Reinforced film, apparatus with reinforced film and method of manufacturing thereof

Abstract

The present invention provides a reinforcing film which is easily peeled off immediately after being attached to an adherend, can be firmly bonded to the adherend by photo-curing of an adhesive after being attached to the adherend, and is less likely to be peeled off by a bending test. The reinforcing film (10) has an adhesive layer (2) of a photo-curable composition containing an acrylic base polymer having a crosslinked structure and a photo-curing agent, which is fixedly laminated on one main surface of a film base material (1). The adhesive layer preferably has a shear storage modulus at -20°C after photo-curing of 1.0 x 10 4 ~ 5.0 x 10 5 Pa and a shear storage modulus at 25°C of 8.0 x 10 3 ~ 5.0 x 10 5 Pa.

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. Background Technology

[0002] Adhesive films are sometimes pasted onto the surfaces of optical devices such as displays and electronic devices for purposes such as surface protection and impact resistance. These adhesive films typically have an adhesive layer laminated on the main surface of a film substrate, which is then used to adhere the film to the surface of the device.

[0003] Before use, such as during assembly, processing, or transportation, an adhesive film can be temporarily pasted onto the surface of the device or its constituent components to prevent scratches and damage to the adhered objects. Patent Document 1 discloses a reinforcing film having an adhesive layer formed of a photocurable adhesive composition on a film substrate.

[0004] This reinforcing film, due to the high gelation rate of its adhesive, exhibits low adhesion immediately after bonding with the substrate, making it easy to peel off. Therefore, it allows for reprocessing of the substrate and selective removal of the reinforcing film from areas where reinforcement is not desired. The adhesive in the reinforcing film bonds firmly to the substrate through photocuring, resulting in a permanent adhesion of the film substrate to the substrate surface. This allows it to be used as a reinforcing material for functions such as surface protection of devices.

[0005] In recent years, organic EL panels using flexible substrates such as resin films have become practical, leading to the development of flexible, bendable displays. Foldable flexible displays (foldable displays) are repeatedly bent at the same location. At the bend, the inner side experiences compressive stress, and the outer side experiences tensile stress, resulting in strain at and around the bend, sometimes causing the adhesive to peel off from the adhered material. Patent document 2 proposes a method to relieve stress and strain at the bend by using a soft adhesive sheet for bonding components in a foldable display (e.g., Patent documents 2, 3).

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2020-41113

[0009] Patent Document 2: Japanese Patent Application Publication No. 2018-45213

[0010] Patent Document 3: Japanese Patent Application Publication No. 2017-119801 Summary of the Invention

[0011] The problem the invention aims to solve

[0012] When the reinforcing film proposed in Patent Document 1 is applied to a flexible device, peeling sometimes occurs at bends due to the hardness of the adhesive after photocuring. In particular, the adhesive's softness and low adhesion retention at low temperatures make it prone to peeling from the substrate at bends during bending tests at low temperatures. While the adhesive sheets described in Patent Documents 2 and 3 suppress peeling from the substrate during repeated bending at low temperatures, the adhesives are not photocurable and exhibit high adhesion immediately after bonding, making peeling and processing difficult.

[0013] In view of the above, the object of the present invention is to provide a reinforcing film that is easy to peel off immediately after being bonded to the substrate, can be firmly bonded to the substrate by photocuring the adhesive after being bonded to the substrate, and is not prone to peeling caused by bending tests.

[0014] Solution for solving the problem

[0015] The reinforced film of the present invention comprises an adhesive layer adhered to one main surface of a film substrate. The adhesive layer is formed of a photocurable composition containing an acrylic-based polymer and a photocuring agent. Preferably, the acrylic-based polymer has a glass transition temperature of -45°C or lower.

[0016] The acrylic base polymer, as a monomer unit, contains one or more monomers selected from the group consisting of hydroxyl-containing monomers and carboxyl-containing monomers. A cross-linked structure is introduced by bonding the hydroxyl and / or carboxyl groups of the base polymer with cross-linking agents such as isocyanate-based cross-linking agents and epoxy-based cross-linking agents. The amount of cross-linking agent relative to 100 parts by weight of the acrylic polymer can be approximately 0.05 to 1 part by weight.

[0017] Preferably, the photocurable composition constituting the adhesive layer 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 be an epoxy-modified polyfunctional (meth)acrylate modified with epoxy alkyl groups such as ethylene oxide or propylene oxide. The functional group equivalent of the photocuring agent is, for example, about 80 to 300 g / eq.

[0018] Preferably, the shear storage modulus of the adhesive layer after photocuring at -20°C is 1.0 × 10⁻⁶. 4 ~5.0×10 5 Pa. Preferably, the shear storage modulus of the photocured adhesive layer at 25°C is 8.0 × 10⁻⁶. 3 ~5.0×10 5 Pa.

[0019] Preferably, the shear storage modulus of the adhesive layer before photocuring is 1.0 × 10⁻⁶ at -20°C. 4 ~5.0×10 5 Pa. Preferably, the shear storage modulus of the adhesive layer before photocuring at 25°C is 5.0 × 10⁻⁶. 3 ~1.0×10 4 Pa.

[0020] Preferably, the shear storage modulus of the photocured adhesive layer at 25°C is 1.2 to 7 times that of the adhesive layer before photocuring at 25°C. More preferably, the shear storage modulus of the photocured adhesive layer at -20°C is 1.2 to 7 times that of the adhesive layer before photocuring at -20°C.

[0021] By bonding the aforementioned reinforcing film to the surface of the device and then photocuring the adhesive layer, a device with the reinforcing film can be obtained. The device can be a flexible, bendable device.

[0022] The effects of the invention

[0023] The reinforcing film of the present invention increases the adhesive strength by forming an adhesive layer from a photocurable composition and then photocuring the adhesive layer after bonding with the substrate. Before photocuring, the adhesive strength is low, making it easy to peel off from the substrate. After photocuring, the adhesive layer of the reinforcing film also has a low storage modulus and high stress-strain relaxation, thus suppressing peeling of the adhesive layer at the bending point in flexible devices, even under repeated bending at the same location, resulting in excellent bonding reliability. Attached Figure Description

[0024] Figure 1 This is a cross-sectional view showing the laminated structure of the reinforcing film.

[0025] Figure 2 This is a cross-sectional view showing the laminated structure of the reinforcing film.

[0026] Figure 3 This is a cross-sectional view showing a device with a reinforcing film attached.

[0027] Figure 4 This is a cross-sectional view showing the device with the reinforcing film attached folded. Detailed Implementation

[0028] Figure 1This is a cross-sectional view showing one embodiment of the reinforcing film. The reinforcing film 10 has an adhesive layer 2 on one main surface of the film substrate 1. The adhesive layer 2 is fixedly laminated on one main surface of the film substrate 1. The adhesive layer 2 is a photocurable adhesive formed from a photocurable composition, which is cured by irradiation with active light such as ultraviolet light, thereby increasing the adhesion strength to the adhered object.

[0029] Figure 2 This is a cross-sectional view of a reinforcing film on which the release film 5 is temporarily pasted on the main surface of the adhesive layer 2. Figure 3 This is a cross-sectional view of a device 100 with a reinforcing film 10 attached to the surface of a foldable device 20. Figure 4 This is a cross-sectional view showing the state of the device after it has been bent around hinge 25.

[0030] 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, before the adhesive layer 2 is photocured, the reinforcing film 10 (adhesive layer 2) is temporarily adhered to the device 20. 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 reinforcing film 10 are fixed together.

[0031] "Fixed" refers to a state where two layers are firmly bonded together, making it impossible or difficult to separate them at the interface. "Temporary bond" refers to a state where the adhesive force between two layers is weak, allowing for easy separation at the interface.

[0032] Figure 2 In the reinforced film shown, the film substrate 1 is fixed to the adhesive layer 2, and the release liner 5 is temporarily adhered to the adhesive layer 2. If the film substrate 1 and the release liner 5 are peeled off, peeling will occur at the interface between the adhesive layer 2 and the release liner 5, maintaining the adhesive layer 2 fixed to the film substrate 1. No adhesive residue will remain on the release liner 5 after peeling.

