Adhesive sheet for flexible display, laminate for flexible display, and flexible display

Abstract

To provide an adhesive sheet for flexible displays that has excellent adhesion to a colorless polyimide film while having excellent recognizability and does not generate floating or peeling after repeated bending, a laminate for flexible displays and, a flexible display.SOLUTION: An adhesive sheet for flexible displays has an adhesive layer, the adhesive layer comprising a (meth)acrylic copolymer (A), a tackifier resin (B), and an isocyanate curing agent (C), the tackifier resin (B) comprising at least one selected from the group consisting of a C5-C9 petroleum resin, a homopolymer of a styrenic monomer, a copolymer between a styrenic monomer and an aromatic monomer other than the styrenic monomer, and a copolymer between the styrenic monomer and an aliphatic monomer, and the adhesive layer having a haze of 3% or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a pressure-sensitive adhesive sheet for a flexible display, a laminate for a flexible display, and a flexible display. [Background technology]

[0002] In recent years, input devices that combine displays such as liquid crystal displays (LCDs) and organic electroluminescence (OLED) displays (OLEDs) with touch panels have become widespread. Transparent conductive films used in touch panels are laminated to members such as supporting glass via an adhesive layer. Polarizing plate films used in displays are attached to liquid crystal modules or OLED modules via an adhesive layer.

[0003] As the display, flat displays using glass substrates have been mainstream, but in recent years, flexible displays such as foldable displays and rollable displays using flexible substrates such as plastic have been developed. Such flexible displays have various advantages such as light weight, thinness, flexibility, and excellent design compared to conventional flat displays using glass substrates.

[0004] The flexible display or the pressure-sensitive adhesive layer used in the flexible display, and the optical film laminate laminated with the pressure-sensitive adhesive layer are required to have good adhesion and to be free from peeling or lifting even when bent, in addition to optical properties and durability.

[0005] Therefore, studies have been conducted to suppress the occurrence of peeling or lifting during bending by improving the adhesion at the joint. For example, Patent Document 1 discloses a pressure-sensitive adhesive composition containing 100 parts by mass of a base agent made of a curable compound such as a (meth)acrylic acid ester monomer, and more than 0 parts by mass and less than 5 parts by mass of a terpene phenol-based tackifier resin having a softening point of 130°C or higher. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2017-95659 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, in recent years, in order to meet the increasing demands on display durability, stricter standards have been required for the property of preventing lifting or peeling when repeatedly bent (dynamic bendability), as well as the property of preventing lifting or peeling when maintained in a bent state for long periods of time (static bendability).

[0008] Furthermore, the use of colorless polyimide films is increasingly being used in flexible displays for their strength and transparency. However, colorless polyimide films are generally low-polarity materials, which means they have low affinity with acrylic adhesives, resulting in poor adhesion.

[0009] When the pressure-sensitive adhesive sheet of Patent Document 1 was adhered to a colorless polyimide film, not only did lifting and peeling of the pressure-sensitive adhesive layer occur, but dynamic and static bending properties were also insufficient.

[0010] Therefore, an object of the present invention is to provide an adhesive sheet for flexible displays, a laminate for flexible displays, and a flexible display, which have excellent visibility while also having good adhesion to colorless polyimide films and do not lift or peel off even after repeated bending. [Means for solving the problem]

[0011] As a result of extensive research, the present inventors have found that the problems of the present invention can be solved in the following manner, and have completed the present invention. That is, the present invention relates to an adhesive sheet for flexible displays having an adhesive layer, the adhesive layer comprising a (meth)acrylic copolymer (A) (wherein the (meth)acrylic copolymer (A) excludes the (meth)acrylic copolymer (D)), a tackifier resin (B) and an isocyanate curing agent (C), the tackifier resin (B) comprising at least one selected from the group consisting of a C5-C9 petroleum resin, a homopolymer of a styrene-based monomer, a copolymer of a styrene-based monomer and an aromatic monomer other than a styrene-based monomer (hereinafter also referred to as a styrene-based monomer / aromatic monomer copolymer), and a copolymer of a styrene-based monomer and an aliphatic monomer (hereinafter also referred to as a styrene-based monomer / aliphatic monomer copolymer), and the haze of the adhesive layer is 3% or less. Effect of the Invention

[0012] According to the present invention having the above configuration, it is possible to provide a highly durable flexible display having a haze of 3% or less, high adhesion to a colorless polyimide film, and excellent visibility and bendability. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] An example of an embodiment to which the present invention is applied will be described below. The numerical values ​​specified in this specification are values ​​obtained by the methods disclosed in the embodiments or examples. Other embodiments are also included in the scope of the present invention as long as they are consistent with the gist of the present invention. The adhesive sheet of the present invention is synonymous with an adhesive film and an adhesive tape. Unless otherwise specified, the various blending components in the adhesive layer can be used independently, either alone or in combination of two or more kinds. Furthermore, (meth)acrylic means acrylic or methacrylic, and (meth)acrylate means acrylate or methacrylate, respectively.

