JP7348761B2 - Adhesive sheet, optical film with adhesive, and method for producing image display device - Google Patents

Description

The present invention relates to an adhesive sheet and an optical film with an adhesive provided with the adhesive sheet. Furthermore, the present invention relates to a method of manufacturing an image display device using an adhesive sheet.

2. Description of the Related Art Liquid crystal display devices and organic EL display devices are widely used as various image display devices such as mobile phones, smartphones, car navigation devices, personal computer monitors, and televisions. In order to prevent damage to the image display panel due to impacts from the outer surface, a front transparent plate (also referred to as a "cover window" etc.) such as a transparent resin plate or glass plate is provided on the viewing side of the image display panel. Sometimes. Furthermore, in recent years, devices equipped with a touch panel on the viewing side of an image display panel have become popular.

In an image display device in which a front transparent member such as a front transparent plate or a touch panel is arranged in front of an image display panel, the image display panel and the front transparent member are bonded together with an adhesive sheet interposed therebetween. A colored layer (decorative printing layer) for the purpose of decoration or light shielding may be formed on the periphery of the front transparent member. When an adhesive is attached to a transparent member having a decorative printing layer, air bubbles are likely to form around the printing steps.

In order to solve the problem caused by the printing level difference on the front transparent member, a soft and thick pressure-sensitive adhesive sheet is used to provide printing level difference absorption properties. For example, in Patent Document 1, there is an example in which an image display panel and a cover window having printing steps are bonded together using a photocurable adhesive sheet containing a base polymer, a photopolymerizable polyfunctional monomer, and a photopolymerization initiator. It is shown. A photocurable adhesive is soft and has excellent step absorption properties before curing, and exhibits excellent adhesion reliability after photocuring.

Photo-curable pressure-sensitive adhesive sheets may undergo photo-curing due to light from sunlight, fluorescent lamps, etc. in the storage environment. As photocuring progresses, the adhesive becomes harder, resulting in insufficient level difference absorption. Patent Document 2 proposes that a UV-absorbing release film is bonded to the surface of the adhesive sheet to suppress photocuring of the adhesive sheet in a storage environment. Furthermore, Patent Document 2 discloses that by adding an ultraviolet absorber to an adhesive composition to impart ultraviolet absorbing properties, it is possible to suppress deterioration of image display device constituent members such as organic EL elements and polarizers due to ultraviolet rays. Are listed.

Japanese Patent Application Publication No. 2014-227453 Japanese Patent Application Publication No. 2017-173479

In the process of manufacturing a pressure-sensitive adhesive sheet, the pressure-sensitive adhesive composition may be photocured with a release film laminated thereon. When ultraviolet rays are irradiated through an ultraviolet absorbing mold release film, the light utilization efficiency is low and it is difficult to sufficiently proceed with photocuring. Therefore, it is necessary to laminate a release film that does not have UV absorption properties during photocuring, and then replace it with a release film that does have UV absorption properties after photocuring, which makes the process complicated and increases costs. It can also be.

In the above-mentioned Patent Document 2, the adhesive layer is made to have ultraviolet absorbing properties, and photocuring inhibition caused by ultraviolet absorbers is suppressed by using a photopolymerization initiator that is sensitive in the visible light region. However, when a photopolymerization initiator with high sensitivity to visible light is used, the pressure-sensitive adhesive sheet is colored due to light absorption of the photopolymerization initiator, which causes a decrease in visibility of an image display device.

In view of the above, an object of the present invention is to provide a photocurable pressure-sensitive adhesive sheet that is suppressed from being photocured by light from a fluorescent lamp or the like in a storage environment and has excellent transparency.

The adhesive sheet of the present invention is a layered adhesive composition containing a base polymer, a photopolymerizable polyfunctional compound, a photopolymerization initiator, and an ultraviolet absorber, and has photocurability. The light transmittance of the adhesive sheet at a wavelength of 420 nm is preferably 85% or more, the light transmittance at a wavelength of 405 nm is preferably 80% or more, and the light transmittance at a wavelength of 380 nm is preferably 4 to 25%. The light transmittance of the adhesive sheet at a wavelength of 365 nm is preferably 1% or less.

As the base polymer, for example, an acrylic polymer is used. The base polymer preferably has a crosslinked structure. For example, a base polymer contains a hydroxy group-containing monomer as a monomer unit, and an isocyanate crosslinking agent is bonded to the hydroxy group to form a crosslinked structure.

The photopolymerizable polyfunctional compound has two or more photopolymerizable functional groups in one molecule, and polyfunctional (meth)acrylate or the like is used. The content of the polymerizable polyfunctional compound in the adhesive composition constituting the adhesive sheet is, for example, about 0.3 to 10 parts by weight based on 100 parts by weight of the base polymer.

The photopolymerization initiator preferably has a low absorption of visible light, and preferably has an absorption coefficient of 50 [mLg ⁻¹ cm ⁻¹ ] or less at a wavelength of 405 nm. The extinction coefficient of the photopolymerization initiator at a wavelength of 380 nm is preferably 80 [mLg ⁻¹ cm ⁻¹ ] or more. The photopolymerization initiator preferably has an absorption maximum in the wavelength range of 310 nm to 370 nm.

The adhesive composition constituting the adhesive sheet preferably contains 0.05 to 1 part by weight of a photopolymerization initiator based on 100 parts by weight of the base polymer. The product εw of the content of the photopolymerization initiator: w (parts by weight) with respect to 100 parts by weight of the base polymer and the extinction coefficient of the photopolymerization initiator at a wavelength of 380 nm: ε (mLg ⁻¹ cm ⁻¹ ) is 10 to 100. is preferred. The product εwT of the light transmittance of the adhesive sheet at a wavelength of 380 nm: T (%) and the above εw is preferably 100 to 1200.

As the ultraviolet absorber, triazine compounds, benzotriazole compounds, etc. are preferably used.

The gel fraction of the adhesive sheet is preferably 30 to 80%. It is preferable that the gel fraction of the pressure-sensitive adhesive sheet after being photocured by irradiation with 1000 mJ/cm ² ultraviolet rays is 5% or more larger than the gel fraction before photocuring.

The pressure-sensitive adhesive sheet can be provided as a pressure-sensitive adhesive sheet with a release film attached to both sides of the pressure-sensitive adhesive sheet. Alternatively, an optical film with an adhesive layer may be formed by laminating the above-mentioned adhesive sheet on one side of an optical film such as a polarizing plate. The optical film with an adhesive layer may be one in which adhesive sheets are laminated on both sides of an optical film.

The above adhesive sheet can be used to form an image display device in which a transparent member having a stepped portion is disposed on the viewing side surface. After bonding the above-mentioned pressure-sensitive adhesive sheet to a transparent member having a stepped portion, the pressure-sensitive adhesive sheet is irradiated with actinic rays to photocure the pressure-sensitive adhesive composition, thereby improving adhesion reliability.

In the adhesive sheet of the present invention, the adhesive is of an ultraviolet curing type. By using a UV-curable adhesive to bond the image display panel to the front transparent member such as the front transparent plate or touch panel, it has high fluidity and level difference absorbing properties during bonding, and also has UV-curable adhesive after bonding. By doing so, the adhesion reliability is increased. By containing the ultraviolet absorber in the adhesive composition, it is possible to suppress a photocuring reaction caused by light such as a fluorescent lamp in a storage environment.

It is a sectional view showing an example of composition of a pressure sensitive adhesive sheet with a release film. FIG. 1 is a cross-sectional view showing a configuration example of an image display device. FIG. 2 is a cross-sectional view showing an example of a laminated structure of an optical film with an adhesive sheet. FIG. 2 is a cross-sectional view showing an example of a laminated structure of an optical film with an adhesive sheet.

FIG. 1 shows a pressure-sensitive adhesive sheet with a release film, in which release films 1 and 2 are temporarily attached to both surfaces of the pressure-sensitive adhesive sheet 5. The adhesive sheet 5 is a layered adhesive composition containing a base polymer having a crosslinked structure, a photopolymerizable polyfunctional compound, and a photopolymerization initiator, and has photocurability. It is preferable that the adhesive sheet has high transparency. The total light transmittance of the adhesive sheet is preferably 85% or more, more preferably 90% or more. The haze of the adhesive sheet is preferably 1.5% or less, more preferably 1% or less.

The thickness of the adhesive sheet is not particularly limited, and may be set depending on the type and shape of the adherend. When the adherend is a member having a printed step, such as a cover window with decorative printing, it is preferable that the thickness of the adhesive sheet is greater than the thickness of the printed step. The thickness of the adhesive sheet used for bonding to an adherend having printed steps such as a cover window is preferably 30 μm or more, more preferably 40 μm or more, and even more preferably 50 μm or more. Increasing the thickness of the pressure-sensitive adhesive sheet tends to increase the level difference absorbency. The upper limit of the thickness of the pressure-sensitive adhesive sheet is not particularly limited, but from the viewpoint of productivity of the pressure-sensitive adhesive sheet, it is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 250 μm or less.

