TWI861068B - Adhesive sheet, manufacturing method thereof, and image display device - Google Patents

Abstract

The present invention provides an adhesive sheet that can achieve both step absorption and dimensional stability without light curing after being bonded to an adherend. The adhesive sheet (5) of the present invention is an adhesive comprising a base polymer having a cross-linked structure formed into a sheet, and has a shear storage modulus of 0.16 MPa or more at a temperature of 25°C and a loss tangent of 0.25 or more at a temperature of 70°C. The glass transition temperature of the adhesive sheet of the present invention is preferably below -3°C. The gel fraction of the adhesive is preferably 30 to 80%, and the polymerization rate (non-volatile component) is preferably 95% or more.

Description

Adhesive sheet and its manufacturing method, and image display device

The present invention relates to an adhesive sheet and a manufacturing method thereof. Furthermore, the present invention relates to an image display device using the adhesive sheet.

Liquid crystal display devices or organic EL (Electroluminescence) display devices are widely used as various image display devices such as mobile phones, smart phones, car navigation system devices, computer monitors, and televisions. In order to prevent damage to the image display panel caused by impact from the outer surface, a transparent front plate (also called a "cover plate") such as a transparent resin plate or a glass plate is provided on the viewing side of the image display panel. In addition, in recent years, devices with a touch panel on the viewing side of the image display panel are becoming increasingly popular.

There is a case where a colored layer (decorative printing layer) is formed around the front transparent member for the purpose of decoration or light shielding. If the adhesive is applied to the transparent member with the decorative printing layer, bubbles are easily generated around the printed step portion. Therefore, a method is adopted to suppress the undesirable situation such as bubble mixing by making the adhesive sheet with a thicker thickness to have step absorption.

In order to impart step absorption, it is proposed to use an adhesive sheet containing a photocurable adhesive composition for bonding the front transparent member. For example, Patent Document 1 shows the following example: a composition in which a multifunctional monomer and a photopolymerization initiator are added to a polymer solution prepared by solution polymerization is applied to a substrate, and the solvent is removed by heating to produce a photocurable adhesive sheet. Patent Document 2 shows the following example: a solvent-free composition containing a low molecular weight polymer, a monofunctional monomer and a multifunctional monomer, and a photopolymerization initiator is applied to a substrate, and the adhesive sheet is produced by photocuring. During light curing, part of the monomer components in the composition remain in an unreacted state, thereby forming an adhesive sheet with high fluidity and excellent gradient absorption.

Since the photocurable adhesive sheet contains a photopolymerizable monomer or oligomer in an unreacted state, the fluidity of the adhesive is high and the gradient absorption is excellent. After being bonded to the adherend, the adhesive sheet is irradiated with active light for photocuring, thereby reducing the fluidity of the adhesive and improving the holding power. [Prior technical literature] [Patent literature]

[Patent document 1] Japanese Patent Publication No. 2014-227453 [Patent document 2] International Publication No. 2013/161666

[The problem the invention is trying to solve]

In an image display device whose size is larger than that of a display panel, a front transparent member such as a cover plate is bonded to a housing by bonding tape or the like in an area outside the periphery of the display panel. That is, the front transparent member is fixed by bonding to the housing by bonding tape or the like and bonding to the surface of the display panel by bonding an adhesive sheet for interlayer filling.

In recent years, with smartphones and other mobile devices as the center, the display device is being promoted to have narrow borders or no borders. With the narrow borders or no borders, image display devices are also developed in which the size of the display panel 10 is equal to or larger than the size of the front transparent component 7. In this structure, the housing 9 and the front transparent component 7 cannot be fixed by adhesive tape or the like, and the front transparent component 7 must be fixed only by the adhesive sheet 5 (see FIG. 2). As a result, the adhesive sheet is required to have a higher adhesive force and not to be peeled off due to impact such as falling.

In addition, as the edges become narrower or borderless, higher dimensional stability is also required during the assembly of image display devices or the transportation of finished products. The photocurable adhesive sheet described in Patent Document 1 or Patent Document 2 has a gradient absorbency to improve the softness of the adhesive. In the state before photocuring, if it is subjected to external force during transportation or processing, the adhesive sheet is easily deformed, so there is a situation where the position of the bonded components is offset. In addition, after bonding with the adherend, it must be photocured, so the manufacturing steps of the image display device are likely to become complicated.

In view of the above, the purpose of the present invention is to provide an adhesive sheet that can simultaneously achieve step absorption and dimensional stability without the need for light curing after being bonded to an adherend and has bonding durability and impact resistance. [Technical means to solve the problem]

One embodiment of the present invention is a double-sided adhesive sheet formed of an adhesive having a cross-linked structure. The shear storage modulus _{G'25 ℃} of the adhesive sheet at a temperature of 25℃ is preferably greater than 0.16 MPa, and the loss tangent _{tanδ70 ℃} at a temperature of 70℃ is preferably greater than 0.25. The glass transition temperature of the adhesive sheet is preferably below -3℃.

The gel fraction of the adhesive sheet is preferably 30 to 80%. The polymerization rate of the adhesive constituting the adhesive sheet is preferably 95% or more. The weight average molecular weight of the gel component of the 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 formed by crosslinking acrylic polymer chains through urethane chain segments. In order to meet the above-mentioned properties, the content of urethane chain segments is preferably 0.3 to 10 parts by weight relative to 100 parts by weight of acrylic polymer chains. The weight average molecular weight of the urethane chain segments is, for example, 5,000 to 30,000.

The base polymer of the adhesive is, for example, an acrylic polymer having a crosslinked structure, or a crosslinked structure introduced into the acrylic polymer chain by a multifunctional (meth)acrylate or (meth)acrylate urethane. With respect to the acrylic polymer chain, it is preferred that the amount of the (meth)acrylate alkyl ester relative to the total amount of the monomer components is 50% by weight or more. In the acrylic polymer chain, the total amount of the hydroxyl-containing monomer and the nitrogen-containing monomer relative to the total amount of the monomer components may also be 15 to 45% by weight.

The base polymer may also include a polymer having a cross-linked structure formed by a urethane chain segment introduced into an acrylic polymer chain. For example, an acrylic polymer having a cross-linked structure formed by a urethane chain segment introduced into an acrylic polymer chain can be obtained by copolymerizing an acrylic monomer constituting an acrylic polymer chain with a multifunctional (meth)acrylic urethane having (meth)acrylic groups at at least two ends.

As the (meth)acrylic urethane, di(meth)acrylic urethane having (meth)acrylic groups at both ends is preferred. The weight average molecular weight of the (meth)acrylic urethane is preferably 5000 to 30000. The glass transition temperature of the (meth)acrylic urethane is preferably below 0°C. The (meth)acrylic urethane may also include polyester (meth)acrylic urethane.

The adhesive sheet comprising a base polymer in which acrylic polymer chains are crosslinked by urethane chain segments is obtained, for example, by coating a composition comprising acrylic monomers and/or partial polymers thereof and (meth)acrylic urethane in a layer on a substrate and then irradiating the composition with active light for photocuring. The adhesive composition preferably contains 0.3 to 10 parts by weight of (meth)acrylic urethane relative to 100 parts by weight of the acrylic monomers and partial polymers thereof.

The adhesive sheet of the present invention is used, for example, for bonding transparent components in an image display device in which the transparent components are arranged on the viewing side surface. For example, the image display device is formed by fixing the front transparent component to the viewing side surface of the image display panel via the above-mentioned adhesive sheet. It is also possible to laminate the adhesive sheet on a transparent film substrate to produce a double-sided adhesive sheet attached to the substrate. [Effect of the invention]

The adhesive sheet of the present invention has a large shear storage modulus at room temperature, excellent bonding reliability and processability, and a large loss tangent at high temperature, so it has excellent step absorption and impact resistance. The adhesive sheet of the present invention is used to attach a cover plate to the image display device on the viewing side surface, and the bonding reliability is excellent, which can cope with narrow edge or frameless.

Fig. 1 shows an adhesive sheet with release films 21 and 22 temporarily attached to both sides of an adhesive sheet 5. Fig. 2 is a cross-sectional view showing an example of the structure of an image display device in which a front transparent plate 7 is fixed using an adhesive sheet.

[Physical properties of adhesive sheet] Adhesive sheet 5 is a double-sided adhesive sheet without a substrate formed in a sheet shape by an adhesive. The adhesive contains a base polymer having a cross-linked structure. The adhesive sheet is preferably highly transparent. The total light transmittance of the adhesive sheet is preferably 85% or more, and more preferably 90% or more. The haze of the adhesive sheet is preferably 1% or less.

From the viewpoint of improving the adhesion retention when the adhesive sheet and the adherend are bonded together and ensuring the processing dimensional stability, the shear storage modulus G'25 _℃ of the adhesive sheet at ₂₅ ℃ is preferably 0.16 MPa or more, more preferably 0.18 MPa or more, further preferably 0.20 MPa or more, and particularly preferably 0.21 MPa or more.

On the other hand, from the perspective of maintaining appropriate viscosity of the adhesive sheet to ensure wettability and to provide gradient absorbency or cushioning against impacts such as falling, the G' _{25 °C} of the adhesive sheet is preferably 0.5 MPa or less, more preferably 0.4 MPa or less, further preferably 0.3 MPa or less, and particularly preferably 0.28 MPa or less.

From the viewpoint of providing the adhesive sheet with step absorbency, the loss tangent tanδ _{70 ℃} of the adhesive sheet at 70 ℃ is preferably 0.25 or more, more preferably 0.30 or more, and further preferably 0.35 or more. tanδ _{70 ℃} may also be 0.40 or more, 0.45 or more, 0.50 or more, or 0.55 or more. From the viewpoint of adhesive retention, tanδ _{70 ℃} is preferably 1.0 or less, more preferably 0.9 or less, and further preferably 0.85 or less. tanδ _{70 ℃} may also be 0.80 or less, 0.75 or less, or 0.70 or less.

The peak value of tanδ of the adhesive sheet is preferably 1.5 or more, more preferably 1.6 or more, and further preferably 1.7 or more. An adhesive sheet with a larger peak value of tanδ tends to have greater adhesive behavior and excellent impact resistance. The upper limit of the peak value of tanδ of the adhesive sheet is not particularly limited, but is generally 3.0 or less. From the perspective of adhesive retention, the peak value of tanδ is preferably 2.7 or less, and 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 adhesive sheet is preferably -20°C or higher, more preferably -15°C or higher, and further preferably -13°C or higher. By setting the glass transition temperature within the above range, the adhesive sheet has appropriate adhesion even in a low temperature region, thereby suppressing the tendency of the adherend to be peeled off due to impact such as falling.

The shear storage modulus G', loss tangent tanδ and glass transition temperature of the adhesive sheet are obtained by viscoelastic measurement at a frequency of 1 Hz. Tanδ is the ratio of storage modulus G' to loss elastic modulus G", G"/G', and the glass transition temperature is the temperature (peak temperature) at which tanδ becomes extremely large. The storage modulus G' is equivalent to the part stored as elastic energy when the material is deformed, and is an indicator of the degree of hardness. The larger the storage modulus of the adhesive sheet, the higher the adhesive retention force, thereby suppressing the tendency to peel off due to deformation. The loss modulus G" is equivalent to the part of the energy lost due to internal friction when the material is deformed, indicating the degree of viscosity. The larger the tanδ, the stronger the viscosity tends to be, the deformation behavior becomes liquid-like and the rebound elastic energy tends to decrease.

