TW202508824A - Reinforcement film, device manufacturing method and reinforcement method - Google Patents

Abstract

The reinforcing film (10) of the present invention has an adhesive layer (2) containing a photocurable composition on one main surface of a film substrate (1), wherein the photocurable composition contains an acrylic base polymer having a crosslinked structure, a photocuring agent, and a photopolymerization initiator, and the reinforcing film (10) contains a multifunctional (meth)acrylate without a urethane bond and a urethane (meth)acrylate as a photocuring agent. The acrylic base polymer contains _C6-9 alkyl (meth)acrylate, _C10-20 alkyl (meth)acrylate, and one or more selected from the group consisting of hydroxyl-containing monomers and carboxyl-containing monomers as monomer components.

Description

Reinforcement film, device manufacturing method and reinforcement method

The present invention relates to a reinforcing film formed by fixing and laminating a film substrate and a photocurable adhesive layer. Furthermore, the present invention relates to a manufacturing method of a device having a reinforcing film attached to a surface, and a reinforcing method of fixing and laminating a reinforcing film on the surface of an adherend.

An adhesive film is sometimes attached to the surface of an optical device or electronic device such as a display for the purpose of protecting the surface or imparting impact resistance. Such an adhesive film usually has an adhesive layer fixedly laminated on the main surface of the film substrate and is attached to the device surface via the adhesive layer.

Before the device is assembled, processed, transported, etc., an adhesive film is temporarily adhered to the surface of the device or its components to prevent damage or destruction of the adherend. Patent document 1 discloses a reinforcing film having a photocurable adhesive layer on a film substrate comprising an acrylic base polymer, a multifunctional (meth)acrylate as a photocuring agent, and a photopolymerization initiator.

The reinforcing film has low adhesion immediately after being bonded to the adherend, so it is easy to peel off from the adherend. Therefore, secondary processing can be performed on the adherend, and the reinforcing film can also be selectively peeled off and removed from the part of the adherend that does not need reinforcement. The adhesive of the reinforcing film is firmly bonded to the adherend by light curing, so that the film substrate is permanently bonded to the surface of the adherend, and can be used as a reinforcing material for surface protection of the device. [Prior technical literature] [Patent literature]

[Patent Document 1] International Publication No. 2022/050009

[The problem the invention is trying to solve]

The reinforcement film attached to the surface of the device is required to have a high adhesion after the adhesive layer is photocured, and not to peel off from the adherend even when the device is deformed or the temperature changes due to bending. Patent document 1 states that by using an acrylic polymer with a low glass transition temperature as the base polymer constituting the photocurable adhesive, the adhesive layer has a lower storage elastic modulus and a higher stress-strain relaxation, so that the peeling of the adhesive layer at the bent portion of the device is suppressed, and the bonding reliability is excellent.

However, if the reinforcing film disclosed in Patent Document 1 is bonded to a flexible device using a film substrate and a temperature cycle test is performed in which the temperature changes from low temperature to high temperature and from high temperature to low temperature, the device may warp or the reinforcing film may peel off. In view of this topic, the purpose of the present invention is to provide a reinforcing film that is not prone to warping of the adherend or peeling off from the adherend even when the temperature changes repeatedly between low temperature and high temperature. [Technical means for solving the problem]

The reinforcing film of the present invention has an adhesive layer fixedly laminated on one main surface of the film substrate. The adhesive layer contains a photocurable composition, which contains an acrylic base polymer with a cross-linked structure, a photocuring agent and a photopolymerization initiator.

The acrylic acid-based polymer comprises C _6-9 alkyl (meth)acrylate having an alkyl group with 6 to 9 carbon atoms, C _10-20 alkyl (meth)acrylate having an alkyl group with 10 to 20 carbon atoms, and one or more selected from the group consisting of hydroxyl-containing monomers and carboxyl-containing monomers as monomer components. The amount of C _6-9 alkyl (meth)acrylate is 40 parts by weight or more relative to 100 parts by weight of the total amount of the monomer components. The amount of C _10-20 alkyl (meth)acrylate can be 1 to 40 parts by weight relative to 100 parts by weight of the total amount of the monomer components.

A crosslinking structure is introduced into the acrylic base polymer. It is preferred to use 0.05 to 1.0 parts by weight of a crosslinking agent relative to 100 parts by weight of the polymer to introduce the crosslinking structure. Examples of the crosslinking agent include isocyanate-based crosslinking agents and epoxy-based crosslinking agents.

The photocurable composition constituting the adhesive layer includes a multifunctional (meth)acrylate having an alkylene oxide chain and no urethane bond, and urethane (meth)acrylate as a photocuring agent. The amount of the multifunctional (meth)acrylate having an alkylene oxide chain and no urethane bond is preferably 2 to 25 parts by weight relative to 100 parts by weight of the acrylic base polymer. The amount of the urethane (meth)acrylate is preferably 0.03 to 5 parts by weight relative to 100 parts by weight of the acrylic base polymer.

The functional group equivalent weight of the (meth)acryl group of the multifunctional (meth)acrylate having an alkylene oxide chain and not having a urethane bond may be 80 to 500 g/eq. The functional group equivalent weight of the (meth)acryl group of the urethane (meth)acrylate may be 80 to 5000 g/eq. The urethane (meth)acrylate may have 4 or more (meth)acryl groups in one molecule.

The adhesive layer has a bonding force _F0 of 0.6 N/25 mm or less at a temperature of 25°C before light curing. The adhesive layer has a bonding force _F1 of 3.0 N/25 mm or more at a temperature of 25°C after light curing, and a bonding force _F2 of 1.0 N/25 mm or more at a temperature of 85°C.

The shear storage modulus of the adhesive layer after light curing at a temperature of 25°C is preferably less than 100 kPa, and the shear storage modulus of the adhesive layer at a temperature of -40°C is preferably less than 1000 kPa.

After the reinforcing film is temporarily attached to the surface of the device as the adherend, the adhesive layer is light-cured to obtain the device with the reinforcing film. The device can be a bendable soft device, and the adherend of the reinforcing film (the object of reinforcement) can be a bendable image display element.

After the reinforcing film is temporarily adhered to the adherend and before the adhesive layer is photocured, the reinforcing film temporarily adhered to the adherend can be cut to remove the reinforcing film from a portion of the adherend (non-reinforcement target area). [Effect of the invention]

The adhesive layer of the reinforcing film of the present invention includes a photocurable composition. After the reinforcing film of the present invention is attached to an adherend, the adhesive layer is photocured to improve the adhesion to the adherend. Before photocuring, the adhesive layer has a relatively low adhesion to the adherend and is easily peeled off from the adherend. The adhesive after photocuring has a relatively high adhesion even at high temperatures, and the change in shear storage elastic modulus caused by temperature changes is relatively small. Therefore, even when the temperature changes repeatedly between low and high temperatures, it is not easy for the adherend to warp or for the reinforcing film to peel off from the adherend.

FIG1 is a cross-sectional view showing an 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 containing a photocurable composition, which is cured by irradiation with active light such as ultraviolet rays, thereby improving the bonding strength with the adherend.

Fig. 2 is a cross-sectional view showing a reinforcing film with a peeling pad 5 temporarily adhered to the main surface of the adhesive layer 2. Fig. 3 is a cross-sectional view showing a state where a reinforcing film 10 is attached to the surface of a device 20.

The peeling pad 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 is in a state of temporarily adhering the reinforcing film 10 (adhesive layer 2) to the device 20 before light curing. By light curing the adhesive layer 2, the bonding force at the interface between the device 20 and the adhesive layer 2 is improved, so that the device 20 and the reinforcing film 10 are fixed.

"Fixed" means that two layers of the stack are firmly attached and cannot or are difficult to peel off at the interface. "Temporary adhesion" means that the adhesion between two layers of the stack is relatively weak and can be easily peeled off at the interface.

In the reinforcement film shown in FIG2 , the film substrate 1 is fixed to the adhesive layer 2, and the peeling pad 5 is temporarily adhered to the adhesive layer 2. If the film substrate 1 and the peeling pad 5 are peeled off, peeling occurs at the interface between the adhesive layer 2 and the peeling pad 5, thereby maintaining the state in which the adhesive layer 2 is fixed to the film substrate 1. No adhesive remains on the peeling pad 5 after peeling.

Regarding the device with the reinforcement film 10 attached as shown in FIG3, before the light curing of the adhesive layer 2, the device 20 and the adhesive layer 2 are temporarily adhered. 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, thereby maintaining the state in which the adhesive layer 2 is fixed to the film substrate 1. No adhesive remains on the device 20, so it is easy to perform secondary processing. After the adhesive layer 2 is photocured, the adhesion between the adhesive layer 2 and the device 20 is enhanced, so it is not easy to peel the reinforcing film 10 from the device 20. If the two are peeled off, there is a possibility that the adhesive layer 2 will be coagulated and destroyed.

[Construction of the reinforcing film] ＜Film substrate＞ The film substrate 1 uses a plastic film. In order to fix the film substrate 1 and the adhesive layer 2, it is preferred not to perform a release treatment on the surface of the film substrate 1 to which the adhesive layer 2 is attached.

The thickness of the film substrate 1 is, for example, about 4 to 500 μm. From the viewpoint of reinforcing the device by imparting rigidity or buffering impact, the thickness of the film substrate 1 is preferably 12 μm or more, more preferably 30 μm or more, and further preferably 45 μm or more. From the viewpoint of making the reinforcing film flexible and improving the handling property, the thickness of the film substrate 1 is preferably 300 μm or less, and more preferably 200 μm or less. In order to make the reinforcing film flexible so that it can be folded, the thickness of the film substrate 1 is preferably 125 μm or less, and more preferably 100 μm or less. From the perspective of both mechanical strength and flexibility, the compressive strength of the film substrate 1 is preferably 100-3000 kg/cm ² , more preferably 200-2900 kg/cm ² , further preferably 300-2800 kg/cm ² , and particularly preferably 400-2700 kg/cm ² .

As the plastic material constituting the film substrate 1, there can be cited polyester resins, polyolefin resins, cyclic polyolefin resins, polyamide resins, polyimide resins, polyetheretherketone resins, etc. In the reinforcing film for optical devices such as displays, the film substrate 1 is preferably a transparent film. In addition, when the adhesive layer 2 is photocured by irradiating active light from the side of the film substrate 1, the film substrate 1 is preferably transparent to the active light used for curing the adhesive layer. In terms of both mechanical strength and transparency, polyester resins such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate are preferably used. When the adhesive layer is cured by irradiating the adherend with active light from the adherend side, the adherend only needs to be transparent to the active light, and the film substrate 1 may not be transparent to the active light.

