TWI847986B - Reinforcement film - Google Patents

Abstract

The reinforcing film (10) of the present invention has an adhesive layer (2) fixedly laminated on one main surface of the film substrate (1). The adhesive layer comprises a photocurable composition containing a base polymer, a photocuring agent, a photoradical initiator, and an antioxidant, and the adhesive layer comprises 10 to 50 parts by weight of the photocuring agent, 0.01 to 1 part by weight of the photoradical initiator, and 0.01 to 2 parts by weight of the antioxidant relative to 100 parts by weight of the base polymer. The reinforcing film of the present invention can arbitrarily set the time from the bonding with the adherend to the enhanced bonding strength.

Description

Reinforcement film

The present invention relates to a reinforcing film which is pasted on the surface of a device.

An adhesive film is sometimes attached to the surface of an optical device or electronic device such as a display for the purpose of surface protection or impact resistance. Such an adhesive film is usually formed by a laminate of an adhesive layer fixed on the main surface of a film substrate and attached to the device surface via the adhesive layer.

Before the device is assembled, processed, transported, etc., it is possible to temporarily adhere an adhesive film to the surface of the device or its components to prevent damage or destruction of the adherend. The adhesive film temporarily adhered to temporarily protect the surface can be easily peeled off from the adherend without leaving any adhesive residue on the adherend.

Patent document 1 discloses an adhesive film which is used for assembly, processing, transportation, etc. of a device and is also used in a state of being adhered to the surface of the device when the device is used. This adhesive film has the function of reinforcing the device by protecting the surface, dispersing the impact of the device, and imparting rigidity to the flexible device.

When an adhesive film is attached to an adherend, there is a possibility of poor attachment, such as bubbles being mixed in or the attachment position being offset. When poor attachment occurs, the following operation (secondary processing) is performed: the adhesive film is peeled off from the adherend and another adhesive film is attached. Adhesive films used as engineering materials are designed based on the premise of peeling off from the adherend, so they are easy to process secondary. On the other hand, reinforcing films based on permanent attachment are generally not assumed to be peeled off from the device but are firmly attached to the surface of the device, so they are difficult to process secondary.

Patent document 2 discloses an adhesive sheet (adhesive layer) designed to have low adhesion immediately after being bonded to an adherend and to increase adhesion over time. The adhesive film having such an adhesive layer fixedly laminated on a film substrate can be easily peeled off from the adherend immediately after being bonded to the adherend and can be firmly bonded to the adherend after a certain period of time, so it can be used as a reinforcing film with secondary processability. [Prior technical document] [Patent document]

[Patent document 1] Japanese Patent Publication No. 2017-132977 [Patent document 2] WO2015/163115 Specification

[The problem the invention is trying to solve]

Regarding the reinforcing film whose adhesion to the adherend changes over time, it is difficult to say that the flexibility of the lead time of the step is sufficient. For example, the reinforcing film with an adhesive layer whose adhesion increases over time must be inspected and processed for adhesion status within a specific time after bonding to the adherend and before the adhesion increases. In addition, when the reinforcing film is removed from a part of the device or device part after the reinforcing film is bonded to the entire surface, the processing must be performed before the adhesion increases.

In view of the above situation, the purpose of the present invention is to provide a reinforcing film that is easy to process after being bonded to an adherend, can arbitrarily set the time from bonding to the adhesion enhancement, and can be firmly bonded to the adherend by enhancing the adhesion. [Technical means to solve the problem]

The reinforcing film of the present invention has an adhesive layer fixedly laminated on one main surface of the film substrate. The adhesive layer includes a photocurable composition containing a base polymer, a photocuring agent, a photoradical initiator, and an antioxidant. The photocurable composition constituting the adhesive layer preferably includes 10 to 50 parts by weight of the photocuring agent, 0.01 to 1 part by weight of the photoradical initiator, and 0.01 to 2 parts by weight of the antioxidant relative to 100 parts by weight of the base polymer. The content of the antioxidant in the photocurable composition is preferably 0.2 to 5 times the content of the photoradical initiator.

From the viewpoint of inhibiting the generation of free radicals from the photoradical initiator by light of a fluorescent lamp or the like, it is preferable to use a photoradical initiator that has an absorption maximum in the wavelength range of 310 nm to 355 nm and does not show an absorption maximum at a wavelength longer than 360 nm.

As the base polymer of the adhesive layer, for example, an acrylic polymer can be used. The acrylic polymer preferably contains 5 to 50% by weight of a monomer component having a glass transition temperature of 40° C. or higher as a homopolymer.

The base polymer of the adhesive layer preferably has a cross-linked structure introduced therein. For example, the base polymer contains hydroxyl-containing monomers and/or carboxyl-containing monomers as monomer units, and the cross-linked structure is introduced by bonding a cross-linking agent such as a multifunctional isocyanate compound or a multifunctional epoxy compound to the functional groups thereof.

The photocuring agent of the adhesive layer is, for example, a multifunctional (meth)acrylate, and the functional group equivalent of the photocuring agent is preferably about 100 to 500 g/eq.

The reinforcement film preferably has a bonding strength of 0.005 to 5 N/25 mm with the polyimide film. After the adhesive layer is photocured, the bonding strength of the reinforcement film with the polyimide film is preferably 6 N/25 mm or more. [Effect of the invention]

The reinforcing film of the present invention increases the adhesion to the adherend by making the adhesive layer contain a photocurable composition and photocuring the adhesive layer after bonding to the adherend. Since the adhesion to the adherend is small before photocuring, secondary processing is easy, and since a higher adhesion is exhibited after photocuring, it helps to reinforce the device and improve reliability. The photocurable adhesive can arbitrarily set the curing time after bonding to the adherend. In addition, the adhesive composition contains an antioxidant in addition to the base polymer, the photocuring agent and the photoradical initiator, thereby suppressing the photocuring reaction caused by light such as fluorescent lamps in the storage environment. Therefore, the reinforcing film can be stored for a long time before being attached to the adherend and before being light-cured after being attached to the adherend. Therefore, the reinforcing film of the present invention has the advantage of being able to flexibly cope with the preparation time of the steps.

FIG1 is a cross-sectional view showing an embodiment of a reinforcing film. The reinforcing film 10 has an adhesive layer 2 on one main surface of a film substrate 1. The adhesive layer 2 is fixedly laminated on one main surface of the substrate film 1. The adhesive layer 2 is a photocurable adhesive containing a photocurable composition, and is cured by irradiation with active light such as ultraviolet rays, thereby increasing the bonding strength with the adherend.

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

The spacer 5 is peeled off from the surface of the adhesive layer 2, and the exposed surface of the adhesive layer 2 is attached to the surface of the device 20, thereby attaching the reinforcing film 10 to the surface of the device 20. This state is a state where the adhesive layer 2 is before light curing and the reinforcing film 10 (adhesive layer 2) is temporarily attached to the device 20. By light curing the adhesive layer 2, the bonding force at the interface between the device 20 and the adhesive layer 2 is increased, so that the device 20 and the reinforcing film 10 are fixed.

The so-called "fixed" refers to the state where two layers of the stack are firmly attached and it is impossible or difficult to peel them off at the interface. The so-called "tentative adhesion" refers to the state where the adhesion between two layers of the stack is relatively small and they can be easily peeled off at the interface.

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

In the device with the reinforcement film 10 attached as shown in FIG. 3, before the light curing of the adhesive layer 2, the device 20 and the adhesive layer 2 are temporarily adhered. If the film substrate 1 and the device 20 are peeled off, peeling occurs at the interface between the adhesive layer 2 and the device 20, and the state of the adhesive layer 2 being fixed on the film substrate 1 is maintained. Since no adhesive remains on the device 20, secondary processing is easy. After the adhesive layer 2 is light cured, since the adhesion between the adhesive layer 2 and the device 20 increases, it is difficult to peel the film 1 from the device 20. If the two are peeled off, there is a possibility that the cohesion of the adhesive layer 2 will be destroyed.

[Film substrate] A plastic film is used as the film substrate 1. In order to fix the film substrate 1 and the adhesive layer 2, it is preferred that the surface of the film substrate 1 to which the adhesive layer 2 is attached is not subjected to mold release treatment.