From the viewpoint of ensuring processing stability by setting _{G'25 ℃} to 0.16 MPa or more and having a suitable degree of softness for imparting step absorbency, the gel fraction of the adhesive sheet is preferably 30 to 80%, more preferably 35 to 70%. The gel fraction may be 40% or more, 45% or more, or 65% or less.

The gel fraction of the adhesive sheet can be obtained as the insoluble component in solvents such as ethyl acetate. Specifically, it is obtained as the weight fraction (unit: weight %) of the insoluble component of the adhesive constituting the adhesive sheet after immersion in ethyl acetate at 23°C for 7 days relative to the sample before immersion. Generally speaking, the gel fraction of a polymer is equal to the degree of crosslinking. The more crosslinked parts in the polymer, the greater the gel fraction. The gel fraction (the amount of crosslinked structure introduced) can be adjusted to the desired range by the method of introducing the crosslinked structure or the type and amount of the crosslinking agent.

The adhesion of the adhesive sheet is preferably 2 N/10 mm or more, more preferably 4 N/10 mm or more, and further preferably 5 N/10 mm or more. By setting the adhesion of the adhesive sheet to the above range, it is possible to prevent the adhesive sheet from peeling off from the adherend when stress is generated due to deformation or impact is generated due to falling, etc. The adhesion is obtained by using a glass plate as the adherend and by a peeling test at a tensile speed of 300 mm/min and a peeling angle of 180°. Unless otherwise specified, the adhesion is a measured value at 25°C.

The thickness of the adhesive sheet is not particularly limited, and can be set according to the type or shape of the adherend. When a component with a printed step is used as the adherend, the thickness of the adhesive sheet is preferably greater than the thickness of the printed step. The thickness of the adhesive sheet used for bonding the front transparent plate (cover plate) is preferably 30 μm or more, more preferably 40 μm or more, and further preferably 50 μm or more. By increasing the thickness of the adhesive sheet, there is a tendency for the step absorption and impact resistance to improve. There is no particular upper limit to the thickness of the adhesive sheet. From the perspective of the productivity of the adhesive sheet, it is preferably 500 μm or less, more preferably 300 μm or less, and further preferably 250 μm or less.

[Composition of adhesive] As long as the adhesive sheet 5 satisfies the above-mentioned characteristics, the composition of the adhesive is not particularly limited, and a polymer such as an acrylic polymer, a silicone polymer, a polyester, a polyurethane, a polyamide, a polyvinyl ether, a vinyl acetate/vinyl chloride copolymer, a modified polyolefin, an epoxy system, a fluorine system, a natural rubber, a synthetic rubber, etc. as a base polymer can be appropriately selected and used. In particular, in terms of excellent optical transparency and showing appropriate adhesive properties such as wettability, cohesion and adhesion, and excellent weather resistance and heat resistance, an acrylic adhesive containing an acrylic polymer can be preferably used as a base polymer.

<Acrylic polymer chain> The acrylic base polymer having a cross-linked structure is one in which a cross-linked structure is introduced into the acrylic polymer chain. The acrylic polymer chain contains (meth)acrylic acid alkyl ester as a main monomer component. In addition, in this specification, the so-called "(meth)acrylic acid" means acrylic acid and/or methacrylic acid.

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

Specific examples of the (meth)acrylate alkyl ester having a chain alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, sec-butyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, oct ... (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, isotridodecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecyl (meth)acrylate,

Specific examples of the alkyl (meth)acrylate having an alicyclic alkyl group include: cycloalkyl (meth)acrylates such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate; (meth)acrylates having a dicyclic aliphatic hydrocarbon ring such as isobutyl (meth)acrylate; and (meth)acrylates having a tricyclic or more aliphatic hydrocarbon ring such as dicyclopentyl (meth)acrylate, dicyclopentyloxyethyl (meth)acrylate, tricyclopentyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate.

The amount of the (meth)acrylic acid alkyl ester relative 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, and further preferably 60% by weight or more. From the viewpoint of setting the glass transition temperature (Tg) of the polymer chain to an appropriate range, the amount of the (meth)acrylic acid alkyl ester having a carbon number of 4 to 10 in the acrylic base polymer relative to the total amount of the monomer components constituting the acrylic polymer chain is preferably 40% by weight or more, more preferably 50% by weight or more, and further preferably 55% by weight or more. Furthermore, the monomer components constituting the acrylic polymer chain are those remaining after removing the monomers used to form the crosslinked structure (the following multifunctional (meth)acrylate, (meth)acrylic acid urethane, etc.) and the crosslinking agent from the total monomer components constituting the polymer.

The acrylic-based polymer may also contain a hydroxyl-containing monomer or a carboxyl-containing monomer as a constituent monomer component. When a crosslinking structure is introduced by an isocyanate crosslinking agent, the hydroxyl group becomes a reaction point with an isocyanate group, and when a crosslinking structure is introduced by an epoxy crosslinking agent, the carboxyl group becomes a reaction point with an epoxy group.

Examples of hydroxyl-containing monomers include (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, or (meth)acrylate (4-hydroxymethylcyclohexyl)-methyl ester. When a cross-linked structure formed by a urethane chain segment is introduced into an acrylic polymer chain, the acrylic base polymer preferably contains a (meth)acrylate having a hydroxyalkyl group having 4 to 8 carbon atoms as a constituent monomer component from the viewpoint of higher compatibility with the urethane chain segment and improved transparency of the adhesive sheet.

By making the acrylic base polymer have a hydroxyl-containing monomer as a constituent monomer component, the transparency of the adhesive sheet is improved, and the whitening in a high temperature and high humidity environment tends to be suppressed. In addition, the hydroxyl group of the hydroxyl-containing monomer can form a physical crosslink based on a hydrogen bond with the acrylic polymer chain or a crosslinking chain segment (such as a urethane chain segment) that crosslinks the acrylic polymer chain. Therefore, by increasing the ratio of the hydroxyl-containing monomer in the monomer component constituting the acrylic polymer chain, there is a tendency for the cohesion to be improved and the G' _{25 ℃} to be increased even when the gel fraction is low. The amount of the hydroxyl-containing monomer is preferably 5 to 30% by weight, more preferably 8 to 25% by weight, and even more preferably 10 to 20% by weight, relative to the total amount of monomer components constituting the acrylic polymer chain.

Examples of the carboxyl group-containing monomer include acrylic acid 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 to connect a touch panel sensor, in order to prevent corrosion of the electrode caused by acid components, the adhesive sheet preferably has a lower acid content. Furthermore, when the adhesive sheet is used to connect a polarizing plate, in order to suppress the polyeneization of the polyvinyl alcohol-based polarizing element caused by the acid components, the adhesive sheet preferably has a lower acid content. Such an acid-free adhesive sheet preferably has an organic acid monomer content of (meth)acrylic acid or less of 100 ppm, more preferably less than 70 ppm, and further preferably less than 50 ppm. The organic acid monomer content of the adhesive sheet is determined by immersing the adhesive sheet in pure water, heating it at 100°C for 45 minutes, and quantifying the acid monomer extracted into the water by ion chromatography.

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

The acrylic-based polymer may also contain a nitrogen-containing monomer as a constituent monomer component. Examples of the nitrogen-containing monomer include vinyl monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperidone, vinylpyrrolidone, vinylpyrrole, vinylimidazole, vinyloxazole, vinyloxaline, (meth)acryloyloxaline, N-vinylcarboxamides, and N-vinylcaprolactam, or cyano-containing acrylic monomers such as acrylonitrile and methacrylonitrile. Among these, N-vinylpyrrolidone is preferred in terms of having a higher effect of improving adhesion by improving cohesive force.

By making the acrylic base polymer contain highly polar monomers such as hydroxyl-containing monomers, carboxyl-containing monomers and nitrogen-containing monomers as constituent monomer components, the cohesive force of the adhesive tends to be improved, _{G'25 °C} increases, and the adhesive retention tends to be improved. On the other hand, if the content of highly polar monomers is too large, there is a situation where the glass transition temperature increases and the impact resistance decreases. Therefore, the amount of highly polar monomers (the total of hydroxyl-containing monomers, carboxyl-containing monomers, and nitrogen-containing monomers) relative to the total amount of monomer components constituting the acrylic polymer chain is preferably 15 to 45% by weight, more preferably 20 to 40% by weight, and further preferably 25 to 37% by weight. It is particularly preferred that the total of hydroxyl-containing monomers and nitrogen-containing monomers is within the above range. The amount of the nitrogen-containing monomer relative to the total amount of the monomer components constituting the acrylic base polymer is preferably 7 to 30 wt %, more preferably 10 to 25 wt %, and even more preferably 12 to 22 wt %.

The acrylic-based polymer may also contain the following as monomer components other than the above: vinyl monomers such as anhydride-containing monomers, caprolactone adducts of (meth)acrylic acid, sulfonic acid-containing monomers, phosphoric acid-containing monomers, vinyl acetate, vinyl propionate, styrene, and α-methylstyrene; cyano-containing acrylic monomers such as acrylonitrile and methacrylonitrile; epoxy-containing monomers such as glycidyl (meth)acrylate; glycol acrylate monomers such as polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, methoxyethylene glycol (meth)acrylate, and methoxypolypropylene glycol (meth)acrylate; acrylate monomers such as tetrahydrofurfuryl (meth)acrylate, fluoro(meth)acrylate, polysilicone (meth)acrylate, or 2-methoxyethyl (meth)acrylate, etc.

The acrylic-based polymer preferably has the highest content of (meth)acrylic acid alkyl ester among the above-mentioned monomer components. The characteristics of the adhesive sheet are easily influenced by the type of the monomer (main monomer) with 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 with a carbon number of 6 or less, there is a tendency for tanδ _{70 ℃} to increase and the step absorption to improve. In particular, when a _C4 alkyl acrylate such as butyl acrylate is the main monomer, there is a tendency for tanδ _{70 ℃} to increase. The amount of (meth)acrylic acid alkyl ester having a chain alkyl group with a carbon number of 6 or less relative to the total amount of the monomer components constituting the acrylic polymer chain is preferably 40 to 85% by weight, more preferably 45 to 80% by weight, and further preferably 50 to 75% by weight. It is particularly preferred that the content of butyl acrylate as a constituent monomer component is within the above range.

The theoretical Tg of the acrylic polymer chain is preferably above -50°C. The theoretical Tg of the acrylic polymer chain is preferably below -10°C, more preferably below -20°C, and further preferably below -25°C. The theoretical Tg is calculated by the following Fox formula based on the glass transition temperature Tg _i of the homopolymer of the monomer components constituting the acrylic polymer chain and the weight fraction _Wi of each monomer component. 1/Tg = Σ( _Wi /Tg _i )

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

<Crosslinking structure> A polymer having a crosslinking structure introduced into an acrylic polymer chain can be obtained, for example, by the following methods: (1) a method of polymerizing an acrylic polymer having a functional group that can react with a crosslinking agent, adding a crosslinking agent, and reacting the acrylic polymer with the crosslinking agent; and (2) a method of introducing a branched structure (crosslinking structure) into a polymer chain by including a polyfunctional compound in the polymerization components of the polymer. These methods can also be used in combination to introduce multiple crosslinking structures into a base polymer.

