Adhesive sheets and films with adhesive layers

Abstract

In a foldable image display device equipped with a touch panel within 500 μm of the touch surface, the present invention provides an adhesive sheet used for bonding components placed between the touch surface and the touch panel, which has high adhesive strength and contributes to suppressing malfunctions of the touch panel. [Solution] The adhesive sheet has a dielectric constant of 4.5 or less at a temperature of 25°C and a frequency of 10kHz. The adhesive sheet is composed of an acrylic adhesive containing an acrylic base polymer, and preferably has a gel fraction of 55-80%. The acrylic base polymer contains (meth)acrylic acid C per 100 parts by weight of the total monomer components. 10-20 Preferably, the product contains 5 to 55 parts by weight of a linear alkyl ester and 2 to 8 parts by weight of a hydroxyl group-containing monomer.

Description

[Technical Field]

[0001] The present invention relates to an adhesive sheet suitably used for bonding components together in a foldable image display device. Furthermore, the present invention relates to an adhesive-layered film in which an adhesive sheet is laminated on at least one surface of a polarizing plate. [Background technology]

[0002] Flat panel displays, such as liquid crystal displays and organic EL displays, are used as image display devices in mobile phones, smartphones, tablet devices, car navigation systems, PC monitors, and televisions. In recent years, organic EL panels using bendable substrates (flexible substrates) such as resin films have been put into practical use, and flexible displays that can be bent have been proposed.

[0003] In flexible displays, in addition to the display panel such as an organic EL panel being bendable, the components such as the housing and touch panel are also bendable, and these components are bonded together via an adhesive sheet (for example, Patent Document 1). In a bendable flexible display (foldable display), the transparent plate (cover window) placed on the viewing surface must also be bendable, and thin materials such as resin film or thin glass are used.

[0004] In foldable displays, bending occurs repeatedly in the same location. At the bending point, compressive stress is applied to the inside and tensile stress to the outside, causing strain in and around the bending point, which raises concerns about device failure. Therefore, it has been proposed to soften the adhesive sheet that bonds the components together to alleviate stress strain (for example, Patent Document 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-2764 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-45213 [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] In recent years, there has been a demand for thinner foldable display devices. In particular, even when the distance from the device surface to the touch panel sensor is small, a highly reliable device with almost no or no malfunction of the touch panel sensor and suppression or prevention of failures and the like is required.

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

[0008] The adhesive sheet of the present invention is used for bonding between members arranged between a touch surface and a touch panel in a foldable image display device provided with a touch panel within a distance of 500 μm from the touch surface. The relative permittivity of the adhesive sheet at a temperature of 25 °C and a frequency of 10 kHz is 4.5 or less.

[0009]

[0010] At a temperature of 25 °C, the ratio of the relative permittivity at 1 kHz to the relative permittivity at 1 MHz of the adhesive sheet is preferably 1.50 or less. The adhesive sheet preferably has a maximum relative permittivity in the temperature range of -40 °C to 80 °C at a frequency of 10 kHz that is 1.4 times or less the minimum value. The ratio of the maximum value to the minimum value of the relative permittivity of the adhesive sheet in the temperature range of -40 °C to 80 °C at a frequency of 1 kHz is preferably 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.

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

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

[0013] The adhesive sheet is composed of an acrylic adhesive containing an acrylic-based polymer. The acrylic-based polymer is composed of (meth)acrylate C per 100 parts by weight of the total monomer components. 10-20 It may also contain 5 to 55 parts by weight of a linear alkyl ester. The acrylic base polymer is (meth)acrylate C 10-20 The linear alkyl ester may include lauryl acrylate.

[0014] The acrylic-based polymer may contain 2 to 15 parts by weight of one or more polar group-containing monomers selected from the group consisting of hydroxyl group-containing monomers, carboxyl group-containing monomers, and nitrogen-containing monomers, per 100 parts by weight of the total monomer components. Preferably, the acrylic-based polymer contains 2 to 10 parts by weight of hydroxyl group-containing monomers per 100 parts by weight of the total monomer components.

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

[0016] 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 per 100 parts by weight of the acrylic base polymer may be 0.1 to 5 parts by weight.

[0017] The adhesive sheet of the present invention may be laminated onto at least one surface of a polarizing plate to form an adhesive-coated film. [Effects of the Invention]

[0018] The image display device using the adhesive sheet of the present invention reduces touch panel malfunctions and exhibits high reliability even when held in a bent state. [Brief explanation of the drawing]

[0019] [Figure 1] This is a cross-sectional view showing an example of the configuration of an adhesive sheet with a release film. [Figure 2] This is a cross-sectional view showing an example of the configuration of an image display device. [Figure 3] This is a cross-sectional view showing an example of the configuration of an image display device. [Figure 4] This is a cross-sectional view showing an example of a laminated polarizing plate with an adhesive sheet. [Figure 5] This is a cross-sectional view showing an example of a laminated polarizing plate with an adhesive sheet. [Modes for carrying out the invention]

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

[0021] In the image display device 101 shown in Figure 2, an organic EL panel 51, a touch panel 41, and a circular polarizing plate 31 are arranged between the housing 75 and the cover window 71. The organic EL panel 51 and the bottom surface of the housing 75 are bonded together via an adhesive sheet 14, the organic EL panel 51 and the touch panel 41 are bonded together via an adhesive sheet 13, the touch panel 41 and the circular polarizing plate 31 are bonded together via an adhesive sheet 12, and the circular polarizing plate 31 and the cover window 71 are bonded together via an adhesive sheet 11. In this way, in a flexible display, multiple components are bonded together via adhesive sheets, resulting in a laminated, integrated structure.

[0022] The cover window 71 is positioned on the viewing surface of the flexible display and constitutes the touch surface. The touch panel 41 is a capacitive touch panel. In a flexible display, a foldable cover window 71 is used, so the thickness of the cover window is small, and consequently, the distance D from the touch surface (the surface of the cover window 71) to the touch panel 41 is also small.

[0023] The image display device 102 shown in Figure 3 is equipped with an organic EL panel 54 with an integrated touch panel, and a circular polarizing plate 31 is attached to the organic EL panel 54 via an adhesive sheet 12. The rest of the configuration is the same as in Figure 2, and the distance from the touch surface to the viewing-side surface of the organic EL panel 54 corresponds to the distance D from the touch surface to the touch panel.

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

[0025] [Characteristics of adhesive sheets] The adhesive sheet of the present invention has a relative permittivity of 4.5 or less at a temperature of 25°C and a frequency of 10kHz. Unless otherwise specified below, the permittivity is measured at a temperature of 25°C. The relative permittivity of the adhesive sheet at a frequency of 10kHz may be 4.0 or less, 3.8 or less, or 3.5 or less. By setting the relative permittivity at a frequency of 10kHz to 4.5 or less, the distance from the touch surface to the touch panel can be reduced, enabling a design that is advantageous for flexibility. Because adhesive sheets with low permittivity have a small capacitance value, it is possible to design sensors with high sensitivity. This enables input methods with a small contact area, such as pen input. Furthermore, by placing an adhesive sheet with low permittivity between the organic EL panel and the touch panel, noise from the organic EL panel can be reduced, thus preventing malfunctions of the touch panel.

[0026] The relative permittivity of the adhesive sheet at a frequency of 1 kHz is preferably 5.0 or less, more preferably 4.8 or less, and may be 4.5 or less, 4.0 or less, or 3.8 or less. The relative permittivity of the adhesive sheet at a frequency of 100 kHz is preferably 4.0 or less, and may be 3.8 or less or 3.5 or less. The relative permittivity of the adhesive sheet at a frequency of 1 MHz is preferably 3.5 or less, and may be 3.3 or less or 3.2 or less.

