JP2024114028A - Reinforcement film and method for manufacturing device with reinforcement film - Google Patents

Abstract

To provide a reinforcing film having an adhesive layer which hardly peels off from an adherend even when applicable to a foldable device and has excellent cut processability before photocuring.SOLUTION: A reinforcing film (10) has an adhesive layer (2) fixed and laminated on one main surface of a film substrate (1). The adhesive layer comprises a photocurable composition including an acrylic base polymer in which a crosslinked structure by a crosslinking agent is introduced, a photocuring agent having two or more photopolymerizable functional groups and a photopolymerization initiator. The adhesive layer has a shear storage modulus of 15 to 60 kPa at 25°C before photocuring. When the adhesive layer of the reinforcing film is laminated to a polyimide film, laser beam cutting is carried out and a cut part is peeled off from the polyimide film, the length ratio of a part in which an adhesive remains on the polyimide film is 80% or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a reinforced film in which a film substrate and a photocurable adhesive layer are bonded together. Furthermore, the present invention relates to a method for manufacturing a device having a surface to which the reinforced film is bonded.

Adhesive films are sometimes applied to the surfaces of optical devices such as displays and electronic devices for the purposes of surface protection, impact resistance, etc. Such adhesive films usually have an adhesive layer fixedly 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 a device component before use, such as during device assembly, processing, or transportation, it is possible to prevent the adherend from being scratched or damaged. Patent Document 1 discloses a reinforcing film that has an adhesive layer made of a photocurable adhesive composition on a film substrate.

The adhesive of this reinforcing film has low adhesion immediately after application to the adherend, making it easy to peel off from the adherend. This allows for rework from the adherend, and also makes it possible to cut the reinforcing film that has been applied to a location on the adherend that does not require reinforcement, and selectively peel and remove the reinforcing film from the surface of the adherend. The adhesive of the reinforcing film adheres firmly to the adherend when photocured, so the film base material is 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 foldable substrates (flexible substrates) such as resin films have been put to practical use, and foldable flexible displays have been proposed. In foldable devices, the same location is repeatedly bent. At the bent location, compressive stress is applied on the inside and tensile stress is applied on the outside, causing distortion at the bent location and its surroundings, which may cause the adhesive to peel off from the adherend.

Patent Document 2 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 the adhesive layer from peeling off from the adherend when the foldable device is repeatedly bent.

JP 2020-41113 A International Publication No. 2022/050009

By using a polymer with a low glass transition temperature as the base polymer for the adhesive, the storage modulus of the adhesive layer after photocuring is small, which makes it possible to suppress peeling of the reinforcing film at bent portions. When a base polymer with a low glass transition temperature is used, the adhesive layer before photocuring also has a small storage modulus, which tends to reduce the cuttability of the adhesive layer. If the cuttability of the adhesive layer is low, when the reinforcing film is peeled off from the adherend, the adhesive is likely to remain on the surface of the adherend in areas where the adhesive layer has not been properly cut. The adhesive remaining on the surface of the adherend will adhere to manufacturing equipment and other components during the device manufacturing process, causing contamination by the adhesive and reduced yields due to components sticking to each other.

In view of the above, the present invention aims to provide a reinforcing film that is unlikely to peel off from an adherend even when applied to a foldable device, and has an adhesive layer that has excellent cuttability before photocuring.

The reinforcing film of the present invention comprises an adhesive layer fixedly laminated on one main surface of a film substrate. The adhesive layer is made of a photocurable composition containing an acrylic base polymer, a photocuring agent, and a photopolymerization initiator. The acrylic base polymer contains, as a monomer unit, one or more monomers selected from the group consisting of hydroxyl group-containing monomers and carboxyl group-containing monomers, and a crosslinked structure is introduced by bonding the hydroxyl group and/or carboxyl group of the base polymer with the crosslinking agent.

The adhesive layer has a shear storage modulus of 15 to 60 kPa at 25°C before photocuring. The adhesive layer preferably has a shear storage modulus of 30 to 80 kPa at 25°C after photocuring.

When the adhesive layer of the reinforcing film is attached to a polyimide film, laser cutting is performed, and the cut portion is peeled off and removed from the polyimide film, it is preferable that the length ratio of the portion where the adhesive remains on the polyimide film is 80% or less.

In the photocurable composition constituting the adhesive layer, the amount of the photocuring agent is preferably 3 to 17 parts by weight per 100 parts by weight of the acrylic base polymer. The photocuring agent is preferably a polyfunctional (meth)acrylate. The polyfunctional (meth)acrylate may have an alkylene oxide chain. The photocuring agent may include a polyfunctional (meth)acrylate having an alkylene oxide chain, and a urethane (meth)acrylate.

The glass transition temperature of the acrylic base polymer is preferably -55°C or lower. The acrylic base polymer may have a nitrogen ratio of 0.1 mol% or less among its constituent elements.

The adhesive strength of the adhesive layer to the polyimide film before photocuring may be 0.5 N/25 mm or less. The adhesive strength of the adhesive layer to the polyimide film after photocuring may be 3 N/25 mm or more.

The above-mentioned reinforcing film is bonded temporarily to the surface of the device as the adherend, and then the adhesive layer is photocured to obtain a device with the reinforcing film. The device may be a foldable flexible device. After the reinforcing film is temporarily attached to the adherend, and before the adhesive layer is photocured, the reinforcing film temporarily attached to the adherend may be cut, and the reinforcing film may be peeled off and removed from a portion of the adherend (a non-reinforced area).

The reinforcing film of the present invention has an adhesive layer made of a photocurable composition, and before the adhesive layer is photocured, the adhesive strength with the adherend is small and the film can be peeled off from the adherend. The adhesive strength with the adherend is increased by photocuring the adhesive layer after adhesion to the adherend. The adhesive layer of the reinforcing film has a small storage modulus and a high stress-strain relaxation property, so the reinforcing film of the present invention can also be suitably used in foldable devices using a resin film substrate.

The reinforcing film of the present invention has an adhesive layer that is excellent in cuttability before photocuring, and after the reinforcing film is temporarily attached to an adherend, the reinforcing film temporarily attached to the adherend is cut before the adhesive layer is photocured, and when the reinforcing film is peeled off and removed from the non-reinforced area on the adherend, little adhesive remains on the adherend surface in the peeled area of the reinforcing film. Therefore, in the device manufacturing process, the adhesive is less likely to adhere to manufacturing equipment or other components, which can contribute to improved yields.

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. 2 is a cross-sectional view showing a device to which a reinforcing film is attached. 13 is a diagram for explaining a method for calculating the ratio of remaining adhesive based on an observation image of a sample in which a cut portion of a reinforcing film has been peeled off and removed from the surface of a polyimide film. FIG.

Figure 1 is a cross-sectional view showing one embodiment of a reinforcing film. The reinforcing film 10 has an adhesive layer 2 on one main surface of a film substrate 1. The adhesive layer 2 is fixedly laminated on one main surface of the film substrate 1. The adhesive layer 2 is a photocurable adhesive made of a photocurable composition, and is cured by irradiation with active light 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 reinforcing film 10 to the surface of the device 20. In this state, the adhesive layer 2 has not yet been photocured, and the reinforcing film 10 (adhesive layer 2) is temporarily attached to the device 20. By photocuring the adhesive layer 2, the adhesive strength at the interface between the device 20 and the adhesive layer 2 increases, and the device 20 and the reinforcing film 10 are fixed together.

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

In the reinforced film shown in Figure 2, the film substrate 1 and the 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 the release liner 5 are peeled away from each other, peeling occurs at the interface between the adhesive layer 2 and the release liner 5, and the adhesive layer 2 remains bonded to the film substrate 1. No adhesive remains on the release liner 5 after peeling.

In the device with the reinforcing film 10 attached as shown in FIG. 3, the device 20 and the adhesive layer 2 are in a temporary bonded state before the adhesive layer 2 is photocured. When the film substrate 1 is peeled off from the device 20, peeling occurs at the interface between the adhesive layer 2 and the device 20, and the adhesive layer 2 remains fixed on the film substrate 1. Since no adhesive remains on the device 20, rework is easy. After the adhesive layer 2 is photocured, the adhesive strength between the adhesive layer 2 and the device 20 increases, making it difficult to peel the film 1 from the device 20, and peeling the two may cause cohesive failure of the adhesive layer 2.

[Configuration of reinforcing film]
<Film substrate>
A plastic film is used as the film substrate 1. In order to bond the film substrate 1 and the pressure-sensitive adhesive layer 2, 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 providing rigidity or shock mitigation, 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 making the reinforcing film flexible and foldable, the thickness of the film substrate 1 is preferably 125 μm or less, more preferably 100 μm or less. From the viewpoint of achieving both mechanical strength and flexibility, the compressive strength of the film substrate 1 is preferably 100 to 3000 kg/cm ² , more preferably 200 to 2900 kg/cm ² , even more preferably 300 to 2800 kg/cm ² , and particularly preferably 400 to 2700 kg/cm ² .

Examples of plastic materials constituting the film substrate 1 include polyester resins, polyolefin resins, cyclic polyolefin resins, polyamide resins, polyimide resins, etc. In a reinforcing film for an optical device such as a display, the film substrate 1 is preferably a transparent film. In addition, when the adhesive layer 2 is photocured by irradiating active light from the film substrate 1 side, the film substrate 1 is preferably transparent to the active light used to cure the adhesive layer. Polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate are preferably used because they have both mechanical strength and transparency. When the adhesive layer is cured by irradiating active light from the adherend side, it is sufficient that the adherend has transparency to active light, and the film substrate 1 does not need to be transparent to active light.