The thickness of the film substrate is, for example, about 4 to 500 μm. From the perspective of reinforcing the device by imparting rigidity or shock mitigation, the thickness of the film substrate 1 is preferably 20 μm or more, more preferably 30 μm or more, and further preferably 45 μm or more. From the perspective of making the reinforcing film flexible and thus improving operability, the thickness of the film substrate 1 is preferably 300 μm or less, and more preferably 200 μm or less. From the perspective of both mechanical strength and flexibility, the compressive strength of the film substrate 1 is preferably 100-3000 kg/cm ² , more preferably 200-2900 kg/cm ² , further preferably 300-2800 kg/cm ² , and particularly preferably 400-2700 kg/cm ² .

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

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

[Adhesive layer] The adhesive layer 2 fixedly laminated on the film substrate 1 is a photocurable composition comprising a base polymer, a photocuring agent, and a photoradical initiator. When the reinforcing film is used in an optical device such as a display, the total light transmittance of the adhesive layer 2 is preferably 80% or more, more preferably 85% or more, and further preferably 90% or more. The haze of the adhesive layer 2 is preferably 2% or less, more preferably 1% or less, further preferably 0.7% or less, and particularly preferably 0.5% or less.

Since the adhesive layer 2 has a weak adhesion to the adherend before light curing, it is easy to perform secondary processing. If the adhesive layer 2 is irradiated with active light such as ultraviolet rays, free radicals are generated from the photo radical initiator, and the adhesion to the adherend is improved through the free radical polymerization reaction (light curing) of the light curing agent. Therefore, when the device is used, the reinforcement film is not easy to peel off from the device surface, and the adhesion reliability is excellent.

Photocurable adhesives are cured by irradiation with ultraviolet rays or the like. Therefore, the adhesive layer 2 comprising a photocurable adhesive composition has the advantage that the curing time can be set arbitrarily and the preparation time of the steps can be flexibly coped with. On the other hand, before the use of the reinforcing film or before the light curing after the reinforcing film is attached to the adherend, there is a possibility that free radicals are generated from the photo-radical initiator due to light from fluorescent lamps or the like in the storage environment.

Since the amount of free radicals generated by light from fluorescent lamps is much smaller than the amount of free radicals generated by ultraviolet irradiation during photocuring, even if the film is left under fluorescent lamps for a short time, the photocuring reaction will hardly proceed. However, if the reinforcing film is kept under fluorescent lamps for a long time, the cumulative amount of free radicals generated from the photoradical initiator due to the light of the fluorescent lamp may increase, and its influence cannot be ignored. Specifically, the photocuring agent may polymerize due to the free radicals generated from the photoradical initiator, and the adhesive force of the adhesive may increase, making it difficult for the reinforcing film to be peeled off from the adherend. In addition, due to the deactivation of the photo-radical initiator caused by long-term storage, photocuring may not proceed even if irradiated with ultraviolet rays, and the bonding strength may not increase.

In the reinforcing film of the present invention, the photocurable composition constituting the adhesive layer 2 contains an antioxidant in addition to the base polymer, the photocuring agent and the photoradical initiator. By using the photoradical initiator and the antioxidant together, even when the reinforcing film is stored under a fluorescent lamp for a long time, the change in the adhesive force is small, and the adhesive force can be appropriately increased when irradiated with light.

<Composition of Adhesive> The following describes the preferred forms of the base polymer, photocuring agent, photoradical initiator, and antioxidant that constitute the photocurable composition.

(Base polymer) The base polymer is the main component of the adhesive composition. The type of base polymer is not particularly limited, and an acrylic polymer, a silicone polymer, a urethane polymer, a rubber polymer, etc. can be appropriately selected. In particular, in terms of excellent optical transparency and adhesion, and easy control of adhesion, the adhesive composition preferably contains an acrylic polymer as the base polymer, and preferably more than 50% by weight of the adhesive composition is an acrylic polymer.

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

As the alkyl (meth)acrylate, an alkyl (meth)acrylate having an alkyl group with a carbon number of 1 to 20 can be preferably used. The alkyl group of the alkyl (meth)acrylate may be a straight chain or may have a branched chain. Examples of the alkyl (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, 1,2-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1,1-dimethoxy-1 isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, isotridodecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecyl (meth)acrylate, aralkyl (meth)acrylate, and the like.

The content of the alkyl (meth)acrylate is preferably 40% by weight or more, more preferably 50% by weight or more, and even more preferably 55% by weight or more, based on the total amount of monomer components constituting the base polymer.

The acrylic base polymer preferably contains a monomer component having a crosslinkable functional group as a copolymer component. As the monomer having a crosslinkable functional group, a hydroxyl-containing monomer or a carboxyl-containing monomer can be listed. The hydroxyl group or carboxyl group of the base polymer becomes a reaction point with the crosslinking agent described below. For example, when an isocyanate crosslinking agent is used, it is preferred that the base polymer contains a hydroxyl-containing monomer as a copolymer component. When an epoxy crosslinking agent is used, it is preferred that the base polymer contains a carboxyl-containing monomer as a copolymer component. By introducing a cross-linked structure into the base polymer, the cohesive force is increased, the adhesion of the adhesive layer 2 is increased, and the amount of paste residue on the adherend during secondary processing tends to be reduced.

Examples of the hydroxyl group-containing monomer include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and 4-(hydroxymethyl)cyclohexylmethyl (meth)acrylate. Examples of the carboxyl group-containing monomer include (meth)acrylic acid, 2-carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid.

Regarding the acrylic-based polymer, the total amount of the hydroxyl-containing monomer and the carboxyl-containing monomer is preferably 1 to 30% by weight, more preferably 3 to 25% by weight, and even more preferably 5 to 20% by weight relative to the total amount of the constituent monomer components. It is particularly preferred that the content of the (meth)acrylate containing a hydroxyl group is within the above range.

The acrylic-based polymer preferably contains nitrogen-containing monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperidone, vinylpyrrol, vinylimidazole, vinyloxazole, vinyloxaline, N-acryloyloxaline, N-vinylcarboxylic acid amides, and N-vinylcaprolactam as constituent monomer components. The acrylic-based polymer containing nitrogen-containing monomer components exhibits moderate water absorption in a humid and hot environment and can inhibit local water absorption of the adhesive, thereby helping to prevent local whitening, local swelling, and peeling of the adhesive layer.

Regarding the acrylic base polymer, the content of the nitrogen-containing monomer is preferably 1 to 30 wt %, more preferably 3 to 25 wt %, and further preferably 5 to 20 wt % relative to the total amount of the constituent monomer components. The acrylic base polymer preferably contains N-vinyl pyrrolidone as the nitrogen-containing monomer within the above range.

When the acrylic base polymer contains both a hydroxyl-containing monomer and a nitrogen-containing monomer as monomer components, the cohesive force and transparency of the adhesive tend to be improved. With respect to the acrylic base polymer, the total amount of the hydroxyl-containing monomer and the nitrogen-containing monomer is preferably 5 to 50% by weight, more preferably 10 to 40% by weight, and further preferably 15 to 35% by weight relative to the total amount of the constituent monomer components.

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

The bonding properties of the adhesive before light curing are easily affected by the composition and molecular weight of the base polymer. The larger the molecular weight of the base polymer, the harder the adhesive tends to become. The weight average molecular weight of the acrylic base polymer is preferably 100,000 to 5 million, more preferably 300,000 to 3 million, and even more preferably 500,000 to 2 million. Furthermore, when a cross-linking structure is introduced into the base polymer, the molecular weight of the base polymer refers to the molecular weight before the cross-linking structure is introduced.

The higher the content of high Tg monomer components in the constituent components of the base polymer, the harder the adhesive tends to become. Furthermore, the so-called high Tg monomer refers to a monomer with a higher glass transition temperature (Tg) of the homopolymer. Examples of monomers with a homopolymer Tg of 40°C or above include: dicyclopentyl methacrylate (Tg: 175°C), dicyclopentyl acrylate (Tg: 120°C), isobutyl methacrylate (Tg: 173°C), isobutyl acrylate (Tg: 97°C), methyl methacrylate (Tg: 105°C), 1-adamantyl methacrylate (Tg: 250°C), 1-adamantyl acrylate (Tg: 153 ℃) and other (meth) acrylic monomers; vinyl monomers containing amide groups such as acrylamide (Tg: 145℃), dimethylacrylamide (Tg: 119℃), diethylacrylamide (Tg: 81℃), dimethylaminopropylacrylamide (Tg: 134℃), isopropylacrylamide (Tg: 134℃), hydroxyethylacrylamide (Tg: 98℃); N-vinylpyrrolidone (Tg: 54℃), etc.

