JP2022022140A - Adhesive sheet and film with adhesive layer - Google Patents

Abstract

To provide an adhesive sheet wherein: in a bendable image display device having a touch panel within a distance of 500 μm from the touch surface, the adhesive sheet is used to bond between members arranged between the touch surface and the touch panel, has a high adhesive strength and can contribute to reduction in erroneous operation of the touch panel.SOLUTION: The relative permittivity of this adhesive sheet at a temperature of 25°C and a frequency of 10 kHz is less than or equal to 4.5. When performing a bending retention test for 240 hours in an environment at a temperature of 60° and relative humidity of 95% in a state where the 35 mm×100 mm adhesive sheet attached to an adherend is held bent along the short side direction at a bending radius of 1.3 mm and a bending angle of 180°, the length of a clearance in the short-side direction between the adhesive sheet and the adherend in the bent part of the sample after the test is preferably 2 mm or less.SELECTED DRAWING: Figure 2

Description

The present invention relates to an adhesive sheet suitably used for bonding members in a foldable image display device. Further, the present invention relates to a film with an adhesive layer in which an adhesive sheet is laminated on at least one surface of a polarizing plate.

Flat panel displays such as liquid crystal displays and organic EL display devices are used as image display devices for mobile phones, smartphones, tablet terminals, car navigation devices, personal computer monitors, televisions, and the like. In recent years, an organic EL panel using a foldable substrate (flexible substrate) such as a resin film has been put into practical use, and a foldable flexible display has been proposed.

In a flexible display, in addition to the display panel such as an organic EL panel being foldable, constituent members such as a housing and a touch panel are also foldable, and these members are bonded via an adhesive sheet (these members are bonded together via an adhesive sheet). For example, Patent Document 1). In a foldable flexible display (foldable display), it is necessary that the transparent plate (cover window) placed on the viewing side surface is also foldable, and a material with a small thickness such as a resin film or thin glass is used. ing.

In a foldable display, bending is repeated at the same place. At the bent part, compressive stress is applied to the inside and tensile stress is applied to the outside, and strain occurs in and around the bent part, so that there is a concern that the device may be destroyed. Therefore, it has been proposed to soften the adhesive sheet that is attached between the members to alleviate the stress strain (for example, Patent Document 2).

Japanese Unexamined Patent Publication No. 2016-2764 Japanese Unexamined Patent Publication No. 2018-45213

In recent years, thinner foldable display devices have been demanded. In particular, even if the distance from the surface of the device to the touch panel sensor is small, the touch panel sensor is required to have almost or no malfunction, and a highly reliable device in which failure or the like is suppressed or prevented.

In view of the above, it is an object of the present invention to provide an adhesive sheet that is suitably used for bonding members of a flexible display, has high reliability, and can contribute to suppressing malfunction of a touch panel.

The adhesive sheet of the present invention is used for bonding members arranged between a touch surface and a touch panel in a bendable image display device provided with a touch panel within a distance of 500 μm from the touch surface. The relative permittivity of the pressure-sensitive adhesive sheet at a temperature of 25 ° C. and a frequency of 10 kHz is 4.5 or less.

The length of the gap between the adhesive sheet and the adherend, which is measured by the procedure of steps A to D below, is 2 mm or less.
Step A: Produce a 35 mm × 100 mm test piece by laminating the adhesive sheet with the adherend Step B: Produce the test piece produced in step A along the short side direction with a bending radius of 1.3 mm and a bending angle. Bending at 180 ° Step C: The test piece bent in step B is held in an environment with a temperature of 60 degrees and a relative humidity of 95% for 240 hours. Step D: The test piece after holding in step C for 240 hours. In the bent portion of the above, the length of the gap portion between the adhesive sheet and the adherend in the short side direction is measured.

At a temperature of 25 ° C., the ratio of the relative permittivity at 1 kHz to the relative permittivity at a frequency of 1 MHz of the pressure-sensitive adhesive sheet is preferably 1.50 or less. The pressure-sensitive adhesive sheet preferably has a maximum relative permittivity of 1.4 times or less of the minimum value in the temperature range of −40 ° C. to 80 ° C. at a frequency of 10 kHz. The adhesive sheet has a ratio of the maximum value to the minimum value of the relative permittivity in the temperature range of -40 ° C to 80 ° C at a frequency of 1 kHz, and the maximum relative permittivity in the temperature range of -40 ° C to 80 ° C at a frequency of 1 MHz. It is preferably 0.8 to 1.2 times the ratio of the value to the minimum value.

The thickness of the pressure-sensitive adhesive sheet is preferably 100 μm or less.

The storage elastic modulus G'25 of the pressure-sensitive adhesive sheet at ₂₅ ° C. and 1 Hz may be 70 kPa or less. The glass transition temperature of the pressure-sensitive adhesive sheet may be −20 ° C. or lower.

The pressure-sensitive adhesive sheet may be composed of an acrylic pressure-sensitive adhesive containing an acrylic-based base polymer. The acrylic base polymer may contain 5 to 55 parts by weight of the (meth) acrylic acid C _10-20 chain alkyl ester with respect to 100 parts by weight of the total monomer component. The acrylic base polymer may contain lauryl acrylate as a (meth) acrylic acid C _10-20 chain alkyl ester.

The acrylic base polymer contains 2 to 15 parts by weight of one or more polar group-containing monomers selected from the group consisting of hydroxy group-containing monomers, carboxy group-containing monomers and nitrogen-containing monomers with respect to a total of 100 parts by weight of the monomer components. It may be contained. The acrylic base polymer may contain 10 parts by weight or less of the hydroxy group-containing monomer with respect to 100 parts by weight of the total monomer components.

The acrylic base polymer may have a crosslinked structure. The crosslinked structure may be one introduced by polyfunctional (meth) acrylate.

The acrylic pressure-sensitive adhesive may further contain an acrylic oligomer having a glass transition temperature of 60 ° C. or higher. The content of the acrylic oligomer per 100 parts by weight of the acrylic base polymer may be 0.1 to 5 parts by weight.

The pressure-sensitive adhesive sheet of the present invention may be laminated on at least one surface of the polarizing plate to form a film with a pressure-sensitive adhesive layer.

The image display device using the adhesive sheet of the present invention can exhibit high reliability even when the malfunction of the touch panel is reduced and the bent state is maintained.

It is sectional drawing which shows the structural example of the adhesive sheet with a release film. It is sectional drawing which shows the structural example of the image display apparatus. It is sectional drawing which shows the structural example of the image display apparatus. It is sectional drawing which shows the laminated structure example of the polarizing plate with an adhesive sheet. It is sectional drawing which shows the laminated structure example of the polarizing plate with an adhesive sheet.

FIG. 1 is a cross-sectional view showing a pressure-sensitive adhesive sheet with a release film to which release films 91 and 92 are temporarily attached to both sides of the pressure-sensitive adhesive sheet 11. 2 and 3 are structural cross-sectional views of a flexible display according to an embodiment.

In the image display device 101 shown in FIG. 2, an organic EL panel 51, a touch panel 41, and a circular polarizing plate 31 are arranged between the housing 75 and the cover window 71. The bottom surface of the organic EL panel 51 and the housing 75 is bonded to each other via the adhesive sheet 14, the organic EL panel 51 and the touch panel 41 are bonded to each other via the adhesive sheet 13, and the touch panel 41 and the circular polarizing plate 31 are bonded to each other. Is bonded via the pressure-sensitive adhesive sheet 12, and the circularly polarizing plate 31 and the cover window 71 are bonded via the pressure-sensitive adhesive sheet 11. As described above, in the flexible display, a plurality of members are laminated and integrated by being bonded to each other via an adhesive sheet.

The cover window 71 is arranged on the visible side surface of the flexible display and constitutes a touch surface. The touch panel 41 is a capacitive touch panel. Since the foldable cover window 71 is used in the flexible display, the thickness of the cover window is small, and the distance D from the touch surface (the surface of the cover window 71) to the touch panel 41 is also reduced accordingly.

The image display device 102 shown in FIG. 3 includes an organic EL panel 54 with an integrated touch panel, and a circular polarizing plate 31 is attached to the organic EL panel 54 via an adhesive sheet 12. The other configurations are the same as in FIG. 2, and the distance from the touch surface to the visible surface of the organic EL panel 54 corresponds to the distance D from the touch surface to the touch panel.

The adhesive sheet of the present invention is used for bonding members arranged between the touch surface and the touch panel in a bendable image display device in which the distance D from the touch surface to the touch panel is within 500 μm.

[Characteristics of adhesive sheet]
The pressure-sensitive adhesive sheet of the present invention has a relative permittivity of 4.5 or less at a temperature of 25 ° C. and a frequency of 10 kHz. In the following, unless otherwise specified, the dielectric constant is a measured value at a temperature of 25 ° C. The relative permittivity of the pressure-sensitive adhesive sheet at a frequency of 10 kHz may be 4.0 or less, 3.8 or less, or 3.5 or less. By setting the relative permittivity at a frequency of 10 kHz to 4.5 or less, the distance from the touch surface to the touch panel can be reduced, so that a design advantageous for flexibility becomes possible. Since the low dielectric constant adhesive sheet has a small capacitance value, it is possible to design a sensor with good sensitivity. This enables an input method having a small contact area such as pen input. Further, by arranging the adhesive sheet having a low dielectric constant between the organic EL panel and the touch panel, noise from the organic EL panel can be reduced, so that the touch panel can be prevented from malfunctioning.

The relative permittivity of the pressure-sensitive adhesive sheet at a frequency of 1 kHz is preferably 5.0 or less, more preferably 4.8 or less, and may be 4.5 or less, 4.0 or less, or 3.8 or less. The relative permittivity of the pressure-sensitive adhesive sheet at a frequency of 100 kHz is preferably 4.0 or less, and may be 3.8 or less or 3.5 or less. The relative permittivity of the pressure-sensitive adhesive sheet at a frequency of 1 MHz is preferably 3.5 or less, and may be 3.3 or less or 3.2 or less.

