Reinforcing film, device with reinforcing film and method for manufacturing same

Abstract

The invention relates to a reinforced film, an apparatus with a reinforced film and a method of making the same. The reinforcing film (10) is provided with an adhesive layer (2) which is adhesively laminated on one main surface of a film substrate (1). The adhesive layer is formed from a photocurable composition containing an acrylic base polymer having a cross-linked structure, a photocuring agent, an acrylic oligomer having a polymerizable functional group at an end of a main chain, and a photopolymerization initiator. The reinforcing film of the present invention is easy to peel immediately after being bonded to an adherend. Furthermore, the reinforcing film according to the present invention can be firmly bonded to an adherend by photocuring the adhesive after being bonded to the adherend, and is less susceptible to deformation such as wrinkles and peeling from the adherend even when repeatedly bent and stretched.

Description

Technical Field

[0001] The present invention relates to a reinforcement film attached to a surface of a device and to a device having the reinforcement film and a method for manufacturing the same. Background Art

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

[0003] By temporarily bonding the adhesive film to the surface of the equipment or its components before use, such as during assembly, processing, and transportation of the equipment, damage or breakage of the adherend can be suppressed. Patent Documents 1 and 2 disclose a reinforcing film having an adhesive layer formed of a photocurable adhesive composition on a film substrate.

[0004] The adhesive of the reinforcement film has a high gel fraction and has low adhesiveness immediately after being attached to the adherend, so it is easy to peel off from the adherend. Therefore, it is possible to rework the adherend, and it is also possible to selectively peel off and remove the reinforcement film from the part of the adherend that does not need to be reinforced. The adhesive of the reinforcement film is firmly bonded to the adherend through photocuring, so that the film substrate is permanently bonded to the surface of the adherend, and can be used as a reinforcement material for surface protection of equipment, etc.

[0005] In recent years, organic EL (Electro-Luminescence) panels using bendable substrates (flexible substrates) such as resin films have been put into practical use, and foldable devices have also been put into practical use. In foldable devices, bending is repeated at the same position. At the bent part, compressive stress is applied to the inside and tensile stress is applied to the outside, so strain is generated at the bent part and its surroundings.

[0006] Patent document 3 proposes: using a photocurable adhesive containing an acrylic base polymer with a low glass transition temperature as an adhesive for a reinforcing film of a foldable device, thereby reducing the shear storage modulus at low temperatures, thereby relaxing the strain at the bending portion and inhibiting the peeling of the adhesive layer at the bending portion of the foldable device.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2020-41113

[0010] Patent Document 2: Japanese Patent Application Publication No. 2020-132851

[0011] Patent Document 3: International Publication No. 2022 / 050009

[0012] In order to suppress peeling and cracking caused by stress at the bending part when the foldable device is repeatedly folded (bent) and stretched in a low temperature environment, the adhesive layer of the reinforcing film is required to have a further low storage modulus. As a method for reducing the shear storage modulus at low temperatures, in addition to lowering the glass transition temperature of the base polymer, it is also considered to add a component such as an oligomer with a low glass transition temperature that acts as a plasticizer.

[0013] However, when oligomers are added, the gel fraction of the adhesive after photocuring is low and the cohesive force is small, so the adhesive force is sometimes insufficient. In addition, since the adhesive to which oligomers are added has low strain recovery, when the foldable device is restored to an open state (stretched state) after being kept in a folded state, the deformation of the adhesive layer cannot be restored and becomes wrinkled, which causes the peeling of the reinforcement film and deformation of the device. Summary of the invention

[0014] The object of the present invention is to provide a reinforcing film which is easy to peel off just after being attached to an adherend, and which can be firmly bonded to the adherend by photocuring the adhesive after being attached to the adherend, and which is not prone to deformation such as wrinkles or peeling even when a foldable device is repeatedly bent and stretched.

[0015] The reinforcing film of the present invention comprises an adhesive layer adhered to one main surface of a film substrate, wherein the adhesive layer is formed of a photocurable composition containing an acrylic base polymer, a photocuring agent, an acrylic oligomer, and a photopolymerization initiator.

[0016] The acrylic base polymer contains one or more monomer units selected from the group consisting of hydroxyl-containing monomers and carboxyl-containing monomers, and a crosslinking structure is introduced into the acrylic base polymer. Preferably, the weight average molecular weight of the acrylic base polymer before the crosslinking structure is introduced is 100,000 or more.

[0017] 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. Preferably, the glass transition temperature of the acrylic oligomer is -30°C or less. Preferably, the content of the acrylic oligomer in the photocurable composition constituting the adhesive layer is 6 to 35 parts by weight relative to 100 parts by weight of the acrylic base polymer.

[0018] As the photocuring agent, polyfunctional (meth)acrylate having an oxyalkylene chain is preferred. As the photocuring agent, in addition to polyfunctional (meth)acrylate having an oxyalkylene chain, urethane (meth)acrylate can also be used.

[0019] It is preferred that the pressure-sensitive adhesive layer have a shear storage modulus of 100 kPa or less at -20°C after photocuring.

[0020] Preferably, the adhesive force of the adhesive layer to the polyimide film before photocuring is 1 N / 25 mm or less. Preferably, the adhesive force of the adhesive layer to the polyimide film after photocuring is 5 N / 25 mm or more.

[0021] By attaching the above-mentioned reinforcement film to the surface of the device and light-curing the adhesive layer, a device with a reinforcement film can be obtained. The device can also be a bendable flexible device.

[0022] Effects of the Invention

[0023] The adhesive layer of the reinforcement film of the present invention is formed of a photocurable composition, and the adhesive layer is photocured after bonding to the adherend to increase the adhesive strength to the adherend. Since the adhesive strength to the adherend is low before photocuring, it is easy to peel off from the adherend.

[0024] The adhesive layer of the reinforcing film has a small shear storage modulus at low temperature, excellent stress relaxation, and high strain recovery, so even when the same part is repeatedly bent and stretched, it is not easy to cause deformation such as wrinkles or peeling from the adherend. Therefore, the reinforcing film of the present invention can also be applied to foldable devices using a resin film substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a cross-sectional view showing the stacked structure of the reinforcing film.

[0026] Figure 2 It is a cross-sectional view showing the stacked structure of the reinforcing film.

[0027] Figure 3 It is a cross-sectional view showing a device with a reinforcement film attached.

[0028] Description of reference numerals:

[0029] 1: film substrate; 2: adhesive layer; 10: reinforcing film; 5: release liner; 20: adherend. DETAILED DESCRIPTION

[0030] Figure 1 1 is a cross-sectional view showing an embodiment of a reinforcing film. The reinforcing film 10 has an adhesive layer 2 on one main surface of a film substrate 1. The adhesive layer 2 is laminated on one main surface of the film substrate 1. The adhesive layer 2 is a photocurable adhesive formed of a photocurable composition, and is cured by irradiation with active light such as ultraviolet rays, thereby increasing the bonding strength with the adherend.

[0031] Figure 2It is a cross-sectional view of a reinforcing film in which a release liner 5 is temporarily bonded to the main surface of the pressure-sensitive adhesive layer 2 . Figure 3 It is a cross-sectional view showing a state where the reinforcing film 10 is attached to the surface of the device 20 .

[0032] The release liner 5 is peeled off 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 is in a state where the reinforcement film 10 (adhesive layer 2) is temporarily attached to the device 20 before photocuring. By photocuring the adhesive layer 2, the adhesive force at the interface between the device 20 and the adhesive layer 2 increases, and the device 20 and the reinforcement film 10 are bonded.

[0033] “Adhesion” refers to a state where two stacked layers are firmly bonded and cannot be peeled off or are difficult to peel off at the interface between the two layers. “Temporary adhesion” refers to a state where the adhesive force between two stacked layers is weak and can be easily peeled off at the interface between the two layers.

[0034] exist Figure 2 In the reinforcement film shown, the film substrate 1 and the adhesive layer 2 are bonded, and the release liner 5 is temporarily bonded to the adhesive layer 2. When the film substrate 1 and the release liner 5 are peeled off, 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 bonded to the film substrate 1. No adhesive remains on the release liner 5 after peeling.

[0035] Figure 3 In the device with a reinforcement film shown, before the adhesive layer 2 is photocured, the device 20 and the adhesive layer 2 are temporarily bonded. When the film substrate 1 and the device 20 are peeled off, the adhesive layer 2 and the device 20 are peeled off at the interface, so that the adhesive layer 2 is kept bonded to the film substrate 1. Since no adhesive remains on the device 20, it is easy to perform peeling operations such as rework and cutting. After the adhesive layer 2 is photocured, the adhesive force between the adhesive layer 2 and the device 20 increases and becomes bonded, so it is not easy to peel the reinforcement film 10 from the device 20.

[0036] [Film base material]

[0037] 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 adhesive layer 2, it is preferred that the surface of the film substrate 1 on which the adhesive layer 2 is to be attached is not subjected to a mold release treatment.