The adhesive sheet may be a single layer or may have a multilayer structure in which a plurality of adhesive layers are laminated. When the adhesive sheet has a multilayer structure, at least one layer may be a photocurable adhesive composition containing an ultraviolet absorber. A photocurable adhesive composition may be used in which multiple adhesive layers contain an ultraviolet absorber.

[Composition of adhesive]
The composition of the adhesive is not particularly limited, and may include acrylic polymer, silicone polymer, polyester, polyurethane, polyamide, polyvinyl ether, vinyl acetate/vinyl chloride copolymer, modified polyolefin, epoxy type, fluorine type, natural rubber, synthetic rubber, etc. A polymer having a base polymer such as a rubber-based polymer can be appropriately selected and used. In particular, it contains an acrylic base polymer with a crosslinked structure, which has excellent optical transparency, exhibits adhesive properties such as moderate wettability, cohesiveness, and adhesion, and has excellent weather resistance and heat resistance. An acrylic pressure-sensitive adhesive is preferably used.

<Base polymer>
The acrylic base polymer contains (meth)acrylic acid alkyl ester as a main constituent monomer component. In addition, in this specification, "(meth)acrylic" means acrylic and/or methacryl.

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

Specific examples of (meth)acrylic acid alkyl esters having a chain alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and (meth)acrylate. s-butyl 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, nonyl (meth)acrylate, 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, (meth) Examples include cetyl acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecyl (meth)acrylate, and aralkyl (meth)acrylate.

Specific examples of (meth)acrylic acid alkyl esters having an alicyclic alkyl group include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, cyclooctyl (meth)acrylate, etc. (meth)acrylic acid cycloalkyl ester; (meth)acrylic acid ester having a bicyclic aliphatic hydrocarbon ring such as isobornyl (meth)acrylate; dicyclopentanyl (meth)acrylate, dicyclopentanyloxy Tricycles such as ethyl (meth)acrylate, tricyclopentanyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, 2-ethyl-2-adamantyl (meth)acrylate, etc. Examples include (meth)acrylic esters having the above aliphatic hydrocarbon rings.

The amount of the (meth)acrylic acid alkyl ester relative to the total amount of monomer components constituting the acrylic base polymer is preferably 40% by weight or more, more preferably 50% by weight or more, and even more preferably 60% by weight or more. From the viewpoint of keeping the glass transition temperature (Tg) of the base polymer in an appropriate range, the amount of (meth)acrylic acid alkyl ester having a chain alkyl group having 4 to 10 carbon atoms in the acrylic base polymer is determined based on the total amount of the constituent monomer components. is preferably 30% by weight or more, more preferably 40% by weight or more, even more preferably 45% by weight or more.

The acrylic base polymer preferably contains an acrylic monomer unit having a crosslinkable functional group as a copolymerization component. Since the base polymer has a crosslinkable functional group, the gel fraction of the adhesive can be adjusted to a desired range by reacting the base polymer with a crosslinking agent.

Examples of the acrylic monomer having a crosslinkable functional group include a hydroxy group-containing monomer and a carboxyl group-containing monomer. For example, when an isocyanate-based crosslinking agent is used, a crosslinked structure is introduced by a reaction between a hydroxyl group and an isocyanate group. When using an epoxy crosslinking agent, a crosslinked structure is introduced by the reaction between a carboxy group and an epoxy group. Among these, it is preferable to use a hydroxy group-containing monomer as a copolymerization component of the base polymer and introduce a crosslinked structure with an isocyanate crosslinking agent. When the base polymer contains a hydroxyl group-containing monomer as a monomer component, the crosslinking properties of the base polymer are increased, and the clouding of the adhesive tends to be suppressed under high temperature and high humidity environments, resulting in a highly transparent adhesive. is obtained.

Hydroxy group-containing monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, and (meth)acrylate. Examples include (meth)acrylic acid esters such as 8-hydroxyoctyl acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)-methyl (meth)acrylate. It will be done. Among these, 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate are preferred because they greatly contribute to improving adhesive strength and can suppress clouding of the pressure-sensitive adhesive sheet in a high-humidity environment.

The amount of the hydroxy group-containing monomer relative to the total amount of monomer components constituting the acrylic base polymer is preferably 3 to 40% by weight, more preferably 5 to 30% by weight, and even more preferably 8 to 25% by weight. By increasing the amount of hydroxy group-containing monomer, the unreacted functional groups of the crosslinking agent are reduced, so the gel fraction is increased with a small amount of crosslinking agent, and the processability and level difference absorption of the adhesive sheet before photocuring are improved. I can do both.

Examples of the carboxy group-containing monomer include acrylic monomers such as (meth)acrylic acid, carboxyethyl (meth)acrylate, and carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid.

When an adhesive sheet is used for adhering a touch panel sensor, it is preferable that the adhesive sheet has a small acid content in order to prevent corrosion of the electrodes due to acid components. In such an acid-free adhesive sheet, the content of organic acid monomers such as (meth)acrylic acid is preferably 100 ppm or less, more preferably 70 ppm or less, and even more preferably 50 ppm or less. . The organic acid monomer content of the adhesive sheet is determined by immersing the adhesive sheet in pure water, heating it at 100° C. for 45 minutes, and quantifying the acid monomer extracted into the water using ion chromatography.

In order to reduce the acid monomer content in the adhesive sheet, it is preferable that the amount of organic acid monomer components such as (meth)acrylic acid in the monomer components constituting the acrylic base polymer is small. Therefore, in order to make the pressure-sensitive adhesive sheet acid-free, it is preferable that the base polymer does not substantially contain an organic acid monomer (carboxy group-containing monomer) as a monomer component. In the acid-free pressure-sensitive adhesive sheet, the amount of the carboxy group-containing monomer relative to the total 100 parts by weight of the monomer components of the base polymer is preferably 0.5 parts by weight or less, more preferably 0.1 parts by weight or less, and 0.05 parts by weight or less. is more preferable, and ideally it is 0.

The acrylic base polymer may contain a nitrogen-containing monomer as a constituent monomer component. Examples of nitrogen-containing monomers include N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, (meth)acryloylmorpholine, and N-vinyl. Examples include carboxylic acid amides, vinyl monomers such as N-vinylcaprolactam, and cyanoacrylate monomers such as acrylonitrile and methacrylonitrile. Among these, N-vinylpyrrolidone is preferred because it has a high effect of improving adhesive strength by improving cohesive strength.

When the acrylic base polymer contains highly polar monomers such as hydroxy group-containing monomers, carboxyl group-containing monomers, and nitrogen-containing monomers as constituent monomer components, the cohesive force of the adhesive is increased and adhesive retention is improved. Tend. The amount of highly polar monomers (total of hydroxyl group-containing monomers, carboxyl group-containing monomers, and nitrogen-containing monomers) based on the total amount of monomer components constituting the acrylic base polymer is preferably 10 to 60% by weight, more preferably 15 to 50% by weight. Preferably, 18 to 40% by weight is more preferable. Further, the amount of the nitrogen-containing monomer relative to the total amount of monomer components constituting the acrylic base polymer is preferably 3 to 25% by weight, more preferably 5 to 20% by weight, and even more preferably 7 to 15% by weight.

The acrylic base polymer contains, as monomer components other than those mentioned above, an acid anhydride group-containing monomer, a caprolactone adduct of (meth)acrylic acid, a sulfonic acid group-containing monomer, a phosphoric acid group-containing monomer, vinyl acetate, vinyl propionate, styrene, Vinyl monomers such as α-methylstyrene; cyanoacrylate monomers such as acrylonitrile and methacrylonitrile; epoxy group-containing monomers such as glycidyl (meth)acrylate; polyethylene glycol (meth)acrylate, polypropylene (meth)acrylate Glycol-based acrylic ester monomers such as glycol, methoxyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate; tetrahydrofurfuryl (meth)acrylate, fluorine (meth)acrylate, silicone (meth)acrylate and (meth)acrylate; ) May contain an acrylic acid ester monomer such as 2-methoxyethyl acrylate.

An acrylic base polymer can be obtained by polymerizing the above monomer components by various known methods such as solution polymerization, emulsion polymerization, and bulk polymerization. Solution polymerization is preferred as the polymerization method from the viewpoint of balance of properties such as adhesive strength and holding power of the pressure-sensitive adhesive and cost. Ethyl acetate, toluene, etc. are generally used as a solvent for solution polymerization. The solution concentration is usually about 20 to 80% by weight. Polymerization initiators include azo initiators, peroxide initiators, redox initiators that combine peroxide and reducing agent (for example, combinations of persulfate and sodium bisulfite, peroxide and ascorbic acid). A thermal polymerization initiator such as a combination of sodium acid and the like is preferably used. The amount of the polymerization initiator used is not particularly limited, but for example, it is preferably about 0.005 to 5 parts by weight, and about 0.02 to 3 parts by weight, based on 100 parts by weight of the total amount of monomer components forming the base polymer. More preferred.

