WO2020184155A1

WO2020184155A1 - Pressure-sensitive adhesive sheet, production method therefor, and image display device

Info

Publication number: WO2020184155A1
Application number: PCT/JP2020/007230
Authority: WO
Inventors: 大器下栗; 崇弘野中; 翔寳田; 理仁丹羽; 祐輔山本
Original assignee: 日東電工株式会社
Priority date: 2019-03-08
Filing date: 2020-02-21
Publication date: 2020-09-17

Abstract

Provided is a pressure-sensitive adhesive sheet which need not be photocured after application to adherends and which can combine irregularity-absorbing properties with dimensional stability. The pressure-sensitive adhesive sheet (5) is a sheet formed from a pressure-sensitive adhesive comprising a base polymer having a crosslinked structure and has a shear storage modulus at 25°C of 0.16 MPa or greater and a loss tangent at 70°C of 0.25 or greater. The pressure-sensitive adhesive sheet has a glass transition temperature of preferably -3°C or lower. The pressure-sensitive adhesive has a gel content of preferably 30-80% and a polymer content (nonvolatile content) of 95% or higher.

Description

Adhesive sheet and its manufacturing method, and image display device

The present invention relates to an adhesive sheet and a method for producing the same. Furthermore, the present invention relates to an image display device using the adhesive sheet.

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. A front transparent plate (also referred to as a "cover window") such as a transparent resin plate or a glass plate is provided on the visual side of the image display panel for the purpose of preventing damage to the image display panel due to an impact from the outer surface. Sometimes. Further, in recent years, a device having a touch panel on the visual side of an image display panel has become widespread.

A colored layer (decorative printing layer) for decoration or light shielding may be formed on the periphery of the front transparent member. When the adhesive is attached to the transparent member having the decorative printing layer, air bubbles are likely to be generated around the printing step portion. Therefore, a method is adopted in which a thick adhesive sheet is used to absorb steps to prevent problems such as air bubbles from being mixed.

Further, in order to impart step absorption, it has been proposed to use an adhesive sheet made of a photocurable adhesive composition for bonding front transparent members. For example, in Patent Document 1, a composition obtained by adding a polyfunctional monomer and a photopolymerization initiator to a polymer solution prepared by solution polymerization is applied onto a substrate, and the solvent is removed by heating to obtain a photocurable adhesion. An example of making a sheet is shown. In Patent Document 2, a solvent-free composition containing a low molecular weight polymer, a monofunctional monomer and a polyfunctional monomer, and a photopolymerization initiator is applied onto a substrate and photocured to prepare an adhesive sheet. An example is shown. By leaving a part of the monomer components in the composition in an unreacted state at the time of photo-curing, an adhesive sheet having high fluidity and excellent step absorption is formed.

Since the photocurable pressure-sensitive adhesive sheet contains a photopolymerizable monomer or oligomer in an unreacted state, the pressure-sensitive adhesive has high fluidity and excellent step absorption. By irradiating the pressure-sensitive adhesive sheet with active light rays and performing photo-curing after bonding with the adherend, the fluidity of the pressure-sensitive adhesive is reduced and the adhesive holding power is improved.

Japanese Unexamined Patent Publication No. 2014-227453 International Publication No. 2013/161666

In an image display device in which the front transparent member such as a cover window is larger than the display panel, the front transparent member and the housing are bonded to each other by an adhesive tape or the like in an area outside the outer peripheral edge of the display panel. .. That is, the front transparent member is fixed by a combination of bonding to the housing with an adhesive tape or the like and bonding to the surface of the display panel with an adhesive sheet for interlayer filling.

In recent years, display devices have become narrower and bezel-less, mainly for mobile devices such as smartphones. With the narrowing of the frame and the bezel-less, an image display device in which the size of the display panel 10 is equal to or larger than the size of the front transparent member 7 has been developed. In such a configuration, the housing 9 and the front transparent member 7 cannot be fixed by an adhesive tape or the like, and the front transparent member 7 needs to be fixed only by the adhesive sheet 5 (see FIG. 2). Along with this, the adhesive sheet is required to have a higher adhesive force and is required not to be peeled off due to an impact such as dropping.

In addition, due to the narrowing of the frame and the bezel-less design, high dimensional stability is required even when assembling the image display device and transporting work-in-process. The photocurable adhesive sheet described in Patent Document 1 and Patent Document 2 enhances the flexibility of the adhesive in order to have step absorption, and in the state before photocuring, an external force is applied during transportation or processing. If is added, the adhesive sheet is easily deformed, so that the bonding members may be displaced from each other. Further, since it is necessary to perform photo-curing after bonding with the adherend, the manufacturing process of the image display device tends to be complicated.

In view of the above, the present invention is an adhesive sheet which does not require photo-curing after being bonded to an adherend, can achieve both step absorption and dimensional stability, and has adhesive durability and impact resistance. For the purpose of providing.

One embodiment of the present invention is a double-sided pressure-sensitive adhesive sheet in which a pressure-sensitive adhesive containing a base polymer having a crosslinked structure is formed in a sheet shape. The shear storage elastic modulus G'25 _° C. of the pressure-sensitive adhesive sheet at a temperature of 25 ° _C. is preferably 0.16 MPa or more, and the loss tangent tan δ _{70 °} C. at a temperature of 70 ° _C. is preferably 0.25 or more. The glass transition temperature of the adhesive sheet is preferably -3 ° C or lower.

The gel fraction of the adhesive sheet is preferably 30 to 80%. The polymerization rate of the pressure-sensitive adhesive constituting the pressure-sensitive adhesive sheet is preferably 95% or more. The weight average molecular weight of the sol of the pressure-sensitive adhesive is, for example, 150,000 to 400,000. The haze of the adhesive sheet is preferably 1% or less.

The base polymer contained in the adhesive sheet includes, for example, a polymer in which an acrylic polymer chain is crosslinked by a urethane segment. In order to satisfy the above characteristics, the content of the urethane-based segment with respect to 100 parts by weight of the acrylic polymer chain is preferably 0.3 to 10 parts by weight. The weight average molecular weight of the urethane-based segment is, for example, 5000 to 30,000.

The base polymer of the pressure-sensitive adhesive is, for example, an acrylic polymer having a crosslinked structure, even if the crosslinked structure is introduced into the acrylic polymer chain by polyfunctional (meth) acrylate or urethane (meth) acrylate. Good. The amount of (meth) acrylic acid alkyl ester in the acrylic polymer chain is preferably 50% by weight or more based on the total amount of the constituent monomer components. In the acrylic polymer chain, the total amount of the hydroxyl group-containing monomer and the nitrogen-containing monomer with respect to the total amount of the constituent monomer components may be 15 to 45% by weight.

The base polymer may contain a polymer in which a crosslinked structure with urethane-based segments is introduced into an acrylic polymer chain. For example, by copolymerizing an acrylic monomer constituting an acrylic polymer chain with a polyfunctional urethane (meth) acrylate having at least two (meth) acryloyl groups at the ends, a crosslinked structure of the acrylic polymer chain with a urethane segment. An acrylic polymer in which is introduced is obtained.

As the urethane (meth) acrylate, urethane di (meth) acrylate having (meth) acryloyl groups at both ends is preferable. The weight average molecular weight of the urethane (meth) acrylate is preferably 5000 to 30,000. The glass transition temperature of urethane (meth) acrylate is preferably 0 ° C. or lower. The urethane (meth) acrylate may contain a polyester urethane (meth) acrylate.

A pressure-sensitive adhesive sheet containing a base polymer in which an acrylic polymer chain is crosslinked with a urethane-based segment has, for example, a composition containing an acrylic monomer and / or a partial polymer thereof, and a urethane (meth) acrylate layered on a substrate. After coating, it is obtained by irradiating the composition with active light and performing photocuring. The pressure-sensitive adhesive composition preferably has a urethane (meth) acrylate content of 0.3 to 10 parts by weight based on a total of 100 parts by weight of the acrylic monomer and its partial polymer.

The adhesive sheet of the present invention is used, for example, for bonding transparent members in an image display device in which a transparent member is arranged on the surface on the viewing side. For example, the image display device is formed by fixing the front transparent member to the visible surface of the image display panel via the above-mentioned adhesive sheet. An adhesive sheet may be laminated on a transparent film substrate to form a double-sided adhesive sheet with a substrate.

The adhesive sheet of the present invention has a large shear storage elastic modulus at room temperature, is excellent in adhesive reliability and workability, and has a large loss tangent at high temperature, so that it has excellent step absorption and impact resistance. The image display device in which a cover window or the like is attached to the surface on the viewing side using the adhesive sheet of the present invention has excellent adhesive reliability, and can be used for narrowing the frame and bezel-less.

It is sectional drawing which shows the structural example of the pressure-sensitive adhesive sheet (base-less double-sided pressure-sensitive adhesive sheet) with a release film. It is sectional drawing which shows the structural example of the image display apparatus. It is sectional drawing which shows the laminated structure example of the optical film with an adhesive sheet. It is sectional drawing which shows the laminated structure example of the optical film with an adhesive sheet. It is sectional drawing which shows the structural example of the adhesive sheet with a release film (double-sided adhesive sheet with a base material). It is sectional drawing which shows the structural example of the image display apparatus. It is a photograph which shows the state of the interlayer adhesion test. It is an observation photograph of a sample in which streak-like bubbles were generated in the interlayer adhesion test. It is a schematic diagram which shows the arrangement of a sample in an impact resistance test.

FIG. 1 shows an adhesive sheet with a release film in which release

films

21 and 22 are temporarily attached to both sides of the adhesive sheet 5. FIG. 2 is a cross-sectional view showing a configuration example of an image display device in which the front transparent plate 7 is fixed by using an adhesive sheet.

[Physical properties of adhesive sheet]
The pressure-sensitive adhesive sheet 5 is a base-less double-sided pressure-sensitive adhesive sheet in which the pressure-sensitive adhesive is formed in the form of a sheet. The pressure-sensitive adhesive contains a base polymer having a crosslinked structure. The adhesive sheet is preferably highly transparent. The total light transmittance of the pressure-sensitive adhesive sheet is preferably 85% or more, more preferably 90% or more. The haze of the adhesive sheet is preferably 1% or less.

From the viewpoint of enhancing the adhesive holding force when the adhesive sheet is attached to the adherend and ensuring the processing dimensional stability, the shear storage elastic modulus G'25 _° C of the adhesive sheet at 25 ° _C is 0.16 MPa or more. Preferably, 0.18 MPa or more is more preferable, 0.20 MPa or more is further preferable, and 0.21 MPa or more is particularly preferable.

On the other hand, the G'25 _{° C.} of the adhesive sheet is 0.5 MPa or less from the viewpoint of ensuring wettability by giving the adhesive sheet an appropriate viscosity and also providing cushioning property against impacts such as dropping. Is more preferable, 0.4 MPa or less is more preferable, 0.3 MPa or less is further preferable, and 0.28 MPa or less is particularly preferable.

From the viewpoint of providing the pressure-sensitive adhesive sheet with step absorption, the loss tangent tan δ _{70 °} C. at 70 ° C. of the pressure-sensitive adhesive sheet is preferably 0.25 or more, more preferably 0.30 or more, still more preferably 0.35 or more. tan δ _{70 ° C.} may be 0.40 or more, 0.45 or more, 0.50 or more, or 0.55 or more. From the viewpoint of adhesive holding power, tan δ _{70 ° C.} is preferably 1.0 or less, more preferably 0.9 or less, and even more preferably 0.85 or less. tan δ _{70 ° C.} may be 0.80 or less, 0.75 or less, or 0.70 or less.