[0027] The dielectric constant changes depending on the polarizability of the material, and the dielectric constant of an adhesive sheet can be controlled by selecting the adhesive material, such as urethane, acrylic, rubber, or silica. Furthermore, since the relative permittivity of air is 1, the dielectric constant of an adhesive sheet can be reduced by adding hollow beads or similar materials to the adhesive. In acrylic adhesives, monomers with long alkyl chains have low polarizability, allowing for low dielectric constant. Using highly polar monomers results in a high polarizability and therefore a high dielectric constant. One method to reduce polarizability is to induce molecular entanglement. Increasing the molecular weight or degree of crosslinking facilitates molecular entanglement, lowering the polarizability and thus enabling low dielectric constant. Since the dielectric constant tends to increase with water content, using materials that do not easily retain moisture can reduce the dielectric constant.

[0028] According to the Clausius-Mossotti equation, the smaller the polarizability of an electric dipole and the smaller the number of electric dipoles per unit volume, the smaller the relative permittivity. To reduce the relative permittivity of an adhesive sheet, the dipole moment of the base polymer constituting the adhesive should be reduced and the molar volume increased. For example, the larger the volume of the side chains of the base polymer, the larger the molar volume tends to be. Also, by selecting monomers with low polarity as monomer components constituting the base polymer, the electron dipoles of the molecule can be reduced.

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

[0030] The adhesive sheet preferably has a small temperature dependence of relative permittivity. Specifically, the ratio (maximum value / minimum value) of the minimum value to the maximum value of the relative permittivity in the temperature range of -40°C to -80°C is preferably close to 1. The ratio X of the maximum value to the minimum value of the relative permittivity at a frequency of 10 kHz 10kHz 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 X of the maximum value to the minimum value of the relative permittivity at a frequency of 1 kHz 1kHz and the ratio X of the maximum value to the minimum value of the relative permittivity at a frequency of 100 kHz 100kHz and the ratio X of the maximum value to the minimum value of the relative permittivity at a frequency of 1 MHz 1MHz are each 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.

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

[0032] The above X 1MHz and X 1kHz and the ratio X of them 1kHz / X 1MHzThe value is preferably 0.8 to 1.2, more preferably 0.85 to 1.15, even more preferably 0.9 to 1.1, and may also be 0.95 to 1.05. 1kHz / X 1MHz The closer this value is to 1, the smaller the change in relative permittivity over a wide temperature and frequency range, thus ensuring operational reliability over a wide temperature and operating frequency range.

[0033] The adhesive strength of the adhesive sheet is preferably 2.7 N / 10 mm or higher, more preferably 2.8 N / 10 mm or higher, and may be 3.0 N / 10 mm or higher. The adhesive strength is determined by a peel test using a polyimide film as the adherend, at a tensile speed of 60 mm / min and a peel angle of 180°. Unless otherwise specified, the adhesive strength is measured at 25°C. By having the adhesive strength of the adhesive sheet within the above range, peeling of the adhesive sheet from the adherend when repeated bending can be prevented.

[0034] In the present invention, the adhesive sheet has a gap length of 2 mm or less, as determined by the bending and holding test described later (i.e., the gap length between the adhesive sheet and the adherend after 240 hours of bending and holding; hereinafter sometimes simply referred to as "gap distance"). When the gap distance is 2 mm or less, even in a foldable image display device equipped with a touch panel within 500 μm of the touch surface, the adhesive sheet can absorb the stress caused by bending when the image display device is bent, thereby improving the reliability of the image display device. Therefore, even if the distance from the device surface to the touch panel sensor is short, malfunctions of the touch panel sensor are almost or completely eliminated, failures are suppressed or prevented, and reliability is high.

[0035] The void distance is preferably 1.5 mm or less, more preferably 1.0 mm or less, and may be 0.8 mm or less, 0.5 mm or less, or 0.3 mm or less. The lower limit of the void distance is not particularly limited and may be 0.

[0036] The void distance is measured according to the following steps A to D. Step A: Prepare a 35mm x 100mm test specimen by attaching an adhesive sheet to the substrate; Step B: Bend the specimen prepared in Step A along the short side with a bending radius of 1.3 mm and a bending angle of 180°; Step C: The specimen bent in Step B is held in a bent state for 240 hours in an environment with a temperature of 60 degrees Celsius and a relative humidity of 95%. Step D: Measure the length of the gap between the adhesive sheet and the adherend in the short-side direction at the bent portion of the test specimen after holding it for 240 hours in Step C.

[0037] In the bending retention test, delamination (voids) is most likely to occur from the end of the bending axis of the test specimen (the end in the short-side direction). If voids occur from both ends or if voids exist in multiple locations, the length of the longest void in the short-side direction shall be used as the void distance. If voids exist in multiple locations, the longest void should be 2 mm or less. Preferably, the sum of the lengths of each void is 2 mm or less, and the sum of the void lengths may be 1.5 mm or less, 1.0 mm or less, 0.8 mm or less, 0.5 mm or less, or 0.3 mm or less, or even 0.

[0038] When a material is bent, the adherend expands and contracts, and if the adhesive cannot keep up with the deformation of the adherend, peeling occurs. For example, the ability of the adhesive to follow the adherend can be adjusted by controlling the storage modulus G' of the adhesive. The smaller the storage modulus of the adhesive, the better its ability to follow the deformation of the adherend. When a bent state is maintained, peeling may occur due to stress concentration associated with the deformation of the bent part. To improve the stress relaxation properties of the adhesive, the loss tangent tanδ should be increased. In addition, to suppress peeling at the interface between the adherend and the adhesive, it is necessary to design the adhesive strength at the bending temperature to be high. Furthermore, in a high-humidity environment, the retention of moisture at the interface between the adhesive and the adherend can cause a decrease in adhesive strength, so it is preferable to use a material that does not easily retain moisture to suppress moisture retention.

[0039] The adhesive sheet has a storage modulus G' at 25°C. 25 It is preferable that the pressure is 70 kPa or less. 25A pressure of 70kPa or less tends to reduce strain when the device is bent, thereby suppressing damage to the device components when repeatedly bent. 25 It is preferable that the pressure be 5 kPa or higher, and within this range, both the adhesive holding strength and strain relaxation of the adhesive sheet can be achieved, thus reducing the void distance. From the viewpoint of more effectively balancing processability, adhesive holding strength, and strain relaxation, the G' of the adhesive sheet 25 The pressure is preferably 10-60 kPa, more preferably 13-50 kPa, and even more preferably 15-40 kPa.

[0040] Storage modulus G' of adhesive sheet at 100°C 100 The pressure is preferably 2 to 50 kPa, more preferably 3 to 40 kPa, and even more preferably 5 to 25 kPa. G' of the adhesive sheet 100 If the above range is maintained, both adhesive holding power and stress relaxation can be achieved even in high-temperature environments, thus reducing the void distance.

[0041] The adhesive sheet has a loss tangent tanδ at 25°C. 25 It is preferable that the value is 0.2 to 0.45. Also, the adhesive sheet has a loss tangent tanδ at 100°C. 100 It is preferable that it is between 0.2 and 0.4. Also, tanδ 25 and tanδ 100 The difference between these values ​​is preferably -0.07 to 0.07. tanδ 25 tanδ may be 0.25 to 0.42. 100 It may be 0.25 to 0.38. Also, tanδ 25 and tanδ 100 The difference may be within ±0.06 or ±0.05. (tanδ of the adhesive sheet) 25 tanδ 100 , and tanδ 25 and tanδ 100 The void distance can be reduced by keeping the difference within the above range. The tanδ of the adhesive sheet can be adjusted by optimizing the material monomer and the degree of crosslinking. For example, the material monomer can be selected so that the glass transition temperature and molecular weight of the base polymer are within an appropriate range.

[0042] The storage modulus G' and loss tangent tanδ of an adhesive sheet are determined by viscoelastic measurement at a frequency of 1 Hz. tanδ is the ratio G'' / G' of the storage modulus G' to the loss modulus G''. The storage modulus G' corresponds to the portion of the material that stores elastic energy when it deforms, and is an indicator of the degree of hardness.

