TW202219225A - Image display device - Google Patents

Abstract

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

Description

video display device

The present invention relates to an image display device which is provided with a touch panel and can be bent.

Flat panel displays such as liquid crystal display devices and organic EL (Electroluminescence) display devices are used in the industry as video display devices such as mobile phones, smart phones, tablet computer terminals, car navigation system devices, personal computer monitors, and televisions. In recent years, an organic EL panel using a bendable substrate (flexible substrate) such as a resin film has been put into practical use, and a bendable flexible display has been proposed.

In flexible displays, in addition to display panels such as organic EL panels, components such as casings and touch panels can be folded, and these components are bonded via an adhesive sheet (eg, Patent Document 1). In a flexible display (foldable display) that can be bent, the transparent plate (cover window) disposed on the visible side surface must also be able to be bent, and a thin material such as a resin film or thin glass is used.

Regarding the foldable display, the bending is repeated in the same place. With regard to the bent portion, compressive stress is applied to the inside and tensile stress is applied to the outside, and strain occurs in the bent portion and its periphery, so there is a risk of damage to the equipment. Therefore, it is proposed in the industry to soften the pressure-sensitive adhesive sheet for bonding the members to relieve stress and strain (for example, Patent Document 2). [Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 2016-2764 [Patent Document 2] Japanese Patent Laid-Open No. 2018-45213

[Problems to be Solved by Invention]

In recent years, the industry has demanded thinner foldable display devices. In particular, even if the distance from the device surface to the touch panel sensor is small, almost or completely no malfunction of the touch panel sensor, failure, etc. can be suppressed or prevented, and a foldable display device with high reliability is required. An object of the present invention is to provide an image display device which can be bent and has high reliability. [Technical means to solve problems]

One embodiment of the present invention is an image display device that includes a touch panel and can be bent. The image display device is provided with a polarizing plate and a cover window in sequence on the visible side of the image display panel, and a touch panel is provided within a distance of 500 μm from the touch surface. The touch panel is disposed between the image display panel and the polarizer, for example. It can also be arranged inside the image display panel.

A first adhesive sheet is arranged on the visible side surface of the polarizer, and a second adhesive sheet is arranged on the side surface of the image display panel of the polarizer. The relative dielectric constants of the first adhesive sheet and the second adhesive sheet at a temperature of 25° C. and 10 kHz are both below 4.5.

Preferably, the ratio of the relative permittivity of the first adhesive sheet and the second adhesive sheet at a temperature of 25° C. and 1 kHz to the relative permittivity at a frequency of 1 MHz is 1.50 or less. Preferably, the maximum value of the relative permittivity of the first adhesive sheet and the second adhesive sheet in the temperature range of -40°C to 80°C at a frequency of 10 kHz is 1.4 times or less than the minimum value. Preferably, the ratio of the maximum value to the minimum value of the relative permittivity of the first adhesive sheet and the second adhesive sheet at a frequency of 1 kHz and a temperature range of -40°C to 80°C is both at a frequency of 1 MHz. 、 The ratio of the maximum value to the minimum value of the relative dielectric constant in the temperature range of -40℃～80℃ is 0.8～1.2 times.

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

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 less than 100 μm.

The storage modulus G' ₂₅ of the first adhesive sheet at 25° C. and 1 Hz may be less than 70 kPa. The glass transition temperature of the adhesive sheet can be below -20°C.

The first adhesive sheet may include an acrylic adhesive containing an acrylic base polymer. The acrylic base polymer may contain 5 to 55 parts by weight of (meth)acrylic acid C _10-20 chain alkyl ester with respect to 100 parts by weight of the total of the monomer components. The acrylic base polymer may also contain lauryl acrylate as the C _10-20 chain alkyl (meth)acrylate.

The acrylic base polymer may contain at least one polar group-containing monomer 2 selected from the group consisting of hydroxyl-containing monomers, carboxyl-containing monomers, and nitrogen-containing monomers with respect to 100 parts by weight of the total monomer components ~ 15 parts by weight. In the acrylic base polymer, the content of the hydroxyl group-containing monomer may be 10 parts by weight or less with respect to 100 parts by weight of the total of the monomer components.

The acrylic base polymer may have a cross-linked structure. The crosslinked structure may be introduced using a polyfunctional (meth)acrylate.

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

The malfunction of the touch panel of the image display device of the present invention is reduced, and high reliability can be exerted.

1 and 2 are cross-sectional views of the structure of a flexible display according to an embodiment.

In the image display device 101 shown in FIG. 1 , the organic EL panel 51 , the touch panel 41 and the circular polarizer 31 are arranged between the casing 75 and the cover window 71 . The organic EL panel 51 and the bottom surface of the casing 75 are bonded via the adhesive sheet 14, the organic EL panel 51 and the touch panel 41 are bonded via the adhesive sheet 13, and the touch panel 41 and the circular polarizer 31 are bonded via the adhesive sheet The material 12 is pasted together, and the circular polarizer 31 and the cover window 71 are pasted through the adhesive sheet 11 . In this way, in a flexible display, lamination integration is performed by bonding a plurality of members through an adhesive sheet.

The cover window 71 is disposed on the visible side surface of the flexible display, and constitutes a touch surface. The touch panel 41 is an electrostatic capacitance type touch panel. Regarding the flexible display, since the cover window 71 that can be bent is used, the thickness of the cover window is small, and accordingly, the distance D from the touch surface (surface of the cover window 71 ) to the touch panel 41 is also reduced. .

The image display device 102 shown in FIG. 2 includes an organic EL panel 54 , which is formed by integrating a touch panel, and the circular polarizer 31 is attached to the organic EL panel 54 via the adhesive sheet 12 . The other structure is the same as that of 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.

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 usually more than 700 μm. On the contrary, in the flexible display, the distance from the touch surface to the touch panel 41 (When the touch panel is an in-cell type, it is the distance from the touch surface to the image display panel) D is 500 μm or less. The distance D from the touch surface to the touch panel may also be 400 μm or less, 350 μm or less, or 300 μm or less.

As shown in FIGS. 1 and 2 , the image display device includes a polarizing plate 31 and a cover window 71 on the viewing side of the image display panels 51 and 54 . The first adhesive sheet 11 is provided on the visible side of the polarizer 31 , and the second adhesive sheet 12 is provided on the polarizer 31 on the side of the image display panels 51 and 54 .

[Dielectric constant of adhesive sheet] The relative permittivity of the adhesive sheets 11 and 12 disposed on the upper and lower surfaces of the polarizer 31 at a temperature of 25° C. and a frequency of 10 kHz is 4.5 or less. Hereinafter, unless otherwise specified, the dielectric constant is a measured value at a temperature of 25°C. The relative permittivity of the adhesive sheet at a frequency of 10 kHz is 4.0 or less, and may also be 3.8 or less or 3.5 or less. By setting the relative permittivity at a frequency of 10 kHz to be 4.5 or less, the distance from the touch surface to the touch panel can be reduced, so that a design favorable for flexibility can be performed. The electrostatic capacitance value of the adhesive sheet with low dielectric constant is small, so it is possible to design a sensor with good sensitivity. Thereby, an input method with a small contact area, such as a stylus input, can be implemented.

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 dielectric constant of the adhesive sheet at a frequency of 100 kHz is preferably 4.0 or less, and may also be 3.8 or less or 3.5 or less. The relative dielectric constant of the adhesive sheet at a frequency of 1 MHz is preferably 3.5 or less, and may also be 3.3 or less or 3.2 or less.

The dielectric constant varies according to the polarizability of the material. By selecting the material of the adhesive such as urethane, acrylic, rubber, silica, etc., the dielectric constant of the adhesive sheet can be controlled. In addition, since the relative permittivity of air is 1, the permittivity of the adhesive sheet can be lowered by adding hollow beads or the like to the adhesive. For acrylic adhesives, monomers with longer alkyl chains have lower polarizability and can be reduced in dielectric. If a monomer with higher polarity is used, the polarizability will be higher, so the dielectric constant will be higher. As a method of reducing the polarizability, there is a method of generating cross-linking of molecules. When the molecular weight is increased or the degree of crosslinking is increased, molecular crosslinking is likely to occur, and the polarizability is lowered, so that the dielectric can be lowered. When the water content increases, the dielectric constant tends to increase, so the dielectric constant can be reduced by using a material that is difficult to retain moisture.

According to the Clausius-Mossotti formula, the smaller the polarizability of the electric dipole and the smaller the number of electric dipoles per unit volume, the smaller the relative permittivity. In order to decrease the relative permittivity of the adhesive sheet, it is only necessary to decrease the dipole moment of the base polymer constituting the adhesive and increase the molar volume. For example, as the volume of the side chain of the base polymer increases, the molar volume tends to increase. Also, by selecting a monomer with a lower polarity as the monomer component constituting the base polymer, the electron dipole of the molecule becomes smaller.

Preferably, the frequency dependence of the relative permittivity of the adhesive sheet at a temperature of 25°C is small. Specifically, the ratio of the relative permittivity of the adhesive sheet at a frequency of 1 kHz to the relative permittivity of the adhesive sheet at a frequency of 1 MHz (1 kHz/1 MHz) is preferably 1.5 or less. The relative dielectric constant is small in a wide frequency range, and the frequency dependence of the relative dielectric constant is small, thereby ensuring the operation reliability for various operating frequencies. As described above, by adjusting the dielectric constant of the adhesive, the frequency dependence of the relative dielectric constant can be reduced.

Preferably, the temperature dependence of the relative permittivity of the adhesive sheet is small. Specifically, it is preferable that the ratio of the minimum value and the maximum value of the relative permittivity in the temperature range of -40°C to -80°C (maximum value/minimum value) be close to 1. The ratio X _{10 kHz} of the maximum value to the minimum value of the relative dielectric constant at a frequency of 10 kHz is preferably 1.4 or less, more preferably 1.3 or less, still more preferably 1.2 or less, and may be 1.1 or less. The ratio of the maximum value to the minimum value of the relative permittivity at a frequency of 1 kHz X _{1 kHz} , the ratio of the maximum value to the minimum value of the relative permittivity at a frequency of 100 kHz X _{100 kHz} , and the relative dielectric constant at a frequency of 1 MHz The ratio X _{1 MHz} between the maximum value and the minimum value of the electrical constant is also preferably 1.4 or less, more preferably 1.3 or less, further preferably 1.2 or less, and may be 1.1 or less.

