Reinforcing film, device with reinforcing film, and method for producing the same

Abstract

To provide a reinforcing film which allows easy peeling immediately after lamination onto an adherend, enables strong bonding to the adherend via photo-curing of an adhesive after lamination onto the adherend, and is resistant to deformation such as wrinkling and peeling from the adherend even under repeated bending and stretching.SOLUTION: A reinforcing film (10) comprises an adhesive layer (2) fixedly laminated on one principal surface of a film base material (1). The adhesive layer is composed of a photocurable composition comprising an acrylic-based base polymer having a crosslinked structure, a photocuring agent, an acrylic-based oligomer having a polymerizable functional group at a terminal of the main chain, and a photopolymerization initiator.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a reinforcing film to be applied to a surface of a device, and further to a device including the reinforcing film and a method for manufacturing the same. [Background technology]

[0002] An adhesive film may be attached to the surface of an optical device such as a display or an electronic device for the purpose of surface protection, imparting impact resistance, etc. Such an adhesive film usually has an adhesive layer fixedly laminated on the main surface of a film substrate, and is attached to the device surface via this adhesive layer.

[0003] By temporarily attaching an adhesive film to the surface of a device or a device component before use, such as during device assembly, processing, transportation, etc., it is possible to prevent the adherend from being scratched or damaged. Patent Documents 1 and 2 disclose reinforcing films that include an adhesive layer made of a photocurable adhesive composition on a film substrate.

[0004] This reinforcing film has a high gel fraction of the adhesive and is low adhesive immediately after bonding to the adherend, so it can be easily peeled off from the adherend. Therefore, it can be reworked from the adherend, and it is also possible to selectively peel off the reinforcing film from areas of the adherend that do not require reinforcement. The adhesive of the reinforcing film is firmly bonded to the adherend by photocuring, so the film base is permanently bonded to the surface of the adherend, and it can be used as a reinforcing material for protecting the surface of devices, etc.

[0005] In recent years, organic electroluminescence (EL) panels using bendable substrates (flexible substrates) such as resin films have been put to practical use, and foldable devices (foldable devices) have also been put to practical use. In foldable devices, the same location is repeatedly bent. At the bent location, compressive stress is applied to the inside and tensile stress is applied to the outside, causing distortion at the bent location and its surroundings.

[0006] Patent Document 3 proposes using a photocurable adhesive containing an acrylic-based polymer with a low glass transition temperature as the adhesive for the reinforcing film of a foldable device, and by reducing the shear storage modulus at low temperatures, it is possible to alleviate distortion at the bending points and suppress peeling of the adhesive layer at the bending points of the foldable device. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2020-41113 A [Patent Document 2] JP 2020-132851 A [Patent Document 3] International Publication No. 2022 / 050009 Summary of the Invention [Problem to be solved by the invention]

[0008] In order to suppress peeling and cracking due to stress at the bending points when a foldable device is repeatedly folded (bent) and unfolded in a low-temperature environment, there is a demand for a further reduction in the storage modulus of the adhesive layer of the reinforcing film. In addition to lowering the glass transition temperature of the base polymer, a possible method for lowering the shear storage modulus at low temperatures is to add a component that acts as a plasticizer, such as an oligomer with a low glass transition temperature.

[0009] However, when oligomers or the like are added, the adhesive may have a low gel fraction and low cohesive strength after photocuring, resulting in insufficient adhesive strength. In addition, since the adhesive to which oligomers or the like are added has low strain recovery, when a foldable device is held in a folded state and then returned to an open state (extended state), the deformation of the adhesive layer does not return to its original state and becomes wrinkled, which causes peeling of the reinforcing film and deformation of the device.

[0010] The present invention aims to provide a reinforcing film that is easy to peel off immediately after being attached to an adherend, can be firmly adhered to the adherend by photocuring an adhesive after being attached to the adherend, and is resistant to deformation such as wrinkles or peeling even when a foldable device is repeatedly bent and extended. [Means for solving the problem]

[0011] The reinforcing film of the present invention includes a pressure-sensitive adhesive layer bonded to one main surface of a film substrate, the pressure-sensitive adhesive layer being made of a photocurable composition containing an acrylic base polymer, a photocuring agent, an acrylic oligomer, and a photopolymerization initiator.

[0012] The acrylic base polymer contains at least one monomer unit selected from the group consisting of a hydroxyl group-containing monomer and a carboxyl group-containing monomer, and has a crosslinked structure introduced therein. The weight average molecular weight of the acrylic base polymer before the introduction of the crosslinked structure is preferably 100,000 or more.

[0013] The acrylic oligomer has a polymerizable functional group at the end of the main chain and has a weight average molecular weight of 1000 to 30000. The glass transition temperature of the acrylic oligomer is preferably −30° C. or lower. The content of the acrylic oligomer in the photocurable composition constituting the pressure-sensitive adhesive layer is preferably 6 to 35 parts by weight based on 100 parts by weight of the acrylic base polymer.

[0014] The photocuring agent is preferably a polyfunctional (meth)acrylate having an alkylene oxide chain. In addition to the polyfunctional (meth)acrylate having an alkylene oxide chain, a urethane (meth)acrylate may also be used as the photocuring agent.

[0015] The pressure-sensitive adhesive layer preferably has a shear storage modulus at -20°C of 100 kPa or less after photocuring.

[0016] The adhesive layer preferably has an adhesive strength to the polyimide film of 1 N / 25 mm or less before photocuring, and preferably has an adhesive strength to the polyimide film of 5 N / 25 mm or more after photocuring.

[0017] The reinforcing film is attached to the surface of the device, and the pressure-sensitive adhesive layer is photocured to obtain a device with the reinforcing film. The device may be a foldable flexible device. Effect of the Invention

[0018] The reinforcing film of the present invention has a pressure-sensitive adhesive layer made of a photocurable composition, and the adhesive strength with the adherend is increased by photocuring the pressure-sensitive adhesive layer after adhesion to the adherend. Since the adhesive strength with the adherend is small before photocuring, the reinforcing film can be easily peeled off from the adherend.

[0019] The adhesive layer of the reinforcing film has a small shear storage modulus at low temperatures, excellent stress relaxation, and high strain recovery, so that even when the film is bent and stretched at the same location, it is unlikely to be deformed, such as wrinkled, or peeled off from the adherend. Therefore, the reinforcing film of the present invention can be suitably used for a foldable device using a resin film substrate. [Brief description of the drawings]

[0020] [Figure 1] FIG. 2 is a cross-sectional view showing a laminated structure of a reinforcing film. [Diagram 2] FIG. 2 is a cross-sectional view showing a laminated structure of a reinforcing film. [Diagram 3] FIG. 2 is a cross-sectional view showing a device to which a reinforcing film is attached. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] 1 is a cross-sectional view showing one embodiment of a reinforced film. The reinforced film 10 has a pressure-sensitive adhesive layer 2 on one main surface of a film substrate 1. The pressure-sensitive adhesive layer 2 is fixedly laminated on one main surface of the film substrate 1. The pressure-sensitive adhesive layer 2 is a photocurable pressure-sensitive adhesive made of a photocurable composition, and is cured by irradiation with active light such as ultraviolet light, thereby increasing the adhesive strength with an adherend.

[0022] Fig. 2 is a cross-sectional view of a reinforced film having a release liner 5 temporarily attached onto the main surface of a pressure-sensitive adhesive layer 2. Fig. 3 is a cross-sectional view showing a state in which a reinforced film 10 is attached to the surface of a device 20.

[0023] The release liner 5 is peeled off and removed from the surface of the adhesive layer 2, and the exposed surface of the adhesive layer 2 is attached to the surface of the device 20, thereby attaching the reinforcement film 10 to the surface of the device 20. In this state, the adhesive layer 2 has not yet been photocured, and the reinforcement film 10 (adhesive layer 2) is temporarily attached onto the device 20. By photocuring the adhesive layer 2, the adhesive strength at the interface between the device 20 and the adhesive layer 2 increases, and the device 20 and the reinforcement film 10 are fixed together.

[0024] "Fixed" means that the two laminated layers are firmly bonded together, making it difficult or impossible to peel them apart at their interface. "Temporary" means that the adhesive strength between the two laminated layers is weak, making them easy to peel apart at their interface.

[0025] In the reinforced film shown in Figure 2, film substrate 1 and adhesive layer 2 are bonded together, and release liner 5 is temporarily attached to adhesive layer 2. When film substrate 1 and release liner 5 are peeled away from each other, peeling occurs at the interface between adhesive layer 2 and release liner 5, and the adhesive layer 2 remains bonded to film substrate 1. No adhesive remains on release liner 5 after peeling.

[0026] In the device with the reinforced film shown in FIG. 3, the device 20 and the adhesive layer 2 are in a temporary bonded state before the adhesive layer 2 is photocured. When the film substrate 1 and the device 20 are peeled off, the peeling occurs at the interface between the adhesive layer 2 and the device 20, so the adhesive layer 2 remains in a bonded state on the film substrate 1. Since no adhesive remains on the device 20, peeling operations such as rework and cutting are easy. After the adhesive layer 2 is photocured, the adhesive strength between the adhesive layer 2 and the device 20 increases and they are in a bonded state, so it is difficult to peel the reinforced film 10 from the device 20.

[0027] [Film base material] A flexible plastic film is used as the film substrate 1 of the reinforcing film 10. In order to bond the film substrate 1 and the pressure-sensitive adhesive layer 2 together, it is preferable that the surface of the film substrate 1 to which the pressure-sensitive adhesive layer 2 is to be attached is not subjected to a release treatment.

[0028] The thickness of the film substrate is, for example, about 4 to 150 μm. From the viewpoint of reinforcing the device by imparting rigidity and cushioning impact, the thickness of the film substrate 1 is preferably 5 μm or more, more preferably 12 μm or more, even more preferably 20 μm or more, and particularly preferably 25 μm or more. From the viewpoint of imparting flexibility to the reinforcing film so that it can be folded, the thickness of the film substrate 1 is preferably 125 μm or less, more preferably 100 μm or less. From the viewpoint of achieving both mechanical strength and flexibility, the compressive strength of the film substrate 1 is 100 to 3000 kg / cm 2 is preferable, and 200 to 2900 kg / cm 2 More preferably, 300 to 2800 kg / cm 2 More preferably, 400 to 2700 kg / cm 2 is particularly preferred.