Regarding the acrylic base polymer, the content of the monomer having a homopolymer Tg of 40°C or more is preferably 5 to 50% by weight, more preferably 10 to 40% by weight, relative to the total amount of the constituent monomer components. In order to form an adhesive layer having a moderate hardness and excellent secondary processability, as the monomer component of the base polymer, it is preferred to include a monomer component having a homopolymer Tg of 80°C or more, and more preferably a monomer component having a homopolymer Tg of 100°C or more. Regarding the acrylic base polymer, the content of the monomer having a homopolymer Tg of 100°C or more is preferably 0.1% by weight or more, more preferably 0.5% by weight or more, further preferably 1% by weight or more, and particularly preferably 3% by weight or more, relative to the total amount of the constituent monomer components. It is particularly preferred that the content of methyl methacrylate is within the above range.

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

(Crosslinking agent) From the perspective of giving the adhesive an appropriate cohesive force, it is preferred to introduce a crosslinking structure into the base polymer. For example, a crosslinking agent is added to a solution of the base polymer after polymerization and heated as necessary to introduce a crosslinking structure. Examples of the crosslinking agent include isocyanate crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, and metal chelate crosslinking agents. These crosslinking agents react with functional groups such as hydroxyl groups or carboxyl groups introduced into the base polymer to form a crosslinking structure. Isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred because they have high reactivity with the hydroxyl or carboxyl groups of the base polymer and can easily introduce a crosslinked structure.

As the isocyanate-based crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule is used. Examples of the isocyanate crosslinking agent include low-order aliphatic polyisocyanates such as butyl diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentyl diisocyanate, cyclohexyl diisocyanate, and isophorone diisocyanate; aromatic isocyanates such as 2,4-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate; trihydroxymethylpropane/toluene diisocyanate trimer adducts (e.g., "Coronate L" manufactured by Tosoh), trihydroxymethylpropane/hexamethylene diisocyanate trimer adducts (e.g., "Coronate 1" manufactured by Tosoh); HL”), trihydroxymethylpropane adduct of xylylene diisocyanate (e.g., “Takenate D110N” manufactured by Mitsui Chemicals, isocyanate adduct of hexamethylene diisocyanate (e.g., “Coronate HX” manufactured by Tosoh), etc.

As the epoxy crosslinking agent, a polyfunctional epoxy compound having two or more epoxy groups in one molecule can be used. The epoxy group of the epoxy crosslinking agent can 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, and the like. Oleyl ether, sorbitol polyglycidyl ether, glycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, polyglycerol polyglycidyl ether, sorbitan polyglycidyl ether, trihydroxymethylpropane polyglycidyl ether, adipate diglycidyl ester, phthalate diglycidyl ester, tri(2-hydroxyethyl)triisocyanate triglycidyl ester, resorcinol diglycidyl ether, bisphenol-S-diglycidyl ether, etc. As the epoxy-based crosslinking agent, commercially available products such as "DENACOL" manufactured by Nagase ChemteX, "Tetrad X" and "Tetrad C" manufactured by Mitsubishi Gas Chemical can also be used.

The amount of the crosslinking agent used can be appropriately adjusted according to the composition or molecular weight of the base polymer. The amount of the crosslinking agent used is about 0.1 to 10 parts by weight relative to 100 parts by weight of the base polymer, preferably 0.3 to 7 parts by weight, more preferably 0.5 to 5 parts by weight, and further preferably 1 to 4 parts by weight. In addition, the value obtained by dividing the amount of the crosslinking agent used relative to 100 parts by weight of the base polymer (parts by weight) by the functional group equivalent (g/eq) of the crosslinking agent is preferably 0.00015 to 0.11, more preferably 0.001 to 0.077, further preferably 0.003 to 0.055, and particularly preferably 0.0045 to 0.044. By using a larger amount of crosslinking agent than that of a conventional acrylic optical transparent adhesive, the adhesive has a moderate hardness, and there is a tendency that the residual adhesive on the adherend during secondary processing is reduced and the secondary processing property is improved.

In order to promote the formation of the crosslinked structure, a crosslinking catalyst may also be used. For example, as a crosslinking catalyst of an isocyanate-based crosslinking agent, there can be cited metal-based crosslinking catalysts (especially tin-based crosslinking catalysts) such as tetrabutyl titanium, tetraisopropyl titanium, iron (III) acetylacetonate, butyltin oxide, and dioctyltin dilaurate. The amount of the crosslinking catalyst used is generally 0.05 parts by weight or less relative to 100 parts by weight of the base polymer.

(Photocuring agent) The adhesive composition constituting the adhesive layer 2 contains a photocuring agent in addition to the base polymer. If the adhesive layer 2 containing the photocurable adhesive composition is photocured after being attached to the adherend, the bonding strength with the adherend is improved.

As the photocuring agent, a compound having two or more ethylenic unsaturated bonds in one molecule is preferred. In addition, the photocuring agent is preferably a compound showing compatibility with the base polymer. In terms of showing appropriate compatibility with the base polymer, the photocuring agent is preferably a liquid at room temperature. By making the photocuring agent compatible with the base polymer and uniformly dispersed in the composition, the contact area with the adherend can be ensured, and an adhesive layer 2 with high transparency can be formed. Furthermore, by making the base polymer and the photocuring agent exhibit appropriate compatibility, a cross-linking structure can be easily and uniformly introduced into the adhesive layer 2 after photocuring, and the bonding strength with the adherend tends to be appropriately increased.

The compatibility of the base polymer and the photocuring agent is mainly affected by the structure of the compound. The structure and compatibility of the compound can be evaluated by the Hansen solubility parameter, for example. The smaller the difference in the solubility parameters of the base polymer and the photocuring agent, the higher the compatibility tends to be.

In terms of high compatibility with acrylic-based polymers, it is preferred to use multifunctional (meth)acrylates as photocuring agents. Examples of multifunctional (meth)acrylates include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, bisphenol A ethylene oxide-modified di(meth)acrylate, bisphenol A propylene oxide-modified di(meth)acrylate, alkanediol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, ethoxylated isocyanuric acid tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol di(meth)acrylate, and the like. ester, trihydroxymethylpropane tri(meth)acrylate, di-trihydroxymethylpropane tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerol di(meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, isoprene (meth)acrylate, etc. Among them, polyethylene glycol di(meth)acrylate or polypropylene glycol di(meth)acrylate is preferred, and polyethylene glycol di(meth)acrylate is particularly preferred, in terms of excellent compatibility with acrylic-based polymers.

The compatibility between the base polymer and the photocuring agent is also affected by the molecular weight of the compound. There is a tendency that the smaller the molecular weight of the photocurable compound is, the higher the compatibility with the base polymer becomes. From the perspective of compatibility with the base polymer, the molecular weight of the photocuring agent is preferably 1500 or less, more preferably 1000 or less, further preferably 500 or less, and particularly preferably 400 or less.

In the adhesive layer 2 before photocuring, the properties of the base polymer are the main controlling factors of the adhesion. Therefore, as long as the base polymer of the adhesive composition is the same, the difference in the adhesion properties of the adhesive layer before photocuring is small even if the type of photocuring agent is different. The type or content of the photocuring agent mainly affects the adhesion of the adhesive layer after photocuring. The smaller the functional group equivalent (i.e., the larger the number of functional groups per unit molecular weight) and the larger the content of the photocuring agent, the greater the difference in the adhesion before and after photocuring can be set.

From the viewpoint of high compatibility with the base polymer and improved adhesion after photocuring, the functional group equivalent (g/eq) of the photocuring agent is preferably 500 or less, more preferably 400 or less, further preferably 300 or less, and particularly preferably 200 or less. On the other hand, if the functional group equivalent of the photocuring agent is too small, the crosslinking point density of the adhesive layer after photocuring may increase and the adhesion may decrease. Therefore, the functional group equivalent of the photocuring agent is preferably 80 or more, more preferably 100 or more, and further preferably 130 or more.

