Reinforcing membrane, apparatus for attaching reinforcing membrane, and method for manufacturing thereof

Abstract

This invention provides a reinforcing film that is easily peeled off immediately after being bonded to an adhesive, and can be firmly bonded to the adhesive by photocuring the adhesive after bonding, and is not easily peeled off due to bending tests. The reinforcing film (10) has an adhesive layer (2) fixedly laminated on one of the main surfaces of a film substrate (1). The adhesive layer contains a photocurable composition comprising an acrylic-based polymer having a cross-linked structure and a photocuring agent. Preferably, the adhesive layer has a shear storage modulus of 1.0 × 10⁻⁶ at -20°C after photocuring. 4~5.0×10 5 Pa, and the shear storage modulus at 25°C is 8.0 × 10⁵ Pa. 3~5.0×10 5Pa.

Description

[Technical Field]

[0001] This invention relates to a reinforcing film applied to the surface of a device. Furthermore, this invention relates to a device having a reinforcing film and a method for manufacturing the same. [Previous Technology]

[0002] Adhesive films are sometimes laminated onto the surface of optical devices or electronic devices such as displays to protect the surface or to provide impact resistance. Such adhesive films typically have an adhesive layer fixedly laminated on the main surface of the film substrate, and are laminated to the surface of the device through the adhesive layer.

[0003] Before the device is assembled, processed, transported, or used, damage or breakage of the adhered object can be suppressed by temporarily adhering an adhesive film to the surface of the device or its constituent parts. Patent Document 1 discloses a reinforcing film having an adhesive layer comprising a photocurable adhesive composition on a film substrate.

[0004] Because the adhesive of this reinforcing film has a high gel content, it has low adhesion immediately after being bonded to the substrate, and therefore easily peels off from the substrate. Therefore, it can be peeled off from the substrate for secondary processing, and the reinforcing film can also be selectively peeled off from areas of the substrate where reinforcement is not required. Since the adhesive of the reinforcing film is firmly bonded to the substrate through photocuring, the film substrate is permanently bonded to the surface of the substrate, thus the aforementioned reinforcing film can be used as a reinforcing material for surface protection of devices, etc.

[0005] In recent years, organic EL (Electroluminescence) panels using flexible substrates such as resin films have begun to be used in practical applications, and flexible displays that can be bent have been proposed. Foldable flexible displays (foldable displays) are repeatedly bent at the same location. At the bent location, compressive stress is applied to the inner side and tensile stress to the outer side, resulting in strain at and around the bent location, which may cause the adhesive to peel off from the adhered body. According to Patent Document 2, a softer adhesive sheet is used in the bonding between components in a foldable display to mitigate the stress and strain at the bent location (e.g., Patent Documents 2 and 3). [Prior Art Documents] [Patent Documents]

[0006] Patent Document 1: Japanese Patent Application Publication No. 2020-41113; Patent Document 2: Japanese Patent Application Publication No. 2018-45213; Patent Document 3: Japanese Patent Application Publication No. 2017-119801 [Summary of the Invention]

[0007] [The problem the invention aims to solve]

[0008] When applying the reinforcing film as described in Patent Document 1 to a flexible device, the photocured adhesive is relatively hard, which may cause peeling at the bending points. In particular, because the adhesive has lower flexibility and adhesion retention at low temperatures, it is easy for the adhesive to peel off from the adherend at the bending points when a bending test is performed at low temperatures. Regarding the adhesive sheets described in Patent Documents 2 and 3, although repeated bending at low temperatures can suppress peeling from the adherend, the adhesive does not have photocurability and immediately exhibits high adhesion after being bonded to the adherend, making it difficult to peel off or process.

[0009] In view of the above, the object of the present invention is to provide a reinforcing film that is easily peeled off immediately after being bonded to the substrate, and can be firmly bonded to the substrate by photocuring the adhesive after bonding, and is not easily peeled off due to bending tests. [Technical Means for Solving the Problem]

[0010] The reinforcing film of the present invention comprises an adhesive layer fixedly deposited on a main surface of a film substrate. The adhesive layer contains a photocurable composition comprising an acrylic-based polymer and a photocuring agent. The glass transition temperature of the acrylic-based polymer is preferably below -45°C.

[0011] 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 cross-linked structure is introduced by bonding the hydroxyl and / or carboxyl groups of the base polymer with a cross-linking agent such as an isocyanate cross-linking agent or an epoxy cross-linking agent. The amount of cross-linking agent is about 0.05 to 1 part by weight relative to 100 parts by weight of the acrylic polymer.

[0012] The photocurable composition constituting the adhesive layer preferably contains 3 to 30 parts by weight of a photocuring agent relative to 100 parts by weight of the acrylic base polymer. For example, a polyfunctional (meth)acrylate can be used as the photocuring agent. The polyfunctional (meth)acrylate can also be an epoxy-modified polyfunctional (meth)acrylate obtained by modifying epoxides such as ethylene oxide and propylene oxide. The functional group equivalent of the photocuring agent is, for example, about 80 to 300 g / eq.

[0013] Preferably, the adhesive layer, after light curing, has a shear storage modulus of 1.0 × 10⁴ to 5.0 × 10⁵ Pa at -20°C. Preferably, the shear storage modulus of the light-cured adhesive layer at 25°C is 8.0 × 10³ to 5.0 × 10⁵ Pa.

[0014] The shear storage modulus of the adhesive layer before light curing at -20°C is preferably 1.0×10⁴ to 5.0×10⁵ Pa. The shear storage modulus of the adhesive layer before light curing at 25°C is preferably 5.0×10³ to 1.0×10⁴ Pa.

[0015] The shear storage modulus of the adhesive layer after light curing at 25°C is preferably 1.2 to 7 times that of the adhesive layer before light curing at 25°C. The shear storage modulus of the adhesive layer after light curing at -20°C is preferably 1.2 to 7 times that of the adhesive layer before light curing at -20°C.

[0016] By attaching the reinforcing film to the surface of the device and allowing the adhesive layer to light-cure, a device with a reinforcing film can be obtained. The device can also be a flexible, bendable device. [Effects of the Invention]

[0017] In the reinforcing film of the present invention, the adhesive layer comprises a photocurable composition. After bonding with the substrate, the adhesion to the substrate is improved by photocuring the adhesive layer. Before photocuring, the adhesion between the reinforcing film and the substrate is relatively small, making it easy to peel off from the substrate. The adhesive layer of the reinforcing film retains a low storage modulus after photocuring and has high stress-strain mitigation properties. Therefore, in a bendable device, even when repeatedly bent at the same location, peeling of the adhesive layer at the bent location can be suppressed, resulting in excellent bonding reliability.

Implementation Method

[0019] FIG1 is a cross-sectional view showing one embodiment of the reinforcing film. The reinforcing film 10 has an adhesive layer 2 on one of the main surfaces of the film substrate 1. The adhesive layer 2 is fixedly deposited on one of the main surfaces of the film substrate 1. The adhesive layer 2 is a photocurable adhesive containing a photocurable composition, which is cured by irradiation with active light such as ultraviolet light, thereby improving the adhesion strength with the adherend.

[0020] Figure 2 is a cross-sectional view of a reinforcing film on which the release film 5 is temporarily adhered to the main surface of the adhesive layer 2. Figure 3 is a cross-sectional view of a device 100 with a reinforcing film attached to the surface of the foldable device 20. Figure 4 is a cross-sectional view showing the state after the device is folded with the hinge 25 as the center.

[0021] The release film 5 is peeled off from the surface of the self-adhesive layer 2, and the exposed surface of the adhesive layer 2 is adhered to the surface of the device 20, thereby attaching the reinforcing film 10 to the surface of the device 20. In this state, the adhesive layer 2 is temporarily adhered to the device 20 with the reinforcing film 10 (adhesive layer 2) before photocuring. By photocuring the adhesive layer 2, the adhesion at the interface between the device 20 and the adhesive layer 2 is increased, thereby fixing the device 20 and the reinforcing film 10 together.

[0022] "Fixed bonding" refers to a state in which the two layers are firmly bonded together and cannot be or is difficult to peel off at the interface. "Temporary adhesion" refers to a state in which the adhesion between the two layers is weak and can be easily peeled off at the interface.

[0023] In the reinforcing membrane shown in Figure 2, the membrane substrate 1 and the adhesive layer 2 are fixedly bonded together, and the release membrane 5 is temporarily bonded to the adhesive layer 2. If the membrane substrate 1 and the release membrane 5 are peeled off, peeling occurs at the interface between the adhesive layer 2 and the release membrane 5, and the state in which the adhesive layer 2 is fixedly bonded to the membrane substrate 1 is maintained. No adhesive residue remains on the release membrane 5 after peeling.

[0024] The reinforcing film device shown in Figure 3 refers to the state in which the device 20 and the adhesive layer 2 are temporarily bonded together before the adhesive layer 2 is photocured. If the film substrate 1 is peeled off from the device 20, peeling occurs at the interface between the adhesive layer 2 and the device 20, thus maintaining the state in which the adhesive layer 2 is fixedly attached to the film substrate 1. Since no adhesive residue remains on the device 20, it is easy to perform peeling operations such as secondary processing or cutting. After the adhesive layer 2 is photocured, the adhesion between the adhesive layer 2 and the device 20 is enhanced and becomes a fixed bond, making it difficult to peel the film 1 off from the device 20.

