B) of the adhesive layer (the adhesive layer sampled above), the mass of the tetrafluoroethylene sheet, and the mass of the kite string. The total mass of the tetrafluoroethylene sheet and the kite string was also measured, and this mass was defined as the wrapping mass (A). Next, the pressure-sensitive adhesive layer is wrapped in a tetrafluoroethylene sheet and tied with kite string (referred to as the "sample"), which is placed in a 50 ml container filled with ethyl acetate and allowed to stand for 7 days at 23° C. Thereafter, the sample (after ethyl acetate treatment) is removed from the container, transferred to an aluminum cup, and dried in a dryer at 130° C. for 2 hours to remove the ethyl acetate. Thereafter, the mass is measured and this mass is defined as the post-immersion mass (C). Then, the gel fraction is calculated using the following formula. Gel fraction [mass%] = 100 × (CA) / B

[0024] The pressure-sensitive adhesive layer of the present invention preferably has a 180° peel adhesion strength against a SUS flat surface at 23°C and 50% RH of 6.3 N / 25 mm or more, more preferably 8.0 N / 25 mm or more, and more preferably 10.0 N / 25 mm or more. When the peel adhesion strength is 6.3 N / 25 mm or more, the pressure-sensitive adhesive sheet is less likely to peel even when repeatedly bent in a high-temperature, high-humidity environment, and has excellent flex resistance. The higher the peel adhesion strength, the less likely peeling occurs, but it is, for example, 50.0 N / 25 mm or less.

[0025] The pressure-sensitive adhesive layer of the present invention preferably has an elongation percentage (tensile elongation percentage) of 100% or less, more preferably 90% or less, and even more preferably 80% or less when stretched at a rate of 50 mm / min and the tensile stress described below is 0.1 MPa. If the tensile elongation percentage is 100% or less, excessive elongation when bent is suppressed, and adhesive extrusion is less likely to occur when the foldable optical component is folded. The tensile elongation percentage is preferably 10% or more, more preferably 20% or more, and even more preferably 30% or more, from the viewpoint of easily following the fold when the foldable optical component is folded and providing excellent adhesion to the adherend. The tensile elongation percentage is, for example, 10% or more, more preferably 20% or more, and even more preferably 30% or more, when the pressure-sensitive adhesive layer is stretched at a rate of 1.0 mm 2 The sample was rolled into a cylindrical shape with a cross-sectional area of ​​1.0 mm, and measured under conditions of a chuck distance of 10 mm, a tensile speed of 50 mm / min, and a tensile direction of 180°, in an environment of 23°C and 50% RH. The value was calculated using the following formula when the tensile stress was 0.1 MPa. Tensile elongation [%] = (length between chucks when stretched - length between chucks before stretching) / length between chucks before stretching × 100 Tensile stress [MPa] = Tensile force (N) / Cross-sectional area (mm 2 )

[0026] The thickness of the pressure-sensitive adhesive layer of the present invention is 5 to 50 μm, preferably 10 to 40 μm, and more preferably 15 to 35 μm. A thickness of 5 μm or more provides excellent conformability and adhesion to adherends, and excellent adhesion even in high-temperature environments, making peeling less likely to occur when the layer is bent. A thickness of 50 μm or less suppresses tensile stress and compressive stress that occur when the layer is bent, making peeling and lifting less likely to occur.

[0027] The pressure-sensitive adhesive layer of the present invention has a structure in which an acrylic polymer is crosslinked with an aromatic polyisocyanate. The pressure-sensitive adhesive layer of the present invention is preferably an acrylic pressure-sensitive adhesive layer containing an acrylic polymer as a base polymer that exhibits adhesion. In this specification, the base polymer refers to a main component among the polymer components in the pressure-sensitive adhesive that constitutes the pressure-sensitive adhesive layer, for example, a polymer component that is contained in an amount of more than 50 mass%.

[0028] The content of the base polymer (particularly, acrylic polymer) in the adhesive layer of the present invention is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more, relative to 100% by mass of the total amount of the adhesive layer.

[0029] The acrylic polymer is a polymer containing an acrylic monomer (a monomer having a (meth)acryloyl group in the molecule) as a monomer component constituting the polymer. That is, the acrylic polymer contains a structural unit derived from the acrylic monomer. Only one type of acrylic polymer may be used, or two or more types may be used. Furthermore, the acrylic polymer may contain only one type of acrylic monomer as a monomer component, or two or more types.

[0030] The acrylic polymer is preferably a polymer constituted (formed) with a (meth)acrylic acid alkyl ester as an essential monomer component. That is, the acrylic polymer preferably contains a (meth)acrylic acid alkyl ester as a constituent unit. In this specification, "(meth)acrylic" refers to "acrylic" and / or "methacrylic" (either one or both of "acrylic" and "methacrylic"), and the same applies to other terms.

[0031] The (meth)acrylic acid alkyl ester is preferably a (meth)acrylic acid alkyl ester having a linear or branched alkyl group. Only one type of (meth)acrylic acid alkyl ester may be used, or two or more types may be used.

[0032] The (meth)acrylic acid alkyl ester having a linear or branched alkyl group is not particularly limited, and examples thereof include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, and methyl (meth)acrylate. Examples of the alkyl (meth)acrylate esters include those having a linear or branched alkyl group having 1 to 20 carbon atoms, such as isononyl acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate (lauryl (meth)acrylate), tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate (stearyl (meth)acrylate), isostearyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate. Of these, the alkyl (meth)acrylate esters having an alkyl group having 2 to 6 carbon atoms are preferred.

[0033] The proportion of the (meth)acrylic acid alkyl ester in the total of all monomer components constituting the acrylic polymer (100% by mass) is not particularly limited, but is preferably 50% by mass or more (e.g., 50 to 100% by mass), more preferably 70% by mass or more, even more preferably 85% by mass or more, and particularly preferably 90% by mass or more. The proportion is preferably less than 100% by mass, more preferably 99.5% by mass or less, even more preferably 98% by mass or less, and particularly preferably 97% by mass or less. When the proportion is within the above range, a good quantitative balance with the copolymerizable monomer is achieved, making it possible to form a pressure-sensitive adhesive layer that has good adhesion even when thin.

[0034] In this specification, the above-mentioned "monomer component" refers to a compound having only one polymerizable functional group, and does not include compounds having two or more polymerizable functional groups such as polyfunctional (meth)acrylates.

[0035] The acrylic polymer may contain a copolymerizable monomer together with the (meth)acrylic acid alkyl ester as a monomer component constituting the polymer. That is, the acrylic polymer may contain a copolymerizable monomer as a structural unit. The copolymerizable monomer may be used alone or in combination of two or more.

[0036] The copolymerizable monomer is preferably a carboxyl group-containing monomer and / or an acid anhydride monomer. Examples of the carboxyl group-containing monomer include (meth)acrylic acid, carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid. Examples of the acid anhydride monomer include maleic anhydride and itaconic anhydride.

