Optical adhesive sheet for foldable devices

Abstract

We provide an optical adhesive sheet for foldable devices that is suitable for preventing peeling from a bendable substrate. [Solution] The optical adhesive sheet S (optical adhesive sheet for foldable devices) of the present invention has an adhesive layer 10. The adhesive layer 10 has a shear storage modulus of 20 to 50 kPa at 25°C. After being attached to an adherend, followed by a heat and pressure treatment at 50°C, 0.5 MPa and 15 minutes, and then left to stand at 25°C for 72 hours, the adhesive layer 10 has a first adhesive strength Xa to the adherend at 25°C. After being attached to an adherend, following the heat and pressure treatment and the standing period, the adhesive layer 10 has a second adhesive strength Xb to the adherend at 60°C. The first adhesive strength Xa and the second adhesive strength Xb satisfy 0.5 ≤ Xb / Xa ≤ 1.0.

Description

Technical Field

[0001] The present invention relates to an optical adhesive sheet for a foldable device.

Background Art

[0002] A display panel has a laminated structure including, for example, a pixel panel, a touch panel, a polarizing plate, and a cover film. In the manufacturing process of such a display panel, for example, a transparent adhesive sheet (optical adhesive sheet) is used for joining the elements included in the laminated structure. On the other hand, for example, for smartphones and tablet terminals, the development of a display panel that can be repeatedly folded (foldable) is in progress. In a foldable display panel, each element in the laminated structure is manufactured to be repeatedly foldable, and an optical adhesive sheet is used for joining such elements. An optical adhesive sheet for a foldable device such as a foldable display panel is described in, for example, Patent Document 1 below.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The optical adhesive sheet in a foldable device is required to exhibit sufficient adhesive reliability to the substrate. However, conventional optical adhesive sheets tend to peel off from the substrate at the bending points of the device. This is because when the device is bent, stresses such as shear stress act locally on the optical adhesive sheet at the bending point. Moreover, conventional optical adhesive sheets exhibit relatively large changes in adhesive strength with temperature changes, so adhesive reliability that prevents the aforementioned peeling over a relatively wide temperature range (for example, including the room temperature range and higher temperature ranges) cannot be obtained. The occurrence of the aforementioned peeling at the bending points is undesirable as it can cause malfunction of the device.

[0005] The present invention provides an optical adhesive sheet for foldable devices that is suitable for suppressing peeling from a bendable substrate. [Means for solving the problem]

[0006] The present invention [1] includes an optical adhesive sheet for a foldable device having an adhesive layer, wherein the adhesive layer has a shear storage modulus of 20 kPa or more and 50 kPa or less at 25°C, the adhesive layer has a first adhesive strength Xa to the adherend at 25°C after being attached to the adherend, followed by a heat and pressure treatment at 50°C, 0.5 MPa and 15 minutes, and then standing at 25°C for 72 hours, the adhesive layer has a second adhesive strength Xb to the adherend at 60°C after being attached to the adherend, the heat and pressure treatment and the standing, and the first adhesive strength Xa and the second adhesive strength Xb satisfy 0.5 ≤ Xb / Xa ≤ 1.0.

[0007] As described above, the adhesive layer of the optical adhesive sheet of the present invention has a shear storage modulus of 20 kPa to 50 kPa at 25°C. An adhesive layer with this degree of softness is suitable for ensuring the cohesive force necessary for bonding between adherends, while also mitigating the stress acting locally on the adhesive layer at the bending point when the adherend to which the adhesive layer is bonded is bent. Therefore, this optical adhesive sheet is suitable for suppressing peeling from a bent adherend.

[0008] As described above, the adhesive layer of this optical adhesive sheet satisfies the following conditions: the first adhesive strength Xa (adhesion in the room temperature range) and the second adhesive strength Xb (adhesion in the temperature range above room temperature) satisfy 0.5 ≤ Xb / Xa ≤ 1. A configuration in which the ratio of the second adhesive strength Xb to the first adhesive strength Xa (Xb / Xa) is 0.5 or greater, in which the second adhesive strength Xb is large enough, is suitable for ensuring stable adhesion in both the room temperature range and higher temperature ranges, thereby suppressing the peeling described above.

[0009] The present invention [2] relates to the optical adhesive sheet for a foldable device according to [1] above, wherein the adhesive layer has a third adhesive strength Xc at 85°C with respect to the adherend after application to the adherend, the heat and pressure treatment, and the standing period, and the first adhesive strength Xa and the third adhesive strength Xc satisfy 0.5 ≤ Xc / Xa ≤ 0.8.

[0010] Such a configuration (where the ratio of the third adhesive force Xc to the first adhesive force Xa is 0.5 or greater, and the third adhesive force Xc is large enough) is preferable for ensuring stable adhesion in both the room temperature range and higher temperature ranges, thereby suppressing the aforementioned peeling.

[0011] The present invention [3] includes an optical adhesive sheet for a foldable device as described in [1] or [2] above, wherein the adhesive layer has a minimum adhesive strength of 10 N / 25 mm or more to the adherend in a temperature range of 25°C to 85°C after application to the adherend, heat and pressure treatment, and standing.

[0012] Such a configuration is preferable for suppressing the aforementioned peeling across both room temperature and higher temperature ranges, and for achieving good adhesive reliability.

[0013] The present invention [4] includes the optical adhesive sheet for foldable devices described in [3] above, wherein the adhesive layer has the minimum adhesive strength at 60°C or above.

[0014] Such a configuration is preferable for achieving good adhesive reliability in the room temperature range.

[0015] The present invention [5] includes an optical adhesive sheet for a foldable device according to any one of [1] to [4] above, wherein the adhesive layer has an adhesive strength to the adherend at 25°C after being attached to the adherend and left to stand at 25°C for 2 minutes, which is 0.5 N / 25 mm or more and 12 N / 25 mm or less.

