JP7572153B2 - Adhesive sheet for use in laminate in flexible image display, laminate for use in flexible image display, and flexible image display - Google Patents

Description

The present invention relates to an adhesive sheet for use in a laminate in a flexible image display device, a laminate for use in a flexible image display device, and a flexible image display device.

Various thin image display devices, such as liquid crystal displays and organic EL displays, have a laminated structure including, for example, an image display panel and an optical film (see, for example, Patent Document 1). Adhesive sheets are generally used to bond the layers that make up the image display device.

JP 2014-157745 A

In recent years, flexible image display devices that can be bent have been put to practical use. According to the study by the present inventors, it has been found that when a flexible image display device having a rollable display unit is used repeatedly, the device leaves marks such as curling tendencies. If an image display device with marks is forcibly returned to a flat state, problems such as peeling of the layers bonded by the adhesive sheet or unevenness in the image display may occur.

After extensive research, the inventors have found that marks such as curling tendencies are caused by distortions that occur in the layers that make up a flexible image display device when the device is bent. It is expected that this distortion can be alleviated by using a flexible adhesive sheet with a low gel fraction. However, it has been found that when the gel fraction of the adhesive sheet is reduced, in a flexible image display device that has a rollable display unit and in which distortion occurs over a relatively wide range, the layers bonded by the adhesive sheet tend to peel off.

The present invention aims to provide an adhesive sheet that can prevent traces of curling and other problems from remaining even when a flexible image display device having a rollable display unit is used repeatedly, and can also prevent the layers that make up the device from peeling off.

The present invention relates to
The 1000% modulus is 0.01 to 0.6 N/ ^mm2 ,
Provided is a pressure-sensitive adhesive sheet for use in a laminate in a flexible image display device having a rollable display section, the sheet having a gel fraction of 30% or more.

Further, the present invention relates to
The above adhesive sheet,
A substrate supporting the pressure-sensitive adhesive sheet;
The present invention provides a laminate for use in a flexible image display device having a rollable display portion, comprising:

Further, the present invention relates to
The laminate,
An image display panel;
Equipped with
The laminate is provided as a flexible image display device having a rollable display section located on the viewing side of the image display panel.

The present invention provides an adhesive sheet that can prevent traces of curling and other problems from remaining even when a flexible image display device having a rollable display unit is used repeatedly, and can prevent the layers that make up the device from peeling off.

1 is a cross-sectional view of a laminate used in a flexible image display device according to one embodiment of the present invention and a flexible image display device. 1 is a cross-sectional view of a laminate and a flexible image display device according to another embodiment of the present invention. FIG. 11A and 11B are diagrams for explaining a method for measuring the amount of misalignment caused by an adhesive sheet of each layer constituting a flexible image display device. 11A and 11B are diagrams for explaining a method for measuring the amount of misalignment caused by an adhesive sheet of each layer constituting a flexible image display device. 11A and 11B are diagrams for explaining a method for measuring the amount of misalignment caused by an adhesive sheet of each layer constituting a flexible image display device. FIG. 1 is a side view showing an example of a flexible image display device. 1A to 1C are diagrams for explaining a method for manufacturing a retardation film. FIG. 13 is a diagram for explaining a continuous winding test.

The present invention will be described in detail below, but the present invention is not limited to the following embodiments, and can be modified as desired without departing from the spirit of the present invention.

(Embodiments of the pressure-sensitive adhesive sheet)
The pressure-sensitive adhesive sheet of the present embodiment is a member used in a laminate in a flexible image display device having a rollable display section, and has a 1000% modulus of 0.01 to 0.6 N/ ^mm2 and a gel fraction of 30% or more.

The 1000% modulus of an adhesive sheet is a characteristic expressed by the value obtained by dividing the stress (tensile stress) generated in the adhesive sheet when the adhesive sheet is stretched 1000% by a tensile force in one direction by the initial cross-sectional area of the adhesive sheet. The 1000% modulus of an adhesive sheet can be evaluated as follows:

The adhesive sheet to be evaluated is cut into a strip of 30 mm x 40 mm. Next, the cut adhesive sheet is rolled in the direction of the long side to prevent air bubbles from being mixed in, to obtain a cylindrical test piece with a height of 30 mm corresponding to the length of the short side. Next, the obtained test piece is set in a tensile tester such as a Tensilon, and a uniaxial tensile test is performed in the height direction to obtain an elongation-stress curve of the adhesive sheet. Note that the preparation of the test piece and the uniaxial tensile test are performed at room temperature (23°C), and the initial chuck distance in the uniaxial tensile test is 10 mm, and the tensile speed is 300 mm/min. From the obtained elongation-stress curve, the stress at 1000% elongation (chuck distance is 110 mm at this time) is calculated, and this is divided by the initial cross-sectional area of the test piece to obtain the 1000% modulus of the adhesive sheet.

Since the 1000% modulus of the adhesive sheet is 0.01 to 0.6 N/mm ² , the adhesive sheet can sufficiently relax the strain applied to the layers constituting the device, particularly the layer bonded to the adhesive sheet, when the flexible image display device is wound up, for example, when the flexible image display device is stored in a state where it is wound around an axial member (roller), while ensuring sufficient cohesive force. In detail, it is assumed that a flexible image display device having a rollable display unit is wound around an axial member when it is used. At this time, a wide range of strain is generated in the display unit. A large stress caused by this strain is generated in the laminate in the flexible image display device, and a large shear stress is also generated in the adhesive sheet. If the 1000% modulus of the adhesive sheet is 0.01 to 0.6 N/mm ² , the above-mentioned strain can be relaxed when a large shear stress is generated in the adhesive sheet. Therefore, the strain applied to the layers constituting the flexible image display device is sufficiently relaxed and the stress generated in the layer is reduced, so that it is possible to sufficiently suppress the device from leaving a mark such as a curling tendency and the layer from peeling off. According to the study by the present inventors, in a foldable image display device having a bendable bent portion, it is presumed that the stress generated at the bent portion is alleviated in the flat portion other than the bent portion. Therefore, for a foldable image display device, the 1000% modulus of the adhesive sheet has hardly been paid attention to.

The 1000% modulus of the pressure-sensitive adhesive sheet is preferably 0.4 N/mm2 or ^less , more preferably 0.25 N/mm2 or ^less , even more preferably 0.2 N/mm2 or ^less , and particularly preferably 0.1 N/mm2 or ^less . The 1000% modulus of the pressure-sensitive adhesive sheet may be 0.05 N/mm2 or ^more , or may be 0.075 N/ ^mm2 or more.

The gel fraction of the adhesive sheet can be evaluated, for example, by the following method. First, a part of the adhesive sheet is scraped off to obtain a small piece. Next, the obtained small piece is wrapped in a stretched porous polytetrafluoroethylene film and tied with a kite string. This results in a test piece. Next, the total weight (weight A) of the adhesive sheet small piece, the stretched porous film, and the kite string is measured. The total weight of the stretched porous film and the kite string used is defined as weight B. Next, the test piece is immersed in a container filled with ethyl acetate and left to stand at 23°C for one week. After standing, the test piece is removed from the container and dried in a dryer set at 130°C for two hours, and then the weight C of the test piece is measured. The gel fraction of the adhesive sheet can be calculated from weight A, weight B, and weight C based on the following formula.
Gel fraction (wt%)=(C−B)/(A−B)×100

When the gel fraction of the adhesive sheet is 30% or more, the adhesive sheet has sufficient cohesive strength. This allows the peeling of the layers bonded by the adhesive sheet to be sufficiently suppressed even in a flexible image display device having a rollable display section and in which distortion occurs over a relatively wide range. The gel fraction of the adhesive sheet may be 40% or more, 50% or more, or 60% or more. The upper limit of the gel fraction of the adhesive sheet is not particularly limited, and may be, for example, 95% or less, 85% or less, or 75% or less. The gel fraction of the adhesive sheet may be 55% or less in some cases.

The 500% modulus of the adhesive sheet is, for example, 0.2 N/mm2 ^or less. The 500% modulus of the adhesive sheet is a characteristic represented by the value obtained by dividing the stress (tensile stress) generated in the adhesive sheet when the adhesive sheet is elongated by 500% by a tensile force in one direction by the initial cross-sectional area of the adhesive sheet. The 500% modulus of the adhesive sheet can be evaluated according to the method described above for the 1000% modulus of the adhesive sheet.

The 500% modulus of the pressure-sensitive adhesive sheet is preferably 0.15 N/mm2 or ^less , more preferably 0.1 N/mm2 or ^less , even more preferably 0.08 N/ ^mm2 or less, and particularly preferably 0.05 N/mm2 or ^less . The lower limit of the 500% modulus of the pressure-sensitive adhesive sheet is not particularly limited, and is, for example, 0.01 N/ ^mm2 or more.

The 700% modulus of the adhesive sheet is, for example, 0.3 N/mm2 ^or less. The 700% modulus of the adhesive sheet is a characteristic represented by the value obtained by dividing the stress (tensile stress) generated in the adhesive sheet when the adhesive sheet is elongated by 700% by a tensile force in one direction by the initial cross-sectional area of the adhesive sheet. The 700% modulus of the adhesive sheet can be evaluated according to the method described above for the 1000% modulus of the adhesive sheet.

The 700% modulus of the pressure-sensitive adhesive sheet is preferably 0.2 N/mm2 ^or less, more preferably 0.15 N/mm2 or ^less , and even more preferably 0.1 N/mm2 or ^less . The lower limit of the 700% modulus of the pressure-sensitive adhesive sheet is not particularly limited, and is, for example, 0.01 N/ ^mm2 or more.

The upper limit of the glass transition temperature (Tg) of the adhesive sheet is preferably 5°C or lower, more preferably -20°C or lower, and even more preferably -25°C or lower. If the Tg of the adhesive sheet is within this range, the adhesive sheet is less likely to harden, and a flexible image display device with excellent stress relaxation properties can be realized.