Specific examples of the crosslinking agent in the method of reacting the base polymer with the crosslinking agent in (1) above include isocyanate crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, metal chelate crosslinking agents, etc. Among them, isocyanate crosslinking agents and epoxy crosslinking agents are preferred because they have high reactivity with the hydroxyl or carboxyl groups of the base polymer and are easy to introduce into the crosslinking structure. These crosslinking agents react with the functional groups such as hydroxyl or carboxyl groups introduced into the base polymer to form a crosslinking structure. In an acid-free adhesive in which the base polymer does not contain a carboxyl group, it is preferred to use an isocyanate crosslinking agent to form a crosslinked structure by reaction between the hydroxyl groups in the base polymer and the isocyanate crosslinking agent.

As the isocyanate-based crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule is used. Examples of isocyanate crosslinking agents include low-order aliphatic polyisocyanates such as butyl diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentyl diisocyanate, cyclohexyl diisocyanate, and isophorone diisocyanate; aromatic isocyanates such as 2,4-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate; trihydroxymethylpropane/toluene diisocyanate trimer adducts (e.g., "Coronate L" manufactured by Tosoh Corporation), trihydroxymethylpropane/hexamethylene diisocyanate trimer adducts (e.g., "Coronate 1" manufactured by Tosoh Corporation); The invention also provides isocyanate adducts such as trihydroxymethylpropane adduct of xylylene diisocyanate (e.g., "Takenate D110N" manufactured by Mitsui Chemicals Co., Ltd.), isocyanurate of hexamethylene diisocyanate (e.g., "Coronate HX" manufactured by Tosoh Corporation), etc. In addition, by using a urethane prepolymer having an isocyanate group at the end as an isocyanate crosslinking agent, a crosslinking structure formed by urethane chain segments can be introduced.

In the method of (2) above in which a polyfunctional compound is included in the polymerization components of the base polymer, the total amount of the monomer components constituting the acrylic base polymer and the polyfunctional compound used to introduce the cross-linking structure can be reacted at once, or the polymerization can be carried out in multiple stages. As a method of carrying out polymerization in multiple stages, the following method is preferred: polymerizing the monofunctional monomers constituting the base polymer (prepolymerization) to prepare a partial polymer (prepolymer composition), adding a polyfunctional compound such as a polyfunctional (meth)acrylate to the prepolymer composition, and polymerizing the prepolymer composition and the polyfunctional monomer (formal polymerization). The prepolymer composition is a polymer containing a low degree of polymerization and a partial polymer of unreacted monomers.

By prepolymerizing the constituents of the acrylic base polymer, the branching points (crosslinking points) formed by the multifunctional compound can be uniformly introduced into the base polymer. Alternatively, a mixture of a low molecular weight polymer or a partial polymer and unpolymerized monomer components (adhesive composition) can be coated on a substrate and then formally polymerized on the substrate to form an adhesive sheet. Low polymer compositions such as prepolymer compositions have low viscosity and excellent coating properties. Therefore, by coating an adhesive composition that is a mixture of a prepolymer composition and a multifunctional compound and then formally polymerizing it on a substrate, the productivity of the adhesive sheet can be improved and the thickness of the adhesive sheet can be made uniform.

As the polyfunctional compound used for introducing the cross-linked structure, there can be cited compounds containing two or more polymerizable functional groups (ethylenic unsaturated groups) having unsaturated double bonds in one molecule. As the polyfunctional compound, polyfunctional (meth)acrylates are preferred in terms of ease of copolymerization with the monomer components of the acrylic base polymer. In the case of introducing a branched (cross-linked) structure by active energy line polymerization (photopolymerization), polyfunctional acrylates are preferred.

Examples of the multifunctional (meth)acrylate include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, poly(1,4-butylene glycol) di(meth)acrylate, bisphenol A ethylene oxide modified di(meth)acrylate, bisphenol A propylene oxide modified di(meth)acrylate, alkane glycol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, ethoxylated isocyanuric acid tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tri(meth)acrylate, The polyfunctional (meth)acrylate may be any of: polyol di(meth)acrylate, trihydroxymethylpropane tri(meth)acrylate, di-trihydroxymethylpropane tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerol di(meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, isoprene (meth)acrylate, etc. In addition, a cross-linked structure formed by a urethane chain segment may be introduced by using a (meth)acryloyl group at the end of the urethane chain as the multifunctional (meth)acrylate.

<Introduction of cross-linked structure formed by urethane chain segments> By cross-linking acrylic polymer chains by urethane chain segments, it is easy to obtain an adhesive that achieves both low glass transition temperature and high adhesive retention. Urethane chain segments are molecular chains with urethane bonds. Both ends of the urethane chain segments are covalently bonded to the acrylic polymer chain, thereby introducing a cross-linked structure formed by the urethane chain segments into the acrylic polymer chain. Urethane chain segments typically include polyurethane chains obtained by reacting diols with diisocyanates.

The molecular weight of the polyurethane chain in the urethane chain segment is preferably 5000-30000, more preferably 6000-23000, and further preferably 7000-20000. The greater the molecular weight of the polyurethane chain in the urethane chain segment, the longer the distance between the crosslinking points of the acrylic polymer chain. If the molecular weight of the polyurethane chain is within the above range, the polymer introduced with the crosslinking structure has appropriate cohesion and fluidity, so that adhesion, step absorption and impact resistance can be achieved at the same time.

When the molecular weight of the polyurethane chain is too small and the distance between crosslinks is short, there is a tendency that tanδ decreases as the cohesion increases, and the step absorption and impact resistance tend to decrease. On the other hand, when the molecular weight of the polyurethane chain is too large and the distance between crosslinks is long, there is a situation where the storage modulus is small and the bonding strength is insufficient. Even when the molecular weight of the polyurethane chain is large, the storage modulus can be increased by increasing the amount of urethane chain segments to increase the gel fraction. However, since the compatibility between polyurethane chains with larger molecular weight and acrylic polymer chains is relatively low, the haze of the adhesive increases as the amount of urethane segments increases, thereby reducing transparency.

If the amount of the urethane chain segment becomes too large, the viscosity of the adhesive may decrease as the gel fraction increases, and the step absorption and impact resistance may decrease. In addition, if the amount of the urethane chain segment becomes too large, the transparency of the adhesive sheet may decrease and the haze may increase. Therefore, the amount of the urethane chain segment in the base polymer is preferably 10 parts by weight or less, more preferably 7 parts by weight or less, and further preferably 5 parts by weight or less, relative to 100 parts by weight of the acrylic polymer chain. On the other hand, from the viewpoint of increasing the gel fraction and having adhesion retention, the amount of the urethane chain segment in the base polymer is preferably 0.3 parts by weight or more, more preferably 0.4 parts by weight or more, and further preferably 0.5 parts by weight or more, relative to 100 parts by weight of the acrylic polymer chain. The amount of the urethane chain segment in the base polymer may be 4 parts by weight or less, or 3 parts by weight or less, or 0.7 parts by weight or more, or 1 part by weight or more, relative to 100 parts by weight of the acrylic polymer chain.

Examples of the diols used to form the polyurethane chain include low molecular weight diols such as ethylene glycol, diethylene glycol, propylene glycol, butanediol, and hexamethylene glycol; and high molecular weight polyols such as polyester polyols, polyether polyols, polycarbonate polyols, acrylic polyols, epoxy polyols, and caprolactone polyols.

Polyether polyols are obtained by ring-opening addition polymerization of alkylene oxides and polyols. Examples of alkylene oxides include ethylene oxide, propylene oxide, butylene oxide, ethylene oxide, tetrahydrofuran, etc. Examples of polyols include the above-mentioned diols or glycerol, trihydroxymethylpropane, etc.

Polyester polyol is a polyester having a hydroxyl group at the end, and is obtained by reacting a polybasic acid with a polyol in such a way that the alcohol equivalent is in excess relative to the carboxylic acid equivalent. The polybasic acid component and the polyol component constituting the polyester polyol are preferably a combination of a dibasic acid and a diol.

Examples of the dibasic acid component include aromatic dicarboxylic acids such as phthalic acid, isophthalic acid, and terephthalic acid; alicyclic dicarboxylic acids such as hexahydrophthalic acid, tetrahydrophthalic acid, 1,3-cyclohexanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid; aliphatic dicarboxylic acids such as oxalic acid, succinic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, dodecanedicarboxylic acid, and octadecanedicarboxylic acid; and anhydrides and lower alcohol esters of these dicarboxylic acids.

The diol component includes ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, neopentyl glycol, pentylene glycol, 1,6-hexanediol, 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, hydrogenated bisphenol A, hydrogenated bisphenol F, and the like.

Examples of polycarbonate polyols include: polycarbonate polyols obtained by subjecting a diol component to a condensation reaction with phosgene; polycarbonate polyols obtained by subjecting a diol component to an ester exchange condensation reaction with a carbonic acid diester such as dimethyl carbonate, diethyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, ethylbutyl carbonate, ethylene carbonate, propylene carbonate, diphenyl carbonate, and dibenzyl carbonate; copolymer polycarbonate polyols obtained by using two or more polyol components in combination; polycarbonate polyols obtained by subjecting the above-mentioned polycarbonate polyols to an esterification reaction with a carboxyl group-containing compound; Acid polyol; a polycarbonate polyol obtained by subjecting the above-mentioned various polycarbonate polyols to an etherification reaction with a hydroxyl-containing compound; a polycarbonate polyol obtained by subjecting the above-mentioned various polycarbonate polyols to an ester compound to an ester exchange reaction; a polycarbonate polyol obtained by subjecting the above-mentioned various polycarbonate polyols to an ester exchange reaction with a hydroxyl-containing compound; a polyester-based polycarbonate polyol obtained by polycondensing the above-mentioned various polycarbonate polyols with a dicarboxylic acid compound; a copolyether-based polycarbonate polyol obtained by copolymerizing the above-mentioned various polycarbonate polyols with an alkylene oxide, and the like.

Polyacrylic polyol is obtained by copolymerizing (meth)acrylate and a monomer component having a hydroxyl group. Examples of the monomer having a hydroxyl group include hydroxyalkyl esters of (meth)acrylate such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxypentyl (meth)acrylate; (meth)acrylate monoesters of polyols such as glycerol and trihydroxymethylpropane; and N-hydroxymethyl (meth)acrylamide. Examples of (meth)acrylate include methyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate.

Polyacrylic acid polyols may also contain monomer components other than those mentioned above as copolymer components. Examples of copolymer monomer components other than those mentioned above include: unsaturated monocarboxylic acids such as (meth) acrylic acid; unsaturated dicarboxylic acids such as maleic acid and their anhydrides and mono- or diesters; unsaturated nitriles such as (meth) acrylonitrile; unsaturated amides such as (meth) acrylamide and N-hydroxymethyl (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; halogenated α,β-unsaturated aliphatic monomers such as vinyl chloride and vinylidene chloride; α,β-unsaturated aromatic monomers such as styrene and α-methylstyrene, etc.

