TWI895250B - Adhesive for repeated bending device, adhesive sheet, repeated bending folding layer component and repeated bending device - Google Patents

Abstract

The present invention provides an adhesive for a repetitive bending device having excellent repetitive bending properties, an adhesive sheet, a repetitive bending layer component, and a repetitive bending device. The adhesive for a repetitive bending device of the present invention is used to bond one repetitive bending component to another repetitive bending component constituting the repetitive bending device; wherein the storage modulus of elasticity (G'(-20)) is greater than 2.5, and the storage modulus of elasticity (G'(85)) is greater than 0.070 MPa and less than 0.200 MPa.

Description

Adhesive for repeated bending device, adhesive sheet, repeated bending folding layer component and repeated bending device

The present invention relates to an adhesive and an adhesive sheet for a repeated bending device, a repeated bending folding layer component, and a repeated bending device.

In recent years, there have been proposals for flexible displays (displays) for electronic devices. In addition to single-curved displays, there have also been proposals for recurved displays that can bend repeatedly.

In the aforementioned reversible display, the bendable component (flexible component) that constitutes one of the reversible displays can be bonded to other reversible components using the adhesive layer of an adhesive sheet. However, using conventional adhesive sheets for reversible displays can cause problems such as lifting and peeling at the interface between the adhesive layer and the adherend.

The adhesive disclosed in Patent Document 1 is designed to prevent the adhesive layer from lifting or peeling even after repeated bending. [Prior Art Document] [Patent Document]

[Patent Document 1] Japanese Patent Publication No. 2016-108555

[The problem that the invention aims to solve]

On the other hand, the aforementioned reversible bend display may have creases formed at the bend portion when it is repeatedly bent. The adhesive described in Patent Document 1 cannot fully solve the problem of reversible bendability.

The present invention was made in light of the above-described circumstances, and its purpose is to provide an adhesive for a repetitive bending device, an adhesive sheet, a repetitive bending layer component, and a repetitive bending device that exhibit excellent repetitive bending properties. [Means for Solving the Problem]

To achieve the above-mentioned purpose, first, the present invention provides an adhesive for a repetitive bending device, which is used to bond one of the repetitive bending components to another repetitive bending component; wherein the storage modulus variation of the quotient of the storage modulus G'(-20) at -20°C divided by the storage modulus G'(85) at 85°C exceeds 2.5; and the storage modulus G'(-20) at -20°C is greater than 0.070 MPa and less than 0.200 MPa (Invention 1).

The adhesive for the repetitive bending device of the above-mentioned invention (Invention 1) has the physical properties related to the above-mentioned storage elastic modulus G'. When a laminate formed by bonding a bendable component to another bendable component using an adhesive layer composed of the adhesive is bent, the laminate has excellent repetitive bendability and is less likely to have creases at the bend portion over a wide temperature range.

In the above invention (Invention 1), the preferred gel fraction is 40% or more and 95% or less (Invention 2).

In the above inventions (Inventions 1 and 2), it is preferred that the adhesive is an acrylic adhesive (Invention 3).

Second, the present invention provides an adhesive sheet (Invention 4) having an adhesive layer for bonding one bendable component constituting a repetitive bending device to another bendable component; wherein the adhesive layer is composed of the adhesive for the repetitive bending device (Inventions 1-3).

In the above invention (Invention 4), it is preferred that the adhesive sheet has an adhesion to polyimide of 6N/25mm or more (Invention 5).

In the above inventions (Inventions 4 and 5), the thickness of the adhesive layer is preferably not less than 1 μm and not more than 300 μm (Invention 6).

In the above inventions (Inventions 4 to 6), it is preferred that the adhesive sheet has two release sheets; and the adhesive layer is held by the release sheets in a state of being adjacent to the release surfaces of the two release sheets (Invention 7).

Third, the repeatedly-bending accumulative layer component provided by the present invention (Invention 8) comprises: a bendable component and another bendable component constituting a repeatedly-bending device, and an adhesive layer for bonding the one bendable component and the other bendable component to each other; wherein the adhesive layer is composed of the adhesive for the repeatedly-bending device (Inventions 1-3).

Fourthly, the present invention provides a repeated bending device (Invention 9) comprising the aforementioned repeated bending folding layer component (Invention 8). [Effects of the Invention]

The adhesive, adhesive sheet, repeatedly bending layer component and repeatedly bending device of the present invention have excellent repeatedly bending properties and are unlikely to have creases at the bending portion under a wide temperature range.

The following describes an embodiment of the present invention. [Adhesive for Repeated Bending Devices] The adhesive for a repeated bending device (hereinafter referred to as "adhesive") of this embodiment is used to bond one bending member to another bending member that constitutes the repeated bending device. The repeated bending device and bending members will be described later.

The adhesive of this embodiment preferably has a storage modulus variation (G'(-20)/G'(85)) of more than 2.5, which is the quotient of the storage modulus G'(-20) at -20°C divided by the storage modulus G'(85) at 85°C. Furthermore, the storage modulus G'(-20) at -20°C is preferably not less than 0.070 MPa and not more than 0.200 MPa. The storage modulus measurement method of this specification is described in the test examples below.

The adhesive of this embodiment, by possessing the aforementioned properties related to the storage modulus G', exhibits excellent repeated bendability when a laminate formed by laminating one bendable component to another using an adhesive layer formed from this adhesive is bent, with little crease formation at the bent portion over a wide temperature range. In particular, when the storage modulus variability exceeds 2.5, excellent adhesion is achieved, suppressing lifting and peeling at the interface between the adhesive layer and the adherend. Furthermore, when the storage modulus G'(-20) is 0.070 MPa or greater, it is estimated that microlifting is less likely to occur at the interface between the adhesive layer and the adherend. Furthermore, especially if the storage elastic modulus G'(-20) is below 0.200 MPa, the adhesive layer's hardness has little effect on the adherend, making it less likely that the adherend itself will crease. By comprehensively utilizing this effect, it is determined that the bends will not crease over a wide temperature range. Specifically, when the laminate is bent 100,000 times at temperatures of 0°C, 25°C, and 80°C, creases will not form at the bends. In particular, when a bendable component of the laminate is a polyimide film or a laminate containing a polyimide film, the above-mentioned excellent repeated bendability can still be exerted.

From the perspective of repeated flexing resistance, the storage modulus variation is preferably 3.0 or greater, more preferably 3.5 or greater, and particularly preferably 4.0 or greater. Furthermore, the storage modulus variation is preferably 50 or less, more preferably 40 or less, particularly preferably 35 or less, and most preferably 27 or less. This allows for easy adjustment of the adhesive strength and storage modulus from low to high temperatures within optimal ranges.

Furthermore, from the perspective of the aforementioned repeated flexing resistance, the -20°C storage modulus G'(-20) is preferably 0.075 MPa or higher, more preferably 0.080 MPa or higher, and particularly preferably 0.090 MPa or higher. Similarly, from the perspective of repeated flexing resistance, the -20°C storage modulus G'(-20) is preferably 0.190 MPa or lower, more preferably 0.180 MPa or lower, particularly preferably less than 0.110 MPa, and most preferably less than 0.100 MPa.