Chain transfer agents may be used to adjust the molecular weight of the base polymer. Examples of chain transfer agents include thiols such as α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, and 2,3-dimercapto-1-propanol. is preferably used.

The viscoelasticity of the pressure-sensitive adhesive sheet before photocuring is easily influenced by the constituent components and molecular weight of the base polymer. The weight average molecular weight of the acrylic base polymer is preferably 100,000 to 2,000,000, more preferably 150,000 to 1,000,000, and even more preferably 200,000 to 700,000. Note that the molecular weight of the base polymer refers to the molecular weight before introducing a crosslinked structure.

<Crosslinked structure of base polymer>
In general, the method for introducing a crosslinked structure into a base polymer is as follows: (1) After polymerizing a base polymer having a functional group that can react with a crosslinking agent, a crosslinking agent is added, and the base polymer and the crosslinking agent are reacted. and (2) a method of introducing a branched structure (crosslinked structure) into the polymer chain by including a photopolymerizable polyfunctional compound in the polymerization components of the base polymer.

The adhesive sheet of the present invention has a crosslinked structure in which the base polymer is introduced by the method (1) above. Furthermore, the adhesive composition constituting the adhesive sheet contains a photopolymerizable polyfunctional compound, and by photocuring after bonding the adhesive sheet and the adherend, the crosslinked structure in (2) above is formed. be introduced. That is, in the adhesive sheet of the present invention, before photocuring, the base polymer has a crosslinked structure with a (thermal) crosslinker such as an isocyanate crosslinker. In the adhesive sheet after photocuring, in addition to a crosslinked structure using an isocyanate crosslinking agent, a crosslinked structure using a photopolymerizable polyfunctional compound (photocrosslinking agent) such as a polyfunctional (meth)acrylate is introduced into the base polymer. ing.

<Crosslinking agent>
A crosslinked structure is introduced into the base polymer by adding a crosslinking agent after polymerizing the base polymer and heating if necessary. In order to distinguish it from the crosslinked structure introduced by a photopolymerizable polyfunctional compound, crosslinking by an isocyanate crosslinking agent etc. is sometimes described as "thermal crosslinking", but the introduction of a crosslinked structure by a crosslinking agent does not involve heating. It can be anything. By introducing a thermally crosslinked structure into the acrylic base polymer, the processability and processing dimensional stability of the pressure-sensitive adhesive sheet before photocuring can be improved.

Examples of the crosslinking agent include compounds that react with functional groups such as hydroxy groups and carboxy groups contained in the base polymer. Specific examples of the crosslinking agent include isocyanate crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, metal chelate crosslinking agents, and the like.

Among these, isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred because they have high reactivity with the hydroxyl group or carboxy group of the base polymer and can easily introduce a crosslinked structure. These crosslinking agents react with functional groups such as hydroxy groups and carboxy groups introduced into the base polymer to form a crosslinked structure. In an acid-free adhesive in which the base polymer does not contain a carboxyl group, it is preferable to use an isocyanate-based crosslinking agent to form a crosslinked structure by reacting the hydroxyl group in the base polymer with the isocyanate crosslinking agent.

As the isocyanate crosslinking agent, polyisocyanate having two or more isocyanate groups in one molecule is used. Examples of the isocyanate-based crosslinking agent include lower aliphatic polyisocyanates such as butylene diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentylene diisocyanate, cyclohexylene diisocyanate, and isophorone diisocyanate; 2,4-tolylene diisocyanate; Aromatic isocyanates such as isocyanate, 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate; trimethylolpropane/tolylene diisocyanate trimer adduct (for example, "Coronate L" manufactured by Tosoh), trimethylolpropane/hexamethylene Diisocyanate trimer adducts (e.g. "Coronate HL" manufactured by Tosoh), trimethylolpropane adducts of xylylene diisocyanate (e.g. "Takenate D110N" manufactured by Mitsui Chemicals), isocyanurates of hexamethylene diisocyanate (e.g. "Coronate HL" manufactured by Tosoh), Examples include isocyanate adducts such as "Coronate HX").

By adjusting the amount of crosslinking agent added, the gel fraction of the pressure-sensitive adhesive sheet can be adjusted within a predetermined range. The amount of the crosslinking agent added is preferably 0.01 to 1 part by weight, more preferably 0.03 to 0.8 parts by weight, and still more preferably 0.05 to 0.5 parts by weight, based on 100 parts by weight of the base polymer. Preferably, 0.08 to 0.3 parts by weight is particularly preferable. The larger the amount of crosslinking agent added, the higher the gel fraction of the adhesive sheet before photocuring tends to be.

<Photopolymerizable polyfunctional compound>
The photopolymerizable polyfunctional compound contained in the adhesive sheet has two or more photopolymerizable functional groups in one molecule. The photopolymerizable functional group may be radically polymerizable, cationically polymerizable, or anionically polymerizable; however, since it has excellent reactivity, a radically polymerizable functional group having an unsaturated double bond (ethylenic unsaturated group) is preferable. is preferred. As the photopolymerizable polyfunctional compound, polyfunctional (meth)acrylate is preferred because of its high compatibility with the acrylic base polymer, and polyfunctional acrylate is particularly preferred because it has high photoradical reactivity.

Examples of polyfunctional (meth)acrylates include hexanediol di(meth)acrylate, nonanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, and 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, Pentaerythster di(meth)acrylate, Neo Bifunctional (meth)acrylic esters such as pentyl glycol di(meth)acrylate and glycerin di(meth)acrylate; pentaerythritol tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, and ethoxylated isocyanuric acid tri(meth)acrylate; Trifunctional (meth)acrylic acid esters such as (meth)acrylates; tetrafunctional (meth)acrylic acid esters such as ditrimethylolpropane tetra(meth)acrylate, ethoxylated pentaerythol tetra(meth)acrylate, and pentaerythritol tetra(meth)acrylate Acrylic esters include (meth)acrylic esters having five or more functional groups such as dipentaerythol penta(meth)acrylate and dipentaerythol hexa(meth)acrylate.

The content of the photopolymerizable polyfunctional compound in the photocurable adhesive sheet is preferably 0.3 to 10 parts by weight, more preferably 0.5 to 5 parts by weight, based on 100 parts by weight of the base polymer. The larger the content of the polyfunctional compound, the larger the gel fraction of the adhesive after photocuring, which tends to improve adhesive reliability. On the other hand, if the content of the polyfunctional compound is too large, the adhesive will become excessively hard and its impact resistance will deteriorate. Adhesion retention at low temperatures may decrease. The amount of the photopolymerizable polyfunctional compound may be 0.7 parts by weight or more or 1 part by weight or more, and may be 4 parts by weight or less or 3 parts by weight or less based on 100 parts by weight of the base polymer. .

<Photopolymerization initiator>
The photocurable adhesive composition contains a photopolymerization initiator. The photopolymerization initiator generates radicals, acids, bases, etc. upon irradiation with actinic rays such as ultraviolet rays, and can be appropriately selected depending on the type of photopolymerizable compound. When using a polyfunctional (meth)acrylate as the photopolymerizable compound, it is preferable to use a photoradical polymerization initiator as the photopolymerization initiator.

The photoradical polymerization initiator is preferably one that is sensitive to ultraviolet light with a wavelength shorter than 405 nm, and includes hydroxyketones, benzyl dimethyl ketals, aminoketones, acylphosphine oxides, benzophenones, and trichloromethyl groups. Containing triazine derivatives and the like can be mentioned. The photoradical polymerization initiators may be used alone or in combination of two or more.

It is preferable that the photopolymerization initiator has low sensitivity to light with a wavelength longer than 400 nm (visible light). The extinction coefficient ε of the photopolymerization initiator at a wavelength of 405 nm is preferably 50 [mLg ^-1 cm ^-1 ] or less, more preferably 20 [mLg ^-1 cm ^-1 ] or less, and 10 [mLg ^-1 cm ^-1 ] or less. is even more preferable. By using a photopolymerization initiator that absorbs little light at a wavelength of 405 nm, the transparency of the pressure-sensitive adhesive sheet can be improved. Further, if a photopolymerization initiator with low sensitivity to visible light is used, photopolymerization of the adhesive due to external light in the storage environment is difficult to proceed, so the storage stability of the adhesive sheet can be improved. The extinction coefficient of the photopolymerization initiator is a value calculated from the absorbance of a 0.001 g/mL acetonitrile solution measured by a visible-ultraviolet spectrophotometer using a quartz cell with an optical path length of 1 cm.