[0014] <Adhesive layer> The pressure-sensitive adhesive layer of the present invention contains a (meth)acrylic copolymer (A), a tackifier resin (B), and an isocyanate curing agent (C), and is laminated with a colorless polyimide film or a glass substrate and used in a bent state as a flexible display.

[0015] <(Meth)acrylic copolymer (A)> The (meth)acrylic copolymer (A) is a copolymer of a monomer mixture containing at least all of the following monomers (a-1) to (a-3). Note that the (meth)acrylic copolymer (A) does not include the (meth)acrylic copolymer (D) described below.

[0016] [Monomer (a-1)] Monomer (a-1) is a (meth)acrylic acid branched alkyl ester monomer having an alkyl group having 6 to 10 carbon atoms, and specific examples thereof include isohexyl (meth)acrylate, isooctyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isononyl (meth)acrylate, and isodecyl (meth)acrylate.

[0017] The acrylic copolymer (A) contains the monomer (a-1) and has an alkyl group with a branched structure in the side chain, which allows the polymers to be appropriately entangled with each other, improving stress relaxation properties and providing a flexible pressure-sensitive adhesive layer, thereby improving adhesion to the substrate.

[0018] Of these monomers (a-1), isooctyl (meth)acrylate and 2-ethylhexyl (meth)acrylate are preferred from the viewpoints of stress relaxation and adhesion.

[0019] The monomer (a-1) is preferably contained in an amount of 30 to 70 mass%, more preferably 40 to 70 mass%, based on 100 mass% of the monomer mixture. When the content is 30 mass% or more, sufficient stress relaxation property is easily obtained. When the content is 70 mass% or less, it is easy to achieve both cohesive strength and stress relaxation property, and therefore dynamic bending property is improved.

[0020] [Monomer (a-2)] Monomer (a-2) is a (meth)acrylic acid alkyl ester monomer having an alkyl group having 12 to 20 carbon atoms, and specific examples thereof include dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, and icosyl (meth)acrylate. Among these (a-2), dodecyl (meth)acrylate, tetradecyl (meth)acrylate, and icosyl (meth)acrylate are preferred from the viewpoint of rubber elasticity, and dodecyl (meth)acrylate is even more preferred from the viewpoint of flexibility. By containing monomer (a-2), the cohesive force of the pressure-sensitive adhesive layer is improved, a tough pressure-sensitive adhesive layer is obtained, and static flexibility is particularly improved.

[0021] The monomer (a-2) is preferably contained in the monomer mixture at 100% by mass, more preferably at 10 to 50% by mass, and even more preferably at 20 to 50% by mass. When the content is 10% by mass or more, sufficient rubber elasticity is easily obtained, and a strong adhesive layer is obtained. When the content is 50% by mass or less, flexibility and rubber elasticity are easily achieved at the same time, and therefore dynamic flexibility and static flexibility can be improved.

[0022] [Monomer (a-3)] Monomer (a-3) may have a monomer having a hydroxyl group, and may further have a monomer having a carboxyl group. The monomer having a hydroxyl group is not limited as long as it is a monomer having a hydroxyl group in the molecule, and specifically includes 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, etc. Among these, 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are preferred from the viewpoint of improving adhesion to the colorless polyimide film.

[0023] The monomer having a carboxy group is not limited as long as it is a monomer having a carboxy group in the molecule, and specific examples thereof include (meth)acrylic acid, p-carboxybenzyl acrylate, β-carboxyethyl acrylate, maleic acid, monoethyl maleic acid, itaconic acid, citraconic acid, fumaric acid, etc. Among these, (meth)acrylic acid is preferred from the viewpoints of cohesive strength and adhesive strength to the colorless polyimide film.

[0024] By including the monomer (a-3), a crosslinked structure is formed with the isocyanate curing agent (C) described later, which improves the cohesive strength of the adhesive layer, provides a tough adhesive layer, and improves adhesion to the colorless polyimide film as well as dynamic and static bending properties.

[0025] The monomer (a-3) is preferably contained in an amount of 0.5 to 2.5 mass% of the monomer mixture, more preferably 0.5 to 2.0 mass%. When the content is 0.5 mass% or more, sufficient cohesive strength is easily obtained. When the content is 2.5 mass% or less, both cohesive strength and stress relaxation properties are achieved, and adhesion to the colorless polyimide film is improved.

