JP7702266B2 - Adhesives and their uses - Google Patents

Description

The present invention relates to a pressure-sensitive adhesive, and more specifically to a pressure-sensitive adhesive containing an acrylic polymer that includes a fluorine-containing acrylic monomer as a monomer unit, and its use.

Generally, adhesives (also called pressure-sensitive adhesives; the same applies below) are in a soft solid (viscoelastic) state at temperatures around room temperature, and have the property of easily adhering to an adherend when pressure is applied. Taking advantage of 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 machines, electrical equipment, and electronic devices. Examples of technical documents related to adhesives include Patent Documents 1 to 4.

Patent No. 6014781 JP 2020-111740 A JP 2019-210343 A JP 2018-193553 A

Patent Documents 1 to 4 propose adhesives that contain (meth)acrylate copolymers that contain structural units derived from fluorine-containing acrylic monomers. All of these adhesives focus on sebum resistance and/or chemical resistance, and do not take into account the refractive index.

The present invention aims to provide an adhesive that has a low refractive index and is flexible. Another related object is to provide a laminated sheet including an adhesive layer made of such an adhesive, and a light-emitting device including the laminated sheet.

According to this specification, a pressure-sensitive adhesive is provided that contains an acrylic polymer (F) that contains a fluorine-containing acrylic monomer (M1) as a monomer unit. The refractive index of the pressure-sensitive adhesive is 1.46 or less. The pressure-sensitive adhesive preferably has a storage modulus G' (hereinafter, sometimes referred to as "storage modulus G' _V2 (25)") at 25°C of 400 kPa or less (for example, 1.0 kPa or more and 400 kPa or less). Such a pressure-sensitive adhesive can be preferably used for purposes such as controlling the behavior of light, for example, in a form in which it is laminated on a layer (which may be a pressure-sensitive adhesive layer) having a higher refractive index. Since the storage modulus G' _V2 (25) of the pressure-sensitive adhesive is limited to a predetermined value or less, it is advantageous in terms of ease of application to an adherend, adhesion, ability to follow the surface shape of the adherend, ability to follow deformation of the adherend, and the like.

In the monomer components constituting the acrylic polymer (F), the content of the fluorine-containing acrylic monomer (M1) is preferably 25% by weight or more. By including an acrylic polymer (F) composed of monomer components of such a composition, it is possible to preferably realize a pressure-sensitive adhesive having a refractive index of 1.46 or less. From the viewpoints of ease of polymerization and the effect of reducing the refractive index, the fluorine-containing acrylic monomer (M1) preferably includes a fluorine atom-containing alkyl (meth)acrylate ester.

In some embodiments, the acrylic polymer (F) contains a hydroxyl-containing monomer as a monomer unit. The monomer unit derived from the hydroxyl-containing monomer can be useful in realizing a pressure-sensitive adhesive that has both the desired flexibility and appropriate cohesiveness.

According to this specification, a laminate sheet is provided that includes a low refractive index adhesive layer formed from any of the adhesives disclosed herein and a high refractive index adhesive layer laminated on the low refractive index adhesive layer. According to a laminate sheet that includes adhesive layers with different refractive indices in a laminated form, the behavior of light passing through the laminate sheet can be controlled by utilizing the refractive index difference between the high refractive index adhesive layer and the low refractive index adhesive layer. The ratio (n1/ _n2 ) of the refractive index _n1 of the high refractive index adhesive layer to the refractive index _n2 of the low refractive index adhesive layer can be, for example, _1.02 or more.

In some embodiments, the refractive index _n1 of the high refractive index pressure-sensitive adhesive layer may be greater than 1.570. A laminate sheet including such a high refractive index pressure-sensitive adhesive layer has a large refractive index difference with the low refractive index pressure-sensitive adhesive layer, and therefore the behavior of light passing through the laminate sheet can be better controlled.

In some embodiments, the high refractive index pressure-sensitive adhesive layer has a storage modulus G' at 25° C. (hereinafter, may be referred to as "storage modulus G' _V1 (25)") of 700 kPa or less. A laminate sheet having a high refractive index pressure-sensitive adhesive layer in which the storage modulus G' _V1 (25) is limited to a predetermined value or less is preferable from the viewpoints of ease of application and flexibility.

In some embodiments, the laminate sheet has a total light transmittance of 86% or more and a haze value of 3.0% or less. Such highly transparent laminate sheets can be preferably used for applications such as bonding optical components and controlling the behavior of light passing through the laminate sheet.

The laminated sheet disclosed herein is suitable, for example, as a component of a light-emitting device. This specification therefore provides a light-emitting device that includes any of the laminated sheets disclosed herein and a self-luminous element.

Note that any suitable combination 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 illustrating a schematic configuration of a pressure-sensitive adhesive sheet according to an embodiment. FIG. 1 is a cross-sectional view showing a schematic configuration of a laminate sheet according to an embodiment of the present invention. 1 is a cross-sectional view illustrating a schematic configuration of a light-emitting device according to an embodiment.

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, the same reference numerals may be used to denote components or parts having the same function, and duplicated descriptions may be omitted or simplified. In addition, 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 product actually provided.

In this specification, the term "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 electroluminescence (EL). The light-emitting device disclosed herein includes such a self-luminous element as a component. Examples of the light-emitting device include, but are not limited to, a light source module device (e.g., a surface light-emitting module) used for illumination, and a display device formed with pixels.

The technology disclosed in this specification may include a low refractive index adhesive and an adhesive composition used to form the same, a high refractive index adhesive layer and an adhesive composition used to form the same, a laminate sheet (adhesive sheet) including a high refractive index adhesive layer and a low refractive index adhesive layer, a laminate sheet with a release liner in which the adhesive surface of the laminate sheet is protected with a release liner, a light emitting device including a high refractive index adhesive layer and a low refractive index adhesive layer, and the like. The technology disclosed in this specification may further include an interlayer sheet including a high refractive index adhesive layer and a low refractive index adhesive layer and disposed between layers of an optical laminate.

<Examples of adhesive sheet configurations>
The adhesive disclosed herein can be used, for example, as an adhesive sheet including a layer (adhesive layer) formed from the adhesive, and is preferably used in the form of an adhesive sheet having an adhesive surface constituted by the adhesive layer. The adhesive sheet may be a substrate-attached adhesive sheet having an adhesive layer on one or both sides of a non-peeling substrate (support substrate), or may be a substrate-less adhesive sheet (i.e., an adhesive sheet without a non-peeling substrate. Typically, the adhesive sheet is made of an adhesive layer) in a form in which the adhesive layer is held by a release liner. The concept of the adhesive sheet here may include those called adhesive tapes, adhesive labels, adhesive films, etc. The adhesive sheet disclosed herein may be in the form of a roll or a sheet. Alternatively, the adhesive sheet may be in the form of an adhesive sheet further processed into various shapes.

An example of the configuration of an adhesive sheet including an adhesive layer formed from the adhesive disclosed herein is shown in FIG. 1. This adhesive sheet 1 is configured as a one-sided adhesive sheet (single-sided adhesive sheet) including an adhesive layer 10 having a first surface 10A that is an attachment surface (adhesive surface) to an adherend, and a support substrate 20 laminated on a second surface 10B of the adhesive layer 10. The second surface 10B of the adhesive layer 10 is bonded to a first surface (non-releasable surface) 20A of the support substrate 20. For example, a plastic film such as a polyester film can be used as the support substrate 20. The support substrate 20 may be an optical film such as a polarizing plate. Before use (before attachment to an adherend), the adhesive sheet 1 may be in the form of an adhesive sheet with a release liner 50, in which the adhesive surface 10A is protected by a release liner 30, at least the adhesive layer side of which is a releasable surface (releasable surface), as shown in FIG. 1. Alternatively, the second surface 20B of the support substrate 20 (the surface opposite the first surface 20A, also called the back surface) may be the release surface, and the adhesive surface 10A may be protected by being wound or laminated so that the adhesive surface 10A abuts against this second surface 20B.

The adhesive layer 10 may have a single layer structure, or a laminate structure in which two or more sub-adhesive layers with different compositions are laminated in direct contact (i.e., not separated by a layer of a non-adhesive material). The refractive indexes of the sub-adhesive layers constituting the laminate structure may be similar or different. In an adhesive layer having a laminate structure including two or more sub-adhesive layers with different refractive indexes, it is preferable that at least one sub-adhesive layer is a low refractive index adhesive layer having a refractive index of 1.46 or less (preferably, further having a storage modulus G'(25) of 400 kPa or less). The refractive index and storage modulus G'(25) of the other sub-adhesive layers are not particularly limited, and for example, the refractive index may be 1.46 or less or may be greater than 1.46, and the storage modulus G'(25) may be 400 kPa or less or may be greater than 400 kPa.

The adhesive sheet disclosed herein may be in the form of a substrateless double-sided adhesive sheet made of an adhesive layer. The adhesive layer constituting the substrateless double-sided adhesive sheet may have a single layer structure, like the adhesive layer 10 in the adhesive sheet 1 shown in FIG. 1, or may have a laminate structure in which two or more sub-adhesive layers with different compositions are laminated in direct contact. FIG. 2 shows an example of the structure of a substrateless double-sided adhesive sheet 2 made of an adhesive layer 10 having a laminate structure including a first adhesive layer 11 and a second adhesive layer 12 with different compositions. The second adhesive layer 12 may be a low refractive index adhesive layer that satisfies a refractive index of 1.46 or less, and further a layer that satisfies a storage modulus G'(25) of 400 kPa or less. The first adhesive layer 11 may be an adhesive layer with a higher refractive index (high refractive index adhesive layer) in relation to the second adhesive layer 12. The adhesive sheet 2 having such a structure may be understood as a laminate sheet having a laminate structure of a high refractive index adhesive layer/low refractive index adhesive layer.

Before use, the substrate-less double-sided adhesive sheet 2 may be in a form in which the first surface (first adhesive surface) 10A and the second surface (second adhesive surface) 10B of the adhesive layer 10 are protected by release liners 31, 32, at least the adhesive layer side of which is a releasable surface (removal surface), as shown in FIG. 2. Alternatively, the back surface of the release liner 31 (the surface opposite to the adhesive side) may be a release surface, and the adhesive surface 10B may be wound or laminated so as to abut against the back surface of the release liner 31, thereby protecting the adhesive surfaces 10A, 10B. Such a substrate-less double-sided adhesive sheet may be used, for example, by bonding a substrate (preferably a light-transmitting substrate, which may be an optical member such as an optical film) to at least one of the first and second adhesive surfaces.

The low refractive index adhesive (adhesive having a refractive index of 1.46 or less) disclosed herein may be used, for example, in a form incorporated in a light-emitting device. One configuration example of such a light-emitting device is shown in FIG. 3. The light-emitting device 100 shown in FIG. 3 includes a self-emitting element 70, a low refractive index adhesive layer 12 arranged on the viewing side of the self-emitting element 70, and a high refractive index adhesive layer 11 laminated in direct contact with the low refractive index adhesive layer 12. The light-emitting device 100 may further include a cover window member 80 arranged on the viewing side of the high refractive index adhesive layer 11. In the light-emitting device 100 shown in FIG. 3, a laminated sheet 10 consisting of a high refractive index adhesive layer 11 and a low refractive index adhesive layer 12 is arranged between the self-emitting element 70 and the cover window member 80. Between the self-emitting element 70 and the low refractive index adhesive layer 12, between the high refractive index adhesive layer 12 and the cover window member 80, and on the viewing side of the cover window member 80, one or more layers not shown may be interposed independently, or may not be interposed. Also, the high refractive index adhesive layer 11 may be disposed on the self-luminous element side, and the low refractive index adhesive layer 12 may be disposed on the cover window member side, in the opposite manner to that shown in FIG. 3. Alternatively, the low refractive index adhesive disclosed herein may be used in a light-emitting device in which the high refractive index adhesive layer 11 in FIG. 3 is omitted or replaced with a high refractive index layer (such as a hard resin layer) that does not have adhesive properties.

The adhesive sheet disclosed herein may be a component of an optical member with an adhesive sheet in which an optical member is bonded to at least one surface of the adhesive layer. For example, the adhesive sheet 1 shown in FIG. 1 may be a component of an optical member with an adhesive sheet in which an optical member is bonded to the first surface 10A of the adhesive layer 10. The optical member may be, for example, a glass plate, a resin film, a metal plate, or the like. In addition, in the adhesive sheet 1 shown in FIG. 1, when the support substrate 20 is an optical member such as an optical film, the adhesive sheet 1 may be understood as an optical member with an adhesive sheet in which an optical member is bonded to the second surface 10B of the adhesive layer 10.

The adhesive sheet disclosed herein may be in the form of a substrate-attached double-sided adhesive sheet (substrate-attached double-sided adhesive sheet) having a support substrate with a non-peeling first surface and a non-peeling second surface, a first adhesive layer fixedly laminated on the first surface, and a second adhesive layer fixedly laminated on the second surface. An example of the configuration of such a substrate-attached double-sided adhesive sheet is a single-sided adhesive sheet 1 shown in FIG. 1, in which the second surface 20B of the support substrate 20 is a non-peeling surface, a second adhesive layer is provided on the second surface 20B, the second surface of the second adhesive layer is bonded to the second surface 20B of the support substrate 20, and the first surface of the second adhesive layer (the surface opposite to the second surface) is the second adhesive surface of the substrate-attached double-sided adhesive sheet. The composition of the adhesive constituting the second adhesive layer may be the same as or different from the composition of the adhesive constituting the first adhesive layer. Before use, the double-sided PSA sheet with substrate may be in a form in which the first and second adhesive surfaces are protected by a release liner, similar to the substrate-less double-sided PSA sheet described above.

For ease of explanation, adhesives with a refractive index of 1.46 or less (low refractive index adhesives) may be described in comparison with layers with higher refractive indexes below, but this is not intended to limit the manner in which the low refractive index adhesives disclosed herein can be used.

<Low refractive index adhesive>
This specification provides a pressure-sensitive adhesive having a refractive index of 1.46 or less. A pressure-sensitive adhesive having such a refractive index can be preferably used for the purpose of controlling the behavior of light, for example, in a form laminated on a layer (which may be a pressure-sensitive adhesive layer) having a higher refractive index. In some embodiments, from the viewpoint of increasing the refractive index difference with the refractive index n ₁ of the high refractive index layer to easily enhance the front brightness improvement effect described later, the refractive index n ₂ of the low refractive index pressure-sensitive adhesive layer is preferably 1.45 or less, preferably 1.44 or less, may be 1.43 or less, may be 1.41 or less, may be 1.40 or less, or may be less than 1.40 (for example, less than 1.39, or even less than 1.38). In addition, from the viewpoint of ease of obtaining materials and ease of compatibility with adhesive properties, in some embodiments, the refractive index n ₂ of the low refractive index pressure-sensitive adhesive may be, for example, 1.35 or more, preferably 1.36 or more, may be 1.38 or more, may be 1.40 or more, or may be 1.42 or more. The technology disclosed herein can be preferably implemented, for example, in an embodiment in which the refractive index _n2 of the low refractive index adhesive is 1.35 or more and 1.46 or less (more preferably 1.36 or more and 1.45 or less).

In this specification, the refractive index of the adhesive refers to the refractive index of the surface (adhesive surface) of the adhesive. The refractive index of the 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. As the Abbe refractometer, for example, the model "DR-M4" manufactured by ATAGO or an equivalent product can be used. As the measurement sample, an adhesive layer made of the adhesive to be evaluated can be used. Specifically, the refractive index of the adhesive can be measured by the method described in the Examples below. The refractive index of the adhesive can be adjusted, for example, by the composition of the adhesive (for example, the composition of the monomer components constituting the base polymer, additives that can be used as necessary, etc.).

The storage modulus G' _V2 (25) of the low refractive index adhesive disclosed herein is suitably 400 kPa or less from the viewpoints of ease of application to an adherend, adhesion, surface shape conformability, deformation conformability, etc. In some embodiments, the storage modulus G' _V2 (25) is preferably 300 kPa or less, more preferably 200 kPa or less (e.g., 180 kPa or less, or 150 kPa or less), and may be 120 kPa or less, 90 kPa or less, or 70 kPa or less. A low storage modulus G' _V2 (25) can be advantageous in terms of enhancing the effects of imparting flexibility and improving deformation conformability, etc., of the low refractive index adhesive layer, for example, in a use embodiment in which the adhesive is laminated to a high refractive index layer. The lower limit of the storage modulus G' _V2 (25) is not particularly limited, and may be, for example, 1.0 kPa or more. In some embodiments, from the viewpoint of easily imparting appropriate cohesiveness to the low refractive index pressure-sensitive adhesive layer, the storage modulus G' _V2 (25) is, for example, suitably 5.0 kPa or more, preferably 10 kPa or more, may be 15 kPa or more, may be 25 kPa or more, may be 35 kPa or more, may be 60 kPa or more, or may be 80 kPa or more. From the viewpoint of easily realizing higher cohesive strength and adhesive properties, in some embodiments, the storage modulus G' _V2 (25) may be 95 kPa or more, may be 110 kPa or more, or may be 140 kPa or more.

In some preferred embodiments, the storage modulus G' at 25°C (i.e., storage modulus _G'V2 (25)) of the low refractive index pressure-sensitive adhesive layer is preferably lower than the storage modulus G' at 25°C of the high refractive index layer that may be disposed adjacent to the low refractive index pressure-sensitive adhesive layer (hereinafter, may be referred to as "storage modulus _G'V1 (25)"). That is, it is preferable that _G'V2 (25)<_G'V1 (25). According to this configuration, by laminating a low refractive index pressure-sensitive adhesive layer having a relatively low storage modulus to a high refractive index layer (e.g., a high refractive index pressure-sensitive adhesive layer) that tends to have a high storage modulus G' as a trade-off for increasing the refractive index, adhesion and flexibility are imparted, thereby improving conformability to steps and curved surfaces, and a laminated sheet (pressure-sensitive adhesive sheet) that can be preferably applied to various device design applications can be realized.

The low refractive index adhesive disclosed herein contains an acrylic polymer (F) that contains a fluorine-containing acrylic monomer (M1) as a monomer unit. The low refractive index adhesive is preferably an acrylic adhesive that contains such an acrylic polymer (F) as a base polymer.

In this specification, the "base polymer" of the adhesive refers to the main component of the rubber-like polymer contained in the adhesive, and is not to be construed in any other limiting sense. The rubber-like polymer refers to a polymer that exhibits rubber elasticity in a temperature range around room temperature. In this specification, the "main component" refers to a component that is contained in an amount of more than 50% by weight, unless otherwise specified.
In addition, 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 in one molecule. Hereinafter, a monomer having at least one (meth)acryloyl group in one molecule is also 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 a polymer in which the proportion of the acrylic monomer in the total monomers used in the synthesis of the acrylic polymer is more than 50% by weight (preferably more than 70% by weight, for example more than 90% by weight).
In this specification, "(meth)acryloyl" refers collectively to acryloyl and methacryloyl. Similarly, "(meth)acrylate" refers collectively to acrylate and methacrylate, and "(meth)acrylic" refers collectively to acrylic and methacrylic. Therefore, the concept of an acrylic monomer here can include both a monomer having an acryloyl group (acrylic monomer) and a monomer having a methacryloyl group (methacrylic monomer).

(Acrylic Polymer (F))
The acrylic polymer (F) in the technology disclosed herein contains a fluorine-containing acrylic monomer (M1) as a monomer component constituting the polymer. That is, the acrylic polymer (F) contains the fluorine-containing acrylic monomer (M1) as a monomer unit. By appropriately using the fluorine-containing acrylic monomer (M1), the refractive index of the acrylic polymer (F) can be reduced.

Here, in the present specification, the term "monomer component constituting an acrylic polymer" refers to a monomer constituting a repeating unit of an acrylic polymer in an adhesive formed from the adhesive composition, regardless of whether the monomer component is contained in the adhesive composition in the form of a preformed polymer (which may be an oligomer) or in the form of an unpolymerized monomer. That is, the monomer component constituting the acrylic polymer may be contained in the adhesive composition in the form of a polymer, an unpolymer, or a partially polymerized product. In view of the ease of preparation of the adhesive composition, etc., in some embodiments, an adhesive composition containing substantially all of the monomer component (for example, 95% by weight or more, preferably 99% by weight or more) in the form of a polymer is preferred. An adhesive composition containing substantially all of the monomer component in the form of a polymer is also preferred from the viewpoint of easily forming an adhesive sheet with little distortion or warping.

The acrylic polymer (F) may be a polymer of a monomer raw material that contains at least a fluorine-containing acrylic monomer (M1) (hereinafter sometimes abbreviated as "monomer (M1)") and may further contain another monomer (copolymerizable monomer) that is copolymerizable with the monomer (M1). The monomer (M1) is not particularly limited as long as it is an acrylic monomer having at least one fluorine atom in the molecule. For example, a fluorine-containing (meth)acrylic acid ester can be preferably used. A suitable example of a fluorine-containing (meth)acrylic acid ester is one having a fluorinated hydrocarbon group at the ester end. Examples of the fluorinated hydrocarbon group include a fluorinated aliphatic hydrocarbon group, a fluorinated alicyclic hydrocarbon group, and a fluorinated aromatic hydrocarbon group. A suitable example of the fluorinated hydrocarbon group is a fluorinated aliphatic hydrocarbon group. Examples of the fluorinated aliphatic hydrocarbon group include a fluorinated alkyl group. In the fluorinated aliphatic hydrocarbon group, the aliphatic hydrocarbon portion may be linear or branched. In addition, in the fluorinated aliphatic hydrocarbon group, the fluorine atom may be bonded to any carbon atom in the aliphatic hydrocarbon group. The number of fluorine atoms bonded to one carbon atom may be either singular or plural. There is no particular restriction on the number of carbon atoms to which fluorine atoms are bonded.

In the fluorinated aliphatic hydrocarbon group (especially the fluorinated alkyl group), the number of carbon atoms in the hydrocarbon group portion is not particularly limited. In some embodiments, taking into consideration compatibility with other copolymerizable monomers, a fluorinated aliphatic hydrocarbon group having, for example, about 1 to 18 carbon atoms (preferably 1 to 12) is preferred. Specific examples of the fluorinated aliphatic hydrocarbon group include fluorinated methyl groups such as trifluoromethyl groups, difluoromethyl groups, and monofluoromethyl groups; fluorinated ethyl groups such as pentafluoroethyl groups, 1,1,2,2-tetrafluoroethyl groups, 1,2,2,2-tetrafluoroethyl groups, 1,1,2-trifluoroethyl groups, 1,2,2-trifluoroethyl groups, 2,2,2-trifluoroethyl groups, 1,1-difluoroethyl groups, 1,2-difluoroethyl groups, 2,2-difluoroethyl groups, 1-monofluoroethyl groups, and 2-monofluoroethyl groups; and the like. Examples of fluorinated alkyl groups having 3 or more carbon atoms include various fluorinated alkyl groups in which one or more fluorine atoms are bonded to one or more carbon atoms in the alkyl group portion, similar to the above-mentioned fluorinated methyl and ethyl groups.

Fluorinated alicyclic hydrocarbon groups include fluorinated cycloalkyl groups. As with the above fluorinated aliphatic hydrocarbon groups, in fluorinated alicyclic hydrocarbon groups, the fluorine atom may be bonded to any carbon atom of the alicyclic hydrocarbon group, and the number of fluorine atoms bonded to one carbon atom may be either single or multiple. Furthermore, the number of carbon atoms to which fluorine atoms are bonded is not particularly limited. Fluorinated alicyclic hydrocarbon groups include, for example, cyclohexyl groups having one fluorine atom, such as 2-fluorocyclohexyl, 3-fluorocyclohexyl, and 4-fluorocyclohexyl groups; cyclohexyl groups having two fluorine atoms, such as 2,4-difluorocyclohexyl and 2,6-difluorocyclohexyl groups; and cyclohexyl groups having three fluorine atoms, such as 2,4,6-trifluorocyclohexyl groups.

The fluorinated hydrocarbon group may or may not have a substituent. Such a substituent is not particularly limited, and examples thereof include hydrocarbon groups such as alkyl groups, alkoxy groups, hydroxy groups, carboxy groups, amino groups, nitro groups, cyano groups, and halogen atoms. The substituents may be used alone or in combination of two or more.

Fluorine atom-containing (meth)acrylic acid esters [fluorinated (meth)acrylates] include, for example, fluorine atom-containing (meth)acrylic acid alkyl esters [fluorinated alkyl (meth)acrylates], fluorine atom-containing (meth)acrylic acid cycloalkyl esters [fluorinated cycloalkyl (meth)acrylates], and fluorine atom-containing (meth)acrylic acid aryl esters [fluorinated aryl (meth)acrylates].

