JP7565162B2 - Image display device - Google Patents

Description

The present invention relates to an image display device including a rigid transparent plate having a curved portion.

In liquid crystal display devices and organic EL display devices, a polarizing plate is provided on the viewing side surface of the image display cell. A hard transparent plate (cover window) made of glass or resin may be provided on the surface of these image display devices to prevent damage to the image display panel due to impact from the outer surface, and this configuration is becoming mainstream for mobile and in-vehicle applications. By bonding the polarizing plate and cover window via an adhesive layer, the refractive index difference at the interface is reduced, preventing the decrease in visibility caused by reflection and scattering, and imparting impact resistance.

In the configuration of an image display device having a polarizing plate on the viewing side surface of an image display cell and a cover window on the viewing side surface of the polarizing plate, it has been proposed to use a double-sided adhesive film having an adhesive layer on one side of the polarizing plate for bonding to the image display cell and an adhesive layer on the other side for bonding to the cover window (see, for example, Patent Document 1).

JP 2014-115468 A

In recent years, it has become possible to produce organic EL cells using flexible substrates such as resin films, and edge displays in which the edges of the screen (longer sides of a rectangle) are curved have been put to practical use. In addition, four-sided edge displays in which all four edges of the screen are curved, and curved displays in which the entire screen is curved or spherical have also been proposed.

In such a display having a curved surface, a flexible optical film (such as a polarizing plate) and an image display cell are bonded to a rigid cover window having a curved surface, so that the optical film and the image display cell are curved to follow the curved surface shape of the cover window, thereby maintaining the curved surface shape. In the curved surface portion, the curvature of the inner surface side (the side of the center of curvature) is smaller than that of the outer surface side, so mechanical distortion occurs at the bonding interface. In particular, a spherical display and corners of the four edges have a three-dimensional curved surface shape, so distortion is large and peeling or wrinkles are likely to occur at the bonding interface.

In view of the above, the present invention aims to provide an optical film with adhesive that is less likely to peel or wrinkle at the bonding interface even when applied to an image display device having a curved surface.

One embodiment of the present invention is an image display device in which at least a portion of the screen is curved. The image display device includes a rigid transparent plate, an optical film, and a flexible image display cell. The transparent plate has a curved portion with a second main surface facing inward, and the first main surface of the optical film is bonded to the second main surface of the transparent plate via a first adhesive layer. The second main surface of the optical film is bonded to the image display cell via a second adhesive layer.

Examples of optical films include optically isotropic films and optically anisotropic films. Examples of optically anisotropic films include polarizers and retardation plates. The optical film may be a circular polarizing plate formed by laminating a polarizer and a retardation plate. The image display cell may be an organic EL cell, and the image display device may be an organic EL display device having a circular polarizing plate as an optical film on the viewing side surface of the organic EL cell.

One embodiment of the present invention is a double-sided adhesive optical film used in forming the image display device described above, which has a first adhesive layer on a first main surface of the optical film and a second adhesive layer on a second main surface of the optical film.

The first pressure-sensitive adhesive layer used to bond the optical film to the transparent plate preferably has a strain cross-sectional area _S1 of 800 ^μm2 or less. The second pressure-sensitive adhesive layer used to bond the optical film to the image display cell preferably has a strain cross-sectional area _S2 of 360 ^μm2 or less. The strain cross-sectional area S is an amount expressed by S=(1/2)×δT, where δ is the amount of strain in the shear direction when a shear load of 10 kPa is applied to the pressure-sensitive adhesive layer for 15 minutes, and T is the thickness of the pressure-sensitive adhesive layer.

The thickness _T1 of the first pressure-sensitive adhesive layer may be 25 to 100 μm, and the thickness _T2 of the second pressure-sensitive adhesive layer may be 5 to 30 μm. The thickness _T1 of the first pressure-sensitive adhesive layer may be greater than the thickness _T2 of the second pressure-sensitive adhesive layer. The strain cross-sectional area _S1 of the first pressure-sensitive adhesive layer may be greater than the strain cross-sectional area _S2 of the second pressure-sensitive adhesive layer.

The first adhesive layer and the second adhesive layer may be acrylic adhesive layers. The adhesive constituting these adhesive layers preferably contains a polymer having a crosslinked structure.

By keeping the strain cross-sectional area of the adhesive layer that bonds the components within a specified range, peeling and wrinkling of flexible components can be suppressed in image display devices that have curved surfaces.

FIG. 1 is a cross-sectional view of an optical film with double-sided pressure-sensitive adhesive according to an embodiment. 1 is a cross-sectional view of an image display device according to an embodiment. FIG. 2 is a cross-sectional view showing an example of a laminated structure of an organic EL cell. FIG. 4 is an explanatory diagram of a strain cross-sectional area of a pressure-sensitive adhesive layer. FIG. 2 is an explanatory diagram for measuring the amount of strain and the strain cross-sectional area using a rotational rheometer.

Figure 1 is a cross-sectional view that shows a schematic representation of an optical film with double-sided adhesive according to one embodiment of the present invention, and Figure 2 is a cross-sectional view that shows a schematic representation of an image display device according to one embodiment of the present invention.

The double-sided adhesive optical film 50 comprises an optical film 10, a first adhesive layer 21 provided on a first main surface of the optical film, and a second adhesive layer 22 provided on a second main surface of the optical film. In FIG. 1, a first release film 41 is releasably attached onto the first adhesive layer 21 of the double-sided adhesive optical film 50, and a second release film 42 is releasably attached onto the second adhesive layer 22.