[0043] To minimize the temperature dependence of tanδ in the range from room temperature to high temperatures, the glass transition temperature of the adhesive sheet is preferably -20°C or lower, more preferably -23°C or lower, and even more preferably -25°C or lower. The glass transition temperature is the temperature at which tanδ is maximum (peak top temperature). Near the glass transition temperature, the temperature dependence of tanδ is large. By setting the glass transition temperature sufficiently lower than the ambient temperature of the device, the temperature dependence of tanδ in the ambient temperature range is reduced. Furthermore, because the glass transition temperature is within the above range, the adhesive sheet maintains adhesive strength even in the low-temperature range, suppressing peeling from the adherend at low temperatures and allowing for a reduction in the void distance.

[0044] The lower limit of the glass transition temperature of the adhesive sheet is not particularly limited, but is generally -80°C or higher. The glass transition temperature of the adhesive sheet is preferably -70°C or higher, more preferably -60°C or higher, and may also be -55°C or higher, or -50°C or higher. By setting the glass transition temperature of the adhesive sheet within the above range, the adhesive holding strength can be effectively increased and the void distance can be reduced.

[0045] The thickness of the adhesive sheet is not particularly limited and can be adjusted as appropriate depending on the type of substrate, the position where the adhesive sheet is placed within the device (lamination configuration), the thickness of the target device, and the required properties of the adhesive sheet. From the viewpoint of increasing the adhesive strength of the adhesive sheet, a thickness of 10 μm or more is preferable. From the viewpoint of thinning the device and suppressing the leakage of adhesive from the edges during processing of the adhesive sheet and bending of the device, the thickness of the adhesive sheet is preferably 100 μm or less, and more preferably 75 μm or less.

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

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

[0048] [Composition and preparation method of adhesive sheets] The adhesive for forming the adhesive sheet of the present invention is not particularly limited as long as the dielectric constant and void distance are within the above range, and examples of base polymers include polymers such as acrylic, silicone, polyester, polyurethane, polyamide, polyvinyl ether, vinyl acetate / vinyl chloride copolymer, modified polyolefin, epoxy, fluorine, and rubber. As the adhesive, an acrylic adhesive mainly composed of an acrylic base polymer is preferred because it is possible to adjust the dielectric constant and void distance to the above predetermined range, as well as control transparency and adhesive strength. The adhesive can be used alone or in combination of two or more types. Furthermore, the adhesive sheet formed by the adhesive may be in the form of a single layer or a laminated form.

[0049] <Acrylic-based polymer> Acrylic-based polymers contain alkyl (meth)acrylate as their main constituent monomer component. In this specification, "(meth)acrylic" means acrylic and / or methacrylic.

[0050] (Meth)acrylate alkyl esters include (meth)acrylate C, where the alkyl group has 1 to 20 carbon atoms. 1-20 Alkyl esters are preferably used. The alkyl group in (meth)acrylate alkyl ester may be in a linear or cyclic form. The linear alkyl group may be a straight-chain alkyl group or a branched alkyl group.

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

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

[0053] The acrylic-based polymer preferably contains 60 to 100 parts by weight of alkyl (meth)acrylate ester per 100 parts by weight of total monomer components, and more preferably 70 to 98 parts by weight.

[0054] The acrylic-based polymer is an alkyl (meth)acrylate, and C (meth)acrylate. 10-20 It is preferable to include a chain-like alkyl ester. By including a long-chain alkyl (meth)acrylate as a monomer component in the acrylic base polymer, the molecular dipole moment can be reduced and the molar volume can be increased, thus lowering the dielectric constant. Furthermore, a homopolymer of alkyl (meth)acrylate having a long-chain alkyl group with 10 or more carbon atoms has a temperature range (plateau region) where the temperature dependence of viscoelasticity is small at temperatures higher than Tg. Therefore, when the base polymer includes a long-chain alkyl (meth)acrylate as a monomer component, the temperature dependence of tanδ can be reduced.

[0055] (meth)acrylic acid C has a wide temperature range in the plateau region and a low storage modulus in the plateau region. 10-20 Among linear alkyl esters, (meth)acrylate C10-16 Alkyl esters are preferred, and (meth)acrylate C 10-13 Alkyl esters are more preferred. In particular, (meth)acrylate C 12 Alkyl esters are preferred, and dodecyl acrylate (lauryl acrylate) is particularly preferred.

[0056] Polymers of long-chain alkyl esters of (meth)acrylic acid are characterized by a wide temperature range in the plateau region and a low storage modulus in the plateau region, but they have high crystallinity and a high glass transition temperature. For example, the glass transition temperature of homopolymers of lauryl acrylate is 0°C. In order to lower the glass transition temperature of the base polymer, (meth)acrylic acid C is used as the monomer component. 10-20 In addition to linear alkyl esters, (meth)acrylate C 1-9 It is preferable that the material contains a linear alkyl ester.

[0057] (meth)acrylic acid C 1-9 Among linear alkyl esters, those with a homopolymer glass transition temperature of -40°C or lower are preferred in order to achieve a low Tg of the base polymer. (meth)acrylic acid C has a homopolymer glass transition temperature of -40°C or lower. 1-9 Specific examples of linear alkyl esters include 2-ethylhexyl acrylate (Tg: -70°C), n-hexyl acrylate (Tg: -65°C), n-octyl acrylate (Tg: -65°C), isononyl acrylate (Tg: -60°C), n-nonyl acrylate (Tg: -58°C), isooctyl acrylate (Tg: -58°C), and butyl acrylate (Tg: -55°C). Among these, butyl acrylate and 2-ethylhexyl acrylate are preferred, and 2-ethylhexyl acrylate is particularly preferred due to its low Tg.

[0058] To obtain an adhesive having the above-mentioned properties, (meth)acrylic acid C is used as the monomer component of the acrylic-based polymer. 10-20 Chain-like alkyl esters and (meth)acrylate C 1-9It is preferable to include both linear alkyl esters and adjust the ratio of the two.

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

[0060] Acrylic-based polymers may contain nitrogen-containing monomers as monomer components. Examples of nitrogen-containing monomers include vinyl monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, (meth)acryloylmorpholine, N-vinyl carboxylic acid amides, and N-vinylcaprolactam, as well as cyanoacrylate monomers such as acrylonitrile and methacrylonitrile. Among these, N-vinylpyrrolidone is preferred because it has a high effect in improving adhesive strength by improving cohesiveness.

[0061] The acrylic base polymer may have a crosslinked structure introduced into it. The crosslinking of the acrylic base polymer allows for high adhesive strength even when the G' of the adhesive sheet is small. To introduce a crosslinked structure into the acrylic base polymer, it is preferable that the acrylic base polymer contains, in addition to the above-mentioned alkyl (meth)acrylate, hydroxyl group-containing monomers and carboxyl group-containing monomers as monomer components. When introducing a crosslinked structure into the acrylic base polymer using isocyanate-based crosslinking agents or epoxy-based crosslinking agents, hydroxyl groups and carboxyl groups serve as points for introducing the crosslinked structure.

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

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

[0064] From the viewpoint of improving the adhesive strength and adhesive retention of the adhesive sheet, the amount of polar group-containing monomer per 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 2 parts by weight or more, and may be 3 parts by weight or more, 4 parts by weight or more, or 5 parts by weight or more. On the other hand, as the content of polar monomer increases, the dipole moment of the base polymer increases and the dielectric constant increases. Also, if the content of polar monomer is excessively high, the glass transition temperature of the polymer increases, and the adhesive strength at low temperatures tends to decrease. For this reason, the amount of polar group-containing monomer per 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 15 parts by weight or less, and may be 13 parts by weight or less, or 10 parts by weight or less.