[0038] The thickness of the film substrate is, for example, about 4 to 150 μm. From the viewpoint of strengthening the device by imparting rigidity, mitigating impact, etc., the thickness of the film substrate 1 is preferably 5 μm or more, more preferably 12 μm or more, further preferably 20 μm or more, and particularly preferably 25 μm or more. From the viewpoint of making the reinforced film flexible and foldable, the thickness of the film substrate 1 is preferably 125 μm or less, more preferably 100 μm or less. From the viewpoint of taking into account both mechanical strength and flexibility, the compressive strength of the film substrate 1 is preferably 100 to 3000 kg / cm 2 , more preferably 200 to 2900 kg / cm 2 , more preferably 300 to 2800 kg / cm 2 , particularly preferably 400 to 2700 kg / cm 2 .

[0039] As the plastic material constituting the film substrate 1, polyester resins, polyolefin resins, cyclic polyolefin resins, polyamide resins, polyimide resins, polyetheretherketone, polyethersulfone, polyarylate resins, aromatic polyamide resins, etc. can be cited. In the reinforcing film for optical devices such as displays, the film substrate 1 is preferably a transparent film. In addition, in the case where the adhesive layer 2 is photocured by irradiating active light from the film substrate 1 side, the film substrate 1 preferably has transparency to the active light used in the curing of the adhesive layer. Because of both mechanical strength and transparency, polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, transparent polyimide, and transparent aromatic polyamide are preferably used. In the case of irradiating active light from the adherend side, as long as the adherend has transparency to the active light, the film substrate 1 can also be opaque to the active light.

[0040] Functional coatings such as an easy-adhesion layer, an easy-slip layer, a release layer, an antistatic layer, a hard coating layer, and an antireflection layer may also be provided on the surface of the film substrate 1. It should be noted that, as described above, in order to make the film substrate 1 and the adhesive layer 2 adhere firmly, it is preferred that a release layer is not provided on the surface of the film substrate 1 on which the adhesive layer 2 is attached.

[0041] [Adhesive layer]

[0042] The adhesive layer 2 fixedly laminated on the film substrate 1 is formed of a photocurable composition. The photocurable composition constituting the adhesive layer 2 contains an acrylic base polymer, a photocuring agent, a photopolymerization initiator, and further contains an acrylic oligomer.

[0043] The adhesive layer 2 has a small adhesive force to the adherends such as equipment and equipment parts before photocuring, so it is easy to peel off. The adhesive layer 2 improves the adhesive force to the adherend through photocuring, so even when the equipment is used, the reinforcement film is not easy to peel off from the equipment surface, and the bonding reliability is excellent.

[0044] Photocurable adhesives are basically not cured under normal storage conditions, but are cured by irradiation with active light such as ultraviolet rays. Therefore, the reinforcing film of the present invention can arbitrarily set the timing of curing the adhesive layer 2, which has the advantage of being able to flexibly cope with the lead time of the process.

[0045] <Base polymer>

[0046] The base polymer is the main constituent of the adhesive composition and is the main factor determining the adhesive layer's adhesive strength, shear storage modulus and other properties. In the present invention, an acrylic polymer is used as the base polymer of the adhesive. Acrylic polymers have excellent optical transparency and adhesion, and are easy to control adhesive strength, shear storage modulus and the like.

[0047] The acrylic base polymer contains an alkyl (meth)acrylate as a monomer component. In addition, in this specification, "(meth)acrylic acid" means acrylic acid and / or methacrylic acid.

[0048] As the alkyl (meth)acrylate, it is preferred to use an alkyl (meth)acrylate having an alkyl group with 1 to 20 carbon atoms. From the viewpoint of suppressing the peeling of the adhesive layer during repeated bending by lowering the glass transition temperature of the acrylic base polymer and lowering the shear storage modulus, it is preferred that the alkyl group of the alkyl (meth)acrylate is a chain alkyl group. The chain alkyl group may be a straight chain or a branched chain.

[0049] Examples of the alkyl (meth)acrylate having a chain alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl (meth)acrylate, tert-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, nonyl (meth)acrylate, and (meth)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, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, and the like.

[0050] Specific examples of the alkyl (meth)acrylate having an alicyclic alkyl group include: cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, cyclooctyl (meth)acrylate and the like; (meth)acrylate having a bicyclic aliphatic hydrocarbon ring such as isobornyl (meth)acrylate; (meth)acrylate having a tricyclic aliphatic hydrocarbon ring such as dicyclopentyl (meth)acrylate, dicyclopentyloxyethyl (meth)acrylate, tricyclopentyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, 2-ethyl-2-adamantyl (meth)acrylate and the like. The alkyl (meth)acrylate having an alicyclic alkyl group may be an alkyl (meth)acrylate having a substituent on the ring such as 3,3,5-trimethylcyclohexyl (meth)acrylate. Furthermore, the (meth)acrylate alkyl ester having an alicyclic alkyl group may be a (meth)acrylate containing a condensed ring of an alicyclic structure and a ring structure having an unsaturated bond, such as dicyclopentenyl (meth)acrylate.

[0051] Among the alkyl (meth)acrylates exemplified, from the viewpoint of lowering the glass transition temperature of the acrylic base polymer, C (meth)acrylate is preferred. 1－9 The alkyl ester is preferably an alkyl (meth)acrylate whose homopolymer has a glass transition temperature of -50°C or less. The glass transition temperature of the homopolymer of the alkyl (meth)acrylate is more preferably -55°C or less, and further preferably -60°C or less. 1－9 Specific 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 them, (meth)acrylic acid C in which the number of carbon atoms of the alkyl group is 6 or more is preferred. 1－9 As the alkyl ester, 2-ethylhexyl acrylate and n-octyl acrylate are particularly preferred.

[0052] The content of the alkyl (meth)acrylate 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, based on 100 parts by weight of the total monomer components constituting the acrylic base polymer.

[0053] In addition to (meth) alkyl acrylate, acrylic acid base polymer also includes monomers with crosslinkable functional groups as constituent monomer components. As monomers with crosslinkable functional groups, hydroxyl-containing monomers and carboxyl-containing monomers can be listed. Acrylic acid base polymer can have both hydroxyl-containing monomers and carboxyl-containing monomers as copolymerization components, or can have only one of them as copolymerization components. By introducing a crosslinking structure into the acrylic acid base polymer, there is a tendency that cohesive force is improved and the peelability of the adhesive layer 2 before photocuring from the adherend is improved.

[0054] Examples of hydroxyl-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, and 4-(hydroxymethyl)cyclohexylmethyl (meth)acrylate. Among them, 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate are preferred because they contribute greatly to improving the adhesive strength of the adhesive after photocuring.

[0055] Examples of the carboxyl group-containing monomer include (meth)acrylic acid, 2-carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, etc. Among them, acrylic acid is particularly preferred because it contributes greatly to improving adhesive strength.

[0056] The amount of the monomer having a crosslinkable functional group (the total amount of the hydroxyl group-containing monomer and the carboxyl group-containing monomer) is preferably 0.5 to 15 parts by weight, more preferably 1 to 10 parts by weight, and further preferably 1.5 to 7 parts by weight relative to a total of 100 parts by weight of the constituent monomer components of the acrylic base polymer. When the acrylic base polymer contains a hydroxyl group, there is a tendency that the strain recovery rate of the adhesive layer is large and the shape recovery is high, 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.

[0057] The acrylic base polymer may contain nitrogen-containing monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, N-acryloylmorpholine, N-vinylcarboxylic acid amides, and N-vinylcaprolactam as constituent monomer components.

[0058] The acrylic base polymer may contain (meth)acrylate having an oxyalkylene chain as a constituent monomer component. The (meth)acrylate having an oxyalkylene chain is represented by the following general formula (1).

[0059] CH2=CR 1 -COO-(R 2 -O) m -R 3 (1)

[0060] R in the general formula (1) 1 is a hydrogen atom or a methyl group, R 1 The compound of formula (1) in which R is a hydrogen atom is an acrylate, 1 The compound of formula (1) which is a methyl group is a methacrylate.

[0061] R in the general formula (1) 2 is an alkylene group such as ethylene, propylene, butylene, etc., -R 2 -O- is an oxyalkylene chain. 2 Specific examples of -O- include ethylene oxide (-CH2CH2-O-), propylene oxide (-CH(CH3)CH2-O-), and butylene oxide (-CH2CH2CH2CH2-O-).

[0062] In the general formula (1), m is the number of repetitions of the oxyalkylene unit and is an integer greater than 1. m is preferably 1 to 5. When m is too large, the (meth)acrylate compound represented by the general formula (1) tends to act as a chain transfer agent, so the molecular weight of the acrylic base polymer as a polymer is not sufficiently increased, and the adhesive strength of the adhesive may be insufficient. 3 R is 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 An alkyl group having 1 to 3 carbon atoms is preferred, and a methyl group or an ethyl group is particularly preferred.

[0063] Specific examples of the compound represented by the general formula (1) include methoxyethyl acrylate, phenoxyethyl acrylate, ethoxyethoxyethyl acrylate, methoxytriethylene glycol acrylate, and the like.