[0037] By using the above carboxy group-containing monomer and / or acid anhydride monomer, the acrylic polymer can easily form a crosslinked structure with an appropriate crosslinking density with the aromatic polyisocyanate, and the adhesive sheet can have excellent adhesion to an adherend such as a stainless steel plate (SUS304BA plate), and can easily maintain a high storage modulus at 60° C. Among the above carboxy group-containing monomer and / or acid anhydride monomer, a carboxy group-containing monomer is preferred, and (meth)acrylic acid is more preferred.

[0038] The proportion of the carboxyl group-containing monomer and / or acid anhydride monomer in the total amount (100% by mass) of all monomer components constituting the acrylic polymer is not particularly limited, but is preferably 0.2% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, and particularly preferably 2% by mass or more. The proportion is preferably 15% by mass or less, more preferably 12% by mass or less, even more preferably 10% by mass or less, and particularly preferably 7% by mass or less. When the proportion is within the above range, a good quantitative balance with the (meth)acrylic acid alkyl ester is achieved, making it possible to form a pressure-sensitive adhesive layer that has good adhesion even when thin.

[0039] The copolymerizable monomer may further contain a functional group-containing monomer for the purpose of introducing crosslinking points into the acrylic polymer or increasing the cohesive strength of the acrylic polymer. Examples of the functional group-containing monomer include hydroxy group-containing monomers, epoxy group-containing monomers, nitrogen atom-containing monomers, keto group-containing monomers, alkoxysilyl group-containing monomers, sulfonic acid group-containing monomers, and phosphoric acid group-containing monomers. The functional group-containing monomers may be used alone or in combination of two or more.

[0040] Examples of the hydroxy group-containing monomer include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)methyl (meth)acrylate; unsaturated alcohols such as vinyl alcohol and allyl alcohol; and polypropylene glycol mono(meth)acrylate.

[0041] Examples of the epoxy group-containing monomer include glycidyl group-containing monomers such as glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, and allyl glycidyl ether.

[0042] Examples of the nitrogen atom-containing monomer include amide group-containing monomers, amino group-containing monomers, cyano group-containing monomers, and monomers having a nitrogen atom-containing ring. Examples of the amide group-containing monomer include (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N-butyl(meth)acrylamide, N-methylol(meth)acrylamide, N-methylolpropane(meth)acrylamide, N-methoxymethyl(meth)acrylamide, and N-butoxymethyl(meth)acrylamide. Examples of the amino group-containing monomer include aminoethyl(meth)acrylate, N,N-dimethylaminoethyl(meth)acrylate, and t-butylaminoethyl(meth)acrylate. Examples of the cyano group-containing monomer include acrylonitrile and methacrylonitrile. Examples of the monomer 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-vinylmorpholine, N-vinylcaprolactam, and N-(meth)acryloylmorpholine.

[0043] Examples of the keto group-containing monomer include diacetone (meth)acrylamide, diacetone (meth)acrylate, vinyl methyl ketone, vinyl ethyl ketone, allyl acetoacetate, and vinyl acetoacetate.

[0044] Examples of the alkoxysilyl group-containing monomer include 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, and 3-(meth)acryloxypropylmethyldiethoxysilane.

[0045] Examples of the sulfonic acid group-containing monomer include styrenesulfonic acid, allylsulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, (meth)acrylamidopropanesulfonic acid, sulfopropyl (meth)acrylate, and (meth)acryloyloxynaphthalenesulfonic acid.

[0046] Examples of the phosphate group-containing monomer include 2-hydroxyethyl acryloyl phosphate.

[0047] The proportion of the functional group-containing monomer in the total (100% by mass) of all monomer components constituting the acrylic polymer may be, for example, 0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, 5% by mass or more, or 10% by mass or more. The proportion may be, for example, 40% by mass or less, 20% by mass or less, 10% by mass or less, 5% by mass or less, or 2% by mass or less, or may be substantially free of the functional group-containing monomer. The phrase "substantially free of" the functional group-containing monomer means that the other monomers are not actively blended in, except in cases where the functional group-containing monomer is inevitably mixed in.

[0048] The copolymerizable monomer may further contain other monomers. Examples of the other monomers include vinyl ester monomers such as vinyl acetate, vinyl propionate, and vinyl laurate; aromatic vinyl compounds such as styrene, substituted styrene (α-methylstyrene, etc.), and vinyltoluene; cycloalkyl(meth)acrylates such as cyclohexyl(meth)acrylate, cyclopentyl(meth)acrylate, dicyclopentanyl(meth)acrylate, and isobornyl(meth)acrylate; aryl(meth)acrylates (e.g., phenyl(meth)acrylate), and aryloxyalkyl(meth)acrylates (e.g., phenoxyethyl(meth)acrylate). acrylate), aromatic ring-containing (meth)acrylates such as arylalkyl(meth)acrylates (for example, benzyl(meth)acrylate); olefin-based monomers such as ethylene, propylene, isoprene, butadiene, and isobutylene; chlorine-containing monomers such as vinyl chloride and vinylidene chloride; isocyanate group-containing monomers such as 2-(meth)acryloyloxyethyl isocyanate; alkoxy group-containing monomers such as methoxyethyl(meth)acrylate and ethoxyethyl(meth)acrylate; and vinyl ether-based monomers such as methyl vinyl ether and ethyl vinyl ether.

[0049] The proportion of the other monomers in the total of all monomer components constituting the acrylic polymer (100% by mass) may be, for example, 0.05% by mass or more, or 0.5% by mass or more. The proportion may be, for example, 20% by mass or less, 10% by mass or less, 5% by mass or less, or 2% by mass or less, or may be substantially free of the other monomers. "Substantially free of" the other monomers means that the other monomers are not actively blended in, except in cases where the other monomers are inevitably mixed in.

[0050] The weight average molecular weight (Mw) of the acrylic polymer is 10 × 10 4 It is preferable that the ratio is 30×10 or more, and more preferably 30×10 4 More preferably, 50 × 10 4 The above Mw is 10 × 104 Above (especially 50 x 10 4 When the Mw is 300×10 or more, a pressure-sensitive adhesive exhibiting good cohesion is easily obtained. 4 It is preferable that the value is equal to or less than 250×10 4 or less, more preferably 200 × 10 4 The above Mw is 300×10 4 If the molecular weight is less than 1000kJ / g, it is easy to form an adhesive that exhibits moderate fluidity (mobility of polymer chains), resulting in superior flex resistance. The weight average molecular weight is measured by gel permeation chromatography (GPC) and calculated in polystyrene equivalent.