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

[0066] Including a hydroxyl group-containing monomer as a polar monomer component improves the adhesive strength of the adhesive sheet and tends to suppress clouding of the adhesive sheet in high humidity environments. Therefore, the amount of hydroxyl group-containing monomer per 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, and may be 2 parts by weight or more. On the other hand, as the content of hydroxyl group-containing monomer (amount of hydroxyl groups in the adhesive) increases, the dielectric constant of the adhesive tends to increase significantly. Therefore, the amount of hydroxyl group-containing monomer per 100 parts by weight of the total monomer components of the acrylic base polymer is preferably 10 parts by weight or less, more preferably 8 parts by weight or less, and may be 6 parts by weight or less. If the amount of hydroxyl group-containing monomer is within the above range, the dielectric constant and void distance can be within the above range.

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

[0068] To prevent corrosion of touch panel electrodes by acidic components, adhesive sheets preferably have a low acid content. Furthermore, when adhesive sheets are used to bond polarizing plates, a low acid content is preferable to suppress polyene formation of polyvinyl alcohol-based polarizers by acidic components. Such acid-free adhesive sheets preferably contain 100 ppm or less of organic acid monomers such as (meth)acrylic acid, more preferably 70 ppm or less, and even more preferably 50 ppm or less. The organic acid monomer content of an adhesive sheet can be determined by immersing the sheet in pure water, heating it at 100°C for 45 minutes, and quantifying the acid monomers extracted into the water using ion chromatography.

[0069] To reduce the acid monomer content in the adhesive sheet, it is preferable that the amount of organic acid monomer components such as (meth)acrylic acid in the monomer components constituting the base polymer is small. Therefore, in order to make the adhesive sheet acid-free, it is preferable that the base polymer substantially does not contain organic acid monomers (carboxyl group-containing monomers) as monomer components. In an acid-free adhesive sheet, the amount of carboxyl group-containing monomers per 100 parts by weight of the total monomer components of the base polymer is preferably 0.5 parts by weight or less, more preferably 0.1 parts by weight or less, even more preferably 0.05 parts by weight or less, and ideally 0.

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

[0071] The theoretical Tg of the acrylic-based polymer is preferably -60 to -20°C. More preferably -23°C or lower, even more preferably -25°C or lower, and particularly preferably -30°C or lower. The theoretical Tg of the acrylic-based polymer may also be -50°C or lower, -45°C or lower, -40°C or lower, or -38°C or lower. If the theoretical Tg of the base polymer is within the above range, the dielectric constant and void distance can be within the above range. The theoretical Tg is the glass transition temperature (Tg) of the homopolymer of the constituent monomer components of the acrylic-based polymer. i And the weight fraction W of each monomer component i Therefore, it is calculated using the following Fox formula. 1 / Tg = Σ(W i / Tg i )

[0072] Tg is the glass transition temperature of the polymer chain (unit: K), W i This is the weight fraction (weight-based copolymerization ratio) of monomer component i that constitutes the segment, Tg iis the glass transition temperature (in K) of the homopolymer of monomer component i. The values ​​listed in the Polymer Handbook, 3rd edition (John Wiley & Sons, Inc., 1989) can be used as the glass transition temperature of homopolymers. For homopolymers of monomers not listed in the above literature, the peak top temperature of tanδ obtained by dynamic viscoelasticity measurement can be used.

[0073] <Cross-linked structure of the base polymer> As described above, the acrylic base polymer may have a crosslinked structure. Introducing a crosslinked structure to the base polymer increases the gel fraction of the adhesive. The gel fraction of the adhesive sheet is preferably 55-85%, more preferably 60-80%, even more preferably 63-77%, and particularly preferably 65-75%. By adjusting the gel fraction to this range, high adhesive holding power can be achieved even when G' is small and the adhesive sheet is soft, thus reducing the void distance mentioned above.

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

[0075] Methods for introducing a crosslinked structure into a base polymer include (1) a method in which a base polymer having a functional group that can react with a crosslinking agent is polymerized, and then a crosslinking agent is added to react the base polymer with the crosslinking agent; and (2) a method in which a branched structure (crosslinked structure) is introduced into the polymer chain by including a polyfunctional compound in the polymerization components of the base polymer. Multiple types of crosslinked structures may also be introduced into the base polymer by using a combination of these methods.

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

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

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

[0079] In the method of including a polyfunctional monomer as the polymerization component of the base polymer described in (2) above, the entire amount of monomer components constituting the acrylic base polymer and the polyfunctional compound for introducing the crosslinking structure may be reacted at once, or polymerization may be carried out in multiple steps. As a method of carrying out polymerization in multiple steps, it is preferable to polymerize the monofunctional monomer constituting the base polymer (prepolymerization) to prepare a partial polymer (prepolymer composition), and then add a polyfunctional compound such as a polyfunctional (meth)acrylate to the prepolymer composition to polymerize the prepolymer composition and the polyfunctional monomer (main polymerization). The prepolymer composition is a partial polymer containing polymers with a low degree of polymerization and unreacted monomers.

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

[0081] Examples of polyfunctional compounds used to introduce crosslinked structures include compounds containing two or more polymerizable functional groups (ethylenically unsaturated groups) having unsaturated double bonds in one molecule. As polyfunctional compounds, polyfunctional (meth)acrylates are preferred because they readily copolymerize with monomer components of acrylic-based polymers. When introducing branched (crosslinked) structures by active energy ray polymerization (photopolymerization), polyfunctional acrylates are preferred.

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

[0083] The molecular weight of polyfunctional compounds such as polyfunctional (meth)acrylates is preferably 1500 or less, and more preferably 1000 or less. The functional group equivalent (g / eq) of the polyfunctional compound is preferably 50 to 500, more preferably 70 to 300, and even more preferably 80 to 200. By having the molecular weight of the polyfunctional compound within this range, the void distance can be reduced.

[0084] <Preparation of base polymer> Acrylic-based polymers can be prepared by known polymerization methods such as solution polymerization, UV polymerization, bulk polymerization, and emulsion polymerization. Solution polymerization or active energy ray polymerization (e.g., UV polymerization) is preferred in terms of transparency, water resistance, and cost of the adhesive. Ethyl acetate and toluene are generally used as solvents for solution polymerization.

[0085] When preparing acrylic-based polymers, polymerization initiators such as photopolymerization initiators and thermal polymerization initiators may be used depending on the type of polymerization reaction. The photopolymerization initiator is not particularly limited as long as it initiates photopolymerization. For example, benzoin ether-based photopolymerization initiators, acetophenone-based photopolymerization initiators, α-ketol-based photopolymerization initiators, aromatic sulfonyl chloride-based photopolymerization initiators, photoactive oxime-based photopolymerization initiators, benzoin-based photopolymerization initiators, benzyl-based photopolymerization initiators, benzophenone-based photopolymerization initiators, ketal-based photopolymerization initiators, thioxanthone-based photopolymerization initiators, and acylphosphine oxide-based photopolymerization initiators can be used. For thermal polymerization initiators, for example, azo-based initiators, peroxide-based initiators, and redox-based initiators combining peroxides and reducing agents (for example, a combination of persulfate and sodium bisulfite, a combination of peroxide and sodium ascorbate, etc.) can be used.

[0086] During polymerization, chain transfer agents and polymerization inhibitors (polymerization retarders) may be used for purposes such as adjusting molecular weight. Examples of chain transfer agents include thiols such as α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, and 2,3-dimercapto-1-propanol, as well as α-methylstyrene dimers.

[0087] The molecular weight of the base polymer can be adjusted by controlling the type and amount of polymerization initiator. For example, in radical polymerization, a larger amount of polymerization initiator leads to a higher radical concentration in the reaction system, resulting in a higher density of reaction initiator sites and a tendency towards a lower molecular weight. Conversely, a smaller amount of polymerization initiator leads to a lower density of reaction initiator sites, allowing the polymer chain to elongate more easily and resulting in a tendency towards a higher molecular weight.

