TWI885051B - Adhesive sheet and image display device - Google Patents

Abstract

The present invention relates to an adhesive sheet and an image display device. The adhesive sheet (15) has a first adhesive layer (51) on one main surface of a transparent film substrate (59), and has a second adhesive layer (53) on the other main surface. The thickness of the first adhesive layer is greater than the thickness of the second adhesive layer, and in the formation of the image display device, the first adhesive layer is disposed in contact with the front transparent plate, and the second adhesive layer is disposed in contact with the image display panel.

Description

Adhesive sheet and image display device

The present invention relates to a double-sided adhesive sheet and an image display device for forming an image display device.

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 devices, personal computer displays, and televisions. In order to prevent damage to the image display panel caused by impact from the outer surface, a transparent front plate such as a transparent resin plate or a glass plate (also called a "cover window") is sometimes provided on the visual side of the image display panel.

Sometimes a colored layer (decorative printed layer) for decoration and light shielding is formed around the front transparent plate. When the adhesive is attached to a transparent member with a decorative printed layer, bubbles are easily generated around the printed step portion. Therefore, the following method is adopted: a thick adhesive sheet is provided with step absorption to suppress undesirable conditions such as bubble mixing (for example, Patent Document 1). As the brightness of the display device increases, there is a tendency to increase the thickness of the decorative printed layer in order to improve the light shielding property. As a result, thicker and softer adhesive sheets are gradually used in the attachment of the front transparent plate.

Patent document 2 proposes the following solution: as an adhesive sheet for bonding a cover window to a polarizing plate of a liquid crystal panel having an embedded or surface-embedded touch sensor, an adhesive sheet with a substrate film is used, which is formed by using an adhesive layer with a specified thickness on both sides of the substrate film.

Previously, the mainstream image display device was a liquid crystal display device, which is a device that places polarizing plates on the front and back of a liquid crystal unit formed by sandwiching a liquid crystal layer between two glass substrates, and is combined with a light source such as a backlight. In recent years, organic EL display devices, which are formed by setting electrodes and a light-emitting layer on a resin film substrate such as a polyimide film, have become practical. Organic EL is a self-luminous type and does not require a light source, so it can achieve thinner, curved, and softer screens.

Existing technical literature Patent Literature

[Patent Document 1] International Publication No. 2013/161666

[Patent Document 2] Japanese Patent Publication No. 2017-160416

In the formation of image display devices, sometimes a pressurization process such as heat pressing or compression welding is performed when or after the image display panel and the front transparent plate are bonded. In image display panels using rigid substrates such as glass substrates, since the substrate is not easily deformed, the image display panel will hardly deform even when subjected to local pressure. On the other hand, in image display panels using soft substrates, if a large pressure is applied locally to the connection parts of the parts during the pressurization process, the substrate will deform locally. If the local deformation does not recover after the pressure is released and a "press mark" remains, it may become a defect in the image display.

By using a double-sided adhesive sheet with a substrate in which adhesive layers are provided on both sides of a film substrate in laminating the image display panel and the front transparent plate, it is possible to suppress the pressure marks caused by deformation of the image display panel caused by local pressure from the back side.

The adhesive sheet of the present invention is a double-sided adhesive sheet with a substrate having a first adhesive layer on one main surface of a transparent film substrate and a second adhesive layer on the other main surface. The thickness of the first adhesive layer is greater than the thickness of the second adhesive layer.

The thickness of the first adhesive layer is preferably 80 μm or more, and the thickness of the second adhesive layer is preferably 150 μm or less. The thickness of the transparent film substrate is preferably 15 μm to 150 μm. The front retardation of the transparent film substrate is preferably 50 nm or less.

The double-sided adhesive sheet with a substrate can be used to form an image display device in which a front transparent plate is arranged on the visual side of the image display panel. In the image display device, the first adhesive layer is bonded to the front transparent plate, and the second adhesive layer is bonded to the image display panel. The image display panel may have a polarizing plate on the visual side surface of the image display unit. In this case, the polarizing plate is bonded to the second adhesive layer.

The image display panel may also be an organic EL display device including an organic EL unit. The organic EL unit may also be a unit having an electrode and an organic light-emitting layer on a resin film substrate.

By bonding the image display unit to the front transparent plate via the adhesive sheet with a substrate of the present invention, the residual pressure marks caused by pressing from the back side can be reduced.

3: Optical film (polarizing plate)

4: Adhesive layer

6: Image display unit

7: Front transparent panel

8: Glass plate

9: Reinforcement substrate

10: Image display panel

15: Double-sided adhesive sheet with substrate

21,23: Release film

24: Release film

51,53: Adhesive layer

59: Transparent film substrate

71: Transparent board

76: Printing layer

83: Taiwan

85:Test samples

87:Metal ball

89:Director

91: Main base board

92: Connecting part (connector)

95:FPC

201: Image display device

Figure 1 is a cross-sectional view showing an example of a laminated structure of a double-sided adhesive sheet with a substrate.

FIG2 is a cross-sectional view showing an example of the structure of an image display device.

Figure 3 is a cross-sectional view showing an example of a laminated structure of an optical film with an adhesive sheet.

Figure 4 is a cross-sectional view showing an example of a laminated structure of an optical film with an adhesive sheet.

Figure 5 is a schematic diagram showing the configuration of the samples in the impact resistance test.

FIG1 is a cross-sectional view of a double-sided adhesive sheet with a substrate. The double-sided adhesive sheet with a substrate 15 has a first adhesive layer 51 on one main surface of a transparent film substrate 59 and a second adhesive layer 53 on the other main surface of the transparent film substrate 59. In the form shown in FIG1, release films 21 and 23 are temporarily adhered to the surfaces of the adhesive layers 51 and 53. FIG2 is a cross-sectional view showing an example of the structure of the following image display device, which is formed by fixing the front transparent plate 7 to the visual side surface of the image display panel using the double-sided adhesive sheet with a substrate 15.

[Image Display Panel]

In the image display device 201 shown in FIG. 2 , the image display panel 10 has an image display unit 6 such as a liquid crystal unit or an organic EL unit.

The image display unit 6 has a functional layer for image display and a substrate supporting the functional layer. For example, in a liquid crystal unit, a liquid crystal layer is provided between two transparent substrates to change the orientation state of the liquid crystal and thus change the polarization state of the transmitted light, thereby adjusting the amount of light passing through the polarizing plate. In an organic EL unit, a pair of electrodes is provided on the substrate to adjust the light emission of the organic light-emitting layer provided between the electrodes.

The substrate of the image display unit 6 can be a rigid substrate such as glass, or a flexible substrate such as a resin film. By using a resin film substrate, thinning and weight reduction can be achieved. In addition, if a resin film substrate is used, a curved display or a flexible display can also be formed.

As an image display unit using a film substrate, a top-emitting type or bottom-emitting type organic EL unit can be cited. The top-emitting type organic EL unit has a metal electrode, an organic light-emitting layer, and a transparent electrode on the substrate in sequence, and emits light from the transparent electrode side (the side opposite to the substrate). The bottom-emitting type organic EL unit has a transparent electrode, an organic light-emitting layer, and a metal electrode on the substrate in sequence, and emits light from the substrate side. In addition to having an organic layer that functions as a light-emitting layer itself, the organic light-emitting layer can also have an electron transport layer, a hole transport layer, etc. A transparent substrate is used in a bottom-emitting type organic EL unit. In a top-emitting type organic EL unit, the substrate does not necessarily have to be transparent.

A reinforcing substrate 9 may also be provided on the back side of the image display unit 6 to protect and reinforce the substrate. As the reinforcing substrate 9, a metal plate, a glass plate, a resin film, etc. may be used. The thickness of the reinforcing substrate is, for example, about 10 μm to about 200 μm.

A flexible printed circuit board (FPC) 95 is connected to the outer peripheral end of the image display unit 6. The FPC 95 is bent so as to wrap around the back of the image display unit 6 and is connected to the main substrate 90 disposed on the back of the image display unit. In FIG2 , the FPC is connected to the main substrate by inserting the terminal of the FPC 95 into the connector 91 provided on the main substrate 90. The FPC can also be connected to the main substrate by welding or the like. In addition, the FPC can also be connected to the main substrate via a wire. At the connection part of the FPC, the thickness of the connector and the welding part is sometimes locally larger to form a convex part.

An optical film 3 may be disposed on the visual side surface of the image display unit 6 via an adhesive layer 4. As an example of the optical film 3 disposed on the visual side surface of the image display unit 6, a polarizing plate may be cited. The polarizing plate includes a polarizing element, and a transparent film as a polarizing element protective film is usually laminated on both sides of the polarizing element. The polarizing element protective film on one or both sides of the polarizing element may also be omitted.

The polarizing plate may also have an optical functional film, which is laminated on one or both surfaces of the polarizing element through an appropriate adhesive layer or adhesive layer as needed. Examples of optical functional films include phase difference plates, viewing angle expansion films, viewing angle limiting (anti-obstruction) films, and brightness enhancement films.

When the image display unit 6 is an organic EL unit, since the metal electrode of the organic EL unit has light reflectivity, if external light enters the interior of the organic EL unit, the light will be reflected by the metal electrode, so that the reflected light looks like a mirror from the outside. By configuring a circular polarizer as an optical film 3 on the visual side surface of the organic EL unit, it is possible to prevent the reflected light from being re-emitted to the outside due to the metal electrode, thereby improving the visibility and design of the screen.

The image display panel 10 can also be an image display panel with an embedded or surface-embedded touch sensor assembled with a touch sensor. When the image display panel 10 has a touch sensor function, there is no need to set a touch sensor on the visual side of the image display panel 10. Therefore, in the image display device 201, the front transparent plate 7 is attached to the visual side surface of the image display panel 10 via a double-sided adhesive sheet 15 with a base material.