[0051] The glass transition temperature (Tg) of the acrylic polymer is preferably 0°C or lower, preferably -10°C or lower, and more preferably -20°C or lower. When the glass transition temperature is 0°C or lower, even when the adhesive sheet is repeatedly bent in a low-temperature environment, it is less likely to develop creases or marks at the bent portion or peel off. The glass transition temperature is preferably -80°C or higher, more preferably -70°C or higher, even more preferably -66°C or higher, and particularly preferably -60°C or higher.

[0052] The glass transition temperature is a value calculated based on the following formula (X) (Fox formula). 1 / Tg=W1 / Tg1+W2 / Tg2+···+Wn / Tgn (X) [In formula (X), Tg represents the glass transition temperature (unit: K) of the polymer, Tgi (i = 1, 2, ... n) represents the glass transition temperature (unit: K) when monomer i forms a homopolymer, and Wi (i = 1, 2, ... n) represents the mass fraction of monomer i in all monomer components.] The above formula (X) is a calculation formula when the polymer is composed of n types of monomer components: Monomer 1, Monomer 2, . . . , Monomer n.

[0053] In this specification, the "glass transition temperature (Tg) when a homopolymer is formed" (sometimes simply referred to as "Tg of the homopolymer") refers to the "glass transition temperature (Tg) of a homopolymer of the monomer," and specific values ​​are listed in "Polymer Handbook" (3rd Edition, John Wiley & Sons, Inc., 1987). The Tg of a homopolymer of a monomer not described in the above document, other than a monomer having a polyorganosiloxane skeleton, refers to a value obtained, for example, by the following measurement method (see JP 2007-51271 A). Specifically, 100 parts by mass of the monomer, 0.2 parts by mass of 2,2'-azobisisobutyronitrile, and 200 parts by mass of ethyl acetate as a polymerization solvent are placed in a reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, and the mixture is stirred for 1 hour while introducing nitrogen gas. After removing oxygen from the polymerization system in this way, the temperature is raised to 63°C and the reaction is allowed to proceed for 10 hours. The mixture was then cooled to room temperature to obtain a homopolymer solution with a solids concentration of 33% by mass. This homopolymer solution was then cast onto a release liner and dried to prepare a test sample (sheet-like homopolymer) with a thickness of approximately 2 mm. This test sample was then punched into a 7.9 mm diameter disk, sandwiched between parallel plates, and measured for viscoelasticity in shear mode using a viscoelasticity tester (product name "ARES" manufactured by Rheometrics) at a temperature range of -70 to 150°C and a heating rate of 5°C / min while applying a shear strain of 1 Hz. The peak top temperature of tan δ was taken as the Tg of the homopolymer.

[0054] The acrylic polymer is obtained by polymerizing a composition containing at least an acrylic monomer. The polymerization method is not particularly limited, but examples thereof include solution polymerization, emulsion polymerization, bulk polymerization, thermal polymerization, and polymerization by active energy ray irradiation (active energy ray polymerization). Among these, bulk polymerization, thermal polymerization, and active energy ray polymerization are preferred in terms of the transparency of the pressure-sensitive adhesive layer, cost, etc. The obtained acrylic polymer may be any of a random copolymer, a block copolymer, a graft copolymer, etc.

[0055] Various common solvents may be used in the polymerization of the monomer components. Examples of the solvent include organic solvents such as esters such as ethyl acetate and n-butyl acetate; aromatic hydrocarbons such as toluene and benzene; aliphatic hydrocarbons such as n-hexane and n-heptane; alicyclic hydrocarbons such as cyclohexane and methylcyclohexane; and ketones such as methyl ethyl ketone and methyl isobutyl ketone. One or more of the solvents may be used.

[0056] The polymerization initiator, chain transfer agent, emulsifier, etc. used in the radical polymerization of the monomer components are not particularly limited and can be appropriately selected and used. The weight-average molecular weight of the polymer can be controlled by the amounts of the polymerization initiator and chain transfer agent used and the reaction conditions, and the amounts used are appropriately adjusted depending on the types of these.

[0057] As the polymerization initiator used for polymerizing the monomer components, a thermal polymerization initiator, a photopolymerization initiator (photoinitiator), etc. can be used depending on the type of polymerization reaction. Only one of the above polymerization initiators may be used, or two or more of them may be used.

[0058] The thermal polymerization initiator is not particularly limited, but examples thereof include azo polymerization initiators, peroxide polymerization initiators (e.g., persulfates such as dibenzoyl peroxide, tert-butyl permaleate, and potassium persulfate, benzoyl peroxide, and hydrogen peroxide), substituted ethane initiators such as phenyl-substituted ethane, aromatic carbonyl compounds, and redox polymerization initiators. Among these, the azo polymerization initiators disclosed in JP-A-2002-69411 are preferred. Examples of the azo polymerization initiator include 2,2'-azobisisobutyronitrile, 2,2'-azobis-2-methylbutyronitrile, 2,2'-azobis(2-methylpropionate)dimethyl, and 4,4'-azobis-4-cyanovaleric acid. The amount of the thermal polymerization initiator used may be a normal amount, and can be selected, for example, from the range of 0.01 to 5 parts by mass, preferably 0.05 to 3 parts by mass, per 100 parts by mass of the monomer component.

[0059] The photopolymerization initiator is not particularly limited, but examples thereof include benzoin ether-based photopolymerization initiators, acetophenone-based photopolymerization initiators, α-ketol-based photopolymerization initiators, aromatic sulfonyl chloride-based photopolymerization initiators, photoactive oxime-based photopolymerization initiators, benzoin-based photopolymerization initiators, benzyl-based photopolymerization initiators, benzophenone-based photopolymerization initiators, ketal-based photopolymerization initiators, and thioxanthone-based photopolymerization initiators. Other examples include acylphosphine oxide-based photopolymerization initiators and titanocene-based photopolymerization initiators. Examples of the benzoin ether-based photopolymerization initiators include benzoin methyl ether, benzoin ethyl ether, benzoin propyl ether, benzoin isopropyl ether, benzoin isobutyl ether, 2,2-dimethoxy-1,2-diphenylethan-1-one, and anisole methyl ether. Examples of the acetophenone-based photopolymerization initiator include 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexylphenyl ketone, 4-phenoxydichloroacetophenone, and 4-(t-butyl)dichloroacetophenone. Examples of the α-ketol-based photopolymerization initiator include 2-methyl-2-hydroxypropiophenone and 1-[4-(2-hydroxyethyl)phenyl]-2-methylpropan-1-one. Examples of the aromatic sulfonyl chloride-based photopolymerization initiator include 2-naphthalenesulfonyl chloride. Examples of the photoactive oxime-based photopolymerization initiator include 1-phenyl-1,1-propanedione-2-(O-ethoxycarbonyl)-oxime. Examples of the benzoin-based photopolymerization initiator include benzoin. Examples of the benzyl-based photopolymerization initiator include benzil. Examples of the benzophenone-based photopolymerization initiator include benzophenone, benzoylbenzoic acid, 3,3'-dimethyl-4-methoxybenzophenone, polyvinylbenzophenone, α-hydroxycyclohexylphenyl ketone, etc. Examples of the ketal-based photopolymerization initiator include benzyl dimethyl ketal, etc.Examples of the thioxanthone-based photopolymerization initiator include thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, isopropylthioxanthone, 2,4-diisopropylthioxanthone, and dodecylthioxanthone. Examples of the acylphosphine oxide-based photopolymerization initiator include 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide. Examples of the titanocene-based photopolymerization initiator include bis(η5-2,4-cyclopentadien-1-yl)-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)-phenyl)titanium. The amount of the photopolymerization initiator used may be a normal amount, and can be selected, for example, from the range of 0.01 to 5 parts by mass, preferably 0.05 to 3 parts by mass, per 100 parts by mass of the monomer component.