The peak top value of tan δ of the adhesive sheet is preferably 1.5 or more, more preferably 1.6 or more, and even more preferably 1.7 or more. An adhesive sheet having a large peak top value of tan δ tends to have a large viscous behavior and excellent impact resistance. The upper limit of the peak top value of tan δ of the adhesive sheet is not particularly limited, but is generally 3.0 or less. From the viewpoint of adhesive holding power, the peak top value of tan δ is preferably 2.7 or less, more preferably 2.5 or less.

The glass transition temperature of the adhesive sheet is preferably -3 ° C or lower, more preferably -4 ° C or lower. The glass transition temperature of the pressure-sensitive adhesive sheet is preferably −20 ° C. or higher, more preferably −15 ° C. or higher, and even more preferably −13 ° C. or higher. When the glass transition temperature is within the above range, the adhesive sheet has an appropriate viscosity even in a low temperature region, and there is a tendency that peeling of the adherend due to an impact such as dropping is suppressed.

The shear storage elastic modulus G', loss contact tan δ, and glass transition temperature of the adhesive sheet are determined by viscoelasticity measurement at a frequency of 1 Hz. tan δ is the ratio G ″ / G ′ of the storage elastic modulus G ′ and the loss elastic modulus G ″, and the glass transition temperature is the temperature at which tan δ is maximized (peak top temperature). The storage elastic modulus G'corresponds to a portion stored as elastic energy when the material is deformed, and is an index indicating the degree of hardness. The larger the storage elastic modulus of the adhesive sheet, the higher the adhesive holding force and the tendency for peeling due to strain to be suppressed. The loss elastic modulus G ”corresponds to the energy loss portion dissipated by internal friction or the like when the material is deformed, and represents the degree of viscosity. The larger the tan δ, the stronger the tendency of viscosity, and the more liquid the deformation behavior becomes. The rebound resilience energy tends to be small.

The gel fraction of the pressure-sensitive adhesive sheet is preferably 30 to 80% from the viewpoint of ensuring processing stability at G'25 _{° C.} of 0.16 MPa or more and providing appropriate flexibility for imparting step absorption. 35-70% is more preferable. The gel fraction may be 40% or more or 45% or more, 65% or less, or 60% or less.

The gel fraction of the pressure-sensitive adhesive sheet can be determined as an insoluble component in a solvent such as ethyl acetate. Specifically, the insoluble component after immersing the pressure-sensitive adhesive constituting the pressure-sensitive adhesive sheet in ethyl acetate at 23 ° C. for 7 days. Is obtained as a weight fraction (unit: weight%) with respect to the sample before immersion. In general, the gel fraction of a polymer is equal to the degree of cross-linking, and the more cross-linked portions in the polymer, the higher the gel fraction. The gel fraction (introduction amount of the crosslinked structure) can be adjusted to a desired range depending on the introduction method of the crosslinked structure, the type and amount of the crosslinking agent, and the like.

The adhesive strength of the adhesive sheet is preferably 2N / 10 mm or more, more preferably 4N / 10 mm or more, and even more preferably 5N / 10 mm or more. When the adhesive force of the adhesive sheet is within the above range, it is possible to prevent the adhesive sheet from peeling off from the adherend when stress due to strain or impact due to dropping or the like occurs. The adhesive strength is determined by a peel test with a tensile speed of 300 mm / min and a peeling angle of 180 ° using a glass plate as an adherend. Unless otherwise specified, the adhesive strength is a measured value at 25 ° C.

The thickness of the adhesive sheet is not particularly limited, and may be set according to the type and shape of the adherend. When a member having a printing step is used as an adherend, it is preferable that the thickness of the adhesive sheet is larger than the thickness of the printing step. The thickness of the pressure-sensitive adhesive sheet used for bonding the front transparent plate (cover window) is preferably 30 μm or more, more preferably 40 μm or more, still more preferably 50 μm or more. Increasing the thickness of the adhesive sheet tends to increase step absorption and impact resistance. The upper limit of the thickness of the pressure-sensitive adhesive sheet is not particularly limited, but from the viewpoint of the productivity of the pressure-sensitive adhesive sheet, it is preferably 500 μm or less, more preferably 300 μm or less, still more preferably 250 μm or less.

[Composition of adhesive]
The composition of the pressure-sensitive adhesive is not particularly limited as long as the pressure-sensitive adhesive sheet 5 satisfies the above characteristics, and the pressure-sensitive adhesive composition is not particularly limited, and acrylic polymer, silicone-based polymer, polyester, polyurethane, polyamide, polyvinyl ether, vinyl acetate / vinyl chloride copolymer, and modified polyolefin. , Epoxy-based, fluorine-based, natural rubber, synthetic rubber and other rubber-based polymers as the base polymer can be appropriately selected and used. In particular, it is excellent in optical transparency, exhibits adhesive properties such as appropriate wettability, cohesiveness and adhesiveness, and is also excellent in weather resistance and heat resistance. Therefore, acrylic adhesive containing an acrylic polymer as a base polymer. The agent is preferably used.

<Acrylic polymer chain>
The acrylic base polymer having a crosslinked structure is one in which a crosslinked structure is introduced into an acrylic polymer chain. The acrylic polymer chain 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 having an alkyl group having 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 the (meth) acrylic acid alkyl ester having a chain alkyl group include methyl (meth) acrylic acid, ethyl (meth) acrylic acid, butyl (meth) acrylic acid, isobutyl (meth) acrylic acid, and (meth). 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 , (Meta) undecyl acrylate, (meth) dodecyl acrylate, (meth) isotridodecyl acrylate, (meth) tetradecyl acrylate, (meth) isotetradecyl acrylate, (meth) pentadecyl acrylate, (meth) Examples thereof include cetyl acrylate, heptadecyl (meth) acrylate, octadecyl (meth) acrylate, isooctadecil (meth) acrylate, and nonadecil (meth) acrylate.

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

The amount of the (meth) acrylic acid alkyl ester with respect to the total amount of the monomer components constituting the acrylic polymer chain is preferably 50% by weight or more, more preferably 55% by weight or more, still more preferably 60% by weight or more. From the viewpoint of setting the glass transition temperature (Tg) of the polymer chain in an appropriate range, the acrylic base polymer is an amount of (meth) acrylic acid alkyl ester having a chain alkyl group having 4 to 10 carbon atoms with respect to the total amount of constituent monomer components. However, it is preferably 40% by weight or more, more preferably 50% by weight or more, and further preferably 55% by weight or more. The monomer components constituting the acrylic polymer chain include all the monomer components constituting the polymer, the monomers used for forming the crosslinked structure (polyfunctional (meth) acrylate, urethane (meth) acrylate, etc. described later) and crosslinked. It is the one excluding the agent.

The acrylic base polymer may contain a hydroxyl group-containing monomer or a carboxy group-containing monomer as a constituent monomer component. When the crosslinked structure is introduced by the isocyanate cross-linking agent, the hydroxyl group becomes the reaction point with the isocyanate group, and when the crosslinked structure is introduced by the epoxy-based cross-linking agent, the carboxy group becomes the reaction point with the epoxy group.

Examples of the hydroxyl group-containing monomer include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, 6-hydroxyhexyl (meth) acrylate, and (meth) acrylic. Examples thereof include (meth) acrylic acid esters such as 8-hydroxyoctyl acid, 10-hydroxydecyl (meth) acrylic acid, 12-hydroxylauryl (meth) acrylic acid and (4-hydroxymethylcyclohexyl) -methyl (meth) acrylate. .. When a crosslinked structure with a urethane-based segment is introduced into the acrylic polymer chain, the acrylic-based base polymer is used as a constituent monomer component from the viewpoint of high compatibility with the urethane-based segment and improving the transparency of the pressure-sensitive adhesive sheet. , A (meth) acrylic acid ester having a hydroxyalkyl group having 4 to 8 carbon atoms is preferably contained.

When the acrylic base polymer has a hydroxyl group-containing monomer as a constituent monomer component, the transparency of the pressure-sensitive adhesive sheet is improved and white turbidity in a high-temperature and high-humidity environment tends to be suppressed. Further, the hydroxyl group of the hydroxyl group-containing monomer can form a physical crosslink by hydrogen bonding with an acrylic polymer chain or a crosslinked segment (for example, a urethane segment) that crosslinks the acrylic polymer chain. Therefore, by increasing the ratio of the hydroxyl group-containing monomer in the monomer components constituting the acrylic polymer chain, the cohesive force tends to be increased and the G'25 _{° C.} tends to increase even when the gel fraction is low. The amount of the hydroxyl group-containing monomer is preferably 5 to 30% by weight, more preferably 8 to 25% by weight, still more preferably 10 to 20% by weight, based on the total amount of the monomer components constituting the acrylic polymer chain.

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

When the adhesive sheet is used for adhering the touch panel sensor, it is preferable that the adhesive sheet has a small acid content in order to prevent the electrode from being corroded by the acid component. Further, when the pressure-sensitive adhesive sheet is used for adhering a polarizing plate, the pressure-sensitive adhesive sheet preferably has a small acid content in order to suppress polyene formation of the polyvinyl alcohol-based polarizer due to an acid component. The content of the organic acid monomer such as (meth) acrylic acid in such an acid-free pressure-sensitive adhesive sheet is preferably 100 ppm or less, more preferably 70 ppm or less, and further preferably 50 ppm or less. .. The organic acid monomer content of the pressure-sensitive adhesive sheet is determined by immersing the pressure-sensitive adhesive sheet in pure water, heating it at 100 ° C. for 45 minutes, and quantifying the acid monomer extracted in water by ion chromatography.

In order to reduce the acid monomer content in the pressure-sensitive adhesive sheet, it is preferable that the amount of the organic acid monomer component such as (meth) acrylic acid in the monomer component 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 based on 100 parts by weight of the total 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 more preferably 0.05 parts by weight. The following is more preferable, ideally 0.

The acrylic base polymer may contain a nitrogen-containing monomer as a constituent monomer component. Examples of the nitrogen-containing monomer include N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholin, (meth) acryloylmorpholin, and N-vinyl. Examples thereof include vinyl-based monomers such as carboxylic acid amides and N-vinylcaprolactam, and cyano group-containing acrylic monomers such as acrylonitrile and methacrylonitrile. Among these, N-vinylpyrrolidone is preferable because it has a high effect of improving the adhesive force by improving the cohesive force.

When the acrylic-based base polymer contains a highly polar monomer such as a hydroxyl group-containing monomer, a carboxy group-containing monomer, and a nitrogen-containing monomer as a constituent monomer component, the cohesive force of the pressure-sensitive adhesive is enhanced and G'25 _{° C.} is increased. , Adhesive retention tends to improve. On the other hand, if the content of the highly polar monomer is excessively large, the glass transition temperature may increase and the impact resistance may decrease. Therefore, the amount of highly polar monomers (total of hydroxyl group-containing monomer, carboxy group-containing monomer, and nitrogen-containing monomer) with respect to the total amount of the monomer components constituting the acrylic polymer chain is preferably 15 to 45% by weight, preferably 20 to 40% by weight. More preferably, 25 to 37% by weight is further preferable. In particular, it is preferable that the total of the hydroxyl group-containing monomer and the nitrogen-containing monomer is within the above range. The amount of the nitrogen-containing monomer with respect to the total amount of the monomer components constituting the acrylic base polymer is preferably 7 to 30% by weight, more preferably 10 to 25% by weight, still more preferably 12 to 22% by weight.

