Image display device

Abstract

The image display device (101) is provided with a polarizing plate (31) and a cover window (71) in that order on the viewing side of the image display panel (51), and a touch panel (41) is provided within a distance of 500 μm from the touch surface. A first adhesive sheet (11) is provided on the viewing side surface of the polarizing plate, and a second adhesive sheet (12) is provided on the image display panel side surface of the polarizing plate. Both the first adhesive sheet and the second adhesive sheet have a relative permittivity of 4.5 or less at a temperature of 25°C and a frequency of 10 kHz.

Description

Technology Field

[0001] The present invention relates to a foldable image display device having a touch panel. Background Technology

[0002] Flat panel displays, such as liquid crystal displays and organic EL displays, 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 commercialized, and bendable flexible displays have been proposed.

[0003] In flexible displays, in addition to display panels such as organic EL panels being foldable, constituent members such as housings and touch panels are also foldable, and these members are bonded through an adhesive sheet (e.g., Patent Document 1). In a foldable flexible display (foldable display), a transparent plate (cover window) placed on the viewing side surface must also be foldable, and thin materials such as resin films or thin glass are used.

[0004] In foldable displays, bending is performed repeatedly at the same location. At the bending point, compressive stress is applied to the inner side and tensile stress to the outer side, and deformation occurs at the bending point and its surroundings, raising concerns about the failure of the device. Therefore, it has been proposed to make the adhesive sheet bonding the members flexible to relieve stress deformation (e.g., Patent Document 2). Prior art literature

[0005] Japanese Patent Publication No. 2016-2764 and Japanese Patent Publication No. 2018-45213 The problem to be solved

[0006] In recent years, there has been a demand for thinner foldable display devices. In particular, there is a demand for devices with high reliability in which there is little to no malfunction of the touch panel sensor and failures are suppressed or prevented, even when the distance from the device surface to the touch panel sensor is small. The present invention aims to provide a foldable image display device that also possesses high reliability. means of solving the problem

[0007] One embodiment of the present invention is a foldable image display device having a touch panel. The image display device has a polarizing plate and a cover window in that order on the viewing side of the image display panel, and has a touch panel within a distance of 500 μm from the touch surface. The touch panel is, for example, disposed between the image display panel and the polarizing plate. It may also be disposed inside the image display panel.

[0008] A first adhesive sheet is provided on the viewing side surface of the polarizing plate, and a second adhesive sheet is provided on the image display panel side surface of the polarizing plate. Both the first adhesive sheet and the second adhesive sheet have a relative permittivity of 4.5 or less at a temperature of 25°C and 10 kHz.

[0009] It is preferable that the first adhesive sheet and the second adhesive sheet have a ratio of the relative permittivity at 1 kHz to the relative permittivity at 1 MHz at a temperature of 25°C to be 1.50 or less. It is preferable that both the first adhesive sheet and the second adhesive sheet have a maximum relative permittivity in a temperature range of -40°C to 80°C at a frequency of 10 kHz to be 1.4 times or less of the minimum relative permittivity. It is preferable that both the first adhesive sheet and the second adhesive sheet have a ratio of the maximum relative permittivity to the minimum relative permittivity in a temperature range of -40°C to 80°C at a frequency of 1 kHz to be 0.8 to 1.2 times the ratio of the maximum relative permittivity to the minimum relative permittivity in a temperature range of -40°C to 80°C at a frequency of 1 MHz.

[0010] The thickness of the cover window may be 100㎛ or less.

[0011] The thickness of the first adhesive sheet may be greater than the thickness of the second adhesive sheet, and the thickness of the first adhesive sheet may be 100㎛ or less.

[0012] Storage modulus G' of the first adhesive sheet at 25°C and 1 Hz 25 It may be 70 kPa or less. The glass transition temperature of the adhesive sheet may be -20℃ or less.

[0013] The first adhesive sheet may be composed of an acrylic adhesive comprising an acrylic base polymer. The acrylic base polymer comprises (meth)acrylic acid C, with respect to 100 parts by weight of the total monomer components. 10-20 It may contain 5 to 55 parts by weight of a chain alkyl ester. The acrylic base polymer is (meth)acrylic acid C 10-20 As a chain alkyl ester, it 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 hydroxyl group-containing monomers, carboxyl group-containing monomers, and nitrogen-containing monomers, based on 100 parts by weight of the total monomer components. The acrylic base polymer may have a content of hydroxyl group-containing monomers of 10 parts by weight or less based on 100 parts by weight of the total monomer components.

[0015] The acrylic base polymer may have a cross-linked structure. The cross-linked structure may be introduced by a polyfunctional (meth)acrylate.

[0016] The acrylic adhesive may further include an acrylic oligomer having a glass transition temperature of 60°C or higher. The content of the acrylic oligomer per 100 parts by weight of the acrylic base polymer may be 0.1 to 5 parts by weight. Effects of the invention

[0017] The image display device of the present invention has reduced malfunction of the touch panel and can exhibit high reliability. Brief explanation of the drawing

[0018] FIG. 1 is a cross-sectional view illustrating an example of the configuration of an image display device. FIG. 2 is a cross-sectional view illustrating an example of the configuration of an image display device. Specific details for implementing the invention

[0019] FIGS. 1 and FIGS. 2 are cross-sectional views of a flexible display configuration according to one embodiment.

[0020] In the image display device (101) illustrated in FIG. 1, an organic EL panel (51), a touch panel (41), and a circular polarizer (31) are arranged between a housing (75) and a cover window (71). The bottom surface of the organic EL panel (51) and the housing (75) is bonded through an adhesive sheet (14), the organic EL panel (51) and the touch panel (41) are bonded through an adhesive sheet (13), the touch panel (41) and the circular polarizer (31) are bonded through an adhesive sheet (12), and the circular polarizer (31) and the cover window (71) are bonded through an adhesive sheet (11). In this way, in the flexible display, a plurality of members are bonded together through an adhesive sheet, thereby forming a laminated integrated structure.

[0021] The cover window (71) is positioned on the visible side surface of the flexible display and forms a touch surface. The touch panel (41) is a capacitive touch panel. In the flexible display, since a foldable cover window (71) is used, the thickness of the cover window is small, and consequently, the distance D from the touch surface (the surface of the cover window (71)) to the touch panel (41) is also small.

[0022] The image display device (102) illustrated in FIG. 2 is equipped with an organic EL panel (54) in which a touch panel is integrated, and a circular polarizing plate (31) is bonded to the organic EL panel (54) through an adhesive sheet (12). The other configuration is the same as in FIG. 1, and the distance from the touch surface to the visible side surface of the organic EL panel (54) corresponds to the distance D from the touch surface to the touch panel.

[0023] In an image display device using a rigid glass plate as a cover window, the distance from the touch surface to the touch panel is generally 700 μm or more, whereas in a flexible display, the distance D from the touch surface to the touch panel (41) (in the case where the touch panel is an in-cell type, the distance from the touch surface to the image display panel) 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.

[0024] As illustrated in FIGS. 1 and 2, the image display device is provided with a polarizing plate (31) and a cover window (71) on the viewing side of the image display panel (51, 54). A first adhesive sheet (11) is provided on the viewing side of the polarizing plate (31), and a second adhesive sheet (12) is provided on the image display panel (51, 54) side of the polarizing plate (31).

[0025] [Permittivity of adhesive sheet]

[0026] The adhesive sheets (11, 12) placed on the upper and lower surfaces of the polarizing plate (31) have a relative permittivity of 4.5 or less at a temperature of 25°C and a frequency of 10 kHz. Unless otherwise specified, the permittivity is a value measured at a temperature of 25°C. The relative permittivity of the adhesive sheets at a frequency of 10 kHz may be 4.0 or less, 3.8 or less, or 3.5 or less. By making the relative permittivity at a frequency of 10 kHz 4.5 or less, the distance from the touch surface to the touch panel can be reduced, thereby enabling a design advantageous for flexibility. Since the low permittivity adhesive sheets have a small capacitance value, it is possible to design a sensor with good sensitivity. This enables input methods with a small contact area, such as pen input.

[0027] The relative permittivity of the adhesive sheet at a frequency of 1 kHz is preferably 5.0 or less, more preferably 4.8 or less, and may be 4.5 or less, 4.0 or less, or 3.8 or less. The relative permittivity of the adhesive sheet at a frequency of 100 kHz is preferably 4.0 or less, and may be 3.8 or less or 3.5 or less. The relative permittivity of the 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.

[0028] The dielectric constant changes depending on the polarization of the material, and the dielectric constant of the adhesive sheet can be controlled by selecting adhesive materials such as urethane, acrylic, rubber, and silica. Additionally, since the relative dielectric constant of air is 1, the dielectric constant of the adhesive sheet can be lowered by adding hollow beads or similar materials to the adhesive. In acrylic adhesives, monomers with long alkyl chains have low polarization, allowing for low dielectric constant. Using highly polar monomers results in high polarization, which in turn increases the dielectric constant. One method to lower polarization is to induce molecular entanglement. Increasing molecular weight or the degree of crosslinking makes it easier to induce molecular entanglement, which lowers polarization and enables low dielectric constant. Since the dielectric constant tends to increase with increasing water content, the dielectric constant can be reduced by using materials that are resistant to moisture retention.