Functional coatings such as an easy-adhesion layer, an easy-slip layer, a release layer, an antistatic layer, a hard coating layer, and an antireflection layer may also be provided on the surface of the film substrate 1. Furthermore, as described above, in order to fix the film substrate 1 and the adhesive layer 2, it is preferred not to provide a release layer on the surface of the film substrate 1 to which the adhesive layer 2 is attached.

<Adhesive layer> The adhesive layer 2 fixedly laminated on the film substrate 1 includes a photocurable composition, which includes a base polymer, a photocuring agent, and a photopolymerization initiator. The adhesive layer 2 has a weak adhesion to the adherend such as the device or device parts before photocuring, so it is easy to peel off. The adhesive layer 2 has an enhanced adhesion to the adherend by photocuring, so even when the device is used, the reinforcement film is not easy to peel off from the device surface, and the adhesion reliability is excellent.

Photocurable adhesives hardly cure in normal storage environments, but cure by exposure to active light such as ultraviolet rays. Therefore, the reinforcing film of the present invention has the following advantages: the curing time of the adhesive layer 2 can be set arbitrarily, and the preparation time of the steps can be flexibly responded to.

The thickness of the adhesive layer 2 is, for example, about 1 to 300 μm. The thicker the adhesive layer 2 is, the better the adhesion to the adherend. On the other hand, when the thickness of the adhesive layer 2 is too large, the fluidity before photocuring is higher, making it difficult to handle. Therefore, the thickness of the adhesive layer 2 is preferably 3 to 100 μm, more preferably 5 to 50 μm, further preferably 6 to 40 μm, and particularly preferably 8 to 30 μm. From the perspective of thinning, the thickness of the adhesive layer 2 may be less than 25 μm, less than 20 μm, or less than 18 μm.

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 further preferably 90% or more. The haze of the adhesive layer 2 is preferably 2% or less, more preferably 1% or less, further preferably 0.7% or less, and particularly preferably 0.5% or less.

Hereinafter, preferred forms of the components of the photocurable composition constituting the adhesive layer 2 will be described in turn.

(Base polymer) The base polymer is the main component of the adhesive composition and is the main component that determines the adhesion of the adhesive layer. The adhesive composition contains an acrylic polymer as the base polymer. Preferably, 50% by weight or more of the adhesive composition is an acrylic polymer. The acrylic base polymer has excellent optical transparency and adhesion, and the adhesion is easy to control.

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

As the alkyl (meth)acrylate, preferably used is an alkyl (meth)acrylate having an alkyl group with a carbon number of 1 to 20. The alkyl group of the alkyl (meth)acrylate may have a branched chain or a cyclic alkyl group (alicyclic alkyl group).

Specific examples of the alkyl (meth)acrylate having a chain alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl (meth)acrylate, tert-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, nonyl (meth)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, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, and the like.

Specific examples of the alkyl (meth)acrylate having an alicyclic alkyl group include: cycloalkyl (meth)acrylates such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate; (meth)acrylates having a dicyclic aliphatic hydrocarbon ring such as isobutyl (meth)acrylate; and (meth)acrylates having a tricyclic or more aliphatic hydrocarbon ring such as dicyclopentyl (meth)acrylate, dicyclopentyloxyethyl (meth)acrylate, tricyclopentyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate. The alkyl (meth)acrylate having an alicyclic alkyl group may have a substituent on the ring such as 3,3,5-trimethylcyclohexyl (meth)acrylate. The alkyl (meth)acrylate having an alicyclic alkyl group may be a (meth)acrylate having a condensed ring of an alicyclic structure and a ring structure having an unsaturated bond such as dicyclopentenyl (meth)acrylate.

The content of the alkyl (meth)acrylate 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, 90 parts by weight or more, or 95 parts by weight or more, relative to 100 parts by weight of the total monomer components constituting the polymer.

From the viewpoint of lowering the glass transition temperature (Tg) of the acrylic-based polymer and improving the adhesive force in a wider temperature range, the alkyl group of the (meth)acrylic acid alkyl ester is preferably a chain alkyl group. The chain alkyl group may be a straight chain or a branched chain.

In the present invention, the acrylic base polymer comprises, as monomer components, C _6-9 alkyl (meth)acrylate having an alkyl group with 6 to 9 carbon atoms and C _10-20 alkyl (meth)acrylate having an alkyl group with 10 to 20 carbon atoms.

Since the glass transition temperature (Tg) of the homopolymer of _C6-9 alkyl (meth)acrylate is relatively low, the Tg of the acrylic base polymer is lowered by including _C6-9 alkyl (meth)acrylate as a monomer component. From the viewpoint of lowering Tg, among _C6-9 alkyl (meth)acrylate, those having a homopolymer glass transition temperature of -50°C or less are preferred.

Specific examples of (meth)acrylic acid _C6-9 alkyl esters having a homopolymer glass transition temperature of -50°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 them, 2-ethylhexyl acrylate and n-octyl acrylate are preferred in terms of having a lower Tg and being able to achieve a low storage elastic modulus of the adhesive.

With respect to the acrylic-based polymer, it is preferred that the monomer with the largest content (main monomer) among the constituent monomers is C _6-9 alkyl (meth)acrylate. The amount of C _6-9 alkyl (meth)acrylate is preferably 40 parts by weight or more, more preferably 50 parts by weight or more, and may also be 55 parts by weight or more, 60 parts by weight or more, or 65 parts by weight or more, relative to 100 parts by weight of the total amount of the constituent monomer components of the polymer.

The homopolymer of C _10-20 alkyl (meth)acrylate has a temperature range (flat zone range) in which the temperature dependence of viscoelasticity is small at a temperature higher than Tg. Therefore, by making the base polymer contain C _10-20 alkyl (meth)acrylate as a monomer component, the temperature dependence of the storage elastic modulus becomes small, and there is a situation that the warping of the adherend or the peeling of the reinforcing film from the adherend caused by temperature changes is suppressed.

Since the temperature range of the plateau region is wide and the storage elastic modulus in the plateau region is small, among the C _10-20 chain alkyl (meth)acrylates, C _12-18 alkyl (meth)acrylates are preferred. Among them, lauryl (meth)acrylate and isostearyl (meth)acrylate are preferred, and lauryl acrylate (lauryl acrylate) is particularly preferred.

The amount of C _10-20 alkyl (meth)acrylate is preferably 1 to 40 parts by weight, more preferably 2 to 30 parts by weight, further preferably 3 to 25 parts by weight, and may also be 4 to 20 parts by weight or 5 to 15 parts by weight, relative to 100 parts by weight of the total monomer components of the polymer.

The acrylic base polymer may also contain (meth)acrylic acid alkyl esters other than (meth)acrylic acid _C6-9 alkyl esters and (meth)acrylic acid _C10-20 alkyl esters as monomer components. As (meth)acrylic acid alkyl esters other than the above, (meth)acrylic acid _C1-5 alkyl esters such as butyl acrylate are preferred.

In addition to containing (meth) alkyl acrylate as a constituent monomer component, the acrylic base polymer also contains a hydroxyl-containing monomer and/or a carboxyl-containing monomer as a constituent monomer component. The hydroxyl group or carboxyl group of the base polymer is a reaction point with the following crosslinking agent. For example, when using an isocyanate crosslinking agent, it is preferred to contain a hydroxyl-containing monomer as a monomer component of the base polymer. When using an epoxy crosslinking agent, it is preferred to contain a carboxyl-containing monomer as a monomer component of the base polymer. By introducing a crosslinking structure into the base polymer, there is a tendency that the cohesive force is improved, and the releasability of the adhesive layer 2 and the adherend before light curing is improved.

Examples of the hydroxyl group-containing monomer 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. Among them, 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate are preferred in terms of their greater contribution to improving the adhesion of the adhesive after light curing.

Examples of carboxyl group-containing monomers include (meth)acrylic acid, 2-carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, etc. Among them, acrylic acid and methacrylic acid are preferred, and acrylic acid is particularly preferred, in terms of improving the cohesiveness of the adhesive and thereby increasing the adhesion and adhesion retention.

From the viewpoint of appropriately introducing a crosslinking structure formed by a crosslinking agent such as an isocyanate crosslinking agent or an epoxy crosslinking agent, the total amount of the hydroxyl group-containing monomer and the carboxyl group-containing monomer is preferably 0.3 parts by weight or more, more preferably 0.5 parts by weight or more, and may also be 0.7 parts by weight or more or 1.0 parts by weight or more, relative to 100 parts by weight of the total amount of the constituent monomer components of the acrylic base polymer.

Since hydroxyl or carboxyl groups are highly polar, when the amount of hydroxyl-containing monomers or carboxyl-containing monomers in the monomer components of the acrylic-based polymer is relatively large, the cohesive force of the polymer is increased, and the storage modulus of the adhesive layer after light curing tends to be larger. From the viewpoint of reducing the storage modulus of the adhesive layer 2, the total amount of hydroxyl-containing monomers and carboxyl-containing monomers is preferably 20 parts by weight or less, more preferably 15 parts by weight or less, and further preferably 10 parts by weight or less, and may also be 7 parts by weight or less or 5 parts by weight or less, relative to 100 parts by weight of the total amount of the monomer components of the acrylic-based polymer.

The acrylic base polymer contains nitrogen-containing monomers as constituent monomer components. Since nitrogen-containing monomers are highly polar, have high cohesive force, and have a high glass transition temperature of the homopolymer, the adhesive layer after light curing tends to have improved adhesion at high temperatures by making the acrylic base polymer contain nitrogen-containing monomers as monomer components.

Examples of nitrogen atom-containing monomers include N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperidone, vinylpyrrolidone, vinylpyrrole, vinylimidazole, vinyloxazole, vinyloxaline, N-acryloyloxaline, N-vinylcarboxamides, and N-vinylcaprolactam. Among them, N-vinylpyrrolidone is particularly preferred because it has a high cohesive force and contributes greatly to improving the adhesive force at high temperatures.

When the acrylic base polymer contains a nitrogen-containing monomer as a monomer component, from the viewpoint of improving the adhesion of the adhesive layer at high temperature after light curing, the amount of the nitrogen-containing monomer is preferably 0.3 parts by weight or more, more preferably 0.5 parts by weight or more, and can also be 0.7 parts by weight or more relative to 100 parts by weight of the total amount of the monomer components constituting the polymer. Even a small amount of the nitrogen-containing monomer helps to improve the adhesion at high temperature.