The ratio of the maximum value to the minimum value of the relative permittivity within the temperature range of -40°C to -80°C, X is an index of the temperature dependence of the relative permittivity, meaning that the closer X is to 1, the higher the temperature of the relative permittivity. less dependent. As mentioned above, it is preferable that the temperature dependence of the relative permittivity is smaller in the entire frequency range of 1 kHz to 1 MHz, in addition to the smaller temperature dependence of the relative permittivity at a frequency of 10 kHz. When the temperature dependence of the relative permittivity is small, the frequency dependence tends to decrease. In addition, as the frequency increases, the temperature at which the relative permittivity becomes the maximum tends to shift to the high temperature side.

The ratio of X _{1 MHz} to X _{1 kHz} , X _{1 kHz} /X _{1 MHz} , is preferably 0.8 to 1.2, more preferably 0.85 to 1.15, further preferably 0.9 to 1.1, and may also be 0.95 to 1.05. The closer X _{1 kHz} /X _{1 MHz} is to 1, the smaller the change in relative permittivity in a wider temperature range and frequency range, so the operation reliability can be ensured for a wider temperature range and operating frequency.

[First adhesive sheet] Preferably, the length of the void portion of the first adhesive sheet 11 disposed on the visible side of the polarizing plate 31 by the following bending retention test (that is, the adhesive sheet and the adhered sheet after bending after holding for 240 hours) The length of the void portion of the body: hereinafter sometimes abbreviated as "void distance") is 2 mm or less. If the gap distance is 2 mm or less, even if it is an image display device with a touch panel within a distance of 500 μm from the touch surface and can be bent, and even if the image display device is bent, the adhesive sheet 11 can absorb the The stress caused by bending can improve the reliability of the image display device. Therefore, even if the distance from the device surface to the touch panel sensor is short, there is almost or completely no malfunction of the touch panel sensor, failures and the like can be suppressed or prevented, and the reliability is high.

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

In the bending retention test, peeling (voids) easily occurred from the end of the bending axis of the test piece (the end in the short-side direction). When a gap is generated from both ends, or when a gap exists in a plurality of places, the length of the gap with the longest length in the short-side direction is defined as the gap distance. When there are gaps in a plurality of parts, the longest gap should be 2 mm or less. The total length of each gap is preferably 2 mm or less, and the total length of the gaps 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.

When bending, the adherend expands and contracts, and the adhesive cannot follow the deformation of the adherend, so it is peeled off from the adherend. For example, by controlling the storage modulus G' of the adhesive, the followability of the adhesive to the adherend can be adjusted. The smaller the storage modulus of the adhesive, the better the followability to the deformation of the adherend. When the bent state is maintained, the stress due to the deformation of the bent portion is concentrated, so that peeling may occur. In order to improve the stress relaxation properties of the adhesive, it is only necessary to increase the loss tangent tanδ. In addition, in order to suppress peeling at the interface between the adherend and the adhesive, the adhesive force at the bending temperature must also be designed to be high. Furthermore, in a high-humidity environment, moisture retention at the interface between the adhesive and the adherend may lead to a decrease in the adhesive force. Therefore, it is preferable to use a material that is difficult to retain moisture to suppress the retention of moisture.

The adhesion force 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 also be 3.0 N/10 mm or more. Next, the force was obtained by a peel test with a tensile speed of 60 mm/min and a peel angle of 180° using a polyimide film as an adherend. Unless otherwise specified, the adhesive force is the measured value at 25°C. When the adhesive force of the adhesive sheet 11 is in the above-mentioned range, the polarizing plate 31 or the cover window 71 can be prevented from peeling off the adherend when it is repeatedly bent.

The storage modulus G' ₂₅ of the adhesive sheet 11 at 25° C. is preferably 70 kPa or less. When G' ₂₅ is 70 kPa or less, the strain when bending the device tends to be relaxed, and the damage of the device constituting members during repeated bending can be suppressed. G' ₂₅ is preferably 5 kPa or more, and within such a range, the adhesion holding force and strain relaxation properties of the adhesive sheet can be simultaneously achieved, so that the gap distance can be reduced. From the viewpoint of more effectively realizing workability, adhesion, and strain relaxation at the same time, G' ₂₅ of the adhesive sheet 11 is preferably 10 to 60 kPa, more preferably 13 to 50 kPa, and still more preferably 15 to 15 kPa. 40 kPa.

The storage modulus G' ₁₀₀ of the adhesive sheet 11 at 100°C is preferably 2-50 kPa, more preferably 3-40 kPa, and still more preferably 5-25 kPa. If the G' ₁₀₀ of the adhesive sheet is in the above range, the adhesion holding force and the stress relaxation property can be simultaneously achieved in a high temperature environment, so the gap distance can be reduced.

The loss tangent tanδ ₂₅ of the adhesive sheet 11 at 25° C. is preferably 0.2˜0.45. In addition, the loss tangent tanδ ₁₀₀ of the adhesive sheet 11 at 100° C. is preferably 0.2 to 0.4. In addition, the difference between tanδ ₂₅ and tanδ ₁₀₀ is preferably -0.07 to 0.07. tanδ ₂₅ may be 0.25 to 0.42. tanδ ₁₀₀ may be 0.25 to 0.38. Also, the difference between tanδ ₂₅ and tanδ ₁₀₀ may be within ±0.06 or within ±0.05. When tanδ ₂₅ , tanδ ₁₀₀ , and the difference between tanδ ₂₅ and tanδ ₁₀₀ of the adhesive sheet are within the above-mentioned ranges, the gap 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 may be selected so that the glass transition temperature or molecular weight of the base polymer falls within an appropriate range.

The storage modulus G' and the loss tangent tanδ of the adhesive sheet 11 were obtained by viscoelasticity measurement at a frequency of 1 Hz. tanδ is the ratio G''/G' of the storage modulus G' to the loss elastic modulus G''. The storage modulus G' is equivalent to the part of elastic energy stored as elastic energy when the material is deformed, and it is an index indicating the degree of hardness.

In order to reduce the temperature dependence of tanδ in the normal temperature to high temperature range, the glass transition temperature of the adhesive sheet 11 is preferably -20°C or lower, more preferably -23°C or lower, and still more preferably -25°C or lower. The glass transition temperature is the temperature at which tanδ becomes the maximum (peak top temperature). In the vicinity of the glass transition temperature, the temperature dependence of tanδ is large. Since the glass transition temperature is sufficiently lower than the operating ambient temperature of the equipment, the temperature dependence of tanδ within the operating ambient temperature range is reduced. Moreover, since the glass transition temperature is in the above-mentioned range, the adhesive sheet 11 also has an adhesive retention force in a low temperature region, so that peeling from the adherend at a low temperature can be suppressed, and the gap distance can be reduced.

The lower limit of the glass transition temperature of the adhesive sheet 11 is not particularly limited, but is usually -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-mentioned range, the adhesion holding force can be effectively improved and the gap distance can be reduced.

The thickness of the adhesive sheet 11 is not particularly limited, and may be appropriately adjusted according to the thickness of the target equipment, the properties required for the adhesive sheet, and the like. From the viewpoint of improving the adhesive force of the adhesive sheet 11, the thickness is preferably 10 μm or more. 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 viewpoint of having cushioning properties against shocks from the outer surface. The thickness of the adhesive sheet 11 is preferably 100 μm or less, more preferably 75 μm or less, from the viewpoints of reducing the thickness of the device and suppressing the overflow of the adhesive from the end face during processing of the adhesive sheet and bending of the device.

The total light transmittance of the adhesive sheet 11 is preferably 85% or more, more preferably 90% or more, and still more preferably 91% or more. The haze of the adhesive sheet 11 is preferably 1.5% or less, more preferably 1% or less, still more preferably 0.7% or less, and particularly preferably 0.5% or less. It is preferable that in addition to the adhesive sheet 11, the adhesive sheets 12, 13, etc. are arranged on the viewing side of the image display panels 51, 54, and the transparency of the adhesive sheet is higher, and the total light transmittance and haze are preferably higher. degree is within the above range.

As long as the relative permittivity of the adhesive sheet 11 is within the above range, the composition of the adhesive is not particularly limited. , polyamide, polyvinyl ether, vinyl acetate/vinyl chloride copolymer, modified polyolefin, epoxy, fluorine, rubber and other polymers. As the adhesive, an acrylic adhesive containing an acrylic base polymer as a main component is preferable because it can control not only the relative permittivity, the adhesive force, but also the transparency and the adhesive force. The adhesive may be used alone or in combination of two or more. In addition, the adhesive sheet formed from the adhesive may be either a single-layer form or a laminated form.

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

As the alkyl (meth)acrylate, C _1-20 alkyl (meth)acrylate having 1 to 20 carbon atoms in the alkyl group can be suitably used. The alkyl group in the alkyl (meth)acrylate may be in either a chain or a cyclic form. In addition, the alkyl group in the chain form (chain alkyl group) may be a straight chain alkyl group or may have a branch.

Specific examples of (meth)acrylic acid chain alkyl esters include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and isobutyl (meth)acrylate. ester, 2-butyl (meth)acrylate, 3-butyl (meth)acrylate, amyl (meth)acrylate, isoamyl (meth)acrylate, neopentyl (meth)acrylate, (meth)acrylate (methyl)hexyl acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate , and C _1-9 chain alkyl esters such as isononyl (meth)acrylate; and decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, Dodecyl (meth)acrylate, isotridecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, (meth)acrylate Pentadecyl acrylate, cetyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isostearyl (meth)acrylate, And C _10-20 chain alkyl esters such as nonadecyl (meth)acrylate.

Specific examples of (meth)acrylic acid alkyl esters having an alicyclic alkyl group (cyclic alkyl group) include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, (meth)acrylate (meth)acrylic acid cycloalkyl esters such as cycloheptyl acrylate, cyclooctyl (meth)acrylate, etc.; (meth)acrylates having bicyclic aliphatic hydrocarbon rings, such as (meth)acrylic acid isophthalate ; Dicyclopentyl (meth)acrylate, Dicyclopentyloxyethyl (meth)acrylate, Tricyclopentyl (meth)acrylate, 1-adamantyl (meth)acrylate, (meth)acrylate (Meth)acrylates having aliphatic hydrocarbon rings of three or more rings, such as 2-methyl-2-adamantyl acrylate and 2-ethyl-2-adamantyl (meth)acrylate.

