JP7774189B2 - 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 Documents 1 and 2 disclose reinforcing films that have 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 adhesiveness 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 devices have also been put to practical use. Patent Document 4 proposes using a photocurable adhesive containing an acrylic-based polymer with a low glass transition temperature as the adhesive for the reinforcing film of a foldable device, thereby preventing peeling of the adhesive layer at the bending points of the foldable device.

Japanese Patent Application Laid-Open No. 2020-41113 Japanese Patent Application Laid-Open No. 2020-134540 Japanese Patent Application Laid-Open No. 2019-20726 International Publication No. 2022/050009

Foldable devices 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. By designing the adhesive layer of the reinforcing film to be flexible with a low storage modulus, distortion at the bent point is alleviated, preventing peeling of the reinforcing film around the bent point and reducing damage to the display element due to stress.

On the other hand, if the adhesive is soft and has little elasticity, when a foldable device is held in a folded state and then returned to an unfolded (extended) state, the deformation of the adhesive layer will not return to its original shape, causing wrinkles and distorting the shape of the device. To prevent wrinkles and other deformations at bent points, the adhesive of the reinforcing film of a foldable device must have high adhesive strength to the adherend as well as high shape recovery.

As described in Patent Document 3, foldable devices use resin films such as polyimide as the substrate material for the display elements, which are prone to charging and can lead to electrostatic damage to the device due to charging (static electricity). In order to prevent charging of a device to which a reinforcing film is attached, it is effective to incorporate an antistatic agent into the adhesive layer provided in contact with the device, thereby reducing the resistance of the adhesive layer and providing antistatic properties. However, even when an antistatic agent is added to the adhesive of the reinforcing film described in Patent Document 4, the adhesive does not sufficiently reduce its resistance. Increasing the amount of antistatic agent added causes the antistatic agent to bleed out, reducing the adhesive strength between the adhesive layer and the device and contaminating the device surface.

To reduce the resistance of the adhesive layer, it is possible to blend a polar group-containing compound such as a polyol into the adhesive in addition to an antistatic agent. However, because additives such as polyols are low-molecular-weight components, increasing the amount of additive blended to reduce resistance reduces the strain recovery of the adhesive, making it more likely that wrinkles and other deformations will occur at the bent points of the foldable device.

As mentioned above, it is not easy to achieve all of the following properties for the adhesive layer used in foldable devices: low storage elasticity (stress relaxation), strain recovery, and low resistance.

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 photo-curing the adhesive after being attached to the adherend, is less likely to peel off from the adherend when bent, has high strain recovery properties in the adhesive layer, and can contribute to preventing static electricity in devices.

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 is made of a photocurable composition containing an acrylic-based polymer, a photocuring agent, a photopolymerization initiator, and an antistatic agent.

The acrylic base polymer contains, as monomer units, (meth)acrylic acid alkyl esters; (meth)acrylic acid esters having alkylene oxide chains; and monomers having crosslinkable functional groups. Examples of monomers having crosslinkable functional groups include hydroxyl group-containing monomers and carboxyl group-containing monomers. The weight average molecular weight of the acrylic base polymer before the introduction of crosslinking structures is preferably 100,000 or more.

For a total of 100 parts by weight of the monomer units of the acrylic base polymer, the amount of (meth)acrylic acid alkyl ester is preferably 40 to 94 parts by weight, and the amount of (meth)acrylic acid ester having an alkylene oxide chain is preferably 5 to 55 parts by weight.

The (meth)acrylic acid ester having an alkylene oxide chain as a monomer unit of the acrylic base polymer is a compound represented by the general formula (1).
CH ₂ =CR ¹ -COO-(R ² -O) _m -R ³ (1)

^R1 is a hydrogen atom or a methyl group. ^R2 is an alkylene group having 2 to 4 carbon atoms which may be branched. m is an integer of 1 to 5. ^R3 is a hydrogen atom, an alkyl group having 1 to 12 carbon atoms or an aryl group having 6 to 20 carbon atoms.

It is preferable that the (meth)acrylic acid alkyl ester used as the monomer unit of the acrylic base polymer has an alkyl group with 6 or more carbon atoms.

The content of the photocuring agent in the photocurable composition that constitutes the adhesive layer is preferably 3 to 25 parts by weight per 100 parts by weight of the acrylic base polymer.

A polyfunctional (meth)acrylate is preferred as the photocuring agent. The polyfunctional (meth)acrylate may contain an alkylene oxide chain. In addition to a polyfunctional (meth)acrylate containing an alkylene oxide chain, a urethane (meth)acrylate may also be used as the photocuring agent.

The pressure-sensitive adhesive layer preferably has a surface resistance of 5×10 ¹¹ Ω or less.The pressure-sensitive adhesive layer preferably has a shear storage modulus of 1.0×10 ⁴ to 5.0×10 ⁵ Pa at −20° C. after photocuring.

It is preferable that the adhesive layer has an adhesive strength to the polyimide film of 1 N/25 mm or less before photo-curing. It is preferable that the adhesive layer has an adhesive strength to the polyimide film of 5 N/25 mm or more after photo-curing.

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, the film can be easily peeled off from the adherend.

The adhesive layer of the reinforcing film contains an antistatic agent, and the acrylic base polymer contains a (meth)acrylic acid ester with an alkylene oxide chain as a monomer unit, resulting in low resistance. This not only suppresses static buildup in the adhesive layer itself, but also contributes to anti-static properties in the adherend. Furthermore, the adhesive layer has a low shear storage modulus at low temperatures and excellent stress relaxation properties, preventing peeling of the adhesive layer at the bent point even when the film is repeatedly bent at the same location, providing excellent adhesion reliability.

Furthermore, because the adhesive layer has a high strain recovery rate, when the adherend is held in a folded state and then returned to an extended state, the adhesive layer easily restores its shape, making it less likely to develop wrinkles or other deformations. Therefore, the reinforced film of the present invention can also be suitably used in foldable devices that use a resin film substrate.

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.

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 reinforcing film with a release liner 5 temporarily attached to the main surface of the adhesive layer 2. Figure 3 is a cross-sectional view showing the reinforcing film 10 attached to the surface of the device 20.

The release liner 5 is peeled off and removed from the surface of the adhesive layer 2, and the exposed surface of the adhesive layer 2 is attached to the surface of the device 20, thereby attaching the reinforcement film 10 to the surface of the device 20. In this state, the adhesive layer 2 has not yet been photocured, and the reinforcement film 10 (adhesive layer 2) is temporarily attached 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 reinforcement film 10 are fixed together.

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

In the device with reinforced 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 reinforced film 10 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 pressure-sensitive adhesive layer 2, which is fixedly laminated on the film substrate 1, is made of a photocurable composition. The photocurable composition constituting the pressure-sensitive adhesive layer 2 contains an acrylic-based polymer, a photocuring agent, and a photopolymerization initiator, and further contains an antistatic agent.

Before photocuring, the adhesive layer 2 has low adhesive strength to adherends such as devices and device components, making it easy to peel off. Photocuring improves the adhesive strength of the adhesive layer 2 to adherends, making it difficult for the reinforcing film to peel off from the device surface even when the device is in use, providing 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.

<Base polymer>
The base polymer is the main component of the pressure-sensitive adhesive composition and is a key factor determining various properties of the pressure-sensitive adhesive layer, such as adhesive strength and storage modulus. In the present invention, an acrylic polymer is used as the base polymer of the pressure-sensitive adhesive. Acrylic polymers have excellent optical transparency and adhesive properties, and their adhesive strength, storage modulus, etc. can be easily controlled.

The acrylic base polymer contains a (meth)acrylic acid alkyl ester as a monomer component. The acrylic base polymer used in the present invention is a copolymer containing, as monomer components, a (meth)acrylic acid ester having an alkylene oxide chain, and a monomer having a crosslinkable functional group in addition to a (meth)acrylic acid alkyl ester.

((Meth)acrylic acid alkyl ester)
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. From the viewpoint of lowering the glass transition temperature of the acrylic base polymer and reducing the storage modulus, thereby suppressing peeling of the pressure-sensitive adhesive layer when repeatedly flexed, it is preferable that the alkyl group of the (meth)acrylic acid alkyl ester is a chain alkyl group. The chain alkyl group may be linear or branched.

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

Among the exemplified (meth)acrylic acid alkyl esters, from the viewpoint of lowering the glass transition temperature of the acrylic base polymer, (meth)acrylic acid alkyl esters whose homopolymer glass transition temperature is -56°C or less are preferred. To lower the glass transition temperature of the homopolymer, the alkyl group of the (meth)acrylic acid alkyl ester preferably has 6 or more carbon atoms. The alkyl group of the (meth)acrylic acid alkyl ester may also have 20 or less, 12 or less, 10 or less, or 9 or less carbon atoms.

Specific examples of (meth)acrylic acid alkyl esters whose homopolymer glass transition temperature (Tg) is -56°C or lower include 2-ethylhexyl acrylate (Tg: -70°C), n-hexyl acrylate (Tg: -65°C), n-heptyl acrylate, n-octyl acrylate (Tg: -65°C), isononyl acrylate (Tg: -60°C), n-nonyl acrylate (Tg: -58°C), and isooctyl acrylate (Tg: -58°C). Among these, 2-ethylhexyl acrylate, n-heptyl acrylate, and n-octyl acrylate are preferred, with n-octyl acrylate being particularly preferred, as they can reduce the storage modulus around -20°C.