In the combination of acrylic base polymer and multifunctional acrylate photocuring agent, when the functional group equivalent weight of the photocuring agent is small, there is a tendency that the interaction between the base polymer and the photocuring agent is strong, and the initial adhesion is increased. In the use of the present invention, there is a case where an excessive increase in the initial adhesion leads to a decrease in secondary processability. From the perspective of maintaining the adhesion between the adhesive layer 2 and the adherend before photocuring within an appropriate range, it is also preferred that the functional group equivalent weight of the photocuring agent is within the above range.

The content of the photocuring agent in the adhesive composition is preferably 10 to 50 parts by weight relative to 100 parts by weight of the base polymer. By setting the amount of the photocuring agent to the above range, the adhesion between the adhesive layer and the adherend after photocuring can be adjusted to an appropriate range. The content of the photocuring agent is more preferably 15 to 45 parts by weight relative to 100 parts by weight of the base polymer, and further preferably 20 to 40 parts by weight.

(Photoradical initiator) Photoradical initiators generate free radicals by irradiation with active light, and promote the free radical polymerization reaction of the photocuring agent by transferring free radicals from the photoradical initiator to the photocuring agent. As photoradical initiators (photoradical generators), preferably, free radicals are generated by irradiation with visible light or ultraviolet light with a wavelength shorter than 450 nm, and examples thereof include hydroxy ketones, benzoyl dimethyl ketal, amino ketones, acyl phosphine oxides, benzophenones, and trichloromethyl-containing tris derivatives. Photoradical initiators can be used alone or in combination of two or more.

When transparency is required for the adhesive layer 2, the photoradical initiator is preferably less sensitive to light with a wavelength longer than 400 nm (visible light). For example, a photoradical initiator with an absorbance of 1×10 ² [mLg ^-1 cm ^-1 ] or less at a wavelength of 405 nm is preferably used. In addition, if a photoradical initiator with a lower sensitivity to visible light is used, the amount of photoradicals generated due to external light in the storage environment is smaller, thereby improving the storage stability of the reinforcing film.

From the perspective of improving the storage stability of the reinforcing film, it is better to use a photo-radical initiator that does not show a maximum absorption at a wavelength longer than 360 nm. A photo-radical initiator that shows a maximum absorption at a wavelength longer than 360 nm easily absorbs ultraviolet light from fluorescent lamps (mainly mercury rays at 365 nm and 405 nm) to generate photo-radicals. If the maximum wavelength of light absorption of the photo-radical initiator is below 360 nm, the amount of free radicals generated due to light in the storage environment such as fluorescent lamps is less. Therefore, even when the reinforcing film is exposed to fluorescent lamps for a long time, the effective concentration of the photo-radical initiator can be maintained at a higher level. In addition, a small amount of free radicals generated by light in the storage environment such as fluorescent lamps will be captured by the antioxidant, thereby inhibiting photopolymerization in the storage environment.

In order to improve the storage stability and to improve the adhesion to the adherend by light irradiation even after long-term storage, the photo radical initiator contained in the adhesive layer is preferably one with a maximum wavelength of light absorption of 355 nm or less. On the other hand, in order to improve the light curing efficiency by ultraviolet irradiation, the photo radical initiator is preferably one with a maximum light absorption at a wavelength longer than 310 nm. Based on the above situation, in order to improve the storage stability of the reinforcing film, the photo radical initiator contained in the adhesive layer 2 is preferably one that does not have a maximum absorption at a wavelength longer than 360 nm and has a maximum absorption in the wavelength range of 310 to 355 nm. The maximum absorption wavelength of the photo-free radical initiator is more preferably 315 to 350 nm, and further preferably 320 to 340 nm.

The content of the photo radical initiator in the adhesive layer 2 is preferably 0.01 to 1 part by weight, more preferably 0.02 to 0.7 part by weight, and further preferably 0.03 to 0.5 part by weight relative to 100 parts by weight of the base polymer. The content of the photo radical initiator in the adhesive layer 2 is preferably 0.005 to 0.5 part by weight, more preferably 0.01 to 0.4 part by weight, and further preferably 0.02 to 0.3 part by weight relative to 100 parts by weight of the photocuring agent. If the content of the photo radical initiator in the adhesive layer is too small, there is a possibility that the photocuring reaction will not be sufficiently performed even if irradiated with ultraviolet rays. If the content of the photo-radical initiator is too high, even when an antioxidant is used concurrently, the photocuring reaction may proceed under the storage environment, and the adhesion to the adherend may increase, making secondary processing of the reinforcement film difficult.

(Antioxidant) Antioxidants have the function of inhibiting the photocuring reaction of the reinforced film in the storage environment. Even if a small amount of photoradicals are generated from the photoradical initiator due to light from a fluorescent lamp or the like, the antioxidant captures the free radicals to generate stable free radicals, thereby inhibiting the transfer of free radicals to the photocuring agent. Therefore, photocuring (radical polymerization reaction of the photocuring agent) caused by light from a fluorescent lamp or the like can be inhibited.

As the antioxidant, there can be mentioned amine antioxidants, sulfur antioxidants, phosphorus antioxidants, phenol antioxidants and the like.

Examples of sulfur-based antioxidants include dilauryl 3,3'-thiodipropionate, dimyristyl 3,3'-thiodipropionate, distearyl 3,3'-thiodipropionate, etc. Examples of phosphorus-based antioxidants include triphenyl phosphite, isodecyl diphenyl phosphite, and diisodecyl phenyl phosphite.

As amine antioxidants, there can be listed: monoalkyldiphenylamines such as monooctyldiphenylamine and monononyldiphenylamine; dialkyldiphenylamines such as 4,4'-dibutyldiphenylamine, 4,4'-dipentyldiphenylamine, 4,4'-dihexyldiphenylamine, 4,4'-diheptyldiphenylamine, 4,4'-dioctyldiphenylamine, 4,4'-dinonyldiphenylamine; polyalkyldiphenylamines such as tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenylamine, tetranonyldiphenylamine; naphthylamines such as α-naphthylamine, phenyl-α-naphthylamine, butylphenyl-α-naphthylamine, pentylphenyl-α-naphthylamine, hexylphenyl-α-naphthylamine, heptylphenyl-α-naphthylamine, octylphenyl-α-naphthylamine, nonylphenyl-α-naphthylamine, etc.

As phenolic antioxidants, there are: 2,6-di-tert-butyl-p-cresol (dibutylhydroxytoluene; BHT), butylated hydroxyanisole, 2,6-di-tert-butyl-4-ethylphenol, β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate stearyl and other monophenolic antioxidants; 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 4,4'-thiobis(3-methyl-6-tert-butylphenol), 4,4'-butylenebis(3-methyl-6-tert-butylphenol), 3,9-bis[1,1-dimethyl-2-[β-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy] Bisphenol antioxidants such as 2,4,8,10-tetraoxaspiro[5,5]undecane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, tetrakis-[methylene-3-(3',5'-di-tert-butyl)-4-hydroxybenzyl] High molecular weight phenolic antioxidants such as tributyl-4'-hydroxyphenyl) propionate] methane, bis[3,3'-bis-(4'-hydroxy-3'-tert-butylphenyl) butyrate] diol ester, 1,3,5-tris(3',5'-di-tert-butyl-4'-hydroxybenzyl)-S-trioxan-2,4,6-(1H,3H,5H)trione, tocopherol, etc.

From the viewpoint of inhibiting photohardening caused by free radicals generated from a photo-radical initiator by light of a fluorescent lamp or the like, among the antioxidants, those that function as free radical chain inhibitors, such as amine antioxidants and phenolic antioxidants, are preferred, and phenolic antioxidants having a hindered phenol structure are particularly preferred.

