Optical adhesive layer

Abstract

To provide an optical adhesive layer suitable for application to a flexible device.SOLUTION: In an optical adhesive layer 10 of the present invention, a gap part maximum length L is 2 mm or less and an end-to-end distance D is 80 mm or more in a predetermined winding test executed to a test piece. The test piece is a laminate comprising a polyimide film having the thickness of 80 μm, a polarizing plate film having the thickness of 32 μm, a polyethylene terephthalate film having the thickness of 125 μm in this order in a thickness direction. An optical adhesive layer 10 bonds between the polyimide film and the polarizing plate film and between the polarizing plate film and the polyethylene terephthalate film. The gap part maximum length L is the maximum length of an interlayer gap in the laminate. The end-to-end distance D is a distance between both ends in a lengthwise direction of the laminate.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical adhesive layer.

Background Art

[0002] A display panel has a laminated structure including, for example, a pixel panel, a polarizing film, a touch panel, and a cover film. In the manufacturing process of such a display panel, an adhesive layer (optical adhesive layer) formed from a transparent adhesive for optical use is used for joining the elements included in the laminated structure.

[0003] On the other hand, for example, for smartphones and tablet terminals, the development of foldable display panels that can be repeatedly bent is in progress. A foldable display panel is specifically capable of repeatedly deforming between a bent shape and a flat non-bent shape. In such a foldable display panel, each element in the laminated structure is made to be repeatedly foldable, and a thin optical adhesive layer is used for joining such elements. Regarding the optical adhesive layer for flexible devices such as foldable display panels, it is described in, for example, Patent Document 1 below.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In foldable display panels, the optical adhesive layer tends to peel off from the substrate at areas that are repeatedly bent. This is because, when the display panel is bent, stresses such as shear stress act locally on the optical adhesive layer at the bent area. On the other hand, the optical adhesive layer at areas that are repeatedly bent in foldable display panels is required to suppress deformation marks (which occur in areas where the shape has not fully recovered after deformation) when not bent. The optical adhesive layer for foldable display panels is required to achieve a high level of both resistance to peeling off from the substrate when the display is bent and suppression of deformation marks when not bent.

[0006] As for flexible devices, the development of rollable display panels is also progressing. Rollable display panels can be repeatedly deformed between, for example, the wound shape after the entire panel has been wound up and the flat shape after the entire panel has been unwound. In such rollable display panels, each element in the laminated structure is manufactured to be repeatedly deformable, and a thin optical adhesive layer is used to join these elements. The optical adhesive layer for rollable display panels requires a very high level of compatibility between being difficult to peel off from the elements as adherends when the display is in the wound shape and suppressing deformation marks when it is in the flat shape.

[0007] This invention provides an optical adhesive layer suitable for flexible device applications. [Means for solving the problem]

[0008] The present invention [1] includes an optical adhesive layer in which the maximum length L of the gap in a winding test in which the following steps 1 to 6 are performed is 2 mm or less, and the distance D between edges is 80 mm or more.

[0009] Step 1: A laminate is prepared comprising a polyimide film with a thickness of 80 μm, a polarizing plate film with a thickness of 32 μm, and a polyethylene terephthalate film with a thickness of 125 μm, in this order in the thickness direction, wherein the polyimide film and the polarizing plate film, and the polarizing plate film and the polyethylene terephthalate film are joined together by the optical adhesive layer.

[0010] Step 2: The laminate is subjected to heating and pressurizing treatment at 50°C and 0.5 MPa for 15 minutes.

[0011] Step 3: A test specimen is cut from the laminate after the second step. The test specimen has dimensions of 100 mm in length and 25 mm in width, with a first end at one end in the longitudinal direction and a second end at the other end.

[0012] Step 4: The test specimen is wound around a core with a cross-sectional diameter of 20 mm in the following manner, such that the length of the test specimen is aligned with the circumferential direction of the core. First, the polyimide film side of the first end of the test specimen is attached to the core via double-sided tape. Next, the test specimen is wound around the core 1.628 times while being pulled in the length direction, such that the polyimide film side of the test specimen is positioned on the inside of the winding. Next, the second end of the test specimen wound around the core is secured with adhesive tape.

[0013] Step 5: The test specimen with the winding core is stored at a temperature of 23°C for 48 hours.

[0014] Step 6: After the fifth step, within five minutes of releasing the winding of the test specimen from the core, the distance between edges D and the maximum void length L of the test specimen are measured. The distance between edges D is the distance between the two ends of the test specimen in the longitudinal direction. The maximum void length L is the maximum length of the interlayer void in the test specimen.

[0015] The configuration in the above winding test, in which the maximum gap length L is 2 mm or less and the edge-to-edge distance D is 80 mm or more, is suitable for an optical adhesive layer for flexible devices, as it simultaneously suppresses peeling from the adherend during device deformation and suppresses deformation marks when not deformed. Therefore, the optical adhesive layer of the present invention is suitable for flexible device applications.

[0016] The present invention [2] includes an optical adhesive layer as described in [1] above, wherein the same steps as those described in the first to sixth steps are performed in a winding test, except that the temperature condition in the fifth step is -20°C, the maximum length L of the gap is 2 mm or less, and the distance D between the edges is 80 mm or more.

[0017] Such a configuration is preferable for achieving both suppression of delamination from the adherend during device deformation and suppression of deformation marks during non-deformed states in low-temperature environments.

[0018] The present invention [3] includes an optical adhesive layer according to [1] or [2] above, wherein the change in transmittance after 1 hour has elapsed since being wound onto a core with a cross-sectional diameter of 20 mm.

[0019] Such a configuration is preferable for ensuring transparency as an optical adhesive layer for flexible device applications.

[0020] The present invention [4] includes an optical adhesive layer according to any one of [1] to [3] above, wherein the difference between the maximum thickness and the minimum thickness is 3 μm or less.

[0021] Such a configuration is preferable for suppressing stress concentration in the adherend when the adherend in contact with the optical adhesive layer is deformed.

[0022] The present invention [5] includes a low adhesive layer having a first surface and a second surface opposite to the first surface, a first high adhesive layer disposed on the first surface, the first high adhesive layer having a first high adhesive surface on the side opposite to the low adhesive layer, and a second high adhesive layer disposed on the second surface, the second high adhesive layer having a second high adhesive surface on the side opposite to the low adhesive layer, each of the first high adhesive surface and the second high adhesive surface having a peel adhesive force of 5 N / 25 mm or more with respect to the polyimide film under the conditions of a peel angle of 180° and a peel speed of 300 mm / min after 30 minutes have elapsed at 23°C from the bonding to the polyimide film, and the shear storage modulus of the low adhesive layer at -20°C being smaller than the shear storage modulus of the high adhesive layer at -20°C, and includes the optical adhesive layer according to any one of [1] to [4] above.

[0023] The configuration in which each of the first high adhesive surface and the second high adhesive surface in the optical adhesive layer has a peel adhesive force of 5 N / 25 mm or more under predetermined conditions is preferable for ensuring good adhesion of the optical adhesive layer to the adherend by the high adhesive surface, and thus is preferable for suppressing peeling of the optical adhesive layer from the adherend. Further, the configuration in which the shear storage modulus of the low adhesive layer at -20°C is smaller than the shear storage modulus of the high adhesive layer at -20°C is preferable for ensuring the overall softness of the optical adhesive layer and ensuring bending deformability. The softer the optical adhesive layer is, when the adherend to which the optical adhesive layer is bonded is deformed with a relatively large curvature (such as the above-described bending deformation and winding deformation), the optical adhesive layer is likely to be deformed with a large curvature following the deformation of the adherend. The fact that the optical adhesive layer is soft and likely to be deformed with a large curvature (bending deformability) is preferable for realizing good repeated deformation (such as repeated bending deformation and winding deformation) of the flexible device to which the optical adhesive layer is used.

[0024] The present invention [6] includes the optical adhesive layer as described in [5] above, in which the ratio of the thickness of the low-tack adhesive layer to the total thickness of the first high-tack adhesive layer and the second high-tack adhesive layer is 1 or more.

[0025] Such a configuration is preferable for ensuring the flexibility and bendability of the optical adhesive layer.

[0026] The present invention [7] includes the optical adhesive layer as described in [5] or [6] above, in which the ratio of the thickness of the low-tack adhesive layer to the total thickness of the first high-tack adhesive layer and the second high-tack adhesive layer is 30 or less.

[0027] Such a configuration is preferable for ensuring the firmness of the optical adhesive layer and ensuring good handling properties.

[0028] The present invention [8] includes the optical adhesive layer as described in any one of [1] to [7] above, which has a total thickness of 5 μm or more and 150 μm or less.

[0029] A configuration in which the total thickness of the optical adhesive layer is 5 μm or more is preferable for ensuring the cohesive force of the optical adhesive layer and ensuring high adhesive strength. A configuration in which the total thickness of the optical adhesive layer is 150 μm or less is preferable for ensuring good deformability (ease of deformation) of the optical adhesive layer.

Brief Description of the Drawings

[0030] [Figure 1] It is a schematic cross-sectional view of an embodiment of the optical adhesive layer of the present invention. [Figure 2] It is a partially enlarged cross-sectional view when the optical adhesive layer shown in FIG. 1 has a three-layer structure. [Figure 3] It is a schematic cross-sectional view of the laminate in the winding test. [Figure 4] It is a side view of an example of the test piece in the 6th step in the winding test. [Figure 5] It is a schematic plan view of an example of the test piece in the 6th step in the winding test. [Figure 6]This shows an example of how to use the optical adhesive layer of the present invention. Figure 6A shows the step of attaching the optical adhesive layer to a first adherend, Figure 6B shows the step of joining the first adherend and the second adherend via the optical adhesive layer, and Figure 6C shows the aging step. [Modes for carrying out the invention]

[0031] As one embodiment of the optical adhesive layer of the present invention, the optical adhesive layer 10 has a sheet shape of a predetermined thickness, as shown in Figure 1, and extends in a direction perpendicular to the thickness direction H (surface direction). Figure 1 illustrates a state in which the optical adhesive layer 10 is manufactured as an optical adhesive sheet S, and release films L1 and L2 are bonded to both sides of the sheet.