[0033] about Figure 3 The apparatus with the reinforcing film shown has the device 20 and the adhesive layer 2 in a temporarily bonded state before the adhesive layer 2 is photocured. If the film substrate 1 is peeled off from the device 20, peeling will occur at the interface between the adhesive layer 2 and the device 20, thus maintaining the adhesive layer 2 adhered to the film substrate 1. Since the adhesive does not remain on the device 20, peeling operations such as reprocessing and cutting are easy to perform. After the adhesive layer 2 is photocured, the adhesion between the adhesive layer 2 and the device 20 increases, forming a fixed state, making it difficult to peel the film 1 off the device 20.

[0034] [Thin Film Substrate]

[0035] As the film substrate 1 of the reinforcing film 10, a flexible plastic film can be used. In order to fix the film substrate 1 to the adhesive layer 2, the surface of the film substrate 1 where the adhesive layer 2 is attached is preferably not subjected to a demolding treatment.

[0036] The thickness of the thin film substrate is, for example, about 4 to 150 μm. From the perspective of strengthening the device through rigidity imparting, impact relaxation, etc., the thickness of the thin 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 perspective of imparting flexibility to the reinforced film so that it can be folded, the thickness of the thin film substrate 1 is preferably 125 μm or less, more preferably 100 μm or less. From the perspective of balancing mechanical strength and flexibility, the compressive strength of the thin film substrate 1 is preferably 100 to 3000 kg / cm². 2 More preferably, it is 200–2900 kg / cm². 2 Further preferred values ​​are 300–2800 kg / cm². 2 The preferred strength is 400–2700 kg / cm². 2 .

[0037] Examples of plastic materials constituting the film substrate 1 include: polyester resins, polyolefin resins, cyclic polyolefin resins, polyamide resins, polyimide resins, polyetheretherketone, polyethersulfone, polyarylate resins, and aramid resins. In reinforcing films used in optical devices such as displays, the film substrate 1 is preferably a transparent film. Furthermore, when photocuring the adhesive layer 2 by irradiating it with active light from the film substrate 1 side, the film substrate 1 preferably has transparency relative to the active light used in curing the adhesive layer. From the perspective of combining mechanical strength and transparency, polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, transparent polyimides, and transparent aramid fibers are suitable. When irradiating the adherend with active light from the adherend side, the adherend only needs to be transparent relative to the active light, while the film substrate 1 can be opaque relative to the active light.

[0038] Functional coatings such as easy-adhesion layer, easy-slip layer, release layer, antistatic layer, hard coating layer, and anti-reflective layer can be provided on the surface of the thin film substrate 1. It should be noted that, as mentioned above, in order to fix the thin film substrate 1 to the adhesive layer 2, a release layer is preferably not provided on the surface of the thin film substrate 1 where the adhesive layer 2 is attached.

[0039] [Adhesive layer]

[0040] The adhesive layer 2, which is adhered and laminated onto the film substrate 1, is formed from a photocurable composition containing a base polymer and a photocuring agent. Before photocuring, the adhesive layer 2 has low adhesion to the device, device components, or other adhered objects, making it easy to peel off. By photocuring, the adhesive layer 2 improves its adhesion to the adhered objects, thus preventing the reinforcing film from easily peeling off from the device surface during use, resulting in excellent bonding reliability.

[0041] Photocurable adhesives hardly cure under normal storage conditions, but are cured by irradiation with active light such as ultraviolet light. Therefore, the reinforcing film of the present invention allows for arbitrary setting of the curing time of the adhesive layer 2, providing advantages such as flexible handling of process pre-processing time.

[0042] The thickness of adhesive layer 2 is, for example, about 1 to 300 μm. There is a tendency that the greater the thickness of adhesive layer 2, the better the adhesion to the substrate. On the other hand, if the thickness of adhesive layer 2 is too large, the flowability before photocuring is high, sometimes making it difficult to handle. Therefore, the thickness of adhesive layer 2 is preferably 3 to 100 μm, more preferably 5 to 50 μm, even more preferably 6 to 40 μm, and particularly preferably 8 to 30 μm. From the perspective of thinness, the thickness of adhesive layer 2 can be 25 μm or less, 20 μm or less, or 18 μm or less.

[0043] When the reinforced 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.

[0044] The adhesive layer 2 preferably exhibits increased adhesion to the substrate through photocuring, and its shear storage modulus (hereinafter referred to as "storage modulus") remains low even at low temperatures after photocuring. Regarding the storage modulus of the adhesive, it is determined by reading the value at a specified temperature under conditions of 1 Hz, within a temperature range of -50 to 150°C, at a heating rate of 5°C / min, according to the method described in JIS K7244-1 "Plastics - Test methods for dynamic mechanical properties".

[0045] The preferred storage modulus of the photocured adhesive layer at -20°C is 5.0 × 10⁻⁶. 5 Pa or less, more preferably 4.0 × 10 Pa. 5 Pa below, more preferably 3.0 × 10 Pa 5 Below Pa, it can be 2.5 × 10 5 Pa or below or 2.0 × 10 5Pa below. By making the storage modulus of the photocured adhesive layer 2 small at low temperatures, the adhesive layer exhibits strain relaxation in low-temperature environments. Therefore, even when the device with the reinforcing film is repeatedly bent or kept in a bent state for a long time, peeling of the adhesive layer at the bending point can be suppressed.

[0046] On the other hand, if the storage modulus of the photocured adhesive layer is too low, the adhesive layer is prone to plastic deformation, and sometimes the adhesive layer may peel off from the adherend due to insufficient adhesive holding force. Therefore, the storage modulus of the photocured adhesive layer at -20°C is preferably 1.0 × 10⁻⁶. 4 Pa or higher, more preferably 2.0 × 10 Pa. 4 Pa or higher, more preferably 3.00 × 10 Pa. 4 Above Pa, it can be 4.0 × 10⁻⁶. 4 Pa or above, 5.0 × 10 4 Pa or above, 6.0 × 10 4 Pa or above, 7.0 × 10 4 Pa or above or 8.0 × 10 4 Pa or above.

[0047] From the perspective of ensuring adhesion at room temperature and suppressing adhesive layer seepage from the ends, the storage modulus of the UV-cured adhesive layer at 25°C is preferably 8.0 × 10⁻⁶. 3 ~1.5×10 5 Pa, more preferably 1.0 × 10 Pa 4 ~1.0×10 5 Pa, more preferably 1.5 × 10 Pa. 4 ~8.0×10 4 Pa can be 2.0 × 10 4 ~6.0×10 4 Pa.

[0048] The composition of adhesive layer 2 is not particularly limited as long as it contains a base polymer and a photocuring agent and its adhesion to the adherend increases through photocuring. From the perspective of improving the efficiency of curing based on active light irradiation, the adhesive composition (photocurable composition) constituting adhesive layer 2 preferably contains a photopolymerization initiator.

[0049] (Basic Polymer)

[0050] The base polymer is the main component of the adhesive composition and is the primary factor determining the adhesive strength and storage modulus of the adhesive layer before photocuring. Due to its excellent optical transparency and adhesion, and the ease of controlling adhesive strength and storage modulus, the adhesive composition preferably contains an acrylic polymer as the base polymer, and preferably more than 50% by weight of the adhesive composition is an acrylic polymer.

[0051] As an acrylic polymer, polymers containing alkyl (meth)acrylate as the main monomer component are suitable. It should be noted that in this specification, "(meth)acrylate" refers to acrylic acid and / or methacrylic acid.

[0052] As alkyl methacrylates, alkyl methacrylates with 1 to 20 carbon atoms in the alkyl group are suitable. The alkyl group in an alkyl methacrylate can be straight-chain or branched. Examples of alkyl methacrylates include: methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobutyl methacrylate, sec-butyl methacrylate, tert-butyl methacrylate, amyl methacrylate, isoamyl methacrylate, neopentyl methacrylate, hexyl methacrylate, heptyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, isooctyl methacrylate, nonyl methacrylate, and so on. Isononyl acrylate, decyl acrylate, isodecyl acrylate, undecyl acrylate, dodecyl acrylate, isotridecyl acrylate, tetradecyl acrylate, isotetradecyl acrylate, pentadecyl acrylate, cetyl acrylate, heptadecanyl acrylate, octadecyl acrylate, isooctadecyl acrylate, nonadecanyl acrylate, eicosyl acrylate, etc.

