JP7579669B2 - Adhesive sheet - Google Patents

Description

The present invention relates to an adhesive sheet suitable for use in displays, etc.

In recent years, bendable displays have been proposed as displays for electronic devices, which are a type of device. In addition to displays that are curved only once, repeatedly bent displays that are intended to be repeatedly bent (folded) have also been proposed.

In the above-mentioned repeatedly bendable display, it is conceivable that one bendable member (flexible member) constituting the flexible display and another flexible member are bonded together by the adhesive layer of an adhesive sheet. However, when a conventional adhesive sheet is used in a repeatedly bendable display, a problem occurs in that peeling occurs at the interface between the adhesive layer and the adherend in the bent portion.

Patent document 1 discloses an adhesive designed to prevent peeling during repeated bending.

JP 2016-108555 A

On the other hand, in a repeatedly bent display such as the one described above, light leakage may occur around the bent portion after a long period of bending. The adhesive described in Patent Document 1 cannot adequately solve the problem of light leakage.

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide an adhesive sheet that can suppress light leakage caused by a long-term bending state.

In order to achieve the above object, first, the present invention provides an adhesive sheet having at least an adhesive layer, characterized in that the refractive index of the adhesive constituting the adhesive layer is 1.55 or more, and the gel fraction of the adhesive is 30% or more and 100% or less (Invention 1).

According to the above invention (Invention 1), when a laminate in which one member (particularly a flexible member) is bonded to another member (particularly a flexible member) by the pressure-sensitive adhesive layer is left in a bent state for a long period of time, light leakage at the bent portion is suppressed.

In the above invention (Invention 1), it is preferable that the haze value of the pressure-sensitive adhesive layer is 10% or less (Invention 2).

In the above inventions (Inventions 1 and 2), it is preferable that the total light transmittance of the pressure-sensitive adhesive layer is 70% or more (Invention 3).

In the above inventions (Inventions 1 to 3), it is preferable that the storage modulus G' of the adhesive at 23°C is 0.05 MPa or more and 20 MPa or less (Invention 4).

In the above inventions (Inventions 1 to 4), it is preferable that the storage modulus G' of the pressure-sensitive adhesive at 80°C is 0.01 MPa or more and 15 MPa or less (Invention 5).

In the above inventions (Inventions 1 to 5), it is preferable that the weight average molecular weight of the sol portion of the adhesive is 10,000 or more and 2,000,000 or less (Invention 6).

In the above inventions (Inventions 1 to 6), it is preferable that the adhesive contains high refractive index fine particles (Invention 7).

In the above inventions (Inventions 1 to 7), it is preferable that the adhesive is an acrylic adhesive (Invention 8).

In the above invention (Invention 8), it is preferable that the acrylic adhesive contains a (meth)acrylic acid ester polymer, and that the (meth)acrylic acid ester polymer contains an aromatic ring-containing monomer as a monomer unit constituting the polymer (Invention 9).

In the above inventions (Inventions 1 to 9), it is preferable that the adhesive layer is provided with two release sheets and is sandwiched between the release sheets so as to be in contact with the release surfaces of the two release sheets (Invention 10).

The adhesive sheets according to the above inventions (Inventions 1 to 10) are preferably used for displays (Invention 11).

The adhesive sheet of the present invention can suppress light leakage caused by bending for long periods of time.

1 is a cross-sectional view of a pressure-sensitive adhesive sheet according to one embodiment of the present invention. 1 is a cross-sectional view of a display according to an embodiment of the present invention. FIG. 1 is an explanatory diagram (side view) illustrating a static bending test.

Hereinafter, an embodiment of the present invention will be described.
[Adhesive sheet]
The pressure-sensitive adhesive sheet according to one embodiment of the present invention has at least a pressure-sensitive adhesive layer, and preferably has a release sheet laminated on one or both sides of the pressure-sensitive adhesive layer.

The refractive index of the adhesive constituting the adhesive layer is preferably 1.55 or more, and the gel fraction of the adhesive is preferably 30% or more and 100% or less. This suppresses light leakage at the bent portion when a laminate formed by bonding one member (particularly a flexible member) to another member (particularly a flexible member) with the adhesive layer is left in a bent state for a long period of time.

In this specification, the refractive index of the adhesive is a value measured in accordance with JIS K0062-1992. The method for measuring the gel fraction is as shown in the test example described later. In this specification, the haze value is a value measured in accordance with JIS K7136:2000.

From the viewpoint of suppressing light leakage, the refractive index of the adhesive is preferably 1.55 or more, more preferably 1.57 or more, particularly preferably 1.59 or more, and even more preferably 1.60 or more. There is no particular upper limit to the refractive index of the adhesive, but it is usually preferably 3.00 or less, more preferably 2.00 or less, particularly preferably 1.80 or less, and even more preferably 1.70 or less.

From the viewpoint of achieving an excellent appearance after long-term bending, the gel fraction of the adhesive is preferably 30% or more, more preferably 50% or more, particularly preferably 70% or more, and even more preferably 75% or more. From the same viewpoint as above, the gel fraction of the adhesive is preferably 100% or less, more preferably 95% or less, particularly preferably 90% or less, and even more preferably 85% or less.

The haze value of the pressure-sensitive adhesive layer is preferably 10% or less, more preferably 7% or less, particularly preferably 3% or less, and even more preferably 1% or less. If the haze value is as low as above, the pressure-sensitive adhesive layer can suppress changes in appearance due to long-term bending, even though the pressure-sensitive adhesive layer has a very high refractive index. The lower limit of the haze value of the pressure-sensitive adhesive layer is not particularly limited, but from the viewpoint of making it easier to meet the refractive index of the pressure-sensitive adhesive, it is preferably 0.01% or more, more preferably 0.1% or more, particularly preferably 0.3% or more, and even more preferably 0.5% or more.

From the viewpoint of visibility of the display, the total light transmittance of the pressure-sensitive adhesive layer is preferably 70% or more, more preferably 80% or more, particularly preferably 90% or more, and even more preferably 97% or more. The upper limit of the total light transmittance of the pressure-sensitive adhesive layer is not particularly limited, and may be 100% or may be a value slightly exceeding 100% due to the measurement relationship. The total light transmittance in this specification is a value measured in accordance with JIS K7361-1:1997.

On the other hand, the storage modulus G' of the pressure-sensitive adhesive at 23°C is preferably 0.05 MPa or more, more preferably 0.10 MPa or more, particularly preferably 0.30 MPa or more, even more preferably 1.0 MPa or more, and most preferably 2.0 MPa or more. The storage modulus G' of the pressure-sensitive adhesive at 23°C is preferably 20 MPa or less, more preferably 10 MPa or less, particularly preferably 5 MPa or less, and even more preferably 3.5 MPa or less. When the storage modulus G' of the pressure-sensitive adhesive at 23°C is in the above range, the light leakage resistance and the appearance after long-term bending are more excellent. The storage modulus G' in this specification is a value measured by a torsional shear method at a measurement frequency of 1 Hz in accordance with JIS K7244-6. Specifically, it is as shown in the test example described later.

The storage modulus G' of the adhesive at 80°C is preferably 0.01 MPa or more, more preferably 0.03 MPa or more, particularly preferably 0.08 MPa or more, and even more preferably 0.14 MPa or more. The storage modulus G' of the adhesive at 80°C is preferably 15 MPa or less, more preferably 10 MPa or less, particularly preferably 5 MPa or less, and even more preferably 2 MPa or less. When the storage modulus G' of the adhesive at 80°C is in the above range, the light leakage resistance and the appearance after long-term bending are improved.

The weight average molecular weight of the sol content of the pressure-sensitive adhesive is preferably 10,000 or more, more preferably 40,000 or more, particularly preferably 80,000 or more, and even more preferably 100,000 or more. The weight average molecular weight of the sol content of the pressure-sensitive adhesive is preferably 2,000,000 or less, more preferably 1,000,000 or less, particularly preferably 500,000 or less, and even more preferably 300,000 or less. Since the weight average molecular weight of the sol content of the pressure-sensitive adhesive is within the above range and the gel fraction is within the above range, the pressure-sensitive adhesive has a superior appearance after long-term bending and is easy to achieve a low haze value.