As the (meth)acrylic acid alkyl ester, a _C6-9 (meth)acrylic acid alkyl ester in which the alkyl group has 6 to 9 carbon atoms and a _C10-20 (meth)acrylic acid alkyl ester in which the alkyl group has 10 to 20 carbon atoms may be used in combination. Compared to a _C6-9 (meth)acrylic acid alkyl ester, a _C10-20 (meth)acrylic acid alkyl ester has a higher homopolymer Tg, but has a temperature region (plateau region) near the Tg in which the temperature dependence of viscoelasticity is small. Therefore, when the acrylic base polymer contains a _C10-20 (meth)acrylic acid alkyl ester in addition to a _C6-9 (meth)acrylic acid alkyl ester as a monomer component, the temperature dependence of the storage modulus in the low temperature region is reduced, and warping of the adherend due to temperature changes and peeling of the reinforcing film from the adherend may be suppressed.

Among _C10-20 chain alkyl (meth)esters, _C12-18 alkyl (meth)esters are preferred because of their wide temperature range in the plateau region and small storage modulus in the plateau region. Of these, dodecyl (meth)acrylate and isostearyl (meth)acrylate are preferred, with dodecyl acrylate (also known as lauryl acrylate) being particularly preferred. When a _C6-9 alkyl (meth)ester and a _C10-20 alkyl (meth)ester are used in combination as the alkyl (meth)ester, a combination of 2-ethylhexyl acrylate or n-octyl acrylate and dodecyl acrylate is particularly preferred.

((Meth)acrylic acid ester having alkylene oxide chain)
The (meth)acrylic acid ester having an alkylene oxide chain is an ester of (meth)acrylic acid and an alcohol having an alkylene oxide chain, and is a compound represented by the following general formula (1).
CH ₂ =CR ¹ -COO-(R ² -O) _m -R ³ (1)

In general formula (1), ^R1 is a hydrogen atom or a methyl group, and a compound of formula (1) in which ^R1 is a hydrogen atom is an acrylate, and a compound of formula (1) in which ^R1 is a methyl group is a methacrylate. From the viewpoint of lowering the Tg of the acrylic base polymer, ^R1 is preferably a hydrogen atom.

In general formula (1), ^R2 is an alkylene group having 2 to 4 carbon atoms which may have a branch, and ^-R2 -O- is an alkylene oxide chain. When ^R2 has 2 to 4 carbon atoms, the resistance of the pressure-sensitive adhesive is reduced and the compatibility between the acrylic base polymer and the photocuring agent is improved. Specific examples of alkylene oxide ^-R2 -O- in which ^R2 has 2 to 4 carbon atoms include ethylene oxide ( _{-CH2CH2 -} O-), propylene oxide (-CH( _{CH3 ) CH2 -} O-), and butylene oxide ( _{-CH2CH2CH2CH2 -} O-).

In general formula (1), m is the number of repeating alkylene oxide units and is an integer from 1 to 5. When the number of repeating alkylene oxide units, m, is 5 or less, it is easy to obtain an acrylic base polymer with a high molecular weight (for example, a weight average molecular weight of 100,000 or more). If m is excessively large, the (meth)acrylate compound represented by general formula (1) tends to act as a chain transfer agent, which can prevent the molecular weight of the acrylic base polymer as a polymer from becoming sufficiently large, resulting in insufficient adhesive strength of the pressure-sensitive adhesive.

^R3 is a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, or an aryl group having 6 to 20 carbon atoms. From the viewpoint of lowering the Tg of the acrylic base polymer and improving compatibility with the photocuring agent, ^R3 is preferably an alkyl group having 1 to 3 carbon atoms, and particularly preferably a methyl group or an ethyl group.

In the general formula (1), a compound in which m=1 and ^R3 is an alkyl group is a (meth)acrylic acid alkoxyalkyl ester. In the general formula (1), a compound in which m≧2 and ^R3 is an alkyl group is a (meth)acrylic acid polyalkylene glycol ester.

Specific examples of compounds represented by general formula (1) include methoxyethyl (meth)acrylate, phenoxyethyl (meth)acrylate, ethoxyethoxyethyl (meth)acrylate, methoxydipropylene glycol (meth)acrylate, and methoxytriethylene glycol (meth)acrylate. Among these, methoxyethyl acrylate (glass transition temperature of homopolymer: -50°C) is particularly preferred from the perspective of lowering the Tg of the acrylic base polymer and reducing the storage modulus of the adhesive layer at low temperatures.

(Monomers having crosslinkable functional groups)
The monomer having a crosslinkable functional group is specifically a hydroxy group-containing monomer or a carboxy group-containing monomer. The acrylic base polymer may have both a hydroxy group-containing monomer and a carboxy group-containing monomer as monomer components, or may have only one of them. The introduction of a crosslinked structure into the acrylic base polymer tends to improve the cohesive strength and the peelability of the pressure-sensitive adhesive layer 2 from the adherend before photocuring.

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. Of these, 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate are preferred because they contribute significantly to improving the adhesive strength of the pressure-sensitive adhesive after photocuring.

Carboxy group-containing monomers include (meth)acrylic acid, 2-carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid. Of these, acrylic acid is particularly preferred because of its significant contribution to improving adhesive strength.

(monomer ratio)
It is preferable that the amount of the (meth)acrylic acid alkyl ester is 40 to 94 parts by weight and the amount of the (meth)acrylic acid ester having an alkylene oxide chain is 5 to 55 parts by weight relative to 100 parts by weight of the total of the constituent monomer components of the acrylic base polymer.

The (meth)acrylic acid ester having an alkylene oxide chain is a component that contributes to improving the compatibility between the base polymer and the photocuring agent and reducing the resistance of the adhesive; the higher its ratio in the acrylic base polymer, the lower the resistance of the adhesive layer tends to be. The amount of the (meth)acrylic acid ester having an alkylene oxide chain is preferably 5 parts by weight or more, more preferably 8 parts by weight or more, even more preferably 10 parts by weight or more, and may be 15 parts by weight or more, or 20 parts by weight or more, per 100 parts by weight of the total monomer components constituting the acrylic base polymer.

The higher the proportion of (meth)acrylic ester having an alkylene oxide chain, the lower the adhesive resistance tends to be. However, if the proportion of (meth)acrylic ester having an alkylene oxide chain is excessively high, the compatibility with the photocuring agent becomes excessive, resulting in high adhesive strength (initial adhesive strength) between the adhesive layer and the substrate before photocuring, which may make peeling difficult. Furthermore, as the proportion of (meth)acrylic ester having an alkylene oxide chain increases, the proportion of (meth)acrylic acid alkyl ester decreases relatively, which may increase the glass transition temperature of the acrylic base polymer and reduce adhesion at low temperatures. Therefore, the amount of (meth)acrylic ester having an alkylene oxide chain is preferably 55 parts by weight or less, more preferably 50 parts by weight or less, even more preferably 45 parts by weight or less, and may be 40 parts by weight or less, 35 parts by weight or less, 30 parts by weight or less, or 25 parts by weight or less, per 100 parts by weight of the total monomer components constituting the acrylic base polymer.

From the viewpoint of lowering the glass transition temperature of the acrylic base polymer, the amount of (meth)acrylic acid alkyl ester is preferably 40 parts by weight or more, more preferably 50 parts by weight or more, even more preferably 60 parts by weight or more, and may be 65 parts by weight or more, 70 parts by weight or more, 75 parts by weight or more, or 80 parts by weight or more, per 100 parts by weight of the total of the constituent monomer components of the acrylic base polymer.

On the other hand, as the proportion of (meth)acrylic acid alkyl ester increases, the proportion of (meth)acrylic acid ester having an alkylene oxide chain decreases relatively, which may result in a decrease in compatibility between the acrylic base polymer and the photocuring agent. The amount of (meth)acrylic acid alkyl ester is preferably 94 parts by weight or less, more preferably 90 parts by weight or less, even more preferably 87 parts by weight or less, and may be 85 parts by weight or less, 83 parts by weight or less, 80 parts by weight or less, or 77 parts by weight or less, per 100 parts by weight of the total of the constituent monomer components of the acrylic base polymer.

The acrylic base polymer may contain two or more types of (meth)acrylic acid alkyl esters as monomer components. The two or more types of (meth)acrylic acid alkyl esters may include those whose homopolymer glass transition temperature is -56°C or lower and those whose homopolymer glass transition temperature is higher than -56°C, and may include those whose alkyl group has 6 or more carbon atoms and those whose alkyl group has 5 or less carbon atoms. As mentioned above, they may also include those whose alkyl group has 6 to 9 carbon atoms and those whose alkyl group has 10 to 20 carbon atoms.

From the viewpoint of lowering the glass transition temperature of the acrylic base polymer, the amount of the (meth)acrylic acid alkyl ester in which the alkyl group has 6 or more carbon atoms is preferably in the above-mentioned range, the amount of the (meth)acrylic acid _C6-9 alkyl ester may also be in the above-mentioned range, the total of 2-ethylhexyl acrylate, n-heptyl acrylate, and n-octyl acrylate may also be in the above-mentioned range, or the amount of n-octyl acrylate may also be in the above-mentioned range.

