WO2024195839A1 - Optical adhesive sheet - Google Patents

Abstract

An adhesive sheet (10) (optical adhesive sheet) comprises a base polymer and an oligomer having a glass transition temperature of 40°C or higher, and has a shear storage elastic modulus of 100 kPa or less at -10°C. In the adhesive sheet (10), the hydrogen bond term δH₁ of the Hansen solubility parameter of the base polymer and the hydrogen bond term δH₂ of the Hansen solubility parameter of the oligomer satisfy the formula: 0.1 ≤ δH₂ - δH₁ ≤ 1.3.

Description

Optical Adhesive Sheet

The present invention relates to an optical adhesive sheet.

A display panel has a laminated structure that includes elements such as a pixel panel, a polarizing film, a touch panel, and a cover film. In the manufacturing process of such a display panel, for example, a transparent adhesive sheet (optical adhesive sheet) is used to bond the elements included in the laminated structure.

On the other hand, the development of display panels that can be repeatedly folded (foldable) for use in, for example, smartphones and tablet terminals is progressing. Specifically, a foldable display panel can be repeatedly deformed between a curved shape and a flat, non-bent shape. In such a foldable display panel, each element in the laminated structure is made to be repeatedly foldable, and a thin optical adhesive sheet is used to bond such elements together. Optical adhesive sheets for flexible devices such as foldable display panels are described, for example, in Patent Document 1 below.

JP 2018-111754 A

Optical adhesive sheets for flexible devices are required to be highly flexible so that they can conform to the adherend when the device is deformed and have excellent stress relaxation properties. However, with conventional optical adhesive sheets, the softer they are, the weaker their adhesive strength becomes.

　In the past, optical adhesive sheets were prone to peeling off from the elements (adherends) at the bent portions of foldable display panels. This is because when the display panel is folded, a relatively large shear force is applied to the folded portion of the optical adhesive sheet in the direction along the adherend. Peeling is undesirable as it can cause the display to malfunction. There is a high demand for optical adhesive sheets for foldable display panels to resist peeling off from the elements (adherends) when the display is bent.

As a flexible device, development of rollable display panels is also progressing. A rollable display panel can be repeatedly deformed, for example, between a rolled shape after being entirely or partially rolled up, and a flat shape after being entirely unrolled. In such a rollable display panel, each element in the laminate structure is made to be repeatedly deformable, and a thin optical adhesive sheet is used to bond such elements. When the rollable display panel is in a rolled shape, the optical adhesive sheet bonded to the rolled element continues to receive a shear force in a direction along the adherend. Such an optical adhesive sheet is required to be highly unlikely to peel off from the element (adherend) when the display is in a rolled shape.

The present invention provides an optical adhesive sheet suitable for flexible device applications.

The present invention [1] is an optical adhesive sheet comprising a base polymer and an oligomer having a glass transition temperature of 40°C or higher, having a shear storage modulus of 100 kPa or less at -10°C, and wherein the hydrogen bond term _δH1 of the Hansen solubility parameters of the base polymer and the hydrogen bond term _δH2 of the Hansen solubility parameters of the oligomer satisfy 0.1≦ _δH2 - _δH1 ≦1.3.

The present invention [2] includes the optical adhesive sheet described in [1] above, which has a haze of 1% or less.

The present invention [3] includes the optical adhesive sheet described in [1] or [2] above, which has an adhesive strength of 7.6 N/20 mm or more in a peel test under conditions of 25°C, a peel angle of 180°, and a tensile speed of 300 mm/min.

The present invention [4] includes an optical adhesive sheet according to any one of the above [1] to [3], which has an adhesive strength F1 in a peel test under conditions of 25°C, a peel angle of 180°, and a tensile speed of 300 mm/min, and has an adhesive strength F2 in a peel test under conditions of 25°C, a peel angle of 180°, and a tensile speed of 60 mm/min, and the ratio of adhesive strength F2 to adhesive strength F1 is 0.5 or more and 1.1 or less.

The present invention [5] includes an optical adhesive sheet according to any one of [1] to [4] above, in which the ratio of the shear storage modulus at 60°C to the shear storage modulus at -10°C is 0.2 or more and 1.0 or less.

The present invention [6] includes an optical adhesive sheet according to any one of [1] to [5] above, which has a gel fraction of 60% by mass or more and 87% by mass or less.

As described above, the optical adhesive sheet of the present invention has a shear storage modulus of 100 kPa or less at -10°C. Such a soft optical adhesive sheet is suitable for relaxing the stress generated in the optical adhesive sheet and the adherend when the adherend to which the adhesive sheet is attached is deformed (stress relaxation function). The stress relaxation in the optical adhesive sheet is suitable for ensuring the conformability of the optical adhesive sheet to the adherend. The stress relaxation in the adherend is suitable for suppressing damage such as cracking of the adherend. In addition, as described above, the optical adhesive sheet of the present invention includes a base polymer and an oligomer having a glass transition temperature (Tg) of 40°C or higher, and the hydrogen bond term δH ₁ of the Hansen solubility parameter (HSP) of the base polymer and the hydrogen bond term δH ₂ of the HSP of the oligomer satisfy 0.1≦δH ₂ -δH ₁ ≦1.3. This configuration is suitable for achieving good adhesive strength of the optical adhesive sheet by unevenly distributing oligomers having a Tg of 40 ° C. or more on the surface (adhesive surface) of the optical adhesive sheet and its vicinity while ensuring the overall softness of the optical adhesive sheet. The high adhesive strength of the optical adhesive sheet is suitable for suppressing peeling of the optical adhesive sheet from an adherend that is repeatedly deformed. The optical adhesive sheet as described above is suitable for flexible device applications.

1 is a schematic cross-sectional view of one embodiment of an optical adhesive sheet of the present invention. Fig. 2 shows an example of a method for using the optical adhesive sheet of the present invention. Fig. 2A shows a step of attaching the optical adhesive sheet to a first adherend, Fig. 2B shows a step of joining the first adherend and the second adherend via the optical adhesive sheet, and Fig. 2C shows an aging step.

As shown in FIG. 1, the adhesive sheet 10 as one embodiment of the optical adhesive sheet of the present invention has a sheet shape of a predetermined thickness and extends in a direction perpendicular to the thickness direction (plane direction). The adhesive sheet 10 has an adhesive surface 11 and an adhesive surface 12 opposite to the adhesive surface 11. FIG. 1 exemplarily shows a state in which release liners L1 and L2 are attached to the adhesive surfaces 11 and 12 of the adhesive sheet 10. The release liner L1 is disposed on the adhesive surface 11. The release liner L2 is disposed on the adhesive surface 12. The adhesive sheet 10 is an optically transparent adhesive sheet (optical adhesive sheet) disposed at a location where light passes in a flexible device. Examples of flexible devices include flexible display panels. Examples of flexible display panels include foldable display panels and rollable display panels. The flexible display panel has a laminated structure including elements such as a pixel panel, a polarizing film, a touch panel, and a cover film. The adhesive sheet 10 is used, for example, in the manufacturing process of a flexible display panel to bond elements included in a laminated structure. The release liners L1 and L2 are each peeled off at a predetermined timing when the adhesive sheet 10 is used.

The pressure-sensitive adhesive sheet 10 is formed from a pressure-sensitive adhesive composition. The pressure-sensitive adhesive composition includes a base polymer and an oligomer having a glass transition temperature (Tg) of 40° C. or higher. That is, the pressure-sensitive adhesive sheet 10 includes a base polymer and an oligomer having a Tg of 40° C. or higher. The pressure-sensitive adhesive sheet 10 has a shear storage modulus of 100 kPa or less at −10° C. Furthermore, in the pressure-sensitive adhesive sheet 10, the hydrogen bond term δH ₁ of the Hansen solubility parameter (HSP) of the base polymer and the hydrogen bond term δH ₂ of the HSP of the oligomer satisfy the following formula (1).

0.1≦δH ₂ −δH ₁ ≦1.3 (1)

As described above, the adhesive sheet 10 has a shear storage modulus of 100 kPa or less at -10°C. Such a soft adhesive sheet 10 is suitable for relieving stresses that occur in the adhesive sheet 10 and the adherend when the adherend to which the adhesive sheet 10 is attached deforms (stress relaxation function). The stress relaxation in the adhesive sheet 10 is suitable for ensuring the conformability of the adhesive sheet 10 to the adherend. The stress relaxation in the adherend is suitable for suppressing damage such as cracking of the adherend. Therefore, the adhesive sheet 10 is suitable for achieving good repeated deformation of a flexible device in which the adhesive sheet 10 is used.

In addition, as described above, the pressure-sensitive adhesive sheet 10 includes a base polymer and an oligomer having a Tg of 40° C. or higher, and the hydrogen bond term δH ₁ of the HSP of the base polymer and the hydrogen bond term δH ₂ of the HSP of the oligomer satisfy 0.1≦δH ₂ -δ _h1 ≦1.3. The difference ΔH (=δH ₂ -δH ₁ ) between the hydrogen bond term δH ₁ of the HSP of the base polymer and the hydrogen bond term δH ₂ of the HSP of the oligomer is an index of the compatibility between the base polymer and the oligomer in the pressure-sensitive adhesive sheet 10. Such a difference ΔH of 0.1 or more and 1.3 or less is suitable for realizing a moderately low compatibility between the base polymer and the oligomer and for unevenly distributing the oligomer on the surface (adhesive surfaces 11, 12) of the pressure-sensitive adhesive sheet 10 and in its vicinity. Therefore, a difference ΔH of 0.1 or more and 1.3 or less is suitable for achieving high adhesive strength in the adhesive sheet 10 by unevenly distributing oligomers having a Tg of 40° C. or more on the surface (adhesive surfaces 11, 12) of the adhesive sheet 10 and its vicinity, while ensuring the above-mentioned overall softness of the adhesive sheet 10. High adhesive strength of the adhesive sheet 10 is suitable for suppressing peeling of the adhesive sheet 10 from an adherend that is repeatedly deformed.

The adhesive sheet 10 described above is suitable for flexible device applications. In other words, the adhesive sheet 10 is suitable for achieving good repeated deformation of the flexible device in which the adhesive sheet 10 is used.

The shear storage modulus G1 of the adhesive sheet 10 at -10°C is preferably 90 kPa or less, more preferably 85 kPa or less, and even more preferably 80 kPa or less, from the viewpoint of stress relaxation at the deformed portion when the adhesive sheet 10 is deformed (bent, curved, etc.). The shear storage modulus G1 (-10°C) is preferably 30 kPa or more, more preferably 40 kPa or more, even more preferably 50 kPa or more, and even more preferably 60 kPa or more, from the viewpoint of ensuring the cohesive force of the adhesive sheet 10 in the low temperature range. The shear storage modulus G1 is determined by dynamic viscoelasticity measurement described later in the examples (the same applies to the shear storage modulus G2 and G3 described later). Methods for adjusting the shear storage modulus of the adhesive sheet 10 include, for example, selection of the type of base polymer in the adhesive sheet 10, adjustment of the molecular weight, and adjustment of the blending amount, and selection of the type of crosslinking agent and adjustment of the blending amount (the same applies to the shear storage modulus G2 and G3 described later).

The shear storage modulus G2 of the adhesive sheet 10 at 60°C is preferably 35 kPa or less, more preferably 32 kPa or less, and even more preferably 27 kPa or less, from the viewpoint of the above-mentioned stress relaxation in the adhesive sheet 10. The shear storage modulus G2 (60°C) is preferably 10 kPa or more, more preferably 15 kPa or more, even more preferably 20 kPa or more, and even more preferably 22 kPa or more, from the viewpoint of ensuring the cohesive force of the adhesive sheet 10 in the high temperature region.

