Adhesive sheets and membranes are equipped with a pressure-sensitive adhesive layer.

Abstract

The invention relates to a pressure-sensitive adhesive plate with a relative dielectric constant less than or equal to 4.5 at 25°C and 10 kHz. When performing a bending maintenance test for 240 hours in an environment at 60°C and 95% relative humidity, in which a 35 mm × 100 mm adhesive plate is bonded to a holding surface held bent along the short edge direction at a bending radius of 1.3 mm and a bending angle of 180°, the length of the gap, along the short edge direction, between the adhesive plate and the holding surface in the bent portion of the specimen after the test is better to be less than or equal to 2 mm. Pressure-sensitive adhesive sheets are used to bond the components positioned between the touch surface and the touch panel in flexible image display devices, where the distance between the touch surface and the touch panel is less than or equal to 500μm, and the pressure-sensitive adhesive sheet exhibits high adhesion and can contribute to preventing touch panel malfunction.

Description

Adhesive sheets and films with adhesive layers

[0001] The present invention relates to a pressure-sensitive adhesive sheet suitable for use in bonding members together in a foldable image display device, and further to a film with a pressure-sensitive adhesive layer, in which the pressure-sensitive adhesive sheet is laminated on at least one surface of a polarizing plate.

[0002] Flat panel displays such as liquid crystal display devices and organic EL display devices are used as image display devices for mobile phones, smartphones, tablet terminals, car navigation devices, personal computer monitors, televisions, etc. In recent years, organic EL panels using bendable substrates (flexible substrates) such as resin films have been put to practical use, and bendable flexible displays have been proposed.

[0003] In a flexible display, not only is a display panel such as an organic EL panel bendable, but also components such as a housing and a touch panel are bendable, and these components are bonded together via an adhesive sheet (see, for example, Patent Document 1). In a bendable flexible display (foldable display), the transparent plate (cover window) placed on the viewing side surface must also be bendable, and a thin material such as a resin film or thin glass is used.

[0004] Foldable displays are repeatedly bent at the same location. At the bent portion, compressive stress is applied to the inside and tensile stress to the outside, causing strain at the bent portion and its surroundings, raising concerns about device damage. For this reason, it has been proposed to soften the adhesive sheet used to bond components to alleviate stress strain (see, for example, Patent Document 2).

[0005] JP 2016-2764 A JP 2018-45213 A

[0006] In recent years, there has been a demand for thinner foldable display devices. In particular, there is a demand for highly reliable devices that have little or no malfunction of the touch panel sensor and suppress or prevent breakdowns, even when the distance from the device surface to the touch panel sensor is short.

[0007] In view of the above, an object of the present invention is to provide an adhesive sheet that can be suitably used for bonding components of a flexible display, has high reliability, and can contribute to suppressing malfunction of a touch panel.

[0008] The pressure-sensitive adhesive sheet of the present invention is used to bond components arranged between a touch panel and a touch surface of a foldable image display device having a touch panel located within 500 μm of the touch surface. The pressure-sensitive adhesive sheet has a dielectric constant of 4.5 or less at a temperature of 25° C. and a frequency of 10 kHz.

[0009] The length of the gap between the pressure-sensitive adhesive sheet and the adherend, measured by the procedure of steps A to D below, is 2 mm or less. Step A: A 35 mm x 100 mm test piece is prepared by laminating the pressure-sensitive adhesive sheet to the adherend. Step B: The test piece prepared in step A is bent along the short side direction at 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 its bent state in an environment at a temperature of 60°C and a relative humidity of 95% for 240 hours. Step D: At the bent part of the test piece held in step C for 240 hours, the length of the gap in the short side direction between the pressure-sensitive adhesive sheet and the adherend is measured.

[0010] At a temperature of 25°C, the ratio of the dielectric constant of the pressure-sensitive adhesive sheet at 1 kHz to the dielectric constant at a frequency of 1 MHz is preferably 1.50 or less. The maximum value of the dielectric constant of the pressure-sensitive adhesive sheet at a frequency of 10 kHz in the temperature range of -40°C to 80°C is preferably 1.4 times or less the minimum value. The ratio of the maximum value to the minimum value of the dielectric constant of the pressure-sensitive adhesive sheet at a frequency of 1 kHz in the temperature range of -40°C to 80°C is preferably 0.8 to 1.2 times the ratio of the maximum value to the minimum value of the dielectric constant of the pressure-sensitive adhesive sheet at a frequency of 1 kHz in the temperature range of -40°C to 80°C.

[0011] The thickness of the adhesive sheet is preferably 100 μm or less.

[0012] Storage modulus G' of adhesive sheet at 25°C and 1 Hz ２５ The pressure-sensitive adhesive sheet may have a glass transition temperature of -20°C or lower.

[0013] The pressure-sensitive adhesive sheet may be made of an acrylic pressure-sensitive adhesive containing an acrylic base polymer. The acrylic base polymer may contain (meth)acrylic acid C based on 100 parts by weight of the total of the monomer components. １０－２０ The acrylic base polymer may contain 5 to 55 parts by weight of a chain alkyl ester. １０－２０ The chain alkyl ester may include lauryl acrylate.

[0014] The acrylic base polymer may contain 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, relative to 100 parts by weight of the total of the monomer components. The acrylic base polymer may contain 10 parts by weight or less of the hydroxy group-containing monomer, relative to 100 parts by weight of the total of the monomer components.

[0015] The acrylic base polymer may have a crosslinked structure, which may be introduced by a polyfunctional (meth)acrylate.

[0016] The acrylic pressure-sensitive adhesive may further contain an acrylic oligomer having a glass transition temperature of at least 60° C. The content of the acrylic oligomer may be 0.1 to 5 parts by weight based on 100 parts by weight of the acrylic base polymer.

[0017] The pressure-sensitive adhesive sheet of the present invention may be laminated on at least one surface of a polarizing plate to form a film with a pressure-sensitive adhesive layer.

[0018] An image display device using the pressure-sensitive adhesive sheet of the present invention reduces malfunction of the touch panel and can exhibit high reliability even when maintained in a bent state.

[0019] Fig. 1 is a cross-sectional view showing an example of the configuration of an adhesive sheet with a release film; Fig. 2 is a cross-sectional view showing an example of the configuration of an image display device; Fig. 3 is a cross-sectional view showing an example of the configuration of an image display device; Fig. 4 is a cross-sectional view showing an example of the stacking configuration of a polarizing plate with an adhesive sheet; Fig. 5 is a cross-sectional view showing an example of the stacking configuration of a polarizing plate with an adhesive sheet.

[0020] Fig. 1 is a cross-sectional view showing a pressure-sensitive adhesive sheet with release films, in which release films 91 and 92 are temporarily attached to both sides of a pressure-sensitive adhesive sheet 11. Fig. 2 and Fig. 3 are cross-sectional views showing the configuration of a flexible display according to one embodiment.

[0021] 2, an organic EL panel 51, a touch panel 41, and a circular polarizer 31 are disposed between a housing 75 and a cover window 71. The organic EL panel 51 and the bottom surface of the housing 75 are bonded together via an adhesive sheet 14, the organic EL panel 51 and the touch panel 41 are bonded together via an adhesive sheet 13, the touch panel 41 and the circular polarizer 31 are bonded together via an adhesive sheet 12, and the circular polarizer 31 and the cover window 71 are bonded together via an adhesive sheet 11. In this way, in a flexible display, a plurality of components are bonded together via adhesive sheets, thereby forming an integrated laminate.

[0022] The cover window 71 is disposed on the viewing side surface of the flexible display and constitutes a touch surface. The touch panel 41 is a capacitive touch panel. Since the flexible display uses a foldable cover window 71, the thickness of the cover window is small, and therefore the distance D from the touch surface (the surface of the cover window 71) to the touch panel 41 is also small.

[0023] 3 includes an organic EL panel 54 integrated with a touch panel, and a circular polarizer 31 is attached to the organic EL panel 54 via an adhesive sheet 12. The rest of the configuration is the same as in Fig. 2, and the distance from the touch surface to the viewing side surface of the organic EL panel 54 corresponds to the distance D from the touch surface to the touch panel.

[0024] The pressure-sensitive adhesive sheet of the present invention is used to bond members disposed between a touch surface and a touch panel in a bendable image display device in which the distance D from the touch surface to the touch panel is 500 μm or less.

[0025] [Characteristics of the Adhesive Sheet] The adhesive sheet of the present invention has a dielectric constant of 4.5 or less at a temperature of 25°C and a frequency of 10 kHz. Unless otherwise specified below, the dielectric constant is measured at a temperature of 25°C. The dielectric constant of the 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 dielectric constant at a frequency of 10 kHz to 4.5 or less, the distance from the touch surface to the touch panel can be reduced, enabling a design that is advantageous for flexibility. Since adhesive sheets with low dielectric constants have small capacitance values, sensors with high sensitivity can be designed. This enables input methods with small contact areas, such as pen input. Furthermore, by placing an adhesive sheet with a low dielectric constant between an organic EL panel and a touch panel, noise from the organic EL panel can be reduced, preventing malfunction of the touch panel.

[0026] The dielectric constant 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 dielectric constant 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 dielectric constant 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.

[0027] The dielectric constant of an adhesive sheet varies depending on the polarizability of the material, and the dielectric constant can be controlled by selecting the adhesive material, such as urethane, acrylic, rubber, or silica. Furthermore, since the relative dielectric constant of air is 1, the dielectric constant of an adhesive sheet can be reduced by adding hollow beads or other materials to the adhesive. In acrylic adhesives, monomers with long alkyl chains have low polarizability, making it possible to achieve low dielectric constant. Using highly polar monomers increases the polarizability, resulting in a high dielectric constant. One way to reduce polarizability is to induce molecular entanglement. Increasing the molecular weight or the degree of crosslinking makes it easier for molecular entanglement to occur, which reduces polarizability and allows for a low dielectric constant. Since increased moisture content tends to increase the dielectric constant, using materials that do not easily retain moisture can reduce the dielectric constant.