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 described above, in order to bond the film substrate 1 and the adhesive layer 2, it is preferable that no release layer is provided on the surface of the film substrate 1 to which the adhesive layer 2 is attached.

<Adhesive Layer>
The adhesive layer 2, which is fixedly laminated on the film substrate 1, is made of a photocurable composition containing a base polymer, a photocuring agent, and a photopolymerization initiator. The adhesive layer 2 has low adhesive strength with an adherend such as a device or device parts before photocuring, and is therefore easy to peel off. The adhesive layer 2 has improved adhesive strength with an adherend by photocuring, and therefore the reinforcing film is unlikely to peel off from the device surface even when the device is in use, resulting in excellent adhesive reliability.

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

The thickness of the adhesive layer 2 is, for example, about 1 to 300 μm. The thicker the adhesive layer 2, the more the adhesive strength with the adherend tends to improve. On the other hand, if the adhesive layer 2 is too thick, the fluidity before photocuring is high, and handling may become difficult. Therefore, the thickness of the adhesive layer 2 is preferably 3 to 100 μm, more preferably 5 to 50 μm, even more preferably 6 to 40 μm, and particularly preferably 8 to 30 μm. From the viewpoint of thinning, the thickness of the adhesive layer 2 may be 25 μm or less, 20 μm or less, or 18 μm or less.

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

The shear storage modulus (hereinafter simply referred to as "storage modulus") of the adhesive layer 2 at a temperature of 25°C before photocuring is preferably 15 kPa or more, more preferably 18 kPa or more, even more preferably 20 kPa or more, and may be 25 kPa or more or 30 kPa or more. The greater the storage modulus at room temperature of the adhesive layer before photocuring, the more improved the cutting processability is, and the more likely it is that adhesive residue on the adherend is suppressed when the reinforcing film is peeled off from the adherend.

On the other hand, if the storage modulus of the adhesive layer at room temperature before photocuring is excessively large, the storage modulus of the adhesive layer after photocuring also tends to be large, and the adhesion to the adherend and impact resistance may be insufficient. In addition, if the storage modulus of the adhesive layer before photocuring is large, the storage modulus of the adhesive layer after photocuring also tends to be large, and when applied to a foldable device, the adhesive layer cannot absorb the strain at and around the bending point, and the reinforcing film is likely to peel off from the device. Therefore, the storage modulus of the adhesive layer at a temperature of 25°C before photocuring is preferably 60 kPa or less, more preferably 55 kPa or less, even more preferably 50 kPa or less, and may be 45 kPa or less or 40 kPa or less.

The storage modulus of the adhesive layer is determined by reading the value at a specified temperature when measured at a temperature rise rate of 5°C/min in the range of -70 to 150°C at a frequency of 1 Hz in accordance with the method described in JIS K7244-1 "Plastics - Test method for dynamic mechanical properties." In this specification, unless otherwise specified, the storage modulus is the value at a temperature of 25°C.

From the viewpoint of realizing high adhesive strength with the adherend after photocuring, the storage modulus of the adhesive layer after photocuring at a temperature of 25°C is preferably 30 kPa or more, more preferably 35 kPa or more, and may be 40 kPa or more or 45 kPa or more. On the other hand, if the storage modulus of the adhesive layer after photocuring is excessively large, when applied to a foldable device, the reinforcing film is likely to peel off from the device at the bending point and its surroundings. In addition, if the storage modulus of the adhesive layer after photocuring is excessively large, the adhesion to the adherend and the impact mitigation effect (cushioning property) tend to decrease. Therefore, the storage modulus of the adhesive layer after photocuring at a temperature of 25°C is preferably 80 kPa or less, more preferably 70 kPa or less, and may be 65 kPa or less, 60 kPa or less, 55 kPa or less, or 50 kPa or less.

Below, the preferred forms of each component of the photocurable composition that constitutes the adhesive layer 2 will be described in order.

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

As the acrylic polymer, one containing an alkyl (meth)acrylate ester as the main monomer component is preferably used. In this specification, "(meth)acrylic" means acrylic and/or methacrylic.

As the (meth)acrylic acid alkyl ester, a (meth)acrylic acid alkyl ester in which the alkyl group has 1 to 20 carbon atoms is preferably used. The (meth)acrylic acid alkyl ester may have an alkyl group that is branched, or may have a cyclic alkyl group (alicyclic alkyl group).

Specific 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 (meth) Examples of such acrylates 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.

Specific examples of (meth)acrylic acid alkyl esters having an alicyclic alkyl group include (meth)acrylic acid cycloalkyl esters such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate; (meth)acrylic acid esters having a bicyclic aliphatic hydrocarbon ring such as isobornyl (meth)acrylate; and (meth)acrylic acid esters having a tricyclic or higher aliphatic hydrocarbon ring such as dicyclopentanyl (meth)acrylate, dicyclopentanyloxyethyl (meth)acrylate, tricyclopentanyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate. (Meth)acrylic acid alkyl esters having an alicyclic alkyl group may have a substituent on the ring, such as 3,3,5-trimethylcyclohexyl (meth)acrylate. In addition, the (meth)acrylic acid alkyl ester having an alicyclic alkyl group may be a (meth)acrylic acid ester that includes a condensed ring of an alicyclic structure and a ring structure having an unsaturated bond, such as dicyclopentenyl (meth)acrylate.

The content of the (meth)acrylic acid alkyl ester is preferably 50 parts by weight or more, more preferably 60 parts by weight or more, and may be 70 parts by weight or more, 80 parts by weight or more, or 90 parts by weight or more, based on 100 parts by weight of the total amount of the monomer components constituting the base polymer.

From the viewpoint of suppressing peeling of the adhesive layer when repeatedly bent by lowering the glass transition temperature of the acrylic base polymer and reducing the storage modulus, 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.

From the viewpoint of lowering the glass transition temperature of the acrylic base polymer, the (meth)acrylic acid alkyl ester is preferably one in which the homopolymer has a glass transition temperature of -56°C or lower, and the alkyl group preferably has 6 or more carbon atoms. The alkyl group of the (meth)acrylic acid alkyl ester may have 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-octyl acrylate (Tg: -65°C), isononyl acrylate (Tg: -60°C), n-nonyl acrylate (Tg: -58°C), isooctyl acrylate (Tg: -58°C), etc. Among these, 2-ethylhexyl acrylate is particularly preferred because it has a low Tg and can reduce the storage modulus of the adhesive.

The content of the (meth)acrylic acid alkyl ester having a homopolymer glass transition temperature (Tg) of -56°C or less is preferably 50 parts by weight or more, more preferably 60 parts by weight or more, and may be 70 parts by weight or more, 75 parts by weight or more, 80 parts by weight or more, 85 parts by weight or more, or 90 parts by weight or more, based on 100 parts by weight of the total amount of the monomer components constituting the base polymer. The amount of 2-ethylhexyl acrylate in the acrylic base polymer based on 100 parts by weight of the total amount of the monomer components may be within the above range.

The upper limit of the amount of (meth)acrylic acid alkyl ester having a homopolymer glass transition temperature (Tg) of -56°C or less is not particularly limited. As described below, in order to introduce crosslinking points into the acrylic base polymer, a hydroxyl group-containing monomer or a carboxyl group-containing monomer is used as a copolymerization component of the acrylic base polymer, so the amount of (meth)acrylic acid alkyl ester having a homopolymer glass transition temperature (Tg) of -56°C or less is preferably 99.5 parts by weight or less, more preferably 99 parts by weight or less, and may be 98 parts by weight or less or 97 parts by weight or less, based on 100 parts by weight of the total amount of the monomer components constituting the 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 a homopolymer having a glass transition temperature of -56°C or lower and a homopolymer having a glass transition temperature of higher than -56°C.

The acrylic base polymer preferably contains a monomer component having a crosslinkable functional group as a copolymerization component. By introducing a crosslinked structure into the base polymer, the cohesive strength is improved, the adhesive strength of the adhesive layer 2 is improved, and there is a tendency for the amount of adhesive residue on the adherend to be reduced when the adhesive layer before photocuring is peeled off from the adherend.

Examples of monomers having a crosslinkable functional group include hydroxyl group-containing monomers and carboxyl group-containing monomers. The hydroxyl group or carboxyl group of the base polymer serves as a reaction site with the crosslinking agent described below. For example, when an isocyanate-based crosslinking agent is used, it is preferable to contain a hydroxyl group-containing monomer as a copolymerization component of the base polymer. When an epoxy-based crosslinking agent is used, it is preferable to contain a carboxyl group-containing monomer as a copolymerization component of the base polymer.

Examples of hydroxy group-containing monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, 4-(hydroxymethyl)cyclohexylmethyl (meth)acrylate, etc. Examples of carboxy group-containing monomers include (meth)acrylic acid, 2-carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, etc.

From the viewpoint of appropriately introducing a crosslinked structure by a crosslinking agent such as an isocyanate-based crosslinking agent or an epoxy-based crosslinking agent, the total amount of the hydroxyl group-containing monomer and the carboxyl group-containing monomer relative to 100 parts by weight of the total amount of the constituent monomer components of the acrylic base polymer is preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, and may be 1.5 parts by weight or more, 2 parts by weight or more, 2.5 parts by weight or more, or 3 parts by weight or more.