The diisocyanate used to form the polyurethane chain may be any of aromatic diisocyanate and aliphatic diisocyanate. Examples of the aromatic diisocyanate include 1,5-naphthalene diisocyanate, 4,4'-diphenylmethane diisocyanate (MDI), 4,4'-diphenyldimethylmethane diisocyanate, tetramethyldiphenylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2-chloro-1,4-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, xylylene diisocyanate, 4,4'-diphenyl ether diisocyanate, 4,4'-diphenyl sulfone diisocyanate, 4,4'-diphenyl sulfone diisocyanate, and 4,4'-biphenyl diisocyanate. 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, isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, 1,3-bis(isocyanatemethyl)cyclohexane, and methylcyclohexane diisocyanate.

As diisocyanate, derivatives of isocyanate compounds may also be used. Examples of derivatives of isocyanate compounds include dimers of polyisocyanate, trimers of isocyanate (isocyanurates), polymeric MDI, adducts with trihydroxymethylpropane, biuret modified products, allophanate modified products, urea modified products, etc. As diisocyanate components, urethane prepolymers having an isocyanate group at the end may also be used.

Among the polyurethane chains exemplified, polyether urethane containing polyether polyol as a diol component and/or polyester urethane containing polyester polyol as a diol component are preferred in terms of higher compatibility with acrylic polymer chains. In particular, when a cross-linked structure is introduced through polyester urethane, there is a tendency for the storage modulus at room temperature to increase, and the adhesion retention and processability to improve. One reason for this is that polyester has a rigid molecular structure compared to polyethers and the like. It is believed that if a cross-linked structure is introduced through a rigid chain segment, the movement of the acrylic polymer chain is restricted, thereby improving the storage modulus. On the other hand, the distance between the cross-linking points of the polymer chain is maintained, thereby showing impact resistance and step absorption.

By using a compound having a functional group copolymerizable with a monomer component constituting an acrylic polymer chain at the end of a polyurethane chain, or a compound having a functional group reactive with a carboxyl group, a hydroxyl group, etc. contained in an acrylic polymer chain at the end of a polyurethane chain, a crosslinked structure formed by a urethane chain segment can be introduced into an acrylic polymer chain. From the perspective of easily introducing crosslinking points uniformly into an acrylic polymer chain and having excellent compatibility between an acrylic polymer chain and a urethane chain segment, it is preferred to use a di(meth)acrylic urethane having (meth)acrylic groups at both ends of a polyurethane chain to introduce a crosslinked structure formed by a urethane chain segment. For example, by copolymerizing the monomer components constituting the acrylic polymer chain with urethane di(meth)acrylate, a cross-linked structure formed by urethane chain segments can be introduced into the acrylic polymer chain.

Di(meth)acrylic urethane having (meth)acrylic groups at both ends is obtained, for example, by using a (meth)acrylic compound having a hydroxyl group in addition to a diol component in the polymerization of a polyurethane. From the viewpoint of controlling the chain length (molecular weight) of the urethane chain segment, it is preferred to react a diol with a diisocyanate in such a way that the isocyanate becomes excessive to synthesize an isocyanate-terminated polyurethane, and then add a (meth)acrylic compound having a hydroxyl group to react the terminal isocyanate group of the polyurethane with the hydroxyl group of the (meth)acrylic compound.

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

Examples of the (meth)acrylic acid compound having a hydroxyl group include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, hydroxymethacrylamide, hydroxyethylacrylamide, and the like.

As the (meth) urethane acrylate, commercial products sold by Arakawa Chemical Industries, Shin-Nakamura Chemical Industries, Toagosei, Kyoeisha Chemical, Nippon Kayaku, Nippon Gosei Chemical Industries, Negami Industries, DAICEL-ALLNEX, etc. can also be used. The weight average molecular weight of the (meth) urethane acrylate is preferably 5,000 to 30,000, more preferably 6,000 to 23,000, and even more preferably 7,000 to 20,000.

The glass transition temperature of (meth) urethane acrylate is preferably below 0°C, more preferably below -10°C, and further preferably below -20°C. By using (meth) urethane acrylate with a low Tg, when the cohesive force of the base polymer is improved by introducing a cross-linking structure through the urethane chain segment, an adhesive having excellent low-temperature adhesion can also be obtained. The lower limit of the glass transition temperature of (meth) urethane acrylate is not particularly limited, but from the viewpoint of obtaining an adhesive having excellent high-temperature retention, it is preferably above -100°C, more preferably above -90°C, and further preferably above -80°C.

When a cross-linked structure formed by a urethane chain segment is introduced into an acrylic polymer chain using urethane (meth)acrylate, the glass transition temperature of the urethane chain segment of the base polymer is substantially equal to the glass transition temperature of urethane (meth)acrylate.

<Preparation of base polymer> The polymer having a cross-linked structure formed by a urethane chain segment introduced into the acrylic polymer chain can be polymerized by various known methods. When (meth)acrylic urethane is used as a constituent component of the urethane chain segment, it is sufficient to copolymerize the monomer component constituting the acrylic polymer chain with the (meth)acrylic urethane.

The amount of (meth)acrylic urethane used is preferably 0.3 to 10 parts by weight, more preferably 0.5 to 7 parts by weight, and further preferably 0.7 to 5 parts by weight, relative to 100 parts by weight of the monomer component used to constitute the acrylic polymer chain. By adjusting the amount of (meth)acrylic urethane used, a base polymer having a urethane chain segment content within the above range can be prepared. When the content of the urethane chain segment is too small, the cohesiveness of the base polymer decreases, which tends to reduce the adhesion retention of the adhesive sheet. When the content of the urethane chain segment is too large, the viscosity of the adhesive sheet decreases as the cohesiveness of the base polymer increases, thereby tending to reduce impact resistance and step absorption.

In addition to (meth)acrylic acid urethane, a cross-linking structure may be introduced into the acrylic polymer chain by a multifunctional (meth)acrylic compound other than (meth)acrylic acid urethane, or a multifunctional (meth)acrylic compound other than (meth)acrylic acid urethane may be used instead of (meth)acrylic acid urethane to introduce a cross-linking structure. If the amount of cross-linking structure based on a multifunctional compound other than (meth)acrylic acid urethane introduced increases, the impact resistance and step absorption of the adhesive may decrease. Therefore, the amount of the multifunctional compound other than (meth)acrylic acid urethane is preferably 0.2 parts by weight or less, more preferably 0.1 parts by weight or less, and further preferably 0.05 parts by weight or less, relative to 100 parts by weight of the monomer component used to constitute the acrylic polymer chain.

As a polymerization method for the base polymer, photopolymerization is preferred. In photopolymerization, the polymer can be prepared without using a solvent, so when forming an adhesive sheet, it is not necessary to dry and remove the solvent to uniformly form an adhesive sheet with a large thickness.

In the preparation of the base polymer, the total amount of the monomer components constituting the acrylic polymer chain and the multifunctional compound used to introduce the crosslinking structure can be reacted at once, or the polymerization can be carried out in multiple stages. As a method of carrying out the polymerization in multiple stages, the following method is preferred: the monofunctional monomer constituting the acrylic polymer chain is polymerized to form a prepolymer composition (prepolymerization), and a multifunctional compound such as di(meth)acrylic acid urethane is added to the slurry of the prepolymer composition to polymerize the prepolymer composition and the multifunctional monomer (formal polymerization). The prepolymer composition is a polymer containing a low degree of polymerization and a partial polymer of unreacted monomers.

By pre-polymerizing the components of the acrylic polymer chain, the branching points (cross-linking points) formed by the polyfunctional compounds such as di(meth)acrylate urethane can be uniformly introduced into the acrylic polymer chain. In addition, after coating a low molecular weight polymer or a mixture of a partially polymerized and unpolymerized monomer components (adhesive composition) on a substrate, a formal polymerization can be performed on the substrate to form an adhesive sheet.

Since low-polymerization-degree compositions such as prepolymer compositions have low viscosity and excellent coating properties, the productivity of adhesive sheets can be improved and the thickness of adhesive sheets can be made uniform by coating an adhesive composition that is a mixture of a prepolymer composition and a multifunctional compound and then performing full polymerization on a substrate.

[Adhesive sheet] As described above, a prepolymer composition with a low degree of polymerization is prepared by prepolymerization, an adhesive composition having a multifunctional compound added to the prepolymer composition is applied in layers on a substrate, and the adhesive composition on the substrate is polymerized (formal polymerization) to obtain an adhesive sheet.

<Prepolymerization> The prepolymer composition can be prepared, for example, by polymerizing a composition obtained by mixing monomer components constituting an acrylic polymer chain with a polymerization initiator. The prepolymer-forming composition may also contain a polyfunctional compound (polyfunctional monomer or polyfunctional oligomer). For example, the prepolymer-forming composition may contain a portion of a polyfunctional compound that is a raw material for the polymer, and after the prepolymer is polymerized, the remaining portion of the polyfunctional compound may be added and subjected to formal polymerization.

The prepolymer forming composition preferably contains a photopolymerization initiator. Examples of the photopolymerization initiator include benzoin ether photopolymerization initiators, acetophenone photopolymerization initiators, α-ketol photopolymerization initiators, aromatic sulfonyl chloride photopolymerization initiators, photoactive oxime photopolymerization initiators, benzoin photopolymerization initiators, benzyl photopolymerization initiators, benzophenone photopolymerization initiators, ketal photopolymerization initiators, 9-oxysulfide photopolymerization initiators, and the like. Photopolymerization initiator, acylphosphine oxide photopolymerization initiator, etc.

During polymerization, a chain transfer agent or a polymerization inhibitor (polymerization retardant) may be used to adjust the molecular weight. Examples of the chain transfer agent include thiol groups such as α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, hydroxyacetic acid, 2-hydroxyethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, 2,3-dihydroxy-1-propanol, and α-methylstyrene dimer.

The polymerization rate of the prepolymer is not particularly limited. From the perspective of making a viscosity suitable for coating on a substrate, it is preferably 3 to 50% by weight, and more preferably 5 to 40% by weight. The polymerization rate of the prepolymer can be adjusted to the desired range by adjusting the type or amount of photopolymerization initiator, the irradiation intensity of active light such as UV (ultraviolet) light, and the irradiation time. The polymerization rate of the prepolymer is calculated based on the weight before and after heating at 130°C for 3 hours and by the following formula. The polymerization rate of the adhesive sheet is also calculated by the same method. Polymerization rate (%) = weight after drying / weight before drying × 100

<Preparation of adhesive composition> The adhesive composition is prepared by mixing a polyfunctional compound such as di(meth)acrylate urethane and, if necessary, the remaining monomer components constituting the acrylic polymer chain, a polymerization initiator, a chain transfer agent, other additives, etc. into the above-mentioned prepolymer composition. The adhesive composition preferably has a viscosity suitable for coating on a substrate (e.g., about 0.5 to 20 Pa·s). The viscosity of the adhesive composition can be set to an appropriate range by adjusting the polymerization rate of the prepolymer, the type or amount of the polyfunctional compound, the composition, molecular weight, and amount of other components (e.g., oligomers). In order to adjust the viscosity, a thickening additive can also be used.