The total light transmittance of the adhesive sheet in the visible light wavelength range (in accordance with JIS K7136:2000) is preferably 85% or more, and more preferably 90% or more.

Examples of adhesives constituting the adhesive sheet include acrylic adhesives, rubber adhesives, vinyl alkyl ether adhesives, silicone adhesives, polyester adhesives, polyamide adhesives, urethane adhesives, fluorine adhesives, epoxy adhesives, and polyether adhesives. The adhesives constituting the adhesive sheet are used alone or in combination of two or more. However, from the standpoint of transparency, processability, durability, adhesion, etc., it is preferable to use an acrylic adhesive (composition) containing a (meth)acrylic polymer alone. In other words, it is preferable for the adhesive sheet to contain a (meth)acrylic polymer.

[(Meth)acrylic polymer]
When an acrylic pressure-sensitive adhesive is used as the pressure-sensitive adhesive composition, it is preferable to contain a (meth)acrylic polymer containing, as a monomer unit, a (meth)acrylic monomer having a linear or branched alkyl group having 1 to 30 carbon atoms. In this specification, "(meth)acrylic polymer" means an acrylic polymer and/or a methacrylic polymer, and "(meth)acrylate" means an acrylate and/or a methacrylate.

Specific examples of (meth)acrylic monomers having a linear or branched alkyl group having 1 to 30 carbon atoms that constitute the main skeleton of a (meth)acrylic polymer include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, isobutyl (meth)acrylate, n-pentyl (meth)acrylate, isopentyl (meth)acrylate, n-hexyl (meth)acrylate, isohexyl (meth)acrylate, isoheptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, and iso Examples of such monomers include octyl (meth)acrylate, n-nonyl (meth)acrylate, isononyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl (meth)acrylate, n-dodecyl (meth)acrylate (lauryl (meth)acrylate), n-tridecyl (meth)acrylate, and n-tetradecyl (meth)acrylate. Among these, preferred are (meth)acrylic monomers having a linear or branched alkyl group having 6 to 30 carbon atoms (hereinafter, sometimes referred to as "(meth)acrylic monomers having a long-chain alkyl group"), and more preferred is n-dodecyl (meth)acrylate (lauryl (meth)acrylate). By using a (meth)acrylic monomer having a long-chain alkyl group, entanglement of the polymer is reduced, and the polymer is more likely to deform in response to minute distortion. From the viewpoint of adhesion at low temperatures, it is also preferable to use a (meth)acrylic monomer whose homopolymer has a glass transition temperature (Tg) of -70 to -20°C, and among these, it is more preferable to use 2-ethylhexyl acrylate. One or more types of (meth)acrylic monomers can be used.

The (meth)acrylic monomer having a linear or branched alkyl group having 1 to 30 carbon atoms is the main component of all monomers constituting the (meth)acrylic polymer. Here, the main component means that the (meth)acrylic monomer having a linear or branched alkyl group having 1 to 30 carbon atoms is 50 to 100% by weight of all monomers constituting the (meth)acrylic polymer, more preferably 80 to 100% by weight, even more preferably 90 to 99.9% by weight, and particularly preferably 94 to 99.9% by weight.

The monomer components constituting the (meth)acrylic polymer may contain a copolymerizable monomer in addition to a (meth)acrylic monomer having a linear or branched alkyl group having 1 to 30 carbon atoms. The copolymerizable monomer may be used alone or in combination of two or more kinds.

The copolymerizable monomer is not particularly limited, but is preferably a hydroxyl group-containing monomer having a reactive functional group. By using a hydroxyl group-containing monomer, a pressure-sensitive adhesive sheet with excellent adhesion tends to be obtained. The hydroxyl group-containing monomer is, for example, a compound that contains a hydroxyl group in its structure and also contains a polymerizable unsaturated double bond such as a (meth)acryloyl group or a vinyl group.

Specific examples of hydroxyl group-containing monomers include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, and 12-hydroxylauryl (meth)acrylate, as well as (4-hydroxymethylcyclohexyl)-methyl acrylate. Among the hydroxyl group-containing monomers, 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are preferred from the standpoint of durability and adhesion. Note that one or more types of hydroxyl group-containing monomers can be used.

The copolymerizable monomer may contain a monomer having a reactive functional group, such as a carboxyl group-containing monomer, an amino group-containing monomer, or an amide group-containing monomer. The use of these monomers is preferable from the viewpoint of adhesion in a humid or high-temperature environment.

When an acrylic adhesive is used as the adhesive composition, the adhesive composition can contain a (meth)acrylic polymer containing a carboxyl group-containing monomer having a reactive functional group as a monomer unit. By using a carboxyl group-containing monomer, an adhesive sheet with excellent adhesion in humid and high temperature environments tends to be obtained. A carboxyl group-containing monomer is a compound that contains a carboxyl group in its structure and a polymerizable unsaturated double bond such as a (meth)acryloyl group or a vinyl group.

Specific examples of carboxyl group-containing monomers include (meth)acrylic acid, carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid.

When an acrylic adhesive is used as the adhesive composition, the adhesive composition can contain, as a monomer unit, a (meth)acrylic polymer containing an amino group-containing monomer having a reactive functional group. By using an amino group-containing monomer, an adhesive sheet with excellent adhesion in a humid or high temperature environment tends to be obtained. The amino group-containing monomer is a compound that contains an amino group in its structure and a polymerizable unsaturated double bond such as a (meth)acryloyl group or a vinyl group.

Specific examples of amino group-containing monomers include N,N-dimethylaminoethyl (meth)acrylate and N,N-dimethylaminopropyl (meth)acrylate.

When an acrylic adhesive is used as the adhesive composition, the adhesive composition may contain, as a monomer unit, a (meth)acrylic polymer containing an amide group-containing monomer having a reactive functional group. By using an amide group-containing monomer, an adhesive sheet with excellent adhesion tends to be obtained. An amide group-containing monomer is a compound that contains an amide group in its structure and a polymerizable unsaturated double bond such as a (meth)acryloyl group or a vinyl group.

Specific examples of amide group-containing monomers include acrylamide-based monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N-isopropylacrylamide, N-methyl(meth)acrylamide, N-butyl(meth)acrylamide, N-hexyl(meth)acrylamide, N-methylol(meth)acrylamide, N-methylol-N-propane(meth)acrylamide, aminomethyl(meth)acrylamide, aminoethyl(meth)acrylamide, mercaptomethyl(meth)acrylamide, and mercaptoethyl(meth)acrylamide; N-acryloyl heterocyclic monomers such as N-(meth)acryloylmorpholine, N-(meth)acryloylpiperidine, and N-(meth)acryloylpyrrolidine; and N-vinyl group-containing lactam-based monomers such as N-vinylpyrrolidone and N-vinyl-ε-caprolactam.

Furthermore, examples of copolymerizable monomers include polyfunctional monomers. When a polyfunctional monomer is included, a cross-linking effect can be obtained by polymerization, making it easy to adjust the gel fraction and improve the cohesive strength. This makes cutting easier and improves processability. Furthermore, peeling due to cohesive failure of the adhesive sheet can be better prevented. The polyfunctional monomer is not particularly limited, but examples thereof include polyfunctional acrylates such as hexanediol di(meth)acrylate (1,6-hexanediol di(meth)acrylate), butanediol di(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, allyl (meth)acrylate, vinyl (meth)acrylate, epoxy acrylate, polyester acrylate, and urethane acrylate, and divinylbenzene. Among these, preferred polyfunctional acrylates are 1,6-hexanediol diacrylate and dipentaerythritol hexa(meth)acrylate. The polyfunctional monomers may be used alone or in combination of two or more.

In the monomer units constituting the (meth)acrylic polymer, the blending ratio (total amount) of the monomer having a reactive functional group and the polyfunctional monomer is preferably 20% by weight or less, more preferably 10% by weight or less, even more preferably 0.01 to 8% by weight, particularly preferably 0.01 to 5% by weight, and most preferably 0.05 to 3% by weight, of all the monomers constituting the (meth)acrylic polymer. If it exceeds 20% by weight, the number of crosslinking points increases, and the flexibility of the adhesive (sheet) is lost, which tends to result in poor stress relaxation properties.

When an acrylic adhesive is used as the adhesive composition, other copolymerizable monomers can be introduced as monomer units in addition to the monomers having reactive functional groups and polyfunctional monomers, as long as the effects of the present invention are not impaired.

Other copolymerizable monomers include, for example, (meth)acrylic acid alkoxyalkyl esters [e.g., 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, methoxytriethylene glycol (meth)acrylate, 3-methoxypropyl (meth)acrylate, 3-ethoxypropyl (meth)acrylate, 4-methoxybutyl (meth)acrylate, 4-ethoxybutyl (meth)acrylate, etc.]; epoxy group-containing monomers [e.g., glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, etc.]; sulfonic acid group-containing monomers [e.g., sodium vinyl sulfonate, etc.]; phosphate group-containing monomers; alicyclic hydrocarbon group-containing (meth)alkyl esters [e.g., 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, methoxytriethylene glycol (meth)acrylate, 3-methoxypropyl (meth)acrylate, 3-ethoxypropyl (meth)acrylate, 4-methoxybutyl (meth)acrylate, 4-ethoxybutyl (meth)acrylate, etc.]; p) Acrylic acid esters [e.g., cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, etc.]; (meth)acrylic acid esters having aromatic hydrocarbon groups [e.g., phenyl (meth)acrylate, phenoxyethyl (meth)acrylate, benzyl (meth)acrylate, etc.]; vinyl esters [e.g., vinyl acetate, vinyl propionate, etc.]; aromatic vinyl compounds [e.g., styrene, vinyl toluene, etc.]; olefins or dienes [e.g., ethylene, propylene, butadiene, isoprene, isobutylene, etc.]; vinyl ethers [e.g., vinyl alkyl ether, etc.]; vinyl chloride, etc.