Acrylic-based base polymers include acid anhydride group-containing monomers, (meth) acrylic acid caprolactone adducts, sulfonic acid group-containing monomers, phosphoric acid group-containing monomers, vinyl acetate, vinyl propionate, and styrene, as monomer components other than the above. Vinyl-based monomers such as α-methylstyrene; cyano group-containing acrylic monomers such as acrylonitrile and methacrylonitrile; epoxy group-containing monomers such as glycidyl (meth) acrylate; (meth) polyethylene glycol acrylate, (meth) acrylic Glycol-based acrylic ester monomers such as polypropylene glycol acid, methoxyethylene glycol (meth) acrylate, and methoxypolypropylene glycol (meth) acrylate; tetrahydrofurfuryl (meth) acrylate, fluorine (meth) acrylate, silicone (meth) acrylate, etc. Acrylic acid ester-based monomers such as 2-methoxyethyl (meth) acrylic acid may be contained.

The acrylic base polymer preferably has the highest content of (meth) acrylic acid alkyl ester among the above-mentioned monomer components. The characteristics of the pressure-sensitive adhesive sheet are easily affected by the type of the monomer (main monomer) having the highest content among the constituent monomers of the acrylic polymer chain. For example, when the main monomer of the acrylic polymer chain is a (meth) acrylic acid alkyl ester having a chain alkyl group having 6 or less carbon atoms, tan δ _{70 ° C.} tends to increase and the step absorption tends to be improved. In particular, when an acrylic acid C ₄ alkyl esters, such as butyl acrylate is the major monomer, they tend to tan [delta _{70 ° C.} increases. The amount of the (meth) acrylic acid alkyl ester having a chain alkyl group having 6 or less carbon atoms is preferably 40 to 85% by weight, more preferably 45 to 80% by weight, based on the total amount of the monomer components constituting the acrylic polymer chain. , 50-75% by weight is more preferable. In particular, the content of butyl acrylate as a constituent monomer component is preferably in the above range.

The theoretical Tg of the acrylic polymer chain is preferably −50 ° C. or higher. The theoretical Tg of the acrylic polymer chain is preferably −10 ° C. or lower, more preferably −20 ° C. or lower, and even more preferably −25 ° C. or lower. Theoretical Tg is a glass transition temperature Tg _i of the homopolymer constituent monomer components of the acrylic polymer chain, the weight fraction W _i of each monomer component, is calculated by the Fox equation below.
1 / Tg = Σ ( _Wi / Tg _i )

Tg is the glass transition temperature of the polymer chain (unit: K), _Wi is the weight fraction of the monomer component i constituting the segment (copolymerization ratio based on the weight), and Tg _i is the glass transition temperature of the homopolymer of the monomer component i. (Unit: K). As the glass transition temperature of the homopolymer, the numerical value described in the third edition of the Polymer Handbook (John Wiley & Sons, Inc., 1989) can be adopted. For the Tg of the homopolymer of the monomer not described in the above document, the peak top temperature of the loss tangent (tan δ) by the dynamic viscoelasticity measurement may be adopted.

<Crosslink structure>
For a polymer in which a crosslinked structure is introduced into an acrylic polymer chain, for example, (1) an acrylic polymer having a functional group capable of reacting with a crosslinking agent is polymerized, and then a crosslinking agent is added to obtain the acrylic polymer and the crosslinking agent. (2) By including a polyfunctional compound in the polymer component of the polymer, a branched structure (crosslinked structure) is introduced into the polymer chain, and the like. These may be used in combination to introduce a plurality of crosslinked structures into the base polymer.

Specific examples of the cross-linking agent in the method of reacting the base polymer of the above (1) with the cross-linking agent include isocyanate-based cross-linking agent, epoxy-based cross-linking agent, oxazoline-based cross-linking agent, aziridine-based cross-linking agent, carbodiimide-based cross-linking agent, and metal. Examples include a chelate-based cross-linking agent. Of these, isocyanate-based cross-linking agents and epoxy-based cross-linking agents are preferable because they have high reactivity with the hydroxyl groups and carboxy groups of the base polymer and the cross-linked structure can be easily introduced. These cross-linking agents react with functional groups such as hydroxyl groups and carboxy groups introduced into the base polymer to form a cross-linked structure. In an acid-free pressure-sensitive adhesive in which the base polymer does not contain a carboxy group, it is preferable to use an isocyanate-based cross-linking agent to form a cross-linked structure by reacting a hydroxyl group in the base polymer with the isocyanate cross-linking agent.

As the isocyanate-based cross-linking agent, polyisocyanate having two or more isocyanate groups in one molecule is used. Examples of the isocyanate-based cross-linking 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 / trimylene diisocyanate trimer adduct (eg, "Coronate L" manufactured by Toso), trimethylolpropane / hexamethylene Diisocyanate trimeric adduct (for example, "Coronate HL" manufactured by Toso), trimethylolpropane adduct of xylylene diisocyanate (for example, "Takenate D110N" manufactured by Mitsui Chemicals), isocyanurate of hexamethylene diisocyanate (for example, manufactured by Toso). Examples thereof include isocyanate additives such as "Coronate HX"). Further, by using a urethane prepolymer having an isocyanate group at the terminal as the isocyanate-based cross-linking agent, a cross-linked structure using urethane-based segments can be introduced.

In the method (2) of including the polyfunctional compound in the polymerization component of the base polymer, the monomer component constituting the acrylic base polymer and the total amount of the polyfunctional compound for introducing the crosslinked structure may be reacted at one time. Polymerization may be carried out in multiple stages. As a method of performing polymerization in multiple steps, a monofunctional monomer constituting the base polymer is polymerized (prepolymerized) to prepare a partial polymer (prepolymer composition), and the prepolymer composition is polyfunctional (meth). A method of adding a polyfunctional compound such as acrylate to polymerize (mainly polymerize) the prepolymer composition and the polyfunctional monomer is preferable. The prepolymer composition is a partial polymer containing a polymer having a low degree of polymerization and an unreacted monomer.

By prepolymerizing the constituent components of the acrylic base polymer, branch points (crosslink points) due to the polyfunctional compound can be uniformly introduced into the base polymer. Further, after applying a mixture (adhesive composition) of a low molecular weight polymer or a partial polymer and an unpolymerized monomer component on a base material, the main polymerization is performed on the base material to form a pressure-sensitive adhesive sheet. You can also. Since a low polymerization composition such as a prepolymer composition has a low viscosity and is excellent in coatability, it is a method of performing main polymerization on a substrate after applying a pressure-sensitive adhesive composition which is a mixture of a prepolymer composition and a polyfunctional compound. According to this, the productivity of the pressure-sensitive adhesive sheet can be improved, and the thickness of the pressure-sensitive adhesive sheet can be made uniform.

Examples of the polyfunctional compound used for introducing the crosslinked structure include compounds containing two or more polymerizable functional groups (ethylenically unsaturated groups) having an unsaturated double bond in one molecule. As the polyfunctional compound, polyfunctional (meth) acrylate is preferable because it can be easily copolymerized with the monomer component of the acrylic base polymer. When a branched (crosslinked) structure is introduced by active energy ray polymerization (photopolymerization), a polyfunctional acrylate is preferable.

Examples of the polyfunctional (meth) acrylate include polyethylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, polytetramethylene glycol di (meth) acrylate, bisphenol A ethylene oxide-modified di (meth) acrylate, and bisphenol A propylene oxide. Modified di (meth) acrylate, alcandiol di (meth) acrylate, tricyclodecanedimethanol di (meth) acrylate, ethoxylated isocyanuric acid tri (meth) acrylate, pentaeristol tri (meth) acrylate, pentaeristol di ( Meta) Acrylate, Trimethylol Propanetri (Meta) Acrylate, Ditrimethylol Propanetetra (Meta) Acrylate, Pentaeristol Tetra (Meta) Acrylate, Penta Eristol Tetra (Meta) Acrylate, Dipenta Eristol Poly (Meta) Examples thereof include acrylate, dipentaeryristol hexa (meth) acrylate, neopentyl glycol di (meth) acrylate, glycerin di (meth) acrylate, epoxy (meth) acrylate, butadiene (meth) acrylate, and isoprene (meth) acrylate. Further, by using a urethane (meth) acrylate having a (meth) acryloyl group at the end of the urethane chain as the polyfunctional (meth) acrylate, a crosslinked structure using urethane-based segments can be introduced.

<Introduction of cross-linked structure using urethane segment>
By cross-linking the acrylic polymer chain with the urethane segment, it is easy to obtain an adhesive that can achieve both a low glass transition temperature and a high adhesive holding force. The urethane-based segment is a molecular chain having a urethane bond, and when both ends of the urethane-based segment are covalently bonded to the acrylic polymer chain, a crosslinked structure by the urethane-based segment is introduced into the acrylic polymer chain. Urethane-based segments typically contain a polyurethane chain obtained by reacting a diol with a diisocyanate.

The molecular weight of the polyurethane chain in the urethane-based segment is preferably 5000 to 30000, more preferably 6000 to 23000, and even more preferably 7000 to 20000. The larger the molecular weight of the polyurethane chain in the urethane-based segment, the longer the distance between the cross-linking points of the acrylic polymer chain. When the molecular weight of the polyurethane chain is within the above range, the polymer having the crosslinked structure introduced has appropriate cohesiveness and fluidity, so that both adhesive strength, step absorption and impact resistance can be achieved at the same time.

When the molecular weight of the polyurethane chain is excessively small and the distance between the cross-linking points is short, the tan δ becomes smaller as the cohesive force increases, and the step absorption and impact resistance tend to decrease. On the other hand, when the molecular weight of the polyurethane chain is excessively large and the distance between the cross-linking points is long, the storage elastic modulus may be small and the adhesive holding force may be insufficient. Even when the molecular weight of the polyurethane chain is large, the storage elastic modulus can be increased by increasing the amount of urethane-based segments to increase the gel fraction. However, since the polyurethane chain having a large molecular weight has low compatibility with the acrylic polymer chain, the haze of the pressure-sensitive adhesive may increase as the amount of the urethane-based segment increases, and the transparency may decrease.

If the amount of urethane-based segment becomes excessively large, the viscosity of the adhesive may decrease as the gel fraction increases, and the step absorption and impact resistance may decrease. Further, if the amount of the urethane-based segment is excessively large, the transparency of the adhesive sheet may decrease and the haze may increase. Therefore, the amount of the urethane-based segment in the base polymer is preferably 10 parts by weight or less, more preferably 7 parts by weight or less, still more preferably 5 parts by weight or less, based on 100 parts by weight of the acrylic polymer chain. On the other hand, from the viewpoint of increasing the gel fraction and imparting adhesive holding power, the amount of the urethane-based segment in the base polymer is preferably 0.3 parts by weight or more with respect to 100 parts by weight of the acrylic polymer chain, preferably 0.4. It is more preferably parts by weight or more, and even more preferably 0.5 parts by weight or more. The amount of the urethane-based segment in the base polymer may be 4 parts by weight or less or 3 parts by weight or less, and 0.7 parts by weight or more or 1 part by weight or more with respect to 100 parts by weight of the acrylic polymer chain. May be good.

Examples of the diol used for forming the polyurethane chain include low molecular weight diols such as ethylene glycol, diethylene glycol, propylene glycol, butylene glycol and hexamethylene glycol; polyester polyol, polyether polyol, polycarbonate polyol, acrylic polyol, epoxy polyol, caprolactone polyol and the like. High molecular weight polyols of.

The polyether polyol is obtained by ring-opening addition polymerization of an alkylene oxide on a polyhydric alcohol. Examples of the alkylene oxide include ethylene oxide, propylene oxide, butylene oxide, styrene oxide, and tetrahydrofuran. Examples of the polyhydric alcohol include the above-mentioned diol, glycerin, trimethylolpropane and the like.

The polyester polyol is a polyester having a hydroxyl group at the terminal, and is obtained by reacting a polybasic acid with a polyhydric alcohol so that the alcohol equivalent is excessive with respect to the carboxylic acid equivalent. As the polybasic acid component and the polyhydric alcohol component constituting the polyester polyol, a combination of dibasic acid and diol is preferable.