When the amount of nitrogen-containing monomers in the monomer components of the acrylic base polymer is relatively high, the cohesive force of the base polymer becomes higher, and the glass transition temperature of the adhesive layer tends to be higher. As a result, the storage elastic modulus of the adhesive layer after light curing tends to be larger. In addition, when the amount of nitrogen-containing monomers is relatively high, the adhesive layer 2 before light curing has a higher adhesion force, and the reinforcing film tends to be difficult to peel off from the adherend. This tendency is particularly significant for the adherend that has been subjected to surface activation treatment such as plasma treatment. From the viewpoint of reducing the adhesion of the adhesive before photocuring and reducing the storage elastic modulus of the adhesive after photocuring, the amount of nitrogen atom-containing monomers is preferably 10 parts by weight or less, more preferably 7 parts by weight or less, and may be 5 parts by weight or less or 3 parts by weight or less, relative to 100 parts by weight of the total amount of monomer components constituting the acrylic base polymer.

The acrylic base polymer may also contain monomer components other than the above. For example, the acrylic base polymer may also contain vinyl ester monomers, aromatic vinyl monomers, epoxy-containing monomers, vinyl ether monomers, sulfone-containing monomers, phosphoric acid-containing monomers, acid anhydride-containing monomers, etc. as monomer components.

From the viewpoint of making the adhesive have excellent adhesion, the glass transition temperature of the acrylic base polymer is preferably -30°C or lower, more preferably -40°C or lower, further preferably -50°C or lower, and may be -55°C or lower or -60°C or lower. The glass transition temperature of the acrylic base polymer is usually -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 temperature) at which the loss tangent tanδ in the viscoelastic measurement is maximum. The theoretical glass transition temperature can also be used instead of the glass transition temperature obtained by the viscoelastic measurement. The theoretical Tg is calculated by the following Fox formula based on the glass transition temperature Tg _i of the homopolymer of the monomer components constituting the polymer and the weight fraction _Wi of each monomer component. 1/Tg = Σ( _Wi /Tg _i )

Tg is the theoretical glass transition temperature of the polymer (unit: K), _Wi is the weight fraction of monomer component i (copolymerization ratio based on weight), and _Tgi is the glass transition temperature of the homopolymer of monomer component i (unit: K). The glass transition temperature of the homopolymer can be the value recorded in the Polymer Handbook, 3rd edition (John Wiley & Sons, Inc., 1989). The glass transition temperature of the homopolymer of the monomer not recorded in the above literature can be the peak temperature of tanδ obtained by dynamic viscoelastic measurement.

The above-mentioned monomer components are polymerized by various known methods such as solution polymerization, emulsion polymerization, and bulk polymerization, thereby obtaining an acrylic polymer as a base polymer. From the perspective of the balance of the adhesive's adhesion, retention, and other properties, or the cost, solution polymerization is preferred. Ethyl acetate, toluene, and the like are used as solvents for solution polymerization. The solution concentration is usually around 20 to 80% by weight. The polymerization initiator used for solution polymerization can be azo, peroxide, and other known ones. In order to adjust the molecular weight, a chain transfer agent can also be used. The reaction temperature is usually around 50 to 80°C, and the reaction time is usually around 1 to 8 hours.

The weight average molecular weight of the acrylic base polymer is preferably 200,000 or more, more preferably 300,000 or more, and further preferably 500,000 or more. The larger the weight average molecular weight of the base polymer, the higher the adhesion at high temperature tends to be. The weight average molecular weight of the acrylic base polymer may also be 1 million or more, 1.3 million or more, 1.5 million or more, 1.6 million or more, or 1.7 million or more.

If the weight average molecular weight of the base polymer is too large, the storage elastic modulus of the adhesive will increase, and the adhesion and flexibility will tend to decrease. Therefore, the weight average molecular weight of the acrylic base polymer is preferably 3 million or less, more preferably 2.5 million or less, and may also be 2.2 million or less or 2 million or less. When a cross-linked structure is introduced into the base polymer, the molecular weight of the base polymer refers to the molecular weight before the cross-linked structure is introduced.

(Crosslinking agent) From the perspective of giving the adhesive an appropriate cohesive force, showing adhesion, and ensuring the peelability of the adhesive layer from the adherend before light curing, it is better to introduce a crosslinking structure into the base polymer. For example, a crosslinking agent is added to a solution after the base polymer is polymerized, and heating is performed as needed to introduce a crosslinking structure. The crosslinking agent has two or more crosslinking functional groups in one molecule. The crosslinking agent may also have three or more crosslinking functional groups in one molecule.

Examples of the crosslinking agent include isocyanate crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, and metal chelate crosslinking agents. These crosslinking agents react with functional groups such as hydroxyl groups or carboxyl groups introduced into the acrylic base polymer to form a crosslinking structure. Isocyanate crosslinking agents and epoxy crosslinking agents are preferred because they have a high reactivity with hydroxyl groups or carboxyl groups of the acrylic base polymer and are easy to introduce into the crosslinking structure.

As the isocyanate crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule is used. The isocyanate crosslinking agent may also have three or more isocyanate groups in one molecule. Examples of the isocyanate crosslinking agent include low-order aliphatic polyisocyanates such as butyl diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentyl diisocyanate, cyclohexyl diisocyanate, and isophorone diisocyanate; aromatic isocyanates such as 2,4-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate; trihydroxymethylpropane/toluene diisocyanate trimer adducts (e.g., "Takenate D101E" manufactured by Mitsui Chemicals), trihydroxymethylpropane/hexamethylene diisocyanate trimer adducts (e.g., "Coronate D101E" manufactured by Tosoh Corporation); Isocyanate adducts such as "HL" produced by Mitsui Chemicals), trihydroxymethylpropane adducts of xylylene diisocyanate (e.g. "Takenate D110N" produced by Mitsui Chemicals), isocyanurate of hexamethylene diisocyanate (e.g. "Coronate HX" produced by Tosoh), etc. As the isocyanate crosslinking agent, isocyanate compounds having a biuret group (e.g. "Duranate 24A-100" produced by Asahi Kasei) or isocyanate compounds having an allophanate group can also be used.

As the epoxy crosslinking agent, a polyfunctional epoxy compound having two or more epoxy groups in one molecule is used. The epoxy crosslinking agent may also be one having 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 the epoxy crosslinking agent include: N,N,N',N'-tetraglycidyl meta-xylylenediamine, diglycidylaniline, 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-hexanediol diglycidyl ether, neopentyl glycol diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and the like. Oleyl ether, sorbitol polyglycidyl ether, glycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, polyglycerol polyglycidyl ether, sorbitan polyglycidyl ether, trihydroxymethylpropane polyglycidyl ether, adipate diglycidyl, phthalate diglycidyl, isocyanuric acid triglycidyl-tri(2-hydroxyethyl) ester, resorcinol diglycidyl ether, bisphenol S-diglycidyl ether, etc. As the epoxy-based crosslinking agent, commercially available products such as "Denacol" manufactured by Nagase Chemicals, "Tetrad X" and "Tetrad C" manufactured by Mitsubishi Gas Chemicals, etc. can also be used.

The amount of crosslinking agent used can be appropriately adjusted according to the composition or molecular weight of the acrylic base polymer, and is preferably about 0.01 to 3 parts by weight relative to 100 parts by weight of the acrylic base polymer. The more the amount of crosslinking agent, the higher the crosslinking density of the base polymer, the more appropriate hardness is given to the adhesive layer before light curing, the larger the storage elastic modulus, and the tendency to suppress the paste residue on the adherend when the reinforcing film is peeled off from the adherend. On the other hand, if the amount of crosslinking agent is too much, the adhesion of the adhesive layer after light curing tends to decrease, especially the adhesion at high temperature tends to decrease. Furthermore, when the amount of the crosslinking agent is large, the storage elastic modulus of the adhesive layer after light curing tends to increase at low temperatures, which may cause the adherend to warp due to temperature changes or the reinforcing film to peel off from the adherend due to temperature changes. From the viewpoint of achieving both low adhesion (easy peelability) of the adhesive layer before photocuring and high adhesion of the adhesive layer after photocuring, and reducing the storage elastic modulus, the amount of the crosslinking agent is preferably 0.05 to 1.0 part by weight, further preferably 0.1 to 0.7 part by weight, and may also be 0.15 to 0.6 part by weight, 0.2 to 0.5 part by weight, or 0.25 to 0.4 part by weight, relative to 100 parts by weight of the acrylic base polymer.

(Photocuring agent) The adhesive composition constituting the adhesive layer 2 contains, in addition to the base polymer, a compound having two or more photopolymerizable functional groups in one molecule as a photocuring agent. The adhesive composition containing the photocuring agent has photocuring properties, and if it is photocured after being attached to an adherend, the adhesion to the adherend is improved.

From the perspective 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 a polymerizable group based on a photo-free radical reaction. As the photocuring agent, a compound having two or more ethylenically unsaturated bonds in one molecule is preferred. From the perspective of showing a moderate degree of compatibility with the acrylic base polymer, a multifunctional (meth)acrylate is preferred.

In the present invention, the photocuring agent uses a multifunctional (meth)acrylate without a urethane bond and a multifunctional (meth)acrylate with a urethane bond. That is, the photocurable composition constituting the adhesive layer 2 includes a multifunctional (meth)acrylate without a urethane bond and a multifunctional (meth)acrylate with a urethane bond. Hereinafter, the multifunctional (meth)acrylate with a urethane bond is recorded as "urethane (meth)acrylate". In addition, the multifunctional (meth)acrylate without a urethane bond is sometimes recorded only as "multifunctional (meth)acrylate".

Examples of the multifunctional (meth)acrylates without a urethane bond include compounds having (meth)acryloyl groups at both ends of the polyoxyalkylene chain, such as polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate; bisphenol A di(meth)acrylate, alkylene glycol di(meth)acrylate, glycerol di(meth)acrylate, glycerol tri(meth)acrylate, and tris(meth)acrylate. Esters of polyols and (meth)acrylic acid such as cyclodecane dimethanol di(meth)acrylate, isocyanuric acid tri(meth)acrylate, trihydroxymethylpropane tri(meth)acrylate, di-trihydroxymethylpropane tetra(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, etc.; epoxy (meth)acrylate, etc.