The dielectric constant changes depending on the polarizability of the material, and the dielectric constant of the pressure-sensitive adhesive sheet can be controlled by selecting the material of the pressure-sensitive adhesive such as urethane, acrylic, rubber, and silica. Further, since the relative permittivity of air is 1, the dielectric constant of the pressure-sensitive adhesive sheet can be lowered by adding hollow beads or the like to the pressure-sensitive adhesive. In acrylic pressure-sensitive adhesives, monomers with long alkyl chains have a low polarizability and can be made less dielectric. When a highly polar monomer is used, the polarizability is large and the dielectric constant is high. As a means of lowering the polarizability, there is a method of causing molecular entanglement. When the molecular weight is increased or the degree of cross-linking is increased, entanglement of molecules is likely to occur, and the polarizability is lowered, so that the dielectric can be reduced. Since the dielectric constant tends to increase as the water content increases, the dielectric constant can be reduced by using a material that does not easily retain water.

According to the Clausius-Mossotti equation, the smaller the polarizability of the electric dipole and the smaller the number of electric dipoles per unit volume, the smaller the relative permittivity. In order to reduce the relative permittivity of the pressure-sensitive adhesive sheet, the dipole moment of the base polymer constituting the pressure-sensitive adhesive may be reduced and the molar volume may be increased. For example, the larger the volume of the side chain of the base polymer, the larger the molar volume tends to be. Further, by selecting a monomer having a small polarity as a monomer component constituting the base polymer, the electron dipole of the molecule becomes small.

The pressure-sensitive adhesive sheet preferably has a small frequency dependence of the relative permittivity at a temperature of 25 ° C. Specifically, the ratio of the relative permittivity at a frequency of 1 kHz (1 kHz / 1 MHz) to the relative permittivity at a frequency of 1 MHz of the pressure-sensitive adhesive sheet is preferably 1.5 or less. Since the relative permittivity is small over a wide frequency range and the frequency dependence of the relative permittivity is small, it is possible to ensure operation reliability for various operating frequencies. As described above, the frequency dependence of the relative permittivity can be reduced by adjusting the dielectric constant of the pressure-sensitive adhesive.

The pressure-sensitive adhesive sheet preferably has a small temperature dependence of the relative permittivity. Specifically, it is preferable that the ratio (maximum value / minimum value) of the minimum value and the maximum value of the relative permittivity in the temperature range of −40 ° C. to −80 ° C. is close to 1. The ratio of the maximum value and the minimum value of the relative permittivity at a frequency of 10 kHz X _{10 kHz} is preferably 1.4 or less, more preferably 1.3 or less, further preferably 1.2 or less, and even if it is 1.1 or less. good. The ratio of the maximum value and the minimum value of the relative permittivity at a frequency of 1 kHz X ₁ kHz, the ratio of the maximum value and the minimum value of the relative permittivity at a frequency of 100 kHz X _{100 kHz} , and the maximum and minimum values of the relative permittivity at a frequency of 1 MHz. The ratio of X _{1 MHz} is also preferably 1.4 or less, more preferably 1.3 or less, further preferably 1.2 or less, and may be 1.1 or less, respectively.

The ratio X of the maximum value and the minimum value of the relative permittivity in the temperature range of -40 ° C to -80 ° C is an index of the temperature dependence of the relative permittivity, and the closer X is to 1, the more the temperature depends on the relative permittivity. It means that the sex is small. As described above, in addition to the small temperature dependence of the relative permittivity at a frequency of 10 kHz, it is preferable that the temperature dependence of the relative permittivity is small over the entire frequency range of 1 kHz to 1 MHz. When the temperature dependence of the relative permittivity is small, the frequency dependence also tends to be small. Further, as the frequency becomes larger, the temperature at which the relative permittivity becomes maximum tends to shift to the high temperature side.

The ratio of X 1 _MHz to X _{1 kHz} X _{1 kHz} / X _{1 MHz} is preferably 0.8 to 1.2, more preferably 0.85 to 1.15, still more preferably 0.9 to 1.1, and 0. It may be .95 to 1.05. The closer X 1 _kHz / X _{1 MHz} is to 1, the smaller the change in the relative permittivity over a wide temperature range and frequency range, so that operational reliability can be ensured over a wide temperature range and operating frequency.

The adhesive strength of the pressure-sensitive adhesive sheet is preferably 2.7 N / 10 mm or more, more preferably 2.8 N / 10 mm or more, and may be 3.0 N / 10 mm or more. Adhesive strength is determined by a peel test with a polyimide film as an adherend, a tensile speed of 60 mm / min, and a peeling angle of 180 °. Unless otherwise specified, the adhesive strength is a measured value at 25 ° C. When the adhesive strength of the adhesive sheet is within the above range, it is possible to prevent the adhesive sheet from peeling off from the adherend when bending is repeated.

In the present invention, the pressure-sensitive adhesive sheet is the length of the gap portion according to the bending holding test described later (that is, the gap portion length between the pressure-sensitive adhesive sheet and the adherend after the bending holding for 240 hours: hereinafter simply referred to as "void distance". There is) is 2 mm or less. When the gap distance is 2 mm or less, the adhesive sheet may absorb the stress due to bending even if the image display device is bendable and has a touch panel within a distance of 500 μm from the touch surface, even if the image display device is bent. It is possible to improve the reliability of the image display device. Therefore, even if the distance from the device surface to the touch panel sensor is short, there is almost or no malfunction of the touch panel sensor, failure or the like is suppressed or prevented, and reliability is high.

The gap distance is preferably 1.5 mm or less, more preferably 1.0 mm or less, and may be 0.8 mm or less, 0.5 mm or less, or 0.3 mm or less. The lower limit of the gap distance is not particularly limited and may be 0.

The gap distance is measured by the procedure of steps A to D below.
Step A: A 35 mm × 100 mm test piece is prepared by laminating an adhesive sheet with an adherend;
Step B: The test piece produced in step A is bent along the short side direction with a bending radius of 1.3 mm and a bending angle of 180 °;
Step C: The test piece bent in step B is held in a bent state for 240 hours in an environment with a temperature of 60 degrees and a relative humidity of 95%;
Step D: In the bent portion of the test piece after being held for 240 hours in step C, the length of the gap portion between the adhesive sheet and the adherend in the short side direction is measured.

Peeling (void) in the bending holding test is likely to occur from the end of the bending shaft of the test piece (the end in the short side direction). If there are voids from both ends or if there are voids at multiple locations, the length of the void with the longest length in the short side direction is defined as the void distance. When there are voids at a plurality of locations, the longest void may be 2 mm or less. Preferably, the total length of each void is 2 mm or less, and the total length of the voids is 1.5 mm or less, 1.0 mm or less, 0.8 mm or less, 0.5 mm or less, or 0.3 mm or less. It may be 0 or 0.

At the time of bending, the adherend expands and contracts, and the adhesive cannot follow the deformation of the adherend, so that peeling from the adherend occurs. For example, by controlling the storage elastic modulus G'of the pressure-sensitive adhesive, the followability of the pressure-sensitive adhesive to the adherend can be adjusted. The smaller the storage elastic modulus of the pressure-sensitive adhesive, the better the ability to follow the deformation of the adherend. When the bent state is maintained, peeling may occur due to the concentration of stress associated with the deformation of the bent portion. In order to enhance the stress relaxation property of the pressure-sensitive adhesive, the loss tangent tan δ may be increased. Further, in order to suppress peeling at the interface between the adherend and the adhesive, it is also necessary to design a high adhesive force at the bending temperature. Furthermore, in a high humidity environment, the retention of water at the interface between the adhesive and the adherend can cause a decrease in adhesive strength, so it is possible to suppress the retention of water by using a material that does not easily retain water. preferable.

The pressure-sensitive adhesive sheet preferably has a storage elastic modulus G'25 at ₂₅ ° C. of 70 kPa or less. When _G'25 is 70 kPa or less, the strain when the device is bent tends to be alleviated, and the damage of the device component when the device is repeatedly bent can be suppressed. _G'25 is preferably 5 kPa or more, and if it is in such a range, the adhesive holding force of the pressure-sensitive adhesive sheet and the strain mitigation property can be compatible with each other, so that the gap distance can be reduced. From the viewpoint of more effectively achieving both workability, adhesive holding power and strain mitigation property, _G'25 of the pressure-sensitive adhesive sheet is preferably 10 to 60 kPa, more preferably 13 to 50 kPa, and even more preferably 15 to 40 kPa.

The storage elastic modulus G'100 of the pressure-sensitive adhesive sheet at ₁₀₀ ° C. is preferably 2 to 50 kPa, more preferably 3 to 40 kPa, and even more preferably 5 to 25 kPa. When the _G'100 of the pressure-sensitive adhesive sheet is within the above range, both the adhesive holding force and the stress relaxation property can be achieved even in a high temperature environment, so that the gap distance can be reduced.

The pressure-sensitive adhesive sheet preferably has a loss tangent tan δ ₂₅ at 25 ° C. of 0.2 to 0.45. Further, the pressure-sensitive adhesive sheet preferably has a loss tangent tan δ ₁₀₀ at 100 ° C. of 0.2 to 0.4. The difference between tan δ ₂₅ and tan δ ₁₀₀ is preferably −0.07 to 0.07. tan δ ₂₅ may be 0.25 to 0.42. tan δ ₁₀₀ may be 0.25 to 0.38. Further, the difference between tan δ ₂₅ and tan δ ₁₀₀ may be within ± 0.06 or within ± 0.05. When the difference between the tan δ ₂₅ , tan δ ₁₀₀ , and tan δ ₂₅ and tan δ ₁₀₀ of the adhesive sheet is within the above range, the gap distance can be reduced. The tan δ of the pressure-sensitive adhesive sheet can be adjusted by optimizing the material monomer and the degree of cross-linking. For example, the material monomer may be selected so that the glass transition temperature and molecular weight of the base polymer are in an appropriate range.

The storage elastic modulus G'and the loss tangent tan δ of the pressure-sensitive adhesive sheet are determined by measuring the viscoelasticity at a frequency of 1 Hz. tan δ is the ratio G "/ G'of the storage elastic modulus G'and the loss elastic modulus G". The storage elastic modulus G'corresponds to a portion stored as elastic energy when the material is deformed, and is an index indicating the degree of hardness.