[0088] To obtain an adhesive sheet with excellent adhesion and small void distance, it is preferable that the acrylic base polymer has a high gel fraction with a low crosslinking point density. To increase the gel fraction (the ratio of polymer chains in which crosslinking structures are introduced) with a low crosslinking density, the molecular weight (polymer chain length) of the base polymer should be increased. As mentioned above, in order to increase the molecular weight of the base polymer, it is preferable to reduce the amount of polymerization initiator used when polymerizing the base polymer.

[0089] The amount of polymerization initiator used during base polymer polymerization should be appropriately set according to the type of polymerization reaction, the monomer composition, the type of 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 a small amount of crosslinking agent, the amount of polymerization initiator used is preferably 0.001 to 0.4 parts by weight, more preferably 0.003 to 0.1 parts by weight, and even more preferably 0.005 to 0.05 parts by weight, per 100 parts by weight of the total monomer components constituting the base polymer.

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

[0091] A prepolymer composition can be prepared, for example, by partially polymerizing (prepolymerizing) a composition (referred to as a "prepolymer-forming composition") which is a mixture of monomer components constituting an acrylic-based polymer and a polymerization initiator. Preferably, the monomers in the prepolymer-forming composition are monofunctional monomer components such as alkyl (meth)acrylates or polar group-containing monomers. The prepolymer-forming composition may also contain polyfunctional monomers in addition to monofunctional monomers. For example, a portion of the polyfunctional monomer may be included in the prepolymer-forming composition, and the remainder of the polyfunctional monomer component may be added after prepolymerization to carry out the main polymerization.

[0092] The polymerization rate of the prepolymer is not particularly limited, but from the viewpoint of achieving a viscosity suitable for coating onto a substrate, 3 to 50% by weight is preferred, and 5 to 40% by weight is more preferred. The polymerization rate of the prepolymer can be adjusted to a desired range by adjusting the type and amount of photopolymerization initiator used, the irradiation intensity and duration of active light such as UV light, etc.

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

[0094] The glass transition temperature of the acrylic oligomer is preferably 60°C or higher, more preferably 80°C or higher, even more preferably 100°C or higher, and particularly preferably 110°C or higher. By using a low-Tg acrylic base polymer with a cross-linked structure in combination with a high-Tg acrylic oligomer, the adhesive strength of the adhesive sheet, especially the adhesive retention strength at high temperatures, tends to improve, and the void distance can be reduced. There is no particular upper limit to the glass transition temperature of the acrylic oligomer, but it is generally 200°C or lower, preferably 180°C or lower, and more preferably 160°C or lower. The glass transition temperature of the acrylic oligomer is calculated using the Fox formula described above.

[0095] Acrylic oligomers with a glass transition temperature of 60°C or higher are preferably those that contain (meth)acrylate (linear alkyl(meth)acrylate) having a linear alkyl group and (meth)acrylate (alicyclic alkyl(meth)acrylate) having an alicyclic alkyl group as constituent monomer components. Specific examples of linear alkyl(meth)acrylate and alicyclic alkyl(meth)acrylate are as previously exemplified as constituent monomers of acrylic polymer chains.

[0096] Among the example alkyl (meth)acrylates, methyl methacrylate is preferred as the linear alkyl (meth)acrylate because it has a high glass transition temperature and excellent compatibility with the base polymer. As the alicyclic alkyl (meth)acrylate, dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate are preferred. In other words, the acrylic oligomer is preferably composed of one or more monomer components selected from the group consisting of dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate, along with methyl methacrylate.

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

[0098] The weight-average molecular weight of the acrylic oligomer is preferably 1,000 to 30,000, more preferably 1,500 to 10,000, and even more preferably 2,000 to 8,000. Using an acrylic oligomer having a molecular weight within this range tends to improve the adhesive strength and adhesion retention of the adhesive, and allows for a reduction in void distance.

[0099] Acrylic oligomers are obtained by polymerizing the above monomer components using various polymerization methods. Various polymerization initiators may be used during the polymerization of acrylic oligomers. Chain transfer agents may also be used to adjust the molecular weight.

[0100] The content of acrylic oligomers in the adhesive sheet is not particularly limited, but in order to sufficiently increase the adhesive strength, the amount of acrylic oligomers per 100 parts by weight of the base polymer is preferably 0.5 parts by weight or more, more preferably 0.8 parts by weight or more, and even more preferably 1 part by weight or more. The amount of acrylic oligomers in the adhesive sheet may be 1.3 parts by weight or more, 1.5 parts by weight or more, 1.8 parts by weight or more, 2 parts by weight or more, 2.3 parts by weight or more, or 2.5 parts by weight or more, per 100 parts by weight of the base polymer. The larger the amount of high-Tg acrylic oligomer added, the smaller the void distance tends to be.

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

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

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

[0104] The adhesive composition preferably contains 50% by weight or more of acrylic-based polymer (or prepolymer composition) relative to the total amount of nonvolatile matter, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more.

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

[0106] (Silane coupling agent) A silane coupling agent may be added to the adhesive composition. When a silane coupling agent is added to the adhesive composition, the amount added is usually about 0.01 to 5.0 parts by weight, preferably about 0.03 to 3.0 parts by weight, per 100 parts by weight of the base polymer. If the amount of silane coupling agent is within the above range, the void distance may become smaller.

[0107] (Other additives) In addition to the components exemplified above, the adhesive composition may also contain additives such as tackifiers, plasticizers, softeners, degradation inhibitors, fillers, colorants, UV absorbers, antioxidants, surfactants, and antistatic agents.

[0108] <Formation of adhesive sheet> An adhesive sheet is formed on a substrate by applying an adhesive composition to the substrate, and, if necessary, drying and removing the solvent, and / or performing polymerization by irradiation with active light. Any suitable substrate can be used for forming the adhesive sheet. The substrate may also be a release film having a release layer on the contact surface with the adhesive sheet.

[0109] Various resin materials are used as the film substrate for the release film. Examples of resin materials include polyester resins such as polyethylene terephthalate and polyethylene naphthalate, acetate resins, polyethersulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, (meth)acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, polyarylate resins, and polyphenylene sulfide resins. Among these, polyester resins such as polyethylene terephthalate are particularly preferred. The thickness of the film substrate is preferably 10 to 200 μm, and more preferably 25 to 150 μm. Examples of release layer materials include silicone-based release agents, fluorine-based release agents, long-chain alkyl-based release agents, and fatty acid amide-based release agents. The thickness of the release layer is generally about 10 to 2000 nm.

[0110] Various methods can be used to apply the adhesive composition to the substrate, including roll coating, kiss roll coating, gravure coating, reverse coating, roll brushing, spray coating, dip roll coating, bar coating, knife coating, air knife coating, curtain coating, lip coating, and die coating.

[0111] When the base polymer of the adhesive composition is a solution polymer, it is preferable to dry the solvent after application. Depending on the purpose, an appropriate drying method may be used. 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. An appropriate drying time may be used as appropriate. The drying time is preferably 5 seconds to 20 minutes, more preferably 5 seconds to 15 minutes, and particularly preferably 10 seconds to 10 minutes.

[0112] If the adhesive composition contains a crosslinking agent, the crosslinking reaction may be carried out after the adhesive composition is applied to the substrate. Heating may be performed as needed during the crosslinking reaction. The temperature for the crosslinking reaction is usually in the range of 20°C to 160°C, and the reaction time is approximately 1 minute to 7 days. Heating to dry the solvent after applying the adhesive composition may also serve as heating for crosslinking. After the solvent has dried, it is preferable to attach a cover sheet to protect the surface of the adhesive sheet. As the cover sheet, it is preferable to use a release film that has a release layer on the contact surface with the adhesive sheet, similar to the substrate film.

