JP7760293B2 - Refractive index modifiers and their uses - Google Patents

Description

The present invention relates to a refractive index adjuster and a pressure-sensitive adhesive and pressure-sensitive adhesive sheet that utilizes the refractive index adjuster.

Generally, adhesives (also known as pressure-sensitive adhesives; the same applies hereinafter) are soft solids (viscoelastic bodies) at temperatures near room temperature, and have the property of easily adhering to adherends when pressure is applied. Utilizing these properties, adhesives are widely used for purposes such as joining, fixing, and protection in a variety of industrial fields, from home appliances to automobiles, various types of machinery, electrical equipment, and electronic devices. One example of the use of adhesives is bonding polarizing films, retardation films, cover window components, and various other light-transmitting components to other components in displays such as liquid crystal displays and organic EL displays. Patent documents 1 and 2 are examples of technical literature related to adhesives for optical components.

JP 2014-169382 A Japanese Patent Application Laid-Open No. 2017-128732

Patent Documents 1 and 2 relate to adhesive compositions primarily composed of a (meth)acrylic acid ester polymer, and propose that the (meth)acrylic acid ester polymer contain a predetermined amount of a monomer having multiple aromatic rings as a monomer unit, thereby providing an adhesive with a high refractive index (see paragraph [0011], etc.). For example, some materials to which adhesives are applied, such as optical components, have high refractive indices. It is known that using a typical acrylic adhesive to bond such high-refractive-index materials can cause reflection at the interface due to the difference in refractive index between the two materials. By using an adhesive with a refractive index close to that of the high-refractive-index material to bond the high-refractive-index materials, it is possible to prevent or suppress this interfacial reflection. The refractive index of typical acrylic adhesives is typically around 1.47.

However, the refractive indexes of materials that can be used as adhesive substrates vary, and it is not necessarily efficient to individually design the composition (type and content ratio) of the monomer units that make up the adhesive's base polymer to match the refractive index of each adherend. For example, it would be beneficial if the refractive index of the adhesive could be adjusted using an additive component separate from the base polymer. Patent Documents 1 and 2 cite a refractive index adjuster as an example of an optional component that can be added to the adhesive composition if desired (paragraph [0054]), but do not describe the structure of the refractive index adjuster, and it is unclear whether it is organic or inorganic.

The present invention was created in light of the above circumstances, and aims to provide a refractive index adjuster for adjusting the refractive index of an adhesive. Another object of the present invention is to provide an adhesive containing the refractive index adjuster. Another related object is to provide a compound useful as the refractive index adjuster.

This specification provides a refractive index adjuster for adhesives. The refractive index adjuster is an organic compound with a structure having two or more substituents on a hub ring. Here, the hub ring is a double bond-containing ring, and at least one of the two or more substituents is a substituent having a double bond-containing ring. Compounds that satisfy these structural requirements are used as components of adhesives and are useful as refractive index adjusters that appropriately adjust the refractive index of the adhesive.

In some embodiments of the technology disclosed herein (including refractive index adjusters, adhesive compositions containing the refractive index adjusters, adhesives containing the refractive index adjusters, adhesive sheets having the adhesives, etc.; the same applies hereinafter), the molecular weight of the refractive index adjuster is less than 3,000. A refractive index adjuster with a molecular weight of less than 3,000 is preferred from the standpoint of ease of incorporation into the adhesive, etc.

In some embodiments, the refractive index adjuster has two or more substituents on the hub ring that have a double bond-containing ring. A refractive index adjuster having such a structure can be used as a component of a pressure-sensitive adhesive to efficiently adjust the refractive index of the pressure-sensitive adhesive (e.g., to increase the refractive index).

In some embodiments, the refractive index adjuster is a substituent in which one or more of the substituents on the hub ring have two or more double-bond-containing rings. A refractive index adjuster having such a structure can be used as a component of a pressure-sensitive adhesive to efficiently adjust the refractive index of the pressure-sensitive adhesive (e.g., to increase the refractive index).

A preferred example of the refractive index control agent disclosed herein is one in which the hub ring is a triazine ring. For example, the refractive index control agent is represented by the following formula (I):
(In formula (I), X ¹ , X ² and X ³ are each independently selected from the group consisting of —O—, —S— and —NR ⁴ —, where R ⁴ is a hydrogen atom or an alkyl group;
one or more of R ¹ , R ² and R ³ each independently represent an aromatic ring-containing group selected from the group consisting of an optionally substituted phenyl group and an optionally substituted biphenyl group, wherein the substituent is an alkyl group, an alkoxy group or a cyano group;
When there are two or less aromatic ring-containing groups among R ¹ , R ² and R ³ , the remaining are aliphatic hydrocarbon groups or hydrogen atoms;
A refractive index adjuster having such a structure can be used as a component of a pressure-sensitive adhesive to suitably adjust the refractive index of the pressure-sensitive adhesive (for example, to increase the refractive index).

This specification provides an adhesive containing any of the refractive index adjusters disclosed herein. An adhesive with such a composition can exhibit a desired refractive index adjusted by the refractive index adjuster.

The adhesive disclosed herein preferably contains the refractive index adjuster and an acrylic polymer containing an aromatic ring-containing monomer (m1) as a monomer unit. An adhesive with such a composition makes it easy to achieve a desired high refractive index (e.g., a refractive index of 1.550 or greater).

According to this specification, the compound of formula (I):
(In formula (I), X ¹ , X ² and X ³ are each independently selected from the group consisting of —O—, —S— and —NR ⁴ —, where R ⁴ is a hydrogen atom or an alkyl group;
one or more of R ¹ , R ² and R ³ each independently represent an aromatic ring-containing group selected from the group consisting of an optionally substituted phenyl group and an optionally substituted biphenyl group, wherein the substituent is an alkyl group, an alkoxy group or a cyano group;
When there are two or less aromatic ring-containing groups among R ¹ , R ² and R ³ , the remaining are aliphatic hydrocarbon groups or hydrogen atoms;
The compound is useful, for example, as a refractive index adjuster (particularly, a refractive index adjuster for adhesives).

In addition, appropriate combinations of the elements described in this specification may also be included within the scope of the invention for which patent protection is sought through this patent application.

1 is a cross-sectional view schematically illustrating the configuration of a pressure-sensitive adhesive sheet according to one embodiment. FIG. 10 is a cross-sectional view schematically illustrating the configuration of a pressure-sensitive adhesive sheet according to another embodiment. FIG. 10 is a cross-sectional view schematically illustrating the configuration of a pressure-sensitive adhesive sheet according to another embodiment. FIG. 1 is a diagram showing the ¹ H-NMR spectrum of Compound A1 according to Synthesis Example 1. FIG. 1 is a diagram showing the ¹³ C-NMR spectrum of Compound A1 according to Synthesis Example 1. FIG. 1 is a diagram showing the ¹ H-NMR spectrum of Compound A2 according to Synthesis Example 2. FIG. 1 is a diagram showing the ¹³ C-NMR spectrum of Compound A2 according to Synthesis Example 2. FIG. 1 is a diagram showing the ¹ H-NMR spectrum of Compound A3 according to Synthesis Example 3. FIG. 1 is a diagram showing the ¹³ C-NMR spectrum of Compound A3 according to Synthesis Example 3. FIG. 1 is an enlarged view of a main portion of the ¹ H-NMR spectrum of Compound A4 according to Synthesis Example 4. FIG. 2 is an enlarged view of a main portion of the ¹³ C-NMR spectrum of Compound A4 according to Synthesis Example 4. FIG. 1 is a diagram showing the ¹ H-NMR spectrum of Compound A5 according to Synthesis Example 5. FIG. 1 is a diagram showing the ¹³ C-NMR spectrum of Compound A5 according to Synthesis Example 5. FIG. 1 is a diagram showing the ¹ H-NMR spectrum of compound A6 according to Synthesis Example 6. FIG. 1 is a diagram showing the ¹³ C-NMR spectrum of Compound A6 according to Synthesis Example 6. FIG. 1 is a diagram showing the ¹ H-NMR spectrum of compound A7 according to Synthesis Example 7. FIG. 1 is a diagram showing the ¹³ C-NMR spectrum of Compound A7 according to Synthesis Example 7. FIG. 1 is a diagram showing the ¹ H-NMR spectrum of compound A8 according to Synthesis Example 8. FIG. 1 is a diagram showing the ¹³ C-NMR spectrum of Compound A8 according to Synthesis Example 8.

Preferred embodiments of the present invention will be described below. Matters necessary for carrying out the present invention other than those specifically mentioned in this specification can be understood by those skilled in the art based on the teachings on carrying out the invention described in this specification and the common general technical knowledge at the time of filing. The present invention can be carried out based on the contents disclosed in this specification and the common general technical knowledge in the relevant field.
In the following drawings, components and parts that perform the same function may be denoted by the same reference numerals, and duplicated explanations may be omitted or simplified. Furthermore, the embodiments shown in the drawings are schematic in order to clearly explain the present invention, and do not necessarily accurately represent the size or scale of the actual product.

In this specification, the term "double bond-containing ring" encompasses both conjugated double bond-containing rings and non-conjugated double bond-containing rings, and is preferably a ring that corresponds to at least one of an aromatic ring and a heterocyclic ring (heterocycle). The heterocycle may have a structure encompassed by an aromatic ring (heteroaromatic ring) or a double bond-containing heterocyclic structure different from an aromatic ring. The heteroatoms contained as ring-constituting atoms in the heterocycle may be, for example, one or more selected from the group consisting of nitrogen (N), sulfur (S), and oxygen (O). Non-limiting examples of the double bond-containing ring include carbocyclic rings such as a benzene ring; a fused ring of a naphthalene ring, an indene ring, an azulene ring, an anthracene ring, or a phenanthrene ring; and heterocyclic rings such as a pyridine ring, a pyrimidine ring, a pyridazine ring, a pyrazine ring, a triazine ring, a pyrrole ring, a pyrazole ring, an imidazole ring, a triazole ring, an oxazole ring, an isoxazole ring, a thiazole ring, and a thiophene ring. The double bond-containing ring may be a non-fused ring (such as a benzene ring or triazine ring) or a fused ring. The fused ring may have a structure in which one or more carbon rings are fused with one or more heterocyclic rings, such as a dinaphthothiophene structure or a benzotriazole structure.

Unless otherwise specified, double bond-containing rings in this specification may have one or more substituents on ring-constituting atoms, or may have no substituents. Examples of "substituents" in this specification include alkyl groups (e.g., alkyl groups having 1 to 12 carbon atoms), aryl groups (including phenyl groups, naphthyl groups, biphenyl groups, etc., which may have one or more acyclic substituents on ring-constituting atoms, such as alkyl groups, halogen atoms (fluorine atoms, chlorine atoms, bromine atoms, etc.), hydroxyl groups, amino groups, monoalkylamino groups, dialkylamino groups, and cyano groups), cycloalkyl groups, hydroxyalkyl groups, monoalkylamino groups, dialkylamino groups, glycidyl groups, aralkyl groups (e.g., groups in which one or more hydrogen atoms of an alkyl group having 1 to 12 carbon atoms, preferably 1 to 8 carbon atoms, are substituted with the above-mentioned aryl groups), ethylenically unsaturated groups such as alkenyl groups (e.g., vinyl groups, allyl groups), and (meth)acryloyl groups, and groups containing an ether bond or a thioether bond in the chain structure of these groups. Examples of such groups include, but are not limited to, groups containing one or more bonds selected from the group consisting of an ether bond and an ester bond (e.g., ethoxyethyl, ethoxyethoxyethyl, and phenoxyethyl groups), groups in which some or all of the hydrogen atoms bonded to carbon in these groups have been replaced with halogen atoms (e.g., fluorine, chlorine, and bromine atoms), groups having a heteroatom such as O, S, or N at the end of the double bond-containing ring side of these groups (e.g., alkoxy, aryloxy, cycloalkyloxy, hydroxyalkyloxy, glycidyloxy, alkylthio, and (meth)acryloyloxy groups; when the heteroatom is N, the remaining valence of the N is bonded to a hydrogen atom or another substituent), a hydroxyl group, an amino group, a cyano group, and a halogen atom (e.g., fluorine, chlorine, and bromine atoms). Examples of the above substituents include both those that have a double bond-containing ring (e.g., aryl groups, aralkyl groups, and groups having a heteroatom such as O, S, or N at the end of these groups on the ring-constituting atom side) and those that do not have a double bond-containing ring (e.g., alkyl groups, alkoxy groups, alkylthio groups, etc.).

In this specification, the "base polymer" of a pressure-sensitive adhesive refers to the main rubbery polymer component contained in the pressure-sensitive adhesive. The rubbery polymer is a polymer that exhibits rubber elasticity in a temperature range around room temperature. Furthermore, in this specification, "main component" refers to a component that accounts for more than 50% by weight, unless otherwise specified.

In this specification, the term "acrylic polymer" refers to a polymer containing, as a monomer unit constituting the polymer, a monomer unit derived from a monomer having at least one (meth)acryloyl group per molecule. Hereinafter, a monomer having at least one (meth)acryloyl group per molecule will also be referred to as an "acrylic monomer." Therefore, in this specification, an acrylic polymer is defined as a polymer containing a monomer unit derived from an acrylic monomer. A typical example of an acrylic polymer is an acrylic polymer in which more than 50% by weight (preferably more than 70% by weight, for example more than 90% by weight) of the monomer components constituting the polymer are acrylic monomers.

In addition, in this specification, "acrylic monomer" refers to a monomer having at least one (meth)acryloyl group in one molecule. Here, "(meth)acryloyl group" refers collectively to acryloyl groups and methacryloyl groups. Therefore, the concept of acrylic monomer here can encompass both monomers having an acryloyl group (acrylic monomers) and monomers having a methacryloyl group (methacrylic monomers). Similarly, in this specification, "(meth)acrylic acid" refers collectively to acrylic acid and methacrylic acid, and "(meth)acrylate" refers collectively to acrylate and methacrylate. The same applies to other similar terms.

<Refractive index adjuster>
The refractive index modifier disclosed in this specification is an organic compound having a structure including a hub ring and two or more substituents bonded to constituent atoms of the hub ring. Here, the hub ring is a double-bond-containing ring, and at least one of the two or more substituents is a substituent having a double-bond-containing ring. Compounds that satisfy these structural requirements are useful as refractive index modifiers used as components of pressure-sensitive adhesives to adjust the refractive index of the pressure-sensitive adhesive.

The refractive index adjuster disclosed herein may be a non-polymer, or may be a polymer having two or more repeat units of the above structure (i.e., a structure having a hub ring and two or more substituents bonded to constituent atoms of the hub ring). When the refractive index adjuster is a polymer, the number of repeat units in the polymer is, for example, 2 to 30 or 2 to 10, preferably 2 to 6, and more preferably 2 to 4 (e.g., 2 to 3).

The molecular weight of the refractive index adjuster is not particularly limited and can be appropriately selected within a range that allows it to exhibit the intended function. The molecular weight of the refractive index adjuster may be, for example, less than approximately 10,000, less than 7,000, less than 5,000, less than 3,500, or less than 3,000. In some embodiments, from the standpoint of ease of incorporation into the adhesive, the molecular weight of the refractive index adjuster is advantageously less than 3,000, preferably less than 2,000, more preferably less than 1,500, less than 1,000, less than 800, less than 700, or less than 650. The molecular weight of the refractive index adjuster may also be, for example, 150 or more, 200 or more, or 250 or more. In some embodiments, from the viewpoint of easily performing the function of adjusting the refractive index (e.g., increasing the refractive index), the molecular weight of the refractive index adjuster is suitably 300 or more, preferably 350 or more, more preferably 400 or more, or may be 430 or more, 470 or more, 500 or more, or 550 or more. It is also preferable that the molecular weight of the refractive index adjuster is not too low from the viewpoint of the heat resistance of the pressure-sensitive adhesive and preventing contamination of the adherend.

When the refractive index modifier is a non-polymer or a polymer with a low degree of polymerization (e.g., about dimer to pentamer), the molecular weight can be calculated based on the chemical structure or measured using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). For polymers with a higher degree of polymerization, the weight-average molecular weight (Mw) determined by GPC performed under appropriate conditions can be used. If the manufacturer provides a nominal molecular weight value, that nominal value can be used.

The hub ring in the refractive index adjuster disclosed herein is typically a double-bond-containing ring that corresponds to at least one of an aromatic ring and a heterocyclic ring (preferably a heteroaromatic ring). The hub ring may be a non-fused ring (monocyclic ring) or a fused ring, but is preferably a non-fused ring (e.g., a 3- to 8-membered ring, preferably a 5- to 7-membered ring). In some preferred embodiments, the hub ring is a non-fused heteroaromatic ring. The number of heteroatoms contained as ring-constituting atoms in the heteroaromatic ring may be one or more, for example, 1 to 3. In some embodiments, a heteroaromatic ring in which at least one of the heteroatoms contained as ring-constituting atoms is N is preferred, and a heteroaromatic ring in which all of the heteroatoms are N is more preferred. A triazine ring is a particularly preferred example of a heteroaromatic ring.

The number of substituents bonded to ring-constituting atoms of the hub ring is typically two or more and less than the number of ring-constituting atoms, for example, three or more and less than the number of ring-constituting atoms. The number of substituents having a double bond-containing ring among the above substituents may be one or more, two or more, or three or more. The number of substituents not having a double bond-containing ring among the above substituents may be zero, one or more and less than the total number of the above substituents minus one, or one or more and less than the total number of the above substituents minus two. The total number of substituents refers to the total number of substituents bonded to ring-constituting atoms of the hub ring, regardless of whether the substituent has a double bond-containing ring.

Of the substituents bonded to the ring-constituting atoms of the hub ring, those having a double bond-containing ring can contribute to improving the refractive index. Refractive index enhancers according to some embodiments have two or more substituents having double bond-containing rings on the hub ring. Refractive index adjusters with such a structure can be used as a component of a pressure-sensitive adhesive to efficiently adjust the refractive index of the pressure-sensitive adhesive (e.g., to increase the refractive index).

In the above-mentioned substituent having a double bond-containing ring, the number of double bond-containing rings possessed by the substituent may be one or two or more. Here, "having two or more double bond-containing rings" means having two or more double bond-containing rings that are not fused to each other. Each double bond-containing ring may independently be a non-fused ring or a fused ring. Having a substituent having two or more double bond-containing rings on a hub ring can be advantageous from the perspective of improving the refractive index. In some embodiments, a refractive index enhancer has one or more (preferably two or more, e.g., three) substituents on a hub ring that have two or more double bond-containing rings. A refractive index adjuster with such a structure can be used as a component of a pressure-sensitive adhesive to efficiently adjust the refractive index of the pressure-sensitive adhesive (e.g., to increase the refractive index).

In the substituent having two or more double bond-containing rings, the two or more double bond-containing rings may be chemically bonded directly (i.e., not via any other atom) as in, for example, a biphenyl structure, or may be bonded via a linking group. Examples of the linking group include an oxy group (-O-), a thiooxy group (-S-), an oxyalkylene group (for example, an -O-(CH ₂ ) _n - group, where n is 1 to 3, preferably 1), a thiooxyalkylene group (for example, an -S-(CH ₂ ) _n - group, where n is 1 to 3, preferably 1), a straight-chain alkylene group (i.e., a -(CH ₂ ) _n - group, where n is 1 to 6, preferably 1 to 3), an -NR ⁴ - group (R ⁴ is a hydrogen atom or an alkyl group (for example, an alkyl group having 1 to 3 carbon atoms, preferably 1)), an -NR ⁴ -(CH ₂ ) _n - group (n is 1 to 3, preferably 1, and R ⁴ is a hydrogen atom or an alkyl group (for example, an alkyl group having 1 to 3 carbon atoms, preferably 1)), the oxyalkylene group, the thiooxyalkylene group, the straight-chain alkylene group, and the -NR ⁴ -(CH ₂ ) _n The alkylene group in the - group may be a group in which it is partially halogenated or completely halogenated. The linking group may have an ester bond. The number of atoms in the linking group is, for example, 1 to 6, preferably 1 to 4, and may be 1 to 2, 2, or 1. The number of atoms in the linking group refers to the minimum number of atoms required to reach from one double bond-containing ring to the other double bond-containing ring. For example, when the linking group is a linear alkylene group (i.e., a -(CH ₂ ) _n - group), the number n is the number of atoms in the linking group. Examples of preferred substituents from the viewpoint of increasing the refractive index include substituents containing a structure in which the two or more double bond-containing rings are directly chemically bonded or bonded via an oxy group or a thiooxy group.

In some preferred embodiments, the substituent having the double bond-containing ring is bonded to a ring-constituting atom (e.g., a carbon atom) of the hub ring via a heteroatom (e.g., O, S, or N, preferably O or S) possessed by the substituent. Compounds having such a structure are preferred because they can be suitable for increasing the refractive index of the compound itself or a composition containing the compound. In the substituent having the double bond-containing ring, the heteroatom bonded to the ring-constituting atom of the hub ring may be a ring-constituting atom of the double bond-containing ring (heterocycle), or may be an atom constituting the linking group connecting the double bond-containing ring and the hub ring. Examples of the linking group include a thiooxy group, an oxyalkylene group (for example, an -O-(CH ₂ ) _n - group, where n is 1 to 3, preferably 1), a thiooxyalkylene group (for example, an -S-(CH ₂ ) _n - group, where n is 1 to 3, preferably 1), an -NR ⁴ - group, and an -NR ⁴ -(CH ₂ ) _n - group (where n is 1 to 3, preferably 1, and R ⁴ is a hydrogen atom or an alkyl group (for example, an alkyl group having 1 to 3 carbon atoms, preferably 1)). The number of atoms in the linking group connecting the double bond-containing ring and the hub ring is, for example, 1 to 6, preferably 1 to 4, and may be 1 to 2, 2, or 1. The number of atoms in the linking group refers to the minimum number of atoms required to reach the double bond-containing ring of the substituent from the hub ring.

The double-bond-containing ring of the substituent bonded to a ring-constituting atom of the hub ring is typically an aromatic ring or a heterocyclic ring, and is preferably a non-fused ring. The double-bond-containing ring may have one or more substituents on its ring-constituting atoms, other than the substituent bonded to the hub ring or the substituent bonded to another double-bond-containing ring. Examples of such substituents include acyclic substituents such as alkyl groups, alkoxy groups, alkylthio groups, alkenyl groups, (meth)acryloyl groups, (meth)acryloyloxy groups, halogen atoms, hydroxyl groups, amino groups, monoalkylamino groups, dialkylamino groups, and cyano groups; and substituents having a non-double-bond-containing ring, such as cycloalkyl groups, cycloalkyloxy groups, and norbornyl groups. In the alkyl groups, alkoxy groups, alkylthio groups, alkenyl groups, monoalkylamino groups, and dialkylamino groups, the alkyl group contained in each group may have, for example, 1 to 6 carbon atoms, preferably 1 to 4, and may also have 1 to 2, 2, or 1 carbon atom.