[0016] A configuration in which the above adhesive strength at 25°C is 0.5 N / 25 mm or more is suitable for ensuring easy peelability of the adhesive layer and reworkability during the bonding operation of the optical adhesive sheet to the substrate. A configuration in which the above adhesive strength at 25°C is 12 N / 25 mm or less is suitable for ensuring the adhesive strength required for the bonding operation in the adhesive layer and achieving good temporary fixing of the optical adhesive sheet to the substrate. [Brief explanation of the drawing]

[0017] [Figure 1] This is a schematic cross-sectional view of one embodiment of the optical adhesive sheet of the present invention. [Figure 2] This shows an example of how to use the optical adhesive sheet of the present invention. Figure 2A shows the step of attaching the optical adhesive sheet to a first adherend, Figure 2B shows the step of joining the first adherend and the second adherend via the optical adhesive sheet, and Figure 2C shows the aging step. [Figure 3] This graph shows the adhesive strength of each adhesive sheet in Example 1 and Comparative Example 1. [Modes for carrying out the invention]

[0018] As one embodiment of the optical adhesive sheet for foldable devices of the present invention, the adhesive sheet S comprises an adhesive layer 10, as shown in Figure 1. The adhesive sheet S has a sheet shape of a predetermined thickness and extends in a direction perpendicular to the thickness direction (surface direction). In Figure 1, release films L1 and L2 are laminated to both sides of the adhesive sheet S. Release film L1 is arranged on one side of the adhesive sheet S in the thickness direction T. Release film L2 is arranged on the other side of the adhesive sheet S in the thickness direction T. The adhesive sheet S with release films can take the form of, for example, a roll (not shown).

[0019] Such an adhesive sheet S is a transparent adhesive sheet (optical adhesive sheet) placed in the light-transmitting area of ​​a foldable device. An example of a foldable device is a foldable display panel. A foldable display panel has a laminated structure that includes, for example, a pixel panel, a touch panel, a polarizing plate, and a cover film. The adhesive sheet S is used, for example, in the manufacturing process of a foldable display panel to bond elements included in the laminated structure together.

[0020] The adhesive layer 10 has a shear storage modulus of 20 kPa or more and 50 kPa or less at 25°C, and the first adhesive force Xa and the second adhesive force Xb satisfy the condition 0.5 ≤ Xb / Xa ≤ 1.0.

[0021] The shear storage modulus of the adhesive layer 10 is the storage modulus obtained by the dynamic viscoelasticity measurement described later for the examples (the same applies to the shear storage modulus described later). The first adhesive force Xa is the adhesive force that the adhesive layer 10 has at 25°C with respect to the adherend after the adhesive layer 10 is attached to the adherend, followed by heat and pressure treatment, and then left standing at 25°C for 72 hours. The second adhesive force Xb is the adhesive force that the adhesive layer 10 has at 60°C with respect to the adherend after the adhesive layer 10 is attached to the adherend, followed by heat and pressure treatment, and then left standing at 25°C for 72 hours. The adherend is a polyimide film (the same applies to the adherend described later). The attachment of the adhesive layer 10 to the adherend is attachment by applying a weight of reciprocating a 2 kg roller once in an environment of 25°C (the same applies to the attachment described later). The heat and pressure treatment is a treatment carried out under the conditions of a temperature of 50°C, a pressure of 0.5 MPa, and a treatment time of 15 minutes, and is a treatment started within 3 minutes from the attachment of the adhesive layer 10 to the adherend (the same applies to the heat and pressure treatment described later). The adhesive force is the adhesive force as the peel strength measured by a peel test under the conditions of a predetermined temperature, a peel angle of 180°, and a tensile speed of 300 mm / min (the same applies to the adhesive force described later).

[0022] As described above, the adhesive layer 10 in the adhesive sheet S has a shear storage modulus of 20 kPa or more and 50 kPa or less at 25°C. The adhesive layer 10 having such softness is suitable for relaxing the stress locally acting on the adhesive layer 10 at the bending portion when the adherend to which the adhesive layer 10 is bonded is bent while ensuring the cohesive force necessary for joining between the adherends. Therefore, the adhesive sheet S is suitable for suppressing peeling from the adherend that is bent.

[0023] Also, as described above, the adhesive layer 10 satisfies 0.5 ≦ Xb / Xa ≦ 1, where Xa is the first adhesive force (adhesive force in the normal temperature region) and Xb is the second adhesive force (adhesive force in the temperature region higher than the normal temperature). A configuration in which the second adhesive force Xb is large enough such that the ratio of the second adhesive force Xb to the first adhesive force Xa (Xb / Xa) is 0.5 or more is suitable for ensuring a stable adhesive force in the normal temperature region and a temperature region higher than that, and suppressing the above-described peeling.

[0024] As described above, the adhesive sheet S is suitable for suppressing peeling from the adherent to be bent. In addition, a configuration in which the adhesive layer 10 has a shear storage modulus of 20 kPa or more and 50 kPa or less at 25°C is suitable for suppressing the adhesion of an adhesive piece derived from the adhesive layer 10 to cutting means such as a Thomson blade used during the cutting process in the manufacturing process of the adhesive sheet S. Therefore, the said configuration is suitable for realizing a good processing yield of the adhesive sheet S.

[0025] In the adhesive sheet S, from the viewpoint of obtaining stable adhesion reliability in the normal temperature region and a temperature region higher than that, Xb / Xa is preferably 0.6 or more, more preferably 0.7 or more, and still more preferably 0.75 or more.

[0026] The adhesive layer 10 satisfies 0.5 ≦ Xc / Xa ≦ 0.8, where Xa is the first adhesive force and Xc is the third adhesive force. The third adhesive force Xc is the adhesive force that the adhesive layer 10 has at 85°C with respect to the adherent after the adhesive layer 10 is attached to the adherent, followed by a heat and pressure treatment, and then left standing at 25°C for 72 hours.