Because hydroxyl groups and carboxyl groups are highly polar, when the amount of hydroxyl group-containing monomer or carboxyl group-containing monomer in the constituent monomer components of the acrylic base polymer is large, the cohesive force of the base polymer increases, and the storage modulus of the adhesive layer after photocuring tends to increase. From the viewpoint of reducing the storage modulus of the adhesive layer 2, the total amount of the hydroxyl group-containing monomer and carboxyl group-containing monomer relative to 100 parts by weight of the total amount of the constituent monomer components of the acrylic base polymer is preferably 25 parts by weight or less, more preferably 20 parts by weight or less, even more preferably 15 parts by weight or less, and may be 12 parts by weight or less, 10 parts by weight or less, or 8 parts by weight or less.

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

Because nitrogen-containing monomers are highly polar and have a high glass transition temperature, when the amount of nitrogen-containing monomer in the constituent monomer components of the acrylic base polymer is large, the cohesive strength of the base polymer tends to be high and the glass transition temperature of the adhesive layer tends to be high, and accordingly the storage modulus of the adhesive layer after photocuring tends to be large. In addition, when the amount of nitrogen-containing monomer is large, the adhesive strength of the adhesive layer 2 before photocuring to an adherend that has been subjected to a surface activation treatment such as plasma treatment tends to be high, and peeling of the reinforcing film from the adherend tends to be difficult.

Therefore, it is preferable that the acrylic base polymer has a small ratio of nitrogen-containing monomer in the constituent monomer components. Also, from the viewpoint of lowering the glass transition temperature of the acrylic base polymer, it is preferable that the amount of nitrogen-containing monomer is small. The amount of nitrogen-containing monomer per 100 parts by weight of the total amount of the constituent monomer components of the acrylic base polymer is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, even more preferably 3 parts by weight or less, and may be 1 part by weight or less, 0.5 parts by weight or less, 0.1 parts by weight or less, or 0.05 parts by weight or less.

The acrylic base polymer may not contain a nitrogen-containing monomer as a constituent monomer component. The acrylic base polymer before the introduction of the crosslinking structure may be substantially free of nitrogen atoms. The ratio of nitrogen in the constituent elements of the acrylic base polymer may be 0.1 mol% or less, 0.05 mol% or less, 0.01 mol% or less, 0.005 mol% or less, 0.001 mol% or less, or 0. By using an acrylic base polymer that is substantially free of nitrogen atoms, the increase in the adhesive strength (initial adhesive strength) of the pressure-sensitive adhesive sheet before photocuring when the adherend is subjected to a surface activation treatment tends to be suppressed. By not using a nitrogen atom-containing monomer as a constituent monomer component, a polymer that is substantially free of nitrogen atoms can be obtained. In addition, when a crosslinking structure is introduced into the base polymer, it is sufficient that the polymer before the introduction of the crosslinking structure is substantially free of nitrogen atoms, and the crosslinking agent may contain nitrogen atoms.

The acrylic base polymer may contain monomer components other than those mentioned above. The acrylic base polymer may contain, as monomer components, for example, vinyl ester monomers, aromatic vinyl monomers, epoxy group-containing monomers, vinyl ether monomers, sulfo group-containing monomers, phosphate group-containing monomers, acid anhydride group-containing monomers, etc.

From the viewpoint of imparting excellent adhesive properties to the adhesive and giving the adhesive a low storage modulus, the glass transition temperature of the acrylic base polymer is preferably -55°C or lower, more preferably -60°C or lower, even more preferably -62°C or lower, and may be -64°C or lower or -65°C or lower. The glass transition temperature of the acrylic base polymer is generally -100°C or higher, and may be -80°C or higher or -70°C or higher.

The glass transition temperature (Tg) is the temperature (peak top temperature) at which the loss tangent tan δ in viscoelasticity measurement is maximized. The theoretical glass transition temperature may be applied instead of the glass transition temperature determined by viscoelasticity measurement. The theoretical Tg is calculated 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 by the following Fox formula.
1/Tg=Σ(W _i /Tg _i )

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

The above monomer components are polymerized by various known methods such as solution polymerization, emulsion polymerization, and bulk polymerization to obtain an acrylic polymer as the base polymer. From the viewpoint of the balance of adhesive strength, holding power, and other properties of the adhesive, as well as cost, the solution polymerization method is preferred. Ethyl acetate, toluene, and the like are used as the solvent for solution polymerization. The solution concentration is usually about 20 to 80% by weight. Various known polymerization initiators such as azo-based and peroxide-based initiators can be used as the polymerization initiator for solution polymerization. A chain transfer agent may be used to adjust the molecular weight. The reaction temperature is usually about 50 to 80°C, and the reaction time is usually about 1 to 8 hours.

The weight average molecular weight of the acrylic base polymer is preferably 200,000 to 3,000,000, more preferably 300,000 to 2,500,000, and may be 400,000 to 2,000,000 or 500,000 to 1,800,000. Note that when a crosslinked structure is introduced into the base polymer, the molecular weight of the base polymer refers to the molecular weight before the introduction of the crosslinked structure.

(Crosslinking Agent)
In order to provide the adhesive with an appropriate cohesive force, it is preferable that a crosslinked structure is introduced into the base polymer. For example, a crosslinking agent is added to the solution obtained by polymerizing the base polymer, and the solution is heated as necessary to introduce the crosslinked structure. The crosslinking agent has two or more crosslinkable functional groups in one molecule. The crosslinking agent may have three or more crosslinkable functional groups in one molecule.

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 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 groups and carboxyl groups of the base polymer and allow easy introduction of a crosslinked structure.

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

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

As described above, even if the base polymer does not substantially contain nitrogen atoms, the crosslinking agent may contain nitrogen atoms. For example, a crosslinking structure may be introduced into a base polymer that does not substantially contain nitrogen atoms using an isocyanate crosslinking agent.

The amount of crosslinking agent used may be adjusted appropriately depending on the composition and molecular weight of the base polymer. The more crosslinking agent used, the higher the crosslink density of the base polymer, which imparts appropriate hardness to the adhesive layer before photocuring, increasing the storage modulus, improving processability such as cutting, and tends to suppress adhesive residue on the adherend when the reinforcing film is peeled off from the adherend. On the other hand, if the amount of crosslinking agent is too high, the adhesive layer has a high storage modulus and low stress-strain relaxation, so that when applied to a foldable device, the reinforcing film is likely to peel off from the device at and around the bent points.

From the viewpoint of achieving both cutting processability and flexibility of the adhesive layer, the amount of the crosslinking agent used is preferably 0.1 to 1.5 parts by weight, more preferably 0.15 to 1 part by weight, and even more preferably 0.25 to 75 parts by weight, and may be 0.3 to 0.7 parts by weight or 0.4 to 0.6 parts by weight, relative to 100 parts by weight of the acrylic base polymer.

(Photocuring agent)
The adhesive composition constituting the adhesive layer 2 contains, in addition to the base polymer, a compound having two or more photopolymerizable functional groups in one molecule as a photocuring agent. The adhesive composition containing the photocuring agent has photocurability, and when photocuring is performed after lamination with an adherend, the adhesive strength with the adherend is improved.

From the viewpoint of compatibility with the base polymer, the photocuring agent is preferably a liquid at room temperature. The photopolymerizable functional group of the photocuring agent is preferably one that is polymerizable by a photoradical reaction. As the photocuring agent, a compound having two or more ethylenically unsaturated bonds in one molecule is preferable, and a multifunctional (meth)acrylate is preferable because it shows moderate compatibility with the acrylic base polymer.

A typical example of a polyfunctional (meth)acrylate is an ester of a polyol and (meth)acrylic acid. Specific examples of polyfunctional (meth)acrylates include polyalkylene glycol di(meth)acrylates such as polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, and 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, and pentaerythritol di(meth)acrylate. tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerin di(meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, isoprene (meth)acrylate, etc.

The polyfunctional (meth)acrylate may be an ester of an alkylene oxide-modified polyol and (meth)acrylic acid. Examples of the alkylene oxide include ethylene oxide (EO) and propylene oxide (PO). The alkylene oxide may 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, isocyanuric acid ethylene oxide modified di(meth)acrylate, isocyanuric acid propylene oxide modified di(meth)acrylate, isocyanuric acid ethylene oxide modified tri(meth)acrylate, isocyanuric acid propylene oxide modified tri(meth)acrylate, pentaerythritol ethylene oxide modified tetra(meth)acrylate, pentaerythritol propylene oxide modified tetra(meth)acrylate, etc.

In polyfunctional (meth)acrylates containing an alkylene oxide chain, 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, and particularly preferably (poly)ethylene oxide. The chain length of the alkylene oxide (the number of repeating units of the alkylene oxide) n is about 1 to 15. When multiple alkylene oxide chains are contained in one molecule, the average chain length n is preferably 1 to 15. 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. By adjusting the type and chain length of the alkylene oxide, the compatibility with the acrylic base polymer can be adjusted to an appropriate range.

From the viewpoint of compatibility with the acrylic base polymer, the molecular weight of the polyfunctional (meth)acrylate as the photocuring agent is preferably 1500 or less, more preferably 1000 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 (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 of the polyfunctional (meth)acrylate is excessively small, the crosslinking point density of the adhesive layer after photocuring may increase, and the adhesiveness may decrease, so the functional group equivalent of the photocuring agent is preferably 80 or more, more preferably 100 or more, and may be 120 or more or 130 or more.