The ratio of the shear storage modulus G2 to the shear storage modulus G1 (G2/G1) is preferably 0.2 or more, more preferably 0.25 or more, even more preferably 0.3 or more, from the viewpoint of ensuring a stable stress relaxation function of the adhesive sheet 10 over a wide temperature range, and is preferably 1.0 or less, more preferably 0.5 or less, even more preferably less than 0.4.

The shear storage modulus G3 of the adhesive sheet 10 at -20°C is preferably 150 kPa or less, more preferably 130 kPa or less, and even more preferably 120 kPa or less, from the viewpoint of the above-mentioned stress relaxation in the adhesive sheet 10. The shear storage modulus G3 (-20°C) is preferably 40 kPa or more, more preferably 50 kPa or more, more preferably 70 kPa or more, and even more preferably 90 kPa or more, from the viewpoint of ensuring the cohesive force of the adhesive sheet 10 in the low temperature range.

The difference ΔH (δH ₂ -δH ₁ ) between the hydrogen bond term δH ₁ of the HSP of the base polymer and the hydrogen bond term δH ₂ of the HSP of the oligomer is preferably 0.2 or more, more preferably 0.3 or more, from the viewpoint of appropriately lowering the compatibility between the base polymer and the oligomer and sufficiently unevenly distributing the oligomer on and near the adhesive surfaces 11, 12. The difference ΔH (δH ₂ -δH ₁ ) is preferably 1.26 or less, from the viewpoint of preventing the compatibility between the base polymer and the oligomer from becoming too low. Ensuring the compatibility between the base polymer and the oligomer helps to achieve low haze in the adhesive sheet 10. An example of a method for adjusting the δH ₁ of the base polymer is adjustment of the monomer composition of the base polymer. An example of a method for adjusting the δH ₂ of the oligomer is adjustment of the monomer composition of the oligomer.

The ratio (δH ₂ /δH ₁ ) of the hydrogen bond term δH ₂ of the HSP of the oligomer to the hydrogen bond term δH ₁ of the HSP of the base polymer is preferably 1.04 or more, more preferably 1.06 or more, even more preferably 1.08 or more, and even more preferably 1.10 or more, from the viewpoint of appropriately lowering the compatibility between the base polymer and the oligomer and sufficiently unevenly distributing the oligomer on and near the adhesive surfaces 11, 12. The ratio (δH ₂ /δH ₁ ) is preferably 1.28 or less, more preferably 1.25 or less, and even more preferably 1.22 or less, from the viewpoint of preventing the compatibility between the base polymer and the oligomer from becoming too low. The ratio (δH ₂ /δH ₁ ) is also an index of the compatibility between the base polymer and the oligomer in the adhesive sheet 10.

The Hansen solubility parameter (HSP) is expressed by the following formula (2), where δH is the hydrogen bond term that represents the energy derived from the hydrogen bonding forces between molecules. δD is the dispersion term that represents the energy derived from the dispersion forces between molecules. δP is the polarization term that represents the energy derived from the polar forces between molecules.

HSP=(δD ² +δP ² +δH ² ) ^1/2 (2)

The δH of a polymer is calculated from the molar fraction _xi of the monomer _mi forming the polymer and the hydrogen bond term _δh of the monomer _mi according to the following formula (3). The δH of an oligomer can be calculated in the same manner. The hydrogen bond term δh of a monomer can be calculated, for example, by computer software HSPiP (Hansen Solubility Parameters in Practice). The method of calculating δH is specifically described below in the examples.

δH=Σ x _i ×δh _i (3)

From the viewpoint of increasing the surface adhesion of the soft adhesive sheet 10 as described above, the Tg of the oligomer is preferably 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and is preferably 145°C or lower, more preferably 135°C or lower, even more preferably 130°C or lower. The Tg of the oligomer is preferably higher than the Tg of the base polymer. Methods for adjusting the Tg of the oligomer include adjusting the monomer composition of the oligomer and adjusting the molecular weight. The specific method for measuring the Tg of the oligomer is as described below in the examples.

The glass transition temperature Tg of an oligomer can be determined by the following Fox formula (theoretical value). The Fox formula is a relational expression between the glass transition temperature Tg of a polymer and the glass transition temperature Tg _i of a homopolymer of a monomer constituting the polymer. In the following Fox formula, Tg represents the glass transition temperature (°C) of an oligomer, W _i represents the weight fraction of a monomer m _i constituting the polymer, and Tg _i represents the glass transition temperature (°C) of a homopolymer formed from the monomer m _i . The glass transition temperature of a homopolymer can be determined by a literature value. For example, the glass transition temperatures of various homopolymers are listed in "Polymer Handbook" (4th edition, John Wiley & Sons, Inc., 1999). On the other hand, the glass transition temperature of a homopolymer of a monomer can also be determined by a method specifically described in JP-A-2007-51271.

Fox's formula 1/(273+Tg)=Σ[W _i /(273+Tg _i )]

The haze of the adhesive sheet 10 is preferably 1% or less, more preferably 0.8% or less, even more preferably 0.7% or less, and even more preferably 0.5% or less. The haze is, for example, 0.01% or more. The haze of the adhesive sheet 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 Research Laboratory Co., Ltd. The specific method of measuring the haze is as described below in the examples.

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

The gel fraction of the adhesive sheet 10 is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 73% by mass or more, from the viewpoint of ensuring the cohesive force of the adhesive sheet 10 in the high temperature region. The gel fraction of the adhesive sheet 10 is preferably 87% by mass or less, more preferably 85% by mass or less, and even more preferably 83% by mass or less, from the viewpoint of ensuring the flexibility of the adhesive sheet 10. Methods for adjusting the gel fraction of the adhesive sheet 10 include, for example, selecting the type of base polymer in the adhesive sheet 10, adjusting the molecular weight, and adjusting the blending amount. Methods for adjusting the gel fraction include selecting the type of crosslinking agent and adjusting the blending amount. The method for measuring the gel fraction is as described below in the examples.

The adhesive strength F1 of the adhesive sheet 10 in a peel test (first peel test) under conditions of 25°C, a peel angle of 180°, and a tensile speed of 300 mm/min is preferably 7.6 N/20 mm or more, more preferably 7.8 N/20 mm or more, even more preferably 8.0 N/20 mm or more, even more preferably 8.2 N/20 mm or more, even more preferably 8.4 N/20 mm or more, even more preferably 8.6 N/20 mm or more, and particularly preferably 8.8 N/20 mm or more, from the viewpoint of suppressing peeling of the adhesive sheet 10 from the adherend. The adhesive strength F1 is, for example, 15 N/20 mm or less. The method of measuring the adhesive strength F1 is specifically as described later in the examples. Methods of adjusting the adhesive strength F1 include, for example, selecting the type of base polymer in the adhesive sheet 10, adjusting the molecular weight, and adjusting the amount of the base polymer. The selection of the type of base polymer includes adjusting the composition of the monomer that forms the base polymer. Methods for adjusting the adhesive strength F1 include selecting the type of components other than the base polymer in the adhesive sheet 10 and adjusting the amount of the components. Such components include crosslinking agents, silane coupling agents, and oligomers. The above-mentioned adhesive strength adjustment methods are also applicable to the adhesive strength F2 described below.

The adhesive strength F2 of the adhesive sheet 10 in a peel test (second peel test) under conditions of 25°C, a peel angle of 180°, and a tensile speed of 60 mm/min is preferably 4.1 N/20 mm or more, more preferably 6.0 N/20 mm or more, even more preferably 6.4 N/20 mm or more, even more preferably 6.8 N/20 mm or more, still more preferably 7.2 N/20 mm or more, particularly preferably 7.6 N/20 mm or more, and particularly preferably 8.0 N/20 mm or more, from the viewpoint of suppressing peeling of the adhesive sheet 10 from the adherend. The adhesive strength F2 is, for example, 12 N/20 mm or less.

From the viewpoint of ensuring stable adhesive strength in the adhesive sheet 10, the ratio of adhesive strength F2 to adhesive strength F1 (F2/F1) is preferably 0.5 or more, more preferably 0.6 or more, preferably 0.7 or more, more preferably 0.75 or more, even more preferably 0.8 or more, and is preferably 1.1 or less, more preferably 1.0 or less.

The base polymer is an adhesive component that imparts adhesiveness to the adhesive sheet 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 polymers may be used alone or in combination of two or more types. From the viewpoint of ensuring good transparency and adhesiveness in the adhesive sheet 10, acrylic polymers are preferred as the base polymer.

An acrylic polymer is a polymer of a monomer component (first monomer component) that contains 50% or more by mass of an alkyl (meth)acrylate ester. "(Meth)acrylic" means acrylic and/or methacrylic.

As the (meth)acrylic acid alkyl ester, preferably, a (meth)acrylic acid alkyl ester in which the alkyl group has 1 to 20 carbon atoms is used. Examples of the (meth)acrylic acid alkyl ester include a (meth)acrylic acid alkyl ester having a chain alkyl group (a (meth)acrylic acid chain alkyl ester) and a (meth)acrylic acid alkyl ester having an alicyclic alkyl group (a (meth)acrylic acid alicyclic alkyl ester).

Examples of (meth)acrylic acid chain alkyl esters include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, neopentyl (meth)acrylate, n-hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, and isooctyl (meth)acrylate. Examples of the acrylates include nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, isotridecyl (meth)acrylate, tetradecyl (meth)acrylate, isotetradecyl (meth)acrylate, pentadecyl (meth)acrylate, cetyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate, and nonadecyl (meth)acrylate.

Examples of (meth)acrylic acid alicyclic alkyl esters include (meth)acrylic acid cycloalkyl esters, (meth)acrylic acid esters having a bicyclic aliphatic hydrocarbon ring, and (meth)acrylic acid esters having a tricyclic or higher aliphatic hydrocarbon ring. Examples of (meth)acrylic acid cycloalkyl esters include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, tert-butylcyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate, cyclooctyl (meth)acrylate, and cyclododecyl (meth)acrylate. Examples of (meth)acrylic acid esters having a bicyclic aliphatic hydrocarbon ring include isobornyl (meth)acrylate. Examples of (meth)acrylic acid esters having three or more aliphatic hydrocarbon rings include dicyclopentanyl (meth)acrylate, dicyclopentenyl (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.

As the (meth)acrylic acid alkyl ester, from the viewpoint of achieving a balance between the softness and adhesive strength required for an adhesive sheet for flexible device applications in the adhesive sheet 10, preferably, at least one selected from (meth)acrylic acid alkyl esters having a first alkyl group having 8 to 12 carbon atoms is used, or at least one selected from (meth)acrylic acid alkyl esters having a first alkyl group having 8 to 12 carbon atoms and at least one selected from a second (meth)acrylic acid alkyl ester having an alkyl group having 1 to 4 carbon atoms is used. The first (meth)acrylic acid alkyl ester is preferably at least one selected from the group consisting of 2-ethylhexyl acrylate (2EHA), n-octyl acrylate (NOAA), isononyl acrylate (INAA), and lauryl acrylate (LA). The second (meth)acrylic acid alkyl ester is preferably n-butyl acrylate (BA).