[0027] Such a configuration (a configuration in which the third adhesive force Xc is large enough such that Xc / Xa is 0.5 or more) is preferable for ensuring a stable adhesive force in the normal temperature region and a temperature region higher than that, and suppressing the above-described peeling. From this viewpoint, Xc / Xa is more preferably 0.6 or more, still more preferably 0.65 or more, and particularly preferably 0.68 or more.

[0028] The minimum adhesive strength of the adhesive layer 10 to the adherend in the temperature range of 25°C to 85°C after application to the adherend, subsequent heat and pressure treatment, and subsequent standing at 25°C for 72 hours is preferably 10 N / 25 mm or more, more preferably 11 N / 25 mm or more, and even more preferably 12 N / 25 mm or more. Such a configuration is preferable for suppressing the peeling described above across the room temperature range and higher temperature ranges, and is preferable for achieving good adhesive reliability.

[0029] The adhesive layer 10 has the minimum adhesive strength described above at 60°C or above. Such a configuration is preferable for achieving good adhesive reliability in the room temperature range.

[0030] The adhesive layer 10 has an adhesive strength at 25°C to the adherend after being attached to the adherend and left to stand for 2 minutes at 25°C, preferably between 0.5 N / 25 mm and 12 N / 25 mm. A configuration in which the above adhesive strength at 25°C (initial adhesive strength) is 0.5 N / 25 mm or more is suitable for ensuring the easy peelability of the adhesive layer 10 and ensuring reworkability during the bonding operation of the adhesive sheet S to the adherend. From this viewpoint, the initial adhesive strength is more preferably 1 N / 25 mm or more, even more preferably 3 N / 25 mm or more, and particularly preferably 5 N / 25 mm or more. A configuration in which the initial adhesive strength at 25°C is 12 N / 25 mm or less is suitable for ensuring the adhesive strength required for the bonding operation in the adhesive layer 10 and achieving good temporary fixing of the adhesive sheet S to the adherend. From this perspective, the initial adhesive strength is more preferably 10 N / 25 mm or less, even more preferably 9 N / 25 mm or less, and particularly preferably 8 N / 25 mm or less. Methods for adjusting the initial adhesive strength include, for example, selecting the type of base polymer for the adhesive layer 10, adjusting the molecular weight, and adjusting the amount. The selection of the type of base polymer includes selecting the type (composition) of the main chain in the base polymer, as well as selecting the type and adjusting the amount of functional groups (the same applies to the selection of the type of base polymer described later). Methods for adjusting the minimum adhesive strength Ya include selecting the type of components other than the base polymer and adjusting the amount of such components. Examples of such components include crosslinking agents, silane coupling agents, and oligomers.

[0031] The first adhesive strength Xa is preferably 15 N / 25 mm or more, more preferably 17 N / 25 mm or more, and even more preferably 19 N / 25 mm or more. The first adhesive strength Xa is preferably 30 N / 25 mm or less, more preferably 25 N / 25 mm or less, and even more preferably 23 N / 25 mm or less. These configurations are preferred to ensure the reliability of bonding between adherends by the optical adhesive sheet at or near room temperature. Methods for adjusting the first adhesive strength Xa include, for example, selecting the type of base polymer for the adhesive layer 10, adjusting the molecular weight and the amount of blending, as well as selecting the type and amount of components other than the base polymer (e.g., crosslinking agents, silane coupling agents, and oligomers). The same applies to methods for adjusting the second adhesive strength Xb, the ratio of the second adhesive strength Xb to the first adhesive strength Xa (Xb / Xa), the third adhesive strength Xc, and the ratio of the third adhesive strength Xc to the first adhesive strength Xa (Xc / Xa). Furthermore, the adhesive strengths Xb,Xc, ratio Xb / Xa, and ratio Xc / Xa in the relatively high-temperature range can also be adjusted by adjusting the molecular weight of the base polymer and the elastic modulus of the adhesive layer 10. Specifically, the higher the molecular weight and elastic modulus, the less likely the adhesive strengths Xb,Xc are to decrease from the first adhesive strength Xa in the room-temperature range (i.e., the ratios Xb / Xa and Xc / Xa are less likely to decrease).

[0032] The second adhesive strength Xb is preferably 7N / 25mm or more, more preferably 9N / 25mm or more, and even more preferably 10N / 25mm or more. The second adhesive strength Xb is preferably 30N / 25mm or less, more preferably 25N / 25mm or less, and even more preferably 23N / 25mm or less. These configurations are preferable for ensuring the reliability of bonding between adherends by the optical adhesive sheet at around 60°C.

[0033] The third adhesive strength Xc is preferably 7N / 25mm or more, more preferably 9N / 25mm or more, and even more preferably 10N / 25mm or more. The third adhesive strength Xc is preferably 25N / 25mm or less, more preferably 22N / 25mm or less, and even more preferably 20N / 25mm or less. These configurations are preferable for ensuring the reliability of bonding between adherends by the optical adhesive sheet at around 85°C.

[0034] The shear storage modulus (first storage modulus Ma) of the adhesive layer 10 at 25°C is preferably 25 kPa or higher, more preferably 30 kPa or higher, even more preferably 33 kPa or higher, and particularly preferably 35 kPa or higher, from the viewpoint of ensuring the above-mentioned cohesive force. The first storage modulus Ma is preferably 50 kPa or lower, more preferably 45 kPa or lower, even more preferably 43 kPa or lower, and particularly preferably 40 kPa or lower, from the viewpoint of stress relaxation. Methods for adjusting the first shear storage modulus Ma of the adhesive layer 10 include, for example, selecting the type of base polymer for the adhesive layer 10, adjusting the molecular weight and the amount blended, and selecting the type of crosslinking agent and adjusting the amount blended. The same applies to the methods for adjusting the second storage modulus Mb, the ratio of the second storage modulus Mb to the first storage modulus Ma (Mb / Ma), the third storage modulus Mc, and the ratio of the third storage modulus Mc to the first storage modulus Ma (Mc / Ma), as described below.