As the fluorine atom-containing (meth)acrylic acid ester, fluorinated alkyl (meth)acrylate (particularly fluorinated alkyl acrylate) is preferred. Examples of fluorinated alkyl (meth)acrylates include 2,2,2-trifluoroethyl acrylate (such as "Viscoat 3F" manufactured by Osaka Organic Chemical Industry Co., Ltd.), 2,2,3,3-tetrafluoropropyl acrylate (such as "Viscoat 4F" manufactured by Osaka Organic Chemical Industry Co., Ltd.), 1H,1H,5H-octafluoropentyl acrylate (such as "Viscoat 8F" manufactured by Osaka Organic Chemical Industry Co., Ltd.), 1H,1H,5H-octafluoropentyl methacrylate (such as "Viscoat 8FM" manufactured by Osaka Organic Chemical Industry Co., Ltd.), 2-(heptadecafluorononyl)ethyl acrylate (such as "FA-108" manufactured by Kyoeisha Chemical Co., Ltd.), and 1H,1H,2H,2H-tridecafluorooctyl acrylate (such as "Viscoat 13F" manufactured by Osaka Organic Chemical Industry Co., Ltd.).

The number of carbon atoms of the fluorinated alkyl group in the fluorinated alkyl (meth)acrylate is advantageously 3 or more from the viewpoint of the effect of reducing the refractive index and flexibility, preferably 4 or more, more preferably 5 or more, even more preferably 6 or more or 7 or more, and particularly preferably 8 or more. The number of carbon atoms of the fluorinated alkyl group is advantageously 18 or less from the viewpoint of adhesive performance, preferably 14 or less, more preferably 12 or less, and may be 10 or less or 9 or less. In some embodiments, the number of carbon atoms of the fluorinated alkyl group may be 7 or less or may be 5 or less. In some embodiments, the fluorine atom-containing (meth)acrylic acid ester is preferably a fluorinated alkyl (meth)acrylate in which fluorine is not bonded to the carbon at the 1st position of the alkyl group, and for example, a fluorinated alkyl (meth)acrylate in which fluorine is not bonded to either the carbon at the 1st position or the carbon at the 2nd position of the alkyl group, such as 1H,1H,2H,2H-tridecafluorooctyl acrylate, may be preferably used.

The content of the monomer (M1) in the monomer raw material for preparing the acrylic polymer (F) may be, for example, 10% by weight or more, suitably 25% by weight or more, or may be 35% by weight or more. From the viewpoint of making it easier to realize a pressure-sensitive adhesive with a lower refractive index, the content of the monomer (M1) is preferably 40% by weight or more, more preferably 45% by weight or more, even more preferably 55% by weight or more, may be 60% by weight or more, may be 75% by weight or more, may be 85% by weight or more, may be 90% by weight or more, or may be 95% by weight or more. The upper limit of the content of the monomer (M1) in the monomer raw material is not particularly limited, and may be 100% by weight. In some embodiments, from the viewpoint of the cohesiveness of the pressure-sensitive adhesive, the content of the monomer (M1) is suitably 99.9% by weight or less, preferably 99.5% by weight or less, may be 99% by weight or less, may be 97% by weight or less, or may be 92% by weight or less. The monomer (M1) can be used alone or in combination of two or more types.

The monomer raw material for preparing the acrylic polymer (F) may be a composition containing a copolymerizable monomer in addition to the monomer (M1). The copolymerizable monomer may be useful for introducing crosslinking points into the acrylic polymer or for increasing the cohesive strength of the acrylic polymer. As the copolymerizable monomer, for example, one or more of functional group-containing monomers such as carboxyl group-containing monomers, hydroxyl group-containing monomers, acid anhydride group-containing monomers, amide group-containing monomers, amino group-containing monomers, monomers having a nitrogen atom-containing ring (for example, N-vinyl cyclic amides such as N-vinyl-2-pyrrolidone), sulfonic acid group-containing monomers, and phosphoric acid group-containing monomers can be used. Other examples of the copolymerizable monomer include non-aromatic ring-containing (meth)acrylates such as alkyl (meth)acrylates, cycloalkyl (meth)acrylates, and isobornyl (meth)acrylates, vinyl ester monomers such as vinyl acetate, aromatic vinyl compounds such as styrene, and alkoxy group-containing monomers. Specific examples of copolymerizable monomers include, but are not limited to, the same monomers that can be used in the preparation of the acrylic polymer (A) described below.

In some embodiments, from the viewpoint of improving cohesive strength, etc., an acrylic polymer (F) using one or more monomers selected from the group consisting of hydroxyl group-containing monomers and carboxyl group-containing monomers (hereinafter also referred to as "monomer (M2)") as the copolymerizable monomer is preferred. Examples of the hydroxyl group-containing monomer include the same hydroxyl group-containing monomers exemplified for monomer (m2) described later. Suitable examples of the hydroxyl group-containing monomer include 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate. From the viewpoint of flexibility, 4-hydroxybutyl acrylate is more preferred. Examples of the carboxyl group-containing monomer include the same carboxyl group-containing monomers exemplified for monomer (m2) described later. Suitable examples of the carboxyl group-containing monomer include acrylic acid and methacrylic acid.

The content of monomer (M2) in the monomer raw material for preparing the acrylic polymer (F) is not particularly limited and can be appropriately set so as to achieve the desired effect of use. The content of monomer (M2) can be, for example, 0.01% by weight or more (preferably 0.1% by weight or more, more preferably 0.5% by weight or more). In some embodiments, the content of the monomer (M2) may be 0.7% by weight or more of the monomer raw material, 0.9% by weight or more, or 1.5% by weight or more. The upper limit of the content of monomer (M2) is not particularly limited and can be appropriately set so that the total with the content of other monomers does not exceed 100% by weight. From the viewpoint of relatively increasing the content of monomer (M1) to facilitate a low refractive index, the content of monomer (M2) is, for example, appropriately set to 15% by weight or less or 10% by weight or less, preferably less than 10% by weight, more preferably less than 5% by weight, and may be less than 3% by weight, less than 2.5% by weight, or less than 1.5% by weight. Monomer (M2) does not need to be used.

In some preferred embodiments, the monomer raw material for preparing the acrylic polymer (F) may be a composition containing a hydroxyl group-containing monomer as the monomer (M2). The hydroxyl group-containing monomer may be useful for improving the cohesive force and introducing crosslinking points. Suitable examples of the hydroxyl group-containing monomer include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl acrylate and 4-hydroxybutyl acrylate. From the viewpoint of improving flexibility in the room temperature range, 4-hydroxybutyl acrylate may be more preferably used. The content of the hydroxyl group-containing monomer in the monomer raw material is not particularly limited and may be, for example, 0.01% by weight or more (preferably 0.1% by weight or more, more preferably 0.5% by weight or more). In some embodiments, the content of the hydroxyl group-containing monomer may be 0.7% by weight or more, 0.9% by weight or more, or 1.5% by weight or more of the monomer raw material. The upper limit of the content of the hydroxyl group-containing monomer is not particularly limited and may be, for example, 15% by weight or less or 10% by weight or less. In some embodiments, from the viewpoint of achieving a low refractive index, the content of the hydroxyl group-containing monomer in the monomer raw material is suitably less than 10% by weight, preferably less than 5% by weight, may be less than 3% by weight, may be less than 2.5% by weight, or may be less than 1.5% by weight.

In some embodiments, the monomer raw material for preparing the acrylic polymer (F) may contain a copolymerizable monomer other than the monomer (M2) (i.e., a copolymerizable monomer that does not have a hydroxyl group- and carboxyl group-containing monomer). Hereinafter, such a copolymerizable monomer may be referred to as "monomer (M3)." As the monomer (M3), any of the various copolymerizable monomers described above that does not fall under the category of monomer (M2) may be used.

The content of monomer (M3) in the monomer raw material for preparing the acrylic polymer (F) is not particularly limited and can be appropriately set so that the desired use effect is exhibited. The content of monomer (M3) can be, for example, 0.1% by weight or more (preferably 1% by weight or more, more preferably 5% by weight or more). In some embodiments, the content of the monomer (M2) may be 7% by weight or more of the monomer raw material, 10% by weight or more, or 15% by weight or more. From the viewpoint of improving flexibility, etc., in some embodiments, the content of the monomer (M2) may be 10% by weight or more, 25% by weight or more, 35% by weight or more, 45% by weight or more, more than 55% by weight, or more than 60% by weight. The upper limit of the content of monomer (M3) is not particularly limited and can be appropriately set so that the total with the content of other monomers does not exceed 100% by weight. The content of monomer (M3) may be, for example, 90% by weight or less, or 85% by weight or less. From the viewpoint of relatively increasing the content of monomer (M1) to facilitate a low refractive index, the content of monomer (M3) is preferably 80% by weight or less, and more preferably 70% by weight or less. In some embodiments, from the viewpoint of a low refractive index, the content of monomer (M3) in the monomer raw material is appropriately less than 60% by weight, preferably less than 50% by weight, may be less than 45% by weight, may be less than 35% by weight, may be less than 20% by weight, may be less than 10% by weight, or may be less than 5% by weight. Monomer (M3) may not be used.

Suitable examples of monomers that can be used as monomer (M3) include alkyl (meth)acrylates, monomers having a nitrogen atom-containing ring (for example, N-vinyl cyclic amides such as N-vinyl-2-pyrrolidone), non-aromatic ring-containing (meth)acrylates such as cycloalkyl (meth)acrylates and isobornyl (meth)acrylates, etc. Monomer (M3) may be used alone or in combination of two or more. For example, preferred embodiments include using one type of alkyl (meth)acrylate alone, using two or more types of alkyl (meth)acrylates in combination, using an alkyl (meth)acrylate and an N-vinyl cyclic amide in combination, and using an alkyl (meth)acrylate and a non-aromatic ring-containing (meth)acrylate in combination.

As the alkyl (meth)acrylate, for example, a compound represented by the following formula (1) can be preferably used.
CH ₂ =C(R ¹ )COOR ² (1)
Here, R ¹ in the above formula (1) is a hydrogen atom or a methyl group. R ² is a chain alkyl group having 1 to 20 carbon atoms (hereinafter, such a range of carbon atoms may be referred to as "C _1-20 "). From the viewpoint of the storage modulus of the adhesive, etc., alkyl (meth)acrylates in which R ² is a chain alkyl group having _1-12 (e.g., C _2-10 , typically C _4-8 ) are preferred. The alkyl (meth)acrylates in which R ² is a chain alkyl group having _1-20 may be used alone or in combination of two or more. Preferred alkyl (meth)acrylates include n-butyl acrylate and 2-ethylhexyl acrylate.

In some embodiments, the monomer raw material for preparing the acrylic polymer (F) preferably has a limited content of a carboxyl group-containing monomer from the viewpoint of suppressing coloring or discoloration (e.g., yellowing) of the adhesive. The content of the carboxyl group-containing monomer in the monomer raw material may be, for example, less than 1 wt%, preferably less than 0.5 wt%, more preferably less than 0.3 wt%, and even more preferably less than 0.1 wt% (e.g., less than 0.05 wt%). The fact that the content of the carboxyl group-containing monomer is limited in this manner is also advantageous from the viewpoint of suppressing corrosion of metal materials (e.g., metal wiring or metal films that may be present on an adherend) that may be placed in contact with or in close proximity to the adhesive disclosed herein. The technology disclosed herein may be preferably implemented in an embodiment in which the monomer raw material does not contain a carboxyl group-containing monomer.
For the same reason, in some embodiments, the monomer raw material for preparing the acrylic polymer (F) preferably has a limited content of monomers having acidic functional groups (including carboxy groups, sulfonic acid groups, phosphoric acid groups, etc.). The content of the acidic functional group-containing monomer in the monomer raw material in such an embodiment can be the above-mentioned preferred content of the carboxy group-containing monomer. The technology disclosed herein can be preferably implemented in an embodiment in which the monomer raw material does not contain an acidic group-containing monomer (i.e., an embodiment in which the acrylic polymer (F) is acid-free).

The acrylic polymer (F) can be prepared by appropriately adopting a known polymerization method, similar to the acrylic polymer (A) described later. The weight average molecular weight (Mw) of the acrylic polymer (F) is not particularly limited, and may be, for example, in the range of about 10×10 ⁴ to 500×10 ⁴ , or may be in the range of about 20×10 ⁴ to 200×10 ^4. In some embodiments, from the viewpoint of adhesion to the adherend or an adjacent layer (which may be a high refractive index layer such as a high refractive index adhesive layer), the Mw of the acrylic polymer (F) is suitably 150×10 ⁴ or less, preferably 120×10 ⁴ or less (for example, 95×10 ⁴ or less), and may be 75×10 ⁴ or less, 68×10 ⁴ or less, or 60×10 ⁴ or less. In some embodiments, from the viewpoint of the cohesiveness of the PSA, the Mw of the acrylic polymer (F) may be, for example, 30 × 10 ⁴ or more, 40 × 10 ⁴ or more, or 50 × 10 ⁴ or more. In order to adjust the Mw, a conventionally known chain transfer agent can be used as necessary.

From the viewpoint of adhesiveness, it is advantageous, but not limited to, that the Tg of the acrylic polymer (F) is approximately 0°C or lower, and preferably approximately -5°C or lower (e.g., approximately -15°C or lower, or -25°C or lower). From the viewpoint of the cohesive strength of the adhesive layer, the Tg of the acrylic polymer (F) is approximately -75°C or higher, and preferably approximately -70°C or higher (e.g., -50°C or higher, or even -30°C or higher). The Tg of the acrylic polymer (F) can be adjusted by appropriately changing the monomer composition (i.e., the types and amount ratios of monomers used in the synthesis of the polymer).

When an alkyl (meth)acrylate is used as the monomer (M3), in order to facilitate lowering the Tg of the acrylic polymer (F), in some embodiments, the Tg calculated by Fox's formula based on the amount of only the alkyl (meth)acrylate used is advantageously less than -50°C, preferably less than -55°C, and may be less than -60°C or less than -65°C.

(Method for preparing acrylic polymer (F))
In the technology disclosed herein, the method for obtaining the acrylic polymer (F) composed of the above-mentioned monomer components is not particularly limited, and known polymerization methods such as solution polymerization, emulsion polymerization, bulk polymerization, suspension polymerization, and photopolymerization can be appropriately adopted. In some embodiments, the solution polymerization method can be preferably adopted. The polymerization temperature when carrying out the solution polymerization can be appropriately selected depending on the type of monomer and solvent 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 the solution polymerization can be appropriately selected from conventionally known organic solvents. For example, any one of the following solvents or a mixture of two or more solvents can be used: aromatic compounds such as toluene (typically aromatic hydrocarbons); acetate esters such as ethyl acetate; aliphatic or alicyclic hydrocarbons such as hexane and cyclohexane; halogenated alkanes such as 1,2-dichloroethane; lower alcohols such as isopropyl alcohol (for example, monohydric alcohols having 1 to 4 carbon atoms); ethers such as tert-butyl methyl ether; ketones such as methyl ethyl ketone; etc.

The initiator used for 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; aromatic carbonyl compounds; and the like. Further examples of polymerization initiators include redox-based initiators formed by combining peroxides with reducing agents. The polymerization initiators can be used alone or in combination of two or more. The amount of polymerization initiator used may be a normal amount, and can be selected from the range of, for example, about 0.005 to 1 part by weight (typically about 0.01 to 1 part by weight) per 100 parts by weight of the monomer component.

In the above polymerization, various conventionally known chain transfer agents can be used as 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 a sulfur atom (non-sulfur chain transfer agent) may 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; styrenes such as α-methylstyrene and α-methylstyrene dimer; and the like. The 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, about 0.01 to 1 part by weight per 100 parts by weight of the monomer raw material.

The weight average molecular weight (Mw) of the base polymer is not particularly limited and may be, for example, in the range of about 10×10 ⁴ to 500×10 ^4. From the viewpoint of adhesive performance, the Mw of the base polymer is preferably in the range of about 20×10 ⁴ to 400×10 ⁴ (more preferably about 30×10 ⁴ to 150×10 ⁴ , for example, about 50×10 ⁴ to 130×10 ⁴ ).

Here, the Mw of the 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% by weight (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: 1 "TSKguard column Super HZ-H" + 2 "TSKgel Super HZM-H" (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 adhesive composition used to form the low refractive index adhesive layer may contain a crosslinking agent as necessary for the purpose of adjusting the cohesive strength of the adhesive. As the crosslinking agent, a crosslinking agent known in the field of adhesives, such as an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, an aziridine-based crosslinking agent, an oxazoline-based crosslinking agent, a melamine-based resin, or a metal chelate-based crosslinking agent, may be used. Among them, an isocyanate-based crosslinking agent may be preferably used. Other examples of the crosslinking agent include a monomer having two or more ethylenically unsaturated groups in one molecule, i.e., a polyfunctional monomer. The crosslinking agent may be used alone or in combination of two or more. As for specific examples and amounts of the crosslinking agent, the description of the crosslinking agent that may be used in the adhesive composition for forming the high refractive index adhesive layer described later may also be applied to the adhesive composition for forming the low refractive index adhesive layer.

In an embodiment in which the adhesive composition used to form the low refractive index adhesive layer contains a crosslinking agent, for example, an isocyanate-based crosslinking agent may be preferably used as the crosslinking agent. In some embodiments, the amount of the isocyanate-based crosslinking agent used relative to 100 parts by weight of the acrylic polymer (F) may be, for example, less than 0.5 parts by weight, less than 0.3 parts by weight, less than 0.2 parts by weight, or less than 0.15 parts by weight, from the viewpoint of adhesion to the adherend or adjacent layers. In addition, in some embodiments, from the viewpoint of appropriately exerting the effect of using the crosslinking agent, the amount of the isocyanate-based crosslinking agent used relative to 100 parts by weight of the acrylic polymer (F) 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, or 0.08 parts by weight or more.

(Other additives)
The low refractive index adhesive may contain optional components such as a plasticizing material, a leveling agent, a tackifier, inorganic particles, other additives, etc. These optional components may be appropriately selected from those that may be used in the high refractive index adhesive layer, and may be used in an appropriate amount.

<High refractive index adhesive>
The adhesive (low refractive index adhesive) disclosed herein may be used in a form laminated with an adhesive (high refractive index adhesive) having a relatively higher refractive index in relation to the adhesive. In such a use form, the ratio (n ₁ /n ₂ ) of the refractive index n ₁ of the high refractive index adhesive to the refractive index n ₂ of the low refractive index adhesive layer may be, for example, greater than 1.00, may be approximately 1.01 or more, may be approximately 1.02 or more, or may be approximately 1.03 or more. In some embodiments, the ratio (n ₁ /n ₂ ) is advantageously approximately 1.05 or more, preferably approximately 1.07 or more, more preferably approximately 1.10 or more, and may be approximately 1.11 or more. The upper limit of the ratio (n ₁ /n ₂ ) is not particularly limited. In some embodiments, from the standpoint of the adhesive properties of the adhesive surface formed by the high refractive index adhesive and/or the adhesive surface formed by the low refractive index adhesive, the transparency of each adhesive layer, and ultimately the transparency of the laminated sheet containing the high refractive index adhesive and the low refractive index adhesive, the ratio (n ₁ /n ₂ ) may be, for example, approximately 1.20 or less, approximately 1.18 or less, approximately 1.16 or less, approximately 1.14 or less, or approximately 1.12 or less.

In some embodiments, the difference between the refractive index n ₁ of the high refractive index adhesive and the refractive index n ₂ of the low refractive index adhesive, i.e., the refractive index difference (n ₁ -n ₂ ), may be, for example, greater than 0.00, may be 0.01 or more, and is preferably 0.02 or more, may be 0.03 or more, may be 0.05 or more, may be 0.10 or more, may be 0.15 or more, may be 0.20 or more, or may be 0.25 or more. The upper limit of the refractive index difference (n ₁ -n ₂ ) is not particularly limited. In some embodiments, from the viewpoint of the adhesive properties of the adhesive surface formed by the high refractive index adhesive and/or the adhesive surface formed by the low refractive index adhesive, the transparency of each adhesive layer, and ultimately the transparency of the laminate sheet containing the high refractive index adhesive and the low refractive index adhesive, the refractive index difference (n ₁ -n ₂ ) may be, for example, 0.30 or less, 0.26 or less, 0.21 or less, 0.18 or less, or 0.16 or less.

The preferred lower limit of the refractive index of the high refractive index adhesive is not limited to a specific range because it may vary depending on the refractive index of the low refractive index adhesive. For example, the refractive index of the high refractive index adhesive may be 1.45 or more, 1.47 or more, 1.50 or more, 1.53 or more, 1.55 or more, or 1.56 or more. Using a high refractive index adhesive with a higher refractive index is preferable from the viewpoint of increasing the refractive index difference with the low refractive index adhesive, and is also advantageous from the viewpoint of increasing the design freedom of the low refractive index adhesive.

In some embodiments, the refractive index of the high refractive index adhesive may be, for example, 1.570 or more, advantageously greater than 1.570, preferably greater than 1.580, more preferably greater than 1.585, and even more preferably greater than 1.590 (e.g., greater than 1.595). With a high refractive index adhesive layer having such a refractive index, the behavior of light passing through the low refractive index adhesive layer and/or the high refractive index adhesive layer can be effectively controlled by utilizing the relative refractive index relationship between the high refractive index adhesive layer and the low refractive index adhesive layer as described above. In some embodiments of the technology disclosed herein, the refractive index of the high refractive index adhesive may be, for example, greater than or equal to 1.600, greater than or equal to 1.605, or greater than or equal to 1.610. The preferred upper limit of the refractive index of the high refractive index adhesive is not limited to a specific range because it may vary depending on the refractive index of the adjacent layers, etc. In some embodiments, taking into consideration the balance between adhesive properties and transparency, the refractive index of the high refractive index adhesive may be, for example, 1.700 or less, 1.670 or less, or 1.650 or less.

(Storage Modulus G')
In the technology disclosed herein, the storage modulus G' of the high refractive index adhesive at 25°C (hereinafter also referred to as "storage modulus G' _V1 (25)") is appropriately set according to the purpose of use, the manner of use, etc., and is not limited to a specific range. The storage modulus G' _V1 (25) may be, for example, approximately 700 kPa or less. In some embodiments, from the viewpoint of ease of application to an adherend, etc., the storage modulus G' _V1 (25) is advantageously approximately 600 kPa or less, preferably 500 kPa or less, and more preferably 400 kPa or less (e.g., 350 kPa or less). In some embodiments, from the viewpoint of increasing the flexibility of the high refractive index adhesive in the room temperature range (e.g., 25°C) and making it easier to adhere to an adherend, the storage modulus G' _V1 (25) is advantageously approximately 330 kPa or less, and preferably 300 kPa or less. In some embodiments where application property and flexibility in the room temperature range are more important, the storage modulus G' _V1 (25) may be, for example, less than 270 kPa or less than 250 kPa, advantageously less than 200 kPa, preferably less than 180 kPa, and more preferably less than 160 kPa (e.g., less than 140 kPa). In some embodiments, the storage modulus G' _V1 (25) may be less than 100 kPa or less than 90 kPa. There is no particular lower limit for the storage modulus G' _V1 (25), but from the viewpoint of processability, handling, and the like, it may be, for example, 30 kPa or more, 50 kPa or more, or 70 kPa or more. In some embodiments, taking into consideration a high refractive index, the storage modulus G′ _V1 (25) may be 100 kPa or more, 150 kPa or more, 200 kPa or more, 250 kPa or more, or 300 kPa or more.

In the technology disclosed herein, the storage modulus G' of the high refractive index adhesive at 50°C (hereinafter also referred to as "storage modulus G' _V1 (50)") is not particularly limited, and may be, for example, less than 100 kPa. In some embodiments, the storage modulus G' _V1 (50) is suitably less than 60 kPa, preferably less than 40 kPa, and more preferably less than 38 kPa (e.g., less than 36 kPa). The high refractive index adhesive having such a limited storage modulus G' _V1 (50) can easily increase the adhesion to the adherend by moderately heating as necessary, thereby improving the adhesion to the adherend. The lower limit of the storage modulus G' _V1 (50) is not particularly limited. In some embodiments, from the viewpoint of heat resistance and the like, the storage modulus G' _V1 (50) may be, for example, 10 kPa or more, 15 kPa or more, 20 kPa or more, or 23 kPa or more.

In some embodiments of the technology disclosed herein, the high refractive index adhesive layer satisfies the following conditions:
(a) a storage modulus _G'V1 (25) of 350 kPa or less (preferably less than 200 kPa, e.g., less than 180 kPa); and (b) a storage modulus _G'V1 (50) of less than 60 kPa (preferably less than 50 kPa, more preferably less than 40 kPa, e.g., less than 38 kPa);
It is preferable to satisfy at least one of the above. A high refractive index adhesive layer that satisfies at least the above condition (a) is preferable from the viewpoint of adhesion and flexibility to an adherend in the room temperature range (e.g., 25°C). A high refractive index adhesive layer that satisfies at least the above condition (b) is preferable because it can easily improve adhesion (adhesiveness) to an adherend by heating to a temperature slightly higher than room temperature. An adhesive sheet having a high refractive index adhesive layer that does not satisfy the above condition (a) and satisfies the above condition (b) has good reworkability (repositionability) at the beginning of attachment in the room temperature range, and can be used as a heat-activated adhesive sheet that can effectively increase the peel strength from an adherend by heating to a temperature slightly higher than room temperature. The heat activation may be performed by heating the adhesive sheet to a temperature slightly higher than room temperature when attaching it to an 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).