[0064] The acrylic base polymer may contain monomer components other than those mentioned above. For example, the acrylic base polymer may contain vinyl ester monomers, aromatic vinyl monomers, epoxy group-containing monomers, vinyl ether monomers, sulfone group-containing monomers, phosphoric acid group-containing monomers, acid anhydride group-containing monomers, etc. as monomer components.

[0065] The glass transition temperature of the acrylic base polymer is preferably below -40°C, more preferably below -45°C, and may be below -50°C or below -55°C. By making the glass transition temperature sufficiently lower than the ambient temperature of the device, the shear storage modulus of the adhesive layer within the temperature range of the ambient temperature is small, and there is a tendency for peeling to be suppressed during repeated bending. The lower limit of the glass transition temperature of the acrylic base polymer is not particularly limited, but is generally above -80°C, and may be above -75°C or above -70°C.

[0066] The glass transition temperature is the temperature (peak temperature) at which the loss tangent tanδ in the viscoelasticity measurement reaches a maximum. The theoretical Tg calculated according to the Fox equation can be used instead of the glass transition temperature obtained by the viscoelasticity measurement. The theoretical Tg is calculated according to the following Fox equation, which is the glass transition temperature Tg of the homopolymer of the monomer component of the acrylic base polymer. i and the weight fraction W of each monomer component i to calculate.

[0067] 1 / Tg=Σ(W i / Tg i )

[0068] Tg is the glass transition temperature of the polymer chain (unit: K), W i is the weight fraction of monomer component i constituting the chain segment (copolymerization ratio based on weight), Tg i is the glass transition temperature of the homopolymer of monomer component i (unit: K). As the glass transition temperature of the homopolymer, the value described in the third edition of Polymer Handbook (John Wiley & Sons, Inc., 1989) can be used. The Tg of the homopolymer of the monomer not described in the above literature can be the peak temperature of tan δ obtained by dynamic viscoelasticity measurement.

[0069] The above-mentioned 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 perspective of the balance of characteristics such as the adhesion and holding power of the adhesive, and the cost, solution polymerization is preferred. As a solvent for solution polymerization, ethyl acetate, toluene, etc. can be used. The solution concentration is usually about 20 to 80% by weight. As a polymerization initiator, various known polymerization initiators such as azo and peroxide can be used. In order to adjust the molecular weight, a chain transfer agent can be used. The reaction temperature is usually about 50 to 80°C, and the reaction time is usually about 1 to 8 hours.

[0070] 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 crosslinking structure is introduced into the acrylic base polymer, the molecular weight of the acrylic base polymer refers to the molecular weight before the crosslinking structure is introduced.

[0071] <Cross-linking agent>

[0072] From the viewpoint of making the adhesive have appropriate cohesive force, showing adhesive strength and ensuring that the adhesive layer before light curing is peelable from the adherend, it is preferred to introduce a crosslinked structure into the acrylic base polymer. For example, a crosslinking agent is added to the solution after the acrylic base polymer is polymerized, and heating is performed as required to introduce a crosslinked structure. The crosslinking agent has two or more crosslinking functional groups in one molecule. The crosslinking agent can also be a crosslinking agent having three or more crosslinking functional groups in one molecule.

[0073] As the crosslinking agent, isocyanate crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, metal chelate crosslinking agents, etc. can be cited. These crosslinking agents react with functional groups such as hydroxyl and carboxyl introduced into the acrylic base polymer to form a crosslinked structure. From the aspect of high reactivity with the hydroxyl and carboxyl of the acrylic base polymer and easy introduction of the crosslinked structure, isocyanate crosslinking agents and epoxy crosslinking agents are preferred. When the acrylic base polymer has a hydroxyl group as a crosslinkable functional group, an isocyanate crosslinking agent is preferred, and when the acrylic base polymer has a carboxyl group as a crosslinkable functional group, an epoxy crosslinking agent is preferred.

[0074] As the isocyanate-based crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule is used. The isocyanate-based crosslinking agent may be an isocyanate-based crosslinking agent having three or more isocyanate groups in one molecule. Examples of the isocyanate crosslinking agent include 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-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylene diisocyanate; trimethylolpropane / toluene diisocyanate trimer adducts (e.g., "TAKENATE D101E" manufactured by Mitsui Chemicals), trimethylolpropane / hexamethylene diisocyanate trimer adducts (e.g., "Coronate HL" manufactured by Tosoh), trimethylolpropane adducts of xylene diisocyanate (e.g., "TAKENATE D110N" manufactured by Mitsui Chemicals), isocyanurates of hexamethylene diisocyanate (e.g., "Coronate 101" manufactured by Tosoh), and isocyanurates of toluene diisocyanate (e.g., "Coronate 101" manufactured by Tosoh). As the isocyanate crosslinking agent, an isocyanate compound having a biuret group (for example, "Duranate 24A-100" manufactured by Asahi Chemical Industry Co., Ltd.) or an isocyanate compound having an allophanate group can be used.

[0075] 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 be an epoxy crosslinking agent having 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 the epoxy crosslinking agent include N,N,N',N'-tetraglycidyl-m-xylylenediamine, diglycidyl aniline, 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, phthalic acid diglycidyl ester, triglycidyl-tris(2-hydroxyethyl)isocyanurate, resorcinol diglycidyl ether, and bisphenol-S-diglycidyl ether. As the epoxy crosslinking agent, commercially available products such as "DENACOL" manufactured by Nagase ChemteX, and "TETRAD X" and "TETRAD C" manufactured by Mitsubishi Gas Chemical can also be used.

[0076] The amount of the crosslinking agent used may be appropriately adjusted according to the composition and molecular weight 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 also 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.

[0077] In order to promote the formation of a cross-linked structure, a cross-linking catalyst can be used. As a cross-linking catalyst, organic metal compounds such as organic metal complexes (chelates), compounds of metals and alkoxy groups, and compounds of metals and acyloxy groups; and tertiary amines, etc. can be listed. In particular, from the perspective of suppressing the cross-linking reaction in a solution state at room temperature and ensuring the shelf life of the adhesive composition, organic metal compounds are preferred. As metals of organic metal compounds, iron, tin, aluminum, zirconium, zinc, titanium, lead, cobalt, etc. can be listed. The amount of the cross-linking catalyst used is usually 0.5 parts by weight or less relative to 100 parts by weight of the acrylic base polymer.

[0078] <Light curing agent>

[0079] The adhesive composition constituting the adhesive layer 2 contains, in addition to the acrylic base polymer, a compound having two or more photopolymerizable functional groups in one molecule as a photocuring agent. The adhesive composition containing the photocuring agent has photocurability, and when it is attached to an adherend and then photocured, the adhesive strength to the adherend is improved.

[0080] As the photopolymerizable functional group, a functional group having polymerizability based on a photoradical reaction is preferred. As the photocuring agent, a compound having two or more ethylenically unsaturated bonds in one molecule is preferred. In view of high compatibility with an acrylic base polymer, a polyfunctional (meth)acrylate is preferred.

[0081] The multifunctional (meth)acrylate is typically an ester of a polyol and (meth)acrylic acid. Specific examples of the multifunctional (meth)acrylate include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, alkane diol 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, trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerol di(meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, isoprene (meth)acrylate, and the like.

[0082] The multifunctional (meth)acrylate may be an ester of a polyol and (meth)acrylic acid modified by an oxyalkylene group. Examples of the oxyalkylene group include oxyethylene (EO) and oxypropylene (PO). The oxyalkylene group may be a polyoxyalkylene group such as polyethylene glycol or polypropylene glycol.

[0083] Specific examples of the oxyalkylene-modified multifunctional (meth)acrylates include bisphenol A oxyethylene-modified di(meth)acrylate, bisphenol A oxypropylene-modified di(meth)acrylate, trimethylolpropane oxyethylene-modified tri(meth)acrylate, trimethylolpropane oxypropylene-modified tri(meth)acrylate, isocyanuric acid oxyethylene-modified di(meth)acrylate, isocyanuric acid oxypropylene-modified di(meth)acrylate, isocyanuric acid oxyethylene-modified tri(meth)acrylate, isocyanuric acid oxypropylene-modified tri(meth)acrylate, pentaerythritol oxyethylene-modified tetra(meth)acrylate, and pentaerythritol oxypropylene-modified tetra(meth)acrylate.

[0084] In the polyfunctional (meth)acrylates containing oxyalkylene chains such as polyalkylene glycol di(meth)acrylate and oxyalkylene-modified polyfunctional (meth)acrylate, as oxyalkylene, (poly)oxyethylene or (poly)oxypropylene is preferred, and (poly)oxyethylene is particularly preferred. The chain length n of the oxyalkylene (the number of repeating units of the oxyalkylene) is about 1 to 15. When a plurality of oxyalkylene chains are contained in one molecule, the preferred average chain length n is 1 to 15. The (average) chain length n of the oxyalkylene chain can 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 oxyalkylene, the compatibility with the acrylic base polymer can be adjusted to an appropriate range.

