Optical adhesive layer

Abstract

This invention provides an optical adhesive layer suitable for use in flexible devices. The optical adhesive layer 10 of this invention has a first surface 10a and a second surface 10b opposite to the first surface 10a. The optical adhesive layer 10 has a shear storage modulus G' of less than 180 kPa at -20°C. At least one of the first surface 10a and the second surface 10b of the optical adhesive layer 10, after being bonded to a polyimide film and subjected to 30 minutes at 23°C, exhibits a peel adhesion F of 5 N / 25 mm or more to the polyimide film at a peel angle of 180° and a peel speed of 300 mm / min.

Description

Technical Field

[0001] This invention relates to an optical adhesive layer. Prior Technology

[0002] Display panels, for example, have a multilayer structure including a pixel panel, a touch panel, a polarizing plate, and a cover film. In the manufacturing process of such a display panel, to bond the elements of the multilayer structure together, an adhesive layer (optical adhesive layer) formed from a transparent adhesive for optical purposes can be used. In the manufacturing process of the display panel, for example, an optical adhesive layer is formed by bonding an optical adhesive sheet formed from an optical adhesive composition to a bonding object. Alternatively, an optical adhesive layer is formed by applying an optical adhesive composition onto the bonding object.

[0003] On the other hand, foldable display panels are currently being developed for use in devices such as smartphones and tablets. In foldable display panels, the elements in the laminated structure are made to be foldable, and the bonding between these elements uses an optical adhesive layer. Optical adhesive layers for flexible devices such as foldable display panels are described, for example, in Patent Document 1 below. [Previous Technical Documents] [Patent Literature]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-111754 Summary of the Invention

[0005] [The problem the invention aims to solve]

[0006] Previously, the optical adhesive layer was prone to peeling off from the substrate at the bending points of foldable display panels. This was because when the display panel was bent, shear stress and other stresses acted locally on the optical adhesive layer at the bending point. This peeling could lead to malfunctions in the device, which was undesirable. For optical adhesive layers used in foldable display panels, a higher level of performance is required to balance the ease with which the display could bend and deform along with the substrate when bent, and to suppress peeling off from the substrate.

[0007] As a flexible device, rollable (flexible) display panels are also being developed. For the optical adhesive layer used in rollable display panels, a very high level of balance is required to ensure that the display can easily deform along with the adhered body when rolled up (winding deformation) and to prevent peeling from the adhered body.

[0008] The present invention provides an optical adhesive layer suitable for use in flexible devices. [Technical means to solve the problem]

[0009] The present invention [1] includes an optical adhesive layer having a first side and a second side opposite to the first side, and having a shear storage modulus G' of less than 180 kPa at -20°C. At least one of the first side and the second side, after being bonded to a polyimide film and subjected to 30 minutes at 23°C, has a peel adhesion F of more than 5 N / 25 mm to the polyimide film under the conditions of a peel angle of 180° and a peel speed of 300 mm / min.

[0010] The present invention [2] comprises an optical adhesive layer as described in [1] above, wherein the shear storage modulus G' (kPa) and the peel adhesion F (N / 25 mm) at -20°C satisfy F≧0.4667×G'-58 when G'>135.

[0011] The present invention [3] comprises an optical adhesive layer as described in [1] or [2] above, wherein the ratio of the shear storage modulus G' (kPa) at -20°C to the peel adhesion F (N / 25 mm) is 200 or less.

[0012] The present invention [4] comprises an optical adhesive layer as described in any one of [1] to [3] above, wherein the ratio of the shear storage modulus G' (kPa) at -40°C to the peel adhesion F (N / 25 mm) is 200 or less.

[0013] The present invention [5] comprises an optical adhesive layer as described in any one of [1] to [4] above, wherein the shear storage modulus G' at -40°C is less than 1200 kPa.

[0014] The present invention [6] includes an optical adhesive layer as described in any one of [1] to [5] above, wherein the difference between the maximum thickness and the minimum thickness is less than 3 μm.

[0015] The present invention [7] comprises an optical adhesive layer as described in any one of [1] to [6] above, which, after being wound around a core with a cross-sectional diameter of 20 mm, has a transmittance change of less than 5% after 1 hour. [Effects of the Invention]

[0016] The optical adhesive layer of the present invention, as described above, has a shear storage modulus G' of less than 180 kPa at -20°C. This high degree of flexibility allows the optical adhesive layer to easily follow the deformation of the adhered body with a relatively large curvature (such as the aforementioned bending and coiling deformation) when the adhered body is deformed. The flexibility and ease of deformation with a large curvature (bending deformability) of the optical adhesive layer is suitable for achieving good repeated deformation (repeated bending and coiling deformation) of the flexible device using the optical adhesive layer. Furthermore, as described above, at least one of the first and second surfaces of the optical adhesive layer has a peel adhesion force F of 5 N / 25 mm or more under certain conditions. This configuration is suitable for ensuring good adhesion of the optical adhesive layer to the adhered body, and therefore suitable for suppressing the peeling of the optical adhesive layer from the adhered body. Optical adhesive layers that balance bending deformation resistance and peel resistance in this way are suitable for use in flexible devices. Simple Explanation of the Diagram

[0017] Figure 1 is a cross-sectional schematic diagram of one embodiment of the optical adhesive layer of the present invention. Figure 2 shows the case where the optical adhesive layer shown in Figure 1 has a two-layer structure. Figure 3 shows the case where the optical adhesive layer shown in Figure 1 has a 3-layer structure. Figure 4 illustrates an example of the method of using the optical adhesive layer of the present invention. Figure 4A shows the step of attaching the optical adhesive layer to the first adherend, Figure 4B shows the step of bonding the first adherend and the second adherend via the optical adhesive layer, and Figure 4C shows the curing step. Figure 5 shows the measurement results of the first shear storage modulus G' (horizontal axis) and peel adhesion F (vertical axis) of the optical adhesive layers of Examples 1-4 and Comparative Examples 2 and 3. Implementation

[0018] As shown in FIG1, the optical adhesive layer 10, as one embodiment of the optical adhesive layer of the present invention, is a sheet shape having a specific thickness and extends in a direction orthogonal to the thickness direction H (surface direction). The optical adhesive layer 10 has a first surface 10a and a second surface 10b opposite to the first surface 10a. FIG1 illustratively shows a state in which the optical adhesive layer 10 is made into an optical adhesive sheet S, and release films L1 and L2 are attached to both sides of the sheet. The optical adhesive sheet S with the release film attached is, for example, in roll form (not shown).

[0019] The optical adhesive layer 10 has a single-layer structure or a multi-layer structure of two or more layers (not shown in Figure 1). Figure 2 is a partially enlarged view of an optical adhesive layer 10A, which is an example of a two-layer optical adhesive layer 10. The optical adhesive layer 10A has an adhesive layer 11 (first adhesive layer) and an adhesive layer 12 (second adhesive layer) sequentially along the thickness direction H (two-layer structure). In the optical adhesive layer 10A, the exposed surface of the adhesive layer 11 (the side of the adhesive layer 11 opposite to the adhesive layer 12) is the first surface 10a, and the exposed surface of the adhesive layer 12 (the side of the adhesive layer 12 opposite to the adhesive layer 11) is the second surface 10b. Figure 3 is a partially enlarged view of an optical adhesive layer 10B, which is an example of a three-layer optical adhesive layer 10. The optical adhesive layer 10B comprises, sequentially along the thickness direction H, an adhesive layer 11 (first adhesive layer), an adhesive layer 12 (second adhesive layer), and an adhesive layer 13 (third adhesive layer) (a three-layer structure). In the optical adhesive layer 10B, the exposed surface of the adhesive layer 11 (the side of the adhesive layer 11 opposite to the adhesive layer 12) is the first surface 10a, and the exposed surface of the adhesive layer 13 (the side of the adhesive layer 13 opposite to the adhesive layer 12) is the second surface 10b.

[0020] This optical adhesive layer 10 is a transparent adhesive layer disposed in the light-passing portion of a flexible device. Examples of flexible devices include flexible display panels. Flexible display panels, for example, have a laminated structure including a pixel panel, a touch panel, a polarizing plate, 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, during the manufacturing process of the flexible display panel for bonding the elements included in the aforementioned laminated structure to each other.