[0028] 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 dielectric constant. In order to reduce the relative dielectric constant of a pressure-sensitive adhesive sheet, the dipole moment of the base polymer constituting the pressure-sensitive adhesive can be reduced and the molar volume can be increased. For example, the larger the volume of the side chain of the base polymer, the larger the molar volume tends to be. Furthermore, by selecting a monomer with low polarity as the monomer component constituting the base polymer, the electronic dipole of the molecule can be reduced.

[0029] The pressure-sensitive adhesive sheet preferably has a small frequency dependence of its dielectric constant at a temperature of 25°C. Specifically, the ratio of the dielectric constant at a frequency of 1 kHz to the dielectric constant at a frequency of 1 MHz (1 kHz / 1 MHz) is preferably 1.5 or less. A small dielectric constant over a wide frequency range and a small frequency dependence of the dielectric constant ensure operational reliability at various operating frequencies. As described above, the frequency dependence of the dielectric constant can be reduced by adjusting the dielectric constant of the pressure-sensitive adhesive.

[0030] It is preferable that the temperature dependency of the dielectric constant of the pressure-sensitive adhesive sheet is small. Specifically, it is preferable that the ratio of the minimum value to the maximum value of the dielectric constant (maximum value / minimum value) in the temperature range of -40°C to -80°C is close to 1. The ratio X of the maximum value to the minimum value of the dielectric constant at a frequency of 10 kHz １０ｋＨｚ is preferably 1.4 or less, more preferably 1.3 or less, further preferably 1.2 or less, and may be 1.1 or less. １ｋＨｚ , the ratio X of the maximum and minimum values ​​of the relative dielectric constant at a frequency of 100 kHz １００ｋＨｚ , and the ratio X between the maximum and minimum values ​​of the relative dielectric constant at a frequency of 1 MHz １ＭＨｚ are each preferably 1.4 or less, more preferably 1.3 or less, even more preferably 1.2 or less, and may be 1.1 or less.

[0031] The ratio X of the maximum value to the minimum value of the dielectric constant in the temperature range of -40°C to -80°C is an index of the temperature dependency of the dielectric constant, and the closer X is to 1, the smaller the temperature dependency of the dielectric constant. As described above, in addition to the small temperature dependency of the dielectric constant at a frequency of 10 kHz, it is preferable that the temperature dependency of the dielectric constant is small over the entire frequency range of 1 kHz to 1 MHz. When the temperature dependency of the dielectric constant is small, the frequency dependency also tends to be small. Furthermore, the higher the frequency, the temperature at which the dielectric constant is maximized tends to shift to the higher temperature side.

[0032] The above X １ＭＨｚ and X １ｋＨｚ Ratio to X １ｋＨｚ / X １ＭＨｚ is preferably 0.8 to 1.2, more preferably 0.85 to 1.15, even more preferably 0.9 to 1.1, and may be 0.95 to 1.05. １ｋＨｚ / X １ＭＨｚ The closer to 1, the smaller the change in the relative dielectric constant is over a wide temperature range and frequency range, so that operational reliability can be ensured over a wide temperature range and operating frequency.

[0033] 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. The adhesive strength is determined by a peel test using a polyimide film as the adherend at a tensile speed of 60 mm / min and a peel angle of 180°. Unless otherwise specified, the adhesive strength is measured at 25°C. When the adhesive strength of the pressure-sensitive adhesive sheet is within the above range, peeling of the pressure-sensitive adhesive sheet from the adherend during repeated bending can be prevented.

[0034] In the present invention, the adhesive sheet has a gap length of 2 mm or less in a bending retention test described below (i.e., the gap length between the adhesive sheet and the adherend after 240 hours of bending retention; hereinafter, this may be simply referred to as the "gap distance"). When the gap distance is 2 mm or less, even in a bendable image display device equipped with a touch panel within 500 μm of the touch surface, the adhesive sheet can absorb the stress caused by bending even when the image display device is bent, thereby improving 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 little or no malfunction of the touch panel sensor, and failures and the like are suppressed or prevented, resulting in high reliability.

[0035] 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.

[0036] The gap distance is measured by the following procedure of steps A to D. Step A: A 35 mm x 100 mm test piece is prepared by laminating the pressure-sensitive adhesive sheet to an adherend; Step B: The test piece prepared in step A is bent along the short side direction at 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 this bent state in an environment at a temperature of 60°C and a relative humidity of 95% for 240 hours; Step D: At the bent part of the test piece held in step C for 240 hours, the length in the short side direction of the gap between the pressure-sensitive adhesive sheet and the adherend is measured.

[0037] Peeling (voids) in the bending retention test tend to occur at the end of the bending axis of the test piece (end in the short side direction). When voids occur at both ends or when voids exist in multiple locations, the length of the void with the longest length in the short side direction is taken as the void distance. When voids exist in multiple 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 may be 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, or even 0.

[0038] When bent, the adherend expands and contracts, and the adhesive is unable to follow the deformation of the adherend, resulting in peeling from the adherend. For example, the ability of the adhesive to conform to the adherend can be adjusted by controlling the storage modulus G' of the adhesive. The smaller the storage modulus of the adhesive, the better its ability to conform to the deformation of the adherend. When a bent state is maintained, peeling may occur due to the concentration of stress associated with deformation at the bent portion. In order to improve the stress relaxation properties of the adhesive, the loss tangent tanδ can be increased. Furthermore, to prevent peeling at the interface between the adherend and the adhesive, it is also necessary to design the adhesive to have high adhesive strength at the bending temperature. Furthermore, in a high-humidity environment, retention of moisture at the interface between the adhesive and the adherend can cause a decrease in adhesive strength, so it is preferable to use a material that is less likely to retain moisture to prevent moisture retention.

[0039] The pressure-sensitive adhesive sheet has a storage modulus G' at 25°C. ２５ It is preferable that G' is 70 kPa or less. ２５ By making G' 70 kPa or less, distortion when the device is bent tends to be alleviated, and damage to the device components when the device is repeatedly bent can be suppressed. ２５ In this range, the adhesive sheet can have both adhesive holding power and strain relaxation property, and therefore the gap distance can be reduced. ２５ is preferably 10 to 60 kPa, more preferably 13 to 50 kPa, and even more preferably 15 to 40 kPa.

[0040] Storage modulus G' of pressure-sensitive adhesive sheet at 100°C １００ is preferably 2 to 50 kPa, more preferably 3 to 40 kPa, and even more preferably 5 to 25 kPa. １００ When the thickness is within the above range, both adhesive holding strength and stress relaxation properties can be achieved even in a high temperature environment, and therefore the gap distance can be reduced.

[0041] The pressure-sensitive adhesive sheet has a loss tangent tanδ at 25°C. ２５ The loss tangent tanδ at 100° C. of the pressure-sensitive adhesive sheet is preferably 0.2 to 0.45.１００ is preferably 0.2 to 0.4. ２５ and tan δ １００ The difference between tan δ and tan δ is preferably −0.07 to 0.07. ２５ may be 0.25 to 0.42. １００ may be 0.25 to 0.38. ２５ and tan δ １００ The difference between the tan δ of the pressure-sensitive adhesive sheet and the tan δ of the pressure-sensitive adhesive sheet may be within ±0.06 or ±0.05. ２５ , tanδ １００ , and tan δ ２５ and tan δ １００ By keeping the difference between the tan δ and tan δ within the above range, the gap distance can be reduced. The tan δ of the PSA sheet can be adjusted by optimizing the material monomer and the degree of crosslinking. For example, the material monomer can be selected so that the glass transition temperature and molecular weight of the base polymer are in an appropriate range.

[0042] The storage modulus G' and loss tangent tanδ of the PSA sheet are determined by viscoelasticity measurement at a frequency of 1 Hz. Tanδ is the ratio G" / G' of the storage modulus G' to the loss modulus G". The storage modulus G' corresponds to the portion stored as elastic energy when the material deforms, and is an index representing the degree of hardness.

[0043] In order to reduce the temperature dependence of tan δ in the room temperature to high temperature range, the glass transition temperature of the pressure-sensitive adhesive sheet is preferably -20°C or lower, more preferably -23°C or lower, and even more preferably -25°C or lower. The glass transition temperature is the temperature at which tan δ is maximized (peak top temperature). In the vicinity of the glass transition temperature, the temperature dependence of tan δ is large. When the glass transition temperature is sufficiently lower than the ambient temperature at which the device is used, the temperature dependence of tan δ in the ambient temperature range is reduced. Furthermore, when the glass transition temperature is in the above range, the pressure-sensitive adhesive sheet has adhesive holding power even in low temperature ranges, so peeling from the adherend at low temperatures is suppressed and the gap distance can be reduced.

[0044] 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, it is possible to effectively increase the adhesive holding power and reduce the gap distance.

[0045] The thickness of the adhesive sheet is not particularly limited and may be adjusted as appropriate depending on the type of adherend, the position where the adhesive sheet is placed in the device (lamination configuration), the thickness of the target device, the properties required of the adhesive sheet, etc. From the viewpoint of increasing the adhesive strength of the adhesive sheet, the thickness is preferably 10 μm or more. From the viewpoint of thinning the device and suppressing the adhesive from spilling out from the edge surface during processing of the adhesive sheet and bending of the device, the thickness of the adhesive sheet is preferably 100 μm or less, and more preferably 75 μm or less.

[0046] From the viewpoint of providing cushioning properties against impacts from the outer surface, the thickness of the adhesive sheet 11 used to attach the cover window is preferably 20 μm or more, more preferably 30 μm or more, and may be 35 μm or more or 40 μm or more. The thickness of the adhesive sheets 12, 13 disposed between the image display panels 51, 54 and the polarizing plate 31 is preferably smaller than the thickness of the adhesive sheet 11. The thickness of the adhesive sheets 12, 13 is preferably 25 μm or less, more preferably 20 μm or less.