[0053] Among the exemplified alkyl (meth)acrylates, from the perspective of reducing the Tg of the base polymer, C(meth)acrylate is preferred. 1-9 Alkyl esters, preferably homopolymers, have a glass transition temperature of -50°C or lower. Homopolymers of (meth)acrylate alkyl esters have a glass transition temperature more preferably of -55°C or lower, and even more preferably of -60°C or lower. (Meth)acrylate C, as a homopolymer, has a glass transition temperature of -50°C or lower. 1-9Specific examples of alkyl esters include: 2-ethylhexyl acrylate (Tg: -70℃), n-hexyl acrylate (Tg: -65℃), n-octyl acrylate (Tg: -65℃), isononyl acrylate (Tg: -60℃), n-nonyl acrylate (Tg: -58℃), isooctyl acrylate (Tg: -58℃), and butyl acrylate (Tg: -55℃). Among these, butyl acrylate and 2-ethylhexyl acrylate are preferred, with 2-ethylhexyl acrylate being particularly preferred due to its low Tg.

[0054] The content of alkyl (meth)acrylate relative to the total amount of monomer components constituting the base polymer is preferably 70% by weight or more, more preferably 80% by weight or more, and even more preferably 85% by weight or more, and can be 90% by weight or more, 93% by weight or more, or 95% by weight or more. Among these, C(meth)acrylate is preferred. 1-9 The amount of alkyl ester is within the above range, more preferably the sum of 2-ethylhexyl acrylate and butyl acrylate is within the above range. The amount of 2-ethylhexyl acrylate may be within the above range.

[0055] The acrylic-based base polymer preferably contains monomer components with crosslinkable functional groups as copolymer components. Examples of monomers with crosslinkable functional groups include hydroxyl-containing monomers and carboxyl-containing monomers. The base polymer, as a copolymer component, can contain both hydroxyl-containing and carboxyl-containing monomers, or only one of them. By introducing a crosslinking structure into the base polymer, the cohesive strength is increased, and there is a tendency to improve the peelability of the self-adhesive layer 2 before photocuring.

[0056] Examples of hydroxyl-containing monomers include: 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, 6-hydroxyhexyl methacrylate, 8-hydroxyoctyl methacrylate, 10-hydroxydecyl methacrylate, 12-hydroxylaurate methacrylate, and 4-(hydroxymethyl)cyclohexyl methacrylate. Among these, 2-hydroxyethyl acrylate (Tg: -15℃) and 4-hydroxybutyl acrylate (Tg: -32℃) are preferred from the perspective of their significant contribution to improving the adhesive strength of the photocured adhesive and their low Tg of the homopolymer.

[0057] Examples of carboxyl-containing monomers include: (meth)acrylic acid, 2-carboxyethyl (meth)acrylic acid, carboxypentyl (meth)acrylic acid, 2-(meth)acryloyloxyethyl-succinic acid, 2-(meth)acryloyloxyethyl hexahydrophthalic acid, 2-(meth)acryloyloxyethyl-phthalic acid, 2-(meth)acryloyloxypropyl hexahydrophthalic acid, 2-(meth)acryloyloxypropyl phthalic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, etc.

[0058] In acrylic-based polymers, the total amount of hydroxyl-containing monomers and carboxyl-containing 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, relative to the total amount of constituent monomer components. When a cross-linked structure is introduced into the acrylic-based polymer via an isocyanate-based cross-linking agent, the content of hydroxyl-containing (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 4-hydroxypropyl (meth)acrylate is preferably within the above range.

[0059] Acrylic base polymers may contain nitrogen-containing monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazolium, vinyloxazole, vinylmorpholine, N-acryloylmorpholine, N-vinylcarboxylic amides, and N-vinylcaprolactam.

[0060] Acrylic base polymers may contain monomer components other than those mentioned above. For example, acrylic base polymers may contain vinyl ester monomers, aromatic vinyl monomers, epoxy-containing monomers, vinyl ether monomers, sulfonate-containing monomers, phosphate-containing monomers, anhydride-containing monomers, etc.

[0061] The base polymer before the introduction of the crosslinking structure can be substantially nitrogen-free. The proportion of nitrogen in the constituent elements of the base polymer can be less than 0.1 mol%, less than 0.05 mol%, less than 0.01 mol%, less than 0.005 mol%, less than 0.001 mol%, or 0. By using a base polymer that is substantially nitrogen-free, there is a tendency to suppress the increase in adhesive strength (initial adhesive strength) of the adhesive layer before light curing after surface activating treatment of the adherends. A base polymer that is substantially nitrogen-free can be obtained by not using nitrogen-containing monomers such as cyano monomers, lactam monomers, amide monomers, or morpholine ring monomers as constituent monomer components of the base polymer. The amount of nitrogen-containing monomers relative to the total amount of constituent monomer components of the acrylic base polymer can be less than 1 wt%, less than 0.5 wt%, less than 0.1 wt%, less than 0.05 wt%, or 0.

[0062] The glass transition temperature of the acrylic-based polymer is preferably below -45°C, more preferably below -50°C, even more preferably below -55°C, and particularly preferably below -60°C, but can be below -63°C or below -65°C. There is no particular limitation on the lower limit of the glass transition temperature of the acrylic-based polymer; it can generally be above -80°C, above -75°C, or above -70°C.

[0063] The glass transition temperature is the temperature at which the loss tangent tanδ reaches its maximum (peak temperature) in viscoelasticity measurements. By ensuring that the glass transition temperature is sufficiently lower than the ambient temperature of the device, the storage modulus G' of the adhesive layer is small within the ambient temperature range, which tends to suppress peeling during repeated bending.

[0064] Instead of using viscoelastic measurements to determine the glass transition temperature, the theoretical Tg calculated using the Fox formula can be applied. The theoretical Tg is based on the glass transition temperature Tg of the homopolymer of the acrylic base polymer, considering the monomer composition of the homopolymer. i and the weight fraction W of each monomer component i The following formula from Fox is used to calculate it.

[0065] 1 / Tg=Σ(W i / Tg i )

[0066] Tg is the glass transition temperature of the polymer chain (unit: K), W i Tg represents the weight fraction (weight-based copolymerization ratio) of monomer component i constituting the chain segment. i Let Tg be the glass transition temperature (in K) of the homopolymer of monomer component i. The glass transition temperature of the homopolymer can be the value described in the 3rd edition of the Polymer Handbook (John Wiley & Sons, Inc., 1989). For homopolymers of monomers not described in the above literature, the Tg can be the peak temperature of tanδ determined based on dynamic viscoelasticity.

[0067] Acrylic polymers, serving as the base polymer, can be obtained by polymerizing the aforementioned monomer components using various known methods such as solution polymerization, emulsion polymerization, and bulk polymerization. From the perspective of balancing adhesive properties such as bonding strength and holding power, as well as cost, solution polymerization is preferred. Ethyl acetate, toluene, etc., can be used as solvents for solution polymerization. The solution concentration is typically around 20–80% by weight. Various known initiators, such as azo-based and peroxide-based initiators, can be used as polymerization initiators. Chain transfer agents can 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.

[0068] The weight-average molecular weight of the acrylic-based 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. It should be noted that when a cross-linking structure is introduced into the base polymer, the molecular weight of the base polymer refers to the molecular weight before the introduction of the cross-linking structure.

[0069] (Cross-linking agent)

[0070] From the perspective of imparting appropriate cohesive strength to the adhesive to exhibit bonding strength and ensuring the self-peelability of the adhesive layer 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 the base polymer is polymerized, and the solution is heated as needed to introduce the crosslinking structure. Examples of crosslinking agents include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, carbodiimide-based crosslinking agents, and metal chelate-based crosslinking agents. These crosslinking agents react with functional groups such as hydroxyl and carboxyl groups introduced into the base polymer to form a crosslinking structure. From the perspective of high reactivity with the hydroxyl and carboxyl groups of the base polymer and ease of introducing the crosslinking structure, isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred.

[0071] As isocyanate-based crosslinking agents, polyisocyanates having two or more isocyanate groups per molecule can be used. Examples of polyisocyanate-based crosslinking agents 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 phenylenediamine 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), and trimethylolpropane adducts of phenylenediamine diisocyanate (e.g., "TAKENATE" manufactured by Mitsui Chemicals Co., Ltd.). D110N”, isocyanurate derivatives of hexamethylene diisocyanate (such as CORONATE HX manufactured by Tosoh Corporation), and other isocyanate adducts.