[0029] According to the Clausius-Mossotti equation, the smaller the polarization of electric dipoles and the smaller the number of electric dipoles per unit volume, the smaller the relative permittivity. To reduce the relative permittivity of an adhesive sheet, the dipole moment of the base polymer constituting the adhesive must be reduced, and the molar volume must be increased. For example, the molar volume tends to increase as the volume of the base polymer's side chains increases. Additionally, by selecting monomers with low polarity as the monomer components constituting the base polymer, the electron dipoles of the molecules become smaller.

[0030] It is desirable that the adhesive sheet has a small frequency dependence of its dielectric constant at a temperature of 25°C. Specifically, it is desirable that 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) of the adhesive sheet be 1.5 or less. By having a small dielectric constant over a wide frequency range and a small frequency dependence of the dielectric constant, operational reliability for various operating frequencies can be ensured. As described above, the frequency dependence of the dielectric constant can be reduced by adjusting the dielectric constant of the adhesive.

[0031] It is desirable for the adhesive sheet to have a small temperature dependence of its dielectric constant. Specifically, it is desirable that the ratio of the minimum to the maximum dielectric constant (maximum value / minimum value) in the temperature range of -40°C to -80°C be close to 1. Ratio of the maximum to the minimum dielectric constant X at a frequency of 10 kHz 10㎑ is 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. The ratio X of the maximum and minimum relative permittivity at a frequency of 1 kHz. 1㎑ , the ratio X of the maximum and minimum relative permittivity at a frequency of 100 kHz 100㎑ and the ratio X of the maximum and minimum relative permittivity at a frequency of 1 MHz 1㎒ Each, 1.4 or less is preferable, 1.3 or less is more preferable, 1.2 or less is even more preferable, and 1.1 or less is acceptable.

[0032] The ratio X of the maximum and minimum values ​​of the relative permittivity in the temperature range of -40℃ to -80℃ is an indicator of the temperature dependence of the relative permittivity, and the closer X is to 1, the smaller the temperature dependence of the relative permittivity. As mentioned above, in addition to the small temperature dependence of the relative permittivity at a frequency of 10 kHz, it is desirable that the temperature dependence of the relative permittivity be small over the entire frequency range of 1 kHz to 1 MHz. When the temperature dependence of the relative permittivity is small, the frequency dependence also tends to be small. Furthermore, as the frequency increases, the temperature at which the relative permittivity is maximized tends to shift toward the high-temperature side.

[0033] The above X 1㎒ and X 1㎑ Of X 1㎑ / X 1㎒ The value 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. X 1㎑ / X 1㎒As the value is closer to 1, the change in dielectric constant is small over a wide temperature and frequency range, so operational reliability can be ensured over a wide temperature range and operating frequency.

[0034] [First adhesive sheet]

[0035] The first adhesive sheet (11) placed on the visible side of the polarizing plate (31) preferably has a gap length of 2 mm or less according to the bending maintenance test described later (i.e., the gap length between the adhesive sheet and the adherend after maintaining the bend for 240 hours: hereinafter simply referred to as "gap distance"). If the gap distance is 2 mm or less, even if the image display device is foldable and the touch panel is provided within 500 μm of the distance from the touch surface, the adhesive sheet (11) can absorb the stress caused by bending, 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 almost no malfunction of the touch panel sensor, and failures are suppressed or prevented, resulting in high reliability.

[0036] 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 specifically limited and may be 0.

[0037] Delamination (voids) in a bending test is likely to occur at the end of the bending axis of the test specimen (the end in the direction of the shorter side). If voids occur from both ends or if voids exist in multiple locations, the length of the void with the longest length in the direction of the shorter side is defined as the void distance. If voids exist in multiple locations, the longest void must be 2 mm or less. Preferably, the sum of the lengths of each void is 2 mm or less, and the sum of the lengths 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 0.

[0038] When bending, the adherend expands and contracts, and delamination occurs because the adhesive cannot follow the deformation of the adherend. For example, the adhesive's ability to follow the adherend can be adjusted by controlling the storage modulus G' of the adhesive. The smaller the storage modulus of the adhesive, the better the ability to follow the deformation of the adherend. When maintaining a bent state, delamination may occur due to the concentration of stress associated with the deformation of the bent portion. To increase the stress relaxation of the adhesive, the loss tangent tangent tanδ should be increased. Furthermore, to suppress delamination at the interface between the adherend and the adhesive, it is also necessary to design the adhesive strength at the bending temperature to be high. Additionally, in high-humidity environments, the retention of moisture at the interface between the adhesive and the adherend can cause a decrease in adhesive strength; therefore, it is desirable to suppress moisture retention by using a material that is resistant to moisture retention.

[0039] The adhesive strength of the first adhesive sheet (11) to the polyimide film 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 with a polyimide film as the substrate, at a tensile speed of 60 mm / min and a peel angle of 180°. Unless otherwise noted, the adhesive strength is a value measured at 25°C. By having the adhesive strength of the adhesive sheet (11) within the above range, peeling of the substrate, such as the polarizing plate (31) or cover window (71), can be prevented when bending is repeated.

[0040] The adhesive sheet (11) has a storage modulus G' at 25°C. 25 It is desirable that G' be 70 kPa or less. 25 Since G is 70 kPa or less, deformation tends to be mitigated when the device is bent, thereby suppressing damage to device components when bending is repeated. 25It is preferable that it be 5 kPa or more, and within this range, the adhesive holding power and deformation relaxation of the adhesive sheet can be achieved simultaneously, so the gap distance can be reduced. In terms of more effectively achieving a balance between processability, adhesive holding power, and deformation relaxation, G' of the adhesive sheet (11) 25 10 to 60 kPa is preferred, 13 to 50 kPa is more preferred, and 15 to 40 kPa is even more preferred.

[0041] The adhesive sheet (11) has a storage modulus G' at 100°C. 100 The value is preferably 2 to 50 kPa, more preferably 3 to 40 kPa, and even more preferably 5 to 25 kPa. G' of the adhesive sheet 100 Within this above range, adhesive retention and stress relaxation can be achieved simultaneously even in high-temperature environments, so the gap distance can be reduced.

[0042] loss tangent tangent of the adhesive sheet (11) at 25°C tangent tangent tangent tangent tangent tangent at 25°C 25 It is preferable that is 0.2 to 0.45. In addition, the adhesive sheet (11) has a loss tangent tangent tanδ at 100°C. 100 It is preferable that this is 0.2 to 0.4. Also, tanδ 25 and tanδ 100 It is preferable that the difference be between -0.07 and 0.07. tanδ 25 tanδ may be between 0.25 and 0.42. 100 It may be 0.25 to 0.38. Also, tanδ 25 and tanδ 100 The difference may be within ±0.06 or ±0.05. tanδ of the adhesive sheet 25 , tanδ 100 and tanδ 25 and tanδ 100By keeping the difference within the above range, the void distance can be reduced. The tanδ of the adhesive 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 or molecular weight of the base polymer is within an appropriate range.

[0043] The storage modulus G' and loss tangent tan δ of the adhesive sheet (11) are obtained by viscoelastic measurement at a frequency of 1 Hz. tan δ is the ratio G" / G' of the storage modulus G' and the loss modulus G". The storage modulus G' corresponds to the portion that is stored as elastic energy when the material is deformed and is an indicator of the degree of hardness.

[0044] In order to reduce the temperature dependence of tanδ in the range from room temperature to high temperature, it is preferable that the glass transition temperature of the adhesive sheet (11) be -20°C or lower, more preferable that it be -23°C or lower, and even more preferable that it be -25°C or lower. The glass transition temperature is the temperature at which tanδ is maximized (peak top temperature). Near the glass transition temperature, the temperature dependence of tanδ is large. By making the glass transition temperature sufficiently lower than the operating environment temperature of the device, the temperature dependence of tanδ in the operating environment temperature range is reduced. Furthermore, since the glass transition temperature is within the above range, the adhesive sheet (11) maintains adhesive strength even in the low temperature range, so peeling from the substrate at low temperatures is suppressed, and the gap distance can be reduced.

[0045] The lower limit of the glass transition temperature of the adhesive sheet (11) is not specifically limited, but is generally -80°C or higher. The glass transition temperature of the adhesive sheet (11) 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 adhesive sheet to the above range, the adhesive holding power can be effectively increased and the gap distance can be reduced.

[0046] The thickness of the adhesive sheet (11) is not particularly limited and can be appropriately adjusted according to the thickness of the intended device, characteristics required of the adhesive sheet, etc. From the perspective of increasing the adhesive strength of the adhesive sheet (11), the thickness is preferably 10 μm or more. From the perspective of providing cushioning properties against impacts from the outer surface, the thickness of the adhesive sheet (11) is preferably 25 μm or more, more preferably 30 μm or more, and may be 35 μm or more or 40 μm or more. From the perspective of thinning the device, processing the adhesive sheet, and bending the device, the thickness of the adhesive sheet (11) is preferably 100 μm or less, more preferably 75 μm or less.

[0047] The total light transmittance of the adhesive sheet (11) is preferably 85% or more, more preferably 90% or more, and even more preferably 91% or more. The haze of the adhesive sheet (11) 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. In addition to the adhesive sheet (11), the adhesive sheet placed on the visible side of the image display panel (51, 54), such as the adhesive sheet (12, 13), is preferably highly transparent, and the total light transmittance and haze are preferably within the above ranges.