In the image display device 100 shown in FIG. 2, the first main surface of the optical film 10 is bonded to the second main surface of the cover window 80 via a first adhesive layer 21, and the second main surface of the optical film 10 is bonded to the image display cell 70 via a second adhesive layer 22. The double-sided adhesive-coated optical film 50 and the image display cell 70 are housed in a housing not shown.

At least a portion of the screen of the image display device 100 has a curved shape that is convex on the viewing side (upper side of the figure). In FIG. 2, the entire screen is shown to have a spherical curved surface, but part of the screen may be curved and other parts may be flat. For example, the screen may be rectangular in plan view, the center of the screen may be flat, and the edges of the screen (the sides of the rectangle) may have a curved shape that curves downward.

The screen of the image display device may have a three-dimensional curved shape. A three-dimensional curved surface refers to a shape in which the cross-sectional shape of all surfaces including the normal of the surface is a curved shape. For example, a spherical curved surface is a three-dimensional curved surface in which the cross-section of a surface including the normal (a radial straight line) is an arc. In addition, a display device (edge display) in which the rectangular sides are curved downward has a three-dimensional curved shape at the corners where these two sides intersect when two adjacent sides have curved shapes that are curved downward. For example, a four-sided edge display in which the entire periphery (all four sides) of the screen, which is rectangular in plan view, is curved has four corners with a three-dimensional curved shape. Note that the cross-section of the center of the rectangular side of the edge display is curved in the direction perpendicular to the side, but the cross-section of the direction parallel to the side is straight, so the center of the side is a "two-dimensional" curved shape.

[Image display cell]
The image display cell may be an organic EL cell. In order to form an image display device having a curved surface, it is preferable to use a flexible image display cell. In order to form a flexible image display cell, a flexible substrate is used.

Figure 3 shows an example of a layered structure of an image display cell, showing a top-emission organic EL cell. A top-emission organic EL cell has a metal electrode 73, an organic light-emitting layer 75, and a transparent electrode 77, in that order, on a substrate 71, with the transparent electrode 77 side (the upper side of the figure) being the light-emitting surface. A sealant 79 is layered on the transparent electrode 77. Although not shown, it is preferable that the sealant 79 be provided so as to cover the side surfaces of the electrodes 73, 77, and the organic light-emitting layer 75.

A flexible plastic substrate is preferably used as the substrate 71. In a top-emission organic EL cell, the substrate 71 does not need to be transparent, and a highly heat-resistant film such as a polyimide film may be used as the substrate 71. A flexible glass plate (glass film) may also be used as the substrate 71.

The organic light-emitting layer 75 may include an electron transport layer, a hole transport layer, etc., in addition to the organic layer that itself functions as a light-emitting layer. The transparent electrode 77 is a metal oxide layer or a metal thin film, and transmits light from the organic light-emitting layer 75. The metal electrode 73, the organic light-emitting layer 75, and the transparent electrode 77 are all thin films, and have a thickness that is sufficiently smaller than that of the substrate 71. Therefore, if the substrate 71 is flexible, the entire image display cell 70 is flexible. A back sheet (not shown) may be provided on the back side of the substrate 71 for the purpose of protecting and reinforcing the substrate.

The organic EL cell may be a bottom-emission type in which a transparent electrode, an organic light-emitting layer, and a metal electrode are laminated in this order on a substrate. In a bottom-emission type organic EL cell, a transparent substrate is used, and the substrate is disposed on the viewing side (second adhesive layer 22 side). The image display cell is not limited to an organic EL cell, and may be a liquid crystal cell or an electrophoretic display cell (electronic paper), etc. A touch panel sensor (not shown) may be disposed on the viewing side surface of the image display cell 70.

[Cover Window]
The cover window 80 arranged on the viewing side surface of the image display device is a rigid transparent plate, with a first main surface 81 arranged on the viewing side and a second main surface 82 arranged on the image display cell 70 side. Transparent resins such as acrylic resins and polycarbonate resins, and glass are used as materials for the transparent plate. The thickness of the cover window 80 is, for example, about 50 to 2000 μm. From the viewpoint of increasing durability against external impacts, the thickness of the cover window 80 is preferably 200 μm or more, and more preferably 300 μm or more.

The cover window 80 may be integrated with a touch panel sensor. An anti-reflection layer, a hard coat layer, or the like may be provided on the first main surface 81 of the cover window 80. A light-shielding printed layer may be provided on a portion of the surface of the cover window 80.

At least part of the surface of the cover window 80 has a curved portion where the second main surface 82 is on the inside and the first main surface 81 is convex. The entire cover window 80 may have a curved shape. The curved shape may be three-dimensional. For example, if the entire screen is spherical, the entire cover window has a three-dimensional curved shape. In a cover window for a four-sided edge display, the center of the screen is flat, and at the periphery of the screen, the center of the sides has a two-dimensional curved shape, and the corners have a three-dimensional curved shape.

The rigid cover window 80 having a curved portion and the flexible image display cell 70 are laminated and integrated via the adhesive-backed optical film 50, so that the image display cell 70 has a curved shape that conforms to the second main surface 82 (inner surface) of the cover window 80. Note that the first main surface 81 of the cover window 80 generally has a curved portion like the second main surface 82, but as long as the second main surface 82 has a curved portion, the first main surface may be flat.