[0047] Like the adhesive sheets 11, 12, and 13 in the device 101 shown in Fig. 2, the adhesive sheets arranged on the viewing side of the image display panel 51 preferably have high transparency. The total light transmittance of the adhesive sheet arranged on the viewing side is preferably 85% or more, more preferably 90% or more, and even more preferably 91% or more. The haze of the adhesive sheet arranged on the viewing side is preferably 1.5% or less, more preferably 1% or less, even more preferably 0.7% or less, and particularly preferably 0.5% or less.

[0048] [Composition and Preparation Method of Pressure-Sensitive Adhesive Sheet] The pressure-sensitive adhesive used to form the pressure-sensitive adhesive sheet of the present invention is not particularly limited as long as its dielectric constant and gap distance are within the above-mentioned ranges. Examples of suitable base polymers include acrylic, silicone, polyester, polyurethane, polyamide, polyvinyl ether, vinyl acetate / vinyl chloride copolymer, modified polyolefin, epoxy, fluorine, and rubber polymers. As the pressure-sensitive adhesive, an acrylic pressure-sensitive adhesive containing an acrylic base polymer as the main component is preferred, as it allows the dielectric constant and gap distance to be adjusted to the above-mentioned predetermined ranges and allows control of transparency, adhesive strength, and the like. Pressure-sensitive adhesives can be used alone or in combination of two or more types. Furthermore, the pressure-sensitive adhesive sheet formed from the pressure-sensitive adhesive may be in either a single-layer or laminated form.

[0049] <Acrylic Base Polymer> The acrylic base polymer contains a (meth)acrylic acid alkyl ester as a main constituent monomer component. In this specification, "(meth)acrylic" means acrylic and / or methacrylic.

[0050] The (meth)acrylic acid alkyl ester includes (meth)acrylic acid C 3 having an alkyl group with 1 to 20 carbon atoms. １－２０ Alkyl esters are preferably used. The alkyl group in the (meth)acrylic acid alkyl ester may be either linear or cyclic. The alkyl group in the linear form (linear alkyl group) may be a straight-chain alkyl group or may have a branch.

[0051] Specific examples of the (meth)acrylic acid chain alkyl ester include C esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, and isononyl (meth)acrylate.１－９ chain alkyl esters; and C such as decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, isotridecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, cetyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, and nonadecyl (meth)acrylate. １０－２０ Examples include chain alkyl esters.

[0052] Specific examples of (meth)acrylic acid alkyl esters having an alicyclic alkyl group (cyclic alkyl group) include (meth)acrylic acid cycloalkyl esters such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate; (meth)acrylic acid esters having a bicyclic aliphatic hydrocarbon ring such as isobornyl (meth)acrylate; and (meth)acrylic acid esters having a tricyclic or higher aliphatic hydrocarbon ring such as dicyclopentanyl (meth)acrylate, dicyclopentanyloxyethyl (meth)acrylate, tricyclopentanyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate.

[0053] In the acrylic base polymer, the amount of the (meth)acrylic acid alkyl ester is preferably 60 to 100 parts by weight, more preferably 70 to 98 parts by weight, per 100 parts by weight of the total of the monomer components.

[0054] The acrylic base polymer contains (meth)acrylic acid C as the (meth)acrylic acid alkyl ester. １０－２０It is preferable that the acrylic base polymer contains a long-chain (meth)acrylic acid alkyl ester as a monomer component, which reduces the molecular dipole moment and increases the molar volume, thereby reducing the dielectric constant. Furthermore, a homopolymer of a (meth)acrylic acid alkyl ester having a long-chain alkyl group having 10 or more carbon atoms has a temperature region (plateau region) above Tg where the temperature dependence of viscoelasticity is small. Therefore, when the base polymer contains a long-chain (meth)acrylic acid alkyl ester as a monomer component, the temperature dependence of tan δ can be reduced.

[0055] Since the temperature range of the plateau region is wide and the storage modulus in the plateau region is small, (meth)acrylic acid C １０－２０ Among the chain alkyl esters, (meth)acrylic acid C １０－１６ Alkyl esters are preferred, and (meth)acrylic acid C １０－１３ Among them, (meth)acrylic acid C alkyl ester is more preferable. １２ Alkyl esters are preferred, with dodecyl acrylate (lauryl acrylate) being particularly preferred.

[0056] Polymers of long-chain alkyl (meth)acrylates are characterized by a wide temperature range in the plateau region and a small storage modulus in the plateau region, but have 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, it is necessary to use a monomer component containing (meth)acrylic acid C １０－２０ In addition to the chain alkyl ester, (meth)acrylic acid C １－９ It is preferable that the alkyl ester contains a chain alkyl ester.

[0057] (Meth)acrylic acid C １－９ Among the chain alkyl esters, those having a homopolymer glass transition temperature of −40° C. or lower are preferred in order to lower the Tg of the base polymer. (Meth)acrylic acid C having a homopolymer glass transition temperature of −40° C. or lower １－９Specific examples of chain alkyl esters include 2-ethylhexyl acrylate (Tg: -70°C), n-hexyl acrylate (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), etc. Among these, butyl acrylate and 2-ethylhexyl acrylate are preferred, with 2-ethylhexyl acrylate being particularly preferred due to its low Tg.

[0058] In order to obtain a pressure-sensitive adhesive having the above-mentioned properties, (meth)acrylic acid C is used as a monomer component of the acrylic base polymer. １０－２０ Chain alkyl ester and (meth)acrylic acid C １－９ It is preferable to contain both the linear alkyl ester and the chain alkyl ester and adjust the ratio of the two.

[0059] (Meth)acrylic acid C relative to 100 parts by weight of the total of the monomer components of the acrylic base polymer １０－２０ The amount of the chain alkyl ester is preferably 5 to 55 parts by weight, more preferably 10 to 50 parts by weight, even more preferably 15 to 45 parts by weight, and particularly preferably 20 to 50 parts by weight. In particular, the amount of lauryl acrylate is preferably within the above range. １－９ The amount of the chain alkyl ester is preferably 30 to 80 parts by weight, more preferably 40 to 75 parts by weight, even more preferably 45 to 70 parts by weight, and particularly preferably 50 to 65 parts by weight. In particular, it is preferable that the amount of 2-ethylhexyl acrylate is in the above range.

[0060] The acrylic base polymer may contain a nitrogen-containing monomer as a monomer component. Examples of the nitrogen-containing monomer include vinyl monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, (meth)acryloylmorpholine, N-vinylcarboxylic acid amides, and N-vinylcaprolactam, and cyanoacrylate monomers such as acrylonitrile and methacrylonitrile. Among these, N-vinylpyrrolidone is preferred because of its high effect of improving adhesive strength by improving cohesive strength.

[0061] A crosslinked structure may be introduced into the acrylic base polymer. By crosslinking the acrylic base polymer, high adhesive holding power can be exhibited even when the G' of the pressure-sensitive adhesive sheet is small. In this way, in order to introduce a crosslinked structure into the acrylic base polymer, the acrylic base polymer preferably contains, as a monomer component, a hydroxy group-containing monomer and a carboxy group-containing monomer in addition to the above-mentioned (meth)acrylic acid alkyl ester. When a crosslinked structure is introduced into the acrylic base polymer using an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, or the like, the hydroxy group or carboxy group serves as the introduction point of the crosslinked structure.

[0062] Examples of hydroxy group-containing monomers include (meth)acrylic acid esters such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, and 12-hydroxylauryl (meth)acrylate. Among these, 2-hydroxyethyl acrylate (Tg: -15°C) and 4-hydroxybutyl acrylate (Tg: -32°C) are preferred because they greatly contribute to improving adhesive strength and can suppress clouding of the PSA sheet in high-humidity environments, and 4-hydroxybutyl acrylate is particularly preferred because of its low Tg.

[0063] Examples of the carboxy group-containing monomer include acrylic monomers such as (meth)acrylic acid, carboxyethyl (meth)acrylate, and carboxypentyl (meth)acrylate, as well as itaconic acid, maleic acid, fumaric acid, and crotonic acid.

[0064] From the viewpoint of enhancing the adhesive strength and adhesive retention of the pressure-sensitive adhesive sheet, the amount of polar group-containing monomer per 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 2 parts by weight or more, and may be 3 parts by weight or more, 4 parts by weight or more, or 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 dielectric constant increases. Furthermore, if the content of the polar monomer is excessively large, the glass transition temperature of the polymer tends to increase, and the adhesive strength at low temperatures tends to decrease. Therefore, the amount of polar group-containing monomer per 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.

[0065] The acrylic base polymer preferably contains a hydroxy group-containing monomer and a nitrogen-containing monomer among the above polar monomer components, and the total amount of the hydroxy group-containing monomer and the nitrogen-containing monomer is preferably within the above range.

[0066] By including a hydroxy group-containing monomer as the polar monomer component, the adhesive strength of the pressure-sensitive adhesive sheet is improved and opacity of the pressure-sensitive adhesive sheet in a high-humidity environment tends to be suppressed. Therefore, the amount of the hydroxy group-containing monomer per 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 even 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 dielectric constant of the pressure-sensitive adhesive tends to increase significantly. Therefore, the amount of the hydroxy group-containing monomer per 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 even be 6 parts by weight or less. When the amount of the hydroxy group-containing monomer is within the above range, the dielectric constant and gap distance can be kept within the above range.

[0067] From the viewpoint of achieving both improved adhesive strength and a low dielectric constant, the amount of nitrogen-group-containing monomer per 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 0.5 to 10 parts by weight, more preferably 1 to 8 parts by weight. In particular, the amount of N-vinylpyrrolidone is preferably within the above range because it significantly contributes to improving adhesive strength. If the amount of nitrogen-containing monomer is within the above range, the relative dielectric constant and gap distance can be kept within the above ranges.