[0060] The pressure-sensitive adhesive layer of the present invention contains a structure in which the acrylic polymer is crosslinked with an aromatic polyisocyanate. By using an aromatic polyisocyanate as a crosslinking agent, the crosslinked polymer is less likely to react with a carboxyl group-containing monomer or an acid anhydride monomer at room temperature, and when crosslinked intentionally, a crosslinked structure with a more appropriate crosslink density is more likely to be formed. The aromatic polyisocyanate may be used alone or in combination of two or more.

[0061] The aromatic polyisocyanate is an aromatic ring compound (a polyfunctional isocyanate aromatic ring compound) having an average of two or more isocyanate groups per molecule. Examples of the aromatic polyisocyanate include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 2,2'-diphenylmethane diisocyanate, 4,4'-diphenylether diisocyanate, 2-nitrodiphenyl-4,4'-diisocyanate, 2,2'-diphenylpropane-4,4'-diisocyanate, Examples of the diisocyanate include 3,3'-dimethyldiphenylmethane-4,4'-diisocyanate, 4,4'-diphenylpropane diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, naphthylene-1,4-diisocyanate, naphthylene-1,5-diisocyanate, 3,3'-dimethoxydiphenyl-4,4'-diisocyanate, xylylene-1,4-diisocyanate, and xylylene-1,3-diisocyanate.

[0062] The content of the aromatic polyisocyanate is not particularly limited, but is preferably more than 1 part by mass and less than 10 parts by mass, more preferably 1.4 to 8 parts by mass, and even more preferably 1.7 to 6 parts by mass, relative to 100 parts by mass of the total amount of the monomer components constituting the acrylic polymer. When the content of the aromatic polyisocyanate is within the above range, the gel fraction and storage modulus at 60°C of the pressure-sensitive adhesive layer can easily be within the ranges of the present invention.

[0063] Crosslinking agents other than the aromatic polyisocyanate may be used for crosslinking the acrylic polymer. Examples of the other crosslinking agents include aliphatic isocyanate crosslinking agents, alicyclic isocyanate crosslinking agents, epoxy crosslinking agents, melamine crosslinking agents, peroxide crosslinking agents, urea crosslinking agents, metal alkoxide crosslinking agents, metal chelate crosslinking agents, metal salt crosslinking agents, carbodiimide crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, amine crosslinking agents, hydrazine crosslinking agents, silicone crosslinking agents, silane crosslinking agents (silane coupling agents), and combinations of organic crosslinking agents and polyfunctional metal chelates.

[0064] The content of the aromatic polyisocyanate in the crosslinking agent is not particularly limited, but is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 99% by mass or more, still more preferably 99.5% by mass or more, and particularly preferably 99.8% by mass or more, relative to 100% by mass of the total amount of the crosslinking agent.

[0065] The pressure-sensitive adhesive layer of the present invention may contain a tackifier. When the pressure-sensitive adhesive layer of the present invention contains a tackifier, the pressure-sensitive adhesive sheet has better interfacial adhesion to the adherend. The above-mentioned tackifier may be used alone or in combination of two or more.

[0066] Examples of the tackifier include a tackifying resin and an oligomer. The oligomer may be present as an oligomer in the pressure-sensitive adhesive layer of the present invention, or may be present in the acrylic polymer as a structural moiety derived from the oligomer. That is, the acrylic polymer may have a structural moiety derived from the oligomer.

[0067] Examples of the tackifying resin include phenol-based tackifying resins, terpene-based tackifying resins, rosin-based tackifying resins, hydrocarbon-based tackifying resins, epoxy-based tackifying resins, polyamide-based tackifying resins, elastomer-based tackifying resins, ketone-based tackifying resins, etc. Only one type of the tackifying resins may be used, or two or more types may be used.

[0068] Examples of the phenolic tackifying resin include terpene phenolic resins, hydrogenated terpene phenolic resins, alkylphenolic resins, and rosin phenolic resins. The terpene phenolic resins are polymers containing terpene residues and phenolic residues, such as copolymers of terpenes and phenolic compounds (terpene-phenol copolymer resins) and phenol-modified terpene homopolymers or copolymers. Examples of terpenes constituting the terpene phenolic resins include monoterpenes such as α-pinene, β-pinene, and limonene (d-isomer, l-isomer, d / l-isomer (dipentene)). The hydrogenated terpene phenolic resins are resins having a hydrogenated structure of the terpene phenolic resins. The alkylphenolic resins are resins (oil-based phenolic resins) obtained from alkylphenols and formaldehyde. Examples of the alkylphenolic resins include novolac and resol types. The rosin phenolic resins are phenol-modified products of rosins or various rosin derivatives described below. The rosin phenolic resin can be obtained, for example, by adding phenol to rosins or various rosin derivatives described below in the presence of an acid catalyst and then thermally polymerizing the resulting mixture.

[0069] Examples of the terpene-based tackifying resin include polymers of terpenes (typically monoterpenes) such as α-pinene, β-pinene, d-limonene, l-limonene, and dipentene. The terpene polymer may be a homopolymer of one type of terpene, or a copolymer of two or more types of terpenes. Examples of homopolymers of one type of terpene include α-pinene polymer, β-pinene polymer, and dipentene polymer. The modified terpene-based tackifying resin is a modified terpene resin (modified terpene resin) obtained by modifying the terpene resin. Examples of the modified terpene resin include styrene-modified terpene resin and hydrogenated terpene resin.