[0029] Examples of the plastic material constituting the film substrate 1 include polyester resins, polyolefin resins, cyclic polyolefin resins, polyamide resins, polyimide resins, polyether ether ketone, polyether sulfone, polyarylate resins, and aramid resins. In a reinforcing film for an optical device such as a display, the film substrate 1 is preferably a transparent film. In addition, when the photocuring of the adhesive layer 2 is performed by irradiating the film substrate 1 with active light rays from the film substrate 1 side, the film substrate 1 is preferably transparent to the active light rays used for curing the adhesive layer. Since they have both mechanical strength and transparency, polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, transparent polyimide, and transparent aramid are preferably used. When irradiating active light rays from the adherend side, the adherend only needs to have transparency to the active light rays, and the film substrate 1 does not need to be transparent to the active light rays.

[0030] The surface of the film substrate 1 may be provided with a functional coating such as an easy-adhesion layer, an easy-slip layer, a release layer, an antistatic layer, a hard coat layer, an antireflection layer, etc. As described above, in order to bond the film substrate 1 and the pressure-sensitive adhesive layer 2, it is preferable that no release layer is provided on the surface of the film substrate 1 to which the pressure-sensitive adhesive layer 2 is to be attached.

[0031] [Adhesive layer] The pressure-sensitive adhesive layer 2 that is fixedly laminated on the film substrate 1 is made of a photocurable composition. The photocurable composition that constitutes the pressure-sensitive adhesive layer 2 contains an acrylic base polymer, a photocuring agent, and a photopolymerization initiator, and further contains an acrylic oligomer.

[0032] The adhesive layer 2 has low adhesive strength with adherends such as devices and device parts before photocuring, and is therefore easy to peel off. The adhesive strength with adherends of the adhesive layer 2 is improved by photocuring, so that the reinforcing film is unlikely to peel off from the device surface even when the device is in use, and the adhesive layer 2 has excellent adhesive reliability.

[0033] The photocurable pressure-sensitive adhesive hardly cures in a general storage environment, but cures when exposed to active light such as ultraviolet light. Therefore, the reinforcing film of the present invention has the advantage that the timing of curing the pressure-sensitive adhesive layer 2 can be set arbitrarily, and the process lead time can be flexibly accommodated.

[0034] <Base polymer> The base polymer is the main component of the pressure-sensitive adhesive composition and is a major factor in determining various properties of the pressure-sensitive adhesive layer, such as adhesive strength and shear storage modulus. In the present invention, an acrylic polymer is used as the base polymer of the pressure-sensitive adhesive. Acrylic polymers are excellent in optical transparency and adhesiveness, and adhesive strength, shear storage modulus, and the like can be easily controlled.

[0035] The acrylic base polymer contains an alkyl (meth)acrylate ester as a monomer component. In this specification, "(meth)acrylic" means acrylic and / or methacrylic.

[0036] As the (meth)acrylic acid alkyl ester, a (meth)acrylic acid alkyl ester having an alkyl group with a carbon number of 1 to 20 is preferably used. From the viewpoint of suppressing peeling of the pressure-sensitive adhesive layer when repeatedly bent by lowering the glass transition temperature of the acrylic base polymer and reducing the shear storage modulus, it is preferable that the alkyl group of the (meth)acrylic acid alkyl ester is a chain alkyl group. The chain alkyl group may be linear or branched.

[0037] Examples of (meth)acrylic acid alkyl esters having a chain alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, and (meth)acrylate. Examples of the acrylate include nonyl acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, isotridecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, cetyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate.

[0038] Specific examples of the (meth)acrylic acid alkyl ester having an alicyclic alkyl group include (meth)acrylic acid cycloalkyl esters such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate; (meth)acrylic acid esters having a bicyclic aliphatic hydrocarbon ring such as isobornyl (meth)acrylate; and (meth)acrylic acid esters having a tricyclic or higher aliphatic hydrocarbon ring such as dicyclopentanyl (meth)acrylate, dicyclopentanyloxyethyl (meth)acrylate, tricyclopentanyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate. The (meth)acrylic acid alkyl ester having an alicyclic alkyl group may have a substituent on the ring such as 3,3,5-trimethylcyclohexyl (meth)acrylate. The (meth)acrylic acid alkyl ester having an alicyclic alkyl group may be a (meth)acrylic acid ester containing a condensed ring of an alicyclic structure and a ring structure having an unsaturated bond, such as dicyclopentenyl (meth)acrylate.

[0039] Among the exemplified (meth)acrylic acid alkyl esters, from the viewpoint of lowering the glass transition temperature of the acrylic base polymer, (meth)acrylic acid C 1-9 Alkyl esters are preferred, and those having a homopolymer glass transition temperature of -50°C or lower are preferred. The glass transition temperature of the homopolymer of (meth)acrylic acid alkyl ester is more preferably -55°C or lower, and even more preferably -60°C or lower. (Meth)acrylic acid C having a homopolymer glass transition temperature of -50°C or lower 1-9Specific examples of alkyl esters include 2-ethylhexyl acrylate (Tg: -70°C), n-hexyl acrylate (Tg: -65°C), n-octyl acrylate (Tg: -65°C), isononyl acrylate (Tg: -60°C), n-nonyl acrylate (Tg: -58°C), isooctyl acrylate (Tg: -58°C), butyl acrylate (Tg: -55°C), etc. Among these, those in which the alkyl group has 6 or more carbon atoms are preferred, and 2-ethylhexyl acrylate and n-octyl acrylate are particularly preferred.

[0040] The content of the (meth)acrylic acid alkyl ester is preferably 40 parts by weight or more, more preferably 50 parts by weight or more, and even more preferably 60 parts by weight or more, per 100 parts by weight of the total of the monomer components constituting the acrylic base polymer.

[0041] The acrylic base polymer contains, as a constituent monomer component, a monomer having a crosslinkable functional group in addition to a (meth)acrylic acid alkyl ester. Examples of the monomer having a crosslinkable functional group include a hydroxyl group-containing monomer and a carboxyl group-containing monomer. The acrylic base polymer may have both a hydroxyl group-containing monomer and a carboxyl group-containing monomer as copolymerization components, or may have only one of them. The introduction of a crosslinked structure into the acrylic base polymer tends to improve the cohesive force and improve the peelability of the pressure-sensitive adhesive layer 2 from the adherend before photocuring.

[0042] Examples of hydroxy group-containing monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, 4-(hydroxymethyl)cyclohexylmethyl (meth)acrylate, etc. Among these, 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate are preferred because they contribute greatly to improving the adhesive strength of the adhesive after photocuring.

[0043] Examples of the carboxy group-containing monomer include (meth)acrylic acid, 2-carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, etc. Among these, acrylic acid is particularly preferred because it contributes greatly to improving adhesive strength.

[0044] The amount of monomers having a crosslinkable functional group (total amount of hydroxyl group-containing monomer and carboxyl group-containing monomer) relative to 100 parts by weight of the total of the constituent monomer components of the acrylic base polymer is preferably 0.5 to 15 parts by weight, more preferably 1 to 10 parts by weight, and even more preferably 1.5 to 7 parts by weight. When the acrylic base polymer contains a hydroxyl group, the pressure-sensitive adhesive layer tends to have a high strain recovery rate and high shape restorability, so the acrylic base polymer preferably contains a hydroxyl group-containing monomer as a constituent monomer component, and the content of the hydroxyl group-containing monomer is preferably within the above range.

[0045] The acrylic base polymer may contain, as a constituent monomer component, a nitrogen-containing monomer such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, N-acryloylmorpholine, N-vinylcarboxylic acid amides, or N-vinylcaprolactam.

[0046] The acrylic base polymer may contain, as a constituent monomer component, a (meth)acrylic acid ester having an alkylene oxide chain. The (meth)acrylic acid ester having an alkylene oxide chain is represented by the following general formula (1). CH 2 =CR 1 -COO-(R 2 -O) m -R 3 (1)

[0047] R in general formula (1) 1 is a hydrogen atom or a methyl group, and R 1 The compound of formula (1) in which R is a hydrogen atom is an acrylate. 1 Compounds of formula (1) where is a methyl group are methacrylates.

[0048] R in general formula (1) 2 is an alkylene group such as ethylene, propylene, or butylene, and -R 2 -O- is an alkylene oxide chain. 2 A specific example of -O- is ethylene oxide (-CH 2 CH 2 -O-), propylene oxide (-CH(CH 3 )CH 2 -O-), butylene oxide (-CH 2 CH 2 CH 2 CH 2 -O-).

[0049] In the general formula (1), m is the number of repeating alkylene oxide units and is an integer of 1 or more. m is preferably 1 to 5. If m is too large, the (meth)acrylate compound represented by the general formula (1) is likely to act as a chain transfer agent, so that the molecular weight of the acrylic base polymer as a polymer does not become sufficiently large, and the adhesive strength of the pressure-sensitive adhesive may be insufficient. 3is a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, or an aryl group having 6 to 20 carbon atoms. From the viewpoint of lowering the Tg of the acrylic base polymer and improving the compatibility with the photocuring agent, R 3 is preferably an alkyl group having 1 to 3 carbon atoms, and particularly preferably a methyl group or an ethyl group.

[0050] Specific examples of the compound represented by the general formula (1) include methoxyethyl acrylate, phenoxyethyl acrylate, ethoxyethoxyethyl acrylate, and methoxytriethylene glycol acrylate.

[0051] The acrylic base polymer may contain monomer components other than those described above. The acrylic base polymer may contain, as monomer components, for example, vinyl ester monomers, aromatic vinyl monomers, epoxy group-containing monomers, vinyl ether monomers, sulfo group-containing monomers, phosphoric acid group-containing monomers, acid anhydride group-containing monomers, etc.

[0052] The glass transition temperature of the acrylic base polymer is preferably -40°C or lower, more preferably -45°C or lower, and may be -50°C or lower or -55°C or lower. When the glass transition temperature is sufficiently lower than the temperature of the device's operating environment, the shear storage modulus of the adhesive layer in the temperature range of the operating environment is small, and peeling during repeated bending tends to be suppressed. The lower limit of the glass transition temperature of the acrylic base polymer is not particularly limited, but is generally -80°C or higher, and may be -75°C or higher or -70°C or higher.