[Optical film with double-sided adhesive]
2 , the cover window 80 and the image display cell 70 are laminated together via the double-sided adhesive-coated optical film 50, and the first adhesive layer 21 arranged on the first main surface (viewing side surface) of the optical film 10 is bonded to the second main surface 82 of the cover window 80. The second adhesive layer 22 arranged on the second main surface of the optical film 10 is bonded to the light exit surface of the image display cell 70.

<Optical film>
Examples of the optical film 10 include an optically isotropic film and an optically anisotropic film. The optically isotropic film is preferably a transparent film. The optically anisotropic film is preferably a retardation plate, a polarizer, or the like. The optical film may be a laminate of a plurality of films or an optically functional film having a coating layer on the surface of the film.

An example of a laminate of multiple optical films is a polarizing plate. A polarizing plate includes a polarizer, and if necessary, a transparent film is laminated on one or both sides as a polarizer protection film.

Examples of polarizers include hydrophilic polymer films such as polyvinyl alcohol films, partially formalized polyvinyl alcohol films, and partially saponified ethylene-vinyl acetate copolymer films that have been uniaxially stretched after adsorbing dichroic substances such as iodine or dichroic dyes, and polyene-based oriented films such as dehydrated polyvinyl alcohol and dehydrochlorinated polyvinyl chloride.

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

As the polarizer protective film, a transparent resin film such as a cellulose-based resin, a cyclic polyolefin-based resin, an acrylic-based resin, a phenylmaleimide-based resin, or a polycarbonate-based resin is preferably used. When a polarizer protective film is provided on both sides of the polarizer, the transparent films 13 and 15 may be films made of the same resin material or films made of different resin materials.

Examples of optically functional films include retardation plates, viewing angle widening films, viewing angle limiting (anti-prying) films, and brightness enhancing films. The optical film 10 may have an optically functional film on one or both surfaces of a polarizer. The optical film 10 may include a touch panel sensor as the optically functional film. In a polarizing plate with a touch panel sensor, a transparent film in contact with the polarizer may function as both a polarizer protective film and an optically functional film. The transparent film may also function as an electrode substrate film for the touch panel sensor.

The metal electrode 73 of the organic EL cell 70 is light reflective. Therefore, when external light enters the organic EL cell, the light is reflected by the metal electrode, and the reflected light is visible from the outside as a mirror surface. By disposing a circular polarizing plate as the optical film 10 on the visible surface of the organic EL cell 70, the light reflected by the metal electrode is prevented from being re-emitted to the outside, improving the visibility and design of the screen of the display device.

The circular polarizing plate includes a retardation film on the surface of the polarizer facing the organic EL cell 70. The retardation film may be a transparent film arranged adjacent to the polarizer. When the retardation film has a retardation of λ/4 and the angle between the slow axis direction of the retardation film and the absorption axis direction of the polarizer is 45°, the laminate of the polarizer and the retardation film functions as a circular polarizing plate for suppressing re-emission of reflected light at the metal electrode. The retardation film constituting the circular polarizing plate may be a laminate of two or more layers of film. For example, a polarizer, a λ/2 plate, and a λ/4 plate are laminated so that their optical axes form a predetermined angle, thereby obtaining a wideband circular polarizing plate that functions as a circular polarizing plate over a wide band of visible light. For example, a stretched resin film is used as the retardation film. The retardation film may be an oriented liquid crystal layer.

<Adhesive Layer>
The first pressure-sensitive adhesive layer 21 for bonding the optical film 10 to the cover window 80 and the second pressure-sensitive adhesive layer 22 for bonding the optical film 10 to the image display cell 70 are preferably made of an optically transparent pressure-sensitive adhesive. The first pressure-sensitive adhesive layer 21 and the second pressure-sensitive adhesive layer 22 may each be a laminate of a plurality of pressure-sensitive adhesive layers.

The adhesive layers 21 and 22 are preferably transparent and have low visible light absorption. The total light transmittance of the adhesive layers 21 and 22 is preferably 85% or more, and more preferably 90% or more. The haze of the adhesive layers 21 and 22 is preferably 2% or less, and more preferably 1% or less. The total light transmittance and haze are measured using a haze meter in accordance with JIS K7136.

As the adhesive constituting the adhesive layers 21 and 22, the adhesive constituting the adhesive layer 21 can be appropriately selected and used from those having a base polymer such as an acrylic polymer, a silicone polymer, a polyester, a polyurethane, a polyamide, a polyvinyl ether, a vinyl acetate/vinyl chloride copolymer, a modified polyolefin, an epoxy-based, a fluorine-based, or a rubber-based polymer. In particular, an acrylic adhesive is preferably used because it has excellent optical transparency, exhibits adhesive properties such as moderate wettability, cohesiveness, and adhesion, and is also excellent in weather resistance and heat resistance.

The acrylic adhesive preferably contains 50% by weight or more of the acrylic base polymer relative to the total solid content of the adhesive composition, more preferably 70% by weight or more, and even more preferably 80% by weight or more. The acrylic base polymer preferably has a main skeleton of a monomer unit of (meth)acrylic acid alkyl ester. In this specification, "(meth)acrylic" means acrylic and/or methacrylic.

