JP7796028B2 - Reinforcement film, device with reinforcement film, and method for manufacturing the same - Google Patents

Description

The present invention relates to a reinforcing film that is attached to the surface of a device. Furthermore, the present invention relates to a device equipped with a reinforcing film and a method for manufacturing the same.

Adhesive films are sometimes applied to the surfaces of optical devices such as displays and electronic devices for purposes such as surface protection and impact resistance. Such adhesive films typically have an adhesive layer firmly laminated to the main surface of a film substrate, and are attached to the device surface via this adhesive layer.

By temporarily attaching an adhesive film to the surface of a device or device component before use, such as during device assembly, processing, or transportation, it is possible to prevent scratches and damage to the adherend. Patent Document 1 discloses a reinforcing film that has an adhesive layer made of a photocurable adhesive composition on a film substrate.

This reinforcing film has a high gel fraction in the adhesive and low adhesion immediately after application to the adherend, making it easy to peel off from the adherend. This allows for rework from the adherend, and also makes it possible to selectively peel off the reinforcing film from areas of the adherend that do not require reinforcement. The adhesive in the reinforcing film firmly bonds to the adherend upon photocuring, leaving the film substrate permanently bonded to the surface of the adherend, making it usable as a reinforcing material for protecting the surface of devices, etc.

In recent years, organic EL panels using bendable substrates (flexible substrates) such as resin films have been put to practical use, and foldable flexible displays have been proposed. Foldable flexible displays are repeatedly bent at the same location. At the bent point, compressive stress is applied to the inside and tensile stress to the outside, causing distortion at the bent point and its surrounding area, which can lead to peeling of the adhesive from the adherend. Patent Document 2 proposes using a soft adhesive sheet to bond components together in a foldable display, thereby mitigating stress distortion at the bent point (see, for example, Patent Documents 2 and 3).

Japanese Patent Application Laid-Open No. 2020-41113 JP 2018-45213 A Japanese Patent Application Laid-Open No. 2017-119801

When the reinforcing film proposed in Patent Document 1 is applied to a flexible device, peeling may occur at bending points due to the hardness of the adhesive after photocuring. In particular, the flexibility and adhesive retention of the adhesive are low at low temperatures, so when a bending test is performed at low temperatures, the adhesive is likely to peel from the adherend at bending points. The adhesive sheets described in Patent Documents 2 and 3 suppress peeling from the adherend when repeatedly bent in a low-temperature environment, but the adhesive is not photocurable and exhibits high adhesion immediately after being attached to the adherend, making them difficult to peel or process.

In light of the above, the present invention aims to provide a reinforcing film that is easy to peel off immediately after being attached to an adherend, can be firmly adhered to the adherend by photocuring the adhesive after being attached to the adherend, and is less likely to peel off during a bending test.

The reinforcing film of the present invention comprises an adhesive layer adhered and laminated to one main surface of a film substrate. The adhesive layer comprises a photocurable composition containing an acrylic-based polymer and a photocuring agent. The glass transition temperature of the acrylic-based polymer is preferably -45°C or lower.

The acrylic base polymer contains, as a monomer unit, one or more monomers selected from the group consisting of hydroxyl group-containing monomers and carboxyl group- containing monomers, and a crosslinking agent such as an isocyanate crosslinking agent or an epoxy crosslinking agent is bonded to the hydroxyl group and/or carboxyl group of the base polymer to introduce a crosslinked structure. The amount of the crosslinking agent may be about 0.05 to 1 part by weight per 100 parts by weight of the acrylic polymer.

The photocurable composition that constitutes the adhesive layer preferably contains 3 to 30 parts by weight of a photocuring agent per 100 parts by weight of the acrylic base polymer. For example, a polyfunctional (meth)acrylate is used as the photocuring agent. The polyfunctional (meth)acrylate may be an alkylene oxide-modified polyfunctional (meth)acrylate modified with an alkylene oxide such as ethylene oxide or propylene oxide. The functional group equivalent of the photocuring agent is, for example, approximately 80 to 300 g/eq.

After photocuring, the pressure-sensitive adhesive layer preferably has a shear storage modulus at −20° C. of 1.0×10 ⁴ to 5.0×10 ⁵ Pa. After photocuring, the pressure-sensitive adhesive layer preferably has a shear storage modulus at 25° C. of 8.0×10 ³ to 5.0×10 ⁵ Pa.

The shear storage modulus of the pressure-sensitive adhesive layer at −20° C. before photocuring is preferably 1.0×10 ⁴ to 5.0×10 ⁵ Pa. The shear storage modulus of the pressure-sensitive adhesive layer at 25° C. before photocuring is preferably 5.0×10 ³ to 1.0×10 ⁴ Pa.

The shear storage modulus at 25°C of the adhesive layer after photocuring is preferably 1.2 to 7 times the shear storage modulus at 25°C of the adhesive layer before photocuring. The shear storage modulus at -20°C of the adhesive layer after photocuring is preferably 1.2 to 7 times the shear storage modulus at -20°C of the adhesive layer before photocuring.

A device with a reinforcing film is obtained by laminating the above-mentioned reinforcing film to the surface of the device and photocuring the adhesive layer. The device may be a flexible device that can be bent.

The reinforcing film of the present invention has an adhesive layer made of a photocurable composition, and by photocuring the adhesive layer after adhering to the adherend, the adhesive strength with the adherend increases. Because the adhesive strength with the adherend is low before photocuring, it is easy to peel from the adherend. Even after photocuring, the adhesive layer of the reinforcing film maintains a low storage modulus and high stress-strain relaxation properties. Therefore, even when a bendable device is repeatedly bent at the same location, peeling of the adhesive layer at the bent location is suppressed, resulting in excellent adhesion reliability.

FIG. 2 is a cross-sectional view showing a laminated structure of a reinforcing film. FIG. 2 is a cross-sectional view showing a laminated structure of a reinforcing film. FIG. 10 is a cross-sectional view showing a device to which a reinforcing film is attached. FIG. 10 is a cross-sectional view showing a folded state of the device to which the reinforcing film is attached.

Figure 1 is a cross-sectional view showing one embodiment of a reinforcing film. The reinforcing film 10 comprises an adhesive layer 2 on one main surface of a film substrate 1. The adhesive layer 2 is fixedly laminated onto one main surface of the film substrate 1. The adhesive layer 2 is a photocurable adhesive made from a photocurable composition, and is cured by exposure to actinic rays such as ultraviolet light, thereby increasing the adhesive strength with the adherend.

Figure 2 is a cross-sectional view of a reinforced film with a separator 5 temporarily attached to the main surface of the adhesive layer 2. Figure 3 is a cross-sectional view of a device 100 with a reinforced film in which a reinforced film 10 is attached to the surface of a foldable device 20. Figure 4 is a cross-sectional view showing the device folded around the hinge 25.

The separator 5 is peeled off and removed from the surface of the adhesive layer 2, and the exposed surface of the adhesive layer 2 is attached to the surface of the device 20, thereby attaching the reinforcing film 10 to the surface of the device 20. In this state, the adhesive layer 2 has not yet been photocured, and the reinforcing film 10 (adhesive layer 2) is temporarily attached to the device 20. By photocuring the adhesive layer 2, the adhesive strength at the interface between the device 20 and the adhesive layer 2 increases, and the device 20 and the reinforcing film 10 are firmly attached.

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

In the reinforcing film shown in Figure 2, the film substrate 1 and adhesive layer 2 are bonded together, and the separator 5 is temporarily attached to the adhesive layer 2. When the film substrate 1 and separator 5 are peeled apart, peeling occurs at the interface between the adhesive layer 2 and the separator 5, and the adhesive layer 2 remains bonded to the film substrate 1. No adhesive remains on the separator 5 after peeling.

