JP2024114026A - Reinforcement film, device manufacturing method and reinforcing method - Google Patents

Abstract

[Problem] To provide a reinforced film in which leakage of static electricity to an adherend is suppressed. [Solution] A reinforced film (10) includes an adhesive layer (2) fixedly laminated on one main surface of a film substrate (1). The adhesive layer is made of a photocurable composition containing an acrylic-based polymer, a photocuring agent having two or more photopolymerizable functional groups, and a photopolymerization initiator. The acrylic-based polymer contains, as a monomer component, one or more selected from the group consisting of a hydroxyl-group-containing monomer and a carboxyl-group-containing monomer, and a crosslinked structure is introduced by a crosslinking agent bonded to the hydroxyl group and/or the carboxyl group. The surface resistance of the adhesive layer is 1 x 1015 Ω or more. [Selected Figure] Figure 1

Description

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

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 selectively peel off the reinforcing film from locations on the adherend that do not require reinforcement. 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.

JP 2020-41113 A

Devices in which a reinforcing film with an adhesive layer bonded to a film substrate are bonded are prone to charging because the substrate of the reinforcing film is made of a resin material, and the charge (static electricity) on the substrate can leak to the electronic components that make up the device, causing electrostatic damage.

In view of the above, the present invention aims to provide a reinforcing film that can be peeled off immediately after being attached to an adherend, can be firmly attached to the adherend by photocuring the adhesive after attachment to the adherend, and is unlikely to leak static electricity to the adherend.

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 surface resistance of the adhesive layer is 1× ¹⁰ Ω or more. By increasing the resistance of the adhesive constituting the reinforcing film, it is possible to reduce leakage of static electricity from the resin film substrate into the inside of the device and suppress electrostatic breakdown of the device. The surface free energy of the adhesive layer is preferably 20 mJ/m ² or less.

The photocurable composition constituting the adhesive layer contains 3 to 25 parts by weight of a photocuring agent 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 acrylic base polymer may contain 50 to 99.5 parts by weight of a (meth)acrylic acid alkyl ester having an alkyl group with 6 to 20 carbon atoms per 100 parts by weight of the total monomer components. The acrylic base polymer may have a nitrogen ratio of 0.1 mol % or less among its constituent elements.

The adhesive layer may have a shear storage modulus of 10 to 70 kPa at 25°C before photocuring. The adhesive layer may have a shear storage modulus of 30 to 140 kPa at 25°C after photocuring.

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 temporarily attached to the surface of the device as the adherend, and then the adhesive layer is photocured to obtain a device with the reinforcing film. After the reinforcing film is temporarily attached to the adherend, the reinforcing film temporarily attached to the adherend may be cut before the adhesive layer is photocured, and the reinforcing film may be peeled off and removed from a portion of the adherend (a non-reinforced area). In the device with the reinforcing film, the surface resistance of the adhesive layer is preferably 1×10 ¹⁵ Ω or more.

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. By photocuring the adhesive layer after adhesion to the adherend, the adhesive strength with the adherend increases. Because the adhesive layer of the reinforcing film has high resistance, electrostatic breakdown caused by leakage of static electricity from the film substrate into the adherend can be suppressed.

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.

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 500 μm. From the viewpoint of reinforcing the device by providing rigidity or shock mitigation, the thickness of the film substrate 1 is preferably 12 μm or more, more preferably 30 μm or more, and even more preferably 45 μm or more. From the viewpoint of imparting flexibility to the reinforcing film and improving handling properties, the thickness of the film substrate 1 is preferably 300 μm or less, and more preferably 200 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 surface resistance of the pressure-sensitive adhesive layer 2 after photocuring is 1× ¹⁰ Ω or more. Since the pressure-sensitive adhesive layer has high resistance, even if the film substrate 1 of the reinforcing film is charged, the pressure-sensitive adhesive layer 2 blocks static electricity, so that electrostatic damage due to leakage of static electricity to the device 20 as the adherend can be suppressed.

The surface resistance of the adhesive layer 2 is a value measured by contacting the surface of the adhesive layer after photocuring with a probe of a resistivity meter for high resistance under conditions of an applied voltage of 1000 V and a voltage application time of 10 seconds. Generally, the surface resistance of the adhesive layer hardly changes before and after photocuring, so if the surface resistance of the adhesive layer before photocuring is 1× ¹⁰ Ω or more, the surface resistance of the adhesive layer after photocuring is also 1× ¹⁰ Ω or more.