Furthermore, by simultaneously satisfying the aforementioned ranges for the -20°C storage modulus G'(-20) and the storage modulus variability, not only is a device provided with excellent flexural resistance across the entire temperature range from low to high temperatures, but also excellent handling properties such as adhesive processability at room temperature.

The storage modulus G'(85) at 85°C is preferably 0.050 MPa or less, more preferably 0.040 MPa or less, particularly preferably 0.030 MPa or less, optimally 0.025 MPa or less, and most optimally 0.015 MPa or less. Thus, the storage modulus variation easily falls within the aforementioned range, and the force acting from the adhesive layer on the adherend decreases with bending, thereby achieving a device with excellent bending resistance at high temperatures. On the other hand, the storage modulus G'(85) at 85°C is preferably 0.005 MPa or more, more preferably 0.008 MPa or more, particularly preferably 0.010 MPa or more, and optimally 0.012 MPa or more. If the 85°C storage modulus G'(85) is too low, the adhesive layer will be too soft at high temperatures, resulting in reduced cohesion, and a density difference will occur in the adhesive layer at the bent part and the non-bent part, so that the adhesive layer itself will have creases, whitening, etc. However, by setting the upper limit of the 85°C storage modulus G'(85) to the above value, such undesirable conditions are less likely to occur.

The 25°C storage modulus G'(25) of the adhesive of this embodiment is preferably 0.080 MPa or less, more preferably 0.070 MPa or less, particularly preferably 0.060 MPa or less, and optimally 0.050 MPa or less. This allows for easy development of adhesive properties. On the other hand, the 25°C storage modulus G'(25) is preferably 0.010 MPa or more, more preferably 0.015 MPa or more, particularly preferably 0.020 MPa or more, and optimally 0.025 MPa or more. This allows for excellent repeated bending properties within a standard ambient temperature range and also good processability such as tip cutting.

The gel fraction of the adhesive of this embodiment is preferably 40% or more, more preferably 43% or more, particularly preferably 46% or more, and optimally 50% or more. This allows the adhesive to exhibit superior cohesive strength capable of withstanding repeated bending. As a result, the aforementioned repeated bending resistance is even better. On the other hand, the gel fraction of the adhesive of this embodiment is preferably 95% or less, more preferably 90% or less, particularly preferably 85% or less, optimally 80% or less, and most optimally 77% or less. This is presumably to suppress the destruction of the cross-linked structure in the adhesive due to repeated bending, and to prevent the adhesive layer from becoming cloudy. The method for determining the gel fraction in this specification is described in the test examples below.

The type of adhesive used in this embodiment is not particularly limited, provided it meets the aforementioned physical properties. It may be, for example, any of acrylic, polyester, polyurethane, rubber, and silicone adhesives. Furthermore, the adhesive may be latex-based, solvent-based, or solvent-free, and may be crosslinked or non-crosslinked. Of these, acrylic adhesives are preferred, as they easily meet the aforementioned physical properties and exhibit excellent adhesive and optical properties. Solvent-based acrylic adhesives are even more preferred.

The adhesive of this embodiment is preferably a cross-linked adhesive composition (hereinafter referred to as "adhesive composition P") comprising a (meth)acrylate polymer (A) and a crosslinking agent (B). This adhesive easily satisfies the aforementioned physical properties and readily achieves excellent adhesion. In this specification, "(meth)acrylic acid" refers to both acrylic acid and methacrylic acid. The same applies to other similar terms. Furthermore, the term "polymer" also encompasses "copolymer."

(1) Components of Adhesive Composition P (1-1) (Meth)acrylate Polymer (A) (Meth)acrylate polymer (A) preferably comprises monomer units comprising an alkyl (meth)acrylate and a monomer having a reactive functional group in the molecule (a monomer containing a reactive functional group).

The (meth)acrylate polymer (A) exhibits improved adhesion when its monomer units contain an alkyl (meth)acrylate. The alkyl (meth)acrylate is preferably one having 1 to 20 carbon atoms. The alkyl group may be linear or branched, or may have a cyclic structure.

Examples of alkyl (meth)acrylates having an alkyl group with 1 to 20 carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, n-decyl (meth)acrylate, n-dodecyl (meth)acrylate, myristyl (meth)acrylate, benzyl (meth)acrylate, and stearyl (meth)acrylate. From the perspective of the aforementioned properties related to the storage elastic modulus G', (meth)acrylates having an alkyl group with 1 to 8 carbon atoms are preferred, and (meth)acrylates having an alkyl group with 4 to 8 carbon atoms are more preferred. Specifically, n-butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate are preferred, and n-butyl acrylate and 2-ethylhexyl acrylate are more preferred. In addition, these may be used alone or in combination of two or more.

The (meth)acrylate polymer (A) preferably contains 60% by mass or more, more preferably 80% by mass or more, particularly preferably 90% by mass or more, and most preferably 95% by mass or more of an alkyl (meth)acrylate having 1 to 20 carbon atoms in the alkyl group of the monomer units constituting the polymer. When the alkyl (meth)acrylate content is set above this amount, the (meth)acrylate polymer (A) can be imparted with improved adhesion and the storage modulus G' can be easily adjusted to a lower value. Furthermore, the content of 99.9% by mass or less, more preferably 99.5% by mass or less, and particularly preferably 99.0% by mass or less of an alkyl (meth)acrylate having 1 to 20 carbon atoms in the alkyl group is preferably set below this amount. By setting the alkyl (meth)acrylate content below this amount, it is possible to incorporate desired amounts of other monomer components into the (meth)acrylate polymer (A).

The (meth)acrylate polymer (A) contains a monomer with a reactive functional group in its monomer units. The reactive functional groups derived from the reactive functional group-containing monomer react with the crosslinking agent (B) described below, thereby forming a crosslinked structure (a three-dimensional mesh structure), resulting in an adhesive with the desired cohesive strength. This adhesive can easily meet the aforementioned storage modulus G'-related physical properties (and gel fraction).

In the (meth)acrylate polymer (A), the monomers comprising the polymer preferably contain a reactive functional group, for example, a monomer containing a hydroxyl group within the molecule (a hydroxyl-containing monomer), a monomer containing a carboxyl group within the molecule (a carboxyl-containing monomer), or a monomer containing an amino group within the molecule (an amino-containing monomer). These reactive functional group-containing monomers may be used alone or in combination of two or more.

Among the above-mentioned reactive functional group-containing monomers, hydroxyl-containing monomers or carboxyl-containing monomers are preferred, with hydroxyl-containing monomers being even more preferred. Hydroxyl-containing monomers can easily satisfy the aforementioned physical properties related to the storage modulus G' (and gel fraction). Specifically, while reactive functional group-containing monomers tend to increase the storage modulus G' (particularly the high-temperature storage modulus G') of the resulting adhesive, the use of hydroxyl-containing monomers can suppress this increase, making it easy to fine-tune the storage modulus G' of the resulting adhesive.

Examples of hydroxyl-containing monomers include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. Of these, hydroxyalkyl (meth)acrylates having a hydroxyalkyl group with 1 to 4 carbon atoms are preferred, given their ease in satisfying the aforementioned storage elastic modulus G' (and gel fraction). Specifically, 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are preferred, with 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate being more preferred. These may be used alone or in combination of two or more.