In order to reduce the temperature dependence of tan δ in the normal temperature to high temperature region, the glass transition temperature of the pressure-sensitive adhesive sheet is preferably −20 ° C. or lower, more preferably −23 ° C. or lower, still more preferably −25 ° C. or lower. The glass transition temperature is the temperature at which tan δ becomes maximum (peak top temperature). In the vicinity of the glass transition temperature, the temperature dependence of tan δ is large. Since the glass transition temperature is sufficiently lower than the operating environment temperature of the device, the temperature dependence of tan δ in the operating environment temperature range becomes small. Further, when the glass transition temperature is in the above range, the adhesive sheet has an adhesive holding force even in a low temperature region, so that peeling from the adherend at a low temperature can be suppressed and the void distance can be reduced.

The lower limit of the glass transition temperature of the pressure-sensitive adhesive sheet is not particularly limited, but is generally −80 ° C. or higher. The glass transition temperature of the pressure-sensitive adhesive sheet is preferably −70 ° C. or higher, more preferably −60 ° C. or higher, and may be −55 ° C. or higher, or −50 ° C. or higher. By setting the glass transition temperature of the pressure-sensitive adhesive sheet within the above range, the adhesive holding force can be effectively increased and the gap distance can be reduced.

The thickness of the adhesive sheet is not particularly limited, and depends on the type of adherend, the position where the adhesive sheet is arranged in the device (laminated structure), the thickness of the target device, the characteristics required for the adhesive sheet, and the like. It may be adjusted as appropriate. From the viewpoint of increasing the adhesive strength of the pressure-sensitive adhesive sheet, the thickness is preferably 10 μm or more. The thickness of the pressure-sensitive adhesive sheet is preferably 100 μm or less, more preferably 75 μm or less, from the viewpoint of reducing the thickness of the device, processing the pressure-sensitive adhesive sheet, bending the device, and the like to prevent the pressure-sensitive adhesive from squeezing out from the end face.

From the viewpoint of providing cushioning property against impact from the outer surface, the thickness of the adhesive sheet 11 used for bonding the cover window is preferably 20 μm or more, more preferably 30 μm or more, and even if it is 35 μm or more or 40 μm or more. good. The thickness of the pressure-sensitive adhesive sheets 12 and 13 arranged between the image display panels 51 and 54 and the polarizing plate 31 is preferably smaller than the thickness of the pressure-sensitive adhesive sheet 11. The thickness of the pressure-sensitive adhesive sheets 12 and 13 is preferably 25 μm or less, more preferably 20 μm or less.

Like the adhesive sheets 11, 12, and 13 in the device 101 shown in FIG. 2, the adhesive sheet arranged on the visual side of the image display panel 51 preferably has high transparency. The total light transmittance of the pressure-sensitive adhesive sheet arranged on the visual recognition side is preferably 85% or more, more preferably 90% or more, still more preferably 91% or more. The haze of the pressure-sensitive adhesive sheet arranged on the visual recognition side is preferably 1.5% or less, more preferably 1% or less, further preferably 0.7% or less, and particularly preferably 0.5% or less.

[Composition and preparation method of adhesive sheet]
The pressure-sensitive adhesive for forming the pressure-sensitive adhesive sheet of the present invention is not particularly limited as long as the specific dielectric constant and the void distance are within the above ranges, and the base polymer includes acrylic, silicone-based, polyester, polyurethane, polyamide, polyvinyl ether, and the like. Examples thereof include those containing a vinyl acetate / vinyl chloride copolymer, a modified polyolefin, an epoxy-based polymer, a fluorine-based polymer, a rubber-based polymer, and the like. As the pressure-sensitive adhesive, an acrylic pressure-sensitive adhesive containing an acrylic-based base polymer as a main component is preferable because the relative permittivity and the void distance can be adjusted within the above-mentioned predetermined ranges and the transparency and adhesive strength can be controlled. The pressure-sensitive adhesive can be used alone or in combination of two or more. Further, the pressure-sensitive adhesive sheet formed by the pressure-sensitive adhesive may be in either a single-layer form or a laminated form.

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

As the (meth) acrylic acid alkyl ester, a (meth) acrylic acid C _1-20 alkyl ester having an alkyl group having 1 to 20 carbon atoms is preferably used. The alkyl group in the (meth) acrylic acid alkyl ester may be in either a chain or cyclic form. The alkyl group in the chain form (chain alkyl group) may be a linear alkyl group or may have a branch.

Specific examples of the (meth) acrylic acid chain alkyl ester include methyl (meth) acrylic acid, ethyl (meth) acrylic acid, butyl (meth) acrylic acid, isobutyl (meth) acrylic acid, and (meth) acrylic acid s-. Butyl, t-butyl (meth) acrylate, pentyl (meth) acrylate, isopentyl (meth) acrylate, neopentyl (meth) acrylate, hexyl (meth) acrylate, heptyl (meth) acrylate, (meth) acrylic C _1-9 chain alkyl esters such as 2-ethylhexyl acid, octyl (meth) acrylate, isooctyl (meth) acrylate, nonyl (meth) acrylate, and isononyl (meth) acrylate; and (meth) acrylic acid. Decyl, (meth) isodecyl acrylate, (meth) undecyl acrylate, (meth) dodecyl acrylate, (meth) isotridecyl acrylate, (meth) tetradecyl acrylate, (meth) isotetradecyl acrylate, (meth) acrylic _C10-20 chain alkyl esters such as pentadecyl acid, cetyl (meth) acrylate, heptadecyl (meth) acrylate, octadecyl (meth) acrylate, isooctadecyl (meth) acrylate, and nonadecil (meth) acrylate Can be mentioned.

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

In the acrylic base polymer, the amount of the (meth) acrylic acid alkyl ester with respect to 100 parts by weight of the total monomer components is preferably 60 to 100 parts by weight, more preferably 70 to 98 parts by weight.

The acrylic base polymer preferably contains (meth) acrylic acid C _10-20 chain alkyl ester as the (meth) acrylic acid alkyl ester. When the acrylic base polymer contains a (meth) acrylic acid long-chain alkyl ester as a monomer component, the dipole moment of the molecule can be reduced and the molar volume can be increased, so that the relative permittivity can be lowered. Can be done. Further, the homopolymer of the (meth) acrylic acid alkyl ester having a long-chain alkyl group having 10 or more carbon atoms has a temperature region (plateau region) in which the temperature dependence of viscoelasticity is small at a temperature higher than Tg. Therefore, when the base polymer contains a (meth) acrylic acid long-chain alkyl ester as a monomer component, the temperature dependence of tan δ can be reduced.

Among the (meth) acrylic acid C _10-20 chain alkyl esters, the (meth) acrylic acid C _10-16 alkyl ester is preferable because the temperature range of the plateau region is wide and the storage elastic modulus in the plateau region is small. (Meta) Acrylic acid C _10-13 alkyl ester is more preferable. Of these, (meth) acrylic acid _C12 alkyl ester is preferable, and dodecyl acrylate (lauryl acrylate) is particularly preferable.

The polymer of the (meth) acrylic acid long-chain alkyl ester is characterized by having a wide temperature range in the plateau region and a small storage elastic modulus in the plateau region, but has high crystallinity and a high glass transition temperature. For example, the glass transition temperature of a homopolymer of lauryl acrylate is 0 ° C. In order to lower the glass transition temperature of the base polymer, the (meth) acrylic acid C _1-9 chain alkyl ester should be contained in addition to the (meth) acrylic acid C _10-20 chain alkyl ester as the monomer component. Is preferable.

Among the (meth) acrylic acid C _1-9 chain alkyl esters, those having a glass transition temperature of the homopolymer of −40 ° C. or lower are preferable in order to reduce the Tg of the base polymer. Specific examples of the (meth) acrylic acid C _1-9 chain alkyl ester in which the glass transition temperature of the homopolymer is −40 ° C. or lower are 2-ethylhexyl acrylate (Tg: −70 ° C.) and n-hexyl acrylic acid. (Tg: -65 ° C), n-octyl acrylate (Tg: -65 ° C), isononyl acrylate (Tg: -60 ° C), n-nonyl acrylate (Tg: -58 ° C), isooctyl acrylate (Tg) : −58 ° C.), butyl acrylate (Tg: −55 ° C.) and the like. Among these, butyl acrylate and 2-ethylhexyl acrylate are preferable, and 2-ethylhexyl acrylate is particularly preferable because Tg is low.

In order to obtain a pressure-sensitive adhesive having the above-mentioned properties, (meth) acrylic acid C _10-20 chain alkyl ester and (meth) acrylic acid C _1-9 chain alkyl ester are used as the monomer components of the acrylic base polymer. It is preferable to include both and adjust the ratio of both.

The amount of (meth) acrylic acid C _10-20 chain alkyl ester to 100 parts by weight of the total monomer component of the acrylic base polymer is preferably 5 to 55 parts by weight, more preferably 10 to 50 parts by weight, and 15 to 45 parts by weight. Parts are more preferable, and 20 to 50 parts by weight are particularly preferable. In particular, the amount of lauryl acrylate is preferably in the above range. The amount of (meth) acrylic acid C _1-9 chain alkyl ester to 100 parts by weight of the total monomer component of the acrylic base polymer is preferably 30 to 80 parts by weight, more preferably 40 to 75 parts by weight, and 45 to 70 parts by weight. Parts are more preferable, and 50 to 65 parts by weight are particularly preferable. In particular, the amount of 2-ethylhexyl acrylate is preferably in the above range.

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

A crosslinked structure may be introduced into the acrylic base polymer. Since the acrylic base polymer is crosslinked, high adhesive holding power can be exhibited even when the G'of the pressure-sensitive adhesive sheet is small. As described above, in order to introduce a crosslinked structure into the acrylic base polymer, the acrylic base polymer contains a hydroxy group-containing monomer and a carboxy group-containing monomer in addition to the above-mentioned (meth) acrylic acid alkyl ester as a monomer component. It is preferable to contain it. When a crosslinked structure is introduced into an acrylic base polymer by using an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, or the like, the hydroxy group or the carboxy group becomes the introduction point of the crosslinked structure.