[0032] The optical adhesive layer 10 has a single-layer structure or a multilayer structure (not shown in Figure 1). Figure 2 is a partially enlarged cross-sectional view of an example of a three-layer optical adhesive layer 10. The optical adhesive layer 10 shown in Figure 2 comprises a low-tack layer 11 and two high-tack layers 12 (12A, 12B). Specifically, the optical adhesive layer 10 is a multilayer adhesive layer comprising a high-tack layer 12A, a low-tack layer 11, and a high-tack layer 12B in order in the thickness direction H. The high-tack layer 12 is an adhesive layer with relatively strong surface adhesion, and the low-tack layer 11 is an adhesive layer with relatively low surface adhesion. The low-tack layer 11 has a first surface 11a and a second surface 11b opposite to the first surface 11a. The high-tack layer 12A (first high-tack layer) is arranged on the first surface 11a. The high-tack layer 12A has a high-tack surface 12a (first high-tack surface) on the side opposite to the low-tack layer 11. The high-tack surface 12a is one of the adhesive surfaces of the optical adhesive layer 10. The high-tack layer 12B (second high-tack layer) is positioned on the second surface 11b. The high-tack layer 12B has a high-tack surface 12b (second high-tack surface) on the side opposite to the low-tack layer 11. The high-tack surface 12b of the high-tack layer 12B is the other adhesive surface of the optical adhesive layer 10. The adhesive strength of the high-tack surface 12a of the high-tack layer 12A and the adhesive strength of the high-tack surface 12b of the high-tack layer 12B may be the same or different.

[0033] Such an optical adhesive layer 10 is a transparent adhesive layer placed in the light-transmitting portion of a flexible device. Examples of flexible devices include flexible display panels. A flexible display panel has a laminated structure including, for example, a pixel panel, a touch panel, a polarizing film, and a cover film. Examples of flexible display panels include foldable display panels and rollable display panels. The optical adhesive layer 10 is used, for example, in the manufacturing process of a flexible display panel to bond elements included in the laminated structure together.

[0034] In the wrapping test (first wrapping test) in which the following steps 1 to 6 are performed, the optical adhesive layer 10 has a maximum void length L of 2 mm or less and an edge-to-edge distance D of 80 mm or more.

[0035] Step 1: A laminate W with the laminated structure shown in Figure 3 is prepared. The laminate W comprises a polyimide film F1 with a thickness of 80 μm, a polarizing plate film F2 with a thickness of 32 μm, and a polyethylene terephthalate film F3 with a thickness of 125 μm, arranged in this order in the thickness direction H. In the laminate W, the polyimide film F1 and the polarizing plate film F2, and the polarizing plate film F2 and the polyethylene terephthalate film F3 are joined by an optical adhesive layer 10.

[0036] Step 2: The laminate W is subjected to heating and pressurizing treatment at 50°C and 0.5 MPa for 15 minutes.

[0037] Step 3: A test specimen is cut from the laminate W after the second step. The test specimen Z has dimensions of 100 mm in length and 25 mm in width, with a first end E1 at one end in the longitudinal direction and a second end E2 at the other end.

[0038] Step 4: The test piece Z is wound around a core with a cross-sectional diameter of 20 mm in the following manner, such that the length of the test piece Z is aligned with the circumferential direction of the core. First, the polyimide film F1 side of the first end E1 of the test piece Z is attached to the core via double-sided tape. Next, the test piece Z is wound around the core 1.628 times while being pulled in the length direction, such that the polyimide film F1 side of the test piece Z is positioned on the inside of the winding (the test piece Z is wound around the core without any gaps between the core and the test piece Z, and without any gaps between the test pieces Z in the diameter direction of the core). Next, the second end E2 of the test piece Z wound around the core is fixed with adhesive tape.

[0039] Step 5: Test specimen Z with a coiled core is stored at a temperature of 23°C for 48 hours.

[0040] Step 6: After step 5, within 5 minutes of releasing the winding of the specimen Z from the core, the edge-to-edge distance D and the maximum void length L of the specimen Z are measured. The edge-to-edge distance D is the distance between the two ends of the specimen Z in the longitudinal direction, as shown in Figure 4. The maximum void length L is the maximum length of the interlayer void V (shown in Figure 5) in the specimen Z. The interlayer void V in the specimen includes voids at the interface between the optical adhesive layer 10 (first optical adhesive layer 10) between films F1 and F2 in the specimen Z and the films F1 and F2, voids within the first optical adhesive layer 10, voids at the interface between the optical adhesive layer 10 (second optical adhesive layer 10) between films F2 and F3 and the films F2 and F3, and voids within the second optical adhesive layer 10. The maximum length of the interlayer void V is specifically the maximum dimension (maximum dimension in plan view) of the void V in the flat specimen Z when the interlayer void V is present. If the interlayer void V is not present, the maximum void length L is 0 mm. In this step, preferably, the maximum void length L is measured after measuring the distance D between the edges.

[0041] The configuration in the above winding test, in which the maximum length L of the void is 2 mm or less and the distance D between the edges is 80 mm or more, is suitable for an optical adhesive layer 10 for flexible devices to achieve both suppression of peeling from the adherend during device deformation and suppression of deformation marks when not deformed. Therefore, the optical adhesive layer 10 is suitable for flexible device applications. From the viewpoint of suppressing peeling, the maximum length L of the void is preferably 1.5 mm or less, more preferably 1.0 μm or less, and even more preferably 0.5 mm or less. If a void V extending from the edge of the test piece Z (for example, a void V extending from one edge in the longitudinal direction of the test piece Z toward the other edge) occurs, the length of the void V is preferably 1.5 mm or less, more preferably 1.0 μm or less, and even more preferably 0.5 mm or less. If a void V exists that does not reach the edge of the test specimen Z, the length of the void V is preferably 1.5 mm or less, more preferably 1.0 μm or less, and even more preferably 0.5 mm or less. Furthermore, from the viewpoint of suppressing deformation marks, the distance D between the edges is preferably 82 mm or more, more preferably 85 mm or more, and even more preferably 90 mm or more.

[0042] In the second wrapping test, the optical adhesive layer 10 is subjected to the same steps as the first wrapping test, except that the storage temperature in step 5 is -20°C, and the maximum length L of the void in the second wrapping test is preferably 2 mm or less, more preferably 1.5 mm or less, even more preferably 1.0 μm or less, and particularly preferably 0.5 mm or less. Furthermore, the distance D between the edges is preferably 80 mm or more, more preferably 82 mm or more, even more preferably 85 mm or more, and particularly preferably 90 mm or more. Such a configuration is preferable for achieving both suppression of peeling from the adherend when the device is deformed and suppression of deformation marks when the device is not deformed, in a low-temperature environment.

[0043] The change in transmittance of the optical adhesive layer 10 after 1 hour has elapsed since winding the optical adhesive layer 10 onto a core with a cross-sectional diameter of 20 mm is preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less. Such a configuration is suitable for ensuring transparency in the optical adhesive layer 10 as an optical adhesive layer for flexible device applications. The above change in transmittance of the optical adhesive layer 10 can be measured specifically by the method described later with respect to the examples.

[0044] In the optical adhesive layer 10, the difference between the maximum thickness and the minimum thickness is preferably 3 μm or less, more preferably 2 μm or less, and even more preferably 1 μm or less. This configuration is preferable for suppressing stress concentration in the adherend when the adherend in contact with the optical adhesive layer 10 deforms. Furthermore, this configuration regarding the difference in thickness is also preferable from the viewpoint of visibility of the flexible device (optical device) having the optical adhesive layer 10 in a laminated structure.

[0045] The optical adhesive layer 10 preferably has a multilayer structure as shown in Figure 2. This configuration is preferable for achieving both peel resistance and deformation mark resistance, as described above, by having each adhesive layer in the optical adhesive layer 10 perform a specific function.

[0046] Each of the high-tack surfaces 12a and 12b has a peel adhesive strength F of preferably 5 N / 25 mm or more after 30 minutes at 23°C following lamination of the optical adhesive layer 10 to the polyimide film, under conditions of a peel angle of 180° and a peel speed of 300 mm / min. Lamination of the optical adhesive layer 10 to the polyimide film is performed by applying a load of 2 kg rollers back and forth once in an environment of 23°C. From the viewpoint of ensuring good adhesion to the adherend, the peel adhesive strength F is preferably 7 N / 25 mm or more, more preferably 9 N / 25 mm or more, and even more preferably 11 N / 25 mm or more. The peel adhesive strength F is, for example, 30 N / 25 mm or less. Methods for adjusting the peel adhesive strength F include, for example, selecting the type of base polymer in the adhesive layer, adjusting the molecular weight, and adjusting the blending amount. The selection of the type of base polymer includes adjusting the composition of the monomers that form the base polymer. Methods for adjusting the peel adhesion strength F include selecting the type of components other than the base polymer in the adhesive layer and adjusting the amount of said components. Examples of such components include crosslinking agents, silane coupling agents, and oligomers. Another method for adjusting the peel adhesion strength F of a multilayer optical adhesive layer 10 is to adjust the thickness of each adhesive layer contained in the optical adhesive layer 10.

[0047] The shear storage modulus of the low-tack layer 11 at -20°C is preferably smaller than that of the high-tack layer at -20°C. This configuration is preferable for ensuring the overall softness of the optical adhesive layer 10 and thus ensuring its flexural deformability. The softer the optical adhesive layer 10 is, the easier it is for the optical adhesive layer 10 to deform with a large curvature when the adherend to which the optical adhesive layer 10 is bonded deforms with a relatively large curvature (such as the bending and winding deformations described above). The softness and ease with which the optical adhesive layer 10 deforms with a large curvature (flexural deformability) is preferable for achieving good repeated deformation (such as repeated bending and winding deformations) in flexible devices using the optical adhesive layer 10.

[0048] The shear storage modulus of the optical adhesive layer 10 at -20°C is preferably 180 kPa or less, more preferably 150 kPa or less, even more preferably 130 kPa or less, and particularly preferably 100 kPa or less. Such a configuration is preferable for ensuring flexibility and bendability suitable for flexible devices in the optical adhesive layer 10. The shear storage modulus at -20°C is preferably 30 kPa or more, more preferably 40 kPa or more, even more preferably 50 kPa or more, and particularly preferably 60 kPa or more. This is preferable for ensuring cohesive force of the optical adhesive layer 10. The shear storage modulus of the optical adhesive layer 10 can be measured by a dynamic viscoelasticity measuring device. In the measurement, the measurement mode is set to shear mode, the measurement temperature range is set to -60°C to 150°C, the heating rate is set to 5°C / min, and the frequency is set to 1 Hz. Specifically, this is described later with respect to the examples. Methods for adjusting the shear storage modulus of the adhesive layer include, for example, selecting the type of base polymer in the adhesive layer, adjusting its molecular weight and blending amount, and selecting the type of crosslinking agent that crosslinks the base polymer and adjusting its blending amount. The selection of the type of base polymer includes adjusting the composition of the monomers that form the base polymer. Specifically, the selection of the type of base polymer includes selecting the type of main chain in the base polymer, and selecting the type and blending amount of functional groups. Furthermore, if the optical adhesive layer 10 has a multilayer structure, methods for adjusting the shear storage modulus of the optical adhesive layer 10 include, for example, adjusting the shear storage modulus of each adhesive layer in the optical adhesive layer 10, and adjusting the thickness of each adhesive layer.