[0026] [Other monomers] The (meth)acrylic copolymer (A) may contain other monomers in addition to the monomers (a-1) to (a-3). Examples of the other monomers include (meth)acrylic acid alkyl ester monomers other than the monomers (a-1) to (a-3), (meth)acrylic acid monomers having an epoxy group, (meth)acrylic acid monomers having an amino group, monomers having an alkyleneoxy group, and other vinyl monomers. However, the alicyclic monomer (d-1) described later is excluded.

[0027] <(Meth)acrylic copolymer (D)> The (meth)acrylic copolymer (D) is a copolymer of a monomer mixture containing at least an alicyclic (meth)acrylic acid alkyl ester monomer (d-1), and the monomer mixture may contain monomers (a-1) to (a-3) as necessary. Note that even if the monomer mixture contains all of the monomers (a-1) to (a-3), it is defined as being equivalent to the (meth)acrylic copolymer (D) if it contains the monomer (d-1).

[0028] [Alicyclic monomer (d-1)] The alicyclic monomer (d-1) has a polymerizable functional group having an unsaturated double bond, such as a (meth)acryloyl group or a vinyl group, and also has an alicyclic structure-containing group. Here, the "alicyclic structure-containing group" refers to a portion containing at least one alicyclic structure, and may be referred to as an alicyclic group below. Examples of the alicyclic group include a hydrocarbon group and a hydrocarbonoxy group having an alicyclic structure.

[0029] Examples of the alicyclic monomer (d-1) include isobornyl (meth)acrylate and cyclohexyl (meth)acrylate.

[0030] The alicyclic monomer (d-1) is preferably contained in an amount of 80 to 95 mass % in 100 mass % of the (meth)acrylic copolymer (D).

[0031] The pressure-sensitive adhesive layer preferably contains a (meth)acrylic copolymer (D) in addition to the (meth)acrylic copolymer (A) because the adhesive strength is improved. The alicyclic monomer (d-1) contributes to improving the cohesive strength as a hard segment, improving the peel strength, which is thought to improve the adhesive strength.

[0032] The content of the (meth)acrylic copolymer (D) is preferably 0.1 to 10 mass%, more preferably 0.1 to 5 mass%, in 100 mass% of the pressure-sensitive adhesive layer. If it exceeds 10 mass%, the ratio of the hard segment in the pressure-sensitive adhesive layer increases, which may reduce tack and adhesive strength.

[0033] The pressure-sensitive adhesive layer may further contain other acrylic pressure-sensitive adhesives, urethane pressure-sensitive adhesives, or rubber pressure-sensitive adhesives in addition to the (meth)acrylic copolymer (A). In this case, the (meth)acrylic copolymer (A) may account for 70% by mass or more of the pressure-sensitive adhesive layer (100% by mass).

[0034] <Tackifier resin> The tackifier resin (B) preferably contains at least one selected from the group consisting of C5-C9 petroleum resins, styrene-based monomer homopolymers, styrene-based monomer / aromatic monomer copolymers, and styrene-based monomer / aliphatic monomer copolymers. The above-mentioned tackifier resin (B) group is all resins polymerized from petroleum-based monomers and is derived from non-natural products. On the other hand, resins formed from natural materials such as rosin are derived from natural products and are not preferably contained in the pressure-sensitive adhesive sheet of the present invention. The tackifier resin (B) may be used alone or in combination of two or more kinds. The tackifier resin (B) is preferably contained in an amount of 0.1 to 3.5% by mass, more preferably 1 to 3% by mass, relative to 100% by mass of the (meth)acrylic copolymer (A). When the tackifier resin (B) is contained in an amount of 0.1% by mass or more, the tackifier resin (B) forms a highly cohesive domain in the adhesive layer, increasing the peel strength. In addition, the adhesive layer exhibits good adhesion to a low-polarity colorless polyimide film, and does not lift or peel even after repeated bending. By making the tackifier resin (B) 3.5% by mass or less, the compatibility in the adhesive layer is maintained, and the haze is reduced to 3% or less, so that high transparency can be maintained.

[0035] Examples of C5-C9 petroleum resins include those manufactured by Zeon Corporation under the trade names Quinton N180 (softening point: 80° C.), Quinton U185 (softening point: 86° C.), Quinton U190 (softening point: 90° C.), Quinton S195 (softening point: 94° C.), Quinton DX395 (softening point: 94° C.), Quinton DX390N (softening point: 93° C.), Quinton D100 (softening point: 99° C.), Quinton E200SN (softening point: 102° C.), Quinton D200 (softening point: 106° C.), and Quinton E200SN (softening point: 108° C.). Examples of such products include Petrotack 60 (softening point: 72°C), Petrotack 70 (softening point: 70°C), Petrotack 90 (softening point: 95°C), Petrotack 90V (softening point: 87°C), Petrotack 90HS (softening point: 87°C), and Petrotack 100V (softening point: 96°C), both of which are Tosoh Corporation's product names.