[0072] As an epoxy crosslinking agent, a polyfunctional epoxy compound having two or more epoxy groups in one molecule can be used. An epoxy crosslinking agent can also be a crosslinking agent having three or more epoxy groups in one molecule. The epoxy groups in an 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-diglycidyl-aminomethyl)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 anhydride polyglycidyl ether, trimethylolpropane polyglycidyl ether, diglycidyl adipate, diglycidyl phthalate, triglycidyl tris(2-hydroxyethyl) isocyanurate, resorcinol diglycidyl ether, bisphenol-S-diglycidyl ether, etc. As an epoxy crosslinking agent, commercially available products such as "DENACOL" manufactured by NagaseChemteX Corporation, "TETRAD X" and "TETRAD C" manufactured by Mitsubishi Gas Chemical Co., Ltd. can be used.

[0073] The amount of crosslinking agent can be adjusted appropriately according to the composition and molecular weight of the base polymer. Relative to 100 parts by weight of the base polymer, the amount of crosslinking agent 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.

[0074] To promote the formation of cross-linked structures, cross-linking catalysts can be used. Examples of isocyanate-based cross-linking catalysts include: tetrabutyl titanate, tetraisopropyl titanate, iron acetylacetonate (NāCEM IRON(III)), butyltin oxide, dioctyltin dilaurate, dibutyltin dilaurate, and other metal-based cross-linking catalysts (especially tin-based cross-linking catalysts). Generally, the amount of cross-linking catalyst used is 0.1 parts by weight or less per 100 parts by weight of the base polymer.

[0075] (Photocuring agent)

[0076] The adhesive composition constituting adhesive layer 2 contains a light-curing agent in addition to the base polymer. When adhesive layer 2, formed from the light-curable adhesive composition, is light-cured after being bonded to the substrate, the adhesion strength to the substrate is improved.

[0077] As a photocuring agent, a photocurable monomer or a photocurable oligomer can be used. Preferably, a compound having two or more olefinic unsaturated bonds per molecule is used as the photocuring agent. Furthermore, the photocuring agent preferably exhibits compatibility with the base polymer. From the viewpoint of exhibiting moderate compatibility with the base polymer, the photocuring agent is preferably a liquid at room temperature.

[0078] From the perspective of high compatibility with acrylic-based polymers, polyfunctional (meth)acrylates are preferred 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, alkanediol dimethacrylate, tricyclodecanediethanol dimethacrylate, isocyanurate dimethacrylate, isocyanurate trimethacrylate, pentaerythritol trimethacrylate, pentaerythritol dimethacrylate, trimethylolpropane trimethacrylate, di(trimethylolpropane)tetramethacrylate, pentaerythritol tetramethacrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexamethacrylate, neopentyl glycol dimethacrylate, glycerol dimethacrylate, urethane (meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, isoprene (meth)acrylate, etc.

[0079] Polyfunctional (meth)acrylates can be esters of polyols modified with epoxides and (meth)acrylates. Examples of epoxides include ethylene oxide (EO) and propylene oxide (PO). Epoxides can 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 with acrylic-based polymers can be adjusted to an appropriate range.

[0080] Specific examples of epoxy-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 isocyanurate-modified di(meth)acrylate, propylene oxide isocyanurate-modified di(meth)acrylate, ethylene oxide isocyanurate-modified tri(meth)acrylate, propylene oxide isocyanurate-modified tri(meth)acrylate, pentaerythritol ethylene oxide-modified tetra(meth)acrylate, and pentaerythritol propylene oxide-modified tetra(meth)acrylate.

[0081] Multifunctional (meth)acrylates can be (meth)acrylate metal salts such as zinc (meth)acrylate, magnesium (meth)acrylate, calcium (meth)acrylate, barium (meth)acrylate, strontium (meth)acrylate, nickel (meth)acrylate, copper (meth)acrylate, and aluminum (meth)acrylate.

[0082] The compatibility between the base polymer and the UV-curing agent is also affected by the molecular weight of the UV-curing agent. There is a tendency that the smaller the molecular weight of the UV-curing agent, the higher its compatibility with the base polymer. From the perspective of compatibility with the base polymer, the molecular weight of the UV-curing agent is preferably below 1500, more preferably below 1000, and can be below 800, below 600, below 500, below 450, or below 400.

[0083] Furthermore, there is a tendency for a smaller functional group equivalent (i.e., a larger number of functional groups per unit molecular weight) in the photocuring agent to result in higher compatibility with the base polymer. From the perspective of compatibility with the base polymer, the functional group equivalent (g / eq) of the photocuring agent is preferably 80–300, more preferably 90–200, and even more preferably 100–170, and can be 110–160 or 120–150.

[0084] When the base polymer and the UV-curing agent are not a completely compatible system, the liquid UV-curing agent seeps to the surface and forms a weak boundary layer (WBL) at the bonding interface with the adherend, enhancing the properties of the liquid state. If a WBL is formed, the liquid properties of the surface (bonding interface) are enhanced while maintaining the volumetric properties of the adhesive layer, such as storage modulus, thus tending to result in weaker adhesion to the adherend.

[0085] When the base polymer and the UV-curing agent exhibit moderate compatibility but are not completely compatible systems, a WBL (Wasteless Baseline) will form in the adhesive layer before UV curing, resulting in low initial adhesion and easy peeling from the adherends. On the other hand, after UV curing, the liquid properties of the UV-curing agent disappear, and a photocrosslinking structure is uniformly introduced into the adhesive layer, thus significantly improving the adhesion to the adherends.

[0086] The compatibility between the base polymer and the photocuring agent is primarily influenced by the structure of the compound. The structure and compatibility of the compound can be evaluated, for example, using the Hansen solubility parameter (HSP). The Hansen solubility parameter (HSP) divides the solubility parameter δ of Hildebrand into a dispersion term δ. d Polar term δ p and hydrogen bond term δ h These three components are expressed in three-dimensional space, δ 2 =δ d 2 +δ p2 +δ h 2 The relationship holds. The dispersion term δ d The polar term δ represents the effect caused by the dispersion force. p The hydrogen bond term δ represents the effect caused by the dipole force. h This represents the effect caused by hydrogen bonding forces. The distance Ra between the HSPs of two substances is determined by the difference Δδ between the dispersion terms of the two substances. d The difference Δδ between polarity terms p The difference Δδ between the hydrogen bond term and the hydrogen bond term h Using Ra={4Δδ d 2 +Δδ p 2 +Δδ h 2} 1 / 2 This indicates that the smaller Ra is, the higher the compatibility, and the larger Ra is, the lower the compatibility.

[0087] Details of Hansen's solubility parameters are documented in Charles M. Hansen's *Hansen Solubility Parameters: A Users Handbook* (CRC Publishing, 2007). For substances for which literature values ​​are unknown, the computer software *Hansen Solubility Parameters in Practice* (HSPiP) can be used for calculation.

[0088] As mentioned earlier, in acrylic-based base polymers with low glass transition temperatures, the proportion of (meth)acrylates such as 2-ethylhexyl acrylate is high, while the proportion of highly polar monomers such as hydroxyl-containing monomers, carboxyl-containing monomers, and nitrogen-containing monomers is low. Therefore, when using base polymers with low glass transition temperatures and low polarity, by using polyfunctional (meth)acrylates incorporating epoxides such as ethylene oxide and propylene oxide as photocuring agents, the distance Ra between the HSPs of the base polymer and the photocuring agent can be adjusted to an appropriate range. There is a tendency for the chain length n of the epoxide in epoxide-modified polyfunctional (meth)acrylates to be larger, resulting in a larger HSP distance Ra.

[0089] When the distance Ra between the HSP of the base polymer and the UV-curing agent is too large, although the initial adhesion is low, contamination due to UV curing agent exudation may occur, or the increase in adhesion brought about by UV curing 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 UV-curing 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 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, have low initial adhesion and exhibit high adhesion to the adhered objects after UV curing. From the perspective of suppressing UV curing agent exudation to the adhesive layer surface, the chain length n of EO is particularly preferably 1.