The polymerization initiator used in the main polymerization is not particularly limited, and for example, the above-mentioned photopolymerization initiator can be used. In the case where the polymerization initiator used in the prepolymerization is not deactivated and remains in the prepolymer composition, the addition of the polymerization initiator used in the main polymerization can be omitted.

(Chain transfer agent) During the main polymerization, it is preferred to adjust the molecular weight by including a chain transfer agent in the adhesive composition. The chain transfer agent used in the main 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 adhesive composition is preferably 0.001 to 2 parts by weight, more preferably 0.005 to 1 part by weight, and further preferably 0.01 to 0.5 parts by weight relative to 100 parts by weight of the constituent components of the base polymer. Furthermore, in the case where the chain transfer agent used in the prepolymerization is not deactivated and remains in the prepolymer composition, the addition of the chain transfer agent to the adhesive composition can be omitted.

The chain transfer agent receives free radicals from the growing polymer chain, stopping the elongation of the polymer, and the chain transfer agent that received the free radical attacks the monomer, restarting the polymerization. By using a chain transfer agent, the increase in the molecular weight of the polymer can be suppressed without reducing the free radical concentration in the reaction system.

When the ratio of monofunctional monomers to polyfunctional monomers is fixed, the larger the molecular weight, the higher the probability of crosslinking points (branching points) formed by polyfunctional monomers contained in one polymer chain, and thus the gel fraction tends to be larger. By using a chain transfer agent to inhibit the elongation of the polymer, the molecular weight of the polymer decreases and the increase in the gel fraction tends to be suppressed. Therefore, by making the adhesive composition contain a chain transfer agent, it is easy to form an adhesive sheet with a large tanδ and excellent step absorption.

(Oligomer) In order to adjust the adhesion or viscosity of the adhesive sheet, the adhesive composition may also contain various oligomers. As oligomers, for example, oligomers with a weight average molecular weight of about 1,000 to 30,000 are used. As oligomers, acrylic oligomers are preferred in terms of excellent compatibility with acrylic base polymers.

The acrylic oligomer contains an alkyl (meth)acrylate as a constituent monomer component. Among them, preferably, an alkyl (meth)acrylate having a chain alkyl group (chain alkyl (meth)acrylate) and an alkyl (meth)acrylate having an alicyclic alkyl group (alicyclic alkyl (meth)acrylate) as a constituent monomer component. Specific examples of the chain alkyl (meth)acrylate and the alicyclic alkyl (meth)acrylate are exemplified above as constituent monomers of the acrylic polymer chain.

The glass transition temperature of the acrylic oligomer is preferably above 20°C, more preferably above 40°C. The glass transition temperature of the acrylic oligomer may also be above 60°C, above 80°C, above 100°C, or above 110°C. By introducing a low-Tg acrylic base polymer with a crosslinked structure and a high-Tg acrylic oligomer together, the bonding retention of the adhesive sheet tends to be improved. The upper limit of the glass transition temperature of the acrylic oligomer is not particularly limited, and is generally below 200°C, preferably below 180°C, and more preferably below 160°C. The glass transition temperature of the acrylic oligomer is calculated by the above-mentioned Fox formula.

Among the exemplified (meth)acrylic acid alkyl esters, methyl methacrylate is preferred as the (meth)acrylic acid chain alkyl ester in terms of having a high glass transition temperature and excellent compatibility with the base polymer. Dicyclopentyl acrylate, dicyclopentyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate are preferred as the (meth)acrylic acid cycloalkyl ester. That is, the acrylic oligomer preferably contains one or more selected from the group consisting of dicyclopentyl acrylate, dicyclopentyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate, and methyl methacrylate as constituent monomer components.

The amount of the cyclic alkyl (meth)acrylate relative 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, and further preferably 30 to 70% by weight. The amount of the chain alkyl (meth)acrylate relative 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, and further preferably 30 to 70% by weight.

The weight average molecular weight of the acrylic oligomer is preferably 1000 to 30000, more preferably 1500 to 10000, and further preferably 2000 to 8000. By using an acrylic oligomer having a molecular weight within this range, the adhesion and adhesion retention of the adhesive tend to be improved.

The acrylic oligomer is obtained by polymerizing the above-mentioned monomer components using various polymerization methods. Various polymerization initiators may be used in the polymerization of the acrylic oligomer. In addition, a chain transfer agent may be used to adjust the molecular weight.

When the adhesive composition contains an oligomer component such as an acrylic oligomer, its content is preferably 0.5 to 20 parts by weight, more preferably 1 to 15 parts by weight, and further preferably 2 to 10 parts by weight relative to 100 parts by weight of the above-mentioned base polymer. When the content of the oligomer in the adhesive composition is within the above range, there is a tendency for the adhesion at high temperature and the high temperature retention to be improved.

(Silane coupling agent) In order to adjust the adhesion, a silane coupling agent may be added to the adhesive composition. When a silane coupling agent is added to the adhesive composition, the amount added is generally about 0.01 to 5.0 parts by weight, preferably about 0.03 to 2.0 parts by weight, relative to 100 parts by weight of the base polymer.

(Ultraviolet absorber) The adhesive composition may also contain an ultraviolet absorber. By making the adhesive composition contain an ultraviolet absorber, the adhesive sheet 5 can be given ultraviolet absorption, thereby preventing the polarizing plate 3 or the image display unit 6 from being deteriorated due to ultraviolet rays.

Examples of the ultraviolet absorber include benzotriazole ultraviolet absorbers, benzophenone ultraviolet absorbers, trioxane ultraviolet absorbers, salicylic acid ultraviolet absorbers, cyanoacrylate ultraviolet absorbers, etc. In terms of easily obtaining an acrylic adhesive sheet having high ultraviolet absorbency, excellent compatibility with acrylic polymers, and high transparency, trioxane ultraviolet absorbers are preferred, and among them, trioxane ultraviolet absorbers containing a hydroxyl group are preferred, and hydroxyphenyl trioxane ultraviolet absorbers are particularly preferred.

A commercially available product may also be used as the UV absorber. Commercially available products of tribasic ultraviolet absorbers include: the reaction product of 2-(4,6-bis(2,4-dimethylphenyl)-1,3,5-tribasic-2-yl)-5-hydroxyphenyl and [(alkoxy)methyl]oxirane ("TINUVIN 400" manufactured by BASF), the reaction product of 2-(2,4-dihydroxyphenyl)-4,6-bis-(2,4-dimethylphenyl)-1,3,5-tribasic and (2-ethylhexyl)-glycidate ("TINUVIN 405" manufactured by BASF), (2,4-bis[2-hydroxy-4-butoxyphenyl]-6-(2,4-dibutoxyphenyl)-1,3,5-tribasic ("TINUVIN 460"), 2-(4,6-diphenyl-1,3,5-trioxan-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-trioxan-2-yl)-6-(4-methoxyphenyl)-1,3,5-trioxan-2-yl ("Tinosorb 479" manufactured by BASF), S”), 2-(4,6-diphenyl-1,3,5-trioxan-2-yl)-5-[2-(2-ethylhexanoyloxy)ethoxy]phenol (“ADK STAB LA-46” manufactured by ADEKA Corporation), etc.

When a UV absorber is added to the adhesive composition, the amount added is generally about 0.1 to 10 parts by weight, preferably 0.3 to 5 parts by weight, relative to 100 parts by weight of the base polymer. By setting the content of the UV absorber to the above range, the reduction in transparency caused by the leakage of the UV absorber can be suppressed, and the UV cutoff property of the adhesive sheet can be improved.

(Other additives) In addition to the components listed above, the adhesive composition may also contain additives such as adhesive agents, plasticizers, softeners, anti-degradation agents, fillers, colorants, antioxidants, surfactants, antistatic agents, etc.

<Coating and formal polymerization of adhesive composition> After the adhesive composition is coated on the substrate in a layer, it is photocured by irradiating active light. When performing photocuring, it is preferred to attach a cover sheet to the surface of the coating layer and irradiate the adhesive composition between two sheets to prevent polymerization hindrance caused by oxygen.

Any appropriate substrate can be used as the substrate and cover sheet for forming the adhesive sheet. The substrate and cover sheet can also 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 containing 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, (meth)acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, polyarylate resins, polyphenylene sulfide resins, etc. Among them, polyester resins such as polyethylene terephthalate are particularly preferred. The thickness of the substrate is preferably 10 to 200 μm, more preferably 25 to 150 μm. The material of the release layer may include: polysilicone release agents, fluorine release agents, long-chain alkyl release agents, fatty amide release agents, etc. The thickness of the release layer is generally about 10 to 2000 nm.

As a method of applying the adhesive composition on the substrate, various methods are used, such as roller coating, contact roller coating, gravure coating, reverse coating, roller brush coating, spray coating, dip roller coating, rod coating, doctor blade coating, air knife coating, curtain coating, die lip coating, die nozzle coating, etc.

The adhesive composition coated on the substrate in a layer is irradiated with active light to carry out the main polymerization. In the main polymerization, the unreacted monomer components in the prepolymer composition react with the polyfunctional compounds such as di(meth)acrylate urethane to obtain a polymer with a cross-linked structure introduced into the acrylic polymer chain.

The active light can be selected according to the type of polymerizable components such as monomers or (meth) urethane acrylates or the type of photopolymerization initiators, and generally ultraviolet light and/or short-wave visible light are used. The cumulative light intensity of the irradiated light is preferably about 100 to 5000 mJ/ ^cm2 . The light source used for light irradiation is not particularly limited as long as it can irradiate light in the wavelength range to which the photopolymerization initiator contained in the adhesive composition is sensitive. LED (light-emitting diode) light sources, high-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halide lamps, xenon lamps, etc. can be preferably used. If the amount of unreacted monomers is large, there is a situation where the G' _{25 ℃} of the adhesive sheet decreases and the adhesive retention force decreases. Therefore, the polymerization rate of the adhesive sheet after the formal 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 adhesive sheet may be heated to volatilize the residual monomers and unreacted polymerization initiators.

As mentioned above, the gel fraction of the adhesive sheet is preferably 30-80%, more preferably 35-70%. By making the gel fraction above 30%, the adhesive retention is improved, and it is not easy to produce the paste missing or the position shift between the components during processing, and the processability and processing dimensional stability are excellent. In addition, by making the gel fraction below 80%, excellent step absorption can be exerted.

The weight average molecular weight of the gel component of the adhesive sheet is preferably 150,000 to 450,000, and more preferably 180,000 to 420,000. The gel component is a soluble component obtained by extracting the base polymer using tetrahydrofuran (hereinafter, THF). It is difficult to measure the molecular weight of each polymer chain of a cross-linked polymer (gel component), so the molecular weight of the gel component (uncross-linked polymer) becomes an indicator of the elongation of the polymer chain. When the molecular weight of the gel component is too large, the glass transition temperature increases and the impact resistance decreases. On the other hand, when the molecular weight of the gel component is too small, the bonding retention decreases.