[0049] The ratio of the thickness of the low-tack layer 11 to the sum of the thicknesses of the high-tack layers 12A and 12B is preferably 1 or more, more preferably 3 or more, even more preferably 6 or more, and particularly preferably 9 or more. Such a configuration is preferable for ensuring the flexibility and bendability of the optical adhesive layer 10 as described above.

[0050] The ratio of the thickness of the low-tack layer 11 to the sum of the thicknesses of the high-tack layers 12A and 12B is preferably 30 or less, more preferably 25 or less, and even more preferably 20 or less. Such a configuration is preferable for ensuring the stiffness of the optical adhesive layer 10 and ensuring good handling.

[0051] The thickness of the low-tack layer 11 is preferably 3 μm or more, more preferably 10 μm or more, and more preferably 15 μm or more. This configuration is preferable for ensuring the flexibility and bendability of the optical adhesive layer 10. The thickness of the low-tack layer 11 is preferably 100 μm or less, more preferably 70 μm or less, and even more preferably 50 μm or less. This configuration is preferable for ensuring the stiffness of the optical adhesive layer 10 and ensuring good handling.

[0052] The thickness of the single high-tack layer 12 is preferably 0.1 μm or more, more preferably 0.5 μm or more, and more preferably 1 μm or more. Such a configuration is preferable for ensuring high adhesion by ensuring the cohesive force of the high-tack layer 12. The thickness of the single high-tack layer 12 is, for example, 30 μm or less.

[0053] The total thickness of the optical adhesive layer 10 is preferably 5 μm or more, more preferably 10 μm or more, and more preferably 15 μm or more, from the viewpoint of ensuring cohesive force and high adhesive strength. The total thickness of the optical adhesive layer 10 is preferably 150 μm or less, more preferably 100 μm or less, and more preferably 50 μm or less, from the viewpoint of ensuring good deformability (ease of deformation).

[0054] In the optical adhesive layer 10, the difference between the maximum thickness and the minimum thickness is preferably 3 μm or less, more preferably 2 μm or less, and even more preferably 1 μm or less. This configuration is preferable for suppressing stress concentration in the adherend when the adherend in contact with the optical adhesive layer 10 deforms. Furthermore, this configuration regarding the difference in thickness is also preferable from the viewpoint of visibility of the flexible device (optical device) having the optical adhesive layer 10 in a laminated structure.

[0055] The change in transmittance of the optical adhesive layer 10 after 1 hour has elapsed since winding the optical adhesive layer 10 onto a core with a cross-sectional diameter of 20 mm is preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less. Such a configuration is suitable for ensuring transparency as an optical adhesive layer for flexible device applications. The above change in transmittance of the optical adhesive layer 10 can be measured specifically by the method described later with respect to the examples.

[0056] The optical adhesive layer 10 is a pressure-sensitive adhesive layer formed from an adhesive composition. The optical adhesive layer 10 contains at least a base polymer. When the optical adhesive layer 10 has a low-tack layer 11 and high-tack layers 12A and 12B as shown in Figure 2, each adhesive layer is a pressure-sensitive adhesive layer formed from an adhesive composition. The low-tack layer 11 has a different composition from the high-tack layers 12A and 12B. The high-tack layers 12A and 12B may have the same composition or different compositions. Each adhesive layer contains at least a base polymer.

[0057] The base polymer is an adhesive component that provides tackiness in the adhesive layer. Examples of base polymers include acrylic polymers, silicone polymers, polyester polymers, polyurethane polymers, polyamide polymers, polyvinyl ether polymers, vinyl acetate / vinyl chloride copolymers, modified polyolefin polymers, epoxy polymers, fluoropolymers, and rubber polymers. The base polymer may be used alone or in combination of two or more types. From the viewpoint of ensuring good transparency and tackiness in the adhesive layer, acrylic polymers are preferably used as the base polymer.

[0058] Acrylic polymers are copolymers of monomer components containing 50% or more by mass of alkyl (meth)acrylate. "(meth)acrylic acid" refers to acrylic acid and / or methacrylic acid.

[0059] As the alkyl (meth)acrylate ester, an alkyl (meth)acrylate ester having 1 to 20 carbon atoms in the alkyl group is preferably used. The alkyl (meth)acrylate ester may have a linear or branched alkyl group, or a cyclic alkyl group such as an alicyclic alkyl group.

[0060] Examples of alkyl (meth)acrylates having linear or branched alkyl groups 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, Examples include nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate (i.e., lauryl 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.

[0061] Examples of alkyl (meth)acrylates having an alicyclic alkyl group include cycloalkyl (meth)acrylates, (meth)acrylates having a bicyclic aliphatic hydrocarbon ring, and (meth)acrylates having three or more aliphatic hydrocarbon rings. Examples of cycloalkyl (meth)acrylates include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, and cyclooctyl (meth)acrylate. An example of a (meth)acrylate ester having a bicyclic aliphatic hydrocarbon ring is isobornyl (meth)acrylate. Examples of (meth)acrylic acid esters having three or more aliphatic hydrocarbon rings include 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.

[0062] Preferably, the alkyl (meth)acrylate ester is an alkyl acrylate having an alkyl group with 3 to 15 carbon atoms, and more preferably, at least one selected from the group consisting of n-butyl acrylate, 2-ethylhexyl acrylate, and dodecyl acrylate. Preferably, the alkyl (meth)acrylate ester in the base polymer for the low-tack layer 11 is at least one selected from the group consisting of 2-ethylhexyl acrylate and dodecyl acrylate. Preferably, n-butyl acrylate is used as the alkyl (meth)acrylate ester in the base polymer for the high-tack layer 12.

[0063] The proportion of alkyl (meth)acrylate in the monomer component is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, and particularly preferably 94% by mass or more, from the viewpoint of appropriately exhibiting basic properties such as tackiness in the adhesive layer. This proportion is, for example, 99% by mass or less.

[0064] The monomer component may include copolymerizable monomers that can copolymerize with alkyl (meth)acrylate esters. Examples of copolymerizable monomers include monomers having polar groups. Examples of polar group-containing monomers include monomers containing hydroxyl groups, monomers containing carboxyl groups, and monomers having nitrogen atom-containing rings. Polar group-containing monomers are useful for modifying acrylic polymers, such as introducing crosslinking sites into acrylic polymers and ensuring the cohesive strength of acrylic polymers.

[0065] Examples of hydroxyl group-containing monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-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-hydroxymethylcyclohexyl)methyl (meth)acrylate. Preferably, at least one selected from the group consisting of 4-hydroxybutyl acrylate and 2-hydroxyethyl acrylate is used as the hydroxyl group-containing monomer.

[0066] The proportion of hydroxyl group-containing monomers in the monomer components is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 0.8% by mass or more, from the viewpoint of introducing a crosslinked structure into the acrylic polymer and ensuring cohesive force in the adhesive layer. From the viewpoint of adjusting the polarity of the acrylic polymer (related to the compatibility between various additive components in the adhesive layer and the acrylic polymer), the proportion is preferably 20% by mass or less, and more preferably 10% by mass or less.

[0067] Examples of monomers containing a carboxyl group include acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, and isocrotonic acid.

[0068] The proportion of carboxyl group-containing monomers in the monomer component is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 0.8% by mass or more, from the viewpoint of introducing a crosslinked structure into the acrylic polymer, ensuring cohesive force in the adhesive layer, and ensuring adhesion to the substrate in the adhesive layer. The same proportion is preferably 30% by mass or less, more preferably 20% by mass or less, from the viewpoint of adjusting the glass transition temperature of the acrylic polymer and avoiding the risk of corrosion of the substrate by acid.

[0069] To prevent corrosion of metal elements such as electrodes in flexible devices by acidic components, it is preferable that the adhesive layer has a low acid content. Furthermore, when the adhesive layer is used to bond polarizing films, it is preferable that the adhesive layer has a low acid content to suppress polyene formation of polyvinyl alcohol-based polarizers by acidic components. In such acid-free adhesive layers, the content of organic acid monomers (e.g., (meth)acrylic acid and carboxyl group-containing monomers) is preferably 100 ppm or less, more preferably 70 ppm or less, and even more preferably 50 ppm or less. The organic acid monomer content of the adhesive layer can be determined by immersing the adhesive layer in pure water and heating it at 100°C for 45 minutes, then quantifying the acid monomers extracted into the water using ion chromatography.

[0070] From an acid-free viewpoint, it is preferable that the base polymer in the adhesive layer substantially does not contain organic acid monomers as monomer components. From an acid-free viewpoint, the proportion of organic acid monomers in the monomer components is preferably 0.5% by mass or less, more preferably 0.1% by mass or less, even more preferably 0.05% by mass, and ideally 0% by mass.

[0071] Examples of monomers having a nitrogen atom-containing ring include N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole, N-(meth)acryloyl-2-pyrrolidone, N-(meth)acryloylpiperidine, N-(meth)acryloylpyrrolidine, N-vinylmorpholine, N-vinyl-3-morpholinone, N-vinyl-2-caprolactam, N-vinyl-1,3-oxazin-2-one, N-vinyl-3,5-morpholindione, N-vinylpyrazole, N-vinylisoxazole, N-vinylthiazole, and N-vinylisothiazole. N-vinyl-2-pyrrolidone is preferably used as the monomer having a nitrogen atom-containing ring.

[0072] The proportion of monomers having nitrogen atom-containing rings in the monomer components is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.55% by mass or more, from the viewpoint of ensuring cohesive force in the adhesive layer and ensuring adhesion force to the substrate in the adhesive layer. The same proportion is preferably 30% by mass or less, more preferably 20% by mass or less, from the viewpoint of adjusting the glass transition temperature of the acrylic polymer and adjusting the polarity of the acrylic polymer (related to the compatibility between various additive components in the adhesive layer and the acrylic polymer).