[0113] When the adhesive composition is a photopolymerizable composition containing a prepolymer composition and a polyfunctional compound, photocuring is performed by applying the adhesive composition in layers onto a substrate and then irradiating it with active light. When performing photocuring, it is preferable to attach a cover sheet to the surface of the coated layer and irradiate the adhesive composition with active light while sandwiched between the two sheets to prevent polymerization inhibition by oxygen.

[0114] The active light should be selected according to the type of polymerizable component, such as monomers or polyfunctional (meth)acrylates, and the type of photopolymerization initiator. Generally, ultraviolet light and / or short-wavelength visible light are used. The integrated light intensity of the irradiation is 100 to 5000 mJ / cm². 2A certain degree is preferable. The light source for light irradiation is not particularly limited as long as it can irradiate light in the wavelength range to which the photopolymerization initiator contained in the adhesive composition is sensitive, and LED light sources, high-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halide lamps, xenon lamps, etc., are preferably used.

[0115] [Layer structure of adhesive sheets] By laminating release films 91 and 92 onto the surface of the adhesive sheet 11, an adhesive sheet with release films temporarily attached to both sides is obtained, as shown in Figure 1. The base material or cover sheet used during the formation of the adhesive sheet may also be used as the release films 91 and 92.

[0116] When release films 91 and 92 are provided on both sides of the adhesive sheet 11, the thickness of one release film 91 and the thickness of the other release film 92 may be the same or different. The peeling force when peeling off the release film temporarily attached to one side of the adhesive sheet 11 and the peeling force when peeling off the release film temporarily attached to the other side of the adhesive sheet 11 may be the same or different. When the peeling forces of the two are different, the workability is excellent when peeling off the release film 91 (light release film), which has a relatively smaller peeling force, from the adhesive sheet 11 first and bonding it to the first adherend, and then peeling off the release film 92 (heavy release film), which has a relatively larger peeling force, and bonding it to the second adherend.

[0117] [Image display device] As described above, the adhesive sheet of the present invention is used for bonding components of a foldable image display device equipped with a touch panel. In the image display device 101 shown in Figure 2, a touch panel 41, a circular polarizing plate 31, and a cover window 71 are arranged on the viewing-side surface of an organic EL panel 51 which serves as an image display panel. In a flexible display, all of these components are flexible and foldable. In the image display device 102 shown in Figure 3, a touch panel is integrated into an organic EL panel 54 which serves as an image display panel, and a circular polarizing plate 31 and a cover window 71 are arranged on its surface.

[0118] <Image display panel> An organic EL panel comprises a pair of electrodes and an organic light-emitting layer sandwiched between the electrodes on a substrate. The organic EL panel may be either a top-emission type, in which a metal electrode, an organic light-emitting layer, and a transparent electrode are sequentially laminated on a substrate, or a bottom-emission type, in which a transparent electrode, an organic light-emitting layer, and a metal electrode are sequentially laminated on a transparent substrate. In both bottom-emission and top-emission types, the substrate and sealing members provided on the visible side of the organic light-emitting layer are transparent. The substrate and sealing members provided on the back side of the organic light-emitting layer (the housing 75 side in Figures 2 and 3) do not have to be transparent. In a bottom-emission type flexible organic EL panel, the substrate does not need to be transparent, and polyimide or the like may be used as the substrate material. The substrate material may also be a transparent resin material such as polyetheretherketone or transparent polyimide. A back sheet may be provided on the back side of the substrate for the purpose of protecting or reinforcing the substrate.

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

[0120] <Cover window> A cover window 71 is provided on the outermost surface of the viewing side of the image display device for the purpose of preventing damage to the image display panel due to impact from the outer surface. In flexible displays, a flexible transparent substrate such as transparent polyimide, polyetheretherketone, or polyethylene terephthalate is used as the cover window 71. A flexible glass plate (glass film) may be used as the material for the cover window 71, and the cover window 71 may be a laminated structure of a glass film and a resin film. From the viewpoint of achieving both strength and flexibility, the thickness of the cover window is preferably 20 to 300 μm, more preferably 25 to 250 μm, and even more preferably 30 to 200 μm. The yield point elongation of the cover window is preferably 5% or more because it has excellent recovery properties after being held in a bent state for a long time. A bendable thin glass substrate may be used as the cover window 71. The cover window may be a laminate of two or more layers of transparent material. An anti-reflective layer or a hard coat layer may be provided on the viewing side surface of the cover window.

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

[0122] In flexible displays, the distance D from the touch surface to the touch panel 41 tends to be small due to the small thickness of the cover window. In image display devices using a rigid glass plate as the cover window, the distance from the touch surface to the touch panel is generally 700 μm or more, whereas in flexible displays, the distance D from the touch surface to the touch panel 41 (or the distance from the touch surface to the image display panel if the touch panel is in-cell type) 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.

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

[0124] Generally, polarizing plates are used in which a transparent protective film is laminated to one or both sides of a polarizer as needed. The polarizer is not particularly limited, and various types can be used. Examples of polarizers include hydrophilic polymer films such as polyvinyl alcohol-based films, partially formalized polyvinyl alcohol-based films, and partially saponified ethylene-vinyl acetate copolymer films, to which dichroic substances such as iodine or dichroic dyes are adsorbed and then uniaxially stretched; and polyene-based oriented films such as dehydrated polyvinyl alcohol or dehydrochlorinated polyvinyl chloride.

[0125] Thin polarizers with a thickness of 10 μm or less can also be used as polarizers. Examples of thin polarizers include those described in Japanese Patent Publication No. 51-069644, Japanese Patent Publication No. 2000-338329, WO2010 / 100917, Japanese Patent No. 4691205, and Japanese Patent No. 4751481. Thin polarizers can be obtained, for example, by a manufacturing method that includes the steps of stretching a polyvinyl alcohol-based resin layer and a stretchable resin substrate in a laminated state, and dyeing it with a dichroic material such as iodine.

[0126] For the transparent protective film used as a protective film for the polarizer, it is preferable to use a resin such as cellulose resin, cyclic polyolefin resin, acrylic resin, phenylmaleimide resin, or polycarbonate resin that has excellent transparency, mechanical strength, thermal stability, moisture barrier properties, and optical isotropy. When transparent protective films are provided on both sides of the polarizer, protective films made of the same polymer material may be used on both sides, or protective films made of different polymer materials may be used.

[0127] An optical film may be laminated on one or both sides of the polarizing plate, with an appropriate adhesive layer or tack layer as needed. Such films include those used in the formation of image display devices, such as phase difference plates, viewing angle expanding films, viewing angle limiting (privacy prevention) films, and brightness enhancing films, and the type is not particularly limited. For example, in liquid crystal display devices, an optical compensation film may be used between the image display panel (liquid crystal panel) and the polarizing plate for purposes such as improving viewing angle characteristics by appropriately converting the polarization state of the light emitted from the liquid crystal cell to the viewing side.

[0128] As described above, in an organic EL display device, by providing a circular polarizer with a quarter-wave plate on the organic EL panel side of the polarizer, the emission of ambient light reflected by the metal electrodes to the viewing side can be blocked. By placing a quarter-wave plate on the viewing side of the polarizer and making the emitted light circularly polarized, an appropriate image display can be made visible even to viewers wearing polarized sunglasses. These optical films (optical anisotropic films) may be laminated on the polarizer without other films in between. In this case, the optical film also functions as a protective film for the polarizer.

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

[0130] <Lamination between components using adhesive sheets> Adhesive sheets are used to bond the flexible components described above. In the image display device shown in Figure 2, the organic EL panel 51 and the bottom surface of the housing 75 are bonded via an adhesive sheet 14, the organic EL panel 51 and the touch panel 41 are bonded via an adhesive sheet 13, the touch panel 41 and the circular polarizer 31 are bonded via an adhesive sheet 12, and the circular polarizer 31 and the cover window 71 are bonded via an adhesive sheet 11. In the image display device shown in Figure 3, the touch panel integrated organic EL panel 54 and the circular polarizer 31 are bonded via an adhesive sheet 12, and the circular polarizer 31 and the cover window 71 are bonded via an adhesive sheet 11.