From the viewpoint of improving the storage stability of the pressure-sensitive adhesive sheet, it is preferable to use a photoradical polymerization initiator that does not exhibit an absorption maximum at wavelengths longer than 380 nm. A photoradical polymerization initiator that exhibits an absorption maximum at a wavelength longer than 380 nm is likely to absorb light from a fluorescent lamp (mainly the 405 nm mercury bright line) and generate photoradicals. If the maximum wavelength of light absorption of the photoradical polymerization initiator is 370 nm or less, the amount of radicals generated due to light in a storage environment such as a fluorescent lamp is small. Therefore, even when the adhesive sheet is exposed to fluorescent light for a long period of time, the effective concentration of the photoradical polymerization initiator can be maintained at a high level.

In order to have high storage stability and improve adhesion reliability with the adherend by light irradiation even after long-term storage, the photopolymerization initiator contained in the adhesive sheet must have a maximum light absorption wavelength of 370 nm or less. It is preferable that there be. On the other hand, in order to increase the photocuring efficiency by ultraviolet irradiation, it is preferable that the photopolymerization initiator has a light absorption maximum at a wavelength longer than 310 nm. From the above, in order to achieve both transparency and storage stability of the adhesive sheet, the photopolymerization initiator contained in the adhesive sheet must not have an absorption maximum at a wavelength longer than 380 nm, and must have an absorption maximum at a wavelength of 310 to 370 nm. It is preferable to have an absorption maximum in the range of . More preferably, the photopolymerization initiator has an absorption maximum in the wavelength range of 310 to 355 nm. The absorption maximum wavelength of the photopolymerization initiator is more preferably 315 to 350 nm, particularly preferably 320 to 345 nm.

The photopolymerization initiator preferably has sensitivity to light with a wavelength of 380 nm. By having photosensitivity at a wavelength of 380 nm, even if the adhesive sheet contains an ultraviolet absorber, the photopolymerization initiator can absorb light that is not absorbed by the ultraviolet absorber, so it can appropriately photocure. The extinction coefficient of the photopolymerization initiator at a wavelength of 380 nm is preferably 80 [mLg ^-1 cm ^-1 ] or more, more preferably 100 [mLg ^-1 cm ^-1 ] or more, and 120 [mLg ^-1 cm ^-1 ] or more. More preferred. The extinction coefficient of the photopolymerization initiator at a wavelength of 380 nm is generally 1000 [mLg ^-1 cm ^-1 ] or less, 500 [mLg ^-1 cm ^-1 ] or less, or 300 [mLg ^-1 cm ^-1 ] or less. It's okay.

The content of the photopolymerization initiator in the pressure-sensitive adhesive sheet may be set depending on the absorbance and the like. The product εw of the extinction coefficient ε (mLg ⁻¹ cm ⁻¹ ) of the photopolymerization initiator at a wavelength of 380 nm and the content w (parts by weight) of the photopolymerization initiator relative to 100 parts by weight of the base polymer is 10 to 100. Preferably, 15 to 80 is more preferable. The larger εw is, the more easily photocuring progresses, but on the other hand, the photocuring due to light from a fluorescent lamp in a storage environment also becomes easier to proceed.

When εw is within the above range, the adhesive exhibits sufficient photoreactivity when irradiated with light for photocuring while suppressing photocuring of the adhesive in a storage environment. The product εw of the extinction coefficient ε of the photopolymerization initiator and the content w may be 20 or more, 25 or more, or 30 or more. εw may be 60 or less or 50 or less. The content w of the photopolymerization initiator based on 100 parts by weight of the base polymer is, for example, 0.05 to 1 part by weight, preferably 0.08 to 0.5 part by weight.

<Ultraviolet absorber>
The ultraviolet absorber has the effect of suppressing the photocuring reaction in the storage environment of the adhesive sheet. In a composition containing a UV inhibitor in addition to a photopolymerization initiator, even when irradiated with light at a wavelength to which the photopolymerization initiator is sensitive from a fluorescent lamp, etc., the amount of light absorbed by the UV absorber is large, so polymerization will not occur. The amount of light absorbed by the initiator will be relatively small. Therefore, photocuring (polymerization reaction of a photopolymerizable polyfunctional compound) caused by light from a fluorescent lamp or the like under the storage environment of the adhesive sheet is suppressed.

Examples of the ultraviolet absorber include benzotriazole ultraviolet absorbers, benzophenone ultraviolet absorbers, triazine ultraviolet absorbers, salicylate ultraviolet absorbers, cyanoacrylate ultraviolet absorbers, and the like. Triazine-based ultraviolet absorbers and benzotriazole-based ultraviolet absorbers are preferred because they have high absorption of ultraviolet light in the wavelength range of 320 to 370 nm and excellent compatibility with acrylic base polymers. Among these, triazine-based ultraviolet absorbers containing hydroxyl groups (particularly hydroxyphenyltriazine-based ultraviolet absorbers) and benzotriazole-based ultraviolet absorbers having one benzotriazole skeleton in one molecule are particularly preferred.

Commercially available products may be used as the ultraviolet absorber. Commercially available triazine UV absorbers include 2-(4,6-bis(2,4-dimethylphenyl)-1,3,5-triazin-2-yl)-5-hydroxyphenyl and [(alkyloxy ) Methyl] reaction product with oxirane (“TINUVIN 400” manufactured by BASF), 2-(2,4-dihydroxyphenyl)-4,6-bis-(2,4-dimethylphenyl)-1,3,5- The reaction product of triazine and (2-ethylhexyl)-glycidic acid ester (“TINUVIN 405” manufactured by BASF), (2,4-bis[2-hydroxy-4-butoxyphenyl]-6-(2,4-di butoxyphenyl)-1,3,5-triazine (BASF “TINUVIN 460”), 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-[(hexyl)oxy] -Phenol (“TINUVIN 577” manufactured by BASF), 2-(2-hydroxy-4-[1-octyloxycarbonylethoxy]phenyl)-4,6-bis(4-phenylphenyl)-1,3,5-triazine (BASF “TINUVIN 479”), bisethylhexyloxyphenolmethoxyphenyltriazine (BASF “Tinosorb S”), 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-[ Examples include 2-(2-ethylhexanoyloxy)ethoxy]-phenol (“ADK STAB LA-46” manufactured by ADEKA).

Commercially available benzotriazole UV absorbers include 2-(2H-benzotriazol-2-yl)-6-(1-methyl-1-phenylethyl)-4-(1,1,3,3-tetra Methylbutyl)phenol (“TINUVIN 928” manufactured by BASF), 2-(2-hydroxy-5-tert-butylphenyl)-2H-benzotriazole (“TINUVIN PS” manufactured by BASF), 2-(2H-benzotriazole-2 -yl)-4,6-bis(1-methyl-1-phenylethyl)phenol (BASF "TINUVIN 900"), 2-(2H-benzotriazol-2-yl)-6-(1-methyl-1 -phenylethyl)-4-(1,1,3,3-tetramethylbutyl)phenol (BASF "TINUVIN 928"), 2-(2H-benzotriazol-2-yl)-6-dodecyl-4-methyl Phenol (“TINUVIN571” manufactured by BASF), 2-(2H-benzotriazol-2-yl)-p-cresol (“TINUVIN P” manufactured by BASF), 2-(2H-benzotriazol-2-yl)-4-6 -bis(1-methyl-1-phenylethyl)phenol (BASF "TINUVIN 234"), 2-[5-chloro(2H)-benzotriazol-2-yl]-4-methyl-6-(tert-butyl) ) Phenol (“TINUVIN 326,” manufactured by BASF), 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol (“TINUVIN 328,” manufactured by BASF), 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol (“TINUVIN 328” manufactured by BASF), Benzenepropanoic acid and 3-(2H-benzotriazol-2-yl)-5 -(1,1-dimethylethyl)-4-hydroxy (C7-9 side chain and linear alkyl) ester compound (BASF "TINUVIN384-2"), methyl-3-(3-(2H-benzotriazole) -2-yl)-5-tert-butyl-4-hydroxyphenyl)propionate and polyethylene glycol reaction product (BASF "TINUVIN1 130"), methyl 3-(3-(2H-benzotriazol-2-yl) )-5-tert-butyl-4-hydroxyphenyl)propionate and polyethylene glycol 300 reaction product (“TINUVIN 213” manufactured by BASF), 2-[2-hydroxy-3-(3,4,5,6- Examples include tetrahydrophthalimidomethyl)-5-methylphenyl]benzotriazole ("Sumisorb 250" manufactured by Sumitomo Chemical).