[0021] The optical adhesive layer 10 has a shear storage modulus G' (first shear storage modulus G') of 180 kPa or less at -20°C. In the optical adhesive layer 10, from the viewpoint of ensuring flexibility and bendability suitable for flexible devices, the first shear storage modulus G' is preferably 150 kPa or less, more preferably 130 kPa or less, further preferably 100 kPa or less, and especially preferably 90 kPa or less. From the viewpoint of ensuring the cohesive force of the optical adhesive layer 10, the first shear storage modulus G' is preferably 30 kPa or more, more preferably 40 kPa or more, further preferably 50 kPa or more, and especially preferably 60 kPa or more. The first shear storage modulus G' of the optical adhesive layer 10 can be measured using a dynamic viscoelasticity measuring device. In this measurement, the measurement mode was set to shear mode, the measurement temperature range was set to -60°C to 150°C, the heating rate was set to 5°C / minute, and the frequency was set to 1 Hz (the same applies to the measurement of the second shear storage modulus G' described later). Specifically, as described below with respect to the examples. Examples of methods for adjusting the shear storage modulus G' of the adhesive layer include: selection of the type of base polymer in the adhesive layer, adjustment of the molecular weight and the amount of the formulation, and selection of the type of crosslinking agent used to crosslink the base polymer and adjustment of the amount of the formulation. Selection of the type of base polymer includes selection of the type of main chain in the base polymer, and selection and adjustment of the type and amount of functional groups. Furthermore, when the optical adhesive layer 10 has a multilayer structure, the methods for adjusting the shear storage modulus G' of the optical adhesive layer 10 can be exemplified by adjusting the shear storage modulus G' of each adhesive layer in the optical adhesive layer 10 and adjusting the thickness of each adhesive layer.

[0022] At least one of the first surface 10a and the second surface 10b of the optical adhesive layer 10 (i.e., one or both of the first surface 10a and the second surface 10b) after being bonded to the polyimide film and subjected to 30 minutes at 23°C, exhibits a peel adhesion F of 5 N / 25 mm or more to the polyimide film at a peel angle of 180° and a peel speed of 300 mm / min. The adhesion of the optical adhesive layer 10 to the substrate is performed at 23°C by subjecting a 2 kg roller to one round trip. From the viewpoint of ensuring good adhesion to the substrate, the peel adhesion 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. For example, the peel adhesion F is 30 N / 25 mm or less. Methods for adjusting the peel adhesion F on the surface of the adhesive layer can include, for example, the selection of the type of base polymer in the adhesive layer, the adjustment of its molecular weight, and the adjustment of its formulation amount. Methods for adjusting the peel adhesion F on the surface of the adhesive layer can also include the selection of the type of components other than the base polymer in the adhesive layer, and the adjustment of the formulation amount of those components. Examples of such components include crosslinking agents, silane coupling agents, and oligomers. Furthermore, when the optical adhesive layer 10 has a multilayer structure, methods for adjusting the peel adhesion F of the optical adhesive layer 10 can include, for example, adjusting the peel adhesion F of each adhesive layer in the optical adhesive layer 10, and adjusting the thickness of each adhesive layer.

[0023] As described above, the optical adhesive layer 10 has a shear storage modulus G' of less than 180 kPa at -20°C. This high degree of flexibility allows the optical adhesive layer 10 to easily follow the deformation of the adhered object with a relatively large curvature when the adhered object deforms at such a curvature. Examples of deformations with relatively large curvature include the bending deformation of a foldable display and the deformation (winding deformation) when a rollable display is rolled up. The flexibility and ease with which the optical adhesive layer 10 deforms with a large curvature (bending deformability) is suitable for achieving good repeated deformation (repeated bending deformation and winding deformation, etc.) of flexible devices using the optical adhesive layer 10.

[0024] As described above, the optical adhesive layer 10 has at least one of its first surface 10a and second surface 10b having a peel adhesion force F of 5 N / 25 mm or more under the aforementioned conditions. That is, the optical adhesive layer 10 has either the first configuration or the second configuration regarding adhesion. Specifically, it has a configuration where both the first surface 10a and the second surface 10b have a peel adhesion force F of 5 N / 25 mm or more under the aforementioned conditions (first configuration), and a configuration where one of the first surface 10a and the second surface 10b has a peel adhesion force F of 5 N / 25 mm or more under the aforementioned conditions (second configuration). This type of optical adhesive layer 10, with its first surface 10b and / or second surface 10b having a peel adhesion force F, is suitable for ensuring good adhesion to the adhered object, and therefore suitable for suppressing the peeling of the optical adhesive layer 10 from the adhered object.

[0025] The optical adhesive layer 10, which combines bending deformation resistance and peel resistance in the manner described above, is suitable for use in flexible devices.

[0026] The optical adhesive layer 10 is preferably a multilayer structure with two or more layers, as shown in Figures 2 and 3. This configuration is better for distributing the function among the various adhesive layers included in the optical adhesive layer 10, thereby achieving a balance between the aforementioned bending deformation resistance and peel resistance. For example, as described below.

[0027] The optical adhesive layer 10A shown in Figure 2, for example, has an adhesive layer 11 with a shear storage modulus and adhesive strength higher than that of adhesive layer 12, and an adhesive layer 12 with a shear storage modulus and adhesive strength lower than that of adhesive layer 11. This configuration is suitable for suppressing the overall shear storage modulus of the optical adhesive layers 10 (adhesive layers 11, 12) and ensuring the aforementioned peel adhesion F of the first surface 10a (the exposed surface on the side of adhesive layer 11). The optical adhesive layer 10B shown in Figure 3, for example, has adhesive layers 11, 13 with a shear storage modulus and adhesive strength higher than that of adhesive layer 12, and an adhesive layer 12 with a shear storage modulus and adhesive strength lower than that of adhesive layers 11, 13. This configuration is suitable for suppressing the overall shear storage modulus of the optical adhesive layers 10 (adhesive layers 11, 12, 13) and ensuring the aforementioned peel adhesion F on the first surface 10a (side surface of adhesive layer 11) and the second surface 10b (side surface of adhesive layer 13). As methods for adjusting the shear storage modulus and adhesion of the adhesive layers, adjusting the molecular weight of the base polymer within the adhesive layer, adjusting the glass transition temperature, and adjusting the degree of crosslinking are more effective. A larger molecular weight of the base polymer tends to result in a higher modulus of elasticity and a higher adhesion of the adhesive layer. A lower glass transition temperature of the base polymer tends to result in a lower modulus of elasticity and a lower adhesion of the adhesive layer. A higher degree of crosslinking of the base polymer tends to result in a higher modulus of elasticity of the adhesive layer. Furthermore, the adhesion of the adhesive layer varies according to the degree of crosslinking, reaching a maximum value at a specific degree of crosslinking of the base polymer. Specifically, as described below. The higher the degree of crosslinking of the base polymer, the higher the cohesive force within the adhesive layer and the higher the adhesive force of the adhesive layer before reaching a certain degree of crosslinking. When the degree of crosslinking exceeds the aforementioned certain level, the higher the degree of crosslinking of the base polymer, the more elastic the adhesive layer tends to be, resulting in lower adhesive force.

[0028] In the case where one of the first surface 10a and the second surface 10b of the optical adhesive layer 10 does not have a peel adhesion force F of 5 N / 25 mm or more under the above conditions (low adhesion surface), the adhesion between the optical adhesive layer 10 and the substrate can be ensured by performing a plasma treatment on the surface of the substrate (pre-bonding surface) before attaching the low adhesion surface side of the optical adhesive layer 10 to the substrate. When both the first surface 10a and the second surface 10b have a peel adhesion force F of 5 N / 25 mm or more under the above conditions, it is not necessary to perform this plasma treatment on the surface of the substrate, which is therefore preferable.

[0029] In the optical adhesive layer 10, the shear storage modulus G' (kPa) and peel adhesion F (N / 25 mm) at -20°C are preferably such that F ≥ 0.4667 × G' - 58 when G' > 135. This balance between high flexibility and high adhesion in the optical adhesive layer 10 is beneficial for balancing the flexibility of the optical adhesive layer 10 and the adhesion to the adhered object. Therefore, it is also beneficial for balancing the aforementioned bending deformation and peel resistance.

[0030] The ratio of the shear storage modulus G' (kPa) at -20°C to the peel adhesion F (N / 25 mm) is preferably 30 or less, more preferably 25 or less, and even more preferably 15 or less, and more preferably 1 or more, more preferably 3 or more, and even more preferably 5 or more. This configuration is better for achieving a balance between the adhesion, flexibility, and bendability of the optical adhesive layer 10.

[0031] The ratio of the shear storage modulus G' (kPa) at -40°C to the peel adhesion F (N / 25 mm) is preferably 200 or less, more preferably 180 or less, and even more preferably 100 or less, and more preferably 10 or more, even more preferably 20 or more, and even more preferably 30 or more. This configuration is better for achieving a balance between the adhesion, flexibility, and bendability of the optical adhesive layer 10.