In some preferred embodiments, the double bond-containing ring having a substituent bonded to a ring-constituting atom of the hub ring is an aromatic ring (preferably a carbon aromatic ring, particularly preferably a benzene ring) having or having no substituent. For example, at least one (e.g., one, two, or three) of the ring-constituting atoms of the hub ring has a group represented by the following formula (II):
-X(CH ₂ ) _n -Ar(Y) _m (II)
(In the above formula (II), X is selected from the group consisting of —O—, —S—, and —NR ⁴ — (wherein R ⁴ is a hydrogen atom or an alkyl group), and is preferably —O— or —S—. n is 0 to 3, preferably 0 or 1, and more preferably 0. Ar is an aromatic ring, preferably a carbocyclic ring, and particularly preferably a benzene ring. Y is a substituent on a ring-constituting atom of Ar, and each Y is independently selected from the group consisting of an alkyl group (for example, an alkyl group having 1 to 3 carbon atoms, preferably 1), a cyano group, a hydroxyl group, a phenyl group, a tolyl group, a xylyl group, and a phenoxy group. m is 0 to 3, preferably 0 or 1, and more preferably 1.)
A compound having a structure in which a substituent represented by the following formula is bonded is preferred.

The refractive index adjuster disclosed herein may have only the above-mentioned double bond-containing ring as the substituent bonded to the ring-constituting atoms of the hub ring. Such a structure makes it possible to bond a larger number of double bond-containing ring-containing substituents to the hub ring, making it suitable for achieving a high refractive index. On the other hand, the refractive index adjuster disclosed herein may have both a double bond-containing ring and a double bond-free ring as the substituent bonded to the ring-constituting atoms of the hub ring. In some embodiments, having a double bond-free ring-free substituent on the hub ring can contribute to improved solubility in solvents and compatibility in adhesives.

In an embodiment in which at least one of the substituents bonded to a ring-constituting atom of the hub ring is a substituent not having a double bond-containing ring, the substituent not having a double bond-containing ring can each independently be, for example, an alkyl group, a cycloalkyl group, a hydroxyalkyl group, an alkoxy group (e.g., a methoxy group or an ethoxy group), a hydroxyalkyloxy group, a glycidyloxy group, an alkylthio group (-S-C _n H _2n+1 group), a monoalkylamino group, a dialkylamino group, an alkenyl group, a (meth)acryloyl group, a (meth)acryloyloxy group, a group in which one or more bonds selected from the group consisting of an ether bond, a thioether bond, and an ester bond are inserted in the chain structure of these groups, a group in which some or all of the hydrogen atoms bonded to carbons in these groups have been replaced with halogen atoms (fluorine atoms, chlorine atoms, bromine atoms, etc.), a hydroxyl group, an amino group, a cyano group, or a halogen atom.

Among the above-mentioned examples of substituents that do not have a double bond-containing ring, those that contain an alkyl group in the substituent (such as an alkyl group, an alkoxy group, an alkylthio group, or an ethoxyethyl group) have, for example, 1 to 12 carbon atoms in each alkyl group, and in some embodiments, preferably 2 to 12, 4 to 12, or 6 to 10 carbon atoms, and in other embodiments, preferably 1 to 6, 1 to 4, or 1 to 2 carbon atoms. When the alkyl group has 3 or more atoms, the alkyl group may be linear or branched. Specific examples of branched alkyl groups include a 2-ethylhexyl group, an isooctyl group, and an isononyl group.
Suitable examples of the substituent not having a double bond-containing ring include alkoxy groups (for example, alkoxy groups having 1 to 12, 1 to 8, 1 to 6, 1 to 4, or 1 to 2 carbon atoms) and alkylthio groups (for example, alkylthio groups having 2 to 12, 4 to 12, or 6 to 10 carbon atoms).

A preferred example of the refractive index adjuster disclosed herein is a triazine compound represented by the following formula (I).

In the above formula (I), X ¹ , X ² and X ³ are each independently selected from the group consisting of -O-, -S- and -NR ^4. Here, R ⁴ is a hydrogen atom or an alkyl group (for example, an alkyl group having 1 to 3 carbon atoms). One or more of R ¹ , R ² and R ³ are each independently an aromatic ring-containing group selected from the group consisting of an optionally substituted phenyl group and an optionally substituted biphenyl group. When there are two or fewer aromatic ring-containing groups among R ¹ , R ² and R ³ , the remaining are aliphatic hydrocarbon groups or hydrogen atoms.

In formula (I) above, if the phenyl group or biphenyl group has a substituent, the substituent is preferably each independently selected from the group consisting of an alkyl group (e.g., an alkyl group having 1 to 3 carbon atoms), an alkoxy group (e.g., an alkoxy group having 1 to 3 carbon atoms), and a cyano group. The aliphatic hydrocarbon group is preferably an alkyl group having 1 to 12, 1 to 8, 1 to 6, 1 to 4, or 1 to 2 carbon atoms, or an alkyl group having 2 to 12, 4 to 12, or 6 to 10 carbon atoms.

In the above formula (I), X ¹ , X ² and X ³ may be the same or different. From the viewpoint of increasing the refractive index, it is advantageous that two or more of X ¹ , X ² and X ³ are selected from —O— and —S—, and preferred examples include a compound in which X ¹ , X ² and X ³ are all —O—, a compound in which two or more of X ¹ , X ² and X ³ are —S—, a compound in which X ¹ , X ² and X ³ are all —S—, etc. Among these, a compound in which X ¹ , X ² and X ³ are all —S— is preferred.

In the above formula (I), R ¹ , R ² and R ³ may be the same or different. From the viewpoint of increasing the refractive index, it is preferable that R ¹ , R ² and R ³ are all aromatic ring-containing groups, and from the viewpoint of ease of synthesis, it is preferable that R ¹ , R ² and R ³ are the same aromatic ring-containing group. Among these, compounds in which R ¹ , R ² and R ³ are all biphenyl groups which may have a substituent (for example, compounds in which R ¹ , R ² and R ³ are all biphenyl groups which have no substituent) are particularly preferable from the viewpoint of increasing the refractive index. Note that when one, two or three of R ¹ , R ² and R ³ are biphenyl groups which may have a substituent, it is preferable that the biphenyl group be 2-biphenyl or 4-biphenyl from the viewpoint of ease of synthesis. Here, 2-biphenyl means that the corresponding X ¹ , X ² , or X ³ is bonded to the carbon at the 2-position (ortho-position) of the biphenyl group, and 4-biphenyl means that the corresponding X ¹ , X ² , or X ³ is bonded to the carbon at the 4-position (para-position) of the biphenyl group. Compounds in which R ¹ , R ² , and R ³ in formula (I) are all 2-biphenyl groups are particularly preferred from the viewpoints of increasing the refractive index and solubility in solvents.

The compound represented by formula (I) above can be synthesized, for example, by a method that involves reacting a commercially available halogenated triazine compound (cyanuric chloride, 2-(4-cyanophenyl)amino-4,6-dichloro-1,3,5-triazine, etc.) with an alcohol compound or thiol compound appropriate for the target structure.

The refractive index of the refractive index adjuster disclosed herein is not particularly limited, as long as it can be blended into an adhesive to adjust the refractive index of the adhesive to the desired range. The refractive index of the refractive index adjuster can be, for example, within the range of 1.500 to 2.000. Note that the refractive index of the refractive index adjuster refers to the refractive index measured at a temperature of 25°C and a wavelength of 594 nm. The refractive index of the refractive index adjuster can be measured using a commercially available measuring device (for example, a Metricon Prism Coupler, Model "2010M").

In some embodiments, the refractive index of the refractive index modifier may be, for example, 1.530 or higher, suitably 1.550 or higher, advantageously 1.560 or higher, preferably 1.580 or higher, and more preferably 1.590 or higher. In some preferred embodiments, the refractive index of the refractive index modifier may be, for example, 1.600 or higher, 1.610 or higher, 1.630 or higher, 1.650 or higher, 1.660 or higher, 1.670 or higher, 1.680 or higher, 1.690 or higher, or 1.700 or higher. A refractive index modifier with a higher refractive index can more efficiently achieve the effect of increasing the refractive index when incorporated into an adhesive. Furthermore, from the perspective of the ease of synthesis and cost of the refractive index modifier, in some embodiments, the refractive index of the refractive index modifier may be 1.900 or lower, 1.800 or lower, 1.780 or lower, 1.750 or lower, or 1.730 or lower. In some embodiments of the refractive index adjuster disclosed herein, the refractive index of the refractive index adjuster is, for example, 1.590 or more and 1.750 or less, preferably 1.610 or more and 1.750 or less, more preferably 1.640 or more and 1.750 or less, and may be 1.660 or more and 1.730 or less, or 1.680 or more and 1.730 or less.

The refractive index adjusters disclosed herein are preferably soluble in a solvent (typically an organic solvent) from the standpoint of ease of incorporation into the adhesive. Here, "soluble" means that a solution containing the refractive index adjuster at a concentration of at least 2% by weight or more (preferably 5% by weight or more, more preferably 10% by weight or more, even more preferably 20% by weight or more, and particularly preferably 40% by weight or more) can be prepared. For example, the refractive index adjuster is preferably soluble in at least one of the following solvents: acetate esters such as ethyl acetate and butyl acetate; lower ketones such as acetone and methyl ethyl ketone (MEK); and aromatic hydrocarbon solvents such as benzene and toluene; and is particularly preferably soluble in one or both of ethyl acetate and MEK.

In the refractive index adjusters disclosed herein (e.g., triazine compounds represented by formula (I) above), advantageous structural features from the perspective of solubility in solvents include (i) a non-planar shape and (ii) rotational non-symmetry with respect to the hub ring. Compounds that meet either (i) or (ii) above, or both, are preferred. Suitable examples of compounds that meet (i) above include compounds (A1) and (A4) described in the Examples below. These compounds have a ball-like (non-planar) shape overall due to their molecular structure. Furthermore, for example, compound (A5) described in the Examples below is rotationally symmetric (has a three-fold rotation axis), whereas compound (A6) is rotationally non-symmetric and falls under (ii) above. Compounds that satisfy (i) above and are rotationally symmetric with respect to the hub ring are preferred because they can have a high refractive index and good solubility.

<Adhesive>
The pressure-sensitive adhesive disclosed herein is characterized by containing a refractive index adjuster as described above. The type of pressure-sensitive adhesive is not particularly limited, and may be, for example, an acrylic pressure-sensitive adhesive, a rubber pressure-sensitive adhesive (natural rubber-based, synthetic rubber-based, a mixture thereof, etc.), a silicone pressure-sensitive adhesive, a polyester pressure-sensitive adhesive, a urethane pressure-sensitive adhesive, a polyether pressure-sensitive adhesive, a polyamide pressure-sensitive adhesive, a fluorine-based pressure-sensitive adhesive, etc. The acrylic pressure-sensitive adhesive refers to a pressure-sensitive adhesive having an acrylic polymer as the base polymer. The same applies to other rubber-based pressure-sensitive adhesives. In some embodiments, acrylic pressure-sensitive adhesives are preferred from the viewpoints of ease of adjusting adhesive properties, optical properties, etc.

The adhesives disclosed herein can be formed using an adhesive composition containing a refractive index modifier (typically, an adhesive composition containing at least a base polymer and a refractive index modifier). The form of the adhesive composition is not particularly limited, and can be in various forms, such as a solvent-based adhesive composition containing adhesive-forming components in an organic solvent; an active energy ray-curable adhesive composition prepared to form an adhesive upon curing (converting into a viscoelastic body) with active energy rays such as ultraviolet light or radiation; a water-dispersed adhesive composition in which adhesive-forming components are dispersed in water; or a hot-melt adhesive composition that is applied in a heated, molten state and forms an adhesive upon cooling to near room temperature. While not particularly limited, the adhesives disclosed herein can be preferably formed using a solvent-based adhesive composition from the standpoint of ease of incorporation of the refractive index modifier. Solvent-based adhesive compositions can typically be formed by drying (preferably further crosslinking) the composition to form the adhesive. Active energy ray-curable adhesive compositions are typically formed by irradiating the composition with active energy rays to promote polymerization and/or crosslinking reactions. If the active energy ray-curable adhesive composition requires drying, it is recommended to irradiate it with active energy rays after drying.

The content of the refractive index adjuster in the adhesive is not particularly limited and can be set so as to obtain an adhesive with the desired refractive index. In some embodiments, the amount of refractive index adjuster used per 100 parts by weight of the adhesive base polymer (e.g., acrylic polymer) can be, for example, 0.1 parts by weight or more. From the perspective of achieving greater effectiveness, it is preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, and may be 3 parts by weight or more, 5 parts by weight or more, 7 parts by weight or more, or 9 parts by weight or more. Furthermore, the amount of refractive index adjuster used per 100 parts by weight of the base polymer can be, for example, 80 parts by weight or less. From the perspective of easily obtaining good adhesive properties, it is advantageous to use it per 60 parts by weight or less, preferably 45 parts by weight or less, and may be 30 parts by weight or less, 25 parts by weight or less, 20 parts by weight or less, or 15 parts by weight or less.

The absolute value of the difference between the refractive index n _a of the refractive index adjuster and the refractive index n _T of the adhesive containing the refractive index adjuster, i.e., |n _a -n _T | (hereinafter also referred to as |Δn _A |), can be set to be greater than 0. In some embodiments, |Δn _A | is, for example, 0.010 or more, preferably 0.020 or more, more preferably 0.030 or more, or may be 0.040 or more, 0.060 or more, 0.080 or more, 0.100 or more, 0.150 or more, 0.200 or more, or 0.250 or more. It can be said that an adhesive with a larger value of |Δn _A | has a greater contribution of the refractive index adjuster to the refractive index of the adhesive. Furthermore, in some embodiments, from the viewpoint of compatibility within the adhesive and transparency of the adhesive, |Δn _A | may be, for example, 0.500 or less, 0.400 or less, 0.300 or less, 0.250 or less, less than 0.200, or less than 0.100. The relationship between the refractive index n _a of the refractive index adjuster and the refractive index n _T of the adhesive may be n _a > n _T or n _a < n _T. That is, 0 < Δn _A or 0 > Δn _A may be satisfied. The refractive index adjuster disclosed herein can exhibit a higher refractive index than general adhesives (e.g., acrylic adhesives), and therefore can be preferably used as a refractive index increasing agent for adhesives, for example, in an embodiment where 0 < Δn _A.

The absolute value of the difference between the refractive index n _a of the refractive index modifier and the refractive index n _b of the base polymer, i.e., |n _b - n _a | (hereinafter also referred to as |Δn _B |), is set to be greater than 0. In some embodiments, |Δn _B | is, for example, 0.010 or more, preferably 0.020 or more, more preferably 0.035 or more, or may be 0.045 or more, 0.065 or more, 0.075 or more, 0.090 or more, 0.100 or more, 0.150 or more, 0.200 or more, or 0.250 or more. By selecting the base polymer and refractive index modifier so that the value of |Δn _B | is greater than 0.010, the refractive index modifier effect by use of the refractive index modifier tends to be enhanced. Furthermore, from the viewpoint of compatibility within the PSA and transparency of the PSA, in some embodiments, |Δn _B | may be, for example, 0.550 or less, 0.450 or less, 0.350 or less, 0.300 or less, less than 0.250, less than 0.150, or less than 0.130. The relationship between the refractive index n _a of the refractive index adjuster and the refractive index n _b of the base polymer may be n _a > n _b or n _a < n _b . That is, 0 < Δn _B or 0 > Δn _B may be satisfied. The refractive index adjuster disclosed herein can exhibit a higher refractive index than the base polymer of a general PSA (e.g., an acrylic polymer), and therefore can be preferably used as a refractive index increasing agent for a PSA, for example, in an embodiment where 0 < Δn _B.

The adhesives disclosed herein will be further explained below, primarily using acrylic adhesives as an example, but it is not intended to limit the adhesives disclosed herein to acrylic adhesives.

According to some preferred embodiments, the PSA is an acrylic PSA that uses an acrylic polymer as a base polymer and contains an aromatic ring-containing monomer (m1) as a monomer component constituting the acrylic polymer. The refractive index adjuster disclosed herein can be used in a PSA containing such a base polymer to effectively adjust the refractive index of the PSA (e.g., to increase the refractive index). Herein, the term "monomer component constituting the acrylic polymer" refers to a monomer that constitutes the repeating unit of the acrylic polymer in the PSA formed from the PSA composition, regardless of whether the monomer component is contained in the PSA composition in the form of a preformed polymer (which may be an oligomer) or an unpolymerized monomer. In other words, the monomer component constituting the acrylic polymer may be contained in the PSA composition in the form of a polymer, an unpolymer, or a partially polymerized product. From the standpoint of ease of preparation of the PSA composition, in some embodiments, a PSA composition is preferred in which substantially all (e.g., 95% by weight or more, preferably 99% by weight or more) of the monomer component constituting the acrylic polymer is in the form of a polymer.

(Monomer (m1))
The monomer (m1) is a compound containing at least one aromatic ring and at least one ethylenically unsaturated group in one molecule, and may be used alone or in combination of two or more of such compounds.

Examples of the ethylenically unsaturated group include a (meth)acryloyl group, a vinyl group, and a (meth)allyl group. From the viewpoint of polymerization reactivity, a (meth)acryloyl group is preferred, and from the viewpoints of flexibility and adhesiveness, an acryloyl group is more preferred. From the viewpoint of the flexibility of the adhesive, a compound containing one ethylenically unsaturated group per molecule (i.e., a monofunctional monomer) is preferably used as the monomer (m1).

The number of aromatic rings contained in one molecule of the compound used as monomer (m1) may be 1 or 2 or more. The upper limit of the number of aromatic rings is not particularly limited and may be, for example, 16 or less. In some embodiments, from the viewpoint of ease of preparation of the PSA composition and transparency of the PSA, the number of aromatic rings may be, for example, 12 or less, preferably 8 or less, more preferably 6 or less, or may be 5 or less, 4 or less, 3 or less, or 2 or less.

The aromatic ring contained in the compound used as monomer (m1) may be a carbocyclic ring such as a benzene ring (which may be a benzene ring constituting part of a biphenyl structure or a fluorene structure); a fused ring of a naphthalene ring, an indene ring, an azulene ring, an anthracene ring, or a phenanthrene ring; or a heterocyclic ring such as a pyridine ring, a pyrimidine ring, a pyridazine ring, a pyrazine ring, a triazine ring, a pyrrole ring, a pyrazole ring, an imidazole ring, a triazole ring, an oxazole ring, an isoxazole ring, a thiazole ring, or a thiophene ring. The heteroatoms contained as ring-constituting atoms in the heterocyclic ring may be, for example, one or more selected from the group consisting of nitrogen, sulfur, and oxygen. In some embodiments, the heteroatoms constituting the heterocyclic ring may be either or both nitrogen and sulfur. Monomer (m1) may have a structure in which one or more carbocyclic rings are fused with one or more heterocyclic rings, such as a dinaphthothiophene structure.

The aromatic ring (preferably a carbocyclic ring) may have one or more substituents on the ring-constituting atoms, or may have no substituents. If the aromatic ring has a substituent, examples of the substituent include, but are not limited to, an alkyl group, an alkoxy group, an aryloxy group, a hydroxyl group, a halogen atom (such as a fluorine atom, a chlorine atom, or a bromine atom), a hydroxyalkyl group, a hydroxyalkyloxy group, and a glycidyloxy group. In a carbon atom-containing substituent, the number of carbon atoms contained in the substituent is preferably 1 to 4, more preferably 1 to 3, for example, 1 or 2. In some embodiments, the aromatic ring may have no substituents on the ring-constituting atoms, or may have one or more substituents selected from the group consisting of an alkyl group, an alkoxy group, and a halogen atom (such as a bromine atom). The term "the aromatic ring of monomer (m1) has a substituent on its ring-constituting atom" refers to the aromatic ring having a substituent other than a substituent having an ethylenically unsaturated group.

The aromatic ring and the ethylenically unsaturated group may be bonded directly or via a linking group. The linking group may be, for example, a group containing one or more structures selected from alkylene groups, oxyalkylene groups, poly(oxyalkylene) groups, phenyl groups, alkylphenyl groups, alkoxyphenyl groups, groups in which one or more hydrogen atoms in these groups have been substituted with hydroxyl groups (e.g., hydroxyalkylene groups), oxy groups (-O- groups), thiooxy groups (-S- groups), etc. In some embodiments, aromatic ring-containing monomers in which the aromatic ring and the ethylenically unsaturated group are bonded directly or via a linking group selected from the group consisting of alkylene groups, oxyalkylene groups, and poly(oxyalkylene) groups are preferably used. The number of carbon atoms in the alkylene group and the oxyalkylene group is preferably 1 to 4, more preferably 1 to 3, for example, 1 or 2. The number of repeating oxyalkylene units in the poly(oxyalkylene) group may be, for example, 2 to 3.

Examples of compounds that can be preferably used as monomer (m1) include aromatic ring-containing (meth)acrylates and aromatic ring-containing vinyl compounds. The aromatic ring-containing (meth)acrylates and aromatic ring-containing vinyl compounds can each be used alone or in combination of two or more. One or more aromatic ring-containing (meth)acrylates and one or more aromatic ring-containing vinyl compounds can also be used in combination.

In some embodiments, a monomer having two or more aromatic rings (preferably carbon rings) in one molecule can be used as monomer (m1), as this is more likely to achieve a high refractive index. Examples of monomers having two or more aromatic rings in one molecule (monomers containing multiple aromatic rings) include monomers having a structure in which two or more non-fused aromatic rings are bonded via a linking group, monomers having a structure in which two or more non-fused aromatic rings are chemically bonded directly (i.e., without the aid of other atoms), monomers having a fused aromatic ring structure, monomers having a fluorene structure, monomers having a dinaphthothiophene structure, and monomers having a dibenzothiophene structure. The monomers containing multiple aromatic rings can be used alone or in combination of two or more.

The linking group may be, for example, an oxy group (-O-), a thiooxy group (-S-), an oxyalkylene group (for example, an -O-(CH ₂ ) _n - group, where n is 1 to 3, preferably 1), a thiooxyalkylene group (for example, an -S-(CH ₂ ) _n - group, where n is 1 to 3, preferably 1), a straight-chain alkylene group (i.e., a -(CH ₂ ) _n - group, where n is 1 to 6, preferably 1 to 3), the oxyalkylene group, the thiooxyalkylene group, and the straight-chain alkylene group in which the alkylene group is partially or completely halogenated, etc. From the viewpoint of the flexibility of the PSA, etc., preferred examples of the linking group include an oxy group, a thiooxy group, an oxyalkylene group, and a straight-chain alkylene group. Specific examples of monomers having a structure in which two or more non-fused aromatic rings are bonded via a linking group include phenoxybenzyl (meth)acrylate (e.g., m-phenoxybenzyl (meth)acrylate), thiophenoxybenzyl (meth)acrylate, and benzyl benzyl (meth)acrylate.

The above-mentioned monomer having a structure in which two or more non-fused aromatic rings are directly chemically bonded can be, for example, a biphenyl structure-containing (meth)acrylate, a triphenyl structure-containing (meth)acrylate, a vinyl group-containing biphenyl, etc. Specific examples include o-phenylphenol (meth)acrylate, biphenylmethyl (meth)acrylate, etc.

Examples of monomers having the above-mentioned fused aromatic ring structure include naphthalene ring-containing (meth)acrylates, anthracene ring-containing (meth)acrylates, vinyl group-containing naphthalenes, and vinyl group-containing anthracenes. Specific examples include 1-naphthylmethyl (meth)acrylate (also known as 1-naphthalenemethyl (meth)acrylate), hydroxyethylated β-naphthol acrylate, 2-naphthoethyl (meth)acrylate, 2-naphthoxyethyl acrylate, and 2-(4-methoxy-1-naphthoxy)ethyl (meth)acrylate.