[0035] The shear storage modulus (second storage modulus Mb) of the adhesive layer 10 at 60°C is preferably 18 kPa or more, more preferably 23 kPa or more, and even more preferably 25 kPa or more, from the viewpoint of ensuring the cohesive force necessary for bonding between adherends in the temperature range around 60°C. The second storage modulus Mb is preferably 45 kPa or less, more preferably 43 kPa or less, and even more preferably 40 kPa or less, from the viewpoint of suppressing peeling from the bendable adherend in the temperature range around 60°C.

[0036] The ratio of the second storage modulus Mb to the first storage modulus Ma (Mb / Ma) preferably satisfies 0.6 ≤ Mb / Ma ≤ 1. Such a configuration is preferred from the viewpoint of stabilizing the adhesive properties in the temperature range from room temperature to around 60°C.

[0037] The shear storage modulus (third storage modulus Mc) of the adhesive layer 10 at 85°C is preferably 15 kPa or more, more preferably 18 kPa or more, and even more preferably 20 kPa or more, from the viewpoint of ensuring the cohesive force necessary for bonding between adherends in the temperature range around 85°C. The third storage modulus Mc is preferably 45 kPa or less, more preferably 43 kPa or less, and even more preferably 40 kPa or less, from the viewpoint of suppressing peeling from the bendable adherend in the temperature range around 85°C.

[0038] The ratio of the third storage modulus Mc to the first storage modulus Ma (Mc / Ma) preferably satisfies 0.5 ≤ Mb / Ma ≤ 0.8. Such a configuration is preferred from the viewpoint of stabilizing the adhesive properties in the temperature range from room temperature to around 85°C.

[0039] The adhesive layer 10 is a pressure-sensitive adhesive layer formed from an adhesive composition. The adhesive layer 10 is transparent (transmits visible light). The adhesive layer 10 contains at least a base polymer.

[0040] The base polymer is an adhesive component that provides tackiness in the adhesive layer 10. 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 10, an acrylic polymer is preferably used as the base polymer.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] Preferably, an alkyl acrylate having an alkyl group with 3 to 15 carbon atoms is used as the (meth)acrylate, and more preferably, at least one selected from the group consisting of n-butyl acrylate, 2-ethylhexyl acrylate, and dodecyl acrylate is used.

[0046] The proportion 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, from the viewpoint of appropriately exhibiting basic properties such as tackiness in the adhesive layer 10. This proportion is, for example, 99% by mass or less.

[0047] 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 having nitrogen atom-containing rings, monomers containing hydroxyl groups, and monomers containing carboxyl groups. 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.

[0048] 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.

[0049] 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 10 and ensuring adhesion force to the adherend in the adhesive layer 10. 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 10 and the acrylic polymer).

[0050] 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, 4-hydroxybutyl (meth)acrylate is used as the hydroxyl group-containing monomer, and more preferably, 4-hydroxybutyl acrylate is used.

[0051] The proportion of hydroxyl 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 and ensuring cohesive force in the adhesive layer 10. From the viewpoint of adjusting the polarity of the acrylic polymer (which relates to the compatibility between the various additive components in the adhesive layer 10 and the acrylic polymer), the ratio is preferably 20% by mass or less, and more preferably 10% by mass or less.

[0052] 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.

[0053] 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 10, and ensuring adhesion force to the adherend in the adhesive layer 10. 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 adherend by acid.

[0054] To prevent corrosion of metal elements such as electrodes in foldable devices by acidic components, it is preferable that the adhesive layer 10 of the adhesive sheet S has a low acid content. Furthermore, when the adhesive sheet S is used to bond polarizing plates, it is preferable that the adhesive layer 10 has a low acid content to suppress polyene formation of polyvinyl alcohol-based polarizers by acidic components. In such an acid-free adhesive sheet S, the content of organic acid monomers (e.g., (meth)acrylic acid and carboxyl group-containing monomers) in the adhesive layer 10 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 10 can be determined by immersing the adhesive layer 10 in pure water and heating it at 100°C for 45 minutes, then quantifying the acid monomers extracted into the water using ion chromatography.

[0055] From an acid-free viewpoint, it is preferable that the base polymer in the adhesive layer 10 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.

[0056] 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.

[0057] In this embodiment, the base polymer has a crosslinked structure. Methods for introducing a crosslinked structure to the base polymer include a method in which a base polymer having a functional group that can react 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 10 (first method), and a 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.

[0058] 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.

[0059] 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), and Takenate D110N (trimethylolpropane adduct of xylylene diisocyanate, manufactured by Mitsui Chemicals Corporation).

[0060] 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.

[0061] 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.

[0062] 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 10. Isocyanate crosslinking agents (especially trifunctional isocyanate crosslinking agents) are preferred from the viewpoint of ensuring durability of the adhesive layer 10. 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 10, a combination of a trifunctional isocyanate crosslinking agent and a peroxide crosslinking agent and / or a difunctional isocyanate crosslinking agent is preferred.

[0063] From the viewpoint of ensuring the cohesive force of the adhesive layer 10, 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 10, the amount of crosslinking agent 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, per 100 parts by mass of the base polymer.

[0064] 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).

[0065] 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.

[0066] Examples of polyfunctional (meth)acrylates include difunctional (meth)acrylates, trifunctional (meth)acrylates, and polyfunctional (meth)acrylates with four or more functions.

[0067] 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.

[0068] Examples of trifunctional (meth)acrylates include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and tris(acryloyloxyethyl) isocyanurate.

[0069] 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.

[0070] 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 G' and loss tangent tanδ) by introducing a crosslinking structure in the base polymer.