[0090] The type and amount of UV-curing agent not only affect adhesive strength but also the volumetric 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 UV curing is small, even if the type of UV-curing agent is different. On the other hand, if the content of UV-curing agent increases, the content of the base polymer in the composition becomes relatively small, thus tending to result in a smaller storage modulus of the adhesive layer before UV curing.

[0091] The smaller the functional group equivalent of the UV-curing agent and the higher the content of the UV-curing agent, the higher the crosslinking density brought about by UV curing. Therefore, there is a tendency for the storage modulus of the adhesive layer to increase after UV curing. That is, there is a tendency that the higher the content of the UV-curing agent, the smaller the storage modulus of the adhesive layer before UV curing and the larger the storage modulus of the adhesive layer after UV curing.

[0092] From the perspective of improving the adhesive strength of the adhesive after photocuring and suppressing the excessive increase of the storage modulus, the content of the photocuring agent in the adhesive composition is preferably 3 to 30 parts by weight, more preferably 5 to 20 parts by weight, and can be 6 to 15 parts by weight or 7 to 12 parts by weight, relative to 100 parts by weight of the base polymer.

[0093] Two or more UV curing agents can be used in combination. When two or more UV curing agents are used in combination, the total amount of UV curing agents is preferably within the range mentioned above. For example, by using a UV curing agent with relatively high compatibility with the base polymer and a UV curing agent with relatively low compatibility with the base polymer in combination, the initial adhesive force can be suppressed to a lower level, and the storage modulus of the adhesive at room temperature after UV curing can be increased, thereby adjusting the properties of the adhesive layer before and after UV curing.

[0094] (Photopolymerization initiator)

[0095] The adhesive composition preferably contains a photopolymerization initiator. The photopolymerization initiator generates active species by irradiation with active light, promoting the curing reaction of the photocurable. Depending on the type of photocurable, photocationic initiators (photoacid generators), photoradical initiators, and photoanionic initiators (photoalkali generators) can be used as photopolymerization initiators. When using polyfunctional acrylates as photocurables, photoradical initiators are preferred. Photoradical initiators that generate free radicals by visible light or ultraviolet light with wavelengths shorter than 450 nm are preferred; examples include hydroxy ketones, benzoyl dimethyl ketals, amino ketones, acylphosphine oxides, benzophenones, and triazine derivatives containing trichloromethyl. Photoradical initiators can be used alone or in combination of two or more.

[0096] When transparency of the adhesive layer 2 is required, the photopolymerization initiator (photoradical generator) preferably has low sensitivity to light with wavelengths longer than 400 nm (visible light), for example, it is preferable to use an absorptivity of 1 × 10⁻⁶ at a wavelength of 405 nm. 2 [mLg -1 cm -1 The following photopolymerization initiators are used. Furthermore, if a photopolymerization initiator with low sensitivity to visible light is used, cracking of the photopolymerization initiator caused by external light in the storage environment is less likely to occur, thus improving the storage stability of the reinforced film.

[0097] 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.

[0098] (Oligomers)

[0099] The adhesive composition may contain oligomers in addition to the base polymer. For example, the adhesive composition may contain acrylic oligomers in addition to the base polymer. As oligomers, oligomers with a weight-average molecular weight of approximately 1000 to 30000 can be used. The acrylic oligomers contain alkyl (meth)acrylates as the main constituent monomers. From the perspective of improving the adhesive strength of the adhesive layer 2 after photocuring, the glass transition temperature of the acrylic oligomers is preferably 40°C or higher, more preferably 50°C or higher. The oligomers may contain crosslinkable functional groups similar to those in the base polymer.

[0100] There is no particular limitation on the content of oligomers in the adhesive composition. When the adhesive composition contains acrylic oligomers in addition to an acrylic base polymer, the amount of oligomers relative to 100 parts by weight of the 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, from the perspective of adjusting the adhesive strength to an appropriate range.

[0101] (Other additives)

[0102] In addition to the components exemplified above, the adhesive layer may contain additives such as silane coupling agents, tackifiers, plasticizers, softeners, degradation inhibitors, fillers, colorants, ultraviolet absorbers, antioxidants, surfactants, and antistatic agents, within a range that does not impair the characteristics of the present invention.

[0103] [Fabrication of Reinforced Thin Films]

[0104] A reinforced film can be obtained by laminating a photocurable adhesive layer 2 onto a film substrate 1. The adhesive layer 2 can be formed directly on the film substrate 1, or an adhesive layer formed as a sheet on another substrate can be transferred onto the film substrate 1.

[0105] The adhesive composition described above is applied to a substrate using methods such as roller coating, licking coating, gravure coating, reverse coating, roller brush coating, spray coating, dip roller coating, bar coating, doctor blade coating, air knife coating, curtain coating, lip coating, and die coating. The solvent is then 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.

[0106] When the adhesive composition contains a crosslinking agent, crosslinking is preferably carried out simultaneously with or after solvent drying by heating or curing. The heating temperature and heating time are appropriately set according to the type of crosslinking agent used, and crosslinking is typically carried out by heating for about 1 minute to 7 days in the range of 20°C to 160°C. The heating used to remove the solvent can also be used for crosslinking.

[0107] By introducing a cross-linking structure into the base polymer, there is a tendency for the gelation rate and the storage modulus of the adhesive layer 2 to increase. The higher the gelation rate of the adhesive before photocuring, the harder the adhesive, and the better it can suppress residual adhesive on the substrate when the reinforcing film is peeled off due to reprocessing, etc. The gelation rate of the adhesive layer 2 before photocuring (i.e., the gelation rate of the photocurable composition constituting the adhesive layer) is preferably 25% or more, more preferably 30% or more, and can be 35% or more, 40% or more, or 45% or more. On the other hand, if the gelation rate is too high, the storage modulus increases and the stress-strain relaxation is low, so adhesive peeling may sometimes occur at the bending portion when using a flexible device. Therefore, the gelation rate of the adhesive layer 2 before photocuring is preferably 80% or less, more preferably 75% or less, and even more preferably 70% or less, and can be 65% or less.

[0108] The gel ratio can be determined as the insoluble component in solvents such as ethyl acetate. Specifically, it can be calculated as the weight fraction (in weight %) of the insoluble component after the adhesive layer has been immersed in ethyl acetate at 23°C for 7 days, relative to the sample before immersion. Generally, the gel ratio of a polymer is equal to its degree of crosslinking; the more crosslinked parts in the polymer, the greater the gel ratio.

[0109] After a cross-linking structure is introduced into the polymer using a cross-linking agent, the photocuring agent also remains in an unreacted state. Therefore, a photocurable adhesive layer 2 containing a base polymer and a photocuring agent is formed. When forming the adhesive layer 2 on the film substrate 1, it is preferable to attach a release liner 5 to the adhesive layer 2 for purposes such as protection. Cross-linking can be performed after attaching the release liner 5 to the adhesive layer 2.

[0110] When an adhesive layer 2 is formed on another substrate, after the solvent is dried, the adhesive layer 2 is transferred onto the film substrate 1, thereby obtaining a reinforced film. The substrate used to form the adhesive layer can be used directly as the release liner 5.

[0111] As the release liner 5, a plastic film such as polyethylene, polypropylene, polyethylene terephthalate, or polyester film is preferably used. The thickness of the release liner is typically 3–200 μm, preferably around 10–100 μm. The contact surface between the release liner 5 and the adhesive layer 2 is preferably treated with a release agent based on silicone, fluorine, long-chain alkyl, or fatty acid amide, or silica powder. By performing a release treatment on the surface of the release liner 5, when the film substrate 1 is peeled from the release liner 5, peeling occurs at the interface between the adhesive layer 2 and the release liner 5, maintaining the adhesive layer 2 adhered to the film substrate 1. The release liner 5 can be treated with antistatic agents on either the treated surface or the untreated surface. By performing antistatic treatment on the release liner 5, the static charge generated when the release liner is peeled off from the adhesive layer can be suppressed.

[0112] [Properties and Applications of Reinforced Films]

[0113] The reinforcing film of the present invention is used by bonding it to a device or a component of a device. The adhesive layer 2 of the reinforcing film 10 is fixed to the film substrate 1, and the adhesive force to the substrate is small after bonding to the substrate and before photocuring. Therefore, the reinforcing film is easy to peel off from the substrate before photocuring.