Specific examples of monomers having the above-mentioned fluorene structure include 9,9-bis(4-hydroxyphenyl)fluorene(meth)acrylate and 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene(meth)acrylate. Note that because monomers having a fluorene structure contain a structural portion in which two benzene rings are directly chemically bonded, they are included in the concept of monomers having a structure in which two or more non-fused aromatic rings are directly chemically bonded.

Examples of the monomer having the dinaphthothiophene structure include (meth)acryloyl group-containing dinaphthothiophene, vinyl group-containing dinaphthothiophene, and (meth)allyl group-containing dinaphthothiophene. Specific examples include (meth)acryloyloxymethyldinaphthothiophene (for example, a compound having a structure in which _CH2CH ( ^R1 )C(O) _OCH2- is bonded to the 5th or 6th position of the dinaphthothiophene ring, where ^R1 is a hydrogen atom or a methyl group), (meth)acryloyloxyethyldinaphthothiophene (for example, a compound having a structure in which _CH2CH ( ^R1 )C(O)OCH( _CH3 )- or _CH2CH ( ^R1 )C(O) _OCH2CH2- is bonded to the 5th or 6th position of the dinaphthothiophene ring, where ^R1 is a hydrogen atom or a methyl group), _{vinyldinaphthothiophene} (for example, a compound having a structure in which a vinyl group is bonded to the 5th or 6th position of the naphthothiophene ring), and (meth)allyloxydinaphthothiophene. In addition, a monomer having a dinaphthothiophene structure is included in the concept of a monomer having the above-mentioned fused aromatic ring structure because it contains a naphthalene structure and also because it has a structure in which a thiophene ring and two naphthalene structures are fused together.

Examples of the monomer having the dibenzothiophene structure include (meth)acryloyl group-containing dibenzothiophene, vinyl group-containing dibenzothiophene, etc. Note that the monomer having the dibenzothiophene structure has a structure in which a thiophene ring and two benzene rings are fused, and therefore is included in the concept of the monomer having the fused aromatic ring structure.
Note that neither the dinaphthothiophene structure nor the dibenzothiophene structure corresponds to a structure in which two or more non-fused aromatic rings are directly chemically bonded.

In some preferred embodiments, a monomer having one aromatic ring (preferably a carbon ring) per molecule is used as monomer (m1). A monomer having one aromatic ring per molecule (aromatic ring-single-containing monomer) can be useful, for example, for improving the flexibility of the adhesive, adjusting the adhesive properties, and improving transparency. Aromatic ring-single-containing monomers can be used alone or in combination of two or more. In some embodiments, a monomer having one aromatic ring per molecule may be used in combination with a monomer having multiple aromatic rings, from the perspective of improving the refractive index of the adhesive.

Examples of monomers having one aromatic ring per molecule include carbon-containing aromatic ring (meth)acrylates such as benzyl (meth)acrylate, methoxybenzyl (meth)acrylate, phenyl (meth)acrylate, ethoxylated phenol (meth)acrylate, phenoxypropyl (meth)acrylate, phenoxybutyl (meth)acrylate, cresyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, and chlorobenzyl (meth)acrylate; 2-(4,6-dibromo-2-s-butylphenoxy)ethyl (meth)acrylate, 2-(4,6-dibromo-2-isopropylphenoxy)ethyl (meth)acrylate, and 6- Examples include bromine-substituted aromatic ring-containing (meth)acrylates such as (4,6-dibromo-2-s-butylphenoxy)hexyl (meth)acrylate, 6-(4,6-dibromo-2-isopropylphenoxy)hexyl (meth)acrylate, 2,6-dibromo-4-nonylphenyl acrylate, and 2,6-dibromo-4-dodecylphenyl acrylate; carbon-containing aromatic ring-containing vinyl compounds such as styrene, α-methylstyrene, vinyltoluene, and tert-butylstyrene; and compounds having a vinyl substituent on a heteroaromatic ring such as N-vinylpyridine, N-vinylpyrimidine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, and N-vinyloxazole.

As the monomer (m1), a monomer having a structure in which an oxyethylene chain is interposed between the ethylenically unsaturated group and the aromatic ring in the various aromatic ring-containing monomers described above may be used. Such a monomer having an oxyethylene chain interposed between the ethylenically unsaturated group and the aromatic ring can be understood as an ethoxylated product of the original monomer. The number of repeating oxyethylene units (—CH ₂ CH ₂ O—) in the oxyethylene chain is typically 1 to 4, preferably 1 to 3, more preferably 1 to 2, for example, 1. Specific examples of ethoxylated aromatic ring-containing monomers include ethoxylated o-phenylphenol (meth)acrylate, ethoxylated nonylphenol (meth)acrylate, ethoxylated cresol (meth)acrylate, phenoxyethyl (meth)acrylate, and phenoxydiethylene glycol di(meth)acrylate.

The content of the multiple aromatic ring-containing monomer in monomer (m1) is not particularly limited and may be, for example, 5% by weight or more, 25% by weight or more, or 40% by weight or more. In some embodiments, the content of the multiple aromatic ring-containing monomer in monomer (m1) may be, for example, 50% by weight or more. From the viewpoint of easily achieving a higher refractive index, it is preferably 70% by weight or more, and may be 85% by weight or more, 90% by weight or more, or even 95% by weight or more. Substantially 100% by weight of monomer (m1) may be multiple aromatic ring-containing monomer. In other words, only one or two or more multiple aromatic ring-containing monomers may be used as monomer (m1). Furthermore, in some embodiments, for example, taking into consideration the balance between a high refractive index and flexibility (e.g., viscoelastic properties such as storage modulus), the content of the multiple aromatic ring monomer in monomer (m1) may be less than 100% by weight, 98% by weight or less, 90% by weight or less, 80% by weight or less, 70% by weight or less, 65% by weight or less, 50% by weight or less, 25% by weight or less, or 10% by weight or less. The technology disclosed herein can also be practiced in an embodiment in which the content of the multiple aromatic ring monomer in monomer (m1) is less than 5% by weight. It is not necessary to use a multiple aromatic ring monomer.

The content of the multiple aromatic ring-containing monomer in the monomer components constituting the acrylic polymer is not particularly limited and can be set according to the purpose. The content of the multiple aromatic ring-containing monomer in the monomer components may be, for example, 3% by weight or more, 10% by weight or more, or 25% by weight or more. In some embodiments, to facilitate the realization of a pressure-sensitive adhesive with a higher refractive index, the content of the multiple aromatic ring-containing monomer in the monomer components may be, for example, greater than 35% by weight, advantageously greater than 50% by weight, preferably greater than 70% by weight, or may be 75% by weight or more, 85% by weight or more, 90% by weight or more, or even 95% by weight or more. In consideration of the balance between a high refractive index and flexibility, the content of the multiple aromatic ring-containing monomer in the monomer components is advantageously approximately 99% by weight or less, preferably 98% by weight or less, more preferably 96% by weight or less, or may be 93% by weight or less, 90% by weight or less, 85% by weight or less, 80% by weight or less, or 75% by weight or less. In some embodiments, to facilitate achieving better adhesive properties and/or optical properties (e.g., transparency), the content of the multiple aromatic ring-containing monomer in the monomer component may be 70% by weight or less, 60% by weight or less, 50% by weight or less, 40% by weight or less, 25% by weight or less, 15% by weight or less, or 5% by weight or less. The technology disclosed herein can also be implemented in embodiments in which the content of the multiple aromatic ring-containing monomer in the monomer component is less than 3% by weight.

The content of the aromatic ring unit-containing monomer in monomer (m1) is not particularly limited and may be, for example, 5% by weight or more, 25% by weight or more, or 40% by weight or more. In some embodiments, the content of the aromatic ring unit-containing monomer in monomer (m1) may be, for example, 50% by weight or more. From the viewpoint of easily achieving a higher refractive index, it is preferably 70% by weight or more, and may be 85% by weight or more, 90% by weight or more, or even 95% by weight or more. Substantially 100% by weight of monomer (m1) may be the aromatic ring unit-containing monomer. In other words, only one or two or more aromatic ring unit-containing monomers may be used as monomer (m1). Furthermore, in some embodiments, for example, taking into consideration the balance between high refractive index and flexibility, the content of the aromatic ring unit-containing monomer in monomer (m1) may be less than 100% by weight, 98% by weight or less, 90% by weight or less, 80% by weight or less, 70% by weight or less, 65% by weight or less, 50% by weight or less, 25% by weight or less, or 10% by weight or less. The technology disclosed herein can also be practiced in an embodiment in which the content of the aromatic ring unit-containing monomer in monomer (m1) is less than 5% by weight. It is not necessary to use the aromatic ring unit-containing monomer.

The content of the aromatic ring unit-containing monomer in the monomer components constituting the acrylic polymer is not particularly limited and can be set according to the purpose. The content of the aromatic ring unit-containing monomer in the monomer components may be, for example, 3% by weight or more, 10% by weight or more, or 25% by weight or more. In some embodiments, to facilitate the realization of a pressure-sensitive adhesive with a higher refractive index, the content of the aromatic ring unit-containing monomer in the monomer components may be, for example, greater than 35% by weight, advantageously greater than 50% by weight, preferably greater than 60% by weight, more preferably greater than 70% by weight, 75% by weight or more, 85% by weight or more, 90% by weight or more, 95% by weight or more, or 98% by weight or more. Considering the balance between high refractive index and flexibility, the content of the aromatic ring unit-containing monomer in the monomer component may be approximately 99% by weight or less, preferably 98% by weight or less, more preferably 96% by weight or less, and may be 93% by weight or less, 90% by weight or less, 85% by weight or less, 80% by weight or less, or 75% by weight or less. In some embodiments, to facilitate the realization of higher adhesive properties and/or optical properties (e.g., transparency), the content of the aromatic ring unit-containing monomer in the monomer component may be 70% by weight or less, 60% by weight or less, 50% by weight or less, 40% by weight or less, 25% by weight or less, 15% by weight or less, or 5% by weight or less. The technology disclosed herein may also be implemented in embodiments in which the content of the aromatic ring unit-containing monomer in the monomer component is less than 3% by weight.

In some embodiments of the technology disclosed herein, a high refractive index monomer may be preferably used as at least a portion of the monomer (m1). Here, "high refractive index monomer" refers to a monomer having a refractive index of, for example, approximately 1.510 or more, preferably approximately 1.530 or more, and more preferably approximately 1.550 or more. The upper limit of the refractive index of the high refractive index monomer is not particularly limited, but from the viewpoint of ease of preparation of the pressure-sensitive adhesive composition and ease of achieving compatibility with flexibility suitable for a pressure-sensitive adhesive, it may be, for example, 3.000 or less, 2.500 or less, 2.000 or less, 1.900 or less, 1.800 or less, or 1.700 or less. The high refractive index monomer may be used alone or in combination of two or more.
The refractive index of the monomer is measured using an Abbe refractometer at a wavelength of 589 nm and a temperature of 25°C. The Abbe refractometer may be an ATAGO DR-M4 model or an equivalent. If the manufacturer provides a nominal value for the refractive index at 25°C, that nominal value may be used.

As the high refractive index monomer, compounds having the appropriate refractive index can be appropriately selected from among the compounds encompassed by the concept of aromatic ring-containing monomer (m1) disclosed herein (for example, the compounds and compound groups exemplified above). Specific examples include m-phenoxybenzyl acrylate (refractive index: 1.566, homopolymer Tg: -35°C), 1-naphthylmethyl acrylate (refractive index: 1.595, homopolymer Tg: 31°C), ethoxylated o-phenylphenol acrylate (number of repeating oxyethylene units: 1, refractive index: 1.578), benzyl acrylate (refractive index (nD20): 1.519, homopolymer Tg: 6°C), phenoxyethyl acrylate (refractive index (nD20): 1.517, homopolymer Tg: 2°C), and phenoxydiethylene glycol acrylate (refractive index: 1.510, homopolymer Tg: 1.520). Examples of such thiophenes include, but are not limited to, 6-acryloyloxymethyldinaphthothiophene (6MDNTA, refractive index: 1.75), 6-methacryloyloxymethyldinaphthothiophene (6MDNTMA, refractive index: 1.726), 5-acryloyloxyethyldinaphthothiophene (5EDNTA, refractive index: 1.786), 6-acryloyloxyethyldinaphthothiophene (6EDNTA, refractive index: 1.722), 6-vinyldinaphthothiophene (6VDNT, refractive index: 1.802), and 5-vinyldinaphthothiophene (abbreviation: 5VDNT, refractive index: 1.793).

The content of high refractive index monomers (i.e., aromatic ring-containing monomers having a refractive index of approximately 1.510 or greater, preferably approximately 1.530 or greater, and more preferably approximately 1.550 or greater) in monomer (m1) is not particularly limited and may be, for example, 5% by weight or greater, 25% by weight or greater, 35% by weight or greater, or 40% by weight or greater. In some embodiments, to facilitate obtaining a higher refractive index, the content of high refractive index monomers in monomer (m1) may be, for example, 50% by weight or greater, preferably 70% by weight or greater, 85% by weight or greater, 90% by weight or greater, or 95% by weight or greater. Substantially 100% by weight of monomer (m1) may be high refractive index monomers. Furthermore, in some embodiments, for example, from the viewpoint of achieving a good balance between a high refractive index and flexibility, the content of the high refractive index monomer in monomer (m1) may be less than 100% by weight, 98% by weight or less, 90% by weight or less, 80% by weight or less, or 65% by weight or less.

The content of high-refractive-index monomers in the monomer components constituting the acrylic polymer is not particularly limited and can be set according to the purpose. Furthermore, if necessary, it can be set while taking into consideration compatibility with adhesive properties (e.g., adhesive strength) and/or optical properties (e.g., total light transmittance, haze value, etc.). The content of high-refractive-index monomers in the monomer components may be, for example, 3% by weight or more, 10% by weight or more, or 25% by weight or more. In some embodiments, the content of high-refractive-index monomers in the monomer components constituting the acrylic polymer may be, for example, greater than 35% by weight. To facilitate achieving a higher refractive index, it is advantageous to have a content of greater than 50% by weight, preferably greater than 70% by weight, and may be 75% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. From the viewpoint of achieving a good balance between a high refractive index and flexibility, the content of the high refractive index monomer in the above monomer component is advantageously 99% by weight or less, preferably 98% by weight or less, more preferably 96% by weight or less, and may be 93% by weight or less, 90% by weight or less, 85% by weight or less, 80% by weight or less, or 75% by weight or less.

In some preferred embodiments, an aromatic ring-containing monomer (hereinafter sometimes referred to as "monomer L") having a homopolymer Tg of 10°C or less is used as at least a portion of the monomer (m1). Increasing the content of aromatic ring-containing monomer (m1) in the monomer component (particularly aromatic ring-containing monomer (m1) corresponding to at least one of the above-mentioned multiple aromatic ring-containing monomer, single aromatic ring-containing monomer, and high refractive index monomer) generally tends to increase the storage modulus G' of the PSA. However, by using monomer L as part or all of the monomer (m1), the increase in storage modulus G' can be suppressed. This allows for an improvement in refractive index while better maintaining flexibility suitable for a PSA. The Tg of monomer L may be, for example, 5°C or less, 0°C or less, -10°C or less, -20°C or less, or -25°C or less. There is no particular lower limit for the Tg of monomer L. Taking into consideration the balance with the refractive index improving effect, in some embodiments, the Tg of monomer L may be, for example, -70°C or higher, -55°C or higher, or -45°C or higher. In other embodiments, the Tg of monomer L may be, for example, -30°C or higher, -10°C or higher, 0°C or higher, or 3°C or higher. Monomer L can be used alone or in combination of two or more types.

Monomer L can be selected from compounds encompassed within the concept of aromatic ring-containing monomer (m1) disclosed herein (e.g., the compounds and compound groups exemplified above) that have the appropriate Tg. Suitable examples of aromatic ring-containing monomers that can be used as monomer L include m-phenoxybenzyl acrylate (homopolymer Tg: -35°C), benzyl acrylate (homopolymer Tg: 6°C), phenoxyethyl acrylate (homopolymer Tg: 2°C), and phenoxydiethylene glycol acrylate (homopolymer Tg: -35°C).

The content of monomer L in monomer (m1) is not particularly limited and may be, for example, 5% by weight or more, 25% by weight or more, or 40% by weight or more. In some embodiments, from the viewpoint of easily obtaining a PSA that simultaneously achieves a high refractive index and a high level of flexibility, the content of monomer L in monomer (m1) may be, for example, 50% by weight or more. From the viewpoint of achieving a low modulus of elasticity, the content is preferably 60% by weight or more, and may be 70% by weight or more, 75% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. Monomer (m1) may comprise substantially 100% by weight of monomer L. Furthermore, in some embodiments, from the viewpoint of achieving a good balance between a high refractive index and flexibility, the content of monomer L in monomer (m1) may be less than 100% by weight, 98% by weight or less, 90% by weight or less, 80% by weight or less, or 65% by weight or less.

The content of monomer L in the monomer components constituting the acrylic polymer may be, for example, 3% by weight or more, 10% by weight or more, or 25% by weight or more. In some embodiments, from the viewpoint of easily obtaining a pressure-sensitive adhesive that simultaneously achieves a high refractive index and a high level of flexibility, the content of monomer L in the monomer components may be, for example, greater than 35% by weight. From the viewpoint of improving the refractive index, it is advantageous to have it be greater than 50% by weight, preferably greater than 70% by weight, or it may be 75% by weight or more, 85% by weight or more, 90% by weight or more, or even 95% by weight or more. From the viewpoint of achieving a good balance between a high refractive index and flexibility, the content of monomer L in the above-mentioned monomer components is advantageously approximately 99% by weight or less, preferably 98% by weight or less, more preferably 96% by weight or less, or it may be 93% by weight or less, 90% by weight or less, 85% by weight or less, 80% by weight or less, or 75% by weight or less.

In some embodiments, the glass transition temperature Tg _m1 based on the composition of monomer (m1) is suitably approximately 20°C or lower, preferably 10°C or lower, from the viewpoint of flexibility of the PSA. For example, it may be 5°C or lower, 0°C or lower, -10°C or lower, -20°C or lower, or -25°C or lower. The lower limit of the glass transition temperature Tg _m1 is not particularly limited. In consideration of the balance with the refractive index improvement effect, in some embodiments, the glass transition temperature Tg _m1 may be, for example, -70°C or higher, -55°C or higher, or -45°C or higher. The technology disclosed herein can also be suitably implemented in embodiments in which the glass transition temperature Tg _m1 is, for example, -40°C or higher, -35°C or higher, -33°C or higher, -30°C or higher, or -25°C or higher. In other embodiments, the glass transition temperature Tg _m1 may be, for example, -10°C or higher, 0°C or higher, or 3°C or higher.

Here, the glass transition temperature Tg _m1 based on the composition of monomer (m1) refers to the Tg calculated by the Fox formula described below based on the composition of only monomer (m1) among the monomer components constituting the acrylic polymer. The glass transition temperature Tg _m1 can be calculated by applying the Fox formula to only monomer (m1) among the monomer components constituting the acrylic polymer, from the glass transition temperature of a homopolymer of each aromatic ring-containing monomer used as monomer (m1) and the weight fraction of each aromatic ring-containing monomer in the total amount of monomer (m1). In an embodiment in which only one type of monomer is used as monomer (m1), the Tg of the homopolymer of that monomer and the glass transition temperature Tg _m1 are the same.

In some embodiments, the aromatic ring-containing monomer (m1) can be a combination of monomer L (i.e., an aromatic ring-containing monomer having a homopolymer Tg of 10°C or less) and monomer H having a Tg higher than 10°C. The Tg of monomer H may be, for example, greater than 10°C, greater than 15°C, or greater than 20°C. By using monomer L and monomer H in combination, a PSA having a high content of aromatic ring-containing monomer (m1) in the monomer component can achieve both a high refractive index and flexibility suitable for adhesion to an adherend at a higher level. The ratio of the amounts of monomer L and monomer H used can be set so as to suitably exhibit this effect, and is not particularly limited. For example, it is preferable to set the ratio of the amounts of monomer L and monomer H used so as to satisfy any of the glass transition temperatures Tg _m1 described above.

In some embodiments, the aromatic ring-containing monomer (m1) can be preferably selected from compounds that do not contain a structure in which two or more non-fused aromatic rings are directly chemically bonded (e.g., a biphenyl structure). For example, an acrylic polymer composed of monomer components having a composition in which the content of a compound containing a structure in which two or more non-fused aromatic rings are directly chemically bonded is less than 5 wt % (more preferably less than 3 wt %, and may even be 0 wt %) is preferred. Limiting the amount of the compound containing a structure in which two or more non-fused aromatic rings are directly chemically bonded in this way can be advantageous from the perspective of achieving a pressure-sensitive adhesive that achieves a better balance between a high refractive index and flexibility.

The content of monomer (m1) in the monomer components constituting the acrylic polymer is not particularly limited and can be set depending on the purpose, for example, to achieve a pressure-sensitive adhesive having one or more of the desired refractive index, elastic modulus, adhesive properties (e.g., adhesive strength), and optical properties (e.g., total light transmittance, haze value, etc.). In some embodiments, the content of monomer (m1) in the monomer components may be, for example, 30% by weight or more, preferably 50% by weight or more, and more preferably 60% by weight or more, or even 70% by weight or more. In some preferred embodiments, the content of monomer (m1) in the monomer components constituting the acrylic polymer may be, for example, more than 70% by weight, with 75% by weight or more being appropriate. To facilitate achieving a higher refractive index, the content is preferably 80% by weight or more, and may be 85% by weight or more, 90% by weight or more, or even 95% by weight or more. The content of monomer (m1) in the above-mentioned monomer component is typically less than 100% by weight. From the viewpoint of achieving a good balance between a high refractive index and flexibility, it is advantageous to have it be approximately 99% by weight or less, preferably 98% by weight or less, more preferably 96% by weight or less, and may be 93% by weight or less, or may be 90% by weight or less. In some embodiments, from the viewpoint of easily achieving higher adhesive properties and/or optical properties (e.g., transparency), the content of monomer (m1) in the above-mentioned monomer component may be less than 90% by weight, less than 85% by weight, or less than 80% by weight.

(Monomer (m2))
In some preferred embodiments, the monomer components constituting the acrylic polymer may further contain a monomer (m2) in addition to the monomer (m1). The monomer (m2) is at least one of a monomer having a hydroxyl group (hydroxyl group-containing monomer) and a monomer having a carboxyl group (carboxy group-containing monomer). The hydroxyl group-containing monomer is a compound having at least one hydroxyl group and at least one ethylenically unsaturated group in one molecule. The carboxyl group-containing monomer is a compound having at least one carboxyl group and at least one ethylenically unsaturated group in one molecule. The monomer (m2) can be useful for introducing crosslinking points into the acrylic polymer or imparting appropriate cohesive properties to the PSA. The monomer (m2) can be used alone or in combination of two or more types. The monomer (m2) is typically a monomer that does not contain an aromatic ring.

Examples of the ethylenically unsaturated group contained in the monomer (m2) include a (meth)acryloyl group, a vinyl group, and a (meth)allyl group. From the viewpoint of polymerization reactivity, a (meth)acryloyl group is preferred, and from the viewpoints of flexibility and adhesiveness, an acryloyl group is more preferred. From the viewpoint of the flexibility of the adhesive, a compound containing one ethylenically unsaturated group per molecule (i.e., a monofunctional monomer) is preferably used as the monomer (m2).