Examples of carboxyl group-containing monomers include ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, and citric acid. Of these, acrylic acid is preferred from the perspective of the adhesion of the resulting (meth)acrylate polymer (A). These monomers may be used alone or in combination of two or more.

In the (meth)acrylate polymer (A), the lower limit of the content of the reactive functional group-containing monomer in the monomer units constituting the polymer is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and particularly preferably 1.0% by mass or more. Furthermore, in the (meth)acrylate polymer (A), the upper limit of the content of the reactive functional group-containing monomer in the monomer units constituting the polymer is preferably 10% by mass or less, more preferably 7% by mass or less, and particularly preferably 5% by mass or less. If the (meth)acrylate polymer (A) contains monomer units containing reactive functional groups in the above-mentioned amount, then through the crosslinking reaction with the crosslinking agent (B), the cohesive force of the obtained adhesive can be made appropriate, and the physical properties related to the aforementioned storage modulus G' (and gel fraction) can be easily satisfied, especially the storage modulus G' (85) at 85°C.

The (meth)acrylate polymer (A) preferably does not contain carboxyl-containing monomers in its monomer units. Because carboxyl groups are acidic components, the absence of carboxyl-containing monomers can suppress acid-induced adverse effects (e.g., corrosion, resistance changes, etc.) even when the adhesive is applied to surfaces susceptible to acidic effects (e.g., transparent conductive films such as tin-doped indium oxide (ITO), metal films, and metal meshes).

The term "free of carboxyl group-containing monomers" herein means substantially free of carboxyl group-containing monomers. Besides being completely free of carboxyl group-containing monomers, the presence of carboxyl group-containing monomers is also permitted to the extent that the carboxyl groups do not cause corrosion of transparent conductive films, metal wiring, etc. Specifically, the (meth)acrylate polymer (A) preferably contains carboxyl group-containing monomers in a monomer unit at a concentration of 0.1% by mass or less, more preferably 0.01% by mass or less, and particularly preferably 0.001% by mass or less.

The (meth)acrylate polymer (A) may contain other monomers as needed. Other monomers are monomers that do not hinder the aforementioned effects of the monomer containing a reactive functional group, and preferably do not contain a reactive functional group. Examples of such monomers include N-acryloyl Non-reactive nitrogen-containing monomers such as phenanthene and N-vinyl-2-pyrrolidone; alkoxyalkyl (meth)acrylates such as methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate; vinyl acetate, styrene, etc. These may be used alone or in combination of two or more.

The polymerization form of the (meth)acrylate polymer (A) may be a random copolymer or a block copolymer.

The weight-average molecular weight of the (meth)acrylate polymer (A) is preferably 250,000 or greater, more preferably 300,000 or greater, particularly preferably 400,000 or greater, optimally 500,000 or greater, and most optimally 900,000 or greater. Furthermore, the weight-average molecular weight of the (meth)acrylate polymer (A) is preferably 2.3 million or less, more preferably 2 million or less, particularly preferably 1.7 million or less, and optimally 1.5 million or less. When the weight-average molecular weight of the (meth)acrylate polymer (A) is within the above range, it is easy to satisfy the aforementioned properties related to the storage modulus G' and the gel fraction, particularly the aforementioned storage modulus variability. The "weight-average molecular weight" in this specification is a standard polystyrene-equivalent value measured by gel permeation chromatography (GPC).

In the adhesive composition P, the (meth)acrylate polymer (A) may be used alone or in combination of two or more.

(1-2) Crosslinking Agent (B) The crosslinking agent (B) is triggered by heating the adhesive composition P containing it, for example, to crosslink the (meth)acrylate polymer (A) to form a three-dimensional network structure. This enhances the cohesive strength of the resulting adhesive, making it easier to meet the aforementioned storage modulus G'-related physical properties and gel fraction requirements, particularly the aforementioned storage modulus variability.

The crosslinking agent (B) can be any crosslinking agent as long as it reacts with the reactive groups of the (meth)acrylate polymer (A). Examples include isocyanate crosslinking agents, epoxy crosslinking agents, amine crosslinking agents, melamine crosslinking agents, aziridine crosslinking agents, hydrazine crosslinking agents, aldehyde crosslinking agents, oxazoline crosslinking agents, metal alkoxide crosslinking agents, metal chelate crosslinking agents, metal salt crosslinking agents, and ammonium salt crosslinking agents. Of these, isocyanate crosslinking agents are preferred due to their excellent reactivity with monomers containing reactive functional groups. The crosslinking agent (B) can be used alone or in combination of two or more.

Isocyanate crosslinking agents contain at least a polyisocyanate compound. Examples of polyisocyanate compounds include aromatic polyisocyanates such as toluene diisocyanate, diphenylmethane diisocyanate, and xylylene diisocyanate; aliphatic polyisocyanates such as hexamethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate and hydrogenated diphenylmethane diisocyanate; their biuret and trimer forms; and their reaction products, i.e., adducts, with low-molecular-weight active hydrogen-containing compounds such as ethylene glycol, propylene glycol, neopentyl glycol, trihydroxymethylpropane, and castor oil. Among them, from the viewpoint of reactivity with hydroxyl groups, trihydroxymethylpropane-modified aromatic polyisocyanates are preferred, and trihydroxymethylpropane-modified toluene diisocyanate or trihydroxymethylpropane-modified xylene diisocyanate are more preferred.

The content of the crosslinking agent (B) in the adhesive composition P is preferably 0.01 parts by mass or greater, more preferably 0.06 parts by mass or greater, and particularly preferably 0.12 parts by mass or greater, relative to 100 parts by mass of the (meth)acrylate polymer (A). Furthermore, the content is preferably 2.0 parts by mass or less, more preferably 1.5 parts by mass or less, particularly preferably 1.0 parts by mass or less, and most preferably 0.5 parts by mass or less. When the content of the crosslinking agent (B) is within the above range, the aforementioned properties related to the storage modulus G' and the gel fraction can be easily met, particularly the aforementioned storage modulus variability.

(1-3) Various Additives Adhesive composition P may optionally contain various additives commonly used in acrylic adhesives, such as silane coupling agents, UV absorbers, antistatic agents, tackifiers, antioxidants, light stabilizers, softeners, fillers, and refractive index modifiers. The polymerization solvent and diluent described below are not included in the additives that constitute adhesive composition P.

The adhesive composition P preferably contains the aforementioned silane coupling agent. Thus, the resulting adhesive layer can enhance the adhesion to the flexible component of the adherend, resulting in better adhesion.

The silane coupling agent is preferably an organic silicon compound having at least one alkoxysilyl group in its molecule from the viewpoint of good compatibility with the (meth)acrylate polymer (A) and light transparency.

Examples of the silane coupling agent include: vinyl trimethoxysilane, vinyl triethoxysilane, methacryloyloxypropyl trimethoxysilane and other silane compounds containing polymerizable unsaturated groups; 3-glycidoxypropyl trimethoxysilane, 3-glycidoxypropyl methyl dimethoxysilane, 2-(3,4-epoxycyclohexyl) ethyl trimethoxysilane and other silane compounds with epoxy structures; 3-butyl trimethoxysilane, 3-butyl triethoxysilane, 3-butyl dimethoxymethyl silane and the like. Silicon compounds containing alkyl groups; amino group-containing silicon compounds such as 3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane; 3-chloropropyltrimethoxysilane, and 3-isocyanatepropyltriethoxysilane; or condensates of at least one of these with alkyl group-containing silicon compounds such as methyltriethoxysilane, ethyltriethoxysilane, methyltrimethoxysilane, and ethyltrimethoxysilane. These compounds may be used alone or in combination of two or more.