The smaller the surface free energy of the pressure-sensitive adhesive layer 2, the smaller the degree of freedom of the molecules on the surface (interface with the adherend) (the molecules are less likely to move), and the greater the surface resistance tends to be. From the viewpoint of increasing the resistance, the surface free energy of the pressure-sensitive adhesive layer 2 after photocuring is preferably 20 mJ/m ² or less, more preferably 19 mJ/m ² or less, even more preferably 18 mJ/m ² or less, and may be 17 mJ/m ² or less, 16 mJ/m ² or less, or 15.5 mJ/m ² or less. From the viewpoint of suppressing contamination of the adherend due to bleeding out of the components of the pressure-sensitive adhesive layer, the surface free energy of the pressure-sensitive adhesive layer is preferably within the above range.

On the other hand, when the surface free energy of the adhesive is small, the adhesive layer before photocuring has a large adhesive strength, and the reinforcing film tends to be difficult to peel off from the adherend. Therefore, the surface free energy of the adhesive layer 2 after photocuring is preferably 12 mJ/ ^m2 or more, more preferably 13 mJ/ ^m2 or more, even more preferably 13.5 mJ/ ^m2 or more, and may be 14 mJ/ ^m2 or more.

The surface resistance of the adhesive layer 2 is a value calculated by dropping two kinds of liquids (probe liquids) with known dispersion term _γd , polar term _γp , and hydrogen bond term _γh of the surface free energy on the surface of the adhesive layer after photocuring, measuring the contact angles _θ1 and _θ2 with respect to each liquid, and using the Owens-Wendt formula from the dispersion term _γd , polar term _γp , and hydrogen bond term _γh of the surface free energy of the probe liquid and the contact angle. Water and diiodomethane are used as the probe liquid to measure the surface free energy of the adhesive layer. The surface free energy of water is ( _γd , _γp , _γh ) = (29.1, 1.3, 42.4), and the surface free energy of diiodomethane is ( _γd , _γp , _γh ) = (46.8, 4.0, 0.0) (units are mJ/ ^m2 ).

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 10 kPa or more, more preferably 15 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 processability, such as cutting, 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. Therefore, the storage modulus of the adhesive layer at a temperature of 25°C before photocuring is preferably 70 kPa or less, more preferably 60 kPa or less, even more preferably 55 kPa or less, and may be 50 kPa or less, 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, 45 kPa or more, or 50 kPa or more. On the other hand, if the storage modulus after photocuring is excessively large, the adhesiveness to the adherend tends to be insufficient. In addition, the smaller the storage modulus of the adhesive layer after photocuring, the higher the impact mitigation effect (cushioning property) of the adhesive layer 2 and the more the impact resistance tends to improve. Therefore, the storage modulus of the adhesive layer after photocuring at a temperature of 25°C is preferably 140 kPa or less, more preferably 120 kPa or less, even more preferably 100 kPa or less, and may be 90 kPa, 80 kPa or less, or 70 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 increasing the resistance of the adhesive layer 2, it is preferable that the (meth)acrylic acid alkyl ester used as the monomer component of the acrylic base polymer has an alkyl group with 6 or more carbon atoms. Since the volume of the alkyl group in a (meth)acrylic acid alkyl ester in which the number of carbon atoms is 6 or more is large, the amount of (meth)acryloyl groups per unit volume of the base polymer is small, the surface free energy is small, and the resistance tends to be high.

From the viewpoint of lowering the glass transition temperature of the acrylic base polymer and imparting appropriate flexibility and adhesiveness to the adhesive sheet, 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 and imparting appropriate flexibility to the adhesive sheet, the (meth)acrylic acid alkyl ester is preferably one having a homopolymer glass transition temperature of -56°C or less, and preferably one having 9 or less carbon atoms in the alkyl group. Specific examples of (meth)acrylic acid alkyl esters having 6 or more carbon atoms in the alkyl group and a homopolymer glass transition temperature (Tg) of -56°C or less 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 contributes greatly to high resistance, has a low Tg, and can reduce the storage modulus of the adhesive.

