JP7743643B2 - Reinforcement film, device with reinforcement film, and method for manufacturing the same - Google Patents

Description

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

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

By temporarily attaching an adhesive film to the surface of a device or device component before use, such as during device assembly, processing, or transportation, it is possible to prevent scratches and damage to the adherend. Patent Documents 1 and 2 disclose reinforcing films that have an adhesive layer made of a photocurable adhesive composition on a film substrate.

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

In recent years, organic EL panels using bendable substrates (flexible substrates) such as resin films have been put to practical use, and bendable flexible displays have been proposed. Foldable devices are repeatedly bent at the same location. At the bend, compressive stress is applied to the inside and tensile stress to the outside, causing distortion at the bend and its surroundings, which can lead to peeling of the adhesive from the adherend. Patent Document 4 proposes using a photocurable adhesive containing an acrylic-based polymer with a low glass transition temperature as the adhesive for the reinforcing film of a foldable device.

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

As described in Patent Document 3, foldable devices use resin films as substrate materials, which are prone to charging and can lead to problems with electrostatic breakdown of the device due to charging (static electricity). Patent Document 2 proposes preventing charging of the reinforcing film by providing an antistatic layer on the back surface of the film substrate of the reinforcing film (the surface opposite the surface to which the adhesive layer is attached). However, because the back surface of the reinforcing film substrate is separated from the adherend, providing an antistatic layer on the back surface of the substrate makes it difficult to adequately suppress charging of the device.

In order to suppress static buildup on devices, it is effective to incorporate an antistatic agent into the adhesive layer that is placed in contact with the device, thereby lowering the resistance of the adhesive layer and providing antistatic properties. However, even when an antistatic agent is added to the adhesive of the reinforcing film described in Patent Document 4, the adhesive's resistance does not sufficiently decrease. Increasing the amount of antistatic agent added causes the antistatic agent to bleed out, reducing the adhesive strength between the adhesive layer and the device and causing contamination of the device surface.

In light of the above, the present invention aims to provide a reinforcing film that is less likely to peel off during bending tests, is applicable to foldable devices, and can contribute to preventing static electricity in the device.

The reinforcing film of the present invention comprises an adhesive layer adhered and laminated to one main surface of a film substrate. The adhesive layer comprises a photocurable composition containing an acrylic-based polymer, a photocuring agent, a photopolymerization initiator, an antistatic agent, and a polyol. The glass transition temperature of the acrylic-based polymer is preferably -40°C or lower.

The number average molecular weight of the polyol contained in the photocurable composition is preferably 300 to 30,000. The amount of polyol in the photocurable composition is preferably 5 to 60 parts by weight per 100 parts by weight of the acrylic base polymer. The polyol may be polypropylene glycol or polytetramethylene glycol. The polyol may be diol or triol polypropylene glycol.

The acrylic base polymer contains one or more monomer units selected from the group consisting of hydroxyl group-containing monomers and carboxyl group-containing monomers, and a crosslinking structure is introduced by bonding a crosslinking agent such as an isocyanate crosslinking agent or an epoxy crosslinking agent to the hydroxyl groups and/or carboxyl groups of the acrylic base polymer. The amount of crosslinking agent may be approximately 0.03 to 2 parts by weight per 100 parts by weight of the acrylic polymer.

The photocurable composition that constitutes the adhesive layer preferably contains 3 to 40 parts by weight of a photocuring agent per 100 parts by weight of the acrylic base polymer. For example, a polyfunctional (meth)acrylate is used as the photocuring agent. The polyfunctional (meth)acrylate may be an alkylene oxide-modified polyfunctional (meth)acrylate modified with an alkylene oxide such as ethylene oxide or propylene oxide.

The surface resistance of the pressure-sensitive adhesive layer is preferably 1×10 ¹¹ Ω or less. After photocuring, the pressure-sensitive adhesive layer preferably has a shear storage modulus of 1.0×10 ⁴ to 1.0×10 ⁶ Pa at −20° C. Before photocuring, the pressure-sensitive adhesive layer may have a shear storage modulus of 1.0×10 ⁴ to 2.0×10 ⁵ Pa at −20° C. Before photocuring, the pressure-sensitive adhesive layer may have a shear storage modulus of 5.0×10 ³ to 1.0×10 ⁵ Pa at 25° C.

It is preferable that the adhesive strength of the adhesive layer to the polyimide film be 1 N/25 mm or less before photo-curing. It is preferable that the adhesive strength of the adhesive layer to the polyimide film after photo-curing be at least five times the adhesive strength to the polyimide film before photo-curing.

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

The reinforcing film of the present invention has an adhesive layer made of a photocurable composition, and by photocuring the adhesive layer after adhering to the adherend, the adhesive strength with the adherend increases. Because the adhesive strength with the adherend is low before photocuring, the film can be easily peeled off from the adherend.

The adhesive layer of the reinforcing film contains an antistatic agent and polyol, which reduces its resistance, suppressing static buildup in the adhesive layer itself and contributing to anti-static properties of the adherend. Furthermore, the adhesive layer contains polyol, which provides a low shear storage modulus and high stress-strain relaxation even after photocuring. This prevents peeling of the adhesive layer at the bending point, even when the same area is repeatedly bent, resulting in excellent adhesion reliability. Therefore, the reinforcing film of the present invention is also suitable for use in foldable devices using resin film substrates.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

<Base polymer>
The base polymer is the main component of the pressure-sensitive adhesive composition and is the main factor determining the adhesive strength and storage modulus of the pressure-sensitive adhesive layer before photocuring. The pressure-sensitive adhesive composition preferably contains an acrylic polymer as the base polymer because it has excellent optical transparency and adhesiveness and is easy to control the adhesive strength and storage modulus.

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

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

Among the exemplified (meth)acrylic acid alkyl esters, from the viewpoint of lowering the Tg of the acrylic base polymer and reducing the shear storage modulus at low temperatures, thereby suppressing peeling of the pressure-sensitive adhesive layer during repeated flexing, _C1-9 alkyl (meth)acrylates are preferred, and those having a homopolymer glass transition temperature of −50° C. or lower are preferred. Specific examples of (meth)acrylic acid _C1-9 alkyl esters having a homopolymer glass transition temperature of −50° C. or lower include 2-ethylhexyl acrylate (Tg: −70° C.), n-hexyl acrylate (Tg: −65° C.), n-octyl acrylate (Tg: −65° C.), isononyl acrylate (Tg: −60° C.), n-nonyl acrylate (Tg: −58° C.), isooctyl acrylate (Tg: −58° C.), and butyl acrylate (Tg: −55° C.). Of these, butyl acrylate (BA) and 2-ethylhexyl acrylate are preferred.

From the perspective of lowering the Tg of the acrylic base polymer, 2-ethylhexyl acrylate (2EHA) is particularly preferred. If 2EHA is the most abundant monomer (main monomer) among the constituent monomers of the acrylic base polymer, the glass transition temperature of the acrylic base polymer can be set to -60°C or below, or -65°C or below. Meanwhile, in the pressure-sensitive adhesive composition, the polyol (details described below) acts as a plasticizer, reducing the shear storage modulus at low temperatures and contributing to improved adhesive reliability. Therefore, when the pressure-sensitive adhesive composition contains a polyol, sufficient adhesive reliability can be ensured even if the glass transition temperature of the acrylic base polymer is around -50°C. From the perspective of improved adhesion and compatibility between the acrylic base polymer and the polyol, it is particularly preferred that the main monomer of the acrylic base polymer is butyl acrylate (BA).