[0085] The polyfunctional (meth)acrylate having an oxyalkylene chain tends to have lower compatibility with the acrylic base polymer as the chain length n of the oxyalkylene chain increases, and has lower compatibility with the acrylic base polymer when n is 4 or more than the acrylic oligomer described later. Therefore, the polyfunctional (meth)acrylate having an oxyalkylene chain, and the polyalkylene glycol di(meth)acrylate having an oxyalkylene chain length n of 4 or more, tends to be preferentially present on the surface of the adhesive layer (near the bonding interface with the adherend), and tends to form an adhesion barrier layer (Weak Boundary Layer; WBL) due to the photocuring agent preferentially present at the bonding interface with the adherend.

[0086] When WBL is formed, the liquid properties of the surface (bonding interface) become stronger while maintaining the main properties such as the shear storage modulus of the adhesive layer, so there is a tendency for the adhesive force to become smaller with the adherend, and the adhesive layer before photocuring is easily peeled off from the adherend. When the adhesive layer with WBL formed by photocuring is subjected to photocuring, the curing reaction of the photocuring agent is easy to proceed near the bonding interface where the density of the photocuring agent is high, so the cohesive force near the bonding interface is easy to increase, and the bonding force is easy to increase. In addition, when WBL is formed, there is a tendency for the increase of the shear storage modulus, which is a main property, to be suppressed in the adhesive after photocuring.

[0087] The polyfunctional (meth)acrylate having an oxyalkylene chain with a chain length n of 3 or less is highly compatible with the acrylic base polymer and thus is less likely to form a WBL. On the other hand, the polyfunctional (meth)acrylate having a small chain length n of the oxyalkylene chain has a small functional group equivalent of the (meth)acryloyl group, so the crosslinking point density of the adhesive layer after photocuring is high, and there is a tendency for the adhesive force to increase.

[0088] As described above, as a photocuring agent, when a multifunctional (meth)acrylate having an oxyalkylene chain with a large chain length n (especially n is 4 or more) is used, there is a tendency for the adhesive force of the adhesive layer before photocuring to become smaller, and when a multifunctional (meth)acrylate having an oxyalkylene chain with a small chain length n (especially n is 3 or less) is used, there is a tendency for the adhesive force of the adhesive layer after photocuring to become larger. For the purpose of adjusting the adhesive force of the adhesive layer before and after photocuring, a variety of multifunctional (meth)acrylates having oxyalkylene chains with different chain lengths n can be used. For example, a multifunctional (meth)acrylate having an oxyalkylene chain with a chain length n of 3 or less and a multifunctional (meth)acrylate having an oxyalkylene chain with a chain length n of 4 or more can be used in combination. In addition, two or more multifunctional (meth)acrylates having an oxyalkylene chain with a chain length n of 3 or less can be used, and two or more multifunctional (meth)acrylates having an oxyalkylene chain with a chain length n of 4 or more can also be used.

[0089] From the viewpoint of maintaining appropriate compatibility with acrylic base polymers, the molecular weight of the multifunctional (meth) acrylate as a photocuring agent, the multifunctional (meth) acrylate containing an oxyalkylene chain therein is preferably 1500 or less, more preferably 1000 or less, and can be 800 or less, 500 or less, or 400 or less. From the viewpoint of taking into account the compatibility with acrylic base polymers and the improvement of the adhesive force after photocuring, the functional group equivalent (g / eq) of the multifunctional (meth) acrylate is preferably 500 or less, more preferably 400 or less, and can be 300 or less, 250 or less, 200 or less, 180 or less, or 160 or less. On the other hand, when the functional group equivalent of the multifunctional (meth) acrylate is too small, the crosslinking point density of the adhesive layer after photocuring becomes high, so the shear storage modulus becomes large, and sometimes the adhesion in the low temperature region decreases. Therefore, the functional group equivalent of the photocuring agent is preferably 80 or more, more preferably 100 or more, and can also be 120 or more or 130 or more.

[0090] Two or more photocuring agents may be used in combination. For example, as photocuring agents, two or more multifunctional (meth)acrylates having oxyalkylene chains may be used, or multifunctional (meth)acrylates having oxyalkylene chains and multifunctional (meth)acrylates without oxyalkylene chains may be used. In addition, as photocuring agents, multifunctional (meth)acrylates having different numbers of functional groups (the number of (meth)acryloyl groups in one molecule) may also be used. For example, for the purpose of adjusting the adhesion, shear storage modulus, etc. of the adhesive layer after photocuring, as photocuring agents, difunctional (meth)acrylates and trifunctional or higher multifunctional (meth)acrylates may be used in combination.

[0091] As the photocuring agent, polyfunctional (meth)acrylates and urethane (meth)acrylates having an oxyalkylene chain can be used. Urethane (meth)acrylates are compounds having one or more urethane bonds and two or more (meth)acryloyl groups in one molecule, preferably containing two or more urethane bonds in one molecule.

[0092] By including carbamate (meth) acrylate as a photocuring agent in addition to the multifunctional (meth) acrylate having an oxyalkylene chain, the adhesive force of the adhesive layer before photocuring sometimes becomes smaller, and the adhesive force of the adhesive layer after photocuring becomes larger. It is believed that the compatibility behavior of carbamate (meth) acrylate with the acrylic base polymer is different from that of the multifunctional (meth) acrylate having an oxyalkylene chain, so the inclusion of carbamate (meth) acrylate to promote the formation of WBL and the like contributes to the reduction of the adhesive force of the adhesive layer before photocuring and the improvement of the adhesive force of the adhesive layer after photocuring.

[0093] The carbamate (meth) acrylate having two or more carbamate bonds is obtained, for example, by the reaction of a polyisocyanate with a (meth) acrylic compound having a hydroxyl group, wherein the isocyanate group of the polyisocyanate is bonded to the hydroxyl group of the (meth) acrylic compound to form a carbamate bond. The carbamate (meth) acrylate having two or more carbamate bonds can also be obtained by the following method: a prepolymer having an isocyanate group at the end is prepared by reacting a polyisocyanate with a polyol, and the (meth) acrylic compound having a hydroxyl group is bonded to the isocyanate group at the end of the prepolymer. In addition, a carbamate (meth) acrylate having two or more carbamate 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.

[0094] The polyisocyanate may be any one of an aromatic polyisocyanate, an aliphatic polyisocyanate and an alicyclic polyisocyanate. As the aromatic polyisocyanate, toluene diisocyanate (TDI) is particularly preferred. Toluene diisocyanate may be any one of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, 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.

[0095] The polyisocyanate may be a trifunctional isocyanate having a cyanuric acid skeleton. Examples of the trifunctional isocyanate having a cyanuric acid skeleton include TDI trimer and HDI trimer. The polyisocyanate may be a polyisocyanate having a biuret group or an allophanate group.

[0096] 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.

[0097] Examples of the (meth)acrylic compound having a hydroxyl group include compounds having one hydroxyl group and one (meth)acryloyl group, such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, hydroxymethacrylic acid amide, and hydroxyethylacrylic acid amide; and compounds having one hydroxyl 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.

[0098] The urethane (meth) acrylate obtained by the reaction of 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, and may be 12 or less or 10 or less.

[0099] From the viewpoint of reducing the adhesive force of the adhesive layer before photocuring and increasing the adhesive force 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, further preferably 1000 to 7000, and may be 1200 to 5000 or 1500 to 4000.

[0100] As the urethane (meth)acrylate, commercially available products such as Kyoeisha Chemical, Shin-Nakamura Chemical, Negami Industries, Mitsubishi Chemical, DAICEL-ALLNEX, and Resonac can be used.

[0101] The smaller the functional group equivalent of the photocuring agent and the more the content of the photocuring agent, the higher the crosslinking density based on photocuring, so there is a tendency for the shear storage modulus of the adhesive layer after photocuring to increase. From the viewpoint of improving the bonding strength of the adhesive after photocuring and suppressing the excessive increase in the shear storage modulus, 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 also 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, as long as a WBL is formed on the surface (bonding interface) of the adhesive layer by adjusting the compatibility of the acrylic base polymer and the photocuring agent, even if the photocuring agent is a small amount, the bonding strength is easily increased due to photocuring, and a high bonding strength can be achieved.

[0102] In the case where the photocurable composition constituting the adhesive layer also includes carbamate (meth) acrylate as a photocuring agent in addition to the multifunctional (meth) acrylate (especially the multifunctional (meth) acrylate with an oxyalkylene chain) without carbamate bonds, the content of carbamate (meth) acrylate is preferably 0.01 weight part or more relative to 100 weight parts of acrylic base polymers, more preferably 0.1 weight part or more, further preferably 0.15 weight part or more, and can also be 0.2 weight part or more, 0.3 weight part or more, 0.4 weight part or more or 0.5 weight part or more. In the case where the content of carbamate (meth) acrylate is too much, it may become the cause of the contamination of the adherend caused by the oozing of the photocuring agent to the adhesive layer surface (the bonding interface with the adherend). In addition, in the case where the amount of carbamate (meth) acrylate is too much, there is a tendency that the adhesive strength of the adhesive based on photocuring is insufficient. Therefore, the content of the urethane (meth)acrylate in the 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, based on 100 parts by weight of the acrylic base polymer.