The blending ratio of other copolymerization monomers is not particularly limited, but is preferably 30% by weight or less, more preferably 10% by weight or less, and even more preferably none, of all monomers constituting the (meth)acrylic polymer. If it exceeds 30% by weight, particularly when a monomer other than a (meth)acrylic monomer is used, there will be fewer reaction sites between the pressure-sensitive adhesive sheet and other layers (film, substrate), and the adhesion will tend to decrease.

The adhesive sheet is formed from an adhesive composition, and the adhesive composition may be an adhesive composition having any form, such as an emulsion type, a solvent type (solution type), an active energy ray curing type, or a hot melt type. Of these, preferred examples of the adhesive composition include a solvent type adhesive composition and an active energy ray curing type adhesive composition.

Preferably, the solvent-based adhesive composition is an adhesive composition containing a (meth)acrylic polymer as an essential component. Preferably, the active energy ray-curable adhesive composition is an adhesive composition containing a mixture of monomer components constituting a (meth)acrylic polymer (monomer mixture) or a partial polymer thereof as an essential component. Note that "partial polymer" means a composition in which one or more of the monomer components contained in the monomer mixture are partially polymerized. The "monomer mixture" includes the case where there is only one type of monomer component.

In particular, from the standpoints of productivity, environmental impact, and ease of obtaining a thick adhesive sheet, it is preferable that the adhesive composition be an active energy ray-curable adhesive composition that contains, as an essential component, a mixture of monomer components constituting a (meth)acrylic polymer (monomer mixture) or a partial polymer thereof.

The (meth)acrylic polymer is obtained by polymerizing the monomer components. More specifically, the (meth)acrylic polymer is obtained by polymerizing the monomer components, the monomer mixture, or a partially polymerized product thereof by a known and commonly used method. Examples of the polymerization method include solution polymerization, emulsion polymerization, bulk polymerization, and polymerization by heat or active energy ray irradiation (thermal polymerization, active energy ray polymerization). Among them, solution polymerization and active energy ray polymerization are preferred in terms of transparency, water resistance, cost, etc. In addition, it is preferable to carry out the polymerization while avoiding contact with oxygen in order to suppress polymerization inhibition by oxygen. For example, it is preferable to carry out the polymerization under a nitrogen atmosphere or by blocking oxygen with a release film (separator). The obtained (meth)acrylic polymer may be any of a random copolymer, a block copolymer, a graft copolymer, etc.

The active energy rays irradiated when performing active energy ray polymerization (photopolymerization) include, for example, ionizing radiation such as α rays, β rays, γ rays, neutron rays, and electron beams, and ultraviolet rays, with ultraviolet rays being particularly preferred. There are no particular limitations on the irradiation energy, irradiation time, irradiation method, etc. of the active energy rays, as long as they can activate the photopolymerization initiator and cause a reaction of the monomer components.

When carrying out solution polymerization, various common solvents can be used. Examples of such solvents 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. The solvents may be used alone or in combination of two or more.

When carrying out polymerization, a polymerization initiator such as a photopolymerization initiator (photoinitiator) or a thermal polymerization initiator may be used depending on the type of polymerization reaction. The polymerization initiator may be used alone or in combination of two or more kinds.

The photopolymerization initiator is not particularly limited, but examples 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.

Examples of 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-diphenylethane-1-one, and anisole methyl ether. Examples of acetophenone-based photopolymerization initiators include 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexyl phenyl ketone, 4-phenoxydichloroacetophenone, and 4-(t-butyl)dichloroacetophenone. Examples of α-ketol-based photopolymerization initiators include 2-methyl-2-hydroxypropiophenone and 1-[4-(2-hydroxyethyl)phenyl]-2-methylpropan-1-one. Examples of aromatic sulfonyl chloride-based photopolymerization initiators include 2-naphthalenesulfonyl chloride. Examples of photoactive oxime-based photopolymerization initiators include 1-phenyl-1,1-propanedione-2-(o-ethoxycarbonyl)-oxime. Examples of benzoin-based photopolymerization initiators include benzoin. Examples of benzyl-based photopolymerization initiators include benzil. Examples of benzophenone-based photopolymerization initiators include benzophenone, benzoylbenzoic acid, 3,3'-dimethyl-4-methoxybenzophenone, polyvinylbenzophenone, α-hydroxycyclohexylphenyl ketone. Examples of ketal-based photopolymerization initiators include benzyl dimethyl ketal. Examples of thioxanthone-based photopolymerization initiators include thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, isopropylthioxanthone, 2,4-diisopropylthioxanthone, and dodecylthioxanthone.

The amount of photopolymerization initiator used is not particularly limited, but is preferably 0.01 to 1 part by weight, and more preferably 0.05 to 0.5 parts by weight, per 100 parts by weight of the total amount of monomer components.

Polymerization initiators used in solution polymerization include, for example, azo polymerization initiators, peroxide polymerization initiators (e.g., dibenzoyl peroxide, tert-butyl permaleate, etc.), 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 (AIBN), 2,2'-azobis-2-methylbutyronitrile, 2,2'-azobis(2-methylpropionate) dimethyl, and 4,4'-azobis-4-cyanovaleric acid.

The amount of the azo polymerization initiator used is not particularly limited, but is preferably 0.05 to 0.5 parts by weight, and more preferably 0.1 to 0.3 parts by weight, per 100 parts by weight of the total amount of the monomer components.

The polyfunctional monomer (polyfunctional acrylate) used as the copolymerization monomer can also be used in a solvent-based or active energy ray-curable adhesive composition. For example, when a solvent-based adhesive composition is used by mixing a polyfunctional monomer (polyfunctional acrylate) with a photopolymerization initiator, active energy ray curing is performed after thermal drying.

The weight-average molecular weight (Mw) of the (meth)acrylic polymer used in the solvent-based adhesive composition is usually in the range of 1 million to 2 million. Considering durability, particularly heat resistance, it is preferably 1.2 million to 2 million, more preferably 1.4 million to 1.8 million. If the weight-average molecular weight is less than 1 million, when crosslinking polymer chains to ensure durability, the number of crosslinking points will be greater than in the case of a weight-average molecular weight of 1 million or more, and the flexibility of the adhesive (sheet) will be lost, so that the distortion of the outer side (convex side) and inner side (concave side) of the bend that occurs between each layer (each film) constituting the image display device during winding cannot be alleviated, and each layer may be prone to breakage. If the weight-average molecular weight is greater than 2.5 million, a large amount of dilution solvent is required to adjust the viscosity to a suitable level for coating, which is undesirable because it increases costs, and the entanglement of the polymer chains of the resulting (meth)acrylic polymer becomes complex, resulting in poor flexibility and the breakage of each layer (film) during winding. The weight average molecular weight (Mw) is measured by gel permeation chromatography (GPC) and calculated in terms of polystyrene.

[(Meth)acrylic oligomer]
The pressure-sensitive adhesive composition may contain a (meth)acrylic oligomer. The (meth)acrylic oligomer is preferably a polymer having a weight average molecular weight (Mw) smaller than that of the (meth)acrylic polymer. By using such a (meth)acrylic oligomer, the (meth)acrylic oligomer is present between the (meth)acrylic polymers, reducing entanglement of the (meth)acrylic polymers, and the composition is more likely to deform in response to a small strain.

Examples of monomers constituting (meth)acrylic oligomers include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, and the like. Examples of the acrylate include alkyl (meth)acrylates such as isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, and dodecyl (meth)acrylate; esters of (meth)acrylic acid and alicyclic alcohols such as cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, and dicyclopentanyl (meth)acrylate; aryl (meth)acrylates such as phenyl (meth)acrylate and benzyl (meth)acrylate; and (meth)acrylates obtained from alcohols derived from terpene compounds. These (meth)acrylates can be used alone or in combination of two or more.

As the (meth)acrylic oligomer, it is preferable to include, as a monomer unit, an acrylic monomer having a relatively bulky structure, such as an alkyl (meth)acrylate with a branched alkyl group, such as isobutyl (meth)acrylate or t-butyl (meth)acrylate; an ester of (meth)acrylic acid and an alicyclic alcohol, such as cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, or dicyclopentanyl (meth)acrylate; or an aryl (meth)acrylate, such as phenyl (meth)acrylate or benzyl (meth)acrylate, having a cyclic structure. By imparting such a bulky structure to the (meth)acrylic oligomer, there is a tendency to further improve the adhesiveness of the pressure-sensitive adhesive sheet. In particular, in terms of bulkiness, those having a cyclic structure are highly effective, and those containing multiple rings are even more effective. When ultraviolet light is used to synthesize (meth)acrylic oligomers or to produce adhesive sheets, those with saturated bonds are preferred because they are less likely to cause polymerization inhibition, and alkyl (meth)acrylates in which the alkyl group has a branched structure, or esters with alicyclic alcohols, can be suitably used as monomers that make up the (meth)acrylic oligomers.

From this viewpoint, suitable (meth)acrylic oligomers include, for example, a copolymer of butyl acrylate (BA), methyl acrylate (MA) and acrylic acid (AA), a copolymer of cyclohexyl methacrylate (CHMA) and isobutyl methacrylate (IBMA), a copolymer of cyclohexyl methacrylate (CHMA) and isobornyl methacrylate (IBXMA), a copolymer of cyclohexyl methacrylate (CHMA) and acryloylmorpholine (ACMO), a copolymer of cyclohexyl methacrylate (CHMA) and diethylacrylamide (DEAA), a copolymer of 1-adamantyl acrylate (ADA) and methyl Examples of such copolymers include copolymers of dicyclopentanyl methacrylate (DCPMA) and isobornyl methacrylate (IBXA), copolymers of dicyclopentanyl methacrylate (DCPMA), cyclohexyl methacrylate (CHMA), isobornyl methacrylate (IBXA), copolymers of cyclopentanyl methacrylate (DCPMA) and methyl methacrylate (MMA), and homopolymers of dicyclopentanyl acrylate (DCPA), 1-adamantyl methacrylate (ADMA), and 1-adamantyl acrylate (ADA).