Examples of the dibasic acid component include aromatic dicarboxylic acids such as orthophthalic acid, isophthalic acid, and terephthalic acid; and alicyclics such as hexahydrophthalic acid, tetrahydrophthalic acid, 1,3-cyclohexanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid. Formula dicarboxylic acid; aliphatic dicarboxylic acids such as oxalic acid, succinic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, decandicarboxylic acid, dodecandicarboxylic acid, octadecanedicarboxylic acid; Examples thereof include acid anhydrides of dicarboxylic acids and lower alcohol esters.

Examples of the diol component include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, pentanediol, 1,6-hexanediol, and the like. 1,8-octanediol, 1,10-decanediol, diethylene glycol, triethylene glycol, polyethylene glycol, dipropylene glycol, polypropylene glycol, 1,4-cyclohexanedimethanol, 1,4-cyclohexanediol, bisphenol A, bisphenol F , Hydrophobic bisphenol A, hydrogenated bisphenol F and the like.

The polycarbonate polyol is a polycarbonate polyol obtained by subjecting a diol component and phosgen to a polycondensation reaction; the diol component, dimethyl carbonate, diethyl carbonate, diprovyl carbonate, diisopropyl carbonate, dibutyl carbonate, ethylbutylcarbonate, ethylene carbonate, propylene carbonate, and carbonic acid. Polycarbonate polyol obtained by ester exchange condensation with carbonic acid diesters such as diphenyl and dibenzyl carbonate; copolymerized polycarbonate polyol obtained by using two or more kinds of polyol components in combination; the above-mentioned various polycarbonate polyols and carboxy group-containing compounds are esterified. Polycarbonate polyol obtained by reaction; Polycarbonate polyol obtained by etherifying the various polycarbonate polyols and hydroxyl group-containing compounds; Polycarbonate polyol obtained by ester exchange reaction between the various polycarbonate polyols and ester compounds; Polycarbonate polyol obtained by ester exchange reaction of polyol and hydroxyl group-containing compound; Polycarbonate-based polycarbonate polyol obtained by polycondensation of various polycarbonate polyols and dicarboxylic acid compounds; Copolymerization of various polycarbonate polyols and alkylene oxides Examples thereof include the obtained copolymerized polyether-based polycarbonate polyol.

The polyacrylic polyol is obtained by copolymerizing a (meth) acrylic acid ester with a monomer component having a hydroxyl group. Examples of the monomer having a hydroxyl group include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, 2-hydroxybutyl (meth) acrylate, and (meth). ) Hydroxyalkyl esters of (meth) acrylic acids such as 4-hydroxybutyl acrylate and 2-hydroxypentyl (meth) acrylate; (meth) acrylic acid monoesters of polyhydric alcohols such as glycerin and trimethyl propane; N- Examples thereof include methylol (meth) acrylamide. Examples of the (meth) acrylic acid ester include methyl (meth) acrylate, butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, and cyclohexyl (meth) acrylate.

The polyacrylic polyol may contain a monomer component other than the above as a copolymerization component. Examples of the copolymerized monomer component other than the above include unsaturated monocarboxylic acids such as (meth) acrylic acid; unsaturated dicarboxylic acids such as maleic acid and their anhydrides and mono- or diesters; and unsaturated nitriles such as (meth) acrylonitrile. Classes; unsaturated amides such as (meth) acrylamide and N-methylol (meth) acrylamide; vinyl esters such as vinyl acetate and vinyl propionate; vinyl ethers such as methyl vinyl ether; α-olefins such as ethylene and propylene; Examples thereof include halogenated α, β-unsaturated aliphatic monomers such as vinyl chloride and vinylidene chloride; and α, β-unsaturated aromatic monomers such as styrene and α-methylstyrene.

The diisocyanate used to form the polyurethane chain may be either an aromatic diisocyanate or an aliphatic diisocyanate. Examples of the aromatic diisocyanis include 1,5-naphthalenediocyanis, 4,4'-diphenylmethane diisocyanate (MDI), 4,4'-diphenyldimethylmethane diisocyanate, tetramethyldiphenylmethane diisocyanate, 1,3-phenylenediisocyanate, 1,4-. Phoenix diisocyanis, 2-chloro-1,4-phenyl diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, xylylene diisocyanate, 4,4'-diphenyl ether diisocyanate, 4,4'-diphenyl sulfoxide diisocyanate , 4,4'-diphenylsulfone diisocyanate, 4,4'-biphenyldiisocyanate and the like. Examples of the aliphatic diisocyanate include butane-1,4-diisocyanate, hexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, cyclohexane-1,4-diisocyanate, and isophorone diisocyanate. , Dicyclohexylmethane-4,4'-diisocyanate, 1,3-bis (isocyanatemethyl) cyclohexane, methylcyclohexane diisocyanate and the like.

A derivative of an isocyanate compound can also be used as the diisocyanate. Derivatives of the isocyanate compound include dimer of polyisocyanate, trimer of isocyanate (isocyanurate), polypeptide MDI, adduct with trimethylolpropane, biuret modified product, allophanate modified product, urea modified product and the like. .. As the diisocyanate component, a urethane prepolymer having an isocyanate group at the terminal may be used.

Among the exemplified polyurethane chains, it is preferable to contain a polyether urethane having a polyether polyol as a diol component and / or a polyester urethane having a polyester polyol as a diol component because of its high compatibility with an acrylic polymer chain. In particular, when a crosslinked structure is introduced with polyester urethane, the storage elastic modulus at room temperature tends to increase, and the adhesive holding force and workability tend to improve. One reason is that polyester has a rigid molecular structure as compared with polyether and the like. When the cross-linked structure is introduced by the rigid segment, the storage elastic modulus is increased because the movement of the acrylic polymer chain is restricted, while the distance between the cross-linking points of the polymer chain is maintained, so that it is impact resistant. It is thought that it shows step absorption.

A compound having a functional group copolymerizable with a monomer component constituting the acrylic polymer chain at the end of the polyurethane chain, or a functional group capable of reacting with a carboxy group, a hydroxyl group, etc. contained in the acrylic polymer chain at the end of the polyurethane chain. By using a compound having a group, a crosslinked structure of urethane-based segments can be introduced into the acrylic polymer chain. Urethane di (meth) having (meth) acryloyl groups at both ends of the polyurethane chain because it is easy to uniformly introduce cross-linking points into the acrylic polymer chain and the compatibility between the acrylic polymer chain and the urethane segment is excellent. It is preferable to introduce a crosslinked structure with urethane-based segments using acrylate. For example, by copolymerizing the monomer component constituting the acrylic polymer chain with the urethane di (meth) acrylate, a crosslinked structure by the urethane segment can be introduced into the acrylic polymer chain.

Urethane di (meth) acrylate having (meth) acryloyl groups at both ends can be obtained, for example, by using a (meth) acrylic compound having a hydroxyl group in addition to the diol component in the polymerization of polyurethane. From the viewpoint of controlling the chain length (molecular weight) of the urethane-based segment, a diol and diisocyanate are reacted so as to have an excess of isocyanate to synthesize an isocyanate-terminated polyurethane, and then a (meth) acrylic compound having a hydroxyl group is added. Therefore, it is preferable to react the terminal isocyanate group of polyurethane with the hydroxyl group of the (meth) acrylic compound.

By reacting the polyhydric alcohol with the polyisocyanate compound so that the polyisocyanate compound becomes excessive, a polyurethane chain having an isocyanate group at the terminal can be obtained. In order to obtain an isocyanate-terminated polyurethane, a diol component and a diisocyanate component are used so that the NCO / OH (equivalent ratio) is preferably 1.1 to 2.0, more preferably 1.15 to 1.5. do it. The diisocyanate component may be added after the diol component and the diisocyanate component are mixed in substantially equal amounts and reacted.

Examples of the (meth) acrylic compound having a hydroxyl group include hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate, hydroxyhexyl (meth) acrylate, hydroxymethylacrylamide, and hydroxyethyl. Examples include acrylamide.

As urethane (meth) acrylate, commercially available products sold by Arakawa Chemical Industry, Shin Nakamura Chemical Industry, Toa Synthetic, Kyoeisha Chemical, Nippon Kayaku, Nippon Synthetic Chemical Industry, Negami Kogyo, Daicel Ornex, etc. are used. May be good. The weight average molecular weight of the urethane (meth) acrylate is preferably 5000 to 30000, more preferably 6000 to 23000, and even more preferably 7000 to 20000.

The glass transition temperature of urethane (meth) acrylate is preferably 0 ° C. or lower, more preferably −10 ° C. or lower, and even more preferably −20 ° C. or lower. By using a low Tg urethane (meth) acrylate, a pressure-sensitive adhesive having excellent low-temperature adhesive strength can be obtained even when a crosslinked structure is introduced by a urethane-based segment to enhance the cohesive force of the base polymer. The lower limit of the glass transition temperature of urethane (meth) acrylate is not particularly limited, but from the viewpoint of obtaining an adhesive having excellent high-temperature holding power, -100 ° C or higher is preferable, -90 ° C or higher is more preferable, and -80 ° C or higher is preferable. More preferred.

When a urethane (meth) acrylate is used to introduce a crosslinked structure of urethane-based segments into an acrylic polymer chain, the glass transition temperature of the urethane-based segment of the base polymer is approximately equal to the glass transition temperature of urethane (meth) acrylate. ..

<Preparation of base polymer>
A polymer in which a crosslinked structure of urethane-based segments is introduced into an acrylic polymer chain can be polymerized by various known methods. When urethane (meth) acrylate is used as a constituent component of the urethane-based segment, the monomer component for forming the acrylic polymer chain and urethane (meth) acrylate may be copolymerized.

The amount of urethane (meth) acrylate used is preferably 0.3 to 10 parts by weight, more preferably 0.5 to 7 parts by weight, and 0, based on 100 parts by weight of the monomer component for forming the acrylic polymer chain. .7 to 5 parts by weight is more preferable. By adjusting the amount of urethane (meth) acrylate used, a base polymer having a urethane-based segment content in the above range can be prepared. When the content of the urethane-based segment is excessively small, the adhesive holding force of the pressure-sensitive adhesive sheet tends to decrease due to the decrease in cohesiveness of the base polymer. When the content of the urethane-based segment is excessively large, the viscosity of the pressure-sensitive adhesive sheet tends to decrease as the cohesiveness of the base polymer increases, and the impact resistance and step absorption tend to decrease.

A crosslinked structure can be introduced into the acrylic polymer chain by using a polyfunctional (meth) acrylic compound other than urethane (meth) acrylate in addition to urethane (meth) acrylate or instead of urethane (meth) acrylate. If the amount of the crosslinked structure introduced by the polyfunctional compound other than urethane (meth) acrylate increases, the impact resistance and step absorption of the pressure-sensitive adhesive may decrease. Therefore, the amount of the polyfunctional compound other than the urethane (meth) acrylate is preferably 0.2 parts by weight or less, preferably 0.1 parts by weight or less, based on 100 parts by weight of the monomer component for forming the acrylic polymer chain. More preferably, 0.05 parts by weight or less is further preferable.

Photopolymerization is preferable as the method for polymerizing the base polymer. Since the polymer can be prepared without using a solvent in photopolymerization, it is not necessary to dry and remove the solvent when forming the pressure-sensitive adhesive sheet, and a thick pressure-sensitive adhesive sheet can be uniformly formed.