[0036] As the styrene-based monomer homopolymer, a general-purpose product can be used, and examples of commercially available products include those available under the trade names Piccolastic A5 (styrene homopolymer, softening point 25°C) and Piccolastic A75 (styrene homopolymer, softening point 73°C) manufactured by Eastman Chemical Co., Ltd., those available under the trade name YS Resin SX100 (styrene homopolymer, softening point 100°C) manufactured by Yasuhara Chemical Co., Ltd., and those available under the trade names FTR8100 (styrene monomer homopolymer, softening point 100°C), FTR8120 (styrene monomer homopolymer, softening point 120°C), and FTR0100 (styrene monomer homopolymer, softening point 100°C) manufactured by Mitsui Chemicals, Inc.

[0037] An example of the styrene monomer / aromatic monomer copolymer is FMR0150 (softening point 150° C.) manufactured by Mitsui Chemicals.

[0038] Examples of the styrene monomer / aliphatic monomer copolymer include a styrene monomer / aliphatic monomer copolymer, an α-methylstyrene / aliphatic monomer copolymer, and a styrene monomer / α-methylstyrene / aliphatic monomer copolymer. Examples of commercially available products include trade names FTR6080 (softening point 80° C.), FTR6100 (softening point 95° C.), FTR6110 (softening point 110° C.), FTR6125 (softening point 125° C.), FTR7100 (softening point 100° C.), and FTR7125 (softening point 125° C.), all of which are manufactured by Mitsui Chemicals.

[0039] Among these, styrene-based monomer homopolymers, styrene-based monomer / aliphatic monomer copolymers, and styrene-based monomer / aromatic monomer copolymers are preferred because they are more likely to form highly cohesive domains when included in the (meth)acrylic copolymer (A), and styrene-based monomer homopolymers are particularly preferred from the viewpoint of improving adhesive strength. The softening point of the tackifier resin (B) is preferably 25° C. or higher, more preferably 90° C. or higher, and even more preferably 100° C. or higher. By having a softening point of 25° C. or higher, the adhesive layer can obtain sufficient cohesive strength and improve dynamic bending properties. The upper limit of the softening point is preferably 150° C. or lower.

[0040] The pressure-sensitive adhesive layer in the present invention may further contain a tackifier resin (C2) other than the above-mentioned tackifier resin (B), but is preferably composed only of the tackifier resin (B). Examples of the tackifier resin (C2) include phenol-based resins and ketone-based resins.

[0041] <Isocyanate hardener> The pressure-sensitive adhesive layer contains an isocyanate curing agent (C). The isocyanate curing agent (C) is preferably contained in an amount of 0.1 to 80% by mass relative to 100% by mass of the (meth)acrylic copolymer (A). By setting the amount in this range, a strong pressure-sensitive adhesive layer having high cohesive strength can be formed, so that lifting or peeling of the pressure-sensitive adhesive layer when bent is suppressed, and dynamic and static bending properties are improved. A more preferred range is 0.1 to 30% by mass, and even more preferably 0.1 to 10% by mass.

[0042] The isocyanate curing agent (C) is preferably an aromatic compound, such as tolylene diisocyanate, methylene diphenyl diisocyanate, xylylene diisocyanate, tetramethyl xylylene diisocyanate, and their allophanate forms, biuret forms, isocyanurate forms, prepolymer forms, and adduct forms.

[0043] The pressure-sensitive adhesive layer of the present invention may contain various resins, oils, silane coupling agents, softeners, dyes, pigments, antioxidants, UV absorbers, weather stabilizers, plasticizers, fillers, antioxidants, antistatic agents, and the like as optional components.

[0044] <Gel fraction> The gel fraction of the pressure-sensitive adhesive layer is preferably 50 to 90% by mass, more preferably 55 to 90% by mass, and even more preferably 60 to 80% by mass. When the gel fraction is 50% by mass or more, the cohesive strength of the pressure-sensitive adhesive layer is improved, a tough pressure-sensitive adhesive layer is obtained, and durability is improved, while when the gel fraction is 90% by mass or less, the stress relaxation property of the pressure-sensitive adhesive layer is improved, a flexible pressure-sensitive adhesive layer is obtained, and adhesion to the colorless polyimide film is improved. In general, the gel fraction of a polymer is equal to the degree of crosslinking, and the more crosslinked parts in the polymer, the higher the gel fraction. The gel fraction (amount of crosslinked structure introduced) can be adjusted to a desired range by the method of introducing the crosslinked structure, the type and amount of the curing agent, etc.