By laminating the release films 21, 22 on the surface of the adhesive sheet 5, an adhesive sheet having release films temporarily adhered to both sides is obtained as shown in Fig. 1. The release film used as the base material or the cover sheet when forming the adhesive sheet can also be directly used as the release films 21, 22.

When release films 21 and 22 are disposed on both sides of the adhesive sheet 5, the thickness of one release film 21 may be the same as or different from the thickness of the other release film 22. The peeling force when peeling off the release film temporarily attached to one side of the self-adhesive sheet 5 may be the same as or different from the peeling force when peeling off the release film temporarily attached to the other side of the self-adhesive sheet 5. When the peeling forces of the two are different, the release film 22 (light peeling film) with a relatively smaller peeling force is first peeled off from the adhesive sheet 5 and then bonded to the first adherend, and the release film 21 (heavy peeling film) with a relatively larger peeling force is peeled off and then bonded to the second adherend. In this case, the workability is excellent.

[Image display device] The adhesive sheet 5 can be used to bond various transparent components or opaque components. The type of the adherend is not particularly limited, and various resin materials, glass, metal, etc. can be listed. Since the adhesive sheet 5 has high transparency, it is suitable for bonding optical components such as image display devices. In particular, since the adhesive sheet 5 has excellent step absorption and impact resistance, it can be preferably used to bond transparent components such as a front transparent plate or a touch panel to the viewing side surface of the image display device.

FIG2 is a cross-sectional view showing an example of a laminated structure of an image display device in which a front transparent plate 7 is bonded to the visual side surface of an image display panel 10 via an adhesive sheet 5. The image display panel 10 has a polarizing plate 3, which is bonded to the visual side surface of an image display unit 6 such as a liquid crystal unit or an organic EL unit via an adhesive sheet 4. The front transparent plate 7 is provided with a printed layer 76 on the periphery of one surface of a transparent flat plate 71. The transparent plate 71 is made of, for example, a transparent resin plate such as an acrylic resin or a polycarbonate resin, or a glass plate. The transparent plate 71 may also have a touch panel function. As a touch panel, a touch panel of any type such as a resistive film type, an electrostatic capacitor type, an optical type, an ultrasonic type, etc. may be used.

The polarizing plate 3 disposed on the surface of the image display panel 10 and the printed layer 76 of the front transparent plate 7 are bonded to each other via the adhesive sheet 5. The bonding sequence is not particularly limited, and the adhesive sheet 5 may be bonded to the image display panel 10 first, or may be bonded to the front transparent plate 7 first. In addition, both bondings may be performed simultaneously. From the viewpoint of the workability of bonding, it is preferred to peel off a release film (light-peel release film) 2, bond the exposed surface of the adhesive sheet 5 to the image display panel 10, and then peel off another release film 21 (heavy-peel release film), and bond the exposed surface of the adhesive sheet to the front transparent plate 7.

It is preferred that after the adhesive sheet 5 and the front transparent plate 7 are bonded, defoaming is performed to remove bubbles near the interface between the adhesive sheet 5 and the flat plate 71 of the front transparent plate 7 or the non-flat portion such as the printed layer 76. As a defoaming method, appropriate methods such as heating, pressurization, and decompression can be adopted. For example, it is preferred to bond while suppressing the mixing of bubbles under decompression and heating, and then, in order to suppress delayed bubbles, pressurization is performed simultaneously with heating by autoclave treatment. 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 for sealing. As described above, the adhesive sheet 5 has a large shear storage modulus, so it has excellent connection reliability in a wide temperature range. Therefore, even when the bonding interface of the adhesive sheet is subjected to stress deformation due to temperature changes during sealing by the resin material or the like, the peeling of the bonding interface can be suppressed. In addition, the adhesive sheet 5 has a low glass transition temperature and a large peak value of tanδ, so it has excellent impact resistance in a wide temperature range and is not easily peeled off due to impacts such as falling.

[Optical film attached to adhesive sheet] In addition to the form in which release films are temporarily attached to both sides of the adhesive sheet 5 as shown in FIG1, the adhesive sheet 5 can also be used in the form of an adhesive film attached to an optical film or the like. For example, in the form shown in FIG3, a release film 21 is temporarily attached to one side of the adhesive sheet 5, and a polarizing plate 3 is fixed to the other side of the adhesive sheet 5. In the form shown in FIG4, an adhesive sheet 4 is further provided on the polarizing plate 3, and a release film 24 is temporarily attached thereto.

As described above, in a form where an optical film such as a polarizing plate is previously bonded to the adhesive sheet, it is sufficient to peel off the release film 21 temporarily bonded to the surface of the adhesive sheet 5 and then bond it to the front transparent member.

[Examples of variations of laminated configurations] In FIGS. 1 to 4, the configuration in which the image display panel 10 (polarizing plate 3) and the front transparent plate 7 (cover plate) are bonded together by a double-sided adhesive sheet 5 without a substrate is mainly described, but the types and combinations of adherends are not limited to these. For example, the cover plate and the touch panel sensor may be bonded together by an adhesive sheet 5. In this configuration, another adhesive sheet is used to bond the touch panel sensor to the image display panel.

The adhesive sheet 5 can also be used as one or both adhesive layers of a double-sided adhesive sheet with a substrate. The double-sided adhesive sheet with a substrate 15 shown in FIG5 has a first adhesive layer 51 on one surface layer of a transparent film substrate 59 and a second adhesive layer 53 on the other surface layer of the transparent film substrate 59. Release films 21, 23 are temporarily adhered to the surfaces of the adhesive layers 51, 53.

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 viewing side surface of an image display panel 10 using a double-sided adhesive sheet 15 with a substrate. In the image display device 206, a first adhesive layer 51 is attached to the front transparent plate 7, and a second adhesive layer 53 is attached to the polarizing plate 3 of the image display panel 10.

A transparent film substrate 59 of the double-sided adhesive sheet 15 attached to the substrate is used as a transparent resin film. The total light transmittance of the transparent film substrate 59 is preferably 85% or more , and more preferably 90% or more. The resin material constituting the film substrate is not particularly limited as long as it is transparent, and examples thereof include: polyesters such as polyethylene terephthalate and polyethylene naphthalate; polyolefins such as polyethylene and polypropylene; cyclic polyolefins such as norolefin polymers; cellulose polymers such as diacetyl cellulose and triacetyl cellulose; acrylic polymers; styrene polymers; polycarbonate, polyamide, polyimide, polyetheretherketone, etc.

The thickness of the transparent film substrate 59 is preferably about 15 to 150 μm, more preferably 25 to 120 μm, and further preferably 35 to 100 μm. From the viewpoint of suppressing the coloring of the rainbow pattern (iris phenomenon) when viewing the screen of the image display device, the transparent film substrate 59 is preferably optically isotropic. The in-plane phase 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 adhesive sheet 15 with a substrate preferably has step absorption in addition to adhesion retention, dimensional stability and impact resistance when attached to the front transparent plate. Therefore, it is preferred to use an adhesive sheet 5 having the above-mentioned characteristics as the first adhesive layer 51. From the perspective of ensuring step absorption and impact resistance, the thickness of the first adhesive layer 51 is preferably 30 μm or more, more preferably 40 μm or more, and further preferably 50 μm or more. On the other hand, from the perspective of productivity, the thickness of the first adhesive layer 51 is preferably 500 μm or less, more preferably 300 μm or less, and further preferably 250 μm or less.

The adhesive of the second adhesive layer 53 that constitutes the transparent film substrate 59 and is disposed on the image display panel 10 side is not particularly limited as long as it is a transparent adhesive. The adhesive of the first adhesive layer 51 and the adhesive of the second adhesive layer 53 may be the same or different. From the viewpoint of improving the adhesion retention, dimensional stability and impact resistance, as the second adhesive layer 53, an adhesive sheet 5 having the above-mentioned characteristics may be used in the same manner as the first adhesive layer 51.

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

The thickness of the second adhesive layer 53 is preferably smaller than the thickness of the first adhesive layer 51. The thickness of the second adhesive layer 53 is preferably 0.2 to 0.85 times the thickness of the first adhesive layer 51, more preferably 0.3 to 0.8 times, and further preferably 0.4 to 0.75 times.

In addition to the form shown in FIG5 in which the release films 21, 23 are temporarily adhered to the adhesive layers 51, 53, the double-sided adhesive sheet 15 with a substrate can also be used in the form of an adhesive film with an optical film or the like fixed to the second adhesive layer 53. Also, similar to the form shown in FIG4, it can also be used in the form of an optical film with double-sided adhesives in which an adhesive sheet is further provided on an optical film (polarizing plate) attached to the second adhesive layer. [Example]

The present invention is described in more detail with the following embodiments and comparative examples, but the present invention is not limited to these embodiments.

[Preparation of acrylic oligomer] 60 parts by weight of dicyclopentyl 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 were mixed and stirred at 70°C for 1 hour in a nitrogen environment. Then, 0.2 parts by weight of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator was added, and after reacting at 70°C for 2 hours, the temperature was raised to 80°C and reacted for 2 hours. Thereafter, the reaction solution was heated to 130°C, and toluene, chain transfer agent, and unreacted monomers were dried and removed to obtain a solid acrylic oligomer. The weight average molecular weight of the acrylic oligomer is 5100.

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

(Preparation of photocurable adhesive composition) To the above prepolymer composition, add 3 parts by weight of NVP and 8 parts by weight of 4HBA as monofunctional monomers; 2 parts by weight of polyester diacrylate urethane ("Artresin UN-350" manufactured by Negami Industries) as (meth) urethane; 5 parts by weight of the above acrylic oligomer; 0.05 parts by weight of "Irgacure 184" and 0.55 parts by weight of "Irgacure 651" as photopolymerization initiators; 1.5 parts by weight of α-methylstyrene dimer ("NOFMER" manufactured by NOF Corporation) as chain transfer agent. MSD): 0.2 parts by weight; and "KBM-403" manufactured by Shin-Etsu Chemical Co., Ltd. as a silane coupling agent: 0.3 parts by weight, and the mixtures were uniformly mixed to prepare an adhesive composition.

(Preparation of adhesive sheet) A 75 μm thick polyethylene terephthalate (PET) film ("DIAFOIL MRF75" manufactured by Mitsubishi Chemical Corporation) with a silicone release layer on the surface was used as a base (also serving as a heavy release film), and the above-mentioned photocurable adhesive composition was applied to the base to a thickness of 150 μm to form a coating layer. A 75 μm thick PET film ("DIAFOIL MRE75" manufactured by Mitsubishi Chemical Corporation) with a silicone release treatment on one side was attached to the coating layer as a cover sheet (also serving as a light release film). The laminate was photocured by irradiating ultraviolet light from the cover sheet side using a black light lamp whose position was adjusted so that the irradiation intensity of the irradiation surface directly below the lamp became 5 mW/ ^cm2 , thereby obtaining an adhesive sheet with a thickness of 150 μm and a polymerization rate of 99%.