As with (meth)acrylic polymers, polymerization methods for (meth)acrylic oligomers include solution polymerization, emulsion polymerization, bulk polymerization, emulsion polymerization, and polymerization by heat or active energy ray irradiation (thermal polymerization, active energy ray polymerization). Among these, solution polymerization and active energy ray polymerization are preferred in terms of transparency, water resistance, cost, etc. The resulting (meth)acrylic oligomer may be any of a random copolymer, a block copolymer, a graft copolymer, etc.

The (meth)acrylic oligomer can be used in a solvent-based adhesive composition or an active energy ray-curable adhesive composition, similar to the (meth)acrylic polymer. For example, an active energy ray-curable adhesive composition can be prepared by mixing a mixture of monomer components constituting a (meth)acrylic polymer (monomer mixture) or a partial polymer thereof with a (meth)acrylic oligomer. When the (meth)acrylic oligomer is dissolved in a solvent, the adhesive composition can be thermally dried to volatilize the solvent, and then active energy ray curing can be completed to obtain an adhesive sheet.

The weight average molecular weight (Mw) of the (meth)acrylic oligomer used in the solvent-based adhesive composition is preferably 1000 or more, more preferably 2000 or more, even more preferably 3000 or more, and particularly preferably 4000 or more. The weight average molecular weight (Mw) of the (meth)acrylic oligomer is preferably 30000 or less, more preferably 15000 or less, even more preferably 10000 or less, and particularly preferably 7000 or less. By adjusting the weight average molecular weight (Mw) of the (meth)acrylic oligomer within the above range, for example, when used in combination with a (meth)acrylic polymer, the (meth)acrylic oligomer is interposed between the (meth)acrylic polymers, which reduces the entanglement of the (meth)acrylic polymer, making the adhesive sheet more easily deformed in response to minute distortion, and tends to reduce the distortion applied to other layers constituting the image display device. This tends to further suppress cracking of each layer and peeling between the adhesive sheet and other layers. The weight average molecular weight (Mw) of the (meth)acrylic oligomer is the value measured by GPC (gel permeation chromatography) and calculated in terms of polystyrene, similar to the (meth)acrylic polymer.

When a (meth)acrylic oligomer is used in the adhesive composition, the amount is not particularly limited, but is preferably 70 parts by weight or less, more preferably 1 to 70 parts by weight, even more preferably 2 to 50 parts by weight, and even more preferably 3 to 40 parts by weight, per 100 parts by weight of the (meth)acrylic polymer. By adjusting the amount of the (meth)acrylic oligomer to be used within the above range, the (meth)acrylic oligomer is appropriately interposed between the (meth)acrylic polymers, the entanglement of the (meth)acrylic polymers is reduced, and the adhesive sheet tends to be easily deformed in response to minute distortion. This reduces the distortion applied to other layers constituting the image display device, and tends to suppress cracking of each layer and peeling between the adhesive sheet and other layers.

[Crosslinking agent]
The adhesive composition may contain a crosslinking agent. As the crosslinking agent, an organic crosslinking agent or a polyfunctional metal chelate may be used in either the solvent-based or active energy ray curable adhesive composition. Examples of the organic crosslinking agent include an isocyanate-based crosslinking agent, a peroxide-based crosslinking agent, an epoxy-based crosslinking agent, and an imine-based crosslinking agent. In the polyfunctional metal chelate, a polyvalent metal is covalently bonded or coordinately bonded to an organic compound. Examples of the polyvalent metal atom include Al, Cr, Zr, Co, Cu, Fe, Ni, V, Zn, In, Ca, Mg, Mn, Y, Ce, Sr, Ba, Mo, La, Sn, and Ti. Examples of the atom in the organic compound that is covalently bonded or coordinately bonded include an oxygen atom, and examples of the organic compound include an alkyl ester, an alcohol compound, a carboxylic acid compound, an ether compound, and a ketone compound. In the case of a solvent-based adhesive composition, a peroxide-based crosslinking agent or an isocyanate crosslinking agent is preferred, and among them, a peroxide-based crosslinking agent is preferably used. The peroxide-based crosslinking agent generates radicals by abstracting hydrogen from the side chains of the (meth)acrylic polymer, and promotes crosslinking between the side chains of the (meth)acrylic polymer. Therefore, compared with the case of crosslinking using an isocyanate-based crosslinking agent (e.g., a polyfunctional isocyanate-based crosslinking agent), the crosslinked state is relatively loose, and the cohesive force tends to be increased while maintaining the ease of deformation against minute strain. This tends to suppress cracking of each layer constituting the image display device and peeling between the adhesive sheet and other layers. The isocyanate-based crosslinking agent (particularly, a trifunctional isocyanate-based crosslinking agent) is preferable in terms of durability, and the peroxide-based crosslinking agent and the isocyanate-based crosslinking agent (particularly, a bifunctional isocyanate-based crosslinking agent) are preferable in terms of winding properties. Both the peroxide-based crosslinking agent and the bifunctional isocyanate-based crosslinking agent form flexible two-dimensional crosslinks, whereas the trifunctional isocyanate-based crosslinking agent forms stronger three-dimensional crosslinks. When winding, two-dimensional crosslinking, which is a more flexible crosslinking, is advantageous. However, since two-dimensional crosslinking alone is poor in durability and may easily cause peeling, a hybrid crosslinking of two-dimensional crosslinking and three-dimensional crosslinking is preferable. Therefore, it is a preferred embodiment to use a trifunctional isocyanate crosslinking agent in combination with a peroxide crosslinking agent or a bifunctional isocyanate crosslinking agent. In addition, as an active energy ray curable pressure-sensitive adhesive composition, it is preferable from the viewpoint of productivity and thick film coating to obtain a crosslinking effect by polymerization using a multifunctional monomer. However, the above-mentioned crosslinking agent may be used, or may be used in combination with a multifunctional monomer. For example, a crosslinking agent may be mixed into a mixture (monomer mixture) of monomer components constituting a (meth)acrylic polymer or a partial polymer thereof, and the reaction of the crosslinking agent may be completed by performing heat drying before and after curing of the pressure-sensitive adhesive composition by active energy rays.

The amount of the crosslinking agent used is, for example, preferably 0.1 to 10 parts by weight, more preferably 0.2 to 5 parts by weight, and even more preferably 0.2 to 1 part by weight, per 100 parts by weight of the (meth)acrylic polymer.

When using a peroxide-based crosslinking agent alone, the amount is preferably 0.5 to 5 parts by weight, and more preferably 1 to 3 parts by weight, per 100 parts by weight of the (meth)acrylic polymer. Within the above range, the adhesive sheet tends to maintain its ease of deformation in response to minute distortion while sufficiently increasing its cohesive force and improving its durability.

When a peroxide-based crosslinking agent and an isocyanate-based crosslinking agent are used in combination, the lower limit of the weight ratio of the peroxide-based crosslinking agent to the isocyanate-based crosslinking agent (peroxide-based crosslinking agent/isocyanate-based crosslinking agent) is preferably 0.02 or more, more preferably 1 or more, even more preferably more than 10, and particularly preferably 15 or more. The upper limit of this weight ratio is preferably 500 or less, more preferably 100 or less, even more preferably less than 50, and particularly preferably 40 or less. Within the above range, the adhesive sheet tends to be able to sufficiently increase the cohesive force while maintaining its ease of deformation in response to minute distortion.

[Additives]
Furthermore, the adhesive composition may contain other known additives, for example, various silane coupling agents, polyether compounds of polyalkylene glycols such as polypropylene glycol, colorants, powders such as pigments, dyes, surfactants, plasticizers, tackifiers, surface lubricants, leveling agents, softeners, antioxidants, antiaging agents, light stabilizers, ultraviolet absorbers, polymerization inhibitors, antistatic agents (alkali metal salts, ionic liquids, ionic solids, etc., which are ionic compounds), inorganic or organic fillers, metal powders, particles, foils, etc. may be appropriately added depending on the application. A redox system to which a reducing agent is added within a controllable range may also be used.

The method for preparing the adhesive composition is not particularly limited, and known methods can be used. For example, as described above, a solvent-based acrylic adhesive composition is prepared by mixing a (meth)acrylic polymer and components added as needed (e.g., a (meth)acrylic oligomer, a crosslinking agent, a silane coupling agent, a solvent, additives, etc.). As described above, an active energy ray-curable acrylic adhesive composition is prepared by mixing a monomer mixture or a partial polymer thereof and components added as needed (e.g., a photopolymerization initiator, a multifunctional monomer, a (meth)acrylic oligomer, a crosslinking agent, a silane coupling agent, a solvent, additives, etc.).

It is preferable that the adhesive composition has a viscosity suitable for handling and application. For this reason, it is preferable that the active energy ray curable acrylic adhesive composition contains a partial polymer of the monomer mixture. The polymerization rate of the partial polymer is not particularly limited, but is preferably 5 to 20% by weight, and more preferably 5 to 15% by weight.

The polymerization rate of the partial polymer is calculated as follows. A part of the partial polymer is sampled to obtain a sample. The sample is precisely weighed to obtain its weight, which is defined as the "weight of the partial polymer before drying". Next, the sample is dried at 130°C for 2 hours, and the sample after drying is precisely weighed to obtain its weight, which is defined as the "weight of the partial polymer after drying". Then, the weight of the sample reduced by drying at 130°C for 2 hours is calculated from the "weight of the partial polymer before drying" and the "weight of the partial polymer after drying", which is defined as the "weight reduction" (volatilized content, weight of unreacted monomer). The polymerization rate (wt%) of the partial polymer of the monomer component is calculated from the obtained "weight of the partial polymer before drying" and "weight reduction" according to the following formula.
Conversion rate (wt%) of partial polymer of monomer component=[1-(weight reduction amount)/(weight of partial polymer before drying)]×100

[Formation of Adhesive Sheet]
Examples of methods for forming a pressure-sensitive adhesive sheet include a method of applying a solvent-based pressure-sensitive adhesive composition to a peel-treated separator (release film) or the like, and drying and removing the polymerization solvent to form a pressure-sensitive adhesive sheet; a method of applying a solvent-based pressure-sensitive adhesive composition to a polarizing film or the like, and drying and removing the polymerization solvent to form a pressure-sensitive adhesive sheet on a polarizing film or the like; and a method of applying an active energy ray-curable pressure-sensitive adhesive composition to a peel-treated separator or the like, and forming the sheet by irradiating the separator with active energy rays. If necessary, heat drying may be performed in addition to the irradiation with active energy rays. When applying the pressure-sensitive adhesive composition, a solvent other than the polymerization solvent may be newly added.