In the device with reinforcing film shown in Figure 3, before the adhesive layer 2 is photocured, the device 20 and the adhesive layer 2 are temporarily attached to each other. When the film substrate 1 and the device 20 are peeled apart, the peeling occurs at the interface between the adhesive layer 2 and the device 20, so the adhesive layer 2 remains adhered to the film substrate 1. Because no adhesive remains on the device 20, peeling operations such as rework and cutting are easy. After the adhesive layer 2 is photocured, the adhesive strength between the adhesive layer 2 and the device 20 increases, resulting in a fixed state, making it difficult to peel the film 1 from the device 20.

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

The thickness of the film substrate is, for example, about 4 to 150 μm. From the viewpoint of reinforcing the device by imparting rigidity and shock absorption, the thickness of the film substrate 1 is preferably 5 μm or more, more preferably 12 μm or more, even more preferably 20 μm or more, and particularly preferably 25 μm or more. From the viewpoint of imparting flexibility to the reinforcing film so that it can be folded, the thickness of the film substrate 1 is preferably 125 μm or less, more preferably 100 μm or less. From the viewpoint of achieving both mechanical strength and flexibility, the compressive strength of the film substrate 1 is preferably 100 to 3,000 kg/cm ² , more preferably 200 to 2,900 kg/cm ² , even more preferably 300 to 2,800 kg/cm ² , and particularly preferably 400 to 2,700 kg/cm ² .

Examples of plastic materials constituting the film substrate 1 include polyester resins, polyolefin resins, cyclic polyolefin resins, polyamide resins, polyimide resins, polyether ether ketone, polyethersulfone, polyarylate resins, and aramid resins. In reinforcement films for optical devices such as displays, the film substrate 1 is preferably a transparent film. Furthermore, when photocuring of the adhesive layer 2 is performed by irradiating actinic rays from the film substrate 1 side, the film substrate 1 is preferably transparent to the actinic rays used to cure the adhesive layer. Polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, as well as transparent polyimides and transparent aramids, are preferred because they combine mechanical strength and transparency. When irradiating actinic rays from the adherend side, the adherend only needs to be transparent to actinic rays; the film substrate 1 does not need to be transparent to actinic rays.

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

[Adhesive layer]
The adhesive layer 2, which is fixedly laminated on the film substrate 1, is made of a photocurable composition containing a base polymer and a photocuring agent. Before photocuring, the adhesive layer 2 has low adhesive strength to an adherend such as a device or device component, and is therefore easy to peel off. Because the adhesive strength to the adherend of the adhesive layer 2 improves upon photocuring, the reinforcing film is less likely to peel off from the device surface even when the device is in use, resulting in excellent adhesive reliability.

Photocurable adhesives undergo little curing in typical storage environments, but are cured when exposed to actinic light such as ultraviolet light. Therefore, the reinforcing film of the present invention has the advantage that the timing of curing of the adhesive layer 2 can be set as desired, allowing for flexible response to process lead times, etc.

The thickness of the adhesive layer 2 is, for example, approximately 1 to 300 μm. The thicker the adhesive layer 2, the better the adhesion to the adherend tends to be. On the other hand, if the adhesive layer 2 is excessively thick, the fluidity before photocuring may be high, making handling difficult. Therefore, the thickness of the 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 achieving a thinner adhesive layer, the thickness of the adhesive layer 2 may be 25 μm or less, 20 μm or less, or 18 μm or less.

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

Preferably, the adhesive layer 2 exhibits increased adhesive strength with the adherend upon photocuring and maintains a low shear storage modulus (hereinafter simply referred to as "storage modulus") at low temperatures even after photocuring. The storage modulus of the adhesive is determined by reading the value at a specified temperature when measured at a frequency of 1 Hz, in the range of -50 to 150°C, and at a heating rate of 5°C/min, in accordance with the method described in JIS K7244-1 "Plastics - Test methods for dynamic mechanical properties."

The storage modulus of the pressure-sensitive adhesive layer after photocuring at −20° C. is preferably not more than 5.0×10 ⁵ Pa, more preferably not more than 4.0×10 ⁵ Pa, even more preferably not more than 3.0×10 ⁵ Pa, and may be not more than 2.5×10 ⁵ Pa or not more than 2.0×10 ⁵ Pa. Since the pressure-sensitive adhesive layer 2 after photocuring has a small storage modulus at low temperatures, the pressure-sensitive adhesive layer exhibits strain relaxation properties in a low-temperature environment, and therefore peeling of the pressure-sensitive adhesive layer at the bent portion can be suppressed even when the device to which the reinforcing film is bonded is repeatedly bent or when the bent state is maintained for a long period of time.

On the other hand, if the storage modulus of the pressure-sensitive adhesive layer after photocuring is excessively small, the pressure-sensitive adhesive layer is likely to undergo plastic deformation, and the pressure-sensitive adhesive layer may peel off from the adherend due to insufficient adhesive holding power. Therefore, the storage modulus of the pressure-sensitive adhesive layer after photocuring at -20°C is preferably 1.0 x 10 ⁴ Pa or more, more preferably 2.0 x 10 ⁴ Pa or more, even more preferably 3.00 x 10 ⁴ Pa or more, and may be 4.0 x 10 ⁴ Pa or more, 5.0 x 10 ⁴ Pa or more, 6.0 x 10 ⁴ Pa or more, 7.0 x 10 ⁴ Pa or more, or 8.0 x 10 ⁴ Pa or more.

From the viewpoint of ensuring adhesiveness at room temperature and suppressing protrusion of the adhesive layer from the edges, the storage modulus of the adhesive layer after photocuring at 25°C is preferably 8.0 x 10 ³ to 1.5 x 10 ⁵ Pa, more preferably 1.0 x 10 ⁴ to 1.0 x 10 ⁵ Pa, even more preferably 1.5 x 10 ⁴ to 8.0 x 10 ⁴ Pa, and may be 2.0 x 10 ⁴ to 6.0 x 10 ⁴ Pa.

The adhesive layer 2 contains a base polymer and a photocuring agent, and its composition is not particularly limited as long as the adhesive strength with the adherend increases upon photocuring. From the perspective of increasing the efficiency of curing upon exposure to actinic light, the adhesive composition (photocurable composition) that constitutes the adhesive layer 2 preferably contains a photopolymerization initiator.

(Base polymer)
The base polymer is the main component of the pressure-sensitive adhesive composition and is the main factor determining the adhesive strength and storage modulus of the pressure-sensitive adhesive layer before photocuring. The pressure-sensitive adhesive composition preferably contains an acrylic polymer as the base polymer, and preferably 50 wt % or more of the pressure-sensitive adhesive composition is an acrylic polymer, because it has excellent optical transparency and adhesiveness and is easy to control the adhesive strength and storage modulus.

Acrylic polymers that contain alkyl (meth)acrylate esters as the main monomer component are preferred. In this specification, "(meth)acrylic" refers to acrylic and/or methacrylic.

As the (meth)acrylic acid alkyl ester, a (meth)acrylic acid alkyl ester having an alkyl group having 1 to 20 carbon atoms is preferably used. The alkyl group of the (meth)acrylic acid alkyl ester may be linear or branched. Examples of (meth)acrylic acid alkyl esters include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, and mono (meth)acrylate. Examples thereof include isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, isotridecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, cetyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate.

Among the exemplified (meth)acrylic acid alkyl esters, from the viewpoint of lowering the Tg of the base polymer, _C1-9 (meth)acrylic acid alkyl esters are preferred, and those having a homopolymer glass transition temperature of −50° C. or lower are preferred. The glass transition temperature of a homopolymer of a (meth)acrylic acid alkyl ester is more preferably −55° C. or lower, and even more preferably −60° C. or lower. Specific examples of (meth)acrylic acid _C1-9 alkyl esters having a homopolymer 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.), and butyl acrylate (Tg: −55° C.). Among these, butyl acrylate and 2-ethylhexyl acrylate are preferred, and 2-ethylhexyl acrylate is particularly preferred due to its low Tg.