[0070] Examples of the rosin-based tackifying resin include rosins and rosin derivative resins. Examples of the rosins include unmodified rosins (raw rosins) such as gum rosin, wood rosin, and tall oil rosin; and modified rosins (hydrogenated rosin, disproportionated rosin, polymerized rosin, and other chemically modified rosins) obtained by modifying these unmodified rosins through hydrogenation, disproportionation, polymerization, or the like. Examples of the rosin derivative resin include derivatives of the rosins. Examples of the rosin derivative resin include rosin esters, such as unmodified rosin esters, which are esters of unmodified rosin and alcohols, and modified rosin esters, which are esters of modified rosin and alcohols; unsaturated fatty acid-modified rosins, which are rosin esters, which are unsaturated fatty acid-modified rosin esters, which are rosin esters, which are unsaturated fatty acid-modified rosin esters, which are rosin esters, which are rosin alcohols obtained by reducing the carboxyl groups of rosins or the various rosin derivatives described above; and metal salts of rosins or the various rosin derivatives described above. Specific examples of the rosin esters include methyl esters, triethylene glycol esters, glycerin esters, and pentaerythritol esters of unmodified or modified rosin.

[0071] Examples of the hydrocarbon tackifying resin include aliphatic hydrocarbon resins, aromatic hydrocarbon resins, aliphatic cyclic hydrocarbon resins, aliphatic-aromatic petroleum resins (styrene-olefin copolymers, etc.), aliphatic-alicyclic petroleum resins, hydrogenated hydrocarbon resins, coumarone resins, and coumarone-indene resins.

[0072] The softening point of the tackifier resin is preferably 160°C or lower, more preferably 150°C or lower, and even more preferably 140°C or lower. When the softening point is low, the elastic modulus can be maintained low even with a small amount of addition, and the flex resistance in a low-temperature environment is further improved. The softening point is, for example, 60°C or higher, preferably 70°C or higher.

[0073] The weight-average molecular weight of the oligomer is preferably 2,500 to 10,000, and more preferably 3,000 to 8,000. The weight-average molecular weight can be determined in polystyrene equivalent terms by GPC. For example, it can be measured under the following conditions using a high-speed GPC device "HPLC-8120GPC" manufactured by Tosoh Corporation. Column: TSKgel SuperHZM-H / HZ4000 / HZ3000 / HZ2000 Solvent: tetrahydrofuran Flow rate: 0.6ml / min

[0074] The oligomer is preferably an acrylic oligomer composed of an acrylic monomer as an essential monomer component. Examples of the acrylic monomer that constitutes the acrylic oligomer include those exemplified and explained as the monomer components that constitute the acrylic polymer. The acrylic oligomer preferably contains a (meth)acrylic acid ester having an alicyclic hydrocarbon group as a constituent unit. The acrylic monomer contained as the constituent unit may be one type or two or more types.

[0075] The proportion of the (meth)acrylic acid ester having an alicyclic hydrocarbon group in the total amount (100% by mass) of all monomer components constituting the acrylic oligomer is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 55% by mass or more. The proportion is preferably 99% by mass or less, and may be 97% by mass or less.

[0076] In one embodiment, the acrylic oligomer preferably contains a (meth)acrylic acid alkyl ester as a structural unit. The (meth)acrylic acid alkyl ester is preferably methyl methacrylate (MMA). The proportion of the (meth)acrylic acid alkyl ester in all monomer components constituting the acrylic oligomer is preferably 10% by mass or more, more preferably 20% by mass or more. This proportion is preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 45% by mass or less. The acrylic oligomer may also contain the copolymerizable monomer as a structural unit.

[0077] In one embodiment, the acrylic oligomer preferably contains a functional group-containing monomer as a constituent unit. The functional group-containing monomer is preferably a polar group-containing monomer, more preferably a carboxy group-containing monomer. The proportion of the functional group-containing monomer in all monomer components constituting the acrylic oligomer is preferably 3% by mass or more, more preferably 4% by mass or more. The proportion is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less. The acrylic oligomer may also contain the copolymerizable monomer as a constituent unit.

[0078] The oligomer can be obtained by polymerizing a composition containing the monomer components that constitute the oligomer. Examples of such polymerization methods include those exemplified and described above as polymerization methods for acrylic polymers. Among these, bulk polymerization, thermal polymerization, and active energy ray polymerization are preferred. Various common solvents may be used when polymerizing the monomer components. Examples of the solvent include those exemplified and described above as solvents that may be used in the polymerization of acrylic polymers. Only one of the solvents may be used, or two or more may be used. The polymerization initiator, chain transfer agent, emulsifier, etc. used in the radical polymerization of the monomer components are not particularly limited and may be appropriately selected and used.

[0079] The content of the tackifier in the pressure-sensitive adhesive layer of the present invention is not particularly limited, but is preferably 9 parts by mass or less, more preferably 7 parts by mass or less, relative to 100 parts by mass of the total amount of the acrylic polymer. When the content is 9 parts by mass or less, the storage modulus at 60°C can be maintained high. When the tackifier is contained, the content may be, for example, 0.5 parts by mass or more, 1 part by mass or more, or 2 parts by mass or more.

[0080] The pressure-sensitive adhesive layer of the present invention may contain a colorant. The colorant may be a pigment or a dye. Examples of colorants include black colorants, cyan colorants, magenta colorants, and yellow colorants. From the viewpoint of superior visibility and design, black colorants are preferred. The pressure-sensitive adhesive layer may contain only one type of colorant, or two or more types of colorants.

[0081] Examples of black colorants include carbon black, carbon nanotubes, graphite, copper oxide, manganese dioxide, azo pigments such as azomethine azo black, aniline black, perylene black, titanium black, cyanine black, activated carbon, ferrite, magnetite, chromium oxide, iron oxide, molybdenum disulfide, complex oxide black pigments, anthraquinone organic black dyes, azo organic black dyes, etc. Examples of carbon black include furnace black, channel black, acetylene black, thermal black, lamp black, etc. Examples of black colorants include CI Solvent Black 3, 7, 22, 27, 29, 34, 43, and 70; CI Direct Black 17, 19, 22, 32, 38, 51, and 71; CI Acid Black 1, 2, 24, 26, 31, 48, 52, 107, 109, 110, 119, and 154; CI Disperse Black 1, 3, 10, and 24; and CI Pigment Black 1 and 7.

[0082] Examples of cyan colorants include CI Solvent Blue 25, 36, 60, 70, 93, and 95; CI Acid Blue 6 and 45; CI Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:4, 15:5, 15:6, 16, 17, 17:1, 18, 22, 25, 56, 60, 63, 65, and 66; CI Vat Blue 4; CI Pigment Green 7; and the like.