[0068] In order to prevent corrosion of touch panel electrodes due to acid components, the adhesive sheet preferably has a low acid content. Furthermore, when the adhesive sheet is used to adhere a polarizing plate, the adhesive sheet preferably has a low acid content in order to suppress polyenization of a polyvinyl alcohol-based polarizer due to acid components. In such an acid-free adhesive sheet, the content of organic acid monomers such as (meth)acrylic acid is preferably 100 ppm or less, more preferably 70 ppm or less, and even more preferably 50 ppm or less. The organic acid monomer content of the adhesive sheet can be determined by immersing the adhesive sheet in pure water, heating it at 100°C for 45 minutes, and quantifying the acid monomers extracted into the water using ion chromatography.

[0069] In order to reduce the acid monomer content in the pressure-sensitive adhesive sheet, it is preferable that the amount of organic acid monomer components such as (meth)acrylic acid in the monomer components 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 organic acid monomers (carboxy group-containing monomers) as monomer components. In the acid-free pressure-sensitive adhesive sheet, the amount of carboxy group-containing monomers per 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, even more preferably 0.05 parts by weight or less, and ideally 0.

[0070] The acrylic base polymer may contain, as a monomer component, a monomer other than the above-mentioned (meth)acrylic acid alkyl ester and polar monomer. Examples of the other monomer components include vinyl monomers such as caprolactone adducts of (meth)acrylic acid, sulfonic acid group-containing monomers, phosphate group-containing monomers, vinyl acetate, vinyl propionate, styrene, and α-methylstyrene; cyanoacrylate monomers such as acrylonitrile and methacrylonitrile; epoxy group-containing monomers such as glycidyl (meth)acrylate; glycol-based acrylic ester monomers such as polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, methoxyethylene glycol (meth)acrylate, and methoxypolypropylene glycol (meth)acrylate; and acrylic ester monomers such as tetrahydrofurfuryl (meth)acrylate, fluorine (meth)acrylate, silicone (meth)acrylate, and 2-methoxyethyl (meth)acrylate.

[0071] 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, even more 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 dielectric constant and the gap distance can be set within the above range. The theoretical Tg is the glass transition temperature Tg of a homopolymer of the constituent monomer components of the acrylic base polymer. ｉ and the weight fraction W of each monomer component ｉ It is calculated by the following Fox formula: 1 / Tg=Σ(W ｉ / Tg ｉ )

[0072] Tg is the glass transition temperature of the polymer chain (unit: K), W ｉ is the weight fraction of the monomer component i constituting the segment (copolymerization ratio based on weight), Tg ｉis the glass transition temperature (unit: K) of a homopolymer of monomer component i. The glass transition temperature of a homopolymer can be determined from the values ​​listed in Polymer Handbook, 3rd Edition (John Wiley & Sons, Inc., 1989). The Tg of a homopolymer of a monomer not listed in the above publication can be determined from the peak top temperature of tan δ measured by dynamic viscoelasticity measurement.

[0073] <Crosslinked Structure of Base Polymer> As described above, the acrylic base polymer may have a crosslinked structure. Introducing a crosslinked structure into the base polymer increases the gel fraction of the PSA. The gel fraction of the PSA sheet is preferably 55 to 85%, more preferably 60 to 80%, even more preferably 63 to 77%, and particularly preferably 65 to 75%. By adjusting the gel fraction within this range, high adhesive holding power can be exerted even when G' is small and the PSA sheet is soft, thereby making it possible to reduce the gap distance.

[0074] The gel fraction can be determined as the insoluble content in a solvent such as ethyl acetate. Specifically, it is determined as the weight fraction (unit: wt%) of the insoluble content after immersing a pressure-sensitive adhesive sheet in ethyl acetate at 23°C for 7 days relative to the sample before immersion. Generally, the gel fraction of a polymer is equal to the degree of crosslinking, and the more crosslinked portions in the polymer, the higher the gel fraction. The gel fraction (amount of crosslinked structure introduced) can be adjusted to a desired range by the method for introducing the crosslinked structure, the type and amount of crosslinking agent, etc.

[0075] Methods for introducing a crosslinked structure into a base polymer include (1) a method in which a base polymer having a functional group reactive with a crosslinking agent is polymerized, and then a crosslinking agent is added to react the base polymer with the crosslinking agent; and (2) a method in which a polyfunctional compound is included in the polymerization components of the base polymer to introduce a branched structure (crosslinked structure) into the polymer chain. These methods may be used in combination to introduce multiple types of crosslinked structures into the base polymer.

[0076] In the above-mentioned method (1) of reacting a base polymer with a crosslinking agent, a crosslinking agent is added to the polymerized base polymer and heated as necessary to introduce a crosslinked structure into the base polymer. Examples of crosslinking agents include compounds that react with functional groups contained in the base polymer, such as hydroxyl groups and carboxyl groups. Specific examples of crosslinking agents include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, carbodiimide-based crosslinking agents, and metal chelate-based crosslinking agents.

[0077] Among these, isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred because they have high reactivity with the hydroxyl and carboxyl groups of the base polymer and facilitate the introduction of crosslinked structures. These crosslinking agents react with functional groups such as hydroxyl and carboxyl groups introduced into the base polymer to form crosslinked structures. For acid-free pressure-sensitive adhesives in which the base polymer does not contain carboxyl groups, it is preferable to use an isocyanate-based crosslinking agent to form a crosslinked structure by reacting the hydroxyl groups in the base polymer with the isocyanate crosslinking agent.

[0078] As the isocyanate crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule is used. Examples of the isocyanate crosslinking agent include lower aliphatic polyisocyanates such as butylene diisocyanate and hexamethylene diisocyanate; alicyclic isocyanates such as cyclopentylene diisocyanate, cyclohexylene diisocyanate and isophorone diisocyanate; aromatic isocyanates such as 2,4-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate and xylylene diisocyanate; trimethylolpropane / Examples of such isocyanate adducts include tolylene diisocyanate trimer adducts (e.g., "Coronate L" manufactured by Tosoh), trimethylolpropane / hexamethylene diisocyanate trimer adducts (e.g., "Coronate HL" manufactured by Tosoh), trimethylolpropane adducts of xylylene diisocyanate (e.g., "Takenate D110N" manufactured by Mitsui Chemicals), and isocyanurate of hexamethylene diisocyanate (e.g., "Coronate HX" manufactured by Tosoh).

[0079] In the method (2) of including a polyfunctional monomer in the polymerization components of the base polymer, the monomer components constituting the acrylic base polymer and the polyfunctional compound for introducing a crosslinked structure may all be reacted at once, or the polymerization may be carried out in multiple stages. A preferred method of carrying out the polymerization in multiple stages involves polymerizing (preliminary polymerization) the monofunctional monomers constituting the base polymer to prepare a partial polymer (prepolymer composition), adding a polyfunctional compound such as a polyfunctional (meth)acrylate to the prepolymer composition, and polymerizing (main polymerization) the prepolymer composition and the polyfunctional monomer. The prepolymer composition is a partial polymer containing a polymer with a low degree of polymerization and unreacted monomer.

[0080] By prepolymerizing the components of the acrylic base polymer, branching points (crosslinking points) due to the polyfunctional compound can be uniformly introduced into the base polymer. Alternatively, a pressure-sensitive adhesive sheet can be formed by applying a mixture (pressure-sensitive adhesive composition) of a low-molecular-weight polymer or partially polymerized product and an unpolymerized monomer component to a substrate, followed by main polymerization on the substrate. Low-molecular-weight compositions such as prepolymer compositions have low viscosity and excellent coatability. Therefore, by applying a pressure-sensitive adhesive composition, which is a mixture of a prepolymer composition and a polyfunctional compound, to a substrate and then performing main polymerization, the productivity of pressure-sensitive adhesive sheets can be improved and the thickness of the pressure-sensitive adhesive sheets can be made uniform.

[0081] Examples of polyfunctional compounds used to introduce a crosslinked structure include compounds containing two or more polymerizable functional groups (ethylenically unsaturated groups) having unsaturated double bonds in one molecule. As polyfunctional compounds, polyfunctional (meth)acrylates are preferred because they are easily copolymerized with the monomer components of the acrylic base polymer. When a branched (crosslinked) structure is introduced by active energy ray polymerization (photopolymerization), polyfunctional acrylates are preferred.

[0082] Examples of polyfunctional (meth)acrylates include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, bisphenol A ethylene oxide modified di(meth)acrylate, bisphenol A propylene oxide modified di(meth)acrylate, alkanediol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, ethoxylated isocyanuric acid tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol dimethacrylate. Examples of the di(meth)acrylate include dipentaerythritol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerin di(meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, and isoprene (meth)acrylate.

[0083] The molecular weight of the polyfunctional compound such as a 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 gap distance can be reduced.

[0084] <Preparation of Base Polymer> The acrylic base polymer can be prepared by a known polymerization method such as solution polymerization, UV polymerization, bulk polymerization, emulsion polymerization, etc. In terms of the transparency, water resistance, cost, etc. of the pressure-sensitive adhesive, solution polymerization or active energy ray polymerization (e.g., UV polymerization) is preferred. Ethyl acetate, toluene, etc. are generally used as the solvent for solution polymerization.