Examples of the hydroxy group-containing monomer include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, 6-hydroxyhexyl (meth) acrylate, and (meth). Examples thereof include (meth) acrylic acid esters such as 8-hydroxyoctyl acrylate, 10-hydroxydecyl (meth) acrylic acid, and 12-hydroxylauryl (meth) acrylic acid. Among these, 2-hydroxyethyl acrylate (Tg: -15 ° C) and 4-hydroxybutyl acrylate (Tg: -15 ° C) (Tg: -15 ° C) and 4-hydroxybutyl acrylate (Tg: -15 ° C) because they contribute greatly to the improvement of adhesive strength and can suppress the white turbidity of the adhesive sheet in a high humidity environment. Tg: −32 ° C.) is preferable, and 4-hydroxybutyl acrylate is particularly preferable because Tg is low.

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

From the viewpoint of enhancing the adhesive force and the adhesive holding force of the pressure-sensitive adhesive sheet, the amount of the polar group-containing monomer is preferably 2 parts by weight or more, preferably 3 parts by weight or more and 4 parts by weight, based on 100 parts by weight of the total monomer components of the acrylic base polymer. It may be more than 5 parts by weight or more. On the other hand, as the content of the polar monomer increases, the dipole moment of the base polymer increases and the relative permittivity increases. Further, if the content of the polar monomer is excessively large, the glass transition temperature of the polymer tends to be high, and the adhesive strength at a low temperature tends to decrease. Therefore, the amount of the polar group-containing monomer with respect to 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 15 parts by weight or less, and may be 13 parts by weight or less or 10 parts by weight or less.

The acrylic-based base polymer preferably contains a hydroxy group-containing monomer and a nitrogen-containing monomer among the above-mentioned polar monomer components, and the total of the hydroxy group-containing monomer and the nitrogen-containing monomer is preferably in the above range.

By containing a hydroxy group-containing monomer as the polar monomer component, the adhesive strength of the pressure-sensitive adhesive sheet is improved, and the white turbidity of the pressure-sensitive adhesive sheet tends to be suppressed in a high humidity environment. Therefore, the amount of the hydroxy group-containing monomer with respect to 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, and may be 2 parts by weight or more. On the other hand, as the content of the hydroxy group-containing monomer (the amount of hydroxy groups in the pressure-sensitive adhesive) increases, the relative permittivity of the pressure-sensitive adhesive tends to increase remarkably. Therefore, the amount of the hydroxy group-containing monomer based on 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 10 parts by weight or less, more preferably 8 parts by weight or less, and may be 6 parts by weight or less. When the amount of the hydroxy group-containing monomer is within the above range, the relative permittivity and the void distance can be within the above range.

From the viewpoint of achieving both improvement in adhesive strength and low dielectric constant, the amount of nitrogen group-containing monomer is preferably 0.5 to 10 parts by weight, preferably 1 to 8 parts by weight, based on 100 parts by weight of the total monomer components of the acrylic base polymer. The part is more preferable. In particular, the amount of N-vinylpyrrolidone is preferably in the above range because it contributes greatly to the improvement of adhesive strength. When the amount of the nitrogen-containing monomer is within the above range, the relative permittivity and the void distance can be within the above range.

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

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

The acrylic base polymer may contain a monomer other than the above-mentioned (meth) acrylic acid alkyl ester and polar monomer as a monomer component. Monomer components other than the above include caprolactone adducts of (meth) acrylic acid, sulfonic acid group-containing monomers, phosphoric acid group-containing monomers, vinyl acetate, vinyl propionate, styrene, α-methylstyrene, and other vinyl-based monomers; acrylonitrile. , Cyanoacrylate-based monomers such as methacrylonitrile; Epoxy group-containing monomers such as (meth) glycidyl acrylate; (meth) polyethylene glycol acrylate, (meth) polypropylene glycol (meth) acrylate, (meth) methoxyethylene glycol (meth) acrylate, ( Glycol-based acrylic ester monomers such as methoxypolypropylene glycol (meth); acrylic acid esters such as tetrahydrofurfuryl (meth) acrylate, fluorine (meth) acrylate, silicone (meth) acrylate and 2-methoxyethyl (meth) acrylate. Examples include system monomers.

The theoretical Tg of the acrylic base polymer is preferably −60 to −20 ° C. The theoretical Tg of the acrylic base polymer is more preferably −23 ° C. or lower, further preferably −25 ° C. or lower, and particularly preferably −30 ° C. or lower. The theoretical Tg of the acrylic base polymer may be −50 ° C. or lower, −45 ° C. or lower, −40 ° C. or lower, or −38 ° C. or lower. If the theoretical Tg of the base polymer is within the above range, the relative permittivity and the void distance can be within the above range. The theoretical Tg is calculated by the following Fox formula from the glass transition temperature Tg _i of the homopolymer which is a constituent monomer component of the acrylic base polymer and the weight fraction _Wii of each monomer component.
1 / Tg = Σ ( _{Wi / Tg i )}

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

<Cross-linked structure of base polymer>
As described above, the acrylic base polymer may have a crosslinked structure. The introduction of a crosslinked structure into the base polymer increases the gel fraction of the adhesive. The gel fraction of the pressure-sensitive adhesive sheet is preferably 55 to 85%, more preferably 60 to 80%, further preferably 63 to 77%, and particularly preferably 65 to 75%. By adjusting the gel fraction within this range, even when the G'is small and the adhesive sheet is soft, a high adhesive holding force can be exhibited, so that the above-mentioned void distance can be reduced.

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

The method for introducing the crosslinked structure into the base polymer is as follows: (1) After polymerizing the base polymer having a functional group capable of reacting with the crosslinking agent, the crosslinking agent is added to react the base polymer with the crosslinking agent; (2) A method of introducing a branched structure (crosslinked structure) into a polymer chain by including a polyfunctional compound in the polymerization component of the base polymer, and the like can be mentioned. These may be used in combination to introduce a plurality of crosslinked structures into the base polymer.

In the method (1) of reacting the base polymer with the cross-linking agent, the cross-linking structure is introduced into the base polymer by adding the cross-linking agent to the polymerized base polymer and heating as necessary. Examples of the cross-linking agent include compounds that react with functional groups such as hydroxy groups and carboxy groups contained in the base polymer. Specific examples of the cross-linking agent include isocyanate-based cross-linking agents, epoxy-based cross-linking agents, oxazoline-based cross-linking agents, aziridine-based cross-linking agents, carbodiimide-based cross-linking agents, metal chelate-based cross-linking agents and the like.

Of these, isocyanate-based cross-linking agents and epoxy-based cross-linking agents are preferable because they have high reactivity with the hydroxy group and carboxy group of the base polymer and the introduction of a cross-linked structure is easy. These cross-linking agents react with functional groups such as hydroxy groups and carboxy groups introduced into the base polymer to form a cross-linked structure. In an acid-free pressure-sensitive adhesive in which the base polymer does not contain a carboxy group, it is preferable to use an isocyanate-based cross-linking agent to form a cross-linked structure by reacting the hydroxy group in the base polymer with the isocyanate cross-linking agent.

As the isocyanate-based cross-linking agent, polyisocyanate having two or more isocyanate groups in one molecule is used. Examples of the isocyanate-based cross-linking agent include lower aliphatic polyisocyanates such as butylene diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentylene diisocyanate, cyclohexylene diisocyanate and isophorone diisocyanate; 2,4-tolylene diisocyanate. Aromatic isocyanates such as isocyanate, 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate; trimethylolpropane / tolylene diisocyanate trimer adduct (eg, "Coronate L" manufactured by Toso), trimethylolpropane / hexamethylene. Diisocyanate trimeric adduct (eg, Tosoh's "Coronate HL"), xylylene diisocyanate trimethylolpropane adduct (eg, Mitsui Chemicals' "Takenate D110N", hexamethylene diisocyanate isocyanurate (eg, Tosoh's "" Examples thereof include isocyanate additives such as "Coronate HX").

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

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

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

Examples of the polyfunctional (meth) acrylate include polyethylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, polytetramethylene glycol di (meth) acrylate, bisphenol A ethylene oxide-modified di (meth) acrylate, and bisphenol A propylene oxide. Modified di (meth) acrylate, alkanediol di (meth) acrylate, tricyclodecanedimethanol di (meth) acrylate, ethoxylated isocyanuric acid tri (meth) acrylate, pentaeristol tri (meth) acrylate, pentaeristoldi ( Meta) Acrylate, Trimethylol Propanetri (Meta) Acrylate, Ditrimethylol Propanetetra (Meta) Acrylate, Pentaeristol Tetra (Meta) Acrylate, Penta Eristol Tetra (Meta) Acrylate, Dipenta Eristol Poly (Meta) Examples thereof include acrylate, dipentaerylistol hexa (meth) acrylate, neopentyl glycol di (meth) acrylate, glycerin di (meth) acrylate, epoxy (meth) acrylate, butadiene (meth) acrylate, and isoprene (meth) acrylate.

The molecular weight of the polyfunctional compound such as polyfunctional (meth) acrylate is preferably 1500 or less, more preferably 1000 or less. The functional group equivalent (g / eq) of the polyfunctional compound is preferably 50 to 500, more preferably 70 to 300, and even more preferably 80 to 200. When the molecular weight of the polyfunctional compound is within this range, the void distance can be reduced.

<Preparation of base polymer>
The acrylic base polymer can be prepared by a known polymerization method such as solution polymerization, UV polymerization, bulk polymerization, and emulsion polymerization. A solution polymerization method or an active energy ray polymerization method (for example, UV polymerization) is preferable in terms of the transparency, water resistance, cost and the like of the pressure-sensitive adhesive. Ethyl acetate, toluene and the like are generally used as the solvent for solution polymerization.

In preparing the acrylic base polymer, a polymerization initiator such as a photopolymerization initiator or a thermal polymerization initiator may be used depending on the type of the polymerization reaction. The photopolymerization initiator is not particularly limited as long as it initiates photopolymerization, and is, for example, a benzoin ether-based photopolymerization initiator, an acetophenone-based photopolymerization initiator, an α-ketol-based photopolymerization initiator, and an 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, thioxanthone-based photopolymerization initiator , Acylphosphine oxide-based photopolymerization initiator and the like can be used. Examples of the thermal polymerization initiator include an azo-based initiator, a peroxide-based initiator, and a redox-based initiator in which a peroxide and a reducing agent are combined (for example, a combination of a persulfate and sodium hydrogen sulfite, and peroxidation. A combination of a product and sodium ascorbate, etc.) can be used.