The content of the (meth)acrylic acid alkyl ester 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 be 90% by weight or more, 93% by weight or more, or 95% by weight or more, based on the total amount of monomer components constituting the base polymer. Among these, the amount of the (meth)acrylic acid _C1-9 alkyl ester is preferably within the above range, and the total of 2-ethylhexyl acrylate and butyl acrylate is more preferably within the above range. The amount of 2-ethylhexyl acrylate may also be within the above range.

The acrylic base polymer preferably contains, as a copolymerization component, a monomer component having a crosslinkable functional group. Examples of monomers having a crosslinkable functional group include hydroxyl group-containing monomers and carboxyl group-containing monomers. The base polymer may contain both a hydroxyl group-containing monomer and a carboxyl group-containing monomer as copolymerization components, or only one of them. Introducing a crosslinked structure into the base polymer improves cohesive strength, which tends to improve the releasability of the pressure-sensitive adhesive layer 2 from the substrate before photocuring.

Examples of hydroxy group-containing monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and 4-(hydroxymethyl)cyclohexylmethyl (meth)acrylate. Among these, 2-hydroxyethyl acrylate (Tg: -15°C) and 4-hydroxybutyl acrylate (Tg: -32°C) are preferred due to their significant contribution to improving the adhesive strength of the adhesive after photocuring and their low homopolymer Tg.

Examples of carboxy group-containing monomers include (meth)acrylic acid, 2-carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, 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, and crotonic acid.

The total amount of hydroxy group-containing monomers and carboxy group-containing monomers in the acrylic base polymer is preferably 0.5 to 15% by weight, more preferably 1 to 10% by weight, and even more preferably 2 to 7% by weight, based on the total amount of constituent monomer components. When a crosslinked structure is introduced into the acrylic base polymer using an isocyanate-based crosslinking agent, the content of hydroxy group-containing (meth)acrylic acid esters such as 2-hydroxyethyl (meth)acrylate and 4-hydroxypropyl (meth)acrylate is preferably within the above range.

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

The acrylic base polymer may contain monomer components other than those described above. Examples of monomer components that may be included in the acrylic base polymer include vinyl ester monomers, aromatic vinyl monomers, epoxy group-containing monomers, vinyl ether monomers, sulfo group-containing monomers, phosphate group-containing monomers, and acid anhydride group-containing monomers.

The base polymer before the crosslinking structure is introduced may be substantially free of nitrogen atoms. The proportion of nitrogen 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 0.001 mol% or less. Using a base polymer substantially free of nitrogen atoms tends to suppress an increase in the adhesive strength (initial adhesive strength) of the pressure-sensitive adhesive layer before photocuring when the adherend is subjected to a surface activation treatment. By not using nitrogen-atom-containing monomers such as cyano group-containing monomers, lactam structure-containing monomers, amide group-containing monomers, and morpholine ring-containing monomers as constituent monomer components of the base polymer, a base polymer substantially free of nitrogen atoms can be obtained. The amount of nitrogen-containing monomer 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.

The glass transition temperature of the acrylic base polymer is preferably -45°C or lower, more preferably -50°C or lower, even more preferably -55°C or lower, particularly preferably -60°C or lower, and may be -63°C or lower or -65°C or lower. There is no particular lower limit for the glass transition temperature of the acrylic base polymer, but it is generally -80°C or higher, and may be -75°C or higher or -70°C or higher.

The glass transition temperature is the temperature (peak top temperature) at which the loss tangent tanδ in viscoelasticity measurements is maximized. Because the glass transition temperature is sufficiently lower than the ambient temperature of the device, the storage modulus G' of the adhesive layer within the ambient temperature range tends to be small, which tends to reduce peeling during repeated bending.

Instead of the glass transition temperature determined by viscoelasticity measurement, a theoretical Tg calculated by Fox's formula may be used. The theoretical Tg is calculated by Fox's formula below from the glass transition temperature Tg _i of the homopolymer of the constituent monomer components of the acrylic base polymer and the weight fraction W _i of each monomer component.
1/Tg=Σ(W _i /Tg _i )

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

The above monomer components are polymerized by various known methods, such as solution polymerization, emulsion polymerization, and bulk polymerization, to obtain an acrylic polymer as the base polymer. Solution polymerization is preferred from the perspective of cost and the balance of adhesive strength, holding power, and other properties of the pressure-sensitive adhesive. Ethyl acetate, toluene, etc. are used as solvents for solution polymerization. The solution concentration is typically around 20 to 80% by weight. Various known polymerization initiators, such as azo-based and peroxide-based initiators, can be used. A chain transfer agent may be used to adjust the molecular weight. The reaction temperature is typically around 50 to 80°C, and the reaction time is typically around 1 to 8 hours.

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. Note that if a crosslinked structure is introduced into the base polymer, the molecular weight of the base polymer refers to the molecular weight before the crosslinked structure is introduced.

(Crosslinking agent)
In order to provide the pressure-sensitive adhesive with adequate cohesive strength, develop adhesive strength, and ensure the adhesive layer can be easily peeled from the adherend before photocuring, it is preferable to introduce a crosslinked structure into the base polymer. For example, a crosslinked structure can be introduced by adding a crosslinking agent to a solution obtained after polymerizing the base polymer and heating the solution as needed. 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 hydroxy groups and carboxy groups introduced into the base polymer to form a crosslinked structure. Isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred because they are highly reactive with the hydroxy groups and carboxy groups of the base polymer and facilitate the introduction of a crosslinked structure.

The isocyanate crosslinking agent used is a polyisocyanate with two or more isocyanate groups per molecule. Examples of polyisocyanate 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-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate; and trimethylolpropane. Examples of isocyanate adducts include a xylylene diisocyanate/tolylene diisocyanate trimer adduct (e.g., "Coronate L" manufactured by Tosoh), a trimethylolpropane/hexamethylene diisocyanate trimer adduct (e.g., "Coronate HL" manufactured by Tosoh), a trimethylolpropane adduct of xylylene diisocyanate (e.g., "Takenate D110N" manufactured by Mitsui Chemicals), and an isocyanurate of hexamethylene diisocyanate (e.g., "Coronate HX" manufactured by Tosoh).

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

The amount of crosslinking agent used can be adjusted appropriately depending on the composition and molecular weight of the base polymer. The amount of crosslinking agent used is approximately 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, per 100 parts by weight of base polymer.

A crosslinking catalyst may be used to promote the formation of a crosslinked structure. For example, crosslinking catalysts for isocyanate-based crosslinking agents include metal-based crosslinking catalysts (particularly tin-based crosslinking catalysts) such as tetra-n-butyl titanate, tetraisopropyl titanate, nursem ferric iron, butyltin oxide, dioctyltin dilaurate, and dibutyltin dilaurate. The amount of crosslinking catalyst used is generally 0.1 parts by weight or less per 100 parts by weight of base polymer.

(Photocuring agent)
The pressure-sensitive adhesive composition constituting the pressure-sensitive adhesive layer 2 contains a photocuring agent in addition to a base polymer. When the pressure-sensitive adhesive layer 2 made of a photocurable pressure-sensitive adhesive composition is photocured after being attached to an adherend, the adhesive strength to the adherend is improved.

A photocuring monomer or a photocuring oligomer is used as the photocuring agent. A compound with two or more ethylenically unsaturated bonds in one molecule is preferred. Furthermore, a compound that is compatible with the base polymer is preferred. To ensure adequate compatibility with the base polymer, it is preferable for the photocuring agent to be liquid at room temperature.