[0085] When 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 polymerization reaction. The photopolymerization initiator is not particularly limited as long as it initiates photopolymerization, and examples thereof include benzoin ether-based photopolymerization initiators, acetophenone-based photopolymerization initiators, α-ketol-based photopolymerization initiators, aromatic sulfonyl chloride-based photopolymerization initiators, photoactive oxime-based photopolymerization initiators, benzoin-based photopolymerization initiators, benzyl-based photopolymerization initiators, benzophenone-based photopolymerization initiators, ketal-based photopolymerization initiators, thioxanthone-based photopolymerization initiators, and acylphosphine oxide-based photopolymerization initiators. Examples of thermal polymerization initiators that can be used include azo-based initiators, peroxide-based initiators, and redox-based initiators that combine peroxides and reducing agents (e.g., a combination of persulfate and sodium bisulfite, a combination of peroxide and sodium ascorbate, etc.).

[0086] During the polymerization, a chain transfer agent, a polymerization inhibitor (polymerization retarder), etc. may be used for the purpose of adjusting the molecular weight, etc. 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, and α-methylstyrene dimer.

[0087] The molecular weight of the base polymer can be adjusted by adjusting the type and amount of polymerization initiator. For example, in radical polymerization, the greater the amount of polymerization initiator, the higher the radical concentration in the reaction system, which tends to increase the density of reaction initiation points and decrease the molecular weight. Conversely, the smaller the amount of polymerization initiator, the lower the density of reaction initiation points, which tends to make it easier for the polymer chain to elongate and increase the molecular weight.

[0088] In order to obtain a pressure-sensitive adhesive sheet with excellent adhesive strength and small gap distance, it is preferable that the acrylic base polymer has a high gel fraction at a low crosslinking density.In order to increase the gel fraction (the ratio of polymer chains into which crosslinked structures are introduced) at a low crosslinking density, it is sufficient to increase the molecular weight (the length of the polymer chain) of the base polymer.As mentioned above, in order to increase the molecular weight of the base polymer, it is preferable to reduce the amount of polymerization initiator used when polymerizing the base polymer.

[0089] The amount of polymerization initiator used during base polymer polymerization may be appropriately set depending on the type of polymerization reaction, the monomer composition, the type of polymerization initiator, the target molecular weight, etc. From the viewpoint of increasing the molecular weight of the base polymer and increasing the gel fraction with a small amount of crosslinking agent, the amount of polymerization initiator used is preferably 0.001 to 0.4 parts by weight, more preferably 0.003 to 0.1 parts by weight, and even more preferably 0.005 to 0.05 parts by weight, relative to 100 parts by weight of the total of the monomer components constituting the base polymer.

[0090] When a crosslinked structure is introduced using 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 a crosslinked structure into the base polymer. When a crosslinked structure is introduced using a polyfunctional compound such as a polyfunctional (meth)acrylate, it is preferable to polymerize the base polymer or prepare a prepolymer composition by solution polymerization or active energy ray polymerization, add the polyfunctional compound, and then introduce a crosslinked structure by the polyfunctional compound by active energy ray polymerization.

[0091] The prepolymer composition can be prepared, for example, by partially polymerizing (preliminary polymerization) a composition (referred to as a "prepolymer-forming composition") obtained by mixing monomer components constituting an acrylic-based polymer with a polymerization initiator. The monomer in the prepolymer-forming composition is preferably a monofunctional monomer component such as a (meth)acrylic acid alkyl ester or a polar group-containing monomer. The prepolymer-forming composition may contain a polyfunctional monomer in addition to the monofunctional monomer. For example, a portion of the polyfunctional monomer may be added to the prepolymer-forming composition, and the remainder of the polyfunctional monomer component may be added after the preliminary polymerization to carry out the main polymerization.

[0092] 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 achieving a viscosity suitable for application to 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 used, the irradiation intensity and irradiation time of actinic rays such as UV light, etc.

[0093] <Acrylic Oligomer> The pressure-sensitive adhesive sheet may contain an oligomer in addition to the acrylic base polymer. The acrylic oligomer used has a weight-average molecular weight of about 1,000 to 30,000. The acrylic oligomer contains an alkyl (meth)acrylate ester as the main constituent monomer component.

[0094] The glass transition temperature of the acrylic oligomer is preferably 60°C or higher, more preferably 80°C or higher, even more preferably 100°C or higher, and particularly preferably 110°C or higher. By using a low Tg acrylic base polymer into which a crosslinked structure has been introduced and a high Tg acrylic oligomer in combination, the adhesive strength of the pressure-sensitive adhesive sheet, particularly the adhesive retention at high temperatures, tends to be improved, and the gap distance can be reduced. 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 using the Fox formula described above.

[0095] The acrylic oligomer having a glass transition temperature of 60° C. or higher preferably contains, as a constituent monomer component, an alkyl (meth)acrylate ester having a chain alkyl group (chain alkyl (meth)acrylate) and an alkyl (meth)acrylate ester having an alicyclic alkyl group (alicyclic alkyl (meth)acrylate). Specific examples of the chain alkyl (meth)acrylate and the alicyclic alkyl (meth)acrylate are as given above as examples of the constituent monomers of the acrylic polymer chain.

[0096] Among the exemplified alkyl (meth)acrylates, methyl methacrylate is preferred as the chain alkyl (meth)acrylate because it has a high glass transition temperature and excellent compatibility with the base polymer. Dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate are preferred as the alicyclic alkyl (meth)acrylate. That is, the acrylic oligomer preferably contains, as constituent monomer components, one or more selected from the group consisting of dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate, and methyl methacrylate.

[0097] The amount of the alicyclic alkyl (meth)acrylate relative to the total amount of the monomer components constituting the acrylic oligomer is preferably 10 to 90% by weight, more preferably 20 to 80% by weight, and even more preferably 30 to 70% by weight. The amount of the chain alkyl (meth)acrylate relative to the total amount of the monomer components constituting the acrylic oligomer is preferably 10 to 90% by weight, more preferably 20 to 80% by weight, and even more preferably 30 to 70% by weight.

[0098] The weight average molecular weight of the acrylic oligomer is preferably 1,000 to 30,000, more preferably 1,500 to 10,000, and even more preferably 2,000 to 8,000. By using an acrylic oligomer having a molecular weight within this range, the adhesive strength and adhesive retention of the pressure-sensitive adhesive tend to be improved, and the gap distance can be reduced.

[0099] 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. Furthermore, a chain transfer agent may be used to adjust the molecular weight.

[0100] The content of the acrylic oligomer in the pressure-sensitive adhesive sheet is not particularly limited, but in order to sufficiently increase the adhesive strength, the amount of the acrylic oligomer relative to 100 parts by weight of the base polymer is preferably 0.5 parts by weight or more, more preferably 0.8 parts by weight or more, and even more preferably 1 part by weight or more. The amount of the acrylic oligomer in the pressure-sensitive adhesive sheet may be 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, 2.3 parts by weight or more, or 2.5 parts by weight or more relative to 100 parts by weight of the base polymer. The greater the amount of high-Tg acrylic oligomer added, the smaller the gap distance tends to be.

[0101] On the other hand, if the amount of the acrylic oligomer added is excessively large, the haze of the PSA sheet increases due to a decrease in compatibility, and transparency tends to decrease. Since high transparency is required for PSA sheets arranged on the viewing side of the image display panel, the amount of the acrylic oligomer in the PSA sheet is preferably 5 parts by weight or less, and may be 4 parts by weight or less, or 3 parts by weight or less, relative to 100 parts by weight of the base polymer.

[0102] <Adhesive composition> An adhesive composition is prepared by mixing the above-mentioned acrylic oligomer, a crosslinking agent and / or a polyfunctional compound for introducing a crosslinked structure, other additives, etc., with an acrylic base polymer (or prepolymer composition) as needed. The remainder of the monomer components constituting the acrylic base polymer may also be added to the adhesive composition as needed. A thickening additive, etc., may also be used for the purpose of viscosity adjustment, etc.

[0103] When the pressure-sensitive adhesive composition contains a prepolymer composition and a polyfunctional compound, etc., the pressure-sensitive adhesive composition preferably contains a photopolymerization initiator for main polymerization. After the prepolymerization, the polymerization initiator for main polymerization may be added to the prepolymer composition. If the polymerization initiator from the prepolymerization remains in the prepolymer composition without being deactivated, the addition of the polymerization initiator for main polymerization may be omitted. The pressure-sensitive adhesive composition may contain a chain transfer agent.

[0104] The pressure-sensitive adhesive composition preferably has an acrylic base polymer (or prepolymer composition) content of 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more, relative to the total amount of nonvolatile components.

[0105] The amount of crosslinker and / or polyfunctional compound in the pressure-sensitive adhesive composition may be adjusted so that the gel fraction falls within the above range. As described above, in order to reduce the gap distance, it is preferable to increase the molecular weight of the acrylic base polymer and increase the gel fraction at a low crosslinking density. For example, when a crosslinked structure is introduced using an isocyanate-based crosslinking agent, the amount of the crosslinking agent is preferably 0.005 to 0.5 parts by weight, more preferably 0.01 to 0.3 parts by weight, and even more preferably 0.02 to 0.1 parts by weight, per 100 parts by weight of the acrylic base polymer. When a crosslinked structure is introduced using a polyfunctional (meth)acrylate, the amount of the polyfunctional (meth)acrylate is preferably 0.005 to 0.3 parts by weight, more preferably 0.01 to 0.2 parts by weight, and even more preferably 0.02 to 0.1 parts by weight, per 100 parts by weight of the acrylic base polymer (prepolymer).

[0106] (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 added is usually about 0.01 to 5.0 parts by weight, and preferably about 0.03 to 3.0 parts by weight, per 100 parts by weight of the base polymer. When the amount of silane coupling agent is within the above range, the gap distance may be reduced.

[0107] (Other Additives) In addition to the components exemplified above, the pressure-sensitive adhesive composition may contain additives such as a tackifier, a plasticizer, a softener, an antidegradant, a filler, a colorant, an ultraviolet absorber, an antioxidant, a surfactant, and an antistatic agent.