[Examples 2 to 16, Comparative Examples 1 to 6] The monomer composition added in the polymerization of the prepolymer, the type and amount of the monofunctional monomer and multifunctional compound (urethane acrylate and/or multifunctional acrylate) added to the adhesive composition, and the amount of the chain transfer agent added were changed as shown in Tables 1 and 2. In addition, a photocurable adhesive composition was prepared in the same manner as in Example 1, applied to a substrate and photocured to obtain an adhesive sheet. Furthermore, in the composition added later, the photopolymerization initiator ("Irgacure 184": 0.05 parts by weight, and "Irgacure 651": 0.55 parts by weight) and the silane coupling agent ("KBM-403": 0.3 parts by weight) are the same in all embodiments and comparative examples, so the description of these components is omitted in Tables 1 and 2.

[Evaluation] <Gel fraction> About 0.2 g of adhesive was scraped from the adhesive sheet and wrapped with a porous polytetrafluoroethylene film ("NTF-1122" manufactured by Nitto Denko Corporation) cut into a size of 100 mm × 100 mm and with a pore size of 0.2 μm, and the package was tied with a kite string. The weight of the adhesive sample was calculated by subtracting the weight of the porous polytetrafluoroethylene film and the kite string measured in advance from the weight of the sample (A). The adhesive sample wrapped in the porous polytetrafluoroethylene film was immersed in about 50 mL of ethyl acetate at 23°C for 7 days to allow the gel component of the adhesive to dissolve out of the porous polytetrafluoroethylene film. After immersion, the adhesive wrapped by the porous polytetrafluoroethylene film is taken out, dried at 130°C for 2 hours, and cooled for about 20 minutes before measuring the dry weight (C). The gel fraction of the adhesive is calculated by the following formula. Gel fraction (%) = 100 × (C-A) / B

<Weight average molecular weight of gel component> About 0.2 g of the adhesive was scraped from the adhesive sheet and immersed in a 10 mM tetrahydrofuran phosphate solution for 12 hours to extract the gel component. Taking into account the gel fraction of the adhesive, the amount of tetrahydrofuran phosphate solution was adjusted so that the gel component content of the solution after extraction would be 0.1% by weight. The filtrate obtained by filtering the solution after extraction using a 0.45 μm membrane filter was used as a sample and GPC analysis was performed using a GPC (gel permeation chromatography) device manufactured by Tosoh (product name "HLC-8120GPC") under the following conditions to calculate the weight average molecular weight Mw of the gel component. (Measurement conditions) Column: G7000HXL＋GMHXL＋GMHXL manufactured by Tosoh Corporation Column size: 7.8 mm each ×30 cm (total column length: 90 cm) Column temperature: 40℃ Flow rate: 0.8 mL/min Injection volume: 100 μL Eluent: Tetrahydrofuran Detector: Differential refractometer (RI) Standard sample: Polystyrene

<Storage modulus, loss tangent and glass transition temperature of adhesive sheets> Ten adhesive sheets were stacked to make a sample with a thickness of about 1.5 mm as the measurement sample. The dynamic viscoelasticity measurement was performed using the "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific under the following conditions. (Measurement conditions) Deformation mode: Torsion Measurement frequency: 1 Hz Heating rate: 5°C/min Shape: Parallel plate 7.9 mm

Based on the measurement results, the shear storage modulus _{G'25 ℃} at 25℃ and the loss tangent _{tanδ70 ℃} at 70℃ are read. In addition, the temperature (peak temperature) at which the loss tangent (tanδ) becomes extremely large is taken as the glass transition temperature of the adhesive sheet.

<Adhesion force> After peeling off the light peel film self-adhesive sheet, a 50 μm thick PET film was laminated and cut into 10 mm wide x 100 mm long. Then the heavy peel film was peeled off and pressed onto a glass plate using a 5 kg roller to prepare a specimen for adhesion force measurement. After keeping the specimen for adhesion force measurement at 25°C for 30 minutes, the specimen was peeled off from the glass plate using a tensile tester at a tensile speed of 300 mm/min and a peeling angle of 180°, and the peeling force was measured.

<Haze> The haze was measured using a haze meter ("HM-150" manufactured by Murakami Color Technology Laboratory) using a test piece obtained by laminating the adhesive sheet to an alkali-free glass (total light transmittance 92%, haze 0.4%) with a thickness of 800 μm. The haze of the adhesive sheet was obtained by subtracting the haze of the alkali-free glass (0.4%) from the measured value.

<Gradual absorption> The adhesive sheet was cut into a size of 75 mm × 45 mm, and the light peeling film was peeled off from the adhesive sheet and then laminated to the center of a 125 μm thick PET film cut into 100 mm × 50 mm by a roller laminator (roller pressure: 0.2 MPa, feed speed: 100 mm/min). After that, the heavy peeling film was peeled off and laminated to a 500 μm thick glass plate (100 mm × 50 mm) with black ink (printing thickness: 25 μm or 40 μm) printed in a frame shape on the periphery by a roller laminator (roller pressure: 0.2 MPa, feed speed: 100 mm/min). The ink printing area of the glass plate is 5 mm from both ends in the short side direction and 15 mm from both ends in the long side direction. The black ink layer is in contact with the area 5 mm from the ends of the four sides of the adhesive sheet. After the sample was treated in an autoclave (50°C, 0.5 MPa) for 30 minutes, the boundary of the black ink printing area was observed using a digital microscope with a magnification of 20 times to confirm the presence of bubbles. For each sample with 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 observed throughout the entire circumference ○: Bubbles were observed in one of the four corners, but no bubbles were observed on all four sides ×: Bubbles were observed in more than two of the four corners or more than one of the four sides

<Processability> The light release film was peeled off from the adhesive sheet and attached to a 100 μm thick PET film ("COSMOSHINE A4100" manufactured by Toyobo). The sample was punched out from the side of the PET film using a press to prepare a sample for processability evaluation. The sample was placed in an environment with a temperature of 23°C and a relative humidity of 50% for 1 week, and then the heavy release film was peeled off and the presence of missing adhesive was visually observed. The sample with no missing adhesive was marked as ○, and the sample with missing adhesive was marked as ×.

<Interlayer Adhesion> (Preparation of Test Samples) The adhesive sheet was cut into a size of 75 mm × 45 mm, and the light peeling film was peeled off from the adhesive sheet and then bonded to the center of a 500 μm thick glass plate (100 mm × 50 mm) by a roller laminating machine (roller inter-roller pressure: 0.2 MPa, feed speed: 100 mm/min). After that, the heavy peeling film was peeled off and bonded to a 500 μm thick glass plate (50 mm × 100 mm) with a 30 μm thick black ink printed in a frame shape on the periphery by vacuum pressing (surface pressure 0.3 MPa, pressure 100 Pa). The ink printing area of the glass plate is 5 mm from both ends in the short side direction and 15 mm from both ends in the long side direction. The black ink layer is in contact with the area 5 mm from the ends of the four sides of the adhesive sheet. The sample is treated in an autoclave (50°C, 0.5 MPa) for 30 minutes.

After the above sample was kept at 60°C for 30 minutes, as shown in Figure 7A, a 200 μm thick polystyrene sheet was inserted between two glass plates from the end of the adhesive sheet to a distance of 1 mm and kept for 10 seconds. The end of the adhesive sheet was observed using a digital microscope with a magnification of 20 times. The sample with strip-shaped bubbles (see Figure 7B) or the adhesive sheet peeling off the glass plate was marked as ×, and the sample with no bubbles or peeling was marked as ○.

<Impact resistance> The size of the glass plate without the black ink printing layer was changed to 100 mm × 70 mm. In the same manner as the preparation of the above-mentioned interlayer adhesion test sample, the glass plates were bonded to both sides of the adhesive sheet and autoclaved to prepare the test sample. As shown in FIG8 , the two ends of the short side direction of the test sample 95 were placed on a table 93 arranged at a distance of 60 mm in such a manner that the glass plate 7 with the printing layer 76 was on the lower side, and the upper surface of the end of the glass plate 8 without the printing layer was fixed to the table 93 with an adhesive tape (not shown). The test sample 95 fixed on the table 93 with an adhesive tape was kept in an environment of -5°C for 24 hours, then taken out and placed at room temperature. Within 40 seconds, a metal ball 97 with a mass of 11 g was dropped from a height of 300 mm onto the glass plate 7 to conduct an impact resistance test.

In the impact resistance test, in order to fix the falling position of the metal ball, a cylindrical guide 99 was used to drop the metal ball 97 to a position 10 mm apart in the short and long directions from the corner of the inner edge of the frame of the printing area of the printing layer 76. The test was performed twice, and the case where the glass plate was not peeled off in any of the tests was rated as ○, and the case where the glass plate was peeled off once or twice in the two tests was rated as ×.

[Evaluation results] The preparation of the adhesive composition used to prepare each adhesive sheet and the evaluation results of the adhesive sheet are shown in Tables 1 and 2. In Tables 1 and 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

<Diacrylate Urethane> UN-350: "Artresin UN-350" manufactured by Negami Industries (polyester diacrylate urethane with a weight average molecular weight of about 12,500) UN-350ND: "Artresin UN-350NDTN011" manufactured by Negami Industries (polyester diacrylate urethane with a weight average molecular weight of about 7,600) UN-350MU: "Artresin UN-350MU" manufactured by Negami Industries (polyester diacrylate urethane with a weight average molecular weight of about 25,000) UV-3305B: "Ultraviolet UV-3305B" manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd. (polyether diacrylate urethane with a weight average molecular weight of about 12,000) UT-6957: "Ultraviolet UV-3305B" manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd. UT6957" (polyether diacrylate urethane with a weight average molecular weight of about 15,000) UV-3010B: "UV-3010B" manufactured by Nippon Synthetic Chemical Industry Co., Ltd. (polyester diacrylate urethane with a weight average molecular weight of about 11,000) ＜Monoacrylate urethane＞ UA-2334: "NE OLIGO UA-2334PHB" manufactured by Shin-Nakamura Chemical Industry Co., Ltd. (polyether monoacrylate urethane with a weight average molecular weight of about 20,000) ＜Multifunctional acrylate＞ HDDA: Hexanediol diacrylate