[0053] The glass transition temperature is the temperature (peak top temperature) at which the loss tangent tan δ in viscoelasticity measurement is maximized. Instead of the glass transition temperature determined by viscoelasticity measurement, the theoretical Tg calculated by the Fox formula may be used. The theoretical Tg is the glass transition temperature Tg of the homopolymer of the constituent monomer components of the acrylic base polymer. i and the weight fraction W of each monomer component i It is calculated using the following Fox formula: 1 / Tg=Σ(Wi / Tg i )

[0054] Tg is the glass transition temperature of the polymer chain (unit: K), W i is the weight fraction of the monomer component i constituting the segment (copolymerization ratio by weight), Tg i is the glass transition temperature (unit: K) of a homopolymer of monomer component i. The glass transition temperature of a homopolymer can be determined from the values ​​listed in Polymer Handbook, 3rd Edition (John Wiley & Sons, Inc., 1989). For the Tg of a homopolymer of a monomer not listed in the above literature, the peak top temperature of tan δ measured by dynamic viscoelasticity measurement can be used.

[0055] The monomer components are polymerized by various known methods such as solution polymerization, emulsion polymerization, and bulk polymerization to obtain an acrylic polymer as a base polymer. From the viewpoint of the balance of properties such as adhesive strength and holding power of the pressure sensitive adhesive, and from the viewpoint of cost, the solution polymerization method is preferred. As the solvent for solution polymerization, ethyl acetate, toluene, etc. are used. The solution concentration is usually about 20 to 80% by weight. As the polymerization initiator, various known ones such as azo-based and peroxide-based initiators can be used. A chain transfer agent may be used to adjust the molecular weight. The reaction temperature is usually about 50 to 80°C, and the reaction time is usually about 1 to 8 hours.

[0056] The weight average molecular weight of the acrylic base polymer is preferably 100,000 to 2,000,000, more preferably 200,000 to 1,500,000, and even more preferably 300,000 to 1,000,000. When a crosslinked structure is introduced into the acrylic base polymer, the molecular weight of the acrylic base polymer refers to the molecular weight before the introduction of the crosslinked structure.

[0057] <Crosslinking agent> In order to provide the pressure-sensitive adhesive with an appropriate cohesive strength, develop adhesive strength, and ensure the peelability of the pressure-sensitive adhesive layer from the adherend before photocuring, it is preferable that a crosslinked structure is introduced into the acrylic-based polymer. For example, a crosslinked structure is introduced by adding a crosslinking agent to a solution obtained by polymerizing the acrylic-based polymer, and heating the solution as necessary. The crosslinking agent has two or more crosslinkable functional groups in one molecule. The crosslinking agent may have three or more crosslinkable functional groups in one molecule.

[0058] Examples of the crosslinking agent include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, carbodiimide-based crosslinking agents, and metal chelate-based crosslinking agents. These crosslinking agents react with functional groups such as hydroxyl groups and carboxyl groups introduced into the acrylic base polymer to form a crosslinked structure. Isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred because they are highly reactive with the hydroxyl groups and carboxyl groups of the acrylic base polymer and can easily introduce a crosslinked structure. When the acrylic base polymer has a hydroxyl group as a crosslinkable functional group, an isocyanate-based crosslinking agent is preferred, and when the acrylic base polymer has a carboxyl group as a crosslinkable functional group, an epoxy-based crosslinking agent is preferred.

[0059] As the isocyanate crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule is used. The isocyanate crosslinking agent may have three or more isocyanate groups in one molecule. As the isocyanate crosslinking agent, for example, lower aliphatic polyisocyanates such as butylene diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentylene diisocyanate, cyclohexylene diisocyanate, and isophorone diisocyanate; aromatic isocyanates such as 2,4-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate; trimethylolpropane / trile diisocyanate; Examples of the isocyanate adduct include a xylylene diisocyanate trimer adduct (e.g., "Takenate D101E" manufactured by Mitsui Chemicals), a trimethylolpropane / hexamethylene diisocyanate trimer adduct (e.g., "Coronate HL" manufactured by Tosoh), a trimethylolpropane adduct of xylylene diisocyanate (e.g., "Takenate D110N" manufactured by Mitsui Chemicals), and an isocyanurate of hexamethylene diisocyanate (e.g., "Coronate HX" manufactured by Tosoh). As the isocyanate-based crosslinking agent, an isocyanate compound having a biuret group (e.g., "Duranate 24A-100" manufactured by Asahi Kasei) or an isocyanate compound having an allophanate group may be used.

[0060] As the epoxy crosslinking agent, a multifunctional epoxy compound having two or more epoxy groups in one molecule is used. The epoxy crosslinking agent may have three or more or four or more epoxy groups in one molecule. The epoxy group of the epoxy crosslinking agent may be a glycidyl group. Examples of epoxy crosslinking agents include N,N,N',N'-tetraglycidyl-m-xylylenediamine, diglycidylaniline, 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-hexanediol diglycidyl ether, neopentyl glycol diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, sorbitol polyglycidyl ether, glycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, polyglycerol polyglycidyl ether, sorbitan polyglycidyl ether, trimethylolpropane polyglycidyl ether, adipic acid diglycidyl ester, o-phthalic acid diglycidyl ester, triglycidyl-tris(2-hydroxyethyl)isocyanurate, resorcinol diglycidyl ether, and bisphenol-S-diglycidyl ether. As the epoxy-based crosslinking agent, commercially available products such as "Denacol" manufactured by Nagase ChemteX, and "Tetrad X" and "Tetrad C" manufactured by Mitsubishi Gas Chemical may be used.

[0061] The amount of the crosslinking agent used may be adjusted appropriately depending on the composition, molecular weight, etc. of the acrylic base polymer. The amount of the crosslinking agent used is about 0.01 to 5 parts by weight, preferably 0.03 to 3 parts by weight, more preferably 0.05 to 1 part by weight, and may be 0.08 to 0.8 parts by weight or 0.1 to 0.5 parts by weight, relative to 100 parts by weight of the acrylic base polymer.

[0062] A crosslinking catalyst may be used to promote the formation of a crosslinked structure. Examples of the crosslinking catalyst include organometallic compounds such as organometallic complexes (chelates), compounds of metals and alkoxy groups, and compounds of metals and acyloxy groups; and tertiary amines. In particular, organometallic compounds are preferred from the viewpoint of suppressing the progress of the crosslinking reaction in a solution state at room temperature and ensuring the pot life of the adhesive composition. Examples of metals in organometallic compounds include iron, tin, aluminum, zirconium, zinc, titanium, lead, and cobalt. The amount of the crosslinking catalyst used is generally 0.5 parts by weight or less per 100 parts by weight of the acrylic base polymer.

[0063] <Light curing agent> The adhesive composition constituting the adhesive layer 2 contains, as a photocuring agent, a compound having two or more photopolymerizable functional groups in one molecule in addition to an acrylic base polymer. An adhesive composition containing a photocuring agent has photocuring properties, and when photocuring is performed after lamination with an adherend, the adhesive strength with the adherend is improved.

[0064] As the photopolymerizable functional group, one having polymerizability by a photoradical reaction is preferable, and as the photocuring agent, a compound having two or more ethylenically unsaturated bonds in one molecule is preferable, and polyfunctional (meth)acrylates are preferable because of their high compatibility with acrylic base polymers.

[0065] The polyfunctional (meth)acrylate is typically an ester of a polyol and (meth)acrylic acid. Specific examples of the polyfunctional (meth)acrylate include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, alkanediol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, isocyanuric acid di(meth)acrylate, isocyanuric acid tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol di(meth)acrylate, and trimethylol di(meth)acrylate. Examples of the acrylates include ethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerin di(meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, and isoprene (meth)acrylate.

[0066] The polyfunctional (meth)acrylate may be an ester of an alkylene oxide-modified polyol and (meth)acrylic acid. The alkylene oxide may be ethylene oxide (EO) or propylene oxide (PO). The alkylene oxide may be a polyalkylene oxide such as polyethylene glycol or polypropylene glycol.

[0067] Specific examples of alkylene oxide-modified polyfunctional (meth)acrylates include bisphenol A ethylene oxide-modified di(meth)acrylate, bisphenol A propylene oxide-modified di(meth)acrylate, trimethylolpropane ethylene oxide-modified tri(meth)acrylate, trimethylolpropane propylene oxide-modified tri(meth)acrylate, isocyanuric acid ethylene oxide-modified di(meth)acrylate, isocyanuric acid propylene oxide-modified di(meth)acrylate, isocyanuric acid ethylene oxide-modified tri(meth)acrylate, isocyanuric acid propylene oxide-modified tri(meth)acrylate, pentaerythritol ethylene oxide-modified tetra(meth)acrylate, and pentaerythritol propylene oxide-modified tetra(meth)acrylate.

[0068] In polyfunctional (meth)acrylates containing an alkylene oxide chain, such as polyalkylene glycol di(meth)acrylates and alkylene oxide modified polyfunctional (meth)acrylates, the alkylene oxide is preferably (poly)ethylene oxide or (poly)propylene oxide, and particularly preferably (poly)ethylene oxide. The chain length of the alkylene oxide (the number of repeating units of the alkylene oxide) n is about 1 to 15. When a molecule contains a plurality of alkylene oxide chains, the average chain length n is preferably 1 to 15. The (average) chain length n of the alkylene oxide chain may be 12 or less, 10 or less, 8 or less, 6 or less, 5 or less, 4 or less, or 3 or less. By adjusting the type and chain length of the alkylene oxide, the compatibility with the acrylic base polymer can be adjusted to an appropriate range.

[0069] The polyfunctional (meth)acrylate having an alkylene oxide chain tends to have lower compatibility with acrylic base polymers as the alkylene oxide chain length n increases, and when n is 4 or more, it has lower compatibility with acrylic base polymers compared to the acrylic oligomers described below. Therefore, polyfunctional (meth)acrylates having an alkylene oxide chain, particularly polyalkylene glycol di(meth)acrylates having an alkylene oxide chain length n of 4 or more, tend to be unevenly distributed on the surface of the pressure-sensitive adhesive layer (near the adhesive interface with the adherend), and a weak boundary layer (WBL) is easily formed by the photocuring agent unevenly distributed at the adhesive interface with the adherend.