The antioxidant having a hindered phenol structure is one in which at least one of the carbon atoms adjacent to the carbon atom on the aromatic ring to which the OH group of phenol is bonded is bonded to a group having a large steric hindrance such as a tert-butyl group. As antioxidants with hindered phenol structures, they include butylated hydroxytoluene (BHT), pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] ("Irganox 1010" manufactured by BASF), octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate ("Irganox 1076" manufactured by BASF), 3,3',3'',5,5',5''-hexa-tert-butyl-a,a',a''-(1,3,5-trimethylbenzene-2,4,6-triyl) tri-p-cresol ("Irganox 1330”), 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-tris-2,4,6(1H,3H,5H)-trione (“Irganox 3114” manufactured by BASF), isocyanuric acid tris[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxyethyl]ester (“Irganox 3125” manufactured by BASF), pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (“Adekastab” manufactured by ADEKA), AO-60”), 3,9-bis{2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl}-2,4,8,10-tetraoxaspiro[5.5]undecane (“Adekastab AO-80” manufactured by ADEKA), 2-[1-(2-hydroxy-3,5-di-tert-pentylphenyl)ethyl]-4,6-di-tert-pentylphenyl acrylate (“Sumilizer GS” manufactured by Sumitomo Chemical), 2-tert-butyl-4-methyl-6-(2-hydroxy-3-tert-butyl-5-methylbenzyl)phenyl acrylate (“Sumilizer GM”), 2,2'-dimethyl-2,2'-(2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diyl)dipropane-1,1'-diyl=bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate] (Sumitomo Chemical’s “Sumilizer GA-80”), 1,3,5-tris(3-hydroxy-4-tert-butyl-2,6-dimethylbenzyl)-1,3,5-tris(2,4,6(1H,3H,5H)-trione) (Cyanox 1790, Cytec’s

The content of the antioxidant in the adhesive layer 2 is preferably 0.01 to 2 parts by weight, more preferably 0.03 to 1 part by weight, further preferably 0.04 to 0.7 parts by weight, and particularly preferably 0.05 to 0.5 parts by weight relative to 100 parts by weight of the base polymer. The content of the antioxidant is preferably 0.2 to 5 times the content of the photo-radical initiator, more preferably 0.3 to 3 times, and further preferably 0.5 to 2 times.

(Effect of combined use of photoradical initiator and antioxidant) In a composition containing an antioxidant in addition to a photoradical initiator, even when a small amount of photoradicals are generated from the photoradical initiator due to light from a fluorescent lamp, the antioxidant captures the radicals to generate stable radicals. Therefore, the radical polymerization reaction caused by light from a fluorescent lamp can be suppressed. From the viewpoint of suppressing the polymerization reaction during storage of the reinforced film, a larger content of the antioxidant is preferred. On the other hand, if the content of the antioxidant is too large, even if ultraviolet light is irradiated for photocuring, the antioxidant captures most of the photogenerated radicals, thereby hindering the transfer of radicals from the photoradical initiator to the photocuring agent, resulting in insufficient photocuring. Therefore, in order to suppress photopolymerization in the storage environment and prevent the increase of adhesive strength, and to appropriately carry out the photocuring reaction during light irradiation to increase the adhesive strength, it is preferred that the content of the antioxidant is within the above range.

By using a photo-radical initiator together with an antioxidant, the antioxidant captures the free radicals generated by light from fluorescent lamps, etc., thereby inhibiting the increase in adhesion caused by the photocuring reaction in the storage environment. Therefore, regardless of the type of photo-radical initiator, the photocuring of the adhesive layer in the storage environment can be inhibited by the action of the antioxidant.

On the other hand, if a reinforcing film having an adhesive layer containing a photoradical initiator and an antioxidant is exposed to a fluorescent lamp for a long time, the effective concentration of the photoradical initiator (the concentration of the photoradical initiator capable of generating photoradicals) decreases due to the inactivation of the photoradical initiator. Therefore, even if ultraviolet rays are irradiated for photocuring, the amount of photoradicals generated is small, and the photocuring reaction may not proceed sufficiently.

It is preferred that the reinforcing film is not easily photocured by light from fluorescent lamps in the storage environment even after long-term storage, and that sufficient free radicals are generated when exposed to light, and the adhesion to the adherend is increased through the free radical polymerization reaction of the photocuring agent. In order to fully perform the photocuring reaction even after long-term storage, it is preferred to use a photoradical initiator with a small amount of free radicals generated in the storage environment. Specifically, it is preferred to use the following photoradical initiator: as described above, it has a maximum absorption wavelength below 360 nm, and the amount of photoradicals generated by the mercury line of fluorescent lamps (especially the wavelength of 365 nm) is small.

The amount of free radicals generated from the photo-radical initiator caused by the light of a fluorescent lamp can be evaluated by, for example, irradiating with light of a wavelength of 365 nm at extremely low temperature and quantifying the amount of free radicals using the electron spin resonance (ESR) method as shown in the examples described below. Since the free radicals from the photo-radical initiator have an extremely short life span at room temperature, extremely low temperatures are suitable for evaluating the amount of free radicals generated.

(Other additives) In addition to the components listed above, the adhesive layer may also contain additives such as silane coupling agents, adhesion imparting agents, plasticizers, softeners, anti-degradation agents, fillers, colorants, ultraviolet absorbers, surfactants, antistatic agents, etc. within the range that does not impair the characteristics of the present invention.

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

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

The thickness of the adhesive layer 2 is, for example, about 1 to 300 μm. There is a tendency that the thicker the adhesive layer 2 is, the better the adhesion to the adherend. On the other hand, when the thickness of the adhesive layer 2 is too large, the fluidity before light curing is higher and the operation becomes difficult. Therefore, the thickness of the adhesive layer 2 is preferably 5 to 100 μm, more preferably 8 to 50 μm, further preferably 10 to 40 μm, and particularly preferably 13 to 30 μm.

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

By introducing a cross-linked structure into the base polymer, the gel fraction increases. The higher the gel fraction, the harder the adhesive is, and when the reinforcing film is peeled off from the adherend due to secondary processing, etc., the tendency of the adhesive residue to remain on the adherend can be suppressed. The gel fraction of the adhesive layer 2 before light curing is preferably 30% or more, more preferably 50% or more, further preferably 60% or more, and particularly preferably 65% or more. The gel fraction of the adhesive layer 2 before light curing can also be 70% or more or 75% or more.

Since the adhesive contains unreacted photocuring agent, the gel fraction of the adhesive layer 2 before photocuring is generally 90% or less. If the gel fraction of the adhesive layer 2 before photocuring is too large, the gripping force on the adherend is reduced and the initial adhesion becomes insufficient. Therefore, the gel fraction of the adhesive layer 2 before photocuring is preferably 85% or less, and more preferably 80% or less.

The gel fraction can be obtained in the form of insoluble matter in solvents such as ethyl acetate. Specifically, it is obtained by immersing the adhesive layer in ethyl acetate at 23°C for 7 days and the weight fraction (unit: weight %) of the sample before immersion. Generally speaking, the gel fraction of a polymer is the same as the degree of crosslinking. The more crosslinked parts in the polymer, the greater the gel fraction. In addition, the greater the amount of light curing agent, the smaller the gel fraction.

After the crosslinking structure is introduced into the polymer by the crosslinking agent, the photocuring agent also remains in an unreacted state. Therefore, a photocurable adhesive layer 2 including the base polymer and the photocuring agent is formed. When the adhesive layer 2 is formed on the film substrate 1, it is preferred to attach a spacer 5 to the adhesive layer 2 for the purpose of protecting the adhesive layer 2. Alternatively, the crosslinking may be performed after attaching the spacer 5 to the adhesive layer 2.

When the adhesive layer 2 is formed 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 for forming the adhesive layer can also be directly used as the spacer 5.

As the isolating member 5, a plastic film such as polyethylene, polypropylene, polyethylene terephthalate, polyester film, etc. can be preferably used. The thickness of the isolating member is usually 3 to 200 μm, preferably about 10 to 100 μm. It is preferred that the contact surface of the isolating member 5 with the adhesive layer 2 is subjected to a mold release treatment using a mold release agent such as a polysilicone-based, fluorine-based, long-chain alkyl-based, or fatty amide-based release agent, or silicon dioxide powder, etc. By performing a release treatment on the surface of the isolating member 5, when the film substrate 1 and the isolating member 5 are peeled off, the state in which the adhesive layer 2 and the isolating member 5 are peeled off and the adhesive layer 2 is fixed on the film substrate 1 can be maintained.

[Use of reinforcing film] The reinforcing film of the present invention is used by being attached to a device or a component of a device. The reinforcing film 10 is a film substrate 1 fixed with an adhesive layer 2, and has a relatively small adhesion to the adherend after being attached to the adherend and before being photocured. Therefore, before photocuring, the reinforcing film is easy to peel off from the adherend and has excellent secondary processing properties. In addition, before photocuring, the reinforcing film can be cut and a portion of the surface of the adherend can be removed easily.