[0073] The monomer component may also contain other copolymerizable monomers. Examples of other copolymerizable monomers include acid anhydride monomers, sulfonic acid group-containing monomers, phosphate group-containing monomers, epoxy group-containing monomers, cyano group-containing monomers, alkoxy group-containing monomers, and aromatic vinyl compounds. These other copolymerizable monomers may be used individually or in combination of two or more types.

[0074] The base polymer preferably has a crosslinked structure. Methods for introducing a crosslinked structure to the base polymer include a first method in which a base polymer having a functional group reactive with a crosslinking agent and a crosslinking agent are blended into an adhesive composition and the base polymer and crosslinking agent are reacted in the adhesive layer, and a second method in which a polyfunctional monomer is included in the monomer component that forms the base polymer, and a base polymer in which a branched structure (crosslinked structure) is introduced into the polymer chain is formed by polymerization of the monomer component. These methods may be used in combination.

[0075] Examples of crosslinking agents used in the first method described above include compounds that react with functional groups (such as hydroxyl groups and carboxyl groups) contained in the base polymer. Examples of such crosslinking agents include isocyanate crosslinking agents, peroxide crosslinking agents, epoxy crosslinking agents, oxazoline crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, and metal chelate crosslinking agents. The crosslinking agent may be used alone or in combination of two or more types. As crosslinking agents, isocyanate crosslinking agents, peroxide crosslinking agents, and epoxy crosslinking agents are preferably used because they have high reactivity with hydroxyl groups and carboxyl groups in the base polymer and facilitate the introduction of crosslinked structures.

[0076] Examples of isocyanate crosslinking agents include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, tetramethyl xylylene diisocyanate, naphthalene diisocyanate, triphenylmethane triisocyanate, and polymethylene polyphenyl isocyanate. Derivatives of these isocyanates can also be used as isocyanate crosslinking agents. Examples of such isocyanate derivatives include isocyanurate-modified and polyol-modified derivatives. Examples of commercially available isocyanate crosslinking agents include Coronate L (trimethylolpropane adduct of tolylene diisocyanate, manufactured by Tosoh Corporation), Coronate HL (trimethylolpropane adduct of hexamethylene diisocyanate, manufactured by Tosoh Corporation), Coronate HX (isocyanurate of hexamethylene diisocyanate, manufactured by Tosoh Corporation), Takenate D110N (trimethylolpropane adduct of xylylene diisocyanate, manufactured by Mitsui Chemicals Corporation), and Takenate 600 (1,3-bis(isocyanatomethyl)cyclohexane, manufactured by Mitsui Chemicals Corporation).

[0077] Examples of peroxide crosslinking agents include dibenzoyl peroxide, di(2-ethylhexyl)peroxydicarbonate, di(4-t-butylcyclohexyl)peroxydicarbonate, di-sec-butylperoxydicarbonate, t-butylperoxyneodecanoate, t-hexylperoxypivalate, and t-butylperoxypivalate.

[0078] Examples of epoxy crosslinking agents include bisphenol A, epichlorohydrin-type epoxy resins, ethylene glycidyl ether, polyethylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, 1,6-hexanediol glycidyl ether, trimethylolpropane triglycidyl ether, diglycidylaniline, diamine glycidylamine, N,N,N',N'-tetraglycidyl-m-xylylenediamine, and 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane.

[0079] Isocyanate crosslinking agents (especially difunctional isocyanate crosslinking agents) and peroxide crosslinking agents are preferred from the viewpoint of ensuring appropriate flexibility (and therefore bendability) of the adhesive layer. Isocyanate crosslinking agents (especially trifunctional isocyanate crosslinking agents) are preferred from the viewpoint of ensuring durability of the adhesive layer. In the base polymer, difunctional isocyanate crosslinking agents and peroxide crosslinking agents form more flexible two-dimensional crosslinks, while trifunctional isocyanate crosslinking agents form stronger three-dimensional crosslinks. From the viewpoint of achieving both durability and flexibility of the adhesive layer, a combination of a trifunctional isocyanate crosslinking agent and a peroxide crosslinking agent and / or a difunctional isocyanate crosslinking agent is preferred.

[0080] From the viewpoint of ensuring the cohesive force of the adhesive layer, the amount of crosslinking agent is, for example, 0.01 parts by mass or more, preferably 0.05 parts by mass or more, and more preferably 0.07 parts by mass or more, per 100 parts by mass of the base polymer. From the viewpoint of ensuring good tackiness in the adhesive layer, the amount of crosslinking agent per 100 parts by mass of the base polymer is, for example, 10 parts by mass or less, preferably 5 parts by mass or less, and more preferably 3 parts by mass or less.

[0081] In the second method described above, the monomer components (including polyfunctional monomers and other monomers for introducing crosslinking structures) may be polymerized in a single step or in multiple steps. In the multi-step polymerization method, first, monofunctional monomers for forming the base polymer are polymerized (prepolymerization), thereby preparing a prepolymer composition containing a partially polymerized product (a mixture of a low-degree polymerized product and unreacted monomers). Next, polyfunctional monomers are added to the prepolymer composition, and then the partially polymerized product and the polyfunctional monomer are polymerized (main polymerization).

[0082] Examples of polyfunctional monomers include polyfunctional (meth)acrylates containing two or more ethylenically unsaturated double bonds in one molecule. From the viewpoint of being able to introduce crosslinked structures by active energy ray polymerization (photopolymerization), polyfunctional acrylates are preferred as polyfunctional monomers.

[0083] Examples of polyfunctional (meth)acrylates include difunctional (meth)acrylates, trifunctional (meth)acrylates, and polyfunctional (meth)acrylates with four or more functions.

[0084] Examples of difunctional (meth)acrylates include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol dimethacrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, glycerin di(meth)acrylate, neopentyl glycol di(meth)acrylate, stearic acid-modified pentaerythritol di(meth)acrylate, dicyclopentenyl diacrylate, di(meth)acryloyl isocyanurate, and alkylene oxide-modified bisphenol di(meth)acrylate.

[0085] Examples of trifunctional (meth)acrylates include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and tris(acryloyloxyethyl) isocyanurate.

[0086] Examples of polyfunctional (meth)acrylates with four or more functions include ditrimethylolpropanetetra(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipentaerythritol monohydroxypenta(meth)acrylate, alkyl-modified dipentaerythritol pentaacrylate, and dipentaerythritol hexa(meth)acrylate.

[0087] The molecular weight of the polyfunctional monomer is preferably 1500 or less, more preferably 1000 or less. The functional group equivalent (g / eq) of the polyfunctional monomer is preferably 50 or more, more preferably 70 or more, and even more preferably 80 or more. The functional group equivalent is preferably 500 or less, more preferably 300 or less, and even more preferably 200 or less. These configurations are preferred from the viewpoint of appropriately adjusting the viscoelasticity (e.g., storage modulus and loss tangent) by introducing a crosslinking structure in the base polymer.

[0088] Acrylic polymers can be formed by polymerizing the monomer components described above. Polymerization methods include, for example, solution polymerization, active energy ray polymerization (e.g., UV polymerization), bulk polymerization, and emulsion polymerization. From the viewpoint of transparency, water resistance, and cost of the adhesive layer, solution polymerization and UV polymerization are preferred. For example, ethyl acetate and toluene are used as solvents for solution polymerization. For example, thermal polymerization initiators and photopolymerization initiators are used as polymerization initiators. The amount of polymerization initiator used is, for example, 0.05 parts by mass or more, and for example, 1 part by mass or less, per 100 parts by mass of monomer components.

[0089] Examples of thermal polymerization initiators include azo polymerization initiators and peroxide polymerization initiators. Examples of azo polymerization initiators include 2,2'-azobisisobutyronitrile, 2,2'-azobis-2-methylbutyronitrile, 2,2'-azobis(2-methylpropionic acid)dimethyl, 4,4'-azobis-4-cyanovaleric acid, azobisisovaleronitrile, 2,2'-azobis(2-amidinopropane)dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis(2-methylpropionamidine)disulfate, and 2,2'-azobis(N,N'-dimethyleneisobutylamidine)dihydrochloride. Examples of peroxide polymerization initiators include dibenzoyl peroxide, t-butyl permaleate, and lauroyl peroxide.

[0090] Examples of photopolymerization initiators include benzoin ether-based photopolymerization initiators, acetophenone-based photopolymerization initiators, α-ketol-based photopolymerization initiators, aromatic sulfonyl chloride-based photopolymerization initiators, photoactive oxime-based photopolymerization initiators, benzoin-based photopolymerization initiators, benzyl-based photopolymerization initiators, benzophenone-based photopolymerization initiators, ketal-based photopolymerization initiators, thioxanthone-based photopolymerization initiators, and acylphosphine oxide-based photopolymerization initiators.

[0091] In polymerization, chain transfer agents and / or polymerization inhibitors (polymerization retarders) may be used for purposes such as molecular weight adjustment. Examples of chain transfer agents include α-thioglycerol, lauryl mercaptan, glycidyl mercaptan, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, 2,3-dimercapto-1-propanol, and α-methylstyrene dimers.

[0092] The molecular weight of the base polymer can be adjusted by controlling the type and / or 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 for the formed base polymer to have a smaller 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 for the formed base polymer to have a larger molecular weight.

[0093] The weight-average molecular weight of the base polymer is preferably 100,000 or more, more preferably 300,000 or more, and even more preferably 500,000 or more, from the viewpoint of ensuring cohesive force in the adhesive layer. The same weight-average molecular weight is preferably 5 million or less, more preferably 3 million or less, and even more preferably 2 million or less. The weight-average molecular weight of the base polymer is measured by gel permeation chromatography (GPC) and calculated on a polystyrene basis.

[0094] The glass transition temperature (Tg) of the base polymer is preferably 0°C or lower, more preferably -10°C or lower, and even more preferably -20°C or lower. The glass transition temperature is, for example, -80°C or higher. The glass transition temperature (first glass transition temperature) of the base polymer contained in the low-tack layer 11 is preferably 0°C or lower, more preferably -10°C or lower, even more preferably -20°C or lower, and also, for example, -80°C or higher. The glass transition temperature (second glass transition temperature) of the base polymer contained in the high-tack layer 12 is higher than the first glass transition temperature of the base polymer for the low-tack layer 11. The second glass transition temperature is preferably -80°C or higher, more preferably -75°C or higher, even more preferably -70°C or higher, and also, for example, 10°C or lower, as long as it is higher than the first glass transition temperature.