[0070] When a WBL is formed, the adhesive layer retains its bulk properties such as shear storage modulus while the liquid properties of the surface (adhesive interface) become stronger, so the adhesive strength with the adherend tends to decrease, and the adhesive layer before photocuring is easy to peel off from the adherend. When a photocuring adhesive layer in which the photocuring agent is unevenly distributed near the adhesive interface with the adherend and a WBL is formed is photocured, the curing reaction of the photocuring agent tends to proceed near the adhesive interface where the density of the photocuring agent is high, so the cohesive force near the adhesive interface tends to increase, and the adhesive strength tends to increase. In addition, when a WBL is formed, the increase in the shear storage modulus, which is a bulk property, tends to be suppressed in the adhesive after photocuring.

[0071] Polyfunctional (meth)acrylates having an alkylene oxide chain with a chain length n of 3 or less have high compatibility with acrylic base polymers and are therefore difficult to form WBLs. On the other hand, polyfunctional (meth)acrylates having an alkylene oxide chain with a short chain length n have a small functional group equivalent weight of the (meth)acryloyl group, so the crosslinking density of the pressure-sensitive adhesive layer after photocuring is high and the adhesive strength tends to be strong.

[0072] As described above, when a multifunctional (meth)acrylate having an alkylene oxide chain with a large chain length n (particularly n is 4 or more) is used as a photocuring agent, the adhesive strength of the adhesive layer before photocuring tends to be small, and when a multifunctional (meth)acrylate having an alkylene oxide chain with a small chain length n (particularly n is 3 or less) is used, the adhesive strength of the adhesive layer after photocuring tends to be large. For the purpose of adjusting the adhesive strength of the adhesive layer before and after photocuring, multiple types of multifunctional (meth)acrylates having alkylene oxide chains with different chain lengths n may be used. For example, a multifunctional (meth)acrylate having an alkylene oxide chain with a chain length n of 3 or less and a multifunctional (meth)acrylate having an alkylene oxide chain with a chain length n of 4 or more may be used in combination. In addition, two or more types of multifunctional (meth)acrylates having an alkylene oxide chain with a chain length n of 3 or less may be used, and two or more types of multifunctional (meth)acrylates having an alkylene oxide chain with a chain length n of 4 or more may be used.

[0073] From the viewpoint of maintaining a suitable compatibility with the acrylic base polymer, the molecular weight of the polyfunctional (meth)acrylate as the photocuring agent, particularly the polyfunctional (meth)acrylate containing an alkylene oxide chain, is preferably 1500 or less, more preferably 1000 or less, and may be 800 or less, 500 or less, or 400 or less. From the viewpoint of achieving both compatibility with the acrylic base polymer and improved adhesive strength after photocuring, the functional group equivalent (g / eq) of the polyfunctional (meth)acrylate is preferably 500 or less, more preferably 400 or less, and may be 300 or less, 250 or less, 200 or less, 180 or less, or 160 or less. On the other hand, if the functional group equivalent of the polyfunctional (meth)acrylate is excessively small, the crosslinking point density of the adhesive layer after photocuring increases, so that the shear storage modulus increases, and the adhesiveness in the low temperature region may decrease. Therefore, the functional group equivalent of the photocuring agent is preferably 80 or more, more preferably 100 or more, and may be 120 or more or 130 or more.

[0074] Two or more types of photocuring agents may be used in combination. For example, two or more types of polyfunctional (meth)acrylates having an alkylene oxide chain may be used as the photocuring agent, or a polyfunctional (meth)acrylate having an alkylene oxide chain and a polyfunctional (meth)acrylate having no alkylene oxide chain may be used. In addition, a polyfunctional (meth)acrylate having a different number of functional groups (the number of (meth)acryloyl groups in one molecule) may be used as the photocuring agent. For example, a bifunctional (meth)acrylate and a trifunctional or more polyfunctional (meth)acrylate may be used in combination as the photocuring agent for the purpose of adjusting the adhesive strength and shear storage modulus of the pressure-sensitive adhesive layer after photocuring.

[0075] As the photocuring agent, a polyfunctional (meth)acrylate having an alkylene oxide chain and a urethane (meth)acrylate may be used. The urethane (meth)acrylate is a compound having one or more urethane bonds and two or more (meth)acryloyl groups in one molecule, and preferably contains two or more urethane bonds in one molecule.

[0076] By including urethane (meth)acrylate as a photocuring agent in addition to a polyfunctional (meth)acrylate having an alkylene oxide chain, the adhesive strength of the adhesive layer before photocuring may be small and the adhesive strength of the adhesive layer after photocuring may be large. Since the compatibility behavior of urethane (meth)acrylate with acrylic base polymers is different from that of polyfunctional (meth)acrylate having an alkylene oxide chain, the inclusion of urethane (meth)acrylate is thought to contribute to the reduction of the adhesive strength of the adhesive layer before photocuring and the improvement of the adhesive strength of the adhesive layer after photocuring by promoting the formation of WBL.

[0077] A urethane (meth)acrylate having two or more urethane bonds can be obtained, for example, by reacting a polyisocyanate with a (meth)acrylic compound having a hydroxyl group, and the isocyanate group of the polyisocyanate and the hydroxyl group of the (meth)acrylic compound are bonded to form a urethane bond. A urethane (meth)acrylate having two or more urethane bonds can also be obtained by reacting a polyisocyanate with a polyol to prepare a prepolymer having an isocyanate group at its terminal, and then bonding a (meth)acrylic compound having a hydroxyl group to the isocyanate group at the terminal of the prepolymer. A urethane (meth)acrylate having two or more urethane bonds can also be obtained by reacting a polyisocyanate with a (meth)acrylic compound having a hydroxyl group, and then reacting the reactant with a polyol.

[0078] The polyisocyanate may be any of aromatic polyisocyanate, alicyclic polyisocyanate, and alicyclic polyisocyanate. As the aromatic polyisocyanate, tolylene diisocyanate (TDI) is particularly preferred. As the tolylene diisocyanate, 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate may be used, or a mixture of the two. As the aliphatic polyisocyanate, hexamethylene diisocyanate (HDI) is particularly preferred. As the alicyclic polyisocyanate, isophorone diisocyanate (IPDI) is particularly preferred.

[0079] The polyisocyanate may be a trifunctional isocyanate having a cyanuric acid skeleton. Examples of the trifunctional isocyanate having a cyanuric acid skeleton include a TDI trimer and an HDI trimer. The polyisocyanate may have a biuret group or an allophanate group.

[0080] The polyisocyanate may be an isocyanate-terminated urethane prepolymer obtained by reacting a polyol with a polyisocyanate. The isocyanate-terminated urethane prepolymer may be a high molecular weight product obtained by reacting a high molecular weight polyol such as a polyester polyol, a polycarbonate polyol, or a polyether polyol with a polyisocyanate.

[0081] Examples of the (meth)acrylic compound having a hydroxy group include compounds having one hydroxy group and one (meth)acryloyl group, such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, hydroxymethylacrylamide, and hydroxyethylacrylamide; and compounds having one hydroxy group and two or more (meth)acryloyl groups, such as pentaerythritol tri(meth)acrylate, dipentaerythritol penta(meth)acrylate, trimethylolpropane di(meth)acrylate, and isocyanuric acid di(meth)acrylate.

[0082] A urethane (meth)acrylate obtained by reacting a diisocyanate with a (meth)acrylic compound having one hydroxyl group and two or more (meth)acryloyl groups in one molecule has two urethane bonds and four or more (meth)acryloyl groups in one molecule. The number of (meth)acryloyl groups in the urethane (meth)acrylate may be 6 or more or 8 or more, or 12 or less or 10 or less.

[0083] From the viewpoint of reducing the adhesive strength of the pressure-sensitive adhesive layer before photocuring and increasing the adhesive strength by photocuring, the molecular weight of the urethane (meth)acrylate is preferably 1000 to 50000, more preferably 1500 to 30000, and may be 2000 to 20000, 2500 to 15000, or 3000 to 10000. The functional group equivalent (g / eq) of the (meth)acryloyl group of the urethane (meth)acrylate is preferably 500 to 20000, more preferably 800 to 10000, and further preferably 1000 to 7000, and may be 1200 to 5000 or 1500 to 4000.

[0084] The above urethane (meth)acrylate may be commercially available from Kyoeisha Chemical, Shin-Nakamura Chemical, Negami Chemical Industries, Mitsubishi Chemical, Daicel-Allnex, Resonac, or the like.

[0085] The smaller the functional group equivalent of the photocuring agent and the higher the content of the photocuring agent, the higher the crosslink density due to photocuring, so the shear storage modulus of the adhesive layer after photocuring tends to be higher. From the viewpoint of suppressing an excessive increase in the shear storage modulus while increasing the adhesive strength of the adhesive after photocuring, the content of the photocuring agent in the adhesive composition is preferably 3 to 25 parts by weight, more preferably 5 to 20 parts by weight, and may be 8 to 17 parts by weight or 10 to 15 parts by weight, relative to 100 parts by weight of the acrylic base polymer. As described above, if a WBL is formed on the surface (adhesive interface) of the adhesive layer by adjusting the compatibility between the acrylic base polymer and the photocuring agent, the adhesive strength is easily increased by photocuring even with a small amount of the photocuring agent, and high adhesive strength can be realized.

[0086] When the photocurable composition constituting the adhesive layer contains urethane (meth)acrylate as a photocuring agent in addition to a polyfunctional (meth)acrylate having no urethane bond (particularly a polyfunctional (meth)acrylate having an alkylene oxide chain), the content of the urethane (meth)acrylate is preferably 0.01 parts by weight or more, more preferably 0.1 parts by weight or more, and even more preferably 0.15 parts by weight or more, and may be 0.2 parts by weight or more, 0.3 parts by weight or more, 0.4 parts by weight or more, or 0.5 parts by weight or more, relative to 100 parts by weight of the acrylic base polymer. If the content of the urethane (meth)acrylate is excessively high, it may cause contamination of the adherend due to bleeding out of the photocuring agent to the surface of the adhesive layer (adhesive interface with the adherend). In addition, if the amount of the urethane (meth)acrylate is excessively high, the increase in adhesive strength of the adhesive by photocuring tends to be insufficient. Therefore, the content of the urethane (meth)acrylate in the pressure-sensitive adhesive composition is preferably 15 parts by weight or less, more preferably 10 parts by weight or less, and may be 7 parts by weight or less, 5 parts by weight or less, or 3 parts by weight or less, relative to 100 parts by weight of the acrylic base polymer.