The silane coupling agent content in the adhesive composition P is preferably 0.01 parts by mass or greater, more preferably 0.05 parts by mass or greater, and particularly preferably 0.1 parts by mass or greater, relative to 100 parts by mass of the (meth)acrylate polymer (A). Furthermore, the content is preferably 1 part by mass or less, more preferably 0.5 parts by mass or less, and particularly preferably 0.3 parts by mass or less. When the silane coupling agent content is within this range, the resulting adhesive layer improves adhesion to the flexible component of the adherend, resulting in greater adhesion.

(2) Preparation of Adhesive Composition P The adhesive composition P is prepared by preparing a (meth)acrylate polymer (A), mixing the obtained (meth)acrylate polymer (A) with a crosslinking agent (B), and adding additives as needed.

(Meth)acrylate polymer (A) is a mixture of monomers that constitute the polymer, and can be produced by conventional free radical polymerization. Polymerization of (meth)acrylate polymer (A) is preferably carried out by solution polymerization, using a polymerization initiator as needed. By polymerizing (meth)acrylate polymer (A) by solution polymerization, it is easy to increase the molecular weight of the resulting polymer, adjust its molecular weight distribution, and reduce the formation of low-molecular-weight products. Therefore, even with a low gel fraction and a moderate degree of crosslinking, the adhesive is less likely to become biased due to repeated flexing, making it easy to obtain an adhesive with excellent repeated flexing properties.

The polymerization solvent used in the solution polymerization method may be, for example, ethyl acetate, n-butyl acetate, isobutyl acetate, toluene, acetone, hexane, methyl ethyl ketone, etc., and two or more thereof may be used in combination.

Examples of polymerization initiators include azo compounds and organic peroxides, and two or more types may be used in combination. Examples of azo compounds include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile), dimethyl-2,2'-azobis(2-methylpropionate), 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2-hydroxymethylpropionitrile), and 2,2'-azobis[2-(2-imidazolin-2-yl)propane].

Examples of organic peroxides include benzoyl peroxide, t-butyl perbenzoate, isopropyl hydroperoxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, di(2-ethoxyethyl) peroxydicarbonate, t-butyl peroxyneodecanoate, t-butyl peroxytrimethylacetate, (3,5,5-trimethylhexanoyl) peroxide, dipropyl peroxide, and diacetyl peroxide.

In addition, during the polymerization step, the weight average molecular weight of the obtained polymer can be adjusted by adding a chain transfer agent such as 2-hydroxyethanol.

After obtaining the (meth)acrylate polymer (A), a crosslinking agent (B), and optionally, additives and a diluent are added to the (meth)acrylate polymer (A) solution. After thorough mixing, a solvent-diluted adhesive composition P (coating solution) is obtained.

In addition, when any of the above components is used in a solid state, or when it precipitates when mixed with other components in an undiluted state, the component can be dissolved (diluted) in a diluent before mixing with other components.

Examples of the diluent include aliphatic hydrocarbons such as hexane, heptane, and cyclohexane; aromatic hydrocarbons such as toluene and xylene; halogenated hydrocarbons such as dichloromethane and ethylene chloride; alcohols such as methanol, ethanol, propanol, butanol, and 1-methoxy-2-propanol; ketones such as acetone, methyl ethyl ketone, 2-pentanone, isophorone, and cyclohexanone; esters such as ethyl acetate and butyl acetate; and cellosol-based solvents such as ethylcellosol.

The concentration and viscosity of the prepared coating solution are not particularly limited, as long as they are within the coatable range, and can be appropriately selected based on the application conditions. For example, the adhesive composition P can be diluted to a concentration of 10-60% by mass. Furthermore, the addition of a diluent is not essential when preparing the coating solution. If the adhesive composition P exhibits a coatable viscosity, the addition of a diluent is not necessary. In this case, the adhesive composition P becomes a coating solution using the (meth)acrylate polymer (A) polymerization solvent as the diluent.

(3) Adhesive Preparation The adhesive of this embodiment is preferably formed by crosslinking the adhesive composition P. Crosslinking of the adhesive composition P can generally be performed by heat treatment. Furthermore, the heat treatment can also serve as a drying process when the diluent solvent is evaporated from the film of the adhesive composition P applied to the desired object.

The heating temperature of the heat treatment is preferably 50 to 150° C., more preferably 70 to 120° C. The heating time is preferably 10 seconds to 10 minutes, more preferably 50 seconds to 2 minutes.

After the heat treatment, a curing period of 1-2 weeks at room temperature (e.g., 23°C, 50% RH) can be designed as needed. If this curing period is required, the adhesive is formed after the curing period. If it is not required, the adhesive is formed after the heat treatment is completed.

By utilizing the aforementioned heat treatment (and curing), the (meth)acrylate polymer (A) is fully crosslinked by the crosslinking agent (B) to form a crosslinked structure, thereby obtaining an adhesive.

[Adhesive Sheet] The adhesive sheet of this embodiment includes an adhesive layer for bonding one bendable member and another bendable member constituting the reciprocating bend mechanism. This adhesive layer is composed of the aforementioned adhesive.

Figure 1 shows the specific structure of an adhesive sheet according to one embodiment. As shown in Figure 1, the adhesive sheet 1 according to one embodiment comprises two release sheets 12a and 12b, and an adhesive layer 11. The adhesive layer 11 is held by the release sheets 12a and 12b, adjacent to their respective release surfaces. As used herein, the term "release surface of a release sheet" refers to a releasable surface of the release sheet, including both surfaces that have been subjected to a release treatment and surfaces that remain releasable even without a release treatment.

(1) Constituent Elements (1-1) Adhesive Layer The adhesive layer 11 is composed of the adhesive of the aforementioned embodiment, preferably an adhesive formed by crosslinking the adhesive composition P.

In the adhesive sheet 1 of this embodiment, the lower limit of the thickness of the adhesive layer 11 (measured in accordance with JIS K7130) is preferably 1 μm or greater, more preferably 5 μm or greater, particularly preferably 10 μm or greater, and most preferably 15 μm or greater. This lower limit allows the required adhesive force to be readily exerted, effectively suppressing the formation of creases in the bent portion. Furthermore, the upper limit of the thickness of the adhesive layer 11 is preferably 300 μm or less, more preferably 150 μm or less, and particularly preferably 90 μm or less. To achieve a thinner repetitive bending device, it is most preferably 40 μm or less. If the upper limit of the thickness of the adhesive layer 11 is within the above range, the force acting from the adhesive layer on the adherend during bending is mitigated, thereby easily improving the repeated bending resistance. In addition, the adhesive layer 11 can be formed as a single layer or as a plurality of laminated layers.