[0103] <Acrylic oligomer>

[0104] 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), preferably a vinyl group or a (meth) acryloyl group. From the viewpoint of photocurability, 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 in the same manner as the photocuring agent.

[0105] The acrylic oligomer contains an alkyl (meth)acrylate as a main monomer component, and the acrylic oligomer is a component having a weight average molecular weight smaller than that of the acrylic base polymer. The weight average molecular weight of the acrylic oligomer is about 1,000 to 30,000, preferably 20,000 or less, more preferably 10,000 or less, further preferably 5,000 or less, and may be 3,000 or less, 2,500 or less, or 2,000 or less. The weight average molecular weight of the acrylic oligomer may also be 1,200 or more, 1,400 or more, or 1,500 or more.

[0106] The smaller the molecular weight of acrylic acid oligomer, the stronger the effect as plasticizer, and there is a tendency that the shear storage modulus of the adhesive layer after light curing tapers. In addition, the smaller the molecular weight of acrylic acid oligomer, the smaller the distance between crosslinking points, and therefore there is a tendency that the strain recovery rate under high temperature tapers. On the other hand, when the molecular weight of acrylic acid oligomer is lower than 1000, because the distance between crosslinking points is small, there is a tendency that the viscosity of adhesive is reduced, and the creep deformation rate tapers.

[0107] As the monomer components constituting the acrylic oligomer, the monomers exemplified above as the monomer components constituting the acrylic base polymer can be cited. The acrylic oligomer can contain crosslinkable functional groups in the same manner as the acrylic base polymer. However, when a crosslinking structure is introduced into the hydroxyl group and the carboxyl group of the acrylic oligomer, there is a tendency that the effect as a plasticizer is reduced. In addition, when the amount of polar functional groups such as hydroxyl group and carboxyl group introduced is large, there is a tendency that the cohesive force becomes larger and the shear storage modulus at low temperature becomes larger. Therefore, it is preferred that the amount of polar groups of the acrylic oligomer is small. Relative to a total of 100 parts by weight of the monomer components constituting the acrylic oligomer, the content of the (meth) alkyl acrylate is preferably 80 parts by weight or more, more preferably 90 parts by weight or more, and further preferably 95 parts by weight or more.

[0108] In addition to the ends of the main chain, acrylic oligomers may also have polymerizable functional groups such as (meth)acryloyl groups in the side chains. However, acrylic acid-acrylates containing polymerizable functional groups in the side chains have little effect on reducing the shear storage modulus of the adhesive layer after photocuring because the (meth)acryloyl groups in the side chains participate in the photocuring reaction. Therefore, acrylic oligomers preferably do not have polymerizable functional groups in the side chains.

[0109] From the viewpoint of reducing the shear storage modulus of the adhesive layer after photocuring at low temperature, the glass transition temperature of the acrylic oligomer is preferably -30°C or less, more preferably -40°C or less, further preferably -50°C or less, and may be -55°C or less, -65°C or less, or -70°C or less. The lower limit of the glass transition temperature of the acrylic oligomer is not particularly limited, but is usually -85°C or more, and may be -80°C or more.

[0110] The acrylic oligomer having a (meth)acryloyl group at the end of the main chain is obtained, for example, by a high-temperature continuous polymerization method. In the high-temperature continuous polymerization method, a reactor capable of being pressurized is set at about 150 to 300° C., a reaction solution mixed with a monomer and a solvent as required is continuously supplied to the reactor, a polymerization solution equal to the amount of the reaction solution supplied is extracted from the reactor, and unreacted monomers (and solvents) are removed from the polymerization solution by distillation. In the high-temperature continuous polymerization method, since a thermal polymerization initiator is not required, a polymer having an unreacted polymerizable functional group at the end can be obtained. For details of the high-temperature continuous polymerization method, reference can be made to Japanese Patent Publication No. 57-502171, Japanese Patent Publication No. 59-6207, and Japanese Patent Publication No. 60-215007. As an acrylic oligomer having a (meth)acryloyl group at the end of the main chain, commercially available products such as the "ARUFON UP-1000" series manufactured by Toa Gosei can be used.

[0111] The acrylic oligomer having a polymerizable functional group at the end of the main chain is the same as the above-mentioned multifunctional (meth)acrylate and urethane (meth)acrylate having an oxyalkylene chain as a photocuring agent in that it has 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 the acrylic oligomer are alkyl (meth)acrylates, the acrylic oligomer has a higher compatibility with the acrylic base polymer than the multifunctional (meth)acrylate and urethane (meth)acrylate having an oxyalkylene chain, and has a smaller effect of reducing the adhesive force of the adhesive layer before photocuring.

[0112] Since acrylic acid oligomers have a low glass transition temperature, they have a strong effect as plasticizers and have the effect of reducing the shear storage modulus of the adhesive layer after photocuring at low temperatures. Acrylic acid oligomers have polymerizable functional groups at the ends and participate in the curing reaction together with the photocuring agent, so they become gel components (insoluble components) in the adhesive layer after photocuring. Therefore, even if acrylic acid oligomers are added, the gel fraction of the adhesive layer after photocuring is not easy to decrease, and the strain recovery at high temperatures can be maintained.

[0113] The content of the acrylic oligomer in the 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 also be 12 to 20 parts by weight relative to 100 parts by weight of the acrylic base polymer. The more the amount of acrylic oligomer, the smaller the shear storage modulus of the adhesive layer at low temperature after photocuring, and there is a tendency for the creep deformation rate at high temperature to increase. When the amount of acrylic oligomer is too much, there is a tendency for the strain recovery rate at high temperature to decrease. In addition, when the amount of acrylic oligomer is too much, sometimes the adhesive force of the adhesive layer after photocuring will not fully increase.

[0114] <Photopolymerization initiator>

[0115] 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 according to the type of the photocuring agent. When a multifunctional acrylate is used as a photocuring agent, a photoradical initiator is preferably used. As the photoradical initiator, a photoradical generator that generates free radicals by cleavage of visible light or ultraviolet rays with a wavelength shorter than 450nm is preferred, and hydroxy ketones, benzil dimethyl ketals, amino ketones, acyl phosphine oxides, benzophenones, trichloromethyl-containing triazine derivatives, etc. can be listed. The photoradical generator can be used alone or in combination of two or more.

[0116] The content of the photopolymerization initiator in the adhesive composition is preferably 0.001 to 5 parts by weight, more preferably 0.01 to 3 parts by weight, and further 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 adhesive composition is preferably 0.02 to 20 parts by weight, more preferably 0.05 to 10 parts by weight, and further 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.

[0117] <Other ingredients>

[0118] As described above, the photocurable adhesive composition constituting the adhesive layer 2 includes 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 include components other than these.

[0119] The adhesive composition may contain an antistatic agent. By making the adhesive contain an antistatic agent, the adhesive layer is reduced in resistance, the charge of the adhesive layer is reduced, and the effect of suppressing the charge of the adherend can be given. As the antistatic agent, ionic compounds containing organic cations, alkali metal salts, ion conductive polymers, ion conductive fillers, conductive polymers, etc. can be listed. Among them, ionic compounds containing organic cations and alkali metal salts are preferably used from the aspect of excellent compatibility with acrylic base polymers.

[0120] When the 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 further preferably 0.1 to 0.7 parts by weight relative to 100 parts by weight of the acrylic base polymer. When the amount of the antistatic agent is small, it is sometimes impossible to sufficiently reduce the resistance of the adhesive. When the amount of the antistatic agent is too much, it may become the cause of contamination, corrosion, and reduced adhesion of the adherend caused by the seepage of the antistatic agent.

[0121] In addition to the above components, the adhesive composition may contain additives such as silane coupling agents, tackifiers, plasticizers, softeners, degradation inhibitors, fillers, colorants, ultraviolet absorbers, antioxidants, and surfactants within a range that does not impair the characteristics of the present invention.

[0122] The low molecular weight additives that do not contribute to the photocuring of the adhesive also remain in the form of liquid components after the adhesive is photocured, so when the amount of the low molecular weight additives is large, the gel fraction of the adhesive after photocuring is small, and there is a tendency that the adhesive layer is easily plastically deformed and the strain recovery rate is reduced. Therefore, the amount of the additive is preferably less than 20 weight parts relative to 100 weight parts of the acrylic acid base polymer, more preferably less than 10 weight parts, further preferably less than 5 weight parts, and can also be less than 3 weight parts, less than 2 weight parts or less than 1 weight part. Relative to the total solid content of the adhesive composition, the total amount of the acrylic acid base polymer, the crosslinking agent, the photocuring agent and the acrylic acid oligomer is preferably more than 80 weight %, more preferably more than 90 weight %, more preferably more than 93 weight %, and can also be more than 95 weight %, more than 97 weight % or more than 98 weight %.

[0123] [Production of reinforcement film]

[0124] The reinforcement film is obtained by laminating the photocurable adhesive layer 2 on the film substrate 1. The adhesive layer 2 may be formed directly on the film substrate 1, or an adhesive layer formed in a sheet form on another substrate may be transferred to the film substrate 1.