[0095] For the glass transition temperature (Tg) of the base polymer, the theoretical glass transition temperature (Tg) can be obtained based on Fox's equation below. Fox's equation is a relationship between the glass transition temperature Tg of a polymer and the glass transition temperature Tgi of the homopolymer of the monomers constituting the polymer. In Fox's equation below, Tg represents the glass transition temperature (°C) of the polymer, Wi represents the weight fraction of monomer i constituting the polymer, and Tgi represents the glass transition temperature (°C) of the homopolymer formed from monomer i. For the glass transition temperature of the homopolymer, literature values ​​can be used. For example, "Polymer Handbook" (4th edition, John Wiley & Sons, Inc., 1999) and "New Polymer Library 7: Introduction to Synthetic Resins for Coatings" (by Kyozo Kitaoka, Polymer Publication Association, 1995) list the glass transition temperatures of various homopolymers. On the other hand, the glass transition temperature of the monomer homopolymer can also be determined by the method specifically described in Japanese Patent Publication No. 2007-51271.

[0096] Fox's formula 1 / (273+Tg)=Σ[Wi / (273+Tgi)]

[0097] Effective methods for adjusting the peel adhesion and / or shear storage modulus of each adhesive layer include, for example, adjusting the molecular weight, glass transition temperature, and degree of crosslinking of the base polymer within the adhesive layer. The larger the molecular weight of the base polymer, the more likely the adhesive layer is to have a higher modulus and higher adhesion. The smaller the glass transition temperature of the base polymer, the more likely the adhesive layer is to have a lower modulus and lower adhesion. The higher the degree of crosslinking of the base polymer, the more likely the adhesive layer is to have a higher modulus. Furthermore, the adhesion of the adhesive layer changes according to the degree of crosslinking, such that it has a maximum value at a predetermined degree of crosslinking of the base polymer. Specifically, this is as follows: The higher the degree of crosslinking of the base polymer, the higher the cohesive force inside the adhesive layer, up to a certain degree of crosslinking, which tends to increase the adhesion of the adhesive layer. Beyond the aforementioned certain degree of crosslinking, the higher the degree of crosslinking of the base polymer, the more likely the adhesive layer is to become too elastic and have low adhesion.

[0098] The monomer components that form the base polymer contained in the low-tack layer 11 preferably include an alkyl (meth)acrylate having an alkyl group with 6 to 15 carbon atoms and a monomer containing a hydroxyl group, and more preferably include 2-ethylhexyl acrylate (2EHA), lauryl acrylate (LA), and 4-hydroxybutyl acrylate (4HBA).

[0099] The monomer components that form the base polymer contained in the high-tack layer 12 preferably include an alkyl (meth)acrylate having an alkyl group with 4 to 12 carbon atoms, a hydroxyl group-containing monomer, and a carboxyl group-containing monomer, and more preferably include butyl acrylate (BA), 2-ethylhexyl acrylate (2EHA), and acrylic acid (AA). Furthermore, the number of carbon atoms in the alkyl group of the alkyl (meth)acrylate contained in the monomer components that form the base polymer contained in the high-tack layer 12 is preferably smaller than the number of carbon atoms in the alkyl group of the alkyl (meth)acrylate contained in the monomer components that form the base polymer contained in the low-tack layer 11.

[0100] The adhesive composition may contain one or more oligomers in addition to the base polymer. When an acrylic polymer is used as the base polymer, an acrylic oligomer is preferably used as the oligomer. The acrylic oligomer is a copolymer of monomer components containing 50% by mass or more of alkyl (meth)acrylate, and has a weight-average molecular weight of, for example, 1,000 to 30,000.

[0101] 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. The glass transition temperature of the acrylic oligomer is, for example, 200°C or lower, preferably 180°C or lower, and more preferably 160°C or lower. By using a low-Tg acrylic polymer (base polymer) with a cross-linked structure in combination with a high-Tg acrylic oligomer, the tackiness of the adhesive layer, especially at high temperatures, can be increased. The glass transition temperature of the acrylic oligomer is calculated using Fox's formula described above.

[0102] Acrylic oligomers with a glass transition temperature of 60°C or higher are preferably polymers of monomer components containing a (meth)acrylate (linear alkyl(meth)acrylate) having a chain-like alkyl group and an (meth)acrylate (alicyclic alkyl(meth)acrylate) having an alicyclic alkyl group. Specific examples of these (meth)acrylate alkyl esters include, for example, the above-mentioned (meth)acrylate alkyl esters used as monomer components in acrylic polymers.

[0103] As the linear alkyl (meth)acrylate, methyl methacrylate is preferred due to its 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 a polymer of monomer components containing one or more selected from the group consisting of dicyclopentanyl acrylate, dicyclopentanyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate, and methyl methacrylate.

[0104] The proportion of alicyclic alkyl (meth)acrylate in the monomer component of the acrylic oligomer is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more. The same proportion is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less. The proportion of linear alkyl (meth)acrylate in the monomer component of the acrylic oligomer is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less. The same proportion is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more.

[0105] The weight-average molecular weight of the acrylic oligomer is preferably 1000 or more, more preferably 1500 or more, and even more preferably 2000 or more. The molecular weight is preferably 30000 or less, more preferably 10000 or less, and even more preferably 8000 or less. Such a molecular weight range for the acrylic oligomer is preferable for ensuring the adhesive strength and adhesion retention of the adhesive layer.

[0106] Acrylic oligomers are obtained by polymerizing the monomer components of the acrylic oligomer. Examples of polymerization methods include solution polymerization, active energy ray polymerization (e.g., UV polymerization), bulk polymerization, and emulsion polymerization. In the polymerization of acrylic oligomers, polymerization initiators may be used, and chain transfer agents may be used for the purpose of adjusting the molecular weight.

[0107] To sufficiently enhance the adhesive strength of the adhesive layer, the acrylic oligomer content in the adhesive layer is preferably 0.5 parts by mass or more, more preferably 0.8 parts by mass or more, and even more preferably 1 part by mass or more, per 100 parts by mass of the base polymer. On the other hand, from the viewpoint of ensuring the transparency of the adhesive layer, the acrylic oligomer content in the adhesive layer is preferably 5 parts by mass or less, more preferably 4 parts by mass or less, and even more preferably 3 parts by mass or less, per 100 parts by mass of the base polymer. In the adhesive layer, if the acrylic oligomer content is too high, the haze tends to increase and the transparency tends to decrease due to a decrease in the compatibility of the acrylic oligomer.

[0108] The adhesive composition may contain a silane coupling agent. The content of the silane coupling agent in the adhesive composition is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, per 100 parts by mass of the base polymer. The content is preferably 5 parts by mass or less, more preferably 3 parts by mass or less.

[0109] The adhesive composition may contain other components as needed. Examples of other components include tackifiers, plasticizers, softeners, antioxidants, fillers, colorants, UV absorbers, surfactants, and antistatic agents.

[0110] The single-layer optical adhesive layer 10 can be manufactured, for example, by applying the above-described adhesive composition onto a release film L1 (first release film) to form a coating, and then drying the coating.

[0111] Examples of release films include flexible plastic films. Examples of such plastic films include polyethylene terephthalate films, polyethylene films, polypropylene films, and polyester films. The thickness of the release film is, for example, 3 μm or more, and for example, 200 μm or less. The surface of the release film is preferably treated to release the film.

[0112] Methods for applying the adhesive composition include, for example, 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. The drying temperature of the coating film is, for example, 50°C to 200°C. The drying time is, for example, 5 seconds to 20 minutes.

[0113] A second release film (L2) may be laminated on top of the optical adhesive layer 10 on the first release film L1. The second release film is a flexible plastic film that has undergone a surface release treatment, and the same type as described above for the first release film can be used.

[0114] In this manner, an optical adhesive layer 10 can be manufactured as an optical adhesive sheet S, with the adhesive surface covered and protected by release films L1 and L2. The release films L1 and L2 are peeled off from the optical adhesive sheet S as needed when using it.

[0115] Methods for forming the multilayer optical adhesive layer 10 include the dry-on-dry method, the wet-on-dry method, and the wet-on-wet method. In the dry-on-dry method, for example, a multilayer adhesive layer can be formed by forming each of several adhesive layers by applying and drying an adhesive composition onto a release film, and then bonding the multiple adhesive layers together. In the wet-on-dry method, for example, a multilayer adhesive layer can be formed by applying and drying an adhesive composition onto a release film for each adhesive layer. In the wet-on-wet method, for example, a multilayer adhesive layer can be formed by applying multiple adhesive compositions in multiple stages onto a release film to form a multilayer coating, and then drying the multilayer coating.

[0116] The optical adhesive layer 10, whether single-layer or multi-layer, may be formed by applying and drying an adhesive composition on the adherend to which the optical adhesive layer 10 is bonded.

[0117] From the viewpoint of ensuring sufficient adhesion to the adherend, the thickness of the optical adhesive layer 10 is preferably 10 μm or more, more preferably 15 μm or more. From the viewpoint of the handling properties of the optical adhesive layer 10, the thickness of the optical adhesive layer 10 is preferably 300 μm or less, more preferably 200 μm or less, even more preferably 100 μm or less, and particularly preferably 50 μm or less.

[0118] The haze of the optical adhesive layer 10 is preferably 3% or less, more preferably 2% or less, and more preferably 1% or less. The haze of the optical adhesive layer 10 can be measured using a haze meter in accordance with JIS K7136 (2000). Examples of haze meters include the "NDH2000" manufactured by Nippon Denshoku Industries Co., Ltd. and the "HM-150" manufactured by Murakami Color Technology Laboratory Co., Ltd.

[0119] The total light transmittance of the optical adhesive layer 10 is preferably 60% or more, more preferably 80% or more, and even more preferably 85% or more. The total light transmittance of the optical adhesive layer 10 is, for example, 100% or less. The total light transmittance of the optical adhesive layer 10 can be measured in accordance with JIS K 7375 (2008).

[0120] Figures 6A to 6C show an example of how to use the optical adhesive layer 10.