The content of the (meth)acrylic acid alkyl ester in which the alkyl group has 6 or more carbon atoms 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.

There is no particular upper limit to the amount of the (meth)acrylic acid alkyl ester having 6 or more carbon atoms in the alkyl group. 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. Therefore, the amount of the (meth)acrylic acid alkyl ester having 6 or more carbon atoms in the alkyl group 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 those in which the alkyl group has 6 or more carbon atoms and those in which the alkyl group has 5 or less carbon atoms. The (meth)acrylic acid alkyl esters in which the alkyl group has 6 or more carbon atoms may include those in which the alkyl group has 6 to 9 carbon atoms and those in which the alkyl group has 10 or more carbon atoms.

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 monomers or carboxyl group-containing monomers in the constituent monomer components of the acrylic base polymer is large, the resistance of the adhesive layer 2 tends to be low. From the viewpoint of making the adhesive layer 2 highly resistive, the total amount of the hydroxyl group-containing monomers and carboxyl group-containing monomers 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.

Since the nitrogen-containing monomer is highly polar, when the amount of the nitrogen-containing monomer in the constituent monomer components of the acrylic base polymer is large, the resistance of the adhesive layer 2 tends to be low. In addition, when the amount of the 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 pressure-sensitive adhesive, the glass transition temperature of the acrylic base polymer is preferably -40°C or lower, more preferably -50°C or lower, even more preferably -55°C or lower, and may be -60°C or lower or -62°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 amount of crosslinking agent used is about 0.1 to 5 parts by weight, preferably 0.15 to 3 parts by weight, more preferably 0.2 to 2 parts by weight, and even more preferably 0.25 to 1.5 parts by weight, and may be 0.3 to 1 part by weight, 0.35 to 0.8 parts by weight, or 0.4 to 0.75 parts by weight, per 100 parts by weight of the acrylic base polymer.

The greater the amount of crosslinking agent, 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 excessively large, the adhesive strength to the adherend may not increase sufficiently even if the adhesive is photocured. If the amount of crosslinking agent is within the above range, the adhesive layer 2 has excellent processability and releasability from the adherend before photocuring, and can adhere firmly to the adherend after photocuring.

(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 2 parts by weight or more, more preferably 3 parts by weight or more, even more preferably 4 parts by weight or more, and may be 5 parts by weight or more, 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 force 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. Acrylic polymers whose main component monomers are (meth)acrylic acid alkyl esters whose alkyl groups have 6 or more carbon atoms often have a smaller storage modulus than acrylic polymers whose main component is butyl acrylate. In photocuring adhesives using acrylic base polymers whose main component is (meth)acrylic acid alkyl esters whose alkyl groups have 6 or more carbon atoms for the purpose of reducing resistance, etc., if the amount of photocuring agent is increased, the storage modulus of the adhesive layer before photocuring may be small and cutting and other processability may be poor.

The greater the amount of photocuring agent such as polyfunctional (meth)acrylate, the greater the surface free energy of the adhesive layer and the lower the resistance tends to be. In particular, photocuring agents with alkylene oxide chains have a great effect of lowering the resistance of the adhesive. Furthermore, if there is an excessive amount of photocuring agent, the photocuring agent is likely to bleed out from the adhesive layer before photocuring, and when the reinforcing film is peeled off from the adherend, the bled-out components may transfer to the adherend and cause contamination.

From the viewpoints of increasing the resistance of the adhesive layer, improving the processability of the adhesive layer before photocuring, and preventing contamination of the adherend, the content of the photocuring agent in the photocurable composition constituting the adhesive layer is preferably 35 parts by weight or less, more preferably 30 parts by weight or less, and even more preferably 25 parts by weight or less, and may be 20 parts by weight or less, 15 parts by weight or less, 12 parts by weight or less, 10 parts by weight or less, or 8 parts by weight or less, relative to 100 parts by weight of the acrylic base polymer.