As the (meth)acrylic acid alkyl ester, a (meth)acrylic acid alkyl ester having 1 to 20 carbon atoms in the alkyl group is preferably used. The (meth)acrylic acid alkyl ester may have a branched alkyl group. The content of the (meth)acrylic acid alkyl ester is preferably 40% by weight or more, more preferably 50% by weight or more, and even more preferably 60% by weight or more, based on the total amount of the monomer components constituting the base polymer. The acrylic base polymer may be a copolymer of multiple (meth)acrylic acid alkyl esters. The arrangement of the constituent monomer units may be random or block.

The acrylic base polymer may contain an acrylic monomer having a crosslinkable functional group as a copolymerization component. When the base polymer has a crosslinkable functional group, the gel fraction of the adhesive can be easily increased by thermal crosslinking or photocuring of the base polymer. Examples of acrylic monomers having a crosslinkable functional group include hydroxyl group-containing monomers and carboxyl group-containing monomers.

Examples of hydroxy group-containing monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and (4-hydroxymethyl)cyclohexylmethyl (meth)acrylate.

The acrylic base polymer may contain a nitrogen-containing monomer as a monomer component. Examples of the nitrogen-containing monomer include vinyl monomers such as N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, (meth)acryloylmorpholine, N-vinylcarboxylic acid amides, and N-vinylcaprolactam, and cyanoacrylate monomers such as acrylonitrile and methacrylonitrile. Of these, N-vinylpyrrolidone and (meth)acryloylmorpholine are preferably used.

The monomer components forming the acrylic polymer may contain a polyfunctional polymerizable compound (polyfunctional monomer). Examples of polyfunctional polymerizable compounds include compounds having two or more ethylenically unsaturated groups in one molecule, and compounds having one C=C bond and a polymerizable functional group such as epoxy, aziridine, oxazoline, hydrazine, or methylol. Among them, compounds having two or more ethylenically unsaturated groups in one molecule, such as polyfunctional (meth)acrylates, are preferred. Specific examples of the polyfunctional polymerizable compound include polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, bisphenol A ethylene oxide modified di(meth)acrylate, bisphenol A propylene oxide modified di(meth)acrylate, alkanediol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, ethoxylated isocyanuric acid triacrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol di(meth)acrylate. acrylate, trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol poly(meth)acrylate, dipentaerythritol hexa(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerin di(meth)acrylate, urethane (meth)acrylate, epoxy (meth)acrylate, butadiene (meth)acrylate, isoprene (meth)acrylate, etc. The polyfunctional polymerizable compound may be an oligomer.

The base polymer can be prepared by known polymerization methods such as solution polymerization, UV polymerization, bulk polymerization, and emulsion polymerization. When preparing the base polymer, a polymerization initiator such as a photopolymerization initiator or a thermal polymerization initiator may be used depending on the type of polymerization reaction. A chain transfer agent may be used to adjust the molecular weight of the base polymer.

In order to provide the adhesive with adequate adhesive retention and flexibility, the weight average molecular weight of the base polymer is preferably 200,000 to 1,000,000, and more preferably 250,000 to 800,000. The molecular weight of the base polymer refers to the molecular weight of the polymer before the introduction of a crosslinked structure.

When a polyfunctional monomer is used in addition to a monofunctional monomer as a monomer component forming the base polymer, the monofunctional monomer may be polymerized first to form a prepolymer composition with a low degree of polymerization (prepolymerization), and then the polyfunctional monomer may be added to the syrup of the prepolymer composition to polymerize the prepolymer and the polyfunctional monomer (postpolymerization). In this way, by performing prepolymerization of the prepolymer, a crosslinked structure due to the polyfunctional monomer can be uniformly introduced into the base polymer. In addition, a mixture of the prepolymer composition and the unpolymerized monomer component (adhesive composition) may be applied to a substrate, and then postpolymerization may be performed on the substrate to form an adhesive layer. Since the prepolymer composition has low viscosity and excellent applicability, the method of performing postpolymerization on a substrate after applying the adhesive composition, which is a mixture of the prepolymer composition and the unpolymerized monomer, can increase the productivity of the adhesive layer and make the thickness of the adhesive layer uniform.

The prepolymer composition can be prepared, for example, by partially polymerizing (preliminarily polymerizing) a composition (referred to as a "prepolymer-forming composition") in which a monomer component constituting an acrylic base polymer and a polymerization initiator are mixed. The monomer in the prepolymer-forming composition is preferably a monofunctional monomer such as an alkyl (meth)acrylate ester or a polar group-containing monomer among the monomer components constituting the acrylic polymer. The prepolymer-forming composition may contain a polyfunctional monomer. For example, a portion of the polyfunctional monomer component that is the raw material for the base polymer may be included in the prepolymer-forming composition, and the remaining portion of the polyfunctional monomer component may be added after polymerization of the prepolymer to subject it to post-polymerization.

The prepolymer-forming composition may contain, in addition to the monomer and polymerization initiator, a chain transfer agent, etc., as necessary. There are no particular limitations on the method of polymerization of the prepolymer, but from the viewpoint of adjusting the reaction time to bring the molecular weight (polymerization rate) of the prepolymer into the desired range, polymerization by irradiation with active light such as UV light is preferred. There are no particular limitations on the polymerization initiator and chain transfer agent used in the prepolymerization.