[Table 1] Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Embodiment 5 Embodiment 6 Embodiment 7 Embodiment 8 Embodiment 9 Embodiment 10 Embodiment 11 Embodiment 12 Embodiment 13 Composition Prepolymer composition 2EHA - - - - - - - - - - - - - BA 78 78 78 78 78 78 78 78 78 78 78 78 78 CHA - - - - - - - - - - - - - NVP 16 16 16 16 16 16 16 16 16 16 16 16 16 4HBA 6 6 6 6 6 6 6 6 6 6 6 6 6 Added later Monofunctional monomer 2EHA - - - - - - - - - - - - - NVP 3 3 3 3 3 3 3 3 3 3 3 3 3 4HBA 8 8 8 8 8 8 8 8 8 8 8 8 8 Urethane diacrylate UN-350 2 2 2 2 5 4 1 1 - - - - - UN-350ND - - - - - - - - 4 2 1 - - UN-350MU - - - - - - - - - - - - - UV-3305B - - - - - - - - - - - - - UT-6957 - - - - - - - - - - - 5 3 Urethane Monoacrylate UA-2234 - - - - - - - - - - - - - Multifunctional acrylate HDDA - - - - - - - 0.02 - - - - - Acrylic oligomer 5 5 5 5 5 5 5 5 5 5 5 5 5 Chain transfer agent NOFMER MSD 0.20 0.10 0.05 0.03 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 Reviews Tg(℃) -5.2 -5 -4.5 -3.5 -5.7 -5.5 -4.7 -5.6 -6.2 -5.6 -5.1 -4 -3.2 G' _{25 ℃} (kPa) 200 210 190 200 240 230 220 190 210 200 200 210 200 tanδ _{70 ℃} 0.67 0.60 0.45 0.38 0.36 0.46 0.66 0.61 0.28 0.47 0.56 0.46 0.56 Adhesion force (N/10 mm) 12.3 11.4 8.7 8.0 8.7 9.4 11.3 8.7 10.8 11.5 12.0 8.9 11.8 Gel fraction (%) 44 52 58 67 67 63 47 64 68 54 40 61 42 Gel component Mw (×10 ⁴ ) 20 36 40 40 18 18 20 19 17 18 20 17 18 Fog(%) 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.3 Step Absorption 40 μm ◎ ◎ ◎ ○ ○ ◎ ◎ ◎ ○ ◎ ◎ ◎ ◎ 25 μm ◎ ◎ ◎ ○ ○ ◎ ◎ ◎ ○ ◎ ◎ ◎ ◎ Layer indirectness ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Processability ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Drop impact durability ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○

[Table 2] Embodiment 14 Embodiment 15 Embodiment 16 Comparison Example 1 Comparison Example 2 Comparison Example 3 Comparison Example 4 Comparison Example 5 Comparison Example 6 Composition Prepolymer composition 2EHA - - - - - - - - - BA 78 78 67 78 78 78 78 78 67 CHA - - 14 - - - - - 14 NVP 16 16 - 16 16 16 16 16 - 4HBA 6 6 9 6 6 6 6 6 9 Added later Monofunctional monomer 2EHA 10 10 - - - - - - - NVP 3 3 12 3 3 3 3 3 12 4HBA - - - 8 8 8 8 8 - Urethane diacrylate UN-350 5 3 4 2 0.5 - - - 5 UN-350ND - - - - - - - - - UN-350MU - - - - - - 2 - - UV-3305B - - 1 - - - - - 5 UT-6957 - - - - - - - - - Urethane Monoacrylate UA-2234 - - - - - - - 15 - Multifunctional acrylate HDDA - - - - - 0.1 - - - Acrylic oligomer 5 5 5 5 5 5 5 5 5 Chain transfer agent NOFMER MSD 0.20 0.20 0.20 - 0.20 0.10 0.10 0.10 0.20 Reviews Tg(℃) -6 -5 -4.7 -2.1 -3.5 -4.2 -4.4 -4.7 -5 G' _{25 ℃} (kPa) 200 200 160 200 210 210 220 130 200 tanδ _{70 ℃} 0.48 0.69 0.31 0.20 0.71 0.30 0.56 0.90 0.10 Adhesion force (N/10 mm) 10.5 14.8 14.4 6.4 11.2 5.2 11.7 24.8 10.0 Gel fraction (%) 62 40 65 82 13 83 26 20 81 Gel component Mw (×10 ⁴ ) 18 20 17 43 twenty one 34 36 38 17 Fog(%) 0.4 0.3 0.3 0.2 0.2 0.2 1.8 0.2 0.4 Step Absorption 40 μm ◎ ◎ ○ × ◎ × ◎ ◎ × 25 μm ◎ ◎ ○ ○ ◎ ○ ◎ ◎ × Layer indirectness ○ ○ ○ ○ ○ ○ × × ○ Processability ○ ○ ○ ○ × ○ × × ○ Drop impact durability ○ ○ ○ × ○ ○ ○ ○ ○

As shown in Table 1 and Table 2, the adhesives of the embodiments have excellent adhesion and processability, and also have excellent step absorption and drop impact durability.

In Examples 1 to 4, as the amount of the chain transfer agent added later decreases, the gel fraction and the molecular weight of the gel component increase, and tanδ _{70 °C} decreases. In Example 4, in which the amount of the chain transfer agent added is 0.03 parts by weight, the step absorption is reduced compared to the other examples. In Comparative Example 1, in which no chain transfer agent is added, the gel fraction exceeds 80, the glass transition temperature is high, and the step absorption and drop impact resistance are reduced.

In Example 5 and Example 6, in which the amount of urethane diacrylate added was larger than that in Example 1, the gel fraction increased and tanδ _{70 ℃} decreased. In the comparison between Example 12 and Example 13, in which the type of urethane diacrylate was changed, the gel fraction increased and tanδ _{70 ℃} decreased as the amount of urethane diacrylate added increased. The same tendency was also found in the comparison between Example 16 and Comparative Example 6, in which the polymer composition was different, that in Comparative Example 6, in which the amount of urethane diacrylate added was larger, the step absorption was poor.

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

In Example 8, in which a multifunctional acrylate is used as a multifunctional monomer in addition to diacrylate urethane, the same excellent properties as the other examples are shown. On the other hand, in Comparative Example 3, in which diacrylate urethane is not used but only a multifunctional acrylate is used as a multifunctional monomer, the gel fraction is greatly increased, tanδ _{70 ℃} is reduced, and the step absorption is poor. In Comparative Example 5, in which monoacrylate urethane is used as a post-addition component, although the amount of urethane acrylate used is large, an appropriate cross-linking structure is not formed, so the gel fraction and storage modulus are low, and the interlayer adhesion and processability are poor.

When comparing Examples 9, 10 and 11 using relatively low molecular weight urethane diacrylates with Examples 6, 1 and 7 using relatively high molecular weight urethane diacrylates, it is found that the tanδ _{70 °C} tends to decrease for urethane diacrylates with smaller molecular weights. Furthermore, in Comparative Example 4 using urethane diacrylates with larger molecular weights, interlayer adhesion and processability decrease. In Comparative Example 4, the haze of the adhesive sheet increases, and the transparency is poor. It is believed that the decrease in transparency is caused by the decrease in compatibility between the main chain structure of the base polymer and the urethane chain segments forming the cross-linking structure.

According to the results of the above examples and comparative examples, by adjusting the composition and cross-linking structure of the base polymer, setting the gel fraction, the shear storage modulus G' _{25 ℃} at room temperature, and the loss tangent tanδ _{70 ℃} at high temperature to specific ranges, an adhesive sheet having both step absorption and impact resistance can be obtained.

3: Polarizing plate 4: Adhesive sheet 5: Adhesive sheet 6: Image display unit 7: Front transparent plate 8: Glass plate 9: Housing 10: Image display panel 15: Double-sided adhesive sheet 21: Release film 22: Release film 23: Release film 24: Release film 51: Adhesive sheet 53: Adhesive sheet 59: Transparent film substrate 71: Transparent plate 76: Printing layer 90: Gap 93: Table 95: Test sample 97: Metal ball 99: Guide 202: Image display device 206: Image display device

FIG. 1 is a cross-sectional view showing a configuration example of an adhesive sheet with a release film (a double-sided adhesive sheet without a substrate). FIG. 2 is a cross-sectional view showing a configuration example of an image display device. FIG. 3 is a cross-sectional view showing a configuration example of an optical film with an adhesive sheet. FIG. 4 is a cross-sectional view showing a configuration example of an optical film with an adhesive sheet. FIG. 5 is a cross-sectional view showing a configuration example of an adhesive sheet with a release film (a double-sided adhesive sheet with a substrate). FIG. 6 is a cross-sectional view showing a configuration example of an image display device. FIG. 7A is a photograph showing the situation of a layer-to-layer adhesion test. FIG. 7B is an observation photograph of a sample in which stripe-shaped bubbles are generated in a layer-to-layer adhesion test. FIG. 8 is a schematic diagram showing the arrangement of samples in the impact resistance test.

5: Adhesive sheet

21: Release film

22: Release film

Claims (9)

An adhesive sheet is formed into a sheet by an adhesive agent including an acrylic base polymer having a crosslinking structure and an acrylic oligomer having a weight average molecular weight of 1000-30000, wherein the acrylic base polymer having a crosslinking structure is a polymer having a crosslinking structure formed by a urethane chain segment having a weight average molecular weight of 5000-20000 introduced into the acrylic polymer chain, wherein the amount of (meth) acrylate in the acrylic polymer chain is 50% by weight or more relative to the total amount of constituent monomer components, and wherein the acrylic oligomer contains at least one monomer selected from the group consisting of a hydroxyl-containing monomer, a carboxyl-containing monomer and a nitrogen-containing monomer as a constituent monomer component, wherein the amount of (meth) alkyl ester in the acrylic polymer chain is 50% by weight or more relative to the total amount of constituent monomer components. The amount of (meth) acrylic acid alkyl ester with alicyclic alkyl group is 10-90% by weight, the content of the acrylic oligomer in the adhesive is 0.5-20 parts by weight relative to 100 parts by weight of the acrylic base polymer, the weight average molecular weight of the gel component is 150,000-450,000, the gel component is the soluble component obtained by extracting the adhesive using tetrahydrofuran, the shear storage modulus at 25°C is above 0.16MPa, the loss tangent at 70°C is above 0.25, the glass transition temperature is below -3°C, the gel fraction is 30-80%, and the gel fraction is the weight fraction of the insoluble component after the adhesive is immersed in ethyl acetate at 23°C for 7 days relative to the sample before immersion. For the adhesive sheet in claim 1, the haze is less than 1%. For adhesive sheets in claim 1 or 2, the polymerization rate is above 95%. As in the adhesive sheet of claim 1 or 2, in which the total amount of hydroxyl-containing monomers and nitrogen-containing monomers in the acrylic polymer chain relative to the total amount of the constituent monomer components is 15-45% by weight. As in the adhesive sheet of claim 1 or 2, wherein the urethane chain segment is polyester urethane. As in the adhesive sheet of claim 1 or 2, the content of the urethane chain segment in the above-mentioned acrylic-based polymer with a cross-linked structure is 0.3 to 10 parts by weight relative to 100 parts by weight of the acrylic polymer chain. As in claim 1 or 2, the adhesive sheet, wherein the acrylic-based polymer having a cross-linked structure is a copolymer of an alkyl (meth)acrylate and a di(meth)acrylate urethane having a terminal (meth)acrylic group and a weight average molecular weight of 5000 to 20000. A method for producing an adhesive sheet as claimed in any one of claims 1 to 7, wherein a composition comprising an acrylic monomer and/or a partial polymer thereof, a di(meth)acrylic urethane having (meth)acrylic groups at both ends and a weight average molecular weight of 5000 to 20000, and an acrylic oligomer having an alicyclic alkyl group in an amount of 10 to 90% by weight relative to the total amount of the monomer components and a weight average molecular weight of 1000 to 30000 is applied in a layer on a substrate, and then the composition is irradiated with active light for photocuring, wherein the content of the acrylic oligomer in the composition is 0.5 to 20 parts by weight relative to 100 parts by weight of the total amount of the acrylic monomer and/or a partial polymer thereof. An image display device, which fixes a front transparent component to the viewing side surface of an image display panel by an adhesive sheet as described in any one of claims 1 to 7.