In some embodiments of the technology disclosed herein, the ratio of the storage modulus G' _V1 (50) [kPa] to the storage modulus G' _V1 (25) [kPa], i.e., the storage modulus ratio G' _V1 (50)/G' _V1 (25) is, for example, 70% or less, and may be 40% or less, 30% or less, or 20% or less. A pressure-sensitive adhesive sheet having a high refractive index pressure-sensitive adhesive layer with a small G' _V1 (50)/G' _V1 (25) is suitable for use as the above-mentioned heat-activated pressure-sensitive adhesive sheet. The lower limit of G' _V1 (50)/G' _V1 (25) is not particularly limited. G' _V1 (50)/G' _V1 (25) is, for example, 5% or more, and from the viewpoint of heat resistance, is preferably 10% or more, and may be 12% or more, or may be 15% or more.

The storage moduli _G'V1 (25) and _G'V1 (50) can be determined by dynamic viscoelasticity measurement, and _G'V1 (50)/ _G'V1 (25) can be calculated from the results. Dynamic viscoelasticity measurement can be performed by a conventional method using a commercially available dynamic viscoelasticity measuring device, for example, ARES manufactured by TA Instruments or its equivalent, under the following measurement conditions. As a sample for measurement, a pressure-sensitive adhesive layer to be evaluated is prepared to a thickness of about 1.5 mm by laminating it as necessary, and then used.
[Measurement conditions]
Deformation mode: Torsion Measurement frequency: 1 Hz
Heating rate: 5°C/min. Shape: Parallel plate 7.9mmφ

The storage modulus _G'V1 (25), _G'V1 (50) and the storage modulus ratio ( _G'V1 (50)/ _G'V1 (25)) can be adjusted by selecting the composition of the monomer components constituting the base polymer of the pressure-sensitive adhesive (for example, selecting the type and content of monomer (m1) described below), whether or not a crosslinking agent is used, selecting the type and amount used, whether or not a refractive index enhancer or plasticizing material is used, selecting the type and amount used, etc. For example, by using, as monomer (m1), in addition to a first monomer which is the main component of monomer (m1), a relatively small amount of a second monomer having a chemical structure different from that of the first monomer in combination with the first monomer, _G'V1 (50) can be reduced and _G'V1 (50)/ _G'V1 (25) can be reduced in addition to the case of using the first monomer alone as monomer (m1).

The type of adhesive constituting the high refractive index adhesive layer is not particularly limited. The adhesive may contain one or more of various rubber-like polymers such as acrylic polymers, rubber polymers (natural rubber, synthetic rubber, mixtures thereof, etc.), polyester polymers, urethane polymers, polyether polymers, silicone polymers, polyamide polymers, and fluorine polymers as adhesive polymers (hereinafter also referred to as "base polymers" in the sense of structural polymers that form the adhesive) that can be used in the field of adhesives. From the viewpoint of adhesive performance, cost, etc., an adhesive containing an acrylic polymer or a rubber polymer as a base polymer can be preferably used. Among them, an adhesive having an acrylic polymer as a base polymer (acrylic adhesive) is preferable. The technology disclosed herein is preferably implemented in an embodiment using an acrylic adhesive.

The following mainly describes high refractive index adhesive layers made of acrylic adhesives, but it is not intended to limit the high refractive index adhesive layers in the technology disclosed herein to acrylic adhesive layers.

(Acrylic Polymer (A))
The high refractive index adhesive layer disclosed herein may be an acrylic adhesive layer. The acrylic polymer that is the base polymer of the acrylic adhesive layer preferably contains an aromatic ring-containing monomer (m1) as a monomer component constituting the acrylic polymer. That is, an acrylic polymer that contains an aromatic ring-containing monomer (m1) as a monomer unit is preferred. Hereinafter, such an acrylic polymer is also referred to as "acrylic polymer (A)".

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

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 viewpoint of flexibility and adhesiveness, an acryloyl group is more preferred. From the viewpoint of suppressing a decrease in the flexibility of the adhesive, a compound having one ethylenically unsaturated group in one 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 contained in monomer (m1) is not particularly limited, and may be, for example, 16 or less. In some embodiments, from the viewpoint of ease of preparation of acrylic polymer (A) and transparency of the adhesive, the number of aromatic rings may be, for example, 12 or less, preferably 8 or less, more preferably 6 or less, may be 5 or less, may be 4 or less, may be 3 or less, or may be 2 or less.

The aromatic ring of the compound used as the monomer (m1) may be, for example, a carbon ring such as a benzene ring (which may be a benzene ring constituting a part of a biphenyl structure or a fluorene structure); a condensed ring of a naphthalene ring, an indene ring, an azulene ring, an anthracene ring, or a phenanthrene ring; or may be, for example, 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 heteroatom contained as a ring-constituting atom in the heterocycle may be, for example, one or more selected from the group consisting of nitrogen, sulfur, and oxygen. In some embodiments, the heteroatom constituting the heterocycle may be one or both of nitrogen and sulfur. The monomer (m1) may have a structure in which one or more carbon rings and one or more heterocycles are condensed, such as a dinaphthothiophene structure.

The aromatic ring (preferably a carbon ring) may have one or more substituents on the ring-constituting atoms, or may have no substituents. When 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 substituent containing a carbon atom, the number of carbon atoms contained in the substituent is preferably 1 to 4, more preferably 1 to 3, and may be, 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 aromatic ring of the monomer (m1) having a substituent on its ring-constituting atoms means that the aromatic ring has 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, an alkylene group, an oxyalkylene group, a poly(oxyalkylene) group, a phenyl group, an alkylphenyl group, an alkoxyphenyl group, a group having a structure in which one or more hydrogen atoms in these groups are replaced with hydroxyl groups (e.g., a hydroxyalkylene group), an oxy group (-O- group), a thiooxy group (-S- group), or the like. In some embodiments, an aromatic ring-containing monomer having a structure in which the aromatic ring and the ethylenically unsaturated group are bonded directly or via a linking group selected from the group consisting of an alkylene group, an oxyalkylene group, and a poly(oxyalkylene) group may be 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, and may be, for example, 1 or 2. The number of repetitions of the oxyalkylene unit 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 may be used in combination.

The content of monomer (m1) in the monomer components constituting the acrylic polymer (A) is not particularly limited, and can be set so as to realize a pressure-sensitive adhesive layer that has both the desired refractive index and adhesive properties (e.g., peel strength, flexibility, etc.) and/or 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, 60% by weight or more, or 70% by weight or more. From the viewpoint of making it easier to obtain a higher refractive index, in some preferred embodiments, the content of monomer (m1) may be, for example, more than 70% by weight, 75% by weight or more, 80% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. The upper limit of the content of monomer (m1) in the monomer components is 100% by weight. From the viewpoint of achieving a good balance between a high refractive index and adhesive properties and/or optical properties, it is advantageous to set the content of the monomer (m1) to less than 100% by weight, and for example, it is preferably about 99% by weight or less, more preferably 98% by weight or less, and may be 97% by weight or less, or may be 96% by weight or less. In some embodiments, the content of the monomer (m1) may be 93% by weight or less, 90% by weight or less, 80% by weight or less, or 75% by weight or less. In some embodiments that place more importance on adhesive properties and/or optical properties, the content of the monomer (m1) in the monomer component may be 70% by weight or less, 60% by weight or less, or 45% by weight or less.

In some embodiments of the technology disclosed herein, as the monomer (m1), a monomer having two or more aromatic rings (preferably carbon rings) in one molecule can be preferably used because it is easy to obtain a high refractive index effect. Examples of monomers having two or more aromatic rings in one molecule (hereinafter also referred to as "monomers containing multiple aromatic rings") include monomers having a structure in which two or more non-condensed aromatic rings are bonded via a linking group, monomers having a structure in which two or more non-condensed aromatic rings are chemically bonded directly (i.e., without other atoms), monomers having a condensed aromatic ring structure, monomers having a fluorene structure, monomers having a dinaphthothiophene structure, monomers having a dibenzothiophene structure, etc. 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., an -(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, and the like. From the viewpoint of the flexibility of the adhesive, 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-condensed aromatic rings are bonded via a linking group include phenoxybenzyl (meth)acrylate (e.g., m-phenoxybenzyl (meth)acrylate), thiophenoxybenzyl (meth)acrylate, benzylbenzyl (meth)acrylate, and the like.

The monomer having a structure in which two or more non-condensed 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 condensed aromatic ring structure include naphthalene ring-containing (meth)acrylates, anthracene ring-containing (meth)acrylates, vinyl group-containing naphthalenes, vinyl group-containing anthracenes, etc. 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, 2-(4-methoxy-1-naphthoxy)ethyl (meth)acrylate, etc.

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

Examples of the monomer having the dinaphthothiophene structure include (meth)acryloyl group-containing dinaphthothiophene, vinyl group-containing dinaphthothiophene, (meth)allyl group-containing dinaphthothiophene, and the like. Specific examples include (meth)acryloyloxymethyldinaphthothiophene (for example, a compound having a structure in which CH ₂ CH(R ¹ )C(O)OCH ₂ -- is bonded to the 5th or 6th position of the dinaphthothiophene ring, where R ¹ is a hydrogen atom or a methyl group), (meth)acryloyloxyethyldinaphthothiophene (for example, a compound having a structure in which CH ₂ CH(R ¹ )C(O)OCH(CH ₃ )- or CH ₂ CH(R ¹ )C(O)OCH ₂ CH ₂ -- is bonded to the 5th or 6th position of the dinaphthothiophene ring, where R ¹ 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), (meth)allyloxydinaphthothiophene, and the like. 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. The monomer having the dibenzothiophene structure has a structure in which a thiophene ring and two benzene rings are condensed, and therefore is included in the concept of the monomer having the condensed aromatic ring structure.
It should be noted that neither the dinaphthothiophene structure nor the dibenzothiophene structure corresponds to a structure in which two or more non-condensed aromatic rings are directly chemically bonded.

As the monomer (m1) in the technology disclosed herein, a monomer having one aromatic ring (preferably a carbon ring) in one molecule may be used. A monomer having one aromatic ring in one molecule may be useful, for example, for improving the flexibility of the adhesive, adjusting the adhesive properties, improving the transparency, etc. In some embodiments, a monomer having one aromatic ring in one molecule is preferably used in combination with a monomer containing multiple aromatic rings from the viewpoint of improving the refractive index of the adhesive.

Examples of monomers having one aromatic ring in one molecule include carbon aromatic ring-containing (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 of such compounds 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 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 an ethylenically unsaturated group and an aromatic ring in the various aromatic ring-containing monomers as described above may be used. A monomer having an oxyethylene chain interposed between an ethylenically unsaturated group and an aromatic ring in this way can be understood as an ethoxylated product of the original monomer. The number of repetitions of the oxyethylene unit (-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 aromatic ring-containing monomer in the 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 realizing an adhesive having a higher refractive index, the content of the aromatic ring-containing monomer in the monomer (m1) may be, for example, 50% by weight or more, preferably 70% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. Substantially 100% by weight of the monomer (m1) may be a monomer containing multiple aromatic rings. That is, only one or more types of aromatic ring-containing monomers may be used as the monomer (m1). In addition, in some embodiments, for example, taking into consideration the balance between a high refractive index and adhesive properties and/or optical properties, the content of the aromatic ring-containing monomer in the 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. In some embodiments, taking into consideration the adhesive properties and/or optical properties, the content of the multiple aromatic ring-containing monomer in the monomer (m1) may be 70% by weight or less, 50% by weight or less, 25% by weight or less, or 10% by weight or less. The technology disclosed herein may also be implemented in an embodiment in which the content of the multiple aromatic ring-containing monomer in the monomer (m1) is less than 5% by weight. The multiple aromatic ring-containing monomer may not be used.

The content of the aromatic ring-containing monomer in the monomer component constituting the acrylic polymer is not particularly limited, and can be set so as to realize a pressure-sensitive adhesive layer that has both the desired refractive index and adhesive properties (e.g., peel strength, flexibility, etc.) and/or optical properties (e.g., total light transmittance, haze value, etc.). The content of the aromatic ring-containing monomer in the monomer component 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 making it easier to realize a pressure-sensitive adhesive having a higher refractive index, the content of the aromatic ring-containing monomer in the monomer component may be, for example, more than 35% by weight, preferably more than 50% by weight, more than 70% by weight, 75% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. The content of the aromatic ring-containing monomer in the monomer component may be 100% by weight, but from the viewpoint of achieving a good balance between a high refractive index and adhesive properties and/or optical properties, it is advantageous to make it less than 100% by weight, and it is preferably about 99% by weight or less, more preferably 98% by weight or less, and may be 96% by weight or less, 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, taking into consideration the adhesive properties and/or optical properties, the content of the aromatic ring-containing monomer in the monomer component may be 70% by weight or less, 50% 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 an embodiment in which the content of the aromatic ring-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 employed as at least a part of the monomer (m1). Here, the term "high refractive index monomer" refers to a monomer having a refractive index of, for example, about 1.510 or more, preferably about 1.530 or more, more preferably about 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 adhesive composition and ease of compatibility with flexibility suitable for an 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 types.
The refractive index of the monomer is measured using an Abbe refractometer at a wavelength of 589 nm and a temperature of 25° C. As the Abbe refractometer, a model "DR-M4" manufactured by ATAGO or an equivalent product can be used. When the manufacturer provides a nominal value of the refractive index at 25° C., the nominal value can be used.

As the high refractive index monomer, compounds having the corresponding refractive index can be appropriately selected from among the compounds (e.g., the compounds and compound groups exemplified above) included in the concept of the aromatic ring-containing monomer (m1) disclosed herein. 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 (repeated number of 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), phenoxydiethylene glycol acrylate (refractive index: 1.510, homopolymer Tg: 2°C), and phenoxydiethylene glycol acrylate (refractive index: 1.520, homopolymer Tg: 2°C). -Tg: -35°C), 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), 5-vinyldinaphthothiophene (abbreviation: 5VDNT, refractive index: 1.793), etc., but are not limited to these.

The content of the high refractive index monomer in the monomer (m1) (i.e., an aromatic ring-containing monomer having a refractive index of about 1.510 or more, preferably about 1.530 or more, more preferably about 1.550 or more) is not particularly limited, and may be, for example, 5% by weight or more, 25% by weight or more, 35% by weight or more, or 40% by weight or more. In some embodiments, from the viewpoint of making it easier to obtain a higher refractive index, the content of the high refractive index monomer in the monomer (m1) may be, for example, 50% by weight or more, preferably 70% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. Substantially 100% by weight of the monomer (m1) may be a high refractive index monomer. In some embodiments, for example, from the viewpoint of achieving a good balance between a high refractive index and adhesive properties and/or optical properties, the content of the high refractive index monomer in the 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. In some embodiments, taking into account adhesive properties and/or optical properties, the content of the high refractive index monomer in the monomer (m1) may be 70% by weight or less, 50% by weight or less, 25% by weight or less, 15% by weight or less, or 10% by weight or less. The technology disclosed herein may also be implemented in an embodiment in which the content of the high refractive index monomer in the monomer component (m1) is less than 5% by weight. The high refractive index monomer may not be used.

The content of the high refractive index monomer in the monomer component constituting the acrylic polymer is not particularly limited, and can be set so as to realize a pressure-sensitive adhesive layer that has both the desired refractive index and adhesive properties (e.g., peel strength, flexibility, etc.) and/or optical properties (e.g., total light transmittance, haze value, etc.). The content of the high refractive index monomer in the monomer component 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 making it easier to realize a pressure-sensitive adhesive having a higher refractive index, the content of the high refractive index monomer in the monomer component may be, for example, more than 35% by weight, preferably more than 50% by weight, more than 70% by weight, 75% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. The content of the high refractive index monomer in the monomer component may be 100% by weight, but from the viewpoint of achieving a good balance between the high refractive index and the adhesive properties and/or optical properties, it is advantageous to make it less than 100% by weight, preferably 99% by weight or less, more preferably 98% by weight or less, and may be 96% by weight or less, 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, taking into account the adhesive properties and/or optical properties, the content of the high refractive index monomer in the monomer component may be 70% by weight or less, 50% 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 an embodiment in which the content of the high refractive index monomer in the monomer component is less than 3% by weight.

In some preferred embodiments of the technology disclosed herein, an aromatic ring-containing monomer (hereinafter, sometimes referred to as "monomer L") having a homopolymer Tg of 10°C or less (preferably 5°C or less or 0°C or less, more preferably -10°C or less, even more preferably -20°C or less, for example -25°C or less) is used as at least a part of the monomer (m1). When the content of the aromatic ring-containing monomer (m1) (particularly the aromatic ring-containing monomer (m1) corresponding to one or both of the above-mentioned multiple aromatic ring-containing monomer and high refractive index monomer) in the monomer component is increased, the storage modulus G' of the adhesive generally tends to increase, but by adopting monomer L as a part or all of the monomer (m1), the increase in the storage modulus G' can be suppressed. This makes it possible to improve the refractive index while better maintaining the flexibility suitable for an adhesive. There is no particular limit on the lower limit of the Tg of monomer L. Considering the balance with the refractive index improving effect, in some embodiments, the Tg of the monomer L may be, for example, -70°C or higher, -55°C or higher, or -45°C or higher. The monomer L may be used alone or in combination of two or more types.

As the monomer L, a compound having the corresponding Tg can be appropriately selected from among the compounds included in the concept of the aromatic ring-containing monomer (m1) disclosed herein (for example, the compounds and compound groups exemplified above). One suitable example of an aromatic ring-containing monomer that can be used as the monomer L is m-phenoxybenzyl acrylate (Tg of homopolymer: -35°C). Another suitable example is phenoxydiethylene glycol acrylate (Tg of homopolymer: -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 making it easier to obtain an adhesive that has a high refractive index and flexibility at a higher level, the content of monomer L in monomer (m1) may be, for example, 50% by weight or more, 60% by weight or more, 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. Substantially 100% by weight of monomer (A1) may be monomer L. In some embodiments, for example, from the viewpoint of achieving a good balance between flexibility suitable for a pressure-sensitive adhesive and a high refractive index, 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, 70% by weight or less, 50% by weight or less, 25% by weight or less, or 10% by weight or less. The technology disclosed herein may also be implemented in an embodiment in which the content of monomer L in monomer (m1) is less than 5% by weight. Monomer L may not be used.

The content of monomer L in the monomer component 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 an adhesive that has both a high refractive index and flexibility at a higher level, the content of monomer L in the monomer component may be, for example, more than 35% by weight, preferably more than 50% by weight, more than 70% by weight, 75% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more. The content of monomer L in the above monomer component may be 100% by weight, but in consideration of the balance between the high refractive index and the adhesive properties and/or optical properties, it is advantageous to make it less than 100% by weight, preferably about 99% by weight or less, more preferably 98% by weight or less, may be 96% by weight or less, may be 95% by weight or less, may be 93% by weight or less, may be 90% by weight or less, may be 85% by weight or less, may be 80% by weight or less, or may be 75% by weight or less. In some embodiments, the content of monomer L in the monomer component may be 70% by weight or less, 50% 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 an embodiment in which the content of monomer L in the monomer component is less than 3% by weight.

In some embodiments, the glass transition temperature Tg _m1 based on the composition of the monomer (m1) is advantageously about 20° C. or less from the viewpoint of the flexibility of the adhesive, preferably 10° C. or less (e.g., 5° C. or less), more preferably 0° C. or less, even more preferably −10° C. or less, and may be −20° C. or less, or may be −25° C. or less. 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 more, −55° C. or more, or −45° C. or more. The technology disclosed herein may also be suitably implemented in an embodiment in which the glass transition temperature Tg _m1 is, for example, −40° C. or more, −35° C. or more, −33° C. or more, −30° C. or more, or −25° C. or more.

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

In some embodiments, as the aromatic ring-containing monomer (m1), a monomer L (i.e., an aromatic ring-containing monomer having a homopolymer Tg of 10° C. or less, preferably 5° C. or less or 0° C. or less, more preferably −10° C. or less, even more preferably −20° C. or less, for example, −25° C. or less) and a monomer H having a Tg higher than 10° C. can be used in combination. The Tg of the monomer H may be, for example, higher than 10° C., higher than 15° C., or higher than 20° C. By using the monomer L and the monomer H in combination, for example, in a configuration in which the content of the aromatic ring-containing monomer (m1) in the monomer component is relatively high, the adhesive can have a high refractive index and flexibility at a higher level. The ratio of the amount of the monomer L and the monomer H used can be set so that such an effect is preferably exhibited, and is not particularly limited. For example, it is preferable to set the ratio of the amount of the monomer L and the monomer H used so as to satisfy any one of the glass transition temperatures Tg _m1 described above.

In some embodiments, the aromatic ring-containing monomer (m1) may be preferably selected from compounds that do not contain a structure in which two or more non-condensed 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-condensed aromatic rings are directly chemically bonded is less than 5% by weight (more preferably less than 3% by weight, and may be 0% by weight). In this way, limiting the amount of the compound containing a structure in which two or more non-condensed aromatic rings are directly chemically bonded can be advantageous in terms of realizing an adhesive that has a good balance between flexibility, adhesion, and a high refractive index.

(Monomer (m2))
In some embodiments of the technology disclosed herein, 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 (carboxyl 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 containing at least one carboxyl group and at least one ethylenically unsaturated group in one molecule. The monomer (m2) can be useful for introducing a crosslinking point into the acrylic polymer or imparting appropriate cohesiveness to the adhesive. The monomer (m2) can be used alone or in combination of two or more kinds. The monomer (m2) is typically a monomer that does not contain an aromatic ring.

Examples of ethylenically unsaturated groups 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 viewpoint of flexibility and adhesiveness, an acryloyl group is more preferred. From the viewpoint of suppressing a decrease in the flexibility of the adhesive, a compound having one ethylenically unsaturated group in one 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. Examples of hydroxyl group-containing monomers that can be preferably used include 4-hydroxybutyl acrylate (Tg: -40°C) and 2-hydroxyethyl acrylate (Tg: -15°C). From the viewpoint of improving flexibility in the room temperature range, 4-hydroxybutyl acrylate with 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 the 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 the monomer (m2), the hydroxyl group-containing monomer may be one or more selected from compounds that do not have a methacryloyl group. Suitable examples of hydroxyl group-containing monomers that do not have a methacryloyl group include the various hydroxyalkyl acrylates described above. For example, it is preferable that more than 50% by weight, more than 70% by weight, or more than 85% by weight of the hydroxyl group-containing monomers used as the monomer (m2) are hydroxyalkyl acrylates. By using hydroxyalkyl acrylates, it is possible to introduce hydroxyl groups that are useful for providing crosslinking points and imparting appropriate cohesiveness into the acrylic polymer, and it is easier to obtain an adhesive that has good flexibility and adhesion in the room temperature range 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. Examples of carboxyl group-containing monomers that can be preferably used include acrylic acid and methacrylic acid. The carboxyl group-containing monomers may be used alone or in combination of two or more. A hydroxyl group-containing monomer and a carboxyl group-containing monomer may 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 the 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 obtaining a higher usage effect, in some embodiments, the content of the monomer (A2) is preferably 1% by weight or more, 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 with the content of other monomers does not exceed 100% by weight. In some embodiments, the content of the monomer (m2) is, for example, appropriately 30% by weight or less or 25% by weight or less, and from the viewpoint of relatively increasing the content of monomer (m1) to facilitate a high refractive index, it is preferably 20% by weight or less, more preferably 15% by weight or less, and may be less than 12% by weight, less than 10% by weight, or less than 7% by weight.

In an embodiment in which a hydroxyl-containing monomer is used as monomer (m2), the content of the hydroxyl-containing monomer in the monomer component is not particularly limited and may be, for example, 0.01% by weight or more (preferably 0.1% by weight or more, more preferably 0.5% by weight or more). In some embodiments, the content of the hydroxyl-containing monomer is preferably 1% by weight or more of the monomer component, may be 2% by weight or more, or may be 4% by weight or more. The upper limit of the content of the hydroxyl-containing monomer in the monomer component is set so that the total with the content of other monomers does not exceed 100% by weight, and is suitably, for example, 30% by weight or less or 25% by weight or less, and is preferably 20% by weight or less from the viewpoint of relatively increasing the content of monomer (m1) to facilitate a high refractive index, more preferably 15% by weight or less, and may be less than 12% by weight, less than 10% by weight, or less than 7% by weight.

In an embodiment in which a carboxyl group-containing monomer is used as the monomer (m2), the content of the carboxyl group-containing monomer in the monomer component is not particularly limited and may be, for example, 0.01% by weight or more (preferably 0.1% by weight or more, more preferably 0.3% by weight or more). In some embodiments, the content of the carboxyl group-containing monomer may be 1% by weight or more, 2% by weight or more, or 4% by weight or more. The upper limit of the content of the carboxyl group-containing monomer in the monomer component is set so that the total amount of the monomer used together with the amount of other monomers used does not exceed 100% by weight, and it is appropriate to set it to, for example, 30% by weight or less or 25% by weight or less. From the viewpoint of relatively increasing the content of the monomer (m1) to facilitate a high refractive index, it is preferably set to 20% by weight or less, more preferably 15% by weight or less, and may be less than 12% by weight, or may be less than 10% by weight. In some embodiments, from the viewpoint of improving the flexibility of the adhesive, the content of the carboxyl group-containing monomer is advantageously less than 7 wt%, preferably less than 5 wt%, may be less than 3 wt%, may be less than 1 wt%, or may be less than 0.5 wt%. The technology disclosed herein can be preferably implemented, for example, in an embodiment in which only a hydroxyl group-containing monomer is used as the monomer (m2), i.e., in an embodiment in which no carboxyl group-containing monomer is used.