When the (meth)acrylic acid alkyl ester contains a _C6-9 (meth)acrylic acid alkyl ester and a _C10-20 (meth)acrylic acid alkyl ester, the amount of the _C6-9 (meth)acrylic acid alkyl ester is preferably 15 to 84 parts by weight, and the amount of the _C10-20 (meth)acrylic acid alkyl ester is preferably 10 to 79 parts by weight, relative to 100 parts by weight of the total amount of the constituent monomer components of the acrylic polymer. The amount of the _C6-9 (meth)acrylic acid alkyl ester may be 20 to 70 parts by weight, 30 to 60 parts by weight, 35 to 55 parts by weight, or 40 to 50 parts by weight, and the amount of the _C10-20 (meth)acrylic acid alkyl ester may be 15 to 60 parts by weight, 20 to 50 parts by weight, 25 to 45 parts by weight, or 30 to 40 parts by weight.

The amount of monomers having crosslinkable functional groups (total amount of hydroxy group-containing monomers and carboxy group-containing monomers) per 100 parts by weight of the total of the constituent monomer components of the acrylic base polymer is preferably 0.5 to 15 parts by weight, more preferably 1 to 10 parts by weight, and even more preferably 1.5 to 7 parts by weight. When the acrylic base polymer contains a hydroxy group, the pressure-sensitive adhesive layer tends to have a high strain recovery rate and high shape restorability. Therefore, it is preferable that the acrylic base polymer contain a hydroxy group-containing monomer as a constituent monomer component, and the content of the hydroxy group-containing monomer is preferably within the above range.

The total amount of (meth)acrylic acid alkyl ester and (meth)acrylic acid ester having an alkylene oxide chain relative to a total of 100 parts by weight of the constituent monomer components of the acrylic base polymer is preferably 60 parts by weight or more, more preferably 70 parts by weight or more, even more preferably 80 parts by weight or more, and may be 85 parts by weight or more, 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more.

The total amount of the (meth)acrylic acid alkyl ester and the monomer having a functional group crosslinkable with the (meth)acrylic acid ester having an alkylene oxide chain relative to a total of 100 parts by weight of the constituent monomer components of the acrylic base polymer is preferably 65 parts by weight or more, more preferably 75 parts by weight or more, even more preferably 85 parts by weight or more, and may be 90 parts by weight or more, 95 parts by weight or more, 98 parts by weight or more, 99 parts by weight or more, or 100 parts by weight.

(Other Monomers)
The acrylic base polymer may contain other monomers as constituent monomer components. Examples of other monomers include 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. Examples of other monomers 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.

(glass transition temperature)
The glass transition temperature of the acrylic base polymer is preferably −40° C. or lower, more preferably −45° C. or lower, and may be −50° C. or lower or −55° C. or lower. When the glass transition temperature is sufficiently lower than the ambient temperature of the device, the storage modulus of the pressure-sensitive adhesive layer within the ambient temperature range of the device tends to be small, and peeling during repeated bending tends to be suppressed. The lower limit of the glass transition temperature of the acrylic base polymer is not particularly limited, but is generally −80° C. or higher, and may be −75° C. or higher or −70° C. or higher.

The glass transition temperature is the temperature (peak top temperature) at which the loss tangent tanδ in viscoelasticity measurement is maximized. Instead of the glass transition temperature determined by viscoelasticity measurement, a theoretical Tg calculated using Fox's formula may be used. The theoretical Tg is calculated using 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.

(Polymerization of acrylic polymer)
The above monomer components can be polymerized by various known methods such as solution polymerization, emulsion polymerization, and bulk polymerization to obtain an acrylic polymer as a base polymer. Solution polymerization is preferred from the viewpoints 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 the solvent for solution polymerization. The solution concentration is typically about 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 about 50 to 80°C, and the reaction time is typically about 1 to 8 hours.

From the viewpoint of increasing the adhesive strength of the pressure-sensitive adhesive layer after photocuring, the weight-average molecular weight of the acrylic base polymer is preferably 100,000 or more, more preferably 150,000 or more, even more preferably 200,000 or more, and may be 300,000 or more, 400,000 or more, or 500,000 or more. As mentioned above, when the number of repeating alkylene oxide units (m in general formula (1)) of the (meth)acrylic acid ester having an alkylene oxide chain is small, the monomer's action as a chain transfer agent is small, and a high-molecular-weight acrylic polymer can be easily obtained.

From the standpoint of ease of handling of the pressure-sensitive adhesive composition and compatibility with the photocuring agent, the weight-average molecular weight of the acrylic base polymer is preferably 2,000,000 or less, more preferably 1,500,000 or less, even more preferably 1,200,000 or less, and may be 1,000,000 or less or 900,000 or less. As described below, a crosslinked structure is introduced into the acrylic base polymer, but the molecular weight of the acrylic base polymer refers to the molecular weight before the crosslinked structure is introduced.

<Crosslinking agent>
From the viewpoints of imparting an appropriate cohesive strength to the pressure-sensitive adhesive, thereby enabling the pressure-sensitive adhesive layer to exhibit adhesive strength, and ensuring the peelability of the pressure-sensitive adhesive layer from the adherend before photocuring, it is preferable that a crosslinked structure be introduced into the base polymer. For example, the crosslinked structure is introduced by adding a crosslinking agent to a solution obtained after polymerization of an acrylic base polymer, and then heating the solution as necessary.

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 groups and carboxyl groups introduced into the acrylic base polymer to form a crosslinked structure. Isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred because they are highly reactive with the hydroxyl and carboxyl groups of the acrylic base polymer and facilitate the introduction of crosslinked structures. When the acrylic base polymer has hydroxyl groups as crosslinkable functional groups, isocyanate-based crosslinking agents are preferred. When the acrylic base polymer has carboxyl groups as crosslinkable functional groups, epoxy-based crosslinking agents are preferred.

As an isocyanate-based crosslinking agent, a polyisocyanate having two or more isocyanate groups per molecule is used. The isocyanate-based crosslinking agent may also have three or more isocyanate groups per molecule. Examples of isocyanate-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-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate; trimethylolpropane/trilene diisocyanate; and Examples of the isocyanate adduct include a diisocyanate trimer adduct (e.g., "Takenate D101E" manufactured by Mitsui Chemicals), a trimethylolpropane/hexamethylene diisocyanate trimer adduct (e.g., "Coronate HL" manufactured by Tosoh), a trimethylolpropane adduct of xylylene diisocyanate (e.g., "Takenate D110N" manufactured by Mitsui Chemicals), and an isocyanurate of hexamethylene diisocyanate (e.g., "Coronate HX" manufactured by Tosoh).

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 acrylic base polymer. The amount of crosslinking agent used is approximately 0.01 to 5 parts by weight, preferably 0.03 to 3 parts by weight, more preferably 0.05 to 1 part by weight, and may be 0.08 to 0.8 parts by weight or 0.1 to 0.5 parts by weight, per 100 parts by weight of the acrylic base polymer.

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

<Photocuring agent>
The pressure-sensitive adhesive composition constituting the pressure-sensitive adhesive layer 2 contains, in addition to a base polymer, a compound having two or more photopolymerizable functional groups in one molecule as a photocuring agent. Pressure-sensitive adhesive compositions containing a photocuring agent have photocurability, and when photocuring is performed after lamination with an adherend, the adhesive strength with the adherend is improved.

The photopolymerizable functional group is preferably one that is polymerizable by a photoradical reaction, and the photocuring agent is preferably a compound with two or more ethylenically unsaturated bonds in one molecule. Multifunctional (meth)acrylates are preferred because of their high compatibility with acrylic-based polymers.

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.

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.

In polyfunctional (meth)acrylates containing alkylene oxide chains, such as polyalkylene glycol di(meth)acrylates and alkylene oxide-modified polyfunctional (meth)acrylates, the alkylene oxide is preferably (poly)ethylene oxide or (poly)propylene oxide, with (poly)ethylene oxide being particularly preferred. The chain length (number of repeating units of alkylene oxide) n of the alkylene oxide is approximately 1 to 15. When multiple alkylene oxide chains are contained in one molecule, an average chain length n of 1 to 15 is preferred. The (average) chain length n of the alkylene oxide chain may be 12 or less, 10 or less, 8 or less, 6 or less, 5 or less, 4 or less, or 3 or less. Compatibility with acrylic base polymers can be adjusted within an appropriate range by adjusting the type and chain length of the alkylene oxide.

From the viewpoint of compatibility with the acrylic base polymer, the molecular weight of the polyfunctional (meth)acrylate used as the photocuring agent is preferably 1,500 or less, more preferably 1,000 or less, and may be 800 or less, 500 or less, or 400 or less. From the viewpoint of achieving both compatibility with the base polymer and improved adhesive strength after photocuring, the functional group equivalent weight (g/eq) of the polyfunctional (meth)acrylate is preferably 500 or less, more preferably 400 or less, and may be 300 or less, 250 or less, 200 or less, 180 or less, or 160 or less. On the other hand, if the functional group equivalent weight of the polyfunctional (meth)acrylate is excessively small, the crosslinking density of the pressure-sensitive adhesive layer after photocuring increases, resulting in an increased storage modulus and reduced adhesiveness at low temperatures. Therefore, the functional group equivalent weight of the photocuring agent is preferably 80 or more, more preferably 100 or more, and may be 120 or more or 130 or more.