The proportion of the (meth)acrylic acid alkyl ester in the first monomer component is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 88% by mass or more, and particularly preferably 90% by mass or more, from the viewpoint of appropriately expressing softness and adhesive strength in the adhesive sheet 10. The proportion is, for example, 99.9% by mass or less, 99.5% by mass or less, or 99% by mass or less. When a first (meth)acrylic acid alkyl ester selected from (meth)acrylic acid alkyl esters having an alkyl group with 8 to 12 carbon atoms is used in combination with another second (meth)acrylic acid alkyl ester selected from (meth)acrylic acid alkyl esters having an alkyl group with 1 to 4 carbon atoms, the proportion of the first (meth)acrylic acid alkyl ester in the monomer component is preferably 60% by mass or more, more preferably 65% by mass or more, even more preferably 70% by mass or more, and preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less, and the proportion of the second (meth)acrylic acid alkyl ester in the monomer component is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, and preferably 35% by mass or less, more preferably 30% by mass or less, even more preferably 25% by mass or less.

The first monomer component may contain a copolymerizable monomer that is copolymerizable with the (meth)acrylic acid alkyl ester. Examples of the copolymerizable monomer include a monomer having a polar group. Examples of the polar group-containing monomer include a hydroxy group-containing monomer, a monomer having a nitrogen atom-containing ring, and a carboxy group-containing monomer. The polar group-containing monomer is useful for modifying the acrylic polymer, such as introducing crosslinking points into the acrylic polymer and ensuring the cohesive force of the acrylic polymer. The copolymerizable monomer may be used alone or in combination of two or more types.

Examples of hydroxy 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. The hydroxy group-containing monomer is preferably at least one selected from the group consisting of 2-hydroxyethyl acrylate (2HEA) and 4-hydroxybutyl acrylate (4HBA).

The proportion of the hydroxyl group-containing monomer in the first monomer component is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, from the viewpoint of introducing a crosslinked structure into the acrylic polymer and ensuring the cohesive strength of the adhesive sheet 10. From the viewpoint of adjusting the polarity of the acrylic polymer (related to the compatibility of the various additive components in the adhesive sheet 10 with the acrylic polymer), the proportion is preferably 12% by mass or less, more preferably 10% by mass or less, and even more preferably 9% by mass or less.

　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, 4-acryloylmorpholine, N-vinyl-2-caprolactam, N-vinyl-1,3-oxazin-2-one, N-vinyl-3,5-morpholinedione, N-vinylpyrazole, N-vinylisoxazole, N-vinylthiazole, and N-vinylisothiazole. The monomer having a nitrogen atom-containing ring is preferably N-vinyl-2-pyrrolidone (NVP).

When a monomer having a nitrogen atom-containing ring is used, the proportion of the monomer having a nitrogen atom-containing ring in the first monomer component is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 1.5% by mass or more, from the viewpoint of ensuring the cohesive strength of the adhesive sheet 10 and ensuring the adhesive strength of the adhesive sheet 10 to the adherend. The same proportion is preferably 10% by mass or less, more preferably 6% by mass or less, and even more preferably 4% 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 of the various additive components in the adhesive sheet 10 with the acrylic polymer).

Carboxy group-containing monomers include, for example, acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, and isocrotonic acid.

When a carboxyl group-containing monomer is used, the proportion of the carboxyl group-containing monomer in the monomer components is preferably 0.1 mass% or more, more preferably 0.5 mass% or more, and even more preferably 0.8 mass% or more, from the viewpoints of introducing a crosslinked structure into the acrylic polymer, ensuring the cohesive strength of the adhesive sheet 10, and ensuring the adhesive strength of the adhesive sheet 10 to the adherend. The proportion is preferably 3 mass% or less, more preferably 1 mass% or less, from the viewpoints of adjusting the glass transition temperature of the acrylic polymer and avoiding the risk of corrosion of the adherend by acid.

The first monomer component may contain other copolymerizable monomers. Examples of the other copolymerizable monomers include acid anhydride monomers, sulfonic acid group-containing monomers, phosphoric acid 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 alone or in combination of two or more kinds.

The first monomer component preferably includes a first (meth)acrylic acid alkyl ester having an alkyl group with 8 to 12 carbon atoms, a second (meth)acrylic acid alkyl ester having an alkyl group with 1 to 4 carbon atoms, a hydroxyl group-containing monomer, and a monomer having a nitrogen atom-containing ring. The first monomer component more preferably includes NOAA, BA, NVP, and 4HBA.

The base polymer preferably has a crosslinked structure. Examples of methods for introducing a crosslinked structure into a base polymer include the following first and second methods. In the first method, a base polymer having a functional group capable of reacting with the crosslinking agent and a crosslinking agent are blended into an adhesive composition, and the base polymer and the crosslinking agent are reacted in an adhesive sheet. In the second method, a first monomer component forming the base polymer contains a multifunctional compound as a crosslinking agent, and the first monomer component is polymerized to form a base polymer in which a branched structure (crosslinked structure) has been introduced into the polymer chain. These methods may be used in combination.

The crosslinking agent used in the first method is, for example, a compound that reacts 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, and carbodiimide crosslinking agents. The crosslinking agents in the first method may be used alone or in combination of two or more types. As the crosslinking agent in the first method, an isocyanate crosslinking agent is preferably used because it has high reactivity with the hydroxyl groups and carboxyl groups in the base polymer and is easy to introduce a crosslinked structure.

Examples of isocyanate crosslinking agents include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, tetramethylxylylene diisocyanate, naphthalene diisocyanate, triphenylmethane triisocyanate, and polymethylene polyphenylisocyanate. In addition, examples of isocyanate crosslinking agents include derivatives of these isocyanates. Examples of the isocyanate derivatives include isocyanurate modified products and polyol modified products. Commercially available isocyanate crosslinking agents include, for example, Coronate L (trimethylolpropane adduct of tolylene diisocyanate, manufactured by Tosoh), Coronate HL (trimethylolpropane adduct of hexamethylene diisocyanate, manufactured by Tosoh), Coronate HX (isocyanurate of hexamethylene diisocyanate, manufactured by Tosoh), Takenate D110N (trimethylolpropane adduct of xylylene diisocyanate, manufactured by Mitsui Chemicals), and Takenate 600 (1,3-bis(isocyanatomethyl)cyclohexane, manufactured by Mitsui Chemicals).

Peroxide crosslinking agents include dibenzoyl peroxide, di(2-ethylhexyl) peroxydicarbonate, di(4-t-butylcyclohexyl) peroxydicarbonate, di-sec-butyl peroxydicarbonate, t-butyl peroxyneodecanoate, t-hexyl peroxypivalate, and t-butyl peroxypivalate.

Epoxy crosslinkers include bisphenol A, epichlorohydrin type epoxy resins, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, 1,6-hexanediol glycidyl ether, trimethylolpropane triglycidyl ether, diglycidyl aniline, diamine glycidylamine, N,N,N',N'-tetraglycidyl-m-xylylenediamine, and 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane.

The amount of crosslinking agent in the first method is, for example, 0.01 parts by mass or more, preferably 0.05 parts by mass or more, and more preferably 0.1 parts by mass or more, per 100 parts by mass of base polymer, from the viewpoint of ensuring the cohesive strength of the adhesive sheet 10. From the viewpoint of ensuring good tackiness in the adhesive sheet 10, the amount of crosslinking agent in the first method is, for example, 5 parts by mass or less, preferably 1 part by mass or less, and more preferably 0.2 parts by mass or less, per 100 parts by mass of base polymer.

In the second method, the first monomer component (including a polyfunctional compound and a monofunctional monomer for introducing a crosslinked structure) may be polymerized in one step or in multiple steps. In the multistep polymerization method, first, a monofunctional monomer for forming a base polymer is polymerized (preliminary polymerization), thereby preparing a prepolymer composition containing a partial polymer (a mixture of a polymer with a low degree of polymerization and an unreacted monofunctional monomer). Next, a polyfunctional compound is added as a crosslinking agent to the prepolymer composition, and then a polymerization reaction is allowed to proceed in a reaction system containing the partial polymer and the polyfunctional compound (main polymerization).

Examples of polyfunctional compounds include polyfunctional monomers and polyfunctional oligomers that contain two or more ethylenically unsaturated double bonds in one molecule. Examples of polyfunctional monomers include polyfunctional (meth)acrylates.

Examples of polyfunctional (meth)acrylates include difunctional (meth)acrylates, trifunctional (meth)acrylates, and tetrafunctional or higher polyfunctional (meth)acrylates.

Examples of bifunctional (meth)acrylates include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, glycerin di(meth)acrylate, ethoxylated bisphenol A diacrylate (BPAEODE), and neopentyl glycol di(meth)acrylate.

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

Examples of tetrafunctional or higher polyfunctional (meth)acrylates include ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol monohydroxypenta(meth)acrylate, alkyl-modified dipentaerythritol penta(meth)acrylate, and dipentaerythritol hexa(meth)acrylate.

Examples of polyfunctional oligomers include urethane (meth)acrylate oligomers, polyester (meth)acrylate oligomers, polyether (meth)acrylate oligomers, polyol (meth)acrylate oligomers, epoxy (meth)acrylate oligomers, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate.

The polyfunctional compound used as the crosslinking agent in the second method may be used alone or in combination of two or more kinds. As the polyfunctional compound, a polyfunctional monomer is preferably used, and more preferably at least one selected from the group consisting of 1,9-nonanediol diacrylate, dipentaerythritol hexaacrylate, 1,6-hexanediol diacrylate, and trimethylolpropane triacrylate is used.

The amount of the polyfunctional compound as a crosslinking agent in the first monomer component is preferably 0.02 parts by mass or more, more preferably 0.05 parts by mass or more, and even more preferably 0.07 parts by mass or more per 100 parts by mass of the monofunctional monomer, from the viewpoint of ensuring the cohesive strength of the adhesive sheet 10. The amount of the polyfunctional compound is preferably 3 parts by mass or less, more preferably 2 parts by mass or less, and even more preferably 1 part by mass or less per 100 parts by mass of the monofunctional monomer, from the viewpoint of ensuring good tackiness in the adhesive sheet 10.

The acrylic polymer (base polymer) can be formed by polymerizing the first monomer component described above. Examples of the polymerization method include solution polymerization, emulsion polymerization, and solvent-free photopolymerization (e.g., ultraviolet polymerization). For example, ethyl acetate and toluene are used as the solvent for solution polymerization. A chain transfer agent may be used in the polymerization. For example, a thermal polymerization initiator and a photopolymerization initiator are used as the polymerization initiator. The polymerization initiator may be used alone or in combination of two or more types. The amount of the polymerization initiator used is preferably 0.03 parts by mass or more, more preferably 0.05 parts by mass or more, and even more preferably 0.07 parts by mass or more, and is preferably 1 part by mass or less, more preferably 0.5 parts by mass or less, and even more preferably 0.3 parts by mass or less, per 100 parts by mass of the first monomer component.

Thermal polymerization initiators include, for example, azo polymerization initiators and peroxide polymerization initiators. Azo polymerization initiators include, for example, 2,2'-azobisisobutyronitrile, 2,2'-azobis-2-methylbutyronitrile, 2,2'-azobis(2-methylpropionate)dimethyl, 4,4'-azobis-4-cyanovaleric acid, azobisisovaleronitrile, and 2,2'-azobis(2-amidinopropane)dihydrochloride. Peroxide polymerization initiators include, for example, dibenzoyl peroxide, t-butyl permaleate, and lauroyl peroxide.

Examples of photopolymerization initiators include radical photopolymerization initiators, cationic photopolymerization initiators, and anionic photopolymerization initiators.