In this specification, the weight average molecular weight of the sol component is a value measured by gel permeation chromatography (GPC) in terms of standard polystyrene. The specific measurement method is as shown in the test examples described later.

FIG. 1 shows a specific configuration of one example of the pressure-sensitive adhesive sheet according to this embodiment.
As shown in Fig. 1, an adhesive sheet 1 according to one embodiment is composed of two release sheets 12a and 12b, and an adhesive layer 11 sandwiched between the two release sheets 12a and 12b so as to be in contact with the release surfaces of the two release sheets 12a and 12b. Note that the release surface of the release sheet in this specification refers to a surface of the release sheet that has releasability, and includes both a surface that has been subjected to a release treatment and a surface that exhibits releasability even without being subjected to a release treatment.

1. Component 1-1. Adhesive Layer The adhesive layer 11 is preferably made of an adhesive having the above-mentioned physical properties. The type of adhesive constituting the adhesive layer 11 is not particularly limited, and may be, for example, any of an acrylic adhesive, a polyester adhesive, a polyurethane adhesive, a rubber adhesive, a silicone adhesive, and the like. The adhesive may be any of an emulsion type, a solvent type, or a solventless type, and may be any of a crosslinked type or a non-crosslinked type. Among them, an acrylic adhesive is preferred because it is easy to satisfy the above-mentioned physical properties and has excellent adhesive properties and optical properties, and in particular, a solvent-type acrylic adhesive is preferred, and a crosslinked acrylic adhesive is more preferred.

The adhesive in this embodiment may be either non-curable with active energy rays or curable with active energy rays. Furthermore, the adhesive in this embodiment preferably contains high refractive index fine particles in order to satisfy the refractive index described above. In this specification, high refractive index fine particles refer to fine particles that can impart a higher refractive index than the matrix to the resulting adhesive.

The adhesive in this embodiment is preferably a crosslinked adhesive composition (hereinafter sometimes referred to as "adhesive composition P") containing a (meth)acrylic acid ester polymer (A), a crosslinking agent (B), high refractive index fine particles (C), and optionally an active energy ray curable component (D). In addition, when the active energy ray curable component (D) is contained, it is preferable that the composition is cured by irradiation with active energy rays. The adhesive obtained by crosslinking (and curing) the adhesive composition P is likely to satisfy the above-mentioned physical properties. In this specification, (meth)acrylic acid means both acrylic acid and methacrylic acid. The same applies to other similar terms. In addition, the concept of "copolymer" is also included in "polymer".

(1) Component of Pressure-Sensitive Adhesive Composition P (1-1) (Meth)acrylic Acid Ester Polymer (A)
The (meth)acrylic acid ester polymer (A) preferably contains, as a monomer unit constituting the polymer, a (meth)acrylic acid alkyl ester for exhibiting good adhesion and a monomer having a reactive functional group (reactive functional group-containing monomer) which serves as a crosslinking point. In addition, the (meth)acrylic acid ester polymer (A) preferably further contains, as a monomer unit constituting the polymer, a monomer (high refractive index monomer) which can increase the refractive index of the matrix of the obtained adhesive. By containing a high refractive index monomer, the difference between the refractive index of the matrix of the adhesive and the refractive index of the high refractive index fine particles (C) can be reduced, and the above-mentioned haze value is easily satisfied. Therefore, it is more preferable that the (meth)acrylic acid ester polymer (A) contains, as a monomer unit constituting the polymer, a (meth)acrylic acid alkyl ester, a high refractive index monomer, and a reactive functional group-containing monomer.

The (meth)acrylic acid alkyl ester preferably includes a (meth)acrylic acid alkyl ester having an alkyl group with 1 to 20 carbon atoms. The alkyl group is preferably linear or branched.

Examples of (meth)acrylic acid alkyl esters having an alkyl group with 1 to 20 carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, n-decyl (meth)acrylate, n-dodecyl (meth)acrylate, myristyl (meth)acrylate, palmityl (meth)acrylate, and stearyl (meth)acrylate. Among these, from the viewpoint of the storage modulus G' and adhesive strength described above, (meth)acrylic acid esters having an alkyl group with 1 to 10 carbon atoms are preferred, and (meth)acrylic acid esters having an alkyl group with 4 to 8 carbon atoms are particularly preferred. Specifically, n-butyl acrylate, isooctyl acrylate, or 2-ethylhexyl acrylate are preferred. These may be used alone or in combination of two or more.

From the viewpoint of obtaining a desired adhesive strength or from the viewpoint of lowering the storage modulus G', the (meth)acrylic acid ester polymer (A) preferably contains 5% by mass or more of (meth)acrylic acid alkyl ester having 1 to 20 carbon atoms in the alkyl group, more preferably 10% by mass or more, particularly preferably 15% by mass or more, and even more preferably 20% by mass or more, as a monomer unit constituting the polymer. Also, from the viewpoint of ensuring the blending amount of other monomer components, the (meth)acrylic acid alkyl ester is preferably contained in an amount of 60% by mass or less, more preferably 50% by mass or less, particularly preferably 40% by mass or less, and even more preferably 30% by mass or less.

Preferred examples of high refractive index monomers include monomers having a cyclic structure, and in particular, monomers having an aromatic ring (aromatic ring-containing monomers). Examples of aromatic ring-containing monomers include monomers containing one or more aromatic rings, and monomers having a polycyclic aromatic skeleton. Among these, aromatic ring-containing monomers containing one aromatic ring are preferred from the viewpoint of lowering the storage modulus G' and increasing the adhesive strength.

Examples of aromatic ring-containing monomers containing one aromatic ring include phenyl (meth)acrylate, 2-phenylethyl (meth)acrylate, benzyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, phenoxybutyl (meth)acrylate, styryl (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, and phenoxypolyethylene glycol (meth)acrylate. These may be used alone or in combination of two or more.

The (meth)acrylic acid ester polymer (A) preferably contains 20% by mass or more of aromatic ring-containing monomer as a monomer unit constituting the polymer, more preferably 30% by mass or more, particularly preferably 40% by mass or more, and even more preferably 50% by mass or more. In addition, it is preferable that the aromatic ring-containing monomer is contained in an amount of 90% by mass or less, more preferably 85% by mass or less, particularly preferably 80% by mass or less, and even more preferably 70% by mass or less. By containing the aromatic ring-containing monomer in the above range, it is presumed that the refractive index difference between the obtained (meth)acrylic acid ester polymer (A) and the high refractive index fine particles (C) will be in a suitable range, and it becomes easier to obtain an adhesive having a low haze value and a high refractive index.

The (meth)acrylic acid ester polymer (A) contains a reactive functional group-containing monomer as a monomer unit constituting the polymer, and reacts with the crosslinking agent (B) described below via the reactive functional group derived from the reactive functional group-containing monomer, thereby forming a crosslinked structure (three-dimensional network structure) and obtaining an adhesive with the desired cohesive strength. The adhesive is likely to satisfy the gel fraction and storage modulus G' described above.

Preferable examples of reactive functional group-containing monomers contained in the (meth)acrylic acid ester polymer (A) as monomer units constituting the polymer include monomers having a hydroxyl group in the molecule (hydroxyl group-containing monomers), monomers having a carboxyl group in the molecule (carboxyl group-containing monomers), and monomers having an amino group in the molecule (amino group-containing monomers). These reactive functional group-containing monomers may be used alone or in combination of two or more.

Among the above reactive functional group-containing monomers, from the viewpoint of improving the dispersibility of the high refractive index fine particles (C) and giving the resulting adhesive a low haze value, hydroxyl group-containing monomers or carboxyl group-containing monomers are preferred, and hydroxyl group-containing monomers are particularly preferred.