[0032] The shear storage modulus G' at -40°C is preferably 1200 kPa or less, more preferably 1000 kPa or less, further preferably 800 kPa or less, especially preferably 600 kPa or less, and more preferably 100 or more, more preferably 200 or more, and further preferably 300 or more. This configuration is better for ensuring the flexibility and bendability of the optical adhesive layer 10.

[0033] In the optical adhesive layer 10, the difference between the maximum and 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 better for suppressing stress concentration in the adhered body when the adhered body in contact with the optical adhesive layer 10 deforms. Furthermore, this thickness difference configuration is also better from the perspective of the visibility of the flexible device (optical device) with the optical adhesive layer 10 in the laminated structure.

[0034] After the optical adhesive layer 10 is wound onto a core with a cross-sectional diameter of 20 mm, the change in transmittance of the optical adhesive layer 10 after 1 hour is preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less. This configuration is suitable for ensuring the transparency of the optical adhesive layer used in flexible devices. Specifically, the aforementioned change in transmittance of the optical adhesive layer 10 can be measured by the method described below with reference to the embodiments.

[0035] The optical adhesive layer 10 is a pressure-sensitive adhesive layer formed from an adhesive composition. The optical adhesive layer 10 is transparent (visible light transmittance). The optical adhesive layer 10 contains at least a base polymer.

[0036] The base polymer is the adhesive component that exhibits adhesive properties in the optical adhesive layer 10. Examples of base polymers include: acrylic polymers, silicone polymers, polyester polymers, polyurethane polymers, polyamide polymers, polyethylene ether polymers, vinyl acetate / vinyl chloride copolymers, modified polyolefin polymers, epoxy polymers, fluoropolymers, and rubber polymers. The base polymer can be used alone or in combination of two or more. From the viewpoint of ensuring good transparency and adhesion of the optical adhesive layer 10, it is preferable to use an acrylic polymer as the base polymer.

[0037] Acrylic polymers are copolymers containing monomeric components of alkyl (meth)acrylate at a ratio of 50% or more by mass. "(Meth)acrylate" means acrylic acid and / or methacrylic acid.

[0038] Alkyl methacrylates with 1 to 20 carbon atoms are suitable as alkyl methacrylates. Alkyl methacrylates may have straight-chain or branched alkyl groups, or cyclic alkyl groups such as alicyclic alkyl groups.

[0039] Examples of alkyl (meth)acrylates having straight-chain or branched alkyl groups include: methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, dibutyl (meth)acrylate, tributyl (meth)acrylate, pentyl (meth)acrylate, isoamyl (meth)acrylate, neopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, and methacrylic acid. Nonyl acrylate, isononyl acrylate, decyl acrylate, isodecyl acrylate, undecyl acrylate, dodecyl acrylate (i.e., lauryl acrylate), isotridecyl acrylate, tetradecyl acrylate, isotetradecyl acrylate, pentadecyl acrylate, hexadecyl acrylate, heptadecanyl acrylate, octadecyl acrylate, isooctadecyl acrylate, and nonadecanyl acrylate.

[0040] 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. Examples of meth)acrylates having a bicyclic aliphatic hydrocarbon ring include: isopropyl (meth)acrylate. Examples of meth)acrylates having three or more aliphatic hydrocarbon rings include: dicyclopentyl (meth)acrylate, dicyclopentoxyethyl (meth)acrylate, tricyclopentyl (meth)acrylate, 1-adamantane (meth)acrylate, 2-methyl-2-adamantane (meth)acrylate, and 2-ethyl-2-adamantane (meth)acrylate.

[0041] As an alkyl methacrylate, it is preferred to use an alkyl acrylate having an alkyl group having 3 to 15 carbon atoms, and more preferably to use at least one selected from the group consisting of n-butyl acrylate, 2-ethylhexyl acrylate and dodecyl acrylate.

[0042] From the viewpoint of appropriately exhibiting basic properties such as adhesion in the optical adhesive layer 10, the ratio of alkyl (meth)acrylate in the monomer component is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. This ratio is, for example, 99% by mass or less.

[0043] The monomer component may also include comonomers capable of copolymerizing with alkyl (meth)acrylates. Examples of comonomers include monomers with polar groups. Examples of monomers containing polar groups include monomers with nitrogen-containing rings, hydroxyl-containing monomers, and carboxyl-containing monomers. Monomers containing polar groups help to introduce crosslinking points into acrylic polymers and ensure the cohesiveness of acrylic polymers, thus modifying the acrylic polymers.

[0044] Examples of monomers having a nitrogen-containing ring include: N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperidone, N-vinylpyrrole, N-vinylimidazol, N-vinylpyrazole, N-(meth)propenyl-2-pyrrolidone, N-(meth)propenylpiperidone, N-(meth)propenylpyrrolidone, N-vinylpyrrolidone, N-vinylpyrrolidone, N-vinylpyrrolidone, N-vinylpyrrolidone, N-vinylpyrrolidone, N-vinylpyrrolidone, N-vinyl-3-pyrrolidone, N-vinyl-2-caprolactone, N-vinyl-1,3-pyrrolidone-2-one, N-vinyl-3,5-pyrrolidone, N-vinylpyrazole, N-vinylisopyrazole, N-vinylthiazole, and N-vinylisothiazole. It is preferable to use N-vinyl-2-pyrrolidone as a monomer with a nitrogen-containing ring.

[0045] From the perspective of ensuring the cohesive force of the optical adhesive layer 10 and the adhesion force of the optical adhesive layer 10 to the adherend, the proportion of monomers having nitrogen-containing atomic rings in the monomer component 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 perspective of adjusting the glass transfer temperature of the acrylic polymer and adjusting the polarity of the acrylic polymer (which relates to the compatibility between the various additive components in the optical adhesive layer 10 and the acrylic polymer), this proportion is preferably 30% by mass or less, and more preferably 20% by mass or less.

[0046] Examples of hydroxyl-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 methyl (4-hydroxymethylcyclohexyl)methacrylate. 4-hydroxybutyl (meth)acrylate is preferred as the hydroxyl-containing monomer, and more preferably 4-hydroxybutyl acrylate.

[0047] From the perspective of introducing a cross-linking structure into the acrylic polymer and ensuring the cohesiveness of the optical adhesive layer 10, the proportion of hydroxyl-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 perspective of adjusting the polarity of the acrylic polymer (which relates to the compatibility of various additive components in the optical adhesive layer 10 with the acrylic polymer), this proportion is preferably 20% by mass or less, and more preferably 10% by mass or less.

[0048] Examples of carboxyl-containing monomers include: acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, methylene succinic acid, maleic acid, trans-butenedioic acid, butenoic acid, and isobutenoic acid.

[0049] From the perspective of introducing a cross-linking structure into the acrylic polymer, ensuring the cohesive force of the optical adhesive layer 10, and ensuring the adhesion of the optical adhesive layer 10 to the adherend, the proportion of carboxyl-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 perspective of adjusting the glass transition temperature of the acrylic polymer and avoiding the risk of corrosion of the adherend caused by acid, this proportion is preferably 30% by mass or less, more preferably 20% by mass or less.

[0050] To prevent corrosion of metallic components such as electrodes in flexible devices by acid components, the optical adhesive layer 10 preferably has a low acid content. Furthermore, when the optical adhesive layer 10 is used for bonding polarizing plates, to suppress polyolefin formation in polyvinyl alcohol-based polarizing elements caused by acid components, the optical adhesive layer 10 preferably has a low acid content. In this acid-free optical adhesive layer 10, the content of organic acid monomers (e.g., (meth)acrylic acid and carboxyl-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 optical adhesive layer 10 is determined by immersing the optical adhesive layer 10 in pure water and heating it at 100°C for 45 minutes to extract the acid monomers into the water, and then quantifying the acid monomers using an ion chromatography system.

[0051] From an acid-free perspective, it is preferable that the base polymer in the optical adhesive layer 10 does not substantially contain organic acid monomers as monomer components. From an acid-free perspective, the ratio of organic acid monomers in the monomer components is preferably 0.5% by mass or less, more preferably 0.1% by mass or less, and even more preferably 0.05% by mass, ideally 0% by mass.

[0052] The monomer component may also include other comonomers. Examples of other comonomers include: acid anhydride monomers, sulfonic acid monomers, phosphate monomers, epoxy monomers, cyano monomers, alkoxy monomers, and aromatic vinyl compounds. These other comonomers may be used alone or in combination of two or more.