The content of the ultraviolet absorber in the adhesive sheet is, for example, about 0.05 to 3 parts by weight, preferably about 0.1 to 1 part by weight, based on 100 parts by weight of the base polymer. The larger the amount of the ultraviolet absorber, the more the pressure-sensitive adhesive sheet tends to be inhibited from being photocured by light from a fluorescent lamp or the like in its storage environment. On the other hand, if the amount of ultraviolet absorber is excessively large, even if excitation light (ultraviolet light) is irradiated for photocuring, much of the excitation light will be absorbed by the ultraviolet absorber, and the amount of light absorbed by the photopolymerization initiator will be is small, which may result in insufficient curing.

(oligomer)
The adhesive composition may contain various oligomers for the purpose of adjusting the adhesive force and viscosity of the adhesive sheet. As the oligomer, for example, one having a weight average molecular weight of about 1,000 to 30,000 is used. As the oligomer, acrylic oligomers are preferred because they have excellent compatibility with acrylic base polymers.

(Silane coupling agent)
A silane coupling agent may be added to the adhesive composition for the purpose of adjusting adhesive strength. When a silane coupling agent is added to the adhesive composition, the amount added is usually about 0.01 to 5.0 parts by weight, and 0.03 to 2.0 parts by weight, based on 100 parts by weight of the base polymer. It is preferable that the degree of

(Other additives)
In addition to the above-mentioned components, the adhesive composition may include chain transfer agents, tackifiers, plasticizers, softeners, deterioration inhibitors, fillers, colorants, antioxidants, surfactants, antistatic agents, etc. It may contain additives.

[Formation of adhesive sheet]
The base polymer is mixed with a crosslinking agent, a photopolymerizable polyfunctional compound, a photopolymerization initiator, an ultraviolet absorber, and if necessary, additives such as oligomers and silane coupling agents. Prepare the composition. The pressure-sensitive adhesive composition preferably has a viscosity (for example, about 0.5 to 20 Pa·s) suitable for application onto a substrate. The viscosity of the adhesive composition can be adjusted to an appropriate range by adjusting the molecular weight of the base polymer, the amount of the photopolymerizable polyfunctional compound added, the composition, molecular weight, and amount of other components (e.g., oligomers). can. A thickening additive or the like may be used for the purpose of adjusting viscosity or the like.

A pressure-sensitive adhesive sheet is formed on the base material by applying the pressure-sensitive adhesive composition onto the base material and, if necessary, drying and removing the solvent. Any suitable base material can be used as the base material for forming the pressure-sensitive adhesive sheet. The base material may be a release film that includes a release layer on the surface that contacts the pressure-sensitive adhesive sheet.

Films made of various resin materials are used as the film base material of the release film. Examples of resin materials include polyester resins such as polyethylene terephthalate and polyethylene naphthalate, acetate resins, polyether sulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, (meth)acrylic resins, Examples include polyvinyl chloride resin, polyvinylidene chloride resin, polystyrene resin, polyvinyl alcohol resin, polyarylate resin, polyphenylene sulfide resin, and the like. Among these, polyester resins such as polyethylene terephthalate are particularly preferred. The thickness of the film base material is preferably 10 to 200 μm, more preferably 25 to 150 μm. Examples of the material for the mold release layer include silicone mold release agents, fluorine mold release agents, long chain alkyl mold release agents, fatty acid amide mold release agents, and the like. The thickness of the release layer is generally about 10 to 2000 nm.

Methods for applying the adhesive composition onto the substrate include roll coating, kiss roll coating, gravure coating, reverse coating, roll brushing, spray coating, dip roll coating, bar coating, knife coating, air knife coating, and curtain coating. Various methods such as , lip coat, die coater, etc. are used.

When the adhesive composition contains a solvent, it is preferable to dry the solvent after applying the adhesive composition onto the substrate. As the drying method, an appropriate method may be adopted depending on the purpose. The heat drying temperature is preferably 40°C to 200°C, more preferably 50°C to 180°C, particularly preferably 70°C to 170°C. As the drying time, an appropriate time can be adopted as needed. The drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 15 minutes, particularly preferably 10 seconds to 10 minutes.

After drying the solvent, it is preferable to attach a cover sheet to protect the surface of the pressure-sensitive adhesive sheet. As the cover sheet, it is preferable to use a release film having a release layer on the surface in contact with the pressure-sensitive adhesive sheet, similar to the base film.

After applying the adhesive composition onto the base material, heating is performed as necessary to introduce a crosslinked structure into the base polymer. The heating temperature and heating time may be appropriately set depending on the type of crosslinking agent used, and are usually in the range of 20° C. to 160° C. for about 1 minute to 7 days. The heating for drying the solvent may also serve as the heating for crosslinking. As mentioned above, the introduction of the crosslinked structure does not necessarily have to involve heating.

A crosslinking catalyst may be used to promote the formation of crosslinked structures. For example, as a crosslinking catalyst for an isocyanate-based crosslinking agent, metal-based crosslinking catalysts (especially tin-based crosslinking catalysts) such as tetra-n-butyl titanate, tetraisopropyl titanate, ferric nathem, butyltin oxide, dioctyltin dilaurate, dibutyltin dilaurate, etc. etc.

By laminating the release films 1 and 2 on the surface of the adhesive sheet 5, as shown in FIG. 1, a pressure-sensitive adhesive sheet having release films temporarily attached to both sides is obtained. The base material and cover sheet used when forming the adhesive sheet may be used as the release films 1 and 2 as they are.

When the release films 1 and 2 are provided on both sides of the adhesive sheet 5, the thickness of one release film 1 and the thickness of the other release film 2 may be the same or different. The peeling force when peeling the release film temporarily attached to one side from the adhesive sheet 5 and the peeling force when peeling the release film temporarily attached to the other side from the adhesive sheet 5 may be the same. May be different. If the peeling forces of the two are different, release film 2 (light release film), which has a relatively small peeling force, is peeled off from the adhesive sheet 5 first and bonded to the first adherend. The workability is excellent when the release film 1 (heavy release film) having a large release force is peeled off and then bonded to the second adherend.

[Physical properties of adhesive sheet]
By introducing a crosslinked structure using an isocyanate crosslinking agent or the like into the base polymer, the gel fraction increases. The gel fraction of the adhesive sheet before photocuring is preferably 30 to 80%. When the gel fraction is 80% or less, the adhesive can easily follow printing steps such as a cover window, and voids near the steps can be suppressed. Further, if the gel fraction is 30% or more, the adhesive sheet has excellent handling properties and processability. The gel fraction of the adhesive sheet before photocuring is more preferably 35 to 70%, and even more preferably 40 to 65%.

The gel fraction of a pressure-sensitive adhesive sheet can be determined as the insoluble content in a solvent such as ethyl acetate. Specifically, the gel fraction of the pressure-sensitive adhesive sheet can be determined as the insoluble content after immersing the pressure-sensitive adhesive forming the pressure-sensitive adhesive sheet in ethyl acetate at 23°C for 7 days. It is determined as the weight fraction (unit: weight %) of the sample before immersion. Generally, the gel fraction of a polymer is equal to the degree of crosslinking, and the more crosslinked parts in the polymer, the higher the gel fraction. The gel fraction (amount of the crosslinked structure introduced) can be adjusted to a desired range by the method of introducing the crosslinked structure, the type and amount of the crosslinking agent, etc.

When a photocurable adhesive sheet is photocured by irradiating active light such as ultraviolet rays, the gel fraction of the adhesive sheet increases due to photopolymerization of the photocurable polyfunctional compound, which improves the adhesion reliability to the adherend. is enhanced. The gel fraction of the adhesive sheet after being photocured by irradiation with 1000 mJ/cm ² of ultraviolet rays is preferably 40% or more, more preferably 50% or more. The gel fraction of the adhesive sheet after photocuring may be 55% or more or 60% or more. The gel fraction of the adhesive sheet after photocuring may be 100% or less, 95% or less, or 90% or less.

The gel fraction of the pressure-sensitive adhesive sheet after being photocured by irradiation with 1000 mJ/cm ² of ultraviolet light is preferably 5% or more greater than the gel fraction of the pressure-sensitive adhesive sheet before photocuring. Adhesion reliability tends to be improved by increasing the gel fraction by 5% or more.

When an adhesive sheet is left under a fluorescent lamp for 7 days and then photocured by irradiating it with 1000 mJ/ ^cm2 ultraviolet rays, the gel fraction after photocuring is as follows: It is preferable that it is 5% or more larger than the gel fraction. As will be described later, by appropriately selecting and adjusting the type and amount of the photopolymerization initiator and ultraviolet absorber, it is possible to prevent photocuring under fluorescent lamps from proceeding and to make it possible to use excitation light (ultraviolet light) for curing. Since photocuring proceeds appropriately during irradiation, it is possible to make the difference in gel fraction before and after photocuring 5% or more.