As mentioned above, polyfunctional (meth)acrylates have excellent compatibility with acrylic-based polymers. However, polyfunctional (meth)acrylates with alkylene oxide chains have higher polarity than polyfunctional (meth)acrylates without alkylene oxide chains, and therefore have lower compatibility with acrylic polymers, particularly with acrylic polymers with a low amount of polar functional groups.

(Meth)acrylic acid alkyl esters with an alkyl group having 6 or more carbon atoms have a relatively small volume ratio of alkylene oxide chains and are therefore less polar than (meth)acrylic acid alkyl esters with a smaller alkyl group carbon number (for example, butyl acrylate, with an alkyl group having 4 carbon atoms).

Acrylic polymers with a high proportion of (meth)acrylic acid alkyl esters, where the alkyl group has six or more carbon atoms, have low polarity and therefore low compatibility with polyfunctional (meth)acrylates with alkylene oxide chains. Therefore, when an acrylic polymer with a high proportion of (meth)acrylic acid alkyl esters, where the alkyl group has six or more carbon atoms, is blended with a polyfunctional (meth)acrylate with an alkylene oxide chain as a light curing agent, problems such as bleeding of the light curing agent may occur.

As mentioned above, the acrylic base polymer contains, as a monomer component, a (meth)acrylic acid ester having an alkylene oxide chain in addition to a (meth)acrylic acid alkyl ester with an alkyl group having six or more carbon atoms, which increases polarity and makes the adhesive compatible with multifunctional (meth)acrylates having alkylene oxide chains. Furthermore, the presence of alkylene oxide chains in both the acrylic base polymer and the photocuring agent tends to reduce the resistance of the adhesive.

By adjusting the ratio of (meth)acrylic acid esters with alkylene oxide chains in the constituent monomer components of the acrylic base polymer, it is possible to adjust compatibility with light-curing agents with alkylene oxide chains. As mentioned above, if the polymer does not contain (meth)acrylic acid esters with alkylene oxide chains, or if the ratio of (meth)acrylic acid esters with alkylene oxide chains is low, compatibility with the light-curing agent will be low, which can cause the light-curing agent to bleed out.

On the other hand, if the acrylic base polymer has a polarity that makes it incompatible with the photocuring agent (a (meth)acrylic acid ester having an alkylene oxide chain), the photocuring agent tends to be unevenly distributed on the surface of the adhesive layer (near the adhesive interface with the adherend), and even if the amount of photocuring agent mixed is small, the photocuring agent unevenly distributed at the adhesive interface with the adherend is likely to form an adhesion-inhibiting layer (Weak Boundary Layer; WBL).

When a WBL is formed, the adhesive layer retains its bulk properties, such as its storage modulus, while the liquid properties of the surface (adhesive interface) become stronger, tending to reduce the adhesive strength with the adherend. Therefore, the adhesive layer before photo-curing is easy to peel from the adherend. When an adhesive layer in which a WBL has been formed, with the photo-curing agent unevenly distributed near the adhesive interface with the adherend, is photo-cured, the curing reaction of the photo-curing agent is more likely to proceed near the adhesive interface where the photo-curing agent is present at a higher density, which tends to improve the adhesive strength. Furthermore, when a WBL is formed, the increase in the storage modulus, a bulk property, due to photo-curing tends to be suppressed, making this material highly applicable to foldable devices.

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 upon photocuring, and therefore the higher the storage modulus of the pressure-sensitive adhesive layer after photocuring. To increase the adhesive strength of the pressure-sensitive adhesive after photocuring while suppressing an excessive increase in the storage modulus, the content of the photocuring agent in the pressure-sensitive adhesive composition is preferably 3 to 25 parts by weight, more preferably 5 to 20 parts by weight, or even 8 to 17 parts by weight or 10 to 15 parts by weight, per 100 parts by weight of the acrylic base polymer. As described above, by adjusting the compatibility between the acrylic base polymer and the photocuring agent, if a WBL is formed on the surface (adhesion interface) of the pressure-sensitive adhesive layer, the adhesive strength is likely to increase upon photocuring, even with a small amount of photocuring agent, and high adhesive strength can be achieved.

Two or more types of photocuring agents may be used in combination. For example, two or more types of multifunctional (meth)acrylates having alkylene oxide chains may be used as the photocuring agent, or a multifunctional (meth)acrylate having an alkylene oxide chain and a multifunctional (meth)acrylate without an alkylene oxide chain may be used. Furthermore, multifunctional (meth)acrylates with different numbers of functional groups (the number of (meth)acryloyl groups per molecule) may be used as the photocuring agent. For example, a bifunctional (meth)acrylate and a trifunctional or higher multifunctional (meth)acrylate may be used in combination as the photocuring agent to adjust the adhesive strength and storage modulus of the pressure-sensitive adhesive layer after photocuring.

Polyfunctional (meth)acrylates with alkylene oxide chains and urethane (meth)acrylates may also be used as photocuring agents. Urethane (meth)acrylates are compounds containing one or more urethane bonds and two or more (meth)acryloyl groups per molecule, preferably two or more urethane bonds per molecule. Urethane (meth)acrylates with two or more urethane bonds can be obtained, for example, by reacting a polyisocyanate with a (meth)acrylic compound containing a hydroxy group. The isocyanate groups of the polyisocyanate bond to the hydroxy groups of the (meth)acrylic compound to form a urethane bond. Urethane (meth)acrylates with two or more urethane bonds can also be obtained by reacting a polyisocyanate with a polyol to prepare a prepolymer with isocyanate groups at its terminals, and then bonding a (meth)acrylic compound with a hydroxy group to the isocyanate groups at the terminals of the prepolymer. Alternatively, a urethane (meth)acrylate having two or more urethane bonds can be obtained by reacting a polyisocyanate with a (meth)acrylic compound having a hydroxy group, and then reacting the reaction product with a polyol.

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

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

Urethane (meth)acrylates obtained by reacting diisocyanates with (meth)acrylic compounds having one hydroxy group and two or more (meth)acryloyl groups per molecule have two urethane bonds and four or more (meth)acryloyl groups per molecule. The number of (meth)acryloyl groups in the urethane (meth)acrylate may be six or more, eight or more, or 12 or fewer, or 10 or fewer.

The above urethane (meth)acrylates may be commercially available from Kyoeisha Chemical, Shin-Nakamura Chemical, Negami Chemical Industries, Mitsubishi Chemical, Daicel Allnex, Resonac, etc.

By including a urethane (meth)acrylate as the photocuring agent in addition to a polyfunctional (meth)acrylate that does not have a urethane bond, the adhesive strength of the adhesive layer before photocuring may be low and the adhesive strength of the adhesive layer after photocuring may be high. One possible reason for this is that the urethane (meth)acrylate promotes the formation of WBL by polyfunctional (meth)acrylates that do not have a urethane bond, particularly polyfunctional (meth)acrylates that have alkylene oxide chains.

When the photocurable composition constituting the adhesive layer contains a urethane (meth)acrylate as a photocuring agent in addition to a polyfunctional (meth)acrylate that does not have a urethane bond, the content of the urethane (meth)acrylate is preferably 0.01 parts by weight or more, more preferably 0.05 parts by weight or more, and may be 0.08 parts by weight or more or 0.1 parts by weight or more, per 100 parts by weight of the acrylic base polymer.

<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), etc. are used depending on the type of photocuring agent, etc. When a polyfunctional acrylate is used as the photocuring agent, it is preferable to use a photoradical initiator. As the photoradical initiator, a photoradical generator that is cleaved by visible light or ultraviolet light having a wavelength shorter than 450 nm to generate a radical is preferred, and examples of the photoradical initiator include hydroxyketones, benzyl dimethyl ketals, aminoketones, acylphosphine oxides, benzophenones, trichloromethyl group-containing triazine derivatives, etc. The photoradical generator may be used alone or in combination of two or more.

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

<Antistatic Agent>
The PSA composition contains an antistatic agent. When the PSA contains an antistatic agent, the PSA layer has a low resistance, reducing static electricity in the PSA layer and also providing the effect of suppressing static electricity on the adherend.

Examples of antistatic agents include ionic compounds containing organic cations, alkali metal salts, ion-conductive polymers, ion-conductive fillers, and conductive polymers. Among these, ionic compounds containing organic cations and alkali metal salts are preferred due to their excellent compatibility with acrylic-based polymers.

Ionic compounds containing organic cations may be ionic liquids, which are liquid at room temperature, or ionic solids, which are solid at room temperature. Ionic compounds containing organic cations are preferably composed of an organic anion or a fluoroinorganic anion and an onium cation. The organic anion may be a fluoroorganic anion containing a fluorine atom, or an organic anion not containing a fluorine atom.

Examples of onium cations include nitrogen-containing onium cations, sulfur-containing onium cations (e.g., trialkylsulfonium cations), and phosphorus-containing onium cations (e.g., tetraalkylphosphonium cations). Of these, nitrogen-containing onium cations are preferred.

Examples of nitrogen-containing onium cations include pyridinium cations, pyrrolidinium cations, piperidinium cations, cations having a pyrroline skeleton, cations having a pyrrole skeleton, imidazolium cations, tetrahydropyrimidinium cations, dihydropyrimidinium cations, pyrazolium cations, pyrazolinium cations, and tetraalkylammonium cations.