[0053] In this embodiment, the base polymer has a cross-linked structure. Examples of methods for introducing a cross-linked structure into the base polymer include: a method in which a base polymer having functional groups capable of reacting with a cross-linking agent and a cross-linking agent are formulated into an adhesive composition, and the base polymer and the cross-linking agent react in the optical adhesive layer 10 (method 1); and a method in which a multifunctional monomer is included in the monomer component forming the base polymer, and a base polymer with a branched structure (cross-linked structure) introduced into the polymer chain is formed by the polymerization of the monomer component (method 2). These methods may also be used in combination.

[0054] As a crosslinking agent used in the first method described above, examples include compounds that react with the functional groups (hydroxyl and carboxyl groups, etc.) contained in the base polymer. Examples of such crosslinking agents include: isocyanate crosslinking agents, peroxide crosslinking agents, epoxy crosslinking agents, aziridine crosslinking agents, carbodiimide crosslinking agents, and metal chelate crosslinking agents. Crosslinking agents can be used alone or in combination of two or more. Considering the high reactivity with the hydroxyl and carboxyl groups in the base polymer and the ease with which crosslinking structures can be introduced, isocyanate crosslinking agents, peroxide crosslinking agents, and epoxy crosslinking agents are preferred as crosslinking agents.

[0055] Examples of isocyanate crosslinking agents include: toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, phenyl diisocyanate, hydrogenated phenyl diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, tetramethylphenyl diisocyanate, naphthalene diisocyanate, triphenylmethane triisocyanate, and polymethylene polyphenyl isocyanate. Furthermore, derivatives of these isocyanates can also be cited as isocyanate crosslinking agents. Examples of such isocyanate derivatives include isocyanurate-modified compounds and polyol-modified compounds. Commercially available isocyanate crosslinking agents include, for example: Coronate L (trimethylolpropane adduct of toluene diisocyanate, manufactured by Tosoh), Coronate HL (trimethylolpropane adduct of hexamethylene diisocyanate, manufactured by Tosoh), Coronate HX (isocyanurate of hexamethylene diisocyanate, manufactured by Tosoh), and Takenate D110N (trimethylolpropane adduct of phenyl diisocyanate, manufactured by Mitsui Chemicals).

[0056] Examples of peroxide crosslinking agents include: benzoyl peroxide, di(2-ethylhexyl) peroxide dicarbonate, di(4-tert-butylcyclohexyl) peroxide dicarbonate, di-second butyl peroxide dicarbonate, tert-butyl peroxydecanoate, tert-hexyl peroxypentanoate, and tert-butyl peroxypentanoate.

[0057] Examples of epoxy crosslinking agents include: bisphenol A, epichlorohydrin type epoxy resin, ethylene glycidyl ether, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether, glycerol triglycidyl ether, 1,6-hexanediol glycidyl ether, trimethylolpropane triglycidyl ether, diglycidyl aniline, diamine glycidylamine, N,N,N',N'-tetraglycidyl m-phenylenediamine, and 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane.

[0058] From the viewpoint of ensuring adequate flexibility (bendability) of the optical adhesive layer 10, isocyanate crosslinking agents (especially difunctional isocyanate crosslinking agents) and peroxide crosslinking agents are preferred. From the viewpoint of ensuring the durability of the optical adhesive layer 10, isocyanate crosslinking agents (especially trifunctional isocyanate crosslinking agents) are preferred. In the base polymer, difunctional isocyanate crosslinking agents and peroxide crosslinking agents form a softer two-dimensional crosslink, while trifunctional isocyanate crosslinking agents form a stronger three-dimensional crosslink. From the viewpoint of balancing the durability and flexibility of the optical adhesive layer 10, it is preferable to use a combination of trifunctional isocyanate crosslinking agents, peroxide crosslinking agents, and / or difunctional isocyanate crosslinking agents.

[0059] From the viewpoint of ensuring the cohesive force of the optical adhesive layer 10, the amount of crosslinking agent relative to 100 parts by mass of the base polymer 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. From the viewpoint of ensuring good adhesion in the optical adhesive layer 10, the amount of crosslinking agent relative to 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.

[0060] In the second method described above, the monomer components (including multifunctional monomers and other monomers used to introduce the crosslinking structure) can undergo either single-stage polymerization or multi-stage polymerization. In the multi-stage polymerization method, firstly, the monofunctional monomers used to form the base polymer are polymerized (prepolymerization), thereby preparing a prepolymer composition containing a portion of the polymer (a mixture of low-degree polymers and unreacted monomers). Then, after adding the multifunctional monomers to the prepolymer composition, the portion of the polymer is polymerized with the multifunctional monomers (formal polymerization).

[0061] As a multifunctional monomer, an example is a multifunctional (meth)acrylate containing two or more vinyl unsaturated double bonds in one molecule. Considering that a cross-linked structure can be introduced through active energy line polymerization (photopolymerization), multifunctional monomers are preferred.

[0062] Examples of polyfunctional (meth)acrylates include: difunctional (meth)acrylates, trifunctional (meth)acrylates, and polyfunctional (meth)acrylates with four or more functions.

[0063] Examples of difunctional (meth)acrylates include: ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanediol dimethacrylate, glycerol dimethacrylate, neopentyl glycol dimethacrylate, stearic acid-modified pentaerythritol dimethacrylate, dicyclopentenyl diacrylate, dimethacrylate isocyanurate, and epoxide-modified bisphenol dimethacrylate.

[0064] Examples of trifunctional (meth)acrylates include: trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and tri(acryloxyethyl) isocyanurate.

[0065] Examples of polyfunctional (meth)acrylates with four or more functions include: di-trimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol monohydroxy penta(meth)acrylate, alkyl-modified dipentaerythritol pentaacrylate, and dipentaerythritol hexa(meth)acrylate.

[0066] The molecular weight of the multifunctional monomer is preferably 1500 or less, more preferably 1000 or less. Furthermore, the functional group equivalent (g / eq) of the multifunctional monomer is preferably 50 or more, more preferably 70 or more, and even more preferably 80 or more. This 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 G' and loss tangent tanδ) in the base polymer by introducing crosslinking structures.

[0067] Acrylic polymers can be formed by polymerizing the aforementioned monomer components. Examples of polymerization methods include solution polymerization, active energy line polymerization (e.g., UV (ultraviolet) polymerization), bulk polymerization, and emulsion polymerization. Considering the transparency, water resistance, and cost of the optical adhesive layer 10, solution polymerization and UV polymerization are preferred. For example, ethyl acetate and toluene are used as solvents for solution polymerization. Furthermore, thermal polymerization initiators and photopolymerization initiators are used as polymerization initiators. The amount of polymerization initiator used relative to 100 parts by mass of the monomer component is, for example, 0.05 parts by mass or more, and for example, 1 part by mass or less.

[0068] 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 ester, 4,4'-azobis-4-cyanovaleric acid, azobisisovalerate, 2,2'-azobis(2-amidinylpropane) dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis(2-methylpropanediamine) disulfide, and 2,2'-azobis(N,N'-dimethyleneisobutylamidine) dihydrochloride. Examples of peroxide polymerization initiators include benzoyl peroxide, tributyl maleate peroxide, and lauryl peroxide.

[0069] Examples of photopolymerization initiators include: benzoin ether-based photopolymerization initiators, acetophenone-based photopolymerization initiators, α-keto alcohol-based photopolymerization initiators, aromatic sulfonyl chloride-based photopolymerization initiators, photoactive oxime-based photopolymerization initiators, benzoin-based photopolymerization initiators, diphenylethylene glycol-based photopolymerization initiators, benzophenone-based photopolymerization initiators, ketal-based photopolymerization initiators, and 9-oxosulfuron-methyl-2-ethylhexylene ... It is a photopolymerization initiator and a phosphine oxide photopolymerization initiator.

[0070] In polymerization, chain transfer agents and / or polymerization inhibitors (polymerization retarders) may also be used for purposes such as molecular weight adjustment. Examples of chain transfer agents include: α-thioglycerol, lauryl thiol, glycidyl thiol, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolic acid, 2,3-dimercapto-1-propanol, and α-methylstyrene dimer.

[0071] The molecular weight of the base polymer can be adjusted by changing the type and / or amount of the polymerization initiator. For example, in free radical polymerization, the more polymerization initiator used, the higher the concentration of free radicals in the reaction system, thus tending to result in a higher density of reaction initiation points and a smaller molecular weight of the formed base polymer. Conversely, the less polymerization initiator used, the lower the density of reaction initiation points, thus tending to result in easier polymer chain elongation and a larger molecular weight of the formed base polymer.