Examples of hydroxyl group-containing monomers include, but are not limited to, hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-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. Preferred examples of hydroxyl group-containing monomers include 4-hydroxybutyl acrylate (Tg: -40°C) and 2-hydroxyethyl acrylate (Tg: -15°C). From the perspective of improving flexibility at room temperature, 4-hydroxybutyl acrylate, which has a lower Tg, is more preferred. In a preferred embodiment, 50% by weight or more (e.g., more than 50% by weight, more than 70% by weight, or more than 85% by weight) of monomer (m2) may be 4-hydroxybutyl acrylate. The hydroxyl group-containing monomers can be used alone or in combination of two or more.

In some embodiments in which a hydroxyl group-containing monomer is used as monomer (m2), the hydroxyl group-containing monomer may be one or more compounds selected from compounds that do not contain a methacryloyl group. Suitable examples of hydroxyl group-containing monomers that do not contain a methacryloyl group include the various hydroxyalkyl acrylates described above. For example, it is preferable that more than 50 wt%, more than 70 wt%, or more than 85 wt% of the hydroxyl group-containing monomers used as monomer (m2) are hydroxyalkyl acrylates. The use of hydroxyalkyl acrylates allows for the introduction of hydroxy groups into the acrylic polymer, which are useful for providing crosslinking points and imparting appropriate cohesion, and also makes it easier to obtain a PSA with good flexibility and adhesion at room temperature compared to using only the corresponding hydroxyalkyl methacrylate.

Examples of carboxyl group-containing monomers include, but are not limited to, acrylic monomers such as (meth)acrylic acid, carboxyethyl (meth)acrylate, and carboxypentyl (meth)acrylate, as well as itaconic acid, maleic acid, fumaric acid, crotonic acid, and isocrotonic acid. Preferred examples of carboxyl group-containing monomers include acrylic acid and methacrylic acid. One type of carboxyl group-containing monomer may be used alone, or two or more types may be used in combination. Hydroxyl group-containing monomers and carboxyl group-containing monomers may also be used in combination.

The content of monomer (m2) in the monomer components constituting the acrylic polymer is not particularly limited and can be set according to the purpose. In some embodiments, the content of monomer (m2) may be, for example, 0.01% by weight or more, 0.1% by weight or more, or 0.5% by weight or more. From the viewpoint of achieving a higher usage effect, in some embodiments, the content of monomer (m2) is preferably 1% by weight or more, or may be 2% by weight or more, or may be 4% by weight or more. The upper limit of the content of monomer (m2) in the monomer components is set so that the total content of monomer (m2) and monomer (m1) does not exceed 100% by weight. In some embodiments, the content of monomer (m2) is suitably, for example, 30% by weight or less or 25% by weight or less. From the viewpoint of relatively increasing the content of monomer (m1) to facilitate a high refractive index, the content is preferably 20% by weight or less, more preferably 15% by weight or less, and may be less than 12% by weight, 10% by weight, or 7% by weight. In some embodiments, the content of the monomer (m2) may be less than 5% by weight, less than 3% by weight, or 1.5% by weight or less.

The total content of monomer (m1) and monomer (m2) in the monomer components constituting the acrylic polymer may be, for example, 31% by weight or more, preferably 51% by weight or more, or 61% by weight or more, or even 71% by weight or more. In some embodiments, the total content of monomer (m1) and monomer (m2) in the monomer components constituting the acrylic polymer may be, for example, 76% by weight or more, preferably 81% by weight or more, or 86% by weight or more, or 91% by weight or more, or 96% by weight or more, or 99% by weight or more, or even substantially 100% by weight, from the viewpoint of making it easier for these monomers to exhibit their effects.

(Monomer m3)
In some preferred embodiments, the monomer components constituting the acrylic polymer may further contain, in addition to the monomer (m1), an alkyl(meth)acrylate (hereinafter also referred to as "monomer (m3)"). The monomer (m3) may be useful for improving the flexibility of the PSA. It may also be useful for improving the compatibility of additives in the PSA and adhesive properties such as adhesive strength. The monomer (m3) may be used alone or in combination of two or more.

As the monomer (m3), an alkyl(meth)acrylate having a linear or branched alkyl group having 1 to 20 carbon atoms (i.e., C _1-20 ) at the ester terminal can be preferably used. Specific examples of the C _1-20 alkyl(meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and isooctyl (meth)acrylate. [0043] Examples of the acrylate include, but are not limited to, butyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate.

In some embodiments, an alkyl (meth)acrylate having a homopolymer Tg of -20°C or lower (more preferably -40°C or lower, e.g., -50°C or lower) can be preferably used as at least a portion of the monomer (m3). Such low-Tg alkyl (meth)acrylates can be useful for reducing the elastic modulus of the PSA. They can also be useful for improving adhesive properties such as adhesive strength. The lower limit of the Tg of the alkyl (meth)acrylate is not particularly limited, and may be, for example, -85°C or higher, -75°C or higher, -65°C or higher, or -60°C or higher. Specific examples of the low-Tg alkyl (meth)acrylates include n-butyl acrylate (BA), 2-ethylhexyl acrylate (2EHA), and isononyl acrylate (iNA).

In some embodiments using monomer (m3), it is preferable to use a _C4-8 alkyl(meth)acrylate as monomer (m3). Of these, it is more preferable to use a _C4-8 alkyl(meth)acrylate. The _C4-8 alkyl(meth)acrylate can be used alone or in combination of two or more. The use of a _C4-8 alkyl(meth)acrylate tends to facilitate improvement in the flexibility of the PSA and facilitate the attainment of good adhesive properties (such as adhesive strength). In embodiments using a _C4-8 alkyl(meth)acrylate as monomer (m3), the proportion of the _C4-8 alkyl(meth)acrylate in the alkyl(meth)acrylate contained in the monomer component is suitably 30% by weight or more, preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 90% by weight or more, and may even be substantially 100% by weight.

In some embodiments using monomer (m3), C _1-6 alkyl(meth)acrylate may be preferably used as monomer (m3). Use of C _1-6 alkyl(meth)acrylate makes it possible to adjust the storage modulus in each temperature range. For example, it is possible to set the storage modulus in the high temperature range relatively high, or to prevent the difference in storage modulus between the low and high temperature ranges from increasing. C _1-6 alkyl(meth)acrylate also tends to have excellent copolymerizability with monomer (m1). C _1-6 alkyl(meth)acrylates may be used alone or in combination of two or more. As the C _1-6 alkyl(meth)acrylate, C _1-6 alkyl acrylate is preferred, C _2-6 alkyl acrylate is more preferred, and C _4-6 alkyl acrylate is even more preferred. In some other embodiments, the C _1-6 alkyl (meth)acrylate is preferably a C _1-4 alkyl (meth)acrylate, more preferably a C _2-4 alkyl (meth)acrylate, and even more preferably a C _2-4 alkyl acrylate. A suitable example of the C _1-6 alkyl (meth)acrylate is BA.

The content of the C _1-6 alkyl (meth)acrylate in the monomer components constituting the acrylic polymer may be, for example, 1% by weight or more, 3% by weight or more, 5% by weight or more, or 8% by weight or more. In some embodiments, the content of the C _1-6 alkyl (meth)acrylate may be 10% by weight or more, 15% by weight or more, 20% by weight or more, or 25% by weight or more (e.g., 30% by weight or more) from the viewpoints of lowering the elastic modulus, adhesive strength, etc. The upper limit of the content of the C _1-6 alkyl (meth)acrylate in the monomer components is, for example, less than 50% by weight or may be less than 35% by weight. In some embodiments, from the viewpoint of maintaining a high refractive index, the content of the C _1-6 alkyl (meth)acrylate is, for example, 24% by weight or less, preferably less than 20% by weight, more preferably less than 17% by weight, and may be less than 12% by weight, less than 7% by weight, less than 3% by weight, or less than 1% by weight. The technology disclosed herein can also be practiced in an embodiment in which substantially no C _1-6 alkyl (meth)acrylate is used.

In some other embodiments using monomer (m3), a C _7-12 alkyl(meth)acrylate may be preferably used as monomer (m3). Use of a C _7-12 alkyl(meth)acrylate can preferably reduce the storage modulus. The C _7-12 alkyl(meth)acrylate may be used alone or in combination of two or more. As the C _7-12 alkyl(meth)acrylate, a C _7-10 alkyl acrylate is preferred, a C _7-9 alkyl acrylate is more preferred, and a C ₈ alkyl acrylate is even more preferred. A suitable example of a C _7-12 alkyl(meth)acrylate is 2EHA.

The content of the C _7-12 alkyl (meth)acrylate in the monomer component constituting the acrylic polymer may be, for example, 1% by weight or more, 3% by weight or more, 5% by weight or more, or 8% by weight or more. In some embodiments, the content of the C _7-12 alkyl (meth)acrylate may be 10% by weight or more, 15% by weight or more, 20% by weight or more, or 25% by weight or more (e.g., 30% by weight or more) from the viewpoints of lowering the elastic modulus, adhesive strength, etc. The upper limit of the content of the C _7-12 alkyl (meth)acrylate in the monomer component is, for example, less than 50% by weight, or may be less than 35% by weight. In some embodiments, from the viewpoint of maintaining a high refractive index, the content of the C _7-12 alkyl (meth)acrylate is, for example, 24% by weight or less, preferably less than 20% by weight, more preferably less than 17% by weight, and may be less than 12% by weight, less than 7% by weight, less than 3% by weight, or less than 1% by weight. The technology disclosed herein can also be practiced in an embodiment in which substantially no C _7-12 alkyl (meth)acrylate is used.

In some embodiments using monomer (m3), from the viewpoint of achieving a low modulus of elasticity, it is preferable that at least a portion of the monomer (m3) is an alkyl acrylate. The use of an alkyl acrylate is also advantageous in terms of adhesive properties such as adhesive strength. For example, it is preferable that 50% by weight or more of the monomer (m3) is an alkyl acrylate, and the proportion of alkyl acrylate in the monomer (m3) is more preferably 75% by weight or more, and even more preferably 90% by weight or more. In some embodiments, substantially 100% by weight of the monomer (m3) may be an alkyl acrylate. In some embodiments, only one or more alkyl acrylates are used as the monomer (m3), and no alkyl methacrylate is used.

In embodiments in which the monomer component includes an alkyl (meth)acrylate, the content of the alkyl (meth)acrylate in the monomer component can be set so as to appropriately achieve the intended effect. In some embodiments, the content of the alkyl (meth)acrylate may be, for example, 1% by weight or more, 3% by weight or more, 5% by weight or more, or 8% by weight or more. The upper limit of the content of monomer (m3) in the monomer component is set so that the sum of the contents of monomers (m1) and (m2) does not exceed 100% by weight, and may be, for example, less than 50% by weight or less than 35% by weight. In some embodiments, the content of monomer (m3) may be, for example, 24% by weight or less. Because alkyl (meth)acrylates generally have a relatively low refractive index, in order to achieve a high refractive index, it is advantageous to limit the content of monomer (m3) in the monomer component and increase the content of monomer (m1) relatively. From this perspective, the content of monomer (m3) is suitably less than 23% by weight of the monomer components, preferably less than 20% by weight, more preferably less than 17% by weight, and may be less than 12% by weight, less than 7% by weight, less than 3% by weight, or less than 1% by weight. The technology disclosed herein can also be preferably implemented in an embodiment in which monomer (m3) is not substantially used.

(Other Monomers)
The monomer components constituting the acrylic polymer may contain, as necessary, monomers other than the above-mentioned monomers (m1), (m2), and (m3) (hereinafter referred to as "other monomers"). The above-mentioned other monomers can be used for purposes such as adjusting the Tg of the acrylic polymer, adjusting the adhesive performance, and improving compatibility within the adhesive layer. The above-mentioned other monomers can be used alone or in combination of two or more.

Examples of the other monomers include monomers containing functional groups other than hydroxyl groups and carboxyl groups (functional group-containing monomers). For example, other monomers that can improve the cohesive strength and heat resistance of the adhesive include sulfonic acid group-containing monomers, phosphate group-containing monomers, and cyano group-containing monomers. Furthermore, examples of monomers that can introduce functional groups that can serve as crosslinking base points into acrylic polymers or that can contribute to improving adhesion to the adherend or compatibility within the adhesive include amide group-containing monomers (e.g., (meth)acrylamide, N-methylol (meth)acrylamide, etc.), amino group-containing monomers (e.g., aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, etc.), monomers with nitrogen atom-containing rings (e.g., N-vinyl-2-pyrrolidone, N-(meth)acryloylmorpholine, etc.), imide group-containing monomers, epoxy group-containing monomers, keto group-containing monomers, isocyanate group-containing monomers, and alkoxysilyl group-containing monomers. Note that some monomers having a nitrogen atom-containing ring, such as N-vinyl-2-pyrrolidone, also fall under the category of amide group-containing monomers. The same applies to the relationship between the above-mentioned nitrogen atom-containing ring-containing monomers and amino group-containing monomers.

Other monomers that can be used in addition to the functional group-containing monomers listed above include vinyl ester-based monomers such as vinyl acetate; non-aromatic ring-containing (meth)acrylates such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate; olefin-based monomers such as ethylene, butadiene, and isobutylene; chlorine-containing monomers such as vinyl chloride; alkoxy group-containing monomers such as methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, and ethoxyethoxyethyl (meth)acrylate; and vinyl ether-based monomers such as methyl vinyl ether. One suitable example of an other monomer that can be used to improve the flexibility of the adhesive is ethoxyethoxyethyl acrylate (also known as ethyl carbitol acrylate, homopolymer Tg: -67°C).

When the above-mentioned other monomers are used, their amount is not particularly limited and can be appropriately set so long as the total amount of the monomer components does not exceed 100% by weight. From the perspective of making it easier to achieve the refractive index-enhancing effect of using monomer (m1), the content of the above-mentioned other monomers in the monomer components can be, for example, approximately 35% by weight or less, suitably approximately 25% by weight or less (e.g., 0 to 25% by weight), or approximately 20% by weight or less (e.g., 0 to 20% by weight), advantageously approximately 10% by weight or less (e.g., 0 to 10% by weight), and preferably approximately 5% by weight or less, for example, approximately 1% by weight or less. The technology disclosed herein can be preferably implemented in an embodiment in which the monomer components are substantially free of the above-mentioned other monomers.

In some embodiments, the monomer components constituting the acrylic polymer may have a composition in which the amount of methacryloyl group-containing monomer used is kept below a predetermined amount. The amount of methacryloyl group-containing monomer used in the monomer components may be, for example, less than 5 wt %, less than 3 wt %, less than 1 wt %, or less than 0.5 wt %. Limiting the amount of methacryloyl group-containing monomer used in this way can be advantageous from the perspective of realizing a pressure-sensitive adhesive that balances flexibility, adhesion, and a high refractive index. The monomer components constituting the acrylic polymer may have a composition that does not contain methacryloyl group-containing monomer (for example, a composition consisting only of acryloyl group-containing monomer).

In some embodiments, the amount of carboxyl group-containing monomer used in the monomer component constituting the acrylic polymer is limited in order to suppress coloration or discoloration (e.g., yellowing) of the PSA. The amount of carboxyl group-containing monomer used in the monomer component may be, for example, less than 1 wt%, less than 0.5 wt%, less than 0.3 wt%, less than 0.1 wt%, or less than 0.05 wt%. Such a limited amount of carboxyl group-containing monomer is also advantageous in terms of suppressing corrosion of metal materials that may be in contact with or adjacent to the PSA disclosed herein (e.g., metal wiring or metal films that may be present on an adherend). The technology disclosed herein can be implemented in an embodiment in which the monomer component constituting the acrylic polymer does not contain a carboxyl group-containing monomer.
For the same reason, in some embodiments, it is preferable that the amount of monomers containing acidic functional groups (including carboxy groups, sulfonic acid groups, phosphate groups, etc.) used in the monomer components constituting the acrylic polymer is limited. The amount of the acidic functional group-containing monomer used in the monomer components in such embodiments can be the same as the preferred amount of the carboxy group-containing monomer used above. The technology disclosed herein can be preferably implemented in an embodiment in which the monomer components do not contain acidic group-containing monomers (i.e., an embodiment in which the acrylic polymer is acid-free).

The refractive index of the acrylic polymer is, for example, 1.460 or higher, preferably 1.500 or higher, more preferably 1.530 or higher, even more preferably 1.550 or higher, and may be 1.560 or higher, 1.570 or higher, or 1.580 or higher. The refractive index adjuster disclosed herein is preferably incorporated into an adhesive having an acrylic polymer having a refractive index of a predetermined level or higher as the base polymer, and used to further increase the refractive index of the adhesive. There is no particular upper limit to the refractive index of the acrylic polymer. In some embodiments, from the standpoint of adhesive properties and optical properties, the refractive index of the acrylic polymer is, for example, 1.700 or lower, preferably 1.650 or lower, or may be 1.630 or lower, 1.610 or lower, or 1.600 or lower.

The refractive index of the base polymer is measured as the refractive index of an adhesive made from the base polymer. In this specification, the refractive index of an adhesive refers to the refractive index of the surface (adhesive surface) of the adhesive. The refractive index of an adhesive can be measured using a commercially available refractive index measuring device (Abbe refractometer) at a measurement wavelength of 589 nm and a measurement temperature of 25°C. For example, an ATAGO DR-M4 model or an equivalent can be used as the Abbe refractometer. The measurement sample can be an adhesive layer made from the adhesive to be evaluated. Specifically, the refractive index of an adhesive can be measured using the method described in the Examples below.

Furthermore, the adhesive containing the refractive index adjuster disclosed herein is not limited to an acrylic adhesive having as its base polymer an acrylic polymer composed of a monomer component containing a certain amount of the monomer (m1) (for example, 30% by weight or more of the monomer component) as described above, but may be, for example, an acrylic adhesive having as its base polymer an acrylic polymer composed of a monomer component that does not contain the above-mentioned monomer (m1) or has a relatively small content of the above-mentioned monomer (m1).
the lower limit of the content of the monomer (m3) is 50% by weight, 60% by weight, 70% by weight, or 80% by weight, and the upper limit is 99.9% by weight, 99.5% by weight, 98% by weight, 95% by weight, or 90% by weight;
the lower limit of the content of the monomer (m2) is 0.1 wt%, 0.5 wt%, 1 wt%, 2 wt%, or 3 wt%, and the upper limit is 30 wt%, 20 wt%, 15 wt%, 10 wt%, 8 wt%, or 5 wt%;
as an optional component, the monomer (m1) is contained in an amount of less than 30% by weight, 20% by weight or less, 10% by weight or less, or 5% by weight or less, and more than 0% by weight, 1% by weight or more, 3% by weight or more, 8% by weight or more, 12% by weight or more, or 18% by weight or more, or the monomer (m1) is not contained;
as optional components, the other monomers are contained in an amount of less than 30% by weight, 20% by weight or less, 10% by weight or less, or 5% by weight or less, and more than 0% by weight, 0.1% by weight or more, 0.5% by weight or more, 1% by weight or more, 3% by weight or more, 8% by weight or more, 12% by weight or more, or 18% by weight or more, or the other monomers are not contained;
The refractive index adjuster disclosed herein can be preferably incorporated into a pressure-sensitive adhesive having an acrylic polymer composed of monomer components having the following composition as a base polymer. The refractive index of the acrylic polymer may be, for example, less than 1.550, 1.530 or less, 1.510 or less, 1.490 or less, or 1.480 or less. The refractive index adjuster disclosed herein can be preferably incorporated into a pressure-sensitive adhesive having an acrylic polymer having such a refractive index as a base polymer, thereby effectively adjusting (typically increasing) the refractive index of the pressure-sensitive adhesive. The lower limit of the refractive index of the acrylic polymer is not particularly limited. The refractive index of the acrylic polymer may be, for example, 1.400 or more, 1.440 or more, 1.460 or more, or 1.470 or more.

(Method for preparing acrylic polymer)
In the technology disclosed herein, the method for obtaining an acrylic polymer composed of such monomer components is not particularly limited, and various polymerization methods known as synthesis methods for acrylic polymers, such as solution polymerization, emulsion polymerization, bulk polymerization, suspension polymerization, and photopolymerization, can be appropriately employed. For example, solution polymerization can be preferably employed. The polymerization temperature during solution polymerization can be appropriately selected depending on the types of monomers and solvents used, the type of polymerization initiator, and the like, and can be, for example, about 20°C to 170°C (typically about 40°C to 140°C).

The solvent (polymerization solvent) used in solution polymerization can be appropriately selected from conventionally known organic solvents. For example, any one solvent selected from the following can be used, or a mixture of two or more solvents can be used: aromatic compounds (typically aromatic hydrocarbons) such as toluene; acetate esters such as ethyl acetate; aliphatic or alicyclic hydrocarbons such as hexane or cyclohexane; halogenated alkanes such as 1,2-dichloroethane; lower alcohols (e.g., monohydric alcohols having 1 to 4 carbon atoms) such as isopropyl alcohol; ethers such as tert-butyl methyl ether; and ketones such as methyl ethyl ketone.

The initiator used in the polymerization can be appropriately selected from conventionally known polymerization initiators depending on the type of polymerization method. For example, one or more azo-based polymerization initiators such as 2,2'-azobisisobutyronitrile (AIBN) can be preferably used. Other examples of polymerization initiators include persulfates such as potassium persulfate; peroxide-based initiators such as benzoyl peroxide and hydrogen peroxide; substituted ethane-based initiators such as phenyl-substituted ethane; and aromatic carbonyl compounds. Still other examples of polymerization initiators include redox-based initiators formed by combining a peroxide with a reducing agent. One polymerization initiator can be used alone, or two or more polymerization initiators can be used in combination. The amount of polymerization initiator used is only required to be a typical amount, and can be selected, for example, from the range of approximately 0.005 to 1 part by weight (typically approximately 0.01 to 1 part by weight) per 100 parts by weight of the monomer components.

Various conventional chain transfer agents can be used in the above polymerization, if necessary. For example, mercaptans such as n-dodecyl mercaptan, t-dodecyl mercaptan, thioglycolic acid, and α-thioglycerol can be used. Alternatively, a chain transfer agent that does not contain sulfur atoms (non-sulfur chain transfer agent) can be used. Examples of non-sulfur chain transfer agents include anilines such as N,N-dimethylaniline and N,N-diethylaniline; terpenoids such as α-pinene and terpinolene; and styrenes such as α-methylstyrene and α-methylstyrene dimer. Chain transfer agents can be used alone or in combination of two or more. When a chain transfer agent is used, the amount used can be, for example, approximately 0.01 to 1 part by weight per 100 parts by weight of the monomer components.

(Base polymer glass transition temperature Tg _T )
The monomer components constituting the base polymer (e.g., acrylic polymer) of the PSA disclosed herein preferably have a composition such that the glass transition temperature (Tg ₎ based on the composition of the monomer components is approximately 15°C or lower. In some embodiments, the glass temperature (Tg ₎ is preferably 10°C or lower, more preferably 5°C or lower, even more preferably 1°C or lower, and may be 0°C or lower. In other embodiments, the glass temperature (Tg ₎ may be -10°C or lower, -20°C or lower, -25°C or lower, -30°C or lower, or -35°C or lower. A low glass transition temperature (Tg _{) can be advantageous from the viewpoint of reducing the elastic modulus of the PSA. Furthermore, the glass transition temperature (Tg) may} be, for example, -80°C or lower, -70°C or higher, or -60°C or higher. From the viewpoint of facilitating the creation of a high refractive index PSA, in some embodiments, the glass transition temperature (Tg ₎ is preferably -50°C or higher, more preferably higher than -45°C, and may even be higher than -40°C. In some preferred embodiments, the glass temperature Tg _T may be greater than -30°C, greater than -20°C, greater than -10°C, or even greater than -5°C.