[0045] <Haze> The haze of the pressure-sensitive adhesive layer of the present invention, measured in accordance with JIS K 7136, is 3% or less, preferably 2% or less, and more preferably 1% or less, from the viewpoint of suitability for bonding optical components.

[0046] <Adhesion to colorless polyimide film> The adhesive strength of the pressure-sensitive adhesive layer of the present invention to the colorless polyimide film is preferably 15 to 50 N / 25 mm, more preferably 15 to 45 N / 25 mm. By setting it within the above range, lifting or peeling at the interface between the colorless polyimide film and the pressure-sensitive adhesive layer when folded can be suppressed. In order to set the adhesive strength within the above range, the surface of the colorless polyimide film may be subjected to a corona treatment or plasma treatment.

[0047] <Colorless polyimide film> The pressure-sensitive adhesive sheet for flexible displays of the present invention is preferably laminated to a colorless polyimide film. The colorless polyimide film is a polyimide film having a transmittance of 80% or more at 400 nm to 700 nm and a glass transition temperature of 250° C. or more. The thickness is preferably 10 to 200 μm, more preferably 20 to 100 μm.

[0048] <Adhesive sheets for flexible displays> The adhesive sheet for flexible displays (hereinafter also referred to as adhesive sheet) of the present invention is preferably made of an adhesive layer and has a separator laminated thereon. Either a form in which a separator is laminated on one side of the adhesive layer, or a form in which both sides of the adhesive layer are sandwiched between separators is preferable, and among these, a form in which both sides are sandwiched is preferable. The adhesive layer is preferably a single layer, but a form in which a core material is provided between the layers is also preferable. As the core material, polyester, polyimide, colorless polyimide, polyamide, epoxy resin, glass, triacetyl cellulose, etc. can be suitably used.

[0049] The adhesive sheet can be produced by applying an adhesive containing the (meth)acrylic copolymer (A), the (meth)acrylic copolymer (D), a tackifier resin (B), an isocyanate curing agent (C), a solvent, etc., to a separator, and then drying and removing the solvent, etc. to form an adhesive layer.

[0050] The separator has a release layer formed by applying a release agent to a substrate such as paper, plastic film, synthetic paper, etc. Examples of the release agent include silicone, alkyd resin, melamine resin, fluororesin, acrylic resin, etc. The thickness of the separator is not particularly limited, but is about 10 to 200 μm.

[0051] The drying temperature for the above-mentioned solvent is preferably 40 to 200° C., more preferably 50 to 180° C., and particularly preferably 70 to 170° C. By setting the drying temperature within the above range, a pressure-sensitive adhesive layer having excellent adhesive properties can be obtained.

[0052] The drying time can be appropriately selected from among various drying times, and is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 10 minutes, and particularly preferably 10 seconds to 5 minutes.

[0053] The pressure-sensitive adhesive can be applied by various methods, specifically, for example, roll coating, kiss roll coating, gravure coating, reverse coating, roll brushing, spray coating, dip roll coating, bar coating, knife coating, air knife coating, curtain coating, lip coating, extrusion coating using a die coater, etc.

[0054] The thickness of the pressure-sensitive adhesive layer of the present invention is preferably 5 to 150 μm, more preferably 15 to 100 μm. The pressure-sensitive adhesive layer may be a single layer, or pressure-sensitive adhesive layers having different compositions may be laminated. If it is within the above range, bending is not hindered, and it is also a preferred embodiment in terms of adhesion. If it exceeds 150 μm, the polymer chains in the pressure-sensitive adhesive layer become more mobile during repeated bending, causing severe deterioration, which may cause peeling, and if it is less than 5 μm, the stress during bending cannot be alleviated, which may cause breakage.

[0055] As an example of using the adhesive sheet for flexible displays in a flexible display, first, the separator on one side is peeled off, and a colorless polyimide film is laminated to the exposed adhesive layer. Next, the other opposing separator is peeled off, and the exposed adhesive layer is laminated to a polarizing plate or a transparent conductive film to form a laminate for flexible displays. In addition, a hard coat layer or an easy-adhesion coating layer may be laminated on the laminated surface of the pressure-sensitive adhesive layer and the colorless polyimide film.