The polyfunctional (meth)acrylate without a urethane bond may be an ester of an alkylene oxide-modified polyol and (meth)acrylic acid. Examples of the ester of an alkylene oxide-modified polyol and (meth)acrylic acid include bisphenol A alkylene oxide-modified di(meth)acrylate, isocyanuric acid alkylene oxide-modified tri(meth)acrylate, trihydroxymethylpropane alkylene oxide-modified tri(meth)acrylate, pentaerythritol alkylene oxide-modified di(meth)acrylate, pentaerythritol alkylene oxide-modified tri(meth)acrylate, dipentaerythritol alkylene oxide-modified poly(meth)acrylate, and the like.

Among the above, from the viewpoint of reducing the adhesion of the adhesive layer before light curing to facilitate peeling and improving the adhesion of the adhesive layer after light curing, a multifunctional (meth)acrylate having an alkylene oxide chain is preferred. Examples of the multifunctional (meth)acrylate having an alkylene oxide chain include compounds having (meth)acryloyl groups at both ends of the polyalkylene oxide chain such as polyethylene glycol di(meth)acrylate and polypropylene glycol di(meth)acrylate, and esters of polyols modified with alkylene oxide and (meth)acrylic acid.

In the multifunctional (meth)acrylate containing an alkylene oxide chain, the alkylene oxide is preferably (poly)ethylene oxide or (poly)propylene oxide, and is particularly preferably (poly)ethylene oxide. The chain length n of the alkylene oxide (the number of repeating units of the alkylene oxide) is about 1 to 15. When a plurality of 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-based polymer can be adjusted.

The alkylene oxide is preferably (poly)ethylene oxide or (poly)propylene oxide, and the chain length (number of repeating units: n) of the alkylene oxide is preferably about 1 to 15. When a plurality of 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-based polymer can be adjusted.

As the photocuring agent, two or more multifunctional (meth)acrylates without urethane bonds may be used in combination. For example, a multifunctional (meth)acrylate with relatively low compatibility with an acrylic base polymer and a multifunctional (meth)acrylate with relatively high compatibility with an acrylic base polymer may be used in combination. For example, a multifunctional (meth)acrylate without an alkylene oxide chain has higher compatibility with an acrylic base polymer than a multifunctional (meth)acrylate with an alkylene oxide chain.

From the viewpoint of compatibility with acrylic-based polymers, the molecular weight of the multifunctional (meth)acrylate used as a 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 both compatibility with acrylic-based polymers and improved adhesion after photocuring, the functional group equivalent (g/eq) of the multifunctional (meth)acrylate is preferably 500 or less, more preferably 400 or less, further preferably 300 or less, particularly preferably 200 or less, and may be 180 or less or 160 or less.

On the other hand, if the functional group equivalent weight of the multifunctional (meth)acrylate is too small, the crosslinking point density of the adhesive layer after light curing may increase, and the adhesion may decrease. Therefore, the functional group equivalent weight of the multifunctional (meth)acrylate is preferably 80 or more, more preferably 100 or more, and may also be 120 or more or 130 or more.

By including a multifunctional (meth)acrylate having an alkylene oxide chain and a urethane (meth)acrylate as a photocuring agent, the adhesion (initial adhesion) of the adhesive layer before photocuring tends to be reduced. In addition, by including a urethane (meth)acrylate as a photocuring agent, the adhesion, especially the adhesion at high temperature, can be improved without excessively increasing the storage elastic modulus of the adhesive layer after photocuring.

Urethane (meth)acrylate is a compound having one or more urethane bonds and two or more (meth)acrylic groups in one molecule, preferably two or more urethane bonds in one molecule. Urethane (meth)acrylate having two or more urethane bonds can be obtained, for example, by the reaction of polyisocyanate with a (meth)acrylic compound having a hydroxyl group, wherein the isocyanate group of the polyisocyanate is bonded with the hydroxyl group of the (meth)acrylic compound to form a urethane bond.

The polyisocyanate may be any one of an aromatic polyisocyanate, an alicyclic polyisocyanate, and an alicyclic polyisocyanate. Among them, aromatic polyisocyanates and aliphatic polyisocyanates are preferred. As the aromatic polyisocyanate, toluene diisocyanate (TDI) is particularly preferred. Toluene diisocyanate may be any one of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, or a mixture thereof. As the aliphatic polyisocyanate, hexamethylene diisocyanate (HDI) is particularly preferred.

The polyisocyanate may be a trifunctional isocyanate having a cyanuric acid skeleton. Examples of the trifunctional isocyanate having a cyanuric acid skeleton include TDI trimer and HDI trimer. The polyisocyanate may also have a biuret group or an allophanate group.

The polyisocyanate may be an isocyanate-terminated urethane prepolymer obtained by reacting a polyol with a polyisocyanate. The isocyanate-terminated urethane prepolymer may be a high molecular weight product obtained by reacting a high molecular weight polyol such as a polyester polyol, a polycarbonate polyol, or a polyether polyol with a polyisocyanate.

Examples of the (meth)acrylic compound having a hydroxyl group include compounds having one hydroxyl group and one (meth)acryl group, such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, hydroxymethacrylamide, hydroxyethylacrylamide, and the like; and compounds having one hydroxyl group and two or more (meth)acryl groups, such as pentaerythritol tri(meth)acrylate, dipentaerythritol penta(meth)acrylate, trihydroxymethylpropane di(meth)acrylate, and isocyanuric acid di(meth)acrylate.

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

The urethane (meth)acrylate obtained by the reaction of a diisocyanate with a (meth)acrylic compound having one hydroxyl group and two or more (meth)acrylic groups in one molecule has two urethane bonds and four or more (meth)acrylic groups in one molecule. The number of (meth)acrylic groups in the urethane (meth)acrylate may be 6 or more or 8 or less, or 12 or less or 10 or less.

As the urethane (meth)acrylate, commercially available products such as Kyoeisha Chemical, Shin-Nakamura Chemical, Negami Industry, Mitsubishi Chemical, DAICEL-ALLNEX, and Resonac can be used.

From the viewpoint of adjusting the compatibility with the acrylic base polymer and the adhesion of the adhesive layer, the molecular weight of the urethane (meth) acrylate is preferably 300-10000, more preferably 500-2000, and may be 600-1500 or 700-1200. From the same viewpoint, the functional group equivalent (g/eq) of the (meth) acryl group of the urethane (meth) acrylate is preferably 80-5000, more preferably 90-1000, further preferably 100-300, and may be 110-250 or 120-200.

The content of the photocuring agent in the adhesive composition (the total of the multifunctional (meth)acrylate without urethane bond and the urethane (meth)acrylate) is preferably 2 parts by weight or more, more preferably 3 parts by weight or more, further preferably 5 parts by weight or more, and may also be 7 parts by weight or more, 8 parts by weight or more, or 10 parts by weight or more, relative to 100 parts by weight of the acrylic base polymer. By setting the amount of the photocuring agent to the above range, a reinforcing film can be obtained that is easily peeled off from the adherend before photocuring and can be firmly bonded to the adherend by photocuring.

The more photocuring agent is used, the smaller the adhesion between the adhesive and the adherend before photocuring, and the better the peeling property. On the other hand, when the amount of photocuring agent is too much, the viscosity of the adhesive is low, and the adhesion or flexibility tends to be insufficient. In addition, the more photocuring agent is used, the greater the storage elastic modulus of the adhesive layer after photocuring, especially the storage elastic modulus at low temperature, and the reinforcing film and the adherend with the reinforcing film are more likely to warp due to temperature changes. Furthermore, when the amount of the photocuring agent is too much, the photocuring agent is likely to seep out, and when the reinforcing film is peeled off from the adherend, the seeped components are transferred to the adherend and become the cause of contamination. Therefore, the content of the photocuring agent is preferably 30 parts by weight or less, more preferably 25 parts by weight or less, and may also be 20 parts by weight or less, 15 parts by weight or less, 12 parts by weight or less, or 10 parts by weight or less, relative to 100 parts by weight of the base polymer.

From the viewpoint of adjusting the adhesion between the adhesive layer and the adherend before and after photocuring and the storage elastic modulus of the adhesive layer after photocuring to an appropriate range, the content of the multifunctional (meth)acrylate without a urethane bond is preferably 2 to 25 parts by weight, more preferably 3 to 20 parts by weight, further preferably 4 to 15 parts by weight, and may also be 5 to 13 parts by weight, 6 to 12 parts by weight, or 7 to 10 parts by weight, relative to 100 parts by weight of the acrylic base polymer.

From the viewpoint of adjusting the adhesion between the adhesive layer and the adherend before and after photocuring and the storage elastic modulus of the adhesive layer after photocuring to an appropriate range, the content of urethane (meth) acrylate is preferably 0.03 to 5 parts by weight, more preferably 0.05 to 4 parts by weight, further preferably 0.08 to 3 parts by weight, and may also be 0.1 to 2.5 parts by weight or 0.15 to 2 parts by weight, relative to 100 parts by weight of the acrylic base polymer.

Urethane (meth)acrylates have low compatibility with acrylic-based polymers. Therefore, urethane (meth)acrylates tend to be concentrated on the surface of the adhesive layer (near the interface with the adherend). In addition, compared with multifunctional (meth)acrylates without alkylene oxide chains, multifunctional (meth)acrylates with alkylene oxide chains have low compatibility with acrylic-based polymers. These photocuring agents tend to be concentrated on the surface of the adhesive layer (near the interface with the adherend), and the photocuring agents concentrated on the interface with the adherend easily form a weak boundary layer (WBL).

If a WBL is formed, the liquid properties of the surface (bonding interface) become stronger while maintaining the bulk properties such as the storage elastic modulus of the adhesive layer, so the bonding force with the adherend tends to decrease. Therefore, the adhesive layer before photocuring is easily peeled off from the adherend. If the photocuring agent is concentrated near the interface with the adherend and the adhesive layer with a WBL is photocured, the curing reaction of the photocuring agent is easy to proceed near the bonding interface where the density of the photocuring agent is high, so the cohesive force near the bonding interface tends to become larger and the bonding force tends to increase. In addition, when a WBL is formed, the increase in the storage elastic modulus, which is a bulk property, tends to be suppressed in the adhesive after photocuring.