As described above, the photocuring agent has the effect of lowering the adhesive strength of the adhesive layer to the adherend before photocuring, making it easier to peel it off from the adherend, and also of increasing the adhesive strength of the adhesive layer to the adherend after photocuring. 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 multifunctional (meth)acrylates without alkylene oxide chains, multifunctional (meth)acrylates with alkylene oxide chains have low compatibility with acrylic base polymers and tend to be unevenly distributed on the surface of the pressure-sensitive adhesive layer (near the adhesive interface with the adherend). When a multifunctional (meth)acrylate with alkylene oxide chains is used as a photocuring agent, even if the amount is small, 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 while the liquid properties of the surface (adhesive interface) become stronger, which tends to reduce the adhesive strength with the adherend. 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 has been 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, which tends to improve the adhesive strength. On the other hand, the increase in the storage modulus, which is a bulk property, tends to be suppressed.

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 due to photocuring. In addition, when a multifunctional acrylate having three or more (meth)acryloyl groups per molecule is used, the storage modulus tends to increase due to photocuring compared to when a multifunctional acrylate having two (meth)acryloyl groups per molecule is used.

As described above, a highly resistant adhesive layer can be formed by using an acrylic polymer in which the main component of the constituent monomer is an alkyl (meth)acrylate with an alkyl group having 6 or more carbon atoms as the base polymer, using a polyfunctional (meth)acrylate with an alkylene oxide chain as the light curing agent, and reducing the amount of light curing agent used. By using a polyfunctional (meth)acrylate with an alkylene oxide chain as the light curing agent, WBL is easily formed at the adhesion interface with the adherend even if the amount of light curing agent added is small (for example, about 10 parts by weight or less per 100 parts by weight of the acrylic base polymer). When WBL is formed with a small amount of light curing agent, the adhesive layer before light curing has a high storage modulus and is excellent in processability such as cutting, and is easy to peel from the adherend because of its small adhesive strength. On the other hand, the adhesive layer after light curing can be firmly adhered to the adherend, while the increase in storage modulus as a bulk property is suppressed, and it has excellent impact resistance.

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.

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 onto 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 adhesive strength with the adherend before and after photocuring to an appropriate range and from the viewpoint of suppressing contamination of the adherend due to bleeding 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 substrate to which the reinforcing film is attached is not particularly limited, and examples include various electronic devices, optical devices and their component parts.

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). Alternatively, the reinforcing film may be attached to the entire area requiring reinforcement (area to be reinforced) and the area not requiring reinforcement (area to be non-reinforced), and then the reinforcing film attached to the area not requiring reinforcement may be cut and removed. If the adhesive has not yet been photocured, the reinforcing film is in a state where it is temporarily attached to the surface of the adherend, and therefore the reinforcing film can be easily peeled off and removed from the surface of the adherend. The reinforcing film may be attached to the area to be reinforced and the area not requiring reinforcement, and the area to be reinforced may be selectively irradiated with light to photocur the adhesive, and then the reinforcing film may be selectively peeled off and removed from the area not requiring reinforcement where the adhesive is not yet cured.

By laminating the reinforcing film, appropriate rigidity is imparted, which is expected to improve handling and prevent breakage. 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 improving processability and suppressing adhesive residue when peeled off from an adherend, the storage modulus of the adhesive layer 2 at 25°C before photocuring is preferably 10 kPa or more, more preferably 15 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 adhesive layer 2 is irradiated with active light rays to photocure the adhesive layer. Examples of active light rays include ultraviolet light, visible light, infrared light, X-rays, α-rays, β-rays, and γ-rays. UV light is preferred as the active light rays 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 rays may be set appropriately depending on the composition and thickness of the adhesive layer. The adhesive layer 2 may be irradiated with active light rays 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 keep the adhesive strength before photocuring (initial adhesive strength) low and increase the adhesive strength of the adhesive after photocuring.

As described above, from the viewpoint of imparting a shock absorbing effect to the adhesive layer 2, the storage modulus of the adhesive layer 2 at 25°C after photocuring is preferably 140 kPa or less, more preferably 120 kPa or less, even more preferably 100 kPa or less, and may be 90 kPa, 80 kPa or less, or 70 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 80 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 the area to be reinforced is also easy.

After the adhesive layer is photocured, it exhibits high adhesion to the adherend, the reinforcing film is difficult to peel off from the device surface, and it has excellent adhesion reliability and high impact resistance. Therefore, even if an external force is suddenly applied during use of the completed device due to the device being dropped, a heavy object being placed on the device, or the device being hit by a flying object, the reinforcing film attached to the device can prevent damage to the device.