[0108] <Formation of Pressure-Sensitive Adhesive Sheet> A pressure-sensitive adhesive composition is applied to a substrate, and if necessary, the solvent is dried and removed, and / or main polymerization is carried out by irradiation with actinic rays, thereby forming a pressure-sensitive adhesive sheet on the substrate. Any appropriate substrate can be used as the substrate used to form the pressure-sensitive adhesive sheet. The substrate may also be a release film having a release layer on the surface that comes into contact with the pressure-sensitive adhesive sheet.

[0109] Films made of various resin materials are used as the film substrate of the release film. Examples of resin materials include polyester resins such as polyethylene terephthalate and polyethylene naphthalate, acetate resins, polyethersulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, (meth)acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, polyarylate resins, and polyphenylene sulfide resins. Among these, polyester resins such as polyethylene terephthalate are particularly preferred. The thickness of the film substrate is preferably 10 to 200 μm, more preferably 25 to 150 μm. Examples of materials for the release layer include silicone-based release agents, fluorine-based release agents, long-chain alkyl-based release agents, and fatty acid amide-based release agents. The thickness of the release layer is generally about 10 to 2,000 nm.

[0110] As a method for applying the pressure-sensitive adhesive composition onto a substrate, various methods such as roll coating, kiss roll coating, gravure coating, reverse coating, roll brush, spray coating, dip roll coating, bar coating, knife coating, air knife coating, curtain coating, lip coating, and die coater can be used.

[0111] 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 adopted depending on the purpose. The heat 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 can be adopted. 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.

[0112] When the pressure-sensitive adhesive composition contains a crosslinking agent, a crosslinking reaction may be carried out after the pressure-sensitive adhesive composition is applied to a substrate. Heating may be performed, if necessary, during crosslinking. The temperature for the crosslinking reaction is typically in the range of 20°C to 160°C, and the crosslinking reaction time is approximately 1 minute to 7 days. After applying the pressure-sensitive adhesive composition, the heating for drying the solvent may also serve as the heating for crosslinking. After drying the solvent, a cover sheet is preferably attached to protect the surface of the pressure-sensitive adhesive sheet. As with the substrate film, a release film having a release layer on the surface that comes into contact with the pressure-sensitive adhesive sheet is preferably used as the cover sheet.

[0113] When the pressure-sensitive adhesive composition is a photopolymerizable composition containing a prepolymer composition and a polyfunctional compound, etc., the pressure-sensitive adhesive composition is applied in the form of a layer on a substrate, and then photocured by irradiating with actinic rays. When photocuring, it is preferable to attach a cover sheet to the surface of the applied layer and irradiate with actinic rays while the pressure-sensitive adhesive composition is sandwiched between the two sheets, thereby preventing polymerization inhibition by oxygen.

[0114] The actinic rays may be selected depending on the type of polymerizable component, such as a monomer or a polyfunctional (meth)acrylate, the type of photopolymerization initiator, etc., and generally, ultraviolet light and / or short-wavelength visible light is used. The cumulative light amount of the irradiated light is 100 to 5000 mJ / cm. ２ The light source for light irradiation is not particularly limited as long as it can irradiate light within a wavelength range to which the photopolymerization initiator contained in the pressure-sensitive adhesive composition is sensitive, and preferably used are an LED light source, a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a metal halide lamp, a xenon lamp, etc.

[0115] [Laminated Structure of Adhesive Sheet] By laminating release films 91, 92 onto the surface of adhesive sheet 11, an adhesive sheet having release films temporarily attached to both sides is obtained, as shown in Fig. 1. The substrate and cover sheet used when forming the adhesive sheet may be used as release films 91, 92 as they are.

[0116] When release films 91, 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. The peel force when peeling off the release film temporarily attached to one side of the pressure-sensitive adhesive sheet 11 and the peel force when peeling off the release film temporarily attached to the other side of the pressure-sensitive adhesive sheet 11 may be the same or different. When the two release forces are different, excellent workability is achieved when the release film 91 (light release film) with a relatively small release force is peeled off from the pressure-sensitive adhesive sheet 11 first to be bonded to the first adherend, and the release film 92 (heavy release film) with a relatively large release force is peeled off to be bonded to the second adherend.

[0117] [Image display device] As described above, the pressure-sensitive adhesive sheet of the present invention is used to bond components of a foldable image display device equipped with a touch panel. The image display device 101 shown in Fig. 2 has a touch panel 41, a circular polarizer 31, and a cover window 71 arranged on the viewing side surface of an organic EL panel 51 serving as an image display panel. In a flexible display, all of these components are flexible and foldable. The image display device 102 shown in Fig. 3 has a touch panel integrated with an organic EL panel 54 serving as an image display panel, and has a circular polarizer 31 and a cover window 71 arranged on its surface.

[0118] <Image Display Panel> An organic EL panel includes a pair of electrodes on a substrate and an organic light-emitting layer sandwiched between the electrodes. The organic EL panel may be either a top-emission type in which a metal electrode, an organic light-emitting layer, and a transparent electrode are sequentially stacked on a substrate, or a bottom-emission type in which a transparent electrode, an organic light-emitting layer, and a metal electrode are sequentially stacked on a transparent substrate. In both bottom-emission and top-emission types, the substrate and sealing member provided on the viewing side of the organic light-emitting layer are transparent. The substrate and sealing member provided on the back side of the organic light-emitting layer (the housing 75 side in Figures 2 and 3) do not need to be transparent. In bottom-emission flexible organic EL panels, the substrate does not need 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 polyether ether ketone or transparent polyimide. A backsheet may be provided on the back side of the substrate for the purpose of protecting and reinforcing the substrate.

[0119] The image display panel is not limited to an organic EL panel, but may be a liquid crystal panel, an electrophoretic display panel (electronic paper), etc. For example, a bendable liquid crystal panel can be formed by using flexible substrates such as resin substrates as transparent substrates that sandwich the liquid crystal layer.

[0120] <Cover Window> A cover window 71 is provided on the outermost surface on the viewing side of the image display device for the purpose of preventing damage to the image display panel due to impact from the outer surface. In a flexible display, a flexible transparent substrate such as transparent polyimide, polyether ether ketone, or polyethylene terephthalate is used as the cover window 71. A flexible glass plate (glass film) may be used as the material for the cover window 71, or 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. To achieve excellent recovery after maintaining a bent state for a long period of time, the yield point elongation of the cover window is preferably 5% or more. A bendable thin glass substrate may be used as the cover window 71. The cover window may be a laminate of two or more layers of transparent material. An anti-reflection layer, a hard coat layer, or the like may be provided on the viewing side surface of the cover window.

[0121] <Touch Panel> The image display device includes a capacitive touch panel on the viewing side surface of the image display panel. The capacitive touch panel detects the touch position based on a change in the amount of electricity when an operator's finger, a touch pen, or the like touches the touch surface. In the configuration of Fig. 2, the touch panel 41 is disposed between the circular polarizer 31 and the organic EL panel 51. In the configuration of Fig. 3, the touch panel is provided inside the image display panel 54. The touch panel may be disposed between the circular polarizer 31 and the cover window 71.

[0122] In a flexible display, the thickness of the cover window is small, so 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 the 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 D from the touch surface to the touch panel 41 (the distance from the touch surface to the image display panel if the touch panel is an in-cell type) 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.

[0123] <Polarizing Plate> A polarizing plate 31 is generally provided on the viewing side of the image display panel. For example, in a liquid crystal display device, the polarizing plate provided on the viewing side of the liquid crystal panel adjusts the transmittance according to the polarization state of light transmitted through the liquid crystal cell. In an organic EL display device, by providing a circular polarizing plate 31 on the viewing side of the organic EL panel 51, it is possible to block external light reflected by the metal electrodes of the organic EL panel from emitting to the viewing side, thereby improving the visibility of the display.

[0124] A polarizing plate generally has a polarizer with an appropriate transparent protective film attached to one or both sides as needed. The polarizer is not particularly limited, and various types can be used. Examples of polarizers include hydrophilic polymer films such as polyvinyl alcohol films, partially formalized polyvinyl alcohol films, and partially saponified ethylene-vinyl acetate copolymer films, which are uniaxially stretched after adsorbing a dichroic substance such as iodine or a dichroic dye, and polyene-based oriented films such as dehydrated polyvinyl alcohol and dehydrochlorinated polyvinyl chloride.

[0125] The polarizer may be a thin polarizer having a thickness of 10 μm or less. Examples of thin polarizers include those described in JP-A-51-069644, JP-A-2000-338329, WO 2010 / 100917, Japanese Patent No. 4691205, and Japanese Patent No. 4751481. The thin polarizer can be obtained, for example, by a manufacturing method including a step of stretching a polyvinyl alcohol-based resin layer and a resin substrate for stretching in a laminate state, and a step of dyeing the laminate with a dichroic material such as iodine.

[0126] The transparent protective film used as the protective film for the polarizer is preferably a resin excellent in transparency, mechanical strength, thermal stability, moisture blocking property, and optical isotropy, such as a cellulose-based resin, a cyclic polyolefin-based resin, an acrylic-based resin, a phenylmaleimide-based resin, a polycarbonate-based resin, etc. When transparent protective films are provided on both sides of the polarizer, the protective films on the front and back may be made of the same polymer material, or protective films made of different polymer materials may be used.

[0127] An optical film may be laminated on one or both surfaces of the polarizing plate, if necessary, via an appropriate adhesive layer or pressure-sensitive adhesive layer. As such a film, a retardation plate, a viewing angle widening film, a viewing angle limiting (peeping prevention) film, a brightness improving film, or the like, which are used in forming an image display device, are used, and the type thereof is not particularly limited. For example, in a liquid crystal display device, an optical compensation film may be used between the image display panel (liquid crystal panel) and the polarizing plate for the purpose of appropriately converting the polarization state of light emitted from the liquid crystal cell to the viewing side, thereby improving the viewing angle characteristics, etc.