[0121] In this method, first, as shown in Figure 6A, the optical adhesive layer 10 is bonded to one side of the first member 21 (adherend) in the thickness direction H. The first member 21 is, for example, one element in the laminated structure of a flexible display panel. Examples of such elements include a pixel panel, a touch panel, a polarizing film, and a cover film (the same applies to the second member 22 described later). Through this step, an optical adhesive layer 10 for bonding with other members is provided on the first member 21.

[0122] Next, as shown in Figure 6B, one side of the first member 21 in the thickness direction H and the other side of the second member 22 in the thickness direction H are joined via the optical adhesive layer 10 on the first member 21. The second member 22 is, for example, another element in the laminated structure of a flexible display panel.

[0123] Next, as shown in Figure 6C, the optical adhesive layer 10 between the first member 21 and the second member 22 is aged. Aging promotes the crosslinking reaction of the base polymer in the optical adhesive layer 10, increasing the bonding strength between the first member 21 and the second member 22. The aging temperature is, for example, 20°C to 160°C. The aging time is, for example, 1 minute to 21 days. When aging is performed by autoclave treatment (heat and pressure treatment), the temperature is, for example, 30°C to 80°C, the pressure is, for example, 0.1 to 0.8 MPa, and the treatment time is, for example, 15 minutes or more.

[0124] The optical adhesive layer 10 used in the manufacturing process of flexible devices as described above has a maximum gap length L of 2 mm or less and an edge-to-edge distance D of 80 mm or more in the first winding test, as described above. Such an optical adhesive layer 10 is suitable as an optical adhesive layer for flexible devices for achieving both suppression of peeling from the adherend during device deformation and suppression of deformation marks when not deformed, and is therefore suitable for flexible device applications. [Examples]

[0125] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these examples. Furthermore, the specific numerical values ​​such as the amounts (contents), physical properties, and parameters described below can be substituted with the upper limits (numerical values ​​defined as "less than or equal to" or "less than") or lower limits (numerical values ​​defined as "greater than or equal to" or "greater than") of the corresponding amounts (contents), physical properties, and parameters described in the "Modes for Carrying Out the Invention" above.

[0126] <Preparation of Polymer P1> In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube, a mixture containing 56 parts by mass of 2-ethylhexyl acrylate (2EHA), 39 parts by mass of lauryl acrylate (LA), 5 parts by mass of 4-hydroxybutyl acrylate (4HBA), 0.1 parts by mass of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator, and ethyl acetate as a solvent (solid content concentration 33% by mass) was stirred at 58°C for 5 hours under a nitrogen atmosphere (polymerization reaction). This yielded a solution containing an acrylic polymer (polymer P1). Subsequently, ethyl acetate was added to this solution to adjust the polymer concentration to 30% by mass. This yielded a first polymer solution containing polymer P1. The weight-average molecular weight of polymer P1 was approximately 830,000.

[0127] <Preparation of Polymer P2> In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube, a mixture containing 94.8 parts by mass of butyl acrylate (BA), 0.2 parts by mass of 2-hydroxyethyl acrylate (HBA), 5 parts by mass of acrylic acid (AA), 0.1 parts by mass of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator, and ethyl acetate as a solvent (solid content concentration 30% by mass) was stirred at 55°C for 8 hours under a nitrogen atmosphere (polymerization reaction). This yielded a solution containing an acrylic polymer (polymer P2) (second polymer solution). The weight-average molecular weight of the second polymer was approximately 2.2 million.

[0128] [Example 1] <Preparation of the first adhesive composition> To the first polymer solution, 0.4 parts by mass of a crosslinking agent (product name "Takenate 600", isocyanate-based crosslinking agent, manufactured by Mitsui Chemicals, Inc.) was added per 100 parts by mass of the acrylic polymer (polymer P1) in the polymer solution and mixed. Then, ethyl acetate was added to adjust the solid content concentration to 20% by mass to obtain the first adhesive composition.

[0129] <Formation of the first adhesive layer> A first adhesive composition was applied to the release-treated surface of a first release film (product name "JT-50Wa", polyester film, thickness 50 μm, manufactured by Nitto Denko Corporation), which had one side treated with silicone release, to form a coating film. Next, the coating film on the first release film was dried by heating at 60°C for 1 minute, followed by heating at 130°C for 1 minute, to form an adhesive layer. Next, the release-treated surface of a second release film (product name "MRQ25T100J", polyester film, thickness 25 μm, manufactured by Mitsubishi Chemical Corporation), which had one side treated with silicone release, was bonded to the adhesive layer on the first release film. After this, an aging treatment was performed at 50°C for 48 hours to promote the crosslinking reaction in the adhesive layer. In this way, a first adhesive layer (thickness 26 μm) with double-sided release films was formed. The first adhesive layer is a low-tack layer with lower tackiness than the second adhesive layer described later.

[0130] <Preparation of the second adhesive composition> To the second polymer solution, 0.6 parts by mass of a crosslinking agent (product name "Coronate L", trimethylolpropane adduct of tolylene diisocyanate, manufactured by Tosoh Corporation) was added per 100 parts by mass of the acrylic polymer (polymer P2) in the polymer solution and mixed. Then, ethyl acetate was added to adjust the solid content concentration to 10% by mass to obtain the second adhesive composition.

[0131] <Formation of the second adhesive layer> A second adhesive composition was applied to the release-treated surface of a first release film (product name "JT-50Wa", manufactured by Nitto Denko Corporation) which had one side treated with silicone release treatment, to form a coating film. Next, the coating film on the second release film was dried by heating at 130°C for 1 minute to form an adhesive layer. Then, the release-treated surface of a second release film (product name "MRQ25T100J", manufactured by Mitsubishi Chemical Corporation), which had one side treated with silicone release treatment, was bonded to the adhesive layer on the first release film. After this, an aging treatment was performed at 50°C for 48 hours to promote the crosslinking reaction in the adhesive layer. In this way, two second adhesive layers (thickness 12 μm) with double-sided release films were formed. The second adhesive layer is a high-adhesion layer with higher adhesive strength than the first adhesive layer described above.

[0132] <Preparation of optical adhesive layer> First, one release film was peeled off from the second adhesive layer with double-sided release film, and the exposed second adhesive layer was bonded to the polyimide film F0. The other release film was peeled off from the second adhesive layer on the polyimide film F0, and the exposed surface was corona-treated. Next, one release film was peeled off from the first adhesive layer with double-sided release film, and the exposed surface was corona-treated. Next, the exposed surface of the first adhesive layer and the exposed surface of the second adhesive layer on the polyimide film F0 were bonded together. In this bonding process, the first and second adhesive layers were pressed together by running a 2kg roller back and forth once in an environment of 23°C (the bonding described later was carried out similarly). Next, the other release film was peeled off from the first adhesive layer on the second adhesive layer, and the exposed surface was corona-treated. Meanwhile, one release film was peeled off from another second adhesive layer with double-sided release film, and the exposed surface was corona-treated. Next, the exposed surface of the second adhesive layer and the exposed surface of the first adhesive layer on the second adhesive layer were bonded together. In this manner, the optical adhesive layer of Example 1 was manufactured as an optical adhesive sheet with double-sided film. The optical adhesive sheet comprises a polyimide film F0, a second adhesive layer (thickness 12 μm), a first adhesive layer (thickness 26 μm), a second adhesive layer (thickness 12 μm), and a release film in order in the thickness direction, and the adhesive layer itself has a three-layer structure.

[0133] [Comparative Example 1] First, a first adhesive layer with double-sided release film was formed in the same manner as the first adhesive layer with double-sided release film described above in Example 1, except that the thickness of the first adhesive layer was changed from 26 μm to 50 μm. Next, one of the release films was peeled off from the first adhesive layer with double-sided release film, and the exposed first adhesive layer was bonded to the polyimide film F0. In this way, the optical adhesive layer of Comparative Example 1 was prepared as an optical adhesive sheet with double-sided film. This optical adhesive sheet comprises a polyimide film F0, a first adhesive layer (thickness 50 μm), and a release film in order in the thickness direction, and the adhesive layer itself has a single-layer structure.

[0134] [Comparative Example 2] First, two second adhesive layers with double-sided release films were formed in the same manner as the second adhesive layer with double-sided release films described above in Example 1, except that the thickness of the second adhesive layer was changed from 12 μm to 25 μm. Next, one release film was peeled off from one of the second adhesive layers with double-sided release films, and the exposed second adhesive layer was bonded to the polyimide film F0. Next, the other release film was peeled off from the second adhesive layer on the polyimide film F0, and the exposed surface was corona treated. Next, one release film was peeled off from the other second adhesive layer with double-sided release films, and the exposed surface was corona treated. Next, the exposed surface of the second adhesive layer was bonded to the exposed surface of the second adhesive layer on the polyimide film F0. In this way, the optical adhesive layer of Comparative Example 2 was produced as an optical adhesive sheet with double-sided films. The optical adhesive sheet comprises a polyimide film, a second adhesive layer (50 μm thick), and a release film in order in the thickness direction, and the adhesive layer itself has a single-layer structure.

[0135] <Thickness of the adhesive layer> The thickness of the optical adhesive layer in each optical adhesive sheet of Example 1 and Comparative Examples 1 and 2 was investigated. Specifically, first, a piece of adhesive sheet with double-sided film (short side 25 mm × long side 100 mm) was cut from the optical adhesive sheet with double-sided film. Next, the release film was peeled off the piece of adhesive sheet with double-sided film to obtain a piece of adhesive sheet with single-sided film. Then, the thickness of each of the five measurement points on the piece of adhesive sheet with single-sided film (total thickness of the sheet) was measured using a dial gauge. The five measurement points were five points that divided the center of the adhesive sheet in the width direction into six equal parts along the long side. Then, the thickness of the polyimide film F0, which was measured separately using a dial gauge, was subtracted from each measured thickness. This gave the thickness of the optical adhesive layer at the five measurement points. The maximum thickness T1 (μm) and minimum thickness T2 (μm) of the adhesive layer thickness at the five measurement points are shown in Table 1. The difference between the maximum thickness and the minimum thickness (T1-T2) is also shown in Table 1.

[0136] <Peelability> The peel adhesion strength of each optical adhesive layer in Example 1 and Comparative Examples 1 and 2 was investigated by peel tests.