A silicone release liner is preferably used as the release-treated separator. When a pressure-sensitive adhesive composition is applied to such a liner and dried to form a pressure-sensitive adhesive sheet, a suitable method may be adopted as a method for drying the pressure-sensitive adhesive, depending on the purpose. A method of heating and drying the coating film is preferably used. For example, when preparing an acrylic pressure-sensitive adhesive using a (meth)acrylic polymer, the heating and drying temperature is preferably 40 to 200°C, more preferably 50 to 180°C, and particularly preferably 70 to 170°C. By setting the heating temperature within the above range, a pressure-sensitive adhesive sheet with excellent adhesive properties tends to be obtained.

Any suitable drying time can be used. For example, when preparing an acrylic adhesive using a (meth)acrylic polymer, the drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 10 minutes, and particularly preferably 10 seconds to 5 minutes.

Various methods can be used to apply the adhesive composition. Specific examples include 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, and extrusion coating using a die coater, etc.

The thickness of the adhesive sheet is preferably 1 to 200 μm, more preferably 5 to 150 μm, and even more preferably 10 to 100 μm. The adhesive sheet may be a single layer or may have a laminated structure. If it is within the above range, it will not hinder the winding of the flexible image display device, and is also preferable from the standpoint of adhesion (retention resistance).

The following methods can be used to prepare the adhesive sheet of this embodiment:

First, the methods using a solvent-based pressure-sensitive adhesive composition (solvent-based methods 1 to 4) will be described. In the solvent-based method 1, a (meth)acrylic polymer obtained by copolymerizing a (meth)acrylic monomer having a long-chain alkyl group is used. Here, in the solvent-based method 1, it is preferable that the (meth)acrylic monomer having a long-chain alkyl group accounts for 40% by weight or more and 99% by weight of all monomers.

In the second solvent-based method, a peroxide-based crosslinking agent is used alone as the crosslinking agent, and 0.1 to 5 parts by weight are added per 100 parts by weight of the (meth)acrylic polymer.

In the third solvent-based method, a peroxide-based crosslinking agent and an isocyanate-based crosslinking agent are used in combination as crosslinking agents, the weight ratio of the peroxide-based crosslinking agent to the isocyanate-based crosslinking agent (peroxide-based crosslinking agent/isocyanate-based crosslinking agent) is 0.02 or more and 500 or less, and the amount of the peroxide-based crosslinking agent added is 0.1 parts by weight or more per 100 parts by weight of the (meth)acrylic polymer.

In the fourth solvent-based method, 1 part by weight or more and 70 parts by weight or less of a (meth)acrylic oligomer is further added to 100 parts by weight of the (meth)acrylic polymer in the second or third solvent-based method, as described above.

Next, the methods of using an active energy ray curable pressure-sensitive adhesive composition (first and second active energy ray curable methods) will be described. In the first active energy ray curable method, a mixture (monomer mixture) containing a (meth)acrylic monomer having a long-chain alkyl group as the main component and a monomer having an alkyl group or a functional group having 1 to 5 carbon atoms or a partial polymer of the mixture is polymerized together with a polyfunctional monomer.

In the second active energy ray curing method, in the first active energy ray curing method, a mixture (monomer mixture) of a (meth)acrylic monomer having an alkyl group with 10 to 30 carbon atoms and a (meth)acrylic monomer having an alkyl group with 6 to 9 carbon atoms, or a partial polymer of the mixture, is used as the (meth)acrylic monomer having a long-chain alkyl group.

Here, in the first and second active energy ray curing methods, the content of the (meth)acrylic monomer having a long-chain alkyl group in all monomers is preferably 60% by weight or more, more preferably 70% by weight or more, while it is preferably 100% by weight or less, more preferably 99% by weight or less. Furthermore, when using a mixture of a (meth)acrylic monomer having an alkyl group with 10 to 30 carbon atoms and a (meth)acrylic monomer having an alkyl group with 6 to 9 carbon atoms, the mixing ratio is preferably ((meth)acrylic monomer having an alkyl group with 10 to 30 carbon atoms):((meth)acrylic monomer having an alkyl group with 6 to 9 carbon atoms)=40:60 to 90:10.

(Embodiments of flexible image display devices and laminates)
As shown in FIG. 1 , a flexible image display device 100 of the present embodiment includes a laminate 10 and an image display panel 3 , and the laminate 10 is disposed on the viewer side of the image display panel 3 .

[Laminate]
The laminate 10 is a member used in the flexible image display device 100, and includes the above-mentioned adhesive sheet 1 and substrate 2. The substrate 2 supports the adhesive sheet 1, and is in contact with the adhesive sheet 1, for example. The substrate 2 does not need to be in direct contact with the adhesive sheet 1. The laminate 10 is attached to the image display panel 3 by the adhesive sheet 1. The laminate 10 does not include, for example, a polarizing film described below.

[Substrate]
The substrate 2 also functions as a protective film that protects the members included in the laminate 10. In Fig. 1, the substrate 2 is located on the outermost side of the laminate 10, and therefore also functions as a window, for example.

The substrate 2 is made of, for example, a transparent resin. Examples of transparent resins include cycloolefin resins such as norbornene resins, olefin resins such as polyethylene and polypropylene, polyester resins, (meth)acrylic resins, and polyimide resins.

The thickness of the substrate 2 is, for example, 5 to 60 μm, preferably 10 to 40 μm, and more preferably 10 to 30 μm. If the thickness of the substrate 2 is within the above range, the substrate 2 is unlikely to impede the winding of the flexible image display device. The substrate 2 may be subjected to surface treatment such as anti-glare, anti-reflection, and anti-static.

[Image display panel]
The image display panel 3 constitutes a display unit of the flexible image display device 100. In the flexible image display device 100, the display unit constituted by the image display panel 3 can be rolled up. The image display panel 3 is typically an organic EL display panel. A touch sensor may be incorporated in the image display panel 3. When the image display panel 3 has a built-in touch sensor, the flexible image display device 100 is a so-called in-cell type flexible image display device.

[Transparent conductive layer]
The laminate 10 may further include a transparent conductive layer constituting a touch sensor. The transparent conductive layer may be located between the adhesive sheet 1 and the substrate 2, or between the adhesive sheet 1 and the image display panel 3. The transparent conductive layer is configured to be bendable, for example.

The material constituting the transparent conductive layer is not particularly limited, and at least one metal or metal oxide selected from the group consisting of indium, tin, zinc, gallium, antimony, titanium, silicon, zirconium, magnesium, aluminum, gold, silver, copper, palladium, tungsten, and molybdenum, or an organic conductive polymer such as polythiophene, may be used. The metal oxide may further contain a metal atom shown in the above group, as necessary. For example, indium oxide (ITO) containing tin oxide, tin oxide containing antimony, etc. are preferably used, and ITO is particularly preferably used. ITO preferably contains 80 to 99% by weight of indium oxide and 1 to 20% by weight of tin oxide.

ITO can be crystalline ITO or non-crystalline (amorphous) ITO. Crystalline ITO can be obtained by increasing the temperature during sputtering or by further heating amorphous ITO.

The thickness of the transparent conductive layer is preferably 0.005 to 10 μm, more preferably 0.01 to 3 μm, and even more preferably 0.01 to 1 μm. If the thickness of the transparent conductive layer is less than 0.005 μm, the change in the electrical resistance value of the transparent conductive layer tends to be large. On the other hand, if the thickness exceeds 10 μm, the productivity of the transparent conductive layer decreases, the cost increases, and the optical properties also tend to deteriorate.

The total light transmittance of the transparent conductive layer is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more.

The density of the transparent conductive layer is preferably 1.0 to 10.5 g/cm ³ , and more preferably 1.3 to 3.0 g/cm ³ .

The surface resistance of the transparent conductive layer is preferably 0.1 to 1000 Ω/□, more preferably 0.5 to 500 Ω/□, and even more preferably 1 to 250 Ω/□.

The method for forming the transparent conductive layer is not particularly limited, and any conventionally known method can be used. Specific examples include vacuum deposition, sputtering, and ion plating. In addition, an appropriate method can be used depending on the required film thickness.

If necessary, an undercoat layer, an oligomer prevention layer, etc. may be further provided between the transparent conductive layer and the substrate 2.

[Conductive layer (antistatic layer)]
The laminate 10 may further have a layer having electrical conductivity (conductive layer, antistatic layer). The laminate 10 may have a very thin thickness, and is therefore susceptible to damage by weak static electricity generated during the manufacturing process, etc. When the laminate 10 has a conductive layer, the load caused by static electricity during the manufacturing process, etc. tends to be greatly reduced.

When a flexible image display device 100 including the laminate 10 is continuously wound, static electricity may be generated in the wound portion. When the laminate 10 has a conductive layer, static electricity generated by winding tends to be quickly removed.

The conductive layer may be an undercoat layer having a conductive function, an adhesive containing an ionic compound that is a conductive component or an antistatic agent, or a surface treatment layer containing a conductive component. The conductive layer may be formed, for example, from an antistatic agent composition containing a conductive polymer such as polythiophene and a binder. The laminate 10 preferably has one or more conductive layers, and may include two or more layers.