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

The prepolymer composition is mixed with the remainder of the monomer components constituting the acrylic base polymer (post-polymerization monomer), and, if necessary, a polymerization initiator, a chain transfer agent, a silane coupling agent, a crosslinking agent, etc. to form a pressure-sensitive adhesive composition. The post-polymerization monomer preferably contains a polyfunctional monomer. In addition to the polyfunctional monomer, a monofunctional monomer may also be added as the post-polymerization monomer.

It is preferable that the base polymer of the adhesive has a cross-linked structure. When the base polymer has a cross-linked structure, the creep rate C of the adhesive becomes smaller, and therefore the strain cross-sectional area described below tends to become smaller.

For example, as described above, a base polymer having a crosslinked structure can be obtained by using a polyfunctional polymerizable compound as a monomer component forming the base polymer. After polymerizing the base polymer, a crosslinked structure can also be formed by adding a crosslinking agent. As the crosslinking agent, a commonly used one such as an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, an oxazoline-based crosslinking agent, an aziridine-based crosslinking agent, a carbodiimide-based crosslinking agent, or a metal chelate-based crosslinking agent can be used. The content of the crosslinking agent is usually in the range of 0.01 to 5 parts by weight, preferably 0.05 to 3 parts by weight, and more preferably 0.07 to 2.5 parts by weight, per 100 parts by weight of the base polymer. The base polymer may contain both a crosslinked structure due to a polyfunctional polymerizable compound and a crosslinked structure due to a crosslinking agent such as polyisocyanate.

The adhesive may contain an ultraviolet absorber. Examples of ultraviolet absorbers include benzotriazole-based ultraviolet absorbers, benzophenone-based ultraviolet absorbers, triazine-based ultraviolet absorbers, salicylate-based ultraviolet absorbers, and cyanoacrylate-based ultraviolet absorbers. Triazine-based ultraviolet absorbers and benzotriazole-based ultraviolet absorbers are preferred because they have high ultraviolet absorption and excellent compatibility with acrylic polymers, making it easy to obtain a highly transparent acrylic adhesive sheet. Among these, triazine-based ultraviolet absorbers containing a hydroxyl group and benzotriazole-based ultraviolet absorbers having one benzotriazole skeleton per molecule are preferred.

In addition to the above-listed components, the adhesive may contain additives such as a silane coupling agent, a tackifier, a plasticizer, a softener, an anti-degradation agent, a filler, a colorant, an antioxidant, a surfactant, and an antistatic agent.

The adhesive composition is applied in layers onto the substrate, and the adhesive layer is formed by drying the solvent and crosslinking/curing the base polymer as necessary. The thickness of the adhesive layers 21 and 22 is not particularly limited, and is generally about 5 to 500 μm. When a decorative printing layer is provided on the second main surface of the cover window 80, the thickness of the first adhesive layer 21 is preferably 25 μm or more, and more preferably 30 μm or more, in order to provide step absorption for printing steps. The thickness of the first adhesive layer 21 may be 35 μm or more, 40 μm or more, or 45 μm or more.

In the curved portion of the cover window 80, the curvature of the image display cell 70 located on the inside of the curve is the largest (the radius of curvature is the smallest), and the distortion is large. Therefore, peeling and wrinkling are likely to occur in the portion of the flexible image display cell 70 where distortion is large. In particular, when the surface has a three-dimensional curved shape, distortion occurs in all directions, and it is difficult to release the distortion, so peeling and wrinkling are likely to occur.

In the present invention, the first pressure-sensitive adhesive layer 21 and the second pressure-sensitive adhesive layer 22 of the double-sided pressure-sensitive adhesive optical film have a predetermined creep characteristic, so that peeling between layers and the occurrence of wrinkles in a display device having a curved surface portion can be suppressed. The strain cross-sectional area _S1 of the first pressure-sensitive adhesive layer 21 is preferably 800 μm2 or ^less , and the strain cross-sectional area _Ss of the second pressure-sensitive adhesive layer ²² is preferably 360 μm2 or less. The strain cross-sectional area S is an amount expressed by S = (1/2) × δT, where δ is the amount of strain in the shear direction when a shear load (stress) of 10 kPa is applied to the pressure-sensitive adhesive layer for 15 minutes, and T is the thickness of the pressure-sensitive adhesive layer.

Figure 4 is an explanatory diagram of the concept of strain cross-sectional area of an adhesive layer, and shows a schematic cross section of an adhesive layer 2 with one surface of the end attached to an adherend 1 and a shear load applied in the direction of the arrow. The cross section of the adhesive layer 2 before the shear load is applied is rectangular (dashed line in Figure 4), whereas when a constant shear load is applied with the bottom surface of the adhesive layer 2 fixed to the adherend 1, the top surface deforms the most and the cross section of the adhesive layer deforms into a parallelogram (solid line in Figure 4).

If the amount of strain on the upper surface is δ and the thickness of the adhesive layer is T, the amount of deformation in the cross section of the adhesive layer is expressed as the area of a triangle S = (1/2) x δT. This cross-sectional area S is the strain cross-sectional area. The creep rate C is the ratio of the amount of strain δ to the thickness T, and is expressed as C = δ/T, so the strain cross-sectional area can be rewritten as S = (1/2) ^CT2 .