[0087] <Acrylic oligomer> The adhesive composition constituting the adhesive layer 2 contains, in addition to the above-mentioned acrylic base polymer and photocuring agent, an acrylic oligomer having a polymerizable functional group at the end of the main chain. The polymerizable functional group is an ethylenically unsaturated group (C=C double bond), and is preferably a vinyl group or a (meth)acryloyl group. From the viewpoint of photocuring, an acryloyl group is particularly preferred as the polymerizable functional group of the acrylic oligomer. The acrylic oligomer having a polymerizable functional group at the end of the main chain participates in the photocuring reaction, similar to the photocuring agent.

[0088] The acrylic oligomer contains a (meth)acrylic acid alkyl ester as a main constituent monomer component, and is a component having a smaller weight average molecular weight than the above-mentioned acrylic base polymer. The weight average molecular weight of the acrylic oligomer is about 1000 to 30000, preferably 20000 or less, more preferably 10000 or less, and even more preferably 5000 or less, and may be 3000 or less, 2500 or less, or 2000 or less. The weight average molecular weight of the acrylic oligomer may be 1200 or more, 1400 or more, or 1500 or more.

[0089] The smaller the molecular weight of the acrylic oligomer, the stronger the action as a plasticizer, and the smaller the shear storage modulus of the adhesive layer after photocuring tends to be. Also, the smaller the molecular weight of the acrylic oligomer, the smaller the distance between crosslinking points, and therefore the higher the strain recovery rate at high temperatures. On the other hand, when the molecular weight of the acrylic oligomer is below 1000, the smaller the distance between crosslinking points, the lower the viscosity of the adhesive and the smaller the creep deformation rate.

[0090] Examples of monomer components constituting the acrylic oligomer include the monomers exemplified above as monomer components constituting the acrylic base polymer. The acrylic oligomer may contain a crosslinkable functional group, similar to the acrylic base polymer. However, when a crosslinked structure is introduced into the hydroxyl group or carboxyl group of the acrylic oligomer, the action as a plasticizer tends to decrease. In addition, when the amount of polar functional groups such as hydroxyl group or carboxyl group introduced is large, the cohesive force tends to be large and the shear storage modulus at low temperature tends to be large. Therefore, it is preferable that the acrylic oligomer has a small amount of polar groups. The content of (meth)acrylic acid alkyl ester relative to the total of 100 parts by weight of the monomer components constituting the acrylic oligomer is preferably 80 parts by weight or more, more preferably 90 parts by weight or more, and even more preferably 95 parts by weight or more.

[0091] The acrylic oligomer may have a polymerizable functional group such as a (meth)acryloyl group on the side chain in addition to the end of the main chain. However, the acrylic acrylate containing a polymerizable functional group on the side chain has a small effect of reducing the shear storage modulus of the adhesive layer after photocuring, since the (meth)acryloyl group on the side chain participates in the photocuring reaction. Therefore, it is preferable that the acrylic oligomer does not have a polymerizable functional group on the side chain.

[0092] From the viewpoint of reducing the shear storage modulus at low temperatures of the pressure-sensitive adhesive layer after photocuring, the glass transition temperature of the acrylic oligomer is preferably −30° C. or lower, more preferably −40° C. or lower, even more preferably −50° C. or lower, and may be −55° C. or lower, −65° C. or lower, or −70° C. or lower. The lower limit of the glass transition temperature of the acrylic oligomer is not particularly limited, but is generally −85° C. or higher, and may be −80° C. or higher.

[0093] The acrylic oligomer having a (meth)acryloyl group at the end of the main chain can be obtained, for example, by a high-temperature continuous polymerization method. In the high-temperature continuous polymerization method, a pressurizable reactor is set to about 150 to 300°C, a reaction liquid in which a monomer and, if necessary, a solvent are mixed is continuously fed to the reactor, and the polymerization liquid in an amount equal to the amount of the reaction liquid fed is extracted from the reactor, and unreacted monomers (and solvent) are removed from the polymerization liquid by distillation. Since the high-temperature continuous polymerization method does not require a thermal polymerization initiator, a polymer having an unreacted polymerizable functional group at the end can be obtained. For details of the high-temperature continuous polymerization method, refer to JP-A-57-502171, JP-A-59-6207, JP-A-60-215007, etc. As the acrylic oligomer having a (meth)acryloyl group at the end of the main chain, a commercially available product such as the "ARUFON UP-1000" series manufactured by Toagosei Co., Ltd. may be used.

[0094] Acrylic oligomers having a polymerizable functional group at the end of the main chain are common to the polyfunctional (meth)acrylates and urethane (meth)acrylates having an alkylene oxide chain described above as photocuring agents in that they have a polymerizable functional group such as a (meth)acryloyl group at the end. On the other hand, since most of the monomer components constituting the main chain of acrylic oligomers are (meth)acrylic acid alkyl esters, they have higher compatibility with acrylic base polymers than polyfunctional (meth)acrylates and urethane (meth)acrylates having alkylene oxide chains, and have less effect of reducing the adhesive strength of the pressure-sensitive adhesive layer before photocuring.

[0095] Acrylic oligomers have a low glass transition temperature, so they act as a plasticizer and reduce the shear storage modulus of the photocured adhesive layer at low temperatures. Acrylic oligomers have polymerizable functional groups at their ends and participate in the curing reaction together with the photocuring agent, so they become gel fraction (insoluble component) in the photocured adhesive layer. Therefore, even if acrylic oligomers are added, the gel fraction of the photocured adhesive layer is unlikely to decrease, and strain recovery at high temperatures can be maintained.

[0096] The content of the acrylic oligomer in the pressure-sensitive adhesive composition is preferably 6 to 35 parts by weight, more preferably 8 to 30 parts by weight, further preferably 10 to 25 parts by weight, and may be 12 to 20 parts by weight, based on 100 parts by weight of the acrylic base polymer. The larger the amount of the acrylic oligomer, the smaller the shear storage modulus at low temperatures and the larger the creep deformation rate at high temperatures of the pressure-sensitive adhesive layer after photocuring. When the amount of the acrylic oligomer is excessively large, the strain recovery rate at high temperatures tends to decrease. In addition, when the amount of the acrylic oligomer is excessively large, the adhesive strength of the pressure-sensitive adhesive layer after photocuring may not increase sufficiently.

[0097] <Photopolymerization initiator> The photopolymerization initiator generates active species by irradiation with active light rays, and promotes the curing reaction of the photocuring agent and the acrylic oligomer having a polymerizable functional group at the end. As the photopolymerization initiator, a photocation initiator (photoacid generator), a photoradical initiator, a photoanion initiator (photobase generator), etc. are used depending on the type of the photocuring agent. When a polyfunctional acrylate is used as the photocuring agent, it is preferable to use a photoradical initiator. As the photoradical initiator, a photoradical generator that is cleaved by visible light or ultraviolet light having a wavelength shorter than 450 nm to generate a radical is preferable, and examples of the photoradical initiator include hydroxyketones, benzyl dimethyl ketals, aminoketones, acylphosphine oxides, benzophenones, trichloromethyl group-containing triazine derivatives, etc. The photoradical generator may be used alone or in a mixture of two or more types.

[0098] The content of the photopolymerization initiator in the pressure-sensitive adhesive composition is preferably 0.001 to 5 parts by weight, more preferably 0.01 to 3 parts by weight, and even more preferably 0.03 to 1 part by weight, relative to 100 parts by weight of the acrylic base polymer. The content of the photopolymerization initiator in the pressure-sensitive adhesive composition is preferably 0.02 to 20 parts by weight, more preferably 0.05 to 10 parts by weight, and even more preferably 0.1 to 7 parts by weight, relative to 100 parts by weight of the total of the photocuring agent and the acrylic oligomer.

[0099] <Other ingredients> As described above, the photocurable adhesive composition constituting the adhesive layer 2 contains an acrylic base polymer, a photocuring agent, an acrylic oligomer having a polymerizable functional group at the end of the main chain, and a photopolymerization initiator. The adhesive composition may contain other components besides these.

[0100] The adhesive composition may contain an antistatic agent. When the adhesive contains an antistatic agent, the adhesive layer has a low resistance, the charge of the adhesive layer is reduced, and the adherend can be prevented from being charged. Examples of the antistatic agent include ionic compounds containing organic cations, alkali metal salts, ion-conductive polymers, ion-conductive fillers, conductive polymers, and the like. Among these, ionic compounds containing organic cations and alkali metal salts are preferred because of their excellent compatibility with acrylic-based polymers.

[0101] When the pressure-sensitive adhesive composition contains an antistatic agent, the amount of the antistatic agent is about 0.01 to 3 parts by weight, preferably 0.03 to 2 parts by weight, more preferably 0.05 to 1 part by weight, and even more preferably 0.1 to 0.7 parts by weight, based on 100 parts by weight of the acrylic base polymer. If the amount of the antistatic agent is small, the resistance of the pressure-sensitive adhesive may not be sufficiently reduced. If the amount of the antistatic agent is excessively large, the antistatic agent may bleed out, causing contamination or corrosion of the adherend, or a decrease in adhesive strength.

[0102] In addition to the above-mentioned components, the pressure-sensitive adhesive composition may contain additives such as a silane coupling agent, a tackifier, a plasticizer, a softener, an anti-degradation agent, a filler, a colorant, an ultraviolet absorber, an antioxidant, a surfactant, etc., within the range that does not impair the properties of the present invention.