[0072] From the perspective of ensuring the cohesiveness of the optical adhesive layer 10, 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. This 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 was determined using gel permeation chromatography (GPC) and calculated using polystyrene.

[0073] The glass transition temperature (Tg) of the base polymer is preferably below 0°C, more preferably below -10°C, and even more preferably below -20°C. This glass transition temperature is, for example, above -80°C.

[0074] The glass transition temperature (Tg) of the basic polymer can be obtained using the theoretical value based on the following Fox formula. The Fox formula is the relationship between the glass transition temperature Tg of the polymer and the glass transition temperature Tgi of the homopolymer of the monomers constituting the polymer. In the Fox formula, 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. The glass transition temperature of the homopolymer can be obtained from literature. For example, the glass transition temperatures of various homopolymers are listed in "Polymer Handbook" (4th edition, John Wiley & Sons, Inc., 1999) and "New Polymer Library 7: Introduction to Synthetic Resins for Coatings" (by Kyozo Kitaoka, Polymer Publishing Association, 1995). Alternatively, the glass transition temperature of the homopolymer of the monomer can also be obtained using the method specifically described in Japanese Patent Application Publication No. 2007-51271.

[0075] Fox formula 1 / (273+Tg)=Σ[Wi / (273+Tgi)]

[0076] The adhesive composition may also contain one or more oligomers in addition to the base polymer. When using an acrylic polymer as the base polymer, it is preferable to use an acrylic oligomer as the oligomer. The acrylic oligomer is a copolymer containing a monomer component of alkyl (meth)acrylate at a ratio of 50% by mass or more, and has a weight average molecular weight, for example, between 1,000 and 30,000.

[0077] The glass transition temperature of the acrylic oligomer is preferably above 60°C, more preferably above 80°C, further preferably above 100°C, and even more preferably above 110°C. The glass transition temperature of the acrylic oligomer is, for example, below 200°C, preferably below 180°C, and more preferably below 160°C. By using a low-Tg acrylic polymer (base polymer) with a cross-linked structure and a high-Tg acrylic oligomer, the adhesion of the optical adhesive layer 10, especially the adhesion at high temperatures, can be improved. The glass transition temperature of the acrylic oligomer is calculated using the Fox formula described above.

[0078] Acrylic oligomers with a glass transition temperature of 60°C or higher are preferably polymers comprising monomer components of alkyl (meth)acrylates having chain alkyl groups ((meth)acrylate chain alkyl esters) and alkyl (meth)acrylates having alicyclic alkyl groups ((meth)acrylate alicyclic alkyl esters). Specific examples of such alkyl (meth)acrylates include, for instance, the alkyl (meth)acrylates described above as monomer components of acrylic polymers.

[0079] Considering the high glass transition temperature and excellent compatibility with the base polymer, methyl methacrylate is preferred among (meth)acrylate chain alkyl esters. Dicyclopentyl acrylate, dicyclopentyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate are preferred among (meth)acrylate alicyclic alkyl esters. That is, the acrylic oligomer is preferably a polymer comprising one or more monomers selected from the group consisting of dicyclopentyl acrylate, dicyclopentyl methacrylate, cyclohexyl acrylate, and cyclohexyl methacrylate, and methyl methacrylate.

[0080] The proportion of (meth)acrylate cycloalkyl esters 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. This 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 (meth)acrylate chain alkyl esters 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. This proportion is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more.

[0081] 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. This molecular weight is preferably 30000 or less, more preferably 10000 or less, and even more preferably 8000 or less. This molecular weight range of the acrylic oligomer is preferable for ensuring the adhesion and adhesion retention of the optical adhesive layer 10.

[0082] Acrylic oligomers are obtained by polymerizing the monomeric components of the acrylic oligomer. Examples of polymerization methods include solution polymerization, active energy line polymerization (e.g., UV polymerization), bulk polymerization, and emulsion polymerization. In the polymerization of acrylic oligomers, polymerization initiators can be used, and chain transfer agents can be used for molecular weight adjustment purposes.

[0083] To sufficiently improve the adhesion of the optical adhesive layer 10, the content of acrylic oligomer in the optical adhesive layer 10 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, relative to 100 parts by mass of the base polymer. On the other hand, from the viewpoint of ensuring the transparency of the optical adhesive layer 10, the content of acrylic oligomer in the optical adhesive layer 10 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, relative to 100 parts by mass of the base polymer. When the content of acrylic oligomer in the optical adhesive layer 10 is too high, the compatibility of the acrylic oligomer decreases, which tends to lead to increased haze and decreased transparency.

[0084] The adhesive composition may also contain a silane coupling agent. The content of the silane coupling agent in the adhesive composition is preferably 0.1 parts by weight or more, more preferably 0.2 parts by weight or more, relative to 100 parts by weight of the base polymer. This content is preferably 5 parts by weight or less, more preferably 3 parts by weight or less.

[0085] The adhesive composition may also contain other components as needed. Examples of other components include, for instance, adhesive agents, plasticizers, softeners, antioxidants, fillers, colorants, UV absorbers, antioxidants, surfactants, and antistatic agents.

[0086] The single-layer optical adhesive layer 10 can be manufactured, for example, by coating the adhesive composition onto the release film L1 (first release film) to form a coating film, and then drying the coating film.

[0087] As a release film, examples 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. Preferably, the surface of the release film has undergone a release treatment.

[0088] Examples of coating methods for adhesive compositions include: roller coating, contact roller coating, gravure coating, reverse coating, roller brush coating, spraying, dip roller coating, rod coating, doctor blade coating, air knife coating, curtain coating, die lip coating, and nozzle 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.

[0089] Alternatively, a release film L2 (second release film) may be laminated onto the optical adhesive layer 10 on the first release film L1. The second release film is a flexible plastic film that has undergone surface peeling treatment, and may be the same as the film described above regarding the first release film.

[0090] In the manner described above, an optical adhesive layer 10, which is an optical adhesive sheet S with its adhesive surface covered and protected by release films L1 and L2, can be manufactured. When using the optical adhesive sheet S, the release films L1 and L2 are peeled off from the optical adhesive sheet S as needed.

[0091] Examples of methods for forming the multilayer optical adhesive layer 10 include: dry-on-dry method, wet-on-dry method, and wet-on-wet method. In the dry-on-dry method, for example, the multilayer adhesive layer can be formed by coating an adhesive composition onto a release film and drying it to form multiple adhesive layers, followed by bonding the multiple adhesive layers together. In the wet-on-dry method, for example, the multilayer adhesive layer can be formed by coating each adhesive layer onto the release film with an adhesive composition and drying it to form an adhesive layer. In the wet-on-wet method, for example, the multilayer adhesive layer can be formed by coating multiple layers of adhesive composition onto the release film in multiple stages to form a multilayer coating film, followed by drying the multilayer coating film.

[0092] The optical adhesive layer 10, whether single-layer or multi-layer, can also be formed by applying an adhesive composition to the bonding object using the optical adhesive layer 10 and allowing it to dry.

[0093] 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 operability 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, further preferably 100 μm or less, and particularly preferably 50 μm or less.

[0094] The haze of the optical adhesive layer 10 is preferably 3% or less, more preferably 2% or less, and even more preferably 1% or less. The haze of the optical adhesive layer 10 can be measured using a haze meter according to JIS K7136 (2000). Examples of haze meters include the "NDH2000" manufactured by Nippon Denshoku Kogyo Co., Ltd., and the "HM-150" manufactured by Murakami Color Technology Research Institute Co., Ltd.

[0095] 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 according to JIS K 7375 (2008).

[0096] Figures 4A to 4C show one example of how the optical adhesive layer 10 is used.

[0097] In this method, firstly, as shown in FIG4A, an optical adhesive layer 10 is bonded to one side of the first component 21 (the adherend) in the thickness direction H. The first component 21 is, for example, an element in the multilayer structure of a flexible display panel. Examples of such elements include: a pixel panel, a touch panel, a polarizing plate, and a cover film (the same applies to the second component 22 described later). Through this step, an optical adhesive layer 10 for bonding with other components is provided on the first component 21.

[0098] Subsequently, as shown in FIG4B, the optical adhesive layer 10 on the first member 21 is used to bond one side of the first member 21 in the thickness direction H to the other side of the second member 22 in the thickness direction H. The second member 22 is, for example, another element in the laminated structure of a flexible display panel.