[0131] It is preferable to use the adhesive sheet of the present invention described above for one or both of the adhesive sheets 11 and 12 that are positioned on the viewing side of the touch panel. From the viewpoint of preventing the cover window 71 from peeling off at the bent portion when the bent state is maintained or when bending is repeated, it is preferable to use the adhesive sheet of the present invention as adhesive sheet 11. Furthermore, it is preferable that both adhesive sheets 11 and 12 have the aforementioned dielectric constant.

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

[0133] The order in which the components are bonded together is not particularly limited. The touch panel 41, the circular polarizing plate 31, and the cover window 71 may be laminated on the image display panel 51 in that order, or a laminate of two or more components bonded together in advance via an adhesive sheet may be bonded onto the image display panel 51.

[0134] [Films and laminates with adhesive layers] In addition to the form in which a release film is temporarily attached to both sides of the adhesive sheet of the present invention, it can also be used to form an image display device as an adhesive-layered polarizing plate in which the adhesive sheet is fixedly laminated to the surface of a polarizing plate. For example, as shown in Figure 4, the adhesive-layered polarizing plate 5 may have a release film 93 temporarily attached to one side of the adhesive sheet 11 and a circular polarizing plate 31 fixedly laminated to the other side of the adhesive sheet 11. In the form shown in Figure 5, the adhesive sheet 11 is fixedly laminated to one side of the circular polarizing plate 31 and the adhesive sheet 12 is fixedly laminated to the other side of the circular polarizing plate 31. As shown in Figure 5, in the double-sided adhesive-layered film 6, it is preferable that the release films 93 and 94 are temporarily attached to the surfaces of the adhesive sheets 11 and 12.

[0135] In a configuration where polarizing plates or the like are pre-laminated onto an adhesive sheet, one release film 93 temporarily attached to the surface of the adhesive sheet 11 is peeled off and bonded to the substrate, and the other release film 94 is peeled off and bonded to the other substrate. By pre-laminating the flexible members constituting the image display device with the adhesive sheet, the process of forming the image display device can be simplified.

[0136] In the double-sided adhesive film 6 shown in Figure 5, the thicknesses of the adhesive sheet 11 laminated on one side of the polarizing plate 31 and the adhesive sheet 12 laminated on the other side of the polarizing plate 31 may be the same or different. For example, if the adhesive sheet 12 is used to bond the polarizing plate 31 to a touch panel or image display panel, and the adhesive sheet 11 is used to bond the polarizing plate 31 to a cover window 71, it is preferable that the thickness of the adhesive sheet 11 is greater than the thickness of the adhesive sheet 12.

[0137] By relatively increasing the thickness of the adhesive sheet 11 provided on the viewing side, cushioning against impacts from the outer surface can be provided, preventing damage to the image display panel. The thickness of the adhesive sheet 11 is preferably 25 to 100 μm, and more preferably 30 to 75 μm. The thickness of the adhesive sheet 11 may be 35 μm or more, or 40 μm or more. The thickness of the adhesive sheet 12 provided on the image display panel side is preferably 10 to 25 μm, and may be 20 μm or less.

[0138] In the film 6 with adhesive layers on both sides, it is preferable that both adhesive sheets 11 and 12 have the aforementioned dielectric constant. Furthermore, it is preferable that both adhesive sheets 11 and 12 are the adhesive sheets of the present invention (i.e., adhesive sheets having a gap distance of 2 mm or less after a predetermined bending and holding test). [Examples]

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

[0140] [Preparation of acrylic oligomers] 60 parts by weight of dicyclopentanyl methacrylate (DCPMA) and 40 parts by weight of methyl methacrylate (MMA) were mixed as monomer components, 3.5 parts by weight of α-thioglycerol was added as a chain transfer agent, and 100 parts by weight of toluene was added as a polymerization solvent. The mixture was stirred at 70°C for 1 hour under a nitrogen atmosphere. Next, 0.2 parts by weight of 2,2'-azobisisobutyronitrile (AIBN) was added as a thermal polymerization initiator, and the mixture was reacted at 70°C for 2 hours, then the temperature was raised to 80°C and the reaction was continued for 2 hours. After that, the reaction mixture was heated to 130°C to dry and remove toluene, chain transfer agent, and unreacted monomers, yielding a solid acrylic oligomer. The weight-average molecular weight of the acrylic oligomer was 5100, and the glass transition temperature (Tg) was 130°C.

[0141] [Example 1] (Polymerization of prepolymers) 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 7 parts by weight of N-vinyl-2-pyrrolidone (NVP), along with 0.015 parts by weight of IGM Resins' "Omnirad 184" as a photopolymerization initiator, were blended and polymerization was carried out by irradiation with ultraviolet light to obtain a prepolymer composition (polymerization rate: approximately 10%).

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

[0143] (Making adhesive sheets) A 75 μm thick polyethylene terephthalate (PET) film (Mitsubishi Chemical's "Diafoil MRF75") with a silicone-based release layer on its surface was used as a base material (heavy-release film). The above-mentioned photocurable adhesive composition was applied to the base material to a thickness of 50 μm to form a coating layer. A 75 μm thick PET film (Mitsubishi Chemical's "Diafoil MRE75") with one side treated with silicone release treatment was laminated onto this coating layer as a cover sheet (light-release film). An irradiation intensity of 5 mW / cm² was applied to the irradiation surface directly beneath the lamp from the cover sheet side of this laminate. 2 Using a black light positioned to achieve this, ultraviolet light was irradiated to perform photocuring, resulting in an adhesive sheet with a thickness of 50 μm.

[0144] [Examples 2-5, Comparative Examples 1-3] The monomer composition used in the polymerization of the prepolymer, the amount of polyfunctional monomer (HDDA) added, and the amount of oligomer added were changed as shown in Table 1. Otherwise, the photocurable adhesive composition was prepared in the same manner as in Example 1, applied to a substrate, and photocured to obtain an adhesive sheet.

[0145] [Preparation of adhesive sheet A] In a reaction vessel, 99 parts by weight of butyl acrylate (BA) and 1 part by weight of 4-hydroxybutyl acrylate (4HBA) as monomers, and 0.3 parts of 2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator were added together with ethyl acetate, and the reaction was carried out at 60°C for 4 hours under a nitrogen gas stream. Subsequently, ethyl acetate was added to the reaction solution to obtain a solution of an acrylic polymer with a weight-average molecular weight of 1.65 million. To this solution, 0.3 parts by weight of dibenzoyl peroxide ("Nippon Oil & Fats Co., Ltd. "Nippon Oil & Fats Co., Ltd.") and 0.1 parts by weight of trimethylolpropane xylylene diisocyanate ("Takenate D110N" manufactured by Mitsui Chemicals Co., Ltd.) as crosslinking agents, and 3 parts by weight of silane coupling agent ("KBM403" manufactured by Shin-Etsu Chemical Co., Ltd.) were added per 100 parts by weight of the polymer to obtain adhesive composition A.

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

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

[0148] <Storage modulus, loss tangent, and glass transition temperature> A sample of adhesive sheets, laminated to a thickness of approximately 1.5 mm, was used for measurement. Dynamic viscoelasticity measurements were performed using the "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific under the following conditions. From the measurement results, the storage modulus G' and loss tangent tanδ were read at each temperature. The temperature at which tanδ was maximum was defined as the glass transition temperature of the adhesive sheet.