Due to their high compatibility with acrylic-based polymers, it is preferable to use polyfunctional (meth)acrylates as photocuring agents. Polyfunctional (meth)acrylates are typically esters of polyols and (meth)acrylic acid. Specific examples of polyfunctional (meth)acrylates include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, alkanediol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, isocyanuric acid di(meth)acrylate, isocyanuric acid tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol di(meth)acrylate, and trimethylolpropane. Examples of the acrylate include ethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerin di(meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, and isoprene (meth)acrylate.

The polyfunctional (meth)acrylate may be an ester of an alkylene oxide-modified polyol and (meth)acrylic acid. Examples of alkylene oxides include ethylene oxide (EO) and propylene oxide (PO). The alkylene oxide may also be a polyalkylene oxide such as polyethylene glycol or polypropylene glycol. The chain length n of the alkylene oxide is approximately 1 to 10. By adjusting the type and chain length of the alkylene oxide, the compatibility with the acrylic base polymer can be adjusted to an appropriate range.

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 isocyanuric acid-modified di(meth)acrylate, propylene oxide isocyanuric acid-modified di(meth)acrylate, ethylene oxide isocyanuric acid-modified tri(meth)acrylate, propylene oxide isocyanuric acid-modified tri(meth)acrylate, pentaerythritol ethylene oxide-modified tetra(meth)acrylate, and pentaerythritol propylene oxide-modified tetra(meth)acrylate.

The polyfunctional (meth)acrylate may be a metal (meth)acrylate such as zinc (meth)acrylate, magnesium (meth)acrylate, calcium (meth)acrylate, barium (meth)acrylate, strontium (meth)acrylate, nickel (meth)acrylate, copper (meth)acrylate, or aluminum (meth)acrylate.

The compatibility between the base polymer and the photocuring agent also depends on the molecular weight of the photocuring agent. The smaller the molecular weight of the photocuring agent, the higher the compatibility with the base polymer tends to be. From the standpoint of compatibility with the base polymer, the molecular weight of the photocuring agent is preferably 1500 or less, more preferably 1000 or less, and may also be 800 or less, 600 or less, 500 or less, 450 or less, or 400 or less.

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

If the base polymer and photocuring agent are not completely compatible, the liquid photocuring agent will bleed out to the surface, forming an adhesion-preventing layer (Weak Boundary Layer; WBL) at the adhesive interface with the adherend, strengthening its liquid-like properties. When a WBL is formed, the adhesive layer's bulk properties, such as storage modulus, are maintained while the liquid-like properties of the surface (adhesive interface) are strengthened, which tends to weaken the adhesive strength with the adherend.

When the base polymer and photocuring agent are moderately compatible but not completely compatible, a WBL is formed in the adhesive layer before photocuring, resulting in low initial adhesive strength and easy peeling from the substrate. On the other hand, after photocuring, the liquid properties of the photocuring agent disappear and a photocrosslinking structure is uniformly introduced into the adhesive layer, significantly improving adhesive strength with the substrate.

The compatibility between the base polymer and the photocuring agent is mainly affected by the structure of the compound. The structure and compatibility of a compound can be evaluated, for example, by the Hansen solubility parameter (HSP). The Hansen solubility parameter (HSP) is a three-dimensional representation of the Hildebrand solubility parameter δ, which is divided into three components: the dispersion term δ _d , the polar term δ _p , and the hydrogen bonding term δ _h . The relationship δ ² = δ _d ² + δ _p ² + δ _h ² holds. The dispersion term δ _d indicates the effect of dispersion forces, the polar term δ _p indicates the effect of dipole-dipole forces, and the hydrogen bonding term δ _h indicates the effect of hydrogen bonding forces. The HSP distance Ra of two substances is expressed as Ra = {4Δδ _d ^{2 +Δδ p 2} +Δδ _h 2 _} ^1/2 , where Δδ _d is the difference in dispersion terms between the two substances, Δδ _p is the difference in polar terms, and Δδ _h is the difference in hydrogen bond terms. The smaller the Ra ^, the higher the compatibility, and the larger ^the Ra, the lower the compatibility.

Details of Hansen solubility parameters are described in Hansen Solubility Parameters: A Users Handbook by Charles M. Hansen (CRC Press, 2007), and for substances for which literature values are unknown, they can be calculated using the computer software Hansen Solubility Parameters in Practice (HSPiP).

As mentioned above, acrylic base polymers with low glass transition temperatures have a high proportion of (meth)acrylic acid esters, such as 2-ethylhexyl acrylate, in their constituent monomer components, and a low proportion of highly polar monomers, such as hydroxyl group-containing monomers, carboxyl group-containing monomers, and nitrogen-containing monomers. Thus, when using a base polymer with a low glass transition temperature and low polarity, the HSP distance Ra between the base polymer and the photocuring agent can be adjusted within an appropriate range by using a multifunctional (meth)acrylate incorporating an alkylene oxide, such as ethylene oxide or propylene oxide, as the photocuring agent. The longer the chain length n of the alkylene oxide in the alkylene oxide-modified multifunctional (meth)acrylate, the greater the HSP distance Ra tends to be.

If the distance Ra between the HSPs of the base polymer and the photocuring agent is excessively large, although the initial adhesive strength is low, contamination due to bleed-out of the photocuring agent may occur, and the increase in adhesive strength upon photocuring may be insufficient. Therefore, the chain length n of the alkylene oxide is preferably 1 to 5, more preferably 1 to 3. For example, if the photocuring agent is trimethylolpropane ethylene oxide-modified triacrylate, the chain length n of the ethylene oxide (EO) attached to each of the three methylol groups of the trimethylolpropane is preferably 1 or 2. A pressure-sensitive adhesive containing a low-Tg acrylic base polymer whose main constituent monomer is 2-ethylhexyl acrylate and a trimethylolpropane EO-modified triacrylate with a chain length n of 1 or 2 exhibits low initial adhesive strength and high adhesive strength to the adherend after photocuring. From the perspective of suppressing bleed-out of the photocuring agent onto the surface of the adhesive layer, the chain length n of the EO is particularly preferably 1.

The type and amount of photocuring agent affect not only adhesive strength but also the bulk properties of the adhesive. If the base polymer of the adhesive composition is the same, the change in storage modulus of the adhesive layer before photocuring is small, even if the type of photocuring agent is different. On the other hand, as the content of photocuring agent increases, the content of base polymer in the composition becomes relatively smaller, which tends to decrease the storage modulus of the adhesive layer before photocuring.

The smaller the functional group equivalent weight of the photocuring agent and the higher the content of the photocuring agent, the higher the crosslink density due to photocuring, and therefore the greater the storage modulus of the adhesive layer after photocuring. In other words, the greater the content of the photocuring agent, the smaller the storage modulus of the adhesive layer before photocuring and the greater the storage modulus of the adhesive layer after photocuring.

From the viewpoint of increasing the adhesive strength of the adhesive after photocuring while suppressing an excessive increase in 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, or may be 6 to 15 parts by weight or 7 to 12 parts by weight per 100 parts by weight of the base polymer.

Two or more types of photocuring agents may be used in combination. When two or more types of photocuring agents are used in combination, it is preferable that the total amount of photocuring agents be within the above range. For example, by combining a photocuring agent that has relatively high compatibility with the base polymer with a photocuring agent that has relatively low compatibility with the base polymer, it is possible to adjust the properties of the adhesive layer before and after photocuring, such as keeping the initial adhesive strength low while increasing the storage modulus of the adhesive at room temperature after photocuring.