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

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

Considering that the temperature range in the plateau region is wider and the storage modulus in the plateau region is smaller, among the C _10-20 chain alkyl esters of (meth)acrylate, C _10- (meth)acrylate is preferred. ₁₆ alkyl ester, more preferably C _10-13 alkyl (meth)acrylate. Among them, C ₁₂ alkyl (meth)acrylate is preferred, and dodecyl acrylate (lauryl acrylate) is particularly preferred.

The polymer of long-chain alkyl (meth)acrylate has the characteristics that the temperature range of the plateau region is wider and the storage modulus in the plateau region is smaller, but the crystallinity is higher and the glass transition temperature is higher. 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, as a monomer component, it is preferable to contain a (meth)acrylic acid C _1-9 chain alkyl group in addition to the (meth)acrylic acid C _10-20 chain alkyl ester ester.

In order to reduce the Tg of the base polymer, among the C _1-9 chain alkyl (meth)acrylates, the glass transition temperature of the homopolymer is preferably -40°C or lower. As a specific example of the C _1-9 chain alkyl ester of (meth)acrylate whose glass transition temperature of the homopolymer is -40°C or lower, 2-ethylhexyl acrylate (Tg: -70°C) may, for example, be mentioned. , n-hexyl acrylate (Tg: -65°C), n-octyl acrylate (Tg: -65°C), isononyl acrylate (Tg: -60°C), n-nonyl acrylate (Tg: -58°C), isononyl acrylate (Tg: -58°C) Octyl ester (Tg: -58°C), butyl acrylate (Tg: -55°C), etc. Among these, butyl acrylate and 2-ethylhexyl acrylate are preferred, and 2-ethylhexyl acrylate is particularly preferred from the viewpoint of low Tg.

In order to obtain an adhesive having the above-mentioned properties, it is preferable to contain both (meth)acrylate C _10-20 chain alkyl ester and (meth)acrylate C _1-9 chain alkyl ester as the acrylic base polymer monomer composition, and adjust the ratio of the two.

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

The acrylic base polymer may contain a nitrogen-containing monomer as a monomer component. Examples of nitrogen-containing monomers include N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperidine, vinylpyrrolidone, Vinyl-based monomers such as vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, (meth)acryloylmorpholine, N-vinylcarboxyamide, N-vinylcaprolactamide, etc. ; Or cyanoacrylate monomers such as acrylonitrile, methacrylonitrile, etc. Among these, N-vinylpyrrolidone is preferable from the viewpoint of high adhesion improvement effect due to improvement in cohesion.

A crosslinked structure can also be introduced into the acrylic base polymer. The cross-linking by the acrylic base polymer can exert high adhesive retention force even when the G' of the adhesive sheet is small. In this way, in order to introduce a cross-linked structure into the acrylic base polymer, it is preferable that the acrylic base polymer further contains a hydroxyl group-containing monomer and a carboxyl group-containing monomer in addition to the above-mentioned alkyl (meth)acrylate as Monomer components. When a cross-linked structure is introduced into the acrylic base polymer by an isocyanate-based cross-linking agent, an epoxy-based cross-linking agent, or the like, a hydroxyl group or a carboxyl group becomes an introduction point of the cross-linked structure.

As a hydroxyl group-containing monomer, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6- (meth)acrylate (Meth)acrylates such as hydroxyhexyl, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, and 12-hydroxylauryl (meth)acrylate. Among them, 2-hydroxyethyl acrylate (Tg: -15° C.) and acrylic acid are preferred in terms of contributing greatly to the improvement of the adhesive force and suppressing the cloudiness of the adhesive sheet 11 in a high-humidity environment 4-hydroxybutyl acrylate (Tg: -32°C) is particularly preferably 4-hydroxybutyl acrylate from the viewpoint of low Tg.

Examples of carboxyl group-containing monomers include acrylic monomers such as (meth)acrylic acid, carboxyethyl (meth)acrylate, and carboxypentyl (meth)acrylate, itaconic acid, maleic acid, trans Butenedioic acid, butenoic acid, etc.

From the viewpoint of improving the adhesive force and adhesive retention force of the adhesive sheet 11, the amount of the polar group-containing monomer is preferably 2 parts by weight or more with respect to 100 parts by weight of the total monomer components of the acrylic base polymer. It may be 3 parts by weight or more, 4 parts by weight or more, or 5 parts by weight or more. On the other hand, as the content of polar monomer increases, the dipole moment of the base polymer increases, and the relative permittivity increases. In addition, when the content of the polar monomer is too large, the glass transition temperature of the polymer tends to increase, and the adhesive force at low temperature tends to decrease. Therefore, the amount of the polar group-containing monomer is preferably 15 parts by weight or less with respect to 100 parts by weight of the total monomer components of the acrylic base polymer, and may be 13 parts by weight or less or 10 parts by weight or less.

It is preferable that the acrylic base polymer contains the hydroxyl-containing monomer and the nitrogen-containing monomer in the polar monomer component, and it is preferable that the sum of the hydroxyl-containing monomer and the nitrogen-containing monomer is in the above-mentioned range.

By including a hydroxyl group-containing monomer as a polar monomer component, the adhesive force 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 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 with respect to 100 parts by weight of the total monomer components of the acrylic base polymer. On the other hand, when the content of the hydroxyl group-containing monomer (the amount of hydroxyl groups in the adhesive) increases, the relative permittivity of the adhesive tends to increase remarkably. Therefore, the amount of the hydroxyl group-containing monomer 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 with respect to 100 parts by weight of the total monomer components of the acrylic base polymer. If the amount of the hydroxyl group-containing monomer is within the above range, the relative permittivity and the void distance can be set within the above range.

From the viewpoint of simultaneously achieving the improvement of the adhesive force and the lowering of the dielectric constant, the amount of the nitrogen-containing monomer is preferably 0.5 to 10 parts by weight, more preferably 100 parts by weight in total of the monomer components of the acrylic base polymer. 1 to 8 parts by weight. In particular, it is preferable that the amount of N-vinylpyrrolidone is within the above-mentioned range from the viewpoint of a large contribution to the improvement of the adhesion force. If the amount of the nitrogen-containing monomer is within the above range, the relative permittivity and the void distance can be set within the above range.

In order to prevent corrosion of the electrodes of the touch panel caused by acid components, it is preferable that the acid content of the adhesive sheet 11 is small. Moreover, in order to suppress the polyolefinization of the polyvinyl alcohol-type polarizing element by an acid component, it is preferable that the content of the acid of the adhesive sheet 11 is small. In such an acid-free adhesive sheet, the content of organic acid monomers such as (meth)acrylic acid is preferably 100 ppm or less, more preferably 70 ppm or less, and still more preferably 50 ppm or less. The organic acid monomer content of the adhesive sheet 11 can be obtained by the following method: the adhesive sheet is immersed in pure water, heated at 100° C. for 45 minutes, and the acid monomer extracted into the water is analyzed by ion chromatography. Quantify.

In order to reduce the acid monomer content in the adhesive sheet 11, it is preferable that the amount of organic acid monomer components such as (meth)acrylic acid in the monomer components constituting the base polymer is small. Therefore, in order to make the adhesive sheet free of acid, it is preferable that the base polymer does not substantially contain an organic acid monomer (carboxyl group-containing monomer) as a monomer component. In the acid-free adhesive sheet, the amount of the carboxyl group-containing monomer is preferably 0.5 part by weight or less, more preferably 0.1 part by weight or less, and still more preferably 0.05 part by weight relative to 100 parts by weight of the total monomer components of the base polymer. The weight part or less is preferably 0.

The acrylic base polymer may contain monomers other than the above-mentioned alkyl (meth)acrylate and polar monomers as monomer components. Examples of monomer components other than the above include: (meth)acrylic acid caprolactone adduct; sulfonic acid group-containing monomer; phosphoric acid group-containing monomer; vinyl acetate, vinyl propionate, styrene, Vinyl monomers such as α-methylstyrene; cyanoacrylate monomers such as acrylonitrile and methacrylonitrile; epoxy group-containing monomers such as glycidyl (meth)acrylate; (meth)acrylic polymer Glycol acrylate monomers such as ethylene glycol ester, polypropylene glycol (meth)acrylate, methoxyethylene glycol (meth)acrylate, and methoxypolypropylene glycol (meth)acrylate; (methyl) ) Acrylate monomers such as tetrahydrofurfuryl acrylate, fluoro(meth)acrylate, polysiloxane (meth)acrylate or 2-methoxyethyl (meth)acrylate.

The theoretical Tg of the acrylic base polymer is preferably -60 to -20°C. The theoretical Tg of the acrylic base polymer is more preferably -23°C or lower, further preferably -25°C or lower, and particularly preferably -30°C or lower. The theoretical Tg of the acrylic base polymer may be -50°C or lower, -45°C or lower, -40°C or lower, or -38°C or lower. If the theoretical Tg of the base polymer is within the above range, the relative permittivity and the void distance can be set within the above range. The theoretical Tg is calculated by the following Fox formula based on the glass transition temperature Tg _i of the homopolymer constituting the monomer components of the acrylic base polymer and the weight fraction W _i of each monomer component. 1/Tg=Σ(W _i /Tg _i )

Tg is the glass transition temperature of the polymer chain (unit: K), Wi is the weight fraction of the monomer component _i constituting the segment (copolymerization ratio on a weight basis), Tg _i is the glass of the homopolymer of the monomer component i Transfer temperature (unit: K). As the glass transition temperature of the homopolymer, the value described in the 3rd edition of the Polymer Handbook (John Wiley & Sons, Inc., 1989) can be used. The Tg of the homopolymer of the monomer not described in the above documents may be the peak top temperature of tan δ measured by dynamic viscoelasticity.

<Crosslinked structure of base polymer> 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 55-85%, more preferably 60-80%, further preferably 63-77%, particularly preferably 65-75%. By adjusting the gel fraction to this range, when G' is small and the adhesive sheet is soft, a high adhesion holding force can be exhibited, so that the above-mentioned gap distance can be reduced.

The gel fraction can be obtained as the insoluble content in a solvent such as ethyl acetate. Obtained in the form of the weight fraction (unit: % by weight) of the sample. Generally, the gel fraction of a polymer is equal to the degree of cross-linking, and the more the cross-linked portion in the polymer is, the higher the gel fraction is. The gel fraction (introduction amount of the crosslinking structure) can be adjusted to a desired range by the method of introducing the crosslinking structure, the type and amount of the crosslinking agent, and the like.