Examples of radical photopolymerization initiators include acylphosphine oxide photopolymerization initiators, benzoin ether photopolymerization initiators, and acetophenone photopolymerization initiators.

Examples of acylphosphine oxide photopolymerization initiators include bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-di-n-butoxyphenylphosphine oxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide. Examples of benzoin ether photopolymerization initiators include benzoin methyl ether, benzoin ethyl ether, benzoin propyl ether, benzoin isopropyl ether, benzoin isobutyl ether, and 2,2-dimethoxy-1,2-diphenylethan-1-one. Examples of acetophenone-based photopolymerization initiators include 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexyl phenyl ketone, 4-phenoxydichloroacetophenone, and 4-(t-butyl)dichloroacetophenone.

The weight average molecular weight of the base polymer is preferably 100,000 or more, more preferably 300,000 or more, and even more preferably 500,000 or more, from the viewpoint of ensuring the cohesive force of the adhesive sheet 10. The weight average molecular weight of the base polymer is measured by gel permeation chromatography (GPC) and calculated in terms of polystyrene.

The glass transition temperature (Tg) of the base polymer is preferably 0°C or lower, more preferably -10°C or lower, and even more preferably -20°C or lower. The glass transition temperature is, for example, -80°C or higher. The glass transition temperature (Tg) of the base polymer can be the glass transition temperature (theoretical value) calculated based on the above Fox formula.

When an acrylic polymer is used as the base polymer, the oligomer is preferably an acrylic oligomer. The acrylic oligomer is a copolymer of a monomer component (second monomer component) containing 50% or more by mass of an alkyl (meth)acrylate ester, and has a weight average molecular weight of, for example, 1,000 or more and 30,000 or less. The oligomer may be used alone or in combination of two or more types.

When two or more oligomers are used in combination, at least one of the oligomers only needs to satisfy the specified parameters (e.g., glass transition temperature, difference ΔH between hydrogen bond term δH ₁ of HSP of base polymer and hydrogen bond term δH ₂ of HSP of oligomer (= δH ₂ - δH ₁ )). In other words, when two or more oligomers are used in combination, oligomers that do not satisfy the specified parameters (e.g., glass transition temperature, difference ΔH between hydrogen bond term δH ₁ of HSP of base polymer and hydrogen bond term δH ₂ of HSP of oligomer (= δH ₂ - δH ₁ )) may also be included within a range that does not impair the effects of the present invention. Preferably, all oligomers used in combination satisfy the specified parameters.

Examples of the (meth)acrylic acid alkyl ester in the second monomer component include (meth)acrylic acid alicyclic alkyl ester and (meth)acrylic acid chain alkyl ester.

The (meth)acrylic acid alicyclic alkyl ester in the second monomer component may be, for example, the (meth)acrylic acid alicyclic alkyl ester described above with respect to the first monomer component. The (meth)acrylic acid alicyclic alkyl ester in the second monomer component is preferably at least one selected from the group consisting of cyclohexyl methacrylate (CHMA), methylcyclohexyl methacrylate, tert-butylcyclohexyl methacrylate, cyclododecyl methacrylate, isobornyl methacrylate (IBXMA), dicyclopentanyl methacrylate (DCPMA), dicyclopentenyl methacrylate, and 1-adamantyl methacrylate (ADMA), and more preferably at least one selected from the group consisting of DCPMA, CHMA, IBXMA, and ADMA.

The proportion of the (meth)acrylic acid alicyclic alkyl ester in the second monomer component is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 40% by mass or more, from the viewpoint of increasing the Tg of the oligomer. The proportion of the (meth)acrylic acid alicyclic alkyl ester in the second monomer component is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 65% by mass or less, from the viewpoint of the polymerizability of the second monomer component.

The (meth)acrylic acid chain alkyl ester in the second monomer component may be, for example, the (meth)acrylic acid chain alkyl ester described above in relation to the first monomer component. The (meth)acrylic acid chain alkyl ester in the second monomer component is preferably a (meth)acrylic acid alkyl ester having an alkyl group with 1 to 6 carbon atoms, and more preferably methyl methacrylate (MMA). MMA is preferred because it has a high glass transition temperature of the homopolymer and is relatively compatible with the base polymer.

The proportion of the (meth)acrylic acid chain alkyl ester in the second monomer component is preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, even more preferably 30% by mass or more, from the viewpoint of ensuring a high Tg of the oligomer and adjusting the compatibility of the oligomer with the base polymer, and is preferably 60% by mass or less, more preferably 55% by mass or less, even more preferably 50% by mass or less.

The mass ratio of the (meth)acrylic acid alicyclic alkyl ester to the (meth)acrylic acid linear alkyl ester in the second monomer component is preferably 0.6 or more, more preferably 0.8 or more, from the viewpoint of increasing the Tg of the oligomer and adjusting the compatibility of the oligomer with the base polymer, and is also preferably 9.0 or less, more preferably 5.0 or less, and even more preferably 2.0 or less.

The second monomer component may contain a copolymerizable monomer that is copolymerizable with the (meth)acrylic acid alkyl ester. Examples of the copolymerizable monomer include hydrophilic monomers. Examples of the hydrophilic monomer include hydroxyl group-containing monomers, monomers having a nitrogen atom-containing ring, carboxyl group-containing monomers, and ether group-containing monomers, and preferably at least one selected from the group consisting of hydroxyl group-containing monomers, monomers having a nitrogen atom-containing ring, carboxyl group-containing monomers, and ether group-containing monomers. The hydrophilic monomers may be used alone or in combination of two or more kinds.

The hydroxyl group-containing monomer in the second monomer component may be, for example, the hydroxyl group-containing monomer described above with respect to the first monomer component. The hydroxyl group-containing monomer in the second monomer component is preferably at least one selected from the group consisting of 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl methacrylate (HPMA), and 4-hydroxybutyl acrylate (4HBA).

The proportion of the hydroxyl group-containing monomer in the second monomer component is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 5% by mass or more, from the viewpoint of adjusting the compatibility of the oligomer with the base polymer, and is preferably 18% by mass or less, more preferably 15% by mass or less, even more preferably 12% by mass or less.

The monomer having a nitrogen atom-containing ring in the second monomer component may be, for example, the monomer having a nitrogen atom-containing ring described above with respect to the first monomer component. The monomer having a nitrogen atom-containing ring in the second monomer component is preferably 4-acryloylmorpholine (ACMO).

The proportion of the monomer having a nitrogen atom-containing ring in the second monomer component is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, from the viewpoint of adjusting the compatibility of the oligomer with the base polymer. The proportion of the monomer having a nitrogen atom-containing ring in the second monomer component is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 22% by mass or less, from the viewpoint of controlling the molecular weight of the oligomer.

The carboxyl group-containing monomer in the second monomer component may be, for example, the carboxyl group-containing monomer described above with respect to the first monomer component. The carboxyl group-containing monomer in the second monomer component is preferably acrylic acid (AA).

The proportion of the carboxyl group-containing monomer in the second monomer component is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 5% by mass or more, from the viewpoint of adjusting the compatibility of the oligomer with the base polymer, and is preferably 18% by mass or less, more preferably 15% by mass or less, even more preferably 12% by mass or less.

Examples of ether group-containing monomers include ethyl carbitol (meth)acrylate, 2-methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, phenoxyethyl (meth)acrylate, and vinyloxyethoxyethyl (meth)acrylate. The ether group-containing monomer is preferably ethyl carbitol acrylate (CBA).

The proportion of the ether group-containing monomer in the second monomer component is preferably 3% by mass or more, more preferably 5% by mass or more, even more preferably 7% by mass or more, from the viewpoint of adjusting the compatibility of the oligomer with the base polymer, and is preferably 40% by mass or less, more preferably 35% by mass or less, even more preferably 32% by mass or less.

The second monomer component preferably contains a (meth)acrylic acid alicyclic alkyl ester, a (meth)acrylic acid chain alkyl ester, and a hydrophilic monomer. In other words, the acrylic oligomer is preferably a copolymer of the second monomer component containing a (meth)acrylic acid alicyclic alkyl ester, a (meth)acrylic acid chain alkyl ester, and a hydrophilic monomer.

The proportion of hydrophilic monomer in the second monomer component is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, from the viewpoint of increasing the Tg of the oligomer and adjusting the compatibility of the oligomer with the base polymer, and is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 32% by mass or less.

As described above, the acrylic oligomer may be used alone or in combination of two or more kinds. When two or more kinds of acrylic oligomers are used in combination, preferably, at least one of the oligomers is a copolymer of a second monomer component containing a (meth)acrylic acid alicyclic alkyl ester, a (meth)acrylic acid chain alkyl ester, and a hydrophilic monomer. Specifically, examples of the copolymer include a copolymer of a second monomer component containing a (meth)acrylic acid alicyclic alkyl ester and a (meth)acrylic acid chain alkyl ester, and a copolymer of a second monomer component containing a (meth)acrylic acid alicyclic alkyl ester, a (meth)acrylic acid chain alkyl ester, and a hydrophilic monomer.

When two or more types of acrylic oligomers are used in combination, the content ratio of each copolymer (each acrylic oligomer) in the total amount of acrylic oligomer is not particularly limited, and is appropriately adjusted within a range in which the two or more types of acrylic oligomers used in combination satisfy the specified parameters. Specifically, the content ratio of the copolymer (one acrylic oligomer) of the second monomer component containing a (meth)acrylic acid alicyclic alkyl ester, a (meth)acrylic acid chain alkyl ester, and a hydrophilic monomer in the total amount of acrylic oligomer is preferably 50 mass% or more, more preferably 55 mass% or more, and even more preferably 58 mass% or more.

The acrylic oligomer is obtained by polymerizing the second monomer component of the acrylic oligomer. Examples of the polymerization method include solution polymerization, emulsion polymerization, and solvent-free photopolymerization (e.g., ultraviolet polymerization). For example, ethyl acetate and toluene are used as the solvent for solution polymerization. In the polymerization, a chain transfer agent may be used to adjust the molecular weight. For example, a thermal polymerization initiator and a photopolymerization initiator are used as the polymerization initiator. The polymerization initiator may be used alone or in combination of two or more kinds. The amount of the polymerization initiator used is preferably 0.03 parts by mass or more, more preferably 0.05 parts by mass or more, and even more preferably 0.07 parts by mass or more, and is preferably 1 part by mass or less, more preferably 0.5 parts by mass or less, and even more preferably 0.3 parts by mass or less, per 100 parts by mass of the second monomer component.

The weight average molecular weight Mw of the oligomer is preferably 4300 or more, more preferably 4500 or more, and even more preferably 4700 or more, from the viewpoint of increasing adhesion on the surface (adhesive surfaces 11, 12) of the adhesive sheet 10. The weight average molecular weight Mw of the oligomer is preferably 10,000 or less, more preferably 8,000 or less, and even more preferably 6,000 or less, from the viewpoint of uneven distribution of the oligomer on the surface of the adhesive sheet 10 and its vicinity (mobility to the surface). The method for measuring the weight average molecular weight Mw of the oligomer is specifically as described below in the examples.

The content of the acrylic oligomer in the adhesive sheet 10 is preferably 0.2 parts by mass or more, more preferably 0.3 parts by mass or more, and even more preferably 0.5 parts by mass or more, relative to 100 parts by mass of the base polymer, in order to sufficiently increase the adhesive strength of the adhesive sheet 10. From the viewpoint of ensuring the transparency of the adhesive sheet 10, the content of the acrylic oligomer in the adhesive sheet 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. If the content of the acrylic oligomer in the adhesive sheet 10 is too large, the haze tends to increase and the transparency tends to decrease due to a decrease in the compatibility of the acrylic oligomer.