[0099] Subsequently, as shown in Figure 4C, the optical adhesive layer 10 between the first component 21 and the second component 22 is cured. Through curing, the optical adhesive layer 10 undergoes a cross-linking reaction of the basic polymer, thereby enhancing the bonding force between the first component 21 and the second component 22. The curing temperature is, for example, 20°C to 160°C. The curing time is, for example, 1 minute to 21 days. When curing is performed by autoclaving (heating and pressurizing), the temperature is, for example, 30°C to 80°C, the pressure is, for example, 0.1 to 0.8 MPa, and the processing time is, for example, 15 minutes or more.

[0100] The optical adhesive layer 10, used in the manufacturing process of flexible devices as described above, has a shear storage modulus G' of less than 180 kPa at -20°C, and at least one of the first surface 10a and the second surface 10b has a peel adhesion F of more than 5 N / 25 mm under the above conditions. This optical adhesive layer 10, as described above, is suitable for balancing flexural deformation resistance and peel inhibition, and is therefore suitable for use in flexible devices. [Example]

[0101] The present invention will be specifically described below with reference to the embodiments shown. However, the present invention is not limited to the embodiments. Furthermore, the specific values ​​of the dosage (content), physical property value, parameters, etc., described below can replace the upper limit (defined as "below" or "not reached") or lower limit (defined as "above" or "exceeded") of the dosage (content), physical property value, parameters, etc., corresponding to those described in the above "Embodiment".

[0102] <Preparation of Polymer P1> In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet, a mixture containing 89 parts by mass of 2-ethylhexyl acrylate (2EHA), 10 parts by mass of N-vinyl-2-pyrrolidone (NVP), 1 part 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 and toluene as solvents (solid content concentration 50% by mass, toluene ratio in solvent 5% by mass) was stirred at 55°C under a nitrogen atmosphere for 6 hours (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, thereby obtaining a first polymer solution containing the acrylic polymer (polymer P1). The weight average molecular weight of the acrylic polymer in the first polymer solution was 2.19 million.

[0103] <Preparation of Polymer P2> In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet, a mixture (solid content concentration of 33% by mass) comprising 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 was stirred at 58°C under a nitrogen atmosphere for 5 hours (polymerization reaction). This yielded a solution containing acrylic polymer (polymer P2). Subsequently, ethyl acetate was added to this solution to adjust the polymer concentration to 30% by mass, thereby obtaining a second polymer solution containing acrylic polymer (polymer P2). The weight average molecular weight of the acrylic polymer in the second polymer solution was 830,000.

[0104] <Preparation of Polymer P3> In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet, a mixture containing 70.3 parts by weight of 2-ethylhexyl acrylate (2EHA), 8.0 parts by weight of lauryl acrylate (LA), 20.1 parts by weight of n-butyl acrylate (BA), 1.0 part by weight of 4-hydroxybutyl acrylate (4HBA), 0.6 parts by weight of N-vinyl-2-pyrrolidone (NVP), 0.1 parts by weight of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator, and ethyl acetate as a solvent (solids concentration of 47% by weight) was stirred at 56°C under a nitrogen atmosphere for 6 hours (polymerization reaction). This yielded a solution containing an acrylic polymer (polymer P3). Subsequently, ethyl acetate was added to this solution to adjust the polymer concentration to 24% by weight. This yielded a third polymer solution containing the acrylic polymer (polymer P3). The weight average molecular weight of the acrylic polymer in the third polymer solution was 2.1 million.

[0105] Preparation of Polymer P4 In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet, a mixture containing 97.3 parts by mass of 2-ethylhexyl acrylate (2EHA), 1.7 parts by mass of N-vinyl-2-pyrrolidone (NVP), 1.0 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 and toluene as solvents (solid content concentration of 50% by mass, toluene ratio in solvent of 5% by mass) was stirred at 55°C under a nitrogen atmosphere for 6 hours (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 fourth polymer solution containing an acrylic polymer (polymer P4). The weight average molecular weight of the acrylic polymer in the fourth polymer solution was 2 million.

[0106] <Preparation of the First Acrylic Acid Oligomer> In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet, a mixture comprising 60 parts by mass of dicyclopentyl methacrylate (DCPMA), 40 parts by mass of methyl methacrylate (MMA), 3.5 parts by mass of α-thioglycerol as a chain transfer agent, and 100 parts by mass of toluene as a solvent was stirred at 70°C under a nitrogen atmosphere for 1 hour. Subsequently, 0.2 parts by mass of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator was added to the mixture to prepare a reaction solution, which was reacted at 70°C under a nitrogen atmosphere for 2 hours, followed by a reaction at 80°C under a nitrogen atmosphere for 2 hours (formation of the first acrylic acid oligomer). This yielded a solid first acrylic acid oligomer. The weight average molecular weight of the first acrylic acid oligomer was 5100. The glass transition temperature (Tg) of the first acrylic acid oligomer was 130°C.

[0107] <Preparation of the Second Acrylic Acid Oligomer> In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet, a mixture comprising 95 parts by mass of cyclohexyl methacrylate (CHMA), 5 parts by mass of acrylic acid (AA), 10 parts by mass of α-methylstyrene dimer (product name "Nofmer MSD", manufactured by Nippon Yushi Co., Ltd.) as a chain transfer agent, and 120 parts by mass of toluene as a solvent was stirred at room temperature under a nitrogen atmosphere for 1 hour. Subsequently, 10 parts by mass of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator were added to the mixture to prepare a reaction solution, which was reacted under a nitrogen atmosphere at 85°C for 2 hours, followed by a reaction under a nitrogen atmosphere at 86°C for 1.5 hours (formation of the second acrylic acid oligomer). This yielded a solid second acrylic acid oligomer. The weight average molecular weight of the second acrylic acid oligomer was 4000. The glass transition temperature (Tg) of the second acrylic acid oligomer was 67°C.

[0108] [Example 1] <Preparation of the First Adhesive Composition> Ethyl acetate was added to the first polymer solution to adjust the solid content concentration to 10% by mass, thus obtaining the first adhesive composition.

[0109] <Preparation of the Second Adhesive Composition> In the second polymer solution, for every 100 parts by weight of the acrylic polymer (polymer P2), 0.5 parts by weight of a crosslinking agent (product name "Nyper BMT 40SV", benzoyl peroxide, manufactured by Nippon Oils & Fats Co., Ltd.) is added and mixed. Then, ethyl acetate is added to adjust the concentration of solids to 23% by weight to obtain the second adhesive composition.

[0110] <Formation of Optical Adhesive Layer> On the release-treated surface of a first release film (product name "JT-50Wa", polyester film, 50 μm thick, manufactured by Nitto Denko Corporation) that has undergone silicone release treatment on one side, a first adhesive composition (lower position) and a second adhesive composition (upper position) are applied by a wet-on-wet method. Specifically, the first adhesive composition is applied to the first release film to form a coating film (2 μm thick after drying), and the second adhesive composition is applied to the coating film to form a coating film (23 μm thick after drying) (formation of a multilayer coating film). Subsequently, the multilayer coating film on the first release film is dried by heating at 155°C for 2 minutes to form a multilayer adhesive layer with a thickness of 25 μm. Subsequently, the peeled side of a second release film (product name "MRQ25T100J", polyester film, 25 μm thick, manufactured by Mitsubishi Chemical Co., Ltd.), which has undergone silicone peeling treatment on one side, is bonded to a multilayer adhesive layer on the first release film. Then, a curing treatment is performed at 50°C for 48 hours to allow a crosslinking reaction to occur in the adhesive layer. The optical adhesive sheet (optical adhesive layer) of Example 1 is prepared in the manner described above. The optical adhesive sheet of Example 1 has a two-layer structure: a first layer (2 μm thick) formed from the first adhesive composition as the first adhesive layer, and a second layer (23 μm thick) formed from the second adhesive composition as the second adhesive layer. The monomer composition of the polymer and the composition of the adhesive layer in the optical adhesive sheet of Example 1 are shown in Table 1, with units expressed as parts by mass (the same applies to the examples and comparative examples described later).

[0111] [Example 2] Except as described below, the optical adhesive sheet of Example 2 was prepared in the same manner as the optical adhesive sheet of Example 1. When preparing the second adhesive composition, the amount of crosslinking agent (product name "Nyper BMT 40SV") was set to 1.0 parts by weight instead of 0.5 parts by weight.