The content of the (meth)acrylic acid alkyl ester is preferably 70% by weight or more, more preferably 80% by weight or more, and even more preferably 85% by weight or more, and may be 90% by weight or more, 93% by weight or more, or 95% by weight or more, based on the total amount of monomer components constituting the acrylic base polymer. Among these, the amount of the (meth)acrylic acid _C1-9 alkyl ester is preferably in the above-mentioned range, and the total of 2-ethylhexyl acrylate and butyl acrylate is more preferably in the above-mentioned range, and the amount of butyl acrylate may be in the above-mentioned range.

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

Hydroxy group-containing monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and 4-(hydroxymethyl)cyclohexylmethyl (meth)acrylate. Of these, 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate are preferred because they contribute significantly to improving the adhesive strength of the pressure-sensitive adhesive after photocuring.

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

The acrylic base polymer preferably contains 0.5 to 15% by weight of the hydroxy group-containing monomer and carboxy group-containing monomer relative to the total amount of constituent monomer components, more preferably 1 to 10% by weight, and even more preferably 2 to 7% by weight. When the acrylic base polymer contains a carboxy group, the acrylic base polymer and polyol exhibit high compatibility. Therefore, the acrylic base polymer preferably contains a carboxy group-containing monomer such as (meth)acrylic acid as a constituent monomer component, and the content of the carboxy group-containing monomer is preferably within the above range.

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

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

The acrylic base polymer before the introduction of a crosslinking structure may be substantially free of nitrogen atoms. The proportion of nitrogen in the constituent elements of the acrylic base polymer may be 0.1 mol% or less, 0.05 mol% or less, 0.01 mol% or less, 0.005 mol% or less, 0.001 mol% or less, or 0.001 mol% or less. Using an acrylic base polymer substantially free of nitrogen atoms tends to suppress an increase in the adhesive strength (initial adhesive strength) of the pressure-sensitive adhesive layer before photocuring when the adherend is subjected to a surface activation treatment. By not using nitrogen-atom-containing monomers such as cyano group-containing monomers, lactam structure-containing monomers, amide group-containing monomers, and morpholine ring-containing monomers as constituent monomer components of the acrylic base polymer, an acrylic base polymer substantially free of nitrogen atoms can be obtained. The amount of nitrogen-containing monomer relative to the total amount of constituent monomer components of the acrylic base polymer may be 1 wt% or less, 0.5 wt% or less, 0.1 wt% or less, 0.05 wt% or less, or 0.

The glass transition temperature of the acrylic base polymer is preferably -40°C or lower, and may be -45°C or lower. The glass transition temperature is the temperature (peak top temperature) at which the loss tangent tanδ in viscoelasticity measurement is maximized. When the glass transition temperature is sufficiently lower than the ambient temperature of the device, the shear storage modulus G' of the adhesive layer within the ambient temperature range of the device is small, which tends to suppress peeling during repeated bending.

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

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

The lower limit of the glass transition temperature of the acrylic base polymer is not particularly limited, but is generally -80°C or higher, and may be -70°C or higher, -65°C or higher, -60°C or higher, or -55°C or higher. From the perspective of reducing the shear storage modulus at low temperatures and suppressing peeling during repeated flexing, the lower the glass transition temperature of the acrylic base polymer, the more preferable. However, if the pressure-sensitive adhesive composition contains a polyol, even if the glass transition temperature of the acrylic base polymer is -55°C or higher, the pressure-sensitive adhesive after photocuring will have excellent adhesive reliability and will be able to suppress peeling from the adherend during repeated flexing at low temperatures.

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

The weight-average molecular weight of the acrylic base polymer is preferably 100,000 to 2,000,000, more preferably 200,000 to 1,500,000, and even more preferably 300,000 to 1,000,000. Note that when a crosslinked structure is introduced into the acrylic base polymer, the molecular weight of the acrylic base polymer refers to the molecular weight before the crosslinked structure is introduced.

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

Examples of crosslinking agents include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, carbodiimide-based crosslinking agents, and metal chelate-based crosslinking agents. These crosslinking agents react with functional groups such as hydroxyl groups and carboxyl groups introduced into the acrylic-based polymer to form a crosslinked structure. Isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred because they are highly reactive with the hydroxyl groups and carboxyl groups of the acrylic-based polymer and allow for the ease of introducing a crosslinked structure.

As mentioned above, from the viewpoint of compatibility with polyols, it is preferable that the acrylic base polymer has a carboxy group derived from a carboxy group-containing monomer such as acrylic acid. It is preferable to introduce a crosslinked structure into the acrylic base polymer with a carboxy group using an epoxy crosslinking agent.

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

As an isocyanate-based crosslinking agent, a polyisocyanate having two or more isocyanate groups per molecule is used. Examples of isocyanate-based crosslinking agents include lower aliphatic polyisocyanates such as butylene diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentylene diisocyanate, cyclohexylene diisocyanate, and isophorone diisocyanate; aromatic isocyanates such as 2,4-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate; trimethylolpropane/trilene diisocyanate; Examples of the isocyanate adduct include a diisocyanate trimer adduct (e.g., "Takenate D101E" manufactured by Mitsui Chemicals), a trimethylolpropane/hexamethylene diisocyanate trimer adduct (e.g., "Coronate HL" manufactured by Tosoh), a trimethylolpropane adduct of xylylene diisocyanate (e.g., "Takenate D110N" manufactured by Mitsui Chemicals), and an isocyanurate of hexamethylene diisocyanate (e.g., "Coronate HX" manufactured by Tosoh).

The amount of crosslinking agent used can be adjusted appropriately depending on the composition and molecular weight of the acrylic base polymer. The amount of crosslinking agent used is approximately 0.03 to 2 parts by weight, preferably 0.05 to 1 part by weight, more preferably 0.08 to 0.8 parts by weight, and may be 0.1 to 0.5 parts by weight, per 100 parts by weight of the acrylic base polymer.

A crosslinking catalyst may be used to promote the formation of a crosslinked structure. The amount of crosslinking catalyst used is generally 0.5 parts by weight or less per 100 parts by weight of the acrylic base polymer.

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

A photocuring monomer or a photocuring oligomer is used as the photocuring agent. A compound with two or more ethylenically unsaturated bonds in one molecule is preferred. Furthermore, a compound that is compatible with acrylic-based polymers and polyols is preferred. Since the photocuring agent has adequate compatibility with acrylic-based polymers, it is preferable that the photocuring agent be liquid at room temperature.

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

The polyfunctional (meth)acrylate may be an ester of an alkylene oxide-modified polyol and (meth)acrylic acid. Examples of alkylene oxides include ethylene oxide (EO) and propylene oxide (PO). The alkylene oxide may also be a polyalkylene oxide such as polyethylene glycol or polypropylene glycol.