The photocuring agent has the effect of increasing the storage modulus of the adhesive by curing, and increasing the adhesive strength with the adherend. The greater the amount of photocuring agent, the greater the storage modulus of the adhesive layer after photocuring tends to be. From the viewpoint of sufficiently increasing the adhesive strength of the adhesive layer with the adherend after photocuring, the content of the photocuring agent in the photocurable composition constituting the adhesive layer is preferably 3 parts by weight or more, more preferably 4 parts by weight or more, even more preferably 5 parts by weight or more, and may be 6 parts by weight or more or 7 parts by weight or more, relative to 100 parts by weight of the acrylic base polymer.

In the adhesive layer before photocuring, the liquid photocuring agent reduces the cohesive strength of the base polymer, so the greater the amount of photocuring agent, the smaller the storage modulus of the adhesive layer before photocuring and the smaller the adhesive strength with the adherend. In addition, the greater the amount of photocuring agent, the greater the liquidity of the adhesive due to the reduced cohesive strength of the base polymer, so the cuttability tends to decrease. If the adhesive layer has poor cuttability, when the reinforcing film is cut with a laser or the like and the cut portion of the reinforcing film is peeled off and removed from the adherend, the adhesive that was not cut properly remains on the surface of the adherend, causing contamination of the equipment by the adhesive and reduced yield due to parts sticking.

From the viewpoint of improving the cutting processability of the adhesive layer before photocuring, the content of the photocuring agent in the photocurable composition constituting the adhesive layer is preferably 15 parts by weight or less, more preferably 12 parts by weight or less, and even more preferably 11 parts by weight or less, and may be 10 parts by weight or less, 9 parts by weight or less, or 8 parts by weight or less, relative to 100 parts by weight of the acrylic base polymer. The amount of the photocuring agent is preferably in the above range from the viewpoint of suppressing contamination of the adherend due to bleed-out of the photocuring agent from the adhesive layer before photocuring.

As described above, the photocuring agent acts to reduce the adhesive strength of the adhesive layer to the adherend before photocuring, making it easier to peel it off from the adherend, and to increase the adhesive strength of the adhesive layer to the adherend after photocuring. On the other hand, if the amount of photocuring agent is excessively large, it may cause a decrease in cutting processability and contamination of the adherend. In particular, for adhesives that use a small amount of crosslinking agent to achieve a low storage modulus (few crosslinking points introduced into the base polymer) for the purpose of application to foldable devices, etc., the polymer has a small cohesive force, so in order to ensure cutting processability, it is necessary to reduce the amount of photocuring agent.

In order to reduce the adhesive strength of the adhesive layer before photocuring and increase the adhesive strength of the adhesive layer after photocuring with a small amount of photocuring agent, it is preferable to use a polyfunctional (meth)acrylate having an alkylene oxide chain as the photocuring agent.

Compared to polyfunctional (meth)acrylates without alkylene oxide chains, polyfunctional (meth)acrylates with alkylene oxide chains have low compatibility with acrylic-based polymers, especially with polymers with a small amount of polar functional groups, and therefore tend to be unevenly distributed on the surface of the adhesive layer (near the adhesive interface with the adherend). When polyfunctional (meth)acrylates with alkylene oxide chains are included as a photocuring agent, even in small amounts, the photocuring agent unevenly distributed at the adhesive interface with the adherend tends to form an adhesion-inhibiting layer (Weak Boundary Layer; WBL).

When a WBL is formed, the adhesive layer retains its bulk properties such as storage modulus, but the liquid properties of the surface (adhesive interface) become stronger, so the adhesive strength with the adherend tends to decrease. Therefore, the adhesive layer before photocuring has a high storage modulus, excellent workability such as cutting, and is easy to peel from the adherend. When an adhesive layer in which a WBL is formed with the photocuring agent unevenly distributed near the adhesive interface with the adherend is photocured, the curing reaction of the photocuring agent is likely to proceed near the adhesive interface where the density of the photocuring agent is high, so the adhesive strength is likely to improve. In addition, when a WBL is formed, the increase in the storage modulus, which is a bulk property, due to photocuring tends to be suppressed, so the adhesive has excellent applicability to foldable devices.

The larger the average chain length n of the alkylene oxide chain of the polyfunctional (meth)acrylate having an alkylene oxide chain, the lower the compatibility with the acrylic base polymer, and the smaller the adhesive strength of the adhesive layer before photocuring tends to be. On the other hand, if the compatibility between the acrylic base polymer and the photocuring agent is excessively low, the liquid properties of the WBL formed on the surface of the adhesive layer are high, making it difficult to shear the photocuring agent, and the bleed-out photocuring agent may cause contamination of the adherend. From the viewpoint of having a suitable compatibility with the acrylic base polymer, the average chain length n of the alkylene oxide chain in the polyfunctional (meth)acrylate having an alkylene oxide chain is preferably 10 or less, more preferably 8 or less, even more preferably 6 or less, and may be 5 or less, 4 or less, or 3 or less.

As described above, since WBL is easily formed by adding a small amount, the photocuring agent is preferably a polyfunctional (meth)acrylate having an alkylene oxide chain and two or more (meth)acryloyl groups in one molecule. The polyfunctional (meth)acrylate having an alkylene oxide chain may have three or more (meth)acryloyl groups in one molecule.

When the photocuring agent has three or more (meth)acryloyl groups per molecule, the adhesive strength of the adhesive layer is likely to increase by photocuring. On the other hand, when a multifunctional acrylate having three or more (meth)acryloyl groups per molecule is used, the storage modulus of the adhesive layer after photocuring may be larger and the flexibility may decrease compared to when a multifunctional acrylate having two (meth)acryloyl groups per molecule is used.

Two or more types of photocuring agents may be used in combination. For example, two or more types of polyfunctional (meth)acrylates having an alkylene oxide chain may be used as the photocuring agent, or a polyfunctional (meth)acrylate having an alkylene oxide chain and a polyfunctional (meth)acrylate having no alkylene oxide chain may be used. In addition, a polyfunctional (meth)acrylate having a different number of functional groups (the number of (meth)acryloyl groups in one molecule) may be used as the photocuring agent. For example, a bifunctional (meth)acrylate and a trifunctional or more polyfunctional (meth)acrylate may be used in combination as the photocuring agent for the purpose of adjusting the adhesive strength and storage modulus of the adhesive layer after photocuring.

As the photocuring agent, a polyfunctional (meth)acrylate having an alkylene oxide chain and a urethane (meth)acrylate may be used. The urethane (meth)acrylate is a compound having one or more urethane bonds and two or more (meth)acryloyl groups in one molecule, and preferably contains two or more urethane bonds in one molecule. The urethane (meth)acrylate having two or more urethane bonds is obtained, for example, by reacting a polyisocyanate with a (meth)acrylic compound having a hydroxy group, and the isocyanate group of the polyisocyanate and the hydroxy group of the (meth)acrylic compound are bonded to form a urethane bond.

The polyisocyanate may be any of aromatic polyisocyanates, alicyclic polyisocyanates, and alicyclic polyisocyanates. Among them, aromatic polyisocyanates and aliphatic polyisocyanates are preferred. As the aromatic polyisocyanate, tolylene diisocyanate (TDI) is particularly preferred. As the tolylene diisocyanate, 2,4-tolylene diisocyanate or 2,6-tolylene diisocyanate may be used, or a mixture of the two may be used. As the aliphatic polyisocyanate, hexamethylene diisocyanate (HDI) 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, hydroxymethylacrylamide, and hydroxyethylacrylamide; 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.

Among these, the (meth)acrylic compound having a hydroxy group is preferably a compound having one hydroxy group and two or more (meth)acryloyl groups, and specific examples thereof include compounds having a pentaerythritol skeleton, such as pentaerythritol tri(meth)acrylate and dipentaerythritol penta(meth)acrylate.

A urethane (meth)acrylate obtained by reacting a diisocyanate with a (meth)acrylic compound having one hydroxy group and two or more (meth)acryloyl groups in one molecule has two urethane bonds and four or more (meth)acryloyl groups in one molecule. The number of (meth)acryloyl groups in the urethane (meth)acrylate may be six or more, eight or more, or twelve or ten or less.

The above urethane (meth)acrylates may be commercially available from Kyoeisha Chemical, Shin-Nakamura Chemical, Negami Chemical Industry, Nippon Synthetic Chemical, Daicel Allnex, Showa Denko Materials, etc.

From the viewpoint of compatibility with the acrylic base polymer and adjustment of the adhesive strength of the pressure-sensitive adhesive layer, the molecular weight of the urethane (meth)acrylate is preferably 500 to 1500, and may be 600 to 1000, or 700 to 900. From the same viewpoint, the functional group equivalent (g/eq) of the (meth)acryloyl group of the urethane (meth)acrylate is preferably 80 to 200, and may be 100 to 150, 110 to 140, or 120 to 130.

By including urethane (meth)acrylate as the photocuring agent in addition to a multifunctional (meth)acrylate that does not have a urethane bond, the adhesive layer tends to have a smaller adhesive strength before photocuring and a larger adhesive strength after photocuring. One possible reason for this is that the urethane (meth)acrylate promotes the formation of WBL by multifunctional (meth)acrylates that do not have a urethane bond, particularly multifunctional (meth)acrylates that have an alkylene oxide chain.