Examples of hydroxyl group-containing monomers include (meth)acrylic acid hydroxyalkyl esters such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. Among the above, from the viewpoint of ease of satisfying the physical property related to the storage modulus G' described above, a (meth)acrylic acid hydroxyalkyl ester having a hydroxyalkyl group having 1 to 4 carbon atoms is preferred. Specifically, for example, 2-hydroxyethyl acrylate or 4-hydroxybutyl acrylate is preferred. These may be used alone or in combination of two or more.

Examples of carboxyl group-containing monomers include ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, and citraconic acid. Among these, acrylic acid is preferred from the viewpoint of the adhesive strength of the resulting (meth)acrylic acid ester polymer (A). These may be used alone or in combination of two or more.

The (meth)acrylic acid ester polymer (A) preferably contains 2% by mass or more of reactive functional group-containing monomer as a monomer unit constituting the polymer, more preferably 4% by mass or more, particularly preferably 8% by mass or more, and even more preferably 12% by mass or more. The (meth)acrylic acid ester polymer (A) preferably contains 34% by mass or less of reactive functional group-containing monomer as a monomer unit constituting the polymer, more preferably 28% by mass or less, particularly preferably 24% by mass or less, and even more preferably 18% by mass or less. When the (meth)acrylic acid ester polymer (A) contains the reactive functional group-containing monomer as a monomer unit in the above amount, the cohesive force of the resulting adhesive becomes appropriate due to the crosslinking reaction with the crosslinking agent (B), and the gel fraction and storage modulus G' described above are more easily satisfied. In addition, although the reason is unclear, it is presumed that the dispersibility of the high refractive index fine particles (C) is improved, and the haze value described above is more easily satisfied.

It is also preferable that the (meth)acrylic acid ester polymer (A) does not contain a carboxyl group-containing monomer as a monomer unit constituting the polymer. If a carboxyl group-containing monomer is contained, the coating liquid of the adhesive composition may gel in relation to the high refractive index fine particles (C), resulting in poor coating.

Here, "free of carboxyl group-containing monomers" means that the polymer does not substantially contain carboxyl group-containing monomers, and in addition to not containing any carboxyl group-containing monomers at all, it is acceptable to contain carboxyl group-containing monomers to the extent that gelation does not occur. Specifically, it is acceptable to contain carboxyl group-containing monomers as monomer units in the (meth)acrylic acid ester polymer (A) in an amount of 0.1% by mass or less, preferably 0.01% by mass or less, and more preferably 0.001% by mass or less.

If desired, the (meth)acrylic acid ester polymer (A) may contain other monomers as monomer units constituting the polymer. As the other monomer, a monomer that does not contain a reactive functional group is preferable so as not to inhibit the above-mentioned action of the reactive functional group-containing monomer. Examples of such monomers include non-reactive nitrogen atom-containing monomers such as N-acryloylmorpholine and N-vinyl-2-pyrrolidone, (meth)acrylic acid alkoxyalkyl esters such as methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate, vinyl acetate, and styrene. These may be used alone or in combination of two or more.

The polymerization form of the (meth)acrylic acid ester polymer (A) may be a random copolymer or a block copolymer.

The weight average molecular weight of the (meth)acrylic acid ester polymer (A) is preferably 200,000 or more, more preferably 400,000 or more, particularly preferably 600,000 or more, and even more preferably 750,000 or more. The weight average molecular weight of the (meth)acrylic acid ester polymer (A) is preferably 2,000,000 or less, more preferably 1,500,000 or less, particularly preferably 1,000,000 or less, and even more preferably 900,000 or less. When the weight average molecular weight of the (meth)acrylic acid ester polymer (A) is within the above range, the gel fraction, the weight average molecular weight of the sol, the storage modulus G', and the adhesive force described below are easily satisfied. In addition, since the upper limit of the weight average molecular weight of the (meth)acrylic acid ester polymer (A) is as described above, the haze value described above is easily satisfied. The weight average molecular weight of the polymer in this specification is a value measured by gel permeation chromatography (GPC) in terms of standard polystyrene.

The refractive index of the (meth)acrylic acid ester polymer (A) is preferably 1.40 or more, more preferably 1.46 or more, particularly preferably 1.48 or more, and even more preferably 1.50 or more. This makes it easier to meet the above-mentioned haze value. The upper limit of the (meth)acrylic acid ester polymer (A) is not particularly limited, but is usually preferably 2.00 or less, more preferably 1.80 or less, particularly preferably 1.60 or less, and even more preferably 1.55 or less. If the refractive index of the (meth)acrylic acid ester polymer (A) is within the above range, it is estimated that the refractive index difference with the high refractive index fine particles (C) is optimized, and it is easy to obtain a pressure-sensitive adhesive with low haze and high refractive index. The refractive index of the (meth)acrylic acid ester polymer (A) in this specification is a value measured in accordance with JIS K0062-1992.

In the adhesive composition P, the (meth)acrylic acid ester polymer (A) may be used alone or in combination of two or more.

(1-2) Crosslinking agent (B)
The crosslinking agent (B) crosslinks the (meth)acrylic acid ester polymer (A) and forms a three-dimensional network structure when the pressure-sensitive adhesive composition P containing the crosslinking agent (B) is heated or the like, which is a trigger. This improves the cohesive strength of the resulting pressure-sensitive adhesive, and makes it easier to satisfy the above-mentioned gel fraction, storage modulus G', and molecular weight of the sol fraction.

The crosslinking agent (B) may be any that reacts with the reactive group of the (meth)acrylic acid ester polymer (A), and examples thereof include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, amine-based crosslinking agents, melamine-based crosslinking agents, aziridine-based crosslinking agents, hydrazine-based crosslinking agents, aldehyde-based crosslinking agents, oxazoline-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, and ammonium salt-based crosslinking agents. Among the above, it is preferable to use an isocyanate-based crosslinking agent that has excellent reactivity with the reactive functional group-containing monomer. The crosslinking agent (B) may be used alone or in combination of two or more.

The isocyanate-based crosslinking agent contains at least a polyisocyanate compound. Examples of polyisocyanate compounds include aromatic polyisocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate, and xylylene diisocyanate, aliphatic polyisocyanates such as hexamethylene diisocyanate, alicyclic polyisocyanates such as isophorone diisocyanate and hydrogenated diphenylmethane diisocyanate, and their biurets, isocyanurates, and adducts which are reactants with low-molecular active hydrogen-containing compounds such as ethylene glycol, propylene glycol, neopentyl glycol, trimethylolpropane, and castor oil. Among these, trimethylolpropane-modified aromatic polyisocyanates, particularly trimethylolpropane-modified tolylene diisocyanate or trimethylolpropane-modified xylylene diisocyanate, are preferred from the viewpoint of reactivity with hydroxyl groups.

The content of the crosslinking agent (B) in the adhesive composition P is preferably 0.001 parts by mass or more, more preferably 0.01 parts by mass or more, and even more preferably 0.1 parts by mass or more, per 100 parts by mass of the (meth)acrylic acid ester polymer (A). The content is also preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 1 part by mass or less. When the content of the crosslinking agent (B) is within the above range, the gel fraction, storage modulus G', and molecular weight of the sol fraction described above are more easily satisfied.

(1-3) High refractive index fine particles (C)
The high refractive index fine particles (C) are not particularly limited as long as they can give the obtained pressure sensitive adhesive a higher refractive index than the (meth)acrylic acid ester polymer (A) and can satisfy the above-mentioned refractive index.Such high refractive index fine particles (C) include metal oxide fine particles, as well as fine particles using high refractive index resin such as styrene resin and sulfur-containing resin.Among these, metal oxide fine particles are preferred from the viewpoint of ease of satisfying the above-mentioned refractive index, and metal oxide fine particles containing transition metal are particularly preferred.

Preferred examples of metal oxide fine particles containing a transition metal include fine particles of zirconium oxide, titanium oxide, barium titanate, tantalum oxide, zinc oxide, hafnium oxide, cerium oxide, and niobium oxide. These may be used alone or in combination of two or more. Among the above, zirconium oxide fine particles are particularly preferred because they easily satisfy the refractive index described above.