At the time of polymerization, a chain transfer agent, a polymerization inhibitor (polymerization retarder) or the like may be used for the purpose of adjusting the molecular weight. Examples of the chain transfer agent include thiols such as α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, and 2,3-dimercapto-1-propanol. Examples thereof include α-methylstyrene dimer.

The molecular weight of the base polymer can be adjusted by adjusting the type and amount of the polymerization initiator. For example, in radical polymerization, the larger the amount of the polymerization initiator, the higher the radical concentration of the reaction system, so that the density of the reaction starting point tends to be high and the molecular weight tends to be small. On the contrary, as the amount of the polymerization initiator is smaller, the density of the reaction initiation point is smaller, so that the polymer chain tends to be easily elongated and the molecular weight tends to be larger.

In order to obtain a pressure-sensitive adhesive sheet having excellent adhesive strength and a small gap distance, it is preferable that the acrylic base polymer has a high gel fraction with a small cross-linking point density. In order to increase the gel fraction (ratio of polymer chains into which the crosslinked structure is introduced) with a small crosslink density, the molecular weight of the base polymer (length of the polymer chain) may be increased. As described above, in order to increase the molecular weight of the base polymer, it is preferable to reduce the amount of the polymerization initiator used when polymerizing the base polymer.

The amount of the polymerization initiator used during the polymerization of the base polymer may be appropriately set according to the type of the polymerization reaction, the composition of the monomer, the type of the polymerization initiator, the target molecular weight and the like. From the viewpoint of increasing the molecular weight of the base polymer and increasing the gel fraction with a small amount of cross-linking agent, the amount of the polymerization initiator used is 0.001 to 0 with respect to 100 parts by weight of the total monomer components constituting the base polymer. .4 parts by weight is preferable, 0.003 to 0.1 parts by weight is more preferable, and 0.005 to 0.05 parts by weight is further preferable.

When the crosslinked structure is introduced by an isocyanate-based crosslinking agent or the like, it is preferable to polymerize the base polymer by solution polymerization, add the crosslinking agent, and heat as necessary to introduce the crosslinked structure into the base polymer. When introducing a crosslinked structure with a polyfunctional compound such as polyfunctional (meth) acrylate, polymerize the base polymer or prepare a prepolymer composition by solution polymerization or active energy ray polymerization, and after adding the polyfunctional compound. , It is preferable to introduce a crosslinked structure by a polyfunctional compound by active energy beam polymerization.

The prepolymer composition is prepared, for example, by partially polymerizing (prepolymerizing) a composition (referred to as "composition for forming a prepolymer") in which a monomer component constituting an acrylic base polymer and a polymerization initiator are mixed. can. The monomer in the composition for forming a prepolymer is preferably a monofunctional monomer component such as a (meth) acrylic acid alkyl ester or a polar group-containing monomer. The composition for forming a prepolymer may contain a polyfunctional monomer in addition to the monofunctional monomer. For example, a part of the polyfunctional monomer may be contained in the composition for forming a prepolymer, and the rest of the polyfunctional monomer component may be added after the prepolymerization to carry out the main polymerization.

The polymerization rate of the prepolymer is not particularly limited, but is preferably 3 to 50% by weight, more preferably 5 to 40% by weight, from the viewpoint of obtaining a viscosity suitable for coating on a substrate. The polymerization rate of the prepolymer can be adjusted to a desired range by adjusting the type and amount of the photopolymerization initiator, the irradiation intensity and irradiation time of active light such as UV light, and the like.

<Acrylic oligomer>
The pressure-sensitive adhesive sheet may contain an oligomer in addition to the acrylic base polymer. As the acrylic oligomer, those having a weight average molecular weight of about 1000 to 30,000 are used. The acrylic oligomer contains a (meth) acrylic acid alkyl ester as a main constituent monomer component.

The glass transition temperature of the acrylic oligomer is preferably 60 ° C. or higher, more preferably 80 ° C. or higher, further preferably 100 ° C. or higher, and particularly preferably 110 ° C. or higher. By using a low Tg acrylic base polymer having a crosslinked structure introduced and a high Tg acrylic oligomer in combination, the adhesive strength of the pressure-sensitive adhesive sheet, especially the adhesive holding power at high temperature, tends to be improved, and the gap distance can be reduced. Can be made smaller. The upper limit of the glass transition temperature of the acrylic oligomer is not particularly limited, but is generally 200 ° C. or lower, preferably 180 ° C. or lower, and more preferably 160 ° C. or lower. The glass transition temperature of the acrylic oligomer is calculated by the above-mentioned Fox formula.

Acrylic oligomers having a glass transition temperature of 60 ° C. or higher have a (meth) acrylic acid alkyl ester (chain alkyl (meth) acrylate) having a chain alkyl group and an alicyclic alkyl group as constituent monomer components (the chain alkyl (meth) acrylate). Those containing a meta) acrylic acid alkyl ester (aliphatic alkyl (meth) acrylate) are preferable. Specific examples of the chain alkyl (meth) acrylate and the alicyclic alkyl (meth) acrylate are as exemplified above as the constituent monomers of the acrylic polymer chain.

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

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

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

The acrylic oligomer can be obtained by polymerizing the above-mentioned monomer components by various polymerization methods. Various polymerization initiators may be used in the polymerization of the acrylic oligomer. Further, a chain transfer agent may be used for the purpose of adjusting the molecular weight.

The content of the acrylic oligomer in the pressure-sensitive adhesive sheet is not particularly limited, but in order to sufficiently enhance the adhesive force, the amount of the acrylic oligomer is preferably 0.5 part by weight or more, preferably 0.8 part by weight, based on 100 parts by weight of the base polymer. More than 1 part by weight is more preferable, and 1 part by weight or more is further preferable. The amount of acrylic oligomer in the pressure-sensitive adhesive sheet is 1.3 parts by weight or more, 1.5 parts by weight or more, 1.8 parts by weight or more, 2 parts by weight or more, or 2.3 parts by weight with respect to 100 parts by weight of the base polymer. It may be more than 2.5 parts by weight or more than 2.5 parts by weight. The larger the amount of the high Tg acrylic oligomer added, the smaller the void distance tends to be.

On the other hand, if the amount of the acrylic oligomer added is excessively large, the haze of the pressure-sensitive adhesive sheet tends to increase due to the decrease in compatibility, and the transparency tends to decrease. Since the adhesive sheet arranged on the visual side of the image display panel is required to have high transparency, the amount of the acrylic oligomer in the adhesive sheet is preferably 5 parts by weight or less with respect to 100 parts by weight of the base polymer. It may be less than a part by weight or less than 3 parts by weight.

<Adhesive composition>
If necessary, the acrylic base polymer (or prepolymer composition) is mixed with the above acrylic oligomer, a cross-linking agent and / or a polyfunctional compound for introducing a cross-linked structure, and other additives to adhere. Prepare the agent composition. If necessary, the balance of the monomer components constituting the acrylic base polymer may be added to the pressure-sensitive adhesive composition. A thickening additive or the like may be used for the purpose of adjusting the viscosity.

When the pressure-sensitive adhesive composition contains a prepolymer composition, a polyfunctional compound and the like, the pressure-sensitive adhesive composition preferably contains a photopolymerization initiator for the main polymerization. After the prepolymerization, a polymerization initiator for the main polymerization may be added to the prepolymer composition. If the polymerization initiator during the prepolymerization remains in the prepolymer composition without being deactivated, the addition of the polymerization initiator for the main polymerization may be omitted. The pressure-sensitive adhesive composition may contain a chain transfer agent.

The content of the acrylic base polymer (or prepolymer composition) in the pressure-sensitive adhesive composition is preferably 50% by weight or more, more preferably 70% by weight or more, and 80% by weight or more, based on the total amount of the non-volatile content. Is more preferable, and 90% by weight or more is particularly preferable.

The amount of the cross-linking agent and / or the polyfunctional compound in the pressure-sensitive adhesive composition may be adjusted so that the gel fraction is within the above range. As described above, in order to reduce the void distance, it is preferable to increase the molecular weight of the acrylic base polymer and increase the gel fraction with a small crosslink point density. For example, when the cross-linking structure is introduced by an isocyanate-based cross-linking agent, the amount of the cross-linking agent is preferably 0.005 to 0.5 parts by weight, preferably 0.01 to 0.3 parts by weight, based on 100 parts by weight of the acrylic base polymer. By weight is more preferred, and 0.02 to 0.1 by weight is even more preferred. When the crosslinked structure is introduced by the polyfunctional (meth) acrylate, the amount of the polyfunctional (meth) acrylate is preferably 0.005 to 0.3 parts by weight with respect to 100 parts by weight of the acrylic base polymer (prepolymer). 0.01 to 0.2 parts by weight is more preferable, and 0.02 to 0.1 parts by weight is further preferable.

(Silane coupling agent)
A silane coupling agent may be added to the pressure-sensitive adhesive composition. When a silane coupling agent is added to the pressure-sensitive adhesive composition, the amount of the silane coupling agent added is usually about 0.01 to 5.0 parts by weight with respect to 100 parts by weight of the base polymer, and 0.03 to 3.0 parts by weight. It is preferably about a part by weight. When the amount of the silane coupling agent is within the above range, the void distance may be reduced.

(Other additives)
In addition to the above-exemplified components, the pressure-sensitive adhesive composition includes a pressure-sensitive adhesive, a plasticizer, a softening agent, a deterioration inhibitor, a filler, a colorant, an ultraviolet absorber, an antioxidant, a surfactant, an antistatic agent, and the like. May contain the additive of.

<Formation of adhesive sheet>
A pressure-sensitive adhesive sheet is formed on the base material by applying the pressure-sensitive adhesive composition on the base material, drying and removing the solvent as necessary, and / or performing the main polymerization by irradiation with active light rays. As the base material used for forming the pressure-sensitive adhesive sheet, any suitable base material is used. As the base material, a release film having a release layer on the contact surface with the pressure-sensitive adhesive sheet may be used.