Urethane (meth)acrylate has the effect of increasing the cohesive strength of the acrylic-based polymer, and furthermore, the acrylic-based polymer and the urethane (meth)acrylate form hydrogen bonds. Therefore, the urethane (meth)acrylate is easily incorporated into the bulk portion of the adhesive layer. When the urethane (meth)acrylate is incorporated into the bulk portion of the adhesive layer, the multifunctional (meth)acrylate that does not have a urethane bond tends to be unevenly distributed near the surface (adhesive interface) of the adhesive layer, which is thought to promote the formation of WBL.

When the photocurable composition constituting the adhesive layer contains, as a photocuring agent, a urethane (meth)acrylate 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.

As described above, since urethane (meth)acrylate is easily incorporated into the bulk portion of the adhesive layer, when the amount of urethane acrylate is large, the increase in the storage modulus, which is a bulk property, due to photocuring tends to be significant. In addition, if the content of urethane (meth)acrylate is excessively high, the photocuring agent (polyfunctional (meth)acrylate and/or urethane (meth)acrylate having no urethane bond) is likely to bleed out to the surface of the adhesive layer (adhesive interface with the adherend), and the bleed-out components may cause contamination of the adherend. Therefore, the content of urethane (meth)acrylate is preferably 5 parts by weight or less, more preferably 3 parts by weight or less, and even more preferably 1 part by weight or less, and may be 0.5 parts by weight or less, 0.4 parts by weight or less, 0.3 parts by weight or less, 0.25 parts by weight or less, or 0.2 parts by weight or less, relative to 100 parts by weight of the base polymer.

When the photocurable composition contains a urethane (meth)acrylate as a photocuring agent in addition to a polyfunctional (meth)acrylate having no urethane bond, it is preferable that the content of the polyfunctional (meth)acrylate having no urethane bond is relatively large from the viewpoint of adjusting the storage modulus before and after photocuring and the adhesive strength with the adherend to an appropriate range, and from the viewpoint of suppressing contamination of the adherend due to bleeding out of the photocuring agent. The content of the urethane (meth)acrylate is preferably 0.2 times or less, more preferably 0.1 times or less, and may be 0.05 times or less, 0.03 times or less, or 0.02 times or less, by weight, relative to the content of the polyfunctional (meth)acrylate having no urethane bond. The content of the urethane (meth)acrylate may be 0.001 times or more, 0.005 times or more, 0.008 times or more, or 0.01 times or more, by weight, relative to the content of the polyfunctional (meth)acrylate having no urethane bond.

(Photopolymerization initiator)
The photopolymerization initiator generates active species by irradiation with active light rays, and promotes the curing reaction of the photocuring agent. As the photopolymerization initiator, a photocation initiator (photoacid generator), a photoradical polymerization initiator, a photoanion initiator (photobase generator), etc. are used depending on the type of the photocuring agent. When an ethylenically unsaturated compound such as a polyfunctional acrylate is used as the photocuring agent, it is preferable to use a photoradical polymerization initiator as the polymerization initiator.

The photoradical polymerization initiator generates radicals when irradiated with active light rays, and promotes the radical polymerization reaction of the photohardener by the transfer of radicals from the photoradical polymerization initiator to the photohardener. As the photoradical polymerization initiator (photoradical generator), one that generates radicals when irradiated with visible light or ultraviolet light having a wavelength shorter than 450 nm is preferable, and examples of the photoradical polymerization initiator include hydroxyketones, benzyl dimethyl ketals, aminoketones, acylphosphine oxides, benzophenones, trichloromethyl group-containing triazine derivatives, etc. The photoradical polymerization initiator may be used alone or in a mixture of two or more types.

The content of the photopolymerization initiator in the adhesive layer 2 is preferably 0.01 to 5 parts by weight, more preferably 0.02 to 3 parts by weight, and even more preferably 0.03 to 2 parts by weight, relative to 100 parts by weight of the base polymer. The content of the photopolymerization initiator in the adhesive layer 2 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, relative to 100 parts by weight of the photocuring agent.

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

Examples of crosslinking accelerators include organometallic compounds such as organometallic complexes (chelates), compounds of metals and alkoxy groups, and compounds of metals and acyloxy groups; and tertiary amines. From the viewpoint of suppressing the progress of the crosslinking reaction in a solution state at room temperature and ensuring the pot life of the adhesive composition, organometallic compounds are preferred. In addition, organometallic compounds that are liquid at room temperature are preferred as crosslinking accelerators, since they are easy to introduce a uniform crosslinked structure throughout the thickness direction of the adhesive layer. Examples of metals in organometallic compounds include iron, tin, aluminum, zirconium, zinc, titanium, lead, cobalt, zinc, etc.

Examples of crosslinking retarders include β-ketoesters such as methyl acetoacetate, ethyl acetoacetate, octyl acetoacetate, oleyl acetoacetate, lauryl acetoacetate, and stearyl acetoacetate; β-diketones such as acetylacetone, 2,4-hexanedione, and benzoylacetone; and alcohols such as tert-butyl alcohol.

[Preparation of Reinforcement 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 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 necessary to form an adhesive layer. As the drying method, any appropriate method may be adopted. 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 adhesive composition contains a crosslinking agent, it is preferable to proceed with 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 usually achieved by heating at a temperature in the range of 20°C to 160°C for about 1 minute to 7 days. The heating for drying and removing the solvent may also serve as the heating for crosslinking.

Even after the crosslinking structure is introduced into the polymer by the crosslinking agent, the photocuring agent remains unreacted. Therefore, a photocurable adhesive layer 2 containing a high molecular weight component and a photocuring agent is formed. When forming the adhesive layer 2 on the film substrate 1, it is preferable to attach a release liner 5 onto the adhesive layer 2 for the purpose of protecting the adhesive layer 2, etc. Crosslinking may be performed after attaching the release liner 5 onto the 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 be used as the release liner 5 as it is.

As the release liner 5, a plastic film such as polyethylene, polypropylene, polyethylene terephthalate, or polyester film is preferably used. The thickness of the release liner is usually 3 to 200 μm, preferably about 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 agent, peeling occurs at the interface between the adhesive layer 2 and the release liner 5, and the adhesive layer 2 is maintained in a state of being fixed to the film substrate 1. The release liner 5 may be treated with an antistatic treatment on either or both of the release-treated surface and the non-treated surface. By treating the release liner 5 with an antistatic treatment, it is possible to suppress charging when the release liner is peeled from the adhesive layer.

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

The adherend to which the reinforcing film is attached is not particularly limited, and examples thereof include various electronic devices, optical devices, and components thereof. 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 arbitrarily, and the device may be bent (folded) at 180°. When the device is a display device, the reinforcing film may be attached to the surface on the screen side, or the reinforcing film may be attached to the back side (housing). In a flexible device that is configured to be bendable at a predetermined location such as a hinge portion, bending and stretching are repeated at the same location when the device is in use.

The reinforcing film may be attached to the entire surface of the adherend, or may be selectively attached only to the area requiring reinforcement (area to be reinforced). After the reinforcing film is attached to the entire area requiring reinforcement (area to be reinforced) and the area not requiring reinforcement (area not to be reinforced), the reinforcing film is cut and peeled off from the area not to be reinforced, thereby producing a device in which the reinforcing film is attached only to the area to be reinforced. If the adhesive is not yet photocured, the reinforcing film is in a state where it is temporarily attached to the surface of the adherend, so that the reinforcing film can be easily peeled off and removed from the surface of the adherend. The adhesive layer before photocuring has a low storage modulus and excellent cutting processability, so that when the cut reinforcing film is peeled off and removed from the surface of the device, little adhesive remains in the area not to be reinforced of the device, improving the process yield.

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

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

If the initial adhesive strength is excessively large, it may be difficult to peel off the reinforcing film attached to the non-reinforcement area. As described above, since the base polymer is substantially free of nitrogen atoms, an excessive increase in the initial adhesive strength to an adherend whose surface has been activated can be suppressed. The adhesive strength between an adherend that has been surface-activated and the adhesive layer before photocuring is preferably 2.5 times or less, more preferably 2 times or less, even more preferably 1.5 times or less, and may be 1.4 times or less, or 1.3 times or less, of the adhesive strength between an adherend that has not been surface-activated and the adhesive layer before photocuring.

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 (initial adhesive strength) between the adhesive layer 2 and the adherend before photocuring is preferably 0.5 N/25 mm or less, more preferably 0.3 N/25 mm or less, even more preferably 0.25 N/25 mm or less, and may be 0.2 N/25 mm or less or 0.15 N/25 mm or less. From the viewpoint of preventing peeling of the reinforcing film during storage or handling, the adhesive strength between the adhesive layer 2 and the adherend before photocuring is preferably 0.005 N/25 mm or more, more preferably 0.01 N/25 mm or more.

The adhesive strength is determined by a peel test using a polyimide film as the adherend, with a tensile speed of 300 mm/min and a peel angle of 180°. Unless otherwise specified, the adhesive strength is measured at 25°C. The adhesive strength between the adhesive layer and the adherend before photocuring is measured using a sample that has been left to stand at 25°C for 30 minutes after lamination.

As described above, from the viewpoint of cutting processability, the storage modulus of the adhesive layer 2 at 25°C before photocuring is preferably 15 kPa or more, more preferably 18 kPa or more, even more preferably 20 kPa or more, and may be 25 kPa or more or 30 kPa or more. The storage modulus of the adhesive layer before photocuring depends on the composition of the base polymer, the amount of crosslinking agent introduced, the content of the photocuring agent, etc. The greater the amount of crosslinking agent introduced, the greater the storage modulus tends to be. The greater the amount of photocuring agent, the smaller the storage modulus tends to be.