[0025] [Membrane Substrate] The membrane substrate 1, which serves as the reinforcing membrane 10, can be a flexible plastic film. In order to fix the membrane substrate 1 to the adhesive layer 2, it is preferable not to perform release treatment on the surface of the membrane substrate 1 where the adhesive layer 2 is attached.

[0026] The thickness of the membrane substrate is, for example, about 4 to 150 μm. To reinforce the device by imparting rigidity or mitigating impact, the thickness of the membrane 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. To make the reinforcing membrane flexible and foldable, the thickness of the membrane substrate 1 is preferably 125 μm or less, more preferably 100 μm or less. To combine mechanical strength and flexibility, the compressive strength of the membrane substrate 1 is preferably 100 to 3000 kg / cm², more preferably 200 to 2900 kg / cm², further preferably 300 to 2800 kg / cm², and particularly preferably 400 to 2700 kg / cm².

[0027] Examples of plastic materials constituting the film substrate 1 include: polyester resins, polyolefin resins, cyclic polyolefin resins, polyamide resins, polyimide resins, polyetheretherketone, polyether ether, polyarylate resins, and aromatic polyamide resins. In reinforcing films used in optical devices such as displays, the film substrate 1 is preferably a transparent film. Furthermore, when the adhesive layer 2 is photocured by irradiating active light from the film substrate 1 side, the film substrate 1 is preferably transparent relative to the active light used to cure the adhesive layer. To achieve both mechanical strength and transparency, suitable materials include: polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; transparent polyimides; and transparent aromatic polyamides. When the active light is irradiated from the side of the adherend, the adherend only needs to be transparent relative to the active light, and the film substrate 1 can also be opaque relative to the active light.

[0028] Functional coatings such as an easy-adhesion layer, an easy-slip layer, a release layer, an antistatic layer, a hard coating layer, and an anti-reflective layer may also be provided on the surface of the membrane substrate 1. Furthermore, as described above, in order to fix the membrane substrate 1 to the adhesive layer 2, it is preferable not to provide a release layer on the surface of the membrane substrate 1 where the adhesive layer 2 is attached.

[0029] [Adhesive Layer] The adhesive layer 2, fixedly laminated on the film substrate 1, contains a photocurable composition comprising a base polymer and a photocuring agent. Since the adhesive layer 2 has relatively low adhesion to the adherend, such as a device or device parts, before photocuring, it is easy to peel off. Photocuring enhances the adhesion between the adhesive layer 2 and the adherend, thus preventing the reinforcing film from easily peeling off from the device surface when the device is used, resulting in excellent adhesion reliability.

[0030] Photocurable adhesives hardly harden under normal storage conditions, but harden by irradiation with active light such as ultraviolet light. Therefore, the reinforcing film of the present invention has the following advantages: the curing time of the adhesive layer 2 can be set arbitrarily, and the preparation time of the steps can be flexibly matched.

[0031] The thickness of the adhesive layer 2 is, for example, about 1 to 300 μm. The greater the thickness of the adhesive layer 2, the higher the adhesion to the substrate tends to be. On the other hand, when the thickness of the adhesive layer 2 is too large, the flowability before photocuring is high, which may lead to difficulties in operation. Therefore, the thickness of the adhesive layer 2 is preferably 3 to 100 μm, more preferably 5 to 50 μm, and even more 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 also be 25 μm or less, 20 μm or less, or 18 μm or less.

[0032] When the reinforcing film is used in optical devices such as displays, the total light transmittance of the adhesive layer 2 is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. The haze of the adhesive layer 2 is preferably 2% or less, more preferably 1% or less, even more preferably 0.7% or less, and particularly preferably 0.5% or less.

[0033] Preferably, the adhesive layer 2 has improved adhesion to the substrate through light curing, and even after light curing, the shear storage modulus at low temperature (hereinafter referred to as "storage modulus") remains relatively small. The storage modulus of the adhesive is determined by the following method: according to the method described in JIS K7244-1 "Plastics - Test method for dynamic mechanical properties", the value corresponding to a specific temperature is read when the measurement is carried out at a frequency of 1 Hz in the range of -50 to 150°C at a heating rate of 5°C / min.

[0034] The storage modulus of the photocured adhesive layer at -20°C is preferably 5.0 × 10⁵ Pa or less, more preferably 4.0 × 10⁵ Pa or less, and even more preferably 3.0 × 10⁵ Pa or less, or 2.5 × 10⁵ Pa or less. Since the storage modulus of the photocured adhesive layer 2 is small at low temperatures, the adhesive layer exhibits strain mitigation at low temperatures. Therefore, even when the device with the reinforcing film is repeatedly bent, or when it is kept in a bent state for a long time, the peeling of the adhesive layer at the bent part can be suppressed.

[0035] On the other hand, when the storage modulus of the adhesive layer after light curing is too small, the adhesive layer is prone to plastic deformation, which may lead to insufficient adhesion and peeling of the adhesive layer from the adherend. Therefore, the storage modulus of the adhesive layer after light curing at -20°C is preferably 1.0 × 10⁴ Pa or more, more preferably 2.0 × 10⁴ Pa or more, and even more preferably 3.00 × 10⁴ Pa or more, and may also be 4.0 × 10⁴ Pa or more, 5.0 × 10⁴ Pa or more, 6.0 × 10⁴ Pa or more, 7.0 × 10⁴ Pa or more, or 8.0 × 10⁴ Pa or more.

[0036] In order to ensure adhesion at room temperature and suppress the adhesive layer from protruding from the end, the storage modulus of the light-cured adhesive layer at 25°C is preferably 8.0×103~1.5×105 Pa, more preferably 1.0×104~1.0×105 Pa, and even more preferably 1.5×104~8.0×104 Pa, or 2.0×104~6.0×104 Pa.

[0037] The composition of the adhesive layer 2 is not particularly limited as long as it contains a base polymer and a photocuring agent, and the adhesion to the adhered body is improved by photocuring. In order to improve the curing efficiency obtained by irradiation with active light, the adhesive composition (photocurable composition) constituting the adhesive layer 2 preferably contains a photopolymerization initiator.

[0038] (Base Polymer) The base polymer is the main component of the adhesive composition, and it is the main factor determining the adhesion or storage modulus of the adhesive layer before photocuring. For excellent optical transparency and adhesion, and easy control of adhesion or storage modulus, the adhesive composition preferably contains an acrylic polymer as the base polymer, and preferably 50% by weight or more of the adhesive composition is an acrylic polymer.

[0039] As an acrylic polymer, it is suitable to use those containing alkyl (meth)acrylate as the main monomer component. Furthermore, in this specification, "(meth)acrylate" refers to acrylic acid and / or methacrylic acid.

[0040] As an alkyl methacrylate, it is suitable to use an alkyl methacrylate having 1 to 20 carbon atoms in the alkyl group. The alkyl group of the alkyl methacrylate can be straight-chain or branched. Examples of alkyl methacrylates include: methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobutyl methacrylate, dibutyl methacrylate, terbutyl methacrylate, pentyl methacrylate, isoamyl methacrylate, neopentyl methacrylate, hexyl methacrylate, heptyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, isooctyl methacrylate, nonyl methacrylate, and propyl methacrylate. Isononyl acrylate, decyl acrylate, isodecyl acrylate, undecyl acrylate, dodecyl acrylate, isotridecyl acrylate, tetradecyl acrylate, isotetradecyl acrylate, pentadecyl acrylate, hexadecyl acrylate, heptadecanyl acrylate, octadecyl acrylate, isooctadecyl acrylate, nonadecanyl acrylate, eicosyl acrylate, etc.

[0041] Among the alkyl methacrylates exemplified, to achieve a low Tg of the base polymer, it is preferable to use C1-9 alkyl methacrylates, and preferably to use homopolymers with a glass transition temperature of -50°C or lower. The glass transition temperature of the homopolymer of the alkyl methacrylate is more preferably -55°C or lower, and even more preferably -60°C or lower. Specific examples of C1-9 alkyl methacrylates with a glass transition temperature of -50°C or lower 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, butyl acrylate and 2-ethylhexyl acrylate are preferred, and 2-ethylhexyl acrylate is particularly preferred for achieving a lower Tg.

[0042] Relative to the total amount of monomer components constituting the basic polymer, the content of alkyl (meth)acrylate is preferably 70% by weight or more, more preferably 80% by weight or more, and even more preferably 85% by weight or more, and may also be 90% by weight or more, 93% by weight or more, or 95% by weight or more. Preferably, the amount of C1-9 alkyl (meth)acrylate is within the above range, and more preferably, the total amount of 2-ethylhexyl acrylate and butyl acrylate is within the above range. The amount of 2-ethylhexyl acrylate may also be within the above range.

[0043] The acrylic base polymer preferably contains a monomer component with crosslinkable functional groups as a copolymer component. Examples of monomers with crosslinkable functional groups include hydroxyl-containing monomers and carboxyl-containing monomers. The base polymer may contain both hydroxyl-containing and carboxyl-containing monomers as copolymer components, or only one of them as a copolymer component. By introducing a crosslinking structure into the base polymer, the cohesive force is improved, thereby increasing the peelability of the adhesive layer 2 from the adherend before photocuring.