[0083] Examples of magenta colorants include CI Solvent Red 1, 3, 8, 23, 24, 25, 27, 30, 49, 52, 58, 63, 81, 82, 83, 84, 100, 109, 111, 121, and 122; CI Disperse Red 9; CI Solvent Violet 8, 13, 14, 21, and 27; and CI Disperse Red 9. CI Basic Red 1, 2, 9, 12, 13, 14, 15, 17, 18, 22, 23, 24, 27, 29, 32, 34, 35, 36, 37, 38, 39, 40; CI Basic Violet 1, 3, 7, 10, 14, 15, 21, 25, 26, 27, 28. Examples of magenta colorants include CI Pigment Red 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 23, 30, 31, 32, 37, 38, 39, 40, 41, 42, and 48:1. 48:2, 48:3, 48:4, 49, 49:1, 50, 51, 52, 52:2, 53:1, 54, 55, 56, 57:1, 58, 60, 60:1, 63, 63:1, 63:2, 64, 64:1, 67, 68, 81, 83, 87, 88, 89, 90, 92, 101 , 104, 105, 106, 108, 112, 114, 122, 123, 139, 144, 146, 147, 149, 150, 151, 163, 166, 168, 170, 171, 172, 175, 176, 177, 178, 179, 184, 185, 187, 190 , 193, 202, 206, 207, 209, 219, 222, 224, 238, 245; CI Pigment Violet 3, 9, 19, 23, 31, 32, 33, 36, 38, 43, 50; CI Bat Red 1, 2, 10, 13, 15, 23, 29, 35, etc.

[0084] Examples of yellow colorants include CI Solvent Yellow 19, 44, 77, 79, 81, 82, 93, 98, 103, 104, 112, and 162; CI Pigment Orange 31 and 43; CI Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16, 17, 23, 24, 34, 35, 37, 42, 53, 55, 65, 73, and 74. , CI 75, 81, 83, 93, 94, 95, 97, 98, 100, 101, 104, 108, 109, 110, 113, 114, 116, 117, 120, 128, 129, 133, 138, 139, 147, 150, 151, 153, 154, 155, 156, 167, 172, 173, 180, 185, 195; CI Bat Yellow 1, 3, 20, etc.

[0085] The pressure-sensitive adhesive layer of the present invention may further contain, as necessary, additives such as crosslinking accelerators, antioxidants, antioxidants, plasticizers, softeners, surfactants, antistatic agents, surface lubricants, leveling agents, light stabilizers, UV absorbers, polymerization inhibitors, foil-like materials, and rust inhibitors, within the range that does not impair the effects of the present invention. Only one of the above additives may be used, or two or more of them may be used.

[0086] The rust inhibitor is a compound that prevents metal rust and corrosion. When the adherend is metal, rust and corrosion can be inhibited when the pressure-sensitive adhesive sheet is attached. Examples of the rust inhibitor include amine compounds, benzotriazole compounds, and nitrites. Other examples include ammonium benzoate, ammonium phthalate, ammonium stearate, ammonium palmitate, ammonium oleate, ammonium carbonate, dicyclohexylamine benzoate, urea, urotropine, thiourea, phenyl carbamate, and cyclohexylammonium-N-cyclohexylcarbamate (CHC). One or more of the rust inhibitors may be used.

[0087] The content of the rust inhibitor is not particularly limited, but is preferably 0.02 to 15 parts by mass relative to 100 parts by mass of the base polymer. When the content is 0.02 parts by mass or more, good corrosion prevention performance is easily obtained. When the content is 15 parts by mass or less, transparency is easily ensured.

[0088] Among these, benzotriazole-based compounds are preferred as the rust inhibitors, since they can achieve a well-balanced and high level of compatibility with the base polymer, adhesive reliability, transparency, and corrosion prevention properties, and can also achieve excellent appearance.

[0089] The content of the benzotriazole compound is not particularly limited, but is preferably 0.02 to 3 parts by mass, more preferably 0.02 to 2.5 parts by mass, and even more preferably 0.02 to 2 parts by mass, per 100 parts by mass of the base polymer.

[0090] The pressure-sensitive adhesive layer constituting the pressure-sensitive adhesive sheet of the present invention may be in any form, for example, an emulsion type, a solvent type (solution type), an active energy ray curable type, a heat-melt type (hot melt type), etc. Among these, a solvent type or an active energy ray curable type pressure-sensitive adhesive layer is preferred because it is easy to obtain a pressure-sensitive adhesive layer with excellent productivity.

[0091] Examples of the active energy rays include ionizing radiation such as α rays, β rays, γ rays, neutron rays, and electron beams, as well as ultraviolet rays, and ultraviolet rays are particularly preferred. That is, the active energy ray-curable pressure-sensitive adhesive layer is preferably an ultraviolet-curable pressure-sensitive adhesive layer.

[0092] The pressure-sensitive adhesive layer can be produced, for example, by applying (coating) a pressure-sensitive adhesive composition for forming the pressure-sensitive adhesive layer onto a release liner and drying and curing the resulting pressure-sensitive adhesive composition layer, or by applying (coating) the pressure-sensitive adhesive composition onto a release liner and irradiating the resulting pressure-sensitive adhesive composition layer with active energy rays to cure it. If necessary, the resulting pressure-sensitive adhesive composition may be further dried by heating.

[0093] (adhesive sheet) The pressure-sensitive adhesive sheet of the present invention may have a release liner attached to the surface (adhesive surface) of the pressure-sensitive adhesive layer until use. Each of the adhesive surfaces on both sides of the pressure-sensitive adhesive sheet may be protected by two release liners, or may be protected by a single release liner with release surfaces on both sides in a rolled form (rolled body). The release liner is used as a protective material for the pressure-sensitive adhesive layer and is peeled off when the sheet is attached to an adherend. However, the release liner is not necessarily provided.

[0094] The release liner can be a conventional release paper, and is not particularly limited. Examples include substrates with a release treatment layer, low-adhesion substrates made of fluoropolymers, and low-adhesion substrates made of non-polar polymers. Examples of substrates with a release treatment layer include plastic films and papers surface-treated with release agents such as silicone-based, long-chain alkyl-based, fluorine-based, and molybdenum sulfide. Examples of fluorine-based polymers in the low-adhesion substrates made of fluoropolymers include polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymers, and chlorofluoroethylene-vinylidene fluoride copolymers. Examples of non-polar polymers include olefin-based resins (e.g., polyethylene, polypropylene, etc.). The release liner can be formed by known or conventional methods. The thickness of the release liner is also not particularly limited.

[0095] The pressure-sensitive adhesive sheet is used to bond components of a foldable optical member (foldable optical member) together. The pressure-sensitive adhesive sheet is particularly preferably used to bond components provided in a foldable optical member to both adhesive surfaces of the pressure-sensitive adhesive sheet. The pressure-sensitive adhesive sheet may be used to fix the components together or to temporarily fix them together.

[0096] Examples of the optical member include electrical and electronic devices. The term "electrical and electronic devices" refers to devices that fall into at least either an electrical device or an electronic device. Examples of the electrical and electronic devices include image display devices (foldable displays) such as liquid crystal displays, electroluminescent displays, and plasma displays, as well as portable electronic devices.

[0097] Examples of the portable electronic device 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, and 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, and portable modems. In this specification, "portable" does not simply mean being portable, but also means having a level of portability that allows an individual (average adult) to carry it relatively easily. The pressure-sensitive adhesive sheet is used, for example, so that the pressure-sensitive adhesive layer adheres to components of the portable electronic device.