After the reinforcing film is attached to the adherend, the film substrate 1 and adhesive layer 2 of the reinforcing film 10 are selectively cut (half cut) at the boundary between the area to be reinforced and the area to be non-reinforced so as not to cut the adherend 20, and the reinforcing film 10 temporarily attached to the area to be non-reinforced of the adherend 20 is peeled off and removed.

The method for cutting the reinforcing film is not particularly limited, but cutting using a laser is preferred. By selecting the wavelength and output of the laser, it is possible to selectively cut only the reinforcing film 10 without damaging the adherend. For example, when the adherend is a polyimide film serving as a substrate for the device 20, a carbon dioxide laser is used and the laser is irradiated from the film base 1 side of the reinforcing film 10, thereby cutting the film base 1 and adhesive layer 2 of the reinforcing film 10 while suppressing damage to the polyimide film.

When the laser output is reduced to prevent damage to the adherend, the cutting ability of the adhesive layer 2 tends to decrease; however, as described above, the reinforcing film of the present invention has excellent cutting processability in the adhesive layer 2, so processing defects can be prevented. Therefore, when the reinforcing film after cutting is peeled off from the surface of the adherend, little adhesive remains in the non-reinforced areas of the adherend, which can contribute to improving the process yield.

The cutting processability of the reinforcement film is evaluated by laminating the adhesive layer of the reinforcement film to a polyimide film, performing laser cutting, peeling and removing the cut portion from the polyimide film, observing the surface of the polyimide film after peeling the reinforcement film, and calculating the length ratio of the portion where the adhesive remains on the polyimide film. Specific conditions are as described in the Examples below.

The length ratio of the portion where the adhesive remains (adhesive remaining ratio) is preferably 80% or less, more preferably 70% or less, even more preferably 60% or less, and may be 55% or less, 50% or less, 45% or less, or 40% or less. The smaller the adhesive remaining ratio, the more preferable, and ideally it is 0. On the other hand, it is not easy to bring the adhesive remaining ratio close to 0 under laser processing conditions that do not damage the polyimide film.

By increasing the storage modulus of the adhesive, it is possible to reduce the adhesive remaining ratio, but in that case, the adhesive will lack flexibility for application to a foldable device. In the range in which the adhesive layer has a storage modulus applicable to a foldable device, the adhesive remaining ratio is generally 15% or more. The adhesive remaining ratio may be 20% or more, 25% or more, 30% or more, or 35% or more.

After laminating the reinforcing film to the adherend and cutting and peeling the reinforcing film, the adhesive layer 2 of the reinforcing film laminated to the adherend is irradiated with active light to photocure the adhesive layer. Examples of active light include ultraviolet light, visible light, infrared light, X-rays, α-rays, β-rays, and γ-rays. UV light is preferred as the active light because it can suppress curing of the adhesive layer during storage and is easy to cure. The irradiation intensity and irradiation time of the active light may be appropriately set according to the composition and thickness of the adhesive layer. The adhesive layer 2 may be irradiated with active light from either the film substrate 1 side or the adherend side, or from both sides.

As the adhesive layer is photocured, the adhesive strength of the adhesive layer to the adherend increases. From the viewpoint of adhesive reliability during practical use of the device, the adhesive strength between the adhesive layer 2 and the adherend after photocuring is preferably 3 N/25 mm or more, more preferably 4 N/25 mm or more, even more preferably 5 N/25 mm or more, and may be 6 N/25 mm or more, 7 N/25 mm or more, 8 N/25 mm or more, 9 N/25 mm or more, or 10 N/25 mm or more. It is preferable that the adhesive layer of the reinforcing film after photocuring has an adhesive strength in the above range to the polyimide film.

The adhesive strength between the adhesive layer 2 and the adherend after photocuring is preferably 5 times or more, more preferably 10 times or more, even more preferably 30 times or more, and may be 50 times or more, 75 times or more, or 100 times or more than the adhesive strength between the adhesive layer 2 and the adherend before photocuring. As described above, by adjusting the type (compatibility with the base polymer) and the amount of photocuring agent added, it is possible to increase the adhesive strength of the adhesive after photocuring while keeping the adhesive strength before photocuring (initial adhesive strength) low and maintaining cuttability.

As described above, in order to provide the adhesive layer 2 with an impact absorbing effect and to prevent peeling of the reinforcing film from the foldable device at and around the bends, the storage modulus of the adhesive layer 2 at 25°C after photocuring is preferably 80 kPa or less, more preferably 70 kPa or less, and may be 65 kPa or less, 60 kPa or less, 55 kPa or less, or 50 kPa or less.

The storage modulus at 25°C of the adhesive layer after photocuring is preferably 5 times or less, more preferably 3 times or less, and may be 2.5 times or less, or 2 times or less, of the storage modulus at 25°C of the adhesive layer before photocuring.

From the viewpoint of suppressing warping or damage to the adherend due to the cure shrinkage of the adhesive when the adhesive layer is photocured in a state where the adhesive layer is attached to the adherend, the increase in the storage modulus of the adhesive layer 2 at 25°C due to photocuring is preferably 60 kPa or less, more preferably 50 kPa or less, even more preferably 40 kPa or less, and may be 30 kPa or less or 25 kPa or less.

The smaller the amount of crosslinking agent and the amount of photocuring agent, the smaller the storage modulus of the adhesive layer after photocuring tends to be, and the smaller the amount of photocuring agent, the smaller the increase in the storage modulus of the adhesive layer due to photocuring tends to be. In addition, by using a multifunctional (meth)acrylate having an alkylene oxide chain as the photocuring agent, it is possible to increase the adhesive strength with the adherend while suppressing the increase in the storage modulus of the adhesive layer due to photocuring.

As described above, by laminating a reinforcing film, the adherend is given an appropriate rigidity and stress is alleviated and dispersed, which suppresses various defects that may occur during the manufacturing process, improves production efficiency, and improves yield. Before the adhesive layer is photocured, the reinforcing film can be easily peeled off from the adherend, so rework is easy even if lamination or lamination defects occur. In addition, processing such as selectively removing the reinforcing film from areas other than those to be reinforced is also easy, and because the adhesive layer before photocuring has excellent cuttability, little adhesive remains on the adherend surface near the cut point.

After the adhesive layer is photocured, it exhibits high adhesion to the adherend, the reinforcing film is not easily peeled off from the device surface, and it is endowed with excellent adhesion reliability and high impact resistance. Therefore, even if an external force is suddenly applied during use of the completed device due to dropping the device, placing a heavy object on the device, or being hit by a flying object, the reinforcing film attached thereto can prevent damage to the device. Furthermore, in a device with a reinforcing film in which the reinforcing film of the present invention is attached to a flexible device using a resin substrate, the reinforcing film is not easily peeled off at the bending points even when the device is repeatedly bent.

The following examples are provided for further explanation, but the present invention is not limited to these examples.

[Preparation of base polymer]
<Polymer A>
In a reaction vessel equipped with a thermometer, a stirrer, a reflux condenser and a nitrogen gas inlet tube, 95 parts by weight of butyl acrylate (BA) and 5 parts by weight of acrylic acid (AA) as monomers, 0.2 parts by weight of azobisisobutyronitrile (AIBN) as a thermal polymerization initiator, and 233 parts by weight of ethyl acetate as a solvent were charged, and nitrogen gas was introduced and nitrogen substitution was performed for about 1 hour while stirring. After that, the mixture was heated to 60°C and reacted for 7 hours to obtain a solution of acrylic polymer A with a weight average molecular weight of 600,000.

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

The monomer ratios charged for the acrylic polymers A to F, as well as the weight average molecular weights (Mw) and glass transition temperatures (Tg) of the polymers are listed in Table 1. The glass transition temperatures were calculated from the monomer ratios charged according to the Fox formula. The weight average molecular weights (polystyrene equivalent) were measured using GPC (Tosoh's "HLC-8220GPC") under the following conditions.
Sample concentration: 0.2% by weight (tetrahydrofuran solution)
Sample injection volume: 10 μL
Eluent: THF
Flow rate: 0.6mL/min
Measurement temperature: 40°C
Sample column: TSKguardcolumn SuperHZ-H (1 column) + TSKgel SuperHZM-H (2 columns)
Reference column: TSKgel SuperH-RC (1 column)

In Table 1, the monomers are described by the following abbreviations.
BA: Butyl acrylate 2EHA: 2-ethylhexyl acrylate MMA: Methyl methacrylate LA: Dodecyl acrylate 2HEA: 2-hydroxyethyl acrylate 4HBA: 4-hydroxybutyl acrylate AA: Acrylic acid NVP: N-vinylpyrrolidone

[Preparation of Reinforcement Film]
<Preparation of Pressure-Sensitive Adhesive Composition>
A crosslinking agent, a multifunctional acrylate (having no urethane bond) and a urethane acrylate as a photocuring agent, and a photopolymerization initiator were added to the acrylic polymer solution and mixed uniformly to prepare the pressure-sensitive adhesive compositions shown in Tables 2 to 4. As the base polymer, polymer B was used in samples 1 to 21, polymer C was used in samples 31 to 41, polymer D was used in samples 51 to 59, polymer F was used in samples 71 to 82, polymer A was used in samples 91 to 93, and polymer E was used in sample 96.