The surface of the metal oxide fine particles may be chemically modified with an organic compound or the like. The shape of the metal oxide fine particles may be spherical or non-spherical, but from the viewpoint of easily satisfying the refractive index described above, spherical shape is preferable.

The average particle size of the high refractive index particles (C) is preferably 1000 nm or less, and more preferably 700 nm or less. This makes it easier to meet the refractive index and haze values described above. The average particle size of the high refractive index particles (C) is preferably 2 nm or more, and more preferably 10 nm or more. This improves dispersibility and makes it easier to meet the storage modulus G' described above. The average particle size of the high refractive index particles (C) is determined by measuring the primary particle size using a zeta potential measurement method.

It is preferable to use the high refractive index fine particles (C) as a dispersion solution, taking into consideration the dispersibility in the adhesive composition P. The dispersion solvent used in this case is preferably a hydrophobic solvent, and it is preferable that the dispersion solvent does not contain a hydrophilic solvent or contains only a small amount of a hydrophilic solvent. This makes it easier to reduce the haze value of the resulting adhesive.

The hydrophobic solvent preferably has an SP value (solubility parameter) of 12 (cal/cm ³ ) ^1/2 or less. By using such a solvent, the high refractive index fine particles (C), particularly the metal oxide fine particles, can be well dispersed in the adhesive composition P. The SP value of the hydrophobic solvent is more preferably 10 (cal/cm ³ ) ^1/2 or less, and particularly preferably 9.5 (cal/cm ³ ) ^1/2 or less. The lower limit of the SP value of the hydrophobic solvent is not particularly limited, but is preferably 6.5 (cal/cm ³ ) ^1/2 or more from the viewpoint of availability.

Examples of solvents having an SP value of 12 (cal/cm ³ ) ^1/2 or less include amides such as dimethylformamide (12.0); nitriles such as acetonitrile (11.8); ketones such as acetone (10.0), methyl ethyl ketone (9.3), methyl propyl ketone (8.7), and methyl isopropyl ketone (8.5); esters (e.g., carboxylate esters) such as di-n-butyl phthalate (9.4), ethyl acetate (9.1), n-butyl acetate (8.5), isopropyl acetate (8.4), and isobutyl acetate (8.3); dioxane (9.9), tetrahydrofuran (9.1), and diethyl ether (9.2). Examples of the solvent include linear and cyclic ethers such as benzene (9.2), toluene (8.9), xylene (8.8), ethylbenzene (8.8), cyclohexane (8.2), n-octane (7.6), n-hexane (7.3), n-pentane (7.0), and other aromatic and aliphatic hydrocarbons; halogenated hydrocarbons (e.g., halogenated aliphatic hydrocarbons) such as methylene chloride (9.7), chloroform (9.3), trichloroethylene (9.2), carbon tetrachloride (8.6), and other carbon sulfides such as carbon disulfide (10.0). The numerical values in parentheses next to the solvent names refer to the SP values ((cal/cm ³ ) ^1/2 ).

When the dispersion solvent for the high refractive index fine particles (C) contains a hydrophilic solvent (having an SP value of more than 12 (cal/cm ³ ) ^1/2 ), the content thereof is preferably 15 mass % or less, more preferably 10 mass % or less, and even more preferably 8 mass % or less.

A dispersant may be used to further improve the dispersibility of the high refractive index fine particles (C). From the viewpoint of compatibility with the (meth)acrylic acid ester polymer (A), the dispersant preferably contains an acrylic component. As an example, it is also preferable to use the active energy ray curable component (D) (and a photopolymerization initiator) described below as the dispersant. When the high refractive index fine particles (C) are used as a dispersion solution containing such a dispersant, the dispersion solution is called a high refractive index component.

From the viewpoint that the resulting adhesive easily meets the refractive index described above, the refractive index of the high refractive index particles (C) is preferably 1.5 or more, more preferably 1.6 or more, particularly preferably 1.7 or more, and even more preferably 1.8 or more. The upper limit of the refractive index of the high refractive index particles (C) is not particularly limited, but is usually preferably 5.0 or less, particularly preferably 3.0 or less, and even more preferably 2.0 or less. The refractive index of the high refractive index particles (C) is a value measured in accordance with JIS K7142:2014, Method B.

The refractive index of the coating film of the high refractive index component is preferably 1.4 or more, more preferably 1.5 or more, particularly preferably 1.6 or more, and even more preferably 1.7 or more, from the viewpoint that the resulting adhesive easily meets the refractive index described above. The upper limit of the refractive index of the high refractive index component is not particularly limited, but it is usually preferably 3.0 or less, particularly preferably 2.0 or less, and even more preferably 1.8 or less. The refractive index of the coating film of the high refractive index component is a value measured in accordance with JIS K0062-1992.

The content of the high refractive index fine particles (C) in the adhesive composition P is preferably 5% by mass or more, more preferably 20% by mass or more, particularly preferably 35% by mass or more, and even more preferably 45% by mass or more. The content is preferably 90% by mass or less, more preferably 80% by mass or less, particularly preferably 70% by mass or less, and even more preferably 65% by mass or less. When the content of the high refractive index fine particles (C) is within the above range, the refractive index, storage modulus G', and haze value described above are easily satisfied, and good adhesiveness is maintained. In addition, when the high refractive index fine particles (C) are blended in the adhesive composition P as a high refractive index component, the above "content of the high refractive index fine particles (C) in the adhesive composition P" is the "content of the solid content of the high refractive index component in the adhesive composition P (the matrix such as the high refractive index fine particles (C) and the dispersant)".

(1-4) Active energy ray-curable component (D)
In the case where the adhesive composition P in the present embodiment contains the active energy ray curable component (D), in the adhesive obtained by thermally crosslinking and curing the adhesive composition P with active energy rays, it is presumed that the active energy ray curable components (D) polymerize with each other, and the polymerized active energy ray curable components (D) are entangled with the crosslinked structure (three-dimensional network structure) of the (meth)acrylic acid ester polymer (A). An adhesive having such a higher order structure is likely to satisfy the above-mentioned gel fraction and storage modulus G'.

The active energy ray curable component (D) is not particularly limited as long as it is a component that cures by irradiation with active energy rays and provides the above-mentioned effects, and may be any of a monomer, oligomer, or polymer, or a mixture thereof. Among them, a polyfunctional acrylate monomer, oligomer, or polymer is preferable, and a mixture thereof may be used. In addition, by containing at least a polyfunctional acrylate monomer, the active energy ray curable component (D) can be used as a dispersant for the high refractive index fine particles (C).

(1-5) Various Additives Various additives that are commonly used in acrylic adhesives, such as silane coupling agents, photopolymerization initiators, tackifiers, UV absorbers, antistatic agents, antioxidants, light stabilizers, softeners, and fillers, may be added to the adhesive composition P as desired. The active energy ray curable component (D) that is not included in the refractive index increasing component may be included in the additives referred to here. The polymerization solvent and dilution solvent described below are not included in the additives that constitute the adhesive composition P.

It is preferable that the adhesive composition P contains the above-mentioned silane coupling agent. This improves the adhesion of the resulting adhesive layer to the flexible member that is the adherend, resulting in more preferable adhesive strength.

As the silane coupling agent, an organosilicon compound having at least one alkoxysilyl group in the molecule, which has good compatibility with the (meth)acrylic acid ester polymer (A) and has optical transparency, is preferred.

Examples of such silane coupling agents include polymerizable unsaturated group-containing silicon compounds such as vinyltrimethoxysilane, vinyltriethoxysilane, and methacryloxypropyltrimethoxysilane; silicon compounds having an epoxy structure such as 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, and 3-mercaptopropyldimethoxymethylsilane; Examples of such compounds include capryl group-containing silicon compounds, amino group-containing silicon compounds such as 3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane, 3-chloropropyltrimethoxysilane, and 3-isocyanatepropyltriethoxysilane, as well as condensates of at least one of these compounds with alkyl group-containing silicon compounds such as methyltriethoxysilane, ethyltriethoxysilane, methyltrimethoxysilane, and ethyltrimethoxysilane. These compounds may be used alone or in combination of two or more.