[0071] 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 10, 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] The weight-average molecular weight of the acrylic 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 10. The same weight-average molecular weight is preferably 5,000,000 or less, more preferably 3,000,000 or less, and even more preferably 2,000,000 or less. The weight-average molecular weight of the acrylic polymer is measured by gel permeation chromatography (GPC) and calculated on a polystyrene basis.

[0077] 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.

[0078] 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.

[0079] Fox's formula 1 / (273+Tg)=Σ[Wi / (273+Tgi)]

[0080] 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.

[0081] 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 crosslinked structure in combination with a high-Tg acrylic oligomer, the adhesive strength of the adhesive layer 10, especially the adhesive strength at high temperatures, can be increased. The glass transition temperature of the acrylic oligomer is calculated using the Fox formula described above.

[0082] 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.

[0083] 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.

[0084] The proportion of alicyclic alkyl (meth)acrylate in the monomer component of the acrylic oligomer is preferably 10% by weight or more, more preferably 20% by weight or more, and even more preferably 30% by weight or more. The same proportion is preferably 90% by weight or less, more preferably 80% by weight or less, and even more preferably 70% by weight or less. The proportion of linear alkyl (meth)acrylate in the monomer component of the acrylic oligomer is preferably 90% by weight or less, more preferably 80% by weight or less, and even more preferably 70% by weight or less. The same proportion is preferably 10% by weight or more, more preferably 20% by weight or more, and even more preferably 30% by weight or more.

[0085] 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 adhesive retention of the adhesive layer 10.

[0086] 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.

[0087] To sufficiently enhance the adhesive strength of the adhesive layer 10, the acrylic oligomer content in the 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, per 100 parts by mass of the base polymer. On the other hand, from the viewpoint of ensuring the transparency of the adhesive layer 10, the acrylic oligomer content in the 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, per 100 parts by mass of the base polymer. In the adhesive layer 10, 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.

[0088] 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.

[0089] The adhesive composition may contain other components as needed. Examples of other components include tackifiers, plasticizers, softeners, degradation inhibitors, fillers, colorants, UV absorbers, antioxidants, surfactants, and antistatic agents.

[0090] The adhesive sheet S can be manufactured, for example, by applying the above-mentioned adhesive composition onto a release film L1 (first release film) to form a coating, and then drying the coating.

[0091] 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.

[0092] 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.

[0093] A release film L2 (second release film) may be laminated on top of the 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.

[0094] In this manner, an adhesive sheet S can be manufactured in which the adhesive surface is covered and protected by release films L1 and L2. The release films L1 and L2 are peeled off from the adhesive sheet S as needed when using it.

[0095] From the viewpoint of ensuring sufficient adhesion to the adherend, the thickness of the adhesive layer 10 is preferably 10 μm or more, more preferably 15 μm or more. From the viewpoint of the handling properties of the adhesive sheet S, the thickness of the 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.

[0096] The haze of the adhesive layer 10 is preferably 3% or less, more preferably 2% or less, and more preferably 1% or less. The haze of the 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.

[0097] The total light transmittance of the 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 adhesive layer 10 is, for example, 100% or less. The total light transmittance of the adhesive layer 10 can be measured in accordance with JIS K 7375 (2008).

[0098] Figure 2 shows an example of how to use the adhesive sheet S.

[0099] In this method, first, as shown in Figure 2A, an adhesive sheet S is attached to one side of the first member 21 (adhered object) in the thickness direction T. The first member 21 is, for example, one element in the laminated structure of a flexible 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 member 22 described later). This step provides an adhesive layer 10 for bonding with other members on the first member 21.

[0100] If a bonding defect occurs during this process (for example, misalignment of the adhesive sheet S on the first member 21), the adhesive sheet S is peeled off the first member 21. Then, the bonding process is repeated with a replacement adhesive sheet S.

[0101] Next, as shown in Figure 2B, one side of the first member 21 in the thickness direction T and the other side of the second member 22 in the thickness direction T are joined via an adhesive layer 10 on the first member 21. The second member 22 is, for example, another element in the laminated structure of the flexible panel.

[0102] Next, as shown in Figure 2C, the 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 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.

[0103] The adhesive sheet S used in the manufacturing process of foldable devices as described above has an adhesive layer 10 that has a shear storage modulus of 20 kPa or more and 50 kPa or less at 25°C, and the first adhesive force Xa and the second adhesive force Xb satisfy 0.5 ≤ Xb / Xa ≤ 1. Such an adhesive sheet S is suitable for suppressing peeling from a bendable adherend, as described above. [Examples]

[0104] The present invention will be specifically described below with reference to examples. The present invention is not limited to these examples. Furthermore, the specific numerical values ​​such as the amounts (content), 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 (content), physical properties, and parameters described in the "Modes for Carrying Out the Invention" above.

[0105] In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube, a mixture containing 95 parts by mass of cyclohexyl methacrylate (CHMA), 5 parts by mass of acrylic acid (AA), 10 parts by mass of α-methylstyrene dimer 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 was added to the mixture to prepare a reaction solution, which was reacted under a nitrogen atmosphere at 85°C for 5 hours (formation of the first acrylic oligomer). This yielded an oligomer solution containing the first acrylic oligomer (solid content concentration 50% by mass). The weight-average molecular weight of the first acrylic oligomer was 4300. The glass transition temperature (Tg) of the first acrylic oligomer was 84°C.

[0106] <Example 2 of Acrylic Oligomer Preparation> In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube, a mixture containing 60 parts by mass of dicyclopentanyl 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 for 1 hour under a nitrogen atmosphere. 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 under a nitrogen atmosphere at 70°C for 1 hour, and then at 80°C for 2 hours (formation of the second acrylic oligomer). The reaction solution was then heated to 130°C to volatilize and remove toluene, the chain transfer agent, and unreacted monomers. This yielded a solid acrylic oligomer (second acrylic oligomer). The weight-average molecular weight of the second acrylic oligomer was 5100. The glass transition temperature (Tg) of the second acrylic oligomer was 130°C.