(Photopolymerization initiator)
The pressure-sensitive adhesive composition preferably contains a photopolymerization initiator. The photopolymerization initiator generates active species upon irradiation with actinic rays and promotes the curing reaction of the photocuring agent. As the photopolymerization initiator, a photocationic initiator (photoacid generator), a photoradical initiator, a photoanionic initiator (photobase generator), or the like is used depending on the type of photocuring agent. When a polyfunctional acrylate is used as the photocuring agent, a photoradical initiator is preferably used. As the photoradical initiator, a photoradical generator that is cleaved by visible light or ultraviolet light having a wavelength shorter than 450 nm to generate radicals is preferred, and examples of the photoradical initiator include hydroxyketones, benzyl dimethyl ketals, aminoketones, acylphosphine oxides, benzophenones, and trichloromethyl group-containing triazine derivatives. The photoradical generators may be used alone or in combination of two or more.

When transparency is required for the pressure-sensitive adhesive layer 2, the photopolymerization initiator (photoradical generator) preferably has low sensitivity to light (visible light) with a wavelength longer than 400 nm, and for example, a photopolymerization initiator having an absorption coefficient of 1 x ¹⁰² [mLg ^- 1cm ^-1 ] or less at a wavelength of 405 nm is preferably used. Furthermore, if a photopolymerization initiator with low sensitivity to visible light is used, cleavage of the photopolymerization initiator due to external light in the storage environment is unlikely to occur, and the storage stability of the reinforcing film can be improved.

The content of the photopolymerization initiator in the adhesive composition is preferably 0.001 to 5 parts by weight, more preferably 0.01 to 3 parts by weight, and even more preferably 0.03 to 1 part by weight, per 100 parts by weight of the base polymer.

(oligomer)
The pressure-sensitive adhesive composition may contain an oligomer in addition to the base polymer. For example, the pressure-sensitive adhesive composition may contain an acrylic oligomer in addition to the acrylic base polymer. The oligomer used has a weight-average molecular weight of approximately 1,000 to 30,000. The acrylic oligomer contains a (meth)acrylic acid alkyl ester as a main constituent monomer component. From the viewpoint of increasing the adhesive strength of the pressure-sensitive adhesive layer 2 after photocuring, the glass transition temperature of the acrylic oligomer is preferably 40°C or higher, more preferably 50°C or higher. The oligomer may contain a crosslinkable functional group, similar to the base polymer.

The content of oligomer in the pressure-sensitive adhesive composition is not particularly limited. When the pressure-sensitive adhesive composition contains an acrylic oligomer in addition to an acrylic base polymer, the amount of oligomer per 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, from the perspective of adjusting the adhesive strength within an appropriate range.

(Other additives)
In addition to the components exemplified above, the adhesive layer may contain additives such as a silane coupling agent, a tackifier, a plasticizer, a softener, an anti-degradant, a filler, a colorant, an ultraviolet absorber, an antioxidant, a surfactant, and an antistatic agent, within a range that does not impair the properties of the present invention.

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

The above-mentioned adhesive composition is applied to a substrate by roll coating, kiss roll coating, gravure coating, reverse coating, roll brush, spray coating, dip roll coating, bar coating, knife coating, air knife coating, curtain coating, lip coating, die coating, or the like, and the solvent is dried and removed as needed to form an adhesive layer. Any appropriate drying method can be used. 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.

When the pressure-sensitive adhesive composition contains a crosslinking agent, it is preferable to promote crosslinking by heating or aging simultaneously with or after drying of the solvent. The heating temperature and heating time are set appropriately depending on the type of crosslinking agent used, and crosslinking is typically achieved by heating at a temperature in the range of 20°C to 160°C for approximately 1 minute to 7 days. The heating used to dry and remove the solvent may also serve as the heating used for crosslinking.

The introduction of a crosslinked structure into the base polymer increases the gel fraction and tends to increase the storage modulus of the adhesive layer 2. The higher the gel fraction of the adhesive before photocuring, the harder the adhesive becomes, and the less adhesive residue tends to be left on the adherend when the reinforcing film is peeled from the adherend during rework, etc. 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 be 35% or more, 40% or more, or 45% or more. On the other hand, if the gel fraction is excessively high, the storage modulus increases and stress-strain relaxation is poor, which may result in adhesive peeling at bent portions during use of a flexible device. Therefore, the gel fraction of the adhesive layer 2 before photocuring is preferably 80% or less, more preferably 75% or less, even more preferably 70% or less, and may be 65% or less.

The gel fraction can be determined as the insoluble portion in a solvent such as ethyl acetate. Specifically, it is determined as the weight fraction (unit: wt%) of the insoluble portion after immersing the adhesive layer in ethyl acetate at 23°C for 7 days relative to the sample before immersion. Generally, the gel fraction of a polymer is equal to the degree of crosslinking; the more crosslinked portions in the polymer, the higher the gel fraction.

Even after the crosslinking agent introduces a crosslinked structure into the polymer, the photocuring agent remains unreacted. As a result, 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 separator 5 to the adhesive layer 2 for purposes such as protecting the adhesive layer 2. Crosslinking may also be performed after attaching the separator 5 to the adhesive layer 2.

When forming the adhesive layer 2 on another substrate, the reinforcing film can be obtained by transferring the adhesive layer 2 onto the film substrate 1 after drying the solvent. The substrate used to form the adhesive layer can also be used as the separator 5 as is.

Preferably, the separator 5 is a plastic film such as polyethylene, polypropylene, polyethylene terephthalate, or polyester film. The thickness of the separator is typically 3 to 200 μm, preferably approximately 10 to 100 μm. The surface of the separator 5 that comes into contact with the adhesive layer 2 is preferably treated with a release agent such as a silicone-based, fluorine-based, long-chain alkyl-based, or fatty acid amide-based release agent, or with silica powder. By treating the surface of the separator 5 with a release agent, when the film substrate 1 and the separator 5 are peeled apart, peeling occurs at the interface between the adhesive layer 2 and the separator 5, maintaining the adhesive layer 2 adhered to the film substrate 1. The separator 5 may be antistatically treated on either or both of the release-treated and untreated surfaces. Treating the separator 5 with an antistatic agent reduces static buildup when the separator is peeled from the adhesive layer.

[Characteristics of the reinforcing film and use of the reinforcing film]
The reinforced film of the present invention is used by being attached to a device or a device component. The reinforced film 10 has a pressure-sensitive adhesive layer 2 fixed to a film substrate 1, and has low adhesive strength to the adherend after being attached to the adherend and before being photocured. Therefore, the reinforced film can be easily peeled from the adherend before being photocured.

The substrate to which the reinforcement film is attached is not particularly limited, and examples include various electronic devices, optical devices, and their component parts. In one embodiment, the reinforcement film is attached to the surface of a foldable flexible device. The foldable device has a hinge portion 25 as shown in Figure 3, and can be folded around this hinge portion. The folding angle can be set arbitrarily, and the device may be bent (folded) 180° as shown in Figure 4. Note that in Figure 4, the device is folded so that the adhesive surface of the reinforcement film 10 is on the inside, but it may also be bent with the reinforcement film 10 on the outside. If the device is a display device, the reinforcement film may be attached to the surface facing the screen, or to the back side (housing). As shown in Figures 3 and 4, a flexible device that is configured to be bendable at a predetermined location, such as the hinge portion 25, repeatedly bends and stretches at the same location during use.

The reinforcing film may be applied to the entire surface of the adherend, or may be selectively applied only to areas requiring reinforcement (reinforcement target areas). Alternatively, the reinforcing film may be applied to the entire areas requiring reinforcement (reinforcement target areas) and areas not requiring reinforcement (non-reinforcement target areas), and then the reinforcing film applied to the non-reinforcement target areas may be cut and removed. Before the adhesive is photocured, the reinforcing film is temporarily attached to the surface of the adherend, making it easy to peel and remove the reinforcing film from the surface of the adherend. The reinforcing film may be applied to the areas requiring reinforcement and the areas not requiring reinforcement, and the areas requiring reinforcement may be selectively irradiated with light to photocure the adhesive, after which the reinforcing film may be selectively peeled and removed from the non-reinforcement target areas where the adhesive is not yet cured.