[Front transparent panel]

The front transparent plate 7 is provided with a printed layer 76 on the periphery of one surface of the transparent plate 71. By configuring the front transparent plate on the visual side surface of the image display panel, it is possible to prevent the image display panel 10 from being damaged by impact from the external surface. In addition, by providing the printed layer 76, the wiring components such as the FPC 95 cannot be visually seen from the outside, thereby improving the design of the image display device.

The transparent plate 71 can be made of, for example, a transparent resin plate such as acrylic resin or polycarbonate resin or a glass plate. The transparent plate 71 can be rigid or flexible. To improve the protection of the image display panel 10, the transparent plate 71 is preferably a rigid substrate, and the thickness of the transparent plate 71 is preferably 200 μm or more, and more preferably 300 μm or more. The thickness of the printed layer 76 is about 10 μm to about 100 μm.

[Double-sided adhesive sheet with substrate]

As shown in FIG. 2 , in the double-sided adhesive sheet 15 with a substrate, the first adhesive layer 51 disposed on one main surface of the transparent film substrate 59 is bonded to the front transparent plate 7, and the second adhesive layer 53 bonded to the other main surface of the transparent film substrate 59 is bonded to the image display panel 10.

The adhesion of the first adhesive layer 51 of the adhesive sheet 15 to the glass is preferably 2N/10mm or more, more preferably 4N/10mm or more, and further preferably 5N/10mm or more. By making the adhesion within the above range, it is possible to prevent the adhesive sheet from being peeled off from the adherend when stress caused by strain or impact caused by falling, etc. is generated. The second adhesive layer 53 is also preferably high in adhesion as the first adhesive layer. The adhesion is obtained by a peeling test with a glass plate as the adherend and a tensile speed of 300mm/minute and a peeling angle of 180°. Unless otherwise specified, the adhesion is a measured value at 25°C.

The total light transmittance of the adhesive sheet 15 is preferably 85% or more, more preferably 90% or more. The haze of the adhesive sheet 15 is preferably 1% or less. The thickness of the adhesive sheet 15 (the total thickness of the transparent film substrate 59 and the adhesive layers 51 and 53) is preferably 120μm to 1000μm, more preferably 150μm to 500μm, and further preferably 180μm to 400μm.

The adhesive sheet 15 preferably has ultraviolet shielding properties. By having ultraviolet shielding properties, the polarizing plate 3 of the image display panel 10 and the organic layer (e.g., organic light-emitting layer) contained in the image display unit 6 can be prevented from being degraded by ultraviolet rays. The transmittance of the adhesive sheet 15 at a wavelength of 380 nm is preferably 20% or less, more preferably 15% or less, and further preferably 10% or less. In order to make the adhesive sheet 15 have ultraviolet shielding properties, it is sufficient to make any one or more of the first adhesive layer 51, the transparent film substrate 59, and the second adhesive layer 53 have ultraviolet shielding properties. For example, by using transparent polyimide, polyethylene naphthalate or other resin materials with ultraviolet shielding properties as the transparent film substrate 59, ultraviolet shielding properties can be imparted. In addition, ultraviolet absorbers can be contained in any one or more of the transparent film substrate 59, adhesive layers 51, 53.

<Transparent film substrate>

烯系聚合物等環狀聚烯烴；二乙醯纖維素、三乙醯纖維素等纖維素系聚合物；丙烯酸系聚合物；苯乙烯系聚合物；聚碳酸酯、聚醯胺、聚醯亞胺、聚醚醚酮等。 As the transparent film substrate 59, a transparent resin film can be used. The total light transmittance of the transparent film substrate 59 is preferably 85% or more, more preferably 90% or more. The resin material constituting the transparent film substrate 59 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 olefin polymers; cellulose polymers such as diacetyl cellulose and triacetyl cellulose; acrylic polymers; styrene polymers; polycarbonate, polyamide, polyimide, polyetheretherketone, etc.

The transparent film substrate 59 may have functional layers such as an easy-to-adhesive layer and an anti-static layer on the surface. The transparent film substrate 59 may have a function as a touch sensor. When the transparent film substrate 59 has a function as a touch sensor, an electrode for position detection is provided on the surface of the film. On the other hand, when the image display panel 10 has a touch sensor function, it is not necessary to provide a touch sensor on the visual side of the image display panel 10. Therefore, the transparent film substrate 59 may not have an electrode for position detection.

By disposing a transparent film substrate between the two adhesive layers 51 and 53, the overall elasticity of the adhesive sheet 15 can be improved, thereby increasing the resilience to deformation. Therefore, even when the image display panel is deformed due to pressure from the back side of the image display panel 10, the plastic deformation of the adhesive sheet 15 is small, and the pressure mark residue can be reduced by the resilience of the film substrate. The thickness of the transparent film substrate 59 is preferably about 15μm to about 150μm, more preferably 25μm to 120μm, and further preferably 35μm to 100μm.

In order to suppress rainbow-like coloring (iris phenomenon) when viewing the screen of the image display device, the transparent film substrate 59 preferably has optical isotropy. The front retardation of the transparent film substrate 59 at a wavelength of 590nm is preferably less than 50nm, more preferably less than 30nm, further preferably less than 10nm, and particularly preferably less than 5nm.

<Adhesive layer>

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

By making the thickness of the first adhesive layer 51 relatively large, there is a tendency to improve the step absorption of the printed layer 76 of the front transparent plate 7. To ensure the step absorption and impact resistance, the thickness of the first adhesive layer 51 is preferably 80μm or more. The thickness of the first adhesive layer 51 can be 100μm or more or 120μm or more. When the first adhesive layer 51 is attached to the front transparent plate with the printed layer 76, the thickness of the first adhesive layer 51 is preferably greater than the thickness of the printed layer 76.

There is no particular upper limit on the thickness of the first adhesive layer 51. From the perspective of productivity and processing dimensional stability, it is preferably less than 500μm, more preferably less than 300μm, and even more preferably less than 250μm.

By making the thickness of the second adhesive layer 53 relatively small, the plastic deformation caused by the pressure from the side of the image display panel 10 is reduced. Even when the image display panel 10 is deformed due to the pressure from the back side, the plastic deformation of the adhesive sheet 15 is small, thereby reducing the residual pressure mark.

To reduce plastic deformation and reduce indentation, the thickness of the second adhesive layer 53 is preferably less than 150μm, and more preferably less than 120μm. The thickness of the second adhesive layer 53 can be less than 100μm or less than 80μm. To provide impact resistance, the thickness of the second adhesive layer 53 is preferably more than 30μm, and more preferably more than 50μm.

When the image display panel is attached to the front transparent plate via a single layer of substrate-free adhesive sheet, when the thickness of the adhesive sheet is small, the step absorption of the printing step of the front transparent plate is insufficient, and when the thickness of the adhesive sheet is large, the plastic deformation of the adhesive sheet is large, so it is easy to leave a pressure mark caused by pressing from the side of the panel. The adhesive sheet with a substrate having a transparent film substrate 59 between the two adhesive layers 51 and 53 can ensure the step absorption by the first adhesive layer 51 with a relatively large thickness. In addition, the second adhesive layer 53 with a relatively small thickness has a smaller plastic deformation. The deformation of the second adhesive layer 53 caused by the pressure from the back side can be easily recovered by the elastic recovery force of the transparent film substrate 59, thereby suppressing the residual pressure mark.

In order to improve the adhesion retention when the adhesive sheet 15 is bonded to the adherend and ensure the processing dimensional stability, the shear storage modulus G'25 _°C of the first adhesive layer 51 at 25°C is preferably above 0.16 MPa, more preferably above 0.18 MPa, further preferably above 0.20 MPa, and particularly preferably above 0.21 MPa.

On the other hand, in order to make the adhesive sheet 15 have an appropriate viscosity to ensure wettability, and have gradient absorbency and cushioning properties against impacts such as falling, the G' _25°C of the first adhesive layer 51 is preferably below 0.5 MPa, more preferably below 0.4 MPa, further preferably below 0.3 MPa, and particularly preferably below 0.28 MPa.

In order to make the adhesive sheet 15 have a step-absorbency, the loss tangent tanδ _70℃ of the first adhesive layer 51 at 70℃ is preferably 0.25 or more, more preferably 0.30 or more, and further preferably 0.35 or more. Tanδ _70℃ can be 0.40 or more, 0.45 or more, 0.50 or more, or 0.55 or more. From the viewpoint of the adhesive retention force, tanδ _70℃ is preferably 1.0 or less, more preferably 0.9 or less, and further preferably 0.85 or less. _{Tanδ 70℃} can be 0.80 or less, 0.75 or less, or 0.70 or less.

The peak value of tanδ of the first adhesive layer 51 is preferably 1.5 or more, more preferably 1.6 or more, and further preferably 1.7 or more. Adhesives with a larger peak value of tanδ tend to have greater adhesive behavior and excellent impact resistance. There is no particular upper limit on the peak value of tanδ of the first adhesive layer 51, and it is usually 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 first adhesive layer 51 is preferably below -3°C, more preferably below -4°C. The glass transition temperature of the first adhesive layer 51 is preferably above -20°C, more preferably above -15°C, and further preferably above -13°C. By keeping the glass transition temperature within the above range, the adhesive has an appropriate viscosity even in a low temperature area, and the tendency of the adherend to be peeled off due to impact such as falling is suppressed.