The adherend to which the reinforcing film is applied is not particularly limited, and various electronic devices, optical devices and their components can be listed. The reinforcing film can be applied to the entire surface of the adherend, or selectively applied only to the portion that needs reinforcement. In addition, after the reinforcing film is applied to the entire surface of the adherend, the reinforcing film on the portion that does not need reinforcement can be cut off and the reinforcing film can be peeled off and removed. Before light curing, the reinforcing film is temporarily adhered to the surface of the adherend, so the reinforcing film can be easily peeled off and removed from the surface of the adherend.

<Characteristics of the adhesive layer before photocuring> (Adhesion) From the perspective of facilitating peeling from the adherend and preventing the adhesive residue of the reinforcing film from remaining in the adherend after peeling, the adhesion between the adhesive layer 2 and the adherend before photocuring is preferably 5 N/25 mm or less, more preferably 2 N/25 mm or less, and further preferably 1.3 N/25 mm or less. From the viewpoint of preventing the reinforcing sheet from peeling off during storage or handling, the adhesion between the adhesive layer 2 and the adherend before light curing is preferably 0.005 N/25 mm or more, more preferably 0.01 N/25 mm or more, further preferably 0.1 N/25 mm or more, and particularly preferably 0.3 N/25 mm or more.

The reinforcing film preferably has a bonding strength to the polyimide film in the above range before the adhesive layer is photocured. In flexible display panels, flexible printed wiring boards (FPCs), devices integrating display panels and wiring boards, etc., flexible substrate materials are used, and polyimide films are generally used from the viewpoint of heat resistance or dimensional stability. The reinforcing film having the above bonding strength to the polyimide film as the substrate is easy to peel off before the adhesive is photocured, and has excellent bonding reliability after photocuring.

(Storage modulus) The shear storage modulus _G'i of the adhesive layer 2 at 25°C before light curing is preferably 1×10 ⁴ to 1.2×10 ⁵ Pa. The shear storage modulus (hereinafter simply referred to as "storage modulus") is obtained by reading the value at a specific temperature when measuring in accordance with the method described in JIS K7244-1 "Plastics - Test methods for dynamic mechanical properties" at a frequency of 1 Hz in the range of -50 to 150°C at a heating rate of 5°C/min.

In materials that exhibit viscoelastic properties such as adhesives, the storage modulus G' is used as an indicator of the degree of hardness. The storage modulus of the adhesive layer has a high correlation with the cohesive force, and there is a tendency that the higher the cohesive force of the adhesive, the greater the gripping force on the adherend. As long as the storage modulus of the adhesive layer 2 before photocuring is 1×10 ⁴ Pa or more, the adhesive has sufficient hardness and cohesive force, so it is not easy to produce paste residues on the adherend when the reinforcing film is peeled off from the adherend. In addition, when the storage modulus of the adhesive layer 2 is large, the adhesive can be suppressed from overflowing from the end surface of the reinforcing film. As long as the storage modulus of the adhesive layer 2 before light curing is less than 1.2×10 ⁵ Pa, the adhesive layer 2 and the adherend can be easily peeled off at the interface, and it is not easy to cause coagulation and destruction of the adhesive layer or paste residue on the surface of the adherend during secondary processing.

From the viewpoint of improving the secondary processability of the reinforcing sheet and thereby suppressing the residual paste on the adherend during the secondary processing, the storage modulus _G'i of the adhesive layer 2 at 25°C before light curing is more preferably 3×10 ⁴ to 1×10 ⁵ Pa, and further preferably 4×10 ⁴ to 9.5×10 ⁴ Pa.

<Photocuring of adhesive layer> After the reinforcing film is attached to the adherend, the adhesive layer 2 is irradiated with active light to photocure the adhesive layer. Examples of active light include ultraviolet light, visible light, infrared light, X-rays, α-rays, β-rays, and γ-rays. Ultraviolet light is preferred as active light because it can inhibit the curing of the adhesive layer in storage and is easy to cure. The irradiation intensity or irradiation time of the active light can be appropriately set according to the composition or thickness of the adhesive layer. The irradiation of the adhesive layer 2 with active light can be carried out from either the film substrate 1 side or the adherend side, or from both sides.

<Characteristics of the adhesive layer after photocuring> (Adhesion) From the perspective of adhesion reliability during practical use of the device, the adhesion between the adhesive layer 2 after photocuring and the adherend is preferably 6 N/25 mm or more, more preferably 10 N/25 mm or more, further preferably 12 N/25 mm or more, and particularly preferably 14 N/25 mm or more. The reinforcing film preferably has an adhesion within the above range for the adhesive layer after photocuring to the polyimide film. The adhesion between the adhesive layer 2 after photocuring and the adherend is preferably 4 times or more, more preferably 8 times or more, and further preferably 10 times or more of the adhesion between the adhesive layer 2 before photocuring and the adherend.

The adhesive layer 2 preferably has a storage modulus _G'f of 1.5×10 ⁵ Pa or more at 25°C after light curing. If the storage modulus of the adhesive layer 2 after light curing is 1.5×10 ⁵ Pa or more, as the cohesive force increases, the adhesion to the adherend increases, and a higher adhesion reliability can be obtained. On the other hand, when the storage modulus is too large, the adhesive is difficult to wet and expand, and the contact area with the adherend becomes smaller. In addition, the stress dispersibility of the adhesive is reduced, so there is a tendency for the peeling force to be easily transmitted to the bonding interface, and the adhesion to the adherend is reduced. Therefore, the storage modulus _G'f of the adhesive layer 2 at 25°C after photocuring is preferably 2×10 ⁶ Pa or less. From the viewpoint of improving the bonding reliability of the reinforcing sheet after photocuring the adhesive layer, _G'f is more preferably 1.8×10 ⁵ to 1.2×10 ⁶ Pa, and further preferably 2×10 ⁵ to 1×10 ⁶ Pa.

The ratio of the storage modulus of the adhesive layer 2 at 25°C before and after photocuring, _G'f / _G'i , is preferably 2 or more. If _G'f is more than twice _G'i , the increase in G' caused by photocuring is greater, and both the secondary processability before photocuring and the bonding reliability after photocuring can be taken into account. _G'f / _G'i is more preferably 4 or more, further preferably 8 or more, and particularly preferably 10 or more. There is no particular upper limit to _G'f / _G'i . When _G'f /G'i _is too large, it is easy to cause poor initial bonding due to small G' before photocuring, or reduce bonding reliability due to excessive G' after photocuring. Therefore, _G'f / _G'i is preferably 100 or less, more preferably 40 or less, further preferably 30 or less, and particularly preferably 25 or less.

The adherend with the reinforcement film may be subjected to a heat treatment such as autoclave treatment for the purpose of improving the affinity of the laminated interface of multiple laminated components, or heat pressing for bonding circuit components. When such a heat treatment is performed, it is preferred that the adhesive between the reinforcement film and the adherend does not flow from the end surface.

From the viewpoint of suppressing the overflow of the adhesive during high temperature heating, the storage modulus of the adhesive layer 2 after light curing at 100°C is preferably 5×10 ⁴ Pa or more, more preferably 8×10 ⁴ Pa or more, and further preferably 1×10 ⁵ Pa or more. From the viewpoint of preventing the overflow of the adhesive during heating and preventing the decrease in adhesion during heating, the storage modulus of the adhesive layer 2 after light curing at 100°C is preferably 60% or more of the storage modulus at 50°C, more preferably 65% or more, further preferably 70% or more, and particularly preferably 75% or more.

[Usage form of reinforcing film] The reinforcing film of the present invention is used by laminating components (semi-finished products) of various devices or finished devices. Since appropriate rigidity can be imparted by laminating the reinforcing film, it can be expected that the operability can be improved or the damage prevention effect can be achieved. In the manufacturing step of the device, when laminating the reinforcing film to the semi-finished product, the reinforcing film can be laminating to the semi-finished product that is large before being cut into the product size. The reinforcing film can also be laminating the reinforcing film roll-to-roll to the mother roll of the device manufactured by the roll-to-roll process (role to role process).