Here, unless otherwise specified, the glass transition temperature Tg ( _T) refers to the glass transition temperature calculated by the Fox equation based on the composition of the above-mentioned monomer components. The Fox equation, as shown below, is a relational expression between the Tg of a copolymer and the glass transition temperature Tgi of a homopolymer obtained by homopolymerizing each of the monomers constituting the copolymer.
1/Tg=Σ(Wi/Tgi)
In the above Fox formula, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi represents the weight fraction of monomer i in the copolymer (copolymerization ratio on a weight basis), and Tgi represents the glass transition temperature (unit: K) of a homopolymer of monomer i.
The glass transition temperature of a homopolymer used to calculate Tg is the value described in publicly available documents such as "Polymer Handbook" (3rd Edition, John Wiley & Sons, Inc., 1989). For monomers for which multiple values are described in the Polymer Handbook, the highest value is used. If the Tg of a homopolymer is not described in publicly available documents, the value obtained by the measurement method described in JP 2007-51271 A is used.

(Mw of base polymer)
The weight-average molecular weight (Mw) of the base polymer (e.g., acrylic polymer) of the PSA disclosed herein is not particularly limited, and is suitably, for example, approximately 50 x 10 ⁴ or more, and may be approximately 70 x 10 ⁴ or more, or may be approximately 80 x 10 ⁴ or more. By using a base polymer with an Mw of a predetermined value or more, it is easy to obtain an appropriate cohesive force that can exhibit the desired adhesive properties. The upper limit of the Mw of the base polymer is, for example, approximately 500 x 10 ⁴ or less, and from the viewpoint of adhesive performance, it is preferably in the range of approximately 400 x 10 ⁴ or less (more preferably approximately 150 x 10 ⁴ or less, for example, approximately 130 x 10 ⁴ or less).

Here, the Mw of the base polymer (e.g., acrylic polymer) can be determined in terms of polystyrene by gel permeation chromatography (GPC). Specifically, it can be determined by measuring under the following conditions using a GPC measuring device "HLC-8220GPC" (manufactured by Tosoh Corporation).
[GPC measurement conditions]
Sample concentration: 0.2 wt% (tetrahydrofuran solution)
Sample injection volume: 10 μL
Eluent: tetrahydrofuran (THF)
Flow rate (flow rate): 0.6 mL/min Column temperature (measurement temperature): 40°C
column:
Sample column: one "TSKguard column Super HZ-H" product name + two "TSKgel Super HZM-H" product name (manufactured by Tosoh Corporation)
Reference column: one "TSKgel Super H-RC" (manufactured by Tosoh Corporation)
Detector: differential refractometer (RI)
Standard sample: polystyrene

(Crosslinking agent)
In the technology disclosed herein, the pressure-sensitive adhesive composition used to form the pressure-sensitive adhesive may contain a crosslinking agent as needed for purposes such as adjusting the cohesive strength of the pressure-sensitive adhesive. Examples of crosslinking agents that can be used include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, aziridine-based crosslinking agents, oxazoline-based crosslinking agents, melamine-based resins, and metal chelate-based crosslinking agents. Of these, isocyanate-based crosslinking agents and epoxy-based crosslinking agents are preferred. Other examples of crosslinking agents include monomers having two or more ethylenically unsaturated groups in one molecule, i.e., polyfunctional monomers. The crosslinking agents can be used alone or in combination of two or more.

As an isocyanate-based crosslinking agent, a bifunctional or higher isocyanate compound can be used, such as aliphatic polyisocyanates such as trimethylene diisocyanate, butylene diisocyanate, hexamethylene diisocyanate (HDI), and dimer acid diisocyanate; alicyclic isocyanates such as cyclopentylene diisocyanate, cyclohexylene diisocyanate, isophorone diisocyanate (IPDI), and 1,3-bis(isocyanatomethyl)cyclohexane; aromatic isocyanates such as 2,4-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and xylylene diisocyanate (XDI); and polyisocyanate modifications obtained by modifying the above isocyanate compounds with allophanate bonds, biuret bonds, isocyanurate bonds, uretdione bonds, urea bonds, carbodiimide bonds, uretonimine bonds, oxadiazinetrione bonds, etc. Examples of commercially available products include Takenate 300S, Takenate 500, Takenate 600, Takenate D165N, and Takenate D178N (all manufactured by Takeda Pharmaceutical Company Limited), Sumidur T80, Sumidur L, and Desmodur N3400 (all manufactured by Sumika Bayer Urethane Co., Ltd.), and Millionate MR, Millionate MT, Coronate L, Coronate HL, and Coronate HX (all manufactured by Tosoh Corporation). Isocyanate compounds can be used alone or in combination of two or more. A bifunctional isocyanate compound and a trifunctional or higher isocyanate compound may also be used in combination.

Examples of epoxy crosslinking agents include bisphenol A, epichlorohydrin-type epoxy resins, ethylene glycidyl ether, polyethylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, 1,6-hexanediol glycidyl ether, trimethylolpropane triglycidyl ether, diglycidylaniline, diamine glycidylamine, N,N,N',N'-tetraglycidyl-m-xylylenediamine, and 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane. These may be used alone or in combination of two or more.

Examples of polyfunctional monomers include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ethylene glycol di(meth)acrylate, 1,6-hexa(meth)acrylate, Examples of polyfunctional monomers include sandiol di(meth)acrylate, 1,12-dodecanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, allyl (meth)acrylate, vinyl (meth)acrylate, divinylbenzene, bisphenol A di(meth)acrylate, epoxy acrylate, polyester acrylate, urethane acrylate, butyldiol (meth)acrylate, and hexyldiol di(meth)acrylate. Polyfunctional monomers can be used alone or in combination of two or more.

When a crosslinking agent (which may be a polyfunctional monomer) is used, its amount is not particularly limited and can be, for example, in the range of approximately 0.001 to 5.0 parts by weight per 100 parts by weight of the above-mentioned monomer components. From the viewpoint of improving adhesion to the adherend, in some embodiments, the amount of crosslinking agent used per 100 parts by weight of the monomer components is preferably 3.0 parts by weight or less, more preferably 2.0 parts by weight or less, and may be 1.0 part by weight or less, 0.5 parts by weight or less, or 0.2 parts by weight or less. Furthermore, from the viewpoint of appropriately achieving the effects of using the crosslinking agent, in some embodiments, the amount of crosslinking agent used per 100 parts by weight of the monomer components may be, for example, 0.005 parts by weight or more, 0.01 parts by weight or more, 0.05 parts by weight or more, 0.08 parts by weight or more, 0.1 parts by weight or more, 0.2 parts by weight or more, or 0.4 parts by weight or more.

A crosslinking catalyst may be used to promote the crosslinking reaction more effectively. Examples of crosslinking catalysts include metal-based crosslinking catalysts such as tetra-n-butyl titanate, tetraisopropyl titanate, nursem ferric, butyltin oxide, and dioctyltin dilaurate. Among these, tin-based crosslinking catalysts such as dioctyltin dilaurate are preferred. The amount of crosslinking catalyst used is not particularly limited. The amount of crosslinking catalyst used per 100 parts by weight of the monomer component can be, for example, in the range of approximately 0.0001 to 1 part by weight, preferably 0.001 to 0.5 parts by weight, taking into account the balance between the crosslinking reaction rate and the pot life of the adhesive composition.

The PSA composition can contain a compound that undergoes keto-enol tautomerization as a crosslinking retarder. This can extend the pot life of the PSA composition. For example, a compound that undergoes keto-enol tautomerization can be preferably used in a PSA composition containing an isocyanate-based crosslinking agent. Various β-dicarbonyl compounds can be used as the compound that undergoes keto-enol tautomerization. For example, β-diketones (acetylacetone, 2,4-hexanedione, etc.) and acetoacetic esters (methyl acetoacetate, ethyl acetoacetate, etc.) can be preferably used. The compound that undergoes keto-enol tautomerization can be used alone or in combination of two or more. The amount of the compound that undergoes keto-enol tautomerization can be, for example, 0.1 to 20 parts by weight, alternatively 0.5 to 10 parts by weight, or alternatively 1 to 5 parts by weight, per 100 parts by weight of the monomer components.

(High refractive index particles)
The adhesive disclosed herein may contain high refractive index particles as an optional component. Herein, high refractive index particles refer to particles that can increase the refractive index of an adhesive when contained in the adhesive. Hereinafter, high refractive index particles may be referred to as "particles P _HRI ". HRI stands for high refractive index.

The particles P _HRI may be, for example, one or more types of particles made of a material having a refractive index of 1.60 or more, preferably 1.65 or more, more preferably 1.70 or more (it may be 1.80 or more, 1.90 or more, or even 2.00 or more). The upper limit of the refractive index of the material making up the particles P _HRI is not particularly limited, and may be, for example, 3.00 or less, 2.80 or less, 2.50 or less, 2.20 or less, or 2.00 or less. The refractive index of the material making up the particles P _HRI is the refractive index measured for a single layer film of the material (a film thickness that allows refractive index measurement) using a commercially available spectroscopic ellipsometer at a measurement wavelength of 589 nm and a measurement temperature of 25°C. As the spectroscopic ellipsometer, for example, the product name "EC-400" (manufactured by JA. Woollam Co., Ltd.) or an equivalent product may be used.

The type of particles P _HRI is not particularly limited, and one or more materials capable of improving the refractive index of the PSA can be selected and used from metal particles, metal compound particles, organic particles, and organic-inorganic composite particles. In some embodiments, inorganic oxides (e.g., metal oxides) capable of improving the refractive index of the PSA sheet can be preferably used as the particles P _HRI . Suitable examples of materials constituting the particles P _HRI include inorganic oxides (specifically, metal oxides) such as titania (titanium oxide, TiO ₂ ), zirconia (zirconium oxide, ZrO ₂ ), aluminum oxide, zinc oxide, tin oxide, copper oxide, barium titanate, and niobium oxide (Nb ₂ O _5, etc.). Particles made of these inorganic oxides (e.g., metal oxides) can be used alone or in combination of two or more. Among these, particles made of titania or zirconia are preferred, and particles made of zirconia are particularly preferred. Furthermore, as metal particles, for example, iron-based, zinc-based, tungsten-based, and platinum-based materials can have a high refractive index. As organic particles, particles made of resins such as styrene-based resins, phenolic resins, polyester-based resins, and polycarbonate-based resins have a relatively high refractive index. Examples of organic-inorganic composite particles include composites of the above-mentioned inorganic and organic materials, and inorganic particles coated with organic materials such as resins. As particles P _HRI , from the viewpoint of compatibility with the pressure-sensitive adhesive components, the above-mentioned organic and inorganic particles may be surface-treated with a surface treatment agent.

The average particle size of the particles P _HRI is not particularly limited, and particles of an appropriate size that can achieve the desired refractive index improvement when contained in an adhesive can be used. The average particle size of the particles P _HRI can be, for example, approximately 1 nm or more, and is suitably approximately 5 nm or more. From the viewpoint of refractive index improvement and handleability, the average particle size of the particles P _HRI is preferably approximately 10 nm or more, may be approximately 20 nm or more, or may be approximately 30 nm or more. Furthermore, from the viewpoint of maintaining adhesive properties, the upper limit of the average particle size is suitably, for example, approximately 300 nm or less. From the viewpoint of refractive index improvement, it is preferably approximately 100 nm or less, more preferably approximately 70 nm or less, even more preferably approximately 50 nm or less, and may be approximately 35 nm or less (e.g., approximately 25 nm or less).

The average particle size of the particles P _HRI refers to the volume average particle size, and specifically refers to the particle size at 50% of the cumulative value in the particle size distribution measured for the particle P _HRI dispersion using a particle size distribution measuring device based on a laser scattering/diffraction method (50% volume average particle size; hereinafter, sometimes abbreviated as D ₅₀ ). As the measuring device, for example, a product name "Microtrac MT3000II" manufactured by Microtrac Bell or an equivalent product can be used.

The content of particles P _HRI in the PSA is not particularly limited. The content of particles P _HRI can vary depending on the desired refractive index. For example, the content of particles P _HRI can be appropriately set so as to achieve a predetermined refractive index or higher, taking into account the required adhesive properties, etc. In some embodiments, the content of particles P _HRI in the PSA can be, for example, approximately 75% by weight or less, and may be approximately 50% by weight or less, or approximately 30% by weight or less, from the viewpoint of adhesive properties and transparency. The lower limit of the content of particles P _HRI is not particularly limited, and may be, for example, more than 0% by weight, 1% by weight or more, or 5% by weight or more. In other embodiments, the content of particles P _HRI in the PSA is, for example, less than 10% by weight, less than 1% by weight, or less than 0.1% by weight. The technology disclosed herein can be implemented in an embodiment in which the PSA is substantially free of particles P _HRI .

The content of particle P _HRI in the PSA can also be determined by its relative relationship with the amount of base polymer contained in the PSA. The content of particle P _HRI can be, for example, approximately 100 parts by weight or less relative to 100 parts by weight of base polymer, and may be approximately 60 parts by weight or less, or approximately 40 parts by weight or less, in terms of adhesive properties and transparency. The lower limit of the content of particle P _HRI relative to 100 parts by weight of base polymer is not particularly limited, and may be, for example, more than 0 parts by weight, 1 part by weight or more, or 5 parts by weight or more. In some embodiments, the content of particle P _HRI is, for example, less than 30 parts by weight, less than 10 parts by weight, less than 1 part by weight, or less than 0.1 parts by weight relative to 100 parts by weight of base polymer.

(Plasticizer)
The adhesives disclosed herein may contain a plasticizer as an optional component. The use of a plasticizer can reduce the elastic modulus of the adhesive. Furthermore, when molded into an adhesive sheet, flexibility and adaptability to deformation can be improved. As the plasticizer, any suitable material that can contribute to reducing the elastic modulus of the adhesive can be used. One type of plasticizer can be used alone, or two or more types can be used in combination.

In some embodiments, the plasticizer is preferably a compound that is liquid at 30°C. In this specification, "liquid" means exhibiting fluidity and refers to a liquid state of matter. Such compounds include compounds with a melting point of 30°C or lower. By being liquid at 30°C, the plasticizing effect of the plasticizer is optimally exerted, effectively achieving a low elastic modulus of the adhesive. The plasticizer is preferably a compound that is liquid at 25°C, and more preferably a compound that is liquid at 20°C.

From the viewpoint of suppressing a decrease in the refractive index of the adhesive due to the incorporation of a plasticizer, in some embodiments, a compound having two or more double-bond-containing rings per molecule (excluding compounds having a structure corresponding to the refractive index adjusters described above) can be preferably used as the plasticizer. From the viewpoint of exerting a plasticizing effect, the number of double-bond-containing rings possessed by the plasticizer is preferably 6 or less, and may be 4 or less, or 3 or less.

The double bond-containing ring of the plasticizer may be a conjugated double bond-containing ring (typically an aromatic ring) or a non-conjugated double bond-containing ring. The plasticizer may have at least one ring selected from an aromatic ring and a heterocycle (heterocycle) as the double bond-containing ring. The double bond-containing ring (typically an aromatic ring, preferably a carbocycle) may have one or more substituents on the ring-constituting atoms, or may have no substituents. In the carbon atom-containing substituent, the number of carbon atoms contained in the substituent is preferably 1 to 4, more preferably 1 to 3, for example, 1 or 2. In some embodiments, the double bond-containing ring may be an aromatic ring having no substituents on the ring-constituting atoms, or having one or more substituents selected from the group consisting of alkyl groups, alkoxy groups, ethylenically unsaturated groups (e.g., (meth)acryloxy groups), hydroxy groups, and hydroxyalkyl groups (preferably alkyl groups, alkoxy groups, and hydroxyalkyl groups).

In some embodiments, a compound that does not have an ethylenically unsaturated group can be preferably used as the plasticizer. This can suppress deterioration of the pressure-sensitive adhesive composition due to heat or light (progression of gelation or decrease in leveling ability due to increased viscosity) and improve storage stability. The use of a plasticizer that does not have an ethylenically unsaturated group is also preferable from the perspective of suppressing changes in elastic modulus, dimensional changes and deformation (warping, waviness, etc.), optical distortion, etc., caused by reactions of ethylenically unsaturated groups in a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer that contains the plasticizer.

From the viewpoint of achieving a low elastic modulus while suppressing a decrease in the refractive index of the PSA, in some embodiments, a plasticizer having a refractive index of approximately 1.50 or more (preferably approximately 1.51 or more, more preferably approximately 1.53 or more, and even more preferably approximately 1.55 or more) can be preferably used. Plasticizers having a refractive index of approximately 1.56 or more, approximately 1.58 or more, approximately 1.60 or more, or approximately 1.62 or more may also be used. In some embodiments, from the viewpoint of ease of preparation of the PSA composition, compatibility within the PSA, etc., the refractive index of the plasticizer is suitably 2.50 or less, advantageously 2.00 or less, or may be 1.90 or less, 1.80 or less, or 1.70 or less.
The refractive index of the plasticizer, like the refractive index of the monomer, is measured using an Abbe refractometer at a wavelength of 589 nm and a temperature of 25° C. If the manufacturer or other source provides a nominal value for the refractive index at 25° C., that nominal value can be used.

The molecular weight of the plasticizer is not particularly limited, but one having a molecular weight smaller than that of the base polymer is usually used. From the viewpoint of easily exhibiting the plasticizing effect, the molecular weight of the plasticizer is suitably 30,000 or less, advantageously 25,000 or less, and may be less than 10,000 (e.g., less than 5,000), or may be less than 3,000. In some embodiments, the molecular weight of the plasticizer is preferably 2,000 or less, more preferably 1,200 or less, even more preferably 900 or less, and may be 600 or less, 500 or less, 400 or less, 300 or less, or 250 or less (e.g., 220 or less). It may be advantageous for the molecular weight of the plasticizer not to be too large to improve compatibility within the pressure-sensitive adhesive layer. Furthermore, from the viewpoint of easily exerting a sufficient plasticizing effect, the molecular weight of the plasticizer is suitably 100 or more, preferably 130 or more, more preferably 150 or more, and may be 170 or more, 200 or more, 220 or more, or 250 or more. It is also preferable that the molecular weight of the plasticizer is not too low from the viewpoint of the heat resistance of the PSA sheet and suppression of contamination of the adherend.
The molecular weight of the plasticizer is calculated based on its chemical structure. If the manufacturer provides a nominal value for the molecular weight, that value can be used.

In some embodiments, an ethylene glycol-based compound having two or more double bond-containing rings per molecule can be used as the plasticizer. The number of oxyethylene units (i.e., —(C ₂ H ₄ O)— units) contained in the ethylene glycol-based compound is, for example, 1 to 10, or may be 1 to 6, or may be 2 to 4. The ethylene glycol-based compound may be a compound having a structure in which two or more non-condensed double bond-containing rings are linked via oxyethylene units (e.g., 1 to 10, preferably 1 to 6, typically 2 to 4 oxyethylene units) as linking groups. Such compounds may have one or more ester groups. Examples of the ethylene glycol-based compound include compounds having a structure in which two or more benzoic acids are linked to ethylene glycol, diethylene glycol, triethylene glycol, or polyethylene glycol via ester bonds.

In some other embodiments, liquid rosins, such as liquid rosin esters, can be used as plasticizers. The liquid rosins (e.g., liquid rosin esters) may fall under the category of compounds having a condensed double bond-containing ring structure.

Other examples of plasticizers that can be used in the adhesives disclosed herein include known plasticizers other than those mentioned above (e.g., phthalate esters, terephthalate esters, adipic acid esters, adipic acid polyesters, benzoic acid glycol esters, etc.), as well as plasticizing materials such as liquid camphenephenol.

The amount of plasticizer used is not particularly limited and can be set according to the purpose. From the perspective of reducing the elastic modulus of the adhesive, the amount of plasticizer used per 100 parts by weight of base polymer may be, for example, 1 part by weight or more, 10 parts by weight or more, 15 parts by weight or more (e.g., more than 15 parts by weight), 20 parts by weight or more, 30 parts by weight or more (e.g., more than 30 parts by weight), 40 parts by weight or more, 50 parts by weight or more, 60 parts by weight or more, 75 parts by weight or more, or 90 parts by weight or more. Furthermore, from the perspective of achieving a good balance between a high refractive index and a low elastic modulus of the adhesive, the amount of plasticizer used per 100 parts by weight of base polymer is suitably approximately 200 parts by weight or less, preferably 150 parts by weight or less, more preferably 120 parts by weight or less, and may be 100 parts by weight or less, 80 parts by weight or less, 70 parts by weight or less, 45 parts by weight or less, or 35 parts by weight or less.

(Additive ( _H2RO ))
The PSA disclosed herein may optionally contain an organic material having a higher refractive index than the base polymer (excluding compounds having a structure corresponding to the refractive index adjuster). Hereinafter, such an organic material may be referred to as "additive (H _RO )." Herein, "H _RO " refers to an organic material with a high refractive index. By using the additive (H _RO ) in combination with a refractive index adjuster, a PSA that more favorably balances refractive index and adhesive properties (peel strength, flexibility, etc.) can be realized. The organic material used as the additive (H _{RO) may be a polymer or a non-polymer. It may or may not have a polymerizable functional group. In this specification, the additive (H RO} ) is defined as being different from the compound used as the plasticizer described above. Therefore, the additive (H _RO ) is not specifically liquid at 30° _C (e.g., 25°C or 20°C). The additives (H _{2 RO} 3 ) can be used alone or in combination of two or more.

The refractive index of the additive (H _RO ) is not limited to a specific range, and can be set within an appropriate range relative to the refractive index of the base polymer. The refractive index of the additive (H _RO ) can be selected from a range that is, for example, greater than 1.55, greater than 1.56, or greater than 1.57, and higher than the refractive index of the base polymer. From the viewpoint of increasing the refractive index of the pressure-sensitive adhesive, in some embodiments, the refractive index of the additive (H _RO ) is advantageously 1.58 or higher, preferably 1.60 or higher, more preferably 1.63 or higher, and may be 1.65 or higher, 1.70 or higher, or 1.75 or higher. The upper limit of the refractive index of the additive (H _RO ) is not particularly limited, but from the viewpoints of compatibility within the adhesive, ease of achieving both a high refractive index and flexibility suitable for the adhesive, etc., the upper limit may be, for example, 3.000 or less, or may be 2.500 or less, 2.000 or less, 1.950 or less, 1.900 or less, or 1.850 or less.
The refractive index of the additive (H _RO ), like the refractive index of the monomer, is measured using an Abbe refractometer at a measurement wavelength of 589 nm and a measurement temperature of 25° C. When a nominal value of the refractive index at 25° C. is provided by a manufacturer or the like, that nominal value can be used.