Since the pressure-sensitive adhesive sheet of the present invention contains a photopolymerization initiator and an ultraviolet absorber, it is unlikely to be photocured by light from a fluorescent lamp or the like in the storage environment of the pressure-sensitive adhesive sheet. The intensity of light in the storage environment, such as light from a fluorescent lamp, is sufficiently low compared to the excitation light used when photo-curing adhesive sheets, so even if the adhesive sheet is left under fluorescent light for a short time, the photo-curing reaction will not occur. hardly progresses. However, if the adhesive sheet is stored under fluorescent light for a long period of time, the cumulative amount of photoexcited components (e.g. photo radicals) generated from the photopolymerization initiator that absorbed light from the fluorescent light increases, and the effect of this can no longer be ignored. There is. Specifically, the photopolymerization of the photopolymerizable polyfunctional compound progresses due to the photoradicals generated from the photopolymerization initiator, and the gel fraction of the adhesive increases, making the adhesive harder, which causes uneven absorption. There may be a decrease in sexual performance.

The pressure-sensitive adhesive sheet of the present invention is characterized in that the photocurable pressure-sensitive adhesive composition constituting the pressure-sensitive adhesive sheet contains a UV absorber in addition to a base polymer, a photocuring agent, and a photoinitiator, and the light absorption band of the photopolymerization initiator is Contains an ultraviolet absorber that absorbs light (ultraviolet light) in a wavelength range that overlaps with the (excitation wavelength). By adjusting the type and/or amount of ultraviolet absorber added according to the type of photoinitiator (light absorption characteristics), fluorescence can be improved while maintaining light transmittance in the wavelength range to which the photoinitiator is sensitive. It is possible to suppress photo-curing of the adhesive sheet due to light from a lamp or the like.

Typical fluorescent lamps emit mercury bright lines with a wavelength of 365 nm, so if you reduce the light transmittance of the adhesive sheet at a wavelength of 365 nm by adding an ultraviolet absorber, even if the adhesive sheet is stored for a long period of time under fluorescent lamps. , it is possible to suppress photo-curing of the adhesive sheet in the storage environment. From the viewpoint of suppressing photocuring of the adhesive sheet in a storage environment, the light transmittance at a wavelength of 365 nm is preferably 1% or less, more preferably 0.5% or less, and even more preferably 0.3% or less. The light transmittance of the adhesive sheet at a wavelength of 365 nm may be 0.2% or less or 0.1% or less.

From the viewpoint of increasing visible light transparency, the light transmittance of the adhesive sheet at a wavelength of 420 nm is preferably 85% or more, more preferably 88% or more. The light transmittance of the adhesive sheet at a wavelength of 405 nm is preferably 80% or more, more preferably 83% or more. As mentioned above, the photopolymerization initiator has low photosensitivity at a wavelength of 405 nm (or has no photosensitivity), so even if the adhesive sheet has a high light transmittance at a wavelength of 405 nm, it will not be exposed to external light in the storage environment. There is almost no photo-curing of the adhesive sheet caused by light from fluorescent lamps or the like.

As mentioned above, the adhesive sheet has a low transmittance at a wavelength of 365 nm, and most of the light around the wavelength of 365 nm is absorbed by the ultraviolet absorber. Further, the photopolymerization initiator has low photosensitivity (or has no photosensitivity) at a wavelength of 405 nm. Therefore, even if the adhesive sheet is irradiated with light with a wavelength of 365 nm and/or light with a wavelength of 405 nm, only a small amount of light can be used by the photopolymerization initiator, and photocuring hardly progresses.

By adjusting the light transmittance of the adhesive sheet so that the photopolymerization initiator can efficiently absorb light in the wavelength range longer than 365 nm and shorter than 405 nm, photocuring in the storage environment is possible. is difficult to proceed, and it is possible to appropriately proceed with photocuring during excitation light irradiation. Specifically, it is preferable that the photopolymerization initiator contained in the pressure-sensitive adhesive sheet can efficiently utilize light with a wavelength of around 380 nm. Therefore, the transmittance of the adhesive sheet at a wavelength of 380 nm is preferably 4% or more, more preferably 5% or more.

The higher the light transmittance at a wavelength of 380 nm, the higher the curing efficiency tends to be when photo-curing by irradiating the adhesive sheet with excitation light. On the other hand, if the light transmittance at a wavelength of 380 nm is excessively high, the photocuring of the pressure-sensitive adhesive sheet may proceed due to external light in the storage environment, and the level difference absorbability may decrease. In addition, when adding UV absorbers to provide UV absorption, if the light transmittance at a wavelength of 380 nm is high, the light transmittance at a wavelength of 365 nm also tends to be high. In some cases, the resulting photocuring of the adhesive sheet cannot be sufficiently suppressed. Therefore, the light transmittance of the adhesive sheet at a wavelength of 380 nm is preferably 25% or less, more preferably 20% or less.

When looking at the light transmission spectrum of the adhesive sheet from the short wavelength side to the long wavelength side, the wavelength at which the transmittance is 10% or more (hereinafter sometimes referred to as "10% cut-on wavelength") is 370~ Preferably, the wavelength is within the range of 385 nm. If the added amount of the ultraviolet absorber is increased, the 10% cut-on wavelength will shift to the shorter wavelength side, and if the added amount of the ultraviolet absorber is decreased, the 10% cut-on wavelength will be moved to the longer wavelength side.

From the viewpoint of suppressing photocuring of the adhesive sheet in the storage environment and increasing the photocuring efficiency during excitation light irradiation, the light absorption characteristics of the adhesive sheet can be adjusted as described above by adding an ultraviolet absorber. In addition, it is preferable to adjust the amount of photopolymerization initiator added depending on the light transmittance of the pressure-sensitive adhesive sheet. When considering the light transmittance T (%) of the pressure-sensitive adhesive sheet at a wavelength of 380 nm, if T is large, the amount of light that can be absorbed by the photopolymerization initiator will decrease even if the amount of photopolymerization initiator is small. Since the amount is large, photocuring efficiency during excitation light irradiation can be ensured. On the other hand, when T is small, even if the amount of photopolymerization initiator is increased, photocuring by light such as a fluorescent lamp under a storage environment is unlikely to occur.

Therefore, if the light transmittance T of the pressure-sensitive adhesive sheet at a wavelength of 380 nm is small, it is preferable to relatively increase the amount of photopolymerization initiator, and if T is large, the amount of photopolymerization initiator should be relatively small. It is preferable to do so. As described above, the effective amount of the photopolymerization initiator in the pressure-sensitive adhesive sheet can be evaluated by the product εw of the content of the photopolymerization initiator and the extinction coefficient ε.

From the viewpoint of suppressing the photocuring of the adhesive sheet in the storage environment and increasing the photocuring efficiency during excitation light irradiation, the light transmittance T (%) of the adhesive sheet at a wavelength of 380 nm and 100 parts by weight of the base polymer are It is preferable that the product εwT of the content w (parts by weight) of the photopolymerization initiator and the extinction coefficient ε (mLg ⁻¹ cm ⁻¹ ) of the photopolymerization initiator at a wavelength of 380 nm is within a predetermined range.

The εwT of the adhesive sheet is preferably 100 to 1200, more preferably 110 to 1000. εwT may be 120 or more, 130 or more, 140 or more, or 150 or more. εwT may be 700 or less, 600 or less, or 500 or less. The larger εwT is, the higher the photocuring efficiency during excitation light irradiation tends to be, and the smaller εwT is, the more the adhesive sheet tends to be suppressed from being photocured in the storage environment.

[Image display device]
The adhesive sheet of the present invention can be used for bonding various transparent members and opaque members. The type of adherend is not particularly limited, and examples thereof include various resin materials, glass, metals, and the like. Since the adhesive sheet of the present invention has high transparency, it is suitable for bonding optical members such as image display devices. In particular, the pressure-sensitive adhesive sheet of the present invention has excellent step absorbability and impact resistance, and is therefore suitably used for laminating transparent members such as front transparent plates and touch panels to the viewing side surface of image display devices.

FIG. 2 is a cross-sectional view showing an example of a laminated structure of an image display device in which a front transparent plate 7 is bonded to the viewing side surface of the image display panel 10 with an adhesive sheet 5 interposed therebetween. The image display panel 10 includes a polarizing plate 3 bonded to the viewing side surface of an image display cell 6 such as a liquid crystal cell or an organic EL cell with an adhesive sheet 4 interposed therebetween. In the front transparent plate 7, a printed layer 76 is provided on the periphery of one side of a transparent flat plate 71. As the transparent plate 71, for example, a transparent resin plate such as acrylic resin or polycarbonate resin, a glass plate, or the like is used. The transparent plate 71 may have a touch panel function. As the touch panel, any type of touch panel such as a resistive type, a capacitive type, an optical type, an ultrasonic type, etc. is used.