[0044] 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-hydroxylaurate (meth)acrylate, and 4-(hydroxymethyl)cyclohexylmethyl (meth)acrylate. Among these, 2-hydroxyethyl acrylate (Tg: -15°C) and 4-hydroxybutyl acrylate (Tg: -32°C) are preferred to significantly improve the adhesion of the adhesive after photocuring.

[0045] Examples of monomers containing carboxyl groups include: (meth)acrylic acid, 2-carboxyethyl (meth)acrylic acid, carboxypentyl (meth)acrylic acid, 2-(meth)acryloxyethyl succinate, 2-(meth)acryloxyethyl hexahydrophthalic acid, 2-(meth)acryloxyethyl phthalic acid, 2-(meth)acryloxypropyl hexahydrophthalic acid, 2-(meth)acryloxypropyl phthalic acid, itconic acid, maleic acid, trans-butenedioic acid, butenoic acid, etc.

[0046] In acrylic-based polymers, the total amount of hydroxyl-containing monomers and carboxyl-containing monomers relative to the total amount of constituent monomers is preferably 0.5 to 15% by weight, more preferably 1 to 10% by weight, and even more preferably 2 to 7% by weight. When an isocyanate-based crosslinking agent is used to introduce a crosslinking structure into the acrylic-based polymer, the content of hydroxyl-containing (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 4-hydroxypropyl (meth)acrylate is preferably within the above range.

[0047] Acrylic base polymers may also contain nitrogen-containing monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperidine, vinylpyridine, vinylpyrrole, vinylimidazolium, vinylpyrazole, vinylpyridine, vinylpyrrolidone, N-acrylamide, N-vinylcarboxylate, and N-vinylcaprolactam as constituent monomer components.

[0048] Acrylic base polymers may also contain monomer components other than those mentioned above. Acrylic base polymers may also contain, for example, vinyl ester monomers, aromatic vinyl monomers, epoxy-containing monomers, vinyl ether monomers, sulfonate-containing monomers, phosphate-containing monomers, and anhydride-containing monomers as monomer components.

[0049] The base polymer prior to the introduction of the crosslinking structure may substantially be nitrogen-free. The nitrogen content in the constituent elements of the base polymer may be 0.1 mol% or less, 0.05 mol% or less, 0.01 mol% or less, 0.005 mol% or less, 0.001 mol% or less, or 0. By using a base polymer that is substantially nitrogen-free, the increase in adhesion force (initial adhesion force) of the adhesive layer before photocuring is suppressed when the adherend undergoes surface activation treatment. By not using nitrogen-containing monomers such as cyano-containing monomers, monomers containing amide structures, monomers containing amide groups, and monomers containing α-porphyrin rings as constituent monomer components of the base polymer, a base polymer that is substantially nitrogen-free can be obtained. The amount of nitrogen-containing monomers relative to the total amount of constituent monomer components of the acrylic base polymer may be 1 wt% or less, 0.5 wt% or less, 0.1 wt% or less, 0.05 wt% or less, or 0.

[0050] The glass transition temperature of acrylic-based polymers is preferably below -45°C, more preferably below -50°C, further preferably below -55°C, and particularly preferably below -60°C. It can also be below -63°C or below -65°C. There is no particular limitation on the lower limit of the glass transition temperature of acrylic-based polymers; generally, it is above -80°C, but it can also be above -75°C or above -70°C.

[0051] The glass transition temperature is the temperature at which the loss tangent tanδ becomes maximum in viscoelasticity measurement (peak temperature). By making the glass transition temperature sufficiently lower than the ambient temperature of the device, the storage modulus G' of the adhesive layer within the ambient temperature range is reduced, thus inhibiting peeling when repeatedly bent.

[0052] The theoretical Tg calculated based on the Fox equation can also be used instead of the glass transition temperature obtained by viscoelastic measurement. The theoretical Tg is calculated based on the glass transition temperature Tgi of the homopolymer of the constituent monomer components of the acrylic base polymer and the weight fraction Wi of each monomer component, according to the following Fox equation: 1 / Tg=Σ(Wi / Tgi)

[0053] Tg is the glass transition temperature of the polymer chain (unit: K), Wi is the weight fraction of monomer component i constituting the chain segment (weight-based copolymerization ratio), and Tgi is the glass transition temperature of the homopolymer of monomer component i (unit: K). The glass transition temperature of the homopolymer can be the value recorded in the Polymer Handbook 3rd edition (John Wiley & Sons, Inc., 1989). For the Tg of homopolymers of monomers not recorded in the aforementioned literature, the peak temperature of tanδ obtained by dynamic viscoelasticity measurement can be used.

[0054] The above-mentioned monomer components can be polymerized by various known methods such as solution polymerization, emulsion polymerization, and bulk polymerization to obtain an acrylic polymer as the base polymer. From the perspective of the balance of adhesive properties such as adhesion and holding power, or cost, solution polymerization is preferred. Solvents for solution polymerization can include ethyl acetate and toluene. The solution concentration is typically around 20–80% by weight. Various known polymerization initiators, such as azo and peroxide initiators, can be used as polymerization initiators. Chain transfer agents can also be used to adjust the molecular weight. The reaction temperature is typically around 50–80°C, and the reaction time is typically around 1–8 hours.

[0055] 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. Furthermore, when a crosslinking structure is introduced into the base polymer, the molecular weight of the base polymer refers to the molecular weight before the crosslinking structure is introduced.

[0056] (Crosslinking Agent) To ensure that the adhesive possesses adequate cohesive force, exhibits adhesion, and guarantees the peelability of the adhesive layer from the adherend before photocuring, it is preferable to introduce a crosslinking structure into the base polymer. For example, a crosslinking agent is added to the solution after polymerization of the base polymer, and heating is applied as needed to introduce the crosslinking structure. Examples of crosslinking agents include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, aziridine-based crosslinking agents, carbodiimide-based crosslinking agents, and metal chelate-based crosslinking agents. These crosslinking agents react with functional groups such as hydroxyl and carboxyl groups introduced into the base polymer to form a crosslinking structure. Isocyanate-based and epoxy-based crosslinking agents are preferred for higher reactivity with the hydroxyl or carboxyl groups of the base polymer and easier introduction of the crosslinking structure.

[0057] As an isocyanate-based crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule can be used. Examples of polyisocyanate crosslinking agents include: lower aliphatic polyisocyanates such as butyl diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentyl diisocyanate, cyclohexyl diisocyanate, and isophorone diisocyanate; aromatic isocyanates such as 2,4-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and phenyl dimethyl diisocyanate; trimethylolpropane / toluene diisocyanate trimer adducts (e.g., "Coronate L" manufactured by Tosoh Corporation), trimethylolpropane / hexamethylene diisocyanate trimer adducts (e.g., "Coronate HL" manufactured by Tosoh Corporation), trimethylolpropane adducts of phenyl diisocyanate (e.g., "Takenate D110N" manufactured by Mitsui Chemicals), and isocyanurates of hexamethylene diisocyanate (e.g., "Coronate" manufactured by Tosoh Corporation). Isocyanate adducts such as HX; etc.

[0058] As an epoxy crosslinking agent, a polyfunctional epoxy compound having two or more epoxy groups in one molecule can be used. The epoxy crosslinking agent can also be one having three or four or more epoxy groups in one molecule. The epoxy groups of the epoxy crosslinking agent can be glycidyl groups. Examples of epoxy-based crosslinking agents include: N,N,N',N'-tetraglycidyl-m-phenylenediamine, 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, diglycidyl adipate, diglycidyl phthalate, tri(2-hydroxyethyl)isocyanurate triglycidyl ether, resorcinol diglycidyl ether, bisphenol-S-diglycidyl ether, etc. As an epoxy crosslinking agent, commercially available products such as "Denacol" manufactured by Nagase Chemical Co., Ltd., and "Tetrad X" and "Tetrad C" manufactured by Mitsubishi Gas Chemical Co., Ltd. can also be used.

[0059] The amount of crosslinking agent used can be adjusted appropriately according to the composition or molecular weight of the base polymer. Relative to 100 parts by weight of the base polymer, the amount of crosslinking agent used is about 0.03 to 2 parts by weight, preferably 0.05 to 1 part by weight, more preferably 0.08 to 0.8 parts by weight, and even more preferably 0.1 to 0.5 parts by weight.

[0060] Crosslinking catalysts may also be used to promote the formation of crosslinked structures. Examples of crosslinking catalysts that are isocyanate-based crosslinking agents include: tetrabutyl titanate, tetraisopropyl titanate, iron triacetone, butyltin oxide, dioctyltin dilaurate, dibutyltin dilaurate, and other metal-based crosslinking catalysts (especially tin-based crosslinking catalysts). The amount of crosslinking catalyst used is generally 0.1 parts by weight or less per 100 parts by weight of the base polymer.

[0061] (Photocuring agent) The adhesive composition constituting adhesive layer 2 contains a photocuring agent in addition to the base polymer. If the adhesive layer 2 containing the photocurable adhesive composition is photocured after being bonded to the substrate, the adhesion to the substrate is improved.

[0062] As a photocuring agent, a photocurable monomer or a photocurable oligomer can be used. Preferably, the photocuring agent is a compound having two or more vinyl unsaturated bonds per molecule. Furthermore, the photocuring agent is preferably a compound exhibiting compatibility with the base polymer. To exhibit adequate compatibility with the base polymer, the photocuring agent is preferably a liquid at room temperature.