[0112] The optical adhesive sheet of Example 2 has a two-layer structure consisting of a first layer (2 μm thick) formed from a first adhesive composition as the first adhesive layer, and a second layer (23 μm thick) formed from a second adhesive composition as the second adhesive layer. The optical adhesive sheet of Example 2 differs from that of Example 2 in the degree of crosslinking of the polymer components in the first and second adhesive layers. As a result, the optical adhesive sheets of Examples 1 and 2 exhibit differences in peel adhesion F, which will be described later. This is because, during the wet-on-wet process in the manufacturing of the optical adhesive sheet, a portion of the crosslinking agent diffuses from the second adhesive composition (upper layer) coated on the first adhesive composition (lower layer) into the first adhesive composition.

[0113] [Example 3] <Preparation of the Third Adhesive Composition> In the third polymer solution, for every 100 parts by weight of the acrylic polymer (polymer P3), 0.28 parts by weight of a crosslinking agent (product name "Nyper BMT 40SV", benzoyl peroxide, manufactured by Nippon Yushu Co., Ltd.), 3 parts by weight of the first acrylic oligomer, and 0.3 parts by weight of an antioxidant (product name "Irganox 1010", manufactured by BASF) are added and mixed. Then, ethyl acetate is added to adjust the solid content concentration to 15% by weight to obtain the third adhesive composition. The first acrylic oligomer is obtained by the following method.

[0114] <Formation of Optical Adhesive Layer> A third adhesive composition was coated onto the release-treated surface of a first release film (product name "JT-50Wa", polyester film, 50 μm thick, manufactured by Nitto Denko Corporation), which had undergone silicone release treatment on one side, to form a coating film (25 μm thick after drying). Then, the coating film on the first release film was dried by heating at 155°C for 2 minutes to form an adhesive layer with a thickness of 25 μm. Next, the release-treated surface of a second release film (product name "MRQ25T100J", polyester film, 25 μm thick, manufactured by Mitsubishi Chemical Corporation), which had undergone silicone release treatment on one side, was adhered to the adhesive layer on the first release film. Subsequently, a crosslinking reaction was carried out in the adhesive layer by curing at 50°C for 48 hours. The optical adhesive sheet of Example 3 (a single-layer optical adhesive layer) was produced in the manner described above.

[0115] [Example 4] In the aforementioned third polymer solution, for every 100 parts by weight of the acrylic polymer (polymer P3) in the polymer solution, 0.26 parts by weight of the first crosslinking agent (product name "Nyper BMT 40SV", benzoyl peroxide, manufactured by Nippon Yushu Co., Ltd.), 0.02 parts by weight of the second crosslinking agent (product name "Coronate L", trimethylolpropane adduct of toluene diisocyanate, manufactured by Tosoh Co., Ltd.), 1.5 parts by weight of the second acrylic oligomer, and 0.3 parts by weight of the silane coupling agent (product name "KBM403", manufactured by Shin-Etsu Chemical Co., Ltd.) are added and mixed. Then, ethyl acetate is added to adjust the solid content concentration to 23% by weight to obtain the fourth adhesive composition. Subsequently, in the formation of the optical adhesive layer, the fourth adhesive composition was used instead of the third adhesive composition. Otherwise, the optical adhesive sheet of Example 4 (a single-layer optical adhesive layer) was made in the same manner as the optical adhesive sheet of Example 3.

[0116] [Comparative Example 1] In the first polymer solution described above, for every 100 parts by weight of the acrylic polymer (polymer P1), 0.5 parts by weight of a crosslinking agent (product name "Nyper BMT 40SV", benzoyl peroxide, manufactured by Nippon Yushu Co., Ltd.) was added and mixed. Then, ethyl acetate was added to adjust the solid content concentration to 15% by weight to obtain the fifth adhesive composition. Subsequently, in the formation of the optical adhesive layer, the fifth adhesive composition was used instead of the third adhesive composition. Otherwise, the optical adhesive sheet of Comparative Example 1 (a single-layer optical adhesive layer) was prepared in the same manner as the optical adhesive sheet of Example 3.

[0117] [Comparative Example 2] In the formation of the optical adhesive layer, the second adhesive composition described above was used instead of the third adhesive composition. Otherwise, the optical adhesive sheet of Comparative Example 2 (single-layer optical adhesive layer) was made in the same manner as the optical adhesive sheet of Example 3.

[0118] [Comparative Example 3] Ethyl acetate was added to the fourth polymer solution to adjust the solid content concentration to 10% by mass, thus obtaining the sixth adhesive composition. Subsequently, in the formation of the optical adhesive layer, the sixth adhesive composition was used instead of the first adhesive composition. Otherwise, the optical adhesive sheet of Comparative Example 3 was prepared in the same manner as the optical adhesive sheet of Example 1. The optical adhesive sheet of Comparative Example 3 has a two-layer structure: a first layer (2 μm thick) formed from the sixth adhesive composition as the first adhesive layer, and a second layer (23 μm thick) formed from the second adhesive composition as the second adhesive layer.

[0119] <Thickness of Adhesive Layer> The thickness of each optical adhesive sheet in Examples 1-4 and Comparative Examples 1-3 was investigated. Specifically, firstly, adhesive sheets (25 mm short side × 100 mm long side) were cut from the optical adhesive sheets. Then, the thickness at five measurement points on the adhesive sheet was measured using a pin gauge. The five measurement points were five points dividing the width direction of the adhesive sheet into six equal parts along its long side. The maximum thickness T1 (μm) and the minimum thickness T2 (μm) among the thickness measurements at the five measurement points are shown in Table 1. The difference between the maximum and minimum thickness (T1 - T2) is also shown in Table 1.

[0120] <Peeling Adhesion> For each optical adhesive sheet of Examples 1-4 and Comparative Examples 1-3, the peel adhesion was investigated by peel test.

[0121] First, test samples were prepared for each optical adhesive sheet. In the preparation of the test samples for each optical adhesive sheet in Examples 1 and 2, firstly, the second release film was peeled off from the optical adhesive sheet, and a plasma-treated polyimide substrate (product name "Upilex 25RN", thickness 25 μm, manufactured by Ube Industries, Ltd.) was bonded to the exposed surface to obtain a laminate. Next, a test piece (25 mm wide × 100 mm long) was cut from this laminate (polyimide substrate / adhesive layer / first release film). Then, the first release film was peeled off from the adhesive layer of the test piece, and a polyimide film (product name "Upilex 50S", thickness 50 μm, manufactured by Ube Industries, Ltd.) was bonded to the exposed surface (the first layer side surface in the multilayer adhesive layers). The bonding system was performed at 23°C by pressing the test piece onto the polyimide film using a 2 kg hand roller that moved back and forth once. Test samples for the optical adhesive sheets of Examples 1, 2, and Comparative Example 3 were prepared in the manner described above. For the test samples for the optical adhesive sheets of Examples 3, 4, and Comparative Examples 1, 2, a polyimide substrate without plasma treatment (product name "Upilex 25RN", thickness 25 μm, manufactured by Ube Industries, Inc.) was used instead of the aforementioned polyimide substrate with plasma treatment. Otherwise, the samples were prepared in the same manner as the test samples in Examples 1, 2, and Comparative Example 3.

[0122] Subsequently, after the test specimens were allowed to stand at room temperature for 30 minutes, a peel test was performed to separate the specimen from the polyimide film in the test specimens, and the peel strength was measured. In the test specimens of Examples 1, 2, and Comparative Example 3, the peel strength of the first layer surface from the polyimide film surface was measured. This measurement was performed using a tensile testing machine (product name "Autograph AG-50NX plus"), manufactured by Shimadzu Corporation. In this measurement, the test temperature was set to 25°C, the peel angle of the specimen from the polyimide film was set to 180°, the tensile speed of the specimen was set to 300 mm / min, and the peel length was set to 50 mm. The average value of the measured peel strengths is shown in Table 1 as the peel adhesion F (N / 25 mm). The peel adhesion F of the adhesive layer in the multilayer structure (first layer / second layer) is the adhesion of the exposed surface of the first layer.

[0123] <Cut Storage Modulus> For each optical adhesive sheet (optical adhesive layer) of Examples 1-4 and Comparative Examples 1-3, the shear storage modulus was measured in the manner described below.

[0124] First, the test sample was prepared. Specifically, multiple adhesive sheets cut from optical adhesive sheets were laminated to create an adhesive sheet approximately 1 mm thick. This sheet was then punched to obtain cylindrical particles (9 mm in diameter) for use as the test sample. Next, the test sample was fixed to a jig with a parallel plate of 8 mm diameter using a dynamic viscoelasticity measuring device (product name "ARES-G2", manufactured by TA Instruments) for dynamic viscoelasticity measurement. In this measurement, the measurement mode was set to shear mode, the measurement temperature range was set to -60°C to 150°C, the heating rate was set to 5°C / min, and the frequency was set to 1 Hz. The first shear storage modulus G' (kPa) at -20°C and the second shear storage modulus G' (kPa) at -40°C were read from the measurement results. The results are shown in Table 1. Table 1 also shows the ratio of the first shear storage modulus G' (kPa) to the peel adhesion F (N / 25 mm) and the ratio of the second shear storage modulus G' (kPa) to the peel adhesion F (N / 25 mm).