The polarizing plate 3 provided on the surface of the image display panel 10 and the surface on which the printed layer 76 of the front transparent plate 7 is formed are bonded together via the adhesive sheet 5. The order of bonding is not particularly limited, and the adhesive sheet 5 may be bonded to the image display panel 10 first, or the pressure sensitive adhesive sheet 5 may be bonded to the front transparent plate 7 first. good. Moreover, both can be bonded at the same time. From the viewpoint of lamination workability, etc., after peeling off one release film (light release film) 2 and laminating the exposed surface of the adhesive sheet 5 to the image display panel 10, the other release film 1 It is preferable to peel off the (heavy release film) and bond the exposed surface of the adhesive sheet to the front transparent plate 7.

After bonding the adhesive sheet 5 and the front transparent plate 7, defoaming is performed to remove air bubbles at the interface between the adhesive sheet 5 and the flat plate 71 portion of the front transparent plate 7, and near non-flat areas such as the printed layer 76. is preferably carried out. At this time, since the adhesive sheet has not yet been photocured and the adhesive has high fluidity, the adhesive can easily follow the shape of uneven parts such as steps, and air bubbles can be easily removed. Furthermore, even after the adhesive sheet is stored for a long period of time in an environment such as under a fluorescent lamp, the adhesive sheet is suppressed from being photocured, so that the level difference absorbability can be maintained.

As a defoaming method, an appropriate method such as heating, pressurization, depressurization, etc. may be employed. For example, it is preferable that the bonding is performed under reduced pressure and heat while suppressing the inclusion of air bubbles, and then, for the purpose of suppressing delay bubbles, pressurization is performed at the same time as heating by autoclave treatment or the like. When defoaming is performed by heating, the heating temperature is generally about 40 to 150°C. When pressurization is performed, the pressure is generally about 0.05 MPa to 2 MPa.

After the pressure-sensitive adhesive sheet before photo-curing is bonded to an adherend such as a front transparent plate, the pressure-sensitive adhesive composition constituting the pressure-sensitive adhesive sheet is photo-cured. The amount of light irradiated during photocuring is not particularly limited as long as it is within a range that allows photocuring of the pressure-sensitive adhesive sheet, and is, for example, about 50 to 10,000 mJ/cm ² in cumulative light amount.

Due to the light irradiation, the polymerization reaction of the photopolymerizable polyfunctional compound progresses, and accordingly, the gel fraction of the adhesive sheet increases, and the reliability of adhesion between the adhesive sheet 5 and the front transparent plate 7 is improved, and it is possible to avoid high-temperature environments. It is possible to suppress the generation of bubbles at the bonding interface when exposed to water for a long time. By adjusting the ultraviolet absorbency of the adhesive sheet and the type and amount of the photopolymerization initiator as described above, if the light absorption band of the photopolymerization initiator and the light absorption wavelength of the ultraviolet absorber overlap. However, since the photocuring reaction efficiency of the adhesive sheet can be maintained, it is possible to form an image display device with excellent adhesion reliability of the front transparent member.

[Optical film with adhesive sheet]
The pressure-sensitive adhesive sheet of the present invention can be used not only in a form in which a release film is temporarily attached to both sides as shown in FIG. 1, but also as a film with a pressure-sensitive adhesive in which the pressure-sensitive adhesive sheet is fixed to an optical film or the like. For example, in the form shown in FIG. 3, the release film 1 is temporarily attached to one surface of the adhesive sheet 5, and the polarizing plate 3 is adhered to the other surface of the adhesive sheet 5. In the form shown in FIG. 4, an adhesive sheet 4 is further provided on the polarizing plate 3, and a release film 2 is temporarily attached thereon.

In this case, in the case where an optical film such as a polarizing plate is attached to the adhesive sheet in advance, the release film 1 temporarily attached to the surface of the adhesive sheet 5 is peeled off and the adhesive sheet 5 is attached to the front transparent member. Then, the adhesive sheet 5 may be photocured.

The present invention will be further explained below with reference to Examples, but the present invention is not limited to these Examples.

[Example 1]
(Preparation of base polymer)
In a reaction vessel equipped with a thermometer, a stirrer, a cooler, and a nitrogen gas inlet tube, monomer components such as 2-ethylhexyl acrylate: 63 parts by weight, N-vinylpyrrolidone (NVP): 15 parts by weight, and hydroxyethyl acrylate: 13 parts by weight of ethyl acetate, 9 parts by weight of methyl methacrylate, and 0.2 parts by weight of azobisisobutyronitrile as a thermal polymerization initiator were added together with 233 parts by weight of ethyl acetate, and the mixture was heated under a nitrogen atmosphere at 23°C. The mixture was stirred for hours and replaced with nitrogen. Thereafter, the mixture was reacted at 65° C. for 5 hours and then at 70° C. for 2 hours to prepare a solution of an acrylic base polymer having a weight average molecular weight of 450,000.

(Preparation of adhesive composition)
To the acrylic base polymer solution obtained above, 2 parts of polypropylene glycol (#400) diacrylate ("NK Ester APG-400" manufactured by Shin Nakamura Chemical Industry Co., Ltd.) as a polyfunctional monomer were added to 100 parts by weight of the base polymer. Parts by weight: Trimethylolpropane adduct of xylylene diisocyanate ("Takenate D110N" manufactured by Mitsui Chemicals): 0.16 parts by weight; Silane coupling agent ("KBM-403" manufactured by Shin-Etsu Chemical) 0. 3 parts by weight; as a photopolymerization initiator, 2,2-dimethoxy 1,2-diphenylethane-1-one ("Irgacure 651" manufactured by BASF, maximum absorption wavelength: 250 nm and 340 nm; extinction coefficient ε at wavelength 380 nm = 1. 7×10 ² [mLg ⁻¹ cm ⁻¹ ]): 0.2 parts by weight; and as a UV absorber, 0.7 parts by weight of bisethylhexyloxyphenolmethoxyphenyltriazine (“Tinosorb S” manufactured by BASF) were added. Thereafter, they were mixed uniformly to prepare a pressure-sensitive adhesive composition.

(Preparation of adhesive sheet)
The above adhesive composition was applied to the release-treated surface of a 75-μm-thick release film (a polyester film whose surface was subjected to silicone release treatment) so that the thickness after drying would be 100 μm, and the mixture was heated at 100°C for 30 minutes. After drying for minutes to remove the solvent, a release film with a thickness of 50 μm was laminated on the surface of the adhesive layer, and crosslinking was performed by aging treatment for 3 days in an atmosphere at 25° C. to obtain an adhesive sheet.

[Examples 2 to 6 and Comparative Examples 1 to 3]
In preparing the adhesive composition, the amounts of the ultraviolet absorber and photopolymerization initiator were changed as shown in Table 1. A pressure-sensitive adhesive sheet was obtained in the same manner as in Example 1 except for the above.

[Example 7, 8]
In the preparation of the adhesive composition, dipentaerythritol hexaacrylate (DPHA) was added as a polyfunctional monomer in place of polypropylene glycol diacrylate in the amount shown in Table 1. A pressure-sensitive adhesive sheet was obtained in the same manner as in Example 1 except for the above.

[evaluation]
<Light transmittance of adhesive sheet>
After peeling off the release film on one side of the adhesive sheet and bonding the adhesive sheet to an alkali-free glass plate, peel off the release film on the other side, and use an ultraviolet-visible spectrophotometer (Hitachi High-Tech's "U- 4100'') to measure the transmission spectrum. From the obtained spectrum, the light transmittance at wavelengths of 365 nm, 380 nm, and 405 nm was read.

<Gel fraction of adhesive immediately after preparation>
About 0.2 g of adhesive was scraped off from the adhesive sheet after 3 days of aging treatment, and this was used as a sample for gel fraction measurement. The sample was wrapped in a porous polytetrafluoroethylene membrane ("NTF-1122" manufactured by Nitto Denko) cut into a size of 100 mm x 100 mm with a pore diameter of 0.2 μm, and the wrapped opening was tied with octopus string. The weight (B) of the adhesive sample was calculated by subtracting the total weight (A) of the porous polytetrafluoroethylene membrane and the octopus thread, which had been measured in advance, from the weight of this sample. An adhesive sample wrapped in a porous polytetrafluoroethylene membrane was immersed in approximately 50 mL of ethyl acetate at 23°C for 7 days, and the sol component of the adhesive was eluted out of the porous polytetrafluoroethylene membrane. . After immersion, the adhesive wrapped in the porous polytetrafluoroethylene membrane was taken out, dried at 130° C. for 2 hours, allowed to cool for about 20 minutes, and then its dry weight (C) was measured. The gel fraction of the adhesive was calculated using the following formula.
Gel fraction (%) = 100 x (CA)/B

<Gel fraction of adhesive sheet after photocuring>
Immediately after the preparation, the pressure-sensitive adhesive sheet was irradiated with ultraviolet rays with a cumulative light intensity of about 1000 mJ/cm ² from the release film attachment surface using a UV lamp with a UVA energy density of 300 mW/cm ² to photocure the pressure-sensitive adhesive. About 0.2 g of the adhesive was scraped off from the photocured adhesive sheet, and the gel fraction was measured in the same manner as above.