Specific examples of alkylene oxide-modified polyfunctional (meth)acrylates include bisphenol A ethylene oxide-modified di(meth)acrylate, bisphenol A propylene oxide-modified di(meth)acrylate, trimethylolpropane ethylene oxide-modified tri(meth)acrylate, trimethylolpropane propylene oxide-modified tri(meth)acrylate, ethylene oxide isocyanuric acid-modified di(meth)acrylate, propylene oxide isocyanuric acid-modified di(meth)acrylate, ethylene oxide isocyanuric acid-modified tri(meth)acrylate, propylene oxide isocyanuric acid-modified tri(meth)acrylate, pentaerythritol ethylene oxide-modified tetra(meth)acrylate, and pentaerythritol propylene oxide-modified tetra(meth)acrylate.

In alkylene oxide-modified polyfunctional (meth)acrylates, the chain length n of the alkylene oxide is approximately 1 to 10. By adjusting the type and chain length of the alkylene oxide, compatibility with the acrylic base polymer can be adjusted within an appropriate range.

Multifunctional (meth)acrylates with alkylene oxide chains (ether chains) have excellent compatibility with acrylic-based polymers, as well as with polyols such as polyethylene glycol and polypropylene glycol. Furthermore, multifunctional (meth)acrylates with alkylene oxide chains can also contribute to lowering the resistance of adhesives.

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

Furthermore, the smaller the functional group equivalent weight of the photocuring agent (i.e., the greater the number of functional groups per unit molecular weight), the higher its compatibility with the acrylic-based polymer tends to be. From the standpoint of compatibility with the acrylic-based polymer, the functional group equivalent weight (g/eq) of the photocuring agent is preferably 80 to 500, more preferably 90 to 400, and may be 100 to 300, 110 to 250, or 120 to 200.

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

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

From the viewpoint of increasing the adhesive strength of the adhesive after photocuring while suppressing an excessive increase in the shear storage modulus, the content of the photocuring agent in the adhesive composition is preferably 3 to 40 parts by weight, more preferably 5 to 35 parts by weight, or may be 7 to 30 parts by weight or 10 to 25 parts by weight per 100 parts by weight of the acrylic base polymer.

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

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

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

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

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

The ionic compound containing an organic cation may be an ionic liquid, which is liquid at room temperature, or an ionic solid, which is solid at room temperature. The ionic compound containing an organic cation is preferably composed of a fluoroorganic anion or a fluoroinorganic anion and an onium cation.

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

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

The fluoroorganic anions that constitute ionic compounds containing organic cations may be completely fluorinated (perfluorinated) or partially fluorinated. Examples of fluoroorganic anions include perfluoroalkylsulfonates, bis(fluorosulfonyl)imides, and bis(perfluoroalkanesulfonyl)imides. More specific examples include trifluoromethanesulfonate, pentafluoroethanesulfonate, heptafluoropropanesulfonate, nonafluorobutanesulfonate, bis(fluorosulfonyl)imides, and bis(trifluoromethanesulfonyl)imides. Examples of fluoroinorganic anions include hexafluorophosphate and tetrafluoroboric acid.

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

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

<Polyol>
The pressure-sensitive adhesive composition contains a polyol. When the pressure-sensitive adhesive contains a polyol in addition to an antistatic agent, the resistance of the pressure-sensitive adhesive layer is further reduced. Furthermore, when the pressure-sensitive adhesive composition contains a polyol, flexibility is imparted to the pressure-sensitive adhesive layer, and peeling from the adherend when repeatedly bent at low temperatures can be suppressed.

Examples of polyols include polyols (diols) with two hydroxy groups per molecule, polyols (triols) with three hydroxy groups per molecule, polyols (tetraols) with four hydroxy groups per molecule, polyols (pentaols) with five hydroxy groups per molecule, and polyols (hexaols) with six hydroxy groups per molecule.

High molecular weight polyols are preferred as polyols, and the number-average molecular weight of the polyol is preferably 300 to 30,000. If the molecular weight of the polyol is less than 300, the polyol is likely to bleed out onto the surface of the adhesive layer, causing contamination of the adherend. If the molecular weight of the polyol is excessively high, it may have poor compatibility with the acrylic base polymer and photocuring agent, resulting in reduced transparency of the adhesive (clouding).

The larger the molecular weight of the polyol, the lower the adhesive strength of the pressure-sensitive adhesive layer 2 to the adherend before photocuring, the higher the adhesive strength after photocuring, and the greater the rate of increase in adhesive strength before and after photocuring. From the perspective of the adhesive properties of the pressure-sensitive adhesive layer before and after photocuring, the number-average molecular weight of the polyol is preferably 400 or more, more preferably 500 or more, even more preferably 700 or more, particularly preferably 1000 or more, and may be 2000 or more, 3000 or more, 4000 or more, or 5000 or more. From the perspective of compatibility, the number-average molecular weight of the polyol is preferably 25,000 or less, more preferably 20,000 or less, and may be 15,000 or less, or 12,000 or less.

Examples of high molecular weight polyols include polyether polyols, polyester polyols, polycarbonate polyols, and caprolactone polyols. Among these, polyether polyols are preferred due to their excellent compatibility with acrylic-based polymers and photocuring agents.

Polyether polyols are obtained by ring-opening addition polymerization of alkylene oxides to low-molecular-weight polyols. Examples of low-molecular-weight diols include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, pentanediol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol, diethylene glycol, triethylene glycol, polyethylene glycol, dipropylene glycol, 1,4-cyclohexanedimethanol, 1,4-cyclohexanediol, bisphenol A, bisphenol F, hydrogenated bisphenol A, and hydrogenated bisphenol F. Examples of low-molecular-weight triols include glycerin and trimethylolpropane.

Examples of alkylene oxides include ethylene oxide, propylene oxide, butylene oxide, and tetrahydrofuran (tetramethylene oxide). Diol-type polyether polyols are obtained by ring-opening addition polymerization of alkylene oxides with low-molecular-weight diols, and triol-type polyether polyols are obtained by ring-opening addition polymerization of ethylene oxide with low-molecular-weight triols.

The polyol is preferably a liquid at room temperature, as it exhibits adequate compatibility with the acrylic base polymer. By using a polyol that is liquid at room temperature, the polyol is more likely to act as a plasticizer, reducing the shear storage modulus of the adhesive before and after photocuring, and tends to inhibit peeling from the adherend when repeatedly flexed at low temperatures. From the perspective of increasing the flex resistance of the adhesive strength at low temperatures, it is preferable that the polyol have a low freezing point. The freezing point of the polyol is preferably 0°C or below, and may be -5°C or below, -10°C or below, -15°C or below, -20°C or below, or -25°C or below.

Polyethylene glycol with a molecular weight of 500 or more is solid at room temperature. On the other hand, polypropylene glycol and polytetramethylene glycol have a lower freezing point than polyethylene glycol. For example, polypropylene glycol is liquid at room temperature even with a molecular weight of 10,000 or more, and has a significant effect of reducing the shear storage modulus of the adhesive. For this reason, polypropylene glycol or polytetramethylene glycol is preferred as the polyol, and diol or triol type polypropylene glycol is particularly preferred.