(Modifications of flexible image display device and laminate)
The laminate 10 of the flexible image display device 100 may include a plurality of adhesive sheets 1 and a plurality of substrates 2, and may further include an optical film. The laminate 11 of the flexible image display device 110 shown in FIG. 2 includes a first adhesive sheet 1a and a second adhesive sheet 1b as the adhesive sheet 1. The laminate 11 includes a first substrate 2a and a second substrate 2b as the substrate 2. The laminate 11 further includes an optical film 20. Except for the above, the structure of the laminate 11 is the same as the structure of the laminate 10 of the flexible image display device 100. Therefore, the same reference symbols are given to elements common to the laminate 10 of the flexible image display device 100 and the laminate 11 of this embodiment, and their description may be omitted. In this specification, the laminate 11 may be referred to as an optical film with an adhesive layer.

As described below, the first substrate 2a is a member included in the optical film 20. The second substrate 2b is, for example, located closer to the viewing side than the optical film 20 and is located on the outermost side of the laminate 11. The first substrate 2a and the second substrate 2b may be the same as each other or different from each other.

The first adhesive sheet 1a is located between the optical film 20 and the image display panel 3, and bonds these components together. The second adhesive sheet 1b is located between the second substrate 2b and the optical film 20, and bonds these components together. The first adhesive sheet 1a and the second adhesive sheet 1b may be the same as or different from each other.

[Optical film]
The optical film 20 has, for example, a first substrate 2a, a polarizing film 4, and a retardation film 5, and the polarizing film 4 is located between the first substrate 2a and the retardation film 5. The first substrate 2a is located, for example, closer to the viewing side than the polarizing film 4 and functions as a protective film for the polarizing film 4. The polarizing film 4 and the retardation film 5 generate, for example, circularly polarized light for preventing light incident inside the polarizing film 4 from the viewing side from being internally reflected and emitted to the viewing side, thereby compensating for the viewing angle.

The thickness of the optical film 20 is preferably 92 μm or less, more preferably 60 μm or less, and even more preferably 10 to 50 μm. Within the above range, the optical film 20 is unlikely to interfere with the winding of the flexible image display device 110.

As long as the properties required for the polarizing film 4 are maintained, the polarizing film 4 and the first substrate 2a may be bonded together by an adhesive layer (not shown). Examples of adhesives constituting the adhesive layer include isocyanate-based adhesives, polyvinyl alcohol-based adhesives, gelatin-based adhesives, vinyl latex, and water-based polyester. The adhesive is usually used as an aqueous solution, and has a solids concentration of, for example, 0.5 to 60% by weight. Examples of adhesives constituting the adhesive layer include ultraviolet-curing adhesives and electron beam-curing adhesives. Electron beam-curing adhesives exhibit suitable adhesion to the first substrate 2a. The adhesive may contain a metal compound filler.

[Polarizing film]
The polarizing film 4 may be made of, for example, a polyvinyl alcohol (PVA) resin that has been stretched by a stretching process such as air stretching (dry stretching) or stretching in boric acid water and has been iodine-oriented.

A representative method for producing the polarizing film 4 is described in JP 2004-341515 A, which includes a step of dyeing a monolayer of PVA-based resin and a step of stretching it (monolayer stretching method). Examples of methods for producing the polarizing film 4 include the methods described in JP 51-069644 A, JP 2000-338329 A, JP 2001-343521 A, WO 2010/100917 A, JP 2012-073563 A, and JP 2011-2816 A, which include a step of stretching a laminate of a PVA-based resin layer and a resin substrate for stretching and a step of dyeing it. With this method, since the PVA-based resin layer is supported by the resin substrate for stretching, defects such as breakage due to stretching can be suppressed even if the PVA-based resin layer is thin.

Examples of manufacturing methods that include a step of stretching the laminate and a step of dyeing include the air-stretching (dry stretching) methods described in the above-mentioned JP-A-51-069644, JP-A-2000-338329, and JP-A-2001-343521. In terms of being able to stretch at a high ratio and being able to easily improve the polarization performance, the manufacturing methods described in WO 2010/100917 and JP-A-2012-073563 that include a step of stretching in an aqueous boric acid solution are preferred, and in particular the manufacturing method (two-stage stretching method) described in JP-A-2012-073563 in which a step of auxiliary air-stretching is performed before stretching in an aqueous boric acid solution is preferred. A method (excessive dyeing and decoloring method) described in JP 2011-2816 A is also preferred, in which the laminate of the PVA-based resin layer and the resin substrate for stretching is stretched, the PVA-based resin layer is excessively dyed, and then decolorized. Examples of the polarizing film 4 include a polarizing film made of the above-mentioned polyvinyl alcohol-based resin oriented with iodine and stretched in a two-stage stretching process consisting of auxiliary air stretching and stretching in boric acid water. Examples of the polarizing film 4 include a polarizing film made of the above-mentioned polyvinyl alcohol-based resin oriented with iodine and produced by excessively dyeing a laminate of the stretched PVA-based resin layer and the resin substrate for stretching, and then decolorizing it.

The thickness of the polarizing film 4 is, for example, 20 μm or less, preferably 12 μm or less, more preferably 9 μm or less, even more preferably 1 to 8 μm, and particularly preferably 3 to 6 μm. If it is within the above range, the winding of the laminate 11 is hardly hindered.

[Retardation film]
A film obtained by stretching a polymer film or a film obtained by aligning and fixing a liquid crystal material can be used as the retardation film 5. In this specification, the retardation film 5 has birefringence in the in-plane and/or thickness direction.

Examples of the retardation film 5 include anti-reflection retardation films (see JP 2012-133303 A [0221], [0222], [0228]), viewing angle compensation retardation films (see JP 2012-133303 A [0225], [0226]), and tilted alignment retardation films for viewing angle compensation (see JP 2012-133303 A [0227]).

As the retardation film 5, any known retardation film can be used as long as it has substantially the above-mentioned functions, and there are no particular limitations on, for example, the retardation value, arrangement angle, three-dimensional birefringence, single layer or multilayer, etc.

The thickness of the retardation film 5 is preferably 20 μm or less, more preferably 10 μm or less, even more preferably 1 to 9 μm, and particularly preferably 3 to 8 μm. If it is within the above range, the winding of the laminate 11 is hardly hindered.

The retardation film 5 is, for example, a retardation film composed of two layers, a quarter-wave plate and a half-wave plate, in which liquid crystal material is oriented and fixed.

(Evaluation of Adhesive Sheets in Flexible Image Display Devices)
As described above, the adhesive sheet 1 included in the flexible image display device 100 (or 110) can sufficiently relieve the distortion applied to the layers constituting the device even when the device is wound up. The degree of the relaxation of the distortion of each layer by the adhesive sheet 1 can be evaluated by the amount of displacement of each layer by the adhesive sheet when the flexible image display device 100 (or 110) is wound around a roller. In the following, as an example, a method for measuring the amount of displacement of each layer by the adhesive sheet 1 (1a and 1b) in the flexible image display device 110 will be described. First, the laminate 11 included in the flexible image display device 110 is attached to the support film 55 via the first adhesive sheet 1a, and cut into a strip of 320 mm x 50 mm to obtain a test piece 15. FIG. 3A shows a cross-sectional view of the test piece 15. For the support film 55, for example, a resin film such as a polyethylene terephthalate (PET) film can be used.

Next, the test piece 15 is wound around the roller 45 with the support film 55 in contact with the roller 45. FIG. 3B is a side view of the test piece 15 completely wound up by the roller 45. Next, the vicinity of the end (area C) of the test piece 15 completely wound up by the roller 45 is observed by a microscope or the like. FIG. 3C is an enlarged view of the area C in FIG. 3B. Next, the distance between the end face of the support film 55 and the end face of the optical film 20 in the winding direction X of the test piece 15 is specified as the amount of deviation L1 due to the first adhesive sheet 1a. The distance between the end face of the optical film 20 and the end face of the second substrate 2b in the winding direction X is specified as the amount of deviation L2 due to the second adhesive sheet 1b. The sum of the amount of deviation L1 and the amount of deviation L2 is specified as the amount of deviation L of the member due to the adhesive sheet 1 in the laminate 11. The larger the amount of deviation L, the more the adhesive sheet 1 tends to alleviate the distortion of each layer. It is also possible to use a similar method to measure the amount of misalignment of each layer caused by the adhesive sheet 1 in the flexible image display device 100.

When the radius r of the circle defined by the side surface of the roller 45 in contact with the test piece 15 is 5 mm, the amount of deviation L is, for example, 0.7 mm or more, preferably 1.0 mm or more, and more preferably 1.5 mm or more. The upper limit of the amount of deviation L is not particularly limited, and is, for example, 3.0 mm or less. The radius r corresponds to the minimum bending radius of the test piece 15. In this specification, the minimum bending radius means the radius of curvature determined by the curved surface that gives the maximum curvature on the inner surface of the test piece 15 when the test piece 15 is curved.

When the radius r is 10 mm, the deviation L is, for example, 0.5 mm or more, preferably 0.8 mm or more, and more preferably 1.0 mm or more. The upper limit of the deviation L is not particularly limited, and is, for example, 2.0 mm or less. When the radius r is 15 mm, the deviation L is, for example, 0.3 mm or more, and preferably 0.5 mm or more. The upper limit of the deviation L is not particularly limited, and is, for example, 1.0 mm or less.

(Example of flexible image display device usage)
As described above, in the flexible image display device 100 (or 110), the display unit configured with the image display panel 3 can be rolled up. The flexible image display device 100 (or 110) having a rollable display unit can be designed to be wound around a roller 45 and pulled in while being bent, for example, as shown in FIG. 4. The flexible image display device 100 in FIG. 4 is a so-called rollable image display device. Note that when the flexible image display device 100 is pulled in further than in FIG. 4, the device 100 will be rolled up in a spiral shape.