[0048] As long as the dielectric constant of the adhesive sheet (11) is within the above range, the composition of the adhesive is not particularly limited, and as a base polymer, examples include polymers such as acrylic, silicone, polyester, polyurethane, polyamide, polyvinyl ether, vinyl acetate / vinyl chloride copolymer, modified polyolefin, epoxy, fluorine, and rubber. As for the adhesive, an acrylic adhesive having an acrylic base polymer as the main component is preferred in that it allows for control of transparency and adhesive strength in addition to dielectric constant and adhesive strength. The adhesive may be used alone or in combination of two or more types. Furthermore, the adhesive sheet formed by the adhesive may be in a single layer form or a laminated form.

[0049] Acrylic-based base polymer

[0050] The acrylic base polymer contains (meth)acrylate alkyl esters as the main constituent monomer components. Additionally, in this specification, "(meth)acrylate" means acrylic and / or methacrylate.

[0051] As an alkyl (meth)acrylic acid ester, (meth)acrylic acid C having 1 to 20 carbon atoms in the alkyl group. 1-20 Alkyl esters are suitably used. The alkyl group in the (meth)acrylate alkyl ester may be in the form of a chain or a cyclic group. In addition, the alkyl group in the form of a chain (chain alkyl group) may be a straight-chain alkyl group or may have branches.

[0052] Specific examples of (meth)acrylate chain alkyl esters include C of (meth)acrylate 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, and (meth)acrylate isononyl.1-9 Chain alkyl esters; and C of (meth)acrylate decyl, (meth)acrylate isodecyl, (meth)acrylate undecyl, (meth)acrylate dodecyl, (meth)acrylate isotridecyl, (meth)acrylate tetradecyl, (meth)acrylate isotetradecyl, (meth)acrylate pentadecyl, (meth)acrylate cetyl, (meth)acrylate heptadecyl, (meth)acrylate octadecyl, (meth)acrylate isooctadecyl and (meth)acrylate nonadecyl, etc. 10-20 Examples include chain alkyl esters.

[0053] Specific examples of (meth)acrylate alkyl esters having a dicyclic alkyl group (cyclic alkyl group) include (meth)acrylate cycloalkyl esters such as (meth)acrylate cyclopentyl, (meth)acrylate cyclohexyl, (meth)acrylate cycloheptyl, (meth)acrylate cyclooctyl, etc.; (meth)acrylate esters having a bicyclic aliphatic hydrocarbon group such as (meth)acrylate isobornyl, etc.; Examples include (meth)acrylic acid esters having three or more aliphatic hydrocarbon rings, such as dicyclofentanyl (meth)acrylate, dicyclofentanyloxyethyl (meth)acrylate, tricyclofentanyl (meth)acrylate, 1-adamantyl (meth)acrylate, 2-methyl-2-adamantyl (meth)acrylate, and 2-ethyl-2-adamantyl (meth)acrylate.

[0054] In the acrylic base polymer, the amount of (meth)acrylate alkyl ester per 100 parts by weight of the total monomer components is preferably 60 to 100 parts by weight, and more preferably 70 to 98 parts by weight.

[0055] The acrylic base polymer is an (meth)acrylic acid alkyl ester, (meth)acrylic acid C 10-20It is preferable to include chain alkyl esters. By including (meth)acrylic acid long-chain alkyl esters as monomer components in an acrylic base polymer, the molecular dipole moment is reduced, and the molar volume can be increased, thereby allowing the relative permittivity to be lowered. In addition, homopolymers of (meth)acrylic acid alkyl esters having long-chain alkyl groups with 10 or more carbon atoms have a temperature region (plateau region) at a temperature higher than Tg in which the temperature dependence of viscoelasticity is small. Therefore, when the base polymer includes (meth)acrylic acid long-chain alkyl esters as monomer components, the temperature dependence of tanδ can be reduced.

[0056] In terms of the wide temperature range of the plateau region and the low storage modulus in the plateau region, (meth)acrylic acid C 10-20 Among chain alkyl esters, (meth)acrylic acid C 10-16 An alkyl ester is preferred, and (meth)acrylic acid C 10-13 Alkyl esters are more preferable. Among them, (meth)acrylic acid C 12 Alkyl esters are preferred, and dodecyl acrylate (lauryl acrylate) is particularly preferred.

[0057] Polymers of (meth)acrylic acid long-chain alkyl esters are characterized by a wide temperature range in the plateau region and a low storage modulus in the plateau region, but they 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, (meth)acrylic acid C as a monomer component 10-20 In addition to chain alkyl esters, (meth)acrylic acid C 1-9 It is preferable to include a chain alkyl ester.

[0058] (Met)acrylic acid C 1-9Among chain alkyl esters, in order to achieve a low Tg of the base polymer, it is preferable that the glass transition temperature of the homopolymer be -40°C or lower. (Met)acrylic acid C having a homopolymer glass transition temperature of -40°C or lower 1-9 Specific 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), and butyl acrylate (Tg: -55°C). Among these, butyl acrylate and 2-ethylhexyl acrylate are preferred, and 2-ethylhexyl acrylate is particularly preferred due to its low Tg.

[0059] In order to obtain an adhesive having the above-described characteristics, (meth)acrylic acid C as a monomer component of an acrylic base polymer 10-20 Chain alkyl ester and (meth)acrylic acid C 1-9 It is desirable to include both chain alkyl esters and adjust the ratio of the two.

[0060] (Met)acrylic acid C per 100 parts by weight of the total monomer components of the acrylic base polymer 10-20 The amount of 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, it is preferable that the amount of lauryl acrylate be within the above range. (Met)acrylic acid C per 100 parts by weight of the total monomer components of the acrylic base polymer 1-9 The amount of 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 acrylic acid be within the above range.

[0061] The acrylic base polymer may include a nitrogen-containing monomer as a monomer component. Examples of nitrogen-containing monomers 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, or cyanoacrylate monomers such as acrylonitrile and methacrylonitrile. Among these, N-vinylpyrrolidone is preferred due to its high effect of improving adhesion strength through improved cohesion.

[0062] A cross-linked structure may be introduced into the acrylic base polymer. By cross-linking the acrylic base polymer, high adhesive retention can be achieved even when the G' of the adhesive sheet is small. In order to introduce a cross-linked structure into the acrylic base polymer in this manner, it is preferable that the acrylic base polymer contain hydroxyl group-containing monomers and carboxyl group-containing monomers as monomer components, in addition to the above-mentioned (meth)acrylate alkyl esters. When a cross-linked structure is introduced into the acrylic base polymer using isocyanate-based cross-linking agents or epoxy-based cross-linking agents, the hydroxyl group or carboxyl group serves as the introduction site for the cross-linked structure.

[0063] Examples of hydroxyl group-containing monomers include (meth)acrylic acid esters such as (meth)acrylic acid 2-hydroxyethyl, (meth)acrylic acid 2-hydroxypropyl, (meth)acrylic acid 4-hydroxybutyl, (meth)acrylic acid 6-hydroxyhexyl, (meth)acrylic acid 8-hydroxyoctyl, (meth)acrylic acid 10-hydroxydecyl, and (meth)acrylic acid 12-hydroxylauryl. Among these, 2-hydroxyethyl acrylic acid (Tg: -15°C) and 4-hydroxybutyl acrylic acid (Tg: -32°C) are preferred because they contribute significantly to improving adhesion and can suppress cloudiness of the adhesive sheet (11) under high humidity environments, and 4-hydroxybutyl acrylic acid is particularly preferred because it has a low Tg.

[0064] Examples of carboxyl group-containing monomers include acrylic monomers such as (meth)acrylic acid, (meth)acrylic acid carboxyethyl, (meth)acrylic acid carboxypentyl, itaconic acid, maleic acid, fumaric acid, and crotonic acid.

[0065] In order to increase the adhesive strength and adhesion retention of the adhesive sheet (11), the amount of a 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. Meanwhile, as the content of the polar monomer increases, the dipole moment of the base polymer increases, and the dielectric constant increases. Also, if the content of the polar monomer is excessively large, the glass transition temperature of the polymer increases, and the adhesive strength at low temperatures tends to decrease. Therefore, the amount of a 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.

[0066] The acrylic base polymer preferably comprises a hydroxyl group-containing monomer and a nitrogen-containing monomer among the polar monomer components above, and preferably the sum of the hydroxyl group-containing monomer and the nitrogen-containing monomer is within the above range.

[0067] By including a hydroxyl group-containing monomer as a polar monomer component, the adhesive strength of the adhesive sheet (11) is improved, and the cloudiness of the adhesive sheet (11) in a high-humidity environment tends to be suppressed. Therefore, the amount of the hydroxyl 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 be 2 parts by weight or more. On the other hand, as the content of the hydroxyl group-containing monomer (the amount of hydroxyl groups in the adhesive) increases, the dielectric constant of the adhesive tends to increase significantly. Therefore, the amount of the hydroxyl 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 be 6 parts by weight or less. If the amount of hydroxyl group-containing monomer is within the above range, the dielectric constant and void distance can be within the above-described range.

[0068] In order to achieve both improved adhesion and low dielectric constant, the amount of nitrogen-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, and more preferably 1 to 8 parts by weight. In particular, it is preferable that the amount of N-vinylpyrrolidone be within the above range, as it contributes significantly to the improvement of adhesion. If the amount of nitrogen-containing monomer is within the above range, the dielectric constant and pore distance can be within the aforementioned ranges.