In the adhesive sheet 1 of this embodiment, the total light transmittance of the adhesive layer 11 (measured in accordance with JIS K7361-1:1997) is preferably 80% or greater, more preferably 90% or greater, particularly preferably 95% or greater, and most preferably 99% or greater. This total light transmittance provides high transparency and is suitable for optical applications (such as for folding displays).

(1-2) Peel Sheet Peel sheets 12a and 12b protect the adhesive layer 11 until the adhesive sheet 1 is used. They are peeled off when the adhesive sheet 1 (adhesive layer 11) is used. The adhesive sheet 1 of this embodiment does not necessarily require either or both of the peel sheets 12a and 12b.

The release sheets 12a and 12b may be made of, for example, polyethylene film, polypropylene film, polybutylene film, polybutadiene film, polymethylpentene film, polyvinyl chloride film, vinyl chloride copolymer film, polyethylene terephthalate film, polyethylene naphthalate film, polybutylene terephthalate film, polyurethane film, ethylene-vinyl acetate film, ionomer resin film, ethylene-(meth)acrylic acid copolymer film, ethylene-(meth)acrylate copolymer film, polystyrene film, polycarbonate film, polyimide film, or fluororesin film. Furthermore, crosslinked films or laminated films of these materials may also be used.

The release surfaces of the release sheets 12a and 12b (particularly the surface adjacent to the adhesive layer 11) are preferably subjected to a release treatment. Examples of release agents used in the release treatment include alkyd, silicone, fluorine, unsaturated polyester, polyolefin, and wax-based release agents. Furthermore, it is preferable that one of the release sheets 12a and 12b be a heavy-peel type with a greater release force, while the other be a light-peel type with a lesser release force.

There is no particular limitation on the thickness of the peeling sheets 12a and 12b, and the thickness is usually about 20 to 150 μm.

(2) Adhesion The adhesive strength of the adhesive sheet 1 of this embodiment to polyimide preferably has a lower limit of 6 N/25 mm or more, more preferably 8 N/25 mm or more, particularly preferably 10 N/25 mm or more, and most preferably 12 N/25 mm or more. If the lower limit of the adhesive strength of the adhesive sheet 1 to polyimide is as described above, the occurrence of creases at the bent portion can be more effectively suppressed when a polyimide film or the like is used as an adherend. On the other hand, regarding the upper limit of the adhesive strength described above, there are cases where the adhesive is insufficiently cross-linked, and cases where the adhesive strength is relatively high. Accordingly, when the adhesive is insufficiently cross-linked, a density distribution is generated in the adhesive due to repeated bending, resulting in the occurrence of creases in the adhesive. From this perspective, the adhesive force is preferably 50 N/25 mm or less, more preferably 40 N/25 mm or less, particularly preferably 30 N/25 mm or less, and most preferably 20 N/25 mm or less. Furthermore, the term "adhesion" in this specification refers to the adhesive force measured using the 180-degree pull-peel method according to JIS Z0237:2009. The specific test method is described in the test examples below.

The adhesive strength of the adhesive sheet 1 of this embodiment to sodium calcium glass preferably has a lower limit of 1.0 N/25 mm or greater, more preferably 2.0 N/25 mm or greater, particularly preferably 3.0 N/25 mm or greater, and most preferably 5.0 N/25 mm or greater. If the lower limit of the adhesive strength of the adhesive sheet 1 to sodium calcium glass is as described above, the formation of creases at the bent portion can be more effectively suppressed when components made of various materials are used as adherends. On the other hand, the upper limit of the adhesive strength described above is not particularly limited, but is generally preferably 50 N/25 mm or less, more preferably 40 N/25 mm or less. From the perspective of reworkability, allowing the adhesive sheet to be reattached in the event of an adhesive bonding error, it is particularly preferably 30 N/25 mm or less, and most preferably 20 N/25 mm or less.

The adhesive strength of the adhesive sheet 1 of this embodiment to alkali-free glass preferably has a lower limit of 1.0 N/25 mm or greater, more preferably 2.0 N/25 mm or greater, particularly preferably 3.0 N/25 mm or greater, and most preferably 5.0 N/25 mm or greater. If the adhesive sheet 1 has the above lower limit, the formation of creases at the bent portion can be more effectively suppressed when components made of various materials are used as adherends. On the other hand, the upper limit of the adhesive strength is not particularly limited, but is generally preferably 50 N/25 mm or less, more preferably 40 N/25 mm or less. From the perspective of reworkability, allowing the adhesive sheet to be reattached in the event of an adhesive bonding error, it is particularly preferably 30 N/25 mm or less, and most preferably 15 N/25 mm or less.

(3) Production of Adhesive Sheet A production example of an adhesive sheet 1 will be described using the adhesive composition P described above. A coating liquid of the adhesive composition P is applied to the release surface of one release sheet 12a (or 12b), and a heat treatment is applied to thermally crosslink the adhesive composition P to form a coating layer. The release surface of another release sheet 12b (or 12a) is then superimposed on the coating layer. If a curing period is required, the coating layer is then converted into the adhesive layer 11. If a curing period is not required, the coating layer is directly converted into the adhesive layer 11. Thus, the adhesive sheet 1 is obtained. The relevant heat treatment and curing conditions are as described above.

Another example of manufacturing the adhesive sheet 1 is to apply a coating liquid of the adhesive composition P to the release surface of one release sheet 12a, and then heat-treat the release surface to thermally crosslink the adhesive composition P to form a coating layer, thereby obtaining release sheet 12a having a coating layer. Furthermore, the coating liquid of the adhesive composition P is applied to the release surface of another release sheet 12b, and then heat-treat the release surface to thermally crosslink the adhesive composition P to form a coating layer, thereby obtaining release sheet 12b having a coating layer. Next, the peeling sheet 12a with a coating layer and the peeling sheet 12b with a coating layer are bonded together so that the coating layers are in contact with each other. If a curing period is required, the deposited coating layer becomes the adhesive layer 11 after the curing period. If a curing period is not required, the deposited coating layer becomes the adhesive layer 11 directly. Thus, the adhesive sheet 1 is obtained. This manufacturing example allows for stable production even when the adhesive layer 11 is relatively thick.

The coating liquid of the adhesive composition P may be applied by, for example, rod coating, knife coating, roller coating, doctor blade coating, die coating, gravure coating, etc.

[Repeatedly Folded Layer Member] As shown in Figure 2, the repeatedly folded layer member 2 of this embodiment comprises a first foldable member 21 (one foldable member), a second foldable member 22 (the other foldable member), and an adhesive layer 11 positioned therebetween to bond the first foldable member 21 and the second foldable member 22 to each other.

The adhesive layer 11 of the repeatedly folded layer component 2 is the adhesive layer 11 of the adhesive sheet 1 .

The repetitively folding layer component 2 is the repetitively folding device itself, or a component that constitutes a portion of the repetitively folding device. The repetitively folding device is preferably a repetitively foldable (including curved) display, but is not limited thereto. Examples of such repetitively folding devices include organic electroluminescent (EL) displays, electrophoretic displays (electronic paper), liquid crystal displays using plastic substrates (films), foldable displays, and touch panels.