Shear storage modulus G', loss tangent tanδ and glass transition temperature can be obtained by viscoelastic measurement at a frequency of 1Hz. Tanδ is the ratio of loss modulus G" to storage modulus G', G"/G', and glass transition temperature is the temperature (peak temperature) when tanδ reaches a maximum. Storage modulus G' is equivalent to the part of the material that is stored in the form of elastic energy when it is deformed, and is an indicator of the degree of hardness. The larger the storage modulus, the higher the holding force, and the more it tends to suppress the peeling caused by strain. The loss modulus G" is equivalent to the part of the energy lost due to internal friction when the material is deformed, and it indicates the degree of viscosity. The larger the tanδ, the stronger the viscosity tendency, and the deformation behavior becomes liquid deformation behavior, so there is a tendency for the rebound energy to become smaller.

In order to make G'25 _℃ be 0.16MPa or more to ensure processing stability and at the same time have appropriate softness for imparting graded absorbency, the gel fraction of the adhesive is preferably 30% to 80%, more preferably 35% to 70%. The gel fraction can be 40% or more, and 45% or more, and 65% or less, or 60% or less.

The gel fraction of the adhesive can be obtained as the insoluble component in a solvent such as ethyl acetate. Specifically, the weight fraction (unit: weight %) of the insoluble component of the adhesive after immersion in ethyl acetate for 7 days at 23°C relative to the sample before immersion is obtained. Generally, 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, the type and amount of the crosslinking agent, etc.

The second adhesive layer 53 is not particularly limited as long as it is a transparent adhesive layer having the above-mentioned thickness. The adhesive of the first adhesive layer 51 and the adhesive of the second adhesive layer 53 may be the same or different. In order to improve the adhesion retention, dimensional stability and impact resistance, the shear storage modulus G'25 _℃ at 25℃, the loss tangent tanδ70 _℃ at 70℃, the peak value of tanδ, the glass transition temperature and the gel fraction of the second adhesive layer 53 are preferably within the above-mentioned ranges described for the first adhesive layer 51.

<Composition of adhesive>

The composition of the adhesive constituting the first adhesive layer 51 and the second adhesive layer 53 is not particularly limited, and an adhesive having a base polymer such as acrylic polymer, silicone polymer, polyester, polyurethane, polyamide, polyvinyl ether, vinyl acetate/vinyl chloride copolymer, modified polyolefin, epoxy, fluorine, natural rubber, synthetic rubber, etc. can be appropriately selected. In order to achieve excellent optical transparency, and to exhibit appropriate adhesive properties such as wettability, cohesion and adhesion, and also excellent weather resistance and heat resistance, it is preferred to use an acrylic adhesive containing an acrylic base polymer with a cross-linked structure.

The acrylic-based polymer contains alkyl (meth)acrylate as the main monomer component. In addition, in this specification, "(meth)acrylic acid" refers to acrylic acid and/or methacrylic acid.

As the (meth)acrylic acid alkyl ester, it is preferred to use an (meth)acrylic acid alkyl ester having an alkyl group with a carbon number of 1 to 20. The (meth)acrylic acid alkyl ester may have a branched chain or a cyclic alkyl group.

Specific examples of the (meth)acrylic acid 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, ) Nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, isotridecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, nonadecyl (meth)acrylate, etc.

基酯等具有二環式脂肪族烴環之(甲基)丙烯酸酯；(甲基)丙烯酸二環戊酯、(甲基)丙烯酸二環戊氧基乙酯、(甲基)丙烯酸三環戊酯、(甲基)丙烯酸1-金剛烷基酯、(甲基)丙烯酸2-甲基-2-金剛烷基酯、(甲基)丙烯酸2-乙基-2-金剛烷基酯等具有三個以上脂肪族烴環之(甲基)丙烯酸酯。 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; isobutyl (meth)acrylate;

(meth)acrylates having a dicyclic aliphatic hydrocarbon ring such as (meth)acrylates; (meth)acrylates having three or more aliphatic hydrocarbon rings 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 is preferably 50% by weight or more, more preferably 55% by weight or more, and further preferably 60% by weight or more relative to the total amount of monomer components constituting the acrylic base polymer. In order to make the glass transition temperature (Tg) within an appropriate range, the amount of the (meth)acrylic acid alkyl ester having a carbon number of 4 to 10 chain alkyl group in the acrylic base polymer is preferably 40% by weight or more, more preferably 50% by weight or more, and further preferably 55% by weight or more relative to the total amount of monomer components constituting the acrylic base polymer. Furthermore, the monomer components constituting the acrylic base polymer refer to the monomer components after removing the monomers used to form the cross-linked structure (the multifunctional (meth)acrylate, (meth)acrylic acid urethane, etc. described later) and the crosslinking agent from all the monomer components constituting the polymer.

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

Examples of hydroxyl-containing monomers include (meth)acrylate esters 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, and (meth)acrylate (4-hydroxymethylcyclohexyl)methyl ester. When a cross-linked structure based on a carbamate chain segment is introduced into an acrylic-based polymer, in order to improve the compatibility with the carbamate chain segment and the transparency of the adhesive, the acrylic-based polymer preferably contains a (meth)acrylate ester having a hydroxyalkyl group with 4 to 8 carbon atoms as a constituent monomer component.

By making the acrylic-based polymer have a hydroxyl-containing monomer as a constituent monomer component, the transparency of the adhesive tends to be 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 and the crosslinking chain segment. Therefore, by increasing the ratio of the hydroxyl-containing monomer in the monomer component constituting the acrylic-based polymer, the cohesive force tends to be improved and the G' _25°C tends to increase. The amount of the hydroxyl-containing monomer relative to the total amount of the monomer component constituting the acrylic-based polymer is preferably 5% to 30% by weight, more preferably 8% to 25% by weight, and even more preferably 10% to 20% by weight.

Examples of carboxyl group-containing monomers include acrylic acid monomers such as (meth)acrylic acid, carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, etc.

、乙烯基吡

、乙烯基吡咯、乙烯基咪唑、乙烯基

唑、乙烯基

啉、(甲基)丙烯醯基

啉、N-乙烯基羧酸醯胺類、N-乙烯基己內醯胺等乙烯基系單體；丙烯腈、甲基丙烯腈等含氰基之丙烯酸系單體等。其中，要想由提高凝集力帶來之接著力提高效果較高，較佳為N-乙烯基吡咯啶酮。 The acrylic base polymer may contain a nitrogen-containing monomer as a constituent monomer component. Examples of the nitrogen-containing monomer include N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperidone, and vinylpyrimidine.

, vinyl pyridine

, vinylpyrrole, vinylimidazole, vinyl

Azole, Vinyl

Phytophenone, (meth)acryloyl

Vinyl monomers such as phenoxyethylene, N-vinyl carboxylic acid amides, and N-vinyl caprolactam; acrylic monomers containing cyano groups such as acrylonitrile and methacrylonitrile, etc. Among them, N-vinyl pyrrolidone is preferred in order to achieve a higher effect of improving adhesion by increasing cohesive force.

By making the acrylic-based 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 is improved, G'25 _°C is increased, and the adhesive retention tends to be improved. On the other hand, when the content of highly polar monomers is too large, the glass transition temperature is sometimes increased and the impact resistance is reduced. 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-based polymer 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 is preferably 7 wt % to 30 wt %, more preferably 10 wt % to 25 wt %, and even more preferably 12 wt % to 22 wt % relative to the total amount of monomer components constituting the acrylic base polymer.

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

Among the above-mentioned monomer components of the acrylic base polymer, it is preferred that the content of alkyl (meth)acrylate is the highest. The characteristics of the adhesive are easily affected by the type of the monomer (main monomer) with the highest content among the constituent monomers of the base polymer. For example, when the main monomer is an alkyl (meth)acrylate having a chain alkyl group with a carbon number of 6 or less, there is a tendency for tanδ _70°C to increase and for the step absorption to improve. In particular, when _C4 alkyl acrylates such as butyl acrylate are the main monomers, there is a tendency for tanδ _70°C to increase. Relative to the total amount of monomer components constituting the acrylic base polymer, the amount of alkyl (meth)acrylate having a chain alkyl group with a carbon number of 6 or less is preferably 40% by weight to 85% by weight, more preferably 45% by weight to 80% by weight, and further preferably 50% by weight to 75% by weight. In particular, the content of butyl acrylate as a constituent monomer component is preferably within the above range.

The theoretical Tg of the acrylic base polymer is preferably -50°C or higher. The theoretical Tg of the acrylic base polymer is preferably -10°C or lower, more preferably -20°C or lower, and further preferably -25°C or lower. The theoretical Tg is calculated by the following Fox formula from the glass transition temperature Tg _i of the homopolymer of the monomer components constituting the acrylic base polymer and the weight fraction _Wi of each monomer component.

1/Tg=Σ(W _i /Tg _i )

Tg is the glass transition temperature of the polymer (unit: K), _Wi is the weight fraction of monomer component i constituting the chain segment (copolymerization ratio based on weight), and _Tgi 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.

A polymer having a crosslinked structure introduced therein can be obtained, for example, by the following methods: (1) polymerizing a polymer having a functional group capable of reacting with a crosslinking agent, then adding a crosslinking agent to react the polymer with the crosslinking agent; and (2) introducing a branched structure (crosslinked structure) by making the polymer components of the polymer include a multifunctional compound; etc. These methods can also be used in combination to introduce a variety of crosslinked structures into the base polymer.