[0114] There are no particular limitations on the substrate to which the reinforcing film is adhered; various electronic devices, optical devices, and their components can be included. In one embodiment, the reinforcing film is adhered to the surface of a flexible, bendable device. A bendable device is, for example... Figure 3 As shown, it has a hinge portion 25, around which it can be bent. The bending angle can be set arbitrarily, as shown in the figure. Figure 4 It can be bent (folded) 180° as shown. It should be noted that... Figure 4 In this case, the device is 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 to the back side (casing). Figure 3 , 4 As shown, in a flexible device constructed in a manner that allows bending at a specified location such as the hinge portion 25, bending and stretching are repeatedly performed at the same location during its use.

[0115] The reinforcing film can be adhered to the entire surface of the substrate or selectively adhered only to the areas requiring reinforcement (the areas to be reinforced). Alternatively, the reinforcing film can be adhered to both the areas requiring reinforcement (the areas to be reinforced) and the areas not requiring reinforcement (the areas not to be reinforced), and then the reinforcing film adhered to the non-reinforced areas can be cut and removed. As long as the adhesive is not light-cured, the reinforcing film is in a temporary bonded state to the surface of the substrate, and can therefore be easily peeled off from the surface of the substrate. Alternatively, the reinforcing film can be adhered to both the areas to be reinforced and the non-reinforced areas, and after selectively irradiating the areas to be reinforced to cure the adhesive, the reinforcing film in the uncured non-reinforced areas can be selectively peeled off.

[0116] By laminating a reinforcing film, appropriate rigidity can be imparted, thus improving handleability and preventing breakage for thin components such as flexible devices. In the device manufacturing process, when laminating the reinforcing film onto a semi-finished product, the reinforcing film can be laminated onto a large-format semi-finished product before it is cut to product size. The reinforcing film is then laminated to the master roll of the device manufactured using a roll-to-roll process in a roll-to-roll manner.

[0117] Before laminating the reinforcing film, the surfaces of the substrates can be activated for purposes such as cleansing. Examples of surface activation treatments include plasma treatment, corona treatment, and glow discharge treatment. Activated substrates contain numerous active groups such as hydroxyl, carbonyl, and carboxyl groups. Through intermolecular interactions with the polar functional groups of the adhesive's base polymer, the adhesive strength is easily increased. Especially when the substrate is polyimide, the activation treatment activates the amyl acid, terminal amino groups, and carboxyl groups (or carboxylic anhydride groups), resulting in strong interactions with the polar functional groups of the base polymer. Therefore, sometimes the initial adhesive strength can increase significantly through activation treatment.

[0118] If the initial adhesive force becomes too high, peeling operations such as reprocessing can sometimes become difficult. As mentioned earlier, by making the base polymer substantially nitrogen-free, it is possible to suppress the excessive increase in the initial adhesive force on the surface-activated substrates.

[0119] From the perspective of facilitating easy peeling from the adherend and preventing adhesive residue on the adherend after peeling off the reinforcing film, the adhesion strength between the adhesive layer 2 before photocuring and the adherend is preferably 1 N / 25 mm or less, more preferably 0.5 N / 25 mm or less, even more preferably 0.3 N / 25 mm or less, and can be 0.1 N / 25 mm or less or 0.05 N / 25 mm or less. From the perspective of preventing peeling of the reinforcing film during storage and handling, the adhesion strength between the adhesive layer 2 before photocuring and the adherend 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 with a tensile speed of 300 mm / min and a peel angle of 180°, using a polyimide film as the adherend. Unless otherwise stated, the adhesion strength is the value measured at 25°C.

[0120] The preferred storage modulus of the adhesive layer 2 before photocuring at 25°C is 5.0 × 10⁻⁶. 3 ~1.0×10 5 Pa. From the perspective of suppressing residual adhesive on the substrate when the self-adhesive peels off the reinforcing film, the storage modulus of the adhesive layer 2 before photocuring at 25°C is preferably 7.0 × 10⁻⁶. 3 Pa or higher, more preferably 9.0 × 10 Pa. 3 Above Pa, it can be 1.0 × 10⁻⁶. 4 Pa or above or 1.5 × 10 4 Pa or higher. From the perspective of imparting flexibility to the adhesive layer, the storage modulus of the adhesive layer 2 before photocuring at 25°C is preferably 7.0 × 10⁻⁶. 4 Pa or less, more preferably 5.0 × 10 Pa. 4 Below Pa, it can be 4.0 × 10 4Pa or below or 3.0 × 10 4 Below Pa.

[0121] The preferred storage modulus of the adhesive layer 2 before photocuring at -20°C is 1.0 × 10⁻⁶. 4 ~2.0×10 5 Pa, more preferably 2.0 × 10 Pa 4 ~1.0×10 5 Pa can be 3.0 × 10 4 ~9.0×10 4 Pa or 4.0 × 10 4 ~9.0×10 4 Pa. By setting the storage modulus of the adhesive layer before photocuring to the range described above at low temperatures, there is a tendency for the storage modulus of the adhesive layer to remain low even after photocuring.

[0122] 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. There is a tendency for the following: the more crosslinking agent introduced, the higher the gelation rate and the greater the storage modulus. Conversely, the more photocuring agent is used, the relatively smaller the content of the base polymer in the composition becomes, thus tending towards a smaller storage modulus.

[0123] The storage modulus of the adhesive changes drastically near the glass transition temperature of the base polymer. As mentioned earlier, by making the glass transition temperature of the base polymer sufficiently lower than -20°C, an adhesive with a low 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 the storage modulus at 25°C, more preferably less than 5 times, and can 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 can be more than 1.5 times, more than 1.8 times, or more than 2.0 times the storage modulus at 25°C.

[0124] After the reinforcing film is bonded to the substrate, the adhesive layer 2 is irradiated with active light, thereby photocuring the adhesive layer. Examples of active light include ultraviolet light, visible light, infrared light, X-rays, alpha rays, beta rays, and gamma rays. From the perspective of suppressing curing of the adhesive layer in its storage state and facilitating curing, ultraviolet light is preferred as the active light. The irradiation intensity and duration of the active light can be appropriately set according to the composition and thickness of the adhesive layer. Irradiation of the adhesive layer 2 with active light can be performed from either the film substrate 1 side or the substrate side, or from both sides.

[0125] As the photopolymer cures, the storage modulus of the adhesive layer increases, and its adhesive strength to the adhered object also increases. From the perspective of adhesive reliability during practical use of the device, the adhesive strength between the photopolymerized adhesive layer 2 and the adhered object is preferably 2 N / 25 mm or more, more preferably 2.5 N / 25 mm or more. From the perspective of suppressing adhesive peeling when the flexible device is repeatedly bent at the same location, the adhesive strength between the photopolymerized adhesive layer 2 and the adhered object can 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 adhesive strength between the photopolymerized adhesive layer 2 and the adhered object is preferably 10 times or more than the adhesive strength between the photopolymerized adhesive layer 2 and the adhered object before photopolymerization, more preferably 30 times or more, and can be 50 times or more, 100 times or more, or 200 times or more. As mentioned earlier, by adjusting the type (compatibility with the base polymer) and amount of the UV curing agent, the adhesive strength before UV curing (initial adhesive strength) can be suppressed to a low level, and the adhesive strength of the adhesive after UV curing can be increased.

[0126] As mentioned above, the preferred storage modulus of the photocured adhesive layer 2 at -20°C is 1.0 × 10⁻⁶. 4 ~5.0×10 5 The energy storage modulus at Pa and 25℃ is 8.0 × 10⁻⁶. 3 ~1.5×10 5 Pa. By making the storage modulus of the photocured adhesive layer low in the low-temperature region, there is a tendency to suppress the peeling of the adhesive layer when repeatedly bent at low temperatures.

[0127] The energy storage modulus of the adhesive layer after photocuring at -20°C is preferably less than 7 times that at 25°C, more preferably less than 5 times, and can be less than 4 times, less than 3.5 times, or less than 3 times. The energy storage modulus of the adhesive layer before photocuring at -20°C can be more than 1.5 times, more than 1.8 times, or more than 2.0 times that at 25°C.

[0128] The storage modulus of the photocured adhesive layer at -20°C is preferably less than 7 times that of the adhesive layer before photocuring at -20°C, and can be less than 5 times, less than 4 times, or less than 3.5 times. The storage modulus of the photocured adhesive layer at -20°C can 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 photocuring at -20°C.