The first bendable component 21 and the second bendable component 22 are components that can be repeatedly bent (including bending), and can be, for example: a covering film, a barrier film, a hard coating film, a polarizing film (polarizing plate), a polarizing element, a phase difference film (phase difference plate), a viewing angle compensation film, a brightness enhancement film, a contrast enhancement film, a diffusion film, a semi-transmitting reflective film, an electrode film, a transparent conductive film, a metal screen film, a thin film sensor (touch sensing film), a liquid crystal polymer film, a light-emitting polymer film, a thin film liquid crystal module, an organic EL module (organic EL film, organic EL element), an electronic paper module (thin film electronic paper), a TFT (Thin Film Transistor) substrate, etc.

In the above, at least one of the first bendable member 21 or the second bendable member 22 is preferably a polyimide film, or a laminate of a polyimide film provided on the side of the adhesive layer 11. While polyimide film generally has poor adhesion to the adhesive layer, the adhesive layer 11 of this embodiment exhibits excellent flexing properties even when the polyimide film is the adherend, and creases are less likely to form at the bend over a wide temperature range.

The Young's modulus of the first and second bendable members 21 and 22 is preferably 0.1 to 10 GPa, more preferably 0.5 to 7 GPa, and particularly preferably 1 to 5 GPa, respectively. With the Young's modulus of the first and second bendable members 21 and 22 within this range, each bendable member can be easily and repeatedly bent.

The thickness of the first and second bending members 21 and 22 is preferably 10-3000 μm, more preferably 25-1000 μm, and particularly preferably 50-500 μm, respectively. When the thickness of the first and second bending members 21 and 22 falls within this range, each bending member can be easily and repeatedly bent.

When manufacturing the repeatedly folded layer component 2, for example, one of the peeling sheets 12a of the adhesive sheet 1 is first torn off, and then the adhesive layer 11 exposed on the adhesive sheet 1 is attached to the other side of the first foldable component 21.

Then, another peeling sheet 12b is torn off from the adhesive layer 11 of the adhesive sheet 1, and the exposed adhesive layer 11 of the adhesive sheet 1 is bonded to the second bendable member 22 to obtain the repeatedly bent layer member 2. In another example, the bonding order of the first bendable member 21 and the second bendable member 22 can be reversed.

[Repeated Bending Device] The repeated bending device of this embodiment includes the aforementioned repeatedly-bending accumulative layer member 2. It may consist solely of the repeatedly-bending accumulative layer member 2, or it may include one or more repeatedly-bending accumulative layer members 2 and another foldable member. When laminating one repeatedly-bending accumulative layer member 2 with another repeatedly-bending accumulative layer member 2, or when laminating a repeatedly-bending accumulative layer member 2 with another foldable member, it is preferred that the laminated layers be laminated with the adhesive layer 11 of the adhesive sheet 1 interposed therebetween.

The repeated bending device of this embodiment can suppress the occurrence of creases in the bent portion even when the device is repeatedly bent (for example, 100,000 times) at a wide range of temperatures (for example, 0°C, 25°C, 80°C, etc.) because the adhesive layer is composed of the aforementioned adhesive.

The repeated bending device of one embodiment of this invention is shown in FIG3. In addition, the repeated bending device of the present invention is not limited to this repeated bending device.

As shown in Figure 3, the repetitive bending device 3 of this embodiment is constructed by laminating, from the top, a cover film 31, a first adhesive layer 32, a polarizing film 33, a second adhesive layer 34, a touch-sensitive film 35, a third adhesive layer 36, an organic EL element 37, a fourth adhesive layer 38, and a TFT substrate 39. These cover film 31, polarizing film 33, touch-sensitive film 35, organic EL element 37, and TFT substrate 39 are considered bendable components.

At least one of the first adhesive layer 32, the second adhesive layer 34, the third adhesive layer 36, and the fourth adhesive layer 38 is the adhesive layer 11 of the adhesive sheet 1. Preferably, at least two of the first adhesive layer 32, the second adhesive layer 34, the third adhesive layer 36, and the fourth adhesive layer 38 are the adhesive layer 11 of the adhesive sheet 1. More preferably, all of the adhesive layers 32, 34, 36, and 38 are the adhesive layer 11 of the adhesive sheet 1.

Cover film 31 is preferably a polyimide film, or a laminate having a polyimide film disposed on the side of first adhesive layer 32. In this case, at least first adhesive layer 32 is preferably adhesive layer 11 of adhesive sheet 1. Furthermore, for example, when TFT substrate 39 includes a polyimide film, and particularly when a polyimide film is disposed on the side of fourth adhesive layer 38, at least fourth adhesive layer 38 is preferably adhesive layer 11 of adhesive sheet 1.

The above-mentioned repeated bending device 3 can suppress the occurrence of creases in the bent portion even when repeated bending is performed (for example, 100,000 times) at a wide range of temperatures (for example, 0°C, 25°C, 80°C, etc.), at least in the laminated portion of the adhesive sheet 1 consisting of the adhesive layer 11 and the bendable component bonded by the adhesive layer.

The embodiments described above are provided to facilitate understanding of the present invention and are not intended to limit the present invention. Therefore, the various elements disclosed in the embodiments described above also encompass all design changes and equivalents within the technical scope of the present invention.

For example, either or both of the release sheets 12a and 12b of the adhesive sheet 1 may be omitted. Alternatively, the release sheets 12a and/or 12b may be replaced with a flexible member required for lamination. [Example]

Hereinafter, the present invention will be described in more detail using embodiments, etc. However, the scope of the present invention is not limited to these embodiments, etc.

[Example 1] 1. Preparation of (Meth)acrylate Polymer (A) (Meth)acrylate Polymer (A) was prepared by copolymerizing 54 parts by mass of n-butyl acrylate, 45 parts by mass of 2-ethylhexyl acrylate, and 1 part by mass of 4-hydroxybutyl acrylate via a solution polymerization method. The molecular weight of (meth)acrylate Polymer (A) was measured according to the method described below, and the weight-average molecular weight (Mw) was 800,000.

2. Preparation of the Adhesive Composition Mix 100 parts by weight (solids basis; the same applies hereinafter) of the (meth)acrylate polymer (A) obtained in Step 1 above, 0.25 parts by weight of trihydroxymethylpropane-modified xylene diisocyanate (XDI; Soken Chemical Co., Ltd., product name "TD-75") as a crosslinker (B), and 0.20 parts by weight of 3-glycidoxypropyltrimethoxysilane (Si1) as a silane coupling agent. Stir thoroughly and dilute with methyl ethyl ketone to obtain a coating solution of the adhesive composition.

3. Adhesive Sheet Preparation The resulting adhesive composition coating solution was applied using a knife coater onto the release-treated surface of a heavy-duty release sheet (Lintec, product name "SP-PET752150"), which had been treated on one side with a silicone release agent. The coating layer was then heated at 90°C for one minute to form a coating.

Next, the coating layer on the heavy-peel release sheet obtained above and the light-peel release sheet (Lintec, product name "SP-PET381130") which was prepared by peeling a polyethylene terephthalate film on one side with a silicone release agent were placed on the light-peel release sheet. The release sheet was laminated with the release-treated surface in contact with the coating layer and cured for 7 days at 23°C and 50% RH to produce an adhesive sheet with a 25μm thick adhesive layer. This adhesive sheet consisted of a heavy-peel release sheet, an adhesive layer (25μm thick), and a light-peel release sheet. The adhesive layer thickness was measured in accordance with JIS K7130 using a constant-pressure thickness gauge (TECLOCK, product name "PG-02").