As a method for introducing a cross-linked structure into the base polymer, for example: (1) polymerizing a base polymer having a functional group capable of reacting with a cross-linking agent, and then adding a cross-linking agent to make the base polymer and the cross-linking agent react with each other. A method of reacting a linking agent; and (2) a method of introducing a branched structure (cross-linked structure) into the polymer chain by including a polyfunctional compound in the polymerization component of the base polymer, and the like. These may be used in combination to introduce a plurality of crosslinked structures into the base polymer.

In the method of reacting the base polymer and the crosslinking agent in the above (1), a crosslinking structure is introduced into the base polymer by adding the crosslinking agent to the base polymer after polymerization and heating as necessary. As a crosslinking agent, the compound which reacts with functional groups, such as a hydroxyl group and a carboxyl group contained in a base polymer, is mentioned. Specific examples of the cross-linking agent include isocyanate-based cross-linking agents, epoxy-based cross-linking agents, oxazoline-based cross-linking agents, aziridine-based cross-linking agents, and carbodiimide-based cross-linking agents. , metal chelate cross-linking agent, etc.

Among them, an isocyanate-based crosslinking agent and an epoxy-based crosslinking agent are preferred from the viewpoint of high reactivity with the hydroxyl group or carboxyl group of the base polymer and easy introduction of a crosslinking structure. These cross-linking agents react with functional groups such as hydroxyl groups or carboxyl groups introduced into the base polymer to form a cross-linked structure. Regarding the acid-free adhesive whose base polymer does not contain a carboxyl group, it is preferable to use an isocyanate-based cross-linking agent, and to utilize the reaction of the hydroxyl group in the base polymer and the isocyanate cross-linking agent to form a cross-linked structure.

As the isocyanate-based crosslinking agent, a polyisocyanate having two or more isocyanate groups in one molecule can be used. Examples of the isocyanate-based crosslinking agent include lower aliphatic polyisocyanates such as butylene diisocyanate and hexamethylene diisocyanate; cyclopentylene diisocyanate, cyclohexylene diisocyanate, isophorone diisocyanate, and the like. Alicyclic isocyanates such as isocyanates; aromatic isocyanates such as 2,4-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate; trimethylolpropane/toluene diisocyanate trimer Adducts (such as "Coronate L" manufactured by Tosoh), trimethylolpropane/hexamethylene diisocyanate trimer adducts (such as "Coronate HL" manufactured by Tosoh), xylylene Trimethylolpropane adduct of diisocyanate (such as "Takenate D110N" manufactured by Mitsui Chemicals, isocyanurate of hexamethylene diisocyanate (such as "Coronate HX" manufactured by Tosoh), etc. things etc.

In the method of including a polyfunctional monomer in the polymerization component of the base polymer in the above (2), the monomer component constituting the acrylic base polymer and the polyfunctional compound for introducing the crosslinking structure can be all reacted at one time, The polymerization can also be carried out in multiple stages. As a method of carrying out the polymerization in multiple stages, preferably, a method of polymerizing (prepolymerizing) a monofunctional monomer constituting a base polymer, preparing a partial polymer (prepolymer composition), and adding a prepolymer to the A polyfunctional compound such as a polyfunctional (meth)acrylate is added to the composition, and the prepolymer composition and the polyfunctional monomer are polymerized (mainly polymerized). The prepolymer composition is a partial polymer comprising a low degree of polymerization polymer and unreacted monomers.

By prepolymerizing the constituent components of the acrylic base polymer, the branch points (crosslinking points) of the polyfunctional compound can be uniformly introduced into the base polymer. In addition, a low molecular weight polymer or a mixture of a partial polymer and an unpolymerized monomer component (adhesive composition) can also be applied to a substrate, and then the substrate is fully polymerized to form an adhesive sheet. Since oligomeric compositions such as prepolymer compositions have low viscosity and are excellent in coatability, the productivity of the adhesive sheet can be improved and the thickness of the adhesive sheet can be made uniform according to the following method. After the cloth is used as the adhesive composition of the mixture of the prepolymer composition and the polyfunctional compound, the formal polymerization is carried out on the substrate.

As a polyfunctional compound for introducing a crosslinked structure, a compound containing two or more polymerizable functional groups (ethylenically unsaturated groups) having an unsaturated double bond in one molecule can be mentioned. The polyfunctional compound is preferably a polyfunctional (meth)acrylate from the viewpoint of being easy to copolymerize with the monomer component of the acrylic base polymer. In the case of introducing a branched (crosslinked) structure by active energy ray polymerization (photopolymerization), a polyfunctional acrylate is preferred.

As the polyfunctional (meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate may, for example, be mentioned. , bisphenol A ethylene oxide modified di(meth)acrylate, bisphenol A propylene oxide modified di(meth)acrylate, alkanediol di(meth)acrylate, tricyclodecane two Methanol di(meth)acrylate, ethoxylated isocyanuric acid tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol di(meth)acrylate, trimethylolpropane tri(meth)acrylate base) acrylate, di-trimethylolpropane tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, Dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerol di(meth)acrylate, epoxy(meth)acrylate, butadiene(meth)acrylate, Isoprene (meth)acrylate, etc.

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

<Preparation of base polymer> The acrylic base polymer can be prepared by known polymerization methods such as solution polymerization, UV polymerization, bulk polymerization, and emulsion polymerization. From the viewpoints of transparency, water resistance, cost, etc. of the adhesive, solution polymerization or active energy ray polymerization (eg, UV polymerization) is preferred. As a solvent for solution polymerization, ethyl acetate, toluene, or the like can be generally used.

系光聚合起始劑、醯基氧化膦系光聚合起始劑等。作為熱聚合起始劑，例如可使用偶氮系起始劑、過氧化物系起始劑、將過氧化物與還原劑組合所得之氧化還原系起始劑(例如過硫酸鹽與亞硫酸氫鈉之組合、過氧化物與抗壞血酸鈉之組合等)。When preparing the acrylic base polymer, a polymerization initiator such as a photopolymerization initiator or a thermal polymerization initiator may also be used according to the type of the polymerization reaction. The photopolymerization initiator is not particularly limited as long as it initiates photopolymerization. For example, a benzoin ether-based photopolymerization initiator, an acetophenone-based photopolymerization initiator, and an α-keto alcohol-based photopolymerization initiator can be used. Initiator, aromatic sulfonyl chloride-based photopolymerization initiator, photoactive oxime-based photopolymerization initiator, benzoin-based photopolymerization initiator, benzoin-based photopolymerization initiator, benzophenone-based photopolymerization initiator Polymerization initiator, ketal-based photopolymerization initiator, 9-oxysulfur 𠮿

It is a photopolymerization initiator, an acylphosphine oxide-based photopolymerization initiator, and the like. As the thermal polymerization initiator, for example, an azo-based initiator, a peroxide-based initiator, and a redox-based initiator obtained by combining a peroxide with a reducing agent (for example, persulfate and hydrogen sulfite) can be used. combination of sodium, combination of peroxide and sodium ascorbate, etc.).

During the polymerization, in order to adjust the molecular weight and the like, a chain transfer agent, a polymerization inhibitor (polymerization retarder), or the like may also be used. As the chain transfer agent, α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, thioglycolic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, 2, Thiols such as 3-dimercapto-1-propanol; or α-methylstyrene dimer, etc.

The molecular weight of the base polymer can be adjusted by adjusting the type or amount of the polymerization initiator. For example, in the case of radical polymerization, the higher the amount of the polymerization initiator, the higher the concentration of radicals in the reaction system, so that the density of the reaction starting point increases and the high molecular weight tends to decrease. Conversely, the smaller the amount of the polymerization initiator, the smaller the density of the reaction starting point, so the polymer chain tends to be elongated and the molecular weight tends to increase.

In order to obtain an adhesive sheet with excellent adhesion and a small void distance, it is preferable that the acrylic base polymer has a high gel fraction with a small cross-linking point density. In order to increase the gel fraction (ratio of polymer chains having a cross-linked structure introduced) with a smaller cross-linking density, it is only necessary to increase the molecular weight of the base polymer (the length of the polymer chain). As described above, in order to increase the molecular weight of the base polymer, it is preferable to reduce the amount of the polymerization initiator used when the base polymer is polymerized.

The amount of the polymerization initiator used in the polymerization of the base polymer may be appropriately set according to the type of polymerization reaction, the composition of the monomers, the type of the polymerization initiator, and the target molecular weight. From the viewpoint of increasing the molecular weight of the base polymer and increasing the gel fraction with less cross-linking agent, the amount of the polymerization initiator to be used is preferably based on 100 parts by weight of the total monomer components constituting the base polymer. It is 0.001-0.4 weight part, More preferably, it is 0.003-0.1 weight part, More preferably, it is 0.005-0.05 weight part.

In the case of introducing a cross-linked structure using an isocyanate-based cross-linking agent or the like, it is preferable to polymerize the base polymer by solution polymerization, then add the cross-linking agent, and heat as necessary to introduce the cross-linking into the base polymer. structure. When using polyfunctional compounds such as polyfunctional (meth)acrylates to introduce the cross-linked structure, it is preferable to carry out the polymerization of the base polymer or the preparation of the prepolymer composition by solution polymerization or active energy ray polymerization. And after adding the polyfunctional compound, the cross-linked structure of the polyfunctional compound is introduced by active energy ray polymerization.

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

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

<Acrylic oligomer> The adhesive sheet 11 may contain an oligomer in addition to the acrylic base polymer. As the acrylic oligomer, those having a weight average molecular weight of about 1,000 to 30,000 can be used. The acrylic oligomer contains an alkyl (meth)acrylate as a main constituent monomer component.

The glass transition temperature of the acrylic oligomer is preferably 60°C or higher, more preferably 80°C or higher, further preferably 100°C or higher, particularly preferably 110°C or higher. By using together a low-Tg acrylic base polymer with a cross-linked structure and a high-Tg acrylic oligomer, the adhesive force of the adhesive sheet, especially the adhesive retention force at high temperature, tends to improve, and the voids can be reduced. distance. The upper limit of the glass transition temperature of the acrylic oligomer is not particularly limited, but is usually 200°C or lower, preferably 180°C or lower, and more preferably 160°C or lower. The glass transition temperature of the acrylic oligomer is calculated by the above-mentioned Fox formula.