As devices become more highly integrated, smaller, lighter, and thinner, there is a tendency for the thickness of components that make up the devices to become smaller. As components become thinner, bending or curling due to stress at the layer interface is more likely to occur. Also, due to thinness, deflection due to its own weight is more likely to occur. Since the adherend can be given rigidity by laminating a reinforcing film, bending, curling, deflection, etc. caused by stress or its own weight can be suppressed, and operability is improved. Therefore, defects or adverse conditions during transportation or processing caused by automated equipment can be prevented by laminating a reinforcing film to the semi-finished product during the manufacturing step of the device.

During automatic transport, it is inevitable that the semi-finished product being transported will come into contact with the transport arm or pins. In addition, there are cases where the semi-finished product is cut to adjust the shape or remove useless parts. When a highly integrated, small, light, and thin device comes into contact with a transport device or undergoes a cutting process, it is easy for damage caused by local stress concentration to occur. In the manufacturing process of a device formed by stacking multiple components, not only are the components stacked in sequence, but also a part of the component or engineering material is peeled off and removed from the semi-finished product. When the component is too thin, stress is locally concentrated on the peeled portion and its vicinity, causing damage or dimensional changes. Since the reinforcing film has stress dispersion properties obtained through the adhesive layer, it can be applied to the transported and processed objects to impart appropriate rigidity, and the stress can be relieved and dispersed, thereby suppressing undesirable conditions such as cracking, rupture, peeling, and dimensional changes.

Thus, by attaching the reinforcing film of the present invention, the semi-finished product as the adherend can be given appropriate rigidity, and the stress can be relieved and dispersed, so that various undesirable conditions that may occur in the manufacturing steps can be suppressed, so that the production efficiency is improved and the yield rate is improved. In addition, since the reinforcing film is easier to peel off from the adherend before the adhesive layer is light-cured, it is also easier to perform secondary processing during lamination or when poor lamination occurs.

The adhesive layer 2 of the reinforcing film of the present invention is photocurable, and the curing time can be set arbitrarily. Since the secondary processing or processing of the reinforcing film can be carried out at any time between the time when the adherend is attached to the reinforcing film and before the adhesive is photocured, the preparation time of the manufacturing steps of the device can be flexibly coped with. As mentioned above, since the adhesive layer contains an antioxidant in addition to the photocuring agent and the photoradical initiator, it is not easy to be photocured by light such as a fluorescent lamp. Therefore, even if the reinforcing film is attached to the adherend and stored for a long time, the reinforcing film can be easily peeled off from the adherend before photocuring.

When the device is used after completion, if it is accidentally subjected to external forces due to the device falling, heavy goods being placed on the device, or flying objects colliding with the device, the device can be prevented from being damaged by affixing the reinforcing film. In addition, since the reinforcing film is firmly attached to the device after the adhesive is photocured, it is not easy to peel off during long-term use, and the reliability is excellent. [Example]

The invention is further described with reference to the following embodiments, but the invention is not limited to these embodiments.

[Preparation of reinforcing film] <Polymerization of base polymer> In a reaction vessel equipped with a thermometer, a stirrer, a reflux condenser and a nitrogen inlet tube, 63 parts by weight of 2-ethylhexyl acrylate (2EHA) as monomers, 15 parts by weight of N-vinyl pyrrolidone (NVP), 9 parts by weight of methyl methacrylate (MMA), and 13 parts by weight of hydroxyethyl acrylate (HEA), 0.2 parts by weight of azobisisobutyronitrile as a thermal polymerization initiator, and 233 parts by weight of ethyl acetate as a solvent were added, and nitrogen was flowed in, and nitrogen replacement was performed for about 1 hour while stirring. Thereafter, the mixture was heated to 60°C and reacted for 7 hours to obtain a solution of an acrylic polymer with a weight average molecular weight (Mw) of 1.2 million.

<Preparation of adhesive composition> Mitsui Chemicals' "Takenate D110N" (75% ethyl acetate solution of trihydroxymethylpropane adduct of xylylene diisocyanate) as a crosslinking agent, Shin-Nakamura Chemical Industries' "A-200" (polyethylene glycol #200 (n=4) diacrylate; molecular weight 308, functional group equivalent 154 g/eq) as a multifunctional acrylic monomer, a photoradical initiator, and an antioxidant were added to the acrylic polymer solution and uniformly mixed to prepare an adhesive composition. The amount of the crosslinking agent (solid content) was set to 2.5 parts by weight relative to 100 parts by weight of the base polymer, and the amount of the multifunctional acrylic monomer was set to 30 parts by weight relative to 100 parts by weight of the base polymer. As an antioxidant, pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] ("Irganox 1010" manufactured by BASF) was prepared in the amount shown in Table 1. As a photo-radical initiator, the following was prepared in the amount shown in Table 1.

(Photoradical initiator) IRG184: 1-Hydroxycyclohexylphenyl ketone ("Irgacure 184" manufactured by BASF, absorption peak wavelengths: 246 nm, 280 nm, 333 nm) IRG651: 2,2-dimethoxy-1,2-diphenylethane-1-one ("Irgacure 651" manufactured by BASF, absorption peak wavelengths: 250 nm, 340 nm) IRG819: Bis(2,4,6-trimethylbenzyl)-phenylphosphine oxide ("Irgacure 819" manufactured by BASF, absorption peak wavelengths: 295 nm, 370 nm)

<Adhesive solution application and cross-linking> The adhesive composition was applied to a 75 μm thick polyethylene terephthalate film ("Lumirror S10" manufactured by Toray) without surface treatment using a slot roll so that the thickness after drying was 25 μm. The solvent was removed by drying at 130°C for 1 minute, and then the release-treated surface of the spacer (25 μm thick polyethylene terephthalate film with silicone release treatment) was attached to the adhesive-coated surface. Afterwards, an aging treatment was performed at 25° C. for 4 days to allow crosslinking to proceed, thereby obtaining a reinforcement film in which a photocurable adhesive sheet was fixedly laminated on a film substrate and a spacer was temporarily adhered thereon.

[Measurement of adhesion force] A polyimide film with a thickness of 12.5 μm ("Kapton 50EN" manufactured by DU PONT-TORAY) was attached to a glass plate via a double-sided tape ("No.531" manufactured by Nitto Denko) to obtain a polyimide film substrate for measurement. The isolation piece was peeled off from the surface of the reinforcement film cut to a width of 25 mm × a length of 100 mm, and then attached to the polyimide film substrate for measurement using a hand roller as a test sample before light curing. The reinforcing film side (PET film side) of the test sample before light curing was irradiated with ultraviolet light with a cumulative light amount of 400 mJ/ ^cm2 using a LED light source with a wavelength of 365 nm to light cure the adhesive layer, and the obtained product was used as the test sample after light curing. Using these test samples, the ends of the polyethylene terephthalate film as the reinforcing film were held by a chuck, and the reinforcing film was peeled off 180° at a tensile speed of 300 mm/min, and the peeling strength was measured.

<Evaluation of the adhesion of the reinforcement film after storage for a certain period of time> The reinforcement sheet was placed in a bright room at room temperature, and after 1 week and 4 weeks, it was bonded to the polyimide film substrate for measurement. The 180° peel strength of the test samples before and after light curing was measured in the same way as above.

The composition of the adhesive of each reinforcing film and the test results of the adhesion before and after light curing are shown in Table 1.

[Table 1]

For any sample, the adhesion force before light curing was in the range of 0.2 to 0.4 N/25 mm immediately after fabrication, and the sample was easily peeled off from the adherend. After light curing, the adhesion force increased to more than 15 N/25 mm, and the sample was firmly bonded to the adherend.

In the case of sample 10 using an adhesive without an antioxidant, the adhesive strength before light curing increased significantly after 4 weeks of storage, and it was difficult to peel off from the adherend. On the other hand, in the case of sample 11 to which 0.1 parts by weight of an antioxidant was added, the adhesive strength before light curing increased slightly after 4 weeks of storage, but it was sufficient to peel off from the adherend. In addition, the adhesive strength after light curing of sample 11 after 4 weeks of storage was equivalent to that of the sample just after production.