The fluoroorganic anions constituting ionic compounds containing organic cations may be completely fluorinated (perfluorinated) or partially fluorinated. Examples of fluoroorganic anions include perfluoroalkylsulfonates, bis(fluorosulfonyl)imides, and bis(perfluoroalkanesulfonyl)imides. More specific examples include trifluoromethanesulfonate, pentafluoroethanesulfonate, heptafluoropropanesulfonate, nonafluorobutanesulfonate, bis(fluorosulfonyl)imides, and bis(trifluoromethanesulfonyl)imides. Examples of organic anions that do not contain fluorine atoms include sulfonate anions such as p-toluenesulfonate, borate anions, and dicyanamide anions. Examples of fluoroinorganic anions include hexafluorophosphate and tetrafluoroboric acid.

The alkali metal salt preferably comprises the above organic anion or fluoroinorganic anion and an alkali metal cation, which is Li ⁺ , Na ⁺ or K ⁺ , with Li ⁺ being preferred.

The content of the antistatic agent in the pressure-sensitive adhesive composition is approximately 0.01 to 3 parts by weight, preferably 0.03 to 2 parts by weight, more preferably 0.05 to 1 part by weight, and even more preferably 0.1 to 0.7 parts by weight, per 100 parts by weight of the acrylic base polymer. The content of the antistatic agent in the pressure-sensitive adhesive layer 2 is approximately 0.01 to 2% by weight, preferably 0.03 to 1% by weight, more preferably 0.05 to 0.7% by weight, and even more preferably 0.1 to 0.5% by weight, and may be 0.15 to 0.4% by weight. If the amount of antistatic agent is too small, the resistance of the pressure-sensitive adhesive may not be sufficiently reduced. If the amount of antistatic agent is too large, bleed-out of the antistatic agent may cause contamination or corrosion of the adherend, or a decrease in adhesive strength.

As described above, the acrylic base polymer of the adhesive composition contains a (meth)acrylic acid ester having an alkylene oxide chain as a monomer component, which reduces the resistance of the adhesive, making it possible to reduce the resistance of the adhesive even with a small amount of antistatic agent added. This makes it possible to provide an adhesive layer with low surface resistance and excellent antistatic properties, while suppressing problems such as antistatic agent bleed-out.

<Other ingredients>
As described above, the photocurable pressure-sensitive adhesive composition constituting the pressure-sensitive adhesive layer 2 contains an acrylic-based polymer, a photocuring agent, a photopolymerization initiator, and an antistatic agent. The pressure-sensitive adhesive composition may contain components other than these.

For example, the pressure-sensitive adhesive composition may contain an oligomer with a lower molecular weight than the base polymer. For example, the pressure-sensitive adhesive composition may contain, in addition to the acrylic base polymer, an acrylic oligomer with a weight-average molecular weight of approximately 1,000 to 30,000. The acrylic oligomer contains a (meth)acrylic acid alkyl ester as the 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 acrylic oligomer may contain crosslinkable functional groups, similar to the acrylic base polymer.

In addition to the above components, the adhesive composition may contain additives such as silane coupling agents, tackifiers, plasticizers, softeners, anti-degradants, fillers, colorants, UV absorbers, antioxidants, surfactants, etc., to the extent that the properties of the present invention are not impaired.

Additives may be used to lower the resistance of the adhesive or to adjust the compatibility between the acrylic base polymer and the photocuring agent. For example, adding polyether polyols such as polyethylene glycol and polypropylene glycol tends to lower the resistance of the adhesive. However, low-molecular-weight additives such as polyols do not contribute to the photocuring of the adhesive and remain as liquid components even after the adhesive is photocured. Therefore, if the amount of low-molecular-weight additive is large, the gel fraction of the adhesive after photocuring tends to be small, the adhesive layer tends to be prone to plastic deformation, and the strain recovery rate tends to be low. Therefore, the amount of additive is preferably 20 parts by weight or less, more preferably 10 parts by weight or less, even more preferably 5 parts by weight or less, and may be 3 parts by weight or less, 2 parts by weight or less, or 1 part by weight or less per 100 parts by weight of the acrylic base polymer.

The total amount of the acrylic base polymer, crosslinker, and photocuring agent relative to the total solids content of the adhesive composition is preferably 80% by weight or more, more preferably 90% by weight or more, even more preferably 93% by weight or more, and may be 95% by weight or more, 97% by weight or more, or 98% by weight or more.

[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 acrylic-based polymer increases the gel fraction and 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 substrate when the reinforcing film is peeled from the substrate 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 of the adhesive after photocuring increases, resulting in poor stress-strain relaxation, which may cause the adhesive to peel off at bent portions when a flexible device is used. Therefore, the gel fraction of the adhesive layer 2 before photocuring is preferably 80% or less, and may be 75% or less, 70% or less, or 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. Therefore, the pressure-sensitive adhesive layer 2 contains an acrylic-based polymer with a crosslinked structure introduced, a photocuring agent, a photopolymerization initiator, and an antistatic agent. When forming the pressure-sensitive adhesive layer 2 on the film substrate 1, it is preferable to apply a release liner 5 to the pressure-sensitive adhesive layer 2 for purposes such as protecting the pressure-sensitive adhesive layer 2. Crosslinking may also be carried out after the release liner 5 is applied to the pressure-sensitive adhesive layer 2.

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

Preferably, the release liner 5 is a plastic film such as polyethylene, polypropylene, polyethylene terephthalate, or polyester film. The thickness of the release liner is typically 3 to 200 μm, preferably approximately 10 to 100 μm. The surface of the release liner 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 release liner 5 with a release treatment, when the release liner 5 is peeled from the film substrate 1, peeling occurs at the interface between the adhesive layer 2 and the release liner 5, maintaining the adhesive layer 2 adhered to the film substrate 1. The release liner 5 may be antistatically treated on either or both of the release-treated and untreated surfaces. Treating the release liner 5 with an antistatic treatment reduces static buildup when the release liner is peeled from the adhesive layer.

[Physical properties of adhesive layer]
The thickness of the pressure-sensitive adhesive layer 2 is, for example, about 1 to 300 μm. The thicker the pressure-sensitive adhesive layer 2, the more likely it is to have improved adhesion to an adherend. On the other hand, if the pressure-sensitive adhesive layer 2 is excessively thick, the fluidity before photocuring may be high, making handling difficult. Therefore, the thickness of the pressure-sensitive 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 viewpoint of achieving a thinner layer, the thickness of the pressure-sensitive 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.

The surface resistance of the pressure-sensitive adhesive layer 2 is preferably 5.0×10 ¹¹ Ω or less, more preferably 1.0×10 ¹¹ Ω or less, and may be 7.0×10 ¹⁰ Ω or less, 5.0×10 ¹⁰ Ω or less, or 3.0×10 ¹⁰ Ω or less. As described above, the pressure-sensitive adhesive composition contains an antistatic agent, and the acrylic base polymer further contains a (meth)acrylic acid ester having an alkylene oxide chain as a monomer component, thereby making it possible to reduce the resistance of the pressure-sensitive adhesive layer 2.

The low resistance of the adhesive layer 2 can suppress electrical damage to the adherend due to static electricity, etc., when the reinforcing film is peeled off from the adherend. It is preferable that the surface resistance of the adhesive layer 2 remains within the above range even after photo-curing. Generally, the surface resistance of the adhesive layer changes very little before and after photo-curing. When the adhesive layer after photo-curing has the above surface resistance, static electricity on the adherend to which the adhesive layer 2 is attached is removed via the adhesive layer 2, thereby suppressing charging of the adherend. This can suppress problems caused by static electricity, such as electrostatic breakdown, in devices to which the reinforcing film is attached.

The shear 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. The shear storage modulus of the pressure-sensitive adhesive layer (hereinafter simply referred to as "storage modulus") is determined by reading the value at a predetermined temperature when measured at a temperature rise rate of 5°C/min in the range of -50 to 150°C at a frequency of 1 Hz in accordance with the method described in JIS K7244-1 "Plastics - Test methods for dynamic mechanical properties."

From the viewpoints of improving the processability of the pressure-sensitive adhesive layer, suppressing the pressure-sensitive adhesive from spilling out from the edge face of the reinforcing film, and preventing contamination of the adherend due to adhesive residue when the reinforcing film is peeled from the adherend, the storage modulus of the pressure-sensitive adhesive layer 2 at 25°C before photocuring is preferably 7.0 x ¹⁰³ Pa or more, more preferably 9.0 x ¹⁰³ Pa or more, and may be 1.0 x ¹⁰⁴ Pa or more or 1.5 x ¹⁰⁴ 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 ¹⁰⁴ Pa or less, more preferably 5.0 x ¹⁰⁴ Pa or less, and may be 4.0 x ¹⁰⁴ Pa or less or 3.0 x ¹⁰⁴ 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 1.5×10 ⁴ to 1.0×10 ⁵ Pa, and may be 2.0×10 ⁴ to 7.0×10 ⁴ Pa or 3.0×10 ⁴ to 7.0×10 ⁴ Pa. When the storage modulus of the pressure-sensitive adhesive layer at low temperature before photocuring is within the above range, the storage modulus of the pressure-sensitive adhesive layer tends to be maintained low even after photocuring.