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.

The adhesive composition may contain other components as required. Examples of the other components include solvents, tackifiers, plasticizers, softeners, antioxidants, fillers, colorants, UV absorbers, antioxidants, surfactants, and antistatic agents. Examples of the solvent include polymerization solvents used as required during polymerization of the acrylic polymer, and solvents added to the polymerization reaction solution after polymerization. Examples of the solvents that are used include ethyl acetate and toluene.

The adhesive sheet 10 can be produced, for example, by applying the above-mentioned adhesive composition onto a release liner L1 (first release liner) to form a coating film, and then irradiating the coating film with ultraviolet light or drying the coating film. The adhesive sheet 10 can also be produced by applying the above-mentioned adhesive composition onto a release liner L1 (first release liner) to form a coating film, laminating a release liner L2 (second release liner) onto the coating film, and then irradiating the coating film between the release liners with ultraviolet light or drying the coating film.

The release liner L1 may be, for example, a flexible plastic film. Examples of the plastic film include polyester films such as polyethylene terephthalate films, polyethylene films, and polypropylene films. The thickness of the release liner L1 is, for example, 3 μm or more and, for example, 200 μm or less. The surface of the release liner L1 is preferably subjected to a release treatment.

　Examples of methods for applying the adhesive composition include 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 for the coating film is, for example, 50°C to 200°C. The drying time is, for example, 5 seconds to 20 minutes.

The release liner L2 is preferably a flexible plastic film with a release-treated surface. The plastic films described above for the release liner L1 can be used as the release liner L2.

In this manner, an adhesive sheet 10 can be manufactured in which the adhesive surfaces 11, 12 are covered and protected by the release liners L1, L2.

Figures 2A to 2C show an example of how to use the adhesive sheet 10.

In this method, first, as shown in FIG. 2A, the adhesive sheet 10 is attached to one side of the first member 21 (adherend) in the thickness direction H. The first member 21 is, for example, one element in a laminated structure of a flexible display panel. Examples of such elements include a pixel panel, a polarizing film, a touch panel, and a cover film (the same applies to the second member 22 described below). Through this process, the adhesive sheet 10 for bonding to other members is provided on the first member 21.

Next, as shown in FIG. 2B, one side of the first member 21 in the thickness direction H is bonded to the other side of the second member 22 in the thickness direction H via the adhesive sheet 10 on the first member 21. The second member 22 is, for example, another element in the laminated structure of the flexible display panel.

Next, as shown in FIG. 2C, the adhesive sheet 10 between the first member 21 and the second member 22 is aged. Aging increases the bonding strength between the adhesive sheet 10 and the members 21 and 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 using 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.

The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples. Furthermore, the specific numerical values of the compounding amounts (contents), physical properties, parameters, etc. described below can be replaced with the upper limits (numerical values defined as "equal to or less than") or lower limits (numerical values defined as "equal to or more than") of the corresponding compounding amounts (contents), physical properties, parameters, etc. described in the above-mentioned "Form for carrying out the invention."

Preparation of First Prepolymer Composition
In a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, 70 parts by mass of n-octyl acrylate (NOAA), 20 parts by mass of n-butyl acrylate (BA), 8 parts by mass of 4-hydroxybutyl acrylate (4HBA), and 2 parts by mass of N-vinyl-2-pyrrolidone (NVP) were added with 0.05 parts by mass of a first photopolymerization initiator (product name "Omnirad 184", 1-hydroxycyclohexyl phenyl ketone, manufactured by IGM Resins) and 0.05 parts by mass of a second photopolymerization initiator (product name "Omnirad 651", 2,2-dimethoxy-1,2-diphenylethan-1-one, manufactured by IGM Resins), and then the mixture was irradiated with ultraviolet light under a nitrogen atmosphere to polymerize a portion of the monomer components in the mixture to obtain a first prepolymer composition. A black light was used for the ultraviolet irradiation. The ultraviolet irradiation was continued until the viscosity of the composition reached 10 to 20 Pa·s. This viscosity was measured using a Brookfield viscometer (TVB-10M, manufactured by Toki Sangyo Co., Ltd.) with rotor No. 22, rotor rotation speed of 6 rpm, and temperature of 30° C. (The same applies to the viscosity described below). The obtained first prepolymer composition was a partial polymer containing acrylic polymer P1 and a monomer component (residual monomer) that had not undergone a polymerization reaction. The weight-average molecular weight of the acrylic polymer P1 in the first prepolymer composition was about 4.3 million.

Preparation of Second Prepolymer Composition
In a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, 0.05 parts by mass of a first photopolymerization initiator (Omnirad 184) and 0.05 parts by mass of a second photopolymerization initiator (Omnirad 651) were added to a monomer mixture of 48 parts by mass of lauryl acrylate (LA), 51 parts by mass of 2-ethylhexyl acrylate (2EHA), and 1 part by mass of 4-hydroxybutyl acrylate (4HBA), and then the mixture was irradiated with ultraviolet light under a nitrogen atmosphere to polymerize a part of the monomer components in the mixture to obtain a second prepolymer composition. A black light was used for the ultraviolet irradiation. The ultraviolet irradiation was continued until the viscosity of the composition reached 10 to 20 Pa·s. The obtained second prepolymer composition was a partial polymer containing the acrylic polymer P2 and a monomer component (residual monomer) that had not undergone a polymerization reaction. The weight average molecular weight of the acrylic polymer P2 in the second prepolymer composition was about 4.8 million.

Preparation of Third Prepolymer Composition
In a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, 70 parts by mass of n-octyl acrylate (NOAA), 20 parts by mass of n-butyl acrylate (BA), 30 parts by mass of lauryl acrylate (LA), 8 parts by mass of 4-hydroxybutyl acrylate (4HBA), and 2 parts by mass of N-vinyl-2-pyrrolidone (NVP) were added with 0.05 parts by mass of a first photopolymerization initiator (Omnirad 184) and 0.05 parts by mass of a second photopolymerization initiator (Omnirad 651), and then the mixture was irradiated with ultraviolet light under a nitrogen atmosphere to polymerize a portion of the monomer components in the mixture to obtain a third prepolymer composition. A black light was used for the ultraviolet irradiation. The ultraviolet irradiation was continued until the viscosity of the composition reached 10 to 20 Pa·s. The obtained third prepolymer composition was a partial polymer containing the acrylic polymer P3 and a monomer component (residual monomer) that had not undergone a polymerization reaction. The weight average molecular weight of the acrylic polymer P3 in the third prepolymer composition was about 5 million.

Preparation of Acrylic Oligomer M1
First, in a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, a mixture containing 45 parts by mass of isobornyl methacrylate (IBXMA), 45 parts by mass of methyl methacrylate (MMA), 10 parts by mass of carbitol acrylate (CBA), 3 parts by mass of α-thioglycerol as a chain transfer agent, 0.3 parts by mass of 2,2'-azobisisobutyronitrile (AIBN) as a thermal polymerization initiator, and ethyl acetate as a solvent (solid content concentration 26% by mass) was reacted at 72°C to 74°C for 6 hours under a nitrogen atmosphere (polymerization reaction). Next, the reaction solution was heated at 90°C for 12 hours to volatilize and remove ethyl acetate, the chain transfer agent, and unreacted monomers. As a result, a solid acrylic oligomer M1 was obtained. The weight average molecular weight (Mw) of the acrylic oligomer M1 was 4880. The glass transition temperature (Tg) of the acrylic oligomer M1 was 98.5°C.

Preparation of Acrylic Oligomer M2
A solid acrylic oligomer M2 was obtained in the same manner as in the preparation of the acrylic oligomer M1, except that the amount of IBXMA was 35 parts by mass, the amount of MMA was 35 parts by mass, and the amount of CBA was 30 parts by mass. The Mw of the acrylic oligomer M2 was 5400. The Tg of the acrylic oligomer M2 was 50.9°C.

Preparation of Acrylic Oligomer M3
First, in a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, a mixture containing 45 parts by mass of dicyclopentanyl methacrylate (DCPMA), 45 parts by mass of MMA, 10 parts by mass of CBA, 3 parts by mass of α-thioglycerol as a chain transfer agent, 0.3 parts by mass of AIBN as a thermal polymerization initiator, and ethyl acetate as a solvent (solid content concentration 26% by mass) was reacted at 72°C to 74°C for 6 hours under a nitrogen atmosphere (polymerization reaction). Next, the reaction solution was heated at 90°C for 12 hours to volatilize and remove ethyl acetate, the chain transfer agent, and unreacted monomers. As a result, a solid acrylic oligomer M3 was obtained. The Mw of the acrylic oligomer M3 was 5230. The Tg of the acrylic oligomer M3 was 97.7°C.

Preparation of Acrylic Oligomer M4
A solid acrylic oligomer M4 was obtained in the same manner as for the acrylic oligomer M1, except that the blending amount of IBXMA was 40 parts by mass, the blending amount of MMA was 40 parts by mass, and 20 parts by mass of 4-acryloylmorpholine (ACMO) was used instead of 10 parts by mass of CBA. The Mw of the acrylic oligomer M4 was 4940. The Tg of the acrylic oligomer M4 was 129.2°C.

Preparation of Acrylic Oligomer M5
A solid acrylic oligomer M5 was obtained in the same manner as in the acrylic oligomer M3, except that the blending amount of DCPMA was 40 parts by mass, the blending amount of MMA was 40 parts by mass, and 20 parts by mass of ACMO was used instead of 10 parts by mass of CBA. The Mw of the acrylic oligomer M5 was 5110. The Tg of the acrylic oligomer M5 was 128.3°C.

Preparation of Acrylic Oligomer M6
A solid acrylic oligomer M6 was obtained in the same manner as in the preparation of the acrylic oligomer M1, except that 10 parts by mass of acrylic acid (AA) was used instead of 10 parts by mass of CBA. The Mw of the acrylic oligomer M6 was 5670. The Tg of the acrylic oligomer M6 was 121.9°C.

Preparation of Acrylic Oligomer M7
A solid acrylic oligomer M7 was obtained in the same manner as for the acrylic oligomer M3, except that 10 parts by mass of 2-hydroxyethyl methacrylate (HEMA) was used instead of 10 parts by mass of CBA. The Mw of the acrylic oligomer M7 was 5600. The Tg of the acrylic oligomer M7 was 115.8°C.

Preparation of Acrylic Oligomer M8
A solid acrylic oligomer M8 was obtained in the same manner as for the acrylic oligomer M3, except that the amount of DCPMA was 60 parts by mass, the amount of MMA was 30 parts by mass, and 10 parts by mass of HEMA was used instead of 10 parts by mass of CBA. The Mw of the acrylic oligomer M8 was 5570. The Tg of the acrylic oligomer M8 was 123.9°C.

Preparation of Acrylic Oligomer M9
First, in a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, a mixture containing 45 parts by mass of cyclohexyl methacrylate (CHMA), 45 parts by mass of MMA, 10 parts by mass of HEMA, 3 parts by mass of α-thioglycerol as a chain transfer agent, 0.3 parts by mass of AIBN as a thermal polymerization initiator, and ethyl acetate as a solvent (solid content concentration 26% by mass) was reacted at 72°C to 74°C for 6 hours under a nitrogen atmosphere (polymerization reaction). Next, the reaction solution was heated at 90°C for 12 hours to volatilize and remove ethyl acetate, the chain transfer agent, and unreacted monomers. As a result, a solid acrylic oligomer M9 was obtained. The Mw of the acrylic oligomer M9 was 5920. The Tg of the acrylic oligomer M9 was 85.4°C.