唑啉系交聯劑、氮丙啶系交聯劑、碳二醯亞胺系交聯劑、金屬螯合物系交聯劑等。其中，要想與基礎聚合物之羥基、羧基之反應性較高而容易導入交聯結構，較佳為異氰酸酯系交聯劑及環氧系交聯劑。該等交聯劑藉由與導入至基礎聚合物 中之羥基、羧基等官能基反應而形成交聯結構。對於基礎聚合物不含羧基之無酸之黏著劑而言，較佳為使用異氰酸酯系交聯劑，藉由基礎聚合物中之羥基與異氰酸酯交聯劑之反應而形成交聯結構。 Specific examples of the crosslinking agent in the method (1) of reacting the base polymer with the crosslinking agent include isocyanate crosslinking agents, epoxy crosslinking agents,

Azoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, metal chelate crosslinking agents, etc. Among them, isocyanate crosslinking agents and epoxy crosslinking agents are preferred in order to have a high reactivity with the hydroxyl and carboxyl groups of the base polymer and to easily introduce a crosslinking structure. These crosslinking agents react with functional groups such as hydroxyl and carboxyl groups introduced into the base polymer to form a crosslinking structure. For acid-free adhesives whose base polymers do not contain carboxyl groups, it is preferred to use isocyanate crosslinking agents to form a crosslinking structure by the reaction of the hydroxyl groups in the base polymer with the isocyanate crosslinking agent.

As the isocyanate-based crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule can be used. Examples of the isocyanate crosslinking agent include lower 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); HL”), trihydroxymethylpropane adduct of xylylene diisocyanate (e.g., “Takenate D110N” manufactured by Mitsui Chemicals Co., Ltd.), isocyanurate form of hexamethylene diisocyanate (e.g., “Coronate HX” manufactured by Tosoh Corporation), and other isocyanate adducts. In addition, by using a urethane prepolymer having an isocyanate group at the end as an isocyanate-based crosslinking agent, a crosslinking structure based on a urethane-based chain segment can be introduced.

In the above method (2) of including a polyfunctional compound in the polymerization component of the base polymer, the monomer components constituting the acrylic base polymer and the polyfunctional compound used to introduce the cross-linking structure can be reacted in full 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 monomer 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 (main polymerization). The prepolymer composition is a partial polymer containing a polymer with a low degree of polymerization and unreacted monomers.

By prepolymerizing the constituents of the acrylic base polymer, the branching points (crosslinking points) of the multifunctional compound can be uniformly introduced into the base polymer. In addition, the adhesive layer can be formed by coating a mixture of a low molecular weight polymer or a partial polymer and an unpolymerized monomer component (adhesive composition) on a substrate and then performing a main polymerization on the substrate. Since low polymer compositions such as prepolymer compositions have low viscosity and excellent coating properties, the productivity of the adhesive layer can be improved and the thickness of the adhesive layer can be made uniform by performing a main polymerization on the substrate after coating an adhesive composition that is a mixture of a prepolymer composition and a multifunctional compound.

As a polyfunctional compound for introducing a cross-linked structure, a compound containing two or more polymerizable functional groups (ethylenic unsaturated groups) with unsaturated double bonds in one molecule can be cited. As a polyfunctional compound, if copolymerization with the monomer component of the acrylic base polymer is easy, a polyfunctional (meth) acrylate is preferred. When a branched (cross-linked) structure is introduced by active energy line polymerization (photopolymerization), a polyfunctional acrylate is preferred.

Examples of the multifunctional (meth)acrylate include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, bisphenol A ethylene oxide-modified di(meth)acrylate, bisphenol A propylene oxide-modified di(meth)acrylate, alkanediol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, ethoxylated isocyanuric acid tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol Di(meth)acrylate, 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.

By using (meth)acrylic urethane with (meth)acrylic group at the end of urethane chain as multifunctional (meth)acrylate, a cross-linked structure based on urethane chain segment can be introduced. By cross-linking acrylic-based polymer using urethane chain segment, an adhesive having both low glass transition temperature and high adhesive retention can be easily obtained. Urethane chain segment is a molecular chain having urethane bond, and a cross-linked structure based on urethane chain segment is introduced by covalently bonding both ends of urethane chain segment with acrylic-based polymer. Urethane chain segment typically includes a polyurethane chain obtained by reacting diol with diisocyanate.

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 larger the molecular weight of the polyurethane chain in the urethane chain segment, the longer the distance between the crosslinking points. 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 it can take into account both adhesion and step absorption and impact resistance.

When the molecular weight of the polyurethane chain is too small and the distance between crosslinks is short, as the cohesive force increases, tanδ decreases, and there is a tendency for step absorption and impact resistance to decrease. On the other hand, when the molecular weight of the polyurethane chain is too large and the distance between crosslinks is long, the storage modulus is sometimes small and the adhesion retention is insufficient. Even when the molecular weight of the polyurethane chain is large, the gel fraction can be increased by increasing the amount of urethane chain segments, thereby increasing the storage modulus. However, the compatibility of polyurethane chains with large molecular weights and acrylic polymers is low, so sometimes as the amount of urethane chain segments increases, the fogging of the adhesive increases and the transparency decreases.

If the amount of the urethane chain segment becomes too large, the viscosity of the adhesive sometimes decreases with the increase of the gel fraction, thereby reducing the step absorption and impact resistance. In addition, if the amount of the urethane chain segment becomes too large, the transparency of the adhesive sometimes decreases and the haze increases. 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. On the other hand, in order to increase the gel fraction to have 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. Relative to 100 parts by weight of the acrylic polymer, the amount of the urethane chain segment in the base polymer can be less than 4 parts by weight or less than 3 parts by weight, and can be more than 0.7 parts by weight or more than 1 part by weight.

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

The diisocyanate used to form the polyurethane chain can be any of aromatic diisocyanate and aliphatic diisocyanate. As diisocyanate, derivatives of isocyanate compounds can also be used. As derivatives of isocyanate compounds, there can be cited: dimers of polyisocyanate, trimers of isocyanate (isocyanurates), polymeric MDI, adducts with trihydroxymethylpropane, biuret modified products, allophanate modified products, urea modified products, etc. As a diisocyanate component, urethane prepolymers having an isocyanate group at the end can 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 for higher compatibility with acrylic polymers. In particular, when a cross-linked structure based on polyester urethane is introduced, the storage modulus at room temperature tends to increase, and the adhesion retention and processability are improved. One of the reasons is that polyester has a rigid molecular structure compared to polyether. It is believed that if a cross-linked structure is introduced through a rigid chain segment, the movement of the polymer will be restricted, so the storage modulus will increase. On the other hand, since the distance between the cross-linking points of the polymer is maintained, it shows impact resistance and step absorption.

By using a compound having a functional group capable of copolymerizing with a monomer component constituting an acrylic polymer at the end of a polyurethane chain, or a compound having a functional group capable of reacting with a carboxyl group, a hydroxyl group, etc. contained in an acrylic polymer at the end of a polyurethane chain, a cross-linked structure based on a urethane chain segment can be introduced into an acrylic polymer. In order to easily and uniformly introduce cross-linking points into an acrylic polymer and to make the compatibility between the acrylic polymer and the urethane chain segment excellent, it is preferred to use a urethane di(meth)acrylate having (meth)acrylic groups at both ends of a polyurethane chain as the above-mentioned multifunctional (meth)acrylate to introduce a cross-linked structure based on a urethane chain segment.

Urethane di(meth)acrylate having (meth)acrylic groups at both ends can be obtained, for example, by using a (meth)acrylic compound having a hydroxyl group in addition to a diol component in the polymerization of polyurethane. In order to control 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.

A polyurethane chain having an isocyanate group at the end is obtained by reacting a polyol with a polyisocyanate compound in such a way that the polyisocyanate compound becomes excessive. In order to obtain an isocyanate-terminated polyurethane, a diol component and a diisocyanate component are 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 mixing approximately equal amounts of the diol component and the diisocyanate component and reacting them.

Examples of (meth) acrylic compounds having a hydroxyl group include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, hydroxymethacrylamide, hydroxyethylacrylamide, etc.

As the (meth)acrylic urethane, commercial products sold by Arakawa Chemical Industries, Shin-Nakamura Chemical Industries, Toa Gosei, Kyoeisha Chemical, Nippon Kayaku, Nippon Gosei Chemical Industries, Negami Industries, Daicell-Allnex, etc. can be used. The weight average molecular weight of the (meth)acrylic urethane is preferably 5000 to 30000, more preferably 6000 to 23000, and even more preferably 7000 to 20000.

The glass transition temperature of (meth)acrylic urethane is preferably below 0°C, more preferably below -10°C, and further preferably below -20°C. By using (meth)acrylic urethane with low Tg, an adhesive with excellent low-temperature adhesion can be obtained even when a cross-linked structure based on urethane chain segments is introduced to improve the cohesive force of the base polymer. There is no particular restriction on the lower limit of the glass transition temperature of (meth)acrylic urethane. To obtain an adhesive with excellent high-temperature retention, it is preferably above -100°C, more preferably above -90°C, and further preferably above -80°C.

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

<Adhesive composition>

The adhesive composition containing the base polymer is applied to the substrate and the solvent is dried, the base polymer is crosslinked and hardened as needed to form an adhesive layer.

The base polymer can be prepared by known polymerization methods such as solution polymerization, photopolymerization (UV polymerization), bulk polymerization, and emulsion polymerization. Considering the transparency, water resistance, and cost of the adhesive, solution polymerization or photopolymerization is preferred. Since the polymer can be prepared without using a solvent in the case of photopolymerization, there is no need to dry and remove the solvent when forming the adhesive layer, and a thicker adhesive layer can be formed uniformly.

系光聚合起始劑、醯基氧化膦系光聚合起始劑等。作為熱聚合起始劑，可以使用偶氮系起始劑、過氧化物系起始劑、將過氧化物與還原劑組合而成之氧化還原系起始劑(例如，過硫酸鹽與亞硫酸氫鈉之組合、過氧化物與抗壞血酸鈉之組合等)。 When preparing the base polymer, a polymerization initiator such as a photopolymerization initiator or a thermal polymerization initiator can be used according to the type of polymerization reaction. As the photopolymerization initiator, benzoin ether-based photopolymerization initiator, acetophenone-based photopolymerization initiator, α-ketol-based photopolymerization initiator, aromatic sulfonyl chloride-based photopolymerization initiator, photoactive oxime-based photopolymerization initiator, benzoin-based photopolymerization initiator, benzyl-based photopolymerization initiator, benzophenone-based photopolymerization initiator, ketal-based photopolymerization initiator, 9-oxysulfide-based photopolymerization initiator, etc. can be used.