[0098] Examples of usage modes of the pressure-sensitive adhesive sheet in the foldable optical member include bonding between members at bent portions, and as an impact-absorbing sheet disposed inside a housing (e.g., the back of a display), etc. Furthermore, when the pressure-sensitive adhesive sheet has light-blocking properties, it can be preferably applied to the back of a display when the display is an OLED display.

[0099] The shock-absorbing sheet to be placed on the rear surface of the display is used by adhering one adhesive surface of the adhesive sheet to the inner surface of the housing (inner surface on the rear side) and the other adhesive surface to the rear surface of the display.

[0100] The pressure-sensitive adhesive sheet may be used to bond at least one adhesive surface to a brittle material such as graphite. It may also be used to bond at least one adhesive surface to a material harder than the brittle material (e.g., a metal plate such as a SUS plate). It is particularly preferred that one adhesive surface (particularly the adhesive surface bonded first) be used to bond to a brittle material such as graphite. It is also preferred that the adherend bonded to the other adhesive surface (particularly the adhesive surface bonded later) is a material harder than the brittle material (e.g., a metal plate such as a SUS plate). [Example]

[0101] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0102] Preparation Example 1 (Preparation of Acrylic Polymer A) A mixture (solids concentration 47% by mass) containing 95 parts by mass of n-butyl acrylate (BA) and 5 parts by mass of acrylic acid (AA) as monomer components, 0.2 parts by mass of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator, and ethyl acetate as a polymerization solvent was stirred in a reaction vessel equipped with a stirrer, thermometer, nitrogen gas inlet tube, reflux condenser, and dropping funnel at 56°C for 6 hours under a nitrogen atmosphere (polymerization reaction). This resulted in a polymer solution containing acrylic polymer A. The acrylic polymer A in this polymer solution had a weight-average molecular weight of approximately 800,000 and a glass transition temperature of -50°C.

[0103] Preparation Example 2 (Preparation of Acrylic Polymer B) A mixture (solids concentration 47% by mass) containing 97 parts by mass of n-butyl acrylate (BA) and 3 parts by mass of acrylic acid (AA) as monomer components, 0.2 parts by mass of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator, and ethyl acetate as a polymerization solvent was stirred in a reaction vessel equipped with a stirrer, thermometer, nitrogen gas inlet tube, reflux condenser, and dropping funnel at 56°C for 6 hours under a nitrogen atmosphere (polymerization reaction). This resulted in a polymer solution containing acrylic polymer B. The acrylic polymer B in this polymer solution had a weight-average molecular weight of approximately 1,000,000 and a glass transition temperature of -52°C.

[0104] Preparation Example 3 (Preparation of Acrylic Polymer C) A reaction vessel equipped with a stirrer, thermometer, nitrogen gas inlet tube, reflux condenser, and dropping funnel was charged with 95 parts by mass of 2-ethylhexyl acrylate (2EHA) and 5 parts by mass of acrylic acid (AA) as monomer components, and 199 parts by mass of ethyl acetate as a polymerization solvent, and the mixture was stirred for 2 hours while introducing nitrogen gas. After removing oxygen from the polymerization system in this way, 0.2 parts by mass of benzoyl peroxide was added as a polymerization initiator, and solution polymerization was carried out at 60°C for 6 hours to obtain a solution of acrylic polymer C. This acrylic polymer C had a weight-average molecular weight of approximately 1 million and a glass transition temperature of -65°C.

[0105] Preparation Example 4 (Preparation of Oligomer A) 60 parts by weight of dicyclopentanyl methacrylate and 40 parts by weight of methyl methacrylate as monomer components, 3.5 parts by weight of α-thioglycerol as a chain transfer agent, and 100 parts by weight of toluene as a polymerization solvent were mixed and stirred at 70°C for 1 hour under a nitrogen atmosphere. Next, 0.2 parts by weight of AIBN was added as a thermal polymerization initiator, and the mixture was reacted at 70°C for 2 hours, then heated to 80°C and reacted for 2 hours. The reaction solution was then heated to 130°C, and the toluene, chain transfer agent, and unreacted monomer were dried and removed to obtain a solid acrylic oligomer (oligomer A). Oligomer A had a weight-average molecular weight of 5100.

[0106] Preparation Example 5 (Preparation of Oligomer B) 95 parts by mass of cyclohexyl methacrylate and 5 parts by mass of AA as monomer components, 10 parts by mass of α-methylstyrene dimer as a chain transfer agent, 10 parts by mass of AIBN as a polymerization initiator, and 120 parts by mass of toluene as a polymerization solvent were mixed and reacted at 85°C for 5 hours under a nitrogen atmosphere to obtain a solution of Oligomer B with a solids concentration of 50% by mass. Oligomer B had a weight average molecular weight of 4,300.

[0107] Example 1 (Preparation of Pressure-Sensitive Adhesive Composition) To the solution of acrylic polymer A prepared in Preparation Example 1, 3 parts by mass of crosslinking agent A (trade name "Takenate D101E", tolylene diisocyanate, manufactured by Mitsui Chemicals, Inc.) was added as a crosslinking agent per 100 parts by mass of acrylic polymer A, and the mixture was stirred and mixed to prepare a pressure-sensitive adhesive composition.

[0108] (Preparation of adhesive sheet) The above adhesive composition was applied to the release layer of a 75 μm thick polyethylene terephthalate film (product name "Diafoil MRF", manufactured by Mitsubishi Chemical Corporation) with one side treated for release with silicone, so that the adhesive layer was 15 μm thick, and dried at 110°C for 3 minutes to form an adhesive layer.A release layer of a 38 μm thick polyethylene terephthalate film (product name "Diafoil MRE", manufactured by Mitsubishi Chemical Corporation) with one side treated for release with silicone was then superimposed on the above adhesive layer to produce the double-sided adhesive sheet of Example 1.

[0109] Examples 2 to 5, Comparative Examples 1 to 3 The pressure-sensitive adhesive compositions and pressure-sensitive adhesive sheets of each example were prepared in the same manner as in Example 1, except that the formulation of the pressure-sensitive adhesive composition was changed as shown in Table 1. The crosslinking agent B was an epoxy-based crosslinking agent (trade name "Tetrad C", manufactured by Mitsubishi Gas Chemical Company, Inc.).

[0110] <Evaluation> The pressure-sensitive adhesive sheets obtained in the examples and comparative examples were evaluated as follows, and the results are shown in the table below.