[0063] To achieve higher compatibility with acrylic-based polymers, it is preferable to use polyfunctional (meth)acrylates as photocuring agents. Representative polyfunctional (meth)acrylates are esters of polyols and (meth)acrylates. Specific examples of polyfunctional (meth)acrylates include: polyethylene glycol dimethacrylate, polypropylene glycol dimethacrylate, polytetramethylene glycol dimethacrylate, alkyl glycol dimethacrylate, tricyclodecanediethanol dimethacrylate, isocyanurate dimethacrylate, isocyanurate trimethacrylate, pentaerythritol trimethacrylate, pentaerythritol dimethacrylate, trimethylolpropane trimethacrylate, di-trimethylolpropane tetramethacrylate, pentaerythritol tetramethacrylate, dipentaerythritol polymethacrylate, dipentaerythritol hexamethacrylate, neopentyl glycol dimethacrylate, glycerol dimethacrylate, urethane (meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, isoprene (meth)acrylate, etc.

[0064] The polyfunctional (meth)acrylate can also be an ester of polyol obtained by modifying an epoxide with (meth)acrylate. Examples of epoxides include ethylene oxide (EO) and propylene oxide (PO). Epoxides can also be polyepoxides such as polyethylene glycol and polypropylene glycol. The chain length n of the epoxide is approximately 1 to 10. By adjusting the type and chain length of the epoxide, the compatibility between the polyfunctional (meth)acrylate and the acrylic-based polymer can be adjusted to an appropriate range.

[0065] Specific examples of alkylene oxide-modified polyfunctional (meth)acrylates include: bisphenol A ethylene oxide-modified di(meth)acrylate, bisphenol A propylene oxide-modified di(meth)acrylate, trimethylolpropane ethylene oxide-modified tri(meth)acrylate, trimethylolpropane propylene oxide-modified tri(meth)acrylate, ethylene oxide isocyanate-modified di(meth)acrylate, propylene oxide isocyanate-modified di(meth)acrylate, ethylene oxide isocyanate-modified tri(meth)acrylate, propylene oxide isocyanate-modified tri(meth)acrylate, pentaerythritol ethylene oxide-modified tetra(meth)acrylate, pentaerythritol propylene oxide-modified tetra(meth)acrylate, etc.

[0066] The polyfunctional (meth)acrylates may also be: zinc (meth)acrylate, magnesium (meth)acrylate, calcium (meth)acrylate, barium (meth)acrylate, strontium (meth)acrylate, nickel (meth)acrylate, copper (meth)acrylate, aluminum (meth)acrylate and other (meth)acrylate metal salts.

[0067] The compatibility between the base polymer and the photocuring agent is also affected by the molecular weight of the photocuring agent. The smaller the molecular weight of the photocuring agent, the higher the compatibility with the base polymer. From the viewpoint of compatibility with the base polymer, the molecular weight of the photocuring agent is preferably below 1500, more preferably below 1000, and can also be below 800, 600, 500, 450, or 400.

[0068] Furthermore, the smaller the functional group equivalent of the photocuring agent (i.e., the larger the number of functional groups per unit molecular weight), the higher the compatibility with the base polymer. From the viewpoint of compatibility with the base polymer, the functional group equivalent (g / eq) of the photocuring agent is preferably 80 to 300, more preferably 90 to 200, and even more preferably 100 to 170, and may also be 110 to 160 or 120 to 150.

[0069] When the base polymer and the photocuring agent are not a completely compatible system, the liquid photocuring agent will seep to the surface and form an adhesion barrier layer (WBL, Weak Boundary Layer) at the bonding interface with the adherend, thus strengthening the liquid properties. If a WBL is formed, the liquid properties of the surface (bonding interface) become stronger while maintaining the overall properties of the adhesive layer such as the storage modulus, thus the adhesion to the adherend tends to decrease.

[0070] When the base polymer and the photocuring agent exhibit moderate compatibility but are not completely compatible systems, the adhesive layer before photocuring has a low initial adhesion due to the formation of WBL, making it easy to peel off from the adherend. On the other hand, after photocuring, the liquid properties of the photocuring agent disappear, and the photocrosslinking structure is uniformly introduced into the adhesive layer, thus significantly improving the adhesion to the adherend.

[0071] The compatibility between the base polymer and the photocuring agent is mainly affected by the compound structure. Compound structure and compatibility can be evaluated, for example, using the Hansen solubility parameter (HSP). The Hansen solubility parameter (HSP) divides the Hildebrand solubility parameter δ into three components: dispersion term δd, polar term δp, and hydrogen bonding term δh, and represents them in three-dimensional space, with the relationship δ² = δd² + δp² + δh² holding true. The dispersion term δd represents the effect based on dispersion forces, the polar term δp represents the effect based on dipole forces, and the hydrogen bonding term δh represents the effect based on hydrogen bonding forces. The distance Ra between the HSPs of two substances is represented by Ra = {4Δδd² + Δδp² + Δδh²}¹ / ², based on the difference in dispersion term Δδd, polar term Δδp, and hydrogen bonding term Δδh between the two substances. A smaller Ra indicates higher compatibility, and a larger Ra indicates lower compatibility.

[0072] Details of Hansen’s solubility parameters are recorded in Charles M. Hansen’s Hansen Solubility Parameters: A Users Handbook (CRC Press, 2007). For substances whose literature values ​​are unknown, the computer software Hansen Solubility Parameters in Practice (HSPiP) can be used for calculation.

[0073] As described above, in acrylic-based base polymers with lower glass transition temperatures, the proportion of (meth)acrylates such as 2-ethylhexyl acrylate in the constituent monomer components is higher, while the proportion of highly polar monomers such as hydroxyl-containing monomers, carboxyl-containing monomers, and nitrogen-containing monomers is lower. Therefore, when using base polymers with lower glass transition temperatures and lower polarity, by using polyfunctional (meth)acrylates incorporating epoxides such as ethylene oxide or propylene oxide as photocuring agents, the HSP distance Ra between the base polymer and the photocuring agent can be adjusted to a suitable range. The larger the chain length n of the epoxide in the epoxide-modified polyfunctional (meth)acrylate, the greater the HSP distance Ra tends to be.

[0074] When the distance Ra between the base polymer and the HSP of the photocuring agent is too large, although the initial adhesion is low, contamination may occur due to the exudation of the photocuring agent, or the adhesion improvement under photocuring may become insufficient. Therefore, the chain length n of the epoxide is preferably 1 to 5, more preferably 1 to 3. For example, when the photocuring agent is trimethylolpropane ethylene oxide modified triacrylate, the chain length n of the ethylene oxide (EO) added to the three hydroxymethyl groups of trimethylolpropane is preferably 1 or 2. Adhesives containing the following components have lower initial adhesion and exhibit higher adhesion to the substrate after photocuring: a low Tg acrylic base polymer whose main monomer component is 2-ethylhexyl acrylate, and trimethylolpropane EO modified triacrylate with a chain length n of 1 or 2. To suppress the exudation of the photocuring agent to the adhesive layer surface, the chain length n of EO is preferably 1.

[0075] The type and amount of photocuring agent not only affect the bonding strength but also the overall properties of the adhesive. If the base polymer of the adhesive composition is the same, the change in the storage modulus of the adhesive layer before photocuring is relatively small, even if the type of photocuring agent is different. On the other hand, if the content of the photocuring agent increases, the content of the base polymer in the composition will relatively decrease, and therefore the storage modulus of the adhesive layer before photocuring tends to decrease.

[0076] Because the smaller the functional group equivalent of the photocuring agent and the higher the content of the photocuring agent, the higher the crosslinking density generated by photocuring becomes. Therefore, the storage modulus of the adhesive layer after photocuring tends to increase. That is, the higher the content of the photocuring agent, the following trend occurs: the smaller the storage modulus of the adhesive layer before photocuring, the larger the storage modulus of the adhesive layer after photocuring.

[0077] In order to improve the adhesion of the adhesive after light curing and suppress the excessive increase of the storage modulus, the content of the light curing agent in the adhesive composition is preferably 3 to 30 parts by weight, more preferably 5 to 20 parts by weight, or 6 to 15 parts by weight or 7 to 12 parts by weight, relative to 100 parts by weight of the base polymer.

[0078] Two or more photocuring agents may be used in combination. When two or more photocuring agents are used in combination, it is preferable that the total amount of photocuring agents is within the range described above. For example, by using a photocuring agent with relatively high compatibility with the base polymer and a photocuring agent with relatively low compatibility with the base polymer in combination, the properties of the adhesive layer before and after photocuring can be adjusted, such as suppressing the initial adhesion to a lower level and increasing the storage modulus of the photocured adhesive at room temperature.

[0079] (Photopolymerization Initiator) The adhesive composition preferably includes a photopolymerization initiator. The photopolymerization initiator generates active species by irradiation with active light, thereby promoting the curing reaction of the photocuring agent. Depending on the type of photocuring agent, photopolymerization initiators can be: photocationic initiators (photoacid generators), photoradical initiators, photoanionic initiators (photoalkali generators), etc. When using polyfunctional acrylates as photocuring agents, photoradical initiators are preferred. As photoradical initiators, photoradical generators that generate free radicals through cleavage by short-wavelength visible light or ultraviolet light (shorter than 450 nm) are preferred; examples include: hydroxy ketones, benzopyrene dimethyl ketals, amino ketones, phosphine oxides, benzophenones, and trichloromethyl trichlorotrimethylamine derivatives, etc. Photoradical generators can be used alone or in combination of two or more.