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, 61% by weight or more, or 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, 86% by weight or more, 91% by weight or more, 96% by weight or more, 99% by weight or more, or substantially 100% by weight, from the viewpoint of making it easier to favorably exert the effects of these monomers.

(Monomer m3)
The monomer components constituting the acrylic polymer may contain monomers other than the above monomers (m1) and (m2) as necessary. An example of such an optional component is alkyl (meth)acrylate (hereinafter also referred to as "monomer (m3)"). The monomer (m3) can be useful for adjusting the flexibility of the adhesive and improving compatibility within the adhesive.

As the monomer (m3), an alkyl(meth)acrylate having a straight or branched alkyl group having 1 to 20 carbon atoms (i.e., _C1-20 ) at the ester end can be preferably used. Specific examples of _C1-20 alkyl(meth)acrylates 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, isooctyl(meth)acrylate, and isooctyl(meth)acrylate. [0113] Examples of the acrylates include, but are not limited to, ethyl (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 less (more preferably -40°C or less, for example -50°C or less) may be preferably used as at least a part of the monomer (m3). Such a low Tg alkyl (meth)acrylate may be useful for improving the flexibility of the adhesive. The lower limit of the Tg of the alkyl (meth)acrylate is not particularly limited, and may be, for example, -85°C or more, -75°C or more, -65°C or more, or -60°C or more. Specific examples of the low Tg alkyl (meth)acrylate include n-butyl acrylate (BA), 2-ethylhexyl acrylate (2EHA), isononyl acrylate (iNA), etc.

In some embodiments using monomer (m3), from the viewpoint of flexibility, adhesion, etc., it is preferable that at least a part of the monomer (m3) is an alkyl acrylate. For example, it is preferable that 50% by weight or more (more preferably 75% by weight or more, and even more preferably 90% by weight or more) of the monomer (m3) is an alkyl acrylate. There may also be an embodiment in which only one or more types of alkyl acrylate are used as monomer (m3), and no alkyl methacrylate is used.

In an embodiment 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 that the effect of use is appropriately exhibited. 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. In some embodiments, the content of the alkyl (meth)acrylate may be, for example, 15% by weight or more, 30% by weight or more, or 45% by weight or more. The upper limit of the content of the monomer (m3) in the monomer component is set so that the total with the content of other monomers does not exceed 100% by weight, and may be, for example, less than 50% by weight. In some embodiments, the content of the monomer (m3) may be, for example, less than 35% by weight. In general, the refractive index of alkyl (meth)acrylate is relatively low, so in order to increase the refractive index, it is advantageous to limit the content of the monomer (m3) in the monomer component and relatively increase the content of the monomer (m1). From this viewpoint, the content of monomer (m3) is advantageously 24% by weight or less of the monomer components, preferably less than 23% by weight, more preferably less than 20% by weight, may be less than 17% by weight, may be less than 12% by weight, may be less than 7% by weight, may be less than 3% by weight, or may be less than 1% by weight. Monomer (m3) may not be substantially used.

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

Examples of the other monomers include monomers having functional groups other than hydroxyl groups and carboxy groups (functional group-containing monomers). For example, examples of other monomers that can improve the cohesive strength and heat resistance of the adhesive include sulfonic acid group-containing monomers, phosphoric acid group-containing monomers, and cyano group-containing monomers. In addition, examples of monomers that can introduce functional groups that can serve as crosslinking base points into acrylic polymers or that can contribute to improving peel strength and compatibility within the adhesive layer 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 having 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. In addition, 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 monomers having a nitrogen atom-containing ring and amino group-containing monomers.

Other monomers that can be used in addition to the functional group-containing monomers include vinyl ester monomers such as vinyl acetate; non-aromatic ring-containing (meth)acrylates such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate; olefin 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; vinyl ether monomers such as methyl vinyl ether; and the like. One suitable example of other monomers that can be used for the purpose of improving the flexibility of the adhesive is ethoxyethoxyethyl acrylate (also known as ethyl carbitol acrylate, homopolymer Tg: -67°C).

When the other monomers are used, the amount used is not particularly limited and may be appropriately set within a range in which the total amount of the monomer components does not exceed 100% by weight. In some embodiments, from the viewpoint of easily exerting the refractive index improving effect by using the monomer (m1), the content of the other monomers in the monomer components may be, for example, about 35% by weight or less, and it is appropriate to set it to about 25% by weight or less (e.g., 0 to 25% by weight), and it may be about 20% by weight or less (e.g., 0 to 20% by weight), about 10% by weight or less, about 5% by weight or less, for example, about 1% by weight or less. The technology disclosed herein may be preferably implemented in an embodiment in which the monomer components do not substantially contain the 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 suppressed to a predetermined amount or less. 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 manner may be advantageous from the viewpoint of realizing a pressure-sensitive adhesive that has a good balance between flexibility, adhesion, and a high refractive index. The monomer components constituting the acrylic polymer may have a composition that does not contain a methacryloyl group-containing monomer (for example, a composition consisting of only an acryloyl group-containing monomer).

In some embodiments, the monomer component constituting the base polymer (e.g., acrylic polymer) of the high refractive index adhesive layer is preferably limited in the amount of carboxyl group-containing monomer used in order to suppress coloring or discoloration (e.g., yellowing) of the adhesive layer. The amount of carboxyl group-containing monomer used in the monomer component may be, for example, less than 1 wt%, preferably less than 0.5 wt%, more preferably less than 0.3 wt%, may be less than 0.1 wt%, or may be less than 0.05 wt%. The fact that the amount of carboxyl group-containing monomer used is limited in this way is also advantageous in terms of suppressing corrosion of metal materials (e.g., metal wiring or metal film that may be present on the adherend) that may be placed in contact with or in close proximity to the high refractive index adhesive layer. The technology disclosed herein can be preferably implemented in an embodiment in which the monomer component does not contain a carboxyl group-containing monomer.
For the same reason, in some embodiments, the monomer component constituting the base polymer of the high refractive index adhesive layer is preferably limited in the amount of monomer having an acidic functional group (including a carboxy group, a sulfonic acid group, a phosphoric acid group, etc.). The amount of the acidic functional group-containing monomer in the monomer component of such an embodiment can be the above-mentioned preferred amount of the carboxy group-containing monomer. The technology disclosed herein can be preferably implemented in an embodiment in which the monomer component does not contain an acidic group-containing monomer (i.e., an embodiment in which the base polymer of the high refractive index adhesive layer is acid-free).

(Glass transition temperature _TgT of base polymer)
In some embodiments, the base polymer of the pressure-sensitive adhesive layer (e.g., an acrylic polymer) has a glass transition temperature Tg _T based on the composition of the monomer components constituting the polymer of about 20° C. or less, preferably about 10° C. or less, more preferably 0° C. or less, and may be −10° C. or less, −20° C. or less, −25° C. or less, −28° C. or less, or −30° C. or less. A low glass transition temperature Tg _T can be advantageous in terms of improving the flexibility of the pressure-sensitive adhesive. In addition, the glass transition temperature Tg _T may be, for example, −60° C. or more, and from the viewpoint of facilitating the high refractive index of the pressure-sensitive adhesive, it is preferably −50° C. or more, more preferably greater than −45° C., may be greater than −40° C., may be greater than −35° C., may be greater than −25° C., may be −15° C. or more, or may be −5° C. or more.

Here, unless otherwise specified, the glass transition temperature _Tg of a polymer refers to the glass transition temperature calculated by the Fox formula based on the composition of the monomer components constituting the polymer. The Fox formula 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, as shown below.
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 the 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 adopted. If the Tg of the homopolymer is not described in publicly available documents, the value obtained by the measurement method described in JP-A-2007-51271 is used.

(Method of Preparing the Base Polymer)
In the technology disclosed herein, the method for obtaining the base polymer of the high refractive index pressure-sensitive adhesive layer (e.g., the acrylic polymer (A) composed of the above-mentioned monomer components) is not particularly limited. For example, the above description of the preparation method of the acrylic polymer (F) can also be applied to the preparation method of the base polymer of the high refractive index pressure-sensitive adhesive layer.

(Refractive index enhancer)
In some embodiments of the technology disclosed herein, the high refractive index adhesive layer (e.g., an acrylic adhesive layer) may contain a refractive index enhancer as necessary in addition to the base polymer. Here, in this specification, the refractive index enhancer refers to a material that can increase the refractive index of the adhesive layer by its use. As the refractive index enhancer, a material having a higher refractive index than the refractive index of the adhesive layer containing the refractive index enhancer can be preferably used. In addition, as the refractive index enhancer, a material having a higher refractive index than the base polymer (e.g., an acrylic polymer (A)) of the adhesive layer containing the refractive index enhancer can be preferably used. By appropriate use of the refractive index enhancer, a higher refractive index and practical adhesive performance can be suitably achieved at the same time. In some embodiments, the refractive index enhancer is preferably an organic material. The organic material used as the refractive index enhancer may be a polymer or a non-polymer. In addition, it may or may not have a polymerizable functional group. The refractive index enhancer may be used alone or in combination of two or more types.

The refractive index of the refractive index enhancer (for example, the additive (H _RO ) described later) can be set in an appropriate range relative to the refractive index of the base polymer, and is not limited to a specific range. The refractive index of the refractive index enhancer can be selected from a range of, for example, more than 1.55, more than 1.56, or more than 1.57, and higher than the refractive index of the base polymer. From the viewpoint of increasing the refractive index of the adhesive, in some embodiments, the refractive index of the refractive index enhancer is advantageously 1.58 or more, preferably 1.60 or more, more preferably 1.63 or more, may be 1.65 or more, may be 1.70 or more, or may be 1.75 or more. A refractive index enhancer with a higher refractive index can achieve the desired refractive index even with the use of a smaller amount of the refractive index enhancer. This is preferable from the viewpoint of suppressing the deterioration of adhesive properties and optical properties. The upper limit of the refractive index of the refractive index enhancer is not particularly limited, but from the viewpoint of compatibility within the adhesive, ease of achieving both a high refractive index and flexibility suitable for an adhesive, and the like, the upper limit is, 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 may be 1.850 or less.

In some embodiments, the difference between the refractive index n _b of the refractive index enhancer (for example, the additive (H _RO ) described later) and the refractive index n _a of the base polymer, i.e., n _b -n _a (hereinafter also referred to as "Δn _A "), is set to be greater than 0. In some embodiments, Δn _A is, for example, 0.02 or more, may be 0.05 or more, may be 0.07 or more, may be 0.10 or more, may be 0.15 or more, may be 0.20 or more, or may be 0.25 or more. By selecting the base polymer and the refractive index enhancer so that Δn _A is larger, the refractive index enhancing effect by using the refractive index enhancer tends to be higher. In addition, from the viewpoint of compatibility in the pressure-sensitive adhesive layer, transparency of the pressure-sensitive adhesive layer, and the like, in some embodiments, Δn _A may be, for example, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.35 or less.

In some embodiments, the difference between the refractive index n _b of the refractive index enhancer (for example, the additive (H _RO ) described later) and the refractive index n _T of the adhesive layer containing the refractive index enhancer, i.e., n _b -n _T (hereinafter also referred to as "Δn _B "), is set to be greater than 0. In some embodiments, Δn _B is, for example, 0.02 or more, may be 0.05 or more, may be 0.07 or more, may be 0.10 or more, may be 0.15 or more, may be 0.20 or more, or may be 0.25 or more. By selecting the composition of the adhesive layer and the refractive index enhancer so that Δn _B is larger, the refractive index enhancing effect by using the refractive index enhancer tends to be higher. In addition, from the viewpoint of compatibility in the adhesive layer, transparency of the adhesive layer, and the like, in some embodiments, Δn _B may be, for example, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.35 or less.

The amount of the refractive index enhancer used relative to 100 parts by weight of the base polymer (when multiple types of refractive index enhancers are used, the total amount of them) is not particularly limited and can be set according to the purpose. From the viewpoint of increasing the refractive index of the adhesive, the amount of the refractive index enhancer 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, may be 7 parts by weight or more, may be 10 parts by weight or more, may be 15 parts by weight or more, or may be 20 parts by weight or more. In some embodiments, the amount of the refractive index enhancer used relative to 100 parts by weight of the base polymer can be, for example, 80 parts by weight or less, and from the viewpoint of achieving a good balance between increasing the refractive index of the adhesive and suppressing the deterioration of the adhesive properties and optical properties, it is advantageous to set it to 60 parts by weight or less, and preferably to set it to 45 parts by weight or less. In some embodiments where the adhesive properties and optical properties are more important, the amount of the refractive index enhancer 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, 10 parts by weight or less, 5 parts by weight or less, or 3 parts by weight or less. The technology disclosed herein can also be preferably implemented in an embodiment where the amount of the refractive index enhancer used per 100 parts by weight of the base polymer in the adhesive layer is less than 1 part by weight, or where the refractive index enhancer is not substantially used. Here, "substantially not used" means that the refractive index enhancer is not used at least intentionally.

(Additive ( _HRO ))
In some embodiments, an organic material having a higher refractive index than the base polymer may be preferably used as the refractive index enhancer. Hereinafter, such an organic material may be referred to as "additive (H _RO )". Herein, the above "H _RO " represents an organic material having a high refractive index. By using a base polymer (e.g., an acrylic polymer, preferably an acrylic polymer (A)) in combination with the additive (H _RO ), a pressure-sensitive adhesive that more suitably balances the refractive index with the adhesive properties (peel strength, flexibility, etc.) and/or the optical properties (total light transmittance, haze value, etc.) can be realized. The organic material used as the additive (H _RO ) may be a polymer or a non-polymer. In addition, it may or may not have a polymerizable functional group. The additive (H _RO ) may be used alone or in combination of two or more types.

The refractive index of the additive (H _RO ), like that 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 may be selected according to the purpose. The molecular weight of the additive (H _RO ) may be selected, for example, from a range of 30,000 or less. In addition, the additive (H _RO ) is preferably a polymer or non-polymer having a lower molecular weight than the base polymer. 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 adhesives), in some embodiments, the molecular weight of the additive (H _RO ) is suitably less than about 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 in the adhesive layer that the molecular weight of the additive (H _RO ) is not too large. In addition, the molecular weight of the additive (H _RO ) may be, for example, 130 or more, or 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 10,000 (e.g., 1000 or more and less than 5000) can be used as the additive (H _RO ).
As the molecular weight of the additive (H _RO ), for a non-polymer or a polymer with a low degree of polymerization (e.g., about dimer to pentamer), a molecular weight calculated based on the chemical structure can be used. When the additive (H _RO ) is a polymer with a higher degree of polymerization, a 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 may be selected as the additive ( _HRO ) include, but are not limited to, organic compounds having aromatic rings, organic compounds having heterocyclic rings (which may be aromatic or non-aromatic heterocyclic rings), and the like.

The aromatic ring contained in the organic compound having an aromatic ring (hereinafter also referred to as an "aromatic ring-containing compound") used as the additive ( _HRO ) can be selected from the same aromatic rings contained in the compound used as the monomer (m1).

The aromatic ring may have one or more substituents on the ring-constituting atoms, or may have no substituents. In the case where 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 the substituent containing a carbon atom, the number of carbon atoms contained in the substituent is, for example, 1 to 10, advantageously 1 to 6, preferably 1 to 4, and more preferably 1 to 3, and may be, 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).

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 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 not having an ethylenically unsaturated group (e.g., 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.) from a compound that can be used as monomer (m1). Non-limiting specific examples of aromatic ring-containing compounds that can be used as the additive (H _RO ) may include aromatic ring-containing monomers such as benzyl acrylate, m-phenoxybenzyl acrylate, 2-(o-phenylphenoxy)ethyl acrylate, phenoxyethyl acrylate, phenoxydiethylene glycol acrylate, phenoxypolyethylene glycol acrylate, 2-hydroxy-3-phenoxypropyl acrylate, the above-mentioned monomers having a fluorene structure, monomers having a dinaphthothiophene structure, and monomers having a dibenzothiophene structure; aromatic ring-containing compounds that do not have an ethylenically unsaturated group, such as 3-phenoxybenzyl alcohol, dinaphthothiophene and derivatives thereof (for example, compounds having a structure in which one or more of one or more substituents selected from hydroxyl groups, methanol groups, diethanol groups, glycidyl groups, and the like are bonded to a dinaphthothiophene ring); and the like. The aromatic ring-containing compound may be an oligomer (preferably an oligomer having a molecular weight of about 5000 or less, more preferably about 1000 or less; for example, a low polymer of about dimer to pentamer) containing such an aromatic ring-containing monomer as a monomer unit. The oligomer may be, for example, a homopolymer of an aromatic ring-containing monomer; a copolymer of one or more aromatic ring-containing monomers; a copolymer of one or more aromatic ring-containing monomers with another monomer; or the like. As the other monomer, one or more monomers having no aromatic ring may be used.

In some embodiments, as the additive (H _RO ), 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 because it is easy to obtain a high refractive index-increasing effect. The multiple aromatic ring-containing compound may or may not have a polymerizable functional group such as an ethylenically unsaturated group. In addition, the multiple aromatic ring-containing compound may be a polymer or a non-polymer. In addition, the polymer may be an oligomer containing a multiple aromatic ring-containing monomer as a monomer unit (preferably an oligomer having a molecular weight of about 5000 or less, more preferably about 1000 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 types of multiple aromatic ring-containing monomers; a copolymer of one or more types of multiple aromatic ring-containing monomers and 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 monomers, a monomer that does not have an aromatic ring, or a combination thereof.

Non-limiting examples of compounds containing multiple aromatic rings include compounds having a structure in which two or more non-condensed aromatic rings are bonded via a linking group, compounds having a structure in which two or more non-condensed aromatic rings are chemically bonded directly (i.e., without other atoms), compounds having a condensed aromatic ring structure, compounds having a fluorene structure, compounds having a dinaphthothiophene structure, compounds having a dibenzothiophene structure, etc. Compounds containing multiple aromatic rings can be used alone or in combination of two or more.

Specific examples of compounds having the above-mentioned fluorene structure include the above-mentioned monomers having the fluorene structure, oligomers which are homopolymers or copolymers of such monomers, as well as 9,9-bisphenylfluorene and its derivatives, such as 9,9-bis(4-hydroxyphenyl)fluorene (refractive index: 1.68), 9,9-bis(4-aminophenyl)fluorene (refractive index: 1.73), 9,9-bis(4-hydroxy-3-methylphenyl)fluorene (refractive index: 1.68), and 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene (refractive index: 1.65).

Specific examples of compounds having the above-mentioned dinaphthothiophene structure include the above-mentioned monomers having the dinaphthothiophene structure, oligomers which are homopolymers or copolymers of such monomers, as well as dinaphthothiophene (refractive index: 1.808); hydroxyalkyl dinaphthothiophenes such as 6-hydroxymethyl dinaphthothiophene (refractive index: 1.766); dihydroxy dinaphthothiophenes such as 2,12-dihydroxy dinaphthothiophene (refractive index: 1.750); 2,12- Examples of dinaphthothiophenes and their derivatives include dihydroxyalkyloxydinaphthothiophenes such as dihydroxyethyloxydinaphthothiophene (refractive index: 1.677); diglycidyloxydinaphthothiophenes such as 2,12-diglycidyloxydinaphthothiophene (refractive index: 1.723); dinaphthothiophenes having two or more ethylenically unsaturated groups such as 2,12-diallyloxydinaphthothiophene (abbreviation: 2,12-DAODNT, refractive index: 1.729); etc.

Specific examples of compounds having the above dibenzothiophene structure include the above-mentioned monomers having the dibenzothiophene structure, oligomers which are homopolymers or copolymers of such monomers, as well as dibenzothiophene (refractive index: 1.607), 4-dimethyldibenzothiophene (refractive index: 1.617), 4,6-dimethyldibenzothiophene (refractive index: 1.617), etc.

Examples of organic compounds having a heterocycle (hereinafter, also referred to as heterocycle-containing organic compounds) that can be selected as the additive (H _RO ) include thioepoxy compounds, compounds having a triazine ring, and the like. Examples of thioepoxy compounds include bis(2,3-epithiopropyl) disulfide and its polymer (refractive index 1.74) described in Japanese Patent No. 3712653. Examples of compounds having a triazine ring include compounds having at least one triazine ring (for example, 3 to 40, preferably 5 to 20) in one molecule. Since the triazine ring has aromaticity, compounds having a triazine ring are also included in the concept of the aromatic ring-containing compound, and compounds having multiple triazine rings are also included in the concept of the aromatic ring-containing compound.

In some embodiments, a compound having no ethylenically unsaturated group may be preferably used as the additive (H _RO ). This can suppress the deterioration of the pressure-sensitive adhesive composition due to heat or light (progression of gelation or decrease in leveling due to increase in 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 the _occurrence of dimensional changes or deformations (warping, waviness, etc.) and optical distortions caused by the reaction of ethylenically unsaturated groups in the pressure-sensitive adhesive layer containing the additive (H RO ) or in a laminate (e.g., laminate sheet) containing the pressure-sensitive adhesive layer.

In the embodiment in which an oligomer is used as the additive (H _RO ), the oligomer can be obtained by polymerizing the corresponding monomer component by a known method. When the oligomer is produced by radical polymerization, a polymerization initiator, a chain transfer agent, an emulsifier, etc. used in radical polymerization can be appropriately added to the monomer component to perform polymerization. The polymerization initiator, the chain transfer agent, the emulsifier, etc. used in the radical polymerization are not particularly limited and can be appropriately selected and used. The weight average molecular weight of the oligomer can be controlled by the amount of the polymerization initiator and the chain transfer agent used and the reaction conditions, and the amount used is appropriately adjusted depending on the type of these.
Examples of the chain transfer agent include lauryl mercaptan, glycidyl mercaptan, mercaptoacetic acid, 2-mercaptoethanol, α-thioglycerol, thioglycolic acid, 2-ethylhexyl thioglycolate, and 2,3-dimercapto-1-propanol. The chain transfer agent may be used alone or in combination of two or more. The amount of the chain transfer agent used can be set so as to obtain an oligomer having a desired weight average molecular weight, depending on the composition of the monomer components used in the synthesis of the oligomer and the type of the chain transfer agent. In some embodiments, the amount of the chain transfer agent used relative to 100 parts by weight of the total amount of the monomers used in the synthesis of the oligomer is suitably about 15 parts by weight or less, may be 10 parts by weight or less, or may be about 5 parts by weight or less. The lower limit of the amount of the chain transfer agent used relative to 100 parts by weight of the total amount of monomers used in the synthesis of the oligomer is not particularly limited, but may be, for example, 0.01 part by weight or more, 0.1 part by weight or more, 0.5 part by weight or more, or 1 part by weight or more.

In the embodiment in which the additive (H _RO ) is used as a refractive index enhancer, the amount of the additive (H _RO ) used relative to 100 parts by weight of the base polymer (when a plurality of types of compounds are used, the total amount of them) is not particularly limited and can be set according to the purpose. From the viewpoint of increasing the refractive index of the adhesive, the amount of the additive (H _RO ) 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, may be 7 parts by weight or more, may be 10 parts by weight or more, may be 15 parts by weight or more, or may be 20 parts by weight or more. In some embodiments, the amount of the additive (H _RO ) used relative to 100 parts by weight of the base polymer can be, for example, 80 parts by weight or less, and from the viewpoint of achieving a good balance between increasing the refractive index of the adhesive and suppressing the deterioration of adhesive properties and optical properties, it is advantageous to set it to 60 parts by weight or less, and preferably to set it to 45 parts by weight or less. In some embodiments in which greater importance is placed on adhesive properties and optical properties, the amount of additive (H _RO ) used relative to 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.

(Plasticized material)
In some embodiments, the high refractive index adhesive layer may contain a plasticizing material having a lower molecular weight than the base polymer of the adhesive layer. The use of the plasticizing material can increase the flexibility of the high refractive index adhesive layer, and improve adhesion to the adherend, overall flexibility, and conformability to deformation. As the plasticizing material, an organic material can be preferably used from the viewpoint of compatibility and transparency in the adhesive layer. The plasticizing material may be a material that can also be used as the above-mentioned refractive index enhancer (for example, the above-mentioned additive (H _RO )).

The molecular weight of the plasticizing material is not particularly limited as long as it is lower than that of the base polymer. In some embodiments, the molecular weight of the plasticizing material may be 30,000 or less, 25,000 or less, less than 10,000, preferably less than 5,000, more preferably less than 3,000 (e.g., less than 1,000), less than 800, less than 600, less than 500, or less than 400, from the viewpoint of easily exerting the plasticizing effect. It may be advantageous from the viewpoint of improving compatibility in the adhesive layer, etc., if the molecular weight of the plasticizing material is not too large. In addition, in some embodiments, the molecular weight of the plasticizing material is suitably 130 or more, preferably 150 or more, from the viewpoint of easily exerting a sufficient plasticizing effect, may be 170 or more, 200 or more, 250 or more, or 300 or more. In some embodiments, the molecular weight of the plasticizing material may be 500 or more, 1000 or more, or 2000 or more. It is preferable that the molecular weight of the plasticizing material is not too low from the viewpoint of the heat resistance of the pressure-sensitive adhesive layer and the prevention of contamination of the adherend.