As the thermal polymerization initiator, an azo initiator, a peroxide initiator, a redox initiator in which a peroxide and a reducing agent are combined (for example, a combination of persulfate and sodium bisulfite, a combination of peroxide and sodium ascorbate, etc.) can be used.

In order to adjust the molecular weight of the base polymer, a chain transfer agent can also be used. Examples of chain transfer agents include: α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, hydroxyacetic acid, 2-hydroxyethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, 2,3-dihydroxy-1-propanol and other thiols; α-methylstyrene dimer, etc.

When a multifunctional monomer is used as a monomer component for forming a base polymer in addition to a monofunctional monomer, the monofunctional monomer may be polymerized first to form a prepolymer composition with a low degree of polymerization (prepolymerization), and a multifunctional monomer may be added to the slurry of the prepolymer composition to polymerize the prepolymer and the multifunctional monomer (main polymerization). By performing the prepolymer prepolymerization in this way, the branch points generated by the multifunctional monomer component can be uniformly introduced into the base polymer. Alternatively, a mixture of the prepolymer composition and the unpolymerized monomer component (adhesive composition) may be applied to a substrate, and then main polymerization may be performed on the substrate to form an adhesive layer. Since the prepolymer composition has low viscosity and excellent coating properties, the productivity of the adhesive layer can be improved and the thickness of the adhesive layer can be made uniform by coating a mixture of the prepolymer composition and unpolymerized monomers, i.e., an adhesive composition, and then performing main polymerization on the substrate.

The prepolymer composition can be prepared, for example, by partially polymerizing (prepolymerizing) a composition formed by mixing monomer components constituting an acrylic base polymer with a polymerization initiator (referred to as a "prepolymer forming composition"). Furthermore, the monomer in the prepolymer forming composition is preferably a monofunctional monomer such as an alkyl (meth)acrylate or a polar group-containing monomer in the monomer components constituting an acrylic polymer. The prepolymer forming composition may also contain a polyfunctional monomer. For example, the prepolymer forming composition may contain a portion of the polyfunctional monomer component as a raw material of the base polymer, and after the prepolymer is obtained by polymerization, the remaining polyfunctional monomer component is added and provided for the main polymerization.

In addition to monomers and polymerization initiators, the prepolymer-forming composition may also contain chain transfer agents as needed. There is no particular restriction on the polymerization method of the prepolymer. To adjust the reaction time so that the molecular weight (polymerization rate) of the prepolymer is within the desired range, it is better to perform polymerization by irradiating active light such as UV light. There is no particular restriction on the polymerization initiator and chain transfer agent used for prepolymerization. For example, the above-mentioned photopolymerization initiator and chain transfer agent can be used.

There is no particular restriction on the polymerization rate of the prepolymer. To make the viscosity suitable for coating on the 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 and amount of the photopolymerization initiator, the irradiation intensity and irradiation time of active light such as UV light, etc. The polymerization rate of the prepolymer is calculated by the following formula based on the weight before and after heating at 130°C for 3 hours. The polymerization rate of the adhesive is also calculated by the same method.

Polymerization rate (%) = weight after drying / weight before drying × 100

The remaining monomer components constituting the acrylic-based polymer (post-added monomers) and the polymerization initiator, chain transfer agent, silane coupling agent, crosslinking agent, etc. as required are mixed into the above prepolymer composition to form an adhesive composition. The post-added monomer preferably contains a multifunctional monomer. When using urethane di(meth)acrylate to introduce a crosslinking structure based on a urethane chain segment, it is preferred to add urethane di(meth)acrylate as a post-added monomer.

There is no particular limitation on the photopolymerization initiator and chain transfer agent used in the main polymerization. For example, the above-mentioned photopolymerization initiator and chain transfer agent can be used. In the case where the polymerization initiator during the prepolymerization is not deactivated and remains in the prepolymer composition, the addition of the polymerization initiator used for the main polymerization can be omitted.

In 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.

Chain transfer agents accept free radicals from the growing polymer chain to stop the elongation of the polymer, and after accepting the free radicals, the chain transfer agent will attack the monomer to start the polymerization again. By using chain transfer agents, the increase of 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 that a polymer chain contains crosslinking points (branching points) formed by polyfunctional monomers, so there is a tendency for the gel fraction to increase. By using a chain transfer agent to inhibit the elongation of the polymer, there is a tendency for the molecular weight of the polymer to decrease and the gel fraction to increase. Therefore, by making the adhesive composition contain a chain transfer agent, it is easy to form an adhesive with a large tanδ and excellent step absorption.

(UV absorber)

系紫外線吸收劑、水楊酸酯系紫外線吸收劑、氰基丙烯酸酯系紫外線吸收劑等。要想紫外線吸收性較高且與丙烯酸系聚合物之相容性優異，容易獲得高透明性之丙烯酸系黏著劑，較佳為三

系紫外線吸收劑及苯并三唑系紫外線吸收劑，其中，較佳為含有羥基之三

系紫外線吸收劑、及於一分子中具有一個苯并三唑骨架之苯并三唑系紫外線吸收劑。 In order to make the adhesive layers 51 and 53 have ultraviolet light absorption properties, an ultraviolet light absorber may be used. Examples of the ultraviolet light absorber include benzotriazole ultraviolet light absorbers, benzophenone ultraviolet light absorbers, and tris(III) ultraviolet light absorbers.

UV absorbers, salicylate UV absorbers, cyanoacrylate UV absorbers, etc. To obtain an acrylic adhesive with high UV absorbency and excellent compatibility with acrylic polymers, it is better to use three

UV absorbers and benzotriazole-based UV absorbers, preferably containing hydroxyl tris

A benzotriazole-based ultraviolet absorber having one benzotriazole skeleton in one molecule.

系紫外線吸收劑之 市售品，可以例舉：2-(4,6-雙(2,4-二甲基苯基)-1,3,5-三

-2-基)-5-羥基苯基與[(烷氧基)甲基]環氧乙烷之反應產物(BASF公司製造之「TINUVIN 400」)、2-(2,4-二羥基苯基)-4,6-雙(2,4-二甲基苯基)-1,3,5-三

與縮水甘油酸(2-乙基己基)酯之反應產物(BASF公司製造之「TINUVIN 405」)、2,4-雙(2-羥基-4-丁氧基苯基)-6-(2,4-二丁氧基苯基)-1,3,5-三

(BASF公司製造之「TINUVIN 460」)、2-(4,6-二苯基-1,3,5-三

-2-基)-5-[(己基)氧基]-苯酚(BASF公司製造之「TINUVIN 577」)、2-(2-羥基-4-[1-辛氧基羰基乙氧基]苯基)-4,6-雙(4-苯基苯基)-1,3,5-三

(BASF公司製造之「TINUVIN 479」)、2,4-雙[{4-(4-乙基己氧基)-4-羥基}-苯基]-6-(4-甲氧基苯基)-1,3,5-三

(BASF公司製造之「Tinosorb S」)、2-(4,6-二苯基-1,3,5-三

-2-基)-5-[2-(2-乙基己醯氧基)乙氧基]-苯酚(ADEKA公司製造之「ADK STAB LA-46」)等。 As the ultraviolet absorber, a commercially available product can be used.

It is a commercially available product of ultraviolet absorber, and examples thereof include: 2-(4,6-bis(2,4-dimethylphenyl)-1,3,5-tri

-2-yl)-5-hydroxyphenyl and [(alkoxy)methyl] oxirane reaction product ("TINUVIN 400" manufactured by BASF), 2-(2,4-dihydroxyphenyl)-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine

The reaction product with 2-ethylhexyl glycidate ("TINUVIN 405" manufactured by BASF), 2,4-bis(2-hydroxy-4-butoxyphenyl)-6-(2,4-dibutoxyphenyl)-1,3,5-tri

("TINUVIN 460" manufactured by BASF), 2-(4,6-diphenyl-1,3,5-triphenyl)

-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-tris(2-hydroxy-4-[1-octyloxycarbonylethoxy]phenyl)-

("TINUVIN 479" manufactured by BASF), 2,4-bis[{4-(4-ethylhexyloxy)-4-hydroxy}-phenyl]-6-(4-methoxyphenyl)-1,3,5-triazine

("Tinosorb S" manufactured by BASF), 2-(4,6-diphenyl-1,3,5-triphenyl)

-2-yl)-5-[2-(2-ethylhexanoyloxy)ethoxy]-phenol ("ADK STAB LA-46" manufactured by ADEKA Corporation), etc.