[0103] Since the low molecular weight additives that do not contribute to the photocuring of the adhesive remain as liquid components even after the adhesive is photocured, when the amount of the low molecular weight additives is large, the gel fraction of the adhesive after photocuring is small, the adhesive layer is easily plastically deformed, and the strain recovery rate tends to decrease. Therefore, the amount of the additive is preferably 20 parts by weight or less, more preferably 10 parts by weight or less, and even more preferably 5 parts by weight or less, and may be 3 parts by weight or less, 2 parts by weight or less, or 1 part by weight or less, relative to 100 parts by weight of the acrylic base polymer. The total amount of the acrylic base polymer, the crosslinking agent, the photocuring agent, and the acrylic oligomer relative to the total solid content of the adhesive composition is preferably 80% by weight or more, more preferably 90% by weight or more, more preferably 93% by weight or more, and may be 95% by weight or more, 97% by weight or more, or 98% by weight or more.

[0104] [Preparation of reinforcing film] A reinforcing film is obtained by laminating a photocurable pressure-sensitive adhesive layer 2 on a film substrate 1. The pressure-sensitive adhesive layer 2 may be formed directly on the film substrate 1, or a pressure-sensitive adhesive layer formed in a sheet form on another substrate may be transferred onto the film substrate 1.

[0105] The above-mentioned adhesive composition is applied to a substrate by roll coating, kiss roll coating, gravure coating, reverse coating, roll brush, spray coating, dip roll coating, bar coating, knife coating, air knife coating, curtain coating, lip coating, die coating, or the like, and the solvent is dried and removed as necessary to form an adhesive layer. As the drying method, an appropriate method may be adopted as appropriate. The heating and drying temperature is preferably 40°C to 200°C, more preferably 50°C to 180°C, and even more preferably 70°C to 170°C. The drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 15 minutes, and even more preferably 10 seconds to 10 minutes.

[0106] When the pressure-sensitive adhesive composition contains a crosslinking agent, it is preferable to proceed with crosslinking by heating or aging simultaneously with or after drying of the solvent. The heating temperature and heating time are appropriately set depending on the type of crosslinking agent used, and crosslinking is usually performed by heating in the range of 20°C to 160°C for about 1 minute to 7 days. The heating for drying and removing the solvent may also serve as the heating for crosslinking.

[0107] Even after the crosslinking structure is introduced into the polymer by the crosslinking agent, the photocuring agent and the acrylic oligomer remain in an unreacted state. Therefore, the adhesive layer 2 contains an acrylic base polymer with a crosslinking structure introduced therein, a photocuring agent, an acrylic oligomer, and a photopolymerization initiator. When the adhesive layer 2 is formed on the film substrate 1, it is preferable to provide a release liner 5 on the adhesive layer 2 for the purpose of protecting the adhesive layer 2, etc. Crosslinking may be performed after providing the release liner 5 on the adhesive layer 2.

[0108] When the pressure-sensitive adhesive layer 2 is formed on another substrate, the solvent is dried, and then the pressure-sensitive adhesive layer 2 is transferred onto the film substrate 1 to obtain a reinforcing film. The substrate used to form the pressure-sensitive adhesive layer may be used as the release liner 5 as it is.

[0109] As the release liner 5, a plastic film such as polyethylene, polypropylene, polyethylene terephthalate, or polyester film is preferably used. The thickness of the release liner is usually 3 to 200 μm, preferably about 10 to 100 μm. The surface of the release liner 5 that comes into contact with the adhesive layer 2 is preferably subjected to a release treatment using a release agent such as a silicone-based, fluorine-based, long-chain alkyl-based, or fatty acid amide-based release agent, or silica powder. By subjecting the surface of the release liner 5 to a release treatment, when the release liner 5 is peeled from the film substrate 1, peeling occurs at the interface between the adhesive layer 2 and the release liner 5, and the adhesive layer 2 is maintained in a state of being fixed to the film substrate 1. The release liner 5 may be subjected to an antistatic treatment on either or both of the release-treated surface and the non-treated surface. By subjecting the release liner 5 to an antistatic treatment, charging when the release liner is peeled from the adhesive layer can be suppressed.

[0110] [Physical properties of adhesive layer] The thickness of the pressure-sensitive adhesive layer 2 is, for example, about 1 to 300 μm. The thicker the pressure-sensitive adhesive layer 2, the more the adhesiveness to the adherend tends to improve. On the other hand, if the pressure-sensitive adhesive layer 2 is too thick, the fluidity before photocuring is high, and handling may become difficult. Therefore, the thickness of the pressure-sensitive adhesive layer 2 is preferably 3 to 100 μm, more preferably 5 to 50 μm, further preferably 6 to 40 μm, and particularly preferably 8 to 30 μm. From the viewpoint of thinning, the thickness of the pressure-sensitive adhesive layer 2 may be 25 μm or less, 20 μm or less, or 18 μm or less.

[0111] When the reinforcing film is used in an optical device such as a display, the total light transmittance of the pressure-sensitive adhesive layer 2 is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. The haze of the pressure-sensitive adhesive layer 2 is preferably 2% or less, more preferably 1% or less, even more preferably 0.7% or less, and particularly preferably 0.5% or less.

[0112] From the viewpoint of facilitating peeling from the adherend and preventing adhesive residue on the adherend after peeling of the reinforcing film, the adhesive strength between the adhesive layer and the adherend before photocuring is preferably 1 N / 25 mm or less, more preferably 0.7 N / 25 mm or less, even more preferably 0.5 N / 25 mm or less, and may be 0.4 N / 25 mm or less, or 0.3 N / 25 mm or less. From the viewpoint of preventing peeling of the reinforcing film during storage or handling, the adhesive strength between the adhesive layer and the adherend before photocuring is preferably 0.01 N / 25 mm or more, more preferably 0.02 N / 25 mm or more, and may be 0.03 N / 25 mm or more, 0.04 N / 25 mm or more, or 0.05 N / 25 mm or more.

[0113] The adhesive strength is determined by a peel test using a polyimide film as the adherend at a tensile speed of 300 mm / min and a peel angle of 180°. Unless otherwise specified, the adhesive strength is measured at 25°C.

[0114] When the pressure-sensitive adhesive layer 2 is photocured, the photocuring agent undergoes a curing reaction, increasing the adhesive strength with the adherend. It is preferable that the pressure-sensitive adhesive layer 2 has a small shear storage modulus at low temperatures after photocuring.

[0115] The shear storage modulus of the pressure-sensitive adhesive layer after photocuring at -20°C is preferably 100 kPa or less, more preferably 90 kPa or less, even more preferably 80 kPa or less, and may be 75 kPa or less, 70 kPa or less, or 65 kPa or less. The shear storage modulus of the pressure-sensitive adhesive layer (hereinafter simply referred to as "storage modulus") is determined by measuring the viscoelasticity at a frequency of 1 Hz and a heating rate of 5°C / min in accordance with the method described in JIS K7244-1 "Plastics - Test method for dynamic mechanical properties" and reading the value at a predetermined temperature.

[0116] Since the pressure-sensitive adhesive layer 2 after photocuring has a small storage modulus at low temperature, the pressure-sensitive adhesive layer exhibits strain relaxation properties in a low-temperature environment, and therefore peeling of the pressure-sensitive adhesive layer at the bent portion can be suppressed even when the device to which the reinforcing film is attached is repeatedly bent or when the bent state is maintained for a long period of time. As described above, since the pressure-sensitive adhesive composition contains an acrylic oligomer having a polymerizable functional group at the end of the main chain, the acrylic oligomer acts as a plasticizer, and the storage modulus of the pressure-sensitive adhesive layer after photocuring at low temperature tends to be small.

[0117] If the storage modulus of the pressure-sensitive adhesive layer after photocuring is too small, the pressure-sensitive adhesive layer may peel off from the adherend due to insufficient adhesive retention. Therefore, the storage modulus of the pressure-sensitive adhesive layer after photocuring at -20°C is preferably 30 kPa or more, more preferably 40 kPa or more, even more preferably 45 kPa or more, and may be 50 kPa or more.

[0118] The photocured pressure-sensitive adhesive layer preferably has a maximum strain (creep deformation rate) of 15% or more when a shear stress of 2 kPa is applied for 10 minutes at 60° C. in a creep test using a rotational rheometer. The creep deformation rate is more preferably 20% or more, further preferably 23% or more, and may be 25% or more.

[0119] The strain recovery rate when the stress is released after the creep test and the specimen is allowed to stand for 10 minutes to recover from the strain is preferably 88% or more, more preferably 90% or more, even more preferably 91% or more, and may be 92% or more. The strain recovery rate is the maximum strain S 1 and the minimum strain at stress release S 2 It is calculated from the following formula: Strain recovery rate (%) = 100 × (S 1 -S 2 ) / S 1

[0120] When a certain amount of stress is applied to a viscoelastic material such as an adhesive, a phenomenon called creep occurs in which the strain (deformation rate) increases over time. The strain of a viscoelastic material when a certain amount of stress is applied includes an elastic component that occurs instantaneously when the stress is applied, a viscoelastic component that is expressed as an increasing function of time and reaches a constant value after a long time, and a viscous component that increases in proportion to time. When the stress is released, the elastic and viscoelastic components of the strain recover, while the viscous component does not recover and remains.

[0121] When a foldable device is folded from an open state, compressive stress is applied to the inside and tensile stress is applied to the outside, causing distortion at the bent portion and its surroundings. If the creep deformation rate of the adhesive layer is large, the adhesive layer deforms in response to the distortion, suppressing deformation of the device due to stress concentration and peeling of the adhesive layer at the bent portion.

[0122] The strain recovery rate is an index showing the degree to which the adhesive layer returns to its original shape when stress is applied to the adhesive layer to cause deformation (distorted state) and the stress is released; the closer to 100%, the higher the shape recovery rate. When a foldable device is returned from a folded state to an extended state, the stress is released. When the strain recovery rate is high, the adhesive layer is more likely to restore its shape when the stress is released, making it less likely to develop wrinkles or other deformations.

[0123] As described above, the larger the creep deformation rate of the adhesive layer after photocuring, the higher the ability to follow stress, and the larger the strain recovery rate, the higher the ability to recover the shape when stress is released. Therefore, the larger the creep deformation rate and the strain recovery rate, the easier it is for the shape of the adhesive layer to follow the deformation of the device when the foldable device is repeatedly bent and extended, and wrinkles at the bending points and peeling of the adhesive layer tend to be suppressed.