The content of the silane coupling agent in the adhesive composition P is preferably 0.01 parts by mass or more, more preferably 0.05 parts by mass or more, and even more preferably 0.1 parts by mass or more, per 100 parts by mass of the (meth)acrylic acid ester polymer (A). The content is also preferably 10 parts by mass or less, more preferably 1 part by mass or less, and even more preferably 0.5 parts by mass or less. When the content of the silane coupling agent is within the above range, the adhesive layer obtained has improved adhesion to the flexible member that is the adherend, and has greater adhesive strength.

When the adhesive composition P contains an active energy ray-curable component (D) and ultraviolet light is used as the active energy ray for curing the adhesive composition P, it is preferable that the adhesive composition P further contains a photopolymerization initiator. As the photopolymerization initiator, a conventionally known one can be used. The content of the photopolymerization initiator in the adhesive composition P is about 0.1 to 30 parts by mass per 100 parts by mass of the active energy ray-curable component (D).

It is also preferable that the adhesive composition P contains a tackifier. This is because the inclusion of a tackifier makes it easier to obtain a desired adhesive strength even in a thin adhesive layer.

Examples of tackifiers include natural resins such as rosin resins and polyterpene resins, petroleum resins such as C5, C9, and dicyclopentadiene resins, and synthetic resins such as coumarone-indene resins and xylene resins. These may be used alone or in combination of two or more.

When the adhesive composition P contains a tackifier, the content of the tackifier in the adhesive composition P is preferably 1 part by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, per 100 parts by mass of the (meth)acrylic acid ester polymer (A). The content is also preferably 90 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 30 parts by mass or less.

(2) Production of adhesive composition P The adhesive composition P can be produced by producing a (meth)acrylic acid ester polymer (A), mixing the obtained (meth)acrylic acid ester polymer (A) with a crosslinking agent (B) and high refractive index fine particles (C) (or a component for increasing refractive index), and adding additives as desired.

The (meth)acrylic acid ester polymer (A) can be produced by polymerizing a mixture of monomers constituting the polymer by a normal radical polymerization method. The polymerization of the (meth)acrylic acid ester polymer (A) is preferably carried out by a solution polymerization method using a polymerization initiator as desired. By polymerizing the (meth)acrylic acid ester polymer (A) by a solution polymerization method, it becomes easy to increase the molecular weight of the obtained polymer and adjust the molecular weight distribution, and it becomes possible to reduce the production of low molecular weight substances. Therefore, even if the gel fraction is relatively small and the degree of crosslinking is relaxed, the adhesive is unlikely to become biased due to bending, and an adhesive with excellent peel resistance is easily obtained.

Polymerization solvents used in the solution polymerization method include, for example, ethyl acetate, n-butyl acetate, isobutyl acetate, toluene, acetone, hexane, methyl ethyl ketone, etc., and two or more types may be used in combination.

Examples of polymerization initiators include azo compounds and organic peroxides, and two or more of them may be used in combination. Examples of azo compounds include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane 1-carbonitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile), dimethyl 2,2'-azobis(2-methylpropionate), 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2-hydroxymethylpropionitrile), and 2,2'-azobis[2-(2-imidazolin-2-yl)propane].

Examples of organic peroxides include benzoyl peroxide, t-butyl perbenzoate, cumene hydroperoxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, di(2-ethoxyethyl)peroxydicarbonate, t-butyl peroxyneodecanoate, t-butyl peroxypivalate, (3,5,5-trimethylhexanoyl)peroxide, dipropionyl peroxide, and diacetyl peroxide.

In addition, in the above polymerization process, the weight average molecular weight of the resulting polymer can be adjusted by adding a chain transfer agent such as 2-mercaptoethanol.

Once the (meth)acrylic acid ester polymer (A) is obtained, the crosslinker (B), high refractive index fine particles (C) (or high refractive index component), and optionally additives and dilution solvent are added to the solution of the (meth)acrylic acid ester polymer (A) and thoroughly mixed to obtain a solvent-diluted adhesive composition P (coating solution).

When any of the above components is in a solid state, or when precipitation occurs when the component is mixed with other components in an undiluted state, the component may be dissolved or diluted in a dilution solvent before mixing with the other components.

Examples of the dilution solvent include aliphatic hydrocarbons such as hexane, heptane, and cyclohexane; aromatic hydrocarbons such as toluene and xylene; halogenated hydrocarbons such as methylene chloride and ethylene chloride; alcohols such as methanol, ethanol, propanol, butanol, and 1-methoxy-2-propanol; ketones such as acetone, methyl ethyl ketone, 2-pentanone, isophorone, and cyclohexanone; esters such as ethyl acetate and butyl acetate; and cellosolve-based solvents such as ethyl cellosolve.

The concentration and viscosity of the coating solution thus prepared are not particularly limited as long as they are within the range that allows coating, and can be appropriately selected depending on the situation. For example, the adhesive composition P is diluted so that the concentration is 10 to 60 mass %. Note that the addition of a dilution solvent or the like is not a necessary condition for obtaining the coating solution, and if the adhesive composition P has a viscosity that allows coating, it is not necessary to add a dilution solvent. In this case, the adhesive composition P becomes a coating solution in which the polymerization solvent for the (meth)acrylic acid ester polymer (A) itself serves as the dilution solvent.

(3) Manufacture of adhesive The adhesive constituting the adhesive layer 11 in this embodiment is preferably obtained by crosslinking the adhesive composition P. The crosslinking of the adhesive composition P can usually be carried out by a heat treatment. This heat treatment can also serve as a drying treatment for volatilizing a diluting solvent and the like from a coating film of the adhesive composition P applied to a desired object.

The heating temperature for the heat treatment is preferably 50 to 150°C, and more preferably 70 to 120°C. The heating time is preferably 10 seconds to 10 minutes, and more preferably 50 seconds to 2 minutes.

After the heat treatment, a curing period of about 1 to 2 weeks may be provided at room temperature (e.g., 23°C, 50% RH) if necessary. If this curing period is required, the adhesive will be formed after the curing period has elapsed. If no curing period is required, the adhesive will be formed after the heat treatment has been completed.

By the above heat treatment (and curing), the (meth)acrylic acid ester polymer (A) is sufficiently crosslinked via the crosslinking agent (B) to form a crosslinked structure, and an adhesive is obtained.

When the adhesive composition P contains an active energy ray-curable component (D), the adhesive constituting the adhesive layer 11 is preferably obtained by thermally crosslinking and active energy ray-curing the adhesive composition P. In this case, it is preferable to cure the adhesive composition P by irradiating it with active energy rays after carrying out the heat treatment as described above.

Here, active energy rays refer to electromagnetic waves or charged particle beams that have an energy quantum, and specific examples include ultraviolet rays and electron beams. Among active energy rays, ultraviolet rays are particularly preferred because they are easy to handle.

The irradiation with ultraviolet rays can be performed by a high pressure mercury lamp, a fusion H lamp, a xenon lamp, or the like, and the irradiation amount of the ultraviolet rays is preferably about 50 to 1000 mW/cm ² in terms of illuminance. The light amount is preferably 50 to 10000 mJ/cm ² , more preferably 80 to 5000 mJ/cm ² , and particularly preferably 200 to 2000 mJ/cm ^2. On the other hand, the irradiation with electron beams can be performed by an electron beam accelerator, or the like, and the irradiation amount of the electron beams is preferably about 10 to 1000 krad.

It is presumed that when the adhesive composition P is irradiated with active energy rays, the active energy ray-curable components (D) polymerize with each other, and the polymerized active energy ray-curable components (D) become entangled in the crosslinked structure of the (meth)acrylic acid ester polymer (A).

It is preferable that the high refractive index fine particles (C) are uniformly dispersed in the adhesive layer 11. When the high refractive index fine particles (C) are uniformly dispersed in the adhesive layer 11, the haze value can be reproducibly suppressed to a low value.