Samples 20 and 30 using adhesives without antioxidants showed a significant increase in adhesion before light curing after 4 weeks of storage, similar to Sample 10. The adhesion before light curing also increased to more than 10 N/25 mm for Samples 20 and 30 after 1 week of storage. Samples 21 and 31 to which 0.1 parts by weight of antioxidants were added also showed lower adhesion before light curing after 1 week of storage, and showed higher adhesion after light curing.

According to the comparison of the above results, it can be seen that by using the photo-radical initiator and the antioxidant together, the following reinforced film can be obtained, which has excellent secondary processability of being peeled off from the adherend before photocuring even when stored under fluorescent light for a long time, and exhibits higher adhesion to the adherend through photocuring.

After one week of storage, the rate of increase in adhesion obtained by light curing of sample 12 with 0.5 parts by weight of antioxidant added decreased, and after four weeks of storage, the adhesion hardly increased even after UV irradiation. After one week of storage, the adhesion of sample 13 with 1 part by weight of antioxidant added was not sufficiently increased by UV irradiation. In the comparison of samples 21 to 23 and samples 31 to 33, it can be seen that as the amount of antioxidant added increases, the adhesion does not increase even after UV irradiation.

According to these results, when the amount of antioxidant added is large, there is a tendency for light curing to be difficult as the storage time from the preparation sample increases. The reason is believed to be that as the storage time from the preparation sample under the fluorescent lamp increases, the photo-free radical initiator is inactivated, and even if ultraviolet irradiation is performed, the amount of free radicals generated is small.

If the storage time under fluorescent lamps increases, the effective concentration of photoradical initiators decreases, and the amount of free radicals generated during ultraviolet irradiation decreases. Antioxidants have the function of capturing free radicals generated from photoradical initiators due to light from fluorescent lamps in the storage environment, and inhibiting undesired photoradical polymerization. However, they also have the function of capturing free radicals when irradiating ultraviolet rays for photocuring. Therefore, when the storage time under fluorescent lamps increases and the effective concentration of photoradical initiators is low, the ratio of free radicals captured by antioxidants among the photoradicals generated from photoradical initiators increases, and the ratio of free radicals that contribute to photoradical polymerization decreases. Since the greater the content of the antioxidant, the greater the polymerization inhibition effect (polymerization barrier), it is considered that even if irradiated with ultraviolet rays, the photo radical reaction of the photocuring agent is not easy to proceed, and the bonding strength will not be sufficiently increased.

Based on the above results, it can be seen that by properly adjusting the amount of antioxidant used in combination with the photo-radical initiator, the following reinforced film can be obtained: even when stored under fluorescent light for a long time, the secondary processability of peeling off from the adherend before photocuring is excellent, and by properly performing photocuring by ultraviolet irradiation, a higher adhesion can be achieved.

[Determination and discussion of free radical concentration by electron spin resonance] In order to verify the free radical generation amount and antioxidant effect of the photo-free radical initiator used in the above-mentioned embodiment, light was irradiated at extremely low temperature and the free radical generation amount was evaluated by ESR.

For each of the systems using a photoradical initiator (IRG184, IRG651 or IRG819) alone, an antioxidant (Irganox 1010) alone, and a combination of a photoradical initiator (Irg651 or Irg819) and an antioxidant, a 0.2 mol/L ethyl acetate solution was prepared (the concentration of each in the combination system was set to 0.2 mol/L). 0.1 mL of the sample solution was filled into an ESR sample tube (quartz tube of about 3.5 mmφ), and placed in the ESR chamber, and light irradiation ESR measurement was performed at a temperature of 40 K. The light irradiation lamp used was an ultra-high pressure mercury lamp (manufactured by Ushio), and short wavelength light below 290 nm was cut off by a glass filter, and heat radiation was cut off by a water filter. The illuminance measured at the front of the cavity (wavelength 365 nm) was 18 mW/cm ² . The apparatus and main measurement conditions are shown below.

Device: ESP350E (BRUKER) Accessories: HP5351B microwave frequency counter (HEWLETT PACKARD) ER035M Gaussmeter (BRUKER) ESR910 cryostat (BRUKER) Cavity: TM110, cylindrical Measurement temperature: 40 K Central magnetic field: 3385 G Magnetic field scan width: 400 G Modulation: 100 kHz, 10 G Microwave: 9.49 GHz, 0.16 mW Scan time: 83.89 seconds × 2 times Time constant: 327.68 milliseconds Number of data points: 1024

The amount of free radicals was quantified by the calibration curve method based on the ESR spectra at 5 minutes, 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes, and 60 minutes after the start of light irradiation. For each sample, the light irradiation time and free radical concentration were plotted and the curves are shown in Figure 4.

Among the three photo-radical initiators, Irgacure 819 (IRG819) showed the highest amount of free radical generation. The reason is believed to be that IRG819 shows a high absorption at a wavelength of 370 nm and is more sensitive to irradiated light (wavelength 365 nm). Among Irgacure 184 (IRG184) and Irgacure 651 (IRG651), Irgacure 651 showed a greater amount of free radical generation.

It can be seen that there is a correlation between the amount of free radicals generated and the change in the adhesion before light curing when samples 10, 20, and 30 were stored under fluorescent lamps. That is, it is considered that since the amount of free radicals generated by the light from the fluorescent lamp is small for Irgacure 184, the adhesion before light curing did not increase after one week of storage for sample 10. In contrast, since the amount of photo free radicals generated by the light from the fluorescent lamp is relatively large for sample 20 using Irgacure 651 and sample 30 using Irgacure 819, the adhesion before light curing increased in the samples after one week of storage.

The antioxidant (Irganox 1010) showed a higher free radical concentration than the three photo-radical initiators. The reason is believed to be that the free radicals generated by the antioxidant are more stable and disappear less than those of the photo-radical initiators. Therefore, as the light irradiation time increases, the amount of free radicals accumulated in the system is greater.

In the system of using a photo-radical initiator (IRG651 or IRG819) and an antioxidant together, the free radical concentration was lower than that when the antioxidant was used alone. Since the free radical concentration of the combined system was lower than the sum of the cases of using a photo-radical initiator alone and the case of using an antioxidant alone, it is believed that the free radicals generated from the photo-radical initiator were captured by the antioxidant.

1. Film substrate 2. Adhesive layer 5. Isolation element 10. Reinforcement film 20. Device

FIG1 is a cross-sectional view showing the laminated structure of the reinforcing film. FIG2 is a cross-sectional view showing the laminated structure of the reinforcing film. FIG3 is a cross-sectional view showing a device with the reinforcing film attached thereto. FIG4 is a graph showing the results of measuring the amount of free radicals generated by light irradiation at extremely low temperatures.

Claims (8)

A reinforcing film comprises a film substrate and an adhesive layer fixedly laminated on one main surface of the film substrate; the adhesive layer comprises a photocurable composition containing a base polymer, a photocuring agent, a photoradical initiator, and an antioxidant, and the adhesive layer comprises 10 to 50 parts by weight of the photocuring agent, 0.01 to 1 part by weight of the photoradical initiator, and 0.01 to 2 parts by weight of the antioxidant relative to 100 parts by weight of the base polymer, and the content of the antioxidant is 0.2 to 3 times the content of the photoradical initiator. As in claim 1, the photo-radical initiator has an absorption peak in the wavelength range of 310nm to 355nm, and does not show an absorption peak at wavelengths longer than 360nm. As in claim 1 or 2, the reinforcing film, wherein the base polymer contains one or more monomer units selected from the group consisting of hydroxyl-containing monomers and carboxyl-containing monomers, and a cross-linking structure is introduced by a cross-linking agent bonded to a hydroxyl group or a carboxyl group. The reinforcing film of claim 1 or 2 contains an acrylic polymer as the above-mentioned base polymer. As in claim 4, the reinforcing film, wherein the acrylic polymer contains 5 to 50% by weight of a monomer component whose homopolymer glass transition temperature is above 40°C. As in the reinforcing film of claim 1 or 2, the photocuring agent is a multifunctional (meth)acrylate. For the reinforcing film of claim 1 or 2, the functional group equivalent of the above-mentioned photocuring agent is 100~500g/eq. For the reinforcing film of claim 1 or 2, the adhesion between the adhesive layer and the polyimide film before light curing is 0.005~5N/25mm, and the adhesion between the adhesive layer and the polyimide film after light curing is 6N/25mm or more.