The molecular weight of the organic material used as the additive (H _RO ) is not particularly limited and can be selected depending on the purpose. From the viewpoint of achieving a good balance between the effect of increasing the refractive index and other properties (for example, optical properties such as flexibility and haze suitable for a pressure-sensitive adhesive), in some embodiments, the molecular weight of the additive (H _RO ) is suitably less than approximately 10,000, preferably less than 5,000, more preferably less than 3,000 (for example, less than 1,000), and may be less than 800, less than 600, less than 500, or less than 400. It may be advantageous from the viewpoint of improving compatibility within the pressure-sensitive adhesive if the molecular weight of the additive (H _RO ) is not too large. Furthermore, the molecular weight of the additive (H _RO ) may be, for example, 130 or more, or may be 150 or more. In some embodiments, from the viewpoint of increasing the refractive index of the additive (H _RO ), the molecular weight of the additive (H _RO ) is preferably 170 or more, more preferably 200 or more, and may be 230 or more, 250 or more, 270 or more, 500 or more, 1000 or more, or 2000 or more. In some embodiments, a polymer having a molecular weight of about 1000 to 10000 (for example, 1000 or more and less than 5000) can be used as the additive (H _RO ).
As with the refractive index adjuster, for the molecular weight of the additive (H _RO ), for non-polymers or polymers with a low degree of polymerization (for example, about dimer to pentamer), a molecular weight calculated based on the chemical structure or a value measured using MALDI-TOF-MS can be used. When the additive (H _RO ) is a polymer with a higher degree of polymerization, the weight average molecular weight (Mw) based on GPC performed under appropriate conditions can be used. When a nominal value of the molecular weight is provided by a manufacturer or the like, that nominal value can be used.

Examples of organic materials that can be selected as the additive (H _{2 RO} 2 ) include, but are not limited to, organic compounds having aromatic rings, organic compounds having heterocycles (which may be aromatic or non-aromatic heterocycles), etc.
The aromatic ring contained in the organic compound having an aromatic ring (hereinafter also referred to as "aromatic ring-containing compound") used as the additive (H _RO ) can be selected from the same aromatic rings contained in the compound used as the monomer (m1).
Examples of aromatic ring-containing compounds that can be used as the additive (H _RO ) include, but are not limited to, compounds that can be used as monomers (m1); oligomers containing, as monomer units, compounds that can be used as monomers (m1); compounds in which, from compounds that can be used as monomers (m1), a group having an ethylenically unsaturated group (which may be a substituent bonded to a ring-constituting atom) or a portion of the group that constitutes an ethylenically unsaturated group is replaced with a hydrogen atom or a group that does not have an ethylenically unsaturated group (for example, a hydroxyl group, an amino group, a halogen atom, an alkyl group, an alkoxy group, a hydroxyalkyl group, a hydroxyalkyloxy group, a glycidyloxy group, etc.); and the like, which do not fall under the category of refractive index adjusters disclosed herein or the above-mentioned plasticizers.

In some embodiments, an organic compound having two or more aromatic rings in one molecule (hereinafter also referred to as a "multiple aromatic ring-containing compound") can be preferably used as the additive (H _RO ) because it is likely to achieve a high refractive index. The multiple aromatic ring-containing compound may or may not have a polymerizable functional group such as an ethylenically unsaturated group. The multiple aromatic ring-containing compound may be a polymer or a non-polymer. The polymer may also be an oligomer containing a multiple aromatic ring-containing monomer as a monomer unit (preferably an oligomer having a molecular weight of approximately 5,000 or less, more preferably approximately 1,000 or less; for example, a low polymer of about 2 to 5). The oligomer may be, for example: a homopolymer of a multiple aromatic ring-containing monomer; a copolymer of one or more multiple aromatic ring-containing monomers; a copolymer of one or more multiple aromatic ring-containing monomers with another monomer; or the like. The other monomer may be an aromatic ring-containing monomer that does not fall under the category of multiple aromatic ring-containing monomer, a monomer that does not have an aromatic ring, or a combination thereof.

In some embodiments, a compound having no ethylenically unsaturated group can be preferably used as the additive (H _RO ). This can suppress deterioration of the pressure-sensitive adhesive composition due to heat or light (progression of gelation or decrease in leveling ability due to increased viscosity) and improve storage stability. The use of an additive (H _RO ) having no ethylenically unsaturated group is also preferable from the viewpoint of suppressing dimensional changes and deformations (warping, waviness, etc.), optical distortion _, etc. caused by the reaction of the ethylenically unsaturated group in a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer containing the additive (H RO ).

The amount of additive ( _H2RO ) used per 100 parts by weight of the base polymer (when multiple types of compounds are used, the total amount thereof) is not particularly limited as long as it is more than 0 parts by weight, and can be set according to the purpose. In some embodiments, the amount of additive ( _H2RO ) used per 100 parts by weight of the base polymer can be, for example, 80 parts by weight or less. From the viewpoint of achieving a good balance between increasing the refractive index of the pressure-sensitive adhesive and suppressing a decrease in adhesive properties, it is advantageous to set it to 60 parts by weight or less, and preferably 45 parts by weight or less. In some embodiments where greater importance is placed on adhesive properties and optical properties, the amount of additive ( _H2RO ) used per 100 parts by weight of the base polymer may be, for example, 30 parts by weight or less, 20 parts by weight or less, 15 parts by weight or less, or 10 parts by weight or less. Furthermore, from the viewpoint of increasing the refractive index of the adhesive, the amount of additive ( _H2RO3 ) used relative to 100 parts by weight of the base polymer can be, for example, 1 part by weight or more, advantageously 3 parts by weight or more, preferably 5 parts by weight or more, or may be 7 parts by weight or more, 10 parts by weight or more, 15 parts by weight or more, or 20 parts by weight or more.

(Tackifier)
The PSA disclosed herein may contain a tackifier. Examples of tackifiers that can be used include known tackifier resins such as rosin-based tackifier resins, terpene-based tackifier resins, phenol-based tackifier resins, hydrocarbon-based tackifier resins, ketone-based tackifier resins, polyamide-based tackifier resins, epoxy-based tackifier resins, and elastomer-based tackifier resins. These can be used alone or in combination of two or more. The amount of tackifier resin used is not particularly limited and can be set to achieve appropriate adhesive performance depending on the purpose and application. In some embodiments, from the viewpoints of refractive index and transparency, the amount of tackifier used per 100 parts by weight of base polymer is suitably 30 parts by weight or less, preferably 10 parts by weight or less, and more preferably 5 parts by weight or less. The technology disclosed herein can be preferably implemented in an embodiment in which a tackifier is not used.

(Other additives)
In addition, the adhesives disclosed herein may contain, as needed, known additives that can be used in adhesives, such as leveling agents, softeners, colorants (dyes, pigments, etc.), fillers, antistatic agents, antioxidants, UV absorbers, antioxidants, light stabilizers, preservatives, etc. As these various additives are not particularly characteristic of the present invention, conventionally known additives can be used in the usual way, and detailed description thereof will be omitted.

(Refractive Index)
The refractive index of the adhesive disclosed herein is not particularly limited and can be set according to the purpose (e.g., taking into consideration the refractive index of the adherend). The refractive index of the adhesive disclosed herein can be, for example, approximately 1.300 to 1.900 (preferably approximately 1.450 to 1.800). In some embodiments, the refractive index of the adhesive is higher than that of conventional acrylic adhesives. The technology disclosed herein can provide an adhesive having a refractive index of, for example, 1.550 or higher, an adhesive composition capable of forming the adhesive, and an adhesive sheet containing the adhesive. The refractive index of the adhesive is suitably 1.560 or higher, preferably greater than 1.570. In some embodiments, the refractive index of the adhesive can be 1.575 or higher, 1.580 or higher, or 1.585 or higher. An adhesive having such a refractive index can effectively suppress light reflection at the interface with the adherend when attached to a material with a high refractive index. The preferred upper limit of the refractive index of the adhesive is not limited to a specific range, as it may vary depending on the refractive index of the adherend, etc., and may be, for example, 1.700 or less, 1.670 or less, 1.650 or less, 1.620 or less, or 1.600 or less. The refractive index of the adhesive can be adjusted, for example, by the composition of the adhesive (e.g., the type of base polymer, the type and amount of refractive index adjuster used, and whether or not other optional components are used).

In this specification, the refractive index of an adhesive refers to the refractive index of the surface (adhesive surface) of the adhesive. The refractive index of an adhesive can be measured using a commercially available refractive index measuring device (Abbe refractometer) at a measurement wavelength of 589 nm and a measurement temperature of 25°C. Examples of Abbe refractometers that can be used include the ATAGO DR-M4 model or equivalent. The measurement sample can be an adhesive layer made of the adhesive to be evaluated. Specifically, the refractive index of an adhesive can be measured using the method described in the Examples below.

(Storage modulus G')
The storage modulus G' at 25°C of the PSA disclosed herein (hereinafter also referred to as "storage modulus G'(25)") is appropriately set depending on the intended use and mode of use, and is not limited to a specific range. The storage modulus G'(25) of the PSA may be, for example, approximately 2000 kPa or less. In some embodiments, from the viewpoint of ease of application to an adherend, the storage modulus G'(25) of the PSA is advantageously approximately 1500 kPa or less, preferably 1000 kPa or less, and may even be 800 kPa or less. In some embodiments, from the viewpoint of increasing the flexibility of the PSA at room temperature (e.g., 25°C) and facilitating adhesion to an adherend, the storage modulus G'(25) of the PSA is preferably approximately 650 kPa or less, and may be 550 kPa or less, 450 kPa or less, 350 kPa or less, or 300 kPa or less. In some embodiments where application property and flexibility at room temperature are more important, the storage modulus G'(25) of the PSA may be, for example, less than 270 kPa or less than 250 kPa, less than 200 kPa, less than 180 kPa, or less than 160 kPa (e.g., less than 140 kPa). In some embodiments, the storage modulus G'(25) of the PSA may be less than 100 kPa or less than 90 kPa. There is no particular lower limit for the storage modulus G'(25) of the PSA, but from the viewpoint of processability, handleability, etc., it may be, for example, 30 kPa or more, 50 kPa or more, or 70 kPa or more. In some embodiments, taking into account the high refractive index, the storage modulus G'(25) may be 100 kPa or more, 150 kPa or more, 200 kPa or more, 250 kPa or more, 300 kPa or more, 500 kPa or more, 600 kPa or more, or 700 kPa or more.

The storage modulus G' at 50°C (hereinafter also referred to as "storage modulus G'(50)") of the PSA disclosed herein is not particularly limited and can be, for example, less than 600 kPa. In some embodiments, the storage modulus G'(50) is suitably less than 400 kPa, preferably less than 250 kPa, and may be less than 200 kPa, less than 150 kPa, less than 100 kPa, less than 70 kPa, less than 50 kPa, less than 38 kPa, or less than 36 kPa. PSA with such a limited storage modulus G'(50) can easily increase adhesion to an adherend by applying moderate heating as needed, thereby improving adhesion to the adherend. There is no particular lower limit for the storage modulus G'(50) of the PSA. In some embodiments, from the viewpoint of the heat resistance characteristics of the pressure-sensitive adhesive, the storage modulus G'(50) may be, for example, 10 kPa or more, 15 kPa or more, 20 kPa or more, or 23 kPa or more. Furthermore, in some embodiments, taking into consideration a high refractive index, the storage modulus G'(50) of the pressure-sensitive adhesive may be 40 kPa or more, 70 kPa or more, 120 kPa or more, or 170 kPa or more.

In some embodiments of the adhesives disclosed herein, the adhesives may be applied under the following conditions:
(a) a storage modulus G'(25) at 25°C of 1500 kPa or less (preferably 1000 kPa or less, e.g., 550 kPa or less, 350 kPa or less, or 180 kPa or less); and (b) a storage modulus G'(50) at 50°C of less than 400 kPa (preferably less than 200 kPa, more preferably less than 150 kPa, e.g., less than 100 kPa or less than 50 kPa);
It is preferable that at least one of the above conditions is satisfied. A PSA that satisfies at least the above condition (a) is preferred from the viewpoint of adhesion to an adherend at room temperature (e.g., 25°C). A PSA that satisfies at least the above condition (b) is preferred because its adhesion (adhesion) to an adherend can be easily improved by heating to a temperature slightly higher than room temperature. A PSA that does not satisfy the above condition (a) but satisfies the above condition (b) can be used as a heat-activatable PSA that has good reworkability (repositionability) at the initial stage of application at room temperature and can effectively increase the peel strength from the adherend by heating to a temperature slightly higher than room temperature. The heat activation may be performed by heating the PSA to a temperature slightly higher than room temperature when applying it to the adherend. The temperature slightly higher than room temperature is, for example, about 60°C or lower, preferably about 55°C or lower (e.g., about 50°C or lower).

(glass transition temperature)
The glass transition temperature (Tg) of the pressure-sensitive adhesive is not particularly limited and can be set taking into consideration flexibility in low-temperature ranges and cohesive strength (heat resistance, etc.) in high-temperature ranges. In some embodiments, the Tg of the pressure-sensitive adhesive is, for example, 50°C or lower, and may be 40°C or lower, 30°C or lower, 15°C or lower, or even 5°C or lower. In some preferred embodiments, the Tg of the pressure-sensitive adhesive is 0°C or lower, more preferably -5°C or lower, even more preferably -10°C or lower, and may be -15°C or lower (e.g., -20°C or lower), from the viewpoint of flexibility. The lower the Tg of the pressure-sensitive adhesive, the more excellent its adhesive properties, such as adhesion to the adherend. The lower limit of the Tg of the pressure-sensitive adhesive is, for example, -50°C or higher, suitably -40°C or higher, and may be -30°C or higher. Pressure-sensitive adhesives having the above Tg tend to easily obtain moderate cohesive strength.

The storage modulus G' and glass transition temperature Tg of the pressure-sensitive adhesive at each of the above temperatures are determined by dynamic viscoelasticity measurement. Specifically, a sheet of pressure-sensitive adhesive (which can be prepared, for example, by appropriately laminating pressure-sensitive adhesive layers) with a thickness of approximately 1.5 mm is punched into a 7.9 mm diameter disk as a measurement sample, and dynamic viscoelasticity measurement is performed under the following conditions. From the measurement results, the storage modulus G' [Pa] of the pressure-sensitive adhesive at each temperature can be determined. Furthermore, the glass transition temperature (Tg) [°C] of the pressure-sensitive adhesive is determined as the temperature corresponding to the peak-top temperature of the loss tangent tan δ (loss modulus G"/storage modulus G') in the dynamic viscoelasticity measurement. The measurement device can be an "Advanced Rheometric Expansion System (ARES)" manufactured by Rheometric Scientific, Inc., or an equivalent. The storage modulus G' and glass transition temperature Tg of the adhesive can be adjusted, for example, by selecting the type and amount of refractive index adjuster, selecting the composition of the base polymer (e.g., selecting the type and content of monomer (m1)), whether or not a crosslinking agent is used, and selecting the type and amount used, etc.
[Measurement conditions]
Deformation mode: Torsion Measurement frequency: 1 Hz
Temperature range: -50℃ to 150℃
Temperature increase rate: 5°C/min

<Adhesive sheet>
This specification provides a PSA sheet having any of the PSAs disclosed herein (which may be a PSA formed from any of the PSA compositions disclosed herein, for example, a cured product of the PSA composition), preferably in the form of a PSA layer.
The PSA sheet may be a substrate-attached PSA sheet having the PSA layer on one or both sides of a non-releasable substrate (support substrate), or may be a substrate-less PSA sheet (i.e., a PSA sheet without a non-releasable substrate; typically, a PSA sheet consisting of a PSA layer) in which the PSA layer is held by a release liner. The concept of PSA sheet here may include what are called PSA tapes, PSA labels, PSA films, etc. The PSA sheet disclosed herein may be in the form of a roll or a sheet. Alternatively, it may be a PSA sheet processed into various shapes.

Examples of the configuration of a double-sided adhesive substrateless PSA sheet (substrateless double-sided PSA sheet) are shown in Figures 1 and 2. PSA sheet 1 shown in Figure 1 has a configuration in which both surfaces 21A and 21B of substrateless PSA layer 21 are protected by release liners 31 and 32, with at least the PSA layer side serving as a release surface. PSA sheet 2 shown in Figure 2 has a configuration in which one surface (adhesive surface) 21A of substrateless PSA layer 21 is protected by release liner 31, with both surfaces serving as release surfaces. When rolled up, the other surface (adhesive surface) 21B of PSA layer 21 abuts against the back surface of release liner 31, thereby also protecting the other surface 21B with release liner 31. The technology disclosed herein is preferably implemented in the form of a substrateless PSA sheet made of a PSA layer, from the standpoint of flexibility to conform to an adherend even when repeatedly folded. The substrateless PSA sheet is also preferable from the standpoint of, for example, reducing the thickness of the PSA sheet and increasing its transparency.

The pressure-sensitive adhesive sheet disclosed herein may have, for example, the cross-sectional structure shown schematically in FIG. 3. The pressure-sensitive adhesive sheet 3 shown in FIG. 3 comprises a support substrate 10 and a first pressure-sensitive adhesive layer 21 and a second pressure-sensitive adhesive layer 22 supported on a first surface 10A and a second surface 10B, respectively, of the support substrate 10. Both the first surface 10A and the second surface 10B are non-releasable surfaces (non-releasable surfaces). The pressure-sensitive adhesive sheet 3 is used by attaching the surface (first adhesive surface) 21A of the first pressure-sensitive adhesive layer 21 and the surface (second adhesive surface) 22A of the second pressure-sensitive adhesive layer 22 to an adherend, respectively. In other words, the pressure-sensitive adhesive sheet 3 is configured as a double-sided pressure-sensitive adhesive sheet (double-sided adhesive sheet). Before use, the pressure-sensitive adhesive sheet 3 has a configuration in which the first adhesive surface 21A and the second adhesive surface 22A are protected by release liners 31 and 32, at least the adhesive surface side of which is a releasable surface (release surface). Alternatively, the release liner 32 can be omitted, and a release liner 31 with release surfaces on both sides can be used, and the adhesive sheet 3 can be rolled up so that the second adhesive surface 22A is in contact with the back surface of the release liner 31, thereby resulting in a configuration in which the second adhesive surface 22A is also protected by the release liner 31.

The technology disclosed herein is preferably implemented in the form of the above-mentioned substrate-less or substrate-attached double-sided PSA sheet for fixing or joining components (e.g., optical components). Alternatively, although not specifically shown, the PSA sheet disclosed herein may be in the form of a substrate-attached single-sided PSA sheet having a PSA layer on only one side of a non-releasable substrate (support substrate). An example of a single-sided PSA sheet configuration is one in which either the first PSA layer 21 or the second PSA layer 22 is absent in the configuration shown in Figure 3.

(Adhesive layer)
The PSA layer of the PSA sheet disclosed herein can be formed by applying (e.g., coating) a PSA composition to a suitable surface and then curing the composition. The PSA composition can be applied using a conventional coater such as a gravure roll coater, a reverse roll coater, a kiss roll coater, a dip roll coater, a bar coater, a knife coater, or a spray coater.

The thickness of the adhesive layer is not particularly limited and can be, for example, 3 μm or more. In some embodiments, the thickness of the adhesive layer is suitably, for example, 5 μm or more, and may be 10 μm or more, 15 μm or more, 20 μm or more, 30 μm or more, 50 μm or more, 70 μm or more, or 85 μm or more. Increasing the thickness of the adhesive layer tends to increase adhesive strength. Furthermore, in some embodiments, the thickness of the adhesive layer may be, for example, 300 μm or less, 250 μm or less, 200 μm or less, 150 μm or less, or 120 μm or less. In some preferred embodiments, the thickness of the adhesive layer is 100 μm or less, more preferably 75 μm or less, even more preferably 70 μm or less, and may be 50 μm or less, or 30 μm or less. Having an adhesive layer that is not too thick can be advantageous from the perspective of, for example, reducing the thickness of the adhesive sheet. Furthermore, a thin pressure-sensitive adhesive layer tends to have excellent conformability to the adherend. The technology disclosed herein can be preferably implemented, for example, in an embodiment in which the thickness of the pressure-sensitive adhesive layer is in the range of 3 μm to 200 μm (more preferably 5 μm to 100 μm, and even more preferably 5 μm to 75 μm). In the case of a pressure-sensitive adhesive sheet having a first pressure-sensitive adhesive layer and a second pressure-sensitive adhesive layer on the first and second surfaces of the substrate, the thickness of the pressure-sensitive adhesive layer described above can be applied to at least the thickness of the first pressure-sensitive adhesive layer. The thickness of the second pressure-sensitive adhesive layer can also be selected from a similar range. In the case of a substrate-less pressure-sensitive adhesive sheet, the thickness of the pressure-sensitive adhesive sheet is the same as the thickness of the pressure-sensitive adhesive layer.

(Optical properties)
In some embodiments, the haze value of the pressure-sensitive adhesive layer may be, for example, 5.0% or less, preferably 3.0% or less, more preferably 2.0% or less, even more preferably 1.0% or less, and particularly preferably less than 1.0% (e.g., 0.9% or less). Pressure-sensitive adhesive sheets having such highly transparent pressure-sensitive adhesive layers, with or without a substrate, can be preferably used in applications requiring high light transmittance (e.g., optical applications) or applications requiring good visibility of the adherend through the pressure-sensitive adhesive sheet. The lower limit of the haze value of the pressure-sensitive adhesive layer is not particularly limited, and a smaller haze value is preferable from the viewpoint of improving transparency. Meanwhile, in some embodiments, taking into account the refractive index and adhesive properties, the haze value may be, for example, 0.05% or more, or even 0.10% or more. These haze values for the pressure-sensitive adhesive layer can also be preferably applied to the haze value of a pressure-sensitive adhesive sheet when the technology disclosed herein is implemented in the form of a substrateless pressure-sensitive adhesive sheet (typically, a pressure-sensitive adhesive sheet consisting of a pressure-sensitive adhesive layer).

Here, the "haze value" refers to the ratio of diffuse transmitted light to total transmitted light when visible light is irradiated onto a measurement object. It is also called the cloudiness value. The haze value can be expressed by the following formula:
Th(%)=Td/Tt×100
In the above formula, Th is the haze value (%), Td is the scattered light transmittance, and Tt is the total light transmittance. The haze value can be measured according to the method described in the Examples below. The haze value of the pressure-sensitive adhesive layer can be adjusted, for example, by selecting the composition, thickness, etc. of the pressure-sensitive adhesive layer.

In some embodiments, the haze value of the pressure-sensitive adhesive sheet may be, for example, 5.0% or less, preferably 3.0% or less, more preferably 2.0% or less, even more preferably 1.0% or less, and particularly preferably less than 1.0% (e.g., 0.9% or less). Such highly transparent pressure-sensitive adhesive sheets are preferably used in applications requiring high light transmittance (e.g., optical applications) or applications requiring good visibility of the adherend through the pressure-sensitive adhesive sheet. There is no particular lower limit for the haze value of the pressure-sensitive adhesive sheet, and from the perspective of improving transparency, a smaller haze value is preferable. On the other hand, in some embodiments, taking into account the refractive index and adhesive properties, the haze value may be, for example, 0.05% or more, or even 0.10% or more. The haze value of the pressure-sensitive adhesive sheet can be measured in the same manner as the haze value of the pressure-sensitive adhesive layer described above. The haze value of the pressure-sensitive adhesive sheet can be obtained by selecting the composition of the pressure-sensitive adhesive layer described above, or, in configurations having a substrate, the type and thickness of the substrate.