In the flexible image display device 100 shown in FIG. 4, the minimum bending radius is, for example, 50 mm or less, and may be 30 mm or less, 20 mm or less, or 10 mm or less. The lower limit of the minimum bending radius is not particularly limited, and is, for example, 5 mm or more. In the flexible image display device 100 shown in FIG. 4, the minimum bending radius corresponds to the radius r of the circle defined by the side of the roller 45 that contacts the flexible image display device 100.

In the flexible image display device 100 shown in FIG. 4, the ratio of the area of the rollable display portion to the area of the entire display portion formed by the image display panel 3 is, for example, 50% or more and 90% or less, and preferably more than 50% and 90% or less.

The flexible image display device 100 (or 110) of the present invention can be suitably used as an image display device such as a flexible liquid crystal display device, an organic EL (electroluminescence) display device, or electronic paper. The flexible image display device 100 (or 110) can be used regardless of the type of touch panel, such as a resistive film type or a capacitive type.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to the examples shown below.

[(Meth)acrylic polymer A1]
A monomer mixture containing 99 parts by weight of butyl acrylate (BA) and 1 part by weight of 4-hydroxybutyl acrylate (HBA) was charged into a four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube, and a cooler. Furthermore, 0.1 parts by weight of 2,2'-azobisisobutyronitrile (AIBN) was charged as a polymerization initiator together with ethyl acetate for 100 parts by weight of the monomer mixture, and nitrogen gas was introduced while gently stirring to replace the atmosphere with nitrogen. The liquid temperature in the flask was kept at about 55°C and a polymerization reaction was carried out for 7 hours. Thereafter, ethyl acetate was added to the resulting reaction liquid to prepare a solution of (meth)acrylic polymer A1 having a weight average molecular weight of 1.6 million, with a solid content concentration of 30%.

[(Meth)acrylic polymers A2 and A3]
Except for changing the monomers used as shown in Table 1, a solution of (meth)acrylic polymer A2 and a solution of (meth)acrylic polymer A3 were prepared in the same manner as for (meth)acrylic polymer A1.

[Acrylic Oligomer (Oligomer B1)]
In a four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube, and a cooler, 95 parts by weight of butyl acrylate (BA), 2 parts by weight of acrylic acid (AA), 3 parts by weight of methyl acrylate (MA), 0.1 parts by weight of 2,2'-azobisisobutyronitrile as a polymerization initiator, and 140 parts by weight of toluene were charged, and nitrogen gas was introduced while gently stirring to sufficiently replace the atmosphere with nitrogen. The liquid temperature in the flask was kept at about 70°C, and a polymerization reaction was carried out for 8 hours to prepare an acrylic oligomer (oligomer B1) solution. The weight average molecular weight of the oligomer B1 was 4,500.

The abbreviations in Table 1 are as follows.
2EHA: 2-ethylhexyl acrylate BA: n-butyl acrylate LA: lauryl acrylate MA: methyl acrylate NVP: N-vinylpyrrolidone AA: acrylic acid HBA: 4-hydroxybutyl acrylate AIBN: azo polymerization initiator, 2,2'-azobisisobutyronitrile (Kishida Chemical Co., Ltd.)

[Preparation of adhesive sheet]
(Example 5, Reference Examples 1 to 6, and Comparative Examples 1 and 2)
A solvent-based pressure-sensitive adhesive composition was obtained by mixing a (meth)acrylic polymer, a (meth)acrylic oligomer, a crosslinking agent, and additives so as to obtain the composition shown in Table 2 below. Next, the obtained pressure-sensitive adhesive composition was applied to the surface of a PET film, which was a base film (separator), and then dried for 2 minutes in an air-circulating thermostatic oven set at 155° C. to form pressure-sensitive adhesive sheets of Example 5, Reference Examples 1 to 6 , and Comparative Examples 1 and 2. A fountain coater was used to apply the pressure-sensitive adhesive composition.

The abbreviations in Table 2 are as follows.
D110N: Trimethylolpropane/xylylene diisocyanate adduct (manufactured by Mitsui Chemicals, product name: Takenate D110N)
C/L: Trimethylolpropane/tolylene diisocyanate (manufactured by Nippon Polyurethane Industry Co., Ltd., product name: Coronate L)
Peroxide: Benzoyl peroxide (manufactured by NOF Corporation, product name: Nyper BMT)

[Preparation of Optical Film with Pressure-Sensitive Adhesive Layer]
Next, an optical film formed by laminating a quarter-wave plate (λ/4 plate), a half-wave plate (λ/2 plate), a polarizing film, and a substrate (protective film) in this order was bonded to the laminate (a) of the substrate film and the adhesive sheet obtained in each Example and Comparative Example with the adhesive sheet to obtain an optical film A with an adhesive layer. The optical film A with an adhesive layer has a multilayer structure of substrate film|adhesive sheet|λ/4 plate|λ/2 plate|polarizing film|protective film from the substrate film side. Each layer constituting the optical film and the optical film were prepared as follows.

(λ/4 Plate and λ/2 Plate)
The retardation film, which is a laminate of a λ/4 plate and a λ/2 plate, was prepared using a polymerizable liquid crystal material (Palicolor LC242, manufactured by BASF) that exhibits a nematic liquid crystal phase after the formation of the alignment film. Specifically, the polymerizable liquid crystal material and a photopolymerization initiator (Irgacure 907, manufactured by BASF) were dissolved in toluene, and then 0.1 to 0.5% by weight of a fluorine-based surfactant (Megafac, manufactured by DIC) was further added depending on the liquid crystal thickness in order to improve the coating property, to prepare a coating liquid L. The solid content concentration of the coating liquid L was set to 25% by weight.

Next, the retardation film manufacturing apparatus 200 shown in FIG. 5 was prepared. The manufacturing apparatus 200 includes a supply reel 221 for supplying a strip-shaped PET substrate 214, pressure rollers 224, 234, shaping rollers 230, 240, peeling rollers 226, 236, conveying roller 231, dies 222, 229, 232, 239, and ultraviolet irradiation devices 225, 227, 235, 237 for irradiating ultraviolet rays from a high-pressure mercury lamp. Next, a solution 210 of ultraviolet curable resin was applied to one side of the PET substrate 214 unwound from the supply reel 221 by the die 222. Next, the pressure roller 224 brought the coating film into contact with the shaping roller 230, and the PET substrate 214 was conveyed along the shaping roller 230 while both were in contact with each other, and the ultraviolet irradiation device 225 irradiated ultraviolet rays from the side of the PET substrate 214 to harden the coating film. On the conveying surface of the PET substrate 214 on the shaping roller 230, linear irregularities (extending in a direction at 75° to the MD direction of the PET substrate) are formed, which will form a λ/4 plate when an alignment film of the polymerizable liquid crystal material is further formed, and a cured film of ultraviolet curable resin having a shape corresponding to the irregularities on the exposed surface is formed by the curing. Next, the PET substrate 214 on which the cured film has been formed is peeled off from the shaping roller 230 by the peeling roller 226, and then the coating liquid L is applied to the exposed surface of the cured film by the die 229, and ultraviolet light is irradiated by the ultraviolet irradiation device 227 to align and cure the coating film. In this way, a λ/4 plate (thickness 3 μm) consisting of a cured film of ultraviolet curable resin and an alignment and cured film of the polymerizable liquid crystal material is formed on the PET substrate 214.

Next, the PET substrate 214 on which the λ/4 plate was formed was transported by the transport roller 231, and the solution 212 of the ultraviolet-curable resin was applied to the exposed surface of the λ/4 plate by the die 232 to form a coating film. Next, the pressure roller 234 brought the coating film into contact with the shaping roller 240, and the PET substrate 214 was transported along the shaping roller 240 while both were in contact, and the ultraviolet irradiation device 235 irradiated ultraviolet light from the side of the PET substrate 214 to harden the coating film. On the transport surface of the PET substrate 214 on the shaping roller 240, linear irregularities (extending in a direction at 15° to the MD direction of the PET substrate) that will form the λ/2 plate when an alignment film of the polymerizable liquid crystal material is further formed were formed, and the curing formed a cured film of the ultraviolet-curable resin having a shape corresponding to the irregularities on the exposed surface. Next, the PET substrate 214 on which the cured film was formed was peeled off from the shaping roller 240 by the peeling roller 236, and then the coating liquid L was applied to the exposed surface of the cured film by the die 239, and the coating film was aligned and cured by irradiating it with ultraviolet light by the ultraviolet irradiation device 237. In this way, a λ/2 plate (thickness 3 μm) consisting of a cured film of ultraviolet curable resin and an aligned and cured film of polymerizable liquid crystal material was further formed on the λ/4 plate of the PET substrate 214 to obtain a laminate (b).

(Laminate of polarizing film and protective film)
The laminate of the polarizing film and the protective film was prepared as follows.

As a thermoplastic resin substrate, an amorphous IPA copolymerized PET film (thickness 100 μm) having 7 mol% of isophthalic acid (IPA) units was prepared, and its surface was subjected to corona treatment (58 W/m ² /min). Separately, PVA (polymerization degree 4200, saponification degree 99.2%) to which 1 wt% of acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., Gohsefimer Z200, average polymerization degree 1200, saponification degree 98.5 mol%, acetoacetylation degree 5 mol%) was added was dissolved in water to obtain a PVA coating liquid with a concentration of 5.5 wt%. Next, the above PVA coating liquid was applied to the corona-treated surface of the IPA copolymerized PET film so that the film thickness after drying was 12 μm, and the coating film was dried by hot air drying at 60° C. for 10 minutes to obtain a laminate consisting of a substrate and a PVA layer on the substrate.