[0080] When transparency is required for the adhesive layer 2, the photopolymerization initiator (photoradical generator) is preferably less sensitive to long wavelengths of light (visible light) greater than 400 nm. For example, it is preferable to use a photopolymerization initiator with an absorption coefficient of 1×10² [mLg⁻¹cm⁻¹] or less at a wavelength of 405 nm. Furthermore, if a photopolymerization initiator with less sensitivity to visible light is used, the photopolymerization initiator is less likely to decompose due to external light in the storage environment, thereby improving the storage stability of the reinforcing film.

[0081] The content of 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 even more preferably 0.03 to 1 part by weight, relative to 100 parts by weight of the base polymer.

[0082] (Oligomer) The adhesive composition may also include oligomers in addition to the base polymer. For example, the adhesive composition may also include acrylic oligomers in addition to an acrylic base polymer. As the oligomer, an oligomer with a weight average molecular weight of about 1,000 to 30,000 is used. The acrylic oligomer contains alkyl (meth)acrylate as the main constituent monomer component. In order to improve the adhesion of the adhesive layer 2 after photocuring, the glass transfer temperature of the acrylic oligomer is preferably 40°C or higher, and more preferably 50°C or higher. The oligomer may also contain crosslinkable functional groups in the same way as the base polymer.

[0083] The content of oligomers in the adhesive composition is not particularly limited. When the adhesive composition contains acrylic oligomers in addition to acrylic base polymers, in order to adjust the adhesion to an appropriate range, the amount of oligomers relative to 100 parts by weight of base polymer is preferably 0.1 to 20 parts by weight, more preferably 0.3 to 10 parts by weight, and even more preferably 0.5 to 5 parts by weight.

[0084] (Other Additives) In addition to the components listed above, the adhesive layer may also contain additives such as silane coupling agents, adhesive imparting agents, plasticizers, softeners, anti-deterioration agents, fillers, colorants, ultraviolet absorbers, antioxidants, surfactants, and antistatic agents, without impairing the characteristics of the present invention.

[0085] [Fabrication of reinforcing film] A reinforcing film can be obtained by depositing a photocurable adhesive layer 2 on a film substrate 1. The adhesive layer 2 can be formed directly on the film substrate 1, or an adhesive layer that is formed in sheet form on other substrates can be transferred to the film substrate 1.

[0086] The adhesive composition described above is applied to a substrate by means of roller coating, contact roller coating, gravure coating, reverse coating, roller brush coating, spraying, dip roller coating, rod coating, doctor blade coating, air knife coating, curtain coating, die lip coating, or die nozzle coating, and the solvent is dried and removed as needed, thereby forming an adhesive layer. Appropriate drying methods can be used as needed. The heating and drying temperature is preferably 40°C to 200°C, more preferably 50°C to 180°C, and even more preferably 70°C to 170°C. The drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 15 minutes, and even more preferably 10 seconds to 10 minutes.

[0087] When the adhesive composition contains a crosslinking agent, it is preferable to perform crosslinking by heating or aging simultaneously with or after the drying of the solvent. The heating temperature or heating time is appropriately set according to the type of crosslinking agent used. Generally, crosslinking is performed by heating for about 1 minute to 7 days within the range of 20°C to 160°C. The heating used for drying and removing the solvent can also be used for crosslinking.

[0088] By introducing a cross-linking structure into the base polymer, the following trend occurs: the gel fraction increases, and the storage modulus of the adhesive layer 2 increases. The higher the gel fraction of the adhesive before photocuring, the harder the adhesive becomes, and when the reinforcing film is peeled off from the adherend due to secondary processing, the residue of the paste on the adherend tends to be suppressed. The gel fraction of the adhesive layer 2 before photocuring (i.e., the gel fraction of the photocurable composition constituting the adhesive layer) is preferably 25% or more, more preferably 30% or more, and may also be 35% or more, 40% or more, or 45% or more. On the other hand, when the gel fraction is too high, the storage modulus becomes high, and the stress-strain mitigation is low. Therefore, when using a flexible device, adhesive peeling may occur at the bending part. Therefore, the gel content of the adhesive layer 2 before light curing is preferably 80% or less, more preferably 75% or less, and even more preferably 70% or less, or 65% or less.

[0089] The insoluble fraction in solvents such as ethyl acetate can be determined as the gel fraction. Specifically, after immersing the adhesive layer in ethyl acetate at 23°C for 7 days, the weight fraction (unit: weight %) of the insoluble component relative to the sample before immersion is determined as the gel fraction. Generally speaking, the gel fraction of a polymer is equivalent to its degree of crosslinking; the more crosslinked parts in the polymer, the greater the gel fraction.

[0090] After the cross-linking structure is introduced into the polymer using a cross-linking agent, the photocuring agent remains in an unreacted state. Therefore, a photocurable adhesive layer 2 comprising a base polymer and a photocuring agent is formed. When the adhesive layer 2 is formed on the film substrate 1, it is preferable to attach a release liner 5 to the adhesive layer 2 to, for example, protect the adhesive layer 2. Alternatively, the cross-linking can be performed after attaching the release liner 5 to the adhesive layer 2.

[0091] When the adhesive layer 2 is formed on other substrates, the adhesive layer 2 can be transferred to the film substrate 1 after the solvent is dried, thereby obtaining a reinforcing film. The substrate used in the formation of the adhesive layer can also be used directly as the release film 5.

[0092] Preferably, the release liner 5 is made of plastic film such as polyethylene, polypropylene, polyethylene terephthalate, or polyester film. The thickness of the release liner is typically 3 to 200 μm, preferably around 10 to 100 μm. For the contact surface between the release liner 5 and the adhesive layer 2, it is preferable to use a release agent such as a silicone-based, fluorine-based, long-chain alkyl-based, or fatty acid amide-based release agent, or silica powder, for release treatment. By performing release treatment on the surface of the release liner 5, when the membrane 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 fixed state on the membrane substrate 1. For the release liner 5, either or both of the release-treated and untreated surfaces can be treated with antistatic agents. By performing antistatic treatment on the release liner 5, charging can be suppressed when the release liner is peeled off from the adhesive layer.

[0093] [Characteristics and Use of the Reinforcing Film] The reinforcing film of the present invention is used to adhere to a device or a component of a device. In the reinforcing film 10, the adhesive layer 2 is fixedly bonded to the film substrate 1. After being bonded to the adhered body and before photocuring, the adhesion between the reinforcing film 10 and the adhered body is relatively small. Therefore, before photocuring, the reinforcing film is easily peeled off from the adhered body.

[0094] The substrate to which the reinforcing film is adhered is not particularly limited, and examples include various electronic devices, optical devices, and their constituent parts. In one embodiment, the reinforcing film is adhered to the surface of a flexible device. As shown in FIG3, the flexible device has a hinge portion 25, which allows it to be bent around the hinge portion. The bending angle can be set arbitrarily, and it can also be bent 180° (folded) as shown in FIG4. Furthermore, FIG4 shows the device being bent with the adhesive surface of the reinforcing film 10 as the inside, but it can also be bent with the reinforcing film 10 as the outside. When the device is a display device, the reinforcing film can be adhered to the surface on the screen side, or it can be adhered to the back side (housing). As shown in FIG3 and 4, for a flexible device configured to be bent at a specific location such as the hinge portion 25, its use involves repeatedly bending and unfolding at the same location.

[0095] The reinforcing film can be adhered to the entire surface of the adherend, or selectively adhered only to the portion requiring reinforcement (the area to be reinforced). Alternatively, the reinforcing film can be adhered to both the portion requiring reinforcement (the area to be reinforced) and the area not requiring reinforcement (the area not to be reinforced), and then the reinforcing film adhered to the area not to be reinforced can be cut and removed. If it is an adhesive before light curing, the reinforcing film is temporarily adhered to the surface of the adherend, and therefore can be easily peeled off from the surface of the adherend. Alternatively, the reinforcing film can be adhered to both the area to be reinforced and the area not to be reinforced, and the area to be reinforced can be selectively irradiated with light to light-cur the adhesive, and then the reinforcing film in the uncured area of ​​the area not to be reinforced can be selectively peeled off.

[0096] By applying the reinforcing film, appropriate rigidity is imparted, thus improving operability or preventing breakage for thinner components such as flexible devices. In the device manufacturing process, when applying the reinforcing film to a semi-finished product, the reinforcing film can be applied to a large-sized semi-finished product before it is cut to the finished product size. Alternatively, the reinforcing film can be applied to the master roller of a device manufactured using a roll-to-roll process.

[0097] Before laminating the reinforcing film, the surface of the adherend can also be activated to achieve purification, etc. Examples of surface activation treatments include plasma treatment, corona treatment, and glow discharge treatment. Adhesives with activated surfaces contain more active groups such as hydroxyl, carbonyl, and carboxyl groups, which can easily improve adhesion through intermolecular interactions with the polar functional groups of the base polymer of the adhesive. Especially when the adherend is polyimide, the activation treatment activates amides, terminal amine groups, or carboxyl groups (or carboxylic anhydride groups), resulting in stronger interactions with the polar functional groups of the base polymer. Therefore, the initial adhesion can be significantly improved through activation treatment.