[0125] Furthermore, regarding the optical adhesive sheets (optical adhesive layers) of Examples 1-4 and Comparative Examples 2 and 3, the measurement results of the peel adhesion F and the first shear storage modulus G' are shown in the graph of Figure 5. In the graph of Figure 5, the horizontal axis represents the first shear storage modulus G' (kPa) as the shear storage modulus at -20°C, and the vertical axis represents the peel adhesion F (N / 25 mm). In Figure 5, point E1 represents the measurement result in Example 1, point E2 represents the measurement result in Example 2, point E3 represents the measurement result in Example 3, point E4 represents the measurement result in Example 4, point C2 represents the measurement result in Comparative Example 2, and point C3 represents the measurement result in Comparative Example 3 (the measurement result of Comparative Example 1 falls outside the range of the vertical and horizontal axes, and is therefore not shown). Furthermore, in Figure 5, point chain line R1 represents the line where the peel adhesion F = 5 N / 25 mm when G' ≦ 135, and point chain line R2 represents the line where F = 0.4667 × G' - 58 when G' > 135. When the peel adhesion F is 5 N / 25 mm or more when G' ≦ 135, and the peel adhesion F satisfies F ≧ 0.4667 × G' - 58 when G' > 135 (points E1~E4), a balance between the flexibility and high adhesion of the optical adhesive layer can be achieved.

[0126] <Changes in Transmission Rate> For each optical adhesive sheet (optical adhesive layer) of Examples 1-4 and Comparative Examples 1-3, the transmittance variation was investigated in the manner described below.

[0127] First, a sample for testing is prepared. Specifically, after the second release film is peeled off from the optical adhesive sheet, the exposed adhesive layer is bonded to a PET substrate (product name "T100C50", thickness 50 μm, manufactured by Mitsubishi Chemical Co., Ltd.). Next, after the first release film is peeled off from the optical adhesive sheet (adhesive layer), the exposed adhesive layer surface is bonded to the PET substrate (product name "T100C50", thickness 50 μm, manufactured by Mitsubishi Chemical Co., Ltd.). This yields a laminate consisting of a PET substrate / optical adhesive layer / PET substrate. Then, a sample for testing, with dimensions of 25 mm width × 100 mm length, is cut from this laminate.

[0128] Next, the transmittance (T1) of light at a wavelength of 550 nm was measured on the sample (first transmittance measurement). The measurement was performed using a transmittance measuring device (product name "U4100 spectrophotometer", manufactured by Hitachi High-Technologies).

[0129] Next, the sample used for the first transmittance measurement was wound along the circumference of the core in the length direction of the sample onto a 20 mm diameter core (approximately 1.6 turns). The sample wound in this manner was then stored at 23°C for 1 hour. Subsequently, the transmittance (T2) of light at a wavelength of 550 nm was measured on the stored sample using the same method as the first transmittance measurement (second transmittance measurement).

[0130] Furthermore, the rate of change of transmittance T2 relative to transmittance T1 is calculated based on the following formula. Its value is shown in Table 1.

[0131] Rate of change of transmittance (%) = [(T2-T1) / T] × 100

[0132] [Table 1] Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Level 1 2nd floor Level 1 2nd floor Level 1 2nd floor acrylic polymer 2EHA 89 56 89 56 70.3 70.3 89 56 97.3 56 LA - 39 - 39 8.0 8.0 - 39 - 39 BA - - - - 20.1 20.1 - - - - 4HBA 1.0 5.0 1.0 5.0 1.0 1.0 1.0 5.0 1.0 5.0 NVP 10 - 10 - 0.6 0.6 10 - 1.7 - Crosslinking agent Nyper BMT 40SV - 0.5 - 1.0 0.28 0.26 0.5 0.5 - 0.5 Coronate L - - - - - 0.02 - - - - First acrylic oligomer - - - - 3.0 - - - - - Second acrylic oligomer - - - - - 1.5 - - - - Antioxidant (Irganox 1010) - - - - 0.3 - - - - - Thickness (μm) 2 twenty three 2 twenty three 25 25 25 25 2 twenty three Maximum thickness T1 (μm) 26 26 25 25 25 25 25 Minimum thickness T 2 (μm) twenty four twenty four 25 25 25 25 25 T1-T2(μm) 2 2 0 0 0 0 0 Peel adhesion force F (N / 25 mm) 11.5 - 5.2 - 10.6 5.2 20 2.4 3.9 - Shear storage modulus G' (kPa) at -20℃ 78 77 140 128 614 73 78 Shear storage modulus G' (kPa) at -40℃ 511 503 1054 874 27139 393 404 G'[-20℃] / F 6.78 14.8 13.2 24.6 30.7 30.4 20 G'[-40℃] / F 44.4 96.7 99.4 168 1,357 164 104 Change in transmittance (%) -0.05 -0.06 0.03 -0.08 0.03 -0.06 0.15

[0133] 10: Optical adhesive layer 10a: Page 1 10b: Page 2 11: First adhesive layer 12: Second adhesive layer 13: Second adhesive layer 21: Component 1 22: Component 2 H: Thickness direction L1: Peel-off membrane L2: Peel-off membrane S: Optical adhesive sheet

Claims

1. An optical adhesive layer having a first surface and a second surface opposite to the first surface, having a shear storage modulus G' of 30 kPa to 140 kPa at -20°C and a shear storage modulus G' of 503 kPa to 1200 kPa at -40°C, wherein at least one of the first surface and the second surface, after being adhered to a polyimide film and subjected to a peel adhesion F of 5 N / 25 mm to 30 N / 25 mm at 23°C for 30 minutes, at a peel angle of 180° and a peel speed of 300 mm / min, has a peel adhesion F to the polyimide film of 5 N / 25 mm to 30 N / 25 mm.

2. The optical adhesive layer of claim 1, wherein the shear storage modulus G' (kPa) and the peel adhesion F (N / 25 mm) at -20°C satisfy F≧0.4667×G'-58 when G'>135.

3. The optical adhesive layer of claim 1 has a shear storage modulus G' (kPa) at -20°C as a ratio of the peel adhesion F (N / 25 mm) to the above-mentioned peel adhesion force of 30 or less.

4. The ratio of the shear storage modulus G' (kPa) at -40°C to the peel adhesion F (N / 25 mm) of the optical adhesive layer of claim 1 is 200 or less.

5. For any of the optical adhesive layers in claims 1 to 4, the difference between the maximum and minimum thicknesses, as determined by the following procedure, is less than 3 μm. The procedure for determining the maximum and minimum thicknesses is as follows: cut an adhesive sheet (25 mm on the short side × 100 mm on the long side) from the optical adhesive layer; then, use a pin gauge to measure the thickness at 5 measurement points on the adhesive sheet; the 5 measurement points are 5 points that divide the width of the adhesive sheet into 6 equal parts along the long side; the maximum and minimum thicknesses are determined from the thickness measurements at the 5 measurement points.

6. For any of the optical adhesive layers in claims 1 to 4, the change in transmittance of light at a wavelength of 550 nm after one hour, as measured by the following procedure, when wound around a core with a cross-sectional diameter of 20 mm, is less than 5%. The procedure for measuring the change in transmittance is as follows: A test sample with a width of 25 mm and a length of 100 mm is cut from a laminate consisting of a PET substrate / optical adhesive layer / PET substrate; then, the transmittance of light at a wavelength of 550 nm (T1) (first transmittance measurement) is measured on the test sample; the measurement is performed using a transmittance measuring device (product name "U4100 type spectrophotometer", manufactured by Hitachi High-Technologies); then, the test sample that has undergone the first transmittance measurement is wound around a core with a cross-sectional diameter of 20 mm in the length direction of the sample along the circumference of the core. A core of mm (wound approximately 1.6 times) is wound around the core; then, the sample wound in this manner is stored at 23°C for 1 hour; then, for the sample used for measurement after storage, the transmittance (T2) of light at a wavelength of 550 nm is measured in the same manner as the first transmittance measurement (second transmittance measurement); the rate of change of transmittance T2 relative to transmittance T1 is calculated based on the following formula: Rate of change of transmittance (%) = [(T2 - T1) / T1] × 100.

Images