[0137] First, a sample for measurement was prepared for each optical adhesive layer. In preparing the sample for measurement, a polyimide substrate (product name "UPIREX 25RN", thickness 25 μm, manufactured by Ube Industries, Ltd.) with a plasma-treated surface was prepared first. Then, for each of Example 1 and Comparative Examples 1 and 2, an optical adhesive sheet with a double-sided film was prepared in the same manner as the optical adhesive sheet with a double-sided film described above, except that the polyimide substrate was used instead of the polyimide film F0. Next, a test piece (width 25 mm x length 100 mm) was cut from the optical adhesive sheet with a double-sided film (polyimide substrate / optical adhesive sheet / release film). Next, the release film was peeled off from the optical adhesive sheet on this test piece, and a polyimide film (product name "UPIREX 50S", thickness 50 μm, manufactured by Ube Industries, Ltd.) was laminated to the exposed surface. In this lamination, the test piece was pressed against the polyimide film by running a 2 kg hand roller back and forth once in an environment of 23 °C. The sample for measurement was prepared in the manner described above.

[0138] Next, after allowing the sample to stand at room temperature for 30 minutes, a peel test was performed to peel the test piece from the polyimide film of the sample, and the peel strength was measured. A tensile testing machine (product name "Autograph AG-50NX plus", manufactured by Shimadzu Corporation) was used for this measurement. In this measurement, the measurement temperature was set to 23°C, the peel angle of the test piece from the polyimide film was set to 180°, the tensile speed of the test piece was set to 300 mm / min, and the peel length was set to 50 mm. The average value of the measured peel strength is shown in Table 1 as the peel adhesion force F (N / 25 mm).

[0139] <Shear storage modulus> The shear storage modulus was measured for the first adhesive layer and the second adhesive layer in Example 1 and Comparative Examples 1 and 2 as follows.

[0140] First, a sample for measurement was prepared. Specifically, an adhesive sheet with a thickness of approximately 1 mm was created by laminating adhesive layers, and then this sheet was punched out to obtain cylindrical pellets (9 mm in diameter) which were to be used as the measurement sample. Then, dynamic viscoelasticity measurements were performed on the measurement sample using a dynamic viscoelasticity measuring device (product name "ARES-G2", manufactured by TA Instruments) after fixing it to a parallel plate jig with a diameter of 8 mm. In this measurement, the measurement mode was set to shear mode, the measurement temperature range to -60°C to 150°C, the heating rate to 5°C / min, and the frequency to 1 Hz. From the measurement results, the shear storage modulus (kPa) at -20°C was read. The results are shown in Table 1.

[0141] <Changes in transmittance> The change in transmittance was investigated for each optical adhesive layer in Example 1 and Comparative Examples 1 and 2 as follows.

[0142] First, a sample for measurement was prepared. In preparing the sample for measurement, a PET substrate (product name "T100C50", thickness 50 μm, manufactured by Mitsubishi Chemical Corporation) was prepared first. Then, an optical adhesive sheet with a double-sided film (PET substrate / optical adhesive sheet / release film) was prepared in the same manner as the optical adhesive sheet with a double-sided film described above for each of Example 1 and Comparative Examples 1 and 2, except that the PET substrate was used instead of the polyimide film F0. Next, the release film was peeled off the optical adhesive sheet, and the exposed optical adhesive layer was bonded to the PET substrate (product name "T100C50", thickness 50 μm, manufactured by Mitsubishi Chemical Corporation). This resulted in a laminate with a PET substrate / optical adhesive layer / PET substrate laminate structure. Next, a sample for measurement with dimensions of 25 mm width x 100 mm length was cut from this laminate.

[0143] Next, the transmittance (T1) of the sample was measured using light at a wavelength of 550 nm (first transmittance measurement). A transmittance measuring device (product name "U4100 type spectrophotometer", manufactured by Hitachi High-Technologies Corporation) was used for the measurement.

[0144] Next, the sample used for measurement, which had undergone the first transmittance measurement, was wound around a 20 mm diameter core such that its length was aligned with the circumferential direction of the core (the sample was wound around the core 1.628 times while being pulled in the length direction). Next, the sample wound around the core in this manner was stored at 23°C for 1 hour. Then, the transmittance (T2) of the sample after storage was measured using light at a wavelength of 550 nm in the same manner as the first transmittance measurement (second transmittance measurement).

[0145] The rate of change of transmittance T2 relative to transmittance T1 was then calculated using the following formula. The values ​​are shown in Table 1.

[0146] Change in transmittance (%) = [(T2-T1) / T1] × 100

[0147] <First Wrapping Test> For each optical adhesive sheet (optical adhesive layer) of Example 1 and Comparative Examples 1 and 2, the following steps 1 to 6 were performed to measure the maximum void length L and the distance between edges D (first winding test). The results are shown in Table 1.

[0148] Step 1: First, a laminate was prepared comprising an 80 μm thick polyimide film, a 32 μm thick polarizing film, and a 125 μm thick polyethylene terephthalate (PET) film in this order in the thickness direction, with the polyimide film and the polarizing film, and the polarizing film and the PET film, each bonded together by an optical adhesive layer. Specifically, it is as follows:

[0149] First, a first optical adhesive sheet with double-sided film (transparent polyimide film / optical adhesive sheet / release film) was prepared for each Example 1 and Comparative Examples 1 and 2 in the same manner as the optical adhesive sheet with double-sided film described above, except that a transparent polyimide film with a thickness of 80 μm (product name "CPI", manufactured by Kolon Corporation) was used instead of polyimide film F0. Next, a second optical adhesive sheet with double-sided film (PET film / optical adhesive sheet / release film) was prepared for each Example 1 and Comparative Examples 1 and 2 in the same manner as the optical adhesive sheet with double-sided film described above, except that a PET film with a thickness of 125 μm (product name "T912E125", manufactured by Mitsubishi Chemical Corporation) was used instead of polyimide film F0. Finally, the release film was peeled off the first optical adhesive sheet with double-sided release film, and the exposed surface was bonded to one side of a 32 μm thick polarizing plate film (the protective layer side described later) (the polarizing plate film was prepared as described later). Next, the release film was peeled off from the second optical adhesive sheet with double-sided release film, and the exposed surface was bonded to the other side of the polarizing plate film (the surface on the second phase difference layer side described later). In this way, the above laminate was obtained.

[0150] Step 2: The laminate was subjected to heating and pressurizing treatment at 50°C and 0.5 MPa for 15 minutes.

[0151] Step 3: A test specimen was cut from the laminate after the second step. The test specimen had dimensions of 100 mm in length and 25 mm in width, with a first end at one end in the longitudinal direction and a second end at the other end.

[0152] Step 4: A test specimen was wound around a metal core with a cross-sectional diameter of 20 mm, such that the length of the test specimen aligned with the circumference of the core, as follows: First, the transparent polyimide film side of the first end of the test specimen was attached to the core using double-sided tape. Next, the test specimen was wound around the core 1.628 times while being pulled in the length direction, such that the transparent polyimide film side of the test specimen was positioned on the inside of the wound core. Specifically, the test specimen was wound around the core without any gaps between the core and the test specimen, and without any gaps between the test specimens in the diameter direction of the core. Next, the second end of the test specimen wound around the core was fixed with adhesive tape.

[0153] Step 5: Test specimens with coiled cores were stored at a temperature of 23°C for 48 hours.

[0154] Step 6: After step 5, within 5 minutes of releasing the specimen from the core, the edge-to-edge distance D and the maximum void length L were measured in the specimen. The edge-to-edge distance D is the distance between the two ends of the specimen in the longitudinal direction. The maximum void length L is the maximum length of the interlayer void in the specimen. In this step, the maximum void length L was measured after the measurement of the edge-to-edge distance D.

[0155] <Second Wrapping Test> For each optical adhesive sheet (optical adhesive layer) in Example 1 and Comparative Examples 1 and 2, steps 1 to 6 were performed in the same manner as in the first wrapping test, except for the following, and the maximum void length L and edge-to-edge distance D were measured (second wrapping test). The storage temperature condition in step 5 was -20°C. The measurement results are shown in Table 1.

[0156] [Table 1]

[0157] The polarizing film used to create the laminate in the winding test was prepared as follows.

[0158] <Fabrication of polarizing films> First, a long, amorphous isophthalic copolymer polyethylene terephthalate film (100 μm thick, 0.75% water absorption, glass transition temperature approximately 75°C) was prepared as a thermoplastic resin substrate. One side of this resin substrate was subjected to corona treatment. Meanwhile, 100 parts by mass of polyvinyl alcohol (PVA) resin, 13 parts by mass of potassium iodide, and water were mixed to prepare a PVA aqueous solution (coating solution). The aforementioned PVA resin is a mixture of PVA with a degree of polymerization of 4200 and a degree of saponification of 99.2 mol%, and acetoacetyl-modified PVA (product name "Gosephymer Z410", manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) in a mass ratio of 9:1. Then, the above PVA aqueous solution was applied to the corona-treated surface of the resin substrate and dried at 60°C to form a 13 μm thick PVA resin layer. This obtained a laminate (resin substrate / PVA resin layer).

[0159] Next, the resulting laminate was uniaxially stretched 2.4 times in the longitudinal direction between rolls with different peripheral speeds in an oven at 130°C (air-assisted stretching). Next, the laminate was immersed for 30 seconds in an insolubilization bath at a liquid temperature of 40°C (a boric acid aqueous solution obtained by mixing 4 parts by mass of boric acid with 100 parts by mass of water) (insolubilization treatment). Next, the laminate was immersed for 60 seconds in a staining bath at a liquid temperature of 30°C (a iodine aqueous solution obtained by mixing iodine and potassium iodide in a mass ratio of 1:7 with 100 parts by mass of water), while adjusting the concentration so that the transmittance (Ts) and unit absorbance at a wavelength of 210 nm of the final polarizing film would be the desired values ​​(staining treatment). Next, the laminate was immersed for 30 seconds in a crosslinking bath at a liquid temperature of 40°C (a boric acid aqueous solution obtained by mixing 3 parts by mass of potassium iodide and 5 parts by mass of boric acid with 100 parts by mass of water) (crosslinking treatment). Next, the laminate was immersed in a boric acid aqueous solution at a liquid temperature of 70°C (boric acid concentration of 4.0% by mass) and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds to a total stretch ratio of 5.5 times (underwater stretching treatment). Next, the laminate was immersed in a washing bath at a liquid temperature of 20°C (a aqueous solution obtained by mixing 4 parts by mass of potassium iodide with 100 parts by mass of water) (washing treatment). Next, the laminate was dried in an oven maintained at 90°C while being brought into contact with a SUS heated roll with a surface temperature maintained at 75°C for about 2 seconds (drying shrinkage treatment). The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment was 2%. As described above, a polarizing film with a thickness of 4.6 μm was formed on the resin substrate.