In the preparation of the base polymer, the monomer components constituting the acrylic polymer chain and the total amount of the polyfunctional compound for introducing the crosslinked structure may be reacted at once, or the polymerization may be carried out in multiple steps. As a method of performing polymerization in multiple steps, a monofunctional monomer constituting an acrylic polymer chain is polymerized to form a prepolymer composition (prepolymerization), and urethane di (meth) acrylate is contained in the syrup of the prepolymer composition. A method of polymerizing (mainly polymerizing) the prepolymer composition and the polyfunctional monomer by adding a polyfunctional compound such as the above is preferable. The prepolymer composition is a partial polymer containing a polymer having a low degree of polymerization and an unreacted monomer.

By prepolymerizing the constituent components of the acrylic polymer chain, branch points (crosslink points) due to a polyfunctional compound such as urethane di (meth) acrylate can be uniformly introduced into the acrylic polymer chain. Further, after applying a mixture (adhesive composition) of a low molecular weight polymer or a partial polymer and an unpolymerized monomer component on a base material, the main polymerization is performed on the base material to form a pressure-sensitive adhesive sheet. You can also.

Since a low-polymerization degree composition such as a prepolymer composition has low viscosity and excellent coatability, a method of performing main polymerization on a substrate after applying a pressure-sensitive adhesive composition which is a mixture of a prepolymer composition and a polyfunctional compound. According to the above, the productivity of the pressure-sensitive adhesive sheet can be improved, and the thickness of the pressure-sensitive adhesive sheet can be made uniform.

[Adhesive sheet]
As described above, a prepolymer composition having a low degree of polymerization is prepared by prepolymerization, and a pressure-sensitive adhesive composition obtained by adding a polyfunctional compound or the like to the prepolymer composition is applied in a layered manner on the base material, and then on the base material. A pressure-sensitive adhesive sheet can be obtained by polymerizing the pressure-sensitive adhesive composition (main polymerization).

<Prepolymerization>
The prepolymer composition can be prepared, for example, by polymerizing a composition in which a monomer component constituting an acrylic polymer chain and a polymerization initiator are mixed. The composition for forming a prepolymer may contain a polyfunctional compound (polyfunctional monomer or polyfunctional oligomer). For example, a part of the polyfunctional compound which is a raw material of the polymer may be contained in the composition for forming the prepolymer, and the rest of the polyfunctional compound may be added after the prepolymer is polymerized to be subjected to the main polymerization.

The composition for forming a prepolymer preferably contains a photopolymerization initiator. Examples of the photopolymerization initiator include a benzoin ether-based photopolymerization initiator, an acetophenone-based photopolymerization initiator, an α-ketol-based photopolymerization initiator, an aromatic sulfonyl chloride-based photopolymerization initiator, and a photoactive oxime-based photopolymerization initiator. Examples thereof include benzoin-based photopolymerization initiators, benzyl-based photopolymerization initiators, benzophenone-based photopolymerization initiators, ketal-based photopolymerization initiators, thioxanthone-based photopolymerization initiators, and acylphosphine oxide-based photopolymerization initiators.

At the time of polymerization, a chain transfer agent, a polymerization inhibitor (polymerization delaying agent) or the like may be used for the purpose of adjusting the molecular weight. Examples of the chain transfer agent 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. Examples thereof include α-methylstyrene dimer.

The polymerization rate of the prepolymer is not particularly limited, but is preferably 3 to 50% by weight, more preferably 5 to 40% by weight, from the viewpoint of obtaining a viscosity suitable for coating on a substrate. The polymerization rate of the prepolymer can be adjusted to a desired range by adjusting the type and amount of the photopolymerization initiator, the irradiation intensity and irradiation time of active light such as UV light, and the like. The polymerization rate of the prepolymer is calculated by the following formula from the weight before and after heating when heated at 130 ° C. for 3 hours. The polymerization rate of the pressure-sensitive adhesive sheet is also calculated by the same method.
Polymerization rate (%) = weight after drying / weight before drying x 100

<Preparation of adhesive composition>
In the prepolymer composition, a polyfunctional compound such as urethane di (meth) acrylate, and if necessary, the rest of the monomer components constituting the acrylic polymer chain, a polymerization initiator, a chain transfer agent, other additives, and the like are added. Mix to prepare a pressure-sensitive adhesive composition. The pressure-sensitive adhesive composition preferably has a viscosity suitable for coating on a substrate (for example, about 0.5 to 20 Pa · s). By adjusting the polymerization rate of the prepolymer, the type and amount of the polyfunctional compound, the composition of other components (for example, oligomers), the molecular weight, the amount of addition, etc., the viscosity of the pressure-sensitive adhesive composition can be adjusted to an appropriate range. it can. A thickening additive or the like may be used for the purpose of adjusting the viscosity.

The polymerization initiator used in the main polymerization is not particularly limited, and for example, the above-mentioned photopolymerization initiator can be used. If the polymerization initiator used in the prepolymerization remains in the prepolymer composition without being deactivated, the addition of the polymerization initiator for the main polymerization may be omitted.

(Chain transfer agent)
In this polymerization, it is preferable to adjust the molecular weight by including a chain transfer agent in the pressure-sensitive adhesive composition. The chain transfer agent used in the present polymerization is not particularly limited, and for example, the above-mentioned chain transfer agent can be used. The amount of the chain transfer agent in the pressure-sensitive adhesive composition is preferably 0.001 to 2 parts by weight, more preferably 0.005 to 1 part by weight, and 0.01 to 0.01 parts by weight, based on 100 parts by weight of the constituent components of the base polymer. 0.5 parts by weight is more preferable. If the chain transfer agent used in the prepolymerization remains in the prepolymer composition without being inactivated, the addition of the chain transfer agent to the pressure-sensitive adhesive composition may be omitted.

The chain transfer agent receives radicals from the growing polymer chain to stop the elongation of the polymer, and the chain transfer agent that receives the radicals attacks the monomer to restart the polymerization. By using a chain transfer agent, an increase in the molecular weight of the polymer can be suppressed without lowering the radical concentration in the reaction system.

When the ratio of the monofunctional monomer to the polyfunctional monomer is constant, the larger the molecular weight, the higher the probability that one polymer chain contains a cross-linking point (branching point) due to the polyfunctional monomer, so that the gel fraction increases. Tend. By suppressing the elongation of the polymer by using a chain transfer agent, the molecular weight of the polymer tends to be reduced and the increase in gel fraction tends to be suppressed. Therefore, when the pressure-sensitive adhesive composition contains a chain transfer agent, a pressure-sensitive adhesive sheet having a large tan δ and excellent step absorption is easily formed.

(Oligomer)
The pressure-sensitive adhesive composition may contain various oligomers for the purpose of adjusting the adhesive force of the pressure-sensitive adhesive sheet, adjusting the viscosity, and the like. As the oligomer, for example, one having a weight average molecular weight of about 1000 to 30,000 is used. As the oligomer, an acrylic oligomer is preferable because it has excellent compatibility with an acrylic base polymer.

Acrylic oligomer contains (meth) acrylic acid alkyl ester as a main constituent monomer component. Among them, (meth) acrylic acid alkyl ester having a chain alkyl group (chain alkyl (meth) acrylate) and (meth) acrylic acid alkyl ester having an alicyclic alkyl group (alicyclic alkyl) as constituent monomer components. Those containing (meth) acrylate) are preferable. Specific examples of the chain alkyl (meth) acrylate and the alicyclic alkyl (meth) acrylate are as exemplified above as the constituent monomers of the acrylic polymer chain.

The glass transition temperature of the acrylic oligomer is preferably 20 ° C. or higher, more preferably 40 ° C. or higher. The glass transition temperature of the acrylic oligomer may be 60 ° C. or higher, 80 ° C. or higher, 100 ° C. or higher, or 110 ° C. or higher. By using a low Tg acrylic base polymer introduced with a crosslinked structure and a high Tg acrylic oligomer in combination, the adhesive holding power of the pressure-sensitive adhesive sheet tends to be improved. The upper limit of the glass transition temperature of the acrylic oligomer is not particularly limited, but is generally 200 ° C. or lower, preferably 180 ° C. or lower, and more preferably 160 ° C. or lower. The glass transition temperature of the acrylic oligomer is calculated by the Fox formula described above.

Among the exemplified (meth) acrylic acid alkyl esters, methyl methacrylate is preferable as the chain alkyl (meth) acrylate because it has a high glass transition temperature and excellent compatibility with the base polymer. As the alicyclic alkyl (meth) acrylate, dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate are preferable. That is, the acrylic oligomer contains one or more selected from the group consisting of dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate as constituent monomer components, and methyl methacrylate. Is preferable.

The amount of the alicyclic alkyl (meth) acrylate is preferably 10 to 90% by weight, more preferably 20 to 80% by weight, still more preferably 30 to 70% by weight, based on the total amount of the monomer components constituting the acrylic oligomer. The amount of the chain alkyl (meth) acrylate with respect to the total amount of the monomer components constituting the acrylic oligomer is preferably 10 to 90% by weight, more preferably 20 to 80% by weight, still more preferably 30 to 70% by weight.

The weight average molecular weight of the acrylic oligomer is preferably 1000 to 30,000, more preferably 1500 to 10000, and even more preferably 2000 to 8000. By using an acrylic oligomer having a molecular weight in the above range, the adhesive strength and adhesive holding strength of the pressure-sensitive adhesive tend to be improved.

Acrylic oligomers are obtained by polymerizing the above monomer components by various polymerization methods. Various polymerization initiators may be used for the polymerization of the acrylic oligomer. Further, a chain transfer agent may be used for the purpose of adjusting the molecular weight.

When the pressure-sensitive adhesive composition contains an oligomer component such as an acrylic oligomer, the content thereof is preferably 0.5 to 20 parts by weight, more preferably 1 to 15 parts by weight, based on 100 parts by weight of the above base polymer. More preferably, 2 to 10 parts by weight. When the content of the oligomer in the pressure-sensitive adhesive composition is in the above range, the adhesiveness at high temperature and the holding power at high temperature tend to be improved.

(Silane coupling agent)
A silane coupling agent may be added to the pressure-sensitive adhesive composition for the purpose of adjusting the adhesive strength. When a silane coupling agent is added to the pressure-sensitive adhesive composition, the amount added is usually about 0.01 to 5.0 parts by weight with respect to 100 parts by weight of the base polymer, and 0.03 to 2.0 parts by weight. It is preferably about.

(UV absorber)
The pressure-sensitive adhesive composition may contain an ultraviolet absorber. When the pressure-sensitive adhesive composition contains an ultraviolet absorber, it is possible to impart ultraviolet absorbability to the adhesive sheet 5 and prevent deterioration of the polarizing plate 3 and the image display cell 6 due to ultraviolet rays.

Examples of the ultraviolet absorber include benzotriazole-based ultraviolet absorbers, benzophenone-based ultraviolet absorbers, triazine ultraviolet absorbers, salicylate-based ultraviolet absorbers, cyanoacrylate-based ultraviolet absorbers, and the like. A triazine-based UV absorber is preferable because it has high UV absorption, excellent compatibility with an acrylic polymer, and a highly transparent acrylic pressure-sensitive adhesive sheet can be easily obtained. Among them, a triazine-based UV absorber containing a hydroxyl group is preferable. Agents are preferred, and hydroxyphenyltriazine-based UV absorbers are particularly preferred.

A commercially available product may be used as the ultraviolet absorber. Commercially available triazine-based UV absorbers include 2- (4,6-bis (2,4-dimethylphenyl) -1,3,5-triazine-2-yl) -5-hydroxyphenyl and [(alkyloxy). ) Methyl] Reaction product with oxylane (BASF "TINUVIN 400"), 2- (2,4-dihydroxyphenyl) -4,6-bis- (2,4-dimethylphenyl) -1,3,5 -Reaction product of triazine and (2-ethylhexyl) -glycidate ester (BASF "TINUVIN 405"), (2,4-bis [2-hydroxy-4-butoxyphenyl] -6- (2,4) -Dibutoxyphenyl) -1,3,5-triazine (BASF "TINUVIN 460"), 2- (4,6-diphenyl-1,3,5-triazine-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"), 2,4-bis- [{4- (4-ethylhexyloxy) -4-hydroxy} -phenyl] -6- (4-methoxyphenyl) -1, 3,5-Triazine (BASF "Tinosorb S"), 2- (4,6-diphenyl-1,3,5-triazine-2-yl) -5- [2- (2-ethylhexanoyloxy) ethoxy ] -Phenol ("ADK STAB LA-46" manufactured by ADEKA) and the like can be mentioned.