[0098] If the initial adhesion force becomes too large, it may be difficult to perform peeling operations such as secondary processing. As mentioned above, by making the base polymer substantially free of nitrogen atoms, the excessive increase in the initial adhesion force to the surface-activated adherend can be suppressed.

[0099] To facilitate easy peeling from the substrate and prevent paste residue from remaining on the substrate after peeling off the reinforcing film, the adhesion strength between the adhesive layer 2 before light curing and the substrate is preferably 1 N / 25 mm or less, more preferably 0.5 N / 25 mm or less, and even more preferably 0.3 N / 25 mm or less, or 0.1 N / 25 mm or less, or 0.05 N / 25 mm or less. To prevent the reinforcing film from peeling off during storage or handling, the adhesion strength between the adhesive layer 2 before light curing and the substrate is preferably 0.005 N / 25 mm or more, more preferably 0.01 N / 25 mm or more. The adhesion strength is determined by a peel test using a polyimide film as the substrate at a tensile speed of 300 mm / min and a peel angle of 180°. Unless otherwise specified, the adhesion strength refers to the value measured at 25°C.

[0100] The storage modulus of the adhesive layer 2 before photocuring at 25°C is preferably 5.0 × 10³ to 1.0 × 10⁵ Pa. To prevent paste residue from remaining on the substrate when the reinforcing film is peeled off from the substrate, the storage modulus of the adhesive layer 2 before photocuring at 25°C is preferably 7.0 × 10³ Pa or more, more preferably 9.0 × 10³ Pa or more, or 1.0 × 10⁴ Pa or more, or 1.5 × 10⁴ Pa or more. To ensure the adhesive layer has flexibility, the storage modulus of the adhesive layer 2 before photocuring at 25°C is preferably 7.0 × 10⁴ Pa or less, more preferably 5.0 × 10⁴ Pa or less, or 4.0 × 10⁴ Pa or less, or 3.0 × 10⁴ Pa or less.

[0101] The storage modulus of the adhesive layer 2 before light curing at -20°C is preferably 1.0×10⁴ to 2.0×10⁵ Pa, more preferably 2.0×10⁴ to 1.0×10⁵ Pa, and may also be 3.0×10⁴ to 9.0×10⁴ Pa or 4.0×10⁴ to 9.0×10⁴ Pa. By making the storage modulus of the adhesive layer before light curing within the above range at low temperature, the following trend is observed: even after light curing, the storage modulus of the adhesive layer is kept relatively low.

[0102] The storage modulus of the adhesive layer before photocuring depends on the composition of the base polymer, the amount of crosslinking agent introduced, and the content of the photocuring agent. The more crosslinking agent introduced, the higher the gel fraction and the greater the storage modulus. The more photocuring agent, the less base polymer is in the composition, and therefore the storage modulus tends to decrease.

[0103] The storage modulus of the adhesive changes drastically near the glass transition temperature of the base polymer. As described above, by making the glass transition temperature of the base polymer sufficiently lower than -20°C, an adhesive with a lower storage modulus at low temperatures can be prepared. The storage modulus of the adhesive layer before photocuring at -20°C is preferably less than 7 times that at 25°C, more preferably less than 5 times, and may also be less than 4 times, less than 3.5 times, or less than 3 times. The storage modulus of the adhesive layer before photocuring at -20°C may also be more than 1.5 times, more than 1.8 times, or more than 2.0 times that at 25°C.

[0104] After the reinforcing film is bonded to the substrate, the adhesive layer 2 is photocured by irradiating it with active light. Examples of active light include ultraviolet light, visible light, infrared light, X-rays, alpha rays, beta rays, and gamma rays. To inhibit the curing of the adhesive layer under storage conditions and facilitate curing, ultraviolet light is preferred. The irradiation intensity or duration of the active light can be appropriately set according to the composition or thickness of the adhesive layer. When irradiating the adhesive layer 2 with active light, it can be done from either the film substrate 1 side or the substrate side, or from both sides.

[0105] With photocuring, not only does the storage modulus of the adhesive layer increase, but the adhesion to the substrate also increases. From the viewpoint of adhesion reliability in practical use of the device, the adhesion between the photocured adhesive layer 2 and the substrate is preferably 2 N / 25 mm or more, and more preferably 2.5 N / 25 mm or more. In order to suppress adhesive peeling when the same part of the flexible device is repeatedly bent, the adhesion between the photocured adhesive layer 2 and the substrate can also be 3 N / 25 mm or more, 4 N / 25 mm or more, 5 N / 25 mm or more, 6 N / 25 mm or more, 7 N / 25 mm or more, 8 N / 25 mm or more, 9 N / 25 mm or more, or 10 N / 25 mm or more. The adhesion between the adhesive layer 2 after photocuring and the substrate is preferably more than 10 times, more preferably more than 30 times, and can also be more than 50 times, more than 100 times, or more than 200 times. As described above, by adjusting the type (compatibility with the base polymer) and amount of photocuring agent, the adhesion before photocuring (initial adhesion) can be suppressed to a lower level, and the adhesion of the adhesive after photocuring can be increased.

[0106] As described above, the adhesive layer 2 after photocuring preferably has a storage modulus of 1.0 × 10⁴ to 5.0 × 10⁵ Pa at -20°C and a storage modulus of 8.0 × 10³ to 1.5 × 10⁵ Pa at 25°C. By making the storage modulus of the adhesive layer in the low-temperature region lower, the peeling of the adhesive layer tends to be suppressed when it is repeatedly bent at low temperature.

[0107] The storage modulus of the adhesive layer after light curing at -20°C is preferably less than 7 times, more preferably less than 5 times, and may also be less than 4 times, less than 3.5 times, or less than 3 times. The storage modulus of the adhesive layer before light curing at -20°C may also be more than 1.5 times, more than 1.8 times, or more than 2.0 times the storage modulus at 25°C.

[0108] The storage modulus of the adhesive layer after light curing at -20°C is preferably less than 7 times, or less than 5 times, 4 times, or 3.5 times that of the adhesive layer before light curing at -20°C. The storage modulus of the adhesive layer after light curing at -20°C may also be more than 1.1 times, 1.3 times, 1.5 times, or 1.7 times that of the adhesive layer before light curing at -20°C.

[0109] The storage modulus of the adhesive layer after light curing at 25°C is preferably less than 7 times that of the adhesive layer before light curing at 25°C, and may also be less than 5 times, less than 4 times, or less than 3.5 times. The storage modulus of the adhesive layer after light curing at -20°C may be more than 1.1 times, more than 1.3 times, more than 1.5 times, or more than 1.7 times that of the adhesive layer before light curing at -20°C.

[0110] As described above, by attaching the reinforcing film of the present invention, the adherend is given appropriate rigidity, and stress is relieved and dispersed. Therefore, various abnormalities that may occur during the manufacturing process can be suppressed, production efficiency can be improved, and yield can be increased. Furthermore, the reinforcing film is easy to peel off from the adherend before the adhesive layer is light-cured, so even if lamination or poor adhesion occurs, secondary processing is easy. In addition, it is also easy to selectively remove the reinforcing film outside the area to be reinforced.

[0111] When using the completed device, even in the event of accidental external loads such as the device being dropped, heavy objects being placed on it, or impacts from flying objects, the reinforcing film prevents damage to the device. Furthermore, because the reinforcing film, after the adhesive has been light-cured, is firmly bonded to the device, it is not prone to peeling even after long-term use, exhibiting excellent reliability. Moreover, the adhesive layer of the reinforcing film has a low storage modulus even at low temperatures, therefore, even when used in flexible devices, adhesive peeling is less likely to occur at bent portions, demonstrating excellent bonding reliability. [Example]

[0112] Hereinafter, examples and comparative examples will be given to further illustrate the invention, but the invention is not limited to these examples.

[0113] [Polymerization of Acrylic Polymers] <Polymer A> 96 parts by weight of 2-ethylhexyl acrylate (2EHA) and 4 parts by weight of 2-hydroxyethyl acrylate (2HEA) as monomers, 0.2 parts by weight of azobisisobutyronitrile (AIB) as polymerization initiator, and 233 parts by weight of ethyl acetate as solvent were added to a reaction vessel equipped with a thermometer, stirrer, reflux condenser, and nitrogen inlet. Nitrogen gas was then introduced, and the mixture was stirred while undergoing nitrogen purging for approximately 1 hour. The mixture was then heated to 60°C and reacted for 7 hours to obtain a solution of acrylic polymer A with a weight average molecular weight (Mw) of 550,000.

[0114] <Polymers B~I> The amount of monomers added was changed as shown in Table 1. Otherwise, the polymerization was carried out in the same manner as that of polymer A to obtain solutions of polymers B~I.