[0107] [Example 1] <Preparation of acrylic-based polymers> In a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube, a mixture (solid content 47% by mass) containing 70 parts by mass of 2-ethylhexyl acrylate (2EHA), 20 parts by mass of n-butyl acrylate (BA), 8 parts by mass of lauryl acrylate (LA), 1 part by mass of 4-hydroxybutyl acrylate (4HBA), 0.6 parts by mass of N-vinyl-2-pyrrolidone (NVP), 0.1 parts by mass of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator, and ethyl acetate as a solvent was stirred at 56°C for 6 hours under a nitrogen atmosphere (polymerization reaction). This yielded a polymer solution containing an acrylic-based polymer. The weight-average molecular weight of the acrylic-based polymer in this polymer solution was approximately 2 million.

[0108] <Preparation of adhesive composition> Adhesive composition C1 was prepared by adding 1.5 parts by mass of a first acrylic oligomer, 0.26 parts by mass of a first crosslinking agent (product name "Nipper BMT-40SV", dibenzoyl peroxide, manufactured by Nippon Oil & Fats Co., Ltd.), 0.02 parts by mass of a second crosslinking agent (product name "Coronate L", trimethylolpropane / tolylene diisocyanate trimer adduct, manufactured by Tosoh Co., Ltd.), and 0.3 parts by mass of a silane coupling agent (product name "KBM403", manufactured by Shin-Etsu Chemical Co., Ltd.) per 100 parts by mass of solid content of the polymer solution and mixing them together.

[0109] <Formation of the adhesive layer> Next, adhesive composition C1 was applied to the release-treated surface of a first release film, which had one side treated with silicone release treatment, to form a coating film. The first release film was a polyethylene terephthalate (PET) film (product name "Diafoil MRF#75", thickness 75 μm, manufactured by Mitsubishi Chemical Corporation) with one side treated with silicone release treatment. Next, the release-treated surface of a second release film, which had one side treated with silicone release treatment, was laminated to the coating film on the first release film. The second release film was a PET film (product name "Diafoil MRF#75", thickness 75 μm, manufactured by Mitsubishi Chemical Corporation) with one side treated with silicone release treatment. Next, the coating film on the first release film was dried by heating at 100°C for 1 minute and then at 150°C for 3 minutes to form a transparent adhesive layer with a thickness of 50 μm. In this way, an adhesive sheet of Example 1 having a transparent adhesive layer (thickness 50 μm) was prepared. Table 1 shows the monomer composition of the acrylic-based polymer and the adhesive layer composition of the adhesive sheet in Example 1, expressed in parts by mass (the same applies to the examples and comparative examples described later).

[0110] [Examples 2-4] The adhesive sheets for Examples 2 to 4 were prepared in the same manner as the adhesive sheet for Example 1, except that the monomer composition of the acrylic-based polymer was changed as shown in Table 1.

[0111] [Examples 5, 6] The adhesive sheets of Examples 5 and 6 were prepared in the same manner as the adhesive sheet of Example 1, except that the thickness of the formed adhesive layer was changed from 50 μm to 25 μm (Example 5) or 100 μm (Example 6).

[0112] [Comparative Example 1] A mixture containing 56 parts by mass of 2-ethylhexyl acrylate (2EHA), 34 parts by mass of lauryl acrylate (LA), 7 parts by mass of 4-hydroxybutyl acrylate (4HBA), 2 parts by mass of N-vinyl-2-pyrrolidone (NVP), and 0.015 parts by mass of a photopolymerization initiator (trade name "Omnirad 184," manufactured by IGM Resins) was irradiated with ultraviolet light (polymerization reaction) to obtain a prepolymer composition (polymerization rate of approximately 10%) (the prepolymer composition contains monomer components that have not undergone the polymerization reaction).

[0113] Next, 100 parts by mass of the prepolymer composition, 0.08 parts by mass of 1,6-hexanediol diacrylate (HDDA), 1 part by mass of the second acrylic oligomer, and 0.3 parts by mass of a silane coupling agent (trade name "KBM403", manufactured by Shin-Etsu Chemical Co., Ltd.) were mixed to prepare a photocurable adhesive composition C2.

[0114] Next, adhesive composition C2 was applied to the release surface of a first release film, which had one side treated with silicone release agent, to form a coating. The first release film was a polyethylene terephthalate (PET) film (product name "Diafoil MRF#75", thickness 75 μm, manufactured by Mitsubishi Chemical Corporation) with one side treated with silicone release agent. Next, the release surface of a second release film (which had one side treated with silicone release agent) was laminated onto the coating on the first release film. The second release film was a PET film (product name "Diafoil MRF#75", thickness 75 μm, manufactured by Mitsubishi Chemical Corporation) with one side treated with silicone release agent. Next, ultraviolet light was irradiated onto the coating through the second release film to cure the coating with ultraviolet light. A black light was used for ultraviolet irradiation. The ultraviolet irradiation intensity was 5 mW / cm². 2 Thus, the adhesive sheet (thickness 50 μm) of Comparative Example 1 was prepared.

[0115] [Comparative Example 2] The adhesive sheet for Comparative Example 2 was prepared in the same manner as the adhesive sheet for Comparative Example 1, except that the monomer composition of the acrylic-based polymer was changed as shown in Table 1.

[0116] [Comparative Example 3] The adhesive sheet of Comparative Example 3 was prepared in the same manner as the adhesive sheet of Example 1, except that the monomer composition of the acrylic-based polymer was changed as shown in Table 1.

[0117] <Adhesive strength> The adhesive strength of each adhesive sheet in Examples 1-6 and Comparative Examples 1-3 was investigated by peel tests.