By including urethane (meth)acrylate as a photocuring agent, the adhesion of the adhesive layer before photocuring tends to be reduced. One reason for this is believed to be that the combined use of urethane (meth)acrylate and a multifunctional (meth)acrylate having no urethane bond promotes the formation of WBL.

It is believed that in the adhesive layer after photocuring, the acrylic-based polymer having a crosslinked structure and the polymer formed by curing the photocuring agent form a polymer network, and the polar groups of the acrylic-based polymer and the polar groups of the urethane (meth)acrylate interact with each other to improve the cohesive force, thereby improving the adhesion. In addition, as described above, it is believed that if the photocuring is performed in a state where the WBL is formed, the cohesive force near the bonding interface is likely to increase, so the adhesion at the interface and the high-temperature adhesion are likely to increase.

(Photopolymerization initiator) The photopolymerization initiator generates active species by irradiating active light, thereby promoting the curing reaction of the photocuring agent. As the photopolymerization initiator, it is preferred to use a photoradical polymerization initiator (photoradical generator).

As the photo-radical polymerization initiator, preferably, one that generates free radicals by irradiation with visible light or ultraviolet light of a wavelength shorter than 450 nm, examples of which include hydroxy ketones, benzoyl dimethyl ketal, amino ketones, acyl phosphine oxides, benzophenones, tris(III) derivatives containing a trichloromethyl group, etc. The photo-polymerization initiator may be used alone or in combination of two or more.

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 further 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 further preferably 0.1 to 7 parts by weight relative to 100 parts by weight of the photocuring agent.

<Other components> As described above, the photocurable adhesive composition constituting the adhesive layer 2 includes an acrylic base polymer, a photocuring agent, and a photopolymerization initiator. The adhesive composition may also include components other than these.

For example, the adhesive composition may also include an oligomer with a lower molecular weight than the base polymer. For example, the adhesive composition may include an acrylic oligomer with a weight average molecular weight of about 1,000 to 30,000 in addition to the acrylic base polymer.

In addition to the above-mentioned components, the adhesive composition may also contain additives such as silane coupling agents, adhesion imparting agents, crosslinking promoters, crosslinking delayers, plasticizers, softeners, antioxidants, anti-degradation agents, fillers, colorants, ultraviolet absorbers, surfactants, antistatic agents, etc. within the range that does not impair the characteristics of the present invention.

As crosslinking promoters (crosslinking catalysts), organic metal compounds such as organic metal complexes (chelates), compounds of metals and alkoxy groups, and compounds of metals and acyloxy groups; and tertiary amines, etc. are exemplified. From the perspective of suppressing the crosslinking reaction in a solution state at room temperature and ensuring the shelf life of the adhesive composition, organic metal compounds are particularly preferred. In addition, from the perspective of easily introducing a uniform crosslinking structure throughout the thickness direction of the adhesive layer, organic metal compounds that are liquid at room temperature are preferred as crosslinking promoters. Metals of organic metal compounds include iron, tin, aluminum, zirconium, zinc, titanium, lead, cobalt, zinc, etc.

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

[Production of reinforcing film] A reinforcing film can be obtained by laminating a photocurable adhesive layer 2 on a film substrate 1. The adhesive layer 2 can be formed directly on the film substrate 1, or an adhesive layer formed in a sheet form on another substrate can be transferred to the film substrate 1.

The adhesive composition is applied to the substrate by roller coating, contact roller coating, gravure coating, reverse coating, roller brush coating, spray coating, dip roller coating, rod coating, scraper coating, air knife coating, curtain coating, die lip coating, die nozzle coating, etc., and the solvent is dried and removed as needed to form an adhesive layer. As a drying method, an appropriate method can be appropriately adopted. The heating drying temperature is preferably 40°C to 200°C, more preferably 50°C to 180°C, and further preferably 70°C to 170°C. The drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 15 minutes, and further preferably 10 seconds to 10 minutes.

When the adhesive composition contains a crosslinking agent, it is preferred to perform crosslinking by heating or aging while or after the solvent is dried. The heating temperature or heating time is appropriately set according to the type of crosslinking agent used, and crosslinking is usually performed by heating at a temperature in the range of 20°C to 160°C for 1 minute to 7 days. The heating used to dry out the solvent can also serve as the heating for crosslinking.

After the crosslinking structure is introduced into the polymer by the crosslinking agent, the photocuring agent also remains in an unreacted state. Therefore, the adhesive layer 2 includes a photocurable adhesive composition, which contains an acrylic base polymer with a crosslinking structure introduced, a photocuring agent, and a photopolymerization initiator. When the adhesive layer 2 is formed on the film substrate 1, it is preferred to attach a peeling pad 5 to the adhesive layer 2 for the purpose of protecting the adhesive layer 2. It is also possible to attach the peeling pad 5 to the adhesive layer 2 before crosslinking.

When the adhesive layer 2 is formed on another substrate, after the solvent is dried, the adhesive layer 2 is transferred to the film substrate 1, thereby obtaining a reinforcement film. The substrate used to form the adhesive layer can be directly used as the peeling pad 5.

As the release pad 5, a plastic film such as polyethylene, polypropylene, polyethylene terephthalate, polyester film, etc. is preferably used. The thickness of the release pad is usually 3 to 200 μm, preferably about 10 to 100 μm. It is preferred to subject the release pad 5 and the adhesive layer 2 to a release treatment using a release agent such as a silicone, fluorine, long-chain alkyl, or fatty amide, or a silicon dioxide powder. By subjecting the surface of the peeling pad 5 to a release treatment, when the film substrate 1 and the peeling pad 5 are peeled off, peeling occurs at the interface between the adhesive layer 2 and the peeling pad 5, and the adhesive layer 2 is kept fixed to the film substrate 1. Antistatic treatment may be applied to either or both of the release-treated surface and the non-treated surface of the peeling pad 5. By subjecting the peeling pad 5 to an antistatic treatment, static electricity generated when the peeling pad is peeled off from the adhesive layer can be suppressed.

[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 component of the device. The adhesive layer 2 of the reinforcing film 10 is fixed to the film substrate 1, and the adhesion between the reinforcing film 10 and the adherend is relatively small before being photocured after being attached to the adherend. Therefore, the reinforcing film can be easily peeled off from the adherend before being photocured.

The adherend to which the reinforcing film is bonded is not particularly limited, and examples thereof include various electronic devices, optical devices, and components thereof. In one embodiment, the reinforcing film is bonded to the surface of a bendable flexible device. The bendable device has a hinge portion, and can be bent around the hinge portion. The bending angle can be set arbitrarily, and can also be bent (folded) 180°. When the device is a display device, the reinforcing film can be bonded to the surface on the screen side, and can also be bonded to the back side (casing). A flexible device that is configured to bend at a predetermined position such as a hinge, and that repeatedly bends and unfolds at the same position when in use.

The reinforcing film can be attached to the entire surface of the adherend, or can be selectively attached to the portion that needs to be reinforced (reinforcement target area). After attaching the reinforcing film to the entire portion that needs to be reinforced (reinforcement target area) and the area that does not need to be reinforced (non-reinforcement target area), the reinforcing film is cut and peeled off from the non-reinforcement target area, thereby making a device with the reinforcing film attached only to the reinforcement target area. If the adhesive is before light curing, the reinforcing film is temporarily adhered to the surface of the adherend, and the bonding force is relatively small, so the reinforcing film can be easily peeled off from the surface of the adherend.

By laminating the reinforcing film, appropriate rigidity is given, so for thin components such as soft devices, it is expected to improve the handling or damage prevention effect. In the manufacturing step of the device, when the reinforcing film is laminated to the semi-finished product, the reinforcing film can be laminated to the large-sized semi-finished product before being cut into the product size. The reinforcing film can also be laminated to the mother roll of the device manufactured by the roll-to-roll process in a roll-to-roll manner.

For example, after the reinforcing film is attached to the mother roll of the device or its semi-finished product in a roll-to-roll manner, the mother roll attached with the reinforcing film is cut and separated into individual products, and the reinforcing film is only cut by half-cutting, and the reinforcing film in the non-reinforcement target area is peeled off, thereby obtaining a device with the reinforcing film attached to the reinforcement target area. The cutting method is not particularly limited, and a suitable cutting method such as a rotary cutter, a press-in knife (such as a Thomson knife), a laser cutter, etc. can be used.

Before attaching the reinforcing film, the surface of the adherend may be activated for the purpose of purification. Examples of surface activation treatments include plasma treatment, corona treatment, and fluorescent discharge treatment.

From the viewpoint of easy peeling from the adherend and preventing the paste residue from remaining on the adherend after peeling off the reinforcing film, the adhesion force (initial adhesion force) between the adhesive layer 2 and the adherend before light curing is preferably 0.6 N/25 mm or less, more preferably 0.5 N/25 mm or less, and further preferably 0.4 N/25 mm or less, and may also be 0.3 N/25 mm or less, 0.25 N/25 mm or less, or 0.2 N/25 mm or less. From the viewpoint of preventing the reinforcing film from peeling off during storage or handling, the adhesion force between the adhesive layer 2 and the adherend before light curing is preferably 0.005 N/25 mm or more, and more preferably 0.01 N/25 mm or more.

Adhesion is measured by peeling test with polyimide film as adherend at a tensile speed of 300 mm/min and a peeling angle of 180°. Adhesion is measured at 25°C unless otherwise specified. Adhesion between adhesive layer and adherend before light curing is measured by using samples that were left at 25°C for 30 minutes after bonding.

The shear storage modulus of the adhesive layer 2 at a temperature of 25°C before light curing (hereinafter, simply recorded as "storage modulus") is preferably 15 kPa or more, more preferably 20 kPa or more, and may be 25 kPa or more or 30 kPa or more. The larger the storage modulus of the adhesive layer at room temperature before light curing, the more the cutting processability is improved, and there is a tendency to suppress the paste residue on the adherend when the reinforcing film is peeled off from the adherend.

On the other hand, if the storage elastic modulus of the adhesive layer at room temperature before light curing is too large, the storage elastic modulus of the adhesive layer after light curing tends to be larger, and there is a case where the adhesion to the adherend or the impact resistance is insufficient. In addition, if the storage elastic modulus of the adhesive layer before light curing is relatively large, the storage elastic modulus of the adhesive layer after light curing tends to be larger, and when used in a foldable device, the adhesive layer cannot absorb the strain of the bent part and its surroundings, and the reinforcing film is easily peeled off from the device. Therefore, the storage elastic modulus of the adhesive layer before light curing at a temperature of 25° C. is preferably 50 kPa or less, more preferably 45 kPa or less, and further preferably 40 kPa or less.