The polyol content in the PSA composition is preferably 5 to 60 parts by weight per 100 parts by weight of the acrylic base polymer. If the amount of polyol is too small, the PSA may not be able to sufficiently reduce its resistance. If the amount of polyol is too large, it may result in a decrease in the adhesive strength of the PSA and contamination of the adherend. The polyol content in the PSA composition is more preferably 10 to 50 parts by weight per 100 parts by weight of the acrylic base polymer, and may also be 15 to 45 parts by weight, 20 to 40 parts by weight, or 25 to 35 parts by weight.

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

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

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

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

The above-mentioned adhesive composition is applied to a substrate by roll coating, kiss roll coating, gravure coating, reverse coating, roll brush, spray coating, dip roll coating, bar coating, knife coating, air knife coating, curtain coating, lip coating, die coating, or the like, and the solvent is dried and removed as needed to form an adhesive layer. Any appropriate drying method can be used. The heating and drying temperature is preferably 40°C to 200°C, more preferably 50°C to 180°C, and even more preferably 70°C to 170°C. The drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 15 minutes, and even more preferably 10 seconds to 10 minutes.

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

The introduction of a crosslinked structure into the acrylic-based polymer increases the gel fraction, which tends to increase the shear storage modulus of the adhesive layer 2. The higher the gel fraction of the adhesive before photocuring, the harder the adhesive becomes, and the less adhesive residue tends to be left on the substrate when the reinforcing film is peeled from the substrate during rework, etc. The gel fraction of the adhesive layer 2 before photocuring (i.e., the gel fraction of the photocurable composition constituting the adhesive layer) is preferably 25% or more, more preferably 30% or more, and may be 35% or more, 40% or more, or 45% or more. On the other hand, if the gel fraction is excessively high, the shear storage modulus increases and stress-strain relaxation is poor, which may result in adhesive peeling at bent portions during use of a flexible device. Therefore, the gel fraction of the adhesive layer 2 before photocuring is preferably 80% or less, and may be 75% or less, 70% or less, or 65% or less.

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

Even after the crosslinking agent introduces a crosslinked structure into the polymer, the photocuring agent remains unreacted. Therefore, the pressure-sensitive adhesive layer 2 contains an acrylic-based polymer with a crosslinked structure introduced, a photocuring agent, a photopolymerization initiator, an antistatic agent, and a polyol. When forming the pressure-sensitive adhesive layer 2 on the film substrate 1, it is preferable to apply a release liner 5 to the pressure-sensitive adhesive layer 2 for purposes such as protecting the pressure-sensitive adhesive layer 2. Crosslinking may also be carried out after the release liner 5 is applied to the pressure-sensitive adhesive layer 2.

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

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

[Physical properties of adhesive layer]
The thickness of the pressure-sensitive adhesive layer 2 is, for example, about 1 to 300 μm. The thicker the pressure-sensitive adhesive layer 2, the more likely it is to have improved adhesion to an adherend. On the other hand, if the pressure-sensitive adhesive layer 2 is excessively thick, the fluidity before photocuring may be high, making handling difficult. Therefore, the thickness of the pressure-sensitive adhesive layer 2 is preferably 3 to 100 μm, more preferably 5 to 50 μm, even more preferably 6 to 40 μm, and particularly preferably 8 to 30 μm. From the viewpoint of achieving a thinner layer, the thickness of the pressure-sensitive adhesive layer 2 may be 25 μm or less, 20 μm or less, or 18 μm or less.

When the reinforcing film is used in an optical device such as a display, the total light transmittance of the adhesive layer 2 is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. The haze of the adhesive layer 2 is preferably 2% or less, more preferably 1% or less, even more preferably 0.7% or less, and particularly preferably 0.5% or less. As described above, the acrylic base polymer, photocuring agent, and polyol have adequate compatibility, which makes the adhesive less likely to become cloudy and results in the formation of an adhesive layer with low haze.

The surface resistance of the pressure-sensitive adhesive layer 2 is preferably 1.0× ¹⁰ Ω or less, more preferably 5.0× ¹⁰ Ω or less, and may be 3.0× ¹⁰ Ω or less or 2.0× ¹⁰ Ω or less. As described above, the pressure-sensitive adhesive composition contains an antistatic agent and a polyol, thereby making it possible to reduce the resistance of the pressure-sensitive adhesive layer 2.

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

The shear storage modulus of the pressure-sensitive adhesive layer 2 at 25°C before photocuring is preferably 5.0 x ¹⁰ to 1.0 x ¹⁰ Pa. The shear storage modulus of the pressure-sensitive adhesive layer (hereinafter simply referred to as "storage modulus") is determined by reading the value at a predetermined temperature when measured at a temperature rise rate of 5°C/min in the range of -50 to 150°C at a frequency of 1 Hz in accordance with the method described in JIS K7244-1 "Plastics - Test methods for dynamic mechanical properties."

From the viewpoints of improving the processability of the pressure-sensitive adhesive layer, suppressing the pressure-sensitive adhesive from spilling out from the edge face of the reinforcing film, and preventing contamination of the adherend due to adhesive residue when the reinforcing film is peeled from the adherend, the storage modulus of the pressure-sensitive adhesive layer 2 at 25°C before photocuring is preferably 7.0 x ¹⁰³ Pa or more, more preferably 9.0 x ¹⁰³ Pa or more, and may be 1.0 x ¹⁰⁴ Pa or more or 1.5 x ¹⁰⁴ Pa or more. From the viewpoint of imparting flexibility to the pressure-sensitive adhesive layer, the storage modulus of the pressure-sensitive adhesive layer 2 at 25°C before photocuring is preferably 7.0 x ¹⁰⁴ Pa or less, more preferably 5.0 x ¹⁰⁴ Pa or less, and may be 4.0 x ¹⁰⁴ Pa or less or 3.0 x ¹⁰⁴ Pa or less.

From the same viewpoint as above, the storage modulus of the pressure-sensitive adhesive layer 2 at 60°C before photocuring is preferably 5.0 x ¹⁰ to 8.0 x ¹⁰ Pa, more preferably 7.0 x ¹⁰ to 5.0 x ¹⁰ Pa, even more preferably 8.0 x ¹⁰ to 4.0 x ¹⁰ Pa, and may be 1.0 x 10 to 3.0 x ¹⁰ Pa or 1.5 ^{x 10} to 2.5 x ¹⁰ Pa. The storage modulus of the pressure-sensitive adhesive layer 2 at 60°C before photocuring is preferably 0.3 times or more, more preferably 0.5 times or more, and may be 0.6 times or more or 0.7 times or more of the storage modulus at 25°C. The storage modulus at 60°C is generally 1.1 times or less, and may be 1.0 times or less or 0.9 times or less of the storage modulus at 25°C.

The storage modulus of the pressure-sensitive adhesive layer 2 at −20° C. before photocuring is preferably 1.0×10 ⁴ to 2.0×10 ⁵ Pa, more preferably 1.5×10 ⁴ to 1.0×10 ⁵ Pa, and may be 2.0×10 ⁴ to 7.0×10 ⁴ Pa or 3.0×10 ⁴ to 7.0×10 ⁴ Pa. When the storage modulus of the pressure-sensitive adhesive layer at low temperature before photocuring is within the above range, the storage modulus of the pressure-sensitive adhesive layer tends to be maintained low even after photocuring.