Preparation of Acrylic Oligomer M10
A solid acrylic oligomer M10 was obtained in the same manner as for the acrylic oligomer M9, except that 10 parts by mass of 4HBA was used instead of 10 parts by mass of HEMA. The Mw of the acrylic oligomer M10 was 5740. The Tg of the acrylic oligomer M10 was 72.2°C.

Preparation of Acrylic Oligomer M11
A solid acrylic oligomer M11 was obtained in the same manner as in the preparation of the acrylic oligomer M9, except that 10 parts by mass of 2-hydroxypropyl methacrylate (HPMA) was used instead of 10 parts by mass of HEMA. The Mw of the acrylic oligomer M11 was 5840. The Tg of the acrylic oligomer M11 was 82.4°C.

Preparation of Acrylic Oligomer M12
Acrylic oligomer M12 was obtained in the same manner as in the preparation of acrylic oligomer M1, except that 10 parts by mass of 4HBA was used instead of 10 parts by mass of CBA. The Mw of the acrylic oligomer M12 was 5320. The Tg of the acrylic oligomer M12 was 99.2°C.

Preparation of Acrylic Oligomer M13
First, in a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, a mixture containing 50 parts by mass of IBXMA, 50 parts by mass of NVP, 3 parts by mass of α-thioglycerol as a chain transfer agent, 0.3 parts by mass of AIBN as a thermal polymerization initiator, and ethyl acetate as a solvent (solid content concentration 26% by mass) was reacted at 72°C to 74°C for 6 hours under a nitrogen atmosphere (polymerization reaction). Next, the reaction solution was heated at 90°C for 12 hours to volatilize and remove ethyl acetate, the chain transfer agent, and unreacted monomers. As a result, a solid acrylic oligomer M13 was obtained. The Mw of the acrylic oligomer M13 was 10200. The Tg of the acrylic oligomer M13 was 193°C.

Preparation of Acrylic Oligomer M14
First, in a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet tube, a mixture containing 50 parts by mass of n-pentyl acrylate (NPA) (trade name "LIMA", manufactured by Osaka Organic Chemical Industry Co., Ltd.), 50 parts by mass of MMA, 3 parts by mass of α-thioglycerol as a chain transfer agent, 0.3 parts by mass of AIBN as a thermal polymerization initiator, and ethyl acetate as a solvent (solid content concentration 26% by mass) was reacted at 72°C to 74°C for 6 hours under a nitrogen atmosphere (polymerization reaction). Next, the reaction solution was heated at 90°C for 12 hours to volatilize and remove ethyl acetate, the chain transfer agent, and unreacted monomers. As a result, a solid acrylic oligomer M14 was obtained. The Mw of the acrylic oligomer M14 was 5220. The Tg of the acrylic oligomer M14 was 20.6°C.

Preparation of Acrylic Oligomer M15
A solid acrylic oligomer M15 was obtained in the same manner as in the preparation of the acrylic oligomer M1, except that the amount of IBXMA was 90 parts by mass and no MMA was added. The Mw of the acrylic oligomer M15 was 4230. The Tg of the acrylic oligomer M15 was 123.4°C.

Preparation of Acrylic Oligomer M16
A solid acrylic oligomer M16 was obtained in the same manner as in the preparation of the acrylic oligomer M3, except that the amount of DCPMA was 40 parts by mass, the amount of MMA was 40 parts by mass, and 20 parts by mass of 4HBA was used instead of 10 parts by mass of CBA. The Mw of the acrylic oligomer M16 was 5350. The Tg of the acrylic oligomer M16 was 76.8°C.

Preparation of Acrylic Oligomer M17
Acrylic oligomer M17 was obtained in the same manner as Acrylic Oligomer M1, except that 10 parts by mass of lauryl methacrylate (LMA) was used instead of 10 parts by mass of CBA. The Mw of Acrylic Oligomer M17 was 5780. The Tg of Acrylic Oligomer M17 was 125.5°C.

Preparation of Acrylic Oligomer M18
A solid acrylic oligomer M18 was obtained in the same manner as for the acrylic oligomer M1, except that the blending amount of IBXMA was 75 parts by mass, the blending amount of MMA was 20 parts by mass, and 5 parts by mass of HEMA was used instead of 10 parts by mass of CBA. The Mw of the acrylic oligomer M18 was 4610. The Tg of the acrylic oligomer M18 was 141.0°C.

Preparation of Acrylic Oligomer M19
A solid acrylic oligomer M19 was obtained in the same manner as in the preparation of the acrylic oligomer M7, except that 10 parts by mass of 4HBA was used instead of 10 parts by mass of HEMA. The Mw of the acrylic oligomer M19 was 5430. The Tg of the acrylic oligomer M19 was 99.2°C.

Preparation of Acrylic Oligomer M20
A solid acrylic oligomer M20 was obtained in the same manner as in the preparation of the acrylic oligomer M1, except that the amount of IBXMA was 80 parts by mass, the amount of CMA was 20 parts by mass, and no MMA was added. The Mw of the acrylic oligomer M20 was 3820. The Tg of the acrylic oligomer M20 was 80.8°C.

Preparation of Acrylic Oligomer M21
A solid acrylic oligomer M21 was obtained in the same manner as for the acrylic oligomer M1, except that the blending amount of IBXMA was 70 parts by mass, the blending amount of MMA was 20 parts by mass, and the blending amount of CBA was 10 parts by mass. The Mw of the acrylic oligomer M21 was 4060. The Tg of the acrylic oligomer M21 was 109.3°C.

Preparation of Acrylic Oligomer M22
A solid acrylic oligomer M22 was obtained in the same manner as for the acrylic oligomer M8, except that the blending amount of DCPMA was 47.5 parts by mass, the blending amount of MMA was 47.5 parts by mass, and the blending amount of HEMA was 5 parts by mass. The Mw of the acrylic oligomer M22 was 5150. The Tg of the acrylic oligomer M22 was 120.0°C.

Preparation of Acrylic Oligomer M23
A solid acrylic oligomer M23 was obtained in the same manner as for the acrylic oligomer M8, except that the blending amount of DCPMA was 70 parts by mass, the blending amount of MMA was 20 parts by mass, and the blending amount of HEMA was 10 parts by mass. The Mw of the acrylic oligomer M23 was 5840. The Tg of the acrylic oligomer M23 was 130.8°C.

Preparation of Acrylic Oligomer M24
A solid acrylic oligomer M24 was obtained in the same manner as for the acrylic oligomer M23, except that 70 parts by mass of 1-adamantyl methacrylate (ADMA) was used instead of 70 parts by mass of DCPMA. The Mw of the acrylic oligomer M24 was 5150. The Tg of the acrylic oligomer M24 was 160.7°C.

Preparation of Acrylic Oligomer M25
A solid acrylic oligomer M25 was obtained in the same manner as in the preparation of the acrylic oligomer M8, except that the amount of DCPMA was 60 parts by mass, the amount of MMA was 40 parts by mass, and HEMA was not used. The Mw of the acrylic oligomer M25 was 4940. The Tg of the acrylic oligomer M25 was 130.6°C.

Example 1
Preparation of Pressure-Sensitive Adhesive Composition
To the first prepolymer composition, 3.0 parts by mass of acrylic oligomer M1 and 0.07 parts by mass of a crosslinker (trade name "Viscoat #260", 1,9-nonanediol diacrylate, manufactured by Osaka Organic Chemical Industry Co., Ltd.) were added per 100 parts by mass of monomer components in the composition (monomer components forming the base polymer in the adhesive layer described below in this example), and mixed to prepare a first adhesive composition. The relative number of parts of the acrylic oligomer per 100 parts by mass of the base polymer in the adhesive layer described below in this example is shown in Tables 1 and 2 as "parts".

<Formation of Pressure-Sensitive Adhesive Layer>
Next, the first pressure-sensitive adhesive composition was applied onto the release-treated surface of the first release liner, one side of which had been treated with silicone release, to form a coating film. The first release liner was a polyethylene terephthalate (PET) film (product name: "Diafoil MRE#75", thickness 75 μm, manufactured by Mitsubishi Chemical Corporation) with one side treated with silicone release. Next, the release-treated surface of the second release liner, one side of which had been treated with silicone release, was attached to the coating film on the first release liner. The second release liner was a PET film (product name: "Diafoil MRF#75", thickness 75 μm, manufactured by Mitsubishi Chemical Corporation) with one side treated with silicone release. Next, the coating film between the release liners was irradiated with ultraviolet light, and the coating film was photocured to form an adhesive layer (thickness 50 μm). In the ultraviolet light irradiation, a black light was used as the irradiation light source, the irradiation intensity was about 2.5 mW/cm ² , and the irradiation time was 16 minutes. In this manner, a pressure-sensitive adhesive sheet (thickness: 50 μm) with a release liner of Example 1 was prepared.

[Examples 2 to 12]
In preparing the pressure-sensitive adhesive composition, the pressure-sensitive adhesive sheets with release liner of Examples 2 to 12 were produced in the same manner as the pressure-sensitive adhesive sheet with release liner of Example 1, except that the type and amount of the acrylic oligomer added was changed as shown in Tables 1 and 2.

Comparative Example 1
A pressure-sensitive adhesive sheet with a release liner of Comparative Example 1 was produced in the same manner as the pressure-sensitive adhesive sheet with a release liner of Example 1, except that no acrylic oligomer was added in the preparation of the pressure-sensitive adhesive composition.

[Comparative Examples 2 to 6]
In preparing the pressure-sensitive adhesive composition, the pressure-sensitive adhesive sheets with release liners of Comparative Examples 2 to 6 were produced in the same manner as the pressure-sensitive adhesive sheet of Example 1, except that the type and amount of acrylic oligomer added was changed as shown in Table 2.

Example 13
Preparation of Pressure-Sensitive Adhesive Composition
To the second prepolymer composition, 1.0 part by mass of acrylic oligomer M18 and 0.07 part by mass of a crosslinker (trade name "Viscoat #260", 1,9-nonanediol diacrylate, manufactured by Osaka Organic Chemical Industry Co., Ltd.) were added per 100 parts by mass of monomer components in the composition (monomer components forming the base polymer in the adhesive layer described below in this example), and mixed to prepare a second adhesive composition. The relative number of parts of the acrylic oligomer per 100 parts by mass of the base polymer in the adhesive layer described below in this example is shown in Table 3 as "parts".

<Formation of Pressure-Sensitive Adhesive Layer>
Except for using the second adhesive composition instead of the first adhesive composition, an adhesive layer having a thickness of 50 μm was formed between release liners in the same manner as in the formation of the adhesive layer described above for Example 1. In this manner, an adhesive sheet (thickness 50 μm) with a release liner of Example 13 was produced.

Comparative Example 7
A pressure-sensitive adhesive sheet with a release liner of Comparative Example 7 was produced in the same manner as the pressure-sensitive adhesive sheet with a release liner of Example 13, except that no acrylic oligomer was added in preparing the pressure-sensitive adhesive composition.

[Comparative Examples 8 and 9]
In preparing the adhesive composition, the adhesive sheets with release liner of Comparative Examples 8 and 9 were prepared in the same manner as the adhesive sheet with release liner of Example 13, except that the type of acrylic oligomer used was changed as shown in Table 3.