The storage elastic modulus of the adhesive layer is obtained as follows: According to the method described in JIS K7244-1 "Plastics - Test methods for dynamic mechanical properties", the value at the specified temperature is read when the temperature is increased at a rate of 5°C/min within the range of -70 to 200°C at a frequency of 1 Hz. In this manual, unless otherwise specified, the storage elastic modulus is the value at a temperature of 25°C.

After the reinforcing film is attached to the adherend, the adhesive layer 2 is irradiated with active light to photo-harden the adhesive layer. Ultraviolet light is preferably used as the active light. The irradiation intensity and irradiation time of the active light can be appropriately set according to the composition and thickness of the adhesive layer 2. The adhesive layer 2 can be irradiated with active light from either the film substrate 1 side or the adherend side, or from both sides.

If the adhesive layer 2 is photocured, the photocuring agent undergoes a curing reaction, and the adhesion to the adherend increases. From the perspective of adhesion reliability during actual use of the device, the adhesion _F1 between the adhesive layer and the adherend after photocuring at a temperature of 25°C is preferably 3 N/25 mm or more, more preferably 5 N/25 mm or more, and may be 7 N/25 mm or more, 9 N/25 mm or more, or 10 N/25 mm or more. The adhesion _F1 between the adhesive layer and the adherend after photocuring is preferably 10 times or more of the adhesion F0 between the adhesive layer and the adherend before photocuring, _more preferably 15 times or more, further preferably 20 times or more, and may be 30 times or more or 50 times or more.

At a temperature of 85°C, the adhesion force _F2 between the adhesive layer after light curing and the adherend is preferably 1 N/25 mm or more, more preferably 2 N/25 mm or more, further preferably 3 N/25 mm or more, and can also be 4 N/25 mm or more, 5 N/25 mm or more, or 6 N/25 mm or more.

As described above, by including urethane (meth)acrylate as a photocuring agent, the cohesive force at the bonding interface is improved, and the bonding force of the adhesive layer after photocuring tends to be increased.

From the perspective of achieving high adhesion to the adherend after photocuring, the storage modulus of the adhesive layer after photocuring at a temperature of 25°C is preferably 40 kPa or more, more preferably 50 kPa or more, and may be 55 kPa or more or 60 kPa or more. On the other hand, when the storage modulus of the adhesive layer after photocuring is too large, the adhesion or impact cushioning effect (cushioning property) on the adherend tends to be reduced. Furthermore, when the storage modulus of the adhesive layer after photocuring is too large, when applied to a foldable device, the reinforcing film is easily peeled off from the device at the bent portion and its periphery. Therefore, the storage elastic modulus of the adhesive layer after light curing at a temperature of 25° C. is preferably 100 kPa or less, more preferably 90 kPa or less, and further preferably 80 kPa or less.

The smaller the amount of the crosslinking agent and the smaller the amount of the photocuring agent, the smaller the storage elastic modulus of the adhesive layer after photocuring tends to be, and the smaller the amount of the photocuring agent, the smaller the increase in the storage elastic modulus of the adhesive layer caused by photocuring tends to be. In addition, by including a multifunctional (meth)acrylate and a urethane (meth)acrylate without a urethane bond as the photocuring agent, the increase in the storage elastic modulus of the adhesive layer caused by photocuring is suppressed, and the adhesion to the adherend tends to be improved.

The storage elastic modulus of the adhesive layer after light curing at a temperature of -40°C is preferably 1000 kPa or less, more preferably 900 kPa or less, and may be 800 kPa or less, 750 Pa or less, or 700 kPa or less. The smaller the storage elastic modulus at -40°C, the smaller the change in the storage elastic modulus caused by temperature change, and the more the warp of the reinforcement film and the adherend with the reinforcement film after the temperature cycle test is suppressed. The storage elastic modulus of the adhesive layer after light curing at a temperature of -40°C may be 300 kPa or more, 400 kPa or more, 450 kPa or more, or 500 kPa or more.

From the perspective of improving the adhesion at high temperature and ensuring the adhesive layer's ability to follow deformation such as bending of the adherend, the storage elastic modulus of the adhesive layer after light curing at a temperature of 85°C is preferably 40 to 100 kPa, more preferably 50 to 90 kPa, and may also be 55 to 85 kPa or 60 to 80 kPa.

The difference between the storage elastic modulus of the adhesive layer after light curing at -40°C and the storage elastic modulus at 85°C is preferably 900 kPa or less, more preferably 800 kPa or less, and further preferably 700 kPa or less. The storage elastic modulus of the adhesive layer after light curing at -40°C is preferably 10 times or less, more preferably 9.5 times or less, and further preferably 9 times or less of the storage elastic modulus at 85°C. The smaller the difference between the storage elastic modulus at low temperature (-40°C) and high temperature (85°C), the more the warping of the reinforcing film and the adherend bonded with the reinforcing film after the temperature cycle test is suppressed.

Similar to the storage elastic modulus at room temperature (25°C), the smaller the amount of the crosslinking agent and the smaller the amount of the photocuring agent, the smaller the storage elastic modulus of the adhesive layer after photocuring at low temperature (-40°C) and high temperature (85°C). In addition, by including a multifunctional (meth)acrylate and urethane (meth)acrylate without a urethane bond as the photocuring agent, the increase in the storage elastic modulus of the adhesive layer after photocuring at low temperature is suppressed, and the bonding strength with the adherend tends to be improved. Furthermore, the main monomer of the acrylic-based polymer is C _6-9 alkyl (meth)acrylate, and in addition to C _6-9 alkyl (meth)acrylate as a monomer component, it also contains C _10-20 alkyl (meth)acrylate as a monomer component, thereby having a tendency to reduce the temperature dependence of the storage elastic modulus of the adhesive layer after light curing.

The difference between the storage elastic modulus of the adhesive layer after light curing at 25°C and the storage elastic modulus at 85°C is preferably 50 kPa or less, more preferably 30 kPa or less, further preferably 20 kPa or less, and may be 15 kPa or less, 10 kPa or less, or 5 kPa or less. The storage elastic modulus of the adhesive layer after light curing at 85°C may also be less than the storage elastic modulus at 25°C. The storage elastic modulus of the adhesive layer after light curing at 85° C. is preferably 0.8 to 1.2 times, more preferably 0.85 to 1.15 times, and may also be 0.9 to 1.1 times, 0.93 to 1.07 times, or 0.95 to 1.05 times the storage elastic modulus at 25° C.

The smaller the difference between the storage elastic modulus at room temperature (25°C) and high temperature (85°C), the more the warping of the reinforcing film and the adherend with the reinforcing film after the temperature cycle test tends to be suppressed. In addition, when the reinforcing film is applied to a foldable device, the peeling of the reinforcing film caused by the deformation of the bending part (hinge) and its vicinity tends to be suppressed. The lower the glass transition temperature of the base polymer, the lower the temperature dependence of the viscoelasticity (flat area range) will be, so there is a tendency for the difference in the storage elastic modulus at room temperature and high temperature to become smaller.

By laminating the reinforcing film, the adherend will be given appropriate rigidity, and the stress will be relieved and dispersed, thus suppressing various adverse conditions that may occur in the manufacturing steps, improving production efficiency and yield.

The adhesive layer 2 of the reinforcing film of the present invention is photocurable, and the curing time can be set arbitrarily. Before the adhesive layer is photocured, the reinforcing film of the present invention has a relatively small adhesion to the surface-treated adherend, so it is easy to peel off from the adherend, and even if lamination or poor adhesion occurs, it is easy to perform secondary processing. In addition, it is also relatively easy to selectively remove the reinforcing film from the non-reinforcement target area.

After the adhesive layer is photocured, it exhibits a higher adhesion to the adherend, so the reinforcing film is not easy to peel off from the device surface. For a device with a reinforcing film formed by attaching the reinforcing film of the present invention to a soft device using a resin substrate, even when the device is repeatedly bent, the reinforcing film is not easy to peel off at the bent part. In addition, since the change in the storage elastic modulus of the adhesive layer caused by temperature changes is small, the warping of the device is suppressed, the display characteristics can be maintained, and the reinforcing film is not easy to peel off due to temperature changes, and the reliability is excellent. [Example]

The following is a further description with reference to embodiments and comparative examples, but the present invention is not limited to these embodiments.

[Preparation of base polymer] ＜Polymer A＞ Into a reaction vessel equipped with a thermometer, a stirrer, a reflux condenser and a nitrogen inlet tube, 20 parts by weight of butyl acrylate (BA) as monomers, 70 parts by weight of 2-ethylhexyl acrylate (2EHA), 8 parts by weight of lauryl acrylate (LA), 1 part by weight of 4-hydroxybutyl acrylate (4HBA), and 1 part by weight of N-vinyl-2-pyrrolidone (NVP), 0.1 parts by weight of azobisisobutyronitrile (AIBN) as a thermal polymerization initiator, and 233 parts by weight of ethyl acetate as a solvent were added, and nitrogen was introduced and nitrogen replacement was performed for about 1 hour while stirring. Thereafter, 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 1.8 million.

<Polymer B~D> The amount of monomer added was changed as shown in Table 1. In the polymerization of polymers B and D, the amount of thermal polymerization initiator (AIBN) was changed to 0.2 parts by weight. Except for these changes, solutions of polymers B~D were obtained in the same manner as the polymerization of polymer A.

Table 1 shows the ratio of added monomers of acrylic polymers A to D and the weight average molecular weight (Mw) of the polymers. The weight average molecular weight (polystyrene conversion) was measured using GPC (HLC-8220GPC manufactured by Tosoh Corporation) under the following conditions. Sample concentration: 0.2 wt% (tetrahydrofuran solution) Sample injection volume: 10 μL Eluent: THF Flow rate: 0.6 mL/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 recorded by the following abbreviations. BA: butyl acrylate 2EHA: 2-ethylhexyl acrylate NOAA: n-octyl acrylate LA: lauryl acrylate 2HEA: 2-hydroxyethyl acrylate 4HBA: 4-hydroxybutyl acrylate AA: acrylic acid NVP: N-vinyl-2-pyrrolidone

[Table 1] Monomer ratio (parts by weight) M BA 2EHA NOAA LA 2HEA 4HBA AA NVP A 20 70 - 8 - 1 - 1 1.8 million B - 96 - - 4 - - - 550,000 C 20 - 70 - - 10 - - 1.8 million D 95 - - - - - 5 - 600,000

[Preparation of reinforcing film] <Preparation of adhesive composition> A crosslinking agent, a multifunctional acrylate (without urethane bond) and a urethane acrylate as a photocuring agent, and a photopolymerization initiator were added to a solution of an acrylic polymer and uniformly mixed to prepare an adhesive composition shown in Table 2.