[0125] By using roller coating, roller lick coating, gravure coating, reverse coating, roller brush, spray coating, dip roller coating, rod coating, blade coating, air knife coating, curtain coating, die lip coating, die coating, etc., the above-mentioned adhesive composition is applied to the substrate, and the solvent is dried and removed as needed to form an adhesive layer. As a drying method, an appropriate method can be appropriately adopted. The heating and drying temperature is preferably 40°C to 200°C, more preferably 50°C to 180°C, and 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 more preferably 10 seconds to 10 minutes.

[0126] When the adhesive composition contains a crosslinking agent, it is preferred to crosslink by heating or aging at the same time as the solvent drying or after the solvent drying. The heating temperature and heating time are appropriately set according to the type of crosslinking agent used, and crosslinking is usually carried out by heating for about 1 minute to 7 days in the range of 20°C to 160°C. The heating for drying and removing the solvent can also serve as the heating for crosslinking.

[0127] After the crosslinking structure is introduced into the polymer by the crosslinking agent, the photocuring agent and the acrylic oligomer also remain in an unreacted state. Therefore, the adhesive layer 2 contains an acrylic base polymer with a crosslinking structure introduced, a photocuring agent, an acrylic oligomer, and a photopolymerization initiator. When the adhesive layer 2 is formed on the film substrate 1, it is preferred to attach a release liner 5 to the adhesive layer 2 for the purpose of protecting the adhesive layer 2. It is also possible to attach the release liner 5 to the adhesive layer 2 before crosslinking.

[0128] 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 to the film substrate 1 to obtain a reinforcement film. The substrate used for forming the pressure-sensitive adhesive layer can be used as the release liner 5 as it is.

[0129] As the release liner 5, a plastic film such as polyethylene, polypropylene, polyethylene terephthalate, polyester film, etc. is preferably used. The thickness of the release liner is usually 3 to 200 μm, preferably about 10 to 100 μm. It is preferred that the contact surface of the release liner 5 with the adhesive layer 2 is subjected to a release treatment using a release agent such as a silicone system, a fluorine system, a long-chain alkyl system or a fatty acid amide system, or silica powder, etc. By subjecting the release liner 5 to a release treatment on the surface, when the film substrate 1 and the release liner 5 are peeled off, peeling occurs at the interface between the adhesive layer 2 and the release liner 5, and the state in which the adhesive layer 2 is adhered to the film substrate 1 is maintained. As far as the release liner 5 is concerned, an antistatic treatment can be applied to either or both of the release-treated surface and the non-treated surface. By subjecting the release liner 5 to an antistatic treatment, the charge when the release liner is peeled off from the adhesive layer can be suppressed.

[0130] [Physical properties of adhesive layer]

[0131] The thickness of the adhesive layer 2 is, for example, about 1 to 300 μm. There is a tendency that the greater the thickness of the adhesive layer 2, the higher the adhesion to the adherend. On the other hand, when the thickness of the adhesive layer 2 is too large, the fluidity before photocuring is high, and it is sometimes difficult to handle. Therefore, the thickness of the 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 adhesive layer 2 can be less than 25 μm, less than 20 μm, or less than 18 μm.

[0132] When the reinforcing film is used in an optical device such as a display, the total light transmittance of the adhesive layer 2 is preferably 80% or more, more preferably 85% or more, and further preferably 90% or more. The haze of the adhesive layer 2 is preferably 2% or less, more preferably 1% or less, further preferably 0.7% or less, and particularly preferably 0.5% or less.

[0133] From the viewpoint of easy peeling from the adherend and preventing the adhesive residue on the adherend after peeling the reinforcing film, the adhesive force between the adhesive layer and the adherend before photocuring is preferably 1N / 25mm or less, more preferably 0.7N / 25mm or less, further preferably 0.5N / 25mm or less, and may also be 0.4N / 25mm or less or 0.3N / 25mm or less. From the viewpoint of preventing the peeling of the reinforcing film during storage and handling, the adhesive force between the adhesive layer and the adherend before photocuring is preferably 0.01N / 25mm or more, more preferably 0.02N / 25mm or more, and may also be 0.03N / 25mm or more, 0.04N / 25mm or more, or 0.05N / 25mm or more.

[0134] The adhesive strength was determined by a peel test using a polyimide film as an adherend at a tensile speed of 300 mm / min and a peel angle of 180°. The adhesive strength is a value measured at 25° C. unless otherwise specified.

[0135] When the adhesive layer 2 is photocured, the photocuring agent undergoes a curing reaction, and the adhesive force to the adherend increases. The adhesive layer 2 preferably has a small shear storage modulus at a low temperature after photocuring.

[0136] The shear storage modulus of the adhesive layer after photocuring at -20°C is preferably 100 kPa or less, more preferably 90 kPa or less, further 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 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 according to the method described in JIS K7244-1 "Plastics - Test methods for dynamic mechanical properties", and reading the value at a specified temperature.

[0137] By reducing the storage modulus of the adhesive layer 2 at low temperature after photocuring, the adhesive layer exhibits strain relaxation in a low temperature environment, so that even when the device with the reinforcement film attached is repeatedly bent or the bent state is maintained for a long time, peeling of the adhesive layer at the bent portion can be suppressed. As described above, by making the adhesive composition contain an acrylic oligomer having a polymerizable functional group at the end of the main chain, the acrylic oligomer acts as a plasticizer, and there is a tendency for the storage modulus of the adhesive layer at low temperature after photocuring to become small.

[0138] When the storage modulus of the adhesive layer after photocuring is too small, the adhesive layer may be peeled off from the adherend due to insufficient adhesive holding force. Therefore, the storage modulus of the adhesive layer after photocuring at -20°C is preferably 30 kPa or more, more preferably 40 kPa or more, further preferably 45 kPa or more, and may also be 50 kPa or more.

[0139] The maximum value of the strain (creep deformation rate) of the adhesive layer after photocuring when a shear stress of 2 kPa is applied at 60° C. for 10 minutes in a creep test using a rotational rheometer is preferably 15% or more. The creep deformation rate is more preferably 20% or more, further preferably 23% or more, and may be 25% or more.

[0140] After the creep test, the stress is released, and the strain recovery rate when the strain is allowed to recover after standing for 10 minutes is preferably 88% or more, more preferably 90% or more, further preferably 91% or more, and may be 92% or more. The strain recovery rate is calculated by the following formula from the maximum strain S1 when the stress is applied and the minimum strain S2 when the stress is released.

[0141] Strain recovery rate (%) = 100 × (S1-S2) / S1

[0142] When a specified stress acts on a viscoelastic body such as an adhesive, a phenomenon (creep) occurs in which the strain (deformation rate) increases over time. The strain of a viscoelastic body when a specified stress is applied includes an elastic component that is generated instantaneously with the application of the stress, a viscoelastic component that is expressed as a function of time increase and reaches a specified 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.

[0143] When the foldable device is folded from an open state, compressive stress is applied to the inside and tensile stress is applied to the outside, thereby generating strain at the bent portion and its surroundings. When the creep deformation rate of the adhesive layer is large, the adhesive layer deforms following the strain, thereby suppressing deformation of the device due to stress concentration and peeling of the adhesive layer at the bent portion.

[0144] The strain recovery rate is an indicator of how much the adhesive layer recovers to its original shape when stress is applied to it (strain state) and when the stress is released. The closer it is to 100%, the higher the shape recovery. When the foldable device is restored from the folded state to the stretched state, the stress is released. When the strain recovery rate is large, the shape of the adhesive layer is easy to recover when the stress is released, so it is not easy to produce deformation such as wrinkles.

[0145] As described above, the greater the creep deformation rate of the adhesive layer after photocuring, the higher the ability to follow the stress, and the greater the strain recovery rate, the higher the recovery of the shape when the stress is released. Therefore, the greater the creep deformation rate and the greater 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 stretched, and there is a tendency that wrinkles at the bent part and peeling of the adhesive layer are suppressed.

[0146] The adhesive composition includes an acrylic oligomer having a polymerizable functional group at the end of the main chain, whereby the acrylic oligomer acts as a plasticizer, and thus there is a tendency for the creep deformation rate to increase. On the other hand, when the amount of the component acting as a plasticizer increases, there is a tendency for the strain recovery rate 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 adhesive layer after photocuring, thereby increasing the creep deformation rate and suppressing the reduction of the strain recovery rate.

[0147] From the perspective of bonding reliability during practical use of the device, the bonding strength between the photocured adhesive layer and the adherend is preferably 5N / 25mm or more, more preferably 7N / 25mm or more, further preferably 8N / 25mm or more, and may also be 9N / 25mm or more or 10N / 25mm or more.

[0148] The bonding strength between the adhesive layer after photocuring and the adherend is preferably 5 times or more, more preferably 10 times or more, further preferably 15 times or more, and may be 20 times or more, 25 times or more, or 30 times or more.

[0149] [Use of reinforcement film]

[0150] The reinforcing film of the present invention is used by being attached to a device or a component of the device. The adhesive layer 2 of the reinforcing film 10 is bonded to the film substrate 1, and the adhesive force to the adherend is small after being attached to the adherend and before being photocured. Therefore, the reinforcing film is easily peeled off from the adherend before photocuring.