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

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

When the base polymer of the pressure-sensitive adhesive composition is a solution-polymerized polymer, it is preferable to dry the solvent after application. As the drying method, an appropriate method can be appropriately adopted depending on the purpose. The heating and drying temperature is preferably 40 ° C. to 200 ° C., more preferably 50 ° C. to 180 ° C., and particularly preferably 70 ° C. to 170 ° C. As the drying time, an appropriate time may be adopted as appropriate. The drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 15 minutes, and particularly preferably 10 seconds to 10 minutes.

When the pressure-sensitive adhesive composition contains a cross-linking agent, a cross-linking reaction may be carried out after the pressure-sensitive adhesive composition is applied onto the substrate. At the time of cross-linking, heating may be performed if necessary. The temperature of the crosslinking reaction is usually in the range of 20 ° C. to 160 ° C., and the time of the crosslinking reaction is about 1 minute to 7 days. After applying the pressure-sensitive adhesive composition, heating for drying the solvent may also serve as heating for crosslinking. After the solvent is dried, it is preferable to attach a cover sheet to protect the surface of the pressure-sensitive adhesive sheet. As the cover sheet, it is preferable to use a release film having a release layer on the contact surface with the pressure-sensitive adhesive sheet, as in the case of the base film.

When the pressure-sensitive adhesive composition is a photopolymerizable composition containing a prepolymer composition, a polyfunctional compound, or the like, the pressure-sensitive adhesive composition is applied in layers on a substrate, and then light is irradiated by irradiating with active light. Curing is done. When photo-curing, a cover sheet may be attached to the surface of the coating layer, and the pressure-sensitive adhesive composition may be sandwiched between two sheets and irradiated with active light to prevent polymerization inhibition due to oxygen. preferable.

The active light may be selected according to the type of polymerizable component such as a monomer or polyfunctional (meth) acrylate, the type of photopolymerization initiator, etc., and generally, ultraviolet rays and / or short wavelength visible light are used. Be done. The integrated light amount of the irradiation light is preferably about 100 to 5000 mJ / cm ² . The light source for light irradiation is not particularly limited as long as the photopolymerization initiator contained in the pressure-sensitive adhesive composition can irradiate light in a sensitive wavelength range, and is an LED light source, a high-pressure mercury lamp, and ultra-high pressure mercury. Lamps, metal halide lamps, xenon lamps and the like are preferably used.

[Laminated structure of adhesive sheet]
By laminating the release films 91 and 92 on the surface of the pressure-sensitive adhesive sheet 11, as shown in FIG. 1, a pressure-sensitive adhesive sheet in which the release films are temporarily attached to both sides can be obtained. The base material and the cover sheet at the time of forming the adhesive sheet may be used as they are as the release films 91 and 92.

When the release films 91 and 92 are provided on both sides of the pressure-sensitive adhesive sheet 11, the thickness of one release film 91 and the thickness of the other release film 92 may be the same or different. Even if the peeling force for peeling the release film temporarily attached to one surface from the adhesive sheet 11 and the release force for peeling the release film temporarily attached to the other surface from the adhesive sheet 11 are the same. It may be different. If the peeling forces of the two are different, the release film 91 (light peeling film) having a relatively small peeling force is peeled off from the adhesive sheet 11 first and bonded to the first adherend, and then relative to each other. It is excellent in workability when the release film 92 (heavy release film) having a large peeling force is peeled off and bonded to the second adherend.

[Image display device]
As described above, the adhesive sheet of the present invention is used for bonding members of a bendable image display device provided with a touch panel. In the image display device 101 shown in FIG. 2, a touch panel 41, a circular polarizing plate 31 and a cover window 71 are arranged on the visible side surface of the organic EL panel 51 as an image display panel. In a flexible display, all of these members are flexible and bendable. In the image display device 102 shown in FIG. 3, a touch panel is integrated with an organic EL panel 54 as an image display panel, and a circular polarizing plate 31 and a cover window 71 are arranged on the surface thereof.

<Image display panel>
The organic EL panel includes a pair of electrodes and an organic light emitting layer sandwiched between the electrodes on a substrate. The organic EL panel is either a top emission type in which a metal electrode, an organic light emitting layer and a transparent electrode are sequentially laminated on a substrate, or a bottom emission type in which a transparent electrode, an organic light emitting layer and a metal electrode are sequentially laminated on a transparent substrate. But it may be. In both the bottom emission type and the top emission type, the substrate and the sealing member provided on the visual side of the organic light emitting layer are transparent. The substrate, sealing member, and the like provided on the back surface side (the housing 75 side in FIGS. 2 and 3) of the organic light emitting layer do not have to be transparent. In the bottom emission type flexible organic EL panel, the substrate does not have to be transparent, and polyimide or the like may be used as the substrate material. The substrate material may be a transparent resin material such as polyetheretherketone or transparent polyimide. A back sheet may be provided on the back surface side of the substrate for the purpose of protecting or reinforcing the substrate.

The image display panel is not limited to the organic EL panel, and may be a liquid crystal panel, an electrophoresis type display panel (electronic paper), or the like. For example, a bendable liquid crystal panel can be formed by using a flexible substrate such as a resin substrate as a transparent substrate that sandwiches the liquid crystal layer.

<Cover window>
A cover window 71 is provided on the outermost surface of the image display device on the visual recognition side for the purpose of preventing damage to the image display panel due to an impact from the outer surface. In the flexible display, a flexible transparent substrate such as transparent polyimide, polyetheretherketone, or polyethylene terephthalate is used as the cover window 71. As the material of the cover window 71, a flexible glass plate (glass film) may be used, and the cover window 71 may have a laminated structure of a glass film and a resin film. From the viewpoint of achieving both strength and flexibility, the thickness of the cover window is preferably 20 to 300 μm, more preferably 25 to 250 μm, and even more preferably 30 to 200 μm. The yield point elongation of the cover window is preferably 5% or more because it is excellent in recoverability after holding the bent state for a long time. A foldable thin glass substrate may be used as the cover window 71. The cover window may be a stack of two or more layers of transparent materials. An antireflection layer, a hard coat layer, or the like may be provided on the visible side surface of the cover window.

<Touch panel>
The image display device is provided with a capacitance type touch panel on the visible side surface of the image display panel. The capacitive touch panel detects the touch position based on the change in the amount of electricity when the operator's finger, stylus, or the like touches the touch surface. In the configuration of FIG. 2, the touch panel 41 is arranged between the circularly polarizing plate 31 and the organic EL panel 51. In the configuration of FIG. 3, a touch panel is provided inside the image display panel 54. A touch panel may be arranged between the circularly polarizing plate 31 and the cover window 71.

In a flexible display, since the thickness of the cover window is small, the distance D from the touch surface to the touch panel 41 tends to be small. In an image display device using a rigid glass plate as a cover window, the distance from the touch surface to the touch panel is generally 700 μm or more, whereas in a flexible display, the distance from the touch surface to the touch panel 41 (the touch panel is an in-cell type). In the case of, the distance D from the touch surface to the image display panel) D is 500 μm or less. The distance D from the touch surface to the touch panel may be 400 μm or less, 350 μm or less, or 300 μm or less.

<Polarizer>
Generally, a polarizing plate 31 is provided on the visual side of the image display panel. For example, in a liquid crystal display device, a polarizing plate provided on the visual side of a liquid crystal panel adjusts the transmittance according to the polarization state of the light transmitted through the liquid crystal cell. In the organic EL display device, by providing a circular polarizing plate 31 on the visible side of the organic EL panel 51, the external light reflected by the metal electrode of the organic EL panel is shielded from being emitted to the visible side to improve the visibility of the display. can.

As the polarizing plate, one in which an appropriate transparent protective film is bonded to one side or both sides of the polarizing element, if necessary, is generally used. The polarizing element is not particularly limited, and various types can be used. As the extruder, for example, a hydrophilic polymer film such as a polyvinyl alcohol-based film, a partially formalized polyvinyl alcohol-based film, an ethylene / vinyl acetate copolymerization system partially saponified film, and two colors such as iodine and a dichroic dye are used. Examples thereof include a uniaxially stretched film by adsorbing a sex substance, a polyene-based oriented film such as a dehydrated product of polyvinyl alcohol and a dehydrogenated product of polyvinyl chloride.

As the substituent, a thin polarizing element having a thickness of 10 μm or less can also be used. Examples of the thin polarizing element include the splitters described in JP-A No. 51-06644, JP-A-2000-338329, WO2010 / 100917, Patent No. 4691,205, and Patent No. 4751481. Can be done. The thin polarizing element is obtained, for example, by a manufacturing method including a step of stretching a polyvinyl alcohol-based resin layer and a stretching resin base material in a laminated state, and a step of dyeing with a dichroic material such as iodine.

The transparent protective film as a protective film for the polarizing element includes transparency, mechanical strength, thermal stability, and moisture blocking of cellulose-based resin, cyclic polyolefin-based resin, acrylic resin, phenylmaleimide-based resin, polycarbonate-based resin, etc. Those having excellent properties and optical isotropic properties are preferably used. When a transparent protective film is provided on both sides of the polarizing element, a protective film made of the same polymer material may be used on the front and back sides thereof, or a protective film made of a different polymer material or the like may be used.

An optical film may be laminated on one or both surfaces of the polarizing plate via an appropriate adhesive layer or adhesive layer, if necessary. As such a film, a film used for forming an image display device such as a retardation plate, a viewing angle expanding film, a viewing angle limiting (peeping prevention) film, and a brightness improving film is used, and the type thereof is not particularly limited. .. For example, in a liquid crystal display device, an image display panel (liquid crystal panel) and a polarizing plate are used for the purpose of appropriately converting the polarization state of light emitted from a liquid crystal cell to the visual recognition side to improve viewing angle characteristics. An optical compensation film may be used in between.