As a photopolymerization initiator, 0.3 parts by weight of "Omnirad 651" manufactured by IGM Resins was added relative to 100 parts by weight of the solid content of the acrylic polymer. The crosslinking agent and the photocuring agent (multifunctional acrylate and urethane acrylate) were added in the amounts shown in Table 2. The addition amounts in Table 2 are relative to 100 parts by weight of the acrylic polymer (parts by weight of the solid content). The details of the crosslinking agent and the photocuring agent are as follows.

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

(Multifunctional acrylate) A200: "NK Ester A200" manufactured by Shin-Nakamura Chemical Industry (polyethylene glycol #200 (n=4) diacrylate; molecular weight 308, functional group equivalent 154 g/eq) M350: trihydroxymethylpropane EO modified (n=1) triacrylate ("ARONIX M-350" manufactured by Toa Gosei, functional group equivalent 143 g/eq) ADPHE: ethoxylated dipentatriol polyacrylate ("NK ESTER A-DPH-12E" manufactured by Shin-Nakamura Chemical Industry, functional group equivalent about 200 g/eq)

(Urethane acrylate) UA306: Pentaerythritol triacrylate-toluene diisocyanate adduct ("UA-306T" manufactured by Kyoeisha Chemical, functional group equivalent: 128 g/eq) UA510: Dipentaerythritol pentaacrylate-hexamethylene diisocyanate adduct ("UA-510H" manufactured by Kyoeisha Chemical, functional group equivalent: 181 g/eq) UF-X9-83: Urethane acrylate containing a polycarbonate skeleton ("UF-X9-83" manufactured by Kyoeisha Chemical, functional group number 2 to 3, weight average molecular weight 8400)

<Application and cross-linking of adhesive solution> The adhesive composition was applied to a 50 μm thick polyethylene terephthalate film substrate ("DIAFOIL T100" manufactured by Mitsubishi Chemical) without surface treatment using a slot roller in a manner such that the thickness after drying was 15 μm. After drying at 130°C for 1 minute to remove the solvent, the release-treated surface of a release pad (polyethylene terephthalate film with a thickness of 25 μm treated with silicone release) was attached to the adhesive-coated surface. Afterwards, an aging treatment was carried out in an environment of 25°C for 4 days to allow crosslinking to proceed, and a reinforcement film was obtained in which an adhesive sheet was fixedly laminated on the film substrate and a peel-off liner was temporarily adhered thereto.

[Evaluation] ＜Adhesion to polyimide film＞ (Adhesion of adhesive layer before light curing) A polyimide film (Upilex 25S, manufactured by UBE) with a thickness of 25 μm was attached to a glass plate via a double-sided adhesive tape (No.531, manufactured by Nitto Denko) to obtain a polyimide film substrate for measurement. The peeling pad was removed from the surface of the reinforcement film cut into 25 mm wide x 100 mm long, and attached to the polyimide film substrate for measurement using a manual roller.

After the sample was left at 25°C for 30 minutes, a 180° peeling test was performed at a tensile speed of 300 mm/min by holding the end of the film substrate of the reinforcing film with a suction cup to measure the peeling strength (adhesion force F ₀ of the reinforcing film to the polyimide film).

(Adhesion of the adhesive layer after photocuring) After the reinforcing film was attached to the polyimide film substrate for measurement, the adhesive layer was photocured by irradiating ultraviolet light with a cumulative light amount of 4000 mJ/ ^cm2 from the reinforcing film side (film substrate side) using a 365 nm LED light source. The adhesion _F1 was measured by a 180° peeling test at a temperature of 25°C in the same manner as above. Furthermore, the test sample after the adhesive layer was photocured was stored in an oven at a temperature of 85°C for 5 minutes, and then a 180° peeling test was performed in an environment at a temperature of 85°C to measure the adhesion _F2 .

<Storage elastic modulus> On a peeling pad, the adhesive composition was coated and cross-linked in the same manner as above to produce an adhesive sheet (before light curing). A peeling pad was attached to the surface of the adhesive layer of the adhesive sheet before light curing to block oxygen, and the adhesive sheet was photocured by irradiating 4000 mJ/ ^cm2 of ultraviolet light using a 365 nm LED lamp. The adhesive sheet before and after light curing was laminated to prepare a test specimen with a thickness of about 1.5 mm. A rotational rheometer ("Discovery-HR2" manufactured by TA Instruments) was used to perform dynamic viscoelasticity measurement under the following conditions, and the shear storage modulus G' was read at -40℃, 25℃ and 85℃. (Measurement conditions) Deformation mode: torsion Measurement frequency: 1 Hz Heating rate: 5℃/min Measurement temperature: -70～200℃ Shape: parallel plate 8.0 mmφ

<Temperature Cycle Test> The peel-off pad was peeled off from the surface of the self-reinforcement film and laminated to a 25 μm thick polyimide film ("Upilex 25S" manufactured by UBE) using a manual roller. The self-reinforcement film side (film substrate side) was irradiated with ultraviolet light with a cumulative light intensity of 4000 mJ/ ^cm2 using a 365 nm LED light source to photoharden the adhesive layer. The test sample was cut into a size of 20 mm × 100 mm and subjected to the temperature cycle test. The temperature cycle is to introduce the sample into the test tank, keep it at -40℃ for 30 minutes, raise the temperature to 85℃ (6.25℃/min), keep it at 85℃ for 30 minutes, and cool it down to -40℃ (6.25℃/min). This operation is set as one cycle, and 200 cycles are implemented. The sample after the temperature cycle test is placed on a horizontal table in a way that it is connected to the long side, and the curling amount at the end is measured.

<Adhesive contamination> Remove the peelable liner from the surface of the reinforcing film cut into 25 mm wide x 100 mm long, and use a manual roller to stick it to the polyimide film substrate for measurement. After standing at 25°C for 30 minutes or 24 hours, peel the reinforcing film from the polyimide film and visually observe the surface of the polyimide film under a fluorescent light to confirm whether there is any contamination. Evaluate the contamination of the adherend using the following criteria. A: No contamination was observed in the sample peeled off after standing for 24 hours B: Contamination was observed in the sample peeled off after standing for 24 hours, but no contamination was observed in the sample peeled off after standing for 30 minutes C: Contamination was observed in the sample peeled off after standing for 30 minutes

The composition of the adhesive of each reinforcement film (type of base polymer, amount of crosslinking agent added, type and amount of photocuring agent added), and the evaluation results are shown in Table 2.

[Table 2] Composition Adhesion force (N/25 mm) G'(kPa) Pollution polymer Crosslinking agent Light Hardener Before hardening After hardening Before hardening 25℃ After hardening Curl(mm) Type Type quantity Multifunctional acrylate Urethane Acrylate F ₀ 25℃ F ₁ 85℃ F ₂ -40℃ 25℃ 85℃ Type quantity Type quantity Embodiment 1 A C/HX 0.3 M350 2 UA306 0.15 0.574 10.01 1.33 41 433 58 59 2 A Embodiment 2 A C/HX 0.3 M350 5 UA306 0.15 0.378 16.01 2.97 37 521 62 65 4 A Embodiment 3 A C/HX 0.3 M350 8 UA306 0.15 0.243 17.26 4.49 35 658 65 68 4 A Embodiment 4 A C/HX 0.3 M350 10 UA306 0.15 0.160 17.90 6.25 30 743 69 71 6 A Embodiment 5 A C/HX 0.3 M350 15 UA306 0.15 0.019 16.72 5.78 25 883 83 85 7 A Embodiment 6 A C/HX 0.3 M350 10 UA306 0.30 0.120 15.71 8.66 31 753 71 75 5 A Embodiment 7 A C/HX 0.3 M350 10 UA306 1.00 0.101 14.13 9.16 30 758 72 73 5 B Embodiment 8 A C/HX 0.3 M350 10 UA306 3.00 0.087 12.53 9.46 29 763 74 76 5 C Embodiment 9 A C/HX 0.1 M350 10 UA306 0.15 0.318 21.19 11.03 26 632 56 53 5 A Embodiment 10 A C/HX 0.2 M350 10 UA306 0.15 0.269 17.47 8.18 28 693 61 61 5 A Embodiment 11 A C/HX 0.5 M350 10 UA306 0.15 0.099 12.94 3.26 38 903 89 93 7 A Embodiment 12 A C/HX 0.3 M350 10 UA510 0.15 0.138 12.71 7.14 31 711 71 73 3 A Embodiment 13 A C/HX 0.3 M350 10 UF-X9-83 0.15 0.219 16.63 5.02 29 743 66 69 3 A Embodiment 14 A C/HX 0.3 A200 10 UA306 0.15 0.029 5.01 2.33 28 613 62 63 3 B Embodiment 15 A C/HX 0.3 ADPHE 10 UA306 0.15 0.137 10.69 4.23 33 812 78 83 6 C Comparison Example 1 A C/HX 0.3 M350 2 - 1.004 9.99 0.96 42 342 56 54 4 A Comparison Example 2 A C/HX 0.3 - UA306 0.15 2.102 2.38 0.76 39 412 41 40 3 A Comparison Example 3 A C/HX 0.3 M350 30 UA306 0.15 0.005 2.33 3.49 15 1508 113 118 12 C Comparison Example 4 B C/HX 0.3 M350 10 UA306 0.15 0.141 14.52 1.29 twenty three 1175 45 46 9 A Comparison Example 5 C C/HX 0.3 M350 10 UA306 0.15 0.129 13.79 2.20 28 891 66 68 7 A Comparative Example 6 D T/C 0.5 A200 30 UA306 0.15 0.147 5.40 1.56 68 275934 193 154 13 A

In Comparative Example 1, in which the adhesive composition was prepared with 2 parts by weight of multifunctional acrylate (M350) as a photocuring agent relative to 100 parts by weight of polymer A, the adhesion force _F0 to the polyimide film exceeded about 1 N/25 mm, showing a larger initial adhesion force. Comparative Example 2, in which 0.15 parts by weight of urethane acrylate (UA306) was prepared as a photocuring agent, showed a larger initial adhesion force than Comparative Example 1.