The amount of strain δ of the adhesive layer can be measured using a rotational rheometer. As shown in Figure 5, when a constant torsional force is applied to parallel plates of diameter D, the upper plate rotates by θ°. The amount of rotation of the outer periphery of the parallel plates at this time, i.e., the length of the arc πD (θ/360), is the amount of strain δ, and the creep rate C, which is the ratio of thickness T to the amount of strain δ, is C = πDθ/360T.

The creep rate C does not change even if the thickness of the adhesive layer changes. For example, if two identical adhesive layers are laminated and the same stress (twice the shear load) is applied as when there is only one adhesive layer, the thickness of the adhesive layer will be 2T and the amount of strain will be 2δ, so the creep rate C is 2δ/2T = δ/T, which is the same as when there is only one adhesive layer. In other words, it can be said that the creep rate is a value specific to the adhesive material.

As described above, the strain cross-sectional area S is proportional to the creep rate C and the square of the thickness ^T2 . Therefore, when the material of the adhesive layer is the same, the larger the thickness, the larger the strain cross-sectional area. In order to reduce the strain cross-sectional area S of the adhesive layer and suppress the occurrence of peeling and wrinkles, it is preferable that the thickness T of the adhesive layer is small. In particular, the second adhesive layer 22 used for bonding the optical film 10 and the image display cell 70 is a layer of flexible members bonded together, so that a large strain caused by the curved surface is likely to cause peeling and wrinkles. Therefore, the thickness _T2 of the second adhesive layer 22 is preferably 30 μm or less, more preferably 25 μm or less, even more preferably 20 μm or less, and particularly preferably 15 μm or less. The strain cross-sectional area _S2 of the second adhesive layer 22 is preferably 250 μm ^or less, more preferably 150 μm ^or less, and even more preferably 100 μm ^or less. _S2 may be 70 μm2 or ^less , 50 μm2 ^or less, 40 ^μm2 or less, or 30 μm2 ^or less.

From the viewpoint of suppressing wrinkles and peeling at the curved surface, it is preferable that the strain cross-sectional area _S2 of the second adhesive layer 22 is as small as possible. The smaller _S2 is, the more peeling and wrinkles can be suppressed even when used to bond a curved surface with a large curvature (small radius of curvature). On the other hand, if the strain cross-sectional area of the second adhesive layer 22 is excessively small, peeling may occur due to insufficient adhesive force. Therefore, the strain cross-sectional area _S2 of the second adhesive layer 22 is preferably 1 μm2 or ^more , more preferably ² μm2 or more.

The creep rate _C2 of the second adhesive layer is preferably 80% or less, more preferably 50% or less, and even more preferably 40% or less. As described above, the smaller the thickness T of the adhesive layer and the smaller the creep rate C, the smaller the strain cross-sectional area S. The higher the molecular weight of the base polymer and the higher the cross-linking degree of the adhesive, the smaller the creep rate tends to be. Therefore, by increasing the amount of cross-linking agent or polyfunctional monomer, an adhesive with a small creep rate can be obtained. In addition, the higher the glass transition temperature of the base polymer, the greater the interaction between molecular chains, such as entanglement of molecular chains, and therefore the creep rate at room temperature tends to be smaller.

In the curved surface portion of the image display device, the first pressure-sensitive adhesive layer 21 has a smaller curvature (larger radius of curvature) than the second pressure-sensitive adhesive layer 22. In addition, since the first pressure-sensitive adhesive layer 21 bonds the optical film 10 to the cover window 80, which is a rigid member, the pressure-sensitive adhesive layer does not deform at the interface on the cover window 80 side, and is less likely to cause wrinkles than the second pressure-sensitive adhesive layer 22. Therefore, the strain cross-sectional area _S1 of the first pressure-sensitive adhesive layer 21 may be larger than the strain cross-sectional area _S2 of the second pressure-sensitive adhesive layer 22. In addition, the thickness _T1 of the first pressure-sensitive adhesive layer 21 may be larger than the thickness _T2 of the second pressure-sensitive adhesive layer 22.

As described above, by reducing the thickness _T2 of the second pressure-sensitive adhesive layer 22 and reducing the strain cross-sectional area _S2 , it is possible to suppress peeling and wrinkles of the optical film 10 and the image display cell 70 at the curved surface shape portion. In addition, by using a pressure-sensitive adhesive layer having a relatively large thickness _T1 and a large strain cross-sectional area _S1 as the first pressure-sensitive adhesive layer 21, it is possible to provide impact resistance against impact from the viewing side surface and step absorbency for printing steps such as a decorative printing layer provided on the second main surface 82 of the cover window 80.

On the other hand, if the strain cross-sectional area _S1 of the first pressure-sensitive adhesive layer 21 is excessively large, this may cause peeling or wrinkling of the optical film 10 and the image display cell 70. Therefore, as described above, the strain cross-sectional area _S1 of the first pressure-sensitive adhesive layer 21 is preferably 800 μm2 or ^less . _S1 is more preferably 600 μm2 or ^less , and even more preferably 500 μm2 or ^less . In order to set the strain cross-sectional area _S1 within the above range, the thickness _T1 of the first pressure-sensitive adhesive layer is preferably 100 μm or less, more preferably 90 μm or less, and even more preferably 80 μm or less. _T1 may be 70 μm or less, 60 μm or less, or 50 μm or less.