[0124] The pressure-sensitive adhesive composition contains an acrylic oligomer having a polymerizable functional group at the end of the main chain, and the acrylic oligomer acts as a plasticizer, so the creep deformation rate tends to increase. On the other hand, when the amount of the component acting as a plasticizer increases, the strain recovery rate tends to decrease. As described above, the acrylic oligomer having a polymerizable functional group at the end of the main chain exists as a gel component in the pressure-sensitive adhesive layer after photocuring, so that the creep deformation rate is increased while the decrease in the strain recovery rate is suppressed.

[0125] From the viewpoint of adhesive reliability during practical use of the device, the adhesive strength between the adhesive layer and the adherend after photocuring is preferably 5 N / 25 mm or more, more preferably 7 N / 25 mm or more, even more preferably 8 N / 25 mm or more, and may be 9 N / 25 mm or more or 10 N / 25 mm or more.

[0126] The adhesive strength between the adhesive layer and the adherend after photocuring is preferably at least 5 times, more preferably at least 10 times, even more preferably at least 15 times, and may be at least 20 times, at least 25 times, or at least 30 times, the adhesive strength between the adhesive layer and the adherend before photocuring.

[0127] [Use of reinforcing film] The reinforced film of the present invention is used by being attached to a device or a device component. The reinforced film 10 has an adhesive layer 2 fixed to a film substrate 1, and has a low adhesive strength to an adherend before being photocured after being attached to the adherend. Therefore, the reinforced film is easy to peel off from the adherend before being photocured.

[0128] The adherend to which the reinforcing film is attached is not particularly limited, and examples thereof include various electronic devices, optical devices, and components thereof. In one embodiment, the reinforcing film is attached to the surface of a foldable flexible device such as a foldable device or a rollable device. A foldable device has a hinge portion and can be folded around the hinge portion. When the device is a display device, the reinforcing film may be attached to the surface on the screen side, or the reinforcing film may be attached to the back side (housing). In a flexible device that is configured to be foldable at a predetermined location such as a hinge portion, bending and stretching are repeatedly performed at the same location when the device is in use.

[0129] The reinforcing film may be attached to the entire surface of the adherend, or may be selectively attached only to the portion requiring reinforcement (reinforcement target area). In addition, the reinforcing film may be attached to the entire portion requiring reinforcement (reinforcement target area) and the area not requiring reinforcement (non-reinforcement target area), and then the reinforcing film attached to the non-reinforcement target area may be cut and removed. If the adhesive has not yet been photocured, the reinforcing film is in a state of being temporarily attached to the surface of the adherend, and therefore the reinforcing film can be easily peeled and removed from the surface of the adherend. The reinforcing film may be attached to the reinforcement target area and the non-reinforcement target area, light may be selectively irradiated to the reinforcement target area to photocur the adhesive, and then the reinforcing film in the non-reinforcement target area where the adhesive is not yet cured may be selectively peeled and removed.

[0130] By laminating the reinforcing film, appropriate rigidity is imparted, and therefore, effects such as improved handleability and damage prevention are expected for thin members such as flexible devices. When the reinforcing film is laminated to a work-in-progress in the device manufacturing process, the reinforcing film may be laminated to a large-sized work-in-progress before being cut to the product size. The reinforcing film may be laminated by roll-to-roll to a mother roll of a device manufactured by a roll-to-roll process.

[0131] After the reinforcing film is attached to the adherend, the pressure-sensitive adhesive layer 2 is irradiated with active light rays to photocure the pressure-sensitive adhesive layer. Examples of active light rays include ultraviolet light, visible light, infrared light, X-rays, α-rays, β-rays, and γ-rays. UV light is preferred as the active light rays because it can suppress curing of the pressure-sensitive adhesive layer during storage and is easy to cure. The irradiation intensity and irradiation time of the active light rays may be appropriately set depending on the composition and thickness of the pressure-sensitive adhesive layer. The pressure-sensitive adhesive layer 2 may be irradiated with active light rays from either the film substrate 1 side or the adherend side, or may be irradiated with active light from both sides.

[0132] As described above, by laminating the reinforced film of the present invention, an appropriate rigidity is imparted to the adherend, and stress is relaxed and dispersed, thereby suppressing various defects that may occur in the manufacturing process, improving production efficiency, and improving yield. Before the adhesive layer is photocured, the reinforced film is easily peeled off from the adherend, so that rework is easy even if lamination or lamination failure occurs. In addition, processing such as selectively removing the reinforced film from areas other than the area to be reinforced is also easy.

[0133] When the completed device is in use, the device can be prevented from being damaged by the application of the reinforcing film even if an external force is suddenly applied due to the device being dropped, a heavy object being placed on the device, the device being hit by a flying object, etc. Furthermore, since the reinforcing film is firmly attached to the device after the adhesive is photocured, the reinforcing film is unlikely to peel off even during long-term use, providing excellent reliability.

[0134] In a device with a reinforced film in which the reinforced film of the present invention is bonded to a flexible device using a resin substrate, even when the device is repeatedly bent and extended, or when the bent state is maintained for a long period of time, the reinforced film is unlikely to deform, such as wrinkles, at the bent points, or peel off from the device, demonstrating excellent adhesion reliability. EXAMPLES

[0135] The present invention will be further explained below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0136] [Examples 1 to 6, Comparative Examples 1 to 5] <Polymerization of acrylic polymer> In a reaction vessel equipped with a thermometer, a stirrer, a reflux condenser and a nitrogen gas inlet tube, 68.1 parts by weight of n-octyl acrylate (NOAA), 1.9 parts by weight of 4-hydroxybutyl acrylate (4HBA) and 30 parts by weight of methoxyethyl acrylate (MEA) as monomers, 0.2 parts by weight of azobisisobutyronitrile (AIBN) as a polymerization initiator and 233 parts by weight of ethyl acetate as a solvent were charged, and nitrogen gas was passed through and substituted with nitrogen for about 1 hour while stirring. After that, the mixture was heated to 60°C and reacted for 7 hours to obtain a solution of acrylic polymer A (glass transition temperature: -60°C) with a weight average molecular weight (Mw) of 700,000.

[0137] <Preparation of Pressure-Sensitive Adhesive Composition> The solution of acrylic polymer A (100 weight percent of polymer solids) was mixed with 0.1 weight percent of a trifunctional isocyanate-based crosslinking agent (Tosoh's "Coronate HX") as a crosslinking agent, 0.02 weight percent of iron acetylacetonate (Nihon Kagaku Sangyo's "Nacem Ferric") as a crosslinking catalyst, 12.5 weight percent of polyethylene glycol #200 (n=4) diacrylate (Shin-Nakamura Chemical Co., Ltd.'s "NK Ester A-200", functional group equivalent 151 g / eq), 1.0 weight percent of polyethylene glycol #400 (n=9) diacrylate (Shin-Nakamura Chemical Co., Ltd.'s "NK Ester A-400", functional group equivalent 263 g / eq), and 2.0 weight percent of a polycarbonate skeleton-containing urethane diacrylate (Negami Chemical Industries' "Art Resin UN-9200A", weight average molecular weight 15,000) as a photopolymerization initiator, and "Omnirad" (IGM Resins' "Omnirad" (Omnirad)) as a photopolymerization initiator. 0.3 parts by weight of "651" and 0.15 parts by weight of 1-ethyl-3-methylimidazolium bisfluorosulfonylimide ("ELEXEL AS-110" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as an antistatic agent were added, and further, an oligomer shown in Table 1 was added and mixed uniformly to prepare a pressure-sensitive adhesive composition.

[0138] Details of the oligomers are as follows: In Comparative Example 1, no oligomer was added. UP-1021: An acrylic oligomer with a weight-average molecular weight of 1600 that has an acryloyl group at the end of the main chain ("ARUFON UP-1021" manufactured by Toagosei, glass transition temperature: -77°C) UP-1000: Acrylic oligomer with a weight average molecular weight of 3000 and an acryloyl group at the end of the main chain ("ARUFON UP-1021" manufactured by Toagosei, glass transition temperature: -71°C) OAP-5000: Acrylic acrylate with a weight average molecular weight of 20,000 ("ART CURE OAP-5000" manufactured by Negami Chemical Industries, functional group equivalent: 2000g / eq, glass transition temperature: 90℃) S-3011: Triol type polypropylene glycol with a weight average molecular weight of 10,000 (AGC "Preminol S-3011")

[0139] <Preparation of reinforcing film> The above adhesive composition was applied to a polyethylene terephthalate film having a thickness of 50 μm using a fountain roll so that the thickness after drying was 18 μm. After drying at 130° C. for 1 minute to remove the solvent, the release-treated surface of a release liner (a polyethylene terephthalate film having a thickness of 25 μm, both sides of which were antistatically treated and one side of which was silicone release-treated) was attached to the adhesive-coated surface. Then, aging treatment was performed for 4 days in an atmosphere of 25° C. to promote crosslinking, and a reinforced film was obtained in which a photocurable adhesive sheet was fixed and laminated on a polyethylene terephthalate film substrate, and a release liner was temporarily attached thereon.

[0140] [evaluation] The reinforced films obtained in the above Examples and Comparative Examples were evaluated as follows.

[0141] <Storage modulus of adhesive layer> The adhesive composition was applied and crosslinked on the release liner in the same manner as in each of the above Examples and Comparative Examples to prepare an adhesive sheet (before photocuring). A release liner was attached to the surface of the adhesive layer of the adhesive sheet before photocuring to isolate it from oxygen, and the adhesive sheet was exposed to 1000 mJ / cm 2 of a 365 nm LED lamp. 2 The adhesive sheets were laminated after photocuring to prepare a measurement sample having a thickness of about 0.8 mm, and dynamic viscoelasticity measurements were performed under the following conditions using a rotational rheometer (TA Instruments' "Discovery-HR2") to read the shear storage modulus G' at -20°C. Deformation mode: Torsion Measurement frequency: 1Hz Heating rate: 5℃ / min Measurement temperature: -30~220℃ Shape: Parallel plate 8.0mmφ

[0142] <Creep deformation rate and strain recovery rate of adhesive layer> A measurement sample was prepared in the same manner as above. Using a rotational rheometer, a shear stress of 2 kPa was applied to the measurement sample for 10 minutes under the following conditions, and the maximum shear strain S 1 The creep deformation rate (creep rate) was determined. After that, the stress was released (stress: 0 kPa) and the specimen was left to stand for 10 minutes to recover the strain. The maximum strain S 1 and the minimum strain at stress release S 2 From the above, strain recovery rate (%) = 100 × (S 1 -S 2 ) / S 1 was calculated. (Measurement conditions) Deformation mode: Torsion Measurement temperature: 60℃ Shape: Parallel plate 8.0mmφ

[0143] <Adhesive strength> A 25 μm thick polyimide film (UBE "Upilex 25S") was attached to a glass plate via a double-sided adhesive tape (Nitto Denko "No. 531") to obtain a polyimide film substrate for measurement. The release liner was peeled off from the surface of a reinforced film cut to a width of 25 mm x length of 100 mm, and the film was attached to the polyimide film substrate for measurement using a hand roller to obtain a test sample before photocuring. The test sample before photocuring was irradiated with ultraviolet light from the reinforced film side (PET film substrate side) to photocur the adhesive layer, thereby obtaining a test sample after photocuring. Using these test samples, the end of the film substrate of the reinforced film was held with a chuck, and the reinforced film was peeled at 180° at a pulling speed of 300 mm / min, and the peel strength was measured.