In Example 1, which contains 2 parts by weight of multifunctional acrylate (M350) and 0.15 parts by weight of urethane acrylate (UA306) as the photocuring agent relative to 100 parts by weight of polymer A, the initial adhesion force _F0 is smaller than that of Comparative Examples 1 and 2, and a larger adhesion force is exhibited after photocuring. In addition, in Example 1, the storage elastic modulus of the adhesive after photocuring is less than 60 kPa, and has the flexibility that can be applied to foldable devices, etc.

In Examples 2 to 4, where the amount of multifunctional acrylate is larger than that of Example 1, it is observed that the larger the amount of multifunctional acrylate, the smaller the initial adhesion force _F0 and the larger the adhesion force _F1 after photocuring. On the other hand, in Example 5, where the amount of multifunctional acrylate is further increased than that of Example 4, the adhesion force after photocuring is smaller than that of Example 4. In Comparative Example 3, where the amount of multifunctional acrylate is further increased, the adhesion force after photocuring is even smaller. In Comparative Example 3, when the reinforcing film is peeled off before the photocuring of the adhesive, contamination of the adherend is observed.

In Examples 1 to 5 and Comparative Example 3, as the amount of multifunctional acrylate increases, the storage modulus of the adhesive layer before light curing decreases, and the storage modulus of the adhesive after light curing tends to increase, especially the increase in the storage modulus at low temperature (-40°C) is more obvious. As the storage modulus of the adhesive layer after light curing increases at low temperature, the difference between the storage modulus at low temperature (-40°C) and high temperature (85°C) increases, and the curling of the sample after the temperature cycle test tends to increase.

In Examples 14 and 15 in which the type of multifunctional acrylate was changed, the initial adhesion force _F0 to the adherend was smaller as in Example 4, and the adhesion force _F2 of the adhesive layer after light curing at high temperature showed a larger value. In Example 15, the curling amount of the sample after the temperature cycle test was equal to that of Example 4. In Example 14 using a bifunctional acrylate (A200), the storage elastic modulus at each temperature was smaller than that of Examples 4 and 15, and the curling of the sample after the temperature cycle test was reduced.

In Examples 6 to 8, where the amount of urethane acrylate is greater than that of Example 4, the initial adhesion force _F0 to the adherend is smaller, the adhesion force _F2 of the adhesive layer after light curing at high temperature is larger, and the storage elastic modulus is low, and the curling of the sample after the temperature cycle test is reduced. In Examples 6 to 8, a tendency of the adhesion force _F2 at high temperature to increase is observed with the increase in the amount of urethane acrylate. In addition, as the amount of urethane acrylate increases, a tendency is observed for the storage modulus of the adhesive layer after light curing to increase, but the storage modulus increases only slightly, and no difference is observed in the amount of curling of the specimens after the temperature cycle test.

In Examples 12 and 13 using urethane acrylate different from that in Example 4, similarly to Example 4, the initial adhesion force _F0 is small, and the adhesion force _F2 of the adhesive layer after photocuring at a high temperature shows a larger value.

In Example 12 using 10-functional urethane acrylate (UA510), the initial adhesion _F0 is smaller than that of Example 4 using 6-functional urethane acrylate (UA306), and the adhesion _F2 at high temperature after light curing is larger than that of Example 4. On the other hand, in Example 13 using high molecular weight urethane acrylate (UF-X9-83), _F0 is larger and _F2 is smaller than that of Examples 4 and 12. From these results, it can be seen that the larger the number of functional groups of urethane acrylate and the smaller the functional group equivalent, the greater the effect of reducing the initial adhesion and improving the adhesion.

When comparing Example 1 with Examples 9 to 11, it is observed that the greater the amount of crosslinking agent, the smaller the adhesion of the adhesive layer before and after photocuring, and the greater the storage elastic modulus. It is also observed that as the storage elastic modulus increases, the curling amount of the sample after the temperature cycle test tends to increase.

In Comparative Example 4 using polymer B having a composition of 2EHA/2HEA=96/4 as the base polymer of the adhesive, the adhesive force _F2 of the adhesive layer after light curing at high temperature becomes smaller than that of Example 4. In addition, in Comparative Example 4, the storage elastic modulus of the adhesive layer after light curing at low temperature is larger, and accordingly, the difference between the storage elastic modulus at low temperature and high temperature (85°C) becomes larger, so the curling of the sample after the temperature cycle test becomes larger.

In Comparative Example 4 using polymer B having a composition of 2EHA/2HEA=96/4 as the base polymer of the adhesive, the adhesive force _F2 of the adhesive layer after light curing at high temperature becomes smaller than that of Example 4. In addition, in Comparative Example 4, the storage elastic modulus of the adhesive layer after light curing at low temperature is larger, and accordingly, the difference between the storage elastic modulus at low temperature and high temperature (85°C) becomes larger, and the curling of the sample after the temperature cycle test becomes larger.

In Comparative Example 5 using polymer C having a composition of BA/NOAA/4HBA=20/70/10 as the base polymer of the adhesive, the adhesion force _F2 at high temperature shows a smaller value than that of Comparative Example 4. In Comparative Example 5, the storage elastic modulus of the adhesive layer after light curing at low temperature is smaller than that of Comparative Example 4, but the storage elastic modulus at low temperature and the difference between the storage elastic modulus at low temperature and high temperature are larger than those of Example 4, and the curling value of the sample after the temperature cycle test is also larger than that of Example 1.

In Comparative Example 6, in which polymer D having a composition of BA/AA=95/5 was used as the base polymer of the adhesive and the amount of the multifunctional acrylate was set to 30 parts by weight, the adhesion _F2 of the adhesive layer after photocuring at high temperature was smaller, and the curling of the sample after the temperature cycle test was a larger value.

These results show that by using a polymer A including lauryl acrylate as a _C12 alkyl acrylate as a monomer component of a base polymer and using a multifunctional (meth)acrylate and a urethane (meth)acrylate having no urethane bond as photocuring agents, the adhesive layer after photocuring can achieve both high adhesion at high temperature and reduced curling during temperature cycling caused by a low storage elastic modulus.

1: Film substrate 2: Adhesive layer 5: Peel-off pad 10: Reinforcement film 20: Adhesive

FIG1 is a cross-sectional view showing a laminated structure of a reinforcing film. FIG2 is a cross-sectional view showing a laminated structure of a reinforcing film. FIG3 is a cross-sectional view showing a device with a reinforcing film attached.

1: Membrane substrate

2: Adhesive layer

10: Reinforcement film

Claims (14)

A reinforcing film comprises a film substrate and an adhesive layer fixedly laminated on one main surface of the film substrate. The adhesive layer comprises a photocurable composition, which comprises an acrylic-based polymer, a photocurable agent having two or more photopolymerizable functional groups, and a photopolymerization initiator. The acrylic-based polymer comprises a (meth)acrylic acid C _6-9 alkyl ester having an alkyl group with 6 to 9 carbon atoms, a (meth)acrylic acid C _{10-20 alkyl ester having an alkyl group with 10 to 20 carbon atoms, and one or more selected from the group consisting of a hydroxyl-containing monomer and a carboxyl-containing monomer as monomer components, and the amount of the (meth)acrylic acid C 6-9 alkyl} ester is 40 parts by weight or more relative to 100 parts by weight of the total amount of the monomer components. A cross-linked structure is introduced into the acrylic-based polymer. The photocurable composition comprises 2 to 25 parts by weight of a multifunctional (meth)acrylate having no urethane bond relative to 100 parts by weight of the acrylic base polymer, and 0.03 to 5 parts by weight of a urethane (meth)acrylate relative to 100 parts by weight of the acrylic base polymer as the photocuring agent. The reinforcing film of claim 1, wherein the acrylic-based polymer is introduced with a crosslinking structure using 0.05 to 1.0 parts by weight of a crosslinking agent relative to 100 parts by weight of the polymer. As in claim 2, the reinforcing film, wherein the crosslinking agent is an isocyanate crosslinking agent or an epoxy crosslinking agent. The reinforcing film of claim 1, wherein the amount of the (meth)acrylic acid _C10-20 alkyl ester is 1 to 40 parts by weight relative to 100 parts by weight of the total monomer components of the acrylic base polymer. In the reinforcing film of claim 1, the functional group equivalent of the (meth)acryl group of the multifunctional (meth)acrylate is 80 to 500 g/eq. In the reinforcing film of claim 1, the functional group equivalent of the (meth)acryl group of the urethane (meth)acrylate is 80 to 5000 g/eq. The reinforcing film of claim 1, wherein the urethane (meth)acrylate has 4 or more (meth)acryl groups in one molecule. A reinforcing film as claimed in any one of claims 1 to 7, wherein the adhesive layer has an adhesion force _F0 of less than 0.6 N/25 mm to the polyimide film at a temperature of 25°C before photocuring, an adhesion force _F1 of more than 3.0 N/25 mm to the polyimide film at a temperature of 25°C after photocuring, and an adhesion force _F2 of more than 1.0 N/25 mm to the polyimide film at a temperature of 85°C. A reinforcing film as claimed in any one of claims 1 to 7, wherein the shear storage modulus of the adhesive layer at a temperature of 25° C. after light curing is less than 100 kPa. A reinforcing film as claimed in any one of claims 1 to 7, wherein the shear storage modulus of the adhesive layer at a temperature of -40°C after light curing is less than 1000 kPa. A method for manufacturing a device having a reinforcing film attached to the surface thereof, wherein the method comprises: temporarily adhering the adhesive layer of the reinforcing film as in any one of claims 1 to 10 to the surface of an adherend, irradiating the adhesive layer with active light to photo-harden the adhesive layer, thereby increasing the bonding strength between the reinforcing film and the adherend. The manufacturing method of the device of claim 11 comprises cutting the reinforcing film temporarily adhered to the adherend after the reinforcing film is temporarily adhered to the adherend and before the adhesive layer is photocured, thereby peeling off and removing the reinforcing film from a portion of the adherend. A method for manufacturing a device as claimed in claim 11 or 12, wherein the adherend is a bendable image display element. A reinforcing method is to adhere a reinforcing film to the surface of an adherend, wherein the reinforcing method is to temporarily adhere the adhesive layer of the reinforcing film as in any one of claims 1 to 10 to the surface of the adherend, and to irradiate the adhesive layer with active light to photo-harden the adhesive layer, thereby increasing the bonding strength between the reinforcing film and the adherend.