When the adhesive layer 2 is photocured, the photocuring agent undergoes a curing reaction, increasing the storage modulus and the adhesive strength with the adherend. It is preferable that the adhesive layer 2 maintain a small storage modulus at low temperatures even after photocuring.

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 3.0×10 ⁵ Pa, even more preferably not more than 2.0×10 ⁵ Pa, and may be not more than 1.5×10 ⁵ Pa or not more than 1.0×10 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 bending points can be suppressed even when a device to which a reinforcing film is bonded is repeatedly bent or when the bent state is maintained for a long period of time.

As mentioned above, the lower the glass transition temperature of the acrylic base polymer that makes up the adhesive layer and the smaller the amount of photocuring agent used, the smaller the storage modulus of the adhesive layer at -20°C after photocuring tends to be. In particular, when the acrylic base polymer contains n-octyl acrylate as a monomer component, a plateau region of storage modulus (a region where the temperature dependence of the storage modulus is small) exists in the low temperature range around -20°C, making it easier to obtain an adhesive with a low storage modulus at -20°C.

If the storage modulus of the pressure-sensitive adhesive layer after photocuring is excessively small, 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 1.0×10 ⁴ Pa or more, more preferably 3.0×10 ⁴ Pa or more, and may be 5.0×10 ⁴ Pa or more or 7.0×10 ⁴ Pa or more.

The strain recovery rate of the pressure-sensitive adhesive layer at a temperature of 60° C. is preferably 85% or more, more preferably 90% or more, even more preferably 91% or more, and may be 92% or more. The strain recovery rate is calculated by the following formula from the maximum strain _S1 when a shear stress of 2 kPa is applied to the pressure-sensitive adhesive layer using a rotational rheometer for 10 minutes, and then the stress is released and left to stand for 10 minutes to allow the strain to recover, and the minimum strain _S2 when the stress is released.
Strain recovery rate (%) = 100 × (S ₁ - S ₂ ) / S ₁

The strain recovery rate is an index that indicates the extent to which the adhesive layer returns to its original shape when stress is applied to it, causing it to deform (strained state), and then the stress is released; the closer it is to 100%, the higher the shape recovery rate. If the strain recovery rate is within the above range, when a foldable device is held in a folded state (a state in which the adhesive layer has been subjected to elongation strain), and then the device is returned to an extended state and the stress on the adhesive layer is released, the adhesive layer easily restores its shape, making it less likely to develop wrinkles or other deformations.

As mentioned above, by introducing an alkylene oxide structure into an acrylic-based polymer to reduce resistance without using additives such as polyols (or using only small amounts), the amount of low-molecular-weight liquid components contained in the adhesive after photocuring is small and the gel fraction of the adhesive is large, making it possible to increase the strain recovery rate.

From the viewpoint of facilitating peeling from the adherend and preventing adhesive residue on the adherend after peeling of the reinforcing film, the adhesive strength between the pressure-sensitive adhesive layer 2 and the adherend before photocuring is preferably 1 N/25 mm or less, more preferably 0.7 N/25 mm or less, even more preferably 0.5 N/25 mm or less, and may be 0.4 N/25 mm or less, or 0.3 N/25 mm or less. From the viewpoint of preventing peeling of the reinforcing film during storage or handling, the adhesive strength between the pressure-sensitive adhesive layer 2 and the adherend before photocuring is preferably 0.01 N/25 mm or more, more preferably 0.02 N/25 mm or more, and may be 0.03 N/25 mm or more, 0.04 N/25 mm or more, or 0.05 N/25 mm or more.

Adhesion 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, adhesion strength is measured at 25°C.

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

The adhesive strength between the adhesive layer and the adherend after photocuring is preferably at least 5 times, more preferably at least 10 times, even more preferably at least 15 times, and may be at least 20 times, at least 30 times, or at least 50 times the adhesive strength between the adhesive layer and the adherend before photocuring. As mentioned above, by adjusting the compatibility between the two based on the composition of the acrylic base polymer and the type of photocuring agent, it is possible to keep the adhesive strength before photocuring (initial adhesive strength) low and increase the adhesive strength of the adhesive after photocuring.

[Use of reinforcing film]
The reinforced film of the present invention is used by being attached to a device or a device component. The reinforced film 10 has 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 adherend to which the reinforcing film is attached is not particularly limited, and examples include various electronic devices, optical devices, and their component parts. In one embodiment, the reinforcing film is attached to the surface of a foldable flexible device. The foldable device has a hinge portion and can be folded around this hinge portion. The folding angle can be set as desired, and the device may be bent (folded) 180 degrees. When the device is a display device, the reinforcing film may be attached to the surface on the screen side, or to the back side (housing). Flexible devices that are configured to be bendable at a predetermined location, such as a hinge portion, repeatedly bend and extend at the same location when in use.

The reinforcing film may be adhered to the entire surface of the adherend, or may be selectively adhered only to areas requiring reinforcement (reinforcement target areas). Alternatively, the reinforcing film may be adhered to the entire areas requiring reinforcement (reinforcement target areas) and areas not requiring reinforcement (non-reinforcement target areas), and then the reinforcing film adhered 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 adhered 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.

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 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. 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. Furthermore, processing such as selectively removing the reinforcing film from areas other than those to be reinforced is also easy.

When the completed device is in use, the reinforcing film prevents damage to the device even if it is suddenly subjected to external force, such as if it is dropped, a heavy object is placed on the device, or it is hit by a flying object. Furthermore, because the reinforcing film firmly adheres to the device after the adhesive is photo-cured, the reinforcing film is less likely to peel off even during long-term use, providing excellent reliability.

In a device with a reinforced film in which the reinforced film of the present invention is bonded to a flexible device using a resin substrate, the reinforced film is less likely to peel off at the bent point, even when the device is repeatedly bent or maintained in a bent state for a long period of time. Furthermore, even when the device is extended after being maintained in a bent state for a long period of time, the adhesive layer has a high strain recovery rate and high shape recovery, making it less likely to develop wrinkles or other deformations. Furthermore, the adhesive layer has low resistance, making it possible to remove static electricity from the device's resin substrate and other components via the adhesive layer, thereby suppressing problems such as electrostatic breakdown of the device caused by charging (static electricity).

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 76 parts by weight of 2-ethylhexyl acrylate (2EHA), 4 parts by weight of 4-hydroxybutyl acrylate (4HBA), and 20 parts by weight of methoxyethyl acrylate (MEA) as monomers, 0.2 parts by weight of azobisisobutyronitrile (AIBN) as a polymerization initiator, and 233 parts by weight of ethyl acetate as a solvent, 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 700,000.

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

[Preparation of reinforcing film]
Example 1
(Preparation of Pressure-Sensitive Adhesive Composition)
To a solution of acrylic polymer A (100 weight percent of the polymer solids), 0.1 parts by weight of a trifunctional isocyanate crosslinking agent ("Coronate HX" manufactured by Tosoh Corporation) as a crosslinking agent, 0.02 parts by weight of iron acetylacetonate ("Nacem Ferric" manufactured by Nippon Chemical Industry Co., Ltd.) as a crosslinking catalyst, 0.3 parts by weight of "Omnirad 651" manufactured by IGM Resins as a photopolymerization initiator, and 0.2 parts by weight of 1-butyl-3-methylpyridinium bistrifluoromethanesulfonylimide ("CIL-312" manufactured by Nippon Carlit Co., Ltd.) as an antistatic agent A were added, and the mixture was mixed uniformly to prepare a pressure-sensitive adhesive composition shown in Table 1.

(Application of adhesive solution and crosslinking)
The above-mentioned adhesive composition was applied to a 50 μm-thick polyethylene terephthalate film using a fountain roll so that the dried thickness was 25 μm. After drying at 130 ° C for 1 minute to remove the solvent, the release-treated surface of a release liner (a 25 μm-thick polyethylene terephthalate film with antistatic treatment on both sides and silicone release treatment on one side) was attached to the adhesive-coated surface. After that, an aging treatment was performed for 4 days in an atmosphere of 25 ° C to promote crosslinking, and a reinforced film was obtained in which a photocurable adhesive sheet was fixedly laminated on a polyethylene terephthalate film substrate and a release liner was temporarily attached thereon.

<Examples 2, 4 to 11, and Comparative Examples 1 to 5>
Pressure-sensitive adhesive compositions were prepared and reinforcing films were produced in the same manner as in Example 1, except that the type of acrylic polymer and the type and amount of photocuring agent added were changed as shown in Table 1.

Example 3
A pressure-sensitive adhesive composition was prepared in the same manner as in Example 2, except that 0.2 parts by weight of antistatic agent B was used instead of antistatic agent A, and a reinforcing film was produced.

Example 12
(Preparation of Pressure-Sensitive Adhesive Composition)
To a solution of acrylic polymer H (100 weight parts as polymer solids), 0.5 parts by weight of a tetrafunctional epoxy crosslinking agent ("Tetrad C" manufactured by Mitsubishi Gas Chemical Company, Ltd.) as a crosslinking agent, 0.2 parts by weight of zirconium tetraacetylacetonate ("ZC-150" manufactured by Matsumoto Fine Chemical Co., Ltd.) as a crosslinking catalyst, a polyfunctional acrylate and a urethane acrylate shown in Table 1 as photocuring agents, 0.3 parts by weight of "Omnirad 651" manufactured by IGM Resins as a photopolymerization initiator, and 0.2 parts by weight of "CIL-312" manufactured by Nippon Carlit Co., Ltd. as an antistatic agent were added, and the mixture was mixed uniformly to prepare a pressure-sensitive adhesive composition.