(4) Thickness of the Adhesive Layer The thickness of the adhesive layer 11 in the adhesive sheet 1 according to the present embodiment (measured according to JIS K7130) is preferably 1 μm or more as a lower limit, more preferably 5 μm or more, particularly preferably 10 μm or more, and even more preferably 20 μm or more. When the lower limit of the thickness of the adhesive layer 11 is within the above range, the desired adhesive strength is easily exhibited.

The thickness of the adhesive layer 11 is preferably 1000 μm or less as an upper limit, more preferably 300 μm or less, particularly preferably 80 μm or less, and even more preferably 35 μm or less. When the upper limit of the thickness of the adhesive layer 11 is as described above, the stress generated in the adhesive layer due to bending can be prevented from becoming too large, and light leakage resistance can be improved. The adhesive layer 11 may be formed as a single layer, or multiple layers may be laminated.

The release sheets 12a and 12b protect the adhesive layer 11 until the adhesive sheet 1 is used, and are peeled off when the adhesive sheet 1 (adhesive layer 11) is used. In the adhesive sheet 1 according to this embodiment, one or both of the release sheets 12a and 12b are not necessarily required.

Examples of the release sheets 12a and 12b that can be used include polyethylene films, polypropylene films, polybutene films, polybutadiene films, polymethylpentene films, polyvinyl chloride films, vinyl chloride copolymer films, polyethylene terephthalate films, polyethylene naphthalate films, polybutylene terephthalate films, polyurethane films, ethylene vinyl acetate films, ionomer resin films, ethylene-(meth)acrylic acid copolymer films, ethylene-(meth)acrylic acid ester copolymer films, polystyrene films, polycarbonate films, polyimide films, and fluororesin films. Crosslinked films of these films can also be used. Furthermore, laminated films of these films can also be used.

It is preferable that the release surfaces of the release sheets 12a and 12b (particularly the surfaces in contact with the adhesive layer 11) are subjected to a release treatment. Examples of release agents used in the release treatment include alkyd-based, silicone-based, fluorine-based, unsaturated polyester-based, polyolefin-based, and wax-based release agents. It is preferable that one of the release sheets 12a and 12b is a heavy release type release sheet with a large release force, and the other is a light release type release sheet with a small release force.

There are no particular limitations on the thickness of the release sheets 12a and 12b, but it is usually about 20 to 150 μm.

2. Physical properties (adhesive strength)
The adhesive strength of the adhesive sheet 1 according to the present embodiment to soda lime glass is preferably 0.01 N/25 mm or more as a lower limit, more preferably 0.05 N/25 mm or more, particularly preferably 0.1 N/25 mm or more, and even more preferably 1.0 N/25 mm or more. When the lower limit of the adhesive strength of the adhesive sheet 1 to soda lime glass is as described above, the appearance after long-term bending is more excellent. On the other hand, the upper limit of the adhesive strength is not particularly limited, but reworkability may be required. From this viewpoint, the adhesive strength is preferably 50 N/25 mm or less, more preferably 40 N/25 mm or less, and particularly preferably 20 N/25 mm or less. In addition, the adhesive strength in this specification basically refers to the adhesive strength measured by a 180-degree peeling method in accordance with JIS Z0237:2009, and the specific test method is as shown in the test example described later.

3. Manufacturing of adhesive sheet As an example of manufacturing the adhesive sheet 1, the case where the adhesive composition P is used will be described. A coating solution of the adhesive composition P is applied to the release surface of one release sheet 12a (or 12b), and the adhesive composition P is thermally crosslinked by heat treatment to form a coating layer, and then the release surface of the other release sheet 12b (or 12a) is superimposed on the coating layer, and active energy rays are irradiated as desired. If a curing period is required, a curing period is provided, and if no curing period is required, the coating layer becomes the adhesive layer 11 as it is. This results in the adhesive sheet 1. The conditions of the heat treatment and curing are as described above.

In another example of the production of the adhesive sheet 1, a coating solution of the adhesive composition P is applied to the release surface of one of the release sheets 12a, and a heat treatment is performed to thermally crosslink the adhesive composition P to form a coating layer, thereby obtaining a release sheet 12a with a coating layer. Also, a coating solution of the adhesive composition P is applied to the release surface of the other release sheet 12b, and a heat treatment is performed to thermally crosslink the adhesive composition P to form a coating layer, thereby obtaining a release sheet 12b with a coating layer. Then, the release sheet 12a with the coating layer and the release sheet 12b with the coating layer are bonded together so that both coating layers are in contact with each other, and active energy rays are irradiated as desired. If a curing period is required, a curing period is provided, and if no curing period is required, the laminated coating layer becomes the adhesive layer 11 as it is. This results in the above-mentioned adhesive sheet 1. According to this production example, stable production is possible even when the adhesive layer 11 is relatively thick.

Methods for applying the coating solution of the adhesive composition P include, for example, bar coating, knife coating, roll coating, blade coating, die coating, and gravure coating.

In this embodiment, the adhesive layer 11 is formed as described above, so that the high refractive index fine particles (C) are uniformly dispersed throughout the layer.

[Display body]
By using the pressure-sensitive adhesive sheet 1, various optical members and optical products can be manufactured, and in particular, displays can be manufactured. Examples of the types of displays include organic electroluminescence (organic EL) displays, electrophoretic displays (electronic paper), liquid crystal displays using a plastic substrate (film) as a substrate, and touch panels are also possible. Among the above displays, organic EL displays are particularly preferred. Light extraction efficiency is often required in organic EL displays, and the pressure-sensitive adhesive layer 11 of the pressure-sensitive adhesive sheet 1 has a high refractive index and therefore has excellent light extraction efficiency, making the pressure-sensitive adhesive sheet 1 particularly useful for organic EL displays.

The display may be a bendable display or a display that can be repeatedly bent (foldable display).

An example of a display, specifically an organic EL display, is shown in FIG. 2. The display 3 shown in FIG. 2 is constructed by laminating, from the top, a cover film 31, a first adhesive layer 32, a polarizing film 33, a second adhesive layer 34, a touch sensor film 35, a third adhesive layer 36, an organic EL element 37, a fourth adhesive layer 38, and a TFT substrate 39. The cover film 31, polarizing film 33, touch sensor film 35, organic EL element 37, and TFT substrate 39 may be flexible members that can be bent.

At least one of the first adhesive layer 32, the second adhesive layer 34, the third adhesive layer 36, and the fourth adhesive layer 38 is the adhesive layer 11 of the adhesive sheet 1 described above. It is preferable that two or more of the first adhesive layer 32, the second adhesive layer 34, the third adhesive layer 36, and the fourth adhesive layer 38 are the adhesive layer 11 of the adhesive sheet 1 described above, and it is most preferable that all of the adhesive layers 32, 34, 36, and 38 are the adhesive layer 11 of the adhesive sheet 1.

The display 3 suppresses light leakage due to bending even when left in a bent state for a long period of time.

The above-described embodiments are described to facilitate understanding of the present invention, and are not described to limit the present invention. Therefore, each element disclosed in the above embodiments is intended to include all design modifications and equivalents that fall within the technical scope of the present invention.

For example, either or both of the release sheets 12a and 12b in the adhesive sheet 1 may be omitted, and a desired member (e.g., a flexible member) may be laminated in place of the release sheets 12a and/or 12b.

The present invention will be explained in more detail below with reference to examples, but the scope of the present invention is not limited to these examples.

Example 1
1. Preparation of (meth)acrylic acid ester polymer (A) 25 parts by mass of n-butyl acrylate, 60 parts by mass of 2-phenoxyethyl acrylate, and 15 parts by mass of 2-hydroxyethyl acrylate were copolymerized by a solution polymerization method to prepare a (meth)acrylic acid ester polymer (A). The molecular weight of this (meth)acrylic acid ester polymer (A) was measured by the method described below, and the weight average molecular weight (Mw) was 800,000.