By laminating a reinforcing film, appropriate rigidity is imparted, which is expected to improve handling and prevent breakage for thin components such as flexible devices. When a reinforcing film is laminated to a work-in-progress during the device manufacturing process, the reinforcing film may be laminated to a large-format work-in-progress before it is cut to product size. The reinforcing film may also be laminated roll-to-roll to the mother roll of a device manufactured by a roll-to-roll process.

Before laminating the reinforcing film, the surface of the adherend may be subjected to an activation treatment for the purpose of cleaning, etc. Examples of surface activation treatments include plasma treatment, corona treatment, and glow discharge treatment. An adherend whose surface has been activated contains many active groups such as hydroxyl groups, carbonyl groups, and carboxyl groups , which tend to increase adhesive strength due to intermolecular interactions with the polar functional groups of the base polymer of the adhesive. In particular, when the adherend is a polyimide, the activation treatment activates amide acids, terminal amino groups, carboxyl groups (or carboxylic anhydride groups), etc., which interact strongly with the polar functional groups of the base polymer, and therefore the activation treatment may significantly increase the initial adhesive strength.

If the initial adhesive strength is excessively high, peeling operations such as rework may become difficult. As mentioned above, the base polymer contains substantially no nitrogen atoms, which prevents excessive increases in the initial adhesive strength when applied to adherends whose surfaces have been activated.

To facilitate peeling from the adherend and prevent adhesive residue on the adherend after peeling the reinforcing film, the adhesive strength between the adhesive layer 2 and the adherend before photocuring 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 may be 0.1 N/25 mm or less, or even 0.05 N/25 mm or less. To prevent peeling of the reinforcing film during storage or handling, the adhesive strength between the adhesive layer 2 and the adherend before photocuring is preferably 0.005 N/25 mm or more, more preferably 0.01 N/25 mm or more. The adhesive strength is determined by a peel test using a polyimide film as the adherend, at a tensile speed of 300 mm/min and a peel angle of 180°. Unless otherwise specified, the adhesive strength is measured at 25°C.

The storage modulus of the pressure-sensitive adhesive layer 2 at 25°C before photocuring is preferably 5.0 x ¹⁰ to 1.0 x ¹⁰ Pa. From the viewpoint of suppressing adhesive residue on the adherend when peeling the reinforcing film from the adherend, the storage modulus of the pressure-sensitive adhesive layer 2 at 25°C before photocuring is preferably 7.0 x ¹⁰ Pa or more, more preferably 9.0 x ¹⁰ Pa or more, and may be 1.0 x ¹⁰ Pa or more or 1.5 x ¹⁰ Pa or more. From the viewpoint of imparting flexibility to the pressure-sensitive adhesive layer, the storage modulus of the pressure-sensitive adhesive layer 2 at 25°C before photocuring is preferably 7.0 x ¹⁰ Pa or less, more preferably 5.0 x ¹⁰ Pa or less, and may be 4.0 x ¹⁰ Pa or less or 3.0 x ¹⁰ Pa or less.

The storage modulus of the pressure-sensitive adhesive layer 2 at −20° C. before photocuring is preferably 1.0×10 ⁴ to 2.0×10 ⁵ Pa, more preferably 2.0×10 ⁴ to 1.0×10 ⁵ Pa, and may be 3.0×10 ⁴ to 9.0×10 ⁴ Pa or 4.0×10 ⁴ to 9.0×10 ⁴ Pa. When the storage modulus at low temperature of the pressure-sensitive adhesive layer before photocuring is within the above range, the storage modulus of the pressure-sensitive adhesive layer tends to be maintained low even after photocuring.

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 photocuring agent. The more crosslinking agent introduced, the higher the gel fraction and the greater the storage modulus. The greater the amount of photocuring agent, the relatively smaller the content of base polymer in the composition, and the smaller the storage modulus tends to be.

The storage modulus of the adhesive changes rapidly near the glass transition temperature of the base polymer. As mentioned above, 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 at -20°C before photocuring is preferably 7 times or less, more preferably 5 times or less, and may be 4 times or less, 3.5 times or less, or 3 times or less, of the storage modulus at 25°C. The storage modulus of the adhesive layer at -20°C before photocuring may be 1.5 times or more, 1.8 times or more, or 2.0 times or more of the storage modulus at 25°C.

After laminating the reinforcing film to the adherend, the pressure-sensitive adhesive layer 2 is photo-cured by irradiating it with actinic rays. Examples of actinic rays include ultraviolet light, visible light, infrared light, X-rays, alpha rays, beta rays, and gamma rays. Ultraviolet light is preferred as it can inhibit curing of the pressure-sensitive adhesive layer during storage and facilitates curing. The irradiation intensity and irradiation time of the actinic rays can be set appropriately depending on the composition and thickness of the pressure-sensitive adhesive layer. The pressure-sensitive adhesive layer 2 can be irradiated with actinic rays from either the film substrate 1 side or the adherend side, or from both sides.

As the adhesive layer is photocured, its storage modulus increases, and its adhesive strength to the adherend also increases. From the viewpoint of adhesive reliability during practical use of the device, the adhesive strength between the adhesive layer 2 and the adherend after photocuring is preferably 2 N/25 mm or greater, and more preferably 2.5 N/25 mm or greater. From the viewpoint of suppressing adhesive peeling when the flexible device is repeatedly bent at the same location, the adhesive strength between the adhesive layer 2 and the adherend after photocuring may be 3 N/25 mm or greater, 4 N/25 mm or greater, 5 N/25 mm or greater, 6 N/25 mm or greater, 7 N/25 mm or greater, 8 N/25 mm or greater, 9 N/25 mm or greater, or 10 N/25 mm or greater. The adhesive strength between the adhesive layer 2 and the adherend after photocuring is preferably 10 times or greater, more preferably 30 times or greater, and may be 50 times or greater, 100 times or greater, or 200 times or greater than the adhesive strength between the adhesive layer 2 and the adherend before photocuring. As mentioned above, by adjusting the type of photocuring agent (compatibility with the base polymer) and the amount added, it is possible to keep the adhesive strength before photocuring (initial adhesive strength) low and increase the adhesive strength of the adhesive after photocuring.

As described above, the pressure-sensitive adhesive layer 2 after photocuring preferably has a storage modulus of 1.0×10 ⁴ to 5.0×10 ⁵ Pa at −20° C. and a storage modulus of 8.0×10 ³ to 1.5×10 ⁵ Pa at 25° C. The low storage modulus in the low temperature range of the pressure-sensitive adhesive layer after photocuring tends to suppress peeling of the pressure-sensitive adhesive layer when repeatedly flexed at low temperatures.

The storage modulus of the pressure-sensitive adhesive layer at -20°C after photocuring is preferably 7 times or less, more preferably 5 times or less, and may be 4 times or less, 3.5 times or less, or 3 times or less, of the storage modulus at 25°C. The storage modulus of the pressure-sensitive adhesive layer at -20°C before photocuring may be 1.5 times or more, 1.8 times or more, or 2.0 times or more of the storage modulus at 25°C.

The storage modulus at -20°C of the adhesive layer after photocuring is preferably 7 times or less, and may be 5 times or less, 4 times or less, or 3.5 times or less, of the storage modulus at -20°C of the adhesive layer before photocuring. The storage modulus at -20°C of the adhesive layer after photocuring may be 1.1 times or more, 1.3 times or more, 1.5 times or more, or 1.7 times or more of the storage modulus at -20°C of the adhesive layer before photocuring.