[0129] The storage modulus of the photocured adhesive layer at 25°C is preferably less than 7 times that of the adhesive layer before photocuring at 25°C, and can be less than 5 times, 4 times, or 3.5 times. The storage modulus of the photocured adhesive layer at -20°C can be more than 1.1 times, 1.3 times, 1.5 times, or 1.7 times that of the adhesive layer before photocuring at -20°C.

[0130] As described above, by applying the reinforcing film of the present invention, the adherend can be given appropriate rigidity and stress can be relaxed / dispersed, suppressing various adverse conditions that may occur during the manufacturing process, improving production efficiency, and increasing yield. Furthermore, the reinforcing film is easily peeled off from the adherend before the adhesive layer is photocured, thus facilitating reprocessing in cases of lamination or poor adhesion. Additionally, selective removal of the reinforcing film outside the reinforced area is also easily performed.

[0131] In the use of the completed device, even under accidental external forces such as the device falling, carrying heavy objects, or being struck by flying objects, damage to the device can be prevented by the bonding of the reinforcing film. Furthermore, since the reinforcing film, after photocuring the adhesive, is firmly bonded to the device, it is not easily peeled off during long-term use, demonstrating excellent reliability. Moreover, because the adhesive layer of the reinforcing film has a low storage modulus in low-temperature regions, even when used in flexible devices, adhesive peeling at bending points is not likely to occur, exhibiting excellent bonding reliability.

[0132] Example

[0133] The following examples and comparative examples are provided for further explanation, but the present invention is not limited to these examples.

[0134] Polymerization of acrylic polymers

[0135] <Polymer A>

[0136] 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 introduced and nitrogen purging was carried out for about 1 hour with stirring. Then, the mixture was 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.

[0137] <Polymer B~I>

[0138] The monomer dosage was changed as shown in Table 1. Otherwise, the polymerization was carried out in the same manner as polymer A to obtain solutions of polymers B through I.

[0139] Table 1 shows the monomer ratios and glass transition temperatures of acrylic polymers A through I. It should be noted that the glass transition temperatures are calculated based on the Fox equation using the monomer ratios.

[0140] 2EHA 2-Ethylhexyl Acrylate

[0141] BA butyl acrylate

[0142] MMA (methyl methacrylate)

[0143] NVP N-vinylpyrrolidone

[0144] 2HEA 2-Hydroxyethyl Acrylate

[0145] 4HBA (4-Hydroxybutyl Acrylate)

[0146] AA Acrylic Acid

[0147] β-CEA (2-Carboxyethyl Acrylate)

[0148] HOA-MS 2-Acryloyloxyethyl succinate

[0149] [Table 1]

[0150]

[0151] [Fabrication of Reinforced Thin Films]

[0152] <Preparation of Adhesive Compositions>

[0153] A crosslinking agent and a UV-curing agent were added to an acrylic polymer solution and mixed thoroughly to prepare the adhesive compositions shown in Table 2. The amounts of crosslinking agent and UV-curing agent in Table 2 are the amounts added relative to 100 parts by weight of the solid component of the base polymer. Details of the crosslinking agent and UV-curing agent are described below. Examples 7-9 used two UV-curing agents, and the amounts of each are shown in Table 2.

[0154] (Cross-linking agent)

[0155] TAKENATE D110N: A 75% ethyl acetate solution of the trimethylolpropane adduct of diphenylmethylene diisocyanate ("TAKENATE D110N" manufactured by Mitsui Chemicals Co., Ltd.)

[0156] C-HX: The isocyanurate form of hexamethylene diisocyanate ("CORONATE HX" manufactured by Tosoh Corporation).

[0157] TC: N,N,N',N'-Tetraglycidyl m-phenylenediamine (a 4-functional epoxy compound, manufactured by Mitsubishi Gas Chemical Co., Ltd. as "TETRAD C")

[0158] (Photocuring agent)

[0159] M350: Trimethylolpropane EO-modified (n=1) triacrylate ("ARONIX M-350" manufactured by Toa Synthetic Co., Ltd., functional group equivalent 144g / eq)

[0160] M360: Trimethylolpropane EO-modified (n=2) triacrylate ("ARONIX M-360" manufactured by Toa Synthetic Co., Ltd., functional group equivalent 186g / eq)

[0161] M310: Trimethylolpropane PO modified (n=1) triacrylate ("ARONIX M-310" manufactured by Toa Synthetic Co., Ltd., functional group equivalent 158g / eq)

[0162] M321: Trimethylolpropane PO modified (n=2) triacrylate ("ARONIX M-321" manufactured by Toa Synthetic Co., Ltd., functional group equivalent 200g / eq)

[0163] APG700: Polypropylene glycol #700 (n=12) diacrylate; (NKEster APG700 manufactured by Shin-Nakamura Chemical Industry Co., Ltd., functional group equivalent 404g / eq)

[0164] A200: Polyethylene glycol #200 (n=4) diacrylate (NKEster A200 manufactured by Shin-Nakamura Chemical Industry Co., Ltd., functional group equivalent 154g / eq)

[0165] TMPT: Trimethylolpropane triacrylate (NK EsterTMPT manufactured by Shin-Nakamura Chemical Industry Co., Ltd., functional group equivalent 99g / eq)

[0166] ZnAc: Zinc acrylate (manufactured by Nisshoku Techno Fine Chemical Co., Ltd., functional group equivalent 104 g / eq)

[0167] (Photopolymerization initiator)

[0168] In addition to those shown in Table 2, Examples 1-18 and Comparative Examples 1-3, 7, and 8 contained 0.2 parts by weight of 2,2-dimethoxy-1,2-diphenylethane-1-one ("Omnirad 651" manufactured by IGM Resins) as a photopolymerization initiator, relative to 100 parts by weight of the solids component of the base polymer. Comparative Examples 4-6 contained 0.1 parts by weight of 1-hydroxycyclohexylphenyl ketone ("Omnirad 184" manufactured by IGM Resins) as a photopolymerization initiator, relative to 100 parts by weight of the solids component of the base polymer.

[0169] <Coating and Crosslinking of Adhesive Solutions>

[0170] The adhesive composition described above was applied to an untreated transparent film substrate using a fountain roll, resulting in a dried thickness of 13 μm. After drying at 130°C for 1 minute to remove the solvent, a release film (a 25 μm thick polyethylene terephthalate film with one side treated with silicone release and both sides treated with antistatic agents) was laminated onto the adhesive-coated side. Then, a curing treatment was performed at 25°C for 4 days to allow cross-linking, thereby fixing the photocurable adhesive sheet onto the transparent film substrate, resulting in a reinforced film with a release film temporarily adhered thereon. Examples 1-5, 7-18, and Comparative Examples 1-8 used a 75 μm thick polyethylene terephthalate film (“Lumirror S10” manufactured by Toray Industries, Inc.) as the transparent film substrate. Example 6 uses a 50 μm thick ultraviolet-transparent transparent polyimide film (NEOPULIM S-100 manufactured by Mitsubishi Gas Chemical Co., Ltd.) as the transparent film substrate.

[0171] [evaluate]

[0172] <Energy Storage Modulus>

[0173] The adhesive composition was coated and crosslinked on the release liner in the same manner as described above to produce an adhesive sheet (before photocuring). An release liner was then attached to the surface of the adhesive layer of the adhesive sheet before photocuring to isolate it from oxygen, and the sheet was irradiated with a 365nm LED lamp at 2000mJ / cm². 2 UV curing was performed. The adhesive sheet before and after UV curing were stacked separately to prepare a test sample approximately 1.5 mm thick. 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' at -20°C and 25°C was read.

[0174] (Measurement conditions)

[0175] Deformation mode: Torsion

[0176] Measurement frequency: 1Hz

[0177] Heating rate: 5℃ / minute

[0178] Measurement temperature: -50~150℃

[0179] Shape: Parallel plate 8.0mmφ

[0180] <Adhesion>

[0181] A 12.5 μm thick polyimide film ("Kapton50EN" manufactured by Du Pont-Toray Co., Ltd.) was adhered to a glass plate using double-sided adhesive tape ("No. 531" manufactured by Nitto Denko Co., Ltd.) to obtain a polyimide film substrate for testing. The release liner was removed from the surface of a reinforcing film cut into 25 mm wide x 100 mm long sections. These sections were then adhered to the polyimide film substrate for testing using a hand roller, serving as test samples before photocuring. Samples obtained by photocuring the adhesive layer by irradiating the reinforcing film side (PET film substrate side) of the pre-curing test samples with ultraviolet light were used as post-curing test samples. Using these test samples, the ends of the reinforcing film substrate were held in a clamp, and the reinforcing film was peeled 180° at a stretching speed of 300 mm / min. The peel strength was then measured.