The acrylic oligomer having a glass transition temperature of 60° C. or higher preferably contains a (meth)acrylic acid alkyl ester ((meth)acrylic acid chain alkyl ester) having a chain alkyl group and an alicyclic alkyl ester The base (meth)acrylic acid alkyl ester ((meth)acrylic acid alicyclic alkyl ester) is used as a constituent monomer component. The specific example of (meth)acrylic-acid chain alkyl ester and (meth)acrylic-acid alicyclic alkyl ester is illustrated above as a structural monomer of an acryl-type polymer chain.

Among the exemplified alkyl (meth)acrylates, as the chain alkyl (meth)acrylate, the glass transition temperature is high and the compatibility with the base polymer is excellent, and methyl is preferred. Methyl acrylate. As the alicyclic alkyl (meth)acrylate, dicyclopentyl acrylate, dicyclopentyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate are preferred. That is, the acrylic oligomer preferably contains at least one selected from the group consisting of dicyclopentyl acrylate, dicyclopentyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate, and methyl methacrylate. Methyl methacrylate as a constituent monomer component.

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

The weight average molecular weight of the acrylic oligomer is preferably 1,000 to 30,000, more preferably 1,500 to 10,000, and still more preferably 2,000 to 8,000. By using an acrylic oligomer having a molecular weight in this range, the adhesive force or the adhesive holding force tends to increase, and the void distance can be reduced.

The acrylic oligomer can be obtained by polymerizing the above-mentioned monomer components by various polymerization methods. In the polymerization of the acrylic oligomer, various polymerization initiators can be used. Moreover, in order to adjust molecular weight, a chain transfer agent can also be used.

The content of the acrylic oligomer in the adhesive sheet 11 is not particularly limited. In order to sufficiently improve the adhesive force, the amount of the acrylic oligomer is preferably 0.5 parts by weight or more relative to 100 parts by weight of the base polymer, and more It is preferably 0.8 part by weight or more, and more preferably 1 part by weight or more. With respect to 100 parts by weight of the base polymer, the amount of the acrylic oligomer in the adhesive sheet 11 may also 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. As the addition amount of the high-Tg acrylic oligomer increases, the void distance tends to decrease.

On the other hand, when the addition amount of an acrylic oligomer is too large, since compatibility falls, there exists a tendency for the haze of an adhesive sheet to rise and transparency to fall. Higher transparency is required for the adhesive sheet disposed on the viewing side of the image display panel, so the amount of the acrylic oligomer in the adhesive sheet 11 is preferably 5 weight parts relative to 100 parts by weight of the base polymer part or less, may be 4 parts by weight or less or 3 parts by weight or less.

<Adhesive composition> In the acrylic base polymer (or prepolymer composition), the above-mentioned acrylic oligomer, crosslinking agent and/or multifunctional compound for introducing the crosslinking structure, other additives, etc., are mixed as required to prepare the adhesive combination. The remainder of the monomer components constituting the acrylic base polymer may also be added to the adhesive composition as required. In order to adjust the viscosity etc., a viscosity increasing additive etc. can also be used.

When the adhesive composition includes a prepolymer composition and a polyfunctional compound, etc., the adhesive composition preferably includes a photopolymerization initiator for formal polymerization. After the prepolymerization, a polymerization initiator for formal polymerization can also be added to the prepolymer composition. When the polymerization initiator remains in the prepolymer composition without being deactivated during the prepolymerization, the addition of the polymerization initiator used for the main polymerization can also be omitted. The adhesive composition may also contain a chain transfer agent.

In the adhesive composition, the content of the acrylic base polymer (or prepolymer composition) relative to the total amount of non-volatile components is preferably 50% by weight or more, more preferably 70% by weight or more, and more preferably 80% by weight % or more, particularly preferably 90% by weight or more.

The amount of the crosslinking agent and/or the polyfunctional compound in the adhesive composition may be adjusted so that the gel fraction falls within the above-mentioned range. As described above, in order to reduce the void distance, it is preferable to increase the molecular weight of the acrylic base polymer, and to increase the gel fraction with a smaller crosslinking point density. For example, when an isocyanate-based cross-linking agent is used to introduce a cross-linked structure, the amount of the cross-linking agent is preferably 0.005 to 0.5 parts by weight, more preferably 0.01 to 0.3 parts by weight relative to 100 parts by weight of the acrylic base polymer , and more preferably 0.02 to 0.1 parts by weight. In the case where the cross-linked structure is introduced by using a polyfunctional (meth)acrylate, the amount of the polyfunctional (meth)acrylate is preferably 0.005 to 0.3 with respect to 100 parts by weight of the acrylic base polymer (prepolymer). The weight part is more preferably 0.01 to 0.2 weight part, and still more preferably 0.02 to 0.1 weight part.

(Silane coupling agent) A silane coupling agent may also be added to the adhesive composition. In the case of adding a silane coupling agent to the adhesive composition, the addition amount is usually about 0.01 to 5.0 parts by weight, preferably about 0.03 to 3.0 parts by weight, relative to 100 parts by weight of the base polymer. When the amount of the silane coupling agent is within the above range, the void distance may be reduced.

(other additives) In addition to the components exemplified above, the adhesive composition may contain an adhesion imparting agent, a plasticizer, a softener, an anti-deterioration agent, a filler, a colorant, an ultraviolet absorber, an antioxidant, a surfactant, an antistatic agent additives, etc.

<Formation of adhesive sheet> The adhesive composition is coated on the substrate, and if necessary, the solvent is dried and removed, and/or the formal polymerization is performed by irradiation with active light rays, thereby forming an adhesive sheet on the substrate. Any appropriate substrate can be used as the substrate for forming the adhesive sheet. As a base material, it can also be a release film, which has a release layer on the contact surface with the adhesive sheet.

As the film base of the release film, films containing various resin materials can be used. The resin material may, for example, include polyester-based resins such as polyethylene terephthalate and polyethylene naphthalate, acetate-based resins, polyether-based resins, polycarbonate-based resins, and polyamides. Amine resin, polyimide resin, polyolefin resin, (meth)acrylic resin, polyvinyl chloride resin, polyvinylidene chloride resin, polystyrene resin, polyvinyl alcohol resin, Polyarylate resin, polyphenylene sulfide resin, etc. Among them, polyester-based resins such as polyethylene terephthalate are particularly preferred. The thickness of the film substrate is preferably 10-200 μm, more preferably 25-150 μm. As the material of the release layer, a polysiloxane-based mold release agent, a fluorine-based mold release agent, a long-chain alkyl-based mold release agent, an aliphatic amide-based mold release agent, and the like can be exemplified. The thickness of the release layer is usually about 10 to 2000 nm.

As a method of applying the adhesive composition to the substrate, roll coating, touch roll coating, gravure coating, reverse coating, roll brush coating, spray coating, dip roll coating, bar coating can be used , blade coating, air knife coating, curtain coating, die lip coating, die mouth coating and other methods.

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

When the adhesive composition contains a cross-linking agent, the cross-linking reaction can also be performed after the adhesive composition is applied to the substrate. During crosslinking, heating may be performed as necessary. The temperature of the cross-linking reaction is usually in the range of 20°C to 160°C, and the time of the cross-linking reaction is about 1 minute to 7 days. After the adhesive composition is applied, the heating for drying the solvent may also serve as the heating for cross-linking. After drying the solvent, in order to protect the surface of the adhesive sheet, a cover sheet is preferably attached. As the cover sheet, like the base film, it is preferable to use a release film having a release layer on the contact surface with the adhesive sheet.

When the adhesive composition is a photopolymerizable composition including a prepolymer composition and a polyfunctional compound, the adhesive composition is applied to the substrate in a layered form, and then irradiated with actinic rays. Light hardening. When photohardening is performed, it is preferable to attach a cover sheet on the surface of the coating layer, and to irradiate an active ray with the adhesive composition sandwiched between two sheets to prevent polymerization inhibition caused by oxygen.

The actinic light may be selected according to the type of polymerizable components such as monomers or polyfunctional (meth)acrylates, or the type of photopolymerization initiator, and ultraviolet rays and/or short-wavelength visible light are usually used. The cumulative light intensity of the irradiation light is preferably about 100 to 5000 mJ/cm ² . The light source for light irradiation is not particularly limited as long as it can irradiate light in a wavelength range with a sensitivity of the photopolymerization initiator contained in the adhesive composition, and an LED light source and a high-pressure mercury lamp can be preferably used. , Ultra-high pressure mercury lamps, metal halide lamps, xenon lamps, etc.

[Second adhesive sheet] The composition of the second adhesive sheet 12 disposed on the image display panel side surface of the polarizing plate 31 is not particularly limited as long as it has the above-mentioned relative permittivity. The impact resistance of the second adhesive sheet 12 such as that of the first adhesive sheet 11 arranged on the viewing side is not required. From the viewpoint of thinning, the thickness of the second adhesive sheet 12 is preferably 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.

As long as the relative permittivity of the second adhesive sheet 12 is within the above range, the composition of the adhesive is not particularly limited. As the base polymer, examples include acrylic, polysiloxane, polyester, polyurethane Acid esters, polyamides, polyvinyl ethers, vinyl acetate/vinyl chloride copolymers, modified polyolefins, epoxy-based, fluorine-based, and rubber-based polymers.

The second adhesive sheet 12 preferably contains an acrylic adhesive containing an acrylic base polymer from the viewpoint of being able to control transparency or adhesive force in addition to relative permittivity and adhesive force. As described above, in order to reduce the relative permittivity of the adhesive sheet, it is effective to reduce the amount of the polar group-containing monomer in the monomer component constituting the base polymer.

When the second adhesive sheet 12 is an acrylic adhesive sheet, the amount of the polar group-containing monomer is preferably 15 parts by weight or less, more preferably 100 parts by weight in total of the monomer components of the base polymer. 10 parts by weight or less, 5 parts by weight or less or 3 parts by weight or less may be sufficient. The amount of the hydroxyl group-containing monomer 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 with respect to 100 parts by weight of the total monomer components of the base polymer.