[0128] As described above, in an organic EL display device, by providing a circular polarizing plate with a quarter-wave plate on the surface of the polarizer facing the organic EL panel, it is possible to block external light reflected by the metal electrode from emitting to the viewing side. By providing a quarter-wave plate on the viewing side of the polarizer and circularly polarizing the emitted light, it is possible to allow a viewer wearing polarized sunglasses to view an appropriate image display. These optical films (optically anisotropic films) may be laminated on the polarizer without any other film in between. In this case, the optical film also functions as a protective film for the polarizer.

[0129] The thickness of a polarizing plate is generally about 10 to 200 μm. From the viewpoint of providing flexibility, the thickness of a polarizing plate used in a flexible display is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 70 μm or less. When optical films such as a quarter-wave plate are laminated on the polarizing plate, the total thickness including these films is preferably within the above range.

[0130] <Lamination of Members with Adhesive Sheet> An adhesive sheet is used to bond the flexible members together. In the image display device shown in Fig. 2, the organic EL panel 51 and the bottom surface of the housing 75 are bonded together via an adhesive sheet 14, the organic EL panel 51 and the touch panel 41 are bonded together via an adhesive sheet 13, the touch panel 41 and the circular polarizer 31 are bonded together via an adhesive sheet 12, and the circular polarizer 31 and the cover window 71 are bonded together via an adhesive sheet 11. In the image display device shown in Fig. 3, the touch panel-integrated organic EL panel 54 and the circular polarizer 31 are bonded together via an adhesive sheet 12, and the circular polarizer 31 and the cover window 71 are bonded together via an adhesive sheet 11.

[0131] It is preferable to use the above-mentioned adhesive sheet of the present invention for either or both of adhesive sheets 11 and 12, which are arranged on the viewing side of the touch panel. From the viewpoint of preventing peeling of cover window 71 at the bent portion when the bent state is maintained or when bending is repeated, it is preferable to use the adhesive sheet of the present invention as adhesive sheet 11. It is also preferable that both adhesive sheets 11 and 12 have the above-mentioned relative dielectric constant.

[0132] As mentioned above, the low dielectric constant of the adhesive sheet reduces malfunction of the touch panel even when the distance D from the touch surface to the touch panel is small. Furthermore, since the adhesive sheet has a gap distance of 2 mm or less after a predetermined bending retention test, failures are suppressed or prevented even when the device is kept in a folded state for a long period of time or when it is folded repeatedly, resulting in high reliability.

[0133] The order in which each component is bonded is not particularly limited, and the touch panel 41, circular polarizer 31, and cover window 71 may be laminated in that order on the image display panel 51, or a laminate in which two or more components are pre-laminated via an adhesive sheet may be bonded on the image display panel 51.

[0134] [Adhesive-Attached Film and Laminate] The adhesive sheet of the present invention can be used to form an image display device, in addition to a form in which release films are temporarily attached to both sides, as an adhesive-attached polarizing plate in which the adhesive sheet is firmly laminated to the surface of a polarizing plate. For example, as in the adhesive-attached polarizing plate 5 shown in Figure 4, a release film 93 may be temporarily attached to one side of an adhesive sheet 11, and a circular polarizing plate 31 may be firmly laminated to the other side of the adhesive sheet 11. In the form shown in Figure 5, an adhesive sheet 11 is firmly laminated to one side of a circular polarizing plate 31, and an adhesive sheet 12 is firmly laminated to the other side of the circular polarizing plate 31. As shown in Figure 5, in the double-sided adhesive-attached film 6, release films 93 and 94 are preferably temporarily attached to the surfaces of the adhesive sheets 11 and 12.

[0135] In a configuration in which a polarizing plate or the like is laminated on the adhesive sheet in advance, one release film 93 temporarily attached to the surface of the adhesive sheet 11 is peeled off to bond it to an adherend, and the other release film 94 is peeled off to bond it to another adherend. By laminating the flexible member and adhesive sheet that constitute the image display device in advance, the process of forming the image display device can be simplified.

[0136] 5 , the thicknesses of the adhesive sheet 11 laminated on one side of the polarizing plate 31 and the adhesive sheet 12 laminated on the other side of the polarizing plate 31 may be the same or different. For example, when the adhesive sheet 12 is used to bond the polarizing plate 31 to a touch panel or an image display panel, and the adhesive sheet 11 is used to bond the polarizing plate 31 to a cover window 71, it is preferable that the thickness of the adhesive sheet 11 be greater than the thickness of the adhesive sheet 12.

[0137] By making the thickness of the adhesive sheet 11 provided on the viewing side relatively larger, it is possible to provide cushioning against impacts from the outer surface and prevent damage to the image display panel. The thickness of the adhesive sheet 11 is preferably 25 to 100 μm, more preferably 30 to 75 μm. The thickness of the 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.

[0138] In the double-sided pressure-sensitive adhesive film 6, both of the pressure-sensitive adhesive sheets 11 and 12 preferably have the above-mentioned relative dielectric constant. In addition, both of the pressure-sensitive adhesive sheets 11 and 12 are preferably pressure-sensitive adhesive sheets of the present invention (i.e., pressure-sensitive adhesive sheets having a gap distance of 2 mm or less after a predetermined bending retention test).

[0139] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0140] [Preparation of Acrylic Oligomer] 60 parts by weight of dicyclopentanyl methacrylate (DCPMA) and 40 parts by weight of methyl methacrylate (MMA) as monomer components, 3.5 parts by weight of α-thioglycerol as a chain transfer agent, and 100 parts by weight of toluene as a polymerization solvent were mixed and stirred at 70°C for 1 hour under a nitrogen atmosphere. 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, then heated to 80°C and reacted for 2 hours. The reaction solution was then heated to 130°C, and the toluene, chain transfer agent, and 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.

[0141] 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 7 parts by weight of N-vinyl-2-pyrrolidone (NVP) were blended as prepolymer-forming monomer components, and 0.015 parts by weight of "Omnirad 184" manufactured by IGM Resins was used as a photopolymerization initiator. The mixture was irradiated with ultraviolet light to polymerize, yielding a prepolymer composition (polymerization rate: approximately 10%).

[0142] (Preparation of Pressure-Sensitive Adhesive Composition) To 100 parts by weight of the prepolymer composition, 0.07 parts by weight of 1,6-hexanediol diacrylate (HDDA), 3 parts by weight of the oligomer, and 0.3 parts by weight of a silane coupling agent ("KBM403" manufactured by Shin-Etsu Chemical Co., Ltd.) were added as post-added components, and then these were mixed uniformly to prepare a pressure-sensitive adhesive composition.

[0143] (Preparation of Pressure-Sensitive Adhesive Sheet) A 75 μm thick polyethylene terephthalate (PET) film ("Diafoil MRF75" manufactured by Mitsubishi Chemical) having a silicone-based release layer on its surface was used as a substrate (double-duty release film), and the above-mentioned photocurable pressure-sensitive adhesive composition was applied to the substrate to a thickness of 50 μm to form a coating layer. Onto this coating layer, a 75 μm thick PET film ("Diafoil MRE75" manufactured by Mitsubishi Chemical) with one side treated with silicone release was attached as a cover sheet (double-duty release film). This laminate was then exposed from the cover sheet side to a lamp with an irradiation intensity of 5 mW / cm at the irradiated surface. ２ The adhesive sheet was then photocured by irradiating it with ultraviolet light using a black light whose position was adjusted so that the adhesive sheet had a thickness of 50 μm.

[0144] [Examples 2 to 5, Comparative Examples 1 to 3] The monomer composition charged in the polymerization of the prepolymer, the blend amount of the polyfunctional monomer (HDDA), and the blend amount of the oligomer were changed as shown in Table 1. Except for these, photocurable pressure-sensitive adhesive compositions were prepared in the same manner as in Example 1, and then coated on a substrate and photocured to obtain pressure-sensitive adhesive sheets.

[0145] [Preparation of Pressure-Sensitive Adhesive Sheet A] 99 parts by weight of butyl acrylate (BA) and 1 part by weight of 4-hydroxybutyl acrylate (4HBA) as monomers, and 0.3 parts of 2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator were added to a reaction vessel together with ethyl acetate, and the mixture was allowed to react for 4 hours at 60°C under a nitrogen gas stream. Ethyl acetate was then added to the reaction solution to obtain a solution of an acrylic polymer having a weight-average molecular weight of 1.65 million. This solution was blended with 0.3 parts by weight of dibenzoyl peroxide (manufactured by NOF Corp., "Niper BMT") and 0.1 parts by weight of trimethylolpropane xylylene diisocyanate (manufactured by Mitsui Chemicals, "Takenate D110N") as crosslinking agents, and 3 parts by weight of a silane coupling agent (manufactured by Shin-Etsu Chemical Co., Ltd., "KBM403"), per 100 parts by weight of the polymer, to obtain a pressure-sensitive adhesive composition A.

[0146] The above-mentioned adhesive composition A was applied to the release-treated surface of a 38 μm thick PET film ("MRF38" manufactured by Mitsubishi Chemical) with a silicone-based release layer on its surface, and dried and crosslinked at 150°C to obtain an adhesive sheet A with a thickness of 15 μm.