[0069] In order to prevent corrosion of the electrodes of the touch panel by acid components, it is preferable that the adhesive sheet (11) has a low acid content. In addition, in order to suppress polyenification of the polyvinyl alcohol-based polarizer by acid components, it is preferable that the adhesive sheet (11) has a low acid content. In such an acid-free adhesive sheet, it is preferable that the content of an organic acid monomer, such as (meth)acrylic acid, is 100 ppm or less, more preferable that it is 70 ppm or less, and even more preferable that it is 50 ppm or less. The organic acid monomer content of the adhesive sheet (11) is determined by immersing the adhesive sheet in pure water, heating it at 100°C for 45 minutes, and quantifying the acid monomer extracted into the water using an ion chromatograph.

[0070] In order to reduce the acid monomer content in the adhesive sheet (11), it is desirable 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 adhesive sheet acid-free, it is desirable that the base polymer does not substantially contain organic acid monomers (carboxyl group-containing monomers) as monomer components. In the acid-free adhesive sheet, the amount of carboxyl 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.

[0071] The acrylic base polymer may include, as a monomer component, monomers other than the above-mentioned (meth)acrylate alkyl esters and polar monomers. As monomer components other than those mentioned above, vinyl monomers such as caprolactone adducts of (meth)acrylic acid, monomers containing sulfonic acid groups, monomers containing phosphate groups, vinyl acetate, vinyl propionate, styrene, and α-methylstyrene; cyanoacrylate monomers such as acrylonitrile and methacrylonitrile; epoxy group-containing monomers such as glycidyl (meth)acrylate; and glycol-based acrylic ester monomers such as polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, methoxyethylene glycol (meth)acrylate, and methoxypolypropylene glycol (meth)acrylate. Examples of acrylic acid ester monomers include tetrahydrofurfuryl (meth)acrylate, fluorine (meth)acrylate, silicon (meth)acrylate, or 2-methoxyethyl (meth)acrylate.

[0072] 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, 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 dielectric constant and pore distance can be within the aforementioned ranges. The theoretical Tg is the glass transition temperature Tg of the homopolymer of the constituent monomer component of the acrylic base polymer. i Wow, weight fraction W of each monomer component i From, it is calculated by the following Fox formula.

[0073] 1 / Tg=Σ(W i / Tg i )

[0074] Tg is the glass transition temperature of the polymer chain (unit: K), W iis the weight fraction of monomer component i constituting the segment (copolymerization ratio on a weight basis), Tg i Tg is the glass transition temperature (unit: K) of the homopolymer of monomer component i. As the glass transition temperature of the homopolymer, the values ​​listed in the Polymer Handbook, 3rd edition (John Wiley & Sons, Inc., 1989) may be used. For the homopolymer Tg of monomers not listed in the above literature, the peak top temperature of tanδ obtained by dynamic viscoelasticity measurement may be used.

[0075] <Cross-linked structure of base polymer>

[0076] As described above, the acrylic base polymer may have a cross-linked structure. By introducing a cross-linked structure into the base polymer, the gel fraction of the adhesive increases. The gel fraction of the adhesive sheet (11) 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 to this range, high adhesive holding power can be achieved even when G' is small and the adhesive sheet is flexible, so the above-mentioned gap distance can be reduced.

[0077] 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: weight%) of the insoluble component after immersing an 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 equivalent to the degree of crosslinking, and the gel fraction increases as the crosslinked portion within the polymer increases. The gel fraction (amount of crosslinked structure introduced) can be adjusted to a desired range by the method of introducing the crosslinked structure, the type and amount of the crosslinking agent, etc.

[0078] Methods for introducing a crosslinked structure into a base polymer include: (1) a method of adding a crosslinking agent after polymerizing a base polymer having functional groups capable of reacting with a crosslinking agent, thereby reacting the base polymer with the crosslinking agent; and (2) a method of introducing a branched structure (crosslinked structure) into the polymer chain by including a polyfunctional compound in the polymerization component of the base polymer. Multiple types of crosslinked structures may be introduced into the base polymer by using these methods in combination.

[0079] In the method of reacting the base polymer and the crosslinking agent of (1) above, a crosslinking agent is added to the base polymer after polymerization, and a crosslinking structure is introduced into the base polymer by heating as needed. Examples of crosslinking agents include compounds that react with functional groups such as hydroxyl groups or carboxyl groups contained in the base polymer. 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, metal chelate-based crosslinking agents, etc.

[0080] Among these, isocyanate-based and epoxy-based crosslinking agents are preferred due to their high reactivity with the hydroxyl or carboxyl groups of the base polymer and the ease of introducing a crosslinked structure. These crosslinking agents react with functional groups, such as hydroxyl or carboxyl groups, introduced into the base polymer to form a crosslinked structure. In acid-free adhesives where the base polymer does not contain carboxyl groups, it is preferable to use an isocyanate-based crosslinking agent to form a crosslinked structure through the reaction between the hydroxyl groups in the base polymer and the isocyanate crosslinking agent.

[0081] As an isocyanate-based crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule is used. Examples of isocyanate-based crosslinking agents 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; Examples of isocyanate adducts include trimethylolpropane / tolylene diisocyanate trimer adducts (e.g., the solvent "Coronate L"), trimethylolpropane / hexamethylene diisocyanate trimer adducts (e.g., the solvent "Coronate HL"), trimethylolpropane adducts of xylylene diisocyanate (e.g., Mitsui Chemicals' "Takenate D110N"), and isocyanurate adducts of hexamethylene diisocyanate (e.g., the solvent "Coronate HX").

[0082] In the method of including a polyfunctional monomer in the polymerization component of the base polymer of (2) above, the entire amount of the monomer component constituting the acrylic base polymer and the polyfunctional compound for introducing a cross-linking structure may be reacted at once, or polymerization may be performed in multiple stages. As a method of performing polymerization in multiple stages, it is preferable to polymerize (pre-polymerize) the monofunctional monomer constituting the base polymer to prepare a partial polymer (prepolymer composition), add a polyfunctional compound such as a polyfunctional (meth)acrylate to the prepolymer composition, and polymerize (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 monomers.

[0083] By performing prepolymerization of the constituent components of an acrylic base polymer, branching points (crosslinking points) formed by a polyfunctional compound can be uniformly introduced into the base polymer. Additionally, a mixture of a low molecular weight polymer or a partial polymer and an unpolymerized monomer component (adhesive composition) can be applied onto a substrate, and then main polymerization can be performed on the substrate to form an adhesive sheet. Since low-molecular-weight compositions such as prepolymer compositions have low viscosity and excellent applicability, by applying an adhesive composition, which is a mixture of a prepolymer composition and a polyfunctional compound, and then performing main polymerization on the substrate, the productivity of the adhesive sheet can be improved, and the thickness of the adhesive sheet can be made uniform.

[0084] Examples of polyfunctional compounds used for introducing a cross-linked structure include compounds containing two or more polymerizable functional groups (ethylenically unsaturated groups) having unsaturated double bonds in one molecule. As for polyfunctional compounds, polyfunctional (meth)acrylates are preferred because copolymerization with the monomer component of an acrylic base polymer is easy. When introducing a branched (cross-linked) structure by active energy beam polymerization (photopolymerization), polyfunctional acrylates are preferred.

[0085] As polyfunctional (meth)acrylates, 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, tricyclodecanedimethanol di(meth)acrylate, ethoxylated isocyanuric acid tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, and dipentaerythritol poly(meth)acrylate. Examples include dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerin di(meth)acrylate, epoxy(meth)acrylate, butadiene(meth)acrylate, isoprene(meth)acrylate, etc.

[0086] The molecular weight of a polyfunctional compound, such as a polyfunctional (meth)acrylate, is preferably 1,500 or less, and more preferably 1,000 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. By keeping the molecular weight of the polyfunctional compound within this range, the pore distance can be reduced.

[0087] Preparation of Base Polymer

[0088] Acrylic base polymers can be prepared by known polymerization methods such as solution polymerization, UV polymerization, bulk polymerization, and emulsion polymerization. Solution polymerization or active energy beam polymerization (e.g., UV polymerization) is preferred in terms of the transparency, water resistance, and cost of the adhesive. Ethyl acetate, toluene, etc. are generally used as solvents for solution polymerization.

[0089] When preparing acrylic base polymers, polymerization initiators such as photopolymerization initiators or thermal polymerization initiators may be used depending on the type of polymerization reaction. As for photopolymerization initiators, there are no particular limitations as long as they initiate photopolymerization, and for example, 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, acylphosphine oxide-based photopolymerization initiators, etc. may be used. As a thermal polymerization initiator, for example, an azo-based initiator, a peroxide-based initiator, or a redox-based initiator combining a peroxide and a reducing agent (for example, a combination of persulfate and sodium bisulfite, a combination of peroxide and sodium ascorbate, etc.) may be used.

[0090] During polymerization, chain transfer agents or polymerization inhibitors (polymerization retarders) may be used for purposes such as adjusting molecular weight. Examples of chain transfer agents include thiols such as α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolic acid, and 2,3-dimercapto-1-propanol, or α-methylstyrene dimers.

[0091] The molecular weight of the base polymer can be controlled by adjusting the type or amount of the polymerization initiator. For example, in radical polymerization, the higher the amount of polymerization initiator, the higher the radical concentration in the reaction system; consequently, the density of reaction initiation sites tends to be higher and the molecular weight tends to decrease. Conversely, the lower the amount of polymerization initiator, the lower the density of reaction initiation sites; consequently, the polymer chains tend to elongate more easily and the molecular weight tends to increase.