The strain relaxation properties of the second adhesive sheet 12 as in the first adhesive sheet 11 are not required. Therefore, the storage modulus G' ₂₅ of the second adhesive sheet 12 at 25° C. can also be greater than the G′ ₂₅ of the first adhesive sheet. G' ₂₅ of the second adhesive sheet 12 may be 70 kPa or more, 75 kPa or more, or 80 kPa or more. By increasing G' ₂₅ of the second adhesive sheet 12, the adhesion holding force can be improved.

The second adhesive sheet 12 is also the same as the first adhesive sheet 11, and the gap distance is preferably 2 mm or less. Since the gap distance is 2 mm or less and has the above-mentioned relative permittivity, reliability can be improved even in an image display device that has a touch panel within a distance of 500 μm from the touch surface and can be bent. Therefore, even if the distance from the device surface to the touch panel sensor is short, there is almost or completely no malfunction of the touch panel sensor, failures and the like can be suppressed or prevented, and the reliability is high.

[Video display device] As described above, the first adhesive sheet 11 and the second adhesive sheet 12 are used for bonding the polarizing plate 31 and other optical components in an image display device having a touch panel and capable of bending.

The image display device 101 shown in FIG. 1 is provided with the touch panel 41 , the circular polarizer 31 and the cover window 71 on the visible side surface of the organic EL panel 51 as the image display panel. In a flexible display, these components are flexible and can be bent. The image display device 102 shown in FIG. 2 integrates a touch panel on the organic EL panel 54 as an image display panel, and has a circular polarizer 31 and a cover window 71 arranged on the surface thereof.

<Video display panel> The organic EL panel includes a pair of electrodes on a substrate, and an organic light-emitting layer sandwiched between the electrodes. The organic EL panel can be either a top emission type or a bottom emission type. The top emission type has a metal electrode, an organic emission layer and a transparent electrode sequentially laminated on a substrate, and the bottom emission type is sequentially laminated on a transparent substrate. There are transparent electrodes, organic light-emitting layers and metal electrodes. In either of the bottom emission type and the top emission type, the substrate, the sealing member, and the like provided on the visible side of the organic light-emitting layer are all transparent. The substrate, the sealing member, and the like provided on the back side (the casing 75 side in FIGS. 1 and 2 ) rather than the organic light-emitting layer may be opaque. Regarding the flexible organic EL panel of bottom emission type, the substrate does not need to be transparent, and as the substrate material, polyimide or the like can be used. The substrate material can also be a transparent resin material such as polyether ether ketone or transparent polyimide. For the protection or reinforcement of the substrate, a primer sheet can also be provided on the back side of the substrate.

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

＜Covered window＞ In order to prevent breakage of the image display panel due to impact from the outer surface, etc., a cover window 71 is provided on the outermost surface of the viewable side of the image display device. In a flexible display, a flexible transparent substrate such as transparent polyimide, polyether ether ketone, polyethylene terephthalate, etc. can be used as the cover window 71 . As the material of the cover window 71, a flexible glass plate (glass film) can be used, and the cover window 71 can be formed by laminating a glass film and a resin film. From the viewpoint of achieving both strength and flexibility, the thickness of the cover window is preferably 20 to 300 μm, more preferably 25 to 250 μm, and still more preferably 30 to 200 μm. The yield point elongation of the covering window is preferably 5% or more, from the viewpoint of excellent recovery properties after being kept in a bent state for a long time. As the cover window 71, a bendable thin glass substrate can also be used. The cover window may be a layer of transparent materials having two or more layers. An anti-reflection layer or a hard coating layer can also be provided on the visible side surface of the cover window.

＜Touch panel＞ The image display device is provided with an electrostatic capacitive touch panel on the visible side surface of the image display panel. The electrostatic capacitance type touch panel detects the touch position based on the change of electric quantity when the operator's finger or stylus is in contact with the touch surface. In the configuration of FIG. 1 , the touch panel 41 is arranged between the circular polarizer 31 and the 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 disposed between the circular polarizer 31 and the cover window 71 .

＜Polarizer＞ The polarizing plate 31 is arranged on the viewing side of the image display panel. For example, in a liquid crystal display device, the polarizing plate provided on the viewing side of the liquid crystal panel adjusts the transmittance according to the polarization state of the light passing through the liquid crystal cell. In the organic EL display device, by disposing the circular polarizer 31 on the visible side of the organic EL panel 51, the external light reflected by the metal electrodes of the organic EL panel can be shielded from exiting toward the visible side, thereby improving the visibility of the display.

As a polarizing plate, a suitable transparent protective film is usually attached to one side or both sides of the polarizing element as required. The polarizing element is not particularly limited, and various polarizing elements can be used. Examples of polarizers include adsorption of iodine or dichroism on hydrophilic polymer films such as polyvinyl alcohol-based films, partially formalized polyvinyl alcohol-based films, and ethylene-vinyl acetate copolymer-based partially saponified films. Dichroic substances such as dyes are uniaxially stretched; polyene-based alignment films such as dehydration-treated products of polyvinyl alcohol or dehydrochloric acid-treated products of polyvinyl chloride, etc.

As the polarizer, a thin polarizer with a thickness of 10 μm or less can also be used. Examples of thin polarizers include those described in Japanese Patent Laid-Open No. 51-069644, Japanese Patent Laid-Open No. 2000-338329, WO2010/100917, Japanese Patent No. 4691205, and Japanese Patent No. 4751481. the polarizing element. A thin polarizer can be obtained by, for example, a manufacturing method including the steps of extending a polyvinyl alcohol-based resin layer and a resin substrate for stretching in a laminate state, and dyeing with a dichroic material such as iodine.

As the transparent protective film of the protective film of the polarizer, transparent protective films such as cellulose-based resins, cyclic polyolefin-based resins, acrylic-based resins, phenylmaleimide-based resins, and polycarbonate-based resins can be preferably used. Those with excellent properties, mechanical strength, thermal stability, moisture barrier properties and optical isotropy. Furthermore, when a transparent protective film is provided on both sides of the polarizing element, a protective film containing the same polymer material or a protective film containing different polymer materials can be used on the front and back sides of the polarizer.

An optical film can also be laminated on one side or both sides of the polarizing plate with an appropriate adhesive layer or adhesive layer as needed. As such a film, a retardation plate, a viewing angle widening film, a viewing angle limiting (privacy protection) film, a brightness enhancement film, or the like used for the formation of an image display device is used, and the type thereof is not particularly limited. For example, in a liquid crystal display device, an optical compensation film is sometimes used between an image display panel (liquid crystal panel) and a polarizing plate in order to appropriately convert the polarization state of light emitted from the liquid crystal cell to the viewing side and improve viewing angle characteristics.

As described above, in the organic EL display device, the following circular polarizing plate is provided, the external light reflected by the metal electrode can be shielded from being emitted to the viewing side, and the circular polarizing plate is arranged on the surface of the organic EL panel side of the polarizing element 1/2 4-wavelength plate. By arranging the 1/4 wavelength plate on the viewing side of the polarizing element, the outgoing light becomes circularly polarized, so that the viewer wearing the polarized sunglasses can also see the appropriate image display. These optical films (optically anisotropic films) may also be laminated on the polarizer without intervening other films. In this case, the optical film also functions as a protective film for the polarizing element.

The thickness of the polarizing plate is usually about 10-200 μm. From the viewpoint of having flexibility, the thickness of the polarizing plate used in the flexible display is preferably 100 μm or less, more preferably 80 μm or less, and still more preferably 70 μm or less. In the case where optical films such as quarter-wavelength plates are laminated on the polarizing plate, it is preferable that the total thickness including these films is within the above-mentioned range.

<Lamination between members using adhesive sheets> An adhesive sheet is used for bonding the above-mentioned flexible members. In the image display device shown in FIG. 1 , the organic EL panel 51 and the bottom surface of the casing 75 are bonded via the adhesive sheet 14 , the organic EL panel 51 and the touch panel 41 are bonded via the adhesive sheet 13 , and the touch panel 41 and the circular polarizing plate 31 are attached through the second adhesive sheet 12 , and the circular polarizing plate 31 and the cover window 71 are attached through the 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 together via the second adhesive sheet 12 , and the circular polarizer 31 and the cover window 71 are bonded via the first adhesive sheet 11 fit.

Since both the second adhesive sheet 12 and the first adhesive sheet 11 disposed on the visible side of the touch panel 41 have low dielectric constants, the distance D from the touch surface to the touch panel is smaller when the distance D is smaller. In this case, the malfunction of the touch panel can also be reduced.

When the image display device of the present invention is subjected to a bending test, it is preferable that the peeling between the components of the curved portion is small, especially the peeling at the interface between the polarizing plate 31 and the cover window 71 is small. The bending test is based on the bending radius Under the bending state of 1.3 mm and bending angle of 180°, it can be kept for 240 hours in a high temperature and high humidity environment with a temperature of 60 degrees and a relative humidity of 95%. The peeling at the interface in the bending retention test can be quantified as the length of the void portion (void distance), which is obtained by bending a 35 mm × 100 mm sample with the short side direction as the bending axis. The length of the void portion (gap distance) along the short side direction of the bending axis at the time of the test was maintained. The gap distance of the image display device after the bending retention test is preferably 2 mm or less, preferably 1.5 mm or less, more preferably 1.0 mm or less, and may also be 0.8 mm or less, 0.5 mm or less, or 0.3 mm or less. The lower limit value of the gap distance is not particularly limited, and may be 0.

As described above, by adjusting the composition of the first adhesive sheet 11, etc., the gap distance of the image display device can be reduced. The void distance of the adhesive sheet can also be evaluated by the same method as described above. Specifically, it is sufficient to measure the length (void distance) of the void portion between the adhesive sheet and the adherend in the order of the following steps A to D.

Step A: Make a 35 mm×100 mm test piece made by laminating the adhesive sheet and the adherend Step B: Bend the test piece produced in Step A along the short side with a bending radius of 1.3 mm and a bending angle of 180° Step C: Keep the bent test piece in Step B in a bent state for 240 hours in an environment with a temperature of 60 degrees and a relative humidity of 95% Step D: The length in the short-side direction of the gap between the adhesive sheet and the adherend was measured at the bent portion of the test piece after holding for 240 hours in Step C.

In the bending retention test, peeling (voids) easily occurred from the end of the bending axis of the test piece (the end in the short-side direction). When a gap is generated from both ends, or when a gap exists in a plurality of places, the length of the gap with the longest length in the short-side direction is defined as the gap distance. When there are gaps in a plurality of parts, the longest gap should be 2 mm or less. The total length of each gap is preferably 2 mm or less, and the total length of the gaps 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.