The storage modulus of an adhesive changes rapidly near the glass transition temperature of the base polymer. As mentioned above, a low glass transition temperature of the base polymer allows the preparation of an adhesive with a low storage modulus at low temperatures. Furthermore, the inclusion of a high molecular weight polyol in the adhesive composition tends to reduce the storage modulus at low temperatures. The storage modulus of the adhesive layer 2 at -20°C before photocuring is preferably 5 times or less, more preferably 3 times or less, and may be 2.5 times or less, or 2 times or less, of the storage modulus at 25°C. The storage modulus at -20°C is generally 1.0 times or more, and may be 1.1 times or more, 1.3 times or more, or 1.5 times or more, of the storage modulus at 25°C.

From the viewpoint of facilitating peeling from the adherend and preventing adhesive residue on the adherend after peeling of the reinforcing film, the adhesive strength between the pressure-sensitive adhesive layer 2 and the adherend before photocuring is preferably 1 N/25 mm or less, more preferably 0.5 N/25 mm or less, even more preferably 0.3 N/25 mm or less, and may be 0.2 N/25 mm or less, or 0.1 N/25 mm or less. From the viewpoint of preventing peeling of the reinforcing film during storage or handling, the adhesive strength between the pressure-sensitive adhesive layer 2 and the adherend before photocuring is preferably 0.01 N/25 mm or more, more preferably 0.02 N/25 mm or more, and may be 0.03 N/25 mm or more, 0.04 N/25 mm or more, or 0.05 N/25 mm or more.

Adhesion strength is determined by a peel test using a polyimide film as the adherend, at a tensile speed of 300 mm/min and a peel angle of 180°. Unless otherwise specified, adhesion strength is measured at 25°C.

The storage modulus and adhesive strength of the pressure-sensitive adhesive layer before photocuring depend on the composition of the base polymer, the amount of crosslinking agent introduced, the type and content of the photocuring agent and polyol, etc. The greater the amount of crosslinking agent introduced, the higher the gel fraction and the greater the storage modulus tends to be. The greater the amount of photocuring agent and polyol used, the relatively smaller the content of base polymer in the composition, and the smaller the storage modulus tends to be.

If the base polymer, photocuring agent, and polyol are not completely miscible, the liquid photocuring agent will bleed out to the surface, forming an adhesion-preventing layer (Weak Boundary Layer; WBL) at the adhesive interface with the adherend, strengthening its liquid properties. When a WBL is formed, the adhesive layer retains its bulk properties, such as storage modulus, while strengthening the liquid properties of the surface (adhesive interface), which tends to weaken the adhesive strength with the adherend. If the base polymer, photocuring agent, and polyol are moderately miscible enough not to impair transparency but are not completely miscible, the formation of a WBL in the adhesive layer before photocuring is thought to weaken its adhesive strength with the adherend.

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

The storage modulus at −20° C. of the pressure-sensitive adhesive layer after photocuring is preferably 1.0×10 ⁶ Pa or less, more preferably 5.0×10 ⁶ Pa or less, even more preferably 4.0×10 ⁵ Pa or less, and may be 3.0×10 ⁵ Pa or less, 2.5×10 ⁵ Pa or less, or 2.0×10 ⁵ Pa or less. Since the pressure-sensitive adhesive layer 2 after photocuring has a small storage modulus at low temperatures, the pressure-sensitive adhesive layer exhibits strain relaxation properties in a low-temperature environment, and therefore peeling of the pressure-sensitive adhesive layer at the bending portion can be suppressed even when a device to which a reinforcing film is bonded is repeatedly bent or when the bent state is maintained for a long period of time.

On the other hand, if the storage modulus of the pressure-sensitive adhesive layer after photocuring is excessively small, the pressure-sensitive adhesive layer is prone to plastic deformation, and the pressure-sensitive adhesive layer may peel off from the adherend due to insufficient adhesive holding power. Therefore, the storage modulus of the pressure-sensitive adhesive layer after photocuring at −20° C. is preferably 1.0×10 ⁴ Pa or more, more preferably 3.0×10 ⁴ Pa or more, even more preferably 5.0×10 ⁴ Pa or more, and may be 7.0×10 ⁴ Pa or more or 1.0×10 ⁵ Pa or more.

The storage modulus at -20°C of the pressure-sensitive adhesive layer after photocuring is preferably 13 times or less, more preferably 10 times or less, and may be 7 times or less, 5 times or less, or 4.5 times or less, of the storage modulus at -20°C of the pressure-sensitive adhesive layer before photocuring. The storage modulus at -20°C of the pressure-sensitive adhesive layer after photocuring may be 1.1 times or more, 1.5 times or more, 2.0 times or more, or 2.5 times or more of the storage modulus at -20°C of the pressure-sensitive adhesive layer before photocuring.

The storage modulus of the pressure-sensitive adhesive layer at -20°C after photocuring is preferably 7 times or less, more preferably 5 times or less, and may be 4.5 times or less or 4 times or less, of the storage modulus at 25°C. The storage modulus of the pressure-sensitive adhesive layer at -20°C before photocuring may be 1.5 times or more, 2.0 times or more, or 2.5 times or more, of the storage modulus at 25°C.

From the viewpoint of ensuring adhesiveness at room temperature and suppressing the adhesive from spilling out from the edge surface, the storage modulus of the adhesive layer after photocuring at 25°C is preferably 5.0 x 10 ³ to 1.5 x 10 ⁵ Pa, more preferably 1.0 x 10 ⁴ to 1.0 x 10 ⁵ Pa, even more preferably 1.5 x 10 ⁴ to 8.0 x 10 ⁴ Pa, and may be 2.0 x 10 ⁴ to 7.0 x 10 ⁴ Pa or 3.0 x 10 ⁴ to 5.0 x 10 ⁴ Pa.

The storage modulus at 25°C of the adhesive layer after photocuring is preferably 7 times or less, and may be 5 times or less, 4 times or less, or 3 times or less, of the storage modulus at 25°C of the adhesive layer before photocuring. The storage modulus at 25°C of the adhesive layer after photocuring may be 1.1 times or more, 1.3 times or more, 1.5 times or more, or 1.7 times or more of the storage modulus at 25°C of the adhesive layer before photocuring.

From the same viewpoint as above, the storage modulus at 60°C of the pressure-sensitive adhesive layer after photocuring is preferably 5.0×10 ³ to 1.0×10 ⁵ Pa, more preferably 8.0×10 ³ to 8.0×10 ⁴ Pa, even more preferably 1.0×10 ⁴ to 6.0×10 ⁴ Pa, and may be 2.0×10 ⁴ Pa to 5.0×10 ⁴ Pa or 2.5×10 ⁴ Pa to 4.5×10 ⁴ Pa.

The storage modulus of the pressure-sensitive adhesive layer after photocuring at 60°C is preferably 0.3 times or more, more preferably 0.5 times or more, and may be 0.6 times or more or 0.7 times or more, of the storage modulus at 25°C. The storage modulus at 60°C may be 1.1 times or less, 1.0 times or less, 0.9 times or less, or 0.8 times or less of the storage modulus at 25°C.

The storage modulus at 60°C of the adhesive layer after photocuring is preferably 7 times or less, and may be 5 times or less, 4 times or less, or 3 times or less, of the storage modulus at 60°C of the adhesive layer before photocuring. The storage modulus at 60°C of the adhesive layer after photocuring may be 1.1 times or more, 1.3 times or more, 1.5 times or more, or 1.7 times or more of the storage modulus at 60°C of the adhesive layer before photocuring.