Next, the obtained laminate was free-end stretched (air-assisted stretching) at a stretch ratio of 1.8 times at 130°C in air to obtain a stretched laminate. Next, the stretched laminate was immersed in a boric acid insolubilizing aqueous solution at a liquid temperature of 30°C for 30 seconds to insolubilize the PVA layer. The content of boric acid in the boric acid insolubilizing aqueous solution was 3 parts by weight per 100 parts by weight of water. Next, the stretched laminate with the insolubilized PVA layer was dyed to obtain a colored laminate. Dyeing was performed by immersing the stretched laminate in a dyeing solution containing iodine and potassium iodide at a liquid temperature of 30°C. In the above dyeing, the PVA layer contained in the stretched laminate is dyed with iodine. The dyeing time was adjusted so that the single transmittance of the PVA layer constituting the finally obtained polarizing film was in the range of 40 to 44%. For the dyeing solution, an aqueous solution with an iodine concentration of 0.1 to 0.4% by weight and a potassium iodide concentration of 0.7 to 2.8% by weight was used. The ratio of the potassium iodide concentration to the iodine concentration in the dye solution was 7. Next, the colored laminate was immersed in a boric acid crosslinking aqueous solution at a liquid temperature of 30°C for 60 seconds to perform a crosslinking treatment to form a crosslinked structure between PVA molecules in the PVA layer that adsorbed iodine. The boric acid content and potassium iodide content in the boric acid crosslinking aqueous solution were both 3 parts by weight per 100 parts by weight of water.

Next, the colored laminate after the crosslinking treatment was stretched in an aqueous boric acid solution at a stretching temperature of 70°C and a stretching ratio of 3.05 times (stretching in boric acid water) to obtain a stretched laminate with a final stretching ratio of 5.50 times. The stretching direction of the stretching in boric acid water was the same as the stretching direction of the first air-assisted stretching. Next, the stretched laminate after stretching was taken out of the aqueous boric acid solution, and the boric acid attached to the surface of the PVA layer was washed with a potassium iodide solution (potassium iodide content: 4 parts by weight per 100 parts by weight of water). Next, the stretched laminate after washing was dried with hot air at 60°C to obtain a laminate of the substrate and the polarizing film (thickness: 5 μm) formed on the substrate.

Next, a stretched film (thickness 20 μm, moisture permeability 160 g/ ^m2 ) of a methacrylic resin having a glutarimide ring unit was prepared as a protective film. The prepared protective film was then bonded to the exposed surface of the polarizing film in the laminate prepared above to obtain a laminate (c) of the substrate and the polarizing film having the polarizing film and protective film. A known acrylic adhesive was used to bond the polarizing film and the protective film.

Next, the optical film A with a pressure-sensitive adhesive layer was prepared as follows using the laminate (a), laminate (b), and laminate (c) prepared above. First, the substrate was peeled off from the laminate (c) to expose the polarizing film. Next, the exposed polarizing film and the λ/2 plate of the laminate (b) were bonded with a known acrylic adhesive. Next, the PET substrate 214 was peeled off from the laminate (b) to expose the λ/4 plate. Next, the exposed λ/4 plate and the laminate (a) were bonded with the adhesive sheet of the laminate (a) to obtain an optical film A with a pressure-sensitive adhesive layer.

From the optical film A with adhesive layer prepared using the adhesive sheet of each Example and Comparative Example, an optical film B with adhesive layer having a multilayer structure of PET layer | adhesive sheet | λ/4 plate | λ/2 plate | polarizing film | protective film | adhesive sheet | polyimide (PI) layer was obtained as follows. First, the base film (separator) was peeled off from the optical film A with adhesive layer to expose the adhesive sheet. Next, a PET layer (corona treated) having a thickness of 125 μm was bonded to the exposed adhesive sheet. Next, a PI layer (thickness 50 μm, corona treated) was bonded to the protective film (corona treated), which is the exposed surface on the opposite side, using the same adhesive sheet as the adhesive sheet used between the PET layer and the λ/4 plate, to obtain an optical film B with adhesive layer.

[evaluation]
<Weight average molecular weight (Mw) of (meth)acrylic polymer and acrylic oligomer>
The weight average molecular weight (Mw) of the resulting (meth)acrylic polymer and acrylic oligomer was measured by GPC (gel permeation chromatography).
Analytical equipment: Tosoh Corporation, HLC-8120GPC
Column: Tosoh Corporation, G7000H _XL + GMH _XL + GMH _XL
Column size: 7.8 mm diameter x 30 cm, total 90 cm
Column temperature: 40°C
・Flow rate: 0.8ml/min
Injection volume: 100 μl
Eluent: Tetrahydrofuran Detector: Differential refractometer (RI)
・Standard sample: polystyrene

<Thickness>
The thickness of the adhesive sheet etc. was measured using a dial gauge (manufactured by Mitutoyo).

<Gel Fraction>
The gel fraction of the prepared adhesive sheet was evaluated by the above-mentioned method. The weight of the small piece obtained by scraping off a part of the adhesive sheet was about 0.2 g. The expanded porous membrane of polytetrafluoroethylene used was NTF1122 (average pore size 0.2 μm) manufactured by Nitto Denko Corporation.

<Modulus>
The 1000% modulus of the prepared pressure-sensitive adhesive sheet was evaluated by the method described above. The tensile tester used was an AG-IS made by Shimadzu Corporation. The pressure-sensitive adhesive sheet was rolled while being peeled off from the base film.

<Continuous winding test>
A continuous winding test was conducted using the pressure-sensitive adhesive layer-attached optical film B as an evaluation sample. The continuous winding test was conducted as follows using a test machine manufactured by Yuasa System Equipment. As shown in Figures 6(A) and (B), the test machine has a mechanism for repeatedly winding up the test piece 15B, which is a planar body, and the bending radius can be changed by adjusting the radius of the circle defined by the side of the roller 45 in contact with the test piece 15B. The continuous winding test was conducted with a load 54 of 100 gf applied vertically downward to the test piece 15B.

The test piece 15B was a strip cut from the pressure-sensitive adhesive layer-attached optical film B of 320 mm x 50 mm. The test piece 15B was fixed to the roller 45 with a double-sided adhesive tape so that it was wound in the long side direction and the PET layer was on the inside. The test was carried out under conditions of a bending radius of 10 mm and a winding speed of 5.7 m/min in an atmosphere of 25°C. The test piece 15B was wound so that the roller 45 rotated 3.3 times from the state of FIG. 6(A). The continuous winding test was evaluated based on the number of windings until peeling occurred in the layers constituting the pressure-sensitive adhesive layer-attached optical film B or until a noticeable winding habit remained in the pressure-sensitive adhesive layer-attached optical film B. Note that the test was terminated when the number of windings reached 100,000 times.

The criteria for judgment are as follows:
AA: The number of times the tape can be wound before peeling or significant curling occurs is 100,000 or more (no problem in practical use).
A: The number of times the tape is wound before peeling or significant curling occurs is between 50,000 and 100,000 (no practical problems).
B: The number of windings until peeling or significant curling occurs is less than 30,000 to 50,000 times (no problem in practical use).
C: The number of windings until peeling or significant curling occurs is less than 10,000 to 30,000 times (no problem in practical use)
D: The number of windings until peeling or significant curling occurs is less than 10,000 (problems in practical use)

As can be seen from Table 3, in the pressure-sensitive adhesive layer-attached optical film B (Example 5 and Reference Examples 1 to 6) having a pressure-sensitive adhesive sheet with a 1000% modulus of 0.01 to 0.6 N/mm2 and a gel fraction of 30% or more, the occurrence of significant curling and peeling of layers could be sufficiently suppressed in the continuous winding test. In contrast, in the pressure-sensitive adhesive layer-attached optical film B of Comparative Example 1 ^having a pressure-sensitive adhesive sheet with a 1000% modulus significantly exceeding 0.6 N/ ^mm2 , significant curling occurred at an early stage of the continuous winding test. In the pressure-sensitive adhesive layer-attached optical film B of Comparative Example 2 having a pressure-sensitive adhesive sheet with a gel fraction of less than 30%, peeling occurred in the layers constituting the pressure-sensitive adhesive layer-attached optical film B at an early stage of the continuous winding test.

The adhesive sheet of the present invention can be suitably used in flexible image display devices having a rollable display section.

Reference Signs List 1: Adhesive sheet 2: Substrate 3: Image display panel 4: Polarizing film 5: Retardation film 10, 11: Laminate for flexible image display device 20: Optical film 100, 110: Flexible image display device

Claims (5)

A pressure-sensitive adhesive sheet formed from a pressure-sensitive adhesive composition containing a (meth)acrylic polymer and a crosslinking agent,
The (meth)acrylic polymer contains a hydroxyl group-containing monomer as a monomer component,
the blending ratio of the hydroxyl group-containing monomer to all monomers constituting the (meth)acrylic polymer is 0.01 to 8% by weight,
The pressure-sensitive adhesive sheet is
The 1000% modulus is 0.01 to 0.6 N/ ^mm2 ,
A pressure-sensitive adhesive sheet for use in a laminate in a flexible image display device having a rollable display section, the gel fraction of which is 50% or more and less than 75%.
However, the pressure-sensitive adhesive sheet is
A pressure-sensitive adhesive for repeatedly bending devices for bonding one flexible member and another flexible member that constitute a device that is repeatedly bent, comprising:
a ratio of a shear stress 60 seconds after a 1000% displacement to a maximum shear stress when one surface and the other surface of a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive are displaced 1000% in opposite directions to the maximum shear stress is 72% or less;
The gel fraction is 40% or more and 90% or less.
The present invention excludes pressure-sensitive adhesives for repeated flexion devices, which are characterized by the above. 2. The pressure-sensitive adhesive sheet according to claim 1, wherein the 1000% modulus is 0.1 N/ ^mm2 or less. The pressure-sensitive adhesive sheet according to claim 1 or 2;
A substrate supporting the pressure-sensitive adhesive sheet;
A laminate for use in a flexible image display device having a rollable display portion, comprising: The laminate of claim 3, further comprising a polarizing film. A laminate according to claim 3 or 4;
An image display panel;
Equipped with
The laminate is located on a viewer side of the image display panel, and the laminate has a rollable display section.

Images