When an ultraviolet absorber is added to the pressure-sensitive adhesive composition, the amount of the ultraviolet absorber added is usually about 0.1 to 10 parts by weight, preferably 0.3 to 5 parts by weight, based on 100 parts by weight of the base polymer. By setting the content of the ultraviolet absorber in the above range, it is possible to improve the ultraviolet blocking property of the adhesive sheet while suppressing the decrease in transparency due to the bleed-out of the ultraviolet absorber or the like.

(Other additives)
In addition to the above-exemplified components, the pressure-sensitive adhesive composition includes additives such as pressure-sensitive adhesives, plasticizers, softeners, deterioration inhibitors, fillers, colorants, antioxidants, surfactants, and antistatic agents. It may be included.

<Application of adhesive composition and main polymerization>
After the pressure-sensitive adhesive composition is applied in layers on the substrate, photocuring is performed by irradiating with active light rays. When photo-curing, a cover sheet is attached to the surface of the coating layer, and the pressure-sensitive adhesive composition is sandwiched between the two sheets and irradiated with active light to prevent polymerization inhibition by oxygen. preferable.

Any suitable base material is used as the base material and cover sheet used for forming the adhesive sheet. The base material and the cover sheet may be a release film having a release layer on the contact surface with the adhesive sheet.

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

As a method of applying the adhesive composition on the substrate, roll coat, kiss roll coat, gravure coat, reverse coat, roll brush, spray coat, dip roll coat, bar coat, knife coat, air knife coat, curtain coat , Lip coat, die coater and the like are used.

This polymerization is carried out by irradiating the pressure-sensitive adhesive composition coated in layers on the base material with active light. In this polymerization, the unreacted monomer component in the prepolymer composition reacts with a polyfunctional compound such as urethane di (meth) acrylate to obtain a polymer in which a crosslinked structure is introduced into an acrylic polymer chain.

The active light may be selected according to the type of polymerizable component such as a monomer or urethane (meth) acrylate, the type of photopolymerization initiator, and the like, and generally, ultraviolet rays and / or visible light having a short wavelength are used. .. The integrated light amount of the irradiation light is preferably about 100 to 5000 mJ / cm ² . The light source for light irradiation is not particularly limited as long as the photopolymerization initiator contained in the pressure-sensitive adhesive composition can irradiate light in a wavelength range having sensitivity, and is an LED light source, a high-pressure mercury lamp, and ultra-high-pressure mercury. Lamps, metal halide lamps, xenon lamps and the like are preferably used. If the residual amount of unreacted monomer is large, the G'25 _{° C.} of the pressure-sensitive adhesive sheet may decrease and the adhesive holding power may decrease. Therefore, the polymerization rate of the pressure-sensitive adhesive sheet after the main polymerization is preferably 95% or more, more preferably 97% or more, further preferably 98% or more, and particularly preferably 99% or more. In order to increase the polymerization rate, the pressure-sensitive adhesive sheet may be heated to volatilize the residual monomer, the unreacted polymerization initiator and the like.

As described above, the gel fraction of the pressure-sensitive adhesive sheet is preferably 30 to 80%, more preferably 35 to 70%. When the gel content is 30% or more, the adhesive holding force of the adhesive is enhanced, glue chipping during processing and positional deviation between members are unlikely to occur, and the processability and processing dimensional stability are excellent. .. Further, when the gel fraction is 80% or less, excellent step absorption can be exhibited.

The weight average molecular weight of the sol content of the adhesive sheet is preferably 150,000 to 450,000, more preferably 180,000 to 420,000. The sol content is a soluble component obtained by extracting the base polymer with tetrahydrofuran (hereinafter, THF). Since it is difficult to measure the molecular weight of individual polymer chains of a crosslinked polymer (gel), the molecular weight of a sol (non-crosslinked polymer) is an index indicating the degree of elongation of the polymer chain. If the molecular weight of the sol is excessively large, the glass transition temperature may increase and the impact resistance may decrease. On the other hand, if the molecular weight of the sol content is excessively small, the adhesive holding power may decrease.

By adhering the

release films

21 and 22 to the surface of the adhesive sheet 5, as shown in FIG. 1, an adhesive sheet in which the release films are temporarily attached to both sides can be obtained. The release film used as the base material or the cover sheet at the time of forming the adhesive sheet may be used as it is as the

release films

21 and 22.

When the

release films

21 and 22 are provided on both sides of the adhesive sheet 5, the thickness of one release film 21 and the thickness of the other release film 22 may be the same or different. Even if the peeling force for peeling the release film temporarily attached to one surface from the adhesive sheet 5 and the release force for peeling the release film temporarily attached to the other surface from the adhesive sheet 5 are the same. It may be different. If the peeling forces of the two are different, the release film 22 (light peeling film) having a relatively small peeling force is peeled off from the adhesive sheet 5 first and bonded to the first adherend, and is relatively relative. Excellent workability when the release film 21 (heavy release film) having a large peeling force is peeled off and bonded to the second adherend.

[Image display device]
The adhesive sheet 5 can be used for bonding various transparent members and opaque members. The type of the adherend is not particularly limited, and examples thereof include various resin materials, glass, and metal. Since the adhesive sheet 5 has high transparency, it is suitable for bonding optical members such as an image display device. In particular, since the adhesive sheet 5 is excellent in step absorption and impact resistance, it is suitably used for bonding a transparent member such as a front transparent plate or a touch panel to the visible side surface of an image display device.

FIG. 2 is a cross-sectional view showing an example of a laminated configuration of an image display device in which a front transparent plate 7 is attached to the visible side surface of the image display panel 10 via an adhesive sheet 5. The image display panel 10 includes a polarizing plate 3 attached to the visible surface of the image display cell 6 such as a liquid crystal cell or an organic EL cell via an adhesive sheet 4. The front transparent plate 7 is provided with a print layer 76 on the peripheral edge of one surface of the transparent flat plate 71. As the transparent plate 71, for example, a transparent resin plate such as an acrylic resin or a polycarbonate resin, a glass plate, or the like is used. The transparent plate 71 may have a touch panel function. As the touch panel, an arbitrary type touch panel such as a resistive film type, a capacitance type, an optical method, and an ultrasonic method is used.

The polarizing plate 3 provided on the surface of the image display panel 10 and the printing layer 76 forming surface of the front transparent plate 7 are bonded to each other 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 adhesive sheet 5 may be bonded to the front transparent plate 7 first. Good. In addition, both can be bonded at the same time. From the viewpoint of bonding workability and the like, one release film (light release film) 2 is peeled off, the surface of the exposed adhesive sheet 5 is bonded to the image display panel 10, and then the other release film 21 is bonded. It is preferable to peel off the (heavy release film) and attach the exposed surface of the adhesive sheet to the front transparent plate 7.

After the adhesive sheet 5 and the front transparent plate 7 are bonded together, defoaming is performed to remove air bubbles near the interface between the adhesive sheet 5 and the flat plate 71 portion of the front transparent plate 7 and the non-flat portion such as the printing layer 76. Is preferably performed. As the defoaming method, an appropriate method such as heating, pressurization, or depressurization can be adopted. For example, it is preferable that the bonding is performed under reduced pressure and heating while suppressing the mixing of bubbles, and then the pressurization is performed at the same time as heating by an autoclave treatment or the like for the purpose of suppressing delay bubbles. 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.

When there is a gap 90 between the housing 9 and the front transparent plate 7, it is preferable to fill the gap 90 with a resin material or the like to seal the gap 90. As described above, since the adhesive sheet 5 has a large shear storage elastic modulus, it is excellent in adhesive reliability in a wide temperature range. Therefore, even if stress strain occurs at the bonding interface of the pressure-sensitive adhesive sheet due to a temperature change during sealing with a resin material or the like, peeling at the bonding interface can be suppressed. Further, since the adhesive sheet 5 has a low glass transition temperature and a large peak top value of tan δ, it has excellent impact resistance in a wide temperature range and is unlikely to be peeled off due to an impact such as dropping.

[Optical film with adhesive sheet]
As shown in FIG. 1, the pressure-sensitive adhesive sheet 5 can be used as a film with a pressure-sensitive adhesive in which a release film is temporarily attached to both sides, and 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 21 is temporarily attached to one surface of the adhesive sheet 5, and the polarizing plate 3 is fixed 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 the release film 24 is temporarily attached on the adhesive sheet 4.

In this way, in the form in which an optical film such as a polarizing plate is bonded to the pressure-sensitive adhesive sheet in advance, the release film 21 temporarily attached to the surface of the pressure-sensitive adhesive sheet 5 is peeled off and bonded to the front transparent member. Just do it.

[Modification example of laminated form]
In FIGS. 1 to 4, the form in which the image display panel 10 (polarizing plate 3) and the front transparent plate 7 (cover window) are bonded to each other by the base material-less double-sided adhesive sheet 5 has been mainly described, but the types of adherends have been described. And combinations are not limited to these. For example, the cover window and the touch panel sensor may be attached to each other via the adhesive sheet 5. In this form, another adhesive sheet is used for bonding the touch panel sensor and the image display panel.

The adhesive sheet 5 can also be used as an adhesive layer for one or both of the double-sided adhesive sheets with a base material. In the double-sided pressure-sensitive adhesive sheet 15 with a base material shown in FIG. 5, the first pressure-sensitive adhesive layer 51 is laminated on one surface of the transparent film base material 59, and the second pressure-sensitive adhesive layer 53 is formed on the other side of the transparent film base material 59. It is laminated.

Release films

21 and 23 are temporarily attached to the surfaces of the pressure-sensitive adhesive layers 51 and 53.

FIG. 6 is a cross-sectional view showing a configuration example of an image display device 206 in which a front transparent plate 7 is fixed to a visible surface of an image display panel 10 using a double-sided adhesive sheet 15 with a base material. In the image display device 206, the first pressure-sensitive adhesive layer 51 is bonded to the front transparent plate 7, and the second pressure-sensitive adhesive layer 53 is bonded to the polarizing plate 3 of the image display panel 10.

A transparent resin film is used as the transparent film base material 59 of the double-sided pressure-sensitive adhesive sheet 15 with a base material. The total light transmittance of the transparent film base material 59 is preferably 85% or more, more preferably 90% or more. The resin material constituting the film substrate is not particularly limited as long as it has transparency, and polyesters such as polyethylene terephthalate and polyethylene naphthalate; polyolefins such as polyethylene and polypropylene; cyclic polyolefins such as norbornene-based polymers; diacetyl cellulose, Cellular polymers such as triacetyl cellulose; acrylic polymers; styrene polymers; polycarbonate, polyamide, polyimide, polyether ether ketones and the like can be mentioned.

The thickness of the transparent film base material 59 is preferably about 15 to 150 μm, more preferably 25 to 120 μm, and even more preferably 35 to 100 μm. From the viewpoint of suppressing the coloring of the iris pattern (iris phenomenon) when the screen of the image display device is visually recognized, the transparent film base material 59 preferably has optical isotropic properties. The in-plane retardation of the transparent film substrate 59 at a wavelength of 590 nm is preferably 50 nm or less, more preferably 30 nm or less, further preferably 10 nm or less, and particularly preferably 5 nm or less.