From the viewpoint of adhesive reliability during practical use of the device, the adhesive strength between the adhesive layer and the adherend after photocuring is preferably 1.5 N/25 mm or more, and more preferably 2.0 N/25 mm or more. From the viewpoint of suppressing adhesive peeling when the flexible device is repeatedly bent at the same location, the adhesive strength between the adhesive layer and the adherend after photocuring may be 2.5 N/25 mm or more or 3.0 N/25 mm or more.

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

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

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

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

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

Before laminating the reinforcing film, the surface of the adherend may be activated for cleaning purposes. Examples of surface activation treatments include plasma treatment, corona treatment, and glow discharge treatment. Adherends with activated surfaces contain many active groups, such as hydroxyl groups, carbonyl groups, and carboxyl groups, which tend to increase adhesive strength due to intermolecular interactions with the polar functional groups of the adhesive's base polymer. In particular, when the adherend is a polyimide, activation treatment activates amide acids, terminal amino groups, and carboxyl groups (or carboxylic anhydride groups), etc., which interact strongly with the polar functional groups of the base polymer, resulting in a significant increase in initial adhesive strength.

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

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

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

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

In a device with a reinforced film in which the reinforced film of the present invention is bonded to a flexible device using a resin substrate, the reinforced film is less likely to peel off at the bent portion, even when the device is repeatedly bent or maintained in a bent state for a long period of time. Furthermore, since static electricity on the device's resin substrate can be removed via the adhesive layer, problems such as electrostatic breakdown of the device caused by charging (static electricity) can be suppressed.

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

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

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

The monomer ratios used for acrylic polymers A to C and the glass transition temperatures of the polymers are listed in Table 1. The glass transition temperatures were calculated from the monomer ratios used based on Fox's formula.
BA: Butyl acrylate 2EHA: 2-ethylhexyl acrylate AA: Acrylic acid 2HEA: 2-hydroxyethyl acrylate

[Preparation of reinforcing film]
Example 1
(Preparation of Pressure-Sensitive Adhesive Composition)
To a solution of acrylic polymer A (100 weight parts as polymer solids), 0.1 parts by weight of a tetrafunctional epoxy crosslinking agent ("Tetrad C" manufactured by Mitsubishi Gas Chemical Company, Ltd.) as a crosslinking agent, 0.2 parts by weight of zirconium tetraacetylacetonate ("ZC-150" manufactured by Matsumoto Fine Chemical Co., Ltd.) as a crosslinking catalyst, 20 parts by weight of polyethylene glycol #600 diacrylate ("NK Ester A-600" manufactured by Shin-Nakamura Chemical Co., Ltd.) as a photocuring agent, 0.3 parts by weight of "Omnirad 651" manufactured by IGM Resins as a photopolymerization initiator, 0.2 parts by weight of 1-butyl-3-methylpyridinium bistrifluoromethanesulfonylimide ("CIL-312" manufactured by Nippon Carlit Co., Ltd.) as an antistatic agent, and 30 parts by weight of polypropylene glycol ("Preminol S3011" manufactured by AGC Co., Ltd.) as a polyol were added and mixed uniformly to prepare a pressure-sensitive adhesive composition having the composition shown in Table 2.

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

<Comparative Example 1>
(Preparation of Pressure-Sensitive Adhesive Composition)
To a solution of acrylic polymer C (100 weight percent of the polymer solids), 0.25 parts by weight of a trifunctional isocyanate crosslinking agent ("Coronate HX" manufactured by Tosoh) as a crosslinking agent, 0.03 parts by weight of iron acetylacetonate ("Nacem Ferric" manufactured by Nippon Chemical Industry Co., Ltd.) as a crosslinking catalyst, 10 parts by weight of trimethylolpropane EO-modified (n = 1) triacrylate ("Aronix M-350" manufactured by Toagosei Co., Ltd.) as a photocuring agent, 0.3 parts by weight of "Omnirad 651" manufactured by IGM Resins as a photopolymerization initiator, and 0.2 parts by weight of 1-butyl-3-methylpyridinium bistrifluoromethanesulfonylimide ("CIL-312" manufactured by Nippon Carlit Co., Ltd.) as an antistatic agent were added and mixed uniformly to prepare a pressure-sensitive adhesive composition having the composition shown in Table 2.

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

<Examples 2 to 14, Comparative Examples 2 to 6>
Pressure-sensitive adhesive compositions were prepared by changing the type of acrylic polymer, the type and amount of crosslinking agent, the type and amount of photocuring agent, the type of antistatic agent, and the type and amount of polyol as shown in Table 2. In Examples 2 to 11 and Comparative Examples 3 to 6, 0.2 parts by weight of zirconium tetraacetylacetonate was used as the crosslinking catalyst, as in Example 1, and in Comparative Example 2, 0.25 parts by weight of iron acetylacetonate was used as the crosslinking catalyst, as in Comparative Example 1. Using a pressure-sensitive adhesive composition having the composition shown in Table 2, coating and crosslinking were carried out in the same manner as in Example 1 to prepare a reinforcing film. In Table 2, the amounts of crosslinking agent, photocuring agent, antistatic agent, and polyol are the amounts added per 100 parts by weight of the solids content of the acrylic polymer. Details of the crosslinking agent, photocuring agent, antistatic agent, and polyol are as follows:

(Crosslinking agent)
TC: N,N,N',N'-tetraglycidyl-m-xylylenediamine (tetrafunctional epoxy compound, "Tetrad C" manufactured by Mitsubishi Gas Chemical Company)
C-HX: Isocyanurate of hexamethylene diisocyanate ("Coronate HX" manufactured by Tosoh)

(Photocuring agent)
A200: polyethylene glycol #200 (n=4) diacrylate ("NK Ester A200" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent weight 151 g/eq)
A600: polyethylene glycol #600 (n=14) diacrylate ("NK Ester A600" manufactured by Shin-Nakamura Chemical Co., Ltd., functional group equivalent weight 371 g/eq)
M350: Trimethylolpropane EO-modified (n=1) triacrylate ("Aronix M-350" manufactured by Toagosei Co., Ltd., functional group equivalent weight 143 g/eq)

(Antistatic Agent)
CIL312: 1-butyl-3-methylpyridinium bistrifluoromethanesulfonylimide ("CIL-312" manufactured by Nippon Carlit)
AS110: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ("Elexcel AS-110" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)

(Polyol)
S3011: Polypropylene glycol, triol type (AGC "Preminol S-3011", Mn = 10,000)
GP3000: Polypropylene glycol, triol type ("Sannyx GP-3000" manufactured by Sanyo Chemical Industries, Ltd., Mn = 3000)
GP1000: Polypropylene glycol, triol type ("Sannyx GP-1000" manufactured by Sanyo Chemical Industries, Ltd., Mn=1000)
GP400: Polypropylene glycol, triol type ("Sannyx GP-400" manufactured by Sanyo Chemical Industries, Ltd., Mn=400)
GP250: Polypropylene glycol, triol type ("Sannyx GP-250" manufactured by Sanyo Chemical Industries, Ltd., Mn = 250)
PP3000: Polypropylene glycol, diol type (Sanyo Chemical Industries, Ltd. "Sannyx PP-3000", Mn = 3000)
PTGL3000: Polytetramethylene glycol, triol type ("PTGL3000" manufactured by Hodogaya Chemical Co., Ltd., Mn = 3000)

Example 15
A film substrate having an antistatic layer of a conductive polymer (PEDOT/PSS) on one side of a 50 μm-thick polyethylene terephthalate film was used, and a pressure-sensitive adhesive layer was formed on the antistatic layer-forming surface of the film substrate using the same pressure-sensitive adhesive composition as in Example 13. The antistatic layer (thickness 30 nm) was formed by applying an antistatic treatment solution with a solids content of about 0.4% to one side of the polyethylene terephthalate film using a bar coater, and then heating and drying at 130°C for 2 minutes.