Non-limiting examples of compounds that can be selected as plasticizing materials include compounds that can be used as monomers (m1) (e.g., (meth)acrylates having aromatic rings such as benzyl groups, phenoxy groups, and naphthyl groups, monomers having a fluorene structure, monomers having a dinaphthothiophene structure, and monomers having a dibenzothiophene structure); oligomers containing compounds that can be used as monomers (m1) as monomer units; compounds in which the ethylenically unsaturated group-containing portion of a compound that can be used as monomers (m1) is removed and replaced with a hydrogen atom or a group that does not have an ethylenically unsaturated group (e.g., 3-phenoxybenzyl alcohol); and the like. From the viewpoint of improving flexibility, oligomers containing compounds that can be used as monomers (m1) as monomer units may be copolymerized with low Tg monomers such as n-butyl acrylate and 2-ethylhexyl acrylate. As the plasticizing material, one or more of known plasticizers (e.g., phthalate esters, terephthalate esters, adipic acid esters, adipic acid polyesters, benzoic acid glycol esters, etc.) may be used.

In some embodiments, an organic material having a refractive index of about 1.50 or more (more preferably 1.53 or more) may be preferably used as the plasticizing material. Specific examples of compounds that may be selected as the plasticizing material include diethylene glycol dibenzoate (refractive index 1.55), dipropylene glycol dibenzoate (refractive index 1.54), 3-phenoxytoluene (refractive index 1.57), 3-ethylbiphenyl (refractive index 1.59), 3-methoxybiphenyl (refractive index 1.61), 4-methoxybiphenyl (refractive index 1.57), polyethylene glycol dibenzoate, 3-phenoxybenzyl alcohol (refractive index 1.59), triphenyl phosphate (refractive index 1.56), benzyl benzoate (refractive index 1.57), 4-(tert-butyl) Examples of the refractive index include, but are not limited to, phenyl diphenyl phosphate (refractive index 1.56), trimethyl phenyl phosphate (refractive index 1.55), butyl benzyl phthalate (refractive index 1.54), rosin methyl ester (refractive index 1.53), alkyl benzyl phthalate (refractive index 1.53), butyl (phenylsulfonyl) amine (refractive index 1.53), trimethyl trimellitate (refractive index 1.52), benzyl phthalate (refractive index 1.52), 2-ethylhexyl diphenyl phosphate (refractive index 1.51), tris (2,4-di-tert-butylphenyl) phosphite, and the like. From the viewpoint of refractive index and compatibility, for example, diethylene glycol dibenzoate may be preferably adopted. The upper limit of the refractive index of the plasticizing material is not particularly limited, and may be, for example, 3.00 or less. In some embodiments, from the standpoint of ease of preparation of the pressure-sensitive adhesive composition, compatibility within the pressure-sensitive adhesive, and the like, the refractive index of the plasticizing material 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 plasticizing material is measured, like that of the monomer, using an Abbe refractometer at a wavelength of 589 nm and a 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.

In the embodiment in which the plasticizing material is used, the amount of the plasticizing material used relative to 100 parts by weight of the base polymer is not particularly limited and can be set according to the purpose. From the viewpoint of enhancing the plasticizing effect, the amount of the plasticizing material used relative to 100 parts by weight of the base polymer may be, for example, 0.1 parts by weight or more, or 0.5 parts by weight or more, and from the viewpoint of obtaining a higher plasticizing effect, it is preferable to set it to 1 part by weight or more, more preferably 3 parts by weight or more, and it may be 5 parts by weight or more, 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. In addition, from the viewpoint of achieving a good balance between the high refractive index of the adhesive and the transparency and plasticizing effect, it is appropriate that the amount of the plasticizing material used relative to 100 parts by weight of the base polymer is approximately 100 parts by weight or less, preferably 80 parts by weight or less, more preferably 60 parts by weight or less, and may be 45 parts by weight or less, 35 parts by weight or less, or 25 parts by weight or less. In some embodiments where greater importance is placed on adhesive properties and optical properties, the amount of plasticizing material used per 100 parts by weight of base polymer may be 15 parts by weight or less, 10 parts by weight or less, or 5 parts by weight or less.

(Leveling Agent)
In some embodiments, the adhesive composition used to form the adhesive layer may contain a leveling agent as necessary for the purpose of improving the appearance of the adhesive layer formed from the composition (for example, improving the uniformity of the thickness) and improving the coatability of the adhesive composition. Non-limiting examples of the leveling agent include acrylic leveling agents, fluorine leveling agents, and silicone leveling agents. The leveling agent may be, for example, an appropriate one selected from commercially available leveling agents and used in a conventional manner.

In some embodiments, a polymer (hereinafter also referred to as "polymer (B)") that is a polymerization product of a monomer having a polyorganosiloxane skeleton (hereinafter also referred to as "monomer S1") and a monomer raw material containing an acrylic monomer (hereinafter also referred to as "monomer raw material B") can be preferably used as the leveling agent. Polymer (B) can be said to be a copolymer of monomer S1 and an acrylic monomer. Polymer (B) can be used alone or in combination of two or more types.

The monomer S1 is not particularly limited, and any monomer containing a polyorganosiloxane skeleton can be used. As the monomer S1, a monomer having a structure having a polymerizable reactive group at one end can be preferably used. In particular, a monomer S1 having a structure having a polymerizable reactive group at one end and not having a functional group at the other end that causes a crosslinking reaction with the base polymer (e.g., acrylic polymer) of the adhesive composition to which the leveling agent is blended can be preferably used. Examples of commercially available products include one-end reactive silicone oil (e.g., product numbers such as X-22-174ASX, X-22-2426, X-22-2475, KF-2012, etc.) manufactured by Shin-Etsu Chemical Co., Ltd. Monomer S1 can be used alone or in combination of two or more types.

The functional group equivalent of the monomer S1 may be, for example, about 100 g/mol to 30,000 g/mol. In some preferred embodiments, the functional group equivalent is, for example, 500 g/mol or more, 800 g/mol or more, 1500 g/mol or more, or 2000 g/mol or more. The functional group equivalent may be, for example, 20,000 g/mol or less, less than 10,000 g/mol, 7000 g/mol or less, or 5,500 g/mol or less. When the functional group equivalent of the monomer S1 is within the above range, a good leveling effect is easily exhibited.
In addition, when two or more types of monomers having different functional group equivalents are used as monomer S1, the functional group equivalent of monomer S1 can be the sum of the products of the functional group equivalents of each type of monomer and the weight fraction of the monomer.

Here, the term "functional group equivalent" refers to the weight of the main skeleton (e.g., polydimethylsiloxane) bonded to one functional group. The unit g/mol is calculated based on 1 mol of functional group. The functional group equivalent of the monomer S1 can be calculated, for example, from the spectrum intensity of ¹ H-NMR (proton NMR) based on nuclear magnetic resonance (NMR). The calculation of the functional group equivalent (g/mol) of the monomer S1 based on the spectrum intensity of ¹ H-NMR can be performed based on a general structural analysis method related to ¹ H-NMR spectrum analysis, with reference to the description of Japanese Patent No. 5951153 if necessary. In the functional group equivalent of the monomer S1, the functional group refers to a polymerizable functional group (e.g., an ethylenically unsaturated group such as a (meth)acryloyl group, a vinyl group, or an allyl group).

The content of monomer S1 in monomer raw material B can be an appropriate value within a range in which the desired effect is achieved using monomer S1, and is not limited to a specific range. In some embodiments, the content of monomer S1 in monomer raw material B may be, for example, 5 to 60% by weight, 10 to 50% by weight, or 15 to 40% by weight.

The monomer raw material B contains, in addition to the monomer S1, an acrylic monomer copolymerizable with the monomer S1. This can improve the compatibility of the polymer (B) in the pressure-sensitive adhesive layer. Examples of acrylic monomers that can be used for the monomer raw material B include acrylic acid alkyl esters. The term "alkyl" used here refers to a chain-like (including linear and branched) alkyl (group) and does not include the alicyclic hydrocarbon group described below. In some embodiments, the monomer raw material B can contain at least one type of (meth)acrylic acid C _4-12 alkyl ester (preferably a (meth)acrylic acid C _4-10 alkyl ester, for example a (meth)acrylic acid C _6-10 alkyl ester). In some other embodiments, the monomer raw material B can contain at least one type of methacrylic acid C _1-18 alkyl ester (preferably a methacrylic acid C _1-14 alkyl ester, for example a methacrylic acid C _1-10 alkyl ester). Monomer raw material B may contain, as an acrylic monomer, for example, one or more selected from methyl methacrylate (MMA), n-butyl methacrylate (BMA), and 2-ethylhexyl methacrylate (2EHMA).

Other examples of the acrylic monomers include (meth)acrylic acid esters having an alicyclic hydrocarbon group. For example, cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, dicyclopentanyl (meth)acrylate, 1-adamantyl (meth)acrylate, etc. can be used. It is not necessary to use a (meth)acrylic acid ester having an alicyclic hydrocarbon group.

The content of the (meth)acrylic acid alkyl ester and the (meth)acrylic acid ester having an alicyclic hydrocarbon group in the monomer raw material B may be, for example, 10% by weight or more and 95% by weight or less, 20% by weight or more and 95% by weight or less, 30% by weight or more and 90% by weight or less, 40% by weight or more and 90% by weight or less, or 50% by weight or more and 85% by weight or less.

Other examples of monomers that may be included in monomer raw material B together with monomer S1 include the carboxy group-containing monomers, acid anhydride group-containing monomers, hydroxyl group-containing monomers, epoxy group-containing monomers, cyano group-containing monomers, isocyanate group-containing monomers, amide group-containing monomers, monomers having a nitrogen atom-containing ring, aminoalkyl (meth)acrylates, vinyl esters, vinyl ethers, olefins, (meth)acrylic acid esters having aromatic hydrocarbon groups, and halogen atom-containing (meth)acrylates, which are exemplified above as monomers that may be used in acrylic polymers.

The Mw of polymer (B) may be, for example, 5,000 or more, preferably 10,000 or more, or may be 15,000 or more. The Mw of polymer (B) may be, for example, 200,000 or less, preferably 100,000 or less, or may be 50,000 or less, or may be 30,000 or less. By setting the Mw of polymer (B) within an appropriate range, suitable compatibility and leveling properties can be exhibited.

The polymer (B) can be prepared, for example, by polymerizing the above-mentioned monomers by a known method such as solution polymerization, emulsion polymerization, bulk polymerization, suspension polymerization, or photopolymerization.
In order to adjust the molecular weight of the polymer (B), a chain transfer agent can be used as necessary. Examples of the chain transfer agent to be used include compounds having a mercapto group such as t-dodecyl mercaptan, mercaptoethanol, and α-thioglycerol; thioglycolic acid esters such as thioglycolic acid and methyl thioglycolate; and α-methylstyrene dimer. The amount of the chain transfer agent used is not particularly limited, and can be appropriately set so as to obtain a polymer (B) having a desired molecular weight. In some embodiments, the amount of the chain transfer agent used relative to 100 parts by weight of the monomer may be, for example, 0.1 to 5 parts by weight, 0.2 to 3 parts by weight, or 0.5 to 2 parts by weight.

The amount of polymer (B) used relative to 100 parts by weight of base polymer (e.g., acrylic polymer) can be, for example, 0.001 parts by weight or more, and from the viewpoint of obtaining a higher usage effect, it may be 0.01 parts by weight or more, or it may be 0.03 parts by weight or more. The amount of polymer (B) used may be, for example, 3 parts by weight or less, and from the viewpoint of reducing the influence on the refractive index, it is appropriate to set it to 1 part by weight or less, and it may be 0.5 parts by weight or less, or it may be 0.1 parts by weight or less.

(Inorganic particles)
The technology disclosed herein can be preferably implemented in an embodiment in which inorganic particles are not substantially used as a refractive index enhancer. However, in some embodiments, the use of inorganic particles as a refractive index enhancer is acceptable to the extent that the application effect of the technology disclosed herein is not significantly impaired. Examples of inorganic particles that can be used as a refractive index enhancer include inorganic particles composed of 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.). The average particle size (meaning the 50% volume average particle size based on the laser scattering and diffraction method) of the inorganic particles can be selected, for example, from a range of about 10 nm to 100 nm. The refractive index of the inorganic particles is measured using a commercially available spectroscopic ellipsometer at a measurement wavelength of 589 nm and a measurement temperature of 23° C. for a single layer film (having a film thickness that allows refractive index measurement) of the material constituting the inorganic particles. As the spectroscopic ellipsometer, for example, a product named "EC-400" (manufactured by JA. Woolam Co., Ltd.) or an equivalent product can be used. When inorganic particles are used as a refractive index enhancer, the amount used is preferably less than 5 parts by weight, and more preferably less than 1 part by weight, per 100 parts by weight of the base polymer. In an embodiment in which an additive (H _RO ) is used, the amount of the inorganic particles used is preferably not more than twice the amount of the additive (H _RO ) used, and more preferably not more than 1 time or not more than 0.5 times, by weight.

(Crosslinking Agent)
In the technology disclosed herein, the adhesive composition used to form the adhesive layer may contain a crosslinking agent as necessary for the purpose of adjusting the cohesive strength of the adhesive. As the crosslinking agent, a crosslinking agent known in the field of adhesives, such as an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, an aziridine-based crosslinking agent, an oxazoline-based crosslinking agent, a melamine-based resin, or a metal chelate-based crosslinking agent, may be used. Among them, an isocyanate-based crosslinking agent may be preferably used. Another example of the crosslinking agent is a monomer having two or more ethylenically unsaturated groups in one molecule, i.e., a polyfunctional monomer. The crosslinking agent may be used alone or in combination of two or more.

As the isocyanate-based crosslinking agent, a bifunctional or higher isocyanate compound can be used, for example, 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 modified products obtained by modifying the above isocyanate compounds with allophanate bonds, biuret bonds, isocyanurate bonds, uretdione bonds, urea bonds, carbodiimide bonds, uretonimine bonds, and oxadiazinetrione bonds. Examples of commercially available products include Takenate 300S, Takenate 500, Takenate 600, Takenate D165N, Takenate D178N (all manufactured by Takeda Pharmaceutical Co., Ltd.), Sumidur T80, Sumidur L, Desmodur N3400 (all manufactured by Sumika Bayer Urethane Co., Ltd.), Millionate MR, Millionate MT, Coronate L, Coronate HL, Coronate HX (all manufactured by Tosoh Corporation). The 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 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 can 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-hexanediol di(meth)acrylate, acrylate, 1,12-dodecanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, allyl(meth)acrylate, vinyl(meth)acrylate, divinylbenzene, bisphenoxyethanolfluorenedi(meth)acrylate, bisphenol A di(meth)acrylate, epoxy acrylate, polyester acrylate, urethane acrylate, butyldiol(meth)acrylate, hexyldiol di(meth)acrylate, etc. The 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, the amount used is not particularly limited, and can be, for example, in the range of about 0.001 to 5.0 parts by weight per 100 parts by weight of the base polymer. From the viewpoint of improving the flexibility of the adhesive, in some embodiments, the amount of the crosslinking agent used per 100 parts by weight of the base polymer is preferably 3.0 parts by weight or less, more preferably 2.0 parts by weight or less, and may be 1.0 parts by weight or less, 0.5 parts by weight or less, or 0.2 parts by weight or less. In addition, from the viewpoint of appropriately exerting the effect of using the crosslinking agent, in some embodiments, the amount of the crosslinking agent used per 100 parts by weight of the base polymer 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, or 0.08 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 the crosslinking catalyst used is not particularly limited. The amount of the crosslinking catalyst used per 100 parts by weight of the base polymer can be in the range of, for example, approximately 0.0001 parts by weight to 1 part by weight, taking into consideration the balance between the speed of the crosslinking reaction and the length of the pot life of the adhesive composition, and is preferably in the range of 0.001 parts by weight to 0.5 parts by weight.

The adhesive composition may contain a compound that generates keto-enol tautomerism as a crosslinking retarder. This can achieve the effect of extending the pot life of the adhesive composition. For example, a compound that generates keto-enol tautomerism may be preferably used in an adhesive composition containing an isocyanate-based crosslinking agent. As the compound that generates keto-enol tautomerism, various β-dicarbonyl compounds may be used. For example, β-diketones (acetylacetone, 2,4-hexanedione, etc.) and acetoacetate esters (methyl acetoacetate, ethyl acetoacetate, etc.) may be preferably used. The compound that generates keto-enol tautomerism may be used alone or in combination of two or more. The amount of the compound that generates keto-enol tautomerism may be, for example, 0.1 parts by weight or more and 20 parts by weight or less, or may be 0.5 parts by weight or more and 10 parts by weight or less, or may be 1 part by weight or more and 5 parts by weight or less, relative to 100 parts by weight of the base polymer.

(Tackifier)
The adhesive layer in the technology disclosed herein may contain a tackifier. As the tackifier, 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 can be used. These can be used alone or in combination of two or more. The amount of the tackifier resin used is not particularly limited, and can be set so that appropriate adhesive performance is exhibited depending on the purpose and application. In some embodiments, from the viewpoint of refractive index and transparency, the amount of the tackifier used is appropriately 30 parts by weight or less, preferably 10 parts by weight or less, and more preferably 5 parts by weight or less, relative to 100 parts by weight of the base polymer of the adhesive layer. The technology disclosed herein can be preferably implemented in an embodiment in which a tackifier is not used.

(Other additives)
In the technology disclosed herein, the adhesive composition used to form the adhesive layer may contain, as necessary, known additives that can be used in adhesive compositions, such as plasticizers, softeners, colorants, antistatic agents, antiaging agents, UV absorbers, antioxidants, light stabilizers, preservatives, etc., to the extent that the effects of the present invention are not significantly hindered. As for such various additives, conventionally known ones can be used in the usual manner, and they do not particularly characterize the present invention, so detailed explanations will be omitted.

<Preparation of Adhesive Layer>
In the technology disclosed herein, the adhesive constituting the adhesive layer (which may be a high refractive index adhesive layer and/or a low refractive index adhesive layer; the same applies below) may be an adhesive obtained by curing a solvent-based, active energy ray-curable, water-dispersible, hot melt, or other adhesive composition by drying, crosslinking, polymerization, cooling, or the like, that is, a cured product of the adhesive composition. The curing means (e.g., drying, crosslinking, polymerization, cooling, etc.) of the adhesive composition may be applied in one type only, or in two or more types simultaneously or in multiple stages. In the case of a solvent-based adhesive composition, the adhesive can typically be formed by drying (preferably further crosslinking) the composition. In the case of an active energy ray-curable adhesive composition, the adhesive is typically formed by irradiating the composition with active energy rays to promote a polymerization reaction and/or a crosslinking reaction. When it is necessary to dry the active energy ray-curable adhesive composition, it is preferable to irradiate the composition with active energy rays after drying.

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

The adhesive layer in the technology disclosed herein may be an adhesive layer having post-curing properties, or may be an adhesive layer having no post-curing properties. Here, the adhesive layer having post-curing properties refers to an adhesive layer that can be further cured by irradiation with heat or active energy rays (e.g., ultraviolet rays). Examples of an adhesive layer having post-curing properties include an adhesive layer having an unreacted ethylenically unsaturated group in the side chain of the base polymer, and an adhesive layer containing an unreacted multifunctional monomer. In some embodiments, it is preferable that the adhesive layer does not have post-curing properties. An adhesive layer having no post-curing properties does not cause dimensional changes associated with the post-curing reaction (i.e., has good dimensional stability), so that it is easy to suppress warping of the adhesive layer or the adherend to which the adhesive layer is attached. The absence of dimensional changes (e.g., cure shrinkage) due to post-curing can also be advantageous from the viewpoint of suppressing optical distortion of the adhesive layer.

The thickness of the adhesive layer is not particularly limited, and may be, for example, 3 μm or more, and is preferably 5 μm or more. With an adhesive layer having a thickness of 5 μm or more, good adhesive properties are easily obtained. In addition, an adhesive layer having such a thickness is easy to absorb unevenness that may exist on the adherend surface and to be bonded to the adherend with good adhesion. It is also preferable that the thickness of the adhesive layer (for example, the thickness of the high refractive index adhesive layer) is 5 μm or more from the viewpoint of preventing coloring and color unevenness due to light interference. In some embodiments, the thickness of the adhesive layer may be 10 μm or more, 20 μm or more, 30 μm or more, 50 μm or more, 70 μm or more, or 85 μm or more. 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. It is advantageous for the thickness of the pressure-sensitive adhesive layer not to be too large in terms of thinning a laminate sheet or a light-emitting device including the pressure-sensitive adhesive layer, etc. 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).
In some embodiments, the thickness of the adhesive layer described above may be applied to at least the thickness _T1 of the high refractive index adhesive layer. The thickness _T2 of the low refractive index adhesive layer may also be selected from the same range. The thickness _T1 of the high refractive index adhesive layer and the thickness _T2 of the low refractive index adhesive layer may be similar or different. The ratio ( _T1 / _T2 ) of the thickness _T1 of the high refractive index adhesive layer to the thickness _T2 of the low refractive index adhesive layer may be, for example, 0.1 or more, 0.3 or more, 0.5 or more, 0.8 or more, 1.2 or more, or 1.5 or more. In addition, the ratio ( _T1 / _T2 ) may be, for example, 20 or less, 10 or less, 5 or less, or 3 or less. In some embodiments, the ratio ( _T1 / _T2 ) may be less than 2, less than 1.5, or less than 1.

Methods for obtaining a structure (laminate sheet) in which a low refractive index adhesive layer and a high refractive index layer (typically a high refractive index adhesive layer) are laminated include, for example, a method of forming a high refractive index adhesive layer and a low refractive index adhesive layer on a release surface (for example, the release surface of a release liner) and laminating them together, a method of laminating a low refractive index adhesive layer formed on a release surface to a non-adhesive high refractive index layer, a method of applying an adhesive composition for forming a low refractive index adhesive layer on a high refractive index layer and curing it, and a method of applying a composition for forming a high refractive index layer on a low refractive index adhesive layer and curing it, but are not limited to these. When laminating a high refractive index layer and a low refractive index adhesive layer, if necessary, a treatment for promoting adhesion between the layers may be performed. For example, autoclave treatment, roll press treatment, etc. can be performed, but are not limited to these.

(Peel Strength)
In some embodiments, the peel strength of the adhesive layer disclosed herein against a glass plate is suitably about 1.0 N/25 mm or more (e.g., 1.5 N/25 mm or more), preferably 2 N/25 mm or more, more preferably 3 N/25 mm or more, and may be 4 N/25 mm or more, 6 N/25 mm or more, 8 N/25 mm or more, 10 N/25 mm or more, or 12 N/25 mm or more. The upper limit of the peel strength is not particularly limited, and may be, for example, 30 N/25 mm or less, 25 N/25 mm or less, or 20 N/25 mm or less.

Here, the peel strength is measured by pressing the adhesive to an alkaline glass plate as an adherend, leaving it in an environment of 23 ° C. and 50% RH for 30 minutes, then placing it in a pressure degassing device (autoclave) and autoclaving it at a temperature of 50 ° C. and a pressure of 0.5 MPa for 30 minutes, and leaving it in an atmosphere of 23 ° C. and 50% RH for 24 hours, and then measuring the 180 ° peel adhesive strength under the conditions of a peel angle of 180 degrees and a tensile speed of 300 mm / min. When measuring, if necessary, a suitable backing material (for example, a polyethylene terephthalate (PET) film with a thickness of about 25 μm to about 50 μm) can be attached to the measurement object to reinforce it. More specifically, the peel strength can be measured according to the method described in the examples described later.
When the low refractive index adhesive layer disclosed herein is laminated with a high refractive index adhesive layer to form a laminated sheet in the form of a double-sided adhesive sheet having a first adhesive surface and a second adhesive surface, in some embodiments, the above-mentioned peel strength is preferably applied to at least the second adhesive surface (the adhesive surface formed by the low refractive index adhesive layer), and more preferably applied to both the first adhesive surface and the second adhesive surface. The peel strength of the first adhesive surface against a glass plate and the peel strength of the second adhesive surface against glass may be the same or different.

<Support base material>
The low refractive index adhesive disclosed herein can be used in the form of a substrate-attached adhesive sheet comprising an adhesive layer formed from the adhesive and a supporting substrate.

The material of the support substrate is not particularly limited, and can be appropriately selected according to the purpose and mode of use. Non-limiting examples of substrates that can be used include polyolefin films mainly composed of polyolefins such as polypropylene (PP) and ethylene-propylene copolymers, polyester films mainly composed of polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN), and plastic films such as polyvinyl chloride films mainly composed of polyvinyl chloride; foam sheets made 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, craft paper, and crepe paper; metal foils such as aluminum foil and copper foil; and the like. Substrates having a composite structure of these may also be used. Examples of such composite substrates include substrates with a structure in which metal foil and the above-mentioned plastic film are laminated, and plastic substrates reinforced with inorganic fibers such as glass cloth.