(Application of adhesive solution and crosslinking)
Using the above pressure-sensitive adhesive composition, coating and crosslinking were carried out in the same manner as in Example 1 to prepare a reinforcing film.

<Examples 13 to 16>
A pressure-sensitive adhesive composition was prepared and a reinforcing film was produced in the same manner as in Example 12, except that the type of acrylic polymer, the amount of crosslinking agent added, the type and amount of photocuring agent added, and the type of antistatic agent were changed as shown in Table 1.

<Comparative Example 6>
A pressure-sensitive adhesive composition was prepared and a reinforcing film was produced in the same manner as in Example 12, except that the type of acrylic polymer, the type and amount of photocuring agent, and the amount of crosslinking agent added were changed as shown in Table 1.

Comparative Example 7
Pressure-sensitive adhesive compositions were prepared and reinforcing films were produced in the same manner as in Example 12, except that the type of acrylic polymer and the type and amount of photocuring agent were changed as shown in Table 1 and 30 parts by weight of triol-type polypropylene glycol ("Preminol S-3011" manufactured by AGC, Mn = 10,000, hereinafter referred to as "PPG") was added as an additive.

<Comparative Example 8>
A pressure-sensitive adhesive composition was prepared in the same manner as in Comparative Example 1, except that 10 parts by weight of PPG was added as an additive, and a reinforcing film was produced from the prepared composition.

[evaluation]
The pressure-sensitive adhesive layers of the reinforcing films obtained in the above Examples and Comparative Examples were evaluated as follows: In Comparative Examples 1 and 2, the liquid photocuring agent oozed onto the surface of the pressure-sensitive adhesive layer, and the layer did not function as a pressure-sensitive adhesive, so no evaluation was performed.

<Weight average molecular weight>
The weight average molecular weight (in terms of polystyrene) of the polymer was measured using GPC ("HLC-8220GPC" manufactured by Tosoh Corporation) under the following conditions.
Sample concentration: 0.2 wt% (tetrahydrofuran solution)
Sample injection volume: 10 μL
Eluent: THF
Flow rate: 0.6mL/min
Measurement temperature: 40℃
Sample column: TSKguardcolumn SuperHZ-H (1 column) + TSKgel SuperHZM-H (2 columns)
Reference column: TSKgel SuperH-RC (1 column)
Detector: RI

<Surface resistance>
The release liner was peeled off from the reinforcing film to expose the pressure-sensitive adhesive layer (before photocuring). A probe (Model 152P-2P manufactured by TREK) was brought into contact with the surface of the pressure-sensitive adhesive layer in an environment of a temperature of 25°C and a relative humidity of 50%, and the surface resistance was measured using a resistivity meter (Model 152-1 manufactured by TREK) under conditions of an applied voltage of 10 V and a voltage application time of 10 seconds.

<Storage modulus>
A pressure-sensitive adhesive composition was applied to a release liner and crosslinked in the same manner as in each of the above Examples and Comparative Examples to prepare a pressure-sensitive adhesive sheet (before photocuring). A release liner was attached to the surface of the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet before photocuring to shield it from oxygen, and the sheet was photocured by irradiating it with 1000 mJ/ ^cm2 of ultraviolet light from a 365 nm LED lamp. The photocured pressure-sensitive adhesive sheets were laminated to prepare a measurement sample with a thickness of approximately 0.8 mm. Dynamic viscoelasticity measurements were performed under the following conditions using a rotational rheometer (TA Instruments, "Discovery-HR2"), and the shear storage modulus G' at -20°C was 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

<Strain recovery rate>
A measurement sample was prepared in the same manner as above. Using a rotational rheometer, a stress of 2 kPa was applied to the measurement sample for 10 minutes, and then the stress was released (stress: 0 kPa) and the sample was allowed to stand for 10 minutes to recover from the strain. The strain recovery rate (%) = 100 × (S ₁ - S ₂ )/S ₁ was calculated from the maximum strain S ₁ when the stress was applied and the minimum strain S ₂ when the stress was released.
(Measurement conditions)
Deformation mode: Torsion Measurement temperature: 60°C
Shape: Parallel plate 8.0 mm diameter

<Adhesive force>
A 25 μm-thick polyimide film (UBE's "Upilex 25S") was attached to a glass plate via double-sided adhesive tape (Nitto Denko's "No. 531") to obtain a polyimide film substrate for measurement. The release liner was peeled off from the surface of a reinforced film cut into a 25 mm wide x 100 mm long piece, and the piece was then attached to the polyimide film substrate for measurement using a hand roller to obtain a pre-photocuring test sample. The pre-photocuring test sample was irradiated with ultraviolet light from the reinforced film side (PET film substrate side) to photocure the adhesive layer, yielding a post-photocuring test sample. 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 to measure peel strength.

<Stainability of adherends>
The release liner was peeled off from the surface of the reinforcing film, and a polyimide film (UBE's "Upilex 25S") was attached to the surface of the pressure-sensitive adhesive layer using a hand roller. After standing at 25°C for 30 minutes, the reinforcing film was peeled off from the polyimide film, and the surface of the polyimide film was visually inspected under a fluorescent lamp to check for the presence or absence of contamination. Those that were found to be contaminated by attached matter were rated x, and those that were not found to be contaminated were rated ◯.

The adhesive compositions (acrylic polymer composition and weight-average molecular weight Mw, amount of photocuring agent, crosslinking agent, antistatic agent, and PPG added) of the reinforcing films of the examples and comparative examples, as well as the evaluation results of adhesive strength, are shown in Tables 1 and 2. In Tables 1 and 2, the amount of each component is the amount added per 100 parts by weight of the acrylic polymer solids, and in all examples, the amount of antistatic agent is 0.2 parts by weight. Details of the constituent monomers of the acrylic polymer, the crosslinking agent, and the photocuring agent are as follows:

(monomer)
2EHA: 2-ethylhexyl acrylate BA: Butyl acrylate NOAA: n-octyl acrylate LA: Lauryl acrylate (dodecyl acrylate)
4HBA: 4-hydroxybutyl acrylate AA: acrylic acid MEA: methoxyethyl acrylate ("Acrix C-1" manufactured by Toagosei)
EEEA: Ethoxyethoxyethyl acrylate ("Viscoat #180" manufactured by Osaka Organic Chemical Industry Co., Ltd.)
AM90G: Methoxypolyethylene glycol #400 acrylate ("NK Ester AM90-G" manufactured by Shin-Nakamura Chemical Co., Ltd.)

(Crosslinking agent)
T-C: 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane (tetrafunctional epoxy compound, "Tetrad C" manufactured by Mitsubishi Gas Chemical Company)
C/HX: Isocyanurate of hexamethylene diisocyanate ("Coronate HX" manufactured by Tosoh)

(Photocuring agent)
A200: Polyethylene glycol #200 (n=4) diacrylate ("NK Ester A-200" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent weight 151 g/eq)
A400: Polyethylene glycol #400 (n=9) diacrylate ("NK Ester A-400" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent weight 263 g/eq)
A600: polyethylene glycol #600 (n=14) diacrylate ("NK Ester A600" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent weight 371 g/eq)
M350: Trimethylolpropane EO-modified (n=1) triacrylate ("Aronix M-350" manufactured by Toagosei Co., Ltd., functional group equivalent weight 143 g/eq)
UA: Polycarbonate skeleton-containing urethane acrylate (Kyoeisha Chemical Company, "UF-X9-83", functional groups 2-3, weight average molecular weight 8400)

(Antistatic Agent)
Antistatic agent A: 1-butyl-3-methylpyridinium bistrifluoromethanesulfonylimide (manufactured by Nippon Carlit Co., Ltd., "CIL-312")
Antistatic agent B: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (abbreviation: EMI-FSI)
Antistatic agent C: 1-ethyl-3-methylimidazolium dicyanamide (abbreviation: EMI-N(CN) ₂ )

In Comparative Example 6, which used an adhesive containing polymer O, whose main monomer component is butyl acrylate (BA), and 30 parts by weight of a photocuring agent (A200), the storage modulus G' of the photocured adhesive layer at -20°C was high and flexibility was poor. In Comparative Example 7, which contained 10 parts by weight of a long-chain photocuring agent (A600) and 30 parts by weight of PPG to reduce resistance, the adhesive layer achieved both a low storage modulus and low resistance, but the strain recovery rate was reduced.

In Comparative Example 1, which used Polymer J, whose main monomer component was 2-ethylhexyl acrylate (2EHA), which has a lower glass transition temperature than butyl acrylate, to achieve a low storage modulus, the compatibility between the polymer and the photocuring agent was low, resulting in the liquid photocuring agent seeping out. The same was true for Comparative Example 2, which used Polymer K, whose main monomer component was n-octyl acrylate (NOAA). Because 2EHA and NOAA have lower polarity than BA, the low compatibility between the acrylic polymer and the photocuring agent, which contains an alkylene oxide chain, is thought to be the cause of the seepage.