Examples of commercially available benzotriazole-based ultraviolet absorbers include 2-(2H-benzotriazole-2-yl)-6-(1-methyl-1-phenylethyl)-4-(1,1,3,3-tetramethylbutyl)phenol ("TINUVIN 928" manufactured by BASF), 2-(2-hydroxy-5-tert-butylphenyl)-2H-benzotriazole ("TINUVIN PS" manufactured by BASF), 2-(2H-benzotriazole-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol ("TINUVIN 900" manufactured by BASF), 2-(2H-benzotriazole-2-yl)-6-(1-methyl-1-phenylethyl)-4-(1,1,3,3-tetramethylbutyl)phenol ("TINUVIN 928”), 2-(2H-benzotriazol-2-yl)-6-dodecyl-4-methylphenol (“TINUVIN 571” manufactured by BASF), 2-(2H-benzotriazol-2-yl)-p-cresol (“TINUVIN P” manufactured by BASF), 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol (“TINUVIN 234” manufactured by BASF), 2-[5-chloro-(2H)- benzotriazol-2-yl]-4-methyl-6-tert-butylphenol (“TINUVIN 326” manufactured by BASF), 2-(2H-benzotriazol-2-yl)-4,6-di-tert-amylphenol (“TINUVIN 328”), 2-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol (“TINUVIN 329” manufactured by BASF), ester compound of phenylpropionic acid and 3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxy (C7-9 side chain and linear alkyl) (“TINUVIN 384-2” manufactured by BASF), reaction product of methyl 3-(3-(2H-benzotriazol-2-yl)-5-tert-butyl-4-hydroxyphenyl)propionate and polyethylene glycol (“TINUVIN 1130”), the reaction product of methyl 3-(3-(2H-benzotriazol-2-yl)-5-tert-butyl-4-hydroxyphenyl) propionate and polyethylene glycol 300 (“TINUVIN 213” manufactured by BASF), 2-[2-hydroxy-3-(3,4,5,6-tetrahydrophthaliminomethyl)-5-methylphenyl]benzotriazole (“Sumisorb250” manufactured by Sumitomo Chemical Co., Ltd.), etc.

When adding a UV absorber to the adhesive composition, the amount of the UV absorber added is generally about 0.05 to about 10 parts by weight, preferably 0.1 to 5 parts by weight, relative to 100 parts by weight of the base polymer. By setting the content of the UV absorber within the above range, the reduction in transparency caused by the seepage of the UV absorber can be suppressed, and the UV shielding property of the adhesive layer can be improved.

(Oligomer)

The adhesive composition may also contain various oligomers to adjust the adhesive's adhesion or viscosity. As an oligomer, for example, an oligomer with a weight average molecular weight of about 1000 to about 30000 may be used. As an oligomer, if the compatibility with the acrylic base polymer is excellent, an acrylic oligomer is preferred.

The acrylic oligomer contains (meth) alkyl acrylate as the main monomer component. Among them, as the monomer component, it is preferred to include (meth) alkyl acrylate having a chain alkyl group ((meth) alkyl acrylate chain alkyl ester) and (meth) alkyl acrylate having an alicyclic alkyl group ((meth) alkyl acrylate alicyclic alkyl ester). Specific examples of (meth) alkyl acrylate chain alkyl ester and (meth) alkyl acrylate alicyclic alkyl ester are as exemplified above as the monomer components of the acrylic polymer.

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 can be above 60°C, above 80°C, above 100°C, or above 110°C. By introducing a low-Tg acrylic base polymer with a cross-linked structure and a high-Tg acrylic oligomer, the adhesive retention tends to be improved. There is no particular limit on the upper limit of the glass transition temperature of the acrylic oligomer, which is usually 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 order to have a higher 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, as the constituent monomer component of the acrylic oligomer, it is preferred to include at least one selected from the group consisting of dicyclopentyl acrylate, dicyclopentyl methacrylate, cyclohexyl acrylate and cyclohexyl methacrylate, and methyl methacrylate.

Relative to the total amount of monomer components constituting the acrylic oligomer, the amount of (meth) acrylate cyclic alkyl ester is preferably 10% to 90% by weight, more preferably 20% to 80% by weight, and further preferably 30% to 70% by weight. Relative to the total amount of monomer components constituting the acrylic oligomer, the amount of (meth) acrylate chain alkyl ester 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 1000-30000, more preferably 1500-10000, and further preferably 2000-8000. By using an acrylic oligomer having a molecular weight within this range, the adhesion or adhesion retention of the adhesive tends to be improved.

Acrylic oligomers can be obtained by polymerizing the above-mentioned monomer components using various polymerization methods. Various polymerization initiators can also be used during the polymerization of acrylic oligomers. In addition, chain transfer agents can also be used to adjust the molecular weight.

When the adhesive composition contains oligomer components such as acrylic oligomers, the content of the oligomer component 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)

A silane coupling agent may also be added to the adhesive composition to adjust the bonding strength. When a silane coupling agent is added to the adhesive composition, the amount of the silane coupling agent added is generally about 0.01 to about 5.0 parts by weight, preferably about 0.03 to about 2.0 parts by weight, relative to 100 parts by weight of the base polymer.

(Other additives)

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

<Formation of adhesive layer>

When the adhesive composition is photocurable, the adhesive composition is coated on a supporting substrate and then photocured by irradiating ultraviolet rays and/or short-wave visible light to form an adhesive layer. When performing photocuring, it is preferred to attach a cover sheet to the surface of the coating layer and irradiate light with the adhesive composition sandwiched between two cover sheets to prevent inhibition caused by oxygen.

As the substrate and cover sheet used in forming the adhesive layer, any appropriate substrate can be used. The substrate and cover sheet can be a release film having a release layer on the contact surface with the adhesive layer. As the substrate or cover sheet, the transparent film substrate 59 of the adhesive sheet 15 can also be used.

As the coating method of the adhesive composition, various methods can be used, such as roller coating, contact roller coating, gravure coating, reverse coating, roller brushing, spray coating, dip roller coating, rod coating, scraper coating, air knife coating, curtain coating, die lip coating, die nozzle coating machine, etc.

The main polymerization is carried out by irradiating the adhesive composition coated on the substrate in a layer with active light. In the main polymerization, the unreacted monomer components in the prepolymer composition react with polyfunctional compounds such as urethane di(meth)acrylate, thereby obtaining a polymer with a cross-linked structure.

The active light can be selected according to the type of polymerizable component, the type of photopolymerization initiator, etc., and ultraviolet light and/or short-wave visible light are usually used. The cumulative light amount of the irradiation light is preferably about 100mJ/ ^cm2 to about 5000mJ/ ^cm2 . As the light source used for light irradiation, there is no particular limitation as long as it can irradiate light of the wavelength range to which the photopolymerization initiator contained in the adhesive composition is sensitive. LED light sources, high-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halide lamps, xenon lamps, etc. can be preferably used. When the amount of unreacted monomer residues is large, the G' _25℃ of the adhesive layer sometimes decreases and the adhesive retention is reduced. Therefore, the polymerization rate of the adhesive after the main polymerization is preferably 95% or more, more preferably 97% or more, further preferably 98% or more, and particularly preferably 99% or more. In order to increase the polymerization rate, the adhesive layer may be heated to volatilize residual monomers, unreacted polymerization initiators, etc.

As mentioned above, the gel fraction of the adhesive is preferably 30% to 80%, and more preferably 35% to 70%. By making the gel fraction above 30%, the adhesive's bonding strength can be improved, and it is not easy to produce paste defects and misalignment between 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 adhesive sol component is preferably 150,000 to 450,000, and more preferably 180,000 to 420,000. The sol component is a soluble component obtained by extracting the base polymer using tetrahydrofuran (hereinafter referred to as THF). Since it is difficult to measure the molecular weight of each polymer chain of the cross-linked polymer (gel component), the molecular weight of the sol component (uncross-linked polymer) becomes an indicator of the elongation of the polymer chain. When the molecular weight of the sol component is too large, the glass transition temperature sometimes increases and the impact resistance decreases. On the other hand, when the molecular weight of the sol component is too small, the retention sometimes decreases.

<Production of double-sided adhesive sheet with substrate>

A double-sided adhesive sheet with a substrate is obtained by laminating the first adhesive layer 51 and the second adhesive layer 53 on the front and back of the transparent film substrate 59, respectively. As described above, an adhesive composition can be applied on the transparent film substrate 59 when forming the adhesive layer, thereby forming a laminate of the transparent film substrate and the adhesive layer.

By laminating the release films 21 and 23 on the surfaces of the first adhesive layer 51 and the second adhesive layer 53, a double-sided adhesive sheet with a substrate having release films temporarily adhered to both sides is obtained as shown in FIG1. The release film used as a substrate or a cover sheet when forming the adhesive layer can be used as the release films 21 and 23 as it is.

[Formation of image display device]

As described above, in the formation of the image display device, the adhesive sheet 15 is suitable for bonding the front transparent plate 7 to the visual side surface of the image display panel 10. The bonding order is not particularly limited, and the adhesive sheet 15 can be bonded to the image display panel 10 first, or the adhesive sheet 15 can be bonded to the front transparent plate 7 first. In addition, the bonding of the adhesive sheet 15 to the image display panel 10 and the bonding of the adhesive sheet 15 to the front transparent plate 7 can also be performed at the same time. It is also possible to bond the second adhesive layer 53 to the polarizing plate 3 and then bond the polarizing plate 3 and the image display unit 6 through the adhesive layer 4.

Regarding the double-sided adhesive sheet 15 with a substrate, in addition to the form in which a release film is temporarily adhered to the adhesive layers 51 and 53 as shown in FIG1, it can also be used as an adhesive film for fixing the second adhesive layer 53 to an optical film such as a polarizing plate. For example, in the form shown in FIG3, a release film 21 is temporarily adhered to the surface of the first adhesive layer 51, and an optical film 3 is fixed to the second adhesive layer 53. In the form shown in FIG4, an adhesive layer 4 is also provided on the polarizing plate 3, and a release film 24 is temporarily adhered to the adhesive layer 4.

After the image display panel 10 and the front transparent plate 7 are bonded together via the adhesive sheet 15, a pressurization process such as heat pressing and compression bonding is performed. At this time, when there is a convex portion with a relatively large local thickness such as the connection portion of the FPC, a relatively large pressure will be locally applied from the back side of the image display panel, causing the image display panel 10 to deform. As described above, by bonding the image display panel 10 and the front transparent plate 7 together via the double-sided adhesive sheet 15 with a substrate, the plastic deformation of the adhesive sheet 15 is relatively small, and the anti-deformation recovery force is brought into play, so the residual pressure marks caused by the pressure from the back side can be reduced.