<Gel fraction after storage under fluorescent light>
After leaving the adhesive sheet with release film on both sides at a distance of 1.5 m from a fluorescent lamp (Panasonic FLR32S/N/N-X; color temperature 5000K) for 7 days, approximately 0.2 g of The adhesive was scraped off, and the gel fraction was measured in the same manner as above.

<Level difference absorption>
After storing the adhesive sheet for one week under fluorescent light, cut it into a size of 75 mm x 45 mm, peel off the release film on one side from the adhesive sheet, and leave a 40 μm thick triacetyl cellulose film on both sides of the polyvinyl alcohol polarizer. It was bonded to the center of the bonded polarizing plates (100 mm x 50 mm) using a roll laminator (pressure between rolls: 0.2 MPa, feed speed: 100 mm/min). After that, the release film on the other side of the adhesive sheet was peeled off, and a 500 μm thick glass plate (100 mm x 50 mm) with a 20 μm thick black ink printed in a frame shape on the periphery was placed in a roll laminator (roll-to-roll pressure : 0.2 MPa, feeding speed: 100 mm/min). The ink printing area of the glass plate was 5 mm from both ends in the short side direction and 15 mm from both ends in the long side direction, and the black ink layer was in contact with an area 5 mm from the ends of the four sides of the adhesive sheet. After processing this sample in an autoclave (50° C., 0.5 MPa) for 30 minutes, it was observed using a digital microscope with a magnification of 20 times to confirm the presence or absence of air bubbles near the boundary of the black ink printing area. Those in which no air bubbles were confirmed were evaluated as OK, and those in which air bubbles were observed were evaluated as NG.

<Adhesion reliability after photocuring>
Using the pressure-sensitive adhesive sheet immediately after preparation, attach a polarizing plate to one side of the pressure-sensitive adhesive sheet and a glass plate with black ink printed in a frame shape on the periphery to the other side, in the same way as in the step absorption evaluation above. A sample was prepared. This sample was irradiated with ultraviolet rays from the glass plate side using a UV lamp with a UVA energy density of 300 mW/cm ² to photocure the adhesive, and then heated to 85°C ^. It was placed in an oven for 72 hours. The samples taken out of the oven were checked for the presence of air bubbles in the same manner as the step absorption test described above, and those with no air bubbles were evaluated as OK, and those with air bubbles observed were evaluated as NG.

Composition of the adhesives of Examples and Comparative Examples (type and amount of polyfunctional monomer added, amount of ultraviolet absorber and photopolymerization initiator added), light transmittance, gel fraction, and level difference absorption and adhesion of the adhesive sheet The reliability evaluation results are shown in Table 1.

The adhesive sheets of Examples 1 to 7 showed a small change in gel fraction after being left standing under a fluorescent lamp for 7 days, and when irradiated with ultraviolet rays of 1000 mJ/cm ² , the gel fraction increased. It had both excellent storage stability and photocuring efficiency. The pressure-sensitive adhesive sheets of Comparative Example 1 and Comparative Example 2 showed a large increase in gel fraction after being left undisturbed for 7 days under a fluorescent lamp, and the level difference absorbency was decreased.

In Comparative Example 2, the amount of ultraviolet absorber added was small and the adhesive sheet had high light transmittance at wavelengths of 380 nm and 365 nm, so it is thought that photocuring progressed in the storage environment. Examples 1 and 2 and Comparative Example 1 had the same amount of ultraviolet absorber added, but in Comparative Example 1, which had a larger amount of photopolymerization initiator added, photocuring occurred in the storage environment. These results show that in order to suppress photocuring in the storage environment, it is important not only to add an ultraviolet absorber but also to adjust the amount of photopolymerization initiator added.

In Comparative Example 3, in which 1.0 of the ultraviolet absorber was added, photocuring under fluorescent lamps was suppressed, but the gel fraction did not increase sufficiently even when irradiated with ultraviolet light at 1000 mJ/ ^cm2 . However, the adhesion reliability was poor. In Comparative Example 3, it is thought that the photocuring efficiency decreased because the ultraviolet absorber absorbed a large amount of excitation light and the photopolymerization initiator absorbed a small amount of light.

Based on the above results, storage stability can be improved by adding a UV absorber to a photocurable adhesive sheet to adjust its absorption characteristics, and by adjusting the type (absorption wavelength) and amount of photopolymerization initiator. It can be seen that it is possible to achieve both high photocuring efficiency.

5 Adhesive sheet 1, 2 Release film 3 Polarizing plate 4 Adhesive sheet 6 Image display cell 10 Image display panel 7 Front transparent plate 100 Image display device

Claims (12)

Selected from the group consisting of an acrylic base polymer having a crosslinked structure, a photopolymerizable multifunctional compound having two or more photopolymerizable functional groups in one molecule, a photopolymerization initiator , and a triazine compound and a benzotriazole compound. A photo-curable pressure-sensitive adhesive sheet comprising a layered pressure-sensitive adhesive composition containing one or more types of ultraviolet absorbers,
The light transmittance at a wavelength of 420 nm is 85% or more, the light transmittance at a wavelength of 405 nm is 80% or more, and the light transmittance at a wavelength of 380 nm is 4 to 25%,
The photopolymerization initiator has an extinction coefficient of 50 [mLg ^-1 cm ^-1 ] or less at a wavelength of 405 nm, and an extinction coefficient ε of 80 [mLg ^-1 cm ^-1 ] or more at a wavelength of 380 nm,
The content of the photopolymerization initiator with respect to 100 parts by weight of the acrylic base polymer is w parts by weight, and the product εw of ε [mLg ⁻¹ cm ⁻¹ ] and w [parts by weight] is 10 to 100.
adhesive sheet. The adhesive sheet according to claim 1, having a light transmittance of 1% or less at a wavelength of 365 nm. The adhesive sheet according to claim 1 or 2, wherein the photopolymerization initiator has an absorption maximum in a wavelength range of 310 nm to 370 nm. The pressure-sensitive adhesive sheet according to any one of claims 1 to 3 , wherein the content w of the photopolymerization initiator is 0.05 to 1 part by weight based on 100 parts by weight of the acrylic base polymer. The light transmittance of the adhesive sheet at a wavelength of 380 nm is T%, the extinction coefficient ε [mLg ⁻¹ cm ⁻¹ ] of the photopolymerization initiator at a wavelength of 380 nm , and the photopolymerization initiation rate for 100 parts by weight of the acrylic base polymer. The pressure-sensitive adhesive sheet according to any one of claims 1 to 4 , wherein the product εwT of the agent content w [parts by weight] and T [%] is 100 to 1200. Any one of claims 1 to 5 , wherein the acrylic base polymer contains a hydroxy group-containing monomer as a monomer unit, and a crosslinked structure is formed by bonding an isocyanate crosslinking agent to the hydroxyl group. Adhesive sheet described in. The adhesive sheet according to any one of claims 1 to 6 , wherein the photopolymerizable polyfunctional compound is a polyfunctional (meth)acrylate. The pressure-sensitive adhesive sheet according to any one of claims 1 to 7 , containing 0.3 to 10 parts by weight of the photopolymerizable polyfunctional compound based on 100 parts by weight of the acrylic base polymer. The pressure-sensitive adhesive sheet according to any one of claims 1 to 8 , having a gel fraction of 30 to 80%. The pressure-sensitive adhesive sheet according to any one of claims 1 to 9 , wherein the gel fraction after photocuring by irradiation with 1000 mJ/cm ² of ultraviolet rays is 5% or more larger than the gel fraction before photocuring. an optical film; a first adhesive sheet laminated on the first main surface of the optical film; and a second adhesive sheet laminated on the second main surface of the optical film,
An optical film with an adhesive layer, wherein the first adhesive sheet is the adhesive sheet according to any one of claims 1 to 10 . A method for manufacturing an image display device in which a transparent member having a stepped portion is disposed on a viewing side surface, the method comprising:
After the adhesive sheet according to any one of claims 1 to 10 and the transparent member are bonded together, the adhesive composition of the adhesive sheet is photocured by irradiating the adhesive sheet with actinic rays. , a method for manufacturing an image display device.

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