Example 14
Preparation of Pressure-Sensitive Adhesive Composition
To the third prepolymer composition, 1.0 part by mass of acrylic oligomer M8 and 0.07 part by mass of a crosslinker (trade name "Viscoat #260", 1,9-nonanediol diacrylate, manufactured by Osaka Organic Chemical Industry Co., Ltd.) were added per 100 parts by mass of monomer components in the composition (monomer components forming the base polymer in the adhesive layer described below in this example), and mixed to prepare a third adhesive composition. The relative number of parts of the acrylic oligomer per 100 parts by mass of the base polymer in the adhesive layer described below in this example is shown in Table 4 as "parts".

<Formation of Pressure-Sensitive Adhesive Layer>
Except for using the third adhesive composition instead of the first adhesive composition, an adhesive layer having a thickness of 50 μm was formed between release liners in the same manner as in the formation of the adhesive layer described above for Example 1. In this manner, an adhesive sheet (thickness 50 μm) with a release liner of Example 14 was produced.

Example 15
In preparing the pressure-sensitive adhesive composition, the type of acrylic oligomer used was changed as shown in Table 4, and the pressure-sensitive adhesive sheet with release liner of Example 15 was produced in the same manner as the pressure-sensitive adhesive sheet with release liner of Example 15.

Comparative Example 10
A pressure-sensitive adhesive sheet with a release liner of Comparative Example 10 was produced in the same manner as the pressure-sensitive adhesive sheet with a release liner of Example 14, except that no acrylic oligomer was added in preparing the pressure-sensitive adhesive composition.

[Comparative Examples 11 and 12]
In preparing the adhesive composition, the adhesive sheets with release liners of Comparative Examples 11 and 12 were prepared in the same manner as the adhesive sheet with release liner of Example 14, except that the type of acrylic oligomer used was changed as shown in Table 4.

Example 16
Preparation of Pressure-Sensitive Adhesive Composition
To the first prepolymer composition, 1.5 parts by mass of acrylic oligomer M22, 0.11 parts by mass of dipentaerythritol hexaacrylate (DPHA), 0.02 parts by mass of additional photopolymerization initiator (trade name "Omnirad 819", bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, manufactured by IGM Resins), and 0.5 parts by mass of silane coupling agent (trade name "KBM-403", 3-glycidoxypropyltrimethoxysilane, manufactured by Shin-Etsu Chemical Co., Ltd.) were added and mixed per 100 parts by mass of monomer components in the composition (monomer components forming the base polymer in the adhesive layer described later in this example), to prepare a fourth adhesive composition. The relative number of parts of the acrylic oligomer per 100 parts by mass of the base polymer in the adhesive layer described later in this example is shown in Table 5 as "parts".

<Formation of Pressure-Sensitive Adhesive Layer>
Except for using the fourth adhesive composition instead of the first adhesive composition, a 50 μm thick adhesive layer was formed between release liners in the same manner as in the formation of the adhesive layer described above for Example 1. In this manner, a PSA sheet (thickness 50 μm) with a release liner of Example 16 was produced.

[Examples 17 to 19]
In preparing the pressure-sensitive adhesive composition, the pressure-sensitive adhesive sheets with release liners of Examples 17 to 19 were produced in the same manner as the pressure-sensitive adhesive sheet with release liner of Example 16, except that the type of acrylic oligomer blended was changed as shown in Table 5. Note that in Example 19, two types of acrylic oligomers were blended (0.5 parts by mass of Acrylic Oligomer M25 and 1.0 part by mass of Acrylic Oligomer M22 were added to the first prepolymer composition per 100 parts by mass of the monomer component in the composition).

<First Measurement Method of Weight Average Molecular Weight>
The weight average molecular weight (Mw) of each of the above-mentioned acrylic polymer and acrylic oligomer not containing a nitrogen-containing monomer was measured by gel permeation chromatography (GPC) under the following first measurement condition, and was calculated in terms of polystyrene. In the measurement, a GPC measuring device (product name "HLC-8120GPC", manufactured by Tosoh Corporation) was used. The sample solution was prepared as follows. First, a tetrahydrofuran (THF) solution (containing 10 mM phosphoric acid) with a sample concentration of 0.20% by mass was prepared using the above-mentioned acrylic polymer or acrylic oligomer not containing a nitrogen-containing monomer as a sample, and the THF solution was left to stand for 20 hours. Next, the THF solution was filtered through a membrane filter with an average pore size of 0.45 μm, and the filtrate was obtained as a sample solution for molecular weight measurement.

[First measurement condition of GPC]
Column: G7000H _XL + GMH _XL + GMH _XL , each TSKgel (manufactured by Tosoh)
Column temperature: 40°C
Eluent: THF solution (phosphoric acid concentration 10 mM)
Flow rate: 0.8 mL/min Sample injection volume: 100 μL
Standard sample: Polystyrene (Agilent)
Detector: Differential refractometer (RI)

<Second Measurement Method of Weight Average Molecular Weight>
The weight average molecular weight (Mw) of each of the above-mentioned acrylic polymer and acrylic oligomer containing a nitrogen-containing monomer was measured by gel permeation chromatography (GPC) under the following second measurement condition, and was calculated in terms of polystyrene. In the measurement, a GPC measuring device (product name "HLC-8120GPC", manufactured by Tosoh Corporation) was used. The sample solution was prepared as follows. First, a dimethylformamide (DMF) solution (salt added) with a sample concentration of 0.20 mass % was prepared using the above-mentioned acrylic polymer or acrylic oligomer not containing a nitrogen-containing monomer as a sample, and the DMF solution was left to stand for 20 hours. Next, the DMF solution was filtered through a membrane filter with an average pore size of 0.45 μm, and the filtrate was obtained as a sample solution for molecular weight measurement.

[Second measurement condition of GPC]
Columns: SuperAWM-H + SuperAW4000 + SuperAW2500, TSKgel (Tosoh)
Column temperature: 40°C
Eluent: DMF solution (salt added)
Flow rate: 0.4 mL/min Sample injection volume: 40 μL
Standard sample: Polystyrene (Agilent)
Detector: Differential refractometer (RI)

<Tg of oligomer>
The glass transition temperatures (Tg) of the acrylic oligomers M1 to M25 were calculated based on the above Fox formula, and the values are shown in Tables 1 to 5.

<Hydrogen bond term of HSP>
The hydrogen bond term of the Hansen solubility parameter (HSP) was determined for each of the acrylic oligomers M1 to M25. Specifically, it is as follows.

First, the hydrogen bond term (δh _i ) of HSP was calculated for each monomer m _i forming the acrylic oligomer by computer software HSPiP (Hansen Solubility Parameters in Practice). Next, the hydrogen bond term (δH) of the acrylic oligomer was calculated by the following formula from the molar fraction x _i of the monomer m _i in the acrylic oligomer and the hydrogen bond term δh _i of the monomer m _i . For example, the hydrogen bond term of the acrylic oligomer M1 was calculated as 5.39 MPa 1/2 by the following formula from the molar fraction of IBXMA (molecular weight 220.0) of 0.289 and the hydrogen bond term of 2.4 MPa ^1/2 , the molar fraction of MMA (molecular weight 100.1) of 0.636 and the hydrogen bond term of 6.6 ^{MPa 1/2} , and the molar fraction of CBA (molecular weight 188.2) of 0.075 and the hydrogen bond term of 6.5 MPa ^1/2 . The hydrogen bond terms of the acrylic oligomers M1 to M25 are shown in Tables 1 to 5 as _δH2 .

δH=Σ x _i ×δh _i

Similarly, the hydrogen bond term (δH ₁ ) of the HSP of the above-mentioned acrylic polymer was determined. For example, in the acrylic polymer consisting of the first prepolymer composition, the value was 5.07 MPa ^1/2 . The ratio of δH ₂ of the acrylic oligomer to δH ₁ of the acrylic polymer in the pressure-sensitive adhesive sheet (δH ₂ /δH ₁ ) and the difference ΔH between δH ₂ and δH ₁ (= δH ₂ - δH ₁ ) are also shown in Tables 1 to 5.

<Gel Fraction>
For each of the pressure-sensitive adhesive sheets of Examples 1 to 19 and Comparative Examples 1 to 12, the gel fraction was measured as follows.

First, about 500 mg of an adhesive sample was taken from the adhesive sheet between the release liners. Next, the mass (W ₁ ) of the adhesive sample was measured. Next, the adhesive sample was immersed in about 40 g of ethyl acetate in a container for 7 days. Next, all components insoluble in ethyl acetate (insoluble parts) were collected. Next, the insoluble parts were dried at 130° C. for 2 hours (removal of ethyl acetate). Next, the mass (W ₂ ) of the insoluble parts was measured. Then, the gel fraction (mass %) of the adhesive sheet after photocuring was calculated based on the following formula. The values are shown in Tables 1 to 5.

Gel fraction (mass%)=( _W2 / _W1 )×100

Haze
For each of the pressure-sensitive adhesive sheets of Examples 1 to 19 and Comparative Examples 1 to 12, the haze was measured as follows.

First, samples for measurement were prepared. Specifically, the first release liner was peeled off from the adhesive sheet, and the sheet was then attached to an alkaline glass (thickness 1.0 mm, total light transmittance 92%, haze 0.4%, manufactured by Matsunami Glass Co., Ltd.). Next, the first release liner was peeled off from the adhesive sheet on the glass. This produced samples for measurement. Next, the haze of each of the adhesive sheets in the samples was measured using a haze meter (model name "HM-150", manufactured by Murakami Color Research Laboratory). The measurement was in accordance with JIS K7136 (2000). In this measurement, the sample was placed in the device so that light was hitting the alkaline glass side of the sample. The measured haze of the adhesive sheet is shown in Tables 1 to 5.

<Shear storage modulus>
For each of the pressure-sensitive adhesive sheets of Examples 1 to 19 and Comparative Examples 1 to 12, the dynamic viscoelasticity was measured (first measurement).

The required number of measurement samples were prepared for each adhesive sheet. Specifically, first, multiple pieces of adhesive sheet cut from the adhesive sheet were glued together to prepare a sample sheet approximately 1.0 mm thick. Next, this sheet was punched out to obtain cylindrical pellets (diameter 7.9 mm) that served as measurement samples.

Then, the measurement samples were fixed to a 7.9 mm diameter parallel plate fixture and subjected to dynamic viscoelasticity measurement using a dynamic viscoelasticity measuring device (name: Advanced Rheometric Expansion System (ARES), manufactured by Rheometric Scientific). In this measurement, the measurement mode was shear mode, the measurement temperature range was -65°C to 200°C, the heating rate was 5°C/min, and the frequency was 1 Hz. From the measurement results, the shear storage modulus at the specified temperatures (-20°C, -10°C, 60°C) was read. The shear storage modulus G1 (kPa) at -10°C, the shear storage modulus G2 (kPa) at 60°C, the shear storage modulus G3 (kPa) at -20°C, and the ratio of the shear storage modulus G2 to the shear storage modulus G1 (G2/G1) are shown in Tables 1 to 5.

Peel test
For each of the pressure-sensitive adhesive sheets of Examples 1 to 19 and Comparative Examples 1 to 12, the adhesive strength to the adherend was examined by a peel test.