2. Preparation of adhesive composition 100 parts by mass (solid content equivalent; the same below) of the (meth)acrylic acid ester polymer (A) obtained in the above step 1, 0.3 parts by mass of trimethylolpropane-modified tolylene diisocyanate (manufactured by Soken Chemical Industries, Ltd., product name "L-45") as a crosslinking agent (B), 100 parts by mass of a high refractive index component containing zirconium oxide fine particles (C1; manufactured by Sakata Inx Corporation, product name "Eco Stage TM RT-HRU-1") as high refractive index fine particles (C), and 0.3 parts by mass of 3-glycidoxypropyltrimethoxysilane (Si1) as a silane coupling agent were mixed, thoroughly stirred, and diluted with methyl ethyl ketone to obtain a coating solution of an adhesive composition.

3. Production of adhesive sheet The obtained coating solution of the adhesive composition was applied by a knife coater to the release-treated surface of a heavy release type release sheet (manufactured by Lintec Corporation, product name "SP-PET382150"), one side of which was a polyethylene terephthalate film treated with a silicone-based release agent. The coating layer was then heat-treated at 90°C for 1 minute to form a coating layer.

Next, a light release type release sheet was laminated on the coating layer so that its release-treated surface was in contact with the coating layer, and ultraviolet light was irradiated through the light release type release sheet under the following conditions to harden the coating layer and form an adhesive layer with a thickness of 25 μm. In this way, an adhesive sheet consisting of heavy release type release sheet/adhesive layer (thickness: 25 μm)/light release type release sheet was produced. The thickness of the adhesive layer was measured using a constant pressure thickness gauge (manufactured by Techlock, product name "PG-02") in accordance with JIS K7130.

<Ultraviolet ray irradiation conditions>
- Uses high pressure mercury lamp - Illuminance 200mW/ ^cm2 , light quantity 200mJ/ ^cm2
・UV illuminance/light intensity meter used is "UVPF-A1" manufactured by iGraphics Co., Ltd.

Here, the formulations (solid content equivalent) of the pressure-sensitive adhesive composition when the (meth)acrylic acid ester polymer (A) is taken as 100 parts by mass (solid content equivalent) are shown in Table 1. The details of the abbreviations and the like shown in Table 1 are as follows.
[(Meth)acrylic acid ester polymer (A)]
BA: n-butyl acrylate 2EHA: 2-ethylhexyl acrylate PhEA: 2-phenoxyethyl acrylate HEA: 2-hydroxyethyl acrylate AA: acrylic acid
[High refractive index particles (C)]
C1: A high refractive index component containing nano-order zirconium oxide particles (manufactured by Sakata Inx Corporation, product name "Ecostage TM RT-HRU-1", a dispersion solution containing zirconium oxide particles, an active energy ray curable component, a photopolymerization initiator and a dispersion solvent, refractive index of a single coating film: 1.73 (measured according to JIS K0062-1992))
C2: Zirconium oxide microparticles (manufactured by Daiken Chemical Co., Ltd., product name "DLZ-007", average particle size: 10 nm, dispersion solvent: methyl ethyl ketone, dispersion solution concentration: 40 mass%, refractive index of microparticles: 1.9 (measured value according to JIS K7142:2014 B method))
C3: Zirconium oxide fine particles (manufactured by Sakai Chemical Industry Co., Ltd., product name "SZR-K", average particle size: 6 nm, dispersion solvent: methyl ethyl ketone (63% by mass) and methanol (7% by mass), dispersion solution concentration: 30% by mass, refractive index of fine particles: 1.9 (measured value according to JIS K7142:2014 B method))
[Silane coupling agents]
Si1: 3-glycidoxypropyltrimethoxysilane Si2: 3-glycidoxypropylmethyldimethoxysilane
[Tackifier]
T1: Rosin-based tackifier (manufactured by Arakawa Chemical Industries, product name "Pine Crystal ME-GH")
T2: Rosin-based tackifier (manufactured by Arakawa Chemical Industries, product name "Pine Crystal ME-G")

[Examples 2 to 8, Comparative Examples 1 to 5]
An adhesive sheet was produced in the same manner as in Example 1, except that the types and ratios of the monomers constituting the (meth)acrylic acid ester polymer (A), the type and amount of the high refractive index fine particles (C), the type of silane coupling agent, and the type and amount of the tackifier were changed as shown in Table 1.

The weight average molecular weight (Mw) mentioned above is a polystyrene-equivalent weight average molecular weight measured by gel permeation chromatography (GPC) under the following conditions (GPC measurement).
<Measurement conditions>
GPC measuring device: Tosoh Corporation, HLC-8020
GPC columns (passed in the following order): TSK guard column HXL-H manufactured by Tosoh Corporation
TSK gel GMHXL (x2)
TSK gel G2000HXL
Measurement solvent: tetrahydrofuran Measurement temperature: 40°C

[Test Example 1] (Evaluation of compatibility)
The adhesive composition coating solution prepared in the examples and comparative examples was dispensed into a vial, and the degree of turbidity of the coating solution was visually judged. In addition, the state of the coating layer formed on the release-treated surface of the heavy release type release sheet in the examples and comparative examples was visually judged. Based on the judgment results, the compatibility of the adhesive composition was evaluated according to the following criteria. The results are shown in Table 2.
◯: The coating solution was transparent, and the coating layer was also transparent.
Δ: The coating solution was cloudy, but the coating layer was transparent.
x: The coating solution was cloudy, and the transparency was not improved even after forming a coating layer.

The adhesive composition coating solutions of Comparative Examples 3 and 4 gelled and could not be applied, and an adhesive layer could not be formed, so no other test examples were performed.

[Test Example 2] (Measurement of gel fraction)
The pressure-sensitive adhesive sheets produced in the Examples and Comparative Examples were cut to a size of 80 mm x 80 mm, the pressure-sensitive adhesive layer was wrapped in a polyester mesh (product name: Tetoron mesh #200), and the mass was measured using a precision balance. The mass of the mesh alone was subtracted to calculate the mass of the pressure-sensitive adhesive alone. This mass was designated M1.

Next, the adhesive wrapped in the polyester mesh was immersed in ethyl acetate at room temperature (23°C) for 24 hours. The adhesive was then removed and air-dried for 24 hours in an environment with a temperature of 23°C and a relative humidity of 50%, and then dried in an oven at 80°C for 12 hours. After drying, the mass was weighed using a precision balance, and the mass of the adhesive alone was calculated by subtracting the mass of the mesh alone. This mass was designated M2. The gel fraction (%) was expressed as (M2/M1) x 100. The results are shown in Table 2.

[Test Example 3] (Measurement of weight average molecular weight of sol content)
The ethyl acetate used in the measurement of the gel fraction in Test Example 2 was concentrated with an evaporator to obtain a sol fraction. The sol fraction was then diluted with tetrahydrofuran to a 0.3% by mass solution, filtered through a 0.45 μm filter, and the weight average molecular weight (Mw) of the sol fraction was measured by GPC measurement. The measurement conditions for the GPC measurement were as described above. The results are shown in Table 2.

Test Example 4 (Measurement of storage modulus G')
The pressure-sensitive adhesive layers of the pressure-sensitive adhesive sheets produced in the Examples and Comparative Examples were laminated together to form a laminate having a thickness of 800 μm. A cylindrical body having a diameter of 8 mm (height of 800 μm) was punched out from the resulting laminate of pressure-sensitive adhesive layers to form a sample.

The storage modulus G' of the above sample was measured under the following conditions by a torsional shear method using a viscoelasticity measuring device (manufactured by Anton Paar, product name "MCR302") in accordance with JIS K7244-1, and the storage modulus G' (MPa) at 23°C and the storage modulus G' (MPa) at 80°C were obtained. The results are shown in Table 2.
Measurement frequency: 1Hz
Measurement temperature: -20℃~150℃
Heating rate: 2°C/min

[Test Example 5] (Measurement of refractive index)
The (meth)acrylic acid ester polymer (A) prepared in the Examples and Comparative Examples was diluted with a solvent and applied in the form of a layer on the release surface of a release film (manufactured by Lintec Corporation, product name "SP-PET382150"). Then, the mixture was heated at 90°C for 1 minute to form a coating layer having a thickness of 25 μm. The refractive index of the formed coating layer was measured in accordance with JIS K0062-1992 using an Abbe refractometer (manufactured by Atago Co., Ltd., product name "Abbe refractometer DR-M2", Na light source, wavelength: 589 nm). The results are shown in Table 1.