The storage modulus at 25°C of the adhesive layer after photocuring is preferably 7 times or less, and may be 5 times or less, 4 times or less, or 3.5 times or less, of the storage modulus at 25°C of the adhesive layer before photocuring. The storage modulus at -20°C of the adhesive layer after photocuring may be 1.1 times or more, 1.3 times or more, 1.5 times or more, or 1.7 times or more of the storage modulus at -20°C of the adhesive layer before photocuring.

As described above, by applying the reinforcing film of the present invention, the adherend is given appropriate rigidity and stress is alleviated and dispersed, thereby suppressing various defects that may occur during the manufacturing process, improving production efficiency, and improving yield. Furthermore, because the reinforcing film is easily peeled from the adherend before the adhesive layer is photocured, rework is easy even if lamination or adhesion defects occur. Processing such as selectively removing the reinforcing film from areas other than those to be reinforced is also easy.

When using a completed device, even if an external force is suddenly applied due to the device being dropped, a heavy object being placed on the device, or a flying object hitting the device, the reinforcing film prevents damage to the device. Furthermore, because the reinforcing film firmly adheres to the device after the adhesive is photocured, the reinforcing film is less likely to peel off even during long-term use, providing excellent reliability. Furthermore, because the adhesive layer of the reinforcing film has a low storage modulus even at low temperatures, the adhesive is less likely to peel off at bent sections, even when used in flexible devices, demonstrating excellent adhesive reliability.

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

[Polymerization of acrylic polymer]
<Polymer A>
A reaction vessel equipped with a thermometer, a stirrer, a reflux condenser, and a nitrogen gas inlet tube was charged with 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 as a polymerization initiator, and 233 parts by weight of ethyl acetate as a solvent, and nitrogen gas was introduced and purged with nitrogen for about 1 hour while stirring. The mixture was then heated to 60°C and reacted for 7 hours to obtain a solution of acrylic polymer A having a weight average molecular weight (Mw) of 550,000.

<Polymers B to I>
The amounts of the monomers charged were changed as shown in Table 1. Solutions of polymers B to I were obtained in the same manner as in the polymerization of polymer A except for the above.

The monomer ratios used and the glass transition temperatures of the acrylic polymers A to I are listed in Table 1. The glass transition temperatures were calculated from the monomer ratios used according to Fox's formula.
2EHA 2-Ethylhexyl acrylate BA Butyl acrylate MMA Methyl methacrylate NVP N-Vinylpyrrolidone 2HEA 2-Hydroxyethyl acrylate 4HBA 4-Hydroxybutyl acrylate AA Acrylic acid β-CEA 2-Carboxyethyl acrylate HOA-MS 2-Acryloyloxyethyl succinic acid

[Preparation of reinforcing film]
<Preparation of Pressure-Sensitive Adhesive Composition>
A crosslinking agent and a photocuring agent were added to the acrylic polymer solution and mixed uniformly to prepare the pressure-sensitive adhesive compositions shown in Table 2. The amounts of crosslinking agent and photocuring agent in Table 2 are the amounts added per 100 parts by weight of the solid content of the base polymer. Details of the crosslinking agent and photocuring agent are as follows. In Examples 7 to 9, two types of photocuring agents were used, and the amounts of each photocuring agent are shown in Table 2.

(Crosslinking agent)
Takenate D110N: 75% ethyl acetate solution of xylylene diisocyanate trimethylolpropane adduct ("Takenate D110N" manufactured by Mitsui Chemicals)
C-HX: Isocyanurate of hexamethylene diisocyanate ("Coronate HX" manufactured by Tosoh)
TC: N,N,N',N'-tetraglycidyl-m-xylylenediamine (tetrafunctional epoxy compound, "Tetrad C" manufactured by Mitsubishi Gas Chemical Company)

(Photocuring agent)
M350: Trimethylolpropane EO-modified (n=1) triacrylate ("Aronix M-350" manufactured by Toagosei Co., Ltd., functional group equivalent weight 144 g/eq)
M360: Trimethylolpropane EO-modified (n=2) triacrylate ("Aronix M-360" manufactured by Toagosei Co., Ltd., functional group equivalent weight 186 g/eq)
M310: Trimethylolpropane PO-modified (n=1) triacrylate ("Aronix M-310" manufactured by Toagosei Co., Ltd., functional group equivalent weight 158 g/eq)
M321: Trimethylolpropane PO-modified (n=2) triacrylate ("Aronix M-321" manufactured by Toagosei Co., Ltd., functional group equivalent weight 200 g/eq)
APG700: Polypropylene glycol #700 (n=12) diacrylate ("NK Ester APG700" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent weight: 404 g/eq)
A200: polyethylene glycol #200 (n=4) diacrylate ("NK Ester A200" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent weight 154 g/eq)
TMPT: Trimethylolpropane triacrylate ("NK Ester TMPT" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent weight 99 g/eq)
ZnAc: zinc acrylate (manufactured by Nisshoku Technofine, functional group equivalent: 104 g/eq)

(Photopolymerization initiator)
In addition to the examples shown in Table 2, 0.2 parts by weight of 2,2-dimethoxy-1,2-diphenylethan-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 to 18 and Comparative Examples 1 to 3, 7, and 8. In Comparative Examples 4 to 6, 0.1 parts by weight of 1-hydroxycyclohexyl phenyl 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.

<Application of adhesive solution and crosslinking>
The above-described pressure-sensitive adhesive composition was applied to an untreated transparent film substrate using a fountain roll to a dried thickness of 13 μm. After drying at 130°C for 1 minute to remove the solvent, the release-treated surface of a separator (a 25 μm-thick polyethylene terephthalate film with one side treated with silicone release and both sides treated with antistatic agents) was bonded to the pressure-sensitive adhesive-coated surface. This was then aged for 4 days in a 25°C atmosphere to promote crosslinking, resulting in a reinforced film in which a photocurable pressure-sensitive adhesive sheet was fixedly laminated onto the transparent film substrate and a separator was temporarily attached thereon. In Examples 1 to 5, 7 to 18, and Comparative Examples 1 to 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-transmitting transparent polyimide film ("Neoprim S-100" manufactured by Mitsubishi Gas Chemical Company, Inc.) was used as the transparent film substrate.

[evaluation]
<Storage modulus>
The adhesive composition was applied and crosslinked on a separator in the same manner as above to prepare an adhesive sheet (before photocuring). A separator was attached to the surface of the adhesive layer of the adhesive sheet before photocuring to isolate it from oxygen, and the adhesive sheet was photocured by irradiating it with 2000 mJ/ ^cm2 of ultraviolet light from a 365 nm LED lamp. The adhesive sheet before photocuring and the adhesive sheet after photocuring were laminated to prepare a measurement sample with a thickness of approximately 1.5 mm. Dynamic viscoelasticity was measured under the following conditions using an "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific, and the shear storage modulus G' values at -20°C and 25°C were read. (Measurement Conditions)
Deformation mode: Torsion Measurement frequency: 1 Hz
Temperature rise rate: 5°C/min Measurement temperature: -50 to 150°C
Shape: Parallel plate 8.0 mm diameter

<Adhesive force>
A 12.5 μm thick polyimide film ("Kapton 50EN" manufactured by Toray DuPont) was attached to a glass plate via double-sided adhesive tape ("No. 531" manufactured by Nitto Denko) to obtain a polyimide film substrate for measurement. The separator was peeled off from the surface of a reinforced film cut to a width of 25 mm and a length of 100 mm, and the film was then attached to the polyimide film substrate for measurement using a hand roller to obtain a test sample before photocuring. The test sample before photocuring was irradiated with ultraviolet light from the reinforced film side (PET film substrate side) to photocure the adhesive layer, thereby obtaining a test sample after photocuring. Using these test samples, the edge of the film substrate of the reinforced film was held with a chuck, and the reinforced film was peeled 180° at a tensile speed of 300 mm/min, and the peel strength was measured.