As described above, in the organic EL display device, by providing a circular polarizing plate in which a 1/4 wave plate is arranged on the surface of the polarizing element on the organic EL panel side, the external light reflected by the metal electrode is emitted to the visual recognition side. Can be shielded. By arranging a 1/4 wave plate on the viewing side of the polarizing element and using circularly polarized light as the emitted light, it is possible to visually recognize an appropriate image display even to a viewer wearing polarized sunglasses. These optical films (optical anisotropic films) may be laminated on the polarizing element without interposing another film. In this case, the optical film also functions as a protective film for the stator.

The thickness of the polarizing plate is generally about 10 to 200 μm. From the viewpoint of imparting flexibility, the thickness of the polarizing plate used in the flexible display is preferably 100 μm or less, more preferably 80 μm or less, still more preferably 70 μm or less. When an optical film such as a 1/4 wave plate is laminated on the polarizing plate, it is preferable that the total thickness including these films is within the above range.

<Laminating between members with adhesive sheet>
An adhesive sheet is used for bonding between the above flexible members. In the image display device shown in FIG. 2, the organic EL panel 51 and the bottom surface of the housing 75 are bonded to each other via the adhesive sheet 14, the organic EL panel 51 and the touch panel 41 are bonded to each other via the adhesive sheet 13, and the touch panel 41 is attached. And the circularly polarizing plate 31 are attached to each other via the adhesive sheet 12, and the circularly polarizing plate 31 and the cover window 71 are attached to each other via the adhesive sheet 11. In the image display device shown in FIG. 3, the touch panel integrated organic EL panel 54 and the circular polarizing plate 31 are bonded to each other via the adhesive sheet 12, and the circular polarizing plate 31 and the cover window 71 are bonded to each other via the adhesive sheet 11. ing.

It is preferable to use the above-mentioned adhesive sheet of the present invention for either or both of the adhesive sheets 11 and 12 arranged on the visual side of the touch panel. It is preferable to use the pressure-sensitive adhesive sheet of the present invention as the pressure-sensitive adhesive sheet 11 from the viewpoint of preventing the cover window 71 from peeling off at the bent portion when the bent state is maintained or when the bending is repeated. Further, it is preferable that both the pressure-sensitive adhesive sheets 11 and 12 have the above-mentioned relative permittivity.

As described above, even when the distance D from the touch surface to the touch panel is small, the low dielectric constant of the adhesive sheet makes it possible to reduce the malfunction of the touch panel. In addition, since the adhesive sheet has a gap distance of 2 mm or less after a predetermined bending and holding test, failure and the like are suppressed or prevented even when the device is held in a folded state for a long time or when the device is repeatedly folded, and is reliable. Highly sex.

The order of bonding the members is not particularly limited, and the touch panel 41, the circular polarizing plate 31 and the cover window 71 may be laminated in order on the image display panel 51, and two or more members may be laminated in advance via an adhesive sheet. The laminated body may be laminated on the image display panel 51.

[Film and laminate with adhesive layer]
The pressure-sensitive adhesive sheet of the present invention is used for forming an image display device as a polarizing plate with a pressure-sensitive adhesive layer in which a pressure-sensitive adhesive sheet is fixedly laminated on the surface of a polarizing plate in addition to a form in which a release film is temporarily attached to both sides. You can also. For example, as in the polarizing plate 5 with an adhesive layer shown in FIG. 4, a release film 93 is temporarily attached to one surface of the adhesive sheet 11, and a circular polarizing plate 31 is fixedly laminated on the other surface of the adhesive sheet 11. You may. In the form shown in FIG. 5, the pressure-sensitive adhesive sheet 11 is fixedly laminated on one surface of the circularly polarizing plate 31, and the pressure-sensitive adhesive sheet 12 is fixedly laminated on the other surface of the circularly polarizing plate 31. As shown in FIG. 5, in the film 6 with the double-sided adhesive layer, it is preferable that the release films 93 and 94 are temporarily attached to the surfaces of the adhesive sheets 11 and 12.

In the form in which a polarizing plate or the like is laminated in advance on the pressure-sensitive adhesive sheet, one release film 93 temporarily adhered to the surface of the pressure-sensitive adhesive sheet 11 is peeled off and bonded to an adherend, and the other release film 94 is attached. It may be peeled off and bonded to another adherend. By laminating the flexible member constituting the image display device and the adhesive sheet in advance, the process of forming the image display device can be simplified.

In the film 6 with the double-sided adhesive layer shown in FIG. 5, the thicknesses of the adhesive sheet 11 laminated on one surface of the polarizing plate 31 and the adhesive sheet 12 laminated on the other surface of the polarizing plate 31 are the same but different. May be good. For example, when the pressure-sensitive adhesive sheet 12 is used for bonding the polarizing plate 31 to a touch panel or an image display panel, and the pressure-sensitive adhesive sheet 11 is used for bonding the polarizing plate 31 and the cover window 71, the thickness of the pressure-sensitive adhesive sheet 11 is adhesive. It is preferably larger than the thickness of the sheet 12.

By relatively increasing the thickness of the adhesive sheet 11 provided on the viewing side, it is possible to provide cushioning property against an impact from the outer surface and prevent damage to the image display panel. The thickness of the pressure-sensitive adhesive sheet 11 is preferably 25 to 100 μm, more preferably 30 to 75 μm. The thickness of the pressure-sensitive adhesive sheet 11 may be 35 μm or more or 40 μm or more. The thickness of the adhesive sheet 12 provided on the image display panel side is preferably 10 to 25 μm, and may be 20 μm or less.

In the film 6 with the double-sided adhesive layer, it is preferable that both the adhesive sheets 11 and 12 have the above-mentioned relative permittivity. Further, it is preferable that both the pressure-sensitive adhesive sheets 11 and 12 are the pressure-sensitive adhesive sheets of the present invention (that is, the pressure-sensitive adhesive sheets having a void distance of 2 mm or less after a predetermined bending holding test).

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

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

[Example 1]
(Polymerization of prepolymer)
43 parts by weight of lauryl acrylate (LA), 44 parts by weight of 2-ethylhexyl acrylate (2EHA), 6 parts by weight of 4-hydroxybutyl acrylate (4HBA), and N-vinyl-2-pyrrolidone (NVP) as monomer components for prepolymer formation. ) 7 parts by weight and 0.015 parts by weight of "Omnirad 184" manufactured by IGM Resins as a photopolymerization initiator are blended and polymerized by irradiating with ultraviolet rays to obtain a prepolymer composition (polymerization rate; about 10%). rice field.

(Preparation of adhesive composition)
To 100 parts by weight of the above prepolymer composition, 0.07 parts by weight of 1,6-hexanediol diacrylate (HDDA), the above oligomer: 3 parts by weight, and a silane coupling agent (Shin-Etsu Chemical Co., Ltd.) as post-addition components. Manufactured by "KBM403"): After adding 0.3 parts by weight, these were uniformly mixed to prepare a pressure-sensitive adhesive composition.

(Making an adhesive sheet)
A polyethylene terephthalate (PET) film (Mitsubishi Chemical "Diafoil MRF75") with a thickness of 75 μm provided with a silicone-based release layer on the surface is used as a base material (cum-heavy release film), and the above-mentioned photocurable adhesive is applied on the base material. The agent composition was applied so as to have a thickness of 50 μm to form a coating layer. A PET film ("Diafoil MRE75" manufactured by Mitsubishi Chemical Corporation) having a thickness of 75 μm, one side of which was treated with silicone peeling, was bonded onto this coating layer as a cover sheet (also a light peeling film). The laminated body is photo-cured by irradiating it with ultraviolet rays from the cover sheet side with a black light whose position is adjusted so that the irradiation intensity on the irradiation surface directly under the lamp is 5 mW / cm ² , and a pressure-sensitive adhesive sheet having a thickness of 50 μm is formed. Obtained.

[Examples 2 to 5, Comparative Examples 1 to 3]
The composition of the charged monomer in the polymerization of the prepolymer, the blending amount of the polyfunctional monomer (HDDA), and the blending amount of the oligomer were changed as shown in Table 1. A photocurable pressure-sensitive adhesive composition was prepared in the same manner as in Example 1 except for the above, and coated on a substrate and photo-cured to obtain a pressure-sensitive adhesive sheet.

[Preparation of adhesive sheet A]
Butyl acrylate (BA): 99 parts by weight and 4-hydroxybutyl acrylate (4HBA): 1 part by weight as monomers and 2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator in the reaction vessel. ): 0.3 part was added together with ethyl acetate and reacted at 60 ° C. for 4 hours under a nitrogen gas stream. Then, ethyl acetate was added to the reaction solution to obtain a solution of an acrylic polymer having a weight average molecular weight of 1.65 million. In this solution, dibenzoyl peroxide (“Niper BMT” manufactured by Nippon Oil & Fats Co., Ltd.): 0.3 parts by weight and trimethylolpropane xylylene diisocyanate (“Takenate D110N” manufactured by Mitsui Chemicals) as a cross-linking agent with respect to 100 parts by weight of the polymer. : 0.1 part by weight and silane coupling agent (“KBM403” manufactured by Shin-Etsu Chemical Co., Ltd.): 3 parts by weight were blended to obtain a pressure-sensitive adhesive composition A.

The above pressure-sensitive adhesive composition A is applied to a release-treated surface of a release-treated PET film (“MRF38” manufactured by Mitsubishi Chemical Corporation) having a silicone-based release layer on the surface and dried at 150 ° C. And cross-linking treatment was carried out to obtain a pressure-sensitive adhesive sheet A having a thickness of 15 μm.

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

<Store modulus, loss tangent, and glass transition temperature>
A sample having a thickness of about 1.5 mm by laminating adhesive sheets was used as a measurement sample. Dynamic viscoelasticity measurement was performed under the following conditions using the "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific. From the measurement results, the storage elastic modulus G'and the loss tangent tan δ at each temperature were read. Further, the temperature at which tan δ became maximum was defined as the glass transition temperature of the pressure-sensitive adhesive sheet.