The double-sided pressure-sensitive adhesive sheet 15 with a base material preferably has step absorption in addition to adhesive holding power, dimensional stability and impact resistance when bonded to the front transparent plate. Therefore, it is preferable to use the pressure-sensitive adhesive sheet 5 having the above-mentioned various properties as the first pressure-sensitive adhesive layer 51. From the viewpoint of ensuring step absorption and impact resistance, the thickness of the first pressure-sensitive adhesive layer 51 is preferably 30 μm or more, more preferably 40 μm or more, and even more preferably 50 μm or more. On the other hand, from the viewpoint of productivity, the thickness of the first pressure-sensitive adhesive layer 51 is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 250 μm or less.

The pressure-sensitive adhesive constituting the second pressure-sensitive adhesive layer 53 arranged on the image display panel 10 side of the transparent film base material 59 is not particularly limited as long as it is a transparent pressure-sensitive adhesive. The pressure-sensitive adhesive of the first pressure-sensitive adhesive layer 51 and the pressure-sensitive adhesive of the second pressure-sensitive adhesive layer 53 may be the same or different. As the second pressure-sensitive adhesive layer 53, a pressure-sensitive adhesive sheet 5 having the above-mentioned characteristics may be used as the second pressure-sensitive adhesive layer 53 from the viewpoint of enhancing the adhesive holding power, dimensional stability, and impact resistance.

The thickness of the second adhesive layer 53 is not particularly limited. From the viewpoint of imparting impact resistance, the thickness of the second pressure-sensitive adhesive layer 53 is preferably 30 μm or more, more preferably 50 μm or more. On the other hand, since the second pressure-sensitive adhesive layer 53 is not required to have step absorption, the thickness of the second pressure-sensitive adhesive layer 53 is preferably 200 μm or less, more preferably 150 μm or less, from the viewpoint of dimensional stability and productivity. It is more preferably 120 μm or less. The thickness of the second pressure-sensitive adhesive layer 53 may be 100 μm or less or 80 μm or less.

The thickness of the second pressure-sensitive adhesive layer 53 is preferably smaller than the thickness of the first pressure-sensitive adhesive layer 51. The thickness of the second pressure-sensitive adhesive layer 53 is preferably 0.2 to 0.85 times, more preferably 0.3 to 0.8 times, and 0.4 to 0.75 times the thickness of the first pressure-sensitive adhesive layer 51. Is even more preferable.

As shown in FIG. 5, the double-sided pressure-sensitive adhesive sheet 15 with a base material adheres to the second pressure-sensitive adhesive layer 53 with an optical film or the like, in addition to the form in which the

release films

21 and 23 are temporarily attached to the pressure-sensitive adhesive layers 51 and 53. It can also be used as a film with an adhesive. Further, as in the form shown in FIG. 4, it can also be used as an optical film with a double-sided adhesive, which is further provided with an adhesive sheet on the optical film (polarizing plate) bonded to the second pressure-sensitive adhesive layer.

The present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[Preparation of acrylic oligomer]
60 parts by weight of dicyclopentanyl methacrylate (DCPMA), 40 parts by weight of methyl methacrylate (MMA), 3.5 parts by weight of α-thioglycerol as a chain transfer agent, and 100 parts by weight of toluene as a polymerization solvent are mixed and nitrogen is mixed. The mixture was stirred at 70 ° C. for 1 hour in an atmosphere. Next, 0.2 parts by weight of 2,2'-azobisisobutyronitrile (AIBN) was added as a thermal polymerization initiator, reacted at 70 ° C. for 2 hours, and then heated to 80 ° C. for 2 hours. It was reacted. Then, the reaction solution was heated to 130 ° C., and toluene, the chain transfer agent and the unreacted monomer were dried and removed to obtain a solid acrylic oligomer. The weight average molecular weight of the acrylic oligomer was 5100.

[Example 1]
(Polymerization of prepolymer)
78 parts by weight of butyl acrylate (BA), 16 parts by weight of N-vinyl-2-pyrrolidone (NVP), 6 parts by weight of 4-hydroxybutyl acrylate (4HBA), and photopolymerization initiation as monomer components for prepolymer formation. The agent (BASF "Irgacure 184": 0.05 parts by weight and BASF "Irgacure 651": 0.05 parts by weight) is blended and the viscosity (BH viscometer No. 5 rotor, 10 rpm, measurement temperature 30 ° C.). The polymer was polymerized by irradiating with ultraviolet rays until the temperature reached about 20 Pa · s to obtain a prepolymer composition (polymerization rate; about 9%).

(Preparation of photocurable pressure-sensitive adhesive composition)
In the above prepolymer composition, NVP: 3 parts by weight and 4HBA: 8 parts by weight as a monofunctional monomer; polyester urethane diacrylate as a urethane (meth) acrylate (“Art Resin UN-350” manufactured by Negami Kogyo): 2 By weight: parts by weight of the acrylic oligomer: 5 parts by weight; as a photopolymerization initiator, "Irgacure 184": 0.05 parts by weight, and "Irgacure 651": 0.55 parts by weight; as a chain transfer agent, α-methylstyrene. Dimeric (“Nofmer MSD” manufactured by Nichiyu): 0.2 parts by weight; and “KBM-403” manufactured by Shin-Etsu Chemical Co., Ltd .: 0.3 parts by weight as a silane coupling agent, and then mixed uniformly. To prepare a pressure-sensitive adhesive composition.

(Making an adhesive sheet)
A 75 μm-thick polyethylene terephthalate (PET) film (“Diafoil MRF75” manufactured by Mitsubishi Chemical Co., Ltd.) with a silicone-based release layer on the surface is used as a base material (cum-heavy release film), and the above photocurable adhesive is applied on the base material. The agent composition was applied so as to have a thickness of 150 μm to form a coating layer. On this coating layer, a PET film (“Diafoil MRE75” manufactured by Mitsubishi Chemical Corporation) having a thickness of 75 μm, which had one side treated with silicone peeling as a cover sheet (also a light peeling film), was laminated. This laminate was photocured by irradiating it with ultraviolet rays from the cover sheet side with a black light whose position was adjusted so that the irradiation intensity on the irradiation surface directly under the lamp was 5 mW / cm ² , and the thickness was 150 μm and the polymerization rate was 99. %% Adhesive sheet was obtained.

[Examples 2 to 16, Comparative Examples 1 to 6]
Table 1 shows the composition of the charged monomer in the polymerization of the prepolymer, the type and amount of the monofunctional monomer and the polyfunctional compound (urethane acrylate and / or the polyfunctional acrylate) to be added to the pressure-sensitive adhesive composition, and the amount of the chain transfer agent added. And changed as shown in Table 2. A photocurable pressure-sensitive adhesive composition was prepared in the same manner as in Example 1 except for the above, and coated on a substrate and photo-cured to obtain a pressure-sensitive adhesive sheet. In the post-addition composition, a photopolymerization initiator (“Irgacure 184”: 0.05 parts by weight and “Irgacure 651”: 0.55 parts by weight); and a silane coupling agent (“KBM-403”: 0). .3 parts by weight) is the same in all Examples and Comparative Examples, so the description of these components is omitted in Tables 1 and 2.

[Evaluation]
<Gel fraction>
Approximately 0.2 g of the adhesive was scraped from the adhesive sheet and wrapped in a porous polytetrafluoroethylene film (Nitto Denko "NTF-1122") with a pore diameter of 0.2 μm cut into a size of 100 mm × 100 mm. I tied my mouth with octopus thread. The weight (B) of the pressure-sensitive adhesive sample was calculated by subtracting the total weight (A) of the porous polytetrafluoroethylene film and the octopus thread measured in advance from the weight of this sample. The pressure-sensitive adhesive sample wrapped in the porous polytetrafluoroethylene membrane was immersed in about 50 mL of ethyl acetate at 23 ° C. for 7 days to elute the sol component of the pressure-sensitive adhesive out of the porous polytetrafluoroethylene membrane. .. After the immersion, the pressure-sensitive adhesive wrapped in the porous polytetrafluoroethylene film was taken out, dried at 130 ° C. for 2 hours, allowed to cool for about 20 minutes, and then the dry weight (C) was measured. The gel fraction of the pressure-sensitive adhesive was calculated by the following formula.
Gel fraction (%) = 100 x (CA) / B

<Weight average molecular weight of sol>
About 0.2 g of the pressure-sensitive adhesive was scraped from the pressure-sensitive adhesive sheet and immersed in a 10 mM tetrahydrofuran solution of phosphate for 12 hours to extract the sol component. The amount of the phosphoric acid tetrahydrofuran solution was adjusted so that the sol content of the solution after extraction was 0.1% by weight in consideration of the gel fraction of the pressure-sensitive adhesive. Using the filtrate obtained by filtering the extracted solution with a 0.45 μm membrane filter as a sample, use a GPC (gel permeation chromatography) device (product name “HLC-8120 GPC”) manufactured by Tosoh under the following conditions. The analysis was performed and the weight average molecular weight Mw of the sol content was calculated.
(Measurement condition)
Column: Made by Tosoh, G7000HXL + GMHXL + GMHXL
Column size: 7.8 mmφ x 30 cm each (total column length: 90 cm)
Column temperature: 40 ° C, flow rate: 0.8 mL / min
Injection volume: 100 μL
Eluent: tetrahydrofuran Detector: Differential Refrometer (RI)
Standard sample: polystyrene

<Storage modulus of adhesive sheet, loss tangent and glass transition temperature>
A measurement sample was obtained by laminating 10 adhesive sheets to a thickness of about 1.5 mm. Dynamic viscoelasticity measurements were performed using the "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific under the following conditions.
(Measurement condition)
Deformation mode: Torsion measurement frequency: 1Hz
Temperature rise rate: 5 ° C / min Shape: Parallel plate 7.9 mmφ

From the measurement results, the shear storage elastic modulus G'25 ° C. at _{25 ° C.} and the loss tangent tan δ _{70 ° C.} at _{70 ° C.} were read. Further, the temperature at which the loss tangent (tan δ) is maximized (peak top temperature) was defined as the glass transition temperature of the adhesive sheet.

<Adhesive strength>
The light release film is peeled off from the adhesive sheet, a PET film with a thickness of 50 μm is attached, cut into a width of 10 mm and a length of 100 mm, the heavy release film is peeled off, and the heavy release film is pressure-bonded to a glass plate with a 5 kg roller. A sample for force measurement was prepared. After holding the sample for adhesive force measurement in an environment of 25 ° C. for 30 minutes, the test piece is peeled off from the glass plate under the conditions of a tensile speed of 300 mm / min and a peeling angle of 180 ° using a tensile tester. The force was measured.

<Haze>
Using a test piece in which an adhesive sheet is attached to a non-alkali glass (total light transmittance 92%, haze 0.4%) having a thickness of 800 μm, and using a haze meter (“HM-150” manufactured by Murakami Color Technology Research Institute), The haze was measured. The value obtained by subtracting the haze (0.4%) of the non-alkali glass from the measured value was taken as the haze of the adhesive sheet.