[0147] [Evaluation] <Gel Fraction> Approximately 0.2 g of adhesive was scraped off from the adhesive sheet and wrapped in a porous polytetrafluoroethylene membrane ("NTF-1122" manufactured by Nitto Denko) with a pore diameter of 0.2 μm cut into a size of 100 mm x 100 mm, and the opening of the wrap was tied with a string. The weight of the adhesive sample (B) was calculated by subtracting the total weight (A) of the porous polytetrafluoroethylene membrane and the string, which had been measured in advance, from the weight of this sample. The adhesive sample wrapped in the porous polytetrafluoroethylene membrane was immersed in approximately 50 mL of ethyl acetate at 23°C for 7 days, and the sol component of the adhesive was eluted out of the porous polytetrafluoroethylene membrane. After immersion, the adhesive wrapped in the porous polytetrafluoroethylene membrane was removed, dried at 130°C for 2 hours, and allowed to cool for approximately 20 minutes, and then the dry weight (C) was measured. The gel fraction of the adhesive was calculated using the following formula. Gel fraction (%) = 100 × (C - A) / B

[0148] <Storage Modulus, Loss Tangent, and Glass Transition Temperature> A measurement sample was prepared by laminating pressure-sensitive adhesive sheets to a thickness of approximately 1.5 mm. Dynamic viscoelasticity measurements were performed under the following conditions using an Advanced Rheometric Expansion System (ARES) manufactured by Rheometric Scientific. From the measurement results, the storage modulus G' and loss tangent tanδ at each temperature were read. The temperature at which tanδ was maximized was determined as the glass transition temperature of the pressure-sensitive adhesive sheet.

[0149] (Measurement conditions) Deformation mode: torsion Measurement frequency: 1 Hz Heating rate: 5°C / min Shape: parallel plate 7.9 mmφ

[0150] <Total Light Transmittance and Haze> Using a test piece in which the pressure-sensitive adhesive sheet was bonded to alkali-free glass (thickness 0.8 to 1.0 mm, total light transmittance 92%, haze 0.4%), the haze and total light transmittance were measured using a haze meter ("HM-150" manufactured by Murakami Color Research Laboratory). The haze of the pressure-sensitive adhesive sheet was determined by subtracting the haze of the alkali-free glass (0.4%) from the measured value. The measured value was used as is for the total light transmittance. The pressure-sensitive adhesive sheets of all Examples and Comparative Examples had a total light transmittance of 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%.

[0151] <Dielectric Constant> The adhesive sheet was sandwiched between copper foil and an electrode, and the dielectric constant was measured at frequencies of 1 kHz, 10 kHz, 100 kHz, and 1 MHz using an Agilent Technologies Precision Impedance Analyzer 4294A in accordance with JIS K6911 under the following conditions. In Examples 1, 4, 5, and Comparative Example 3, in addition to measurement at a temperature of 25°C, the dielectric constant was also measured at 20°C intervals in the temperature range of -40°C to 80°C. Electrode configuration: 12.1 mmφ, 0.5 mm thick aluminum plate Counter electrode: 3 oz copper plate Measurement environment: temperature 25°C, relative humidity 50%

[0152] <Adhesion Strength to Polyimide Film> The release film on one side of the pressure-sensitive adhesive sheet was peeled off, and a 25 μm-thick PET film was attached. This was then cut to a width of 10 mm and a length of 100 mm to prepare a test specimen. The release film on the other side of the test specimen was peeled off, and the pressure-sensitive adhesive sheet was pressed onto an 80 μm-thick transparent polyimide film (manufactured by Kolon Industries) using a 2 kg roller. The test specimen was peeled off from the polyimide film using a tensile tester at 25°C, a pulling rate of 60 mm / min, and a peel angle of 180°, to measure the peel strength.

[0153] <Bending Retention Test> The release film on one side of the adhesive sheet was peeled off, and a 51 μm thick polarizing plate was attached using a 2 kg roller. The release film on the other side of the adhesive sheet was peeled off, and a 80 μm thick transparent polyimide film was attached using a 2 kg roller. Furthermore, a 125 μm thick PET film was attached to the polarizing plate via a 15 μm thick adhesive sheet A using a 2 kg roller. Before attachment to the adhesive sheet, plasma treatment was performed on the surfaces of the polarizing plate, polyimide film, and PET film.

[0154] This laminate was cut into a 35 mm × 100 mm rectangle so that the absorption axis direction of the polarizing plate was parallel to the long side direction, and autoclaved for 15 minutes at 35 ° C. and 0.35 MPa to obtain a test specimen. Using a sheet-shaped unloaded U-shaped stretch tester (manufactured by Yuasa System Kiki), bending jigs were attached to each end of the long side of the test specimen within 20 mm of each other (a central 60 mm region in the long side direction was left unfixed), and the specimen was held in a bent state with a bending radius of 1.3 mm and a bending angle of 180° so that the PET film side was facing inward, and then held in a constant temperature and humidity chamber at 60 ° C. and 95% relative humidity for 240 hours to perform a bending retention test.

[0155] The test pieces after the bending test were visually inspected to determine whether or not peeling occurred at the interface between the transparent polyimide film and the polarizing plate at the bent portion. In all cases where peeling was observed, peeling (gaps) occurred at the edge of the short side of the sample. For samples where peeling was observed, the length (mm) of the gap in the short side direction of the sample was measured. For samples where peeling was observed along the entire length of the short side of the sample, the gap length (gap distance) was recorded as 35 mm, and for samples where no peeling was observed, the gap distance was recorded as 0. For samples where peeling occurred from both ends in the short side direction, the gap length with the longer length was recorded as the gap distance. In all samples, no peeling was observed at the interface where the PET film and the polarizing plate were bonded.

[0156] [Evaluation Results] Table 1 shows the formulations of the pressure-sensitive adhesive compositions used to prepare the pressure-sensitive adhesive sheets of the Examples and Comparative Examples, and Table 2 shows the evaluation results. Table 2 also shows the measurement results of the dielectric constant of the pressure-sensitive adhesive sheet A used to bond the polarizing plate and the PET film in the sample for the bending retention test. Table 3 shows the measurement results of the dielectric constant of the pressure-sensitive adhesive sheets of Examples 1, 4, and 5 and Comparative Example 3 in the temperature range of -40°C to 80°C, as well as the ratio X of the minimum and maximum relative dielectric constant at each frequency, and the value of X at a frequency of 1 kHz (X １ｋＨｚ ) and the value of X at a frequency of 1 MHz (X １ＭＨｚ ) and the ratio X １ｋＨｚ / X １ＭＨｚ The figures show:

[0157] In Table 1, the components are 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

[0158]

[0159]

[0160] The pressure-sensitive adhesive sheets of Examples 1 to 5 all had a dielectric constant of 4.5 or less and a gap distance of 2 mm or less at a temperature of 25°C and a frequency of 10 MHz. Table 3 shows that the pressure-sensitive adhesive sheets of the Examples have a small dielectric constant, and the temperature dependency and frequency dependency of the dielectric constant are also small.

[0161] REFERENCE SIGNS LIST 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 (circularly polarizing plate) 51 Image display panel (organic EL panel) 54 Image display panel (touch panel integrated organic EL panel) 41 Touch panel 71 Cover window 75 Housing 101, 102 Image display device

Claims

1. In a foldable image display device having a touch panel within 500 μm from a touch surface, an adhesive sheet used for bonding members disposed between the touch surface and the touch panel, having a relative permittivity of 4.5 or less at a temperature of 25° C. and a frequency of 10 kHz, and having a length of a gap between the adhesive sheet and an adherend measured by the following procedures of Steps A to D of 2 mm or less: Adhesive sheet: Step A: Prepare a 35 mm × 100 mm test piece by bonding the adhesive sheet to the adherend. Step B: Bend the test piece prepared in Step A along the short side direction with a bending radius of 1.3 mm and a bending angle of 180°. Step C: Hold the test piece bent in Step B in an environment of a temperature of 60° C. and a relative humidity of 95% for 240 hours. Step D: Measure the length in the short side direction of the gap between the adhesive sheet and the adherend at the bent portion of the test piece after holding for 240 hours in Step C.

2. The 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.

3. The 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.

4. The adhesive sheet according to any one of claims 1 to 3, wherein 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 0.8 to 1.2 times 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 MHz.

5. The adhesive sheet according to any one of claims 1 to 4, having a thickness of 100 μm or less.

6. Storage elastic modulus G' at 25°C and 1 Hz ２５ The pressure-sensitive adhesive sheet according to any one of claims 1 to 5, wherein the storage elastic modulus G' is 70 kPa or less and the glass transition temperature is -20°C or less.

7. The adhesive sheet according to any one of claims 1 to 6, comprising an acrylic adhesive containing an acrylic base polymer.

8. The acrylic-based polymer contains 5 to 55 parts by weight of a chain alkyl ester of (meth)acrylic acid with respect to 100 parts by weight in total of monomer components. １０－２０ The pressure-sensitive adhesive sheet according to claim 7.

9. The acrylic-based polymer contains lauryl acrylate as the (meth)acrylic acid C １０－２０ chain alkyl ester, and the pressure-sensitive adhesive sheet according to claim 8.

10. 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 a hydroxy group-containing monomer, a carboxy group-containing monomer, and a nitrogen-containing monomer with respect to 100 parts by weight in total of the monomer components. The adhesive sheet according to any one of claims 7 to 9.

11. The pressure-sensitive adhesive sheet according to any one of claims 7 to 10, wherein the acrylic-based polymer has a content of a hydroxy group-containing monomer of 10 parts by weight or less with respect to 100 parts by weight in total of monomer components.

12. The pressure-sensitive adhesive sheet according to any one of claims 7 to 11, wherein the acrylic-based polymer has a crosslinked structure.

13. The pressure-sensitive adhesive sheet according to claim 12, wherein the crosslinked structure is a crosslinked structure introduced by a polyfunctional (meth)acrylate.

14. The pressure-sensitive adhesive sheet according to any one of claims 7 to 13, wherein the acrylic-based pressure-sensitive adhesive further contains an acrylic-based oligomer having a glass transition temperature of 60°C or higher.

15. The pressure-sensitive adhesive sheet according to claim 14, wherein the content of the acrylic-based oligomer with respect to 100 parts by weight of the acrylic-based polymer is 0.1 to 5 parts by weight.

16. A film with an adhesive layer, wherein the pressure-sensitive adhesive sheet according to any one of claims 1 to 15 is laminated on at least one surface of a polarizing plate.

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