From the viewpoint of providing the first pressure-sensitive adhesive layer 21 with appropriate impact resistance and flexibility, the storage modulus G' of the first pressure-sensitive adhesive layer 21 at 25°C is preferably 0.35 MPa or less, more preferably 0.30 MPa or less, and even more preferably 0.25 MPa or less. On the other hand, if the pressure-sensitive adhesive layer 21 is excessively soft, the creep rate _C1 increases, and the strain cross-sectional area _S1 increases accordingly, which may cause peeling or wrinkles at the curved surface portion. Therefore, the storage modulus of the first pressure-sensitive adhesive layer 21 at 25°C is preferably 0.05 MPa or more, and more preferably 0.10 MPa or more. From the same viewpoint, the storage modulus of the second pressure-sensitive adhesive layer 22 at 25°C is preferably 0.05 MPa or more, and more preferably 0.10 MPa or more.

<Release film>
As shown in Fig. 1, the double-sided adhesive-attached optical film preferably has release films 41, 42 temporarily attached to the surfaces of the adhesive layers 21, 22 until it is used to form an image display cell. As the release films 41, 42, those having a release layer on the surface of the film substrate are preferably used. Examples of materials for the release layer include silicone-based release agents, fluorine-based release agents, long-chain alkyl-based release agents, and fatty acid amide-based release agents. Silicone release agents are preferred because they can achieve both adhesion to acrylic adhesives and releasability.

The film substrate of the release film is preferably a resin film having transparency. Examples of the resin material include polyester-based resins such as polyethylene terephthalate and polyethylene naphthalate, acetate-based resins, polyethersulfone-based resins, polycarbonate-based resins, polyamide-based resins, polyimide-based resins, polyolefin-based resins, (meth)acrylic-based resins, polyvinyl chloride-based resins, polyvinylidene chloride-based resins, polystyrene-based resins, polyvinyl alcohol-based resins, polyarylate-based resins, and polyphenylene sulfide-based resins. Among these, polyester-based resins such as polyethylene terephthalate (PET) are particularly preferred. The thickness of the release film is preferably 10 to 200 μm, and more preferably 25 to 80 μm. The thickness of the first release film 41 and the thickness of the second release film 42 may be the same or different.

[Formation of image display device]
The image display device 100 is formed by bonding the cover window 80 to the first adhesive layer 21 and bonding the image display cell 70 to the second adhesive layer 22. The order of bonding is not particularly limited, and the bonding of the cover window 80 may be performed first, the bonding of the image display cell 70 may be performed first, or both may be performed simultaneously.

After the cover window 80 and the image display cell 70 are bonded to the adhesive layers 21 and 22, heat, pressure, depressurization, and other treatments may be performed to remove air bubbles at the bonding interface and near the printing steps. Autoclave treatment may also be performed to suppress delay bubbles, etc.

As described above, since the first adhesive layer 21 and the second adhesive layer 22 have predetermined properties, peeling between layers and the occurrence of wrinkles in the optical film 10 and the image display cell 70 can be suppressed even in an image display device in which the cover window 80 has a curved portion.

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.

[Preparation of Pressure-Sensitive Adhesive Sheets and Measurement of Creep]
<Adhesives A to N>
Monomers (total 100 parts by weight) were charged in a reaction vessel in the weight ratio shown in Table 1, 0.1 parts by weight of a photopolymerization initiator (BASF's "Irgacure 184") was added, and ultraviolet light was irradiated under a nitrogen atmosphere to obtain a prepolymer composition with a polymerization rate of about 10%. To 100 parts by weight of this prepolymer composition, 0.2 parts by weight of a photopolymerization initiator (BASF's "Irgacure 651"), 1,6-hexanediol diacrylate (HDDA) in the amount shown in Table 1 as a polyfunctional monomer, and 0.3 parts by weight of a silane coupling agent (Shin-Etsu Chemical's "KBM-403") were added and mixed uniformly to prepare a pressure-sensitive adhesive composition.

The above-mentioned pressure-sensitive adhesive composition was applied to the release-treated surface of a release film (a polyester film with one side treated with silicone for release) to a thickness of 50 μm to form a coating layer, and the release-treated surface of another release film was attached to the coating layer. Then, polymerization was carried out by irradiating UV rays with a black light whose position was adjusted so that the irradiation intensity on the irradiated surface directly under the lamp was 5 mW/ ^cm2 until the accumulated light amount reached 3000 mJ/ ^cm2 , thereby obtaining a pressure-sensitive adhesive sheet with release films temporarily attached to both sides.

<Adhesives O and P>
Monomers (total 100 parts by weight), 233 parts by weight of ethyl acetate, and 0.2 parts by weight of azobisisobutyronitrile (AIBN) as a thermal polymerization initiator were added to a reaction vessel in the weight ratio shown in Table 1, and the mixture was stirred for 1 hour under a nitrogen atmosphere at 23°C, followed by nitrogen substitution. The mixture was then reacted at 65°C for 5 hours, and then reacted at 70°C for 2 hours to prepare a solution of an acrylic polymer having a weight average molecular weight of about 450,000. 0.27 parts by weight of trimethylolpropane xylylene diisocyanate (Mitsui Chemicals'"TakenateD110N") was added to this solution as an isocyanate crosslinking agent to prepare a pressure-sensitive adhesive composition.