As a photopolymerization initiator, 0.3 parts by weight of "Omnirad 651" manufactured by IGM Resins was added per 100 parts by weight of the solid content of the acrylic polymer. The crosslinking agent and multifunctional acrylate were added so as to obtain the compositions shown in Tables 2 to 4. The amounts added in Tables 2 to 4 are the amounts (parts by weight of solid content) per 100 parts by weight of the base polymer. Details of the crosslinking agent and photocuring agent are as follows.

(Crosslinking Agent)
TC: N,N,N',N'-tetraglycidyl-m-xylylenediamine ("Tetrad C" manufactured by Mitsubishi Gas Chemical Company)
C-HX: Isocyanurate of hexamethylene diisocyanate ("Coronate HX" manufactured by Tosoh)
D110N: Trimethylolpropane adduct of xylylene diisocyanate ("Takenate D110N" manufactured by Mitsui Chemicals)

(Multifunctional acrylate)
A200: "NK Ester A200" manufactured by Shin-Nakamura Chemical Co., Ltd. (polyethylene glycol #200 (n=4) diacrylate; molecular weight 308, functional group equivalent 154 g/eq)
A600: "NK Ester A600" manufactured by Shin-Nakamura Chemical Co., Ltd. (polyethylene glycol #600 (n=14) diacrylate; molecular weight 708, functional group equivalent 354 g/eq)
M350: Trimethylolpropane EO modified (n=1) triacrylate ("Aronix M-350" manufactured by Toagosei Co., Ltd., functional group equivalent weight 143 g/eq)
APG700: APG700: Polypropylene glycol #700 (n=12) diacrylate; ("NK Ester APG700" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent: 404 g/eq)
ADPHE: ethoxylated dipentaerythritol polyacrylate ("NK Ester A-DPH-12E" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent of about 200 g/eq)
TMPT: Trimethylolpropane triacrylate ("NK Ester A-TMPT" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent: 99 g/eq)
(Urethane acrylate)
UA306: Pentaerythritol triacrylate-tolylene diisocyanate adduct ("UA-306T" manufactured by Kyoeisha Chemical Co., Ltd., functional group equivalent: 128 g/eq)

<Application of adhesive solution and crosslinking>
The above adhesive composition was applied to a 75 μm-thick polyethylene terephthalate film substrate (Mitsubishi Chemical's "Diafoil T100-75S") that had not been surface-treated, using a fountain roll, so that the thickness after drying would be 13 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 whose surface was silicone release-treated) was attached to the adhesive-coated surface. Thereafter, aging treatment was performed for 4 days in an atmosphere at 25° C. to promote crosslinking, and a reinforcing film was obtained in which an adhesive sheet was fixed and laminated on the film substrate, and a release liner was temporarily attached thereon.

[evaluation]
<Adhesive strength to polyimide film>
(Adhesive strength of pressure-sensitive adhesive layer before photocuring)
A 25 μm-thick polyimide film (Ube Industries, Ltd., "Upilex 25S") was attached to a glass plate via a double-sided adhesive tape (Nitto Denko, Ltd., "No. 531") to obtain a polyimide film substrate for measurement. The release liner was peeled off and removed from the surface of a reinforcing film cut to a size of 25 mm wide x 100 mm long, and the film was attached to the polyimide film substrate for measurement using a hand roller.

After leaving this sample at 25°C for 30 minutes, the edge of the film substrate of the reinforcing film was held with a chuck and a 180° peel test was performed at a pulling speed of 300 mm/min to measure the peel strength (adhesion strength of the reinforcing film to the polyimide film without plasma treatment).

While the polyimide film substrate for measurement was transported at a transport speed of 3 m/min, a normal pressure plasma treatment machine was used to perform plasma treatment on the surface of the polyimide film under conditions of an electrode voltage of 160 V. A reinforcing film was attached to the plasma-treated polyimide film using a hand roller, and the adhesive strength to the plasma-treated polyimide film was measured using a 180° peel test in the same manner as above.

From the results obtained, the ratio of adhesive strength with plasma treatment to adhesive strength without plasma treatment (the rate of increase in adhesive strength due to plasma treatment) was calculated.

(Adhesive strength of adhesive layer after photocuring)
After laminating the reinforcing film to the polyimide film substrate for measurement (not plasma-treated), the adhesive layer was photocured by irradiating the reinforcing film side (film substrate side) with ultraviolet light at an integrated light quantity of 4000 mJ/ ^cm2 using an LED light source with a wavelength of 365 nm. Using this test sample, the adhesive strength was measured by a 180° peel test in the same manner as above.

From the results obtained, the ratio of adhesive strength after photocuring to that before photocuring (the rate of increase in adhesive strength due to photocuring) was calculated.

<Storage modulus>
The adhesive composition was applied and crosslinked on the release liner in the same manner as above to prepare an adhesive sheet (before photocuring). A release liner was attached to the surface of the adhesive layer of the adhesive sheet before photocuring to isolate it from oxygen, and the adhesive sheet was photocured by irradiating it with 4000 mJ/ ^cm2 of ultraviolet light from a 365 nm LED lamp. The adhesive sheet before photocuring and the adhesive sheet after photocuring were laminated to prepare a measurement sample with a thickness of about 1.5 mm, and a dynamic viscoelasticity measurement was performed under the following conditions using a rotational rheometer ("ADiscovery-HR2" manufactured by TA Instruments), and the value of the shear storage modulus G' at 25 ° C was read.
(Measurement conditions)
Deformation mode: Torsion Measurement frequency: 1 Hz
Heating rate: 5°C/min Measurement temperature: -70 to 150°C
Shape: Parallel plate 8.0mmφ

<Cutting processability>
The release liner was peeled off from the surface of the reinforcement film, and a 50 μm thick polyimide film (Ube Industries'"Upilex50S") was attached to the surface of the adhesive layer using a hand roller. A carbon dioxide laser (frequency: 30 kHz, output: 11 W) was irradiated from the reinforcement film side of this laminate, and the reinforcement film was cut by scanning at a scanning speed of 300 mm/min to form a cutting line with a length of 180 mm. Another cutting line was formed parallel to this cutting line at an interval of 1 mm, and the reinforcement film (width 1 mm) in the area surrounded by the two cutting lines was peeled off from the surface of the polyimide film at a peeling speed of 300 mm/min. As a result, a sample was obtained in which there was a slit-like area on the polyimide film with a width of 1 mm and a length of 180 mm where the reinforcement film was not attached.

The sample was observed using a three-dimensional measuring machine (Eastern Electronics' "MicroVu"), and the length of the portion of the adhesive layer that was not properly cut and remained on the surface of the polyimide film was measured to the nearest 0.1 mm in an observation area of 2.5 mm along the length of the slit line (the area where the reinforcing film was peeled off) (see Figure 4). In Figure 4, the white two-dot chain line (imaginary line) corresponds to the cutting line made by the laser, and the black portion observed below this imaginary line is the remaining adhesive portion. In Figure 4, four remaining adhesive portions were confirmed, with their respective lengths being 0.3 mm, 0.7 mm, 0.5 mm, and 0.6 mm. Thirty portions (total length 75 mm) were observed along the slit line, and the ratio (%) of the total length of the remaining adhesive portions to the observation length (75 mm) was taken as the remaining adhesive portion ratio.

<Substrate Contamination>
The release liner was peeled off from the surface of the reinforcing film, and a polyimide film (Ube Industries'"Upilex25S") was attached to the surface of the adhesive layer using a hand roller. After leaving it at 25°C for 30 minutes or standing, 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 confirm the presence or absence of contamination. The contamination of the adherend was evaluated according to the following criteria.
A: No contamination was observed in the sample peeled off after leaving it still for 24 hours. B: Contamination was observed in the sample peeled off after leaving it still for 24 hours, but not in the sample peeled off after leaving it still for 30 minutes. C: Contamination was observed in the sample peeled off after leaving it still for 30 minutes.

The adhesive composition of each reinforcing film (type of base polymer, type and amount of crosslinking agent, type and amount of photocuring agent) and the evaluation results are shown in Tables 2 to 4.

Sample 91, which uses an adhesive composition containing 10 parts by weight of a photocuring agent for 100 parts by weight of polymer A, had a storage modulus of the adhesive layer before photocuring of over 60 kPa, and a storage modulus of the adhesive layer after photocuring of 153 kPa. Sample 91 had an adhesive layer with poor flexibility, making it unsuitable for use in foldable devices.

In sample 92, in which the amount of photocuring agent was increased to 20 parts by weight, the storage modulus of the adhesive image before photocuring was reduced compared to sample 91, but the storage modulus after photocuring was greater than that of sample 91, and similar to sample 91, the adhesive layer had poor flexibility. In addition, in sample 92, the adhesive layer before photocuring had a remaining adhesive ratio of more than 90% when cut by laser, and the adhesive had poor cutting processability. Sample 93, in which 0.15 parts by weight of urethane acrylate was added as a photocuring agent, had a slightly higher adhesive strength of the adhesive layer after photocuring compared to sample 92, but other properties were not significantly different from sample 92, and similar to sample 92, the storage modulus before and after photocuring was large and the cutting processability was poor.

In samples 5, 17-21, 32, 37-41, 52, 55-59, and 72, 77-82, in which the amount of photocuring agent was changed in the range of 2-20 parts by weight using polymers B, C, D, and F, the storage modulus of the adhesive layer before photocuring tended to be larger and the storage modulus of the adhesive layer after photocuring tended to be smaller as the amount of photocuring agent was smaller, but in all cases the storage modulus of the adhesive layer before photocuring was smaller than 60 kPa, and the adhesive layer had appropriate flexibility.