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 22, polymer C in samples 31 to 43, polymer D in samples 51 to 63, polymer F in samples 71 to 83, polymer A in samples 91 and 92, and polymer E 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

<Surface resistance>
The adhesive layer was photocured by irradiating ultraviolet light with an integrated light quantity of 4000 mJ/ ^cm2 from the film substrate side of the reinforcing film using an LED light source with a wavelength of 365 nm. The release liner was peeled off and removed from the surface of the adhesive layer after photocuring, and the probe of a resistivity meter ("Hirester UX MCP-HT800" manufactured by Nitto Seiko Analytec) was brought into contact with the surface of the adhesive layer in an environment of a temperature of 25°C and a relative humidity of 50%, and the surface resistance was measured under the conditions of an applied voltage of 1000 V and a voltage application time of 10 seconds.

<Surface free energy>
A 1.0 μL droplet of the probe liquid was dropped onto the surface of the photocured adhesive layer, and the contact angle was measured 500 milliseconds after the drop using an automatic contact angle calculator (manufactured by Kyowa Interface Science, "CA-V" model). Water and diiodomethane were used as the probe liquid, and the surface free energy γ of the adhesive layer was calculated using the Owens-Wendt equation from the measured surface free energies and contact angles of each.

<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 used an adhesive composition containing 100 parts by weight of polymer A and 30 parts by weight of a photocuring agent, had a surface free energy of the photocured adhesive layer of more than 20 mJ/ ^m2 and a resistance of the order of ¹⁰ Ω. The same was true for sample 96, which used polymer E. Sample 92, which contained 0.15 parts by weight of urethane acrylate as a photocuring agent, had a slightly higher adhesive strength of the photocured adhesive layer than sample 91, but other properties were not significantly different from those of sample 91.

Samples 21, 42, 62, and ⁸² , in which 30 parts by weight of a multifunctional acrylate and 0.15 parts by weight of a urethane acrylate were blended with polymers B, C, D, and E, had a surface free energy of the adhesive layer after photocuring of less than 20 mJ/m2 and a resistance of more than 1× ¹⁰ Ω, which was higher than that of samples 91, 92, and ^96. On the other hand, samples 22, 43, 63, and 83, in which the blending amount of the multifunctional acrylate was changed to 40 parts by weight, had a surface free energy of the adhesive layer after photocuring of less than 20 mJ/m2, but a resistance of the order ^{of 10} Ω.

In samples 91 and 92, it is believed that the adhesive layer had low resistance because polymer A, whose main constituent monomer is butyl acrylate, has a lower resistance than polymers B, C, D, and F, whose main constituent monomer is 2-ethylhexyl acrylate. In sample 96, it is believed that the adhesive layer had low resistance because polymer E contains a large amount of highly polar monomers as a constituent monomer component.

In addition, because polymer E contains NVP, a highly polar monomer containing nitrogen atoms, as a constituent monomer component, the adhesive strength of the pressure-sensitive adhesive layer before photocuring to the plasma-treated polyimide film in sample 96 was high. The adhesive strength of samples 71 to 85, which used polymer F, to the plasma-treated polyimide film was also more than twice as high as that 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.

Samples 22, 43, 63, and 83 are thought to have low resistance of the adhesive layer due to the large amount of light curing agent. A comparison of samples 5 and 16-22, which used polymer B, samples 32 and 37-43, which used polymer C, samples 52 and 57-63, which used polymer D, and samples 72 and 77-83, which used polymer F, showed a tendency for the surface free energy of the adhesive layer to increase as the amount of light curing agent increased.

These results show that by using a low-resistance acrylic-based polymer and reducing the amount of photocuring agent, the surface free energy of the adhesive layer can be reduced, resulting in high resistance.

In addition, a comparison of samples in which the amount of photocuring agent mixed in with the same base polymer was varied showed that the smaller the amount of photocuring agent mixed in, the less likely the adherend would be contaminated when the pre-light-curing adhesive layer was peeled off from the adherend. In addition to increasing resistance, a smaller amount of photocuring agent is preferable from the standpoint of contamination resistance as well.