In some embodiments, the total light transmittance of the pressure-sensitive adhesive layer is preferably 85.0% or more (e.g., 88.0% or more, 90.0% or more, or greater than 90.0%). Pressure-sensitive adhesive sheets having such highly transparent pressure-sensitive adhesive layers, with or without a substrate, are preferably used in applications requiring high light transmittance (e.g., optical applications) or applications requiring good visibility of the adherend through the pressure-sensitive adhesive sheet. The upper limit of the total light transmittance may be, for example, approximately 98% or less, approximately 96% or less, or approximately 95% or less in practical terms. In some embodiments, taking into account the refractive index and adhesive properties, the total light transmittance of the pressure-sensitive adhesive layer may be approximately 94% or less, approximately 93% or less, or approximately 92% or less. The total light transmittance is measured using a commercially available transmittance meter in accordance with JIS K 7136:2000. The transmittance meter used may be a product manufactured by Murakami Color Research Laboratory under the trade name "HAZEMETER HM-150" or an equivalent. Total light transmittance can be measured according to the method described in the Examples below. The total light transmittance of the pressure-sensitive adhesive layer can be adjusted, for example, by selecting the composition and thickness of the pressure-sensitive adhesive layer.

In some embodiments, the total light transmittance of the PSA sheet is preferably 85.0% or higher (e.g., 88.0% or higher, 89.0% or higher, or 90.0% or higher). Such highly transparent PSA sheets are preferably used in applications requiring high light transmittance (e.g., optical applications) or applications requiring good visibility of the adherend through the PSA sheet. The upper limit of the total light transmittance may be, for example, approximately 98% or lower, approximately 96% or lower, or approximately 95% or lower in practice. In some embodiments, taking into account the refractive index and adhesive properties, the total light transmittance of the PSA sheet may be approximately 94% or lower, approximately 93% or lower, or approximately 92% or lower. The total light transmittance of the PSA sheet can be measured in the same manner as for measuring the total light transmittance of the PSA layer described above. The total light transmittance of the PSA sheet can be obtained by selecting the composition of the PSA layer described above, or, in configurations having a substrate, the type and thickness of the substrate.

The chromaticity b ^* of the pressure-sensitive adhesive layer as defined in the L ^* a ^* b ^* color system is not particularly limited, but in some embodiments (for example, embodiments expected to be applied to optical applications), it is appropriate to set it in the range of ±15, preferably in the range of ±10, and more preferably in the range of ±5.0 (for example, in the range of ±1.0). In this specification, the term "range of ±X" is used to mean the range of -X to +X.
Here, the L ^* a ^* b ^* color system in this specification conforms to the standard recommended by the International Commission on Illumination in 1976 or the standard of JIS Z 8729. Specifically, L ^* a ^* b ^* can be measured at multiple locations (for example, five or more locations) on the surface of the pressure-sensitive adhesive layer using a color difference meter (product name "CR-400" manufactured by Minolta Co., Ltd.), and the average value of the measurements can be used. The same applies to the examples described below.

(peel strength)
The peel strength of the pressure-sensitive adhesive sheet against a glass plate is not particularly limited.In some embodiments, the peel strength of the pressure-sensitive adhesive sheet against a glass plate is, for example, 0.1N/25mm or more, preferably 0.5N/25mm or more, and may be 1.0N/25mm or more, 1.5N/25mm or more, 2.0N/25mm or more, or 3.0N/25mm or more.The upper limit of the peel strength is not particularly limited, and may be, for example, 30N/25mm or less, 25N/25mm or less, or 20N/25mm or less.
Here, the peel strength is determined by pressing the sheet against an alkaline glass plate as an adherend, leaving it in an environment of 23°C and 50% RH for 30 minutes, and then measuring the peel strength under conditions of a peel angle of 180° and a pulling speed of 300 mm/min. When measuring, if necessary, the pressure-sensitive adhesive sheet to be measured can be reinforced by attaching an appropriate backing material (for example, a polyethylene terephthalate (PET) film with a thickness of about 25 μm to 50 μm).

(Adhesive sheet thickness)
The thickness of the PSA sheet (substrate-less PSA sheet or substrate-attached PSA sheet) disclosed herein may be, for example, 1000 μm or less, 350 μm or less, 200 μm or less, 120 μm or less, 75 μm or less, or 50 μm or less. From the viewpoint of handleability, etc., the thickness of the PSA sheet may be, for example, 5 μm or more, 10 μm or more, 25 μm or more, 80 μm or more, or 130 μm or more.
The thickness of the PSA sheet refers to the thickness of the portion that is attached to the adherend. For example, in the PSA sheet 3 configured as shown in Fig. 3, the thickness refers to the thickness from the first adhesive surface 21A to the second adhesive surface 22A, and does not include the thickness of the release liners 31 and 32.

<Support base material>
PSA sheets according to some embodiments may be in the form of a substrate-attached PSA sheet having a PSA layer on one or both sides of a support substrate. The material of the support substrate is not particularly limited and can be appropriately selected depending on the intended use and manner of use of the PSA sheet. Non-limiting examples of usable substrates include plastic films such as polyolefin films primarily composed of polyolefins such as polypropylene (PP) and ethylene-propylene copolymers, polyester films primarily composed of polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN), and polyvinyl chloride films primarily composed of polyvinyl chloride; foam sheets composed of foams such as polyurethane foam, polyethylene (PE) foam, and polychloroprene foam; woven and nonwoven fabrics made by spinning various fibrous materials alone or in combination; papers such as Japanese paper, fine paper, kraft paper, and crepe paper; and metal foils such as aluminum foil and copper foil. Substrates having a composite configuration of these materials may also be used. Examples of such composite substrates include substrates having a structure in which a metal foil and the above-mentioned plastic film are laminated together, and plastic substrates reinforced with inorganic fibers such as glass cloth.

In some embodiments, various film substrates can be preferably used. The film substrate can be a porous substrate such as a foam film or a nonwoven fabric sheet, or a nonporous substrate, or a substrate having a structure in which a porous layer and a nonporous layer are laminated. In some embodiments, the film substrate can preferably be one that includes a resin film that can independently maintain its shape (freestanding or independent) as a base film. Here, "resin film" refers to a resin film that has a nonporous structure and typically contains substantially no air bubbles (void-free). Therefore, the resin film is a concept that is distinct from foam films and nonwoven fabrics. The resin film can preferably be one that can independently maintain its shape (freestanding or independent). The resin film can have a single-layer structure or a multi-layer structure of two or more layers (e.g., a three-layer structure).

Examples of resin materials that can be used to form the resin film include polyester; polyolefin; polycycloolefin derived from a monomer having an alicyclic structure such as a norbornene structure; polyamide (PA) such as nylon 6, nylon 66, and partially aromatic polyamide; polyimide (PI) such as transparent polyimide (CPI); polyamideimide (PAI); polyetheretherketone (PEEK); polyethersulfone (PES); polyphenylene sulfide (PPS); polycarbonate (PC); polyurethane (PU); ethylene-vinyl acetate copolymer (EVA); fluororesins such as polytetrafluoroethylene (PTFE); acrylic resin; cellulose-based polymers such as triacetyl cellulose (TAC); polyarylate; polystyrene; polyvinyl chloride; and polyvinylidene chloride.

The resin film may be formed using a resin material containing one of these resins alone, or may be formed using a resin material containing a blend of two or more of these resins. The resin film may be unstretched or stretched (e.g., uniaxially or biaxially stretched). For example, PET film, PBT film, PEN film, unstretched polypropylene (CPP) film, biaxially oriented polypropylene (OPP) film, low-density polyethylene (LDPE) film, linear low-density polyethylene (LLDPE) film, PP/PE blend film, cycloolefin polymer (COP) film, CPI film, TAC film, etc. are preferred. Examples of resin films that are preferred from the standpoint of strength and dimensional stability include PET film, PEN film, PPS film, and PEEK film. PET film and PPS film are particularly preferred from the standpoint of availability, with PET film being particularly preferred.

The resin film can be blended with known additives such as light stabilizers, antioxidants, antistatic agents, colorants (dyes, pigments, etc.), fillers, slip agents, and antiblocking agents, as needed, provided that the effects of the present invention are not significantly impaired. The amount of additives blended is not particularly limited and can be set appropriately depending on the intended use of the PSA sheet, etc.

The method for producing the resin film is not particularly limited. For example, conventional resin film molding methods such as extrusion molding, inflation molding, T-die casting, and calendar roll molding can be used as appropriate.

The substrate may be substantially composed of such a base film. Alternatively, the substrate may include an auxiliary layer in addition to the base film. Examples of such auxiliary layers include optical property adjusting layers (e.g., colored layers, anti-reflection layers), printed layers or laminate layers for imparting a desired appearance to the substrate, antistatic layers, undercoat layers, release layers, and other surface treatment layers.

In some embodiments, a substrate having optical transparency (hereinafter also referred to as an optically transparent substrate) can be preferably used as the support substrate. This makes it possible to construct a pressure-sensitive adhesive sheet with an optically transparent substrate. The total light transmittance of the optically transparent substrate may be, for example, greater than 50%, or may be 70% or greater. In some preferred embodiments, the total light transmittance of the support substrate is 80% or greater, more preferably 90% or greater, and may even be 95% or greater (e.g., 95 to 100%). The total light transmittance is measured in accordance with JIS K 7136:2000 using a commercially available transmittance meter. The transmittance meter used may be a product manufactured by Murakami Color Research Laboratory under the trade name "HAZEMETER HM-150" or an equivalent. A suitable example of the optically transparent substrate is a resin film having optical transparency. The optically transparent substrate may also be an optical film.

The thickness of the substrate is not particularly limited and can be selected depending on the intended use and mode of use of the pressure-sensitive adhesive sheet. The thickness of the substrate may be, for example, 500 μm or less, and from the viewpoint of the handleability and processability of the pressure-sensitive adhesive sheet, it is preferably 300 μm or less, and may be 150 μm or less, 100 μm or less, 50 μm or less, 25 μm or less, or 10 μm or less. As the thickness of the substrate decreases, its ability to conform to the surface shape of the adherend tends to improve. Furthermore, from the viewpoint of handleability and processability, the thickness of the substrate may be, for example, 2 μm or more, 10 μm or more, or 25 μm or more.

The surface of the substrate on which the adhesive layer is to be laminated may be subjected to conventional surface treatments, such as corona discharge treatment, plasma treatment, ultraviolet irradiation treatment, acid treatment, alkali treatment, or the formation of an undercoat layer by applying a primer. Such surface treatments may be intended to improve the anchoring ability of the adhesive layer to the substrate. The composition of the primer used to form the undercoat layer is not particularly limited and can be appropriately selected from known primers. There are no particular restrictions on the thickness of the undercoat layer, but a thickness of approximately 0.01 μm to 1 μm is typically appropriate, with approximately 0.1 μm to 1 μm being preferred. Other treatments that may be applied to the substrate as needed include antistatic layer formation treatment, colored layer formation treatment, printing treatment, etc. These treatments may be applied alone or in combination.

<Adhesive sheet with release liner>
The PSA sheet disclosed herein can take the form of a PSA product in which the surface (adhesive surface) of a PSA layer is in contact with the release surface of a release liner. Accordingly, this specification provides a PSA sheet with a release liner (adhesive product) comprising any of the PSA sheets disclosed herein and a release liner having a release surface in contact with the adhesive surface of the PSA sheet.

The release liner is not particularly limited, and examples include release liners having a release layer on the surface of a liner substrate such as a resin film or paper (which may be paper laminated with a resin such as polyethylene), and release liners made of a resin film formed from a low-adhesion material such as a fluorine-based polymer (such as polytetrafluoroethylene) or a polyolefin resin (such as polyethylene or polypropylene). Due to their excellent surface smoothness, release liners having a release layer on the surface of a resin film as a liner substrate and release liners made of a resin film formed from a low-adhesion material are preferred. The resin film is not particularly limited as long as it is capable of protecting the adhesive layer, and examples include polyethylene (PE) film, polypropylene (PP) film, polybutene film, polybutadiene film, polymethylpentene film, polyvinyl chloride film, vinyl chloride copolymer film, polyester film (such as PET film or PBT film), polyurethane film, and ethylene-vinyl acetate copolymer film. To form the release layer, known release treatment agents can be used, such as silicone-based release treatment agents, long-chain alkyl-based release treatment agents, olefin-based release treatment agents, fluorine-based release treatment agents, fatty acid amide-based release treatment agents, molybdenum sulfide, and silica powder.

<Application>
The material (adherend material) to which the pressure-sensitive adhesive sheet disclosed herein can be attached is not particularly limited, and examples thereof include metal materials such as copper, silver, gold, iron, tin, palladium, aluminum, nickel, titanium, chromium, zinc, etc., or alloys containing two or more of these metals, as well as polyimide resins, acrylic resins, polyethernitrile resins, polyethersulfone resins, polyester resins (PET resins, polyethylene naphthalate resins, etc.), polyvinyl chloride resins, polyphenylene sulfide resins, polyetheretherketone resins, polyamide resins (so-called aramid resins, etc.), polyarylate resins, polycarbonate resins, cellulose polymers such as diacetyl cellulose and triacetyl cellulose, vinyl butyral polymers, liquid crystal polymers, and various other resin materials (typically plastic materials), alumina, zirconia, alkali glass, alkali-free glass, quartz glass, carbon, and other inorganic materials. The pressure-sensitive adhesive sheet disclosed herein can be attached to a member (e.g., an optical member) made of the above material and used.

The components or materials to which the adhesive sheet disclosed herein is attached (in the case of a double-sided adhesive sheet, at least one of the adherends) may be made of a material with a higher refractive index than typical acrylic adhesives. The refractive index of the adherend material is, for example, 1.50 or higher, with some having a refractive index of 1.55 or higher, 1.58 or higher, and even 1.62 or higher (e.g., approximately 1.66). Such high-refractive-index adherend materials are typically resin materials. More specifically, they may be polyester resins such as PET, polyimide resins, aramid resins, polyphenylene sulfide resins, polycarbonate resins, etc. The effect of using the adhesive sheet disclosed herein with such materials (suppression of light reflection due to refractive index differences) can be effectively achieved. The upper limit of the refractive index of the above-mentioned adherend material is, for example, 1.80 or lower, and may be 1.70 or lower. The adhesive sheet disclosed herein is preferably used in a form in which it is attached to such high-refractive-index adherends (e.g., components). A suitable example of such an adherend is a resin film with a refractive index of 1.50 to 1.80 (preferably 1.55 to 1.75, e.g., 1.60 to 1.70). The refractive index can be measured in the same manner as the refractive index of the pressure-sensitive adhesive.

The component or material to which the PSA sheet is attached (in the case of a double-sided PSA sheet, at least one of the adherends) may be optically transparent. Such adherends are more likely to benefit from the technology disclosed herein (suppression of light reflection at the interface between the adherend and the PSA sheet). The total light transmittance of the adherend may be, for example, greater than 50%, preferably 70% or greater. In some preferred embodiments, the total light transmittance of the adherend is 80% or greater, more preferably 90% or greater, and may be 95% or greater (e.g., 95-100%). The PSA sheets disclosed herein are preferably used in embodiments where they are attached to adherends (e.g., optical components) with a total light transmittance of a predetermined value or greater. The total light transmittance is measured in accordance with JIS K 7136:2000 using a commercially available transmittance meter. The transmittance meter used may be a "HAZEMETER HM-150" model manufactured by Murakami Color Research Laboratory, or an equivalent.

In some preferred embodiments, the adherend (e.g., a member) to which the PSA sheet is attached may have the refractive index described above and the total light transmittance described above. Specifically, the PSA sheets disclosed herein can be preferably used in embodiments where they are attached to an adherend, e.g., a member, having a refractive index of 1.50 or higher (e.g., 1.55 or higher, 1.58 or higher, 1.62 or higher, or approximately 1.66, etc.) and a total light transmittance of more than 50% (e.g., 70% or higher, preferably 80% or higher, more preferably 90% or higher, or even 95% or higher). In embodiments where the PSA sheet is attached to such a member, the effects of the technology disclosed herein are particularly favorably exhibited.

One example of a preferred application is optical applications. More specifically, the pressure-sensitive adhesive sheets disclosed herein can be preferably used as optical pressure-sensitive adhesive sheets used, for example, for bonding optical components (for bonding optical components) or for manufacturing products using the optical components (optical products).

The optical component refers to a component that has optical properties (e.g., polarization, light refraction, light scattering, light reflectivity, light transmittance, light absorption, light diffraction, optical rotation, visibility, etc.). The optical component is not particularly limited as long as it has optical properties. Examples include components that constitute devices (optical devices) such as display devices (image display devices) and input devices, or components used in such devices. Examples include polarizing plates, wavelength plates, retardation plates, optical compensation films, brightness enhancement films, light guide plates, reflective films, anti-reflection films, hard coat (HC) films, impact absorbing films, antifouling films, photochromic films, light control films, transparent conductive films (ITO films), design films, decorative films, surface protection plates, prisms, lenses, color filters, transparent substrates, and components in which these are laminated (these may be collectively referred to as "functional films"). The above terms "plate" and "film" are intended to include plate-like, film-like, sheet-like, and other forms. For example, "polarizing film" includes "polarizing plate" and "polarizing sheet," and "light guide plate" includes "light guide film" and "light guide sheet." Furthermore, the above term "polarizing plate" is intended to include circular polarizing plates.

Examples of the display device include liquid crystal display devices, organic EL display devices, micro LEDs (μLEDs), mini LEDs (miniLEDs), PDPs, electronic paper, etc. Furthermore, examples of the input device include touch panels, etc.

The optical component is not particularly limited, but examples include components (e.g., sheet-, film-, or plate-shaped components) made of glass, acrylic resin, polycarbonate, PET, thin metal films, etc. In this specification, "optical component" also includes components that decorate or protect the display device or input device while maintaining its visibility (such as decorative films, ornamental films, and surface protection films).

The technology disclosed herein can be preferably used, for example, to bond optical films such as films having one or more functions such as light transmission, reflection, diffusion, waveguiding, focusing, and diffraction, or fluorescent films, to other optical components (which may be other optical films). In particular, when bonding optical films having at least one function of light guiding, focusing, and diffraction, it is desirable for the entire bulk of the bonding layer to have a high refractive index, making this a preferred application of the technology disclosed herein.

The adhesives disclosed herein can be preferably used for bonding optical films such as light-guiding films, diffusion films, fluorescent films, color-tuning films, prism sheets, lenticular films, and microlens array films. In these applications, the trend toward miniaturization and high performance of optical components demands thinner films and improved light extraction efficiency. The adhesives disclosed herein can be preferably used as adhesives that can meet these demands. More specifically, for example, when bonding light-guiding films or diffusion films, adjusting the refractive index of the adhesive layer used as the bonding layer (e.g., increasing the refractive index) can contribute to thinner films. When bonding fluorescent films, appropriately adjusting the refractive index difference between the fluorescent emitter and the adhesive can improve light extraction efficiency (which can also be understood as luminous efficiency). When bonding color-tuning films, appropriately adjusting the refractive index of the adhesive to minimize the refractive index difference with the color-tuning pigment can reduce scattered components and contribute to improved light transmittance. When bonding prism sheets, lenticular films, microlens array films, etc., by appropriately adjusting the refractive index of the adhesive, it is possible to control the diffraction of light, contributing to improved brightness and/or viewing angle.

The pressure-sensitive adhesive sheet disclosed herein is preferably used in an embodiment in which it is attached to a high-refractive index substrate (which may be a high-refractive index layer, member, etc.), thereby suppressing interfacial reflection with the substrate. As described above, the pressure-sensitive adhesive sheet used in such an embodiment preferably has a small refractive index difference with the substrate and high adhesion at the interface with the substrate. Furthermore, from the viewpoint of enhancing the homogeneity of the appearance, it is preferable that the pressure-sensitive adhesive layer has a highly uniform thickness, and for example, the adhesive surface preferably has high surface smoothness. When the high-refractive index substrate has a relatively small thickness (e.g., 5 μm or less, 4 μm or less, or 2 μm or less), suppressing interfacial reflection is particularly meaningful from the viewpoint of suppressing coloring and color unevenness due to interference of reflected light. One example of such a usage embodiment is an embodiment in which the pressure-sensitive adhesive sheet is used to bond the polarizer to the first retardation layer and/or the first retardation layer to the second retardation layer in a polarizing plate with a retardation layer, which has a polarizer, a first retardation layer, and a second retardation layer in this order.

In addition, because the adhesive sheet disclosed herein is suitable for achieving a high refractive index, it can be preferably used in an embodiment in which it is attached to an emitting layer of an optical semiconductor or the like (for example, a highly refractive emitting layer composed primarily of inorganic materials). By reducing the difference in refractive index between the emitting layer and the adhesive layer, reflection at the interface between them can be suppressed, improving light extraction efficiency. Adhesive sheets used in such an embodiment preferably have an adhesive layer with a high refractive index. Furthermore, from the perspective of improving brightness, it is preferable that the adhesive sheet has low coloration. This can also be advantageous from the perspective of suppressing unintentional coloration caused by the adhesive sheet.

The adhesive sheet disclosed herein can be preferably used in a light-emitting device that includes a self-luminous element as a component, in a configuration in which the adhesive sheet is arranged on the viewing side of the self-luminous element. Here, "self-luminous element" refers to a light-emitting element whose luminance can be controlled by the value of the current flowing through it. The self-luminous element may be composed of a single element or an aggregate. Specific examples of self-luminous elements include, but are not limited to, light-emitting diodes (LEDs) and organic electroluminescent devices. Examples of light-emitting devices that include the self-luminous element as a component include, but are not limited to, light source module devices used for lighting (e.g., surface light-emitting module) and display devices formed with pixels.

The adhesives disclosed herein are preferably used in microlenses and other lens components (e.g., microlenses constituting microlens array films, lens components such as microlenses for cameras) used as components of cameras, light-emitting devices, etc., as coating layers covering the lens surfaces, bonding layers to components facing the lens surfaces (e.g., components having a surface shape corresponding to the lens surfaces), filling layers filled between the lens surfaces and the components, and the like. Because the adhesives disclosed herein are suitable for achieving a high refractive index, they can reduce the refractive index difference with even high-refractive-index lenses (e.g., lenses made of high-refractive-index resins or lenses having a surface layer made of high-refractive-index resins). This is advantageous from the perspective of reducing the thickness of the lenses and products incorporating the lenses, and can also contribute to suppressing aberrations and improving the Abbe number. The adhesives disclosed herein can also be used as lens resins themselves, for example, by filling recesses or voids in an appropriate transparent component.

The mode in which optical components are bonded using the pressure-sensitive adhesive sheet disclosed herein is not particularly limited, and may include, for example, (1) a mode in which optical components are bonded to each other via the pressure-sensitive adhesive sheet disclosed herein, (2) a mode in which an optical component is bonded to a component other than an optical component via the pressure-sensitive adhesive sheet disclosed herein, or (3) a mode in which the pressure-sensitive adhesive sheet disclosed herein includes an optical component and the pressure-sensitive adhesive sheet is bonded to an optical component or a component other than an optical component. In the mode (3), the pressure-sensitive adhesive sheet including an optical component may be, for example, a pressure-sensitive adhesive sheet whose support is an optical component (e.g., an optical film). Such a pressure-sensitive adhesive sheet containing an optical component as a support may also be understood as a pressure-sensitive adhesive optical component (e.g., a pressure-sensitive adhesive optical film). Furthermore, when the pressure-sensitive adhesive sheet disclosed herein is a pressure-sensitive adhesive sheet having a support and the above-mentioned functional film is used as the support, the pressure-sensitive adhesive sheet disclosed herein may also be understood as a "pressure-sensitive adhesive functional film" having the pressure-sensitive adhesive layer disclosed herein on at least one side of the functional film.