[0149] (Measurement conditions) Transformation mode: Twist Measurement frequency: 1Hz Heating rate: 5°C / min Shape: Parallel plate 7.9mmφ

[0150] <Total light transmittance and haze> Test specimens were prepared by laminating an adhesive sheet onto alkali-free glass (thickness 0.8-1.0 mm, total light transmittance 92%, haze 0.4%). A haze meter (HM-150, manufactured by Murakami Color Technology Laboratory) was used to measure the haze and total light transmittance. The haze of the adhesive sheet was defined as the measured value minus the haze of the alkali-free glass (0.4%). The measured total light transmittance was used as is. The total light transmittance of the adhesive sheets in all examples and comparative examples was 92%. The haze of the adhesive sheet in Comparative Example 1 was 0.5%, while the haze of the adhesive sheets in the other examples and comparative examples was 0.3%.

[0151] <Relative permittivity> An adhesive sheet was placed between the copper foil and the electrode, and the relative permittivity was measured at frequencies of 1 kHz, 10 kHz, 100 kHz, and 1 MHz at Agilent Technologies' "Precision Impedance Analyzer 4294A" in accordance with JIS K6911, under the following conditions. In Examples 1, 4, 5 and Comparative Example 3, in addition to measurements at 25°C, the relative permittivity was measured at 20°C intervals within the temperature range of -40°C to 80°C. Electrode configuration: 12.1mm diameter, 0.5mm thick aluminum plate Counter electrode: 3oz copper plate Measurement environment: Temperature 25°C, relative humidity 50%

[0152] <Adhesion strength to polyimide film> A test specimen was prepared by peeling the release film from one side of an adhesive sheet, laminating it to a 25 μm thick PET film, and cutting it to a 10 mm wide x 100 mm long piece. The release film from the other side of the test specimen was peeled off, and the adhesive sheet was pressed onto an 80 μm thick transparent polyimide film (manufactured by Kolon Industries) using a 2 kg roller. Using a tensile testing machine, the test specimen was peeled from the polyimide film at a tensile speed of 60 mm / min and a peel angle of 180° in a 25°C environment, and the peel force was measured.

[0153] <Flexibility test> The release film was peeled from one side of the adhesive sheet, and a 51 μm thick polarizing plate was bonded to it using a 2 kg roller. The release film was peeled from the other side of the adhesive sheet, and an 80 μm thick transparent polyimide film was bonded to it using a 2 kg roller. Furthermore, a 125 μm thick PET film was bonded onto the polarizing plate via a 15 μm thick adhesive sheet A, using a 2 kg roller. Prior to bonding with the adhesive sheet, plasma treatment was performed on the surfaces of the polarizing plate, polyimide film, and PET film.

[0154] This laminate was cut into a 35mm x 100mm rectangle so that the absorption axis of the polarizer was parallel to the long side, and a test specimen was obtained by autoclaving at 35°C and 0.35MPa for 15 minutes. Using a planar unloaded U-shaped stretch tester (manufactured by Yuasa System Equipment), bending fixtures were attached and fixed to a range of 20mm from each end of the long side of the test specimen (the central 60mm area of ​​the long side was not fixed), and the specimen was held in a bent state with a bending radius of 1.3mm and a bending angle of 180°, with the PET film side facing inward, and held in a constant temperature and humidity chamber at 60°C and 95% relative humidity for 240 hours to perform a bending retention test.

[0155] After the bending and holding test, the test specimens were visually inspected to check for delamination at the interface between the transparent polyimide film and the polarizing plate at the bent portion. In all cases where delamination was observed, the delamination (void) occurred from the edge in the short-side direction of the sample. For those where delamination was observed, the length (mm) of the void in the short-side direction of the sample was measured. For samples where delamination was observed along the entire short-side, the void length (void distance) was set to 35 mm, and for samples where no delamination was observed at all, the void distance was set to 0. For samples where delamination occurred from both ends in the short-side direction, the void distance was set to the longer of the two void lengths. In all samples, no delamination was observed at the bonding interface between the PET film and the polarizing plate.

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

[0157] In Table 1, each component is listed using the following abbreviations. LA: Lauryl acrylate 2EHA: 2-ethylhexyl acrylate BA: Butyl acrylate CHA: Cyclohexyl Acrylate 4HBA: 4-hydroxybutyl acrylate 2HEA: 2-hydroxyethyl acrylate NVP: N-vinyl-2-pyrrolidone

[0158] [Table 1]

[0159] [Table 2] [Table 3]

[0160] The adhesive sheets of Examples 1 to 5 all had a relative permittivity of 4.5 or less and a gap distance of 2 mm or less at a temperature of 25°C and a frequency of 10 MHz. From Table 3, it can be seen that the adhesive sheets of the examples have a low relative permittivity, and that the temperature dependence and frequency dependence of the relative permittivity are also small. [Explanation of Symbols]

[0161] 11, 12, 13, 14 Adhesive sheets 91,92 Release film (heavy release film) 93,94 Release film 1. Adhesive sheet with release film 5,6 Film with adhesive layer 31. Polarizing plate (circular polarizer) 51 Image display panel (OLED panel) 54 Image display panel (touch panel integrated organic EL panel) 41 Touch panel 71 Cover window 75 cabinets 101,102 Image display device

Claims

1. In a foldable image display device equipped with a touch panel within 500 μm of the touch surface, an adhesive sheet is used for bonding members placed between the touch surface and the touch panel, It is composed of an acrylic adhesive containing an acrylic base polymer, The acrylic-based polymer is composed of (meth)acrylic acid C in proportion to 100 parts by weight of the total monomer components. １０－２０ It contains 5 to 55 parts by weight of a linear alkyl ester and 2 to 8 parts by weight of a hydroxyl group-containing monomer. The gel fraction of the aforementioned acrylic adhesive is 55 to 80%. An adhesive sheet having a relative permittivity of 4.5 or less at a temperature of 25°C and a frequency of 10 kHz.

2. The adhesive sheet according to claim 1, wherein, at a temperature of 25°C, the ratio of the relative permittivity at a frequency of 1 kHz to the relative permittivity at a frequency of 1 MHz is 1.50 or less.

3. The adhesive sheet according to claim 1 or 2, wherein the maximum value of the relative permittivity in the temperature range of -40°C to 80°C at a frequency of 10 kHz is 1.4 times or less the minimum value.

4. The adhesive sheet according to any one of claims 1 to 3, wherein the ratio of the maximum and minimum values ​​of the relative permittivity in the temperature range of -40°C to 80°C at a frequency of 1 kHz is 0.8 to 1.2 times the ratio of the maximum and minimum values ​​of the relative permittivity in the temperature range of -40°C to 80°C at a frequency of 1 MHz.

5. An adhesive sheet according to any one of claims 1 to 4, wherein the thickness is 100 μm or less.

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

7. The acrylic-based polymer is the (meth)acrylic acid C １０－２０ An adhesive sheet according to any one of claims 1 to 6, comprising lauryl acrylate as a linear alkyl ester.

8. The adhesive sheet according to any one of claims 1 to 7, wherein the acrylic base polymer has a total amount of hydroxyl group-containing monomers, carboxyl group-containing monomers, and nitrogen-containing monomers of 2 to 15 parts by weight per 100 parts by weight of the total monomer components.

9. The adhesive sheet according to any one of claims 1 to 8, wherein the acrylic-based polymer contains 0.5 to 8 parts by weight of nitrogen-containing monomers relative to 100 parts by weight of the total monomer components.

10. The adhesive sheet according to any one of claims 1 to 9, wherein the acrylic-based polymer has a crosslinked structure introduced by a polyfunctional (meth)acrylate.

11. The adhesive sheet according to any one of claims 1 to 10, wherein the acrylic adhesive further comprises an acrylic oligomer having a glass transition temperature of 60°C or higher.

12. The adhesive sheet according to claim 11, wherein the content of the acrylic oligomer relative to 100 parts by weight of the acrylic base polymer is 0.1 to 5 parts by weight.

13. An adhesive film having an adhesive sheet according to any one of claims 1 to 12 laminated on at least one surface of a polarizing plate.

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