[Implementation example]

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

[Production of acrylic oligomers]

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 under a nitrogen atmosphere. Then, 0.2 parts by weight of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator was added, reacted at 70°C for 2 hours, and then heated to 80°C and reacted for 2 hours. Then, the reaction solution was heated to 130°C to dry and remove toluene, chain transfer agent and unreacted monomers, thereby obtaining a solid acrylic oligomer. The weight average molecular weight of the acrylic oligomer is 5100.

[Preparation of prepolymer composition]

<Prepolymer composition 1>

70 parts by weight of butyl acrylate (BA), 17 parts by weight of N-vinyl-2-pyrrolidone (NVP) and 13 parts by weight of 4-hydroxybutyl acrylate (4HBA) as monomer components for prepolymer formation, and photopolymerization initiators (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, and polymerization was performed by irradiating ultraviolet light until the viscosity (BH viscometer, No. 5 rotor, 10 rpm, measuring temperature 30°C) reached about 20 Pa. s, thereby obtaining prepolymer composition 1 (polymerization rate; about 9%).

<Prepolymer composition 2>

The monomer components for forming the prepolymer were changed to 68 parts by weight of 2-ethylhexyl acrylate (2EHA), 17 parts by weight of NVP, and 17 parts by weight of 4-hydroxybutyl acrylate (4HBA), and polymerization was carried out in the same manner as above to obtain prepolymer composition 2.

[Preparation of photocurable adhesive composition]

<Adhesive composition A>

To 100 parts by weight of the above prepolymer composition 1, add 2 parts by weight of polyester urethane diacrylate with a weight average molecular weight of 12,500 ("Art Resin UN-350" manufactured by Negami Industries); 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; 0.07 parts by weight of α-methylstyrene dimer ("Nofmer MSD" manufactured by NOF Corporation) as a chain transfer agent; and 0.3 parts by weight of "KBM-403" manufactured by Shin-Etsu Chemical Co., Ltd. as a silane coupling agent, and then uniformly mix them to prepare an adhesive composition A.

<Adhesive composition B>

0.70 parts by weight of ultraviolet absorber ("Tinosorb S" manufactured by BASF) was added, and as a photopolymerization initiator, 0.05 parts by weight of "Irgacure 184" manufactured by BASF, 0.05 parts by weight of "Irgacure 651" manufactured by BASF, and 0.40 parts by weight of "Irgacure 819" manufactured by BASF were prepared. Except for these changes, adhesive composition B was prepared in the same manner as adhesive composition A.

<Adhesive composition C>

Adhesive composition C was prepared in the same manner as adhesive composition B except that the amount of chain transfer agent was changed to 0.03 parts by weight.

<Adhesive composition D>

Adhesive composition D was prepared in the same manner as adhesive composition B except that 0.1 parts by weight of hexanediol diacrylate (HDDA) was added instead of polyester urethane diacrylate as the multifunctional monomer.

<Adhesive composition E>

To 100 parts by weight of the prepolymer composition 2, 0.1 parts by weight of HDDA, 5 parts by weight of the acrylic oligomer, 0.04 parts by weight of "Irgacure 184" as a photopolymerization initiator, 0.04 parts by weight of "Irgacure 651" and 0.12 parts by weight of "Irgacure 819", and 0.3 parts by weight of a silane coupling agent were added, and then they were uniformly mixed to prepare an adhesive composition E.

[Preparation of adhesive layer]

<Adhesive layer A1>

A 75μm thick polyethylene terephthalate (PET) film with a silicone release layer on the surface was used as a substrate, and the above-mentioned photocurable adhesive composition was applied on the substrate to a thickness of 150μm to form a coating layer. A 75μm thick PET film with a silicone release treatment on one side was attached to the coating layer as a cover sheet. The laminate was photocured by irradiating ultraviolet rays from the cover sheet side using a black light lamp that was adjusted in position so that the irradiation intensity of the irradiation surface directly below the lamp reached 5mW/ ^cm2 , thereby obtaining an adhesive layer A1 with a thickness of 220μm and a polymerization rate of 99%.

<Adhesive layer A2~A4>

The thickness of the adhesive layer was changed to 150μm, 100μm, and 70μm. Adhesive layers A2, A3, and A4 were prepared in the same manner as adhesive layer A1.

<Adhesive layer B1~B4, C2, C3, D2, D3, E1~E3>

The adhesive layer was prepared in the same manner as the adhesive layer A1 except that the type and thickness of the adhesive composition were changed as shown in Table 1.

[Evaluation of adhesive layer]

For each of the above adhesive layers, the glass transition temperature, storage modulus, loss tangent, gel fraction and molecular weight of the sol component were measured by the following method.

<Storage modulus, loss tangent and glass transition temperature>

Multiple adhesive layers were laminated after removing the release films (substrate and cover sheet) on the front and back sides to a thickness of about 1.5 mm, and the laminated layers were used as the test samples. Dynamic viscoelasticity measurements were performed under the following conditions using the Advanced Rheometric Expansion System (ARES) manufactured by Rheometric Scientific.

(Measurement conditions)

Deformation mode: twist

Measuring frequency: 1Hz

Heating rate: 5℃/min

Shape: Parallel plate 7.9mm

The shear storage modulus G'25 _℃ at 25℃ and the loss tangent tanδ70 _℃ at 70℃ are read from the measurement results. In addition, the temperature (peak temperature) when the loss tangent (tanδ) reaches a maximum is taken as the glass transition temperature of the adhesive layer.

<Gel fraction>

Scrape about 0.2g of adhesive from the adhesive layer, wrap it with a porous polytetrafluoroethylene membrane (NTF-1122 manufactured by Nitto Denko Corporation) cut into 100mm×100mm size and 0.2μm pore size, and tie the package mouth with a kite line. Subtract the total weight of the porous polytetrafluoroethylene membrane and the kite line (A) measured in advance from the weight of the sample to calculate the weight of the adhesive sample (B). The adhesive sample wrapped with the porous polytetrafluoroethylene membrane is immersed in about 50mL of ethyl acetate at 23℃ for 7 days to dissolve the sol component of the adhesive outside the porous polytetrafluoroethylene membrane. After immersion, take out the adhesive wrapped with a porous polytetrafluoroethylene film, dry it at 130℃ for 2 hours, cool it naturally for about 20 minutes, and then measure the dry weight (C). The gel fraction of the adhesive is calculated according to the following formula.

Gel fraction (%) = 100 × (C-A) / B

<Weight average molecular weight of sol component>

Scrape about 0.2g of adhesive from the adhesive layer and immerse it in a 10mM tetrahydrofuran phosphate solution for 12 hours to extract the sol component. Considering the gel fraction of the adhesive, adjust the amount of tetrahydrofuran phosphate solution so that the sol content in the extracted solution is 0.1% by weight. Filter the extracted solution using a 0.45μm membrane filter, and use the obtained filtrate as a sample. GPC analysis is performed under the following conditions using a GPC (gel permeation chromatography) device manufactured by Tosoh Corporation (product name "HLC-8120GPC") to calculate the weight average molecular weight Mw of the sol component.

(Measurement conditions)

Pipe column: manufactured by Tosoh Corporation, G7000HXL+GMHXL+GMHXL

×30cm(合計管柱長度：90cm) Column size: 7.8mm each

×30cm (Total pipe length: 90cm)

Column temperature: 40°C, flow rate: 0.8mL/min

Injection volume: 100μL

Solvent: Tetrahydrofuran

Detector: Differential Refractometer (RI)

Standard sample: polystyrene

The composition and thickness of each adhesive layer (adhesive composition) and the evaluation results are shown in Table 1.

[Implementation Example 1]

Adhesive layer A2 as the first adhesive layer is laminated on one main surface of a 50μm thick cycloolefin polymer (COP) film ("ZEONOR Film ZF16" manufactured by Zeonor Co., Ltd. of Japan; front retardation 3nm), and adhesive layer A3 as the second adhesive layer is laminated on the other main surface, thereby manufacturing a double-sided adhesive sheet with a film substrate having adhesive layers on both sides of the film substrate.

[Example 2]

A double-sided adhesive sheet with a film substrate was prepared in the same manner as in Example 1 except that a 50 μm thick biaxially stretched polyethylene terephthalate (PET) film ("Lumirror S10" manufactured by Toray Industries, Inc.; front retardation 920 nm) was used as the film substrate.

[Examples 3 to 9, Comparative Examples 3 to 5]

As shown in Table 2, the type of film substrate and the type of adhesive layer are changed to produce a double-sided adhesive sheet with a film substrate.

[Comparative Examples 1, 2, 6, 7]

Use single-layer adhesive layers A1, B1, E1, and E3 as substrate-free adhesive sheets.

[Evaluation]

The adhesive sheets of the embodiments and comparative examples were evaluated for adhesion, step absorption, drop impact resistance, and indentation using the following methods.

<Continue>

The release film was peeled off from the second adhesive layer, and a PET film with a thickness of 50μm was attached. The film was cut into a width of 10mm×length of 100mm, and then the release film was peeled off from the first adhesive layer and pressed onto a glass plate using a 5kg roller to prepare a sample for adhesion measurement. In Comparative Examples 1, 2, 6, and 7 using a single-layer adhesive sheet without a substrate, the release film on one side was peeled off and a PET film was attached, and then the release film on the other side was peeled off and attached to a glass plate (the same applies to other subsequent evaluations).

Keep the specimen for adhesion test at 25°C for 30 minutes, then use a tensile testing machine to peel the specimen off the glass plate at a tensile speed of 300 mm/min and a peeling angle of 180° to measure the peeling force.