[0160] <Fabrication of polarizing films with protective layers> An acrylic film (surface refractive index 1.50, thickness 20 μm) was bonded to the surface of the polarizing film (the side opposite to the resin substrate) as a protective layer via an ultraviolet-curing adhesive. Specifically, an ultraviolet-curing adhesive was applied to the surface of the polarizing film to form an adhesive coating with a thickness of 1.0 μm, and then the polarizing film and the protective layer (acrylic film) were bonded together via this adhesive coating using a roll machine. Next, ultraviolet light was irradiated onto the adhesive coating through the protective layer to cure the adhesive coating (forming the first adhesive layer). Then, by peeling the resin substrate from the polarizing film, a long protective layer-attached polarizing film (width 1300 mm) with a laminated structure of protective layer / first adhesive layer / polarizing film was obtained. The transmittance of this polarizing film with a protective layer was 43.5%, the unit absorbance at a wavelength of 210 nm was 0.91, the ratio of the orthogonal absorbance A470 at a wavelength of 470 nm to the orthogonal absorbance A600 at a wavelength of 600 nm (A470 / A600) was 0.87, and the orthogonal b value was -3.00. This orthogonal b value can be measured, for example, by an ultraviolet-visible spectrophotometer (product name "V7100", manufactured by JASCO Corporation).

[0161] <Formation of the first phase difference layer> A liquid crystal composition was prepared by mixing 10 parts by mass of a photopolymerizable liquid crystal compound having nematic liquid crystal properties (product name "Paliocolor LC242", represented by the following structural formula, manufactured by BASF), 3 parts by mass of a photopolymerization initiator (product name "Irgacure 907", manufactured by BASF), and 40 parts by mass of toluene as a solvent.

[0162] [ka]

[0163] On the other hand, the surface of a polyethylene terephthalate (PET) film (38 μm thick) was rubbed with a rubbing cloth (orientation treatment). The rubbing direction was such that, after bonding the first liquid crystal orientation solidification layer (described later) to the polarizing film side of the polarizing film with the protective layer described above, the orientation direction of the liquid crystal compound in the layer was 15° clockwise with respect to the absorption axis of the polarizing film, as viewed from the polarizing film side. Next, the liquid crystal composition described above was applied to the orientation-treated surface of the PET film using a bar coater to form a coating film. Next, the coating film on the PET film was dried by heating at 90°C for 2 minutes to form a liquid crystal layer. In this liquid crystal layer, the liquid crystal compound is oriented along the rubbing direction of the PET film surface. Next, the liquid crystal layer on the PET film was irradiated with ultraviolet light to photo-cure the liquid crystal layer and form a first liquid crystal orientation solidification layer (first phase difference layer) with a thickness of 2.5 μm. For ultraviolet irradiation, a metal halide lamp was used as the irradiation light source, and the integrated irradiation light amount was 1 mJ / cm². 2 The in-plane phase difference Re(550) of the first liquid crystal alignment solidification layer was 270 nm. The first liquid crystal alignment solidification layer had a refractive index distribution of nx>ny=nz.

[0164] <Formation of the second phase difference layer> A second liquid crystal alignment solidification layer (second phase difference layer) was formed in the same manner as the first liquid crystal alignment solidification layer, except for the thickness and orientation direction of the liquid crystal compound. The thickness of the second liquid crystal alignment solidification layer was 1.5 μm. The orientation direction of the liquid crystal compound in the second liquid crystal alignment solidification layer was set to a direction that is 75° clockwise with respect to the absorption axis of the polarizing film, as viewed from the polarizing film side, after bonding the second liquid crystal alignment solidification layer to the polarizing film side of the polarizing film with a protective layer. The in-plane phase difference Re(550) of the second liquid crystal alignment solidification layer was 140 nm. The second liquid crystal alignment solidification layer had a refractive index distribution of nx>ny=nz.

[0165] <Fabrication of polarizing film> First, an ultraviolet-curing adhesive was applied to the exposed surface of the polarizing film with the protective layer described above to form an adhesive coating (1 μm thick). Then, using a roll machine, the polarizing film and the first liquid crystal alignment solidification layer on the PET film were bonded together via this adhesive coating. At this time, the bonding was performed so that the angle between the absorption axis of the polarizing film and the orientation direction (slow axis) of the liquid crystal compound in the first liquid crystal alignment solidification layer was 15°. Next, ultraviolet light was irradiated from the PET film side onto the adhesive coating between the polarizing film and the first liquid crystal alignment solidification layer to cure the adhesive coating (forming a second adhesive layer). This bonded the first liquid crystal alignment solidification layer to the polarizing film. After this, the PET film was peeled off from the first liquid crystal alignment solidification layer. This resulted in obtaining an intermediate laminate having a laminated structure of a protective layer, a first adhesive layer, a polarizing film, a second adhesive layer, and a first liquid crystal alignment solidification layer (first phase difference layer).

[0166] Next, an ultraviolet-curing adhesive was applied to the exposed surface of the first liquid crystal alignment solidification layer of the intermediate laminate to form an adhesive coating (1 μm thick). Then, using a roll machine, the first liquid crystal alignment solidification layer and the second liquid crystal alignment solidification layer on the PET film were bonded together via this adhesive coating. At this time, the bonding was performed so that the angle between the absorption axis of the polarizing film and the orientation direction (slow axis) of the liquid crystal compound in the second liquid crystal alignment solidification layer was 75°. Next, ultraviolet light was irradiated from the PET film side onto the adhesive coating between the first and second liquid crystal alignment solidification layers to cure the adhesive coating (a third adhesive layer was formed). This bonded the second liquid crystal alignment solidification layer to the first liquid crystal alignment solidification layer. After this, the PET film was peeled off from the second liquid crystal alignment solidification layer. This resulted in a polarizing plate film (a polarizing film with a protective layer and a phase difference layer) having a laminated structure consisting of a protective layer, a first adhesive layer, a polarizing film, a second adhesive layer, a first liquid crystal alignment solidification layer (first phase difference layer), a third adhesive layer, and a second liquid crystal alignment solidification layer (second phase difference layer). The thickness of this polarizing plate film was 32 μm. [Explanation of symbols]

[0167] S Optical Adhesive Sheet 10 Optical adhesive layer 11 Low adhesive layer 11a 1st page 11b Side 2 12A High adhesive layer (1st high adhesive layer) 12a High adhesive side (1st high adhesive side) 12B High adhesive layer (2nd high adhesive layer) 12b High adhesive side (second high adhesive side) H thickness direction L1, L2 release film 21 First Member 22 Second Member

Claims

1. A first high-tack layer, a low-tack layer, and a second high-tack layer are provided in order in the thickness direction, The first high-tack layer, the low-tack layer, and the second high-tack layer each contain an acrylic polymer. An optical adhesive layer in which the maximum length L of the gap in a wrapping test, in which the following steps 1 to 6 are performed, is 2 mm or less, and the distance D between edges is 80 mm or more. Step 1: A laminate is prepared comprising a polyimide film with a thickness of 80 μm, a polarizing plate film with a thickness of 32 μm, and a polyethylene terephthalate film with a thickness of 125 μm, in this order in the thickness direction, wherein the polyimide film and the polarizing plate film, and the polarizing plate film and the polyethylene terephthalate film are joined together by the optical adhesive layer. Step 2: The laminate is subjected to heating and pressurizing treatment at 50°C and 0.5 MPa for 15 minutes. Step 3: A test specimen is cut from the laminate after the second step. The test specimen has dimensions of 100 mm in length and 25 mm in width, and has a first end at one end in the longitudinal direction and a second end at the other end. Step 4: The test specimen is wound around a core with a cross-sectional diameter of 20 mm in the following manner, such that the length of the test specimen is aligned with the circumferential direction of the core. First, the polyimide film side of the first end of the test specimen is attached to the core using double-sided tape. Next, with the polyimide film side of the test piece positioned on the inside of the winding, the test piece is wrapped around the core 1.628 times while being pulled in the length direction. Then, the second end of the test piece wrapped around the core is secured with adhesive tape. Step 5: The test specimen with the winding core is stored at a temperature of 23°C for 48 hours. Step 6: After step 5, within 5 minutes of releasing the winding of the test piece from the core, the distance D between the edges and the maximum length L of the gap in the test piece are measured. The edge-to-edge distance D is the distance between the two edges of the test specimen in the longitudinal direction. The maximum void length L is the maximum length of the interlayer void in the test specimen.

2. The optical adhesive layer according to claim 1, wherein the same steps as those in the first to sixth steps are performed in a wrapping test, except that the temperature condition in the fifth step is -20°C, the maximum length L of the void is 2 mm or less, and the distance D between the edges is 80 mm or more.

3. The optical adhesive layer according to claim 1, wherein the change in transmittance after 1 hour has elapsed since being wound onto a core with a cross-sectional diameter of 20 mm is 5% or less.

4. The optical adhesive layer according to claim 1, wherein the difference between the maximum thickness and the minimum thickness is 3 μm or less.

5. A low-tack layer having a first surface and a second surface opposite to the first surface, A first high-tack layer disposed on the first surface, the first high-tack layer having a first high-tack surface on the side opposite to the low-tack layer, The second high-tack layer is disposed on the second surface and has a second high-tack surface on the side opposite to the low-tack layer, Each of the first and second high-adhesion surfaces has a peel adhesion strength of 5 N / 25 mm or more to the polyimide film after 30 minutes at 23°C following lamination to the polyimide film, under conditions of a peel angle of 180° and a peel speed of 300 mm / min. The optical adhesive layer according to claim 1, wherein the shear storage modulus at -20°C of the low-tack layer is smaller than the shear storage modulus at -20°C of the high-tack layer.

6. The optical adhesive layer according to claim 5, wherein the ratio of the thickness of the low-adhesive layer to the sum of the thicknesses of the first high-adhesive layer and the second high-adhesive layer is 1 or more.

7. The optical adhesive layer according to claim 5, wherein the ratio of the thickness of the low-adhesive layer to the sum of the thicknesses of the first high-adhesive layer and the second high-adhesive layer is 30 or less.

8. An optical adhesive layer according to any one of claims 1 to 7, having a total thickness of 5 μm or more and 150 μm or less.

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