[0151] The adherend to which the reinforcing film is adhered is not particularly limited, and various electronic devices, optical devices and their components can be listed. In one embodiment, the reinforcing film is adhered to the surface of a foldable flexible device such as a foldable device and a rollable device. The foldable device has a hinge portion and can be bent with the hinge portion as the center. In the case where the device is a display device, the reinforcing film can be adhered to the surface of the screen side, and can also be adhered to the back side (housing). In a flexible device that is configured to be foldable at a specified position such as a hinge portion, bending and stretching are repeated at the same position when it is in use.

[0152] The reinforcement film can be attached to the entire surface of the adherend, or it can be selectively attached only to the portion that needs to be reinforced (reinforcement target area). In addition, after the reinforcement film is attached to the entire portion that needs to be reinforced (reinforcement target area) and the area that does not need to be reinforced (non-reinforcement target area), the reinforcement film attached to the non-reinforcement target area can be cut and removed. If the adhesive is before light curing, the reinforcement film is in a state of temporary adhesion to the surface of the adherend, so the reinforcement film can be easily peeled off from the surface of the adherend. It is also possible to attach a reinforcement film to the reinforcement target area and the non-reinforcement target area, selectively irradiate light to the reinforcement target area to photocure the adhesive, and then selectively peel off the reinforcement film in the non-reinforcement target area where the adhesive is not cured.

[0153] By attaching the reinforcing film, appropriate rigidity can be imparted, so for thin components such as flexible devices, it can be expected to improve handling and prevent damage. In the manufacturing process of the device, when the reinforcing film is attached to the semi-finished product, the reinforcing film can also be attached to the semi-finished product of a large size before cutting into the product size. The reinforcing film can also be attached in a roll-to-roll manner on the mother roll of the device manufactured by the roll-to-roll process.

[0154] After the adherend is attached to the reinforcing film, the adhesive layer 2 is irradiated with active light to photocure the adhesive layer. As active light, ultraviolet rays, visible light, infrared rays, X-rays, α rays, β rays and γ rays can be listed. From the perspective of being able to inhibit the curing of the adhesive layer in the storage state and being easy to cure, ultraviolet rays are preferred as active light. The irradiation intensity and irradiation time of the active light can be appropriately set according to the composition, thickness, etc. of the adhesive layer. The irradiation of the active light of the adhesive layer 2 can be implemented from either side of the film substrate 1 side and the adherend side, or the irradiation of the active light can be performed from both sides.

[0155] As described above, by laminating the reinforcing film of the present invention, the adherend is given appropriate rigidity, and the stress is relaxed and dispersed, thereby suppressing various undesirable situations that may occur in the manufacturing process, improving production efficiency, and improving the yield rate. The reinforcing film is easy to peel off from the adherend before the adhesive layer is photocured, so it is easy to rework even if lamination or poor lamination occurs. In addition, it is also easy to perform processing such as selectively removing the reinforcing film from outside the reinforcement target area.

[0156] When the completed device is used, even if an external force is accidentally applied due to the device being dropped, a heavy object being placed on the device, or a flying object colliding with the device, the device can be prevented from being damaged by the bonding of the reinforcement film. In addition, the reinforcement film is firmly bonded to the device after the adhesive is photocured, so even if it is used for a long time, the reinforcement film is not easy to peel off, and the reliability is excellent.

[0157] In a device with a reinforcing film in which the reinforcing film of the present invention is adhered to a flexible device using a resin substrate, even when the device is repeatedly bent and stretched or kept in a bent state for a long time, the reinforcing film at the bent portion is not easily deformed, such as wrinkles, or peeled off from the device, thereby showing excellent bonding reliability.

[0158] The present invention will be further described below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[0159] [Examples 1 to 6, Comparative Examples 1 to 5]

[0160] <Polymerization of acrylic acid-based polymer>

[0161] In a reaction vessel equipped with a thermometer, a stirrer, a reflux condenser and a nitrogen inlet pipe, 68.1 parts by weight of n-octyl acrylate (NOAA) as monomers, 1.9 parts by weight of 4-hydroxybutyl acrylate (4HBA) and 30 parts by weight of methoxyethyl acrylate (MEA), 0.2 parts by weight of azobisisobutyronitrile (AIBN) as a polymerization initiator, and 233 parts by weight of ethyl acetate as a solvent were added, and nitrogen was introduced and nitrogen replacement was performed for about 1 hour while stirring. Then, 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.

[0162] <Preparation of Adhesive Composition>

[0163] To a solution of acrylic polymer A (based on 100 parts by weight of the solid content of the polymer) were added 0.1 parts by weight of a trifunctional isocyanate crosslinking agent ("Coronate HX" manufactured by Tosoh) as a crosslinking agent, 0.02 parts by weight of ferric acetylacetonate ("NACEM Iron (III)" manufactured by Nippon Chemical Industry) as a crosslinking catalyst, 12.5 parts by weight of polyethylene glycol #200 (n=4) diacrylate ("NK ESTETR A-200" manufactured by Shin-Nakamura Chemical Industry, functional group equivalent weight 151 g / eq) as a photocuring agent, 1.0 parts by weight of polyethylene glycol #400 (n=9) diacrylate ("NK ESTETR A-400" manufactured by Shin-Nakamura Chemical Industry, functional group equivalent weight 263 g / eq) as a photocuring agent, and urethane diacrylate having a polycarbonate skeleton ("ART RESIN UN-9200A", weight average molecular weight 15000) 2.0 weight parts, IGM Resins "Omnirad 651" as a photopolymerization initiator 0.3 weight parts, 1-ethyl-3-methylimidazolium bisfluorosulfonyl imide (Daiichi Kogyo Seiyaku "ELEXCEL AS-110") as an antistatic agent 0.15 weight parts, and further added the oligomer shown in Table 1, and uniformly mixed to prepare an adhesive composition.

[0164] The details of the oligomer are as follows. In Comparative Example 1, no oligomer was added.

[0165] UP-1021: an acrylic oligomer having a weight average molecular weight of 1,600 and having an acryloyl group at the terminal of the main chain (“ARUFON UP-1021” manufactured by Toagosei, glass transition temperature: −77° C.).

[0166] UP-1000: an acrylic oligomer having a weight average molecular weight of 3,000 and having an acryloyl group at the terminal of the main chain (“ARUFON UP-1021” manufactured by Toagosei, glass transition temperature: −71° C.).

[0167] OAP-5000: acrylic acid-acrylate having a weight average molecular weight of 20,000 ("ART CURE OAP-5000" manufactured by Negami Industries, functional group equivalent: 2,000 g / eq, glass transition temperature: 90°C).

[0168] S-3011: triol type polypropylene glycol having a weight average molecular weight of 10,000 ("PREMINOL S-3011" manufactured by AGC).

[0169] <Production of reinforcement film>

[0170] On a polyethylene terephthalate film with a thickness of 50 μm, the above-mentioned adhesive composition was applied 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 with a thickness of 25 μm, which was antistatically treated on both sides and silicone release treated on one side) was attached to the coated surface of the adhesive. Then, an aging treatment was performed for 4 days in an atmosphere of 25°C, and cross-linking was performed to obtain a reinforcing film in which a photocurable adhesive sheet was laminated on a polyethylene terephthalate film substrate and a release liner was temporarily bonded to the photocurable adhesive sheet.

[0171] [evaluate]

[0172] The following evaluations were performed on the reinforcement films obtained in the above-mentioned Examples and Comparative Examples.

[0173] <Storage modulus of adhesive layer>

[0174] On the release liner, the adhesive composition was coated and cross-linked in the same manner as in the above-mentioned embodiments and comparative examples to prepare an adhesive sheet (before photocuring). A release pad was attached to the surface of the adhesive layer of the adhesive sheet before photocuring to isolate it from oxygen and irradiate it with a 365nm LED lamp at 1000mJ / cm 2 The photocured adhesive sheets were laminated to prepare a measurement sample with a thickness of about 0.8 mm, and the dynamic viscoelasticity was measured using a rotational rheometer ("Discovery-HR2" manufactured by TA Instruments) under the following conditions to read the shear storage modulus G' value at -20°C.

[0175] Deformation mode: twist.

[0176] Measuring frequency: 1Hz.

[0177] Heating rate: 5°C / min.

[0178] Measuring temperature: -30~220℃.

[0179] Shape: parallel plate 8.0mmφ.

[0180] <Creep deformation rate and strain recovery rate of adhesive layer>

[0181] The test sample was prepared in the same manner as above. Using a rotational rheometer, a shear stress of 2 kPa was applied to the test sample for 10 minutes under the following conditions, and the maximum value S1 of the shear strain during this period was taken as the creep deformation rate (creep rate). Then, the stress was released (stress: 0 kPa) and the strain was allowed to recover after standing for 10 minutes. Based on the maximum strain S1 when the stress was applied and the minimum strain S2 when the stress was released, the strain recovery rate (%) was calculated as 100 × (S1-S2) / S1.