[0111] (1) Storage modulus G' (60°C) The release liners on both sides of the double-sided PSA sheets produced in the Examples and Comparative Examples were peeled off, and the double-sided PSA sheets were laminated to produce test samples approximately 1 mm thick. These test samples were punched out into disks with a diameter of 7.9 mm, sandwiched between parallel plates, and viscoelasticity was measured in shear mode using a viscoelasticity tester (product name "ARES", manufactured by Rheometrics) at a temperature range of -50 to 150°C and a heating rate of 5°C / min while applying a shear strain of 1 Hz, to obtain G' (dynamic shear storage modulus) at 60°C.

[0112] (2) Gel fraction Approximately 0.2 g of adhesive was scraped from the adhesive sheet and wrapped in a porous polytetrafluoroethylene membrane (trade name "NTF-1122" manufactured by Nitto Denko Corporation) with a pore size of 0.2 μm cut to a size of 100 mm x 100 mm. The top of the wrap was then tied with a string. The mass of the adhesive sample (B) was calculated by subtracting the total mass (A) of the porous polytetrafluoroethylene membrane and string, which had been measured in advance, from the mass of this sample. The adhesive sample wrapped in the porous polytetrafluoroethylene membrane was immersed in approximately 50 mL of ethyl acetate at 23°C for 7 days to allow the sol component of the adhesive to elute out of the porous polytetrafluoroethylene membrane. After immersion, the adhesive wrapped in the porous polytetrafluoroethylene membrane was removed and dried at 130°C for 2 hours. After allowing to cool for approximately 20 minutes, the dry mass (C) was measured. The gel fraction of the adhesive was calculated using the following formula: Gel fraction (%) = 100 × (CA) / B

[0113] (3) 180° peel adhesive strength (under high temperature and humidity conditions) In a measurement environment of 23°C and 50% RH, the release liners on both sides of the double-sided PSA sheets prepared in the Examples and Comparative Examples were peeled off, and a 25 μm thick PET film treated with a primer (product name "N-200NT" manufactured by 3M Innovative Properties Company) was attached to one adhesive surface of the double-sided PSA sheet to form a backing. The sheet was then cut to a size of 20 mm wide and 100 mm long to prepare a measurement sample. The exposed adhesive surface of the prepared measurement sample was then pressed against the surface of a stainless steel plate (SUS304BA plate) using a 2 kg roller, moving back and forth once, in an environment of 23°C and 50% RH. This sheet was left in the same environment for 24 hours, and then left in an environment of 60°C and 90% RH for 10 minutes. The 180° peel strength (adhesive strength) [N / 25 mm] was measured using a universal tension and compression tester at a tension speed of 300 mm / min, a peel angle of 180°, and an environmental condition of 60°C and 90% RH. The universal tension and compression tester used was a tensile tester equipped with a constant temperature and humidity chamber (product name "AG-X", manufactured by Shimadzu Corporation).

[0114] (4) Tensile elongation The double-sided PSA sheets prepared in the examples and comparative examples were 1.0 mm thick. 2 The specimen was rolled into a cylindrical shape having a cross-sectional area of ​​1.0 mm, and set in a tensile testing machine (product name "AG-IS", manufactured by Shimadzu Corporation) with a chuck distance of 10 mm in the longitudinal direction. The specimen was stretched in a 180° direction at a rate of 50 mm / min, and the value calculated by the following formula when the tensile stress was 0.1 MPa was taken as the tensile stress. Tensile elongation [%] = (length between chucks when stretched - length between chucks before stretching) / length between chucks before stretching × 100 Tensile stress [MPa] = Tensile force (N) / Cross-sectional area (mm 2 )

[0115] [Table 1]

[0116] As shown in Table 1, the double-sided PSA sheets of the examples had high adhesive strength at 60°C and 90% RH humidity, resulting in excellent adhesion to adherends in high-temperature, high-humidity environments. They also had a tensile elongation of 100% or less when subjected to a stress of 0.1 MPa, indicating that excessive elongation upon bending was suppressed. On the other hand, when the storage modulus at 60°C was low (Comparative Example 1), the adhesive strength at 60°C and 90% RH humidity was low, indicating poor adhesion to adherends in high-temperature, high-humidity environments. Furthermore, the tensile elongation upon application of a stress of 0.1 MPa was high, indicating that excessive elongation upon bending may occur. When the gel fraction was low (Comparative Example 2), the tensile elongation upon application of a stress of 0.1 MPa was high, indicating that excessive elongation upon bending may occur. When the gel fraction was high (Comparative Example 3), the adhesive strength at 60°C and 90% RH humidity was low, indicating poor adhesion to adherends in high-temperature, high-humidity environments.

[0117] Variations of the invention according to the present disclosure are described below. [Appendix 1] An adhesive sheet used to bond components of foldable optical components together, A pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer that includes a structure in which an acrylic polymer is crosslinked with an aromatic polyisocyanate, and that has a storage modulus of 40 to 100 kPa at 60°C when measured at a frequency of 1 Hz, a gel fraction of 20 to 65%, and a thickness of 5 to 50 μm. [Appendix 2] The pressure-sensitive adhesive sheet according to Appendix 1, wherein the content of the acrylic polymer in the pressure-sensitive adhesive layer is 90% by mass or more. [Appendix 3] The pressure-sensitive adhesive sheet according to appendix 1 or 2, wherein the acrylic polymer contains, as monomer components, a (meth)acrylic acid alkyl ester and (meth)acrylic acid. [Appendix 4] The pressure-sensitive adhesive sheet according to Appendix 3, wherein the (meth)acrylic acid alkyl ester is a (meth)acrylic acid alkyl ester having an alkyl group having 2 to 6 carbon atoms. [Appendix 5] An electrical / electronic device comprising the adhesive sheet according to any one of Appendices 1 to 4, wherein the adhesive sheet has both adhesive surfaces that bond and fix components provided in the foldable optical component. [Explanation of symbols]

[0118] 1 double-sided adhesive sheet 2. Adhesive layer 3,4 Release liner

Claims

1. An adhesive sheet used to bond components of a foldable optical component together, A pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer that includes a structure in which an acrylic polymer is crosslinked with an aromatic polyisocyanate, has a storage modulus of 40 to 100 kPa at 60°C when measured at a frequency of 1 Hz, a gel fraction of 20 to 65%, and a thickness of 5 to 50 μm.

2. The pressure-sensitive adhesive sheet according to claim 1 , wherein the content of the acrylic polymer in the pressure-sensitive adhesive layer is 90% by mass or more.

3. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the acrylic polymer contains, as monomer components, an alkyl (meth)acrylate and (meth)acrylic acid.

4. 4. The pressure-sensitive adhesive sheet according to claim 3, wherein the (meth)acrylic acid alkyl ester is a (meth)acrylic acid alkyl ester having an alkyl group having 2 to 6 carbon atoms.

5. An electric / electronic device comprising the adhesive sheet according to claim 1 or 2, wherein both adhesive surfaces of the adhesive sheet bond and fix components provided in the foldable optical member to each other.

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