The above adhesive composition was applied to the release-treated surface of a release film so that the thickness after drying would be 50 μm, and the adhesive was heated at 130°C for 3 minutes to remove the solvent, after which the release-treated surface of another release film was superimposed on the adhesive layer to obtain an adhesive sheet with release films temporarily attached to both sides.

<Measurement of creep amount and storage modulus>
The release film was peeled off and the pressure-sensitive adhesive layer was laminated to a thickness of about 1.5 mm to prepare a measurement sample. Creep measurement and dynamic viscoelasticity measurement were performed under the following conditions using a rotational rheometer (Rheometric Scientific's "Advanced Rheometric Expansion System (ARES)") equipped with a parallel plate of 8.0 mmφ. The creep rate C was calculated from the strain amount δ in the creep measurement, and the storage modulus at 25°C was read from the results of the dynamic viscoelasticity measurement.
(Creep measurement)
Stress: 10 kPa
Measurement temperature: 25°C
(Dynamic viscoelasticity measurement)
Measurement frequency: 1Hz
Heating rate: 5° C./min

The compositions of adhesives A to P, as well as the measurement results of the creep rate and storage modulus of a 50 μm thick adhesive sheet, are shown in Table 1.

[Preparation of polarizing plate with double-sided adhesive]
Using a roll laminator, adhesive sheet 1 was attached to one side (on the hard coat layer) of a polarizing plate having a thickness of about 75 μm, and adhesive sheet 2 was attached to the other side to obtain a long polarizing plate with double-sided adhesive (samples 1 to 42). The polarizing plate used was one in which a triacetyl cellulose film (32 μm) provided with a hard coat layer was attached to one side of a polarizer made of a stretched polyvinyl alcohol film having a thickness of 12 μm impregnated with iodine, and a triacetyl cellulose film (31 μm) provided with a coating retardation layer was attached to the other side of the polarizer.

The adhesive sheets 1 and 2 used were those shown in Table 2. The adhesive types A to P shown in Table 2 were the same as the adhesives used in the adhesive sheets A to P in the adhesive sheet production examples described above, and the thicknesses were changed as shown in Table 2. The strain cross-sectional area S of the adhesive sheet was calculated based on the adhesive sheet thickness T and the creep amount C measured using adhesive sheets A to P with a thickness of 50 μm, according to the following formula.
S=(1/2)×CT ²

<Bonding evaluation>
The release film was peeled off from the adhesive sheet 2 of samples 1 to 42, and a polyimide film ("Kapton EN100" manufactured by Toray DuPont) having a thickness of 25 μm was laminated by roll-to-roll. The sample was cut into a square of 40 mm x 40 mm, the release film was peeled off from the adhesive sheet 1, and the adhesive sheet 1 was laminated to the center of the inner surface of a spherical glass having a thickness of 2 mm, a diameter of 65 mm, and an inner surface curvature radius of 100 mm. The lamination was performed using a vacuum laminator under conditions of vacuum pressure of 100 Pa, pressure of 0.2 MPa, and pressure time of 10 seconds at room temperature. Then, the sample was treated in an autoclave at 50°C and 0.5 MPa for 15 minutes.

The sample after autoclaving was placed in a heating oven at 85° C. for 240 hours, and then the sample was taken out and the state of bonding was visually inspected and evaluated according to the following criteria.
◯: The adhesion state of each layer was maintained. △: Wrinkles less than 5 mm in width were observed. ×: Wrinkles or peeling more than 5 mm in width were observed.

The adhesive sheets used in samples 1 to 42 (type of adhesive, thickness of adhesive sheet, and strain cross-sectional area) and the results of the bonding evaluation are shown in Table 2.

As shown in Table 2, the small thickness of Adhesive Sheet 1 and Adhesive Sheet 2 reduces the strain cross-sectional area, which can suppress peeling and wrinkling when attached to a curved rigid member.

10 Optical film (circular polarizing plate)
21, 22 Adhesive layer 41, 42 Release film 50 Optical film with double-sided adhesive 70 Image display cell (organic EL cell)
71 Flexible substrate 80 Transparent plate (cover window)
100 Image display device

Claims (3)

An image display device in which at least a part of a screen has a three-dimensional curved shape,
a rigid transparent plate having a first main surface and a second main surface, the second main surface being on the inside, and having a three-dimensional curved portion;
an optical film having a first major surface and a second major surface;
A flexible image display cell;
a first pressure-sensitive adhesive layer bonding a first main surface of the optical film to a second main surface of the transparent plate; and a second pressure-sensitive adhesive layer bonding the second main surface of the optical film to the image display cell,
The first pressure-sensitive adhesive layer has a strain cross-sectional area _S1 of 800 μm2 or ^less ,
The second pressure-sensitive adhesive layer has a strain cross-sectional area _S2 of 360 μm2 or ^less .
Image display device:
Here, the strain cross-sectional area S is an amount expressed by S = (1/2) × δT using the amount of strain δ in the shear direction when a shear load of 10 kPa is applied to the pressure-sensitive adhesive layer for 15 minutes and the thickness T of the pressure-sensitive adhesive layer,
A three-dimensional curved surface shape is a shape in which the cross-sectional shape of every plane including the normal line of the plane is a curve. The image display device according to claim 1 , wherein the optical film comprises a polarizer. The image display device according to claim 2 , wherein the optical film includes a circular polarizing plate, and the image display cell is an organic EL cell.

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