When forming the image display device, the bonding order of each component is not particularly limited. The touch panel 41 , the circular polarizer 31 and the cover window 71 can be sequentially laminated on the image display panel 51 , and the laminated body can also be laminated on the image. On the display panel 51, the lamination system is formed by laminating two or more members through an adhesive sheet in advance. [Example]

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

[Example 1] <First adhesive sheet> (Preparation of Acrylic Oligomers) 60 parts by weight of dicyclopentyl 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 a polymerization solvent 100 parts by weight of toluene was mixed, and stirred in a nitrogen atmosphere at 70° C. for 1 hour. Next, 0.2 weight part of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator was put in, and after making it react at 70 degreeC for 2 hours, it heated up to 80 degreeC and made it react for 2 hours. Then, the reaction liquid was heated to 130 degreeC, toluene, a chain transfer agent, and an unreacted monomer were dried and removed, and the solid acrylic oligomer was obtained. The weight average molecular weight of the acrylic oligomer was 5100, and the glass transition temperature (Tg) was 130°C.

(polymerization of prepolymer) As monomer components for prepolymer formation, 43 parts by weight of lauryl acrylate (LA), 44 parts by weight of 2-ethylhexyl acrylate (2EHA), 6 parts by weight of 4-hydroxybutyl acrylate (4HBA), and N - 7 parts by weight of vinyl-2-pyrrolidone (NVP) and 0.015 part by weight of "Omnirad 184" manufactured by IGM Resins as a photopolymerization initiator, irradiated with ultraviolet rays to conduct polymerization to obtain a prepolymer composition (polymerization rate; about 10%).

(Preparation of Adhesive Composition) To 100 parts by weight of the above-mentioned prepolymer composition, 0.07 parts by weight of 1,6-hexanediol diacrylate (HDDA), 3 parts by weight of the above-mentioned oligomer, and a silane coupling agent ("Shin-Etsu Chemical Co., Ltd. product") were added. KBM403"): 0.3 parts by weight was used as a post-addition component, and then these were uniformly mixed to prepare an adhesive composition.

(Manufacture of adhesive sheet) A 75 μm thick polyethylene terephthalate (PET) film (“DIAFOIL MRF75” manufactured by Mitsubishi Chemical Corporation) with a polysiloxane-based release layer on the surface was used as a base material (double weight) A peeling film), the said photocurable adhesive composition was apply|coated on a base material so that thickness might become 50 micrometers, and a coating layer was formed. On the coating layer, a PET film with a thickness of 75 μm (“DIAFOIL MRE75” manufactured by Mitsubishi Chemical Co., Ltd.) treated with polysiloxane peeling on one side was pasted as a cover sheet (also a light peeling film). The layered body was irradiated with ultraviolet rays from the cover sheet side with a black light lamp whose position was adjusted so that the irradiation intensity of the irradiation surface directly under the lamp became 5 mW/cm ² , and photocuring was performed to obtain an adhesive sheet with a thickness of 50 μm.

<Second adhesive sheet> Butyl acrylate (BA) as a monomer: 99 parts by weight, 4-hydroxybutyl acrylate (4HBA): 1 part by weight, and 2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator ): 0.3 part was put into the reaction container together with ethyl acetate, and the reaction was carried out at 60° C. for 4 hours under nitrogen flow. Then, ethyl acetate was added to the reaction liquid, and the solution of the acrylic polymer with a weight average molecular weight of 1,650,000 was obtained. In this solution, with respect to 100 parts by weight of the polymer, dibenzyl peroxide (“Nyper BMT” manufactured by NOF Oil Co., Ltd.) as a crosslinking agent was prepared: 0.3 parts by weight and trimethylolpropane xylylene dimethyldiene Isocyanate ("Takenate D110N" by Mitsui Chemicals): 0.1 part by weight, and silane coupling agent ("KBM403" by Shin-Etsu Chemical Industry Co., Ltd.): 3 parts by weight to obtain adhesive composition A.

The above-mentioned adhesive composition A was coated on the release-treated surface of a PET film (“MRF38” manufactured by Mitsubishi Chemical) with a thickness of 38 μm after the surface was provided with a polysiloxane-based release layer and subjected to release treatment. Dry and cross-link at 150°C to obtain an adhesive sheet with a thickness of 15 μm.

<Preparation of sample for bending hold test> The release film on one side was peeled off from the first adhesive sheet, and a 51 μm-thick polarizing plate was pasted with a 2 kg roller. Peel off the release film on the other side of the adhesive sheet, and use a 2 kg roller to laminate a transparent polyimide film with a thickness of 80 μm. Furthermore, a PET film with a thickness of 125 μm was pasted on the polarizing plate using a 2 kg roll through the second adhesive sheet. During lamination, plasma treatment is performed on the surface of the polarizer, polyimide film and PET film before lamination with the adhesive sheet.

This laminate was cut into a rectangle of 35 mm×100 mm so that the absorption axis direction of the polarizing plate was parallel to the longitudinal direction, and autoclaved at 35° C. and 0.35 MPa for 15 minutes to obtain a sample for evaluation.

[Examples 2 to 5, Comparative Examples 1 to 3] When producing the first adhesive sheet, the composition of the monomers added in the polymerization of the prepolymer, the compounding amount of the polyfunctional monomer (HDDA), and the compounding amount of the oligomer were changed as shown in Table 1. Except for this, a photocurable adhesive composition was prepared in the same manner as in Example 1, and the coating on the base material and the photocuring were performed to obtain a first adhesive sheet. Using the obtained first adhesive sheet, in the same manner as in Example 1, a polyimide film, a first adhesive sheet, a polarizing plate, a second adhesive sheet and a PET film were laminated in this order to produce evaluation sample.

[Evaluation of the first adhesive sheet] <Gel fraction> About 0.2 g of adhesive was scraped from the adhesive sheet and wrapped with a porous PTFE membrane (“NTF-1122” manufactured by Nitto Denko) with a pore size of 0.2 μm cut into a size of 100 mm×100 mm. And use kite string to fasten the package mouth. The weight (B) of the adhesive sample was calculated by subtracting the total (A) of the weights of the porous PTFE membrane and the kite string measured in advance from the weight of the sample. The adhesive sample wrapped with the porous polytetrafluoroethylene film was immersed in about 50 mL of ethyl acetate at 23°C for 7 days, so that the sol component of the adhesive was dissolved out of the porous polytetrafluoroethylene film. After the immersion, the adhesive wrapped with the porous polytetrafluoroethylene film was taken out, dried at 130° C. for 2 hours, left to cool for about 20 minutes, and then the dry weight (C) was measured. The gel fraction of the adhesive was calculated by the following formula. Gel fraction (%)=100×(C-A)/B

<Storage modulus, loss tangent, and glass transition temperature> The adhesive sheets were laminated so as to have a thickness of about 1.5 mm as a sample for measurement. The dynamic viscoelasticity measurement was performed under the following conditions using "Advanced Rheometric Expansion System (ARES, Advanced Rheometric Expansion System)" manufactured by Rheometric Scientific. According to the measurement results, the storage modulus G' and the loss tangent tanδ of each temperature are read. In addition, the temperature at which tan δ becomes the maximum is set as the glass transition temperature of the adhesive sheet.

(measurement conditions) Deformation Mode: Torsion Measurement frequency: 1 Hz Heating rate: 5℃/min Shape: Parallel plate 7.9 mmϕ

<Total light transmittance and haze> A test piece made by laminating an adhesive sheet to an alkali-free glass (thickness 0.8 to 1.0 mm, total light transmittance 92%, haze 0.4%) was used. -150"), measure haze and total light transmittance. The value obtained by subtracting the haze of the alkali-free glass (0.4%) from the measured value was defined as the haze of the adhesive sheet. The total light transmittance was directly used as the measured value. The total light transmittance of the first adhesive sheet in any of the Examples and Comparative Examples was 92%. The haze of the first adhesive sheet of Example 5 was 0.7%, and the haze of the first adhesive sheet of the other Examples and Comparative Examples was 0.3%.

<Relative permittivity> The adhesive sheet was sandwiched between the copper foil and the electrode, and the "Precision Impedance Analyzer 4294A" manufactured by Agilent Technology was used. According to JIS K6911, the measurement frequencies were 1 kHz, 10 kHz, and 100 kHz under the following conditions. and relative permittivity at 1 MHz. Regarding the first adhesive sheets of Examples 1, 4, 5 and Comparative Example 3, in addition to the measurement at a temperature of 25°C, the relative permittivity was measured at every 20°C in the temperature range of -40°C to 80°C. Electrode composition: 12.1 mmΦ, 0.5 mm thick aluminum plate Counter electrode: 3 oz copper plate Measurement environment: temperature 25°C, relative humidity 50%

<Adhesion to Polyimide Film> The release film on one side of the self-adhesive sheet was peeled off, and the PET film with a thickness of 25 μm was attached to it, and the obtained product was cut into a width of 10 mm × a length of 100 mm as a test piece. The release film on the other side was peeled off from the test piece, and a 2 kg roller was used to press the adhesive sheet to a transparent polyimide film with a thickness of 80 μm (manufactured by Kolon Industries, Ltd., Korea). Using a tensile tester, the test piece was peeled off from the polyimide film under the conditions of a tensile speed of 60 mm/min and a peeling angle of 180° at a temperature of 25°C, and the peeling force was measured.

[Bend retention test] Using a planar unloaded U-shaped stretching tester (manufactured by YUASA SYSTEM), bending jigs were installed and fixed from both ends of the longitudinal direction of the evaluation samples produced in the examples and comparative examples within a range of 20 mm. (The central 60 mm area in the longitudinal direction is not fixed), so that the surface on the side of the PET film becomes the inner side, it is held in a curved state with a bending radius of 1.3 mm and a bending angle of 180°, at a temperature of 60°C, relative to It was kept in a constant temperature and humidity tank with a humidity of 95% for 240 hours, and the bending retention test was carried out.

The sample after the bending retention test was visually observed, and the presence or absence of peeling at the interface between the transparent polyimide film and the polarizing plate in the bending portion was confirmed. It was confirmed that peeling (voids) occurred from the ends in the short-side direction of the samples in all those who peeled. When peeling was confirmed, the length (mm) of the void portion in the short-side direction of the sample was measured. When peeling was observed in the entire length of the short side of the sample, the length of the void portion (void distance) was set to 35 mm, and when no peeling was observed at all, the void distance was set to 0. About the thing which peeled from both ends of the transversal direction, let the length of a void part be larger as a void distance. In addition, peeling was not confirmed at the bonding interface of the PET film and the polarizing plate in any of the samples.