<Step absorption>
The adhesive sheet was cut into a size of 75 mm × 45 mm, the light release film was peeled off from the adhesive sheet, and a roll laminator (pressure between rolls: 0.2 MPa, feed rate: feed rate:) was placed in the center of the PET film having a thickness of 125 μm cut out to 100 mm × 50 mm. It was bonded at 100 mm / min). After that, the heavy release film was peeled off, and a glass plate (100 mm × 50 mm) having a thickness of 500 μm on which black ink (printing thickness: 25 μm or 40 μm) was printed in a frame shape on the peripheral edge was used as a roll laminator (pressure between rolls: 0). .2 MPa, feed rate: 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 the area 5 mm from the four side ends of the adhesive sheet. After treating this sample in an autoclave (50 ° C., 0.5 MPa) for 30 minutes, the vicinity of the boundary of the printing area of the black ink was observed with a digital microscope at a magnification of 20 times to confirm the presence or absence of air bubbles. For each of the samples having a printing thickness of 25 μm and 40 μm of black ink, the step absorption was evaluated according to the following criteria.
⊚: No bubbles were generated all around 〇: Bubbles were found in one of the four corners, but no bubbles were found on any of the four sides ×: Bubbles found in two or more of the four corners, or one or more of the four sides

<Workability>
The light release film was peeled off from the adhesive sheet, attached to a PET film having a thickness of 100 μm (Toyobo's "Cosmo Shine A4100"), and punched from the PET film side using a press machine to prepare a sample for processability evaluation. .. This sample was left in an atmosphere of a temperature of 23 ° C. and a relative humidity of 50% for 1 week, then the heavy-release film was peeled off, and the presence or absence of adhesive chipping was visually observed. Those without glue chipping were marked with 〇, and those with glue chipping were marked with x.

<Interlayer adhesiveness>
(Preparation of test sample)
The adhesive sheet is cut into a size of 75 mm × 45 mm, the light release film is peeled off from the adhesive sheet, and a roll laminator (pressure between rolls: 0.2 MPa, feed rate: 100 mm) is centered on a glass plate (100 mm × 50 mm) having a thickness of 500 μm. / Minutes). After that, the heavy release film was peeled off, and a glass plate (50 mm × 100 mm) having a thickness of 500 μm on which black ink having a thickness of 30 μm was printed in a frame shape on the peripheral portion was vacuum-bonded (surface pressure 0.3 MPa, pressure 100 Pa). I pasted them together. 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 the area 5 mm from the four side ends of the adhesive sheet. This sample was treated in an autoclave (50 ° C., 0.5 MPa) for 30 minutes.

After holding the above sample in an environment of 60 ° C. for 30 minutes, as shown in FIG. 7A, a polystyrene sheet having a thickness of 200 μm was inserted between two glass plates at a distance of 1 mm from the end of the adhesive sheet. It was held for 10 seconds. The edge of the adhesive sheet was observed with a digital microscope at a magnification of 20 times. Those in which streak-shaped bubbles (see FIG. 7B) or the adhesive sheet was peeled off from the glass plate were evaluated as x, and those in which neither bubbles nor peeling occurred were evaluated as 〇.

<Impact resistance>
Except for changing the size of the glass plate without the black ink printing layer to 100 mm × 70 mm, the glass plates are attached to both sides of the adhesive sheet in the same manner as in the preparation of the sample for the interlayer adhesion test described above. , Autoclave treatment was performed to prepare a test sample. As shown in FIG. 8, both ends of the test sample 95 in the short side direction are arranged on a table 93 with an interval of 60 mm so that the glass plate 7 provided with the printing layer 76 is on the lower side. The upper surface of the end portion of the glass plate 8 not provided with the printing layer was fixed on the table 93 with an adhesive tape (not shown). The test sample 95 fixed on the table 93 with an adhesive tape was held in an environment of -5 ° C for 24 hours, and then within 40 seconds after being taken out to room temperature, a metal ball 97 having a mass of 11 g was placed on the glass plate 7. Was dropped from a height of 300 mm to perform an impact resistance test.

In the impact resistance test, a tubular guide 99 is used to keep the falling position of the metal ball constant, and the print layer 76 is separated from the corner of the inner edge of the frame of the print area by 10 mm in each of the short side direction and the long side direction. The metal ball 97 was dropped at the above position. Two tests were performed, and those in which the glass plate did not peel off in any of the tests were evaluated as 0, and those in which peeling of the glass plate occurred in either or both of the two tests were evaluated as x.

[Evaluation results]
Tables 1 and 2 show the composition of the pressure-sensitive adhesive composition used for producing each pressure-sensitive adhesive sheet and the evaluation results of the pressure-sensitive adhesive sheet. In addition, in Table 1 and Table 2, each component is described by the following abbreviations.
<Acrylic monomer>
2EHA: 2-ethylhexyl acrylate BA: butyl acrylate CHA: cyclohexyl acrylate NVP: N-vinyl-2-pyrrolidone 4HBA: 4-hydroxybutyl acrylate

<Urethane diacrylate>
UN-350: "Art Resin UN-350" manufactured by Negami Kogyo (polyester urethane diacrylate with a weight average molecular weight of about 12500)
UN-350ND :: "Art Resin UN-350NDTN011" manufactured by Negami Kogyo (polyester urethane diacrylate with a weight average molecular weight of about 7600)
UN-350MU: "Art Resin UN-350MU" manufactured by Negami Kogyo (polyester urethane diacrylate with a weight average molecular weight of about 25,000)
UV-3305B: "Shikou UV-3305B" manufactured by Nippon Synthetic Chemical Industry Co., Ltd. (polyether urethane diacrylate with a weight average molecular weight of about 12000)
UT-6957 :: Nippon Synthetic Chemical Industry Co., Ltd. "Shikou UT6957" (polyether urethane diacrylate with a weight average molecular weight of about 15,000)
UV-3010B: "UV-3010B" manufactured by Nippon Synthetic Chemical Industry Co., Ltd. (polyester urethane diacrylate with a weight average molecular weight of about 11000)
<Urethane monoacrylate>
UA-2334: "NE Oligo UA-2334PHB" manufactured by Shin Nakamura Chemical Industry Co., Ltd. (polyether urethane monoacrylate with a weight average molecular weight of about 20000)
<Polyfunctional acrylate>
HDDA: Hexanediol diacrylate

As shown in Tables 1 and 2, the adhesives of the examples were excellent in adhesiveness and workability, and also had excellent step absorption and drop impact durability.

In Examples 1 to 4, as the amount of the chain transfer agent added afterwards was reduced, the gel fraction and the molecular weight of the sol component increased, and tan δ _{70 ° C.} decreased. In Example 4 in which the amount of the chain transfer agent added was 0.03 parts by weight, the step absorption was lower than in the other examples. In Comparative Example 1 in which the chain transfer agent was not added, the gel fraction was over 80, the glass transition temperature was high, and the step absorption and drop impact resistance were lowered.

In Examples 5 and 6 in which the amount of urethane diacrylate added was larger than that in Example 1, the gel fraction increased and tan δ _{70 ° C.} decreased. Also in the comparison between Example 12 and Example 13 in which the type of urethane diacrylate was changed, the gel fraction tended to increase and the tan δ _{70 ° C.} tended to decrease as the amount of urethane diacrylate added increased. It was. The same tendency was observed in the comparison between Example 16 and Comparative Example 6 having different polymer compositions, and Comparative Example 6 in which the amount of urethane diacrylate added was large was inferior in step absorption.

In Example 7 in which the amount of urethane diacrylate added was small, the gel fraction was lower and tan δ _{70 ° C.} was higher than in Example 1. In Comparative Example 2 in which the amount of urethane diacrylate added was even smaller, the gel fraction was reduced to 13%, and the processability was insufficient.

In Example 8 in which the polyfunctional acrylate was used in combination with the urethane diacrylate as the polyfunctional monomer, the same excellent characteristics as those of the other examples were exhibited. On the other hand, in Comparative Example 3 in which urethane diacrylate was not used as the polyfunctional monomer and only polyfunctional acrylate was used, the gel fraction was significantly increased, tan δ _{70 ° C.} was small, and the step absorption was inferior. In Comparative Example 5 in which urethane monoacrylate was used as a post-addition component, although an appropriate amount of urethane acrylate was used, an appropriate crosslinked structure was not formed, so that the gel fraction and storage elastic modulus were low, and the interlayer adhesiveness and interlayer adhesiveness were improved. The workability was inferior.

Example 9, Example 10 and Example 11 using a relatively low molecular weight urethane diacrylate, and Example 6, Example 1 and Example 7 using a relatively high molecular weight urethane diacrylate. In comparison, the smaller the molecular weight of the urethane diacrylate, the smaller the tan δ _{70 ° C.} tended to be. Further, in Comparative Example 4 in which urethane diacrylate having a large molecular weight was used, deterioration in interlayer adhesiveness and processability was observed. Further, in Comparative Example 4, the haze of the adhesive sheet increased and the transparency was inferior. The decrease in transparency is considered to be due to the decrease in compatibility between the main chain structure of the base polymer and the urethane segment forming the crosslinked structure.

From the results of the above Examples and Comparative Examples, the composition and crosslinked structure of the base polymer are adjusted, and the gel fraction, the shear storage elastic modulus at room temperature G'25 _{° C.} , and the high temperature loss tangent tan δ _{70 ° C.} are set within the predetermined ranges. As a result, it can be seen that an adhesive sheet having characteristics such as step absorption and impact resistance can be obtained.

5,51,53

Adhesive sheet

21, 22, 23,24 Release film 59 Transparent film base material 3 Polarizing plate 4 Adhesive sheet 6 Image display cell 10 Image display panel 7 Front transparent plate 9

Housing

202, 206 Image display device

Claims

A pressure-sensitive adhesive sheet in which a pressure-sensitive adhesive containing a base polymer having a crosslinked structure is formed in a sheet shape.
The shear storage elastic modulus at a temperature of 25 ° C. is 0.16 MPa or more.
The loss tangent at a temperature of 70 ° C. is 0.25 or more.
The glass transition temperature is -3 ° C or less,
Adhesive sheet with a gel fraction of 30-80%.
The adhesive sheet according to claim 1, wherein the haze is 1% or less.
The adhesive sheet according to claim 1 or 2, wherein the polymerization rate is 95% or more.
The adhesive sheet according to any one of claims 1 to 3, wherein the weight average molecular weight of the sol is 150,000 to 450,000.
The pressure-sensitive adhesive sheet according to any one of claims 1 to 4, wherein the base polymer is an acrylic polymer containing an acrylic polymer chain having a crosslinked structure.
In the acrylic polymer chain, the amount of (meth) acryl alkyl ester is 50% by weight or more based on the total amount of the constituent monomer components, and the total amount of the hydroxyl group-containing monomer and the nitrogen-containing monomer is 15 to 45% by weight. , The adhesive sheet according to claim 5.
The pressure-sensitive adhesive sheet according to claim 5 or 6, wherein the base polymer contains an acrylic polymer in which a crosslinked structure of urethane-based segments is introduced into an acrylic polymer chain.
The adhesive sheet according to claim 7, wherein the urethane-based segment has a weight average molecular weight of 5,000 to 30,000.
The adhesive sheet according to claim 7 or 8, wherein the urethane-based segment is polyester urethane.
The pressure-sensitive adhesive sheet according to any one of claims 7 to 9, wherein the acrylic polymer has a urethane-based segment content of 0.3 to 10 parts by weight with respect to 100 parts by weight of the acrylic polymer chain.
The invention according to any one of claims 7 to 10, wherein the acrylic polymer contains a copolymer of a (meth) acrylic acid alkyl ester and a urethane (meth) acrylate having a (meth) acryloyl group at the terminal. Adhesive sheet.
The method for manufacturing an adhesive sheet according to any one of claims 5 to 11.
A composition containing an acrylic monomer and / or a partial polymer thereof and a polyfunctional monomer having two or more (meth) acryloyl groups in one molecule is applied in a layer on a substrate and then activated on the composition. A method for manufacturing an adhesive sheet, which is photocured by irradiating with light rays.
The method for producing an adhesive sheet according to claim 12, wherein the polyfunctional monomer is a urethane di (meth) acrylate having (meth) acryloyl groups at both ends.
An image display device in which a front transparent member is fixed to the visible side surface of an image display panel by the adhesive sheet according to any one of claims 1 to 11.