[0115] The monomer ratios and glass transition temperatures of acrylic polymers A to I are summarized in Table 1. Furthermore, the glass transition temperatures are calculated based on the Fox formula using the monomer ratios. 2EHA 2-Ethylhexyl acrylate BA Butyl acrylate MMA Methyl methacrylate NVP N-Vinylpyrrolidone 2HEA 2-Hydroxyethyl acrylate 4HBA 4-Hydroxybutyl acrylate AA β-CEA 2-Carboxyethyl acrylate HOA-MS 2-Acryloyloxyethyl succinate

[0116] [Table 1] Monomer components (weight %) Tg (°C) 2EHA BA MMA NVP 2HEA 4HBA SECOND β-CEA HOA-MS A 96 - - - 4 - - - - -68 B 95 - - - 5 - - - - -68 C 91 - - - - 9 0.02 - - -67 D 98.5 - - - - 1.5 - - - -70 ITS - 95 - - 5 - - -50 F 63 - 9 15 13 - - - - -34 G 96 - - - - - 4 - - -66 H 96 - - - - - - 4 - -67 I 96 - - - - - - - 4 -69

[0117] [Preparation of Reinforcing Film] <Preparation of Adhesive Composition> A crosslinking agent and a photocuring agent were added to an acrylic polymer solution and mixed uniformly to prepare the adhesive composition shown in Table 2. The amounts of crosslinking agent and photocuring agent in Table 2 are relative to 100 parts by weight of the solid content of the base polymer. Details of the crosslinking agent and photocuring agent are described below. In Examples 7-9, two photocuring agents were used, and the amounts of each photocuring agent are shown in Table 2.

[0118] (Crosslinking agent) Takenate D110N: 75% ethyl acetate solution of trimethylolpropane adduct of phenyl diisocyanate (Mitsui Chemicals Co., Ltd. "Takenate D110N") C-HX: Isocyanurate form of hexamethylene diisocyanate (Tosoh Co., Ltd. "Coronate HX") TC: N,N,N',N'-tetraglycidyl-m-phenylenediamine (a 4-functional epoxy compound, Mitsubishi Gas Chemical Co., Ltd. "Tetrad C")

[0119] (Light curing agent) M350: Trimethylolpropane EO modified (n=1) triacrylate ("ARONIX M-350" manufactured by Dong-A Synthetic Co., Ltd., functional group equivalent 144 g / eq) M360: Trimethylolpropane EO modified (n=2) triacrylate ("ARONIX M-360" manufactured by Dong-A Synthetic Co., Ltd., functional group equivalent 186 g / eq) M310: Trimethylolpropane PO modified (n=1) triacrylate ("ARONIX M-310" manufactured by Dong-A Synthetic Co., Ltd., functional group equivalent 158 ​​g / eq) M321: Trimethylolpropane PO modified (n=2) triacrylate ("ARONIX M-321" manufactured by Dong-A Synthetic Co., Ltd., functional group equivalent 200 g / eq) APG700: Polypropylene glycol #700 (n=12) diacrylate (NK ESTER APG700 manufactured by Shin-Nakamura Chemical Industry Co., Ltd., functional group equivalent 404 g / eq) A200: Polyethylene glycol #200 (n=4) diacrylate (NK ESTER A200 manufactured by Shin-Nakamura Chemical Industry Co., Ltd., functional group equivalent 154 g / eq) TMPT: Trimethylolpropane triacrylate (NK ESTER TMPT manufactured by Shin-Nakamura Chemical Industry Co., Ltd., functional group equivalent 99 g / eq) ZnAc: Zinc acrylate (manufactured by Nisshoku Techno Fine Co., Ltd., functional group equivalent 104 g / eq)

[0120] (Photopolymerization Initiator) In addition to the components shown in Table 2, 0.2 parts by weight of 2,2-dimethoxy-1,2-diphenylethane-1-one (Omnirad 651 manufactured by IGM Resins) was added as a photopolymerization initiator relative to 100 parts by weight of the solid content of the base polymer in Examples 1-18 and Comparative Examples 1-3, 7, and 8. In Comparative Examples 4-6, 0.1 parts by weight of 1-hydroxycyclohexylphenyl ketone (Omnirad 184 manufactured by IGM Resins) was added as a photopolymerization initiator relative to 100 parts by weight of the solid content of the base polymer.

[0121] <Coating and Crosslinking of Adhesive Solution> On a transparent film substrate without surface treatment, the above-mentioned adhesive composition was coated with a grooved roller to a thickness of 13 μm after drying. After drying at 130°C for 1 minute to remove the solvent, the release-treated side of the release film (a 25 μm thick polyethylene terephthalate film with one side treated with silicone release and both sides treated with antistatic treatment) was attached to the coated side of the adhesive. Then, crosslinking was performed by aging at 25°C for 4 days, thereby obtaining a reinforcing film on which a photocurable adhesive sheet was fixedly laminated on a transparent film substrate, and on which the release film was temporarily adhered. In Examples 1-5, 7-18, and Comparative Examples 1-8, a 75 μm thick polyethylene terephthalate film (“Lumirror S10” manufactured by Toray Industries, Inc.) was used as the transparent film substrate. In Example 6, a 50 μm thick ultraviolet-transparent transparent polyimide film (Neopulim S-100 manufactured by Mitsubishi Gas Chemical) was used as the transparent film substrate.

[0122] [Evaluation] <Storage Modulus> An adhesive sheet (before photocuring) was prepared by coating and crosslinking the adhesive composition onto the release liner in the same manner as described above. An release liner was attached to the surface of the adhesive layer of the adhesive sheet before photocuring to block oxygen, and it was photocured by irradiating it with 2000 mJ / cm2 ultraviolet light using a 365 nm LED (Light-Emitting Diode) lamp. The adhesive sheet before photocuring and the adhesive sheet after photocuring were laminated to prepare a test sample with a thickness of about 1.5 mm. Dynamic viscoelasticity was measured using the "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific under the following conditions, and the shear storage modulus G' values ​​at -20°C and 25°C were read. (Measurement Conditions) Deformation Mode: Torsion Measurement Frequency: 1 Hz Heating Rate: 5℃ / min Measurement Temperature: -50~150℃ Shape: Parallel plate 8.0 mm ϕ

[0123] <Adhesion> A 12.5 μm thick polyimide film ("Kapton 50EN" manufactured by Toray DuPont) was attached to a glass plate using double-sided adhesive tape ("No. 531" manufactured by Nitto Denko Corporation) to obtain a polyimide film substrate for testing. The release liner was peeled off from the surface of a reinforcing film cut into 25 mm wide x 100 mm long sections, and then adhered to the polyimide film substrate for testing using a hand roller to obtain a test sample before photocuring. The adhesive layer was photocured by irradiating the reinforcing film side (PET film substrate side) of the test sample before photocuring, and the obtained product after photocuring was used as the test sample after photocuring. Using these test samples, the end of the reinforcing film substrate was held by a clamp, and the reinforcing film was peeled 180° at a stretching speed of 300 mm / min to measure the peel strength.

[0124] <Bending Test> The release liner is peeled off from the surface of the reinforcing film, and a polyimide film ("Kapton 50EN" manufactured by Toray DuPont) is bonded to the surface of the adhesive layer using a hand roller. The laminate is cut into pieces with a width of 25 mm and a length of 100 mm. The adhesive layer is photocured by irradiating ultraviolet light from the reinforcing film side (PET film substrate side) to obtain a test piece. Using a planar unloaded U-shaped stretch tester (manufactured by YUASA SYSTEM), a bending jig is mounted on the short side of the test piece, and repeated bending tests are performed in a constant temperature bath at -20°C or 25°C and 50% relative humidity, with the reinforcing film side (film substrate side) as the inside, under the following conditions. Test pieces where no peeling or bulging occurs between the reinforcing film and the adhered material at the bent portion after repeated bending tests are evaluated as OK, and test pieces where peeling or bulging occurs are evaluated as NG. (Test Conditions) Bending radius: 3 mm; Bending angle: 180°; Bending speed: 1 second / bending cycle; Number of bending cycles: 200,000

[0125] The composition of the adhesive for each reinforcing film, the shear storage modulus G' and adhesion force of the adhesive layer before and after light curing, and the evaluation results of the bending test are shown in Table 2.