[Evaluation Results] In the Examples and Comparative Examples, the composition of the adhesive composition used in the production of the first adhesive sheet is shown in Table 1, and the evaluation results of the first adhesive sheet and the evaluation results of the bending retention test are shown in Table 1. shown in Table 2. Table 2 also shows the measurement results of the dielectric constant of the second adhesive sheet used for bonding the polarizing plate and the PET film in the sample for the bending retention test. Table 3 shows the measurement results of the dielectric constants of the first adhesive sheets of Examples 1, 4, 5 and Comparative Example 3 in the temperature range of -40°C to 80°C, and the relative dielectric constants at various frequencies The ratio X of the minimum value to the maximum value, and the value of the ratio X _{1 kHz} /X _{1 MHz} of the value of X at a frequency of 1 kHz (X _{1 kHz} ) to the value of X at a frequency of 1 MHz (X _{1 MHz} ).

In Table 1, each component is described by the following abbreviation. LA: Lauryl Acrylate 2HEA: 2-ethylhexyl acrylate BA: Butyl Acrylate CHA: cyclohexyl acrylate 4HBA: 4-hydroxybutyl acrylate 2HEA: 2-hydroxyethyl acrylate NVP: N-vinyl-2-pyrrolidone

[Table 1] Prepolymer (base polymer) composition added after LA 2EHA BA CHA 4HBA 2HEA NVP HDDA Oligomer Example 1 43 44 - - 6 - 7 0.07 3 Example 2 43 44 - - 6 - 7 0.07 1 Example 3 43 44 - - 6 - 7 0.07 - Example 4 34 57 - - 8 - 1 0.08 0.5 Example 5 36 61 - - 2 - 1 0.08 0.5 Comparative Example 1 38 46 - - 11 - 5 0.07 3 Comparative Example 2 43 44 - - 6 - 7 0.14 3 Comparative Example 3 - - 57 12 twenty three 8 - 0.13 -

[Table 2] Adhesive sheet physical properties Subsequent evaluation Gel fraction (%) Tg (°C) G'(kPa) Tanδ Relative dielectric constant (25℃) Adhesion force (N/10 mm) Clearance distance (mm) 25℃ 100℃ 25℃ 100℃ Difference 1 kHz 10kHz 100kHz 1 MHz 1KHz/1MHz Example 1 72 -33 30 10 0.34 0.34 0.00 4.6 4.4 3.8 3.3 1.41 3.3 1.0 Example 2 71 -34 30 11 0.33 0.33 0.00 4.6 4.4 3.8 3.3 1.41 2.8 1.0 Example 3 73 -34 29 10 0.33 0.33 0.00 4.6 4.4 3.8 3.3 1.41 2.2 2.0 Example 4 69 -43 twenty four 11 0.41 0.36 0.05 3.7 3.7 3.4 3.1 1.20 4.2 0 Example 5 67 -48 19 6 0.49 0.51 -0.02 3.3 3.2 3.1 2.8 1.17 3.8 0.3 Comparative Example 1 74 -35 27 9 0.35 0.34 0.01 5.1 4.7 4.0 3.4 1.50 3.0 0.5 Comparative Example 2 87 -33 31 18 0.30 0.15 0.15 4.6 4.4 3.8 3.3 1.41 2.2 30 Comparative Example 3 88 -twenty one 130 81 0.22 0.15 0.07 7.3 7.2 6.8 5.8 1.26 6.0 35 second adhesive sheet 4.2 4.2 4.1 3.9 1.08

[table 3] frequency -40℃ -20℃ 0℃ 20℃ 25℃ 40℃ 60℃ 80℃ X X _{1 kHz} /X _{1 MHz} Example 1 1 kHz 4.4 4.6 4.6 4.7 4.6 4.5 4.3 4.1 1.13 0.92 10kHz 4.2 4.2 4.3 4.4 4.4 4.5 4.3 4.1 1.08 100kHz 3.6 3.6 3.7 3.8 3.8 4.1 4.1 4.1 1.16 1 MHz 3.1 3.1 3.2 3.3 3.3 3.5 3.6 3.8 1.23 Example 4 1 kHz 3.8 3.8 3.8 3.7 3.7 3.8 3.8 3.8 1.03 1.00 10kHz 3.7 3.7 3.7 3.7 3.7 3.7 3.8 3.7 1.03 100kHz 3.4 3.4 3.4 3.4 3.4 3.5 3.5 3.5 1.03 1 MHz 3.0 3.0 3.0 3.1 3.1 3.1 3.1 3.1 1.03 Example 5 1 kHz 3.4 3.4 3.5 3.6 3.3 3.3 3.3 3.2 1.10 1.01 10kHz 3.3 3.3 3.5 3.5 3.2 3.3 3.2 3.2 1.09 100kHz 3.1 3.1 3.2 3.3 3.1 3.1 3.2 3.2 1.08 1 MHz 2.8 2.8 2.9 3.0 2.8 2.8 2.9 3.0 1.09 Comparative Example 3 1 kHz 5.2 7.0 7.9 7.4 7.3 7.0 6.7 6.4 1.53 0.97 10kHz 4.9 6.1 7.9 7.3 7.2 7.0 6.6 6.2 1.63 100kHz 4.4 5.4 5.9 6.8 6.8 6.8 6.5 6.2 1.54 1 MHz 4.0 4.8 4.9 5.7 5.8 6.3 6.3 6.1 1.58

In Examples 1 to 5, the relative dielectric constants of the first adhesive sheet and the second adhesive sheet at a frequency of 10 MHz were both 4.5 or less, and the void distances were both 2 mm or less. It can be seen from Table 3 that the relative permittivity of the first adhesive sheet of the embodiment is small, and the temperature dependence and frequency dependence of the relative permittivity are also small.

11: Adhesive sheet 12: Adhesive sheet 13: Adhesive sheet 14: Adhesive sheet 31: polarizer (circular polarizer) 41: Touch panel 51: Image display panel (organic EL panel) 54: Image display panel (touch panel integrated organic EL panel) 71: Cover window 75: Shell 101: Video display device 102: Video display device

FIG. 1 is a cross-sectional view showing a configuration example of an image display device. FIG. 2 is a cross-sectional view showing a configuration example of an image display device.

11: Adhesive sheet

12: Adhesive sheet

13: Adhesive sheet

14: Adhesive sheet

31: polarizer (circular polarizer)

41: Touch panel

51: Image display panel (organic EL panel)

71: Cover window

75: Shell

101: Video display device

Claims (18)

An image display device, which is provided with a touch panel within a distance of 500 μm from a touch surface and can be bent, and A polarizing plate and a cover window are provided in sequence on the viewing side of the image display panel. A first adhesive sheet is arranged on the visible side surface of the polarizer, and a second adhesive sheet is arranged on the side surface of the image display panel of the polarizer, The relative dielectric constants of the first adhesive sheet and the second adhesive sheet at a temperature of 25° C. and a frequency of 10 kHz are both below 4.5. The image display device of claim 1, wherein the ratio of the relative permittivity of the first adhesive sheet and the second adhesive sheet at a temperature of 25°C and 1 kHz to the relative permittivity at a frequency of 1 MHz is average is 1.50 or less. The image display device according to claim 1 or 2, wherein the maximum values of the relative permittivity of the first adhesive sheet and the second adhesive sheet at a frequency of 10 kHz and within a temperature range of -40°C to 80°C are both 1.4 times or less of the minimum value. The image display device according to claim 1 or 2, wherein the maximum value of the relative permittivity of the first adhesive sheet and the second adhesive sheet at a frequency of 1 kHz and within a temperature range of -40°C to 80°C is equal to The ratio of the minimum value is 0.8 to 1.2 times the ratio of the maximum value to the minimum value of the relative permittivity in the temperature range of -40°C to 80°C at a frequency of 1 MHz. The image display device according to claim 1 or 2, wherein the thickness of the cover window is 100 μm or less. The image display device of claim 1 or 2, wherein the thickness of the first adhesive sheet is greater than the thickness of the second adhesive sheet, The thickness of the above-mentioned first adhesive sheet is 100 μm or less. The image display device according to claim 1 or 2, wherein the storage modulus G'25 of the first adhesive sheet at ₂₅ °C and 1 Hz is below 70 kPa, and the glass transition temperature is below -20°C. The image display device of claim 1, wherein the first adhesive sheet comprises an acrylic adhesive containing an acrylic base polymer. The image display device according to claim 8, wherein the acrylic base polymer contains 5 to 55 parts by weight of (meth)acrylic acid C _10-20 chain alkyl ester with respect to 100 parts by weight of the total monomer components. The image display device according to claim 9, wherein the above-mentioned acrylic base polymer comprises lauryl acrylate as the above-mentioned C _10-20 chain alkyl ester of (meth)acrylate. The image display device according to any one of claims 8 to 10, wherein the acrylic base polymer contains a monomer selected from the group consisting of a hydroxyl group-containing monomer, a carboxyl group-containing monomer, and a nitrogen-containing monomer relative to 100 parts by weight of the total monomer components. One or more kinds of polar group-containing monomers in the formed group contain 2 to 15 parts by weight of the monomer. The image display device according to any one of claims 8 to 10, wherein in the acrylic base polymer, the content of the hydroxyl group-containing monomer is 10 parts by weight or less with respect to 100 parts by weight of the total monomer components. The image display device according to any one of claims 8 to 10, wherein the acrylic base polymer has a cross-linked structure. The image display device according to claim 13, wherein the cross-linked structure is a cross-linked structure introduced by polyfunctional (meth)acrylate. The image display device according to any one of claims 8 to 10, wherein the acrylic adhesive further comprises an acrylic oligomer having a glass transition temperature of 60°C or higher. The image display device according to claim 15, wherein the content of the acrylic oligomer is 0.1 to 5 parts by weight relative to 100 parts by weight of the acrylic base polymer. The image display device according to claim 1 or 2, wherein a touch panel is provided inside the image display panel. The image display device according to claim 1 or 2, wherein a touch panel is provided between the image display panel and the polarizing plate.

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