Specifically, first, the required number of measurement samples were prepared for each adhesive sheet. In preparing the measurement samples, first, the first release liner was peeled off from the adhesive sheet, and the exposed surface was attached to a polyethylene terephthalate (PET) film (product name "Lumirror S10", thickness 25 μm, manufactured by Toray) whose surface had been plasma-treated to obtain a laminate. In the plasma treatment, a plasma irradiation device (product name "AP-TO5", manufactured by Sekisui Kogyo Co., Ltd.) was used, and the voltage was set to 160 V, the frequency to 10 kHz, and the treatment speed to 5000 mm/min (the same applies to the plasma treatment described below). Next, a test piece (width 20 mm x length 100 mm) was cut out from the laminate (PET film/adhesive sheet/second release liner). Next, the second release liner was peeled off from the adhesive sheet of this test piece, and the exposed surface was attached to a glass plate (acrylic glass manufactured by Matsunami Glass Co., Ltd.). Next, the glass plate with the adhesive sheet (test piece) was heated and pressurized at a temperature of 50°C and a pressure of 0.5 MPa for 15 minutes. This caused the test piece to be pressed against the glass plate. In this way, the measurement sample was prepared.

Next, the measurement sample was left to stand at room temperature for 30 minutes, and then a 180° peel test was conducted to peel the test piece from the glass plate of the measurement sample, and the force required for peeling (peel strength) was measured (first peel test). A tensile tester (product name "Autograph AG-50NXplus", manufactured by Shimadzu Corporation) was used for this measurement. In this measurement, the measurement temperature was 25°C, the relative humidity was 55%, the peel angle of the test piece from the glass plate was 180°, the pulling speed of the test piece was 300 mm/min, and the peel length was 50 mm. The average measured peel strength is shown in Tables 1 to 5 as adhesive strength F1 (N/20 mm).

On the other hand, a peel test was conducted under the same conditions as the first peel test, except that the pulling speed was changed to 60 mm/min (second peel test). The measurement results are shown in Tables 1 to 5 as adhesive strength F2 (N/20 mm). The ratio of adhesive strength F2 to the above-mentioned adhesive strength F1 (F2/F1) is also shown in Tables 1 to 5.

〔evaluation〕
The adhesive sheet of Comparative Example 1 is a soft adhesive sheet having a shear storage modulus G1 of 100 kPa or less at -10°C, but does not contain oligomers with a Tg of 40°C or higher, and has small adhesive strengths F1 and F2.

The pressure-sensitive adhesive sheet of Comparative Example 2 contains an oligomer (acrylic oligomer M13) with a Tg of 40° C. or higher, but the difference ΔH (= δH ₂ - δH ₁ ) is less than 0.1, and the adhesive strengths F1 and F2 are small.
In the pressure-sensitive adhesive sheet of Comparative Example 2, the acrylic oligomer M13 is not unevenly distributed on the pressure-sensitive adhesive surface or in its vicinity.

The adhesive sheet of Comparative Example 3 does not contain oligomers with a Tg of 40°C or higher, and has low adhesive strengths F1 and F2.

The pressure-sensitive adhesive sheet of Comparative Example 4 contains an oligomer (acrylic oligomer M15) with a Tg of 40° C. or higher, but the difference ΔH (= δH ₂ - δH ₁ ) is less than 0.1, and the adhesive strengths F1 and F2 are small.
In the pressure-sensitive adhesive sheet of Comparative Example 4, the acrylic oligomer M15 is not unevenly distributed on the pressure-sensitive adhesive surface or in its vicinity.

The pressure-sensitive adhesive sheet of Comparative Example 5 contains an oligomer (acrylic oligomer M16) with a Tg of 40° C. or higher, but the difference ΔH (= δH ₂ - δH ₁ ) exceeds 1.3, and the haze is too large. In the pressure-sensitive adhesive sheet of Comparative Example 5, the acrylic oligomer M16 has low compatibility with the base polymer, forming domains that cause light scattering.

The pressure-sensitive adhesive sheet of Comparative Example 6 contains an oligomer (acrylic oligomer M17) with a Tg of 40° C. or higher, but the difference ΔH (= δH ₂ - δH ₁ ) is less than 0.1, and the adhesive strengths F1 and F2 are small. In the pressure-sensitive adhesive sheet of Comparative Example 6, the acrylic oligomer M17 is not sufficiently distributed on the adhesive surface and its vicinity.

In contrast, each of the pressure-sensitive adhesive sheets of Examples 1 to 12 was soft, with a shear storage modulus G1 at -10°C of 100 kPa or less, contained an oligomer with a Tg of 40°C or more, and the _δH1 of the HSP of the base polymer and the _δH2 of the HSP of the oligomer satisfied 0.1≦ _δH2 - _δH1 ≦1.3. The pressure-sensitive adhesive sheets of Examples 1 to 12 had a large adhesive strength F1 of 8.0 N/20 mm or more. In each of the pressure-sensitive adhesive sheets of Examples 1 to 12, the oligomer with a Tg of 40°C or more is sufficiently unevenly distributed on the adhesive surface and in its vicinity.

The adhesive sheet of Comparative Example 7 is a soft adhesive sheet with a shear storage modulus G1 of 100 kPa or less at -10°C, but does not contain an oligomer with a Tg of 40°C or more, and has small adhesive forces F1 and F2. The adhesive sheet of Comparative Example 8 contains an oligomer with a Tg of 40°C or more (acrylic oligomer M19), but the difference ΔH (= _δH2 - _δH1 ) exceeds 1.3, the haze is too large, and the adhesive forces F1 and F2 are also small. The adhesive sheet of Comparative Example 9 contains an oligomer with a Tg of 40°C or more (acrylic oligomer M20), but the difference ΔH (= _δH2 - _δH1 ) is less than 0.1, and the adhesive forces F1 and F2 are small. In the adhesive sheet of Comparative Example 9, the acrylic oligomer M20 is not unevenly distributed on the adhesive surface and its vicinity. In comparison with the pressure-sensitive adhesive sheets of Comparative Examples 7 to 9, the pressure-sensitive adhesive sheet of Example 13 is soft with a shear storage modulus G1 of 100 kPa or less at -10°C, contains an oligomer with a Tg of 40°C or more, and the _δH1 of the HSP of the base polymer and the _δH2 of the HSP of the oligomer satisfy 0.1≦ _δH2 - _δH1 ≦1.3. The adhesive strength F1 of such a pressure-sensitive adhesive sheet of Example 13 is 7.7 N/20 mm, which is greater than the adhesive strength F1 of the pressure-sensitive adhesive sheets of Comparative Examples 7 to 9 (the base polymer is the same as that of Example 13). In each pressure-sensitive adhesive sheet of Example 13, the oligomer with a Tg of 40°C or more is sufficiently unevenly distributed on the adhesive surface and in the vicinity thereof. The pressure-sensitive adhesive sheet of Example 13 is very soft with a shear storage modulus G1 of 40 kPa or less at -10°C, and both flexibility and adhesive strength are achieved.

The adhesive sheet of Comparative Example 10 is a soft adhesive sheet having a shear storage modulus G1 of 100 kPa or less at -10°C, but does not contain an oligomer having a Tg of 40°C or more, and has small adhesive forces F1 and F2. The adhesive sheet of Comparative Example 11 contains an oligomer having a Tg of 40°C or more (acrylic oligomer M19), but the difference ΔH (= _δH2 - _δH1 ) exceeds 1.3, and the adhesive forces F1 and F2 are small. The adhesive sheet of Comparative Example 12 contains an oligomer having a Tg of 40°C or more (acrylic oligomer M21), but the difference ΔH (= _δH2 - _δH1 ) is less than 0.1, and the adhesive forces F1 and F2 are small. In the adhesive sheet of Comparative Example 12, the acrylic oligomer M21 is not unevenly distributed on the adhesive surface and its vicinity. In comparison with the pressure-sensitive adhesive sheets of Comparative Examples 10 to 12, the pressure-sensitive adhesive sheets of Examples 14 and 15 were soft, with a shear storage modulus G1 at -10°C of 100 kPa or less, contained oligomers with Tg of 40°C or more, and the _δH1 of the HSP of the base polymer and the _δH2 of the HSP of the oligomer satisfied 0.1≦ _δH2 - _δH1 ≦1.3. The adhesive strength F1 of the pressure-sensitive adhesive sheets of Examples 14 and 15 was 7.6 N/20 mm or more, which was greater than the adhesive strength F1 of the pressure-sensitive adhesive sheets of Comparative Examples 10 to 12 (the base polymer was the same as in Examples 14 and 15). In each of the pressure-sensitive adhesive sheets of Examples 14 and 15, the oligomers with Tg of 40°C or more were sufficiently unevenly distributed on the adhesive surface and in the vicinity thereof.

Moreover, each of the pressure-sensitive adhesive sheets of Examples 16 to 19 was soft, with a shear storage modulus G1 at -10°C of 100 kPa or less, contained an oligomer with a Tg of 40°C or more, and the δH1 of the HSP of the base polymer and the δH2 of the HSP of the oligomer satisfied 0.1≦ _δH2 - _δH1 ≦1.3. Such pressure-sensitive adhesive sheets of Examples 16 to 19 had a large adhesive force F1 of 7.9 N/20 mm or more. In each of the pressure-sensitive adhesive sheets of Examples 16 to 19, the oligomer with a Tg of 40°C or more was sufficiently unevenly distributed on the adhesive surface and in the vicinity thereof. The pressure-sensitive adhesive sheet of Example 19 contained two types of oligomers, each of which had a Tg of 40°C or more, and the δH1 of the HSP of the base polymer and the δH2 of the HSP of each oligomer satisfied 0.1≦ _δH2 - _δH1 ≦1.3.

The above invention is provided as an exemplary embodiment of the present invention, but this is merely an example and should not be interpreted as limiting. Modifications of the present invention that are obvious to those skilled in the art are intended to be included in the scope of the claims below.

The optical adhesive sheet of the present invention is suitable for use in light passing areas in flexible devices (e.g., flexible display panels such as foldable display panels and rollable display panels).

10. Adhesive sheet (optical adhesive sheet)
11 First surface 12 Second surface L1, L2 Release liner 21 First member 22 Second member H Thickness direction

Claims (6)

An optical adhesive sheet,
Contains a base polymer and an oligomer having a glass transition temperature of 40° C. or higher,
having a shear storage modulus of 100 kPa or less at -10°C;
The hydrogen bond term δH ₁ of the Hansen solubility parameter of the base polymer and the hydrogen bond term δH ₂ of the Hansen solubility parameter of the oligomer are
An optically adhesive sheet that satisfies 0.1≦δH ₂ - δH ₁ ≦1.3. The optical adhesive sheet according to claim 1, having a haze of 1% or less. The optical adhesive sheet according to claim 1, which has an adhesive strength of 7.6 N/20 mm or more in a peel test under conditions of 25°C, a peel angle of 180°, and a tensile speed of 300 mm/min. In a peel test under conditions of 25°C, a peel angle of 180° and a tensile speed of 300 mm/min, the adhesive strength is F1.
In a peel test under conditions of 25°C, a peel angle of 180° and a tensile speed of 60 mm/min, the adhesive strength is F2.
The optical adhesive sheet according to claim 1 , wherein the ratio of adhesive strength F2 to adhesive strength F1 is 0.5 or more and 1.1 or less. The optical adhesive sheet according to claim 1, in which the ratio of the shear storage modulus at 60°C to the shear storage modulus at -10°C is 0.2 or more and 1.0 or less. The optical adhesive sheet according to any one of claims 1 to 5, having a gel fraction of 60% by mass or more and 87% by mass or less.