[0056] <Flexible display> A flexible display has a bending function as one of its major features, and includes the above-mentioned laminate for flexible displays and an organic EL display panel configured to be bendable, and the laminate for flexible displays is disposed on the viewing side of the organic EL display panel and configured to be bendable. In addition, instead of the organic EL display panel, a micro LED, a liquid crystal panel, or an electronic paper module may be used, and further, a window may be disposed on the viewing side of the laminate for flexible displays.

[0057] The flexible display of the present invention can be suitably used as a flexible liquid crystal display device, an organic EL (electroluminescence) display device, a PDP (plasma display panel), electronic paper, etc. In addition, it can be used regardless of the type of touch panel, such as a resistive film type or a capacitive type. EXAMPLES

[0058] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples. In the examples, unless otherwise specified, "parts" and "%" are values ​​based on "parts by mass" and "% by mass", respectively.

[0059] <Measurement of weight average molecular weight (Mw)> The weight average molecular weight (Mw) was measured using a GPC "LC-GPC system" manufactured by Shimadzu Corporation. The weight average molecular weight (Mw) was determined by conversion using polystyrene with a known molecular weight as the standard substance. Device name: Shimadzu Corporation, LC-GPC system "Prominence" Column: Four GMHXL columns manufactured by Tosoh Corporation and one HXL column manufactured by Tosoh Corporation were connected together. Mobile phase solvent: Tetrahydrofuran Flow rate: 1.0ml / min Column temperature: 40℃

[0060] <Production Example of Acrylic Copolymer (A)> (Acrylic Copolymer (A-1)) A reaction vessel (hereinafter simply referred to as the "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping device, and a nitrogen inlet tube was charged with 68 parts of 2-ethylhexyl acrylate (EHA), 30 parts of dodecyl acrylate (DOA), 1 part of acrylic acid (AA), 1 part of 4-hydroxybutyl acrylate (HBA), and 0.2 parts of 2,2'-azobisisobutyronitrile (hereinafter simply referred to as "AIBN") as an initiator, and the atmosphere in the reaction vessel was replaced with nitrogen gas. Then, the mixture was heated to 50°C while stirring under a nitrogen atmosphere to start the reaction. The reaction solution was then reacted at 50°C for 4 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain a copolymer (A-1) solution with a non-volatile content of 30% and a viscosity of 5000 mPa·s. The weight average molecular weight of the obtained copolymer (A-1) was 1.4 million.

[0061] (Acrylic Copolymer (A-2)) Copolymer (A-2) was synthesized in the same manner as in the production of acrylic copolymer (A-1), except that the composition and blending amounts (parts by mass) were changed to those shown in Table 1.

[0062] <Production Example of Acrylic Copolymer (D)> (Acrylic copolymer (D-1)) In a reaction vessel (hereinafter simply referred to as the "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping device, and a nitrogen inlet tube, 5 parts of dodecyl acrylate (DOA), 94 parts of isobornyl acrylate (IBXA), 1 part of dimethylaminoethyl methacrylate (DM), and 2 parts of AIBN were charged, and the atmosphere in the reaction vessel was replaced with nitrogen gas. Then, the mixture was heated to 50°C while stirring under a nitrogen atmosphere to start the reaction. The reaction solution was then reacted at 50°C for 4 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain a copolymer (D-1) solution with a non-volatile content of 35% and a viscosity of 100 mPa·s. The weight average molecular weight of the obtained copolymer (D-1) was 25,000.

[0063] (Acrylic copolymers (D-2 to D-4)) Copolymers (D-2 to D-4) were synthesized in the same manner as in the production of the acrylic copolymer (D-1), except that the compositions and blending amounts (parts by mass) were changed to those shown in Table 1.

[0064] The weight average molecular weights (Mw) of the resulting copolymers (A-1 to A-2) and (D-1 to D-4) are shown in Table 1. The blanks in the table indicate that no compound was added.

[0065] [Table 1]

[0066] The abbreviations in the table are as follows. EHA: 2-Ethylhexyl acrylate DOA: dodecyl acrylate HBA: 4-hydroxybutyl acrylate MA: Methyl acrylate BA: Butyl acrylate AA: Acrylic acid IBXA: Isobornyl acrylate (alicyclic) DM: 2-(dimethylamino)ethyl methacrylate