In some embodiments, various film substrates can be preferably used. The film substrate may be a porous substrate such as a foam film or a nonwoven sheet, or may be a nonporous substrate, or may be a substrate having a structure in which a porous layer and a nonporous layer are laminated. In some embodiments, the film substrate may preferably be one that includes a resin film that can independently maintain its shape (self-supporting or independent) as a base film. Here, the term "resin film" refers to a resin film that has a nonporous structure and typically does not substantially contain air bubbles (voidless). Therefore, the resin film is a concept that is distinguished from foam films and nonwoven fabrics. As the resin film, one that can independently maintain its shape (self-supporting or independent) can be preferably used. The resin film may be a single-layer structure, or a multi-layer structure of two or more layers (for example, a three-layer structure).

Materials that make up the resin film include, for example, polyester-based resins whose main component is polyester, such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN); polyolefin-based resins whose main component is polyolefin, such as polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, and ethylene-butene copolymer; cellulose resins such as triacetyl cellulose; acetate-based resins; polysulfone-based resins; polyethersulfone-based resins; polycarbonate-based resins; polyamide (PA)-based resins such as nylon 6, nylon 66, and partially aromatic polyamide; and polyimide (PI)-based resins. Examples of such resins include transparent polyimide resins, polyamideimide (PAI), polyether ether ketone (PEEK), polyether sulfone (PES), cyclic polyolefin resins such as norbornene-based resins, (meth)acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, ethylene-vinyl acetate copolymer resins, ethylene-vinyl alcohol copolymer resins, polyarylate resins, polyphenylene sulfide (PPS) resins, polyurethane (PU), ethylene-vinyl acetate copolymer (EVA), fluorine-based resins such as polytetrafluoroethylene (PTFE) and fluorinated polyimides, etc.

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 two or more of these resins blended together. 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, etc. may be preferably used. Examples of resin films that are preferable from the viewpoint of strength and dimensional stability include PET film, PEN film, PPS film, and PEEK film. From the viewpoint of availability, etc., PET film and PPS film are particularly preferable, and PET film is particularly preferable.

The resin film may contain known additives such as light stabilizers, antioxidants, antistatic agents, colorants (dyes, pigments, etc.), fillers, slip agents, and antiblocking agents, as necessary, to the extent that the effects of the present invention are not significantly impeded. The amount of additives to be added is not particularly limited, and can be set appropriately depending on the application of the pressure-sensitive adhesive 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 appropriately used.

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 the auxiliary layer include an optical property adjusting layer (e.g., a coloring layer, an anti-reflection layer), a printing layer or a lamination layer for imparting a desired appearance to the substrate, an antistatic layer, an undercoat layer, a release layer, and other surface treatment layers.

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

The thickness of the substrate is not particularly limited and may be selected according to the purpose and mode of use. The thickness of the substrate may be, for example, 500 μm or less, and from the viewpoint of ease of handling and processability, 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, the conformability to the surface shape of the adherend tends to improve. Also, from the viewpoint of ease of handling 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 laminated may be subjected to a conventionally known surface treatment, such as a corona discharge treatment, a plasma treatment, an ultraviolet irradiation treatment, an acid treatment, an alkali treatment, or the formation of an undercoat layer by applying an undercoat agent (primer), as necessary. Such a surface treatment may be a treatment for improving 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 ones. The thickness of the undercoat layer is not particularly limited, but is usually appropriate to be about 0.01 μm to 1 μm, and preferably about 0.1 μm to 1 μm. Other treatments that may be applied to the substrate as necessary include an antistatic layer formation treatment, a colored layer formation treatment, a printing treatment, etc. These treatments can be applied alone or in combination.

In the technology disclosed herein, when the low refractive index adhesive layer constitutes a substrate-attached adhesive sheet, the thickness of the adhesive sheet 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. In addition, from the viewpoint of handleability, etc., the thickness of the adhesive sheet may be, for example, 10 μm or more, 25 μm or more, 80 μm or more, or 130 μm or more.
The thickness of the adhesive sheet refers to the thickness of the portion attached to the adherend. For example, in the substrate-less double-sided adhesive sheet 2 having the configuration shown in Fig. 2, the thickness refers to the thickness from the first surface (first adhesive surface) 10A to the second surface (second adhesive surface) 10B of the adhesive layer, and does not include the thickness of the release liners 31 and 32.

<Adhesive sheet>
The low refractive index adhesive disclosed herein can be used in the form of an adhesive sheet that includes an adhesive layer (low refractive index adhesive layer) formed from the adhesive and may further include other layers (high refractive index adhesive layer, supporting substrate, etc.).

(Total light transmittance)
In the technology disclosed herein, the total light transmittance of the low refractive index adhesive layer is suitably 86% or more, preferably 88% or more, more preferably 90% or more (e.g., more than 90.0%), and may be 90.5% or more, 93% or more, or 95% or more. The upper limit of the total light transmittance is theoretically a value obtained by subtracting the light loss (Fresnel loss) due to reflection at the air interface from 100%, and in practice may be approximately 98% or less, approximately 96% or less, or approximately 95% or less. In some embodiments, taking into account the refractive index and adhesive properties, the total light transmittance of the low refractive index 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. As the transmittance meter, a product name "HAZEMETER HM-150" manufactured by Murakami Color Research Laboratory or an equivalent product is used. More specifically, the total light transmittance of the low refractive index pressure-sensitive adhesive layer can be measured, for example, according to the examples described later. The total light transmittance of the low refractive index pressure-sensitive adhesive layer can be adjusted, for example, by selecting the composition, thickness, etc. of the low and high refractive index pressure-sensitive adhesive layers.
The above description regarding the total light transmittance of a low refractive index adhesive layer can also be applied to the total light transmittance of an adhesive sheet including a low refractive index adhesive layer and a supporting substrate, the total light transmittance of a high refractive index layer, the total light transmittance of a laminated sheet (adhesive sheet) including a low refractive index adhesive layer and a high refractive index layer (typically a high refractive index adhesive layer), etc.

(Haze value)
The haze value of the low refractive index pressure-sensitive adhesive layer is suitably 3.0% or less, preferably 2.0% or less, more preferably 1.0% or less, and even more preferably 0.9% or less. A low haze value of the low refractive index pressure-sensitive adhesive layer is advantageous in applications requiring high light transmittance (e.g., optical applications) and applications requiring good visibility of the adherend through the low refractive index pressure-sensitive adhesive layer. In some embodiments, the haze value of the low refractive index pressure-sensitive adhesive layer may be 0.8% or less, 0.5% or less, or 0.3% or less. The lower limit of the haze value of the low refractive index pressure-sensitive adhesive layer is not particularly limited, and from the viewpoint of improving transparency, the smaller the haze value, the more preferable it is. On the other hand, in some embodiments, taking into consideration the refractive index and adhesive properties, the haze value of the low refractive index pressure-sensitive adhesive layer may be, for example, 0.05% or more, 0.1% or more, 0.2% or more, 0.3% or more, or 0.4% or more.
The above description regarding the haze value of the low refractive index adhesive layer can also be applied to the haze value of an adhesive sheet including a low refractive index adhesive layer and a supporting substrate, the haze value of a high refractive index layer, the haze value of a laminated sheet (adhesive sheet) including a low refractive index adhesive layer and a high refractive index layer (typically a high refractive index adhesive layer), etc.

Here, the "haze value" refers to the ratio of diffuse transmitted light to the total transmitted light when a measurement target is irradiated with visible light. 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.

(Surface smoothness of adhesive surface)
In some embodiments, it is preferable that the surface (adhesive surface) of the pressure-sensitive adhesive layer has high surface smoothness.

For example, the adhesive surface is preferably limited to a predetermined value or less in its arithmetic mean roughness Ra. A configuration having an adhesive surface designed to have a low arithmetic mean roughness Ra is preferable from the viewpoint of optical homogeneity. By limiting the arithmetic mean roughness Ra, for example, in a usage mode in which light is extracted through the adhesive surface (such as an adhesive sheet arranged on the viewing side of the self-luminous element in a light-emitting device), it is possible to exert an effect of suppressing the occurrence of luminance unevenness due to the surface condition of the adhesive layer. A low arithmetic mean roughness Ra of the adhesive surface is also advantageous in suppressing optical distortion, and suppression of optical distortion also contributes to improving optical homogeneity. When the adhesive sheet disclosed herein is in the form of a double-sided adhesive sheet having a first adhesive surface and a second adhesive surface (for example, in the form of a laminated sheet including a high refractive index adhesive layer and a low refractive index adhesive layer), it is preferable that the arithmetic mean roughness Ra of at least the first adhesive surface is limited to a predetermined value or less, and it is more preferable that the arithmetic mean roughness Ra of both adhesive surfaces is limited to a predetermined value or less. By having each adhesive surface of the double-sided pressure-sensitive adhesive sheet have high surface smoothness, adhesion with excellent optical uniformity can be preferably achieved.

In some embodiments, the arithmetic mean roughness Ra of the adhesive surface is preferably about 70 nm or less, more preferably about 65 nm or less, and even more preferably about 55 nm or less, and may be less than 50 nm, less than 45 nm, or less than 40 nm. From the viewpoint of production efficiency, etc., in some embodiments, the arithmetic mean roughness Ra of the adhesive surface may be, for example, about 10 nm or more, about 20 nm or more, or about 30 nm or more (e.g., about 40 nm or more). In an embodiment in which the adhesive sheet has a first adhesive surface and a second adhesive surface, the arithmetic mean roughness Ra of the first adhesive surface and the arithmetic mean roughness Ra of the second adhesive surface may be about the same or different.

For example, the adhesive surface is preferably limited to a maximum height Rz of a predetermined value or less. A configuration having an adhesive surface designed to have a low maximum height Rz is preferable from the viewpoint of optical homogeneity. By limiting the maximum height Rz, for example, in a usage mode in which light is extracted through the adhesive surface as described above, it is possible to suppress the occurrence of uneven brightness due to the surface condition of the adhesive layer. A low maximum height Rz of the adhesive surface is also advantageous in suppressing optical distortion. When the adhesive sheet disclosed herein is in the form of a double-sided adhesive sheet having a first adhesive surface and a second adhesive surface, it is preferable that the maximum height Rz of at least the first adhesive surface is limited to a predetermined value or less, and it is more preferable that the maximum heights Rz of both adhesive surfaces are both limited to a predetermined value or less. By having each adhesive surface of the double-sided adhesive sheet have high surface smoothness, adhesion with excellent optical homogeneity can be preferably realized.

In some embodiments, the maximum height Rz of the adhesive surface is preferably about 600 nm or less, more preferably about 500 nm or less, even more preferably about 450 nm or less, and particularly preferably about 400 nm or less, and may be less than 350 nm, less than 300 nm, or less than 250 nm. From the viewpoint of production efficiency, etc., in some embodiments, the maximum height Rz of the adhesive surface may be, for example, about 10 nm or more, about 50 nm or more, about 100 nm or more, or about 200 nm or more. In an embodiment having a first adhesive surface and a second adhesive surface, the maximum height Rz of the first adhesive surface and the maximum height Rz of the second adhesive surface may be about the same or different.

The arithmetic mean roughness Ra and maximum height Rz of the adhesive surface are measured using a non-contact surface roughness measuring device. As a non-contact surface roughness measuring device, a light interference type surface roughness measuring device can be used, for example, a 3D optical profiler (product name "NewView7300", manufactured by ZYGO) or an equivalent product. Specifically, the arithmetic mean roughness Ra and maximum height Rz can be measured, for example, by the following measurement method, or by setting the measurement operation and measurement conditions so as to obtain results equivalent to or corresponding to those obtained by the measurement method.

That is, in an environment of 23°C and 50% RH, a three-dimensional optical profiler (product name "NewView7300", manufactured by ZYGO Corporation) is used to measure the surface shape of the measurement sample under the following conditions. From the measured data, the arithmetic surface roughness Ra is calculated in accordance with JIS B 0601-2001. The maximum height Rz is calculated as the sum of the height Rp of the highest peak above the average line of the roughness curve obtained by the above measurement (roughness curve) and the depth Rv of the deepest valley below the average line. The measurement is performed five times (i.e., N=5), and the average value thereof is used.
The above-mentioned measurement sample can be prepared, for example, by cutting the pressure-sensitive adhesive layer to be measured or the pressure-sensitive adhesive sheet containing the pressure-sensitive adhesive layer to a size of about 150 mm in length and 50 mm in width. If the adhesive surface is protected by a release liner, the release liner is gently peeled off (for example, under conditions of a pulling speed of 300 mm/min and a peel angle of 180°) to expose the adhesive surface. It is desirable to leave the exposed adhesive surface for about 30 minutes before carrying out the measurement.
[Measurement conditions]
Measurement area: 5.62mm x 4.22mm
(Objective lens: 2.5x, internal lens: 0.5x)
Analysis mode:
Remove: Cylinder
Data Fill: ON (Max: 25)
Remove Spikes: ON (xRMS: 1)
Filter: OFF

The arithmetic mean roughness Ra and maximum height Rz of the adhesive surface can be adjusted by the composition and properties (viscosity, leveling properties, etc.) of the adhesive composition used to form the adhesive layer, and the properties of the surface (release surface) of the release liner that protects the adhesive surface.

(Water absorption rate)
In some embodiments in which the low refractive index adhesive disclosed herein is used together with a high refractive index adhesive layer, the high refractive index adhesive layer preferably has a water absorption rate limited to a predetermined value or less. By limiting the water absorption rate of the high refractive index adhesive layer, the dimensional change of the high refractive index adhesive layer due to the fluctuation of the water content in the high refractive index adhesive layer (for example, absorption and release of water such as moisture in the environment) tends to be suppressed. This makes it possible to suppress warping of the high refractive index adhesive layer or an optical laminate including the high refractive index adhesive layer caused by the mismatch of the dimensional change between the high refractive index adhesive layer and the adjacent layer (which may be a low refractive index adhesive layer, a support substrate, a release liner, an adherend, etc.). It is also preferable from the viewpoint of maintaining the flatness, transparency, refractive index, etc. of the high refractive index adhesive layer constant to be able to suppress the fluctuation of the water content in the high refractive index adhesive layer. In addition, a high refractive index adhesive layer with a low water absorption rate is less likely to occlude water, and is therefore suitable as a component of a member or product including an element that dislikes water, such as an organic EL element.

In some embodiments, the water absorption rate of the high refractive index adhesive layer is suitably about 1.0% or less, preferably 0.7% or less, more preferably 0.5% or less (e.g., less than 0.5%), and may be 0.4% or less, 0.3% or less, or 0.2% or less. The lower limit of the water absorption rate of the high refractive index adhesive layer is not particularly limited, but from the practical viewpoint of compatibility with adhesive properties, it may be, for example, 0.01% or more, 0.05% or more, 0.1% or more, 0.15% or more, or 0.25% or more. The water absorption rate of the low refractive index adhesive layer may be the same as that of the high refractive index adhesive layer, or may be different. From the viewpoint of obtaining a higher effect, it is more preferable that the water absorption rates of both the high refractive index adhesive layer and the low refractive index adhesive layer are limited to a predetermined value or less.

The water absorption rate (also referred to as moisture content) of the high refractive index pressure-sensitive adhesive layer is measured by the following method: The water absorption rate of the low refractive index pressure-sensitive adhesive layer is also measured by the same method.
[Moisture content measurement]
The adhesive layer to be evaluated is cut into a size of 4 cm x 5 cm (area: 20 cm ² ) together with two release liners arranged on one side and the other side, the release liner on one side is removed, and the piece is attached to aluminum foil that has been weighed in advance. The release liner on the other side of the adhesive layer is then removed, and the piece is placed in a thermohygrostat at a temperature of 60°C and a relative humidity of 90%, and taken out after 72 hours. The test piece in which the adhesive layer and the aluminum foil are laminated is weighed, and then the moisture content is measured by Karl Fischer coulometric titration under the following conditions using a moisture meter (Mitsubishi Chemical Analytech CA-200 type) equipped with a heating vaporizer (Mitsubishi Chemical Analytech VA-200 type).
Anolyte: Aquamicron AKX (manufactured by Mitsubishi Chemical)
Catholyte: Aquamicron CXU (Mitsubishi Chemical)
Heat vaporization temperature: 150°C

(Gel Fraction)
The gel fraction of the adhesive layer disclosed herein is appropriately set according to the purpose of use, the mode of use, etc., and is not limited to a specific range. The gel fraction is, for example, about 99% or less, and is suitably about 97% or less. From the viewpoint of favorably achieving both a low refractive index and adhesive properties, in some preferred embodiments, the gel fraction may be about 95% or less, more preferably about 92% or less (for example, about 90% or less). It is also preferable that the gel fraction is not too high from the viewpoint of appropriately following and adhering to unevenness that may exist on the adherend surface (for example, unevenness structure provided for the purpose of improving light extraction efficiency in a light emitting device). In some embodiments, the gel fraction may be about 88% or less, about 75% or less, or about 65% or less. In addition, from the viewpoint of imparting appropriate cohesiveness to the adhesive and appropriately expressing adhesive properties, the gel fraction is, for example, about 10% or more, and is suitably about 20% or more, and may be about 30% or more. From the viewpoint of deformation resistance of the pressure-sensitive adhesive layer (prevention of bubbles due to protrusion caused by pressure or inclusion of foreign matter, etc.), the gel fraction is preferably about 30% or more, more preferably about 40% or more, and may be about 45% or more, about 50% or more, about 65% or more, or about 75% or more. The gel fraction of a pressure-sensitive adhesive sheet including a high refractive index pressure-sensitive adhesive layer and a low refractive index pressure-sensitive adhesive layer (for example, a laminated sheet in the form of a substrateless double-sided pressure-sensitive adhesive sheet consisting of a high refractive index pressure-sensitive adhesive layer and a low refractive index pressure-sensitive adhesive layer) is also preferably within the range exemplified above. The gel fraction can be adjusted by the molecular weight, molecular structure, concentration, degree of crosslinking, etc. of the base polymer. The gel fraction is measured by the following method.

[Measurement of gel fraction]
A given amount of adhesive sample (weight _Wg1 ) is wrapped in a porous polytetrafluoroethylene film (weight _Wg2 ) with an average pore size of 0.2 μm in the shape of a purse, and the opening is tied with string (weight _Wg3 ). As the porous polytetrafluoroethylene (PTFE) film, "Nitoflon (registered trademark) NTF1122" (average pore size 0.2 μm, porosity 75%, thickness 85 μm) available from Nitto Denko Corporation or an equivalent product is used.
The package is immersed in a sufficient amount of ethyl acetate and kept at room temperature (typically 23° C.) for 7 days to allow only the sol content in the adhesive to elute out of the film, after which the package is taken out and the ethyl acetate adhering to the outer surface is wiped off, the package is dried at 130° C. for 2 hours, and the weight of the package ( _Wg4 ) is measured. The gel fraction of the adhesive layer is determined by substituting each value into the following formula.
Gel fraction (%)=[( _Wg4 − _{Wg2− Wg3} )/ _Wg1 ]×100

The gel fraction of the high refractive index adhesive layer and the gel fraction of the low refractive index adhesive layer may be similar or different. In some embodiments, the gel fraction of the low refractive index adhesive layer can be lower than the gel fraction of the high refractive index adhesive layer. With this configuration, the low refractive index adhesive layer, which has a relatively low gel fraction, contributes to increasing the overall flexibility. This allows a good balance between a high refractive index and flexibility.

In some embodiments of the technology disclosed herein, the peak temperature of tan δ of the adhesive constituting the high refractive index adhesive layer is preferably about -50°C or higher, and about 50°C or lower. Here, tan δ (loss tangent) of the adhesive refers to the ratio of the loss modulus G" to the storage modulus G' of the adhesive. In other words, tan δ = G"/G'. The tan δ of the adhesive is calculated from the storage modulus G' (Pa) and loss modulus G" (Pa) at that time by the following formula: tan δ = G"/G'; when a disk-shaped adhesive sample having a thickness of about 2 mm and a diameter of 7.9 mm is sandwiched between parallel plates, and a temperature dispersion test of the adhesive is performed in shear mode under the conditions of a measurement temperature range of -60°C to 60°C and a heating rate of 5°C/min while applying a shear strain of a frequency of 1 Hz using a viscoelasticity tester. From the change in tan δ in the above temperature range, the peak temperature of tan δ of the adhesive (hereinafter sometimes referred to as Tpeak) can be determined. As a viscoelasticity tester, an ARES manufactured by TA Instruments or an equivalent product can be used.

In some embodiments, the Tpeak of the high refractive index adhesive layer is advantageously 45°C or less or 35°C or less, preferably 30°C or less (e.g., 25°C or less), and may be 20°C or less or 15°C or less. An adhesive with a lower Tpeak tends to be more likely to provide good initial adhesion and adhesion in the room temperature range. On the other hand, it is preferable that the Tpeak of the adhesive is not too low from the viewpoint of imparting moderate cohesiveness to the adhesive, and tends to be suitable for achieving compatibility with a high refractive index. From this viewpoint, in some embodiments, the Tpeak of the adhesive may be, for example, -40°C or more, -30°C or more, -20°C or more, -5°C or more, 5°C or more, 15°C or more, or even 25°C or more. An adhesive with a relatively high Tpeak may be preferably used in an embodiment in which, when attached to an adherend, one or both of the adhesive and the adherend are heated to a temperature slightly higher than room temperature as necessary. The Tpeak of the adhesive can be adjusted by selecting the composition of the adhesive (for example, the composition of the monomer components constituting the base polymer, whether or not a refractive index enhancer or plasticizing material is used, and selection of the type and amount used).
The Tpeak of the above-mentioned adhesive is preferably applied to at least the high refractive index adhesive layer, more preferably to both the high refractive index adhesive layer and the low refractive index adhesive layer. The Tpeak of the high refractive index adhesive layer and the Tpeak of the low refractive index adhesive layer may be the same or different.

<Laminated sheet with release liner>
The high refractive index adhesive layer and the low refractive index adhesive layer disclosed herein may take the form of an adhesive product (a laminated sheet with a release liner) in which the adhesive surface of a laminated sheet including a high refractive index adhesive layer and a low refractive index adhesive layer is abutted against the release surface of a release liner before being incorporated into a light emitting device. Thus, this specification provides a laminated sheet with a release liner (adhesive product) including a laminated sheet of a high refractive index adhesive layer and a low refractive index adhesive layer, and a release liner having a release surface abutting against the adhesive surface of the laminated sheet.

The release liner is not particularly limited, and examples thereof include a release liner having a release treatment layer on a release liner substrate such as a resin film or paper (which may be paper laminated with a resin such as polyethylene), and a release liner made of a resin film formed from a low-adhesion material such as a fluorine-based polymer (polytetrafluoroethylene, etc.) or a polyolefin-based resin (polyethylene, polypropylene, etc.). The release treatment layer may be formed by surface treating the release liner substrate with a release treatment agent. The release treatment agent may be a known release treatment agent such as a silicone-based release treatment agent, a long-chain alkyl-based release treatment agent, a fluorine-based release treatment agent, or molybdenum (IV) sulfide. In some embodiments, a release liner having a release treatment layer made of a silicone-based release treatment agent may be preferably used. The thickness and method of forming the release treatment layer are not particularly limited, and can be set so that the adhesive surface of the release liner exhibits appropriate releasability.

In some embodiments, from the viewpoint of smoothness of the adhesive surface, etc., a release liner (hereinafter also referred to as a release film) having a release treatment layer on a resin film (hereinafter also referred to as a release film substrate) as a release liner substrate may be preferably used. As the release film substrate, various plastic films can be used. In this specification, a plastic film is typically a non-porous sheet, and is a concept that is distinguished from, for example, nonwoven fabrics (i.e., does not include nonwoven fabrics).

Examples of materials for the plastic film include polyester resins such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN); polyolefin resins such as polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, and ethylene-butene copolymer; cellulose resins such as triacetyl cellulose; acetate resins, polysulfone resins, polyethersulfone resins, polycarbonate resins, polyamide resins, polyimide resins, and cyclic polyolefin resins such as norbornene resins; (meth)acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, ethylene-vinyl acetate copolymer resins, ethylene-vinyl alcohol copolymer resins, polyarylate resins, and polyphenylene sulfide resins. Release film substrates formed from one or a mixture of two or more of these resins can be used. Among these, a polyester resin film (e.g., a PET film) formed from a polyester resin can be given as a preferred release film substrate.

The plastic film used as the release film substrate described above may be any of a non-stretched film, a uniaxially stretched film, and a biaxially stretched film. The plastic film may have a single layer structure or a multilayer structure including two or more sublayers. The plastic film may contain known additives that can be used in release film substrates for pressure-sensitive adhesive sheets, such as antioxidants, antiaging agents, heat stabilizers, light stabilizers, ultraviolet absorbers, colorants such as pigments and dyes, lubricants, fillers, antistatic agents, and nucleating agents. In a multilayer plastic film, each additive may be contained in all sublayers, or only in some of the sublayers.