[0118] First, for each adhesive sheet, the required number of test pieces were prepared for peel tests before and after autoclaving, as described below. In preparing the test pieces, the second release film was first peeled off from the adhesive sheet, and a PET film (25 μm thick) was bonded to the exposed surface of the adhesive layer to obtain a laminate. Next, test pieces (25 mm wide x 100 mm long) were cut from this laminate. Then, the first release film was peeled off from the adhesive layer of the test piece, and the exposed surface was plasma treated. Meanwhile, the polyimide film (product name "GV200D", 80 μm thick, manufactured by SKC Kolon PI Co., Ltd.) used as the adherend was also plasma treated. For each plasma treatment, a plasma irradiation device (product name "AP-TO5", manufactured by Sekisui Co., Ltd.) was used, with a voltage of 160 V, a frequency of 10 kHz, and a processing speed of 5000 mm / min. Then, the exposed surface of the adhesive layer of the test specimen and the plasma-treated surface of the polyimide film were bonded together. In this bonding process, the test specimen was pressed against the substrate by moving a 2kg roller back and forth once in an environment of 25°C.

[0119] [Peel test before autoclaving] Following the bonding described above, a peel test was performed by peeling the test specimen from the polyimide film after standing at 25°C for 2 minutes, and the peel strength was measured as the adhesive force. A tensile testing machine (product name "Autograph AGS-J", manufactured by Shimadzu Corporation) was used for this measurement. In this measurement, the measurement temperature was set to 25°C, the peel angle of the test specimen relative to the adherend was set to 180°, the tensile speed of the test specimen was set to 300 mm / min, and the peel length was set to 50 mm (measurement conditions for the peel test). The measured adhesive force is shown in Table 1 as the initial adhesive force (N / 25mm).

[0120] [Peel test after autoclaving] Within 3 minutes of the bonding described above, autoclaving (heating and pressurizing) of the test specimen with the adherend attached was started. The autoclaving was performed at a temperature of 50°C, a pressure of 0.5 MPa, and a processing time of 15 minutes. After autoclaving, the specimens were left to stand at 25°C for 72 hours. Then, a peel test (measurement temperature 25°C) was performed under the same measurement conditions as the peel test before autoclaving, and the adhesive strength was measured. The measured adhesive strength is shown in Table 1 as adhesive strength Xa (N / 25mm). Meanwhile, the autoclaving and peel tests were performed on the test specimens in the same manner, except that the measurement temperature in the peel test was changed to a predetermined temperature (60°C, 70°C, 85°C), and the adhesive strength was measured. Table 1 shows the adhesive strength Xb (N / 25mm) at 60°C, Xd (N / 25mm) at 70°C, Xc (N / 25mm) at 85°C, the ratio of Xb to Xa (Xb / Xa), the ratio of Xd to Xa (Xd / Xa), and the ratio of Xc to Xa (Xc / Xa). Figure 3 shows the measurement results of the peel test after autoclaving. In this graph, at each measurement time, the bar on the left represents the adhesive strength of the adhesive sheet of Comparative Example 1, and the bar on the right represents the adhesive strength of the adhesive sheet of Example 1.

[0121] <Storage modulus, loss tangent, and glass transition temperature> Dynamic viscoelasticity measurements were performed on the adhesive layers of each adhesive sheet in Examples 1-6 and Comparative Examples 1-3. The samples for measurement were prepared as follows: First, multiple adhesive layer pieces were bonded together to create an adhesive sheet with a thickness of approximately 1.5 mm. Next, this sheet was punched out to obtain cylindrical pellets (7.9 mm in diameter) for measurement. Then, dynamic viscoelasticity measurements were performed on the measurement samples using a dynamic viscoelasticity measuring device (product name "Advanced Rheometric Expansion System (ARES)", manufactured by Rheometric Scientific Inc.) after fixing them to a parallel plate jig with a diameter of 7.9 mm. In this measurement, the measurement mode was set to torsion mode, the measurement temperature range was set to -50°C to 150°C, the heating rate was set to 5°C / min, and the frequency was set to 1 Hz. From the measurement results, the storage modulus G' (shear storage modulus) and loss tangent tanδ were read at each temperature (shown in Table 1). Table 1 shows the storage modulus Ma (kPa) at 25°C, Mb (kPa) at 60°C, Mc (kPa) at 85°C, the ratio of storage modulus Mb to storage modulus Ma (Mb / Ma), and the ratio of storage modulus Mc to storage modulus Ma (Mc / Ma). The temperature at which the loss tangent tanδ is maximum was defined as the glass transition temperature of the adhesive sheet. This glass transition temperature (°C) is also shown in Table 1.

[0122] <Haze and total light transmittance> The haze and total light transmittance of the adhesive layers of each adhesive sheet in Examples 1-6 and Comparative Examples 1-3 were investigated as follows. First, a sample for haze measurement was prepared. Specifically, after peeling off the second release film from the adhesive sheet, the adhesive layer side of the sheet (first release film, adhesive layer) was bonded to alkali-free glass (thickness 0.8-1.0 mm, total light transmittance 92%, haze 0.4%, manufactured by Matsunami Glass Co., Ltd.), and the first release film was peeled off from the adhesive layer on the glass. This prepared a sample for measurement. Next, the haze and total light transmittance of the adhesive layer in the sample were measured using a haze measuring device (product name "HM-150", manufactured by Murakami Color Technology Laboratory). In this measurement, the sample was placed in the device so that light shone on the sample from the alkali-free glass side. In this measurement, the measurement results obtained by measuring only the alkali-free glass under the same conditions were used as the baseline. The haze and total light transmittance of the adhesive layer obtained in this way are shown in Table 1.