<Bending test>
The separator was peeled off from the surface of the reinforcement film, and a polyimide film ("Kapton 50EN" manufactured by Toray DuPont) was attached to the surface of the adhesive layer using a hand roller. This laminate was cut into a size of 25 mm wide x 100 mm long, and the adhesive layer was photocured by irradiating ultraviolet light from the reinforcement film side (PET film substrate side) to obtain a test specimen. Using a planar body no-load U-shaped stretch tester (manufactured by Yuasa System Instruments), a bending jig was attached to the short side of the test specimen, and repeated bending tests were performed under the following conditions in a thermostatic chamber at a temperature of -20 ° C or 25 ° C and a relative humidity of 50%, with the reinforcement film side (film substrate side) facing inward. After the repeated bending test, samples that showed no peeling or lifting between the reinforcement film and the adherend were rated as OK, and samples that showed peeling or lifting were rated as NG.
(Test conditions)
Bending radius: 3 mm
Bending angle: 180°
Bending speed: 1 second/time Bending times: 200,000 times

The adhesive composition of each reinforcing 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.

In Comparative Example 2, which used Polymer F, which has a glass transition temperature of -34°C, as the base polymer, the storage modulus at -20°C was high both before and after photocuring of the adhesive layer, and peeling of the adhesive layer was observed after repeated bending tests. In Comparative Example 3, in which the type of crosslinker was changed, the crosslinking material was reduced, and the amount of photocuring agent was also reduced, the storage modulus of the adhesive layer was lower than in Comparative Example 2, but the storage modulus at -20°C was still high, and peeling of the adhesive layer was observed after repeated bending tests. In the other Comparative Examples, the storage modulus at -20°C was also high, and peeling of the adhesive layer was observed after repeated bending tests.

On the other hand, in Examples 1 to 18, in which the storage modulus of the adhesive layer at -20°C after photocuring was small, no peeling of the adhesive layer was observed after repeated bending tests, demonstrating good adhesive reliability.

A comparison of Examples 1 to 3 with Comparative Example 7 shows that the smaller the amount of isocyanate crosslinking agent, the smaller the storage modulus of the adhesive layer before and after photocuring tends to be. In Examples 14 to 16, in which a crosslinked structure using an epoxy-based crosslinking agent was introduced into base polymers G, H, and I, which have carboxy groups, the adhesive strength before photocuring was low and the adhesive strength after photocuring was high, similar to Examples 2 and 12, which used an isocyanate-based crosslinking agent. A comparison of Examples 4 to 6 shows that the greater the amount of photocuring agent, the smaller the initial adhesive strength tends to be.

Comparing Examples 2, 10, 11, 17, and 18 with Comparative Example 8, when the type of photocuring agent was changed, there was no significant difference in the storage modulus of the adhesive layer before photocuring, but differences were observed in the initial adhesive strength. Furthermore, in these examples, differences were also observed in the adhesive strength and storage modulus of the adhesive after photocuring. Of these examples, Example 2, which used M350 as the photocuring agent, exhibited low initial adhesive strength and high adhesive strength after photocuring, demonstrating excellent properties.

In Comparative Example 8, which used only trimethylolpropane triacrylate (TMPT) as the light curing agent, the increase in adhesive strength before and after light curing was small, but the storage modulus of the adhesive layer at -20°C after light curing increased significantly, and peeling was observed after the flex resistance test. In contrast, in Examples 7 and 8, which used a combination of TMPT and M350 as light curing agents, the increase in storage modulus at -20°C after light curing was small, and the adhesive layer after light curing exhibited excellent adhesive properties.

These results show that not only the composition of the base polymer, but also the type and amount of crosslinker and the type and amount of photocuring agent significantly affect the properties of the adhesive layer before and after photocuring. In particular, when the type of photocuring agent was changed, the change in the physical properties of the adhesive layer before and after photocuring was significant. The compatibility between the base polymer and the photocuring agent changes as the photocuring agent structure changes, and when the two show adequate compatibility, the adhesive exhibits low initial adhesive strength and excellent adhesive properties after photocuring, as well as a low storage modulus at low temperatures, which prevents the adhesive from peeling even when repeatedly flexed.

Claims (9)

A film substrate and a pressure-sensitive adhesive layer fixedly laminated on one main surface of the film substrate,
the pressure-sensitive adhesive layer is made of a photocurable composition containing an acrylic-based polymer and a photocuring agent;
the acrylic base polymer contains, as a monomer unit, one or more monomer units selected from the group consisting of a hydroxy group-containing monomer and a carboxy group- containing monomer, and has a glass transition temperature of −55° C. or lower;
The acrylic base polymer is a crosslinked polymer into which a crosslinked structure has been introduced 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 an alkylene oxide-modified polyfunctional (meth)acrylate having a (poly)alkylene oxide chain with 1 to 5 repeating units of alkylene oxide,
the pressure-sensitive adhesive layer has, after photocuring, a shear storage modulus of 1.0×10 ⁴ to 5.0×10 ⁵ Pa at −20° C. and a shear storage modulus of 8.0×10 ³ to 1.5×10 ⁵ Pa at 25° C.;
Reinforcement film. 2. The reinforcing film according to 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 according to claim 1 , wherein the photocuring agent has a functional group equivalent of 80 to 300 g/eq. The reinforcing film according to any one of claims 1 to 3 , wherein the crosslinking agent is an isocyanate-based crosslinking agent or an epoxy-based crosslinking agent. The reinforced film according to any one of claims 1 to 4, wherein the pressure-sensitive adhesive layer has a shear storage modulus of 1.0 x 10 ⁴ to 5.0 x 10 ⁵ Pa at -20°C and a shear storage modulus of 5.0 x 10 ³ to 1.0 x 10 ⁵ Pa at 25°C before photocuring. The reinforced film according to any one of claims 1 to 5 , wherein the shear storage modulus at 25°C after photocuring of the pressure-sensitive adhesive layer is 1.2 to 7 times the shear storage modulus at 25°C before photocuring. The reinforced film according to any one of claims 1 to 6 , wherein the shear storage modulus at -20°C of the pressure-sensitive adhesive layer after photocuring is 1.2 to 7 times the shear storage modulus at -20°C of the pressure-sensitive adhesive layer before photocuring. A device with a reinforcing film, in which a reinforcing film is attached to the surface of a foldable device,
The reinforcing film includes a film substrate and a pressure-sensitive adhesive layer fixedly laminated on one main surface of the film substrate,
the pressure-sensitive adhesive layer is attached to a device surface,
the pressure-sensitive adhesive layer is made of a photocured product obtained by photocuring a photocurable pressure-sensitive adhesive composition containing an acrylic-based polymer and a photocuring agent;
In the photocurable pressure-sensitive adhesive composition,
the acrylic base polymer contains, as a monomer unit, one or more monomer units selected from the group consisting of a hydroxy group-containing monomer and a carboxy group- containing monomer, and has a glass transition temperature of −55° C. or lower;
The acrylic base polymer is a crosslinked polymer into which a crosslinked structure has been introduced 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 an alkylene oxide-modified polyfunctional (meth)acrylate having a (poly)alkylene oxide chain with 1 to 5 repeating units of alkylene oxide,
the pressure-sensitive adhesive layer has a shear storage modulus of 1.0×10 ⁴ to 5.0×10 ⁵ Pa at −20° C. and a shear storage modulus of 8.0×10 ³ to 5.0×10 ⁵ Pa at 25° C.;
Device with reinforced film. A method for manufacturing a device with a reinforcing film in which a reinforcing film is attached to a surface of a foldable device, comprising:
The pressure-sensitive adhesive layer of the reinforcing film according to any one of claims 1 to 7 is attached to a surface of a foldable device,
photocuring the pressure-sensitive adhesive layer;
A method for manufacturing a device with a reinforcing film.