[0144] <Bending test and flexing test> (Preparation of test specimens) The release liner was peeled off from the surface of the reinforcing film, and a 12.5 μm thick polyimide film (UBE's "Upilex 25RN") was attached to the surface of the adhesive layer using a hand roller. This laminate was cut into a size of 25 mm wide x 100 mm long, and the adhesive layer was photocured by irradiating ultraviolet light from the reinforcing film side (PET film substrate side) to obtain a test specimen.

[0145] (Low temperature bending test) In an environment with an ambient temperature of -20°C, the test piece was folded at the center of the long side with a bending radius of 6 mm around the short side as the bending axis, and then completely unfolded (to an extended state) 100 times.

[0146] (High temperature repeated bending test) Using a planar object no-load U-shaped expansion and contraction tester (manufactured by Yuasa System Equipment), a bending jig was attached to the short side of the test piece, and repeated bending tests were performed under the following conditions in a thermostatic chamber at a temperature of 60°C and a relative humidity of 50%, with the reinforcing film side (the PET film substrate side) facing inward. Bending radius: 3mm Bending angle: 180° Test speed: 1 second / cycle (60 rpm) Number of flexes: 200,000

[0147] (evaluation) After the test, the sample was left in a stretched state for 24 hours in an environment of 25° C. and 50% relative humidity, and then the test area was visually observed and evaluated according to the following criteria. A: No change from before the test, no creases found in the bent area B: Slight creases were observed in the bent area, but the reinforcing film was not peeled off. C: Slight creases were observed in the bent area, and the reinforcing film was peeled off at the end of the bent area. D: Clear creases are observed over the entire surface of the bent part, and the reinforcing film has peeled off along the creases.

[0148] Table 1 shows the type and amount of oligomer added to the pressure-sensitive adhesive of the reinforcing films of the Examples and Comparative Examples (amount of oligomer per 100 parts by weight of solid content of polymer A), and the evaluation results.

[0149] [Table 1]

[0150] In Comparative Example 1, which used an oligomer-free adhesive, the storage modulus at low temperature exceeded 100 kPa, and bending resistance at low temperature was insufficient. In addition, in Comparative Example 1, the creep deformation rate of the adhesive layer was small, and bending resistance at high temperature was also insufficient.

[0151] Comparative Example 4, which used a pressure-sensitive adhesive containing an oligomer (S-3011) having no polymerizable functional group, had a smaller storage modulus at low temperatures and better bending resistance at low temperatures than Comparative Example 1. However, in Comparative Example 4, the strain recovery rate of the pressure-sensitive adhesive layer was low, so bending resistance at high temperatures was insufficient.

[0152] Examples 1 to 5, which used a pressure-sensitive adhesive containing an acrylic oligomer (UP-1021) having an acryloyl group at the end of the main chain, had a smaller storage modulus at low temperatures and better bending resistance at low temperatures than Comparative Example 1. Furthermore, Examples 1 to 5 had a larger creep deformation rate than Comparative Example 1, a higher strain recovery rate than Comparative Example 4, and better bending resistance at high temperatures.

[0153] Comparative Example 2, in which the amount of the acrylic oligomer was 5 parts by weight, was insufficient in both bending resistance at low temperature and bending resistance at high temperature, similar to Comparative Example 1. Comparative Example 3, in which the amount of the acrylic oligomer was 40 parts by weight, was excellent in bending resistance at low temperature, but had a low strain recovery rate and insufficient bending resistance at high temperature. Furthermore, Comparative Example 3 showed a lower adhesive strength of the pressure-sensitive adhesive layer after photocuring than Examples 1 to 5.

[0154] Comparing Examples 1 to 5 with Comparative Examples 1 to 3, it was found that the greater the amount of acrylic oligomer (UP-1021) having an acryloyl group at the end of the main chain, the smaller the storage modulus at low temperatures, the better the bending resistance at low temperatures, and the larger the creep deformation rate at high temperatures and the smaller the strain recovery rate. The acrylic oligomer having an acryloyl group at the end of the main chain has a plasticizer-like effect of lowering the storage modulus at low temperatures and increasing the creep deformation rate at high temperatures, and is involved in photocuring together with the photocuring agent, so it is believed that the reduction in the strain recovery rate is suppressed compared to Comparative Example 4.

[0155] Example 6, which used an acrylic oligomer (UP-1000) with a large molecular weight, was excellent in bending resistance at low temperatures and flex resistance at high temperatures, similar to Examples 1 to 5. Comparing Example 6 with Example 3, it is clear that an acrylic oligomer with a smaller molecular weight has a greater effect of reducing the storage modulus at low temperatures and is advantageous in improving bending resistance.

[0156] In Comparative Example 5, which used an acrylic acrylate (OAP-5000) having an acryloyl group in the side chain as the oligomer, the creep deformation rate at high temperatures was increased compared to Comparative Example 1, which did not contain an oligomer, but the storage modulus at low temperatures was larger than that of Comparative Example 1, and the bending resistance at low temperatures and bending resistance at high temperatures were inferior. Since the acrylic acrylate has a large number of acryloyl groups and a small distance between crosslinking points, the movement of the molecular chain at low temperatures is suppressed in the adhesive layer after photocuring, and it is considered that the plasticization effect at low temperatures is smaller than when an oligomer having a polymerizable functional group at the end of the main chain is used.

[0157] From the above results, it can be seen that a pressure-sensitive adhesive composition containing, in addition to an acrylic base polymer and a photocuring agent, a predetermined amount of an acrylic oligomer having a polymerizable functional group at the end of the main chain has low adhesive strength before photocuring and exhibits excellent adhesive properties after photocuring, and further has a low storage modulus in the low temperature range and a high strain recovery rate, making it suitable as a reinforcing film for foldable devices. [Explanation of symbols]

[0158] 1 Film substrate 2 Adhesive layer 10 Reinforcement film 5 Release Liner 20 Adherent

Claims

1. The adhesive tape comprises a film substrate and a pressure-sensitive adhesive layer fixedly laminated on one main surface of the film substrate, the pressure-sensitive adhesive layer is made of a photocurable composition including an acrylic base polymer, a photocuring agent having two or more photopolymerizable functional groups, an acrylic oligomer having a weight-average molecular weight of 1,000 to 30,000, and a photopolymerization initiator; the acrylic base polymer includes, as a monomer unit, one or more selected from the group consisting of a hydroxy group-containing monomer and a carboxy group-containing monomer, and a crosslinked structure is introduced into the acrylic base polymer; The acrylic oligomer has a polymerizable functional group at an end of a main chain, The photocurable composition contains 6 to 35 parts by weight of the acrylic oligomer based on 100 parts by weight of the acrylic base polymer. Reinforcement film.

2. The reinforced film according to claim 1 , wherein the photocuring agent comprises a polyfunctional (meth)acrylate having an alkylene oxide chain.

3. The reinforced film of claim 2 , wherein the light hardener further comprises a urethane (meth)acrylate.

4. The reinforced film according to any one of claims 1 to 3, wherein the acrylic base polymer has a weight average molecular weight of 100,000 or more before the introduction of a crosslinked structure.

5. The reinforced film according to any one of claims 1 to 3, wherein the acrylic oligomer has a glass transition temperature of -30°C or lower.

6. The reinforcing film according to any one of claims 1 to 3, wherein the photocurable composition contains 3 to 25 parts by weight of the photocuring agent based on 100 parts by weight of the acrylic base polymer.

7. The reinforcing film according to any one of claims 1 to 3, wherein the adhesive layer has an adhesive strength to a polyimide film of 1 N / 25 mm or less before photocuring.

8. The reinforcing film according to any one of claims 1 to 3, wherein the adhesive layer has an adhesive strength to a polyimide film of 5 N / 25 mm or more after photocuring.

9. The reinforced film according to any one of claims 1 to 3, wherein the pressure-sensitive adhesive layer has a shear storage modulus of 100 kPa or less at -20°C after photocuring.

10. A method for producing a device with a reinforced film, in which a reinforced film is attached to a surface of a foldable device, comprising the steps of: The pressure-sensitive adhesive layer of the reinforcing film according to any one of claims 1 to 3 is attached to a surface of a foldable device, The pressure-sensitive adhesive layer is photocured. A method for manufacturing a device with a reinforcing film.

11. A device with a reinforcing film, in which a reinforcing film is attached to a surface of a foldable device, The reinforcing film includes a film substrate and a pressure-sensitive adhesive layer fixedly laminated on one main surface of the film substrate, The pressure-sensitive adhesive layer is attached to a surface of a device, the pressure-sensitive adhesive layer is a photocured product obtained by photocuring a photocurable pressure-sensitive adhesive composition including an acrylic base polymer, a photocuring agent having two or more photopolymerizable functional groups, and an acrylic oligomer having a weight-average molecular weight of 1,000 to 30,000 and a polymerizable functional group at an end of the main chain; The acrylic base polymer contains, as a monomer unit, one or more selected from the group consisting of a hydroxy group-containing monomer and a carboxy group-containing monomer, and a crosslinked structure is introduced into the acrylic base polymer. Device with reinforcement film.

12. The device with a reinforced film according to claim 11, wherein the pressure-sensitive adhesive layer has a shear storage modulus of 100 kPa or less at -20°C.

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