[0067] Details of the materials used in the examples and comparative examples are given below. B-1 Styrene monomer homopolymer (softening point: 120°C, non-naturally derived, Mitsui Chemicals, FTR8120) B-2 Styrene monomer homopolymer (softening point: 25°C, non-natural origin, Eastman Chemical, Picolastic A5) B-3 Styrene monomer homopolymer (softening point: 100°C, non-naturally derived, Mitsui Chemicals, FTR0100) B-4 Styrene monomer / aliphatic monomer copolymer (softening point: 125°C, non-naturally derived, Mitsui Chemicals, FTR6125) B-5 Styrene monomer / aliphatic monomer copolymer (softening point: 100°C, non-naturally derived, Mitsui Chemicals, FTR6100) B-6 Styrene monomer / aromatic monomer copolymer (softening point: 150°C, non-naturally derived, Mitsui Chemicals, FMR0150) B-7 C5-C9 petroleum resin (softening point: 96°C, non-natural origin, Tosoh Corporation, Petrotack 100V) B-8 Styrene monomer homopolymer (softening point: 100°C, non-naturally derived, Yasuhara Chemical Co., Ltd., YS Resin SX100) B-9 Alicyclic C9 petroleum resin (hydrogenated petroleum resin containing cyclohexane ring) (softening point: 125°C, derived from non-natural products, manufactured by Arakawa Chemical Industries, Alcon P125) B-10 Hydrogenated rosin ester resin (softening point: 94-104°C, derived from natural products, hydroxyl value 42mgKOH / g, manufactured by Arakawa Chemical Industries, Pine Crystal KE-359) B-11 Terpene phenol resin (softening point: 130℃, derived from natural products, Yasuhara Chemical Co., Ltd., YS Polyster TH130) B-12 Xylene resin (liquid substance: non-naturally derived, manufactured by Fudow Co., Ltd., hydroxyl value 29 mg KOH / g, Nikanol H-80) C-1 Tolylene diisocyanate trimethylolpropane adduct

[0068] Example 1 <Adhesive adjustment> A pressure-sensitive adhesive was obtained by blending 100 parts by mass of the nonvolatile content of the acrylic copolymer (A-1) with 3 parts by mass of the tackifier (B-1) and 0.1 parts by mass of the isocyanate curing agent (C-1), and further adding toluene to adjust the nonvolatile content to 20%.

[0069] <Manufacturing of adhesive sheets for flexible displays> The adhesive was applied to a 75 μm thick polyethylene terephthalate separator so that the thickness after drying was 50 μm, and the adhesive layer was formed by drying with hot air at 100° C. for 3 minutes. Next, a 50 μm thick polyethylene terephthalate separator was attached to the adhesive layer to produce a "separator / adhesive layer / separator" laminate. The resulting laminate was aged for 1 week at a temperature of 40° C. to obtain an adhesive sheet for flexible displays.

[0070] (Examples 2 to 19, Comparative Examples 1 to 7) As shown in Table 2, a pressure-sensitive adhesive sheet was obtained in the same manner as in Example 1, except that the types and amounts (parts by mass) of the (meth)acrylic copolymer (A), the (meth)acrylic copolymer (D), the tackifier resin (B), and the isocyanate curing agent (C) were changed.

[0071] <Measurement and evaluation of physical properties of adhesive sheets> The resulting pressure-sensitive adhesive sheet was used to evaluate the gel fraction, adhesion to a colorless polyimide film, dynamic bending, static bending, and haze.

[0072] <Measurement of gel fraction> The adhesive sheet was cut into a size of 30 mm x 100 mm, the separator on one side was peeled off, and the exposed adhesive layer was attached to a weighed 300 mesh stainless steel wire mesh (weight W0), and the other separator was peeled off to prepare a sample. The weight of the sample was weighed and designated W1, and the sample was left in ethyl acetate for 24 hours, dried at 100°C for 1 hour, and weighed and designated W2, and the weight was calculated using the following formula. Gel fraction (%) = {(W2-W0) / (W1-W0)} x 100

[0073] <Adhesive strength> The separator on one side of the adhesive sheet was peeled off, and the adhesive layer was attached to a 50 μm PET film using a laminator. Next, the other separator was peeled off, and the adhesive layer exposed by the laminator was attached to the corona-treated surface of a 50 μm-thick colorless polyimide film (manufactured by KOLON Co., Ltd.) that had been corona-treated on one side at an output of 300 W. Then, the adhesive layer was held in an autoclave at 50° C. and 5 atm for 20 minutes to allow each member to adhere to each other, thereby obtaining a measurement sample. After leaving the measurement sample at 23° C. for one day, the adhesive strength was measured at a peel speed of 300 mm / min and a peel angle of 180° in an environment of 23° C. and 50% relative humidity using a tensile tester (Orientec Co., Ltd. "Tensilon"). The evaluation criteria are as follows. +++: Adhesive strength is 20N / 25mm or more. Excellent. ++: Adhesive strength is 15N / 25mm or more and less than 20N / 25mm. Good. +: Adhesive strength is 10N / 25mm or more, but less than 15N / 25mm. Practical use is possible. NG: Adhesive strength is less than 10N / 25mm. Not practical.