[0092] In order to obtain an adhesive sheet with excellent adhesion and a small pore distance, it is desirable for the acrylic base polymer to have a high gel fraction with a low density of crosslinking points. To increase the gel fraction (the ratio of polymer chains to which crosslinking structures have been introduced) with a low density of crosslinking, the molecular weight (length of the polymer chains) of the base polymer must be increased. As described above, in order to increase the molecular weight of the base polymer, it is desirable to reduce the amount of polymerization initiator used when polymerizing the base polymer.

[0093] The amount of polymerization initiator used during base polymer polymerization can be appropriately set according to the type of polymerization reaction, the composition of the monomer, the type of polymerization initiator, and the desired molecular weight. From the perspective of increasing the molecular weight of the base polymer and increasing the gel fraction with less 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, based on 100 parts by weight of the total monomer components constituting the base polymer.

[0094] When introducing a cross-linked structure using isocyanate-based cross-linking agents, it is preferable to polymerize the base polymer by solution polymerization, add the cross-linking agent, and, if necessary, heat the polymer to introduce the cross-linked structure into the base polymer. When introducing a cross-linked structure 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 beam polymerization, add the polyfunctional compound, and then introduce the cross-linked structure by the polyfunctional compound through active energy beam polymerization.

[0095] A prepolymer composition can be prepared by partially polymerizing (pre-polymerizing) a composition (referred to as "composition for forming a prepolymer") in which a monomer component constituting an acrylic base polymer is mixed with a polymerization initiator, for example. In addition, the monomer in the composition for forming a prepolymer is preferably a monofunctional monomer component, such as an alkyl meth)acrylate ester or a monomer containing a polar group. The composition for forming a prepolymer may contain polyfunctional monomers in addition to monofunctional monomers. For example, a portion of the polyfunctional monomer may be included in the composition for forming a prepolymer, and the remainder of the polyfunctional monomer component may be added after pre-polymerization to perform the main polymerization.

[0096] The polymerization rate of the prepolymer is not particularly limited, but from the perspective of achieving a viscosity suitable for coating onto a substrate, 3 to 50 weight% is preferred, and 5 to 40 weight% is more preferred. The polymerization rate of the prepolymer can be adjusted to a desired range by adjusting the type and amount of photopolymerization initiator, the irradiation intensity and irradiation time of active light such as UV light, etc.

[0097] Acrylic oligomers

[0098] The adhesive sheet (11) may include an oligomer in addition to an acrylic base polymer. As for the acrylic oligomer, one having a weight average molecular weight of about 1,000 to 30,000 is used. The acrylic oligomer contains (meth)acrylate alkyl ester as a main constituent monomer component.

[0099] The glass transition temperature of the acrylic oligomer is preferably 60°C or higher, more preferably 80°C or higher, more preferably 100°C or higher, and particularly preferably 110°C or higher. By using a low-Tg acrylic base polymer with a cross-linked structure in combination with a high-Tg acrylic oligomer, the adhesive strength of the adhesive sheet, particularly the adhesion retention strength at high temperatures, tends to improve, allowing the pore distance to be reduced. The upper limit of the glass transition temperature of the acrylic oligomer is not particularly limited, but generally it is 200°C or lower, preferably 180°C or lower, and more preferably 160°C or lower. The glass transition temperature of the acrylic oligomer is calculated by the aforementioned Fox formula.

[0100] It is preferable that an acrylic oligomer having a glass transition temperature of 60°C or higher comprises, as constituent monomer components, an alkyl ester of (meth)acrylate having a chain-like alkyl group (chain-like alkyl (meth)acrylate) and an alkyl ester of (meth)acrylate having a dicyclic alkyl group (dicyclic alkyl (meth)acrylate). Specific examples of the chain-like alkyl (meth)acrylate and the dicyclic alkyl (meth)acrylate are as previously exemplified as constituent monomers of the acrylic polymer chain.

[0101] Among the alkyl (meth)acrylate esters exemplified, methyl methacrylate is preferred as a chain alkyl (meth)acrylate because it has a high glass transition temperature and excellent compatibility with the base polymer. As a dicyclic alkyl (meth)acrylate, dicyclofentanyl acrylate, dicyclofentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate are preferred. That is, the acrylic oligomer preferably comprises, as constituent monomer components, one or more selected from the group consisting of dicyclofentanyl acrylate, dicyclofentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate, and methyl methacrylate.

[0102] The amount of alicyclic alkyl (meth)acrylate relative to the total amount of monomer components constituting the acrylic oligomer is preferably 10 to 90 weight%, more preferably 20 to 80 weight%, and even more preferably 30 to 70 weight%. The amount of chain-like alkyl (meth)acrylate relative to the total amount of monomer components constituting the acrylic oligomer is preferably 10 to 90 weight%, more preferably 20 to 80 weight%, and even more preferably 30 to 70 weight%.

[0103] 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 or adhesion retention strength of the adhesive tends to improve, allowing the pore distance to be reduced.

[0104] Acrylic oligomers are obtained by polymerizing the above monomer components by various polymerization methods. When polymerizing acrylic oligomers, various polymerization initiators may be used. In addition, chain transfer agents may be used for the purpose of adjusting the molecular weight.

[0105] The content of the acrylic oligomer in the adhesive sheet (11) is not particularly limited, but in order to sufficiently increase the adhesive strength, the amount of acrylic oligomer per 100 parts by weight of 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 acrylic oligomer in the adhesive sheet (11) 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, or 2.3 parts by weight or more, or 2.5 parts by weight or more, relative to 100 parts by weight of base polymer. As the amount of high-Tg acrylic oligomer added increases, the pore distance tends to decrease.

[0106] Meanwhile, if the amount of acrylic oligomer added is excessively large, the haze of the adhesive sheet increases due to reduced compatibility, and the transparency tends to decrease. Since high transparency is required for the adhesive sheet placed on the viewing side rather than the image display panel, the amount of acrylic oligomer in the adhesive sheet (11) is preferably 5 parts by weight or less per 100 parts by weight of the base polymer, and may be 4 parts by weight or less or 3 parts by weight or less.

[0107] <Adhesive Composition>

[0108] 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, and other additives, etc., into an acrylic base polymer (or prepolymer composition) as needed. If necessary, the remainder of the monomer components constituting the acrylic base polymer may be added to the adhesive composition. Thickening additives, etc., may be used for the purpose of adjusting viscosity, etc.

[0109] When the adhesive composition comprises a prepolymer composition and a polyfunctional compound, it is preferable that the adhesive composition includes a photopolymerization initiator for the main polymerization. After the prepolymerization, a polymerization initiator for the main polymerization may be added to the prepolymer composition. If the polymerization initiator from the prepolymerization remains in the prepolymer composition without being deactivated, the addition of the polymerization initiator for the main polymerization may be omitted. The adhesive composition may also include a chain transfer agent.

[0110] The adhesive composition preferably has an acrylic base polymer (or prepolymer composition) content of 50% by weight or more relative to the total amount of non-volatile components, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more.

[0111] The amount of the crosslinking agent and / or polyfunctional compound in the adhesive composition can be adjusted so that the gel fraction is within the above range. As described above, in order to reduce the pore distance, it is desirable to increase the molecular weight of the acrylic base polymer and increase the gel fraction with a low crosslinking point density. For example, when introducing a crosslinked structure by 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 introducing a cross-linked structure by a polyfunctional (meth)acrylate, the amount of 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 an acrylic base polymer (prepolymer).

[0112] (Silanic coupling agent)

[0113] A silane coupling agent may be added to the adhesive composition. When a silane coupling agent is added to the adhesive composition, the amount added is typically 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 the silane coupling agent is within the above range, the pore distance may be reduced.

[0114] (Other additives)

[0115] In addition to each component exemplified above, the adhesive composition may include additives such as tackifiers, plasticizers, softeners, deterioration inhibitors, fillers, colorants, UV absorbers, antioxidants, surfactants, and antistatic agents.

[0116] <Formation of Adhesive Sheet>

[0117] An adhesive sheet is formed on a substrate by applying an adhesive composition onto the substrate and, if necessary, performing the polymerization by drying and removing the solvent and / or irradiating with active light. Any suitable substrate is used as the substrate for forming the adhesive sheet. As the substrate, it may be a release film having a release layer on the contact surface with the adhesive sheet.

[0118] As the film substrate of the release film, a film made of various resin materials is used. 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, polyphenylene sulfide resins, etc. Among these, polyester resins such as polyethylene terephthalate are particularly preferred. The thickness of the film substrate is preferably 10 to 200 μm, and 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, fatty acid amide-based release agents, etc. The thickness of the release layer is generally about 10 to 2000 nm.

[0119] Various methods such as roll coat, kiss roll coat, gravure coat, reverse coat, roll brush, spray coat, dip roll coat, bar coat, knife coat, air knife coat, curtain coat, rip coat, and die coater are used as methods for applying an adhesive composition onto a substrate.

[0120] When the base polymer of the adhesive composition is a solution polymer, it is preferable to dry the solvent after application. As a drying method, an appropriate and suitable method may be employed depending on the purpose. The heating and drying temperature is preferably 40°C to 200°C, more preferably 50°C to 180°C, and particularly preferably 70°C to 170°C. An appropriate and suitable drying time may be employed. 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.