In Comparative Example 8, which used an adhesive in which 10 parts by weight of PPG was added to the composition of Comparative Example 1, no seepage of the photocuring agent was observed as in Comparative Example 1, but the surface of the polyimide film was contaminated after the reinforcing film was peeled off. In Comparative Example 8, the PPG acted as a compatibilizer, suppressing the seepage of the photocuring agent, but the photocuring agent and/or PPG were not sufficiently compatible in the composition, and it is believed that the bled-out components were the cause of contamination of the adherend.

In Example 1, where Polymer A containing methoxyethyl acrylate (MEA) as a monomer component was used instead of Polymer J in Comparative Example 1, G' was as small as in Comparative Example 8, and no contamination of the adherend due to seepage or bleed-out of the photocuring agent was observed. The adhesive layer in Example 1 had a high strain recovery rate of 93%, and combined low G', low resistance, and high recovery.

Examples 2, 4 to 10, which used polymers B to G containing methoxyethyl acrylate (MEA) as a monomer component instead of polymer K in Comparative Example 2, also achieved low G', low resistance, and high recovery, just like Example 1. This was also the case for Example 3, which used a different type of antistatic agent. Example 3 had lower surface resistance than Example 2, demonstrating that antistatic agent B (EMI-FSI) exhibited superior antistatic properties.

In Examples 1 to 10, the acrylic polymer contained MEA, an acrylic acid ester containing an alkylene oxide chain, as a constituent monomer. This is thought to have increased the polarity of the polymer, improved the compatibility between the acrylic polymer and the photocuring agent, and suppressed the seepage of the photocuring agent. Furthermore, it is thought that Examples 1 to 10 exhibited a high strain recovery rate because the adhesive layer after photocuring did not contain low-molecular-weight components such as PPG.

Example 11, which used Polymer H containing ethoxyethoxyethyl acrylate (EEEA) as a monomer component, also exhibited excellent properties, similar to Examples 1 to 10. Example 12, which used Polymer I containing acrylic acid as a monomer component, exhibited excellent properties, but the strain recovery rate was lower than that of Examples 1 to 11.

Examples 13 to 16, which used 2-ethylhexyl acrylate (2EHA), a _C8 alkyl ester, and lauryl acrylate (LA), a _C12 alkyl ester, as the (meth)acrylic acid alkyl ester monomer component, exhibited higher adhesive strength after photocuring and lower surface resistance, thus exhibiting superior properties, compared to Example 12. A comparison between Examples 13 and 14, and a comparison between Examples 15 and 16, shows that antistatic agent C (EMI-N(CN) ₂ ), which does not contain fluorine atoms, exhibits antistatic properties equal to or better than antistatic agent B (EMI-FSI).

Comparative Example 3, which used Polymer L with a weight-average molecular weight of 60,000, had low surface resistance and excellent antistatic properties, but photocuring did not sufficiently increase adhesive strength, resulting in insufficient adhesive strength for the adhesive layer after photocuring. Methoxypolyethylene glycol acrylate (AM90G), used as a monomer component of Polymer L, has a long polyethylene glycol chain length, which makes it excellent at reducing resistance. However, it acts as a chain transfer agent during polymer polymerization, which is thought to prevent the molecular weight of the polymer from increasing sufficiently.

Comparative example 4, which used polymer M, in which the proportion of MEA in the constituent monomer components of the acrylic polymer was 4%, had a small G' and a large strain recovery rate, but had high surface resistance and insufficient antistatic properties.

Comparing Examples 2, 5, and 8 with Comparative Example 4, it can be seen that the surface resistance of the adhesive layer tends to decrease as the proportion of MEA in the constituent monomer components of the acrylic polymer increases. These results demonstrate that copolymerizing an acrylic acid ester containing an alkylene oxide chain as a monomer component not only improves the compatibility between the acrylic polymer and the photocuring agent, but also reduces the resistance of the adhesive layer.

A comparison of Examples 5, 6, and 7 reveals that the surface resistance of the adhesive layer tends to decrease as the amount of photocuring agent increases, suggesting that the photocuring agent containing an alkylene oxide chain also contributes to the reduction in resistance. Furthermore, a comparison of Examples 2 and 4 reveals that the resistance of the adhesive layer tends to decrease as the amount of monomer component containing an alkylene oxide chain and photocuring agent containing an alkylene oxide chain increases.

In Comparative Example 5, which used Polymer N, an acrylic polymer with a 60% MEA content in its constituent monomer components, the adhesive layer had high adhesive strength before photocuring and poor peelability. Furthermore, in Comparative Example 5, photocuring did not sufficiently increase adhesive strength, resulting in insufficient adhesive strength in the adhesive layer after photocuring. In Comparative Example 5, the proportion of acrylic acid esters containing alkylene oxide chains in the monomer components making up the acrylic polymer was high, which excessively increased the compatibility between the acrylic polymer and the photocuring agent, making it difficult for the photocuring agent to function as a WBL and resulting in a high initial adhesive strength.

These results demonstrate that photocurable adhesive compositions containing acrylic-based polymers with a specified composition have low resistance and antistatic properties, as well as low initial adhesive strength and excellent adhesive properties after photocuring. Furthermore, they have a low storage modulus in the low temperature range and a high strain recovery rate, making them suitable as reinforcing films for foldable devices.

REFERENCE SIGNS LIST 1 film substrate 2 pressure-sensitive adhesive layer 10 reinforcing film 5 release liner 20 adherend

Claims (14)

A film substrate and a pressure-sensitive adhesive layer fixedly laminated on one main surface of the film substrate,
the pressure-sensitive adhesive layer is made of a photocurable composition including an acrylic-based polymer, a photocuring agent, a photopolymerization initiator, and an antistatic agent;
The acrylic base polymer contains, as monomer units, one or more selected from the group consisting of a (meth)acrylic acid alkyl ester; a (meth)acrylic acid ester represented by the following general formula (1); and a hydroxy group-containing monomer and a carboxy group-containing monomer, wherein the amount of the (meth)acrylic acid alkyl ester is 40 to 94 parts by weight and the amount of the (meth)acrylic acid ester represented by the following general formula (1) is 5 to 55 parts by weight relative to 100 parts by weight of the total of the monomer units,
A crosslinked structure is introduced into the acrylic base polymer.
Reinforcement film:
CH ₂ =CR ¹ -COO-(R ² -O) _m -R ³ (1)
In general formula (1),
^R1 is a hydrogen atom or a methyl group;
^R2 is an optionally branched alkylene group having 2 to 4 carbon atoms,
m is an integer from 1 to 5;
^R3 is a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, or an aryl group having 6 to 20 carbon atoms. The reinforcing film described in claim 1, wherein the alkyl group of the (meth)acrylic acid alkyl ester has 6 or more carbon atoms. A reinforced film as described in claim 1 or 2, wherein the acrylic base polymer has a weight average molecular weight of 100,000 or more before the introduction of a crosslinked structure. A reinforcing film as described in claim 1 or 2, wherein the photocurable composition contains 3 to 25 parts by weight of the photocuring agent per 100 parts by weight of the acrylic base polymer. A reinforcing film as described in claim 1 or 2, wherein the photocuring agent is a multifunctional (meth)acrylate. The reinforced film described in claim 5, wherein the photocuring agent comprises a multifunctional (meth)acrylate containing an alkylene oxide chain. The reinforced film described in claim 6, wherein the light curing agent further contains urethane (meth)acrylate. A reinforcing film as described in claim 1 or 2, wherein the adhesive layer has an adhesive strength to the polyimide film of 1 N/25 mm or less before photocuring. A reinforcing film as described in claim 1 or 2, wherein the adhesive layer has an adhesive strength to a polyimide film of 5 N/25 mm or more after photocuring. 3. The reinforced film according to claim 1, wherein the pressure-sensitive adhesive layer has a shear storage modulus at −20° C. of 1.0×10 ⁴ to 5.0×10 ⁵ Pa after photocuring. The reinforced film according to claim 1 or 2, wherein the pressure-sensitive adhesive layer has a surface resistance of 5×10 ¹¹ Ω or less. 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 claim 1 or 2 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. 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 a photocured product obtained by photocuring a photocurable pressure-sensitive adhesive composition containing an acrylic-based polymer and a photocuring agent, and contains an antistatic agent;
The acrylic base polymer contains, as monomer units, one or more selected from the group consisting of a (meth)acrylic acid alkyl ester; a (meth)acrylic acid ester represented by the following general formula (1); and a hydroxy group-containing monomer and a carboxy group-containing monomer, wherein the amount of the (meth)acrylic acid alkyl ester is 40 to 94 parts by weight and the amount of the (meth)acrylic acid ester represented by the following general formula (1) is 5 to 50 parts by weight relative to 100 parts by weight of the total of the monomer units,
a crosslinked structure is introduced into the acrylic base polymer,
the pressure-sensitive adhesive layer has a shear storage modulus at −20° C. of 1.0×10 ⁴ to 5.0×10 ⁵ Pa;
Device with stiffening film:
CH ₂ =CR ¹ -COO-(R ² -O) _m -R ³ (1)
In general formula (1),
^R1 is a hydrogen atom or a methyl group;
^R2 is an optionally branched alkylene group having 2 to 4 carbon atoms,
m is an integer from 1 to 5;
^R3 is a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, or an aryl group having 6 to 20 carbon atoms. The device with a reinforced film according to claim 13, wherein the pressure-sensitive adhesive layer has a surface resistance of 5×10 ¹¹ Ω or less.