[0126] [Table 2] Adhesive composition G'(×10 4 Pa) Following force (N / 25 mm) Bending test polymer Crosslinking agent Light curing agent Before hardening After hardening type Tg (°C) type quantity type quantity -20℃ 25℃ -20℃ 25℃ Before hardening After hardening -20℃ 25℃ Example 1 A -68 C-HX 0.10 M350 10 5.8 0.98 9.8 2.1 0.05 10.7 OK OK Example 2 A -68 C-HX 0.25 M350 10 6.2 1.9 13 3.7 0.02 11.5 OK OK Example 3 A -68 C-HX 0.50 M350 10 7.4 3.4 25 7.7 0.02 10.5 OK OK Example 4 A -68 C-HX 0.25 M350 5.0 6.0 1.7 12 3.4 0.40 16.0 OK OK Example 5 A -68 C-HX 0.25 M350 7.5 6.1 1.8 13 3.6 0.10 15.1 OK OK Example 6 A -68 C-HX 0.25 M350 10 6.2 1.9 13 3.7 0.02 11.5 OK OK Example 7 A -68 C-HX 0.25 M350 7.5 6.5 2.0 twenty two 6.1 0.08 17.8 OK OK TMPT 5.0 Example 8 A -68 C-HX 0.25 M350 7.5 6.4 2.0 31 8.0 0.11 13.7 OK OK TMPT 10 Example 9 A -68 C-HX 0.25 M350 10 6.2 1.9 18 7.8 0.15 18.5 OK OK ZnAc 0.5 Example 10 A -68 C-HX 0.25 A200 10 6.2 1.9 13 4.0 0.32 3.9 OK OK Example 11 A -68 C-HX 0.25 M360 10 6.2 1.9 13 3.7 0.02 3.5 OK OK Example 12 C -67 C-HX 0.25 M350 10 6.1 1.9 12 3.6 0.10 14.1 OK OK Example 13 D -70 C-HX 0.25 M350 10 5.9 1.8 11 3.6 0.02 13.6 OK OK Example 14 G -66 TC 0.25 M350 10 7.2 2.4 19 3.8 0.02 9.8 OK OK Example 15 H -67 TC 0.25 M350 10 6.9 2.3 15 3.7 0.02 9.5 OK OK Example 16 I -69 TC 0.25 M350 10 6.2 2.3 13 3.7 0.02 9.9 OK OK Example 17 A -68 C-HX 0.25 M310 10 6.2 1.9 twenty one 4.2 0.02 6.8 OK OK Example 18 A -68 C-HX 0.25 M321 10 6.2 1.9 14 3.7 0.02 6.9 OK OK Comparative Example 1 E -50 TC 0.50 A200 30 8.8 4.8 130 15 0.10 11.3 NG OK Comparative Example 2 F -34 D110N 2.5 APG700 30 6000 7.8 6800 35 0.33 6.9 NG OK Comparative Example 3 F -34 C-HX 0.25 M350 10 100 9.5 280 38 0.85 4.2 NG OK Comparative Example 4 B -68 D110N 2.0 TMPT 20 79 6.9 98 twenty two 0.53 15.4 NG OK Comparative Example 5 B -68 D110N 2.0 APG700 20 10 0.4 54 1.7 0.29 0.80 NG OK Comparative Example 6 A -68 C-HX 4.0 APG700 20 13 0.85 55 6.3 0.43 0.81 NG OK Comparative Example 7 A -68 C-HX 1.5 M350 10 11 9.4 65 twenty one 0.01 0.40 NG OK Comparative Example 8 A -68 C-HX 0.25 TMPT 10 6.2 1.9 52 9.5 0.85 1.1 NG OK

[0127] Regarding Comparative Example 2, which uses polymer F with a glass transition temperature of -34°C as the base polymer, the storage modulus at -20°C was relatively large both before and after photocuring of the adhesive layer, and peeling of the adhesive layer was observed after repeated bending tests. Regarding Comparative Example 3, which changed the type of crosslinking agent, reduced the amount of crosslinking material, and reduced the amount of photocuring agent, although the storage modulus of the adhesive layer was smaller than that of Comparative Example 2, the storage modulus at -20°C was still relatively large, and peeling of the adhesive layer was observed after repeated bending tests. Regarding the other comparative examples, similarly, the storage modulus at -20°C was relatively large, and peeling of the adhesive layer was observed after repeated bending tests.

[0128] On the other hand, in Examples 1 to 18, which had a smaller storage modulus of the adhesive layer after light curing at -20°C, no peeling of the adhesive layer was found after repeated bending tests, and good adhesion reliability was demonstrated.

[0129] As can be seen from the comparison between Examples 1-3 and Comparative Example 7, the less isocyanate crosslinking agent used, the smaller the storage modulus of the adhesive layer before and after photocuring. Regarding Examples 14-16, which introduced crosslinking structures into carboxyl-containing base polymers G, H, and I using epoxy crosslinking agents, similarly to Examples 2 and 12, which used isocyanate crosslinking agents, the adhesion before photocuring was lower, and the adhesion after photocuring was higher. As can be seen from the comparison between Examples 4-6, the more photocuring agent used, the lower the initial adhesion.

[0130] A comparison of Examples 2, 10, 11, 17, and 18 with Comparative Example 8 shows that, when the type of photocuring agent was changed, although the storage modulus of the adhesive layer before photocuring did not differ significantly, the initial adhesion strength did differ. Furthermore, in these examples, the adhesion strength and storage modulus of the adhesive after photocuring also differed. In these examples, Example 2, which used M350 as the photocuring agent, exhibited a lower initial adhesion strength and a higher adhesion strength after photocuring, demonstrating excellent characteristics.

[0131] Regarding Comparative Example 8, which used only trimethylolpropane triacrylate (TMPT) as a photocuring agent, although the rate of increase in adhesion before and after photocuring was low, the storage modulus of the adhesive layer after photocuring increased significantly at -20°C, and peeling was observed after the bending test. On the other hand, regarding Examples 7 and 8, which used both TMPT and M350 as photocuring agents, the increase in storage modulus at -20°C caused by photocuring was smaller, and the adhesive layer after photocuring exhibited excellent adhesion properties.

[0132] Based on the above results, it can be seen that not only the composition of the base polymer, but also the type and amount of crosslinking agent, or the type and amount of photocuring agent, have a significant impact on the properties of the adhesive layer before and after photocuring. When the type of photocuring agent is changed, the changes in the physical properties of the adhesive layer before and after photocuring are particularly obvious. With the change of the structure of the photocuring agent, the compatibility between the base polymer and the photocuring agent changes. When the two exhibit moderate compatibility, the initial adhesion is low, and excellent adhesion characteristics are exhibited after photocuring. Furthermore, the storage modulus at low temperatures is low, thus it is believed that even when repeatedly bent, adhesive peeling can be suppressed. [Simplified Explanation of the Diagram]

[0018] Figure 1 is a cross-sectional view showing the laminated structure of the reinforcing film. Figure 2 is a cross-sectional view showing the laminated structure of the reinforcing film. Figure 3 is a cross-sectional view showing the device with the reinforcing film attached. Figure 4 is a cross-sectional view showing the device with the reinforcing film attached in a folded state.

Claims

1. A reinforcing film comprising a film substrate and an adhesive layer fixedly laminated on a main surface of the film substrate, wherein the adhesive layer contains a photocurable composition comprising an acrylic-based polymer and a photocuring agent, wherein the acrylic-based polymer contains one or more monomer units selected from the group consisting of hydroxyl-containing monomers and carboxyl-containing monomers, and has a glass transition temperature of -55°C or below, wherein the acrylic-based polymer is a crosslinked polymer with a crosslinked structure introduced by using 0.05 to 1 part by weight of a crosslinking agent relative to 100 parts by weight of the polymer, and contains an epoxy-modified polyfunctional (meth)acrylate as the photocuring agent, wherein the epoxy-modified polyfunctional (meth)acrylate has a (poly)epoxy chain with 1 to 5 repeating units of epoxy, and the adhesive layer, after photocuring, has a shear storage modulus of 1.0 × 10⁴ to 5.0 × 10⁵ at -20°C. Pa, and its shear storage modulus at 25°C is 8.0×10³~1.5×10⁵ Pa.

2. The reinforcing film of claim 1, wherein the photocurable composition contains 3 to 30 parts by weight of the photocuring agent relative to 100 parts by weight of the acrylic base polymer.

3. The reinforcing film as claimed in claim 1 or 2, wherein the functional group equivalent of the aforementioned photocuring agent is 80 to 300 g / eq.

4. The reinforcing film as claimed in claim 1 or 2, wherein the crosslinking agent is an isocyanate-based crosslinking agent or an epoxy-based crosslinking agent.

5. The reinforcing film as claimed in claim 1 or 2, wherein the adhesive layer has a shear storage modulus of 1.0×10⁴ to 5.0×10⁵ Pa at -20°C and a shear storage modulus of 5.0×10³ to 1.0×10⁵ Pa at 25°C before photocuring.

6. The reinforcing film as claimed in claim 1 or 2, wherein the shear storage modulus of the adhesive layer after photocuring at 25°C is 1.2 to 7 times that of the shear storage modulus at 25°C before photocuring.

7. The reinforcing film as claimed in claim 1 or 2, wherein the shear storage modulus of the adhesive layer after photocuring at -20°C is 1.2 to 7 times that of the shear storage modulus at -20°C before photocuring.

8. An apparatus with a reinforcing film, wherein the reinforcing film is affixed to the surface of a bendable apparatus, and the reinforcing film comprises a film substrate and an adhesive layer fixedly deposited on a main surface of the film substrate, the adhesive layer being adhered to the surface of the apparatus, the adhesive layer comprising a photocurable adhesive composition formed by photocuring a photocurable adhesive composition comprising an acrylic base polymer and a photocuring agent; wherein in the photocurable adhesive composition, the acrylic base polymer contains one or more monomer units selected from the group consisting of hydroxyl-containing monomers and carboxyl-containing monomers, and has a glass transition temperature of -55°C or below, and the acrylic base polymer is a crosslinked polymer with a crosslinked structure introduced by using 0.05 to 1 part by weight of a crosslinking agent relative to 100 parts by weight of the polymer. The photocuring agent contains epoxy-modified polyfunctional (meth)acrylate, which has a (poly)epoxide chain with 1 to 5 repeating units of epoxy; and the adhesive layer has a shear storage modulus of 1.0 × 10⁴ to 5.0 × 10⁵ Pa at -20°C and a shear storage modulus of 8.0 × 10³ to 5.0 × 10⁵ Pa at 25°C.

9. A method for manufacturing a device with a reinforcing film, wherein the reinforcing film is attached to the surface of a bendable device, and the method for manufacturing a device with a reinforcing film involves attaching the adhesive layer of the reinforcing film as claimed in any one of claims 1 to 7 to the surface of the bendable device, and then photocuring the adhesive layer.

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