[0121] If the adhesive composition contains a crosslinking agent, a crosslinking reaction may be performed after the adhesive composition is applied onto the substrate. During crosslinking, heating may be performed as necessary. The temperature of the crosslinking reaction is typically in the range of 20°C to 160°C, and the time of the crosslinking reaction is approximately 1 minute to 7 days. After applying the adhesive composition, heating to dry the solvent may also serve as heating for crosslinking. After drying the solvent, it is preferable to install a cover sheet to protect the surface of the adhesive sheet. As the cover sheet, it is preferable to use a release film having a release layer on the contact surface with the adhesive sheet, similar to the substrate film.

[0122] When the adhesive composition is a photopolymerizable composition including a prepolymer composition and a polyfunctional compound, etc., photocuring is performed by applying the adhesive composition in layers on a substrate and then irradiating it with an active light. When performing photocuring, it is preferable to place a cover sheet on the surface of the coating layer and irradiate the active light while the adhesive composition is sandwiched between two sheets to prevent inhibition of polymerization by oxygen.

[0123] The active light can be selected according to the type of polymerizable component, such as monomers or polyfunctional (meth)acrylates, or the type of photopolymerization initiator, and generally, ultraviolet light and / or short-wavelength visible light is used. The integrated light intensity of the irradiation light is preferably about 100 to 5000 mJ / cm². As for the light source for light irradiation, it is not particularly limited as long as it can irradiate light within a wavelength range to which the photopolymerization initiator included in the adhesive composition is sensitive, and LED light sources, high-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halide lamps, xenon lamps, etc. are preferably used.

[0124] [Second Adhesive Sheet]

[0125] The second adhesive sheet (12) disposed on the image display panel side of the polarizing plate (31) has the aforementioned dielectric constant, and its composition is not particularly limited. The second adhesive sheet (12) does not require impact resistance to the same degree as the first adhesive sheet (11) disposed on the viewing side. From the perspective of thinning, it is preferable that the thickness of the second adhesive sheet (12) be smaller than the thickness of the first adhesive sheet (11). The thickness of the second adhesive sheet (12) is preferably 10 to 25 μm, and may be 20 μm or less.

[0126] If the dielectric constant of the second adhesive sheet (12) is within the above range, the composition of the adhesive is not particularly limited, and as a base polymer, it may include an acrylic, silicone, polyester, polyurethane, polyamide, polyvinyl ether, vinyl acetate / vinyl chloride copolymer, modified polyolefin, epoxy, fluorine, rubber, etc.

[0127] In addition to dielectric constant and adhesive strength, transparency and adhesive strength can be controlled, so it is preferable that the second adhesive sheet (12) be composed of an acrylic adhesive containing an acrylic base polymer. As mentioned above, in order to reduce the dielectric constant of the adhesive sheet, it is effective to reduce the amount of a polar group-containing monomer in the monomer component constituting the base polymer.

[0128] When the second adhesive sheet (12) is an acrylic adhesive sheet, the amount of a polar group-containing monomer per 100 parts by weight of the total monomer components of the base polymer is preferably 15 parts by weight or less, more preferably 10 parts by weight or less, and may be 5 parts by weight or less or 3 parts by weight or less. The amount of a hydroxyl group-containing monomer per 100 parts by weight of the total monomer components of the base polymer is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, and may be 3 parts by weight or less.

[0129] The second adhesive sheet (12) is not required to have the same degree of deformation relaxation as the first adhesive sheet (11). Therefore, the storage modulus G' of the second adhesive sheet (12) at 25°C 25 is G' of the first adhesive sheet 25 It may be larger than G' of the second adhesive sheet (12). 25 It may be 70 kPa or more, 75 kPa or more, or 80 kPa or more. G' of the second adhesive sheet (12) 25 By making it larger, the adhesive retention force can be improved.

[0130] It is preferable that the second adhesive sheet (12), like the first adhesive sheet (11), has a gap distance of 2 mm or less. By having a gap distance of 2 mm or less and also having the above dielectric constant, reliability can be improved even for a foldable image display device having a touch panel within a distance of 500 μm from the touch surface. Therefore, even if the distance from the device surface to the touch panel sensor is short, there is almost no malfunction of the touch panel sensor, and failures are suppressed or prevented, resulting in high reliability.

[0131] [Image display device]

[0132] As described above, the first adhesive sheet (11) and the second adhesive sheet (12) are used for bonding a polarizing plate (31) and other optical members in a foldable image display device having a touch panel.

[0133] In the image display device (101) illustrated in FIG. 1, a touch panel (41), a circular polarizer (31), and a cover window (71) are arranged on the visible 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 can be folded. In the image display device (102) illustrated in FIG. 2, a touch panel is integrated with an organic EL panel (54) serving as an image display panel, and a circular polarizer (31) and a cover window (71) are arranged on the surface thereof.

[0134] <Image Display Panel>

[0135] An organic EL panel comprises a pair of electrodes and an organic light-emitting layer sandwiched and supported between the electrodes on a substrate. 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 stacked sequentially on a substrate, or a bottom emission type in which a transparent electrode, an organic light-emitting layer, and a metal electrode are stacked sequentially on a transparent substrate. In either the bottom emission type or the top emission type, the substrate and sealing member provided on the visible side of the organic light-emitting layer are transparent. The substrate and sealing member provided on the back side (the housing (75) side in FIG. 1 and FIG. 2) of the organic light-emitting layer do not need to be transparent. In a bottom emission type flexible organic EL panel, 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 polyetheretherketone or transparent polyimide. A back sheet may be provided on the back side of the substrate for the purpose of protecting or reinforcing the substrate.

[0136] The image display panel is not limited to organic EL panels and may be a liquid crystal panel or an electrophoretic display panel (electronic paper), etc. For example, a foldable liquid crystal panel can be formed by using a flexible substrate, such as a resin substrate, as a transparent substrate that supports the liquid crystal layer.

[0137] <Cover Window>

[0138] A cover window (71) is provided on the outermost surface of 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, polyetheretherketone, or polyethylene terephthalate is used as the cover window (71). As the material for the cover window (71), a flexible glass plate (glass film) may be used, and the cover window (71) may be composed of a laminate of a glass film and a resin film. From the perspective 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. In terms of excellent recovery after maintaining a bent state for a long time, the yield point elongation of the cover window is preferably 5% or more. A thin glass substrate that can be folded may be used as the cover window (71). The cover window may be a laminate of two or more layers of transparent material. The visible surface of the cover window may be provided with an anti-reflective layer or a hard coat layer.

[0139] <Touch Panel>

[0140] The image display device is equipped with a capacitive touch panel on the visible side surface of the image display panel. The capacitive touch panel detects a touch position based on a change in the amount of electricity when an operator's finger or a stylus comes into contact with the touch surface. In the configuration of FIG. 1, a touch panel (41) is placed between a circular polarizer (31) and an organic EL panel (51). In the configuration of FIG. 2, a touch panel is provided inside the image display panel (54). A touch panel may also be placed between a circular polarizer (31) and a cover window (71).

[0141] Polarizer

[0142] A polarizing plate (31) is positioned on the viewing side of an image display panel. For example, in a liquid crystal display device, a polarizing plate provided on the viewing side of the liquid crystal panel adjusts the transmittance according to the polarization state of the light that has passed 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), the emission of external light reflected from the metal electrode of the organic EL panel to the viewing side can be blocked, thereby improving the visibility of the display.

[0143] As a polarizing plate, a suitable transparent protective film is generally used, bonded to one or both sides of the polarizer 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-based films, partially formalized polyvinyl alcohol-based films, and partially saponified ethylene-vinyl acetate copolymer-based films, which are uniaxially stretched by adsorbing dichromatic substances such as iodine or dichromatic dyes, and polyene-based oriented films such as dehydrated polyvinyl alcohol or dehydrochlorinated polyvinyl chloride.

[0144] As a polarizer, a thin polarizer with a thickness of 10 μm or less may be used. Examples of thin polarizers include the polarizers described in Japanese Patent Publication No. 51-069644, Japanese Patent Publication No. 2000-338329, WO2010 / 100917, Japanese Patent No. 4691205, and Japanese Patent No. 4751481. A thin polarizer is obtained, for example, by a manufacturing method comprising a process of stretching a polyvinyl alcohol-based resin layer and a stretching resin substrate in a laminated state, and a process of dyeing with a dichromatic material such as iodine.

[0145] As a transparent protective film for a polarizer, it is preferable to use a cellulose-based resin, a cyclic polyolefin-based resin, an acrylic-based resin, a phenyl maleimide-based resin, a polycarbonate-based resin, etc., which has excellent transparency, mechanical strength, thermal stability, moisture barrier properties, and optical isotropy. In addition, when a transparent protective film is provided on both sides of a polarizer, a protective film made of the same polymer material on both the front and back sides may be used, or a protective film made of a different polymer material may be used.

[0146] An optical film may be laminated on one or both sides of a polarizer through an appropriate adhesive layer or pressure-sensitive adhesive layer as needed. As such a film, one used in the formation of an image display device, such as a phase difference plate, a viewing angle enlargement film, a viewing angle limiting (anti-peeping) film, or a brightness enhancement film, is 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 an image display panel (liquid crystal panel) and a polarizer for purposes such as appropriately converting the polarization state of light emitted from a liquid crystal cell toward the viewing side to improve viewing angle characteristics.