The refractive index of the adhesive layer of the adhesive sheets produced in the examples and comparative examples was also measured in the same manner as above. The results are shown in Table 2.

In addition, the adhesive layers in Comparative Examples 1 and 2 had turned white, so the refractive index could not be measured.

[Test Example 6] (Measurement of haze value)
The adhesive layer of the adhesive sheet produced in the examples and comparative examples was attached to glass to prepare a measurement sample. After background measurement was performed on the glass, the haze value (%) of the measurement sample was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., product name "NDH5000") in accordance with JIS K7136:2000. The results are shown in Table 2.

[Test Example 7] (Measurement of total light transmittance)
The pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet produced in the Examples and Comparative Examples was attached to glass to prepare a measurement sample. After background measurement was performed on the glass, the total light transmittance (%) of the measurement sample was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., product name "NDH5000") in accordance with JIS K7361-1:1997. The results are shown in Table 2.

[Test Example 8] (Measurement of adhesive strength)
The light release type release sheet was peeled off from the pressure-sensitive adhesive sheet produced in the Examples and Comparative Examples, and the exposed pressure-sensitive adhesive layer was attached to an easy-adhesion layer of a polyethylene terephthalate (PET) film (manufactured by Toyobo Co., Ltd., product name "PET A4300", thickness: 100 μm) having an easy-adhesion layer, to obtain a laminate of heavy release type release sheet/pressure-sensitive adhesive layer/PET film. The obtained laminate was cut into a width of 25 mm and a length of 110 mm.

In an environment of 23°C and 50% RH, the heavy release type release sheet was peeled off from the laminate, and the exposed adhesive layer was attached to one side of a soda lime glass plate (manufactured by Nippon Sheet Glass Co., Ltd., product name "Soda Lime Glass", thickness: 1.1 mm), and pressurized for 20 minutes at 0.5 MPa and 50°C in an autoclave manufactured by Kurihara Seisakusho Co., Ltd. After that, it was left for 24 hours under conditions of 23°C and 50% RH, and then the adhesive strength (N/25 mm) was measured when the laminate of the PET film and the adhesive layer was peeled off from the adherend using a tensile tester (manufactured by Orientec Co., Ltd., Tensilon) at a peel speed of 300 mm/min and a peel angle of 180 degrees. The measurements were performed under conditions other than those described here in accordance with JIS Z0237:2009. The results are shown in Table 2.

[Test Example 9] (Evaluation of light leakage resistance)
A plurality of COP polarizing plates (thickness 60 μm) each having a triacetyl cellulose (TAC) film (thickness 25 μm), a polyvinyl alcohol (PVA) film polarizer (thickness 12 μm), and a cycloolefin polymer (COP) film (thickness 23 μm) laminated in that order were prepared. The surface of each COP polarizing plate that was to be bonded to the pressure-sensitive adhesive layer was subjected to a corona treatment.

The light release type release sheet was peeled off from the adhesive sheet produced in the examples and comparative examples, and the exposed adhesive layer was attached to the TAC film side of one COP polarizing plate. Next, the heavy release type release sheet was peeled off, and the exposed adhesive layer was attached to the COP film side of another COP polarizing plate in a cross-Nicol state. Then, after pressurizing at 0.5 MPa and 50°C for 20 minutes in an autoclave manufactured by Chiyoda Electric Industry Co., Ltd., it was left for 24 hours under conditions of 23°C and 50% RH. The laminate consisting of the COP polarizing plate/adhesive layer/COP polarizing plate obtained in this way was cut into a width of 50 mm and a length of 200 mm, and this was used as a sample.

The obtained sample was held in a bent state between two holding plates (distance between each plate: 3 mm) made of two upright glass plates as shown in Figure 3 using a durability tester (manufactured by Yuasa System Co., Ltd., product name "Planar body unloaded U-shaped expansion and contraction tester, model: CL09-typeD01-FSC90") for 1000 hours (test conditions are as follows). The bent state was then released and the sample was left for 24 hours in an environment of 23°C and 50% RH. Light leakage was then visually confirmed using transmitted light from a backlight (manufactured by Dentsu Sangyo Co., Ltd., product name "HF-SL-A312LC"), and light leakage resistance was evaluated according to the following criteria. The results are shown in Table 2.

<Test conditions>
Bending direction: Bending so that the TAC film of one COP polarizing plate is on the outside and the COP film of the other COP polarizing plate is on the inside. Minimum bending diameter: 3 mmφ
Bending time: 1000 hours Test environment: 60°C, 50% RH
<Evaluation Criteria for Light Leakage Resistance>
No light leakage occurred at the bent portion.
◯: A slight amount of light leakage occurred at the bent portion, but was at a level that did not cause any practical problems.
△: Light leakage occurred at the bent portion.
×: Light leakage occurred at the bent portion, and the light leakage spread to the periphery of the bent portion.

[Test Example 10] (Evaluation of appearance after long-term bending)
The test was carried out in the same manner as in Test Example 9, except that the test temperature was changed to 80° C., and then the appearance after long-term bending was evaluated according to the following criteria.
◎: No changes were observed in the bent portion or its surroundings.
◯: A few air bubbles were found either at the bent portion or in the vicinity thereof, but no peeling was found.
Δ: A series of air bubbles were observed either at the bent portion or in its vicinity, but no peeling was observed.
×: Large bubbles or peeling was observed either at the bent portion or in the vicinity thereof.

As can be seen from Table 2, the adhesive layer of the adhesive sheet of the embodiment had excellent light leakage resistance when two flexible members were attached and left in a bent state for a long period of time. In addition, the adhesive layer of the adhesive sheet of the embodiment also had excellent appearance after being bent for a long period of time.

The present invention is suitable for bonding one flexible member that constitutes a display body to another flexible member.

REFERENCE SIGNS LIST 1... Adhesive sheet 11... Adhesive layer 12a, 12b... Release sheet 3... Display body 31... Cover film 32... First adhesive layer 33... Polarizing film 34... Second adhesive layer 35... Touch sensor film 36... Third adhesive layer 37... Organic EL element 38... Fourth adhesive layer 39... TFT substrate S... Test piece P... Holding plate

Claims (7)

A pressure-sensitive adhesive sheet having at least a pressure-sensitive adhesive layer,
The refractive index of the adhesive constituting the adhesive layer is 1.55 or more,
The pressure-sensitive adhesive has a gel fraction of 30% or more and 100% or less,
The haze value of the pressure-sensitive adhesive layer is 10% or less,
the pressure-sensitive adhesive is an acrylic pressure-sensitive adhesive containing high refractive index fine particles,
the acrylic pressure-sensitive adhesive contains a (meth)acrylic acid ester polymer,
The (meth)acrylic acid ester polymer contains an aromatic ring-containing monomer as a monomer unit constituting the polymer.
A pressure-sensitive adhesive sheet characterized by: The pressure-sensitive adhesive sheet according to claim 1 , wherein the pressure-sensitive adhesive layer has a total light transmittance of 70% or more. 3. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive has a storage modulus G' at 23° C. of 0.05 MPa or more and 20 MPa or less. 4. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive has a storage modulus G' at 80° C. of 0.01 MPa or more and 15 MPa or less. 5. The pressure-sensitive adhesive sheet according to claim 1, wherein the weight-average molecular weight of the sol component of the pressure-sensitive adhesive is 10,000 or more and 2,000,000 or less. Two release sheets;
The pressure-sensitive adhesive sheet according to any one of claims 1 to 5 , further comprising the pressure-sensitive adhesive layer sandwiched between the two release sheets so as to be in contact with the release surfaces of the release sheets. The pressure-sensitive adhesive sheet according to any one of claims 1 to 6 , which is used for a display.

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