(Measurement condition)
Deformation mode: Torsion measurement frequency: 1Hz
Temperature rise rate: 5 ° C / min Shape: Parallel plate 7.9 mmφ

<Total light transmittance and haze>
Using a test piece in which an adhesive sheet is attached to non-alkali glass (thickness 0.8 to 1.0 mm, total light transmittance 92%, haze 0.4%), a haze meter (Murakami Color Technology Research Institute "HM-" 150 ”) was used to measure haze and total light transmittance. The value obtained by subtracting the haze (0.4%) of the non-alkali glass from the measured value was taken as the haze of the adhesive sheet. For the total light transmittance, the measured value was adopted as it is. The total light transmittance of the pressure-sensitive adhesive sheets of both Examples and Comparative Examples was 92%. The haze of the pressure-sensitive adhesive sheet of Comparative Example 1 was 0.5%, and the haze of the pressure-sensitive adhesive sheets of the other Examples and Comparative Examples was 0.3%.

<Relative permittivity>
An adhesive sheet is sandwiched between a copper foil and an electrode, and the relative permittivity at frequencies of 1 kHz, 10 kHz, 100 kHz, and 1 MHz is measured using Agilent Technologies "Precision Impedance Analyzer 4294A" under the following conditions according to JIS K6911. did. In Examples 1, 4 and 5 and Comparative Example 3, in addition to the measurement at a temperature of 25 ° C., the relative permittivity was measured every 20 ° C. in the temperature range of −40 ° C. to 80 ° C.
Electrode configuration: 12.1 mmΦ, 0.5 mm thick aluminum plate Opposite electrode: 3oz copper plate Measurement environment: temperature 25 ° C, relative humidity 50%

<Adhesive strength to polyimide film>
The release film on one side was peeled off from the pressure-sensitive adhesive sheet, a PET film having a thickness of 25 μm was attached, and the test piece was cut into a width of 10 mm and a length of 100 mm. The release film on the other surface was peeled off from the test piece, and the adhesive sheet was pressure-bonded to a transparent polyimide film (manufactured by Coron Industry) having a thickness of 80 μm using a 2 kg roller. Using a tensile tester, the test piece was peeled from the polyimide film under the conditions of a tensile speed of 60 mm / min and a peeling angle of 180 ° in an environment of 25 ° C., and the peeling force was measured.

<Bending retention test>
The release film on one side was peeled off from the pressure-sensitive adhesive sheet, and a polarizing plate having a thickness of 51 μm was attached using a 2 kg roller. The release film on the other side of the pressure-sensitive adhesive sheet was peeled off, and a transparent polyimide film having a thickness of 80 μm was bonded using a 2 kg roller. Further, a PET film having a thickness of 125 μm was bonded onto the polarizing plate via a pressure-sensitive adhesive sheet A having a thickness of 15 μm using a roller of 2 kg. At the time of bonding, plasma treatment was performed on the surfaces of the polarizing plate, the polyimide film and the PET film before bonding with the pressure-sensitive adhesive sheet.

This laminate was cut into a rectangle of 35 mm × 100 mm so that the absorption axis direction of the polarizing plate was parallel to the long side direction, and autoclaved at 35 ° C. and 0.35 MPa for 15 minutes to obtain a test piece. Using a planar unloaded U-shaped expansion / contraction tester (manufactured by Yuasa System Equipment), attach and fix a bending tool within a range of 20 mm from each end in the long side direction of the test piece (the area of 60 mm in the center in the long side direction is (Not fixed), hold in a bent state with a bending radius of 1.3 mm and a bending angle of 180 ° so that the surface on the PET film side is on the inside, and a constant temperature and humidity chamber with a temperature of 60 ° C and a relative humidity of 95%. The bending holding test was carried out by holding the film in the room for 240 hours.

The test piece after the bending retention test was visually confirmed, and the presence or absence of peeling at the interface between the transparent polyimide film and the polarizing plate at the bending portion was confirmed. In all cases where peeling was confirmed, peeling (voids) occurred from the end in the short side direction of the sample. For those in which peeling was confirmed, the length (mm) of the void portion in the short side direction of the sample was measured. The length of the void (void distance) of 35 mm was set to 35 mm for the sample in which peeling was confirmed over the entire length of the short side of the sample, and the void distance was set to 0 in the case where no peeling was confirmed. For those that had peeled off from both ends in the short side direction, the one with the larger gap length was defined as the gap distance. In any of the samples, no peeling was confirmed at the bonding interface between the PET film and the polarizing plate.

[Evaluation results]
The formulations of the pressure-sensitive adhesive compositions used to prepare the pressure-sensitive adhesive sheets of Examples and Comparative Examples are shown in Table 1, and the evaluation results are shown in Table 2. Table 2 also shows the measurement results of the dielectric constant of the pressure-sensitive adhesive sheet A used for bonding the polarizing plate and the PET film in the bending retention test sample. Table 3 shows the measurement results of the dielectric constants of the pressure-sensitive adhesive sheets of Examples 1, 4, 5 and Comparative Example 3 in the temperature range of -40 ° C to 80 ° C, and the minimum and maximum values of the relative permittivity at each frequency. The ratio X and the ratio of the value of X at a frequency of 1 kHz (X _{1 kHz} ) to the value of X at a frequency of 1 MHz (X _{1 MHz} ) are shown as a value of X 1 _kHz / X _{1 MHz} .

In Table 1, each component is described by the following abbreviations.
LA: Lauryl acrylate 2HEA: 2-ethylhexyl acrylate BA: Butyl acrylate CHA: Cyclohexyl acrylate 4HBA: 4-Hydroxybutyl acrylate 2HEA: 2-Hydroxyethyl acrylate NVP: N-vinyl-2-pyrrolidone

All of the pressure-sensitive adhesive sheets of Examples 1 to 5 had a relative permittivity of 4.5 or less and a void distance of 2 mm or less at a temperature of 25 ° C. and a frequency of 10 MHz. From Table 3, it can be seen that the pressure-sensitive adhesive sheet of the example has a small relative permittivity, and the temperature dependence and frequency dependence of the relative permittivity are also small.

11,12,13,14 Adhesive sheet 91,92 Release film (heavy release film)
93,94 Release film 1 Adhesive sheet with release film 5,6 Film with adhesive layer 31 Polarizing plate (circular polarizing plate)
51 Image display panel (organic EL panel)
54 Image display panel (organic EL panel with integrated touch panel)
41 Touch panel 71 Cover window 75 Housing 101,102 Image display device

Claims (16)

An adhesive sheet used for bonding members arranged between a touch surface and a touch panel in a bendable image display device provided with a touch panel within a distance of 500 μm from the touch surface.
The relative permittivity at a temperature of 25 ° C. and a frequency of 10 kHz is 4.5 or less.
The length of the gap between the adhesive sheet and the adherend, which is measured by the procedure of steps A to D below, is 2 mm or less.
Adhesive sheet:
Step A: Produce a 35 mm × 100 mm test piece by laminating the adhesive sheet with the adherend Step B: Produce the test piece produced in step A along the short side direction with a bending radius of 1.3 mm and a bending angle. Bending at 180 ° Step C: The test piece bent in step B is held in an environment with a temperature of 60 degrees and a relative humidity of 95% for 240 hours. Step D: The test piece after holding in step C for 240 hours. In the bent portion of the above, the length of the gap portion between the adhesive sheet and the adherend in the short side direction is measured. The pressure-sensitive adhesive sheet according to claim 1, wherein the ratio of the relative permittivity at a frequency of 1 kHz to the relative permittivity at a frequency of 1 MHz at a temperature of 25 ° C. is 1.50 or less. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the maximum value of the relative permittivity in the temperature range of −40 ° C. to 80 ° C. at a frequency of 10 kHz is 1.4 times or less the minimum value. The ratio of the maximum value to the minimum value of the relative permittivity in the temperature range of -40 ° C to 80 ° C at a frequency of 1 kHz is the maximum and minimum values of the relative permittivity in the temperature range of -40 ° C to 80 ° C at a frequency of 1 MHz. The pressure-sensitive adhesive sheet according to any one of claims 1 to 3, which is 0.8 to 1.2 times the ratio of the above. The pressure-sensitive adhesive sheet according to any one of claims 1 to 4, which has a thickness of 100 μm or less. The storage elastic modulus G'25 at ₂₅ ° C. and 1 Hz is 70 kPa or less.
The glass transition temperature is -20 ° C or lower,
The adhesive sheet according to any one of claims 1 to 5. The pressure-sensitive adhesive sheet according to any one of claims 1 to 6, which is composed of an acrylic pressure-sensitive adhesive containing an acrylic-based base polymer. The pressure-sensitive adhesive sheet according to claim 7, wherein the acrylic base polymer contains 5 to 55 parts by weight of (meth) acrylic acid C _10-20 chain alkyl ester with respect to 100 parts by weight of the total monomer component. The pressure-sensitive adhesive sheet according to claim 8, wherein the acrylic base polymer contains lauryl acrylic acid as the (meth) acrylic acid C _10-20 chain alkyl ester. The acrylic-based base polymer contains 2 to 15 weights of one or more polar group-containing monomers selected from the group consisting of hydroxy group-containing monomers, carboxy group-containing monomers, and nitrogen-containing monomers with respect to a total of 100 parts by weight of the monomer components. The adhesive sheet according to any one of claims 7 to 9, which is contained in a portion. The pressure-sensitive adhesive sheet according to any one of claims 7 to 10, wherein the acrylic base polymer has a hydroxy group-containing monomer content of 10 parts by weight or less based on a total of 100 parts by weight of the monomer components. The pressure-sensitive adhesive sheet according to any one of claims 7 to 11, wherein the acrylic base polymer has a crosslinked structure. The pressure-sensitive adhesive sheet according to claim 12, wherein the cross-linked structure is a cross-linked structure introduced by a polyfunctional (meth) acrylate. The pressure-sensitive adhesive sheet according to any one of claims 7 to 13, wherein the acrylic pressure-sensitive adhesive further contains an acrylic-based oligomer having a glass transition temperature of 60 ° C. or higher. The pressure-sensitive adhesive sheet according to claim 14, wherein the content of the acrylic oligomer is 0.1 to 5 parts by weight with respect to 100 parts by weight of the acrylic base polymer. A film with an adhesive layer, wherein the adhesive sheet according to any one of claims 1 to 15 is laminated on at least one surface of the polarizing plate.

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