[0182] <Bending Test>

[0183] The release liner is peeled off from the surface of the self-reinforcing film, and a polyimide film ("Kapton 50EN" manufactured by Du Pont-Toray Co., Ltd.) is bonded to the surface of the adhesive layer using a hand roller. This laminate is cut into pieces 25mm wide and 100mm long, and the adhesive layer is photocured by irradiating it with 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 Co., Ltd.), a bending fixture 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. After repeated bending tests, the sample is evaluated as OK if there is no peeling or lifting between the reinforcing film and the adherend, and NG if peeling or lifting occurs.

[0184] (Experimental conditions)

[0185] Bending radius: 3mm

[0186] Bending angle: 180°

[0187] Bending speed: 1 second / bend

[0188] Bending cycles: 200,000

[0189] The composition of the adhesive for each reinforced film, the shear storage modulus G' and adhesive strength of the adhesive layer before and after photocuring, and the evaluation results of the bending test are shown in Table 2.

[0190] [Table 2]

[0191]

[0192] In Comparative Example 2, which used polymer F with a glass transition temperature of -34°C as the base polymer, the adhesive layer exhibited a high storage modulus at -20°C both before and after photocuring, resulting in adhesive layer delamination after repeated bending tests. In Comparative Example 3, which changed the type of crosslinking agent, reduced the amount of crosslinking material, and reduced the amount of photocuring agent, the adhesive layer had a lower storage modulus compared to Comparative Example 2, but the storage modulus at -20°C remained high, leading to adhesive layer delamination after repeated bending tests. Similarly, in other comparative examples, the high storage modulus at -20°C resulted in adhesive layer delamination after repeated bending tests.

[0193] On the other hand, in Examples 1 to 18, where the photocured adhesive layer had a low storage modulus at -20°C, no peeling of the adhesive layer occurred after repeated bending tests, demonstrating good bonding reliability.

[0194] A comparison of Examples 1-3 with Comparative Example 7 shows that a lower amount of isocyanate crosslinking agent results in a lower storage modulus of the adhesive layer before and after photocuring. In Examples 14-16, which incorporate epoxy-based crosslinking structures into carboxyl-based polymers G, H, and I, similar to Examples 2 and 12 using isocyanate-based crosslinking agents, the adhesive strength before photocuring is low, but high adhesive strength is observed after photocuring. A comparison of Examples 4-6 shows that a higher amount of photocuring agent results in a lower initial adhesive strength.

[0195] A comparison of Examples 2, 10, 11, 17, and 18 with Comparative Example 8 shows that, despite changes in the type of UV-curing agent, the storage modulus of the adhesive layer before UV curing did not differ significantly, but the initial adhesive strength did. Furthermore, in these examples, the adhesive strength and storage modulus of the adhesive after UV curing also differed. Among these examples, Example 2, which used M350 as the UV-curing agent, exhibited low initial adhesive strength and high adhesive strength after UV curing, demonstrating excellent properties.

[0196] In Comparative Example 8, which used only trimethylolpropane triacrylate (TMPT) as a UV curing agent, although the rate of increase in adhesive strength before and after UV curing was low, the storage modulus of the UV-cured adhesive layer increased significantly at -20°C, and peeling occurred after the flexural test. On the other hand, in Examples 7 and 8, which used a combination of TMPT and M350 as UV curing agents, the increase in storage modulus at -20°C due to UV curing was small, and the UV-cured adhesive layer exhibited excellent adhesive properties.

[0197] These results show that not only the composition of the base polymer, but also the type and amount of crosslinking agent and the type and amount of UV-curing agent significantly affect the properties of the adhesive layer before and after UV curing. Particularly when the type of UV-curing agent is changed, the changes in the physical properties of the adhesive layer before and after UV curing are significant. It can be considered that with changes in the structure of the UV-curing agent, the compatibility between the base polymer and the UV-curing agent changes. When both exhibit moderate compatibility, the initial adhesive strength is low, but excellent adhesive properties are observed after UV curing, and the storage modulus is low in the low-temperature region, which also suppresses adhesive peeling under repeated bending.

Claims

1. A reinforced film comprising a film substrate and an adhesive layer adhered to a main surface of the film substrate. The adhesive layer is formed from a photocurable composition containing an acrylic-based polymer and a photocuring agent. The acrylic-based polymer has a glass transition temperature below -55°C. The acrylic-based polymer, as a monomer unit, contains one or more monomers selected from the group consisting of hydroxyl-containing monomers and carboxyl-containing monomers. A crosslinking agent is used to introduce a crosslinked structure into the acrylic-based polymer, and the crosslinking agent is used at 0.05 to 1 part by weight per 100 parts by weight of the polymer. The reinforced film contains epoxy-modified polyfunctional (meth)acrylate as the photocuring agent, wherein the chain length n of the epoxy alkyl group in the epoxy-modified polyfunctional (meth)acrylate is 1~5. The shear storage modulus of the adhesive layer at -20°C after photocuring is 1.0 × 10⁻⁶. 4 ~5.0×10 5 Pa, and the shear storage modulus at 25℃ is 8.0 × 10⁻⁶. 3 ~1.0×10 5 Pa, The photocurable composition contains 3 to 30 parts by weight of the photocuring agent relative to 100 parts by weight of the acrylic-based polymer. The crosslinking agent is an isocyanate-based crosslinking agent or an epoxy-based crosslinking agent.

2. The reinforced film according to claim 1, wherein, The functional group equivalent of the photocuring agent is 80~300g / eq.

3. The reinforced film according to claim 1 or 2, wherein, The shear storage modulus of the adhesive layer at -20°C before photocuring is 1.0 × 10⁻⁶. 4 ~5.0×10 5 Pa, and the shear storage modulus at 25℃ is 5.0 × 10⁻⁶. 3 ~1.0×10 5 Pa.

4. The reinforced film according to claim 1 or 2, wherein, The shear storage modulus of the adhesive layer at 25°C after photocuring is 1.2 to 7 times that at 25°C before photocuring.

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

6. A device with a reinforcing film, wherein the reinforcing film is attached to the surface of a flexible device. The reinforced film comprises a film substrate and an adhesive layer fixedly laminated on one main surface of the film substrate. The adhesive layer is bonded to the surface of the device. The adhesive layer is formed from a photocurable material, which is obtained by photocuring a photocurable adhesive composition containing an acrylic-based polymer and a photocuring agent. The acrylic-based polymer has a glass transition temperature below -55°C. The acrylic-based polymer, as a monomer unit, contains one or more monomers selected from the group consisting of hydroxyl-containing monomers and carboxyl-containing monomers. A crosslinking agent is used to introduce a crosslinked structure into the acrylic-based polymer, and the crosslinking agent is used at 0.05 to 1 part by weight per 100 parts by weight of the polymer. The reinforced film contains epoxy-modified polyfunctional (meth)acrylate as the photocuring agent, wherein the chain length n of the epoxy alkyl group in the epoxy-modified polyfunctional (meth)acrylate is 1~5. The shear storage modulus of the adhesive layer at -20°C is 1.0 × 10⁻⁶. 4 ~5.0×10 5 Pa, and the shear storage modulus at 25℃ is 8.0 × 10⁻⁶. 3 ~1.0×10 5 Pa, The photocurable adhesive composition contains 3 to 30 parts by weight of the photocuring agent relative to 100 parts by weight of the acrylic-based polymer. The crosslinking agent is an isocyanate-based crosslinking agent or an epoxy-based crosslinking agent.

7. A method for manufacturing a device with a reinforcing film, comprising a method for manufacturing a device with a reinforcing film in which a reinforcing film is attached to the surface of a flexible device, wherein, The adhesive layer of the reinforcing film according to any one of claims 1 to 5 is adhered to the surface of the flexible device. The adhesive layer is then photocured.

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