The greater the amount of photocuring agent, the smaller the adhesive strength of the adhesive layer to the polyimide film before photocuring, the greater the rate of increase in adhesive strength due to photocuring, and the greater the adhesive strength of the adhesive layer after photocuring. In addition, the greater the amount of photocuring agent, the smaller the storage modulus of the adhesive layer before photocuring, the greater the rate of increase in storage modulus due to photocuring, and the greater the storage modulus of the adhesive layer after photocuring.

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.

These results show that by adjusting the amounts of photocuring agent and crosslinking agent, it is possible to adjust the adhesive strength and storage modulus of the pressure-sensitive adhesive layer before and after photocuring to a range suitable for use as a reinforcing film.

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 contained an ethylene oxide chain. TMPT, which does not contain an ethylene oxide chain, is highly compatible with acrylic polymers and does not easily form WBL, whereas multifunctional acrylates that contain ethylene oxide chains have lower compatibility with acrylic polymers than TMPT and are more likely to form WBL, which is thought to result in a lower initial adhesive strength and a higher rate of increase in adhesive strength due to light curing.

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, showed a larger increase in the storage modulus due to photocuring than samples 6 to 8, which used a bifunctional acrylate, and the storage modulus of the adhesive layer after photocuring was larger. In particular, sample 10 showed a more significant increase in the storage modulus due to photocuring than the other examples. As mentioned above, TMPT used as the photocuring agent in sample 10 is highly compatible with acrylic polymers and does not easily form WBL, so it is easily incorporated into the bulk part of the adhesive layer, and the photocuring agent cures uniformly throughout the thickness direction of the adhesive layer, which is thought to have caused a large increase in the storage modulus, which is a bulk property.

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 (15)

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,
A reinforcing film, wherein the pressure-sensitive adhesive layer has a surface resistance of 1×10 ¹⁵ Ω or more. The reinforcing film according to claim 1 , wherein the pressure-sensitive adhesive layer has a surface free energy of 20 mJ/m ² or less after photocuring. The reinforced film according to claim 1, wherein the photocurable composition contains 3 to 25 parts by weight of the photocuring agent per 100 parts by weight of the acrylic base polymer. 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 acrylic base polymer contains 50 to 99.5 parts by weight of a (meth)acrylic acid alkyl ester having an alkyl group with 6 to 20 carbon atoms per 100 parts by weight of the total monomer components. 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 pressure-sensitive adhesive layer has a shear storage modulus of 10 to 70 kPa at 25°C before photocuring. The reinforced film according to claim 1, wherein the pressure-sensitive adhesive layer has a shear storage modulus of 30 to 140 kPa at 25°C after photocuring. 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 device with a reinforcing film, the device having a reinforcing film attached to a surface thereof,
The reinforcing film includes a film substrate and a pressure-sensitive adhesive layer fixedly laminated on one main surface of the film substrate,
The pressure-sensitive adhesive layer is attached to a surface of a device,
the pressure-sensitive adhesive layer is made of a photocured product obtained by photocuring a photocurable pressure-sensitive adhesive composition including an acrylic-based polymer and a photocuring agent;
the acrylic base polymer contains, as a monomer unit, one or more monomer units selected from the group consisting of a hydroxyl group-containing monomer and a carboxyl group-containing monomer, and a crosslinked structure is introduced into the acrylic base polymer;
The surface resistance of the pressure-sensitive adhesive layer is 1×10 ¹⁵ Ω or more.
Device with reinforcement film. A method for producing a device with a reinforced film having a surface to which a reinforced film is bonded, comprising the steps of:
After the pressure-sensitive adhesive layer of the reinforcing film according to any one of claims 1 to 12 is temporarily attached to the surface of an adherend,
A method for producing a device with a reinforced film, comprising: irradiating the pressure-sensitive adhesive layer with active light rays to photocure the pressure-sensitive adhesive layer, thereby increasing the adhesive strength between the reinforced film and the adherend. A reinforcing method for bonding a reinforcing film to a surface of an adherend, comprising the steps of:
The pressure-sensitive adhesive layer of the reinforcing film according to any one of claims 1 to 12 is temporarily attached to a surface of an adherend,
The reinforcing method includes irradiating the pressure-sensitive adhesive layer with active light rays to photocure the pressure-sensitive adhesive layer, thereby increasing the adhesive strength between the reinforcing film and the adherend.

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