(A) As a binder, a 25% aqueous dispersion of saturated copolymer polyester resin ("Binalol MD-1480" manufactured by Toyobo Co., Ltd.): 30 parts by weight in terms of solid content. (B) As a binder, an aqueous dispersion of acrylic resin obtained by emulsion polymerization of methyl methacrylate/butyl acrylate/hydroxyethyl methacrylate in a weight ratio of 85/10/5: 70 parts by weight in terms of solid content. (C) As a conductive polymer, an aqueous solution containing 0.5% poly(3,4-ethylenedioxythiophene) and 0.8% polystyrene sulfonate having a weight average molecular weight of 150,000 (Bytron P.H.C. manufactured by Stark): 20 parts by weight in terms of solid content. (D) Melamine-based crosslinking agent ("Sumimal M-50W" manufactured by Sumitomo Chemical Co., Ltd.): 5 parts by weight in terms of solid content.

[evaluation]
<Appearance>
The reinforcing film was visually observed, and those that were cloudy were rated as NG, and those that were transparent were rated as OK. For Comparative Example 2 in which cloudiness was observed, further evaluation was not carried out.

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

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

<Adhesive strength>
A 25 μm-thick polyimide film (Ube Industries, "Upilex 25S") was attached to a glass plate via double-sided adhesive tape (Nitto Denko, "No. 531") to obtain a polyimide film substrate for measurement. The release liner was peeled off from the surface of a reinforced film cut into a 25 mm wide x 100 mm long piece, and the piece was then attached to the polyimide film substrate for measurement using a hand roller to obtain a pre-photocuring test sample. The pre-photocuring test sample was irradiated with ultraviolet light from the reinforced film side (PET film substrate side) to photocure the adhesive layer, resulting in a photocured test sample. Using these test samples, the edge of the film substrate of the reinforced film was held with a chuck, and the reinforced film was peeled 180° at a tensile speed of 300 mm/min to measure peel strength.

<Bending test>
The release liner was peeled off from the surface of the reinforced film, and a 12.5 μm thick polyimide film (Ube Industries'"Upilex12.5SN") was laminated to the surface of the adhesive layer using a hand roller. This laminate was cut into a size of 25 mm wide x 100 mm long, and the adhesive layer was photocured by irradiating ultraviolet light from the reinforced film side (PET film substrate side) to obtain a test specimen. Using a planar no-load U-shaped stretch tester (Yuasa System Instruments), a bending jig was attached to the short side of the test specimen, and repeated bending tests were performed under the following conditions in a thermostatic chamber at a temperature of -20 °C or 25 °C and a relative humidity of 50%, with the reinforced film side (film substrate side) facing inward. After the repeated bending test, samples that showed no peeling or lifting between the reinforced film and the adherend were rated as OK, while samples that showed peeling or lifting were rated as NG.
(Test conditions)
Bending radius: 3 mm
Bending angle: 180°
Bending speed: 1 second/bending number: 200,000 times

<Stainability of adherends>
The release liner was peeled off from the surface of the reinforcing film, and a polyimide film (Ube Industries, Ltd.'s "Upilex 25S") was attached to the surface of the adhesive layer using a hand roller. After leaving the film standing at 25°C for 30 minutes, the reinforcing film was peeled off from the polyimide film, and the surface of the polyimide film was visually inspected under a fluorescent lamp to check for contamination. Those that showed contamination due to attached matter were rated as NG, and those that showed no contamination were rated as OK.

The adhesive composition of each reinforcing film and the evaluation results are shown in Table 2.

In Examples 1 to 11, in which the pressure-sensitive adhesive composition contained an antistatic agent and a polyol (triol-type polypropylene glycol) in addition to a base polymer, a crosslinking agent, a photocuring agent, and a photopolymerization initiator, the surface resistance of the pressure-sensitive adhesive was 2 x 10 ¹⁰ Ω or less, and no contamination of the adherend was observed. Furthermore, photocuring increased the adhesive strength to polyimide film by more than 10 times, the shear storage modulus G' at -20°C after photocuring was 5 x 10 ⁵ Pa or less, and no peeling of the pressure-sensitive adhesive layer was observed after repeated bending tests, demonstrating good adhesive reliability. Similarly to Example 1, Example 12, in which a diol-type polypropylene glycol was used as the polyol, also produced a pressure-sensitive adhesive with low resistance and good adhesive reliability in bending tests.

Example 13, which used an antistatic agent containing bis(fluorosulfonyl)imide, a non-perfluorinated fluoroorganic anion, had reduced surface resistance compared to Example 1. Example 13 had lower initial adhesive strength compared to Example 1, but after photocuring, exhibited high adhesive strength equivalent to that of Example 1. Example 15, in which an adhesive layer was formed on a substrate with an antistatic layer, had lower surface resistance of the adhesive layer compared to Example 13, and the storage modulus and adhesive strength were similar to those of Example 13.

Example 14, which used triol-type polytetramethylene glycol as the polyol, produced a pressure-sensitive adhesive with low resistance and good adhesion reliability in bending tests, but exhibited higher surface resistance than Example 13. Comparing Example 6 and Example 13, which used polyols with equivalent molecular weights, shows that polypropylene glycol contributes more to lowering resistance than polytetramethylene glycol.

In Comparative Example 3, in which the adhesive composition did not contain polyol, the surface resistance of the adhesive was higher than in Example 5, and the G' of the adhesive before and after photocuring was also high. In Comparative Example 3, the G' of the adhesive layer after photocuring was large at -20°C, so the adhesive layer peeled off after repeated bending tests at -20°C, resulting in poor adhesion reliability. In Comparative Example 4, in which the amount of crosslinker was reduced to 0.1 parts by weight, like Comparative Example 3, the G' of the adhesive layer after photocuring was large at -20°C, and the adhesive layer peeled off after repeated bending tests at -20°C.

Comparative example 1, which used polymer C with a low glass transition temperature, had a small G' before and after photocuring, and no peeling of the adhesive layer was observed during the bending test, but the surface resistance was high, and the effect of the antistatic agent in reducing resistance was not observed.

These results show that when an adhesive composition contains a polyol in addition to an antistatic agent, the adhesive's resistivity is reduced, imparting antistatic properties, and the polyol acts as a plasticizer, reducing G' at low temperatures, thereby improving adhesion reliability in bending tests.

In Comparative Example 2, in which 30 parts by weight of polyol was added to the composition of Comparative Example 1, the adhesive became cloudy. The cloudiness is thought to be due to the polyol's low compatibility with the other components of the adhesive composition.