[0147] As described above, in an organic EL display device, by providing a circular polarizer with a quarter-wave plate placed on the organic EL panel side of the polarizer, the emission of external light reflected from the metal electrode toward the viewer side can be blocked. By placing a quarter-wave plate on the viewer side of the polarizer to make the emitted light circularly polarized, a suitable image display can be seen even by a viewer wearing polarized sunglasses. Such an optical film (optical anisotropic film) may be laminated onto the polarizer without passing through another film. In this case, the optical film serves as a protective film for the polarizer.

[0148] The thickness of the polarizing plate is generally about 10 to 200 μm. From the perspective of providing flexibility, the thickness of the 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 an optical film such as a quarter-wave plate is laminated onto the polarizing plate, it is preferable that the total thickness including these films is within the above range.

[0149] Lamination of components by adhesive sheet

[0150] An adhesive sheet is used for bonding between the above flexible members. In the image display device shown in FIG. 1, the bottom surface of the organic EL panel (51) and the housing (75) is bonded through an adhesive sheet (14), the organic EL panel (51) and the touch panel (41) are bonded through an adhesive sheet (13), the touch panel (41) and the circular polarizer (31) are bonded through a second adhesive sheet (12), and the circular polarizer (31) and the cover window (71) are bonded through a first adhesive sheet (11). In the image display device shown in FIG. 2, the touch panel integrated organic EL panel (54) and the circular polarizer (31) are bonded through a second adhesive sheet (12), and the circular polarizer (31) and the cover window (71) are bonded through a first adhesive sheet (11).

[0151] Since both the second adhesive sheet (12) and the first adhesive sheet (11), which are positioned on the visible side rather than the touch panel (41), have low dielectric constant, even if the distance D from the touch surface to the touch panel is small, the malfunction of the touch panel can be reduced.

[0152] In the image display device of the present invention, when a bending test is performed for 240 hours in a high temperature and high humidity environment of 60 degrees and 95% relative humidity in a bent state with a bending radius of 1.3 mm and a bending angle of 180°, it is desirable that the delamination between members in the bent portion is small, and in particular, it is desirable that the delamination at the interface between the polarizing plate (31) and the cover window (71) is small. The delamination at the interface during the bending test can be quantified as the length of the gap portion (gap distance) in the direction of the short side along the bending axis when a sample of size 35 mm × 100 mm is subjected to a bending test with the short side direction as the bending axis. The gap distance of the image display device after the bending test is preferably 2 mm or less, 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.

[0153] As described above, the gap distance of the image display device can be reduced by adjusting the composition of the first adhesive sheet (11), etc. The gap distance of the adhesive sheet can also be evaluated by the same method as above. Specifically, the length of the gap portion (gap distance) between the adhesive sheet and the adherend can be measured by following the steps A to D below.

[0154] Step A: Prepare a 35 mm × 100 mm test specimen by bonding the adhesive sheet to the substrate.

[0155] Step B: The test specimen prepared in Step A is bent along the shorter side direction with a bending radius of 1.3 mm and a bending angle of 180°.

[0156] Step C: Maintain the test specimen bent in Step B in the bent state for 240 hours in an environment of 60°C and 95% relative humidity.

[0157] Step D: In the bending portion of the test specimen after maintaining for 240 hours in Step C, the length in the direction of the shorter side of the gap between the adhesive sheet and the adherend is measured.

[0158] Delamination (voids) in a bending test is likely to occur at the end of the bending axis of the test specimen (the end in the direction of the shorter side). If voids occur from both ends or if voids exist in multiple locations, the length of the void with the longest length in the direction of the shorter side is defined as the void distance. If voids exist in multiple locations, the longest void must be 2 mm or less. Preferably, the sum of the lengths of each void is 2 mm or less, and the sum of the lengths 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 0.

[0159] In forming an image display device, the bonding order of each component is not particularly limited, and a touch panel (41), a circular polarizer (31), and a cover window (71) may be laminated in order on an image display panel (51), or a laminate formed by laminating two or more components in advance through an adhesive sheet may be bonded on the image display panel (51).

[0160] Examples

[0161] 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.

[0162] [Example 1]

[0163] <First Adhesive Sheet>

[0164] (Preparation of acrylic oligomer)

[0165] 60 parts by weight of dicyclofentanyl 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) were added as a thermal polymerization initiator, and the mixture was reacted at 70°C for 2 hours, then the temperature was raised to 80°C and the mixture was reacted for 2 hours. Afterward, the reaction mixture was heated to 130°C to dry and remove toluene, the chain transfer agent, and unreacted monomers, thereby obtaining a solid acrylic oligomer. The weight average molecular weight of the acrylic oligomer was 5100, and the glass transition temperature (Tg) was 130°C.

[0166] (Polymerization of prepolymer)

[0167] As monomer components for forming a 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), and 0.015 parts by weight of "Omnirad 184" from IGM Resins as a photopolymerization initiator were mixed, and polymerization was carried out by irradiating with ultraviolet light to obtain a prepolymer composition (polymerization rate: about 10%).

[0168] (Preparation of adhesive composition)

[0169] To 100 parts by weight of the above prepolymer composition, 0.07 parts by weight of 1,6-hexanediol diacrylate (HDDA), 3 parts by weight of the above oligomer, and 0.3 parts by weight of a silane coupling agent (Shin-Etsu Kagaku Kogyo “KBM403”) were added as post-added components, and then these were uniformly mixed to prepare an adhesive composition.

[0170] (Production of adhesive sheets)

[0171] A polyethylene terephthalate (PET) film with a thickness of 75 μm ("Diafoil MRF75" manufactured by Mitsubishi Chemical), having a silicone-based release layer on its surface, was used as a substrate (and also a medium-release film). The above-described photocurable adhesive composition was applied to the substrate to a thickness of 50 μm to form a coating layer. On this coating layer, a PET film with a thickness of 75 μm ("Diafoil MRE75" manufactured by Mitsubishi Chemical), having one side treated with silicone release, was bonded as a cover sheet (and also a light-release film). Photocuring was performed on this laminate by irradiating it with ultraviolet light using a black light positioned so that the irradiation intensity on the irradiation surface directly below the lamp from the cover sheet side was 5 mW / cm², thereby obtaining an adhesive sheet with a thickness of 50 μm.

[0172] <Second Adhesive Sheet>

[0173] In a reaction vessel, 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 together with ethyl acetate, and the reaction was carried out at 60°C for 4 hours under a nitrogen gas stream. Afterward, ethyl acetate was added to the reaction mixture to obtain a solution of an acrylic polymer with a weight-average molecular weight of 1.65 million. To this solution, 0.3 parts by weight of dibenzoyl peroxide (Nihon Yushi "Niper BMT") as a crosslinking agent, 0.1 parts by weight of trimethylolpropanexylylene diisocyanate (Mitsui Kagaku "Takenate D110N"), and 3 parts by weight of a silane coupling agent (Shin-Etsu Kagaku Kogyo "KBM403") were added to 100 parts by weight of polymer to obtain adhesive composition A.

[0174] The above adhesive composition A was applied to the release treatment surface of a 38㎛ thick PET film (Mitsubishi Chemical “MRF38”) that has been released and has a silicone-based release layer provided on its surface, and dried and crosslinked at 150℃ to obtain an adhesive sheet with a thickness of 15㎛.

[0175] <Preparation of Samples for Flexibility Retention Tests>

[0176] A release film on one side of the first adhesive sheet was peeled off, and a polarizing plate with a thickness of 51 μm was bonded using a 2 kg roller. A release film on the other side of the adhesive sheet was peeled off, and a transparent polyimide film with a thickness of 80 μm was bonded using a 2 kg roller. Additionally, a PET film with a thickness of 125 μm was bonded onto the polarizing plate through the second adhesive sheet using a 2 kg roller. During bonding, plasma treatment was performed on the surfaces of the polarizing plate, the polyimide film, and the PET film prior to bonding with the adhesive sheet.

[0177] This laminate was cut into a 35 mm × 100 mm rectangle so that the absorption axis direction of the polarizer was parallel to the long side direction, and an evaluation sample was obtained by autoclaving at 35°C and 0.35 MPa for 15 minutes.

[0178] [Examples 2 to 5, Comparative Examples 1 to 3]

[0179] In the preparation of the first adhesive sheet, the input monomer composition, the amount of polyfunctional monomer (HDDA), and the amount of oligomer in the polymerization of the prepolymer were changed as shown in Table 1. A photocurable adhesive composition was prepared in the same manner as in Example 1, and the first adhesive sheet was obtained by applying it onto a substrate and performing photocuring. Using the obtained first adhesive sheet, an evaluation sample was prepared in which a polyimide film, the first adhesive sheet, a polarizing plate, a second adhesive sheet, and a PET film were laminated in sequence in the same manner as in Example 1.

[0180] [Evaluation of the first adhesive sheet]

[0181] Gel fraction

[0182] Approximately 0.2 g of adhesive was scraped from an adhesive sheet, wrapped in a porous polytetrafluoroethylene membrane (Nitto Denko “NTF-1122”) with a pore diameter of 0.2 μm and cut to a size of 100 mm × 100 mm, and the wrapped opening was tied with a string. The weight of the adhesive sample (B) was calculated by subtracting the sum of the weights of the porous polytetrafluoroethylene membrane and the string (A), 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 to elute the sol component of the adhesive to the outside of the porous polytetrafluoroethylene membrane. After immersion, the adhesive wrapped in the porous polytetrafluoroethylene membrane was removed, dried at 130°C for 2 hours, cooled for approximately 20 minutes, and then the dry weight (C) was measured. The gel fraction of the adhesive was calculated by the following formula.