Here, when the (meth)acrylate polymer (A) is taken as 100 parts by mass (solid content conversion value), the various blending amounts (solid content conversion value) of the adhesive composition are shown in Table 1. In addition, the details of the codes etc. described in Table 1 are as follows. [(meth)acrylate polymer (A)] BA: n-butyl acrylate 2EHA: 2-ethylhexyl acrylate 4HBA: 4-hydroxybutyl acrylate IBXA: isoborneol acrylate ACMO: N-acryloyl HEA: 2-Hydroxyethyl acrylate MMA: Methyl methacrylate [Crosslinking agent (B)] XDI: Trihydroxymethylpropane-modified xylene diisocyanate (Soken Chemical Co., Ltd., product name "TD-75") TDI: Trihydroxymethylpropane-modified toluene diisocyanate (Toyochem, product name "BHS8515") [Silane coupling agent] Si1: 3-Glycyloxypropyltrimethoxysilane Si2: 3-Glycyloxypropylmethyldimethoxysilane

[Examples 2-12, Comparative Examples 1-3] Adhesive sheets were prepared in the same manner as in Example 1, except that the types and ratios of the monomers constituting the (meth)acrylate polymer (A), the weight-average molecular weight (Mw) of the (meth)acrylate polymer (A), the type and amount of the crosslinking agent (B), the type of the silane coupling agent, and the thickness of the adhesive layer were changed as shown in Table 1.

[Test Example 1] (Gel Fraction Determination) The adhesive sheets produced in the Examples and Comparative Examples were cut into 80 mm x 80 mm sheets. The adhesive layer was wrapped with a polyester mesh (Tetoron Mesh #200). The mass was measured using a precision balance. The mass of the adhesive alone was deducted from the mass of the mesh to calculate the mass. This mass was designated M1.

Next, the adhesive wrapped in the polyester mesh was immersed in ethyl acetate at room temperature (23°C) for 24 hours. The adhesive was then removed and air-dried at 23°C and 50% relative humidity for 24 hours, followed by drying in an 80°C oven for 12 hours. After drying, the mass was measured using a precision scale. The mass of the mesh was subtracted to calculate the mass of the adhesive alone. This mass was designated M2. The gel fraction (%) was expressed as (M2/M1) × 100. The results are shown in Table 2.

[Test Example 2] (Storage Modulus (G') Measurement) The adhesive layers of the adhesive sheets produced in the Examples and Comparative Examples were laminated multiple times to form a 3mm thick laminate. A cylinder with an 8mm diameter (3mm height) was punched through the laminate to create a sample.

For the above samples, the storage modulus G' was measured using a viscoelasticity tester (Dynamic Analayzer, manufactured by REOMETRIC) using the torsional shear method under the following conditions in accordance with JIS K7244-6. The storage modulus G'(-20) at -20°C, the storage modulus G'(25) at 25°C, and the storage modulus G'(85) at 85°C (MPa) were obtained. The results are shown in Table 2. Measurement frequency: 1 Hz Measurement temperature: -20°C to 140°C

Furthermore, based on the obtained storage modulus G'(-20) and storage modulus G'(85), the storage modulus variation is calculated as the quotient of storage modulus G'(-20) divided by storage modulus G'(85). The results are shown in Table 2.

[Test Example 3] (Adhesion Measurement) The light-peel release sheet was removed from the adhesive sheets obtained in the Examples and Comparative Examples. The exposed adhesive layer was then laminated to the easily adhesive layer of a polyethylene terephthalate (PET) film (manufactured by Toyobo Co., Ltd., product name "PET A4300," thickness: 100 μm) with an easily adhesive layer. This resulted in a heavy-peel release sheet/adhesive layer/PET film laminate. The laminate was cut into pieces 25 mm wide and 110 mm long.

On the other hand, the following three types of adherends were prepared. (1) Sodium calcium glass plate (manufactured by Nippon Sheet Glass Co., Ltd., product name "Sodium Calcium Glass", thickness: 1.1 mm) (2) A laminate formed by laminating a polyimide film (manufactured by DuPont Toray, product name "KAPTON 100PI", polyimide film thickness: 25 μm, adhesive layer thickness: 5 μm) with an adhesive layer on one side of the soda calcium glass plate (manufactured by Nippon Sheet Glass Co., Ltd., product name "Sodium Calcium Glass", thickness: 1.1 mm) (the polyimide film side was the adherend surface) (3) Alkali-free glass plate (manufactured by NSG Precision Co., Ltd., product name "Corning Glass EAGLE XG", thickness: 0.7 mm)

In an environment of 23°C and 50% RH, a heavy-peel release sheet was removed from the laminate, and the exposed adhesive layer was then applied to each of the adherends. Pressurization was performed in a Kurihara Seisakusho hot press at 0.5 MPa and 50°C for 20 minutes. After 24 hours at 23°C and 50% RH, the adhesion force (N/25mm) of the laminated PET film and adhesive layer when peeled from the adherend was measured using a tensile tester (Orientec) at a peel speed of 300 mm/min and a peel angle of 180 degrees. All conditions other than those listed here were measured according to JIS Z0237:2009. The results are shown in Table 2.

[Test Example 4] (Evaluation of Repeated Bending Properties) In an environment of 23°C and 50% RH, the light-peel release sheet was removed from the adhesive sheets produced in the Examples and Comparative Examples. The exposed adhesive layer was then laminated to one side of a polyimide (PI) film (manufactured by DuPont Toray, product name "KAPTON 100PI," thickness: 25 μm, Young's modulus: 3.4 GPa). Next, the heavy-peel release sheet was removed, and the exposed adhesive layer was laminated to one side of a polyethylene terephthalate (PET) film (manufactured by Toyobo Co., Ltd., product name "COSMOSHINE A4300," thickness: 100 μm, Young's modulus: 4.5 GPa). The samples were then pressurized in a Kurihara Seisakusho hot press at 0.5 MPa and 50°C for 20 minutes and then placed at 23°C and 50% RH for 24 hours. The resulting laminate consisting of PI film/adhesive layer/PET film was cut into pieces 50 mm wide and 200 mm long to serve as samples.

The samples were subjected to repeated bending under the following conditions using a durability testing machine (manufactured by Yuasa System Instruments Co., Ltd., product name: "Planar Body Unloaded U-Shaped Elongation Testing Machine Model: CL09-typeD01-FSC90"). The state of the bent portion was then visually inspected, and the repeated bending resistance at each test temperature was evaluated according to the following criteria. The results are shown in Table 2.

<Test Conditions> Bending Direction: Bend toward the side of the PET film Minimum Bend Diameter: 3mm Bending Cycles: 100,000 Test Temperatures: 0°C, 25°C, and 80°C <Evaluation Criteria for Repeated Bending Resistance> ◎…No crease at the bend. ○…A crease at the bend is present, but it is barely noticeable when viewed from the front. △…A crease at the bend is noticeable even when viewed from the front, but there is no white turbidity. ×…A crease at the bend is present, and the turbidity is noticeable.