[0074] <Dynamic bending test> On a PET film (product name: A4300, manufactured by Toyobo Co., Ltd.) having a thickness of 188 μm, the separator on one side of the adhesive sheet was peeled off, and the adhesive layer was attached using a hand laminator. Next, the other separator was peeled off, and the exposed adhesive layer was placed on the corona-treated surface of a colorless polyimide film (manufactured by KOLON Co., Ltd.) having a thickness of 50 μm, and attached using a hand laminator. After that, the components were kept in an autoclave at 50°C and 5 atm for 20 minutes to adhere to each other, thereby obtaining a measurement sample. As a result, a laminate for flexible displays of "PET film / adhesive layer / colorless polyimide film" was obtained. The obtained laminate for flexible displays was folded with the PET film surface facing inward using a folding tester (manufactured by Yuasa System Co., Ltd.) at 25°C and a relative humidity of 50%, with the inner diameter (diameter) set to 4 mm, and 200,000 cycles were repeated, with folding and opening at 180° being one cycle. The appearance of the laminate was visually inspected to see whether peeling, lifting, wrinkles, etc. occurred between the pressure-sensitive adhesive layer and the PET film or colorless polyimide film.

[0075] <Static bending test> A laminate made in exactly the same manner as the "PET film / adhesive layer / colorless polyimide film" laminate used in the dynamic bending test was sandwiched between two glass plates with a spacer to adjust the gap so that the inner diameter (diameter) was 4 mm when folded with the PET film side on the inside, and the appearance of the laminate after standing for 10 days in an atmosphere of 60°C and 90% relative humidity was evaluated using the same evaluation method as for the dynamic bending test. The evaluation criteria for both the dynamic bending test and the static bending test are as follows. +++: No peeling, lifting, wrinkles, etc. Good. ++: No peeling occurred, but wrinkles occurred in the adhesive layer. Practical use possible. NG: Peeling occurred. Not usable.

[0076] <Haze measurement of adhesive layer> The separator on one side of the adhesive sheet was peeled off, and the exposed adhesive surface was attached to a cover glass (manufactured by Matsunami Glass Co., Ltd.). Then, the other separator was peeled off from the adhesive sheet, and another cover glass was attached to the exposed adhesive surface to prepare a laminate of "cover glass / adhesive layer / cover glass". In this state, the laminate was kept in an autoclave at 50°C and 5 atm for 20 minutes and adhered to a glass plate to obtain a measurement sample. The haze of the measurement sample was measured using an integrating sphere light transmittance measuring device (manufactured by Nippon Denshoku Industries Co., Ltd., NDH-8000) in accordance with JIS K 7136.

[0077] The above measurement results are shown in Tables 2 to 5.

[0078] [Table 2]

[0079] [Table 3]

[0080] [Table 4]

[0081] [Table 5]

Claims

1. An adhesive sheet for a flexible display having an adhesive layer, wherein the adhesive layer contains a (meth)acrylic copolymer (A) (provided that the (meth)acrylic copolymer (A) excludes the (meth)acrylic copolymer (D)) and an adhesion - imparting resin (B), the adhesion - imparting resin (B) comprises at least one selected from the group consisting of C5 - C9 petroleum resins, styrene - based monomer homopolymers, copolymers of styrene - based monomers and aromatic monomers other than styrene - based monomers, and copolymers of styrene - based monomers and aliphatic monomers, an adhesive sheet for a flexible display, characterized in that the haze of the adhesive layer is 3% or less.

2. The adhesive sheet for a flexible display according to Claim 1, wherein the softening point of the adhesion - imparting resin (B) is 90°C or higher and 150°C or lower.

3. The adhesive sheet for a flexible display according to Claim 2, which contains 0.1 to 3.5% by mass of the adhesion - imparting resin (B) with respect to 100% by mass of the (meth)acrylic copolymer (A).

4. The adhesive sheet for a flexible display according to Claim 3, wherein the adhesive layer further contains a (meth)acrylic copolymer (D) which is a copolymer of a monomer mixture containing an alicyclic (meth)acrylate monomer.

5. The adhesive sheet for a flexible display according to any one of Claims 1 to 4, wherein the adhesive layer has an adhesive force to a colorless polyimide film of 15 to 50 N / 25 mm.

6. An adhesive sheet for a flexible display according to Claim 5 and a laminate for a flexible display comprising a colorless polyimide.

7. A flexible display comprising the adhesive sheet for a flexible display according to Claim 5.