<Haze>

After peeling off the release film from the second adhesive layer and attaching it to alkali-free glass with a thickness of 800μm (total light transmittance of 92%, haze 0.4%), peel off the release film from the first adhesive layer and use the obtained sample as a test piece. The haze is measured using a haze meter ("HM-150" manufactured by Murakami Color Technology Laboratory Co., Ltd.). The value obtained by subtracting the haze of the alkali-free glass (0.4%) from the measured value is taken as the haze of the adhesive sheet.

<Step-by-step absorption>

The adhesive sheet was cut into a size of 75 mm × 45 mm, the release film was peeled off from the second adhesive layer, and it was attached to the center of a PET film cut into 100 mm × 50 mm and having a thickness of 125 μm using a roller laminating press (pressure between rollers: 0.2 MPa, feed speed: 100 mm/min). Then, the release film was peeled off from the first adhesive layer, and it was attached to a glass plate (100 mm × 50 mm) having a thickness of 500 μm and having black ink (printing thickness: 25 μm or 40 μm) printed in a frame shape on the periphery using a roller laminating press (pressure between rollers: 0.2 MPa, feed speed: 100 mm/min). The ink printing area of the glass plate is 5mm from both ends in the short side direction and 15mm from both ends in the long side direction, and the black ink layer is in contact with the adhesive sheet within 5mm from the four sides. The sample was treated in an autoclave (50℃, 0.5MPa) for 30 minutes, and then the boundary of the black ink printing area was observed with a digital microscope with a magnification of 20 times to confirm the presence of bubbles. For each sample with a black ink printing thickness of 25μm and 40μm, the step absorption was evaluated according to the following standards.

◎: No bubbles were observed during the entire cycle

○: Bubbles were observed at one of the four corners, but no bubbles were observed on the four sides.

×: Bubbles are observed at two or more of the four corners or at one or more of the four sides

<Impact resistance>

The adhesive sheet was cut into a size of 75 mm × 45 mm, the release film was peeled off from the second adhesive layer, and it was bonded to the center of a 500 μm thick glass plate (100 mm × 70 mm) using a roller laminating press (pressure between rollers: 0.2 MPa, feed speed: 100 mm/min). Then, the release film was peeled off from the first adhesive layer, and it was 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, and the black ink layer is in contact with the adhesive sheet in an area 5 mm from the four edges. The sample was treated in a high pressure autoclave (50°C, 0.5MPa) for 30 minutes.

As shown in FIG5 , the two ends of the short side direction of the test sample 85 are placed on a table 83 arranged at a distance of 60 mm, with the glass plate 7 provided with the printed layer 76 as the lower side, and the upper surface of the end of the glass plate 8 without the printed layer is fixed to the table 83 with an adhesive tape (not shown). The test sample 85 fixed to the table 83 with the adhesive tape is kept in an environment of -5°C for 24 hours, and then taken out to a room temperature environment. Within 40 seconds, a metal ball 87 with a mass of 11g is dropped from a height of 300mm onto the glass plate 7 to conduct an impact resistance test.

In the impact resistance test, a cylindrical guide 89 is used to fix the falling position of the metal ball, so that the metal ball 87 falls to a position 10 mm away from the corner of the inner edge of the frame of the printing area of the printing layer 76 in the short side direction and the long side direction. Two tests are conducted, and the situation where the glass plate does not peel off in all tests is evaluated as ○, and the situation where the glass plate peels off in any one or both of the two tests is evaluated as ×.

<Indentation test>

The release film was peeled off from the second adhesive layer and bonded to a 50μm thick PET film using a roller laminating press (pressure between rollers: 0.2MPa, feed speed: 100mm/min). Then, the release film was peeled off from the first adhesive layer and bonded to a 500μm thick glass plate using a roller laminating press (pressure between rollers: 0.2MPa, feed speed: 100mm/min). The sample was treated in a high pressure autoclave (50℃, 0.5MPa) for 30 minutes.

Using SAICAS manufactured by Daipla Wintes, a steel ball indenter with a diameter of 1 mm was pressed into the PET film side of the sample at a speed of 5 μm/s until the vertical load reached 1N, and then held for 180 seconds, and then the indenter was lifted at a speed of 5 μm/s. Six hours after the test, the surface shape of the PET film was measured using a non-contact interference microscope (WYKO), and the depth of the indentation of the sample was determined and evaluated according to the following standards.

◎: The depth of the indentation is less than 3μm

○: The depth of the indentation is greater than 3μm and less than 4μm

×: The depth of the indentation is greater than 4μm

[Evaluation results]

The layered structure and evaluation results of each adhesive sheet are shown in Table 1 and Table 2. The values in parentheses of the first adhesive layer, film substrate, and second adhesive layer in Table 2 are the thickness of each layer (unit: μm).

The double-sided adhesive sheets with substrates of Examples 1 to 4, which have a relatively thick adhesive layer on the visual side of the transparent film substrate and a relatively thin adhesive layer on the image display panel side of the transparent film substrate, have excellent step absorption, and less residual pressure marks caused by deformation when the pressure head is pressed into the pressure head from the second adhesive layer side, showing good characteristics. On the other hand, the double-sided adhesive sheet without substrate of Comparative Example 1, which includes an adhesive layer A1 with a thickness of 220 μm, has good step absorption, but easily retains pressure marks caused by deformation when the pressure head is pressed into the adhesive layer, resulting in pressure mark defects.

The same tendency was observed in Example 5, the comparison between Example 6 and Comparative Example 2, and the comparison between Example 9 and Comparative Example 6, in which the composition of the adhesive was changed. In Example 7 and Example 8, in which the composition of the adhesive was changed, as in the other examples, the step absorption was excellent and it was not easy to leave residual pressure marks, showing good characteristics.

The double-sided adhesive sheets with substrates of Comparative Examples 3 to 5, in which an adhesive layer B1 with a thickness of 220 μm as the second adhesive layer is provided on one main surface of the film substrate, and adhesive layers B2, B3, and B4 with relatively smaller thicknesses as the first adhesive layer are provided on the other main surface of the film substrate, also had poor pressure marks as did the adhesive sheet without substrate of Comparative Example 2 including the adhesive layer B2.

Although the result of the indentation test of the substrate-free adhesive sheet of Comparative Example 7, which includes an adhesive layer E3 with a thickness of 100 μm, is good, the thickness of the adhesive sheet is smaller, so the step absorption is poor.

According to the above results, it can be seen that by using a double-sided adhesive sheet with a substrate having a relatively thick adhesive layer on the visual side of the transparent film substrate as the first adhesive layer and a relatively thin adhesive layer on the image display panel side of the transparent film substrate as the second adhesive layer, the step absorption of the printing step of the front transparent plate is excellent, and the defects caused by the pressure mark generated by the pressure from the image display panel side can be suppressed.

3: Optical film (polarizing plate)

4: Adhesive layer

6: Image display unit

7: Front transparent panel

9: Reinforcement substrate

10: Image display panel

15: Double-sided adhesive sheet with substrate

51,53: Adhesive layer

59: Transparent film substrate

71: Transparent board

76: Printing layer

90: Main base board

91: Connector

95:FPC

201: Image display device

Claims (14)

An image display device is a device in which an image display panel is bonded to a front transparent plate disposed on a visual side surface of the image display panel via an adhesive sheet, wherein the adhesive sheet comprises: a transparent film substrate; a first adhesive layer fixed and laminated on a first main surface of the transparent film substrate; and a second adhesive layer fixed and laminated on a second main surface of the transparent film substrate, wherein the first adhesive layer is bonded to the front transparent plate, and the second adhesive layer is bonded to the image display panel, and the first adhesive layer and the second adhesive layer are bonded to each other. The layers respectively include acrylic polymers, the thickness of the first adhesive layer is 80~500μm, the thickness of the second adhesive layer is 30~150μm, the thickness of the first adhesive layer is greater than the thickness of the second adhesive layer, and the adhesion of the second adhesive layer to the glass is greater than 2N/10mm at a temperature of 25°C through a peeling test at a tensile speed of 300mm/min and a peeling angle of 180°. The image display panel includes a touch sensor, and the front transparent plate does not include a touch sensor. As in claim 1, the image display device, wherein the thickness of the transparent film substrate is 15μm~150μm. As in claim 1, the image display device, wherein the front retardation of the transparent film substrate is less than 50nm. As in claim 1, the image display device, wherein the shear storage modulus of the first adhesive layer at a temperature of 25°C is greater than 0.16 MPa. As in claim 1, the image display device, wherein the loss tangent of the first adhesive layer at a temperature of 70°C is greater than 0.25. As in claim 1, the image display device, wherein the glass transition temperature of the first adhesive layer is below -3°C. An image display device as claimed in any one of claims 1 to 6, wherein in the first adhesive layer, the acrylic polymer has a cross-linked structure. As in claim 7, the image display device, wherein the gel fraction of the first adhesive layer is 30% to 80%. As in claim 7, the image display device, wherein the polymerization rate of the first adhesive layer is greater than 95%. As in claim 7, the image display device, wherein in the first adhesive layer, a cross-linked structure based on a urethane chain segment is introduced into the acrylic polymer. An image display device as claimed in any one of claims 1 to 6, wherein the adhesion of the first adhesive layer to the glass obtained by a peeling test at a temperature of 25°C with a tensile speed of 300 mm/min and a peeling angle of 180° is 2N/10mm or more. An image display device as claimed in any one of claims 1 to 6, wherein the image display panel has a polarizing plate on the visual side surface of the image display unit, and the polarizing plate is bonded to the second adhesive layer. An image display device as claimed in any one of claims 1 to 6, wherein the image display panel comprises an organic EL unit. As in claim 13, the organic EL unit has an electrode and an organic light-emitting layer on a resin film substrate.