[0182] (Measurement conditions)

[0183] Deformation mode: twist.

[0184] Measurement temperature: 60℃.

[0185] Shape: parallel plate 8.0mmφ.

[0186] <Adhesion>

[0187] A polyimide film with a thickness of 25 μm ("Upilex25S" manufactured by UBE) was attached to a glass plate with the aid of a double-sided tape ("No. 531" manufactured by Nitto Denko) to obtain a polyimide film substrate for measurement. The release liner was peeled off and removed from the surface of the reinforcing film cut into a width of 25 mm × a length of 100 mm, and affixed to the polyimide film substrate for measurement using a hand roller as a test sample before photocuring. The sample was obtained by irradiating ultraviolet rays from the reinforcing film side (PET film substrate side) of the test sample before photocuring to photocure the adhesive layer, and the sample was used as a test sample after photocuring. Using these test samples, the end of the film substrate of the reinforcing film was held with a chuck, and the reinforcing film was peeled off 180° at a tensile speed of 300 mm / min to measure the peel strength.

[0188] <Bending test and bending test>

[0189] (Preparation of test pieces)

[0190] The release liner was peeled off from the surface of the reinforcing film, and a polyimide film ("Upilex 25RN" manufactured by UBE) with a thickness of 12.5 μm was adhered to the surface of the adhesive layer using a hand roller. The laminate was cut into a size of 25 mm in width × 100 mm in length, and ultraviolet rays were irradiated from the reinforcing film side (PET film substrate side) to photocure the adhesive layer to obtain a test piece.

[0191] (Low temperature bending test)

[0192] The test piece was bent at a central portion of the longitudinal direction of the test piece with the lateral direction as the bending axis at an ambient temperature of -20°C with a bending radius of 6 mm, and then fully extended (stretched) was repeated 100 times.

[0193] (High temperature repeated bending test)

[0194] A planar unloaded U-shaped expansion and contraction tester (manufactured by YUASA SYSTEM CO., LTD.) was used to install a bending fixture on the short side of the test piece. In a constant temperature chamber at a temperature of 60°C and a relative humidity of 50%, a repeated bending test was performed under the following conditions with the surface on the reinforcing film side (PET film substrate side) as the inner side.

[0195] Bending radius: 3mm.

[0196] Bending angle: 180°.

[0197] Test speed: 1 second / time (60 rpm).

[0198] Bending times: 200,000 times.

[0199] (evaluate)

[0200] The test specimens were placed in a stretched state and left to stand for 24 hours in an environment with a temperature of 25° C. and a relative humidity of 50%. The test sites were then visually observed and evaluated according to the following criteria.

[0201] A: No change from before the test, and no crease was confirmed at the bent portion.

[0202] B: A slight crease was observed at the bent portion, but no peeling of the reinforcing film occurred.

[0203] C: A slight crease was observed at the bent portion, and the reinforcing film was peeled off at the end of the bent portion.

[0204] D: A clear fold was observed on the entire surface of the bent portion, and the reinforcing film was peeled off along the fold.

[0205] Table 1 shows the types and amounts of oligomers added to the adhesives of the reinforcement films of Examples and Comparative Examples (amount of oligomer relative to 100 parts by weight of the solid content of polymer A) and the evaluation results.

[0206] [Table 1]

[0207]

[0208] In Comparative Example 1 using an adhesive containing no oligomer, the storage modulus at low temperature exceeded 100 kPa, and the bending resistance at low temperature was insufficient. In addition, in Comparative Example 1, the creep deformation rate of the adhesive layer was small, and the bending resistance at high temperature was also insufficient.

[0209] Compared with Comparative Example 1, Comparative Example 4 using an adhesive containing an oligomer (S-3011) having no polymerizable functional group has a smaller storage modulus at low temperature and excellent bending resistance at low temperature. However, in Comparative Example 4, the strain recovery rate of the adhesive layer is low, so the bending resistance at high temperature is insufficient.

[0210] Compared with Comparative Example 1, Examples 1 to 5 using an adhesive containing an acrylic oligomer (UP-1021) having an acryloyl group at the end of the main chain have a smaller storage modulus at low temperature and are excellent in bending resistance at low temperature. In addition, in Examples 1 to 5, the creep deformation rate is larger than that of Comparative Example 1, and the strain recovery rate is higher than that of Comparative Example 4, and the bending resistance at high temperature is also excellent.

[0211] Comparative Example 2, in which the amount of acrylic oligomer is 5 parts by weight, is similar to Comparative Example 1, in which both the bending resistance at low temperature and the bending resistance at high temperature are insufficient. Comparative Example 3, in which the amount of acrylic oligomer is 40 parts by weight, has excellent bending resistance at low temperature, but has a low strain recovery rate and insufficient bending resistance at high temperature. In addition, the adhesive strength of the adhesive layer after photocuring in Comparative Example 3 shows a low value compared to Examples 1 to 5.

[0212] From the comparison between Examples 1 to 5 and Comparative Examples 1 to 3, it can be observed that the greater the amount of the acrylic oligomer (UP-1021) having an acryloyl group at the end of the main chain, the smaller the storage modulus at low temperature, the higher the bending resistance at low temperature, and the tendency that the creep deformation rate at high temperature becomes larger and the strain recovery rate becomes smaller. It can be considered that the acrylic oligomer having an acryloyl group at the end of the main chain has the function of a plasticizer that reduces the storage modulus at low temperature and increases the creep deformation rate at high temperature, and participates in photocuring together with the photocuring agent, so the reduction in the strain recovery rate is suppressed compared with Comparative Example 4.

[0213] Example 6 using a large molecular weight acrylic oligomer (UP-1000) has excellent bending resistance at low temperatures and bending resistance at high temperatures, as in Examples 1 to 5. From the comparison between Example 6 and Example 3, it can be observed that when the molecular weight of the acrylic oligomer is small, the effect of reducing the storage modulus at low temperatures is high, which is beneficial to improving the bending resistance.

[0214] Compared with Comparative Example 1 containing no oligomer, in Comparative Example 5 using acrylic acid-acrylate (OAP-5000) having an acryloyl group in the side chain as an oligomer, the creep deformation rate at high temperature increased, but the storage modulus at low temperature was greater than that of Comparative Example 1, and the bending resistance at low temperature and the bending resistance at high temperature were poor. It is believed that since acrylic acid-acrylate has a large number of acryloyl groups and a small distance between crosslinking points, the molecular chain movement at low temperature is suppressed in the adhesive layer after photocuring, and the plasticizing effect at low temperature is small compared with the case of using an oligomer having a polymerizable functional group at the end of the main chain.

[0215] From the above results, it can be seen that in the adhesive composition that contains a specified amount of an acrylic oligomer having a polymerizable functional group at the end of the main chain in addition to the acrylic base polymer and the photocuring agent, the adhesion force before photocuring is low and it exhibits excellent adhesion properties after photocuring. Furthermore, the storage modulus in the low temperature region is low and the strain recovery rate is high, which is suitable as a reinforcement film for foldable devices.

Claims

1. A reinforcement film, wherein: The reinforcing film comprises a film substrate and an adhesive layer adhered and laminated on one main surface of the film substrate. The adhesive layer is formed 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 contains one or more monomer units selected from the group consisting of a hydroxyl group-containing monomer and a carboxyl group-containing monomer, and a cross-linked structure is introduced into the acrylic base polymer. The acrylic oligomer has a polymerizable functional group at the end of the main chain. The photocurable composition includes 6 to 35 parts by weight of the acrylic oligomer based on 100 parts by weight of the acrylic base polymer.

2. The reinforcement film according to claim 1, wherein: The photocuring agent includes a multifunctional (meth)acrylate having an oxyalkylene chain.

3. The reinforcement film according to claim 2, wherein: The photocuring agent further comprises urethane (meth)acrylate.

4. The reinforcing 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 a cross-linking structure is introduced.

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

6. The reinforcement 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 reinforcement film according to any one of claims 1 to 3, wherein The adhesive force of the adhesive layer to the polyimide film before photocuring is 1 N / 25 mm or less.

8. The reinforcement film according to any one of claims 1 to 3, wherein The adhesive force of the adhesive layer to the polyimide film after photocuring is 5 N / 25 mm or more.

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

10. A method for manufacturing a device with a reinforcement film, wherein a reinforcement film is attached to the surface of a bendable device, wherein: The adhesive layer of the reinforcement film according to any one of claims 1 to 9 is attached to the surface of a bendable device, The adhesive layer is photocured.

11. A device with a reinforcement film, wherein the reinforcement film is attached to the surface of a bendable device, wherein: The reinforcing film comprises a film substrate and an adhesive layer adhered and laminated on one main surface of the film substrate. The adhesive layer is attached to the surface of the device. The adhesive layer is a photocured product obtained by photocuring a photocurable adhesive composition comprising 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 having a polymerizable functional group at the end of the main chain. The acrylic base polymer includes, as monomer units, one or more selected from the group consisting of a hydroxyl group-containing monomer and a carboxyl group-containing monomer, and a crosslinking structure is introduced into the acrylic base polymer.

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

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