In some preferred embodiments, the release film substrate (typically a plastic film) may be one in which the content of particles such as inorganic particles (e.g., pigments, lubricants, fillers, etc.) in the layer on the release surface side is limited, or one that is substantially free of such particles. Here, "substantially free" means that the amount of particles (e.g., inorganic particles) in the layer is less than 1% by weight, preferably less than 0.1% by weight (e.g., 0 to 0.01% by weight). A release film having such a release film substrate tends to have a low arithmetic mean roughness Ra and maximum height Rz of the release surface. When the release film substrate (typically a plastic film) has a multilayer structure, the particle content in the layer on the release surface side may be 1/10 or less (e.g., 1/50 or less) of the particle content in layers other than the layer on the release surface side.

In a laminate sheet with a release liner having a release liner on each of the first adhesive surface and the second adhesive surface, the release liner placed on one adhesive surface (hereinafter also referred to as the first release liner) and the release liner placed on the other adhesive surface (hereinafter also referred to as the other release liner) may be of the same material and configuration, or may be of different materials and configurations.

The thickness of the release liner (preferably the release film) is not particularly limited and may be, for example, about 10 μm to 500 μm. From the viewpoint of the strength and dimensional stability of the release liner, the thickness of the release liner is suitably 20 μm or more, preferably 30 μm or more, may be 35 μm or more, may be 40 μm or more, or may be 45 μm or more. Also, from the viewpoint of the handleability of the release liner (e.g., ease of rolling), etc., the thickness of the release liner is suitably 300 μm or less, preferably 250 μm or less, may be 200 μm or less, may be 150 μm or less, or may be 130 μm or less. In some preferred embodiments, the thickness of the release liner is approximately 125 μm or less, may be approximately 115 μm or less, may be approximately 105 μm or less, may be approximately 90 μm or less, or may be approximately 70 μm or less. By making the thickness of the release liner below a certain value, it is less likely to leave marks when rolled up, it can be removed more smoothly from the adhesive sheet, and it is easier to obtain a high level of surface smoothness on the adhesive surface after removing the release liner.

In a laminate sheet with release liner having one release liner and another release liner, the thicknesses of the release liners may be the same or different. In some embodiments, from the viewpoint of ease of peeling, it is preferable that one release liner and the other release liner have different thicknesses. For example, it is preferable that the thickness of the thicker release liner is approximately 1.1 times or more (e.g., approximately 1.25 times or more; there is no particular upper limit, but it is preferably 5 times or less) the thickness of the thinner release liner.

(Arithmetic mean roughness Ra of adhesive side surface)
In some embodiments, the release liner (preferably the release film) is preferably limited to a predetermined value or less (e.g., about 100 nm or less, or even less than 50 nm) on the adhesive side surface, from the viewpoint of realizing an adhesive surface with high surface smoothness. In some embodiments, the arithmetic mean roughness Ra of the adhesive side surface of the release liner is preferably, for example, about 30 nm or less, more preferably about 25 nm or less, and may be about 20 nm or less, or may be about 18 nm or less. In addition, from the viewpoint of ease of manufacture and handling of the release liner, in some embodiments, the arithmetic mean roughness Ra may be, for example, about 5 nm or more, about 10 nm or more, or about 15 nm or more. In a laminate sheet with release liner in a form in which a release liner is disposed on each of the first adhesive surface and the second adhesive surface, it is preferable that the adhesive side surfaces of both release liners satisfy any of the above-mentioned arithmetic mean roughness Ra. The arithmetic mean roughness Ra of the adhesive side surfaces of both release liners may be approximately the same or different.

(Maximum height Rz of adhesive surface)
In some embodiments, the release liner (preferably the release film) preferably has a maximum height Rz of the adhesive surface side of 700 nm or less, from the viewpoint of realizing an adhesive surface with high surface smoothness. In some embodiments, the maximum height Rz of the adhesive surface side of the release liner is preferably about 600 nm or less, may be about 500 nm or less, may be about 400 nm or less, or may be about 300 nm or less. In addition, from the viewpoint of ease of production and handling of the release liner, in some embodiments, the above-mentioned maximum height Rz may be, for example, about 50 nm or more, may be about 80 nm or more, may be about 100 nm or more, may be about 200 nm or more, or may be about 300 nm or more. In a laminated sheet with release liners in a form in which release liners are arranged on the first adhesive surface and the second adhesive surface, it is preferable that the adhesive surface sides of both release liners satisfy any of the maximum heights Rz described above. The maximum heights Rz of the adhesive surface sides of both release liners may be about the same or different.

(Surface properties of the back side)
The arithmetic mean roughness Ra and maximum height Rz of the back surface (opposite the pressure-sensitive adhesive layer) of the release liner (preferably the release film) are not particularly limited. The arithmetic mean roughness Ra of the back surface of the release liner may be, for example, more than 30 nm (e.g., more than 35 nm, or even about 50 nm or more) from the viewpoint of productivity, etc. The maximum height Rz of the back surface of the release liner may be, for example, more than 400 nm (e.g., about 500 nm or more) or more than 800 nm (e.g., 1000 nm or more) from the viewpoint of productivity, etc.

The arithmetic mean roughness Ra and maximum height Rz of the release film surface can be adjusted by the selection of film material, the molding method, surface treatment such as release treatment, etc. Examples include adjusting the smoothness of the layers that make up the release surface (anti-blocking layer, hard coat layer, oligomer prevention layer, etc.), reducing or eliminating the use of filler particles in the surface layer or release film substrate (particle-free), and adjusting the stretching conditions.

The arithmetic mean roughness Ra and maximum height Rz of the release liner (preferably release film) surface are measured using a non-contact surface roughness measuring device. As a non-contact surface roughness measuring device, a light interference type surface roughness measuring device is used, for example, a three-dimensional optical profiler (product name "NewView7300", manufactured by ZYGO) or an equivalent product can be used. For example, a glass plate (a soda lime glass plate manufactured by MATSUNAMI, thickness 1.3 mm) is attached and fixed with an adhesive to the surface opposite to the measurement surface of the release liner, and the surface shape can be measured using a three-dimensional optical profiler (product name "NewView7300", manufactured by ZYGO) in an environment of 23°C and 50% RH.

<Application>
The adhesive (e.g., low refractive index adhesive) disclosed herein can be used by being attached to various adherends. The constituent material of the adherend (adherend material) is not particularly limited, but may be, for example, metal materials such as copper, silver, gold, iron, tin, palladium, aluminum, nickel, titanium, chromium, indium, zinc, etc., or alloys containing two or more of these, or various resin materials (typically plastic materials) such as polyimide resins, acrylic resins, polyethernitrile resins, polyethersulfone resins, polyester resins (PET resins, polyethylene naphthalate resins, etc.), polyvinyl chloride resins, polyphenylene sulfide resins, polyether ether ketone resins, polyamide resins (so-called aramid resins, etc.), polyarylate resins, fluorine resins, polycarbonate resins, cellulose polymers such as diacetyl cellulose and triacetyl cellulose, vinyl butyral polymers, liquid crystal polymers, and carbon materials such as graphene, etc., alumina, zirconia, titania, SiO ₂ Examples of such materials include metal oxides and mixtures thereof, such as ITO (indium tin oxide) and ATO (antimony-doped tin oxide), nitrides and composites thereof, such as aluminum nitride, silicon nitride, titanium nitride, gallium nitride, and indium nitride, and inorganic materials, such as alkali glass, non-alkali glass, quartz glass, borosilicate glass, and sapphire glass carbon. The adhesive disclosed herein can be used by being attached to a member (e.g., an optical member) having at least a surface made of the above material.

The adhesive disclosed herein can be used in an application mode that does not require heating to a temperature higher than room temperature (e.g., 20°C to 35°C) after application to an adherend. In addition, if permissible depending on the type of adherend, heat treatment may be performed at least at any one of the following times: after application to the adherend, at the time of application, and before application. Heat treatment can be performed for the purpose of improving the adhesiveness of the adhesive to the adherend and promoting adhesion. The heat treatment temperature can be set appropriately so as to obtain the desired effect, within the range permissible depending on the constituent material of the adhesive sheet and the type of adherend, taking into account the surface condition of the adherend, and may be, for example, about 100°C or less, 80°C or less, 60°C or less, or 50°C or less.

The member or material to which the adhesive is applied may be light-transmitting. With such an adherend, the advantage of the adhesive disclosed herein being highly transparent is easily obtained. The total light transmittance of the adherend may be, for example, more than 50%, or may be 70% or more. In some preferred embodiments, the total light transmittance of the adherend is 80% or more, more preferably 90% or more, and even more preferably 95% or more (e.g., 95 to 100%). The adhesive disclosed herein may be preferably used in an embodiment in which it is applied to an adherend (e.g., an optical member) having a total light transmittance of a predetermined value or more. The total light transmittance is measured using a commercially available transmittance meter in accordance with JIS K 7136:2000. As the transmittance meter, a product named "HAZEMETER HM-150" manufactured by Murakami Color Research Laboratory or an equivalent product is used.

The refractive index of the adherend and the refractive index of the adhesive layer (typically a low refractive index adhesive layer, or a high refractive index adhesive layer in a form having a high refractive index adhesive layer) arranged in contact with the adherend may be similar or different. For example, by increasing the refractive index of the adhesive layer relative to the refractive index of the adherend, light incident on the adhesive layer from the adherend side at an angle equal to or less than the critical angle can be refracted to the front side, thereby increasing the front brightness. In this case, the refractive index of the adherend may be, for example, 1.55 or less, 1.50 or less, 1.48 or less, 1.45 or less, or may be less than 1.45, and may be, for example, 1.10 or more, 1.20 or more, 1.30 or more, or 1.35 or more. In addition, an adherend with a relatively high refractive index relative to the adhesive layer can refract light incident on the adherend from the adhesive layer side to the front side, thereby increasing the front brightness. In this case, the refractive index of the adherend may be, for example, 1.60 or more, 1.65 or more, or 1.70 or more, and may be, for example, 3.00 or less, 2.50 or less, or 2.00 or less. On the other hand, by reducing the difference in refractive index between the adhesive layer and the adherend, light reflection at the interface can be suppressed. In this case, the refractive index of the adherend may be about 1.55 to 1.80, about 1.55 to 1.75, or about 1.60 to 1.70. The refractive index of the adherend can be measured in the same manner as the refractive index of the adhesive.

In some preferred embodiments, the adherend may have any of the refractive indices described above and any of the total light transmittances described above. The effects of the technology disclosed herein are particularly favorable in optical products (e.g., light-emitting devices) in which a high refractive index adhesive layer and/or a low refractive index adhesive layer is attached or laminated to such an adherend.

The low refractive index adhesive layer and the high refractive index adhesive layer disclosed herein can be used in the form of a laminate sheet containing them, by being attached to various adherends as described above. An example of a preferred application is optical applications. More specifically, the laminate sheet disclosed herein can be preferably used as an optical adhesive sheet used, for example, for bonding optical members (for bonding optical members) or for manufacturing products (optical products) using the optical members. The laminate sheet used in such an embodiment can also be understood as an interlayer sheet disposed between layers of an optical laminate.

The optical member refers to a member having optical properties (e.g., polarization, light refraction, light scattering, light reflectivity, light transmission, light absorption, light diffraction, optical rotation, visibility, etc.). The optical member is not particularly limited as long as it has optical properties, but examples thereof include components constituting devices (optical devices) such as display devices (image display devices) and input devices, or components used in these devices, such as polarizing plates, wave plates, retardation plates, optical compensation films, brightness enhancement films, light guide plates, reflective films, anti-reflective 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 further components in which these are laminated (these may be collectively referred to as "functional films"). The above "plate" and "film" are intended to include plate-like, film-like, sheet-like, and other shapes. 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 "polarizing plate" is intended to include a circular polarizing plate.

Examples of the display device include a liquid crystal display device, an organic electroluminescence (EL) display device, a micro LED (μLED), a mini LED, a plasma display panel (PDP), and electronic paper. Examples of the input device include a touch panel.

The optical member is not particularly limited, but examples thereof include members (e.g., sheet-like, film-like, or plate-like members) made of glass, acrylic resin, polycarbonate, polyethylene terephthalate, metal thin film, etc. In this specification, the term "optical member" also includes members that play a role in decorating or protecting the display device or input device while maintaining its visibility (design films, decorative films, surface protection films, etc.).

The low refractive index adhesive layer disclosed herein (which may be in the form of a laminated sheet with a high refractive index layer) can be used, for example, in an embodiment in which it is disposed between an optical film, such as a film having one or more functions such as light transmission, reflection, diffusion, waveguiding, light collection, and diffraction, or a fluorescent film, and another optical member (which may be another optical film), and can be preferably used to bond the optical film to the other optical member. In particular, in bonding optical films having at least one function of light guiding, light collection, and diffraction, it is desirable that the entire bulk of the bonding layer has a high refractive index, and the technology disclosed herein can be a preferred application target when the technology disclosed herein is implemented in an embodiment including a high refractive index layer (typically a high refractive index adhesive layer).

The adhesive layer 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, from the viewpoint of the trend toward miniaturization and high performance of optical components, there is a demand for thinning and improved light extraction efficiency. The adhesive layer disclosed herein can be preferably used as an adhesive layer that can meet such demands. More specifically, for example, in bonding of light-guiding films and diffusion films, the refractive index of the adhesive layer as a bonding layer can be adjusted (for example, to have a high refractive index) to contribute to thinning. In bonding of fluorescent films, the light extraction efficiency (which can also be understood as luminous efficiency) can be improved by appropriately adjusting the refractive index difference between the fluorescent emitter and the adhesive. In bonding of color-tuning films, the refractive index of the adhesive can be appropriately adjusted so that the refractive index difference with the color-tuning pigment is small, thereby reducing scattered components and contributing to improved light transmittance. When bonding prism sheets, lenticular films, microlens array films, etc., the refractive index of the adhesive can be appropriately adjusted to control the diffraction of light, contributing to improved brightness and/or viewing angle.

The high refractive index adhesive layer disclosed herein is preferably used in the form of a laminated sheet with a low refractive index adhesive layer, for example, attached to a high refractive index adherend (which may be a high refractive index layer or member, etc.), and can suppress interfacial reflection with the adherend. The high refractive index adhesive layer used in such an embodiment preferably has a small refractive index difference with the high refractive index adherend as described above, and has high adhesion at the interface with the adherend. In addition, from the viewpoint of improving the homogeneity of the appearance, it is preferable that the thickness of the adhesive layer is highly uniform, and for example, the surface smoothness of the adhesive surface is preferably high. When the thickness of the high refractive index adherend is relatively small (for example, 5 μm or less, 4 μm or less, or 2 μm or less), it is particularly meaningful to suppress reflection at the interface from the viewpoint of suppressing coloring and color unevenness due to interference of reflected light. One example of such a usage mode is a polarizing plate with a retardation layer that includes a polarizer, a first retardation layer, and a second retardation layer in this order, in which the film is used to bond the polarizer to the first retardation layer and/or the first retardation layer to the second retardation layer.

The high refractive index adhesive layer disclosed herein can be preferably used in a form in which it is attached to a light emitting layer of an optical semiconductor or the like (for example, a high refractive index light emitting layer mainly composed of inorganic materials). By reducing the difference in refractive index between the light emitting layer and the high refractive index adhesive layer, reflection at the interface therebetween can be suppressed and light extraction efficiency can be improved. In addition, from the viewpoint of preventing deterioration of the self-luminous element due to moisture, it is preferable that the water absorption rate of the high refractive index adhesive layer is low. From the viewpoint of improving brightness, it is preferable that the high refractive index adhesive layer is low colored. This can also be advantageous from the viewpoint of suppressing unintentional coloring caused by the high refractive index adhesive layer.

The high refractive index adhesive layer disclosed herein can be preferably used as a coating layer covering the lens surface, a bonding layer with a member facing the lens surface (e.g., a member having a surface shape corresponding to the lens surface), a filling layer filled between the lens surface and the member, etc., in microlenses and other lens members (e.g., microlenses constituting a microlens array film, lens members such as microlenses for cameras) used as components of cameras, light emitting devices, etc. The high refractive index adhesive layer disclosed herein can reduce the refractive index difference with the lens even when placed in contact with a high refractive index lens (e.g., a lens made of a high refractive index resin, or a lens having a surface layer made of a high refractive index resin). This is advantageous from the viewpoint of thinning the lens and products equipped with the lens, and can also contribute to suppressing aberrations and improving the Abbe number. In the technology disclosed herein, the adhesive (viscoelastic material) constituting the high refractive index adhesive layer can be used itself as a lens resin, for example, in a form filled in a recess or void of an appropriate transparent member.

The embodiment of bonding optical members using the adhesive layer disclosed herein (a high refractive index adhesive layer and/or a low refractive index adhesive layer, preferably a low refractive index adhesive layer that may be laminated on a high refractive index layer) is not particularly limited, but may be, for example, (1) an embodiment in which optical members are bonded to each other via the adhesive layer disclosed herein, (2) an embodiment in which an optical member is bonded to a member other than an optical member via the adhesive layer disclosed herein, or (3) an embodiment in which the adhesive layer disclosed herein is in the form of an adhesive sheet containing an optical member, and the adhesive sheet is bonded to an optical member or a member other than an optical member. In the above embodiment (3), the adhesive sheet in the form containing an optical member may be, for example, an adhesive sheet whose support is an optical member (e.g., an optical film). An adhesive sheet in the form containing an optical member as a support in this way may also be understood as an adhesive-type optical member (e.g., an adhesive-type optical film). In addition, when the adhesive layer disclosed herein constitutes an adhesive sheet having a support, and the functional film is used as the support, the adhesive sheet can also be understood as an "adhesive-type functional film" having the adhesive layer disclosed herein on at least one side of the functional film.

As described above, the technology disclosed herein provides an optical laminate comprising the adhesive layer disclosed herein and a member (e.g., a resin film such as an optical film) to which the adhesive sheet is attached. The member to which the adhesive layer is attached may have the refractive index of the adherend material described above. In addition, the difference (refractive index difference) between the refractive index of the adhesive layer and the refractive index of the member may be the refractive index difference between the adherend and the adhesive layer described above. The members constituting the laminate are as described above as the members, materials, and adherends, and therefore redundant description will not be repeated.

As will be understood from the above description and the following examples, the matters disclosed in this specification include the following:

[101] A pressure-sensitive adhesive comprising an acrylic polymer (F) containing a fluorine-containing acrylic monomer (M1) as a monomer unit,
A pressure-sensitive adhesive having a refractive index of 1.46 or less and a storage modulus G' at 25°C of 1.0 kPa or more and 400 kPa or less.
[102] The pressure-sensitive adhesive according to [101] above, wherein the content of the fluorine-containing acrylic monomer (M1) in the monomer components constituting the acrylic polymer (F) is 25 wt % or more.
[103] The pressure-sensitive adhesive according to the above-mentioned [101] or [102], wherein the above-mentioned fluorine-containing acrylic monomer (M1) contains a fluorine atom-containing alkyl (meth)acrylate ester.
[104] The pressure-sensitive adhesive according to any one of [101] to [103] above, wherein the acrylic polymer (F) contains a hydroxyl group-containing monomer as a monomer unit.
[105] A low refractive index pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive according to any one of [101] to [104] above;
a high refractive index adhesive layer laminated on the low refractive index adhesive layer;
A laminated sheet comprising:
[106] The laminate sheet according to [105] above, wherein the ratio (n ₁ /n ₂ ) of the refractive index n ₁ of the high refractive index pressure-sensitive adhesive layer to the refractive index n ₂ of the low refractive index pressure-sensitive adhesive layer is 1.02 or more.
[107] The laminate sheet according to [105] or [106] above, wherein the refractive index _n1 of the high refractive index pressure-sensitive adhesive layer is greater than 1.570.
[108] The laminate sheet according to any one of [105] to [107] above, wherein the high refractive index pressure-sensitive adhesive layer has a storage modulus G' at 25°C of 700 kPa or less.
[109] The laminate sheet according to any one of the above [105] to [108], which has a total light transmittance of 86% or more and a haze value of 3.0% or less.
[110] A self-luminous element;
A laminate sheet according to any one of the above items [105] to [109],
the laminate sheet is disposed on a more visible side than the self-luminous elements.

[1] A pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer,
The adhesive surface is constituted by the adhesive layer,
The pressure-sensitive adhesive layer has a refractive index of more than 1.570, a total light transmittance of 86% or more, and a haze value of 3.0% or less.
[2] The pressure-sensitive adhesive sheet according to [1] above, wherein the pressure-sensitive adhesive layer has a thickness of 5 μm or more.
[3] The pressure-sensitive adhesive sheet according to the above [1] or [2], which has a peel strength (adhesive strength) to a glass plate of 3 N/25 mm or more.
[4] The pressure-sensitive adhesive sheet according to any one of [1] to [3] above, wherein the pressure-sensitive adhesive surface has an arithmetic mean roughness Ra of 100 nm or less.
[5] The pressure-sensitive adhesive sheet according to any one of [1] to [4] above, wherein the pressure-sensitive adhesive layer has a water absorption rate of 1.0% or less.
[6] The pressure-sensitive adhesive sheet according to any one of [1] to [5] above, which is configured as a laminate including the pressure-sensitive adhesive layer and a light-transmitting substrate.
[7] The pressure-sensitive adhesive sheet according to [6] above, wherein the light-transmitting substrate is a resin film.
[8] The pressure-sensitive adhesive sheet according to any one of [1] to [5] above, which is a double-sided pressure-sensitive adhesive sheet comprising the pressure-sensitive adhesive layer.
[9] The pressure-sensitive adhesive sheet according to any one of [1] to [8] above,
a release liner disposed on the adhesive surface of the pressure-sensitive adhesive sheet; and
A release liner and an adhesive sheet comprising:
[10] A pressure-sensitive adhesive composition used to form a pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet according to any one of [1] to [8] above.

[11] An acrylic polymer (A) containing an aromatic ring-containing monomer (m1) as a monomer unit;
an additive (H _RO ) which is an organic material having a higher refractive index than the acrylic polymer (A);
A pressure-sensitive adhesive composition comprising:
[12] The pressure-sensitive adhesive composition according to the above-mentioned [11], wherein the additive (H 2 _RO ) has a refractive index of 1.60 or more.
[13] The pressure-sensitive adhesive composition according to the above [11] or [12], wherein the content of the additive (H 2 _RO ) per 100 parts by weight of the acrylic polymer (A) is more than 0 part by weight and not more than 60 parts by weight.
[14] The pressure-sensitive adhesive composition according to any one of the above [11] to [13], wherein the additive (H _RO ) comprises at least one compound selected from the group consisting of aromatic ring-containing compounds and heterocycle-containing compounds.
[15] The pressure-sensitive adhesive composition according to any one of the above [11] to [14], wherein the additive (H 2 _RO ) contains a compound having two or more aromatic rings in one molecule.
[16] The additive (H _RO ) is a compound having two or more aromatic rings in one molecule,
(i) a structure in which two non-fused aromatic rings are directly chemically bonded, and (ii) a structure in which two aromatic rings are fused.
The pressure-sensitive adhesive composition according to the above-mentioned [15], comprising a compound satisfying at least one of the above.
[17] The pressure-sensitive adhesive composition according to any one of [11] to [16], wherein the content of the aromatic ring-containing monomer (m1) in the monomer components constituting the acrylic polymer (A) is 50% by weight or more.
[18] In the monomer components constituting the acrylic polymer (A), the content of the aromatic ring-containing monomer (m1) is more than 70% by weight and less than 100% by weight,
The pressure-sensitive adhesive composition according to any one of the above [11] to [17], wherein 50% by weight or more of the aromatic ring-containing monomers (m1) are monomers having a glass transition temperature of a homopolymer of 10° C. or lower.
[19] The pressure-sensitive adhesive composition according to any one of [11] to [18], wherein the monomer component constituting the acrylic polymer (A) further contains a monomer (m2) having at least one of a hydroxyl group and a carboxy group.
[20] The pressure-sensitive adhesive composition according to any one of [11] to [18] above, which is used to form a pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet according to any one of [1] to [8] above.
[21] A pressure-sensitive adhesive formed from the pressure-sensitive adhesive composition according to any one of [11] to [20] above, the pressure-sensitive adhesive having a refractive index of higher than 1.570.
[22] A pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer constituted by a pressure-sensitive adhesive formed from the pressure-sensitive adhesive composition according to any one of [11] to [20] above.
[23] The pressure-sensitive adhesive sheet according to [22] above, wherein the pressure-sensitive adhesive layer has a haze value of 1.0% or less.