[0183] Gel fraction (%) = 100 × (CA) / B

[0184] Storage Modulus, Loss Tangent, and Glass Transition Temperature

[0185] A laminated adhesive sheet with a thickness of approximately 1.5 mm was prepared as a sample for measurement. Dynamic viscoelasticity measurements were performed under the following conditions using the "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific. From the measurement results, the storage modulus G' and the loss tangent tangent tanδ at each temperature were read. Additionally, the temperature at which tanδ is maximized was defined as the glass transition temperature of the adhesive sheet.

[0186] (Measurement conditions)

[0187] Transformation Mode: Twist

[0188] Measurement frequency: 1 Hz

[0189] Heating rate: 5℃ / min

[0190] Shape: Parallel plate 7.9 mm φ

[0191] <Total Light Transmittance and Haze>

[0192] A test specimen was used in which an adhesive sheet was bonded to alkali-free glass (thickness 0.8 to 1.0 mm, total light transmittance 92%, haze 0.4%), and haze and total light transmittance were measured using a haze meter (Murakami Shikisai Kijutsu Genkyujo "HM-150"). The haze of the adhesive sheet was determined by subtracting the haze of the alkali-free glass (0.4%) from the measured value. The total light transmittance was adopted as the measured value. The total light transmittance of the first adhesive sheet in all examples and comparative examples was 92%. The haze of the first adhesive sheet in Example 5 was 0.7%, and the haze of the first adhesive sheets in the other examples and comparative examples was 0.3%.

[0193] Non-permittivity

[0194] An adhesive sheet was placed between a copper foil and an electrode, and using an Agilent Technologies "Precision Impedance Analyzer 4294A" in accordance with JIS K6911, the dielectric constant at frequencies of 1 kHz, 10 kHz, 100 kHz, and 1 MHz was measured under the following conditions. For the first adhesive sheet of Examples 1, 4, 5 and Comparative Example 3, in addition to the measurement at a temperature of 25°C, the dielectric constant was measured at every 20°C in the temperature range of -40°C to 80°C.

[0195] Electrode configuration: 12.1 mm Φ, 0.5 mm thick aluminum plate

[0196] Counter electrode: 3oz copper plate

[0197] Measurement environment: Temperature 25℃, Relative humidity 50%

[0198] Adhesion to Polyimide Film

[0199] A release film on one side of the adhesive sheet was peeled off, a PET film with a thickness of 25 μm was bonded, and a piece cut to a width of 10 mm × a length of 100 mm was prepared as a test specimen. A release film on the other side of the test specimen was peeled off, and the adhesive sheet was pressed onto a transparent polyimide film (manufactured by Kollong Industries) with a thickness of 80 μm using a 2 kg roller. Using a tensile testing machine, the test specimen was peeled from the polyimide film under conditions of a tensile speed of 60 mm / min and a peel angle of 180° in an environment of 25°C, and the peel strength was measured.

[0200] [Flexibility Retention Test]

[0201] Using a planar body no-load U-shaped elongation tester (manufactured by Yuasa System Kiki), a bending jig was installed and fixed within a range of 20 mm from each end of the long side of the evaluation sample prepared in the example and comparative example (the central 60 mm area in the long side direction was not fixed), and the sample was maintained in a bent state with a bending radius of 1.3 mm and a bending angle of 180° so that the PET film side face inward, and a bending holding test was performed by maintaining it for 240 hours in a constant temperature and humidity chamber at a temperature of 60°C and a relative humidity of 95%.

[0202] After the bending test, the samples were visually inspected to check for the presence or absence of delamination at the interface between the transparent polyimide film and the polarizer in the bending portion. In all cases where delamination was confirmed, delamination (voids) occurred from the ends along the short side of the sample. For cases where delamination was confirmed, the length (mm) of the void along the short side of the sample was measured. For cases where delamination was confirmed along the entire length of the short side of the sample, the void length (void distance) was set to 35 mm, and for cases where no delamination was confirmed at all, the void distance was set to 0. For cases where delamination occurred from both ends along the short side, the longer void length was set as the void distance. Furthermore, in none of the samples was delamination confirmed at the bonding interface between the PET film and the polarizer.

[0203] [Evaluation Results]

[0204] In the examples and comparative examples, the formulation of the adhesive composition used to produce the first adhesive sheet is shown in Table 1, and the evaluation results of the first adhesive sheet and the evaluation results of the flexural retention test are shown in Table 2. Table 2 also shows the measurement results of the dielectric constant of the second adhesive sheet used for bonding the polarizer and the PET film in the sample for the flexural retention test. Table 3 shows the measurement results of the dielectric constant of the first adhesive sheet of Examples 1, 4, 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 values ​​at each frequency and the value of X at a frequency of 1 kHz (X 1㎑ ) and the value of X at a frequency of 1 MHz (X 1㎒ Ratio X of ) 1㎑ / X 1㎒ It shows the value of.

[0205] In Table 1, each component is listed by the following abbreviations.

[0206] LA: Lauryl acrylic acid

[0207] 2HEA: 2-ethylhexyl acrylate

[0208] BA: Butyl acrylate

[0209] CHA: Cyclohexyl acrylate

[0210] 4HBA: 4-hydroxybutyl acrylate

[0211] 2HEA: 2-hydroxyethyl acrylate

[0212] NVP: N-vinyl-2-pyrrolidone

[0213]

[0214]

[0215]

[0216] In Examples 1 to 5, the dielectric constant of the first adhesive sheet and the second adhesive sheet at a frequency of 10 MHz was 4.5 or less, and the gap distance was 2 mm or less. From Table 3, it can be seen that the first adhesive sheet of the example has a low dielectric constant, and also has low temperature and frequency dependence of the dielectric constant. Explanation of the symbols

[0217] 11, 12, 13, 14: Adhesive sheets 31: Polarizer (circular polarizer) 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

Claim 1 A foldable image display device having a touch panel within a distance of 500 μm from a touch surface, wherein a polarizing plate and a cover window are provided in this order from the image display panel side on the viewing side of the image display panel, a first adhesive sheet is provided on the viewing side surface of the polarizing plate, and a second adhesive sheet is provided on the image display panel side surface of the polarizing plate, wherein both the first adhesive sheet and the second adhesive sheet have a relative permittivity of 4.5 or less at a temperature of 25°C and a frequency of 10 kHz, and the first adhesive sheet is composed of an acrylic adhesive including an acrylic base polymer, and the acrylic base polymer has a content of hydroxyl group-containing monomer of 10 parts by weight or less per 100 parts by weight of the total monomer components. Claim 2 An image display device according to claim 1, wherein the first adhesive sheet and the second adhesive sheet both have a ratio of the relative permittivity at 1 kHz to the relative permittivity at 1 MHz at a temperature of 25°C, which is 1.50 or less. Claim 3 An image display device according to claim 1 or 2, wherein the first adhesive sheet and the second adhesive sheet both have a maximum relative permittivity of 1.4 times or less than the minimum value in a temperature range of -40°C to 80°C at a frequency of 10 kHz. Claim 4 An image display device according to claim 1 or 2, wherein the first adhesive sheet and the second adhesive sheet, in a temperature range of -40°C to 80°C at a frequency of 1 kHz, have a ratio of the maximum and minimum values ​​of the relative permittivity that is 0.8 to 1.2 times the ratio of the maximum and minimum values ​​of the relative permittivity that is in a temperature range of -40°C to 80°C at a frequency of 1 MHz. Claim 5 An image display device according to claim 1 or 2, wherein the thickness of the cover window is 100㎛ or less. Claim 6 An image display device according to claim 1 or 2, wherein the thickness of the first adhesive sheet is greater than the thickness of the second adhesive sheet, and the thickness of the first adhesive sheet is 100 μm or less. Claim 7 In claim 1 or 2, the first adhesive sheet has a storage modulus G' at 25°C and 1 Hz. 25 An image display device having a kPa of 70 kPa or less and a glass transition temperature of -20°C or less. Claim 8 delete Claim 9 In claim 1 or 2, the acrylic base polymer comprises (meth)acrylic acid C with respect to 100 parts by weight of the total monomer components. 10-20 An image display device containing 5 to 55 parts by weight of a chain alkyl ester. Claim 10 In claim 9, the acrylic base polymer is the (meth)acrylic acid C 10-20 An image display device comprising lauryl acrylate as a chain alkyl ester. Claim 11 An image display device according to claim 1 or 2, wherein 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 hydroxyl group-containing monomers, carboxyl group-containing monomers, and nitrogen-containing monomers, based on a total of 100 parts by weight of monomer components. Claim 12 delete Claim 13 An image display device according to claim 1 or 2, wherein the acrylic base polymer has a cross-linked structure. Claim 14 An image display device according to claim 13, wherein the cross-linking structure is a cross-linking structure introduced by a polyfunctional (meth)acrylate. Claim 15 An image display device according to claim 1 or 2, wherein the acrylic adhesive further comprises an acrylic oligomer having a glass transition temperature of 60°C or higher. Claim 16 An image display device according to claim 15, wherein the content of the acrylic oligomer is 0.1 to 5 parts by weight per 100 parts by weight of the acrylic base polymer. Claim 17 An image display device according to claim 1 or 2, comprising a touch panel inside the image display panel. Claim 18 An image display device according to claim 1 or 2, comprising a touch panel between the image display panel and the polarizing plate.

Images