As described above, the technology disclosed herein provides a laminate comprising the adhesive sheet disclosed herein and a member to which the adhesive sheet is attached. The member to which the adhesive sheet is attached may have the refractive index of the adherend material described above. Furthermore, the difference (refractive index difference) between the refractive index of the adhesive sheet and the member may be the refractive index difference between the adherend and the adhesive sheet described above. The members that make up the laminate are as described above for the member, material, and adherend, so redundant description will not be repeated.

As will be understood from the above description and the following examples, the matters disclosed by this specification include the following.
[1] A refractive index adjuster for a pressure-sensitive adhesive,
The refractive index adjuster is an organic compound having a structure with two or more substituents on a hub ring, wherein the hub ring is a double bond-containing ring, and at least one of the two or more substituents is a substituent having a double bond-containing ring.
[2] The refractive index adjuster according to [1] above, having a molecular weight of less than 3,000.
[3] The refractive index adjuster according to [1] or [2] above, wherein two or more of the substituents on the hub ring are substituents having a double bond-containing ring.
[4] The refractive index adjuster according to any one of [1] to [3] above, wherein at least one of the substituents on the hub ring is a substituent having two or more double bond-containing rings.
[5] The refractive index adjuster according to any one of [1] to [4] above, wherein the hub ring is a triazine ring.

[6] The following formula (I):
(In formula (I), X ¹ , X ² and X ³ are each independently selected from the group consisting of —O—, —S— and —NR ⁴ —, where R ⁴ is a hydrogen atom or an alkyl group;
one or more of R ¹ , R ² and R ³ each independently represent an aromatic ring-containing group selected from the group consisting of an optionally substituted phenyl group and an optionally substituted biphenyl group, wherein the substituent is an alkyl group, an alkoxy group or a cyano group;
When there are two or less aromatic ring-containing groups among R ¹ , R ² and R ³ , the remaining are aliphatic hydrocarbon groups or hydrogen atoms;
A compound represented by the formula:
[7] The compound according to [6] above, wherein two or more of X ¹ , X ² and X ³ are each independently selected from —O— and —S—.
[8] The compound according to [6] above, wherein X ¹ , X ² and X ³ are all —O—.
[9] The compound according to [6] above, wherein X ¹ , X ² and X ³ are all —S—.
[10] The compound according to any one of [6] to [9] above, wherein R ¹ , R ² and R ³ are each independently 2-biphenyl which may or may not have a substituent.
[11] A refractive index adjuster comprising the compound according to any one of [6] to [10] above.

[12] A pressure-sensitive adhesive comprising the refractive index adjuster according to any one of [1] to [5] and [11] above.
[13] The pressure-sensitive adhesive according to the above [12], further comprising an acrylic polymer containing an aromatic ring-containing monomer (m1) as a monomer unit.
[14] The pressure-sensitive adhesive according to [12] or [13], wherein the monomer components constituting the acrylic polymer contain, in addition to the aromatic ring-containing monomer (m1), a monomer (m2) having at least one of a hydroxyl group and a carboxyl group.
[15] The pressure-sensitive adhesive according to any one of [12] to [14], wherein the content of the aromatic ring-containing monomer (m1) in the monomer components is 60% by weight or more.
[16] The pressure-sensitive adhesive according to any one of [12] to [15], wherein 50% by weight or more of the aromatic ring-containing monomers (m1) are monomers having a glass transition temperature of 10°C or lower as a homopolymer.
[17] The pressure-sensitive adhesive according to any one of [12] to [16], wherein the content of the refractive index adjuster relative to 100 parts by weight of the acrylic polymer is 0.1 parts by weight or more and 80 parts by weight or less.
[18] The pressure-sensitive adhesive according to any one of [12] to [17] above, having a refractive index of 1.550 or more.

[19] A pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive according to any one of [12] to [18] above.
[20] The pressure-sensitive adhesive sheet according to [19] above, wherein the pressure-sensitive adhesive layer has a thickness in the range of 5 to 75 μm.
[21] The pressure-sensitive adhesive sheet according to the above [19] or [20], which has a total light transmittance of 85% or more.
[22] The pressure-sensitive adhesive sheet according to any one of [19] to [21] above, having a haze value of 3% or less.
[23] The pressure-sensitive adhesive sheet according to any one of [19] to [22] above, wherein the chromaticity b ^* defined in the L ^* a ^* b ^* color system is in the range of ±5.

The following describes several examples of the present invention, but it is not intended that the present invention be limited to these specific examples. In the following description, "parts" and "%" representing amounts used and contents are by weight unless otherwise specified.

In the following synthesis examples, the refractive index of each compound was measured as follows. A 5% ethyl acetate solution or a 5% MEK solution of the compound to be measured was prepared. This solution was applied to a glass substrate to a dry film thickness of 10 μm, and the substrate was dried by heating at 130°C for 5 minutes. The refractive index of the compound to be measured thus formed on the glass substrate was measured using a prism coupler (Metricon, Model "2010M") at a measurement temperature of 25°C and a measurement wavelength of 594 nm.

Synthesis Example 1: Synthesis of Compound A1

Under a nitrogen atmosphere, biphenyl-2-ol (5.28 g, 31 mmol), acetone (50 mL), and potassium hydroxide (1.74 g, 31 mmol) were charged into a reaction vessel and stirred at room temperature for 30 minutes. A solution of cyanuric chloride (1.84 g, 10 mmol) in acetone (20 mL) was added dropwise over 15 minutes, and the mixture was stirred at room temperature for 2 hours, followed by stirring at 55°C for 8 hours. The reaction solution was poured into 800 mL of distilled water, resulting in the formation of an oily precipitate. The supernatant was removed by decantation, and the mixture was dried under reduced pressure at 100°C for 4 hours to obtain a glassy solid (2.7 g). This solid was dissolved in acetone (20 mL) and poured into hexane (100 mL). After allowing to stand at room temperature for 24 hours, the supernatant was removed, and the oily precipitate was collected and dried under reduced pressure at 80°C for 4 hours to obtain 2.25 g (3.84 mmol) of Compound A1. The yield was 38.4%. Compound A1 exhibited solubility such that 50% solutions could be prepared in both methyl ethyl ketone (MEK) and ethyl acetate. The refractive index was 1.645. The ¹ H-NMR (400 MHz, CDCl ₃ ) and ¹³ C-NMR (400 MHz, CDCl ₃ ) spectra of Compound A1 are shown in Figures 4 and 5, respectively.

<Synthesis Example 2: Synthesis of Compound A2>

Under a nitrogen atmosphere, biphenyl-4-ol (5.28 g, 31 mmol), acetone (50 mL), potassium hydroxide (1.74 g, 31 mmol), and water (20 mL) were charged into a reaction vessel and stirred at room temperature for 30 minutes. A solution of cyanuric chloride (1.84 g, 10 mmol) in acetone (20 mL) was added dropwise over 15 minutes, and the mixture was stirred at room temperature for 24 hours, resulting in the precipitation of a white solid. The reaction solution was poured into 800 mL of distilled water, and the white powdery precipitate was collected by filtration and dried under reduced pressure at 80°C for 6 hours to obtain a white solid (5 g). This was recrystallized from acetone to obtain 4.85 g (7.94 mmol) of Compound A2. The yield was 79.4%. The ¹ H-NMR (400 MHz, CDCl ₃ ) and ¹³ C-NMR (400 MHz, CDCl ₃ ) spectra of Compound A2 are shown in Figures 6 and 7, respectively.

Synthesis Example 3: Synthesis of Compound A3

Under a nitrogen atmosphere, biphenyl-4-ol (5.28 g, 31 mmol), acetone (50 mL), potassium hydroxide (1.74 g, 31 mmol), and water (20 mL) were charged into a reaction vessel and stirred at room temperature for 30 minutes. A solution of 2,4-dichloro-6-methoxytriazine (1.80 g, 10 mmol) in acetone (30 mL) was added to the reaction vessel and stirred at room temperature for 24 hours. The reaction solution was poured into 800 mL of distilled water, and the resulting white powdery precipitate was collected by filtration, dried under reduced pressure at 80°C for 5 hours, and recrystallized from acetone to obtain 1.81 g (2.96 mmol) of Compound A3. The yield was 30.2%. Compound A3 exhibited solubility sufficient to prepare a 7% solution in MEK. ^{The 1} H-NMR (400 MHz, CDCl ₃ ) spectrum and ¹³ C-NMR (400 MHz, CDCl ₃ ) spectrum of Compound A3 are shown in FIGS. 8 and 9, respectively.

Synthesis Example 4: Synthesis of Compound A4

Under a nitrogen atmosphere, biphenyl-2-thiol (19.8 g, 106.3 mmol) and acetone (100 mL) were charged into a reaction vessel, and an 85% aqueous solution of potassium hydroxide (7.1 g, 67.3 mmol) was added. The mixture was stirred at room temperature for 1 hour. The system was cooled to 10°C, and a solution of cyanuric chloride (6 g, 34.3 mmol) in acetone (60 mL) was added dropwise over 15 minutes, followed by stirring at room temperature for 6 hours. Water was then added, the mixture was neutralized with 0.1 M hydrochloric acid, extracted with ethyl acetate, dried over sodium sulfate, filtered, and concentrated. The mixture was purified by silica gel column chromatography using a 10/1 (volume ratio) ethyl acetate:heptane eluent to obtain 13 g (20.5 mmol) of Compound A4. The yield was 59.8%. Compound A4 exhibited solubility sufficient to prepare 50% solutions in both methyl ethyl ketone (MEK) and ethyl acetate. The refractive index was 1.702. ^{The 1} H-NMR (400 MHz, CDCl ₃ ) spectrum and ¹³ C-NMR (400 MHz, CDCl ₃ ) spectrum of Compound A4 are shown in FIGS. 10 and 11, respectively.

Synthesis Example 5: Synthesis of Compound A5

Under a nitrogen atmosphere, cyanuric chloride (9.28 g, 50 mmol), N-methylpyrrolidone (100 mL), p-toluenethiol (20.65 g, 166 mmol), and triethylamine (17.8 g, 176 mmol) were charged into a reaction vessel and stirred at 80°C for 24 hours. Water (1500 mL) was then added, the mixture was neutralized with 0.1 M hydrochloric acid, extracted with ethyl acetate (1 L), and dried over sodium sulfate (200 g). The mixture was then filtered and the solvent was evaporated to give a paste-like mixture. This mixture was washed with acetone (100 mL) to give 7.35 g of crude product, which was recrystallized by heating in acetone to give 6.5 g (14.5 mmol) of Compound A5. The yield was 29%. Compound A5 exhibited sufficient solubility in MEK to prepare a 5% solution. The refractive index was 1.672. The ¹ H-NMR (400 MHz, DMSO-d ₆ ) spectrum and ¹³ C-NMR (400 MHz, DMSO-d ₆ ) spectrum of Compound A5 are shown in FIGS. 12 and 13, respectively.

Synthesis Example 6: Synthesis of Compound A6

Under a nitrogen atmosphere, 2-ethylhexanethiol (20 mmol), tetrahydrofuran (20 mL), and potassium hydroxide (1.24 g, 22 mmol) were charged into a reaction vessel and stirred at room temperature for 2 hours, then at 60°C for 1 hour, and allowed to cool to room temperature. A solution of cyanuric chloride (4.06 g, 20 mmol) in tetrahydrofuran (20 mL) was added dropwise over 30 minutes, and the mixture was stirred at room temperature for 24 hours. The sodium chloride was then removed by filtration, and the solvent was distilled off to obtain an oily compound (2,4-dichloro-6-(2-ethylhexylthio)triazine). N-methylpyrrolidone (50 mL), p-toluenethiol (5.47 g, 44 mmol), and triethylamine (5.06 g, 50 mmol) were added to the mixture, and the mixture was stirred at 80°C for 24 hours. Water (1500 mL) was then added, and the precipitated oil was collected and purified by silica gel column chromatography using hexane:dichloromethane = 3:1 (volume ratio) as a developing solvent to obtain 0.57 g (1.2 mmol) of Compound A6. The yield was 6%. Compound A6 exhibited solubility in MEK and ethyl acetate, allowing the preparation of 50% solutions. The refractive index was 1.620. The ¹ H-NMR (400 MHz, CDCl ₃ ) spectrum and ¹³ C-NMR (400 MHz, CDCl ₃ ) spectrum of Compound A6 are shown in Figures 14 and 15, respectively.

Synthesis Example 7: Synthesis of Compound A7

Under a nitrogen atmosphere, 2-ethylhexanethiol (20 mmol), tetrahydrofuran (20 mL), and potassium hydroxide (1.24 g, 22 mmol) were charged to a reaction vessel and stirred at room temperature for 2 hours, then at 60°C for 1 hour, and then allowed to cool to room temperature. A solution of 2-(4-cyanophenyl)amino-4,6-dichloro-1,3,5-triazine (2.66 g, 10 mmol) in tetrahydrofuran (10 mL) was added dropwise over 30 minutes, and the mixture was stirred at room temperature for 24 hours. The sodium chloride was removed by filtration, and the solvent was evaporated to give a viscous oil. The resulting oil was purified by silica gel column chromatography using a 1:1 (volume ratio) hexane:dichloromethane eluent to obtain 2.62 g (5.4 mmol) of Compound A7 as a white viscous oil. The yield was 54%. Compound A7 exhibited solubility sufficient to prepare 50% solutions in both MEK and ethyl acetate. The refractive index of Compound A7 was 1.594. The ¹ H-NMR (400 MHz, CDCl ₃ ) spectrum and ¹³ C-NMR (400 MHz, CDCl ₃ ) spectrum of Compound A7 are shown in Figures 16 and 17, respectively. The refractive index of Compound A7 was 1.645.

Synthesis Example 8: Synthesis of Compound A8

Methanol (110 mL), water (14 mL), and sodium bicarbonate (18.2 g, 217 mmol) were charged into a reaction vessel, and cyanuric chloride (20.0 g, 108 mmol) was added in 10 approximately 2 g portions over approximately 30 minutes. After stirring at 30°C for 1 hour, water (50 mL) was added, and the resulting white precipitate was collected by filtration, washed with water (50 mL), and then with methanol (30 mL). After drying under reduced pressure at room temperature for 12 hours, 14.6 g (80 mmol) of 2,4-dichloro-6-methoxytriazine was obtained. The yield was 74.6%.
Under a nitrogen atmosphere, biphenyl-4-thiol (2.00 g, 10.7 mmol), acetone (40 mL), potassium hydroxide (0.60 g, 10.7 mmol), and water (15 mL) were charged into a reaction vessel and stirred at room temperature for 30 minutes. To this was added a solution of 2,4-dichloro-6-methoxytriazine (0.90 g, 5.0 mmol) in acetone (30 mL), followed by stirring at room temperature for 24 hours. The reaction solution was then poured into 800 mL of distilled water. The resulting white precipitate was collected by filtration and dried under reduced pressure at 80°C for 5 hours. It was then reprecipitated from a hexane:acetone = 5:1 (volume ratio) mixed solvent and purified by silica gel column chromatography using a hexane:methylene chloride mixed solvent (volume ratio gradient from 3:1 to 1:1) as the electrolyte, yielding 1.42 g of Compound A8. The yield was 59.2%. Compound A8 exhibited solubility sufficient to prepare a 2.5% solution in MEK. ^{The 1} H-NMR (400 MHz, CDCl ₃ ) spectrum and ¹³ C-NMR (400 MHz, CDCl ₃ ) spectrum of Compound A8 are shown in FIGS. 18 and 19, respectively.

<Example 1>
(Preparation of Acrylic Polymer Solution)
A four-neck flask equipped with a stirring blade, thermometer, nitrogen gas inlet tube, and condenser was charged with 95 parts of m-phenoxybenzyl acrylate (manufactured by Kyoeisha Chemical Co., Ltd., trade name "Light Acrylate POB-A", refractive index: 1.566, homopolymer Tg: -35°C; hereinafter referred to as "POB-A") as monomer components, 5 parts of 4-hydroxybutyl acrylate (4HBA), 0.2 parts of 2,2'-azobisisobutyronitrile as a polymerization initiator, and 150 parts of ethyl acetate as a polymerization solvent. Nitrogen gas was introduced while gently stirring, and the liquid temperature in the flask was maintained at around 60°C, allowing the polymerization reaction to proceed for 6 hours, yielding a 40% solution of acrylic polymer P1. The Mw of the acrylic polymer P1 was 500,000.

(Preparation of Pressure-Sensitive Adhesive Composition)
The above-mentioned solution (40%) of acrylic polymer P1 was diluted to 20% with ethyl acetate, and to 500 parts of this solution (100 parts non-volatile content), 20 parts of a 50% ethyl acetate solution of compound A1 (10 parts non-volatile content), 10 parts (0.1 part non-volatile content) of a 1% ethyl acetate solution of an isocyanurate of hexamethylene diisocyanate (manufactured by Tosoh Corporation, trade name "Coronate HX", a trifunctional isocyanate compound) as a crosslinking agent, 2 parts of acetylacetone as a crosslinking retarder, and 1 part (0.01 part non-volatile content) of a 1% ethyl acetate solution of nursem ferric as a crosslinking catalyst were added and mixed by stirring to prepare a pressure-sensitive adhesive composition according to this example.

(Preparation of adhesive sheet)
The acrylic pressure-sensitive adhesive composition prepared above was applied to the silicone-treated surface of a polyethylene terephthalate (PET) film R1 (50 μm thick) that had been silicone-treated on one side, and heated at 130°C for 2 minutes to form a 20 μm thick pressure-sensitive adhesive layer. Next, the silicone-treated surface of a PET film R2 (25 μm thick) that had been silicone-treated on one side was bonded to the surface of the pressure-sensitive adhesive layer. In this way, a substrateless double-sided pressure-sensitive adhesive sheet consisting of the pressure-sensitive adhesive layer was obtained. Both sides of this pressure-sensitive adhesive sheet were protected by PET films (release liners) R1 and R2.

<Examples 2 to 4>
Except for using compound A4 (Example 2), compound A5 (Example 4), or compound A6 (Example 4) instead of compound A1, the PSA compositions of each example were prepared in the same manner as the preparation of the PSA composition in Example 1. Except for using each of the obtained PSA compositions, PSA sheets of Examples 2 to 4 (substrate-less double-sided PSA sheets comprising a PSA layer) were produced in the same manner as the production of the PSA sheet in Example 1.

<Example 5>
Except for using compound B1 (diethylene glycol dibenzoate, refractive index 1.535) instead of compound A1, pressure-sensitive adhesive compositions according to this example were prepared in the same manner as the preparation of the pressure-sensitive adhesive composition in Example 1. Except for using each of the resulting pressure-sensitive adhesive compositions, pressure-sensitive adhesive sheets according to this example (substrate-less double-sided pressure-sensitive adhesive sheets comprising a pressure-sensitive adhesive layer) were produced in the same manner as the production of the pressure-sensitive adhesive sheet in Example 1.

<Example 6>
Except for not using compound A1, PSA compositions according to this example were prepared in the same manner as the preparation of the PSA composition in Example 1. PSA sheets according to this example (substrate-less double-sided PSA sheets comprising a PSA layer) were produced in the same manner as the production of the PSA sheet in Example 1, except for using each of the obtained PSA compositions.

<Evaluation>
(Refractive index of adhesive)
The refractive index of the pressure-sensitive adhesive layer (substrate-less double-sided pressure-sensitive adhesive sheet) according to each example was measured using an Abbe refractometer (manufactured by ATAGO, model "DR-M4") under conditions of a measurement wavelength of 589 nm and a measurement temperature of 25°C.

(Total light transmittance and haze)
The pressure-sensitive adhesive layer according to each example was attached to alkali-free glass (thickness 0.8 to 1.0 mm, total light transmittance 92%, haze 0.4%) to form a test piece, which was then measured for total light transmittance and haze using a haze meter ("HM-150" manufactured by Murakami Color Research Laboratory). The total light transmittance and haze of the alkali-free glass were subtracted from the measured value to obtain the total light transmittance [%] and haze [%] of the pressure-sensitive adhesive (layer). For a substrate-less pressure-sensitive adhesive sheet comprising the pressure-sensitive adhesive layer, the total light transmittance [%] and haze [%] of the pressure-sensitive adhesive layer are equivalent to the total light transmittance [%] and haze [%] of the pressure-sensitive adhesive sheet.

(Measurement of chromaticity b ^* )
The chromaticity b ^* of the pressure-sensitive adhesive layer according to each example was measured by the method described above. The measurement was carried out at five points on the surface of the pressure-sensitive adhesive layer, and the average value was calculated.

An overview of the adhesive for each example and the evaluation results are shown in Table 1.

As shown in Table 1, the PSA's of Examples 1 to 4 formed from PSA compositions in which Compounds A1 and A4 to A6, each having a higher refractive index than Compound B1 used in Example 5, were blended with the PSA composition of Example 6, all exhibited higher refractive indices than the PSA's of Examples 5 and 6. The PSA's of Examples 1 to 4 had good light transmittance, haze, and chromaticity b ^* at levels that allowed them to be used in optical applications.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

1, 2, 3 Pressure-sensitive adhesive sheet 10 Support substrate 10A First surface 10B Second surface 21 Pressure-sensitive adhesive layer, first pressure-sensitive adhesive layer 21A Pressure-sensitive adhesive surface, first pressure-sensitive adhesive surface 21B Pressure-sensitive adhesive surface 22 Second pressure-sensitive adhesive layer 22A Second pressure-sensitive adhesive surface 31, 32 Release liner

Claims (3)

A refractive index adjuster for a pressure-sensitive adhesive,
The following formula (I):
(In formula (I), X ¹ , X ² and X ³ are all —O— or all —S—;
R ¹ , R ² and R ³ are
Both are aromatic ring-containing groups, or
two are aromatic ring-containing groups and the remaining one is an alkyl group having 1 to 12 carbon atoms;
The aromatic ring-containing groups are the same groups and are selected from the group consisting of optionally substituted phenyl groups and optionally substituted biphenyl groups, and when the phenyl group or the biphenyl group has a substituent, the substituent is selected from an alkyl group having 1 to 3 carbon atoms;
is a triazine compound represented by the formula:
The pressure-sensitive adhesive is an acrylic pressure-sensitive adhesive having an acrylic polymer as a base polymer, and a monomer component constituting the acrylic polymer containing an aromatic ring-containing monomer (m1), and the content of the aromatic ring-containing monomer (m1) in the monomer component is 75% by weight or more . A pressure- sensitive adhesive comprising the refractive index adjuster according to claim 1 ,
the pressure-sensitive adhesive is an acrylic pressure-sensitive adhesive having an acrylic polymer as a base polymer, and a monomer component constituting the acrylic polymer including an aromatic ring-containing monomer (m1),
A pressure-sensitive adhesive, wherein the content of the aromatic ring-containing monomer (m1) in the monomer components is 75% by weight or more . The pressure-sensitive adhesive according to claim 2 , having a refractive index of 1.550 or more.