[0123] <Bending Test> The adhesion properties (the degree to which peeling from the adherend is suppressed) of each adhesive sheet in Examples 1-6 and Comparative Examples 1-3 to a bendable substrate were investigated. Specifically, the results are as follows:

[0124] First, laminate samples were prepared for each adhesive sheet. In preparing the laminate samples, the second release film was first peeled off the adhesive sheet, and the exposed surface (first exposed surface) was plasma-treated. Meanwhile, the exposed surface of the polarizing plate of the first adherend, a 66 μm thick polarizing plate with an adhesive layer (having a laminated structure of a 51 μm thick polarizing plate and a 15 μm thick adhesive layer), was also plasma-treated. For each plasma treatment, a plasma irradiation device (product name "AP-TO5", manufactured by Sekisui Co., Ltd.) was used, with a voltage of 160 V, a frequency of 10 kHz, and a processing speed of 5000 mm / min (the same was used for the plasma treatment described later). Then, the first exposed surface of the adhesive sheet and the plasma-treated surface of the polarizing plate were bonded together. In this bonding process, the polarizing plate and the adhesive sheet were pressed together by moving a 2 kg roller back and forth once in an environment of 25 °C. Next, the first release film was peeled off from the adhesive sheet on the polarizing plate, and the exposed surface (second exposed surface) was plasma-treated. Meanwhile, the polyimide film (product name "GV200D", thickness 80 μm, manufactured by SKC Kolon PI Co., Ltd.) as the second adherend was also plasma-treated. Then, the second exposed surface of the adhesive sheet and the plasma-treated surface of the polyimide film were bonded together. In this bonding process, the polyimide film and the adhesive sheet were pressed together by running a 2 kg roller back and forth once in an environment of 25°C. Next, a plasma-treated 125 μm thick PET film was bonded to the adhesive layer surface of the polarizing plate with the adhesive layer by running a 2 kg roller back and forth once. A laminated sample was prepared in the manner described above. The laminated sample has a laminated structure consisting of a PET film, an adhesive layer, a polarizing plate, an adhesive sheet (an adhesive sheet according to one of the examples or comparative examples), and a polyimide film.

[0125] Next, the laminated sample was cut into a 35 mm x 100 mm rectangle so that the absorption axis of the polarizer was parallel to the long side. Then, the laminated sample was autoclaved (heated and pressurized) at 50°C, 0.5 MPa, and for 15 minutes. Next, the autoclaved laminated sample was subjected to a bending test using a planar unloaded U-shaped stretch tester (manufactured by Yuasa System Equipment). In this test, bending fixtures were attached to each end of the long side of the laminated sample within a range of 20 mm from the sample edge, and the laminated sample was fixed to the tester (the central 60 mm area of ​​the long side of the laminated sample was not fixed). In this test, the laminated sample was held in a bent state with a bending radius of 1.3 mm and a bending angle of 180°, with the PET film side of the laminate facing inward. The sample in this state was then held for 240 hours in a constant temperature and humidity chamber at a temperature of 25°C and a relative humidity of 95% (first bending test).

[0126] The laminated samples after this first bending test were visually inspected to check for delamination between the polyimide film and the polarizing plate at the bent portion. In all samples where delamination was confirmed, the delamination (void) occurred from the edge in the short-side direction of the laminated sample. For laminated samples in which delamination was confirmed, the length (mm) of the void in the short-side direction of the sample was measured. The adhesion of the adhesive sheet to the bendable substrate (the degree to which delamination from the substrate is suppressed) was evaluated as "excellent" if the void length was less than 2 mm, and as "poor" if the void length was 2 mm or more (in this evaluation, if delamination (void) extending along the entire length in the short-side direction of the laminated sample was confirmed, the length of the void was set to 35 mm, and if no delamination was confirmed at all, the void length was set to 0 mm). The evaluation results are shown in Table 1.

[0127] Furthermore, the bending test was conducted in the same manner as the first bending test, except that the holding temperature in the constant temperature and humidity chamber was changed from 25°C to 85°C (second bending test). After the second bending test, the laminated sample was visually inspected to check for any delamination between the polyimide film and the polarizing plate at the bent portion. Then, the adhesion of the adhesive sheet to the bendable substrate was evaluated using the same criteria as in the first bending test. The evaluation results are shown in Table 1.

[0128] [Table 1] [Explanation of Symbols]

[0129] S Adhesive Sheet (Optical Adhesive Sheet for Foldable Devices) T (thickness direction) 10 Adhesive layer L1, L2 release film 21 First Member 22 Second Member

Claims

1. An optical adhesive sheet for foldable devices having an adhesive layer, The adhesive layer has a shear storage modulus of 20 kPa or more and 50 kPa or less at 25°C. The adhesive layer, after being attached to the adherend, subjected to a heat and pressure treatment at 50°C, 0.5 MPa and 15 minutes, and then left to stand at 25°C for 72 hours, has a first adhesive strength Xa to the adherend at 25°C. The adhesive layer, after being attached to the adherend, subjected to heat and pressure treatment, and allowed to stand, has a second adhesive strength Xb to the adherend at 60°C. The first adhesive force Xa and the second adhesive force Xb satisfy 0.5 ≤ Xb / Xa ≤ 1.0, The adhesive layer, after being attached to the adherend, subjected to heat and pressure treatment, and allowed to stand, has a third adhesive strength Xc to the adherend at 85°C. An optical adhesive sheet for a foldable device, wherein the first adhesive force Xa and the third adhesive force Xc satisfy 0.5 ≤ Xc / Xa ≤ 0.

8.

2. The optical adhesive sheet for a foldable device according to claim 1, wherein the minimum adhesive strength of the adhesive layer to the adherend in a temperature range of 25°C to 85°C after application to the adherend, heating and pressurizing treatment, and standing is 10 N / 25 mm or more.

3. The optical adhesive sheet for a foldable device according to claim 2, wherein the adhesive layer has the minimum adhesive strength at 60°C or higher.

4. The optical adhesive sheet for a foldable device according to any one of claims 1 to 3, wherein the adhesive layer has an adhesive strength of 0.5 N / 25 mm or more and 12 N / 25 mm or less to the adherend at 25°C after being attached to the adherend and left to stand at 25°C for 2 minutes.

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