Polymers B, C, D, and F are acrylic polymers whose main constituent monomer is 2-ethylhexyl acrylate, and it is believed that the adhesive layer had excellent flexibility due to the low glass transition temperature of the base polymer. On the other hand, samples 91 to 93 use polymer A whose main constituent monomer is butyl acrylate, and have a higher glass transition temperature than polymers B, C, D, and F, and because the base polymer has high elasticity, they have a high storage modulus both before and after photocuring, and are therefore considered to have lacked flexibility compared to the examples using polymers B, C, D, and F.

In sample 96, in addition to the high glass transition temperature of polymer E, it contains a large amount of highly polar monomer components and has a large cohesive force, so it is believed that the storage modulus before and after photocuring was higher than that of sample 91 using polymer A. In addition, since polymer E contains NVP, a highly polar monomer containing a nitrogen atom, as a constituent monomer component, in sample 96, the adhesive strength of the pressure-sensitive adhesive layer before photocuring to the plasma-treated polyimide film was high. In samples 71 to 82 using polymer F, the adhesive strength to the plasma-treated polyimide film was more than twice as high as the adhesive strength to the non-plasma-treated polyimide film, and there was a tendency for the initial adhesive strength to increase significantly due to the plasma treatment of the adherend.

As described above, samples using polymers B, C, D, and F had a lower storage modulus and showed excellent flexibility compared to samples using polymers A and E, but as the amount of photocuring agent increased, the storage modulus of the adhesive layer before photocuring decreased and the proportion of adhesive remaining at the time of laser cutting tended to increase. Samples 16, 41, 59, and 82, which contained 20 parts by weight of photocuring agent, had a proportion of adhesive remaining above 80%, similar to samples 92 and 93.

In samples 92 and 93, although the storage modulus of the adhesive layer before photocuring exceeded 60 kPa, the proportion of remaining adhesive was large and cutting processability was poor. Therefore, when the amount of photocuring agent is large, in addition to the small storage modulus of the adhesive layer before photocuring, the adhesive is highly liquid due to the large amount of photocuring agent, which is thought to be the cause of the poor cutting processability.

Samples 16, 41, 59, and 82 were rated as C for contamination, and contamination was observed on the adherend after the reinforcing film was peeled off. The cause of contamination is thought to be excess photocuring agent that bled out onto the surface of the adhesive layer and was transferred to the adherend. From the standpoint of preventing contamination of the adherend, as well as the cuttability of the adhesive layer, it can be said that it is preferable to use a small amount of photocuring agent, as long as the adhesive layer after photocuring has sufficient adhesion.

On the other hand, samples 16, 37, 57, and 77, which contain 2 parts by weight of photocuring agent, had good cuttability of the adhesive layer before photocuring, but had high adhesion to the polyimide film and were poorly removable from the adherend.

Comparing samples 5 and 11-15, which used polymer B, samples 31-36, which used polymer C, samples 51-56, which used polymer D, and samples 71-76, which used polymer F, it was found that the greater the amount of crosslinker blended, the smaller the adhesive strength of the adhesive layer before photocuring and the greater the storage modulus of the adhesive layer before and after photocuring, and therefore the proportion of adhesive remaining after laser cutting tended to be smaller.

On the other hand, samples 16, 36, 54, and 77, which contained a large amount of crosslinking agent, had a storage modulus of the adhesive layer before photocuring of more than 60 kPa, and the storage modulus of the adhesive layer after photocuring was also large, resulting in poor flexibility. These results show that cutting processability can be improved by increasing the amount of crosslinking agent to the extent that flexibility suitable for application to foldable devices can be ensured.

Samples 5 to 10, which used polymer B and changed the type of multifunctional acrylate used as the light curing agent, showed differences in characteristics depending on the type of light curing agent. Sample 10, which used TMPT, a light curing agent that does not contain an ethylene oxide chain, had a higher adhesive strength of the adhesive layer before light curing and a lower adhesive strength of the adhesive layer after light curing, compared to samples 5 to 9, which used light curing agents that did contain an ethylene oxide chain.

Samples 5 to 10 did not show a large difference in the storage modulus of the adhesive layer before photocuring, but samples 5, 9, and 10, which used a multifunctional acrylate with three or more functional groups as the photocuring agent, tended to show a greater increase in storage modulus due to photocuring than samples 6 to 8, which used a bifunctional acrylate. In particular, sample 10 showed a more significant increase in storage modulus due to photocuring than the other examples, and its curing shrinkage was also greater than the other examples.

TMPT, which does not contain ethylene oxide chains, is highly compatible with acrylic polymers and does not easily form WBLs, so it is easily incorporated into the bulk of the adhesive layer, and the photocuring agent cures uniformly throughout the thickness of the adhesive layer, which is thought to have significantly increased the storage modulus, a bulk property. In contrast, multifunctional acrylates containing ethylene oxide chains have lower compatibility with acrylic polymers than TMPT and are more likely to form WBLs, so samples 5 to 9 have smaller initial adhesive strength than sample 10, and it is thought that the adhesive strength can be increased while suppressing the increase in storage modulus due to photocuring.

Samples 5, 6, and 10 had excellent contamination resistance, with little contamination of the adherend when the pre-light-curing adhesive layer was peeled off from the adherend. On the other hand, samples 7 to 9 had poor contamination resistance. In samples 7 to 9, the ethylene oxide chain length n of the multifunctional acrylate used as the light curing agent was long and had low compatibility with the acrylic base polymer, which is thought to have caused the light curing agent to bleed out easily, resulting in contamination of the adherend.

No significant difference was observed in the adhesive strength of the adhesive layer before photocuring for samples 5 to 9, but sample 5 showed a greater increase in adhesive strength due to photocuring than the other samples, and the adhesive strength of the adhesive layer after photocuring was greater. The photocuring agent (M350) used in sample 5 is a trifunctional acrylate containing an ethylene oxide chain, which is prone to the formation of WBL and increases the crosslinking density near the adhesive interface after photocuring, which is thought to be why the adhesive strength after photocuring was greater.

In samples 1 to 5, which used polymer B, contained 8 parts by weight of M350 as the multifunctional acrylate, and varied the amount of urethane acrylate, there was a tendency for the adhesive strength before light curing to decrease as the amount of urethane acrylate was increased. On the other hand, there was a tendency for the adhesive layer before light curing to become more likely to contaminate the adherend as the amount of urethane acrylate was increased. From these results, it can be seen that by adding a small amount of urethane acrylate as a light curing agent, in addition to a multifunctional acrylate that does not contain urethane bonds, but not enough to contaminate the adherend, a reinforcing film with high adhesive strength to the adherend and excellent adhesive reliability can be obtained after the adhesive is light cured.

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

Claims (12)

The adhesive tape comprises 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 base polymer, a photocuring agent having two or more photopolymerizable functional groups, and a photopolymerization initiator;
The acrylic base polymer contains, as a monomer component, at least one monomer selected from the group consisting of a hydroxy group-containing monomer and a carboxy group-containing monomer, and a crosslinked structure is introduced into the acrylic base polymer by a crosslinking agent bonded to the hydroxy group and/or the carboxy group,
the pressure-sensitive adhesive layer has a shear storage modulus of 15 to 60 kPa at 25° C. before photocuring;
the pressure-sensitive adhesive layer of the reinforcing film is attached to a polyimide film, and a laser cutting process is performed; when the cut portion is peeled off and removed from the polyimide film, the length ratio of the portion in which the pressure-sensitive adhesive remains on the polyimide film is 80% or less;
Reinforcement film. The reinforced film according to claim 1, wherein the pressure-sensitive adhesive layer has a shear storage modulus of 30 to 80 kPa at 25°C after photocuring. The reinforced film according to claim 1, wherein the photocurable composition contains 3 to 17 parts by weight of the photocuring agent per 100 parts by weight of the acrylic base polymer. The reinforcing film according to claim 1, which contains a multifunctional (meth)acrylate as the photocuring agent. The reinforcing film according to claim 1, which contains a polyfunctional (meth)acrylate having an alkylene oxide chain as the photocuring agent. The reinforcing film according to claim 1, wherein the photocuring agent includes a multifunctional (meth)acrylate having an alkylene oxide chain and a urethane (meth)acrylate. The reinforced film according to claim 1, wherein the glass transition temperature of the acrylic base polymer is -55°C or lower. The reinforced film according to claim 1, wherein the acrylic base polymer has a nitrogen content of 0.1 mol% or less among its constituent elements. The reinforced film according to claim 1, wherein the acrylic base polymer has a crosslinked structure introduced therein using 0.2 to 1.5 parts by weight of a crosslinking agent per 100 parts by weight of the polymer. The reinforced film according to claim 1, in which the adhesive layer has an adhesive strength with the polyimide film before photocuring of the adhesive layer of 0.5 N/25 mm or less. The reinforcing film according to claim 1, in which the adhesive layer has an adhesive strength with the polyimide film after photocuring of the adhesive layer is 3 N/25 mm or more. A method for producing a device with a reinforced film, in which a reinforced film is attached to a surface of a foldable device, comprising the steps of:
The pressure-sensitive adhesive layer of the reinforcing film according to any one of claims 1 to 11 is temporarily attached to a surface of a foldable device,
With the reinforcing film temporarily attached to the surface of the device, the reinforcing film is cut, and the reinforcing film temporarily attached to the non-reinforcement target area of the device is peeled off and removed from the surface of the device.
The pressure-sensitive adhesive layer is photocured.
A method for manufacturing a device with a reinforcing film.

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