[24] An interlayer sheet for use by being disposed between layers of a laminate in optical applications, comprising:
A viscoelastic layer _V1 having a refractive index _n1 of 1.570 or more, and a total light transmittance of 86% or more;
A haze value of 1.0% or less; and
The storage modulus G' at 25°C is 30 kPa to 700 kPa;
Meet the interlayer sheet.
[25] The interlayer sheet according to [24] above, having a thickness of 5 μm or more.
[26] The interlayer sheet according to [24] or [25], wherein the viscoelastic layer _V1 contains a main polymer and a plasticizing material having a lower molecular weight than the main polymer.
[27] The interlayer sheet according to [26] above, wherein the weight average molecular weight of the plasticizing material is 30,000 or less.
[28] The viscoelastic layer _V2 is laminated on the viscoelastic layer _V1 ,
The interlayer sheet according to any one of the above [24] to [27], wherein the storage modulus _G'V2 of the viscoelastic layer _V2 at 25°C is lower than the storage modulus _G'V1 of the viscoelastic layer _V1 at 25°C.
[29] The interlayer sheet according to [28] above, wherein the refractive index _n2 of the viscoelastic layer _V2 is lower than the refractive index _n1 of the viscoelastic layer _V1 .
[30] The interlayer sheet according to any one of [24] to [29] above, wherein the viscoelastic layer _V1 is a layer formed from the pressure-sensitive adhesive composition according to any one of [11] to [18] above.
[31] The interlayer sheet according to any one of [24] to [29] above, wherein the viscoelastic layer _V1 is a pressure-sensitive adhesive layer in the pressure-sensitive adhesive sheet according to any one of [1] to [5] above.
[32] The interlayer sheet according to any one of [24] to [31] above,
A resin film laminated on the interlayer sheet;
An optical laminate comprising:
[33] The interlayer sheet according to any one of [24] to [31] above,
a release liner covering at least one surface of the interlayer sheet;
An interlayer sheet with a release liner comprising:

[34] A self-luminous element;
a low refractive index layer disposed on a viewing side of the self-luminous element;
a high refractive index pressure-sensitive adhesive layer laminated in direct contact with the low refractive index layer;
Including,
The high refractive index pressure-sensitive adhesive layer has a refractive index _n1 of more than 1.570, a total light transmittance of 86% or more, and a haze value of 3.0% or less.
[35] The light emitting device according to the above [34], wherein the ratio (n ₁ /n ₂ ) of the refractive index n ₁ of the high refractive index adhesive layer to the refractive index n ₂ of the low refractive index layer is 1.05 or more.
[36] The light emitting device according to the above [35] or [36], wherein the high refractive index adhesive layer has an arithmetic mean roughness Ra of a surface of 100 nm or less.
[37] The light emitting device according to any one of the above [34] to [36], wherein the ratio (T ₁ /T ₂ ) of the thickness T ₁ of the high refractive index adhesive layer to the thickness T ₂ of the low refractive index layer is 0.5 to 5.
[38] The light emitting device according to any one of [34] to [37] above, wherein the high refractive index adhesive layer has a thickness _T1 of 5 μm or more.
[39] The light-emitting device according to any one of [34] to [38] above, wherein the laminate sheet composed of the high refractive index adhesive layer and the low refractive index layer has a total light transmittance of 86% or more and a haze value of 3.0% or less.
[40] The light-emitting device according to any one of [34] to [39] above, wherein the high refractive index adhesive layer is a layer formed from the adhesive composition according to any one of [11] to [18] above.
[41] The light emitting device according to any one of [34] to [40] above, wherein the high refractive index adhesive layer is an adhesive layer in the adhesive sheet according to any one of [1] to [5] above.
[42] The light emitting device according to any one of [34] to [41] above, wherein the low refractive index layer is a layer constituted by the pressure-sensitive adhesive according to any one of [101] to [104] above.
[43] The interlayer sheet according to any one of [24] to [31] above, wherein the viscoelastic layer _V2 is a layer constituted by the pressure-sensitive adhesive according to any one of [101] to [104] above.

Below, we will explain some experimental aspects related to the present invention. In the following explanation, "parts" and "%" representing the amount used and the content are based on weight unless otherwise specified.

<Preparation of Pressure-Sensitive Adhesive Composition C1>
In a four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube and a cooler, 99.0 parts of 1H,1H,2H,2H-tridecafluorooctyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., trade name "Viscoat 13F") as a monomer component, 1.0 parts of 4-hydroxybutyl acrylate (4HBA), 0.2 parts of 2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator, and 200 parts of ethyl acetate as a polymerization solvent were charged, and nitrogen gas was introduced while gently stirring, and the liquid temperature in the flask was kept at around 60 ° C. for 9 hours to carry out a polymerization reaction, and a solution of acrylic polymer P1 (33%) was prepared. The weight average molecular weight (Mw) of this acrylic polymer P1 was 800,000.
The above-mentioned solution of acrylic polymer P1 (33%) was diluted with ethyl acetate to 30%. 10 parts (0.1 parts non-volatile content) of a 1% ethyl acetate solution of an isocyanurate of hexamethylene diisocyanate (manufactured by Tosoh Corporation, product name "Coronate HX", a trifunctional isocyanate compound) as a crosslinking agent was added per 100 parts non-volatile content (solid content), and the mixture was stirred and mixed to prepare a pressure-sensitive adhesive composition C1.

<Preparation of Pressure-Sensitive Adhesive Compositions C2 to C10>
Solutions of acrylic polymers P2 to P10 were prepared in the same manner as in the preparation of the solution of acrylic polymer P1, except that the composition of the monomer components was changed as shown in Table 1.
Pressure-sensitive adhesive compositions C2 to C10 were prepared in the same manner as for the preparation of pressure-sensitive adhesive composition C1, except that the solution of acrylic polymer P1 was replaced with the solutions of acrylic polymers P2 to P10, respectively.

In the composition of the monomer components shown in Table 1, "V13F" represents 1H,1H,2H,2H-tridecafluorooctyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., product name "Viscoat 13F"), "V8F" represents 1H,1H,5H-octafluoropentyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., product name "Viscoat 8F"), and "V3F" represents 2,2,2-trifluoroethyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., product name "Viscoat 3F"). 2EHA represents 2-ethylhexyl acrylate, NVP represents N-vinyl-2-pyrrolidone, IBXA represents isobornyl acrylate, and CHA represents cyclohexyl acrylate.

<Preparation of Adhesive Sheet (1)>
(Example 1)
The pressure-sensitive adhesive composition C1 prepared above was applied to the silicone-treated surface of a polyethylene terephthalate (PET) film R1 (thickness 50 μm) with one side silicone-treated, and heated at 130° C. for 2 minutes to form a pressure-sensitive adhesive layer with a thickness of 25 μm. The silicone-treated surface of a PET film R2 (thickness 38 μm) with one side silicone-treated was attached to the surface of the pressure-sensitive adhesive layer. In this way, a pressure-sensitive adhesive layer (substrate-less double-sided pressure-sensitive adhesive sheet) was obtained in which both sides were protected by PET films (release liners) R1 and R2. The release liner R2 has a relatively light release compared to the release liner R1.

(Examples 2 to 10)
PSA layers (substrate-less double-sided PSA sheets) according to Examples 2 to 10 were prepared in the same manner as in Example 1, except that PSA compositions C2 to C10 were used instead of PSA composition C1, respectively.

The resulting adhesive sheet was allowed to fully acclimate to an environment of 23°C and 50% RH before being used for the following measurements and evaluations.

<Measurement and Evaluation (1)>
(Refractive Index)
The refractive index of each pressure-sensitive adhesive layer was measured using an Abbe refractometer (manufactured by ATAGO, model "DR-M4") at a measurement wavelength of 589 nm and a measurement temperature of 25° C. The results are shown in Table 1.

(Storage Modulus G')
Each pressure-sensitive adhesive layer was laminated to a thickness of about 1.5 mm to prepare a measurement sample. Dynamic viscoelasticity measurement was performed under the following conditions using an ARES manufactured by TA Instruments. From the measurement results, the storage modulus G' at 25°C was read. The results are shown in Table 1.
[Measurement conditions]
Deformation mode: Torsion Measurement frequency: 1 Hz
Heating rate: 5°C/min. Shape: Parallel plate 7.9mmφ

(Total light transmittance and haze value)
The pressure-sensitive adhesive sheet 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 specimen, and the total light transmittance and haze of the test specimen were measured in a measurement environment of 23° C. using a haze meter (manufactured by Murakami Color Research Laboratory, product name "HAZEMETER HM-150"). The total light transmittance and haze of the alkali-free glass were subtracted from the measured values to obtain the total light transmittance and haze value of the pressure-sensitive adhesive sheet. The results are shown in Table 1.

(Glass plate peel strength)
In a measurement environment of 23°C and 50% RH, the release liner was peeled off from one side of the pressure-sensitive adhesive sheet according to each example, and a 50 μm thick PET film was attached to the backing, and then the sheet was cut to a size of 25 mm wide and 100 mm long to prepare a test piece. The release liner on the other side was peeled off from the test piece, and the sheet was pressed against the surface of an alkaline glass plate (manufactured by Matsunami Glass Industry Co., Ltd., thickness 1.35 mm, blue plate edge polished product) as an adherend by moving a 2 kg roller back and forth once. This was left in the same environment for 30 minutes, then placed in a pressure degassing apparatus (autoclave) and autoclaved for 30 minutes under conditions of a temperature of 50°C and a pressure of 0.5 MPa, and further left for 24 hours in an atmosphere of 23°C and 50% RH. After that, a universal tension and compression tester was used to measure the peel strength (adhesive strength) [N/25 mm] in accordance with JIS Z 0237:2000 at a tension speed of 300 mm/min and a peel angle of 180 degrees. As the universal tension and compression tester, a "Tension and compression tester, TG-1kN" manufactured by Minebea Co., Ltd. was used.

As shown in Table 1, the pressure-sensitive adhesive sheets of Examples 1 to 9 all had a refractive index of 1.46 or less and a storage modulus G'(25) of 400 kPa. These pressure-sensitive adhesive sheets exhibited high transparency and a practical peel strength suitable for bonding optical components.

<Preparation of Pressure-Sensitive Adhesive Composition C11>
In a four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube and a cooler, 65 parts of 2EHA as monomer components, 30 parts of 1H,1H,5H-octafluoropentyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., trade name: Viscoat 8F), 3 parts of N-vinyl-2-pyrrolidone (NVP, manufactured by Nippon Shokubai) and 2 parts of 4HBA, 0.2 parts of AIBN as a polymerization initiator, and 200 parts of ethyl acetate as a polymerization solvent were charged, and nitrogen gas was introduced while gently stirring, and the liquid temperature in the flask was kept at around 60 ° C. for 9 hours to carry out a polymerization reaction, and a solution (33%) of acrylic polymer P11 was prepared. The weight average molecular weight (Mw) of the acrylic polymer P11 was 550,000.
The above-mentioned solution (33%) of the acrylic polymer P11 was diluted with ethyl acetate to 30%, and 10 parts (0.1 parts non-volatile content) of a 1% ethyl acetate solution of an isocyanurate of hexamethylene diisocyanate (manufactured by Tosoh Corporation, product name "Coronate HX", a trifunctional isocyanate compound) as a crosslinking agent was added per 100 parts non-volatile content (solid content), and the mixture was stirred and mixed to prepare a pressure-sensitive adhesive composition C11.

<Preparation of Pressure-Sensitive Adhesive Composition C12>
In a four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube and a cooler, 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 abbreviated as "POB-A") as a monomer component, 5 parts of 4HBA, 0.2 parts of AIBN as a polymerization initiator, and 100 parts of toluene as a polymerization solvent were charged, and nitrogen gas was introduced while gently stirring. The liquid temperature in the flask was kept at around 60°C, and a polymerization reaction was carried out for 6 hours to prepare a solution (50%) of acrylic polymer P12. The weight average molecular weight (Mw) of this acrylic polymer P12 was 500,000. The acrylic polymer P12 has a Tg based on the composition of the monomer components (ie, Tg _T ) of −35° C., and a Tg based on the composition of the aromatic ring-containing monomer (ie, Tg _m1 ) of −35° C.
The above-mentioned solution (50%) of acrylic polymer P12 was diluted to 30% with ethyl acetate, and 334 parts of this solution (100 parts non-volatile content) was mixed with 10 parts (0.1 parts non-volatile content) of a 1% ethyl acetate solution of an isocyanurate of hexamethylene diisocyanate (manufactured by Tosoh Corporation, product name "Coronate HX", a trifunctional isocyanate compound) as a crosslinking agent, 2 parts of acetylacetone as a crosslinking retarder, and 1 part (0.01 parts non-volatile content) of a 1% ethyl acetate solution of ferric nursem as a crosslinking catalyst, and the mixture was stirred to prepare adhesive composition C12.

<Preparation of Pressure-Sensitive Adhesive Composition C13>
In a four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube and a cooler, 72 parts of POB-A as monomer components, 23 parts of 1-naphthyl methyl acrylate (manufactured by Kyoeisha Chemical Co., Ltd., trade name "Light Acrylate NMT-A", refractive index: 1.595, homopolymer Tg: 31 ° C., hereinafter abbreviated as "NMT-A"), 5 parts of 4HBA, 0.2 parts of AIBN as a polymerization initiator, and 100 parts of toluene as a polymerization solvent were charged, and nitrogen gas was introduced while gently stirring, and the liquid temperature in the flask was kept at around 60 ° C. for 6 hours to carry out a polymerization reaction, to prepare a solution (50%) of acrylic polymer P13. The weight average molecular weight (Mw) of this acrylic polymer P13 was 500,000.
Into a separable flask equipped with a thermometer, a stirrer, a reflux condenser, and a nitrogen gas inlet tube, 20 parts of POB-A as monomer components, 80 parts of NMT-A, 0.2 parts of AIBN as a polymerization initiator, 3.5 parts of α-thioglycerol as a chain transfer agent, and 67 parts of methyl ethyl ketone were charged, and then nitrogen gas was introduced and nitrogen substitution was performed for about 1 hour while stirring. Thereafter, the flask was heated to 70°C and reacted for 12 hours to obtain an acrylic oligomer (oligomer B) having a weight average molecular weight (Mw) of 4000 and a refractive index of 1.63.
The above-mentioned acrylic polymer C13 solution (50%) was diluted to 30% with ethyl acetate, and 334 parts of this solution (100 parts non-volatile content) was mixed with 20 parts of the above-prepared oligomer B, 10 parts (0.1 parts non-volatile content) of a 1% ethyl acetate solution of an isocyanurate of hexamethylene diisocyanate (manufactured by Tosoh Corporation, product name "Coronate HX", a trifunctional isocyanate compound) as a crosslinking agent, 2 parts of acetylacetone as a crosslinking retarder, and 1 part (0.01 parts non-volatile content) of a 1% ethyl acetate solution of ferric nursem as a crosslinking catalyst, and the mixture was stirred to prepare a pressure-sensitive adhesive composition C13.

<Preparation of Adhesive Sheet (2)>
(Example 11)
The pressure-sensitive adhesive composition C11 prepared above was applied to the silicone-treated surface of a release liner R1 (thickness 50 μm) and heated at 130° C. for 2 minutes to form a pressure-sensitive adhesive layer having a thickness of 10 μm. The silicone-treated surface of a release liner R2 (thickness 38 μm) was attached to the surface of the pressure-sensitive adhesive layer. In this way, a pressure-sensitive adhesive layer (substrate-less double-sided pressure-sensitive adhesive sheet) formed from the pressure-sensitive adhesive composition C11 was obtained, the both sides of which were protected by PET films (release liners) R1 and R2.

(Example 12)
The pressure-sensitive adhesive composition C12 prepared above was applied to the silicone-treated surface of the release liner R1 and heated at 130° C. for 2 minutes to form a pressure-sensitive adhesive layer having a thickness of 25 μm. The silicone-treated surface of the release liner R2 was attached to the surface of the pressure-sensitive adhesive layer. In this way, a pressure-sensitive adhesive layer (substrate-less double-sided pressure-sensitive adhesive sheet) formed from the pressure-sensitive adhesive composition C12 was obtained, with both sides protected by PET films (release liners) R1 and R2.
The release liner R2 was peeled off from each of the obtained adhesive layer and the adhesive layer prepared in Example 11, and the adhesive surfaces were attached to each other and pressed with a hand roller. This laminate was autoclaved for 30 minutes under conditions of 50°C and 0.60 MPa, and then aged in an environment of 50°C for 48 hours. In this way, a laminate sheet (substrate-less double-sided adhesive sheet) consisting of a two-layer structure of an adhesive layer (high refractive index adhesive layer) formed from adhesive composition C12/an adhesive layer (low refractive index adhesive layer) formed from adhesive composition C11 was obtained. The surface of this adhesive sheet is protected by two release liners R1,

(Example 13)
A laminated sheet (substrate-less double-sided adhesive sheet) consisting of a two-layer structure of a high refractive index adhesive layer/a low refractive index adhesive layer was obtained in the same manner as in Example 12, except that the type of adhesive composition used to form each adhesive layer and the thickness of each adhesive layer were changed as shown in Table 2.

(Examples 14 and 15)
In the same manner as in Example 13, single-layered pressure-sensitive adhesive layers were prepared from each of the pressure-sensitive adhesive compositions C12 and C13 and had the thicknesses shown in Table 2, to give pressure-sensitive adhesive sheets according to Examples 14 and 15.

<Measurement and Evaluation (2)>
The pressure-sensitive adhesive sheets obtained in Examples 11 to 15 were thoroughly acclimatized to an environment of 23°C and 50% RH, and then each item was measured and evaluated in the same manner as in "Measurement and Evaluation (1)" described above. For substrate-less double-sided pressure-sensitive adhesive sheets having a two-layer structure of a high refractive index pressure-sensitive adhesive layer/a low refractive index pressure-sensitive adhesive layer, the peel strength of the high refractive index pressure-sensitive adhesive layer side was measured. The results are shown in Table 2.

As shown in Table 2, the adhesive sheets of Examples 11 to 13 contained an adhesive layer with a refractive index of 1.46 or less, and exhibited high transparency. These adhesive sheets exhibited practical peel strength suitable for bonding optical components.

<Evaluation of front luminance improvement effect>
The adhesive sheets according to each of Examples 11 to 15 were attached onto a white LED light source, and the light source was turned on and stabilized for 30 minutes or more in a darkroom environment, and then the front luminance of the portion where the adhesive sheet was attached was measured using a spectroradiometer SR-UL1R (manufactured by Topcon Technohouse Corporation). Using the average value of the luminance measured three times, those that had a luminance improvement effect of 10% or more compared to the luminance of the light source without the adhesive sheet attached were evaluated as G (Good), and those that had a luminance improvement of less than 10% were evaluated as P (Poor). The results are shown in Table 3.

As shown in Table 3, the adhesive sheets (laminate sheets) of Examples 12 and 13, which have a laminated adhesive layer combining a low refractive index adhesive layer formed from adhesive composition C11 and a high refractive index adhesive layer formed from adhesive composition C12 or C13, showed a front brightness improvement effect of 10% or more compared to when the adhesive sheet was not used. In the adhesive sheets of Examples 11, 14, and 15, which have a single-layer adhesive layer, no front brightness improvement effect was observed due to the adhesive sheet alone.

<Preparation of Pressure-Sensitive Adhesive Composition C14>
In a four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube and a cooler, 79 parts of POB-A as monomer components, 20 parts of n-butyl acrylate, 1 part of 4HBA, 0.2 parts of AIBN as a polymerization initiator, and 100 parts of toluene as a polymerization solvent were charged, and nitrogen gas was introduced while gently stirring. The liquid temperature in the flask was kept at about 60°C, and a polymerization reaction was carried out for 6 hours to prepare a solution (50%) of acrylic polymer P14. The Mw of this acrylic polymer P14 was 520,000.
The above-mentioned solution of acrylic polymer P14 (50%) was diluted to 30% with ethyl acetate, and 334 parts of this solution (100 parts non-volatile content) was mixed with 10 parts of a 1% ethyl acetate solution of Coronate HX as a crosslinking agent (0.1 parts non-volatile content), 2 parts of acetylacetone as a crosslinking retarder, and 1 part of a 1% ethyl acetate solution of Nursem Ferric as a crosslinking catalyst (0.01 parts non-volatile content) and stirred to prepare adhesive composition C14.

<Preparation of Pressure-Sensitive Adhesive Composition C15>
A solution of acrylic polymer P15 (50%) was prepared in the same manner as in the preparation of the solution of acrylic polymer P14, except that the composition (weight ratio) of the monomer components was changed to POB-A/ethyl carbitol acrylate (CBA)/4HBA=79/20/1. The Mw of this acrylic polymer P15 was 460,000. A pressure-sensitive adhesive composition C15 was prepared in the same manner as in the preparation of pressure-sensitive adhesive composition C14, except that a solution of acrylic polymer P15 was used instead of the solution of acrylic polymer P14.

<Preparation of Pressure-Sensitive Adhesive Composition C16>
A solution of acrylic polymer P16 (50%) was prepared in the same manner as the preparation of the solution of acrylic polymer P14, except that the composition (weight ratio) of the monomer components was changed to P2H-A/4HBA=99/1. "P2H-A" in the composition of the monomer components represents phenoxydiethylene glycol acrylate (manufactured by Kyoeisha Chemical Co., Ltd., product name "Light Acrylate P2H-A", refractive index: 1.510, Tg of homopolymer: -35°C). The Mw of this acrylic polymer P16 was 1 million.
A solution (50%) of acrylic polymer P16 was diluted to 30% with ethyl acetate, and 334 parts of this solution (100 parts non-volatile content) was mixed with 20 parts of 6- _{ethylacrylate} -dinaphtho[2,1-b:1',2'-d]thiophene (6-acryloyloxyethyldinaphthothiophene, abbreviation: 6EDNTA, refractive index: 1.722, manufactured by Sugai Chemical Industry Co., Ltd.) as an additive (H RO ), 10 parts of a 1% ethyl acetate solution of Coronate HX as a crosslinking agent (0.1 parts non-volatile content), 2 parts of acetylacetone as a crosslinking retarder, and 1 part of a 1% ethyl acetate solution of Nursem Ferric as a crosslinking catalyst (0.01 parts non-volatile content), and the mixture was stirred to prepare a pressure-sensitive adhesive composition C16.

<Preparation of adhesive sheet>
(Examples 16 to 18)
A laminate sheet (substrate-less double-sided pressure-sensitive adhesive sheet) having a two-layer structure of a high refractive index pressure-sensitive adhesive layer/a low refractive index pressure-sensitive adhesive layer was obtained in the same manner as in Example 12, except that the type of pressure-sensitive adhesive composition used to form each pressure-sensitive adhesive layer and the thickness of each pressure-sensitive adhesive layer were as shown in Table 4. The Mw of the acrylic polymer P4, which is the base polymer of pressure-sensitive adhesive composition C4, is 550,000.

<Measurement and Evaluation (3)>
The PSA sheets obtained in Examples 16 to 18 were thoroughly acclimatized to an environment of 23° C. and 50% RH, and then the respective items were measured and evaluated in the same manner as in “Measurement and Evaluation (2)” described above. The results are shown in Table 4.

As shown in Table 4, the laminated sheets of Examples 16 to 18, which have a configuration in which a low refractive index adhesive layer formed from adhesive composition C4 is laminated with an adhesive layer having a refractive index 0.01 or more (more specifically, 0.10 or more) higher than that of the low refractive index adhesive layer, all exhibited high transparency. These adhesive sheets exhibited practical peel strength suitable for bonding optical components.

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 Adhesive sheet 2 Adhesive sheet (laminate sheet)
10 Adhesive layer 10A First surface (first adhesive surface)
10B Second surface (second adhesive surface)
11 First pressure-sensitive adhesive layer (high refractive index pressure-sensitive adhesive layer)
12 Second pressure-sensitive adhesive layer (low refractive index pressure-sensitive adhesive layer)
70 Self-luminous element 80 Cover window member 100 Light-emitting device

Claims (8)

a low refractive index pressure-sensitive adhesive layer formed from a pressure-sensitive adhesive containing an acrylic polymer (F) containing a fluorine-containing acrylic monomer (M1) and a hydroxyl group-containing monomer as monomer units;
A high refractive index adhesive layer laminated on the low refractive index adhesive layer,
the content of the hydroxyl group-containing monomer in the monomer components constituting the acrylic polymer (F) is 0.5% by weight or more and 15% by weight or less,
The refractive index of the adhesive is 1.46 or less, and
The pressure-sensitive adhesive has a storage modulus G' at 25°C of 1.0 kPa or more and 400 kPa or less. 2. The laminate sheet according to claim 1, wherein the content of the fluorine-containing acrylic monomer (M1) in the monomer components constituting the acrylic polymer (F) is 25% by weight or more. The laminate sheet according to claim 1 or 2, wherein the fluorine-containing acrylic monomer (M1) comprises a fluorine atom-containing alkyl (meth)acrylate ester. The laminate sheet according to any one of claims 1 to 3 , wherein the ratio (n ₁ /n ₂ ) of the refractive index n ₁ of the high refractive index pressure-sensitive adhesive layer to the refractive index n 2 of the low refractive index pressure-sensitive adhesive layer is 1.02 or more. The laminate sheet according to any one of claims 1 to 4 , wherein the refractive index _n1 of the high refractive index pressure-sensitive adhesive layer is greater than 1.570. The laminate sheet according to any one of claims 1 to 5 , wherein the adhesive constituting the high refractive index adhesive layer has a storage modulus G' at 25°C of 700 kPa or less. The laminate sheet according to any one of claims 1 to 6 , which has a total light transmittance of 86% or more and a haze value of 3.0% or less. A self-emitting element;
A laminate sheet according to any one of claims 1 to 7 ,
the laminate sheet is disposed on a more visible side than the self-luminous elements.

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