In Example 11, which used M350 as the photocuring agent, as in Comparative Example 2, no clouding was observed. Therefore, in Comparative Example 2, the clouding is believed to be due to low compatibility between the acrylic base polymer and/or crosslinker and the polyol. In Example 10, which used a combination of a low-glass transition temperature polymer B containing 2EHA as the main monomer component and an epoxy-based crosslinker, no clouding of the adhesive was observed. Therefore, in Comparative Example 2, the base polymer does not contain acrylic acid, a carboxyl group-containing monomer, but contains 2-hydroxyethyl acrylate, a hydroxyl group-containing monomer, and contains an isocyanate crosslinker. In other words, in the Examples, the base polymer contains a carboxyl group-containing monomer as a monomer component, and a crosslinked structure is introduced by the epoxy-based crosslinker. This results in excellent compatibility with the polyol, and therefore, the adhesive is highly transparent.

In Comparative Example 5, in which the amount of polyol added was increased compared to Examples 1, 3, 4, and 9, the surface of the polyimide film was contaminated after the reinforcing film was peeled off. Furthermore, in Comparative Example 5, the surface resistance of the adhesive was higher than in Examples 1, 3, 4, and 9, indicating that adding an excess amount of polyol did not have the effect of reducing resistance.

Comparing Examples 1, 3, 4, and 9 with Comparative Example 5, it was found that the greater the amount of polyol added, the lower the adhesive strength of the adhesive before photocuring and the greater the rate of increase in adhesive strength due to photocuring. In these examples, it is believed that the polyol formed WBL in the adhesive before photocuring, strengthening the liquid properties of the surface of the adhesive layer (the adhesive interface with the adherend), thereby reducing the adhesive strength with the adherend. In Comparative Example 7, excessive WBL was formed, and the polyol precipitated on the surface is thought to have caused contamination of the adherend.

In Comparative Example 6, which used a low molecular weight polyol, contamination of the adherend was observed, similar to Comparative Example 5. Low molecular weight polyols have little interaction with the base polymer and photocuring agent of the adhesive composition, and are prone to bleed out onto the surface of the adhesive layer, which is thought to be the cause of contamination of the adherend.

Furthermore, in Comparative Example 6, the adhesive strength of the adhesive before photocuring was greater, the adhesive strength after photocuring was smaller, and the rate of increase in adhesive strength before and after photocuring was smaller compared to Examples 1, 6, 7, and 8. Comparing Examples 1, 6, 7, and 8 with Comparative Example 6, it was found that the larger the molecular weight of the polyol, the smaller the adhesive strength of the adhesive before photocuring, the larger the adhesive strength after photocuring, and the greater the rate of increase in adhesive strength before and after photocuring. It is believed that low-molecular-weight polyols have little effect as WBLs before photocuring the adhesive layer, but tend to precipitate near the surface of the adhesive layer and act as WBLs after photocuring the adhesive layer.

The above results show that when a photocurable adhesive composition contains an antistatic agent and a polyol of a specified molecular weight, the adhesive has low resistance and antistatic properties, low initial adhesive strength, and excellent adhesive properties after photocuring. Furthermore, the shear storage modulus in the low temperature range is low, and peeling of the adhesive is suppressed even when repeatedly bent.

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

Claims (15)

A film substrate and a pressure-sensitive adhesive layer fixedly laminated on one main surface of the film substrate,
the pressure-sensitive adhesive layer is made of a photocurable composition including an acrylic-based polymer, a photocuring agent, a photopolymerization initiator, an antistatic agent, and a polyol;
the acrylic base polymer contains a carboxyl group-containing monomer as a monomer unit, and a crosslinked structure is introduced using 0.03 to 2 parts by weight of an epoxy crosslinking agent per 100 parts by weight of the polymer ;
The polyol has a number average molecular weight of 300 to 30,000,
The photocurable composition contains 5 to 60 parts by weight of the polyol relative to 100 parts by weight of the acrylic base polymer.
Reinforcement film. The reinforced film according to claim 1, wherein the polyol is polypropylene glycol or polytetramethylene glycol. The reinforced film according to claim 1, wherein the polyol is a diol or triol polypropylene glycol. The reinforced film according to any one of claims 1 to 3, wherein the acrylic base polymer has a glass transition temperature of -40°C or lower . The reinforced film according to any one of claims 1 to 3, wherein the photocurable composition contains 3 to 40 parts by weight of the photocuring agent per 100 parts by weight of the acrylic base polymer. The reinforcing film according to any one of claims 1 to 3, wherein the photocuring agent is a polyfunctional (meth)acrylate. The reinforcing film according to any one of claims 1 to 3, wherein the photocuring agent is a polyfunctional (meth)acrylate containing an alkylene oxide chain. 4. The reinforced film according to claim 1, wherein the pressure-sensitive adhesive layer has a shear storage modulus of 1.0×10 ⁴ to 1.0×10 ⁶ Pa at −20° C. after photocuring. The pressure-sensitive adhesive layer has a shear storage modulus of 1.0×10 ⁴ to 2.0×10 ⁵ Pa at −20° C. and a shear storage modulus of 5.0×10 ³ to 1.0×10 ⁵ Pa at 25° C. before photocuring. The reinforced film according to any one of claims 1 to 3. The reinforced film according to any one of claims 1 to 3, wherein the adhesive layer has an adhesive strength to the polyimide film of 1 N/25 mm or less before photocuring. The reinforced film according to any one of claims 1 to 3, wherein the adhesive strength of the pressure-sensitive adhesive layer to the polyimide film after photocuring is at least five times the adhesive strength to the polyimide film before photocuring. The reinforced film according to any one of claims 1 to 3, wherein the pressure-sensitive adhesive layer has a surface resistance of 1 x 10 ¹¹ Ω or less. A method for manufacturing a device with a reinforcing film in which a reinforcing film is attached to a surface of a foldable device, comprising:
The pressure-sensitive adhesive layer of the reinforcing film according to any one of claims 1 to 3 is attached to a surface of a foldable device,
photocuring the pressure-sensitive adhesive layer;
A method for manufacturing a device with a reinforcing film. A device with a reinforcing film, in which a reinforcing film is attached to the surface of a foldable device,
The reinforcing film includes a film substrate and a pressure-sensitive adhesive layer fixedly laminated on one main surface of the film substrate,
the pressure-sensitive adhesive layer is attached to a device surface,
the pressure-sensitive adhesive layer is a photocured product obtained by photocuring a photocurable pressure-sensitive adhesive composition containing an acrylic-based polymer and a photocuring agent, and further contains an antistatic agent and a polyol having a number average molecular weight of 300 to 20,000;
the acrylic base polymer contains a carboxyl group-containing monomer as a monomer unit , and a crosslinked structure is introduced using 0.03 to 2 parts by weight of an epoxy crosslinking agent per 100 parts by weight of the polymer ;
the pressure-sensitive adhesive layer has a shear storage modulus at −20° C. of 1.0×10 ⁴ to 1.0×10 ⁶ Pa;
Device with reinforced film. The device with a reinforced film according to claim 14 , wherein the pressure-sensitive adhesive layer has a surface resistance of 1×10 ¹¹ Ω or less.