[Test Example 5] (Measurement of Total Light Transmittance) The adhesive layers of the adhesive sheets obtained in the Examples and Comparative Examples were attached to glass to serve as measurement samples. Background measurements were performed on the glass, and the total light transmittance (%) of the measurement samples was measured using a haze meter (NDH-5000, manufactured by Nippon Denshoku Industries) in accordance with JIS K7361-1:1997. The results are shown in Table 2.

[Table 1] (Meth)acrylate polymer (A) Crosslinking agent (B) Silane coupling agent Adhesive layer thickness (μm) Composition Mw Kind Mass Kind Mass Example 1 BA/2Em/4HBA=54/45/1 800,000 XDI 0.25 Si1 0.20 25 Example 2 0.15 0.20 25 Example 3 0.40 0.20 25 Example 4 400,000 0.35 0.20 25 Example 5 1.2 million 0.15 0.20 25 Example 6 0.15 0.20 10 Example 7 0.15 Si2 0.20 25 Example 8 TDI 0.15 Si1 0.20 25 Example 9 XDI 0.30 0.20 25 Example 10 0.30 0.20 50 Example 11 BA/2EHA/4HBA=52/45/3 800,000 0.25 0.20 25 Example 12 BA/2EHA/4HBA=50/45/5 750,000 0.25 0.20 25 Comparative example 1 2EHA/IBXA/ACMO/HEA=60/10/10/20 500,000 TDI 0.20 0.20 25 Comparative example 2 2EHA/MMA/HEA=60/20/20 800,000 TDI 0.25 0.20 25 Comparative example 3 BA/2EHA/4HBA=54/45/1 1.2 million XDI 2.00 0.20 25

[Table 2] Gel fraction (%) Storage elastic modulus G'(MPa) Store elastic modulus variation Adhesion force (N/25mm) Repeated inflection evaluation Total light transmittance (%) G'(-20) G'(25) G'(85) Sodium calcium glass polyimide Alkali-free glass 0℃ 25℃ 80℃ Example 1 70 0.096 0.031 0.009 10.9 8.8 7.0 6.9 ○ ◎ ◎ 99 Super Example 2 66 0.106 0.033 0.009 12.1 16.3 23.5 11.4 ○ ◎ ○ 99 Super Example 3 72 0.101 0.035 0.014 7.1 3.3 8.9 2.7 ○ ◎ ◎ 99 Super Example 4 53 0.108 0.025 0.006 19.6 6.7 12.0 6.0 ○ ◎ ○ 99 Super Example 5 73 0.096 0.047 0.023 4.1 6.5 13.5 6.2 ◎ ◎ ◎ 99 Super Example 6 73 0.096 0.047 0.023 4.1 5.0 7.9 4.7 ○ ◎ ◎ 99 Super Example 7 72 0.097 0.045 0.022 4.4 6.8 9.7 5.7 ◎ ◎ ◎ 99 Super Example 8 73 0.097 0.046 0.022 4.4 8.5 10.0 7.2 ◎ ◎ ◎ 99 Super Example 9 78 0.098 0.052 0.031 3.2 3.0 8.6 2.6 ○ ◎ ○ 99 Super Example 10 78 0.098 0.052 0.031 3.2 4.9 14.3 5.5 ◎ ◎ ○ 99 Super Example 11 74 0.110 0.034 0.012 9.1 5.9 9.6 3.7 ○ ◎ ◎ 99 Super Example 12 75 0.119 0.035 0.012 9.8 9.1 10.9 5.0 ○ ◎ ◎ 99 Super Comparative example 1 51 18.0 0.067 0.009 2000.0 36.5 35.9 34.6 × ◎ △ 99 Super Comparative example 2 72 80.60 0.130 0.028 2878.6 16.0 17.6 16.6 × ○ ◎ 99 Super Comparative example 3 93 0.102 0.065 0.058 1.76 2.2 2.2 1.2 × △ × 98

Table 2 shows that the adhesive layer of the adhesive sheet of the embodiment can suppress the formation of creases at the bent portion when two bendable components are bonded together and repeatedly bent at various temperatures ranging from low to high. [Industrial Applicability]

The present invention is suitable for bonding a bendable component (especially a polyimide film or a laminate containing a polyimide film) constituting a repetitive bending device to another bendable component.

1: Adhesive sheet 11: Adhesive layer 12a, 12b: Release sheet 2: Repeated bending layer component 21: First bending component 22: Second bending component 3: Repeated bending device 31: Cover film 32: First adhesive layer 33: Polarizing film 34: Second adhesive layer 35: Touch sensor film 36: Third adhesive layer 37: Organic EL element 38: Fourth adhesive layer 39: TFT substrate

Figure 1 is a cross-sectional view of an adhesive sheet according to an embodiment of the present invention; Figure 2 is a cross-sectional view of a repeatedly bent folding layer component according to an embodiment of the present invention; and Figure 3 is a cross-sectional view of a repeatedly bent folding device according to an embodiment of the present invention.

1: Adhesive sheet

11: Adhesive layer

12a, 12b: Peeling film

Claims (9)

An adhesive for a repetitive bending device is used to bond one of the repetitive bending components to another repetitive bending component. The adhesive has a storage modulus variation of the quotient of the storage modulus G'(-20) at -20°C and a measurement frequency of 1 Hz divided by the storage modulus G'(85) at 85°C and a measurement frequency of 1 Hz of 3.5 or more and 10.9 or less. The storage modulus G'(-20) at -20°C and a measurement frequency of 1 Hz of 0.070 MPa or more and 0.119 MPa or less. The gel fraction is 40% or more and 85% or less. An adhesive for a repetitive bending device is used to bond one of the repetitive bending components to another repetitive bending component. The adhesive has a storage modulus variation of more than 2.5, which is the quotient of the storage modulus G'(-20) at -20°C and a measured frequency of 1 Hz divided by the storage modulus G'(85) at 85°C and a measured frequency of 1 Hz. The storage modulus G'(-20) at -20°C and a measured frequency of 1 Hz is greater than 0.070 MPa and less than 0.119 MPa. The gel fraction is greater than 70% and less than 77%. For example, the adhesive for a repetitive bending device as claimed in claim 1 or 2, wherein the adhesive is an acrylic adhesive. An adhesive sheet having an adhesive layer for bonding one bendable component of a repetitive bending device to another bendable component; wherein the adhesive layer is composed of the adhesive for a repetitive bending device as described in claim 1 or 2. For example, the adhesive sheet of claim 4 has an adhesion strength to polyimide of not less than 6 N/25 mm and not more than 50 N/25 mm. For example, in the adhesive sheet of claim 4, the thickness of the adhesive layer is greater than 1 μm and less than 300 μm. The adhesive sheet of claim 4 includes two release sheets; The adhesive layer is held by the release sheets so as to be adjacent to the release surfaces of the two release sheets. A repeatedly bending layer component comprises: a first bending component and another bending component constituting a repeatedly bending device; and an adhesive layer for bonding the first bending component and the other bending component to each other, wherein the adhesive layer is composed of the adhesive for the repeatedly bending device as described in claim 1 or 2. A repeated bending device comprising a repeated bending folding layer component as described in claim 8.